Manipulating Host-Guest Charge Transfer of a Water-Soluble Double-Cavity Cyclophane for NIR-II Photothermal Therapy

Water-soluble small organic photothermal agents (PTAs) over NIR-II biowindow (1000–1350 nm) are highly desirable, but the rarity greatly limits their applications. Based on a water-soluble double-cavity cyclophane GBox-4⁴⁺, we report a class of host–guest charge transfer (CT) complexes as structurally uniform PTAs for NIR-II photothermal therapy. As a result of its high electron-deficiency, GBox-4⁴⁺ can bind different electron-rich planar guests with a 1 : 2 host/guest stoichiometry to readily tune the CT absorption band that extends to the NIR-II region. When using a diaminofluorene guest substituted with an oligoethylene glycol chain, the host–guest system realized both good biocompatibility and enhanced photothermal conversion at 1064 nm, and was then exploited as a high-efficiency NIR-II PTA for cancer cell and bacterial ablation. This work broadens the potential applications of host–guest cyclophane systems and provides a new access to bio-friendly NIR-II photoabsorbers with well-defined structures.