STRONGLY EXOTHERMIC ELECTRON-TRANSFER REACTION IN THE EXCITED SINGLET-STATE OF ALKYLCARBAZOLE POLYNITROFLUORENE AND ALKYLCARBAZOLE POLYNITROFLUORENONE BICHROMOPHORIC SYSTEMS .1. CORRELATION BETWEEN THE PROBABILITY OF CHARGE SEPARATION, PHOTOACTIVITY, AND PICOSECOND LASER PHOTOLYSIS STUDIES ON THE PHOTOINDUCED CHARGE RECOMBINATION OF ION-PAIR STATE PRODUCED IN SOME MEDIA

The present study was undertaken to investigate the parameters controlling the rates of forward and backward electron-transfer reactions in strongly exothermic bichromophoric systems of the alkylcarbazole-polynitrofluorene and -polynitrofluorenone types. Electronic absorption spectra of these bichromophoric systems in acetonitrile have shown the appearance of charge-transfer bands from which the electronic matrix elements for electron transfer is evaluated (approximately 40 meV). From the redox properties of the D-S-A couples the rates for charge separation (CS) is calculated using the semi-quantum-mechanical theory applied to these nonadiabatic electron-transfer reactions. The time constants (tau(CS)) of the photoinduced CS of these systems vary from 200 fs to about 20 ps, so that they are considerably longer than the solvent (acetonitrile) dielectric relaxation time tau(L) except that of B4, which is close to tau(L). These rates are compared to those obtained by the time-resolved absorption spectra of contact ion pairs (CIP) with picosecond laser spectroscopy. The xerographic activity is also measured for the bichromophore encased in polymeric Lexan matrices and for polysebacate and polysuccinate oligomers containing these bichromophores as pendant groups. It is shown that the square of the rate constant for charge separation (k(CS)2) correlates well with the xerographic gains measured in photoactivity experiments. More detailed measurements of the photoinduced charge recombination of the ion pairs formed by irradiation of these systems with the third harmonic of the YAG laser at 355 nm have shown that the first-order rate constants for the subsequent relaxation (k(d)) of these contact ion pairs (CIP) to the solvent-separated ion pairs (SSIP) and for the charge recombination (k(CR)) to the original ground state of these systems are about of the same order of magnitude (approximately 10(8) s-1) and are nearly solvent independent. The intrinsic properties of these systems favor good electron transport in various media that can be controlled by modifying the electronic coupling and the nature of the nuclear modes coupled to the two electronic states (Franck-Condon factors).