Calorimetric study of triphenylethene: observation of homogeneous-nucleation-based crystallization

Heat capacities of triphenylethene were measured in the range 6 < (T/K) < 354 using an adiabatic calorimeter, and the thermodynamic functions were evaluated. The temperature, molar enthalpy, and molar entropy of fusion were determined to be (340.95 +/- 0.02) K, (20.35 +/- 0.02)kJ.mol(-1) and (59.67 +/- 0.06)J.K-1.mol(-1), respectively. A glass transition was found at T-g = (248 +/- 1) K, and the residual entropy of the glass formed by precooling at the rate of 0.167 K.s(-1) was estimated to be (18.6 +/- 0.1)J.K-1.mol(-1). An anomalous heat-evolution effect was observed around T = 254 K, immediately above T-g for the partly crystallized sample, and found from the comparison between the room temperature powder X-ray diffraction patterns of crystals formed at T = 254 K and T = 297 K to be due to another crystallization which gives the same crystal structure as the ordinary crystal growth proceeding above T = 258 K. The anomalous crystallization appeared to cease above T = 257 K, and was interpreted to proceed by a homogeneous-nucleation-based mechanism in which the crystal embryos formed homogeneously in the liquid are stabilized by their coalescence with the pre-existent crystal. (C) 1998 Academic Press Limited.