Thin-Film Molecular Materials Based on Tetrametallic “Squares”:  Nanoscale Porosity and Size-Selective Guest Transport Characteristics

Abstract

Described are the preparation and functional characterization of nanocrystalline and/or amorphous thin films comprising of neutral "molecular squares" of the form [Re(CO) 3 (Cl)(µ-L)] 4 (L ) difunctional imine or azine ligand). The films are strongly adherent, stable in aqueous media, and characterized by comparatively few pinhole defects. Electrochemical transport experiments show that the materials are exceptionally porous with respect to sufficiently small solution-phase permeants but blocking toward larger permeants. 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 electrochemical probe experiments, indicate that (1) membranelike permeation via pores or tunnels of about nanometer diameter is the primary mode of transport of molecular and ionic species through thin films and (2) the transportrelevant pore or tunnel diameter is defined by the cavity dimensions for the component molecular square. The crystal structure of a single isomer of [Re(CO) 3 (Cl)(µ-4,4′-bipyridine)] 4 is also reported. A packing view down the c axis of the tetragonal unit cell shows that the molecules, which are significantly puckered in the crystalline state, are arranged with cavities aligned to generate infinite zeolite-like channels.

Shape-selective transport through rectangle-based molecular materials: Thin-film scanning electrochemical microscopy studies

“…6 4Ϫ yielded values of 6 ϫ 10 Ϫ9 and 2 ϫ 10 Ϫ9 cm 2 ⅐s Ϫ1 , respectively. These values are roughly three orders of magnitude smaller than the corresponding values of solutionphase diffusion coefficients (16). They are roughly two orders of magnitude smaller than PD f values observed for transport through films composed of intermediate-sized molecular squares (11,16,(18)(19)(20).…”

“…Sieving behavior has been examined spectrally by using dye molecules together with films supported on macroporous membranes (13). Sieving has also been probed electrochemically by using redox-active molecules together with films supported on electrodes (14,15); molecules that successfully traverse a film yield a redox response at the underlying electrode (16)(17)(18). Consistent with sieving behavior that is primarily defined by the cavities contained within individual host molecules, sharp size cutoffs corresponding to minimum cavity sizes are generally observed.…”

Molecular Materials and Devices: Developing New Functional Systems Based on the Coordination Chemistry Approach

“…5,19 Nanoporous materials have been generated from discrete molecular building blocks, as exemplified by the series of nanoscale cavity-containing coordination compounds, reported by Belanger et al. 8 In this particular aspect, the coordination approach is very effective, circumventing two of the most conventional problems found in solid state chemistry. The first one is the collapse of the channels upon removal of the solvent from the porous solid.…”

“…Most of the strategies rely upon the preprogramming of synthons, in order to allow a directed-assembly approach. For instance, in the assembly of square cavity structures (Figure 2), synthons have been employed 8 consisting of octahedral complexes displaying rigid or semi-rigid bifunctional edges in cis-configuration, a detail which allows them to coordinate sequentially.…”

“…Consequently, with the rise of molecular nanotechnology, molecular materials are becoming a major focus of interest for the design of nanostructures and devices. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] In fact, functional nanostructured molecular materials are being currently pursued in the academy and industries, relying on the bottom up approach, as envisaged by Erix Drexler, 3 but using molecular building blocks and supramolecular chemistry as tools. 5 Supramolecular chemistry provides, perhaps, the most important strategy toward molecular nanotechnology.…”