Although the self-assembly of organic ligands on gold has been dominated by sulfur-based ligands for decades, a new ligand class, N-heterocyclic carbenes (NHCs), has appeared as an interesting ...alternative. However, fundamental questions surrounding self-assembly of this new ligand remain unanswered. Herein, we describe the effect of NHC structure, surface coverage, and substrate temperature on mobility, thermal stability, NHC surface geometry, and self-assembly. Analysis of NHC adsorption and self-assembly by scanning tunneling microscopy and density functional theory have revealed the importance of NHC-surface interactions and attractive NHC-NHC interactions on NHC monolayer structures. A remarkable way these interactions manifest is the need for a threshold NHC surface coverage to produce upright, adatom-mediated adsorption motifs with low surface diffusion. NHC wingtip structure is also critical, with primary substituents leading to the formation of flat-lying NHC
Au complexes, which have high mobility when isolated, but self-assemble into stable ordered lattices at higher surface concentrations. These and other studies of NHC surface chemistry will be crucial for the success of these next-generation monolayers.
The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors ...typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol(-1)) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.
Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of ...these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold-carbon bond relative to that of a gold-sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs.
In this paper, we show that simultaneous noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM) is a powerful tool for molecular discrimination on the Si(111)–7 × 7 ...surface, even at room temperature. Using density functional theory modeling, we justify this approach and show that the force response allows us to distinguish straightforwardly between molecular adsorbates and common defects, such as vacancies. Finally, we prove that STM/nc-AFM method is able to determine attachment sites of molecules deposited on semiconductor surface at room temperature.
Templating an organic array with Si(111)-7×7 Weymouth, Alfred J; Miwa, Roberto H; Edge, Graham J A ...
Chemical communications (Cambridge, England),
07/2011, Letnik:
47, Številka:
28
Journal Article
Recenzirano
We demonstrate that nearest neighbor molecular adsorption can be sterically hindered on the Si(111)-7×7 surface reconstruction. This breaks the energetic equivalence of corner and edge di-σ ...attachment geometries and allows a translationally ordered organic layer to be templated directly on the 7×7 reconstruction.
Two novel star molecules functionalized with 7‐azaindolyl and 2,2′‐dipyridylamino groups have been synthesized. Both molecules possess a sixfold rotation symmetry. Molecule L 1 is based on the ...hexaphenylbenzene core with the formula of hexap‐(7‐azaindolyl)phenylbenzene, while molecule L 2 is based on the hexakis(biphenyl)benzene core with the formula of hexap‐(2,2′‐dipyridylamino)biphenylbenzene. Both compounds have been characterized by single‐crystal X‐ray diffraction analyses. Molecule L 1 forms extended two‐dimensional layered structure, while L 2 forms interpenetrating columnar stacks in the solid state, as revealed by X‐ray diffraction analyses. Nanowire structures based on columnar stacks through self‐assembly of L 2 on a graphite surface were revealed by an STM study. Molecules L 1 and L 2 are capable of binding to metal ions, resulting in unusual structural motifs. Two AgI complexes with the formulae of (AgNO3)2(L 1) (1) and (AgNO3)3(L 1) (2) were isolated from the reactions of AgNO3 with L 1. Compound 1 displays extended intermolecular π–π stacking interactions that are responsible for its extended two‐dimensional structure in the crystal lattice. Complex 2 has a “bowl” shape and forms polar stacks in the crystal lattice. A CuII complex with the formula of {Cu(NO3)2}6(L 2) (3) was isolated from the reaction of Cu(NO3)2 with compound L 2. The six CuII ions in 3 are chelated by the 2,2′‐dipyridylamino groups of the star ligand L 2. Intermolecular CuO (nitrate) bonds lead to the formation of an extended two‐dimensional coordination network of 3. Both L 1 and L 2 are blue luminescent. Their interactions with AgI or CuII cause drastic quenching of emission. In addition, the luminescence of L 1 and L 2 is sensitive to the presence of protons, which cause a reduction of emission intensity and a red shift of the emission energy.
Luminescent molecular stars with a diameter as large as 3 nm, such as that shown here, are able to self‐assemble to nanowires on graphite surface, to form supramolecular network with metal ions, and to display drastic luminescent quenching in response to the presence of metal ions and protons.
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