Ultrafast Photoinduced Electron Transfer Between Donor (Eosin‐Y) and Acceptor (Naphthoquinone) in a Supramolecular Array Based on a Coordination Cage Host

An octanuclear M8L12 coordination cage host (HPEG) in aqueous solution binds neutral electron‐accepting guests such as naphthoquinone (NQ) in its central cavity via the hydrophobic effect, and multiple anionic photosensitisers such as Eosin‐Y (EY2‐) around the exterior surface due to 16+ charge on the complex cation: this is confirmed by both solution titration experiments and X‐ray crystallography. In the three‐component assembly HPEG / EY2‐ / NQ, photoexcitation of EY2‐ at 525 nm results in ultrafast (ca. 1 ps) photoinduced EY2‐ ® NQ electron transfer from surface‐bound EY2‐ to cavity‐bound NQ (which have been brought into close proximity by their interactions with different sites on the host cage) to give the charge‐separated pair EY•– / NQ•–, identified by transient absorption spectroscopy: back‐ET results in charge recombination in ≈70 ps. The assembly of donor and acceptor components via orthogonal interactions using the host cage as a scaffold presents a promising route into spatial control of multiple components in supramolecular arrays for photophysical applications.