Molecular Design, Synthesis And Light-emitting Properties Of Novel Ionic Iridium Complexes

Light-emitting electrochemical cells (LECs) and organic light-emitting diodes(OLEDs) based on ionic iridium complexes can be used as new-type displays or lightingsources. On the basis of the research progress of ionic iridium complexes in LECs andOLEDs, the paper focuses mainly on the development of novel ionic iridium complexes,especially those blue-emitting ones, explores the photophysical and electrochemicalproperties of the complexes and the performances of the complexes in LECs andOLEDs.Conventional ionic iridium complexes adopt bpy (2,2'-bipyridine)-type ancillaryligands, which makes it difficult to achieve blue light emission with high color purity.In the paper, an effective molecular design strategy to achieve blue-emitting ioniciridium complexes has been proposed, which is to insert electron-donating atoms intothe ancillary ligands to increase the LUMO (lowest unoccupied molecular orbital) levelsof the complexes. With pyrazole-type ancillary ligands, blue-green and blue-emittingcomplexes have been achieved. Compared to conventional bpy-type ancillary ligands,pyrazole-type ancillary ligands increase the LUMO levels of the complexes by0.27-0.31eV and shift the emission spectra of the complexes toward blue by more than100nm. The LEC based on the blue-emitting complex shows an electroluminescencepeak at460nm and CIE (Commission Internationale de l'Eclairage) coordinates of (0.20,0.28), indicating much better color purity than that of other reported blue LECs basedon ionic iridium complexes.With imidazole-type ancillary ligands, new ionic iridium complexes have beendesigned and synthesized. Imidazole-type ancillary ligands also significantly shift theemission spectra of the complexes toward the blue region of the spectrum. Bycontrolling the conjugation-length of imidazole-type ancillary ligands, a new series ofblue-green to red-emitting ionic iridium complexes has been achieved. Blue-green,green, yellow and orange-red LECs show peak current efficiencies of8.4,18.6,12.7and13.2cd/A, respectively. Because of the bulky side group attached to the complex,the efficiency of the blue-green LEC has been dramatically enhanced to18.3cd/A,which is the highest among blue-green LECs reported so far. By doping a small amount of red-emitting complexes into blue-green LECs, white LECs have been fabricated,which show CIE coordinates around (0.37,0.41), CRI (color rendering index) valuesaround80, and a peak current efficiency, external quantum efficiency and powerefficiency of11.4cd/A,5.6%and11.2lm/W, respectively. The efficiencies are thehighest among white LECs reported so far.The performances of ionic iridium complexes as phosphorescent dopants insolution-processed OLEDs have been systematically investigated. The peak currentefficiencies of blue-green, green, yellow and red OLEDs reach24.3,25.3,20.5and4.2cd/A, respectively. With ionic iridium complexes as the dopants, white OLEDs havebeen fabricated, which show a peak current efficiency, external quantum efficiency andpower efficiency of20.7cd/A,9.6%and8.5lm/W, respectively.