Oxygen-Independent Activatable Photosensitizers Based on the Control of Intersystem Crossing with a Tetrad-BODIPY Scaffold.

A compact photosensitizer platform has been developed to enable precise and tunable control over intersystem crossing (ISC) without the use of heavy atoms. The tetrad-based molecular architecture incorporates a simple electron-withdrawing substituent, allowing ISC to be switched OFF and the photosensitizer to remain inactive until reactivated. This design achieves an exceptionally long excited-state lifetime of 142 μs, facilitating low-dose operation with phototoxic efficacy that is virtually independent of oxygen concentration. The heavy-atom-free structure minimizes dark toxicity and enhances intracellular distribution. By overcoming key limitations of conventional photosensitizers─such as reliance on heavy atoms, poor selectivity, and lack of spatiotemporal control─the platform establishes a universal scaffold for biomedical applications. Beyond photodynamic therapy, the system provides a foundational molecular tool for optobiology, enabling light-mediated regulation of biological functions with high precision. These features position the photosensitizer as a versatile agent for advancing therapeutic strategies and light-regulated life sciences.