Iminophosphorane-naphthalimide derivatives as multi-responsive fluorescent sensors for Sn(II) ion detection and intracellular pH imaging.

A series of iminophosphorane-functionalized naphthalimide derivatives (4a-4e) were synthesized and evaluated as fluorescent sensors. Among them, the trimethylammonium-functionalized derivative 4e exhibited pronounced solvent-dependent fluorescence, characteristic of an intramolecular charge transfer (ICT) process. In acetonitrile, 4e showed excellent selectivity toward Sn(II) ions, accompanied by significant fluorescence quenching and a spectral shift resulting from preferential coordination of Sn(II) to the iminophosphorane nitrogen atom. Fluorescence titration experiments revealed sensitive detection of Sn(II) with a limit of detection of 86.3 nM and a 1:1 binding stoichiometry. These findings were supported by Job's plot analysis and Stern-Volmer studies, which indicated a static quenching mechanism. In addition, probe 4e displayed marked fluorescence enhancement over the acidic pH range of 3.0-6.0, which was attributed to protonation-regulated photoinduced electron transfer (PET). Complementary DFT and TD-DFT calculations provided molecular-level insight into the sensing mechanisms by elucidating the Sn(II)-induced ICT inhibition and protonation-regulated PET modulation. The probe also exhibited good stability, high pH selectivity, and low cytotoxicity toward RAW264.7 macrophage cells. Fluorescence imaging studies further demonstrated its ability to monitor intracellular pH variations, with fluorescence being suppressed in lipopolysaccharide-stimulated macrophages as a result of increased intracellular acidification. These results highlight the potential of 4e as a multifunctional fluorescent probe for environmental sensing and biological imaging.