Advances towards synthetic machines at the molecular and nanoscale level

Abstract

The fabrication of increasingly smaller machines to the nanometer scale can be achieved by either a "top-down" or "bottom-up" approach. While the former is reaching its limits of resolution, the latter is showing promise for the assembly of molecular components, in a comparable approach to natural systems, to produce functioning ensembles in a controlled and predetermined manner. In this review we focus on recent progress in molecular systems that act as molecular machine prototypes such as switches, motors, vehicles and logic operators.

Keywords: catenanes; devices; molecular machines; nanovehicles; rotary motors; rotaxanes; switches.

Figure 1.

Schematic representation of two different types of interlocked molecules that have shown potential as molecular level machinery components. (a) Interlocked ring structures referred to as catenanes and (b) interlocked ring and thread terminated by bulky stoppers are termed rotaxanes.

Figure 2.

Sauvage’s copper(I) template synthesis of an interlocked catenane in two sequential steps: (i) coordination of the disubstituted phenanthroline ligand to the metal; (ii) mechanical interlocking by a covalent bond forming reaction; (iii) Demetallation leads to a specific conformational change.

Figure 3.

A model for a catenane-based molecular switch that is redox-switchable by control of the copper oxidation state [29].

Figure 4.

A representation of the desymmetrized donor-acceptor [2]catenane [34].

Figure 5.

(a) Stoddart and Yaghi’s catenane used in a metal-organic framework, (b) Side on view of a portion of the MOF along the a-axis [37].

Figure 6.

Rotaxanes are able to ‘shuttle’ between two different states indicating switching (ON/OFF) or binary processes (0/1). Shuttling is a form of translational isomerism. The challenge is controlling this motion.

Figure 7.

Copper-complexing [2]rotaxane comprising two different bidentate stations [43]. The diphenylbiisoquinoline ligand (dpbiiq) site in the macrocyclic ring structure is complementary to either the diphenylphenanthroline (dpp) or bipyridine (bipy) sites along the threading molecule and shuttling is possible upon electrochemically-driven complexation events with copper.

Figure 8.

Schematic representation of Sauvage’s “artificial muscle” using transition metal complexation to interchange between (a) stretched and (b) contracted geometries.

Figure 9.

Template-assisted, one pot synthesis of a [3]rotaxane. Two, successive covalent bonds are formed (via the CuAAC click reaction) assisted by ligation to the ring yielding an interlocked structure consisting of two threaded components [53].

Figure 10.

Schematic representation of a [2]rotaxane molecular machine that is comprised of an oligopeptide axle that results in a helical structure.

Figure 11.

A schematic diagram of the unthreading and threading of a [2]pseudorotaxane, corresponding to an XOR logic function [58].

Figure 12.

A representation of the push-button molecular switch [59].

Figure 13.

A representation of the squaraine-based [2]rotaxane molecular switch of Chiu [60].

Figure 14.

Schematic representation of the general design components of the Feringa rotary molecular motor with evolutionary design [61].

Figure 15.

Schematic representation of a molecular rotary motor with self-complexing lock. The well known complementarity between crown ethers and ammonium salts provides a non-covalent interaction to produce a (a) locked and (b) unlocked state for the molecular rotary motor.

Figure 16.

The concept of a “top-down” approach for the construction of vehicles at the molecular level as developed by the Tour group. Using a three component design strategy analogues of macroscopic transportation vehicles (e.g., cars, trucks, trains) have been created such as nanocars and nanotrucks. R = solubilizing groups.

Figure 17.

Teaching molecules to walk. The use of a scaffold containing functionality designed to make disulfide and hydrazone links has been used to allow a molecule to transverse the walkway in a controlled fashion.