Photoinduced Fluorescence Switching in Molecular Aggregates by Topological [2+2] Cycloaddition

The optical modulation of fluorescence characteristics in molecular aggregates, which mainly involves diverse molecular stacking and the consequent intermolecular interactions, remains a significant challenge for potential applications in optical anticounterfeiting, data storage, and imaging. Here we successfully realize in situ fluorescence switching in molecular aggregates of thianaphthene-dioxide derivatives by topological 2+2] photocycloaddition, which represents a promising way to regulate the molecular stacking and alter photophysical processes. Notably, 2-(3,5-bis-trifluoromethylphenyl)benzob]thiophene-dioxide (BTO-TF) in both crystal and powder forms exhibits a unique switching from an initial nonfluorescent state to a highly fluorescent state (Φ_PL=0.46) upon UV irradiation, because of the destruction of the 2+2] cycloaddition process by volume expansion of the photodimer. Furthermore, we demonstrate such a 2+2] photocycloaddition can occur when 2-(4-carboxypheny)benzob]thiophene-dioxide (BTO-OH) is doped within selective polymer matrixes, and can be utilized for the visualization of macrophase separation in polymer blends.