Photoswitching, aggregation, and negative photochromism in ionic arylazoisoxazolium photoswitches

The direct introduction of ionic character in azoheteroarenes led to the generation of azopyridinium, azoimidazolium, and azopyrazolium photoswitches, which have previously garnered intriguing application prospects due to their varied Z-E thermal relaxation properties. We leveraged this foundation to design and synthesize three azoisoxazolium-based ionic photoswitches, aiming to expand their application scope and tailor their properties. The investigations on their photoswitching characteristics in different solvents, including water, revealed a solvent-dependent aggregation that competes with isomerization, although their thermal relaxations slow down the aggregation. Also, spectroscopic and dynamic light scattering (DLS) studies showed that the azoisoxazolium ionic photoswitches can exhibit negative photochromism, light-induced disaggregation at low concentrations in one of the derivatives, and significantly fluorescence emission in water. In addition, microscopic studies using scanning electron microscope (SEM), transmission electron microscope (TEM), polarized optical microscope (POM), and confocal microscope revealed the size and structural morphology and mesophase changes of the aggregates. Overall, our comprehensive investigation has positioned azoisoxazolium salts as a new class of ionic photoswitches characterized by several intriguing properties and a pronounced tendency to aggregate.