Spectroscopy of the benzil–bibenzyl mixed crystal system: High resolution optical studies

Abstract

The S0→S1 excitation and T1→S0 emission spectra of a benzil doped bibenzyl mixed crystal at temperatures ranging from 300 to 2 K have been measured. Two kinds of spectra have been found and assigned to the benzil molecule trapped in two different molecular configurations. One of the observed sets of spectra is nearly identical to those of the benzil single crystal. The other, red shifted with respect to those of the benzil single crystal, is similar to the spectra of systems known to contain a trans-planar 1,2-dicarbonyl fragment. Such a geometry change is consistent with the manner in which the benzil molecule would most probably substitute into the bibenzyl lattice. The red shifted spectra are broad, beyond the extremely sharp 0–0, and this broadness is attributed to the strong excitation–phonon coupling expected for molecules that undergo large geometrical changes upon photoexcitation.