Molecular-scale devices

There is little doubt that diverse physical properties common to polyoxometalates places them in an almost unmatched class of materials which could be extremely useful as hybrid materials and nanocomposites [31]. In this section we will focus on POM systems with properties that could be exploited in the development of molecular-scale devices. 

The gap between concepts in molecular design to produce polyoxometalate integrated nanosystems or molecular-scale devices is vast due to the problem of fabrication and control of molecular orientation. Molecule-by-molecule assembly is... 

Key conceptual influence on nanotechnology can be traced back over 30 years ago to physicist and Nobel laureate R. Feynman [183]. Like many contemporary nanotechnologists such as Drexler [207] and others, Feynman credited his inspiration to the molecular-scale devices and information systems observed in the biological world. 

Khanna, V.K. (2004) Emerging Trends in Ultra-miniaturized CMOS (Complementary Metal-Oxide-Semiconductor) Transistors, Single-Electron and Molecular-Scale Devices ... 

Seideman, T. (2003) Current-driven dynamics in molecular-scale devices. /. Phys. Condens. Matter, 15, R521-R549. 

It should be noted, however, that the molecular-level devices described in this chapter operate in solution, that is, in an incoherent fashion. For most kinds of applications, they need to be interfaced with the macroscopic world by ordering them in some way, for example, at an interface or on a surface [103, 149], so that they can behave coherently, either in parallel or in series. Research on this topic is developing at a fast-growing rate [6, 7]. Furthermore, addressing a single molecular-scale device by instruments working at the nanometer level is no longer a dream [150, 151]. 

Catenanes have attracted considerable current interest particularly owing to their recently explored potential as molecular-scale devices. The incorporation of a coordination bond into catenane frameworks makes it possible to realize a quick molecular motion, which is somewhat similar to that of well-known magic rings under thermodynamic conditions a catenane framework rapidly arises from two preformed molecular rings. In the following sections, we describe the self-assembly, chemical manipulation, and characterization of several self-assembled catenanes. 

The ultimate aim of nanotechnology is the development of self-assembling molecular-scale devices that can themselves perform specific, precisely controlled operations at the molecular and atomic level. Current methods nsing natnral molecular machines — proteins, enzymes, antibodies, and the like — or synthetic molecnlar forms still rely to a large degree on bulk processes. They provide us with rudimentary devices that operate at the molecular and atomic level, but at present they lack the precision and positional control reqnired to develop more advanced nanotechnologies. 

The study of molecular scale devices has created the need for new theoretical tools which could be used for predictions of their structures and properties and to probe their new designs. Electronic devices are open systems with respect to electron flow, and a theoretical description of such devices should be done in terms of statistically mixed states which cast the problem in terms of quantum kinetic theory [100]. The only completely adequate theory that could currently address this task is the non-equilibrium Grin s function formulation of many-body theory. 

One of the potential applications of mechanically interlocked molecules is construction of molecular-scale devices such as molecular machines and switches. [2J(Pseudo)rotaxanes containing CB 6] were studied along... 

Synthesis of Molecular Scale Devices with Heteroatomic Functionalities... 

Described here are the syntheses of functionalized molecular scale devices which are designed to have nonlinear I(V) responses by adding heterofunctionalities to modulate the n-electron system. Some of the systems... 

The cyclic voltammetry characteristics of the nitroaniline-containing molecular scale devices were determined to help elucidate the transport mechanism. It was therefore necessary to synthesize thioether 73 that is more stable to hydrolysis and subsequent oxidation than the thioacetate-terminated system (Scheme 3.34). For the synthesis of thioether 73, intermediate 69 was deprotected and Pd/Cu-catalyzed coupled to 72 to form thioether terminated 73. This compound was subjected to cyclic voltammetry that confirmed that the compound was being reduced at -1.7 V and again reduced at -2.3 V (Ag/AgNOs reference electrode, 1.0 M n-tetrabutylammonium tetrafluoroborate in DMF at a scan rate of 100 mV/sec). Of course, there can be no correlation of absolute reduction potentials between the solution-phase and SAM experiments since the environments are grossly different. However, that 73 could undergo a reversible 2-electron reduction was useful in the development of a hypothesis of a mechanism of the transport effect. ... 

A two terminal molecular scale device 83 that is similar to compound 70 has been synthesized according to Scheme 3.38 although this bears a,co-alligator clips. 

Currently, these molecular systems are studies as SAMs on a metal surface. An additional method of preparing ordered monolayers of molecular devices is the use of Langmuir-Blodgett (LB) films. Therefore, a compound with hydrophilic and hydrophobic subunits with the central nitroaniline core similar to 70 was synthesized as in Scheme 3.40. n-Hexylbenzene was easily brominated on neutral alumina and coupled to TMSA followed by silyl removal and coupling to the nitroacetanilide core intermediate, 68, to afford 88. The methyl ester, intermediate 90, was synthesized by the coupling of methyl 4-ethynylbenzoate (89) to 88. The amine was unmasked and the methyl ester was saponified with lithium hydroxide to afford molecular scale device 91. Compound 91 is suitable for the formation of a LB film due to its hydrophilic carboxylic acid end-group and the hydrophobic n-hexyl end-group. 

Recently it has been discovered that the use of an isonitrile as the contact between the organic molecular scale wire and a palladium probe would significantly reduce the conduction barrier, and would allow an increase in the conductivity of the molecular scale wires. Therefore molecular scale devices with an isonitrile attachment moiety was synthesized. ... 

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