Effect Of Pressure On Electron-phonon Coupling Constants Ofβ-Carotene

The chain of conjugated(straight) polyene molecules have the properties of bignonlinear coefficient and fast optoelectronic response, these optic properties will bechanged and improved in the doping external field. Carotenoids are conjugatedpolyenes biomolecules, which have important applications in the development of thephotovoltaic device and functional materials. The Raman spectroscopy can provide awealth of information about the molecular structure and the high pressure filed canenlarge the interactions of intermolecular or intramolecular. The electron-phononcoupling constant can be characterized by the Raman spectral intensity, π electrondelocalization degree, scattering cross section, the effective conjugation length andwidth size. So it is necessary for studying the electron-phonon coupling of the β-carotene to use the Raman spectroscopy and UV absorption spectroscopy. At thesame time, it provides the theoretical basis for studying the typical π-electronconjugated system of the molecular structure.In this paper, we used the diamond anvil cell technology to measure up to0.60GPa in-situ high pressure Raman spectra of β-carotene which dissolves in CS2and water respectively and compared the both of them the Raman shift and CC bondof the full width at half maximum (FWHM) of the Raman spectra. We also calculatethe electron-phonon coupling constant and Huang-Rhys factor of β-carotenedissolved in CS2solution. So far we have made some innovative results.1、We get the results by studying the Raman spectroscopy of β-carotenedissolved in CS2solution and water under high pressure that both of the samples’Raman shift moved to the high wave number and the full width at half maximumincreased depending of the pressure. The experiment phenomena were interpreted bythe theory of the conjugated polyene molecules “coherent weakly dampedelectronic-lattice vibration model” and the “effect conjugation length model”. The mechanism is that the β-carotene is compressed and has the lower structure order,shorte the effective conjugation length, decreased Raman active, weaker the coherentweakly damping CC bond vibration in high pressure. Because of the CC bond lengthbecome short, so the Raman spectra are found to blueshift. The CC bond of the fullwidth at half maximum (FWHM) of the Raman spectra increased is attributed to theincrease of difference in C-C and C=C bond lengths. Moreover, due to dissolving innon-polar CS2solvent, the β-carotene encounters the interaction of the surroundingsolvent molecules. So the dispersion force interaction between solute and solvent ismore sensitive to pressure. Then it makes that the slop of Raman shift and the fullwidth at half maximum in the CS2solution are faster than dissolved in water withincreasing pressure.2、By studying the effect of pressure on the electron-phonon couping constant ofβ-carotene molecule dissolved in CS2solution, the results show that the visibleabsorption spectra of β-carotene in nonpolar solven carbon disulfide are red-shiftedwith pressure increased, but the frequency shifts towards higher frequencies in theRaman spectra, the Raman scattering cross section decreases, Huang-Rhys factorincreases, the electron-phonon coupling constants of CC bond vibration modesincreases. The mechanism is that all-trans-β-carotene caused by compressed and adecrease in the structurally ordered properties of the molecules leads to narrow downenergy gap of the π, shorten effective conjugation length, hinder delocalization ofπ-electron, decrease in the Raman scattering cross section, increase the Huang-Rhysfactor and the electron-phonon coupling constants.