Scanning Tunneling Microscopy Examination of Molecules with Fused Thiophene and Pyrrole Groups Adsorbed on the Electrified Au(111) Interface

Molecules with a fused pyrrole and thiophene backbone can be harnessed into organic semiconductors and optoelectronics devices. Closely related to these applications is contact with a frequently used gold electrode. This organic/metal interface has been examined with scanning tunneling microscopy (STM) at the molecular level, revealing the adsorption orientation and spatial structure of the Au(111) electrode. The current study focused on the adsorption of 1,4-bis(4H-dithieno[3,2-b:2′,3′-d]pyrrol-4-yl)benzene (BDTP) on an ordered Au(111) electrode. The BDTP adlayer was adsorbed onto a Au crystal by immersion in 0.01 mM BDTP/benzene dosing solution for 2 min after the Au electrode was pretreated with the annealing-and-quenching process. The STM experiment was performed in 0.1 M HClO₄ or H₂SO₄ under potential control. The cyclic voltammetric results indicate that the BDTP adlayer was stable between −0.2 and 0.7 V (versus Ag/AgCl) in both acids before desorption and oxidation commenced at negative and positive potentials. High-quality STM imaging was achieved to provide insights into the interactions between BDTP admolecules and with the Au support. The van der Waals interaction between BDTP and Au(111) was strong enough to partially lift the Au(111)-(√3 × 22) reconstructed phase. The three-dimensional adsorption configuration of the BDTP admolecule on the Au(111) electrode is inferred from the corrugated features in the STM image. Potential control affected the interfacial interactions, the coadsorption of anions, and finally the spatial molecular arrangement on the Au electrode.