Rationalization of pH-dependent absorption spectrum of O-methyl red in aqueous solutions: TD-DFT calculation and experiment study

Abstract

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 timedependent 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 solvent effects 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.