Temperature effects on excitation relaxation dynamics of the carotenoid β-carotene and its analogue β-apo-8′-carotenal, probed by femtosecond fluorescence spectroscopy

Femtosecond time-resolved fluorescence spectra were measured on the carotenoid beta-carotene and its analogue beta-apo-8'-carotenal in 2-methyltetrahydrofuran at 295 and 80 K. beta-Carotene showed intramolecular vibrational redistribution (IVR) with time constants of 40-80 fs depending on the wavelengths, followed by vibrational relaxation with it time constant of 220 fs. At 80 K, the former process was resolved; however, the latter was not necessarily clear because of large spectral changes. However, beta-apo-8'-carotenal showed relaxation processes different from those of beta-carotene; the IVR process was not resolved under both temperature conditions, and at 80 K, the decay constants were prolonged at all wavelengths measured. Changes in the time-resolved fluorescence spectra were discernible but small. These findings indicate that the IVR process of beta-apo-8'-carotenal was very fast, that it was shorter than our time resolution (30 fs), and that the slow relaxation process involved an interaction between a solute and a solvent, especially under the low-temperature condition.