Aggregation-induced Emission Phenomenon Research Articles

Endowing an amphiphilic aggregation-induced emission molecule with turn-on fluorescence in both latent fingerprints development and antibiotics detection

The water-soluble aggregation-induced emission (AIE) molecule is desirable as a turn-on fluorescence probe, but it is challenging because most AIE-based molecules are lipophilic and can emit strong fluorescence in water. To address the problem, tetraphenylethene (TPE) backbones are decorated with hydrophilic pyridine ethanol-related groups, and a series of novel amphiphilic TPE-based pyridinium salts (denoted as TPE-Py-OH, TPE-2Py-2OH and TPE-4Py-4OH) are synthesized. Encouragingly, TPE-2Py-2OH aqueous solution which is almost non-emissive can be used as a bifunctional turn-on fluorescent probe for developing latent fingerprints (LFPs) and detecting antibiotics. Level 3 details in LFPs are developed via soaking/ spraying method, owing to that AIE phenomenon is activated by the combination of lipophilic part in TPE-2Py-2OH and fatty acid residues in fingerprints. The method is worthy to promote, because it displays the advantages of water solubility, low working concentration, ease of operation, good stability, high safety, high contrast, high sensitivity, and high selectivity. It can also response to penicillin potassium (PNG) with a detection limit of 0.18 μM because the electrostatic interaction between TPE-2Py-2OH and PNG can mainly induce the enhanced fluorescence. This discovery provides a facile approach to design more promising amphiphilic AIE-based turn-on fluorescence probes.

A novel approach to supramolecular Aggregation-Induced emission using tetracationic tetraphenylethylene and sulfated β-Cyclodextrin

The combination of aggregation-induced emission-based luminogens (AIEgens) with supramolecular chemistry has gained significant attention due to its ability to produce intense emission even at low concentrations. In this study, we report a supramolecular arrangement characterized by aggregation-induced emission through the integration of tetracationic tetraphenylethylene with a macrocyclic polyanionic derivative of β-cyclodextrin. Despite the exploration of ionic derivatives of cyclodextrins in pharmaceutical and host–guest supramolecular chemistry, the phenomenon of supramolecular aggregation-induced emission based on non-covalent interactions between the ionic derivative of β-cyclodextrin and derivatives of tetraphenylethyelene, a class of AIEgens, has received limited investigation. Our work explores AIE shown by the allylimidazolium derivative of tetracationic tetraphenylethyelene (ALITPE) induced by complexation with polyanionic sulfated β-cyclodextrin (SCD). The effectiveness of the emission intensity of this supramolecular assembly based on AIE is confirmed by steady-state and time-resolved fluorescence measurements, which are supported by ground-state absorption measurements. We also investigate the effect of changes in external environment, such as temperature, pH, and ionic strength, on the supramolecular assembly. Moreover, the aggregation behavior of ALITPE with SCD is comparatively studied with neutral native β-cyclodextrin, highlighting the contribution of high negative charge density on the surface of SCD for efficient aggregation with ALITPE molecules. Additionally, Molecular Docking calculations are employed to provide insights into the aggregation behavior of ALITPE molecules induced by SCD. Overall, our findings demonstrate the potential applicability of the ALITPE/SCD supramolecular assembly as a potential fluorescent sensing probe for various biological applications.

Aggregation-induced emission for the detection of peptide ligases with improving ligation efficiency

BackgroundMonitoring peptide ligase activity is of great significance for biological research, medical diagnosis, and drug discovery. The current methods for the detection of peptide ligases suffer from the limitations of high background signal, elaborate design of substrate, and high reversibility of ligation reaction. In this work, we proposed a simple and sensitive method for ligase detection with reducing ligation reversibility on the basis of aggregation-induced emission (AIE) mechanism. ResultsThe peptide probes labeled with AIE luminogens (AIEgens) were water-soluble and emitted weak fluorescence. After ligation reaction, the enzymatic products with AIEgens showed high hydrophobicity and could readily assembly into aggregates, thus lighting up the fluorescence. More interestingly, the formation of aggregates pushed the equilibrium to the generation of the desired ligation products, thus improving the catalytic efficiency by driving the reaction towards completion. The ligation reaction conversion rate (>80 %) is significantly higher than that without blocking the reversibility with additional treatment. With sortase A (SrtA) as the analyte example, the detection limit of this method was found to be 0.01 nM with a linear range of 0–50 nM. The system was applied to evaluate the inhibition efficiency of berberine chloride and quercetin and determine the activity of SrtA in serum, lysate and Staphylococcus aureus with satisfactory results. SignificanceThis study indicated that the ligation efficiency and detection sensitivity can be improved by reducing ligation reversibility through AIE phenomenon. The proposed strategy could be used for the detection of other peptide ligases by adopting sequence-specific peptide substrates.

Dual-Emissive Rectangular Supramolecular Pt(II)-p-Biphenyl with 4,4'-Bipyridine Derivative Metallacycles: Stepwise Synthesis and Photophysical Properties.

Mixed-ligand tetranuclear supramolecular coordination complexes (SCCs) of Pt(II)-p-biphenyl and bridging ligands derivatives of 4,4'-bypiridine (8)-(10), were synthesized and characterized. The SCCs were synthesized stepwise, starting from the Pt-p-biphenyl -Pt core. The crystal structure of complex {[Pt(2,2'-bpy)]4(μ-bph)2(μ-(4,4'-bpy)2}{PF6}4 (2,2'-bpy = 2,2'-bipyridine, bph = p-biphenyl and 4,4'-bpy = 4,4' bipyridine), was determined using single-crystal diffraction methods. The emission profile of the tetranuclear complexes (8)-(10) was influenced by the length of the bridging ligands and was found to depend on solvent polarity. Dual-emission patterns in methanol-water mixtures were observed only in the cases of complexes (9) and (10), attributed to aggregation-induced emission phenomena.

Photoactivable AIEgen-based Lipid-Droplet-Specific Drug Delivery Model for Live Cell Imaging and Two-Photon Light-Triggered Anticancer Drug Delivery.

Lipid droplets (LDs) are dynamic complex organelles involved in various physiological processes, and their number and activity are linked to multiple diseases, including cancer. In this study, we have developed LD-specific near-infrared (NIR) light-responsive nano-drug delivery systems (DDSs) based on chalcone derivatives for cancer treatment. The reported nano-DDSs localized inside the cancer microenvironment of LDs, and upon exposure to light, they delivered the anticancer drug valproic acid in a spatiotemporally controlled manner. The developed systems, namely, 2'-hydroxyacetophenone-dimethylaminobenzaldehyde-valproic (HA-DAB-VPA) and 2'-hydroxyacetophenone-diphenylaminobenzaldehyde-valproic (HA-DPB-VPA) ester conjugates, required only two simple synthetic steps. Our reported DDSs exhibited interesting properties such as excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) phenomena, which provided advantages such as AIE-initiated photorelease and ESIPT-enhanced rate of photorelease upon exposure to one- or two-photon light. Further, colocalization studies of the nano-DDSs by employing two cancerous cell lines (MCF-7 cell line and CT-26 cell line) and one normal cell line (HEK cell line) revealed LD concentration-dependent enhanced fluorescence intensity. Furthermore, systematic investigations of both the nano-DDSs in the presence and absence of oleic acid inside the cells revealed that nano-DDS HA-DPB-VPA accumulated more selectively in the LDs. This unique selectivity by the nano-DDS HA-DPB-VPA toward the LDs is due to the hydrophobic nature of the diphenylaminobenzaldehyde (mimicking the LD core), which significantly leads to the aggregation and ESIPT (at 90% volume of fw, ΦF = 20.4% and in oleic acid ΦF = 24.6%), respectively. Significantly, we used this as a light-triggered anticancer drug delivery model to take advantage of the high selectivity and accumulation of the nano-DDS HA-DPB-VPA inside the LDs. Hence, these findings give a prototype for designing drug delivery models for monitoring LD-related intracellular activities and significantly triggering the release of LD-specific drugs in the biological field.

Controlling ESIPT-based AIE effects for designing optical materials with single-component white-light emission

Aggregation-induced emission (AIE) molecules have gained significant importance in various fields such as biological imaging and organic light-emitting diodes. Here, we focused on exploring the process of the induction and control of AIE effects by modifying intramolecular hydrogen bonds. Firstly, inspired by the AIE characteristics of our reported amino-type excited-state intramolecular proton transfer (ESIPT)-based molecules, qualitative and quantitative relationships between the acidity of the amino group and the ESIPT process were investigated. They were successfully revealed through artificial intelligence modeling and quantum chemical calculations. Based on this, eight compounds, (2-(2′-aminophenyl) benzothiazole (1) and its derivatives), were synthesized. Then, their mechanism of structural and optical phenomena (AIE or aggregation-caused quenching (ACQ)) was further verified by quantum chemical calculations and experiments. It has been proved that multistage photochemical reactions could lead to the AIE phenomenon. Finally, the eight compounds were applied to biological imaging and white light material applications, benefiting from tunable dual-emission spectroscopic properties of amino compounds with ESIPT properties. This study provided extensive guidance on developing molecules with AIE properties and opened up new avenues for AIE-based principles, as well as single-component white-light emission optical materials.

Dangling Water Molecules Bridge for ESIPT in Aggregated TMP: A Theoretical Study.

We present a theoretical study on the occurrence of excited-state proton transfer in an aggregated structure of 2-(benzo[d]thiazol-2-yl)-6-methoxyphenol (TMP) exclusively in water among polar solvents, as reported in a recent experiment (Bhattacharyya, A. New J. Chem. 2019, 43, 15087). Our extensive investigation of the TMP monomer and dimer implementing density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, in three different solvents, namely, water, methanol, and dimethyl sulfoxide (DMSO), with explicit inclusion of solvent molecules demonstrated the existence of both enol and keto forms of the TMP dimer in the excited state, but only in water; this confirmed the experimental emission spectra completely and simultaneously validated the aggregation-induced emission phenomenon. Further analysis of various parameters such as potential energy scan (PES) of the hydroxyl (O-H) bond involved in hydrogen bonding, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP), and infrared (IR) stretching frequencies of both the monomer and dimer forms of TMP in different solvents clearly indicated the geometry of the dimer, with the arrangement of the solvent molecules to be the sole reason for the excited-state charge transfer. The bridging alignment of water molecules in between the stacked units of the TMP dimer results in intermolecular interactions, ultimately leading to intermolecular proton transfer in the excited state.

Stimuli-responsive aggregation-induced emission of molecular probes by electrostatic and hydrophobic interactions: Effect of organic solvent content and application for probing of alkaline phosphatase activity

We suggest that aggregation-induced emission (AIE) molecular probes with single charged/reactive group can exist in the formation of nanostructures but not monomers at extremely low organic solvent content. The nanoaggregates show good dispersivity and emit week emission. Stimuli-responsive assembly of nanoaggregates by electrostatic interactions can turn on the fluorescence, facilitating the design of biosensors with single-charged molecular probes as the AIE fluorogens. To prove the concept, tetraphenylethene-substituted pyridinium salt (TPE-Py) was used as the AIE fluorogen for probing of alkaline phosphatase (ALP) activity with pyrophosphate ion (PPi) as the enzyme substrate. The dynamic light scattering and transmission electron microscope experiments demonstrated that TPE-Py probes existed in aqueous solution at nanometer size and morphology. Stimuli such as the negatively charged PPi, citrate, ATP, ADP, NADP and DNA could trigger the aggregation of the positively charged TPE-Py nanoparticles, thus enhancing the fluorescence via AIE effect. ALP-enzymatic hydrolysis of PPi into two phosphate ions (Pi) limited the aggregation of TPE-Py nanoparticles. The strategy was used for the assay of ALP with a low detection limit (1 U/L) and wide linear range (1–200 U/L). We also investigated the effect of organic solvent content on the AIE process and found that high concentration of organic solvent can prevent the hydrophobic interaction between AIE molecules but show no essential influence on the electrostatic interaction-mediated assembly. The work should be evaluable for understanding AIE phenomenon and developing novel, simple and sensitive biosensors using a molecular probe with single charged/reactive group as the signal reporter.

Development of meso-Five-Membered Heterocycle BODIPY-Based AIE Fluorescent Probes for Dual-Organelle Viscosity Imaging.

Fluorescent rotors with aggregation-induced emission (AIE) and organelle-targeting properties have attracted great attention for sensing subcellular viscosity changes, which could help understand the relationships of abnormal fluctuations with many associated diseases. Despite the numerous efforts spent, it remains rare and urgent to explore the dual-organelle targeting probes and their structural relationships with viscosity-responsive and AIE properties. Therefore, in this work, we reported four meso-five-membered heterocycle-substituted BODIPY-based fluorescent probes, explored their viscosity-responsive and AIE properties, and further investigated their subcellular localization and viscosity-sensing applications in living cells. Interestingly, the meso-thiazole probe 1 showed both good viscosity-responsive and AIE (in pure water) properties and could successfully target both mitochondria and lysosomes, further imaging cellular viscosity changes by treating lipopolysaccharide and nystatin, attributing to the free rotation and potential dual-organelle targeting ability of the meso-thiazole group. The meso-benzothiophene probe 3 with a saturated sulfur only showed good viscosity-responsive properties in living cells with the aggregation-caused quenching effect and no subcellular localization. The meso-imidazole probe 2 showed the AIE phenomenon without an obvious viscosity-responsive property with a C═N bond, while the meso-benzopyrrole probe 4 displayed fluorescence quenching in polar solvents. Therefore, for the first time, we investigated the structure-property relationships of four meso-five-membered heterocycle-substituted BODIPY-based fluorescent rotors with viscosity-responsive and AIE properties, and among these, 1 with a C═N bond and a saturated sulfur on the meso-thiazole, potentially contributing to their corresponding AIE and viscosity-responsive properties, served as a sensitive AIE fluorescent rotor for imaging dual-organelle viscosity in both mitochondria and lysosomes.

Unraveling the Mechanism of ACQ-to-AIE Transformation of Fluorescein Derivatives.

Although fluorescein derivatives have excellent properties and strong practicability, they are typical aggregation-induced quenching (ACQ) molecules, which are not conducive to working in the solid state. Recently, the fluorescein derivative Fl-Me with aggregation-induced emission (AIE) property was synthesized, which brought a new dawn for the research and development of fluorescein-based materials. In this study, the AIE mechanism of Fl-Me was investigated based on time-dependent density functional theory and the ONIOM method. The results revealed that an effective dark-state deactivation pathway leads to the fluorescence quenching of Fl-Me in a solution environment. Accordingly, the AIE phenomenon originates from the closure of the dark-state quenching channel. It is worth emphasizing that we found that the carbonyl group of molecular Fl-Me has intermolecular hydrogen bonding interaction with the adjacent molecules, which caused the increase of the dark-state energy in the crystalline state. Moreover, the restriction of the rotational motion and the nonexistence of the π-π stacking interaction are beneficial to the enhancement of fluorescence upon aggregation. Finally, the ACQ-to-AIE transformation mechanisms of fluorescein derivatives have been discussed. This work provides deeper insight into the photophysical mechanism for the fluorescein derivatives Fl-Me with AIE feature and eventually is expected to help researchers to develop more fluorescein-based AIE materials with remarkable properties for various fields.

Understanding AIE and ACQ phenomenon of organometallic iridium(III) complexes by simple cationization engineering

Understanding the relationship between structure and properties is critical to the development of solid-state luminescence materials with desired characteristics and performance optimization. In this work, we elaborately designed and synthesized a pair of mononuclear iridium(III) complexes with similar structures but different degrees of cationization. [Ir2-f][2PF6] with two counterions is obtained by simple N-methylation of the ancillary ligand of [Ir1-f][PF6] which is a classic cationic iridium(III) complex. Such a tiny modification results in tremendously different optical properties in dilute solutions and powders. [Ir1-f][PF6] exhibits weak light in solution but enhanced emission in solid-state as well as poly(methyl methacrylate) matrix, indicative of its aggregation-induced emission (AIE) activity. On the sharp contrary, [Ir2-f][2PF6] is an aggregation-caused quenching (ACQ) emitter showing strong emission in the isolated state but nearly nonemissive in aggregation states. Benefiting from the appealing characteristics of mechanochromic luminescence and AIE behavior, [Ir1-f][PF6] has been successfully applied in reversible re-writable data recording and cell imaging. These results might provide deep insights into AIE and ACQ phenomenon of iridium(III) complexes and facilitate the development of phosphorescent materials with promising properties.

Regulating donor-acceptor system toward highly efficient dual-state emission for sensitive response of nitroaromatic explosives

Dual-state emission luminogens (DSEgens) as fluorophores emit efficiently in solution and solid forms have gained increasing concern in the field of chemical sensing. Recent efforts by our group led to the identification of DSEgens as an easy-to-visualize nitroaromatic explosives (NAEs) detection platform. However, none of the previously studied NAEs probes show effective improvement in sensitivity. Here, we designed a series of benzoxazole-based DSEgens through multiple strategies driven by theoretical calculations, revealing their improved detecting performance on NAEs. Compounds 4a-4e exhibit thermal- and photo-stability, large Stokes shift as well as sensitivity solvatochromism (except for 4a and 4b). A subtle balance between rigid conjugation and distorted conformation endows these D-A type fluorophores 4a-4e with DSE properties. Furthermore, 4d and 4e show aggregation-induced emission phenomenon caused by distorted molecular conformation and restricted intramolecular rotation. Interestingly, DSEgen 4e displays anti-interference and sensitivity towards NAEs with a detection limit of 10-8 M. It can be applied for expedient and distinct visual identification of NAEs not only in solution but also on filter paper and film, supporting this new DSEgen as reliable NAEs chemoprobe.

Controllable Aggregation-Induced Emission and Förster Resonance Energy Transfer Behaviors of Bistable [c2] Daisy Chain Rotaxanes for White-Light Emission and Temperature-Sensing Applications.

Bistable [c2] daisy chain rotaxanes with respective extended and contracted forms of [c2]A and [c2]B containing a blue-emissive anthracene (AN) donor and orange-emissive indandione-carbazole (IC) acceptor were successfully synthesized via click reaction. Tunable-emission bistable [c2] daisy chain rotaxanes with fluorescence changes from blue to orange, including bright-white-light emissions, could be modulated by the aggregation-induced emission (AIE) characteristics and Förster resonance energy transfer (FRET) processes through altering water fractions and shuttling processes (i.e., acid/base controls). Accordingly, as a result of excellent fine-tuning AIE (at 60% water content of H2O/THF) and FRET (with a compatible energy transfer of EFRET = 33.2%) behaviors after the shuttling process (by adding base), the brightest white-light emission at CIE (0.31, 0.37) with a quantum yield of Φ = 15.64% was obtained in contracted [c2]B with good control of molecular shuttling to possess higher photoluminescence (PL) quantum yields and better energy transfer efficiencies (i.e., the manipulation of reduced PET and enhanced FRET processes) due to their intramolecular aggregations of blue AN donors and orange IC acceptors with a proper water content of 60% H2O. Furthermore, dynamic light-scattering (DLS) and time-resolved photoluminescence (TRPL) measurements, along with theoretical calculations, were utilized to investigate and confirm AIE and FRET phenomena of bistable [c2] daisy chain rotaxanes. Especially, both bistable [c2] daisy chain rotaxanes [c2]A and [c2]B and noninterlocked monomer M could be exploited for the applications of ratiometric fluorescence temperature sensing due to the temperature effects on the AIE and FRET features. Based on these desirable bistable [c2] daisy chain rotaxane structures, this work provides a potential strategy for the future applications of tunable multicolor emission and ratiometric fluorescence temperature-sensing materials.

Tetraphenylethene-based macrocycles: Visualized monitoring the hydrolysis of silicon-oxygen bond and their tunable luminescent properties

The preparation of silicone material is generally carried out by hydrolytic condensation, and various methods have been tried so far to study the kinetics of silicon-oxygen bond hydrolysis. However, the process and kinetics of silicon-oxygen bond hydrolysis cannot be efficiently monitored in real-time due to the limitations of traditional testing methods. In the present study, we have successfully designed and synthesized a series of circular tetraphenylethylene (TPE) derivatives connected with different lengths of the siloxanes chain. As the chain length gets longer, the restriction on phenyl rotation in TPE derivatives becomes weaker, which induced fluorescence quenching gradually at the molecular level. Hence, it provides the most direct and convincing evidence for the effect of chain lengths in the popular restricted intramolecular rotation (RIR) mechanism with aggregation-induced emission (AIE) phenomenon. The strong fluorescent molecule was used to monitor the kinetics of silicon-oxygen bond hydrolysis in real-time successfully by the AIE method with the RIR mechanism. The high sensitivity and efficiency of fluorescence provide an unparalleled “visualization” window for real-time monitoring of the kinetic control of Si-O bond hydrolysis. Furthermore, the half-life and reaction kinetic data of Si-O bond-breaking reaction by different pH are obtained to provide a reference for scientists and engineers not only in fundamental research but also in industrial production. Finally, the aggregation-induced color-tunable emission was obtained with the different aggregation forms of molecules, exhibiting a promising potential for intelligent luminescent materials.

Cationic Polymer Functionalized Copper Nanocluster-Based Fluorescent Probe for the Selective and Sensitive Detection of S2– Ions

The sensitive and selective detection of sulfide (S2–) ions in water is of major interest due to its toxicity and physiological effects on organisms. Herein, we have designed a fluorescent probe, poly(allylamine) hydrochloride (PAH) functionalized copper nanocluster (Cu NC@PAH), for the detection of S2– ions. The synthesis of the probe is based on the electrostatic interaction between negatively charged Cu NCs and the positively charged polymer, PAH. The drastic enhancement of the photoluminescence (PL) intensity of Cu NCs and the quantum yield (QY) enhancement from 0.3 to 6% are evident after functionalizing with a cationic polymer matrix of PAH. The steady-state and time-resolved fluorescence studies support the aggregation-induced emission (AIE) phenomenon for PL QY enhancement. In addition, the Cu NCs@PAH exhibit excellent selectivity toward aqueous S2– ions. The probe displays a reasonable quenching response for S2– ions over a concentration range of 0–20 μM with a detection limit of 2.39 μM. We believe that this facile and economical synthesis methodology of Cu NCs@PAH with interesting AIE properties will extend the scope of previously available techniques for the detection of the S2– ion.

Aggregation-Induced Emission-Active Nanostructures: Beyond Biomedical Applications

The discovery of aggregation-induced emission (AIE) phenomenon in 2001 has had a significant impact on materials development across different research disciplines. AIE-active materials have been widely exploited for various applications in optoelectronics, sensing, biomedical, and stimuli-responsive systems, etc. This is made possible by integrating AIE features with other fields of science and engineering, such as nanoscience and nanotechnology. AIE has been extensively employed, particularly for biomedical applications, such as biosensing, bioimaging, and theranostics. However, development of AIE-based nanotechnology for other applications is comparatively less, although there have been increasing research activities in recent years. Given the significance and potential of the marriage between AIE hallmark and nanotechnology in AIE-active materials development, this review article summarizes and showcases the latest research efforts in AIE-based nanomaterials, including nanomaterials synthesis and their nonbiomedical applications, such as sensing, optoelectronics, functional coatings, and stimuli-responsive systems. A perspective on the outlook of AIE-based nanostructured materials and relevant nanotechnology for nonbiomedical applications will be provided, giving an insight into how to design AIE-active nanostructures as well as their applications beyond the biomedical domain.

High‐Efficiency Solid‐State Luminescence from Hydrophilic Carbon Dots with Aggregation‐Induced Emission Characteristics

AbstractCarbon dots (CDs) have excellent properties and have made great progress in synthesis and applications. However, it is still a challenge to realize convenient preparation and highly emissive solid‐state luminescence without the assistance of matrix. In this study, high‐efficiency hydrophilic yellow emissive CDs (Y‐CDs) with aggregation‐induced emission (AIE) characteristics are synthesized by a one‐pot microwave method. Y‐CDs show weak yellow emission with a photoluminescence quantum yield (PLQY) of 6.14% in aqueous solutions, but exhibit a significantly enhanced PLQY of 58.35% in the solid state. The AIE characteristic is confirmed by the phenomenon that the fluorescence emission intensity of Y‐CDs continues to increase with the gradual increase of the poor solvent fraction. The AIE phenomenon of Y‐CDs is derived from the suppression of surface piperazine groups motions and decrease of the non‐radiative rate, as is verified by a series of experiments. Finally, based on the high PLQY and AIE property, Y‐CDs are applied as a color‐converting layer on blue‐light‐emitting chips to fabricate white‐light‐emitting diodes with color coordinates of (0.31, 0.35), while Y‐CDs are also explored for their potential applications in anti‐counterfeiting, encryption and bioimaging.

Restriction of photoinduced electron transfer as a mechanism for the aggregation-induced emission of a trityl-functionalised maleimide fluorophore.

The restriction of intramolecular motion (RIM) and restricted access to a conical intersection (RACI) have been accepted as general working mechanisms for aggregation-induced emission (AIE) phenomena. However, as the family of AIE molecules grows, the RIM and RACl mechanisms cannot be used to fully understand some AIE phenomena. Herein, the restriction of the photoinduced electron transfer (RPET) state is proposed to rationalize the AIE phenomena of trityl-functionalised maleimide molecule based on density functional theory calculations. The "state-crossing from a locally excited to an electron transfer state" (SLEET) model was employed to predict the ON/OFF molecular PET in solution and solid states. According to the SLEET model, we showed that a non-emissive electron transfer excited state leads to the fluorescence quenching of trityl-functionalised maleimide in solution. However, due to the reduced polarity of the environment in aggregates, the electron transfer state is thermodynamically inaccessible, and a low-lying locally excited state exhibits intense emission. These findings provide a theoretical foundation to understand the working mechanisms of AIE molecules and the design of new AIEgens. We expect that the RPET mechanism can be used to screen potential AIEgens using the SLEET model.

Structural isomerism induces pH dependent AIE-coupled ESIPT: an in-depth spectroscopic exploration with application in amine vapor sensing.

A novel excited state intramolecular proton transfer probe 3-(benzo[d]thiazol-2-yl)-4-hydroxy-5-methoxybenzaldehyde (3BTHMB) was synthesized and characterized using NMR, ESIMS and single crystal diffraction studies. The steady state and time resolved studies using the time correlated single photon counting (TCSPC) technique revealed that 3BTHMB shows ESIPT reaction with a time constant of 0.10-0.15 ns and longer local emission and anion emissions of time constants 0.5-2.0 ns and 3.0-4.0 ns, respectively, that co-exist in polar solution. In a polar protic solution like methanol, the ESIPT process is damped. Interestingly, 3BTHMB shows aggregation induced emission (AIE) solely in an acidic environment, evidenced by the enhanced quantum yield of emission (∼35%) in acidic solution as well as a long lifetime component of 7.4 ns that corresponds to the lifetime in the solid state. The AIE phenomenon was explored using the DLS technique where enhancement of the hydrodynamic radius in an acidic medium was observed. The judicious positioning of the formyl group in 3BTHMB gives the mentioned probe to show AIE in an acidic medium, as opposed to its previously reported structural isomer BTHMB, which showed no AIE phenomenon as well as its parent molecule 2-(benzo[d]thiazol-2-yl)-6-methoxyphenol (TMP), which showed AIE in neutral pH. The AIE property of 3BTHMB was exploited by successful and rapid detection of amine vapor in the solid state, qualitative ratiometric detection of aqueous pH, by observing the colour change under UV light in acidic (yellow-green) and basic to neutral (blue) medium. The above results pave the way for multiutility fluorescent probes for environmental purposes along with a deep understanding of the underlying photophysical processes which bestow the probes with such outstanding utilities.

Improved electroluminescence efficiency derived from functionalized decoration of 1,3,4-oxadiazole (OXD)-based Ir(III) complexes.

The performance of organic light emitting devices (OLEDs) fabricated using Ir(III) complexes bearing 1,3,4-oxadiazole (OXD)-based cyclometallic ligands still needs to be improved. In this work, Ir3+ was coordinated with a 2-(9,9-diethyl-9H-fluoren-2-yl)-1,3,4-oxadiazole (F-OXD) fragment, which was modified with various functionalized substituents, including fluorenyl, OXD and carbazolyl groups. Three complexes, named Ir-Flu, Ir-OXD and Ir-Cz, were synthesized successfully and their photophysical, electrochemical and electroluminescence properties were investigated in detail. All these complexes exhibited yellow-orange emission in solution and a distinct aggregation-induced phosphorescent emission (AIPE) phenomenon was observed. Monochrome OLEDs were fabricated using these phosphorescent dopants, and the turn-on voltage (V), luminance (L) and current efficiency (CE) showed significant improvement compared to analogous OXD-based Ir(III) complexes reported before. In particular, the device with Ir-OXD as the dopant achieved the highest maximum brightness of 25 014 cd m-2 and the lowest efficiency roll-off (42.6%) at the maximum luminance among all the devices. These results provided a proven strategy of functionalized decoration of OXD-based complexes to achieve superior luminous efficiency devices.