Aggregation‐Induced Emission of Tetraphenylethene–Hexaphenylbenzene Adducts: Effects of Twisting Amplitude and Steric Hindrance on Light Emission of Nonplanar Fluorogens

A series of nonplanar tetraphenylethene (TPE)–hexaphenylbenzene (HPB) adducts was designed and synthesized by Diels–Alder reaction of the acetylene precursors and tetraphenylcyclopentadienone. All of the adducts showed aggregation‐induced emission features. The twisting amplitude and steric hindrance of the TPE and HPB units were found to play a crucial role in their fluorescence behaviors in the aggregated state.     

Water-soluble NIR fluorescent probes based on squaraine and their application for protein labeling.

Water-soluble near-infrared (NIR) fluorescent labeling probes, named KSQ-3 and -4, which are based on a squaraine backbone, were synthesized and applied to biological labeling. The presented results demonstrate that the large, planar and hydrophobic squaraine dye becomes fully soluble in aqueous solution by the introduction of several sulfo group terminated alkyl substituents. Especially KSQ-4, which is substituted with four sulfo groups, exhibited perfect water solubility and significant fluorescence emission at the NIR region (817 nm) in the presence of bovine serum albumin (BSA). BSA was covalently labeled with KSQ-4, and the conjugate showed a strong absorption peak at 787 nm, which indicates compatibility with commercially available NIR laser diodes used for exciting the fluorophore. Furthermore, strong fluorescence emission was observed at 812 nm (phi = 0.08).     

Deoxyribonucleoside‐Modified Squaraines as Near‐IR Viscosity Sensors

Deoxyribonucleoside‐modified squaraines were synthesized by Sonogashira coupling reactions using an unsymmetrical, terminal alkynylated benzothiazolium squaraine dye. These non‐natural nucleosides exhibited fluorescent ‘turn‐on’ properties in viscous conditions with an enhancement of >300‐fold. The viscosity‐dependent fluorescence enhancement was attributed to a combination of hampering both molecular aggregation and intramolecular bond rotation of the squaraine probe. Fluorescence microscopy allowed visualization of highly viscous regions during various stages of cellular mitosis.     

An Unsymmetrical Squaraine-Based Activatable Probe for Imaging Lymphatic Metastasis by Responding to Tumor Hypoxia with MSOT and Aggregation-Enhanced Fluorescent Imaging

Optoacoustic imaging has great potential for preclinical research and clinical practice, and designing robust activatable optoacoustic probes for specific diseases is beneficial for its further development. Herein, an activatable probe has been developed for tumor hypoxia imaging. For this probe, indole and quinoline were linked on each side of an oxocyclobutenolate core to form an unsymmetrical squaraine. A triarylamine group was incorporated to endow the molecule with the aggregation enhanced emission (AEE) properties. In aqueous media, the squaraine chromophore aggregates into the nanoprobe, which specifically responds to nitroreductase and produces strong optoacoustic signals due to its high extinction coefficient, as well as prominent fluorescence emission as a result of its AEE feature. The nanoprobe was used to image tumor metastasis via the lymphatic system both optoacoustically and fluorescently. Moreover, both the fluorescence signals and three-dimensional multispectral optoacoustic tomography signals from the activated nanoprobe allow us to locate the tumor site and to map the metastatic route.     

A graphene binding-promoted fluorescence enhancement for bovine serum albumin recognition

Chemically converted graphene (CCG) was found to greatly enhance the fluorescence response of squaraine (SQ) dyes to bovine serum albumin (BSA). Addition of BSA to the SQ-CCG solution, where squaraine dyes absorbed on the CCG surface, raised the fluorescence intensity by as much as 80 fold.     

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.     

Reevaluating Protein Photoluminescence: Remarkable Visible Luminescence upon Concentration and Insight into the Emission Mechanism

It is a textbook knowledge that protein photoluminescence stems from the three aromatic amino acid residues of tryptophan(Trp), tyrosine (Tyr), and phenylalanine (Phe), with predominant contributions from Trp. Recently, inspired by the intrinsic emission of nonaromatic amino acids and poly(amino acids) in concentrated solutions and solids, we revisited protein light emission using bovine serum albumin (BSA) as a model. BSA is virtually nonemissive in dilute solutions (≤0.1 mg mL^?1), but highly luminescent upon concentration or aggregation, showing unique concentration‐enhanced emission and aggregation‐induced emission (AIE) characteristics. Notably, apart from well‐documented UV luminescence, bright blue emission is clearly observed. Furthermore, persistent room‐temperature phosphorescence (p‐RTP) is achieved even in the amorphous solids under ambient conditions. This visible emission can be rationalized by the clustering‐triggered emission (CTE) mechanism. These findings not only provide an in‐depth understanding of the emissive properties of proteins, but also hold strong implications for further elucidating the basis of tissue autofluorescence.     

Tunable fluorescence conjugated copolymers consisting of tetraphenylethylene and fluorene units: From aggregation‐induced emission enhancement to dual‐channel fluorescence response

A series of new conjugated polymers PTPE_xF_y, which consist of tetraphenylethylene (TPE) units and fluorene (F) units, have been designed and synthesized by Suzuki cross‐coupling polymerization. The polymers PTPE_xF_y exhibited aggregation‐induced emission enhancement and dual‐channel fluorescence response (DCFR) when they were aggregated in solution, and these properties are related with their TPE‐to‐F ratio in the polymer backbone. For PTPE and PTPE_0.5F_0.5 , the fluorescence emission was enhanced by aggregation when water was added into their THF solutions. For the copolymers PTPE_0.3F_0.7 , PTPE_0.2F_0.8 , and PTPE_0.1F_0.9 , the DCFRs were observed when they were aggregated by adding water into their solution, which can be attributed to the different responses of fluorene segments and TPE segments to aggregation. The fluorene segments have an aggregation‐caused quenching characteristic, whereas the TPE segments have an aggregation‐induced emission characteristic. According to the fluorescence lifetime and quantum yield data of the polymer solutions, we have discovered that the polymer's natural life time increases as its TPE content increases. In the solid film, PTPE_0.3F_0.7 and PTPE_0.2F_0.8 showed better quantum yield than other polymers, due to the combination of the excellent fluorescent property of the fluorene groups and the nonplanar conformation of the TPE groups. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

A Fluorescent Probe for Hg2+ Based on Gold(I) Complex with An Aggregation‐Induced Emission Feature

A gold(I) complex that exhibited aggregation‐induced emission in acetonitrile‐water mixtures was designed. It showed high selectivity and sensitivity for Hg²⁺ in acetonitrile‐H₂O (1:1, V:V) solution. Dynamic light scattering measurements were conducted to verify that the addition of Hg²⁺ changed the particle size and induced fluorescence quenching.     

Newly synthesized polybenzoxazole derivative with an adjacent hydroxyphenyl ring for optical sensing

A photoluminescent polymer with a 2‐(2′‐hydroxyphenyl)benzoxazole unit in the molecular main chain was synthesized through the deprotection reaction of a precursor polymer. The amorphous, conjugated polymer emitted green light, both in solution and as a solid, with a fluorescence emission maximum at 518 nm from an excited keto tautomer because of excited‐state intramolecular proton transfer. The polymer showed different fluorescence emission spectra in solvents with different polarities because of intramolecular hydrogen‐bond interruption. The intramolecular hydrogen‐bond‐induced emission change was successfully used as a sensitive sensing signal for metal cations as expected, the polymer acting as a fluorescence‐based chemosensor. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1397–1403, 2005     

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.     

Photoactivated Fluorescence Enhancement in F,N‐Doped Carbon Dots with Piezochromic Behavior

Photoactivation in CdSe/ZnS quantum dots (QDs) on UV/Vis light exposure improves photoluminescence (PL) and photostability. However, it was not observed in fluorescent carbon quantum dots (CDs). Now, photoactivated fluorescence enhancement in fluorine and nitrogen co‐doped carbon dots (F,N‐doped CDs) is presented. At 1.0 atm, the fluorescence intensity of F,N‐doped CDs increases with UV light irradiation (5 s–30 min), accompanied with a blue‐shift of the fluorescence emission from 586 nm to 550 nm. F,N‐doped CDs exhibit photoactivated fluorescence enhancement when exposed to UV under high pressure (0.1 GPa). F,N‐doped CDs show reversible piezochromic behavior while applying increasing pressure (1.0 atm to 9.98 GPa), showing a pressure‐triggered aggregation‐induced emission in the range 1.0 atm–0.65 GPa. The photoactivated CDs with piezochromic fluorescence enhancement broadens the versatility of CDs from ambient to high‐pressure conditions and enhances their anti‐photobleaching.     

Hetero‐oligophenylene‐Based AIEE Material as a Multiple Probe for Biomolecules and Metal Ions to Construct Logic Circuits: Application in Bioelectronics and Chemionics

New hetero‐oligophenylene derivative ( 2 ) was synthesized which exhibits aggregation‐induced emission enhancement (AIEE) in H₂O/THF (80:20). The aggregates serve as a biological probe for three different proteins, that is bovine serum albumin (BSA), cytochrome c, and lysozyme, and DNA in contrasting modes. Further, among 29 metal ions tested, the contrasting fluorescence behavior of aggregates of 2 is observed with only Pb²⁺ and Pd²⁺ ions. Multiple output logic circuits based upon the fluorescence behavior between BSA and cytochrome c and between Pb²⁺ and Pd²⁺ ions are constructed.     

White‐Light Emission Strategy of a Single Organic Compound with Aggregation‐Induced Emission and Delayed Fluorescence Properties

A novel white‐light‐emitting organic molecule, which consists of carbazolyl‐ and phenothiazinyl‐substituted benzophenone (OPC) and exhibits aggregation‐induced emission‐delayed fluorescence (AIE‐DF) and mechanofluorochromic properties was synthesized. The CIE color coordinates of OPC were directly measured with a non‐doped powder, which presented white‐emission coordinates (0.33, 0.33) at 244 K to 252 K and (0.35, 0.35) at 298 K. The asymmetric donor–acceptor–donor′ (D‐A‐D′) type of OPC exhibits an accurate inherited relationship from dicarbazolyl‐substituted benzophenone (O2C, D‐A‐D) and diphenothiazinyl‐substituted benzophenone (O2P, D′‐A‐D′). By purposefully selecting the two parent molecules, that is, O2C (blue) and O2P (yellow), the white‐light emission of OPC can be achieved in a single molecule. This finding provides a feasible molecular strategy to design new AIE‐DF white‐light‐emitting organic molecules.     

Studies on the energy transfer system of terbium-norfloxacin chelate and its interaction with serum albumins

The energy absorbed by norfloxacin could be transferred to terbium(Ⅲ) through chela-tion of norfloxacin with terbium(Ⅲ),then the characteristic fluorescence emission could be observed.The interaction of serum albumins with norfloxacin have been investigated in this paper.The results showed that HSA could inhibit the energy transfer between norfloxacin and terbium(Ⅲ).But,BSA could not.It was shown that the binding properties of norfloxacin to HSA and BSA were totally different.     

Study of interaction of Potassium Dodecatangestato Cobaltate(III) with Bovine Serum Albumin using Fluorescence Spectroscopy

The binding of potassium dodecatangestato cobaltate(III) (PDC) as a water-soluble polyoxometal with bovine serum albumin (BSA) as a major transporting protein of plasma, has been investigated at pH 7.2, 5?mM phosphate buffer, 27°C and various ionic strength by fluorescence spectroscopy.

The results show that the binding of PDC to BSA quenches BSA emission and the Stern–Volmer linear relationship can be applied for the quenching process.

The values of Stern–Volmer constant, K _sv, which can be considered as a binding constant for formation of 1:1 complex at 0.01, 0.1 and 0.2?M NaCl are 8.56 × 10⁵, 5.72 × l0⁵ and 9.60 × 10⁵, respectively. The interpretation of the results represents that binding affinity depends on both electrostatic forces and conformational stability of BSA. A step-by-step aggregation model, which has been developed by Borissevich et al., was used to determine the average aggregation number of BSA, ?J?, from the fluorescence quenching. The results show that the binding of PDC to BSA does not induce any considerable aggregation in BSA molecules. Therefore, it can be concluded that there are no considerable conformational changes in BSA molecules during its interaction with PDC.     

Highlights on the Road towards Highly Emitting Solid‐State Luminophores: Two Classes of Thiazole‐Based Organoboron Fluorophores with the AIEE/AIE Effect

Developing a novel, small‐sized molecular building block that may be capable of emitting light in the solid state is a challenging task and has rarely been reported in the literature. BF₂‐containing dyes seem to be promising candidates towards this aim. Two series of new N^NBF₂ complexes showing aggregation‐induced emission (AIE) and aggregation‐induced emission enhancement (AIEE) were designed and synthesized by means of a new protocol, which improved on the traditional method by employing microwave irradiation. The optical and photophysical properties of the BF₂ complexes were investigated in depth. The synthesized complexes showed fluorescence in both solution and the solid state and, in a mixture of tetrahydrofuran/water, may aggregate into fluorescent nanoparticles. The experimental investigation was supported by quantum mechanical calculations. Their availability, stability, large Stokes shifts, and aggregation capabilities, along with their solid‐state emission capability, render this new class of BF₂ complexes promising AIEE/AIE fluorophores for further applications in the fields of fluorescence imaging and materials science.     

Squaraine Rotaxane as a Reversible Optical Chloride Sensor

A mechanically interlocked squaraine rotaxane is comprised of a deep‐red fluorescent squaraine dye inside a tetralactam macrocycle. NMR studies show that Cl^? binding to the rotaxane induces macrocycle translocation away from the central squaraine station, a process that is completely reversed when the Cl^? is removed from the solution. Steady‐state fluorescence and excited‐state lifetime measurements show that this reversible machine‐like motion modulates several technically useful optical properties, including a three‐fold increase in deep‐red fluorescence emission that is observable to the naked eye. The excited states were characterized quantitatively by time‐correlated single photon counting, femtosecond transient absorption spectroscopy, and nanosecond laser flash photolysis. Cl^? binding to the rotaxane increases the squaraine excited singlet state lifetime from 1.5 to 3.1 ns, and decreases the excited triplet state lifetime from >200 to 44 μs. Apparently, the surrounding macrocycle quenches the excited singlet state of the encapsulated squaraine dye and stabilizes the excited triplet state. Prototype dipsticks were prepared by adsorbing the lipophilic rotaxane onto the ends of narrow, C18‐coated, reverse‐phase silica gel plates. The fluorescence intensity of a dipstick increased eighteen‐fold upon dipping in an aqueous solution of tetrabutylammonium chloride (300 mM ) and was subsequently reversed by washing with pure water. It is possible to develop the dipsticks for colorimetric determination of Cl^? levels by the naked eye. After dipping into aqueous tetrabutylammonium chloride, a dipstick’s color slowly fades at a rate that depends on the amount of Cl^? in the aqueous solution. The fading process is due primarily to hydrolytic bleaching of the squaraine chromophore within the rotaxane. That is, association of Cl^? to immobilized rotaxane induces macrocycle translocation and exposure of the electrophilic C₄O₂ core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product.     

Bioorthogonal Turn‐On Probe Based on Aggregation‐Induced Emission Characteristics for Cancer Cell Imaging and Ablation

Bioorthogonal turn‐on probes have been widely utilized in visualizing various biological processes. Most of the currently available bioorthogonal turn‐on probes are blue or green emissive fluorophores with azide or tetrazine as functional groups. Herein, we present an alternative strategy of designing bioorthogonal turn‐on probes based on red‐emissive fluorogens with aggregation‐induced emission characteristics (AIEgens). The probe is water soluble and non‐fluorescent due to the dissipation of energy through free molecular motion of the AIEgen, but the fluorescence is immediately turned on upon click reaction with azide‐functionalized glycans on cancer cell surface. The fluorescence turn‐on is ascribed to the restriction of molecular motion of AIEgen, which populates the radiative decay channel. Moreover, the AIEgen can generate reactive oxygen species (ROS) upon visible light (λ=400–700 nm) irradiation, demonstrating its dual role as an imaging and phototherapeutic agent.