Conical Pentaaryl[60]fullerene Thiols: Self-Assembled Monolayers on Gold and Photocurrent Generating Property

Conical-shaped penta(aryl)[60]fullerene thiol derivatives bearing one alkyl thiol linker surrounded by five phenyl or biphenyl groups were synthesized in good yield, and characterized by X-ray crystallographic analysis. These conical fullerene thiols were used for formation of self-assembled monolayers (SAMs) on gold surface in anaerobic and aerobic condition. Under nitrogen atmosphere, SAMs on gold were successfully prepared for all thiols (C₆₀Ph₅(CH₂)₃SH, C₆₀Ph₅(CH₂)₄SH, C₆₀Ph₅(CH₂)₆SH, C₆₀(biphenyl)₅(CH₂)₆SH) used in this work. The SAMs of penta(aryl)[60]fullerene thiols exhibited anodic and cathodic photocurrent generation upon light irradiation (λ = 400 nm) in the presence of sacrificial electron-donating and -accepting reagents, respectively. The SAM formation under air gave either SAMs or aggregates on gold depending on the length of alkyl chains of the linker. When the linker is long enough (i.e., C₆₀Ph₅(CH₂)₆SH) so that the disulfide group is not sterically protected (i.e., C₆₀Ph₅(CH₂)₆S−S(CH₂)₆C₆₀Ph₅), the molecules formed SAMs and generated anodic and cathodic photocurrent. Further aggregation of the molecules occurred when the linker is short relative to the arene cone consisting of five aryl groups (i.e., C₆₀Ph₅(CH₂)₃SH). These aggregates showed unusual anodic photocurrent behavior, likely because of diffusion issue for electrolyte and sacrificial reagents. The aggregates were further investigated by cyclic voltammetry, atomic force microscopy, and photoelectron yield spectroscopy.