Thiophene-Based Oligomers Interacting with Silver Surfaces and the Role of a Condensed Benzene Ring.

A comparative study of DTBT (1,3-di(thiophen-2-yl)benzo[c]thiophene), Br-DTBT-Br (1,3-bis(4,5-dibromothiophen-2-yl)benzo[c]thiophene), and a model 3T (2,2′:5′,2-terthiophene) oligothiophenes has been performed by investigating the interactions occurring between these compounds and both polycrystalline and single-crystal Ag surfaces. As a main result we report that the interaction of 3T with the Ag surfaces is followed by a fast, low-temperature polymerization. On the contrary, DTBT-like compounds, less reactive at low temperature, undergo thermally activated dissociation processes which hinder the possibility of a surface-induced polymerization of such compounds as a suitable synthetic strategy. Our results are supported by Raman and surface-enhanced Raman spectroscopy (SERS) measurements of the molecules in contact with a polycrystalline Ag substrate, showing the fingerprint of polythiophene chains in the case of 3T and a characteristic S-S stretching signal compatible with ring-opening processes in the case of DTBT-like compounds, not detected for 3T. Moreover, high-resolution X-ray photoelectron spectroscopy (XPS) measurements, probing carbon 1s and sulfur 2p core levels of the same molecules deposited onto a Ag(110) single crystal, confirm the suggestion of a ring-opening process by a C-S bond cleavage induced by both the interaction with silver and thermal treatments. Near-edge X-ray absorption fine structure (NEXAFS) carbon K-edge measurements complete the information, showing a common tendency of the molecules to lay flat on the Ag surface. Finally, density functional theory (DFT) calculations have been carried out to investigate the interaction and molecular conformation/orientation at the 3T and DTBT/Ag(110) interfaces. Theoretical findings support the results of measurements and suggest that a fine interplay between structural and electronic modifications of the benzo-fused rings of DTBT-like compounds in contact with the Ag surface is responsible for the peculiar reactivity of such molecules.