Two-Dimensional Semiconducting Covalent Organic Frameworks with Unidirectional Fused-Ring Bands

Integrating long-range conjugated fused-ring structures is crucial for developing high-performance conductive covalent organic frameworks (COFs), yet it remains highly challenging. In this work, we present four two-dimensional (2D) COFs containing unidimensional fused-ring structures via metal ion-mediated metal tetraaza[14]annulene (MTAA) ring formation from nonfused-ring-based precursors. The resulting COFs exhibit unique rectangular 2D frameworks comprising interconnected, parallel linear conjugated fused-ring bands. This architecture confers a narrow optical bandgap below 1 eV, high intrinsic conductivity on the order of 10^-3 S cm^-1, and significant carrier mobility reaching up to 10^-1 cm² V^-1 s^-1. Additionally, abundant exposed metal coordination sites within the 2D plane suggest significant potential as high-performance electrocatalysts. CoTAA-COF-1 demonstrates an oxygen reduction reaction (ORR) half-wave potential of 0.84 V in alkaline conditions, among the highest reported for COF-based ORR catalysts. This work provides an effective design strategy for fused-ring COFs while offering new insights for developing narrow-band-gap organic semiconductors and advanced electrocatalysts.