Photoreduction of 3-phenylquinoxalin-2-ones by amines: Transient-absorption and semiempirical quantum-chemical studies

Photoreduction of 3-phenylquinoxalin-2-ones, XNQ, by triethylamine, TEA, gives a metastable semireduced quinoxalin-2-one via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. During the photoreduction, an ion-radical pair, XNQ(-.)/TEA(+.), a neutral-radical pair, XNQH(.)/TEA-H-., and an ion-pair, XNQH(-)/TEA-H+, are formed. We present time-resolved spectroscopic data and quantum mechanical semiempirical AM1, PM3, and ZINDO/S results for the transient species formed during the flash photolysis of quinoxalin-2-ones in the presence of amines. These calculations show that the neutral-radical pair and the ion-pair are similar in energy, and that the calculated spectra of all the transient species should have similar absorption bands near 400 nm in agreement with experimental results. The ZINDO/S calculated spectra of the XNQH(-)/iminium ion pair fit the experimental spectra and explain the lack of visible or near-IR absorption of the metastable compound. Energy changes between the species involved are of interest with regard to the possible use of quinoxalin-2-one/amine systems for light to chemical energy conversion or as temporal data storage devices