Stable photoinduced charge separation in layered viologen compounds

SOLAR energy can be used and stored by the efficient production of long-lived photoinduced charge separation 1–3 —a state achieved in photosynthetic systems by the formation of a long-lived radical pair 4–7 . A number of artificial systems have been reported that efficiently undergo photochemical charge transfer 8–10 ; unfortunately, the thermal back electron transfer often proceeds at an appreciable rate, limiting the utility of these systems. Here we report the photochromic behaviour of novel layered zirconium phosphonate/viologen compounds which show very efficient photo-induced charge transfer, and form a charge-separated state which is long-lived and stable in air. Spectroscopic studies indicate that the photoproduct is the dialkyl viologen radical cation, produced in the inter lamellar region of the zirconium phosphonate. We suggest that the remarkable stability of the charge-separated state arises from structural features which allow for stabilization of the radicals by delocalization and shielding from molecular oxygen.