A Donor‐π‐Acceptor Type Red Thermally Activated Delayed Fluorescence Emitter Featuring Spiro as the Bridge Exhibiting Aggregation‐Induced Emission Activity and Outstanding Efficiencies in Organic Light‐Emitting Diodes

Abstract Constructing donor‐ π ‐acceptor (D‐ π ‐A)‐type molecular structures by employing a phenyl as the π ‐bridge to link donor (D) and acceptor (A) units has been recognized as an effective way to develop highly efficient red thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs). However, flexible and relatively planar structures would open potential energy loss channels, such as nonradiative inactivation and aggregation‐induced triplet quenching processes. Here, a bulky spiro‐9,9′‐bifluorene unit is first implemented to serve as a bridging group to construct a D‐ π ‐A molecule, enabling it to have higher overall rigidity, more sufficient steric hindrance, prolonged molecule length, and obvious aggregation‐induced emission characteristics compared with a common phenyl bridge. As a result, energy dissipation routes are effectively relieved at the unimolecular level, together with mitigated interchromophore quenching, rendering a 100% photoluminescence quantum yield and a larger horizontal dipole ratio of 89%. The OLED based on 3‐(2‐(diphenylamino)‐9,9′‐spirobi[fluoren]‐7‐yl)dibenzo[a,c]phenazine‐11,12‐dicarbonitrile exhibits an excellent external quantum efficiency of nearly 37% at 612 nm, which is over 1.38‐fold enhancement compared with the phenyl bridge‐based control molecule. This work provides an instructive solution to design highly efficient red TADF emitters exploiting D‐ π ‐A‐type molecular structures.