Synthesis and UV−Visible Properties of Soluble Regioregular Oligo(3-octylthiophenes), Monomer to Hexamer

A series of head to tail alkyl-substituted oligothiophenes was synthesized by a stepwise synthesis based on the repetitive use of the Suzuki coupling reaction between a stable thienyl boronic ester and the appropriate chloride-protected precursors catalyzed by Pd[P(C 6 H 5 ) 3 ] 4, under mild conditions. By this method, a very high level of regiospecificity was achieved, as shown in the synthesis of the dimer 3,4‘-dioctyl-5‘-chloro-2,2‘-bithiophene, where no regioisomers could be detected. From the thus obtained chlorinated oligomers, free-ends oligo(3-octylthiophenes) were synthesized by reductive dehalogenation. UV−visible properties were investigated in solution, and the absorption energy was found to depend linearly on the reciprocal of the number of thiophene units. A comparison with the properties of analogous regioregular poly(3-octylthiophenes) gave information on the structure of such polymers in solution.