A COPPER(II) BROMIDE DIMER SYSTEM EXHIBITING PIEZOCHROMIC AND THERMOCHROMIC PROPERTIES - THE CRYSTAL-STRUCTURE AND ELECTRONIC SPECTROSCOPY OF THE 2 ROOM-TEMPERATURE PHASES OF BIS(TETRAPROPYLAMMONIUM) HEXABROMODICUPRATE(II)

Two phases of the compound bis(tetrapropylammonium) hexabromodicuprate(II) were synthesized and their crystal structures determined. The orange phase exhibits piezochromism, with the application of a small force causing it to spontaneously transform to the green phase. The green phase is thermochromic and transforms to the orange phase upon heating to 90-degrees-C. The green triclinic phase, with space group P1BAR, a = 9.612 (2) angstrom, b = 9.602 (2) angstrom, c = 12.006 (3) angstrom, alpha = 102.27 (2)degrees, beta = 104.30 (2) degrees, gamma = 112.79 (1) degrees, V = 929.2 (3) angstrom-3, Z = 1, d(x) = 1.75 g/cm3, and R = 0.0642, is stable at room temperature. The orange phase, metastable at room temperature, is monoclinic with space group C2/c,a = 17.460 (6) angstrom, b = 14.625 (4) angstrom, c = 15.061 (4) angstrom, beta = 103.82 (2) degrees, V = 3735 (2) angstrom-3, Z = 4, d(x) = 1.74 g/cm3, and R = 0.0839. Both phases have isolated Cu2Br6(2-) dimers with very similar geometries. The correlation of polarized absorption spectral data and X-ray crystallography show that the difference in orientation of the CU2Br6(2-) chromophores in the two phases is responsible for the color variance. A prominent feature of the visible absorption spectra is the presence of a so-called ''dimer'' transition at approximately 13 000 cm-1. This assignment gives strong support for ligand to metal character of the dimer transition, and not a simultaneous excitation of d-d transitions on both copper centers. EHMO calculations support this assignment.