Platinum(ii) complexes of benzannulated N⊥N-⊥O-amido ligands: bright orange phosphors with long-lived excited states

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 (NN-perpendicular to O)-N-perpendicular to-coordinating beta-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 similar to 450-575 nm and are phosphorescent in solution at room temperature, emitting bright orange light (lambda(max) similar to 600 nm) with quantum yields of up to 16% and excited-state lifetimes on the order of similar to 20 mu s, representing significant improvements to these photophysical properties compared with many previously reported (NNO)-N-perpendicular to-O-perpendicular to or (NNN)-N-perpendicular to-N-perpendicular to-ligated systems. Computational modelling reveals that the lowest-lying triplet state is populated efficiently thanks 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.

Automated summary

A series of platinum (II) complexes of benzannulated pincer-type diarylamido ligands were synthesized and characterized, showing strong metal-to-ligand charge-transfer absorptions and phosphorescence at room temperature. Computational modeling revealed efficient population of the lowest-lying triplet state in these complexes. The optical properties were minimally affected by substituents in the phenanthridinyl unit, but emission was significantly quenched by molecular distortions in the excited state.