A series of compounds from the tetraceno[2,3-b]thiophene and the anthra[2,3-b]thiophene family of semiconducting molecules has been made. Specifically, synthetic routes to functionalize the parent molecules with bromo and then hexyl groups are shown. The bromo- and hexyl-functionalized tetraceno[2,3-b]thiophene and anthra[2,3-b]thiophene were characterized in the top-contact thin-film transistor (TFT) geometry. They give high mobilities, ranging from 0.12 cm(2) V-1 s(-1) for alpha-n-hexylanthra[2,3-b]thiophene to as high as 0.85 cm(2) V-1 s(-1) for alpha-bromotetraceno[2,3-b]thiophene. Notably, grain size increases, going from the shorter anthra[2,3-b]thiophene core to the longer tetraceno[2,3-b]thiophene core, with a corresponding increase in mobility. The transition from undesirable 3D to desirable 2D thin-film growth is explained by the increase in length of the molecule, in this case by one benzene ring, which results in an increase in intralayer interactions relative to interlayer interactions.