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

Iridium(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 1 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 (π) residing on the fragment CC-C6H4-R to the LUMO (π*) 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 < λex < 430 nm, they show weak emission at ca. 618 nm since 3ILCT 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 3ILCT to the highly emissive 3LLCT/3MLCT 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.