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Walljet Electrochemistry: Quantifying Molecular Transport through Metallopolymeric and Zirconium Phosphonate Assembled Porphyrin Square Thin Films
journal contribution
posted on 2004-05-25, 00:00 authored by Aaron M. Massari, Richard W. Gurney, Craig P. Schwartz, SonBinh T. Nguyen, Joseph T. HuppBy employing redox-active probes, condensed-phase molecular transport through nanoporous thin films
can often be measured electrochemically. Certain kinds of electrode materials (e.g. conductive glass) are
difficult to fabricate as rotatable disks or as ultramicroelectrodesthe configurations most often used for
electrochemical permeation measurements. These limitations point to the need for a more materials-general measurement method. Herein, we report the application of walljet electrochemistry to the study
of molecular transport through model metallopolymeric films on indium tin oxide electrodes. A quantitative
expression is presented that describes the transport-limited current at the walljet electrode in terms of
mass transport through solution and permeation through the film phase. A comparison of the film
permeabilities for a series of redox probes measured using the walljet electrode and a rotating disk electrode
establishes the accuracy of the walljet method, while also demonstrating similar precision for the two
methods. We apply this technique to a system consisting of zirconium phosphonate assembled films of a
porphyrinic molecular square. Transport through films comprising three or more layers is free from significant
contributions from pinhole defects. Surprisingly, transport through films of this kind is 2−3 orders of
magnitude slower than through films constructed via interfacial polymerization of nearly identical
supramolecular square building blocks (Keefe; et al. Adv. Mater. 2003, 15, 1936). The zirconium phosphate
assembled films show good size exclusion behavior. The details of the observed dependence of permeation
rates on probe molecule size can be rationalized with a model that assumes that the walls of the squares
are slightly tilted from a strictly vertical geometry, consistent with atomic force microscopy measurements,
and assumes that the individual wall geometries are locked by rigid interlayer linkages.