Initially, sensor 1 displayed essentially no fluorescence in H2O/CH3CN, 7:3, v/v, indicating that the spirocyclic form was retained in the solution. However, treatment with Cu2+ immediately elicited a dramatic fluorescence turn-on response (<10 min) at room temperature (Fig. 1). A strong new emission band at 550 nm was formed, and up to about 70-fold enhancement in the fluorescence intensity was noted. The large fluorescence turn-on was corroborated by the observation that the emission color of