Chemical Control over Nucleation and Anisotropic Growth of Two-Dimensional Covalent Organic Frameworks

Abstract

Two-dimensional covalent organic frameworks (2D COFs) are composed of structurally precise, permanently porous, layered polymer sheets. 2D COFs have traditionally been synthesized as polycrystalline aggregates with small crystalline domains. Only recently have a small number of 2D COFs been obtained as single crystals, which were prepared by a seeded growth approach via the slow introduction of monomers, which favored particle growth over nucleation. However, these procedures are slow and operationally difficult, making it desirable to develop polymerization methods that do not require the continuous addition of reactants over days or weeks. Here, we achieve the rapid growth of boronate ester-linked COFs by chemically suppressing nucleation via addition of an excess of a monofunctional competitor, 4-tert-butylcatechol (TCAT), into the polymerization. In situ X-ray scattering measurements show that TCAT suppresses colloid nucleation, which enables seeded growth polymerizations in the presence of high monomer concentrations. Kinetic Monte Carlo simulations reveal that TCAT limits oligomers to sizes below the critical nucleus size and that in-plane expansion is restricted compared to out-of-plane oriented attachment of oligomers. The simulations are consistent with transmission electron micrographs, which show that the particles grow predominantly in the stacking direction. This mechanistic insight into the role of the modulators in 2D polymerizations enables the size and aspect ratio of COF colloids to be controlled under operationally simple conditions. This chemically controlled growth strategy will accelerate the discovery and exploration of COF materials and their emergent properties.

Figure 1

Schematic of 2D polymerization approaches. Monomers nucleate to form 2D colloidal seeds (left). Pre-existing COF colloid seeds can be grown via slow addition of monomers to seeds resulting in slow, rate controlled growth (top right) or instantaneous addition of monomers resulting in rapid, chemically controlled growth (bottom right).

Figure 2

Chemically suppressed COF-5 nucleation. (A) Schematic of COF-5 nucleation delayed by the addition of TCAT. (B) In situ SAXS of PBBA/HHTP condensation with variable TCAT loading. (C) In situ WAXS of PBBA/HHTP condensation with variable TCAT loading. (D) WAXS traces of COF-5 monomer with 15 equiv of TCAT loading over time.

Figure 3

Chemically controlled COF-5 colloid growth. (A) Schematic of COF-5 seeded growth with the addition of 15 equiv of TCAT. (B) WAXS traces of COF-5 particles as a function of the amount of added monomer after 48 h of heating. (C) DLS number-average size distributions of traces in part B. (D) WAXS traces of COF-5 particles with 4.5 equiv of monomer added as a function of the amount of time spent heating. (E) DLS number-average size distributions of traces in part D. (F) Nitrogen sorption isotherms recorded at 77 K and BET surface areas of COF-5 growth products in part A. (G) NLDFT incremental pore volume distributions of traces in part F.

Figure 4

Kinetic Monte Carlo simulation of the oligomerization of HHTP and PBBA in the presence of variable TCAT loading. Evolution of (A) the average oligomer size and (B) the size of the largest oligomer (excluding TCAT units) as a function of time.

Figure 5

TEM characterization of COF-5 particles. (A) Low-magnification image of COF-5 seed particle. Inset: FFT of the image. (B) Lattice-resolution TEM image of a COF-5 particle grown from the seeds in part A by adding 4.5 equiv of monomer and 15 equiv of TCAT and heating the reaction for 48 h with consistent lattice fringes extending across the entire particle. (C) FFTs of the three color-coded regions of interest in part B. For additional TEM images at different added monomer equivalents see Figures S24–S28.

Figure 6

Chemically controlled colloid growth of other boronate ester-linked COFs. (A–C) WAXS traces of seeded growth products made from instantaneously adding 9 equiv of monomer and 15 equiv of TCAT to COF seeds as a function of the time spent heating for TP-COF (A, D), DPB-COF (B, E), and COF-10 (C, F), respectively. (D–F) DLS number-average size distributions of seed and final time point traces in parts A–C.