2-Ureido-4[1H]-pyrimidinones are known to dimerize via a strong quadruple hydrogen bond array. A detailed study of the dimerization constant and lifetime of the dimer is presented here. Excimer fluorescence of pyrene-labeled 2-ureido-4[1H]-pyrimidinone 1b was used to determine a dimerization constant K dim of 6 × 10 7 M -1 in CHCl 3 , 1 × 10 7 M -1 in chloroform saturated with water, and 6 × 10 8 M -1 in toluene (all at 298 K). Under these conditions, the preexchange lifetime of the similar dimers of both 1d and 1e is 170 ms in CDCl 3 , 80 ms in wet CDCl 3 , and 1.7 s in toluene-d 8 , as determined by dynamic NMR spectroscopy. Association rate constants were calculated from the K dim values and the preexchange lifetimes. The resulting values are significantly lower than the diffusion-controlled association rate constants calculated using the Stokes-Einstein and the Debeije equations. This difference is ascribed to a tautomeric equilibrium of the monomer between the dimerizing 4[1H]-pyrimidinone and nondimerizing 6[1H]-pyrimidinone tautomers, which is unfavorable for dimerization.

A class of amphiphilic macromolecules has been synthesized by combining well-defined polystyrene (PS) with poly(propylene imine) dendrimers. Five different generations, from PS- dendr -NH 2 up to PS- dendr -(NH 2 ) 32 , were prepared in yields of 70 to 90 percent. Dynamic light scattering, conductivity measurements, and transmission electron microscopy show that in aqueous phases, PS- dendr -(NH 2 ) 32 forms spherical micelles, PS- dendr -(NH 2 ) 16 forms micellar rods, and PS- dendr -(NH 2 ) 8 forms vesicular structures. The lower generations of this class of macromolecules show inverted micellar behavior. The observed effect of amphiphile geometry on aggregation behavior is in qualitative agreement with the theory of Israelachvili et al . The amphiphiles presented here are similar in shape but different in size as compared with traditional surfactants, whereas they are similar in size but different in shape as compared with traditional block copolymers.

Abstract:The protonation behavior of poly(propylene imine) dendrimers and some related oligo amines has been measured using natural abundance 15 N-NMR. The chemical shifts of the different shells of nitrogen nuclei as a function of pH have been directly interpreted as the degree of protonation for that shell. The thus obtained protonation behavior is described systematically using the Ising model which only requires values for the intrinsic protonation constants and for the pair interaction free energies between the different nitrogen nuclei. This subsequently leads to a quantitative description of the titration curves of the individual shells and to a rationalization of the microscopic and macroscopic equilibrium constants for these molecules.

Details of the forces between nanoparticles determine the ways in which the nanoparticles can self-assemble into larger structures. The use of directed interactions has led to new concepts in self-assembly such as asymmetric dendrons, Janus particles, patchy colloids and colloidal molecules. Recent models that include attractive regions or 'patches' on the surface of the nanoparticles predict a wealth of intricate modes of assembly. Interactions between such particles are also important in a range of phenomena including protein aggregation and crystallization, re-entrant phase transitions, assembly of nanoemulsions and the organization of nanoparticles into nanowires. Here, we report the synthesis of 6-nm nanoparticles with dynamic hydrophobic patches and show that they can form reversible self-assembled structures in aqueous solution that become topologically more connected upon dilution. The organization is based on guest-host supramolecular chemistry with the nanoparticles composed of a hydrophobic dendrimer host molecule and water-soluble hydrophilic guest molecules. The work demonstrates that subtle changes in hierarchal composition and/or concentration can dramatically change mesoscopic ordering.