Photoluminescence and Electroluminescence Characterization of High Performance Near-Infrared Emitters Based on 1,5-Naphthyridin-4-ol Containing Heteroleptic Platinum (II) Complexes

Four 1,5-naphthyridin-4-ol-containing platinum complexes, AtFOND, AtFNND, PBSOND, and PBSNND, have been synthesized and characterized for their photoluminescence (PL) and electroluminescence (EL) properties. In addition to supporting the aggregate/excimer formation of the four platinum complexes in the solid state, the absorption and emission spectra of the solution, dopant thin film, and solid state suggest that the metal–metal-to-ligand charge transfer (MMLCT) is the main reason for the very long emission wavelengths of λmax 770 and 774 nm for PL and EL of AtFOND, respectively. The MMLCT is inferable from the single-crystal X-ray structure, namely, the Pt–Pt contacts which are short (3.39 and 3.88 A) and almost normal to the molecular planes of AtFOND. The solid-state PL quantum yield (PLQY) has been determined to be 9%, 58%, 53%, and 33% for AtFOND, AtFNND, PBSOND, and PBSNND, respectively. Although the four platinum complexes show green to yellow (λPLmax 523–546 nm) PL in diluted solution, all non-dopant OLEDs with a thin emissive layer (2 nm) based on AtFOND, AtFNND, PBSOND, and PBSNND display NIR (λELmax 704–774 nm) EL with EQE values of 1.8%, 6.5%, 4.0%, and 10.1%, respectively. The relatively high EQE (10.1%) of the PBSNND NIR OLED can be ascribed to the molecular alignment in the thin film that enhances the light outcoupling of the devices. Together with the single-crystal X-ray structure, the 2D-GIWAXS study provides convincing evidence for the advantageous molecular alignment of PBSNND.