Femtosecond–Picosecond Laser Photolysis Studies on Reduction Process of Excited Benzophenone with Tertiary Aromatic Amines in Acetonitrile Solution

Abstract

Photoreduction processes of benzophenone (BP)-N,N-diethylaniline (DEA) system in acetonitrile solution were studied by means of femtosecond–pecosecond laser photolysis and time resolved transient absorption spectroscopy. The reaction processes including the formation of geminate ion pairs (IP) by electron transfer (ET) between BP* and DEA at encounter followed by intra-IP proton transfer giving the ketyl radical (BPH·) were clearly observed in both triplet and singlet excited state, while the IP produced by excitation of the CT complex between BP and amine formed easily in more or less concentrated solutions did not give BPH·. In addition to the detailed investigations on the BP-DEA system, we made comparative studies on BP–tertiary aromatic amine systems including N,N-diethyl-p-toluidine, N,N-dimethylaniline, and N-methyldiphenylamine. We observed clearly the characteristic tendency that the proton transfer rate in the 3IP decreased with decrease of the oxidation potential of the amine. This result was interpreted as due to the increase of the inter-ionic distance in the 3IP with increase of the free energy gap for the charge separation at encounter.