Optimization on Molecular Restriction for Highly Efficient Thermally Activated Delayed Fluorescence Emitters

Abstract To develop efficient thermally activated delayed fluorescence (TADF) emitters with optimal molecular restrictions for organic light‐emitting diodes (OLEDs), four novel TADF emitters are designed and synthesized: 5′‐(9,9‐dimethylacridin‐10(9 H )‐yl)‐[1,1′:3′,1″‐terphenyl]‐2′,4,4″‐tricarbonitrile (oAcTBC), 5′‐(10 H ‐spiro[acridine‐9,9′‐fluoren]‐10‐yl)‐[1,1′:3′,1″‐terphenyl]‐2′,4,4″‐tricarbonitrile (oSpTBC), 2′‐(9,9‐dimethylacridin‐10(9 H )‐yl)‐[1,1′:3′,1″‐terphenyl]‐4,4″,5′‐tricarbonitrile (mAcTBC), and 2′‐(10 H ‐spiro[acridine‐9,9′‐fluoren]‐10‐yl)‐[1,1′:3′,1″‐terphenyl]‐4,4″,5′‐tricarbonitrile (mSpTBC). The four compounds have similar constituent segments but different molecular restrictions by enhancing either segment rigidity or steric hindrance. The OLEDs using oAcTBC, oSpTBC, mAcTBC, and mSpTBC as the emitter exhibit maximum external quantum efficiencies of 20.9%, 26.8%, 19.2%, and 18.9%, respectively, consistent with their photoluminescence quantum yield values of 83.88%, 93.24%, 76.64%, and 65.02%. The best performance is successfully realized by oSpTBC with the optimal molecular restriction. This work suggests that appropriately enhancing the molecular restrictions with more rigid constituent segments is an optimal approach to improve the efficiencies of TADF emitters.