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Emissive Osmium(II) Complexes Supported by N‑Heterocyclic Carbene-based CCC‑Pincer Ligands and Aromatic Diimines

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posted on 20.08.2012, 00:00 by Lai-Hon Chung, Siu-Chung Chan, Wing-Chun Lee, Chun-Yuen Wong
Osmium­(II) complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis­(1-methylimidazolin-2-ylidene)­phenyl anion (C1CC1) or 1,3-bis­(3-methylbenzimidazolin-2-ylidene)­phenyl anion (C2CC2) and aromatic diimine (2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), or 4,4′-diphenyl-2,2′-bipyridine (Ph2bpy)) in the form of [Os­(CCC)­(NN)­(CO)]+ have been prepared. Crystal structures for these complexes show that the Os–CNHC bonds are essentially single (Os–CNHC distances = 2.079(5)–2.103(7) Å). Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic transition associated with these complexes (λmax = 493–536 nm, εmax = (5–10) × 103 dm3 mol–1 cm–1, solvent = CH3CN) originate from a dπ(OsII) → π*­(NN) metal-to-ligand charge transfer transition, where the dπ(OsII) and π*­(NN) levels contain significant contribution from the CCC ligands. All these complexes are emissive in the red-spectral region (674–731 nm) with quantum yields of 10–4–10–2 and emission lifetimes of around 1–6 μs. Transient absorption spectroscopy and spectroelectrochemical measurements have also been used to probe the nature of the emissive excited-states. Overall, this joint experimental and theoretical investigation reveals that the CCC ligands can be used to modulate the photophysical properties of a [Os­(NN)] core via the formation of the hybrid [Os + CCC] frontier orbitals.