Highly Efficient Long-Wavelength Thermally Activated Delayed Fluorescence OLEDs Based on Dicyanopyrazino Phenanthrene Derivatives.

Highly efficient long-wavelength thermally activated delayed fluorescence (TADF) materials are developed using 2,3-dicyanopyrazino phenanthrene (DCPP) as the electron acceptor (A), and carbazole (Cz), diphenylamine (DPA), or 9,9-dimethyl-9,10-dihydroacridine (DMAC) as the electron donor (D). Because of the large, rigid π-conjugated structure and strong electron-withdrawing capability of DCPP, TADF molecules with emitting colors ranging from yellow to deep-red are realized with different electron-donating groups and π-conjugation length. The connecting modes between donor and acceptor, that is, with or without the phenyl ring as π-bridge, are also investigated to study the π-bridge effect on the thermal, photophysical, electrochemical, and electroluminescent properties. Yellow, orange, red, and deep-red organic light-emitting diodes (OLEDs) based on DCPP derivatives exhibit high efficiencies of 47.6 cd A-1 (14.8%), 34.5 cd A-1 (16.9%), 12.8 cd A-1 (10.1%), and 13.2 cd A-1 (15.1%), with Commission Internationale de L'Eclairage (CIE) coordinates of (0.44, 0.54), (0.53, 0.46), (0.60, 0.40), and (0.64, 0.36), respectively, which are among the best values for long-wavelength TADF OLEDs.