Elucidation of Excited-State Properties of Bimetallic Ir(III)-Pt(II) Complexes with Conjugated Bridging Ligands

Abstract

We investigated excited-state properties of 4',7'-phenanthrolino-5',6':2,3-pyrazine (ppz)-bridged bimetallic complexes, (L)(2)Ir-ppz-PtCl2 (Ir-ppz-Pt) = 2-(4',6'-difluorophenyl)pyridinato-N,C-2 (dfppy) ), and [(L)(2)Ir](2)(ppz) (Ir-ppz-Ir). These properties were compared to those of reported dpp bridging complexes, 2,3-bis(2-pyridyl)pyrazine (dpp)-bridged bimetallic complexes, Ir-dpp-Pt, and Ir-dpp-Ir. Excited-state properties of hetero-bimetallic Ir-Pt systems were very different from those of related Ru-Pt systems with a dpp bridging ligand; Ru-dpp-Pt displayed the spin forbidden metal-to-ligand charge-transfer emission, whereas Ir-dpp-Pt was relatively silent in emission. Steady-state photochemical studies and photodynamic studies were undertaken for each Ir or Pt center within peripheries of bridging ligands to confirm either energy or ET and at the same time its direction. Thermodynamic driving force of the electron-transfer (ET) process from Ir to Pt in the hetero-bimetallic Ir-Pt systems was predicted by the Rehm-Weller equation to be unfavorable for both Ir-ppz-Pt and Ir-dpp-Pt. However, when photoinduced ET in ground state was considered, the complexes could undergo an ET process from Pt to Ir center. Such a prediction is well manifested from selective excitation on the bridging ligand.