Stabilizing the Quinoidal Form of Polycyclic Aromatic Hydrocarbons by O─B←N Units to Give a > 1000 nm Redshift in the Absorption Wavelength

Polycyclic aromatic hydrocarbons (PAHs) have distinct chemical structures and excellent optoelectronic properties. Although PAHs with ultra-large π-conjugated skeletons have been successfully synthesized, ultrasmall bandgap (E_g < 1.2 eV) or short-wavelength infrared (SWIR) light absorption (λ > 1000 nm) is seldom reported for PAHs. In this work, we design and synthesize a PAH by incorporating four O─B←N units into naphthalene diimide (NDI) as the core. The incorporation of four O─B←N units changes the NDI core from an aromatic form to a quinoidal form, leading to significantly enhanced π-electron delocalization. As a result, the optical bandgap of the molecule decreases from 3.10 to 0.88 eV, and the onset absorption wavelength is redshifted from 400 to 1410 nm. The resulting molecule exhibits strong light absorption in the infrared region and high transparency in the visible region. It can be used to develop colorless infrared anti-counterfeiting technology. This work provides a novel molecular design strategy to tune optoelectronic properties of PAHs.