Building molecules molecular engineering

Toute la nature est i mes ordres, nous repondit la Durand, et elle sera tou-jours aux volont6s de ceux qui F6tudieront avec la chimie et la physique on parvient h tout. 

All of nature is at my command, replied Madame Durand, and she will always be at the will of those who study her with chemistry and physics one can achieve anything. 

The usual and most rational route towards the synthesis of new complex molecules is by coupling of smaller molecules. However, we are not going to explore how all the required molecules are obtained, but proceed much in the way chemists would do in the laboratory, that is by starting from available products. A few examples will make this point clear. 

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The observation of SHG from a molecule deprived of vectorial features, opens new perspectives in molecular engineering towards quadratic NLO properties. Non-dipolar non-centrosymmetric molecular moieties could serve as building block for novel types of NLO materials in which the organization is not influenced by dipole-dipole interactions. 

The Langmuir-Blodgett (LB) films are solid organized media, where the studies of the chemical reactivity lead to a better comprehension of molecular interactions in the solid state, and to a development of a true molecular engineering. The LB technique allows the building of different structures at the molecular level, such as mixed layers and alternating layers. These molecular assemblies appear as sets of molecules, which can interlock, to give rise to specific physico-chenucal properties. 

SAMs are ordered molecular assembHes formed by the adsorption (qv) of an active surfactant on a soHd surface (Fig. 6). This simple process makes SAMs inherently manufacturable and thus technologically attractive for building supedattices and for surface engineering. The order in these two-dimensional systems is produced by a spontaneous chemical synthesis at the interface, as the system approaches equiHbrium. Although the area is not limited to long-chain molecules (112), SAMs of functionalized long-chain hydrocarbons are most frequently used as building blocks of supermolecular stmctures. 

Strong intermolecular interactions between active SCO mononuclear building blocks stem from the presence of efficient hydrogen-bonding networks or 7i-7i stacking interactions and have led to abrupt spin transitions [1], sometimes with associated hysteresis [2-4]. Despite the important efforts made by crystal engineers in establishing reliable connections between molecular and supramolecular structures on the basis of intermolecular interactions, the control of such forces is, however, difficult and becomes even more complicated when uncoordinated counter-ions and/or solvent molecules are present in the crystal lattice. 

The paramount advantage of molecular solids over their more classical inorganic counterparts is that their constituents, the building blocks, are molecules or clusters that can be designed and synthesized in other words they can be intentionally modified. Therefore, we can talk about molecular and crystal engineering and the goal is to be able to produce materials with predetermined physical properties. We are not yet at this desired level but the scientific and technical bases are certainly at hand. 

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