Reactions involving the central cavity of porphyrin molecules attached to self-assembled monolayers

Abstract

In this work we studied the metalation, transmetalation and demetalation reactions of protoporphyrin (PPIX) molecules attached to self-assembled monolayers (SAM) grown over Au(111) surfaces at the solid/liquid interface. Using X-ray photoelectron spectroscopy we found that when the molecules attached to the SAM are immersed in Cu2+ or Zn2+ containing solutions, the degree of metalation is larger for Cu2+ than for Zn2+. Furthermore, when ZnPPIX molecules attached to the SAM are placed in contact with a Cu2+ solution the partial exchange of Zn ions with Cu ions at the porphyrin central cavity is observed. This suggests that insertion of Cu2+ to form CuPPIX is both kinetically and thermodynamically favored over the insertion of Zn2+ in agreement with the solution chemistry of the molecules. Finally, unlike the behavior in solution, Zn and CuPPIX molecules attached to the amino terminated SAMs are more stable against demetalation in acidic conditions. We attribute this difference to the competing protonation of the amine terminal groups. Our results could be useful for the design of molecular devices based on porphyrins that are exposed to liquid interfaces.