Giant Enhancement of Carrier Mobility in Bimetallic Coordination Polymers

Electrically conductive metal-organic coordination polymers (CPs) are promising candidates for a variety of technological applications. However, poor energetic and spatial overlap between the sp-electrons of organic ligands and the d-electrons of metal ion often blocks an effective charge transport (mobility) across CPs. Herein, we present a bimetallic design principle for enhancing carrier mobility in CPs. Bimetallic CPs of Fe(III) and Cr(III) ions coordinated to 1,3,5-benzenetricarboxylic acid (BTC) ligand (Fe-BTC-Cr) exhibited remarkably high carrier mobility at the matching mole ratio (1:1) with enhancement factors of 10² and 10⁴ in comparison to those of monometallic parents, Fe-BTC and Cr-BTC, respectively. The observation was substantiated by lowering of the band gap between the valence band and the conduction band upon the formation of a hybrid p-n-type structure in the bimetallic CPs. The direct current conductivity values of the CPs measured by four-probe technique were in good agreement with the alternating current conductivity values obtained from the electrochemical impedance spectroscopy. Our flexible approach of picking and choosing the appropriate combination of metal ions from the periodic table is expected to generate various CPs with desirable semiconducting properties.