Transient optical absorption and time-resolved resonance Raman experiments on covalently linked porphyrin–quinone systems

Light-induced triplet electron transfer (ET) and subsequent triplet radical-pair (RP) recombination in two covalently linked porphyrin–quinone systems in highly viscous ethanol has been investigated by both transient optical absorption and time-resolved resonance Raman spectroscopy with a time resolution of 10 ns. The temperature dependence of the rates is measured between 155 and 200 K and compared with predictions of solvent-controlled adiabatic electron-transfer theory. It is shown that the triplet ET in the normal region (exergonicity ΔG0 < solvent reorganization energy λs) is correctly described. The mechanism of the RP recombination in the inverted region (ΔG0 > λs) depends on the dynamics of the exchange interaction J, on the triplet–singlet mixing of the radical pair states and on the singlet recombination rate. An intermolecular ET process leading to a disproportionation reaction of the quinone moieties is also observed.