The first detection of the 3Ag− state in carotenoids using resonance-Raman excitation profiles

The singlet 3Ag− state that had been theoretically predicted in shorter polyenes [P. Tavan and K. Schulten J. Chem. Phys. 85 (1986) 6602; Phys. Rev. B 36 (1987) 4337] was first identified in bacterial carotenoids by measurements of resonance-Raman excitation profiles. It is almost overlapped with the 1Bu+ state in spheroidene (the number of conjugated double bonds, n=10), and located in-between the 1Bu+ and 1Bu− states in lycopene, anhydrorhodovibrin and spirilloxanthin (n=11–13). The slopes when the 2Ag−-, 1Bu−- and 3Ag−-state energies were expressed as linear functions of 1/(2n+1) exhibited the ratio of 2:3.1:3.8 in excellent agreement with that theoretically predicted, 2:3.1:3.7.