Fluorescence Spectroscopy of All-trans-anhydrorhodovibrin and Spirilloxanthin: Detection of the 1Bu- Fluorescence

Fluorescence spectra of all- trans -anhydrorhodovibrin and spirilloxanthin having the number of conjugated double bonds n = 12 and 13 were recorded in n -hexane solution. The optically forbidden 2A g - → 1A g - fluorescence was observed for carotenoids having such a long conjugated chain, and the optically forbidden 1B u - → 1A g - fluorescence was identified for the first time. The proposed energies of the 2A g -, 1B u -, and 1B u + fluorescence origins are 12 500, 14 900, and 19 200 cm -1 for anhydrorhodovibrin and 11 900, 13 600, and 18 900 cm -1 for spirilloxanthin (at 295 K). The estimated transition dipole moments for fluorescence from the 2A g -, 1B u -, and 1B u + states to the ground 1A g - state are 1.24, 4.76, and 17.4 D for the former and 1.37, 3.72, and 16.8 D for the latter.