The Heat Capacity of 1,2,4,5-Tetracyanobenzene and of its 1 : 1 Charge-Transfer Complex with Pyrene from 10 K to 300 K

Abstract

The heat capacity of 1,2,4,5-tetracyanobenzene and of its 1 : 1 complex with pyrene has been measured form ～ 10 K to ～ 300 K. There is a transition in the complex which appears as an approximately symmetrical anomaly in the heat capacity-temperature curve between ～ 220 K and 250 K, but the heat capacity is not reproducible in this region even if the sample is subjected to the same preliminary thermal treatment. Moreover, the room-temperature form can be undercooled at least down to 150 K. In the two runs in which the heat capacity anomaly was most prominent. C_p reached its maximum at 232 ± 1 K. The largest measured value of the entropy of transition was 9.2 JK^?1 mol^?1. This value is compatible with but does not prove, the exisstence of static disorder above the transition, with one of the components (pyrene, in the light of the structural information) having at least two distinguishable orientations. At all temperatures in the range covered (apart from the transition region), the heat capacity of the complex is less than the sum of that of the pure components, and possible reasons for this are brifly discussed.