Efficiency of Intramolecular Charge Separation from the Second Excited State: Suppression of the Hot Charge Recombination by Electron Transfer to the Secondary Acceptor

Ultrafast intramolecular charge transfer induced by the Soret-band excitation of the donor-acceptor(1)-acceptor(2) molecular triads has been explored within the stochastic point-transition model. It is shown that nonthermal (hot) charge transfer from the primary to the secondary acceptor, assisted by relaxation of solvent polarization, can effectively screen ultrafast back electron transfer into the first excited state of the donor. Ways to increase the quantum yield of the charge-separated states are discussed. The dependencies of the quantum yield of the charge-separated states on the main electron transfer parameters: the free energy gaps, the reorganization energy of the solvent and intramolecular vibrational modes, the electronic couplings, and the solvent relaxation timescale are revealed. The important role of the geometry of the donor-acceptor(1)-acceptor(2) triad in charge separation effectiveness is emphasized. For the zinc-porphyrin-imide(1)-imide(2) triad, the charge-transfer parameters maximizing the quantum yield of the charge separated states are estimated.