Heavy-Atom Free spiro Organoboron Complexes As Triplet Excited States Photosensitizers for Singlet Oxygen Activation

[Image: see text] Herein, we present a new strategy for the development of efficient heavy-atom free singlet oxygen photosensitizers based on rigid borafluorene scaffolds. Physicochemical properties of borafluorene complexes can be easily tuned through the choice of ligand, thus allowing exploration of numerous organoboron structures as potent (1)O(2) sensitizers. The singlet oxygen generation quantum yields of studied complexes vary in the range of 0.55–0.78. Theoretical calculations reveal that the introduction of the borafluorene moiety is crucial for the stabilization of a singlet charge transfer state, while intersystem crossing to a local triplet state is facilitated by orthogonal donor–acceptor molecular architecture. Our study shows that quantitative oxidation of selected organic substrates can be achieved in 20–120 min of irradiation with only 0.05 mol % loading of a photocatalyst.