Deep-Red Luminescence from Platinum(II) Complexes of N^N –^N‑Amido Ligands with Benzannulated N‑Heterocyclic Donor Arms

A synthetic methodology for accessing narrow-band, deep-red phosphorescence from mononuclear Pt­(II) complexes is presented. These charge-neutral complexes have the general structure (N^N – ^N)­PtCl, in which the Pt­(II) centers are supported by benzannulated diarylamido ligand scaffolds bearing substituted quinolinyl and/or phenanthridinyl arms. Emission maxima ranging from 683 to 745 nm are observed, with lifetimes spanning from 850 to 4500 ns. In contrast to the corresponding proligands, benzannulation is found to counterintuitively but markedly blue-shift emission from metal complexes with differing degrees of ligand benzannulation but similar substitution patterns. This effect can be further tuned by incorporation of electron-releasing (Me, tBu) or electron-withdrawing (CF3) substituents in either the phenanthridine 2-position or quinoline 6-position. Compared with symmetric bis(quinoline) and bis(phenanthridine) architectures, “mixed” ligands incorporating one quinoline and one phenanthridine unit present a degree of charge transfer between the N-heterocyclic arms that is more pronounced in the proligands than in the Pt­(II) complexes. The impact of benzannulation and ring-substitution on the structure and photophysical properties of both the proligands and their deep-red emitting Pt­(II) complexes is discussed.