The effect of locking π-conjugation in organoboron moieties in the structures of luminescent tetracoordinate boron complexes

Mono- and Bis-Phosphine Promoted Incorporation of Boron, Nitrogen, and Phosphorus into Heterocycles via Staudinger Reactions of Borafluorene Azides

We report the synthesis and characterization of a series of BNP-incorporated borafluorenate heterocycles formed via thermolysis reactions of pyridylphosphine and bis(phosphine)-coordinated borafluorene azides. The use of diphenyl-2-pyridylphosphine (PyPh2P), trans-1,2-bis(diphenylphosphino)ethylene (Ph2P(H)C═C(H)PPh2), and bis(diphenylphosphino)methane (Ph2PC(H2)PPh2) as stabilizing ligands resulted in Staudinger reactions to form complex heterocycles with four- (BN2P, BNPC, P2N2) and five-membered (BNP2C and BN2PC) rings, which were successfully isolated and fully characterized by multinuclear NMR and X-ray crystallography. However, when bis(diphenylphosphino)benzene (Ph2P-Ph-PPh2) was used as the ligand in a reaction with 9-bromo-9-borafluorene (BF-Br), due to the close proximity of the donor P atoms, the diphosphine-stabilized borafluoronium ion with an unusual borafluorene dibromide anion was formed. Reaction of the borafluoronium ion with trimethylsilyl azide left the cation intact, and the dibromide anion was substituted by a diazide. Density functional theory calculations were used to provide mechanistic insight into the formation of these new boracyclic compounds. This work highlights a new method in which donor phosphine ligands may be used to promote dimerization, cyclization, and ring contraction reactions to produce boracycles via Staudinger reactions.

Difluoroboron Complexes Based on Benzimidazole-Phenolates as Blue Emitters

Novel four-coordinated boron complexes (1-5) were synthesized via a reaction between BF3·CH3OH and benzimidazole-phenolate ligands (L1-L5), which are N,O-donors. These complexes exhibit intense blue emission in the solution and solid states accompanied by notable fluorescence quantum yields (ΦF). The study of the structure-property relation, through theoretical and experimental approaches, revealed a distinctive trend where compounds incorporating electron-donating substituents (methyl and ethoxy groups) in the phenolate moiety manifest shifts in emission wavelengths across the blue spectrum, concomitant with an increase in ΦF. Furthermore, the incorporation of an aromatic ring into the benzimidazole moiety considerably intensifies the rate of radiative relaxation from excited states. Notably, in the solid phase, either as a crystalline powder or loaded into polymer films, these modified complexes maintain or even surpass ΦF values observed in molecular solutions, ranging from 0.18 to 0.57, depending on the substitution. This characteristic makes these compounds attractive for applications in optoelectronics. All of the compounds were characterized using 1H, 13C, 11B, and 19F NMR, elemental analysis, and the molecular structures were corroborated through single-crystal X-ray diffraction analysis. Computational calculations via time-dependent density functional theory further elucidate the tunability of optical bandgaps through group substitution on ligands, aligning well with experimental observations.

Highly electron-deficient 3,6-diaza-9-borafluorene scaffolds for the construction of luminescent chelate complexes

The synthesis and characterization of two fluorinated 3,6-diaza-9-hydroxy-9-borafluorene oxonium acids featuring improved hydrolytic stability and the strong electron-deficient character of the diazaborafluorene core is reported. These boracycles served as precursors of fluorescent spiro-type complexes with (O,N)-chelating ligands which revealed specific properties such as delayed emission, white light emission in the solid state and photocatalytic performance in singlet oxygen-mediated oxidation reactions.

Photochemically and Thermally Generated BN-Doped Borafluorenate Heterocycles via Intramolecular Staudinger-Type Reactions

A series of BN-incorporated borafluorenate heterocycles, bis(borafluorene-phosphinimine)s (11-15), have been formed via intramolecular Staudinger-type reactions. The reactions were promoted by light or heat using monodentate phosphine-stabilized 9-azido-9-borafluorenes (R3P-BF-N3; 6-10) and involve the release of dinitrogen (N2), migration of phosphine from boron to nitrogen, and oxidation of the phosphorus center (PIII to PV). Density functional theory (DFT) calculations provide mechanistic insight into the formation of these compounds. Compounds 11-15 are blue emissive in the solution and solid states with absolute quantum yields (ΦF) ranging from 12 to 68%.

Luminescent hexagonal microtubes prepared through water-induced self-assembly of a polymorphic organoboron compound: formation mechanism and waveguide behaviour

Tetra-coordinated organoboron (TCOB) compounds are promising candidates for developing high-performance optical devices due to their excellent optoelectronic performance. Fabricating TCOB-based nanomaterials of controlled and defined morphology through rapid and easy-to-execute protocols can significantly accelerate their practical utility in the aforesaid applications. Herein, we report water-induced self-assembly (WISA) to convert a polymorphic TCOB complex (HNBI-B, derived from a 2-(2'-hydroxy-naphthyl)-benzimidazole precursor) into two unique nanomorphologies viz. nanodiscoids (NDs) and fluorescent microtubes with hexagonal cross-sections (HMTs). Detailed electron microscopic investigations revealed that oriented assembly and fusion of the initially formed NDs yield the blue emissive HMTs (SSQY = 26.7%) that exhibited highly promising photophysical behaviour. For example, the HMTs outperformed all the crystal polymorphs of HNBI-B obtained from CHCl3, EtOAc and MeOH in emissivity and also exhibited superior waveguide behaviour, with a much lower optical loss coefficient α' = 1.692 dB mm-1 compared to the rod-shaped microcrystals of HNBI-B obtained from MeOH (α' = 1.853 dB mm-1). Thus, this work reports rapid access to high performance optical nanomaterials through WISA, opening new avenues for creating useful nanomaterial morphologies with superior optical performance.

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.

Four-Coordinate Monoboron Complexes with 8-Hydroxyquinolin-5-Sulfonate: Synthesis, Crystal Structures, Theoretical Studies, and Luminescence Properties

8-Hydroxyquinolin-5-sulfonic acid (8HQSA) was combined with 3-pyridineboronic acid (3PBA) or 4-pyridineboronic acid (4PBA) to give two zwitterionic monoboron complexes in crystalline form. The compounds were characterized by elemental analysis, single-crystal X-ray diffraction studies, and IR, 1H NMR, UV-Visible, and luminescence spectroscopy. The analyses revealed compounds with boron atoms adopting tetrahedral geometry. In the solid state, the molecular components are linked by charge-assisted (B)(O-H···−O(S) and N+-H···O(S) hydrogen bonds aside from C-H···O contacts and π···π interactions, as shown by Hirshfeld surface analyses and 2D fingerprint plots. The luminescence properties were characterized in terms of the emission behavior in solution and the solid state, showing emission in the bluish-green region in solution and large positive solvatofluorochromism, caused by intramolecular charge transfer. According to TD-DFT calculations at the M06-2X/6-31G(d) level of theory simulating an ethanol solvent environment, the emission properties are originated from π-π * and n-π * HOMO-LUMO transitions.

Recent advances in the synthesis of luminescent tetra-coordinated boron compounds

Tetra-coordinated boron compounds offer a plethora of luminescent materials. Different chelation around the boron center (O,O-, N,C-, N,O-, and N,N-) has been explored to tune the electronic and photophysical properties of tetra-coordinated boron compounds. A number of fascinating molecules with interesting properties such as aggregation induced emission, mechanochromism and tunable emission by changing the solvent polarity were realised. Owing to their rich and unique properties, some of the molecules have shown applications in making optoelectronic devices, probes and so on. This perspective provides an overview of the recent developments of tetra-coordinated boron compounds and their potential applications.

Boranils: Versatile Multifunctional Organic Fluorophores for Innovative Applications

Multifunctional stimuli-responsive fluorophores showing bright environment-sensitive emissions have fueled intense research due to their innovative applications in the fields of biotechnologies, optoelectronics, and materials. A strong structural diversity is observed among molecular materials, which has been enriched over the years with a growing responsiveness to stimuli. Boron dipyrromethene (BODIPY) dyes have long been the flagship of emissive boron complexes due to their outstanding properties until a decade ago when analogues based on N^O, N^N, or N^C π-conjugated chelates emerged. The finality of developing borate dyes was to compensate for BODIPYs’ lack of solid-state fluorescence and small Stokes shifts while keeping their excellent optical properties in solution. Among them, the borate complexes based on a salicylaldimine ligand, called by the acronym boranils appear as the most promising, owing to their facile synthesis and dual-state emission properties. Boranil dyes have proven to be good alternatives to BODIPY dyes and have been applied in applications such as bioimaging, bioconjugation, and detection of biosubstrates. Meanwhile, ab initio calculations have rationalized experimental results and provided insightful feedback for future designs. This review article aims at providing a concise yet representative overview of the chemistry around the boranil core with the subsequent applications.

Heavy-Atom Free spiro Organoboron Complexes As Triplet Excited States Photosensitizers for Singlet Oxygen Activation

Herein, we present a new strategy for the development of efficient heavy-atom free singlet oxygen photosensitizers based on rigid borafluorene scaffolds. Physicochemical properties of borafluorene complexes can be easily tuned through the choice of ligand, thus allowing exploration of numerous organoboron structures as potent ¹O₂ sensitizers. The singlet oxygen generation quantum yields of studied complexes vary in the range of 0.55-0.78. Theoretical calculations reveal that the introduction of the borafluorene moiety is crucial for the stabilization of a singlet charge transfer state, while intersystem crossing to a local triplet state is facilitated by orthogonal donor-acceptor molecular architecture. Our study shows that quantitative oxidation of selected organic substrates can be achieved in 20-120 min of irradiation with only 0.05 mol % loading of a photocatalyst.

Stabilization of the Elusive 9-Carbene-9-Borafluorene Monoanion

Two-electron reduction of carbene-supported 9-bromo-9-borafluorenes with excess KC₈ , Na, or Li-naphthalenide affords six N-heterocyclic carbene (NHC)- or cyclic(alkyl)(amino) carbene (CAAC)-stabilized borafluorene anions (3-8)-the first isolated and structurally authenticated examples of the elusive 9-carbene-9-borafluorene monoanion. The electronic structure, bonding, and aromaticity of the boracyclic anions were comprehensively investigated via computational studies. Compounds 5 and 8 react with metal halides via salt elimination to give new B-E (E=Au, Se, Ge)-containing materials (9-12). Upon reaction with diketones, the carbene ligand cleanly dissociates from 5 or 8 to yield new B-O containing spirocycles (13-14) that cannot be easily obtained using "normal" valent borafluorene compounds. Collectively, these results support the notion that carbene-stabilized monoanionic borafluorenes may serve as a new platform for the one-step construction of higher-value boracyclic materials.

New luminescent tetracoordinate boron complexes: an in-depth experimental and theoretical characterisation and their application in OLEDs

New luminescent 2-iminopyrrolyl boron complexes with different BX2 moieties are extensively studied via complementary experimental and theoretical methodologies, including application in OLEDs.

(Hetero)arene-fused boroles: a broad spectrum of applications

(Hetero)arene-fused boroles are a class of compounds containing a 5-membered boron diene-ring. Based on their molecular framework, the (hetero)arene-fused boroles can be considered as boron-doped polycyclic antiaromatic hydrocarbons and are thus of great interest. Due to the vacant p_z orbital on the 3-coordinate boron atom, the antiaromaticity and strain of the 5-membered borole ring, (hetero)arene-fused boroles possess strong electron accepting abilities and Lewis acidity. By functionalization, they can be tuned to optimize different properties for specific applications. Herein, we summarize synthetic methodologies, different strategies for their functionalization, and applications of (hetero)arene-fused boroles.

(Hetero)arene-fused boroles, ‘antiaromatic’ 2n-electron π-systems, more stable and more functionalizable than boroles, offer greater potential for a variety of applications.

The Design Strategy for an Aggregation- and Crystallization-Induced Emission-Active Molecule Based on the Introduction of Skeletal Distortion by Boron Complexation with a Tridentate Ligand

We describe here a new design strategy for obtaining boron complexes with aggregation- and crystallization-induced emission (AIE and CIE, respectively) properties based on the introduction of skeletal distortion. According to our recent results, despite the fact that an almost planar structure and robust conjugation were obtained, the boron azomethine complex provided a slight emission in solution and an enhanced emission in aggregation and crystal. Quantum calculation results propose that unexpected emission annihilation in solution could be caused through intramolecular bending in the excited state. Herein, to realize this unique molecular motion and obtain AIE and CIE molecules, the phenyl quinoline-based boron complexes BPhQ and BPhQm with distorted and planar structures were designed and synthesized, respectively. BPhQm showed emission in solution and aggregation-caused quenching (ACQ, BPhQm: ΦF,sol. = 0.21, ΦF,agg. = 0.072, ΦF,cryst. = 0.051), while BPhQ exhibited a typical AIE and CIE (BPhQ: ΦF,sol. = 0.008, ΦF,agg. = 0.014, ΦF,cryst. = 0.017). The optical data suggest that a large degree of molecular motion should occur in BPhQ after photo-excitation because of the intrinsic skeletal distortion. Furthermore, single-crystal X-ray diffraction data indicate that the distorted π-conjugated system plays a positive role in presenting solid-state emission by inhibiting consecutive π–π interactions. We demonstrate in this paper that the introduction of the distorted structure by boron complexation should be a new strategy for realizing AIE and CIE properties.

Organolithium-Mediated Postfunctionalization of Thiazolo[3,2-c][1,3,5,2]oxadiazaborinine Fluorescent Dyes

An effective method for transition-metal-free postfunctionalization of thiazolo[3,2-c][1,3,5,2]oxadiazaborinine dyes via direct lithiation of the 1,3-thiazole ring was developed. The reaction allows valuable regioselective C-H modification of these N,O-chelated organoboron chromophores incorporating different groups, including C-, Hal-, Si-, S-, Se-, and Sn-substituents. As a result, a library of novel fluorescent 1,3-thiazole-based organoboron complexes has been synthesized and characterized. The influence of the donor/acceptor strength of the substituent E on the photophysical properties has been established. The compound with a bulky lipophilic substituent (SnBu₃) exhibits a relatively high solid-state photoluminescence quantum yield of 44%.