Abstract: Ultra-thin two-dimensional (2D) ionic covalent organic frameworks (iCOFs) are highly promising for solar-driven H₂O₂ production, benefiting from their short charge diffusion paths, superior self-dispersion, and ease of exfoliation. Nevertheless, the intrinsic dielectric confinement in these 2D materials gives rise to a pronounced exciton effect, which severely impedes charge separation. Herein, we performed molecular-level functionalization of iCOFs by incorporating electron-donating groups (hydroxyl or methoxy) and comprehensively probed the influence of this modification on dielectric confinement and photogenerated carrier dynamics. The detailed experimental and theoretical investigations demonstrate that the installed donor moieties provoke intense orientational polarization, which maximizes permittivity and yields the most substantial reduction in exciton binding energy (E_b). As a result, Hall electron mobility is considerably improved, and singlet excitons are efficiently dissociated into free charge carriers under irradiation, enabling their engagement in the surface H₂O₂ evolution reaction. Remarkably, the methoxy-functionalized iCOF (TMTG) delivers the highest H₂O₂ yield of 568.9 μmol L^-1 h^-1 in ambient air and pure water, surpassing its hydrogen- and hydroxyl-substituted counterparts (TBTG and TPTG, respectively). Furthermore, evidence from a set of control experiments demonstrates that O₂ activation is facilitated by the introduction of methoxy groups, thereby guaranteeing that H₂O₂ is produced via a two-step single-electron route. This research highlights that accurate tuning of the chemical microenvironment in iCOFs paves the way for designing high-performance organic photocatalysts for solar-to-chemical conversion.

Key words: Photocatalysis, Ionic covalent organic frameworks, H₂O₂ production, Electron-donating groups, Dielectric confinement