Summary
The photochemistry of Carotenoids is determined by the electronic structures of their low energy, excited states. This chapter first relates the optical spectroscopy of carotenoids to current theoretical descriptions of polyene electronic states. Highly detailed spectroscopic information now available for model polyenes leads to a simple, three-levelenergy scheme: S2(I1 Bu)>S1(21Ag)>S0(I1 Ag) Optical studies of polyenes and carotenoids of intermediate length demonstrate that increases in conjugation lengths invariably result in S2→S0 emissions replacing the characteristic S1→S0 emissions of short polyenes. This cross-over to S2 fluorescence can be accounted for by the energy gap law: decreases in S1 energies lead to sharp increases in the rates of S1→S0 nonradiative decay with increasing conjugation. The dominance of S2 emissions in long polyenes means that the S1 states of the carotenoids employed by photosynthetic systems are difficult to locate using fluorescence detection. Even less information is available regarding the energies of carotenoid T1 states due, in large part, to the absence of confirmed phosphorescence in any polyene or carotenoid. This chapter critically examines the use of extrapolation procedures, including applications of the energy gap law, in estimating the S1 and T1 energies of long, non-emissive carotenoids. In spite of the inherent limitations of extrapolations, estimates of S1 energies in molecules such as spheroidene and β-carotene now are adequate for understanding the mechanisms of singlet energy transfer in photosynthetic systems. Outstanding issues still to be addressed include: the development of a more quantitative understanding of the effects of conjugation, substitution, isomeric structure, and solvent environment on the energies and dynamics of S1 and S2 states; the extension of the limited data base of T1 energies to longer polyenes and carotenoids; the evaluation of the effects of conformational disorder on carotenoid spectroscopy and photochemistry; and the consideration of additional low-lying singlet states in the energy level diagram.