Thienylpyridine-based cyclometallated iridium(III) complexes and their use in solid state light-emitting electrochemical cells
The synthesis and characterization of four iridium(III) complexes [Ir(thpy)2(N^N)][PF6] where Hthpy = 2-(2′-thienyl)pyridine and N^N are 6-phenyl-2,2′-bipyridine (1), 4,4′-di-tbutyl-2,2′-bipyridine (2), 4,4′-di-tbutyl-6-phenyl-2,2′-bipyridine (3) or 4,4′-dimethylthio-2,2′-bipyridine (4) are described. The single crystal structures of ligand 4 and the complexes containing the [Ir(thpy)2(1)]+ and [Ir(thpy)2(4)]+ cations have been determined. In [Ir(thpy)2(1)]+, the pendant phenyl ring engages in an intra-cation π-stacking interaction with one of the thienyl rings in the solid state, and undergoes hindered rotation on the NMR timescale in [Ir(thpy)2(1)]+ and [Ir(thpy)2(3)]+. The solution spectra of [Ir(thpy)2(1)][PF6] and [Ir(thpy)2(4)][PF6] show emission maxima around 640 nm and are significantly red-shifted compared with [Ir(thpy)2(2)][PF6] and [Ir(thpy)2(3)][PF6] which have structured emission bands with maxima around 550 and 590 nm. In thin films, the emission spectra of the four complexes are similar with emission peaks around 550 and 590 nm and a shoulder around 640 nm that are reminiscent of the features observed in solution. In solution, quantum yields are low, but in thin films, values range from 29% for [Ir(thpy)2(1)][PF6] to 51% for [Ir(thpy)2(4)][PF6]. Density functional theory calculations rationalize the structured emission observed for the four complexes in terms of the 3LC nature predicted for the lowest-energy triplet states that mainly involve the cyclometallated [thpy]− ligands. Support for this theoretical result comes from the observed features of the low temperature (in frozen MeCN) photoluminescence spectra of the complexes. Photoluminescence and electroluminescence spectra of the complexes in a light-emitting electrochemical cell (LEC) device configuration have been investigated. The electroluminescence spectra are similar for all [Ir(thpy)2(N^N)][PF6] complexes with emission maxima at ≈600 nm, but device performances are relatively poor probably due to the poor charge-transporting properties of the complexes.