Rapid photoinduced electron transfer was observed with supramolecular assemblies of donor and acceptor metal complexes at the galleries of n-butylammonium salt of alpha-zirconium phosphate ((C4H9NH3)(2)Zr(PO4)(2). xH(2)O, abbreviated as BAZrP). Tris(2,2＇-bipyridyl)ruthenium(II)2(+) luminescence, for example, was quenched by tris(2,2＇-bipyridyl)cobalt(III)(3+) with rate constants exceeding 1.6 x 10(12) m(2) mol(-1) s(-1), when the metal complexes were intercalated in the galleries of BAZrP. The quenching rates were unaffected when the viscosity of the BAZrP suspensions was increased (2.1 mixture of ethylene glycol-water) or when the loading of the donor was increased 10-fold. Even when cooled to 77 K, as a rigid glass, the electron transfer reactions between the donor and the acceptor proceed at essentially the same rate as at room temperature indicating a very low activation energy for the electron transfer reaction. These activation-less rate constants are faster than anticipated for diffusion controlled electron transfer rates at BAZrP galleries, at low temperatures. Excited-state energy migration from donor to donor (antenna effect) followed by electron transfer between the nearest donor-acceptor pairs is one explanation proposed to account for the above observations.