In the past decade much attention has been paid to "biradicaloid" species, reactive intermediates characterized by a small HOMO-LUMO energy gap. The study of these compounds is important for insight into the details of thermal and photochemical processes.
In this paper we present high resolution emission (see figure) and excitation spectra of o-durylene, obtained in situ by photolysis of durene at low temperatures.
The main points to be considered are the nature of the lowest excited state (singly or doubly excited in origin), the geometry of the molecule in this state as compared to the ground state and the vibronic features.
Low resolution spectra of o-durylene in organic glasses or in n-hexane at temperatures above 50 K exhibit a Franck Condon forbidden band contour. The intensity patterns and the length of the vibrational progressions in the measured quasilinear spectra, however, show that geometry changes do not play an important role. Furthermore, in the spectra an abhahlJ.tian-emidAhn adynlm&y has been observed for two vibrational modes. From these data, combined with PPP calculations we conclude that the lowest excited state has A1 symmetry with doubly excited character. This conclusion could not been drawn by other investigators due to lack of experimental data.