Tetracoordinate-Boron-Rigidified Heterohelicenes with Fragment-Precise Short-Range Charge Transfer for Narrow...

Organic near-infrared (NIR) emitters with narrow full widths at half-maximum (fwhm) are critically required for high-resolution applications; however, their development is fundamentally hindered by the energy-gap law, which dictates accelerated nonradiative decay at smaller band gaps. Herein, we introduce a design paradigm to overcome this limitation: tetracoordinate-boron-rigidified heterohelicenes featuring alternant donor-acceptor frameworks. This unique architecture achieves ultranarrow NIR emission up to 730 nm with fwhm's as low as 0.09 eV. The incorporation of tetracoordinate boron not only diminishes the optical band gap but also further imparts rigidity to the helicene backbone. Coupled with the rigidified skeleton, the alternating donor-acceptor arrangement enables fragment-precise short-range charge transfer with drastically suppressed nonradiative decay. Crucially, the internal steric repulsion of the helicene framework effectively eliminates the cis/trans isomerism of substituents on boron, breaking a major synthetic bottleneck and enabling the successful isolation of the molecule B6N15 featuring six tetracoordinate boron centers. This work establishes a generalizable strategy for constructing narrowband NIR emitters while simultaneously solving the persistent challenge of multiple-tetracoordinate-boron isomerism.