Reversible Mechanochromic Luminescence Research Articles

Tuning the Reversible Mechanochromic Luminescence of Triphenylethylenes by Rational Introduction of Functionalized Thiophenes Substituents

Tuning the Reversible Mechanochromic Luminescence of Triphenylethylenes by Rational Introduction of Functionalized Thiophenes Substituents

Asymmetric D-A-D’ type diphenylsulfone derivative modified by carbazole and tetraphenylethylene showing high solid-state luminescence efficiency, aggregation-induced emission and reversible mechanochromic luminescence

The new D-A-D’ type diphenylsulfone derivative TPE-DBS-Cz with carbazole and tetraphenylethylene units as the donors and diphenylsulfone moiety as the acceptor was prepared. Photophysical properties in solutions and theoretical calculations indicated that the D-A-D’ type molecule TPE-DBS-Cz adopted a highly twisted molecular conformation, obvious intramolecular charge transfer (ICT) and aggregation-induced emission (AIE) behaviors, which not only endowed it with high solid-state luminescence efficiency but also have an obvious fluorescent response to mechanical force. When applied by the grinding and fuming treatment, the emitting color was continuously switched between bright cyan and green. In other words, effective MCL behavior was obtained. The XRD, DSC, fluorescence lifetimes and the PL spectral analyses in different solid states indicated that the MCL nature of TPE-DBS-Cz was derived from the conversion of the crystalline and amorphous states. Moreover, the bathochromic-shift of the PL peak after grinding could be attributed to the degree of extended conjugation and the conformational planarization under external pressure.

Multistimuli Luminescence and Anthelmintic Activity of Zn(II) Complexes Based on 1H-Benzimidazole-2-yl Hydrazone Ligands

Three novel Zn(II) mononuclear complexes with the general formula ZnL2Cl2 (L = 2-(4-R-phenylmethylene)benzimidazol-2-hydrazines; R-H (1), R-CH3 (2), and R-OCH3 (3)) were synthesized and fully characterized by various means. These complexes demonstrate excitation-dependent emission, which is detected by a change in the emission color (from blue to green) upon an increase in the excitation wavelength. Moreover complex 1 shows reversible mechanochromic luminescence behavior due to the reversible loss of solvated methanol molecules upon the intense grinding of crystals. In addition, 1 exhibits vapochromic properties, which originate from the adsorption methanol vapor on the crystal surface. The strengthening of anthelmintic activity at the transition from free hydrazones to zinc-based complexes is shown.

Multi-component synthesis and enhanced photophysical study of novel azolo[1,5-a]pyrimidine based dyes

A series of novel fluorophores were obtained based on azolo[1,5-a]pyrimidines (APs) through multicomponent and oxidation reactions, starting from aminoazoles, morpholineacrylonitrile, and 4-(dimethylamino)benzaldehyde. Photophysical studies, as well as DFT calculations have been performed for the obtained APs. Aggregation-induced emission (AIE) phenomenon was demonstrated for fluorophores containing an aromatic substituent at the C-2 position of the azolo[1,5-a]pyrimidine core. It was observed that fluorophore 6d exhibited reversible mechanochromic luminescence. Two-photon absorption cross-section was measured for all obtained APs. In addition, cell staining resulted in identical intracellular distribution under excitation at 405 nm, 488 nm and 561 nm.

Reversible mechanochromic luminescence in a binuclear zero-dimensional Cd(II) complex through transformation between exciplex and excimer

In this work, the mechanochromic luminescence (MCL) property of a binuclear zero-dimensional cadmium complex with a variety of weak interactions is investigated. It exhibits a reversible transition between blue (445 nm) and yellow-green (470 nm) emission under the alternating treatments of grinding and CH3CN fumigation. The mechanism of MCL is proposed by combining the structural features, characterization results and theoretical calculations. During the grinding process, the ordered stacking of the structural units is disrupted and the crystalline state is transformed into an amorphous state, which is accompanied by a transition of the luminescent center from exciplex to excimer. In addition to exploring the MCL properties of a binuclear cadmium complex, this work provides feasible ideas for studying the response mechanism.

A Reversible Mechanochromic AIEgen Based on Triphenylamine for the Selective Detection of Vitamin B2 (Riboflavin) in Aqueous Media and Histotoxicity.

(E)-Ethyl 2-cyano-3-(4'-(diphenyl amino)-[1,1'-biphenyl]-4-yl) acrylate (RSJ) is a novel luminogen based on triphenylamine. It has been fully synthesized and characterized, exhibiting an incredible photophysical phenomenon known as aggregation-induced emission (AIE). This work describes a fluorescent sensor that detects vitamin B2 in mixed aqueous media with high selectivity and a low limit of detection as well as a mechanism for reversible mechanochromic luminescence. Moreover, the molecule was validated for its nontoxicity in water using a histotoxicological study. Fish subjected to two different concentrations of the "novel luminogen" that displayed photophysical phenomena during sensing of vitamin B2 (riboflavin) in mixed aqueous media did not exhibit any significant differences in the structural makeup of their liver, kidney, gills, brain, and muscle tissues when compared with the control group.

Self-Assembly and Mechanochromism Behavior of Gold(I) Supramolecular Cluster

Aurophilicity interaction has played a crucial role in the controllable self-assembly of polynuclear clusters with high nuclear structures and tunable photoluminescence properties. We proposed a multi-component synergistic self-assembly strategy for the controlled synthesis of a hexadeca-nuclei gold(I) supramolecular cluster through spontaneous ligand substitution and conformation rearrangement driven by AuI···AuI bonding interactions. In supramolecular cluster 12+·2BF4–, multi-components could be highly accurately aggregated together when thiolate-chloride/phosphine coordination exchange reactivity is partially activated by changing the stoichiometric ratio between digold(I) corners, dppmAu2Cl2, and ditopic linkers, trithiocyanuric salt. Mass spectra confirmed the stepwise self-assembly mechanism in solution, which was also supported by crystallographic data. Supramolecular cluster 12+·2BF4– displayed intense emission properties either in the solid state or in solution. Moreover, cluster 12+·2BF4– exhibited a reversible mechanochromic luminescence behavior in the crystalline state and aggregation-caused blue-shifted emission in solution. Hence, emissive gold(I) cluster with strong aurophilicity interactions has great potential value in the fluorescence imaging for 3D printing hydrogels.

Bisisocyanide cyclometallated platinum(II) complexes: synthesis, structure, photophysical properties, and mechanochromic behavior

A series of cyclometallated platinum(II) complexes [Pt(ppy){CNAr}2]X with two isocyanide ligands (Hppy = 2-phenylpyridine, Ar = C6H2-2,4,6-Me3, C6H3-2-Cl-6-Me , C6H3-2,6-Cl2, C6H4-4-NMe2, C6H4-4-Me, C6H4 4-Cl, C6H4-4-Br, C6H4-4-I, C6H4-4-CF3, C6H4-3-CF3; X = BF4, OTf) was synthesized by the reaction of the [{Pt(ppy)Cl}2] dimer with isocyanides (yield 52-70%). The structure of the resulting complexes was determined using mass spectrometry, 1H, 13C{1H}, 195Pt{1H}, 1H-1H COSY, 1H-1H NOESY, 1H-13C HSQC, and 1H-13C HMBC NMR spectroscopy in solution and solid-state CP/MAS 13C and 195Pt NMR spectroscopy, IR spectroscopy and X-ray diffraction analysis in the solid phase. The photophysical properties of the obtained complexes in the solid phase and the mechanochromic luminescence behavior were studied. In the solid phase, all synthesized compounds phosphoresce in the green or orange range of visible light, while photoluminescence quantum yields reach 26%. Green phosphors exhibit a reversible mechanochromic luminescence change achieved by mechanical grinding (green to orange) and solvent adsorption (orange to green).

Ultralong room temperature phosphorescence and reversible mechanochromic luminescence in ionic crystals with structural isomerism

Photofunctional ionic crystals have become more and more appealing in the applications of light-emitting devices, optical imaging, and anti-counterfeiting. However, it is still limited to achieve ultralong room temperature phosphorescence (RTP) and stimuli-responsive property in ionic crystals due to the lack of valid design principles. Herein, we developed a new class of organic ionic crystals (sodium o-, m-, p-carboxyphenylborate, o-, m-, p-Nacpb) by a structural isomerization strategy, which could exhibit ultralong RTP and mechanochromic emission. Among the three isomers, m-Nacpb crystal exhibits the longest RTP lifetime up to 732.8 ms, which is ca. 120 times longer than that (6.1 ms) of o-Nacpb crystal. Interestingly, o-Nacpb could display mechano-responsive luminescence with tunable fluorescence-phosphorescence emissions and longer phosphorescence lifetime (increased by about 20-fold). Both experimental and theoretical studies indicate that the distinct RTP and mechanochromic luminescence properties of the organic ionic crystals are attributed to the alternation of molecular conformations and spatial packing modes. Moreover, potential applications including data encryption and decoration are demonstrated, benefiting from the varied phosphorescence lifetimes and good processability. Therefore, this work not only provides a rational isomerization route to prepare various organic ionic crystals with ultralong RTP and stimuli-responsive emission, but also supplies a facile way to develop the advanced information encryption applications.

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.

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.

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.

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.

New insight into mechanochromic property of one N,N′-disubstituted-dihydrodibenzo[a‚c]phenazine derivative

We report herein the reversible mechanochromic luminescence property of (dibenzo[a,c]phenazine-9,14-diylbis(4,1-phenylene))dimethanol (DPAC-OH) where the photoluminescene color changes from blue to blue-green upon grinding, which can be restored upon slow solvent evaporation. 9,14-bis(4-(methoxymethyl)phenyl)-9,14-dihydrodibenzo[a,c]phenazine (DPAC-OCH3) is synthesized as the control sample to further demonstrate the role of hydrogen bonding in DPAC-OH. The vibration-induced emission (VIE) mechanism and the presence of hydrogen bonding interactions are utilized to account for the mechanochromic phenomenon.

Naphthaldehyde-based Schiff base dyes: aggregation-induced emission and high-contrast reversible mechanochromic luminescence

This work provides a naphthaldehyde-based Schiff base with reversible multiple stimuli-responsive behaviour of AIEE mechanochromic luminescence, which functions as a sensitive chemosensor for Cu2+ ions.

High‐Efficiency Wideband Excitable Mechanoluminescence from a Yellow MOF Phosphor as White LED and Multicolor Thin Films

AbstractHigh‐efficiency multifunctional metal‐organic framework (MOF) phosphor is highly desired for various practical applications, such as in lighting, anticounterfeiting, sensing, and so on. Herein, a highly yellow‐emitting MOF (Zn‐TPPA) phosphor is prepared via mechanical grinding for application as white light‐emitting diode (LED) and composite films. Under UV or visible‐light excitation, Zn‐TPPA exhibits the intense cyan emission (λem = 497 nm) with a high photoluminescence quantum yield (PLQY). Zn‐TPPA undergoes an intriguing reversible mechanochromic luminescence through mechanical stimuli (grinding). Notably, the grinding not only induces emissive color changes from cyan to yellow but also obviously enhances the luminescence.More significantly, upon excitation with blue light (λex = 460 nm), the PLQY of yellow light approaches as high as 92.7 (±1.8) %, which makes the yellow MOF phosphor potential as high‐performance white LED. In addition, the MOF‐based composite films exhibit dual emission and excitation‐dependent emission color due to the presence of two emission sources (TPPA monomer and aggregation state). Hence, Zn‐TPPA with reversible mechanochromic luminescence would have potentials in the fields of luminescence sensor, display, and anticounterfeiting.

Multistimuli Responsive Solid-State Emission of a Zinc(II) Complex with Multicolour Switching.

The development of smart luminescent materials, especially those stimulus-responsive fluorescent materials that can switch between different colors repeatedly under external stimulation based on a single molecule, is of great significance but a challenge. In this work, a novel zinc(II)-Schiff base complex (ZnL2) was obtained and characterized. Upon exposure to the HCl and NH3 vapors, it displayed remarkable tricolor acidochromic behavior with high contrast and rapid response under the ambient light as well as UV light (365 nm). The XPS analyses of ZnL2 crystals before and after HCl/NH3 fuming show that the acidochromism originates principally from the adsorption of vapor and the gas-solid reaction equilibrium on the crystal surface. The reddish-brown color of the HCl-fumigated ZnL2 crystals could be attributed to the generation of HL at the surface of ZnL2, and red-shifted emission could be ascribed to the self-absorption effect. The single crystal X-ray diffraction data indicate that these processes cause slight changes in the molecular conformation and crystal packing. ZnL2 shows reversible mechanochromic luminescence behavior between yellow and orange emission during the grinding-fuming/heating cycles due to the modulation between amorphous and crystalline states. Moreover, ZnL2 was successfully made into test paper for the rapid detection of HCl/NH3 vapors and mechanical stimuli.

Synthesis and Optical Properties of C,N-Swapped Boranils Derived from Potassium Acyltrifluoroborates

Abstract Novel C,O-chelated bora-heterocycles were synthesized. Reaction of potassium acyltrifluoroborates with tetrahydroquinolin-8-ol furnished C,N-swapped boranils in a single step. All five obtained borates showed photoluminescence in solid state. One of them showed yellow-to-orange reversible mechanochromic luminescence. Further derivatization afforded a B-chiral borate with an O,C,O-tridentate ligand that showed significant thermochromism.

Fine-tuning emission color of aggregation-induced emission-active Ir(III) phosphors through simple ligand modification

The emission color is a crucial parameter to evaluate the performances of luminescent materials, thus seeking out the effective control method for illuminating color is an important issue. To further explore the relationship between emission colors and structure of aggregation-induced emission (AIE)-active phosphors, cationic Ir(III) complexes comprising the same ancillary ligand but different 1-phenyl-1H-pyrazole derivatives as cyclometalated ligands, were designed and synthesized. The experimental results show that all complexes exhibit outstanding AIE characters and their emission colors can be adjusted by simply changing their cyclometalated ligands. However, different from previous strategy, modifying main ligands only changes the energy level of lowest unoccupied molecular orbital rather than that of highest occupied molecular orbital. Combining this result and our reported approach, a strong blue-emitting AIE-active Ir(III) complex is successfully constructed. In addition, we observed that all complexes upon mechanical grinding exhibit remarkable and reversible mechanochromic luminescence behavior.

Fabricating a mechanochromic AIE luminogen into a wearable sensor for volatile organic compound (VOC) detection

Solid-supported sensors and devices benefit greatly from aggregation-induced emission (AIE) molecular systems, on account of their high emission efficiency in the aggregated state. In this study, we present an AIE luminogen, (E)-1-(((4-acetylphenyl)iminio)methyl)naphthalen-2-olate (AINO), decorated with electron donor-acceptor (D-A) units. It has strong emission at solid state, but undergoes sharp decrease accompanied by slight hypochromatic shift in response to mechanical stimuli. After fumigation with volatile organic compound (VOC), the emission intensity recovers, showing reversible mechanochromic luminescence. Crystal analysis reveals its working mechanism, which is generally ascribed to the formation of coupled π-dimers. Based on the vapochromic recovery of the mechanochromic luminescence; a wearable impregnated cellulose sensor is fabricated, capable of being encapsulated into clothes and gloves. It exhibits remarkable emission change with high visual discrimination when exposed to VOC, affording a low detection down to 17.40 ± 6.91 mg/m3 (4.16 ± 1.77 ppm).