Solvent dependence of the ultrafast S2-S1 internal conversion rate of β-carotene

The solvent dependence of the steady-state and time-resolved fluorescence emission from the S-2 (1(1)B(u)) State of an-trans-beta-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 pi* 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.