Excited-state dynamics of overlapped optically-allowed 1Bu + and optically-forbidden 1Bu - or 3Ag - vibronic levels of carotenoids: Possible roles in the light-harvesting function

Abstract

Pump-probe spectroscopy after selective excitation of all-trans Cars (n = 9-13) in nonpolar solvent identified a symmetry selection rule of diabatic electronic mixing and diabatic internal conversion, i.e., '1Bu +-to-1Bu - is allowed But 1Bu +-to-3Ag - is forbidden'. Kerr-gate fluorescence spectroscopy showed that this selection rule breaks down, due to the symmetry degradation when the Car molecules are being excited, and, as a result, the 1Bu +-to-3Ag - diabatic electronic mixing and internal conversion become allowed. On the other hand, pumpprobe spectroscopy after coherent excitation of the same set of Cars in polar solvent identified three stimulatedemission components (generated by the quantum-beat mechanism), consisting of the long-lived coherent cross term from the 1Bu + + 1Bu - or 1Bu + + 3Ag - diabatic pair and incoherent short-lived 1Bu + and 1Bu - or 3Ag - split incoherent terms. The same type of stimulated-emission components were identified in Cars bound to LH2 complexes, their lifetimes being substantially shortened by the Car-to-BChl singlet-energy transfer. Each diabatic pair and its split components appeared with high intensities in the first component. The low-energy shifts of the 1Bu +(0), 1Bu -(0) and 3Ag -(0) levels and efficient triplet generation were also found. © by Pawel Pomorski and Polish Biochemical Society 2009.