Selective Dimerization Control in a Redox-Responsive Ureidopyrimidinone Supramolecular Assembly
Authors: Cedano Mario , Smith Diane K. 
Published: February 2020

Supramolecular polymers can be formed through directional intermolecular interactions between designated molecular subunits in order to achieve dynamic behavior. The strength of these intermolecular interactions can be altered through the use of external signals such as light absorption, changes in temperature and/or pH, or in an almost unexplored caveat, the application of voltage. Such is one of the important goals in supramolecular chemistry: the development of stimuli-responsive systems where the strength of intermolecular interactions can be perturbed through the use of external signals. Applications of supramolecular chemistry include self-healing polymers and gels, controlled release of entrapped molecules, and the development of smart materials.
Ureidopyrimidinones (UPy) – introduced by Meijer and coworkers – can form two self-complementary 4 hydrogen bond arrays with binding motifs AADD or ADAD and have a relatively small dissociation constant of 10-6. Due to their directionality and strength of dimerization, UPys have been widely imployed as polymer cross-linkers in oligomerization and also as capping agents in chain termination. In these cases, association and/or dissociation of UPy hydrogen bonded complexes are influenced by external signals that were previously described. However, the application of voltage to affect binding of the UPy supramolecular complex is a relatively new approach towards dimerization control.
In order for UPy to show electrochemical responsiveness, an electroactive substituent needs to be attached to the UPy framework. We have prepared and characterized such a system in which a ferrocene (Fc) is attached to the pyrimidone ring in the UPy. Concentration and scan-rate-dependent cyclic voltammetry and diffusion coefficient data from bulk electrolysis coupled with steady-state voltammetry indicate a square scheme mechanism for the UPyFc / UPyFc+ redox couple in dichloromethane. The fully reduced UPyFc exists as a dimer. Upon oxidation to UPyFc+, the dimer breaks apart to form the monomer. However, it is unknown how conversion between the AADD and ADAD binding motifs – due to tautomerization – factors into the square scheme mechanism. Nonetheless CV simulations with a simple square scheme mechanism (neglecting tautomerization) provide reasonable fits to the experimental data. Importantly, the overall process appears to be completely chemically reversible even over the relatively long time scale of bulk electrolysis.
Based off of this preliminary work, syntheses of covalently-linked UPyFc oligomers are underway with the goal of creating a redox-responsive supramolecular polymer whose physical properties can be altered upon oxidation. As it has been confirmed that the fully reduced UPyFc exists as a dimer, the physical properties of the material should reflect this dimerization by showing rigidity or in some cases, a highly viscous material. Upon oxidation, dimerization should cease, causing the ferrocene-UPy oligomer to break apart and show physical properties that reflect a loss in rigidness and greater fluid, thereby achieving a more selective process of perturbing electrostatic interactions in supramolecular assemblies.