Knowledge Bank

The absorption and emission spectra of stilbene and azobenzene in single mixed-crystals with dibenzyl were studied at 20$\circ$K in the hope of learning more about the photo-isomerization of these molecules. The analogous transitions in the two trans-molecules showed considerable similarity and important differences. The vibrations were assigned by studying substituted stilbenes and substituted benzenes. A prominent repeating frequency, $208cm-1$ in stilbene, was identified as an in-plane bending of the rings against the ethylenic link. The corresponding progression in azobenzene is much longer, suggesting a greater change of the ring-central link angle in azobenzene than in stilbene. The azobenzene transition is also more diffuse. The main repeating frequency in stilbene absorption $1600cm-1$, and in azobenzene absorption $1340cm-1$, does not appear to be a ring frequency, and is more likely a stretching of the central link. If so, the $\pi$-electron binding in this link is not appreciably decreased upon excitation. Experiments on selective excitation of the various vibrational levels are in progress.