Topological Effects of Two-Dimensional Covalent Organic Polymers on Photocatalytic Water Splitting

Two-dimensional covalent organic polymers (2D COPs) offer a promising platform for metal-free photocatalytic water splitting (PWS), owing to their tunable optoelectronic properties, extended π-conjugation, and structural porosity. However, the impact of their lattice topology on photocatalytic performance remains poorly understood. In this study, we employ density functional theory and nonadiabatic molecular dynamics to investigate eight topological isomers of 2D COPs featuring single-pore rhombic (rmb) and dual-pore kagome (kgm) lattices, constructed from experimentally accessible building blocks, tetrathieno-naphthalene (TTN) and tetrathienylethene (TTE) linked via alkene or imine moieties. Our calculations reveal that optoelectronic properties, including bandgaps (1.03–1.86 eV), optical absorption range, and HER/OER thermodynamics, are primarily dictated by the core-linker chemistry. However, topological structure governs the carrier dynamics in PWS. kgm COPs exhibit pronounced asymmetry in electron-hole transport, spatially separated charge carriers, and substantially prolonged electron-hole recombination lifetimes compared to their rhombic counterparts, which exhibit unidirectional anisotropic transport leading to rapid recombination. Among all candidates, kgm-TTN-Bz-I stands out as a viable overall photocatalyst under neutral pH, with optimal band alignment and extended carrier lifetimes (104.91 ps). Overall, this work highlights the critical interplay between molecular and topological design parameters, offering valuable insights for the development of efficient and sustainable metal-free photocatalysts.