New Insight into the Nature of Cu(TCNQ): Solution Routes to Two Distinct Polymorphs and Their Relationship to Crystalline Films That Display Bistable Switching Behavior
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Syntheses and characterization of two polymorphs of Cu(TCNQ) have been carried out and the results correlated to films of the materials. Reactions of CuI with TCNQ or [Cu(CH3CN)4][BF4] with TCNQ- lead to blue-purple needles of Cu(TCNQ) phase I (1). A slurry of this kinetic product in CH3CN yields a second crystalline phase of Cu(TCNQ), phase II (2), which exhibits a platelet morphology. Powder X-ray diffraction and scanning electron microscopy data revealed that both phases are present in films of Cu(TCNQ) formed by oxidation of Cu foil by TCNQ in CH3CN. X-ray photoelectron spectra of the two phases are indistinguishable from each other and are indicative of the presence of Cu(I). Single-crystal X-ray studies were undertaken on very small crystals of the two samples, the results of which reveal that subtle geometrical changes for the nitrile arrangements around the four-coordinate Cu(I) centers lead to major changes in the architectural framework of the polymers. Phase I was indexed in the tetragonal crystal system, but due to disorder and twinning, the crystal diffracted to only ̃40° in 20. The data were solved and refined in the monoclinic Pn space group. The polymeric motif consists of Cu atoms surrounded by four nitrile lone pairs of independent TCNQ- molecules arranged in a distorted tetrahedral geometry. A quadruply twinned crystal of Cu(TCNQ) phase II was indexed in the monoclinic system and resolved by deconvolution methods. The Cu(I) ions in phase II occupy the tetrahedral positions of a Cooperite structure (PtS), and the TCNQ- radicals occupy the square planar sites. In both phases there are two interpenetrating lattices present. In phase I the TCNQ- units are involved in close π-stacking interactions at ̃3.24 ̊ whereas in phase II the closest approach of the rings is ̃6.8 ̊. In qualitative agreement with these observations are the magnetic properties; 1 is essentially diamagnetic and 2 displays Curie-Weiss behavior down to very low temperatures. The charge-transport properties of the samples revealed that, while they are both semiconductors, 1 is a good semiconductor with a room-temperature conductivity of 0.25 S cm-1 and a band gap of 0.137 eV whereas 2 is a very poor semiconductor with σ(rt) = 1.3 x 10-5 S cm-1 and a band gap of 0.332 eV. Cu(TCNQ) film devices have been found to switch between two states that exhibit very similar conducting properties. © 1999 American Chemical Society.
author list (cited authors)
Heintz, R. A., Zhao, H., Ouyang, X., Grandinetti, G., Cowen, J., & Dunbar, K. R.