A series of ethynyl-substituted molecules with the tetraceno[2,3-è] thiophene and anthra[2,3-b]thiophene core have been synthesized. The aim was to investigate the impact of differently bulky side-chain substituents on the packing of the molecule in thin film and hence its thin-film transistor (TFT) mobility. Three R groups were used, namely, the tri-isopropyl-, triethyl-, and trimethylsilylethynyl groups. We did not observe a direct correlation between substituent size and TFT mobility. However, the 5,12-bis(tri- isopropylsilylethynyl)tetraceno[2,3-b]thiophene has a mobility as high as 1.25 cm²/V·s, the 5,12-bis(trimethylsilylethynyl)tetraceno[2,3-b] thiophene has a mobility of about 0.00616 cm²/V·s, while 5,10-bis(triethylsilylethynyl)anthra[2,3-b]thiophene and 5,10- bis(trimethylsilylethynyl)anthra[2,3-è]thiophene have a mobility of 10^-4cm²/V·s on phenylsilane- and octadecyltrichlorosilane-treated surfaces respectively.