(1-Butyl-4-methyl- pyridinium)[ Cu( SCN)2]: A Coordination Polymer and Ionic Liquid

The compound (C(4)C(1)py)[Cu(SCN)(2)], (C(4)C(1)py=1-Butyl-4-methyl-pyridinium), which can be obtained from CuSCN and the ionic liquid (C(4)C(1)py)(SCN), turns out to be a new organic-inorganic hybrid material as it qualifies both, as a coordination polymer and an ionic liquid. It features linked [Cu(SCN)(2)](-) units, in which the thiocyanates bridge the copper ions in a (1,3)-fashion. The resulting one-dimensional chains run along the aaxis, separated by the C(4)C(1)py counterions. Powder X-ray diffraction not only confirms the single-crystal X-ray structure solution but proves the reformation of the coordination polymer from an isotropic melt. However, the materials shows a complex thermal behavior often encountered for ionic liquids such as a strong tendency to form a supercooled melt. At a relatively high cooling rate, glass formation is observed. When heating this melt in differential scanning calorimetry (DSC) and temperature-dependent polarizing optical microscopy (POM), investigations reveal the existence of a less thermodynamically stable crystalline polymorph. Raman measurements conducted at 10 and 100 degrees C point towards the formation of polyanionic chain fragments in the melt. Solid-state UV/Vis spectroscopy shows a broad absorption band around 18870cm(-1) (530nm) and another strong one below 20000cm(-1) (<500nm). The latter is attributed to the d(Cu-I)*(SCN)-MLCT (metal-to-ligand charge transfer) transition within the coordination polymer yielding an energy gap of 2.4eV. At room temperature and upon irradiation with UV light, the material shows a weak fluorescence band at 15870cm(-1) (630nm) with a quantum efficiency of 0.90(2)% and a lifetime of 131(2)ns. Upon lowering the temperature, the luminescence intensity strongly increases. Simultaneously, the band around 450nm in the excitation spectrum decreases.