Photochemistry of the Stilbenes in Methanol. Trapping the Common Phantom Singlet State

A comparative study of the photochemistry of cis- and trans-stilbene in methanol shows that both isomers undergo methanol photoaddition giving similar yields of α-methoxybibenzyl in competition with cis-trans photoisomerization. Methanol addition occurs primarily following torsional relaxation of the lowest excited singlet states of each isomer, ¹ c* and ¹ t*, to a common twisted singlet excited state intermediate, ¹ p*, initially called the phantom singlet state. The addition is consistent with the zwitterionic character of ¹ p*. Ether forms by direct 1,2-addition of CH₃OH to the central carbon atoms and by 1,1-addition following rearrangement to 1-benzyl-1-phenylcarbene. Use of CD₃OD and GC/MS (gas chromatographic/mass spectroscopic) analysis of the ether products revealed that the ratio of carbene/direct addition pathways is higher starting from cis-stilbene. We conclude that ¹ p* formed from ¹ c* is hotter than ¹ p* formed from ¹ t*. Surprisingly, except for favoring the carbene pathway, the use of higher energy photons (254 vs 313 nm) does not affect the overall ether quantum yield starting from cis-stilbene but significantly enhances both pathways starting from trans-stilbene. It appears that carbene formation and direct methanol addition to higher trans-stilbene excited state(s) compete with relaxation to S₁. Substitution of D for the vinyl Hs of stilbene enhances the direct addition pathway more than 2-fold and strongly suppresses the carbene insertion pathway, revealing a large, k_pc^d0/ k_pe^d2 = 6.3, primary deuterium isotope effect in the carbene rearrangement. The 2-fold increase in the ether quantum yield is due primarily to a 2.75-fold increase in the lifetime of ¹ p* on deuterium substitution of the vinyl hydrogens.