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

Molecular materials based on thin films of the zinc-containing tetraporphyrinic square assembly, 1, can be rapidly and, in many cases, completely functionalized by exposure to aqueous or alkane solutions of good N-donor ligands such as pyridine and imidazole. Modification can also be achieved via direct 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. Vaporphase 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 controlled by both ligand basicity (σ 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 with thermodynamic stability, as indicated by binding constants. Kinetic stability under these conditions, therefore, is a function of both ligand-N/Zn(II) bond strength and ligand solvophobic character. For films in contact with an inert atmosphere (air), kinetic stability could be correlated successfully with simply the ligand-N/Zn(II) bond strength (as inferred from the ligand pK(a)). The combined results support the notion that mesopore derivatization-leading to systematic alteration of component cavity size, shape, and chemical affinity-can be usefully achieved via axial ligation of metalloporphyrins. We suggest that the ready availability of an extended array of derivatized thin film materials could be useful in membrane-based transport applications, catalyst applications, and/or chemical sensing applications. (C) 1999 Elsevier Science S.A.