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

The energies of the optically forbidden 2A g - and 1B u - states of crystalline carotenoids were determined by measurements of resonance-Raman excitation profiles together with those of the optically allowed 1B u + state. The 1B u +, 1B u -, and 2A g - 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 1B u + (S 3 ) > 1B u - (S 2 ) > 2A g - (S 1 ), (ii) all the state energies decrease when n increases, and (iii) the dependence of the state energy on n is the strongest for the 1B u - 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).