Metal-Free Singlet Oxygen Generation via Excited-State Intramolecular Proton-Transfer-Driven Intersystem Crossing in...

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.