Amorphous molecular solid

So far we have outlined the conceptual framework in which we discuss charge transfer in organic semiconductors. It is based on a molecular picture where the molecular unit is considered central, with interactions between molecular units added afterwards. For amorphous molecular solids and for molecular crystals this approach is undisputed. In the case of semiconducting polymers, a conceptually different view has been proposed that starts from a one-dimensional (ID) semiconductor band picture, and that is generally known as the Su-Schrieffer-Heeger (SSH) model [21-24]. 

A novel topological strategy has been examined for designing amorphous molecular solids suitable for optoelectronic applications. In this approach, chromophores were attached to a tetrahedral point of convergence. For instance, stilbenoid units were covalently linked to a tetraphenylmethane core by means of a palladium-catalyzed Suzuki coupling reaction [143]. The optical properties of these compounds were examined. 

Figure 30. Mobility, g, plotted linearly in In, as a function of electric field strength (E), plotted as for holes in tri-/ -tolylamine (TTA) (40 wt.%) in bisphenol-A polycarbonate. The range of field strengths is approximately 10 -10 V cm (1-100 V pm ). The mobility depends exponentially on With increasing temperature, the overall magnitude of/r increases while the dependence on E weakens. These dependences are observed in nearly all amorphous molecular solids. (Reprinted with permission from Ref. [73r].)...

Before presenting some additional experimental observations, we pause to discuss, qualitatively, how the foregoing trends and dependences may be understood. (A quantitative theory of charge transport in amorphous molecular solids will be discussed later.) In any charge transport process, whether the carriers are electronic or ionic, and whether the mediiun is crystalline, amorphous, or fluid, each individual carrier undergoes a random walk in which there is a preference (large or small, de-... 

Figure 38. Schematic of an amorphous molecular solid with both energetic and geometrical disorder. Each randomly oriented hopping site (CTM) is represented by a disk with the number of concentric rulings representing the energy of a charge carrier. (Site A is relatively shallow site B is very deep.) Electric field direction is E. See the text for a detailed explanation.

A simple model, the Gaussian Disorder Model of Bassler and co-workers, has been very useful in rationalizing charge transport data on many amorphous molecular solids [59]. Its present version consists of the following assumptions. 

Equations (7) and (8) have been used to fit the data on many amorphous molecular solids. They reproduce the temperature dependence of the mobility that is often observed, both at zero [Eq. (5)] and nonzero field strength [60a]. The slope of a plot of p 0, T) vs. T (see Figures 31, 35) can be used to evaluate a. Equation (8) also predicts a T dependence for the slope parameter, viz. S T) = [C((t/ b) ]7 -I- constant (see Eq. (4)), and this dependence is consistent with the experimental data. For the example shown in Figure 32, the slope of a plot of 5(T) vs. 2" can be combined with the value of a evaluated from the temperature dependence of p 0, T) to evaluate the proportionality constant C. The result in that case is 2.6 x 10 cm / in remarkable agreement with the predicted value of... 

In this section, we touch briefly on several additional aspects of charge transport in amorphous molecular solids. 

When charge transport fails to reach a steady state during the time available, the most likely reason is that the transit time is dominated by the time required to escape from the slowest site(s) that a carrier encounters as it crosses the sample. Furthermore, the distribution of release times is such that the carrier continues to encounter slower and slower sites as it crosses a sample. Transport under such conditions is called dispersive, and has been the subject of much study since a seminal paper by Scher and Montroll [73a-e]. The term dispersive alludes to the wide dispersion in release times and/or the fact that carriers that are injected simultaneously spread out, disperse, to an anomalous extent as they cross the sample. The literature has several examples of studies of this subject in amorphous molecular solids [66b, 73f-h]. Some materials undergo a transition from essentially dispersive transport at low temperatures to essentially nondispersive transport at higher temperatures, and this dispersive-to-nondispersive transition has been the subject of significant attention [73i-p]. 

Solids can be crystalline, molecular crystals, or amorphous. Molecular crystals are ordered solids with individual molecules still identihable in the crystal. There is some disparity in chemical research. This is because experimental molecular geometries most often come from the X-ray dilfraction of crystalline compounds, whereas the most well-developed computational techniques are for modeling gas-phase compounds. Meanwhile, the information many chemists are most worried about is the solution-phase behavior of a compound. 

Solids can be characterized as amorphous if their particles are randomly arranged or crystalline if their particles are ordered. Crystalline solids can be further characterized as ionic solids if their particles are ions, molecular solids if their particles are molecules, covalent network solids if they consist of a covalently bonded array of atoms without discrete molecules, or metallic solids if their particles are metal atoms. 

Vasanthavada, M., W. Tong, Y. Joshi, and M. S. Kislalioglu. 2004. Phase behavior of amorphous molecular dispersions I Determination of the degree and mechanism of solid soliRfiffipi Res 21 1598-1606. 

Hancock, B.C., Shamblin, S.L., andZogra, G. (1995). Molecular mobility of amorphous pharmaceutical solids below their glass transition temperaturB rm. Res., 12 799-806. 

Two theoretical approaches for calculating NMR chemical shift of polymers and its application to structural characterization have been described. One is that model molecules such as dimer, trimer, etc., as a local structure of polymer chains, are in the calculation by combining quantum chemistry and statistical mechanics. This approach has been applied to polymer systems in the solution, amorphous and solid states. Another approach is to employ the tight-binding molecular orbital theory to describe the NMR chemical shift and electronic structure of infinite polymer chains with periodic structure. This approach has been applied to polymer systems in the solid state. These approaches have been successfully applied to structural characterization of polymers... 

There can be slightly different forces holding particles together within a solid. Ionic solids, metallic solids, network atomic solids, molecular solids, and amorphous solids each use a different force or combination of forces to hold molecules or atoms together. 

The diaryl or aryl alkyl tellurides are dense yellow oils or crystalline solids, which are easier to handle than the dialkyl tellurides of similar molecular weight. Some of the diaryl derivatives are almost odorless solids. The same comments are valid for the diorganoditellurides 4, which are dark red oils (aliphatic derivatives) and dark red solids (aromatic derivatives). It is recommended that solutions of tellurides or ditellurides should not be kept in contact with air, since an amorphous white solid will form after some time. For some compounds, this reaction with oxygen is very fast. Aliphatic derivatives are more air sensitive than the aromatic ones. In view of this fact, it is recommended to bubble nitrogen into the solutions while a column or thin-layer chromatographic separation is performed. Evaporation of the solvent, however, minimizes the air oxidation. Pure liquids or solids can be handled in air with no need for special precautions, but prolonged exposure to air and to ambient light should be avoided. 

Molecular solids may exhibit either crystalline or amorphous structures, depending on the complexity of the individual molecules comprising the bulk material. As with all solids, the more complex the subunits are, the harder it is for them to organize themselves in a repeatable fashion, resulting in an amorphous structure. Unlike purely ionic solids, molecular compounds may be soluble in either nonpolar or polar solvents, as long as the solvent polarity between solute and solvent is matched ( like dissolves like ). 

S. Cunsolo and G. Signorelli. Some Considerations on Spectra Induced by Intermolecular interactions in molecular solids and amorphous systems A workshop report. In G. Birnbaum (ed.), Phenomena Induced by Intermolecular Interactions, Plenum Press, New York, 1985, pp. 609-611. 

The problems associated with freeze drying of peptides and proteins for therapeutic use have also received calorimetric attention recently - particularly, attempts to understand and interpret the dynamics of amorphous solids. Structural relaxation time is a measure of molecular mobility involved in enthalpy relaxation and thus is a measure of the dynamics of amorphous (glassy) solids. These dynamics are important in interpretation of the physicochemical properties and reactivities of drugs in amorphous formulations. The authors conclude that microcalorimetry may provide data useful for rational development of stable peptide and protein formulations and for control of their processing . 

In amorphous state, solid polymers retain the disorder characteristic for liquids, except that the molecular movement in amorphous solid state is restrained. The movement of one molecule versus the other is absent, and some typical liquid properties such as flow are absent. At low stress, polymers display elastic properties, reverting to a certain extent to the initial shape in a relaxation process. However, they can be irreversibly deformed upon application of appropriate force. The deformation and flow of polymers is very important for practical purposes and is studied by a branch of science known as rheology (see e.g. [1]). The combination of mechanical force and increased temperature are commonly applied for polymer molding for their practical applications. The polymers that can be made to soften and take a desired shape by the application of heat and pressure are known as thermoplasts, and most linear polymers have thermoplastic properties. 

IR and Raman are sensitive to the rotation and vibration of molecules in solid phases (crystalline or x-ray amorphous). Molecular units of similar structure and composition absorb IR radiation in the same energy range, usually independent of the larger structure of the material this property makes IR spectroscopy useful for studying molecules in the interfacial region such as surface hydroxyl groups and As oxoanions on mineral surfaces, and for fingerprinting the local environment of As in aystalline... 

If it is not possible to improve the bioavailability of a substance as desired by the addition of a solubilizing agent, this is frequently because the surface area of the crystals of active ingredient exposed to the solvent is too small. It is therefore necessary to increase the surface area, to accelerate dissolution. The first solid dispersions with antibiotics in povidone were described in the literature in about i960 [49,60]. In solid solutions and dispersions the active substance is embedded in a hydrophilic carrier to improve its bioavailability. The difference between a solid solution and a solid dispersion can be defined in terms of the state of the active substance. In a solid solution, it is present in an amorphous molecular form, while in a solid dispersion it is in the form of crystals that must be as fine as possible. 

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