Refine
Has Fulltext
- yes (366)
Is part of the Bibliography
- yes (366)
Year of publication
Document Type
- Journal article (237)
- Doctoral Thesis (113)
- Preprint (14)
- Book article / Book chapter (1)
- Report (1)
Language
- English (366) (remove)
Keywords
- Organische Chemie (68)
- Supramolekulare Chemie (21)
- Selbstorganisation (18)
- self-assembly (16)
- Farbstoff (13)
- perylene bisimide (13)
- fluorescence (12)
- water oxidation (12)
- Merocyanine (11)
- RNA (11)
Institute
- Institut für Organische Chemie (366) (remove)
Schriftenreihe
Sonstige beteiligte Institutionen
- International Max Planck Research School Molecular Biology, University of Göttingen, Germany (2)
- Agricultural Center, BASF SE, 67117 Limburgerhof, Germany (1)
- Center for Nanoscale Microscopy and Molecular Physiology of the Brain (CNMPB), Göttingen, Germany (1)
- Center for Nanosystems Chemistry (1)
- Center for Nanosystems Chemistry (CNC), University of Würzburg (1)
- Center for Nanosystems Chemistry (CNC), Universität Würzburg, Am Hubland, 97074 Würzburg, Germany (1)
- Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16 Prague, Czech Republic (1)
- Cluster of Excellence "Multiscale Bioimaging: from Molecular Machines to Networks of Excitable Cells, Göttingen (1)
- Department of Cellular Biochemistry, University Medical Center Göttingen (1)
- Department of Cellular Biochemistry, University Medical Centre Göttingen (1)
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.
A new twelvefold methoxy-triethyleneglycol-jacketed tetraphenoxy-perylene bisimide (MEG-PBI) amphiphile was synthesized that self-assembles into two types of supramolecular aggregates in water: red-coloured aggregates of low order and with weak exciton coupling among the PBIs and blue-coloured strongly coupled J-aggregates consisting of a highly ordered hydrogen-bonded triple helix of PBIs. At room temperature this PBI is miscible with water at any proportions which enables the development of robust dye aggregates in solution, in hydrogel states and in lyotropic liquid crystalline states. In the presence of 60–95 wt% water, self-standing coloured hydrogels exhibit colour changes from red to blue accompanied by a fluorescence light-up in the far-red region upon heating in the range of 30–50 °C. This phenomenon is triggered by an entropically driven temperature-induced hydrogen-bond-directed slipped stacking arrangement of the MEG-PBI chromophores within structurally well-defined J-aggregates. This versatile aqua material is the first example of a stable PBI J-aggregate in water. We anticipate that this study will open a new avenue for the development of biocompatible functional materials based on self-assembled dyes and inspire the construction of other hydrogen-bonded supramolecular materials in the highly competitive solvent water.
The He (I) photoelectron spectra of 2-bicyclo[2.1.l]hexene (1), 2,3-bis(methylene)bicyclo[2.1.l]hexane (3), and 3,4-bis(methylene)tricyclo[3.l.O.0\(^{2.6}\)]hexane (4) have been investigated. The assignment given is based on a ZDO model and semiempirical calculations. Tagether with the PE data of benzvalene (2), the reported data allow a comparison between 1-2 and 3-4. This yields a measure of the interactions between 8 cyclobutane or 8 bicyclobutane moiety and a double bond system within a ZDO model. The resonance integral found in the case of 1 and 3 amounts to -1.9 eV, that for 2 and 4, to -2.3 eV. The investigations furthermore reveal that the electronic factors which contribute to the higher reactivity of the bicyclobutane compounds amount to 5 kcal/mol.
The He I photoelectron (PE) spectra of octavalene (5) as weil as its hydrogenated products 6-8 have been investigated. The assignment given is based on an empirical comparison of 5-8 with related compounds, a ZDO model, and semiempirical and ab initio calculations. Within the ZDO model the interaction between the buta.diene moiety and the bicyclobutane fragment of 5 is described by a resonance integral of -2.3 eV. The orbitalsequence of 5 is found tobe 2a\(_2\) (\(\pi\)-\(\sigma\)), 9a\(_1\) (\(\sigma\)), 3b1 (\(\pi\) - \(\sigma\)), 1a\(_2\) (\(\sigma\) + \(\pi\)), 2b\(_1\) (\(\sigma\) + \(\pi\)).
A new Ru oligomer of formula {[Ru-\(^{II}\)(bda-\(\kappa\)-N\(^2\)O\(^2\))(4,4'-bpy)]\(_{10}\)(4,4'-bpy)}, 10 (bda is [2,2'-bipyridine]-6,6'-dicarbox-ylate and 4,4'-bpy is 4,4'-bipyridine), was synthesized and thoroughly characterized with spectroscopic, X-ray, and electrochemical techniques. This oligomer exhibits strong affinity for graphitic materials through CH-\(\pi\) interactions and thus easily anchors on multiwalled carbon nanotubes (CNT), generating the molecular hybrid material 10@CNT. The latter acts as a water oxidation catalyst and converts to a new species, 10'(H\(_2\)O)\(_2\)@CNT, during the electrochemical oxygen evolution process involving solvation and ligand reorganization facilitated by the interactions of molecular Ru catalyst and the surface. This heterogeneous system has been shown to be a powerful and robust molecular hybrid anode for electrocatalytic water oxidation into molecular oxygen, achieving current densities in the range of 200 mA/cm\(^2\) at pH 7 under an applied potential of 1.45 V vs NHE. The remarkable long-term stability of this hybrid material during turnover is rationalized based on the supramolecular interaction of the catalyst with the graphitic surface.
A perylene bisimide dye bearing amide functionalities at the imide positions derived from amino acid L-alanine and a dialkoxy-substituted benzyl amine self-assembles into tightly bound dimers by π-π-stacking and hydrogen bonding in chloroform. In less polar or unpolar solvents like toluene and methylcyclohexane, and in their mixtures, these dimers further self-assemble into extended oligomeric aggregates in an anti-cooperative process in which even numbered aggregates are highly favoured. The stepwise transition from dimers into oligomers can not be properly described by conventional K\(_2\)-K model, and thus a new K\(_2\)-K aggregation model has been developed, which interpretes the present anti-cooperative supramolecular polymerization more appropriately. The newly developed K\(_2\)-K model will be useful to describe self-assembly processes of a plethora of other π-conjugated molecules that are characterized by a favored dimer species.
The self-assembly of molecules based on π-π-interactions and hydrogen bonding is of significant importance in nature. These processes enable the formation of complex supramolecular structures with diverse functions. For the transfer of the concepts from nature to artificial supramolecular structures, a basic understanding of those processes is needed. For this purpose, π-conjugated aromatic molecules with an easy synthetic access are suitable as their functionalities can be changed effortless. Perylene bisimide (PBIs) dyes are attractive candidates since they fulfill these requirements owing to their tendency to self-assemble in solution due to their large aromatic π-surfaces. Furthermore, the changes of the optical properties (for instance absorption, emission or circular dichroism) of PBI dyes, caused by their self-assembly, are easy to study experimentally. Structural variations of PBI dyes including additional non-covalent interactions, such as hydro-gen bonding, enable to direct their self-assembly process. Thus, the formation of interesting su-pramolecular structures of PBI dyes could be realized, although, often of undefined size. The aim of this thesis was to develop strategies to restrict the aggregate size of PBI dyes. Therefore, de-fined structural features of PBI molecules were combined and a variation of external influences such as solvent and concentration included. Furthermore, DNA was utilized as a template for the limitation of the aggregate size of PBI dyes.
Chapters 1 and 2 provide general information and describe examples from literature which are necessary to understand the following experimental work. The first chapter is based on the inter-actions of various molecules with DNA. Therefore, DNA is considered as a supramolecular biom-acromolecule containing specific structural and functional features to interact with small mole-cules. Afterwards, the main interaction modes of small molecules with DNA such as electrostatic interaction, intercalation and groove binding with corresponding examples are discussed. Among all techniques applied to study the interaction of ligands with DNA, UV/Vis absorption, fluores-cence and circular dichroism spectroscopy were described in detail. At the end of this chapter, examples of already pre-associated systems showing interactions with DNA are presented.
The second chapter is focused on the determination and mathematic evaluation of the self-assembly processes. The simplest models such as monomer-dimer and isodesmic model are de-scribed and supplemented by examples. Furthermore, the simplest modification of the isodesmic model, the K2-K model, is presented. Additionally, experimental problems, which may arise dur-ing the investigations of the self-assembly processes, are addressed. For the description of the entire self-assembly process, a sufficiently large concentration range and an appropriate measure-ment method that is sensitive in this concentration range is necessary. Furthermore, the full transi-tion from the monomeric to the aggregated species has to be spectroscopically ascertainable. This enables an accurate mathematic evaluation of the self-assembly process and provides meaningful binding constants. The self-assembly pathway can be controlled by the variation of solvent, con-centration or temperature. However, this pathway can also be directed by a rational design of the molecular structure of the considered system. For example, a specific interplay of π-π-interactions and hydrogen bonding may promote isodesmic as well as cooperative growth into large struc-tures.
The main focus of this thesis is to develop strategies to control the aggregate size of PBI dyes (Chapter 3). For this purpose, a PBI scaffold was designed which contains hydrogen bonding amide functions at the imide positions derived from the amino acid L-alanine and solubilizing side groups in the periphery (Figure 81). The variations of the residues R/R’ range from didodecylox-yphenyl, didodecylphenyl, dioligo(ethylene glycol)phenyl to branched and linear alkyl chains.
The most extensive study of the aggregation behavior was performed for the PBI dye 5. Concen-tration-dependent 1H NMR and UV/Vis absorption measurements clearly revealed the formation of dimers in chloroform. Further investigations by means of 2D NMR, VPO and ITC confirmed the exclusive presence of dimer aggregates of PBI 5 in the investigated concentration range. Mo-lecular modelling studies, supported by NMR and FT-IR experiments, provided structural reasons for the absence of further growth into larger aggregates. The specific combination of π-π interac-tions and hydrogen bonds between the NH groups of the amide groups and the carbonyl oxygen atoms of the PBI core are decisive for the formation of the discrete dimer stack (see Figure 82). The investigations of the aggregation behavior of PBIs 6-9 were less extensive but consistent with the results obtained for PBI 5. However, the determined binding constants vary over a considera-ble range of 1.1 x 102 M-1 (PBI 8) to 1.4 x 104 M-1 (PBI 5). These differences could be attributed to structural variations of the dyes. The electron-rich phenyl substituent promoted the aggregation tendency of PBIs 5-7 compared with 8 and 9 that carry only alkyl side chains. Thus, the π-π in-teractions of bay-unsubstituted PBI cores in combination with hydrogen bonding of the amide functions control the formation of discrete dimers of these PBI dyes.
The variation of conditions, such as solvent, change the aggregation behavior of PBI dyes. In the solvents toluene and/or methylcyclohexane, anti-cooperative growth into larger aggregates of PBI 5 was observed (Chapter 4). The important feature of this self-assembly process is the absence of isosbestic points over the whole concentration range in the UV/Vis absorption measurements. The preference for the dimeric species of PBI 5 remained in both solvents as well as in mixtures of them, but upon increasing the concentration these dimers self-assemble into larger aggregates.
An important feature of the self-assembly process is the preferred formation of even-numbered aggregates compared to the odd-numbered ones (see Figure 83). Although, the conventional K2-K model provides plausible binding constants, it is not capable to describe the aggregation behavior adequately, since it considers a continuous size distribution. The gradual aggregation process over dimers, tetramers, hexamers, etc. was therefore analyzed with a newly developed K2-K model for anti-cooperative supramolecular polymerization. By the global analysis of the UV/Vis absorption spectra a very good agreement between the experimental and simulated spectra, which were based on the new K2-K model, was obtained. Furthermore, the calculated UV/Vis absorption spectra of a dimer and an aggregate highlighted the most important structural differences. The absorption spectrum of the dimer still has a pronounced vibronic structure which gets lost in the spectrum of the aggregate.
In another part of this work, a series of water soluble PBI dyes were described which contain similar PBI scaffolds as PBIs 5-8 (Chapter 5). These PBI dyes self-assemble into similar dimer aggregates in water due to their positively charged side chains causing electrostatic repulsion be-tween the molecules (see Figure 84). Here, however, the self-assembly behavior has not been studied thoroughly in water due to the similarities of already reported PBI dyes.
Instead, the focus here is on the characterization of the interactions of these dyes with DNA/RNA. The comprehensive studies using thermal denaturation experiments showed the high stability of these PBI/polynucleotide complexes. The spermine-functionalized PBI dyes having six positive charges showed strong interactions with DNA/RNA which was expressed in a signif-icant increase of the melting temperatures of DNA/RNA (ΔTm values between 7 and > 35 ° C). The dioxa analogues containing only two positive charges had lower enhancement of the melting temperature of DNA/RNA (ΔTm values between 3 and 30 ° C). A similar trend has been observed in the fluorimetric titrations. The spermine-functionalized PBI dyes showed high binding con-stants (log Ks = 9.2 - 9.8), independently of the used polynucleotides. In contrast, the dioxa ana-logues displayed smaller binding constants (log Ks = 6.5 - 7.9) without any correlation between binding affinity and binding strength of the PBI dyes and the applied polynucleotides. The CD-spectroscopic measurements revealed significant differences in the binding properties of the dyes with DNA/RNA. They were dependent on the steric hindrance of the amino acid residues at the imide position and their configuration on one side and the grooves properties of ds-DNA/RNA on the other side. The spectroscopic results confirmed the formation of excitonically coupled PBI dimers in the minor groove of ds-DNA and the major groove of ds-RNA. Depending on the se-quence, the grooves of the polynucleotides provide different amount of space for embedding molecules. The guanine amino groups protrude into the minor groove of the polynucleotide poly(dG-dC)2 increasing the steric hindrance, which is not the case for poly(dA-dT)2. Molecular modeling studies showed that the PBI dimers penetrate deeper into the groove of poly(dA-dT)2 due to the absence of the steric hindrance, in comparison to the groove of poly(dG-dC)2 (see Figure 85).
In the first part of this work we presented the synthesis and photophysical properties of a series of transition metal donor-acceptor Ir(III)complexes of the type [(C^N)2Ir(N^N)][PF6]. The Ir(III) was connected with hole conducting donor-moieties like carbazole (CZ) and triarylamine (TAA) linked via a methylene and ethylene bridge to the cyclometalating C^N ligands phenylpyrazole (ppz) and phenylpyridine (ppy). Bidentate N^N and P^P ligands like 2,2’-bipyridyl (bpy), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp) and cis-1,2-bis(diphenylphosphino)ethylene (bdppe) were used as acceptor units. In order to analyse the influence of the electron density of the bpy ligand, TAA-complexes with acceptor- and donor-substituted bpy acceptor units were synthesised. Therefore, 4,4’-dinitro-2,2’-bipyridyl, 4,4’-dichloro-2,2’-bipyridyl, 4,4’-dimethoxy-2,2’-bipyridyl and 4,4’-dimethylamino-2,2’bipyridyl were used as neutral N^N ligands. In order to compare the photophysical properties, all reference compounds without hole conducting component were syntesised. All the carbazole compounds, except the bdppe complexes, exhibit emission and transient absorption properties similar to their reference compounds that make them interesting for OLED (organic light emitting device) applications. LEC (light emitting electrochemical cell) studies show a red shifted luminescence. The triarylamine compounds do not luminesce at RT but they exhibit an intense, blue-shifted and long-lived luminescence at 77 K in a rigid matrix. The transient absorption spectra differ strongly from that of their reference compounds. The spectra display characteristic features of the spectra of the isolated radical anions and cations supported by spectroelectrochemical measurements. Thus, it can be assumed that the transient states are charge separated (CS) states in which the positive charge is localised at the TAA donor units and the negative charge at the N^N acceptor units. The decays of the transient states are biexponentially what indicates the presence of two transient states, the 1CS and the 3CS state. To understand this behaviour the differently substituted bipyridyl-complexes were synthesised and analysed. Temperature dependent transient absorption measurements showed that all rate constants are indepentend of the temperature, except for the complex with OMe subsituents at the bpy ligand. The equilibrium constant K = k1 / k2 is nearly one for all complexes. For the OMe-compound it decreases with increasing temperature. Plotting the rate constants vs. the free energy differences (determined by cyclovoltammetry measurements) shows that all constants are decreasing with increasing donor strength of the bpy ligand. DFT calculations on the OMe-compound are already in work. In the second part of this work, neutral Ir(III) and Pt(II) complexes of the type [(O^O)Ir(N^N)2] and [(O^O)Pt(N^N)] were introduced. There, TTA was connected directly or via a CH2 bridge to acectylacetonate (acac = O^O) in order to probe the influence of the different kinds of connection on the photophysics of the complexes. As the bidentate N^N ligand 2,2’-bipyridyl (bpy) was chosen. All the corresponding reference compounds without triarylamine were obtained in order to compare with the TAA substituted analoga. Furthermore, the homoleptic fac Ir(N^N)3 complex with triarylamine connected via a methylene and ethylene bridge to phenylpyrazole as introduced in the first part of this work was synthesised. The synthesis of the Ir(III) compound with the TAA substituted acac ligand connected via the CH2 group was not successful. All the neutral triarylamine-substituted -diketonato Pt(II) and Ir(III) complexes do not luminesce at RT, except the Pt(II)-complex with CH2 bridge. This compound shows transient state characteristics that are in good agreement with the luminescence lifetimes at RT and that are similar to the reference compound, what suggests to a 3Pt(N^N)(O^O) state. The complexes without the CH2 bridging unit show no transient signals what may be caused by charge-transfer quenching due to the direct linkage between donor and acceptor unit. The homoleptic fac Ir(N^N)3 complex exhibits no emission at RT and no transient signals. At 77 K it shows a highly structured emission with 14 s lifetime. Compared to the literature-known reference compound this emission is caused by the population of a 3Ir(ppz)3 state. Our findings are important for designing complexes with stronger acceptor units (i.e. naphthaleneimide) for long CS states lifetimes to be used as photosynthesisers in solar cells and other optoelectronic devices. Besides, LEC and OLED studies on the carbazole complexes are still of interest to analyse the degree of triplet-triplet-annihiliation in these devices.
SARS-CoV-2 variants such as the delta or omicron variants, with higher transmission rates, accelerated the global COVID-19 pandemic. Thus, novel therapeutic strategies need to be deployed. The inhibition of acid sphingomyelinase (ASM), interfering with viral entry by fluoxetine was reported. Here, we described the acid ceramidase as an additional target of fluoxetine. To discover these effects, we synthesized an ASM-independent fluoxetine derivative, AKS466. High-resolution SARS-CoV-2–RNA FISH and RTqPCR analyses demonstrate that AKS466 down-regulates viral gene expression. It is shown that SARS-CoV-2 deacidifies the lysosomal pH using the ORF3 protein. However, treatment with AKS488 or fluoxetine lowers the lysosomal pH. Our biochemical results show that AKS466 localizes to the endo-lysosomal replication compartments of infected cells, and demonstrate the enrichment of the viral genomic, minus-stranded RNA and mRNAs there. Both fluoxetine and AKS466 inhibit the acid ceramidase activity, cause endo-lysosomal ceramide elevation, and interfere with viral replication. Furthermore, Ceranib-2, a specific acid ceramidase inhibitor, reduces SARS-CoV-2 replication and, most importantly, the exogenous supplementation of C6-ceramide interferes with viral replication. These results support the hypotheses that the acid ceramidase is a SARS-CoV-2 host factor.
The present thesis encompasses two parts. The first supramolecular part focuses on the development of new flexible self-assembling zwitterions as building blocks for supramolecular polymers. In the second part, the aim was to develop bioorganic receptors for amino acids and dipeptides in aqueous media. Both research projects are based on the guanidiniocarbonyl pyrrole 1 as a new efficient binding motif for the complexation of carboxylates in polar solution.A necessary requirement for the realization of these research projects was to develop an efficient and mild synthetic approach for the cationic guanidiniocarbonyl pyrroles in general. The harsh reaction conditions of the previously used method and the problematic purification of the cationic guanidinocarbonyl pyrroles so far prevented a more extensive exploration in bioorganic and supramolecular research. In the course of this work I successfully developed a new synthesis starting with mono tBoc-protected guanidine that was coupled with a benzyl protected pyrrole carboxylic acid. After deprotection of the benzyl group, a key intermediate in the newly developed synthesis, the tBoc-protected guanidinocarbonyl pyrrole acid, was obtained. This new, mild and extremely efficient synthetic approach for the introduction of acyl guanidines is now the standard procedure in our group for the preparation of both solution and solid-phase guanidiniocarbonyl pyrroles. With this facile method at hand, a new class of flexible zwitterions, in which a carboxylate is linked via an alkyl chain to a guanidiniocarbonyl pyrrole cation was synthesized. The self-aggregation and the influence of the length and therefore flexibility of the alkyl spacer on the structure and stability of the formed aggregates were studied in solution and gas phase. In solution the aggregation was studied by NMR-dilution experiments in DMSO which suggest that flexible zwitterions with n = 1, 3 and 5 form oligomers. For n = 1, highly stable helical aggregates with nanometer size are formed. In the gas phase studies the stability and the fragmentation kinetics of a series of sodiated dimeric zwitterions with n = 2, 3 and 5 were investigated. This was done by infrared multiphoton dissociation Fourier transform ion cyclotron resonance mass spectrometry (IRMPD-FT-ICR-MS). These kinds of studies can be used in the future for a more directed design of supramolecular building blocks The bioorganic research part comprises three different projects. In a first project I synthesized four new arginine analogues which can be implemented in peptides as a substitute for arginine. Therefore, I developed the new multi-step synthesis shown below for these arginine analogues. As a test for their application in normal solid phase synthesis, I successfully prepared a tripeptide sequence Ala-AA1-Val (AA: arginine analogue. In a second project I studied the influence of additional ionic interactions within our binding motif. I synthesized a di-cationic and a tris-cationic receptor and evaluated the binding properties via NMR titration experiments against a variety of amino acids. Especially, the tris-cationic receptor was capable to strongly complex amino acids. The association constants were about a factor of 100 higher than those for the guanidiniocarbonyl pyrroles known so far. Even in 90 %water/10 % DMSO the association constants determined by NMR titration were extremely high with values around Kass = 2000 M-1. In the third project I developed a de-novo designed receptor for C-terminal dipeptides in a beta-sheet conformation based on molecular calculations. This receptor was studied in NMR and also UV titration experiments. In 40 % water/ 60 % DMSO the association constants were too strong to be measured by NMR titration experiments. Therefore, the complexation properties of 12 were studied by UV titration in water (with 10 % DMSO added for solubility reasons) with various dipeptides and amino acids as substrates. The data show that 12 binds dipeptides very efficiently even in water with association constants Kass > 10000 M-1, making 12 one of the most effective dipeptide receptors known so far. In contrast to that, simple amino acids are bound up to ten times less efficiently (Kass > 1000 M-1) than dipeptides. In the series of dipeptides studied the complex stability increases depending on the side chains present in the order Gly < Ala < Val which is a result of the decreasing flexibility of the peptide and the increasing hydrophobicity of the side chains. The binding properties of this receptor are superior to any other dipeptide receptor reported so far. Within my thesis I have not only developed an essential, mild and efficient synthetic approach for guanidiniocarbonyl pyrroles in general, but also a new binding motif for the complexation of amino acids 15, 11 and in addition a dipeptide receptor 12 that is superior to all dipeptides receptors known so far.
Study of the hyperfine coupling constants of the moleculs NH<sub>2</sub>, NHD and ND<sub>2</sub>
(1990)
In the present paper we c:alculate tbe magnetic hyperfine couplina constants (hfcc) ai.ID and A11 of the ground states of the isotopes NH2, NHD and ND2 using truncated MR..CI methods. Differences from other theoretical methocls and shortoominp of the truncated Cl approach in calculating tlj10 are studied. Polarization effects wbich detennirae ailo. as weU as a simple model to describe the dipolar hfcc's, are discussed. All results are in. excellent aareement with experimental data. lt is shown that ab initio methods are able to obtain reliable values for otf-diaaonal values of A41 which are difficult to measure experimentaDy.
The synthesis and characterization of laterally extended azabora[5]‐, ‐[6]‐ and ‐[7]helicenes, assembled from N‐heteroaromatic and dibenzo[g,p]chrysene building blocks is described. Formally, the π‐conjugated systems of the pristine azaborole helicenes were enlarged with a phenanthrene unit leading to compounds with large Stokes shifts, significantly enhanced luminescence quantum yields (Φ) and dissymmetry factors (g\(_{lum}\)). The beneficial effect on optical properties was also observed for helical elongation. The combined contributions of lateral and helical extensions resulted in a compound showing green emission with Φ of 0.31 and |g\(_{lum}\)| of 2.2×10\(^{−3}\), highest within the series of π‐extended azaborahelicenes and superior to emission intensity and chiroptical response of its non‐extended congener. This study shows that helical and lateral extensions of π‐conjugated systems are viable strategies to improve features of azaborole helicenes. In addition, single crystal X‐ray analysis of configurationally stable [6]‐ and ‐[7]helicenes was used to provide insight into their packing arrangements.
A comparative ab initio study of the Si\(_2\)C\(_4\), Si\(_3\)C\(_3\), Si\(_4\)C\(_2\) clusters
(1994)
Various structural possibilities for the Si\(_2\)C\(_4\) and Si\(_4\)C\(_2\) clusters are investigated by employing a basis set of triple-zeta plus polarization quality; electron correlation is generally accounted for by second-order M0ller-Plesset and, in certain instances, by higher-order perturbation (CASPT2) approaches. The building-up principle recently suggested from an analysis of Si\(_3\)C\(_3\) clusters is found to be fully operative for Si\(_2\)C\(_4\) and Si\(_4\)C\(_2\) clusters. A comparison of the structure and stability of various geometrical arrangements in the series C\(_6\) , Si\(_2\)C\(_4\) , Si\(_3\)C\(_3\) , Si\(_4\)C\(_2\), and Si\(_6\) shows that linear and planar structures become rapidly less stable if carbons are replaced by silicons and that the three-dimensional bipyramidal forms become less favorable as soon as silicons are exchanged by carbons in the parent Si\(_6\) structure. The effects can be rationalized in qualitative terms based on differences in silicon and carbon bonding.
In this thesis the syntheses and detailed investigations on two foldable PBI systems were presented. The reversible, solvent-dependet folding/unfolding-behavior was used to study the ground and excited states properties of folda-dimer and folda-trimer by means of different spectroscopic methods as well as theoretical studies. The switching between charge transfer or excimer formation pathways of photoexcited molecules influenced by the spatial arrangement of chromophores within defined dye systems illustrates the impact of conformational preferences on functional properties.
We introduce fluorescence-detected pump–probe microscopy by combining a wavelength-tunable ultrafast laser with a confocal scanning fluorescence microscope, enabling access to the femtosecond time scale on the micrometer spatial scale. In addition, we obtain spectral information from Fourier transformation over excitation pulse-pair time delays. We demonstrate this new approach on a model system of a terrylene bisimide (TBI) dye embedded in a PMMA matrix and acquire the linear excitation spectrum as well as time-dependent pump–probe spectra simultaneously. We then push the technique towards single TBI molecules and analyze the statistical distribution of their excitation spectra. Furthermore, we demonstrate the ultrafast transient evolution of several individual molecules, highlighting their different behavior in contrast to the ensemble due to their individual local environment. By correlating the linear and nonlinear spectra, we assess the effect of the molecular environment on the excited-state energy.
Three novel tetracationic bis‐triarylboranes with 3,4‐ethylenedioxythiophene (EDOT) linkers, and their neutral precursors, showed significant red‐shifted absorption and emission compared to their thiophene‐containing analogues, with one of the EDOT‐derivatives emitting in the NIR region. Only the EDOT‐linked trixylylborane tetracation was stable in aqueous solution, indicating that direct attachment of a thiophene or even 3‐methylthiophene to the boron atom is insufficient to provide hydrolytic stability in aqueous solution. Further comparative analysis of the EDOT‐linked trixylylborane tetracation and its bis‐thiophene analogue revealed efficient photo‐induced singlet oxygen production, with the consequent biological implications. Thus, both analogues bind strongly to ds‐DNA and BSA, very efficiently enter living human cells, accumulate in several different cytoplasmic organelles with no toxic effect but, under intense visible light irradiation, they exhibit almost instantaneous and very strong cytotoxic effects, presumably attributed to singlet oxygen production. Thus, both compounds are intriguing theranostic agents, whose intracellular and probably intra‐tissue location can be monitored by strong fluorescence, allowing switching on of the strong bioactivity by well‐focused visible light.
A starlike heterocyclic molecule containing an electron‐deficient nonaaza‐core structure and three peripheral isoquinolines locked by three tetracoordinate borons, namely isoquinoline‐nona‐starazine (QNSA), is synthesized by using readily available reactants through a rather straightforward approach. This new heteroatom‐rich QNSA possesses a quasi‐planar π‐backbone structure, and bears phenyl substituents on borons which protrude on both sides of the π‐backbones endowing it with good solubility in common organic solvents. Contrasting to its starphene analogue, QNSA shows intense fluorescence with a quantum yield (PLQY) of up to 62 % in dilute solution.
A striking feature of the metabolite profile of \(Ancistrocladus\) \(likoko\) (Ancistrocladaceae) is the exclusive production of 5,8\('\)-linked naphthylisoquinoline alkaloids varying in their OMe/OH substitution patterns and in the hydrogenation degree in their isoquinoline portions. Here we present nine new compounds of this coupling type isolated from the twigs of this remarkable Central African liana. Three of them, the ancistrolikokines E (9), E\(_2\) (10), and F (11), are the first 5,8\('\)-linked naphthyldihydroisoquinolines found in nature with \(R\)-configuration at C-3. The fourth new metabolite, ancistrolikokine G (12), is so far the only representative of the 5,8\('\)-coupling type that belongs to the very rare group of alkaloids with a fully dehydrogenated isoquinoline portion. Moreover, five new \(N\)-methylated naphthyltetrahydroisoquinolines, named ancistrolikokines A\(_2\) (13), A\(_3\) (14), C\(_2\) (5), H (15), and H\(_2\) (16) are presented, along with six known 5,8\('\)-linked alkaloids, previously identified in related African \(Ancistrocladus\) species, now found for the first time in \(A.\) \(likoko\). The structural elucidation was achieved by spectroscopic analysis (HRESIMS, 1D and 2D NMR) and by chemical (oxidative degradation) and chiroptical (electronic circular dichroism) methods. The new ancistrolikokines showed moderate to good preferential cytotoxic activities towards pancreatic PANC-1 cells in nutrient-deprived medium (NDM), without causing toxicity under normal, nutrient-rich conditions, with ancistrolikokine H\(_2\) (16) being the most potent compound.