Near-infrared time-resolved study of the S-1 state dynamics of the carotenoid spheroidene

Using a novel experimental approach based on near-infrared femtosecond abso rption spectroscopy, we have determined the energy of the S-1 state of the carotenoid spheroidene. The energy of this state is 13 400 +/- 90 cm(-1) at both 293 and 186 K, showing that there is no temperature-induced shift of the S-1 level. A discrepancy of about 800 cm(-1) between the S-1 energy det ermined here and that obtained from previous fluorescence and resonance Ram an measurements is explained in terms of the different conformational speci es coexisting in the S-1 excited state. Measurements of kinetics in the nea r-infrared region revealed that at least three relaxation processes take pl ace after excitation of spheroidene into its S-2 state. Ultrafast S-2 --> S -1 internal conversion occurs within the first 300 fs, followed by vibratio nal cooling in the SI state, which occurs on a time scale of similar to 700 fs. The S-1 lifetime is 8 ps at 293 K, in good agreement with previous mea surements of the S-1 --> S-N transition. A somewhat longer S-1 lifetime of 9.5 ps is observed at 186 K.