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-PtCl 2 (Ir-ppz-Pt) (L = 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 metalto-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.

Accumulative Charge Separation in a Modular Quaterpyridine Bridging Ligand Platform and Multielectron Transfer Photocatalysis of π-Linked Dinuclear Ir(III)–Re(I) Complex for CO2 Reduction

“…The BLs ( dpp and bpm ), the related cationic Ir-BL and Ir-BL-Re complexes, [( piq C^N) 2 Ir III (bpy)] + ( piq C^N = 1-phenylisoquinoline), [( piq C^N) 2 Ir III Cl] 2 , , 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole (BIH), 5-bromo-2,2′-bipyridine ( bpy m -Br ), and 4-bromo-2,2′-bipyridine ( bpy p -Br ), 5–(tributylstannyl)-2,2′-bipyridine ( bpy m -SnBu 3 ), and 4–(tributylstannyl)-2,2′-bipyridine ( bpy p -SnBu 3 ) were prepared by modifying synthetic protocols described previously, details of which are described below (see Scheme and the synthetic details in the Supporting Information (SI)).…”

“…On the other hand, the incorporation of substantially π-conjugated aryl substituents limit the ET efficiency toward the CO 2 reduction center requiring at least approximately −1.0 V (vs SCE) owing to the energy lowering of the PS or PS-BL unit caused by extensive π-delocalization. ,, In addition, the strong electronic coupling between components through π-bridging ligands generally leads to undirected ET pathways, such as back ET or metal-to-metal charge transfer (MM′CT), consequently impeding vectorial and irreversible electron transfer from the PS to the Cat. , Indeed, most π-conjugated-BL-based bimetallic photocatalysts exhibit relatively poor photocatalytic activity compared to their alkyl-bridged counterparts, indicating that the adiabatic ET process between components within delocalized dinuclear complex hinders directional ET from the PS to the Cat. , Thus, a new synthetic approach whereby the magnitude of the π-interaction between components is appropriately modulated is required to optimize directional ET in bimetallic photocatalysts.…”