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Keywords:

  • Fluorenes;
  • Fluorescent materials;
  • Light-emitting diodes, organic

Abstract

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-π 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 12 000 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.