Meccano on the Nanoscale—A Blueprint for Making Some of the World's Tiniest Machines

Abstract

Molecular compounds—comprised of mechanically interlocked components—such as rotaxanes and catenanes can be designed to display readily controllable internal movements of one component with respect to the other. Since the weak noncovalent bonding interactions that contribute to the template-directed synthesis of such compounds live on between the components thereafter, they can be activated such that the components move in either a linear fashion (rotaxanes) or a rotary manner (catenanes). These molecules can be activated by switching the recognition elements off and on between components chemically, electrically, or optically, such that they perform motions reminiscent of the moving parts in macroscopic machines. This review will highlight how the emergence of the mechanical bond in chemistry during the last two decades has brought with it a real prospect of integrating a bottom-up approach, based on molecular design and micro- and nanofabrication, to construct molecular electronic devices that store information at very high densities using minimal power. Although most of the research reported in this review on switchable catenanes and rotaxanes has been carried out in the context of solution-phase mechanical processes, recent results demonstrate that relative mechanical movements between the components in interlocked molecules can be stimulated (a) chemically in Langmuir and Langmuir–Blodgett films, (b) electrochemically as self-assembled monolayers on gold, and (c) electronically within the settings of solid-state devices. Not only has reversible, electronically driven switching been observed in devices incorporating a bistable [2]catenane, but a crosspoint random access memory circuit has been fabricated using an amphiphilic, bistable [2]rotaxane. The experiments provide strong evidence that switchable catenanes and rotaxanes operate mechanically in a soft-matter environment and can withstand simple device-processing steps. Studies on single-walled carbon nanotubes used as one of the electrodes in molecular switch tunnel junctions have revealed that interfacial chemical interactions involving electrodes containing carbon, silicon, and oxygen are good choices when carrying out molecular electronics on the class of rotaxane- and catenane-based molecules reported in this review. This conclusion is supported by differential conductance measurements (at 4 K) made with single-molecule transistors using the break-junction method. It transpires that the electronic transport properties in such devices are more sensitive to the chemical nature of the molecule–electrode contacts than the details of the molecules' electronic structure away from the contacts. This result has profound implications for molecular electronics and highlights the importance of also considering the molecules and the electrodes as an integrated system. It all adds up to an integrated systems-oriented approach to nanotechnology that finds its inspiration in the transfer of concepts like molecular recognition from the life sciences into materials science and provides a model for how, in principle, to transfer elements of traditional chemistry to technology platforms that are being developed on the nanoscale. Before there can be any serious prospect of a technology, there has to be some good, sound science in the making. Molecular electronics is very much in its infancy and, as such, it can be expected to give rise to a great deal of intellectually stimulating science before it stands half a chance of becoming a viable companion to silicon-based technology.

The molecular level modification of surfaces: from self-assembled monolayers to complex molecular assemblies

“…sensors and biochips, 2,3 photovoltaics, 4,5 fuel cell, 6 biomaterials, 7 and molecular electronics, 4,8,9 as well as to provide fundamental understanding of electron transfer, 10 controlling reactions at surfaces, 11 and tailoring surface properties. 12 Among the many exciting developments, a number of rather sophisticated molecular assemblies have been developed that not only involve organic and organometallic molecules, but also nanomaterials and/or biomolecules.…”

Molecular machines operated by light

“…In principle, molecular machines can be designed starting from several kinds of molecular and supramolecular systems [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47], including DNA [39,40]. However, for the reasons mentioned below, most of the systems constructed so far are based on interlocked molecular species such as rotaxanes and related species.…”

“…In fact, the majority of artificial molecular machines reported so far [31][32][33][34][35][36][38][39][40][43][44][45][46][47] are powered by chemical energy, most typically supplied by acid-base reactions. It would therefore be interesting to identify viable strategies for using light to operate 'stand alone' pH-driven molecular machines.…”

“…Among the systems reported are molecular tweezers [9], propellers [10], rotors [11], turnstiles [12], gyroscopes [13,14], gears [15], brakes [16], a molecular pedal [17], ratchets [18], rotary motors [19], shuttles [20], elevators [21], muscles [22], valves [23], processive artificial enzymes [24], walkers [25][26][27], vehicles [28], and catalytic self-propelled micro-and nano-objects [29,30]. Several excellent reviews [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] and a monograph [47] dealing with artificial molecular machines and motors are available.…”

Australian Journal of Chemistry Celebrates its Sixtieth Anniversary

“…Although the journal has had an Australasian-focus in the past, the current editorial direction, with an emphasis on ensuring proper coverage of a range of internationally recognized research priorities, means that this is less so now. As a consequence, and as evidenced by looking at the most highly cited papers published in AJC during 2004, [1][2][3] 2005, [4][5][6] and 2006, [7][8][9] the work produced by Australasian authors is receiving greater attention, while increasing numbers of papers arising from groups working in the Northern Hemisphere are now appearing in the journal.…”