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The formation of supramolecular polymers is possible through directional and favorable intermolecular interactions that, unlike conventional covalent bonds, are weaker and purely electrostatic. The electrostatic nature of these interactions grants their respective systems reversibility in binding interactions, allowing molecular systems to achieve dynamic behavior. An important goal in supramolecular chemistry is the development of stimuli-responsive systems where the strengths of these binding interactions can be perturbed. Perturbation of these intermolecular forces is achieved through the use of external signals such as light absorption, changes in temperature and/or pH, etc. However, in an almost unexplored area, the use of an applied voltage as an external signal is expected to perturb intermolecular binding via electrochemical redox reactions. One such system known for self-complementary hydrogen bonding is the Ureidopyrimidinone (UPy). Introduced by Meijer, UPy can dimerize with another UPy along its complementary 4 hydrogen bond frame. Due to their directionality and strength of association, UPy’s have been utilized as cross-linkers in oligomerization and as capping agents in oligomer chain termination. In these cases, association and/or dissociation of UPy dimers can be influenced by external signals that were previously described. However, the use of voltage as a means of dimerization control of the UPy system and other H-bonded systems is virtually unheard of. Preliminary studies on electroactive UPy derivatives – UPy’s with substituents rendering them responsive to electrochemical stimuli – have been explored. One such UPy derivative is UPyFc, where a ferrocene substituent acts as an electrochemical redox center of the UPy derivative. Cyclic voltammetry and bulk electrolysis / steady-state voltammetry studies reveal that dimerization of UPyFc can be electrochemically controlled, and the mechanism of the electrochemical reaction indicates a square scheme 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. Furthermore, this process appears to be completely reversible in dichloromethane. Based off of these preliminary studies, electro-inactive UPy oligomers have been developed with the goal of altering the physical properties of bulk non-polar organic solutions containing these UPy oligomer units and electro-active UPy derivatives. It is expected that, in bulk solution, the formation of UPy hetero-dimers between UPyFc and electro-inactive UPy oligomer units will be observed and the bulk solution will show low viscosity. Oxidizing UPyFc to UPyFc+ will break these hetero-dimers apart and UPy oligomerization will be observed, in which the bulk solution should show an increase in viscosity. Based off of preliminary studies, this should be a reversible process in which physical properties of bulk solutions can be altered by controlling H-bonding interactions via electrochemical redox reactions. |
