Synchronously manipulating absorption and nonradiative transition of croconaine via donor engineering for efficient NIR-II photothermal theranostics

Organic molecules with strong absorption in the near-infrared (NIR)-II region are highly desired for cancer photothermal theranostics. However, it remains challenges to construct NIR-II photothermal agents with desired optical properties. This study reports a donor engineering approach to simultaneously manipulate the absorption and nonradiative transitions of croconaine dyes. By introducing different donor units with increasing electron donating ability to the backbone, the absorption of the synthesized croconaines were gradually red-shifted to NIR-II region, especially the benzo[ c , d ]indolium conjugated croconaine (CR1045). The extensive conjugation of CR1045 had a narrower HOMO-LUMO energy gap and strong NIR-II light responsiveness. Importantly, the steric repulsion between this large planar donor and croconate acceptor of CR1045 promotes the formation of a twisted intramolecular charge transfer (TICT) state, making it more prone to direct the absorbed energy towards heat-producing non-radiative decay rather than other dissipative pathways. And the nanoformulated CR1045 exhibits excellent optoacoustic generation capability and ultrahigh photothermal conversion efficiency (∼84%). In vitro and in vivo investigations confirm good optoacoustic imaging and effective photothermal treatment of tumors. Therefore, this work offers insights into donor regulation for the future development of efficient NIR-II photothermal agents.