Storage of light energy by photoelectron transfer across a sensitized zeolite–solution interface

Abstract

A COMMON strategy for storage of solar energy involves the photoexcitation of a donor molecule D followed by electron transfer to an acceptor A. To exploit this strategy in a practical context, a way must be found to impede the back-reaction in which the electron is transferred from A to D (ref. 1). Previous attempts to achieve long-lived charge separation have involved the use of D–A combinations held in well defined geometries by spacer groups^2,3 or immobilized on supports such as porous media^4–12. Immobilization of the redox species poses problems, however, for their subsequent separation in order to reclaim the stored energy. Here we report a system that achieves efficient D–A electron transfer, a slow back-reaction and easy separation of the products. We trap the photosensitizer donor, trisbipyridine ruthenium(II), in the supercages of zeolite Y, and use as the acceptor a neutral, zwitter-ionic viologen in the surrounding solution. Electron transfer from the ruthenium centre to the viologen is mediated by N,N′-tetramethylene-2,2′-bipyridinium ions loaded into the zeolite by ion exchange. Isolation of the donor within the zeolite from the acceptor in the solution outside makes the photochemically generated products easily accessible. Practical utilization of this trimolecular redox assembly will, however, require improvement of the quantum yield.