Interchain conjugated chains

The charge transport in a conjugated chain and the interchain hopping is explained in terms of conjugation defects (radical or ionic sites), called solitons and polarons. Several possible conjugation defects are demonstrated in Fig. 5.33 on the example of trans-polyacetylene. 

However, in all these calculations, the conjugated chains bear H atoms only. As bulkier side groups are introduced, the overall interchain interaction decreases and it becomes more anisotropic. If chains form stacks, relatively strong two-dimensional coupling will persist and may still have consequences similar to those of three-dimensional coupling. The question remains open. But substituted CPs are generally even more disordered than the simple ones, and we shall see now that disorder will then have more important consequences than the details of interchain interaction. 

These segregated side groups will either shield conjugated chains from one another, or induce a partial two-dimensional ordering of CP chains (i.e., an incipient lamellar structure). In all cases, interchain interactions will be affected. Substitutions have therefore also been attempted to modulate and control interchain interactions and therefore electrical conductivity [66]. 

The electrical conductivity of a material is a macroscopic solid-state property since even in high molecular-weight polymers there is not just one conjugated chain which spans the distance between two electrodes. Then it is not valid to describe the conductivity by the electronic structure of a single chain only, because intra- and interchain charge transport are important. As with crystalline materials, some basic features of the microscopic charge-transport mechanism can be inferred from conductivity measurements [83]. The specific conductivity a can be measured as the resistance R of a piece of material with length d and cross section F within a closed electrical circuit,... 

The conductivity is a solid-state phenomenon, and, as pointed out already, conductivity is not a single chain phenomenon. The band-gap description of a conjugated chain is a one-dimensional model. Additionally, considerably interchain charge transport is necessary to describe a metal-like behavior in the highly doped three-dimensional sample and further transfer mechanisms across the polymer chains have to be discussed. What one actually needs to know in explaining conductivity of organic polymers is how the charge transport proceeds... 

As noted above, because of the reduced interchain electron transfer interaction for conjugated chains in solution, the electronic structure is expected to be more nearly onedimensional. Thus, the possibility of a Peierls transition with the formation of an energy gap in the excitation spectrum might be anticipated. The susceptibility results shown in Figure 9 suggest that this is indeed the case for polyaniline in solution in sulfuric acid. Such a large decrease in % cannot be accounted for in the context of the half-filled band expected for [B-NH-B-NH-]+n- To reduce X by such a large factor would require an increase in the band width by the same factor i.e. to a band width of more than 200 eV ... 

In conducting carbon films there is a juxtaposition of sp islands and an sp network that resembles the conjugated chain in a polymer (linear sp bond) with sp defects (o- defects as in a-Ge). In the case of high molecular weight and relatively few sp defects, even relatively weak interchain coupling is sufficient to avoid onedimensional localization. This phenomenon leads to coherent transport with a mean free path that is limited by the mean separation between chain imperfections or by phonon scattering. 

Yassar A, Gamier F, Deloffre F, Horowitz G, Ricard L (1994) Crystal structure of a,(0-bis(triisopropylsylyl)-sexithiophene unusual conjugated chain distoition induced by interchain steric effects. Adv Mater 6 660-663... 

The three-dimensional charge transport among localized states with flat density of states at the Fermi level Nds( f) is described by the VRH model. The VRH model can be applied for intrachain jumps over defeas intermpting conjugated chain and for description of interchain jumps. At low temperatures the VRH model predias the temperature dependence of conductivity in the well-known form ... 

Figure 4-6. Evolution of the INDO/SCI-calculalcd. splitting between the lowest two oplieal transitions of cofacial dimers formed by two PPV chains as a function of the inverse number of bonds (1/u) along the conjugated backbone of the oligomer. The theoretical results are reported for interchain distances of 4 A (open circles) and C> A (tilled circles).

The introduction of bulky side chains that contain adamantyl groups to poly(p-phenylenevinylene) (PPV), a semiconducting conjugated polymer, decreases the number of interchain interactions. This action will reduce the aggregation quenching and polymer photoluminescence properties would be improved [93]. 

In a comparative study of disulfide reducing agents, it was determined that use of the relatively strong reductants DTT and TCEP required only 3.25 and 2.75 mole equivalents per mole equivalent of antibody molecule to achieve the reduction of two interchain disulfide bonds between the heavy chains of a monoclonal IgG (Sun et al., 2005). This limited reduction strategy retains intact bispecific antibody molecules while providing discrete sites for conjugation to thiols. 

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