Anionic modulation induces molecular polarity in a three-component crown ether system

Abstract

Three-component crown ether phase change materials are characterized by a structural phase change in response to external stimuli such as temperature and electric or magnetic fields, resulting in significant changes in physical properties. In this work, we designed and synthesized two novel host–guest crown ether molecules [(PTFMA)(15-crown-5)ClO₄] (1) and [(PTFMA)(15-crown-5)PF₆] (2), through the reaction of p-trifluoromethylaniline (PTFMA) with 15-crown-5 in perchloric acid or hexafluorophosphoric acid aqueous solution. Compound 1 undergoes a structural change from the non-centrosymmetric space group (P2₁) to the centrosymmetric space group (P2₁/c) with increasing temperature. This transformation is accompanied by a switchable second harmonic generation (SHG) signal, a noticeable dielectric response, and a reversible phase transition in differential scanning calorimetry (DSC). For compound 2, there are reversible phase transitions accompanied by a dielectric response, but it does not exhibit a switchable SHG signal. The difference in properties between the two compounds may be due to the polarity modulation of the anion, providing new ideas for obtaining crown ether complexes with SHG response properties.