n-type conjugated oligoquinoline and oligoquinoxaline with triphenylamine endgroups:: Efficient ambipolar light emitters for device applications

New n-type conjugated oligomers with p-type endgroups, 2,2'-bis(triphenylamine)-4,4'-diphenyl-[6,6']-biquinoline (TPA-2PQ, 3) and 2,2'-bis(triphenylamine)-3,3'-diphenyl-[6,6']biquinoxaline (TPA-2PQx, 6), were synthesized, characterized, and found to be efficient ambipolar blue-green and pure green emitters in organic light-emitting diodes (OLEDs). In toluene solution, oligoquinoline TPA-2PQ emitted blue-green fluorescence with a quantum yield of 62% and a lifetime of 1.3 ns, whereas the oligoquinoxaline TPA-2PQx emitted green fluorescence with a 34% quantum yield and a lifetime of 5.3 ns. Both oligomers showed ambipolar redox properties, identical HOMO levels at 5.31 eV, and LUMO levels of 2.59 eV for TPA-2PQ and 2.92 eV for TPA-2PQx. Bright green electroluminescence (up to 15 330 cd/m(2)) with high efficiency ( up to 7.9 cd/A and 3.0% external quantum efficiency) were achieved from TPA-2PQx as the emitter in OLEDs. Despite its better fluorescence quantum yield, blue-green electroluminescence with far lower brightnesses (up to 6850 cd/m(2)) and efficiencies (up to 2.7 cd/A) were obtained from the oligoquinoline (TPA-2PQ) emitter. The substantially better device performance of the oligoquinoxaline as well as the variation in OLED color among the two oligomers can be understood from their 0.33 eV difference in electron affinity, which favors improved electron injection/transport and confinement in the oligoquinoxaline. These results show that n-type oligomers with p-type endgroups represent a promising approach to efficient ambipolar emitters for OLEDs.