Bithiophene oxidation potential

For example, the rate of oligomerization and polymerization increases when 2,6-di-tert-butylpyridine is added to a solution of bithiophene. However, in the case of monomeric thiophene, the high oxidation potential of the starting species between (>1.6 V versus Ag/AgCl) prevents any formation of conducting polymeric material. 

An alternative way to get better structural regularity might be to begin with bithiophene or terthiophene where some of the inter-ring bonds in the polymer are formed before the polymerization and these monomers have lower oxidation potentials (thiophene 1.6 V, bithiophene 1.2 V and terthiophene 1.0 V v SCE). Polymerizations of both bithiophene 143) and terthiophene 144) have been described but there is some doubt about whether the polymers derived from oligomers are more regular or... 

The majority of the synthetic studies towards the construction of novel photodiodes of fullerene-based double-cable polymers concern either electropolymerization of suitable aromatic monomeric units or copolymerization of two monomeric units, one carrying the fullerene moiety and one designed to improve solubility. Most of the electropolymerized conjugated polymeric materials bearing fullerenes that have been obtained consist of bithiophene units with low oxidation potential that favor electropolymerization [162-164]. On the other hand, the preparation of some photodiodes based on novel chemically synthesized double-cable polymers was recently reported and studied using their photophysical properties [165-170]. For example, the structures of some conju-gated-backbone hybrids covalently linked with organofullerene moieties are shown in Scheme 8. 

Another mechanistic possibility is the attack of the thiophene cation radical (420) upon a neutral thiophene monomer (419) to form a cation-radical dimer (421) [247]. The oxidation and loss of two protons leads to formation of the neutral dimer (422). Once again, rapid oxidation of the dimer occurs upon its formation due to its close proximity to the electrode surface and its lower oxidation potential. The cation-radical dimer (423) which is formed then reacts with another monomer molecule in a similar series of steps to produce the trimer 425. A kinetic study of the electrochemical polymerization of thiophene and 3-alkylthiophenes led to the proposal of this mechanism (Fig. 61) [247]. The rate-determining step in this series of reactions is the oxidation of the thiophene monomer. The reaction is first order in monomer concentration. The addition of small amounts of 2,2 -bithiophene or 2,2 5, 2"-terthiophene to the reaction resulted in a significant increase in the rate of polymerization and in a lowering of the applied potential necessary for the polymerization reaction. In this case the reaction was 0.5 order in the concentration of the additive. 

In order to co-polymerize the IC unit, Suzuki and Stille polymerizations have been used. First, Blouin et al. [94] were able to obtain polyindolo[3,2-fr]-carbazole derivatives with bithiophene or biEDOT as co-monomers. Unfortunately, these studies demonstrated a relatively low oxidation potential for these polymers (especially for P35 and P37), limiting their applications in OFETs and PCs. However, for doped state applications, these polymers may exhibit interesting properties [35]. For instance, when copolymerized with bithiophene, the resulting copolymer shows a good electrical conductivity (as high as 0.7 Scm 1) but a low Seebeck coefficient of 4.3 iV K 1 [35]. Finally, the UV-Vis absorption maxima are similar for poly(2,8-indolocarbazole-a/f-bithiophene) and poly(2,8-indolocarbazole-a/f-bis(3,4-ethylenedioxythiophene)). A broad absorption band is centered at 430 nm whereas, for the 3- and 9-substituted copolymers, the broad band is centered around 490-500 nm [94],... 

A range of alkoxy groups11 5,6, 7 has also been added to the bithiophene starting material to reduce the oxidation potential even further. Attachment of the alkyl group 7 also facilitates the solubility of the polymer in nonpolar organic solvents. 

Other workers have shown that polymerization of functional thiophenes from aqueous media is possible if surfactants are used to help solubilize the monomer.18 19 20 21 In the case of SDS being used as a solubilizing agent for bithiophene,18 it was also found to lower the oxidation potential and inhibit the dissolution of the... 

Alkoxy-substituted bithiophenes have also been successfully polymerized, using Cu(C104)2 as oxidant.50 As with related FeCl3 oxidations of alkyl-substituted bithiophenes,51 these dimeric substrates are easier to oxidize than the corresponding substituted thiophene monomers, due to their lower oxidation potentials. 

For the electrosynthesis of PEDOT and PPy, water was the preferred electrolyte solvent. Although some reports suggest that under special experimental conditions the aqueous electropolymerization of poly(thiophene) derivatives is possible, all attempts conducted in this study failed, including deposition at low pH values and using bithiophene which exhibits a lower oxidation potential than thiophene as the monomeric species [43, 44], Typically, boron trifluoride diethyl etherate and (fairly) anhydrous acetonitrile were used instead [34]. Although both solvents were successfully used to prepare poly(thiophene) inverse opals via the templated synthesis using poly(styrene) microsphere arrays, they tended to destroy the styrenic DG-structured scaffold [2 ]. 

We have recently shown that 3,6-dimethoxythieno[3,2-h]thiophene 21 leads to a polymer presenting low oxidation potential and moderate bandgap (1.7 eV) [74]. The advantage of thienothiophene unit compared to bithiophene one resides in the planar structure and absence of positional isomers. Furthermore, the crystallographic structure of the dimer 22 (Figure 13.1), shows a hilly planar conjugated system stabilized by noncovalent intramolecular sulhir-oxygen interactions. 

Another example 77 of this class of precursors with bithiophene groups has been recently described [ 184]. Electropolymerization in the presence of Li, Na, or Ba shows that the nature of the cation strongly affects the polymerization process. The electrochemical and spectroelectrochemical properties of the resulting polymers show that the presence of Ba in the electrosynthesis medium leads to 300 mV decrease of the oxidation potential of the polymer together with a significant redshift of the absorption maximum providing conclusive evidences for a metal template effect during electropolymerization [183]. 

Swager and coworkers synthesized receptor polymers 102 and 103 [209,210]. The polymers were prepared by copolymerization of 2,5-dibromo-3-decylthiophene or 3,3 -bis(methoxyethoxy)-2,2 -bithiophene with the organozinc derivative of the macrocyclic 3,3 -dialkoxy-2,2 -bithiophene by palladium-catalyzed cross-coupling. Cyclic voltammetry and in situ conductivity measurements on polymer films have evidenced the complexation of paraquat derivatives. However, the complexation between polymer films and acceptors can lead to opposite shifts of oxidation potential of the polymer due to the interplay of donor-acceptor interaction and conformational changes of the polymer. 

To solve these problems, we developed the synthesis of symmetrically disubstituted bipyridine ligands 150, which possess two electropolymerizable bithiophenic groups fixed at an internal (3-position of thiophene by an alkylsuUknyl or an alkoxy spacer [161]. The analysis of the electropolymerization of these compounds shows that the association of low oxidation potential polymerizable groups and two-site precursors allows us to synthesize stable functional polymers. 

In 1995, Swager et al. synthesized the first calixarene-coupled diiodinated bithiophene, which afforded copolymer 2.166 (Chart 1.36) by Stille-type cross-coupling with distannylated 3,3 -bis(methoxyethoxy) bithiophene [263]. Selective recognition of Na+ ions was studied by UV-Vis spectroscopy and cyclic voltammetry. After addition of 0.5 mMNa+, cyclic voltammetric measurements showed a positive shift of the oxidation potential of about -1-100 mV with a simultaneous dramatic decrease in conductivity. This finding was attributed to an electrostatic effect of the Na+ ions and reduced electron-donating ability of the sodium bound oxygen atoms of the calixarene. 

Furthermore, Bauerle and Emge prepared uracil-substimted end-capped terthiophene 2.188 and bithiophene 2.189 (Scheme 1.24) [283, 284]. Compound 2.188 was prepared by reaction of 5-bromopentyl-substituted terthiophene with orotic acid in 38% yield. Bithiophene derivative 2.189 was synthesized by conversion of 3-(5-bromopentyl)-2,2 -bithiophene to the nitrile and carboxylic acid followed by reaction with 6-(chloromethyl)uracil in 36% yield. Molecular recognition properties of 2.188 and 2.189 with complementary acetyl-9-octyladenine and 2,4-diacetamido-6-pentoxypyrimidine were smdied by cyclic voltammetry in CH2CI2 solutions. A positive shift of the oxidation potential (AE = 20-130 mV) was observed upon addition of complementary nucleobases to the electrolyte. 

Shi et al. have developed another method for the electrochemical polymerization of high oxidation potential monomers in boron fluoride ethyl ether (BFEE) which could yield highly conducting PT films (Scheme 9.4) [32]. As observed in the case of the electropolymerization of 3-methylthiophene, bithiophene 2T and terthiophene 3T, such improvement stems from the lower oxidation potentials at which the electropolymerization occurs in BFEE compared with those required in common electrolytes. Recent development of this strategy by the Reynolds group has shown that thiophene, 3-methylthiophene, 3-bromothiophene and 3,4-dibromothiophene can be polymerized in BFEE to yield homogeneous, electroactive polymer films, where their electrochemical polymerization in common electrochemical solvents has proved much more difficult [33],... 

Following this approach, crown-annelated bithiophenes 9 have been recently synthesized and their anodic oxidation has led to the efficient formation of polymers that exhibit affinity with lithium cation. The oxidation potential of the polymer decreases upon addition of lithium cations, suggesting a planarization of the chain [58]. 

Different strategies have been pursued to prepare electrogenerated functional PTs [13, 63, 71]. One of the most efficient approaches consisted in the introduction of a functional group at one internal position of a 2,2 -bithiophene structure 13 (Scheme 9.14). Compared with -substituted thiophene monomers 12, the decrease of the oxidation potentials of bithiophene derivatives 13 allows for the introduction of functional... 

High monomer concentrations (0.1 M or more) are generally used in order to avoid competitive reactions of the radical cations or reactions of the oxidized polymer with nucleophiles present in the medium however, the choice of monomer concentration depends mostly on the oxidation potential of the monomer. If the monomer is oxidized at low potential, the competition of side reactions decreases and even millimolar concentrations may be successfully used, as in the case of bithiophenes. 

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