Reducing Is Easier When Lying Down?

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Science  17 Dec 2004:
Vol. 306, Issue 5704, pp. 2005
DOI: 10.1126/science.306.5704.2005a

The applied potential needed to oxidize or reduce molecules in solution reflects in part the energy needed to stabilize more highly charged species (ions versus neutrals). If the molecules are adsorbed on a metal, the formation of mirror-image charges should reduce the energetic expense of solvating a charged ion, because a dipole is formed instead. The effect of this on the coupling of the real and mirror charges should also drop off with distance. Vesper et al. provide experimental evidence for this effect using two porphyrazine derivatives adsorbed on gold surfaces. Derivative 1 has a single set of sulfur-terminated “legs” so that it self-assembles in “standing-up” geometry, and derivative 2 was designed with two opposing sets so that would lie flat. The molecules were patterned on gold with dip-pen lithography, and the structures were verified by atomic force microscopy. In methylene chloride solution, the molecules showed similar redox behavior. When adsorbed on gold, the first ring-reduction potential of 1 shifted to less negative voltages by 0.43 V, whereas that of 2, whose central ring lies closer to the surface, shifted by 0.80 V. — PDS

J. Am. Chem. Soc. 10.1021/ja045270m (2004).

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