Solvatochromic effects on anionic and protonated forms of methyl orange dye: An experimental and theoretical study

Chemical environment and solvent interactions are critical factors in induced changes in the electronic structure of organic compounds, leading to significant modification in their optical behavior. Through experimental and theoretical approaches, the present study analyzes the solvatochromic effects on the anionic and protonated forms of methyl orange (MO), with emphasis on its light absorption and nonlinear optical response. Considering protic and aprotic polar solvents, absorption spectra of MO were measured in solvents of varying polarities, evaluating how the protonation degree affects the electronic transitions in such azo dye. Additionally, multi-scale theoretical calculations were conducted to estimate the solvent's influence on the MO absorption wavelengths. Experimental data revealed that solvent polarity promotes a broadening and redshift of the main absorption band of methyl orange, which depends on whether the solvent presents a proton-donation or proton-acceptance nature. Moreover, it is verified that the absorbance spectrum of MO is significantly modified by HCl addition, especially in solvents exhibiting the hydroxyl group, like methanol and water. Such results indicate the stabilization of the diazonium form of methyl orange upon protonation, being supported by theoretical modeling of anionic and diazonium forms of MO in different solvents. Using the PCM-MC/QM version of sequential Monte Carlo-Quantum Mechanical (s-MC/QM) methodology, theoretical simulations of absorption band maximum of MO are in good agreement with experimental findings, achieving a Pearson Product-Moment Correlation (PPMC) coefficient of 0.886. Regarding the nonlinear optical response of MO, our results revealed that the environmental conditions significantly impact the third-order susceptibility of such azo dye. Specifically, we verify that solute-solvent interactions may increase or suppress the nonlinear optical response of methyl orange, depending on the proton-donation or proton-acceptance nature of the solvent.