PHOTOINDUCED ELECTRON-TRANSFER IN POLYCHROMOPHORIC SYSTEMS

Combining stationary and time resolved fluorescence spectroscopy with laser-induced opto-acoustic spectroscopy allowed to investigate the in fluence of molecular structure and solvent polarity on the rate of the radiative and radiationless decay processes of exciplexes of omega-ph enyl-alpha-N,N-dimethylaminoalkanes. By making intersystem crossing to the locally excited state less exergonic increasing the solvent polar ity decreased the rate constant for intersystem crossing. The apparent decrease of the fluorescence rate constant in highly polar solvents w as attributed to the direct formation of a solvent separated ion-pair. The formation of solvent separated ion-pairs can also explain why, co ntradicting the predictions of Marcus theory, the rate constant for in ternal conversion decreases sometimes in highly polar solvents. The so lvent dependence of the fluorescence spectra of dialkylamino and diary lamino substituted triphenylbenzene derivatives indicates that in thos e completely symmetric molecules the emission occurs from a highly pol ar excited state. The exponential fluorescence decays suggest that equ ilibration between the different excited species occurs within the tim e resolution of the experimental set-up. The comparison of compounds w ith meta- and para-substitution and the comparison of dialkylamino and diarylamino derivatives suggests that either a conjugated intramolecu lar charge transfer state or a TICT state, in which the amino or the a mino moiety rotates out of the plane of the triphenylbenzene moiety, a re formed. Comparing the room temperature emission spectra with those at 77 K in isopentane glass allowed to obtain information on the geome try of the species emitting in less polar solvents.