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From the leaves of a botanically and phytochemically as yet unexplored Ancistrocladus liana discovered in the rainforests of the Central region of the Democratic Republic of the Congo in the vicinity of the town of Ikela, six new naphthylisoquinoline alkaloids were isolated, viz., two constitutionally unsymmetric dimers, the mbandakamines B\(_3\) (3) and B\(_4\) (4), and four related 5,8′-linked monomeric alkaloids, named ikelacongolines A–D (5a, 5b, 6, and 7). The dimers 3 and 4 are structurally unusual quateraryls comprising two 5,8′-coupled monomers linked via a sterically strongly constrained 6′,1′′-connection between their naphthalene units. These compounds contain seven elements of chirality, four stereogenic centers and three consecutive chiral axes. They were identified along with two known related compounds, the mbandakamines A (1) and B\(_2\) (2), which had so far only been detected in two Ancistrocladus species indigenous to the Northwestern Congo Basin. In addition, five known monomeric alkaloids, previously found in related Central African Ancistrocladus species, were isolated from the here investigated Congolese liana, three of them belonging to the subclass of 5,8′-coupled naphthylisoquinoline alkaloids, whereas two compounds exhibited a less frequently occurring 7,8′-biaryl linkage. The stereostructures of the new alkaloids were established by spectroscopic (in particular HRESIMS, 1D and 2D NMR), chemical (oxidative degradation), and chiroptical (electronic circular dichroism) methods. The mbandakamines B\(_3\) (3) and B\(_4\) (4) displayed pronounced activities in vitro against the malaria parasite Plasmodium falciparum and the pathogen of African sleeping sickness, Trypanosoma brucei rhodesiense.
Rapid multiple-quantum three-dimensional fluorescence spectroscopy disentangles quantum pathways
(2019)
Coherent two-dimensional spectroscopy is a powerful tool for probing ultrafast quantum dynamics in complex systems. Several variants offer different types of information but typically require distinct beam geometries. Here we introduce population-based three-dimensional (3D) electronic spectroscopy and demonstrate the extraction of all fourth- and multiple sixth-order nonlinear signal contributions by employing 125-fold (1⨯5⨯5⨯5) phase cycling of a four-pulse sequence. Utilizing fluorescence detection and shot-to-shot pulse shaping in single-beam geometry, we obtain various 3D spectra of the dianion of TIPS-tetraazapentacene, a fluorophore with limited stability at ambient conditions. From this, we recover previously unknown characteristics of its electronic two-photon state. Rephasing and nonrephasing sixth-order contributions are measured without additional phasing that hampered previous attempts using noncollinear geometries. We systematically resolve all nonlinear signals from the same dataset that can be acquired in 8 min. The approach is generalizable to other incoherent observables such as external photoelectrons, photocurrents, or photoions.
Although known about and investigated since the late 1970’s, the picture of the basic principles governing inhibitor strengths and the structure-activity relationships of the cysteine protease inhibition mechanism is still very incomplete. Computational approaches can be a very useful tool for investigating such questions, as they allow the inspection of single, specific effects in isolation from all others, in a manner very difficult to achieve experimentally. The ab initio treatments of such large systems like proteins are still not feasible. However, there is a vast number of computational approaches capable of dealing with protein structures with reasonable accuracy. This work presents a summary of theoretical investigations into cysteine protease cathepsin B using a range of methods. We have concentrated on the investigation of cysteine protease inhibition by epoxide- and aziridine-based inhibitors in order to obtain better insight into these important topics. Various model systems are simulated by means of pure quantum mechanical methods and by hybrid (QM/MM) methods. Both approaches provide a static picture. Dynamical effects are then accounted for by additional molecular dynamics (MD) simulations, using both classical and QM/MM MD approaches. The quantum mechanical approach was used to study very small model systems consisting only of the electrophilic warhead of the inhibitor (both substitituted and not) and molecular moieties simulating a very simplified protein active site (methylthiolate instead of Cys29 and methylimidazolium instead of His199 residue) and solvent surroundings (two waters or two ammonium ions, in combination with a continuum solvent model). Although simple, such a system provides a good description of the most important interactions involved in the inhibition reaction. It also allows investigation of the influence of the properties of the electrophilic warhead on the reaction rate. Beside the properties of the electrophilic warhead, the protein and solvent environment is also an important factor in the irreversible deactivation of the enzyme active site by the inhibitor. The non-covalent interactions of the inhibitor with the oxyanion hole and other subsites of the enzyme, as well as its interaction with the solvent molecules, need to be explicitly taken into account in the calculations, because of their possible impact on the reaction profile. As molecular modeling methods allow the treatment of such large systems, but lack the possibility of describing covalent interactions, our method of choice was the combined quantum mechanics/molecular modeling approach. By splitting the system into a smaller part that undergoes the bond cleavage/formation process and must be treated quantum mechanically, and a larger part, comprised of the rest of the protein, which could be treated using force fields, we managed to simulate the system at the desired precision. Our investigations concentrated on the role of His199 in the inhibition mechanism as well as on the structure-reactivity relationships between cysteine protease and various inhibitors, yielding new insight into the kinetics, regio- and stereospecificity of the inhibition. In particular, our calculations provide the following insights: i.) an explanation for the regioselectivity of the reaction, and original insight into which interactions affect the stereoselectivity; ii.) a clear model which explains the known structure-activity relationships and connects these effects with the pH-dependency of the inhibition; iii.) our computations question the generally accepted two-step model by showing that substituent effects accelerate the irreversible step to such an extent that the achievement of an equilibrium in the first step is doubtful; iv.) by way of theoretical characterizations of aziridine models, the reasons for similarities and differences in the mode of action of epoxide- and aziridine-based inhibitors are elucidated; and finally, v.) combining our results with experimental knowledge will allow rational design of new inhibitors. To account for dynamical effects as well, molecular dynamics (MD) computations were also performed. In these calculations the potential energy was computed at the force field level. The results not only supported and clarified the QM/MM results, but comparison with previous X-ray structures helped correct existing errors in the available geometrical models and resolved inconsistencies in the weighting of various factors governing the inhibition. In the work the first QM/MM MD calculations on the active site of the cysteine proteases are presented. In contrast to the MD simulations, these calculations used potential energies computed at the QM/MM-level. With the help of these computations we sought to address strongly disputed questions about the reasons for the existence of the active site ion pair and its role in the high activity of the enzyme.
Fluorogenic RNA aptamers are synthetic functional RNAs that specifically bind and activate conditional fluorophores. The Chili RNA aptamer mimics large Stokes shift fluorescent proteins and exhibits high affinity for 3,5-dimethoxy-4-hydroxybenzylidene imidazolone (DMHBI) derivatives to elicit green or red fluorescence emission. Here, we elucidate the structural and mechanistic basis of fluorescence activation by crystallography and time-resolved optical spectroscopy. Two co-crystal structures of the Chili RNA with positively charged DMHBO+ and DMHBI+ ligands revealed a G-quadruplex and a trans-sugar-sugar edge G:G base pair that immobilize the ligand by π-π stacking. A Watson-Crick G:C base pair in the fluorophore binding site establishes a short hydrogen bond between the N7 of guanine and the phenolic OH of the ligand. Ultrafast excited state proton transfer (ESPT) from the neutral chromophore to the RNA was found with a time constant of 130 fs and revealed the mode of action of the large Stokes shift fluorogenic RNA aptamer.
Fluorogenic RNA aptamers are synthetic functional RNAs that specifically bind and activate conditional fluorophores. The Chili RNA aptamer mimics large Stokes shift fluorescent proteins and exhibits high affinity for 3,5-dimethoxy-4-hydroxybenzylidene imidazolone (DMHBI) derivatives to elicit green or red fluorescence emission. Here, we elucidate the structural and mechanistic basis of fluorescence activation by crystallography and time-resolved optical spectroscopy. Two co-crystal structures of the Chili RNA with positively charged DMHBO+ and DMHBI+ ligands revealed a G-quadruplex and a trans-sugar-sugar edge G:G base pair that immobilize the ligand by π-π stacking. A Watson-Crick G:C base pair in the fluorophore binding site establishes a short hydrogen bond between the N7 of guanine and the phenolic OH of the ligand. Ultrafast excited state proton transfer (ESPT) from the neutral chromophore to the RNA was found with a time constant of 130 fs and revealed the mode of action of the large Stokes shift fluorogenic RNA aptamer.
Main objectives of the present dissertation can be divided in two parts. The first part deals with setting up a spectroscopic technique for reliable and accurate measurements of the two-photon absorption (2PA) cross section spectra. In the second part, this firmly established experimental technique together with conventional spectroscopic characterization, quantum-chemical computations and theoretical modelling calculations was combined and therefore used as a tool to gain information for the so-called structure-property relationship through several molecular compounds.
Herein we report a broad series of new trinuclear supramolecular Ru(bda) macrocycles bearing different substituents at the axial or equatorial ligands which enabled investigation of substituent effects on the catalytic activities in chemical and photocatalytic water oxidation. Our detailed investigations revealed that the activities of these functionalized macrocycles in water oxidation are significantly affected by the position at which the substituents were introduced. Interestingly, this effect could not be explained based on the redox properties of the catalysts since these are not markedly influenced by the functionalization of the ligands. Instead, detailed investigations by X-ray crystal structure analysis and theoretical simulations showed that conformational changes imparted by the substituents are responsible for the variation of catalytic activities of the Ru macrocycles. For the first time, macrocyclic structure of this class of water oxidation catalysts is unequivocally confirmed and experimental indication for a hydrogen-bonded water network present in the cavity of the macrocycles is provided by crystal structure analysis. We ascribe the high catalytic efficiency of our Ru(bda) macrocycles to cooperative proton abstractions facilitated by such a network of preorganized water molecules in their cavity, which is reminiscent of catalytic activities of enzymes at active sites.
In light of the rapidly increasing global demand of energy and the negative effects of climate change, innovative solutions that allow an efficient transition to a carbon-neutral economy are urgently needed. In this context, artificial photosynthesis is emerging as a promising technology to enable the storage of the fluctuating energy of sunlight in chemical bonds of transportable “solar fuels”. Thus, in recent years much efforts have been devoted to the development of robust water oxidation catalysts (WOCs) leading to the discovery of the highly reactive Ru(bda) (bda: 2,2’-bipyridine-6,6’-dicarboxylic acid) catalyst family. The aim of this thesis was the study of chemical and photocatalytic water oxidation with functionalized Ruthenium macrocycles to explore the impact of substituents on molecular properties and catalytic activities of trinuclear macrocyclic Ru(bda) catalysts. A further objective of this thesis comprises the elucidation of factors that influence the light-driven water oxidation process with this novel class of supramolecular WOCs.
We have investigated theoretically the importance of the O(\(^3\)P)+CH(a\(^4\sum^-\)) and the O(\(^3\)P)+CH(X\(^2\Pi\)) channels in the collinear chemi-ionization reaction O+CH->HCO\(^+\) +e\(^-\). We have found that both channels may lead to chemi-ionization via favorable Franck-Condon overlaps with the states ofthe ionic species.
A new flavanone-chromone biflavonoid, preussianone (1), has been isolated from the leaves of Garcinia preussii, along with four known biflavonoids. The absolute stereostructures were elucidated by chemical, spectroscopic, and chiroptical methods. The biological properties of the new biflavonoid against several bacterial strains were evaluated.
Detection of metals in different environments with high selectivity and specificity is one of the prerequisites of the fight against environmental pollution with these elements. Pyrenes are well suited for the fluorescence sensing in different media. The applied sensing principle typically relies on the formation of intra- and intermolecular excimers, which is however limiting the sensitivity range due to masking of e. g. quenching effects by the excimer emission. Herein we report a highly selective, structurally rigid chemical sensor based on the monomer fluorescence of pyrene moieties bearing triazole groups. This sensor can quantitatively detect Cu\(^{2+}\), Pb\(^{2+}\) and Hg\(^{2+}\) in organic solvents over a broad concentrations range, even in the presence of ubiquitous ions such as Na\(^{+}\), K\(^{+}\), Ca\(^{2+}\) and Mg\(^{2+}\). The strongly emissive sensor's fluorescence with a long lifetime of 165 ns is quenched by a 1 : 1 complex formation upon addition of metal ions in acetonitrile. Upon addition of a tenfold excess of the metal ion to the sensor, agglomerates with a diameter of about 3 nm are formed. Due to complex interactions in the system, conventional linear correlations are not observed for all concentrations. Therefore, a critical comparison between the conventional Job plot interpretation, the method of Benesi-Hildebrand, and a non-linear fit is presented. The reported system enables the specific and robust sensing of medically and environmentally relevant ions in the health-relevant nM range and could be used e. g. for the monitoring of the respective ions in waste streams.
Nanodiamond (ND) is a versatile and promising material for bio-applications. Despite many efforts, agglomeration of nanodiamond and the non-specific adsorption of proteins on the ND surface when exposed to bio-fluids remains a major obstacle for biomedical applications. An assortment of branched and linear molecules with superior ability to colloidally stabilize nanoparticles in salt and cell media environment, for up to 30 days, was attached to the ND’s surface.
The building box system with azide as external groups offers a huge variety of binding with many molecules, such as drugs, dyes or targeting molecules, is possible. Clicking, for instance, zwitterions moieties to the chain protects ND surface from protein corona forming when the particles get in contact with biofluids containing proteins.
Thermogravimetric analysis results of the ND surface loading show a significant prevention of up to 98 % of the protein adsorption compared with NDs without zwitterionic headgroups and long colloidal stability when tetraethylene glycol (TEG) are attached to the surface.
The versatility of the modular system to bind not only zwitterionic chains but also clickable functional molecules to fluorescent nanodiamonds (fNDs) demonstrates the potential of the system at the nanodiamond. Using defect structures, such as nitrogen-vacancy (NV) centers, diamond particles, due to their widely non-toxic behavior, can be used as fNDs for photostable labeling, bioimaging and nanoscale sensing in living cells and organisms. To functionalize the fND surface a novel milling technique with diazonium salts was established to perform grafting on poorly reactive HPHT fNDs yielding in high surface loading and high negative zeta potential.
Combining the benefits of TEG and zwitterion containing groups with antibody enabled nucleus targeting ability on fND confirms the enhanced colloidal stability in living cells experiments for the first time. Furthermore, the results indicate an improved corona repulsion compared with fND without zwitterion containing headgroups. As a result, the circulation times were enlarged from 4 (fND without zwitterion chain but with antibody) to 17 (with antibody and zwitterion chains) hours.
In non-biomedical applications, the modular system can be used as a probe for heavy metals by binding it to dyes. Detection of metals in different environments with high selectivity and specificity is one of the prerequisites of the fight against environmental pollution with these elements. Pyrenes are well suited and known for fluorescence sensing in different media.
The applied sensing principle typically relies on the formation of intra- and intermolecular excimers, which is however limiting the sensitivity range due to masking of e.g. quenching effects by the excimer emission. This study shows a highly selective, structurally rigid chemical sensor based on the monomer fluorescence of pyrene moieties bearing triazole groups.
This probe can quantitatively detect Cu2+, Pb2+ and Hg2+ in organic solvents over a broad concentration range, even in the presence of ubiquitous ions such as Na+, K+, Ca2+ and Mg2+. The strongly emissive sensor’s fluorescence with a long lifetime of 165 ns is quenched by a 1:1 complex formation upon addition of metal ions in acetonitrile. Upon addition of a tenfold excess of the metal ion to the sensor, agglomerates with a diameter of about 3 nm are formed. Due to complex interactions in the system, conventional linear correlations are not observed for all concentrations. Therefore, a critical comparison between the conventional Job plot interpretation, the method of Benesi-Hildebrand, and a non-linear fit is presented. The reported system enables the specific and robust sensing of medically and environmentally relevant ions in the health-relevant nM range and could be used e.g. for the monitoring of the respective ions in waste streams.
Nonetheless, often these waste streams end up in sensitive aquacultures, where such sensor technology only works if the probe is water-soluble to monitor the spread and formation of environmental damage from heavy metals. Many chemosensors only work quantitatively in specific solvents and under highly pure conditions. In this thesis a method to stabilize water-insoluble chemosensors on nanodiamonds in saline water while maintaining the sensor efficacy and specificityas as well as colloidal stability is presented. Additionally, the sensor capability is retained in organic solvents. This study provides insight into the absorptivity of pyrene derivatives to the nanodiamond surface and a way to reversibly desorb them.
Moreover, the system proves that in presence of 95 % oxygen atmosphere while the fluoresce measurement the results of the do not vary from the one in argon atmosphere. Furthermore, the presence of common ions in water do not disturb the colloidal stability of the NDs and also no influence the sensor functionality and thus is highly promising candidate for measurement without cumbersome preparation steps.
We synthesized new pyrene derivatives with strong bis(para ‐methoxyphenyl)amine donors at the 2,7‐positions and n ‐azaacene acceptors at the K‐region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7‐substituted pyrenes but not for K‐region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin‐Day class‐II and delocalized Robin‐Day class‐III species.
We synthesized a series of new mono‐, di‐, tri‐ and tetra‐substituted perylene derivatives with strong bis(para‐methoxyphenyl)amine (DPA) donors at the uncommon 2,5,8,11‐positions. The properties of our new donor‐substituted perylenes were studied in detail to establish a structure‐property relationship. Interesting trends and unusual properties are observed for this series of new perylene derivatives, such as a decreasing charge transfer (CT) character with increasing number of DPA moieties and individual reversible oxidations for each DPA moiety. Thus, (DPA)‐Per possesses one reversible oxidation while (DPA)\(_{4}\)‐Per has four. The mono‐ and di‐substituted derivatives display unusually large Stokes shifts not previously reported for perylenes. Furthermore, transient absorption measurements of the new derivatives reveal an excited state with lifetimes of several hundred microseconds, which sensitizes singlet oxygen with quantum yields of up to 0.83.
In this thesis, intermolecular acceptor-acceptor interactions in organic solar cells based on new non-fullerene acceptors are addressed. For this purpose, first the reproducibility of organic electronic devices was tested on a new facility for their fabrication. This was followed by the screening for new acceptor materials. Based on this, three molecular systems were investigated with regard to their acceptor-acceptor interactions and their influence on solar cell efficiency.
Bowl-shaped naphthalimide-annulated corannulene as nonfullerene acceptor in organic solar cells
(2020)
An electron-poor bowl-shaped naphthalimide-annulated corannulene with branched alkyl residues in the imide position was synthesized by a palladium-catalyzed cross-coupling annulation sequence. This dipolar compound exhibits strong absorption in the visible range along with a low-lying LUMO level at –3.85 eV, enabling n-type charge transport in organic thin-film transistors. Furthermore, we processed inverted bulk-heterojunction solar cells in combination with the two donor polymers PCE–10 and PM6 to achieve open-circuit voltages up to 1.04 V. By using a blend of the self-assembled naphthalimide-annulated corannulene and PCE–10, we were able to obtain a power conversion efficiency of up to 2.1%, which is to the best of our knowledge the highest reported value for a corannulene-based organic solar cell to date.
A series of novel imide‐functionalized C\(_{64}\) nanographenes is investigated as acceptor components in organic solar cells (OSCs) in combination with donor polymer PM6. These electron‐poor molecules either prevail as a monomer or self‐assemble into dimers in the OSC active layer depending on the chosen imide substituents. This allows for the controlled stacking of electron‐poor and electron‐rich π–scaffolds to establish a novel class of non‐fullerene acceptor materials to tailor the bulk‐heterojunction morphology of the OSCs. The best performance is observed for derivatives that are able to self‐assemble into dimers, reaching power conversion efficiencies of up to 7.1%.
Quantum chemical calculations of circular dichroism (CD) spectra in combination with experimental CD studies are one of the most efficient analytical tools for the elucidation of the three-dimensional structure of a chiral molecule. In the present work 18 chiral compounds of most different molecular structures and origins were investigated using various theoretical methods (the semiempirical CIS methods, the time-dependent DFT and DFT/MRCI approaches). The advantages and limitations of the applied methods were discussed in the context of the studied compounds. Furthermore, the last part of this work deals with the CD investigations of a chiral compound in the crystalline state. A well-known natural product with a specific conformation/CD spectrum behavior was used as a model compound to examine a novel solid-state CD method and to investigate the possibility of its improvement to provide a higher reliability for the assignment of the absolute configuration.
The chirality of the interlocked bay-arylated perylene motif is investigated upon its material prospect and the enhancement of its chiroptical response to the NIR spectral region. A considerable molecular library of inherently chiral perylene bisimides (PBIs) was utilized as acceptors in organic solar cells to provide decent device performances and insights into the structure-property relationship of PBI materials within a polymer blend. For the first time in the family of core-twisted PBIs, the effects of enantiopurity on the device performance was thoroughly investigated. The extraordinary structural sensitivity of CD spectroscopy served as crucial analytical tool to bridge the highly challenging gap between molecular properties and device analytics by proving the excitonic chirality of a helical PBI dimer. The chirality of this perylene motif could be further enhanced on a molecular level by both the expansion and the enhanced twisting of the π-scaffold to achieve a desirable strong chiroptical NIR response introducing a new family of twisted QBI-based nanoribbons. These achievements could be substantially further developed by expanding this molecular concept to a supramolecular level. The geometrically demanding supramolecular arrangement necessary for the efficient excitonic coupling was carefully encoded into the molecular design. Accordingly, the QBIs could form the first J-type aggregate constituting a fourfold-stranded superhelix of a rylene bisimide with strong excitonic chirality. Therefore, this thesis has highlighted the mutual corroboration of experimental and theoretical data from the molecular to the supramolecular level. It has demonstrated that for rylene bisimide dyes, the excitonic contribution to the overall chiroptical response can be designed and rationalized. This can help to pave the way for new organic functional materials to be used for
chiral sensing or chiral organic light-emitting devices.
We report the direct imidization of naphthalene and perylene dicarboxylic anhydrides/esters with bulky ortho,ortho‐diaryl‐ and ortho,ortho‐dialkynylaniline derivatives. This imidization method uses n‐butyllithium as a strong base to increase the reactivity of bulky amine derivatives, proceeds under mild reaction conditions, requires only stoichiometric amounts of reactants and gives straightforward access to new sterically crowded rylene dicarboximides. Mechanistic investigations suggest an isoimide as intermediary product, which was converted to the corresponding imide upon addition of an aqueous base. Single‐crystal X‐ray diffraction analyses reveal dimeric packing motifs for monoimides, while two‐side shielded bisimides crystallize in isolated molecules without close π–π‐interactions. Spectroscopic investigations disclose the influence of the bulky substituents on the optical properties in the solid state.
The present thesis introduce different synthetic strategies towards a variety of polycyclic aromatic dicarboximides (PADIs) with highly interesting and diverse properties. This included tetrachlorinated, tetraaryloxy- and tetraaryl-substituted dicarboximides, fused acceptor‒donor(‒acceptor) structures as well as sterically shielded rylene and nanographene dicarboximides. The properties and thus the disclosure of structure‒property relationships of the resulting dyes were investigated in detail among others with UV‒vis absorption spectroscopy, fluorescence spectroscopy, cyclic voltammetry and single crystal X-ray analysis. For instance, some of the fused and substituted PADIs offer strong absorption of visible and near infrared (NIR) light, NIR emission and low-lying LUMO levels. On the contrary, intriguing optical features in the solid-state characterize the rylene dicarboximides with their bulky N-substituents, while the devised sterically enwrapped nanographene host offered remarkable complexation capabilities in solution.
General and efficient tools for site-specific fluorescent or bioorthogonal labeling of RNA are in high demand. Here, we report direct in vitro selection, characterization, and application of versatile trans-acting 2'-5' adenylyl transferase ribozymes for covalent and site-specific RNA labeling. The design of our partially structured RNA pool allowed for in vitro evolution of ribozymes that modify a predetermined nucleotide in cis (i.e. intramolecular reaction), and were then easily engineered for applications in trans (i.e. in an intermolecular setup). The resulting ribozymes are readily designed for specific target sites in small and large RNAs and accept a wide variety of N6-modified ATP analogues as small molecule substrates. The most efficient new ribozyme (FH14) shows excellent specificity towards its target sequence also in the context of total cellular RNA.
In vitro selected ribozymes are promising tools for site-specific labeling of RNA. Previously known nucleic acid catalysts attached fluorescently labeled adenosine or guanosine derivatives through 2’,5’-branched phosphodiester bonds to the RNA of interest. Herein, we report new ribozymes that use orthogonal substrates, derived from the antiviral drug tenofovir, and attach bioorthogonal functional groups, as well as affinity handles and fluorescent reporter units through a hydrolytically more stable phosphonate ester linkage. The tenofovir transferase ribozymes were identified by in vitro selection and are orthogonal to nucleotide transferase ribozymes. As genetically encodable functional RNAs, these ribozymes may be developed for potential cellular applications. The orthogonal ribozymes addressed desired target sites in large RNAs in vitro, as shown by fluorescent labeling of E. coli 16S and 23S RNAs in total cellular RNA.
Polymer micelles are an attractive means to solubilize water insoluble compounds such as drugs. Drug loading, formulations stability and control over drug release are crucial factors for drug‐loaded polymer micelles. The interactions between the polymeric host and the guest molecules are considered critical to control these factors but typically barely understood. Here, we compare two isomeric polymer micelles, one of which enables ultra‐high curcumin loading exceeding 50 wt.%, while the other allows a drug loading of only 25 wt.%. In the low capacity micelles, steady‐state fluorescence revealed a very unusual feature of curcumin fluorescence, a high energy emission at 510 nm. Time‐resolved fluorescence upconversion showed that the fluorescence life time of the corresponding species is too short in the high‐capacity micelles, preventing an observable emission in steady‐state. Therefore, contrary to common perception, stronger interactions between host and guest can be detrimental to the drug loading in polymer micelles.
The present thesis demonstrates the potential of dipolar aggregation of merocyanine dyes as novel directional and specific supramolecular binding motif for the creation of more elaborate supramolecular architectures beyond simple dimers. Furthermore, the self-assembly studies into bis(merocyanine) nanorods gave new insights into the kinetics of morphogenesis in supramolecular aggregates.
The pseudopeptide backbone provided by N-(2-aminoethyl)-glycine oligomers with attached nucleobases has been widely utilized in peptide nucleic acids (PNAs) as DNA mimics. Here we demonstrate the suitability of this backbone for the formation of structurally defined dye stacks. Toward this goal a series of peptide merocyanine (PMC) dye oligomers connected to a N-(2-aminoethyl)-glycine backbone were prepared through peptide synthesis. Our concentration-, temperature- and solvent-dependent UV/Vis absorption studies show that under the control of dipole–dipole interactions, smaller-sized oligomers consisting of one, two or three dyes self-assemble into defined duplex structures containing two up to six chromophores. In contrast, upon further extension of the oligomer, the chosen peptide backbone cannot direct the formation of a defined duplex architecture anymore due to intramolecular aggregation between the dyes. For all aggregate species a moderate aggregation-induced emission enhancement is observed.
The present thesis demonstrates the importance of the solid state packing of dipolar merocyanine dyes with regard to charge transport and exciton coupling.
Due to the charge transport theory for disordered materials, it is expected that high ground state dipole moments in amorphous thin films lead to low mobility values due to a broadening of the density of states. However, due to their inherent dipolarity, merocyanine dyes usually align in antiparallel dimers in an ordered fashion. The examination of twenty different molecules with ground state dipole moments up to 15.0 D shows that by a high dipolarity and well-defined sterics, the molecules pack in a highly regular two-dimensional brickwork-type structure, which is beneficial for hole transport. Utilization of these molecules for organic thin-film transistors (OTFTs) leads to hole mobility values up to 0.21 cm²/Vs. By fabrication of single crystal field-effect transistors (SCFETs) for the derivative showing the highest mobility values in OTFTs, even hole mobilities up to 2.34 cm²/Vs are achieved. Hence, merocyanine based transistors show hole mobility values comparable to those of conventional p-type organic semiconductors and therefore high ground state dipole moments are not necessarily disadvantageous regarding high mobility applications.
By examination of a different series of ten merocyanine dyes with the same chromophore backbone but different donor substituents, it is demonstrated that the size of the donor has a significant influence on the optical properties of thin films. For small and rigid donor substituents, a hypsochromic shift of the absorption compared to the monomer absorption in solution is observed due to the card stack like packing of the molecules in the solid state. By utilization of sterical demanding or flexible donor substituents, a zig-zag type packing is observed, leading to a bathochromical shift of the absorption. These packing motifs and spectral shifts with an offset of 0.93 eV of the H- and J-bands comply with the archetype examples of H- and J-aggregates from Kasha’s exciton theory.
The aim of the thesis was to develop water soluble poly(2-oxazoline) (POx) copolymers with new side group functionalities, which can be used for the formation of hydrogels in biomedical applications and for the development of peptide-polymer conjugates.
First, random copolymers of the monomer MeOx or EtOx with ButEnOx and EtOx with DecEnOx were synthesized and characterized. The vinyl functionality brought into the copolymer by the monomers ButEnOx and DecEnOx would later serve for post-polymerization functionalization. The synthesized copolymers were further functionalized with thiols via post-polymerization functionalization using a newly developed synthesis protocol or with a protected catechol molecule for hydrogel formation. For the formation of peptide-polymer conjugates, a cyclic thioester, namely thiolactone acrylamide and an azlactone precursor, whose synthesis was newly developed, were attached to the side chain of P(EtOx-co-ButEnOx) copolymers.
The application of the functionalized thiol copolymers as hydrogels using thiol-ene chemistry for cross-linking was demonstrated. The swelling behavior and mechanical properties were characterized. The hydrophilicity of the network as well as the cross-linking density strongly influenced the swelling behavior and the mechanical strength of the hydrogels. All hydrogels showed good cell viability results.
The hydrogel networks based on MeOx and EtOx were loaded with two dyes, fluorescein and methylene blue. It was observed that the uptake of the more hydrophilic dye fluorescein depended more on the ability of the hydrogel to swell. In contrast, the uptake of the more hydrophobic dye methylene blue was less dependent on the swelling degree, but much more on the hydrophilicity of the network.
For the potential application as cartilage glue, (biohybrid) hydrogels were synthesized based on the catechol-functionalized copolymers, with and without additional fibrinogen, using sodium periodate as the oxidizing agent. The system allowed for degradation due to the incorporated ester linkages at the cross-linking points. The swelling behavior as well as the mechanical properties were characterized. As expected, hydrogels with higher degrees of cross-linking showed less swelling and higher elastic modulus. The addition of fibrinogen however increased the elasticity of the network, which can be favorable for the intended application as a cartilage glue. Biological evaluation clearly demonstrated the advantage of degradable ester links in the hydrogel network, where chondrocytes were able to bridge the artificial gap in contrast to hydrogels without any ester motifs.
Lastly, different ways to form peptide-polymer conjugates were presented. Peptides were attached with the thiol of the terminal cysteine group to the vinyl side chain of P(EtOx-co-ButEnOx) copolymers by radical thiol-ene chemistry. Another approach was to use a cyclic thioester, thiolactone, or an azlactone functionality to bind a model peptide via native chemical ligation. The two latter named strategies to bind peptides to POx side chains are especially interesting as one and in the case of thiolactone two free thiols are still present at the binding site after the reaction, which can, for example, be used for further thiol-ene cross-linking to form POx hydrogels.
In summary, side functional poly(oxazoline) copolymers show great potential for numerous biomedical applications. The various side chain functionalities can be introduced by an appropriate monomer or by post-polymerization functionalization, as demonstrated. By their multi-functionality, hydrogel characteristics, such as cross-linking degree and mechanical strength, can be fine-tuned and adjusted depending on the application in the human body. In addition, the presented chemoselective and orthogonal reaction strategies can be used in the future to synthesize polymer conjugates, which can, for example, be used in drug delivery or in tissue regeneration.
N\(^6\)-Isopentenyladenosine in RNA Determines the Cleavage Site of Endonuclease Deoxyribozymes
(2020)
RNA-cleaving deoxyribozymes can serve as selective sensors and catalysts to examine the modification state of RNA. However, site-specific endonuclease deoxyribozymes that selectively cleave posttranscriptionally modified RNA are extremely rare and their specificity over unmodified RNA is low. In this study, we report that the native tRNA modification N\(^6\)-isopentenyladenosine (i\(^6\)A) strongly enhances the specificity and has the power to reconfigure the active site of an RNA-cleaving deoxyribozyme. Using in vitro selection, we identified a DNA enzyme that cleaves i\(^6\)A-modified RNA at least 2500-fold faster than unmodified RNA. Another deoxyribozyme shows unique and unprecedented behaviour by shifting its cleavage site in the presence of the i\(^6\)A RNA modification. Together with deoxyribozymes that are strongly inhibited by i\(^6\)A, these results highlight intricate ways of modulating the catalytic activity of DNA by posttranscriptional RNA modifications.
Deoxyribozymes are emerging as modification-specific endonucleases for the analysis of epigenetic RNA modifications. Here, we report RNA-cleaving deoxyribozymes that differentially respond to the presence of natural methylated cytidines, 3-methylcytidine (m\(^3\)C), N\(^4\)-methylcytidine (m\(^4\)C), and 5-methylcytidine (m\(^5\)C), respectively. Using in vitro selection, we found several DNA catalysts, which are selectively activated by only one of the three cytidine isomers, and display 10- to 30-fold accelerated cleavage of their target m\(^3\)C-, m\(^4\)C- or m\(^5\)C-modified RNA. An additional deoxyribozyme is strongly inhibited by any of the three methylcytidines, but effectively cleaves unmodified RNA. The mXC-detecting deoxyribozymes are programmable for the interrogation of natural RNAs of interest, as demonstrated for human mitochondrial tRNAs containing known m\(^3\)C and m\(^5\)C sites. The results underline the potential of synthetic functional DNA to shape highly selective active sites.
Deoxyribozymes are emerging as modification-specific endonucleases for the analysis of epigenetic RNA modifications. Here, we report RNA-cleaving deoxyribozymes that differentially respond to the presence of natural methylated cytidines, 3-methylcytidine (m\(^3\)C), N\(^4\)-methylcytidine (m\(^4\)C), and 5-methylcytidine (m\(^5\)C), respectively. Using in vitro selection, we found several DNA catalysts, which are selectively activated by only one of the three cytidine isomers, and display 10- to 30-fold accelerated cleavage of their target m\(^3\)C-, m\(^4\)C- or m\(^5\)C-modified RNA. An additional deoxyribozyme is strongly inhibited by any of the three methylcytidines, but effectively cleaves unmodified RNA. The m\(^X\)C-detecting deoxyribozymes are programmable for the interrogation of natural RNAs of interest, as demonstrated for human mitochondrial tRNAs containing known m\(^3\)C and m\(^5\)C sites. The results underline the potential of synthetic functional DNA to shape highly selective active sites.
Deoxyribozymes are artificially evolved DNA molecules with catalytic abilities. RNA-cleaving deoxyribozymes have been recognized as an efficient tool for detection of modifications in target RNAs and provide an alternative to traditional and modern methods for detection of ribose or nucleobase methylation. However, there are only few examples of DNA enzymes that specifically reveal the presence of a certain type of modification, including N6-methyladenosine, and the knowledge about how DNA enzymes recognize modified RNAs is still extremely limited. Therefore, DNA enzymes cannot be easily engineered for the analysis of desired RNA modifications, but are instead identified by in vitro selection from random DNA libraries using synthetic modified RNA substrates. This protocol describes a general in vitro selection stagtegy to evolve new RNA-cleaving DNA enzymes that can efficiently differentiate modified RNA substrates from their unmodified counterpart.
RNA molecules play diverse roles in biological systems. Post-transcriptional RNA modifications and dynamic structures enhance the functional diversity of RNA. A prerequisite for studying their biological significance is the availability of reliable methods for the detection of RNA modifications and structures. Several promising approaches have been developed in the last few decades; however, efficient, and versatile tools are still required to study the dynamic features of RNA. This thesis focuses on the development of nucleic acid catalysts as a tool to address the current needs in studying RNA. The major part of this thesis aimed at the development of deoxyribozymes as a tool for the detection of RNA modifications. Using in vitro selection from a random DNA library, we found deoxyribozymes that are sensitive to N 6 -isopentenyladenosine (i6A), a native tRNA modification and structural analogue of m6A. The in vitro evolution identified three classes of DNA enzymes: AA, AB08, and AC17 DNAzymes that showed distinct response to i6A modification and showed strong discrimination between structural analogues, i.e., m6A and i6A. In the continuation of the project, we attempted to develop RNA-cleaving deoxyribozymes that differentially respond to monomethylated cytidine isomers, 3-methylcytidine (m3C), N4 - methylcytidine (m4C), and 5-methylcytidine (m5C). Several deoxyribozymes were identified from in vitro selection, which are selective for a specific methylated cytidine isomer. The characterization of AL112, AM101, AN05, and AK104 catalysts confirmed the successful evolution of modification-specific and general deoxyribozymes that showed a broad substrate scope. In order to accelerate the DNAzymes discovery, a high throughput sequencing method (DZ-seq) was established that directly quantifies the RNA cleavage activity and cleavage site from deep sequencing data. The libraries contained information about cleavage status, cleavage site and sequence of deoxyribozymes and RNA substrate. The fraction cleaved (FC) data obtained from Dz-seq was validated for a subset of deoxyribozmes using conventional gel based kinetic assay and showed a good linear correlation (R2 = 0.91). Dz-seq possesses a great potential for the discovery of novel deoxyribozymes for the analysis of various RNA modifications in the future. The second objective of the current study was the development of structure-specific RNA labeling ribozymes. Here, we attempted to develop ribozymes that targets RNA of interest by structure-specific interaction rather than base-pairing and focused on a specific RNA G-quadruplex as the target. Two subsequent selection experiments led to the identification of the adenylyltransferase ribozymes AO10.2 and AR9. The partial characterization of these catalysts showed that A010.2 was unable to recognize intact BCL2 structure, but it turned out as the first reported trans-active ribozyme that efficiently labeled uridine in a defined substrate RNA hybridized to the ribozyme. The other ribozyme AR9 was shown to serve as a trans-active, self-labeling ribozyme that catalyzed adenylyl transferase reaction in the presence of the intact BCL2 sequence. Based on these preliminary findings, we envision that AR9 could potentially serve as a reporter RNA by self-labeling in the presence of an RNA G-quadruplex. However, both AO10.2 and AR9 still require more detailed characterization for their potential applications.
Perylene bisimide (PBI) dyes are a widely used class of industrial pigments, and currently have gained significant importance for organic-based electronic and optical devices. Structural modification at the PBI core results in changes of the optical and electronic properties, which enable tailored functions. Moreover, the aggregation behavior of PBIs is alterable and controllable to achieve new materials, among which organogels are of particular interest because of their potential for applications as supramolecular soft materials. In this work, new PBI-based organic gelators were designed, synthesized, and characterized, and the aggregation behaviors under different conditions were intensively studied by various spectroscopic and microscopic methods. In chapter 2, a brief overview is given on the structural and functional features of organogel systems. The definition, formation and reversibility of organogels are introduced. Some examples on dye based organogel are selected, among which PBI-based organogelators reported so far are especially emphasized. Some basic knowledges of supramolecular chirality are also overviewed such as characterization, amplification, and symmetry breaking of the chiral aggregates. According to our former experiences, PBIs tend to form aggregates because the planer aromatic cores interact with one another by pi-pi interaction. In chapter 3, a new PBI molecule is introduced which possesses amide groups between the conjugated core and periphery alkyl chains. It is found that well oriented aggregates are formed by hydrogen bonding and the pi-pi interaction of the cores. These interactions enable the aggregates to grow in one-dimension forming very long fibers, and these fibers further intercross to 3D network structures, e.g., organogels. In comparison to the very few PBI-based gelators reported before, one advantage of this gelator is that, it is more versatile and can gelate a wide range of organic solvents. Moreover, the well-organized fibers that are composed of extended π-stacks provide efficient pathways for n-type charge carriers. Interestingly, AFM studies reveal that the PBI molecules form well-defined helical fibers in toluene. Both left-handed (M) and right-handed (P) helicities can be observed without any preference for one handedness because the building block is intrinsically achiral. In chapter 4, we tried to influence the M/P enantiomeric ratio by applying external forces. For example, we utilized chiral solvents to generate chiral aggregates with a preferential handedness. AFM analysis of the helices showed that a enantiomeric ratio of about 60: 40 can be achieved by aggregation in chiral solvents R- or S-limonene. Moreover, the long aggregated fibres can align at macroscopic level in vortex flows upon rotary stirring In chapter 5, bulky tetra-phenoxy groups are introduced in the bay area of the PBI gelator. The conjugated core of the new molecule is now distorted because of the steric hindrance. UV/Vis studies reveal a J-type aggregation in apolar solvents like MCH due to intermolecular pi-pi-stacking and hydrogen-bonding interactions. Microscopic studies reveal formation of columnar aggregates in apolar solvent MCH, thus this molecule lacks the ability to form gels in this solvent, but form highly fluorescent lyotropic mesophases at higher concentration. On the other hand, in polar solvents like acetone and dioxane, participation of the solvent molecules in hydrogen bonding significantly reduced the aggregation propensity but enforced the gel formation. The outstanding fluorescence properties of the dye in both J-aggregated viscous lyotropic mesophases and bulk gel phases suggest very promising applications in photonics, photovoltaics, security printing, or as fluorescent sensors. In chapter 6, we did some studies on combining PBI molecules with inorganic gold nanorods. Gold nanorods were synthesized photochemically. By virtue of the thioacetate functionalized PBIs, the rods were connected end to end to form gold nanochains, which were characterized by absorption spectra and TEM measurement. Such chromophore-nanorod hybrids might be applied to guide electromagnetic radiation based on optical antenna technology.
Liquid crystal (LC) shape‐amphiphiles with a disc tethered to a fullerene have been intensely studied for the application in photovoltaics, and helical nanosegregation of C\(_{60}\) has been claimed around the π‐stacking disks based on X‐ray results. The most promising materials reported to date have been resynthesized and studied comprehensively by XRS, density measurements, modelling, and electron density reconstruction. In contrast to previous reports, the results indicate that metal phthalocyanine−fullerene mesogens pack in lamellar columnar phases with p2gm symmetry. Fullerenes assemble in layers and are flanked by phthalocyanine columns, thus explaining the balanced charge carrier mobility of electrons and holes. Such variable donor−acceptor structures are promising for organic electronic applications.
Parallel polar dimers in the columnar self‐assembly of umbrella‐shaped subphthalocyanine mesogens
(2021)
The self-assembly of umbrella-shaped mesogens is explored with subphthalocyanine cores and oligo(thienyl) arms with different lengths in the light of their application as light-harvesting and photoconducting materials. While the shortest arm derivatives self-assemble in a conventional columnar phase with a single mesogen as a repeating unit, the more extended derivatives generate dimers that pile up into liquid crystalline columns.
In contrast to the antiparallel arrangement known from single crystals, the present mesogens align as parallel dimers in polar columnar phases as confirmed by X-ray scattering, experimental densities, dielectric spectroscopy, second harmonic generation, alignment, and conductivity studies.
UV–vis and fluorescence spectroscopies reveal a broad absorption in the visible range and only weak emission of the Q-band. Thus, these light-collecting molecules forming strongly polar columnar mesophases are attractive for application in the area of photoconductive materials.
Columnar Liquid Crystals from Star‐Shaped Conjugated Mesogens as Nano‐Reservoirs for Small Acceptors
(2020)
Shape‐persistent conjugated mesogens with oligothiophene arms of different lengths have been synthesized. Such mesogens possess free intrinsic space between their conjugated arms. They form columnar liquid‐crystalline phases, in which the void is filled by dense helical packing in the neat phase similar to an oligo(phenylene vinylene) derivative of equal size. The void can also be compensated by the inclusion of the small acceptor molecule 2,4,7‐trinitrofluorenone. In solution, the acceptor interacts with the core as the largest π‐surface, while in the solid material, it is incorporated between the arms and sandwiched by the star‐shaped neighbours along the columnar assemblies. The TNF acceptors are not nanosegregated from the star‐shaped donors, thus the liquid crystal structure converts to a nano‐reservoir for TNF (endo‐receptor). These host–guest arrangements are confirmed by comprehensive X‐ray scattering experiments and solid‐state NMR spectroscopy. This results in ordered columnar hexagonal phases at high temperatures, which change to helical columnar mesophases or to columnar soft crystals at room temperature.
Energy Transfer Between Squaraine Polymer Sections: From helix to zig-zag and All the Way Back
(2015)
Joint experimental and theoretical study of the absorption spectra of squaraine polymers in solution provide evidence that two different conformations are present in solution: a helix and a zig-zag structure. This unique situation allows investigating ultrafast energy transfer processes between different structural segments within a single polymer chain in solution. The understanding of the underlying dynamics is of fundamental importance for the development of novel materials for light-harvesting and optoelectronic applications. We combine here femtosecond transient absorption spectroscopy with time-resolved 2D electronic spectroscopy showing that ultrafast energy transfer within the squaraine polymer chains proceeds from initially excited helix segments to zig-zag segments or vice versa, depending on the solvent as well as on the excitation wavenumber. These observations contrast other conjugated polymers such as MEH-PPV where much slower intrachain energy transfer was reported. The reason for the very fast energy transfer in squaraine polymers is most likely a close matching of the density of states between donor and acceptor polymer segments because of very small reorganization energy in these cyanine-like chromophores.
The catalytic splitting of water into its elements is an important reaction to establish hydrogen as a solar fuel. The bottle-neck of this process is considered to be the oxidative half reaction generating oxygen, and good catalysts are required to handle the complicated redox chemistry involved. As can be learned from nature, the incorporation of the catalytically active species into an appropriate matrix can help to improve the overall performance. Thus, the aim of the present thesis was to establish novel supramolecular approaches to improve water oxidation catalysis using the catalytically active {Ru(bda)} fragment as key motive (bda = 2,2'-bipyridine-6,6'-dicarboxylate).
First, the synthesis of ruthenium catalysts gathering three {Ru(bda)} water oxidation subunits in a macrocyclic fashion is described. By using bridging bipyridine ligands of different lengths, metallosupramolecular macrocycles with distinct sizes have been obtained. Interestingly, an intermediate ring size has been proven to be optimal for the catalytic water oxidation. Detailed kinetic, spectroscopic, and theoretical studies helped to identify the reaction mechanism and to rationalize the different catalytic activities. Furthermore, solubilizing side chains have been introduced for the most active derivative to achieve full water solubility.
Secondly, the {Ru(bda)} fragment was embedded into supramolecular aggregates to generate more stable catalytic systems compared to a homogeneous reference complex. Therefore, the catalyst fragment was equipped with axial perylene bisimide (PBI) ligands, which facilitate self-assembly. Moreover, the influence of the different accessible aggregate morphologies on the catalytic performance has been investigated.
Eight streptophenazines (A-H) have been identified so far as products of Streptomyces strain HB202, which was isolated from the sponge Halichondria panicea from the Baltic Sea. The variation of bioactivities based on small structural changes initiated further studies on new derivatives. Three new streptophenazines (I-K) were identified after fermentation in the present study. In addition, revised molecular structures of streptophenazines C, D, F and H are proposed. Streptophenazines G and K exhibited moderate antibacterial activity against the facultative pathogenic bacterium Staphylococcus epidermidis and against Bacillus subtilis. All tested compounds (streptophenazines G, I-K) also showed moderate activities against PDE 4B.
In this work the influence of “active” bridge units on the electron transfer (ET) mechanism within organic donor-bridge-electrode arrays in self-assembled monolayers (SAMs) was studied by spectroscopic and electrochemical methods. In the first part of this work ferrocenealkanethiols 1 – 3 and the ferrocenearylthiols 4, 5 were investigated to get experience in the monolayer preparation for measuring ET rates. Cyclic voltammetry of the monolayers indicates that homogeneously mixed monolayers containing redox active molecules and dummy molecules were formed. For the known ferrocenealkanethiols 1 – 3 the ET rates could be confirmed compared to the ones measured by Creager et al. [206]. As expected the ET rate decreases by increasing chain length of the alkane spacer from 2 to 3. Changing the bonding between the redox centre and the alkane spacer with the same bridge lenght, e. g. by using a carboxy-group in case of 1, does not influence the ET behaviour very strong. The aromatic ferrocenethiols 4 and 5 show very high ET rates due to the strong conjugated system although the distance between the redox centre and the electrode is comparable to the C8-alkyl compound 2. The electronic coupling factors all indicate a nonadiabatic ET between the redox centre and the electrode. As expected the electronic coupling factors increase with decreasing spacer length or with an enlarged conjugated system. To sum up, experience in monolayer preparation could be obtained, the measured ET rates for well known ferrocenealkane-compounds 1 - 3 could be verified and the information could be transferred to the conjugated systems 4 and 5. In the second part the triarylamine- 29, 32 and the phenothiazinealkanethiol 35 have been examined relative to their ET behaviour in mixed monolayers. The cyclic voltammograms of the diluted monolayers indicate that homogeneously formed monolayers are present. The ET rates of triarylamine- 29, 32 and phenothiazinealkanethiols 35 are 10 to 100 times higher than compared to ferrocenealkanethiols with equal chain length[183, 206], whereas in a [Ru(bpy)2(pp)]+-containing monolayer the same value was observed [177]. Almost two parameters influence the ET rate constant: the electronic coupling matrix element and the reorganisation energy  [209]. The ET rate in donor substituted alkanethiols is mainly influenced by the reorganisation energy  [177] and even small changes have a dramatic effect on the observed processes, therefore an increasing ET rate from the ferrocene (high reorganisation energy) over the phenothiazine 35 and the [Ru(bpy)2(pp)]+ to the triarylamine chromophores 29 and 32 (low reorganisation energy) is observed. Furthermore the bonding between the redox centres and the alkane spacer plays an important role on the ET rate in case of the triarylamines 29 and 32 opposite to the assumption made by Creager et al. that the connection does not play any role. For the electron rich ether connected compound 29 the ET is not only dominated by the reorganisation energy but also by mesomeric effects where the positive charge of the electron rich derivative 29 is more located at the ether function so that the chain is formally shortend by one atom resulting in higher ET rates than compared to 32. In the third part of the thesis a series of “molecular wires” consisting of methoxy- or chloro-substituted triarylamines and phenothiazines with different bridge units and bridge length between the redox centre and the anchor thiol function have been prepared in order to investigate their ET-behaviour. Cyclic voltammetry and UV/vis-spectroscopy show that the oxidation potential and the energetic states could be controlled very well by introducing different redox centres and bridge units resulting in a decreasing oxidation potential of the redox centres and a bathochromic shift of the absorption bands in the UV/vis-spectra. Also the densitiy of the chromophores in mixed monolayers could be controlled very well for only three compounds (49, 52 and 87) with nitrile-substituted bridges reliable ET rates could be obtained. In these chromophores the ET rate decreases by increasing the density of the redox active molecules in the mixed monolayers indicating that the adsorption geometry changes with coverage with the chromophores tilting to a more upright orientation as the surface becomes more crowded. For all other compounds the measurements were limited by the fast ET rates. Conformational, as well as a very weak distance dependence of the ET resulting in very high ET rates [172] or unfavourable HOMO-LUMO energies of the donor, bridge and the electrode are reasons for this behaviour. The fact that compound 49 shows almost the same rate constant independent of the length (n = 2 or n = 3) may indicate that a hopping process is operating for which a much weaker length dependence is expected than in the case of a superexchange.
Despite their popularity as enzyme engineering targets structural information about Sucrose Phosphorylases remains scarce. We recently clarified that the Q345F variant of Bifidobacterium adolescentis Sucrose Phosphorylase is able to accept large polyphenolic substrates like resveratrol via a domain shift. Here we present a crystal structure of this variant in a conformation suitable for the accommodation of the donor substrate sucrose in excellent agreement with the wild type structure. Remarkably, this conformation does not feature the previously observed domain shift which is therefore reversible and part of a dynamic process rather than a static phenomenon. This crystallographic snapshot completes our understanding of the catalytic cycle of this useful variant and will allow for a more rational design of further generations of Sucrose Phosphorylase variants.
The initial goal was the conversion of Bifidobacterium adolescentis Sucrose Phosphorylase (BaSP) into a polyphenol glucosidase by structure based enzyme engineering. BaSP was chosen because of its ability to utilize sucrose, an economically viable and sustainable donor substrate, and transfer the glucosyl moiety to various acceptor substrates. The introduction of aromatic residues into the active site was considered a viable way to render it more suitable for aromatic acceptor compounds by reducing its polarity and potentially introducing π-π-interactions with the polyphenols. An investigation of the active site revealed Gln345 as a suitable mutagenesis target. As a proof of concept BaSP Q345F was employed in the glycosylation of (+)-catechin, (-)-epicatechin and resveratrol. The variant was selective for the aromatic acceptor substrates and the glucose disaccharide side reaction was only observed after almost quantitative conversion of the aromatic substrates. A crystal structure of BaSP Q345F in complex with glucose was obtained and it displayed an unexpected shift of an entire domain by 3.3 Å. A crystal structure of BaSP D192N-Q345F, an inactive variant in complex with resveratrol-3-α-D-glucosid, the glucosylation product of resveratrol, synthesized by BaSP Q345F was solved. It proved that the domain shift is in fact responsible for the ability of the variant to glycosylate aromatic compounds. Simultaneously a ligand free crystal structure of BaSP Q345F disproved an induced fit effect as the cause of the domain shift. The missing link, a crystal structure of BaSP Q345F in the F-conformation is obtained. This does not feature the domain shift, but is in outstanding agreement with the wildtype structure. The domain shift is therefore not static but rather a step in a dynamic process. It is further conceivable that the domain shifted conformation of BaSP Q345F resembles the open conformation of the wild type and that an adjustment of a conformational equilibrium as a result of the Q345F point mutation is observed. An investigation into the background reaction, the formation of glucose-glucose disaccharides of BaSP Q345F and three further variants that addressed the same region (L341C, D316C-L341C and D316C-N340C) revealed the formation of nigerose by BaSP Q345F.
[60]Fullerene hexakisadducts possessing 12 carboxylic acid side chains form crystalline hydrogen-bonding frameworks in the solid state. Depending on the length of the linker between the reactive sites and the malonate units, the distance of the [60]fullerene nodes and thereby the spacing of the frameworks can be controlled and for the most elongated derivative, continuous channels are obtained within the structure. Stability, structural integrity and porosity of the material were investigated by powder X-ray diffraction, thermogravimetry and sorption measurements.
Abstraction of an allylic hydrogen atom in homobenzvalene (4) either in solurion by photolyticaßy generated tert-butoxyl radicals or in an adamantane matrix by X-rays produces the homobcnzvalenyl radical (5). which tbennally rearranps · to tbe tropylium ndical (1). In solution tbe activation cnergy for the rate determined step of the reaction sequence was detennined· to be 13.4 ± O.S kcal/mol.
A series of bis‐(4’‐pyridylethynyl)arenes (arene=benzene, tetrafluorobenzene, and anthracene) were synthesized and their bis‐N‐methylpyridinium compounds were investigated as a class of π‐extended methyl viologens. Their structures were determined by single crystal X‐ray diffraction, and their photophysical and electrochemical properties (cyclic voltammetry), as well as their interactions with DNA/RNA were investigated. The dications showed bathochromic shifts in emission compared to the neutral compounds. The neutral compounds showed very small Stokes shifts, which are a little larger for the dications. All of the compounds showed very short fluorescence lifetimes (<4 ns). The neutral compound with an anthracene core has a quantum yield of almost unity. With stronger acceptors, the analogous bis‐N‐methylpyridinium compound showed a larger two‐photon absorption cross‐section than its neutral precursor. All of the dicationic compounds interact with DNA/RNA; while the compounds with benzene and tetrafluorobenzene cores bind in the grooves, the one with an anthracene core intercalates as a consequence of its large, condensed aromatic linker moiety, and it aggregates within the polynucleotide when in excess over DNA/RNA. Moreover, all cationic compounds showed highly specific CD spectra upon binding to ds‐DNA/RNA, attributed to the rare case of forcing the planar, achiral molecule into a chiral rotamer, and negligible toxicity toward human cell lines at ≤10 μM concentrations. The anthracene‐analogue exhibited intracellular accumulation within lysosomes, preventing its interaction with cellular DNA/RNA. However, cytotoxicity was evident at 1 μM concentration upon exposure to light, due to singlet oxygen generation within cells. These multi‐faceted features, in combination with its two‐photon absorption properties, suggest it to be a promising lead compound for development of novel light‐activated theranostic agents.
Remdesivir is the only FDA-approved drug for the treatment of COVID-19 patients. The active form of remdesivir acts as a nucleoside analog and inhibits the RNA-dependent RNA polymerase (RdRp) of coronaviruses including SARS-CoV-2. Remdesivir is incorporated by the RdRp into the growing RNA product and allows for addition of three more nucleotides before RNA synthesis stalls. Here we use synthetic RNA chemistry, biochemistry and cryoelectron microscopy to establish the molecular mechanism of remdesivir-induced RdRp stalling. We show that addition of the fourth nucleotide following remdesivir incorporation into the RNA product is impaired by a barrier to further RNA translocation. This translocation barrier causes retention of the RNA 3ʹ-nucleotide in the substrate-binding site of the RdRp and interferes with entry of the next nucleoside triphosphate, thereby stalling RdRp. In the structure of the remdesivir-stalled state, the 3ʹ-nucleotide of the RNA product is matched and located with the template base in the active center, and this may impair proofreading by the viral 3ʹ-exonuclease. These mechanistic insights should facilitate the quest for improved antivirals that target coronavirus replication.
The successful synthesis of a family of donor-iridium complex-acceptor triads (T1–T6, pMV1 and mMV1) and their electrochemical and photophysical properties were presented in this work. Triarylamines (TAA) were used as donors and naphthalene diimide (NDI) as acceptor. A bis-cyclometalated phenylpyrazole iridium dipyrrin complex acts as a photosensitiser. In addition, a molecular structure of T1 was obtained by single crystal X-ray diffraction.
Transient absorption spectroscopy experiments of these triads resembled that upon excitation a photoinduced electron transfer efficiently generates long-lived, charge-separated (CS) states. Thereby, the electron-transfer mechanism depends on the excitation energy.
The presence of singlet and triplet CS states was clarified by magnetic-field dependent transient-absorption spectroscopy in the nanosecond time regime. It was demonstrated that the magnetic field effect of charge-recombination kinetics showed for the first time a transition from the coherent to the incoherent spin-flip regime.
The lifetime of the CS states could be drastically prolonged by varying the spacer between the iridium complex and the NDI unit by using a biphenyl instead of a phenylene unit in T4.
A mixed-valence (MV) state of two TAA donors linked to an iridium metal centre were generated upon photoexcitation of triad pMV1 and mMV1. The mixed-valence character in these triads was proven by the analysis of an intervalence charge-transfer (IV-CT) band in the (near-infrared) NIR spectral region by femtosecond pump-probe experiments. These findings were supported by TD-DFT calculations.
The synthesis of dyads (D1–D4) was performed. Thereby the dipyrrin ligand was substituted with electron withdrawing groups. The electrochemical and photophysical characterisation revealed that in one case (D4) it was possible to generate a CS state upon photoexcitation.
Modified nucleotides in tRNAs are important determinants of folding, structure and function. Here we identify METTL8 as a mitochondrial matrix protein and active RNA methyltransferase responsible for installing m\(^3\)C\(_{32}\) in the human mitochondrial (mt-)tRNA\(^{Thr}\) and mt-tRNA\(^{Ser(UCN)}\). METTL8 crosslinks to the anticodon stem loop (ASL) of many mt-tRNAs in cells, raising the question of how methylation target specificity is achieved. Dissection of mttRNA recognition elements revealed U\(_{34}\)G\(_{35}\) and t\(^6\)A\(_{37}\)/(ms\(^2\))i\(^6\)A\(_{37}\), present concomitantly only in the ASLs of the two substrate mt-tRNAs, as key determinants for METTL8-mediated methylation of C\(_{32}\). Several lines of evidence demonstrate the influence of U\(_{34}\), G\(_{35}\), and the m\(^3\)C\(_{32}\) and t\(^6\)A\(_{37}\)/(ms\(^2\))i\(^6\)A\(_{37}\) modifications in mt-tRNA\(^{Thr/Ser(UCN)}\) on the structure of these mt-tRNAs. Although mt-tRNA\(^{Thr/Ser(UCN)}\) lacking METTL8-mediated m\(^3\)C\(_{32}\) are efficiently aminoacylated and associate with mitochondrial ribosomes, mitochondrial translation is mildly impaired by lack of METTL8. Together these results define the cellular targets of METTL8 and shed new light on the role of m\(^3\)C\(_{32}\) within mt-tRNAs.
The reversible condensation of catechols and boronic acids to boronate esters is a paradigm reaction in dynamic covalent chemistry. However, facile backward hydrolysis is detrimental for stability and has so far prevented applications for boronate-based materials. Here, we introduce cubic boronate ester cages 6 derived from hexahydroxy tribenzotriquinacenes and phenylene diboronic acids with ortho-t-butyl substituents. Due to steric shielding, dynamic exchange at the Lewis acidic boron sites is feasible only under acid or base catalysis but fully prevented at neutral conditions. For the first time, boronate ester cages 6 tolerate substantial amounts of water or alcohols both in solution and solid state. The unprecedented applicability of these materials under ambient and aqueous conditions is showcased by efficient encapsulation and on-demand release of β-carotene dyes and heterogeneous water oxidation catalysis after the encapsulation of ruthenium catalysts.
The present thesis describes the development of a strategy to create discrete finite-sized supramolecular stacks of merocyanine dyes. Thus, bichromophoric stacks of two identical or different chromophores could be realized by folding of bis(merocyanine) dyes and their optical properties were discussed in terms of exciton theory. Quantum chemical calculations revealed strong exciton coupling between the chromophores within the homo- and hetero-π-stacks and the increase of the J-band of the hetero-dimers with increasing energy difference between the excited states of the chromophores could be attributed not only to the different magnitudes of transition dipole moments of the chromophores but also to the increased localization of the excitation in the respective exciton state. Furthermore, careful selection of the length of the spacer unit that defines the interplanar distance between the tethered chromophores directed the self-assembly of the respective bis(merocyanines) into dimers, trimers and tetramers comprising large, structurally precise π-stacks of four, six or eight merocyanine chromophores. It could be demonstrated that the structure of such large supramolecular architectures can be adequately elucidated by commonly accessible analysis tools, in particular NMR techniques in combination with UV/vis measurements and mass spectrometry. Supported by TDDFT calculations, the absorption spectra of the herein investigated aggregates could be explained and a relationship between the absorption properties and the number of stacking chromophores could be established based on exciton theory.
Herein described is the discovery of three novel types of dimeric naphthylisoquinoline alkaloids, named mbandakamines, cyclombandakamines, and spirombandakamines. They were found in the leaves of a botanically as yet unidentified, potentially new Ancistrocladus species, collected in the rainforest of the Democratic Republic of the Congo (DRC). Mbandakamines showed an exceptional 6′,1′′-coupling, in the peri-position neighboring one of the outer axes, leading to an extremely high steric hindrance at the central axis, and to U-turn-like molecular shape, which – different from all other dimeric NIQs, whose basic structures are all quite linear – brings three of the four bicyclic ring systems in close proximity to each other. This created an unprecedented follow-up chemistry, involving ring closure reactions, leading to two further, structurally even more intriguing subclasses, the cyclo- and the spirombandakamines, displaying eight stereogenic elements (the highest total number ever found in naphthylisoquinoline alkaloids). The metabolites exhibited pronounced antiplasmodial and antitrypanosomal activities. Likewise reported in this doctoral thesis are the isolation and structural elucidation of naphthylisoquinoline alkaloids from two further potentially new Ancistrocladus species from DRC. Some of these metabolites have shown pronounced antiausterity activities against human pancreatic cancer PANC-1 cells.
Dye–dye interactions affect the optical and electronic properties in organic semiconductor films of light harvesting and detecting optoelectronic applications. This review elaborates how to tailor these properties of organic semiconductors for organic solar cells (OSCs) and organic photodiodes (OPDs). While these devices rely on similar materials, the demands for their optical properties are rather different, the former requiring a broad absorption spectrum spanning from the UV over visible up to the near‐infrared region and the latter an ultra‐narrow absorption spectrum at a specific, targeted wavelength. In order to design organic semiconductors satisfying these demands, fundamental insights on the relationship of optical properties are provided depending on molecular packing arrangement and the resultant electronic coupling thereof. Based on recent advancements in the theoretical understanding of intermolecular interactions between slip‐stacked dyes, distinguishing classical J‐aggregates with predominant long‐range Coulomb coupling from charge transfer (CT)‐mediated or ‐coupled J‐aggregates, whose red‐shifts are primarily governed by short‐range orbital interactions, is suggested. Within this framework, the relationship between aggregate structure and functional properties of representative classes of dye aggregates is analyzed for the most advanced OSCs and wavelength‐selective OPDs, providing important insights into the rational design of thin‐film optoelectronic materials.
A highly sensitive short-wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well-organized nanocrystalline bulk-heterojunction (BHJ) active layer composed of a dicyanovinyl-functionalized squaraine dye (SQ-H) donor material in combination with PC\(_{61}\)BM. Through thermal annealing, dipolar SQ-H chromophores self-assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full-width at half-maximum of only 85 nm (815 cm\(^{-1}\)) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J-type aggregates. Photoplethysmography application for heart-rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short-range coupled dye aggregates for low-power operating wearable applications.
The transient yellow color observed in the cycloaddition of homobenzvalene (HB) with tetracyanoethylene (TCNE) is associated with the charge-transfer complex [HB, TCNE). The deliberate photoexcitation of [HB,TCNE) affords a mixture of charge-transfer cycloadducts (1, 2, and 3) that differs from that obtained in thermal cycloaddition. The relationship of {HB t TCNE•) radical-ion pair (as the critical reactive intermediate in charge-transfer cycloaddition) to the activation process for thermal cycloaddition is discussed.
In this work the catalytic activity of nanodiamond particles with different dopants and surface terminations and of diamond nanomaterials funtionalized with ruthenium-based photocatalysts was investigated, illustrating materials application in photoredox chemistry and the photo(electro)catalytic reduction of CO2. Regarding the application of diamond nanomaterials in photocatalysis, methods to fabricate and characterize several (un)doped nanoparticles with different surface termination were successfully developed. Various photocatalysts, attached to nanodiamond particles via linker systems, were tested in photoredox catalysis and the photo(electro)catalytic reduction of CO2.
Supramolecular self-assembly of perylene bisimide (PBI) dyes via non-covalent forces gives rise to a high number of different PBI architectures with unique optical and functional properties. As these properties can be drastically influenced by only slightly structural changes of the formed supramolecular ensembles (Chapter 2.1) the controlled self-assembly of PBI dyes became a central point of current research to design innovative materials with a high potential for different applications as for example in the fields of organic electronics or photovoltaics.
As PBI dyes show a strong tendency to form infinite aggregated structures (Chapter 2.2) the aim of this thesis was to precisely control their self-assembly to create small, structurally well-defined PBI assemblies in solution. Chapter 2.3 provides an overview on literature known strategies that were established to realize this aim. It could be demonstrated that especially backbone-directed intra- and intermolecular self-assembly of covalently linked Bis-PBI dyes evolved as one of the most used strategies to define the number of stacked PBI chromophores by using careful designed spacer units with regard to their length and flexibility.
By using conventional spectroscopic methods like UV/Vis and fluorescence experiments in combination with NMR measurements an in-depth comparison of the molecular and optical properties in solution both in the non-stacked and aggregated state of the target compounds could be elucidated to reveal structure-property relationships of different PBI architectures. Thus, it could be demonstrated, that spacer units that pre-organize two PBI chromophores with an inter-planar distance of r < 7 Å lead to an intramolecular folding, whereas linker moieties with a length between 7 to 11 Å result in an intermolecular self-assembly of the respective Bis-PBIs dyes via dimerization to form well-defined quadruple PBI pi-stacks. Hence, if the used spacer units ensure an inter-planar distance r > 14 Å larger oligomeric PBI pi-stacks are generated.
In Chapter 4 a detailed analysis of the exciton coupling in a highly defined H-aggregate quadruple PBI pi-stack is presented. Therefore, bay-tethered PBI dye Bis-PBI 1 was investigated by concentration-dependent UV/Vis spectroscopy in THF and toluene as well as by 2D-DOSY-NMR spectroscopy, ESI mass spectrometry and AFM measurements confirming that Bis-PBI 1 self-assembles exclusively into dimers with four closely pi-stacked PBI chromophores. Furthermore, with the aid of broadband fluorescence upconversion spectroscopy (FLUPS) ensuring broadband detection range and ultrafast time resolution at once, ultrafast Frenkel exciton relaxation and excimer formation dynamics in the PBI quadruple pi-stack within 1 ps was successfully investigated in cooperation with the group of Dongho Kim. Thus, it was possible to gain for the first time insights into the exciton dynamics within a highly defined synthetic dye aggregate beyond dimers. By analysing the vibronic line shape in the early-time transient fluorescence spectra in detail, it could be demonstrated that the Frenkel exciton is entirely delocalized along the quadruple stack after photoexcitation and immediately loses its coherence followed by the formation of the excimer state.
In Chapter 5 four well-defined Bis-PBI folda-dimers Bis-PBIs 2-4 were introduced, where linker units of different length (r < 7 Å) and steric demand were used to gain distinct PBI dye assemblies in the folded state. Structural elucidation based on in-depth UV/Vis, CD and fluorescence experiments in combination with 1D and 2D NMR studies reveals a stacking of the two PBI chromophores upon folding, where geometry-optimized structures obtained from DFT calculations suggest only slightly different arrangements of the PBI units enforced by the distinct spacer moieties. With the resulting optical signatures of Bis-PBIs 2-4 ranging from conventional Hj-type to monomer like absorption features, the first experimental proof of a PBI-based “null-aggregate” could be presented, in which long- and short-range exciton coupling fully compensate each other. Hence, the insights of this chapter pinpoint the importance of charge-transfer mediated short-range exciton coupling that can significantly influence the properties of pi-stacked PBI chromophores
In the last part of this thesis (Chapter 6), spacer-controlled self-assembly of four bay-linked Bis-PBI dyes Bis-PBIs 5-8 into well-defined supramolecular architectures was investigated, where the final aggregate structures are substantially defined by the nature of the used spacer units. By systematically extending the backbone length from 7 to 15 Å defining the inter-planar distance between the tethered chromophores, different assemblies from defined quadruple PBI pi-stacks to larger oligomeric pi-stacks could be gained upon aggregation.
In conclusion, the synthesis of nine covalently linked PBI dyes in combination with a detailed investigation of their spacer-mediated self-assembly behaviour in solution concerning structure-properties-relationships was presented within this thesis. The results confirm a strong exciton coupling in different types of Bis-PBI architectures e.g. folda-dimers or highly defined quadruple pi-stacks, which significantly influences their optical properties upon self-assembly.
Potential energy and spectroscopic constants for the X\(^2 \sum^+ _\mu\) ground state of a;, were calculated by configuration-interaction (Cl) methods, using large basis sets with polarization and diffuse functions. From these CI wavefunctions, the isotropic (a\(_{iso}\)) and dipolar (A\(_{dip}\)) components of the hyperfine coupling constant were obtained. The effects of various s, p basis sets, polarization and diffuse functions, as well as the influence of reference configurations and configuration selection thresholds were investigated. The best values obtained are 35·31 G for a\(_{iso}\) and 29·440 for A\(_{dip}\)• tobe compared with experimental values of 37 ± 1 G and 32 ± 1 G, respectively. It is shown that the contributions to a1so of the K and L shells are opposite in sign, differing by about 4 G. Upon vibrational averaging, both a\(_{iso}\) and A\(_{dip}\) move towards smaller values as v increases. An adiabatic electron affinity of 2·46eV was obtained for CL\(_2\) , and a vertical electron detachment energy of 3·71 eV for Cl;.
The isotropic (a\(_{iso}\)) and dipolar (A\(_{dip}\)) hyperfine coupling constants of 19F2 were obtained from MRD-CI wave functions using a variety of basis sets. In series I, increasing numbers of d functions were added to a 5s4p contracted Huzinaga!Dunning basis. In series II, the 5s3p basis set was uncontracted in several steps until 9s5p was reached, to which were added from one to three d-polarization functions. Cl parameters (selectioo threshoids and the number of reference coofiguratioos) were also varied. A study of the R dependence of aiso and Adip was perfonned. The best values obtained at R\(_e\) are 260 G for a\(_{iso}\) and 308 G for A\(_{dip}\)• compared with experimental values of about 280 G for a;10 and 320 G for A\(_{dip}\)·
The future of water-derived hydrogen as the “sustainable energy source” straightaway bets on the success of the sluggish oxygen-generating half-reaction. The endeavor to emulate the natural photosystem II for efficient water oxidation has been extended across the spectrum of organic and inorganic combinations. However, the achievement has so far been restricted to homogeneous catalysts rather than their pristine heterogeneous forms. The poor structural understanding and control over the mechanistic pathway often impede the overall development. Herein, we have synthesized a highly crystalline covalent organic framework (COF) for chemical and photochemical water oxidation. The interpenetrated structure assures the catalyst stability, as the catalyst’s performance remains unaltered after several cycles. This COF exhibits the highest ever accomplished catalytic activity for such an organometallic crystalline solid-state material where the rate of oxygen evolution is as high as ∼26,000 μmol L\(^{–1}\) s\(^{–1}\) (second-order rate constant k ≈ 1650 μmol L s\(^{–1}\) g\(^{–2}\)). The catalyst also proves its exceptional activity (k ≈ 1600 μmol L s\(^{–1}\) g\(^{–2}\)) during light-driven water oxidation under very dilute conditions. The cooperative interaction between metal centers in the crystalline network offers 20–30-fold superior activity during chemical as well as photocatalytic water oxidation as compared to its amorphous polymeric counterpart.
A donor-acceptor-donor (D-A-D) type naphthalene-diimide (NDI-H) chromophore exhibits highly cooperative J-aggregation leading to nanotubular self-assembly and gelation in n-decane, as demonstrated by UV/Vis, FT-IR, photoluminescence and microscopy studies. Analysis of temperature-dependent UV/Vis spectra using the nucleation-elongation model and FT-IR data reveals the molecular origin of the cooperative nature of the self-assembly. The supramolecular polymerization is initiated by H-bonding up to a degree of polymerization similar to 20-25, which in a subsequent elongation step promotes J-aggregation in orthogonal direction leading to possibly a sheet-like structure that eventually produces nanotubes. Time-resolved fluorescence and absorption measurements demonstrate that such a tubular assembly enables very effective delocalization of excited states resulting in a remarkably prolonged excited state lifetime.
In conclusion, the present thesis demonstrates that the highly desired J-type aggregation of functional perylene bisimide chromophores can be achieved by proper design of monomeric building blocks that direct self-assemble by mutual effects of hydrogen bonding and pi-pi interaction, and on the other hand, are prevented to assemble in columnar stacks owing to their twisted pi-conjugated core and sterically demanding substituents. Furthermore, the self-assembly studies gave new insights into the dynamic aggregation process of low-dimensional extended assemblies with strongly excitonically coupled chromophores. The relationship between commonly known cyanine dye aggregates like that of THIATS and that of the present PBI 1a was investigated by absorption and fluorescence spectroscopy at low temperatures down to 5 K. The formerly unprecedented functional properties of PBI aggregates that are expressed in J-type excitonic coupling hold promise for application in optoelectronic and photovoltaic devices.
Molnupiravir is an orally available antiviral drug candidate currently in phase III trials for the treatment of patients with COVID-19. Molnupiravir increases the frequency of viral RNA mutations and impairs SARS-CoV-2 replication in animal models and in humans. Here, we establish the molecular mechanisms underlying molnupiravir-induced RNA mutagenesis by the viral RNA-dependent RNA polymerase (RdRp). Biochemical assays show that the RdRp uses the active form of molnupiravir, β-d-\(N^4\)-hydroxycytidine (NHC) triphosphate, as a substrate instead of cytidine triphosphate or uridine triphosphate. When the RdRp uses the resulting RNA as a template, NHC directs incorporation of either G or A, leading to mutated RNA products. Structural analysis of RdRp–RNA complexes that contain mutagenesis products shows that NHC can form stable base pairs with either G or A in the RdRp active center, explaining how the polymerase escapes proofreading and synthesizes mutated RNA. This two-step mutagenesis mechanism probably applies to various viral polymerases and can explain the broad-spectrum antiviral activity of molnupiravir.
Microbial studies of the Mediterranean sponge Tethya aurantium led to the isolation of the fungus Bartalinia robillardoides strain LF550. The strain produced a number of secondary metabolites belonging to the chloroazaphilones. This is the first report on the isolation of chloroazaphilones of a fungal strain belonging to the genus Bartalinia. Besides some known compounds (helicusin A (1) and deacetylsclerotiorin (2)), three new chloroazaphilones (helicusin E (3); isochromophilone X (4) and isochromophilone XI (5)) and one new pentaketide (bartanolide (6)) were isolated. The structure elucidations were based on spectroscopic analyses. All isolated compounds revealed different biological activity spectra against a test panel of four bacteria: three fungi; two tumor cell lines and two enzymes.
Modular frameworks featuring well-defined pore structures in microscale domains establish tailor-made porous materials. For open molecular solids however, maintaining long-range order after desolvation is inherently challenging, since packing is usually governed by only a few supramolecular interactions. Here we report on two series of nanocubes obtained by co-condensation of two different hexahydroxy tribenzotriquinacenes (TBTQs) and benzene-1,4-diboronic acids (BDBAs) with varying linear alkyl chains in 2,5-position. n-Butyl groups at the apical position of the TBTQ vertices yielded soluble model compounds, which were analyzed by mass spectrometry and NMR spectroscopy. In contrast, methyl-substituted cages spontaneously crystallized as isostructural and highly porous solids with BET surface areas and pore volumes of up to 3426 m\(^2\) g\(^{-1}\) and 1.84 cm\(^3\) g\(^{-1}\). Single crystal X-ray diffraction and sorption measurements revealed an intricate cubic arrangement of alternating micro- and mesopores in the range of 0.97–2.2 nm that are fine-tuned by the alkyl substituents at the BDBA linker.
Large Stokes shift (LSS) fluorescent proteins (FPs) exploit excited state proton transfer pathways to enable fluorescence emission from the phenolate intermediate of their internal 4 hydroxybenzylidene imidazolone (HBI) chromophore. An RNA aptamer named Chili mimics LSS FPs by inducing highly Stokes-shifted emission from several new green and red HBI analogs that are non-fluorescent when free in solution. The ligands are bound by the RNA in their protonated phenol form and feature a cationic aromatic side chain for increased RNA affinity and reduced magnesium dependence. In combination with oxidative functional-ization at the C2 position of the imidazolone, this strategy yielded DMHBO\(^+\), which binds to the Chili aptamer with a low-nanomolar K\(_D\). Because of its highly red-shifted fluorescence emission at 592 nm, the Chili–DMHBO\(^+\) complex is an ideal fluorescence donor for Förster resonance energy transfer (FRET) to the rhodamine dye Atto 590 and will therefore find applications in FRET-based analytical RNA systems.
The present work consists of two parts. The first one deals with theoretical questions and tests the performance of orbitals obtained from a self-interaction free KS method, the LHFapproach, in multireference ab initio methods. The purpose of this part is to enable a more efficient computation of excitation energies, which is important for the spectroscopic characterization of many organic and bioorganic molecules. The second part focuses on bioorganic questions and studies the base pairing properties of the purine base xanthine in order to explain, e.g., the unusually high stability of selfpairing xanthine alanyl-PNA double strands and the mutagenicity of xanthine formed in DNA. Part1: In contrast to HF- and standard DFT-methods, the LHF-approach leads to a fully bound virtual orbital spectrum, because Coulomb self interactions are exactly canceled in the LHFansatz. Furthermore, the energies of the occupied orbitals are not upshifted, like it is the case for standard DFT-methods, so that Koopmans' theorem remains valid. In line with this, also the occupied LHF-orbitals are somewhat more compact than standard DFT-orbitals. The present work shows that both properties are of great benefit for MR methods. The virtual LHF-orbitals are well optimized and allow an efficient description of excited states and static correlation in both MRCI- and MRPT2-approaches. Furthermore, the higher compactness of the occupied LHF- compared to standard DFT-orbitals leads to a better description of the center ion of Rydberg states. However, for each of the two advantages mentioned at least one example molecule has been found, for which LHF-orbitals actually perform worse than HF-and/or standard DFT-orbitals. This shows, that even though LHF virtual orbitals allow an excellent MRCI- and MRPT2-description for the electronically excited states of a large number of molecules, this cannot be generalized and their performance needs to be tested for each individual case. In the second part of the present work, the base pairing properties of xanthine and xanthine derivatives were studied. The purpose of this part was to find an explanation for the unexpectedly high stability of the xanthine alanyl PNA double strand. Furthermore, it was analyzed, why xanthine, that is formed from guanine in DNA under chemical stress, is able to form mismatched base pairs with the pyrimidine base thymine. Stability of xanthine alanyl PNA: In the first step, the regioisomer present in the considered alanyl PNA was identified to be the N7-regioisomer of xanthine by a theoretical analysis of the 13C-NMR spectrum. To analyze the stability of the xanthine self-pairing, a simplified model was set up, in which the stability of the PNA double strand was explained solely by the energy contributions from H-bonding and base stacking. For that purpose, the dimerization and stacking energies for the xanthine-xanthine, guaninecytosine, adenine-thymine and xanthine-2,6-diaminopurine base pairs were computed using DFT and MP2 methods. Solvent effects were taken into account by the conductor like screening model. The influence of the peptide backbone on the stacking geometry was considered by force field optimizations. While the individual contributions from hydrogen bonding and stacking do not correlate with the melting temperature Tm, the sum of both correlates linearly with Tm. This correlation is somewhat surprising, because this means that the effects of the entropy and the molecular water environment either cancel or are similar for all systems compared. In this model, the stability of the xanthine selfpairing mainly stems from an enlarged stacking interaction, while the H-bonds give only minor contributions to the stability of the xanthine selfpaired double strand of alanyl-PNA. Base pairing properties of N9-Xanthine: The computation of the base pairing properties of N9-xanthine revealed a strong variation in the individual H-bond strengths for the selfpairing of xanthine, that range from -4 to -11 kcal/mol in the gas phase and -2.5 to -5 kcal/mol in polar solvent. By comparison with model systems it was shown that the strong variance of the H-bond strength is mainly due to attractive or repulsive secondary electrostatic interactions. For the homodimer of hypoxanthine it was shown that the increase of aromaticity in the pyrimidine ring upon dimer formation leads to a strengthening of the hydrogen bonds. Mutagenicity of hypoxanthine and xanthine: Several neutral and anionic Watson-Crick base pairs of xanthine were computed with MP2- and DFT-methods in order to explain the mutagenicity of hypoxanthine and xanthine. Also basepairs involving tautomeric forms of xanthine and hypoxanthine were considered. To evaluate the dimerization energies found, the dimers were classified into pairings that have the exact geometry of the canonical base pairs and those that realize a distorted Watson-Crick pairing mode. The computations show that a stable pairing which realizes the exact geometry of a canonical Watson Crick base pairing is only possible for the pairing of xanthine to cytosine, however, the base pairs are only weakly bound. The dimerization energies of both the neutral and the anionic pairing is around 0 kcal/mol, so that the xanthine-cytosine base pairs are incorporated into DNA solely because the base pairs fulfill the geometric demands of DNA polymerase, but it does not profit from any additional stabilization due to hydrogen bonding. The bonding that in the Watson-Crick pairing mode xanthine has almost no affinity to cytosine is in correspondence with the experimental result that the cytosine-xanthine base pair is incorporated into DNA at a much lower rate than the cytosine-guanine base pair, which has a very strong hydrogen bonding. While the affinity of xanthine to cytosine is very low, the computations predict that xanthine is able to form a stable Watson-Crick pairing with thymine. However, the pairing has a somewhat distorted Watson-Crick geometry, so that its high stability is outbalanced by the worsened fit to the binding pocket of DNA-polymerase. As a consequence, the xanthinethymine pairing is incorporated into DNA not at a faster, but only at a rate comparable to that of the xanthine-cytosine pairing.
In the present work the dimethylamino radical ( ( CH\(_3\)) \(_2\)N) and its protonated cation ( ( CH\(_3\))\(_2\)NH\(^+\)) are investigated by means of ab initio methods. The geometries of various conformations of both compounds are obtained with UMP2/6·31 G** calculations, while the hyperfine structure and its dependence on the geometry is studied using the MRD-Cl/B\(_K\) method. The two molecules are compared to study the inftuence of the protonation on geometry and hyperfine structure. The effects of the rotational barriers on the hyperfine structures of (CH\(_3\))\(_2\)N, (CH\(_3\)CH\(_2\))\(_2\)N and ( (CH\(_3\))\(_2\)CH)\(_2\)N will be discussed.
In π-conjugated organic photovoltaic materials, an excimer state has been generally regarded as a trap state which hinders efficient excitation energy transport. But despite wide investigations of the excimer for overcoming the undesirable energy loss, the understanding of the relationship between the structure of the excimer in stacked organic compounds and its properties remains elusive. Here, we present the landscape of structural dynamics from the excimer formation to its relaxation in a co-facially stacked archetypical perylene bisimide folda-dimer using ultrafast time-domain Raman spectroscopy. We directly captured vibrational snapshots illustrating the ultrafast structural evolution triggering the excimer formation along the interchromophore coordinate on the complex excited-state potential surfaces and following evolution into a relaxed excimer state. Not only does this work showcase the ultrafast structural dynamics necessary for the excimer formation and control of excimer characteristics but also provides important criteria for designing the π-conjugated organic molecules.
Background It is well known that carbohydrates play fundamental roles in cell signaling and infection processes as well as tumor formation and progression. However, the interaction pathways and cellular receptors targeted by carbohydrates and glycoconjugates remain poorly examined and understood. This lack of research stems, at least to a major part, from accessibility problems of large, branched oligosaccharides. Results To test glycan - cell interactions in vitro, a variety of tailored oligosaccharides was synthesized chemo-enzymatically. Glycosyltransferases from the GRAS organisms Bacillus megaterium (SacB) and Aspergillus niger (Suc1) were used in this study. Substrate engineering of these glycosyltransferases generally acting on sucrose leads to the controlled formation of novel tailored di-, tri- and tetrasaccharides. Already industrially used as prebiotics in functional food, the immunogenic potential of novel oligosaccharides was characterized in this study. A differential secretion of CXCL8 and CCL2 was observed upon oligosaccharide co-cultivation with colorectal epithelial Caco-2 cells. Conclusion Pure carbohydrates are able to stimulate a cytokine response in human endothelial cells in vitro. The type and amount of cytokine secretion depends on the type of co-cultivated oligosaccharide.
Sialic acids are located at the termini of mammalian cell-surface glycostructures, which participate in essential interaction processes including adhesion of pathogens prior to infection and immunogenicity. Here we present the synthesis and bioorthogonal metabolic incorporation of the sialic acid analogue N-(1-oxohex-5-ynyl)neuraminic acid (Neu5Hex) into the cell-surface glycocalyx of a human larynx carcinoma cell line (HEp-2) and its fluorescence labelling by click chemistry.
In the first part of this work a new approach to measure transient absorption spectra of fluorescent compounds by means of laser flash photolysis technique was presented. Generally, the recorded transient absorption signal consists of transient absorption, fluorescence and ground state bleaching. Thus, for fluorescent chromophores a fluorescence correction is indispensable in order to obtain undisturbed absorption decay curves as well as accurate transient absorption spectra. Due to time response characteristics of the PMT detector the fluorescence contribution cannot be corrected by recording the fluorescence separately. Measuring two transient absorption signals with probe light differing in intensity, compounds with quantum yields up to ~ 35 % can be investigated. This is a major improvement because transient absorption spectroscopy is a powerful method to gain insight into the kinetics and the energy of excited states and information in the time domain of fluorescence are no longer lost. In the second part the synthesis and the photophysical characterisation of redox cascades were reported. These cascades consist of an acridine acceptor and up to three triarylamine donor subunits. The redox potentials of the triarylamines were tuned by adequate substituents in the para-position of the phenyl ring to ensure a directed redox gradient. Upon photoexcitation a locally excited state or a CT state is populated which then injects a hole onto the adjacent donor and consequently results in a CS state. Fluorescence and transient absorption measurements revealed that HT depends strongly on donor strength and solvent polarity. Formation of a CS state was only observed in case of strong terminal donors or polar solvents. A low lying localised triplet state acts as an energy trap and quenches all CS states even in case of the cascade with the strongest terminal donor in very polar solvents. Furthermore, population of a CS state catalyses the formation of this triplet states which results in a shorter lifetime of the CS state compared to the lifetime of the CT state of the corresponding reference compound. Compared to redox cascades already reported in literature, the electronic coupling between the redox centres was decreased by sterical as well as electronic effects. To prolong the lifetime of the CS state saturated spacers on the one hand and a perpendicular orientation of the acceptor and the adjacent donor on the other hand were selected. The twisting of the subunits forming the CT state results in a higher degree of charge separation but its contribution to increase the lifetimes of the CS states is of minor importance. The longer lifetime of the CS states can be ascribed to the saturated spacers. Experimental data in combination with calculated values indicate that charge recombination takes place in the Marcus normal region by a superexchange mechanisms. Although charge recombination of the known cascades is located in the Marcus inverted region, these CS states decay faster than the CS states of the compounds investigated in this work.
In T cells, as in all other cells of the body, sphingolipids form important structural components of membranes. Due to metabolic modifications, sphingolipids additionally play an active part in the signaling of cell surface receptors of T cells like the T cell receptor or the co-stimulatory molecule CD28. Moreover, the sphingolipid composition of their membranes crucially affects the integrity and function of subcellular compartments such as the lysosome. Previously, studying sphingolipid metabolism has been severely hampered by the limited number of analytical methods/model systems available. Besides well-established high resolution mass spectrometry new tools are now available like novel minimally modified sphingolipid subspecies for click chemistry as well as recently generated mouse mutants with deficiencies/overexpression of sphingolipid-modifying enzymes. Making use of these tools we and others discovered that the sphingolipid sphingomyelin is metabolized to ceramide to different degrees in distinct T cell subpopulations of mice and humans. This knowledge has already been translated into novel immunomodulatory approaches in mice and will in the future hopefully also be applicable to humans. In this paper we are, thus, summarizing the most recent findings on the impact of sphingolipid metabolism on T cell activation, differentiation, and effector functions. Moreover, we are discussing the therapeutic concepts arising from these insights and drugs or drug candidates which are already in clinical use or could be developed for clinical use in patients with diseases as distant as major depression and chronic viral infection.
Alzheimer′s disease (AD) is a neurological disorder with still no preventive or curative treatment. Flavonoids are phytochemicals with potential therapeutic value. Previous studies described the flavanone sterubin isolated from the Californian plant Eriodictyon californicum as a potent neuroprotectant in several in vitro assays. Herein, the resolution of synthetic racemic sterubin (1) into its two enantiomers, (R)‐1 and (S)‐1, is described, which has been performed on a chiral chromatographic phase, and their stereochemical assignment online by HPLC‐ECD coupling. (R)‐1 and (S)‐1 showed comparable neuroprotection in vitro with no significant differences. While the pure stereoisomers were configurationally stable in methanol, fast racemization was observed in the presence of culture medium. We also established the occurrence of extracted sterubin as its pure (S)‐enantiomer. Moreover, the activity of sterubin (1) was investigated for the first time in vivo, in an AD mouse model. Sterubin (1) showed a significant positive impact on short‐ and long‐term memory at low dosages.
The precise interplay between the mRNA codon and the tRNA anticodon is crucial for ensuring efficient and accurate translation by the ribosome. The insertion of RNA nucleobase derivatives in the mRNA allowed us to modulate the stability of the codon-anticodon interaction in the decoding site of bacterial and eukaryotic ribosomes, allowing an in-depth analysis of codon recognition. We found the hydrogen bond between the N1 of purines and the N3 of pyrimidines to be sufficient for decoding of the first two codon nucleotides, whereas adequate stacking between the RNA bases is critical at the wobble position. Inosine, found in eukaryotic mRNAs, is an important example of destabilization of the codon-anticodon interaction. Whereas single inosines are efficiently translated, multiple inosines, e.g., in the serotonin receptor 5-HT2C mRNA, inhibit translation. Thus, our results indicate that despite the robustness of the decoding process, its tolerance toward the weakening of codon-anticodon interactions is limited.
The present work deals with the synthesis and the investigation of the photophysical properties of covalently constructed calix[4]arene–perylene bisimide dye arrays containing various PBI units. The obtained conjugates are characterized with respect towards their application in a new, zigzag-type architecture of artificial light-harvesting systems. For this purpose, orange (core-unsubstituted), red (6,7,11,12-tert-butylphenoxy-functionalized) and green (1,7-pyrrolidino-substituted) perylene bisimide building blocks have been attached to the calix[4]arene scaffold. First, the monochromophoric reference systems have been studied, and second, the photophysical properties of a comprehensive series of newly synthesized, multichromophoric calix[4]arene–perylene bisimide conjugates showing efficient energy transfer processes between the individual dye subunits have been investigated. Furthermore, a series of bichromophoric compounds containing identical chromophoric units has been obtained. Towards this goal, a variety of spectroscopic techniques such as UV/vis absorption, steady state and time-resolved fluorescence emission, and femtosecond transient absorption spectroscopy as well as a spectrotemporal analysis of the obtained data has been applied. This work presents a new concept for an artificial light-harvesting system positioning the dye units by means of calix[4]arene spacers along a zigzag chain. The investigations start with the syntheses and optical properties of the monochromophoric building blocks and result in an elaborate study on the energy and electron transfer processes occurring after photoexcitation in a comprehensive series of multichromophoric calix[4]arene–perylene bisimide conjugates. Finally, the photophysical properties of a series of compounds containing each two identical PBI units are discussed.
Self-assembly of multi-stranded perylene dye J-aggregates in columnar liquid-crystalline phases
(2018)
Many discoid dyes self-assemble into columnar liquid-crystalline (LC) phases with packing arrangements that are undesired for photonic applications due to H-type exciton coupling. Here, we report a series of crystalline and LC perylene bisimides (PBIs) self-assembling into single or multi-stranded (two, three, and four strands) aggregates with predominant J-type exciton coupling. These differences in the supramolecular packing and optical properties are achieved by molecular design variations of tetra-bay phenoxy-dendronized PBIs with two N–H groups at the imide positions. The self-assembly is driven by hydrogen bonding, slipped π–π stacking, nanosegregation, and steric requirements of the peripheral building blocks. We could determine the impact of the packing motifs on the spectroscopic properties and demonstrate different J- and H-type coupling contributions between the chromophores. Our findings on structure–property relationships and strong J-couplings in bulk LC materials open a new avenue in the molecular engineering of PBI J-aggregates with prospective applications in photonics.
Thus, the main focus of this thesis was to generate and investigate new one-dimensional LC PBI J-aggregates of an entirely new PBI organization with the transition dipole moments of the chromophores arranged parallel to the columnar axis and in slipped pi-pi stacking fashion to form highly fluorescent J-aggregates. Towards this goal, the tetra-bay substituted PBI 4c bearing free NH functional groups at the imide positions and four dendrons with branched ethylhexyl alkoxy chains at the meta-position of the phenoxy spacer (Figure 8.1a) was synthesized and compared to a literature known reference PBI 1. The mesogenic dendrons ensure LC character of the dye, which was confirmed by POM, DSC and extensive X-ray analysis. Furthermore, the sterically demanding bay-substituents prevent the cofacial assembly of the chromophores and force the dyes into a slipped pi-stacked order with the main transition dipole moments of the dyes oriented parallel to the columnar axis. X-ray analysis revealed that PBI 4c assembles into columnar triple-stranded helices consisting of side-to-side stacked molecules, which organize into a Colh phase (Figure 8.1b). FT-IR experiments of a thin film and aggregates in MCH solution confirmed the formation of H-bonds between the imide moieties. Temperature-dependent investigations furthermore proved a reversible formation of H-bonds and polarized FT-IR experiments finally gave evidence for the direction of the H-bonds along the shearing respective the columnar axis (Figure 8.1c). This was additionally verified by polarized UV-Vis absorption studies of aligned thin films. The changes in the UV-Vis absorption spectra of concentration- and temperature-dependent experiments in MCH are in agreement with the formation of J-aggregates and could be fitted to a nucleation-elongation growth mechanism. Remarkably, fluorescence spectroscopy studies revealed highly emissive aggregates in solution. These various spectroscopic techniques proved the utilization of directional noncovalent forces like hydrogen-bonding and pi-pi interactions in a cooperative manner forcing the PBI molecules in an unprecedented organization of a slipped pi-stacked arrangement with the orientation of the molecular axis and the respective transition dipole moments parallel to the columns of the LC phase. By the group of Dietrich the formation of exciton-polaritons in imprinted LC pillar microcavities as consequent use of the LC 4c was reported for the first time.In the second part of this thesis the hierarchical organization of LC PBIs into defined single-, double-, triple- and quadruple-stranded J-aggregates within crystalline and columnar LC phases, partially arranged in helical supramolecular structures in dependence of the molecular design was demonstrated. This was achieved via the preparation of a library of twelve molecules PBI 3-6(a-c) (Figure 8.2a) that was synthesized by varying the substitution position of the dendrons at the phenoxy-spacer from ortho to meta or para and by introducing an additional methyl group in ortho-position. Also the length and shape of the alkoxy chains was changed. Consequently, the impact of the sterical demand of the bay substituents concerning their phase properties, molecular arrangement and exciton coupling was investigated. POM, DSC and X-ray studies revealed the formation of only crystalline phase for the ortho-substituted PBIs 3a-c, whereas the other derivatives generated SC or LC phases. The main focus was the series with the n-C12-alkoxy chains. For the corresponding PBIs 4-6b columnar LC phases were confirmed. Retrostructural analysis by modelling and simulations gave indications for a single stranded organization for PBI 3b, a double-stranded helix for PBI 6b, a triple-stranded helical arrangement for PBI 5b and a quadruple-stranded helix for PBI 4b (Figure 8.2b-d). For all four derivatives the same molecular orientation within the columns as for PBI 4c was proven by polarized FT-IR and UV-Vis absorption studies in aligned thin films. The organization in helices of different number of strands in the Cr and LC phases of PBI 3b, 4b, 5b and 6b offered a unique possibility to elucidate the influence of particular packing arrangements on dye aggregate interactions with light. In particular, it can be investigated how exciton coupling of the dyes’ transition dipole moments and fluorescence properties are affected. In this context, the spectroscopic properties were investigated in thin film, which revealed a strong bathochromic shift of the absorption maxima compared to the monomers in solution in dependence on the number of strands for PBIs 4-6b in contrast to PBI 3b (Figure 8.2e). The same tendency was observed for the respective aggregates in MCH solution. The spectral changes obtained during concentration- and temperature-dependent UV-Vis absorption studies verified the formation of J-aggregates in MCH solution and solid state. The respective aggregates are highly likely formed via a nucleation-elongation growth mechanism. Appliance of Kasha’s exciton theory on the supramolecular aggregates revealed different contributions of H- and J-type coupling for the oligo-stranded helices. Under these considerations, it delivered an explanation for the absorption and fluorescence properties of the assemblies and declares the “best” J-aggregate for the double stranded arrangement of PBI 6b with purely negative couplings among neighbour molecules and a quantum yield above 74 % of the aggregates in MCH solution. With this H-bonded PBI-based library approach of twelve derivatives it could be shown how molecular engineering of perylene bisimide dyes can be used to design defined, complex supramolecular assemblies with unprecedented packing patterns and concomitant intriguing spectroscopic properties.
So far, the formation of defined liquid crystalline supramolecular structures of tetra-bay substituted PBIs by double H-bonding between free imide moieties and pi-pi interactions between the chromophores was demonstrated. The impact of the H-bonds on the molecular arrangement was investigated in the next part of this thesis. In this regard, PBIs 7 and 8 bearing a methyl or cyclohexyl group at the imide position (Figure 8.3a) were synthesized and compared to PBI 4c. The soft character of the solid state for PBIs 7 and 8 was confirmed by POM, DSC and X-ray analysis. The X-ray studies further revealed for both PBIs a change of the molecular assembly towards helical columnar structures of conventional pi-stacked chromophores (Figure 8.3b) when the directed H-bonds cannot contribute as noncovalent interactions to the assembly formation. Temperature-dependent UV-Vis absorption studies demonstrated the importance of H-bonding in MCH solution in the way that the formation of J-aggregates as for PBI 4c could not be observed for the imide substituted molecules. In the next step, the spectroscopic properties in thin film were investigated. For PBI 7 a J-type band and fluorescence spectra with an enlarged Stokes shift and increased fluorescence lifetime of 11.4 ns, compared to PBI 4c, was obtained, suggesting the generation of excimer type emission by considering the assumed conventional stacking of rotational displaced molecules from X-ray analysis. With polarized UV-Vis absorption experiments the orientation of the molecules perpendicular to the shearing direction and subsequently to the columnar axis was confirmed. These diverse investigations clearly demonstrated the imperative of H-bonds for stable, defined, LC J-aggregates with the transition dipole moments parallel to the columnar axis. With PBIs 7 and 8 it is impressively shown how small changes in the molecular structure influence the molecular arrangement dependent on the cooperation of non-covalent interactions like H-bonding and pi-pi stacking.
In the last part of this thesis the generation of two-dimensional LC arrangements is presented. Since tetra-bay substituted PBIs lead always to twisted cores preventing lamellar arrangement, here 1,7-disubstitution and the simultaneous retention of the free imide positions was chosen to generate LC lamellar phases of PBIs 9a, 9b and 10 (Figure 8.4a). This molecular design was expected to form planar perylene cores that can strongly interact by pi-pi stacking and H-bonding. POM, DSC and X-ray investigations of the compounds suggest lamellar LC phases for PBIs 9a and 9b and a soft phase for PBI 10. In this regard, the goal of the formation of LC lamellar phase of PBIs could be attained. The change from dendrons with n-C12-alkoxy chains to large fork-like mesogens like in 9b clearly changed the phase properties. PBI 9b exhibits the lowest clearing point, high phase stability, least viscosity, easy shearability at room temperature and phase transitions between lamellar and Colh phases dependent on temperature. The formation of H-bonds parallel to the layers was demonstrated by polarized FT-IR experiments for all three PBIs. Concentration-dependent UV-Vis absorption studies revealed the formation of a J-type aggregate, which seems to exhibit an overall two-dimensional structure. With STM investigations the formation of lamellar structures from drop-casted 9a and 10 solutions in 1-phenyloctane on HOPG surface could be observed. Figure 8.4b illustrates a schematic possible arrangement of the molecules in the layers (here exemplarily demonstrated for PBI 9a), which has to be further confirmed by modelling and simulations. Unfortunately, fluorescence investigations of the thin films revealed non- or only slightly emissive LC states, which make them negligible for photonic applications. Nevertheless, the synthesized and analyzed compounds might be an inspiration for further investigations on the path to two-dimensional exciton transport for photonic devices.
Summary The nature of the chemical bond is a topic under constant debate. What is known about individual molecular properties and functional groups is often taught and rationalized by explaining Lewis structures, which, in turn, make extensive use of the valence concept. The valence concept distinguishes between electrons, which do not participate in chemical interactions (core electrons) and those, which do (single, double, triple bonds, lone-pair electrons, etc.). Additionally, individual electrons are assigned to atomic centers. The valence concept is of paramount success: It allows the successful planning of chemical syntheses and analyses, it explains the behavior of individual functional groups, and, moreover, it provides the “language” to think of and talk about molecular structure and chemical interactions. The resounding success of the valence concept may be misleading to forget its approximative character. On the other hand, quantum mechanics provide in principle a quantitative description of all chemical phenomena, but there is no discrimination between electrons in quantum mechanics. From the quantum mechanical point of view there are only indistinguishable electrons in the field of the nuclei, i.e., it is impossible to assign a given electron to a particular center or to ascribe a particular purpose to individual electrons. The concept of indistinguishability of micro particles is founded on the Heisenberg uncertainty relation, which states, that wavepackets diverge in the 6N dimensional phase space, such that individual trajectories can not be identified. Hence it is a deep-rooted and approved physical concept. As an introduction to the present work density partitioning schemes were discussed, which divide the total molecular density into chemically meaningful areas. These partitioning schemes are intimately related to either the concepts of bound atoms in a molecule (as in the Atoms In Molecules theory (AIM) according to Bader or as in the Hirshfeld partitioning scheme) or to the concept of chemical structure in the sense of Lewis structures, which divide the total molecular density into core and valence density, where the valence density is split up again into bonding and non-bonding electron densities. Examples are early and recent loge theories, the topological analysis by means of the Electron Localization Function (ELF), and the Natural Bond Orbital (NBO) approach. Of these partitioning schemes, the theories according to Bader (AIM), to Becke and Edgecomb (ELF) and according to Weinhold (NBO and Natural Resonance Theory, NRT), respectively, were reviewed in detail critically. Points of criticism were explicated for each of the mentioned theories. Since theoretically derived electron densities are to be compared to experimentally derived densities, a brief introduction into the theory of X-ray di®raction experiments was given and the multipole formalism was introduced. The procedure of density refinement was briefly discussed. Various suggestions for improvements were developed: One strategy would be the employment of model parameters, which are to a maximum degree mutually orthogonal, with the object of minimizing correlations among the model parameters, e.g., to introduce nodal planes into the radial functions of the multipole model. A further suggestion involves the guidance of the iterative refinement procedure by an extremum principle, which states, that when di®erent solutions to the least squares minimization problem are available with about the same statistical measures of quality and with about the same residual density, then the solution is to prefer, which yields a minimum density at the bond critical point (BCP) and a maximum polarity in terms of the ratio of distances between the BCP and the nuclei. This suggestion is based on the well known fact, that the bond polarity (in terms of the ratio of distances between the BCP and the respective nuclei) is underestimated in the experiment. Another suggestion for including physical constraints is the explicit consideration of the virial theorem, e.g., by evaluating the integration of the Laplacian over the entire atomic basins and comparing this value to zero and to the value obtained from the integration of the electron gradient field over the atomic surface. The next suggestion was to explicitly use the electrostatic theorem of Feynman (often also denoted as Hellmann-Feynman theorem), which states, that the forces onto the nuclei can be calculated from the purely classical electrostatic forces of the electron distribution and the nuclei distribution. For a stationary system, these forces must add to zero. This also provides an internal quality criterion of the density model. This can be performed in an iterative way during the refinement procedure or as a test of the final result. The use of the electrostatic theorem is expected to reduce significantly correlations among static density parameters and parameters describing vibrations, since it is a valuable tool to discriminate between physically reasonable and artificial static electron densities. All of these mentioned suggestions can be applied as internal quality criteria. The last suggestion is based on the idea to initiate the experimental refinement with a set of model parameters, which is, as much as possible close to the final solution. This can be achieved by performing periodic boundary conditions calculations, from which theoretically created files are obtained, which contain the Miller indices (h, k, l) and the respective intensity I. This file is used for a model parameter estimation (refinement), which excludes vibrations. The resulting parameters can be used for the experimental refinement, where, in a first step, the density parameters are fixed to determine the parameters describing vibrations. For a fine tuning, again the electrostatic theorem and the other above mentioned suggestions could be applied. Theoretical predictions should not be biased by the method of computation. Therefore the dependence of the density analyzing tools on the level of calculation (method of calculation/basis set) and on the substituents in complex chemical bonding situations were evaluated in the second part of the present work. A number of compounds containing formal single and double sulfur nitrogen bonds was investigated. For these compounds, experimental data were also available. The calculated data were compared internally and with the experimental results. The internal comparison was drawn with regard to questions of convergency as well as with regard to questions of consistency: The resulting molecular properties from NBO/NRT analyses were found to be very stable, when the geometries were optimized at the respective level of theory. This stability is valid for variations in the methods of calculation as well as for variations in the basis set. Only the individual resonance weights of the contributing Natural Lewis Structures differed considerably depending on the level of calculation and depending on the substituents. However, the deviations were in both cases to a large extent within a limit which preserves the descending order of the leading resonance structure weights. The resulting bond orders, i.e., the total, covalent and ionic bond order from NRT calculations, were not affected by the shift in the resonance weights. The analysis of the bond topological parameters resulted in a discrimination between insensitive parameters and sensitive parameters. The stable parameters do neither depend strongly on the method of calculation nor on the basis set. Only minor variation occurs in the numerical values of these parameters, when the level of calculation is changed or even when other functional groups (H, Me, or tBu) are employed, as long as the methods of calculation do not drop considerably below a standard level. The bond descriptors of the sulfur nitrogen bonds were found to be also stable with respect to the functional groups R = H, R = Me, and R = tBu. Stable parameters are the bond distance, the density at the bond critical point (BCP) and the ratio of distances between the BCP and the nuclei A and B, which varies clearly when considering the formal bond type. For very small basis sets like the 3-21G basis set, this characteristic stability collapses. The sensitive parameters are based on the second derivatives of the density with respect to the coordinates. This is in accordance with the well known fact, that the total second derivative of the density with respect to the coordinates is a strongly oscillating function with positive as well as negative values. A profound deviation has to be anticipated as a consequence of strong oscillations. lambda3, which describes the local charge depletion in the direction of the interaction line, is the most varying parameter. A detailed analysis revealed that the position of the BCP in the rampant edge of the Laplacian distribution is responsible for the sensitivity of the numerical value of lambda3 in formal double bonds. Since the slope of the Laplacian assumes very high values in its rampant edge, a tiny displacement of the BCP leads already to a considerable change in lambda3. This instability is not a failure of the underlying theory, but it yields de facto to a considerable dependence of sensitive bond topological properties on the method of calculation and on the applied basis sets. Since the total second derivative is important to judge on the nature of the bond in the AIM theory (closed shell interactions versus shared interactions), the changes in lambda3 can lead to differing chemical interpretations. The comparison of theoretically derived bond topological properties of various sulfur nitrogen bonds provides the possibility to measure the self consistency of this data set. All data sets clearly exhibit a linear correlation between the bond distances and the density at the BCP on one hand and between the bond distances and the Laplacian values at the BCP on the other hand. These correlations were almost independent of the basis set size. In this context, the linear regression has to be regarded exclusively as a descriptive statistics tool. There is no correlation anticipated a priori. The formal bond type was found to be readily deducible from the theoretically obtained bond topological descriptors of the model systems. In this sense, the bond topological properties are self consistent despite of the numerical sensitivity of the derivatives, as exemplified above. Often, calculations are performed with the experimentally derived equilibrium geometries and not with optimized ones. Applying this approach, the computationally costly geometry optimizations are saved. Following this approach the bond topological properties were calculated using very flexible basis sets and employing the fixed experimental geometry (which, of course, includes the application of tBu groups). Regression coe±cients similar to those from optimized geometries were obtained for correlations between bond distances and the densities at the BCP as well as for the correlation between bond distances and the Laplacian at the BCP, i.e. the approach is valid. However, the data points scattered less and the coe±cient of correlation was clearly increased when geometry optimizations were performed beforehand. The comparison between data obtained from theory and experiment revealed fundamental discrepancies: In the data set of bond topological parameters from the experiment, the behavior of only 2 out of 3 insensitive parameters was comparable to the behavior of the theoretically obtained values, i.e. theoretical and experimental bond distances as well as theoretical and experimental densities at the BCP correlate. From the theoretically obtained data it was easy to deduce the formal bond type from the position of the BCP, since it changed in a systematic manner. The respective experimentally obtained values were almost constant and did not change systematically. For the SN bonds containing compounds, the total second derivative assumes exclusively negative values in the experiment. Due to the different internal behavior, experimentally and theoretically sensitive bond topological values could not be compared directly. The qualitative agreement in the Laplacian distribution, however, was excellent. In the third and last part of this work, the application to chemical systems follows. Formal hypervalent molecules, i.e. molecules where some atoms are considered to hold more than 8 electrons in their valence shell, were investigated. These were compounds containing sulfur nitrogen bonds (H(NtBu)2SMe, H2C{S(NtBu)2(NHtBu)}2, S(NtBu)2 and S(NtBu)3) and a highly coordinated silicon compound. The set of sulfur nitrogen compounds also contained a textbook example for valence expansion, the sulfur triimide. For these molecules, experimental reference values were available from high resolution X-ray experiments. The experimental results were in the case of the sulfur triimide not unique. Furthermore, from the experimental bond topological data no definite conclusion about the formal bonding type could be drawn. The situation of sulfur nitrogen bonds in the above mentioned set of molecules was analyzed in terms of a geometry discussion and by means of a topological analysis. The methyl-substituted isolated molecules served as model compounds. For the interpretation of the bonding situation additional NBO/NRT calculations were preformed for the sulfur nitrogen compounds and an ELF calculation and analysis was performed for the silicon compound. The ELF analysis included not only the presentation and discussion of the ELF-isosurfaces (eta = 0.85), but also the investigation of populations of disynaptic valence basins and the percentage contributions to these populations of the individual atoms when the disynaptic valence basins are split into atomic contributions according to Bader’s partitioning scheme. The question of chemical interest was whether hypervalency is present in the set of molecules or not. In the first case the octet rule would be violated, in the second case Pauling’s verdict would be violated. While the concept of hypervalency is well established in chemistry, the violation of Pauling’s verdict is not. The quantitative numbers of the sensitive bond topological values from theory and experiment were not comparable, since no systematic relationship between the experimentally and theoretically determined sensitive bond descriptors was found. However, the insensitive parameters are in good agreement and the qualitative Laplacian distribution is, with few exceptions, in excellent agreement. The formal bonding type was deduced from experimental and theoretical topological data by considering the number and shape of valence shell charge concentrations in proximity to the sulfur and nitrogen centers. The results from NBO/NRT calculations confirmed the findings. All employed density analyzing tools AIM, ELF and NBO/NRT coincided in describing the bonding situation in the formally hypervalent molecules as highly polar. A comparison and analysis of experimentally and theoretically derived electron densities led consistently to the result, that regarding this set of molecules, hypervalency has to be excluded unequivocally.
A new perylene bisimide (PBI), with a fluorescence quantum yield up to unity, self‐assembles into two polymorphic supramolecular polymers. This PBI bears four solubilizing acyloxy substituents at the bay positions and is unsubstituted at the imide position, thereby allowing hydrogen‐bond‐directed self‐assembly in nonpolar solvents. The formation of the polymorphs is controlled by the cooling rate of hot monomer solutions. They show distinctive absorption profiles and morphologies and can be isolated in different polymorphic liquid‐crystalline states. The interchromophoric arrangement causing the spectral features was elucidated, revealing the formation of columnar and lamellar phases, which are formed by either homo‐ or heterochiral self‐assembly, respectively, of the atropoenantiomeric PBIs. Kinetic studies reveal a narcissistic self‐sorting process upon fast cooling, and that the transformation into the heterochiral (racemic) sheetlike self‐assemblies proceeds by dissociation via the monomeric state.
The research presented in this thesis illustrates that self-assembly of organic molecules guided by intermolecular forces is a versatile bottom-up approach towards functional materials. Through the specific design of the monomers, supramolecular architectures with distinct spatial arrangement of the individual building blocks can be realized. Particularly intriguing materials can be achieved when applying the supramolecular approach to molecules forming liquid-crystalline phases as these arrange in ordered, yet mobile structures. Therefore, they exhibit anisotropic properties on a macroscopic level. It is pivotal to precisely control the interchromophoric arrangement as functions originate in the complex structures that are formed upon self-assembly. Consequently, the aim of this thesis was the synthesis and characterization of liquid-crystalline phases with defined supramolecular arrangements as well as the investigation of the structure-property relationship. For this purpose, perylene bisimide and diketopyrrolopyrrole chromophores were used as they constitute ideal building blocks towards functional supramolecular materials due to their thermal stability, lightfastness, as well as excellent optical and electronic features desirable for the application in, e.g., organic electronics.
This thesis established the fabrication of organic solar cells of DA dye donors and fullerene acceptors under ambient conditions in our laboratory, however, with reduced power conversion efficiencies compared to inert conditions. It was shown that moisture had the strongest impact on the stability and reproducibility of the solar cells. Therefore, utilization of robust materials, inverted device architectures and fast fabrication/characterization are recommended if processing takes place in air. Furthermore, the dyad concept was successfully explored in merocyanine dye-fullerene dyads and power conversion efficiencies of up to 1.14 % and 1.59 % were measured under ambient and inert conditions, respectively. It was determined that the major drawback in comparison to comparable BHJ devices was the inability of the dyad molecules to undergo phase separation. Finally, two series of small molecules were designed in order to obtain electron transport materials, using the acceptor-core-acceptor motive. By variation of the acceptor units especially the LUMO levels could be lowered effectively. Investigation of the compounds in organic thin film transistors helped to identify promising molecules with electron transport properties. Electron transport mobilities of up to 7.3 × 10−2 cm2 V−1 s−1 (ADA2b) and 1.39 × 10−2 cm2 V−1 s−1 (AπA1b) were measured in air for the ADA and AπA dyes, respectively. Investigation of selected molecules in organic solar cells proved that these molecules work as active layer components, even though power conversion efficiencies cannot compete with fullerene based devices yet. Thus, this thesis shows new possibilities that might help to develop and design small molecules as substitutes for fullerene acceptors.
Understanding relationships between microstructure and electrical transport is an important goal for the materials science of organic semiconductors. Combining high-resolution surface potential mapping by scanning Kelvin probe microscopy (SKPM) with systematic field effect transport measurements, we show that step edges can trap electrons on the surfaces of single crystal organic semiconductors. n-type organic semiconductor crystals exhibiting positive step edge surface potentials display threshold voltages that increase and carrier mobilities that decrease with increasing step density, characteristic of trapping, whereas crystals that do not have positive step edge surface potentials do not have strongly step density dependent transport. A device model and microelectrostatics calculations suggest that trapping can be intrinsic to step edges for crystals of molecules with polar substituents. The results provide a unique example of a specific microstructure–charge trapping relationship and highlight the utility of surface potential imaging in combination with transport measurements as a productive strategy for uncovering microscopic structure–property relationships in organic semiconductors.
Physical properties of active materials built up from small molecules are dictated by their molecular packing in the solid state. Here we demonstrate for the first time the growth of n-channel single-crystal field-effect transistors and organic thin-film transistors by sublimation of 2,6-dichloro-naphthalene diimide in air. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode (\(\beta\)-phase) is obtained that is distinguished from the previously reported herringbone packing motif obtained from solution (\(\alpha\)-phase). We are able to fabricate single-crystal field-effect transistors with electron mobilities in air of up to 8.6 cm\(^{2}\)V\(^{-1}\)s\(^{-1}\) (\(\alpha\)-phase) and up to 3.5 cm\(^{2}\)V\(^{-1}\)s\(^{-1}\) (\(\beta\)-phase) on n-octadecyltriethoxysilane-modified substrates. On silicon dioxide, thin-film devices based on \(\beta\)-phase can be manufactured in air giving rise to electron mobilities of 0.37 cm\(^{2}\)V\(^{-1}\)s\(^{-1}\). The simple crystal and thin-film growth procedures by sublimation under ambient conditions avoid elaborate substrate modifications and costly vacuum equipment-based fabrication steps.
Up to three polychlorinated pyridyldiphenylmethyl radicals bridged by a triphenylamine carrying electron withdrawing (CN), neutral (Me), or donating (OMe) groups were synthesized and analogous radicals bridged by tris(2,6‐dimethylphenyl)borane were prepared for comparison. All compounds were as stable as common closed‐shell organic compounds and showed significant fluorescence upon excitation. Electronic, magnetic, absorption, and emission properties were examined in detail, and experimental results were interpreted using DFT calculations. Oxidation potentials, absorption and emission energies could be tuned depending on the electron density of the bridges. The triphenylamine bridges mediated intramolecular weak antiferromagnetic interactions between the radical spins, and the energy difference between the high spin and low spin states was determined by temperature dependent ESR spectroscopy and DFT calculations. The fluorescent properties of all radicals were examined in detail and revealed no difference for high and low spin states which facilitates application of these dyes in two‐photon absorption spectroscopy and OLED devices.
Fluorescence enhancement of a high-mobility polymer semiconductor is achieved via energy transfer to a higher fluorescence quantum yield squaraine dye molecule on 50 ps timescales. In organic light-emitting diodes, an order of magnitude enhancement of the external quantum efficiency is observed without reduction in the charge-carrier mobility resulting in radiances of up to 5 W str\(^{-1}\) m\(^{-2}\) at 800 nm.
Expansion microscopy (ExM) enables super-resolution imaging of proteins and nucleic acids on conventional microscopes. However, imaging of details of the organization of lipid bilayers by light microscopy remains challenging. We introduce an unnatural short-chain azide- and amino-modified sphingolipid ceramide, which upon incorporation into membranes can be labeled by click chemistry and linked into hydrogels, followed by 4x to 10x expansion. Confocal and structured illumination microscopy (SIM) enable imaging of sphingolipids and their interactions with proteins in the plasma membrane and membrane of intracellular organelles with a spatial resolution of 10-20nm. As our functionalized sphingolipids accumulate efficiently in pathogens, we use sphingolipid ExM to investigate bacterial infections of human HeLa229 cells by Neisseria gonorrhoeae, Chlamydia trachomatis and Simkania negevensis with a resolution so far only provided by electron microscopy. In particular, sphingolipid ExM allows us to visualize the inner and outer membrane of intracellular bacteria and determine their distance to 27.6 +/- 7.7nm. Imaging of lipid bilayers using light microscopy is challenging. Here the authors label cells using a short chain click-compatible ceramide to visualize mammalian and bacterial membranes with expansion microscopy.
New synthetic methodologies for the formation of block copolymers have revolutionized polymer science within the last two decades. However, the formation of supramolecular block copolymers composed of alternating sequences of larger block segments has not been realized yet. Here we show by transmission electron microscopy (TEM), 2D NMR and optical spectroscopy that two different perylene bisimide dyes bearing either a flat (A) or a twisted (B) core self-assemble in water into supramolecular block copolymers with an alternating sequence of (A\(_{m}\)BB)\(_{n}\). The highly defined ultralong nanowire structure of these supramolecular copolymers is entirely different from those formed upon self-assembly of the individual counterparts, that is, stiff nanorods (A) and irregular nanoworms (B), respectively. Our studies further reveal that the as-formed supramolecular block copolymer constitutes a kinetic self-assembly product that transforms into thermodynamically more stable self-sorted homopolymers upon heating.
Perylene bisimide hydrogels and lyotropic liquid crystals with temperature-responsive color change
(2016)
The self-assembly of perylene bisimide (PBI) dyes bearing oligo ethylene glycol (OEG) units in water affords responsive functional nanostructures characterized by their lower critical solution temperature (LCST). Tuning of the LCST is realized by a supramolecular approach that relies on two structurally closely related PBI–OEG molecules. The two PBIs socially co-assemble in water and the resulting nanostructures exhibit a single LCST in between the transition temperatures of the aggregates formed by single components. This permits to precisely tune the transition from a hydrogel to a lyotropic liquid crystal state at temperatures between 26 and 51 °C by adjusting the molar fraction of the two PBIs. Owing to concomitant changes in PBI–PBI interactions this phase transition affords a pronounced color change with “fluorescence-on” response that can be utilized as a smart temperature sensory system.
In summary, it can be stated that the herein studied set of acceptor-substituted squaraine dyes can be seen as potent candidates for OTFTs. Furthermore, their transistor performance can be easily tuned to obtain hole mobilities up to 0.45 cm2/Vs from solution and 1.3 cm2/Vs from sublimation by choosing adequate deposition techniques. In the end, a probable structural model derived from studies of the thin-film morphology by methods such as optical spectroscopy, AFM and X-ray even facilitated the clarification of the observed charge transport behavior.
Paclitaxel (PTX) is one of the leading drugs against breast and ovarian cancer. Due to its low solubility, treatment of the patients with this drug requires a very well-suited combination with a soluble pharmaceutical excipient to increase the bioavailability and reduce the strong side ef-fects. One efficient way to achieve this in the future could be the incorporation of PTX into pol-ymeric micelles composed of poly(2-oxazoline) based triblock copolymers (POL) which ena-bles PTX loadings of up to 50 wt.%. However, structural information at an atomic level and thus the knowledge of interaction sites within these promising but complex PTX-POL formula-tions were not yet available. Such results could support the future development of improved excipients for PTX and suitable excipients for other pharmaceutical drugs. Therefore, a solid-state MAS NMR investigation of these amorphous formulations with different POL-PTX com-positions was performed in this thesis as this gives insights of the local structure at an atomic level in its solid state. NMR in solution showed very broad 13C signals of PTX for this system due to the reduced mobility of the incorporated drug which exclude this as an analytical meth-od.
In a first study, crystalline PTX was structurally characterized by solid-state NMR as no com-plete 13C spectrum assignment and no 1H NMR data existed for the solid state. In addition, the asymmetric unit of the PTX crystal structure consists of two molecules (Z'=2) that can only be investigated in its solid state. As crystalline PTX in total has about 100 different 13C and 1H chemical shifts with very small differences due to Z’=2, and furthermore, its unit cell consisting of more than 900 atoms, accompanying GIPAW (CASTEP) calculations were required for NMR signal assignments. These calculations were performed using the first three available purely hydrous and anhydrous PTX structures, which were determined by XRD and published by Vel-la-Zarb et al. in 2013. Within this thesis, is was discovered that two investigated batches of commercially available PTX from the same supplier both contained an identical and so far un-known PTX phase that was elucidated by PXRD as well as solid-state NMR data. One of the two batches consists of an additional phase that was shown to be very similar to a known hy-drated phase published in 2013.[1] By heating the batch with the mixture of the two phases un-der vacuum, it is transformed completely to the new dry phase occurring in both PTX batches. Since the drying conditions to obtain anhydrous PTX in-situ on the PXRD setup described by Vella-Zarb et. al.[1] were much softer than ours, we identify our dry phase as a relaxed version of their published anhydrate structure. The PXRD data of the new anhydrate phase was trans-ferred into a new structural model, which currently undergoes geometry optimization. Based on solid-state NMR data at MAS spinning frequencies up to 100 kHz, a 13C and a partial 1H signal assignment for the new anhydrous structure were achieved. These results provided sufficient structural information for further investigations of the micellar POL-PTX system.
In a second study, the applicability and benefit of two-dimensional solid-state 14N-1H HMQC MAS NMR spectra for the characterization of amorphous POL-PTX formulations was investi-gated. The mentioned technique has never been applied to a system of similar complexity be-fore and was chosen because around 84% of the small-molecule drugs contain at least one nitrogen atom. In addition, the number of nitrogen atoms in both POL and PTX is much smaller than the number of carbons or hydrogens, which significantly reduces the spectral complexity. 14N has a natural abundance of 99.6% but leads to quadrupolar broadening due to its nuclear spin quantum number I = 1. While this is usually undesirable due to broadening in the resulting 1D 14N NMR spectra, this effect is explicitly used in the 2D 14N-1H HMQC MAS experiment. The indirect 14N measurement can avoid the broadening while maintaining the advantage of the high natural abundance and making use of the much more dispersed signals due to the additional quadrupolar shifts as compared to 15N.
This measurement method could be successfully applied to the complex amorphous POL-PTX mixtures. With increasing PTX loading of the formulations, additional peaks arise as spatial proximities of the amide nitrogens of POL to NH or OH groups of PTX. In addition, the 14N quadrupolar shift of these amide nitrogens decreases with increasing PTX content indicating a more symmetric nitrogen environment. The latter can be explained by a transformation of the trigonal planar coordination of the tertiary amide nitrogen atoms in pure POL towards a more tetrahedral environment upon PTX loading induced by the formation of hydrogen bonds with NH/OH groups of PTX.
In the third and last project, the results of the two abovementioned studies were used and ex-tended by solid state 13C and two-dimensional 1H-13C as well as 1H-1H MAS NMR data with the aim to derive a structural model of the POL-PTX formulations at an atomic level. The knowledge of the NMR signal assignments for crystalline PTX was transferred to amorphous PTX (present in the micelles of the formulations). The 13C solid-state NMR signals were evalu-ated concerning changes in chemical shifts and full widths of half maximum (FWHM) for the different PTX loadings. In this way, the required information about possible interaction sites at an atomic level becomes available. Due to the complexity of these systems, such proximities often cannot be assigned to special atoms, but more to groups of atoms, as the individual de-velopments of line widths and line shifts are mutually dependent. An advantageous aspect for this analysis was that pure POL already forms unloaded micelles. The evaluation of the data showed that the terminal phenyl groups of PTX seem to be most involved in the interaction by the establishment of the micelle for lowest drug loading and that they are likely to react to the change in the amount of PTX molecules as well. For the incorporation of PTX in the micelles, the following model could be obtained: For lowest drug loading, PTX is mainly located in the inner part of the micelles. Upon further increasing of the loading, it progressively extends to-ward the micellar shell. This could be well shown by the increasing interactions of the hydro-phobic butyl chain of POL and PTX, proceeding in the direction of the polymer backbone with rising drug load. Furthermore, due to the size of PTX and the hydrodynamic radius of the mi-celles, even at the lowest loading, the PTX molecules partially reach the core-shell interface of the micelle. Upon increasing the drug loading, the surface coverage with PTX clusters increas-es based on the obtained model approach. The latter result is supported by DLS and SANS data of this system. The abovementioned results of the 14N-1H HMQC MAS investigation of the POL-PTX formulations support the outlined model.
As an outlook, the currently running geometry optimization and subsequently scheduled calcu-lation of the chemical shieldings of the newly obtained anhydrous PTX crystal structure can further improve the solid-state NMR characterization through determination of further spatial proximities among protons using the existing 2D 1H(DQ)-1H(SQ) solid-state MAS NMR spec-trum at 100 kHz rotor spinning frequency. The 2D 14N-1H HMQC MAS NMR experiments were shown to have great potential as a technique for the analysis of other disordered and amor-phous drug delivery systems as well. The results of this thesis should be subsequently applied to other micellar systems with varying pharmaceutical excipients or active ingredients with the goal of systematically achieving higher drug loadings (e.g., for the investigated PTX, the similar drug docetaxel or even different natural products). Additionally, it is planned to transfer the knowledge to another complex polymer system containing poly(amino acids) which offers hy-drogen bonding donor sites for additional intermolecular interactions. Currently, the POL-PTX system is investigated by further SANS studies that may provide another puzzle piece to the model as complementary measurement method in the future. In addition, the use of MD simu-lations might be considered in the future. This would allow a computerized linking of the differ-ent pieces of information with the aim to determine the most likely model.
Water‐soluble cationic perylene diimide dyes as stable photocatalysts for H\(_2\)O\(_2\) evolution
(2023)
Photocatalytic generation of hydrogen peroxide, H\(_2\)O\(_2\), has gained increasing attention in recent years, with applications ranging from solar energy conversion to biophysical research. While semiconducting solid‐state materials are normally regarded as the workhorse for photogeneration of H\(_2\)O\(_2\), an intriguing alternative for on‐demand H\(_2\)O\(_2\) is the use of photocatalytic organic dyes. Herein we report the use of water‐soluble dyes based on perylene diimide molecules which behave as true molecular catalysts for the light‐induced conversion of dissolved oxygen to hydrogen peroxide. In particular, we address how to obtain visible‐light photocatalysts which are stable with respect to aggregation and photochemical degradation. We report on the factors affecting efficiency and stability, including variable electron donors, oxygen partial pressure, pH, and molecular catalyst structure. The result is a perylene diimide derivative with unprecedented peroxide evolution performance using a broad range of organic donor molecules and operating in a wide pH range.
Two different chromophores, namely a dipolar and an octupolar system, were prepared and their linear and nonlinear optical properties as well as their bioimaging capabilities were compared. Both contain triphenylamine as the donor and a triarylborane as the acceptor, the latter modified with cationic trimethylammonio groups to provide solubility in aqueous media. The octupolar system exhibits a much higher two‐photon brightness, and also better cell viability and enhanced selectivity for lysosomes compared with the dipolar chromophore. Furthermore, both dyes were applied in two‐photon excited fluorescence (TPEF) live‐cell imaging.