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

The orientation of a self-assembled monolayer (SAM) of the 5-(4-{8-[(11-mercaptoundecanoyl)ferrocenyl]octyl-oxy}phenyl)-10,15,20-triphenylporphyrin (PC(8)FcC(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(8)FcC(11)SH molecule was determined to be 44 and 29 Angstrom, respectively, showing that while alkyl chains in the ferrocenecarbonylundecanethiol (FcC(11)SH) part of the PC(8)FcC(11)SH SAM should be oriented with a tilt angle of ca. 30 degrees 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.