EPR STUDY OF BITHIOPHENE, TERTHIOPHENE AND QUATERTHIOPHENE RADICAL CATIONS

Radical cations of a series of and-methyl-substituted bi-, ter- and qu aterthiophenes were investigated by EPR and, in part, ENDOR spectrosco py. 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 quan tum-mechanical calculations (MNDO, PM3, RHF-INDO/SP). Sterically non-h indered oligothiophene radical cations are planar and form mixtures of cis and trans conformers, whereas uniform species were observed in th e case of sterically hindered, non-planar radical cations.