Interconvertible multiple photoluminescence color of a gold(i) isocyanide complex in the solid state: solvent-induced blue-shifted and mechano-responsive red-shifted photoluminescence

Triple-chromic (photo-, thermo-, and mechano-chromic) metal complexes containing N-salicylideneaminopyridine ligands

N-Salicylideneaminopyridine (SAP) is a well-known organic chromic compound that shows a reversible colour change upon UV light irradiation (photochromism) and upon cooling (thermochromism). Herein, we report novel multi-chromic metal complexes containing SAP derivatives as ligands, viz. [Ni(NCS)2(3,5-t-Bu-SAP)4] (Ni1) and [Co(NCS)2(3,5-t-Bu-SAP)4] (Co1). The Ni1 crystals exhibited both photo- and thermochromism with new colour variations, which were due to the light absorption of the Ni(II) ions and chromic properties of the SAP ligands. The Co1 crystal also exhibited photo- and thermochromism originating from the SAP ligands. Furthermore, the Co1 crystal exhibited mechanochromism induced by grinding with a mortar, which was considered to be attributable to the change of the Co1 coordination structure. Such a triple-chromic material is rare and very fascinating for the applications of multiple sensors, memory devices, and functional inks.

Phosphorescent Isocyanide-Metal-Carboranyl Complexes of Copper(I) and Gold(I): Synthesis and Radioluminescence

A series of (L)MX (L=isocyanide; M=Cu(I) or Au(I); X=chloride or carboranyl) complexes have been prepared. The first examples of isocyanide Cu(I) chloride complexes with 1 : 1 stoichiometry between isocyanide and CuCl are reported and structurally characterised. The photophysical properties of (RNC)AuCl and (RNC)Au(η1-carboranyl) complexes have been investigated. Complexes (RNC)Au(η1-carboranyl) show no aurophilic Au-Au interactions and emit long-lived deep-blue phosphorescence up to 377 μs. The radioluminescence of these gold complexes was studied to show a superior radioluminescence stability for the gold(carboranyl) complexes compared to gold chloride materials under constant high energy X-ray radiation of 350 keV.

Fluorene-containing binuclear gold(I) complexes: High-contrast mechanochromic fluorescence quenching and information encryption application

A series of fluorene-based binuclear gold(I) complexes I-VI have been successfully synthesized. Their structures were characterized by nuclear magnetic resonance, high-resolution mass spectrometry, and single crystal X-ray diffraction techniques. The fluorescence switching characteristics of complexes I-VI in the solid state were studied by photoluminescence spectroscopy. Luminogens I-VI showed different solid-state fluorescence involving blue-green, yellow-green and yellow colors before mechanical stimulation, which suggests that the solid fluorescent properties of I-VI can be effectively manipulated by positional isomerism and increasing conjugation strategies. Interestingly, their solid fluorescence intensities of all luminogens weakened significantly upon grinding. Impressively, luminogen IV barely displayed macroscopic fluorescence after grinding, indicative of its remarkable high-contrast mechanochromic fluorescence quenching feature. The unique mechanofluorochromic phenomena of I-VI were fully elucidated by analyzing X-ray diffraction patterns changes of I-VI before and after grinding and crystal packing structure of III. Furthermore, an effective dual information encryption system was constructed based on complex IV.

Ultrafast Luminescence Detection with Selective Adsorption of Carbon Disulfide in a Gold(I) Metal-Organic Framework

Although a widely used and important industrial chemical, carbon disulfide (CS2) poses a number of hazards due to its volatility and toxicity. As such, the development of multifunctional materials for the selective capture and easy recognition of CS2 is one of the crucial issues. Herein, we demonstrate completely selective CS2 adsorption among trials involving H2O, alcohols, volatile organic compounds (including thiol derivatives), N2, H2, O2, CH4, CO, NO, and CO2. We also showcase its fine detection using remarkable luminescent response in a gold(I)-based metal-organic framework (MOF) of {ZnII(pz)[AuI(CN)2]2} (pz=pyrazine; 1) with a two-fold interpenetration network. Ex situ single crystal X-ray diffraction for 1 and CS2-accommodated 1 suggested that the Au ⋅⋅⋅ Au atoms are not only luminescent centers but also act as interaction sites for CS2 modulating the Au ⋅⋅⋅ Au contacts. These experiments revealed the specificity of CS2 and how changes in the CS2-induced structure. Based on the obtained structural transformation, 1 exhibited a sensitive detecting ability for CS2 with an ultrafast response time of less than 10 s. Moreover, ex situ time-resolved photoluminescence analyses developed in this work implied that CS2 varied the energetic relaxation at the excited states related to the luminescent efficiency of the resultant MOF system.

Stimuli-Responsive Chiral Cellulose Nanocrystals Based Self-Assemblies for Security Measures to Prevent Counterfeiting: A Review

The proliferation of misleading information and counterfeit products in conjunction with technical progress presents substantial worldwide issues. To address the issue of counterfeiting, many tactics, such as the use of luminous anticounterfeiting systems, have been investigated. Nevertheless, traditional fluorescent compounds have a restricted effectiveness. Cellulose nanocrystals (CNCs), known for their renewable nature and outstanding qualities, present an excellent opportunity to develop intelligent, optically active materials formed due to their self-assembly behavior and stimuli response. CNCs and their derivatives-based self-assemblies allow for the creation of adaptable luminous materials that may be used to prevent counterfeiting. These materials integrate the photophysical characteristics of optically active components due to their stimuli-responsive behavior, enabling their use in fibers, labels, films, hydrogels, and inks. Despite substantial attention, existing materials frequently fall short of practical criteria due to limited knowledge and poor performance comparisons. This review aims to provide information on the latest developments in anticounterfeit materials based on stimuli-responsive CNCs and derivatives. It also includes the scope of artificial intelligence (AI) in the near future. It will emphasize the potential uses of these materials and encourage future investigation in this rapidly growing area of study.

Force-triggered hypso- and bathochromic bidirectional fluorescence switching beyond 120 nm and its anticounterfeiting applications

Achieving high-contrast tricolor emissive regulation of a single-component molecule using a single type of external stimulus is highly desirable but challenging. In the present study, we report a symmetric acceptor-donor-acceptor (A-D-A)-type aggregation-induced emission-active luminogen, which displays a sequential high-contrast fluorescence switching just by anisotropic mechanical grinding. Specifically, upon light grinding, an orange-yellow-to-blue hypsochromic mechanofluorochromic response with a distinct color contrast (change in the maximum emission wavelength, Δλem,max = 122 nm) is noticed, and the slightly ground solid exhibits a blue-to-red high-contrast (Δλem,max = 185 nm) bathochromic mechanofluorochromic conversion upon vigorous grinding. Thus, using a single luminogen developed here, we can realize wide-range (Δλem,max > 100 nm) hypso- and bathochromic fluorescence mechanochromisms simultaneously. The tricolored mechanofluorochromic phenomenon is attributed to two different morphological transitions involving crystalline-to-crystalline and crystalline-to-amorphous states. Furthermore, three information anticounterfeiting systems are developed using the luminogen presented here.

Multistimuli-responsive multicolor solid-state luminescence tuned by NH-dependent switchable hydrogen bonds

Revealing the stimuli-responsive mechanism is the key to the accurate design of stimuli-responsive luminescent materials. We report herein the multistimuli-responsive multicolor solid-state luminescence of a new dicopper(I) complex [{Cu(bpmtzH)}2(μ-dppa)2](ClO4)2 (1), and the multistimuli-responsive mechanism is clarified by investigating its four different solvated compounds 1·2CH3COCH3·2H2O, 1·2DMSO·2H2O, 1·4CH3OH, and 1·4CH2Cl2. It is shown that luminescence mechanochromism is associated with the breakage of the hydrogen bonds of bmptzH-NH with counter-ions such as ClO4- induced by grinding, while luminescence vapochromism is attributable to the breaking and forming of hydrogen bonds of dppa-NH with solvents, such as acetone, dimethylsulfoxide, and methanol, caused by heating and vapor fuming. In addition, those results might provide new insights into the design and synthesis of multistimuli-responsive multicolor luminescent materials by using various structure-sensitive functional groups, such as distinct N-H ones, to construct switchable hydrogen bonds.

E- or Z-Isomers Arising from the Geometries of Ligands in the Mercury Complex of 2-(Anthracen-9-ylmethylene)-N-phenylhydrazine Carbothioamide

An anionic mercury(II) complex of 2-(anthracen-9-ylmethylene)-N-phenylhydrazine carbothioamide (HATU) and two isomers of a neutral mercury(II) complex of the anion of the same ligand (ATU) were reported. The anionic complex [Hg(HATU)2Cl2]·CH2Cl2 had a monodentate HATU ligand (a neutral form of the ligand) and chloride ligands. The two conformational isomers were of the neutral mercury(II) complex Hg(ATU)2·2DMF. The two isomers were from the E or Z geometry of the ligands across the conjugated C=N-N=C-N scaffold of the coordinated ligand. The two isomers of the complex were independently prepared and characterized. The spectroscopic properties of the isomers in solution were studied by 1H NMR as well as fluorescence spectroscopy. Facile conversion of the E-isomer to the Z-isomer in solution was observed. Density functional theory (DFT) calculations revealed that the Z-isomer of the complex was stable compared to the E-isomer by an energy of 14.35 kJ/mol; whereas, E isomer of the ligand was more stable than Z isomer by 8.37 KJ/mol. The activation barrier for the conversion of the E-isomer to the Z-isomer of the ligand was 167.37 kJ/mol. The role of the mercury ion in the conversion of the E-form to the Z-form was discussed. The mercury complex [Hg(HATU)2Cl2]·CH2Cl2 had the E-form of the ligand. Distinct photophysical features of these mercury complexes were presented.

The first solid-state route to luminescent Au(I)-glutathionate and its pH-controlled transformation into ultrasmall oligomeric Au10-12(SG)10-12 nanoclusters for application in cancer radiotheraphy

There is still a need for synthetic approaches that are much faster, easier to scale up, more robust and efficient for generating gold(I)-thiolates that can be easily converted into gold-thiolate nanoclusters. Mechanochemical methods can offer significantly reduced reaction times, increased yields and straightforward recovery of the product, compared to the solution-based reactions. For the first time, a new simple, rapid and efficient mechanochemical redox method in a ball-mill was developed to produce the highly luminescent, pH-responsive Au(I)-glutathionate, [Au(SG)]n. The efficient productivity of the mechanochemical redox reaction afforded orange luminescent [Au(SG)]n in isolable amounts (mg scale), usually not achieved by more conventional methods in solution. Then, ultrasmall oligomeric Au10-12(SG)10-12 nanoclusters were prepared by pH-triggered dissociation of [Au(SG)]n. The pH-stimulated dissociation of the Au(I)-glutathionate complex provides a time-efficient synthesis of oligomeric Au10-12(SG)10-12 nanoclusters, it avoids high-temperature heating or the addition of harmful reducing agent (e.g., carbon monoxide). Therefore, we present herein a new and eco-friendly methodology to access oligomeric glutathione-based gold nanoclusters, already finding applications in biomedical field as efficient radiosensitizers in cancer radiotherapy.

Mechanical and Thermal ON-OFF Switching of the Vapochromic Behavior of a Luminescent Polymorphic Pt(II) Complex

Vapochromic materials that exhibit color/luminescence changes induced by vapor exposure have attracted considerable attention. Herein, we report the grinding- and heating-induced ON-OFF switching of the vapochromic behavior of [Pt(ppyCl₂)(Clacac)] (1; ppyCl₂ = 2-(3-chlorophenyl)-4-chloropyridinato, Clacac = 3-chloroacetylacetonato). 1 formed yellow and orange polymorphs (1-Y and 1-O), and 1-Y could be converted to 1-Og, which showed a very similar crystal structure but with a broadened X-ray diffraction pattern compared with that of 1-O. Moreover, 1-Og can be reversibly transformed into 1-O via heating and grinding. Notably, 1-Og underwent a N,N-dimethylacetamide vapor-induced transformation to 1-Y, whereas 1-O did not undergo such a transformation. These results indicate the ON-OFF switching of vapochromic behavior induced via grinding and heating. This finding will be beneficial for developing intelligent molecular devices.

Solvates of a dianisyl-substituted donor–acceptor-type benzothiadiazole: mechanochromic, vapochromic, and acid-responsive multicolor luminescence

Solvates of a donor–acceptor-type fluorophore exhibited multicolor luminescence that can respond to mechanical stimuli, solvent and acid vapors.

Control of Fluorescence of Organic Dyes in the Solid-State by Supramolecular Interactions

Fluorescent organic dyes play an essential role in the creation of new "smart" materials. Fragments and functional groups capable of free rotation around single bonds can significantly change the fluorescent organic dye's electronic structure under analyte effects, phase state transitions, or changes in temperature, pressure, and media polarity. Dependencies between steric and electronic structures become highly important in transition from a solution to a solid-state. Such transitions are accompanied by a significant increase in the dye molecular structure's rigidity due to supramolecular associates' formation such as H-bonding, π···π and dipole-dipole interactions. Among those supramolecular effects, H-bonding interactions, first of all, lead to significant molecular packing changes between loose or rigid structures, thus affecting the fluorescent dye's electronic states' energy and configuration, its fluorescent signal's position and intensity. All the functional groups and heteroatoms that are met in the organic dyes seem to be involved in the control of fluorescence via H-bonding: C-H···N, C-H···π, S = O···H-C, P = O···H, C-H···O, NH···N, C - H···C, C - H···Se, N-H···O, C - H···F, C-F···H. Effects of molecular packing of fluorescent organic dyes are successfully used in developing mechano-, piezo-, thermo- fluorochromes materials for their applications in the optical recording of information, sensors, security items, memory elements, organic light-emitting diodes (OLEDs) technologies.

Mechanical deformation and multiple thermal restoration of organic crystals: reversible multi-stage shape-changing effect with luminescence-color changes

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.

A tricolor-switchable stimuli-responsive luminescent binuclear Cu(i) complex with switchable NH⋯O interactions

Blue-green-yellow tricolor luminescence conversion is attributed to the loss and recovery of CH2Cl2 solvent molecules and the destruction and restoration of the orderly packing array caused by the breaking and rebuilding of NH⋯O hydrogen bonds.

Tuning the luminescence of transition metal complexes with acyclic diaminocarbene ligands

Organometallics featuring acyclic diaminocarbene ligands have recently emerged as powerful emitters for use in electroluminescent technologies.

Mechanochemical synthesis of mononuclear gold(I) halide complexes of diphosphine ligands with tuneable luminescent properties

A mechanochemical method is reported for the synthesis of Au(diphos)X complexes of diphosphine (diphos = XantPhos and N-XantPhos) ligands and halide ions (X = Cl and I). The Au(XantPhos)X (1: X = Cl; 2: X = I) and Au(N-XantPhos)Cl (3) complexes exhibited either yellowish green (1) or bluish green (2) emission, whereas 3 was seemingly non-emissive in the solid state at room temperature. Blue- (2B) and bluish green (2G) luminescent concomitant solvates of 2 were obtained by recrystallization. Luminescent colour changes from blue (2B) or bluish green (2G) to yellow were observed when these forms were subjected to mechanical stimulus, while the original emission colour can be recovered in the presence of solvent vapours. Moreover, the luminescence of 2B can be reversibly altered between blue and yellow by heating/cooling-cycles. These results demonstrate the power of mechanochemistry in the rapid (4 min reaction time), efficient (up to 98% yield) and greener synthesis of luminescent and stimuli-responsive gold(I) complexes.

Osmaindenes: Synthesis and Reversible Mechanochromism Characteristics

A series of novel osmaindenes 1 - 6 bearing different substituents (CF 3 , H, I, Br, OCH 3 , N(Ph) 2 ) has been synthesized by nucleophilic reaction of water with the corresponding aromatic osmanaphthalyne complexes. All osmaindenes 1 - 6 have been characterized by elemental analysis (EA) and nuclear magnetic resonance (NMR) spectroscopy, although the low solubilities of 3 and 4 precluded the accumulation of their 13 C NMR spectra. Osmaindenes 2 , 3 and 5 have also been characterized by single-crystal X-ray diffraction analysis. Subsequently, through solid-state fluorescence spectroscopy, mechanochromic studies, and powder X-ray diffraction (XRD) analysis, we found that osmaindenes 1 - 6 fluoresce at wavelengths in the range 500-800 nm, while also displaying reversible mechanochromic properties. The solid-state fluorescence emission of 1 after grinding extends into the near-infrared region. This research provides new insight into the design and synthesis of metallic materials with excellent mechanochromic properties.

Regulation of Multicolor Fluorescence Changes Found in Donor-acceptor-type Mechanochromic Fluorescent Dyes

The regulation of multicolor fluorescence changes in mechanochromic fluorescence (MCF) remains a challenging task. Herein, we report the regulation of MCF using a donor-acceptor structure. Two crystal polymorphs, BTD-pCHO(O) and BTD-pCHO(R) produced by the introduction of formyl groups to an MCF dye, respond to a mechanical stimulus, allowing a three-color fluorescence change. Specifically, the orange-colored fluorescence of the metastable BTD-pCHO(O) polymorph changed to a deep-red color in the amorphous-like state to finally give a red color in the stable BTD-pCHO(R) polymorph. This change occurred by mechanical grinding followed by vapor fuming. The two different crystal packing patterns were selectively regulated by the electronic effect of the introduced functional groups. The two types of selectively formed crystals in BTD(F)-pCHO bearing fluorine atoms, and BTD(OMe)-pCHO bearing methoxy groups, respond to mechanical grinding, allowing for the regulation of multicolor MCL from a three-color change to two different types of two-color changes.

Multi-color mechanochromic luminescence of three polymorphic crystals of a donor–acceptor-type benzothiadiazole derivative

The three polymorphic crystals of a donor–acceptor dye exhibited different luminescence colors, which changed in response to mechanical grinding.

A polymorphic pentiptycene-containing gold(I) isocyanide complex: solvent- and conformation-dependent supramolecular luminescence

Four crystalline (pseudo)polymorphs of [Ph-Au-C[triple bond, length as m-dash]N-Phip-OC8H17] (1), where Phip = central phenylene of pentiptycene, reveal that the π-backbone conformation relative to the AuAu bonding axis is important in determining the energy and efficiency of the supramolecular luminescence, which offers mechanistic insights into the luminescence mechanochromism and vapochromism and the solvent-dependent aggregation-induced emission (AIE) of 1.

Mechanofluorochromic carbon dots under grinding stimulation

The exploration of new hotspot nanomaterials to acquire mechanofluorochromic (MFC) properties has drawn substantial interest. However, previously reported MFC nanomaterials have required external pressures on the level of gigapascals, and observing distinct reversible MFC phenomena in nanomaterials under low-pressure conditions is still a challenge. Herein, a kind of reversible MFC-carbon dots (CDs) under low pressure has been reported for the first time. The MFC-CDs exhibited an apparent solid-state fluorescence color change, with emission shifting from green to blue via anisotropic grinding, owing to the alteration of hydrogen bonds and stacking structure among the CDs. Notably, these MFC-CDs exhibited a reversible fluorescence resulting from their being treated with acid vapors. This reversibility was indicated from X-ray diffraction analysis to be due to recovery of the crystalline state. The results highlighted the relationship between reversible MFC properties and structure, and showed the utility of these MFC-CDs as security films for further applications.

Mechanochromic Delayed Fluorescence Switching in Propeller-Shaped Carbazole-Isophthalonitrile Luminogens with Stimuli-Responsive Intramolecular Charge-Transfer Excited States

Herein, the universal design of high-efficiency stimuli-responsive luminous materials endowed with mechanochromic luminescence (MCL) and thermally activated delayed fluorescence (TADF) functions is reported. The origin of the unique stimuli-triggered TADF switching for a series of carbazole-isophthalonitrile-based donor-acceptor (D-A) luminogens is demonstrated based on systematic photophysical and X-ray analysis, coupled with theoretical calculations. It was revealed that a tiny alteration of the intramolecular D-A twisting in the excited-state structures governed by the solid morphologies is responsible for this dynamic TADF switching behavior. This concept is applicable to the fabrication of bicolor emissive organic light-emitting diodes using a single TADF emitter.

The endeavor of vibration-induced emission (VIE) for dynamic emissions

Organic chromophores with large Stokes shifts and dual emissions are fascinating because of their fundamental and applied interest. Vibration-induced emission (VIE) refers to a tunable multiple fluorescence exhibited by saddle-shaped N,N'-disubstituted-dihydribenzo[a,c]phenazines (DHPs), which involves photo-induced configuration vibrations from bent to planar form along the N-N axis. VIE-active molecules show intrinsic long-wavelength emissions in the unconstrained state (planar state) but bright short-wavelength emissions in the constrained state (bent state). The emission response for VIE-active luminogens is highly sensitive to steric hindrance encountered during the planarization process such that a tiny structural variation can induce an evident change in fluorescence. This can often be achieved by tuning the intensity ratio of short- and long-wavelength bands. In some special cases, the alterations in the emission wavelength of VIE fluorophores can be achieved step by step by harnessing the degree of bending angle motion in the excited state. In this perspective, we summarize the latest progress in the field of VIE research. New bent heterocyclic structures, as novel types of VIE molecules, are being developed, and the general features of the chemical structures are also being proposed. Technologically, novel emission color-tuning approaches and VIE-based probes for visualizing biological activity are presented to demonstrate how the dynamic VIE effect can be exploited for cutting-edge applications.

Mixed crystal formation of two gold isocyanide complexes with various ratios for continuous tuning of photophysical properties

Mixed crystals composed of two distinct gold isocyanide complexes are reported. Characterization of the mixed crystal formation and mixing ratios are performed by single-crystal X-ray diffraction and nuclear magnetic resonance spectroscopy analyses. By changing the mixing ratio, we achieve continuous changes in the emission lifetimes and the emission quantum yields.

Multi-Color Photoluminescence Based on Mechanically and Thermally Induced Liquid-Crystalline Phase Transitions of a Hydrogen-Bonded Benzodithiophene Derivative

Controlling assembled structures of π-conjugated liquid-crystalline molecules is of great interest in the development of stimuli-responsive luminescent materials due to their molecular motility in the ordered states. Herein, we describe a mechanoresponsive hydrogen-bonded benzodithiophene liquid-crystalline molecule that exhibits a tricolor photoluminescence switching at ambient temperature. The compound shows a shear-induced phase transition from a rectangular columnar to a metastable optically anisotropic mesophase, which is accompanied by the luminescent color change from yellow to sky-blue. The metastable mesophase exhibits a time-responsive transformation to another metastable mesophase showing a blue-green emission through isothermal aging at room temperature. The luminescent color of aged sample reverts back to the initial yellow color by thermal annealing at 150 °C. These dynamic structural changes accompanied by the emission color changes are governed by distinct π-stacking modes and hydrogen-bonded patterns. The shear-induced luminescent color change from yellow to blue is found to occur above the shear strain of 390 % at which the shear stress is 2.4×10⁵  Pa as determined from dynamic viscoelastic measurements.

Mechanochromic Luminescence from Crystals Consisting of Intermolecular Hydrogen-Bonded Sheets

Introduction of functional groups that can form intermolecular hydrogen bonds into highly-emissive luminophores is a promising way to induce mechanochromic luminescence. Herein, we report that a 9,10-bis(phenylethynyl)anthracene derivative featuring two amide groups forms green-emissive crystals based on two-dimensional hydrogen-bonded molecular sheets. Mechanical grinding changed the emission from green to yellow, owing to a transition from a crystalline to an amorphous phase. Infrared spectroscopy revealed that mechanical stimuli disrupted the linear hydrogen-bonding formation. A thermal treatment recovered the original green photoluminescence.

Tunable multi-color luminescence from a self-assembled cyanostilbene and cucurbit[7]uril in aqueous media

A tunable multicolor luminescent supramolecular system was designed in aqueous media employing the self-assembly of a cationic amphiphilic cyanostilbene and the host–guest chemistry of cucurbit[7]uril.

Mechanochemical changes on cyclometalated Ir(iii) acyclic carbene complexes – design and tuning of luminescent mechanochromic transition metal complexes

A new class of luminescent cyclometalated Ir(iii) complexes with readily tunable mechanochromic properties derived from the mechanically induced trans-to-cis isomerization have been developed.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

Synthesis and Photoluminescence of Tetracyanidonitridorhenium(V) Complexes with Five-Membered N-Heteroaromatic Ligands and Photoluminescence-Intensity Change

Novel tetracyanidonitridorhenium(V) complexes with five-membered N-heteroaromatic ligands, (PPh₄)₂[ReN(CN)₄L] [L = imidazole (Him) (2), 1-methylimidazole (Mim) (3), and pyrazole (pyz) (4)] and (PPh₄)₂[ReN(CN)₄L]·L [L = Him (5) and Mim (6)], were synthesized by the reactions of (PPh₄)₂[ReN(CN)₄] (1) with Him, Mim, and pyz, and their structures were determined by single-crystal X-ray analysis. The complexes 2, 3, 4, and 6 showed intense photoluminescence, with the emission quantum yields (Φ_em) being 0.65-0.75 in the solid state at 296 K. In contrast, the Φ_em and τ_em values of 5 are significantly smaller and shorter, respectively, than the relevant values of 2. The interconversion reactions among 1, 2, and 5 accompanied by large photoluminescence-intensity changes were accomplished by solvent-free reactions and exposure of water. The mechanochemical reaction of 2 with 1 mol equiv of Him in the solid state gave 5. Complex 5 was also obtained by the mechanochemical reaction of 1 with 2 mol equivalents of Him in the solid state. By placing solid of 5 in water, the solid showed intense photoluminescence to give 2. Complex 1 was produced under vacuum at 185 °C from 2 or 5.

Anisotropic strain release in a thermosalient crystal: correlation between the microscopic orientation of molecular rearrangements and the macroscopic mechanical motion

We report the salient effect of the crystal of gold complex that bridges the gap between macroscopic mechanical movements (i.e., jump) and microscopic changes of the crystal structure.

The salient effect, which refers to a jumping phenomenon of organic and organometallic molecular crystals typically triggered by phase transitions in response to external stimuli, has been investigated intensively in the last five years. A challenging topic in this research area is the question of how to characterize the release of microscopic strain accumulated during phase transitions, which generates macroscopic mechanical motion. Herein, we describe the thermosalient effect of the triphenylethenyl gold 4-chlorophenyl isocyanide complex 1, which jumps reversibly at approximately –100 °C upon cooling at 50 °C min^–1 and heating at 30 °C min^–1. Single-crystal X-ray diffraction measurements and differential scanning calorimetric analyses of 1 suggest the occurrence of a thermal phase transition at this temperature. Detailed structural analyses indicate that anisotropic changes to the molecular arrangement occur in 1, whereby the crystallographic a axis contracts upon cooling while the b axis expands. Simultaneously, macroscopic changes of the crystal dimensions occur. This is observed as bending, i.e., as an inclination of the crystal edges, and in the form of splitting, which occurs in a perpendicular direction to the major crystal axis. This study thus bridges the gap between macroscopic mechanical responses that are observed in high-speed photographic images and microscopic changes of the crystal structure, which are evaluated by X-ray diffraction measurements with face indexing.

Mechanochromic luminescence based on a phthalonitrile-bridging salophen zinc(ii) complex

Here, we showcase the impressive stimuli-responsive properties of a luminescent zinc(ii)–salophen complex CN-Zn, highlighting a reversible mechanochromic property.

4-Pyridylisocyanide gold(i) and gold(i)-plus-silver(i) luminescent and mechanochromic materials: the silver role

X-Ray and DFT studies support that the red-shift of luminescence from [AuAr(CNPy-4)] (Ar = C₆F₅, C₆F₃Cl₂-3,5) to [Ag[AuAr(CNPy-4)]₂](BF₄) is not due to non-existent Au⋯Ag interactions but to adoption of structures with shorter Au⋯Au distances.

Reversible tricolour luminescence switching based on a piezochromic iridium(iii) complex

On the basis of rational molecular design, the tricolour luminescence switching of an Ir(iii) complex is achieved for the first time.

Fluorescence response of cruciform D–π–A–π–D phenothiazine derivatives to mechanical force

Three kinds of crystals of two phenothiazine derivatives transformed into similar amorphous powders, in which the short-range π-stacking can be deduced by single-crystal structure.

Reversible Mechanochromic Delayed Fluorescence in 2D Metal-Organic Micro/Nanosheets: Switching Singlet-Triplet States through Transformation between Exciplex and Excimer

Mechanochromic luminescent materials have attracted much attention and present a variety of applications in information security, data recording, and storage devices. However, most of these smart luminescent systems are based on typical fluorescence and/or phosphorescence mechanisms; the mechanochromic delayed fluorescence (MCDF) materials involving switching singlet and triplet states are rarely studied to date. Herein, new 2D layered metal-organic micro/nanosheets, [Cd(9-AC)2(BIM)2] (named as MCDF-1; 9-AC = anthracene-9-carboxylate and BIM = benzimidazole) and its solvate form containing interlayer CH3CN (named as MCDF-2), which exhibit reversible mechanochromic delayed fluorescence characteristics, are presented. With applying the mechanical force, the luminescent center of MCDF-1 can be converted from 9-AC/BIM exciplex to 9-AC/9-AC excimer, resulting in alternations of delayed fluorescence. Such luminescent change can be further recovered by CH3CN fumigation, accompanied by the structural transformation from MCDF-1 to MCDF-2. Furthermore, the force-responsive process also refers to the energy redistribution between singlet and triplet states as inferred by both temperature-dependent photophysics and theoretical calculations. Therefore, this work not only develops new 2D micro/nanosheets as MCDF materials, but also supplies a singlet-triplet energy switching mechanism on their reversible mechanochromic process.

A luminescent benzothiadiazole-bridging bis(salicylaldiminato)zinc(ii) complex with mechanochromic and organogelation properties

Herein, a new bis(salicylaldiminato)Zn(ii) Schiff base complex, BTZn, derived from benzo[c][1,2,5]thiadiazole-5,6-diamine was designed and synthesized. It exhibited unique mechanical force-induced luminescence change characteristic. Upon mechanical grinding, the as-prepared BTZn solid crystalized from an ethanol/dichloromethane solution displays a high-contrast emission-colour variation from yellow (emission maximum λem = 545 nm) to red (λem = 645 nm), and this emission variation can be erased through solvent vapour treatment. The reversible emission colour alteration between yellow and red can be repeatedly performed. Thermal annealing of the as-prepared BTZn solid resulted in a more ordered orange phase with an emission maximum of 575 nm. The multi-stimuli-responsive luminescence mechanism has been investigated via SEM, powder X-ray diffraction (XRD), and thermal analyses. It is demonstrated that mechanical force can induce morphology transformation from the crystalline to the amorphous phase, which is accompanied by a change in the BTZn molecular packing modes. The BTZn-based solids have molecular packing-dependent emission characteristics. The XRD experimental results reveal that for the yellow emissive as-prepared BTZn solid, a columnar square molecular arrangement is adopted. On the other hand, the BTZn complex exhibits the ability to organize into organic luminescent gels constructed by one-dimensional BTZn molecular nanofibrils. The BTZn xerogel also displays mechanochromic properties. Accordingly, BTZn-based solids may be potential candidates for the development of new stimuli-responsive materials.

Developing the structure-property relationship to design solid state multi-stimuli responsive materials and their potential applications in different fields

Establishing the structure–property relationship for multi-stimuli responsive mechanochromic materials based on charge transfer luminogens.

Prediction of multi-stimuli responsive behavior in newly developed luminogens is an appealing yet challenging puzzle, since no concrete design strategy has been developed so far. In this article, we demonstrate a potent strategy to gain a deep understanding of the structure–property relationship to design multi-stimuli responsive mechanochromic materials. To achieve our goal, a variety of new isoindolinone core based charge transfer luminogens exhibiting aggregation-induced emission (AIE) have been prepared through C–H bond activation using a cost-effective ruthenium (Ru) metal catalyzed one-pot synthetic strategy. We have shown that slight tuning of the donor moiety is found to be highly effective in controlling molecular packing and metastable energy states in solid states, and thus, optical properties and multi-stimuli responsive behaviors. The flexibility and twisting of donor moieties afford a loosely bound ‘herringbone’ packing, enabling reversible transformation under multiple mechanical stimuli. The cyclized derivative of the donor exhibits a completely different packing mode (i.e., cross packing), and subsequently, does not give rise to mechanochromism. The Hirshfeld surface analysis from a single crystal infers that non-covalent interactions (specifically C–H···π and π···π) are extremely important to yield mechanochromism under external force. Correlating all solid-state behavior with the molecular structure, we conclude that the synergistic effect between the twisting and conformational flexibility of donor moieties along with numerous non-covalent interactions gives rise to multi-stimuli responsive behaviors. Finally, the newly designed molecules are found to be highly emissive in solution and potentially applicable in fluorescence thermometer construction, lighting up cells, acid–base sensors and rewritable devices.

Dimethylamine substituted bisbenzocoumarins: solvatochromic, mechanochromic and acidochromic properties

Highly fluorescent emissive dimethylamine substituted bisbenzocoumarins (DB-Cns) show reversible fluorescence color changes under mechanical force stimuli and acidochromic properties.

A gold isocyanide complex with a pendant carboxy group: orthogonal molecular arrangements and hypsochromically shifted luminescent mechanochromism

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.

High contrast mechanochromic and thermochromic luminescence switching by a deep red emitting organic crystal

In this study, we report a deep red emissive organic crystal that displays high contrast fluorescence switching under mechanical and thermal stimulation.