Influence of the ancillary ligands on the luminescence of platinum(II) complexes with a triazole-based tridentate C^N^N luminophore

Abstract

The effect of different ancillary ligands and counterions in platinum(II) complexes has been investigated. Based on the previously reported tridentate C^N^N ligand precursor 2-(1-benzyl-1H-1,2,3-triazol-4-yl)-6-phenylpyridine (HL), the photophysical properties of complexes of the type [Pt(L)(X)]n+ have been varied by changing the fourth (monodentate) ligand (X) of the square-planar platinum(II) complexes. Different lifetimes and quantum yields were observed, depending on the identity of this ancillary ligand. The most favorable photophysical properties within this series of complexes were obtained for neutral complexes with the phenylacetylido ligand with a quantum yield of 35% and a lifetime of 2.22 µs, while for cationic complexes bearing nitrile, isonitrile and triphenylphosphane units gave comparable results with quantum yields ranging from 11% to 16% and lifetimes from 3.59 µs to 4.93 µs. Introducing a ferrocene moiety attached to an acetylido ligand, the complex became hardly emissive. The investigated counterions perchlorate, tetrafluoroborate and hexafluorophosphate of positively charged complexes regarding their photophysical properties were found to affect the non-radiative decay rates. To understand the minor effect observed for the emission maxima of the complexes, density functional theory (DFT) was applied. The experimental emission spectra of the complexes were reproduced by using simplified model systems. The distribution of the frontier orbitals used for the description of the emissive T1 state in its optimized geometry mainly involves the tridentate luminophore rather than the ancillary ligand. This explains why the emission is dominated by the pincer unit with perturbative participation of the metal center while excluding significant influence of the ancillary ligand.