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A rhodamine–naphthalimide dyad probe, 1, that selectively responds to the addition of trivalent metal ions (Fe³⁺ or Al³⁺ or Cr³⁺) via ultrafast Förster resonance energy transfer (FRET) from naphthalimide to rhodamine is designed and synthesized. 1 is highly selective to the trivalent metal ions and the presence of other monovalent or divalent metal ions do not affect its detection ability. The probe is highly sensitive and it can respond to the presence of trivalent metal ions even at sub-micromolar levels. 1 is stable over a broad range of pH, non-toxic under experimental conditions and suitable to the fluorescence bio-imaging of live cells exposed to trivalent metal ions. The trivalent metal ion induced ultrafast energy transfer kinetics of 1 is explored using time resolved fluorescence experiments.