In situ scanning tunneling microscopy characterization of thienothiophene-based semiconducting organic molecules adsorbed on a Au(111) electrode

Organic molecules with a thienothiophene core are promising candidates for use in the fabrication of organic thin film transistors. In addition to the nature of the molecule, the adsorption orientation and spatial structure of admolecules at metal electrodes also affects the efficiency of charge injection at the molecule/metal interface. Scanning tunneling microscopy (STM), which is well-known for its sub-nanometer resolution, is particularly suitable in studying the structure of this interface. Dithieno[2,3-b:3,2-d]thiophene diphenyl (C6H5-DTT-C6H5) adsorbed on Au(111) electrode was previously examined. In this study we looked at molecules with perfluorophenyl substituents, 2-pentafluorophenyl-6-phenyldithieno[2,3-b:3′,2′-d]thiophene(C6F5-DTT-C6H5,1) and 2,6-bis(pentafluorophenyl)dithieno[3,2-b;2′,3′-d]thiophene(C6F5-DTT-C6F5, 2. In situ STM results obtained in 0.1 M HClO4 showed that these molecules could be adsorbed in ordered adlattices on a Au(111) electrode from dosing solutions made of 50 μM 1 or 2 in dicholobenzene. The adsorption strength of these molecules on Au(111) varied greatly with the electrochemical potential. They were mostly stable on Au(111) at 0.3 V (vs. reversible hydrogen electrode), but could be displaced by water dipoles and perchlorate anions at E < 0.2 V and E > 0.5 V, respectively. It was possible to take advantage of this result to improve the degree of ordering by setting the potential at 0 V briefly, then switching back to 0.3 V. While 1 was adsorbed in a lamella structure as observed previously with C6H5-DTT-C6H5, 2 was arranged in a very different structure determined largely by the need to optimize the intermolecular interactions.