Effect Of External Field (Environment) On Electron-phonon Coupling Of Carotenes

π-Conjugated linear polymers are highly interesting subjects due to theiroptoelectronic applications, such as in light-emitting diodes, solar cells, andfield-effect transistors. Carotenoids have multiple roles in the photosynthetic process.They harvest incoming photons at wavelengths that chlorophyll molecules do notabsorb and are capable of accepting the energy of chlorophyll triplet states, which caninduce the formation of harmful singlet oxygen species. Its electro-optical propertiesare closely related to the structure ordered, that is π-electron delocalization extent.The researches on electron-phonon coupling of all-trans-β-carotene in resonantRaman scattering contain the important regularity between the electron and atomvibration. At the same time, it has theoretical value and bright application prospect forenriching molecule spectroscopy content and developing the high-quality polyenephotoelectric material. We need have deeper research on electron-phonon coupling,we need develop some practical application on it.We use the resonance Raman and visible absorption spectroscopy technology toresearch following work according to the theory of quantum mechanics and molecularspectrum. We got resonance Raman spectra of the all-trans-β-carotene dissolved in1,2-dichloroethane in the83K to323K temperature range. The fundamental,combination and harmonic modes and width of CC bonds vary with temperature. Wemeasure the Raman and absorption spectra of all-trans-β-carotene in dimethylsulfoxide （DMSO） at the temperature range from25to81oCand analysis the effectof temperature on electron-phonon coupling. The absorption and resonance Ramanspectra of all-trans-β-carotene in nonpolar cyclohexane and polar1,2-dicholoroethaneat the temperature range from20to60oChave been measured to analysis the effectof solvent’s polar on electron-phonon coupling. The effect of pressure onelectron-phonon coupling of all-trans-β-carotene in CS2has been discussed. We gotthe following innovative achievements. 1. We got resonance Raman spectra of the all-trans-β-carotene dissolved in1,2-dichloroethane through varying temperature and obtain the temperature effect onfundamental, combination and harmonic modes and width of CC bonds. The innovateresearch describes that during the solid-liquid phase transition, the Raman ScatteringCross Section（RSCS） of the fundamental CC bonds rapidly decreases but the rate ofrelative intensities increases sharply as the temperature decreases. The observed highRaman activity in segment s can be attributed to the greater electron-phonon couplingthat can rapidly arise in all-trans-β-carotene.The temperature dependence （25-81oC） of the visible absorption and Ramanspectra of all-trans-β-carotene extremely diluted in dimethyl sulfoxide （DMSO） isinvestigated to clarify the effects of temperature on the absorption wavelength,characteristic energy, Huang-Rhys （HR） factor and electron-phonon coupling. Aprominent feature of the temperature evolution of all-trans-β-carotene is theacceleration of molecular movement, which results in a decrease in entropy andcauses the solution to become unstable. Weakening of the coherent weakly dampedCC stretching vibrations of all-trans-β-carotene caused by a decrease in thestructurally ordered properties of the molecules leads to decreases in the RSCS. Wemeasure the absorption spectra of all-trans-β-carotene at different temperatures. Aprominent feature of the all-trans-β-carotene absorption spectrum is the blueshift withincreases in temperature. We interpret the phenomenon with Lorentz-Lorenz function.Therefore, with increasing temperature, the blueshift of the all-trans-β-caroteneabsorption spectrum in the visible region could be interpreted as arising fromconcomitant decreases in the liquid density and refractive index by theLorentz-Lorenz function. The decrease in effective conjugation length （ECL） withincreasing temperature can lead to the blueshift of absorption spectra. Inaddition, we also calculated an important parameter: Huang-Rhys （HR） factor, itdescribes the degree of electron-phonon coupling. The HR factor trend in theabsorption indicates that excitons have strengthened coupling with the primaryphonon with energy of E_p when temperature is increased. The full width at half maximum （FWHM） of the Raman peak increases monotonically with increasingtemperature. We calculate electron–phonon coupling parameter combine the Ramanand absorption spectra. With increasing temperature, the degree of effectiveconjugation length, modifying the lattice configuration, electron–phonon couplingdecreases.（2） We measure the absorption and resonance Raman spectra ofall-trans-β-carotene in nonpolar cyclohexane and polar1,2-dicholoroethane at thetemperature range from20to60oC. We get the effect of solvent polar on structureordered, πelectron delocalization, HR factor and electron–phonon coupling ofall-trans-β-carotene. The few species of interaction force between the molecules makethe structure ordered better in a non-polar solvent cyclohexane. The ECL increases,πelectron delocalization extended and coherent weakly electron-lattice vibrationenhanced lead to the high Raman activity. The electron-phonon coupling constant ofall-trans-β-carotene in non-polar solvent cyclohexane is smaller than that in polarsolvent1,2-dicholoroethane. However, the electrostatic induction ofall-trans-β-carotene in decreases the structural ordered, shorten the ECL, diminishedπelectron delocalization. All-trans-β-carotene has low Raman activity in polar solvent1,2-dicholoroethane.（3） Visible absorption and Raman spectra of all-trans-β-carotene are measured inCS2at pressure range from0.04-0.6Gpa. The results indicated that the visibleabsorption spectra are red-shifted, FWHM increases, the RSCS decreases,Huang-Rhys factor increases, the electron-phonon coupling constants of CC bondvibration modes increases. The reasons for these phenomena are the molecularstructural order of β-carotene decreased and π electron energy gap diminished withthe pressure increased. The visible absorption spectra are red-shifted. Otherwise, thedensity and refractive index increase with the increasing pressure. It makescontribution on the red shift. The FWHM of the0-0peak increases monotonicallywith increasing pressure. The intermolecular electrostatic interactions enhanced withthe increasing pressure. The broader bandwidth of this band in S1relative to its width in S₀is ascribed to the shorter vibrational relaxation time in the S1state. Thevibrational relaxation is due to the intermolecular electrostatic interactions betweenthe transition dipoles of the solute molecules. Weakening of the coherent weaklydamped CC stretching vibrations of all-trans-β-carotene caused by a decrease in thestructurally ordered properties of the molecules leads to decreases in the Ramanscattering cross section. We combine the absorption spectra with the Raman spectra tocalculate the electron-phonon coupling constants. The electron-phonon couplingconstant of CC bond vibration modes which is proposed by R. Tubino increases withincreasing pressure.In summary, we analyse the effect of external field such as temperature, solventpolarity, pressure on electron-phonon coupling parameter. The mechanism ofgenerating optical phenomenon and the interactions between intramolecular andintermolecular have been grasped. It has theoretical value and bright applicationprospect for enriching molecule spectroscopy content and developing the high-qualitypolyene photoelectric material.