Effects of Counterion Mobility, Surface Morphology, and Charge Screening on the Electron-Transfer Rates of Porphyrin Monolayers

By Jiao, Jieying; Nordlund, Eric; Lindsey, Jonathan S. & Bocian, David F.
Published in The Journal of Physical Chemistry C NULL 2008

Abstract

The standard electron-transfer rate constants (k0) for the oxidation of porphyrin monolayers are reported for a number of solvent/electrolyte systems and electroactive surfaces. The goal is to explain the inverse correlation between the electron-transfer rates and the porphyrin surface concentration (Roth et al., J. Phys. Chem. B 2002, 106, 8639-8648). Each porphyrin is a zinc chelate and contains three meso-mesityl groups and a benzyl alcohol or benzyl thiol for surface attachment. The solvent/electrolyte systems include (i) the organic solvent propylene carbonate containing electrolytes with a common cation and anions of different size/mobility (PF6-, ClO4-, and Cl-) and (ii) neat ionic liquids with a common cation and anions of different size/mobility [(CF3SO2)2N- and (NC)2N-]. The substrates include Si(100), Au(111), and TiN. The k0 values observed using electrolytes with PF6-, ClO4-, and (CF3SO2)2N- counterions are similar to one another, whereas those observed using electrolytes Cl- and (NC)2N- counterions are 2-5 times faster. The faster rates for the latter anions are attributed to their smaller size/higher mobility. The k0 values observed for monolayers on Si(100) and Au(111) are similar to one another; the k0 values for monolayers on TiN are ∼5-fold faster. The faster rates for the TiN substrate are attributed to a rougher surface morphology (as determined via atomic force microscopy measurements) which results in an actual surface concentration that is lower than the concentration based on the geometrical area of a planar substrate. The k0 values determined for mixed monolayers where the electroactive porphyrin is co-deposited with an electroinactive porphyrin are dependent only on the concentration of the redox-active species, not on the total porphyrin concentration. This behavior is consistent with space-charge effects being the principal determinant of the inverse correlation between the electron-transfer rates and the porphyrin surface concentration. The space charge effects can be mitigated, but not eliminated, by using smaller, more mobile counterions and rougher surfaces.

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