Internal conversion versus intersystem crossing: what drives the gas phase dynamics of cyclic α,β-enones?

We investigate the competition between intersystem crossing (ISC) and internal conversion (IC) as nonradiative relaxation pathways in cyclic α,β-unsaturated enones following excitation to their lowest lying (1)ππ* state, by means of time-resolved photoelectron spectroscopy and ab initio computation. Upon excitation, the (1)ππ* state of 2-cyclopentenone decays to the lowest lying (1)nπ* state within 120 ± 20 fs. Within 1.2 ± 0.2 ps, the molecule subsequently decays to the triplet manifold and the singlet ground state, with quantum yields of 0.35 and 0.65, respectively. The corresponding dynamics in modified derivatives, obtained by selective methylation, show a decrease in both IC and ISC rates, with the quantum yields of ISC varying between 0.35 and 0.08. The rapid rates of ISC are explained by a large spin orbit coupling of 45-60 cm(-1) over an extended region of near degeneracy between the singlet and triplet state. Furthermore, the rate of IC is depressed by the existence of a well-defined minimum on the (1)nπ* potential energy surface. The nonadiabatic pathways evinced by the present results highlight the fact that these molecular systems conceptually represent "intermediate cases" between ultrafast dynamics mediated by vibrational motions at conical intersections versus those by statistical decay mechanisms.