Supramolecular Chemistry I: Directed Synthesis and Molecular Recognition

Abstract

Early pioneering studies by Iler, reported almost 30 years ago, demonstrated the feasibility of growing multilayers of organic and inorganic polymer colloids on glass by sequential deposition.2 Although such materials bear a resemblance to Langmuir-Blodgett films (both may be viewed as stacks of two-dimensional arrays), they differ, significantly, by being porousand nonuniform, i.e., their supramolecular structure reflects the fact that they are composed of spherical particles of varying diameter.3-4 *In this report, we describe the preparation of "Iler-like" arrays that have been constructed from dendrimer molecules.5-12 The ability to synthesize dendrimers with perfect uniformity and the possibility of introducing chromophores, redox active species, catalytic sites, etc. at defined locations make them unique as building blocks for the construction of novel and exploitable materials.13-14 The procedure that we have developed for growing dendritic multilayers is reminiscent of Mallouk's approach to organometallic thin films.15 In essence, it involves the deposition of amine-terminated dendrimers onto a Pt2+-bearing surface, followed by reactivation with K2PtCl4 (Scheme 1). Solutions of a poly-(amidoamine) dendrimer (la; PAMAM, Generation 6, MW 58 048; Aldrich Chem.) and K2PtCl4 were prepared in freshly distilled DMSO; final concentrations of primary amine and K2-PtCl4 were 2.0 mM. In a typical preparation, a silicon wafer (2 cm2) was first silylated with (3-aminopropyl)triethoxysilane, immersed in a solution of K2PtCU (30 min), and rinsed with water and methanol.16 One complete growth cycle was then carried out by immersion of the wafer in a solution of la (30 min), rinsing with water and methanol, reactivation with K2-PtCl4 (30 min), rinsing, and drying under a stream of nitrogen; (1) Supported by the U.S.. Zasadzinski, J. A.; Viswanathan. R.; Madsen, L.; Garnaes, J.; Schwartz, D. K. Science 1994, 263, 1726. (14) For recent extensions of the Her approach to multilayer construction using organic and inorganic polymers, see: (a) Lvov, Y.; Decher, G.; Sukhorukov.G. Macromolecules 1993,26,5396. (b) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (15) Lee. H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. Scheme 1 repetition of this protocol yielded corresponding multilayers. All stages of growth were carried out at 23 °C under an argon atmosphere. PAMAM, Generation = 0 Ellipsometric film thicknesses that were measured as a function of cycle number are shown in Figure 1; similar multilayers that were prepared using growth periods of 10 and 60 min are also shown. From these results, we conclude that 30 min is the minimum time that is needed to produce a maximum growth of ca. 80 A per cycle; the contribution from each Pt2+ half-cycle was ca. 10 A (not shown).17 Control experiments that were carried out in the absence of Pt2+ half-cycles showed negligible growth of dendritic film. The required presence of Pt2+ for multilayer growth implies that some fraction of the immobilized metal ion is bonded to a minimum of two amino groups, one per neighboring dendrimer in adjoining monolayers. An analogous assembly grown from lb (PAMAM, Generation 4, MW 14 215; Aldrich Chem.) showed a maximum thickness of ca. 50 A per cycle (Figure 2). Previous size-exclusion chromatographic analyses of PAM-AM dendrimers having an ammonia (as opposed to an ethyl-enediamine) core indicate apparent diameters of 40 and 67 A for generations 4 and 6, respectively.6* In view of the conformational flexibility that is inherent in the dendrimer structure, we regard these experimental and estimated values as being in good agreement.6* We also regard this agreement as strong evidence for the existence of dense monolayer coverage by the dendrimer. Examination of 4.5-and 5.0-cycle multilayered assemblies by X-ray photoelectron spectroscopy (XPS, Scienta ESCA-310) (16) The silylation reagent, which was freshly distilled under reduced pressure, was used as a 5 mM "wet" chloroform solution (100 mL of Burdick & Jackson, HPLC + 10 uL of water). Typically, a silicon wafer was first treated for 30 min at room temperature with this solution, followed by washing with CHClj and drying under a stream of nitrogen. Prior to silylation, the silicon wafer was cleaned by immersion in concentrated H2SO4/30% HjO; (7/3 v/v) for 30 min at 90 °C, washed with Milli-Q water and CHjOH, and dried under a stream of nitrogen. Dendrimer samples that were used in this work are estimated by the manufacturer (Dendri tech, Midland, MI) to consist of ca. 80-90% of a single molecular form (gel electrophoresis); 10-20% of each sample contains defects due to incomplete growth. (17) Film thicknesses are based on a refractive index of 1.59, which has been estimated, ellipsometrically, from a film that was grown to 800 A in thickness. (18) Caminati, G.;Turro, N. J.; Tomalia, D. A.