Intramolecular photochemical electron transfer. Part 5.—Solvent dependence of electron transfer in a porphyrin–amide–quinone molecule

Abstract

The compound PAQ, which consists of a tetra-arylporphine attached to methyl-p-benzoquinone via a single amide linkage, exhibits light-induced intramolecular electron transfer from the porphyrin excited-singlet state to the quinone at a rate which is strongly solvent-dependent. The rate constants are found to correlate well with the semiclassical Marcus theory of electron transfer, provided that the solvent effect on both the Gibbs energy change, ΔG°, for the electron-transfer reaction ¹P*AQ → P˙⁺ AQ˙^– and the Marcus reorganisation energy, λ, are considered. The ΔG° values are obtained from direct measurement of redox potentials in each solvent with various work-term corrections for Coulombic interaction in P˙⁺AQ˙^–, and λ is calculated from the optical and dielectric properties of each solvent. For fourteen solvents, reasonable agreement with Marcus theory is found using this approach on uncorrected ΔG° values and those corrected with a solvent-dependent work term; a solvent-independent correction is not successful. For two solvent mixtures (acetonitrile–benzonitrile and acetonitrile–chloroform), excellent agreement with Marcus theory is found using uncorrected ΔG° values and those corrected with a solvent-dependent work term. We have found that Weller's method for calculating ΔG° in various solvents from a single measurement in a reference solvent gives a poor correlation with Marcus theory, primarily because of a poor prediction of the solvent dependence of ΔG°.