CONSPECTUS: Molecular photoswitches with drastically switchable luminescent properties have attracted increasing attention due to their unique advantages in nondestructive and high-contrast read-out systems, which feature promising applications in optoelectronic devices, data storage, and optical imaging. In general, common organic photoswitches, capable of undergoing reversible structural changes between two photoisomers in solution state, exhibit markedly different optical properties in response to light stimuli. However, the photoisomerization dynamics in solid state are often severely inhibited due to the close molecular stacking that limits their structural transformation, resulting in the absence of functionality or the negative implication in practical applications. To address this issue, photoswitches have been introduced into various porous platforms to overcome molecular crowding and acquire sufficient conformational freedom for photoisomerization in condensed phase. Nonetheless, the realization of solid-state photoisomerization in the single-component molecular system remains extremely difficult, probably arising from the difficulties of structural modification and molecular-packing optimization. More importantly, exploring the solid-state switchable luminescent properties of photoswitch materials is highly challenging for molecular design and functional integration, which should not only fulfill the requirement of conformational freedom for efficient photoisomerization but also need to regulate the excited-state dynamics simultaneously.In this Account, we present the recent progress of our research groups in the development of solid-state luminescent molecular photoswitches, mainly focusing on molecular design strategy, switchable luminescence, and potential applications. At the beginning of our research, we proposed a new approach to implementing solid-state photoinduced luminescence switching and optical application. In a flexible covalent coupling strategy, we incorporated twisted luminophores into a common photoswitch, spiropyran (SP), via appropriate flexible covalent linkers to impede tight molecular packing. The modified photoswitch exhibited a unique absorption and luminescence modulation ability in solid state. Then we improved the photoreversibility of photoswitches by increasing molecular distortion and reducing intramolecular interactions. For these luminescent photoswitches, two distinct and selective fluorescent modes can be conveyed into each other in a reversible feature upon irradiation with alternating ultraviolet (UV) and visible light. Following this work, we regulated the photoluminescence and photoisomerization processes separately through the "pump-trigger" strategy, which successfully realized reversible three-color fluorescence switching. The scope of our work was further extended to a series of photoswitch molecules, including diarylethene (DAE) and fulgide, by employing the strategy that utilized the design of nonplanar molecular conformation of luminophores in solid state. To insightfully investigate the relationship between molecular structure and optical properties, we also discussed intrinsic excited-state dynamics and the switching mechanism of different photoswitch systems. Owing to multiple luminescent switching properties of photoswitches, we explored their potential applications in advanced anticounterfeiting, optical memory, information encryption, and super-resolution imaging. Accordingly, our works enable molecular engineering of luminescent photoswitches and their controllable and predictable optical properties in solid state. Future research in the field of luminescent photoswitches will include the development of new molecular design strategies, the in-depth understanding of photoinduced switching mechanisms, and the development of high-performance luminescent photoswitches for multifunctional applications.
