Single-Atom Tuning of Structural and Optoelectronic Properties in Halogenated Anthracene-Based Covalent Organic...

Strategies for tuning structural and (opto-)-electronic properties are fundamental to the rational design of functional materials. Here, we present a molecular-design approach that enables precise modulation of the optoelectronic properties of covalent organic frameworks (COFs) through single-atom halogen substitution on π-extended anthracene linkers. Using a Wurster-type tetratopic amine (W-NH₂) and a series of anthracene-based dialdehydes bearing H, Cl, Br, or I at the 2-position, a family of imine-linked COFs, W-A-X (X = H, Cl, Br, I), was synthesized. The halogen substituent strongly influences framework formation, with brominated COFs forming substantially larger crystalline domains than the unsubstituted analogue. UV-vis absorption and photoluminescence measurements reveal a systematic redshift across the series (H < Cl < Br < I), demonstrating that a single-atom modification tunes the optical response. In addition, the significantly longer excited-state lifetime of W-A-Cl COF highlights the strengthened donor-acceptor interactions induced by the more electron-withdrawing chlorine substituent, further confirming the decisive role of single-atom functionalization in controlling excited-state dynamics. Time-dependent density functional theory calculations on both isolated fragments and extended COF models attribute the observed trends in optical response to halogen-induced changes in the COF band structure and provide a mechanistic understanding of how a single-atom substitution influences the optoelectronic properties of the extended π-framework. Overall, this study establishes single-atom halogen substitution as a powerful and modular tool for tailoring the structural and optical properties of anthracene-based COFs.