Manipulating the Excited States of Cyclometalated Iridium Complexes with β‑Ketoiminate and β‑Diketiminate Ligands

A series of cyclometalated iridium complexes with β-ketoiminate
and β-diketiminate ligands are described. Two different cyclometalating
(C^N) ligands2-phenylpridine (ppy) and 2-phenylbenzothiazole
(bt)are used in concert with three different ancillary (LX)
ligandsa phenyl-substituted β-ketoiminate (acNacMe), a phenyl-substituted β-diketiminate (NacNacMe), and a fluorinated version
of the β-diketiminate (NacNacCF3)to
furnish a suite of six complexes. The complexes are prepared by metathesis
reactions of chloro-bridged dimers [Ir­(C^N)2(μ-Cl)]2 with potassium or lithium salts of the ancillary LX ligand.
Four of the complexes are characterized by X-ray crystallography,
and all six were subjected to in-depth optical and electrochemical
interrogation. Cyclic voltammetry shows both reduction and oxidation
waves, with the latter strongly dependent on the identity of the LX
ligand. The complexes are all luminescent, with the nature of the
emissive excited state and the quantum yield (Φ) dependent on
the identity of both the C^N and LX ligands. Whereas the complexes
Ir­(ppy)2(NacNacMe) and Ir­(ppy)2(acNacMe) are weakly luminescent (Φ ≈ 0.01), the complexes
Ir­(bt)2(NacNacMe) and Ir­(bt)2(acNacMe) are strongly luminescent, with the latter’s quantum
efficiency (Φ = 0.82) among the highest ever observed for cyclometalated
iridium complexes. Fluorination of the NacNac ligand gives rise to
completely disparate emission behavior suggestive of a NacNac-centered
emissive state. The results described here, in comparison with previous
groups’ studies on acetylacetonate (acac) analogues, suggest
that the weaker-field NacNac and acNac ligands raise the energy of
the metal-centered HOMO, with energy of the HOMO increasing in the
order NacNacCF3 < acNacMe <
NacNacMe.