Vibrational relaxation pathways in the electronic excited state of carotenoid

The intra- and inter-molecular vibrational relaxation in the electronic excited state (1Bu+) of spheroidene derivative (the number of conjugated double bonds, image) has been investigated at room temperature by means of femtosecond time-resolved fluorescence spectroscopy based on an optical-Kerr-gate technique. Depending on the photo-excitation either to the image or image vibronic level, remarkable differences were observed in hot luminescence spectra related to the vibrational relaxation process of high- and low-frequency modes. Under the excitation to the image state hot luminescence from the image state was observed as a dominant feature of the time-resolved spectra while the dynamic Stokes shift originating from the low-frequency-modes dynamics was clearly observed under the excitation to the image. These observations of the excitation energy dependence of time-resolved fluorescence spectra were discussed by analyzing the Franck–Condon factors of transitions from image, and 2 levels of high-frequency modes.