High-efficiency TADF materials featuring carbazole-modified spiroacridan-pyrimidine skeletons with an external quantum efficiency exceeding 26% in sky-blue light emission

Abstract

We report the design and synthesis of a novel sky-blue TADF emitter, 4SpAc35CzPy, incorporating spiroacridan as the donor and pyrimidine as the acceptor. Compared to the previously reported 4Ac35CzPy emitter, incorporating the fluorenyl group controlled the twisting angle of the spiroacridan structure, enhancing the ICT (intramolecular charge transfer) effect and improving TADF performance. Introducing the spirofluorenyl group to the acridan suppresses energy loss caused by intramolecular vibrations, further optimizing the TADF characteristics. The electroluminescent device with a light emitting layer of 3% 4SpAc35CzPy doping concentration in o-DiCbzBz falls within the sky-blue light range, exhibiting an emission λ_max of 480 nm at 9.5 V with a CIE of (0.165,0.274) and a brightness of 1677 cd m⁻². An external quantum efficiency (η_EQE) of 20.2%, a current efficiency (η_CE) of 36.9 cd A⁻¹, and a power efficiency (η_PE) of 33.2 lm W⁻¹ were achieved. Adjusting the doping concentration leads to an even more efficient device with an η_EQE of 26.7%, an η_CE of 55.7 cd A⁻¹, and an η_PE of 50.1 lm W⁻¹, outperforming similar acridan-based compounds. Natural orbital analysis reveals that the high TADF-OLED performance of 4SpAc35CzPy may be attributed to the multichannel high-lying reverse intersystem crossing of the hot triplet excitons back to their S₂ and S₃ states for luminescence.