Direct Light Storage with Controlled Dark Release in a Bipyridine-Based Covalent Triazine Framework

Abstract Utilizing bifunctional materials capable of direct light-to-charge storage offers an attractive strategy to overcome solar intermittency and enable reliable power supply under dark conditions. Reported bifunctional materials are predominantly based on metal oxides and metal–organic compounds, whereas fully organic systems remain rare despite their rich structural diversity and tunability, which enable systematic structure–property–function investigations. Here, we reveal the light-induced, reversible electron storage properties of a two-dimensional (2D) 2,2′-bipyridine (bpy)-based crystalline covalent triazine framework (CTF), CTF-Bpy. Mechanistic studies suggest that the photoreduction of CTF-Bpy primarily proceeds through a proton-coupled, one-electron reduction of the bpy units, yielding bpyH• and bpyH2•+ as the identified reduction products. Photoanodes based on CTF-Bpy show a stable photoinduced open-circuit potential (photo-OCP) of −0.92 V vs Ag/AgCl, which is so far the most negative potential reported for an organic bifunctional material. Under oxygen-free conditions, photogenerated electrons remain stable and can be controllably released on-demand in the dark, either by electrical discharge (for aqueous solar battery applications) or by reducing O2 to H2O2. Moreover, this work establishes CTF-Bpy as the first photochargeable covalent organic framework (COF) with stored electrons sufficiently reducing to drive H2 evolution in the dark in the presence of a Pt cocatalyst, highlighting the promise of using COFs for "dark photocatalysis". The systematic investigation of the light-induced electron storage in CTF-Bpy with a set of spectroscopic and (spectro-)electrochemical techniques provides a framework for studying optoionic materials and guiding their rational design for tailored dark photocatalysis.