NMR and ESR investigations of the interaction between a carboxylic acid and an amine at the focal point of l-lysine based dendritic branches

Abstract

This paper reports the characterisation of supramolecular complexes formed between the carboxylic acid group at the focal point of host dendritic branches based on L-lysine building blocks and an amine group on an appropriate guest molecule. ¹H NMR titration investigations indicate that the interaction is relatively weak. Interestingly the dendritic generation appears to have no effect on the thermodynamics of benzylamine recognition – in contrast to previous studies in which charged guests have been bound to dendritic hosts. Control experiments using dendritic branches in which the carboxylic acid is protected as a methyl ester indicate that there is only a small amount of non-specific binding of the amine functionalised guest molecule within the dendritic framework itself. ESR investigations clearly show the binding between the dendritic branch and amine functionalised TEMPO radicals. Most interestingly, rotational correlation times can be determined from the ESR studies and they indicate that the mobility of the TEMPO radical is diminished on binding to the dendritic branch. Notably this effect is generation dependent, with larger dendritic branches having a more dramatic effect on the tumbling of the radical. Control experiments clearly prove the importance of the acid-base interaction and also demonstrate that effective binding only occurs in non-polar solvents. These results therefore illustrate that using host–guest chemistry at the focal point of a dendritic structure is an effective way to control and modify the solution phase properties and mobility of active species such as radicals.