The dynamics of the S₁ excited states of carotenoids

Transient absorption spectroscopy was used to measure the S₁ dynamics of four all-trans-carotenoids: 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene and spheroidene. After excitation into the S₂ state and subsequent relaxation to the S₁ state, the S₁ → S_n electronic absorption bands in the region 460-520 nm were probed and observed to decay with single-exponential kinetics: τ = 407 ± 23 ps for 3,4,7,8-tetrahydrospheroidene, 85 ± 5 ps for 3,4,5,6-tetrahydrospheroidene, 25.4 ± 0.9 ps for 3,4-dihydrospheroidene and 8.7 ± 0.1 ps for spheroidene. These data are discussed in three contexts: (1) as a test of the adherence of carotenoids to the energy gap law; (2) as a means of determining the S₁ state energy of spheroidene, which appears to be at 14100 cm^-1; and (3) in terms of the constraints placed on the efficiencies of carotenoid-to-chlorophyll energy transfer in photosynthetic systems.