  Rigid, cage-like molecules, like diamondoids, show unique self-assembly
behavior, such as templating 1-D nanomaterial assembly via pathways that are
typically blocked for such bulky substituents. We investigate molecular forces
between diamondoids to explore why molecules with high structural rigidity
exhibit these novel assembly pathways. The rigid nature of diamondoids
significantly lowers configurational entropy, and we hypothesize that this
influences molecular interaction forces. To test this concept, we calculated
the distance-dependent impact of entropy on assembly using molecular dynamics
simulations. To isolate pairwise entropic and enthalpic contributions to
assembly, we considered pairs of molecules in a thermal bath, fixed at set
intermolecular separations but otherwise allowed to freely move. By comparing
diamondoids to linear alkanes, we draw out the impact of rigidity on the
entropy and enthalpy of pairwise interactions. We find that linear alkanes
actually exhibit stronger van der Waals interactions than diamondoids at
contact, because the bulky structure of diamondoids induces larger net atomic
separations. Yet, we also find that diamondoids pay lower entropic penalties
when assembling into contact pairs. Thus, the cage-like shape of diamondoids
introduces an enthalpic penalty at contact, but the penalty is counterbalanced
by entropic effects. Investigating the distance dependence of entropic forces
provides a mechanism to explore how rigidity influences molecular assembly. Our
results show that low entropic penalties paid by diamondoids can explain the
effectiveness of diamondoids in templating nanomaterial assembly. Hence, tuning
molecular rigidity can be an effective strategy for controlling the assembly of
functional materials, such as biomimetic surfaces and nanoscale materials.
