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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]

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]

Ferroelasticity has been reported for several types of molecular crystals, which show mechanical‐stress‐induced shape change under twinning and/or spontaneous formation of strain. Aiming to create materials that exhibit both ferroelasticity and light‐emission characteristics, we discovered the first examples of ferroelastic luminescent organometallic crystals. Crystals of arylgold(I)(N‐heterocyclic carbene)(NHC) complexes bend upon exposure to anisotropic mechanical stress. X‐ray diffraction analyses and stress‐strain measurements on these ferroelastic crystals confirmed typical ferroelastic behavior, mechanical twinning, and the spontaneous build‐up of strain. A comparison with single‐crystal structures of related gold‐NHC complexes that do not show ferroelasticity shed light on the structural origins of the ferroelastic behavior. [ABSTRACT FROM AUTHOR]

Chromism—color changes by external stimuli—has been intensively studied to develop smart materials because of easily detectability of the stimuli by eye or common spectroscopy as color changes. Luminescent chromism has particularly attracted research interest because of its high sensitivity. The color changes typically proceed in a one-way, two-state cycle, i.e. a stimulus-induced state will restore the initial state by another stimuli. Chromic systems showing instant, biphasic color switching and spontaneous reversibility will have wider practical applicability. Here we report luminescent chromism having such characteristics shown by mechanically controllable phase transitions in a luminescent organosuperelastic crystal. In mechanochromic luminescence, superelasticity—diffusion-less plastic deformation with spontaneous shape recoverability—enables real-time, reversible, and stepless control of the abundance ratio of biphasic color emissions via a single-crystal-to-single-crystal transformation by controlling a single stimulus, force stress. The unique chromic system, referred to as superelastochromism, holds potential for realizing informative molecule-based mechanical sensing. Color changes by external stimuli, so-called chromism, have been intensively studied to develop smart materials due to the stimulus-responsiveness of chromic materials. Here the authors demonstrate luminescent chromism during a mechanically controllable phase transition in a luminescent organosuperelastic crystal. [ABSTRACT FROM AUTHOR]

Mechanical twinning changes atomic, molecular, and crystal orientations along with directions of the anisotropic properties of the crystalline materials while maintaining single crystallinity in each domain. However, such deformability has been less studied in brittle organic crystals despite their remarkable anisotropic functions. Herein we demonstrate a direction‐dependent mechanical twinning that shows superelasticity in one direction and ferroelasticity in two other directions in a single crystal of 1,3‐bis(4‐methoxyphenyl)urea. The crystal can undergo stepwise twinning and ferroelastically forms various shapes with multiple domains oriented in different directions, thereby affording a crystal that shows superelasticity in multiple directions. This adaptability and shape recoverability in a ferroelastic and superelastic single crystal under ambient conditions are of great importance in future applications of organic crystals as mechanical materials, such as in soft robotics. [ABSTRACT FROM AUTHOR]

As altering permanent shapes without loss of material function is of practical importance for material molding, especially for elastic materials, shape‐rememorization ability would enhance the utility of elastic crystalline materials. Since diffusionless plastic deformability can preserve the crystallinity of materials, the interconversion of diffusionless mechanical deformability between superelasticity and ferroelasticity could enable shape rememorization of superelastic single crystals. This study demonstrates the shape rememorization of an organosuperelastic single crystal of 1,4‐dicyanobenzene through time‐reversible interconversion of superelasticity–ferroelasticity relaxation by holding the mechanically twinned crystal without heating. The shape‐rememorization ability of the organosuperelastic crystal indicates the compatibility of superelasticity (antiferroelasticity) and ferroelasticity as well as the intrinsic workability of organic crystalline materials capable of recovering their crystal functions under mild conditions. [ABSTRACT FROM AUTHOR]

Quinine, one of Cinchona alkaloids, has been of great interest from medical, synthetic, and supramolecular viewpoints. However, unaccountably, the guest-free (GF) crystal of quinine containing no solvent or other molecules has not been reported for nearly three decades, although GF crystals of other Cinchona alkaloids, such as quinidine, cinchonidine, and cinchonine, are already known. In this study, we successfully revealed the crystal structure of quinine, which belongs to the P21 space group with the cell parameters of a=6.0587(1), b=19.2492(5), c=22.2824(7) Å, β=92.1646(11)°, and V=2596.83(12) Å3. Interestingly, the crystal has three crystallographically independent molecules in the cell ( Z′=3) that are connected through a N(quinoline)⋅⋅⋅HO hydrogen bond to form a pseudo three-two-fold (32) double-helical motif. The helical motif is completely different from those observed in GF crystals of other Cinchona alkaloids. Hierarchical comparison on the crystal structures of a series of Cinchona alkaloids including quinine clearly demonstrated that only small structural differences of a molecule, particularly the position of the vinyl group, cause a significant variety of assembly manner in the crystalline state. There have been no reports systematically demonstrating such steric effect in crystals of Cinchona alkaloids, and, therefore, the present system contributes to the design of desired functional crystal structures. [ABSTRACT FROM AUTHOR]