Solvent dependence of photochemical electron-transfer rates in a covalently linked porphyrin–quinone molecule

The solvent dependence of intramolecular photochemical electron-transfer rate constants in a covalently linked porphyrin–amide–quinone molecule (PAQ) has been found in nine solvents to correlate reasonably well with the semiclassical formulation of Marcus theory. In making this correlation it is necessary to take into account the solvent dependence of both ΔG°, the Gibbs energy of the electron-transfer reaction from the excited state of the porphyrin to the quinone, and that of λ, the reorganisation energy. The former was estimated from the redox potentials of PAQ measured in each solvent, and the latter was estimated using Marcus' dielectric continuum model.