Correlating Molecular Structure to Field-Effect Mobility: The Investigation of Side-Chain Functionality in Phenylene−Thiophene Oligomers and Their Application in Field Effect Transistors

Abstract

Short conjugated phenylene (P)−thiophene (T) oligomers with varying α and ω alkyl and alkoxy substitutions were synthesized using Stille and Suzuki coupling reactions to investigate the correlation between end-group structure and electronic properties on P2TP and P3TP conjugated cores. Several soluble oligomers were synthesized with 5,5‘-bis(4-n-octylphenyl)-2,2‘-bithiophene (do-PTTP) showing the highest mobility (μ = 0.18 cm2 V-1 s-1) and on/off ratio of 107 at a substrate temperature of 50 °C. Thin film morphologies analyzed using atomic force microscopy revealed higher nucleation densities for branched alkoxy side-chain molecules compared to those of their straight alkyl side-chain counterparts. Field-effect mobility showed an inverse correlation to side-group bulkiness, indicating a strong morphological component to charge transport in the P2TP and P3TP series.