We use dynamic imine-bond templation to generate new conjugated polymers that are capable of "self-healing": a rupture in the polymer chain may be repaired by reassembly, given the dynamic covalent nature of the bonds linking the monomer units together.
Initial experiments indicated that helicate 1 and o-phenylenediamine reacted in aqueous solution to produce dimeric helicate 2, as shown below. Electronic structure calculations have been carried out by our collaborators Chris J. Cramer and Laura Gagliardi. Their Density Functional Theory (DFT)-minimised structure of 2 reproduced key features observed in the crystal structure solved by Kari Rissanen, such as the Cu-Cu and Cu-N distances, within 0.02 Å. These calculations indicate a high degree of electronic delocalisation among the closely-spaced copper(I) ions at the core of this molecule. A "hole" generated by removing one electron would thus be shared among all metal ions, suggesting that oligomeric or polymeric analogs may serve as true conductive molecular wires. We have published initial work on this system in Chem. Eur. J., 2008, 14, 7180-7185.
The formation of 2 from 1 corresponds to the first step in a step polymerisation reaction. A stumbling block for the use of this methodology to produce longer polymeric structures is the poor solubility of longer oligomers. A clear way forward is thus to construct more soluble analogues of 2 through the preparation of more soluble subcomponents. The rigid, water-soluble conductive polymers thus generated would be of considerable technological interest. The reversible nature of the imine and coordinative bonds holding these structures together means that they are capable of dynamic reconfiguration, allowing them to be built into "self-healing" materials.
By addition of PEG chains to the phenylenediamine subcomponent, we were able to overcome the solubility problem and synthesize a polymer, which was shown to possess several properties of conceptual and practical interest, as we described in J. Am. Chem. Soc. 2012, 135, 19170-19178. (1) Individual double-helical strands appear to further aggregate through entanglement of their side chains to form well-defined superstructures such as nanoscale bow ties and macrocycles, which can be imaged on a surface. (2) The material’s copper(I) ions underwent reversible electrochemical oxidation in solution, whereas analogous model compounds were observed to decompose upon oxidation: the polymer’s greater length appeared to stabilize oxidized states through delocalization or entrapment. (3) Demet Asil in Richard Friend's research group undertook photophysical measurements, which revealed this material to be photo- and electroluminescent, allowing her to use it for the fabrication of electroluminescent devices.
