Achieving near-infrared emission in platinum(ii) complexes by using an extended donor–acceptor-type ligand

A series of C^N ligands with donor-acceptor (D-A) frameworks, i.e. TPA-BTPy, TPA-BTPy-Fl and Fl(TPA-BTPy)2, as well as their mono- and di-nuclear platinum(II) complexes of (TPA-BTPy)Pt(pic), (TPA-BTPy-Fl)Pt(pic) and [Fl(TPA-BTPy)2]Pt2(pic)2 are respectively designed and synthesized, in which triphenylamine (TPA) and fluorene (Fl) are used as the D units, 4-(pyrid-2-yl)benzothiadiazole (BTPy) as the A unit, and the picolinate anion (pic) as the auxiliary ligand. Their thermal, photophysical and electrochemical characteristics were investigated. Compared to mono-nuclear platinum complexes and their free ligands, this dinuclear one of [Fl(TPA-BTPy)2]Pt2(pic)2 shows an obvious interaction from the platinum atom to ligand and dual emission peaks at 828 and 601 nm in thin films. Upon oxidation with antimony pentachloride in dichloromethane, charge transfer transitions between the platinum and ligand are observed for the three complexes. The single-emissive-layer polymer light-emitting devices doped with [Fl(TPA-BTPy)2]Pt2(pic)2 display a strong electroluminescence with dual emission peaks at 780 and 600 nm at a dopant concentration over 4 wt%. A maximum external quantum efficiency of 0.02% with a radiance of 59 μW cm(-2) is obtained in the device at 30 wt% dopant concentration. This work indicates that the use of an extended D-A-type ligand is an effective strategy to achieve NIR emission for platinum complexes in PLEDs.