Temperature-dependent photochemistry of 1,3-diphenylpropenes. The di-pi-methane reaction revisited

The temperature-dependent photochemical behavior of 1,3-diphenylpropene and several of its 3-substituted derivatives has been investigated over a wide temperature range. The singlet state is found to decay via two unactivated processes, fluorescence and intersystem crossing, and two activated proces ses, trans,cis isomerization and phenyl-vinyl bridging. The latter activate d process yields a diradical intermediate which partitions between ground-s tate reactant and formation of the di-pi -methane rearrangement product. Ki netic modeling of temperature-dependent singlet decay times and quantum yie lds of fluorescence, isomerization, di-pi -methane rearrangement, and nonra diative decay provides rate constants and activation parameters for each of the primary and secondary processes. Substituents at the 3-position are fo und to have little effect on the electronic spectra or unactivated fluoresc ence and intersystem crossing pathways. However, they do effect the activat ed primary and secondary processes. Thus, the product ratios are highly tem perature dependent.