Large molecules adsorbed on surfaces can be analyzed by scanning tunneling microscopy (STM) under various environmental conditions: on a dry surface in air or vacuum, and at the solid-liquid interface. However, can measurements under dissimilar conditions be compared, e.g., when sample A was studied at the solid-liquid interface and sample B in a dry environment? Only rarely can the same substance be examined with more than one approach. since completely different set-up and preparations are necessary. Furthermore, few substances are suitable for several methods of sample preparation and characterization. We have chosen a large, flexible. nonplanar molecule. namely an alkoxy-substituted second-generation dendritic compound with a chiral core unit, which is peculiar for its 'hourglass' conformation, The assembly properties have been explored by STM both in solution-cast self-organized monolayers (SOMs) and multilayer films, as well as at the solid-liquid interface, The complexity and limits of the three approaches applied to our hourglass-shaped dendritic compound are discussed. Depending on the approach and environmental conditions, several quality levels of image resolution could be achieved measurements carried out at low temperatures led to highest resolution on the aromatic parts of the molecule. A comparison of equally sized images obtained under these varying conditions reveals not only different packing arrangements. but also spots of unlike shape. Therefore, when the approach, preparation, and/or environmental conditions are not the same, STM measurements of different compounds have to he compared with greatest care.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTCombinatorial Library Approach for the Identification of Synthetic Receptors Targeting Vancomycin-Resistant BacteriaRuo Xu, Guy Greiveldinger, Linda E. Marenus, Alan Cooper, and Jonathan A. EllmanView Author Information Department of Chemistry, University of California Berkeley, California 94720 Department of Chemistry, University of Glasgow Glasgow G12 8QQ, Scotland, UK Cite this: J. Am. Chem. Soc. 1999, 121, 20, 4898–4899Publication Date (Web):May 6, 1999Publication History Received25 January 1999Revised23 March 1999Published online6 May 1999Published inissue 1 May 1999https://pubs.acs.org/doi/10.1021/ja990240ihttps://doi.org/10.1021/ja990240irapid-communicationACS PublicationsCopyright © 1999 American Chemical SocietyRequest reuse permissionsArticle Views762Altmetric-Citations96LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Ligands,Molecular mechanics,Monomers,Peptides and proteins,Receptors Get e-Alerts

Dendrimers are highly symmetrical macromolecules of nanometre scale consisting of branching units attached to a central polyfunctional core. The chiral core or the branching units influence the conformation of the dendrimer, Because dendrimers cannot easily be crystallized for diffraction studies, scanning probe methods are used to investigate the conformation of chiral dendrimers on a metal surface. Dendrimers of different size and symmetry were dissolved in CH2Cl2, film cast onto a Pt(100) surface and investigated by scanning tunnelling microscopy (STM). Measurements performed under ambient conditions reveal a quasi-periodic structure that is dependent on the shape of the investigated dendrimers: there is a distinct difference between dendrimers of the same generation with two or three branches. Furthermore, dendrimers with two branches differing only in chirality seem to pack themselves onto the metal substrate differently, Reference measurements of benzene on the Pt(100) surface suggest that the electronic structure probed by the STM tip is probably due to the phenyl rings inside the dendrimer structure, This assumption is supported by low-temperature (77 K) STM measurements, providing higher resolution. Copyright (C) 1999 John Wiley & Sons, Ltd.

(S)-4,4,4-Trifluoro-3-hydroxybutanoic acid was used as a starting material for the synthesis of dendritic branches (16-21, 30-32) which were attached to a chiral triol (6 or ent-6), derived from (R)-3-hydroxybutanoic acid, to give CF3-substituted dendrimers (33-36, 38-40) and dendritic compounds (37) of 1st and 2nd generation. The key steps in these syntheses are diastereoselective aldol additions of dioxanone enolates (building blocks and intermediates 6-15, 22, 23, and 25-28) and Williamson etherifications of benzylic-branch bromides with triols (intermediates and products 16-21, 28, and 30-40). The surfaces of the dendrimers are covered with MeO (33-35, 38-40) or allyloxy groups (36 and 37). The new dendrimers are characterized by NMR and mass spectroscopy. F-19-NMR Signals of the CF3 groups reveal constitutional heterotopicities caused by substituents which are separated From the F-nuclei by up to 15 bonds (through a 11'-biphenyl-4,4'-diyl spacer!) (Fig. 6).

Chiral triols (which may be considered as derivatives of tris(hydroxymethyl)methane), without (3-5) and with aliphatic (6) or aromatic (7) elongating units, and the 1st- and 2nd-generation benzylic branched bromides, 17, 18, 23, 24, 29, and 30 are subjected to Williamson etherification conditions (NaH in THF). This gave the first ‘fully chiral’ dendrimers, with triple branching and with a stereogenic center at each and every branching point (including the central building block: see 33-42, 44, and 46-49). Higher than 2nd-generation dendrimers of this type could not be prepared. Certain combinations of diastereoisomeric 2nd-generation branched bromides, 23, 24, 29, and 30, and enantiomeric center-piece triols, 3 and 4, would smoothly react to give the desired dendrimers (e.g., 44, and 46-49) and others would not, with the reactions stopping at the dendritic alcohols containing only two branches (e.g., 45, and 50-53; see Schemes 4 and 5). Considering the distance at which the intermediate diastereoisomeric ‘doubly coupled’ dendritic alcohols differ in their configuration, this diastereodifferentiation or molecular recognition phenomenon (discovered by trying to prepare only 8 out of 239 possible diastereoisomers!) is a most surprising result. All compounds were fully characterized, and the 2nd-generation dendrimers, e.g., 38, 40, and 47 with and without elongation were shown to be monodisperse and without defects, by MALDI-TOF mass spectroscopy (cf. Fig. 4). A simple, unambiguous nomenclature for identification of the novel dendritic compounds is proposed and applied in the Exper. Part.

Chiral triols 1-3('tris(hydroxymelhyl)methane' derivatives), prepared from (R)-3-hydroxybutanoic acid and aldehydes, are used as center pieces of dendrimers. The triols may be employed as such or after attachment of spacers containing alkyl or aryl moieties (see 5 and 7). The branches combined with the original or elongated triols are those first reported by Frechet (9-12, benzyl ethers of 3,5-dihydroxybenzyl alcohol and bromide). In this way, 1st-, 2nd-, and 3rd-generation chiral dendrimers without (13-15), or with aliphatic (16-18) or aromatic (19-21) spacers are prepared. The molecular weights range from 447 to 2716 Dalton. Two of the chiral triols, i.e., 2 and 3, are used as center pieces for chiral dendrimers containing 6 NH2, or 6 and 12 NO2 groups on the periphery (22-27), with 3,5-dinitrobenzoyl chloride as the branching unit. All compounds thus synthesized are of course monodisperse and are fully characterized. In some cases, the optical activity of the dendrimers indicates that conformationally chiral substructures might be present. The NH2 and NO2-substituted compounds avidly clathrate smaller molecules; they are sorbents exchanging host molecules through the gas phase.

The Li enolate of (2S,6R)-2-(tert-butyl)-6-trifluoromethyl-1,3-dioxan-4-one (1) reacts with isobutyraldehyde, pivalaldehyde and cyclohexanecarboxaldehyde to give unexpected aldols [3a-5a, (1'R,2R,5R,6R)-2,6-dialkyl-5-(2',2',2'-trifluoro-1'-hydroxyethyl)-1,3-dioxan-4-ones]. The Li enolate of (2R,6R)-2-(tert-butyl)-6-methyl-1,3-dioxan-4-one (2) reacts with pivalaldehyde to give (1'R,2S,5S,6R)-2,6-di-(tert-butyl)-5-(1'-hydroxyethyl)-1,3-dioxan-4-one (7), another ''strange'' aldol-type product resulting from one enolate and two aldehyde molecules. There is an unusual reorganization involving a transacetalization process under basic conditions. The structures of two products (4a and 7) were determined by X-ray crystallography, and a mechanism of formation proposed.

The first dendrimer with a chiral core and chiral branch units (2) is readily available from triols of the type 1. Attachment of spacer groups or direct etherification of 1 with benzylic bromides lead to first, second, and third generation chiral dendrimers. The molecular weights of the fully characterized dendrimers range between 1000 and 3000 Da. The dendrimer 2, built up from four chiral building blocks, is one out of 4096 possible stereoisomers.