Distinct electroluminescent properties of triphenylamine derivatives in blue organic light-emitting devices

Two highly fluorescent triphenylamines (TPA) end-capped respectively with one and three pyrene arms, namely N,N-diphenyl-4-(pyren-1-yl)aniline (PyTPA) and 4,4',4 ''-trispyrenylphenylamine (TPyPA), have been designed, synthesized and applied as hole-transporting emitters in organic light-emitting devices (OLEDs). While the two compounds have similar chemical structures (the only difference being the number of pyrene arms), devices based on them show distinct electroluminescent characteristics. PyTPA-based devices exhibit intense deep-blue emission with Commission Internationale de L'Eclairage coordinates (CIE) of (0.14, 0.11); whilst TPyPA-based devices emit white light with CIE of (0.31, 0.35). It is considered that the three electron-donating pyrene arms in TPyPA increase its electron-donating ability, which facilitates exciplex formation with the electron-transporter. This gives rise to a yellow exciplex emission and thus shifts the electroluminescence from deep-blue to white. More importantly, both devices can operate at record-low driving voltages (<4V at 20 mA cm(-2)), indicating the TPA-cored emitters offer not only high luminance efficiencies, but also good hole-injection and transporting features.