DYNAMICAL ASPECTS OF ULTRAFAST INTERMOLECULAR ELECTRON-TRANSFER FASTER THAN SOLVATION PROCESS - SUBSTITUENT EFFECTS AND ENERGY-GAP DEPENDENCE

We have investigated intermolecular electron transfer (ET) from electr on-donating solvents (aniline and N,N-dimethylaniline) to coumarins in the excited state by means of the femtosecond fluorescence up-convers ion technique. The coumarins we studied have a variety of structures w ith different substituents in the 4- and 7-positions. The ET occurs on a time scale ranging from a few nanoseconds to a couple of hundred fe mtoseconds depending on the structure of the coumarins and solvent. As for the 7-position, as the length of the alkyl chain on the amino gro up is longer, the ET is slower, and when the amino group is fixed by a double-hexagonal ring, it is slowest. When the electron-accepting abi lity of the substituent in the 4-position is increased, the reaction o ccurs faster. The origin of this substituent effect is mainly attribut ed to the variation of the energy gap between the reactant and product states. This is confirmed by theoretical calculations in terms of the extended Sumi-Marcus two-dimensional model. Good agreement between th e experiment and calculation indicates that some of the reactions take place from the relaxed vibrational state of reactant to the excited v ibrational states of high-frequency modes of product states. The simul ated population decays for nonequilibrium configuration of solvents ag reed well with experimental data. In the steady-state fluorescence spe ctra was also observed an effect of very fast fluorescence quenching d ue to ET; i.e., the amount of fluorescence Stokes shift depends on the rate of ET because the excited state is quenched in competition with thermal equilibration of the solvent configuration. We regard this spe ctral shift as the result of the ''chemical timing'' effect in solutio n.