Nopinone-based aggregation-induced emission (AIE)-active difluoroboron β-diketonate complex: photophysical,electrochemical and electroluminescence properties

Four difluoroboron (BF₂) β-diketonate nopinone complexes 3a–3d that exhibited typical aggregation-induced emission (AIE) properties were synthesized using the natural renewable β-pinene derivative nopinone as the starting material. The thermal, photophysical, electrochemical and electroluminescent properties as well as the AIE properties of complexes 3a–3d were analyzed systematically. The data of photophysical and electrochemical demonstrated that compound 3b with a methoxy group exhibited the largest bathochromic shift, the highest absolute photoluminescence quantum yields and narrowest optical bandgap among 3a–3d. Using 3b as the emitter, electroluminescent (EL) device I exhibits blue-green light with CIE coordinates of (0.2774, 0.4531) and showed a better performance with a luminous efficacy (η_p) of 7.09 lm W⁻¹ and correlated color temperature (T_C) of 7028 K. The results demonstrate that new AIE compounds are promising solid-state luminescent materials with practical utility in electroluminescent materials.

Four difluoroboron (BF₂) β-diketonate nopinone complexes 3a–3d which exhibited typical AIE property were synthesized. Owing to high absolute fluorescence quantum yields of 3b, EL device based on 3b was fabricated, which exhibits a blue-green light.     

Aggregation-induced emission enhancement (AIEE)-active tetraphenylethene (TPE)-based chemosensor for Hg2+ with solvatochromism and cell imaging characteristics

An aggregation-induced emission enhancement (AIEE)-active fluorescent sensor based on a tetraphenylethene (TPE) unit has been successfully designed and synthesized. Interestingly, the luminogen could detect Hg²⁺ with high selectivity in an acetonitrile solution without interference from other competitive metal ions, and the detection limit was 7.46 × 10⁻⁶ mol L⁻¹. Furthermore, the luminogen also showed interesting solvatochromic behavior and superior cell imaging performance.

A TPE-based AIEE-active fluorescent sensor for Hg²⁺ was synthesized. Furthermore, it showed solvatochromism and cell imaging characteristics.

The design and synthesis of molecular sensors for the detection of metal ions, especially transition-metal ions, has attracted much attention.^1–3 Among all transition-metal ions, mercury (Hg²⁺) is identified as one of the most dangerous and ubiquitous heavy metals. Indeed, it is not biodegradable, and can cause extreme injury to the environment as well as human health.^4–8 Additionally, it can be accumulated through the food chain in the human body, consequently giving rise to several deleterious effects such as central nervous system defects, kidney damage, endocrine system disease and so on.^9–12 Although many governments around the world have adopted strict regulations to limit Hg²⁺ emission, the global Hg²⁺ pollution caused by human activities is still serious.^13,14 Therefore, it is highly desirable to develop a new method for the detection of Hg²⁺ with high selectivity and sensitivity.^15–17Most traditional fluorescent sensors suffer from a detrimental phenomenon called aggregation-caused quenching (ACQ), which usually results in a poor solid-state emission efficiency. Fortunately, in 2001, Tang et al. reported a fluorescent molecule named 1-methyl-1,2,3,4,5-pentaphenylsilole. Interestingly, the fluorescence emission of this luminogen was induced by aggregation, a phenomenon referred to as aggregation-induced emission (AIE).¹⁸ Subsequently, in 2002, Park et al. reported an interesting phenomenon named aggregation-induced emission enhancement (AIEE).¹⁹ In fact, both AIE and AIEE can achieve highly efficient emission in the solid state or aggregated state.^20–27 In the past years, AIE (or AIEE) phenomenon has attracted considerable research interest owing to the potential applications in a lot of fields, including bioimaging, fluorescence sensors, organic lighting emitting diode (OLED) devices and organic lasers.^28–33 Meanwhile, many stimuli-responsive materials have been reported, including photochromism, mechanochromism, and solvatochromism.^34–46 At present, the solvatochromism materials have been generally used in chemical and biological systems.⁴⁷ To date, many fluorescent chemosensors for Hg²⁺ have been reported. In contrast, the corresponding chemosensors with AIE or AIEE effect are rare, not to mention solvatochromic AIE or AIEE-active fluorescent sensors for Hg²⁺ with good cell imaging behavior. Indeed, preparing such multifunctional sensors is challenging and significative. In this paper, we reported an AIEE-active tetraphenylethene (TPE)-based fluorescent sensor (Scheme 1) for the detection of Hg²⁺. Furthermore, the luminogen also showed remarkable solvatochromism and good cell imaging characteristics.Open in a separate windowScheme 1The synthetic route of luminogen 1.To investigate the AIEE phenomenon of luminogen 1, we initially studied the UV-visible absorption spectra and photoluminescence (PL) spectra in acetonitrile–water mixtures with different water fractions (f_w). The results indicated that the absorption spectra exhibited level-off tails in the long wavelength region as the water content increased (Fig. S1^†). It is well-known that such tails are usually associated with the Mie scattering effect, which is the key signal of nano-aggregate formation.^48,49 As presented in Fig. 1, luminogen 1 showed weak fluorescence and the luminescence quantum yield (Φ) was as low as 1.35%. Interestingly, when the f_w in the acetonitrile solution was increased to 80%, an obvious emission band was formed, and a yellow-green fluorescence was observed. As the water content was increased to 90%, the emission intensity was further increased, and a bright yellow-green luminescence (Φ = 27.81%) with a λ_max at 545 nm could be observed.Open in a separate windowFig. 1(a) Fluorescence spectra of the dilute solutions of 1 (2.0 × 10⁻⁵ mol L⁻¹) in acetonitrile–water mixtures with different water content (0–90%). Excitation wavelength = 380 nm. (b) Changes the emission intensity of 1 at 545 nm in acetonitrile–water mixtures with different volume fractions of water (0–90%). (c) PL images of 1 (2.0 × 10⁻⁵ mol L⁻¹) in acetonitrile–water mixtures with different f_w values under 365 nm UV illumination.Clearly, water is a poor solvent of luminogen 1. As a result, the generation of the yellow-green emission can be attributed to the aggregate formation.^50–52 Moreover, as shown in Fig. 2, the nano-aggregates obtained were verified by dynamic light scatterings (DLS). Therefore, 1 is a typical AIEE-active fluorescent molecule, and its AIEE behavior is caused by the restricted intramolecular rotation. As shown in Fig. S2,^† solid-state compound 1 showed a strong green emission (Φ = 14.50%) with a λ_max at 497 nm, and the corresponding fluorescence lifetime is 2.66 ns (Fig. S3^†).Open in a separate windowFig. 2Size distribution curve of luminogen 1 (2.0 × 10⁻⁵ mol L⁻¹) in acetonitrile–water mixtures with f_w = 90%.On the other hand, the luminogen 1 also displayed interesting solvatochromism effect. As presented in Fig. 3, the absorption spectra and fluorescence spectra of luminogen 1 in different solvents were investigated, respectively. Obviously, the absorption spectra was barely affected by the polarity of solvents. However, the photoluminescence peaks were gradually red-shifted from 515 nm to 625 nm, and thus 1 exhibited remarkable solvatochromism behavior. Obviously, the molecular structure of luminogen 1 is distorted due to the presence of TPE unit, and the conjugation degree of molecule 1 is different in various solvents, and the intramolecular charge transfer (ICT) effect is possibly responsible for the solvatochromism behavior of 1.Open in a separate windowFig. 3(a) UV-Vis absorption spectra and (b) normalized fluorescence spectra (Excitation wavelength = 380 nm) of luminogen 1 in different solvents (2.0 × 10⁻⁵ mol L⁻¹). (c) Photographs of 1 under 365 nm UV illumination in different solvents: tol (toluene); THF (tetrahydrofuran); EA (ethyl acetate); BT (acetone); MeCN (acetonitrile); DMSO (dimethyl sulfoxide).Subsequently, the changes in the fluorescence of luminogen 1 induced by Hg²⁺ were investigated in acetonitrile (2.0 × 10⁻⁵ mol L⁻¹) at room temperature. In the fluorometric titration experiments, as shown in Fig. 4, the emission intensity significantly decreased when the Hg²⁺ concentration increased from 0 to 7.0 equivalents in acetonitrile. Meanwhile, the fluorescent color changed from orange-red to colorless, and followed by a plateau upon further titration (Fig. 5). Remarkably, the emission intensity of luminogen 1 was almost quenched completely. Furthermore, based on the titration experiments, the detection limit of luminogen 1 for Hg²⁺ on the basis of LOD = 3 × σ/B (where σ is the standard deviation of blank sample and B is the slope between the fluorescence intensity versus Hg²⁺ concentration) was 7.46 × 10⁻⁶ M (Fig. S4^†). Moreover, a good linear relationship could be obtained (R = −0.9933) and the quenching constant of luminogen 1 with Hg²⁺ was 1.9 × 10⁴ M⁻¹ (Fig. S5^†). On the other hand, the binding ratio of luminogen 1 for Hg²⁺ was established through Job''s plot and the results showed 1 : 1 stoichiometric complexation (Fig. S6^†). Next, the binding interactions between 1 and Hg²⁺ were further investigated by NMR in acetonitrile-d₃. As presented in Fig. 6, the signal of Ha from 9.42 ppm shifted to 9.62 ppm and the Hb changed from 8.43 ppm to 9.24 ppm. Besides, the Hc or Hd was slightly shifted for 0.04 ppm or 0.06 ppm, respectively. These consequences revealed that the N on the pyridine and the N on the pyrazine (near the N on the pyridine) are the most probable binding with Hg²⁺. Mass spectra were utilized to further demonstrate the binding mode of luminogen 1 toward Hg²⁺. The peak located at m/z = 820.0 was coincided well with the ensemble [1 + Hg²⁺ +2NO₃⁻ + Cl⁻]⁻, confirming the binding ratio of luminogen 1 for Hg²⁺ with 1 : 1 stoichiometry (Fig. S7^†).Open in a separate windowFig. 4Fluorescence titration spectra of luminogen 1 (2.0 × 10⁻⁵ mol L⁻¹) induced by Hg²⁺ (0–7.0 equiv.) in an acetonitrile solution. Excitation wavelength = 380 nm.Open in a separate windowFig. 5The emission intensity changes of luminogen 1 at 625 nm with different equivalents of Hg²⁺.Open in a separate windowFig. 6 ¹H NMR (acetonitrile-d₃, 400 MHz) spectra changes of luminogen 1 in the presence of Hg²⁺.Next, in order to study the selectivity behavior of luminogen 1 as a fluorescent sensor for Hg²⁺, other metal ions, such as Zn²⁺, Cd²⁺, Ba²⁺, Sr²⁺, Mn²⁺, Mg²⁺, Ca²⁺, Pb²⁺, Ni²⁺, Co²⁺, Cu²⁺, Al³⁺, Fe³⁺, Cr³⁺, Ag⁺ and K⁺ were also measured in acetonitrile under the same experimental conditions. The corresponding UV-Vis absorption spectra were shown in Fig. S8.^† Furthermore, as showed in Fig. 7, when these cations were added separately into the solution containing luminogen 1, no obvious fluorescence changes were observed. Indeed, as noticed in Fig. S9,^† no obvious interference was observed when Hg²⁺ (7.0 equiv.) was added with other ions (7.0 equiv.). These results indicated that luminogen 1 could be served as a highly selective fluorescent sensor for detection of Hg²⁺.Open in a separate windowFig. 7(a) Fluorescence spectra of luminogen 1 (2.0 × 10⁻⁵ mol L⁻¹) towards various cations including Zn²⁺, Cd²⁺, Hg²⁺, Ba²⁺, Sr²⁺, Mn²⁺, Mg²⁺, Ca²⁺, Pb²⁺, Ni²⁺, Co²⁺, Cu²⁺, Al³⁺, Fe³⁺, Cr³⁺, Ag⁺ and K⁺. Excitation wavelength = 380 nm. (b) Fluorescence photographs of luminogen 1 after addition of various metal ions (7.0 equiv.) under 365 nm light.Fluorescent probe is a powerful tool for optical imaging, which allows direct visualization of biological analytes.⁵³ Luminogen 1 was AIEE-active due to the restriction of intramolecular rotation in the aggregate state, which is beneficial for cell imaging. Indeed, the viability of HeLa cells incubated with luminogen 1 was evaluated by the standard MTT method (Fig. 8), and the result indicated that compound 1 exhibited low cytotoxicity. Next, cell imaging behavior of luminogen 1 was investigated using a confocal laser scanning microscopy (CLSM). HeLa cells were incubated with luminogen 1 (20 μM) for 30 min at 37 °C and the fluorescence images were obtained by CLSM.Open in a separate windowFig. 8The MTT assay of compound 1 for measuring cell viability.As presented in Fig. 9, an intense yellow-green fluorescence, which was consistent with the fluorescence of luminogen 1 in acetonitrile–water mixture with high water content (80% or 90%), was displayed inside the cells. This result indicated that luminogen 1 tended to aggregate inside the cells, and thus the bright yellow-green luminescence was clearly observed. Furthermore, the merged picture c demonstrated that picture a and picture b overlapped very well. These results indicated that luminogen 1 showed superior cell imaging performance.Open in a separate windowFig. 9Fluorescence images of HeLa cells incubated with luminogen 1 (20 μM) for 30 min at 37 °C: (a) bright field image; (b) fluorescence image; (c) merge image (a and b).In summary, a TPE-based fluorescent molecule was designed and synthesized. The luminogen exhibited obvious AIEE phenomenon. Moreover, luminogen 1 could be used as a highly selective fluorescence turn-off chemosensor for Hg²⁺, and the detection limit was 7.46 × 10⁻⁶ mol L⁻¹. Furthermore, 1 also displayed interesting solvatochromic behavior and superior cell imaging performance. Further studies on the design of new AIE or AIEE-active fluorescent chemosensors are in progress.     

Blue-emissive two-component supergelator with aggregation-induced enhanced emission

Two-component organogels offer several advantages over one-component gels, but their design is highly challenging. Hence, it is extremely important to design new approaches for the crafting of two-component organogels with interesting optical and mechanical properties. Herein, we report the design of a new class of two-component supergelators obtained from the assembly between acid functionalized tetraphenylethylene (TPE)-based dendrons and alkylated melamine. No gelation behaviour is observed for the individual components, but interestingly, remarkable gelation behaviour is observed for their hydrogen-bonded complex. The primary driving force responsible for the gelation is the strong π–π stacking interaction of TPE units. Because of the strong π-stacking of TPEs in the gel state, the C(sp²)–C(sp²) bond rotation of the TPE segment is completely arrested in the gel state, which results in intense fluorescence emission of the gels. Furthermore, excellent elastic response is observed for the gels as evident from their high storage modulus compared to loss modulus values. Our results clearly demonstrate that by the appropriate selection of the molecular components, this approach can be applied for the creation of functional nanomaterials with emergent properties absent in the individual blocks.

Design of a novel class of two-component, highly emissive, low molecular weight supergelator is reported.     

Axially chiral 1,4-dihydropyridine derivatives: aggregation-induced emission in exciplexes and application as viscosity probes

By combining the fluorophores of axially chiral 1,1′-binaphthol (BINOL) and 1,4-dihydropyridine derivatives, axially chiral 1,4-dihydropyridine derivatives ((R)-/(S)-2) with aggregation-induced emission (AIE) in exciplexes were designed and synthesized. (R)-/(S)-2 emitted low fluorescence in THF solutions of their locally excited states; however, they emitted red-shifted fluorescence in the aggregate state upon exciplex formation. Moreover, (R)-/(S)-2 showed linear and multi-exponential relationships between their local excited and exciplex fluorescence intensities and the viscosity of the medium, which allowed us to determine the viscosities of different mixed solvents. In addition, as an axially chiral viscosity probe, (R)-/(S)-2 show excellent CD signals and have potential applications in the fields of chiral recognition and fluorescence imaging, which will broaden the new family of AIE fluorophores. To the best of our knowledge, there are few reports of axially chiral intramolecular exciplex-mediated AIE molecules.

Axially chiral 1,4-dihydropyridine derivatives ((R)-/(S)-2) with aggregation-induced emission (AIE) in exciplex were designed and synthesized. They have excellent CD signals and can determine the different viscosity in mixed solvents.     

Vinylpyrroles: solid-state structures and aggregation-induced emission properties

An aggregation-induced emission chromophore, vinylpyrrole, was prepared from a formylpyrrole derivative, Meldrum''s acid, and 1,3-dimethylbarbituric acid. The optical properties of the chromophore both in the solution and solid states were investigated by UV-vis and fluorescence spectroscopy. Single crystal X-ray diffraction measurements revealed that the dimethylbarbituric acid adduct formed a J-aggregate in the solid and resulted in higher fluorescence quantum yield compared to the Meldrum''s acid adduct. Emission enhancement was found to occur by the restriction of molecular rotation in the solid state.

A cyclic ester and a cyclic amide functionalized monopyrroles show aggregation-induced emission (AIE) by the restriction of intramolecular rotation (RIR) mechanism.     

A fluorescence and UV/vis absorption dual-signaling probe with aggregation-induced emission characteristics for specific detection of cysteine

Biological thiols with similar structures, such as glutathione (GSH), N-acetyl-l-cysteine (NAC), homocysteine (Hcy) and cysteine (Cys), play important roles in human physiology and are associated with different diseases. Thus, the discrimination of these thiols is a great necessity for various biochemical investigations and the diagnosis of related diseases. Herein, we present a new dual-signaling probe consisting of a typical aggregation induced emission fluorogen of a tetraphenylethylene group and 2,4-dinitrobenzenesulfonyl moiety. The probe can be used to selectively and quantitatively detect Cys over a variety of bio-species, including GSH, NAC and Hcy, from both UV/vis absorption and fluorescence channels. The mechanism study showed that the fluorescence and UV/vis absorption were turned on as the probe undergoes displacement of the 2,4-dinitrobenzenesulfonyl group with Cys, where the UV/vis and fluorescence signals originate from the dinitrophenyl-containing compounds and aggregates of TPE-OH, respectively. In addition, the discrimination of Cys was achieved by more rapid intramolecular displacement of sulfur with the amino group of Cys than NAC, Hcy and GSH. Moreover, the probe shows ignorable cytotoxicity against HepG2 cells, which demonstrates the great potential of the probe in selectively detecting Cys in vivo.

A dual-signaling of fluorescence and UV/vis absorption modes for selective and quantitative detection of cysteine over homocysteine, N-acetyl-l-cysteine and glutathione is developed on the basis of aggregation-induced emission (AIE) effect.     

Intra-mitochondrial reaction for cancer cell imaging and anti-cancer therapy by aggregation-induced emission

Controlled intracellular chemical reactions to regulate cellular functions remain a challenge in biology mimetic systems. Herein, we developed an intra-mitochondrial bio-orthogonal reaction to induce aggregation induced emission. In situ carbonyl ligation inside mitochondria drives the molecules to form nano-aggregates with green fluorescence, which leads to depolarization of the mitochondrial membrane, generation of ROS, and subsequently mitochondrial dysfunction. This intra-mitochondrial carbonyl ligation shows great potential for anticancer treatment in various cancer cell lines.

Controlled intracellular chemical reactions to regulate cellular function remain a challenge in biology mimetic systems.     

Diphenylpyrenylamine-functionalized polypeptides: secondary structures,aggregation-induced emission,and carbon nanotube dispersibility

In this study we prepared—through ring-opening polymerization of γ-benzyl-l-glutamate N-carboxyanhydride (BLG-NCA) initiated by N,N-di(4-aminophenyl)-1-aminopyrene (pyrene-DPA-2NH₂)—poly(γ-benzyl-l-glutamate) (PBLG) polymers with various degrees of polymerization (DP), each featuring a di(4-aminophenyl)pyrenylamine (DPA) luminophore on the main backbone. The secondary structures of these pyrene-DPA-PBLG polypeptides were investigated using Fourier transform infrared spectroscopy and wide-angle X-ray diffraction, revealing that the polypeptides with DPs of less than 19 were mixtures of α-helical and β-sheet conformations, whereas the α-helical structures were preferred for longer chains. Interestingly, pyrene-DPA-2NH₂ exhibited weak photoluminescence (PL), yet the emission of the pyrene-DPA-PBLG polypeptides was 16-fold stronger, suggesting that attaching PBLG chains to pyrene-DPA-2NH₂ turned on a radiative pathway for the non-fluorescent molecule. Furthermore, pyrene-DPA-2NH₂ exhibited aggregation-caused quenching; in contrast, after incorporation into the PBLG segments with rigid-rod conformations, the resulting pyrene-DPA-PBLG polypeptides displayed aggregation-induced emission. Transmission electron microscopy revealed that mixing these polypeptides with multiwalled carbon nanotubes (MWCNTs) in DMF led to the formation of extremely dispersible pyrene-DPA-PBLG/MWCNT composites. The fabrication of MWCNT composites with such biocompatible polymers should lead to bio-inspired carbon nanostructures with useful biomedical applications.

PBLG chains to pyrene-DPA-2NH₂ turned on a radiative pathway for the non-fluorescent molecule and TEM revealed these polypeptides with carbon nanotube to form PBLG/MWCNT composite.     

Folic acid functionalized aggregation-induced emission nanoparticles for tumor cell targeted imaging and photodynamic therapy

Recently, molecules with aggregation-induced luminescence (AIE) characteristics have received more and more attention due to the fluorescence of traditional dyes being easily quenched in the aggregated state. AIE molecules have significant advantages, such as excellent light stability, bright fluorescence, high contrast, and large Stokes shift. These characteristics have aroused wide interest of researchers and opened up new applications in many fields, especially in the field of biological applications. However, AIE molecules or their aggregates have certain limitations in multifunctional biological research due to their low specific targeting ability, poor biocompatibility, and poor stability in physiological body fluids. In order to overcome these problems, a novel nanoparticle, FFM1, was fabricated and characterized. FFM1 displayed good water solubility, biocompatibility, and AIE emission properties. It could target HeLa cells specifically by recognizing their folate receptor. Reactive oxygen triggered by light irradiation induced tumor cell apoptosis. Summarily, FFM1 displayed excellent capacity in target imaging and photodynamic killing of HeLa cells. It has shown potential application value in targeted diagnosis and photodynamic therapy of tumors, and has important guiding significance for the treatment of malignant tumors. It paves a way for the development of a novel strategy for tumor theranostics.

Herein, a novel nanoparticle, FFM1, displays good water solubility, biocompatibility and AIE emission properties. It has shown potential application value in targeted diagnosis and photodynamic therapy of tumors by recognizing folate receptor.     

Synthesis,micellar structures and emission mechanisms of an AIE and DDED-featured fluorescent pH- and thermo-meter

A new nanoprobe, the luminescent diblock copolymer PNIPAM(MAh-4)-b-P4VP (PN4P), with pH- and thermo-responsive deprotonation-driven emission decay (DDED) and aggregation-induced emission (AIE) features was designed and synthesized. The nanoprobe PN4P can form micellar structures in water with reversible dual-responsive fluorescence (FL) behavior within a wide pH range of 2–11. The critical solution temperature was found at about 32, 30 and 27 °C as the pH switched from 2, 7 to 11. The critical pH value of the probe was about 4.0, and the micelles showed a core–shell inversion in response to pH and thermal stimuli, accompanied by a desirable emission tunability. P4VP as the micellar shell at pH = 2 was more easily dehydrated with the increase in temperature as compared to PNIPAM as the micellar shell at pH > 4. The strongest dehydration of the P4VP shell would make PN4P the most strongly aggregated and the most AIE-active, which supports the 2.10-fold most distinguished thermal-responsive emission enhancement at pH = 2. Moreover, a dramatic acidochromic redshift of the emission band from 450 (pH > 4) to 490 nm (pH = 2) was observed, and the maximum emission at pH = 2 was enhanced by about 2.07-fold as compared with that at pH = 7. Therefore, the probe displays the desired dual responses and good reversibility. AIE and DDED are the two major mechanisms responsible for the dual-responsive emission change, with AIE playing a more important role than DDED. This work offers a promising approach to interpreting temperature (range from 28 to 40 °C) and pH changes (range from 2 to 7) in water.

A nanoprobe in water features pH- and thermal-responsive micellar/clustering structures, deprotonation-driven emission decay (DDED) and aggregation-induced emission (AIE).     

A coumarin-containing Schiff base fluorescent probe with AIE effect for the copper(ii) ion

A novel coumarin-derived Cu²⁺-selective Schiff base fluorescent “turn-off” chemosensor CTPE was successfully obtained, which showed an AIE effect. It could identify Cu²⁺ by quenching its fluorescence. The lower limit of detection was 0.36 μM. CTPE can act as a highly selective and sensitive fluorescence probe for detecting Cu²⁺.

A novel coumarin-derived Schiff base fluorescent “turn-off” chemosensor with AIE effect showed selectivity towards Cu²⁺. The recognition mechanism is presented.

The specific detection of transition metal ions has received considerable attention because of their significant roles in the fields of biological, chemical, medical and environmental processes.¹ Among these metal ions, copper(ii), acting as one of the most important micronutrients, is of particular interest due to its essential role in a variety of fundamental physiological processes. In spite of the fact that copper(ii) is essential in living systems, accumulation of copper(ii) can lead to serious environmental and health problems.² For instance, the metabolic balance of Cu²⁺ in the body being destroyed can lead to neurodegenerative diseases, such as Alzheimer''s, Parkinson''s, Menkes, and Wilson''s disease.^3–8 Besides, free Cu²⁺ is regarded as a significant environmental pollutant.^9,10 Therefore, an increasingly convenient and fast method to detect Cu²⁺ ion existing in environmental and biological resources is currently of considerably important. By far there existing many methods make it possible to detect and quantify Cu²⁺ ion, such as atomic absorption spectrometry,^11,12 inductively coupled plasma mass spectrometry,¹³ inductively coupled plasma-atomic emission spectrometry¹⁴ and voltammetry,^15,16etc. However, these methods require tedious sample preparation procedures and complex instrumentation, which limit their prosperous applications. Alternatively, more and more attention has been attracted to the analytical methods based on fluorescent chemosensors for the detection of the metal ions owing to their high sensitivity, selectivity, and simplicity.¹⁷ As a result, various types of fluorescent probes such as organic dyes, magnetic nanoparticles (MNPs), semiconductor quantum dots (QDs), carbon dots (CDs), fluorescent metal nanoclusters (NCs), and fluorescent metal organic frameworks (MOFs) have been designed and prepared to determine Cu²⁺ ion.^18–27 Since first reported in 2000, MNPs are extensively used in the field of nanotechnology for its nonhazardous feature, strong magnetization values, superparamagnetic property, active surface that can easily assembled of biological soluble structure and targeting, imaging, and therapeutic molecules.²² Unlike organic fluorescent dyes, QDs are semiconductor particles which exhibit high photochemical stability, excellent resistance to chemical degradation, outstanding photodegradation and extremely large Stokes shift.²³ CDs is a comprehensive term for various nanosized carbon materials including graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs)²⁴ which shows excellent properties including high stability, bio-compatibility, low photo bleaching and toxicity and so on.²⁵ Despite successfully applying in detecting Cu²⁺, traditional fluorescent probes are still puzzled by the toxicity, poor solubility, “aggregation-caused quenching” (ACQ) effect, etc. Thus, preparing fluorescent probes that can compensate for these deficiencies is still imminently desired in the Cu²⁺ ion detection.Coumarin derivatives are well known excellent fluorophores in the fluorescent probe design and synthesis with the advantages of high fluorescence intensity, excellent solubility, efficient cell permeation, high quantum yield and ease of preparation.^28,29 As a result, an increasing number of papers concerning coumarin derivatives fluorescent chemosensors for copper(ii) ion have emerged.^30–33 In 2009, Lee Jin Yong and co-workers developed a novel coumarin-based fluorogenic probe, which can act as a fluorescent chemosensor with high selectivity and suitable affinity in biological systems toward Cu²⁺ (ref. 34). Besides, in 2019, Yin Jiqiu and co-workers obtained a fluorescent chemosensor concerning coumarin derivatives for copper(ii) which exhibited good sensitivity, fast response time and high selectivity for Cu²⁺ ion in the presence of other important relevant metal ions.³⁵In 2001, Tang Benzhong and co-workers proposed the concept of “aggregation-induced emission” (AIE).³⁶ Due to their effective in circumventing the ACQ effect, AIE-active materials provide a new path for the design and synthesis of fluorescent probes. Tetraphenylethene (TPE), one of the most commonly used aggregation-induced emission luminogens (AIEgens), has been widely favored by researchers.^37,38 Owing to its simple structure, simple synthesis, easy modification, and obvious AIE effect, TPE is usually used as an ideal model for construction of various fluorescent sensors.³⁹ In 2019, Zhao Feng and co-workers designed and synthesized a new tetraphenylethene-based Schiff base ligand with AIE effect, and it could be utilized as optical recording materials.⁴⁰ The same year, Ni Zhonghai and co-workers obtained two different polymorphs of a new tetraphenylethene-based Schiff base, which exhibited totally different photochromic and fluorescence properties.⁴¹Many Schiff base ligands have been synthesized for the detection of Cu²⁺, which have general fluorescence characteristics. To solve the ACQ problem, we are going to design and synthesize a Schiff base fluorescent probe with AIE effect. For this purpose, a Schiff base fluorescent probe (CTPE) incorporating the TPE group into coumarin framework has been designed and synthesized, which exhibits fast response time, simple synthetic step, lower cost and AIE effect. And it can rapidly recognize Cu²⁺ in a mixed THF/H₂O system. Scheme 1 was the synthetic route for CTPE. Other compounds and intermediates were labelled and displayed in Scheme S1 in the ESI.^† As shown in Scheme 1, CTPE was prepared in high yield via Maillard reaction of 8-formyl-7-hydroxy-4-methylcoumarin (M1) and 1-(4′-aminophenyl)-1,2,2-triphenylethene (TPE-NH₂) in absolute ethanol. M1, TPE-NH₂ and CTPE were prepared according to the literatures.^42,43 Their chemical structures were confirmed by nuclear magnetic resonance (NMR), high resolution mass spectrometry (HRMS) and Fourier transform infrared (FTIR) (Fig. S1–S10, ESI^†). In Fig. S10,^† compared with TPE-NH₂ and M1, the spectrum of CTPE appeared an in-plane bending vibration absorption peak of C N bond at 1620 cm⁻¹, and a C O bond stretching vibration absorption peak on the aldehyde group at 1740 cm⁻¹, indicating that CTPE has been successfully synthesized.Open in a separate windowScheme 1Synthesis and structure of CTPE.As an AIE-based fluorescent probe, the AIE effect of CTPE was confirmed by its fluorescence spectra in THF/H₂O mixtures with various water contents. CTPE was in a good dispersion state in THF solvent, but CTPE gradually aggregated as the volume fraction of water (f_w) increased. As shown in Fig. 1A and B, CTPE had good dispensability and exhibited weak luminescence in dilute THF solution (f_w = 0%). The weak emissive nature of the luminogens in THF and aqueous mixtures with low f_w values should be ascribed to active intramolecular rotations of the genuinely dissolved compounds, which effectively consumed the energy of the excitons non-radiatively.⁴⁴ When f_w increased from 10% to 70%, the fluorescence intensity of CTPE gradually decreased, which was attributed to the intramolecular charge transfer (ICT) effect with the increasing polarity of the solution for CTPE.^40,45 When f_w was above 70%, CTPE displayed a significantly sudden increase in the fluorescence intensity owing to molecular aggregation. The fluorescence intensity at 565 nm was about 60 in THF/H₂O (30/70, v/v) mixtures, while it increased to about 1288 in THF/H₂O (1/99, v/v) mixtures with about 21-fold enhancement. Simultaneously, and the maximum emission wavelength showed a slight red shift from 565 nm to 575 nm with the increase of fluorescence intensity. The fluorescence enhancement phenomenon could be ascribed to the restriction of the intramolecular rotations. In the aggregated state, intramolecular motion (mainly including C N isomerization and rotation of the benzene ring) was limited, and the attenuation of non-radiative energy was correspondingly blocked, so that a clear enhancement and a slight red shift of the fluorescence of CTPE were observed.⁴⁰ The above results clearly implied that CTPE exhibited a significant AIE effect.Open in a separate windowFig. 1(A) PL spectra of CTPE in THF/H₂O mixtures with different f_w. (B) Plots of PL intensity of CTPEversus f_w in THF/H₂O mixtures at 565 nm. Inset: photograph of CTPE in THF/H₂O mixtures under a hand-held UV lamp illumination (λ_ex = 365 nm). (C) PL spectra of CTPE after adding Cu²⁺ in THF/H₂O (1/99, v/v) at different reaction time. Inset: relationship between fluorescence intensities of CTPE in THF/H₂O (1/99, v/v) at 575 nm and time of addition of Cu²⁺. (D) UV-vis absorption and (E) fluorescence spectra of CTPE with different metal ions in THF/H₂O (10/90, v/v). (F) Fluorescence responses at 565 nm of CTPE to various metal cations in THF/H₂O (10/90, v/v) solution.The Cu²⁺-specific binding of CTPE with various competitive metal ions was then investigated under the same experimental condition. Different from these AIE sensors that worked in the aggregated state, there was not complete fluorescence quenching even after about 1 h of adding 20 equivalent Cu²⁺ in THF/H₂O (1/99, v/v) solution (shown in Fig. 1C). However, at the moment of adding Cu²⁺, the fluorescence was immediately quenched in THF/H₂O (10/90, v/v) solution. Therefore the THF/H₂O (10/90, v/v) system was adopted to study the selectivity of CTPE to all metal ions.Metal ion selectivity studies were performed on absorption and fluorescence spectra. Absorption spectra of CTPE recorded after the addition of each metal ion (20 eq.) was shown in Fig. 1D. In Fig. 1D, upon addition of a constant amount (20 eq.) of Cu²⁺ ion to CTPE, a significant hypsochromic shift from 395 nm to 371 nm in absorption spectrum was observed. No other metal ion induced any change under the identical conditions except Fe³⁺ and Fe²⁺ ions. These results demonstrated the specificity of the CTPE for selective binding interaction with Cu²⁺ ion.To further investigate the selectivity of CTPE to Cu²⁺, we also studied the fluorescence response of CTPE in THF/H₂O (10/90, v/v) solution. The behaviour of fluorescence “turn off” merely shown by Cu²⁺ was observed in the Fig. 1E. Surprisingly, the fluorescence was immediately quenched in THF/H₂O (10/90, v/v) solution at the moment of adding Cu²⁺. And the fluorescence intensities decreased after adding Fe³⁺ and Fe²⁺, respectively. No obvious changes were observed upon addition of 20 eq. other competitive metal ions. The interferences from the other metal ion with CTPE in its response to Cu²⁺ were performed. As shown in Fig. 1F, the red bars represent the emission changes of CTPE in the presence of metal ions of interest (all are 20 eq.). The black bars represent the changes of the emission that occurs upon the subsequent addition of Cu²⁺ to the above solution. The fluorescent intensity of CTPE in the presence of any of the other metal ions tested after adding Cu²⁺ was decreased significantly, demonstrating little interference from the other metal ions.Fluorescent titration experiments of CTPE in THF/H₂O (10/90, v/v) solution with Cu²⁺ ion were carried out and the changes in the fluorescence intensity at 565 nm of CTPE solution with the concentration of Cu²⁺ ion were measured and showed in Fig. 2A. It was found that the fluorescence intensity at 565 nm of CTPE solution decreased with the increase in the concentration of Cu²⁺ ion. As showed in Fig. 2B, F₀ and F were the fluorescence intensities at 565 nm without adding Cu²⁺ and adding different concentrations of Cu²⁺, respectively. In addition, when the Cu²⁺ concentration in the solution to be tested was less than 1.2 μM, the F₀/F of CTPE at 565 nm had a good linear relationship with the concentration of Cu²⁺.Open in a separate windowFig. 2(A) Fluorescence titration spectra of CTPE (10 μM) in THF/H₂O (10/90, v/v) solution. Inset: fluorescence intensities of CTPE (10 μM) at 565 nm as a function of Cu²⁺ concentration (0–3.2 μM). (B) Linear relationship between F₀/F and Cu²⁺ concentration.By taking that change in fluorescence intensity in micro molar range we have calculated the lower limit of detection (LOD) from standard deviation and the slope of calibration plot (Fig. 2B) using the equation.⁴⁶ It was deduced from the fluorescence titration profile that the LOD of CTPE toward Cu²⁺ ion reached 0.36 μM.To gain a better understanding the sensing mechanism of CTPE towards Cu²⁺, ¹H-NMR titration experiments were performed in DMSO-d₆ at room temperature (Fig. 3A). The ¹H-NMR spectra of CTPE showed considerable variation with the increasing of Cu²⁺ in DMSO-d₆ solvent. Cu²⁺ is a paramagnetic ion that affects the NMR resonance frequency of protons that are close to the Cu²⁺ binding site.⁴⁷ The downfield value of H_a (of –OH̲) at δ = 14.86 ppm in CTPE was due to the intramolecular hydrogen bond between the imine-N atom of CTPE with H_a forming a six-membered transition state.⁴⁸ On addition of Cu²⁺ ion, the intramolecular hydrogen bonding was disturbed.⁴⁹ As Cu²⁺ gradually increased, the proton signal of H_a almost disappeared. The proton signal of H_b (of –CH̲ N–) at δ = 9.16 ppm significantly decreased indicating the participation of the nitrogen atom in the binding with Cu²⁺ ion. The proton signals of the aromatic ring became broader and weaker with the increasing amount of Cu²⁺. These results confirmed that Cu²⁺ binded to the CTPE chemosensor through the N atom of the imine and the O atom of phenolic hydroxyl, which were directly connected to the aromatic rings.⁵⁰ Thus, combined with the FTIR spectra of CTPE before and after the addition of Cu²⁺ (Fig. S11^†), it was clearly illustrated that the fluorescence “turn off” of CTPE was attributed to the chelation between imine-N atom, phenolic hydroxyl-O atom and Cu²⁺ ion. The possible coordination modes of Cu²⁺ and CTPE were shown in Fig. 3B.Open in a separate windowFig. 3(A) ¹H-NMR data of CTPE in DMSO-d₆ solution in the absence and presence of Cu²⁺. (B) Proposed mechanism for CTPE upon addition of Cu²⁺.In summary, a novel Schiff base fluorescent probe CTPE based on coumarin and TPE showing an AIE effect has been successfully synthesized and characterized. The fluorescence of CTPE was rapidly quenched by Cu²⁺, but no significant influence was observed for other metal ions tested. CTPE can distinguish Cu²⁺ from other metal ions via the fluorescence “turn off”. The LOD of CTPE for Cu²⁺ can reach 0.36 μM. Recognition mechanism between CTPE and Cu²⁺ was given. Therefore, CTPE can act as a potential fluorescence probe to selectively and rapidly identify Cu²⁺.     

Synthesis,characterization, aggregation-induced emission,solvatochromism and mechanochromism of fluorinated benzothiadiazole bonded to tetraphenylethenes

Compounds consisting of unsubstituted, monofluoro and difluoro substituted benzothiadiazole bonded to two tetraphenylethenes were successfully prepared by palladium catalyzed Suzuki–Miyaura cross-coupling reaction of their corresponding co-monomers. All compounds exhibited aggregation-induced emission characteristics when the water fraction was higher than 60% in the THF/water mixtures. The emission maximum for the three compounds was blue-shifted when the water content reached 90% compared to that in THF solution. The intensity of emission maximum of difluorinated benzothiadiazole linked with two tetraphenylethenes was 2.5 times higher in 90% water compared to those in THF solution. Surprisingly, two liquid crystal phases with two distinct emission colors were observed only for the compound containing difluorinated benzothiadiazole bonded to two tetraphenylethene. All compounds showed remarkable solvatochromic properties in selected solvents with different polarities. The powder XRD results and mechanochromism of the compounds suggested that the solid state structures can change from one form to another by grinding, fuming or annealing processes.

Fluorinated benzothiadiazole bonded to two tetraphenylethenes were synthesized. The compounds exhibited remarkable aggregation-induced emission, solvatochromism and mechanochromism.     

A sensitive fluorescent probe for alkaline phosphatase and an activity assay based on the aggregation-induced emission effect

In this work, we report a fluorescent probe (TPEQN-P) for detecting alkaline phosphatase (ALP) with high sensitivity and monitoring its activity based on the specific aggregation-induced emission (AIE) effect. TPEQN-P can be constructed by conjugating the phosphate moiety into an AIE molecule (TPE-QI) through a reactive p-hydroxybenzyl group, which exhibits very weak emission in aqueous media due to its good water solubility. In the presence of ALP, TPEQN-P undergoes dephosphorylation and releases the hydrolysis product TPE-QI, which is intensely fluorescent because of its poor water solubility. The detection limit for ALP using TPEQN-P can be as low as 0.0077 U L⁻¹ with a linear range of 0–30 mU mL⁻¹ in solution. TPEQN-P also shows excellent applicability in serum samples, demonstrating potential applications in clinical diagnosis and biomedical research. TPEQN-P also can be applied for the determination of ALP activity and in ALP inhibitor screening.

A sensitive fluorescent probe (TPEQN-P) was designed and synthesized for detecting alkaline phosphatase and monitoring its enzymatic activity based on the specific aggregation-induced emission effect.     

Synthesis of fluorescent polycarbonates with highly twisted N,N-bis(dialkylamino)anthracene AIE luminogens in the main chain

A synthetic route to embed aggregation-induced-emission-(AIE)-active luminophores in polycarbonates (PCs) in various ratios is reported. The AIE-active monomer is based on the structure of 9,10-bis(piperidyl)anthracene. The obtained PCs display good film-forming properties, similar to those observed in poly(bisphenol A carbonate) (Ba-PC). The fluorescence quantum yield (Φ) of the PC with 5 mol% AIE-active monomer was 0.04 in solution and 0.53 in solid state. Moreover, this PC is also miscible with commercially available Ba-PC at any blending ratio. A combined analysis by scanning electron microscopy and differential scanning calorimetry did not indicate any clear phase separation. These results thus suggest that even engineering plastics like polycarbonates can be functionalized with AIE luminogens without adverse effects on their physical properties.

Fluorescent polycarbonates were synthesized by embedding AIE-active diol monomers with simple structures in the polymer chain.     

Facile construction of a hyperbranched poly(acrylamide) bearing tetraphenylethene units: a novel fluorescence probe with a highly selective and sensitive response to Zn2+

Thermo-responsive hyperbranched copoly(bis(N,N-ethyl acrylamide)/(N,N-methylene bisacrylamide)) (HPEAM-MBA) was synthesized by using reversible addition–fragmentation chain-transfer polymerization (RAFT). Interestingly, the zinc ion (Zn²⁺) was found to have a crucial influence on the lowest critical solution temperature (LCST) of the thermo-responsive polymer. The tetraphenylethylene (TPE) unit was then introduced onto the backbone of the as-prepared thermo-responsive polymer, which endows a Zn²⁺-responsive “turn-off” effect on the fluorescence properties. The TPE-bearing polymer shows a highly specific response over other metal ions and the “turn-off” response can even be tracked as the concentration of Zn²⁺ reduces to 2 × 10⁻⁵ M. The decrement of fluorescence intensity was linearly dependent on the concentration of Zn²⁺ in the range of 4–18 μmol L⁻¹. The flexible, versatile and feasible approach, as well as the excellent detection performance, may generate a new type of Zn²⁺ probe without the tedious synthesis of the moiety bearing Zn²⁺ recognition units.

A novel fluorescent HPEAM-TPEAH, possessing a highly selective and sensitive response to Zn²⁺, was synthesized using RAFT.     

Structure and emission properties of dinuclear copper(i) complexes with pyridyltriazole

A new series of five highly emissive binuclear heteroleptic pyridyltriazole-Cu(i)-phosphine complexes 1–5 was synthesized and examined by different experimental (IR, elemental and thermogravimetric analysis, single crystal X-ray diffraction technique, UV-vis and fluorescence spectroscopy) and quantum chemical aproaches. Complexes 1–5 exhibited excellent stimuli-responsive photoluminescent performance in the solid state at room temperature (quantum yield (QY) = 27.5–52.0%; lifetime (τ) = 8.3–10.7 μs) and when the temperature was lowered to 77 K (QY = 38.3–88.2; τ = 17.8–134.7 μs). The highest QY was examined for complex 3 (52%) that can be explained by the small structural changes between the ground S₀ and exited S₁ and T₁ states leading to the small S₁–T₁ triplet gap and efficient thermally-activated delayed fluorescence. Moreover, complex 4 demonstrates reversible mechanochromic and excitation dependent luminescence.

New highly emissive copper(i) complexes based on 3/4-pyridyltriazole have been synthesized and fully characterized. Photophysical properties and the mechanism of photo- and mechanochromic and excitation dependent luminescence are discussed.     

8,8′-(Benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(quinolin-4(1H)-one): a twisted photosensitizer with AIE properties

A new benzothiadiazole (BTZ) luminogen is prepared via the Suzuki–Miyaura Pd-catalysed C–C cross-coupling of 8-iodoquinolin-4(1H)-one and a BTZ bispinacol boronic ester. The rapid reaction (5 min) affords the air-, thermo-, and photostable product in 97% yield as a yellow precipitate that can be isolated by filtration. The luminogen exhibits aggregated-induced emission (AIE) properties, which are attributed to its photoactive BTZ core and nonplanar geometry. It also behaves as a molecular heterogeneous photosensitizer for the production of singlet oxygen under continuous flow conditions.

A new benzothiadiazole (BTZ) luminogen is prepared via the Suzuki–Miyaura Pd-catalysed C–C cross-coupling of 8-iodoquinolin-4(1H)-one and a BTZ bispinacol boronic ester.