Heptazine-Assisted Multi-Resonance TADF Emitters With Fast Reverse Intersystem Crossing for Efficient Solution-Processed OLEDs.

Abstract

The development of solution-processed organic light-emitting diodes (SP-OLEDs) with narrowband emission, high brightness, and minimal efficiency roll-off remains a significant challenge. Herein, we report a series of heptazine-assisted multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters for high-performance SP-OLEDs. By integrating a heptazine core with one to three MR units (tBuCzBN), we synthesized three emitters (HAP-CzBN, HAP-2CzBN, and HAP-3CzBN) exhibiting excellent photoluminescence quantum yields and progressively reduced singlet-triplet energy gaps (ΔE_ST) with increasing MR content. Among these, HAP-2CzBN and HAP-3CzBN show green and yellow emission at 537 and 556 nm with small full widths at half maximum (FWHM) of 43 and 45 nm in toluene, respectively. Notably, both emitters exhibit concentration- and temperature-dependent dual emission with TADF behavior in n-hexane, suggesting potential for optical temperature sensing. In the solid state, HAP-3CzBN achieves the shortest reported delayed lifetime of 0.92 µs and a record-high reverse intersystem crossing (RISC) rate constant of 1.19 × 10⁶ s^-1 among MR-TADF emitters without heavy atoms with long-wavelength emission. SP-OLEDs based on HAP-3CzBN exhibit a peak external quantum efficiency (EQE) of 15.5%, negligible roll-off at 1000 cd m^- ² (EQE₁₀₀₀ = 15.4%), and maintain 11.2% EQE at 10,000 cd m^-2, setting a new benchmark for high brightness MR-TADF devices via solution processing. An OLED employing a trap-free transport layer configuration further increases the EQE_max to 20.3%, along with exceptional current efficiencies of 90.2 cd A^-1.