SUBPICOSECOND TRANSIENT ABSORPTION STUDY OF INTERMOLECULAR ELECTRON-TRANSFER BETWEEN SOLUTE AND ELECTRON-DONATING SOLVENTS

Nile blue A experiences ultrafast fluorescence quenching in diffusionless, electron-donating solvent systems such as N,N-dimethylaniline (DMA) or aniline (AN), in which the shortest fluorescence decay time reaches 10(-13) s, as revealed by femtosecond fluorescence up-conversion studies [Kobayashi, T.; et al. Chem. Phys. Lett. 1991, 180, 416]. In the present paper, the reaction mechanism is studied in more detail by subpicosecond transient absorption spectroscopy. By directly detecting the absorption of the products, namely, reduced nile blue (470 nm) and the solvent cation (470 nm, DMA+; 405 nm, AN+), we established that the fluorescence quenching is, as previously proposed, due to an electron-transfer reaction. In the present systems of small free energy difference for forward and backward electron transfer, the reverse electron transfer occurs on a picosecond time scale, i.e., 4.0 ps in DMA and 2.7 ps in AN, and the present systems perform a full cycle of charge separation and recombination in less than 10 ps.