Redox-Responsive 4 H-Bond Arrays for Supramolecular Applications

Abstract

Due to the ease of synthesis and strong H-bonding, Meijer’s ureidopyrimidone (UPy) 4 H-bonding array has been extensively investigated for use in a variety of functional supramolecular assemblies and polymers. However, somewhat surprisingly, there are relatively few examples of electroactive UPy’s, particularly ones in which electron transfer has a significant effect on the dimerization of the self-complementary AADD or ADAD arrays. Recently we have described an electroactive UPy that contains a phenylenediamine redox couple formed by attachment of a p-dimethylaminophenyl group to R1. In this system the AADD tautomer is favored. Two electron oxidation of the phenylenediamine leads to a sufficient increase in the acidity of one of the urea H donor sites such that proton transfer occurs, resulting in breakup of the dimer. In more recent work we have been investigating the effect of redox couples attached at the R2 position. In one example, based on initial studies by Alexander et al. (Chem Comm 2007, 22, 2246), a ferrocene is attached to this position. CV studies indicate that like the phenylenediamine system, oxidation of the ferrocene leads to break up of the dimer. In another example, an alkyl pyridinium group is attached to the R2 position to create a “monoquat” (N-alkyl-4,4'-bipyridinium) type redox couple. In this case, the weaker binding DADA array is favored in the starting oxidation state. Reduction of the pyridinium should increase the negative charge on the pyrimidinol leading to stronger rather than weaker H-bonding as observed in the other cases. Initial CV studies suggest that this is happening, although the voltammetry is much more complicated than that observed in the other examples. More detailed investigations, currently in progress, will help to ascertain whether the binding strength is indeed increasing upon reduction. Figure 1