Geometric and Electronic Structures of Boron(III)‐Cored Dyes Tailored by Incorporation of Heteroatoms into Ligands

Complexation of a boron atom with a series of bidentate heterocyclic ligands successfully gives rise to corresponding BF₂‐chelated heteroarenes, which could be considered as novel boron(III)‐cored dyes. These dye molecules exhibit planar structures and expanded π‐conjugated backbones due to the locked conformation with a boron center. The geometric and electronic structures of these BF₂ complexes can be tailored by embedding heteroatoms in the unique modes to form positional isomer and isoelectronic structures. The structure–property relationship is further elucidated by studying the photophysical properties, electrochemical behavior and quantum‐chemical calculations.     

Synthesis and Photovoltaic Properties of Boron β‐Ketoiminate Dyes Forming a Linear Donor‐π‐Acceptor Structure

Organoboron complexes are of interest as chromophores for dye sensitizers owing to their light‐harvesting and carrier‐transporting properties. In this study, compounds containing boron β‐ketoiminate (BKI) as a chromophore were synthesized and used as dye sensitizers in dye‐sensitized solar cells. The new dyes were orange or red crystals and showed maximum absorptions in the 410–450 nm wavelength region on titanium dioxide substrates. These electrodes exhibited maximum efficiencies of over 80% in incident photon‐to‐current conversion efficiency spectra, suggesting that the continuous process of light absorption‐excitation‐electron injection was effectively performed. Open‐circuit photovoltages were relatively high owing to the large dipole moments of the BKI dyes with a linear molecular structure. Thus, a maximum power conversion efficiency of 5.3% was successfully observed. Comparison of BKI dyes with boron β‐diketonate dyes revealed certain differences in solution stability, spectral properties, and photovoltaic characteristics.     

Ring Size Effects on Multi‐Stimuli Responsive Luminescent Properties of Cyclic Amine Substituted β‐Diketones and Difluoroboron Complexes

Emissive β‐diketones (bdks) and difluoroboron complexes (BF₂bdks) show multi‐stimuli responsive luminescence in both solution and the solid state. A series of bdk ligands and boron coordinated dyes were synthesized with different cyclic amine substituents in the 4‐position to explore ring size effects on various luminescent properties, including solvatochromism, viscochromism, aggregation‐induced emission (AIE), mechanochromic luminescence (ML) and halochromism. Red‐shifted absorption and emission were observed in CH₂Cl₂ for both bdk ligands and boron dyes with increasing substituent ring size. The compounds displayed bathochromic emission in more polar solvents, and higher fluorescence intensity in more viscous media. The AIE compounds exhibited enhanced emission when aggregated. For solid‐state properties, a large emission wavelength shift was shown for the piperidine substituted bdk after melt quenching on weighing paper. Large blue‐shifted emissions were observed in all the boron dye spin cast films after trifluoroacetic acid vapor annealing, and the original emissions were partially recovered after triethylamine vapor treatment.     

BODIPY chromophores as efficient green light sensitizers for lanthanide-induced near-infrared emission

A new boron dipyrromethene (BODIPY) modified 8-hydroxylquinoline ligand (8-HOQ-BODIPY) is synthesized for the sensitization of near-infrared emission of lanthanide(III) ions. The BODIPY unit, as revealed by single-crystal X-ray diffraction analysis, aligns almost perpendicularly to the 8-HOQ unit. The ligand exhibits strong absorption at ~506 nm and fluorescence at 510 nm in organic solvents with quantum yields ranging from ~0.45 in dichloromethane to 0.015 in ethanol. It forms stable ytterbium(III), erbium(III) and neodymium(III) complexes with 3:1 ligand-to-metal molar ratios. Upon excitation (~522 nm), the neodymium(III) and erbium(III) complexes emit weakly at 1060 and 1382 nm, respectively, whereas the ytterbium(III) complex exhibits strong emission at 976 and 1003 nm. The results demonstrate the potential of BODIPY dyes as efficient and robust visible light sensitizers for lanthanide-based NIR emitters in medical diagnosis.     

Synthesis of luminescent 2-(2'-hydroxyphenyl)benzoxazole (HBO) borate complexes

Complexation of boron trifluoride by a series of electron donor/acceptor substituted 2-(2'-hydroxy phenyl)benzoxazole (HBO) derivatives yields luminescent B(III) complexes with an emission wavelength ranging from 385 to 425 nm in dichloromethane or toluene. Appropriate chemical functionalization of these new dyes allows connection to different photoactive subunits (Boranil, BODIPY), endowing an efficient cascade energy transfer.     

Synthesis of Boron(III)‐Coordinated Subchlorophins and Their Peripheral Modifications

A pyrrole‐cleaving modification to transform boron(III) meso ‐triphenylsubporphyrin into boron(III) meso ‐triphenylsubchlorophin has been developed. Boron(III) subchlorophins thus synthesized show absorption and fluorescence spectra that are roughly similar to those of boron(III) subchlorins, but B ‐methoxy boron(III) subchlorophin showed considerably intensified fluorescence and a small Stokes shift. Peripheral modification reactions of B ‐phenyl boron(III) subchlorophin such as regioselective nitration with Cu(NO₃)₂⋅3 H₂O, ipso ‐substitution reactions of boron(III) α‐nitrosubchlorophin with CsF and CsCl, and Pd‐catalyzed cross‐coupling reactions of boron(III) α‐chlorosubchlorophin with arylacetylenes, have been also explored to tune the optical properties of subchlorophins.     

Efficient NIR‐Light Emission from Solid‐State Complexes of Boron Difluoride with 2′‐Hydroxychalcone Derivatives

This article describes a series of nine complexes of boron difluoride with 2′‐hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor–acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5 b , which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the π‐stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene‐containing dye 4 , for example, a fluorescence quantum yield of about 15 % with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3 a and acetonaphthone‐based compounds 1 b , 2 b , and 3 b showed no evidence of degradation as solutions in CH₂Cl₂ that contained EtOH. In particular, solutions of brightly fluorescent compound 3 a (brightness: ε×Φ_f=45 000 M ^?1 cm^?1) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid‐state NIR fluorophores for applications in materials science.     

1,3,2‐(2H)‐dioxaborine polymethine dyes on the base of dehydroacetic acid — effective fluorescent amine probes

A number of new polymethine dyes based on the boron difluoride complexes of dehydroacetic acid and 3‐acetyl‐4‐hydroxypyridone has been synthesized. Their spectral‐luminescent properties have been investigated. The reaction of obtained merocyanines with primary and secondary amines has been studied. These low fluorescent dyes gave highly fluorescent open chain products, which on further heating cyclized to weakly fluorescent pyridone dyes.     

BF2‐Chelate Complexes of 6‐(4‐Iodophenyl)‐2,3,4,8,9,10‐hexamethyldipyrrin and 2‐(4‐Iodobenzoyl)‐3,4,5‐trimethylpyrrole: Fluorescent Dyes with a Chemical Anchor Group

The one‐pot condensation/coordination reaction of 4‐iodobenzoylchloride, 2,3,4‐trimethylpyrrole and BF₃ × Et₂O yields the BF₂ chelate complexes of the 1:1 condensation product 2‐(4‐iodobenzoyl)‐3,4,5‐trimethylpyrrole and of the 1:2 product 6‐(4‐iodophenyl)‐2,3,4,8,9,10‐hexamethyldipyrrin, as separable compounds in 6 and 38 % yield, respectively. Both new boron derivatives are fluorescent already upon exitation with ambient light. While the fluorescence quantum yield of the benzoyl derivative is very low, this value is significantly higher for the related boron dipyrrin (BODIPY) derivative. Single crystal X‐ray diffraction studies of both compounds reveal that the reason for these deviating physical properties are structural in nature. For the BODIPY an essentially flat structure of the fluorophor has been established, in addition to restricted rotation of the 4‐iodophenyl substituent, so that no conformational dynamic facilitates radiationless deactivations. The 1:1 condensation product on the other hand allows a fast equilibration of the photophysical exitation by dynamic processes and therefore exhibits a low fluorescence quantum yield. Both luminophores contain an iodoaryl moiety with potential uses for further functionalization and bioconjugation.     

Synthesis of 1‐(p‐Sulphophenyl)‐3‐methyl‐5‐pyrazolone Based Acid Dyes and Their Applications on Leather

A new series of pyrazolone based azo acid dyes (3a–g) has been synthesized starting from 1‐(p‐sul‐phophenyl)‐3‐methyl‐5‐pyrazolone (1). The synthetic methodology included the nitrosation of p‐sulphophenyl methyl pyrazolone followed by reduction, diazotization and coupling with Naphthol AS derivatives (2a–f), in alkaline medium to yield different acid dyes. Multichromic metal complexes of these dyes (5a–f, 6a–f and 7a–f) with 3d transition metals Chromium, Iron and Copper were also synthesized. The structures of all of newly synthesized compounds were confirmed by analytical data and spectroscopic techniques. The synthesized dyes were applied on leather to assess their light fastness, wash fastness and rubbing fastness and were shown to exhibit high values of 4–5 for majority of dyes.     

New 3‐(Heteroaryl)‐2‐iminocoumarin‐based Borate Complexes: Synthesis,Photophysical Properties,and Rational Functionalization for Biosensing/Biolabeling Applications

Members of a series of boron difluoride complexes with 3‐(heteroaryl)‐2‐iminocoumarin ligands bearing both a phenolic hydroxyl group (acting as a fluorogenic center) and an N‐aryl substituent (acting as a stabilizing moiety) have been synthesized in good yields by applying a straightforward two‐step method. These novel fluorogenic dyes belong to the family of “Boricos” (D. Frath et al., Chem. Commun.­ 2013 , 49, 4908–4910) and are the first examples of phenol‐based fluorophores of which the photophysical properties in the green‐yellow spectral range are dramatically improved by N,N‐chelation of a boron atom. Modulation of their fluorescence properties through reversible chemical modification of their phenol moieties has been demonstrated by the preparation of the corresponding 2,4‐dinitrophenyl (DNP) ethers, which led to a dramatic “OFF‐ON” fluorescence response upon reaction with thiols. Additionally, to expand the scope of these “7‐hydroxy‐Borico” derivatives, particularly in biolabeling, amine or carboxylic acid functionalities amenable to (bio)conjugation have been introduced within their scaffold. Their utility has been demonstrated in the preparation of fluorescent bovine serum albumin (BSA) conjugates and “Borico”‐DOTA‐like scaffolds in an effort to design novel monomolecular multimodal fluorescence‐ radioisotope imaging agents.     

Dramatic Substituent Effects on the Photoluminescence of Boron Complexes of 2‐(Benzothiazol‐2‐yl)phenols

Substituents can induce dramatic changes in the photoluminescence properties of N,O‐chelated boron complexes. Specifically, the boron complexes of 2‐(benzothiazol‐2‐yl)phenols become bright deep blue‐ and orange‐red‐emitting materials depending on amino substituents at the 5‐ and 4‐positions of 2‐(benzothiazol‐2‐yl)phenol, respectively. Absorption and emission data show that the resulting boron complexes have little or small overlap between the absorption and emission spectra and, furthermore, X‐ray crystal structures for both the blue and orange‐red complexes indicate the absence of π–π stacking interaction in the crystal‐packing structures. These features endow the boron complexes with bright and strong photoluminescence in the solid state, which distinguishes itself from the typical boron complexes of dipyrromethenes (BODIPYs). A preliminary study indicates that the blue complexes have promising electro‐optical characteristics as dopant in an organic light‐emitting diode (OLED) device and show chromaticity close to an ideal deep blue. The substituent effects on the photoluminescent properties may be used to tune the desired emission wavelength of related boron or other metal complexes.     

Synthesis of Luminescent Ethynyl‐Extended Regioisomers of Borate Complexes Based on 2‐(2′‐Hydroxyphenyl)benzoxazole

A series of thirteen luminescent tetrahedral borate complexes based on the 2‐(2′‐hydroxyphenyl)benzoxazole (HBO) core is presented. Their synthesis includes the incorporation of an ethynyl fragment by Sonogashira cross‐coupling reaction, with the goal of extending the conjugation and consequently redshifting their emission wavelength. Different regioisomers, substituted in the 3‐, 4‐, or 5‐position of the phenolate side of the HBO core, were studied in order to compare their photophysical properties. The complexes were characterized by X‐ray diffraction and NMR, UV/Vis, and emission spectroscopy in solution and in the solid state. In all cases, complexation to boron leads to a donor–acceptor character that impacts their photophysical properties. Complexes with a 3‐ or 5‐substituted fragment display mild to pronounced internal charge transfer (ICT), a feature strengthened by the presence of p‐dibutylaminophenylacetylene in the molecular structure, protonation of the nitrogen atom of which leads to a significant blueshift and an increase in quantum yield. On the contrary, when the ethynyl module is grafted on the 4‐position, narrow, structured, symmetrical absorption/emission bands are observed. Moreover, the fact that protonation has little effect on the emission maximum wavelength reveals singlet excited‐state decay. Solid‐state emission properties reveal a redshift compared to solution, explained by tight packing of the π‐conjugated systems and the high planarity of the dyes. Subsequent connection of these complexes to other photoactive subunits (BODIPY, Boranil) provides dyads in which efficient cascade energy transfer is observed.     

Color‐Tunable Solid‐State Fluorescence Emission from Carbazole‐Based BODIPYs

Several carbazole‐based boron dipyrromethene (BODIPY) dyes were synthesized by organometallic approaches. Thiazole, benzothiazole, imidazole, benzimidazole, triazole, and indolone substituents were introduced at the 1‐position of the carbazole moiety, and boron complexation of each dipyrrin generated the corresponding compounds 1 , 2 a , and 3 – 6 . The properties of these products were investigated by UV/Vis and fluorescence spectroscopy, cyclic voltammetry, X‐ray crystallography, and DFT calculations. These compounds exhibited large Stokes shifts, and compounds 1 , 2 a , and 3 – 5 fluoresced both in solution and in the solid state. Complex 2 a showed the highest fluorescence quantum yield (Φ_F) in the solid state, therefore boron complexes of the carbazole–benzothiazole hybrids 2 b – f , which had several different substituents, were prepared and the effects of the substituents on the photophysical properties of the compounds were examined. The fluorescence properties showed good correlation with the results of crystal‐packing analyses, and the dyes exhibited color‐tunable solid‐state fluorescence.     

Ring Enlargement of Three‐Membered Boron Heterocycles upon Reaction with Organic π Systems: Implications for the Trapping of Borylenes

New low‐energy pathways for the reaction between substituted boriranes and borirenes with unsaturated hydrocarbons (ethyne or ethene) were discovered using density functional and coupled cluster theory. The interaction between the π bond of the hydrocarbon and the empty p orbital of the boron center leads to ring expansion of the three‐membered to a five‐membered boron heterocycle. The reactions are strongly exothermic and have low or even no barriers. They involve intermediates with a pentacoordinate boron center with two hydrocarbon molecules coordinating to boron akin to metal‐olefin complexes. These borylene complexes are shallow minima on the potential energy surfaces. But significantly higher barriers for ring formation are computed for 1,5‐cyclooctadiene and dibenzocyclooctatetraene complexes of borylenes, making these complexes likely detectable under appropriate experimental conditions. Our computational findings have implications for the interpretation of trapping experiments of thermally generated small borylenes with excess of small π systems. Because of very low barriers for reactions of three‐membered boron heterocycles with π systems and the at least locally large excess of the latter under such conditions, formation of five‐membered boron heterocycles should be considered.     

Boron complexes of porphyrins and related polypyrrole ligands: unexpected chemistry for both boron and the porphyrin

The coordination of boron to a range of polypyrrole-containing ligands is explored in this feature article. The boron dipyrromethenes are well-known as laser dyes and fluorescent labels in biology. Subphthalocyanine and subporphyrin macrocycles containing only three pyrrole rings can exist only when templated by a central boron atom. Boron complexes of expanded porphyrins (six or eight pyrroles) can complex boron in dipyrromethene (but not bipyrrole) sites. The primary focus of the article is on boron porphyrin and corrole complexes, where the tight fit of two boron atoms within the very constrained coordination site gives rise to unexpected chemistry at both boron and the porphyrin ligand. These unusual features are described and reasons for their occurrence postulated.     

Reversible Reduction of Oxatriphyrin(3.1.1)—Adjusting the Coordination Abilities to the Central Ion

Furan‐fused oxatriphyrin(3.1.1) acts as a ligand towards boron(III), phosphorous(III), and phosphorous(V) ions. The coordination abilities are adjusted by changes of the oxidation state. Two coordination modes are possible, depending on the central‐ion entrapped in the oxatriphyrin(3.1.1) environment. A free base, structurally similar to the complexes, can be easily obtained by regioselective reduction. The resulting molecules are highly emissive after conversion into a phlorin‐like oxatriphyrin(3.1.1) frame (NON donors), and show chiral activity caused by the presence of a tetrahedral carbon.     

Metal-Boron Compounds – Problems and Perspectives

“True” metal-boron compounds have been known for about ten years. The bonding between the metal and boron atoms can vary widely in nature: Not only simple covalent bonds are encountered but also coordinate bonds and boron-metal multicenter bonds. Furthermore, π complexes of transition metals with boron-nitrogen systems and metal complexes containing boron(I) compounds as ligands have recently also been prepared.     

Ferrocenyl-substituted Schiff base complexes of boron: synthesis, structural, physico-chemical and biochemical aspects

Biological important complexes of boron(III) derived from 1-acetylferrocenehydrazinecarboxamide (L1H), 1-acetylferrocenehydrazinecarbothioamide (L2H) and 1-acetylferrocene carbodithioic acid (L3H) have been prepared and investigated using a combination of microanalytical analysis, melting point, electronic, IR, 1H NMR and 13C NMR spectral studies, cyclic voltammetry and X-ray powder diffraction studies. Boron isopropoxide interacts with the ligands in 1:1, 1:2 and 1:3 molar ratios (boron:ligand) resulting in the formation of coloured products. On the basis of conductance and spectral evidences, tetrahedral structures for boron(III) complexes have been assigned. The ligands are coordinated to the boron(III) via the azomethine nitrogen atom and the thiolic sulfur atom/enolic oxygen atom. On the basis of X-ray powder diffraction study one of the representative boron complex was found to have orthorhombic lattice, having lattice parameters: a=9.9700, b=15.0000 and c=7.0000. Both the ligands and their complexes have been screened for their biological activity on several pathogenic fungi and bacteria and were found to possess appreciable fungicidal and bactericidal properties. Plant growth regulating activity of one of the ligand and its complexes has also been recorded on gram plant, and results have been discussed.