Aggregation-induced Emission Mechanism Research Articles

A Review on 2,6-Diformyl-4-methylphenol Derived Schiff Bases as Fluorescent Sensors

2,6-Diformyl-4-methylphenol fluorophore derived Schiff base sensors detect metal ions such as Zn2+, Al3+, Hg2+, Cd2+, Sn2+ and Cu2+ with high sensitivity and selectivity through changes in fluorescence intensity based on CHEF, ICT, FRET, ESIPT, C=N isomerization and PET mechanism. Several anions viz. HPO4 2–, H2PO4 –, PO4 3–, AsO3 3–, H2AsO4 –, AsO2 –, PPi (pyrophosphate), I–, F– and N3 – were also detected through intermolecular hydrogen bonding (between sensor and anion) based on TICT, PET, CHEF, ESIPT and aggregation induced emission mechanism. Selectivity and sensitivity for these metal ions and anions were achieved by introducing various amines to core fluorophore 2,6-diformyl-4-methylphenol. Majority of these fluorescent sensors were Zn2+ ion selective. Due to the filled d10 electronic configuration of Zn2+ ion usually does not show deactivation of excited state via any electron or energy transfer mechanisms. Both solvent dependent and independent ten multi-ion selective sensors are found. This review will consolidate on 2,6-diformyl-4-methylphenol derived Schiff base fluorescent chemosensors.

Click-formed polymer gels with aggregation-induced emission and dual stimuli-responsive behaviors

Stimuli-responsive polymer gels have recently attracted great attention due to their heat/solvent resistance, dimensional stability, and unique sensitivity to external stimuli. In this work, we synthesized thiol-functionalized tetraphenylethylene (TPE) and constructed polymer gels through thiol-ene click reaction. The synthetic process of the polymer gels could be monitored by fluorescence emission of TPE moieties based on aggregation-induced emission mechanism. In addition, due to the dual redoxand acid responsiveness of the polymer gels, in the presence of dithiothreitol and trifluoroacetic acid, fluorescence quenching of the polymer gels can be observed. This stimuli-responsive characteristics endows the polymer gels with potential applications in fluorescent sensing and imaging, cancer diagnosis and selfhealing materials.

Substitution effect on luminescent property of thermally activated delayed fluorescence molecule with aggregation induced emission: A QM/MM study.

Pure organic molecules with blue emission have attracted much attention due to its important application in organic light emitting diodes (OLEDs), especially for which with aggregation induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties. Theoretical study to reveal the inner luminescent mechanisms can promote its development. In this work, four kinds of molecules with perfluorobiphenyl (PFBP) unit as acceptor, non-substituted and tert-butyl substituted 9,9-dimethyl-9,10-dihydro-acridine (DMAC) unit as donors, are selected and their photophysical properties are studied in detail. The surrounding environment effects in toluene and solid phase are taken into consideration by the polarized continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method respectively. Results show that geometric changes between the first singlet excited state (S1) and ground state (S0) are restricted in solid phase with decreased root-mean squared displacement (RMSD). Moreover, the Huang-Rhys factors and reorganization energies we calculated are all decreased in solid phase, which indicates that the non-radiative energy consumption process of S1 is hindered by enhanced intermolecular interactions in rigid environment, and it brings aggregation induced emission phenomenon. Furthermore, the substitution effect of tert-butyl in donor unit can efficiently decrease the energy gap and increase the spin-orbit coupling (SOC) constant, further promotes the intersystem crossing (ISC) and reverse intersystem crossing (RISC) rates. Meanwhile, molecules with donor-acceptor-donor (D-A-D) configuration have more efficient luminous performance than D-A type molecules due to the enhanced ISC and RISC processes. Thus, tert-butyl substituted D-A-D type molecules have outstanding TADF features. Our investigations provide a theoretical perspective for AIE and TADF mechanisms and propose a design strategy for efficient TADF molecules, which could promote the development of OLEDs.

The Fluorescence Behavior Studies and Applications of Two D‐π‐A Type Imidazolium Analogs

The fluorescence behaviors and applications of two water‐soluble imidazolium analogs TPI‐1 and TPI‐2 with D‐π‐A structures have been studied. Compound TPI‐1 possessed the aggregation‐induced emission (AIE) effect, but TPI‐2 not. Moreover, compound TPI‐1 could serve as a fluorescence turn‐on probe for detecting uridine‐5'‐triphosphate (UTP) in living cells by AIE mechanism, while TPI‐2 was able to penetrate into the cell membranes and light up the living cells.

Restriction of Access to the Dark State: A New Mechanistic Model for Heteroatom-Containing AIE Systems.

Restriction of intramolecular motion (RIM), as the working mechanism of aggregation-induced emission (AIE), cannot fully explain some heteroatom-containing systems. Now, two excited states are taken into account and a mechanism, restriction of access to dark state (RADS), is specified to elaborate RIM and complete the picture of AIE mechanism. A nitrogen-containing molecule named APA is chosen as a model compound; its weak fluorescence in solution is ascribed to the easy access from the bright (π,π*) state to the close-lying dark (n,π*) state. By either metal complexation or aggregation, the dark state is less accessible due to restriction of the molecular motion leading to the dark state and elevation of the dark state energy, thus the bright state emission is restored. RADS is powerful in elucidating the AIE effect of molecules with excited states favoring non-radiative decay, including overlap-forbidden states such as (n,π*) and CT states, spin-forbidden triplet states, and so on.

Deciphering the Mechanism of Aggregation-Induced Emission of a Quinazolinone Derivative Displaying Excited-State Intramolecular Proton-Transfer Properties: A QM, QM/MM, and MD Study.

A combination of excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) has opened new opportunities to develop color-tunable luminescent materials with high quantum yield. Understanding the emission mechanism of these luminophores is essential for the molecular design and construction of a functional system. Herein, we report QM (MS-CASPT2//TD-DFT, MS-CASPT2//CASSCF) and ONIOM (QM/MM) studies on the fluorescence quenching and AIE mechanisms of 2-(2-hydroxy-phenyl)-4(3H)-quinazolinone with typical characteristics of AIE and ESIPT as an example. The computational results indicate that in the tetrahydrofuran solution, once being excited to the S1 state, the molecule tends to undergo an ultrafast, barrierless ESIPT from enol to keto tautomer and then accesses a S1/S0 conical intersection in the vicinity of a C═C bond twisted intramolecular charge-transfer (TICT) intermediate, leading to a nonradiative decay from the excited to ground state. Hence, the TICT-induced nonadiabatic transition, which has been further confirmed by the on-the-fly trajectory surface hopping dynamics simulations, accounts for the fluorescence quenching in solution. In contrast, in the solid state, the nonradiative relaxation pathway via the C═C bond rotation is suppressed due to environmental hindrance, leaving the ESIPT-induced enol-keto tautomerization as the only excited-decay channel, thus the fluorescence is observably enhanced in the crystal.

Understanding Aggregation Induced Emission in a Propeller‐Shaped Blue Emitter

AbstractOrganic fluorophores with an enhanced emission in the condensed phase have great potential for the design of optoelectronic materials. Several propeller‐shaped molecules show aggregation‐induced emission (AIE), in particular, silole derivatives have attracted wide attention because of their significant quantum yields in the solid state. In this contribution, we investigate the mechanism of AIE of a propeller‐shaped blue emitter: 1,2,3,4‐tetraphenyl‐1,3‐cyclopentadiene (TPC). We explore the excited state mechanism in the light of models most commonly used to explain it: restriction of intramolecular motions (RIM) and restricted access to the conical intersection (RACI). Our interpretation is supported by excited state dynamics simulations and the analysis of Huang‐Rhys factors and reorganisation energies. We quantify the effects of intermolecular interactions and exciton couplings. The mechanism for TPC is compared with previous investigations of analogue silole compounds. Our systematic investigation highlights the role of conical intersections on the nonradiative decay mechanisms and complementary descriptions provided by the RIM and RACI models.

An excited-state intramolecular proton transfer (ESIPT)-based aggregation-induced emission active probe and its Cu(II) complex for fluorescence detection of cysteine

In this work, an effective and simple aggregation-induced emission (AIE) active fluorescent probe based on ESIPT luminescence principle, 2-hydroxy-1-naphthal-4-aminoantipyrine (1), was synthesized via a simple chemical reaction. It displayed strong fluorescence enhancement in water. The AIE mechanism of 1 was explored by electron microscopy and computational analysis. We demonstrate the combination effect of intramolecular hydrogen bonds and π-π stacking to endow 1 with such a property. Importantly, the ensemble of 1 with copper ions (Cu-1) as a fluorescent probe enables highly selective and sensitive turn-on fluorescence detection of cysteine (Cys) over other interfering substances. Under optimal detection conditions, this probe has a good linear relationship in the concentration range of Cys between 0 and 8 μM, and the detection limit of Cys was estimated as 84 nM. Furthermore, this assay exhibits good reversibility of 1 to Cu2+ and Cys. Due to its good biocompatibility evaluated by CCK-assay, the probe Cu-1 was finally applied in imaging of Cys within Min6 cells successfully.

Effects of intramolecular and intermolecular interactions on excited state properties of two isomeric Cu complexes with AIE and TADF mechanisms in solid phase: A QM/MM study

Abstract Molecular photophysical properties are not only related to the basic structures but also affected by their packing modes. In this work, excited state properties of two isomeric Cu complexes (Complex 1 and Complex 2) with aggregation induced emission (AIE) and thermally activated delayed fluorescence (TADF) features are theoretically studied. The surrounding environments for molecule in solution and solid phase are considered by polarizable continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method respectively. In addition, the intermolecular interaction and intramolecular interaction are visualized by the reduced density gradient (RDG) function. Our investigations show that the geometrical changes between ground state (S0) and lowest singlet excited state (S1) are hindered in solid phase by the restricted intramolecular rotation (RIR) effect, which brings smaller Huang-Rhys factors. Moreover, the decreased reorganization energy from solution to solid phase, is mainly contributed by the reduction of dihedral angle in low frequency (

Aggregation-Induced Emission (AIE)-Labeled Cellulose Nanocrystals for the Detection of Nitrophenolic Explosives in Aqueous Solutions.

Aggregation-induced emission (AIE) active cellulose nanocrystals (TPE-CNCs) were synthesized by attaching tetraphenylethylene (TPE) to cellulose nanocrystals (CNCs). The structure and morphology of TPE-CNCs were characterized by FT-IR, XRD, ζ-potential measurements, elemental analysis, TEM, atomic force microscopy (AFM), and dynamic laser light scattering (DLS). Fluorescent properties of TPE-CNCs were also further studied. Unlike aggregation-caused quenching (ACQ), TPE-CNCs emitted weak fluorescence in the dilute suspensions, while emitting efficiently in the aggregated states. The AIE mechanism of TPE-CNCs was attributed to the restriction of an intramolecular rotation (RIR) process in the aggregated states. TPE-CNCs displayed good dispersity in water and stable fluorescence, which was reported through the specific detection of nitrophenolic explosives in aqueous solutions by a fluorescence quenching assay. The fluorescence emissions of TPE-CNCs showed quantitative and sensitive responses to picric acid (PA), 2,4-dinitro-phenol (DNP), and 4-nitrophenol (NP), and the detection limits were 220, 250, and 520 nM, respectively. Fluorescence quenching occurred through a static mechanism via the formation of a nonfluorescent complex between TPE-CNCs and nitrophenolic analytes. A fluorescence lifetime measurement revealed that the quenching was a static process. The results demonstrated that TPE-CNCs were excellent sensors for the detection of nitrophenolic explosives in aqueous systems, which has great potential applications in chemosensing and bioimaging.

An Aggregation-Induced Emission-Based Indirect Competitive Immunoassay for Fluorescence "Turn-On" Detection of Drug Residues in Foodstuffs.

A new fluorescent “turn-on” probe-based immunosensor for detecting drug residues in foodstuffs was established by combining the mechanism of aggregation-induced emission (AIE) and an indirect competitive enzyme-linked immunosorbent assay (ELISA). In this study, a luminogen, with negligible fluorescence emission (TPE-HPro), aggregated in the presence of H2O2, and exhibited astrong yellow emission based on its AIE characteristics. This AIE process was further configured into an immunoassay for analyzing drug residues in foodstuffs. In this approach, glucose oxidase (GOx) was used as an enzyme label for the immunoassay and triggered GOx/glucose-mediated H2O2 generation, which caused oxidation of TPE-HPro and a “turn-on” fluorescence response at 540 nm. To quantitatively analyze the drug residues in foodstuffs, we used amantadine (AMD) as an assay model. By combining the AIE-active “turn-on” fluorescent signal generation mechanism with conventional ELISAs, quantifying AMD concentrations in chicken muscle samples was realized with an IC50 (50% inhibitory concentration) value of 0.38 ng/mL in buffer and a limited detection of 0.06 μg/kg in chicken samples. Overall, the conceptual integration of AIE with ELISA represents a potent and sensitive strategy that broadens the applicability of the AIE-based fluorometric assays.

Consideration of Molecular Structure in the Excited State to Design New Luminogens with Aggregation-Induced Emission.

Aggregation-induced emission (AIE) is a photoluminescence phenomenon in which an AIE luminogen (AIEgen) exhibits intense emission in the aggregated or solid state but only weak or no emission in the solution state. Understanding the mechanism of AIE requires consideration of excited state molecular geometry (for example, a π twist). This Minireview examines the history of AIEgens with a focus on the representative AIEgen, tetraphenylethylene (TPE). The mechanisms of solution-state quenching are reviewed and the crucial role of excited-state molecular transformations for AIE is discussed. Finally, recent progress in understanding the relationship between excited state molecular transformations and AIE is overviewed for a range of different AIEgens.

Grinding-Triggered Single Crystal-to-Single Crystal Transformation of a Zinc(II) Complex: Mechanochromic Luminescence and Aggregation-Induced Emission Properties.

We first report single crystal X-ray analysis of ground crystals of mechanochromic luminescence (MCL) that shows single crystal-to-single crystal transformation (SCSCT). Single crystals of [ZnL2] (1-SG, HL = 2-[[[4-(2-benzoxazolyl)phenyl]imino]-methyl]-5-(diethylamino)-phenol) were obtained upon slight grinding of single crystals of [ZnL2]·0.5CH3OH (1), both of which were characterized by single crystal X-ray diffraction. Crystals of 1 showed emission centered at 647 nm (red color), while crystals of 1-SG showed emission band at 624 nm (orange-red color) under UV light, indicating MCL property of the Zn(II) complex. Reversible MCL property with emission color change between red and yellow for 1 was observed upon high grinding and recrystallization. Single crystal X-ray analysis suggested that it is due to the alteration of molecular conformation of ligands in ZnL2 instead of weak intermolecular interaction that 1 exhibits MCL. Investigation of the control Zn(II) complexes (2-4) indicated that flexible substituents and rotated aromatic rings are desirable to generate the MCL-active complexes. In addition, 1 was highly fluorescent in THF solution, but its fluorescence quenched upon addition of water. DFT calculations suggested that this is due to the formation of the excited hydrated ZnL2 species via Zn-O coordination bond, which results in electron-driven proton transfer (EDPT). Aggregates formed as water fraction ( fw) in THF/H2O (v/v) reached 70%, and fluorescence emission was enhanced. This phenomenon continued until fw was 90%, indicating aggregation-induced emission (AIE) property. The mechanism of AIE of ZnL2 in THF/H2O is the restriction of intramolecular rotation (RIR).

AIE-active Metal-organic Coordination Complexes Based on Tetraphenylethylene Unit and Their Applications

Tetraphenylethylene (TPE) and its derivatives, as the widely used aggregation-induced emission (AIE) fluorophores, have attracted rapidly growing interest in the fields of material science and biological technology due to their unique light-emitting mechanism—they are nearly non-emissive in dilute solution but emit brilliant fluorescence in the aggregate state because of the restriction of intramolecular motion. Coordination-driven self-assembly, which provides a highly effective method to put the individual chromophores together, is consistent with the AIE mechanism of TPE. During the past few years, some AIE-active metal-organic coordination complexes have been successfully constructed via coordination-driven self-assembly, and their AIE properties and applications have been investigated. In this review, we survey the recent progress on TPE-based metal-organic coordination complexes and their applications in fluorescence sensors, cell imaging, and light-emitting materials. We will introduce them from three different types of structures: metallacycles, metallacages, and metal-organic frameworks (MOFs).

Aggregation induced non-emissive-to-emissive switching of molecular platinum clusters.

We show here for the first time the Aggregation Induced Emission (AIE) mechanism and solvatochromic impact on Pt-SG (SG-deprotonated glutathione) nanoclusters. In this work, the AIE properties of Pt-SG clusters were investigated through computational and spectroscopic investigations. Computational data established that aggregation triggers a distinct change in the frontier molecular orbitals (FMOs) from metal d-orbital centered FMOs in the monomer to metal-thiolate and thiolate centered FMOs in the dimer improving the radiative decay process. Solvent dependent photoluminescence studies proved that a Lewis-acidic environment can significantly perturb the metal-thiolate and thiolate centered FMOs that are involved in the electronic transitions as predicted by our computational work. These semiconducting clusters exhibit a large Stokes shift and zero spectral overlap between absorption and emission which makes this Pt-SG cluster an excellent material for solar concentrators and solid-state light emitters. This AIE-OFF-ON emission was utilized to delineate a proof-of-concept sensor device that is sensitive to temperature and an acid/base.

Rhodamine-naphthalimide demonstrated a distinct aggregation-induced emission mechanism: elimination of dark-states via dimer interactions (EDDI).

Rhodamine B-naphthalimide (RhB-Naph) demonstrated a distinct aggregation-induced emission (AIE) mechanism, different from the restriction of intramolecular rotations or vibrations as in traditional AIE molecules. The monomers of RhB-Naph were non-emissive, due to the presence of a dark S1 state. Upon molecular aggregation, intermolecular interactions significantly altered the electronic properties of RhB-Naph, leading to the formation of a bright S1 state and endowing RhB-Naph with notable AIE properties. Besides, we demonstrated that RhB-Naph enabled the development of a solid-state three-color fluorescent switch upon multi-external stimuli.

Erratum on “Galactose functionalized diketopyrrolopyrrole as NIR fluorescent probes for lectin detection and HepG2 cell targeting based on aggregation-induced emission mechanism”

Erratum on “Galactose functionalized diketopyrrolopyrrole as NIR fluorescent probes for lectin detection and HepG2 cell targeting based on aggregation-induced emission mechanism”

Insight into structural influences on the optical properties and heteroenantiomeric self-assembly of racemic C6-unsubstituted tetrahydropyrimidines with strong aggregation-induced emission

Racemic C-6 unsubstituted tetrahydropyrimidines (THPs), the products of an efficient five-component reaction (5CR), are novel fluorophores with strong aggregation-induced emission (AIE). Here, the influences of molecular structures on the optical properties and heteroenantiomeric self-assembly of THPs have been investigated in details via the design and synthesis of 66 THPs with different substituents R1−R4, the analysis of 22 X-ray diffraction single crystals of 16 THPs and the theoretically calculated HOMOs/LUMOs based on the conformations in single crystals. All THPs with different R1−R4 in n-hexane solutions showed no emission and had similar absorbance wavelengths (λab) (315–320 nm) except R2 = R4 = alkyl (λab = 293 nm). However, upon aggregation, 64 THPs with aromatic R2 and R4 were emissive and had very different solid-state fluorescence quantum yields (ΦF) (3–100%), excitation and emission wavelengths (340–409 nm and 436–492 nm, respectively), except two non-emissive THPs with aliphatic R2 and R4. It was also found that the influences of all R1−R4 depended on their combinations, and the solid-state ΦF values of THPs were greatly enhanced by simply changing the combinations of R1−R4 with larger R1/smaller R3 or smaller R1/larger R3 combining suitable R2 and R4 optimal. 20 THPs with ΦF > 80% were mainly from various aryls except 4-OHPh and rigid naphthalenyl as R3 combining with suitable R1 (Et/Me), R2 and R4 (Ph/4-Cl/4-CF3/3-CF3/3-BrPh). The R- and S-enantiomer alignments of THPs in the 22 crystals were classified into four molecular packing modes (MPMs), whose formation mainly depended on R3 on chiral carbon. The through-space conjugation existing in the HOMOs and LUMOs of THPs with aryl R2 and R4 also correlated with R3: aromatic R3 and R4 when R3 = 4-CNPh, or only R4 when R3 = other aryls participating in conjugation through space. The mechanisms of structural effects on the solid-state fluorescence properties and heteroenantiomeric self-assembly of THPs are discussed based on the obtained experimental results and our previous work on AIE mechanism. The simple and efficient 5CR built a large library of diverse compounds, the relationship between substituent on chiral carbon and heteroenantiomeric self-assembly, the strong and useful AIE characteristics, and moreover, the firstly reported efficient method of enhancing solid-state fluorescence via simply changing the combinations of substituents will be expected to make racemic THPs potential materials and attract great interest in the development and applications of new interesting racemic fluorophores.

Overriding Phthalate Decomposition When Exploring Mycophenolic Acid Intermediates as Selenium-Based ROS Biological Probes.

Hypochlorous (OCl–) acid is the most well-known bacterial oxidant to be produced by neutrophils. Excess amounts of OCl– can cause various disorders in living systems. Herein, we have designed, synthesized, and characterized two novel organoselenium-based target molecules (Probe-1 and Probe-OCl) based on a synthetic intermediate of mycophenolic acid for the aqueous detection of OCl–. Probe 1 has been recently reported (Org. Lett. 2018, 20, 3557–3561); both probes show immediate “turn-on” fluorescence (<1 s) upon the addition of OCl–, display an increase in the fluorescence quantum yield (3.7-fold in Probe-1 and 11.6-fold in Probe-OCl), and are completely soluble in aqueous media without the help of any cosolvent. However, a decrease in the “turn-on” intensity with the oxidized version of Probe-1 in cell assays due to the anhydride/phthalate functionality suggests that probe degradation occurs based on hydrolytic action (a probe degradation half-life of ∼1500 s at 15 μM Probe-1 and 150 μM OCl). Thus, the change of “anhydride” to “methylamide” begets Probe-OCl, which possesses more stability without sacrificing its water solubility properties and responses at short times. Further studies suggest that Probe-OCl is highly stable within physiological pH (pH = 7.4). Surprisingly, in live cell experiments involving U-2 OS cells and HeLa cells, Probe-OCl accumulated and aggregated in lipid droplets and gives a “turn-on” fluorescence response. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays confirmed that Probe-OCl is not toxic. Cuvette aggregation studies were also performed (tetrahydrofuran/H2O) to demonstrate aggregation-induced fluorescence at longer times. Our current hypothesis is that the “turn-on” fluorescence effect is caused by the aggregation-induced emission mechanism available for Probe-OCl. In this case, in tandem, we reanalyzed the Mes-BOD-SePh derivative to compare and contrast cell localization as imaged by confocal microscopy; fluorescence emission occurs in the absence of, or prior to, Se oxidation.

From Tetraphenylfurans to Ring-Opened (Z)-1,4-Enediones: ACQ Fluorophores versus AIEgens with Distinct Responses to Mechanical Force and Light.

Two aryl-substituted tetraphenylfurans (TPF-1 and TPF-2) and the corresponding ring-opened (Z)-1,4-enedione derivatives (TPBD-1 and TPBD-2) have been successfully synthesized. Although all the molecules adopt propeller-like configurations, experimental results revealed that the tetraphenylfurans (TPFs) were aggregation-caused quenching (ACQ) fluorophores whereas the 1,4-enediones (TPBDs) were aggregation-induced emission (AIE) active. Additionally, the TPFs and TPBDs exhibited contrasting "turn-on" mechanochromic behavior, with an unusual blueshift of mechanochromic fluorescence for the TPFs and a remarkable redshifted mechanochromism for the TPBDs. Both of these solid-state optical properties were proven to be highly dependent on the mode of molecular packing and substituent effects. In particular, in addition to the tunability of the fluorescent properties through aggregation and mechanical force, ground TPF-1 was found to be efficiently sensitive to light. On the basis of these findings we constructed a smart film with ground TPF-1 and demonstrated its utility in data encryption and decryption well controlled by UV light. Therefore, this work not only provides an insight into the mechanism of AIE as well as mechanochromic effects, but also paves the way towards the development of multifunctional stimuli-responsive materials and devices.