邻位甲基红水溶液的光谱性质随pH值的变化：含时密度泛函理论计算与实验研究

探明影响甲基红光谱性质的各种因素，有助于拓宽偶氮苯衍生物在有机光电器件中的应用。采用密度泛函理论和实验相结合的方法研究了溶液酸碱性和溶剂水对邻位甲基红水溶液光谱的影响。溶液pH从13.1逐渐降低至0.5，邻位甲基红水溶液的最大吸收波长从430 nm红移至520 nm。在不同酸碱条件下，主要有三种物种共存于甲基红水溶液中，它们分别是双质子化的甲基红o-H₂MR⁺ (强酸性条件下)，单质子化的甲基红o-HMR (弱酸条件下)和碱性甲基红o-MR^- (碱性条件下)，通过密度泛函理论计算研究了三种不同形式的电子结构特征。采用含时密度泛函理论计算了甲基红偶极跃迁允许的最低激发能，分别采用连续介质模型和分子簇模型研究水溶剂对甲基红电子结构和光谱性质的影响。在酸性条件下，o-H₂MR⁺和o-HMR分子内氢键导致π共轭体系平面性增强，因而光谱红移。而在碱性条件下，溶剂对o-MR^-的光谱有显著影响：极性o-HMR和o-MR^-与水分子的偶极-偶极相互作用导致光谱进一步红移。

Rationalization of pH-Dependent Absorption Spectrum of o-Methyl Red in Aqueous Solutions: TD-DFT Calculation and Experiment Study

The understanding of factors that affect the optical properties of azo dyes sheds insight to the design of novel optoelectronic devices. The effect of the acidity or alkalinity and the solvent on the absorption spectra of ortho-methyl red (o-MR) aqueous solutions was investigated using UV/Vis experiments and density functional theory (DFT) calculations. The spectra of o-MR aqueous solutions showed a red shift of the maximum absorption peak from 430 nm to 520 nm when the pH of the solution was decreased from 13.1 to 0.5. In various acidity or alkalinity conditions, three main forms of o-MR coexisted in the aqueous solutions, i.e., diprotic o-H₂MR⁺ (strong acid condition), nonionic o-HMR (weak acid condition), and o-MR^- (basic condition), whose electronic structures were studied by DFT. The lowest dipole-allowed excitation energies of o-MR in aqueous solutions have been estimated by performing time-dependent density functional theory (TD-DFT) calculations. Both polarized continuum model (PCM) and explicit water cluster model were applied to study the solvent effects on the electronic structures and calculated spectra. The intramolecular hydrogen bond increases the planarity of o-H₂MR⁺ and o-HMR, leading to the enhancement of π-conjugation and, hence, a red shift in the spectra. Significant solventeffects on the calculated UV/Vis spectra of o-MR^- (under basic condition) were revealed. Strong dipole-dipole interactions between the polar o-MR^- and solvent water molecules may contribute to the red shift in the spectra.