Chemical synthesis of organoborane polymers and their applications is discussed. The past decade has seen a renewal of interest and development of main group organometallic chemistry.
The past decade has witnessed tremendous advances in the synthesis of polymers that contain elements from the main groups beyond those found in typical organic polymers. Unique properties that arise ...from dramatic differences in bonding and molecular geometry, electronic structure, and chemical reactivity, are exploited in diverse application fields. Herein we highlight recent advances in inorganic backbone polymers, discuss how Lewis acid/base functionalization of polymers results in unprecedented reactivity, and survey conjugated hybrids with unique electronic structures for sensor and device applications.
Polymers go main group! This Review shows how the incorporation of the full range of available main‐group elements into polymers leads to new functional hybrid materials with potential use in diverse application fields ranging from advanced elastomers, responsive gels, biodegradable materials, to organic electronics, imaging agents, sensors, and supported catalysts.
The functionalization of polymeric materials with Lewis acidic boron sites has led to exciting new opportunities in diverse areas, ranging from new polymeric catalysts to the preparation of highly ...luminescent materials and the design of efficient sensor systems for nucleophiles. Recent advances in the preparation of these Lewis acidic boron polymers, their coordination behavior, and some of their intriguing applications are reviewed.
We demonstrate that the modification of anthracene with B ← N Lewis pairs at their periphery serves as a highly effective tool to modify the electronic structure with important ramifications on the ...generation and reactivity toward singlet oxygen. A series of BN-fused dipyridylanthracenes with Me groups in different positions of the pyridyl ring have been prepared via directed electrophilic borylation. The steric and electronic effects of the substituents on the structural features and electronic properties of the isomeric borane-functionalized products have been investigated in detail, aided by experimental tools and computational studies. We find that BDPA-2Me, with Me groups adjacent to the pyridyl N, has the longest B–N distance and shows overall less structural distortions, whereas BDPA-5Me with the Me group close to the anthracene backbone experiences severe distortions that are reflected in the buckling of the anthracene framework and dislocation of the boron atoms from the planes of the phenyl rings they are attached to. The substitution pattern also has a dramatic effect on the self-sensitized reactivity of the acenes toward O2 and the thermal release of singlet oxygen from the respective endoperoxides. Kinetic analyses reveal that BDPA-2Me rapidly reacts with O2, whereas BDPA-5Me is converted only very slowly to its endoperoxide. However, the latter serves as an effective singlet oxygen sensitizer, as demonstrated in the preferential formation of the endoperoxide of dimethylanthracene in a competition experiment. These results demonstrate that even relatively small modifications in the substitution of the pyridyl ring of BN-fused dipyridylanthracenes change the steric and electronic structure, resulting in dramatically different reactivity patterns. Our findings provide important guidelines for the design of highly effective sensitizers for singlet oxygen on one hand and the realization of materials that readily form endoperoxides in a self-sensitized manner and then thermally release singlet oxygen on demand on the other hand.
Classical Lewis pairs (LPs) between unhindered electron-poor Lewis acids (LAs) and electron-rich Lewis bases (LBs) present an overlooked motif with tremendous potential as dynamic cross-links in ...transient polymer networks (TPNs) for self-healing and stimuli-responsive applications. We demonstrate that simple and intuitive matching of weak/strong organoborane LA and amine LB pairs offers access to a large set of binding equilibrium constants, K eq, that span ∼6 orders and dissociation rate constants, k diss, that span ∼7 orders of magnitude. The implementation in polystyrene (PS)/polydimethylsiloxane (PDMS) blends results in dynamically cross-linked networks with bulk thermomechanical properties that are directly correlated with the strength and kinetic parameters for the LP interactions. The LP dynamic cross-link design is highly versatile and broadly applicable to different polymer architectures as demonstrated in the formation of reprocessable elastomers from Lewis base-decorated high molecular weight PDMS in combination with Lewis acid-decorated PS when reinforced with fumed silica as a filler.
The extension of conjugated organoboranes from monomeric species to oligomers, macrocycles, and polymers offers access to a plethora of fascinating new materials. The p–π* conjugation between empty ...orbitals on boron and the conjugated linkers not only affects the electronic structure and optical properties, but also enables mutual interactions between electron‐deficient boron centers. The unique properties of these electron‐deficient π‐conjugated systems are exploited in highly luminescent materials, organic optoelectronic devices, and sensing applications.
The development of π‐extended organoboranes is systematically discussed from both theoretical and experimental perspectives. An analysis of the p–π* conjugation and of mutual interactions between electron‐deficient boron centers across π‐conjugated linkers allows for important structure–property relationships to be deduced. It is also illustrated how the unique properties of these boron‐containing π‐conjugated systems are exploited in the field of organic optical and optoelectronic device materials.
Covering an exceptionally wide range of bond strengths, the dynamic nature and facile tunability of dative B−N bonds is highly attractive when it comes to the assembly of supramolecular polymers and ...materials. This Minireview offers an overview of advances in the development of functional materials where Lewis pairs (LPs) play a key role in their assembly and critically influence their properties. Specifically, we describe the reversible assembly of linear polymers with interesting optical, electronic and catalytic properties, discrete macrocycles and molecular cages that take up diverse guest molecules and undergo structural changes triggered by external stimuli, covalent organic frameworks (COFs) with intriguing interlocked structures that can embed and separate gases such as CO2 and acetylene, and soft polymer networks that serve as recyclable, self‐healing, and responsive thermosets, gels and elastomeric materials.
Boron‐nitrogen Lewis pairs (LPs) serve as versatile motifs that not only facilitate self‐assembly but also endow macromolecules with unique characteristics. The dynamic nature of the dative bonding is exploited in the generation of new macrocycles, molecular cages, 1D polymers, and 3D networks with interesting optical and electronic properties, host–guest characteristics, stimuli‐responsiveness, and shape‐shifting properties.
Two novel luminescent triarylborane polymeric Lewis acids and the corresponding molecular model compounds were prepared, studies on their photophysical properties performed, and applications as ...supported catalysts in Lewis acid-catalyzed hydrosilylation reactions explored. Variations of the substituents in the ortho-position of a phenylene linker between the Lewis acidic borane and the polystyrene framework lead to steric and electronic fine-tuning while retaining the high Lewis acidity of the boron center. The polymeric Lewis acids serve as effective catalysts in the hydrosilylation of aldehyde, ketone, and imine compounds and because of their distinct solubility characteristics are readily amenable to recycling. In addition, as a result of the twisted biphenyl donor−π-acceptor structure, both the styrene copolymers and model compounds display strong luminescence in solution and the solid state, encompassing prompt fluorescence and in the case of the chlorophenyl-linked derivative also longer-lived room temperature phosphorescence (RTP). Consistent with quantum-chemical calculations, the greater donor strength of the 2,6-dimethylphenyl in comparison to the 2-chlorophenyl linker leads to a red-shifted emission, while the chloro substituent leads to a larger gap between singlet and triplet excited states. In the solid state, distinctly different emission properties are observed for the polymers in comparison to the molecular compounds because of the site isolation of the chromophores embedded in the polymer matrix.
Although widely used in catalysis, the multistep syntheses and high loadings typically employed are limiting broader implementation of highly active tailor-made arylborane Lewis acids and Lewis ...pairs. Attempts at developing recyclable systems have thus far met with limited success, as general and versatile platforms are yet to be developed. We demonstrate a novel approach that is based on the excellent control and functional group tolerance of ring-opening metathesis polymerization (ROMP). The ROMP of highly Lewis acidic borane-functionalized phenylnorbornenes afforded both a soluble linear copolymer and a cross-linked organogel. The polymers proved highly efficient as recyclable catalysts in the reductive N-alkylation of arylamines under mild conditions and at exceptionally low catalyst loadings. The modular design presented herein can be readily adapted to other finely tuned triarylboranes, enabling wide applications of ROMP-borane polymers as well-defined supported organocatalysts.
B–N-fused dianthracenylpyrazine derivatives are synthesized to generate new low gap chromophores. Photophysical and electrochemical, crystal packing, and theoretical studies have been performed. Two ...energetically similar conformers are identified by density functional theory calculations, showing that the core unit adopts a curved saddle-like shape (x-isomer) or a zig-zag conformation (z-isomer). In the solid state, the z-isomer is prevalent according to an X-ray crystal structure of a C6F5-substituted derivative (4-Pf), but variable-temperature nuclear magnetic resonance studies suggest a dynamic behavior in solution. B–N fusion results in a large decrease of the HOMO–LUMO gap and dramatically lowers the LUMO energy compared to the all-carbon analogues. 4-Pf in particular shows significant absorbance at greater than 700 nm while being almost transparent throughout the visible region. After encapsulation in the biodegradable polymer DSPE-mPEG2000, 4-Pf nanoparticles (4-Pf-NPs) exhibit good water solubility, high photostability, and an excellent photothermal conversion efficiency of ∼41.8%. 4-Pf-NPs are evaluated both in vitro and in vivo as photothermal therapeutic agents. These results uncover B–N Lewis pair functionalization of PAHs as a promising strategy toward new NIR-absorbing materials for photothermal applications.
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