Characterization of the Ultraviolet-B Absorption Band of Carotenoids Using Solvent-dependent Shifts in Steady-State and Transient Absorption Spectra.

The versatile functions of carotenoids in biological systems are associated with the extended π-electron conjugation system. Strong visible absorption resulting from the optically allowed S 2 (1B u + ) state and the low-lying optically forbidden S 1 (2A g - ) state examined. Carotenoids also exhibit an absorption band in the ultraviolet-B region; however, the origin of this band (hereafter referred to as S uv state) is not well characterized. The S uv state is a candidate for the destination level of the well-known S 1 → S n transient absorption; however, an obvious energy mismatch has been observed. In this study, we examined the steady-state and picosecond transient absorption spectra of lycopene in various solvents. The S uv absorption of carotenoids with diverse conjugation lengths was also examined. The dependence of the energies on solvent polarizability and conjugation length revealed that both S uv and S n are the "second" B u + state. The absorption spectrum for lycopene at 200 K revealed an additional vibrational band, which may be the vibrational origin of the S 0 → S uv band. Considering the slow vibrational relaxation of the 2A g - state, the S 1 → S n transition may represent the 2A g - ( v = 1) → 2B u + ( v = 0) transition, and the energetic contradiction can be resolved.