N2 adsorption-desorption experiments were conducted to investigate the porosity of FeCN@NU-1000. As indicated in the isotherms shown in Figure , although the porosity of NU-1000 decreases after exposure to ferrocyanide, the resulting FeCN@NU-1000 is still mesoporous. The Brunauer-Emmett-Teller (BET) surface areas of NU-1000 and FeCN@NU-1000 are 2,100 and 1,460 m 2 /g, respectively. Density functional theory (DFT) pore size distributions obtained from the nitrogen isotherms are shown in Figure . A slight decrease in mesopore width accompanies shrinking pore size of mesopores can be observed after the sensitization of ferrocyanide ions; a similar change in mesopore width was observed in an earlier study of polyoxometalate installation in NU-1000. Figure shows diffuse reflectance infrared Fourier transform spectra (DRIFTS) of NU-1000 and FeCN@NU-1000. A peak at 3,674 cm -1 , which is associated in the unmodified MOF with terminal and bridging -OH groups on the nodes, is substantially less in FeCN@NU-1000, as are peaks at ca. 2,551 and 2,745 cm -1 , associated in NU-1000 with H-bonded H2O and -OH. These observations are broadly consistent with the aforementioned single crystal X-ray structure which indicates recruitment of the oxygen atoms of terminal and bridging hydroxo groups for binding cyano-ferrate species. A broad band at around 3,000-3,600 cm -1 attributable to physisorbed water also appears in the spectrum of FeCN@NU-1000. A strong and broad peak centered around 2,088 cm -1 , attributable to C≡N stretching is present in the spectrum of FeCN@NU-1000. For comparison, the C≡N stretch in the IR spectrum of ferrocyanide appears at 2,040 cm -1 , while that for ferricyanide appears at 2,114 cm -1 . Examining the 2,088 cm -1 peak for FeCN@NU-1000 more closely, it appears to consist of multiple overlapping, but unresolved, peakspossibly resulting from the presence of two chemically distinct cyano-ferrate species, as evinced by the single crystal X-ray structure. Alternatively, the cause could be the reduction in symmetry that accompanies ligand-substitution-based attachment of the iron complex to the node. Notably, a reduction in symmetry (albeit, without ligand substitution on iron) is readily evident in the infrared and Raman spectra for ferrocyanide bound to nanoparticulate TiO2. Diffuse reflectance UV-visible (UV-vis) spectra of both NU-1000 and FeCN@NU-1000 were also measured, as shown in Figure . A broad absorbance in the visible region, mostly overlapping with the absorbance from the pyrene-based linkers in NU-1000 but tailing out to 750 nm, can be observed in the UV-vis spectrum of FeCN@NU-1000. Subtraction of the NU-1000 spectrum from the spectrum of FeCN@NU-1000 results in a broad band with an apparent maximum at ~430 nm (although the accuracy of the maximum may be compromised by the large absorbances below 430 nm in the parent spectra). The observed band is reminiscent of the known charge-transferabsorbance for ferrocyanide in combination with TiO2 nanoparticles. It is tempting to offer a similar assignment here, i.e. Fe(II)-to-Zr(IV) charge transfer, although a somewhat short maximum wavelength might be expected in view of the much more negative reduction potential for Zr(IV) relative to Ti(IV). Finally, as shown in Figure , the pyrene-linker-based fluorescence of the MOF is quenched for FeCN@NU-1000presumably due to electron transfer from the linker excited state to the cyano-ferrate complex. To explore the electrochemical behavior of FeCN@NU-1000, thin films were formed on conducting glass platforms (fluorine-doped tin oxide (FTO)) by spin coating, following a procedure reported previously. As shown in Figure , cyclic voltammetry yields an anodic and cathodic peak pair centered at roughly 0.2 V and a weak second pair centered at roughly 0.38 V, versus Ag/AgCl/KCl (3 M) (the full-scale CV curves are shown in Figure ). We ascribe these to cyano-ferrate(III/II) couples, and note that the single crystal X-ray structure shows that two chemically distinct cyano-ferrate species are present in FeCN@NU-1000. To quantify the amount of cyano-ferrate that is electrochemically accessible (on the tens of seconds timescale), chronoamperometry measurements were made (Figure ), with the total loading of cyano-ferrate per square centimeter of the film being obtained by UV-vis measurements as reported previously. From the measurements only ~2% of the installed cyano-ferrate complexes are electrochemically addressable on the timescale of the chronoamperometry measurement. Nevertheless, such a low percentage is consistent with previous observations for NU-1000-supported clusters of cobalt oxide, manganese oxide, and copper oxide. Application of the Cottrell equation to the chronoamperometry an apparent charge diffusion coefficient of 7 × 10 -14 cm 2 /s for redox hopping via cyano-ferrate sites. (See Figure and the discussion in Supporting Information for details.)