An investigation of the structure and properties of layered copper thiolates

Abstract

Copper(I) n-alkane thiolate systems have been characterized using powder X-ray diffraction, IR spectroscopy, X-ray photoelectron spectroscopy and thermal analyses. The diffraction pattern is composed of a series of peaks, which are indexed to (0k0) reflections of a layered structure. The tilt angle, θ, of the alkyl chains is estimated to be 13°, distinctly different from the silver thiolates, where the chains are near normal to the silver plane. The layers interpenetrate to a small (or negligible) extent. The alkyl chain assembly is different from that of crystalline alkanes. An increase in the alkyl chain order with increase in chain length is evident in the infrared spectra. The chains are essentially all-trans, but gauche conformations are seen near the sulfur end. Upon cooling, an orientational transition occurs resulting in the freezing of the local symmetry around the methyl group. The materials show high thermal stability and desorption is seen only above 500 K. High temperature phase transitions of the materials have been studied by variable temperature powder XRD, variable temperature IR and differential scanning calorimetry. Two distinct transitions; one from an original gauche-trans (g-t) to a gauche (g) phase and another subsequent one, to a columnar mesophase, have been identified. The transitions are not fully reversible, conformational changes are observed upon heating and cooling. Thiolates appear to be model systems for the investigation of molecular self-assembly on copper surfaces.