Erik Schwartz

Helical Chromophoric Nanowires (Matthieu Koepf & Erik Schwartz)

Polyisocyanides are most commonly prepared by nickel(II) induced polymerisation of isocyanide monomer units (Figure A). A special characteristic of polyisocyanides is the fact that every carbon atom in the polymer backbone bears a substituent, leading to considerable steric hindrance. The C-C bonds of the backbone twist and adopt a helical structure in order to minimise the steric hindrance. Polyisocyanides have a well-defined 41 helical conformation (i.e., 4 repeat units per helical turn), which is only stable when sterically demanding side-chains are present. Introduction of chiral peptide substituents leads to a preference for a left (M) or a right (P) handed helical arrangement of the polymer. With Atomic Force Microscopy (AFM) techniques it was found that the well-defined hydrogen bonding network between the peptide side chains at position n and (n+4), with a stacking distance of 4.6 Å, led to extremely stiff and long polymers with a persistence length of 76 nm.

Figure A: Representation of the resonance structure of isocyanides and the conversion to polyisocyanides leading to a helical polymer

Research Projects:

1) In our group we have demonstrated that, using the concept of polyisocyanides, chromophoric molecules such as porphryrins and perylenes (Figure B) could be organised over a large distance resulting in highly ordered functional polymers. The synthesis of these molcules is, however, sometimes difficult and laborious. Therefore, we synthesised and characterised acetylene containing isocyanopeptides to use as building blocks for the construction of functionalised polyisocyanides, making use of the copper-catalysed cycloaddition with derivatised azides. At current, we are aming to incorporate various azides (chromophoric, magnetic, radical, biological) groups into our polymers.

Figure B: The polymerization of a perylene functionalized isocyanide could be followed by just looking at the colour

2) In addition to the use of the copper catalyzed cycloaddition to post-functionalize the polymers' backbone, we aim to take advantage of the high efficiency of this reaction to expand the structural diversity of polyisocyanides i.e. building star-shaped and other branched macromolecules. The use of an pre-functionalized Ni(II) initiator has already proved to be an efficient way to introduce azide groups and subsequently dyes at one end of the polymer chains. This strategy will now be expanded with the use of such azide-ended polymers as bulding blocks to prepare branched structures (figure c). By the incorporation of fluorescent labels within the polymers' chains (either by direct co-polymerization of suitable monomers or by the development of a selective "double click" strategy) the dynamics of these structures in various environments will be investigated by fluorescence microscopy.

Figure C: (left) Preparation of star shaped structures via "click' chemistry". (right) AFM picture of polymers prepared with Ni(II) initiatiator.