New organic light-emitting materials: Synthesis, thermal, photophysical, electrochemical, and electroluminescent properties

A new series of organic-light-emitting materials, 6,7-dimethyl-2,3-di-(4'-diphenylamino-biphenyl-4-yl)quinoxaline (MAPQ), 6,7-dimethyl-2,3-di-[4-(9,9-dibutyl-9H-fluoren-2-yl)-phenyll-quinoxaline (MFPQ), 2,3dicyano-5,6-di-[4-(9,9-dibutyl-9H-fluoren-2-yl)-phenyl]-pyrazine (CFPP), and 6,7-dicyano-2,3-di-[4-(9,9dibutyl-9H-fluoren-2-yl)-phenyl]-quinoxaline (CFPQ), have been synthesized in high yields and fully characterized. These compounds have high thermal stability and show bright-light-emission varying from blue to green owing to the different strengths of the donor and acceptor. Moreover, good reversible oxidation or reduction waves were observed except for compound MFPQ due to the potential limitation of the solvent we used, which suggests these compounds have potential applications for hole/electron transportation. Organic light-emitting diodes were fabricated in a facile nondoped configuration based on these materials. Compared to MFPQ, CFPP, and CFPQ, the higher lying HOMO level of MAPQ facilitates more efficient hole injection/ transport and a higher charge-recombination rate; thus, the device based on MAPQ shows the highest luminous efficiency. For compounds CFPP and CFPQ, the LUMO levels are obviously decreased because of the incorporation of electron-accepting cyano group, so the devices based on these two compounds display better electron transportation/injection properties and better performances than those of MFPQ. These results demonstrate that high-performance light-emitting devices can be achieved from intramolecular charge-transfer emission.