Chapter - 14

MOLECULAR NANOMACHINES

Figure 1. Concept of bistability. The system is stable in two states and they can be interconverted

by the action of external stimulus.

Figure 2. Various kinds of molecular switches. The state on the left can be considered as ‘0’ and

that on the right as ‘1’. Various kinds of stimuli are indicated. Cases listed in A correspond to

molecular systems while B corresponds to a supramolecular system. In B, the metal ion is

released out of the crown ether cage in the reverse direction.

Figure 3. Schematic illustration of a macroscopic ratchet and a proposed molecular analogue.

The helicene acts as both the pawl and the spring and the triptycene acts as the wheel.

Figure 3. Schematic representations of a rotaxane and a catenane. There are two interconnected

units in both and that makes the prefix [2].

Figure 4. Synthetic approaches used for rotaxanes. In step 2, the de-threaded state exists in

equilibrium with the threaded state. Three strategies are shown of which 3 involves proper design

of components so that starting material is assembled to some extent.

Figure 5. Schematic illustration of a [2]rotaxane structure in which two stations are incorporated

on the rod. The bead can slide over the thread and can be held at station 1 or station 2, depending

upon the strength of the interaction as shown in the potential energy diagram below. Deeper

potential well at station 2 makes that isomer to form preferentially. When the potential is

unfavorable at that position, due to repulsive interaction between the station and the bead (as in

the case of a electrochemical oxidation), the potential energy surface gets modified and the

minimum becomes a maximum. As a result the bead slides over to the other station. When the

oxidized state is reduced, original situation returns and the bead returns.

Figure 6. A rotaxane in which molecular motion has been demonstrated. The neutral molecule

predominantly exists in the structure shown (86% occupancy) and when the benzidine unit is

oxidized either electrochemically or chemically, the bead shifts to the other station. The structures

have to be rotated a bit.