Functional supramolecular polymers

Abstract

Supramolecular polymers can be random and entangled coils with the mechanical properties of plastics and elastomers, but with great capacity for processability, recycling, and self-healing due to their reversible monomer-to-polymer transitions. At the other extreme, supramolecular polymers can be formed by self-assembly among designed subunits to yield shape-persistent and highly ordered filaments. The use of strong and directional interactions among molecular subunits can achieve not only rich dynamic behavior but also high degrees of internal order that are not known in ordinary polymers. They can resemble, for example, the ordered and dynamic one-dimensional supramolecular assemblies of the cell cytoskeleton and possess useful biological and electronic functions.

Figure 1

Molecular representation of four different monomers and the corresponding supramolecular polymers formed after their aggregation through specific interactions. The first pair (A and B) is a ureidopyrimidinone monomer capable of forming quadruple hydrogen bonds to assemble a random coil supramolecular polymer. The second pair (C and D) is a peptide amphiphile monomer composed of three segmental domains, a sequence bearing a biological signal, a domain containing amino acids with a strong tendency to form β sheets, and a hydrophobic alkyl tail. The idealized structure of this ordered supramolecular polymer formed by these monomers is a cylindrical aggregate in which twisted β sheets (red) collapse through hydrophobic interactions among alkyl chains, thus displaying high densities of the biological signal. The blue regions represent water domains present in the interior of the supramolecular assembly (in real dynamic structures the peptide amphiphile and water domains are not expected to have a characteristic periodicity). The third monomer is based on the fluorophore oligo(phenylene vinylene), substituted by alkyl groups for solubility and also chiral centers. One terminus of the monomer is capable of forming quadruple hydrogen bonds to create stable dyads. The ordered supramolecular polymer takes the form of a twisted ribbon with defined chirality (E and F). The last monomer is based on a hexabenzocoronene core which can behave as an electron donor, substituted by phenyl triethylene glycol and dodecyl chains (G and H). The ordered supramolecular polymer formed from this monomer is a nanotube with a wall thickness defined by the dimension of monomeric units.

Figure 2

Supramolecular polymers based on random coil chains with excellent mechanical properties. A. Rubbery material of a small molecule based on two ureidopyrimidinone moieties separated by a flexible spacer. B. Plots of storage modulus (G′) and loss modulus (G″) as a function of frequency for a material similar to that shown in A. but containing a stiff and short spacer between ureidopyrimidinone units. C. Atomic force micrograph of supramolecular polymers that form nanofibers due to lateral hydrogen bonding of a urea group adjacent to a ureidopyrimidinone unit; these materials behave mechanically as thermoplastic elastomers. D. Light-emitting films based on supramolecular copolymers formed by monomers with molecular segments having different fluorescent properties. A large variety of films can be obtained and even white light emission is possible in these systems when a specific ratio of three different components leads to association of monomers into a random terpolymer.

Figure 3

A. Schematic representation of a bioactive supramolecular filament displaying high densities of biological signals that bind to receptors on the cell surface. The high signal density and stiffness of the ordered supramolecular polymer should be effective at recruiting and co-localizing large numbers of receptors for effective signaling. B. Strong interactions between molecular signals and cell receptors could reconstruct a non-covalent supramolecular assembly with relatively weak bonds to match a raft of receptors. C. Cryo electron micrograph image of peptide amphiphile nanofibers. D. Plot of the number of new neurons developed from a population of neural stem cells upon contact with supramolecular nanofibers containing various mole per cents of monomer displaying the bioactive pentapeptide signal IKVAV (the number of cells is obtained counting β-tubulin III positive cells, a marker for neuronal lineage); the inset reveals a cluster of neural stem cells undergoing differentiation into neurons (the green color indicates the presence in the cells of β-tubulin III). The dotted curve in D. shows that adding various mole per cents of the soluble IKVAV signal to a network of non-bioactive supramolecular polymers has no effect on differentiation of the neural stem cells E. A rat cornea showing the growth of many new blood vessels 7–10 days after placing in the tissue peptide amphiphile nanofibers that display heparin and bind angiogenic growth factors.

Figure 4

A. Schematic representation of the vision for photovoltaic devices containing large bundles of aligned semiconducting ordered supramolecular polymers with built-in p/n-heterojunctions. B. Molecular graphics representation of the monomer and the resulting supramolecular nanotube containing a co-axial p/n-heterojunction. The donor moiety in the monomer is hexabenzocoronene (HBC, in blue) and the acceptor is trinitrofluorenone (TNF, in green); other parts of the monomer structure include triethylene glycol (TEG) and dodecyl (C₁₂) chains for solubility. C. Current-voltage profiles of supramolecular coaxial nanotubes films at ambient temperature with photoirradiation (orange) and without (green) [λ = 300 to 650 nm]; the inset shows the modulation of electric current using photoirradiation of a cast film of photoconductive nanotubes at room temperature. A large current increase is observed with illumination (orange) versus without (green).