Red-emitting fluorenes as efficient emitting hosts for non-doped, organic red-light-emitting diodes

Rare red-fluorescent fluorene derivatives have been designed and synthesized. The long-wavelength red fluorescence is achieved by incorporating a di(4-tolyl) amino or diphenylamino electron donor and a dicyanovinyl electron acceptor. The single-crystal X-ray structures of the di(4-tolyl)amino (pTSPDCV) and diphenylamino (PhSPDCV) compounds indicate only weak non-pi van der Waals contacts in addition to long-distance dipole-dipole interactions of the red-emitting fluorene molecules in the solid state. The aggregation of the dipolar fluorene is largely suppressed by introducing bulky 9,9-substituents (spiro-fused bifluorene) as well as a non-planar di (4-tolyl) amino or diphenylamino group. In the solid state, these fluorene derivatives all show red fluorescence that is much brighter than with the red dopants Nile Red and DCM (4-(dicyanomethylene)-2-methyl-6-[4-(dimethylaminostyryl)-4H-pyran]). The unique photophysical properties of red-emitting fluorene derivatives differ from other known red dopants and facilitate the fabrication of non-doped red organic light-emitting diodes (OLEDs). Authentic red (CIE, x=0.65, y=0.35) electroluminescence with a brightness of more than 12000 cd m(-2) (greater than 600 cd m(-2) at 20 mA cm(-2)) and a remarkable external quantum efficiency as high as 3.6% have been observed for the red-emitting OLEDs with pTSPDCV or PhSPDCV as the sole emitting host.