Amcom24 1895..1897

Abstract

There has been tremendous recent interest in the assembly or self-assembly of inorganic and organometallic amolecular squareso: cyclic structures typically featuring metal-ion corners and difunctional bridging ligands as edges. Motivations include solution-phase catalysis, chemical sensing, and more general host/guest chemistry. Here we describe a solidstate application that may lead to further application in separations processes: size-selective molecular transport from a guest-containing solution to one initially free of guest species. Electrochemical molecular sieving (size-selective transport of redox-active species from solution to an underlying electrode rather than a receiving solution) has previously been demonstrated using thin films of molecular squares. Molecular sieving or nanofiltration has also been demonstrated with mesoporous polycarbonate membranes that have had metallic gold deposited via electroless plating to yield roughly nanometer-sized channels, in addition to more common zeolite and carbon molecular sieves. In this work, size-selective transport through novel amolecular squareo membranes has been demonstrated by measuring the permeabilities of various sized molecules through thin films of pyrazineand zinc-dipyridyl porphyrin molecular squares on polyester membrane supports. For sieving or nanofiltration, thin-film assemblies of neutral molecular squares are appropriate, since any charge compensating ions in non-neutral assemblies might block the pores. From the literature, neutral-square cavities range in size (minimum van der Waals diameter) from roughly 3 Š (pyrazine edges) to 18 Š (alkyl-functionalized dipyridyl porphyrins as edges). A pair of squares, 1 and 2, that lie at the extremes of the cavity-size distribution have been examined in this work. Thin-films of squares were evaporatively cast on commercially available, polyester membranes (see Experimental section) by using suspensions of 1 in 3:1 chloroform:acetonitrile or solutions of 2 in pure chloroform. In fabricating films of 1, vacuum grease was used to mount the polyester membranes on glass slides, each featuring a 0.32 cm diameter hole. The membrane/slide combination was then exposed to water to fill the pores. Casting of the films at the organic-aqueous interface was accomplished after allowing the exposed membrane surface to dry and then adding the suspension of 1 dropwise to the center of the membranes. Films of 2 were cast by simply adding the solution dropwise to the center of the membranes lying dry on glass slides. The active areas of the membranes were approximately 0.32 cm in diameter. Film thicknesses (1 = 8 ± 1 lm, 2 = 16 ± 1 lm) were determined by recording cross sectional brightness and luminescence images of coated membranes that had been microtomed after embedding in a polymer; see Figure 1. The cross sectional images additionally revealed that 2 effectively infiltrates the membrane pores yielding a total thickness

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Cite this paper

@inproceedings{CzaplewskiAmcom241, title={Amcom24 1895..1897}, author={Kenneth F. Czaplewski and Joseph T Hupp} }