Electrically conductive [Fe4S4]-based organometallic polymers.

Tailoring the molecular components of hybrid organic-inorganic materials enables precise control over their electronic properties. Designing electrically conductive coordination materials, e.g. metal-organic frameworks (MOFs), has relied on single-metal nodes because the metal-oxo clusters present in the vast majority of MOFs are not suitable for electrical conduction due to their localized electron orbitals. Therefore, the development of metal-cluster nodes with delocalized bonding would greatly expand the structural and electrochemical tunability of conductive materials. Whereas the cuboidal [Fe4S4] cluster is a ubiquitous cofactor for electron transport in biological systems, few electrically conductive artificial materials employ the [Fe4S4] cluster as a building unit due to the lack of suitable bridging linkers. In this work, we bridge the [Fe4S4] clusters with ditopic N-heterocyclic carbene (NHC) linkers through charge-delocalized Fe-C bonds that enhance electronic communication between the clusters. [Fe4S4Cl2(ditopic NHC)] exhibits a high electrical conductivity of 1 mS cm-1 at 25 °C, surpassing the conductivity of related but less covalent materials. These results highlight that synthetic control over individual bonds is critical to the design of long-range behavior in semiconductors.

Gram-scale synthesis of MIL-125 nanoparticles and their solution processability.

Although metal-organic framework (MOF) photocatalysts have become ubiquitous, basic aspects of their photoredox mechanisms remain elusive. Nanosizing MOFs enables solution-state techniques to probe size-dependent properties and molecular reactivity, but few MOFs have been prepared as nanoparticles (nanoMOFs) with sufficiently small sizes. Here, we report a rapid reflux-based synthesis of the photoredox-active MOF Ti8O8(OH)4(terephthalate)6 (MIL-125) to achieve diameters below 30 nm in less than 2 hours. Whereas MOFs generally require ex situ analysis by solid-state techniques, sub-30 nm diameters ensure colloidal stability for weeks and minimal light scattering, permitting in situ analysis by solution-state methods. Optical absorption and photoluminescence spectra of free-standing colloids provide direct evidence that the photoredox chemistry of MIL-125 involves Ti3+ trapping and charge accumulation onto the Ti-oxo clusters. Solution-state potentiometry collected during the photochemical process also allows simultaneous measurement of MOF Fermi-level energies in situ. Finally, by leveraging the solution-processability of these nanoparticles, we demonstrate facile preparation of mixed-matrix membranes with high MOF loadings that retain the reversible photochromism. Taken together, these results demonstrate the feasibility of a rapid nanoMOF synthesis and fabrication of a photoactive membrane, and the fundamental insights they offer into heterogeneous photoredox chemistry.