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 S2 (11Bu) 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 S2 (11Bu) state. The lifetime of the S2 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 S2 lifetimes, the π* solvatochromic parameter, and the ratio of the absorption fine structure. A correlation between the rate of internal conversion from the S2 state and the previously reported S1 (21Ag) state Raman shifts of the CC stretching mode supports the contention that the S2 and S1 states are vibronically coupled. The implications of these results for light harvesting in photosynthesis are also discussed.