Presence and absence of electronic mixing in shorter-chain and longer-chain carotenoids: Assignment of the symmetries of 1Bu- and 3Ag- states located just below the 1Bu+ state

In spheroidene (having the number of conjugated double bonds n = 10), stimulated emission was observed from the mixed vibronic levels of 1Bu+(0)+1Bu−(2) and 1Bu+(1)+1Bu−(3), whereas in lycopene, anhydrorhodovibrin and spirilloxanthin (n = 11-13), stimulated emission, from the pure vibronic levels of 1Bu+(0) and 1Bu+(1). Thus, the 1Bu+ state can mix with the 1Bu− state but not with the 3Ag− state, both being located just below the 1Bu+ state. The presence and absence of the mixing of the neighboring diabatic states support the symmetries of the next low-lying 1Bu− and 3Ag− states.

The vibronic levels of 1Bu+ mix with those of 1Bu−, but not with those of 3Ag−, due to the symmetries of the relevant singlet-excited states of carotenoids. The results support the symmetry notation based on the slopes of the linear relations, i.e., 1Bu−:3Ag−≈3:4.