Polymer molecule, electronic states

Chemical alternation of the surface layer and deposition of a new layer on top of the silicone mbber can be achieved by physical techniques. For the inert surface of silicone rubber, the former requires the generation of high-energy species, such as radicals, ions, or molecules in excited electronic states. In the latter case, coatings of atoms or atomic clusters are deposited on polymer surfaces using technique such as plasma (sputtering and plasma polymerization) or energy-induced sublimation, like thermal or electron beam-induced evaporation. 

Alternatively, the electron, or the polymer anion, may react with an existing cation radical producing an excited state of the polymer molecule, P. For example. 

Structural information at the molecular level can be extracted using a number of experimental techniques which include, but are not restricted to, optical rotation, infra-red and ultra-violet spectroscopy, nuclear magnetic resonance in the solid state and in solution, diffraction using electrons, neutrons or x-rays. Not all of them, however, are capable of yielding structural details to the same desirable extent. By far, experience shows that x-ray fiber diffraction (2), in conjunction with computer model building, is the most powerful tool which enables to establish the spatial arrangement of atoms in polymer molecules. 

The second reason for delocalization of energy losses is the collective nature of excited states. This collectivity may exist even for excited electronic states of a single molecule. The simplest example is the excitation of zr-electron states, which are delocalized along the molecule. When a fast electron excites such a molecule, it transfers its energy to the whole ensemble of tt electrons. As a result, the energy absorption is delocalized along the molecule, and the latter can be a long (e.g., a polymer molecule). 

Because of periodic symmetry, the electronic excitation states in a molecular crystal are also of collective nature. These are the well-studied exciton states.81 82 Their energy is close to that of discrete electronic states of isolated molecules (4-8 eV), but the excitation envelops a large group of molecules, migrating efficiently up to 100 nm along the crystal.82 In the same manner, because of efficient migration, the excitation of a fragment of a polymer chain rapidly spreads over the whole molecule.37... 

Thus, the excitation of discrete electronic states (AEn < /,) in polymer molecules and molecular crystals is certainly delocalized. 

The change that converts the polyacetylene molecule from a nonconductive to a conductive state involves the addition of some foreign material, a dopant, to the polymer. Two kinds of dopants are used those that attract electrons and remove them from the bonds that make up a polymer molecule, and those that donate electrons to the molecule. In either case, the normal electronic structure of the molecule is disrupted, and individual electrons within the molecule become more mobile. As their mobility increases, they tend to flow through a molecule and from one molecule to the next when an external electrical potential is applied to the polymer. 

The extraordinarily strong chiral properties of [nfhelicenes provide an impetus for the development of synthetic approaches to nonracemic [nfhelicenes for applications as organic materials. From this point of view, asymmetric syntheses of functionalized long [n]helicenes (n > 7), and also [n]helicene-like molecules and polymers with novel electronic structures and material properties, are important. The properties of helicenes related to materials are relatively unexplored, compared with the more synthetically accessible n-conjugated molecules and polymers. Notably, redox states of helicenes are practically unknown [33, 34]. Assembly of helicenes on surfaces, their uses as liquid crystals, chiral sensors, ligands or additives for asymmetric synthesis and helicene-biomolecule interactions are in the exploratory stages [35-43],... 

Lindsay, G. A. and Singer, K. D. (eds) (1995). Polymers for Second-Order Nonlinear Optics. ACS Symposium Series. American Chemical Society, Washington, DC Liptay, W. (1969). Angew. Chem. 81, 195 Angew. Chem. Int. Ed. Engl. 8, 177 Liptay, W. (1974). Excited States, Vol. 1. Dipole Moments and Polarizabilities of Molecules in Excited Electronic States (ed. E. C. Lim). Academic Press, New York, p. 129... 

There are a number of different theoretical approaches to the calculation of the band structures of polymers. These are extensions of the methods employed to calculate the electronic states of molecules, which obtain molecular orbitals from a linear combination of atomic orbitals. In this case the states in question are those of an infinitely long molecule, which is approximated by a finite length system with cyclic boundary conditions, i.e. the right-hand end of the chain is, in effect, joined to the left-hand end of the chain. This is the method used for band... 

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