Adsorption characteristics of tripodal thiol-functionalized porphyrins on gold

X-ray photoelectron and Fourier transform infrared spectroscopy studies are reported for self-assembled monolayers (SAMs) of two tripodal thiol-functionalized metalloporphyrins (Zn and Cu) and three benchmark tripods on gold substrates. The tripodal unit common to all five molecules is 1-(phenyl)-1,1,1-tris(4-mercaptomethylphenyl)methane (Tpd). Both porphyrins contain S-acetyl-protected thiols and are linked to the 4-position of the phenyl ring of Tpd via a phenylethyne group. The benchmark molecules include (1) two tripods containing a bromine atom at the 4-position of the apical phenyl ring, one a free thiol and the other its S-acetyl-protected analogue, and (2) a S-acetyl-protected tripod containing a phenylethyne unit at the 4-position of the apical phenyl group. Together, the spectroscopic studies reveal that none of the five tripodal molecules bond to the gold surface via all three sulfur atoms. Instead, the average number of bound thiols ranges from 1.5 to 2, with the porphyrinic molecules generally falling at the middle to upper end of the range and the smallest benchmark tripods falling at the lower end. Similar surface binding is found for the S-acetyl-protected and free benchmark tripods, indicating that the presence of the protecting group does not influence binding. Furthermore, the surface binding characteristics of the SAMs are not sensitive to deposition conditions such as solvent type, deposition time, or temperature of the solution.