New 9-fluorene-type trispirocyclic compounds for thermally stable hole transport materials in OLEDs

A novel trispirocyclic hydrocarbon having three 9-fluorene moieties around the core of truxene (5) was prepared readily via coupling of truxenone with 2-bromobiphenyl; 5 was a high melting (>500 °C) solid. For the application of 5 to an effective hole transport material (HTM) in the OLED, a triphenylamine derivative carrying six diphenylamino groups at the 2- and 7-positions of each 9-fluorene moiety (6) was designed in order to get high thermal stability as an improved material of the TPD type HTM. The synthesis of 6 was easily achieved using 4,4′-bis(diphenylamino)-2-bromobiphenyl (9). The trispirocyclic 6 was found to show a glass transition temperature as high as 170 °C. It effects the formation of its stable cation radical upon electrooxidation in solution, and amorphous thin films in solid. A multi-layered EL device for 6 as an HTM using Alq3 as an electron transporting emitter showed good EL characteristics such as the maximum luminance of 37 000 cd m−2 at 14 V. Thus, the hexakis(diphenylamino)substituted trispirocycle 6 (TX-F6S) can be used as an efficient and thermally stable HTM in OLEDs.