A Convenient Synthesis of 2,5-Thienylene Oligomers; Some of Their Spectroscopic and Electrochemical Properties

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Abstract
A convenient synthesis of a number of oligomeric thiophcnes and oligomeric methyl-, and ethyl-substituted thiophenes as building blocks for novel conducting polymers is reported. The synthesis is based on a NiCl2(dppp) (dppp = Ph2PCH2CH2CH2;PPh2) catalyzed coupling reaction between appropriately substituted bromothiophenes and Grignard compounds derived of them. The λmax of the UV spectra of the unsubstituted oligomeric thiophenes increases, as expected, with the number of thiophene units in the oligomers. Also, as expected, the oxidation potential Emax of these compounds decreases with increasing the number of thiophene units. In the series of the methyl- or ethyl-substituted oligomeric thiophenes such a clear cut relationship is not observed. This behavior is discussed on the basis of diminishing coplanarity due to steric hindrances by the alkyl groups in the 3,3′-positions, which results in a blue shift of the λmax of the UV spectra and a decreased stability of the radical cation formed upon electrooxidation, manifesting itself by a higher oxidation potential.