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 β-carotene and its analogue β-apo-8‘-carotenal in 2-methyltetrahydrofuran at 295 and 80 K. β-Carotene showed intramolecular vibrational redistribution (IVR) with time constants of 40−80 fs depending on the wavelengths, followed by vibrational relaxation with a 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, β-apo-8‘-carotenal showed relaxation processes different from those of β-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 β-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.