Question

Explain why [Cu(deen)2](ClO4)2 in ethanol solution does not show thermochromic behaviour while the solid can change colour upon heating above 35 degrees celsius

Answer 1

Thermochromism is the property of substances to change colour due to a change in temperature.The coordination geometry around Cu(II) is responsible for the colour change from red to violet. Solid state UV the broad absorption bands are assigned to the d-d transitions of the copper(II) ion, and the absorption νmax varied depending on the axial ligands . The unsolvated samples were slightly different from those expected.

In [Cu(deen)2](ClO4)2 the colors of the solutions gradually changed from blue to yellow-red on heating . The color change rate on heating appeared to be faster than that during cooling. In the UV-Vis spectra for this solutions, the change accompanied an increase of a peak at around 25000 cm−1 , whereas the d-d absorption peak at around 16500 cm−1 remained the same. This is also oberved in [Cu(Cl-acac)(diamine)]ClO4 in pyridine, except that the intensity of the d-d absorption changes as a function of temperature