Revealing Sequential Stages of Excited-State Water Chemistry on a Monolayer Covalent Triazine Framework

Direct spectroscopic identification of the earliest stages of interfacial water activation remains a major challenge in photocatalysis, as the first reaction intermediates are short-lived and rapidly evolve into more stable products. Here, we investigate water activation on an electronically excited two-dimensional covalent triazine framework (CTF) grown on Au(111) using localized water dosing and synchrotron X-ray photoelectron spectroscopy (XPS). Room-temperature measurements reveal persistent chemical species consistent with products of the reaction at the water/CTF interface, whereas low-temperature experiments under an ice overlayer reveal a spectroscopic signature assigned to protonation of triazine nitrogen. This transient feature, likely stabilized by the ice environment, may represent the earliest experimentally accessible chemical consequence of the excited-state water−CTF interaction. Its spectroscopic fingerprint is consistent with the initial proton-transfer step proposed in excited-state proton-coupled electron-transfer (ES-PCET) descriptions of water activation. These results provide direct spectroscopic access to transient interfacial species involved in the initial phase of photocatalytic water activation and establish experimental constraints for mechanistic descriptions based on PCET.