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Cis- or trans-stilbene were isomerized in solution by irradiation within the singlet-triplet absorption band in the presence of O2. No photocyclization to phenanthrene was found. The quantum yields for cis → trans and trans → cis isomerization are 0.22 ± 0.04 and 0.42 ± 0.03, respectively. Known experimental evidence is interpreted as supporting an existence of two non-equivalent energy minima, in twisted configurations, of the lowest triplet state of stilbene.
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Bylina et al. (1969) studied this question.
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