A series of low-cost phthalimide end-capped oligothiophene small molecules with variations to the terminal alkyl chain and number of thiophene units in the conjugated core have been synthesized and investigated. All molecules exhibit H-aggregation in the solid-state but different crystal structures and electronic properties, showing that subtle chemical modifications can result in dramatic changes to molecular self-assembly. Field-effect transistors display high electron mobilities of up to 0.2 cm² V⁻¹ s⁻¹.