Non-classical temperature-dependent phase sequence: long-lived metastable piezoelectric helical columnar liquid crystalline polymorph

Abstract

Polymorphism is an intriguing research topic, because multiple functionalities can be accessed from a single material. Herein, we report the unusual polymorphic behavior of a liquid crystalline (LC) naphthalene derivative (1) based on 1,2,3-triazolyl linkages. Two individual polymorphs corresponding to metastable helical columnar (Col_hel) LC and thermodynamically stable monoclinic crystalline (Cry) phases can be obtained at room temperature by mild thermal treatments. According to the thermal, dynamic, and structural analyses of 1, the triazolyl H-bonded network along the columnar axis plays a key role in the formation of the polar helical columnar order and its longevity. As a consequence of the remarkable stability of the metastable Col_hel, a non-classical phase sequence, that is, Col_hel–liquid–cry–liquid upon heating, was observed for the first time, although it was also predicted theoretically in monotropic LC polymorphism. Interestingly, piezoelectric measurement of the poled Col_hel polymorph showed a noticeable increase in open-circuit voltage upon the application of force, which was more than five times greater than the measured voltage of the Cry polymorph.