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

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 λ_max 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 E_max 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 λ_max of the UV spectra and a decreased stability of the radical cation formed upon electrooxidation, manifesting itself by a higher oxidation potential.