Metal–organic frameworks (MOFs) are nanoporous crystalline materials made from metal ions connected to organic linkers, and are typically of interest in applications such as gas storage, catalysis and sensing. Because of the insulating nature of the linker molecules, and the fact that there is little orbital overlap between the linkers and the metal ions, MOFs are usually poor electrical conductors. Alec Talin, Mark Allendorf and colleagues at Sandia National Laboratories, the US National Institute of Standards and Technology, and the University of Maryland have now shown that conducting MOFs can be created by adding conjugated guest molecules to the pores of the material.
The researchers began by growing thin films of a MOF known as HKUST-1, which is made from copper ions and the organic molecule benzene-1,3,5-tricarboxylic acid, on a patterned electrode surface. The films were then infiltrated with the redox-active molecule 7,7,8,8-tetracyanoquinododimethane (TCNQ), which was found to significantly increase the conductivity of the films. By altering the TCNQ exposure time, the conductivity of the thin-film devices could be varied over six orders of magnitude and conductivities as high as 7 siemens per metre could be achieved.
With the help of spectroscopic data and modelling, Allendorf and colleagues determined that the conductivity is probably due to the TCNQ guest molecules binding to copper subunits in the MOF and forming a strong electronic coupling between them. They also suggest that the conducting MOFs could have applications in electronic devices and reconfigurable electronics.
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