High-contrast metal-responsive fluorescent probes based on synergistic electronic and conformational switching
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We have prepared and characterized a Mg(II)-responsive fluorescent probe, constructed using a large tetradentate, 15-membered crown ether ligand and a 1,3,5-triaryl-substituted pyrazoline fluorophore. The photoinduced electron transfer (PET) driving force between the ligand and fluorophore controls the fluorescence contrast ratio when an analyte is bound to the probe. Previous studies have shown that the maximum obtainable fluorescence probes based on macrocyclic ligands was impaired due to ternary complex formation of the metal-ligand complex with the solvent molecules. We redesigned the metal ion receptor by incorporating the aniline ring into the ligand. We selected four fluoro substituents to increase the electron withdrawing ability of the 1-aryl-ring, yielding a very high fluorescence enhancement upon saturation with Mg(II) in acetonitrile. The quantum yield of this pyrazoline probe was found to be 79%.We expect that the described approach should be applicable for rationally designing high-contrast pyrazoline-based PET probes selective towards other metal cations as well.