Effect Of Complex Formation On The Resonance Raman Spectra Of All-trans-β-carotene In Iodine Solution

Complex has a wide range of applications as a kind of common compounds inthe daily life, industrial production, life science and other fields. Two molecules,atoms or groups with a large charge density difference can form a charge transfercomplex by the charge transfer interaction. The charge transfer complex was used inanalytical chemistry, biological medicine, semiconductor science to learn scientificresearch fields due to charge transfer interactions leading to changes in molecularsolubility, stability, viscosity, molar volume.Linear polyene has a wide application in the field of optoelectronic devices dueto its unique property. Carotenoids exist in nature widely as a special kind of linearpolyene molecules. β-carotene, lycopene and canthaxanthin are the most widelystudied of the three ones of carotenoids. β–carotene is in use with light energycollection, energy transfer and quenching oxygen during photosynthesis, which makesits one of research hot spot in physics, chemistry and biology.All-trans-β-carotene is a linear polyene molecule with9conjugated doublebonds, the RSCS of its CC bonds can reach10-23—10-20cm2mol-1Sr-1at514.5nmexcitation from an Ar ion laser. It makes the charge transfer complex form that β–carotene becomes molecular isomerization and iodine is attached to the conjugateddouble bond. The absorption spectra show that the visible absorption band of thecomplex formed by all-trans-β-carotene and iodine shifts from460nm to1000nm. Inthis paper, we measured the resonance Raman spectra of the all-trans-β-carotenehaving not formed the complex.We use the resonance Raman and visible absorption spectroscopy technology toresearch following work according to the theory of molecular spectrum. The Ramanscattering cross section of beta carotene has been studied in water by Teflon-AFfiber. The resonance Raman spectra of the fundamental, combination, and secondharmonic modes around the C–C and C=C stretches of all-trans-β-carotene in1,2-dichloroethane with iodine solution are obtained in the range of293K to83K. We got the range of Raman scattering cross section of CC bonds of all-trans-β-carotenenot formed in complex, the ratio between the Raman spectral intensities of thecombinations (overtones), its full bandwidth broadens at different temperatures,compared with the one without iodine.The UV-vis absorption and resonance Ramanspectra of all-trans-β-carotene in polar solvent1,2-dichloroethane with iodine weremeasured at293K. We got the range of Raman scattering cross section of CC bondsof all-trans-β-carotene not formed in complex, its full bandwidth broadens andelectron-phonon parameter with different concentrations. We got the followinginnovative achievements.(1) The Raman scattering cross section of β-carotene has been studied in waterby Teflon-AF fiber. It shows that the RSCS of its C=C and C-C bonds can reach10-23—10-20cm2mol-1Sr-1during the reducing of concentration. Its mechanism is asthe concentration of β-carotene is reduced in the water, the Intermolecular Forcesdecrease, which makes its molecular structure order and effective conjugation lengthincrease, the Raman active is sharply higher, the Raman scattering cross section ofβ-carotene increases.(2) The resonance Raman spectra of the fundamental, combination, and secondharmonic modes around the C–C and C=C stretches of all-trans-β-carotene in1,2-dichloroethane with iodine solution are obtained in the range of293K to83K. TheRaman scattering cross section of the fundamentals in the liquid (293K to213K) andsolid phases (188K to83K) generally increases as the temperature decreases, exceptfor the liquid-solid phase transition, which exhibits a decreasing trend; The ratiobetween the Raman spectral intensities of the combinations (overtones) and thefundamentals of CC bonds increases with decreasing temperature, especially in theliquid-solid phase transition. The Raman bandwidths of the CC bonds graduallybecome narrow, but then reach a maximum in the phase transition. The Ramanscattering cross section of the CC bonds in solution with iodine decreases and theRaman bandwidths broaden, compared with the one without iodine. This phenomenonis attributed to the enhanced coherent weakly damped electron-lattice vibrationsassociated with the increase in the structural order. Resonant effects also lead to a high Raman intensity for the CC bonds in fundamental modes. The decreasedstructural order induces the RSCS of the fundamental modes to decrease rapidlyduring the liquid-solid phase transition. However, the strong electron-phononcoupling causes the relative Raman intensity of the combination and harmonic modesto rapidly increase as the temperature decreases. The bandwidths of CC bondsbecome narrow while abnormal in phase transition period. The molecular structuralorder decreases, the RSCS of CC bonds decreases, the bandwidth become broaden asthe complexes are formed in the solution.(3) The Uv-vis absorption and resonance Raman spectra of all-trans-β-carotenein polar solvent1,2-dichloroethane with iodine were measured at293K. The resultsindicated that the absorption peak of all-trans-β-carotene in the complex disappears at460nm and a new peak of the complex formed by all-trans-β-carotene and iodine isfound at1000nm, so that the all-trans-β-carotene within the complex cannot producethe resonance Raman spectrum by514.5nm excitation laser. The Raman scatteringcross section of CC bonds of all-trans-β-carotene not formed in complex decreases, itsfull bandwidth broadens and electron-phonon parameter increases with the increasingconcentration of the complex, because when the concentration of the complexincreases the disorder increases in the solution and the molecular structural orderdecreases.In summary, we analyse the effect of complex formation on the resonance ramanspectra of all-trans-β-carotene in iodine solution. The mechanism of generatingoptical phenomenon and the interactions between intramolecular and intermolecularhave been grasped. It has theoretical value for understanding the effect of complexformation on the structure and optical property of all-trans-β-carotene and theinfluence of electronic-phonon interaction, provides reference information in thestudy of the linear polyene as well.