Red emitting [Ir(C^N)2(N^N)]+ complexes employing bidentate 2,2′:6′,2′′-terpyridine ligands for light-emitting electrochemical cells

Abstract

2,2′:6′,2′′-Terpyridine (tpy), 4′-(4-HOC₆H₄)-2,2′:6′,2′′-terpyridine (1), 4′-(4-MeOC₆H₄)-2,2′:6′,2′′-terpyridine (2), 4′-(4-MeSC₆H₄)-2,2′:6′,2′′-terpyridine (3), 4′-(4-H₂NC₆H₄)-2,2′:6′,2′′-terpyridine (4) and 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine (4) act as N^N chelates in complexes of the type [Ir(C^N)₂(N^N)][PF₆] in which the cyclometallating ligand, C^N, is derived from 2-phenylpyridine (Hppy) or 3,5-dimethyl-1-phenyl-1H-pyrazole (Hdmppz). The single crystal structures of eight complexes have been determined, and in each iridium(III) complex cation, the non-coordinated pyridine ring of the tpy unit is involved in a face-to-face π-stacking interaction with the cyclometallated ring of an adjacent ligand. Solution NMR spectra of the [Ir(ppy)₂(N^N)]⁺ complexes are consistent with the presence of a non-classical hydrogen bond between the non-coordinated N-donor of the tpy domain and a CH unit of one pyridine ring of an adjacent ppy⁻ ligand; the presence of the N⋯HC interaction was confirmed in one of the solid-state structures. The pendant pyridine ring of the coordinated tpy undergoes hindered rotation on the NMR timescale at 295 K. In CH₂Cl₂, the complexes are orange or red emitters, with λ^em_max in the range 580 to 642 nm; photoluminescence quantum yields (PLQY) are <10%, and lifetimes range from 54 to 136 ns. N-Methylation of the pendant 4′-(4-pyridyl) group in [Ir(dmppz)₂(pytpy)][PF₆] essentially quenches the emission. Light-emitting electrochemical cells (LECs) have been fabricated in a thin film configuration; the emission spectra of the LECs are red-shifted with respect to the PL spectra of the corresponding complex in thin film configuration. For the device incorporating [Ir(ppy)₂(pytpy)][PF₆], the PL to EL red-shift is extremely large and this is indicative of a different emitting state being involved. The most efficient devices used [Ir(ppy)₂(1)][PF₆], [Ir(ppy)₂(2)][PF₆] or [Ir(ppy)₂(3)][PF₆] in the emissive layer; the devices exhibited rapid turn-on times, but showed relatively low efficiencies in accordance with the solid state photoluminescence quantum yields.