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The ability to make large changes in properties against small external stimuli is one of the key factors in sensing materials. Molecular domino transformation, i.e., polymorphic transformation starting at a stimulated point and extending to the whole crystal, is an attractive phenomenon from this viewpoint. We recently found such a transformation for the first time in an organic crystal of 4-nitro-N-salicylideneaniline as one of the Schiff bases. In this study, quantitative evaluations were conducted on the mechanical stimulus and emission properties in the transformation of the crystal. Our results demonstrate the potential applicability of the crystal to the detection of even less than a few μN mechanical stimuli as an emission color change. A molecular level transformation mechanism revealed by microcrystal electron diffraction also contributes to the future development of the transformation-based materials. [ABSTRACT FROM AUTHOR]

Molecular crystals of Pt(II) complexes with metallophilic interactions can provide bright assembly-induced luminescence with colour tunability. However, the brittleness of many of these crystals makes their application in flexible optical materials difficult. Herein, we have achieved the elastic deformation of crystals of polyhalogenated Pt(II) complexes exhibiting bright assembly-induced luminescence. A crystal of [Pt(bpic)(dFppy)] (Hbpic = 5-bromopicolinic acid, HdFppy = 2-(2,4-difluorophenyl)pyridine) and a co-crystal of [Pt(bpic)(dFppy)] and [Pt(bpic)(ppy)] (Hppy = 2-phenylpyridine) were found to exhibit significant elastic deformation due to their highly anisotropic interaction topologies. While the crystal of [Pt(bpic)(dFppy)] exhibited monomer-based ligand-centred 3ππ* emission with an emission quantum yield of 0.40, the co-crystal exhibited bright, triplet metal–metal-to-ligand charge transfer (3MMLCT) emission owing to Pt⋯Pt interactions, thereby achieving a significantly higher emission quantum yield of 0.94. [ABSTRACT FROM AUTHOR]

Solid-state grinding can provide "mechano-distinctive" co-crystals that are not accessible from solutions. Herein, we demonstrate the structure determination of a powdered mechano-distinctive co-crystal of 2-aminopyrimidine and succinic acid in a 2 : 1 molar ratio using microcrystal electron diffraction. [ABSTRACT FROM AUTHOR]

Mechanical twinning, which enables crystalline solids to deform reversibly beyond their elastic limit, has been getting much attention in recent progressive studies on flexible organic crystals. In addition to the deformability, organic single crystals can obtain new functionalities or directions of their functions with anisotropy can be controlled based on mechanical twinning. Owing to these features, mechanical twinning will be one of the key mechanisms in developing the next-generation flexible organic crystalline materials. This highlight describes recent advances in mechanical twinning in organic crystals to help clarify the potential and challenges faced for future developments. For this purpose, the areas of focus are the molecular mechanisms and functions in addition to qualitative and quantitative aspects of the deformability. [ABSTRACT FROM AUTHOR]

A unique mechanical property of superelasticity is observed not only in so-called shape-memory alloys but also in molecular crystals. In contrast, superelasticity can coexist with various attractive functions derived from molecules in the latter. Herein we demonstrate a photoluminescent organosuperelastic crystal of a pure organic compound: 7-amino-4-methylcoumarin. UV light irradiation causes a negligible effect on a stress–strain curve with a typical superelastic hysteresis loop in the crystal. The emission color of yellow-green is visually unchanged during the superelastic deformation because of the twinning nature. Superelasticity coexisting with photoluminescence properties is unprecedented in the conventional SMAs and indicates promising practical applications of molecular crystals as the next generation superelastic materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A chiral crystallization mechanism is proposed for crystals of achiral ammonium carboxylates on the basis of non-parallel face contacts between alkyl and naphthalene groups. [ABSTRACT FROM AUTHOR]

Molecular crystals of Pt(II) complexes with metallophilic interactions can provide bright assembly-induced luminescence with colour tunability. However, the brittleness of many of these crystals makes their application in flexible optical materials difficult. Herein, we have achieved the elastic deformation of crystals of polyhalogenated Pt(II) complexes exhibiting bright assembly-induced luminescence. A crystal of [Pt(bpic)(dFppy)] (Hbpic = 5-bromopicolinic acid, HdFppy = 2-(2,4-difluorophenyl)pyridine) and a co-crystal of [Pt(bpic)(dFppy)] and [Pt(bpic)(ppy)] (Hppy = 2-phenylpyridine) were found to exhibit significant elastic deformation due to their highly anisotropic interaction topologies. While the crystal of [Pt(bpic)(dFppy)] exhibited monomer-based ligand-centred 3 ππ* emission with an emission quantum yield of 0.40, the co-crystal exhibited bright, triplet metal-metal-to-ligand charge transfer ( 3 MMLCT) emission owing to Pt⋯Pt interactions, thereby achieving a significantly higher emission quantum yield of 0.94.

Shape-memory materials can be mechanically deformed and subsequently reverse the deformation upon changing the temperature. Shape-memory materials have attracted considerable attention for basic research and industrial applications, and polymer and alloy shape-memory materials have been well studied; however, it is formidably challenging to develop functional shape-memory materials, such as materials with multi-stage and anisotropic shape changes and shape changes accompanied by changes in color and light emission. Here, we found a reversible multi-stage shape-changing effect after mechanical deformation in a molecular crystal induced by multi-step thermal phase transitions with reversible shape changes and luminescence-color changes. Using single-crystal structure and thermal analyses as well as mechanical property measurements, we found that the reversible multi-stage shape-changing effect was achieved by a combination of a twinning deformation and multi-step thermal phase transitions. The changes in the crystal shape and luminescence suggest novel strategies for imparting known shape-memory materials with additional functionalities., Competing Interests: The authors declare no competing financial interests., (This journal is © The Royal Society of Chemistry.)

Molecular assemblies based on helical motifs are of substantial interest from the view point of fundamental science as well as application. In this study, we propose a new class of organic crystal, that is, heteroH-MOC (multi-component organic crystal containing different kinds of helical motifs consisted of different components), and describe successful construction of heteroH-MOCs with P2(1) and P2(1)2(1)2(1) space groups by using steroidal bile acids and cinchona alkaloids. In the P2(1) crystals, two kinds of helices composed of the steroid and alkaloid are arranged in a parallel fashion, while, in the P2(1)2(1)2(1) crystals, those are in a perpendicular fashion. It is remarkable that, in such systems, particularly in the latter crystals, components ingeniously achieved highly-ordered synchronization of periodicity (helical pitches r and periodic distances in the array of helices p), which is first demonstrated in this study through hierarchical interpretation of the crystal structures.

A universal method to determine handedness of 2(1) helical assemblies composed of planar aromatic molecules is proposed and demonstrated by taking P2(1)/c and Pbca crystals of benzene, the simplest aromatic molecule, as examples., (This journal is © The Royal Society of Chemistry 2012)