Highly Effective Thermally Activated Delayed Fluorescence Emitters Based on Symmetry and Asymmetry Nicotinonitrile Derivatives.

In this study, we developed two thermally activated delayed fluorescence (TADF) emitters, ICzCN and ICzCYP , to apply to organic light-emitting diodes (OLEDs). These emitters involve indolocarbazole (ICz) donor units and nicotinonitrile acceptor units with a twisted donor-acceptor-donor (D-A-D) structure for small singlet (S 1 ) and triplet (T 1 ) state energy gap (Δ E ST ) to enable efficient exciton transfer from the T 1 to the S 1 state. Depending on the position of the cyano-substituent, ICzCN has a symmetric structure by introducing donor units at the 3,5-position of isonicotinonitrile, and ICzCYP has an asymmetric structure by introducing donor units at the 2,6-position of nicotinonitrile. These emitters have different properties, such as the maximum luminance ( L max ) value. The L max of ICzCN reached over 10000 cd m -2 . The external quantum efficiency ( η ext ) was 14.8% for ICzCN and 14.9% for ICzCYP , and both achieved a low turn-on voltage ( V on ) of less than 3.4 eV.