A thermo-responsive structural switch and colossal anisotropic thermal expansion in a chiral organic solid

Mechanical Transitions in Crystals: The Low-Temperature Thermosalient Transition of a Mesogenic Polyphenyl

Thermosalient transitions are a subset of single-crystal-to-single-crystal (SCSC) transitions, in which the change of lattice parameters is highly anisotropic and very fast. As a result, crystals at the transition undergo macroscopic dynamical effects (hopping, jumping, and shattering). These transitions feature a conversion of heat to mechanical energy that can be exploited in the realization of advanced materials. Most thermosalient transitions are observed at temperatures higher than room temperature. Examples of low-temperature thermosalient transitions are rare. We describe a new example of a low-temperature thermosalient transition in a sexiphenyl compound. At about -40 °C, the parent single crystal (phase I) shatters into single crystal fragments of the new phase (phase II). The two phases have been studied by single-crystal X-ray analysis using a synchrotron source, variable-temperature Raman spectroscopy, and computational analysis of lattice normal vibration modes. A mechanism of the transition is proposed. We confirm colossal thermal expansion coefficients and supercells as reliable features of thermosalient transitions and add as a third feature a low-frequency principal optical vibration of the crystal lattice prompting the transition. Based on this, a roadmap for the automated prediction of thermosalient transitions in molecular crystals is also outlined.

Structural Evolution Leading to the Thermosalient Phase Transition of Oxitropium Bromide

This study investigates the thermosalient effect in oxitropium bromide, with a focus on the role of anisotropic thermal expansion, elastic properties, and sound propagation in driving this phenomenon. Variable-temperature X-ray powder diffraction (VTXRPD) revealed significant anisotropic thermal expansion, including negative thermal expansion (NTE) along the c-axis in the low-temperature Form A. Density functional theory (DFT) calculations were used to analyze elastic properties of oxitropium bromide and confirmed that it does not exhibit negative compressibility, emphasizing thermal anisotropy as the primary factor in the phase transition. Studies of elastic constants and sound propagation demonstrated a preferred pathway for energy transfer along the z-direction, enabling rapid strain release during the phase transition. These findings confirmed that the thermosalient effect arises from cooperative molecular motion, resulting in an abrupt and energetic transformation driven by the interplay of structural anisotropy and elastic properties.

Amplifying Colossal Thermal Expansion of a Martensitic Molecular Crystal through Interlayer Shear-Induced Side-Chain Liberation

Molecular crystals capable of colossal thermal expansion (TE) are fascinating owing to their substantial and continuous volume changes and reasonably linear responses to temperature. This makes them promising candidates for micromachine applications. Macroscopic motion is driven by subtle yet cooperative movements of molecules that respond to the thermal motions of dynamic functional units. The study of p-TIPS-DSB presented here offers a compelling case highlighting the relationship between the degree of dynamicity of functional units and TE behavior. In its α-phase, the p-TIPS-DSB crystal undergoes an irreversible martensitic transition to the β-phase, accompanied by significant cooperative interlayer shear. This process substantially enhances the mobility of the side-chains driven by the increased free volume surrounding them. This nearly doubles the volumetric TE coefficient from 255.3 (10) to 444.9 (32) MK-1, particularly in the actuation direction from 175.0 (7) to 291.7 (20) MK-1, enabling about 4.5 % elongation/contraction. As demonstrated here, p-TIPS-DSB exhibits a decent force density (>1.4×107 N m-3) and precise motion control capabilities due to its hysteresis-free and non-abrupt TE nature. Furthermore, we demonstrated the limited operating distance of colossal TE materials can be amplified by utilizing levers, highlighting the high potential of these materials for use in micromachines.

Specific Noncovalent Association of Truncated exo-Functionalized Triangular Homochiral Isotrianglimines through Head-to-Head, Tail-to-Tail, and Honeycomb Supramolecular Motifs

Chiral isotrianglimines were synthesized by the [3 + 3] cyclocondensation of (R,R)-1,2-diaminocyclohexane with C5-substituted isophthalaldehyde derivatives. The substituent's steric and electronic demands and the guest molecules' nature have affected the conformation of individual macrocycles and their propensity to form supramolecular architectures. In the crystal, the formation of a honeycomb-like packing arrangement of the simplest isotrianglimine was promoted by the presence of toluene or para-xylene molecules. A less symmetrical solvent molecule might force this arrangement to change. Polar substituents present in the macrocycle skeleton have enforced the self-association of isotrianglimines in the form of tail-to-tail dimers. These dimers could be further arranged in higher-order structures of the head-to-head type, which were held together by the solvent molecules. Non-associating isotrianglimine formed a container that accommodated acetonitrile molecules in its cavity. The calculated dimerization energies have indicated a strong preference for the formation of tail-to-tail dimers over those of the capsule type.

Use of a Diels–Alder reaction to modify thermal expansion properties in charge-transfer cocrystals

A strategy for modifying thermal expansion properties in dichroic, charge-transfer cocrystals is described.

Inherent Ethyl Acetate Selectivity in a Trianglimine Molecular Solid

Ethyl acetate is an important chemical raw material and solvent. It is also a key volatile organic compound in the brewing industry and a marker for lung cancer. Materials that are highly selective toward ethyl acetate are needed for its separation and detection. Here, we report a trianglimine macrocycle (TAMC) that selectively adsorbs ethyl acetate by forming a solvate. Crystal structure prediction showed this to be the lowest energy solvate structure available. This solvate leaves a metastable, “templated” cavity after solvent removal. Adsorption and breakthrough experiments confirmed that TAMC has adequate adsorption kinetics to separate ethyl acetate from azeotropic mixtures with ethanol, which is a challenging and energy‐intensive industrial separation.

Controlling thermal expansion within mixed cocrystals by tuning molecular motion capability

Thermal expansion behavior is tuned by incorporating motion-capable or -incapable molecules into organic solids.

Controlling thermal expansion (TE) behaviors of organic materials is challenging because several mechanisms can govern TE, such as noncovalent interaction strength and structural motions. Here, we report the first demonstration of tuning TE within organic solids by using a mixed cocrystal approach. The mixed cocrystals contain three unique molecules, two of which are present in variable ratios. These two molecules either lack or exhibit the ability to undergo molecular motion in the solid state. Incorporation of higher ratios of motion-capable molecules results in larger, positive TE along the motion direction. Addition of a motion-incapable molecule affords solids that undergo less TE. Fine-tuned TE behavior was attained by systematically controlling the ratio of motion-capable and -incapable molecules in each solid.

Extraordinary anisotropic thermal expansion in photosalient crystals

Although a plethora of metal complexes have been characterized, those having multifunctional properties are very rare. This article reports three isotypical complexes, namely [Cu(benzoate)L 2], where L = 4-styryl-pyridine (4spy) (1), 2'-fluoro-4-styryl-pyridine (2F-4spy) (2) and 3'-fluoro-4-styryl-pyridine (3F-4spy) (3), which show photosalient behavior (photoinduced crystal mobility) while they undergo [2+2] cyclo-addition. These crystals also exhibit anisotropic thermal expansion when heated from room temperature to 200°C. The overall thermal expansion of the crystals is impressive, with the largest volumetric thermal expansion coefficients for 1, 2 and 3 of 241.8, 233.1 and 285.7 × 10-6 K-1, respectively, values that are comparable to only a handful of other reported materials known to undergo colossal thermal expansion. As a result of the expansion, their single crystals occasionally move by rolling. Altogether, these materials exhibit unusual and hitherto untapped solid-state properties.

Halogen-bonded cocrystallization with phosphorus, arsenic and antimony acceptors

The formation of non-covalent directional interactions, such as hydrogen or halogen bonds, is a central concept of materials design, which hinges on using small compact atoms of the 2nd period, notably nitrogen and oxygen, as acceptors. Heavier atoms are much less prominent in that context, and mostly limited to sulfur. Here, we report the experimental observation and theoretical study of halogen bonds to phosphorus, arsenic and antimony in the solid state. Combining 1,3,5-trifluoro-2,4,6-triiodobenzene with triphenylphosphine, -arsine, and -stibine provides cocrystals based on I···P, I···As and I···Sb halogen bonds. The demonstration that increasingly metallic pnictogens form halogen bonds sufficiently strong to enable cocrystal formation is an advance in supramolecular chemistry which opens up opportunities in materials science, as shown by colossal thermal expansion of the cocrystal involving I···Sb halogen bonds.

Halogen bonding can be exploited for the design of functional supramolecular materials, but heavier elements that are known to accept a halogen bond remain limited. Here, the authors demonstrate the formation of two-component cocrystals based on halogen bonds with phosphorus, arsenic and antimony.

Influence of molecular width on the thermal expansion in solids

It has been shown that the thermal expansion would be higher in a direction along which the molecular width is shorter and it would be smaller if the molecular width is longer along that direction.

Cooling-rate dependent single-crystal-to-single-crystal phase transition in an organic co-crystal

A reversible single-crystal-to-single-crystal phase transition is achieved via slow cooling; flash cooling locks the crystal in a kinetically trapped phase.

Sterically Crowded Trianglimines-Synthesis, Structure, Solid-State Self-Assembly, and Unexpected Chiroptical Properties

The chiral, triangular-shape hexaimine macrocycles (trianglimines), bearing bulky alkynyl or aryl substituents were synthesized and studied by means of experimental and theoretical methods. The macrocyclization reactions are driven by the extraordinary stability of the trianglimine ring and provided products with high yields. Electrostatic repulsion between imine nitrogen atoms and the substituents forced an anti conformation of the aromatic linkers. Although the DFT-optimized structure of 7 is D₃ symmetrical, in the crystal, the macrocycle adopts a bowl-like molecular shape. The macrocycle self-assembles into tail-to-tail dimers by mutual interdigitation of aromatic moieties. In contrast, macrocycle 8 adopts a rigid pillararene-like conformation. The nature of the substituent significantly affects the electronic properties of the linker. As a result, unexpectedly high exciton Cotton effects are observed in the electronic circular dichroism (ECD) spectra. The origin of these effects was subject of an in-depth study.

Thermal expansion along one-dimensional chains and two-dimensional sheets within co-crystals based on halogen or hydrogen bonds

Co-crystals assembled via halogen or hydrogen bonds yield minimal thermal expansion along 1D chains and greater expansion in 2D sheets.

Reversible thermosalience of 4-aminobenzonitrile

Crystals of 4-aminobenzonitrile grown by sublimation undergo a reversible thermosalient phase change during cooling and subsequent heating. Single-crystal diffraction studies have been carried out at 20 K intervals during cooling from 300 to 100 K in order to explain the structural change that occurs.

Consistent supramolecular assembly arising from a mixture of components – self-sorting and solid solutions of chiral oxygenated trianglimines

The formation of trianglimines and their aggregates is stereoselective, and uniformly chiral macrocycles differing in chemical composition crystallize as solid solutions.