Recent advances in photoresponsive fluorescent materials based on [2+2] photocycloaddition reactions

Abstract

The development of light responsive fluorescent materials with nondestructive and high-contrast signaling properties has attracted much attention due to their potential use in a variety of areas, such as optical sensing, data storage, smart display, and advanced anti-counterfeiting. In general, most light responsive fluorescent materials are built on intramolecular Z/E photoisomerisation and photocyclisation of a single molecule, making them more suitable for solution-based systems due to the aggregation-caused quenching effect. Recent advances demonstrated that intermolecular [2+2] photocycloaddition reactions could be exploited to construct a new class of light responsive fluorescent materials whose solid-state fluorescence can be dramatically photoactivated. Such a conversion of solid-state emission from quenched to enhanced emission provides a promising solution to overcome the notorious aggregation-caused quenching effect, making these smart molecules more suitable for surface/interface-involved applications. This review systematically summarized recent advances in photoresponsive fluorescent molecules based on intermolecular [2+2] photocycloaddition reactions. The molecules were classified into four groups based on their molecular skeletons: diarylethenes, cyanodiarylethenes, benzoheterocycles, and diaryl acetylenes. We highlighted the structural design of these smart molecules, controlled molecular aggregation in different environments, including ordered crystals, disordered polymers, order-dependent liquid crystals, and caves of specific supramolecules, and their distinct photoactivation mechanisms of solid-state fluorescence. Finally, we analyse the challenges and opportunities for the future development of light responsive fluorescent materials based on intermolecular [2+2] photocycloaddition as a promising class of smart materials.