Figure 14.

Rhodamine-crown ether conjugate 32 was prepared as a mixture of regioisomers (as indicated), and found to be a selective probe for Cr³⁺; this substrate elicits a turn-on response from the rhodamine fluorophore that is thought to arise via metal-mediated opening of the spirolactam [109]. Triazolyl-coumarin-aza-crown ether 33 responds strongly to Hg²⁺, Fe³⁺ and Cr³⁺, with solvent-dependent variations in response: in water, Fe³⁺ triggers a strong enhancement of the fluorescence emission at 475 nm, with weaker responses to Hg²⁺ and Cr³⁺, while in ethanol it is Hg²⁺ that elicits the strongest response, and smaller responses are seen to Fe³⁺ and Cr³⁺ [110]. Fluorenyl-imine derivative 34 is a very sensitive and selective probe for Cu²⁺, which triggers a ca. 54-fold fluorescence enhancement thought to arise due to inhibition of both PET and C=N isomerisation quenching processes [111].