Rigid Molecular Tripod with an Adamantane Framework and Thiol Legs. Synthesis and Observation of an Ordered Monolayer on Au(111)

Tripod-shaped trithiols 1−3, containing CH2SH groups at the three bridgehead positions of the adamantane
framework and a halogen-containing group [Br (1), p-BrC6H4 (2), or p-IC6H4 (3)] at the fourth bridgehead,
were synthesized, and self-assembled monolayers (SAMs) were prepared on atomically flat Au(111)
surfaces. The three-point chemisorption of these tripods was confirmed by polarization modulation infrared
reflection absorption spectroscopy, which showed the absence of a S−H stretching band. Scanning
tunneling microscopy of the SAM of 1 exhibited a hexagonal arrangement of the adsorbed molecule
with a lattice constant of 8.7 Å. A unidirectionally oriented, head-to-tail array of 1, which allows the
close approach of neighboring molecules, is proposed as a reasonable model of the two-dimensional
crystal, where the adsorbed sulfur atoms form a quasi-(√3 × √3)R30° lattice. The charge of the
electrochemical reductive desorption of the SAM of 1 was in good agreement with the expected surface
coverage, while the SAMs of 2 and 3 showed somewhat less (ca. 70%) charge. The large negative reduction
peak potentials, observed for the SAM of 1, are taken to indicate a tight anchoring of this tripod by three
sulfur atoms.