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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.
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.
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.