Benzo-extended N^N^N-chelated tetracoordinate boron hetero[8]helicene featuring an inner N-B-N helical rim for circularly polarized TADF

The synthesis of tetracoordinate boron helicenes bearing B-N units aligned along the inner rim of the helical backbone remains exceedingly rare due to steric hindrance. We present the design and synthesis, and comprehensive characterization of a novel benzo-fused N^N^N-chelated tetracoordinate boron-containing hetero[8]helicene (Hel-BNN), which exhibits a highly twisted inner N-B-N helical rim. The fused benzene rings at the termini of the tridentate N^N^N ligand impart enhanced molecular rigidity and thermal stability (T_d = 460 °C), along with tunable electronic structures with a moderate bandgap (∼2.6 eV). Density functional theory (DFT) calculations reveal a helically distorted geometry and a spatially separated frontier orbital distribution, resulting in a small singlet-triplet splitting energy (ΔE_ST = 0.22 eV) conducive to efficient thermally activated delayed fluorescence (TADF). Hel-BNN exhibits strong circular dichroism and circularly polarized luminescence (CPL), with dissymmetry factors (g_abs and g_PL) on the order of ∼10⁻³. When employed as a dopant in organic light-emitting diodes (OLEDs), Hel-BNN delivers bright yellow electroluminescence with a peak external quantum efficiency of 21.9% and distinct circularly polarized electroluminescence (CPEL) signals from resolved enantiomers. This work demonstrates the potential of benzo-fused tetracoordinate boron hetero-helicenes as promising chiral emitters for high-performance CPL-TADF optoelectronic applications.