The Effect of Proton Transfer on Redox-Responsive Dimerization in Electroactive 4 H-Bond Arrays
Authors: Smith Diane K. , Darzi Ghazwan M 
Published: February 2020

The design of stimuli-responsive systems is an important topic in supramolecular chemistry. These are systems in which an external signal, such as a change in light, temperature, pH or voltage on an electrode, results in structural changes in the supramolecular assembly. Applications include self-healing polymers and gels, controlled release of entrapped molecules and smart materials. Over the years our group has investigated a number of simple systems in which electron transfer is used to perturb the strength of H-bonding interactions. More recently we have expanded the research to include larger H-bond arrays. In particular we have been investigating several different electroactive ureidopyrimidones or UPy’s, which are capable of strong self-dimerization through the formation of 4 linear H-bonds. Non-electroactive UPy’s have already been widely used in the construction of supramolecular polymers and gels capable of self-repair; introduction of electroactive UPy’s in which dimerization can be controlled electrochemically will make even more types of applications possible. These systems are also of interest from a fundamental standpoint in that they provide a well-understood platform in which to investigate the role of intermolecular H-bonding (and proton transfer) on electron transfer.
The principle behind redox-responsive H-bonding is straightforward: an oxidation that decreases the negative charge on a H-acceptor (A) or reduction that decreases the positive charge on a H-donor (D) will weaken a H-bond. Alternatively, reduction that increases the negative charge on a H-acceptor or oxidation that increases the positive charge on a H-donor will increase the strength of a H-bond. However, in the latter case, it is possible that oxidation or reduction could also lead to full proton transfer. If this occurs across the H-bond, the primary H-bonds will stay the same, but the secondary H-bonds will change, possibly in a way that has an undesired effect. It is also possible that proton transfer occurs between non-H-bonded sites. This can lead to a H-bonding pattern that is no longer self-complementary. An example of this is found with a UPy we have made that contains a pyridinium redox couple attached to the pyrimidine ring in the UPy, UPyH(MeP+). This UPy was expected and confirmed by NMR to exhibit relatively weak dimerization in CH2Cl2 in the oxidized state due to the electrostatic repulsion and a weaker DADA H-bond motif. Dimerization strength was expected to increase upon reduction to the uncharged radical due to the loss of electrostatic repulsion and increased basicity of the H-acceptor sites. However, the voltammetry observed in CH2Cl2 is not consistent with this result. Furthermore, very similar voltammetry is observed in CH3CN, even though NMR shows only the monomer is present at the same concentrations in this solvent. Studies with model compounds strongly suggest that the reason for the failure to dimerize upon reduction is that the initial reduction product, an uncharged radical, is basic enough to be protonated by the oxidized starting material, resulting in a H-bonding motif that is no longer self-complementary. While this result negates the original goal of strengthening dimerization through reduction, it also opens up the intriguing possibility of switching binding partners by including a guest molecule that is not complementary to the oxidized UPyH(MeP+) form, but is complementary to the reduced, protonated UPyH2(MeP)+ form.