Sandra Arias

Nanospheres, Nanotubes, Toroids, and Gels with Controlled Macroscopic Chirality†

The interaction of a highly dynamic poly(aryl acetylene) (poly-1) with Li(+), Na(+), and Ag(+) leads to macroscopically chiral supramolecular nanospheres, nanotubes, toroids, and gels. With Ag(+), nanospheres with M helicity and tunable sizes are generated, which complement those obtained from the same polymer with divalent cations. With Li(+) or Na(+), poly-1 yields chiral nanotubes, gels, or toroids with encapsulating properties and M helicity. Right-handed supramolecular structures can be obtained by using the enantiomeric polymer. The interaction of poly-1 with Na(+) produces nanostructures whose helicity is highly dependent on the solvation state of the cation. Therefore, structures with either of the two helicities can be prepared from the same polymer by manipulation of the cosolvent. Such chiral nanotubes, toroids, and gels have previously not been obtained from helical polymer-metal complexes. Chiral nanospheres made of poly(aryl acetylene) that were previously assembled with metal(II) species can now be obtained with metal(I) species.

The leading role of cation–π interactions in polymer chemistry: the control of the helical sense in solution

Cation–π interactions determine the helical sense adopted by a polyphenylacetylene bearing (R)-α-methoxy-α-phenylacetamide as a pendant group (poly-1). These interactions take place when small amounts of Li+, Na+ and Ag+ salts dissolved in donor cosolvents are added to the polymer dissolved in a non-donor solvent and the corresponding helical polymer–metal complexes (HPMCs) are formed. Extensive spectroscopic studies (i.e., 7Li, 23Na, 1H NMR and IR among others) indicate that the pendants act as dipodal receptors for metal cations, where cation–π interactions play a major role in controlling the M or P helicity of the polymer. Although all the cations tested generate cation–π binding, each metal presents its own particular and distinctive behaviour, based on polymer/cosolvent ratios and the type of cosolvent. Manipulation of these variables allows the selective disruption or activation of the cation–π interactions (i.e., a controlled switch on/off mechanism) and the selection of the helical sense on an a la carte basis. The 3/1 helical structure of poly-1 can cause a novel “triple cascade effect” of cooperative cation–(π)n–π interactions, which further induce the high chiral amplification response found in these HPMCs. These non-covalent interactions are not found when divalent cations are used instead.

Simultaneous Adjustment of Size and Helical Sense of Chiral Nanospheres and Nanotubes Derived from an Axially Racemic Poly(phenylacetylene)

Nanospheres and nanotubes with full control of their size and helical sense are obtained in chloroform from the axially racemic chiral poly(phenylacetylene) poly-(R)-1 using either Ag+ as both chiral inducer and cross-linking agent or Na+ as chiral inducer and Ag+ as cross-linking agent. The size is tuned by the polymer/ion ratio while the helical sense is modulated by the polymer/cosolvent (i.e., MeCN) ratio. In this way, the helicity and the size of the nanoparticles can be easily interconverted by very simple experimental changes.

A general route to chiral nanostructures from helical polymers: P/M switch via dynamic metal coordination

A general method for the conversion of a polyphenylacetylene into its P or M helically oriented polymer, and into its corresponding P/M helical nanoparticles via dynamic metal coordination is presented. Addition of Mn+ ions (e.g., Ca2+, Zn2+, Ag+, among others) to poly(phenylacetylene)s (poly-2 to poly-6), bearing OMe protected amino acids, produces complex I (chelation of the two carbonyl groups; sp conformation) associated with a helix inversion of the polymer. Ulterior addition of a small amount of MeOH transforms complex I into complex II (metal ion coordinated only to the ester group; ap conformation), where a second helix inversion is produced. This dynamic coordination process between complex I and complex II works also at the nanoscale level, and therefore chiral nanoparticles with either P or M helices can be generated or transformed into each other by controlling the polymer/metal/methanol ratio in the helical polymer metal complex (HPMC).

Unexpected Chiro-Thermoresponsive Behavior of Helical Poly(phenylacetylene)s Bearing Elastin-Based Side-Chains

The thermoresponsive behavior of an elastin-based polymer can be altered by the polymeric macromolecular conformation. Thus, when the elastin basic amino acid sequence VPGVG is used as a pendant group of a poly(phenylacetylene) (PPA) its thermoresponsive behavior in water can be remotely detected through conformational changes on the formed helix. Circular dichroism at different temperatures shows an inversion of the first Cotton effect (450 nm) at 25.8 °C that matches with the cloud point temperature. The elastin-based side-chain poly(phenylacetylene) shows an upper critical solution temperature with low pH and concentration dependency, not expected in elastin-based polymers. It was found that the polymer self-assembles in water into spherical nanoparticles with hydrodynamic diameters of 140 nm at the hydrophobic state.