Co-Ca Phosphonate Showing Humidity-Sensitive Single Crystal to Single Crystal Structural Transformation and Tunable Proton Conduction Properties

A combination of humidity-dependent single crystal to single crystal (SC-SC) structural transformation and single crystal proton conductivity measurements is essential to elucidate the underlying proton transport mechanism in metal-organic framework materials. Herein, we report a new layered Co-Ca phosphonate [(CoCaII)-Ca-III(notpH(2))(H2O)(2)]ClO4 center dot nH(2)O [abbreviated as CoCa center dot nH(2)O, where notpH(6) = 1,4,7-triazacydononane-1,4,7-triyl-tris(methylenephosphonic acid), C9H18N3(PO3H2)(3)]. CoCa center dot nH(2)O undergoes a reversible relative humidity (RH) dependent SC-SC structural transformation between CoCa center dot 2H(2)O and CoCa center dot 4H(2)O at room temperature. Accordingly the continuous hydrogen bond network observed in CoCa center dot 4H(2)O (95% RH) is interrupted in CoCa center dot 2H(2)O (40% RH), leading to a drastic decrease in proton conductivity by similar to 5 orders of magnitude. The process is reversible; hence, the proton conductivity is tunable simply through humidity control. The AC impedance measurements using single crystals of CoCa center dot nH(2)O reveal that the [010] direction of H-bond extension is the preferred proton conduction pathway showing the greatest conductivity of 1.00 X 10(-3) S cm(-1) at 25 degrees C and 95% RH. Although the [20-1] direction, which involves the phosphonate oxygen atoms in the H-bond network shows the lowest conductivity of 4.35 x 10(-8) S cm(-1) at 25 degrees C and 95% RH, the ClO4- anions play a key role in not only connecting the lattice water molecules into a continuous hydrogen bond network but also assisting the proton diffusion between the lattice water molecules. This work provides a rare example of a proton conductive MOF with a well-illustrated proton conduction mechanism and is a promising humidity sensor for future applications.