Varying the electrochemical potential and thickness of porphyrazine SAMs by molecular design

A series of multithiol-functionalized free-base and Zn-coordinated porphyrazines (pz's) have been prepared and characterized as self-assembled monolayers (SAMs) on Au. The synthetic flexibility of the pz's provides a unique opportunity to tune their electronic and chemical characteristics and to control the distance of the redox-active pz macrocycle from the Au surface. This allows us to study the reduction potentials of these surface-bound pz's as a function of film thickness and molecular charge distribution using angle-resolved X-ray photoelectron spectroscopy and cyclic voltammetry. Upon SAM formation, the reduction potentials of all pz's show a significant positive shift from their formal potentials when free in solution (up to ∼ +1 V), with the magnitude of the shift inversely related to the Au-pz distance as determined from the film thickness of the pz SAM (thicknesses ranging from 3.5 to 11.8 Å). When the pz lies down on the surface, in a SAM of thickness ∼3.5 Å, the charge distribution within a pz macrocycle also plays a role in determining the potential shift. These observations are consistent with our originally proposed mechanism for potential shifts upon binding to a metal surface based on image charge effects and with the analysis of Liu and Newton (J.Phys. Chem. 1994, 98, 7162).