Polythiophenes

Polythiophenes (including oligothiophenes) are one of the most studied and important classes of linear conjugated polymers [444,445], Versatile synthetic approaches to PTs (chemical [446] and electrochemical [447]), easy functionalization and unique, widely tunable electronic properties have been the source of tremendous interest in this class of polymers. 

SCHEME 2.59 Synthesis of polythiophene via metal-catalyzed couplings. 

Besides the oligomers based on thiophene units, polymers with similar backbones have become very represented in literature since the late nineties. Due to the fact that pure thiophene polymer chains without any substitutes show very poor solubility, mainly alkyl substituted polythiophenes are used. 

Polymers of thiophene carboxylic acids (9) undergo different chromatic transitions upon the addition of a variety of cations (McCullough et al. 1997). Although the acid form of the polymer was not water soluble, addition of various ammonium or metal 

PT disassembled by larsie metal (fetecfron, disordered and not conjugated COLOR-YELLOW color tunable-yellow, orange, red 

Seff-assemtded PT tSsassembling by large metal detection -very r rSy losing con/ugat/on 

PT in metat-driven self-assembled state, after detection of small metal, very highly amjugated COLOR-PURPLE 

Polythiophenes functionalized with monosaccharides have been evaluated for their ability to detect the influenza virus and E. coli (Baek et al. 2000). Copolymers of thiophene acetic acid 10 and carbohydrate-modified thiophenes 11 have been prepared via iron(III) chloride mediated polymerization. Addition of influenza virus to a sialic acid containing copolymer resulted in a blue shift of the polymer absorption maximum, resulting in an orange to red chromatic transition. Mannose-containing polythiophenes underwent color changes upon the addition of the lectin ConA or E. coli cells that contain cell surface mannose-binding receptors. A similar biotinylated pol5hhiophene afforded a streptavidin responsive material (Paid and Leclerc 1996). 

Tuning of the EL was also obtained by preparing copolymers of 3-alkyIthiophene and unsubstituted thiophenes in different molar ratios [149]. 

Heeger and co-workers [157] found that high EL efficiencies can be reached by dispersing surfactant-like additives in the active layer. They reported that a device prepared with surfactant-like compounds in poly(3-octylthiophene) gives, with an aluminium electrode, 0.03% external efficiency, much higher than the corresponding device without additives but with calcium as the cathode. The procedure reported in [157] has been successfully applied to other electroluminescent polymers by the same authors. 

As in the case of polymers of the phenyl family, a great amount of work has been done concerning the introduction of electron/hole transporter molecules in the main conjugated backbone and by modifying the architecture of the devices [164-167]. Introduction of 

Alq3 between the PAT thin film and the cathode increases the EL efficiency up to 0.05%, using aluminium as the cathode [167]. It was also observed that a certain tuning of the EL by applying different electric fields may be reached [168]. 

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Conductivities of polymers of technological interest such as polypyrrole and polythiophene are typically 1000 cm in the doped state, and the conductivity can be tuned by reversibly doping and undoping the polymer. Derivatives of these and other polymers have achieved even higher conductivities. 

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Other polymers ia this category iaclude CJ-conjugated polygermylenes (20) and TT-conjugated poly acetylene, polythiophene, and poly(p-phenylenevinylene). The photoconductivity of many TT-conjugated polymers can be enhanced by dopiag with fuUerenes (21). 

AppHcations of polythiophenes being considered utilize either the electrical properties of the doped conducting state with either anionic or cationic... 

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. 

In all cases of electrochemicaHy or chemically polymerized unsubstituted polypyrrole, the final polymer is intractable in both the conducting and insulating forms. In contrast, a broad number of substituted polythiophenes have been found to be processible both from solution and in the melt. The most studied of these systems ate the poly(3-alkylthiophenes) (P3AT). 

Polythiophenes with substituents other than alkyl groups at the 3 position have been prepared by the polymerization of substituted monomers. Many of these polymers have been substituted alkylthiophenes (8) where example side chains are (R =) —(86—89), —OCH (68), —NHC(0) (CH2) qCH (6 )) —0502(0112)30112 (90). Ohiral side chains have also been employed (91,92). Poly(3-alkoxythiophenes) (9) (93—95) and... 

Tetrathia- and tetraselenafulvalenes, polythiophenes and related compounds, polypyridines as organic electronic conductors 97YGK410. 

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

Polymers, large molecules made up of smaller molecules in a repeating pattern, are used for many electrochromic materials. Conjugating polymers, which have alternating single and double bonds, are particularly suitable. Figure B shows the electrochemical oxidation of the conjugated polymer, polythiophene. Oxidation (in which electrons are removed) produces a semiconductive polymer. The neutral (unoxidized) polythiophene is red in color, whereas the semiconductive polythiophene (oxidized) is blue. In their neutral... 

Different color transitions are obtained by simply changing the molecular structure of the polythiophene and then oxidizing or reducing it. Figure C. p. 93. shows the different colors obtained by taking the neutral form... 

Neutral polythiophene (left) is red in color and becomes blue and semiconductive when electrons are removed. The process can be reversed by adding electrons to the blue polymer to make it transition back to red. 

Polysaccharide, 617,619 Polythiophene, 93 Polyunsaturated fats, 604 Polyvinyl chloride (PVC), 612-613 Positive integer, 643... 

In the following sections, results from various photoelectron spectroscopy studies of poly(p-pheny enevinylenc), polythiophene, and polyaniline, and their interaction with different metals will be discussed. The intention is not to cover the whole existing literature, but still give a relatively extensive overview, and, where appropriate, give references for further reading. 

Polythiophene [78] is a promising material for certain future electronic applications, due to its relatively high stability and processability in the substituted form [79-81]. Upon substitution, with e.g. alkyl side-chains [79, 80], polythiophene exhibit properties such as solvalochromism [82] and thermochromism [83]. Presently, a large variety of substituted polythiophenes with various band gaps exists (for example see Ref. [81 ]). 

T. Granlund. M. Theander, M. Beiggren, M. Andersson, A. Ruzcckas, V. Sundstrom, G. Bjork, M. Granstrom, O. lnganas, A polythiophene microcavity lasct Chem. Phys. Leu. 1998, 288, 879. 

The polaron is characterized by the reversal of bond alternation, which, in the case of polythiophene, extends over five monomer units [30-32], and the appearance of two localized stales in the band gap ). These states have been indeed observed by UV-V1S absorption of both oligomers and polymers, in solution [33— 40] and in the solid state [41-45]. 

Several attempts to use otganic polymeric semiconductors as the active component in photovoltaic devices have been reported during the last two decades. Interest in the photovoltaic properties of conjugated polymers like polyacelylcne, various derivatives of polythiophenes and poly(para-phenylene vinylene)s arose from... 

The photovoltaic properties of PPV and PPV based soluble polymers have been quantitatively confirmed also for polythiophenes. The IN characteristics of ITO/ P30T/Au [60] and of ITO/P3HT/Au [61] diodes showed excellent rectification behavior and a high photosensitivity under reversed bias. 

C. Taliani, W. Gebaucr in Handbook of Oligo- and Polythiophenes (D. Fiehou Ed.), Wiley-VCH, Weinheiin 1999, 361. 

Analysis of the spin-1/2 2-PADMR spectrum is more complicated as it contains two features with increased and decreased transmission. As the photoexcitation density is proportional to -AT, we describe these as PA-quenching (Sn<0) and PA-enhancing (Sn >0) bands. We defer detailed discussion of the spectrum to the following section, where we compare the doping, PA, and PADMR studies of a-sexithiophene (u-6T) and polythiophene. Magnetic resonance is found to enhance... 

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