Recent advances in small-molecule fluorescent photoswitches with photochromism in diverse states

Photochromic materials have been widely used in various areas including data encryption, biological imaging and anti-counterfeiting because of their excellent contrast, responsiveness and fatigue resistance. Molecular materials that use fluorescence as a readout signal generally exhibit superior sensitivity and accuracy compared to those that rely solely on absorbance signals. Therefore, integrating the photoluminescence property into photochromic molecules is an increasing trend in achieving dual-channel signal response and improving the sensitivity. The molecules with a reversible photoswitching characteristic and high-contrast fluorescent response constitute an emerging class of smart molecules named small-molecule fluorescent photoswitches. Relying on their working environments, these small-molecule fluorescent photoswitches can be divided into two groups, one of which can switch fluorescence in the solution state through controllable photoirradiation, making them suitable for super-resolution fluorescence imaging. Solution-state small-molecule fluorescent photoswitches were developed mainly based on traditional diarylethenes and emerging hydrozone derivatives with photochromism. Differing from the first group, the second group of fluorescent photoswitches is capable of switching their solid-state fluorescence, making them more suited for surface/interface-involved applications such as information storage and data encryption. Solid-state small-molecule fluorescent photoswitches were achieved by combining conventional photochromic switches and aggregation-induced emission luminogens (AIEgens). This review systematically introduces the most recent advances in the molecular structure, optical properties, and practical applications of small-molecule fluorescent photoswitches in diverse states, as well as the molecular design principle and working mechanisms. Finally, we outline some existing problems and challenges to promote follow-up development in this field. This review summarizes recent advances in small-molecule fluorescent photoswitches based on classic photochromic molecules in diverse states, and provides some existing problems and challenges to promote follow-up development in this field.