Copper(I)-responsive fluorescent probes based on photoinduced electron transfer (PET) switching consistently display incomplete recovery of emission upon Cu(I) binding compared to the corresponding isolated fluorophores, raising the question of whether Cu(I) might engage in adverse quenching pathways. To address this question, we performed detailed photophysical studies on a series of Cu(I)-responsive fluorescent probes that are based on a 16-membered thiazacrown receptor ([16]aneNS3) tethered to 1,3,5-triarylpyrazoline-fluorophores. The fluorescence enhancement upon Cu(I) binding, which is mainly governed by changes in 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 29-fold fluorescence enhancement upon saturation with Cu(I) in methanol and a greater than 500-fold enhancement upon protonation with trifluoroacetic acid. Time-resolved fluorescence decay data for the Cu(I)-saturated probe indicated the presence of three distinct emissive species in methanol. Contrary to the notion that Cu(I) might engage in reductive electron transfer quenching, femtosecond time-resolved pump-probe experiments provided no evidence for formation of a transient Cu(II) species upon photoexcitation. Variable temperature 1H NMR experiments revealed a dynamic equilibrium between the tetradentate NS3-coordinated Cu(I) complex and a ternary complex involving coordination of a solvent molecule, an observation that was further supported by quantum chemical calculations. The combined photophysical, electrochemical, and solution chemistry experiments demonstrate that electron transfer from Cu(I) does not compete with radiative deactivation of the excited fluorophore, and hence, that the Cu(I)-induced fluorescence switching is kinetically controlled.

The photophysical properties of 1,3,5-triarylpyrazolines are strongly influenced by the nature and position of substituents attached to the aryl-rings, rendering this fluorophore platform well suited for the design of fluorescent probes utilizing a photoinduced electron transfer (PET) switching mechanism. To explore the tunability of two key parameters that govern the PET thermodynamics, the excited state energy ΔE 00 and acceptor potential E(A/A − ), a library of polyfluoro-substituted 1,3-diaryl-5-phenyl-pyrazolines was synthesized and characterized. The observed trends for the PET parameters were effectively captured through multiple Hammett linear free energy relationships (LFER) using a set of independent substituent constants for each of the two aryl rings. Given the lack of experimental Hammett constants for polyfluoro substituted aromatics, theoretically derived constants based on the electrostatic potential at the nucleus (EPN) of carbon atoms were employed as quantum chemical descriptors. The performance of the LFER was evaluated with a set of compounds that were not included in the training set, yielding a mean unsigned error of 0.05 eV for the prediction of the combined PET parameters. The outlined LFER approach should be well suited to design and optimize the performance of cation-responsive 1,3,5-triarylpyrazolines.

The design of fluorescent probes for the detection of redox-active transition metals such as Cu(I/II) is challenging due to potentially interfering metal-induced non-radiative deactivation pathways. By using a ligand architecture with a built-in conformational switch that maximizes the change in donor potential upon metal binding and an electronically decoupled tunable pyrazoline fluorophore as acceptor, we systematically optimized the photoinduced electron transfer (PET) switching behavior of a series of Cu(I)-selective probes and achieved an excellent fluorescence enhancement of greater than 200-fold. Crystal structure analysis combined with NMR solution studies revealed significant conformational changes of the ligand framework upon Cu(I) coordination. The photophysical data are consistent with a kinetically controlled PET reaction involving only the ligand moiety, despite the fact that Cu(I)-mediated reductive quenching would be thermodynamically preferred. The study demonstrates that high-contrast ratios can be achieved even for redox-active metal cations, providing that the metal-initiated quenching pathways are kinetically unfavorable.

We have prepared and characterized a Cu(I)-responsive fluorescent probe, constructed using a large tetradentate, 16-membered thiazacrown ligand ([16]aneNS3) 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 ΔE00, 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.