Functional Naphthalene‐Cored Reversible Fluorescent Probes for Picomolar Fe³⁺ Sensing in Environmentaland Nutritional Samples

New xylene‐bridged naphthalene‐based fluorescent chemosensing system consisting of two structurally homologous probes, OHN and MHN , is synthesized for the selective and sensitive sensing of Fe 3+ ions in a MeOH:H 2 O (1:1, v/v) 4‐(2‐hydroxyethyl)‐1‐piperazineethanesulfonic acid buffer solution through fluorimetric analysis. Both probes are highly intrinsically fluorescent, which is greatly quenched after coordination with Fe 3+ ions. The probe alone exhibits strong fluorescence emission. However, upon binding with Fe 3+ , a significant quenching of fluorescence is observed. This quenching can be attributed to three key mechanisms: i) inhibition of photoinduced electron transfer, ii) restriction of CO bond rotation, and iii) electron transfer facilitated by the paramagnetic nature of the Fe 3+ ion. Job's plot analysis substantiates a 1:2 (ligand:metal) binding stoichiometry for both OHN and MHN to Fe 3+ ions. The binding constants ( Ksv ) are calculated, and the complex formation is further confirmed by electrospray ionization time‐of‐flight mass spectrometry and density functional theory calculations. Both complexes display outstanding sensitivity, with detection limits in the picomolar range. The utility of these probes in a practical sense is showcased by a smartphone‐based colorimetric assay by using the Color Grab app and utilized effectively for detection of Fe 3+ ions detection in environmental and nutritional samples.