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As part of our interest in the self-assembly of materials into higher order structures we have synthesized bisterpyridine monomers that facilitate the assembly of benzenoid architectures based on terpyridine-metal-terpyridine connectivity. The space filling and wire frame models were created using Ceruis2 molecular modeling software from Accelrys (formerly Molecular Simulations, Inc.). Self-assembly, using ruthenium as the metal glue, was accomplished via reaction of an equimolar mixture of bismetallated ligand and uncomplexed bisterpyridine monomer. Structure verification was supported by synthesis of the identical structure via a directed approach as well as traditional methods. Construction of Fe-based hexamers has also been investigated along with mixed Fe-Ru constructs. Pertinent dimensions of the hexaruthenium ring include a diameter of 37.5 angstroms, as an internal void distance 17.5 angstroms, and a Ru to adjacent Ru distance of 13.5 angstroms. Continuing studies of this unique organic-inorganic hybrid include periphery modification to facilitate its use as a building block for the preparation of higher order, nanoscale architectures.
Construction of these bisterpyridine building blocks has led to the synthesis of the first, non-dendritic, fractal metallomacromolecule comprised of 36 Ruthenium and 6 Iron ions arranged in a nearly planar array of repeating hexagonal shapes. The iterative, hexameric self-similarity exhibited by this new material led to its description as a "Sierpinski Hexagonal Gasket" based on the work of Polish mathematician Vaclav Seirpinski that was later expanded on by B. B. Mandelbrot. Structure characterization was accomplished by standard methods, such as NMR, along with XPS, TEM, MS, AFM, and ultrahigh-vacuum low-temperature STM, which resulted in beautiful images of the hexameric gasket architecture.
"Nanoassembly of a Fractal Polymer: A Molecular Sierpinsky 'Hexagonal Gasket" George R. Newkome, Pingshan Wang, Charles N. Moorefield, Tae Joon Cho, Prabhu P. Mohapatra, Sinan Li, Seok-Ho Hwang, Olena Lukoyanova, Luis Echegoyen, Judith A. Palagallo, Violeta Iancu, Saw-Wai Hla Science 2006 312, 1782-1785.
Composite materials have been prepared by employing dendrimers as counterions to our bisterpyridine-based, metallomacrocylces. The result is the automorphogenic and stoichiometric self-assembly of nanoscale fibers possessing photonic and energy storage potential. These ionically-paired superstructures are examples of new architectures based on the removal of the relative positional randomness exhibited by singly-charged counterions. Fiber generation is predicated on the simple addition of 1st generation, 12 carboxylate-terminated dendrimer to an acetonitrile solution of cationic, Ru(II)-based metallohexamer. The result is a reddish, precipitated fiber that can be easily observed in the TEM. Selected area electron diffraction (SAED) revealed structural odering within the fiber that suggests the presence of adjacent columns of alternating dendrimer-hexamer-type stacking.
Wang, P.; Moorefield, C. N.; Jeong, K.-U.; Hwang, S.-H.; Li, S.; Cheng, S. Z. D.; Newkome, G. R. "Dendrimer-Metallomacrocycle Composites: Nanofiber Formation by Multi-Ion Pairing" Adv. Mater. 2008, 20, 1381-1385.
H-Bonding-based molecular recognition on the interior of a dendritic framework has also been investigated. Donor-acceptor H-bonding sites, provided by 2,6-diamidopyridine moieties capable of molecular recognition of imide-based groups, were constructed onto a dendrimers core region. Incorporation of the binding sites was accomplished by use of a 3-component, single-step reaction that facilitated the construction of the requisite pyridine-based, homologated monomer Based on 1H NMR chemical shift data, these dendritic hosts were shown to bind barbituric acid and analogous imide-based derivatives such as the potent anticancer drug AZT. This demonstrated the potential to use site-specific molecular recognition for guest(s) encapsulation and solubilization.
Our method of combinatorial dendritic construction was initially founded on the preparation of a new class of modular, isocyanate-based, 1 to 3 branched building blocks. Whereas "traditional" combinatorial-type methods focus on screening biologically active molecules (library preparation and deconvolution), and while, dendrimers have recently been employed and touted as vehicles to access small molecule libraries, our procedure relies on mixtures of AB3-type monomers for sequential tier formation and dendritic construction. Fundamental characteristics of the monomers used for combintorial construction include the potential to create branched structures with differing, yet mutually compatible, functionality as well as the ability of each member of the series to react at an equal (or nearly so) rate. Mutual compatibility is required from the standpoint of reactivity between mixtures of monomers (i.e., they must be inert towards reaction with each other); "equality of reactivity" is not strictly required, but removes a degree of freedom from synthetic consideration and helps to ensure the 'uniform introduction of functional heterogeneity' (while this feature is less of a concern when combinatorially coating less dense surfaces, it is anticipated that as surface group density increases, with the limiting case at or near dense packing, equal rates of monomer reactivity will become more desirable). Further, it is envisioned that the attributes of dendritic combinatorial construction can be applied to many other chemistries discussed herein as well as those outside the "dendrimer domain" such as in surface coatings, and (molecular) electronics applications, to mention but a couple.
For a review of the majority of our publications relating specifically to dendrimers please see our "selected publications".
© 2013 Center for Molecular Design and Recognition
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