Synthesis and Electrical Properties of Covalent Organic Frameworks with Heavy Chalcogens

S research has been devoted to capturing the Sun’s energy with both inorganic and organic materials. Although the challenges are myriad, transformative discoveries in the latter class fall squarely in the realm of synthetic chemistry. Despite being lightweight, mechanically flexible, as well as easily and affordably processed, organic photovoltaics suffer from lower efficiencies and lifetimes than commercial inorganic solar cells. Such limitations stem both from morphological defects and from weak electronic coupling coupled to strong electron− phonon interactions. These work together to affect exciton formation, migration, and dissociation, as well as charge transfer and separation at the electrodes. Controlling the way donor and acceptor moieties are assembled is thus key to improving the performance of photovoltaics and other organic electronic devices. One class of materials that can address this challenge is covalent organic frameworks (COFs), whose periodicity, dimensionality, and rigidity allow for optimization of molecular architecture and of intraand intermolecular interactions. Although the majority of COFs lack significant bulk electrical conductivity, their potential to harvest light and conduct electricity is beginning to be tapped. Most relevantly, Jin et al. explored charge dynamics in donor−acceptor COFs and showed that the inherent bicontinuous heterojunction allows direct coupling of light absorption, exciton generation, and charge separation. In 2013, chalcogenophene-based COFs were reported with the use of thiophene, dithiophene, and thienothiophene diboronic acids. These materials engage in unique charge-transfer interactions with electron acceptors such as TCNQ, a first requirement for developing charge and exciton-diffusion materials, and can be used as active materials for organic photovoltaic devices. Even more recently, reports on COFs based on benzodithiophene and tetrathiafulvalene were published. Here, we report our efforts to leverage the structural features of COFs and introduce the first heavier chalcogen-based materials in the form of fused benzodiselenophenes and benzoditellurophenes. The motivation behind the design of chalcogenophene-containing fused-aromatic COFs stems from the leading role of fused chalcogenophenes in organic conductors, narrow band gap polymers, and field-effect transistors. Use of heavy chalcogen atoms has been shown to increase charge mobility in organic photovoltaic materials, presumably through the enhanced orbital overlap afforded by the 3p and 4p orbitals of selenium and tellurium atoms, respectively, or through enhanced spin−orbit coupling effects. Indeed, whereas thiophene-based polymers have larger optical band gaps (1.9 eV in polythiophene vs. 1.5 eV in CdTe) and much smaller charge mobility values than inorganic semiconductors(0.1 cm V−1 s in polythiophene vs. 100−1000 cm V−1 s in indium zinc oxide), replacing sulfur with selenium reduces the optical band gap to 1.6 eV. The close stacking of selenium and tellurium is also expected to reduce the impact of disorder in the self-assembly process required for COF synthesis and generate more disperse bands to enhance interchain packing of the framework. This should lead to increased conductivity for the heavier chalcogen materials without significantly changing the structure or the unit cell size of the resulting extended networks. Herein, we report the synthesis and characterization of the first COFs that incorporate selenium and tellurium atoms into the backbone and show that the presence of the heavier chalcogens indeed leads to superior electrical properties relative to the thiophene analogue. Treatment of trimethylsilyl-protected benzo[1,2-b:4,5-b′]diselenophene and benzo[1,2-b:4,5-b′]ditellurophene with BBr3, followed by basic hydrolysis and subsequent acidification afforded the new diboronic acids benzo[1,2-b:4,5-b′]diselenophene (H2BDSe) and benzo[1,2-b:4,5-b′]ditellurophene (H2BDTe) in good yields (Scheme 1). For