Redox-Responsive Dimerization in a Ferrocene-Ureidopyrimidinone Supramolecular Assembly

Abstract

One of the important goals in supramolecular chemistry is the development of stimuli-responsive systems, where the strength of the intermolecular interactions can be altered by external signals such as changes in light, temperature, pH, or voltage of an electrode. Applications include self-healing polymers and gels, controlled release of entrapped molecules, and smart materials. In this paper, the results of our electrochemical studies with a ferrocene-containing ureidopyrimidone (UPy) will be described. The UPy system, introduced by Meijer, can form two self-complementary 4 H-bond arrays (AADD or ADAD) which dimerize in relatively non-polar solvents. By attaching an electroactive ferrocene, it may be possible to control the strength of interaction to create a redox-responsive UPy. Following earlier studies by Graham and co-workers, we have prepared a UPy with a ferrocene on the pyrimidinone side, UPy(Fc). Concentration and scan-rate-dependent cyclic voltammetric studies indicate a square scheme type mechanism in methylene chloride. In the starting ferrocene state, the AADD dimer is preferred. Oxidation to UPy(Fc+) results in conversion to another form with a less positive E1/2. Preference for this new form increases at lower concentrations suggesting that it is a monomeric species, which would indicate that oxidation breaks apart the dimer as desired. Alternatively it is possible that the observed concentration dependence results from rate-limiting dissociation of the dimer, but the monomer then tautomerizes to the ADAD form which re-dimerizes. To distinguish between these possibilities, the relative diffusion coefficients (D’s) of UPy(Fc+) to UPy(Fc) were measured by steady-state voltammetry at a microdisk electrode and compared to the relative D’s of ferrocenium to ferrocene. The results of these studies support the conclusion that UPy(Fc+) is monomeric at mM concentrations in methylene chloride, meaning that oxidation of the UPy(Fc) does break apart the dimer. Furthermore, the process appears to be completely reversible. Currently, CV simulations are being performed to determine if this mechanism can quantitatively explain the observed voltammetry. In addition, the synthesis of covalently linked UPy(Fc) oligomers is being initiated, with the goal of creating a redox-responsive supramolecular polymer using this system. Results from these on-going studies will also be reported.