Three-dimensional protonic conductivity in porous organic cage solids

Abstract

Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells. To maximize proton conduction, three-dimensional conduction pathways are preferred over one-dimensional pathways, which prevent conduction in two dimensions. Many crystalline porous solids to date show one-dimensional proton conduction. Here we report porous molecular cages with proton conductivities (up to 10−3 S cm−1 at high relative humidity) that compete with extended metal-organic frameworks. The structure of the organic cage imposes a conduction pathway that is necessarily three-dimensional. The cage molecules also promote proton transfer by confining the water molecules while being sufficiently flexible to allow hydrogen bond reorganization. The proton conduction is explained at the molecular level through a combination of proton conductivity measurements, crystallography, molecular simulations and quasi-elastic neutron scattering. These results provide a starting point for high-temperature, anhydrous proton conductors through inclusion of guests other than water in the cage pores.

Porous Molecular Solids and Liquids

“…78,96 An obvious exception here would be conjugated COFs, 97 where there are as yet no molecular analogues; a worthy future target. For some applications, such as the separation of krypton and xenon, molecular crystals were demonstrated to have better selectivity than extended frameworks, 70 at least transiently.…”

Oriented Two‐Dimensional Porous Organic Cage Crystals

“…Also, unlike covalent organic frameworks (COFs), POCs are crystallized without any bond‐forming reactions; hence, while single‐crystalline COFs are very rare,4 it is relatively easy to grow high‐quality single‐crystal POCs. POCs have been explored in various applications such as sensing,5 gas storage,2 molecular separations (for example, xylene isomers,6 noble gases,7 and chiral molecules8), and proton conductivity 9. As discrete, soluble molecules, POCs can be processed in organic solvents in a way that cannot be achieved with insoluble porous frameworks.…”

“…[3] Also,u nlike covalent organic frameworks (COFs), POCs are crystallized without any bondforming reactions;h ence,w hile single-crystalline COFs are very rare, [4] it is relatively easy to grow high-quality singlecrystal POCs.P OCs have been explored in various applications such as sensing, [5] gas storage, [2] molecular separations (for example,x ylene isomers, [6] noble gases, [7] and chiral molecules [8] ), and proton conductivity. [9] As discrete,s oluble molecules,P OCs can be processed in organic solvents in aw ay that cannot be achieved with insoluble porous frameworks.F or example,m odular mix and match assembly strategies have been used to form binary and ternary cocrystals, [10] and cage crystals can be incorporated into polymers to form composite membranes. [11] Thef abrication of functional materials into thin films, membranes,a nd oriented crystals on substrates is of importance for applications in sensors,catalysts,electronic devices, and electrodes for fuel cells.…”

Modular assembly of porous organic cage crystals: isoreticular quasiracemates and ternary co-crystal

“…This can impart guest selectivity [12][13][14] and confinement. [15][16][17] The accessibility of the intrinsic pore of an isolated cage is dictated by the dimensions of the cage window, but the PD is ultimately determined by how the cages pack in the solid state. Therefore, the function of crystalline POCs is related to the crystal packing of the cages as well as the isolated chemical structure of the cage.…”