No abstract

Stimuli-responsive polymers and assemblies are viable candidates for the so-called "smart" materials. In this communication, we report a new class of amphiphilic homopolymers that forms micelle-like structures in polar solvents and inverted micelle-like structures in apolar solvents. We demonstrate that these superstructures are the result of the changes in the molecular-level conformations in the monomer.

Amphiphilic homopolymers containing a hydrophilic and a hydrophobic functionality in each monomer unit have been shown to form polar or apolar containers depending on the solvent environment. When presented with a mixture of solvents, these polymeric containers are capable of releasing certain guest molecules. The fundamental mechanism behind these properties is investigated, and the utility of these assemblies in separations has been demonstrated with an example.

Amphiphilic dendrimers with a custom-designed biaryl repeat unit have been synthesized. These dendrimers afford hydrophobic nanocontainers in polar solvents and hydrophilic nanocontainers in apolar solvents. These container properties were investigated using dye incorporation studies. The aggregation states of these dendrimers are also reported using DLS studies.

One of the more promising entries into molecular nanotechnology has followed in the wake of the advent of the mechanical bond [1] in chemistry. Mechanically interlocked molecules [2] (MIMs), such as bistable [2]catenanes [3] and [2]rotaxanes, [4] have been integrated into nanoelectromechanical systems [5] (NEMs) in the context of, for example, molecular electronic devices [6] (MEDs) with a future generation of computers in mind, and mechanized silica nanoparticles [7] (MSNPs) with finely tuned and precisely targeted drug delivery capsules in prospect. The initial forays into the making of the mechanical bond relied on either statistical events [8] intervening during the course of classical chemical reactions or on the meticulously scripted and synthetically demanding use of covalent templation [9] in the midst of multistep reaction sequences. These approaches to the synthesis of MIMs are often limited in their practical utility on account of low yields of products and the tedious nature of the synthetic protocols, not to mention the close to total absence of any discrete and potentially useful intramolecular intercomponent interactions existing between their mechanically interlocked components.The emergence of noncovalent templates, which rely upon metal-ligand coordination, [10] donor-acceptor interactions, involving both charged [11] and neutral [12] recognition sites, hydrophobic forces [13] expressed in aqueous solutions, hydrogen bonding also operative in neutral [14] as well as charged [15] settings, electrostatic forces, [16] and anion binding [17] to provide [18] the supramolecular assistance [19] to covalent and hence mechanical bond formation, have transformed the chemical landscape and brought mechanostereochemistry [20] to the forefront as a means of introducing integrated nanosystems based on "smart" molecules into nano-and biotechnology. These templation processes, which make use of noncovalent bonding interactions, however, have not relied previously upon the remarkable stabilization which can ensue from the radical-pairing interactions.In the domain of donor-acceptor MIMs, viologen-(1,1'-dialkyl-4,4'-bipyridinium) dications [21] V 2+ -and cyclobis-(paraquat-p-phenylene) (CBPQT 4+ ) [22] have been investigated [23] extensively as examples of p-electron-deficient guests and hosts, respectively. Recently, we have demonstrated [24] that viologen radical cations (VC + ) form strong inclusion complexes with the tetracationic cyclophane in its reduced diradical dicationic state (CBPQT 2( C +) ) as a consequence of radical-pairing interactions. In view of the remarkable stabilization [25] associated with this supramolecular entity containing three [p···p]-stacked bipyridinium radical cations (BIPYC + ), [26] we set out to assemble a [2]rotaxane incorporating a viologen derivative and the tetracationic cyclophane in its dumbbell and ring components, respectively, by means of a radical template-directed protocol. Assuming this objective can be met, we can contemplate a [2]rotaxane existing in a rela...