Aggregation‐Induced Emission: A Rising Star in Chemistry and Materials Science

Fluorophore multimerization on a PEG backbone as a concept for signal amplification and lifetime modulation

Fluorescent labels have strongly contributed to many advancements in bioanalysis, molecular biology, molecular imaging, and medical diagnostics. Despite a large toolbox of molecular and nanoscale fluorophores to choose from, there is still a need for brighter labels, e.g., for flow cytometry and fluorescence microscopy, that are preferably of molecular nature. This requires versatile concepts for fluorophore multimerization, which involves the shielding of dyes from other chromophores and possible quenchers in their neighborhood. In addition, to increase the number of readout parameters for fluorescence microscopy and eventually also flow cytometry, control and tuning of the labels' fluorescence lifetimes is desired. Searching for bright multi-chromophoric or multimeric labels, we developed PEGylated dyes bearing functional groups for their bioconjugation and explored their spectroscopic properties and photostability in comparison to those of the respective monomeric dyes for two exemplarily chosen fluorophores excitable at 488 nm. Subsequently, these dyes were conjugated with anti-CD4 and anti-CD8 immunoglobulins to obtain fluorescent conjugates suitable for the labeling of cells and beads. Finally, the suitability of these novel labels for fluorescence lifetime imaging and target discrimination based upon lifetime measurements was assessed. Based upon the results of our spectroscopic studies including measurements of fluorescence quantum yields (QY) and fluorescence decay kinetics we could demonstrate the absence of significant dye-dye interactions and self-quenching in these multimeric labels. Moreover, in a first fluorescence lifetime imaging (FLIM) study, we could show the future potential of this multimerization concept for lifetime discrimination and multiplexing.

Fluorescence-Encoded Time-Domain Coherent Raman Spectroscopy in the Visible Range

Fluorescence-encoded vibrational spectroscopy has attracted increasing attention by virtue of its high sensitivity and high chemical specificity. We recently demonstrated fluorescence-encoded time-domain coherent Raman spectroscopy (FLETCHERS), which enables low-frequency vibrational spectroscopy of low-concentration fluorophores using near-infrared (800-900 nm) light excitation. However, the feasibility of this study was constrained by the scarcity of excitable molecules in the near-infrared range. Consequently, the broader applicability of FLETCHERS has not been investigated. Here we extend the capabilities of FLETCHERS into the visible range by employing a noncollinear optical parametric amplifier as a light source, significantly enhancing its versatility. Specifically, we use the method, which we refer to as visible FLETCHERS (vFLETCHERS), to individually acquire Raman spectra from five visible fluorophores that have absorption peaks in the 600-700 nm region. These results not only confirm the versatility of vFLETCHERS for a wide range of molecules but also allude to its widespread applicability in biological research through highly sensitive supermultiplexed imaging.

Clickable Polymerization-Induced Emission Luminogens Toward Color-Tunable Modification of Non-Traditional Intrinsic Luminescent Polymers

Non-traditional intrinsic luminescent (NTIL) polymer is an emerging field, and its color-tunable modification is highly desirable but still rarely investigated. Here, a click chemistry approach for the color-tunable modifications of NTIL polymers by introducing clickable polymerization-induced emission luminogen (PIEgen), is demonstrated. Through Cu-catalyzed azide-alkyne cycloaddition click chemistry, a series of PIEgens is successful prepared, which is further polymerized via reversible addition-fragmentation chain transfer (RAFT) polymerization. Interestingly, after clickable modification, these monomers are nonemissive in both solution and aggregation states; while, the corresponding polymers exhibit intriguing aggregation-induced emission (AIE) characteristics, confirming their PIEgen characteristics. By varying alkynyl substitutions, color-tunable NTIL polymers are achieved with emission wavelength varying from 448 to 498 nm, revealing a series of PIEgens and verifying the importance of modification of NTIL polymers. Further luminescence energy transfer application is carried out as well. This work therefore designs a series of clickable PIEgens and opens a new avenue for the modification of NTIL polymers via click chemistry, which may cause inspirations to the research fields including luminescent polymer, NTIL, click chemistry, AIE and modification.

Targeted and Long-Term Fluorescence Imaging of Plant Cytomembranes Using Main-Chain Charged Polyelectrolytes with Aggregation-Induced Emission

Fluorescent polyelectrolytes have attracted tremendous attention due to their unique properties and wide applications. However, current research objects of fluorescent polyelectrolytes mainly focus on side-chain charged polyelectrolytes, and the applications of polyelectrolytes in plant cytomembrane imaging with long time and high specificity still remain challenging. Herein, long-time and targeted fluorescence imaging of plant cytomembranes was achieved for the first time using main-chain charged polyelectrolytes (MCCPs) with aggregation-induced emission (AIE). A series of MCCPs were designed and synthesized, among which the red-emissive and AIE-active MCCP with a triphenylamine linker and a cyano group around the cationic ring-fused heterocyclic core showed the best fluorescence imaging performance of plant cells. Unlike other MCCPs and its neutral form of polymer, this cyano-substituted conjugated polyelectrolyte can specifically target the cytomembrane of plant cells within a short staining time with many advantages, including wash-free staining, high photostability and imaging integrity, excellent durability (at least 12 h), and low biotoxicity. In addition to onion epidermal cells, this AIE fluorescence probe also shows good imaging capabilities for other kinds of plant cells such as Glycine max and Vigna radiata. Such an AIE-active MCCP-based imaging system provides an effective design strategy to develop fluorescence probes with high specificity and long-term imaging ability toward plant plasma membranes.

Barbier Polymerization-Induced Emission towards Fully Substituted Polyethylene Analogues with Non-Traditional Intrinsic Luminescence

The properties of polyethylene are highly dependent on the variety and quantity of substitutions. Generally, polyethylene can only be fully substituted with fluorine atoms, mainly e. g., polytetrafluoroethylene and nafion, because atomic radius of fluorine atom is small enough. The preparation of fully substituted polyethylene analogues (FSPEA) and their non-traditional intrinsic luminescence (NTIL) are attractive, especially for substitutions with relatively larger atomic radii than a fluorine atom. Here, Barbier polymerization-induced emission (PIE) is demonstrated as a universal method for the molecular design of NTIL type FSPEAs with intriguing aggregation-induced emission (AIE) behaviors. Through Barbier polymerization of diphenyldichloromethane and different peroxyesters in the presence of Mg in one pot, a series of FSPEAs, including polytriphenylethanol (PTPE), polydiphenylfurylethanol (PDPFE), polydiphenylthiophenylethanol (PDPTE) and polydiphenylnaphthylethanol (PDPNE) have been successfully prepared. Further potential applications for explosive detection, artificial light-harvesting system and white phosphor-converted light-emitting diode are investigated. Therefore, this work opens up a new approach for the molecular design of FSPEA with non-conjugated luminescence, which may cause inspirations to different research fields like polyolefin and luminescent materials.

Pd(II)- and Pt(II)-Assisted P-C Activation/Cyclization Reactions with a Luminescent α-Aminophosphine

There is unceasing interest toward transformations of phosphine derivatives, which are facilitated by transition metals. We report a facile Pd(II)- and Pt(II)-assisted P-C bond cleavage in a luminescent 2-phenylbenzothiazole-based α-methylaminophosphine (PCN, 1). Specifically, reactions between 1 and [M(COD)Cl2] (M = Pd, Pt; COD = cycloocta-1,5-diene) in different solvents (methylene chloride, acetonitrile, pyridine, toluene) resulted in the formation of PPh2-, captured either as a bridging ligand in binuclear complexes with a {M2(PPh2)2} moiety or as an adduct to COD in [Pt2(PPh2COD)2Cl2]. The heterocyclic part transforms to annulated c-CN+ species with a 1,2-dihydroquinazoline cycle formed. In the presence of pyridine as a base, annulated form c-CN+ destabilizes and undergoes reverse cyclization transforming to deprotonated CN form. Quantum-chemical density functional theory (DFT) calculations predict that a crucial step in the reactions involves proton transfer from the N atom of the amino group of PCN to a neighboring molecule. A combination of high photophysical sensitivity of c-CN+ toward its immediate environment and rich structural capabilities in assembling (c-CN)22+ pairs in different crystal packings in a family of phases with the general formula (c-CN)2[M2(PPh2)2Cl4] allows one to fine-tune the luminescence properties of the latter. The results were rationalized as a variation of π-π intercationic spacings, which tunes the degree of excited-state charge transfer between c-CN+ cations. As a result, compounds with relatively short interplanar π-π-separation between the cations show a stronger charge-transfer-mediated bathochromic shift.

Synergistically Boosting the Circularly Polarized Luminescence of Functionalized Pillar[5]arenes by Polymerization and Aggregation

Supramolecular polymers based on chiral macrocycles have attracted increasing attention in the field of circularly polarized luminescence (CPL) owing to their unique properties. However, the construction of macrocyclic supramolecular polymers with highly efficient CPL properties in aggregate states still remains challenging. Herein, w e constructed a class of macrocycle-based coordination polymers by combining the planar chiral properties of pillar[5]arene with the excellent fluorescence properties of aggregation-induced emission luminogens. The formation of polymers enhances both the fluorescence and chiral properties, resulting in chiral supramolecular polymers with remarkable CPL properties. Increasing the aggregation degree of the polymers can further improve their CPL properties, as evidenced by a 21-fold increase in the dissymmetry factor and an over 25-fold increase in the fluorescence quantum yield in the aggregate state compared to the solution state. Such a synergistic effect of polymerization- and aggregation-enhanced CPL can be explained by the restriction of intramolecular motions and aggregation-induced conformation confinement. This work provides a promising method for developing highly efficient CPL supramolecular polymers.

Meta-CF3-Substituted Analogues of the GFP Chromophore with Remarkable Solvatochromism

In this work, we have shown that the introduction of a trifluoromethyl group into the me-ta-position of arylidene imidazolones (GFP chromophore core) leads to a dramatic increase in their fluorescence in nonpolar and aprotic media. The presence of a pronounced solvent-dependent gradation of fluorescence intensity makes it possible to use these substances as fluorescent polarity sensors. In particular, we showed that one of the created compounds could be used for selective labeling of the endoplasmic reticulum of living cells.

Dual-Responsive and Reusable Optical Sensors Based on 2,3-Diaminoquinoxalines for Acidity Measurements in Low-pH Aqueous Solutions

This work is focused on the age-old challenge of developing optical sensors for acidity measurements in low-pH aqueous solutions (pH < 5). We prepared halochromic (3-aminopropyl)amino-substituted quinoxalines QC1 and QC8 possessing different hydrophilic-lipophilic balance (HLB) and investigated them as molecular components of pH sensors. Embedding the hydrophilic quinoxaline QC1 into the agarose matrix by sol-gel process allows for fabrication of pH responsive polymers and paper test strips. The emissive films thus obtained can be used for a semi-quantitative dual-color visualization of pH in aqueous solution. Being exposed to acidic solutions with pH in the range of 1-5, they rapidly give different color changes when the analysis is performed in daylight or under irradiation at 365 nm. Compared with classical non-emissive pH indicators, these dual-responsive pH sensors allow for an increase in the accuracy of pH measurements, particularly in complex environmental samples. pH indicators for quantitative analysis can be prepared by the immobilization of amphiphilic quinoxaline QC8 using Langmuir-Blodgett (LB) and Langmuir-Schäfer (LS) techniques. Compound QC8 possessing two long alkyl chains (n-C₈H₁₇) forms stable Langmuir monolayers at the air-water interface, and these monolayers can be successfully transferred onto hydrophilic quartz and hydrophobic polyvinylchlorid (PVC) substrates using LB and LS techniques, respectively. The 30-layer films thus obtained are emissive, reveal excellent stability, and can be used as dual-responsive pH indicators for quantitative measurements in real-world samples with pH in the range of 1-3. The films can be regenerated by immersing them in basic aqueous solution (pH = 11) and can be reused at least five times.

Aggregation-Induced emission photosensitizer with lysosomal response for photodynamic therapy against cancer

Photosensitizers play a key role in bioimaging and photodynamic therapy (PDT) of cancer. However, conventional photosensitizers usually do not achieve the desired efficacy in PDT due to their poor photostability, targeting ability, and responsiveness. Herein, we designed a series of photosensitizers with aggregation-induced emission (AIE) effect using benzothiazole- triphenylamine (BZT-triphenylamine) as the parent nucleus. The synthesized compound SIN ((E)-2-(4-(diphenylamino)styryl)-3-(4-iodobutyl)benzo[d]thiazol-3-ium) exhibits good biocompatibility, photostability, and bright emission in the near-infrared range (600-800 nm). The fluorescence emission intensity is responsive to viscosity, with significant fluorescence enhancement (48 times) and high fluorescence quantum yield (4.45 %) at high viscosity. Moreover, SIN has particular lysosome targeting properties with a Pearson correlation coefficient (PCC) of 0.97 and has good ¹O₂ generation ability under white light irradiation, especially in a weak acidic environment. Thus, SIN can realize good bioimaging ability and photodynamic therapeutic efficacy under the highly viscous and weakly acidic environment of lysosomes in the tumor cells. This study indicates that SIN has potential as a multifunctional organic photosensitizer for bioimaging and PDT of tumor.

Multifunctional Fluorescent Main-Chain Charged Polyelectrolytes Synthesized by Cascade C-H Activation/Annulation Polymerizations

Fluorescent polyelectrolytes have attracted enormous attention as functional polymer materials. In contrast with the widely studied conjugated polyelectrolytes with ionic groups in side chains, fluorescent main-chain charged polyelectrolytes (MCCPs) have rarely been explored due to the large synthetic difficulty. Herein, we develop a facile and atom-economical N-heterocyclic carbene-directed cascade C-H activation/annulation polymerization strategy that can transform readily available imidazolium substrates and internal diynes into multifunctional fluorescent MCCPs with complex structures and high molecular weights (absolute Mn up to 135 600) in nearly quantitative yields. The presence of multisubstituted polycyclic N-heteroaromatic cations in polymer backbones endow the obtained MCCPs with excellent solution processability, high thermal stability, and dual-state efficient fluorescence in both solution and aggregate states. Benefiting from the strong electron-withdrawing capability of the cationic heterocycles in main chains, multicolored aggregate-state fluorescence can be readily achieved by modifying the substituents around the cationic ring-fused core. Taking advantage of the good photosensitivity of the fluorescent MCCP thin films, multiscale and high-resolution fluorescent photopatterns with different colors can be facilely prepared with potential applications in optical display devices and anticounterfeiting systems. Moreover, the strong electrostatic interactions of these cationic MCCPs with anionic polyelectrolytes enable them to form multicolored fluorescent interfacial polyelectrolyte complexation microfibers with directly visualized internal structures. Such flexible microfibers can be further made into diversified forms of fiber-based macroscopic patterns or painting.

Catalyst-free synthesis of diverse fluorescent polyoxadiazoles for the facile formation and morphology visualization of microporous films and cell imaging

The development of facile polymerizations toward functional heterocyclic polymers is of great significance for chemistry and materials science. As an important class of heterocyclic polymers, polyoxadiazoles (PODs) have found applications in various fields. However, the synthetic difficulties of PODs greatly restrict their structural diversity and property investigation. Herein, we report a series of catalyst-free multicomponent polymerizations (MCPs) that can facilely synthesize functional PODs with well-defined and diversified topological structures from commercially available or readily accessible aldehydes, carboxylic acids, secondary amines, and (N-isocyanimino)triphenylphosphorane at room temperature. Unlike conventional Ugi polycondensations, the present Ugi-type MCPs can in situ generate oxadiazole moieties in polymer backbones. The obtained PODs possess good solubility, high thermal and morphological stability, and excellent film-forming ability. The introduction of aggregation-induced emission (AIE) moieties together with the inherent structural features of PODs endow these polymers with multiple functionalities. The AIE-active linear PODs can form fluorescent microporous films with stable and ordered structures based on the simple breath figure patterning method, and the self-assembly morphologies can be directly visualized by fluorescence microscopy in a high-contrast and sensitive manner. Moreover, both the linear and hyperbranched AIE-active PODs possess excellent biocompatibility, good lysosome specificity, and excellent photobleaching resistance, which enable them to serve as promising lysosome-specific fluorescent probes in biological imaging.

Stimuli-Responsive Electrospun Fluorescent Fibers Augmented with Aggregation-Induced Emission (AIE) for Smart Applications

This review addresses the latest advancements in the integration of aggregation-induced emission (AIE) materials with polymer electrospinning, to accomplish fine-scale electrospun fibers with tunable photophysical and photochemical properties. Micro- and nanoscale fibers augmented with AIE dyes (termed AIEgens) are bespoke composite systems that can overcome the limitation posed by aggregation-caused quenching, a critical deficiency of conventional luminescent materials. This review comprises three parts. First, the reader is exposed to the basic concepts of AIE and the fundamental mechanisms underpinning the restriction of intermolecular motions. This is followed by an introduction to electrospinning techniques pertinent to AIE-based fibers, and the core parameters for controlling fiber architecture and resultant properties. Second, exemplars are drawn from latest research to demonstrate how electrospun nanofibers and porous films incorporating modified AIEgens (especially tetraphenylethylene and triphenylamine derivatives) can yield enhanced photostability, photothermal properties, photoefficiency (quantum yield), and improved device sensitivity. Advanced applications are drawn from several promising sectors, encompassing optoelectronics, drug delivery and biology, chemosensors and mechanochromic sensors, and innovative photothermal devices, among others. Finally, the outstanding challenges together with potential opportunities in the nascent field of electrospun AIE-active fibers are presented, for stimulating frontier research and explorations in this exciting field.

Construction of Multi-color fluorescent carbon dots by Aggregation-Induced emission

Aggregation-induced emission luminogens (AIEgens) have garnered significant attention because of their outstanding photophysical characteristics. AIEgens are used in fluorescence imaging, sensors, tumor treatment, and other related fields. However, the synthese of these AIEgens are relatively complicated and requires expensive raw materials. These drawbacks limit their applications and development to a certain extent. In this study, using cheap and convenient materials, we developed a new type of carbon dots (O-CDs) using a one-step solvothermal method, which has the potential to become a new AIEgen. O-CDs exhibit different fluorescence colors in different solvents, and they exist as monomers in ethylic acid and, ethanol alcohol, etc., exhibiting blue fluorescence. After adding water, the fluorescence of O-CDs gradually turns orange red, because the internal rotation of the disulfide bond molecules is restricted and the AIE effect occurs. Using the unique AIE performance of O-CDs, we fabricated an anti-counterfeiting luminous ink, that can be used for encryption in the reversible double switch mode.

Development of Yellow-to-Orange Photoluminescence Molecules Based on Alterations in the Donor Units of Fluorinated Tolanes

Since the aggregation-induced emission (AIE) phenomenon was first reported by Tang et al., much effort has been devoted to the development of solid-state luminescent molecules by chemists worldwide. Our group successfully developed fluorinated tolanes as novel compact π-conjugated luminophores with blue photoluminescence (PL) in the crystalline state. Moreover, we reported the yellow-green PL molecules based on their electron-density distributions. In the present study, we designed and synthesized fluorinated tolanes with various amine-based donors and evaluated their photophysical properties. The carbazole-substituted fluorinated tolane exhibited strong PL in the solution state, whereas piperidine- or phenothiazine-substituted fluorinated tolanes showed a dramatic decrease in PL efficiency. Notably, fluorinated tolanes with piperidine or phenothiazine substituents displayed yellow-to-orange PL in the crystalline state; this may have occurred because these tolanes exhibited tightly packed structures formed by intermolecular interactions, such as H···F hydrogen bonds, which suppressed the non-radiative deactivation process. Moreover, fluorinated tolanes with amine-based donors exhibited AIE characteristics. We believe that these yellow-to-orange solid PL molecules can contribute to the development of new solid luminescent materials.

Aliphatic Polyesters with White-Light Clusteroluminescence

Single-molecule white-light emission (SMWLE) has many advantages in practical applications; however, the fabrication of SMWLE from nonconjugated luminescent polymers, namely, clusteroluminogens (CLgens), is still a big challenge. Herein, the first example of linear nonconjugated polyesters with SMWLE is reported. Twenty-four kinds of nonconjugated aliphatic polyesters with tunable clusteroluminescence (CL) colors and efficiency were synthesized by the copolymerization of six epoxides and four anhydrides. Experimental and calculation results prove that, at the primary structure level, the balance of structural flexibility and rigidity via adjusting the side-chain length significantly enhances the efficiency of CL without wavelength change. However, altering the chemical structures of the monomer from succinic anhydride to trans-maleic anhydride (MA), cis-MA, and citraconic anhydride (CA), secondary structures of these polyesters change from helix to straight and folding sheet accompanied by gradually red-shifted CL from 460 to 570 nm due to the increase in through-space n-π* interactions, as demonstrated by the computational and experimental results. Then, pure SMWLE with CIE coordination (0.30, 0.32) based on overlapped short-wavelength and long-wavelength CL is achieved in CA-based polyesters. This work not only provides further insights into the emission mechanism of CL but also provides a new strategy to manipulate the properties of CL by regulating the hierarchical structures of CLgens.

Aggregation-Induced Emission (AIE) Photosensitizer Combined Polydopamine Nanomaterials for Organelle-Targeting Photodynamic and Photothermal Therapy by the Recognition of Sialic Acid

The construction of organelle-targeting nanomaterials is an effective way to improve tumor imaging and treatment. Here, a new type of composite nanomaterial named as PTTPB is developed. PTTPB is composed of organelle-targeting aggregation-induced emission photosensitizer TTPB and polydopamine nanomaterials. With the functional modification of TTPB, PTTPB can recognize sialic acid on the cell membrane and present mitochondrial targeted capabilities. The intake of PTTPB in cancerous cells can be increased by the recognition process of cell membrane. PTTPB can generate singlet oxygen for photodynamic therapy (PDT), and present good photothermal conversion ability with irradiation. The PTTPB with organelle-targeting imaging-guided can realize the tumor ablation with the synergistic effect of PDT and photothermal therapy.

Aggregation-induced emission: An emerging concept in brain science

As an emerging concept in brain science, aggregation-induced emission (AIE) has captivated much interest by virtue of the unique superiority of AIE fluorophores in terms of emission intensity, imaging resolution, biocompatibility and photosensitivity. This review mainly overviews the current state-of-art advances of AIE fluorophores achieving the superb performance in brain imaging and therapy, which facilitate deep tissue penetration, high contrast to autofluorescence and efficient blood-brain barrier (BBB) crossing by rational molecular design and functionalized strategies. We expect this review serve as a modest spur to push forward the blooming growth of research in this fertile field.

Aggregation-Induced Emission Boosting the Study of Polymer Science

The past one hundred years witness the great development of polymer science. The advancement of polymer science is closely related with the developing of characterization techniques and methods, from viscometry in molecular weight determination to advanced techniques including differential scanning calorimetry, nuclear magnetic resonance and scanning electron microscopy. However, these techniques are normally constrained to tedious sample preparation, high cost, harsh experimental condition, or ex-situ characterization. Fluorescence technology has the merits of high sensitivity and direct visualization. Contrary to conventional aggregation-causing quenching fluorophores, those dyes with aggregation-induced emission characteristic show high emission efficiency in aggregate states. Based on the restriction of intramolecular motions for AIE properties, the AIE materials are very sensitive to the surrounding microenvironments owing to the twisted propeller-like structures and therefore reveal great potentials in polymer's study. The AIE concept has been successfully used in polymer's study and provides us a deeper understanding on polymer structure and properties. In this review, the applications of AIEgens in polymer science for visualizing polymerization, glass transition, dissolution, crystallization, gelation, self-assembly, phase separation, cracking and self-healing were exemplified and summarized. Lastly, the challenges and perspectives in the study of polymer science using AIEgens are addressed. This article is protected by copyright. All rights reserved.

Covalent Attachment of Aggregation-Induced Emission Molecules to the Surface of Ultrasmall Gold Nanoparticles to Enhance Cell Penetration

Three different alkyne-terminated aggregation-induced emission molecules based on a para-substituted di-thioether were attached to the surface of ultrasmall gold nanoparticles (2 nm) by copper-catalyzed azide–alkyne cycloaddition (click chemistry). They showed a strong fluorescence and were well water-dispersible, in contrast to the dissolved AIE molecules. The AIE-loaded nanoparticles were not cytotoxic and easily penetrated the membrane of HeLa cells, paving the way for an intracellular application of AIE molecules, e.g., for imaging.

Metal-Enhanced Aggregation-Induced Emission Strategy for the HIV-I RNA-Binding Ligand Assay

The HIV-Ι trans-activation responsive (TAR) RNA-trans-activator of transcription (Tat) protein complex is crucial for the efficient transcription of the integrated human immunodeficiency virus-I genome and is an established therapeutic target for AIDS diagnosis and treatment. Developing a sensitive strategy for the TAR RNA-binding ligand assay could provide antiviral leads with a radically new mechanism for the treatment of AIDS. Herein, a new TAR RNA-binding ligand assay platform was established using a signal amplification strategy that combines aggregation-induced emission (AIE) with a metal-enhanced fluorescence (MEF) concept. The tetraphenylethylene (TPE) derivative was labeled on the Tat peptide as a fluorescent molecule, while the TAR RNA was immobilized on the surface of the Fe₃O₄@Au@Ag@SiO₂ nanoparticles (NPs) to specifically bind the TPE-Tat peptide. The TPE-Tat peptide was weakly emissive itself while emitting strongly in the NP-TAR-TPE-Tat complex by the AIE and MEF signal amplification effect. It was confirmed by known Tat peptide competitors that this system could be applied to the screening and detection of TAR RNA-binding ligands because they could replace the TPE-Tat peptide from the complex and make the system fluorescence decrease. When this system was adopted to test four candidate ligands, it was found that bisantrene had a favorable TAR RNA-binding ability. The proposed AIE-MEF strategy not only provides a sensitive and reliable method for the TAR RNA-binding ligand assay but also can avoid the influence of ligands on fluorescent detection in the conventional displacement assay.

Just Add the Gold: Aggregation-Induced-Emission Properties of Alkynylphosphinegold(I) Complexes Functionalized with Phenylene-Terpyridine Subunits

A series of organometallic complexes containing an alkynylphosphinegold(I) fragment and a phenylene-terpyridine moiety connected together by flexible linker have been prepared using the specially designed terpyridine ligands. The compounds were studied crystallographically to reveal that all of them contain a linearly coordinated Au(I) atom and a free terpyridine moiety. The different orientations of the molecules relative to each other in the solid state determine the multiple noncovalent interactions such as antiparallel ππ stacking, CH-π, and CH-Au, but no aurophilic interactions are realized. The organometallic Au(I) complexes obtained show fluorescence in the solution and dual singlet-triplet emission in the solid state. This means that their photophysical behavior is determined by both intermolecular lattice-defined interactions and Au(I) atom introduction. Density functional theory computational analysis supported the assignment of emission to intraligand electronic transitions only inside the phenylene-terpyridine part with no Au(I) involved. In addition, a study of the nature of the excited states for the "dimer" with an antiparallel orientation of the terpyridine fragment showed that this orientation leads to the generation of abstracted singlet and triplet states, lowering their energy in comparison with the monomer complex. Thus, the complexes obtained can be qualified as examples of Au(I)-containing organometallic aggregation-induced-emission luminogens.

Highly Efficient Multifunctional Organic Photosensitizer with Aggregation-Induced Emission for In Vivo Bioimaging and Photodynamic Therapy

Photosensitizers play a critical role in photodynamic therapy (PDT). Multifunctional organic nanoparticles (NPs) that possess bright fluorescence in aggregates, high singlet oxygen (¹O₂) quantum yield, near-infrared (NIR) absorption and emission, large Stokes shift, two-photon bioimaging, specific organelle targeting, high PDT efficiency, as well as good biocompatibility and photostability are ideal candidate photosensitizers for image-guided PDT. Due to its enhanced fluorescence and high ¹O₂ generation efficiency in aggregate states, photosensitizers with aggregation-induced emission (AIE) characteristics have attracted increasing interest in PDT. In this study, a new AIE-active Schiff base 5-(((5-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)methylene)amino)-3-methylthiophene-2,4-dicarbonitrile (TBTDC) based on a D-A-π-A skeleton has been designed and synthesized, and it can be readily encapsulated by Pluronic F-127 to form uniform nanoparticles. TBTDC NPs exhibit bright NIR emission at 825 nm with a Stokes shift up to 300 nm, impressive two-photon bioimaging capability with tissue penetration deep into 300 μm, high ¹O₂ generation quantum yield (0.552), specific targeting to lysosome, as well as good biocompatibility and photostability. Furthermore, TBTDC NPs present remarkable cytotoxicity for tumor cells and suppression of tumor growth in nude mice through reactive oxygen species generation upon white light irradiation. These results reveal that TBTDC NPs have great potential to become excellent candidates for multifunctional organic photosensitizers for two-photon bioimaging and image-guided PDT and are promising in future clinical applications.

A triple-stimulus responsive melanin-based nanoplatform with an aggregation-induced emission-active photosensitiser for imaging-guided targeted synergistic phototherapy/hypoxia-activated chemotherapy

Multimodal synergistic therapy has gained increasing attention in cancer treatment to overcome the limitations of monotherapy and achieve high anticancer efficacy. In this study, a synergistic phototherapy and hypoxia-activated chemotherapy nanoplatform based on natural melanin nanoparticles (MPs) loaded with the bioreduction prodrug tirapazamine (TPZ) and decorated with hyaluronic acid (HA) was developed. A self-reporting aggregation-induced emission (AIE)-active photosensitizer (PS) (BATTMN) was linked to the prepared nanoparticles by boronate ester bonds. The MPs and BATTMN-HA played roles as quenchers for PS and cancer targeting/photodynamic moieties, respectively. As a pH sensitive bond, the borate ester bonds between HA and BATTMN are hydrolysed in the acidic cancer environment, thereby separating BATTMN from the nanoparticles and leading to the induction of fluorescence for imaging-guided synergistic phototherapy/hypoxia-activated chemotherapy under dual irradiation. TPZ can be released upon activation by pH, near-infrared (NIR) and hyaluronidase (Hyal). Particularly, the hypoxia-dependent cytotoxicity of TPZ was amplified by oxygen consumption in the tumor intracellular environment induced by the AIE-active PS in photodynamic therapy (PDT). The nanoparticles developed in our research showed favorable photothermal conversion efficiency (η = 37%), desired cytocompatibility, and excellent synergistic therapeutic efficacy. The proposed nanoplatform not only extends the application scope of melanin materials with AIE-active PSs, but also offers useful insights into developing multistimulus as well as multimodal synergistic tumor treatment.

Unsymmetrical Hexafluorocyclopentene-Linked Twisted π-Conjugated Molecules as Dual-State Emissive Luminophores

Dual-state emissive (DSE) luminophores, which can luminesce both in solution and in solid states, have recently attracted significant attention because of their broad applications. However, their development is difficult due to the difference in molecular design between solution- and solid-state luminophores. In this study, DSE luminophores based on unsymmetrical hexafluorocyclopentene-linked twisted π-conjugated structures carrying various substituents to tune the electron-density were designed and synthesized in a single-step reaction from heptafluorocyclopentene or perfluoro-1,2-diphenylcyclopentene derivatives. The twisted π-conjugated luminophores exhibited absorption in the UV region at approximately 330 nm, along with several signals in the high-energy region. Upon irradiating the luminophore solution (wavelength 330 nm), light-green to yellow photoluminescence (PL) was observed in the range of 422–471 nm with high PL efficiency. Theoretical calculations revealed that excitation from ground to excited states altered the structural shape of the luminophores from twisted to planar, leading to red-shifted PL and high PL efficiency (ΦPL). The intense blue PL exhibited by the luminophores in the crystalline state was attributed to their twisted molecular structures that suppressed non-radiative deactivation via the effective blocking of π/π stacking interactions.

Zinc (II) and AIEgens: The "Clip Approach" for a Novel Fluorophore Family. A Review

Aggregation-induced emission (AIE) compounds display a photophysical phenomenon in which the aggregate state exhibits stronger emission than the isolated units. The common term of "AIEgens" was coined to describe compounds undergoing the AIE effect. Due to the recent interest in AIEgens, the search for novel hybrid organic-inorganic compounds with unique luminescence properties in the aggregate phase is a relevant goal. In this perspective, the abundant, inexpensive, and nontoxic d10 zinc cation offers unique opportunities for building AIE active fluorophores, sensing probes, and bioimaging tools. Considering the novelty of the topic, relevant examples collected in the last 5 years (2016-2021) through scientific production can be considered fully representative of the state-of-the-art. Starting from the simple phenomenological approach and considering different typological and chemical units and structures, we focused on zinc-based AIEgens offering synthetic novelty, research completeness, and relevant applications. A special section was devoted to Zn(II)-based AIEgens for living cell imaging as the novel technological frontier in biology and medicine.

Switching energy dissipation pathway: in situ proton-induced transformation of AIE-active self-assemblies to boost photodynamic therapy

With the morphological transformation of fluorescent self-assembled nanostructures, their functions can be varied simultaneously. However, little attention has been paid to the function variation in this process. Herein, we present aggregation-induced emission (AIE)-active self-assembled nanospheres to investigate the transformation-induced function variation by switching the energy dissipation pathway. The self-assembled nanospheres showed strong emission under neutral conditions, indicating that radiative decay dominates the energy dissipation. Under acidic conditions, the spheres transformed to vesicles and nanotubes, in which the excited energy was largely consumed by the intersystem crossing pathway and highly efficient reactive oxygen species (ROS) generation was afforded. In particular, this morphological transformation and function variation can smoothly proceed in acidic lysosomes, thus drastically boosting photodynamic cancer therapy.