Mechanochromic Luminescent Materials Research Articles

Designing High-Sensitivity Mechanochromic Luminescent Materials Through Friction-Induced Crystallization Strategy.

Despite recent significant breakthroughs in novel mechanochromic luminescent (ML) materials, developing a high-sensitivity ML material is still challengeable. Herein, a "friction-induced crystallization" strategy is proposed to realize highly sensitive transformations of luminescent signal, through an integration of polymeric chains and an aggregation-sensitive luminescent core, which act as mechanical sensors and fluochromic actuators, respectively. The coupling of these two components enables the material to crystallize in response to shear friction, thereby exhibiting blue-shift fluorescence due to a more restricted relaxation pathway. This study underscores a high-sensitivity ML material based on the precise regulation of molecular-scale motions, and also expands the scope and potential of ML materials toward user-friendly, interactive wearable devices.

Mechanochromic luminescent material with high quantum yield for multi-mode anti-counterfeiting applications

The multi-mode anti-counterfeiting materials, constructed by mechanochromic luminescent materials with high quantum yield, can effectively prevent illegal counterfeiting, but their design and construction still present significant challenges. Herein, a twisting D-A-D conjugated molecule, FBtF, is obtained with a special hybridized local and charge transfer excited state, and shows approximately 100 % photoluminescence quantum yield in the coating film. The self-assembled FBtF microplates exhibit bathochromic-shifted emission from green to orange, accompanied by the photoluminescence quantum yield increase from 47.8 % to 89.0 % upon grinding, which is the highest in reported MCL materials, to the best of our knowledge. In contrast, a D-π-D type molecule (FBenF) and a D-D’-D molecule (FAnF) maintain the blue-emission after grinding. Through mechanistic investigation, the force-induced molecular conformational planarization and the breakage of the intermolecular charge transfer collaboratively lead to the emission-color change and photoluminescence quantum yield increase. The FBtF microplates are designed as ink for anti-counterfeiting applications based on multi-mode output signals, including the UV light-excited image and the grinding-induced emission color change together with the photoluminescence quantum yield increase. The written tags exhibit high stability in various harsh environments, and demonstrate superior anti-counterfeiting performance in practical applications, such as medicine anti-counterfeiting.

Organic Mechanochromic Luminescent Materials with Self-Recovering Characters.

Recently, applied research on stimuli-responsive materials with luminescence-switching characteristics has been conducted in various fields. A representative phenomenon of stimuli-responsive luminescent materials is mechanochromic luminescence (MCL), which exhibits luminescent color change induced by mechanical stimuli such as grinding. These materials are among the most prominent candidates for security and sensing applications. Interestingly, some mechanochromic luminescent materials have shown self-recovery character, in which their original luminescent color can be recovered by just standing under ambient conditions after grinding. Although there are more and more reports of such materials in recent years, the fundamental principles of molecular design still remain elusive. In this concept, we summarize distinctive advances in mechanochromic luminescent materials with self-recovery according to the core structures of luminescent molecules. Controlling amorphous state by introducing substituents such as alkyl or polar groups is effective method to provide self-recovering properties.

AIE-active dyes based on Tröger's base: Synthesis and mechanochromic luminescence

Three fluorescent dyes (DI-TB, ICZ-TB and ITPA-TB) were synthesized based on 1,3-indindanedione substituted Tröger's base (TB). All the dyes exhibited AIE properties in the mixed system of chloroform and n-hexane. The compounds showed reversible MCL properties with obvious fluorescence changes, which were confirmed by XRD and DSC. The fluorescence of ITPA-TB showed high contrast (68 nm) before and after grinding, and the fluorescence changed from yellow (546 nm) to red (614 nm). In addition, the three solid compounds showed good thermal stability by thermogravimetric analysis. The TB dyes further opened up new ideas for the development of TB-based mechanochromic luminescent materials.

Investigation of Mechanochromic Luminescence of Pyrene-based Aggregation-Induced Emission Luminogens: Correlation between Molecular Packing and Luminescence Behavior.

To better understand the correlation between molecular structure and optical properties such as aggregation-induced emission (AIE) and mechanochromic luminescence (MCL) emission, two new pyrene-based derivatives with substitutions at the 4- and 5-positions (1HH) and at the 4-, 5-, 9-, and 10-positions (2HH) were designed and synthesized. Cyano groups were introduced at the periphery of the synthesized compounds (1HCN, 1OCN, 1BCN, 2HCN, 2OCN, and 2BCN) to investigate the influence of these groups on the emission properties of the pyrene derivatives both in solution and in the solid state. The fluorescence emission performance of these compounds in water/acetone mixtures was simultaneously studied, revealing outstanding aggregation-induced emission properties. The typical shift in emission maxima to higher values was attributed to J-aggregate formation in the aggregate state. Careful investigation of the crystal structures demonstrated abundant and intense intermolecular interactions, such as C-H…π and C-H…N hydrogen bonds, contributing to the remarkable mechanochromic luminescence performance of these compounds. The MCL properties of all the compounds were investigated using powder X-ray diffraction, and the remarkable mechanochromic properties were attributed to J-aggregate phenomena in the solid state. These results provide valuable insights into the structure-property relationship of organic MCL materials, guiding the design of efficient organic MCL materials.

Visualization and Depth Estimation of Inner Crack in High‐Pressure Equipment Based on Organic Mechanochromic Luminescent Materials

AbstractInvisible hydrogen‐induced cracking or stress corrosion cracking easily appears in high‐pressure equipment in service. Mechanoresponsive luminogens (MRLs) can convert mechanical force into visible luminescence emission. Thus, MRL‐based detection methods to damage of structures have been paid extensive attention in recent years. However, the structural damage‐induced luminescence response and mechanism are still not fully comprehended. In this study, organic mechanochromic luminescent materials (1,1,2,2‐tetrakis (4‐nitrophenyl) ethene (TPE‐4N)) are proposed to detect the invisible inner crack in high‐pressure equipment. Because the high‐pressure equipment in service is subjected to tensile loading, the inner crack‐induced fluorescence response and mechanism under tension are investigated. The inner crack‐induced local strain concentration can be transformed into a visible green fluorescence, which can be easily observed from the outside. According to the appearance of fluorescence, the position of the inner crack can be detected. Moreover, the depth level of the inner crack can be quantitatively estimated using applied tensile loading and the ratio of fluorescent area. The investigations may provide a new idea of non‐destructive evaluation material and method for invisible damage in high‐pressure equipment.

Dibenzthiophene and carbazole modified dicyanoethylene derivative exhibiting aggregation-induced emission enhancement and mechanochromic luminescence

A novel D-A type dicyanoethylene derivative DBT-Cz-DCN was designed and synthesized using dibenzthiophene, dicyanoethylene and carbazole as raw materials. Photophysical studies indicated that the luminogen DBT-Cz-DCN possessed unique intramolecular charge transfer (ICT) characteristic from electron donating carbazole unit to electron accepting dicyanoethylene moiety and aggregation-induced emission enhancement (AIEE) property. Theoretical calculations indicated that DBT-Cz-DCN exhibited a highly twisted conformation due to the steric hindrance effect of the dicyanoethylene unit. It was worth noting that DBT-Cz-DCN also exhibited distinct mechanochromic luminescence property. Specifically, when the pristine powder of DBT-Cz-DCN underwent grinding, the emitting color shifted from yellow to orange-yellow. The emitting color and wavelength could be restored to the pristine state after fuming with dichloromethane vapor. Powder X-ray diffraction confirmed that the transformation between the amorphous state and the crystalline state under external force stimuli was the reason for the MCL (mechanochromic luminescent) behavior of DBT-Cz-DCN. The current work not only developed a novel molecular structure with AIEE characteristic and MCL behavior, but also laid the foundation for the development of MCL materials with high contrast, high luminescence efficiency and good reversibility.

Mononuclear copper(Ⅰ) complexes with mechanochromic thermally activated delayed fluorescence behaviour based on switchable hydrogen bonds

Four neutral mononuclear copper(I) complexes (1–4) with formula of [(N^P)Cu(PPh3)2] (1–2) and [(N^P)Cu(Xantphos)] (3–4) based on NH–deprotonated (2–(1H–benzimidazole)phenyl) diphenylphosphine ligands (N^P) and phosphine ancillary ligands have been synthesized. The copper(I) complexes exhibit bright thermally activated delayed fluorescence at room temperature with photoluminescence quantum yield up to 43.5%. Upon grinding, the complexes display apparent bathochromic shift in the emission maxima of up to 50 nm (1). Meanwhile, the complexes display similar luminescence profiles to those of the origin samples exposed to CH3OH vapor. Single crystal X–ray and PXRD diffraction results demonstrate that the reversible mechanochromic luminescent (MCL) properties are ascribed to the transformation of crystalline and amorphous state dependent on the disruption and restoration of hydrogen bonds during external stimuli, which have been rarely observed in the copper(I)-based MCL materials hitherto. These findings clearly suggest that the introduction of switchable secondary interactions in mononuclear copper(I) complex is a large-scale synthesis strategy for multifunctional luminescent materials.

Pressure-shortened delayed fluorescence lifetime of solid-state thermally activated delayed fluorescent 4CzIPN: the structure evolution.

The incorporation of mechanochromic luminescence into thermally activated delayed fluorescence (TADF) molecules is a promising strategy for developing multifunctional mechanochromic luminescent materials. Nevertheless, due to the difficulties in systematic design, it is still challenging to controllably exploit the versatility of TADF molecules. In this work, we were surprised to find that the delayed fluorescence lifetime of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene crystals was continuously shortened with increasing pressure, which was ascribed to the increasing HOMO/LUMO overlap by planarization of the molecular conformation, as well as the pressure-induced emission enhancement and obvious multicolor emission at high pressure (green to red), owing to the formation of new interactions and the part-planarization of the molecular conformation, respectively. This study not only developed a new function of TADF molecules, but also provided a route to shorten the delayed fluorescence lifetime, which is beneficial for designing TADF-OLEDs with small efficiency roll-off.

Aggregation induced emission and mechanochromic luminescence by cyanostilbene-based organic luminophores

In this manuscript, we report novel cyanostilbene based luminogens, 1CF3 and 1F2 that exhibit pronounced “aggregation induced emission” (AIE) characteristics and mechanochromic luminescence (MCL) switching in solid state. A poorly emissive methanol solution of 1CF3 or 1F2 can be highly emissive after addition of poor solvent. Our morphological study reveals that the molecules self-assemble to higher crystalline state at during AIE process. When the single crystals of 1CF3 or 1F2 were subjected to mechanical pressure, a prominent mechanochromic switching from greenish yellow emission to red emission was observed. The red emissive material can be switched back to its initial greenish yellow emissive material by solvent vapour fumigation. Powder X-ray diffraction and single crystal X-ray diffraction study revealed that the mechanochromic colour switching is due to the packing change induced by mechanical pressure. The practical applicability of the MCL materials was demonstrated for ink-free writing and solvent vapour erasing.

Highly twisted luminogen based on asymmetric D-A-D’ type dicyanovinyl derivative decorated dibenzofuran and carbazole with aggregation-induced emission enhancement and obvious mechanochromic behavior

A novel asymmetric D-A-D’ type dicyanovinyl derivative DBF-DCN-Cz, which was constructed with a dibenzofuran unit and a carbazole moiety bridged by a dicyanovinyl spacer, was designed and synthesized. It was found that the luminogen DBF-DCN-Cz showed unique intramolecular charge transfer (ICT) process from electron-donating carbazole unit to electron-accepting dicyanovinyl moiety. Optical studies and theoretical calculations indicated that DBF-DCN-Cz possessed highly twisted conformation and aggregation-induced emission enhancement (AIEE) property because of the presence of the large steric hindrance. Interestingly, the emitting color of solid powder of DBF-DCN-Cz changed from initial bright yellow to orange after grinding, showing obvious mechanochromic luminescence (MCL). XRD and DSC experiments confirmed that the MCL phenomenon originated from the phase transition from crystalline to amorphous states. This work might broaden the knowledge for AIE-active MCL materials and laid the foundation for their extensive applications.

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

Mechanoresponsive luminogen (MRL)-based real-time and visible detection method for biaxial fatigue crack propagation

Fatigue cracks under complex stress state is crucial for safety assessment. Owing to the characteristics of visualization, rapid response and nondestructive measurement, mechanoresponsive luminogen (MRL) based methods have a great potential for stress sensing and damage detection. In this study, fatigue crack propagation in aluminum alloys under biaxial loadings were detected using a pure organic mechanochromic luminescent material. Both fatigue crack propagation behavior and luminescence emission were investigated. Real-time fatigue crack length and crack propagation path can be successfully detected using the fluorescence distribution. The MRL-based methods may open up new opportunities for nondestructive evaluation and structural damage evaluation.

Large steric hindrance amide modified benzothiadiazole: High contrast mechanochromic luminescence and volatile acid vapor response

Three mechanochromic luminescent (MCL) materials were obtained by modifying the 5, 6 and 4, 7 positions of benzothiadiazole (BTD) with two sterically hindered amides and two aromatic groups, respectively. The Stokes shifts of the compounds were larger in different solvents (>6000 cm−1), especially 3c in dimethyl sulfoxide up to 8900 cm−1. Upon response to mechanical force, the fluorescence of 3c had high contrast (75 nm) before and after milling, and the emission changed from bright yellow light (533 nm) to orange-red light (608 nm). Meanwhile, strong fluorescence emission intensities were observed before and after grinding (quantum yield: 56%, 46% respectively). The MCL reasons of the compounds were explained in detail by theoretical calculation, single crystal X-ray diffraction (XRD), powder XRD and differential scanning calorimetry (DSC) analysis. In addition, the MCL materials also showed fluorescence responsiveness to the vapor of volatile acids.

Recent Progress on Designable Hybrids with Stimuli-Responsive Optical Properties Originating from Molecular Assembly Concerning Polyhedral Oligomeric Silsesquioxane.

In this review, we describe recent progress on stimuli-responsive hybrid materials based on polyhedral oligomeric silsesquioxane (POSS) and their applications as a chemical sensor. In particular, we explain the unique functions originating from molecular assembly concerning POSS-containing soft materials mainly from our studies. POSS has an inorganic cubic core composed of silicon-oxygen (Si-O) bonds and organic substituents at each vertex. Owing to intrinsic properties of POSS, such as high thermal stability, rigidity, and low chemical reactivity, various robust hybrid materials have been developed. From the numerous numbers of POSS hybrids, we herein focus on the environment-sensitive optical materials in which molecular assembly of POSS itself and functional units connected to POSS should be a key factor for expressing material properties. We also explain the mechanisms of chemical sensors originating from these stimuli-responsive optical properties. Stimuli-responsive excimer emission and pollutant detectors, nanoplastic sensors with the water-dispersive POSS networks, trans fatty acid sensors, turn-on luminescent sensors for aerobic condition and fluoride anion sensors are described. We also mention the mechanochromic polyurethane hybrids and the thermally-durable mechanochromic luminescent materials. The roles of the unique optical properties from soft materials composed of rigid POSS, which doesn't have significant light-absorption and emission properties in the visible region, are surveyed.

Highly emissive D-A-π-D type aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE)-active benzothiadiazole derivatives with contrasting mechanofluorochromic features

Mechanochromic luminophors with strong solid-state emission are promising candidates for high-contrast mechanochromic luminescence materials. Meanwhile, mechanically responsive luminogenic molecules with tricolor switching are highly desirable but are seldom reported. In this work, three anthracene-based donor–acceptor-π-donor (D-A-π-D) type benzothiadiazole derivatives were designed and synthesized. These luminogens showed remarkable aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE) effect. Furthermore, these luminogens exhibited bright and different solid state fluorescence involving yellow-green, yellow and orange colors, and the fluorescence of their solids could be effectively regulated by mechanical grinding. For luminogen 1, its solid displayed reversible two-color mechanofluorochromic property. As for luminogens 2 and 3, their solids displayed fluorescent colors change from yellow to yellow-green upon slight grinding, and the yellow-green light-emitting solids were converted into orange fluorescent solids after heavy grinding, demonstrating interesting three-color mechanofluorochromism features.

New Rofecoxib-Based Mechanochromic Luminescent Materials and Investigations on Their Aggregation-Induced Emission, Acidochromism, and LD-Specific Bioimaging.

Development of new mechanochromic luminescent (MCL) materials from aggregation-induced emission luminogens (AIEgens) has attracted wide attention due to their potential application in multiple areas. However, rational design and crafting of new MCL materials from the simple AIEgens skeleton is still a big challenge because of the undesirable concentration quenching effect. In this study, we have constructed a new class of MCL materials by adding one phenyl as a new rotator and incorporating one pair of electron donor (D) and acceptor (A) into the system of rofecoxib skeleton. This strategy endowed the compounds (Y1-Y8) with tunable emission behavior and some of them with the AIE effect and reversible MCL behavior. These properties may be caused by the highly twisted conformation and loosely molecular packing modes, which were elucidated clearly by analyzing the data of single-crystal X-ray diffraction, powder X-ray diffraction, and differential scanning calorimetry. Further investigation revealed that Y7 displayed acidochromic property due to the protonation of the nitrogen atom. Moreover, Y7, as a typical compound, showed its potential applications in the area of anticounterfeiting, pH sensor, and LD-specific bioimaging.

A Precise Molecular Design to Achieve ACQ‐to‐AIE Transformation for Developing New Mechanochromic Material by Regio‐Isomerization Strategy**

AbstractHerein, we have designed and synthesized a series of MeO‐rofecoxib based fluorophore (MORF1–6) for exploiting new promising mechanochromic luminescent (MCL) materials. Among them, MORF1 and MORF3 exhibit aggregation‐induced emission (AIE) effects when the substituent is located at the ortho‐position on the benzene ring of b. Meanwhile, by changing the type and position of the substituent, the corresponding compounds MORF2, MORF4, MORF5, MORF6 show aggregation caused‐quenching (ACQ) behavior. These results demonstrate that the position and type of the substituent have a great effect on their fluorescent properties. Furthermore, an X‐ray crystal structure analysis of MORF3 revealed that the intermolecular π‐π stacking was suppressed and intramolecular hydrogen bonding were formed, which could effectively restrict the molecular motions, thus causing typical AIE effect and mechanochromic property. Finally, a re‐writable data recording device was fabricated using MORF3 as the active material. Our molecular design strategy may provide a new avenue for achieving efficient MCL materials.

Twisted aggregation-induced emission luminogens (AIEgens) contribute to mechanochromism materials: a review

Mechanochromic luminescence (MCL) materials have several promising advantages, such as controllable response, high sensing ability and recyclable utilization, and thus can be widely used as file encryption, security inks, stress sensors, etc.

Intense mechanoluminescence in an organic donor-acceptor crystal: Grinding induced crystal-to-crystal phase transformation

Mechanochromic luminescent (MCL) materials with sensitive and reversible responses have attracted extensive attention in many fields. Herein, we report a donor-acceptor (D-A) phenothiazine derivative (CzS-CN), whose crystals show obvious hypochromatic shifts upon anisotropic grinding. In this work, three different crystals based on the phenothiazine derivative were prepared by solvent evaporation method. Due to the different solvents used during crystallization, these three crystals show different molecular arrangements and stacking methods, and thus they present different shapes, including needle-like (crystal-A), flake-like (crystal-B) and block-like (crystal-C). In addition, these three kinds of crystals showed mechanochromic luminescent property with a remarkable change in emission colors from yellow to yellow-green after grinding. To our excitement, these crystals can be partially recovered by heating at 120 °C for 5min or recrystallized in solvents. Detailed spectroscopic and structural analyses revealed that the hypochromatic shifted emission under grinding and the recovery of fluorescence under recrystallization or heating are attributed to the crystal-to-crystal transformation. On the basis of these crystals, reversible crystal-to-crystal phase transitions in response to external stimuli (such as heating and recrystallization) are achieved, which is ultimately attributed to the stacking mode and molecular structure. This work reports a kind of MCL material, which shows the phase transition from crystal to crystal, and reveals some important information to better understand the relationship between molecular packing and mechanochromic behaviors.