A porphyrin-based molecular square (1), prepared by reaction of a dipyridylporphyrn species with Re(CO)5Cl and subsequently with Zn(II) acetate, was functionalized with 2-(methylene-15-crown-5-ether)-nicotinoyl ester (2). Functionalization was achieved by axially ligating each of the four available Zn(II) sites within the porphyrins. (2) was synthesized by reaction of 2-(hydroxymethyl)-15-crown-5-ether and nicotinoyl chloride in CH2Cl2. Functionalization renders the squares fluorescence emission intensity responsive to added Na(+) and, to a lesser extent, K(+).