Heat Capacity of the Discotic Mesogen, 2,3,6,7,10,11-Hexa-n octanoyloxy triphenylene: A Complex Solid State Polymorphismt

Abstract

Heat capacity of the discotic mesogen, 2,3,6,7,10,11-hexa-n-octanoyloxy triphenylene (HAT-C8) has been measured between 12 and 425 K by adiabatic calorimetry. HAT-C8 exhibited a variety of melting phenomena depending on its thermal history. The solids well annealed at around 361 and 349 K melted at 362.6 and 359.7 K, respectively, while a fresh sample obtained by recrystallization from an ethanol solution melted at 348 K. These three solid modifications have been here designated as solids I, II and III in the order of higher melting point. The enthalpy and entropy of melting for the solids I, II and III were 24.21 kJ mol⁻¹/66.76 J K⁻¹ mol⁻¹, 25.44/71.12 and 34.77/99.91, respectively: the lower the melting point is, the larger the melting entropy is. The transition from the columnar mesophase to isotropic liquid occurred at 402.16 K accompanied by enthalpy and entropy changes of 3.626 kJ mol⁻¹ and 9.02 J K⁻¹ mol⁻¹. All the solid forms obeyed the third law of thermodynamics: that is, no residual entropies at 0 K within the present experimental error. A surprising fact is that the solid III, characterized by the lowest melting point among the three, is the most stable crystalline phase at 0 K. Its Gibbs free energy crosses over those of the solids I and II at around 290 K, although the present calorimetric study failed to realize completely this phase transition. The expected entropy associated with this hidden transition is as large as 52.8 J K⁻¹ mol⁻¹. Some reasons for the unsuccessful observation of this transition have been discussed in relation to different molecular conformations in each solid phase. The thermal behavior of HAT-C8 has been compared with that of the benzene-core discotic mesogen having the same alkyl chains. A remarkable difference is that the entropy gain due to the transition from the columnar phase to the isotropic liquid is extremely small for HAT-C8 (9.02 J K⁻¹ mol⁻¹) in comparison with the benzene-core (59.93 J K⁻¹ mol⁻¹).

Keywords:

• heat-capacity
• discotic triphenylene
• polymorphism