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 e xcitation into the S2 state and subsequent relaxation to the S1 state, the S1-->S(n) electronic absorption bands in the region 460-520 nm we re probed and observed to decay with single-exponential kinetics: tau = 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 spheroi dene, which appears to be at 14100 cm-1; and (3) in terms of the const raints placed on the efficiencies of carotenoid-to-chlorophyll energy transfer in photosynthetic systems.