Synthesis of a luminescent macrocycle and its crystalline structure-adaptive transformation

We report that the marriage of macrocycle chemistry and crystal engineering provides interesting macrocycle crystals with switchable luminescence and structure-adaptive transformation. A 2-(diphenylmethylene)malononitrile based luminescent macrocycle (MC), connected by methylenes, is designed and synthesized. Two luminescent crystals of MC, where one is yellow-emissive and the other is orange-emissive, are successfully produced. Luminescent switching between orange and yellow emissions has been reversibly realized through the adsorption and desorption of aromatic volatile organic compounds (VOCs). In contrast, no fluorescence change was observed upon using crystals of the corresponding monomer. Mechanistic studies by powder and single-crystal X-ray diffraction analyses verify that the reversible switching is due to a single-crystal-to-single-crystal transformation triggered by vapor uptake and activation. The macrocyclic framework plays a critical role in these stimuli-responsive crystalline materials, serving to reduce the packing density, induce structural flexibility, and supply sufficient spaces for motion and adsorption.