Tailoring Optoelectronic Properties of Phenanthroline‐Based Thermally Activated Delayed Fluorescence Emitters through Isomer Engineering

Three isomeric phenanthroline compounds, namely o‐PXZP, m‐PXZP, and p‐PXZP, are constructed by incorporating electron‐donor phenoxazine unit into ortho‐, meta‐, or para‐positions of a rigid electron‐acceptor phenanthroline core. This approach functionalizes these compounds to exhibit thermally activated delayed fluorescence (TADF) characteristics with microsecond‐scale photoluminescence (PL) lifetimes. Their thermal, electrochemical properties, emissive characteristics, and energy levels can be finely tailored through isomer engineering. The meta‐ and para‐linking compounds (m‐PXZP and p‐PXZP) possess higher decomposition temperatures, deeper lowest unoccupied molecular orbital levels, higher PL quantum efficiencies, smaller singlet–triplet energy splitting, and reduced DF lifetimes relative to their ortho‐disposition isomer (o‐PXZP). Consequently, a green fluorescence organic light‐emitting diode (OLED) based on m‐PXZP achieves a maximum external quantum efficiency of 18.9%, and has a slow efficiency roll‐off of 28% at the luminance of 1000 cd m⁻². Notably, m‐PXZP‐based device exhibits a nearly 100% utilization efficiency of electro‐generated singlet and triplet excitons. These results demonstrate for the first time that phenanthroline derivatives can be employed as TADF emitters for high‐efficiency OLEDs. The isomer engineering for TADF emitters provides a simple method to extend structure diversity of TADF emitters, and moreover it provides a flexible method to optimize optoelectronic properties and thereby manipulate electroluminescence performance.