Formal hydride transfer mechanism for photoreduction of 3-phenylquinoxalin-2-ones by amines. Association of 3-phenylquinoxalin-2-one with aliphatic amines

The photophysical and photochemical behavior of 1-methyl-3-phenylquinoxalin -2-one (MeNQ) and 3-phenylquinoxalin-2-one (HNQ) in the presence of amines is reported. While HNQ fluorescence shows an auxochromic effect and a batho chromic shift with added amines, explained by association of HNQ with amine in the ground state and emission from both excited species HNQ* and [HNQ-a mine]*, both MeNQ and HNQ are photoreduced efficiently on irradiation in th e presence of amines, leading to the semireduced quinoxalin-2-ones, MeNQH(- ) and HNQH(-), respectively, via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. The semireduced quinoxal in-2-ones XNQH(-) (X = H, Me) revert almost quantitatively to the parent XN Q in a dark thermal reaction with an activation free energy for MeNQH(-) of 17.4 and 25.9 kcal/mol in acetonitrile and benzene, respectively. Kinetic and spectroscopic (UV and NMR) evidence supports the proposed reaction mech anism for the reversible photoreduction.