Photokinetic study on remarkable excimer phosphorescence from heteroleptic cyclometalated platinum(ii) complexes bearing a benzoylated 2-phenylpyridinate ligand

Abstract

Novel heteroleptic cyclometalated platinum(II) complexes consisting of 5′-benzoylated 2-phenylpyridinate (ppy) cyclometalated and acetylacetonate ancillary ligands were synthesized, and their photoluminescence (PL) properties were investigated. The 5′-benzoylated complex without any other substituents exhibited phosphorescence-based monomer emission at 479 nm in dichloromethane (10 μM, rt) with a PL quantum yield of 0.28. On the other hand, in poly(methyl methacrylate) (PMMA) film, remarkable excimer emission additionally emerged at ca. 600 nm with a relatively high PL quantum yield of 0.47 as the doping level increased to 0.20 mmmol g⁻¹, which was comparably intense in comparison with the monomer emission. In the case of the complexes with unsubstituted, 4′-benzoylated, and 5′-fluorinated ppy cyclometalated ligands, excimer emission was modestly generated at the same doping level, and thus the introduction of a benzoyl group to the 5′-position is effective to obtain remarkable excimer emission. The combination of benzoyl and fluoro groups was more effective at inducing excimer emission, and the intensity of excimer emission of the 2-(5-benzoyl-4,6-difluorophenyl)pyridinate-based complex was 3.5 times larger than that of monomer emission at a doping level of 0.20 mmmol g⁻¹ in PMMA. From the analysis of PL lifetimes at varying concentrations, photokinetic profiles were fully analyzed according to the model system for the irreversible excimer formation, and the excimer formation rate constant of the 5′-benzoylated complex was determined in dichloromethane as 2.2 × 10⁹ M⁻¹ s⁻¹, which is 4.4 times larger than that of the unsubstituted complex. We also fabricated an organic light-emitting diode using the 2-(5-benzoyl-4,6-difluorophenyl)pyridinate-based complex as a single emitter. The device exhibited pseudo-white EL with the Commission internationale de l’éclairage chromaticity coordinates of (0.42, 0.42).