Disordered conjugated polymers

A frequently asked question is what is the relevance of band structure calculations to real polymers For, however sophisticated the calculation is, it is only performed on an ideal structure. The actual polymer materials in question have much more complex structures with a high density of defects, as described in Chapter 1. Two basic models have been used to try to take real structures into account and to model the consequences of defects on single 

The theoretical analysis could have started from the premise that the electron-electron interaction (electron correlation) is more important than the electron-phonon interaction. The Hubbard Hamiltonian would then be more appropriate than the SSH one. The Hubbard Hamiltonian has the form  

The appearance of one or more CT-excitons below the conduction band of the conjugated chain may not appear to be of major importance for the properties of the material. In fact the consequences of the occurrence of excitons are significant. Slater and Shockley (1936) demonstrated that the descriptions of the system by Bloch functions, i.e. the band model, and by localised excitations, i.e. excitons, were related to one another by a unitary transformation. They were also the first to consider the impact of the 

Whether conjugated polymers are best described by a band model, such as the SSH model, or an exciton model, will depend crucially on the relative strengths of the electron-phonon and electron-electron interactions. After the discovery of highly conductive polymers, the band model was widely accepted and applied to the interpretation of experimental data. Gradually since that time, evidence that suggests that an exciton picture is more appropriate has been accumulating. Comparison of experimental results with the models described above has been used to estimate the relative importance of the two types of interaction. This will be discussed in the following sections. 

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Fig. 21 A schematic view of the hierarchy of charge carrier hopping in a network of disordered conjugated polymer chains. 1 depicts ultra-fast motion within an ordered segment of the chain while 2 and 3 illustrate intra-and interchain hopping processes...

Dykstra TE, Kovalevskij V, Yang X, Scholes GD (2005) Excited state dynamics of a conformationally disordered conjugated polymer a comparison of solutions and film. Chem Phys 318 21... 

Unveiling Electronic Phase Relaxation in a Strongly Disordered Conjugated Polymer... 

Unveiling electronic pluise relaxation in a strongly disordered conjugated polymer... 

PEDOT PSS can be described as a heterogeneously disordered conjugated polymer. The large disorder in PEDOT PSS is a major difference to other conductive polymers such as polyacetylene or polypyrrole. The temperature dependence of conductivity in PEDOT PSS films is discussed in Chapter 9 with a particular focus on the results obtained using the variable range hopping model. There are different conclusions about the dimensionality of the hopping mode.i The electronic states in PEDOT are also discussed in Chapter 9... 

In this contribution, we discussed effects of disorder on the electronic properties of quasi-one-dimensional Peierls systems, like the conjugated polymer fraus-poly-acetylene. Since polymer materials generally are rather disordered and the effect of disorder on any quasi-one-dimensional system is strong, a proper description of these materials requires consideration of such effects. 

The role of disorder in the photophysics of conjugated polymers has been extensively described by the work carried out in Marburg by H. Bassler and coworkers. Based on ultrafast photoluminescence (PL) (15], field-induced luminescence quenching [16J and site-selective PL excitation [17], a model for excited state thermalizalion was proposed, which considers interchain exciton migration within the inhomogenously broadened density of states. We will base part of the interpretation of our results in m-LPPP on this model, which will be discussed in some detail in Sections 8.4 and 8.6. 

The proposed scenario is mainly based on the molecular approach, which considers conjugated polymer films as an ensemble of short (molecular) segments. The main point in the model is that the nature of the electronic state is molecular, i.e. described by localized wavefunctions and discrete energy levels. In spite of the success of this model, in which disorder plays a fundamental role, the description of the basic intrachain properties remains unsatisfactory. The nature of the lowest excited state in m-LPPP is still elusive. Extrinsic dissociation mechanisms (such as charge transfer at accepting impurities) are not clearly distinguished from intrinsic ones, and the question of intrachain versus interchain charge separation is not yet answered. 

In the via precursor method, however, it is difficult to prepare the ji-conjugated polymers with ideally developed -conjugation system the -conjugated polymer chains contain many conformational defects because the jc-conjugated chains are caused to develop from disordered precursor polymer, which form random coil conformation, in solid state. For the preparation of polymers with well-developed jc-conjugation system by the via precursor method, accordingly, it is necessary to introduce orientational and conformational orderliness of the precursor polymers in the films. 

Dobrosavljevic V, Stratt R (1987) Role of conformational disorder in the electronic-structure of conjugated polymers substituted polydiacetylenes. Phys Rev B 35 2781-2794... 

Grozema FC, van Duijnen PT, Berlin YA, Ratner MA, Siebbeles LDA (2002) Intramolecular charge transport along isolated chains of conjugated polymers effect of torsional disorder and polymerization defects. J Phys Chem B 106 7791... 

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