Linear and Nonlinear Spectroscopic Insights Into the Synergistic Solvation of Rhodamine-6G in Methanol-Dichloromethane Binary Solvent Mixture.

This study shows that intermolecular interactions in methanol-dichloromethane (MeOH-DCM) binary solvent mixtures play a crucial role in governing the solvation dynamics of the fluorescent probe rhodamine-6G (R6G). By employing both linear (UV-Vis absorption and steady-state fluorescence) and nonlinear (two-photon-induced fluorescence, TPIF) spectroscopic techniques, we demonstrate that the combined effects of solute-solvent and solvent-solvent interactions significantly modify the local environment around R6G. Methanol, a polar protic solvent, and dichloromethane, a polar aprotic solvent, interact through hydrogen bonding and dipole-dipole interactions, creating a microenvironment with an effective polarity higher than that of either pure solvent. As a result, the absorption spectrum exhibits a noticeable red shift, accompanied by enhanced TPIF intensity and a maximum two-photon excitation action cross-section in the mixed solvent system. These findings highlight that carefully choosing and tailoring the solvent provides a simple yet practical way to effectively tune two-photon absorption behavior. Such an approach offers promising opportunities for applications in high-sensitivity bioimaging, optical materials, and photonic devices, where strong nonlinear optical responses are essential.