Rapid derivatization of mesoporous thin-film materials based on Re(I) zinc-porphyrin 'molecular squares': selective modification of mesopore size andshape by binding of aromatic nitrogen donor ligands

Molecular materials based on thin films of the zinc-containing tetraporphyr inic square assembly, 1, can be rapidly and, in many cases, completely func tionalized by exposure to aqueous or alkane solutions of good N-donor ligan ds such as pyridine and imidazole. Modification can also be achieved via di rect vapor-phase exposure of films to volatile ligands. In both experiments modification is a consequence of simple Zn(II) coordination chemistry and is facilitated by the exceptional mesoporosity of the parent material. Vapo r-phase quartz crystal microbalance experiments indicated an average ligand /component square binding stoichiometry of 2.5, in fair agreement with the stoichiometry of 4 implied by absorbance measurements and expected from the number of Zn(II) sites per assembly. Systematic studies with 9 of the more than 40 total ligands examined, show that ligand binding strength is contr olled by both ligand basicity (sigma electron donating ability) and ligand solvophobic phenomena. In several instances the film modification chemistry was found to be reasonably persistant; in a few instances the modification was demonstrably permanent. For modified mesoporous films in contact with liquid environment, kinetic stability could be qualitatively correlated wit h thermodynamic stability, as indicated by binding constants. Kinetic stabi lity under these conditions, therefore, is a function of both ligand-N/Zn(I I) bond strength and ligand solvophobic character. For films in contact wit h an inert atmosphere (air), kinetic stability could be correlated successf ully with simply the ligand-N/Zn(II) bond strength (as inferred from the li gand pK(a)). The combined results support the notion that mesopore derivati zation-leading to systematic alteration of component cavity size, shape, an d chemical affinity-can be usefully achieved via axial ligation of metallop orphyrins. We suggest that the ready availability of an extended array of d erivatized thin film materials could be useful in membrane-based transport applications, catalyst applications, and/or chemical sensing applications. (C) 1999 Elsevier Science S.A. All rights reserved.