Thin-film molecular materials based on tetrametallic "squares": Nanoscale porosity and size-selective guest transport characteristics

Described are the preparation and functional characterization of nanocrysta lline and/or amorphous thin films comprising of neutral "molecular squares" of the form [Re(CO)(3)(Cl)(mu-L)](4) (L = difunctional imine or azine liga nd). The films are strongly adherent, stable in aqueous media, and characte rized by comparatively few pinhole defects. Electrochemical transport exper iments show that the materials are exceptionally porous with respect to suf ficiently small solution-phase permeants but blocking toward larger permean ts. Related thin-film experiments based on monometallic "corner" materials indicate efficient exclusion of all candidate permeant molecules evaluated. For the title materials, these experiments, together with additional elect rochemical probe experiments, indicate that (1) membranelike permeation via pores or tunnels of about nanometer diameter is the primary mode of transp ort of molecular and ionic species through thin films and (2) the transport -relevant pore or tunnel diameter is defined by the cavity dimensions for t he component molecular square. The crystal structure of a single isomer of [Re(CO)(3)(Cl)(mu-4,4'-bipyridine)](4) is also reported. A packing view dow n the c axis of the tetragonal unit cell shows that the molecules, which ar e significantly puckered in the crystalline state, are arranged with caviti es aligned to generate infinite zeolite-like channels.