Energy State Adjustment of Multi-Carbazole TADF Emitter by Ortho-Biphenyl Substitution

Thermally activated delayed fluorescence (TADF) materials has been researched to replace heavy-atom phosphorescent emitters and blue fluorescent emitters. In this study, we report a novel TADF materials substituted by ortho-biphenyl unit in multi-carbazole TADF molecule. The new TADF emitter, 4mCz-BP is bearing four dimethylcarbazole donors in benzonitrile acceptor core to trigger TADF effect. The emitter exhibits a bluish-green light of a 491 nm wavelength as a peak intensity and a high photoluminescence quantum yield (PLAQ) of 95% in mCBP host. We used a time dependent density theory (TD-DFT) calculation method to define ortho-biphenyl substitution effect by comparing with similar multi-carbazole TADF materials 4CzBN and 4mCzBN. A local excited triplet state (³LE) with an energy (2.81 eV) close to the lowest singlet charge-transfer state (¹CT) can forms at the biphenyl of 4mCzBN-BP. However, because of the planarization of the biphenyl unit at triplet state and its large steric caused by nearby carbazole donors, ³LE at the biphenyl unit can be formed by a subset of feasible conformations of 4mCzBN-BP molecules in the mCBP host, leading to a multiexponential decay of the delayed fluorescence. The device also record a long operational lifetime (LT50) of 750 hours with organic light emitting diodes based on 4mCzBN-BP.