Ultrafast Time-Resolved Spectroscopy of Xanthophylls at Low Temperature

Abstract

Many of the spectroscopic features and photophysical properties of xanthophylls and their role in energy transfer to chlorophyll can be accounted for on the basis of a three-state model. The characteristically strong visible absorption of xanthophylls is associated with a transition from the ground state S0 (1(1)Ag-) to the S2 (1(1)Bu+) excited state. The lowest lying singlet state denoted S1 (2(1)Ag-), is a state into which absorption from the ground state is symmetry forbidden. Ultrafast optical spectroscopic studies and quantum computations have suggested the presence of additional excited singlet states in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+). One of these is denoted S* and has been suggested in previous work to be associated with a twisted molecular conformation of the molecule in the S1 (2(1)Ag-) state. In this work, we present the results of a spectroscopic investigation of three major xanthophylls from higher plants: violaxanthin, lutein, and zeaxanthin. These molecules have systematically increasing extents of pi-electron conjugation from nine to eleven conjugated carbon-carbon double bonds. All-trans isomers of the molecules were purified by high-performance liquid chromatography (HPLC) and studied by steady-state and ultrafast time-resolved optical spectroscopy at 77 K. Analysis of the data using global fitting techniques has revealed the inherent spectral properties and ultrafast dynamics of the excited singlet states of each of the molecules. Five different global fitting models were tested, and it was found that the data are best explained using a kinetic model whereby photoexcitation results in the promotion of the molecule into the S2 (1(1)Bu+) state that subsequently undergoes decay to a vibrationally hot S1 (1(1)Ag-) state and with the exception of violaxanthin also to the S* state. The vibrationally hot S1 (1(1)Ag-) state then cools to a vibrationally relaxed S1 (2(1)Ag-) state in less than a picosecond. It was also found that a portion of the S* population is converted into S1 (2(1)Ag-) during deactivation, but this process and the relative yield of S* was found to depend on temperature, consistent with it being associated with a twisted conformation of the xanthophyll. The results of the global fitting suggest that subpopulations of twisted conformers of xanthophylls already exist in the ground state prior to photoexcitation.

Singlet and triplet excited state properties of natural chlorophylls and bacteriochlorophylls

“…They suggested that the mobility of residual oxygen present in the solvent is greatly reduced upon freezing and leads to enhancement of the triplet lifetime. This explanation sounds very reasonable, however, recently a similar effect was observed for the radiationless decay of the first excited singlet state of carotenoids (Cong et al 2008;Niedzwiedzki et al 2007). For these molecules, the lifetime of the first excited singlet state increases also with lowering of the environmental temperature (up to 50%) but the presence and mobility of oxygen do not play any role.…”

Carotenoid deactivation in an artificial light-harvesting complex via a vibrationally hot ground state

“…zeaxanthin and lutein) relax into two different conformers after being excited to the S 2 state [5,21]. This interpretation was also supported by work where transient signals of long-chain xanthophylls were recorded at 77 K, indicating a temperature dependence of the S * yield [22]. A simplified representation of the model proposed in these studies with the added purpurin energy-levels is shown in Fig.…”

“…S 1 pathway appears slow, up to 120 ps, compared to the S * 8.8 ps life time found in the dyad [24]. Given that polyene chain length and substituents are essential factors for the observations of Cong et al [22], one should be cautious in comparing such a small effects between different classes of linear polyenes; carotenoids and xanthophylls. Thus, the S * ?…”

“…These discrepancies between theory and experiments have prompted other interpretations for carotenoid deactivation. The suggested schemes include ground state heterogeneity [4], an ultrafast isomerization [20,5,21,22], and a vibrationally hot ground state [23,16,3].…”

Spectroscopic Investigation of Carotenoids Involved in Non-Photochemical Fluorescence Quenching

“…At low temperatures, the vibrational resolution increases further, and the 0-1 band is often split to reveal the individual C-C and C = C mode features (Cong et al 2008 ), which allows an estimation of the frequencies of the C-C and C = C stretches in the S 2 state. The precise values of these stretching frequencies in the S 2 state are not known, but low-temperature absorption spectra confi rm that vibrational frequencies do not differ much in the S 0 and S 2 states.…”