| Linear polyene molecule is a chain structural molecule with carbon-carbon(CC) double bond. Since it has so many excellent properties such as high nonlinear coefficient and rapid optoelectronic response, linear polyene molecule has significant applications on high-quality semiconductors, molecular wire, high-speed optical switch and dry battery. β-carotene is an ideal molecular to study polyene’s π-conjugated property because of its short straight chain structure. Meanwhile, electron-photon coupling is a key interaction in electron transition and Raman scattering, the coupling coefficient of which is able to characterize many properties of polyene molecule such as ECL(Effective conjugation length), π-electron delocalization, Raman scattering intensity, Raman shifts and its linewidth. It’s known to all that physics process, chemical process and biological process are easy to be influenced by temperature. Therefore, it is valuable for further study on optoelectronic properties of linear polyene by doing research and making progress on β-carotene’s electron-phonon coupling properties at different temperatures.In this paper, we use the ECL, weakly damped coherent electron-lattice vibration and amplitude models, and use the absorption and resonance Raman spectroscopy technology to calculate the Huang-Rhys(HR) factor and electronic-phonon coupling coefficient in liquid, solid and solid-liquid phase-transition. We respectively investigated: the HR factor and dimensional electron-phonon coupling coefficient of β-carotene at different temperatures(68-26℃) and different solvents( polar and non-polar); The spectrum’s temperature features of β-carotene solutions in polar and nonpolar solvents, and in liquid, solid and phase-transition environments; We also measured the first and second order non-dimensional electron-phonon coupling coefficients and discussed their effects on spectral intensity, width, frequency shift, line shape, etc. Additionally, further study of the effects of phase transition on β-carotene under electron-phonon coupling by measuring the UV absorption and resonance Raman spectra of β-carotene solution. Innovative achievements are as follows:1. External temperature and solvent effects have significant influence on structural order of carotenoids. Molecular structure ordered better when π-electron delocalized and external conditions do influence the π-electron delocalization, HR factor and electron-phonon coupling coefficient. We measured the UV absorption and resonance Raman spectra at a temperature range from 68-26 ℃. As the temperature decreases, molecular thermal disorder is reduced, which makes the structural order of molecules increased, effective conjugation length increases, so that the UV absorption spectra redshift. With the weak damping coherent electron-lattice vibrations enhancing, the Raman scattering cross section increases, while HR factor and electron-phonon coupling coefficient of C-C and C=C decreases. Moreover, different optical properties were showed in polar and nonpolar solvents of different structural order. In non-polar solvents, the intermolecular interaction forces are weak, the structural ordering of the molecules is increased, and the effective conjugation length of the molecule becomes large, the π-electron delocalization expansion enhanced so that the electron-phonon coupling reduced, the coherence weakly damped electron-lattice vibrations are enhanced. Hence the strong Raman activity is produced. When molecules are in polar solvents, electrostatic interactions between the solute and solvent degraded the structural order of β-carotene molecule, the effective conjugation length is shortened, and π-electron delocalization is hindered, electron- phonon coupling increases. Therefore, the Raman activity is deteriorated.2. The UV absorption and resonant Raman spectra of β-carotene were measured under phase-transition condition dissolved in polar solvents and non-polar solvents, and the absorption spectra of the samples in liquid, solid and phase-transition were compared, and so were the change of fundamental, combination and harmonic modes in the Raman spectra of the samples with temperature. From the measured data, when β-carotene molecule was in the non-polar solvent 1, 2-dichloroethane between liquid and solid phase, with the increase of molecule structural ordered, π-electron delocalization extended and coherent weakly electron-lattice vibration enhanced, leading to the resonant effect weakened, and Raman cross section reduced. When the β-carotene molecule is in the polar solvent cyclohexanol, with the decrease of temperature to the phase-transition point, the solution is rapidly transferred from the liquid to the solid state. Because the characteristic value of energy to change the conformation ε of the polyene molecule is different, which means the value is bigger in solid phase than in liquid phase, leading to the result that effective conjugation length of molecule increase in solid phase, then the variation of the π-electron delocalization of molecule increase in solid phase and the moderation on CC bond is strengthened. Thus, with temperature decreasing, the variation of parameters related to electronic band gap including electron-vibration coupling coefficient and Raman cross section is larger in solid phase than in liquid phase.3. The influence of electron-phonon coupling effect on molecular spectra(including UV, fluorescence, IR, and Raman) is related to the structure of alkene and Peierls band gap, which control all the spectrum properties like intensity, full width at half maximum(FWHM), frequency shift, and linear. The value of first-order electron-phonon coupling coefficient effect the variation of fundamental mode, and the second-order electron-phonon coupling coefficient influence combination and harmonic modes variation. By measuring the UV absorption and resonant Raman spectra of β-carotene dissolved in cyclohexanol solution in the 345-295 K temperature range, the first- and second-order electron-photon coupling coefficients can be calculated, and the variation of temperature dependent second-order electron-photon coupling coefficient is larger than that of first-order. The Raman spectra are related to the moderation of π electronic energy gap on CC bond vibration. Therefore, with decreasing temperature, the variation of the red shift, peak intensity, and FWHM of CC bond harmonic modes is bigger than that of fundamental modes. By measuring the UV absorption spectra and resonant Raman spectra of β-carotene dissolved in cyclohexanol solution in the 65-2℃temperature range, with the influence of electron-photon coupling, the effect of the phase change of solvent on molecule is also studied. The lower the temperature is, after phase change the stronger the electron-phonon coupling action and the moderation of electronic energy gap on CC bond become. Thus, after phase change, the speed of red frequency turn faster, the Raman cross section have a substantial increase, and FWHM narrow quickly.With the rapid development of material, electron and computer science, there is an urgent need for this conjugated electron polyene molecules to develop. Therefore, by comparing the absorption and resonant Raman spectra of β-carotene in liquid, solid and phase-transition, discussing how phase-transition influence the electron-photon coupling, and the variation of molecular spectra under the influence of electron-photon coupling, have the function of enriching the content of molecular spectroscopy and help to provide the corresponding reference for the development of high performance optic electronic materials. |
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