Modulation of Steric Hindrance to Achieve Highly Efficient Pure Blue Thermally Activated Delayed Fluorescent Emitters

Improving the device efficiency and color purity of blue thermally activated delayed fluorescent (TADF) emitters is a significant and difficult issue. However, traditional D-A-type TADF molecules usually present a strong charge-transfer state and are thus easily influenced by the molecular polarity environment, which affects the photoluminescence quantum yield (PLQY) and color purity of blue emitters. Here, we designed and synthesized three highly efficient TADF emitters, Cz-PFTX, tCz-PFTX, and SPAc-PFTX, by introducing a large block group of PF to suppress the interaction between molecules. The distribution of frontier molecular orbitals and natural transition orbitals (NTOs) demonstrates that the large block group does not participate in intramolecular charge transfer, and the single-crystal structure reduced pi-pi interactions via the large block group, which results in a smaller full width at half-maximum (FWHM) than those of traditional TADF emitters and a high PLQY of over 80% for these compounds. Moreover, the wavelength of emission spectra can be adjusted easily by the large block group of PF and by modulating the electron-donating ability of donors. The optimized OLEDs based on these compounds showed the maximum EQE of 17.5% with the EL peak at 456 nm for Cz-PFTX, 19.1% with the EL peak at 484 nm for tCz-PFTX, and 18.3% with the EL peak at 516 nm for SPAc-PFTX, respectively. Especially, the Cz-PFTX-based OLEDs exhibit a pure blue emission spectrum with a CIE of (0.15, 0.14) and FWHM of 64 nm. This study provides a valid strategy to realize a highly efficient D-A-type blue TADF emitters with high color purity.