An important goal 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 supramolecular polymers and gels, controlled release of entrapped molecules, and smart materials. In this paper, the results of our recent 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 such as methylene chloride. By introducing 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, UPy(Fc). Concentration-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. If UPy(Fc) is a dimer in both oxidation states, the DUPy(Fc+)/DUPy(Fc) ratio should be similar to that observed for ferrocene (DFc+/DFc), which will be a monomer in both oxidation states. However, if UPy(Fc+) is a monomer, the DUPy(Fc+)/DUPy(Fc) ratio will be larger than DFc+/DFc, since the D of the UPy(Fc+) monomer will be relatively larger than the D of the UPy(Fc) dimer due to its smaller size. The latter is observed, supporting the conclusion that oxidation breaks apart the UPy(Fc) dimer at mM concentrations in methylene chloride.

Redox-responsive dimerization in a ferrocene-containing four hydrogen-bond supramolecular assembly

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.