Copper(II)-benzimidazole complexes as efficient fluorescent probes for l-cysteine in water

Copper(ii) complexes [Cu(L1)(H₂O)₂](SO₃CF₃)₂1 and [Cu(L2)(H₂O)₂](SO₃CF₃)₂2 based on 2,6-bis(benzimidazolyl)pyridine were synthesized and are reported herein as highly selective "turn-on" optical probes for l-cysteine. The Cu(ii)/Cu(i) redox potential of probe 1 (0.14 V vs. NHE) was lower than that of 2 (0.233 V vs. NHE) in water. The molecular structure of 2 adopted a square pyramidal geometry (τ = 0.2545), with the Cu-N_py bond (1.958 Å) of its middle pyridine unit being shorter than the other two Cu-N_benzim bonds (Cu-N, 1.995, 2.000 Å). The axial Cu-O2 bond distance (2.247 Å) was slightly longer than the equatorial Cu-O1 bond distance (1.953 Å). The square-based geometry was further supported by the A_∥ value of 156 × 10^-4 cm^-1 in EPR at 70 K. The d-d and ligand-based transitions appeared at 662 and 314-356 nm for 1 and 651 and 313-360 nm for 2, respectively, in HEPES buffer at pH 7.34. These probes showed selective and efficient "turn-on" fluorescence behaviour towards Cys over other natural amino acids with a binding constant for 1 of 5.4 × 10⁴ and 1.30 × 10⁴ M^-1 for 2 and a limit of detection of 2.9 × 10^-8 M and 3.32 × 10^-8 M, respectively, for 1 and 2 at pH 7.34. The quantum yield for the detection of Cys by 1 (14.7%) was much lower than by 2 (23%). The fluorescence intensity of 1 and 2 were also slightly enhanced by histidine, but at a relatively lower level than that exhibited by Cys.