Conjugated polymers description

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 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. 

Fig. 16 (a) Description of the detection of amyloid fibrils in proteins with an anionic conjugated polymer, PTAA. (b) Emission spectra (bottom) of PTAA-Native bovine insulin (filled square) and PTAA-amyloid fibrillar bovine insulin (x). (c) Kinetics of insulin amyloid fibril formation monitored by PTAA fluorescence [29]... 

Although conjugated polymers can be both n-doped and p-doped - and thus, in principle, be capable of behaving either as negative or as positive electrodes - the majority of applications have been confined to the p-doping, positive side. Conductive polymers have been proposed and tested in a variety of advanced electrochemical devices. Due to lack of space, we will confine our attention to the description of the most illustrative examples which are rechargeable lithium batteries and multi-chromic optical displays. 

Below, the one-dimensional band structure of simple linear conjugated polymers is outlined, followed by a brief description of the use of some model 7t-conjugated molecules for conjugated polymers. Then, to close out the chapter, the description of the properties of conjugated polymers is extended to issues of optical absorption and photo-luminescence. 

A full quantum mechanical description of the third order non-linearity requires forty-eight terms, which are more complex than those describing second order effects, see for example Boyd (2003). The simple model of molecular non-linearity considered above (Fig. 3.22) indicates that the delocalised 7r-electron systems of conjugated polymers will have large third order non-linearity. Other factors governing the third order non-linearity of conjugated polymers will be discussed in Chapter 9, Section 9.4.2. 

The initial application of quantum mechanics to the electronic states of a perfect linear conjugated chain, as in the Htickel model discussed in Section 4.2.5 and above, led to a model of a one-dimensional semiconductor with well-defined valence and conduction bands. This labelling of the electronic states is widespread in the literature. On the other hand, when electron correlation is included, the electronic states are more localised and an exciton description is more appropriate. The disorder present in all but a few exceptional cases inevitably leads to the conclusion that the electronic states must be localised by chain defects. The extent to which the electronic states of conjugated polymers are localised, i.e. deviate from the band model, has been a matter of debate. There is a growing body of experimental and theoretical evidence, discussed in Sections 9.4.2 and 9.4.3 below, that suggests that the exciton description is closer to the truth. 

The most important drawback in the electronic description of conjugated polymers is the occurrence of structural defects such as sp -hybridized carbon (inters in an extended 7t-chain [58, 59]. This leads to an interruption of 7i-conjugation which may determine the effective conjugation length within the polymer. The scientific literature is full of examples in which one does not pay proper attention to a complete structural elucidation and to the synthesis of defect-free, homogeneous polymeric materials instead, the consideration of conjugated jt-systems is often restricted to idealized, oversimplified or even partially decomposed structures. 

Section 3.3 clearly documents the limitations of a molecular description of electrical conductivity. Nevertheless, the molecular and supramolecular architecture of defect-free oligomers provides an easier access to a description of a much more complex situation in polymers. The conclusion to be drawn is that the intrinsic conductivity of conjugated polymers will be even higher than reached up to now, if synthetic procedures succeed in avoiding structural defects (see Sect. 9). 

Although this very elementary description of conjugated polymers in terms of energy band theory explains how conjugation can lead to semiconducting and metallic properties, it is incomplete for three important reasons ... 

We Initiate here the microscopic description of PDA within the ir-electron framework of Parlser-Parr-Pople (PPP) theory. Quite aside from the crystallinity and Interesting nonlinear optical properties of PDAs, we are convinced that related it-electron descriptions should apply to PA, PDA, and other conjugated polymers. Furthermore, the nature of the low-lying excited states of polymers Is a prerequisite for understanding their relaxation and dynamics. In sharp contrast to ir-electron models, a more realistic treatment of triple bonds leads to Important and previously overlooked Coulomb correlations. We focus below on the novel aspects of excitations In ene-yne systems. 

---