Recent Advances in Mechanism of AIE Mechanochromic Materials

Organic mechanochromic materials(also known as piezochromic materials),whose color or emission changes under mechanical force,have attracted great interest owing to their potential applications in pressure sensors,rewritable materials,optical storage,and security ink.Organic mechanochromic materials with aggregation-induced emission(AIE)features have better development prospects and research value owing to their excellent optical properties.To date,mechanochromism has mostly been realized by means of mechanical grinding.Nevertheless,the magnitude of the grinding force is usually uncontrollable and its direction is anisotropic,making it awkward to study the mechanism of mechanochromic materials.On the contrary,hydrostatic pressure,whose magnitude and direction are controllable,is a more valid and governable method to investigate the mechanism of mechanochromic materials,which can help us to construct a meaningful structure-property relationship and understand the latent origin of the mechanochromism.Furthermore,it is conducive to developing other mechanochromic material systems with desired chemical and physical properties.In this review,we focus on the recent progress in the mechanism of organic mechanochromic materials with AIE features under hydrostatic pressure.Four types of mechanisms are included:intermolecular interaction change,intramolecular conformation change,transformation from locally excited state to intramolecular charge-transfer state,and intra-and inter-molecular effects induced by hydrostatic pressure,respectively.     

Mechanochromic Luminescence of Copper Iodide Clusters

Luminescent mechanochromic materials are particularly appealing for the development of stimuli‐responsive materials. Establishing the mechanism responsible for the mechanochromism is always an issue owing to the difficulty in characterizing the ground phase. Herein, the study of real crystalline polymorphs of a mechanochromic and thermochromic luminescent copper iodide cluster permits us to clearly establish the mechanism involved. The local disruption of the crystal packing induces changes in the cluster geometry and in particular the modification of the cuprophilic interactions, which consequently modify the emissive states. This study constitutes a step further toward the understanding of the mechanism involved in the mechanochromic luminescent properties of multimetallic coordination complexes.     

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.     

Dimethylamine substituted bisbenzocoumarin amides with solvatochromic and mechanochromic properties

Stimuli responsive luminescent materials have attracted increasing attention for their potential application in many fields. In this work, dimethylamine substituted bisbenzocoumarins amides (DBCE and DBCP) are synthesized and their optical properties are investigated. These molecules show solvatochromic properties. The orange fluorescence emission of DBCE in crystalline state is blue-shifted to yellow emission upon grinding. The orange color could be recovered by recrystallization process. Powder wide-angle X-ray diffraction and DSC experiments reveal that the transformation from crystalline states to amorphous states under external stimuli is responsible for the mechanochromic properties. This work developed a new kind of binaphthane-type luminescent materials with blue-shifted mechanochromic properties.     

Organic soft crystals exhibiting spontaneously reversible mechano-responsive luminescence

Mechano-responsive luminescence, or mechanochromic luminescence (MCL), is a type of luminescence that can be reversibly controlled by the addition of mechanical stimuli. Organic materials exhibiting MCL have been an ongoing area of development since the early 2000s, and the number of reports into such materials has been steadily increasing. While the majority of MCL systems rely on the brittle nature of organic crystalline solids, there is a growing interest in "flexible" organic crystals that exhibit mechanical bending or shape deformation owing to their elasticity/plasticity. Such non-destructive deformed crystals may exhibit a new type of MCL that can be controlled by the magnitude of the force stress. In this review, we describe MCL systems capable of the spontaneous recovery of changes in their luminescent properties in response to the loading/unloading of mechanical stress. We particularly focus on the MCL of flexible crystals based on the density gradient of molecular packing (i.e., elastic and plastic crystals) and an emerging system known as "superelastochromism,” which is based on spontaneously reversible crystal polymorphism. This emerging research area has the potential to play an important role in the promotion of next-generation soft crystals.     

Mechanochromic luminescence of soft crystals: Recent systematic studies in controlling the molecular packing and mechanoresponsive properties

Soft crystals are a class of smart materials that can switch their photophysical or mechanical properties in response to gentle external stimuli. A representative stimuli-responsive behavior of soft crystals is mechanochromic luminescence (MCL), i.e., a reversible color change of solid-state photoluminescence induced by external mechanical stimuli. Together with the rapid growth in the area of solid-state photoluminescence including fluorescence, room-temperature phosphorescence (RTP), thermally activated delayed fluorescence (TADF), white-light emission (WLE), and circularly polarized luminescence (CPL), a number of soft crystals that exhibit MCL behaviors have been reported during the past decade. In the typical MCL of soft crystals, the emission color switches in the bathochromic direction upon amorphization by grinding and recovers to the original color upon recrystallization by heating or exposure to organic solvents. Relatively few are known to exhibit hypsochromically shifted MCL, two-step MCL, self-recovering MCL, or mechanical-stimuli-induced single-crystal-to-single-crystal (SCSC) transitions. Rational design guidelines to control the mechanoresponsive properties of soft crystals have not yet been established. This review summarizes the systematic studies on the substituent effect to control the MCL properties of soft crystals. Recent studies provide useful insights into the effects of electronic and steric differences of substituents on crystal structure, luminescence properties, and mechanoresponsive behaviors.     

Recent Advancements in and the Future of Organic Emitters: TADF‐ and RTP‐Active Multifunctional Organic Materials

Organic emitting compounds that are based on π‐conjugated skeletons have emerged as promising next‐generation materials for application in optoelectronic devices. In this Minireview, recent advances in the development of organic emitters that irradiate room‐temperature phosphorescence and/or thermally activated delayed fluorescence with extraordinary luminescence properties, such as aggregation‐induced emission, mechanochromic luminescence, and circularly polarized luminescence, are discussed.     

Mechanochromic Luminescence (MCL) of Purely Organic Two-Component Dyes: Wide-Range MCL over 300 nm and Two-Step MCL by Charge-Transfer Complexation

Despite recent extensive studies on mechanochromic luminescence (MCL), rational control over the magnitude of the emission-wavelength shift in response to mechanical stimuli remains challenging. In the present study, a two-component donor-acceptor approach has been applied to create a variety of organic MCL composites that exhibit remarkable emission-wavelength switching. Dibenzofuran-based bis(1-pyrenylmethyl)diamine and typical organic fluorophores have been employed as donor and acceptor dyes, respectively. Outstanding wide-range MCL with an emission-wavelength shift of over 300 nm has been achieved by mixing the diamine with 3,4,9,10-perylenetetracarboxylic diimide. Unprecedented two-step MCL in response to mechanical stimuli of different intensity has also been realized for a two-component mixture with 9,10-anthraquinone. Fluorescence microscopy observations at the single-particle level revealed that the segregation and mixing of the two-component dyes contribute to the stimuli-responsive emission-color switching of the MCL composites.     

Piezochromism in Dynamic Three-Dimensional Covalent Organic Frameworks

Piezochromic materials with pressure-dependent photoluminescence tuning properties are important in many fields, such as mechanical sensors, security papers, and storage devices. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials (CPMs) with structural dynamics and tunable photophysical properties, are suitable for designing piezochromic materials, but there are few related studies. Herein, we report two dynamic three-dimensional COFs based on aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) chromophores, termed JUC-635 and JUC-636 (JUC=Jilin University China), and for the first time, study their piezochromic behavior by diamond anvil cell technique. Due to the various luminescent groups, JUC-635 has completely different solvatochromism and molecular aggregation behavior in the solvents. More importantly, JUC-635 with AIE effect exhibits a sustained fluorescence upon pressure increase (≈3 GPa), and reversible sensitivity with high-contrast emission differences (Δλ_em=187 nm) up to 12 GPa, superior to other CPMs reported so far. Therefore, this study will open a new gate to expand the potential applications of COFs as exceptional piezochromic materials in pressure sensing, barcoding, and signal switching.     

In-situ vibrational optical rotatory dispersion of molecular organic crystals at high pressures

Organic structures respond to pressure with a variety of mechanisms including degradation, intramolecular transformation and intermolecular bonding. The effects of pressure on chiral organic structures are of particular interest because of the potential steric controls on the fate of pressurized molecules. Despite representing a range of opportunities, the simultaneous study of high pressures on different forms of chiral structures is poorly explored. We have combined synchrotron-source vibrational optical rotatory dispersion, micro-Fourier transform infrared spectroscopy and the use of a diamond anvil cell to simultaneously monitor the effects of pressure on the two enantiomers of the simple amino acid, alanine.     

Manipulation of Molecular Aggregation States to Realize Polymorphism,AIE, MCL,and TADF in a Single Molecule

Multifunctional emitting materials are scarce and need to be further explored. Now, a newly anthraquinone derivative, 2‐(phenothiazine‐10‐yl)‐anthraquinone (PTZ‐AQ) was designed and synthesized and found to demonstrate polymorphism, multi‐color emission, aggregation‐induced emission (AIE), mechanochromic luminescence (MCL), and thermally activated delayed fluorescence (TADF) in its different solid forms. It is shown for the first time that TADF properties of a compound can be systematically tuned via its aggregation state. The optimized PTZ‐AQ crystal shows a small singlet–triplet energy splitting of 0.01 eV and exhibits red TADF with a photoluminescence quantum yield as high as 0.848. This study shows that the unique multiple functions can be integrated into one single compound through controlling the aggregation states, which provides a new strategy for the investigation and application of multifunctional organic materials.     

A Mechanochromic Luminescent Dye Exhibiting On/Off Switching by Crystalline–Amorphous Transitions

A mechanochromic luminescent dye based on a simple aminomaleimide skeleton was readily synthesized in a one‐pot process. It exhibited an on/off mechanochromic luminescent switching property dependent on external stimuli, unlike a traditional mechanochromic color change. The green emission was turned on by grinding in a mortar and turned off by heating or treatment with dichloromethane. In the crystalline state, two molecules were stacked by cofacial π–π interactions, which caused concentration self‐quenching. The crystalline‐to‐amorphous transition induced by grinding removed cofacial π–π stacking, which led to intensive emission. Crystallizing processes recovered the cofacial π–π stacking, resulting in elimination of the emission. Theoretical calculations and X‐ray diffraction analyses revealed that the dye molecule was distorted in the crystalline state; thus even a mechanical stimulus caused the crystalline‐to‐amorphous transition.     

Manipulation of Molecular Aggregation States to Realize Polymorphism,AIE, MCL,and TADF in a Single Molecule

Multifunctional emitting materials are scarce and need to be further explored. Now, a newly anthraquinone derivative, 2‐(phenothiazine‐10‐yl)‐anthraquinone (PTZ‐AQ) was designed and synthesized and found to demonstrate polymorphism, multi‐color emission, aggregation‐induced emission (AIE), mechanochromic luminescence (MCL), and thermally activated delayed fluorescence (TADF) in its different solid forms. It is shown for the first time that TADF properties of a compound can be systematically tuned via its aggregation state. The optimized PTZ‐AQ crystal shows a small singlet–triplet energy splitting of 0.01 eV and exhibits red TADF with a photoluminescence quantum yield as high as 0.848. This study shows that the unique multiple functions can be integrated into one single compound through controlling the aggregation states, which provides a new strategy for the investigation and application of multifunctional organic materials.     

Bioinspired Healable Mechanochromic Function from Fluorescent Polyurethane Composite Film

Camouflage and wound healing are two vital functions for cephalopods to survive from dangerous ocean risks. Inspired by these dual functions, herein, we report a new type of healable mechanochromic (HMC) material. The bifunctional HMC material consists of two tightly bonded layers. One layer is composed of polyvinyl alcohol (PVA) and titanium dioxide (TiO₂) for shielding. Another layer contains supramolecular hydrogen bonding polymers and fluorochromes for healing. The as-synthesized HMC material exhibits a tunable and reversible mechanochromic function due to the strain-induced surface structure of composite film. The mechanochromic function can be further restored after damage because of the incorporated healable polyurethane. The healing efficiency of the damaged HMC materials can even reach 98 % at 60 °C for 6 h. The bioinspired HMC material is expected to have potential applications in the information encryption and flexible displays.     

Luminescent Mechanochromic Porous Coordination Polymers

Three 2D luminescent isomeric porous coordination polymers are synthesized and characterized. Their luminescence properties can be modified by grinding and they can act as mechanochromic materials and their properties are probably related to the weak interactions of cuprophilicity and π–π interactions.     

Mechanically Induced Multicolor Change of Luminescent Materials

Mechanofluorochromic or piezochromic fluorescence chemistry involves the switching and tuning of the luminescent properties of solid‐state materials induced by exogenous forces, such as grinding, shearing, compression, tension, and so forth. Up until now, most reported mechanochromic systems, including liquid crystals, organic molecules, organometallic compounds, polymers, and dye‐doped polymers, have displayed reversible two‐color changes, which arise from either supramolecular or chemical structure transformations. However, fluorescent materials that undergo mechanically induced multicolor changes remain rare; this Minireview is focused on such materials. Topics are categorized according to the different applied forces that are required to induce the multicolor change, including mechanical control of either the supramolecular structures or the chemical structures, and mechanical control of both the supramolecular structures and chemical structures.     

Recent advances in organic pressure-responsive luminescent materials

Pressure is a powerful tool to regulate the molecule aggregation and intermolecular interactions. Organic luminescent materials under high pressure can produce rich phenomena,which have many potential applications.     

Recent advances in organic pressure-responsive luminescent materials

Organic luminescent materials are very sensitive to external stimuli,such as pressure,temperature,and electric field.The luminescent properties of some organic luminescent materials significantly change under high pressure.Some materials may show luminescence discoloration,whereas some may exhibit luminescence enhancement.These properties have many potential applications in anticounterfeiting,force sensor,data recording and storage,and luminescent devices,thereby greatly attracting the attention of scientists.In this review,the progress of research on these materials at high pressure in recent years is summarized.     

FTIR spectroscopy of biodegraded historical textiles

Fungal attack is a common and severe problem in the storage rooms of museums. Fungi can damage different materials; organic materials are especially sensitive. In this work two different FTIR spectroscopy methods (micro-spectroscopy with diamond anvil cell and ATR) were used to investigate structural changes on biodeteriorated and non-affected textile fibres obtained from different Slovene museums and sacred objects. Several structural changes were observed in spectra of biodeteriorated as well as of non-affected cellulosic fibres, whereas no changes were observed in proteinaceous fibres. In the scope of spectral analysis crystallinity index has also been calculated by comparing two different band ratios. The research showed that the crystallinity index, calculated from the band intensity ratio I₁₃₇₂/I₂₉₀₀ groups fibres into two groups; biodeteriorated fibres predominantly have lower crystallinity index.     

Computational Design,Synthesis, and Mechanochromic Properties of New Thiophene‐Based π‐Conjugated Chromophores

The possibility of exploiting supramolecular architectures for the preparation of innovative mechanochromic devices has been extended by designing novel thienyl‐substituted 1,4‐bis(ethynyl)benzene dyes, which are characterized by a conjugated, rigid, rodlike core structure. This new family of chromophores was synthesized according to a simple two‐step sequential cross‐coupling reaction, and the optical properties were investigated in solution and in a polymeric matrix. To tune the mechanochromic performances in smart polymer materials, a virtual screening was set up that was able to select a derivative with optimal spectral features. The effective combination of experimental and computational investigations allowed us to spot those homologues with already potential anisotropic and aggregachromic features and characterized by the best spectral properties and luminescent response. The best candidate was synthesized and dispersed into a polyethylene matrix, indeed achieving an “in silico designed” mechanochromic material. Besides the specific applications of this novel material, the integration of computational and experimental techniques reported here defines an efficient protocol that can be applied to make a selection among similar dye candidates, which constitute the essential responsive part of such supramolecular devices.