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.