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

The self-stacking in water of a series of naphthalene-1,8: 4,5-bis(dicarbox imide)s (also known as naphthalene diimides, NDIs) bearing cationic side ch ains has been studied using NMR techniques. The position of the charge in t he side chain has a strong effect on the propensity of the NDI to self-stac k. Examples with a cationic center three atoms away from the NDI ring in ge neral do not self-stack; those with cationic centers further out on the sid e chains are prone to self-stack at NMR concentrations. The size of the sid e 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. I ncreasing the ionic strength of the solution can also induce stacking. Deri vatives with aromatic side chains can show intramolecular self-stacking of the side chain with the central NDI ring. This is significant for side chai ns with isoquinoline and bipyridine groups but not significant for side cha ins with pyridine groups. An example of an NDI that undergoes both intramol ecular and intermolecular stacking is the derivative bearing side chains en ding in Ru(bpy)(3)(2+) moieties. In methanol, the shift patterns of the aro matic resonances of the bipyridine rings are very close to those of a model compound. However, a more complicated chemical shift pattern is seen in wa ter. This indicates that conformations with the Ru(bpy)(3)(2+) moieties lyi ng at least partly within the shielding cone of the aromatic NDI system are favored. Molecular modeling indicates that conformations in which the bipy ridine ring interacts with the NDI ring are readily achievable. The tempera ture dependence of the chemical shifts for this molecule indicates that bot h intramolecular interactions of the bipyridine rings with the NDI ring and self-stacking of the NDI rings are significant.