Substituent effects on dynamics at conical intersections:: α,β-enones

Femtosecond time-resolved photoelectron spectroscopy and high-level theoretical calculations were used to study the effects of methyl substitution on the electronic dynamics of the alpha,beta-enones acrolein (2-propenal), crotonaldehyde (2-butenal), methylvinylketone (3-buten-2-one), and methacrolein (2-methyl-2-propenal) following excitation to the S-2(pi pi*) state at 209 and 200 nm. We determine that following excitation the molecules move rapidly away from the Franck-Condon region, reaching a conical intersection promoting relaxation to the S-1(n pi*) state. Once on the S-1 surface, the trajectories access another conical intersection, leading them to the ground state. Only small variations between molecules are seen in their S-2 decay times. However, the position of methyl group substitution greatly affects the relaxation rate from the S-1 surface and the branching ratios to the products. Ab initio calculations used to compare the geometries, energies, and topographies of the S-1/S-0 conical intersections of the molecules are not able to satisfactorily explain the variations in relaxation behavior. We propose that the S-1 lifetime differences are caused by specific dynamical factors that affect the efficiency of passage through the S-1/S-0 conical intersection.