Aggregation Structure and Luminescent Properties of Hydrophobic Organic Molecules in Water-THF Mixed Solvent

Hydrophobic organic molecules are known to form aggregates in a mixture of water and tetrahydrofuran (THF). Changing the water-THF ratio in such solutions generally leads to property changes. However, systematic studies on the exact mechanism governing the change in aggregation state and its effect on the property change remain underdeveloped. In this study, we performed small-angle neutron scattering (SANS), wide-angle X-ray scattering (WAXS), and dynamic light scattering (DLS) measurements along with fluorescence microscope observations on originally developed hydrophobic luminescent molecules in water-THF mixtures with varying water content. These measurements revealed that the hydrophobic solute molecules form "loose aggregates" at about 50% water volume fraction and transition to "tight aggregates" as the water fraction is increased. The correlation between the change in aggregation structure and luminescence property, particularly aggregation-caused quenching, was also clarified. These findings will be helpful in advancing technologies to control the formation of organic molecular aggregates and their properties in applications such as organic light-emitting diodes as well as drug delivery systems.