Acceptor/π-bridge planarization and donor rotation manipulation for designing an NIR-II AIEgen with high photothermal conversion efficiency to enhance cancer phototherapy

Aggregation-induced emission luminogens (AIEgens) with second near infrared (NIR-II) fluorescence and photothermal therapy (PTT) have recently attracted great research interest. The formidable challenge, the competing balance between radiation-mediated NIR-II fluorescence imaging and non-radiation NIR-PTT, prevents the development of efficient NIR-II AIEgens. In this study, a quite strong photothermal agent TPTQ is developed via rational design, which involves planarizing acceptor and π-bridge and rotating donor. The strong electron-withdrawing moiety 6, 7-diphenyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline is selected as the acceptor and thiophene is fabricated as the π-bridge. The planarization of π-bridge and acceptor units can enhance the intramolecular charge transfer effect, which makes TPTQ possess a long imaging wavelength longer than 1300 nm. Moreover, phenothiazine is utilized as the donor and triphenylamine is engineered on the donor part as a rotor to optimize the AIE-active characteristic and photothermal conversion efficiency of TPTQ. Notably, the TPTQ nanoparticles (NPs) achieve the highest photothermal conversion efficiency of 73.32% among the reported photothermal agents with NIR-II fluorescence imaging capability. Extensive tests in vivo indicate that the constructed TPTQ NPs can not only serve as a superior NIR-II fluorescence imaging (FLI) for the blood vessels of mice but also be successfully applied in 4T1 tumor-xenografted mice for the outstanding photothermal imaging (PTI), demonstrating distinguished FLI-guided photothermal tumoricidal capability. Herein, the smart design of acceptor/π-bridge planarization and donor rotation provide a new approach of developing highly effective NIR-II photothermal therapeutic agents.