Nanosized Self-assembling Rotor Systems

In a collaboration with the experimental research group of Prof. Dirk Volkmer at the University Augsburg we have performed a feasibility study on self-assembling rotor systems. The aim of the project was to investigate whether self-assembled supra-molecular complexes could act as molecular rotors, using substituted tribenzotriquinacenes as stators and fullerene -derivatives.

From the outset it was clear that nano-sized molecular rotors - and molecular machines built thereof - behave entirely different from their macroscopic congeners. For the majority of molecular rotors, the two fragments which are intended to rotate against each other are connected by a covalent bond, serving as “axle”. This can be rotatable single bonds or even double bonds for photochemically driven rotors. In contrast to that, within this project the aim was to investigate if and how rotor and stator should be held together by non-bonded forces like dispersive interactions, allowing the machine to self-assemble instead of having to generate it by a chemical reaction, forming a bond. In order to make this work it is clear that completely different design rules apply, as compared to established “bonded” molecular rotors. In particular, the van-der-Waals interaction between rotor and stator must be large enough to allow sufficient binding strength, which implies a large contact area. On the other hand this contact area must be maintained during a rotational motion and a shape complementarity needs to be realized. The starting point of the project was to realize that these boundary conditions could be met by using spherical Fullerene-derivatives as rotors and “bowl-shaped” -symmetric tribenzotriquinacene derivatives as fullerene receptors and stators.

Primarily three different receptor systems RA, RB and RC haven been synthesized by the Volkmer group and investigate by us using both dispersion corrected DFT methods and MM force fields (in the end we have parametrized MOF-FF to describe these systems).

A very important finding was to realize that for flexible receptor systems entropy contributions are crucial to determine the host-guest binding energy. Extensive MD sampling was necessary to explain the tendencies observed experimentally for the binding energies. Overall, large but rigid receptors are better suited for a strong binding, whereas an “adaptive” binding is not achieved via large and flexible host systems.

In an additional investigation, we studied a chiral receptor system with respect to the possibility to achieve unidirectional rotoation:

The figure shows a chiral rotor system, consisting of a rotor with a dipolar head group and a axial chiral stator with symmetry synthesized recently in the Volkmer group (the joint work has been published in Chemistry Eur. J.).

By force field MD simulations we found that this system indeed shows an asymmetric energy profile for the rotation of the rotor. The difference is however too small to expect this to be observable experimentally.