Photoalkylation of 10-alkylacridinium ion via a charge-shift type of photoinduced electron transfer controlled by solvent polarity

A charge-shift type of photoinduced electron-transfer reactions from various electron donors to the singlet excited state of 10-decylacridinium cation (DeAcrH⁺) in a nonpolar solvent (benzene) is found to be as efficient as those of 10-methylacridinium cation (MeAcrH⁺) and DeAcrH⁺ in a polar solvent (acetonitrile). Irradiation of the absorption bands of MeAcrH⁺ in acetonitrile solution containing tetraalkyltin compounds (R₄Sn) results in the efficient and selective reduction of MeAcrH⁺ to yield the 10-methyl-9-alkyl-9, 10-dihydroacridine (AcrHR). The same type of reaction proceeds in benzene when MeAcrH⁺ is replaced by DeAcrH⁺ which is soluble in benzene. The photoalkylation of R′AcrH⁺ (R′ = Me and De) also proceeds in acetonitrile and benzene using 4-tert-butyl-1-benzyl-1,4-dihydronicotinamide (Bu^tBNAH) instead of R₄Sn, yielding MeAcrHBu^t. The quantum yield determinations, the fluorescence quenching of R′AcrH⁺ by electron donors, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the photoalkylation of R′AcrH⁺ in benzene as well as in acetonitrile proceeds via photoinduced electron transfer from the alkylating agents (R₄Sn and Bu^tBNAH) to the singlet excited states of R′AcrH⁺. The limiting quantum yields are determined by the competition between the back electron-transfer process and the bond-cleavage process in the radical pair produced by the photoinduced electron transfer. The rates of back electron transfer have been shown to be controlled by the solvent polarity which affects the solvent reorganization energy of the back electron transfer. When the free energy change of the back electron transfer (Δ^G0_bet) in a polar solvent is in the Marcus inverted region, the rate of back electron transfer decreases with decreasing the solvent polarity, leading to the larger limiting quantum yield for the photoalkylation reaction. In contrast, the opposite trend is obtained when the ΔG⁰_bet value is in the normal region: the limiting quantum yield decreases with decreasing the solvent polarity.