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.