A new fluorescent probe X based on 2-pyridin-2-ylethanamine and benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid was designed and synthesized for the detection of Fe³⁺ and PPi. X showed high sensitivity and selectivity for Fe³⁺ in the presence of other competing metal ions in DMF/H₂O buffer solution. The limit of detection for Fe³⁺ was calculated from the titration curve to be 6.7 × 10⁻⁷ M. The Job's plot confirmed a 1 : 1 stoichiometry between the probe and Fe³⁺, and the complexation stability constant was calculated to be 7.46 × 10³ M⁻¹. Moreover, the fluorescence intensity of the probe was recovered after addition of the PPi. X could act as a fluorescence “ON–OFF–ON” probe for the sequential detection of Fe³⁺ and PPi. The binding mode and sensing mechanism of X with Fe³⁺ was verified by DFT/TDDFT calculations using Gaussian 09.