Rectangular nanostructuring of Au(111) surfaces by self-assembly of size-selected thiacrown ether macrocycles.

Surface modification by self-assembled monolayers (SAMs) has continuously attracted considerable attention because of several potential applications in domains such as nanopatterning, biosensors and chemical sensors, and microelectronics.1,2 Organo-sulfur derivatives such as n-alkanethiols R-SH and dialkyl disulfides R-SS-R′ are the most extensively studied SAMs adsorbed on various metal surfaces such as gold, silver, or copper.3,4 In comparison, very few studies have been reported on SAMs based on thioethers R-S-R′. However, in an early report Troughton et al. studied the spontaneous self-assembly of symmetrical and unsymmetrical dialkyl sulfides onto gold substrates.5 More recently, Noh et al. investigated the adsorption of dioctadecyl sulfide SAMs on Au(111) by scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy.6 It has been observed in particular that the C-S-C motif of dioctadecyl sulfide remains intact after adsorption on gold, that is, without cleavage of the C-S bonds. Among thioethers, thiacrown ethers (TCEs) are cyclic thioethers consisting of an alternance of sulfur atoms and (CH2)n methylene bridges.7,8 Saturated TCEs can be viewed as the sulfur counterparts of the well-known crown ethers and contain a variable number of sulfur atoms, each separated by a variable number of CH2 groups. TCEs can behave as soft Lewis bases to interact selectively with metal ions such as Ag+, Hg2+, Cu+, and Pd2+ and form inclusion complexes.9,10 If the basic process involved in coordination of TCEs with metal ions is well-understood,11-13 interaction of TCEs with metal substrates has not been explored yet. Because of the cyclic geometry of the TCE molecules, it can be expected that their adsorption on metal surfaces would take place via sulfur-metal interactions and result in self-organized domains of cavities with various sizes and shapes. Such a size-selected nanostructuring of metallic surfaces could then be used to form highly ordered 2D inclusion networks and investigate electron-transfer processes between a metal substrate on one hand and various systems such as (bio)macromolecules on the other hand. Recently, Stang et al. reported ordered cavity structures relating to the self-organization of supermolecular metallacyclic rectangles on Au(111).14,15 Besides, Yoshimoto et al. studied the formation of crown ether substituted phthalocyanine arrays on Au substrate.16 Although ordered cavities can be realized by self-organization, the interaction with the substrate may be too weak to allow further application. In this Communication, we investigate the formation of ordered SAMs of two families of TCEs (Figure 1), namely tetrathiacrown ethers 1 (12S4) and 2 (14S4) and hexathiacrown ether 3 (18S6), by means of STM at the liquid/solid interface.17-19 Figure 2 shows typical STM images obtained by depositing solutions of TCE 1 and 3 in n-tetradecane (∼0.05 mg/mL) on an annealed Au(111) substrate. For the two molecules, large-scale STM images reveal ordered domains rotated at 120°, reflecting the threefold symmetry of Au(111). These domains have an average lateral size of several tens of nanometers. In both cases, the ordered domains are separated by apparently disordered regions. Randomly distributed depressions are observed at the domain boundaries as well as in the ordered phases. These pits exhibit an average depth of ∼2.4 Å, which corresponds to the thickness of a monatomic layer of the Au(111) substrate, showing that individual Au atoms have been removed from the surface (see Supporting Information). The occurrence of etched pits implies that strong Au-S chemical † SPCSI/DRECAM. ‡ UMR 7611 CNRS-Université P&M Curie. § National Center for Nanoscience and Technology (NCNST). Figure 1. Molecular structures of the three thiacrown ethers (TCE) used in this study.