ARTICLE pubs.acs.org/Langmuir
Surface-Confined Conformers and Coassembly-Induced Conformer Resolution Cui-Zhong Guan,†,‡ Ting Chen,† Jing-Yi Wu,† Qing Chen,† Dong Wang,*,† Peter J. Stang,§ and Li-Jun Wan*,† †
Institute of Chemistry, Chinese Academy of Sciences (CAS), and Beijing National Laboratory for Molecular Sciences, Beijing 100190, China ‡ Graduate University, Chinese Academy of Sciences (CAS) § Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States
bS Supporting Information ABSTRACT: Stereoisomerism is a fundamental chemistry issue and has been intensively investigated because of its importance in organic chemistry, biology, and pharmacology. Molecules with freely rotatable single bonds have many interconvertable conformers. Herein, we report the surface-adsorption-induced conformer resolution by employing azobenzene-3,3-dicarboxylic acid (ADA-33) as a model compound. Two linear assembly phases composed of trans conformers on a highly oriented pyrolytic graphite (HOPG) surface are observed by scanning tunneling microscopy. With the codeposition with 1-octanoic acid (OA), only one trans conformer of ADA-33 can be recognized by OA to form a two-component assembly with alternately arranged ADA-33 and OA stripes, which can be attributed to the epitaxial assembly of ADA33 and OA on the HOPG surface, and weak hydrogen bonding exists between conformer I and OA molecules. The results are of significance with respect to the discrimination and resolution of conformers on a solid surface and provide molecular insights into the coadsorption assembly on the surface.
’ INTRODUCTION Stereoisomeric molecules and crystals have been widely investigated because of their importance in organic chemistry, biology, medicine, and pharmacology.16 Different stereoisomers usually have different properties. For example, the toxicity of L-nicotine is much greater than that of D-nicotine.7 The trans and cis isomers of azobenzene derivatives show different conductivities and have the great potential to be used as molecular photoswitches.810 Stereoisomerism can be divided into configurational isomerism and conformational isomerism. The configurational isomerism can be further categorized into enantioisomerism and cistrans isomerism, where the isomers cannot be interconverted without breaking the covalent bond to the stereocenter. The configurational isomers are structurally stable and can be isolated and discriminated by spectroscopy or imaged by submolecular-resolution scanning tunneling microscopy (STM).1118 As a sharp contrast, the most important feature of the conformational isomerism is that the isomers can be interconverted exclusively by rotations around formally single bonds. Generally, the energy differences and energy barriers among conformers are always very small; the conformers can interconvert between each other rapidly. Only under some special conditions, such as at low temperature to freeze the molecular rotation process, conformers can coexist in a balanced system in certain composition proportions. In addition, people can restrict the single-bond rotation to access a single conformer with unique properties by steric interactions19 or an intramolecular r 2011 American Chemical Society
cationπ interaction.20 For example, by rational molecular design to freeze the single-bond rotation, the molecules exhibit a unique aggregation-induced emission property.21 The discrimination of conformers can be achieved by some spectroscopic means, such as X-ray diffraction and IR and NMR spectroscopy.22,23 In addition, the advanced theoretical calculation supplies important insight into the energy landscape of different conformers, which can verify and guide the experimental study.2429 Azobenzene-3,3-dicarboxylic acid (ADA-33) is an asymmetrically substituted azobenzene. Each molecule has two terminal carboxyl groups. The similar asymmetrically substituted azobenzene molecules have been taken as a model compound in conformational isomerism studies.24,30 Among many intermediate conformers, ADA-33 has four preferred planar conformers, as shown in Figure 1. These four conformers can be interconverted to each other through the rotation around two single bonds of the azobenzene moiety. When confined to a 2D surface, two trans conformers I and IV cannot be superimposed through any surface symmetry operation. However, two cis conformers II and III can be correlated to each other through a C2 rotational operation normal to the molecular plane; therefore, they are stereochemically the same. Received: June 2, 2011 Revised: June 28, 2011 Published: July 01, 2011 9994
dx.doi.org/10.1021/la202076z | Langmuir 2011, 27, 9994–9999
Langmuir
ARTICLE
Figure 1. Chemical structures of four planar conformers for ADA-33. They can be interconverted exclusively by rotations around two NC single bonds.
Herein, we report the adsorption-induced conformer resolution of ADA-33 on a highly oriented pyrolytic graphite (HOPG) surface. Two trans conformers of ADA-33, conformers I and IV, are found to self-assemble into two different adlayer structures on the HOPG surface. The conformations of two conformers are identified by using STM and theoretical simulations. Furthermore, the codeposition of ADA-33 with 1-octanoic acid (OA) results in the formation of a uniform single-phase adlayer. Detailed analysis indicates that only conformer I can coadsorb with OA and form a long-range-ordered lamellar structure, whereas conformer IV observed in the pure ADA-33 adlayer disappears in the coadsorbed adlayer. The formation of the coadsorption phase is attributed to the epitaxial assembly of ADA-33 and OA on the HOPG surface, and weak hydrogen bonding exists between conformer I and OA molecules. The results are of significance for the discrimination and resolution of conformers on a solid surface and provide molecular insights into coadsorption assembly on the surface.
’ EXPERIMENTAL SECTION STM measurements were performed with a Nanoscope IIIa SPM (Digital Instruments) under ambient conditions. STM tips were mechanically cut Pt/Ir wires (90/10). All of the images were recorded in constant-current mode and are shown without further processing. Specific tunneling conditions of each figure are given in the corresponding figure captions. ADA-33 and all of the chemicals used in this study were purchased from TCI and used as received. The adlayers were prepared by the solution deposition (∼2 μL) of dilute (∼105 M) solutions on freshly cleaved HOPG surfaces. Theoretical calculations were performed by using dispersion-corrected density functional theory (DFT-D) provided by the DMol3 code.3133 We used the standard parameter set of Grimme without any parameter optimization. The atomic orbitals were represented by a double-numeric-quality basis set with d-polarization functions (DNP). All of the calculations were allelectron ones and were performed with an extra-fine mesh. A selfconsistent field procedure was used with a convergence criterion of 105 au on the energy and electron density.
’ RESULTS Adlayer of ADA-33 Conformers. An adlayer of ADA-33 is prepared by dropping ∼2 μL of ADA-33 solution (dissolved in THF,
