NIR-II Absorbing Monodispersed Oligomers Based on N-B←N Unit.

Organic molecules with near-infrared II (NIR II) light absorption are essential for many biological and opto-electronic applications. Herein, we report monodispersed oligomers as NIR II light absorber using a new molecular design strategy of resonant N-B←N unit, i.e. balanced resonant boron-nitrogen covalent bond (B-N) and boron-nitrogen coordination bond (B←N). We synthesize a series of monodispersed oligomers with thiophene-fused 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (TB), which contains resonant N-B←N unit, as the repeating unit. The TB pentamer exhibits the maximum absorption wavelength of 1169 nm, which is the longest for oligomers reported so far. Organic photodetectors (OPDs) with the TB tetramer as the electron acceptor shows the specific detectivity of 2.98×10 Jones at 1180 nm under zero bias. This performance is among the best for NIR II OPDs. These results indicate a new kind of NIR II absorbing molecules as excellent opto-electronic materials.