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‐4 4+ , 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‐4 4+ 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.