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Self-assembly of Helical Functional Nanomaterials

Final Report Summary - SAHNMAT (Self-assembly of Helical Functional Nanomaterials)

Up until recently self-assembly in dilute aqueous environments has predominantly dealt with linear amphiphiles that form closed structures, such as spherical or cylindrical micelles, and vesicles. Morphological control in objects of defined size or shape is increasingly well understood. Surprisingly, however, the generality of these concepts has not been translocated into another area of increasing interest, namely, the self-assembly of one-dimensional ordered arrays. In that context the development of discotic monomers has proven to be a valuable route to allow for the synthesis of rod-like ordered supramolecular polymers, whose potential applications in functional soft matter include electronics, sensing or regenerative medicine. Considering the enormous interest in such systems, it is surprising that efforts to control the size and shape of nano- and micrometer size one-dimensional objects are rare. In order to manipulate the growth of aqueous one-dimensional supramolecular polymers, we have developed a strategy that utilizes electrostatic repulsive contributions in analogy to surfactant type self-assembly [1, 2, 3].
The molecular design of our self-assembling unit is based on the C3-symmetrical benzene-1,3,5-tricarboxamide (BTA) core (depicted in green, Fig. 1) that directs the self-assembly into triple hydrogen bonded helices. This moiety was extended with a fluorinated L-phenylalanine and an aminobenzoate spacer (depicted in blue, Fig. 1), creating a hydrophobic pocket in the core of the discotic to shield the triple hydrogen-bonding motif. By increasing the ionic character of the peripheral Gd(III) complexes in the charge neutral discotic 1 to the negatively charged discotic 2, we aimed at introducing frustration in the one-dimensional growth of the stacks. Cryo-TEM imaging at the Eindhoven University of Technology and Small Angle X-ray Scattering experiments at the ESRF (European Synchrotron Radiation Facilities) in Grenoble revealed that it is possible to switch from elongated, rod-like assemblies to small and discrete objects (Fig. 1 and Fig. 2), by balancing out attractive non-covalent forces within the hydrophobic core of the polymerizing building blocks with electrostatic repulsive interactions on the hydrophilic rim.

Figure 1. Schematic representation of the self-assembly of the discotic amphiphiles 1 and 2.

The order in the self-assembled objects and their growth mechanism were also characterized using circular dichroism, UV/Vis and 1H-NMR spectroscopy. In line with our continuous efforts in elucidating the mechanisms of supramolecular polymerizations, we have focused on correlating the morphological properties of the produced materials with the appropriate mechanistic details of the self-assembly pathways: cooperative growth of monomer 1 leads to very high molecular weight supramolecular polymers, whereas frustrated growth of discotic 2 and the resulting anticooperativity, results in the formation of small and discrete objects, without compromising on their thermodynamic stability.

Figure 2. SAXS profiles of the self-assembled discotics 1 (left graph) and 2 (right graph) at different concentrations.

This is a unique example for directional self-assembly in water whereby the supramolecular polymer shape and size can be dictated by Coulombic interactions. In analogy to many systems found in biology, mechanistic details of the self-assembly process emphasise the importance of cooperativity as a key feature that dictates the physical properties of the supramolecular polymers. We have shown that the latter are very promising building blocks for the development of paramagnetic self-assembled nanoparticles as supramolecular MRI contrast agents. Further investigations are well under way and target molecular imaging studies of cardiovascular diseases.