Synthesis and Fluorescence Properties of Thiazole–Boron Complexes Bearing a β-Ketoiminate Ligand

Construction of Boron Difluoride Complexes with Asymmetric N,N'-Bidentate Ligands

Boron difluoride (BF2) complexes with asymmetrical N,N'-bidentate ligands have received increasing attention due to their fascinating properties and broad applications. They are generally constructed in two steps: ligand formation, followed by boron complexation. This review focuses on categorizing these BF2 complexes based on the key synthetic strategies that have been applied in the ligand formation steps. The post-functionalization, properties and applications of different types of BF2 complexes are presented. Their challenges and opportunities are also discussed. This should help the future rational design and synthesis of BF2 complexes with intriguing properties and practical applications.

Phosphine vs DBU-Catalyzed Annulation Reactions of β'-Acetoxy Allenoates with Acyl-Tethered Benzothiazole Bisnucleophiles: (4 + 3) or (4 + 1) vs (3 + 3) Annulation

Dearomative annulation reaction of acyl-tethered benzothiazole bisnucleophiles with β'-acetoxy allenoates by switching the Lewis base is developed. The DBU-catalyzed reaction gives benzothiazole-fused 1,4-dihydropyridine carboxylates by (3 + 3) annulation chemoselectively. By contrast, the PR3-catalyzed reaction gives benzothiazole-fused azepines by (4 + 3) annulation and cyclopentene carboxylates by (4 + 1) annulation; the ratio of the latter two products depends on the solvent. A possible rationale for the difference in the reactivity, based on the 1,4/1,5-addition of the 2-acyl-tethered benzothiazole to the key phosphonium intermediate, is provided.

Mechanistic insights into diversified photoluminescence behaviours of BF2 complexes of N-benzoyl 2-aminobenzothiazoles

Many BF2 complexes of heteroaromatics are well known for their dual-state emission (DSE) properties. However, AIE and ACQ effects have also been observed in certain cases. To date, no rational explanations have been proposed for these uncommon photoluminescence (PL) behaviours. The current research prepared four BF2 complexes of N-benzoyl 2-aminobenzothiazoles with diversified photoluminescence (PL) properties as model compounds and utilized quantum chemical calculation tools to address this issue. Theoretical calculations revealed that the electron-donating groups (EDGs) at the para-position of the exocyclic phenyl ring exert significant influence on their ground-state electronic structures and vertical excitation features. Potential energy curve (PEC) analysis showed that the exocyclic phenyl ring and NMe2 could not function as effective rotors due to elevated energy barriers. Only the NPh2 of BFBB-3 could spontaneously rotate ∼60° to induce the formation of an emissive twisted intramolecular charge transfer (TICT) state. The two-channel model involving both vibronic relaxation and S0/S1 surface crossing revealed that the drastic narrowing of the S1/S0 energy gap in the region approaching minimun energy conical intersection (MECI) led to the generation of a dark state in BFBB-1. The small energy barrier to access the dark-state region makes the resulting fast internal conversion a competitive channel for excited-state deactivation. In contrast, the presence of EDGs in BFBB-2 and 4 inhibits this pathway, thereby resulting in intense fluorescence emissions in solution. In addition, crystallographic analysis illustrated that the F atoms perpendicular to the polyheterocycle promoted a slipped face-to-face packing mode and enhanced intermolecular interactions. The efficiencies of their solid-state emissions are mainly affected by the degree of π-π overlaps.

Synthesis, Characterizations, and Quantum Chemical Investigations on Imidazo[1,2-a]pyrimidine-Schiff Base Derivative: (E)-2-Phenyl-N-(thiophen-2-ylmethylene)imidazo[1,2-a]pyrimidin-3-amine

In this study, (E)-2-phenyl-N-(thiophen-2-ylmethylene)imidazo[1,2-a]pyrimidin-3-amine (3) is synthesized, and detailed spectral characterizations using 1H NMR, 13C NMR, mass, and Fourier transform infrared (FT-IR) spectroscopy were performed. The optimized geometry was computed using the density functional theory method at the B3LYP/6-311++G(d,p) basis set. The theoretical FT-IR and NMR (1H and 13C) analysis are agreed to validate the structural assignment made for (3). Frontier molecular orbitals, molecular electrostatic potential, Mulliken atomic charge, electron localization function, localized orbital locator, natural bond orbital, nonlinear optical, Fukui functions, and quantum theory of atoms in molecules analyses are undertaken and meticulously interpreted, providing profound insights into the molecular nature and behaviors. In addition, ADMET and drug-likeness studies were carried out and investigated. Furthermore, molecular docking and molecular dynamics simulations have been studied, indicating that this is an ideal molecule to develop as a potential vascular endothelial growth factor receptor-2 inhibitor.

Synthesis, Characterization, Antimicrobial, Density Functional Theory, and Molecular Docking Studies of Novel Mn(II), Fe(III), and Cr(III) Complexes Incorporating 4-(2-Hydroxyphenyl azo)-1-naphthol (Az)

This work synthesized three new CrAz₂, MnAz₂, and FeAz₂ complexes and investigated them using IR, mass, UV spectroscopy, elemental analysis, conductivity and magnetic tests, and thermogravimetric analysis. The azo-ligand, 4-(2-hydroxyphenylAzo)-1-naphthol (Az), couples with metal ions via its nitrogen (in -N=N- bonds) and oxygen (in hydroxyl group) atoms, according to the IR spectra of these complexes. Through thermal examination (TG/TGA), the number and location of water in the complexes were also determined. Density functional theory (DFT) theory is applied to ameliorate the structures of the ligand (Az) and metal complexes and analyze the quantum chemical characteristics of these complexes. The antifungal and antibacterial activity of the ligand and its complexes opposed to several hazardous bacteria and fungi was investigated in vitro. Metal complexes were discovered to have a higher inhibitory impact on some organisms than the free ligand. The MnAz₂ complex exhibited the best activity among the studied materials, whereas the CrAz₂ complex had the lowest. The compounds' binding affinity to the E. coli (PDB ID: 1hnj) structure was predicted using molecular docking. Binding energies were calculated by analyzing protein-substrate interactions. These encouraging findings imply that these chemicals may have physiological effects and may be valuable for a variety of medical uses in the future.

Recent Advances in the Synthesis of Borinic Acid Derivatives

Borinic acids [R2B(OH)] and their chelate derivatives are a subclass of organoborane compounds used in cross-coupling reactions, catalysis, medicinal chemistry, polymer or optoelectronics materials. In this paper, we review the recent advances in the synthesis of diarylborinic acids and their four-coordinated analogs. The main strategies to build up borinic acids rely either on the addition of organometallic reagents to boranes (B(OR)3, BX3, aminoborane, arylboronic esters) or the reaction of triarylboranes with a ligand (diol, amino alcohol, etc.). After general practical considerations of borinic acids, an overview of the main synthetic methods, their scope and limitations is provided. We also discuss some mechanistic aspects.

The Synthesis of Picolinamide-Supported Tetracoordinated Organoboron Complexes with Aggregation-Induced Emission Property

The picolinamide-supported tetracoordinated organoboron complexes containing diaryl boronyl segments have been synthesized for the first time. Aryl trifluoroborates were utilized as the BAr₂ sources to introduce different aryl motifs with diverse functional groups. The optical experiments discovered these five-membered boron-containing complexes were aggregation-induced emission (AIE) active, thus affording a new class of AIE molecules.

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.

AIE-Active Difluoroboron Complexes with N,O-Bidentate Ligands: Rapid Construction by Copper-Catalyzed C-H Activation

The development of organic materials with high solid-state luminescence efficiency is highly desirable because of their fundamental importance and applicability in optoelectronics. Herein, a rapid construction of novel BF2 complexes with N,O-bidentate ligands by using Cu(BF4 )2 •6H2 O as a catalyst and BF2 source is disclosed, which avoids the need for pre-composing the N,O-bidentate ligands and features a broad substrate scope and a high tolerance level for sensitive functional groups. Moreover, molecular oxygen is employed as the terminal oxidant in this transformation. A library of 36 compounds as a new class of BF2 complexes with remarkable photophysical properties is delivered in good to excellent yields, showing a substituent-dependency on the photophysical properties, derived from the π-π* character of the photoexcited state. In addition, aggregation-induced emission (AIE) is observed and quantified for the brightest exemplars. The excited state properties are fully investigated in solids and in THF/H2 O mixtures. Hence, a new series of photofunctional materials with variable photophysical properties is reported, with potential applications for sensing, bioimaging, and optoelectronics.

Substituent and Solvent Polarity on the Spectroscopic Properties in Azo Derivatives of 2-Hydroxynaphthalene and Their Difluoroboranes Complexes

Novel fluorescent dyes such as difluoroborane complexes of 1-phenylazonaphthalen-2-ol derivatives were successfully synthesized and characterized with a focus on the influence of a substituent and a solvent on the basic photophysical properties. ¹H, ¹¹B, ¹³C, ¹⁵N, and ¹⁹F nuclear magnetic resonance (NMR) spectra of substituted 1-phenylazonaphthalen-2-ol difluoroboranes and their parent azo dyes were recorded and discussed. The absorption and emission properties of synthesized compounds were investigated in solvents of varying polarity. They were found to be fluorescent despite the presence of the azo group. The azo group rotation was blocked by complexing with -BF₂ to get a red shift in absorption. Solvent-dependent spectral properties of compounds were investigated using Lipper-Mataga and Bakhshiev plot. The calculated DFT energies and Frontier Molecular Orbitals calculations of the studied compounds were proved to be consistent with the experimental observations.

Supramolecular polymorphism in aggregates of a boron-difluoride complex of peri-naphthoindigo via solvent- and pathway-dependent self-assembly

Supramolecular polymorphism, a rare phenomenon, has been demonstrated using BF2-coordinated peri-naphthoindigo.

A novel boron ketoiminate-based conjugated polymer with large Stokes shift: AIEE feature and cell imaging application

A novel π-extended boron ketoiminate-based conjugated polymer with a typical AIEE feature has been successfully synthesized and used for cell imaging application.

Substituted 2-Phenacylbenzoxazole Difluoroboranes: Synthesis, Structure and Properties

Novel fluorescent dyes such as benzoxazole-boron complexes, bearing β-ketoiminate ligands, have been synthesized and characterized with a focus on the influence of a substituent on the basic photophysical properties. ¹H, ¹¹B, ¹³C, ¹⁵N, and ¹⁹F nuclear magnetic resonance (NMR) spectra of substituted 2-phenacylbenzoxazole difluoroboranes have been recorded and discussed. It is worth mentioning that a high correlation coefficient was found between ¹⁵N-NMR parameters and substituent constants. The photophysical properties of these new dyes have been investigated by fluorescence and ultraviolet-visible (UV-Vis) absorption spectroscopy. The geometry optimization, vibrational spectra, and the HOMO and LUMO energies were calculated based on density functional theory with the use of the B3LYP functional and 6-311++G(d,p) basis set.

Density Functional Studies on Photophysical Properties of Boron-Pyridyl-Imino-Isoindoline Dyes: Effect of the Fusion

In this work, to make out the aryl-fusion effect on the photophysical properties of boron-pyridyl-imino-isoindoline dyes, compounds 1-5 were theoretically studied through analyses of their geometric and electronic structures, optical properties, transport abilities, and radiative (k r) and non-radiative decay rate (k nr) constants. The highest occupied molecular orbitals of aryl-fused compounds 2-5 are higher owing to the extended conjugation. Interestingly, aryl fusion in pyridyl increases the lowest unoccupied molecular orbital (LUMO) level, while isoindoline decreases the LUMO level; thus, 4 and 5 with aryl fusion both in pyridyl and isoindoline exhibit a similar LUMO to 1. Compounds 4 and 5 show relatively low ionization potentials and high electron affinities, suggesting a better ability to inject holes and electrons. Importantly, the aryl fusion is conducive to the decrease of k IC. The designed compound 5 exhibits a red-shifted emission maximum, low λh, and low k IC, which endow it with great potential for applications in organic electronics. Our investigation provides an in-depth understanding of the aryl-fusion effect on boron-pyridyl-imino-isoindoline dyes at molecular levels and demonstrates that it is achievable.

Synthesis, photophysical and electrochemical properties of donor–acceptor type hydrazinyl thiazolyl coumarins

A practical green MCR strategy has been developed for the synthesis of D–A type hydrazinyl thiazolyl coumarins catalyzed by an environmentally compatible MMT K10 clay in water at RT.

Recent Progress of BODIPY Dyes With Aggregation-Induced Emission

With the development of organic optoelectronic materials and bioimaging technology, to exploit organic luminescent materials with high luminescent efficiency in aggregation-state has become a research hotspot. BODIPYs have become one of the research objects of this kind of material because of their obvious advantages. This review focuses on the design and synthesis of AIE-type BODIPYs, the mechanism of AIE properties and their applications in recent years. Through classification, analysis, and summary, this review aims to explore the structure-activity relationship of AIE-type BODIPYs and to provide ideas for the further design and potential applications of AIE-active fluorescent materials.

Fluorescent Probes from Aromatic Polycyclic Nitrile Oxides: Isoxazoles versus Dihydro-1λ3,3,2λ4-Oxazaborinines

Anthracenenitrile oxide undergoes 1,3‐dipolar cycloaddition reaction with propargyl bromide affording the expected isoxazole as single regioisomer, suitably synthetically elaborated and functionalized with a protected triple bond. The introduction of a bromine atom at the position C10 of the anthracene moiety allows for inserting a variety of aromatic and heterocyclic substituents through Suzuki coupling. A two‐way synthetic route can lead to simple isoxazole derivatives or, after N−O bond reductive cleavage and BF₃ complexation, enamino ketone boron complexes. The photophysical properties of both the substituted isoxazoles and the corresponding boron complexes were investigated to show the potentialities for the employment as fluorescent tags in imaging techniques. The quite good quantum yield values confirm the suitability of these compounds in the cellular environment. Scope and limitations of the methodology are discussed.

Scope and Limitations of Boron Fluorescent Complexes from Stable Nitrile Oxides in ABPP Assays

Anthracenenitrile oxide undergoes 1,3-dipolar cycloaddition reactions with 1-substituted-4-(prop-2-yn-1-yloxy)benzene affording the expected isoxazoles in good yields and as single regioisomers. N-O Bond cleavage and boron complexation afforded the corresponding boron complexes, derivatized with either a triple bond for click-chemistry applications or an oxime group for nitrile oxide 1,3-dipolar cycloaddition. The optical properties of these compounds show potential for employment as fluorescent tags in imaging techniques. The activity-based protein profiling assays showed good reactivity for the synthesized probes even with short peptide chains (epoxomicin). Scope and limitations are discussed in the light of the obtained results.

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.

Benzo[4,5]thiazolo[3,2- c][1,3,5,2]oxadiazaborinines: Synthesis, Structural, and Photophysical Properties

A family of highly emissive benzo[4,5]thiazolo[3,2- c][1,3,5,2]oxadiazaborinines, conjugated with the donor 4-dimethylaminophenyl group, was designed and synthesized. Their photophysical, both in solution and in the solid state, and structural properties were investigated. The influence of donor and acceptor substituents (R) in the benzothiazole unit on photophysical properties of complexes was found out. The tetrafluorobenzothiazole analogue exhibits nonbonded nuclear spin-spin coupling between fluorines from the BF₂ group and α-fluorine atom at the benzene ring. Additionally, this boron complex demonstrates a comparatively high solid-state fluorescence quantum yield (Φ = 0.34).

Centrosymmetric Binuclear Boron Compounds Derived from Dithiooxamides: Synthesis, Characterization, and Their Photophysical Properties

In this paper, we report the synthesis and characterization of new boron compounds derived from dithiooxamides. The compounds were characterized by NMR (1H and 13C), UV-vis, fluorescence spectroscopy, and high resolution mass spectrometry. The crystal structure of the mononuclear boron compound was determined by single-crystal X-ray diffraction analysis. The photophysical properties of the boron compounds were investigated, and we found moderate fluorescence emission (compound 2 ΦF: 4.07% and compound 4 ΦF: 2.89%). We also observed that the mononuclear complex presented greater stability. Compound 4 showed interesting luminescent properties; in solid state, it exhibited an increase in fluorescence by mechanostimuli by changing to a bright red color, and also in solution, it showed a decrease in fluorescence intensity when oxygen and air were supplied to the solution.

N,O π-Conjugated 4-Substituted 1,3-Thiazole BF2 Complexes: Synthesis and Photophysical Properties

A series of 1,3-thiazole-based organoboron complexes has been designed and synthesized by acylation of 2-amino 4-subsituted 1,3-thiazoles with (4-dimethylamino)benzoyl chloride and the subsequent BF₂ complexation reaction. The influence of substituents in position 4 of the thiazole ring on photophysical properties of the complexes has been investigated. Synthesized thiazolo[3,2-c][1,3,5,2]oxadiazaborinines mainly showed intensive fluorescence in solutions. Complex with a 4,5-unsubstituted thiazole unit demonstrated an aggregation induced emission (AIE) effect and a very high fluorescent quantum yield (94%) in the solid state because of the inhibition of π-π/π-n interactions in the molecular packing.

The inherent mechanism of mechanochromism under different stress: electron cloud density distribution, J-type stacking, pore structure and collapse of J-type stacking

TPEDKBF2OMe, TPEDKBF2OBu and TPEDKBF2OHe exhibit high contrast mechanochromism after being ground and smashed.

Access to Fully Substituted Thiazoles and 2,3-Dihydrothiazoles via Copper-Catalyzed [4 + 1] Heterocyclization of α-(N-Hydroxy/aryl)imino-β-oxodithioesters with α-Diazocarbonyls

An efficient chemoselective practical route to fully substituted thiazoles and 2,3-dihydrothiazoles has been devised by [4 + 1] heterocyclization of α-(N-hydroxy/aryl)imino-β-oxodithioesters with in situ generated Cu-carbenoids of diazocarbonyls. The α-(N-hydroxy/aryl)imino-β-oxodithioesters are readily accessible by the reaction of β-oxodithioesters with nitrous acid/nitrosoarenes. The overall transformation involves sequential N-O/C-N bonds cleavage followed by cascade C-N/C-S bonds formation in one-pot. This new strategy allows full control over the introduction of various sensitive functional groups at different positions of the thiazole ring, broadening the arsenal of synthetic methods to obtain such scaffolds.

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.

Hexaphenylbenzene-based fluorescent aggregates for detection of zinc and pyrophosphate ions in aqueous media: tunable self-assembly behaviour and construction of a logic device

The aggregates of HPB derivative 7 exhibited “on–on” response towards Zn²⁺ ions and this in situ prepared 7-Zn2+ ensemble was utilized as a “not quenched” probe for detection of PPi ions in aqueous media.

Tetraphenylethene modified β-ketoiminate boron complexes bearing aggregation-induced emission and mechanofluorochromism

Two new thiazole-based β-ketoiminate boron complexes, namelyTTPE-HandTTPE-CN, were designed and successfully synthesized. The two compounds exhibited strong aggregation-induced emission (AIE) and evident mechanofluorochromic (MFC) properties.

Synthesis, computational, and spectroscopic analysis of tunable highly fluorescent BN-1,2-azaborine derivatives containing the N-BOH moiety

Methods to tune the luminescence wavelength and the quantum yields by controlling the power and location of electron-donor and acceptor substituents on the ring system.

Synthesis and fluorescence properties of novel squarylium–boron complexes

Novel squarylium–boron complexes showed a significantly enhanced fluorescence quantum yield compared to the corresponding uncomplexed squarylium dyes.

Thienyl Difluoroboron β-Diketonates in Solution and Polylactide Media

Difluoroboron β-diketonates have impressive optical properties in both solution and the solid state. In particular, both fluorescence and room-temperature phosphorescence are present when the dyes are confined to a rigid matrix, such as poly(lactic acid) (PLA). To expand current knowledge and color range capabilities of this unique type of multi-emitting chromophore, a series of thienyl-substituted BF₂bdk complexes have been synthesized. The photophysical properties were investigated in methylene chloride solution and in the solid state as dye/PLA blends. By varying donor ability, i.e. methyl, phenyl, methoxyl, and thienyl substituents, and by changing the dye loading in the PLA media (0.1-10% dye loading) red-shifted emission was achieved, important for biological imaging applications. In dilute CH₂Cl₂ solution, complexes exhibited absorptions ranging from 350 - 420 nm, solid-state fluorescence in PLA ranging from 390 - 500 nm, and oxygen sensitive phosphorescence ranging from 540 - 585 nm in PLA blends. Promising candidates as dye/PLA blends serve as models for dyepolymer conjugates for application as biological oxygen nanoprobes.

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.

High-contrast mechanochromism and polymorphism-dependent fluorescence of difluoroboron β-diketonate complexes based on the effects of AIEE and halogen

A series of AIEE-active difluoroboron β-diketonate complexes derivatives exhibit high-contrast mechanochromism, polymorphism-dependent fluorescence, and extremely high fluorescent quantum yields which could be as promising stimuli-responsive materials.

Fluorophores based on a minimal thienylthiazole core: towards multifunctional materials with solid state red emissions, solvatochromism and AIE behaviour

Minimal architecture solid red emissive D–A–D–A tetrads were built on a thienylthiazole core and their photophysical properties were investigated.

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.