Rational Molecular\nDesign of Phenanthroimidazole-Based\nFluorescent Materials toward High-Efficiency Deep-Blue OLEDs by Molecular\nIsomer Engineering

Organic light-emitting diodes (OLEDs) have been extensively\ninvestigated\nin full-color displays and energy-saving lighting owing to their unique\nadvantages. However, deep-blue OLEDs based on nondoped emitting layers\nwith a satisfactory external quantum efficiency (EQE) are still rare\nfor applications. In this work, six hot exciton materials, PPIM-12F,\nPPIM-22F, PPIM-13F, PPIM-23F, PPIM-1CN, and PPIM-2CN, are designed\nand synthesized via an isomer engineering design strategy and their\nphotophysical properties and OLED performance are systematically investigated.\nThese emitters all possess wide band gaps (3.53–3.69 eV), hybrid\nlocal and charge transfer (HLCT) characteristics, and good thermal\nstabilities. The C2 series compounds, PPIM-22F, PPIM-23F, and PPIM-2CN,\nall show redder emission peaks than the N1 series counterparts of\nPPIM-12F, PPIM-13F, and PPIM-1CN. In addition, the LUMO energy levels\ndecrease consecutively in the sequence of PPIM-22F < PPIM-23F <\nPPIM-2CN and are all lower than their respective N1 series position\nisomers of PPIM-12F, PPIM-13F, and PPIM-1CN. The CV measurements indicate\nthat such a design strategy renders the fine-tuning of LUMO energy\nlevels, and the incorporation of electron acceptors at the extended\nC2 position of the PI unit is a better choice to improve the electron\ninjection ability. Theoretical simulations indicate that they may\nharvest the triplet exciton through an upper-level reverse intersystem\ncrossing process, which decreases the gathering of triplet excitons\nand allows the OLEDs to be fabricated by nondoping technology. Among\nthem, PPIM-22F with a difluorobenzene substituent at the C2 position\nmanifests the best performance in OLEDs, which exhibits the maximum\nEQE of 7.87% and Commission Internationale de ĺEclairage (CIE)\ncoordinates of (0.16, 0.10). This work demonstrates an effective strategy\nfor considerable improvement in device performance by a subtle change\nin the molecular structure through isomer engineering.