Achieving NIR Emission for Donor–Acceptor Type Platinum(II) Complexes by Adjusting Coordination Position with Isomeric Ligands

Four platinum­(II) complexes Pt-1, Pt-2, Pt-3, and Pt-4 with the isomeric donor–acceptor (D–A) conjugated ligand framework are designed and prepared, and their thermal, photophysical, and electrochemical characteristics investigated. Crystal structures for Pt-1 and Pt-4 are determined with single-crystal X-ray diffraction analysis, showing distorted and nonplanar geometry. Complex Pt-4 exhibits much greater distortion, attributed to the steric interactions between benzothiadiazole and naphthalene. Remarkably different photophysical, electrochemical, and electroluminescent properties are found for these platinum­(II) complexes. Photoluminescence wavelengths of these complexes range from 590 to 800 nm with bandgaps of 1.7–2.0 eV. Coordination with [1,2,5]­thiadiazolo­[3,4-c]­pyridine and triphenylamine can enhance D–A interactions, reducing the bandgap and producing near-infrared emission for Pt-3. Organic light-emitting devices (OLEDs) display electroluminescence with emission peaks at 626, 645, 826, and 571 nm, with maximum external quantum efficiencies of 0.13%, 0.04%, 0.49%, and 0.22% for Pt-1, Pt-2, Pt-3, and Pt-4 doped OLEDs, respectively. Thus, adjusting the coordination position with the isomeric conjugation framework ligand is an appropriate strategy to tune the light-emitting properties of platinum complexes in OLEDs.