Picosecond Spectroscopic Studies on Photoreduction of Benzophenone by Triethylamine

Abstract

Picosecond spectroscopy on benzophenone (BP)-triethylamine (TEA) system was performed for the study of the elementary process of hydrogen abstraction by BP in the lowest excited triplet state (³BP∗). TEA was used as solvent since (i) it did not form a ground state complex with ³BP (ii) the rate of hydrogen abstraction was not controlled by the diffusion of hydrogen donor in solvent, and (iii) charge transfer from TEA to BP in the excited singlet state (¹BP∗) was inefficient because the polarity of TEA was lower than that of acetonitrile.
An excitation light source was a second harmonic of a mode-locked ruby lase r (347 mm, 30 ps). Time-resolved spectra and kinetics of the transient absorption were measured with an OMA dual spectrum mode apparatus and with an echelon system, respectively.
The absorption spectrum of ³BP∗ was measured at 0 ps (Fig.4). The lifetime of ³BP∗ was determined to be 50 ps (Fig.7). Ketyl radical was completely formed within 150 ps (Fig.6). Anion radical of BP was not observed. There are two interpretations: (i) the rapid proton transfer from TEA+ to the anion radical of BP or (ii) the direct hydrogen transfer (both electron and proton).³BP∗ abstracts hydrogen from TEA (in less polar media) with the time constant of 50 ps.