Tetraphenylethene-Induced Free Volumes for the Isomerization of Spiropyran toward Multifunctional Materials in the Solid State

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.