Giant anisotropic thermal expansion actuated by thermodynamically assisted reorientation of imidazoliums in a single crystal

Abstract

Materials demonstrating unusual large positive and negative thermal expansion are fascinating for their potential applications as high-precision microscale actuators and thermal expansion compensators for normal solids. However, manipulating molecular motion to execute huge thermal expansion of materials remains a formidable challenge. Here, we report a single-crystal Cu(II) complex exhibiting giant thermal expansion actuated by collective reorientation of imidazoliums. The circular molecular cations, which are rotationally disordered at a high temperature and statically ordered at a low temperature, demonstrate significant reorientation in the molecular planes. Such atypical molecular motion, revealed by variable-temperature single crystal X-ray diffraction and solid-state NMR analyses, drives an exceptionally large positive thermal expansion and a negative thermal expansion in a perpendicular direction of the crystal. The consequent large shape change (~10%) of bulk material, with remarkable durability, suggests that this complex is a strong candidate as a microscale thermal actuating material.

Giant Single‐Crystal Shape Transformation with Wide Thermal Hysteresis Actuated by Synergistic Motions of Molecular Cations and Anions

“…The single crystal of 1 is structurally analogous to the previously reported compound (Himd) 2 [CuCl 4 ], however, the mechanical responses of these two compounds are obviously different. In contrast to the abrupt shape transformation of 1 that manifests as a remarkable wide thermal hysteresis, the mechanical response of the chloride analogue is sluggish (the thickness of crystal gradually expands 10 % upon heating from 123 to 393 K) and the corresponding thermal hysteresis is negligibly small (less than 3 K).…”

“…Recently, we discovered an organic‐inorganic hybrid Cu II complex, (Himd) 2 [CuCl 4 ] (Himd + =imidazolium) that performs a giant shape transformation in response to a temperature variation . However, because the intact CuCl 4 2− counter anions impair the rapid propagation of the molecular motions of cations in the whole crystal, the shape transformation in this chloride compound is sluggish and without thermal hysteresis.…”

“…The 9 % elongation upon heating is similar to that which was observed in the chloride analogue of (Himd) 2 [CuCl 4 ], whereas the concurrent 9 % contraction is extremely large in solid‐state materials. Such dramatic shape change represents one of the largest mechanical responses observed in thermal dynamic single‐crystal materials, suggesting the outstanding structural flexibility of 1 (see Table S2) …”

Humidity-Sensing Properties of an 1D Antiferromagnetic Oxalate-Bridged Coordination Polymer of Iron(III) and Its Temperature-Induced Structural Flexibility

“…It is noteworthy that all three polymorphs, but especially HT-1 , exhibited large linear thermal expansion coefficients along the b- and c -directions ( Figure 1 b). Molecular crystals usually exhibit a thermal expansion coefficient in the range of 20 × 10 −6 K −1 , but large thermal expansions have recently been reported for (phenylazophenyl)palladium hexaflouracetyacetonate (260 × 10 −6 K −1 ) [ 36 ], N ′-2-propylidene-4-hydroxybenzohydrazide (360 × 10 −6 K −1 ) [ 37 ], and a copper(II) complex with imidazoliums (346 × 10 −6 K −1 ) [ 38 ].…”

Statics and dynamics of ferroelectric domains in molecular multiaxial ferroelectric (Me3NOH)2[KCo(CN)6]

“…However, unlike TMC-1, the cell volume of TMC-4 shrinks 0.45% (from 1877.5 Å 3 at 230 K to 1869.0 Å 3 at 450 K) during the bond-switching transition, indicating a negative thermal expansion. [60][61][62]…”