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Venkatachalapathy S K Balagurusamy

Publications and source records attributed to Venkatachalapathy S K Balagurusamy.

12 recordsLinked to original sources
PubMed2007

Self-assembly of semifluorinated minidendrons attached to electron-acceptor groups into pyramidal columns.

The synthesis and self-assembly of twelve semifluorinated first-generation dendrons or minidendrons attached to electron-acceptor (n-type) groups generated from various combinations of eight acceptors and three dendrons are reported. Dendrons attached to small electron-acceptor molecules mediate their self-assembly into pi-stacks located in the center of a supramolecular helical pyramidal column with the long axis of the acceptor perpendicular to the long axis of the column. Dendrons attached to large electron-acceptor molecules, such as perylene bisimide, mediate the assembly of their acceptors in an unprecedented arrangement of pi-stacks that have the long axis of the acceptors parallel to the long axis of the supramolecular pyramidal column. All supramolecular columns self-organize into various periodic columnar arrays that exhibit liquid-crystalline phases, crystalline phases, or a liquid-crystalline phase with enhanced intracolumnar order. The present study demonstrates the simplicity and the versatility of the concept of assembly of n-type electroactive groups mediated by semifluorinated dendrons and assesses the scope and limitations of this supramolecular strategy.

Journal Article↗
PubMed2006

Self-assembly of semifluorinated dendrons attached to electron-donor groups mediates their pi-stacking via a helical pyramidal column.

Semifluorinated first-generation self-assembling dendrons attached via a flexible spacer to electron-donor molecules induce pi-stacking of the donors in the center of a supramolecular helical pyramidal column. These helical pyramidal columns self-organize in various columnar liquid crystal phases that mediate self-processing of large single crystal liquid crystal domains of columns and self-repair their intracolumnar structural defects. In addition, all supramolecular columns exhibit a columnar phase at lower temperatures that maintains the helical pyramidal columnar supramolecular structure and displays higher intracolumnar order than that in the liquid crystals phases. The results described here demonstrate the universality of this concept, the power of the fluorous phase or the fluorophobic effect in self-assembly and the unexpected generality of pyramidal liquid crystals.

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PubMed2006

Exploring and expanding the structural diversity of self-assembling dendrons through combinations of AB, constitutional isomeric AB2, and AB3 biphenyl-4-methyl ether building blocks.

General, efficient and inexpensive methods for the synthesis of dendritic building blocks methyl 3',4'-dihydroxybiphenyl-4-carboxylate, 3',5'-dihydroxybiphenyl-4-carboxylate, and methyl 3',4',5'-trihydroxybiphenyl-4-carboxylate were elaborated. In all syntheses the major step involved an inexpensive Ni(II)-catalyzed Suzuki cross-coupling reaction. These three building blocks were employed together with methyl 4'-hydroxybiphenyl-4-carboxylate in a convergent iterative strategy to synthesize seven libraries containing up to three generations of 3',4'-, 3',5'-, and 3',4',5'-substituted biphenyl-4-methyl ether based amphiphilic dendrons. These dendrons self-assemble into supramolecular dendrimers that self-organize into periodic assemblies. Structural and retrostructural analysis of their assemblies demonstrated that these dendrons self-assemble into hollow and non-hollow supramolecular dendrimers exhibiting dimensions of up to twice those reported for architecturally related dendrons based on benzyl ether repeat units. These new dendrons expand the structural diversity and demonstrate the generality of the concept of self-assembling dendrons based on amphiphilic arylmethyl ethers.

Journal Article↗
PubMed2006

Synthesis, structural analysis, and visualization of a library of dendronized polyphenylacetylenes.

A library of eleven high cis-content cis-transoidal polyphenylacetylenes (PPAs) dendronized with self-assembling dendrons was prepared from a library of fifteen convergently synthesized macromonomers. Using [Rh(C triple bond CPh)(nbd)(PPh(3))(2)] (nbd=2,5-norbornadiene) in the presence of 10 equiv of N,N-dimethylaminopyridine, predictive control over molecular weight and narrow molecular weight distribution are obtained. The PPA backbone serves as a helical scaffold for the self-assembling dendrons. The dendron primary structure dictates the diameter of the cylindrical PPAs in bulk, both in the self-organized hexagonal columnar (Phi(h)) lattice determined by X-ray diffraction (XRD) and in monolayers on highly ordered pyrolytic graphite (HOPG) and mica visualized by atomic force microscopy (AFM). Thermal and bulk phase characteristics of the cylindrical PPAs reinforces the generality that flexible polymer backbones adopt a helical conformation within the cylindrical macromolecules generated by polymers jacketed with self-assembling dendrons.

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PubMed2006

Surface crystallization in a liquid AuSi alloy.

X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au82Si18, at temperatures above the alloy's melting point. Surface-induced atomic layering, the hallmark of liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.

Journal Article↗
PubMed2005

Thermoreversible cis-cisoidal to cis-transoidal isomerization of helical dendronized polyphenylacetylenes.

High cis content (81-99%) cis-transoidal polyphenylacetylene (PPA) jacketed with amphiphilic self-assembling dendrons, poly[(3,4-3,5)mG2-4EBn] with m = 8, 10, 12, 14, 16, and (S)-3,7-dimethyloctyl, were synthesized by Rh(C triple bond CPh)(nbd)(PPh(3))(2) (nbd = 2,5-norbornadiene)/N,N-(dimethylamino)pyridine (DMAP) catalyzed polymerization of macromonomers. The resulting cylindrical PPAs self-organize into hexagonal columnar lattices with intracolumnar order (Phi(h)(io)) and without (Phi(h)). The polymers with m = 12, 14, and 16 exhibit also a hexagonal columnar crystal phase (Phi(h,k)). The reversible Phi(h,k)-to-Phi(h)(io)-to- Phi(h) phase transition in these dendronized PPAs was analyzed by a combination of differential scanning calorimetry and small and wide-angle X-ray diffraction experiments performed on powder and oriented fibers. In the Phi(h,k) and Phi(h)(io) phases, the dendronized PPAs form helical porous columns. The helical pore disappears in the Phi(h) phase. This change is accompanied by a decrease of the external column diameter that is induced by stretching of the polymer backbone along the axis of the cylinder. The helix sense of the porous PPA is selected by homochiral alkyl dendritic tails. This transition is generated by an unprecedented conversion of the PPA backbone from the cis-cisoidal conformation in the Phi(h,k) and Phi(h)(io) phases to the cis-transoidal conformation in the Phi(h) phase. Under the same conditions, the pristine cis-PPA undergoes cis-trans isomerization and irreversible intramolecular 6pi electrocyclization of 1,3-cis,5-hexatriene sequences followed by chain cleavage. These processes are eliminated in the dendronized cis-PPA below its decomposition temperature.

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PubMed2004

Self-assembly of amphiphilic dendritic dipeptides into helical pores.

Natural pore-forming proteins act as viral helical coats and transmembrane channels, exhibit antibacterial activity and are used in synthetic systems, such as for reversible encapsulation or stochastic sensing. These diverse functions are intimately linked to protein structure. The close link between protein structure and protein function makes the design of synthetic mimics a formidable challenge, given that structure formation needs to be carefully controlled on all hierarchy levels, in solution and in the bulk. In fact, with few exceptions, synthetic pore structures capable of assembling into periodically ordered assemblies that are stable in solution and in the solid state have not yet been realized. In the case of dendrimers, covalent and non-covalent coating and assembly of a range of different structures has only yielded closed columns. Here we describe a library of amphiphilic dendritic dipeptides that self-assemble in solution and in bulk through a complex recognition process into helical pores. We find that the molecular recognition and self-assembly process is sufficiently robust to tolerate a range of modifications to the amphiphile structure, while preliminary proton transport measurements establish that the pores are functional. We expect that this class of self-assembling dendrimers will allow the design of a variety of biologically inspired systems with functional properties arising from their porous structure.

Biological Transport↗
PubMed2004

Designing libraries of first generation AB3 and AB2 self-assembling dendrons via the primary structure generated from combinations of (AB)(y)-AB3 and (AB)(y)-AB2 building blocks.

Structural analysis of three libraries of up to five generations of self-assembling dendrons based on AB(3), AB(2), and combinations of AB(3) with AB(2) building blocks (Percec et al. J. Am. Chem. Soc. 2001, 123, 1302) facilitated the discovery of several nanoscale lattices previously unknown for organic compounds (3-D Pm3n cubic, 3-D P4(2)/mnm tetragonal, and a crystallographically forbidden 12-fold symmetry liquid quasicrystal) and provided fundamental correlations between the molecular structure of the dendron and the shape and the diameter of the supramolecular dendrimers which, in these experiments, were limited to less than 75 A. That study concluded that alternative design principles should be elaborated for the assembly of supramolecular dendrimers of larger dimensions. Here we report design principles, synthesis and analysis of first and higher generations AB(3) and AB(2) self-assembling dendrons, based on various primary structures, and combinations of (AB)(y)-AB(3) and (AB)(y)-AB(2) (i.e., from nondendritic AB where y = 1 to 11 and dendritic AB(3) and AB(2)) building blocks that produced the largest structural (including six new lattices) and dimensional (100 to 217 A diameter) diversity of supramolecular dendrimers.

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PubMed2003

Application of isomorphous replacement in the structure determination of a cubic liquid crystal phase and location of counterions.

A second generation monodendron with dodecyl end-groups based on the AB(3) monomer 3,4,5-trihydroxy benzoate has previously been shown to form a thermotropic cubic phase with Pmthremacr;n symmetry (Balagurusamy et al., J. Am. Chem. Soc. 1997, 119, 1539). A structure consisting of spherical "micelles" was proposed originally, but an alternative choice of structure factor phases, giving a structure of interlocked squashed columns, could not be ruled out by diffraction data on the original material alone. We have therefore synthesized two selectively fluorinated equivalent compounds, the carboxylic acid and its Rb salt, to be able to apply a variant of the isomorphous replacement crystallographic technique. On the basis of the electron density maps of the new labeled compounds, reconstructed using small-angle X-ray diffraction intensities, the interlocking columns model is unequivocally rejected and the spheres model is upheld. Furthermore, the location of the metal cation in the center of the "micelles" is directly confirmed. Micellar diameter was shown to decrease on fluorination of the dodecyl chain ends, and increase significantly on introduction of Rb. This is interpreted in terms of changes in the number of wedge-shaped dendrons fitting into a spherical micelle due to their changing taper angle. It was found that the Rb-rich regions at the centers of six out of eight "micelles" in the unit cell are elongated in the direction of their closest packing. This adds support to the suggestion of a partial "column-like" character of stacked rows of such micelles, consistent with the position of the Pmthremacr;n phase next to the columnar phase in the phase sequence of most taper-shape compounds. The results illustrate the potential of isomorphous replacement, used a great deal in protein crystallography, in structure investigation of liquid crystals and supramolecular soft matter.

Journal Article↗
PubMed2003

Hierarchical self-assembly, coassembly, and self-organization of novel liquid crystalline lattices and superlattices from a twin-tapered dendritic benzamide and its four-cylinder-bundle supramolecular polymer.

The synthesis and structural analysis of the twin-dendritic benzamide 10, based on the first-generation, self-assembling, tapered dendrons 3,4,5-tris(4'-dodecyloxybenzyloxy)benzoic acid and 3,4,5-tris(4'-dodecyloxybenzyloxy)-1-aminobenzene, and the polymethacrylate, 20, which contains 10 as side groups, are presented. Benzamide 10 self-assembles into a supramolecular cylindrical dendrimer that self-organizes into a columnar hexagonal (Phi(h)) liquid crystalline (LC) phase. Polymer 20 self-assembles into an imperfect four-cylinder-bundle supramolecular dendrimer, and creates a giant vesicular supercylinder that self-organizes into a columnar nematic (N(c)) LC phase which displays short-range hexagonal order. In mixtures of 20 and 10, 10 acts as a guest and 20 as a host to create a perfect four-cylinder-bundle host-guest supramolecular dendrimer that coorganizes with 10. A diversity of Phi(h), simple rectangular columnar (Phi(r-s)) and centered rectangular columnar (Phi(r-c)), superlattices are produced at different ratios between 20 and 10. This diversity of LC lattices and superlattices is facilitated by the architecture of the twin-dendritic building block, polymethacrylate, the host-guest supramolecular assembly, and by hydrogen bonding along the center of the supramolecular cylinders generated from 10 and 20.

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