Pure aromatic hydrocarbons with meta-linked phenyl-core and perihedral fluorene substitutions with/without inert groups of tert-butyl: bipolar hosts for blue phosphorescence

Two pure hydrocarbon molecules of 1,3,5-tris(9-phenyl-9H-fluoren-9-yl)benzene (m TPFB) and 1,3,5-tris(2-tert-butyl-9-phenyl-9H-fluoren-9-yl)benzene (t Bu-m TPFB) were synthesized. Due to the conjugation blocked connection mode and rigid/bulky substitutions, these two materials possess high triplet energy, enabling them as good hosts for blue phosphor in PhOLEDs. By studying their thermal, electrochemical, electronic absorption and photoluminescent properties, it was found that the influence of the inert tert-butyl group on material photoelectrical properties is negligible. For instance, m TPFB and t Bu-m TPFB showed very similar absorption and emission profiles, with almost the same bandgap, triplet energy and energy levels. However, the encapsulation of tert-butyl on the 2-position of 9-phenylfluorene enhanced material thermal stability. Most importantly, carrier transport properties were improved dramatically, as proved by the mono carrier device. Blue phosphorescent OLEDs hosted by t Bu-m TPFB showed external quantum efficiency of 15.2% and current efficiency of 23.0 cd/A, which were much higher than that of the OLEDs based on m TPFB with the analogous structure.