Scanning the Potential Energy Surface for Synthesis of Dendrimer-Wrapped Gold Clusters: Design Rules for True Single-Molecule Nanostructures

The formation of true single-molecule complexes between organic ligands and nanoparticles is challenging and requires careful design of molecules with size, shape, and chemical properties tailored for the specific nanoparticle. Here we use computer simulations to describe the atomic-scale structure, dynamics, and energetics of ligand-mediated synthesis and interlinking of 1 nm gold clusters. The models help explain recent experimental results and provide insight into how multidentate thioether dendrimers can be employed for synthesis of true single-ligand–nanoparticle complexes and also nanoparticle–molecule–nanoparticle “dumbbell” nanostructures. Electronic structure calculations reveal the individually weak thioether–gold bonds (325 ± 36 meV), which act collectively through the multivalent (multisite) anchoring to stabilize the ligand–nanoparticle complex (∼7 eV total binding energy) and offset the conformational and solvation penalties involved in this “wrapping” process. Molecular dynamics simulations show that the dendrimer is sufficiently flexible to tolerate the strained conformations and desolvation penalties involved in fully wrapping the particle, quantifying the subtle balance between covalent anchoring and noncovalent wrapping in the assembly of ligand–nanoparticle complexes. The computed preference for binding of a single dendrimer to the cluster reveals the prohibitively high dendrimer desolvation barrier (1.5 ± 0.5 eV) to form the alternative double-dendrimer structure. Finally, the models show formation of an additional electron transfer channel between nitrogen and gold for ligands with a central pyridine unit, which gives a stiff binding orientation and explains the recently measured larger interparticle distances for particles synthesized and interlinked using linear ligands with a central pyridine rather than a benzene moiety. The findings stress the importance of organic–inorganic interactions, the control of which is central to the rational engineering and eventual large-scale production of functional building blocks for nano(bio)electronics

Synthesis of Molecular Tripods Based on a Rigid 9,9′-Spirobifluorene Scaffold

The efficient synthesis of a new tripodal platform based on a rigid 9,9′-spirobifluorene with three acetyl protected thiol groups in the positions 2, 3′ and 6′ for deposition on Au(111) surfaces is reported. The modular 9,9′-spirobifluorene platform provides both a vertical arrangement of the molecular rod in position 7 and its electronic coupling to the gold substrate. To demonstrate the validity of the molecular design, the model compound <b>24</b> exposing a <i>para</i>-cyanophenylethynyl rod is synthesized. Our synthetic approach is based on a metal–halogen exchange reaction of 2-iodobiphenyl derivative and his subsequent reaction with 2,7-disubstituted fluoren-9-one to afford the carbinol <b>16</b>. Further electrophilic cyclization and separation of regioisomers provided the corresponding 2,7,3′,6′-tetrasubstituted 9,9′-spirobifluorene <b>17</b> as the key intermediate. The molecular structure of <b>17</b> was determined by single-crystal X-ray diffraction crystallography. The self-assembly features of the target compound <b>24</b> were analyzed in preliminary UHV-STM experiments. These results already demonstrated the promising potential of the concept of the tripodal structure to stabilize the molecule on a Au(111) surface in order to control the spatial arrangement of the molecular rod

Adatom Coadsorption with Three-Dimensional Cyclophanes on Ag(111)

The structure of molecular adlayers is of great interest for surface functionalization. As molecular complexity increases, the subtle interplay of the relevant interactions becomes more difficult to unravel. Here, we present a scanning tunneling microscope (STM) and atomic force microscope study along with free-energy calculations using density functional theory on two closely related NDI-cyclophane molecules. These three-dimensional double-decker molecules are designed to attach to the substrate with one subunit while the other functional moiety is exposed to the environment. The molecular arrangements obtained on Ag(111) are rationalized by the inclusion of adatoms from the substrate into the structure. The presence of adatoms is identified by a drastic change in corrugation of the STM images that takes place at moderate bias voltages. Our calculations using density functional theory of the system’s free-energy yield that two adatoms favorably coadsorb with the molecules

Polymer Library Comprising Fluorene and Carbazole Homo- and Copolymers for Selective Single-Walled Carbon Nanotubes Extraction

To date, (<i>n</i>, <i>m</i>) single-walled carbon nanotubes (SWNTs) cannot be selectively synthesized. Therefore, postprocessing of SWNTs including solubilization and sorting is necessary for further applications. Toward this goal, we have synthesized a polymer library consisting of fluorene- and carbazole-based homo- and copolymers. Variations of the connection of these aromatics together with the incorporation of further conjugated monomers give access to a broad diversity of polymers. Their ability to selectively wrap specific (<i>n</i>, <i>m</i>) species is investigated toward HiPco SWNTs raw material which contains more than 40 (<i>n</i>, <i>m</i>) species. Absorption and fluorescence spectroscopies were used to analyze SWNTs/polymer suspensions. These results provide evidence for selective SWNTs/polymer interactions and allow a more detailed assessment of polymer structure–property relationships, thus paving the way toward custom synthesis of polymers for single (<i>n</i>, <i>m</i>) SWNTs extraction

Deltoid versus Rhomboid: Controlling the Shape of Bis-ferrocene Macrocycles by the Bulkiness of the Substituents

Precise structural control of heteroannularly disubstituted ferrocene (Fc) structures is very challenging as the high rotational mobility of the Fc unit allows a large conformational diversity. Herein we present the syntheses, characterization, and electrochemical investigation of two complementary bis-ferrocene macrocycles, built up via Sonogashira cross coupling and intramolecular ring-closing reaction. While the X-ray structure of 1,2-ethynylbenzene bridged bis-ferrocene complex <b>1</b> shows a deltoidal conformation, a stretched oriented rhomboidal bis-ferrocene metallacycle <b>2</b> is formed when the peripheral benzene rings are decorated with bulky <i>tert</i>-butylsulfanyl groups. VT-NMR spectroscopy is used to assign the rotation of the embedded Fc units in rhomboid <b>2</b>. Moreover, cyclic voltammetry (CV) of deltoid <b>1</b> and rhomboid <b>2</b> indicate that electronic communication between both ferrocenyl groups can be neglected, while the electrostatic through space coupling is significant

Monofunctionalized Gold Nanoparticles Stabilized by a Single Dendrimer Form Dumbbell Structures upon Homocoupling

The assembly of dumbbell structures as organic–inorganic hybrid materials is presented. Gold nanoparticles (NPs) with a mean diameter of 1.3 nm were synthesized in very good yields using a stabilizing dendrimer based on benzylic thioether subunits. The extended dendritic ligand covers the NP surface and contains a peripheral protected acetylene, providing coated and monofunctionalized NPs. These NPs themselves can be considered as large molecules, and thus, applying a wet-chemical deprotection/oxidative acetylene coupling protocol exclusively provides dimers of NPs interlinked by a diethynyl bridge. The concept not only enables access to novel organic/inorganic hybrid architectures but also promises new approaches in labeling technology

Tuning Charge Transport Properties of Asymmetric Molecular Junctions

Charge transport characteristics of asymmetric molecules containing a 9,9′-spirobifluorene platform coupled covalently to a phenylene ethynylene linker capped with either a thiol or a nitrile end group are investigated by break junction techniques. It is shown that the platform provides very good electronic coupling with metallic leads and the differences in the charge transport depend solely on the type of the anchoring group at the opposite end of the molecule. The SH-terminated molecule has 1 order of magnitude higher conductance compared to the CN-terminated one, and the charge transport path depends on the end group utilized. By a combined experimental break junction techniques and theoretical DFT calculations, it was demonstrated that in molecules containing SH-terminated phenylene ethynylene wire attached to the 9,9′-spirobifluorene platform the charge is transported through fluorene unit and covalently coupled phenylene ethynylene linker. For CN-terminated molecules the charge is transported through the thiolate termini of the 9,9′-spirobifluorene tripod. These studies demonstrate the potential of spirobifluorene platform for the bottom-up approach to molecular architectures by its immobilization with all three thiol groups to one of the electrodes without compromising charge transport via the conjugated backbone

Tuning Charge Transport Properties of Asymmetric Molecular Junctions

Charge transport characteristics of asymmetric molecules containing a 9,9′-spirobifluorene platform coupled covalently to a phenylene ethynylene linker capped with either a thiol or a nitrile end group are investigated by break junction techniques. It is shown that the platform provides very good electronic coupling with metallic leads and the differences in the charge transport depend solely on the type of the anchoring group at the opposite end of the molecule. The SH-terminated molecule has 1 order of magnitude higher conductance compared to the CN-terminated one, and the charge transport path depends on the end group utilized. By a combined experimental break junction techniques and theoretical DFT calculations, it was demonstrated that in molecules containing SH-terminated phenylene ethynylene wire attached to the 9,9′-spirobifluorene platform the charge is transported through fluorene unit and covalently coupled phenylene ethynylene linker. For CN-terminated molecules the charge is transported through the thiolate termini of the 9,9′-spirobifluorene tripod. These studies demonstrate the potential of spirobifluorene platform for the bottom-up approach to molecular architectures by its immobilization with all three thiol groups to one of the electrodes without compromising charge transport via the conjugated backbone