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

Abstract

An octanuclear M₈L₁₂ coordination cage host (H^PEG ⋅ M) 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 (EY^2-) 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 H^PEG ⋅ Cd/EY^{2 -}/NQ, photoexcitation of EY^{2 -} at 525 nm results in ultrafast (ca. 1 ps) photoinduced EY^2-→NQ electron transfer from surface-bound EY^{2 -} 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.