Tuning the photophysical properties and energy levels by linking spacer and topology between the benzimidazole and carbazole units: Bipolar host for highly efficient phosphorescent OLEDs

A new series of benzimidazole/carbazole hybrids with different linking spacers or linking topologies between the benzimidazole and carbazole moieties were facilely prepared, and their thermal, photophysical, and electrochemical properties were investigated. With the incorporation of rigid benzimidazole moiety, these compounds possess excellent thermal stability with high glass-transition temperatures (T) of 137-186 °C and the thermal-decomposition temperatures (T) of 479-544 °C. 2 and 3 with the m-terphenyl unit as the linking spacer between the carbazole and the benzimidazole moieties exhibit significant blue shifts as compared to 1 and 4 with the phenyl unit because the longer linking spacer alleviate intramolecular charge transfer. Their HOMO and LUMO energy levels vary in the range of 5.50-5.63 and 2.02-2.35 eV, respectively. Devices employing the new compounds as the host for the green emitter of Ir(ppy)3 were fabricated with the configurations of ITO/MoO (10 nm)/NPB (80 nm)/Host: 9 wt % Ir(ppy) (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al(100 nm). Their EL efficiencies follow the order of 3 > 2 > 1 φ 4, which correlates with their triplet energy and the separation of HOMO and LUMO distributions at hole-and electron-transporting moieties. A maximum current efficiency of 70.2 cd A and a maximum power efficiency of 73.4 lm W were achieved when 3 was used as the host. A facile strategy to manipulate the spatial distribution of energy levels and triplet energy of hosts by changing linking spacers or linking topologies is demonstrated. © 2010 American Chemical Society.