A temperature-induced order–disorder phase transition in a 4-substituted 4,2′:6′,4′′-terpyridine

Abstract

Crystals of 4′-(isoquinolin-4-yl)-4,2′:6′,4′′-terpyridine ( iqtp ), C 24 H 16 N 4 , grown from an ethanol solution, undergo a reversible first-order single-crystal to single-crystal phase transition at T c in the range 273–275 K, from a disordered higher-temperature phase [form (I)] in the space group P 2 1 / c , with one single molecule in the asymmetric unit, to an ordered lower-temperature one [form (II)] in the space group P 2 1 / n , with two independent molecules in the asymmetric unit. There is a group–subgroup relationship linking (I)–(II), due to cell doubling and the disappearance of a number of symmetry operations. In addition to X-ray diffraction, the transition has been monitored by Raman spectroscopy and differential scanning calorimetry, the latter disclosing an enthalpy change of 0.72 (6) kJ mol −1 . Variations of the unit-cell parameters with temperature between 170 and 293 K are presented. The evolution of diffraction spots in the vicinity of the transition temperature shows the coexistence of both phases, confirming the first-order character of the transition. Structural details of both phases are analyzed and intermolecular interactions compared in order to investigate the mechanism of the phase transition. A three-dimensional Hirshfeld surface analysis was performed to corroborate the significant changes in the intermolecular features.