Macrocycle Co‐Crystals Showing Vapochromism to Haloalkanes

Organic co‐crystal engineering is a promising method to make multifunctional materials. Here, the marriage of macrocyclic chemistry and co‐crystal engineering provides a smart strategy to build vapochromic materials. The macrocycle co‐crystals (MCCs) were constructed from π‐electron rich pillar[5]arene (P5) and an electron‐deficient pyromellitic diimide derivative (PDI) on a 10 g scale. MCCs of P5‐PDI are in red owing to the formation of a charge‐transfer (CT) complex. After solvent removal, a white crystalline solid with a new structure (P5‐PDI α ) is yielded, which exhibits selective vapochromic responses to volatile organic compounds (VOCs) of haloalkanes, accompanied by color changes from white to red or orange. Powder and single‐crystal X‐ray diffraction analyses reveal that the color changes are attributed to the vapor‐triggered solid‐state structural transformation to form CT co‐crystals. Coating films of P5 and PDI on glass showed a visible vapochromic behavior with good reversibility.