Significant differences in photochemical and photophysical behavior of trans-a-methylstilbene and trans-stilbene have been attributed to structural changes caused by the steric requirements of the methyl group. We present here the X-ray structures of cis- and trans-a-methylstilbene (c- and t-MeSt). The solid state packing of t-MeSt resembles that of trans-stilbene in that both exhibit disorder with bicycle pedal relationship of nearest neighbors. The difference is that the bicycle pedal disorder is dynamic in trans-stilbene but is static in t-MeSt. The X-ray structures are compared with calculated structures at the B3LYP/6-311++g(3df,2p), M062X/6-311++g(3df,2p) and MN15/aug-cc-pVTZ levels of theory. Deviations from planarity imposed by the methyl substitution account for chirality in t-MeSt, destabilize its ground state and lowest singlet and triplet excited states and explain its shorter lifetimes and fluorescence yields. While the free space available in the trans-stilbene crystal allows the bicycle pedal motion about the central double bond to occur easily, methyl substitution prevents bicycle pedal conformer equilibration. Photochemical observations on the MeSt isomers in alcohols reveal protonation of the twisted excited singlet state leading to ether formation and rearrangement to 2,3-diphenylpropene.

Photophysics and photochemistry of the a-methylstilbenes

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.