This highlight gives an overview of recent advances in mechanically flexible molecular crystals, with qualitative and quantitative studies performed on different molecular systems. The diverse methods for the tuning of mechanical properties via crystal engineering have gained immense attention in the field of materials science. Such studies render support in establishing the structure to properties to function parallels. The understanding of intermolecular interactions helps in a systematic placement of different molecular crystals in the elastic-plastic spectrum. This overview helps in emphasizing the potential as well as challenges faced in predicting/designing mechanically compliant crystalline compounds.

Potential and challenges of engineering mechanically flexible molecular crystals

Stimuli-responsive crystals have aroused tremendous interest due to their potential as smart materials. However, endowing mono-component crystals with multiple stimuli-responsive and multifunction properties is still a great challenge. Here, we report an anthracene derivative, 9-anthraldehyde, which can respond to mechanical force and light. It possesses two-dimensional (2D) elastic bending, plastic twisting and photoinduced bending properties. Unusual elastic bending of the plastic twisted crystal was achieved in one crystal. The crystal obtained in this work is a multi-dimensional elastic crystal, which is different from reported twistable materials. To reveal the mechanisms of the properties of 2D elastic bending, plastic twisting and photoinduced bending, nanoindentation tests, energy framework analysis and Hirshfeld surface analysis were used to study the property–structure relationship at the molecular level. It was found that these properties can be ascribed to the unique intermolecular interactions and types of molecular arrangement.

Multiple Stimuli-Responsive Flexible Crystal with 2D Elastic Bending, Plastic Twisting and Photoinduced Bending Capabilities

Single crystals showing elastic flexibility have attracted increasing attention as next-generation soft materials. Herein, we successfully developed a series of polyhalogenated platinum(II)–bipyridine complexes showing solvent-of-crystallisation-dependent elastic flexibility. Notably, methanol-solvated crystals were the first examples of flexible crystals showing assembly-induced luminescence arising from the triplet metal–metal-to-ligand charge transfer (3MMLCT) excited state. Furthermore, factors inducing elastic deformability based on the packing structures of the crystals were explored.

Elastic deformability and luminescence of crystals of polyhalogenated platinum(II)–bipyridine complexes

Crystals of a family of six one-dimensional (1D) coordination polymers of cadmium(II) with cyanopyridines [[CdX2L2]n, where X = Cl, Br, or I and L = 3-cyanopyridine (3-CNpy) or 4-cyanopyridine (4-CNpy)] presented a variety of morphologies and mechanical responses with dominant two-dimensional (2D) anisotropic flexibility, which has not been previously reported. All mechanically adaptable crystals were 2D flexible and displayed a variety of direction-dependent responses; in addition to 2D isotropic flexibility observed for solely elastic materials, 2D anisotropic flexibility was noticed for both elastic and elastic → plastic crystals. The consequences of fine and controlled structural variations on mechanical behavior were additionally explored via microfocus single-crystal X-ray diffraction and complementary theoretical studies, revealing that the relative strength and direction of the hydrogen bonding interactions were the key parameters in delivering a specific mechanical response.

https://pubs.acs.org/doi/pdf/10.1021/acs.chemmater.2c00062

Two-Dimensional Anisotropic Flexibility of Mechanically Responsive Crystalline Cadmium(II) Coordination Polymers

In this communication, an organic crystal of the polar space group Pc that is capable of plastic bending in response to external mechanical stress is reported, and its high dielectric constant and strong second-order harmonic generation (SHG) effect have been demonstrated.

A plastically bendable and polar organic crystal

The generation of elastic crystalline fibers from a nonfibrous crystal of metal complex is demonstrated. Applying mechanical stimuli to a platelike crystal of NiII(salophen) [1; H2salophen = N,N′-b...

Akira Sugimoto

Papers

Elastic Crystalline Fibers Composed of a Nickel(II) Complex.

A plastically bendable and polar organic crystal