β-胡萝卜素分子的黄琨因子的温度特性

Temperature Characteristics of Huang-Ryes Factor of All-Trans-β-Carotene

β-胡萝卜素具有光采集、光防护功能,又是重要的光电材料,它在外场下的分子结构和性能变化既有理论意义也有应用价值。测量了β-胡萝卜素在环己醇中68～26℃温度范围内的紫外-可见吸收、拉曼光谱。实验结果表明随着温度的降低,黄琨因子和碳碳键每个振动模的电子-声子耦合常数减小,紫外-可见吸收光谱红移,碳碳键拉曼散射截面增加。用线性链状多烯分子的"相干弱阻尼电子-晶格振动模型"、"有效共轭长度模型"等理论给予了解释。随着温度的降低,β-胡萝卜素分子的热无序减小,分子结构有序性增加,π电子离域扩展,有效共轭长度增加,导致紫外-可见吸收光谱红移和强的拉曼活性。相干弱阻尼电子-晶格振动增强,使碳碳键拉曼散射截面增加。引用带有量纲的电子-声子相互作用常数,既可以与黄昆因子建立关系式,计算出碳碳键每个振动模的数值,也可以表征分子的有效共轭长度,π电子离域程度及拉曼散射截面的大小等。

A Visible absorption and Raman spectra of all-trans-β-carotene was measured in cyclohexanol solution in the temperature range from 68℃ to 26℃.The results indicated that the visible absorption spectra are red-shifted,Raman scattering cross section increases,Huang-Ryes factor and electron-phonon coupling constants of CC bond vibration modes decreases with the temperature decreasing.The changes are interpreted using the theory of＂coherent weakly damped electronic-lattice vibration model＂and＂effective conjugation length model＂.The red shift of the absorption spectra and intensity of the Raman active are attributed to the thermal conformational change-induced increase in the effective conjugation length in all-trans-β-carotene chains.All-trans-β-carotene has strong coherent weakly damped CC bonds vibrational properties,which lead to large Raman scattering cross section in the solvent of low temperature.The electron-phonon coupling constants with dimension are used,which can easily establish relation with the Huang-Rhys factor and calculate the electron-phonon coupling constants of CC bond vibration modes.Effective conjugation length,theπ-electron delocalization range and the Raman scattering cross section are described by the electron-phonon coupling constants.