Photoluminescence and electroluminescence characterization of high-performance near-infrared emitters based on 1,5-naphthyridin-4-ol-containing heteroleptic platinum(ii) complexes

Abstract

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 λ_max 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 Å) 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 (λ^PL_max 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 (λ^EL_max 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.