Thallium(III) Trifluoroacetate−Trifluoroacetic Acid in the Chemistry of Polythiophenes. 2. Treatment of 3-Alkylthiophenes and Electron Paramagnetic Resonance Results

Abstract

The treatment of thiophene and 3-alkylthiophenes with thallium(III) trifluoroacetate (TTFA) in trifluoroacetic acid (TFA) gives insoluble and dark powdery solids with oxygen content and electrical conductivities ranging from 10-4 to 10-6 Ω-1 cm-1. Polar and short fractions are negligible. All of them show semiconductivity (10-3−10-6 Ω-1 cm-1) when doped in iodine atmosphere. Electron paramagnetic resonance (EPR) spectra of either as-synthesized or I2-treated solids display characteristic single and broad lines (ΔHpp, 1.84−7.4 G) with Lorentzian shapes and g-values in the range 2.0028−2.0038. Infrared spectra show characteristic C−H out-of-plane deformations (780 cm-1 for polythiophene and 820−825 cm-1 for poly(3-alkylthiophenes)) in addition to a strong peak at 1650−1690 cm-1 which has not been conclusively assigned. EPR spectra of some disubstituted and tetrasubstituted 2,2‘-bithiophene radical cations have been observed and their g-values and coupling constants assigned when the corresponding parent compounds are photolyzed with ultraviolet light in TFA. Photolysis of 3-alkylthiophenes in TFA in the EPR instrument gave the radical cations of 4,4‘-dialkyl-2,2‘-bithiophenes. In no case, were EPR signals of the isomeric 3,3‘-dialkyl- or 3,4‘-dialkyl-2,2‘-bithiophene radical cations observed, indicating that dimerization of 3-alkylthiophenes occurs through the less sterically hindered 5-position. The presence of two doublet species corresponding to both conformers, syn and anti, in the radical cations is associated with a large barrier to rotation about the C(2)−C(2‘) bond.

Radical Ions from 3,3′′′,3′′′″‐Tris(butylsulfanyl)‐2,2′:5′,2″:5″,2′′′,5′′′,2′′′′:5′′′′,2′′′″‐sexithiophene: An Experimental and Theoretical Study of the p‐ and n‐Doped Oligomer

“…Stable radical cations are usually generated from oligothiophenes when the reactive terminal a positions are blocked through the insertion of a variety of substituents in order to prevent further oligomerisation. [1±8] One of the most popular methods for the generation of the radical cations of small thiophene systems consists in the addition of trifluoroacetic acid (TFA) to a CH 2 Cl 2 (or CHCl 3 ) solution of the substrate in the presence of Hg(II ) or Tl(III ) trifluoroacetate. [3,4,9,10] As a consequence of the lower oxidation potential with respect to that of thiophene, radical cations of longer oligothiophenes can be easily generated by TFA also in the absence of metal ions.…”

“…[1±8] One of the most popular methods for the generation of the radical cations of small thiophene systems consists in the addition of trifluoroacetic acid (TFA) to a CH 2 Cl 2 (or CHCl 3 ) solution of the substrate in the presence of Hg(II ) or Tl(III ) trifluoroacetate. [3,4,9,10] As a consequence of the lower oxidation potential with respect to that of thiophene, radical cations of longer oligothiophenes can be easily generated by TFA also in the absence of metal ions. [3,11] Radical anions and dianions of alkyl-end-capped bithiophenes are instead unstable, [9] unless the alkyl groups are replaced by trimethylsilyl moieties.…”

“…[3,4,9,10] As a consequence of the lower oxidation potential with respect to that of thiophene, radical cations of longer oligothiophenes can be easily generated by TFA also in the absence of metal ions. [3,11] Radical anions and dianions of alkyl-end-capped bithiophenes are instead unstable, [9] unless the alkyl groups are replaced by trimethylsilyl moieties. [12] Negative ions are more easily accessible in the presence of cyano and [13] nitro groups [14] or in the case of longer end-capped oligothiophenes.…”

Thallium(III) in Organic Synthesis

“…Eberson and co-workers, [205][206][207][208][209][210][211] as well as Davies and coworkers [212][213][214][215][216] have used 4-tolyl thallium(III)-bis(trifluoroacetate) and TTFA for the generation of stable radical cations. Juliá and co-workers have also studied the behaviour of the system TTFA/TFA for the generation of radical cations from 2,2'-bithienyl 217,218 and 2,2'-bifuranyl derivatives, 219 as well as for polymers preparation. 218,220 In a study dealing with the chemistry of push-pull substituted stilbenes, their radical cations were generated by using TTFA in CH 2 Cl 2 .…”

“…Juliá and co-workers have also studied the behaviour of the system TTFA/TFA for the generation of radical cations from 2,2'-bithienyl 217,218 and 2,2'-bifuranyl derivatives, 219 as well as for polymers preparation. 218,220 In a study dealing with the chemistry of push-pull substituted stilbenes, their radical cations were generated by using TTFA in CH 2 Cl 2 . 221 Radical cations can also be generated using TTA instead of TTFA.…”