Naphthalene Diimide-Based Molecular Salts: Tuning Molecular Arrangements for Efficient Electron Transport

Abstract

The development of electron-transporting n-type organic semiconductors (OSCs) has lagged that of hole-transporting p-type OSCs, owing to the limited number of molecules that combine air stability with high electron mobility. The electron mobility of n-type OSCs is governed by the spatial extent of the lowest unoccupied molecular orbital (LUMO), particularly the orbital overlap between adjacent molecules. Therefore, controlling arrangements is essential for achieving high electron mobility. The current work focused on molecular salts composed of naphthalenediimides (NDIs), with air stability and relatively high electron mobility, modulating the molecular arrangement and elucidating its relationship with electronic properties. An NDI dicarboxylic acid was synthesized by introducing carboxyl groups with a methylene spacer to minimize the influence on the electronic state of NDIs. As the carboxyl group has low acidity, molecular salts were prepared using strongly basic amines. Depending on the type of amines, the molecular arrangement of NDIs was modulated through strong hydrogen bonds. As a result, a slipped-parallel stacking arrangement with varied interplanar and centroid-to-centroid distances between adjacent NDIs was obtained. In addition, a quasi-orthogonal slipped-stacking arrangement, unprecedented for NDI derivatives, was realized. Calculated transfer integrals between adjacent molecules revealed that slipped-parallel stacking is more favorable for electron transport than quasi-orthogonal slipped-stacking. In addition, it was revealed that, within similar slipped-parallel stacking arrangements, the arrangement with a larger overlap of π-conjugation is more favorable for efficient electron transport. The current work demonstrates an NDI molecular arrangement that enables efficient electron transport by modulating the molecular arrangement and analyzing the intermolecular interactions.