Sp2 Carbon Linked Covalent Organic Framework: Synthesis, Structural Engineering, And Advanced Applications In Energy And Catalysis

Molecular Composition And Structural Characteristics Of Sp2 Carbon Linked Covalent Organic Framework

Sp2 carbon linked covalent organic frameworks are distinguished by their fully conjugated backbones constructed through irreversible C=C linkage formation, typically via Knoevenagel condensation or aldol condensation reactions14. The fundamental architecture comprises planar aromatic monomers—such as 1,3,5-triformylphloroglucinol (TFP), 2,4,6-trimethyl-1,3,5-triazine (TMT), or pyrene derivatives—that undergo condensation with activated methylene or nitrile-bearing co-monomers under solvothermal conditions (120–160°C, 48–96 h)14. For instance, the synthesis of TFPPy-PDAN involves reacting 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPPy) with 1,5-naphthalenediamine (PDAN) in a mesitylene/1,4-dioxane mixture at 90°C, yielding a red crystalline material with hexagonal pore geometry (pore diameter ~2.8 nm) and Brunauer-Emmett-Teller (BET) surface area exceeding 1200 m²/g2.

The sp2 hybridization of carbon atoms within the linkage ensures maximal orbital overlap, facilitating delocalized π-electron clouds across the entire framework. X-ray diffraction (XRD) patterns of sp2-C COFs typically exhibit sharp reflections at 2θ = 3–5° (corresponding to (100) planes with d-spacing ~25–30 Å) and broader peaks at 25–27° indicative of interlayer π-π stacking (interlayer distance ~3.4–3.6 Å)13. Fourier-transform infrared (FTIR) spectroscopy confirms the absence of aldehyde C=O stretches (~1690 cm⁻¹) and emergence of olefinic C=C vibrations (~1580–1620 cm⁻¹), while solid-state ¹³C NMR reveals diagnostic signals at 120–140 ppm corresponding to sp2 carbons46.

Key structural parameters include:

• Pore Size Distribution: Tunable from 1.0 to 8.0 nm by varying monomer geometry; smaller pores (≤2 nm) favor selective gas adsorption, whereas larger pores (≥3 nm) enhance catalytic site accessibility812.
• Layer Stacking Mode: Predominantly AA or AB stacking with interlayer distances of 3.4–3.6 Å, as determined by powder XRD and transmission electron microscopy (TEM)110.
• Crystallinity: Quantified by full-width at half-maximum (FWHM) of XRD peaks; high-quality sp2-C COFs exhibit FWHM < 0.5° for the (100) reflection, indicating long-range order exceeding 50 nm610.

The incorporation of heteroatoms (N, S, B) into the aromatic backbone further modulates electronic properties. For example, triazine-based sp2-C COFs (e.g., Cu-TMT) integrate nitrogen atoms that activate adjacent methyl groups for aldol condensation, while simultaneously introducing Lewis basic sites for metal coordination35. Sulfur-doped variants (thioether-functionalized COFs) demonstrate red-shifted optical absorption (λmax ~550 nm vs. 480 nm for undoped analogs) and enhanced singlet oxygen generation quantum yields (ΦΔ = 0.68)14.

Precursors And Synthesis Routes For Sp2 Carbon Linked Covalent Organic Framework

Knoevenagel Condensation Pathway

The Knoevenagel reaction between aromatic aldehydes and activated nitriles (e.g., malononitrile, arylacetonitrile) constitutes the most widely adopted route for sp2-C COF synthesis14. A representative protocol involves:

1. Monomer Selection: C3-symmetric aldehyde (e.g., 1,3,5-triformylbenzene, TFP) and dicyano-functionalized linker (e.g., 2,2'-([2,2'-bipyridine]-5,5'-diyl)diacetonitrile)1.
2. Solvent System: Mesitylene/1,4-dioxane (1:1 v/v) with 6 M NaOH aqueous solution (catalyst)12.
3. Degassing: Three freeze-pump-thaw cycles under liquid nitrogen to remove dissolved oxygen, followed by flame-sealing in Pyrex tubes14.
4. Heating: Solvothermal treatment at 140–160°C for 72–96 h; higher temperatures (≥150°C) accelerate kinetics but may induce defect formation46.
5. Purification: Centrifugal separation, sequential washing with water and tetrahydrofuran (THF), Soxhlet extraction in THF for 48 h, and vacuum drying at 80°C12.

Critical process parameters include:

• Base Concentration: NaOH molarity of 4–6 M optimizes deprotonation of active methylene groups; lower concentrations (<3 M) result in incomplete conversion, while higher concentrations (>8 M) promote side reactions24.
• Monomer Stoichiometry: Aldehyde-to-nitrile molar ratio of 1:1.5 to 1:2 ensures excess nitrile availability, compensating for volatility losses during heating14.
• Reaction Time: Extended durations (≥72 h) improve crystallinity (FWHM reduction from 0.8° to 0.4°) but exhibit diminishing returns beyond 96 h6.

Aldol Condensation Pathway

Aldol condensation between triazine-activated methyl groups and aromatic aldehydes generates unsubstituted olefin linkages (-CH=CH-), offering superior π-conjugation compared to cyano-substituted analogs36. The synthesis of Cu-TMT exemplifies this approach:

1. Metal-Organic Precursor: Cu3(PyCA)3·H2O (PyCA = 4-pyridinecarboxaldehyde) serves as both aldehyde source and metal dopant3.
2. Co-Monomer: 2,4,6-Trimethyl-1,3,5-triazine (TMT) provides activated methyl groups adjacent to electron-withdrawing nitrogen atoms3.
3. Reaction Conditions: Mesitylene/1,4-dioxane (1:1 v/v), trifluoroacetic acid (TFA, 0.5 mL), acetonitrile (0.2 mL), 150°C, 72 h36.
4. Product Isolation: Centrifugation, washing with dimethylformamide (DMF) and ethanol, vacuum drying at 100°C3.

The resulting Cu-TMT framework exhibits:

• Optical Bandgap: 1.85 eV (λonset = 670 nm), significantly narrower than imine-linked COFs (Eg ~2.4 eV)3.
• Photocurrent Density: 12.3 μA·cm⁻² under simulated solar irradiation (AM 1.5G, 100 mW·cm⁻²), threefold higher than metal-free TMT-based COFs3.
• Singlet Oxygen Quantum Yield: ΦΔ = 0.72, enabling rapid degradation of chemical warfare agent simulants (2-chloroethyl ethyl sulfide, CEES) with 98% conversion in 30 min3.

Solid-Phase Mechanochemical Synthesis

Room-temperature mechanochemical grinding offers a solvent-free alternative for sp2-C COF preparation, particularly advantageous for large-scale production13. The protocol involves:

1. Monomer Mixing: Stoichiometric amounts of aldehyde and nitrile monomers are combined with catalytic NaOH (10 mol%) in a ball-mill jar13.
2. Grinding: Planetary ball milling at 400 rpm for 2–4 h with intermittent cooling (10 min grinding, 5 min rest)13.
3. Post-Treatment: The crude product is washed with water and ethanol, then annealed at 120°C under vacuum for 12 h to enhance crystallinity13.

Mechanochemically synthesized COFs exhibit comparable surface areas (950–1100 m²/g) to solvothermally prepared analogs but with reduced particle sizes (200–500 nm vs. 1–5 μm), beneficial for catalytic applications requiring high external surface accessibility13.

Physical And Chemical Properties Of Sp2 Carbon Linked Covalent Organic Framework

Thermal And Chemical Stability

Sp2-C COFs demonstrate exceptional thermal stability, with thermogravimetric analysis (TGA) revealing negligible mass loss (<5%) up to 400°C under nitrogen atmosphere14. Decomposition onset temperatures (Td,5%) range from 420°C (cyano-substituted COFs) to 480°C (unsubstituted olefin-linked COFs), surpassing imine-linked COFs (Td,5% ~300°C) by >100°C36. This enhanced stability stems from the high bond dissociation energy of C=C linkages (610 kJ·mol⁻¹) compared to C=N bonds (305 kJ·mol⁻¹)4.

Chemical resistance tests demonstrate:

• Acidic Conditions: Retention of >95% crystallinity after immersion in 6 M HCl for 7 days at 25°C, as confirmed by XRD12.
• Basic Conditions: Stable in 6 M NaOH for 7 days; slight pore size expansion (<5%) observed due to framework swelling24.
• Oxidative Environments: Exposure to 30% H2O2 for 24 h results in <10% surface oxidation (detected by X-ray photoelectron spectroscopy, XPS), with core structure remaining intact46.
• Organic Solvents: Insoluble in common solvents (DMF, THF, chloroform, toluene) even under reflux conditions, confirming robust covalent cross-linking12.

Electronic And Optical Properties

The extended π-conjugation in sp2-C COFs imparts semiconducting behavior with tunable bandgaps (1.6–2.8 eV) depending on monomer selection and linkage type34. Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) reveals broad absorption across the visible spectrum (400–700 nm), with Tauc plot analysis yielding direct bandgaps:

• Cyano-Substituted COFs: Eg = 2.2–2.5 eV (λonset = 500–560 nm)14.
• Unsubstituted Olefin-Linked COFs: Eg = 1.8–2.1 eV (λonset = 590–690 nm)36.
• Heteroatom-Doped COFs: Eg = 1.6–1.9 eV (λonset = 650–775 nm), with sulfur doping inducing the most pronounced red-shift14.

Electrochemical impedance spectroscopy (EIS) and Hall effect measurements quantify charge transport properties:

• Electrical Conductivity: 10⁻⁶ to 10⁻³ S·cm⁻¹ for pristine sp2-C COFs; post-carbonization at 600°C under N2 increases conductivity to 10⁻¹ S·cm⁻¹513.
• Charge Carrier Mobility: 0.5–8.1 cm²·V⁻¹·s⁻¹ (electrons) and 0.2–3.4 cm²·V⁻¹·s⁻¹ (holes), measured via time-resolved microwave conductivity (TRMC)310.
• Exciton Binding Energy: 0.3–0.5 eV, lower than imine-linked COFs (0.6–0.8 eV), facilitating charge separation under illumination46.

Photoluminescence (PL) spectroscopy reveals emission maxima at 520–650 nm with quantum yields (ΦPL) of 5–15%, indicating moderate radiative recombination rates suitable for photocatalytic applications314.

Porosity And Surface Area

Nitrogen adsorption-desorption isotherms at 77 K exhibit Type IV behavior with H2-type hysteresis loops, characteristic of mesoporous materials12. Key porosity metrics include:

• BET Surface Area: 800–2100 m²/g, with highest values achieved in frameworks featuring large aromatic cores (e.g., pyrene, porphyrin)210.
• Pore Volume: 0.6–1.8 cm³·g⁻¹, correlating with pore diameter and framework density112.
• Pore Size Distribution: Narrow distributions (FWHM < 1 nm) centered at 1.5–4.5 nm, determined by non-local density functional theory (NLDFT) analysis24.

The hierarchical porosity—comprising micropores (<2 nm) within framework walls and mesopores (2–8 nm) between stacked layers—enhances mass transport in catalytic and adsorption applications1012.

Functionalization And Post-Synthetic Modification Of Sp2 Carbon Linked Covalent Organic Framework

Amino Group Grafting

Surface modification with amino groups enhances hydrophilicity and introduces reactive sites for further derivatization2. The protocol involves:

1. Hydroxylamine Treatment: Suspending sp2-C COF (100 mg) in anhydrous ethanol (20 mL) containing NH2OH·HCl (200 mg) and triethylamine (0.5 mL)2.
2. Heating: Stirring at 85°C for 12 h under N2 atmosphere2.
3. Purification: Centrifugation, washing with ethanol, vacuum drying at 60°C2.

Characterization by XPS reveals N 1s peaks at 399.2 eV (amino nitrogen) and 401.5 eV (residual nitrile), with amino content quantified at 2.5–4.0 mmol·g⁻¹2. Amino-functionalized COFs (TFPPy-PDAN-AO) exhibit enhanced adsorption capacity for perfluorooctanoic acid (PFOA, 185 mg·g⁻¹ vs. 120 mg·g⁻¹ for pristine COF) due to electrostatic interactions between protonated amines and anionic PFOA2.

Metal Coordination

Incorporation of metal ions (Cu²⁺, Zn²⁺, Ni²⁺) into sp2-C COFs generates single-atom catalysts with atomically dispersed active sites313. Two