Molecule@MOF: A New Class of Electronic Materials

Conducting organic polymers are highly attractive electronic materials, having the advantages of low-cost fabrication routes, tunable electronic properties, and mechanical flexibility. Unfortunately, their charge mobility relative to traditional inorganic semiconductors is poor due in part to microstructural disorder. A new class of synthetically flexible, crystalline materials known as Metal-Organic Frameworks (MOFs) could solve the disorder problem, providing within a single material the highly ordered structure of inorganic conductors with the tunable properties and low cost of organics. MOFs are inorganic-organic hybrids consisting of metal cations linked by organic groups such as carboxylates and amines, forming a crystalline, nanoporous structure. With record-setting surface areas (>7000 m²/g), they are ideal for applications such as CO₂sequestration and catalysis. While efforts to develop MOFs for these applications made major strides over the past decade, electrically conducting frameworks would open up a host of other promising avenues, such as novel electronic devices, photovoltaics, supercapacitors, and electrocatalysts.  Recently, we discovered that infiltrating the pores of the copper-containing MOF HKUST-1 with redox-active guest molecules, such as TCNQ (7,7,8,8-tetracyanoquinododimethane), increases the electrical conductivity of thin film devices by as much as seven orders of magnitude [1]. This emergent property results from a novel donor-bridge-acceptor geometry, in which TCNQ binds to two Cu(II) dimer units within the MOF pore. In this presentation I will describe the synthesis and characterization of Molecule@MOF materials. Coupling these results with first-principles electronic structure calculations demonstrates that conductivity occurs via a hopping mechanism facilitated by the bridging TCNQ. From these results it will be clear that Molecule@MOF represents a novel class of electronic materials with the potential to bridge the properties gap between inorganic and organic conductors, providing a high degree of electronic tailorability combined with long-range order for high charge mobility.

Figure 1 Process for fabrication of conductive metal-organic framework thin film devices, and electrical characterization data. (A) A thin film of metal-organic framework is grown on an insulating wafer pre-patterned with electrodes. Molecules are then infiltrated by exposing the thin film device to a solution containing the molecules. (B) A TCNQ molecule shown above a Cu₃(BTC)₂ MOF with arrow pointing to the pore. Atom color code: white – hydrogen, blue – nitrogen, cyan – carbon, red – oxygen, ochre (light brown) – copper. (C) SEM image of MOF thin film device with optical images of devices before and after TCNQ infiltration (the rectangular features are Au electrodes overcoated with the MOF film (D) Current-voltage characteristics before and after TCNQ infiltration indicating an increase in conductivity of ×10⁷ or larger. Trace colors represent different electrode gaps

[1] A. A. Talin, A. Centrone, A. Ford, M. E. Foster, V. Stavila, P. Haney, R. A. Kinney, V. Szalai, F. El Gabaly, H. P. Yoon, F. Léonard, M. D. Allendorf, Science 343, 66 (2014)