Intramolecular photochemical electron transfer

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, AGO, for the electron-transfer reaction 'P*AQ -+ P'+AQ'- and the Marcus reorganisation energy, A, are considered. The AGO values are obtained from direct measurement of redox potentials in each solvent with various work-term corrections for Coulombic interaction in P'+AQ'-, and 1 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 AGO 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 AGO values and those corrected with a solvent-dependent work term. We have found that Weller's method for calculating AGO 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 AGO