BODIPY-Helicene Based Heavy-Atom-Free Photocatalyst for Oxidative Coupling of Amines and Photooxidation of Sulfides.

To develop heavy-atom-free triplet photosensitizers (PSs) based photocatalysts, we designed and synthesized two BODIPY-helicene dyes by fusing the BODIPY core and modified [5]helicene structures. These BODIPY-helicenes structures are twisted and their twisting angles are increased by the developed synthetic method. The BODIPY-helicenes have broad absorption bands over UV-visible region with high triplet conversions and long triplet lifetimes as compared to planar BODIPY dye, PM567. Consequently, these dyes are also highly efficient in generating O by transferring their triplet energy to O. All these are confirmed by dye-sensitised photooxidation reaction, nanosecond transient absorption spectroscopy study, phosphorescence measurement and DFT calculations. Finally, photocatalytic activity of the highest O generating BODIPY-helicene (4 b) was checked. 4 b is highly efficient in photocatalytic oxidative coupling of differently substituted amines through aerobatic oxidation using O generated by its photosensitization. It is also highly efficient photocatalyst for aerobatic oxidation of sulfides to sulfoxides. Importantly, the photocatalyst could be quantitatively recovered and reused for several cycles. All these results confirmed the potential use of the BODIPY-helicenes as PSs for photocatalytic organic reactions and the design strategy will be useful for the future development of heavy-atom-free photocatalyst.