Abstract
Electronic charge transport through crystalline metal-organic frameworks (MOFs) can be accomplished by site-to-site electron (or hole) hopping, provided that redox-active sites, such as easily reducible or oxidizable MOF linkers, are present. If the framework itself is redox-inert, solvent-assisted ligand incorporation of redox-active moieties can serve to enable hopping-based charge transport. Here we have studied the redox hopping process within Ru-bpy@NU-1008, where Ru-bpy is a carboxylate-functionalized derivative, i.e., a node-ligating derivative, of the well-known chromophore Ru(2,2′-bipyridine)32+, and NU-1008 is a redox-inert MOF featuring hierarchical porosity and csq topology. Chronoamperometry experiments with electrode-supported thin films of Ru-bpy@NU-1008 show that charge transport is feasible through portions of the MOF, with other portions being inaccessible. Possible confounding features are the undersized c-pores that cross-connect 1D mesoporous channels, as ingress and egress of charge-compensating anions is believed to accompany the net oxidation of Ru(II) to Ru(III) and the reduction of Ru(III) to Ru(II). Phenomenologically, transport through the electroactive portion of the films is diffusion-like, with the magnitude of the apparent diffusion coefficient being 6 × 10−12 cm2/s.
Highlights
The increasingly compelling demand for renewable, carbon-neutral sources of energy offers an incentive to develop catalytic materials that can seamlessly execute reactions analogous to those comprising naturally occurring photosynthesis, e.g., water oxidation and chemical-energy-storing reduction of carbon dioxide
The metal-organic frameworks (MOFs) is of csq topology and offers cross-connected triangular and hexagonal 1D channels of ~12 Å and 30 Å diameter, respectively
The cross-connecting pores, i.e., c- and c′-pores, have a minimum van der Waals width of ~2 × 7 and 2.6 × 10 Å2, respectively. They are significantly narrower than the crossconnecting pores in NU-1000 and PCN-222—closely related MOFs characterized by csq topology
Summary
The increasingly compelling demand for renewable, carbon-neutral sources of energy offers an incentive to develop catalytic materials that can seamlessly execute reactions analogous to those comprising naturally occurring photosynthesis, e.g., water oxidation and chemical-energy-storing reduction of carbon dioxide. Morris et al incorporated Ru(bpy)32+ within a UiO-67 type framework by mixed ligand approach and examined the immobilized compound as a light absorber for dyesensitized solar cells (Maza et al, 2016) and as an emitter for electro-generated chemiluminescense (Cai et al, 2018). In both cases, charge transport (i.e., transport of redox equivalents) is necessary. We have quantified diffusive charge transport within electrode-supported thin films of the functionalized MOF by introducing large or small perturbations of the applied potential via chronoamperometry
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