Superacid-resistant macrocyclic BODIPYs

Abstract Boron-dipyrromethenes (BODIPYs) are versatile fluorophores with intense fluorescence and broad applications in bioimaging and sensing. However, they undergo deboronation under acidic conditions, which causes fluorescence degradation. Herein, we designed exceptionally acid-stable BODIPYs by harnessing the synergistic boron-chelation effect of calix3pyrrole-like macrocycles. We show that their characteristic optical properties are retained in strongly acidic media, including superacids, without undergoing deboronation. Macrocyclic BODIPYs exhibit sharp absorption and protonation-induced fluorescence switching, with quantum yields of up to 0.90 and narrow Stokes shifts. Notably, no deboronation was observed even in non-diluted fluorosulfuric acid, and visible fluorescence was sustained for over a day. Beyond their unusual acid resistance, the macrocyclic BODIPYs had higher thermal- and photostability compared with conventional BODIPYs. Peripheral substitution allowed the modulation of absorption and emission wavelengths, and fluorous-tagging through axial ligand exchange enabled fluorescence switching in response to perfluorooctanoic acid in a fluorous solvent. We used superacid-resistant BODIPYs as acid indicators for the fluorescence staining of Nafion beads and sulfonylated gels, which are too acidic to sustain the fluorescence emission of conventional BODIPYs. Our findings expand the scope of BODIPYs into strongly acidic, non-aqueous environments, opening opportunities for fluorescence imaging and sensing in materials and biological systems.