2D Self-Assembly of Fused Oligothiophenes: Molecular Control of Morphology

We report the synthesis and properties of two pi-functional heteroaromatic tetracarboxylic acids (isomeric tetrathienoanthracene derivatives 2-TTATA and 3-TTATA) and their self-assembly on highly oriented pyrolytic graphite. Using scanning tunneling microscopy at the liquid-solid interface we show how slight geometric differences between the two isomers (position of sulfur in the molecule) lead to dramatic changes in monolayer structure. While 3-TTATA self-assembles exclusively in a highly ordered porous network via dimeric R-2(2)(8) hydrogen-bonding connection (synthon), 2-TTATA is polymorphic, forming a less ordered porous network via R-2(2)(8) synthons as well as a close-packed network via rare tetrameric R-4(4)(16) synthons. Density functional theory calculations show that the self-assembly direction is governed by the angle between the carboxylic groups and secondary interactions with sulfur atoms.