Ultraviolet Spectra of Stilbene, p-Monohalogen Stilbenes, and Azobenzene and the trans to cis Photoisomerization Process

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Abstract
In order to improve our understanding of the electronic states and photochemical reactions of stilbene, we have carried out a spectroscopic and photochemical investigation of stilbene, some substituted stilbenes, and azobenzene. High resolution absorption and fluorescence spectra of the singlet—singlet transition in dilute mixed crystal have been analyzed, and from them it is estimated that the potential barrier to trans‐cisisomerization in the first excited singlet state is about 40 kcal/mole. The absorptionspectrum of the first singlet—triplet transition has been observed by enhancement with a heavy atom solvent and is interpreted as showing that the central bond in the lowest triplet state has a very substantial barrier to rotation. An electronic energy level scheme for stilbene has been constructed by treating the molecule as one ethylene molecule interacting with two toluene molecules. This treatment suggests that as many as four triplet states may be of lower energy than the first excited singlet and would then be possible pathways for photoisomerization. A study of the deactivation processes of photoexcited stilbene has included the temperature dependences of fluorescence and of isomerization efficiencies. The first showed an activated quenching process with a frequency factor of 1012, which makes it a less forbidden process than is common for singlet—triplet crossings. The second showed that the activated process has the major isomerization yield. In p‐bromostilbene an unactivated process has the major isomerization yield, indicating that there is an atomic number effect on the rate of the unactivated process. Thus the direct isomerization in the first excited singlet or first excited triplet states has been ruled out; two other paths, one with a small activation energy and the other apparently a singlet to excited triplet crossing, have been found.

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