Formation of dimer and higher aggregates of methylene blue in alcohol

Abstract

This study investigates MB aggregation in propanol, ethanol, butanol, and methanol using UV–Visible, steady-state, and time-resolved fluorescence spectroscopy at room temperature. This work reveals the presence of monomers, dimers, and higher aggregates across various concentrations, affecting both the absorption spectra and fluorescence intensity. Characteristic peaks for monomeric, dimeric, and higher aggregates of MB are identified, with shifts in emission peak intensity and lifetime as a function of concentration, solvent polarity, and viscosity. A strong correlation between fluorescence lifetime and solvent properties, such as dielectric constant and viscosity, suggests that solvent polarity and viscosity significantly influence the stabilization of excited states and the deactivation dynamics of methylene blue. The critical concentration for MB aggregation is identified for each solvent and is found to be higher in solvents with a greater dielectric constant, likely due to Van der Waals forces dominating over Coulombic interactions in the aggregation process. The findings indicate that MB aggregation in alcohols differs from that in aqueous solutions, underscoring the importance of the solvent environment in MB’s aggregation and fluorescence behavior, with implications for optimizing its use in laser and photomedicine applications.