Thiophene dimers and trimers

The thiophene dimer and trimer have been electrochemically polymerized to grow poly thiophene films at lower potentials [248]. The dimer—2,2 -bithiophene—polymerizes at 1.2 V (versus SCE) to produce polythiophene with conductivities up to I S cm" [249]. The trimer— -terthiophene (2,2 5, 2"-terthienyl)—electrochemically polymerizes at 1.0 V (versus SCE) to yield films with conductivities up to 10" S cm" [201]. A mixture with sulfuric acid or use of tetrabutylammonium tetrafluoroborate as the supporting electrolyte in acetonitrile with the trimer produced no polymer, only gels or powder, respectively [250]. 

There are several examples of the copolymerization of pyrrole with bithiophene [77-80] and pyrrole with a-terthiophene [78,81]. The lowered oxidation potentials of the thiophene dimers and trimers facilitates copolymerization with pyrrole. Nevertheless, early research showed that bithiophene content in polymers of pyrrole/ bithiophene was not linearly related to the bithiophene... 

The free y hydroxy-acetylenic acids in the thiophene series are very unstable and trimerize very readily. The ease of dimerization and trimerization of acetylenic compounds in the presence of a hydroxyl group, conjugated double bond or a second triple bond is very characteristic and was observed with certain acetylenic alcohols and diacetylenic glycols in this series. 

Elmaci and Yurtsever used DPT to study the relationship between thermochromism and internal rotation in poly thiophenes [59b]. For head-to-tail methyl-substituted thiophenes, rotational potential maps of the dimer and trimer were calculated. Using a time-dependent DFT methodology, the UV-Vis absorption spectra of these oligomers were calculated as functions of the torsional angles. Theoretical temperature-dependent UV-Vis spectra were generated from the distribution of torsional angles. The results show that the internal rotation alone is not sufficient to explain the mechanism of thermochromism. 

Thiophene oligomers (dimer and trimer) separated by an insulating CH2-CH2 bridge (13) were electropolymerized [146]. In the latter case the product, resulting from dimerization, displays a lower conductivity than... 

Mixed thiophene-furan dimer and trimers have been synthesized and electropolymerized [104], although the polymer films were not fully characterized. 

Table V. Yields of Dimers and Trimers from Styrene and 2-Vlnyl thiophene (7)...

At 26.7 mbar pressure 40% weight toss main product is 2,4-diphenyl thiophene at least 11 unidentified minor products CO2, H2O, butene, isobutene, dimethyl ketene, styrene, methacrylic acid, succinic-type 5-membered cyclic anhydrides Chlorotrifluoroethylene, styrene, HQ, chloropentafluoropropene, ethene, chloroethene, totuene, a-melhylstyiene, dimer and trimer structures with some unsaturation S1F4 (fiom reaction of HF with glass). Distribution of products varies with polymer composition CO, CO2, propene, isobutene, dimethyl ketene, acrolein, allyl alcohol, toluene, styrene, cl-methylstyrene, ethylbenzene, glycidol, glycidylmethacrylate product distribution depends on copolymer composition... 

Many substituted thiophenes have also been electrochemically polymerised [19,54,399-405] (Table 4) as have thiophene dimers [21,37,55,251,400,406], trimers [21, 83,407], and tetramers [256,406], with the thiophene dimer giving rise to higher quality films than does the monomer [37, 395,408]. Several polycyclic monomers including a thiophene ring have also been polymerised [408-416], as have a series of compounds consisting of two thiophene rings linked by a polyene chain (Fig. 23c). The polymerisation of dithieno-thiophene (Fig. 23d) results in a polymer which shows remarkable similarity to polythiophene in its properties [409,410,414],... 

Starting from 3,6-dimethylthieno[3,2-3]thiophene 71a and 3,6-dimethylselenolo[3,2-/ ]selenophene 71b <1994H(38)143>, their dimers, trimers, and tetramers, in which each thieno[3,2-3]thiophene unit and selenolosele-nophene unit was regularly connected at their a-positions, were satisfactorily synthesized (Scheme 4) <1996X471 >. 

H NMR, NMR, gas chromatography (GC) IR, and GC MS. The solution of S-monomer at room temperature gave a mixture of the SS-dimer, SSS-trimer, and a polymer (Scheme 38). Evidence for formation of an SSSS-tetramer (M 440) was only obtained by GC MS. Similar results were obtained on FVP of (5-ethyl-2-thiophene-yl)methyl benzoate <1997JOC8980>. 

Methods for preparing crown ethers containing thiophene dimers, trimers, and higher analogues is described (5)... 

Besides the possibility of inserting fluorescent substituents in only one monomer unit, such as pyrrole or thiophene, it is noteworthy that suitable fluorescent groups have been appended in dimeric or trimeric monomer systems. Using this approach, Cihaner and Algi have initiated a programme aimed at the design and synthesis of photo- and electroactive polymer materials based on 2,5-di(2-thienyl)pyrroles (SNS). This system consist of thiophene and pyrrole rings interconnected by their a-positions and exhibit intense blue emission upon irradiation. They have reported the synthesis and characterisation of a series of novel fluorescent and electrochromic polymers based on N-substituted... 

The mechanism is quite complex however, a largely accepted mechanism is shown in Scheme 3. Thiophene dimer formation is promoted by an electrochemical oxidation and coupling, followed by the loss of two protons. The neutral dimer is then oxidized and coupled with another unit producing a trimer the mechanism is repeated and a polymer with n thiophene rings is formed [39]. Several studies have... 

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. 

Richardeau et al. investigated thiophene adsorption from liquid solutions containing hydrocarbons over HFAU zeolites in a stirred batch system at room temperature.144 They found that the maximum number of thiophene molecules adsorbed per gram of zeolites is equal to their concentration of acidic sites and considered that the acidic sites are the adsorption sites. They further found that the presence of toluene causes a large decrease in the removal of thiophene, and when the concentration of thiophene is high (27.7 wt%), an acid-catalyzed condensation of thiophene occurs to form dimers, trimers, and tetramers, which remain trapped on the zeolite. They concluded that thiophene removal by adsorption on acidic zeolites could only be carried out from diluted solutions containing no olefinic compounds. 

As in the cases of pyrrole and aniline polymers, both chemical and electrochemical procedures have been employed in the synthesis of polythiophene (PTH) and its derivatives. The thiophene polymers exhibit remarkable stability in air and water [119], The alkyl-substituted derivatives also exhibit a high degree of processability [120,121]. PTH has also been synthesized from bithiophane (the dimer) or terthiophene (the trimer). The resulting poly(2,2 -bithiophene) (PbTH) and polyterthiophene (PtTH) are more ordered than PTH and appear to have the same basic structure as the starting monomer [117,122],... 

The UV spectra of 3,6-dimethylthieno[3,2-Z)]thiophene, 3,6-dimethylselenolo [3,2-Z ]selenophene, their dimers, trimers and tetramers (96T471), 2,2,5,5-tetra-methylthieno[3,2-Z ]thiophene-3,6-dione (68TCA247, 77CB1421), and derivatives of benzo[ )]thieno[3,2-Z ]benzo[ )]thiophene 121 (80ZOR425) and compounds 300-305 (93JOC5209) were measured and analysed. 

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