10.1002/chem.202302333.

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.

A new class of borate luminophores has been synthesized by a simple two‐step reaction using potassium acyltrifluoroborates (KATs) as starting materials. The hydrazones obtained from reactions between KATs and 2‐hydrazinopyridines followed by a cyclization resulted in the unprecedented formation of C,N‐chelated six‐membered bora‐heterocycles. Under consideration of the results of DFT and TD‐DFT calculations, four luminophores based on such bora‐heterocycles are designed and synthesized, which exhibit a tunable fluorescence range from blue to red in the solid state. Moreover, one of the luminophores exhibits mechanofluorochromism from blue to yellow/green. As a result of the aforementioned mechanochromism of one of these luminophores, white‐color emission was achieved by simply mixing the four luminophores.

Herein we report a crystalline molecular rotor with rotationally modulated triplet emission that displays macroscopic dynamics in the form of crystal moving and/or jumping, also known as salient effects. Molecular rotor 2 with a central 1,4‐diethynyl‐2,3‐difluorophenylene rotator linked to two gold(I) nodes, crystalizes as infinite 1D chains through intermolecular gold(I)–gold(I) interactions. The rotational motion changes the orientation of the central phenylene, changing the electronic communication between adjacent chromophores, and thus the emission intensities. Crystals of 2 showed the large and reversible thermal expansion/compression anisotropy, which accounts for 1) a nonlinear Arrhenius behavior in molecular‐level rotational dynamics, which correlates with 2) changes in emission, and determines 3) the macroscopic crystal motion. A molecular rotor analogue 3 has properties similar to those of 2, suggesting a generalized way to control mechanical properties at molecular and macroscopic scales.

Two gold N-heterocyclic carbene complexes show contrasting luminescent mechanochromism behavior with hypso- and bathochromic spectral shifts. The distinct emission shifts upon mechanical stimulation can be explained by the aggregate-to-monomer transformation and intensified intermolecular interactions, respectively.

Two gold N-heterocyclic carbene complexes show contrasting luminescent mechanochromism behavior with hypso- and bathochromic spectral shifts. The distinct emission shifts upon mechanical stimulation can be explained by the aggregate-to-monomer transformation and intensified intermolecular interactions, respectively.

We theoretically investigated phenyl(phenyl isocyanide) gold(I) (PhNC)Au(Ph) 1 and phenyl(dimethylphenyl isocyanide) gold(I) (dimPhNC)Au(Ph) 2 in crystal using our periodic quantum mechanics/molecular mechanics (QM/MM) method based on the self-consistent point charges to elucidate interesting mechano-chemical changes of absorption and emission spectra of 1 and 2 in crystal. To characterize 1 and 2 in crystal, their absorption and emission spectra in crystal were compared to those in gas phase and CHCl3 solvent, where a three-dimensional reference interaction site model self-consistent field (3D-RISM-SCF) was employed to incorporate solvation effect. To investigate the phosphorescence spectrum in crystal, we optimized the geometry of the molecule at the triplet state in crystal which had ground-state geometry because the population of the excited state is generally very small. The QM/MM calculations showed that 1 formed two polymorphs 1b and 1y, 2 formed two polymorphs 2b and 2g, and 1y was more stable than 1b, which agree with the experimental findings. In 1b and 2b, the ligand-to-ligand charge transfer state is the lowest-energy excited state in the absorption, and the π–π* locally excited state on the PhNC moiety is the lowest-energy triplet state in the emission. In 1y and 2g, on the other hand, the metal–metal-to-ligand charge transfer (MMLCT) state is the lowest-energy excited state in both absorption and emission. These characteristic differences between two crystal structures arise from the geometrical features that the Au–Au distance is much shorter in 1y and 2g than in 1b and 1y and the intermolecular torsion angle η between two Au-PhNC moieties is much smaller in 1y and 2g than in 1b and 2b, respectively; the short Au–Au distance raises the energy level of antibonding orbital consisting of two Au dσ orbitals, and the small η angle lowers the energy level of bonding orbital consisting of two PhNC π* orbitals, leading to the presence of a lower-energy MMLCT state. The QM/MM calculations also disclosed intramolecular torsion angle τ between the Ph and PhNC planes and CH−π interaction of the Ph plane significantly influence absorption spectrum. Based on those computational results, discussion is presented on the differences in absorption and emission spectra among gas, solution, and crystal, the assignments of experimentally observed excitation and emission spectra in crystal, and their energy shifts induced by single-crystal-to-single-crystal phase transition.

Correction: A gold isocyanide complex with a pendant carboxy group: orthogonal molecular arrangements and hypsochromically shifted luminescent mechanochromism
https://pubs.rsc.org/en/content/articlelanding/2018/cc/c8cc90488j

The epistatic double hydrogen bonds that arise from the presence of a pendant carboxy group in a gold isocyanide complex result in strong aurophilic interactions in a magenta-emitting polymorph. This is due to the lack of the typically antiparallel dimer arrangements, which often prevent the formation of strong aurophilic interactions. Furthermore, this polymorph shows hypsochromically shifted luminescence mechanochromism.

Functional organic solids with switchable photoluminescence properties in response to external stimulation have attracted much interest because of their potential applications in sensors and imaging devices. Among these organic solids, luminescent mechanochromic compounds are emerging materials that can switch their photoluminescence properties when mechanically perturbed. Our group has intensively studied the solid-state emission properties and luminescent mechanochromism of a series of aryl gold isocyanide complexes. Here, we prepared bent-shaped binuclear gold isocyanide complex 2, which exhibits different pseudopolymorphs with different emission properties when prepared using different crystallization procedures. X-ray diffraction measurements and NMR spectroscopy indentified inclusion of solvent molecules in the crystalline lattices of 2 except for one polymorph. Complex 2 also shows luminescent mechanochromism based on a crystalline-to-amorphous phase transition. The ground phases of all of the pseudopolymorphs revert to the polymorph without solvent molecules upon thermal treatment.

Luminescence alterations in solid-state materials upon external stimulations have attracted much attention due to their potential for the development of highly functional devices or sensors. We have previously reported the first examples of mechano-induced single-crystal-to-single-crystal (SCSC) phase transitions of gold(I) isocyanide complexes under concomitant emission-color changes. However, the reverse phase transitions of the crystals obtained after mechanical stimulation have not yet been achieved. Herein, a reversible change of the luminescence based on two SCSC phase transitions via mechanical cutting and solvent-vapor adsorption is described. Crystallization of a gold(I) complex that bears CF3 and biaryl moieties from CH2Cl2/MeOH afforded a green-emitting single crystal packed in a polar space group (Pna21). The green-emitting single crystals included MeOH molecules. Upon cutting the crystal under MeOH vapor at 22 °C, the green-emitting single crystal spontaneously changed into a centrosymmetric orange-emitting single crystal (P1̅) under concomitant release of MeOH. Remarkably, the initial green-emitting crystal could be recovered from the orange-emitting crystal by a solvent-induced SCSC transition under saturated MeOH vapor. The combination of two different types of SCSC phase transitions enables the reversible structural and photoluminescent alternations.

Here we present a structural design aimed at the control of phosphorescence emission as the result of changes in molecular rotation in a crystalline material. The proposed strategy includes the use of aurophilic interactions, both as a crystal engineering tool and as a sensitive emission probe, and the use of a dumbbell-shaped architecture intended to create a low packing density region that permits the rotation of a central phenylene. Molecular rotor 1, with a central 1,4-diethynylphenylene rotator linked to two gold(I) triphenylphosphane complexes, was prepared and its structure confirmed by single-crystal X-ray diffraction, which revealed chains mediated by dimeric aurophilic interactions. We showed that green-emitting crystals exhibit reversible luminescent color changes between 298 and 193 K, which correlate with changes in rotational motion determined by variable-temperature solid-state 2H NMR spin–echo experiments. Fast two-fold rotation with a frequency of ca. 4.00 MHz (τ = 0.25 μs) at 298 K becomes essentially static below 193 K as emission steadily changes from green to yellow in this temperature interval. A correlation between phosphorescence lifetimes and rotational frequencies is interpreted in terms of conformational changes arising from rotation of the central phenylene, which causes a change in electronic communication between the gold-linked rotors, as suggested by DFT studies. These results and control experiments with analogue 2, possessing a hindered tetramethylphenylene that is unable to rotate in the crystal, suggest that the molecular rotation can be a useful tool for controlling luminescence in the crystalline state.

The lifetime of the transient phases of luminescent mechanochromic gold isocyanide complexes could be tuned through the modification of the termini of flexible side chains. Upon grinding, these complexes initially showed an emission color change from blue to yellow. The yellow emission spontaneously changed to blue or green, but with different lifetimes of the yellow-emitting transient phases depending on the substituents. Details of the mechano-induced structure changes were also described.

Herein, a novel mechano-responsive luminescent (MRL) material based on crystal-to-crystal phase transitions between crystals of a chiral and those of a centrosymmetric space group, accompanied by a change of emission properties, is described. Initially, a gold complex containing a biphenyl moiety, which exhibits an achiral structure in solution, afforded an orange-emitting amorphous phase together with a viscous isotropic oil after evaporation of the solvent. Upon pricking, the orange-emitting oil spontaneously crystallized either in a centrosymmetric or in a chiral space group while simultaneously changing the emission properties. Remarkably, grinding the chiral crystals induced a solid-state phase transition to the achiral crystals under concomitant changes of the emission properties.

The low-temperature-selective mechanochromism of a thienyl gold(I) isocyanide complex is reported. The as-prepared powder of this complex did not show any luminescent color changes upon grinding at room temperature. When cooled below −50 °C, the powder showed a blue emission in the absence of a phase transition. Upon grinding this powder below −50 °C, a green emission was observed, which indicates notable mechanochromism that only occurs at low temperature.

Upon mechanical stimulation, 9-anthryl gold(I) isocyanide complex 3 exhibited a bathochromic shift of its emission color from the visible to the infrared (IR) region, which is unprecedented in its magnitude. Prior to exposure to the mechanical stimulus, the polymorphs 3α and 3β exhibit emission wavelength maxima (λem,max) at 448 and 710 nm, respectively. Upon grinding, the λem,max of 3αground and 3βground are bathochromically shifted to 900 nm, i.e., Δλem,max (3α) = 452 nm or 1.39 eV. Polymorphs 3α and 3β thus represent the first examples of mechanochromic luminescent materials with λem,max in the IR region.

Luminescent compounds that are sensitive to volatile organic solvents are useful for detection of harmful gases. Although such compounds have been reported, discrimination of various types of volatile organic compounds using one compound remains challenging. We reported a series of gold isocyanide complexes that form various crystalline structures with distinct emission properties, which can be interconverted by mechanical stimulation and solvent addition. Here, we report that introduction of a biphenyl unit into a gold isocyanide scaffold (denoted complex 3) enables discrimination of various volatile organic compounds by forming 11 solvent-containing crystal structures 3/solvent [solvent can be CHCl3, pyridine (Py), CH2Cl2, CH2Br2, dimethylacetamide (DMA), acetaldehyde (AcH), CH3CN, DMF, (S)-propylene oxide (SPO), rac-propylene oxide (racPO), or acetone] with different emission properties (emission maxima of 490–580 nm). Mechanical stimulation of 3/solvent affords amorphous 3ground without solvent inclusion. The resulting 3ground can again detect volatile compounds by forming 3/solvent with concomitant emission color changes. We also afforded a dozen single crystals of 3, which include 11 solvated 3/solvent and one solvent-free 3/none. The molecular arrangements of 3 in 3/solvent and 3/none are all different. Comparison of various crystallographic parameters of 3/solvent and 3/none with their corresponding optical properties indicates that a combination of various structural properties of 3 affects the optical properties of 3. This study reveals that the introduction of a biphenyl moiety could be a useful design to develop versatile indicators for solvents through the formation of multiple luminescent crystal structure.

Mechanoinduced phase transitions of emissive organic crystalline materials have received much attention. Although a variety of such luminescent mechanochromic compounds have been reported, it is challenging to develop mechanochromic compounds with crystal-to-crystal phase transitions in which precise structural information about molecular arrangements can be obtained. Here, we report a screening approach to explore mechanochromic compounds exhibiting a crystal-to-crystal phase transition. We prepared 48 para-substituted (R1) phenyl[para-substituted (R2) phenyl isocyanide]gold(I) complexes designated R1–R2 (six R1 and eight R2 substituents) and then performed three-step screening experiments. The first screening step was selection of emissive complexes under UV light, which gave 37 emissive R1–R2 complexes. The second screening step involved evaluation of the mechanochromic properties by emission spectroscopy. Twenty-eight complexes were found to be mechanochromic. The third screening step involved preparation of single crystals, reprecipitated powders, and ground powders of the 28 mechanochromic R1–R2 complexes. The changes in the powder diffraction patterns of these complexes induced by mechanical stimulation were investigated. Two compounds exhibited a crystal-to-crystal phase transition upon mechanical stimulation, including the previously reported H–H complex. Single crystals of the as-prepared and ground forms of the newly discovered CF3–CN complex were obtained. Density functional theory calculations indicated that the mechanoinduced red-shifted emission of CF3–CN is caused by formation of aurophilic interactions. Comparison of the crystal structures of CF3–CN with those of the other complexes suggests that the weaker intermolecular interactions in the as-prepared form are an important structural factor for the observed mechanoinduced crystal-to-crystal phase transition.

Ball milling of blue-emitting crystal 1B of gold(I) isocyanide complex 1 induces a phase transition to yellow-emitting powder 1YG, which has the same solid structure as the previously reported 1Y obtained upon photoirradiation of 1B. This is the first example of mechanical stimulation inducing a phase change to a photo-accessible crystalline phase.

Photoluminescent materials that exhibit tunable emission properties when subjected to mechanical stimuli have numerous potential applications. Although many organic/inorganic and organometallic compounds display this property, called mechanochromic luminescence, most of these materials undergo a crystalline-to-amorphous (C → A) phase transition; examples of crystalline-to-crystalline (C1 → C2) transformation are rare. Single-crystal X-ray diffraction may allow direct analysis of the molecular packing of mechanochromic luminescence materials before and after C1 → C2 transformation, which may help to understand the underlying mechanism of this transformation. Reported herein is a mechanochromic luminescence material that displays an unprecedented type of C1 → C2 transformation mediated by a transient amorphous phase (C1 → [A] → C2). This mechanochromic luminescence material was developed by introducing soft triethylene glycol side chains in a crystalline gold(I) complex that exhibits mechanochromic luminescence based on a C → A phase transition. When this new gold(I) complex bearing triethylene glycol chains was subjected to a mechanical or thermal stimulus, dynamic phase changes were observed with irreversible luminescence color changes from blue to yellow to green in both the cases. The crystallinity of the mechanically generated C2 phase was lower than that of the thermally generated C2 phase. This is because the mechanically induced C1 → [A] → C2 process was finished within seconds, whereas the thermal C1 → [A] → C2 process occurred over a few minutes. To control the C1 → [A] → C2 transformation, we doped the complex with an inactive soft component. This successfully made the transformation reversible (from green to blue) upon thermal annealing of the mechanically obtained C2 phase. This approach allowed the development of an imaging process involving invisible information storage even under UV illumination.

The mechanochromic compound 1 forms green-emitting crystals (1g) and blue-emitting crystals (1b) that exhibit two intriguing responses to mechanical stimulation. Upon mechanical stimulation, the emission color of 1g does not change but its crystalline structure does. Conversely, 1b shows a clear emission shift, but it retains its molecular arrangement upon grinding.

In this study, we report the interconvertible tetracolored solid state photoluminescence of gold(I) isocyanide complex 2 upon various external stimuli through solid state structural changes. Soaking complex 2 in acetone yields blue emission as a result of the formation of 2B. The subsequent removal of acetone yields 2G through a crystal-to-crystal phase transition, which exhibits green emission. This green-emitting solid 2G exhibits stepwise emission color changes to yellow and then to orange upon mechanical stimulation by ball-milling, which corresponds to the formation of 2Y and 2O, respectively. 2B could be recovered upon the addition of acetone to 2G, 2Y, and 2O. Thus, these four emitting solid states of 2 can be switched between repeatedly by means of acetone soaking and the application of mechanical stimulation. Importantly, single crystal and powder X-ray diffraction (PXRD) studies fully show the detailed molecular arrangements of 2B, 2G, and 2Y. This is the first mechanochromic compound to show interconvertible four color emission in the solid state. We also present the first example of using PXRD measurements and the Rietveld refinement technique for the structural analysis of a ground powder in a luminescence mechanochromism study. We obtained complete molecular-level structural information of the crystalline states of 2B, 2G, 2Y, and 2O. In comparison with a more solvophobic analogue 1, we suggest that the weak interaction of 2 with acetone in the solid state would allow a solvent inclusion/release mode, which is an important structural factor for the unprecedented multicolor mechanochromic luminescence.

We report the first photoinduced single-crystal-to-single-crystal (SCSC) phase transition of a gold complex that involves the shortening of intermolecular aurophilic bonds. This is also the first solid state photochromism of a gold complex. Upon UV irradiation, the blue-emitting crystals of the gold(I) isocyanide complex 1 (1B) transform into the weakly yellow-emitting polymorph 1Y. X-ray diffraction analyses reveal that this phase transition proceeds in an SCSC manner. After phase transition from 1B to 1Y, the intermolecular Au⋯Au separation decreases from 3.5041(14) to 3.2955(6) Å, resulting in a red-shifted emission. The photoinduced shortening of the aurophilic bond in the excited state initiates the change in the crystal structure from 1B to 1Y. Moreover, the crystal 1B showed a photosalient effect: the 1B crystals jump upon irradiation with strong UV light owing to the phase transition into 1Y. The aurophilic bond formation in the crystal generates the mechanical movement of the crystal.

Green and blue polymorphs: A single‐crystal‐to‐single‐crystal (SCSC) phase transition of phenyl(3,5‐dimethylphenyl isocyanide)gold(I) was triggered by mechanical picking or solid seeding and propagated spontaneously with a domino‐like mechanism (see picture). As a result, one phase with intense green emission was transformed to another phase with weaker blue emission.

We report the luminescent color tuning of a new complex, 2‐benzothiophenyl(4‐methoxyphenyl isocyanide)gold(I) (1), by using a new “polymorph doping” approach. The slow crystallization of the complex 1 afforded three different pure polymorphic crystals with blue, green, and orange emission under UV‐light irradiation. The crystal structures of pure polymorphs of 1 were investigated in detail by means of single‐crystal X‐ray analyses. Theoretical calculations based on the single‐crystal structures provided qualitative explanation of the difference in the excited energy‐levels of the three polymorphs of 1. In sharp contrast, the rapid precipitation of 1, with the optimized conditions reproducibly afforded homogeneous powder materials showing solid‐state white‐emission with Commission Internationale de l’Éclairage (CIE) 1931 chromaticity coordinates of (0.33, 0.35), which is similar to pure white. New “polymorphic doping” has been revealed to be critical to this white emission through spectroscopic and X‐ray diffraction analyses. The coexistence of the multiple polymorphs of 1 within the homogeneous powder materials and the ideal mixing of multiple luminescent colors gave its white emission accompanied with energy transfer from the predominant green‐emitting polymorph to the minor orange‐emitting polymorph.

Numerous studies have focused on the mechanical control of solid structures and phase changes in molecular crystals. However, the molecular-level understanding of how macroscopic forces affect the molecules in a solid remains incomplete. Here we report that a small mechanical stimulus or solid seeding can trigger a single-crystal-to-single-crystal transformation from a kinetically isolated polymorph of phenyl(phenyl isocyanide)gold(I) exhibiting blue photoluminescence to a thermodynamically stable polymorph exhibiting yellow emission without the need for heating or solvent. The phase transformation initiates at the location of the mechanical stimulation or seed crystal, extends to adjacent crystals, and can be readily monitored visually by the accompanying photoluminescent colour change from blue to yellow. The transformation was characterized using single crystal X-ray analysis. Our results suggest that the transformation proceeds through self-replication, causing the complex to behave as ‘molecular dominoes’.