Water-Induced Crystal-to-Crystal Transformation: Switchable Mechanical and Luminescent Properties

Endowing luminescent crystals with adjustable mechanical flexibility and rigidity is a challenge. In this work, we successfully achieve water-vapor-regulated mechanical and luminescent properties in a single system based on a novel ethoxy-substituted cyanostilbene derivative (DEA). Specifically, the pristine crystal of DEA exhibits elasticity with a yellow fluorescence. Upon water-assisted vapor fuming, a rigid crystal (DEA-w) with an orange fluorescence was obtained. The analysis of crystal structures shows that water can form multiple hydrogen bonds with the DEA molecule, further compelling the whole packing structure (crystal-to-crystal) transformation. Experimental and theoretical investigations reveal that the red-shifted fluorescence is ascribed to the enhanced intermolecular overlap, which favors an excimer emission. The “soft-to-rigid” transition is attributed to the packing transformation from a one-dimensional π column to a two-dimensional strong hydrogen bond network, which provides stronger resistance to external deformation. In addition, the DEA-w crystal undergoes a typical brittle fracture under cutting, offering excellent processability. This study provides an inspired method for in situ adjustment of the mechanical properties of the crystal.