Photo-induced electron transfer in polychromophoric systems

Combining stationary and time resolved fluorescence spectroscopy with laser-induced opto-acoustic spectroscopy allowed to investigate the influence of molecular structure and solvent polarity on the rate of the radiative and radiationless decay processes of exciplexes of w-phenyl-a-N,N-dimethylaminoalkanes. By making intersystem crossing to the locally excited state less exergonic increasing the solvent polarity decreased the rate constant for intersystem crossing. The apparent decrease of the fluorescence rate constant in highly polar solvents was attributed to the direct formation of a solvent separated ion-pair. The formation of solvent separated ion-pairs can also explain why, contradicting the predictions of Marcus theory, the rate constant for internal conversion decreases sometimes in highly polar solvents. The solvent dependence of the fluorescence spectra of dialkylamino and diarylamino substituted triphenylbenzene derivatives indicates that in those completely symmetric molecules the emission occurs from a highly polar excited state. The exponential fluorescence decays suggest that equilibration between the different excited species occurs within the time resolution of the experimental set-up. The comparison of compounds with meta- and para-substitution and the comparison of dialkylamino and diarylamino derivatives suggests that either a conjugated intramolecular charge transfer state or ti TICT state, in which the amino or the anilino moiety rotates out of the plane of the triphenylbenzene moiety, arc formed. Comparing the room temperature emission spectra with those at 77 K in isopentane glass allowed to obtain information on the geometry of the species emitting in less polar solvents.