Internal-conversion of carotenoids with 9, 10 and 11 conjugated double bonds upon excitation to differenct vibronic 1B_u⁺(v = 0, 1, 2) excited states as revealed by subpicosecond time-resolved absorption spectroscopy

Abstract

Carotenoids (Cars) are group of pigments complementary to bacteriochlorophyll (BChl), which play important functions of light-harvesting, which include capturing light energy by Car followed by transferring its singlet energy to bacteriochloropyll. In this paper, subpicosecond time resolved absorption spectra of carotenoids having 9-11 conjugated double bonds were recorded in the visible region, upon excitation at different vibronic excited states, 1B_u⁺(v = 0,1,2). The spectral data matrices were analyzed by singular-value decomposition (SVD) followed by global fitting by the use of a sequential model. SVD was used to extract all significant components and the decay time constants determined by global fitting. The lifetimes (τ) of the 1B_u⁺, 1B_u^- and 2A_g^- states shortened with the number of conjugated double bonds (n) among neurosporene (9), spheroidene (10) and lycopene (11). The linear dependence of lnτ as a function of 1/(2n+1) was explained in term of the Englman-Jortner energy-gap law.