Excited-State Potential Surface Crossings in Acrolein: A Model for Understanding the Photochemistry and Photophysics of α,β-Enones

The interplay between the ground-and the three low-lying singlet/triplet excited-state surfaces (S1, T2, and T1) in s-trans-and s-cis-acrolein has been studied using CASSCF computations at the 6-31G* level. The objective is to provide a model for understanding α,β-enone photochemistry and photophysics. Two different photochemically active relaxation paths starting from a planar S11(n-π*) excited state minimum have been documented. The first of these pathways involves a radiationless decay via intersystem crossing to the triplet manifold, leading to production of a short-lived T13(π-π*) twisted intermediate. This intermediate then decays, via a second intersystem crossing, to the ground state, leading to isomerization of the acrolein double-bond. The second relaxation path involves the singlet manifold only. In this case relaxation to S0occurs via a single decay channel which corresponds to a S1/S0conical intersection. This conical intersection lies 15 and 10 kcal mol-1above the 1(n-π*) s-trans-and 1(n-π*) s-cis-acrolein, respectively. Production of oxetene is found to occur via the singlet path starting exclusively from the S11(n-π*) s-cis-acrolein. The computational results agree well with the available experimental data. The existence of a T13(π-π*) intermediate is supported by the observation of a 280-310-nm transient absorption in both acyclic and cyclic α,β-enones. Further, the existence of a barrier to production of oxetene is in agreement with the fact that this product accumulates when acyclic α,β-enones are irradiated with light near 250 nm, but it is not detected when α,β-enones are irradiated near 300 nm. © 1994, American Chemical Society. All rights reserved.