The 1B_u⁺, 1B_u^-, and 2A_g^- Energies of Crystalline Lycopene, β-Carotene, and Mini-9-β-carotene as Determined by Resonance-Raman Excitation Profiles: Dependence of the 1B_u^- State Energy on the Conjugation Length

Abstract

The energies of the optically forbidden 2Ag- and 1Bu- states of crystalline carotenoids were determined by measurements of resonance-Raman excitation profiles together with those of the optically allowed 1Bu+ state. The 1Bu+, 1Bu-, and 2Ag- state energies (for the 0 ← 0 vibronic transition) were 18 600, 15 770, and 13 200 cm-1 in lycopene; 19 150, 16 550, and 14 670 cm-1 in β-carotene; and 20 900, 19 700 and 15 750 cm-1 in mini-9-β-carotene, respectively. Comparison between lycopene (the number of the conjugated double bonds, n = 11) and spheroidene (n = 10) (Sashima et al. Chem. Phys. Lett. 1999, 299, 187) as well as between β-carotene (n = 11) and mini-9-β-carotene (n = 9) lead us to the following conclusions: (i) the ordering of the singlet states is 1Bu+ (S3) > 1Bu- (S2) > 2Ag- (S1), (ii) all the state energies decrease when n increases, and (iii) the dependence of the state energy on n is the strongest for the 1Bu- state. All of these observations agree with extrapolation of theoretical prediction by Tavan and Schulten for shorter polyenes (Tavan, P.; Schulten, K. J. Chem. Phys. 1986, 85, 6602).