Research On Second Order Electron-phonon Coupling Of β-carotene Under Environmental And Molecular Phase Transition

In the past ten years,π-conjugated organic molecules have continued to arouse the research interest of researchers due to their potential in display technology,photovoltaic research,and wearable devices.A linear polyene molecule is a molecule with a conjugated carbon-carbon double bond linear structure.It has a large nonlinear coefficient and fast photoelectric response.Therefore,it is one of the typical molecules for studyingπ-conjugated systems.Carotenoids contain many types of isomers,the most common of which isβ-carotene,which is widely distributed in nature and is an ideal candidate molecule for studyingπ-conjugated organic systems.Intermolecular interactions have always been a hot field in physical chemistry.Intermolecular interactions have a variety of forms,such as interactions between species,phase transitions,π-πstacking,etc.Intermolecular interactions have significant effects on the electronic energy gap（absorption spectrum）and vibrational transitions（Raman,infrared spectrum）ofπ-conjugated molecules.The study of intermolecular interactions is of great value for fully exploiting the photoelectric properties ofπ-conjugated molecules.Electron-phonon coupling is one of the important physical quantities in the study of electron transition and molecular vibration.The electron-phonon coupling coefficient can reflect the displacement of the potential energy surface of the carotenoid molecule from the potential energy surface of the ground state,and the effective conjugate length of the carotenoid molecule is one of the key parameters affecting the coefficient.Therefore,studying the electron-phonon coupling when the electron-vibration coupling coefficient changes with the molecular phase of the environment is of great value for studying the photoelectric function of carotenoid molecules.This paper combines the effective conjugate length model,coherent weakly damped electron-lattice vibration model,and amplitude mode theory.Absorption spectrum,resonance Raman spectrum,temperature-variant Raman spectrum,high-pressure Raman spectrum are used as the technical means,negative second derivative fitting method and two-dimensional correlation analysis as the analysis methods.Takingβ-carotene as the research object,the change law of electron-phonon coupling during solution phase transition and molecular aggregation was studied.The effects of solvent polarizability on the electron-phonon coupling coefficients and the change law of the electron-phonon coupling coefficients during temperature-induced solvent phase changes are studied separately.Effect of high pressure on the electron-phonon coupling coefficient ofβ-carotene molecules in the liquid phase and the solid phase.Varying temperature Raman scattering ofβ-carotene aggregates and comparison withβ-carotene molecules in organic solutions.Finally,β-carotene was prepared.By applying an electric field,the effect of increasing the electric field on the electron-phonon coupling ofβ-carotene was calculated,and a feasible scheme for electric field modulation ofβ-carotene was discussed.The main research progress achieved is as follows:（1）Combined with visible light absorption and resonance Raman scattering,β-carotene molecules with different polarizability solvents and solvent phase transitions were studied.Combined with two-dimensional correlation analysis,it is difficult to observe that the Raman intensity at the 1528 cm^-1 Raman frequency changes with the polarizability of the solvent in the one-dimensional spectrum.By comparing different calculation methods,it is considered that Tubino’s calculation method is more suitable for solutions,and the amplitude mode theory is more suitable for solids.The phase transition of the solution will cause an inflection point in the electron-phonon coupling coefficient,and the absorption spectrum will undergo a red-shift-blue-shift and then red-shift characteristic as a whole.The asymmetric change of the half-height width of the double-bond vibration mode evidenced the conclusion that there is a second double-bond vibration mode in the 2D correlation analysis results.（2）A resonance Raman spectroscopy experiment of theβ-carotene film under electric field was performed.By flexibly using electrode materials and preparing theβ-carotene film,about¹0⁶ V/cm was successfully applied.Theβ-carotene film exhibits a blue shift of the double bond vibration mode and a red shift of the single bond vibration mode.By calculating the electron-phonon coupling coefficient and comparing the line width variation of the CC mode,we obtained the modulation effect ofβ-carotene film being"solvated"and that the application of an external electric field can indeed adjust the bond length alternation.It indicates that the direction of the applied electric field,the morphology of the molecule or the lattice structure will affect the regulation of the bond length and the electron-phonon coupling by the electric field.（3）The resonance Raman spectrum of theβ-carotene aggregate solution at 77K-273K was measured,and it was found that the 1157cm^-1（ν₁）and 1522cm^-1（ν₃）modes had little change with temperature.After fitting the peaks of theν₃ mode,it was found that there should be two types of double bond modes to contribute.It is found that as the energy gap decreases,the effective conjugate length decreases,the spectral blue shift component increases,the composition of the H-type aggregate increases,and electron delocalization is hindered.As the temperature decreases,the damping coefficient decreases,so the Raman intensity of the second-order resonance increases significantly,which results in the generation of coherent-weakly damped electronic lattice vibrations.（4）It was found that there are three possible phase transition points ofβ-carotene during the solid-phase pressurization.By calculating the electron-phonon coupling coefficient,evidence of phase transition is obtained,that is,the phase exists near 7Gpa and 10Gpa.The"conjugation length change"that occurs in the low-pressure region is not essentially a phase change.In this pressure range,the damping coefficient rises sharply,andβ-carotene loses the coherent-weakly damped electronic lattice vibration,which causes a sharp drop in the first-and second-order resonance Raman intensity.Studying the effects of environmental and molecular phase transitions onβ-carotene electron-phonon coupling can enrich the spectroscopy results ofπ-conjugated molecules.The results show that some physical parameters of the photoelectric properties ofπ-conjugated molecules can be quickly confirmed using second order electron-phonon coupling.By studying the regulation of electron-phonon coupling under different conditions,it provides a strategy for the application of the scheme to convert the photoelectric properties ofπ-conjugated molecules in practice.The environmental conditions under which coherent weakly damped electron-lattice vibration may occur are discussed,which provides ideas for the study of excited states ofπ-conjugated molecules and provides reference for how carotenoid molecules play a role in photosynthesis.