Robust free-base and metalated corrole radicals with reduction-induced emission

Preparing free-base porphyrinoid radicals that can function as coordination ligands is a challenging task. Here we report the synthesis of a stable, free-base benzocorrole (BC) radical containing only two inner NH protons via a retro-Diels-Alder conversion. The radical character of BC was fully supported by crystallographic analysis, spectroscopic evidence, and theoretical calculations. This neutral radical ligand allowed easy insertion of Zn(II), Ga(III), and Pd(II) ions to produce radical complexes. All these radicals exhibited luminescence-on responses under weak reducing atmosphere, corresponding to the conversion to their aromatic anions. The red fluorescence was observed for BC and its Zn(II) and Ga(III) complexes, and the near-infrared phosphorescence (>900 nm) was detected for Pd(II) complex at room temperature. Furthermore, Ga(III) corrole exhibited a variation in fluorescence in response to axial coordination. Our findings provide a promising radical platform for coordination and developing novel functional materials with switchable spin and emission.