Design and performance of sulfur and selenium-substituted triarylboron D3-A TADF emitters for OLED applications

This study presents the design, synthesis, and comprehensive theoretical and photophysical analysis of two new D₃-A type thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diode (OLED) applications. Utilizing a triarylboron core as the electron-accepting group and phenothiazine (PTZ) or phenoselenazine (PSZ) as electron-donating units, the molecules BTP-S and BTP-Se were developed. The D₃-A structure supports the separation of frontier molecular orbitals (FMOs), leading to minimized singlet-triplet energy gaps (ΔE_ST), which are crucial for the TADF mechanism. Density functional theory (DFT) calculations presented that BTP-S and BTP-Se exhibit band gaps (E_g) of 2.52 and 3.23 eV, respectively, with BTP-S showing an ΔE_ST value as low as 0.007 eV for the S₁-T₁ transition at the lowest energy conformation. Photophysical studies revealed high photoluminescence quantum yields (PLQYs) for both compounds, with BTP-S achieving up to 85 % in mCP films and BTP-Se up to 59 %. In vacuum-processed OLEDs, BTP-S achieved a maximum external quantum efficiency (EQE) of 25.3 %, a current efficiency (ηc) of 195.8 cd/A, and a maximum luminance (Lmax) of 17356 cd/m², while BTP-Se reached an EQE of 7.5 %, an ηc of 132.19 cd/A, and an Lmax of 16826 cd/m² likely limited by the contributions of a folded-donor conformer enabled by the Se substitution. These findings underscore the impact of donor unit selection and conformation on the TADF characteristics, and provide valuable insights for designing high-performance OLED materials.