Tuning the photophysical properties of cationic iridium(iii) complexes containing cyclometallated 1-(2,4-difluorophenyl)-1H-pyrazole through functionalized 2,2′-bipyridineligands: blue but not blue enough

Abstract

Four new heteroleptic iridium(III) complexes in the family [Ir(dfppz)₂(N^N)]⁺, where Hdfppz = 1-(2,4-difluorophenyl)-1H-pyrazole and N^N = 6-phenyl-2,2′-bipyridine (1), 4,4′-(di-tert-butyl)-6-phenyl-2,2′-bipyridine (2), 4,4′-(di-tert-butyl)-6,6′-diphenyl-2,2′-bipyridine (3) and 4,4′-bis(dimethylamino)-2,2′-bipyridine (4), have been synthesized as the hexafluoridophosphate salts and fully characterized. Single crystal structures of ligand 3 and the precursor [Ir₂(dfppz)₄(μ-Cl)₂] have been determined, along with the structures of the complexes 4{[Ir(dfppz)₂(1)][PF₆]}·3CH₂Cl₂, [Ir(dfppz)₂(3)][PF₆]·CH₂Cl₂ and [Ir(dfppz)₂(4)][PF₆]·CH₂Cl₂. The role of inter- and intramolecular face-to-face π-stacking in the solid state is discussed. In the [Ir(dfppz)₂(N^N)]⁺ (N^N = 1–3) cations, the phenyl substituent in ligands 1, 2 or 3 undergoes hindered rotation on the NMR timescale at 298 K in solution and the systems have been studied by variable temperature NMR spectroscopy. Acetonitrile solutions of [Ir(dfppz)₂(N^N)][PF₆] (N^N = 1–3) exhibit similar absorption spectra arising from ligand-based transitions; absorption intensity is enhanced on going to [Ir(dfppz)₂(4)][PF₆] and the spectrum extends further into the visible region. Acetonitrile solutions of the complexes are blue emitters with λ_em = 517, 505, 501 and 493 nm for N^N = 1, 2, 3 and 4, respectively (λ_exc = 280–310 nm). The redox behaviours of [Ir(dfppz)₂(N^N)][PF₆] (N^N = 1–3) are similar, and the introduction of the electron-donating NMe₂ substituents onto the N^N ligand shifts the metal-centred oxidation to less positive potentials. Theoretical calculations predict a mixed metal-to-ligand/ligand-to-ligand charge transfer (MLCT/LLCT) character for the emitting triplet state in agreement with the broad and unstructured character of the emission bands. The NMe₂ substituents enlarge the HOMO–LUMO gap and blue-shifts the emission of [Ir(dfppz)₂(4)]⁺ that is centred on the ancillary ligand. These complexes, when processed into a thin film and sandwiched between two electrodes, lead to very low voltage operating electroluminescent devices. No additional components are needed, which demonstrates their electron and hole transport abilities in conjunction with the luminescent properties.