Kinetically Controlled Photoinduced Electron Transfer Switching in Cu(I)-Responsive Fluorescent Probes
摘要:
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]aneNS(3)) 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 H-1 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.
Electronically tuned 1,3,5-triarylpyrazolines as Cu(i)-selectivefluorescent probes
作者:Manjusha Verma、Aneese F. Chaudhry、M. Thomas Morgan、Christoph J. Fahrni
DOI:10.1039/b918311f
日期:——
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.
Kinetically Controlled Photoinduced Electron Transfer Switching in Cu(I)-Responsive Fluorescent Probes
作者:Aneese F. Chaudhry、Manjusha Verma、M. Thomas Morgan、Maged M. Henary、Nisan Siegel、Joel M. Hales、Joseph W. Perry、Christoph J. Fahrni
DOI:10.1021/ja908326z
日期:2010.1.20
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]aneNS(3)) 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 H-1 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.