Internal-Conversion, Vibrational Relaxation and Vibrational Redistribution Processes of the Singlet-Excited States of Photosynthetic Carotenoids Revealed by Kerr-Gate Fluorescence Spectroscopy

Abstract

In the light-harvesting system, carotenoids capture light-energy to be the optically-allowed 1B_u⁺ state, and transfer it to the bacteriochlorophyll. The singlet-energy transfer can carry out from not only the 1B_u⁺ state but also the other low-lying optically-forbidden states, e.g., 3A_g⁻, 1B_u⁻ and 2A_g⁻ states. The rates of internal-conversion among these singlet states are too fast to complete the vibrational-relaxation and -redistribution. Since the ordering of the state energies drastically dependent on the conjugation length, internal-conversion pathway should change one carotenoid to another. Kerr-gate fluorescence spectroscopy having high spectral resolution is a powerful technique to determine precise transition energies of such a complicated system. We try to analyze Kerr-gate fluorescence spectra of a set of carotenoids having different conjugation length by simulating the internal conversion with Franck-Condon factor and the rate of vibrational relaxations as parameters and including vibrational redistribution between the C=C and the C-C stretching modes.