Photo-and mechanoluminescent materials that exhibit tunable emission properties when subjected to external stimuli have a wide variety of applications. However, most mechanoresponsive materials have a mechanoinduced structural transition from crystalline to amorphous phase, and there are only few reports on the crystalline to crystalline transformation. This study reports an amide-linked dyad P1 containing spiropyran and naphthalimide chromophores with dual sensitivity of photochromism and mechanochromism. Under light and mechanical stimuli, P1 performs different color transition. With mechanical force, the morphologies of P1 change from microfiber to nanosphere and the amide group in P1 plays a vital role in these transition processes. Mechanical force can induce the morphology change of P1 through enhancing π-π stacking and destroying hydrogen bonds. These results demonstrate the feasibility of the design strategy for new mechanoresponsive switching materials: both π−π stacking and hydrogen bonding of the dyad contribute the mechano-induced crystalline/crystalline transformation.

Solid-state photochromic materials with reversible and adjustable optical properties are very appealing because of their wide prospects in advanced functional materials. Yet, it remains a significant challenge to develop such materials in the solid state. In this study, a tetraphenylethene derivative (SP-TPE-SP)-based solid-state photoswitch, which exhibits reversible photochromism in the solid state, was constructed. Efficient photoswitching between SP-TPE-SP and its photoisomer MC-TPE-MC is assisted by the large free volumes caused by the nonplanar molecular structures of the TPE1 moieties and the intramolecular π-π stackings between the two MC moieties. The free volumes are large enough to allow for the transport of HCl gas molecules for an acidochromic response. Furthermore, the morphology of the SP-TPE-SP solid surface can be regulated by ultraviolet light irradiation. The contact angles of the SP-TPE-SP solid surface can be decreased, changing from 97 to 82°. Therefore, SP-TPE-SP with a rather simple molecular structure is appealing for advanced multifunctional materials.

Organic luminescent materials with the ability to reversibly switch the luminescence when subjected to external stimuli have attracted considerable interest in recent years. However, the examples of luminescent materials that exhibit multiresponsive properties are rarely reported. In this work, a new stimuli‐responsive dye P1 is designed and synthesized with two identical chromophores of naphthalimide, one at each side of an amidoamine‐based spacer. This amide‐rich molecule offers many possibilities for forming intra‐ and intermolecular hydrogen bond interactions. Particularly, P1 has an intrinsic property of cocrystallizing with methanol. Compared with the pristine P1 sample, the as‐prepared two‐component cocrystalline material displays an exceptive deep‐blue emission, which is extremely rare among naphthalimide‐based molecules in the solid state. Furthermore, the target material exhibits an obvious mechanochromic fluorescent behavior and a large spectral shift under force stimuli. On the other hand, the cocrystalline material shows an unusual “turn off” thermochromic luminescence accompanied by solvent evaporation. Moreover, using external stimuli to reversibly manipulate fluorescent quantum yields is rarely reported to date. The results demonstrate the feasibility of a new design strategy for solid‐state luminescence switching materials: the incorporation of solvents into organic compounds by cocrystallization to obtain a crystalline state luminescence system.

A novel Rhodamine 6G-naphthalimide molecule is used for high-contrast fluorescence change upon acid and force stimuli in the solid state.

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