Mechanochromic Luminescence Research Articles

A Schiff base with excited-state intramolecular proton transfer and its Zinc(II) complex: Mechanochromic luminescence and acid-base stimuli response

A new Schiff base compound (HL·CH3COOH) with D-A structure and its zinc(II) complex (1) were successfully obtained. Due to the introduction of diethylamino and rotatable aromatic rings, both HL·CH3COOH and the complex adopt loosely packing patterns. HL·CH3COOH exhibits keto-form emission due to the excited-state intramolecular proton transfer (ESIPT) process, while complex 1 shows enol-form emission. They all show reversible mechanochromic luminescence (MCL) behavior, which is caused by the phase transformation from crystalline state to amorphous state revealed by Powder X-ray diffraction (PXRD) studies, and is accompanied by the departure of CH3COOH molecules for HL·CH3COOH. HL·CH3COOH and complex 1 can be protonated under the fumigation of HCl, resulting in the acidochromic properties, accompanied with the phase transformation. HCl also destroyed the interactions between CH3COOH and Schiff base molecules, leading to the departure of acetic acid molecules. More interestingly, HL·CH3COOH also exhibited NH3-stimuli response properties due to the prohibition of the ESIPT process.

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.

Aggregation-induced emission and distinct mechanochromic luminescence based on symmetrical D-A-D type and unsymmetrical D-A type carbazole functionalized dicyanovinyl derivatives

The π-conjugated luminophores CZ-DPMN and DCZ-DPMN, which are composed of electron-rich carbazole group and electron-deficient dicyanovinyl unit, are designed and synthesized. Spectral studies and theoretical calculations affirm that both compounds show unique ICT (intramolecular charge transfer) process and donor-acceptor (D-A) type molecular structure with highly distorted conformation. CZ-DPMN and DCZ-DPMN also exhibit AIE (aggregation-induced emission) characteristics at high water fraction in THF/water mixtures. Importantly, CZ-DPMN and DCZ-DPMN display remarkable fluorescent response to mechanical force. The as-synthesized solids of CZ-DPMN and DCZ-DPMN emit bright green and yellow fluorescence, and the emitting-colors change into yellow and orange-yellow after grinding, respectively. The spectral shifts are 62 nm and 22 nm upon the treatment of grinding. This MCL (mechanochromic luminescence) phenomenon is reversible during repeating grinding-fuming treatment. PXRD diffraction patterns indicate that the change of molecular internal structure from crystalline state to amorphous state after the treatment of grinding is the direct cause of MCL phenomenon.

Self-Assembly of Mechanoluminochromic Ladder-Shaped Gold(I) Clusters Promoted Using Cooperative Aurophilicity.

The self-assembly of mechanoluminochromic polynuclear gold(I) complexes has attracted more and more attention in the field of supramolecular gold(I) chemistry. In this work, we adopted a stepwise self-assembly strategy to precisely synthesize two polynuclear gold(I) supramolecular clusters. Through cooperative AuI···AuI and Au-N interactions, the gold(I) clusters 1+•BF4- and 24+•4BF4- with Au4 and Au16 cores, respectively, were successfully constructed. In these supramolecular clusters, (dppm)Au2Cl2 coordination motifs and trithiocyanuric linkers were stepwise assembled via sequential thiolate-chloride/phosphine coordination substitution and Au-S/Au-N coordination bond rearrangement. Two well-defined gold(I) supramolecular clusters displayed intense emission both in the solid state and in solution. Furthermore, the ladder-shaped cluster 24+•4BF4- exhibited reversible mechanochromic luminescence behavior in the solid state as well as aggregation-caused redshifted emission in solution. Upon mechanical grinding, the emission of the cluster 24+•4BF4- changed from yellow at 582 nm to red at 612 nm. The initial emission could be fully recovered by treatment with acetonitrile.

Crystal Structure of a 4,7-Bis(phenylethynyl)-2,1,3-benzothiadiazole-Based Cyclophane and the Mechanoresponsive Luminescence

Abstract The photophysical properties of luminescent cyclophanes have been traditionally examined in an isolated state in solution. In contrast, the behavior of luminescent cyclophanes in response to external stimuli in the solid state has not yet been fully understood. Herein, we report on the crystal structure of a cyclophane featuring one 4,7-bis(phenylethynyl)-2,1,3-benzothiadiazole group and its mechanochromic luminescence. Mechanical grinding of the crystals results in a phase transition to an amorphous state, which is concomitant with an emission color change from green to yellow. Annealing the amorphous phase at 100 °C induces a phase transition to another crystalline state, and green emission is observed again. On subsequent heating, the benzothiadiazole-based cyclophane forms a nematic liquid-crystalline phase above 191 °C. These results indicate that the introduction of π-extended planar luminophores within cyclic molecular structures is a promising approach for the design of stimuli-responsive luminescent materials.

Solvent-induced polymorphism, thermally activated delayed fluorescence, and mechanochromic luminescence of a single compound

A newly synthesized organic compound, 4,5,6,7-tetrakis-carbazol-9-yl-2-methyl-isoindole-1,3-dione (4CzPTANMe) exhibits polymorphism, mechanochromic luminescence, and thermally activated delayed fluorescence (TADF), simultaneously. 4CzPTANMe exists six solvent-induced aggregation states including five crystalline forms with fluorescence emission from green to red. Interestingly, 4CzPTANMe shows typical TADF characteristics in crystalline states while no TADF property in disordered amorphous state. Crystal structure analysis together with photophysical measurements reveal that the polymorph-dependent TADF of 4CzPTANMe can be facilely tuned by the solvents in the crystals. In addition, a doped light-emitting device based on 4CzPTANMe as an emitter exhibits greenish-yellow emission with a peak at 538 nm, a maximum current efficiency of 15.9 cd/A and a decent external quantum efficiency of 5.96%.

Chiral Macrocyclic AIEgens from BINOL and Tetraphenylethene via Tailoring Chirality of Luminescent Molecular Propellers

Comprehensive SummaryHerein, a pair of chiral macrocyclic luminogens derived from chiral 1,1‐bi(2‐naphthol) (BINOL) and tetraphenylethenes (TPEs) have been prepared with high yield via intramolecular McMurry coupling, in which the preferential helicity of propeller‐shaped TPE units can be tuned via choosing proper chiral BINOL tethers. The preferential chiral geometry of the macrocyclic luminogen was unambiguously confirmed by X‐ray crystal crystallography and theoretical calculation, whereas the variable‐temperature NMR spectra and chiral HPLC analysis further verified the intramolecular motions. These two chiral macrocyclic luminogens both exhibit obvious aggregation‐induced emission behavior and mechanochromic luminescence properties. Furthermore, aggregation attenuated CD signals and circularly polarized luminescence were observed for these two chiral macrocylic luminogens in THF‐water mixture and PMMA matrix. These results had paved a new approach toward constructing chiral aggregation‐induced emission luminogens (AIEgens) and their structural engineering.

A self-assembled aza-BODIPY linked dicyanostilbenzene with a large Stokes shift, AIE, mechanochromism and singlet oxygen yield

A novel polycatenar liquid crystal containing two α-cyanostilbene (CS) interconnected by an aza-BODIPY core and three alkoxy chains at each end has been prepared via Suzuki coupling and Knoevenagel reactions as key steps. This compound can self-assemble into the enantiotropic columnar phase with p4mm lattice and organogels with different morphologies. Such NIR compound displays a large Stokes shift and AIE effect. It possesses a certain singlet oxygen yield. The CS units also endow such compound mechanochromic luminescence (MCL) responsive property. These properties have been verified by theoretical calculation.

Imidazobenzothiadiazole: A new multifunction dyes for mechanochromic luminescence, acid responsing and mitochondrial staining

To develop new luminophores, three imidazobenzothiadiazole (ImBTD) fluorescent dyes (4a, 4b, and 4c) have been successfully synthesized and characterized. Interestingly, the introduction of imidazole groups at the 5- and 6-positions of benzothiadiazole can effectively enhance the fluorescence in both solid and aqueous solution and lead to a fluorescence response to acid. Modification of different aromatic groups at position 4 and 8 of ImBTD can produce tunable mechanochromic luminescence (MCL), which has been studied by theoretical calculations, single crystal X-ray diffraction (XRD), powder XRD analysis, and differential scanning calorimeter (DSC). More importantly, the neutral dyes 4a-c show mitochondrial staining comparable to commercially positively charged dyes. As a novel multifunctional fluorescent dye, it has application value in the fields of solid-state fluorescence and biological imaging.

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.

A copper(I) thiolate coordination polymer with thermochromic and mechanochromic luminescence

A luminescent copper(I) thiolate coordination polymer named [Cu(pot)]n has been synthesized by the employment of the thiol ligand of 5-(3-pyridyl)-1,3,4-oxadiazole-2-thiol (Hpot). The maximum emission peak of [Cu(pot)]n at room temperature is located at 609 nm. The compound shows superb luminescent stability toward acidic/alkaline conditions and organic solvents. [Cu(pot)]n exhibits an interesting thermochromic behavior with the luminescence changing from saffron at 77 K to orange at 297 K and yellowish green at 497 K. Meanwhile, its luminescent color changes from orange to red after grinding. Thus, the compound represents a rare example of copper(I) thiolate coordination polymers possessing thermochromic and mechanochromic luminescence simultaneously.

Design, synthesis, and mechanochromic luminescence of twisted donor–acceptor bisarylic methanone derivatives

Two bisarylic methanone derivatives, DBF-BZ-POZ and DBF-BZ-PTZ, which have phenoxazine or phenothiazine as electron donors and carbonyl group as electron acceptor are presented. Both molecules have highly distorted molecular conformation and exhibit obvious intramolecular charge transfer (ICT), effective solid-state emissions, and high-contrast mechanofluorochromic (MFC) properties. Both DBF-BZ-POZ and DBF-BZ-PTZ have good solid-state luminescence, efficiencies that reach 33.5% and 38.9%, respectively. The fluorescence colors of the as-prepared powders for both molecules are bright blue-green. The colors change to bright yellow with external mechanical stimulation and red shifts of 104 and 82 nm are observed in the photoluminescence (PL) spectra. Moreover, the MFC behavior of DBF-BZ-POZ and DBF-BZ-PTZ are reversible when their ground samples are subjected to fuming with the DCM vapor. Powder X-ray diffraction results indicate that the MFC phenomenon of DBF-BZ-POZ and DBF-BZ-PTZ may be attributed to crystalline-to-amorphous transitions in the material. The red shifts observed in the PL spectra after the material is ground are due to the smaller band gap induced by the planar intramolecular charge transfer (PICT) and the increased π-π interactions, the increase of exciton coupling, as well as the increased orbital overlap between neighboring molecules.

Zn(II) Complexes Based on a Schiff Base: Mechanochromism- and Solvent Molecule-Dependent Acidochromism

A new Schiff base molecule (HL) with a triphenylamine group and its three zinc(II) complexes (1, 1-THF, and 1-DMF) were synthesized and characterized. As expected, three kinds of loosely packed structures that are susceptible to mechanical stimulation were adjusted through twisted conformations and weak intermolecular interactions (C–H···π, π···π, and hydrogen bonds). They all exhibit remarkable mechanochromic luminescence (MCL) behavior with high contrast upon mechanical grinding due to the phase transition between a crystalline and an amorphous state, as confirmed by powder X-ray diffraction (PXRD) studies. Especially, grinding of 1-THF was accompanied by the departure of lattice tetrahydrofuran (THF) molecules. Further investigation revealed that 1-THF and 1-DMF showed outstanding acidochromic behavior. The PXRD and thermogravimetric analysis (TGA) results proved that fumigation with HCl could destroy the interaction between the solvent and ligand molecules, resulting in the phase transition from the crystalline state to the amorphous state, which is essential for their acidochromic properties. The acidochromic mechanism is different from the reported protonation and deprotonation process. Both HL and complex 1 exhibit no acidochromism. This work will shed light on the mechanism of acidochromic properties of Schiff base zinc(II) complexes and expand the potential solid-state stimuli-responsive luminescence materials.

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.

Insoluble Salt of Memantine with a Unique Fluorescence Phenomenon.

Alzheimer's disease is a chronic disease, and the long-term treatment of chronic diseases has always been a concern. Memantine (Mem) is approved by the US Food and Drug Administration for the treatment of moderate to severe Alzheimer's disease. In this study, reactions of memantine (Mem) with pamoic acid (Pam) were carried out to form insoluble salts (Mem-Pam). Four polymorphic forms (Forms I-IV) of Mem-Pam were successfully obtained through polymorphic screening, which were systematically characterized by X-ray powder diffraction (PXRD), thermal analysis (TGA and DSC), single-crystal X-ray diffraction (SXRD), and solid-state fluorescence. Compared with the hydrochloride form, the dissolution and release rates of these four forms are lower. The presence of pamoic acid reduces the release rate of memantine and makes it possible to achieve a sustained release of the drug. Interestingly, because of the presence of memantine, each polymorphic solid crystal of Mem-Pam has unique fluorescence emission. Therefore, memantine and pamoic acid have a synergistic effect on the fluorescence performance and can be expected to be used for real-time monitoring in continuous and controlled release drug delivery systems. In addition, the polymorphic solid crystals also exhibit reversible mechanochromic luminescence under the fumigation of acetonitrile vapor, which has a guiding role in the fluorescence design and synthesis of Pam substances and is expected to be used for information security, visual inspection of organic substances, etc.

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.

Room temperature phosphorescence, thermally activated delayed fluorescence and multicolor mechanochromic luminescence of emitters through molecular interaction and conformational modulations

Two novel heavy atom-free pure organic pyrimidine-based asymmetric donor-acceptor type emitters were designed and synthesized. The twisted D-A conformation was favored to produce small ΔEst and effective ISC. Excitingly, PMBT and CzPMBT showed precious fluorescence-RTP-TADF triple emission. Furthermore, PMBT responded to external stimuli of volatile solvents fumigation and different intensity grinding to generate a multicolor switching system, which included charming white light due to the dual emission of fluorescence and phosphorescence. Crystallographic analysis was conducted to explain the reasons for the differences in photophysical properties produced by the two compounds. This research provides insight into the mechanism of action of RTP, TADF and multicolor MCL through intermolecular interactions and conformational modulation.

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.

Development of Rofecoxib-Based Fluorophores from ACQ to AIE by Positional Regioisomerization.

The development of aggregation-induced emission luminogens (AIEgens) has attracted increasing attention due to their potential applications in various areas in recent years. In this study, a facile conversion from aggregation-caused quenching (ACQ) to aggregation-induced emission (AIE) was achieved by an efficient regioisomerization strategy based on the rofecoxib scaffold. Two compounds, named PYR2 and PYR4, were identified as regioisomers of rofecoxib derivatives to show dramatically different fluorescent properties. Compound PYR2 with an ortho-substituted piperidine group showed typical AIE activity while compound PYR4 with a para-piperidine group exhibited typical ACQ behavior. Notably, compound PYR2 showed polymorphism with two forms of crystals. It was also endowed with reversible mechanochromic luminescence and acidochromic properties. The different fluorescent properties were elucidated by UV/Vis absorption spectroscopy, powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analyses. Its application as a security ink and in lipid droplets imaging have been demonstrated.