Steady-State And Ultrafast Spectroscopy Of β-Carotene Under High Pressure

Carotenoids are one of the most abundant pigments found in nature. They are present in most organisms including humans. Carotenoids play an important role in photosynthesis, protection against various diseases in humans, etc. The energy levels and dynamics of carotenoid excited states have been extensively investigated using steady-state and ultrafast time-resolved spectroscopy as well as theoretical analysis. These investigations have provided valuable information for elucidating the biological functions of carotenoids. However, lots of problems are still needed to be resolved, such as whether some intermediate electronic or vibrational states are really existent, how about their relaxation dynamics and environment dependence, and so on. Therefore, the physical mechanisms of the biological functions of carotenoids in nature have not been well and truly understood.The combination of high-pressure conditions and ordinary spectroscopic technique was proposed, in this dissertation, to clarify the aforementioned problems. Some intermolecular interactions can be amplified when being pressured, so the investigation on the steady-state spectroscopies under high pressure can reveal the enverimental effects on the electronic and vibrational levels. Some transient species can be correctely assigned under high pressure because of their different behavior when being pressed, so ultrafast spectroscopies under high pressure can be expected to clarify some puzzling problems in the ultrafast dynamics of carotenoids. For these reasons, the steady-state and femtosecond time-resolved transient absorption spectra ofβ-carotene, the most extensively investigated carotenoid at ambient condition, were measured under high pressure and theoretical analysis were also performed. This work illustrated the internal and external factors that affect the steady-state and ultrafast spectroscopies of carotenoids and proposed a new energy relaxation pathway, and therefore provided some novel insights for elucidating the biological functions of carotenoids in nature.In order to understand the results under high pressure, the solvent effect on the absorption spectra ofβ-carotene at ambient condition were firstly investigated. The absorption spectra in 32 solvents were measured and the time-domain formula was used to analyze the absorption spectra. The 0-0 band wavenumber and bandwidth depend mainly on polarizability and slightly on polarity of solvent, and polarity of solvents contributes much more to the bandwidth than to the wavenumber of 0-0 band. Besides the polarizability and polarity, other microcosmic factors, such as the size and movement actions of the solvent molecules, can also affect the absorption spectra. It is essential to take the microscopic characteristics of the solvent molecules into account in the investigation of the environment effect on carotenoids.Steady-state absorption and Raman spectra ofβ-carotene in hexane and CS2 solventions were investigated under high pressure. Both the red shift and broadening of the absorption spectra are stronger in CS2 than that in n-hexane because of the more sensitive pressure dependence of dispersive interactions in CS2. This was ascribed to the large polarizability and small size of CS2 molecule. The opposite pressure dependent behavior of the S0→S2 transition moment in these two solventions was explained with the effective solvent molecules model, which confirmed that the light-harvesting function of carotenoids can be influenced by the microcosmic factors of the solvent molecules, such as relative dimension, location and orientations. The diverse pressure dependences of several representatives Raman bands were explained using a competitive mechanism involving bond length changes and vibronic coupling. This model shows that the in-phase C=C stretching mode plays an essential role in the internal conversion from S1 to S0 states in carotenoids. It can also be concluded from the Raman spectra thatβ-carotene molecules have undergone a small structural torsion under high pressure. This conclusion offers us valuable information for analyzing the transient absorption spectra ofβ-carotene under high pressure.Density functional theory as implemented in the Gaussian 03 program package was used to investigate the effect ofβ-rings rotation on the potential energy surface and vibrational spectroscopic characteristics ofβ-carotene. It can be found from the calculation that C6-C7 bond ofβ-carotene molecule is easily to be twisted; two stable isomers (cis and trans) having Ci symmetry can be obtained by this rotation; the energy barrier for trans→cis isomerization is quite low. Although the small structural torsion ofβ-carotene molecule under high pressure can not result in the isomerization, it can make the potential energy surface more flat.The high-pressure transient absorption spectra ofβ-carotene in hexane solvention were finally investigated based upon the above experimental and theoretical analysis. We developed a new high-pressure cell that was suitable for the transient spectroscopy, built the femtosecond time-resolved transient absorption spectroscopic system under high pressure, and measured the high-pressure transient absorption spectra ofβ-carotene using this system. The time-resolved spectral data were analyzed by singular value decomposition followed by global fitting. Comparing the pressure dependences of the energy levels and the kinetics behavior of different spectral components, it can be concluded that cis→trans isomerization takes place at S1 state. The rate constant of the radiationless S1→S0 internal conversion process is affected by both the energy gap between S1 and S0 states and the viscosity of solvent. This conclusion can offer important insights into the efficient energy-transfer functions of carotenoids in natural pigment-protein complexes.This work realized the combination of high-pressure conditions and femtosecond time-resolved transient spectroscopic technique. The high-pressure transient absorption spectra ofβ-carotene in the whole white light region were measured for the first time. This is a new technique to investigate ultrafast processes, and therefore opens a fire-new approach to deeply explore physical and chemical issues.