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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.
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.
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.
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.
The thesis enhances the strategy of non-destructive fluorescence read-out in rylene bisimide-diarylethene containing photochromic systems. The fluorescence of the emitter unit is quenched by a photoinduced electron transfer only to one of the isomeric forms of the photochrome. The driving force of the fuorescence-quenching electron transfer was calculated by the help of the Rehm-Weller equation. The novel photochromic systems satisfy the necessary requirements for non-destructive read-out in write/read/erase fluorescent memory devices.
In the first part of this thesis, the synthesis of a series of bistriarylamine (bisTAA) compounds was presented. On the one hand, the substitution pattern of the TAA at the benzene bridging unit was varied from meta- to para-position (pX and mX), on the other hand, the energetic position of the bridging unit was tuned by use of two electron-donating or electron-accepting substituents X (with X = OMe, Me, Cl, CN, NO2) in 2,5-position. In case of the meta-series, compounds with X in 4,6-position were synthesized (mX46). The photophysical and electrochemical properties of the neutral compounds were investigated.
The cationic mixed valence (MV) bisTAA compounds could be generated by oxidation. Thermally induced hole transfer (HT) in the groud state was investigated by temperature depending ESR spectroscopy. While the HT rate k and HT barrier ΔG in mX are unaffected by the substituents X, k and ΔG in the pX series increase simultaneously with increasing electron-donating strength of X. This, at first contradictory observation can be explained by an increasingly important solvent dynamic effect and an additional, effective barrier. The optically induced HT was examined by UV/Vis/NIR spectroscopy. The pX-series revealed an increase of the electronic coupling V, and correspondingly a decrease of ΔG, with an increase of the electron donating character of X. For mX, a spectroscopic determination of these parameters was not possible. mX46 showed an intermediate behavior, MV compounds with strong electron-donating X, obtained coupling of similar magnitude as pX, which could be explained by means of DFT calculations, with regard to the molecular orbitals.
In the second part of this work, the synthesis of a series of dyads with triarylamine (TAA) as a donor and naphthalene diimide (NDI) as an acceptor was presented. Again, the substitution pattern of the redox centers at the benzene bridging unit was varied in the form of a meta- or para-position (pXNDI or mXNDI) and the energetic position of the bridging unit was varied by X (with X = OMe, Me, Cl, CN, NO2) attached in the 2,5-position. Additionally, compound mOMe46NDI with methoxy substitution in 4,6-position was synthesized. The photophysical and electrochemical properties of these compounds were investigated. The electron transfer (ET) processes of charge separation (CS) and charge recombination (CR) of these were investigated by means of transient absorption (TA) spectroscopy in toluene. This was not possible for the nitro-compounds p-/mNO2NDI, since they decomposed under irradiation. In addition to that, the CR of pXNDI was not detectable by ns-setup, which is why the focus was given to the mXNDI series (with X = OMe–CN).The CS was examined by fs-TA spectroscopy, where the formation of a CS state could be detected. The rise time of the CS states decreases with increasing electron-withdrawing substituents X. CR was examined with ns-TA spectroscopy and shows a biexponential decay behavior, which is caused by singlet-triplet equilibrium in the CS state. By applying an external magnetic field, the decay behavior was decisively changed and the singlet-triplet splitting could be determined. This finding could also be confirmed by simulating the decay curves.
In both parts of this work, the decisive influence of the benzene bridging unit on the appearing ET processes became obvious. For the HT in the ground state of the MV compound, as well as for the ET in the exited states of the DA compounds, the highest transfer rates were found for the para-series pX and pXNDI, and much smaller rates for the meta-series mX and mXNDI. The meta46-compounds mX46 and mOMeNDI46 showed an intermediate behavior in both parts of this work.
In the course of this work, a total of three photocatalytically active dyads for proton reduction could be synthesized together with the associated individual components. Two of them, D1 and D2, comprised a [Ru(bpy)3]2+ photosensitizer and D3 an [Ir(ppy)2bpy]+ photosensitizer. A Ppyr3-substituted propyldithiolate [FeFe] complex was used as catalyst in all systems. The absorption spectroscopic and electrochemical investigations showed that an inner-dyadic electronic coupling is effectively prevented in the dyads due to conjugation blockers within the bridging units used. The photocatalytic investigations exhibited that all dyad containing two-component systems (2CS) showed a significantly worse performance than the corresponding bimolecular three-component systems (3CS). Transient absorption spectroscopy showed that the 2CS behave very similarly to the associated multicomponent systems during photocatalysis. The electron that was intended for the intramolecular transfer from the photosensitizer unit to the catalyst unit within the dyads remains at the photosensitizer for a relatively long time, analogous to the 3CS and despite the covalently bound catalyst. It is therefore assumed that this intramolecular electron transfer is likely to be hindered as a result of the weak electronic coupling caused by the bridge units used. Instead, the system bypasses this through an intermolecular transfer to other dyad molecules in the immediate vicinity. In addition, with the help of emission quenching experiments and electrochemical investigations, it could be clearly concluded that all investigated systems proceed via the reductive quenching mechanism during photocatalysis.
This work is concerned with the syntheses and photophysical properties of para-xylylene bridged macrocycles nPBI with ring sizes from two to nine PBI units, as well as the complexation of polycyclic aromatic guest compounds.
With a reduced but substantial fluorescence quantum yield of 21% (in CHCl3) the free host 2PBI(4-tBu)4 can be used as a dual fluorescence probe. Upon encapsulation of rather electron-poor guests the fluorescence quenching interactions between the chromophores are prevented, leading to a significant fluorescence enhancement to > 90% (“turn-on”). On the other hand, the addition of electron-rich guest molecules induces an electron transfer from the guest to the electron-poor PBI chromophores and thus quenches the fluorescence entirely (“turn-off”). The photophysical properties of the host-guest complexes were studied by transient absorption spectroscopy. These measurements revealed that the charge transfer between guest and 2PBI(4-tBu)4 occurs in the “normal region” of the Marcus-parabola with the fastest charge separation rate for perylene. In contrast, the charge recombination back to the PBI ground state lies far in the “inverted region” of the Marcus-parabola.
Beside complexation of planar aromatic hydrocarbons into the cavity of the cyclophanes an encapsulation of fullerene into the cyclic trimer 3PBI(4-tBu)4 was observed. 3PBI(4-tBu)4 provides a tube-like structure in which the PBI subunits represent the walls of those tubes. The cavity has the optimal size for hosting fullerenes, with C70 fitting better than C60 and a binding constant that is higher by a factor of 10. TA spectroscopy in toluene that was performed on the C60@3PBI(4-tBu)4 complex revealed two energy transfer processes. The first one comes from the excited PBI to the fullerene, which subsequently populates the triplet state. From the fullerene triplet state a second energy transfer occurs back to the PBI to generate the PBI triplet state.
In all cycles that were studied by TA spectroscopy, symmetry-breaking charge separation (SB-CS) was observed in dichloromethane. This process is fastest within the PBI cyclophane 2PBI(4-tBu)4 and slows down for larger cycles, suggesting that the charge separation takes place through space and not through bonds. The charges then recombine to the PBI triplet state via a radical pair intersystem crossing (RP-ISC) mechanism, which could be used to generate singlet oxygen in yields of ~20%.
By changing the solvent to toluene an intramolecular folding of the even-numbered larger cycles was observed that quenches the fluorescence and increases the 0-1 transition band in the absorption spectra. Force field calculations of 4PBI(4-tBu)4 suggested a folding into pairs of dimers, which explains the remarkable odd-even effect with respect to the number of connected PBI chromophores and the resulting alternation in the absorption and fluorescence properties. Thus, the even-numbered macrocycles can fold in a way that all chromophores are in a paired arrangement, while the odd-numbered cycles have open conformations (3PBI(4-tBu)4, 5PBI(4-tBu)4, 7PBI(4-tBu)4) or at least additional unpaired PBI unit (9PBI(4-tBu)4).
With these experiments we could for the first time give insights in the interactions between cyclic PBI hosts and aromatic guest molecules. Associated with the encapsulation of guest molecules a variety of possible applications can be envisioned, like fluorescence sensing, chiral recognition and photodynamic therapy by singlet oxygen generation. Particularly, these macrocycles provide photophysical relaxation pathways of PBIs, like charge separation and recombination and triplet state formation that are hardly feasible in monomeric PBI dyes. Furthermore, diverse compound specific features were found, like the odd-even effect in the folding process or the transition of superficial nanostructures of the tetrameric cycle influenced by the AFM tip. The comprehensive properties of these macrocycles provide the basis for further oncoming studies and can serve as an inspiration for the synthesis of new macrocyclic compounds.