Electronically tuned 1,3,5-triarylpyrazolines as Cu(I)-selective fluorescent probes

We have prepared and characterized a Cu(I)-responsive fluorescent probe, constructed using a large tetradentate, 16-membered thiazacrown ligand ([16]aneNS(3)) and 1,3,5-triaryl-substituted pyrazoline fluorophores. The fluorescence contrast ratio upon analyte binding, which is mainly governed by changes of the photoinduced electron transfer (PET) driving force between the ligand and fluorophore, was systematically optimized by increasing the electron withdrawing character of the 1-aryl-ring, yielding a maximum 50-fold fluorescence enhancement upon saturation with Cu(I) in methanol and a greater than 300-fold enhancement upon protonation with trifluoroacetic acid. The observed fluorescence increase was selective towards Cu(I) over a broad range of mono-and divalent transition metal cations. Previously established Hammett LFERs proved to be a valuable tool to predict two of the PET key parameters, the acceptor potential (E(A/A(-)) and the excited state energy Delta E-00, and thus to identify a set of pyrazolines that would best match the thermodynamic requirements imposed by the donor potential E(D+/D) of the thiazacrown receptor. The described approach should be applicable for rationally designing high-contrast pyrazoline-based PET probes selective towards other metal cations.