B–N Lewis Pair Functionalization of Anthracene: Structural Dynamics, Optoelectronic Properties, and O₂ Sensitization

The judicial placement of main group elements in conjugated structures is emerging as a key route to novel functional hybrid materials. We demonstrate here that the formation of B–N Lewis pairs at the periphery of anthracene leads to buckling of the backbone while also dramatically lowering the LUMO energy. The resulting BN-substituted contorted polycyclic aromatic hydrocarbons show large bathochromic shifts in the absorption and emission relative to all-carbon analogs. In the presence of light, they rapidly react with O₂ without the need for an external photosensitizer, resulting in selective and reversible formation of the corresponding endoperoxides

Stereoselective Ortho Borylation of Pyridylferrocenes

In an effort to develop new routes to planar chiral Lewis acids and Lewis pairs, the borylation of the pyridylferrocenes (p<i>S</i>)-1-stannyl-2-(3,5-dimethylpyrid-2-yl)­ferrocene and (p<i>S</i>)-1-mercurio-2-(3,5-dimethylpyrid-2-yl)­ferrocene with PhBCl₂ was investigated. In both cases, clean conversion to 2-(chlorophenylboryl)-1-(3,5-dimethylpyrid-2-yl)­ferrocene, a novel boron–nitrogen heterocycle, was observed. The structures of the products were confirmed by multinuclear and 2D NOESY NMR, high-resolution MS, and single-crystal X-ray diffraction analysis. While all NMR data proved to be identical, the single-crystal X-ray structures revealed retention of stereochemistry for the mercury precursor but inversion of stereochemistry for the organotin precursor. The opposite chirality of the products was further confirmed by chiral HPLC analyses and optical rotation measurements. These results imply that the reaction of PhBCl₂ with 1-mercurio-2-(3,5-dimethylpyrid-2-yl)­ferrocene proceeded as expected via an ipso-borodemercuration process, whereas a stereoselective rearrangement occurred in the reaction with 1-stannyl-2-(3,5-dimethylpyrid-2-yl)­ferrocene. A mechanism is proposed in which the Lewis basic pyridyl group in the stannylferrocene precursor acts as an ortho-directing group that facilitates highly selective attack of a borenium intermediate at the adjacent C–H position. Subsequent proton migration and elimination of Me₃SnCl result in a single isomer of the product with inverted stereochemistry. Reactivity studies were performed with Brønsted and Lewis acids to support the proposed mechanism

B–N Lewis Pair Functionalization of Anthracene: Structural Dynamics, Optoelectronic Properties, and O₂ Sensitization

The judicial placement of main group elements in conjugated structures is emerging as a key route to novel functional hybrid materials. We demonstrate here that the formation of B–N Lewis pairs at the periphery of anthracene leads to buckling of the backbone while also dramatically lowering the LUMO energy. The resulting BN-substituted contorted polycyclic aromatic hydrocarbons show large bathochromic shifts in the absorption and emission relative to all-carbon analogs. In the presence of light, they rapidly react with O₂ without the need for an external photosensitizer, resulting in selective and reversible formation of the corresponding endoperoxides

Luminescent Boron Quinolate Block Copolymers via RAFT Polymerization

The preparation of well-defined luminescent organoboron quinolate block copolymers via sequential RAFT polymerization is reported. Boron-containing block copolymers with PS, P­(St-<i>alt</i>-MAh), and PNIPAM as the second block were successfully synthesized. The photophysical properties of the block copolymers were studied by UV–vis and fluorescence spectroscopy. Independent of the second block, the boron quinolate block copolymers that contain the parent 8-hydroxyquinolato ligand (PM1-<i>b</i>-PS, PM1-<i>b</i>-PNIPAM, PM1-<i>b</i>-P­(St-<i>alt</i>-MAh)) are green luminescent, whereas a polymer with 5-(4-dimethylaminophenyl)-8-hydroxyquinolate as the ligand (PM2-<i>b</i>-PS) shows red luminescence. The P­(St-<i>alt</i>-MAh)-based block copolymer was further modified with photoactive azobenzene groups. The self-assembly behavior of the amphiphilic block copolymers was studied by transmission electron microscopy (TEM) and dynamic light scattering (DLS). In water, PM1-<i>b</i>-PNIPAM forms spherical micelles. The azobenzene-modified P­(St-<i>alt</i>-AbMA)-<i>b</i>-PM1 exhibits a solvent-dependent self-assembly behavior in basic solutions, and large spindle-shaped aggregates and spherical micelles were observed

Electron-Deficient Triarylborane Block Copolymers: Synthesis by Controlled Free Radical Polymerization and Application in the Detection of Fluoride Ions

Luminescent triarylborane homo and block copolymers with well-defined chain architectures were synthesized via reversible addition–fragmentation chain transfer polymerization of a vinyl-functionalized borane monomer. The Lewis acid properties of the polymers were exploited in the luminescent detection of fluoride ions. A dual-responsive fluoride sensor was developed by taking advantage of the reversible self-assembly of a PNIPAM-based amphiphilic block copolymer. Anion detection in aqueous solution was realized by introducing positively charged pyridinium moieties along the polymer chain

B–N Lewis Pair Functionalization of Anthracene: Structural Dynamics, Optoelectronic Properties, and O₂ Sensitization

The judicial placement of main group elements in conjugated structures is emerging as a key route to novel functional hybrid materials. We demonstrate here that the formation of B–N Lewis pairs at the periphery of anthracene leads to buckling of the backbone while also dramatically lowering the LUMO energy. The resulting BN-substituted contorted polycyclic aromatic hydrocarbons show large bathochromic shifts in the absorption and emission relative to all-carbon analogs. In the presence of light, they rapidly react with O₂ without the need for an external photosensitizer, resulting in selective and reversible formation of the corresponding endoperoxides

Stereoselective Ortho Borylation of Pyridylferrocenes

In an effort to develop new routes to planar chiral Lewis acids and Lewis pairs, the borylation of the pyridylferrocenes (p<i>S</i>)-1-stannyl-2-(3,5-dimethylpyrid-2-yl)­ferrocene and (p<i>S</i>)-1-mercurio-2-(3,5-dimethylpyrid-2-yl)­ferrocene with PhBCl₂ was investigated. In both cases, clean conversion to 2-(chlorophenylboryl)-1-(3,5-dimethylpyrid-2-yl)­ferrocene, a novel boron–nitrogen heterocycle, was observed. The structures of the products were confirmed by multinuclear and 2D NOESY NMR, high-resolution MS, and single-crystal X-ray diffraction analysis. While all NMR data proved to be identical, the single-crystal X-ray structures revealed retention of stereochemistry for the mercury precursor but inversion of stereochemistry for the organotin precursor. The opposite chirality of the products was further confirmed by chiral HPLC analyses and optical rotation measurements. These results imply that the reaction of PhBCl₂ with 1-mercurio-2-(3,5-dimethylpyrid-2-yl)­ferrocene proceeded as expected via an ipso-borodemercuration process, whereas a stereoselective rearrangement occurred in the reaction with 1-stannyl-2-(3,5-dimethylpyrid-2-yl)­ferrocene. A mechanism is proposed in which the Lewis basic pyridyl group in the stannylferrocene precursor acts as an ortho-directing group that facilitates highly selective attack of a borenium intermediate at the adjacent C–H position. Subsequent proton migration and elimination of Me₃SnCl result in a single isomer of the product with inverted stereochemistry. Reactivity studies were performed with Brønsted and Lewis acids to support the proposed mechanism

Synthesis, Structures, and Hydroboration of Oligo- and Poly(3-alkynylthiophene)s

3-Alkynyl-substituted terthiophenes and polythiophenes were synthesized, and their properties and behavior toward hydroboration with Mes₂BH were investigated. The alkynyl-substituted terthiophene was found to crystallize in an intriguing layered structure that mimics an interdigitated polymer wherein the terthiophenes form tightly π-stacked linear pseudopolymer strands. The heptynyl side chains of neighboring stacks interlock ideally, almost completely filling the void in-between substituents and thus allowing for minimal spacing between neighboring pseudopolymer strands. Poly­(3-alkynylthiophene)­s were accessed through Grignard metathesis polymerization (GRIM) of 2,5-dibromo-3-heptynylthiophene, which yielded polymers of moderate molecular weights (<i>M</i>_n = 6.3; PDI = 1.73) in yields of up to 82%. Further modification by hydroboration with Mes₂BH resulted in a material that is partially functionalized with vinylborane groups and as a result exhibits charge transfer bands in the UV–vis spectra. A single-crystal X-ray structure of the corresponding hydroborated terthiophene species shows extensive intermolecular interactions, resulting in π-stacks of π-dimers, despite the steric bulk of the dimesitylborane moieties

Luminescent Main-Chain Organoborane Polymers: Highly Robust, Electron-Deficient Poly(oligothiophene borane)s via Stille Coupling Polymerization

A series of polymers (<b>PB<i>n</i>T</b>, <i>n</i> = 2–5) with boron atoms incorporated into the conjugated polythiophene main chain have been prepared via Pd-catalyzed coupling of stannylated thienylborane monomers. The polymers exhibit excellent long-term chemical stability to air and moisture and remarkable thermal stability with decomposition temperatures reaching over 300 °C. The high stability is achieved by placing very bulky pendant groups, 2,4,6-tri-<i>tert</i>-butylphenyl (Mes*) and 2,4,6-tris­(trifluoro­methyl)­phenyl (^FMes), on boron that prevent attack by nucleophiles. All these polymers display strong absorptions in the visible region and intense fluorescence in both solution and the solid state with quantum yields of up to 38% and fast radiative decay constants (<i>k</i>_r) of up to 3.3 × 10⁸ s^–1. Density functional theory (DFT) studies on diborylated oligothiophene model compounds suggest that the strong absorption of the polymers results from π–π* transitions on the oligothiophene borane main chain with significant charge transfer to boron. The unusually intense luminescence in the solid state is favored by the rigid planar skeleton and steric shielding of the bulky pendent groups. The emission color can be tuned from blue to deep orange by varying the length of the π-conjugated oligothiophene spacer between the boron atoms. Spectroelectrochemical studies on a dimeric model compound in THF solution reveal reversible two-step reductions to give highly colored species, while the corresponding polymeric material precipitates at higher potentials after undergoing an initial reversible reduction. The LUMO energy levels of the polymers can be effectively lowered by introduction of electron-withdrawing pendent groups on boron, affording a versatile approach for development of electron-deficient boron-containing polymers with controllable electronic structures and photophysical properties. The facile modular synthetic approach combined with the exceptional stability opens the door to broad adoption of electron-deficient organoboranes in conjugated materials design and development

Tris(2-pyridylborate) (Tpyb) Metal Complexes: Synthesis, Characterization, and Formation of Extrinsically Porous Materials with Large Cylindrical Channels

Sandwich-like metal complexes (Tpyb)₂M (M = Mg, Fe, Mn) that are based on the novel <i>t</i>-butylphenyltris­(2-pyridyl)­borate ligand were prepared and fully characterized by multinuclear NMR spectroscopy, high-resolution matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, and single crystal X-ray crystallography. The unique steric and electronic nature of the Tpyb ligand led to structural parameters and properties that are quite different to those of previously reported tris­(pyrazolyl)­borate and tris­(2-pyridyl)­aluminate complexes. Most importantly, depending on the crystallization procedure, supramolecular structures could be generated with relatively smaller (ca. 4–5 Å) or larger (ca. 8 Å) diameter pores propagating throughout the crystal lattice. Although the supramolecular structures are held together only by weak intermolecular C–H···π interactions, the solvent in the larger channels could be completely removed without any loss of crystallinity or degradation of the framework. Surface area and gas uptake measurements on the Mg complex further confirmed the permanent porosity, while the calculated non-localized density functional theory (NLDFT) pore diameter of 8.6 Å proved to be in excellent agreement with that obtained from single crystal X-ray crystallography. Our new materials are remarkably thermally stable as degradation did not occur up to about 400 °C based on thermogravimetric analysis (TGA), and a sample of the Mg complex showed no loss of crystallinity even after heating to 140 °C under high vacuum for 72 h according to single crystal X-ray diffraction data