New Molecular Devices:  In Search of a Molecular Ratchet

Abstract

The triptycene-substituted [3]- and [4]helicenes 1 and 2 were examined as possible molecular versions of mechanical ratchets, where the triptycene serves as the ratchet wheel and the helicenes as pawl and spring. The syntheses of 1 and 2b are described. 1H NMR was employed to examine rotation around the triptycene/helicene single bond; at 20 °C rotation is frozen for both 1 and 2b, but the NMR of 1 revealed a plane of symmetry, indicating that 1 cannot function as a unidirectional ratchet. In contrast, NMR revealed that, like a ratchet, triptycyl[4]helicene 2b lacks the symmetry of 1 and has a barrier to rotation of 24.5 kcal/mol, but spin polarization transfer NMR experiments indicated the triptycene in 2b nonetheless rotates equally in both directions. That outcome is rationalized from the standpoint of thermodynamics.

Molecular Barbed Wire: Threading and Interlocking for the Mechanical Reinforcement of Polymers

“…Studies show that the mechanical properties still follow the stiffness/ductility tradeoff [14][15][16][17] and even sometimes sacrifice both. [18] One particular structure that has been gaining interest is triptycene (T), [19][20][21][22][23][24][25] which is characterized by its rigid, shape-persistent geometry. It differs from typical pendant groups because it contains a structural characteristic called the internal molecular-free volume (IMFV), defined as "the difference in volume between that which is generated by the geometry of a structure and that which is occupied by the structure itself" (Fig.…”

Artificial molecular motors

“…The group of Kelly developed several ratchet-type molecules, 12,66,67 leading up to their report of a chemically driven rotor, capable of performing 120° unidirectional rotation around a single bond in two chemical steps. 68,69 This device, much like their previous designs, consists of a triptycene 'wheel' and a helicene 'brake', connected by a single bond.…”

Photoisomerization of a rotaxane hydrogen bonding template: Light-induced acceleration of a large amplitude rotational motion

“…Here we demonstrate that the rate of rotation of the interlocked components of fumaramide-derived [2]rotaxanes can be accelerated, by >6 orders of magnitude, by isomerizing them to the corresponding maleamide [2]rotaxanes by using light. molecular machines ͉ dynamics L arge amplitude internal rotations that resemble to some extent processes found in authentic machinery have recently inspired analogic molecular versions of gears (1), turnstiles (2), brakes (3), ratchets (4,5), rotors (6), and unidirectional spinning motors (7)(8)(9)(10) and are an inherent characteristic of many catenanes and rotaxanes (11)(12)(13). Establishing methods for controlling aspects of such movements is a prerequisite for the development of artificial devices that function through rotary motion at the molecular level.…”