Luminescent Re(I) sensors for copper ion detection in the lifetime based mode

Abstract

A series of diphosphine Re(I) complexes Re(PPh₃)(P-bpy), Re(PPh₃)(P-terpy), Re(P-OEG)(P-bpy) and Re(P-OEG)(P-terpy) have been designed via decoration of the archetypal {Re(CO)₂(NN-OEG)} core with the phosphine ligands bearing bipyridyl (P-bpy) and terpyridyl (P-terpy) functions, which are capable to bind Cu(I) and Cu(II) ions. The presence of the bipyridyl (NN-OEG) and the phosphine (P-OEG) ligands functionalized with oligo(ethylene glycol) fragments is sufficient to impart water-solubility to the target complexes Re(P-OEG)(P-bpy) and Re(P-OEG)(P-terpy) that makes possible to study their photophysical characteristics both in organic and aqueous media. The obtained complexes demonstrate phosphorescence from ³MLCT excited state typical for this type of chromophores and display weak sensitivity to molecular oxygen in methanol, which becomes almost negligible in aqueous and model physiological media. Addition of copper ions (Cu⁺ and Cu²⁺) to the solutions of Re(PPh₃)(P-bpy), Re(PPh₃)(P-terpy), Re(P-OEG)(P-bpy) and Re(P-OEG)(P-terpy) results in a dramatic decrease in their emission intensity accompanied by strong reduction of the excited state lifetime, while the other biologically relevant ions do not display a distinct effect onto the photophysical characteristics of the chromophores. The complexes response onto variations in the copper ions concentration was quantified by building up the lifetime-concentration calibration curves. To gain a deeper understanding of the sensing processes, DFT calculations were performed, which showed the substantial involvement of the orbitals of {(P-bpy/terpy)Cu} fragment into formation of the emissive excited states. Cellular experiments with Chinese hamster ovary (CHO–K1) cells demonstrate cytoplasmic localization of the PPh₃-containing Re(I) probes.