Intermolecular contributions to the isotope effect on excited state lifetimes: Benzene

Publisher Website
Google Scholar
Abstract
The phosphorescence lifetimes of several deuterium isotopes of benzene are measured. The lifetimes of C6H5D and p‐C6H4D2 are found to depend upon the orientation of the molecule in the site field for C6D6, sym‐C6H3D3, and borazine host crystals. This variation is measurable because the six or three possible orientations of the respective isotopic guests group into two energetically different sets of sites. Measurements of the lifetimes of each component of the resulting spectral doublets show an orientational difference in their lifetimes that is a significant fraction of the difference between the lifetimes of C6H6 and C6D6 in the borazine and, presumably, the benzene host crystals. In fact, the lifetimes of C6H6, C6H5D, p‐C6H4D2, sym‐C6H3D3, and C6D6 in borazine can all be fit by a model which assumes that the isotope effect is entirely of intermolecular origin. It was found that one orientation gave a much larger isotope effect than the other. None of the existing theoretical models for interpreting isotope effects on excited state lifetimes is consistent with these data. Thus, much of the work on benzene which attempts to interpret changes in the phosphorescence decay rates in terms of basically intramolecular theoretical models must be reconsidered. Arguments are given which suggest that at least part, if not all, of the isotope effect observed in the benzene and borazine hosts is in the radiative decay channel.