Self-stacking of naphthalene bis(dicarboximide)s probed by NMR†

Abstract

The self-stacking in water of a series of naphthalene-1,8∶4,5-bis(dicarboximide)s (also known as naphthalene diimides, NDIs) bearing cationic side chains has been studied using NMR techniques. The position of the charge in the side chain has a strong effect on the propensity of the NDI to self-stack. Examples with a cationic center three atoms away from the NDI ring in general do not self-stack; those with cationic centers further out on the side chains are prone to self-stack at NMR concentrations. The size of the side chain per se does not appear to be a significant controlling factor; even a derivative with biotin side chains shows no evidence of self-stacking. Increasing the ionic strength of the solution can also induce stacking. Derivatives with aromatic side chains can show intramolecular self-stacking of the side chain with the central NDI ring. This is significant for side chains with isoquinoline and bipyridine groups but not significant for side chains with pyridine groups. An example of an NDI that undergoes both intramolecular and intermolecular stacking is the derivative bearing side chains ending in Ru(bpy)₃²⁺ moieties. In methanol, the shift patterns of the aromatic resonances of the bipyridine rings are very close to those of a model compound. However, a more complicated chemical shift pattern is seen in water. This indicates that conformations with the Ru(bpy)₃²⁺ moieties lying at least partly within the shielding cone of the aromatic NDI system are favored. Molecular modeling indicates that conformations in which the bipyridine ring interacts with the NDI ring are readily achievable. The temperature dependence of the chemical shifts for this molecule indicates that both intramolecular interactions of the bipyridine rings with the NDI ring and self-stacking of the NDI rings are significant.