Introduction

Single-crystal-to-single-crystal (SCSC) transformations in molecular crystals are known to be induced by various external perturbations, such as heat, light, static pressure and exposure to small molecules1,2,3,4,5,6,7,8,9,10,11. The crystal structures can be compared before and after the structure change using single-crystal X-ray diffraction (XRD) analysis, providing a molecular-level understanding of the solid-state reactions and phase transformations12,13. Anisotropic addition of a bulk force, such as shearing, ball-milling or grinding, can also alter the solid structure of the molecular crystals, and numerous studies have suggested that this transformation can include direct crystal-to-crystal conversion14,15,16,17,18,19,20,21. However, anisotropic forces also induce crystal collapse, making them unsuitable for single-crystal X-ray analysis. To date, the observation of a SCSC transformation induced by a mechanical stimulus has not yet been reported.

In this paper, we report the first observation of the SCSC transformation of phenyl(phenyl isocyanide)gold(I) (1), which was triggered by applying a small mechanical force to the crystal surface or by contact with a crystal seed of the opposite polymorph. The phase change first occurred at the initial contact area and subsequently progressed spontaneously throughout the entire crystal. Single crystal X-ray analysis was performed on the resulting crystals, providing molecular-level information on the phase transformation. Distinctive structural changes were observed, including a transition from CH/π to aurophilic intermolecular interactions23,24,25. The phase change was accompanied by a drastic luminescence colour change caused by the switching of the intermolecular interactions. This colour change allowed the progression of the phase change to be clearly visualized.

Results

Synthesis of 1 and its polymorph Ib

During the course of our studies on the luminescence mechanochromism of gold(I) complexes14, we synthesized 1 in high yield (89%) through the reaction of chloro(phenyl isocyanide)gold(I) with phenyllithium for 1 h at −78 °C (Fig. 1a). Careful but rapid crystallization from a hexane/dichloromethane solution produced crystals of 1 in the Ib phase. Blue photoluminescence was observed from Ib crystals under ultraviolet irradiation with emission maxima at 460 and 490 nm (quantum yield Φem=0.15, lifetime: τ1=1.47 μs (A1=0.92), τ2=72.1 μs (A1=0.08), λex=355 nm) (Fig. 1b, blue solid line; Fig. 2a). The excitation spectrum of Ib displayed a peak at 315 nm (Fig. 1b, blue dotted line). The crystal structure of Ib was determined by the XRD of a single crystal (Fig. 2a–d; Supplementary Fig. S1; Supplementary Tables S1 and 2). Ib was found to be triclinic (P-1, a=6.0214(5) Å, b=9.0729(8) Å, c=11.4498(10) Å, α=102.159(7)°, β=101.468(7)°, γ=102.358(7)° at 123 K, Z=2, R1=3.3%, wR2= 8.2%, goodness of fit (GOF)=1.056, calculated density: 2.169 g cm−3). X-ray analysis, elemental analysis, thermal gravimetric analysis and 1H NMR measurements of Ib indicated that there was no solvent inclusion in the crystals (Supplementary Figs S2 and S3; Supplementary Table S3; Supplementary Note 1). Each of the molecules in Ib formed a herringbone-like structure with a head-to-tail arrangement of the phenyl ligand on the gold atoms, and the isocyanide phenyl moiety twisted with a dihedral angle of 71.6° (Fig. 2b)22. There were large distances (>4.65 Å) between the gold atoms of adjacent molecules, indicating the absence of aurophilic interactions, which typically possess Au···Au distances of <3.5 Å in solid-state structures23,24. Time-dependent density functional theory calculations were performed based on the crystal structure of Ib, affording insight into its excitation by metal-perturbed ligand-to-ligand charge transfer (Supplementary Figs S4–S8; Supplementary Tables S4–S6).

Figure 1: Structural and optical properties of 1.
figure 1

(a) Synthesis, crystallization and phase conversions of 1. (b) Emission and excitation spectra, displayed as solid and dotted lines, respectively. Blue: Ib (excited at 315 nm for the emission spectrum, observed at 460 nm for the excitation spectrum), red: IIy (excited at 432 nm, observed at 565 nm), black: IIy(ground Ib) (excited at 385 nm, observed at 565 nm). (c) Powder diffraction of IIy(ground Ib) (black line), and simulated diffraction patterns for IIy (red line) and Ib (blue line).

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Figure 2: Photographs and crystal structures of polymorphs Ib and IIy.
figure 2

(a) Photographs of polymorph Ib under ambient and ultraviolet (UV, 365 nm) light. (b) Oak Ridge Thermal Ellipsoid Program (ORTEP) drawing (50% probability level) of the crystal structure of Ib. (c) Crystal structure of Ib depicted as stick and space filling models viewed along the c axis. (d) Crystal structure of Ib viewed along the a axis. (e) Photographs of the polymorph IIy under ambient and UV (365 nm) light. (f) ORTEP drawing (50% probability level) of the crystal structure of IIy. (g,h) Crystal structures of IIy.

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Solution phase preparation of polymorph IIy

The slow crystallization of 1 from a hexane/dichloromethane solution produced the polymorphic crystals IIy, which exhibited a significantly different crystal structure compared with that of Ib. The IIy crystals exhibited a strong yellow emission (λmax=571 nm) under ultraviolet irradiation (365 nm) (Fig. 1b, red solid line and Fig. 2e). A higher emission quantum yield (Φem=0.43) and shorter emission lifetime (τ1=0.17 μs (A1=0.20), τ2=0.65 μs (A2=0.80), λex=355 nm) were observed compared with those for Ib. The excitation spectrum of IIy displayed a peak at a longer wavelength (432 nm) than that of Ib (Fig. 1b, red dotted line). The crystal structure of IIy was in the tetragonal system with a space group of I-42d (Fig. 2f–h, a=13.4781(4) Å, c=24.8079(8) Å at 123 K, Z=16, R1=2.5%, wR2=5.4%, GOF=1.093, calculated density: 2.224 g cm−3, Supplementary Fig. S9; Supplementary Tables S1 and S2). X-ray analysis, elemental analysis and 1H NMR measurements of IIy indicated that there was no solvent inclusion in the crystals (Supplementary Fig. S2; Supplementary Table S3; Supplementary Note 1). The molecules of IIy adopted a nearly flat conformation with a dihedral angle between the isocyanide phenyl group and the gold-coordinated phenyl group of 3.1°. Oblique head-to-tail dimers contained a short Au···Au distance of 3.177 Å, suggesting the presence of significant aurophilic interactions23,24. Each dimer interacted with adjacent dimers through isocyanide carbons separated by just 3.186 Å to form 1-D chain-like structures comprising dimer units (Supplementary Fig. S9). Time-dependent density functional theory calculations of a model structure based on the IIy crystals indicated a metal–metal to ligand charge transfer characteristic, which was facilitated by the short Au···Au distance (Supplementary Figs S4, S10 and S11; Supplementary Tables S4 and S7)23,24.

Luminescent mechanochromism of Ib

The blue luminescent Ib displayed a significant luminescence mechanochromism (Fig. 3a)14,15,16,17. The ball-milling of Ib for 3 min afforded the powder IIy(ground Ib), which was identical to crystalline IIy in terms of its photoluminescence and solid-state structure. Similar to IIy, IIy(ground Ib) exhibited a luminescence maximum at 567 nm, an emission quantum yield of 0.38, and emission lifetimes of τ1=0.27 μs (A1=0.34) and τ2=0.68 μs (A1=0.66) at an excitation wavelength of 355 nm (Fig. 2b, black solid line) with an excitation peak at 400nm (Fig. 2b, black dotted line). The powder XRD pattern of the ball-milled sample (IIy(ground Ib), Fig. 1c, black solid line) matched the simulated diffraction pattern based on the single crystal X-ray analysis of IIy (Fig. 1c, red solid line), and no trace of Ib was found in the ball-milled sample (Fig. 1c, blue solid line). These results indicate that the ball-milling process induced direct crystal-to-crystal transformation of Ib to IIy.

Figure 3: Photographs of the phase transformation of Ib to IIy.
figure 3

(a) Luminescence mechanochromism of Ib. Yellow luminescent IIy(ground Ib) was generated by grinding the blue luminescent Ib powder with a spatula. (b) Mechanical stimulus-triggered phase transformation (scale bar, 200 μm). A small pit (left, white arrow) was first produced by pricking the fixed sample with a needle. The phase transformation gradually spread over the entire crystal after 9 h. (c) Solid seeding-triggered phase transformation (scale bar, 200 μm). The phase transformation of Ib initiated from the area in contact with the seed crystal of IIy (upper left photograph) and spread over the entire crystal after 15 h (lower right photograph). (d) Progression of the phase transformation over the crystals (scale bar, 200 μm). Partial decomposition of the IIy phase was observed (white triangles). (e) Progression of phase transformation from the seed crystals (white triangles; scale bar, 200 μm). (f) Crystal structure of IIyscsc. These mechanical- and seeding-triggered phase transformation experiments could be replicated many times; however, the conversion rate varied considerably from crystal to crystal.

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SCSC transformation

The ball-milling described above involves a considerable mechanical stimulus; we next conducted the mechanical triggering experiment with a much smaller stimulus. A small pit was formed on the surface of an Ib crystal using a needle under atmospheric conditions (Fig. 3b). A yellow luminescent spot was initially observed at the location of the small pit. Subsequently, the domain exhibiting yellow emission gradually increased to nearly the entire crystal after 9 h at room temperature. The transformation could also be triggered by contacting a seed crystal of IIy with a crystal of Ib (Fig. 3c). The luminescent colour change initiated at the point of contact and extended to the entire crystal after 15 h. This colour change propagated through the contacting surfaces of adjacent crystals; eventually, the blue luminescent crystals that were in contact with neighbouring yellow luminescent crystals became yellow, maintaining their transparency (Fig. 3d, Supplementary Fig. 12). This solid-seeding phase change represents a self-replicating progression of phase transformation. It should be noted that the rate of conversion was highly variable: some crystals changed rapidly, whereas others were slower.

Single crystal X-ray analysis was performed after the mechanical stimulus-triggered SCSC transformation of Ib. A yellow luminescent crystal was prepared by the mechanical pricking of a small region of one Ib crystal. After the initial contact area was removed to avoid affecting the quality of the XRD data, the yellow emissive crystal (denoted IIyscsc) was subjected to single crystal X-ray analysis. The crystal structure of IIyscsc obtained via SCSC was identical to that of IIy crystallized from the solution phase (Fig. 3f; Supplementary Fig. S13; Supplementary Tables S1 and S2; IIyscsc: a=13.4075(19) Å, c=24.9673(19) Å at 123 K, Z=16, R1=8.8%, wR2=21.7%, GOF=1.316, calculated density: 2.233 g cm−3).

Discussion

The mechanical- or solid seeding-triggered phase change described in this study occurred at room temperature, whereas a thermal phase transformation of Ib to IIy required elevated temperatures of 64.5–74.9 °C (Supplementary Fig. S3). No reverse phase change was observed when the sample was cooled. The Ib crystal could only be regenerated by the dissolution and recrystallization of IIy. The thermal analysis suggested that Ib was a metastable, kinetically isolated form of 1, and IIy was the thermodynamically favoured form (Supplementary Fig. S14). The thermodynamic analysis and the sample crystallization process revealed that 1 is a good example of the Ostwald rule: the less stable polymorphs (Ib) crystallize first, and the more stable polymorphs (IIy) form later1,2,3. In some reports, mechanical pricking or the presence of defects can accelerate the phase change near the thermal phase change temperature, resulting in polycrystalline formation1,3,13,25. However, the phase transformation of Ib occurred at much lower temperatures, suggesting that the transformation observed in our study was triggered purely by mechanical stimulation. To our knowledge, this is the first clear example of the mechanical- or solid seeding-trigged SCSC transformation of molecular crystals. In addition, the emergence of metallophilic interactions after the SCSC transformation of mother crystals lacking metallophilic interactions has not been reported previously.

The solid-state structures of Ib and IIy varied greatly (triclinic P-1 and tetragonal I-42d) (see Supplementary Fig. S15). Thus, the significant structural transformation cannot be explained by a simple conformational change or molecular reorientation. The phase change may proceed by a similar mechanism to the epitaxial mechanism proposed for the thermal phase transformation3,25. A small portion of the IIy daughter phase forms initially in the Ib mother phase through mechanical stress or contact with a seed crystal of IIy (Fig. 4a). The molecules in the thermodynamically unstable Ib phase diffuse across the narrow gap between phases Ib and IIy, and rearrange to the IIy phase through the formation of intermolecular aurophilic interactions (Fig. 4c).

Figure 4: Proposed mechanism for the mechanical stimulus-triggered phase transformation.
figure 4

(a) Mechanical stimulation of the metastable Ib phase (blue rectangles). (b) Generation of the IIy phase. (c,d) The thermodynamically stable IIy phase extends by absorbing molecules from the metastable Ib phase.

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This phase transformation resembles dominoes, in that a mechanical stimulus (pushing) triggers a state change in the entire assembly. The self-replicative nature of this molecular domino may provide a new way to control nano or molecular structures through macroscopic mechanical stimulation. The domino-like amplification of the optical response to the small mechanical stimulus (from the small pit to the entire crystal) can be applied to the highly sensitive detection of mechanical stimulation.

Methods

Synthesis of 1

Chloro(phenyl isocyanide)gold(I) was prepared from chloro(tetrahydrothiophene)gold(I) and phenyl isocyanide using a method similar to that reported previously26. Chloro(phenyl isocyanide)gold(I) (100.7 mg, 0.30 mmol) and a magnetic stir bar were placed in a vial and sealed with a rubber septum. After the reaction vial was connected to a vacuum manifold through a needle, it was evacuated and then back filled with nitrogen. Dry THF (0.44 ml) was added via a syringe to dissolve the chloro(phenyl isocyanide)gold(I). A butyl ether solution of phenyllithium (0.164 ml, 1.83 M, 0.30 mmol) was added at −78 °C under stirring. After 30 min, the reaction was quenched by the addition of a saturated aqueous solution of NH4Cl (2 ml), extracted three times with CH2Cl2, washed with brine and then dried over MgSO4. After filtration, the volatiles were removed under reduced pressure. Crude phenyl(phenyl isocyanide)gold(I) (1) was obtained as a pale purple solid (106.7 mg, 89%). The yield was determined by 1H NMR spectroscopy using Cl2CHCHCl2 as an internal standard. 1 was recrystallized from CH2Cl2/hexane after filtration to remove an insoluble purple solid. 1 was fairly stable in air at room temperature, but readily decomposed at elevated temperatures (>80 °C), with metal deposition occurring on its surface. In solution, 1 remained intact in the dark for a short period (<1 h) but readily decomposed under light. The data for 1H and 13C NMR and HRMS were included in the Supplementary Methods.

Crystal and powder sample preparation (Ib, IIy)

Typical preparation procedures are as follows. On the surface of a solution of 1 (prepared from Ib or IIy, 20 mg) in CH2Cl2 (1.0 ml)/hexane (2.0 ml) in a small glass vial, a stream of nitrogen gas was flowed through a needle connected to a nitrogen line for a few minutes to produce crystals of Ib. IIy crystals were obtained by the slow crystallization (typically 10 h) of a solution of 1 in CH2Cl2/hexane placed in a refrigerator. The ground sample of IIy(ground Ib) was prepared with a ball mill (Taitec Bead Crusher μT-01, Koshigaya, Saitama, Japan). Typically, Ib (50 mg) was treated in a micro tube (φ13 × 49 mm) with a screw cap at a shaking speed of 4,600 r min−1 for 3 min (60 s three times) using a stainless bead (1/8 inch).

Single crystal and powder X-ray analysis

Measurements for Ib, IIy and IIyscsc were made on a Rigaku R-AXIS RAPID diffractometer using graphite monochromatic Mo-Kα radiation. The diffraction data were collected at −150 °C. The structure was solved by direct methods and expanded using Fourier techniques. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. All calculations were performed using the CrystalStructure crystallographic software package, except for refinement, which was performed using SHELXL-97. The powder diffraction data for IIy(ground Ib) were recorded at room temperature on a Rigaku SmartLab diffractometer with Cu-Kα radiation and a D/teX Ultra detector covering 5–50° (2θ). The diffraction data were collected at 20 °C. The powder simulation patterns for Ib and IIy were generated from the single crystal X-ray structures collected at 20 °C. The powder patterns for Ib and IIy displayed in Fig. 1b were generated using Mercury 3.0.

Additional information

Accession codes: The X-ray crystallographic coordinates for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers CCDC 897541, 897542 and 909344. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

How to cite this article: Ito, H. et al. Mechanical stimulation and solid seeding trigger single-crystal-to-single-crystal molecular domino transformations. Nat. Commun. 4:2009 doi: 10.1038/ncomms3009 (2013).