Femtosecond transient-grating spectroscopy with optical heterodyne detection was employed to observe separately the time evolution of the absorption and dispersion components of the third-order nonlinear optical signal following resonant excitation of the S2 (1Bu+) states of �-carotene in benzonitrile and peridinin in methanol using 40-fs pulses centered at 520 nm. The absorption and dispersion components exhibit distinctively different time profiles owing to the population of intermediate states. An initial intermediate state is populated on an ultrashort (<30 fs) time scale in both carotenoids. Owing to the fast red-shifting of the stimulated emission part of the S2-state transient grating signal, we suggest that the intermediate state arises from vibrational displacements on the S2-state potential surface that eventually yield twisted or bent conformations. Motions of the molecule of this type in the S2-state would contribute to a mixing of the diabatic S2 and S1 electronic states and would impact the efficiency of energy transfer from the S1 state to the (B)Chl Qy state in photosynthetic light-harvesting proteins. The time-resolved transient-grating spectra obtained for peridinin in the peridinin�chlorophyll a protein from Amphidinium carterae suggest a more rapid formation of the intermediate than for peridinin in methanol. This finding suggests that the conformation of the peridinin chromophore is controlled in the binding site to optimize the formation of the intermediate upon excitation of the S2 state. (Supported by the Department of Energy under Award Number DE-SC0010847.)

Femtosecond nonlinear optical studies of radiationless decay in carotenoids and in the peridinin�chlorophyll <i>a</i> protein

247th National Meeting (2014)

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Femtosecond nonlinear optical studies of radiationless decay in carotenoids and in the peridinin�chlorophyll a protein