Conjugated model molecules

Magnetic criteria of aromaticity are based on the model of interatomic teratomic ring currents induced in conjugated cyclic molecules by external magnetic fields. 

The flat model requires the introduction of an energy either of physical adsorption or of chemisorption by the formation of a a bond. The other models require the introduction of the resonance of the 77 electrons. Thus we obtain a conjugated model if the metal atoms are in the plane of the ethylene molecule. We obtain... 

T able 4.1 Some typical model molecules for conjugated polymers ... 

It is one of the standard results of theoretical organic chemistry that benzenoid hydrocarbons (and polycyclic conjugated molecules in general) have no unique classical structural formula. The conjugation models of their 7r-electrons must be visualized by means of several so-called Kekule structures [22, 25, 32, 33], none of which correctly describes the actual situation. An attempt to overcome this century-long problem was proposed by Clar in the 1950 s and eventually elaborated in due detail in his seminal book [18]. 

Electron correlation probably also plays an important role in the theory of unsaturated or conjugated organic molecules. Unfortunately, accurate numerical data are so far lacking, and we must discuss simplified models in order to understand the electron correlation in n electron systems. Although different contributions to electron correlation should be considered, namely the horizontal and the vertical correlation of the o and n systems and the intergroup correlation, so far the correlation of the n system alone has been most studied. As has been mentioned, the horizontal correlation of the ti system can be taken into account within the framework of the a—n separation. 

Before going on to something as complex as an atom, let s look at a model problem in some detail. The first one is the one-dimensional particle-in-a-box problem. This turns out to be an excellent conceptual model for conjugated dye molecules (see Chapter 21) and also a model for trapped charged particles. The problem and its solutions are similar to the vibrating string just discussed. The potential term is shown graphically and mathematically in Fig. 7.1. 

The Oudar-Chemla equation has been tested for many donor-acceptor TT-conjugated organic molecules. The two-state model works quite well in most cases. However, it should not be left unmentioned that Are values of are usually overestimated in comparison to the more advanced quantum chemical calculations [3, 31, 38—40]. Hence, the two-state model should be treated only as a rather rough approximation to the SOS method. On the other hand, for the most applications, the relative values of the first-order hyperpolarizabilies are of the higher importance. The two-state model allows to establish the structure-NLO properties relationship in terms of relatively simple spectroscopic parameters. 

In this paper, we report a theoretical characterisation of the metal/conjugated polymer Interaction involving Al and Ca. The two metals, which are of current Interest in device epplications, are likely to interact differently with the polymer since Ca is divalent (4s ), whereas Al is trivalent (3s 3p ). Benzene and hexatriene are chosen to model the conjugated materials. These model molecules represent fundamental units of important n-conjugated hydrocarbon polymers used in device preparation polyacetylene, poly(p-phenylene), and poly(p-phenylene-vinylene) ... 

Table 1. Structural data for the optimized model molecules and some important conjugated polymers. All units in (Aj.

Experimental studies of the electronic structure in conjugated molecules during metallization with Al and Ca has recently been performed by means of photoelectron spectroscopy chemically pure (oxygen free) thin films of a model molecule for t ra Ji s-polyacetylene shown in Figure 6, a,ai-diphenyltetradecaheptaene (hereafter denoted DPT), were prepared under ultra-high vacuum conditions. The evolution of the valence leotronlo levels in this molecule could than be monitored in situ by ultraviolet photoelectron spectroscopy (OPS) upon vapour deposition, basically in monolayer steps of Al or Ca. 

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