Abstract: Polymerizing monomers into periodic two-dimensional (2D) networks provides structurally precise, layered macromolecular sheets linked by robust, covalent bonds. These materials exhibit desirable mechanical, optoelectrotronic, and molecular transport properties derived from their designed structure and permanent porosity. 2D covalent organic frameworks (COFs) offer broad monomer scope but are generally isolated as insoluble powders comprised of aggregated nanometer-scale crystallites. We overcome this limitation by controlling 2D COF formation using a two-step procedure. In the first step, 2D COF nanoparticle seeds are prepared with approximate diameters of 30 nm. Next, monomers are slowly added to suppress new nucleation while promoting epitaxial growth on the existing seeds to sizes of several microns. The resulting 2D COFs are of exceptional and unprecedented quality, isolated as single crystalline materials with micron-scale domain sizes. Transient absorption spectroscopy of the dispersed, single-crystalline COF nanoparticles enables two to three orders of magnitude improvement of the optical transient absorption signal-to-noise ratio by minimizing scattering photons that plague these measurements on 2D COF powders. These measurements also show evidence for exciton diffusion as a function of crystallite size at longer length scales than available within the small, aggregated crystallites obtained solvothermally. These findings advance the controlled synthesis of 2D COFs and will enable a broad exploration of synthetic 2D polymer structures and properties.
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