Potential-Controlled Organization of 2,3-Diphenyl-5,7-di(thiophen-2-yl)thieno[3,4- b]pyrazine Adsorbed on Au(111) and Au(100) Electrodes

The 2,3-diphenyl-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine (DPDTTP) molecule is an organic molecule that is often used to produce photovoltaics and light-emitting diodes. The orientation of an organic semiconductor molecule at the metallic interface can affect the charge injection efficiency. The current study employed in situ scanning tunneling microscopy (STM) to examine the adsorption orientation of DPDTTP molecules on an ordered Au(111) electrode in 0.1 M HClO₄, H₂SO₄, and HCl. DPDTTP molecules were found to irreversibly adsorbed onto the Au electrode from a 10 μM dosing solution. Molecular-resolution STM images were obtained to reveal their spatial structure as functions of the chemical identity of the supporting electrolyte, the atomic structure of the Au(111) substrate, and the potential control. Only the reconstructed Au(111) electrode afforded ordered DPDTTP adlattices between -0.1 and 0.4 V (vs Ag/AgCl) in HClO₄and H₂SO₄, and the DPDTTP adlayer became more compact with more positive potential. In 0.1 M HCl, pre-adsorbed DPDTTP molecules on the Au(111) electrode were displaced by chloride anions at E > 0.2 V, as evidenced by a well-ordered hexagonal array with a nearest-neighbor spacing of 3.8 ± 0.1 Å. The DPDTTP admolecule desorbed at E < -0.1 V in all acids. High-quality STM images were acquired to reveal two kinds of molecular conformations, as also found in the bulk single crystal of DPDTTP.