System , conjugated

In contrast to the two main groups described above the charge carrier transport in conjugated systems occurs via the polymer backbone. Various substituents are used to tailor the mechanical properties and the processibility. For instance, the chain can be a skeleton with quasi-(T-conjugation like in polysilanes [36, 38-42] or polygermylenes [39]. 

The second group of main chain polymers contains a 7i-conjugated backbone like polyacetylene, polythiophene, polypyrrole, polyphenylene, poly(phenylenevinylene) (PPV), and their derivatives. For a review see [43, 44]. 

We investigated the charge transport properties of the rc-conjugated polymer poly (para-phenylenevinylene) and its soluble substituted derivative DPOP-PPV (Fig. 1.2). In 

In the following we will show that the charge carrier transport in these materials can be described by the conventional models mentioned before, which have been developed for the characterization of disordered molecular systems. 

Based on the MT model a method has been developed to calculate the distribution of capture rates from the measured photocurrent [68, 69]. In contrast to Ref [34], the method is based on a Fourier transform tecbnique with better numerical stability as compared to the inverse Laplace transform used in Ref [34]. 

Molecular Structure Understanding Steric and Electronic Effects from Molecular Mechanics, 

In the early days of quantum mechanics, before computers, it was practically impossible to deal in any meaningful way with systems as large and complicated as butadiene and benzene. So the physicists and others investigating the quantum mechanics of such molecules followed what to chemists would have been the logical path and divided the molecule into a and n components. Ethylene contains a total of 16 electrons and 6 nuclei, while benzene contains 42 electrons and 12 nuclei. The former, in terms of the number of particles, represented a really sizable problan to hand calculation, whereas the latter represented an almost impossible problan, unless we were fortunate enough to have symmetry cancel out most of the work. That actually happens in the particular case of benzene but not in compounds as simple as substituted benzenes. But, if we look only at the Ji electrons, ethylene contains two, and benzene contains six, and both of these are reasonable problems to deal with for hand calculations. 

The idea of o-n separation though is quite useful because it permits what would otherwise be a very complicated problem to be reduced in essentials to a relatively simple problem. And even having computers available for the heavy work, a great deal of understanding, and ideas that prove to be important for molecular mechanics, can 

In general, then, the use of transferable constants as parameters in molecular mechanics is a first approximation. Often the constants are not really constants (although they may appear to be when one is looking at a limited set of structmes), but rather they are functions of variables, the nature of which depend on the case at hand. Thus, the simple logic behind molecular mechanics remains intact, but the details are somewhat more complicated than originally envisioned, if one wishes to exploit the inherent accuracy and generality of the method. 

The standard way of treating conjugated systems using the o-n separation approximation in the 1930s and into the 1960s was the so-called Hiickel methody In this method, the electron-electron interactions are not explicitly considered. Rather, the positions of the nuclei are fixed, and the electrons move in the field of the nuclei. Much of the error that results from neglecting the interactions of the electrons with one another can be circumvented with proper adjustment of empirical parameters. These parameters are also adjusted to allow (approximately) for the interaction from the o system, which is thus taken into account without specific calculations. This method is crude but does often give qualitative results that enable rationalization of many chemical phenomena of interest. It was a powerful and useful tool in its time. A better approximation is the Hartree-Fock or self-consistent field (SCF) method in which the electron-electron interactions are explicitly considered (Self-Consistent Field Method in Chapter 3). The quantum mechanical calculations on the n system in this case are carried out in a 

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Salem L 1966 Molecular Orbital Theory of Conjugated Systems (Reading, MA Benjamin)... 

Soret band of the macrocycle, a ti to tt transition, is excited instead. It has been found that the vinyl groups do not participate in the conjugated system [4]. This is based on the fact that the vinyl C=C stretch does not... 

The treatment of conjugated systems in terms of electron systems that extend smoothly over all atoms allows the treatment of a variety of structural phenomena, as may be explained with a spedes that shows hindered rotation and with the nitro group. 

Then the Huckel matrix for the conjugated i -system is constructed. The a-values of the Huckel matrix of each atom i of the conjugated system are adjusted to the <7-chaige distribution by Eq. (13). 

The initial values, a, , are derived by correlations with dipole moments of a series of conjugated systems. The exchange integrals are taken from Abraham and Hudson [38] and are considered as being independent of charge. The r-charges are then calculated from the orbital coefficients, c,j, of the HMO theory according to Eq. (14). 

The quality of the r-charge values thus obtained has been demonstrated by the calculation of dipole moments of a series of 80 conjugated systems [39],... 

The dimension of the matrix is the number of atoms in the n conjugated system. Let us take the three-carbon system allyl as our next step. Concentrate on the end... 

Salem, L., 1966. The Molecular Orbital Theory of Conjugated Systems. Benjamin, New York. Saunders, M., 1987. 7. Amer. Chem. Soc. 109, 3150. 

Tertiary amines capable of eliminating a secondary amine to form a conjugated system can react with hydrogen cyanide to form y-keto nitriles by amine replacement. Thus (I) yields p-benzoylpropionitrile (IV) ... 

The former exhibits absorption tjrpical of an isolated keto group, whereas the latter shows a high intensity -band associated with the conjugated system HO—C=C—C=0. The proportions of the two forms under various conditions are readily determined from the ultraviolet spectra. The ultraviolet spectra in various solvents are shown in Fig. A, 7, 2. Since the absorption of the keto form is negligible, the percentage of enol present is 100(em/e ), where e is the observed extinction at 245 mp. and that of the pure enol. It was shown that in alcoholic solution is 1900 and the percentage of enol is 12. Thus e is ca. 16000, and use of this value permits the approximate evaluation of the enol content in different solvents. The results are collected in Table XII. 

The condensation of aldehydes or ketones with secondary amines leads to "encunines via N-hemiacetals and immonium hydroxides, when the water is removed. In these conjugated systems electron density and nudeophilicity are largely transferred from the nitrogen to the a-carbon atom, and thus enamines are useful electroneutral d -reagents (G.A. Cook, 1969 S.F. Dyke, 1973). A bulky heterocyclic substituent supports regio- and stereoselective reactions. 

The pyridine-like nitrogen of the 2H-pyrrol-2-yiidene unit tends to withdraw electrons from the conjugated system and deactivates it in reactions with electrophiles. The add-catalyzed condensations described above for pyrroles and dipyrromethanes therefore do not occur with dipyrromethenes. Vilsmeier formylation, for example, is only successful with pyrroles and dipyrromethanes but not with dipyrromethenes. 

However, benzylidene derivatives show a strong bathochromic shift in comparison with alkylidene derivatives. Thus absorption is a result of the whole conjugated system that is comparable to that of the quinoid dyes. The color of this type of compound is sensitive to acids and bases. 

Conjugare is a Latin verb meaning to link or yoke together and allylic carbocations allylic free radicals and conjugated dienes are all examples of conjugated systems In this chapter we 11 see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone... 

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates m chemical reactions The third type of conjugated system that we will examine conjugated dienes, consists of stable molecules... 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons He pointed out that cycloheptatrienyl cation also called tropyhum ion contained a completely conjugated closed shell six tt electron sys tern analogous to that of benzene... 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

Huckel s rule is now taken to apply to planar monocyclic completely conjugated systems generally not just to neutral hydrocarbons... 

This electron delocalization stabilizes a conjugated system Under conditions cho sen to bring about their mterconversion the equilibrium between a p 7 unsaturated ketone and an a p unsaturated analog favors the conjugated isomer... 

Carotenoids are natural pigments characterized by a tail to tail linkage between two C20 units and an extended conjugated system of double bonds They are the most widely dis tributed of the substances that give color to our world and occur m flowers fruits plants insects and animals It has been estimated that biosynthesis from acetate produces approximately a hundred million tons of carotenoids per year The most familiar carotenoids are lycopene and (3 carotene pigments found m numerous plants and easily isolable from npe tomatoes and carrots respectively... 

Ultraviolet visible (UV VIS) spectroscopy (Section 13 21) An alytical method based on transitions between electronic en ergy states in molecules Useful in studying conjugated systems such as polyenes... 

It is apparent from the size of the conjugated system here that numerous resonance possibilities exist in this species in both the radical and the molecular form. Styrene also has resonance structures in both forms. On the principle that these effects are larger for radicals than monomers, we conclude that the difference ep. - ej > 0 for both hemin and styrene. On the principle that greater resonance effects result from greater delocalization, we expect the difference to be larger for hemin than for styrene. According to Eq. (7.23), r j oc > 1. According to Eq. (7.24), i2 < 1. 

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