M. Skupin et al., Methylammonium groups at the solid walls of nanometer-sized, water-filled monolayer gaps as binding sites for a tetraanionic porphyrin, J AM CHEM S, 123(15), 2001, pp. 3454-3461
Long-chain hydrosulfides containing two secondary amide functions and eithe
r electron-poor or electron-rich carbon-carbon double bonds were self-assem
bled on gold surfaces around a flat-lying, octaanionic porphyrin. Rigid and
reactive surface monolayers with 2 nm-wide, porphyrin-based gaps were thus
obtained. The gold electrodes were then immersed in water, and the double
bonds on the gaps' surfaces reacted with methylamine. It was added to the d
ouble bonds either by Michael addition or by bromination with hypobromite f
ollowed by methylamine substitution. Only the double bonds at the border of
the gaps were accessible to methylamine dissolved in the bulk water volume
and could react. The walls of the rigid membrane gaps now contained methyl
ammonium groups at the sites of the double bonds in defined heights. A tetr
acationic copper(II) porphyrinate could not diffuse any more into the gap a
nd did not quench the fluorescence of the octaanionic porphyrin on the bott
om of the gap. A tetraanionic porphyrin, on the other hand, was fixated by
the ring of ammonium groups. The bound porphyrin then acted as molecular co
ver for the gap with respect to ferricyanide transport from bulk water to t
he electrode. It was removed by raising the pH to a value of 12, where the
methylammonium groups were neutralized to amines. Lowering the pH to 7 agai
n and addition of more of the anionic porphyrin reclosed the gap. The porph
yrin "cover" should be localized at distances of 8-10 and 20 Angstrom from
the bottom porphyrin by multiple charge interactions. The 8-10 Angstrom dis
tance is ideal for studies of photoinduced electron transfer between two po
rphyrin monomers of different redox potential. Furthermore it was found, th
at redox-active tyrosine could be trapped in the water volume above the por
phyrin on gold.