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

Singlet oxygen (1O2) is a key reactive species in photodynamic therapy and organic synthesis. Conventional generation of 1O2 relies on metal-containing sensitizers to promote intersystem crossing (ISC) and thereby activate molecular oxygen (3O2), which limits biocompatibility and scalability. Here, we report a metal-free strategy leveraging excited-state intramolecular-proton-transfer (ESIPT) to enhance 1O2 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 1O2 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 1O2 generation, expanding the scope of ESIPT in photocatalytic applications.