β‐Iminoenamine‐BF 2 Complexes: Aggregation‐Induced Emission and Pronounced Effects of Aliphatic Rings on Radiationless Deactivation

Molecular design and application of luminescent materials composed of group 13 elements with an aggregation-induced emission property

Complexation of π-conjugated ligands by metal or semimetal ions leads to the enhancement of the planarity and rigidity of π-conjugated systems. Boron, especially, has played a central role in the design of luminescent main-group complexes. However, these complexes still suffer the disadvantage of aggregation-caused quenching as well as typical organic fluorophores. It has recently been reported that some types of boron complexes exhibit the aggregation-induced emission (AIE) property. Moreover, AIE behavior from complexes and organometallic compounds composed of the other group 13 elements, such as aluminum and gallium, has emerged in this decade. These observations greatly encourage us to develop advanced functional materials based on the group 13 elements. Indeed, recent research has demonstrated that these classes of materials are potentially versatile scaffolds for constructing chromic luminophores, efficiently emissive π-conjugated polymers and so on. This review mainly describes AIE-active group 13 complexes with four-coordinate structures and their application as photo-functional materials. Proposed mechanisms of the origins of AIE behavior are briefly discussed.

Aggregation-Induced Emission Probes for Specific Turn-On Quantification of Bovine Serum Albumin

A series of aggregation-induced emission (AIE) fluorescence probes, coined 4H-pyrimido[2,1-b]benzothiazole derivatives, has been synthesized by Biginelli reactions. Utilizing spectroscopic techniques, their photophysical properties have been comprehensively investigated in working environment both in vitro and in vivo. Density functional theory (DFT) calculations were performed to better understand the mechanism of these probes. The interactions between 4H-pyrimido[2,1-b]benzothiazoles with different substituents and bovine serum albumin (BSA) were analyzed using UV-vis and fluorescence spectroscopy, synchronous fluorescence, and docking analysis. Studies found that 4H-pyrimido[2,1-b]benzothiazole could bind to bovine serum albumin (BSA) through a hydrogen bond and hydrophobic interactions, resulting in enhancement of fluorescence emission of probes 1a-6f and fluorescence quenching of BSA. Combined with cell imaging experiments, these provide information on potential effects of benzothiazoles on disease treatment.

Design, Synthesis, and Biological Evaluation of (E)-N'-((1-Chloro-3,4-Dihydronaphthalen-2-yl)Methylene)Benzohydrazide Derivatives as Anti-prostate Cancer Agents

Prostate Cancer (PCa) is the most frequently diagnosed cancer in men in their late '50s. PCa growth is mainly due to the activation of the androgen receptor by androgens. The treatment for PCa may involve surgery, hormonal therapy, and oral chemotherapeutic drugs. A structural based molecular docking approach revealed the findings of (E)-N'-((1-chloro-3,4-dihydronaphthalen-2-yl)methylene)benzohydrazide derivatives, where the possible binding modes of the compounds with protein (PDB ID: 3V49) are shown. The compounds (6a-k) were synthesized and characterized by using conventional methods. The compounds, 6g, 6j, and 6k were reconfirmed through single crystal X-ray diffraction (XRD). Further, the compounds (6a-k) and standard drug were evaluated against human prostate cancer cell lines, LNCaP and PC-3 and the non-cancerous cell line, 3T3. Among these compounds, 6g and 6j showed higher cytotoxicity, and 6g exhibited dose-dependent activity and reduced cell viability. The mechanism of action was observed through the induced apoptosis and was further confirmed by western blot and ELISA. Molecular dynamics simulation studies were carried out to calculate the interaction and the stability of the protein-ligand complex in motion. ADME properties were predicted for all the tested compounds. These findings may give vital information for further development.

Cooling-Induced NIR Emission Enhancement and Targeting Fluorescence Imaging of Biperylene Monoimide and Glycodendrimer Conjugates

Under high concentrations, strong pressure, and low temperature, fluorophores usually exhibit the fluorescence quenching phenomenon. Of significance, the development of aggregation-induced emission (AIE) and pressure-induced emission (PIE) fluorophores has perfectly prevented fluorescence quenching under high concentrations and strong pressure. However, cooling-induced fluorescence quenching in water is still an urgent problem. In this paper, cooling-induced emission (CIE) enhancement based on a biperylene monoimide (BPMI) derivative, BPMI-18Lac, with a conjugated lactose-based glycodendrimer was developed. BPMI-18Lac, as a non-AIE molecule, exhibited the CIE phenomenon with a fluorescent intensity increasing 7-fold when the temperature decreased from 80 to -40 °C. The mechanism was due to the inhibition of the intramolecular electron interactions between the perylene monoimide moieties linked by the C-C single bond. In addition, BPMI-18Lac, as a multivalent glycodendrimer, showed selective fluorescence imaging for HepG 2 cells through the ASGP receptor on the cell surface. Importantly, this work developed a water-soluble CIE molecule for potential application below freezing temperature.

Optoelectronic, Aggregation, and Redox Properties of Double-Rotor Boron Difluoride Hydrazone Dyes

We develop the chemistry of boron difluoride hydrazone dyes (BODIHYs) bearing two aryl substituents and explore their properties. The low-energy absorption bands (λ_max =427-464 nm) of these dyes depend on the nature of the N-aryl groups appended to the BODIHY framework. Electron-donating and extended π-conjugated groups cause a redshift, whereas electron-withdrawing groups result in a blueshift. The title compounds were weakly photoluminescent in solution and strongly photoluminescent as thin films (λ_PL =525-578 nm) with quantum yields of up to 18 % and lifetimes of 1.1-1.7 ns, consistent with the dominant radiative decay through fluorescence. Addition of water to THF solutions of the BODIHYs studied causes molecular aggregation which restricts intramolecular motion and thereby enhances photoluminescence. The observed photoluminescence of BODIHY thin films is likely facilitated by a similar molecular packing effect. Finally, cyclic voltammetry studies confirmed that BODIHY derivatives bearing para-substituted N-aryl groups could be reversibly oxidized (E_ox1 =0.62-1.02 V vs. Fc/Fc⁺ ) to their radical cation forms. Chemical oxidation studies confirmed that para-substituents at the N-aryl groups are required to circumvent radical decomposition pathways. Our findings provide new opportunities and guiding principles for the design of sought-after multifunctional boron difluoride complexes that are photoluminescent in the solid state.

Aggregation-induced emission enhancement (AIEE)-active boron-difluoride dyes with reversible mechanochromic fluorescence

A novel class of solid-emissive boron-difluoride derivatives, using phenanthrenequinone hydrazone as ligands, were designed and efficiently synthesized. These dyes exhibit weak fluorescence in dilute solutions, but much higher fluorescence efficiency in aggregate states with a large stokes shift (over 70 nm) due to the their aggregation-induced emission enhancement (AIEE) characteristics. According to their photophysical properties and X-ray single crystal structure analysis, the AIEE was ascribed to the H(J)-aggregate formation aided by multiple intermolecular interactions to restrict intramolecular motion in the solid state. Moreover, their solid emissions could be reversibly tuned between “on” and “off” by mechanical grinding and recrystallization, due to the stacking model transition between H(J)-aggregation with loose molecular packing and J-aggregation with intense intermolecular interactions.

A class of AIEE-active mechanochromic boron-difluorides were reported, and the mechanochromism was ascribed to the transition between H(J)-aggregation with loose molecular packing and J-aggregation with intense intermolecular interactions.

Nickel catalyzed synthesis of 4,4′-bichromenes/4,4′-bithiochromenes and their Atropisomerism

4,4′-bichromenes and 4,4′-bithiochromenes have been synthesized by a Ni catalyzed homocoupling method and their Atropisomeric behavior has been studied.

Iminoenamine based novel androgen receptor antagonist exhibited anti-prostate cancer activity in androgen independent prostate cancer cells through inhibition of AKT pathway

Treatment by androgen receptor (AR) antagonists is one of the regimens for prostate cancer. The prolonged treatment with AR antagonist leads to the expression of point mutation in the ligand binding domain of the AR. This point mutation causes resistance to AR antagonist by converting them into an agonist. The T887A mutated AR was frequently expressed in androgen independent prostate cancer (AIPC) patients. Through literature survey and molecular modelling, we have identified a novel AR antagonist having a bulky β-iminoenamine BF2 complex scaffold. The tested and standard ligands were screened in AR positive (LNCaP, MCF-7 and MDA-MB-453), AR negative (PC3), and non-cancerous (3T3) cell lines through anti-proliferation assay. The ligand, ARA3 was the most potent molecule among all the tested ligands and was 7.6 folds selective for AR positive cell lines. The mechanism of anti-prostate cancer activity of ARA3 was confirmed by western blot, qPCR, and apoptotic assays in LNCaP (T887A positive AR) cells. Structural activity relationship was derived by correlating the in-vitro and in-silico data. Consequently, we have identified the essential functional groups that could prevent the resistance concerning mutant AR. The ARA3 induces the apoptosis in AIPC cells by preventing the AR mediated activation of AKT pathway. The bicalutamide did not induce the apoptosis because it failed to prevent the AR mediated activation of AKT.

Aggregation induced emission enhancement (AIEE) characteristics of quinoline based compound - A versatile fluorescent probe for pH, Fe(III) ion, BSA binding and optical cell imaging

Novel benzimidazoquinoline derivative (AVT) was synthesized through a substitution reaction and characterized by various spectral techniques. Analyzing the optical properties of AVT under absorption and emission spectral studies in different environments exclusively with respect to solvents and pH, intriguing characteristics viz. aggregation induced emission enhancement (AIEE) in the THF solvent and 'On-Off' pH sensing were found at neutral pH. Sensing nature of AVT with diverse metal ions and bovine serum albumin (BSA) was also studied. Among the metal ions, Fe³⁺ ion alone tunes the fluorescence intensity of AVT probe in aqueous medium from "turn-on" to "turn-off" through ligand (probe) to metal charge transfer (LMCT) mechanism. The probe AVT in aqueous medium interacts strongly with BSA due to Fluorescence Resonance Energy Transfer (FRET) and the conformational change in BSA was further analyzed using synchronous fluorescence techniques. Docking study of AVT with BSA reveals that the active site of binding is tryptophan residue which is also supported by the experimental results. Interestingly, fluorescent AVT probe in cells was examined through cellular imaging studies using BT-549 and MDA-MB-231 cells. Thus, the single molecule probe based detection of multiple species and stimuli were described.

Boron Complexes of Glyoxal-Derived Ugi Adducts as a New Class of Aggregation-Induced Emission Photoluminescent Materials

A series of O,O-chelated boron complexes was prepared through a four-component Ugi reaction followed by complexation of the resulting 1,3-dicarbonyl compounds with boron trifluoride diethyl etherate. The optical properties of these novel luminophores were investigated by UV/Vis spectroscopy and spectrofluorometry, revealing pronounced aggregation-induced emission (AIE) features.

Development of solid-state emissive o-carboranes and theoretical investigation of the mechanism of the aggregation-induced emission behaviors of organoboron “element-blocks”

This paper presents the aggregation-induced emission (AIE) properties of o-carborane derivatives and proposes a potential strategy for constructing AIE-active organoboron complexes via the enhancement of freedom of intramolecular mobility. Initially, the optical properties of o-carborane derivatives with or without the fused ring structure at the C–C bond in o-carborane in which elongation should be induced by photo-excitation according to theoretical calculations were compared. Accordingly, it was shown that large mobility at the C–C bond in o-carborane should be responsible for the annihilation of emission in solution, leading to the AIE property. From this result, it was presumed that by enhancing the freedom of intramolecular mobility in conventional luminescent organoboron complexes, the deactivation of the excited state in solution and emission recovery in the aggregate can be induced. Based on this idea, we have performed several studies and introduce two representative results. Firstly, the decrease in luminescent properties of boron dipyrromethene (BODIPY) in solution by introducing a movable functional group is explained. Next, the AIE behaviors of boron ketoiminates and the potential mechanism concerning conformational changes for the deactivation of the excited state in the solution state are illustrated. It is proposed that enhancement of the freedom of mobility in the excited state of luminescent organoboron complexes could be a potential strategy for realizing AIE behaviors.

Nitrogen-Containing Analog of Dibenzoylmethanate of Boron Difluoride: Luminescence, Structure, Quantum Chemical Modeling, and Delay Fluorescence

Boron difluoride of 3-amino-1,3-diphenyl-2-propene-1-onate (1) has been synthesized and its crystal structure has been determined. The comparative studies of 1 and its oxygen analog 1,3-diphenyl-1,3-dionate (dibenzoylmethanate) of boron difluoride (2) have been performed using the methods of stationary and time-resolved spectroscopy and quantum chemical modeling. It was established that at the transition from solutions to crystals, a bathochromic shift of the spectra and a significant increase of luminescence intensity of 1 take place. The luminescent properties of solutions of 1 and 2 are similar. The peculiarities of crystal packings of 1 and 2 are responsible for differences in crystals luminescent properties. For crystals of 2, one observes the luminescence of J-aggregates and excimers, while for 1, in which a dimer is an elementary structural fragment, only the excimer luminescence is registered. A delayed excimer fluorescence of the P-type was observed for crystals of 1 and 2 at room temperature. The intensity of the delayed fluorescence of 1 is 300-fold higher than that of 2. Graphical Abstract Luminescence of J-aggregates and the formation of excimers in crystals of 1 and 2.

New AIE-active pyrimidine-based boronfluoride complexes with high solid-state emission and reversible mechanochromism luminescence behavior

New BF₂ complexes (PB-1–4) exhibiting intense fluorescence in their solid-state were developed. PB-4 exhibits a reversible mechanochromic response to grinding, and a distinct red-shift of emission is observed.

Detection of cyanide by a novel probe with a V-shaped structure based on aggregation of the probe adduct

A novel V-shaped probe is reported with extremely rapid response, high selectivity and very low detection limit to cyanide anions in aqueous solution based on the intensely emissive aggregates of the probe adduct.

Benzothiazole–pyrimidine-based BODIPY analogues: promising luminophores with fluorescence sensing and imaging ability and asymmetrization-induced solid-state emission

A series of boron difluoride dyes were demonstrated to possess intense emission both in solution and solid-state. Their fluorogenic sensing and imaging abilities were also investigated.

Blue highly fluorescent boron difluoride complexes based on phthalazine–pyridine

Synthesis and characterization of three blue fluorescent phthalazine–pyridine boron complexes with quantum yields up to 79%.

Tailoring Oxygen Sensitivity with Halide Substitution in Difluoroboron Dibenzoylmethane Polylactide Materials

The dual-emissive properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdkPLA) materials have been utilized for biological oxygen sensing. In this work, BF2dbm(X)PLA materials were synthesized, where X = H, F, Cl, Br, and I. The effects of changing the halide substituent and PLA polymer chain length on the optical properties in dilute CH2Cl2 solutions and solid-state polymer films were studied. These luminescent materials show fluorescence, phosphorescence, and lifetime tunability on the basis of molecular weight, as well as lifetime modulation via the halide substituent. Short BF2dbm(Br)PLA (6.0 kDa) and both short and long BF2dbm(I)PLA polymers (6.0 or 20.3 kDa) have fluorescence and intense phosphorescence ideal for ratiometric oxygen sensing. The lighter halide-dye polymers with hydrogen, fluorine, and chlorine substitution have longer phosphorescence lifetimes and can be utilized as ultrasensitive oxygen sensors. Photostability was also analyzed for the polymer films.

Oxygen Sensing Difluoroboron Dinaphthoylmethane Polylactide

Dual emissive luminescence properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdk-PLA) materials have been utilized as biological oxygen sensors. Dyes with red-shifted absorption and emission are important for multiplexing and in vivo imaging, thus hydroxyl-functionalized dinaphthoylmethane initiators and dye-PLA conjugates BF2dnm(X)PLA (X = H, Br, I) with extended conjugation were synthesized. The luminescent materials show red-shifted absorbance (~435 nm) and fluorescence tunability by molecular weight. Fluorescence colors range from yellow (~530 nm) in 10 - 12 kDa polymers to green (~490 nm) in 20 - 30 kDa polymers. Room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) are present under a nitrogen atmosphere. For the iodine-substituted derivative, BF2dnm(I)PLA, clearly distinguishable fluorescence (green) and phosphorescence (orange) peaks are present, making it ideal for ratiometric oxygen-sensing and imaging. Bromide and hydrogen analogues with weaker relative phosphorescence intensities and longer phosphorescence lifetimes can be used as highly sensitive, concentration independent, lifetime-based oxygen sensors or for gated emission detection. BF2dnm(I)PLA nanoparticles were taken up by T41 mouse mammary cells and successfully demonstrated differences in vitro ratiometric measurement of oxygen.

Study on the solution and solid-state fluorescence of novel BF2 complexes with (Z)-2-[phenanthridin-6(5H)-ylidene]-1-phenylethanone and its derivatives as ligands

A family of novel N,O-chelated BF₂ complexes with intense solution and solid-state emission were synthesized and their photophysical properties were studied.

High solid-state luminescence in propeller-shaped AIE-active pyridine–ketoiminate–boron complexes

Two pyridine-ketoiminate-based organoboron complexes were demonstrated to possess aggregation-induced emission, large Stokes shift and high quantum yield in the solid-state, which were rationalized through X-ray crystal analysis and electronic structure calculations.

Dipyrrolylquinoxaline difluoroborates with intense red solid-state fluorescence

Dipyrrolylquinoxalines and their BF₂ complexes were synthesized and characterized by their X-ray structural analysis, optical and electrochemical properties. These dyes exhibit strong broad absorption in the visible region and intensely tunable solid-state fluorescence.

How is the AIE mechanism profoundly changed in an ESIPT family: the novel introduction of a tetraphenylethene group onto (Z)-3-(quinolin-2-ylmethylene)-3,4-dihydroquinoxalin-2(1H)-one

The first example showing that the AIE mechanism can be completely changed from restriction of TICT to RIR by the introduction of a tetraphenylethene group onto a molecule with ESIPT character is reported herein.

Synthesis, structures and electrochemical and photophysical properties of anilido-benzoxazole boron difluoride (ABB) complexes

Four-ring-fused anilido-benzoxazole boron difluoride (ABB) dyes were synthesized, and exhibited very bright luminescence in solution (Φ_f = 0.45–0.96 in CH₂Cl₂) and in the solid-state (Φ_f = 0.07–0.37).

Facile synthesis of highly fluorescent BF2 complexes bearing isoindolin-1-one ligand

A new class of isoindolin-1-one based BF₂ complexes containing pyridine or benzothiazole groups has been prepared from a “one-pot” reaction.