The development of responsive, luminescent lifetime probes based upon axially functionalised fac-[Re(CO)3(di-imine)(L)]+ complexes

The syntheses of four new ditopic ligands (L1–4), based on N,N-functionalised 4-aminomethylpyridine, was achieved via a reductive amination methodology. The ligands react with fac-[Re(CO)3(di-imine)(MeCN)](OTf) to give the mixed-ligand species fac-[Re(CO)3(di-imine)- (L1–4)](OTf). X-Ray crystallography has been used to structurally characterise one of the complexes confirming that the axial ligand (L4) coordinates via the pyridine unit: by adopting this binding mode the axial ligand therefore provides an additional binding site for metal cations. Each of the complexes displayed room temperature phosphorescence from 3MLCT excited states. In addition the complexes based upon L1–3 also possessed ligand-centred fluorescence, originating from either the quinoline or 8-hydroxyquinoline units of the axial ligands. The luminescent properties of the complexes were monitored in the presence of metal dications of physiological and toxicological importance (Cu(II), Zn(II) and Hg(II)). As well as emission intensity changes the results show that these probes firstly, display highly modulated 3MLCT lifetimes thus allowing differentiation between the different dications and secondly, can demonstrate selectivity in an ionic mixture, as judged by 3MLCT lifetime measurements