Scanning electrochemical microscopy assessment of rates of molecular transport through mesoporous thin-films of porphyrinic "molecular squares"

Scanning electrochemical microscopy (SECM), in combination with electrode platform micropatterning, has been used for qualitative characterization of molecular sieving and exceptionally precise quantitative measurement of molecular permeation through thin films of meso- and microporous materials composed of porphyrinic "molecular squares". SECM measurements reveal that modification of the squares, to reduce cavity sizes and alter the size cutoff for permeation, has only a modest effect upon the permeabilities of those probe molecules small enough to pass through the modified films. Additional experiments as a function of electrolyte composition show that the ability to observe sieving behavior electrochemically is determined not only by the relative sizes of redox-active permeant molecules and film-based cavity openings, but also by the steric properties of ions needed for charge compensation - even when the permeant molecule is initially neutral. As expected, when the size of the opening is altered the steric requirements for the charge-compensating ion are similarly altered. The experiments also show that charge-compensating ion transport can be convoluted to a small but measurable extent with the probe-molecule transport measurement. The identity of the component of the electrolyte not required for charge compensation, however, does not detectably affect the SECM-based permeation measurement.