Platinum(II) Complexes of Benzannulated N^N–^O-Amido Ligands: Bright Orange Phosphors With Long-Lived Excited States

Abstract

The synthesis, structural characterization and photophysical properties of a series of platinum(II) complexes of benzannulated pincer-type diarylamido ligands are described. The ligands all contain tricyclic phenanthridine (3,4-benzoquinoline) rings as donor arms, which were elaborated into N^N–^O-coordinating β-enaminoketonato chelates via partial condensation with acetylacetone. The proligands are easily deprotonated, and metallation can be achieved under mild conditions using simple Pt(II) salts and Ag2O as a base. The resulting Pt(II) complexes exhibit strong metal-to-ligand charge-transfer absorptions in the region of ~450-575 nm and are phosphorescent in solution at room temperature, emitting bright orange light (λmax ~ 600 nm) with quantum yields of up to 16% and excited-state lifetimes on the order of ~20 μs, representing significant improvements to these photophysical properties compared with previously reported N^N^O or N^N^N-ligated systems. Computational modelling reveals that the lowest-lying triplet state is populated efficiently due to strong coupling between singlet and triplet excited state manifolds, as in cyclometallated compounds of Pt(II). Substituents (CH3, tBu, or CF3) in the 2-position of the phenanthridinyl unit are found to have little influence on the optical properties, but the emission is severely quenched when a methyl substituent is introduced ortho to the coordinating nitrogen. Molecular distortions in the excited state are shown to be primarily responsible for the quenching in this complex.