Conversion from ILCT to LLCT/MLCT Excited State by Heavy Metal Ion Binding in Iridium(III) Complexes with Functionalized 2,2'-Bipyridyl Ligands

Iridiurn(III) complexes with two ppy (ppy = 2-phenylpyridine) and one functionalized 2,2'-bipyridyl ligand containing a bidentate or terdentate binding group were synthesized, and their photophysical and luminescence sensing properties to heavy metal ions were studied. Complex I was also Structurally characterized by single-crystal X-ray crystallography. These complexes display a low-energy absorption band with the maximum at 410-430 nm ascribed to an intraligand charge transfer (ILCT) transition from the HOMO (pi) residing on the fragment C C-C6H4-R to the LUMO (pi) localized on the 2,2'-bipyridyl (bpy) in the functionalized 2,2'-bipyridyl ligand bpyC C-C6H4-R, as revealed from DFT calculations. Upon irradiation at 350 nm < lambda(ex), < 430 nm, they show weak emission at ca. 618 nm since 3 ILCT transition is the lowest lying excited state. The ion-binding properties of these iridium(III) complexes to heavy metal ions were investigated by UV-vis and emission spectroscopy. Upon complexation with metal ions through the binding sites in the functionalized 2,2'-bipyridyl ligand, the absorption band at 410-430 nm is significantly blue-shifted to 350-390 nm, resulting most likely from a conversion of ILCT to LLCT/MLCT transition as the lowest energy excited state, as verified from DFT calculations. Interestingly, a significant luminescence enhancement was detected in these iridium(III) complexes upon binding to specific metal ions due to a conversion of the lowest energy state from (ILCT)-I-3 to the highly emissive (LLCT)-L-3/(3) MLCT triplet excited state. The sensing properties of these iridium(III) complexes to heavy metal ions were modulated by modifying the binding sites in the functionalized 2,2'-bipyridyl ligand.