ABSTRACT
Three thermochromic phases (α, green; ß, red; γ, yellow) and six polymorphic modifications (α(1), monoclinic, P2(1)/n, Z = 2; ß(1), monoclinic, P2(1)/c, Z = 4; ß(2), triclinic, P1[overline], Z = 4; ß(3), monoclinic, P2(1)/n, Z = 4; γ(1) and γ(2), tetragonal, P4(2)/n, Z = 4) have been found and structurally characterized for copper(II) diiminate Cu[CF(3)-C(NH)-CFâC(NH)-CF(3)](2) (1). The α phase is stable under normal conditions, whereas the high-temperature ß and γ phases are metastable at room temperature and transform slowly into the more stable α phase over several days or even weeks. X-ray diffraction study revealed that the title molecules adopt different conformations in the α, ß, and γ phases, namely, staircase-like, twisted, and planar, respectively. The investigation of the α, ß, and γ phases by differential scanning calorimetry showed that the three endothermic peaks in the range 283, 360, and 438 K are present on their thermograms upon heating/cooling. The two peaks at 283 and 360 K correspond to the solid-solid phase transitions, and the high-temperature peak at 438 K belongs to the melting process of 1. The temperature and thermal effect of all the observed transitions depend on the prehistory of the crystalline sample obtained. A reversible thermochromic single-crystal-to-single-crystal α(1)<--> ß(1) phase transition occurring within a temperature interval of 353-358 K can be directly observed using a CCD video camera of the X-ray diffractometer. A series of other solid-solid α(1)âγ(1), ß(2)âγ(1), ß(3)âγ(1), and γ(1)<-->γ(2) phase transitions can be triggered in 1 by temperature. It has been suggested that, under equilibrium conditions, the α(1)âγ(1) and ß(2)âγ(1) phase transitions should proceed stepwise through the α(1)âß(1)âß(2)âß(3)âγ(1) and ß(2)âß(3)âγ(1) stages, respectively. The mechanism of the phase transitions is discussed on the basis of experimental and theoretical data.