Electronic Structure of a Self-Assembled Monolayer with Two Surface Anchors: 6-Mercaptopurine on Au(111)

Abstract

The electronic structure of aromatic and aliphatic thiols on Au(111) has been extensively studied in relation to possible applications in molecular electronics. In this work, the effect on the electronic structure of an additional anchor to the S-Au bond using 6-mercaptopurine as a model system has been investigated. Results from X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory (DFT) confirm that this molecule adsorbs on Au(111) with S-Au and iminic N-Au bonds. Combined ultraviolet photoelectron spectroscopy and DFT data reveal that formation of the 6MP self-assembled monolayer generates a molecular dipole perpendicular to the surface, with negative charges residing at the metal/monolayer interface and positive charges at the monolayer/vacuum interface, which lowers the substrate work function. Scanning tunneling microscopy shows two surface molecular domains: a well-ordered rectangular lattice where molecules are tilted on average 30° with respect to the substrate and aligned 6MP islands where molecules are standing upright. Finally, we found a new electronic state located at -1.7 eV with respect to the Fermi level that corresponds to a localized π molecular state, while the state corresponding to the N-Au bond is hybridized with Au d electrons and stabilized at much lower energies (-3 eV).

Theoretical Investigation of 6-Mercaptopurine Isomers’ Adsorption on the Au(001) Surface: Revealing the Fate of Different Isomers

“…For the thione isomers, according to refs16, 17, there are four predominant isomers of 6-MP(1,7), 6-MP(1,9), 6-MP (3,9), and 6-MP (3,7). For thiol isomers, there are two isomers for consideration: 6-MP(7,10) and 6-MP (9,10). We tried our best to check all of the possible adsorption modes, including monodentate and bridging bidentate, as illustrated in Scheme 1.…”

“…Based on whether one hydrogen atom is linked to the S atom, the 6-MP isomers can be categorized into two isomers, thione isomers and thiol isomers. For the thione isomers, according to refs16, 17, there are four predominant isomers of 6-MP(1,7), 6-MP(1,9), 6-MP (3,9), and 6-MP (3,7). For thiol isomers, there are two isomers for consideration: 6-MP(7,10) and 6-MP (9,10).…”

“…The selected geometric parameters of adsorbed 6-MP isomers are also listed in Table 1, and the corresponding adsorption energies with inclusion of the vdW correction are also given in Table 2. As shown in Figure 3, the 6-MP (1,9) isomer can also adsorb onto the Au(001) surface via two potential modes, monodentate for configurations mb-2, mb-3, and mb-4 and bridging bidentate for mb-5. In these five configurations, the most stable one is mb-5, with an adsorption configurations.…”

“…6-Mercaptopurine ( 6-MP ) is one of the thiol family for which its interaction with Au surfaces has been studied theoretically − and experimentally. − Maza et al investigated the self-assembled monolayers (SAMs) of 6-MP on the Au(100) – (1 × 1) and Au(111) – (1 × 1) surfaces to understand the interplay of molecule–substrate interactions for heterocyclic thiols capable of binding to the surface by two anchors, which spontaneously form a highly disordered film on Au(111). Carro et al reported a different scenario for 6-MP on the Au(001) – (5 × 20) reconstructed surface and showed that 6-MP lifts the reconstruction upon adsorption and ejects a large excess of gold adatoms.…”

Adsorption/Desorption Behaviors and SERS Chemical Enhancement of 6-Mercaptopurine on a Nanostructured Gold Surface: The Au20 Cluster Model

“…Therefore, a deeper insight into the metal substrate and molecule binding mechanisms is of critical importance for a more comprehensive interpretation of SERS spectra. Several reports on the interactions of 6MP with nanostructured metal surfaces in terms of both experimental and theoretical aspects are available in the recent literature [43][44][45][46]. In order to elucidate the nature of these interactions, validate relevant thermodynamic parameters, as well as interpret or predict spectroscopic properties from the vibrational, electronic absorption spectra, density functional theory (DFT) approaches are commonly selected.…”