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

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.