Reversible Phase Transition of the 1:1 Complexes of 18-Crown-6 with 4-Ethoxyanilinium Perchlorate

The solid structure of the crystalline 1:1 complexes of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclo-octadecane) with 4-ethoxyanilinium perchlorate [1] has been determined at 298 and 93 K, respectively. The measurements of the unit cell parameters versus temperature show that the values of a axis and crystal volume change abruptly and remarkably at ∼163 K, indicating that this phase character may be a first-order. The crystal structural analysis of both two temperatures shows that in addition to van der Waals’ forces, conventional N−H···O, C−H···O hydrogen bonds, and C−H···π interaction are the main molecular interactions. The most distinct differences between the structures in room-temperature phase and low-temperature phase are of the twist of molecules viewing along a-axis. DSC measurements further confirm that this inclusion compound undergoes a reversible structural phase transition at about 163 K with a thermal hysteresis of 3.6 K in which relatively large latent heat (H) makes 1 a good candidate for PCMs (phase change materials). As usual, the physical properties, such as dielectric constant and specific heat (Cp), will change a lot near phase transition temperature (Tc). No distinct dielectric constant anomaly was observed in frequency and temperature ranges of 10−1000 kHz and 100−298 K.