Tridentate C^N^N ligands as cyclometalated luminophores for phosphorescent Pt(II) complexes – A case study concerning the influence of the substitution pattern and the co-ligands on the excited state properties

Abstract

In this work, the synthesis as well as the structural and photophysical characterization encompassing a series of Pt(II) complexes bearing cyclometalating pyrazole-based C^N^N ligands as tridentate luminophores are reported. By varying the substitution pattern of the main luminophore and by exchanging the ancillary co-ligand, different Pt(II) complexes were synthesized and compared regarding their excited state properties. The complexes C1-C8 bear differently decorated luminophores and a chlorido co-ligand; compounds C9–15 possess stronger ancillary ligands, such as cyanido and substituted acetylido derivates. While all complexes showed luminescence from metal-perturbed ligand-centered triplet states (3MP-LC), there are significant differences regarding the photoluminescence lifetimes (τ) and quantum yields (ΦL), both resulting from variations in the average radiative (kr) and radiationless (knr) deactivation rate constants. Although the chlorido complexes generally show lower ΦL, the OH-substituted compound C3 reaches a surprisingly high value of 36 %. Complex C9 with a cyanido co-ligand shows the longest τ of the series with 15.76 µs at room temperature in an Ar-purged solution, whereas all chlorido complexes C1-C8 possess lifetimes below 1.90 µs. The highest ΦL is achieved with phenylacetylido as the co-ligand. In general, the insertion of electron-withdrawing moieties at the main luminophore on the meta-position (with respect to the cyclometallation site) has a detrimental impact on the overall performance, whereas electron donors improve the photoluminescence efficiency. The t-butyl group on the main luminophore enhances the processability without affecting the excited state properties.