Orbicular-Donor-Acceptor System in N-doped Nanographene for Highly Efficient NIR-II Photothermal Therapy

Developing stable and efficient photothermal agents (PTAs) for the second near-infrared window (NIR-II, 1 000-1700 nm) photothermal therapy (PTT) is highly desirable but remains challenging. Herein, a facile strategy to prepare NIR-II nano-PTA based on the ionic N-doped nanographene hexapyrrolohexaazacoronene (HPHAC) is reported featuring a specific orbicular-donor-acceptor (O-D-A) structure. Oxidizing HPHAC 1 to dication 1²⁺ causes a substantial decrease in its band gap, leading to a shift in absorption from the confined UV region to a broad absorption range that reaches up to 1400 nm. The dication 1²⁺ exhibits global aromaticity and excellent stability. Theoretical investigation demonstrates that the strong NIR-II absorption of 1²⁺ is attributed to a distinct inner-to-outer intramolecular charge transfer. Encapsulating 1²⁺ with amphiphilic polymers results in water-soluble 1²⁺ NPs with retained optical characteristics. The 1²⁺ NPs exhibit exceptional biocompatibility, intense photoacoustic responses, and a high photothermal conversion efficiency of 72% under the 1064 nm laser irradiation, enabling efficient PTT of cancer cells. The "O-D-A" system on HPHAC, which is created by a simple redox approach, provides a novel strategy to construct efficient NIR-II photothermal materials through molecular engineering of nanographenes.