Charge-transfer-induced photoreduction of azoalkanes by amines

Abstract

The unique DBH-type azoalkanes 1, which exhibit high intersystem crossing quantum yields, have made possible the exploration of the bimolecular photoreduction of the n,π* triplet-excited azo chromophore. In the laser-flash photolysis, amines were found to quench the triplet azoalkane 1a with high rate constants (k_q ca. 10⁸ M⁻¹ s⁻¹). Steady-state photolysis of the azoalkanes 1a and 1b (φ_ISC ca. 0.5) in the presence of primary, secondary, and tertiary aliphatic amines gave high chemical yields of the corresponding hydrazines 4a and 4b in competition with the unimolecular products, namely the housanes 2 and the aziranes 3. In contrast, the azoalkane 1c undergoes appreciable photoreduction only in neat amines, while the azoalkane 1d (φ_ISC ca. 0.10) is not reduced even under such conditions. Except for N,N-dimethylbenzylamine, the amine oxidation products of the azoalkane photoreduction are analogous to those obtained from the reactions of amines with triplet benzophenone. In marked contrast, the absolute quantum yields of photoreduction for azoalkanes 1 are substantially lower (0.01-0.06) than for benzophenone (0.3-1.0). Efficient deactivation of the triplet-excited states by charge-transfer (k_H^CT), which competes with hydrogen atom abstraction (k_H^CT), is postulated to account for the low quantum yields. The efficiencies of photoreduction follow the trend primary ≈ tertiary ≫ secondary amines observed with benzophenone, for which secondary amines also display the poorest efficiency. Electron transfer to triplet-excited azoalkanes, analogous to benzophenone, is observed for amines with low oxidation potentials. Thus, when triphenylamine (E_ox = 0.85 V versus SCE) is used, the formation of its radical cation can be readily detected by laser-flash photolysis.