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 Fluorenyl‐Substituted Ethylenes

It has been reported several times that some organic luminogens with aggregation‐induced emission (AIE) characteristics exhibit the abnormal phenomenon of crystallization‐induced blueshift fluorescence, which makes them suitable for utilization as luminescence color‐switching materials. Because of the attractive application potential and the numerous underlying structure–property relationships in such materials, we investigated a series of fluorenyl‐containing tetrasubstituted ethylenes for their novel optical properties and structural features. The dyes show morphology‐dependent luminescence. Their emission color can be switched between green and blue by means of mechanical grinding and solvent fuming. The transformation between crystalline and amorphous accounts for the luminescence changing. Through single‐crystal and X‐ray diffraction (XRD) analysis, the twisted molecular geometries and loose packing motifs in the crystalline samples are believed to be the intrinsic origin of the external‐stimuli‐induced structural transformation.     

有机荧光分子2-氰基-3-(3, 4-二甲氧基苯基)-2-丙烯酰胺的可逆压致变色和质子刺激响应性能

设计合成了3-芳基-2-腈基丙烯酰胺类有机发光小分子2-氰基-3-(3, 4-二甲氧基苯基)-2-丙烯酰胺(CDMPA)。经研究发现, CDMPA化合物具有明显的压致变色和酸致变色现象。在外力刺激下,化合物CDMPA荧光最大发射峰发生20 nm的红移,经过加热或蒸汽处理后可恢复初始状态。对样品研磨前后粉末的X射线衍射图谱及荧光寿命衰减曲线进行测试分析得出, CDMPA压致变色现象归因于分子构型由晶态到无定形态的转化。另外,在酸刺激下CDMPA发光颜色由蓝光红移至黄光,最大发射波长红移33 nm。经过二甲基甲酰胺(DMF)处理后可恢复到初始状态。由测试得到的红外光谱及分子轨道理论计算推测,酸致变色现象是由氨基取代基的质子化影响了CDMPA前线分子轨道引起的。本研究可使人们深入了解这种类型材料的多刺激响应发光机制,且显著的颜色变化性能使CDMPA在传感器和检测装置方面具有潜在的应用前景。     

Dinuclear ZnII Complexes Exhibiting Thermally Activated Delayed Fluorescence and Luminescence Polymorphism

Zn^II complexes exhibiting strong emission in the solid state remain scarce, and most of them exhibit only prompt fluorescence. Herein the synthesis, structures, and photoluminescence properties of two Zn^II complexes containing new donor–acceptor ligands is reported. The new Zn^II complexes have dinuclear structures in which each metal ion adopts a distorted square-pyramidal geometry. The Zn^II complexes show strong emission in the solid state with quantum yields up to 50 %. Variable-temperature transient photoluminescence studies revealed an emission mechanism involving prompt and thermally activated delayed fluorescence (TADF). DFT calculations showed well-separated HOMO and LUMO in the ground state and small excited singlet–triplet energy splitting, accounting for the TADF. The complexes also exhibit different emission colors in the as-synthesized powder state and in single crystals, that is, they exhibit luminescence polymorphism. The single-crystal emission is responsive to mechanical grinding and was characterized by powder XRD.     

N‐Boc‐Indolylbenzothiadiazole Derivatives: Efficient Full‐Color Solid‐State Fluorescence and Self‐Recovering Mechanochromic Luminescence

Herein, the solid‐state emission with good fluorescence quantum yields of N‐Boc‐indolylbenzothiadiazoles as a new class of fluorophores is described. Their solid‐state emission covers the wide range of the visible spectrum and the emission color can be tuned easily by changing the substituents on the two heteroaromatic rings. Among these, 3‐methylindolyl derivatives exhibit moreover autonomously self‐recovering mechanochromic luminescence, whereby the original solid‐state emission could be recovered spontaneously at room temperature after exposure to a mechanical stimulus. The emission color, as well as the recovery time for the color change could be tuned via the introduction of different substituents on the benzothiadiazole ring. We propose that the mechanism of the autonomously self‐recovering mechanochromic luminescence of 3‐methylindolylbenzothiadiazoles is based on a partial amorphization of the crystals upon exposure to the mechanical stimulus, followed by autonomous recovering in the form of recrystallization.     

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.     

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.     

Modulating Photo‐ and Electroluminescence in a Stimuli‐Responsive π‐Conjugated Donor–Acceptor Molecular System

Functional organic materials that display reversible changes in fluorescence in response to external stimuli are of immense interest owing to their potential applications in sensors, probes, and security links. While earlier studies mainly focused on changes in photoluminescence (PL) color in response to external stimuli, stimuli‐responsive electroluminescence (EL) has not yet been explored for color‐tunable emitters in organic light‐emitting diodes (OLEDs). Here a stimuli‐responsive fluorophoric molecular system is reported that is capable of switching its emission color between green and orange in the solid state upon grinding, heating, and exposure to chemical vapor. A mechanistic study combining X‐ray diffraction analysis and quantum chemical calculations reveals that the tunable green/orange emissions originate from the fluorophore's alternating excited‐state conformers formed in the crystalline and amorphous phases. By taking advantage of this stimuli‐responsive fluorescence behavior, two‐color emissive OLEDs were produced using the same fluorophore in different solid phases.     

X-Ray storage luminescence of BaFCl:Eu2+ single crystals

Temperature behaviors of X-ray luminescence (XL), photoluminescence (PL), photostimulated luminescence, and thermoluminescence (TL) were studied in BaFCl:Eu2+ single crystals from room temperature to liquid nitrogen temperature. Six emissions at 275, 315, 365, 385, 435, and 500 nm were observed in the XL spectra and are attributed to Cl excitons, V(k)(Cl2-), the 4f65d1 (2e(g)) --> 4f7 (8S(7/2)) transition of Eu2+, and oxygen vacancies, respectively. Three emission peaks at 315, 365, and 390 nm were observed in the PL and TL measurements. These three emissions are from the transitions of 4f7(6I(7/2)) --> 4f7(8S(7/2)), 4f7(6P(7/2)) --> 4f7(8S(7/2)), and 4f65d1 (2e(g)) --> 4f7(8S(7/2)) of Eu2+, respectively. In our measurements, we observed that the emission of Eu2+ increases in intensity upon beta-irradiation and did not see any signals related to Eu3+ ions, which indicates that Eu2+ ions might not be oxidized to Eu3+ upon X-ray or beta-irradiation. Instead, the color centers, Cl excitons, and oxygen defects are created and are stable at room temperature, and they might play a key role in the storage luminescence.     

Soft Crystals: Flexible Response Systems with High Structural Order

A new material concept of soft crystals is proposed. Soft crystals respond to gentle stimuli such as vapor exposure and rubbing but maintain their structural order and exhibit remarkable visual changes in their shape, color, and luminescence. Various interesting examples of soft crystals are introduced in the article. By exploring the interesting formation and phase-transition phenomena of soft crystals through interdisciplinary collaboration, new materials having both the characteristics of ordered hard crystals and those of flexible soft matter are expected.     

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.     

Multistimuli-Responsive Luminescence of Naphthalazine Based on Aggregation-Induced Emission

Stimuli-responsive luminescent materials, which are dependent on changes in physical molecular packing modes, have attracted more and more interest over the past ten years. In this study, 2,2-dihydroxy-1,1-naphthalazine was synthesized and shown to exhibit different fluorescence emission in solution and solid states with characteristic aggregation-induced emission (AIE) properties. A remarkable change in the fluorescence of 2,2-dihydroxy-1,1-naphthalazine occurred upon mechanical grinding, heating, or exposure to solvents. According to the characterization by solid-state fluorescence spectroscopy, X-ray crystallography, differential scanning calorimetry, and X-ray powder diffraction, the fluorescence change could be attributed to transitions between two structurally different polymorphs. These significant properties could also give 2,2-dihydroxy-1,1-naphthalazine more potential applications as a multifunctional material.     

Piezofluorochromic and Aggregation‐Induced‐Emission Compounds Containing Triphenylethylene and Tetraphenylethylene Moieties

New fluorescent compounds containing triphenylethylene and tetraphenylethylene moieties were synthesized, and their piezofluorochromic and aggregation‐induced emission behaviors were investigated. The results show that all compounds exhibit aggregation‐induced emission characteristics and only the crystalline compound possesses piezofluorochromic properties. The color, emission spectra, and morphological structures of the one piezofluorochromic compound exhibit reversibility upon grinding and annealing (or fuming) treatments. The piezofluorochromic behaviors are caused by a change between different modes of solid state molecular packing under external pressure. The single crystal X‐ray diffraction analysis reveals that the twisted conformation of the aggregation‐induced emission compound leads to the formation of metastable crystal lattice with cavity which is readily destroyed under external pressure. A possible mechanism of piezofluorochromic phenomenon has been proposed.     

Multistimuli‐Responsive Luminescence Switching of Pyrazine Derivative Based Donor–Acceptor–Donor Luminophores

Three symmetrical donor–acceptor–donor (D–A–D) luminophores ( C1 , C2 , and C3 ) with pyrazine derivatives as electron‐withdrawing groups have been developed for multistimuli‐responsive luminescence switching. For comparison, reference compounds R1 and R2 without the pyrazine moiety have also been synthesized. Intramolecular charge transfer (ICT) interactions can be found for all D–A–D luminophores owing to the electron‐withdrawing properties of the two imine nitrogen atoms in the pyrazine ring and the electron‐donating properties of the other two amine nitrogen atoms in the two triphenylamine units. Moreover, luminophores C1 , C2 , and C3 exhibit “on–off–on” luminescence switching properties in mixtures of water/tetrahydrofuran with increasing water content, which is different from the “on–off” switching for typical aggregation‐caused quenching (ACQ) materials and “off–on” switching for traditional aggregation‐induced emission (AIE) materials. Additionally, upon grinding the pristine samples, luminophores C1 , C2 , and C3 display bathochromically shifted photoluminescence maxima that can be recovered by either solvent fuming or thermal annealing treatments. The piezofluorochromic (PFC) properties are more pronounced than those for reference compounds R1 and R2 , which indicates that D–A molecules have the ability to amplify the PFC effect by tuning the ICT interactions upon tiny structural changes under pressure. Furthermore, the target luminophores demonstrate acid‐responsive photoluminescence spectra that can be recovered in either basic or ambient environments. These results suggest that D–A complexes are potential candidates for multistimuli‐responsive luminescence switching because their ICT profiles can be facilely tuned with tiny external stimuli.     

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

In this study, we report the interconvertible tetracolored solid state photoluminescence of gold(i) isocyanide complex 2 upon various external stimuli through solid state structural changes. Soaking complex 2 in acetone yields blue emission as a result of the formation of 2B. The subsequent removal of acetone yields 2G through a crystal-to-crystal phase transition, which exhibits green emission. This green-emitting solid 2G exhibits stepwise emission color changes to yellow and then to orange upon mechanical stimulation by ball-milling, which corresponds to the formation of 2Y and 2O, respectively. 2B could be recovered upon the addition of acetone to 2G, 2Y, and 2O. Thus, these four emitting solid states of 2 can be switched between repeatedly by means of acetone soaking and the application of mechanical stimulation. Importantly, single crystal and powder X-ray diffraction (PXRD) studies fully show the detailed molecular arrangements of 2B, 2G, and 2Y. This is the first mechanochromic compound to show interconvertible four color emission in the solid state. We also present the first example of using PXRD measurements and the Rietveld refinement technique for the structural analysis of a ground powder in a luminescence mechanochromism study. We obtained complete molecular-level structural information of the crystalline states of 2B, 2G, 2Y, and 2O. In comparison with a more solvophobic analogue 1, we suggest that the weak interaction of 2 with acetone in the solid state would allow a solvent inclusion/release mode, which is an important structural factor for the unprecedented multicolor mechanochromic luminescence.     

Photomechanical Luminescence from Through-Space Conjugated AIEgens

Transforming molecular motions into the macroscopic scale is a topic of great interest to nanoscience. The photomechanical effect is a promising strategy to achieve this goal. Herein, we report an intriguing photomechanical luminescence driven by the photodimerization of 2-phenylbenzo[b]thiophene 1,1-dioxide (P-BTO) in molecular crystals and elucidate the working mechanism and substituent effect through crystallographic analysis and theoretical calculations. Striking splitting, hopping, and bending mechanical behaviors accompanied by a significant blue fluorescence enhancement are observed for P-BTO crystals under UV light, which is attributed to the formation of photodimer 2P-BTO. Although 2P-BTO is poorly π-conjugated because of the central cyclobutane ring, it exhibits prominent through-space conjugation and aggregation-induced emission (AIE), affording strong solid-state blue fluorescence at 415 nm with an excellent quantum yield of up to 96.2 %.     

Ultrasound‐Assisted Construction of Halogen‐Bonded Nanosized Cocrystals That Exhibit Thermosensitive Luminescence

Multi‐component organic nanocrystals that are comprised of two or more supramolecular building blocks can be used to extend the design and assembly scope of solid molecular materials. Herein, we report the use of ultrasonication to prepare halogen‐bonded stilbene‐based nano‐cocrystals that exhibit different photoemission properties, including one‐ and two‐phonon emission and fluorescence lifetimes, relative to those of macrodimensional crystals. The structural transformation from nano‐cocrystals into nanocrystals upon heating results in a luminescence red‐shift from greenish blue to yellow. The temperature‐dependent ratiometric luminescence may allow such nano‐cocrystals to be used as fluorescent sensors and thermosensitive materials.     

Anion‐, Solvent‐, Temperature‐, and Mechano‐Responsive Photoluminescence in Gold(I) Diphosphine‐Based Dimers

A series of [Au₂(nixantphos)₂](X)₂ (nixantphos=4,6‐bis(diphenylphosphino)‐phenoxazine; X=NO₃, 1 ; CF₃COO, 2 ; CF₃SO₃, 3 ; [Au(CN)₂], 4 ; and BF₄, 5 ) complexes that exhibit intriguing anion‐switchable and stimuli‐responsive luminescent photophysical properties have been synthesized and characterized. Depending on their anions, these complexes display yellow ( 3 ), orange ( 4 and 5 ), and red ( 1 and 2 ) emission colors. They exhibit reversible thermo‐, mechano‐, and vapochromic luminescence changes readily perceivable by the naked eye. Single‐crystal X‐ray studies show that the [Au₂(nixantphos)₂]²⁺ cations with short intramolecular Au ??? Au interactions are involved as donors in an infinite N?H ??? X (X=O and N) hydrogen‐bonded chain formation with CF₃COO^? ( 2 C ) and aurophilically linked [Au(CN)₂]^? counterions ( 4 C ). Both crystals show thermochromic luminescence; their room temperature red ( 2 C ) and orange ( 4 C ) emission turns into yellow upon cooling to 77 K. They also exhibit reversible mechanochromic luminescence by changing their emission color from red to dark ( 2 C ), and orange to red ( 4 C ). Compounds 1 – 5 also display reversible mechanochromic luminescence, altering their emission colors between orange ( 1 ) or red ( 2 ) to dark, as well as between yellow ( 3 ) or orange ( 4 and 5 ) to red. Detailed photophysical investigations and correlation with solid‐state structural data established the significant role of N?H ??? X interactions in the stimuli‐responsive luminescent behavior.     

A Thermo‐ and Mechanoresponsive Cyano‐Substituted Oligo(p‐phenylene vinylene) Derivative with Five Emissive States

Multiresponsive materials that display predefined photoluminescence color changes upon exposure to different stimuli are attractive candidates for advanced sensing schemes. Herein, we report a cyano‐substituted oligo(p‐phenylene vinylene) (cyano‐OPV) derivative that forms five different solvent‐free solid‐state molecular assemblies, luminescence properties of which change upon thermal and mechanical stimulation. Single‐crystal X‐ray structural analysis suggested that tolyl groups introduced at the termini of solubilizing side‐chains of the cyano‐OPV play a pivotal role in its solid‐state arrangement. Viewed more broadly, this report shows that the introduction of competing intermolecular interactions into excimer‐forming chromophores is a promising design strategy for multicolored thermo‐ and mechanoresponsive luminescent materials.     

Photoresponsive aggregation-induced emission polymer film for anti-counterfeiting

The development of solid-state smart materials, in particular those showing photoresponsive luminescence, is highly desirable for their cutting edge applications in displays, sensors, data-storage, and anti-counterfeiting. However, to achieve both excellent photoresponsive performance and bright luminescence in solid state remains challenge. Herein, we integrate a novel photochromic fluorophore YL into flexible polymer chains, thereby enabling the resultant polymer PYL with reversible photoisomerization upon aggregation. Remarkably, the polymer PYL possesses excellent photochromic properties and aggregation-induced emission (AIE) activity, which can be attributed to the photoactive YL moiety. Upon light exposure, its film exhibits reversibly off-to-on fluorescent modulation with quick response, high emission efficiency and signal contrast, sharply different from the weak emission in solution. The novel photoresponsive AIE polymer with invisible/visible color and fluorescence transformation allows for advanced anti-counterfeiting applications. This work provides an efficient platform for constructing solid-state photocontrollable luminescent materials.