Heat capacity and crystal polymorph of the organic charge-transfer complex, tetrathiafulvalene-p-chloranil

Heat capacities of the mixed stack organic charge-transfer complex tetrathiafulvalen-p-chloranil have been measured at temperatures between 8 and 300 K. Three kinds of crystal polymorphs are obtained depending on the method of synthesis. One of the two polymorphs obtained by recrystallization from acetonitrile solution exhibits a small heat capacity peak due to the neutral-to-ionic transition (NI transition) at 83.4 K, while the other shows no NI transition. On the other hand, a large peak in the N-I transition is observed at 82.5 K in crystals prepared by the co-sublimation method. Its enthalpy and entropy changes are (504 ± 5) J mol−1 and (6.12 ± 0.06) J K−1 mol−1, respectively. The crystals with small N-I transition are affected by thermal cycling in that the peak intensity decreases and eventually vanishes. In contrast, the crystals made by the co-sublimation method do not exhibit such a thermal effect. The thermodynamic stability of these polymorphs and the mechanism of the N-I transition are discussed.