Conjugated polymer molecular structures

Conjugated polymers are generally poor conductors unless they have been doped (oxidized or reduced) to generate mobile charge carriers. This can be explained by the schematic band diagrams shown in Fig. I.23 Polymerization causes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the monomer to split into n and n bands. In solid-state terminology these are the valence and conduction bands, respectively. In the neutral forms shown in Structures 1-4, the valence band is filled, the conduction band is empty, and the band gap (Eg) is typically 2-3 eV.24 There is therefore little intrinsic conductivity. 

An important challenge in the design of novel conjugated polymers is the synthesis of materials with tailor-made solid-state electronic properties. This section outlines the synthesis of the most significant classes of poly(para-phenylenevinylene)s (PPVs), poly(para-phenylene)s (PPPs), and related structures. Furthermore, this review demonstrates that the chromophoric and electronic properties of conjugated rr-systems are sensitive to their molecular and supra-molecular architecture. 

Fig. 5. A schematic band structure and molecular orbital diagram for a conjugated polymer containing no mid-gap states.

Figure 15 Molecular structures of conjugated polymers studied by Birgerson and co-workers [108],...

FIGURE 1.1 Molecular structures of popular conjugated polymers. 

Abstract Conjugated polymers have many unique photophysical properties that make them useful for a variety of applications within the fields of chemistry, molecular biology, and medicine, specifically their ability to produce a conformation-dependant spectral signature reflective of changes in their local environment. This physical property makes conjugated polymers an indispensible tool in the toolbox of fluorescent reporters, and within this chapter, their utilization as molecular probes for studying protein structure and conformation is emphasized. 

Control of alignment of n-conjugated polymers on the substrate is important for excellent performance of the polymer in electronic devices (e.g., higher mobility of carrier in field-effect transistors [134,136]). Details of the molecular structure and molecular assembly of PAEs will be discussed in other chapters. 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

Table 2 shows the present state-of-the-art for the electrical conductivity of doped conjugated polymers. The magnitude of the electrical conductivity in polymers is a complex property determined by many structural aspects of the system. These include main-chain structure and 7T-overlap, molecular... 

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