Hexaphenylbenzene‐Based, π‐Conjugated Snowflake‐Shaped Luminophores: Tunable Aggregation‐Induced Emission Effect and Piezofluorochromism

In this work, two rigid, multiple tetraphenylethene (TPE)‐substituted, π‐conjugated, snowflake‐shaped luminophores BT and BPT were facilely synthesized by using a 6‐fold Suzuki coupling reaction. These molecules are constructed based on the nonplanar structure of propeller‐shaped hexaphenylbenzene (HPB) or benzene as core groups and TPE as end groups. As a result, they reserve the intrinsic aggregation‐induced emission (AIE) property of the TPE moiety. Meanwhile, both fluorescence quantum yield and piezochromic behavior in the solid state can be tuned or switched by inserting the phenyl bridges through changing the twisting conformation. The more extended structure BPT showed a much stronger AIE effect and higher Φ_F,f in the solid state in comparison with that of BT. Furthermore, an excellent optical waveguide application of these molecules was achieved. However, the revisable piezofluorochromic behavior has only appeared when BT was ground using a pestle and treated with solvent.     

Synthesis and optical properties of π-conjugated polymers and oligomers bearing hexaphenylbenzene and tetraphenyl ethene units

π-Conjugated polymers (CPs) and oligomers (COs) bearing hexaphenylbenzene (HPB) and tetraphenyl ethene (TPE) units were synthesized by Sonogashira and Suzuki–Miyaura coupling reactions. The optical properties of the CPs and COs were investigated by UV–vis, photoluminescence (PL), and fluorescence lifetime (τ) measurements. The PL intensities of the solutions of the CPs and COs synthesized by Sonogashira coupling were reduced upon addition of poor solvent like water, which is attributed to aggregation-caused quenching. In contrast, the CP and COs bearing the TPE unit synthesized by Suzuki–Miyaura coupling exhibited aggregation-induced emission enhancement in solution. This difference in fluorescence behavior is discussed herein in terms of the molecular sizes, conformations in solution, and τ values of the CPs and COs.     

Aggregation‐Induced Emission Characteristics of o‐Carborane‐Functionalized Tetraphenylethylene Luminogens: The Influence of Carborane Cages on Photoluminescence

Tetraphenylethylene (TPE)–carborane hybrids are constructed, and the impact of carborane substituents on the aggregation‐induced emission (AIE) characteristics of TPE‐cores has been investigated. When altering the 2‐R‐group on the carborane unit with ‐H, ‐CH₃ or phenyl group, the luminescent quantum yield of the corresponding TPE derivatives can be manipulated from 0.18 to 0.63 in the solid state. The emission color exhibits an obvious 100 nm shift (from blue to yellow).     

Fluorescent Dendritic Organogels Based on 2‐(2′‐Hydroxyphenyl)benzoxazole: Emission Enhancement and Multiple Stimuli‐Responsive Properties

A new highly efficient and versatile poly(benzyl ether) dendritic organogelator HPB‐G₁ with 2‐(2′‐hydroxyphenyl)benzoxazole (HPB) at the focal point has been designed and synthesized. HPB‐G₁ can form stable organogels toward various apolar and polar organic solvents. Further studies revealed that intermolecular multiple π–π stacking interactions are the main driving forces for the formation of the organogels. Notably, dendron HPB‐G₁ exhibited a significantly enhanced emission in the gel state in contrast to weak emission in solution. Most interestingly, these dendritic organogels exhibited multiple stimuli‐responsive behaviors upon exposure to environmental stimuli, including temperature, sonication, shear stress, and the presence of anions, metal cations, acids/bases, thus leading to reversible sol–gel phase transitions.     

A Versatile Naphthalimide–Sulfonamide‐Coated Tetraphenylethene: Aggregation‐Induced Emission Behavior,Mechanochromism, and Tracking Glutathione in Living Cells

A tetraphenylethene (TPE) derivative substituted with a sulfonyl‐based naphthalimide unit ( TPE‐Np ) was designed and synthesized. Its optical properties in solution and in the solid state were investigated. Photophysical properties indicated that the target molecule, TPE‐Np , possessed aggregation‐induced emission (AIE) behavior, although the linkage between TPE and the naphthalimide unit was nonconjugated. Additionally, it exhibited an unexpected, highly reversible mechanochromism in the solid state, which was attributed to the change in manner of aggregation between crystalline and amorphous states. On the other hand, a solution of TPE‐Np in a mixture of dimethyl sulfoxide/phosphate‐buffered saline was capable of efficiently distinguishing glutathione (GSH) from cysteine and homocysteine in the presence of cetyltrimethylammonium bromide. Furthermore, the strategy of using poly(ethylene glycol)–polyethylenimine (PEG‐PEI) nanogel as a carrier to cross‐link TPE‐Np to obtain a water‐soluble PEG‐PEI/ TPE‐Np nanoprobe greatly improved the biocompatibility, and this nanoprobe could be successfully applied in the visualization of GSH levels in living cells.     

A 1,3‐Indandione‐Functionalized Tetraphenylethene: Aggregation‐Induced Emission,Solvatochromism, Mechanochromism,and Potential Application as a Multiresponsive Fluorescent Probe

A tetraphenylethene (TPE) derivative substituted with the electron‐acceptor 1,3‐indandione (IND) group was designed and prepared. The targeted IND‐TPE reserves the intrinsic aggregation‐induced emission (AIE) property of the TPE moiety. Meanwhile, owing to the decorated IND moiety, IND‐TPE demonstrates intramolecular charge‐transfer process and pronounced solvatochromic behavior. When the solvent is changed from apolar toluene to highly polar acetonitrile, the emission peak redshifts from 543 to 597 nm. IND‐TPE solid samples show an evident mechanochromic process. Grinding of the as‐prepared powder sample induces a redshift of emission from green (peak at 515 nm) to orange (peak at 570 nm). The mechanochromic process is reversible in multiple grinding–thermal annealing and grinding–solvent‐fuming cycles, and the emission of the solid sample switches between orange (ground) and yellow (thermal/solvent‐fuming‐treated) colors. The mechanochromism is ascribed to the phase transition between amorphous and crystalline states. IND‐TPE undergoes a hydrolysis reaction in basic aqueous solution, thus the red‐orange emission can be quenched by OH^? or other species that can induce the generation of sufficient OH^?. Accordingly, IND‐TPE has been used to discriminatively detect arginine and lysine from other amino acids, due to their basic nature. The experimental data are satisfactory. Moreover, the hydrolyzation product of IND‐TPE is weakly emissive in the resultant mixture but becomes highly blue‐emissive after the illumination for a period by UV light. Thus IND‐TPE can be used as a dual‐responsive fluorescent probe, which may extend the application of TPE‐based molecular probes in chemical and biological categories.     

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.     

Decomposition‐Assembly of Tetraphenylethylene Nanoparticles With Uniform Size and Aggregation‐Induced Emission property

Tetraphenylethylene (TPE)‐substituted poly(allylamine hydrochloride) (PAH‐g‐TPE) is synthesized by a Schiff base reaction between PAH and TPE‐CHO. The PAH‐g‐TPE forms micelles in water at pH 6, which are further transformed into pure TPE‐CHO nanoparticles (NPs) with a diameter of ≈300 nm after incubation in a solution of low pH value. In contrast, only amorphous precipitates are obtained when TPE‐CHO methanol solution is incubated in water. The aggregation‐induced emission feature of the TPE molecule is completely retained in the TPE NPs, which can be internalized into cells and show blue fluorescence. Formation mechanism of the TPE NPs is proposed by taking into account the guidance effect of linear and charged PAH molecules, and the propeller‐stacking manner between the TPE‐CHO molecules.     

Stress–strain behavior of low‐density polyethylene/poly(methyl methacrylate) blends with modulated interfaces with a hydrogenated polybutadiene‐block‐poly(methyl methacrylate) diblock copolymer

The stress–strain diagrams and ultimate tensile properties of uncompatibilized and compatibilized hydrogenated polybutadiene‐block‐poly(methyl methacrylate) (HPB‐b‐PMMA) blends with 20 wt % poly(methyl methacrylate) (PMMA) droplets dispersed in a low‐density polyethylene (LDPE) matrix were studied. The HPB‐b‐PMMA pure diblock copolymer was prepared via controlled living anionic polymerization. Four copolymers, in terms of the molecular weights of the hydrogenated polybutadiene (HPB) and PMMA sequences (22,000–12,000, 63,300–31,700, 49,500–53,500, and 27,700–67,800), were used. We demonstrated with the stress–strain diagrams, in combination with scanning electron microscopy observations of deformed specimens, that the interfacial adhesion had a predominant role in determining the mechanism and extent of blend deformation. The debonding of PMMA particles from the LDPE matrix was clearly observed in the compatibilized blends in which the copolymer was not efficiently located at the interface. The best HPB‐b‐PMMA copolymer, resulting in the maximum improvement of the tensile properties of the compatibilized blend, had a PMMA sequence that was approximately half that of the HPB block. Because of the much higher interactions encountered in the PMMA phase in comparison with those in HPB (LDPE), a shorter sequence of PMMA (with respect to HPB but longer than the critical molecular weight for entanglement) was sufficient to favor a quantitative location of the copolymer at the LDPE/PMMA interface. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 22–34, 2005     

From Dark to Light to Fluorescence Resonance Energy Transfer (FRET): Polarity‐Sensitive Aggregation‐Induced Emission (AIE)‐Active Tetraphenylethene‐Fused BODIPY Dyes with a Very Large Pseudo‐Stokes Shift

The work presented herein is devoted to the fabrication of large Stokes shift dyes in both organic and aqueous media by combining dark resonance energy transfer (DRET) and fluorescence resonance energy transfer (FRET) in one donor–acceptor system. In this respect, a series of donor–acceptor architectures of 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) dyes substituted by one, two, or three tetraphenylethene (TPE) luminogens were designed and synthesised. The photophysical properties of these three chromophore systems were studied to provide insight into the nature of donor–acceptor interactions in both THF and aqueous media. Because the generation of emissive TPE donor(s) is strongly polarity dependent, due to its aggregation‐induced emission (AIE) feature, one might expect the formation of appreciable fluorescence emission intensity with a very large pseudo‐Stokes shift in aqueous media when considering FRET process. Interestingly, similar results were also recorded in THF for the chromophore systems, although the TPE fragment(s) of the dyes are non‐emissive. The explanation for this photophysical behaviour lies in the DRET. This is the first report on combining two energy‐transfer processes, namely, FRET and DRET, in one polarity‐sensitive donor–acceptor pair system. The accuracy of the dark‐emissive donor property of the TPE luminogen is also presented for the first time as a new feature for AIE phenomena.     

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.     

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.     

Aggregation‐Induced Emission and Aggregation‐Promoted Photo‐oxidation in Thiophene‐Substituted Tetraphenylethylene Derivative

Aggregation‐induced emission combined with aggregation‐promoted photo‐oxidation has been reported only in two works quite recently. In fact, this phenomenon is not commonly observed for AIE‐active molecules. In this work, a new tetraphenylethylene derivative (TPE‐4T) with aggregation‐induced emission (AIE) and aggregation‐promoted photo‐oxidation was synthesized and investigated. The pristine TPE‐4T film exhibits strong bluish‐green emission, which turns to quite weak yellow emission after UV irradiation. Interestingly, after solvent treatment, the weakly fluorescent intermediate will become bright‐yellow emitting. Moreover, the morphology of the TPE‐4T film could be regulated by UV irradiation. The wettability of the TPE‐4T microcrystalline surface is drastically changed from hydrophobic to hydrophilic. This work contributes a new member to the aggregation induced photo‐oxidation family and enriches the photo‐oxidation study of tetraphenylethylene derivatives.     

Visualization of Atom Transfer Radical Polymerization by Aggregation‐Induced Emission Technology

Aggregation‐induced emission (AIE) technology has been demonstrated to be a facile approach for in‐situ monitoring atom transfer radical polymerization (ATRP). A series of tertraphenyl ethylene (TPE)‐containing α‐bromo compounds were synthesized and applied as ATRP initiators. The photoluminescent (PL) emission of the polymerization system is proved to be sensitive to the local viscosity owing to the AIE characteristics of TPE. Linear relationships between the resulting molecular weight M_n and PL intensity were observed in several polymerization systems with different monomers, indicating the variability of this technique. Compared to physical blending, the chemical bonding of the TPE group in the chain end has higher sensitivity and accuracy to the polymer segments and the surrounding environment. This work promoted the combination of the AIE technique and controlled living radical polymerization, and introduced such an optical research platform to the ATRP polymerization process.     

Measurement of Solubilization Location in Micelles Using Anchored Aggregation‐Induced Emission Donors

Solubilization locations play a critical role in developing advanced surfactants and improving solubilization power in micelle‐based applications. However, the current polarity‐based techniques for measuring solubilization locations could come to conflicting conclusions. The key challenge is the unpredictable polarities in the micellar microenvironment. Now, an approach that is independent of micellar polarities is used to measure solubilization locations by covalently linking tetraphenylethylene (TPE) to the alkyl chain end of cationic surfactants. The solubilization locations of solubilized acceptors in the TPE‐cored spherical micelles were accurately measured by calculating the Förster resonance energy transfer distance between anchored TPE donors and solubilized acceptors. Solubilization locations of solubilized substances in the micellar interior and at the micellar surface depend on their size and hydrophobicity, respectively.     

Photocontrolled Intramolecular Charge/Energy Transfer and Fluorescence Switching of Tetraphenylethene‐Dithienylethene‐Perylenemonoimide Triad with Donor–Bridge–Acceptor Structure

Photochromic 1,2‐dithienylethene (DTE) derivatives with a high thermal stability and fatigue resistance are appealing for optical switching of fluorescence. Here, we introduce a donor–photochromic bridge–acceptor tetraphenylethene‐dithienylethene‐perylenemonoimide (TPE‐DTE‐PMI) triad, in which TPE acts as the electron donor, PMI as the electron acceptor, and DTE as the photochromic bridge. In this system, the localized and intramolecular charge transfer emission of TPE‐DTE‐PMI with various Stokes shifts have been observed due to the photoinduced intramolecular charge transfer in different solvents. Upon UV irradiation, the fluorescence quenching resulting from photochromic fluorescence resonance energy transfer in TPE‐DTE‐PMI has been demonstrated in solution and in solid films. The fluorescence on/off switching ratio in polymethylacrylate film exceeds 100, a value much higher than in polymethylmethacrylate film, thus indicating that the fluorescence switching is dependent on matrices.     

Discriminatory Detection of Cysteine and Homocysteine Based on Dialdehyde‐Functionalized Aggregation‐Induced Emission Fluorophores

We demonstrate a concept‐proof work of using fluorescence (FL) “turn‐on” probes for the discriminatory detection of cysteine (Cys) over homocysteine (Hcy). The fluorogens are provided with aggregation‐induced emission (AIE) characteristic and functionalized with two aldehyde‐groups (DMTPS‐ALD and TPE‐ALD). All the detections were carried out in a biocompatible medium (10 mM HEPES buffer and DMSO, pH 7.4). In principle, the formation of thiazinane/thiazolidine through the chemical reaction of aldehydes on the probe molecules and the residue of Cys/Hcy determines the selective recognition of Cys and Hcy over other amino acids and glucose. The FL responses originate from the AIE property of thiazinane/thiazolidine resultants, which have low solubility and precipitate (aggregate) in the detection medium. The discrimination between Cys and Hcy comes from the difference in reaction kinetics of TPE‐ALD/DMTPS‐ALD with Cys and Hcy, thereby the FL responses show different time courses and intensity enhancement. It is worth noting that TPE‐ALD outshined the other two probes in performance with fast response, a high FL enhancement up to 16‐fold, high sensitivity, and good specificity and selectivity. Moreover, its FL response threshold at 250 μM is very close to the lower limit of the normal level of Cys in human plasma, which implies that TPE‐ALD could be applied as a potential indicator of Cys deficiency.     

Squaraine Dyes: Interaction with Bovine Serum Albumin to Investigate Supramolecular Adducts with Aggregation‐Induced Emission (AIE) Properties

Bovine serum albumin (BSA)–squaraine supramolecular adducts with aggregation‐induced emission (AIE) properties were prepared and characterized by spectroscopic methods. While squaraine dyes showed a very low fluorescence quantum yield in water, a great enhancement in the fluorescence of the aggregated BSA adducts was achieved due to the abnormal aggregation‐induced emission properties of squaraines. The adducts formation was studied from a kinetic point of view showing unexpected structure–properties relationships.     

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.     

A theoretical investigation about the excited state behavior for 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole: The water‐assisted excited state proton transfer process

In this work, based on density functional theory (DFT) and time‐dependent DFT (TD‐DFT) methods, we theoretically investigate the excited‐state process of the 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole (2HPB) system in acetonitrile and water solvents. Since acetonitrile is an aprotic solvent, it has no effect on the solvent‐assisted excited‐state proton transfer (ESPT) process. Therefore, the 2HPB molecule cannot transfer the proton in acetonitrile, which is consistent with previous experimental observation. On the other hand, 2HPB can combine one water molecule (which is a protic solvent), forming the 2HPB–H₂O complex in the S₀ state. After photoexcitation, the intermolecular hydrogen bonds O1 H2···O3 and O3 H4···N5 both get strengthened in the S₁ state, which leads to the possibility of a water‐assisted ESPT process. Further, the charge redistribution reveals the tendency of ESPT. By exploring the potential energy curves for the 2HPB–H₂O complex in water, we confirm that a stepwise double proton transfer process occurs in the S₁ state. Water‐assisted ESIPT can occur along O1 H2···O3 or O3 H4···N5 because of their similar potential barriers. Based on the stepwise ESPT mechanism, we reinterpret the absorption and fluorescence spectra mentioned in the experiments and confirm the rationality of the water‐assisted ESPT process.