Substituent Variation in Tetraaza[14]Annulene-Based Covalent Organic Frameworks for Modulation of Electronic States

Introducing metal centers into covalent organic frameworks (COFs) is an effective and attractive strategy, but the pool of building blocks offering tunable structures and controllable electronic properties remains limited. Here, we designed two amino-functionalized, tetrafunctional nickel tetraaza[14]annulene (TAA) building blocks and incorporated them into two-dimensional COFs. Notably, the peripheral benzene rings of the TAA moiety bear abundant proximal sites amenable to modification, allowing facile tuning of the central nickel ion's electronic state. To investigate the positional effect of substituents, we constructed a pair of isomeric COFs containing the same number of fluorine atoms either at proximal or distal positions. Combined experimental and computational studies reveal that proximal fluorination more effectively reduces electron density at the nickel sites and enhances electron affinity, substantially improving electrocatalytic oxygen evolution reaction (OER) performance. Consequently, the optimized COF achieves an overpotential of 380 mV at 10 mA cm^-2, significantly outperforming its distal-fluorine and fluorine-free counterparts. This work establishes metal TAA macrocycles as a new class of building blocks for metal-based COFs and demonstrates that precise substituent positioning is an effective strategy for modulating the electronic structure and catalytic performance of crystalline framework materials.