Dual Thermoresponsive Crystals: Synchronous Macroscopic Expansion and Directional Color Switch.

Thermoresponsive crystals linking molecular motion to macroscopic functions pose challenges in smart material integration. This work reports a new thermoresponsive organic crystal, 4-((4-(dimethylamino)phenyl)diazenyl)benzonitrile (Y28) exhibiting reversible thermally induced macroscopic expansion coupled with a sharp thermochromic switch. Through variable-temperature single-crystal X-ray diffraction, spectroscopy, and response behavior analysis, it was found that the crystal underwent continuous elongation upon heating and that a color change from red to orange occurred at a specific temperature, which process is fully reversible. Structural analysis indicates that the slight, reversible adjustment of the twist angle and interlayer spacing is manifested optically as a decrease in the band gap and a color change from red to orange with increasing temperature. At 420 K, the symmetry center disappears, and the entire heating-cooling cycle shows an order-disorder phase transition. By combining variable-temperature single-crystal diffraction data with macroscopic in situ deformation measurements, a complete process from the atomic to the macroscopic scale is constructed. This qualitatively and quantitatively elucidates the physical nature of the anisotropic thermal expansion of the Y28 crystal, along with its excellent cyclic stability. Its multifunctional coupling response mechanism provides new ideas for designing next-generation smart drive and optical sensing materials.