Synthesis and characterization of n-type materials for non-doped organic red-light-emitting diodes

Two compounds, 2,3-dicyano-5,6-di(4′-diphenylamino-biphenyl-4-yl) pyrazine (CAPP) and 6,7-dicyano-2,3-di(4′-diphenylamino-biphenyl-4-yl) quinoxaline (CAPQ), capable of intramolecular charge transfer, have been designed and synthesized in high yield by a convenient procedure. The compounds have been fully characterized spectroscopically. They have a high thermal stability and show bright light emission both in non-polar solvents and in the solid state. Moreover, they exhibit excellent reversible oxidation and reduction waves. The higher energy level of the highest occupied molecular orbital (-5.3 eV) and the triphenylamine group are advantageous for hole-injection/transport. In addition, the high electron affinities of 3.4 eV and the observed reversible reductive process suggest that these compounds enhance electron injection and have potential for use in electron transport. Three types of non-doped red-light-emitting diodes have been studied using CAPP and CAPQ as the electron-transporting and host-light-emitting layers, respectively. The devices exhibit red electroluminescence (EL), and constant Commission Internationale de l'Eclairage coordinates have been observed on increasing the current density. Pure red EL of CAPP, with a maximum brightness of 536 cd m^-2 and an external quantum efficiency of 0.7% in ambient air, was achieved.