Article pubs.acs.org/Langmuir
Cite This: Langmuir XXXX, XXX, XXX−XXX
Strong Hydrogen Bonds at the Interface between Proton-Donating and -Accepting Self-Assembled Monolayers on Au(111) Hiroyuki S. Kato,*,†,‡ Shinya Yoshimoto,‡ Akira Ueda,‡ Susumu Yamamoto,‡ Yusuke Kanematsu,§ Masanori Tachikawa,∥ Hatsumi Mori,‡ Jun Yoshinobu,‡ and Iwao Matsuda*,‡ †
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan The Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa, Chiba 277-8581, Japan § Faculty of Information Sciences, Hiroshima City University, 3-4-1, Hiroshima, Hiroshima 731-3194, Japan ∥ Graduate School of Nanobioscience, Yokohama City University, Yokohama, Kanagawa 236-0027, Japan ‡
S Supporting Information *
ABSTRACT: 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. the films depending of the H+ position as a new conceptual electronic device. In this study, we created a hydrogen-bonding heterogeneous bilayer consisting of H+-donating and -accepting molecules on substrates as the first step. For the H+-acceptor molecular layer, we prepared an imidazole-terminated alkanethiolate self-assembled monolayer (Im-SAM) on the Au(111) substrate (Figure 1a). The imidazole molecule can accept H+ at the imino N site and stabilize it as an imidazolium cation.9,10 At the saturation coverage of alkanethiolate-based SAMs, the molecules are in a closely packed standing-up form and the end group tends toward the open side on the SAM.11−13 For the functional end groups, the molecular orbitals and chemical properties are less perturbed from Au substrates due to the spacer layer of the closely packed alkylene chains in the SAMs. The energy levels of both occupied and unoccupied molecular orbitals for the functional groups are reported to be very similar to those of their molecules.14,15 In the case of Im-SAMs, as expected, the
1. INTRODUCTION Hydrogen bonds are sufficiently strong to fix molecular configurations, for instance, protein folding1 and DNA duplex assembling.2,3 In addition, hydrogen bonds are intrinsically flexible for breaking and creating themselves depending on the surrounding conditions at room temperature. With this strength and flexibility, hydrogen bonds provide a variety of properties and functions for molecular systems. To acquire these abilities of the hydrogen-bonding systems, we designed and created hydrogen-bonding bilayers on substrates. An important property of hydrogen bonds is the effective assistance of the proton (H+) transfer between donor and acceptor sites along the hydrogen bond.4−6 As one H+ transfer occurs between the H+-donating and -accepting molecules, the charge distributions of those molecules are substantially changed. With a balance of H + transfer and charge redistribution, highly conductive pure organic molecular crystals have been realized at the partially deprotonated molecular crystals of catechol-fused ethylenedithiotetrathiafulvalene (H2Cat-EDT-TTF).7,8 By constructing a H+ donor and acceptor bilayer on substrates with a microfabrication technique, we might be able to control the conductivity of © XXXX American Chemical Society
Received: October 6, 2017 Revised: January 4, 2018
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DOI: 10.1021/acs.langmuir.7b03451 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
Au(111) surface was prepared in an ultrahigh vacuum (UHV) chamber by the repetition of Ar+ sputtering (400 eV for 10 min) and annealing at ∼650 K. A clean Au thin film surface consisting of large, flat (111)-faced grains of more than several 100 nm in diameter was obtained by annealing in a butane burner flame for a few seconds. The surface flatness and chemical activity for the self-assembly of alkanethiolate on the Au thin film were confirmed by scanning tunneling microscopy (STM) and IRAS (Figures S1 and S2, respectively, in Supporting Information). Heterogeneous bilayers on the Au(111) surfaces were created by sequential two-step immersions of the Au substrates into individual solutions under a dry N2 atmosphere in a glovebox at room temperature. During the first immersion, the cleaned Au substrate was immersed in the deoxygenated ethanol solution containing 1 mM 1-(11-mercaptoundecyl)-imidazole (Im-C11SH; Aldrich, 96%) for more than 20 h. By this immersion procedure, the well-packed imidazole-terminated undecanethiolate was assembled on Au(111).16−18 The immersed samples were rinsed with a gentle flow of pure ethanol (a few milliliters) followed by drying in a stream of dry N2. For the upper-layer adsorption, the Im-SAM-covered substrates were immersed in deoxygenated chloroform solution containing 1 mM synthesized H2Cat-BMT-TTF19 for more than 20 h to ensure the highest coverage. After the second immersion, the samples were rinsed with a gentle flow of pure chloroform (a few milliliters) followed by drying in a stream of dry N2. The created samples of the heterogeneous layers on Au(111) were transported to individual measurement systems using N2-filled capsules of microtubes or a homemade load−lock cylinder to avoid exposure to air. We confirmed that storage in the microtubes for 3 days followed by exposure to air for ≥10 min does not cause a notable change in the vibrational spectra of the samples. The surface morphology of the prepared samples was examined using an AFM system20 (JEOL: JSPM-4200) under a nitrogen atmosphere at normal air pressure. All AFM images shown in this paper were obtained in tapping mode at room temperature using a rectangular Si3N4-coated cantilever (typically f ≈ 210 kHz). The molecular orientation and adsorption states for the prepared samples were examined by IRAS, according to the selection rule of vibrational excitation.21 As a reference, the infrared (IR) absorption spectra of nonoriented molecules in the liquid phase or powder were measured according to an attenuated total reflection method (ATRIR) using a Ge prism. The IRAS system consists of a main UHV chamber (base pressure
