Benzo-Extended N^N^N-Chelated Tetracoordinate Boron Hetero[8]Helicene Featuring an Inner N‒B‒N Helical Rim for Circularly Polarized TADF

Abstract

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 (Td = 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 (ΔEST = 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 (gabs and gPL) 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.