New insight into the nature of Cu(TCNQ): Solution routes to two distinct polymorphs and their relationship to crystalline films that display bistableswitching behavior

Syntheses and characterization of two polymorphs of Cu(TCNQ have been carri ed out and the results correlated to films of the materials. Reactions of C uI with TCNQ or [Cu(CH3CN)(4)][BF4] with TCNQ(-) lead to blue-purple needle s 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 pla telet 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 t wo phases an indistinguishable from each other and are indicative of the pr esence 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 geome trical changes for the nitrile arrangements around the four-coordinate Cu(I ) centers lead to major changes in the architectural framework of the polym ers. Phase I was indexed in the tetragonal crystal system, but due to disor der and twinning, the crystal diffracted to only similar to 40 degrees in t heta. The data were solved and refined in the monoclinic Pn space group. Th e polymeric motif consists of Cu atoms surrounded by four nitrile lone pair s of independent TCNQ- molecules arranged in a distorted tetrahedral geomet ry. A quadruply twinned crystal of Cu(TCNQ) phase II was indexed in the mon oclinic system and resolved by deconvolution methods. The Cu(I) ions in pha se II occupy the tetrahedral positions of a Cooperite structure (PtS), and the TCNQ(-) radicals occupy the square planar sites. In both phases there a re two interpenetrating lattices present. In phase I the TCNQ- units are in volved in close pi-stacking interactions at similar to 3.24 A whereas in ph ase II the closest approach of the rings is similar to 6.8 A. In qualitativ e agreement with these observations are the magnetic properties; 1 is essen tially diamagnetic and 2 displays Curie-Weiss behavior down to very low tem peratures. The charge-transport properties of the samples revealed that, wh ile they are both semiconductors, 1 is a good semiconductor with a room-tem perature conductivity of 0.25 S cm(-1) and a band gap of 0.137 eV whereas 2 is a very poor semiconductor with sigma(rt) = 1.3 x 10(-5) S cm(-1)and a b and gap of 0.332 eV. Cu(TCNQ) film devices have been found to switch betwee n two states that exhibit very similar conducting properties.