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 beta-carotene; and 20 900, 19 700 and 15 750 cm(-1) in mini-9-beta-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 p-carotene (n = 11) and mini-9-beta-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).