Mechanofluorochromic Materials Research Articles

Fluorescent Enhancement of [AgS4] Microplates by Mechanical Force Induced Crystallinity Breaking.

Mechanofluorochromic materials are a type of "smart" material because of their adjustable fluorescent properties under external mechanical force, making them significant members of the materials family. However, as the fluorescent characteristics of these materials highly depend on their microstructures, the still insufficiently in-depth research linking molecular structures to light emission motivates researchers to explore the fluorescent properties of these materials under external stimuli. In this work, based on synthetic [AgS4] microplates, we explore a fascinating mechanical-induced photoluminescent enhancement phenomenon. By applying mechanical force to solid-state [AgS4] to damage the surface morphology, a significant enhancement in photoluminescence is observed. Moreover, the emitted intensity increases with the extent of damage, which can be attributed to alterations in crystallinity. This work provides valuable insights into the relationship among photoluminescence, crystallinity, and mechanical force, offering new strategies for designing luminescent devices.

Comprehensive undergraduate experiment: synthesis and chromism of 4-[bis(4-methylphenyl)amino] benzaldehyde

To solve the problems of the lack of property research in organic synthesis experiments and the relative independence of instrumental analytical methods in experiments, we designed a comprehensive undergraduate experiment based on mechanofluorochromic materials. In this project, 4-[bis(4-methylphenyl)amino] benzaldehyde was synthesized via the Vilsmeier-Haack reaction using 4,4'-dimethyltriphenylamine as the raw material. The product was then characterized by mass spectrometry, infrared absorption spectroscopy, and nuclear magnetic resonance spectroscopy. The solvatofluorochromism and mechanofluorochromism of the target material were studied using ultraviolet-visible absorption spectroscopy, fluorescence spectroscopy, etc. Furthermore, the mechanism of mechanofluorochromism was determined using powder X-ray diffraction. Organic synthesis and a series of instrumental analytical methods were combined to form an integrated experiment. The experiment is interesting, scientific, and comprehensive for undergraduates as a creative exercise; moreover, it can inspire their interest in chemical research, cultivate a variety of experimental operation abilities, improve creative-thinking skills, and encourage the development of effective solutions to existing problems in chemical experiments.

Reversible Red‐/Blue‐Shifted Mechanofluorochromic Properties of Indole‐Based Difluoroboron Compounds with Aggregation‐Induced Emission

AbstractTo further explore the relationship between structure and property of the mechanofluorochromic materials, two indole‐based aggregation‐induced emission compounds (I−Ph−BF2 and I−PhNp−BF2) with BF2‐core were designed and synthesized. They displayed intramolecular charge transfer characteristics and dissimilar reversible mechanofluorochromic behaviors (red‐shift for I−Ph−BF2 and blue‐shift for I−PhNp−BF2). Based the powder X‐ray diffraction patterns and quantum chemical calculation results, it was responsible for the blue‐shift mechanofluorochromic behavior of I−PhNp−BF2 that the reduced molecular polarity of the excited state and the undermined partially ordered structure. In contrast, the phase change between an ordered arrangement state and a disordered amorphous state of I−Ph−BF2 solid powder, as well as the appropriate molecular polarity, brought out red‐shift mechanofluorochromic effect. It provided a foundation for further exploration of the mechanofluorochromic property.

Tetraphenylethylene fused pyrrolo[3,2-b]pyrrole derivatives with reversible mechanofluorochromic and aggregation-induced emission enhancement characteristics

Modification on typical of molecular unit with aggregation-induced emission (AIE)/aggregation-induced emission enhancement (AIEE) can play a important role in obtaining solid-state fluorescence materials and mechanofluorochromic (MFC) materials. We utilize the multicomponent reaction of TPE-containing aldehyde, arylamine and butane-2,3-dione in the presence of iron(III) perchlorate to design and synthesize two novel MFC materials by combination of the tetraphenylethylene (TPE) and pyrrolo [3,2-b]pyrrole core units. Investigation of CHW-1 and CHW-2 aggregation-induced emission behaviors indicated that these materials all exhibited AIEE properties. Subsequent mechanochromic luminescence tests exhibited reversible mechanochromism natures that involve fluorescent color change from green to yellow (Grinding-induced spectral shift (△λ): 35 nm and 38 nm, respectively). Then the datas (including SEM, PXRD and DSC) revealed that this mechanofluorochromism was ascribed to the transformation of crystal state to the amorphous form. Moreover, CHW-1 and CHW-2 could be applied for optical recording devices and security printing.

Mechanofluorochromic Properties of 1,4-Diphenylanthracene Derivatives with Hypsochromic Shift.

Several types of 1,4-diphenylanthracene derivatives 1-4 were prepared, and their photophysical properties were observed in the solid and solution states. Interestingly, the CN-group-substituted 1,4-diphenylanthracene derivative 2 was found to exhibit a higher fluorescence quantum yield (ϕf = 0.71) in the solid state than in the solution state, probably due to the formation of an intermolecular Ar-CN⋯H-Ar hydrogen bond and antiparallel type locked packing structure in the solid state. Furthermore, for some derivatives, an increase in the fluorescence quantum yield was observed in the PMMA film (1 wt%) over both the solid state and the solution state. More interestingly, some of the 1,4-diphenylanthracene derivatives exhibited unusual mechanofluorochromic properties with a "hypsochromic shift" in luminous color depending on the substituents of the phenyl group, and with the derivatives having CF3, OMe, CN, and two F substituents (1d-1f, 2-4) showing a significant luminous color change with a "hypsochromic shift" after grinding. However, no change in the luminous color was observed for the derivatives having H, Me, and one F substituent (1a-1c), and especially for some of the CN-substituted derivatives, a reversible luminous color change with a "hypsochromic shift" was observed, probably due to the formation of an antiparallel type packing structure. These "hypsochromic" anthracene derivatives could probably be utilized as new mechanofluorochromic materials.

Mechanofluorochromic properties of 1-phenylanthracene derivatives with extremely simple structures

Several types of 1-phenylanthracene derivatives 1–4 were prepared and their photophysical properties were observed in the solution and solid states. Interestingly, the fluorine-substituted 1-phenylanthracene derivative 1c was found to exhibit a higher fluorescence quantum yield (Φf=0.57) in the solid state than in the solution state, probably due to the formation of intermolecular Ar–F•••H–Ar hydrogen bonds. Additionally, some of the 1-phenylanthracene derivatives exhibited mechanofluorochromic properties depending on the substituents of the phenyl group, with the derivatives having CF3, OMe, and CN substituents (1d-1f, 2, 3) showing a significant change in luminous color from a bluish to greenish color at “room temperature” after grinding. However, no change in the luminous color was observed for the derivatives having H, Me, and F substituents (1a-1c) at “room temperature” but luminous color change was observed with low temperature grinding (−78 °C), especially for the CN substituent, in which a significant change in the luminous color was observed at “room temperature” probably due to the formation of Ar-CN•••H–Ar hydorogen bond interactions. Interestingly, for the 1-phenylanthracene derivative 3 substituted with two CN groups, which exhibited a solvatochromic phenomenon, the luminous color recovered with controllable rate from green to bluish when heated and fumed with solvent vapor in the solid state, and the luminous color change with controllable recovery rate was repeatedly successfully observed. These anthracene derivatives have extremely simple structures and could probably be applied as new mechanofluorochromic materials.

Nitrogen Atom Induced Contrast Effect on the Mechanofluorochromic Characteristics of Anthracene-Based Acceptor-Donor-Acceptor Fluorescent Molecules.

The mechanofluorochromic (MFC) characteristics of anthracene-based acceptor-donor-acceptor (A-D-A) fluorescent molecules are explored through a comprehensive investigation of their photophysical behaviors. Six 9,10-diheteroarylanthracene derivatives with varying acceptor groups (pyridin-4-yl, pyridin-3-yl, pyridin-2-yl, pyrimidin-5-yl, pyrazinyl and quinoxalinyl) are synthesized and systematically characterized. The photophysical properties in both solution and solid-state are examined, revealing subtle yet significant influences of the spatial arrangement and number of nitrogen atoms within the acceptor group on fluorescence emission. Single-crystal structures of these compounds provide insights into their steric configurations and intermolecular packing modes, offering valuable insights into the fundamental mechanisms that underlie the observed MFC properties. This study illuminates the intricate interplay between MFC properties and the refined molecular structure, thus presenting promising avenues for the design and advancement of novel MFC materials.

D-A type π-systems containing tetraphenylethene exhibiting aggregation-induced emission and reversible mechanical force-induced luminescent enhancement and chromism

Two novel D-A type tetraphenylethene-based fluorophores, named XA-BZ-TPE and XA-DCE-TPE, were designed and synthesized. Both fluorophores exhibit typical intramolecular charge-transfer features, good solid-state luminescence, and remarkable aggregation-induced emission (AIE) behavior. The AIE factors for XA-BZ-TPE and XA-DCE-TPE were >243 and 12, respectively. Notably, both XA-BZ-TPE and XA-DCE-TPE display high-contrast mechanofluorochromic (MFC) properties. During the grinding of initially prepared solid powders, the emission colors transformed from blue and blue-green to blue-green and yellow, and their maximum emission wavelengths shifted from 449 to 485 nm to 481 and 557 nm, respectively. Importantly, the two fluorophores attained enhancement in luminescence efficiency and chromism when the mechanical force was applied. After grinding, the solid-state luminescence efficiencies were elevated from 0.243 to 0.132 to 0.484 and 0.278, respectively, for XA-BZ-TPE and XA-DCE-TPE. In XA-BZ-TPE and XA-DCE-TPE, according to the analysis using PXRD, fluorescence lifetimes, and spectral data, the MFC behavior can be attributed to the phase transition between the crystalline and amorphous states. The present study contributes to expanding the repertoire of D-A type MFC materials and provides valuable insights for the development of novel MFC materials incorporating tetraphenylethene units, which exhibit excellent responsiveness under solid-state emission stimuli.

Facile Synthesis of Tetraphenylethene (TPE)-Based Fluorophores Derived by π-Extended Systems: Opposite Mechanofluorochromism, Anti-Counterfeiting and Bioimaging.

Although remarkable progresses are achieved in the design and development of the mono-shift in photoluminescence for mechanofluorochromic materials, it is still a severe challenge to explore the opposite mechanofluorochromic materials with both blue- and red-shifted photoluminescence. Herein, two unprecedented 4,5-bis(TPE)-1H-imidazole fused pyridine or quinoline-based fluorophores X-1 and X-2 were designed and synthesized, and X-1 and X-2, exhibit completely opposite mechanofluorochromic behavior. Under UV lamp, the color of pristine X-1 changed from blue to green with reversible redshifted 27 nm in fluorescence emission spectra after ground, while the color of pristine X-2 changed from red to yellow with reversible blue-shifted 74 nm after ground. The detailed characterizations (including PXRD, SEM and DSC) confirmed that this opposite mechanofluorochromism was attributed to the transformation of order-crystalline and amorphous states. The crystal structure analysis and theoretical calculation further explain that opposite mechanofluorochromic behavior take into account different π-π stacking mode by induced π-extended systems. In addition, these TPE-based fluorophores (X-1 and X-2) exhibited excellent bio-compatibility and fluorescence properties for bio-imaging, writable data storage and anti-counterfeiting materials.

One-step facile transformation from rofecoxib to reversible mechanofluorochromic materials with dual-state emission

We have developed a couple of rofecoxib analogues (NOR1 and NOR2) via a one-step reaction, and both of them exhibited DSE properties.

Progress of BODIPY dyes with aggregation-induced emission

<p indent="0mm">With the development of organic optoelectronic materials and bioimaging technology, the investigation of organic light-emitting materials with aggregation-induced emission (AIE) properties has gradually become a hot spot in material science field. The fluoroborondipyrromethenes (BODIPY) dyes have become one of the research objects of AIE materials due to their excellent luminescence properties. This paper reviews the recent advances in the design and synthesis of AIE-type BODIPY derivatives, the mechanism of AIE properties and their promising applications in fluorescence imaging, gas sensing, photodynamic therapy, and mechanofluorochromic materials. It aims to explore the structure-activity relationship of AIE-type BODIPY fluorescent molecules, and provides ideas for the further design, synthesis and potential applications of BODIPY-type aggregated luminescent materials through classification, analysis and summary.

Aggregation-induced emission and mechanofluorochromic materials based on the structural modification of 4,5,6-triaryl-4H-pyran skeleton

Although aggregation-induced emission (AIE)-active molecules with twisted structures have become the mainstream of mechanofluorochromic (MFC) materials, twisted molecular conformations do not necessarily lead to MFC properties. There have been some examples of failure based on 4,5,6-triaryl-4H-pyran skeleton. Herein, a series of 4,5,6-triaryl-4H-pyran derivatives were designed and synthesized to obtain fluorescent materials with both AIE and MFC characteristics. The results reveal that the introduction of triphenylamine endows all these propeller-type compounds with typical AIE properties originating from the restriction of intramolecular rotation, while the introduction of halogen atoms and the imidization of amino group are both conducive to the generation of the MFC activity by reducing the crystallization ability of derivatives. Furthermore, the red-shifted MFC phenomena are demonstrated to be ascribed to the planarization of molecular conformation caused by the crystalline-to-amorphous transition upon external pressure. This work confirms that appropriate crystallinity is very important for the candidate AIE-active compounds to exhibit MFC activities, which can be achieved by precise structural modification.

Nanoconfinement of tetraphenylethylene in zeolitic metal-organic framework for turn-on mechanofluorochromic stress sensing

Mechanofluorochromic materials are of great significance for the fabrication of innovative sensors and optoelectronics. However, efficient mechanofluorochromic materials are rarely explored due to the deficiency of existing design strategies. Here, we demonstrate the incarceration of aggregation-induced emission (AIE) materials within metal-organic framework (MOF) single crystals to construct a composite system with turn-on mechanofluorochromism. A new type of AIE@MOF material was designed: integrating a zeolitic MOF (ZIF-71) and tetraphenylethylene (TPE, a topical AIE material) to generate a TPE@ZIF-71 system with exceptional turn-on type mechanofluorochromism. Using terahertz vibrational spectroscopy, we show the unique fluorochromism mainly emanates from the enhanced nanoconfinement effect exerted by ZIF-71 host on TPE guest under pressure. Compared with pure TPE, we demonstrate the nanoconfinement in AIE@MOF not only changes the TPE's turn-off type sensing behavior to a turn-on type, but boosts the original sensitivity markedly by tenfold. Significantly, because ZIF-71 prevents the spontaneous recrystallization of TPE upon unloading, this allows TPE@ZIF-71 to record the stress history. This is the first demonstration of the Guest@MOF system combining the concepts of AIE and MOF; its promising properties and potential engineering applications will stimulate new directions pertaining to luminescent stress sensors and smart optics.

Mechano-responsive D–A luminogen based on bisarylic methanone derivative with brightly tricolored mechanochromic luminescence

A new luminogen PTZ-BZ-DBF bearing an electron donor phenothiazine unit and an electron acceptor carbonyl group is designed and synthesized. This molecule adopts highly twisted molecular conformation and exhibits intramolecular charge transfer (ICT), effective solid-state luminescence, and high contrast mechanofluorochromic (MFC) properties. The as-prepared powders show bright yellow-green fluorescence. Under conditions of external mechanical stimulation, the color changes to bright orange-red, and a red shift of 87 nm is observed in the emission spectral profile. Meanwhile, the solid-state luminescence efficiency increases from 20.5% to 28.5%. It is different from general MFC materials, when fumed using DCM vapor or annealed at 150 °C, the fluorescence peaks blue shift to approximately 560 nm, and the yellowish-emitting Y-powders are obtained. The ground solids and Y-powders can be converted to the as-prepared solid powders following the process of solvent recrystallization. Thus, tricolor switching is realized. The results obtained using the PXRD and DSC techniques indicate that the MFC phenomenon can be attributed to the process of crystalline-to-amorphous transition. The red shift of the PL spectra is attributed to the planar intramolecular charge transfer (PICT) process induced by conformational planarization in response to external forces.

Construction of Mechanofluorochromic and Aggregation-Induced Emission Materials Based on 4-Substituted Isoquinoline Derivatives.

Although many organic mechanofluorochromic (MFC) materials have been reported, the design strategy and formation mechanism are still unclear, and thus some of their discoveries are based on chance or a result of blind screening of some compounds. Herein, we show a strategy for constructing MFC materials from a multisubstituted isoquinoline compound with excellent crystallinity. The introduction of bromine atom at 4-position realizes the morphological change from crystalline state to amorphous state under the stimulation of external pressure, but it is not enough to generate MFC phenomenon. Similarly, although naphthalene, benzofuran, and benzothiophene endow the resultant isoquinolines with twisted molecular conformations and aggregation-induced emission (AIE) activities, no obvious solid-state emission color changes are observed. However, the introduction of 2-benzylidenemalononitrile and (E)-2-cyano-3-phenylacrylate leads not only to the AIE activities of IQ-M and IQ-A, but also to their outstanding and reversible MFC properties. The MFC activities of these two derivatives are demonstrated to come from more twisted molecular conformations, larger deformation spaces, and stronger intramolecular charge transfer compared with the other isoquinolines. This work offers important reference value for the construction of MFC and AIE materials from traditional fluorophores.

Twisted phenothiazine-terminated bisarylic methanone π-architecture exhibiting high-contrast mechanofluorochromism

Bisarylic methanone π-architecture PTZ-BZ-DBT with carbonyl group as the acceptor and phenothiazine as the donor was synthesized. Theoretical calculations and photophysical properties in solution demonstrated that the obtained donor-acceptor (D-A) type molecule PTZ-BZ-DBT adopts a molecular conformation of high distortion and significant intramolecular charge transfer (ICT) behavior, which endows it not only with intense solid-state luminescence but also with a high-contrast mechanofluorochromic (MFC) feature. The original as-prepared samples of PTZ-BZ-DBT display bright yellow-green fluorescence. When the as-prepared samples are ground, the ground solid powders exhibit orange-red fluorescence are obtained. Accordingly, during the grinding process, the emission wavelength of the solid powders red shifts from 518 nm of the as-prepared samples to 617 nm of the ground samples, and thus a spectral shift of 99 nm is observed. Different from general MFC materials, if the ground solid powders are fumed by DCM vapor or annealed at 150 °C, the fluorescence peaks blue shift to about 564 nm, and the yellow-emitting Y-powders are gained. However, the ground samples can be converted into the as-prepared solid powders through solvent recrystallization; thus tri-color switching is obtained. The PXRD, fluorescence lifetimes, DSC and the spectral analysis demonstrate that the remarkable MFC nature of PTZ-BZ-DBT is attributed to crystalline-to-amorphous transition. Further, the red shift of the fluorescence band after grinding can be ascribed to the extension of conjugate length and subsequent planar intramolecular charge transfer (PICT) induced by conformational planarization under external forces.

Excellent and reversible mechanofluorochromism in donor–acceptor π-systems based on bisarylic methanone derivatives

Mechanofluorochromic (MFC) materials characterized by high-contrast fluorescent colors and/or high intensities have rarely been reported. This can be attributed to the complex mechanism and molecular design associated with MFC phenomenon. Herein, the route followed for the preparation of two D–A type bisarylic methanone fluorescent molecules CAR-BZ-POZ and CAR-BZ-PTZ, has been presented. Results obtained from systematic photophysical experiments reveal that CAR-BZ-POZ and CAR-BZ-PTZ are characterized by twisted intramolecular charge-transfer (TICT) states, highly distorted molecular conformations, good solid-state emission properties, and high-contrast reversible mechanofluorochromism. The as-prepared CAR-BZ-POZ and CAR-BZ-PTZ solids emitted intense yellow-green fluorescence (531 and 550 nm, respectively), and the solid-state luminescence efficiencies recorded were 32.54% and 24.92%, respectively. When the samples were ground, orange-red fluorescence was emitted (602 and 594 nm, for CAR-BZ-POZ and CAR-BZ-PTZ, respectively). Results obtained by conducting mechanistic studies suggest that the MFC phenomenon can be attributed to the phase transition (from crystalline to amorphous phase) of the molecules. The red-shift in the PL spectral profiles, achieved following the process of grinding, could be attributed to the extension of the molecular conjugation length and the planar intramolecular charge transfer (PICT) properties.

CuOx/OMS-2 catalyzed synthesis of 4,5-dicyano-1H-imidazoles moiety: Directly access to d-A system for AIE-active mechanofluorochromic materials

• Efficient methodology for the synthesis of 4,5-dicyano-1H-imidazoles moiety. • TPE-DCI shown a AIE and MFC behavior. • TPE-DCI was successfully applied in cell staining. Although a lot of mechanofluorochromic (MFC) materials have been reported, it still needs to be further explored due to the lack of the universal design principle. The AIE moiety molecules with d -A framework have been widely regarded as the ideal structural models for MFC materials, because their optical properties is simply tuned by changing the molecule packing mode or conformation. Herein, we have developed a new and efficient synthetic methodology for the synthesis of 4,5-dicyano-1H-imidazoles moiety by using CuO x /OMS-2 as catalyst with excellent yields. The DCI moiety is a potential candidate for electron acceptor units and constructing the fluorescence skeleton with donor-acceptor system. Through the above synthesis method, we have constructed the new compound TPE-DCI with donor-acceptor framework by using tetraphenylethene as electron donor and DCI moiety as electron acceptor. As expected, TPE-DCI shown the excellent AIE behavior and significant MFC behavior (Δλ=25 nm). Furthermore, TPE-DCI was also successfully applied in cell staining. The results suggested that TPE-DCI is a promising candidate material for mechanical sensor, security protection and cell imaging.

A Novel Strategy to Design and Construct AIE-active Mechanofluorochromic Materials via Regulation of Molecular Structure.

In this work, we first designed and synthesized tetraphenylene-fused aryl-imidazole derivatives TM-1-4 via regulation of molecular structure, which were consisted of 1H-imidazo[4,5-f][1,10]phenanthroline, 1H-phenanthro[9,10-d]imidazole, 4,5-diphenyl-1H-imidazole, 3,3'-(1H-imidazole-4,5-diyl)dipyridine moieties and AIE-active tetraphenylethene units, respectively. The results illustrated that TM-1-4 exhibited clear AIE characteristics. Meanwhile, TM-2 and TM-3 show excellent solid emission properties (ΦTM-2 =13.73 % and ΦTM-3 =36.21 %), whereas TM-1 and TM-4 exhibit the opposite properties (ΦTM-1 =1.48 % and ΦTM-4 =4.83 %). The multiple rotors (pyridine and benzene ring) causes twisted conformations of the molecule that prevents π-π stacking and enhances solid emission(ΦTM-2<ΦTM-3, ΦTM-1<ΦTM-4). Significantly, TM-2 and TM-3 also exhibited reversible mechanochromic behavior (Emission red shifts: ΔλTM-2 =43 nm and ΔλTM-3 =41 nm) with color changes between blue and green emissions. The powder X-ray diffraction (PXRD) suggested the disordered state of ground sample could be readily returned to an ordered crystalline. Therefore, the mechanochromisms of TM-2 and TM-3 are ascribable to the phase transformation between crystal and amorphous structure. The single crystal X-ray analysis of TM-2 reveals a twisted conformation for TPE moiety and the absence of π-π intermolecular stacking. These excellent optical properties of TM-2 and TM-3 make them potentially applications in mechanochromic materials and imaging agents.

Modulation of mechanofluorochromism based on carboxylic acid esters compounds bearing triphenylamine and pyrene with different substituents

Two new three-armed and four-armed carboxylic acid esters derivatives TPT and TBT have been developed and characterized. The two D-A type compounds (different substituents including triphenylamine and pyrene) exhibit intramolecular charge transfer (ICT) emission process from electron donor (aromatic group) to electron-withdrawing (methyl naphthoate moiety). Interestingly, it was found that TPT and TBT exhibit reversible mechanofluorochromic (MFC) properties with fluorescence color switches from initial cyan and bright yellow light to final bright yellow emissions under mechanical force stimuli. Upon the treatment of grinding, heating or exposure to organic vapor, the emission peak of TPT in as-synthesized solid powder exhibited bathochromic shift from 472 nm to 499 nm, but TBT showed hypsochromic shift from 541 nm to 498 nm, respectively. The X-ray diffraction and DSC measurements suggested that the MFC-chromism of TPT and TBT originated from the reversible different phase morphs between the crystalline (ordered accumulation) and amorphous states (disordered accumulation). The mechanochromic mechanism was given based on the spectral results. Therefore, the carboxylic acid esters derivatives might have potential applications in mechanofluorochromic materials.