A synthetic sequence was developed to provide 3,7-didodecylbenzo[1,2-b:4,5-b′]dithiophene and its dialkylthio and dialkoxy analogues as planar and soluble building-blocks for constructing benzodithiophene-based π-systems. The benzodithiophenes were also transformed to the corresponding 2,6-di(5-phenyl-2-thienyl)-capped derivatives as the representative oligomers. Their absorption and emission spectra as well as CV data showed that the HOMO–LUMO energy gaps of the dialkylthio and dialkoxy derivatives are smaller than that of the dialkyl one, which is attributable to the lower E_LUMO as well as E_HOMO energies of the dialkylthio analogue and the higher E_HOMO energy of the dialkoxy analogue, respectively. The intra- and intermolecular charge-carrier mobilities were then estimated by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) method and solution-processed top contact FET performance. The TRMC method showed similar high intrinsic hole mobilities (0.16–0.24 cm² V⁻¹ s⁻¹) irrespective of the substituents, whereas the dialkyl derivative especially displayed a good field-effect mobility of 0.023 cm² V⁻¹ s⁻¹.