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

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)4BF4 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 2θ. 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 Å whereas in phase II the closest approach of the rings is ∼6.8 Å. 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 × 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.