[Show abstract][Hide abstract] ABSTRACT: We introduce a new pentacene-based organic semiconductor, 5,6,7-trithiapentacene-13-one (TTPO). TTPO is a small-molecule organic semiconductor that is simple to synthesize and purify, readily crystallizes, melts in air from 386–388 °C without decomposition, and is indefinitely stable against degradation in acid-free solution. TTPO has a high molar absorptivity, optical and electrochemical HOMO–LUMO gaps of 1.90 and 1.71 eV, respectively, and can be thermally evaporated to produce highly uniform thin films. Its cyclic voltammogram reveals one reversible oxidation and two reversible reductions between +1.5 and −1.5 V. The crystal structure for TTPO has been solved and its unique parallel displaced, and head-to-tail packing arrangement has been examined and explained using high-level density functional theory. High-resolution scanning tunneling microscopy (STM) was used to image individual TTPO molecules upon assembly on a pristine Au(111) surface in ultrahigh vacuum. STM images reveal that vapor-deposited TTPO molecules nucleate in a unique stacked geometry with a small acute angle with respect to Au(111) surface. Preliminary TTPO-based bilayer photovoltaic devices show increases in short circuit current density upon heating from 25 to 80 °C with a concomitant 4–160-fold increase in power conversion efficiencies. TTPO has the potential to be used in thin-film electronic devices that require operation over a wide range of temperatures such as thin-film transistors, sensors, switches, and solar cells.
The Journal of Physical Chemistry C 10/2014; 118(46):26955. DOI:10.1021/jp505011x · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The novel pentacene derivative 5,6,7-trithiapentacene-13-one (TTPO) is a
robust electron donor candidate for use in high temperature photovoltaic devices.
STM imaging has revealed interesting nanoscale surface structures of TTPO molec-
ular chains as well as an ordered self-assembled monolayer on 3.9nm wide gold (788)
surface terraces. TTPO is a polar species of pentacene with centered oxygen and
sulfur bridge substituents. It is along this sulfur bridge that TTPO arranges itself
laterally with a small cant angle between the molecule and the gold surface. This
lateral assembly varies from the common flat-lying and standing-up phases of pen-
tacene on surfaces. Combining imaging with density functional theory calculations
allows for classification of these self-assembled structures with particular interest be-
ing directed toward the interaction between TTPO and gold at this organic-metallic
interface. Understanding the structure of organic-metal interfaces with molecular
precision potentially allows for the tailoring of those interfaces in order to maximize
charge carrier transport.
[Show abstract][Hide abstract] ABSTRACT: Controlling the molecular structure of the donor-acceptor
interface is essential to overcoming the efficiency bottleneck in
organic photovoltaics. We present a study of self-assembled fullerene
(C60) molecular chains on perfectly ordered
6,13-dichloropentacene (DCP) monolayers forming on a vicinal Au(788)
surface using scanning tunneling microscopy in conjunction with density
functional theory calculations. DCP is a novel pentacene derivative
optimized for photovoltaic applications. The molecules form a brick-wall
patterned centered rectangular lattice with the long axis parallel to
the monatomic steps that separate the 3.9 nm wide Au(111) terraces. The
strong interaction between the C60 molecules and the gold
substrate is well screened by the DCP monolayer. At submonolayer
C60 coverage, the fullerene molecules form long parallel
chains, 1.1 nm apart, with a rectangular arrangement instead of the
expected close-packed configuration along the upper step edges. The
perfectly ordered DCP structure is unaffected by the C60
chain formation. The controlled sharp highly-ordered organic interface
has the potential to improve the conversion efficiency in organic
[Show abstract][Hide abstract] ABSTRACT: A novel and effective technique is developed to make the first determination of shear stress between dielectrophoretically assembled single-walled carbon nanotubes (SWNTs) and surfaces. The results demonstrate that we can vary the shear stress by a factor of 20 by functionalizing a gold surface with different alkanethiols. The interfacial shear stress between a small bundle of SWNTs and a gold surface with and without self-assembled monolayers of alkanethiol (2-phenylethanethiol or 2-aminoethanethiol) is determined. The measurements are based on simple NEMS cantilever beams, a nanomanipulator, and a scanning electron microscope (SEM). It is emphasized that the measured quantity is the slack in the nanotube (not the shear stress) induced by the nanomanipulation. The shear stress is determined from the slack through a mechanics model. An average shear stress of 87MPa between SWNTs and gold surfaces is obtained. For the tests on the self-assembled 2-aminoethanethiol surface, an average shear stress of 142MPa is obtained. For the self-assembled 2-phenylethanethiol surface, the shear stress is determined to be around 7.2MPa with an estimated work of adhesion of 0.5J/m(2).
Journal of Colloid and Interface Science 07/2013; 407. DOI:10.1016/j.jcis.2013.06.021 · 3.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A water soluble pentacene, potassium 3,3′-(pentacene-6,13-diylbis(sulfanediyl))dipropanoate (4), has been synthesized and characterized. The synthesis of 4 is straightforward and scalable, and its isolation does not require time consuming chromatographic separations. UV-vis spectra in several solvents indicate an optical HOMO–LUMO gap of approximately 1.91–1.97 eV. Water soluble pentacene 4 is long-lived in the solution phase and in the solid state. Because it forms stable solutions, inks based on 4 have been formulated and printed onto paper and flexible plastic using an unmodified commercial ink-jet printer. A bi-layer photovoltaic cell using 4 as donor and fullerene as acceptor was fabricated and shown to be active. The crystal structure of the pentacene diacid precursor to water soluble pentacene 4 has been solved and shows a parallel displaced arrangement of pentacene rings, indicative of stabilizing π–π stacking interactions. DFT modeling for 4, however, suggests an unusual, low energy conformation in which both potassium carboxylate moieties are located on the same face (syn) of the pentacene π system. Likewise, calculated two-molecule stacks of 4 suggest a crystal packing arrangement in which potassium carboxylate moieties are intercalated between adjacent pentacene rings.
Journal of Materials Chemistry 01/2013; DOI:10.1039/C3TC00278K · 7.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Pentacene is an organic semiconductor used in a variety of thin-film organic electronic devices. Although at least six separate syntheses of pentacene are known (two from dihydropentacenes, two from 6,13-pentacenedione and two from 6,13-dihydro-6,13-dihydroxypentacene), none is ideal and several utilize elevated temperatures that may facilitate the oxidation of pentacene as it is produced. Here, we present a fast (-2 min of reaction time), simple, high-yielding (≥ 90%), low temperature synthesis of pentacene from readily available 6,13-dihydro-6,13-dihydroxypentacene. Further, we discuss the mechanism of this highly efficient reaction. With this improved synthesis, researchers gain rapid, affordable access to high purity pentacene in excellent yield and without the need for a time consuming sublimation.
[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. DOI:10.1021/jo201921h · 4.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electronic ground states of the recently synthesized stable nonacene derivatives (J. Am. Chem. Soc. 2010, 132, 1261) are open-shell singlets with a polyradical nature instead of closed-shell singlets as originally assumed, according to the unrestricted broken spin-symmetry density functional theory (UBS-DFT) computations (at B3LYP/6-31G*). It is the bulky protecting groups, not the transfer from the open-shell singlet to closed-shell singlet ground state, that stabilizes these longest characterized acenes. Similar analyses also confirmed the open-shell singlet character of the hexacene and teranthene derivatives.
[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. DOI:10.1021/ja9095472 · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[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: 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 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 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: 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 title compound, C20H18, 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)°. The phenyl substituents of molecule B are twisted by −133.3 (2) and 50.9 (3)°. Each molecule is stabilized by a pair of intramolecular C(aryl, sp
2)—H⋯π interactions, as well as by several intermolecular C(methyl, sp
[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.