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.
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 O
without the need for an external photosensitizer, resulting in selective and reversible formation of the corresponding endoperoxides.
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.
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.
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.
We introduce a new boron‐doped cyclophane, the hexabora16cyclophane B6‐FMes, in which six tricoordinate borane moieties alternate with short conjugated p‐phenylene linkers. Exocyclic ...2,4,6‐tris(trifluoromethyl)phenyl (FMes) groups serve not only to further withdraw electron density but at the same time sterically shield the boron atoms, resulting in a macrocycle that is both highly electron‐deficient and stable. The optical and electronic properties are compared with those of related linear oligomers and the electronic structure is further evaluated by computational methods. The studies uncover unique properties of B6‐FMes, including a low‐lying and extensively delocalized LUMO and a wide HOMO–LUMO gap, which arise from the combination of a cyclic π‐system, strong electronic communication between the closely spaced borons, and the attachment of electron‐deficient pendent groups. The binding of small anions to the electron‐deficient macrocycle and molecular model compounds is investigated and emissive exciplexes are detected in aromatic solvents.
B to B: Short phenylene linkers and strongly electron‐withdrawing fluorinated mesityl groups render the title compound highly electron‐deficient yet air‐ and moisture‐stable. Cyclic voltammetry and DFT calculations reveal strong electronic communication between the six boron atoms through a very low‐lying LUMO. Unique optical, electronic, and anion‐binding properties are further substantiated by comparisons with linear oligomers.
A series of polymers (PBnT, n = 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-tert-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 (k 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.
We describe a solid-state material formed from binary assembly of atomically precise molecular clusters. Co₆Se₈(PEt₃)₆C₆₀₂ and Cr₆Te₈(PEt₃)₆C₆₀₂ assembled into a superatomic relative of the cadmium ...iodide (Cdl₂) structure type. These solid-state materials showed activated electronic transport with activation energies of 100 to 150 millielectron volts. The more reducing cluster Ni₉Te₆(PEt₃)₈ transferred more charge to the fullerene and formed a rock-salt—related structure. In this material, the constituent clusters are able to interact electronically to produce a magnetically ordered phase at low temperature, akin to atoms in a solid-state compound.
The functionalization of polycyclic aromatic hydrocarbons (PAHs) via B←N Lewis pair formation offers an opportunity to judiciously fine-tune the structural features and optoelectronic properties, to ...suit the demands of applications in organic electronic devices, bioimaging, and as sensitizers for singlet oxygen generation. We demonstrate that the N-directed electrophilic borylation of 2,6-di(pyrid-2-yl)anthracene offers access to linearly extended acene derivatives Py-BR (R=Et, Ph, C
F
). In comparison to indeno-fused 9,10-diphenylanthracene, the formal "BN for CC" replacement in Py-BR selectively lowers the LUMO, resulting in a much reduced HOMO-LUMO gap. An even more extended conjugated system with seven six-membered rings in a row (Qu-BEt) is obtained by borylation of 2,6-di(quinolin-8-yl)anthracene. Fluorinated Py-BPf shows particularly advantageous properties, including relatively lower-lying HOMO and LUMO levels, strong yellow-green fluorescence, and effective singlet oxygen sensitization, while resisting self-sensitized conversion to its endoperoxide.
Introduced herein is a series of conjugated thienylboranes, which are inert to air and moisture, and even resist acids and strong bases. X‐ray analyses reveal a coplanar arrangement of the thiophene ...rings, an arrangement which facilitates p–π conjugation through the boron atoms despite the presence of highly bulky 2,4,6‐tri‐tert‐butylphenyl (Mes*) or 2,4,6‐tris(trifluoromethyl)phenyl (FMes) groups. Short B⋅⋅⋅F contacts, which lead to a pseudotrigonal bipyramidal geometry in the FMes species, have been further studied by DFT and AIM analysis. In contrast to the Mes* groups, the highly electron‐withdrawing FMes groups do not diminish the Lewis acidity of boron toward F− anions. These compounds can be lithiated or iodinated under electrophilic conditions without decomposition, thus offering a promising route to larger conjugated structures with electron‐acceptor character.
The bigger the better: Sterically demanding 2,4,6‐tri‐tert‐butylphenyl (Mes*) or 2,4,6‐tris(trifluoromethyl)phenyl (FMes) groups do not prevent but rather promote coplanarity and enhance electronic communication within conjugated thienylboranes. FMes exerts a strong electron‐withdrawing effect which results in significant lowering of the LUMO energy level and high Lewis acidity toward fluoride anions, while ensuring stability in air and towards acid or base.
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