Cyanoanthracene-sensitized photooxidations of N,N-dibenzylhydroxylamine and its derivatives: free-energy dependence of back electron-transfer rates within geminate radical ion pairs

Abstract

The fluorescence of 9-cyanoanthracene (CA) and 9,10-dicyanoanthracene (DCA) is quenched by the title hydroxylamine 1 in acetonitrile according to the Stern–Volmer equation. An analysis of the Rehm–Weller plot of the bimolecular quenching rate constant against the free-energy change involved in the electron-transfer (ET) process indicated the operation of an ET mechanism for the fluorescence quenching of CA and DCA by the hydroxylamine quencher. However, solvent deuterium isotope effects on the quantum yields for the CA- and DCA-sensitized oxidations of 1 in acetonitrile established that there is a substantial contribution of a singlet oxygen mechanism to the overall CA-sensitized reaction in the low concentration ranges of 1 but its contribution markedly decreases with an increase in the concentration of 1, thus enabling us to use the limiting quantum yield (Φ_lim) for the estimation of the back electron-transfer (BET) rate within the initially-formed geminate radical ion pair (GRIP), which serves as a key intermediate for a superoxide mechanism. In addition to protic solvent effects on the Φ_lim value, the free-energy dependence of the rate of BET giving the starting 1 and the sensitizer demonstrated that BET within the GRIP obtained by ET from 1 to excited DCA proceeds through a solvent-separated radical ion pair (SSRIP) in the Marcus ‘normal region’ whereas BET within the GRIP formed by ET between 1 and excited CA takes place from a contact radical ion pair (CRIP) in the Marcus ‘inverted region’, except in the CA–N,N-bis(p-methylbenzyl)hydroxylamine system. The BET in this system is suggested to occur not from a CRIP but from a SSRIP in the ‘inverted region’ resulting in a much higher sensitized-oxidation efficiency than expected.