Correlating molecular structure to field-effect mobility: The investigation of side-chain functionality in phenylene - Thiophene oligomers and their application in field effect transistors

Short conjugated phenylene (P)-thiophene (T) oligomers with varying alpha and omega 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 (mu = 0.18 cm(2) V-1 s(-1)) and on/off ratio of 10(7) at a substrate temperature of 50 degrees 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.