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, self-healing polymers and gels, triggered release of entrapped molecules for drug delivery, and “smart” materials. In this study, a four H-bond ureidopyrimidinone (UPy) array with an alkyl-pyridinium, (RP), redox center has been synthesized, UPy(RP), shown below. This array prefers the tautomer that presents an ADAD H-bond motif in the starting oxidation state. Due to electrostatic repulsions and unfavorable secondary H bond interactions, this motif would form a dimer with relatively weak H-bonding. Upon 2e^- reduction, where 1e^- is gained per R-pyridinium redox center, the H-bond strength should increase due to the loss of the repulsive charges, making  the nitrogen a stronger hydrogen acceptor. Because the nitrogen is now a better hydrogen acceptor, there could be a possibility of an intermolecular proton transfer. This would encourage the tautomer to have an AADD motif that will make the H-bonding stronger by increasing the favorable secondary H-bond interactions.

UPy dimers can exist in two tautomeric forms called pyrimidinol and pyrimidinone. In order to more efficiently study the electrochemistry of the two forms, two compounds, 4-acetylpyridinium (AcP) and N-methyl-4,4’-bipyridinium (MeV⁺), were used as model compounds. AcP resembles the pyrimidinone tautomer and MeV⁺ resembles the pyrimidinol tautomer. The cyclic voltammetry (CV) scans for the two model compounds in CH₂Cl₂ and CH₃CN show two widely spaced, reversible redox waves. In contrast, CV’s of UPy(RP) in these solvents show two closely spaced reductions.  The first occurs at a potential very similar to that seen with simple model compounds.  The second is considerably positive of that observed for the model compounds, consistent with strong stabilization of the doubly reduced form by H-bonding.  However, the voltammetry is complex. The first reduction is partially reversible if the scan direction is switched immediately after the peak, but irreversible if the scan direction is switched after the second reduction peak.  Going through the second reduction also leads to the appearance of new oxidation waves at more positive potentials.  This behavior suggests, not surprisingly, that the second reduction induces proton transfer and tautomerization.  It is possible that the new oxidation peaks are due to oxidation of a more strongly H-bonded dimer. Further studies to help elucidate what is actually happening in this system will include concentration dependent CV’s and analysis of binding strength using NMR.