The Effect Of Solution Phase-transition On Resonance Raman Spectra Of β-carotene

Carotenoids are one of important pigments found in nature and play the role ofauxiliary in photosynthesis. Carotenoids widely distribute in organs of animals andplants including human. Carotenoids contained π-electron conjugated doubled bonds(CDB) have many unique biochemical properties and biological activities. Materialproperties and functions are closely related to molecular structure. Molecularspectrum is one of powerful methods of studying molecular structure. In ourprevious work, our researchers had completed the researches about the effect ofconcentration, solvent and pressure on carotenoids molecular structure andproperties using Raman spectrum, UV-visible absorption spectrum and infraredspectrum, and gained many research findings. Recently, we have carried out theresearch on the temperature dependence of molecular structure and properties of β-carotene, and found Raman spectrum and UV-visible absorption spectrum of theliquid β-carotene, their intensity, line shape and frequency are very sensitive to thetemperature. Based on this background, this paper enlarges the temperature range,making our research object are not only the liquidβ-carotene, but also solid evenliquid-solid phase transition.In this paper, we have measured the resonance Raman spectra ofβ-carotene in1,2-dichlorothane in the temperature range of323K-83K. β-carotene sample in thisrange experiences the liquid, the liquid-solid phase change, solid three state.323K-233K is the sample of the liquid temperature range,203K-83K is the sample ofthe solid temperature range, in233K-203K sample changes from liquid to solid. Theresults are as follow:(1) The Raman scattering cross sections (RSCSs) of the CC fundamentalmodes of β-carotene are enhanced with decreasing temperatures in the liquid andsolid temperature range. During the liquid-solid phase transition period, RSCSs of CC fundamental modes rapidly decrease. Two factors, namely, Resonance RamanEffect and the coherent weakly damped electron-lattice vibrations are responsible forresonance RSCSs of CC fundamentals. As the temperature decreases, samplesolution density increases, intermolecular distance get shorter, and the room forall-trans-β-carotene molecular distortion and swing reduces, thus weakeningmolecular thermal disorder, improving molecular structural order, enhancingresonance Raman Effect and coherent weakly damped electron-lattice vibrations.These two enhanced factors lead to the increase of RSCSs of CC fundamental bands.During the period of liquid-solid phase transition, β-carotene molecular structurerecombination, which causes molecular structural order reduce, coherent weeklydamped electron-lattice vibration and resonance Raman effect weaken, thus RSCRsdecrease rapidly at here.(2) Over the whole temperature range, relative intensity of the CC overtoneand combination modes of β-carotene are enhanced with decreasing temperatures,but during the liquid-solid phase transition period, relative intensity of the CCovertone and combination modes enhance more rapidly. Compared to fundamentalbands, the influence of resonance Raman Effect on combination and overtone is veryweak. The main factors affecting combination and overtone are the coherent weaklydamped electron-lattice vibration and the strength of electron-phonon coupling. Astemperature decreases, the improvement of molecular structural order enhances thecoherent weakly damped electron-lattice vibration, leading to the increase ofcombinations and overtones intensities. During the phase transition period,molecular structural order and effective conjugation length decreases sharply. Thestrength of electron-phonon coupling is inversely proportional to the moleculareffective conjugation length. So, the sharply enhanced strength of electron-phononcoupling causes the intensities of combinations and overtones still increase rapidly inphase transition range, although the coherent weakly damped electron-latticevibration become weaker.(3) The Raman bandwidth of C=C band became narrow gradually with thedecreasing temperatures, but appeared abnormal in the phase transition period for Raman bandwidth broadened here and reached its maximum at213K. Lowering thetemperature improves molecular structural order, increases the extent of π-electrondelocalization, and reduces vibratory frequency difference, which is the reason fortemperature-induced decrease of bandwidth of β-carotene. On the contrary, duringthe phase transition period, the lowering of molecular structural order causes theincreases of the band length difference and vibratory frequency difference, thuswidening bandwidth of β-carotene.