Description
The design of stimuli-responsive systems in which supramolecular structure changes in response to external signals, such as the change in voltage of an electrode, is important for many applications, for example, drug delivery. In this study, a four H-bond ureidopyrimidinone (UPy) array with a methyl-pyridinium, (MeP), redox center has been synthesized, UPy(MeP). It was initially thought that this array would dimerize and undergo two 1 e⁻ reductions, and that the dimerization ability would increase after each reduction due to the strengthening of the secondary H-bond interactions. However, UPy(MeP) shows very complicated cyclic voltammograms (CV’s) due to tautomerization resulting from the weak dimerization of the fully oxidized states of UPy(MeP). UPy dimers can exist in three tautomeric forms, and each can have many rotamers. The ¹H NMR data indicates that UPy(MeP) in CD₃CN exists as the folded-one monomer. The CV’s of UPy(MeP) in CH₃CN/NBu₄PF₆ showed three reduction waves along with two major oxidation waves with one of them occurring at very positive potential. This data suggests a tautomerization of UPy during the potential scan. The very positive oxidation wave is time-dependent according to the sweep-step function CV’s, and, furthermore, it occurs in a potential region where 2+ charged species are made when the data is compared to three model compounds. This suggests a protonation mechanism, where the reduced UPy accepts a proton from the fully oxidized UPy. The most stable forms of the deprotonated and protonated UPy(MeP) can form a dimer, but, upon oxidation, the dimer is expected to dissociate. Dimerization can be stronger in CH₂Cl₂ because it is a less polar solvent. The concentration dependent ¹H NMR data in CD₂Cl₂ suggests the presence of a mixture of a 2 H-bond and a 4 H-bond dimer. This explains the greater complexity of the CV’s of UPy(MeP) in CH₂Cl₂/NBu₄TFAB where four reductions are observed. However, the oxidation behavior of UPy(MeP) in CH₂Cl₂/NBu₄TFAB is similar to that in CH₃CN/NBu₄PF₆. The CV’s in CH₂Cl₂ are consistent with a dimer between the deprotonated and protonated UPy(MeP)’s that is more stable and harder to dissociate over a greater concentration range than in CH₃CN.
