Solvent Dependence of the Ultrafast S2−S1 Internal Conversion Rate of β-Carotene

Abstract

The solvent dependence of the steady-state and time-resolved fluorescence emission from the S 2 (1 1 B u ) state of all- trans -β-carotene was investigated in a range of polar and nonpolar solvents and in hexane/carbon disulfide mixtures. The steady-state absorption and fluorescence emission maxima in both polar and nonpolar solvents showed parallel shifts with increasing solvent polarizability, indicating that the emitting state is the S 2 (1 1 B u ) state. The lifetime of the S 2 state was determined using the fluorescence upconversion technique, and lifetimes varying from 120 fs in quinoline to 177 fs in hexane were found. An intramolecular relaxation process occurring on a similar time scale was observed as a broadening in the reconstructed time-dependent emission spectra in hexane and as emission wavelength-dependent lifetimes in all the solvents studied. Correlations were observed between the S 2 lifetimes, the π* solvatochromic parameter, and the ratio of the absorption fine structure. A correlation between the rate of internal conversion from the S 2 state and the previously reported S 1 (2 1 A g ) state Raman shifts of the C C stretching mode supports the contention that the S 2 and S 1 states are vibronically coupled. The implications of these results for light harvesting in photosynthesis are also discussed.