Design of star-shaped molecular architectures based on carbazole and phosphine oxide moieties: towards amorphous bipolar hosts with high triplet energy for efficient blue electrophosphorescent devices

With a carbazole moiety as the electron donor and a phosphine-oxide moiety as the electron acceptor, two novel star-shaped bipolar hosts, 4,4',4 ''-tri(N-carbazolyl)triphenylphosphine oxide (TCTP) and 3,6-bis(diphenylphosphoryl)-9-(4'-(diphenylphosphoryl)phenyl)carbazole (TPCz), have been designed and synthesized. Their topology structure differences are that the phosphine-oxide moiety is located in the molecular centre and the periphery for TCTP and TPCz, respectively. The star-shaped architecture imparts them with high decomposition temperatures (T-d: 497 degrees C for TCTP and 506 degrees C for TPCz) and results in the formation of a stable amorphous glassy state (T-g: 163 degrees C for TCTP and 143 degrees C for TPCz), while the phosphine oxide linkage ensures the disrupted conjugation and the high triplet energy (>3.0 eV). In addition, both TCTP and TPCz possess a bipolar transporting capability. However, TCTP mostly transports holes and TPCz primarily conducts electrons. On the basis of appropriate device configurations, high performance blue electrophosphorescent devices with comparable efficiency (35.0-36.4 cd A(-1), 15.9-16.7%) have been realized using TCTP and TPCz as the host for the blue phosphor, respectively. Compared with the unipolar host, 4,4',4 ''-tri(N-carbazolyl)triphenylamine (TCTA, 15.9 cd A(-1), 7.8%), the efficiency is improved by more than two-fold. As far as the obtained state-of-the-art performance is concerned, we think that these novel materials should provide an avenue for the design of amorphous bipolar hosts with high triplet energy used for blue PhOLEDs on a star-shaped scaffold.