Switchable Guest Molecular Dynamics in a Perovskite‐Like Coordination Polymer toward Sensitive Thermoresponsive Dielectric Materials

Abstract

A new perovskite-like coordination polymer [(CH 3 ) 2 NH 2 ][Cd(N 3 ) 3 ] is reported which undergoes a reversible ferroelastic phase transition. This transition is due to varied modes of motion of the [(CH 3 ) 2 NH 2 ] + guest accompanied by a synergistic deformation of the [Cd(N 3 ) 3 ] À framework. The unusual two-staged switchable dielectric relaxation reveals the molecular dynamics of the polar cation guest, which are well controlled by the variable confined space of the host framework. As the material switches from the ferroelastic phase to the paraelastic phase, a remarkable increase of the rotational energy barrier is detected. As a result, upon heating at low temperature, this compound shows a notable change from a low to a high dielectric state in the ferroelastic phase. This thermoresponsive host-guest system may serve as a model compound for the development of sensitive thermoresponsive dielectric materials and may be key to understanding and modulating molecular/ionic dynamics of guest molecules in confined space.Host-guest systems that can respond to external stimuli, such as temperature, pressure, light, pH, or concentration, have attracted much attention because of their theoretical significance in multiple disciplines and their potential application in a wide variety of fields. [1][2][3][4][5][6][7][8] Among various types of stimuli-responsive host-guest systems, coordination polymers (CPs) which mimic the inorganic ABO 3 -type perovskite structure, as one type of CP-based inclusion compound, have potentially promising applications in modern optoelectronic materials, such as dielectric, ferroelectric, and base materials for high-efficiency photovoltaics. [9][10][11][12][13][14][15][16] These perov-skite-like CPs, with inclusion of guest cations within the wellmatched cage-like host frameworks, can undergo reversible structural phase transitions upon thermal stimulus, which can often lead to a change or a switch of the related physical properties. Additionally, the metal species, bridging ligand, and guest cation component of these systems have designable and tunable characteristics. In combination, perovskite-like CPs may serve as a unique host-guest model to understand and modulate the molecular dynamics of the guest cations in the variable confined space constructed by the host frameworks.To date, the ligands commonly employed for the construction of perovskite-like CPs have contained mostly the monoatomic I À , diatomic CN À , and triatomic HCOO À ions. It was found that the guest cations, rather than the host frameworks, play a key role in their structural phase transitions. However, compared with the host frameworks constructed by the above-mentioned bridging ligands, and in particular the curved HCOO À ion, those constructed using a rodlike N 3 À spacer can play a more important role to induce the structural transitions. The azido coordination framework has more flexible and deformable characteristics, as indicated by the structural analyses from our recent [17] and other related reports, [...

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