Synthesis, photoluminescence and electroluminescence of new 1H-pyrazolo[3,4-b]quinoxaline derivatives

Several new 1H-pyrazolo[3,4-b] quinoxaline derivatives with N,N-dialkylamino electron-donating groups were prepared; their photoluminescence in solution and electroluminescence in thin-film devices have been investigated. They show emission at 520 - 540 nm with a fluorescence quantum yield close to unity in moderately polar solvents. The absorption and emission maxima shift to red with increasing solvent polarity. The fluorescence quantum yield also increases with increasing solvent polarity from non-polar to moderately polar solvents, then decreases slightly with further increase of solvent polarity. It indicates that both "positive'' and "negative'' solvatokinetic effects co-exist. The electroluminescence of OLED devices fabricated by using these compounds as dopants was investigated. All devices with the configuration of indium - tin oxide coated glass/NPB (60 nm)/Alq(3) : Dopant (30 nm)/ Alq(3) (20 nm)/Mg : Ag (200 nm) show bright green emission with efficiencies of 7.5 - 9.7 cd A(-1) and narrow full bandwidth at half intensity of 68 - 75 nm peaking at 536 - 552 nm.