The photoisomerization of the stilbenes was proposed to proceed via a common twisted intermediate (phantom singlet excited state) that decays to give cis- and trans-stilbene (c- and t-St) with similar probability. This intermediate has been referred to as a funnel point or a conical intersection, CI, on the singlet excited state surface. Laarhoven and Pincock independently showed that 1,1-addition occurs via a carbene mechanism in competition with direct 1,2-addition.The mechanism of the 1,2-addition was not established but there was a preference for addition to the longer lived t-St singlet excited state. We measured the photoadditions of methanol to t- and c-St relative to their interconversion. Our quantum yield ratios of photoisomerization to photoaddition starting from t- and c-St in methanol are 184 and 187 respectively, are identical within experimental error. This implicates a twisted zwitter-ionic common intermediate in the photoaddition of methanol to both isomers. This conclusion was confirmed by the observation that the ratios of direct to carbene ether formation from the addition of CD3OD to c- and t-St, 1.50 and 1.44, respectively, are also the same within experimental error. The study of methanol addition to t-stilbene-d2 revealed a large D-isotope effect in carbene formation. Our results are consistent with current theoretical results that predict pyramidalization in one of the benzyl moieties of the twisted intermediate as it reaches the conical S1/S0 CI.

Photoaddition of methanol to the stilbenes: Involvement of a common twisted intermediate

253rd National Meeting (2017)

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