Dipolar disorder

Another class of structural disorder can occur when the crystal structure allows the molecule to assume two different orientations within the lattice with a similar or even identical lattice energy. A nice example is 2,3 dimethylnaphthalene (DMN), 

In some weak tt-tt donor-acceptor crystals, the acceptor lattice is ordered, while the donor has an additional degree of freedom. An example is the anthracene-tetracyano-benzene crystal (cf Fig. 1.6). Here, the donor has two equivalent sites, 

A complete analysis of the nature, number and distribution of all the impurities in a crystal is difficult and not always possible. In practice, one considers only those impurities which are important for a particular investigation, and the detection limit is extended only as far as is necessary and practical with the given detection method. 

The first method is visual examination. For example, in the zone refining of aromatic hydrocarbons, one often observes a brownish discolouration at the end of the zone refining tube, where impurities coUecL Using gas-phase chromatography, when possible combined with a mass spectrometer, the chemical nature of these impurities can often be identified. 

Particularly important are the spectroscopic techniques. Absorption and emission spectroscopy or, in the case of paramagnetic impurities, also ESR spectroscopy in many cases permit the detection and identification of impurities down to very low concentrations. In Sect. 6.9.2, we show that for example using the method of sensitised fluorescence, a detection limit of better than molecules/molecule can be attained. 

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Borsenberger PM, Bassler H (1991) Concerning the role of dipolar disorder on charge transport in molecularly doped polymers. J Chem Phys 95 5327... 

The key parameter of the disorder formalism is the width of the hopping site manifold. There is considerable evidence that a is dependent on the dipole moment of the dopant molecule, as well as the polymer repeat unit, and/or polar additives. This has been described by an argument based on dipolar disorder, originally due to Borsenberger and Bassler (1991). Stated simply, the argument... 

For other treatments of dipolar disorder, see Sugiuchi and Nishizawa (1993), Richert and Loring (1995), Dunlap et al. (1996), Dunlap (1996), Gartstein and Conwcll (1996). and Parris (1996). 

The increase in the width of the DOS with increasing dipole moment was described by a model based on dipolar disorder. The model is premised on the... 

Figure 7 shows the concentration dependencies of a. The width of the DOS was described by the dipolar disorder model and gave van der Waals components of 0.104 eV, independent of the TPM-E concentration. These results differ from the aiylamine doped polymers where the van der Waals component increases with increasing dilution. These results are discussed in Section III. From a plot of log(ju0/p > - Y 14.59 versus p, the wavefunction decay constant was determined as 1.2 A. 

The key result of the study of Borsenberger and Weiss concerns the concentration dependence of the width of the DOS. The results agree with earlier work of Borsenberger (1992) and more recent work of Borsenberger et al., 1996, 1997), Gruenbaum et al., (1996), and Magin et al. (1996). The results were described by the dipolar disorder model. The dipolar disorder model is based on the assumption that the total width a is comprised of a dipolar component Gj and a van der Waals component aV(. Combining Eqs. (1) and (2) yields... 

The width of the DOS was described by a dipolar disorder argument. From the value of 0.060 eV for Gvdw derived by Borsenberger and Fitzgerald (1993), the dipolar component was calculated from Eq. (1). These values were compared to the expression of Dieckmann et al. (1993)... 

The concentration dependence of a was described by the dipolar disorder model. Due to the low dipole moments of ENA-B and the PS repeat unit, Sinicropi et al. argued that the dipolar components can be neglected and the total width of the DOS is determined only by the van der Waals component. The results yield a van der Waals components of 0.079 to 0.090 eV that increase as the ENA-B concentration decreases. The low values of the van der Waals component and the high prefactor mobilities are the reasons for the high mobilities of ENA-B doped PS. The degree of positional disorder and the wave-... 

The dependence of the width of the DOS on the polymer was described by the dipolar disorder model, as described above by Sinicropi et al. (1996). The... 

Yuh and Pai argued that the role of the polymer was related to the activation energy. Borsenberger and Bassler explained their results on a model based on dipolar disorder. According to the model, a is determined by the dipole moment of both the dopant molecule and the polymer repeat unit. The effect of the polymer host is then related to the difference in dipole moments of the dopant molecule and the polymer repeat unit as well as the dopant concentration. Most recent studies have been described by dipolar disorder arguments. 

This differs from the expression for T>TC in that the constant in the exponent is 1/2, rather than 2/3. Describing the T derived from the T >TC data. Borsenberger et al. described the width of the DOS by the dipolar disorder model and concluded that the width was largely determined by the dipolar component. 

Within the framework of current transport theories, the task of the chemist is to prepare donor molecules in which the molecule has the appropriate oxidation potential, orbital delocalization, and solubility. Further, the effects of dipolar disorder and intermolecular dimer sites must be minimized. Finally, the physical and chemical interactions of the transport and generation materials (whether in a single or dual-layer configuration) must promote efficient charge generation and injection. The systematic integration of all of these characteristics is indeed formidable. 

Thus far, electron transport layers have not been used in commercial applications. Positively charged photoreceptors have been either single-layer or have the generation layer uppermost. Because of the influence of dipolar disorder on mobilities, acceptor molecules must have a combination of a low reduction potential, high solubility, and low dipole moment. This combination has thus far proven elusive. 

Since dipolar disorder (or, empirically, the presence of polar eomponents) is a major factor governing the mobility, it is worthwhile to make several additional comments on the subject. 

However, in most experimental systems, the manifestations of the polaronic character of the charge carriers are masked by the effects of disorder. In any solution-deposited thin him, disorder is present and causes the energy of a polaronic charge carrier on a particular site to vary across the polymer network. Variations of the local conformation of the polymer backbone, presence of chemical impurities or structural defects of the polymer backbone, or dipolar disorder due to random orientation of polar groups of the polymer semiconductor or the gate dielectric result in a signihcant broadening of the electronic density of states. 

Lithium compounds are under investigation for a number of additional medical uses. Salzman has reviewed 21 additional potential uses of lithium in psychiatry that have been reported in the literature. Lithium has been shown to inhibit herpes, pox and adenoviruses in vitro and lithium succinate ointment is useful in treating herpes simplex infections. Interestingly, patients with dipolar disorder who are treated with oral lithium carbonate exhibit a reduction in the frequency of labial herpes recurrence. Lithium reduces brain damage in animal models of certain neurodegenerative diseases and has been shown to inhibit expression of apoptotic proteins, p53 and Bax, which leads to the possibility that it may be useful in treating neurodegenerative diseases. ... 

For the formulation of the lattice dynamics in a molecular crystal, only six coupled equations of motion are necessary three for translation and three for rotation of a single molecule, if all the molecules in the unit cell are connected by symmetry operations. (This assumption does not hold e.g. for dipolar disorder in dimethyl-naphthalene or dimethyl-anthracene crystals (see Sect. 5.7).) The translational-rotational displacements u(lk) of the kth molecule in the Ith unit cell are taken to be given by propagating plane waves u(lfe) = U(fk)e F Here, S2(K)... 

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