A superelastochromic crystal

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.

C hromism 1 has been intensively studied with organic materials to exploit their colorability, transparency, designability, and other advantages in response to various stimuli such as photons, heat, electric charge, vapor, and mechanical stress. Fundamental research has a fascination with chromism because color changes based on wavelength shifts in absorption, emission, or reflection correlate with stimuli-responsive structural changes of materials on the subnanometer to micrometer scale. Chromism is also attractive from a practical viewpoint since the color changes are easily detectable by the naked eye or simple spectroscopy.
The process of color changes induced by pressing, shearing, cutting, and other mechanical forces is called mechanochromism [2][3][4][5][6][7] , which is particularly important because the most fundamental stimuli in nature are commonplace mechanical forces that can be generated without any special apparatus. Mechanochromism based on elastic deformation has been demonstrated in amorphous gel materials [8][9][10] and a few molecular crystals under high pressure (3)(4)(5)(6)(7)(8)(9)(10) 11 to show continuous changes in structural periodicity and color, which has led to quantitative mechanical sensing. Mechanically induced defects can also induce mechanochromism 4 . These systems exhibited spontaneously reversible chromism, but most chromic processes demand another stimulus for reverse chromism, i.e., reversion to the initial color.
As we previously reported about the polymorph-dependent luminescence of organic crystals, the luminescence color depends on the difference in molecular packing in crystals, and no chemical reactions are involved. Information about such systems could prove fruitful for elucidating the crystal structure-property relationship 12,13 . Mechanochromism based on a phase transition, e.g., polymorphic 14 or crystal-to-amorphous 2,3 , is spontaneously irreversible but shows a biphasic color change at a high resolution, which is useful for memory storage and sensing tiny forces.
In this context, the focus of this work is organosuperelasticity, which we first discovered in 2014 15 . Elasticity is a common physical property in the spontaneous shape recoverability of materials. Recently, research on the manner of elastic deformation of organic crystals has been intensive 16-20 despite a general perception of their brittleness. In contrast, superelasticity or more specifically plastic deformation with spontaneous shape recoverability is a minor and unusual physical property, except in special kinds of metallic solids called superelastic alloys and shapememory alloys 21,22 , and research is still in its infancy especially in organic crystals 15,[23][24][25][26][27][28][29][30] . In the elastic deformation, the densitygradient distribution of components causes strain to accumulate, whereas the superelastic deformation can accommodate strain through orientation changes of domains upon phase transition or twinning; thus, superelastic deformation has a potential ability to abruptly change physical properties. Since superelastic materials were developed specifically for metal alloys, their electronic physical properties limited coupling to conductivity and, recently, magnetism 31 . In organosuperelasticity, organic solids even in the single-crystal state show spontaneously reversible mechanically induced phase transitions, indicating the potential functionality of superelasticity with dielectric and optical properties due to the organic materials' characteristics. Very recently, organoferroelasticity-diffusion-less plastic deformation leaving spontaneous strain without spontaneous shape recoverability-in luminescent organometallic crystals was reported 32 . The luminescence is stable and unchanged during the organoferroelastic mechanical twinning, giving no mechanochromism.
Ideally, superelastic solids have optical functionality, and the occurrences and amounts of the phase transitions can be easily controlled in real time as material strain caused by mechanical stress, enabling biphasic color switching with excellent controllability in direction, region, timing, and velocity ( Fig. 1). We find that single crystals of the luminescent organic compound 7-chloro-2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine (7Cl) 13 show superelasticity with luminescent color changes.

Results
Polymorphs and solid-state luminescence of 7Cl. Solid-state luminescence from 7Cl derivatives is based on an excited-state intramolecular proton transfer (ESIPT) (Fig. 2a) [33][34][35][36] , resulting in a large Stokes shift and suppression of aggregation-caused quenching. Note that even small changes in solid-state molecular arrangements can cause a large shift of the emission spectra of 7Cl crystals, due to the formation of an unusual environmentsensitive zwitterionic keto form in the excited state 37 . Most ESIPT-luminescent molecules are emitted from a nonionic keto form via enol-to-keto tautomerization when excited. Actually, two polymorphic 7Cl crystals, YG and YO, showed yellow-green and orange fluorescence, respectively, under ultraviolet (UV) light (365 nm) (Fig. 2b, c).
Mechanical deformation of a YG and YO crystal. Superelastic behaviors were observed in YG crystals sheared under an optical microscope equipped with polarizing plates (Supplementary Movies 1 and 2). Under polarized white (PW) light, the mother (α YG ) domain was converted into a daughter (β) domain in  A singlecolor (absorption or emission) material in the mother (α) phase shows biphasic colors superelastically when force is applied, indicated by a black arrow, along with a polymorphic phase transition from the α phase to a stress-induced daughter (β) phase. association with the deformation of a YG crystal ( Supplementary  Fig. 11). The deformed crystal spontaneously reverted to its initial shape when the β domain contracted after the shearing force was removed. Interestingly, the different fluorescence, yellow-green and orange, exhibited by the α YG and β domains, respectively, under UV light suggests that the β domain is the YO (β YO ) crystal (Fig. 3a). The conversion of the α YG domain into the β YO domain in the superelasticity process was also confirmed by in situ fluorescence spectroscopy during the superelastic behavior ( Supplementary Fig. 3) and by single-crystal X-ray diffraction  Fig. 4, Supplementary Table 3). The superelasticity by conversion of the α YG domain into the β YO domain was quite surprising because the transition between YG and YO polymorphs was a thermally non-inducible monotropic one according to differential scanning calorimetry (DSC) measurements ( Supplementary Fig. 9). In general shape-memory alloys and some organosuperelastic materials, on the other hand, superelasticity is based on thermally reversible enantiotropic phase transition and shows a large temperature dependence ( Supplementary Fig. 10). The mechanofluorochromism of YG crystals has spontaneous reversibility and controllability of fluorochromic regions originating from the characteristics of superelasticity in single crystals. Based on these features, the ability to sense mechanical force across a threshold was demonstrated by in situ fluorescence spectroscopy, during the superelasticity process, measuring changes in the ratio of two solidstate emissions correlating to the abundance ratio of the areas of the α YG and β YO domains ( Supplementary Fig. 3).
In the case of a YO crystal, a daughter (β YG ) domain confirmed by single-crystal X-ray diffraction measurements of a YO crystal in the state with coexisting α and β domains ( Supplementary  Fig. 5, Supplementary Table 4) was generated by shearing the mother (α YO ) domain under both PW and UV light (Supplementary Fig. 11c, d). Here, a small β YG domain was also generated and spontaneously grew, suggesting that YG crystals are thermodynamically more stable than YO crystals at ambient conditions. Such clear and irreversible responses-crystal deformation and emission color change induced by a stimulus, a momentary mechanical force exceeding a threshold-are useful for sensing applications.
Crystallographic studies of as-prepared and mechanically deformed crystals of 7Cl. Molecular movement during the shearinduced phase transition between the two polymorphs was then investigated by X-ray crystallographic study of a superelastically deformed YG crystal (Fig. 3b-d). The interface of the two polymorphic domains was indexed as 1 20 ð Þ αYG //(120) βYO (or (120) αYG // 1 20 ð Þ βYO ), resulting in a calculated bending angle of 42.1°, which agrees well with the measurement by optical microscope observation (ca. 42°). The α YG domain transformed into the β YO domain by a 68°(or 61°and 16°) rotation of 7Cl molecules, which also induced their displacive motions of 2.0 and 1.9 Å along the a-and b-axes, respectively, to optimize the herringbone arrangement at the interface (Fig. 3c-e, Supplementary  Figs. S6-S8). The relatively large molecular rotation in comparison with previous examples of organosuperelastic materialse.g., terephthalamide that rotates ca. 10°-32°during thermally or mechanically induced phase transition-is one possible reason for the monotropic nature of a 7Cl crystal. While anisotropic shear stress can effectively trigger the specific molecular movement required for the crystal transition, thermal activation of molecular rotation may not be practically sufficient to induce phase transition in this case. The emission color difference between the YG and YO crystals can be attributed to a difference in the arrangement of 7Cl molecules; for example, the π overlap of 7Cl molecules is larger in a YO crystal than a YG crystal, leading to red-shifted emission spectrum (Fig. 3d, e, Supplementary Fig. 6).

Mechanical characterization of superelasticity in a YG crystal.
Stress-displacement curves recorded in shear tests revealed the effects of temperature and light on the mechanical properties of YG crystals (Fig. 4, Supplementary Figs. 12-17, Supplementary Table 6 Table 3). The σ f and E S values are ca. 3 and 9 times, respectively, those of phase transition-based organosuperelasticity in a terephthalamide crystal 15 are ca. 22 and 47 times, respectively, which are those of twinning-based organosuperelasticity in a 3,5-difluorobenzoic acid crystal 23 (Supplementary Table 7). More importantly, the σ c values of 1.173 MPa at 0°C (Fig. 4 short-dashed line, Supplementary  Fig. 12) and 1.136 MPa at 100°C (Fig. 4 solid line, Supplementary  Fig. 14) are close to those measured at r.t., demonstrating temperature independence of the superelasticity attributable to a mechanically induced phase transition between monotropic polymorphs, which is different from that between enantiotropic polymophs in the superelasticity of shape-memory alloys (Fig. 4 inset, Supplementary Fig. 17). In addition, the mechanical parameters under UV light almost correspond to those measured under PW light, showing UV light independence of the superelasticity (Supplementary Fig. 16).

Discussion
In conclusion, ESIPT-fluorescent crystals of 7Cl, a pure and simple organic molecule, exhibit spontaneously reversible chromism due to a temperature-independent superelasticity, showing biphasic luminescent color switching with an emission intensity dependent on the domain volume and the sensing of small stress; for example, a YG crystal with an intersectional area of ca. 19 μm 2 can detect an Aphaenogaster famelica (ca. 3 mg). Organosuperelasticity offers the advantage of reversible and strict crystal deformation in association with molecular rearrangement against applied force. Mechanochromism can be induced at any position (s) at any time by superelasticity, or mechanically well-regulated structural transition. These characteristics differentiate superelastochromism from conventional chromisms, e.g., superelastochromism can detect in situ mechanical stress quantitatively: detection of the moment when mechanical stress is applied and removed, and how much work is done on a crystal specimen. The advantage of ESIPT molecules is their strong emissions with a color sensitive to solid-state molecular arrangement. Combining these advantages will open up the science of superelastochromism, which can expand the designability and applicability of chromic materials in the future.  Fig. 18). 13  Microscope observations. An optical microscope (SZ61, Olympus Co.) equipped with polarizing plates and a digital camera was used to record mechanical deformation of crystals using tweezers.

Methods
DSC measurements. DSC measurements of 7Cl crystals were carried out using a DSC-60 (Shimadzu Co.) and DSC 7020 (Hitachi High-Technologies Co.) instruments under a nitrogen gas flow (65 ml min −1 ). Crystals in YG and YO form with the size of a few hundreds of micrometers for the measurements were prepared by slow evaporation of a 7Cl solution in 1:1 mixture of THF and toluene and in ethanol, respectively, at r.t. Experimental conditions are summarized in Supplementary Table 1.
Single-crystal X-ray structural analysis. A mechanically deformed YG (coexisting state of α YG and β YO domains) and YO crystals (coexisting state of α YO and β YG domains) were prepared in addition to as-prepared 7Cl single crystals in YG and YO forms. To avoid spontaneous dissipation of a β YO domain, the mechanically deformed YG crystal was partially cleaved at the α YG //β YO interfaces. Singlecrystal X-ray diffraction measurements of the crystals were performed at 298 K (25°C) with a CMOS detector (Bruker Photon III C14) with a nitrogen-flow temperature controller using a rotating anode X-ray source (MoKα radiation (λ = 0.71073 Å)). Multi-scan absorption corrections were applied using the SADABS program. The structure was solved by intrinsic phasing methods (SHELXT-2014/5) and refined by full-matrix least-squares calculations on F 2 (SHELXL-2016/6). Nonhydrogen atoms were refined anisotropically; hydrogen atoms were fixed at calculated positions by riding model approximation. With respect to the mechanically deformed crystals, X-ray diffraction patterns were obtained around the interface of α YG //β YO and α YO //β YG and they were analyzed as twins. Crystal face indexing was carried out using APEX III Ver.2016.1-0 program package with a twin-resolution program ( Supplementary Figs. 4 and 5).
Force measurements. Shear tests were carried out on a universal testing machine. A crystal fixed on a glass base was sheared by a glass jig and observed under an optical microscope equipped with polarizing plates. Light sources: LA-HDF5010 (90W, HAYASHI-REPIC CO., LTD.) for VIS light and HLV-24UV365-4WPCLTL (365 nm, 0.7A/3.3W, CCS Inc.) connected to PD3-5024-4-EI(A) (CCS Inc.) for UV light. The experimental information and schematic representation of the setup are shown in Supplementary Table 2 and Supplementary Fig. 1, respectively.

Data availability
All the data generated or analyzed during this study are included in this published article (and its Supplementary information files) or available from the authors upon reasonable request. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 1969297-1969298. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.