Synthesis of octasubstituted cyclooctatetraenes and their use as electron transporters in organic light emitting diodes

The synthesis and characterization of octasubstituted cyclooctatetraenes (COTs) as well as their use as electron transporting materials in organic LEDs are reported. Tetraaryl-tetraarylethynyl-cyclooctatetrenes[C8Ar4(C=CAr)(4)] were prepared from diaryldiynes with a RuH2(CO)(PPh3)(3) catalyst in good yield (40-80%). Octaaryl-cyclooctatraenes were prepared from diarylacetylenes by treatment with lithium and iodine in 50% yield. Cyclic voltametry indicates that these COTs are reduced in sequential one-electron steps. C8Ar4(C=CAr)(4) and C8Ar8 are thermally stable to sublimation and have wide optical energy gaps [lambda(max)(emission) = 392-412 nm] making them good candidates for use in organic LEDs. These octasubstituted COTs have been used as electron transport layers in single heterostructure organic LEDs, i.e. ITO/NPD 400 Angstrom/octasubstituted COT 400 Angstrom/Mg-Ag (TTO = indium-tin oxide, NPD = N,N'-diphenyl-N,N'-dinaphthylbenzidine). External quantum efficiencies of 0.1-0.2% (photons/electrons) were observed, with turn-on voltages of ca. 6 V. The emission from this device comes exclusively from the NPD hole transporting layer, with lambda(max) of 435 nm. Doping the NPD layer with 1% perylene leads to an increased quantum efficiency of 0.6% and an electroluminescence spectrum indicative of emission solely from the perylene dopant, confirming exclusive emission from the NPD hole transporting layer.