The Impact of Molecular Rigidity on Diamondoid Self-Assembly

Abstract:

Rigid cage-like molecules, like diamondoids, have been used to template the assembly of 1-D nanomaterials, and it has been hypothesized that uniquely strong van der Waals forces may explain this structure-directing behavior. However, the molecular origins of diamondoid assembly has remained largely unexplored. In this work, we computationally investigate diamondoid assembly to determine how structural rigidity influences molecular interactions. We developed a model system to isolate pairwise interactions and parse entropic and enthalpic contributions to the free energy of diamondoid assembly. The model consists of pairs of molecules in a thermal bath, fixed at set intermolecular separations and allowed to freely rotate. By comparing diamondoids to analogous linear alkanes, we draw out the impact of rigidity on pairwise interactions. Due to the bulky structure of diamondoids, we find that linear alkanes actually exhibit stronger van der Waals dispersion interactions than diamondoids at molecular contact. Yet, we also find that diamondoid assembly is more favorable than analogous linear alkanes, as diamondoids pay lower entropic penalties when assembling into contact pairs. Thus, the cage-like shape of diamondoids introduces an enthalpic penalty for molecular assembly, but the penalty is outweighed by favorable entropic effects. These results provide a rigorous understanding of the structure-directing interactions observed in diamondoids and show how molecular rigidity can be engineered to tune the selfassembly of functional materials, such as biomimetic surfaces and nanoscale materials.