Photochemistry of the 1,4-Diphenyl-1,3-butadienes in Ethanol. Trapping Conical Intersections

We report photoisomerization and photoaddition quantum yields in ethanol starting from each 1,4-diphenyl-1,3-butadiene, DPB, isomer. Despite the fact that the trans,trans isomer, tt-DPB, has a significant fluorescence quantum yield and lifetime, whereas no fluorescence is observed from the cis isomers, ether formation occurs with similar efficiency from tt-DPB and ct-DPB and less efficiently from cc-DPB. Photoaddition is about 72 times slower than photoisomerization to the ct- and tt-DPB isomers starting from tt- and ct-DPB, respectively. The results are consistent with addition of alcohol to the common zwitterionic trans-phenallyl cation/benzyl anion intermediate that leads to photoisomerization through a conical intersection. Ether formation from cc-DPB tracks inefficient formation of tt-DPB indicating that the small bicycle pedal cc-DPB → tt-DPB component proceeds stepwise through the same zwitterionic trans-phenallyl cation/benzyl anion intermediate. Previous results concerning the addition of methanol to the stilbenes are similarly interpreted. In contrast to trans-stilbene, the fluorescence and photoisomerization quantum yields of tt-DPB are inconsistent with the assumption of strict complementarity between radiative and torsional relaxation channels of tt-DPB in alcohols.