Remarkable enhancement of optical and electric properties by temperature-controlled solid-phase molecular motion

Abstract

Crown-ether-based molecular rotors, as a significant branch of artificial molecular machines, have garnered substantial attention since the announcement of the 2016 Nobel Prize in Chemistry. However, their optical and electric properties deteriorate generally with increasing temperature due to dynamic molecular motion, which poses a significant hindrance to their widespread commercial application. Herein, under the guidance of precise molecular modification strategies, a molecular rotator, [(Me₂N(CH₂)₂NH₃)(18-crown-6)]ClO₄ ([(N,N-dimethylethylenediammonium)(18-crown-6)]ClO₄), is successfully constructed. Intriguingly, the thermally activated dynamic motions of the molecular rotator lead to an infrequent polar-to-polar (Pca2₁-to-Cmc2₁) phase transition, accompanied by a significant enhancement in the electric and optical properties. Notably, in its high-temperature phase, the second harmonic generation (SHG) intensity even surpasses that of potassium dihydrogen phosphate (KDP), while the piezoelectric coefficient d₃₃ (∼20 pC N⁻¹) outperforms the majority of reported analogs. This phenomenon is comprehensively investigated through crystallographic physics theory and simulation calculations. Furthermore, the energy harvesting device was successfully prepared to validate its piezoelectricity, and one ‘SEU’-shaped light-emitting diode (LED) was successfully lit by harvesting mechanical energy. This work conducts a systematic experiment, in-depth theoretical analysis and first-principles calculation, thus paving the way for designing and constructing further artificial molecular machines with exceptional performance.