Research On The Electronic State And Electronic Excitations Of Carotenoids Under The Environment Modulation

Linear conjugated polyene molecule is a kind of organic molecular material with conjugated skeleton structure.Due to its unique electronic and optical properties,it plays an important role in biology,chemistry and materials science.The uniqueπ-conjugated structure of polyene molecules is closely related to the nature and functions of the molecules.Carotene is a type of natural pigment widely existed in nature.It plays a very important role in photosynthetic,energy capture and transportation and disease prevention.It is one of the typical molecules of studying the linear conjugated polyene system.A comprehensive study of the physical properties of carotenoids and the process of the molecular properties of carotenoids modulated by the external environment is helpful to explore the physical mechanism of carotenoid’s important function in nature.In this paper,the Raman activity,resonant Raman mode,electron energy level structure and oscillator strength ofβ-carotene were studied by Raman spectrum and UV-Vis absorption spectrum.The geometric structure ofβ-carotene in different environments was optimized with the quantum chemistry calculation method.On this basis,vibration analysis,excited state calculation and Franck-Condon analysis were carried out,and experimental results and theoretical results were mutually verified.The mechanism of modulating physical properties ofβ-carotene in polarizability environment was summarized,and the contribution of electron transition modes to the UV-Vis absorption spectrum was analyzed.The sources of various transition modes in the UV-Vis absorption spectrum with vibration resolution were discussed,many phenomena that are difficult to distinguish experimentally were obtained by calculation.Meanwhile,the configuration and wave function information in the theoretical calculation results were extracted by wave function analysis method,the difference of electron density and electron transition density matrix of the low excited states were calculated,and the electron-hole analysis method was applied to the linear conjugate system in a pioneering way to study the electron excitation process ofβ-carotene.The main innovation achievements were listed below:1.The mechanism of modulating the molecular structure and vibration behavior ofβ-carotene by polarizability environment was studied by combining Raman spectroscopy and density functional theory method.β-carotene molecule has a centrosymmetric configuration,and the polarization environment has little effect on the molecular geometry.The carbon-carbon single bond stretching vibration,carbon-carbon double bond stretching vibration and carbon-hydrogen bending vibration ofβ-carotene have high IR and Raman activity,but the carbon-hydrogen stretching vibration only have high IR activity,the Raman response is almost invisible.With the decrease of solvent polarizability,the Raman shift of various vibration modes hardly change,but the combined effect of solvent-induced dipole effect and resonance Raman effect results in a significant decrease in Raman intensity.At the same time,with the decrease of solvent polarizability,the solvent-induced dipole effect weakened,the molecular orbital energy increased,the HOMO-LUMO gap increased,theπ-electron energy gap increased,and the dipole moment and polarizability both weakened.2.The mechanism of modulating the electronic states ofβ-carotene molecules and the electron transition mode ofβ-carotene molecules by the polarization environment were studied by UV-Vis absorption spectra,time-dependent density functional theory calculation and wave function analysis.With the decrease of solvent polarizability,the electron transition density was less affected,and the absorption spectrum intensity had no obvious change,but theπ-electron gap increased gradually,and the position of absorption spectrum showed a trend of blue shift.Under the influence of Franck-Condon effect,the main absorption peak ofβ-carotene is split into three vibration bands,in which the 0-0 peak was derived from the transition between the lowest vibration dynamics and the transition from the lowest vibration dynamics to the first excited state of intermolecular vibration,and the 0-1 peak was derived from the transition from the lowest vibration dynamics to the first excited state of the carbon-carbon single bond stretching vibration,the 0-2 peak was derived from the transition from the lowest vibrational state to the first vibrational excited state of the carbon-carbon double bond stretching vibration.At the same time,the low excited states ofβ-carotene have different excitation processes,S₀→S₁ and S₀→S₃ belong to local excitation,the electron excitation is concentrated in the center of the conjugated long-chain,S₀→S₂ and S₀→S₄ had both local excitation and center-symmetric charge transfer excitation characteristics.In the excitation process of S₀→S₂ and S₀→S₄,the charge tends to transfer from theβ-violonone ring ends to the conjugated chain.3.The molecular structure,electronic and optical properties ofβ-carotene cis-isomers were studied by time-dependent density functional theory.The breakage of symmetry on the vibration and electronic state of beta-carotene was discussed.The non-methylated cis-isomers were unstable due to spatial interactions at the bond pits,and the carbon-carbon bond lengths varied at the isomerized sites,while the methylated cis-isomers overlapped well.The cis-isomerization ofβ-carotene had little effect on the vibration frequency on the carbon skeleton,and the position of vibrational energy level is relatively stable,but the Raman activity changed significantly,especially for the stretching vibration of carbon-carbon single bonds and carbon-carbon double bonds.At the same time,the isomerization ofβ-carotene had little effect on the position of electron energy level,the absorption peak of UV-vis absorption spectrum showed a slightly blue shift trend,but the effect on the oscillator intensity was very significant,there was a transition between"bright"excited state and"dark"excited state.In summary,the electron and optical properties of linear conjugated molecules represented byβ-carotene were studied in this paper by molecular spectroscopy combined with quantum chemical calculation.The mechanism ofβ-carotene molecular structure,vibration and electronic state modulated by polarization environment was discussed.The electron excitation properties ofβ-carotene were studied by using electron-hole analysis method,which promoted the understanding of the unique architecture and physical properties of carotenoid,and provided data references and new ideas for the study of linear conjugate system.