Excited-State Dynamics of Overlapped Optically-Allowed 1B(u)(+) and Optically-Forbidden 1B(u)(−) or 3A(g)(−) Vibronic Levels of Carotenoids: Possible Roles in the Light-Harvesting Function

The unique excited-state properties of the overlapped (diabatic) optically-allowed 1B(u)(+) and the optically-forbidden 1B(u)(−) or 3A(g)(−) vibronic levels close to conical intersection (‘the diabatic pair’) are summarized: Pump-probe spectroscopy after selective excitation with ∼100 fs pulses of all-trans carotenoids (Cars) in nonpolar solvent identified a symmetry selection rule in the diabatic electronic mixing and diabatic internal conversion, i.e., ‘1B(u)(+)-to-1B(u)(−) is allowed but 1B(u)(+)-to-3A(g)(−) is forbidden’. On the other hand, pump-probe spectroscopy after coherent excitation with ∼30 fs of all-trans Cars in THF generated stimulated emission with quantum beat, consisting of the long-lived coherent diabatic cross term and a pair of short-lived incoherent terms.