Dynamic Vapochromic Behaviors of Organic Crystals Based on the Open-Close Motions of S-Shaped Donor-Acceptor Folding Units

The first vapochromic organic crystals are described with respect to their preparation, color change, adsorption/desorption properties, crystal structures, and color-change mechanism. Non-solvatochromic, 1,4,5,8-naphthalene-tetracarboxylic diimide (NDI) derivatives la bearing two pyrrole imine (PO tethers have been used as a motif for the crystal packing template. Red-purple vapochromic solid 3 was prepared by evacuation of orange crystals 2 (equivalent to 1a.2 MeOH), obtained by recrystallization of I a from MeOH. Solid 3 showed high-adsorption ability and unprecedented vapor-dependent color changes upon exposure to a variety of organic vapors, whereas light brown amorphous solid 1a, did not show vapo- or solvatochromic behavior toward any organic solvent. The strong adsorption capability of 3 was confirmed by TGA experiments and adsorption/desorption isotherms. Analysis of the solid-state UV/Vis analysis revealed that the vapor-dependent color changes of 3 were owed to the specific interference of solvent vapors with its broad CT absorbance at lambda = 450-650 nm. Packing structures of la in orange crystals 2. red-purple solid 3, and regenerated orange solid 2 were unequivocally established by single crystal and synchrotron powder X-ray diffraction, respectively. Molecular structures and arrays of 1 a in these materials indicated that 1) unit 1 a had an S-shaped folded conformation in 2 and 3 by intramolecular donor acceptor interactions between NDI and two PI units: 2) inclusion of the guest vapor into the S-shaped template decreased the intramolecular PI-NDI interactions, accompanied by increasing intermolecular NDI-NDI and PI-PI interactions: and 3) such flexible, open close motions of the S-shaped template could be repeated during reversible adsorption/desorption processes without degradation of crystal packing. The adsorption properties and mechanism of molecular shape-dependent vapochromic behavior of 3 are discussed with reference to experimental results, crystallographic data. and theoretical calculations.