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Fluorescence-active chemosensors (L1-L4), comprising a rhodamine scaffold and a pseudo azacrown cation-binding subunit, have been proposed and characterized as a fluorescent chemosensor for Hg(2+). An on-off type fluorescent enhancement was observed by the formation of the ring-opened amide form of the rhodamine moiety, which was induced by the interactions between Hg(2+) and the chemosensor. Upon the addition of Hg(2+), an overall emission change of 350-fold was observed, and the selectivity was calculated to be 300 times higher than Cu(2+) for receptors L2-L4. A polymeric thin film can be obtained by doping poly(methyl methacrylate) or PMMA with chemosensor L2. Such a thin film sensor can be used to detect Hg(2+) with high sensitivity and can be recovered using diluted NaOH.

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A reversible rhodamine-based sensor (L1) capable of undergoing excimer-fluorescent resonance energy transfer (E m -FRET) was designed and synthesized using a ditopic receptor strategy. Addition of Cu 2+ ions to a solution of L1 induced a ring-open conformation of spirolactam (E m -FRET ON), whilst ring closure was induced upon addition of CH 3 COO À (E m -FRET OFF). Fig. 1 Spectral overlap between naphthyl emission (black) and ringopened rhodamine B absorption (red).

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