[show abstract][hide abstract] ABSTRACT: Several o-quinodimethane adducts of fullerene were synthesized and their intramolecular aryl CH-fullerene π interactions were studied using variable temperature-NMR (VT-NMR). Evaluation of the rate constants associated with the first-order transition states for cyclohexene boat-to-boat inversions enables quantification of ΔG(‡) values for each inversion. A comparison between two constitutional isomers, only one of which is capable of intramolecular CH-π interactions, provides a lower limit of 0.95 kcal/mol for each aryl CH-fullerene π interaction.
The Journal of Organic Chemistry 02/2012; 77(3):1308-15. · 4.56 Impact Factor
[show abstract][hide abstract] ABSTRACT: Defining pathways to assemble long-range-ordered 2D nanostructures of specifically designed organic molecules is required in order to optimize the performance of organic thin-film electronic devices. We report on the rapid fabrication of a nearly perfect self-assembled monolayer (SAM) composed of a single-domain 6,13-dichloropentacene (DCP) brick-wall pattern on Au(788). Scanning tunneling microscopy (STM) results show the well-ordered DCP SAM extends over hundreds of nanometers. Combining STM results with insights from density functional theory, we propose that a combination of unique intermolecular and molecule-step interactions drives the DCP SAM formation.
[show abstract][hide abstract] ABSTRACT: Most known chemical methods to prepare hydrogenated fullerenes or fulleranes are described with an emphasis on those
methods that are of historical significance and/or convenient, scalable and high yielding. Separate sections are dedicated
to C60H2, C60H4–16, C60H18, C60H36, and C60H>36. The simplest of all fullerane derivatives, 1,2-C60H2, is prepared in good isolated yield using either a NaBH4 reduction (59% isolated) or a Zn(Cu) couple reduction (66% isolated). A few C60H4 (e.g., 1,2,3,4-C60H4) and C60H6 (e.g., 1,2,33,41,42,50-C60H6) isomers have been formed and identified using structure sensitive analytical techniques but evidence for fulleranes
of formula C60H8–16 is based largely upon structure insensitive mass spectrometry data. Crown shaped C
3v C60H18 is the only fullerane species that can be produced in high yield using multiple synthetic methods. It is a stable, easily
isolable species of limited solubility that persists for years. Of the synthetic methodologies available, the polyamine chemistries
[conventional heating or microwave irradiation] are best because the yields are excellent, the reactions are scalable, only
standard glassware is required, the work-up is simple, and no purification is required. Birch reductions, transfer hydrogenations
and Zn-acid conditions all lead to mixtures of C60H36 isomers in good yield. Of these methods, the transfer hydrogenation reaction gives the cleanest product slate with formation
of three isomers possessing C
1 symmetry (60–70%), C
3 symmetry (25–30%) and T symmetry (2–5%), respectively. The formation of fulleranes with more than 36 hydrogens is well documented but the products
have generally escaped characterization by structure sensitive analytical techniques. No single isomer of formula C60H>36 has ever been isolated.
[show abstract][hide abstract] ABSTRACT: Novel pentacene derivatives that show excellent resistance to photooxidation have been prepared and show potential as semiconductors in active layers of organic thin film electronic devices. The design and fabrication of more efficient organic thin film devices require us to develop an essential understanding of the growth processes of these molecules on various substrates and how the intermolecular and molecule-substrate interactions reflect their final structure formation. Self-assembled monolayers (SAMs) are promising molecular structures with long-term stability in these devices. Here we present a combined experimental and theoretical study by STM and ab-initio calculations of the self-assembly of a photooxidatively resistant pentacene derivative --- 6,13-dichloropentacene (DCP) --- on gold surfaces. On the Au(111) surface, DCP forms self-assembled domains with various high symmetry orientations. The uniformity of the SAM improves greatly when the DCP molecules are deposited on the Au(788) vicinal surface where the presence of parallel atomic steps select only one of the possible SAM orientations due to the molecule-step interaction. Thus we observe the formation of large DCP SAM structures with perfect single domain orientation.
[show abstract][hide abstract] ABSTRACT: A significant technical barrier (i.e., facile oxidative degradation) that has prevented the preparation of large acenes has now been breached. Using a combination of experimentally and theoretically derived substituent effects, the design, synthesis, isolation, and characterization of the first persistent nonacene derivative is described. The molecular design strategy includes placement of arylthio (or alkylthio) substituents on the terminal rings of the nonacene skeleton, effectively converting an open-shell singlet diradical into a closed-shell system. These powerful substituent effects appear to be suitable for the synthesis of other persistent, soluble, large acene derivatives required for advanced thin-film organic semiconductor applications.
Journal of the American Chemical Society 02/2010; 132(4):1261-3. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: In the title compound, C(20)H(16)Br(2), the terminal phenyl groups are twisted away from the central ring by approximately 55 and -125 degrees (average of four dihedral angles each), respectively. The crystal structure is stabilized by a combination of interMolecular and intraMolecular interactions including interMolecular pi-pi stacking interactions [C atoms of closest contact = 3.423 ( 5) angstrom].
[show abstract][hide abstract] ABSTRACT: Self-assembled thin films of novel organic molecules hold the promise of emerging technologies and applications ranging from sensors for biological applications to organic electronics and more efficient organic photovoltaics. Self-assembled monolayers (SAMs) form as a result of a delicate balance between competing molecule-substrate and intermolecular interactions. To control such self-assembly processes, it is mandatory to understand how this balance reflects onto the SAM's final structure. Here we present an ultra-high vacuum scanning tunneling microscopy (STM) study of the self-assembly of novel pentacene derivatives and functionalized fullerenes (F-C60) on metal surfaces. Pentacene is known to exhibit large carrier mobility and has been studied extensively as a semiconductor in organic thin film devices. However, it is subject to facile photo-oxidation that limits device lifetime. We recently synthesized novel pentacene derivatives that show a dramatically increased resistance to photo-oxidation. We identified 6,13-dichloropentacene as a promising candidate for organic electronics. On the compact surface of gold, 6,13-dichloropentacene forms self-assembled domains with various high symmetry orientations. The quality of the SAM is seem to dramatically improve when the 6,13-dichloropentacene are deposited on the (788) vicinal surface of gold where the presence of parallel atomic steps will select only one of the possible SAM orientations due to the molecule-step interaction. Thus we observe the formation of very large self-assembled 6,13-dichloropentacene monolayers with perfect single domain orientation. We have also studied the self-assembly of C60 functionalized with alkyl chains of various lengths (F-C60) on Ag(111). We find that as a function of the alkyl chain length various structures are forming, ranging from zigzag like to linear arrays of C60 fullerene cages. The symmetry and unit cell size of the F-C60 SAMs is dictated by the alkyl-surface and the intermolecular interactions. These results show that C60 molecules can be assembled in 2D and non-compact molecular arrays with a surface density controllable via appropriate chemical functionalization. Those structures show promise as candidates for selfassembled molecular junctions.
[show abstract][hide abstract] ABSTRACT: The tip-based nanofabrication method called field-assisted nanopatterning or FAN has now been extended to the transfer of metals, metal oxides and metal salts onto various receiving substrates including highly ordered pyrolytic graphite, passivated gold and indium-tin oxide. Standard atomic force microscope tips were first dip-coated using suspensions of inorganic compounds in solvent. The films prepared in this manner were non-uniform and contained inorganic nanoparticles. Tip-based nanopatterning on chosen substrates was conducted under high electric field conditions. The same tip was used for both nanofabrication and imaging. Arbitrary patterns were formed with dimensions that ranged from tens of microns to sub-20 nm and were controlled by tuning the tip bias during fabrication. Most tip-based nanopatterning techniques are limited in terms of the type of species that can be deposited and the type of substrates onto which the deposition occurs. With the successful deposition of inorganic species reported here, FAN is demonstrated to be a truly versatile tip-based nanofabrication technique that is useful for the deposition of a wide variety of both organic and inorganic species including small molecules, large molecules and polymers.
[show abstract][hide abstract] ABSTRACT: Substituent effects have been exploited to produce an unusually persistent heptacene derivative. In total, four new heptacene derivatives with varying levels of photooxidative resistance (1 < 2 < 3 < 4) have been synthesized. A combination of p-(t-butyl)thiophenyl substituents at positions 7 and 16 (i.e., arylthio substituents attached to the most reactive ring) and o-dimethylphenyl substituents at positions 5, 9, 14, and 18 (i.e., steric resistance on neighboring rings) make heptacene derivative 4 especially resistant to photooxidation. It persists for weeks as a solid, for 1-2 days in solution if shielded from light, and for several hours in solution when directly exposed to both light and air. Heptacene derivative 4 has been fully characterized. It possesses a small HOMO-LUMO gap of 1.37 eV.
Journal of the American Chemical Society 03/2009; 131(10):3424-5. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present a combined experimental and theoretical study of the self-assembly of C60 molecules functionalized with long alkane chains on the (111) surface of silver. We find that the conformation of the functionalized C60 changes upon adsorption on Ag(111) and that the unit cell size in the self-assembled monolayer is determined by the interactions between the functional groups. We show that C60 molecules can be assembled in ordered 2D arrays with intermolecular distances much larger than those in compact C60 layers, and propose a novel way to control the surface pattern by appropriate chemical functionalization.
[show abstract][hide abstract] ABSTRACT: In the title compound, C(20)H(16)Br(2), the terminal phenyl groups are twisted away from the central ring by approximately 55 and -125° (average of four dihedral angles each), respectively. The crystal structure is stabilized by a combination of inter-molecular and intra-molecular inter-actions including inter-molecular π-π stacking inter-actions [C atoms of closest contact = 3.423 (5) Å].
[show abstract][hide abstract] ABSTRACT: The title compound, C(20)H(18), has two crystallographically independent molecules in the asymmetric unit. The phenyl substituents of molecule A are twisted away from the plane defined by the central benzene ring by 131.8 (2) and -52.7 (3)degrees. The phenyl substituents of molecule B are twisted by -133.3 (2) and 50.9 (3)degrees. Each molecule is stabilized by a pair of intraMolecular C(aryl, sp(2))-H center dot center dot center dot pi interactions, as well as by several interMolecular C(methyl, sp(3))-H center dot center dot center dot pi interactions.
[show abstract][hide abstract] ABSTRACT: The complete mol-ecule of the title compound, C(4)H(2)Cl(2)O(2)S, is generated by crystallographic twofold symmetry, with the S atom lying on the rotation axis. In the crystal, the molecules are linked by C-H⋯O hydrogen bonds..
[show abstract][hide abstract] ABSTRACT: The title compound, C(20)H(18), has two crystallographically independent mol-ecules in the asymmetric unit. The phenyl substituents of mol-ecule A are twisted away from the plane defined by the central benzene ring by 131.8 (2) and -52.7 (3)°. The phenyl substituents of mol-ecule B are twisted by -133.3 (2) and 50.9 (3)°. Each mol-ecule is stabilized by a pair of intra-molecular C(aryl, sp(2))-H⋯π inter-actions, as well as by several inter-molecular C(methyl, sp(3))-H⋯π inter-actions.
[show abstract][hide abstract] ABSTRACT: The complete molecule of the title compound, C4H2Cl2O2S, is generated by crystallographic twofold symmetry, with the S atom lying on the rotation axis. In the crystal, the molecules are linked by C—H...O hydrogen bonds..
[show abstract][hide abstract] ABSTRACT: We combine scanning tunneling microscopy (STM) measurements with ab initio calculations to study the self-assembly of long chain alkanes and related alcohol and carboxylic acid molecules on graphite. For each system, we identify the optimum adsorption geometry and explain the energetic origin of the domain formation observed in the STM images. Our results for the hierarchy of adsorbate-adsorbate and adsorbate-substrate interactions provide a quantitative basis to understand the ordering of long chain alkanes in self-assembled monolayers and ways to modify it using alcohol and acid functional groups.
The Journal of Chemical Physics 04/2008; 128(12):124709. · 3.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: We apply ab initio density functional calculations to study the hydrogenation of graphitic nanocarbons including fullerenes, onions and nanotubes using diethylenetriamine (DETA) as hydrogenation reagent. Our results indicate that transfer of atomic hydrogen from the amine end-group of chemisorbed DETA molecules to nanocarbons is an exothermic reaction. We explore the optimum pathway for the hydrogenation reaction and find the activation energy associated with sigmatropic rearrangement of chemisorbed hydrogen atoms to lie near 1 eV, thus facilitating formation of energetically favorable adsorbate structures by surface diffusion. Chemisorbed hydrogen assists in a local sp^2 to sp^3 bonding conversion of the graphitic nanocarbons, causing large-scale structural changes ranging from local relaxations in nanotubes to shell opening in multi-wall onions.
[show abstract][hide abstract] ABSTRACT: We combine experimental observations with ab initio calculations to study the reversible hydrogenation of single-wall carbon nanotubes using high boiling polyamines as hydrogenation reagents. Our calculations characterize the nature of the adsorption bond and identify preferential adsorption geometries at different coverages. We find the barrier for sigmatropic rearrangement of chemisorbed hydrogen atoms to be approximately 1 eV, thus facilitating surface diffusion and formation of energetically favored, axially aligned adsorbate chains. Chemisorbed hydrogen modifies the structure and stability of nanotubes significantly and increases the inter-tube distance, thus explaining the improved dispersability in solvents like methanol, ethanol, chloroform, and benzene.
Journal of the American Chemical Society 03/2008; 130(7):2296-303. · 10.68 Impact Factor