Electron transfer reaction in solution: Initial charge separation distance distribution and its impact on free ion formation

In this study, the initial charge separation distance and its impact on free radical ion (FRI) formation are investigated in detail. Transient photocurrent techniques together with other spectroscopic methods were used with a series of donors and acceptors in various solvents. The main conclusions include: (i) It is proved that, in solution, direct photo-excitation of longer than contact distance, random donor/acceptor pairs can happen. The ground state random pairs make significant contributions to electron donor/acceptor (EDA) absorption spectra. (ii) For the first time, convincing proof is found of the existence of an energy barrier between solvent-separated radical ion pairs (SSRIPs) and contact radical ion pairs (CRIPs) on the excited state Coulomb potential surface. Temperature dependence experiments imply that the height of the energy barrier is at least as large as that of the free radical ions. Based on this new mechanism, FRIs are believed to form exclusively from SSRIPs rather than CRIPs. The SSRIP charge recombination mechanism was found to favor the through-tunneling channel. Diffusion back to CRIPs position is not necessarily a charge recombination pathway for SSRIPs. (iii) The detailed FRIs recombination mechanism of C60-TTF systems was studied. By examining the rise and decay phases of the photocurrent curves, SSRIPs charge recombination rate constants were obtained. It is proved that the slow charge recombination rate is due to the high probability of FRI regeneration. For the first time, the existence of SSRIPs as geminate ion pairs has been proved with convincing evidence. It was found that ∼36% of the total geminate ion pairs are SSRIPs after quenching.