Calorimetric study of thermochromic complexes. 1. Heat capacity and phase transition of bis( N, N-diethylethylenediamine)-copper(II) tetrafluoroborate

Abstract

The results of heat capacity measurements at constant pressure, C p, on the thermochromic compound bis( N, N-diethylethylenediamine)copper(II) tetrafluoroborate with an adiabatic calorimeter between 13 and 343 K are reported. An appreciable heat capacity effect was found at 302.64 K with a long heat-capacity tail extending down to ∼200 K. The enthalpy and entropy of the phase transition were found to be Δ trs H = 16.62 kJ mol −1 and Δ trs S = 55.35 J K −1 mol −1, respectively. The main part of Δ trs S arises from puckering of the copper-ligand ring and orientational disordering of the ethyl groups bonded to the nitrogen atoms. The phase transition behavior has been simulated by applying the Chesnut exciton model. The experimental results can be well accounted for by this model in spite of its simplicity.