Molecular level devices

Concept The concept of (macroscopic) device can be extended to the molecular level. A molecular-level device can be defined as an assembly of a discrete number of molecular components designed to achieve a... 

Molecular-level devices operate via electronic and/or nuclear rearrangements and, like macroscopic devices, are characterized by (i) the kind of energy input supplied to make them work, (ii) the way in which their operation can be monitored, (in) the possibility to repeat the operation at will (cyclic process), (iv) the time scale needed to complete a cycle, and (v) the performed function. 

Finally, as far as point (v) is concerned, molecular-level devices performing various kinds of functions can be imagined some specific examples will be discussed below. 

In this chapter we will illustrate examples of three families of molecular-level devices (i) devices for the transfer of electrons or electronic energy, (ii) devices capable of performing extensive nuclear motions, often called molecular-level machines, and (Hi) devices whose function implies the occurrence of both electronic and nuclear rearrangements. Most of the examples that will be illustrated refer to devices studied in our laboratories. 

A molecular-level machine is a particular type of molecular-level device in which the component parts can display changes in their relative positions as a result of some external stimulus.13,9 131 Although there are many chemical compounds whose structure and/or shape can be modified by an external stimulus (see, e.g., the photoinduced cis-trans isomerization processes), the term molecular-level machines is only used for systems showing large amplitude movements of molecular components. 

In the two preceding sections, we have illustrated examples of molecular-level devices working on the basis of either electron or nuclear movements. In other devices, which will be described in this section, the function they perform is based on both electronic and nuclear rearrangements, that take place in distinct steps. 

V. Balzani, A. Credi, M. Venturi, Molecular-Level Devices", in Supramolecular Science Where It Is and Where It Is Going (Eds. R. Ungaro, E. Dalcanale), Kluwer, Dordrecht, 1999, pp. 1-22. 

Concept A set of chemical inputs can generate a particular light output from a light-powered molecular-level device. Such output patterns correspond to various members of the logic vocabulary. Besides offering... 

Keywords Luminescence m Fluorescence m Phosphorescence a Sensors a Switches a Logic Gates a Supramolecular Systems a Truth Tables a Photoinduced Electron Transfer a Molecular-Level Devices... 

The concept of dual-mode stimulation can be expanded further. It is possible to devise systems capable of existing in several forms (multistate) that can be intercon-verted by different external stimuli (multifunctional). Such systems can give rise to intricate networks of reactions that, when examined from the viewpoint of molecular-level devices"1311 reveal very interesting properties.1321... 

Synthetic flavylium compounds can exist in several forms (multistate), which can be interconverted by more than one type of external stimulus (multifunctional). The intricate network made up by their reactions, when examined from the viewpoints of molecular level devices and molecular level logic functions , reveals that these systems exhibit very interesting properties. 

Girardeau et al. [53] have described the chain dynamics of PEO within nanotubes of a-cyclodextrin using dueterated PEO (d-PEO) and 2H solid - state NMR spectroscopy. The chain dynamics were explored and compared with the respective unthreaded d-PEO. As these materials are continually proposed for applications in molecular-level devices [54,55] characterization of their molecular dynamics is important since they play a key role in governing bulk physical properties. 

A general approach is provided by the emerging field of heterosupramolecular chemistry, where molecular or supramolecular species are linked to nanoparticles.87 Confinement of molecular-level devices and machines in restricted environments such as those offered by porous materials88 (e.g., zeolites89-91) have also been investigated. 

Interestingly, the bottom-up approach to the construction of molecular level devices and machines was poetically anticipated by Primo Levi in his already-cited book The Monkey s Wrench [55] ... 

Up until now, nobody has succeeded in constructing a chemical system as complex as a microbe or the spore of a mold in recent years, however, a number of very simple molecular-level devices and machines have been built. 

Balzani V, Credi A, Venturi M (2002) The bottom-up approach to molecular-level devices and machines. Chem Eur J 8 55246-55532... 

Gunnlaugsson, T., Donaill, D.A.M., and Parker, D. (2001) Lanthanide macrocyclic quinolyl conjugates as luminescent molecular-level devices. Journal of the American Chemical Society, 123, 12866-12876. 

T. Gunnlaugsson, D. A. M. DonaiU, D. Parker, Lanthanide Macrocyclic Quinolyl Conjugates as Luminescent Molecular-Level Devices , J. Am. Chem. Soc., 123, 12866 (2001)... 

The concept of a device can be extended to the molecular level [1-4]. A molecular-level device can be defined as an assembly of a discrete number of molecular components (i.e., a supramolecular structure) designed to achieve a specific function. Each molecular component performs a single act, while the entire assembly performs a more complex function, which results from the cooperation of the various molecular components (Fig. lb). 

In general, molecular-level devices that perform light-induced functions, that is, in which photons act as an energy supply and/or input/output signals, can be termed photochemical molecular devices (PMDs) [2, 4, 9]. The role of light in reference to the relevant features of molecular-level devices will be discussed in more detail in the following section. 

Molecular-level devices operate via electronic and/or nuclear rearrangements, that is, through some kind of chemical reaction. Like their macroscopic counterpart, they are... 

Figure 1. Schematic representation of the assembly of (a) a macroscopic device and (b) a supramolecular system capable of performing as a molecular-level device.

If a molecular-level device has to work by inputs of chemical energy, it will need addition of fresh reactants ( fuel ) at any step of its working cycle, with the concomitant formation of waste products [11]. Accumulation of such waste products, however, will compromise the cyclic operation of the device unless they are removed from the system, as it happens in our body as well as in macroscopic internal combustion engines. The need to remove waste products introduces noticeable limitations in the design and construction of artificial molecular-level devices based on chemical fuel inputs. In any case, since a molecular device has to work by repeating cycles [point (c)], a fundamental requirement is that any chemical process taking place in the system has to be reversible. 

Chemical fuel, however, is not the only means by which energy can be supplied to operate molecular-level devices. As recalled in the previous section, nature shows that in green plants the energy needed to sustain the machinery of life is supplied by sunlight. Photochemical energy inputs can indeed cause the occurrence of endergonic chemical reactions, which can make a device work without formation of waste products. Currently there is an increasing interest in... 

The operation time scale of molecular-level devices [point (d)] can range from less than picoseconds to seconds, depending on the type of rearrangement (electronic or nuclear) and the nature of the components involved. 

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]. 

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