Effects of a Single Atom Substitution on the Physical Properties, Excited State Dynamics, and Iodine Capture Performance of Emissive Covalent Organic Frameworks

A common strategy for developing emissive covalent organic frameworks (COFs) with varied properties is incorporating diverse chromophoric monomers. Herein, an alternative approach is adopted to demonstrate that a simple alteration in just one atom (oxygen vs. sulfur) in monomer design can result in significant differences in the physical, chemical, and photophysical properties of the resulting COFs. Specifically, monomers with the same symmetry but containing either urea or thiourea functionalities are used to synthesize two crystalline, fully conjugated emissive COFs, COF-SMU-2 (urea-based), and COF-SMU-3 (thiourea-based), with sql type topology. Steady-state (both in solid state and solution), time-resolved, and broadband femtosecond transient absorption spectroscopies reveal the excited-state exciton dynamics of the two COFs, explaining the dramatic differences in their photoluminescence behaviors. Further, density functional theory (DFT) studies are performed, which confirm the occurrence of charge transfer in these systems. A direct impact of the single atom variation is also observed during I₂ adsorption studies. Taken together, this study presents new routes to fabricate COFs with distinct properties by making single-atom modulations, and widens the scope of developing emissive COFs capable of demonstrating excited-state charge transfer, with potential applications in optoelectronics and environmental remediation.