Recent advances in AIEgen-based chemosensors for small molecule detection, with a focus on ion sensing

Since the aggregation-based emission (AIE) phenomenon emerged in 2001, numerous chemical designs have been built around the AIE concept, displaying its utility for diverse applications, including optics, electronics, energy, and biosciences. The present review critically evaluates the broad applicability of AIEgen-based chemical models towards sensing small analytes and the structural design strategies adjusting the mode of action reported since the last decade. Various AIEgen models have been discussed, providing qualitative and quantitative estimation of cationic metal ions and anionic species, as well as biomolecular, cellular, and organelle-specific probes. A systematic overview of the reported structural design and the underlying working mode will pave the way for designing and developing the next generation of AIEgens for specific applications.

Investigating the role of corrole as an excitation energy relay in light-induced processes in closely connected N,N'-bis(biphenyl-4-yl)aniline functionalized corrole donor-acceptor dyad

A photosynthetic antenna-reaction center model, BBA-PFCor comprised of N,N'-bis(biphenyl-4-yl)aniline (BBA) covalently functionalized to bis(pentafluoro)corrole moiety has been prepared and the contribution of the BBA as the photoinduced energy transfer antenna was investigated. UV-visible studies have shown that integrating the electron-rich BBA chromophore into the corrole core has broadened the soret band of the corrole moiety with the absorption spanning from 300 to 700 nm. Electrochemical studies, in corroboration with the computational calculations, revealed that, BBA moiety can act as an electron reservoir and, in the excited state, it would transfer the excited energy to the corrole moiety in the dyad. Steady-state fluorescence studies have demonstrated that, upon photoexcitation of the BBA moiety of BBA-PFCor at 310 nm in solvents of varied polarity, the BBA emission centered at 400 nm was observed to be quenched, with the concomitant appearance of the corrole emission from 500 to 700 nm, indicating the happening of photoinduced energy transfer (PEnT) from 1BBA* to corrole moiety. Parallel control experiments involving the excitation of the corrole moiety at 410 nm did not result in the diminishing of the corrole emission, suggesting that the quenching of the BBA emission in BBA-PFCor is majorly due to intramolecular PEnT from 1BBA* to corrole moiety leading to the formation of singlet excited corrole, that is, 1BBA*-PFCor ➔ BBA-1PFCor*. The free energy changes of PEnT, ΔGEnT, were found to be thermodynamically feasible in all the solvents used for the study. Parallel time-resolved fluorescence studies were congruent with the steady-state fluorescence results and provided further evidence for the occurrence of ultrafast PEnT from 1BBA*➔corrole in the dyad with the rates of energy transfer (kEnT) of ~108 s-1.

Enhancing Fluorescence in Both Solution and Solid States Induced by Imine Cage Formation

Developing luminescent materials that exhibit strong emissions in both solution and solid phases is highly desirable and challenging. Herein, we report imine-bond directed formation of a rigid organic cage (TPE-cage) that was synthesized by [2+4] imine condensation of a TPE-cored tetra-aldehyde (TPE-TA) with a clip-like diamine (XA) to illustrate confinement-induced fluorescence enhancement. Compared to the non-emissive TPE-TA (ϕF =0.26 %) in the dichloromethane (DCM) solution, the TPE-cage achieved a remarkable (~520-fold) emission enhancement (ϕF =70.38 %). In contrast, a monomeric tetra-imine model compound (TPE-model) showed only a minor enhancement (ϕF =0.56 %) in emission compared to the parent tetra-aldehyde TPE-TA. The emission of TPE-cage was further enhanced by ~1.5-fold (ϕF =80.96 %) in the aggregated state owing to aggregation-induced emission enhancement (AIEE). This approach establishes the potential for synthesizing luminescent materials with high emission in both solution and solid-state by employing a single-step imine condensation reaction.

Influence of yttrium doping on the nonlinear optical limiting properties of cadmium molybdate nanostructures

The emerging demand for the production of nonlinear optical materials with high optical limiting performance has an apparent impact in the field of nonlinear optics owing to their wide application in photonic devices. In this regard, transition metal molybdates have received attention owing to their remarkable optical and luminescence characteristics, leading to their extensive use in next generation optoelectronics devices. Herein, we report the nonlinear optical response of yttrium (Y³⁺) doped cadmium molybdate (CdMoO₄) nanostructures synthesized via a co-precipitation technique. The X-ray diffraction and Raman spectroscopy results confirm the formation of CdMoO₄ nanostructures with a tetragonal structure having the I4₁/a space group. High resolution scanning electron microscopy (HRSEM) of the pristine CdMoO₄ exposed the cubic flat edged nature of the nanostructures and that doping results in particle size reduction due to lattice contraction. X-ray photo electron spectroscopy confirmed the chemical state of the elements present in Y³⁺doped CdMoO₄. The optical properties of the samples were studied using UV-Vis Spectroscopy and the bandgap was found to increase upon Y³⁺ doping. The NLO response measured using the open aperture z-scan technique with a Nd: YAG pulsed laser (532 nm, 7 ns, 10 Hz) exhibited a reverse saturable absorption arising from a two photon absorption (2PA) process. An increase in the 2PA coefficient and simultaneous decrease in the onset of the optical limiting threshold clearly suggests the great potential of the yttrium-doped CdMoO₄ nanoparticles for good optical limiting applications.

(a) Polyhedral structure and (b) two photon absorption of CdMoO4 nanostructures.

Silver(I)-Carbene Bond-Directed Rigidification-Induced Emissive Metallacage for Picric Acid Detection

A new triphenylamine-based tetraimidazolium salt L was developed for silver(I)-carbene bond-directed synthesis of tetranuclear silver(I) octacarbene ([Ag₄(L)₂](PF₆)₄) metallacage 1. Interestingly, after assembly formation, metallacage 1 showed a nine-fold emission enhancement in dilute solution while ligand L was weakly fluorescent. This is attributed to the rigidity induced to the system by metal-carbene bond formation where the metal center acts as a rigidification unit. The enhanced emission intensity in dilute solution and the presence of the triphenylamine core made 1 a potential candidate for recognition of picric acid (PA). This recognition can be ascribed to the dual effect of ground-state charge-transfer complex formation and resonance energy transfer between the picrate and metallacage 1. For metallacage 1, a considerable detection limit toward PA was observed. The use of such metal-carbene bond-directed rigidification-induced enhanced emission for PA sensing is noteworthy.

Fluorescence Imaging and Photodynamic Inactivation of Bacteria Based on Cationic Cyclometalated Iridium(III) Complexes with Aggregation-Induced Emission Properties

Antibacterial photodynamic therapy (PDT) is one of the emerging methods for curbing multidrug-resistant bacterial infections. Effective fluorescent photosensitizers with dual functions of bacteria imaging and PDT applications are highly desirable. In this study, three cationic and heteroleptic cyclometalated Ir(III) complexes with the formula of [Ir(CˆN)₂ (NˆN)][PF₆ ] are prepared and characterized. These Ir(III) complexes named Ir(ppy)₂ bP, Ir(1-pq)₂ bP, and Ir(2-pq)₂ bP are comprised of three CˆN ligands (i.e., 2-phenylpyridine (ppy), 1-phenylisoquinoline (1-pq), and 2-phenylquinoline (2-pq)) and one NˆN bidentate co-ligand (bP). The photophysical characterizations demonstrate that these Ir(III) complexes are red-emitting, aggregation-induced emission active luminogens. The substitution of phenylpyridine with phenylquinoline isomers in the molecules greatly enhances their UV and visible-light absorbance as well as the photoinduced reactive oxygen species (ROS) generation ability. All three Ir(III) complexes can stain both Gram-positive and Gram-negative bacteria efficiently. Interestingly, even though Ir(1-pq)₂ bP and Ir(2-pq)₂ bP are constitutional isomers with very similar structures and similar ROS generation ability in buffer, the former eradicates bacteria much more effectively than the other through white light-irradiated photodynamic inactivation. This work will provide valuable information on the rational design of Ir(III) complexes for fluorescence imaging and efficient photodynamic inactivation of bacteria.

Probing the Strong Near-IR Two-Photon Transition in Supramolecular Triphenylamine-based Polymers by Nonlinear Absorption Spectroscopy

Due to their capability of film formation and remarkable optical features, semiconductor polymers with high two-photon absorption (2PA) have been studied as potential candidates for the development of organic photonic platforms. Furthermore, there is a high demand for photonic devices operating in the near-infrared (IR) region. However, the magnitude of the nonlinear optical response of random coil polymers in the IR region is weak due to the loss of molecular structure caused by increasing the π-conjugated backbone. Thus, herein we aim to investigate the molecular structure and 2PA features relationship for four polymers with supramolecular (helical) rodlike structure. Such polymers have a rigid core based on triphenylamine groups connected to the chiral binaphthalene units and a strong electron-withdrawing group (EWG). This kind of structure allows a very high chromophore density, which was responsible for generating 2PA cross-section between 305 GM and 565 GM in the near-IR (900-1300 nm), depending on the EWG strength. in light of the two-level model within the sum-overstates approach, we estimated the degree of intramolecular charge transfer induced by 2PA in the IR region, and values as high as 50-70% were found. Such a critical outcome allows the 2PA cross-section in the IR region to remain high even though the ratio between the visible/IR-band 2PA cross-section increases as a function of EWG strength.

Function-oriented synthesis of two-dimensional (2D) covalent organic frameworks – from 3D solids to 2D sheets

This review provides guidelines for the function-oriented synthesis of 2D COFs from 3D solids to 2D sheets.

Photopolymerization with AIE dyes for solid-state luminophores

The photoinitiating activities of MPAs/ONI were evaluated. The AIE emission of MPAs occurred during photocuring. MPAs showed potential as fluorescent molecular probes to monitor the progress of photopolymerization.

Synthesis, photophysical and electropolymerization properties of thiophene-substituted 2,3-diphenylbuta-1,3-dienes

Electropolymerizable diphenylbuta-1,3-diene derivatives with AIE or AEE properties were synthesized allowing low bandgap polymers to be obtained through electropolymerization processes.

Phenothiazine electrophores immobilized on periodic mesoporous organosilicas by ion exchange

Two different phenothiazines carrying quaternary ammonium groups in the side chain have been synthesized and fully characterized.

Amphiphilic triphenylamine–benzothiadiazole dyes: preparation, fluorescence and aggregation behavior, and enzyme fluorescence detection

Change of aggregate stabilization based on removal of the galactopyranose moiety leads to an emission enhancement to detect β-galactosidase activity.

Photophysics of proton transfer in hydrazides: a combined theoretical and experimental analysis towards OLED device application

Effect of conjugation to support ESIPT with impossible double proton transfer in structurally favored species.

Linkage induced enhancement of fluorescence in metal-carbene bond directed metallacycles and metallacages

Two new 1,4-dihydropyrrolo[3,2-b]pyrrole based aggregation induced emission (AIE)-inactive di- and tetra-imidazolium salts were employed with Ag(i) for the synthesis of a Ag-carbene bond directed metallacycle (1) and metallacage (2), respectively. Transmetalation of these complexes allowed their facile conversion to their respective Au(i)-metallacycle (3) and metallacage (4). The final assemblies exhibit linkage induced enhancement of fluorescence (LIEF). The free ligands are almost non-fluorescent (ΦF = 3.2, 3.4) in comparison to their metal-carbene counterparts (ΦF up to 32.0). Thus, without using any AIEgen, obtaining high emission efficiencies in complexes AgI-CNHC (1, 2) via linkage is a nice approach towards turn-on fluorescence.

Stimuli-responsive fluorescence switching of cyanostilbene derivatives: ultrasensitive water, acidochromism and mechanochromism

A novel donor-π-acceptor structure stimuli-responsive fluorescent material of (Z)-2-(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)-3-(pyridin-2-yl)acrylonitrile (oN-TPA) was designed and synthesized, with the cyano-group and pyridine as the acceptors (A) and triphenylamine as the donor (D). oN-TPA exhibits an obvious solvatochromic effect and the excited state is confirmed to be a hybridized local and charge-transfer (HLCT) state that simultaneously possesses the locally-excited (LE) state and charge transfer (CT) state characters. The LE state ensures relatively high fluorescence efficiency while the CT state provides multi-stimuli responsive fluorescence behaviors because it is easily tuned by the surrounding environment. Firstly, oN-TPA exhibits "on-off-on" fluorescence properties in the mixture of water/tetrahydrofuran (THF) with the increasing water content. For the "on-off" part, a good linear relationship between fluorescence intensity and water fraction is achieved, which is ascribed to the synergistic effect of protons in water and intramolecular charge-transfer (ICT) effect depending on solvent polarity. The "off-on" part demonstrates the aggregation-induced enhanced emission (AIEE) character of oN-TPA. Secondly, oN-TPA can be used as a protonic acid sensor to detect trifluoroacetic acid (TFA) in solvent and HCl vapour in the solid state due to the binding of the proton to the pyridine group. Finally, oN-TPA presents remarkable and reversible mechanochromic fluorescence switching between 552 nm and 642 nm (90 nm red-shift) during the pressurizing-depressurizing process. This work not only comprehensively demonstrates the stimuli-responsive fluorescence behaviors of oN-TPA, but also provides a D-π-A structure fluorescent material possessing potential applications in detection and sensing with remarkable fluorescence changes.

Self-Assembled α-Cyanostilbenes for Advanced Functional Materials

In the specific context of condensed media, the significant and increasing recent interest in the α-cyanostilbene (CS) motif [ArCHC(CN)Ar] is relevant. These compounds have shown remarkable optical features in addition to interesting electrical properties, and hence they are recognized as very suitable and versatile options for the development of functional materials. This progress report is focused on current and future use of CS structures and molecular assemblies with the aim of exploring and developing for the next generations of functional materials. A critical selection of illustrative materials that contain the CS motif, including relevant subfamilies such as the dicyanodistyrylbenzene and 2,3,3-triphenylacrylonitrile shows how, driven by the self-assembly of CS blocks, a variety of properties, effects, and possibilities for practical applications can be offered to the scientific community, through different rational routes for the elaboration of advanced materials. A survey is provided on the research efforts directed toward promoting the self-assembly of the solid state (polycrystalline solids, thin films, and single crystals), liquid crystals, nanostructures, and gels with multistimuli responsiveness, and applications for sensors, organic light-emitting diodes, organic field effect transistors, organic lasers, solar cells, or bioimaging purposes.

Aggregation-induced emission of [3]cumulenes functionalized with heptagon-containing polyphenylenes

[3]Cumulenes became luminescent upon aggregation at room temperature by protection with heptagon-containing polyphenylenes as endgroups.

Design and synthesis of theranostic antibiotic nanodrugs that display enhanced antibacterial activity and luminescence

We report the modular formulation of ciprofloxacin-based pure theranostic nanodrugs that display enhanced antibacterial activities, as well as aggregation-induced emission (AIE) enhancement that was successfully used to image bacteria. The drug derivatives, consisting of ciprofloxacin, a perfluoroaryl ring, and a phenyl ring linked by an amidine bond, were efficiently synthesized by a straightforward protocol from a perfluoroaryl azide, ciprofloxacin, and an aldehyde in acetone at room temperature. These compounds are propeller-shaped, and upon precipitation into water, readily assembled into stable nanoaggregates that transformed ciprofloxacin derivatives into AIE-active luminogens. The nanoaggregates displayed increased luminescence and were successfully used to image bacteria. In addition, these nanodrugs showed enhanced antibacterial activities, lowering the minimum inhibitory concentration (MIC) by more than one order of magnitude against both sensitive and resistant Escherichia coli The study represents a strategy in the design and development of pure theranostic nanodrugs for combating drug-resistant bacterial infections.

A series of water-soluble A-π-A' typological indolium derivatives with two-photon properties for rapidly detecting HSO3 -/SO3 2- in living cells

It is believed that HSO₃ ^- and SO₃ ^2- play important roles in several physiological processes. However, probes with two-photon absorption to detect HSO₃ ^- or SO₃ ^2- in living cells are still limited. Herein, a series of novel indolium derivatives (L1-L4) with an A-π-A' structure was designed and synthesized as ratiometric probes to detect HSO₃ ^-/SO₃ ^2-in vitro. L3 and L4 display a colorimetric and ratiometric fluorescence dual response to HSO₃ ^-/SO₃ ^2- with a very fast (∼15 s) and high specificity, as well as low detection limits (∼22 nM). Furthermore, their detection is also carried out by using a two-photon excited fluorescence method. A nucleophilic addition reaction is proposed for the sensing mechanism, which is supported by MS, ¹H NMR, and density functional theory (DFT) investigations. Importantly, L3 was successfully used for detecting intrinsically generated intracellular HSO₃ ^-/SO₃ ^2- in cancerous cells under one- and two-photon excited fluorescence imaging.

Tuning the structures and photophysical properties of 9,10-distyrylanthrance (DSA) via fluorine substitution

Because of the differences in the position and degree of fluorine substitution, four fluorinated DSA derivatives, namely 4-BFSA, 3-BFSA, BDFSA and BTFSA, display different crystal packing and photophysical properties in the solid state.

E/Z isomerization, solvatachromism and aggregation-induced emission enhancement of donor–acceptor type oligo(p-phenylene vinylene)s

A donor–acceptor molecule, PICNDSB, with cyclopenta[b]indole as the donor and (1,4-phenylene)bis(3-phenylacrylonitrile) as the acceptor, is designed and synthesized. PICNDSB shows typical aggregation-induced emission characteristics. The photoluminescence quantum yield of PICNDSB in dilute THF was only 0.7% and reaches 51.4% in the solid state. Compared with its analogue CzCNDSB, PICNDSB possesses stronger intramolecular charge transfer properties, as revealed from CV measurements, theoretical calculations, and their fluorescence behaviors. Correspondingly, the relatively more efficient electron-donating terminal group of cyclopenta[b]indole endows the molecule with a much higher two-photon absorption cross section of 1286 GM compared to CzCNDSB (357 GM).

A novel aggregation-induced emission platform from 2,3-diphenylbenzo[b]thiophene S,S-dioxide

The steric hindrance and electronic push–pull effect greatly influence the aggregation-induced emission (AIE) properties of 2,3-diphenylbenzo[b]thiophene S,S-dioxide.

A novel aggregation-induced emission enhancement triggered by the assembly of a chiral gelator: from non-emissive nanofibers to emissive micro-loops

A novel aggregation-induced emission enhancement triggered by chiral self-assembled superstructures has been developed in this work.

Tuning the Solid State Emission of the Carbazole and Cyano-Substituted Tetraphenylethylene by Co-Crystallization with Solvents

Solid state emissive materials with high quantum yields and tunable emissions are desirable for various applications. A new TPE derivative (1) with two carbazole moieties and two cyano groups is reported, which shows typical aggregation induced emission behavior. Four crystals 1a, 1b, 1c, and 1d are obtained after crystallization from N,N-dimethylformamid (DMF), trichloromethane (CHCl₃ ), tetrahydrofuran (THF), and dichloromethane (CH₂ Cl₂ ), respectively. Crystal structural analyses reveal that (i) molecules of 1 co-crystallize with DMF, CHCl₃ , THF, and CH₂ Cl₂ in 1a, 1b, 1c, and 1d, respectively, and (ii) conformations of 1 are different within 1a, 1b, 1c, and 1d, and compound 1 within crystal 1a adopts the most twisting conformation. Crystalline solids 1a, 1b, 1c, and 1d exhibit high emission quantum yields up to 0.65, but their emission colors are varied from blue to green. In comparison, the amorphous solid of 1 is yellow-emissive with emission maximum at 542 nm. Moreover, the blue- or green-emissive crystalline solids and the yellow-emissive amorphous solid can be inter-converted by the grinding of crystalline solids and exposure of the amorphous solid to vapors of appropriate solvents. It is also demonstrated that microrods of 1a, 1b, and 1d show typical optical waveguiding behavior.

Structure-activity relationship for the solid state emission of a new family of "push-pull"π-extended chromophores

We report the design, synthesis, molecular optical properties, and solid state emissive behaviour of a series of novel compounds, which, similar to the archetypal AIE luminogen tetraphenylethene, are formed of a central olefin stator and decorated with either three or four rotors. These rotors, being either electron-rich substituted benzenes, or electron-withdrawing functional groups (esters, ketones, cyano groups) confer a "push-pull" character to the overall molecular structure. Building on both new and already published contributions, a comprehensive picture of the properties and the potential of these compounds is provided.

In Situ Spectroelectrochemical Investigations of the Redox-Active Tris[4-(pyridin-4-yl)phenyl]amine Ligand and a Zn(2+) Coordination Framework

An investigation of the redox-active tris[4-(pyridin-4-yl)phenyl]amine (NPy3) ligand in the solution state and upon its incorporation into the solid-state metal-organic framework (MOF) [Zn(NPy3)(NO2)2·xMeOH·xDMF]n (MeOH = methanol and DMF = N,N-dimethylformamide) was conducted using in situ UV/vis/near-IR, electron paramagentic resonance (EPR), and fluorescence spectroelectrochemical experiments. Through this multifaceted approach, the properties of the ligand and framework were elucidated and quantified as a function of the redox state of the triarylamine core, which can undergo a one-electron oxidation to its radical cation. The use of pulsed EPR experiments revealed that the radical generated was highly delocalized throughout the entire ligand backbone. This combination of techniques provides comprehensive insight into electronic delocalization in a framework system and demonstrates the utility of in situ spectroelectrochemical methods in assessing electroactive MOFs.

Photophysical properties of di-Schiff bases: evaluating the synergistic effect of non-covalent interactions and alkyl spacers in enhanced emissions of solids

Di-Schiff bases with alkyl spacer (ethyl, butyl and hexyl) showed enhanced light emitting properties in solid state, while quenching was observed for di-Schiff bases with hydrazine spacer. Packing of the molecules via non covalent interactions along with the flexible spacer played the role in dictating the emission properties.

Design, synthesis, linear and nonlinear photophysical properties of novel pyrimidine-based imidazole derivatives

Novel pyrimidine imidazole derivatives with flexible ether chains have been synthesised and evaluated for their cell imaging performanceviaphotophysical investigations and theoretical calculations.

Multi-color solid-state emission of β-iminoenolate boron complexes tuned by methoxyl groups: aggregation-induced emission and mechanofluorochromism

New β-iminoenolate boron complexes bearing benzoxazole were synthesized, and they exhibited strong luminescence in solid states.

Polyurethane foam functionalized with an AIE-active polymer using an ultrasonication-assisted method: preparation and application for the detection of explosives

We have fabricated PU foams modified with AIE-active polymer PSiTPE nanoparticles on the flexible sponge surface. The as-prepared foams show a high sensitive to PA solution and DNT vapor.