Monolayer Orientation of omega-Substituted Amide-Bridged Alkanethiols on Gold

A new set of aryl-substituted amides of 16-mercaptohexadecanoic acid (R = 4-OCH3; 3,5-di-OCH3) are synthesized using a simple and general procedure. Self-assembled monolayers of these molecules on Au(111) are studied by infrared and visible ellipsometry and contact angle measurements. Model calculations are employed to determine the molecular tilt angle. The SAM methylene chains exist in prevailing all-trans conformation and the tilt of the CCC plane is decreased by approximately 15 degrees in comparison with the correspondent value for n-alkanethiols. Strong hydrogen bonds between the amide proton and the carbonyl oxygen are detected with C=O and N-H dipoles oriented parallel to the gold surface. The wetting of the outermost film surface reveals predominantly OCH3 groups exposed.