Angle-Resolved X-Ray Photoelectron Spectroscopic Study on a Self-Assembled Monolayer of a Porphyrin-Ferrocene-Thiol Linked Molecule on Gold: Evidence for a Highly Ordered Arrangement for Efficient Photoinduced Electron Transfer

摘要

The orientation of a self-assembled monolayer (SAM) of the 5-(4-{8-[(11-mercaptoundecanoyl) ferrocenyl] octyl-oxy} phenyl)-10, 15, 20-triphenylporphyrin (PC_8FcC_(11)SH) molecule on a gold substrate, at which a photoinduced up-hill electron transfer with the highest quantum efficiency (> 10%) ever reported was achieved, was investigated using angle-resolved X-ray photoelectron spectroscopy (ARXPS) combined with an electrochemical coverage determination. The total thickness and the distance between the Fe and the outermost part of the SAM of the PC_8FcC_(11)SH molecule was determined to be 44 and 29 A, respectively, showing that while alkyl chains in the ferrocenecarbonylundecanethiol (FcC_(11)SH) part of the PC_8FcC_(11)SH SAM should be oriented with a tilt angle of ca. 30° normal to the gold surface, the plane of the porphyrin ring should be almost surface-normal. This result suggests that one of the most important reasons for the very high photoconversion efficiency is the large separation between gold and the porphyrin ring, leading to a reduction in the reverse electron transfer and energy transfer quenching of the excited porphyrin state by gold.