Rational Molecular Design of Phenanthroimidazole-Based Fluorescent Materials toward High-Efficiency Deep-Blue OLEDs by Molecular Isomer Engineering

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