Heterocyclic N-Oxides as Small-Molecule Fluorogenic Scaffolds: Rational Design and Applications of Their “On–Off” Fluorescence

Small-molecule fluorescent probes are powerful tools in chemical analysis and biological imaging. However, as the foundation of probe design, the meager existing set of core fluorophores have largely limited the diversity of current probes. Consequently, there is a high demand to discover fluorophores with new scaffolds and optimize the existing fluorophores. Here, we put forward a facile strategy of heterocyclic N-oxidation to address these challenges. The introduced N–O bond reconstructs the electron "push–pull" system of heterocyclic scaffolds and dramatically improves their photophysical properties by red-shifting the spectra and increasing the Stokes shift. Meanwhile, the heterocyclic N–O bond also enables a function of the fluorescence switch. It can turn on the fluorescence of pyridine and increase the fluorescence of quinoline and, conversely, decrease the fluorescence of acridines and resorufin. As a further practical application, we successfully utilized the quinoline N-oxide scaffold to design fluorogenic probes for H2S (8) and formaldehyde (FA, 9). Given their ultraviolet–visible spectra, both probes with high selectivity and sensitivity could be conveniently used in the naked eye detection of target analytes under illumination with a portable UV lamp. More interestingly, the probes could be effectively used in the imaging of nuclear and cytoplasmic H2S or nuclear and perinuclear FA. This potentially overcomes the weaknesses of existing H2S or FA probes that can only work in the cytoplasm. These interesting findings demonstrate the ability to rapidly expand and optimize the existing fluorophore library through heterocyclic N-oxidation.