Interlocking Mechanism between Molecular Gears Attached to Surfaces

Abstract

While molecular machines play an increasingly significant role in nanoscience research and applications, there remains a shortage of investigations and understanding of the molecular gear (cogwheel), which is an indispensable and fundamental component to drive a larger correlated molecular machine system. Employing ab initio calculations, we investigate model systems consisting of molecules adsorbed on metal or graphene surfaces, ranging from very simple triple-arm gears such as PF and NH to larger multiarm gears based on carbon rings. We explore in detail the transmission of slow rotational motion from one gear to the next by these relatively simple molecules, so as to isolate and reveal the mechanisms of the relevant intermolecular interactions. Several characteristics of molecular gears are discussed, in particular the flexibility of the arms and the slipping and skipping between interlocking arms of adjacent gears, which differ from familiar macroscopic rigid gears. The underlying theoretical concepts suggest strongly that other analogous structures may also exhibit similar behavior which may inspire future exploration in designing large correlated molecular machines.

How Does the Flexibility of Molecules Affect the Performance of Molecular Rotors?

“…In summary, our simulations of bending effects in molecular rotors and gears lead to the following observations: (1) longer rotor arms containing a long linear group (like those illustrated in Figure 2) actually produce no stronger driving forces (to adjacent rotors) than shorter arms (like those illustrated in Figure 1); (2) the driving forces generated in all the studied typical rotors are actually mainly contributed by the single bond (C-C6 or F-C6 here) connecting to the aromatic ring, instead of by the C≡N or C≡C triple bond, or any others; (3) there is no substantial difference between various chemical groups in the magnitude of generated resistance/stiffness; (4) we observe almost no deformation of the planar aromatic rings, as bending a π bond is believed to be generally more difficult; 45 (5) we have found no cases in which the rotors are destroyed -although this may indeed happen in certain cases, as observed in previous studies 20,21 . Therefore, we suggest that, in designing molecular machines, aromatic rings be included in the molecules so as to enhance the stiffness of the machine, and conversely: replacing aromatic rings with other groups may reduce the machine's stiffness.…”

“…Each gear is now made of a 6-membered carbon ring substituted with six ethynyl groups, thus the arms in this gear are substantially longer than those shown in Figure 1; on the other hand, we have also increased the gear-gear distance (from 3 to 4 graphene lattice constants), which gives the gear arms a bit more space to move in. Based on the 18-electron principle in organometallic chemistry, we highlight that chromium and its congeners in the same column of the Periodic Table are the only suitable intermediates for a 6-membered carbon ring to stand robustly while rotating nearly freely on a graphene sheet 20 , so we adopted chromium here. While in this system a single driver can successfully drive a single slave through a 60˚ rotation and thus a 360˚ full rotation (see Figure S1 in the supplementary information for details), we find that a longer gear chain containing two slaves starts to become problematic, as illustrated in Figure 2.…”

“…a molecular motor) to a point of application. 20,21 Interestingly, we found in our earlier research that the coupling/interlocking behavior between molecular devices (such as motors and gears) is quite different from that in the macroscopic world, and may sometimes be problematic. 21 This mainly comes from the flexibility of the molecules, viz.…”

A chiral molecular propeller designed for unidirectional rotations on a surface

“…However, our molecular propellers may not be suited for such cascade-gear applications because of their trigonal geometry with approximately 120° angle between the blades. Such a large angle could result in slippage and thus the molecular gear with more teeth would be required 24 . A key demonstration of our work is that the substrate surface can be used to engineer chirality of the molecular propellers and thus not only the internal structure of the molecules but also the substrate should be considered for the design of the molecular machines to be operated on solid surfaces.…”

Fragment motion in motor molecules: basic concepts and application to intra-molecular rotations

“…Examples are combinations of fragment translation and rotation as found in [c2]daisy chain supramolecules 7,8 or the influence of fragment flexibility as identified in molecular dynamics calculations on BTP-BCO. Also details of interlocking rotor molecules, such as those examined recently, 35,36 will be interesting as candidates for the mechanical analysis. Studies along these lines are currently under way.…”