Self-assembled Monolayers of (3-Thienyl)alkanethiols on Gold

Thiophene-containing alkanethiols, Th-(CH₂)_n-SH (Th = 3-thiophene) with n=2, 6, and 12, have been synthesized and self-assembled onto gold-coated Si(111) wafers. The properties of the monolayers have been compared with those of methyl-terminated self-assembled monolayers (SAMs) having thesamenumber of methylene units. X-ray and ultraviolet photoelectron spectroscopies (XPS andUPS)demonstrate formation of surface thiolate bonds and assembly with the thiophene rings at the periphery of the monolayers. Dynamic contact angle measurements using water and hexadecane probe liquids are consistent with this conclusion and, particularly in the case of 12-(3-thienyl)dodecanethiol, indicate a densely packed, wellordered monolayer. Ellipsometry measurements yield thickness values of 6.7, 10.7, and 17.9 Å for n = 2, 6, and 12, respectively. Assuming the alkyl chains are in a fully extended all-trans conformation, these data indicate tilt angles of 42, 41, and 35°, respectively. Quartz crystal microbalance measurements demonstrate that the thienyl-terminated alkanethiol monolayers have similar packing densities compared to methyl-terminated SAMs of similar lengths. Thermal stability measurements using XPS and UPS show that the thienyl-terminated SAMs are stable to at least 100 °C but desorb/decompose on heating to 150 °C.