Emissive Single-Crystalline Boroxine-Linked Colloidal Covalent Organic Frameworks

Abstract

The synthesis of periodic two-dimensional (2D) polymers and characterization of their optoelectronic behaviors are challenges at the forefront of polymer chemistry and materials science. Recently, we showed that layered 2D polymers known as 2D covalent organic frameworks (COFs) can be synthesized as single crystals by preparing COF particles as colloidal suspensions. Here we expand this approach from the condensation of boronic acids and catechols to the dehydrative trimerization of polyboronic acids. The resulting boroxine-linked colloids are the next class of 2D COFs to be obtained as single‑crystalline particles, as demonstrated here for four 2D COFs and one 3D COF. Colloidal stabilization enables detailed structural analysis by synchrotron X-ray diffraction and high-resolution transmission electron microscopy. Solution fluorescence spectroscopy revealed that the COF crystallites are highly emissive compared to their respective monomer solutions. Excitation‑emission matrix fluorescence spectroscopy indicated that the origin of this enhanced emission can be attributed to through-space communication of chromophores between COF sheets. These observations will motivate the development of colloidal COF systems as a platform to organize functional aromatic systems into precise and predictable assemblies with emergent properties.

High‐Sensitivity Acoustic Molecular Sensors Based on Large‐Area, Spray‐Coated 2D Covalent Organic Frameworks

“…Recently, we prepared several 2D COFs as stable colloidal suspensions, which is a form amenable to solution‐processing methods ( Figure A). [ 28–33 ] Using these colloidal suspensions as a starting point, here we demonstrate spray coating of 2D COF thin‐films in a manner that retains their crystallinity, as assessed by X‐ray diffraction (XRD) (Figure 1B). This additive manufacturing approach is demonstrated to be generalizable to five different 2D COF films on metal, polymer, and oxide substrates.…”

“…Boronate‐ester‐linked 2D COFs were prepared in high yields as colloidal nanoparticle solutions to be used as inks for spray coating (Figures 1A and 2A). [ 28,31,38 ] 1 H NMR spectroscopy of the supernatant after removal of the colloids by centrifugation revealed no signals corresponding to residual monomers or oligomers (Figures S60,S61, Supporting Information), indicating efficient COF formation. Dynamic light scattering (DLS, Figure B) and in solvo XRD (Figure 2C) indicate that the solutions contain 2D COFs as crystalline nanoparticles with a typical diameter of 50 nm.…”

Synthesis of Boroxine and Dioxaborole Covalent Organic Frameworks via Transesterification and Metathesis of Pinacol Boronates

“…The corresponding NMR spectral data matched that previously reported. 10 1 H NMR (500 MHz, CDCl3) δ 7.23 (m, 2H), 7.09 (m, 2H), 0.82 (s, 3H). 13 C NMR (126 MHz, CDCl3) δ 148.29, 122.49, 112.17 (H3C-B was not observed).…”

“…1 Despite the limited hydrolytic stability the highly dynamic nature of the boroxine and dioxaborole links makes them very attractive synthons in the design of COFs with excellent structural order (including COF single crystals 6 ) and many optoelectronically functional COFs have been synthesized by condensation of areneboronic acids. [7][8][9][10][11] Notably, despite 15 years of extensive exploration of such COFs, only a few chemical modifications of the original synthetic approach 1 have been reported. [12][13] In both boroxine and dioxaborole synthesis water is produced upon condensation, thus achieving anhydrous conditions during COF formation is impractical.…”