pi-Donor-polyoxyethylene-capped Zn(II)-porphyrins, 1-3, form supramolecular complexes with N,N＇-dimethyl-4,4＇-bipyridinium, MV2+, 4. The association constants of the resulting adducts are 8 x 10(4), 1 x 10(4), and 1 x 10(3) M-1, respectively. The Zn(II)-porphyrins are quenched by MV2+ by two complementary routes that include internal electron transfer quenching in the supramolecular complexes and diffusional electron-transfer quenching of the free chromophore.