Ultrathin, Cationic Covalent Organic Nanosheets for Enhanced CO2 Electroreduction to Methanol.

Metalloporphyrins and metallophthalocyanines emerge as popular building blocks to develop covalent organic nanosheets (CONs) for CO2 reduction reaction (CO2 RR). However, existing CONs predominantly yield CO, posing a challenge in achieving efficient methanol production through multielectron reduction. Here, ultrathin, cationic, and cobalt-phthalocyanine-based CONs (iminium-CONs) are reported for electrochemical CO2 -to-CH3 OH conversion. The integration of quaternary iminium groups enables the formation of ultrathin morphology with uniformly anchored cobalt active sites, which are pivotal for facilitating rapid multielectron transfer. Moreover, the cationic iminium-CONs exhibit a lower activity for hydrogen evolution side reaction. Consequently, iminium-CONs manifest significantly enhanced selectivity for methanol production, as evidenced by a remarkable 711% and 270% improvement in methanol partial current density (jCH3OH ) compared to pristine CoTAPc and neutral imine-CONs, respectively. Under optimized conditions, iminium-CONs deliver a high jCH3OH of 91.7 mA cm-2 at -0.78 V in a flow cell. Further, iminium-CONs achieve a global methanol Faradaic efficiency (FECH3OH ) of 54% in a tandem device. Thanks to the single-site feature, the methanol is produced without the concurrent generation of other liquid byproducts. This work underscores the potential of cationic covalent organic nanosheets as a compelling platform for electrochemical six-electron reduction of CO2 to methanol.

Heteroatom-boron-heteroatom-doped π-conjugated systems: structures, synthesis and photofunctional properties.

The potency of heteroatom-doping in reshaping optoelectronic properties arises from the distinct electronegativity variations between heteroatoms and carbon atoms. By incorporating two heteroatoms with differing electronegativities (e.g., B = N), not only is the architectural coherence of π-conjugated systems retained, but also dipolar traits are introduced, accompanied by unique intermolecular interactions absent in their all-carbon analogs. Another burgeoning doping strategy, featuring the heteroatom-boron-heteroatom motif (X-B-X, where X = N, O), has captured growing attention. This configuration's coexistence of the boron-heteroatom unit and an isolated heteroatom stimulates mutual modulation in the dipole of the boron-heteroatom unit and the heteroatom's electronegativity. In this Feature article, we present an encompassing survey of XBX-doped π-conjugated systems, elucidating how the integration of the X-B-X unit induces transformative structural and property changes within π-conjugated systems.

Multiple Helicenes Defected by Heteroatoms and Heptagons with Narrow Emissions and Superior Photoluminescence Quantum Yields.

The incorporation of heteroatoms and/or heptagons as the defects into helicenes expands the variety of chiroptical materials with novel properties. However, it is still challenging to construct novel boron-doped heptagon-containing helicenes with high photoluminescence quantum yields (PLQYs) and narrow full-width-at-half-maximum (FWHM) values. We report an efficient and scalable synthesis of a quadruple helicene 4Cz-NBN with two nitrogen-boron-nitrogen (NBN) units and a double helicene 4Cz-NBN-P1 bearing two NBN-doped heptagons, the latter could be formed via a two-fold Scholl reaction of the former. The helicenes 4Cz-NBN and 4Cz-NBN-P1 exhibit excellent PLQYs up to 99 % and 65 % with narrow FWHM of 24 nm and 22 nm, respectively. The emission wavelengths are tunable via stepwise titration experiments of 4Cz-NBN-P1 toward fluoride, enabling distinguished circularly polarized luminescence (CPL) from green, orange (4Cz-NBN-P1-F1) to yellow (trans/cis-4Cz-NBN-P1-F2) with near-unity PLQYs and broader circular dichroism (CD) ranges. The five structures of the aforementioned four helicenes were confirmed by single crystal X-ray diffraction analysis. This work provides a novel design strategy for construction of non-benzenoid multiple helicenes exhibiting narrow emissions with superior PLQYs.

Direct Synthesis of Ammonia from Nitrate on Amorphous Graphene with Near 100% Efficiency.

Ammonia is an indispensable commodity in the agricultural and pharmaceutical industries. Direct nitrate-to-ammonia electroreduction is a decentralized route yet challenged by competing side reactions. Most catalysts are metal-based, and metal-free catalysts with high nitrate-to-ammonia conversion activity are rarely reported. Herein, it is shown that amorphous graphene synthesized by laser induction and comprising strained and disordered pentagons, hexagons, and heptagons can electrocatalyze the eight-electron reduction of NO3 - to NH3 with a Faradaic efficiency of ≈100% and an ammonia production rate of 2859 µg cm-2 h-1 at -0.93 V versus reversible hydrogen electrode. X-ray pair-distribution function analysis and electron microscopy reveal the unique molecular features of amorphous graphene that facilitate NO3 - reduction. In situ Fourier transform infrared spectroscopy and theoretical calculations establish the critical role of these features in stabilizing the reaction intermediates via structural relaxation. The enhanced catalytic activity enables the implementation of flow electrolysis for the on-demand synthesis and release of ammonia with >70% selectivity, resulting in significantly increased yields and survival rates when applied to plant cultivation. The results of this study show significant promise for remediating nitrate-polluted water and completing the NOx cycle.

[4]Triangulenes Modified by Three Oxygen-Boron-Oxygen (OBO) Units: Synthesis, Characterizations, and Anti-Kasha Emissions.

Modification of π-conjugated systems using a boron atom as the dopant has become a powerful approach to create new structures and new properties. Herein, we report a facile synthesis of replacing the carbon edges of [4]triangulene by three oxygen-boron-oxygen (OBO) units. The OBO-modified [4]triangulenes are structurally similar to [4]triangulene and isoelectronic to the trianion of [4]triangulene. The structure of OBO-modified [4]triangulene is confirmed by single-crystal X-ray diffraction analysis, revealing an off-plane core with three edge-modified OBO units. These OBO-modified [4]triangulenes exhibit excellent thermal stability. These compounds have phosphorescence with lifetime longer than 1 s at 77 K. Both theoretical calculations and photophysical investigation of OBO-modified [4]triangulenes indicate that this kind of molecules display a rare anti-Kasha fluorescence and phosphorescence emissions from multiple higher excited states.

Atomically Thin, Ionic-Covalent Organic Nanosheets for Stable, High-Performance Carbon Dioxide Electroreduction.

The incorporation of charged functional groups is effective to modulate the activity of molecular complexes for the CO2 reduction reaction (CO2 RR), yet long-term heterogeneous electrolysis is often hampered by catalyst leaching. Herein, an electrocatalyst of atomically thin, cobalt-porphyrin-based, ionic-covalent organic nanosheets (CoTAP-iCONs) is synthesized via a post-synthetic modification strategy for high-performance CO2 -to-CO conversion. The cationic quaternary ammonium groups not only enable the formation of monolayer nanosheets due to steric hindrance and electrostatic repulsion, but also facilitate the formation of a *COOH intermediate, as suggested by theoretical calculations. Consequently, CoTAP-iCONs exhibit higher CO2 RR activity than other cobalt-porphyrin-based structures: an 870% and 480% improvement of CO current densities compared to the monomer and neutral nanosheets, respectively. Additionally, the iCONs structure can accommodate the cationic moieties. In a flow cell, CoTAP-iCONs attain a very small onset overpotential of 40 mV and a stable total current density of 212 mA cm-2 with CO Faradaic efficiency of >95% at -0.6 V for 11 h. Further coupling the flow electrolyzer with commercial solar cells yields a solar-to-CO conversion efficiency of 13.89%. This work indicates that atom-thin, ionic nanosheets represent a promising structure for achieving both tailored activity and high stability.

Access to tetracoordinate boron-doped polycyclic aromatic hydrocarbons with delayed fluorescence and aggregation-induced emission under mild conditions.

Boron-doped polycyclic aromatic hydrocarbons (PAHs) have attracted ongoing attention in the field of optoelectronic materials due to their unique optical and redox properties. To investigate the effect of tetracoordinate boron in PAHs bearing N-heterocycles (indole and carbazole), a facile approach to four-coordinate boron-doped PAHs was developed, which does not require elevated temperature and pre-synthesized functionalized boron reactants. Five tetracoordinate boron-doped PAHs (NBNN-1-NBNN-5) were synthesized with different functional groups. Two of them (NBNN-1 and NBNN-2) could further undergo oxidative coupling reactions to form fused off-plane tetracoordinate boron-doped PAHs NBNN-1f and NBNN-2f. The investigation of photophysical properties showed that the UV/vis absorption and fluorescence emission are significantly red-shifted compared to those of the three-coordinate boron-doped counterparts. In addition, the emission of NBNN-1-NBNN-3 consisted of prompt fluorescence and delayed fluorescence. The compounds NBNN-1f and NBNN-2f showed aggregation-induced emission.

Correction: Suppressing carboxylate nucleophilicity with inorganic salts enables selective electrocarboxylation without sacrificial anodes.

[This corrects the article DOI: 10.1039/D1SC02413B.].

Suppressing carboxylate nucleophilicity with inorganic salts enables selective electrocarboxylation without sacrificial anodes.

Although electrocarboxylation reactions use CO2 as a renewable synthon and can incorporate renewable electricity as a driving force, the overall sustainability and practicality of this process is limited by the use of sacrificial anodes such as magnesium and aluminum. Replacing these anodes for the carboxylation of organic halides is not trivial because the cations produced from their oxidation inhibit a variety of undesired nucleophilic reactions that form esters, carbonates, and alcohols. Herein, a strategy to maintain selectivity without a sacrificial anode is developed by adding a salt with an inorganic cation that blocks nucleophilic reactions. Using anhydrous MgBr2 as a low-cost, soluble source of Mg2+ cations, carboxylation of a variety of aliphatic, benzylic, and aromatic halides was achieved with moderate to good (34-78%) yields without a sacrificial anode. Moreover, the yields from the sacrificial-anode-free process were often comparable or better than those from a traditional sacrificial-anode process. Examining a wide variety of substrates shows a correlation between known nucleophilic susceptibilities of carbon-halide bonds and selectivity loss in the absence of a Mg2+ source. The carboxylate anion product was also discovered to mitigate cathodic passivation by insoluble carbonates produced as byproducts from concomitant CO2 reduction to CO, although this protection can eventually become insufficient when sacrificial anodes are used. These results are a key step toward sustainable and practical carboxylation by providing an electrolyte design guideline to obviate the need for sacrificial anodes.

Formation of an air-stable diborane via a stepwise BH3 addition of pyrido[1,2-a]isoindole with H2 evolution.

A novel and air-stable organo(hydro)diborane featuring a five-membered aryl ring supported bridging B-C-B three-centre-two-electron (3c-2e) bond has been reported. Pyrido[1,2-a]isoindole was found to undergo a stepwise BH3 addition reaction, during which a mono-BH3 adduct was formed from a electrophilic addition at the Cγ in pyrido[1,2-a]isoindole. A molecule of hydrogen was eliminated throughout the second step of addition reaction. DFT calculations indicate that the H2 evolution is concerted to the second BH3 addition rather than forming BC before the second BH3 attack.

Sequential and Diverse Synthesis of BN-Heterocycles and Investigation of Their Photoreactivity.

Substituents modification of BN-heterocycles on the boron atom has proven important to the photoreactivity and optoelectronic properties of BN-heterocycles. We developed a sequential and diverse synthetic strategy toward BN-heterocycles, in which the boron building block can be introduced with fully pre-functionalized substituents (Route A) or the substituents can be partially (Route B) or fully (Route C) modified after borylation. These three routes are complementary to provide more diverse BN-heterocycles, which will find broad applications in manipulating/controlling molecular transformations and the development of new photoresponsive materials.

Epoxidation of Cyclooctene Using Water as the Oxygen Atom Source at Manganese Oxide Electrocatalysts.

Epoxides are useful intermediates for the manufacture of a diverse set of chemical products. Current routes of olefin epoxidation either involve hazardous reagents or generate stoichiometric side products, leading to challenges in separation and significant waste streams. Here, we demonstrate a sustainable and safe route to epoxidize olefin substrates using water as the oxygen atom source at room temperature and ambient pressure. Manganese oxide nanoparticles (NPs) are shown to catalyze cyclooctene epoxidation with Faradaic efficiencies above 30%. Isotopic studies and detailed product analysis reveal an overall reaction in which water and cyclooctene are converted to cyclooctene oxide and hydrogen. Electrokinetic studies provide insights into the mechanism of olefin epoxidation, including an approximate first-order dependence on the substrate and water and a rate-determining step which involves the first electron transfer. We demonstrate that this new route can also achieve a cyclooctene conversion of ∼50% over 4 h.

Doping Polycyclic Arenes with Nitrogen-Boron-Nitrogen (NBN) Units.

With the aid of borylation and oxidative coupling reactions, six new polycyclic aromatic hydrocarbons (PAHs) doped by nitrogen-boron-nitrogen (NBN) units were achieved. The structure-optoelectronic property relationship for this group of compounds was examined. All six compounds are fluorescent with contrasting emission colors and quantum yields.

Identifying (BN)2-pyrenes as a New Class of Singlet Fission Chromophores: Significance of Azaborine Substitution.

Singlet fission converts one photoexcited singlet state to two triplet excited states and raises photoelectric conversion efficiency in photovoltaic devices. However, only a handful of chromophores have been known to undergo this process, which greatly limits the application of singlet fission in photovoltaics. We hereby identify a recently synthesized diazadiborine-pyrene ((BN)2-pyrene) as a singlet fission chromophore. Theoretical calculations indicate that it satisfies the thermodynamics criteria for singlet fission. More importantly, the calculations provide a physical chemistry insight into how the BN substitution makes this happen. Both calculation and transient absorption spectroscopy experiments indicate that the chromophore has a better absorption than pentacene. The convenient synthesis pathway of the (BN)2-pyrene suggests an in situ chromophore generation in photovoltaic devices. Two more (BN)2-pyrene isomers are proposed as singlet fission chromophores. This study sets a step forward in the cross-link of singlet fission and azaborine chemistry.

Substituent Directed Phototransformations of BN-Heterocycles: Elimination vs Isomerization via Selective B-C Bond Cleavage.

Electron-rich and -poor BN-heterocycles with benzyl-pyridyl backbones and two bulky aryls on the boron (Ar = tipp, BN-1, Ar = MesF, BN-2) have been found to display distinct molecular transformations upon irradiation by UV light. BN-1 undergoes an efficient photoelimination reaction forming a BN-phenanthrene with ΦPE = 0.25, whereas BN-2 undergoes a thermally reversible, stereoselective, and quantitative isomerization to a dark colored BN-1,3,5-cyclooctatriene (BN-1,3,5-COT, BN-2a). This unusual photoisomerization persists for other BN-heterocycles with electron-deficient aryls such as BN-3 with a benzyl-benzothiazolyl backbone and Mes(F) substituents or BN-4 with a benzyl-pyridyl backbone and two C6F5 groups on the boron. The photoisomerization of BN-4 goes beyond BN-1,3,5-COT (BN-4a), forming a new species (BN-1,3,6-COT, BN-4b) via C-F bond cleavage and [1,3]-F atom sigmatropic migration. Computational studies support that BN-4a is an intermediate in the formation of BN-4b. This work establishes that steric and electronic factors can effectively control the transformations of BN-heterocycles, allowing access to important and previously unknown BN-embedded species.

1,1-Hydroboration of Fused Azole-Isoindole Analogues as an Approach for Construction of B,N-Heterocycles and Azole-Fused B,N-Naphthalenes.

Three isoelectronic analogues of pyrido[2,1-a]isoindole have been found to undergo a facile 1,1-hydroboration with HBMes2 borane, which provides a new and convenient method for the synthesis of B,N-heterocycles 1a-3a in high yields. Compounds 1a-3a can undergo photoelimination upon irradiation at 300 nm, generating heterocycle-fused B,N-naphthalene molecules 1b-3b, which display distinct yellow-green and blue fluorescent colors, respectively. Compound 1a undergoes thermal elimination, producing 1b at 280 °C, while compound 2a only undergoes partial elimination, forming 2b at 320 °C. Compound 3a is thermally stable up to 320 °C.

In†Situ Solid-State Generation of (BN)2 -Pyrenes and Electroluminescent Devices.

New BN-heterocyclic compounds have been found to undergo double arene photoelimination, forming rare yellow fluorescent BN-pyrenes that contain two BN units. Most significant is the discovery that the double arene elimination can also be driven by excitons generated electrically within electroluminescent (EL) devices, enabling the in situ solid-state conversion of BN-heterocycles to BN-pyrenes and the use of BN-pyrenes as emitters for EL devices. The in situ exciton-driven elimination (EDE) phenomenon has also been observed for other BN-heterocycles.

Thermal and Photolytic Transformation of NHC-B,N-Heterocycles: Controlled Generation of Blue Fluorescent 1,3-Azaborinine Derivatives and 1H-Imidazo[1,2-a]indoles by External Stimuli.

NHC-B,N-heterocyclic compounds have been found to act as convenient precursors for obtaining either 1,3-azaborinine or 1H-imidazo[1,2-a]indole derivatives, which are two different and rare classes of compounds. The formation of these two classes of compounds from the NHC-B,N-heterocycles is highly selective depending on the external stimuli employed, and the resulting products have been studied for their interesting chemical and photophysical properties. The mechanism and possible reaction pathways of the unusual transformation are established by computational studies.

One-Pot Synthesis of Brightly Fluorescent Mes2B-Functionalized Indolizine Derivatives via Cycloaddition Reactions.

Four new BMes2-functionalized indolizine derivatives (Mes = mesityl) have been prepared via the cycloaddition reaction between pyrido[2,1-a]isoindole (A) or pyrrolo[1,2-a]pyridine (B) and BMes2-containing alkynes. All four compounds are brightly blue or blue-green fluorescent with λ(em) = 428-495 nm and Φ = 0.27-0.68, depending on the substitution position of the BMes2 group. Experimental and TD-DFT computational data indicated that the primary electronic transitions responsible for the fluorescence of 1-4 are from HOMO to LUMO (π → π*) rather than charge transfer from N → B, which is in agreement with previous findings suggesting that the lone-pair on N is delocalized throughout the N-heterocycles.

Reversible 1,1-hydroboration: boryl insertion into a C-N bond and competitive elimination of HBR2 or R-H.

Boranes with the general formula of HBR2 have been found to undergo a facile 1,1-hydroboration reaction with pyrido[1,2-a]isoindole (A), resulting in insertion of a BR2 unit into a CN bond and the formation of a variety of BN heterocycles. Investigation on the thermal reactivity of the BN heterocycles revealed that these molecules have two distinct and competitive thermal elimination pathways: HBR2 elimination (or retro-hydroboration) versus RH elimination, depending on the R group on the B atom and the chelate backbone. Mechanistic aspects of these highly unusual reactions have been established from both experimental and computational evidence. Adduct formation between HBR2 and A was found to be the key intermediate in 1,1-hydroboration of A.