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

Small-molecule fluorescent\nprobes are\npowerful tools in chemical analysis and biological imaging. However,\nas the foundation of probe design, the meager existing set of core\nfluorophores have largely limited the diversity of current probes.\nConsequently, there is a high demand to discover fluorophores with\nnew scaffolds and optimize the existing fluorophores. Here, we put\nforward a facile strategy of heterocyclic N-oxidation to address these\nchallenges. The introduced N–O bond reconstructs the electron “push–pull”\nsystem of heterocyclic scaffolds and dramatically improves their photophysical\nproperties by red-shifting the spectra and increasing the Stokes shift.\nMeanwhile, the heterocyclic N–O bond also enables a function\nof the fluorescence switch. It can turn on the fluorescence of pyridine\nand increase the fluorescence of quinoline and, conversely, decrease\nthe fluorescence of acridines and resorufin. As a further practical\napplication, we successfully utilized the quinoline N-oxide scaffold\nto design fluorogenic probes for H2S (8) and\nformaldehyde (FA, 9). Given their ultraviolet–visible\nspectra, both probes with high selectivity and sensitivity could be\nconveniently used in the naked eye detection of target analytes under\nillumination with a portable UV lamp. More interestingly, the probes\ncould be effectively used in the imaging of nuclear and cytoplasmic\nH2S or nuclear and perinuclear FA. This potentially overcomes\nthe weaknesses of existing H2S or FA probes that can only\nwork in the cytoplasm. These interesting findings demonstrate the\nability to rapidly expand and optimize the existing fluorophore library\nthrough heterocyclic N-oxidation.