Small-angle neutron scattering studies of charged carboxyl-terminated dendrimers in solutions.

Small-angle neutron scattering was used to characterize the solution behavior of charged carboxylic acid terminated "cascade" dendrimers (Z-Cascade/methane [4]/3-oxo-6-oxa-2-azaheptylidyne/3-oxo-2-azaheptylidyne/propanoic acids) of third (G3) and fifth (G5) generations as a function of dendrimer concentration, pH, and ionic strength. An increase in dendrimer concentration leads to a single broad peak in the scattering profile arising from interdendrimer interaction. The dissociation of terminal carboxylate groups also gives rise to an interdendrimer interaction peak, which could be suppressed by the addition of excess salt. The results of contrast matching measurements indicate the accumulation of an excess concentration of tetramethylammonium counterions around the surface of these highly charged particles, and the thickness of these counterions lies somewhere between 4 and 6 A. This conclusion is consistent with our previous potentiometric titration (Zhang, H.; et al. J. Phys. Chem. B 1997, 101, 3494) and capillary electrophoresis (Huang, Q. R.; et al. J. Phys. Chem. B 2000, 104, 898) data.

Paramagnetic cobalt(II) as an NMR probe of dendrimer structure: mobility and cooperativity of dendritic arms.

Cobalt(II) has been utilized as an external paramagnetic (1)H NMR probe for the study of the structure of dendrimers that possess specifically located metal recognition sites. The hyperfine-shifted (1)H NMR signals of the Co(II) complexes of several 2,6-diamidopyridine-containing dendrimers have been fully assigned by means of 1D and 2D NMR techniques, including NOE difference, EXSY, COSY, and TOCSY. Temperature-dependent T(1) values of the hyperfine-shifted signals were used to conclude that the Co(II)-dendrimer complexes are in the "liquidlike" regime, indicative of a shell-like structure instead of a "dense-core" structure. The presence of sizable cavities within the dendrimers was observed including a loosely packed conformation for the 2,6-diamidopyridino moiety to bind to potential guest molecules. Cooperativity among the dendritic arms in metal binding is also observed, whereby two dendritic arms bind to the metal center at the same time. In the case of dendrimers with the metal binding site located near the surface of the molecule, such binding cooperativity is still observed despite the large degree of freedom of the metal-binding moiety. Cooperativity among the dendritic arms can thus be considered an intrinsic property, which has to be taken into consideration in future design of functional dendrimers for the purpose of specific recognition and catalysis. The hydrodynamic radii of these dendrimers have been determined by means of nuclear Overhouser effect at low temperature. The study offers a method for the study of the dynamics of dendrimers in solution under different conditions and upon ligand binding and recognition. The study also provides a tool for monitoring systematic variation of the metal binding site in different dendrimer frameworks for specific applications, such as catalysis and molecular recognition.

Dendrimer construction and macromolecular property modification via combinatorial methods.

The design and utility of a family of isocyanate functionalized branched monomers, as well as others, are described. Use of these monomers in logical combinations for the construction of branched architectures leads to the formation of unique, asymmetric dendritic species possessing multiple functionalities. Ramifications of this combinatorial-based, macromolecular construction technique are discussed with respect to functional group positioning and the potential to create dynamic heterogenous surfaces resembling a molecular "Rubik's sphere."

Structure of a 2,6-pyridinophane.

2,5,11,14-Tetraoxa-19,20-diazatricyclo-[13.3.1.1(6,10)] icosa-1(19),6,8,10(20),15,17-hexaene, C14H14N2O4, Mr = 274.3, monoclinic, P2(1)/c, a = 9.354(2), b = 9.011(3), c = 7.954(2) A, beta = 101.75(2) degrees, V = 656.4(6) A3, Z = 2, Dx = 1.388 g cm-3, lambda(Mo K alpha) = 0.71073 A, mu = 0.97 cm-1, F(000) = 288, T = 299 K, R = 0.035 for 834 observations with I greater than 3 sigma(I). The molecule lies on an inversion center and is in the anti or stepped conformation. The ethylene glycol chains are fully extended, and the two N-C-O-C torsion angles about the ring-O bonds are near zero, 1.6(2) and -2.3(2) degrees.