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 (ΔE(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.