Strong Hydrogen Bonds at the Interface between Proton-Donating and -Accepting Self-Assembled Monolayers on Au(111)

Hydrogen-bonding heterogeneous bilayers on substrates have been studied as a base for new functions of molecular adlayers by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), and density functional theory (DFT) calculations. Here, we report the formation of the catechol-fused bis­(methyl­thio)­tetra­thia­ful­valene (H₂Cat-BMT-TTF) adlayer hydrogen bonding with an imidazole-terminated alkanethiolate self-assembled monolayer (Im-SAM) on Au(111). The heterogeneous bilayer is realized by sequential two-step immersions in solutions for the individual Im-SAM and H₂Cat-BMT-TTF adlayer formations. In the measurements by AFM, a grained H₂Cat-BMT-TTF adlayer on Im-SAM is revealed. The coverage and the chemical states of H₂Cat-BMT-TTF on Im-SAM are specified by XPS. On the vibrational spectrum measured by IRAS, the strong hydrogen bonds between H₂Cat-BMT-TTF and Im-SAM are characterized by the remarkably red-shifted OH stretching mode at 3140 cm^–1, which is much lower than that for hydrogen-bonding water (typically ∼3300 cm^–1). The OH stretching mode frequency and the adsorption strength for the H₂Cat-BMT-TTF molecule hydrogen bonding with imidazole groups are quantitatively examined on the basis of DFT calculations.