Three-step photoselection and the assignment of the electronic states of the benzyl radical

A contradiction between assignments of the first excited state of the benzyl radical, originally made by three-step photoselection in rigid solutions, and later by gas-phase rotational analysis, has been resolved. The photochemically produced radical in a rigid solution can be subject to rotational diffusion over the time duration of the experiment. This weakens (or even confuses) the crucial correlation with reference axes of the parent molecule necessary for achieving absolute assignments. By increasing the viscosity of the medium, such randomization can be considerably reduced and photoselection polarization ratios greater than 3/1 (a characteristic of three-step photoselection) are obtained for p-methylbenzyl. The photoselection data for the p-methylbenzyl radical indicate that the first three observed excited states can be assigned 2A2, 2A2, 2B2, respectively. If the 2A2 assignment for the lowest excited state of p-methylbenzyl in a rigid solvent can be applied to the benzyl radical itself, then agreement is achieved with the gas-phase rotational analysis.