Unexpected Propeller-Like Hexakis(fluoren-2-yl)benzene Cores for Six-Arm Star-Shaped Oligofluorenes: Highly Efficient Deep-Blue Fluorescent Emitters and Good Hole-Transporting Materials

Grafting six fluorene units to a benzene ring generates a new highly twisted core of hexakis(fluoren-2-yl)benzene. Based on the new core, six-arm star-shaped oligofluorenes from the first generation T1 to third generation T3 are constructed. Their thermal, photophysical, and electrochemical properties are studied, and the relationship between the structures and properties is discussed. Simple double-layer electroluminescence (EL) devices using T1T3 as non-doped solution-processed emitters display deep-blue emissions with Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.08) for T1, (0.16, 0.08) for T2, and (0.16, 0.07) for T3. These devices exhibit excellent performance, with maximum current efficiency of up to 5.4 cd A1, and maximum external quantum efficiency of up to 6.8%, which is the highest efficiency for non-doped solution-processed deep-blue organic light-emitting diodes (OLEDs) based on starburst oligofluorenes, and is even comparable with other solution-processed deep-blue fluorescent OLEDs. Furthermore, T2- and T3-based devices show striking blue EL color stability independent of driving voltage. In addition, using T0T3 as hole-transporting materials, the devices of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)/T0T3/tris(8-hydroxyquinolinato)aluminium (Alq3)/LiF/Al achieve maximum current efficiencies of 5.516.62 cd A1, which are among the highest for hole-transporting materials in identical device structure.