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

Abstract 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.