Toward white electroluminescence by ruthenium quinoxaline light emitting diodes

Ancillary ligand substitution in the new series [Ru(dicnq)(pyTz)(X)]+ {dicnq = 6,7-dicyanodipyrido[2,2-d:2′,3′-f]quinoxaline and X = 2,2-bipyridine (bpy), 1,10-phenanthroline (phen), 2-pyridine tetrazole (pyTz)} proves to be an effective way to create white electroluminescence with the Commission Internationale d'Eclairage Chromaticity Coordinates x = 0.40 and y = 0.39. These complexes were characterized by UV-visible and FT-IR spectroscopy, 1H-NMR, CHN and Mass spectroscopy analysis. The absorption and emission spectra are consistent with low-lying MLCT excited states, which are typical of Ru(II) polypyridyl complexes. All the complexes exhibiting reversible metal-centered oxidation processes undergo a one-electron oxidation within the potential range +1.30 vs. SCE. Upon reduction, each compound undergoes several reversible or quasi reversible ligand-centered reduction processes (−1.65 V to −2.40 V). It was found that white light emission can be obtained from the device ITO/PEDOT:PSS/PVK/ruthenium complex/PBD/Al by combining Forster emission from Ru(dicnq) and Exciplex emission at the PVK/Ru(dicnq) interface, although the CIE chromaticity coordinates gradually change with applied voltages. The highest luminous efficiency and luminance and lowest turn-on voltage values are obtained from the 8 wt% Ru(dicnq)(pyTz)(bpy) doped device as 1.78 cd A−1 and 1030 cd m−2 at 16 V and 5.2 V, respectively. As an important result, the incorporation of pyTz as the ancillary ligand into [Ru(dicnq)] complexes was found to be the most beneficial ancillary substitution in terms of creating the white electroluminescence in ruthenium quinoxaline complexes.