Crystalline coordination framework endowed with dynamic gate-opening behaviour by being downsized to a thin film

The fabrication of porous coordination frameworks in thin-film forms has been investigated intensively with a view to using their structural response to external stimuli and guests for potential nanotechnological applications, for example as membranes for gas separation. Here we report a coordination framework that exhibits a dynamic guest-sorption behaviour in a nanometre-sized thin-film form (16 nm thick), yet shows no guest uptake in the bulk. Highly oriented crystalline thin films of this coordination framework--which consists of interdigitated two-dimensional layers of {Fe(py)2[Pt(CN)4]} (py, pyridine)--were fabricated through liquid-phase layer-by-layer synthesis. The resulting thin film exhibited a clear guest uptake with a structural transformation of the gate-opening type as characterized by in situ X-ray diffraction. Increasing the film's thickness markedly suppressed this behaviour. We envisage that such a crystal-downsizing effect may be observed with other coordination frameworks, and may be of use to develop functional materials, for example, for switching or sensing devices.