In-situ monitoring of thiazine molecular aggregation in various solvents via a free-standing acoustic levitator

Formation of dimer and higher aggregates of methylene blue in alcohol

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.

Alignment of fibrous J-aggregates and the resulting macroscopic optical anisotropy observed in static solution

J-aggregates, which are supramolecular assemblies that exhibit unique optical properties owing to their excitonic interactions, have potential applications in artificial light-harvesting systems and fluorescence biosensing. Although J-aggregates are formed in solution, in situ observations of their structures and behaviors in solution remain scarce. In this study, we investigated the J-aggregates of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate [DiIC18(3)] in methanol/water (M/W) binary solvents using fluorescence imaging as well as polarized absorption and fluorescence measurements to explore the relationship between their structure and macroscopic optical properties under static conditions. Fluorescence images revealed that the DiIC18(3) J-aggregates have fibrous structures in the M/W = 44/56 (v/v) binary solvent. We measured the polarization-angle dependence of the fluorescence intensity of the fibrous J-aggregates to determine the direction of their transition dipole moment. Furthermore, the J-band absorbance was dependent on the polarization angle of the linearly polarized incident light, even in the absence of an external force such as that generated by a flow or stirring, indicating that the J-aggregates "spontaneously" aligned in solution. We also monitored the time evolution of the degree of alignment of the fibrous J-aggregates, which revealed that the formation and elongation of the fibers induced their alignment, resulting in the observed macroscopic optical anisotropy in solution.