September 2020
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A train of molecule gears consisting of PF3 molecules was studied using semi-empirical ASED+ method to explore the mechanism of rotational transmission along this train. It was observed that a unidirectional rotational transmission occurs between only the first two PF3 molecules for a PF3 molecule train up to six molecule-gears, the four PF3 molecules at the end of the train being used to rigidify the rotation axle of the first two PF3. This demonstrates that in a train of molecule-gears, the rotation of each molecule is resulting from a collective action of many degrees of freedom per molecule. This collective motion is rather fragile against many others possible minimum energy trajectories which can develop on the multidimensional ground state potential energy surface of a molecule-gear train to respond to the increase of the potential energy required to rotate the first molecule-gear of the train.
![The chemical skeleton (left) and three possible optimized conformations of the free PAH-4W molecule using the MOPAC package [19]. The central molecular chassis is in blue with a ‘1’ and the four triptycene molecular wheels in red (the chemical group marked with a ‘2’). Optimized structure (i) is showing how the four triptycene wheels are pushed down the chassis through a steric saddle shape chassis structure, with four wheels down and four tert-butyl groups up, due to deformation induced by the presence of two five-membered rings on the central part of the chassis. In the case of other optimized structure (ii) (or (iii), not found experimentally), the deformation occurs in three wheels and 1 tert-butyl group (two wheels and 2 tert-butyl groups respectively) up and one wheel and 3 tert-butyl groups (two wheels and 2 tert-butyl groups) down.](https://www.researchgate.net/publication/327618497/figure/fig1/AS:11431281208665613@1701433443617/The-chemical-skeleton-left-and-three-possible-optimized-conformations-of-the-free_Q320.jpg)
![(a) Typical scanning tunneling dI/dV spectra recorded at slightly different tip locations on the molecule chassis but near the wheel. The corresponding low voltage experimental image is presented also in (a) with the location of the tip during spectroscopic measurements indicated as blue and red spots (corresponding to the blue and red curves, respectively). The corresponding dI/dV ESQC calculated image is presented in (a). Three distinct peaks are observed and identified by vertical dotted lines. One at positive corresponds to the first virtual reduced state and two at negative bias voltage corresponding to the first and second virtual oxidation states contributing to the tunneling current. (b) The experimental differential conductance (dI/dV) maps recorded at the bias voltage identified in (a) (they realize a projection of the PAH-4W electronic probability density of these molecular electronic states on a two dimensional plane). In order to determine the main mono-electronic molecular orbitals entering the multi-electron virtual states mentioned above and contributing to the experimentally observed electronic probability density maps, molecular orbitals of the free PAH-4W are represented at the bottom row (c) and were calculated using the semi-empirical PM7-MOPAC package [19]. Notice that the electron density of not only HOMO and LUMO but also up to HOMO-7 crowds into the chassis. The first mono-electronic state located on a wheel is arising at HOMO-8. The 4 wheel states are electronically separated from each other and can be found between HOMO-8 to HOMO-11. Only one was represented in (c). Images sizes are 4 nm × 4 nm for topographic image and 5 nm × 5 nm for dI/dV maps.](https://www.researchgate.net/publication/327618497/figure/fig3/AS:11431281208665616@1701433444067/a-Typical-scanning-tunneling-dI-dV-spectra-recorded-at-slightly-different-tip-locations_Q320.jpg)
