Transient photoconductivity caused by dissociation of excited states in solutions. effect of temperature

Kinetics of transient photocurrents caused by dissociation of exciplexes, intermediate excited states, into the geminate pairs of solvent-separated radical ions A^- + D⁺ is computed in the case that the diffusion separation time of the ion pair is comparable to the life time of exciplexes, precursors of the ions. The time dependence of the displacement current of the ion pairs A^- + D⁺ was found from the Smoluchowski equation for the motion of the ions in the combined Coulomb and external electric fields. The obtained time dependence of the photocurrent agrees well with photoconductivity traces observed for typical donor-acceptor systems, like durene-9,10-dicyanoanthracene and pyrene-p-dicyanobenzene in polar solvents. The rise time of photocurrent depends on the diffusion separation time, but weakly depends on temperature. The performed analysis supports mechanism of ion formation in which the ions arise by dissociation of exciplexes.