Thiophene oxidation potential

Pyrroles, furans and thiophenes undergo photoinduced alkylation with diarylalkenes provided that the alkene and the heteroaromatic compound have similar oxidation potentials, indicating that alkylation can occur by a non-ionic mechanism (Scheme 20) (81JA5570). 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

Fig. 8. Relationship between oxidation potential values of substituted monomeric thiophenes and their corresponding polymers...

Another ligand including a thiophene moiety but lacking the C2-symmetry and thus bearing electronically different phosphorus atoms was prepared by these authors, in 2001. The electrochemical oxidative potential was obtained by cyclic voltammetry. The oxidation potential of the phosphine group located on the phenyl ring was found to be 0.74 V (vs. Ag/Ag" ) and the authors attributed a value of 0.91 V to the phosphine attached to the thiophene moiety. This second functionality is a rather electron-poor phosphine. As shown... 

Ag+/Ag, in the range of the oxidation potential of 3-methylthiophene, (14.) due to the irreversible oxidation of the monomeric thiophene unit. Figure 1 shows the typical cyclic voltammetry upon repeatedly scanning the potential of a Pt electrode between 0.0 V and 1.5 V vs. Ag+/Ag in a solution of 0.2 M 1. 

If an increasing number of thiophene rings are inserted into the TPD structure, as in the BMA-rcT series 13a-d, the oxidation potential does not change with... 

Bipolar Molecular Glasses. Recently, bipolar molecular glasses have been described that allow both injection of holes and electrons (Fig. 3.30). 2- 4-[bis(4-methylphenyl)amino]phenyl -5-(dimesitylboryl)thiophene (PhAMB-lT, 68) and 2- 4-[bis(9,9-dimethylfluorenyl)amino]phenyl -5-(dimesitylboryl)thiophene (F1AMB-1T, 69) show oxidation potentials of 0.62 and 0.58 V, and reduction potentials of —2.13 and —2.01 V vs. Ag/0.01 Ag+, respectively [145]. Oxidation as well as reduction leads to stable radical ions. With the conversion rules given above, the HOMO and LUMO levels can be estimated to be approximately at —5.3 and —2.8 eV. In comparison, for the bipolar compound 70, consisting of triarylamine and oxadiazole moieties, the values are —5.5 and — 2.7eV [129]. However, in this case no data on the stability of the radical ions are available. 

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. 

The electrochemical polymerization of thiophene is apparently rather similar to that of pyrrole and studies have been reported by Tourillon and Gamier133) and by Kaneto et al.134,135). Early studies are reviewed by Tourillon136). The oxidation potential of the monomer is significantly higher (1.6 V v SCE) than that of pyrrole and it might be expected that the more reactive cations would lead to greater structural irregularity in the polymer, which appears to be the case. 

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... 

Because their oxidation potentials are similar, substituted pyrroles can be copolymerized with pyrrole, allowing the limiting conductivity of the fully-doped polymer to be varied 194,19S). The oxidation potentials of the monomers, and hence their reactivity ratios, are sensitive to the substituent196). Inganas et al.197) reported the synthesis of pyrrole-thiophene copolymers starting from terthiophene, whose oxidation potential is similar to that of pyrrole. Sundaresan et al.198) copolymerized pyrrole with 3-(pyrrol-l-yl)propanesulphonate to give a polymer in which the sulphonate counter-ion is a part of the polymer structure. 

An x-ray crystallographic analysis of bcnzo[J,2,3- / 4,5,6-cV ]bis(thieno)[2,3-c]thiophene (61) demonstrated that the molecule is planar and symmetrical but has strained bond angles. The crystal structure comprises herring-bone type column stacking with intercolumnar heteroatom interactions. Compound (61) showed the same oxidation potential as perilene and, like perilene, formed an iodine complex with the relatively high electroconductivity of 0.11 S cm-1 <93BCJ2033>. 

Three-ring organosulfur compounds have an ionization potential (IP) that ranges around 8 eV, which is lower than that of methanol (10.85 eV), so that electron abstraction from an organosulfur compound should be favored over abstraction from methanol when an organosulfur compound is present. Thiophene and benzo[3]thiophene have an oxidation potential in excess of -I-1.8V versus NHE and are not oxidized prior to the solvent in the mixture. 

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. 

---