COF-300 synthesis and colloidal stabilization with substituted benzoic acids

Abstract

Colloidal covalent organic framework (COF) synthesis enables morphological control of crystallite size and shape. Despite numerous examples of 2D COF colloids with various linkage chemistries, 3D imine-linked COF colloids are more challenging synthetic targets. Here we report a rapid (15 min-5 day) synthesis of hydrated COF-300 colloids ranging in length (251 nm-4.6 μm) with high crystallinity and moderate surface areas (150 m² g^-1). These materials are characterized by pair distribution function analysis, which is consistent with the known average structure for this material alongside different degrees of atomic disorder at different length scales. Additionally, we investigate a series of para-substituted benzoic acid catalysts, finding that 4-cyano and 4-fluoro substituted benzoic acids produce the largest COF-300 crystallites with lengths of 1-2 μm. In situ dynamic light scattering experiments are used to assess time to nucleation in conjunction with ¹H NMR model compound studies to probe the impact of catalyst acidity on the imine condensation equilibrium. We observe cationically stabilized colloids with a zeta potential of up to +14.35 mV in benzonitrile as a result of the carboxylic acid catalyst protonating surface amine groups. We leverage these surface chemistry insights to synthesize small COF-300 colloids using sterically hindered diortho-substituted carboxylic acid catalysts. This fundamental study of COF-300 colloid synthesis and surface chemistry will provide new insights into the role of acid catalysts both as imine condensation catalysts and as colloid stabilizing agents.

Scheme 1. COF-300 colloid synthesis.

Fig. 1. (A) SEM images of colloidal COF-300 (X = H) particle growth at 90 °C, (B) PXRD: experimental data (top) and simulated hydrated COF-300 pattern (bottom), (C) N₂ adsorption isotherm, and (D) dimensions as determined via SEM measurements.

Fig. 2. (A) Standard one phase crystallographic pair distribution function model (red), data (blue), and difference (green) above and (B) model with separate inter and intra monomer ADPs.

Fig. 3. (A–D, F–H) SEM of COF-300 colloids synthesized at 90 °C for 48 hours made from different benzoic acids. (E) Average colloid length and width distribution data plotted vs. Hammett parameter.

Scheme 2. Carboxylic acid identity affects a series of equilibria: (A) imine condensation and (B) acid/base neutralization.

Fig. 4. (A) ¹H NMR of model compound study displays the ratio of imine (blue diamonds) vs. aldehyde (green circles) quantified relative to a 1,4-dinitrobenzene internal standard (*), (B) the relative concentrations of imine (blue diamonds) to aldehyde (green circles) changes as a function of para-substituted benzoic acid catalyst acidity as quantified via Hammett parameter. The total concentration of imine + aldehyde (yellow triangles) is plotted relative to the expected total concentration (grey line).

Fig. 5. Effect of different benzoic acids on (A) derived mean count rate during synthesis as measured via in situ DLS and (B) nucleation induction delay as a function of Hammett parameter. Error bars come from experiments performed in triplicate.

Scheme 3. Acid plays multiple roles in imine-linked COF colloid synthesis including (A) acid-catalyzed imine condensation, (B) acid-catalyzed imine hydrolysis, (C) acid-catalyzed transimination, and (D) a colloid stabilizing counterion via formation of an acid–base adduct.

Fig. 6. (A) COF-300 colloids that have been fully purified via centrifugation are colloidally stable in acetonitrile only in the presence of added benzoic acid. (B) Z-average size (nm) (represented as filled triangles) and zeta potential (mV) (represented as unfilled triangles) of COF-300 colloids purified via centrifugation and resuspended in benzonitrile as a function of carboxylic acid concentration.

Fig. 7. SEM images comparing COF-300 particle sizes as synthesized with (A–C) para-substituted benzoic acids (X = CH₃, F, CF₃) vs. particles synthesized with (D–F) diortho-substituted benzoic acids (X = CH₃, F, CF₃).