Kinetically Controlled Photoinduced Electron Transfer Switching in Cu(I)-Responsive Fluorescent Probes

Abstract

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.

Synthetic fluorescent probes for studying copper in biological systems

“…104,105 Coordination to the cationic analyte produces an electrostatic attraction between the analyte and an electron in the highest occupied molecular orbital (HOMO) of the receptor, decreasing the oxidation potential (increasing the reduction potential) of the analyte-bound receptor sufficiently to render the thermodynamics of PET no longer favourable (Figure 2). 98,106 This fluorescence switching mechanism offers an opportunity to finely tune the thermodynamic and photophysical properties of fluorescent probes by synthetically altering the reduction and oxidation potentials of its components and the excited-state equilibrium energy, as shown in previous studies 103,107–109 and can be inverted into a donor PET mechanism as well. 110 Chemical structures of various Cu(I) probes are depicted in Figure 3 and key properties are shown in Table 2.…”

“…From this starting point, systematic modifications to the fluorophore scaffold were employed to improve signal-to-noise response and to understand fundamental aspects of electron-transfer chemistry of these dyes in organic and aqueous media. 109,119 In efforts to improve the hydrophilicity of these platforms, four hydroxymethyl groups were appended to the thiazacrown receptor and a sulphonate moiety to the fluorophore portion to produce CTAP-2 (Figure 3, Table 2). 111 This sensor could be dissolved directly in water instead of requiring dilution from an organic co-solvent, and it did not form nano-scale colloidal aggregates that commonly occur with fluorescent dyes.…”

Proton-Coupled Electron Transfer in Tyrosine and a β-Hairpin Maquette: Reaction Dynamics on the Picosecond Time Scale

“…Peptide A (IMDRYRVRNGDRIHIRLR) was synthesized by solid state synthesis and was obtained from Genscript (Piscataway, NJ; >95% purity). 27,28 Transient absorption spectra 32 were acquired with a pumpwhite light continuum probe spectroscopy system (Ultrafast Systems, Helios). The measurement system consists of a regeneratively amplified titanium sapphire laser (Solstice, Spectra-Physics, 800 nm, 3.7 W average power, 100 fs pulse width, 1 kHz repetition rate) and a computer-controlled optical parametric amplifier (OPA) (Spectra-Physics, TOPAS, wavelength range: 266−2600 nm, pulse width: ∼75 fs HW 1/e ) that is pumped by the amplified laser.…”

Rational design of a water-soluble, lipid-compatible fluorescent probe for Cu(i) with sub-part-per-trillion sensitivity

“…Second, significant residual PET quenching occurs in the Cu(I)-bound form due to partial dissociation of the Cu(I)-N bond, presumably driven by ternary complex formation with solvent molecules. 15 As outlined in the following section, we used a knowledge-driven design approach to systematically address both of these challenging issues within a hydrophilic, lipid bilayer-compatible fluorophore platform.…”

“…According to previous studies with N-arylthiazacrown Cu(I)-complexes, steric interference between the aniline ring and ligand backbone is alleviated through displacement of the arylamine nitrogen donor from the Cu(I) center by a solvent molecule, resulting in residual PET quenching. 15 …”