Influence of the chloro substituent position on the triplet reactivity of benzophenone derivatives: a time-resolved resonance Raman and density functional theory study

A nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic investigation of the photoreduction reactions and ability of several chloro-substituted benzophenone (Cl-BP) triplets is described. The TR3 results show that the 3-chlorobenzophenone (3-Cl-BP), 4-chlorobenzophenone (4-Cl-BP) and 4,4'-dichlorobenzophenone (4,4'-dichloro-BP) triplets exhibit similar hydrogen abstraction ability with the parent BP triplet. In 2-propanol, the 3-Cl-, 4-Cl- and 4,4'-dichloro-diphenylketyl (DPK) radicals were observed and they appear to react with dimethylketyl radicals at the para-position to form a light absorption transient species. These transient species were characterized with TR3 spectra, and identified with the help of results from density functional theory calculations. In an acetontitrile/water (MeCN:H2O) 1:1 mixed solvent, these DPK radicals were also observed but with slower formation rates. However, the 2-Cl-DPK radical was observed to form with a lower yield and a significantly slower formation rate than the other chloro-substituted benzophenones examined here in 2-propanol under the same experimental conditions. These results reveal that the 2-chloro substituent reduces the hydrogen abstraction ability of the substituted BP triplet, which was not as expected based on the assumption that the electron-withdrawing group could increase its photoreduction ability. This unusual ortho effect of the chlorine substitution is briefly discussed. Copyright (C) 2011 John Wiley & Sons, Ltd.