Self-assembled monolayers of ω-(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 the same number of methylene units. X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) 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, well-ordered monolayer. Ellipsometry measurements yield thickness values of 6.7, 10.7, and 17.9 Angstrom 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 35degrees, 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 degreesC but desorb/decompose on heating to 150 degreesC.