实验测得核黄素在水、 二甲基亚砜(DMSO)和三氯甲烷三种不同极性溶剂中的稳态吸收光谱、 荧光光谱和时间分辨荧光光谱, 研究了溶剂对核黄素光谱性质的影响。 实验结果表明, 在不同极性的溶剂中, 核黄素的吸收峰位置几乎不变, 而荧光光谱峰值随着溶剂极性的增大而出现红移。 这是由于溶质分子的电子激发及溶剂化效应引起的电子重新分布导致它在极性溶剂中第一激发单重态能级的变化。 在时间分辨荧光光谱实验中, 核黄素在水溶液中荧光寿命也高于在其他两种溶剂中, 荧光寿命的延长可归因于核黄素与氢键对体溶剂之间的分子间氢键相互作用。 应用Gaussian09软件, 采用密度泛函理论和含时密度泛函理论, 结合基于密度的溶剂化模型, 对不同极性溶剂中的核黄素分子进行基态和激发态优化和计算。 通过前线分子轨道分析, 核黄素的受激跃迁属于苯环和含氮杂环上的π电子向苯环及CN, CO共轭双键的反键轨道π*的跃迁。 分子偶极矩的计算结果表明, 核黄素分子的第一激发态偶极矩大于基态偶极矩, 偶极矩的增大, 导致溶质与溶剂分子之间的相互作用的增大。 而溶剂分子与溶质分子基态和激发态的相互作用程度不同, 使得吸收峰和荧光峰出现不同变化情况。 极性越大的溶剂越有利于对激发态的稳定作用, 使激发态能量降低, 相应的发射波长发生红移。 最后, 通过分子表面静电势和弱相互作用分析, 在水溶剂中考虑氢键作用对核黄素分子光谱的影响。 多聚体结构的理论吸收和发射峰值更接近实验结果, 说明多聚体结构合理。 水分子二聚体与核黄素形成的环状结构, 有利于提高核黄素分子的刚性, 有利于荧光的发射, 减少非辐射跃迁的几率, 荧光寿命延长。

The steady-state absorption and fluorescence spectra and time-resolved fluorescence spectra of riboflavin in water, DMSO and chloroform were measured by British Edinburgh FLS920P spectrometer. It was found that the absorption peaks hardly change in different solvents, and the fluorescence peaks show a significant red shift with the increase of the polarity of solvents, because the charge distribution of the excited state leads to the difference between the absorption and fluorescence spectra. Besides, the fluorescence lifetime of riboflavin in water is also longer than that in the other two solvents, which can be attributed to the hydrogen bond interaction between riboflavin and water. Furthermore, the ground and excited states of riboflavin molecules in different solvents were calculated by using Gaussian 09 software. Through frontier molecular orbital analysis, the stimulated transition of riboflavin is the transition of π electrons on benzene ring and nitrogen heterocycle ring to the antibonding orbital π* of benzene ring and CN, CO conjugated double bond. The results show that the charge distribution of riboflavin increases the dipole moment and the solute-solvent intermolecular interaction, which leads to spectral variation and a red shift in fluorescence spectra. Finally, the effect of H-bond on the spectra of riboflavin was considered by the molecular electrostatic potential and noncovalent interaction analysis. The theoretical absorption and emission peaks of the polymer structure are closer to the experimental results, which shows that the results of the polymer are reasonable. The ring structure formed by water dimer and riboflavin can increase the rigidity of riboflavin molecule, which is beneficial to the fluorescence emission, thus decreasing the probability of non-radiative transition and prolonging the fluorescence lifetime.