Ultrafast Dynamics of Long Homologues of Carotenoid Zeaxanthin

Three zeaxanthin homologues with conjugation lengths N of 15, 19, and 23 denoted as Z15, Z19, and Z23 were studied by femtosecond transient absorption spectroscopy, and the results were compared to those obtained for zeaxanthin (Z11). The energies of S2 decrease from 20 450 cm(-1) (Z11) to 18 280 cm(-1) (Z15), 17?095 cm(-1) (Z19), and 16?560 cm(-1) (Z23). Fitting the N dependence of the S-2 energies allowed the estimation of [Formula: see text], the S-2 energy of a hypothetical infinite zeaxanthin, to be similar to 14?000 cm(-1). Exciting the 0-0 band of the S-2 state produces characteristic S-1-S-n spectral profiles in transient absorption spectra with maxima at 556 nm (Z11), 630 nm (Z15), 690 nm (Z19), and 740 nm (Z23). The red shift of the S-1-S-n transition with increasing conjugation length is caused by a decrease in the S-1 state energy, resulting in S-1 lifetimes of 9 ps (Z11), 0.9 ps (Z15), 0.35 ps (Z19), and 0.19 ps (Z23). Essentially the same lifetimes were obtained after excess energy excitation at 400 nm, but S-1-S-n becomes broader, indicating a larger conformation disorder in the S-1 state after 400 nm excitation compared to excitation into the 0-0 band of the S-2 state. An S* signal was observed in all samples, but only for Z15, Z19, and Z23 does the S* signal decay with a lifetime different from that of the S-1 state. The S* lifetimes are 2.9 and 1.6 ps for Z15 and Z19, respectively. In Z23 the S* signal needs two decay components yielding lifetimes of 0.24 and 2.3 ps. The S* signal is more pronounced after 400 nm excitatio