Survey of the literature on mechanoluminescence from 1605 to 2013

Self-recoverable elastico mechanoluminescence of a hybrid metal halide crystal

Materials exhibiting self-recoverable elastico mechanoluminescence (EML) are highly sought after due to their utility in sensing and information encryption. However, the current pool of identified EML materials remains limited, exclusively comprising purely inorganic substances. This study delves into the investigation of the EML properties of a zero-dimensional (0D) organic-inorganic metal halide denoted as [C19H18P]2MnBr4 (where C19H18P+ signifies methyl triphenyl phosphonium). Notably, [C19H18P]2MnBr4 manifests two distinct polymorphs, with the piezoelectric and non-piezoelectric polymorphs exhibiting thermodynamic and kinetic stability, respectively. Despite both compounds showing bright greenish luminescence, only the piezoelectric phase exhibits desirable EML behavior. The EML in this context is distinguished by its high intensity, strong fatigue resistance and prompt self-recovery. The underlying mechanism of EML in the piezoelectric polymorph can be explained by the piezoelectric effect and the stress-induced energy band tilting. Calorimetric and piezoelectric experiments reveal the self-recoverable EML arises from the relaxation of the stress-induced kinetically stable non-piezoelectric to the thermodynamically favored piezoelectric phase. This work paves a new pathway in the exploration of self-recoverable EML materials in the realm of hybrid organic-inorganic crystals.

Engineering Trap Distribution by Doping Rare Earth Ion for Mechanoluminescence Enhancement

Mechanoluminescence materials exhibit fascinating optical properties due to their energy harvesting and controllable release capabilities. SrAl2O4:Eu2+ (SAOE) has been extensively studied as a traditional mechanoluminescence material, however, the luminescence intensity enhancement and the luminescence mechanism of its mechanoluminescence remain an unresolved issue, which hinders the development and widespread application of excellent phosphors. Herein, a promising rare earth (Re3+ = Sm3+, Dy3+, Er3+, and Tm3+) doping strategy was proposed to achieve intense mechanoluminescence of SAOE. By introducing different Re3+ ions to manipulate the energy level positions in SAOE phosphors, the depth and density of electron and hole traps can be tuned, resulting in the maximum mechanoluminescence intensity of SrAl2O4:Eu2+, Tm3+ is about 11-fold higher than that of SAOE. The mechanism governing trap distribution has been unveiled through thermoluminescence glow curve analysis and density functional theory calculations. Our research provides valuable guidance for designing high-performance phosphors and opens up new opportunities for multifunctional applications.

Recent advances in multimodal mechanoluminescent sensors enabled by nanostructure design

Multiple modes of perception have evolved in creatures to help them survive in a highly complex world under different harsh environments. Inspired by this, multimodal sensing materials have been created as one of the most crucial elements to bridge artificial intelligence with reality. The well-organized integration of multiple independent stimuli in a single material rather than simple integration, is expected to increase the accuracy and multifunctional applications of sensing devices. However, achieving multifunction coupling through elaborate nanostructure and supramolecular design, still remains a challenge that attracts great attention. Under the framework of nanostructural design for a multimodal response, the coupling of mechanoluminescence ability and advanced stimulus-response, has been reported to realize comprehensive perception and multifunctional applications for more complex scenarios. Herein, this mini review briefly provides an overview on the latest advances of multimodal mechanoluminescent sensors, concentrating on the nanostructure design strategy for multifunctional coupling, including triboelectric compositing, supramolecular interfacial connection, and band structure modulation; as well as emphatically discussing the advantages of mechanoluminescence coupling with self-powered sensing, piezoresistive response, temperature/chemical detection, and the corresponding advanced tools for heterogeneous output decoupling. Finally, the conclusions and outlook of multimodal mechanoluminescent sensors are presented.

Mechanoluminescence from Organic-Inorganic Metal Halide Perovskite Derivative

Metal halide perovskites and their derivatives are gaining significant attention as photoluminescent materials due to their exceptional light-emitting properties. However, most research has concentrated on electroluminescence and photoluminescence, there remains a substantial gap in the exploration of mechanoluminescence (ML) properties in perovskites, making this field largely uncharted. ML is an ancient and intriguing luminescent phenomenon that occurs when a material is subjected to mechanical forces. Here, the discovery of the first organic-inorganic hybrid terbium (Tb3+)-based metal halide, a type of perovskite derivative, which exhibits notable ML properties is reported. Through material orthogonal design, the critical roles played by molecular geometry and metal-site ions in achieving remarkable ML in organic-inorganic hybrid metal halides are identified.

Achieving Ultrasound-Excited Emission with Organic Mechanoluminescent Materials

Unlike traditional photoluminescence (PL), mechanoluminescence (ML) achieved under mechanical excitation demonstrates unique characteristics such as high penetrability, spatial resolution, and signal-to-background ratio (SBR) for bioimaging applications. However, bioimaging with organic mechanoluminescent materials remains challenging because of the shallow penetration depth of ML with short emission wavelengths and the absence of a suitable mechanical force to generate ML in vivo. To resolve these issues, the present paper reports the achievement of ultrasound (US)-excited fluorescence and phosphorescence from purely organic luminogens for the first time with emission wavelengths extending to the red/NIR region, with the penetrability of the US-excited emission being considerably higher than that of PL. Consequently, US-excited subcutaneous phosphorescence imaging can be achieved using a mechanoluminescent-luminogen-based capsule device with a quantified intensity of 9.15 ± 1.32 × 104 p s-1 cm-2 sr-1 and an SBR of 24. Moreover, the US-excited emission can be adequately tuned using the packing modes of the conjugated skeletons, dipole orientation of mechanoluminescent luminogens, and strength and direction of intermolecular interactions. Overall, this study innovatively expands the kind of excitation sources and the emission wavelengths of organic mechanoluminescent materials, paving the way for practical biological applications based on US-excited emission.

A mechanoluminescent material, ZnS:Mn,Li, with enhanced brightness for visualizing dental occlusion

Mechanoluminescent materials are characterized by high luminescence intensity, high repeatability, no external voltage activation, and a good linear relationship between stress and mechanoluminescence intensity within a certain range. Therefore, mechanoluminescent materials have attracted increasing attention from researchers in the fields of stress sensing, encryption and anti-counterfeiting, structural health monitoring, energy-saving lighting, intelligent wearable devices, and other fields. In this study, ZnS:Mn powders with different Mn2+ ratios and different ion doping were synthesized by a high-temperature solid-phase reaction, and the synthesis of various materials was characterized. Then, the optimal mechanoluminescence effect of the ZnS:1%Mn,1%Li material was obtained. The photoluminescence intensity of ZnS:1%Mn,1%Li was 16.7 times higher than that of the sample without doping with Li+, and the mechanoluminescence intensity was 1.64 times higher. Finally, polyethylene terephthalate (PET) film was combined with ZnS:Mn,Li mechanoluminescent powders to prepare flexible three-layer composite film. Based on this, a feasible strategy for the detection of temporomandibular disorders was proposed. The composite film is easy to use, economical, and safe, and has good mechanoluminescent performance, which has potential application value in the field of occlusal force detection and visualization.

Chromic soft crystals based on luminescent platinum(II) complexes

Platinum(II) complexes of square-planar geometry are interesting from a crystal engineering viewpoint because they exhibit strong luminescence based on the self-assembly of molecular units. The luminescence color changes in response to gentle stimuli, such as vapor exposure or weak mechanical forces. Both the molecular and the crystal designs for soft crystals are critical to effectively generate the chromic luminescence phenomenon of Pt(II) complexes. In this topical review, strategies for fabricating chromic luminescent Pt(II) complexes are described from a crystal design perspective, focusing on the structural regulation of Pt(II) complexes that exhibit assembly-induced luminescence via metal-metal interactions and structural control of anionic Pt(II) complexes using cations. The research progress on the evolution of various chromic luminescence properties of Pt(II) complexes, including the studies conducted by our group, are presented here along with the latest research outcomes, and an overview of the frontiers and future potential of this research field is provided.

Triboluminescence and crystal structure of the complex [Eu(MBA)3 Dipy]2 (HMBA)

The atomic structure of the crystals [Eu(MBA)3 Dipy]2 (HMBA) (MBA-аnion toluic acid, Dipy-2,2'-dipyridyl, HMBA-toluic acid), displaying intensive luminescence and triboluminescence was determined using single crystal X-ray diffraction analysis. The triclinic centrosymmetric crystals have the following parameters: a = 10.620(2), b = 11.849(2), c = 14.9868(2) Å, α = 68.297(1), β = 76.172(1), γ = 80.378(1)°, sp.gr. P - 1, Z = 1, ρcalc.  = 1.537 g/cm3 . The structure belongs to the insular type and is presented as the isolated complex Eu2 -dimeric formations and outer-sphere HMBA. The structural aspects of the observed luminescence and triboluminescence properties of the compound were discussed, and the cleavage plane role in the process of the crystal destruction was revealed.

Biosafety evaluation of BaSi2O2N2:Eu2+/PDMS composite elastomers

In recent years, mechanoluminescent (ML) materials have shown great potential in stress sensing, mechanical energy collection and conversion, so they have attracted wide attention in the field of stomatology. In the early stage of this study, BaSi₂O₂N₂:Eu²⁺ ML phosphors were synthesized by two-step high temperature solid state method, and then mixed with Polydimethylsiloxane (PDMS) in different proportions to obtain BaSi₂O₂N₂:Eu²⁺/PDMS ML composites with different mass fractions (10%,20%,30%,40%,50%). Then its biosafety was evaluated by Cell Counting Kit-8 (CCK-8), Calcein-AM/PI fluorescence staining, hemolysis, oral mucosal irritation, acute and subacute systemic toxicity tests. The experimental results show that the biosafety of BaSi₂O₂N₂:Eu²⁺/PDMS ML composite elastomers with different mass fraction is in line with the existing standards, and other related properties can be further studied.

Chromic triboluminescence of self-assembled platinum(II) complexes

A series of Pt(II) complexes bearing N-heterocyclic carbenes, [Pt(CN)₂(Rim-Mepy)] (Rim-MepyH⁺ = 3-alkyl-1-(4-methyl-(2-pyridinyl))-1H-imidazolium, R = Me, Et, ⁱPr, or ^tBu), exhibits triboluminescence in the visible range from blue to red, as well as the corresponding intense photoluminescence. Remarkably, among the complexes, the ⁱPr-substituted one exhibits chromic triboluminescence behaviour during the process of rubbing and also vapour exposure.

Realizing Photoswitchable Mechanoluminescence in Organic Crystals Based on Photochromism

Organic mechanoluminescent (ML) materials possessing photophysical properties that are sensitive to multiple external stimuli have shown great potential in many fields, including optic and sensing. Particularly, the photoswitchable ML property for these materials is fundamental to their applications but remains a formidable challenge. Herein, photoswitchable ML is successfully realized by endowing reversible photochromic properties to an ML molecule, namely 2-(1,2,2-triphenylvinyl) fluoropyridine (o-TPF). o-TPF shows both high-contrast photochromism with a distinct color change from white to purplish red, as well as bright blue ML (λ_ML  = 453 nm). The ML property can be repeatedly switched between ON and OFF states under alternate UV and visible light irradiation. Impressively, the photoswitchable ML is of high stability and repeatability. The ML can be reversibly switched on and off by conducting alternate UV and visible light irradiation in cycles under ambient conditions. Experimental results and theoretical calculations reveal that the change of dipole moment of o-TPF during the photochromic process is responsible for the photoswitchable ML. These results outline a fundamental strategy to achieve for the control of organic ML and pave the way to the development of expanded smart luminescent materials and their applications.

Two Cholesterol-Containing Pyrene Derivatives: Subtle Spacer Difference, Diverse Stimuli-Responsive Luminescence, Chirality, and Self-Assembly Behaviors

Two chiral molecules 1 and 2 were designed and synthesized with a pyrene moiety directly linked to a chiral cholesterol moiety and connected through a methylene spacer, respectively. Influence of the spacer on their stimuli-responsive luminescence, chirality, and self-assembly behaviors was systematically investigated. Molecules 1 and 2 had similar aggregation-induced emission enhancement (AIEE) in solution, because of carrying the same fluorescence moiety. Both molecules displayed mechanochromism (MC) property but with different color contrast, whereas only 2 showed mechanoluminescence (ML) activity. When doping in liquid crystal molecule 5CB, both molecules induced the formation of chiral nematic liquid crystals (N*-LCs) with strong circularly polarized luminescence (CPL). Molecule 2 induced single handedness signal, irrespective of doping ratios, while 1-doped N*-LCs showed an inversion of CPL signal from negative to positive upon the increase of doping ratios. Molecules 1 and 2 also self-assembled into different coassemblies with 5CB. Their distinct behaviors were attributed to the influence of the methylene spacer, which caused different molecular conformation and steric bulkiness; accordingly, it changed intermolecular interactions and molecular packing of the two molecules and led to diverse chirality and luminescence. This work provided important model molecules to better understand the molecular structure-property relationship and guide the design of novel functional molecules.

Versatile Method of Generating Triboluminescence in Polymer Films Blended with Common Luminophores

Herein we report a versatile method for the preparation of triboluminescent polymer films by physical blending with common luminophores. This method does not require the presence of a crystalline phase or the use of materials known to be triboluminescent. Emission is generated in response to friction of the polymer surface via triboelectrification, either by rubbing directly or through an inert coating layer, even with low applied stress (<0.1 MPa). Our findings offer a convenient and practical method of preparation of triboluminescent, amorphous polymer films with easily tunable emission properties.

Advances in Pure Organic Mechanoluminescence Materials

Mechanoluminescence (ML) is the production of light from materials in response to the external stimulus of mechanical action. For organic compounds, the production of ML is tightly associated with fracture and plastic deformation of materials with piezoelectric effect in crystal lattice, and the ML property is highly dependent on the molecular packing mode, spatial conformation, and intermolecular interaction in the solid state. In the past few years, our group focused on the molecular design of pure organic ML compounds, with an attempt to discover different features of ML in pursuing the inherent emission mechanism and potential practical applications. We successfully found polymorph-dependent ML, ML with a phosphorescent property, conformation-dependent ML, ML with odd-even effect, wearable ML devices applied in heartbeat and pressure detection, etc. In this Perspective, we aim to deepen the understanding of ML and provide some guidance for the molecular design of organic light-emitting materials through the combination of our contributions.

MgF2:Mn2+: novel material with mechanically-induced luminescence

Mechanoluminescent (ML) materials can directly convert external mechanical stimulation into light without the need for excitation from other forms of energy, such as light or electricity. This alluring characteristic makes ML materials potentially applicable in a wide range of areas, including dynamic imaging of force, advanced displays, information code, storage, and anti-counterfeiting encryption. However, current reproducible ML materials are restricted to sulfide- and oxide-based materials. In addition, most of the reported ML materials require pre-irradiation with ultraviolet (UV) lamps or other light sources, which seriously hinders their practical applications. Here, we report a novel ML material, MgF₂:Mn²⁺, which emits bright red light under an external dynamic force without the need for pre-charging with UV light. The luminescence properties were systematically studied, and the piezophotonic application was demonstrated. More interestingly, unlike the well-known zinc sulfide ML complexes reported previously, a highly transparent ML film was successfully fabricated by incorporating MgF₂:Mn²⁺ into polydimethylsiloxane (PDMS) matrices. This film is expected to find applications in advanced flexible optoelectronics such as integrated piezophotonics, artificial skin, athletic analytics in sports science, among others.

Photo- and triboluminescent pyridinophane Cu complexes: New organometallic tools for mechanoresponsive materials

The development of mechanoresponsive polymers has emerged as a new, attractive area of research in which changes at the molecular level exert macrolevel effects in the bulk material, and vice...

Mechanoluminescence Rebrightening the Prospects of Stress Sensing: A Review

The emergence of new applications, such as in artificial intelligence, the internet of things, and biotechnology, has driven the evolution of stress sensing technology. For these emerging applications, stretchability, remoteness, stress distribution, a multimodal nature, and biocompatibility are important performance characteristics of stress sensors. Mechanoluminescence (ML)-based stress sensing has attracted widespread attention because of its characteristics of remoteness and having a distributed response to mechanical stimuli as well as its great potential for stretchability, biocompatibility, and self-powering. In the past few decades, great progress has been made in the discovery of ML materials, analysis of mechanisms, design of devices, and exploration of applications. One can find that with this progress, the focus of ML research has shifted from the phenomenon in the earliest stage to materials and recently toward devices. At the present stage, while showing great prospects for advanced stress sensing applications, ML-based sensing still faces major challenges in material optimization, device design, and system integration.

Photo- and triboluminescent robust 1D polymers made of Mn(ii) halides and meta-carborane based bis(phosphine oxide)

The 1D CPs of [Mn(L)X₂]_n type (X = Cl, Br, I), designed on 1,7-bis(diphenylphosphinyl)-m-carborane (L), show bright phosphorescence and triboluminescence, as well as exhibit thermo- and solvatochromic luminescence.

A family of Mn(ii) complexes exhibiting strong photo- and triboluminescence as well as polymorphic luminescence

New Mn(ii) complexes supported by diphosphine dioxides exhibit strong room-temperature phosphorescence as well as bright triboluminescence and polymorphic luminescence.

Mechanics and physics of a glass/particles photonic sponge

A glass containing mechanoluminescent crystalline particles behaves as a photonic sponge: that is to say it fills up with trapped electrons when exposed to UV light, and it emits light when submitted to a mechanical loading, similar to a sponge soaked with water that is wringed under mechanical action! A major finding of the present study is that the elasto-mechanoluminescence effect showing up on unloading is governed by the deviatoric part of the applied stress (no effect under hydrostatic pressure). Furthermore, the structural source for this phenomenon was elucidated by a detailed density functional theory analysis of the e⁻ energetics at the possible oxygen vacancy sites within the crystalline phase. Both the e⁻ trapping and detrapping processes under load could be explained. An analogy with hydraulic circuits and the rheology of viscoelastic media was successfully introduced to pave the way to a constitutive law for the mechano-optical coupling phenomenon.

Triboluminescence of a new family of CuI-NHC complexes in crystalline solid and in amorphous polymer films

Triboluminescent compounds that generate emission of light in response to mechanical stimulus are promising targets in the development of “smart materials” and damage sensors. Among triboluminescent metal complexes, rare-earth europium and terbium complexes are most widely used, while there is no systematic data on more readily available and inexpensive Cu complexes. We report a new family of photoluminescent Cu–NHC complexes that show bright triboluminescence (TL) in the crystal state visible in ambient indoor light under air. Moreover, when these complexes are blended into amorphous polymer films even at small concentrations, TL is easily observed. Observation of TL in polymer films overcomes the limitation of using crystals and opens up possibilities for the development of mechanoresponsive coatings and materials based on inexpensive metals such as Cu. Our results may also have implications for the understanding of the TL effect's origin in polymer films.

Triboluminescent compounds that generate emission of light in response to mechanical stimulus are promising targets in the development of “smart materials” and damage sensors.

A ZnS/CaZnOS Heterojunction for Efficient Mechanical-to-Optical Energy Conversion by Conduction Band Offset

Actively collecting the mechanical energy by efficient conversion to other forms of energy such as light opens a new possibility of energy-saving, which is of pivotal significance for supplying potential solutions for the present energy crisis. Such energy conversion has shown promising applications in modern sensors, actuators, and energy harvesting. However, the implementation of such technologies is being hindered because most luminescent materials show weak and non-recoverable emissions under mechanical excitation. Herein, a new class of heterojunctioned ZnS/CaZnOS piezophotonic systems is presented, which displays highly reproducible mechanoluminescence (ML) with an unprecedented intensity of over two times higher than that of the widely used commercial ZnS (the state-of-the-art ML material). Density functional theory calculations reveal that the high-performance ML originates from efficient charge transfer and recombination through offset of the valence and conduction bands in the heterojunction interface region. By controlling the ZnS-to-CaZnOS ratio in conjunction with manganese (Mn²⁺ ) and lanthanide (Ln³⁺ ) doping, tunable ML across the full spectrum is activated by a small mechanical stimulus of 1 N (10 kPa). The findings demonstrate a novel strategy for constructing efficient ML materials by leveraging interface effects and ultimately promoting practical applications for ML.

Tribo-Induced Near-Infrared Light Emission between Metal and Quartz

Triboluminescence (TL) refers to the luminescence phenomenon at the material surface under the action of pressure or shear. This fascinating phenomenon can directly convert mechanical energy into light emission without the need for other auxiliary components; therefore, it attracts more and more researchers to conduct research in different wavelength ranges, such as X-ray, ultraviolet, visible light, and terahertz. However, there have been few reports on the study of the near-infrared (NIR) range, which is very important in the integrity of the triboluminescence research. In this research, we found that NIR light with a wavelength ranging from 800 to 1000 nm was generated by friction between solid metals and a quartz crystal. Analysis of the cross section of the quartz disk after friction revealed that the TL phenomenon had a strong relationship with the doping of metal grains into the silica. Density functional theory (DFT) and X-ray photoelectron spectroscopy were also conducted to further identify the results. We infer that such light emission arises from the implantation of metal grains into the surface of the quartz, which forms a metal-insulator junction with amorphous silica. Moreover, electron transition between the metal and the insulator, followed by a transition at the center of the defects, causes near-infrared light emission. Our research reveals the infrared luminescence behavior from a different perspective, the transfer of materials, and perhaps deepens the understanding of the near-infrared emission mechanism.

Stretchable Hybrid Bilayered Luminescent Composite Based on the Combination of Strain-Induced and Triboelectrification-Induced Electroluminescence

High luminescence intensity from materials that are excited by external stimuli is highly desired. In this work, a stretchable hybrid luminescent composite (HLC) that has multiple luminescence modes is reported. The luminescence can be excited either by externally applied mechanical strain or by a moving object that slides against the HLC. When the HLC is deformed, such as being twisted or folded, the ZnS/Cu phosphor experiences mechanical strain that trigger the mechanoluminescence (ML) of the phosphors. Moreover, as the HLC slides against a contact object, the triboelectrification at the contact interface induces the electroluminescence of phosphor. Here, a series of internal and external factors were studied on how they influence the luminescent intensity. It is found that the luminescent intensity from the two modes can be superposed. The HLC material was used to fabricate a fiber-based luminescent device that can be driven by air flow. The overall luminescent intensity is enhanced by over 72% compared to that obtained solely from the ML. The HLC reported in this work has such potential applications as self-powered light sources and sensors as means of detecting dynamic motions and interaction.

Ultralong UV/mechano-excited room temperature phosphorescence from purely organic cluster excitons

Purely organic room temperature phosphorescence (RTP) has attracted wide attention recently due to its various application potentials. However, ultralong RTP (URTP) with high efficiency is still rarely achieved. Herein, by dissolving 1,8-naphthalic anhydride in certain organic solid hosts, URTP with a lifetime of over 600 ms and overall quantum yield of over 20% is realized. Meanwhile, the URTP can also be achieved by mechanical excitation when the host is mechanoluminescent. Femtosecond transient absorption studies reveal that intersystem crossing of the host is accelerated substantially in the presence of a trace amount of 1,8-naphthalic anhydride. Accordingly, we propose that a cluster exciton spanning the host and guest forms as a transient state before the guest acts as an energy trap for the RTP state. The cluster exciton model proposed here is expected to help expand the varieties of purely organic URTP materials based on an advanced understanding of guest/host combinations.

Triboionization: a Novel Ionization Method by Peeling of Cohesive Substances for Mass Spectrometry

A novel ionization/sampling method termed triboionization was developed. Triboionization is an ionization method that only uses cohesive substances, such as food wrap or sticky tape, and does not require an electrode, electric power supply, heat source, light source, radiation, or gas, unlike most other conventional ambient ionization methods. In this study, the sample compound attached to adhesive tape or plastic wrap was quickly peeled off at a distance of approximately 2 cm from the atmospheric interface of a mass spectrometer. All of the five types of food wrap and 13 types of adhesive tape tested successfully ionized caffeine. Nine out of ten model compounds were detected as the corresponding molecular ions in the positive or negative mode by this ionizing contrivance using an oriented polypropylene adhesive tape. The detected molecular ions were typically protonated molecules or sodium adducts in the positive mode or deprotonated molecules in the negative mode. The elemental compositions of the observed ions were confirmed within 5 ppm by high-resolution mass spectrometry. The triboionization phenomenon was considered to depend on physical and electronic events caused by peeling off a cohesive substance. Triboionization is able to provide a compact ion source using only mechanical mechanisms. Additionally, triboionization allows sticky tape to be used as a convenient sampling device for surface analysis.

Fundamental Studies of New Ionization Technologies and Insights from IMS-MS

Exceptional ion mobility spectrometry mass spectrometry (IMS-MS) developments by von Helden, Jarrold, and Clemmer provided technology that gives a view of chemical/biological compositions previously not achievable. The ionization method of choice used with IMS-MS has been electrospray ionization (ESI). In this special issue contribution, we focus on fundamentals of heretofore unprecedented means for transferring volatile and nonvolatile compounds into gas-phase ions singly and multiply charged. These newer ionization processes frequently lead to different selectivity relative to ESI and, together with IMS-MS, may provide a more comprehensive view of chemical compositions directly from their original environment such as surfaces, e.g., tissue. Similarities of results using solvent- and matrix-assisted ionization are highlighted, as are differences between ESI and the inlet ionization methods, especially with mixtures such as bacterial extracts. Selectivity using different matrices is discussed, as are results which add to our fundamental knowledge of inlet ionization as well as pose additional avenues for inquiry. IMS-MS provides an opportunity for comparison studies relative to ESI and will prove valuable using the new ionization technologies for direct analyses. Our hypothesis is that some ESI-IMS-MS applications will be replaced by the new ionization processes and by understanding mechanistic aspects to aid enhanced source and method developments this will be hastened.

Mechanoluminescence of Ce/Tb inorganic salts in methane-acetylene mixtures with inert gases

The mechanoluminescence of cerium (Ce) and terbium (Tb) lanthanide salts is studied in hydrocarbon [methane (CH₄ ) and acetylene (C₂ H₂ )] and inert [helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe)] gaseous mixtures. The lines of *N₂ , *Ln³⁺ , inert gases, *CH, and *C₂ radicals resulted from the mechanochemical decomposition of CH₄ and C₂ H₂ are observed in the emission spectrum. The luminescence intensity of the inert gases decreases with the hydrocarbon gas concentration in the mixture. The intensities of the *CH or *C₂ bands remains almost unchanged within 15-100 vol% of CH₄ or C₂ H₂ in the mixture. When the concentration of CH₄ or C₂ H₂ is lower than 15%, the intensities of the CH or C₂ bands increase achieving their maxima at 0.5-3% of the hydrocarbon. This is probably due to the optimal compositions of the mixtures with the most efficient generation of electrical discharges responsible for mechanoluminescence.

A Review of Mechanoluminescence in Inorganic Solids: Compounds, Mechanisms, Models and Applications

Mechanoluminescence (ML) is the non-thermal emission of light as a response to mechanical stimuli on a solid material. While this phenomenon has been observed for a long time when breaking certain materials, it is now being extensively explored, especially since the discovery of non-destructive ML upon elastic deformation. A great number of materials have already been identified as mechanoluminescent, but novel ones with colour tunability and improved sensitivity are still urgently needed. The physical origin of the phenomenon, which mainly involves the release of trapped carriers at defects with the help of stress, still remains unclear. This in turn hinders a deeper research, either theoretically or application oriented. In this review paper, we have tabulated the known ML compounds according to their structure prototypes based on the connectivity of anion polyhedra, highlighting structural features, such as framework distortion, layered structure, elastic anisotropy and microstructures, which are very relevant to the ML process. We then review the various proposed mechanisms and corresponding mathematical models. We comment on their contribution to a clearer understanding of the ML phenomenon and on the derived guidelines for improving properties of ML phosphors. Proven and potential applications of ML in various fields, such as stress field sensing, light sources, and sensing electric (magnetic) fields, are summarized. Finally, we point out the challenges and future directions in this active and emerging field of luminescence research.

Spontaneous Charge Separation and Sublimation Processes are Ubiquitous in Nature and in Ionization Processes in Mass Spectrometry

Ionization processes have been discovered by which small and large as well as volatile and nonvolatile compounds are converted to gas-phase ions when associated with a matrix and exposed to sub-atmospheric pressure. Here, we discuss experiments further defining these simple and unexpected processes. Charge separation is found to be a common process for small molecule chemicals, solids and liquids, passed through an inlet tube from a higher to a lower pressure region, with and without heat applied. This charge separation process produces positively- and negatively-charged particles with widely different efficiencies depending on the compound and its physical state. Circumstantial evidence is presented suggesting that in the new ionization process, charged particles carry analyte into the gas phase, and desolvation of these particles produce the bare ions similar to electrospray ionization, except that solid particles appear likely to be involved. This mechanistic proposition is in agreement with previous theoretical work related to ion emission from ice. Graphical Abstract ᅟ.

A simple and versatile strategy for realizing bright multicolor mechanoluminescence

Bright multicolor mechanoluminescence (ML) was first realized using a stable organic blue ML emitter, N-phenylcarbazole, as the host.

Bright NUV mechanofluorescence from a terpyridine-based pure organic crystal

Bright NUV mechanofluorescence from a terpyridine-based pure organic crystal with non-centrosymmetric packing and a piezoelectric space group.

Opposite mechanoluminescence behavior of two isomers with different linkage positions

A new strategy for producing pure organic “high molecular weight” ML luminogens was proposed by simply changing linkage positions between aromatic blocks.

Touch-sensitive mechanoluminescence crystals comprising a simple purely organic molecule emit bright blue fluorescence regardless of crystallization methods

Simple N-phenylcarbazole can readily form highly active mechanoluminescence crystals and emit bright blue fluorescence upon gentle grinding.

Mechanistic Investigation of Inducing Triboluminescence in Lanthanide(III) β-Diketonate Complexes

In this work, we synthesized a series of lanthanide(III) β-diketonate complexes to investigate the induction of triboluminescence. Triboluminescence (TL) spectra, solid-state emission spectra, and luminescence lifetimes of the complexes were obtained to prove consistent emitting species for steady-state and triboluminescence measurements. Detailed analyses of the crystal lattice packing were conducted in an attempt to correlate crystal symmetry, gas discharge, and structural arrangements with "triboexcitation", and it is found that either noncentrosymmetric or centrosymmetic compounds can be TL-active. Furthermore, an intensely TL compound, Eu(dbm)₄TMP, was achieved, and its light emission can be seen under daylight upon mechanical stress.

Mechanoluminescence from pure hydrocarbon AIEgen

The pure hydrocarbon molecule of 1,1,2,2-tetrakis(4-ethynylphenyl)ethene (TETPE), with an ideal symmetric molecular geometry, exhibits the character of aggregation-induced emission (AIE) and mechanoluminescence (ML), mainly due to its twist structure and strong intermolecular static electric interaction.