Kinetics of transient photocurrents arising from dissociation of exciplexes in solutions at times comparable to the diffusion separation time of geminate ion pairs

Kinetics of transient photocurrents caused by the electron transfer (ET) reactions in weakly polar solvents is computed on the assumption that ions are formed by dissociation of exciplexes, the primary product of the ET reactions. The rise time of the photocurrent arising from the time dependent polarization of geminate solvated ion pairs in the applied electric field is shown to be less than the fluorescence decay time of exciplexes, the precursors of the ions. The obtained time dependence of the photocurrent agrees well with the photoconductivity traces in the solutions of 9,10-dicyanoanthracene and durene observed by Zhou et al. [J. Zhou, B.R. Findley, T.M. Francis, E.A. Nytko, C.L. Braun, Chem. Phys. Lett. 362 (2002) 63–71; J. Zhou, R.P. Shah, B.R. Findley, C.L. Braun, J. Phys. Chem. A 106 (2002) 12–20]. Their interpretation of the experimental data rests on the alternative assumption that free ions are generated not from exciplexes but from solvent-separated ion pairs originally produced on the ET quenching of the excited states of 9,10-dicyanoanthracene by durene. However, the performed analysis of the photocurrent kinetics supports the conclusion that ions arise by dissociation of exciplexes.