Tuning the emission and exciton utilization mechanisms of pyrazine-based multi-carbazole emitters and their use in organic light-emitting diodes

Abstract

Thermally activated delayed fluorescence (TADF) and hot excitons are two distinct exciton harvesting mechanisms that can lead to 100% internal quantum efficiency in organic light-emitting diodes (OLEDs). Herein, we show that with judicious molecular engineering, the resulting structurally similar compounds emit via distinct photophysical mechanisms, which has a direct consequence on the OLED efficiency. When the pyrazine core is substituted with four carbazoles, the molecule 4CzPyz shows TADF in doped PPT film, with Φ_PL of 75%, ΔE_ST of 0.23 eV and τ_d of 150 μs. The device based on 4CzPyz emits in the sky-blue (λ_EL = 486 nm) with an EQE_max of 24.1%. When one carbazole is replaced by an ortho-biphenyl, the ΔE_ST of 3CzBPz increases to 0.29 eV, the Φ_PL decreases to 56%, and the TADF character is largely suppressed in the PPT film. However, a RISC process between higher-lying triplet excited states and the S₁ state is hypothesized to be operational, supported by a combined photophysical and DFT study, to rationalize how the device with 3CzBPz shows an EQE_max of 9.6% (λ_EL = 464 nm), reflecting that greater than 86% of the excitons are converted into light in the OLED. When two ortho-biphenyl groups are connected to the pyrazine core, the ΔE_ST of 2CzBPz is further increased to 0.34 eV while the Φ_PL is reduced to 45% in the PPT film. The DFT and photophysical studies indicate that 2CzBPz should act as a traditional blue fluorescence emitter. The OLED devices with 2CzBPz bear this out and exhibit an EQE_max 3.2% at a λ_EL of 446 nm. These results show how subtle structural changes modulate the efficiency of the triplet exciton harvesting mechanisms and provide new design directions for highly efficient blue emitters for OLEDs.