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 approximately +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 A). When the pz lies down on the surface, in a SAM of thickness approximately 3.5 A, 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).
Department of Chemistry and International Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208, USA.
Hong
H
Zong
Peng
P
Sun
Chad A
CA
Mirkin
Anthony G M
AG
Barrett
Brian M
BM
Hoffman
D055664:Electrochemical Techniques / Q000379:methods*; D006046:Gold; D008567:Membranes, Artificial; D010084:Oxidation-Reduction; D011166:Porphyrins / Q000737:chemistry*
D008567:Membranes, Artificial; D011166:Porphyrins; D006046:Gold