Molecular functional unit

Lehn, J. M. Supramolecular chemistry - Molecules, supermolecules, and molecular functional units. Angew. Chem. Int. Ed. Engl. 1988, 27, 89-112. 

Can the present-day electronics based on sihcon and related inorganic semiconductor materials, in particular micro- to nanoelectronics, be extended in the not-too-distant future to include electronics with molecules which makes use of molecular functional units and applies their specific molecular properties. Can molecular devices be integrated into circuits based on silicon. The fascinating aspect of such ideas is a conceivable further miniaturisation beyond that possible with inorganic semiconductors, combined with the enormous variability which is offered by organic chemistry, and perhaps also the greater ease of development and fabrication of devices based on such materials. 

Fig. 12.9 The structure formula of a molecular functional unit which can act as a molecular rectifier . A is an electron acceptor group, D a donor group. See also D. Haarer,...

The monolayer technique is extremely significant and has numerous applications. With a system such as a monolayer, we have the realization of a simple molecular functional unit. In other words, it is a system whose properties are determined by an orderty arrangement of molecules, i.e., the molecules form a team and the stem behaves differently from an independoit molecule. Such functional systems are often encovmtered in biological structures, for example, the proteins interact with lipid, polysaccharides and with other interfaciaUy-boimd compoimds. In fact, the whole living body is a functional unit where varied molecules interact and cooperate with each other. 

If the proportion of /-functional units is small (p< l), the condensation presumably will be carried relatively close to completion in order to achieve a substantial average molecular weight. Then p will be near unity and it is permissible, consequently, to take Eqs. 

Figure 9. Variations of uranium (a) and thorium (b) contents in the filtrates of a sample as a function of the filtration size, and relation with the variations of the dissolved organic carbon (DOC). Data sources (1) Viers et al. (1997), (2) Dupre et al. (1999), (3) Porcelh et al. (1997, 2001), (4) Riotte et al. (2003). Filtrates are recovered by tangential ultra-filtration. Low filtration sizes are usually given in Dalton—a molecular weight unit of 1 g/mol—and are ranging between 3 and 300 kD. These filtration sizes have been converted here into an approximate rm pore size.

Nanochemistry has not yet advanced to a level where it can meet crucial technological require ments. The above examples, however, convincingly support the view of molecules as potential basic functional units. For a long time, so-called molecular electronics has been looked at as remote since... 

To the best of our knowledge, only one other example of a carboxylic acid functionalized hyperbranched structure is known in the literature, and this concerns a polyamide [19]. The synthesis reported starts from A2 (aminofunctional) and B3 (carboxylic acid functional) units and leads to low molecular weight products due to low conversion in dilute solution. These conditions were mandatory to prevent gelation [20]. Two different approaches to the synthesis of carboxylic acid functional hyperbranched polyesteramides are presented below [21]. 

The core-first approach is based on the initiation of polymerization by a multifunctional initiator. The number of arms is then defined by the number of functional units present on the core. In order to have a good control of the molecular structure of star-shaped polyesters, the initiation must be quantitative and fast. It is also mandatory to avoid possible side-reactions between the initiating species on the core. 

The molecular dynamics unit provides a good example with which to outline the basic approach. One of the most powerful applications of modem computational methods arises from their usefulness in visualizing dynamic molecular processes. Small molecules, solutions, and, more importantly, macromolecules are not static entities. A protein crystal structure or a model of a DNA helix actually provides relatively little information and insight into function as function is an intrinsically dynamic property. In this unit students are led through the basics of a molecular dynamics calculation, the implementation of methods integrating Newton s equations, the visualization of atomic motion controlled by potential energy functions or molecular force fields and onto the modeling and visualization of more complex systems. 

In general terms, these results indicate that, as the structural complexity increases, the overall properties of the system cannot be easily rationalized solely on the basis of the type and sequence of the functional units incorporated in the molecular framework—that is, its primary structure. Higher level conformational effects, which are reminiscent of those related to the secondary and tertiary structures of biomolecules,4d have to be taken into consideration. The comprehension of these effects constitutes a stimulating scientific problem and a necessary step for the design of novel artificial molecular devices and machines. 

Note that many of these surface reactions involve the conversion of a hydrophophic polymer to one with a hydrophilic surface or vice versa. For example, the replacement of trifluoroethoxy groups at the interface by hydroxyl units changes a non-adhesive, highly hydrophobic surface to an adhesive hydrophilic one. Variations in the reaction conditions allow both the depth of transformation and the ratios of the initial to the new surface groups to be controlled. A possible complication that needs to be kept in mind for all of these surface transformations is that polymer molecular motions may bury the newly introduced functional units if the polymer comes into contact with certain media. For example, a hydrophilic surface on a hydrophobic polymer may become buried when that surface is exposed to dry air or a hydrophobic liquid. But this process can be reversed by exposure to a hydrophilic liquid. 

The following sections are devoted primarily to the introduction of two different types of functional units. A distinction is made between dendrimers with bi-functionalised molecular periphery (Fig. 3.7 Types A, B, C, D) and those in which one function is located in the core and the other in the branching units or in the periphery (Types E, F). Multifunctional dendrimers of type G with different functional units in the core, scaffold, and periphery have so far played only a minor role and will therefore only be treated briefly here, particularly since compounds of this type will be considered in greater depth in Chapter 6. 

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