New Class of Microporous Materials: Porous Metal Phosphonate Thin Films

Solid state metal phosphonates (M(O3P-R-PO3) or M(O3P-R)2 (M = metal)) have layered structures where the metal atoms lie in planar sheets and the intervening R groups take up the interlamellar space. Microporous metal phosphonates can be prepared by reaction of the metal with a mixture of large and small phosphonates (M(O3P-LARGE)x(O3P-SMALL)2-x. The larger group acts as a pillar that holds the layers apart. Void spaces result from the presence of the smaller groups. The porous nature of these solids make them potential candidates for applications as sensors, size- and shape- selective catalysts, and chromatographic materials. Metal diphosphonates (M(O3P-R-PO3) can also be prepared one layer at a time on a surface, resulting in the construction of interesting superstructures that are not accessible through the solid state synthesis. For example, these superstructures can contain different components in sequential layers and may have applications in energy conversion, vectorial electron transport, and NLO devices. The preparation of microporous thin films would combine the desirable potential applications of the porous solids with the interesting parallel superstructures that can be prepared from the thin film assemblies. We report our progress toward the construction of microporous metal phosphonate thin films. The two methods that are currently being developed include: 1) phosphonate exchange of pre-assembled films, and 2) co-deposition of different large and small phosphonates during film assembly.