EPR study of Bi‐, Ter‐ and quaterthiophene radical cations

Abstract

Radical cations of a series of a-methyl-substituted bi-, ter-and quaterthiophenes were investigated by EPR and, in part, ENDOR spectroscopy. In the case of terthiophenes, the electronic and steric effects of methyl substituents were studied. The experimentally determined spin density distributions were rationalized by means of semiempirical quantum-mechanical calculations (MNDO, PM3, RHF-INDO/SP). Sterically noo-hindered oligothiop hene radical cations are planar and form mixtures of cis and trans conformers, whereas uniform species were observed in the case of sterically hindered, non-planar radical cations.

Doped Polymers by Oxidative Polymerization. 4. Oxidative Coupling of Methylated Oligothiophenes by FeCl3·6H2O as a Model Reaction for the Oxidative Polymerization of Thiophene Derivatives

“…With regard to the graduated reactivity of the β-positions, we refer to our EPR studies of 5,5‘‘‘-dimethyl-2,2‘;5‘,2‘‘;5‘‘,2‘‘‘-quaterthiophene . Here, we found remarkably higher electron spin densities for the 3- and 3‘‘‘-positions in relation to the 4- and 4‘‘‘-positions.…”

“…If 9 is treated with FeCl 3 ·6H 2 O, radical cations are formed, which, being stable under these conditions, therefore do not form a dimerization product (tetramer of the octathiophene type). It is remarkable that a graduated regioselectivity exists for both types of β-positions following the different electron spin densities, as can be seen by EPR studies 1 …”

“…Here, we found remarkably higher electron spin densities for the 3- and 3‘‘‘-positions in relation to the 4- and 4‘‘‘-positions. Additional methyl groups in the molecule do not essentially change the hyperfine coupling constants; therefore, the same ratios of electron spin density distributions may also be valid for its 3‘,3‘‘,4‘,4‘‘-tetramethyl derivative 9 . Quantum mechanical calculations (PM3) demonstrated the electron spin density of the radical cation of dimer 9 (Figure ).

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“…It is remarkable that a graduated regioselectivity exists for both types of β-positions following the different electron spin densities, as can be seen by EPR studies. 10 With regard to the graduated reactivity of the β-positions, we refer to our EPR studies of 5,5′′′-dimethyl-2,2′;5′,2′′;5′′,2′′′-quaterthiophene. 10 Here, we found remarkably higher electron spin densities for the 3-and 3′′′-positions in relation to the 4-and 4′′′-positions.…”

The Generation of Organic Radical Cations: A Guide to Current Practice for EPR Spectroscopic Studies

“…70 Phenyliodine(III) bis(trifluoroacetate), PhI(TFA) 2 , is a stronger oxidant with an oxidizing power similar to that of Tl(TFA) 3 , and in HFPA will generate the radical cations from arenes with E o (ArH •+ /ArH) ≤2.1 V (vs Ag/AgCl) . 71,72…”