New red–orange phosphorescent/electroluminescent cycloplatinated complexes of 2,6-bis(2′-indolyl)pyridine

Three novel cyclometalated complexes of 2,6-bis(2′-indolyl)pyridine (H2BIP), Pd(BIP)(Py) (Py = pyridine) (1), Pt(BIP)(SMe2) (2) and Pt(BIP)(Py) (3) have been synthesized and characterized structurally. The Pd and Pt centers in these compounds are four-coordinate with a square planar geometry. The BIP ligand acts as a tridentate chelate to the metal center, and the dimethyl sulfide and the pyridine ligand bind to the metal center as terminal ligands. Compound 1 has no luminescence. Compounds 2 and 3 display bright orange luminescence either at 77 K in a frozen CH2Cl2 solution or at ambient temperature in a polymer matrix (e.g. poly(carbonate) or PVK). The emission maxima for 2 and 3 in the frozen CH2Cl2 solution are at λ = 572 and 589 nm, respectively. The emission spectra of 2 and 3 in the polymer matrix are very similar with λmax = 585 nm. The luminescence of 2 and 3 is phosphorescent as supported by the emission lifetimes of 2 (94(1) μs in CH2Cl2 at 77 K, 22.1(1) μs in poly(carbonate) at 298 K) and 3 (44(1) μs in CH2Cl2 at 77 K, 19(1) μs in poly(carbonate) at 298 K). Based on the luminescence properties of the free ligand and the results of molecular orbital calculations, we propose that the luminescence of 2 and 3 is most likely due to a π → π* transition with a significant dπ orbital contribution from the Pt(II) center to the π level. Electroluminescent devices using compound 3 as the emitter and PVK as the host and hole transport material have been fabricated successfully.