On the yield of intermediates formed in the photoreduction of benzophenone

Abstract

The production of intermediates (light absorbing transients, LAT) has been examined in the photoreduction of benzophenone by different hydrogen donors in inert solvents (acetonitrile and benzene). For 2-propanol in acetonitrile, a branching ratio of 0.3 for the coupling reaction between dimethyl- and diphenylketyl radicals was extracted by computer model simulations considering inner filter effects and the quenching of triplet benzophenone by LAT. Experimental evidence is presented that indicates that LAT could be an efficient H-donor. Steady-state quantum yields for LAT production, Φ LAT, and benzophenone conversion, − Φ Ph2CO, were determined for the donors 2-propanol, n- and c-hexane, benzhydrol and tetrahydrofuran. The relative quantum yield, − Φ LAT/ Φ Ph2CO, which indicates the differential partitioning of the products, shows that 2-propanol and tetrahydrofuran are nearly ten times more efficient than alkanes. From the analysis of the mechanism we derived an upper limit for the rate constant for the coupling reaction of two diphenylketyl radicals to yield LAT: k 6d ⩽ 5 × 10 5 M −1 s −1.