Assignment of Excited Electronic States of the Benzyl Radical by the Method of Three-Step Photoselection

The assignments of three excited states of the benzyl radical and some of its methyl derivatives are obtained by a method which involves an extension of the usual photoselection technique. Rigid solutions at 77°K of the parent methylated benzenes are photolyzed with polarized light to give the corresponding benzyl radical (Step 1), the radical is then itself excited with polarized light (Step 2), and the polarized‐radical emission is analyzed (Step 3). The theory of this three‐step procedure is outlined and applied both qualitatively and quantitatively to the experimental results. Because of the known ${}^1{L}_b$ assignment of the excited state associated with the parent molecule absorption, it is possible to determine the absolute polarization of the transitions in the benzyl radicals. Polarizations of the vibronic bands in the benzyl‐radical emission independently confirm the results. The state assignments which follow (and which are based on a ${}^2{B}_2$ ground state symmetry) fully disagree with the predictions of MO theories (slightly refined). On the other hand they confirm the predictions arising from unrefined valence‐bond and composite systems calculations.