Towards Fused Terthiophene Monomers for Optoelectronic Applications

In recent years, polythiophene derivatives have been on the front line of the conducting polymer industry. The most efficient polymer solar cell and the fastest polymer transistors are both polythiophene derivatives. The presence of the electron rich element sulfur has been suggested as an important pathway towards high performance organic semiconductor. Fused aromatic monomers are of particular interest as the extension of the π system allows for better π-π stacking. This results in a more crystalline structure and improves the charge transport of the resulting polymer. In this work, attempts were made towards an alkyl substituted fused terthiophene monomer. The backbone of the monomer was previously reported by Roncali et al. in 1994 and it has shown promising results. However, Roncali was unable to polymerise the monomer effectively and it continues to be undeveloped. Roncali did not report the details of his synthesis of the fused terthiophene and attempts made in this work to replicate the synthesis based on his brief description were unsuccessful. Alternative routes were explored towards the synthesis of Roncali’s fused terthiophene. The successful route found involved the Baylis-Hillman reaction of 2 thiophenecarboxyaldehye with methyl acrylate, followed by the oxidation of the Baylis-Hillman adduct and immediate Nazarov cyclisation of the resulting divinyl ketone. Ester hydrolysis of the resulting ketoester was able to synthesise thiaindanone in a large scale. The thiaindanone was then brominated and the resulting α-bromoketone was efficiently coupled with the ketoester precursor to synthesise an ester dimer. The ester dimer was hydrolysed to obtained a 1,4 diketone, which was cyclised using Lawesson’s reagent to form Roncali’s fused terthiophene. However, problems with solubility and stability found for this compound will need to be addressed prior to polymerisation.