Conjugated Covalent Organic Frameworks via Michael Addition- Elimination M. Rajeswara Rao, † Yuan Fang, †,‡ Steven De Feyter, ‡ and Dmitrii F. Perepichka* ,† † Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 0B8, Canada ‡ Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven - University of Leuven, Celestijnenlaan 200 F, B-3001 Leuven, Belgium * S Supporting Information ABSTRACT: Dynamic covalent chemistry enables self-assembly of reactive building blocks into structurally complex yet robust materials, such as covalent organic frameworks (COFs). However, the synthetic toolbox used to prepare such materials, and thus the spectrum of attainable properties, is very limited. For π-conjugated COFs, the Schiff base condensation of aldehydes and amines is the only general dynamic reaction, but the resulting imine-linked COFs display only a moderate electron delocalization and are susceptible to hydrolysis, particularly in acidic conditions. Here we report a new dynamic polymerization based on Michael addition-elimination reaction of structurally diverse β-ketoenols with amines, and use it to prepare novel two-dimensional (2D) π-conjugated COFs, as crystalline powders and exfoliated micron-size sheets. π-Conjugation is manifested in these COFs in significantly reduced band gap (1.8-2.2 eV), solid state luminescence and reversible electrochemical doping creating midgap (NIR absorbing) polaronic states. The β-ketoenamine moiety enables protonation control of electron delocalization through the 2D COF sheets. It also gives rise to direct sensing of triacetone triperoxide (TATP) explosive through fluorescence quenching. ■ INTRODUCTION Covalent organic frameworks (COFs) are porous crystalline solids obtained by 2D or 3D polymerization of organic building blocks. 1 They emerge as new promising materials for gas 2 and energy storage, 3 catalysis, 4 and semiconducting device applications. 5 Achieving crystalline order in COFs relies on the reversibility of the used chemical reaction; carrying out the polymerization under dynamic equilibrium conditions allows the system to self-assemble toward thermodynamic minimum, via error-correction mechanism (i.e., unlinking and relinking improperly connected building blocks). 1 However, only a handful of reactions 1e are sufficiently dynamic to enable such mechanism: boronic acid self-condensation and polycondensa- tion with aromatic diols, 1,6 aldehyde/amine condensation (Schiff base reaction), 7 nitroso dimerization 8 and imidation 9 reactions. Among these, Schiff base reaction forming imine bonds is currently the only dynamic covalent chemistry suitable for making π-conjugated COFs. 10 Electron delocalization in conventional, 1D π-conjugated polymers gives rise to their special optoelectronic properties, that are employed in organic light-emitting diodes, photo- voltaics, photodetectors, field-effect transistors, sensors, etc. 11 Even more intriguing opportunities could be created if equally efficient electron delocalization in π-functional materials is extended in the second dimension. 12 These opportunities, however, are barely explored in COFs, although a significant progress has been made in 2D conjugated networks synthesized via surface-catalyzed polymerization and other nonequilibrium reactions. 13 Indeed, due to its high polarization, the imine linkage of Schiff base COFs is not very efficient in supporting π- delocalization between the connected units, and most COFs prepared via the dynamic covalent chemistry approach display a modest (0.2-0.3 eV) reduction in the bandgap in comparison to molecular reference compounds (Table S1). Herein, we present a novel and general synthetic approach toward crystalline COFs with improved π-delocalization, using Michael addition-elimination reaction of β-ketoenols and aromatic amines (Scheme 1). The resulting β-ketoenamine linked COFs display an enhanced hydrolytic stability endowed by the intramolecular CO···HN hydrogen bonding, and can be prepared from a broad variety of electrophilic and nucleophilic building blocks. The electron delocalization in these COFs leads to a pronounced band gap reduction (0.3- 0.6 eV), reversible electrochemical doping, and light-emitting properties. The combination of the π-conjugation with specific host-guest interactions within the pores of the COF, gives rise to a remarkable sensing behavior. The fluorescence of 3BD and 3′PD is effectively quenched by the direct interaction with triacetone triperoxide (TATP). This is a notoriously dangerous explosive, but its detection is more challenging than that of common nitro explosives due to its relative chemical inertness Received: November 28, 2016 Published: January 14, 2017 Article pubs.acs.org/JACS © 2017 American Chemical Society 2421 DOI: 10.1021/jacs.6b12005 J. Am. Chem. Soc. 2017, 139, 2421-2427