Luminescent PtII Complexes of Tridentate Cyclometalating 2,5-Bis(aryl)-pyridine Ligands

Abstract

Bis-cyclometalated PtII complexes of dianionic 2,5-bis(aryl)-pyridine ligands (L1–6)2–, carrying various cyclometalating or pending aryl groups, are synthesised in two steps. The reactions of H2L protoligands with K2[PtCl4] in acetic acid give the mono-cyclometalated complexes [Pt(HL)Cl]2. Heating these complexes in hot DMSO (dimethyl sulfoxide) yields the double-cyclometalated DMSO complexes [Pt(L1–6)(DMSO)]. The reaction of [Pt(L4)(DMSO)] with N,N-dimethylimidazolium iodide in the presence of KOtBu as the base gives the carbene complex [Pt(L4)(Me2Imd)]. Detailed photophysical studies reveal the intense orange luminescence of these complexes in CH2Cl2 solution, with quantum yields up to 0.22, and increased quantum yields of up to 1.00 in glassy frozen CH2Cl2/MeOH (1:1) and up to 0.44 in PMMA matrices. Detailed electrochemistry (including spectroelectrochemistry) reveals reversible ligand-based first reductions at –2.1 to –2.3 V, irreversible Pt-centred oxidations at around 0.8 V and electrochemical band gaps of 2.8–3.0 eV. Further reduction waves at very negative potentials interfere with the solvent (THF with traces of water) discharge and can be traced, with UV/Vis spectroelectrochemistry, to Pt-centred reductions for the DMSO complexes and to a second ligand-centred reduction for the Me2Imd complex from. The photo/electrochemical properties can be roughly correlated with the ligand pattern and suggest their use in optoelectronic applications.