Cationic Magnetically Active Nitrogen-Doped Polycyclic Aromatic Hydrocarbon with Record Low Band Gap

Polycyclic aromatic hydrocarbons (PAHs) have attracted significant interest in material chemistry, particularly if they own extremely low band gaps and magnetic properties. However, challenges remain regarding the synthetic accessibility and energy saturation issues. In this study, we introduce NR-11, which consists of eleven aromatic rings in its main conjugation and is separately doped with two electron-rich nitrogen atoms. This unique structure imparts intriguing oxidation characteristics to NR-11. The cationic radical NR-11⁺⋅ exhibits enhanced stability and demonstrates strong absorption in the range of 1250 nm to 3000 nm, peaking at 2570 nm. As a result, the optical energy gap of NR-11⁺⋅ is one of the lowest reported to date. Additionally, X-ray crystal structure analysis reveals that NR-11⁺⋅ displays unusual symmetry-broken charge separation. For the dication, variable-temperature NMR and variable-temperature EPR studies indicate that NR-11²⁺ exhibits a high diradical character with a ▵E_S-T of approximately -1 kcal/mol. Additionally, its spins are polarized at two ends of the PAH. Meanwhile, its strong absorption in the near-infrared II region suggests promise in photoacoustic (PA) conversion applications. This work underscores the significance of cationic species of extended long PAHs, highlighting their exceptional properties and potential applications.