Original polymorphism in a naphthalene bisimide π-conjugated organogelator: a complex interplay between hydrogen bonding and heterocycle π-stacking

This study focuses on the structure of different supramolecular assemblies formed in the gel phase and in the solid state by a naphthalene-bisimide organogelator: N,N′-di(2-(3,4,5-trioctyloxybenzamido))eth-1-yl naphthalene-1,4,5,8 tetracarboxylic acid bisimide (NDI2). This molecule is formed by a naphthalene bisimide core with symmetric dendrons composed of a flexible alkyl linker, an amide group and a trialkoxyphenyl group. The structure, especially the original polymorphism of this molecule, was correlated with optical and electronic properties in the solid state. Four polymorphs have been prepared in pure form: a dried gel phase, a quenched metastable liquid crystalline phase, a crystalline phase and its sister phase with molten alkyl side chains. The molecular packing of those phases was determined by X-ray diffraction, electron diffraction and high resolution TEM. UV-vis and infrared spectroscopy of the polymorphs uncover the nature of intermolecular π-stacking and H-bonding interactions. In the fibrils of the gel phase, NDI2 molecules form columnar stacks and the fiber axes correspond to the direction of H-bonds between amide groups. The crystalline phase shows an unusual packing where each NDI is π-stacked between two phenyl groups of the adjacent NDI2 molecules. In the metastable LC phase NDI2 molecules assemble into 6/1 supramolecular helices grouped in a frustrated trigonal unit cell. Temperature-dependent TEM, UV-vis and FTIR helped to unveil the molecular reorganization upon transformation between these different polymorphs. Finally, a global phase diagram has been obtained.