Electronic properties of polythiophenes

Having these situations as a background, we describe in this chapter the electronic properties of the polythiophenes. Special attention is directed to (i) the structure and conformation (section 2.2) and (ii) various electronic processes (section 2.3) of these materials. The latter section mainly deals with the electronic properties relevant to the charged states. 

When we deal with the real polymer systems, disorder effects have to be taken into consideration. This is because in the (quasi) one-dimensional system the disorder tends to make the electronic states localized in cooperation with the electron-lattice coupling [12]. If the disorder is severe, the charges will be transported via hopping among the localized states accompanied by disorder potential as in the case of classical amorphous or non-crystalline media. This will be discussed in sections 2.3.2 and 2.3.3 in relation to the charge transport and recombination in the materials. 

---

Mardalen, J. et al.. Structure and electronic-properties of polythiophene and poly(3-alkylthiophene) films electropolymerized on indium tin oxide (Ito) glass at elevated potentials, Macwmol. Chem. Phys. 194, 2483-2495, 1993. 

Barbarella, G., L. Favaretto, G. Sotgiu, M. Zambianchi, C. Arbizzani, A. Bongini, and M. Mas-tragostino. 1999. Controlling the electronic properties of polythiophene through the insertion of nonaromatic thienyl S,S-dioxide units. Chem Mater 11 2533-2541. 

Figure 4,2 CVs at 50 mV s in PC-0.2 M TEABF4 of (a) poly(TOT), (b) poly(TTOTT) and (c) poly(TTTOTTT). The Coulombic efficiencies of the p- and n-doping process from the Wth CV are (a) 98, 96%, (b) 98, 100% and (c) 96, 99 %, respectively. Adapted with permission from Figure 4 of C. Barbarella, L Favaretto, G. Sotgiu, M. Zambianchi, C. Arbizzani, /, Bongini and M. Mastragostino, Controlling the electronic properties of polythiophene through the insertion of nonaromatic thienyl S,S-dioxide units, Chem. Mater., 11, 2533-2541 (1999). Copyright 1999 American Chemical Society...

Michael Bendikov and Sanjio Zade, in Chapter 8, introduce the reader to the tools of quanmm chemical calculation, allowing the rationalization and prediction of the structural and electronic properties of polythiophenes. 

Conversely, due to the defined structure of a-oligothiophenes it was quickly realized that they constitute interesting models of the electronic properties of polythiophene [32]. The first spectral characterization of oligothiophene radical cations and dications in 1989 [32c, 33] and the use of a-sexithiophene 6 as active component in an a//-organic electronic device one year later [34] have triggered a renaissance of interest and intensive research work in the synthesis and characterization of these materials that had been neglected for so many years [35]. 

The development of polythiophenes since the early 1980s has been extensive. Processible conducting polymers are available and monomer derivathation has extended the range of electronic and electrochemical properties associated with such materials. Problem areas include the need for improved conductivity by monomer manipulation, involving more extensive research using stmcture—activity relationships, and improved synthetic methods for monomers and polymers alike, which are needed to bring the attractive properties of polythiophenes to fmition on the commercial scale. 

Related Polymer Systems and Synthetic Methods. Figure 12A shows a hypothetical synthesis of poly (p-phenylene methide) (PPM) from polybenzyl by redox-induced elimination. In principle, it should be possible to accomplish this experimentally under similar chemical and electrochemical redox conditions as those used here for the related polythiophenes. The electronic properties of PPM have recently been theoretically calculated by Boudreaux et al (16), including bandgap (1.17 eV) bandwidth (0.44 eV) ionization potential (4.2 eV) electron affinity (3.03 eV) oxidation potential (-0.20 vs SCE) reduction potential (-1.37 eV vs SCE). PPM has recently been synthesized and doped to a semiconductor (24). 

Optical Properties of Polythiophenes Electronic Band Structure and UV-Visible Spectra... 

In all eases, Al atoms arc found to react with the conjugated systems by forming new Al-carbon covalent bonds. As a consequence, the carbon atoms involved in the bonding adopt an p hybridization. The n electron conjugation is dramatically perturbed and the corresponding wavcfunction delocalization, which constitutes the basis for the electronic properties of these materials, is significantly reduced in polytmcs or even disrupted in polythiophenes. ... 

Aromatic heterocyclic polymers such as polypyrrole and polythiophene have low ionization potentials and low electron affinities [1, 2]. An early theoretical study on the electronic properties of these polymers indicated that electron affinity is mainly affected by substitution on the heteroatom whereas ionization potential is mainly affected by substitution on the backbone [9], Later theoretical studies on polythiophene indicated that chemical transformation of thienyl sulfur to the corresponding thienyl-5-oxide or thienyl-5,5-dioxide would deeply affect the electronic properties of the polymer as a result of the increase in both electron affinity (EA) and ionization potential (IP) [10, 11],... 

Efforts towards the preparation of porphyrin-functionalized polythiophenes have centered on complexes 38-41, in which thiophenes are tethered to porphyrin cores (Chart 5.9) [50], The syntheses employed either porphyrin formation using MacDonald-type conditions (38-40) or Wittig coupling to produce 41. No electronic properties of these materials have yet been reported. 

Because of the ease of chemical modification of the thiophene rings, especially at the a- and fl-positions, these materials acquire good solution-processability and mechanical drawability [10]. This versatility allowed the wide investigation of their structure-property relationships. For example, spectroscopic studies related to thermochromism and solvatochromism [11] revealed that the electronic properties of the polythiophenes are strongly coupled to the backbone conformation. Moreover, polarized IR studies of drawn polythiophene films showed that the charged carrier motion is highly confined along the backbone [12]. 

B. M. Medina, A. Van Vooren, P. Brocorens, J. Gierschner, M. Shkunov, M. Heeney, I. McCulloch, R. Lazzaroni and J. CornU, Electronic structure and charge-transport properties of polythiophene chains containing thienothiophene units a joint experimental and theoretical study. Chem. Mater., 19, 4949-4956 (2007). 

These are regarded as copolymers of thiophene and acetylene. Thus, they should have properties of polythiophene and polyacetylene, for example high conductivity of polyacetylene as well as stability, tunability of the electronic structure and processability of polythiophene. 

Having the above research history as a background, in this chapter we compare the polythiophenes and oligothiophenes from several aspects. The stmctund and spectroscopic features of the materials will be emphasized. These features are closely connected with the electronic properties of the materials and, at the same time, depend considerably on their synthesis and preparation procedures. Several specific topics for tiiis are dealt with by other authors in further detail in subsequent chapters. 

Another remarkable example of conjugated polymer exhibiting a supramolecular structure deeply affecting the physical and electronic properties of the material is provided by the regiorandom and regioregular polythiophene derivatives. 

H. Isotalo, H. Stubb, and J. Saarilahti, Ion implantation of polythiophene, in Electronic Properties of Conjugated Polymers (H. Kuzmany, M. Mehring, and S. Roth, eds.), Springer-Verlag, Berlin, 1987, p. 285. 

---