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

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 λemmax 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.