A New Tetraphenylethylene‐Derived Fluorescent Probe for Nitroreductase Detection and Hypoxic‐Tumor‐Cell Imaging

The fluorescence detection of nitroreductase (NTR) and evaluation of the hypoxia status of tumor cells are vital, not only for clinical diagnoses and therapy, but also for biomedical research. Herein, we report the synthesis and application of a new fluorometric “turn‐on” probe for the detection of NTR ( TPE?NO₂ ) that takes advantage of the aggregation‐induced emission of tetraphenylethylene. TPE?NO₂ can detect NTR at concentrations as low as 5 ng mL^?1 in aqueous solution. The detection mechanism relied on the aggregation and deaggregation of tetraphenylethylene molecules. Moreover, this fluorescent probe can be used to monitor the hypoxia status of tumor cells through the detection of endogenous NTR.     

Monodentate AIEgen Anchored on Metal‐Organic Framework for Fast Fluorescence Sensing of Phosphate

Herein, a novel sensor (TPE‐UiO‐66) was designed via anchoring monodentate tetraphenylethylene (TPE) onto UiO‐66 framework. The combination of the distinct aggregation‐induced emission (AIE) of TPE and the easy replacement of monodentate linker by guest phosphate, makes TPE‐UiO‐66 an ideal platform for sensing HPO₄^2–. Experimental results indicate that TPE‐UiO‐66 can selectively sense HPO₄^2– from other common anions. The limit of detection (LOD) can reach to 5.56 μmol·L^–1 and more importantly, TPE‐UiO‐66 also exhibits an ultra‐fast equilibrium response of 2 min, far faster than those of other sensors especially for UiO‐66‐NH₂. The combination of experimental analysis and density functional theory (DFT) calculations demonstrates that the high selectivity, high sensitivity and fast response of HPO₄^2– detection by TPE‐UiO‐66 can be attributed to the stronger coordination interactions of HPO₄^2– with Zr‐O cluster of UiO‐66 than that of TPE molecule. This study not only provides a potential probe for phosphate, but also represents a novel strategy to design stimuli‐responsive fluorescent MOF‐based sensors via using monodentate AIEgens.     

Peripheral Templation‐Modulated Interconversion between an A4L6 Tetrahedral Anion Cage and A2L3 Triple Helicate with Guest Capture/Release

An anion‐coordination‐based A₄L₆ (“A” denotes anion and “L” is ligand) tetrahedral cage was constructed by a C₂‐symmetric bis‐bis(urea) ligand and phosphate anion, which showed reversible interconversion with the A₂L₃ triple helicate as a response to the template, concentration, or solvent. Notably, an unusual “peripheral” templation was found to be critical to stabilize the tetrahedral structure. This peripheral effect was utilized to assemble an “empty” A₄L₆ cage that allows the multi‐stimuli‐controlled capture/release of biologically important species such as choline and acetylcholine.     

Diels–Alder Polymer Networks with Temperature‐Reversible Cross‐Linking‐Induced Emission

A novel synthetic strategy gives reversible cross‐linked polymeric materials with tunable fluorescence properties. Dimaleimide‐substituted tetraphenylethene (TPE‐2MI), which is non‐emissive owing to the photo‐induced electron transfer (PET) between maleimide (MI) and tetraphenylethene (TPE) groups, was used to cross‐link random copolymers of methyl (MM), decyl (DM) or lauryl (LM) methacrylate with furfuryl methacrylate (FM). The mixture of copolymer and TPE‐2MI in DMF showed reversible fluorescence with “on/off” behavior depending on the Diels–Alder (DA)/retro‐DA process, which is easily adjusted by temperature. At high temperatures, the retro‐DA reaction is dominant, and the fluorescence is quenched by the photo‐induced electron transfer (PET) mechanism. In contrast, at low temperatures, the emission recovers as the DA reaction takes over. A transparent PMFM/TPE‐2MI polymer film was prepared which shows an accurate response to the external temperature and exhibited tunable fluorescent “turn on/off” behavior. These results suggest the possible application in areas including information security and transmission. An example of invisible/visible writing is given.     

Selective Fluorescence Turn‐On Sensing of Phosphate Anion in Water by Tetraphenylethylene Dimethylformamidine

A tetraphenylethylene (TPE) derivative bearing two dimethylformamidine units was synthesized. The dihydrogen chloride salt of this TPE derivative was soluble in water and showed almost no emission. By addition of phosphate anions, the dihydrogen chloride salt could be transformed into the monohydrogen chloride salt, which was barely soluble and emitted strong fluorescence through aggregation‐induced emission (AIE), while many other anions could not bring about a fluorescence enhancement. Meanwhile, the dihydrogen chloride salt and monohydrogen chloride salt could be reversible transformed by addition of acid and base alternately in the presence of phosphate anion, which led to fluorescence turn‐on and turn‐off. Therefore, the TPE dimethylformamidine holds potential for selectively sensing phosphate anions in water and use as fluorescence pH switch. This study provided a new approach to AIEgen sensors by using formamidine groups.     

Turn‐On Fluorescence in Tetra‐NHC Ligands by Rigidification through Metal Complexation: An Alternative to Aggregation‐Induced Emission

Two tetraphenylethylene (TPE) bridged tetraimidazolium salts, [H₄ L ‐Et](PF₆)₄ and [H₄ L ‐Bu](PF₆)₄, were used as precursors for the synthesis of the dinuclear Ag^I and Au^I tetracarbene complexes [Ag₂( L ‐Et)](PF₆)₂, [Ag₂( L ‐Bu)](PF₆)₂, [Au₂( L ‐Et)](PF₆)₂, and [Au₂( L ‐Bu)](PF₆)₂. The tetraimidazolium salts show almost no fluorescence (Φ _F<1 %) in dilute solution while their NHC complexes display fluorescence “turn‐on” (Φ _F up to 47 %). This can be ascribed to rigidification mediated by the restriction of intramolecular rotation within the TPE moiety upon complexation. DFT calculations confirm that the metals are not involved in the lowest excited singlet and triplet states, thus explaining the lack of phosphorescence and fast intersystem crossing as a result of heavy atom effects. The rigidification upon complexation for fluorescence turn‐on constitutes an alternative to the known aggregation‐induced emission (AIE).     

Tetraphenylethylene‐based Glycoconjugate as a Fluorescence “Turn‐On” Sensor for Cholera Toxin

Tetraphenylethylene (TPE)‐based glycoconjugates were easily synthesized by copper(I)‐catalyzed “click reactions” between propargyl‐attached TPE and azido‐functionalized sugars. The TPE compound bearing lactosyl moieties ( Lac‐TPE ) was found to be a fluorescence “turn‐on” sensor for cholera toxin by virtue of aggregation‐induced emission characteristics of the TPE motif owing to the specific interaction of lactose with the cholera toxin B subunit, whilst a cellobiose‐functionalized TPE derivative did not show any response to the toxin. Therefore, Lac‐TPE shows promising applications in the detection of cholera toxin, as well as in the investigation of carbohydrate–protein interaction.     

Anion Coordination of Bis‐bisurea Ligands: Aggregation of Dihydrogen Phosphate Anion into Oligomers and Infinite Chains

A series of alkanediyl‐spaced bis‐bisurea ligands ( L² – L⁴ ) were synthesized and their anion coordination behavior studied. These ligands form interesting complexes with polymeric and oligomeric dihydrogen phosphate aggregates in the solid state. The ligands L² and L³ coordinate with H₂PO₄^– anions to form a unique molecular “necklace” with an infinite (H₂PO₄^–)_n chain and surrounding ligand molecules. Meanwhile, two different dihydrogen phosphate‐water oligomers, (H₂PO₄)₆ · (H₂O)₄ and (H₂PO₄)₄ · (H₂O)₂, were observed in the complexes with the ligands L³ and L⁴ . In addition, solution anion binding properties of the ligands were studied by ¹H NMR and UV/Vis spectroscopy.     

AIE Fluorescent Gelators with Thermo‐, Mechano‐, and Vapochromic Properties

A series of aggregation‐induced emission (AIE) fluorescent gelators (TPE‐C_n‐Chol) were synthesized by attaching tetraphenylethylene (TPE) to cholesterol through an alkyl chain. The properties of the gel, nano‐/microaggregate, and condensed phases were studied carefully. TPE‐C_n‐Chol molecules form AIE fluorescent gels in acetone and in DMF. Their fluorescence can be reversibly switched between the “on” and “off” states by a gel–sol phase transition upon thermal treatment. The AIE properties of aggregated nano‐/microstructures in acetone/water mixtures with different water fractions were studied by using fluorescence spectrometry and scanning electron microscopy (SEM). In different acetone/water mixtures, the TPE‐C_n‐Chol molecules formed different nano‐/microaggregates, such as rodlike crystallites and spherical nanoparticles that showed different fluorescence colors. Finally, the condensed phase behavior of TPE‐C_n‐Chol was studied by using polarizing microscopy (POM), differential scanning calorimetry (DSC), fluorescence spectrometry, fluorescence optical microscopy, and wide‐angle X ray scattering (WAXS). The clover‐shaped TPE unit introduced into the rodlike cholesterol mesogen inhibits not only the formation of a liquid‐crystal phase but also recrystallization upon cooling from the isotropic liquid phase. Very interestingly, TPE‐C_n‐Chol molecules in the condensed state change their fluorescence color under external stimuli, such as melting, grinding, and solvent fuming. The phase transition is the origin of these thermo‐, mechano‐, and vapochromic properties. These findings offer a simple and interesting platform for the creation of multistimuli‐responsive fluorescent sensors.     

The Contrasting Recognition Behavior of β‐Cyclodextrin and Its Sulfobutylether Derivative towards 4′,6‐Diamidino‐2‐phenylindole

The noncovalent interactions between 4′, 6‐diamidino‐2‐phenylindole (DAPI) and sulfobutylether β‐cyclodextrin (SBE₇β‐CD) are evaluated by using photochemical measurements and compared with that of native β‐CD. Contrasting recognition behavior and intriguing modulations in the photochemical behavior of DAPI were observed. In particular, a large enhancement in the fluorescence emission and excited‐state lifetime were seen upon binding to SBE₇β‐CD, with the SBE₇β‐CD inclusion complex being approximately 1000 times stronger than that of β‐CD. The ensuing fluorescence “turn on” was demonstrated to be responsive to chemical stimuli, such as metal ions and adamantylanmine (AD). Upon addition of Ca²⁺/AD, nearly quantitative dissociation of the complex was established to regenerate the free dye and result in fluorescence “turn off”. The SO₃^? groups are believed to be critical for the strong and selective binding of the chromophore and the stimuli‐responsive tuning. This is as an important design criterion for the optimization of host–guest properties through supramolecular association, which is relevant for drug‐delivery applications.     

The Effect of the Spacer of Bis(biurea) Ligands on the Structure of A2L3‐type (A=anion) Phosphate Complexes

By tuning the length and rigidity of the spacer of bis(biurea) ligands L, three structural motifs of the A₂L₃ complexes (A represents anion, here orthophosphate PO₄^3?), namely helicate, mesocate, and mono‐bridged motif, have been assembled by coordination of the ligand to phosphate anion. Crystal structure analysis indicated that in the three complexes, each of the phosphate ions is coordinated by twelve hydrogen bonds from six surrounding urea groups. The anion coordination properties in solution have also been studied. The results further demonstrate the coordination behavior of phosphate ion, which shows strong tendency for coordination saturation and geometrical preference, thus allowing for the assembly of novel anion coordination‐based structures as in transition‐metal complexes.     

A S2O32−‐Enhanced Fluorogenic Chemosensor for Fe3+ in Aqueous Media Based on a Boron–Fluorine Derivative

A fluorescent “turn‐on” probe for Fe³⁺ was investigated in an aqueous system based on a boron 2‐(2′‐pyridyl) imidazole complex (BOPIM‐dma). BOPIM‐dma shows weak or no fluorescence in polar solvents due to twisted intramolecular charge transfer, but the addition of Fe³⁺ to BOPIM‐dma leads to fluorescence switch‐on responses. The binding is highly selective to Fe³⁺ over other metal ions, indicating that BOPIM‐dma is a chemodosimeter for Fe³⁺. Furthermore, the existence of S₂O₃²⁻ could much enhance and stabilize the emission significantly, indicating that the BOPIM‐dma/Fe³⁺/S₂O₃²⁻ complexes are a strong fluorescence system, and can be used as a sensitive detector for Fe³⁺, with the limit of detection of 6.0 × 10⁻⁷ mol L⁻¹.     

Novel Functional Conjugative Hyperbranched Polymers with Aggregation‐Induced Emission: Synthesis Through One‐Pot “A2+B4” Polymerization and Application as Explosive Chemsensors and PLEDs

With the aim to develop new tetraphenylethylene (TPE)‐based conjugated hyperbranched polymer, TPE units, one famous aggregation‐induced emission (AIE) active group, are utilized to construct hyperbranched polymers with three other aromatic blocks, through an “A₂+B₄” approach by using one‐pot Suzuki polycondensation reaction. These three hyperbranched polymers exhibit interesting AIEE behavior and act as explosive chemsensors with high sensitivity both in the nanoparticles and solid states. This is the first report of the AIE activity of the TPE‐based conjugated hyperbranched polymers. Their corresponding PLED devices also demonstrate good performance.     

A novel three‐dimensional AgI coordination polymer based on mixed naphthalene‐1,5‐disulfonate and aminoacetate ligands

The three‐dimensional coordination polymer poly[[bis(μ₃‐2‐aminoacetato)di‐μ‐aqua‐μ₃‐(naphthalene‐1,5‐disulfonato)‐hexasilver(I)] dihydrate], {[Ag₆(C₁₀H₆O₆S₂)(C₂H₄NO₂)₄(H₂O)₂]·2H₂O}_n, based on mixed naphthalene‐1,5‐disulfonate (L1) and 2‐aminoacetate (L2) ligands, contains two Ag^I centres (Ag1 and Ag4) in general positions, and another two (Ag2 and Ag3) on inversion centres. Ag1 is five‐coordinated by three O atoms from one L1 anion, one L2 anion and one water molecule, one N atom from one L2 anion and one Ag^I cation in a distorted trigonal–bipyramidal coordination geometry. Ag2 is surrounded by four O atoms from two L2 anions and two water molecules, and two Ag^I cations in a slightly octahedral coordination geometry. Ag3 is four‐coordinated by two O atoms from two L2 anions and two Ag^I cations in a slightly distorted square geometry, while Ag4 is also four‐coordinated by two O atoms from one L1 and one L2 ligand, one N atom from another L2 anion, and one Ag^I cation, exhibiting a distorted tetrahedral coordination geometry. In the crystal structure, there are two one‐dimensional chains nearly perpendicular to one another (interchain angle = 87.0°). The chains are connected by water molecules to give a two‐dimensional layer, and the layers are further bridged by L1 anions to generate a novel three‐dimensional framework. Moreover, hydrogen‐bonding interactions consolidate the network.     

Fluorescence Sensor with A New ON1–OFF–ON2 Switching Mechanism Using the Excited State Intermolecular Proton Transfer Reaction of An Anthracene‐diurea Compound

A new photoreaction mechanism of “Three‐state molecular switch” fluorescence sensor based on ON¹‐OFF‐ON² sequence was achieved by anthracene‐diurea compound, which was designed using two phenylurea groups and one anthracene, 9,10BtDSPUA. Photochemical properties of 9,10BtDSPUA and interaction between 9,10BtDSPUA and anion were investigated in detail by absorption, ¹H NMR, fluorescence, and fluorescence decay measurements. While the fluorescence of 9,10BtDSPUA in DMSO (ON¹) was quenched in the presence of low concentration of acetate anion (OFF), fluorescence enhancement occurred by the addition of high concentration of acetate anion (ON²). This compound forms complex with acetate anion through hydrogen bonding interaction in the ground state resulted in tautomer formation by excited state intermolecular proton transfer (ESIPT) on irradiation. Whereas single coordination of acetate anion to anthracene‐diurea compound may cause fluorescence quenching, full coordination may cause fluorescence enhancement due to suppressing ESIPT. This suppressing ESIPT was occurred by electron‐donating resonance effect between two urea moieties. This study is the first example of ON¹‐OFF‐ON² fluorescence sensor for concentration detection of acetate anion.     

A Fluorescent Probe with Aggregation‐Induced Emission for Detecting Alkaline Phosphatase and Cell Imaging

Alkaline phosphatase (ALP) is associated with many diseases, and its accurate detection is of great significance. Fluorescent compounds with aggregation‐induced emission (AIE) feature show beneficial advantages for serving as fluorescent probes. Herein, an AIE‐active “turn on” probe for ALP detection was synthesized through incorporating a strong electron‐withdrawing group (cyano) in the middle and the recognition moiety phosphate group at the end, thereby rendering a D–A–D structure with a relatively high conjugation degree and good water solubility. It was found that the probe TPE‐CN‐pho is highly sensitive to ALP in aqueous solution. In the presence of ALP, the hydrophilic phosphate group on the probe is rapidly removed, resulting in a decrease in water solubility and subsequent formation of aggregates, thereby achieving aggregation‐induced emission. Moreover, the probe TPE‐CN‐pho has also been successfully applied to imaging ALP in living cells.     

Stepwise Encapsulation of Sulfate Ions by Ferrocenyl‐Functionalized Tripodal Hexaurea Receptors

Three ferrocenyl‐functionalized tripodal hexaurea anion receptors with ortho‐ ( L² ), meta‐ ( L³ ), and para‐phenylene ( L⁴ ) bridges, which showed strong binding affinities toward sulfate ions, have been designed and synthesized. In particular, meta‐phenylene‐bridged ligand L³ , owing to its trigonal bipyramidal structure, can encapsulate two SO₄^2? ions in its “inner” and “outer” tripodal clefts, respectively, as supported by their clearly distinct NMR resonances and by molecular modeling. The sulfate complex of ortho‐ligand L² , (TBA)₂[SO₄? L² ] ? 2 H₂O ( 1 ), displays a caged tetrahedral structure with an encapsulated sulfate ion that is hydrogen bonded by the six urea groups of ligand L² . CV studies showed two types of electrochemical response of the ferrocene/ferrocenium redox couple upon anion binding, that is, a shift of the wave and the appearance of a new peak. Quantitative binding data were obtained from the NMR and CV titrations.     

Fluorescence Turn‐On Sensing of Lectins with Mannose‐Substituted Tetraphenylethenes Based on Aggregation‐Induced Emission

The synthesis of mannose‐substituted tetraphenylethenes (TPEs) and their aggregation‐induced emission (AIE) behavior, induced by interactions with concanavalin A (Con A), are reported. A mixture of the mannose‐TPE conjugates and Con A in a buffer solution displays an intense blue emission on agglutination within a few seconds, which serves as a “turn‐on” fluorescent sensor for lectins. The sensing is also selective: the conjugates act as a sensor for Con A, but do not sense a galactose‐binding lectin, PNA. Con A‐recognition is not affected even in the presence of other proteins in a mixture. The conjugates also exhibit high sensitivity to detect Con A. An increased sensitivity of the conjugates results if mannopyranoside substituents are linked to the TPE‐core unit with a flexible chain and/or when the number of mannose residues increases.     

Metal‐Ion Induced In Situ Ligand Oxidation for Self‐Assembled Clusters: from Bis(5‐(2‐pyridine‐2‐yl)‐1,2,4‐triazole‐3‐yl)methane to Alcohol or Ketone

Hydrothermal reactions of metal nitrates and ligand bis(5‐(pyridine‐2‐yl)‐1,2,4‐triazol‐3‐yl)methane (H₂L¹) gave three cluster compounds, {Cr₂}, {Zn₁₂} and {Fe₈}. Notably, methylene group of H₂L¹ was in situ oxidized either to hydroxymethylated (L²‐O)³⁻ in the metallo‐ring {Zn₁₂} or to a rigid carbonylated (L³=O)²⁻ in the screw‐type {Fe₈}. In light of comparative experimental results, NO₃⁻ was deduced to be of a catalytic role in the ligand oxidation. Metal ion could be regarded as an “induced” tool for clusters generation in self‐assembly process.     

A New Approach to Prepare Efficient Blue AIE Emitters for Undoped OLEDs

Two aggregation‐induced emission active luminogens (TPE–pTPA and TPE–mTPA) were successfully synthesized. For comparison, another six similar compounds were prepared. Because of the introduced hole‐dominated triphenylamine (TPA), fluorene groups with high luminous efficiency, and unconjugated linkages, the π conjugation length of the obtained luminogens is effectively restricted to ensure their blue emission. The undoped organic light‐emitting diodes based on TPE–pTPA and TPE–mTPA exhibited blue or deep‐blue emissions, low turn‐on voltages (3 V), and high electroluminescence efficiencies with L_max, η_C,max, and η_P,max values of up to 26 697 cd m^?2, 3.37 cd A^?1, and 2.40 Lm W^?1.