Photoresponsive structure transformation and emission enhancement based on a tapered azobenzene gelator

Abstract

A new low molecular mass organic gelator bearing 1,3,4-oxadiazole and azobenzene groups, namely N-(3,4,5-tributoxyphenyl)-N′-4-[(4-hydroxyphenyl)azophenyl] 1,3,4-oxadiazole (AOB-t4), was designed and synthesized in this work. The organogelator shows a great ability to gel moderate polar solvents and form stable organogels with the critical gelation concentration as low as 2.0 mg mL⁻¹, which thus can be considered as a supergelator. The morphology of xerogels demonstrates a strong dependence on the nature of gelling solvents. Due to the photo-induced isomerization of azobenzene unit, the transformations from fiberous to porous structure in AOB-t4 dichloromethane gel, and from fiber to nanoparticle in ethanol solution have been successfully achieved with UV 365 nm irradiation, respectively. Meanwhile, an enhanced fluorescence via J-aggregate molecular arrangement can be observed with the trans-to-cis photoisomerization in both dichloromethane gel and ethanol solution of AOB-t4, and the emission quantum yield can be increased from 10.7 × 10⁻⁵ to 18 × 10⁻² in ethanol, corresponding to 1682 times enhancement. The obtained results would be of significance in developing novel photo-controllable luminescence molecular device and broadening the application fields of azobenzene derivatives.