Metal‐Free Singlet Oxygen Generation via Excited‐State Intramolecular Proton‐Transfer‐Driven Intersystem Crossing in 2D Covalent Organic Frameworks

Abstract Singlet oxygen ( 1 O 2 ) is a key reactive species in photodynamic therapy and organic synthesis. Conventional generation of 1 O 2 relies on metal‐containing sensitizers to promote intersystem crossing (ISC) and thereby activate molecular oxygen ( 3 O 2 ), which limits biocompatibility and scalability. Here, we report a metal‐free strategy leveraging excited‐state intramolecular‐proton‐transfer (ESIPT) to enhance 1 O 2 production. Two classes of ESIPT‐active materials, 1D polymers and 2D covalent organic frameworks (COFs), were systematically compared. Interestingly, while the ESIPT transition in the 1D polymer is incomplete and unstable, 2D COF enables a highly stabilized tautomeric transition, resulting in a persistent metastable state that acts as a gateway to enhanced ISC. This difference is due to a reversed ESIPT pathway dictated by ground‐state geometry. Time‐resolved spectroscopic studies reveal that the ESIPT transition process in the 2D COF triggers ISC, facilitating 1 O 2 generation. Thermodynamic analysis reduces the singlet–triplet energy gap and increases dipole moment changes, while spin–orbit coupling and frontier molecular orbital reorganization indicate kinetic facilitation of ISC. This work highlights the unique advantages of 2D‐COF‐based ESIPT transformations, offering a groundbreaking approach to boosting ISC efficiency and 1 O 2 generation, expanding the scope of ESIPT in photocatalytic applications.