Self-assembly of pyridine-substituted alkanethiols on gold: The electronic structure puzzle in the ortho- and para-attachment of pyridine to the molecular chain

X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, infrared reflection absorption spectroscopy, and electrochemistry were applied to monitor the formation and properties of self-assembled monolayers (SAMs) of 4-(aminomethyl)pyridine-11- mercaptoundecanamide (C10AP1), 2-(aminomethyl) pyridine-11-mercaptoundecanamide (C10AP2), and aminomethyl-di(2-pyridyl)-11-mercaptoundecanamide (C10AP3) prepared on Au(111) substrates. While all three precursors formed well-defined SAMs, their structural and electronic properties were found to be noticeably different depending on (i) the attachment of the terminal pyridine moiety to the molecular chain and (ii) the number of pyridine units in the tail group. In particular, whereas the appearance of intramolecular hydrogen bonds is the most likely scenario in the ortho case, the formation of the intermolecular "cross-linking" network could be proposed in the para case. Accordingly, the branching of the characteristic pre-edge absorption resonances in the C K-edge NEXAFS spectra of the target SAMs is distinctly different for the cases of the ortho- or para-attachment of pyridine, which could be tentatively explained by the different couplings of the electronic and vibrational excitations in each case.