Platinum(II) complexes with dianionic tetradentate ligands as green, red and near infrared emitters : synthesis, characterization and performance in electroluminescence devices

Abstract

Platinum(II) complexes supported by dianionic tetradentate ligands are promising candidates for applications in organic light-emitting diodes (OLEDs) due to the complexes’ high luminescence quantum yields, ease of synthesis, and high thermal and chemical stability. The use of tetradentate ligands in platinum(II) complexes can also prevent the isomerization issue commonly observed for iridium(III) emitters. However, a large number of OLEDs fabricated with these Pt(II) emitters suffer from severe efficiency roll-off when the OLED is operated at high brightness. On the other hand, Near infrared (NIR) emitters have applications in various fields such as telecommunications, night-vision, bio-imaging, sensors and military use, but the possibility of platinum (II)-based NIR emitters has not been extensively explored. In this thesis, I describe the preparation of platinum(II) complexes, supported by sterically hindered tetradentate ligands with high photoluminescence quantum yields (PLQY) and reduced emission self-quenching rate constants (kq), and the performance of OLEDs fabricated with these emitters. The substantially suppressed kq values were achieved by incorporating tert-alkyl groups, bridgeheaded bicyclic groups or sterically congested aromatic groups to the ligand scaffold of platinum(II) complexes. Incorporation of these groups at different positions of platinum(II) complexes were also found to affect the emission color and emission quantum yield. Complex Pt3.06, supported by a O^N^C^N ligand functionalized with a bridgeheaded bicyclic norborene moiety, is a green emitter with a high PLQY of 0.90 and a low kq of 8.8 × 108 dm3 mol-1 s-1. Its corresponding OLED achieved a high maximum current efficiency of 66.7 cd A-1 and a small efficiency roll-off of only 2.4% at 1000 cd m-2. Yellow emission with λmax at 553 nm was achieved with Pt3.10, which shows a high PLQY of 0.82 and a low kq of 3.7 × 108 dm3 mol-1 s-1. Red-emitting platinum(II) Schiff base complexes with high PLQY of up to 0.49 (Pt4.21) were also synthesized. Compared with other red-emitting platinum(II) Schiff base complexes reported in the literature, the PLQY of our complexes was improved by about 63%. Most of these have low kq values of < 3.5 × 108 dm3 mol-1 s-1 and; the lowest kq value achieved was 3 × 107 dm3 mol-1 s-1 (Pt4.30). A Pt4.21-based OLED achieved a high EQE of 22.5% and a relatively small efficiency roll-off of 48% at 1000 cd m-2 for a red OLED. The corresponding CIE coordinates were (0.66, 0.34), which is very close to pure red emission, as stated by the NTSC standard. It was found that by incorporating electron-donating/withdrawing substituents at suitable positions in platinum(II) Schiff base complexes, the emission can be shifted from red to NIR. NIR emissions with λmax ranging from 767 nm to 985 nm were achieved with a good PLQY of up to 0.16. In particular, a Pt5.02-based, spin-coated NIR-OLED achieved a maximum EQE of 2.4%, with a small efficiency roll-off of 29% at a maximum brightness of 0.4 mW cm-2. The efficiency roll-off is 50% smaller compared with that of Pt-porphyrin-based devices reported in the literature.