Solution-graphite Interface Research Articles

Metal Ion and Guest-Mediated Spontaneous Resolution and Solvent-Induced Chiral Symmetry Breaking in Guanine-Based Metallosupramolecular Networks.

Two-dimensional (2D) chirality has been actively studied in view of numerous applications of chiral surfaces such as in chiral resolutions and enantioselective catalysis. Here, we report on the expression and amplification of chirality in hybrid 2D metallosupramolecular networks formed by a nucleobase derivative. Self-assembly of a guanine derivative appended with a pyridyl node was studied at the solution-graphite interface in the presence and absence of coordinating metal ions. In the absence of coordinating metal ions, a monolayer that is representative of a racemic compound was obtained. This system underwent spontaneous resolution upon addition of a coordinating ion and led to the formation of a racemic conglomerate. The spontaneous resolution could also be achieved upon addition of a suitable guest molecule. The mirror symmetry observed in the formation of the metallosupramolecular networks could be broken via the use of an enantiopure solvent, which led to the formation of a globally homochiral surface.

Hydrogen-Bonded Donor-Acceptor Arrays at the Solution-Graphite Interface.

Controlling the nanoscale morphology of organic thin films represents a critical challenge in the fabrication of organic (opto)electronic devices. The morphology of the (multicomponent) thin films in turn depends on the mutual orientation of the molecular components and their supramolecular packing on the surface. Here, it is shown how the surface co-assembly of electron-donating and -accepting building blocks can be controlled via (supra)molecular design. Hexa-peri-hexabenzocoronene (HBC) derivatives with multiple hydrogen-bonding (H-bonding) sites were synthesized and their co-assembly with alkyl-substituted perylene tetracarboxy diimide (PDI) was studied using scanning tunneling microscopy (STM) at the solution-graphite interface. STM data shows that electron-rich HBCs co-assemble laterally with electron deficient PDIs via preprogrammed H-bonding sites with high fidelity. The surface stoichiometry of the two components could be readily tuned by changing the number of H-bonding sites on the HBC derivatives via organic synthesis. This model study highlights the utility of (supra)molecular design in co-assembly of building blocks relevant for organic electronics.

Tracking Invisible Transformations of Physisorbed Monolayers: LDI-TOF and MALDI-TOF Mass Spectrometry as Complements to STM Imaging.

Triphenyleneethynylene (TPEE) derivatives bearing one long aliphatic chain on each terminal aryl ring and two short aliphatic chains on the central aryl ring (core chains) self-assemble single component and 1-D patterned, two-component, crystalline monolayers at the solution-graphite interface. The monolayer morphology directs the core chains off the graphite, making them accessible for chemical reactions but invisible to imaging by scanning tunneling microscopy (STM). This precludes using STM to monitor transformations of the core chains, either by reaction or solution-monolayer exchange of TPEE molecules. Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) successfully identifies TPEE compounds within physisorbed monolayers. The LDI-TOF spectra of TPEE monolayer-graphite samples exhibit strong molecular ion peaks and minimal fragmentation or background. LDI-TOF and STM techniques are combined to evaluate monolayer composition and morphology, track solution-monolayer exchange, to identify reaction products and to measure kinetics of chemical reactions at the solution-monolayer interface. LDI-TOF MS provides rapid qualitative evaluation of monolayer composition across a graphite substrate. Challenges to quantitative composition evaluation by LDI-TOF include compound-specific light absorption, surface desorption/ionization and fragmentation characteristics. For some, but not all, compounds, applying matrix onto a self-assembled monolayer increases molecular ion intensities and affords more accurate assessment of monolayer composition via matrix assisted laser desorption/ionization (MALDI) MS. Matrix addition precludes subsequent chemical or STM studies of the monolayer, whereas reactions and STM may be performed at nonirradiated regions following LDI-TOF measurements. LDI- and MALDI-TOF MS are useful complements to STM and are easily implemented tools for study of physisorbed monolayers.

Deposition-Temperature- and Solvent-Dependent 2D Supramolecular Assemblies of Trimesic Acid at the Liquid–Graphite Interface Revealed by Scanning Tunneling Microscopy

We investigated the self-assembly of trimesic acid (TMA) at the solution–graphite interface by using scanning tunneling microscopy (STM). We show that the polymorphism of the adsorbate structures of TMA can be controlled by the substrate temperature during the deposition of the molecules out of the solution for various solvents of different polarity. TMA was dissolved in phenyloctane, octanoic acid, and undecanol. At elevated substrate temperatures, various periodic assemblies of TMA could be obtained. By increasing the temperature of the preheated substrate, the specific 2D supramolecular network structure and the corresponding packing density can be precisely tuned in each kind of the solvents studied. The results found by STM are explained by the increased concentration of the solution at the preheated substrate as well as the higher mobility of the solute molecules increasing the opportunity of interactions between the molecules, in particular different hydrogen bonding motifs. Our interpretation is s...

Semiquantitative Electron Tunneling Barrier Height Measurements of Molecular Monolayers at the Solution–Graphite Interface: Nonorbital-Mediated Tunneling

Semiquantitative apparent tunneling barrier measurements were collected of stearic acid adsorbed onto highly oriented pyrolytic graphite from 1-phenyloctane. A detailed investigation of the topography revealed an interesting geometric phenomenon, namely, the observation of different conformers within a single monolayer; this phenomenon has not been previously reported, despite the large volume of work on these and similar systems in the literature. Apparent barrier height images showed two separate effects. The apparent barrier height map is dominated by the distribution of surface charge (as tracked by electrostatic potential) in regions where significant polarity exists (i.e., near the carboxyl groups). Conversely, in regions where there is little variation in electrostatic potential (i.e., near the interlaced alkane chains), the manifestation of the topography dominates the apparent barrier height. The electron tunneling involved in imaging these types of monolayers is not mediated by adlayer molecular...

C3-Symmetric Pyridine and Bipyridine Derivatives: Simple Preparation by Cyclocondensation and 2D Self-Assembly at a Solution–Graphite Interface

The efficient preparation of four C3-symmetric (star-shaped) pyridine and bipyridine derivatives is reported. The key steps are Suzuki couplings of 4-pyridyl nonaflates with 4-acetyl-phenylboronic acid followed by an acid-promoted cyclocondensation reaction converting the methyl ketone moiety into the central benzene ring of the target compounds. Based on STM studies at a graphite–solution interface the two-dimensional arrangements of the compounds are discussed, showing the influence of the pyridine substitution pattern.

Desorption Kinetics and Activation Energy for Cobalt Octaethylporphyrin from Graphite at the Phenyloctane Solution–Graphite Interface: An STM Study

Temperature-dependent desorption rates and desorption energies are determined from a monolayer assembly at the solution–solid (SS) interface. Scanning tunneling microscopy (STM) was used to measure molecular-scale temperature-dependent desorption of cobalt(II) octaethylporphyrin (CoOEP) at the phenyloctane solution–highly ordered pyrolytic graphite (HOPG) interface. At lower temperatures, monolayer formation of metal(II) octaethylporphyrin (MOEP) on HOPG from solution was found to be completely controlled by kinetics, and the adlayer formed was stable up to 70 °C. Significant desorption of CoOEP from the HOPG surface was observed above 80 °C on a time scale of hours. CoOEP desorbs from HOPG into phenyloctane at a rate of 0.0055 ± 0.0007 min–1 at 90 °C, 0.013 ± 0.001 min–1 at 100 °C, and 0.033 ± 0.003 min–1 at 110 °C. From these temperature- and time-dependent measurements, assuming an Arrhenius rate law, the activation energy of molecular desorption at the SS interface was determined using studies solely ...

Living on the edge: Tuning supramolecular interactions to design two-dimensional organic crystals near the boundary of two stable structural phases.

One of the benefits of supramolecular assemblies that form at dynamic interfaces is the opportunity to develop condensed phase systems that respond to environmental stimuli. A prerequisite of this responsive behavior is that the supramolecular system be designed to sit very near the stability of two or more crystal structures. We have created such a bi-phasic system with aryl-triazole oligomers by investigating how phase morphology is controlled by the interplay between interactions that involve the oligomer's dipolar cores (Δμ = 3.5 debye), van der Waals contacts of their pendant alkyl chains (C4-C18), and close-contact hydrogen bonding. Scanning tunneling microscopy experiments conducted at the solution-graphite interface allow sub-molecular resolution of the ordered monolayers to unambiguously determine the packing and structure of two principle phases, α and β. The system is balanced very near the edge of phase stability, evidenced by co-existent phases present over short time frames and by the changes in preference between the two 2D supramolecular assemblies that occur with small modifications to the molecular structure. We demonstrate that the bi-phasic behavior can be understood as a balance between electrostatic interactions and van der Waals contacts, two variables within a larger parameter space, allowing synthetic design to move this solution-surface system across the stability boundary of different condensed-phase structures. These findings are a foundation for the development of environmentally responsive 2D supramolecular arrays.

Anion-induced dimerization of 5-fold symmetric cyanostars in 3D crystalline solids and 2D self-assembled crystals

Anions stabilize stacking of C5-symmetric macrocycles, called cyanostars, into dimers in solution and herein we demonstrate this stacking in solid-state crystals. We further show that this guest binding can be applied as a route to bilayer growth at the solution-graphite interface.

Surface behavior and imaging of the lowest unoccupied molecular orbital of indolo[2,1-b]quinazoline-6,12-dione (tryptanthrin) via scanning tunneling microscopy

Indolo[2,1-b]quinazoline-6,12-dione (tryptanthrin) was imaged at the solution-graphite interface using scanning tunneling microscopy. STM images of the molecular monolayers show clear lobe-to-lobe resolution of the lowest unoccupied molecular orbitals of the molecules, as well as a somewhat unusual parallel arrangement of the molecular dipoles. Evaluation of the dipole-dipole interactions reveals that the interactions of the closest molecule pairs are reduced due to a magic angle-like effect. These results represent an important step toward a more in-depth understanding of the behavior of tryptanthrins, which are of interest due to their potential as pharmaceutical compounds.

Dipolar Side Chain Control of Monolayer Morphology: Symmetrically Substituted 1,5-(Mono- and diether) Anthracenes at the Solution−HOPG Interface

Scanning tunneling microscopy (STM) is used to determine the 2-D unit cell parameters of monolayers self-assembled by twelve symmetrical, 1,5-bis(linear aliphatic ether side chain) anthracenes at the solution-graphite interface. The standard morphology assembled by 1,5-bis(alkyloxymethyl) anthracenes consists of single-lamella domains containing columns of anthracene cores alternating with columns of interdigitated, aliphatic side chains. Adjacent side chains within the aliphatic columns adsorb in antiparallel orientations. The terminal methyl (omega-position) of each side chain lies in registration with the 2-positions of its two neighboring chains ((omega <--> 2)-packing). Anthracenes with diether side chains can generate repulsive or attractive dipole-dipole interactions between proximate ethers of adjacent aliphatic chains. Anthracenes bearing even length side chains with oxygens at the 2- and omega-1 positions or at the 3- and omega-2 positions do not assemble (omega <--> 2)-packed monolayers. Repulsive dipolar interactions between ethers in adjacent side chains raise the energy of (omega <--> 2) morphologies. These "self-repulsive" side chains drive assembly of (omega <--> l)- or (omega <--> 3)-packed morphologies, which enjoy stabilizing dipolar interactions between ethers in adjacent side chains. In stark contrast, anthracenes bearing odd length diether side chains assemble (omega <--> 2)-packed morphologies, regardless of whether adjacent chains suffer zero, one, or two sets of proximate dipole-dipole repulsions. The intrinsic energy gap from (omega <--> 2)- to non-(omega <--> 2)-packed morphologies of odd length side chain anthracenes is, apparently, larger than for even length side chain anthracenes. Overall, the twelve compounds self-assemble seven different morphologies. Distinguishing morphologies, understanding polymorphism within the monolayers, and evaluating the morphological consequences of side chain dipolar interactions is facilitated by viewing the monolayers as assemblies of 1-D, molecular tapes.

Scanning tunneling microscopy of indolo[2,1-b]quinazolin-6,12-dione (tryptanthrin) on HOPG: Evidence of adsorption-induced stereoisomerization

Scanning tunneling microscopy was used to observe monolayers of indolo[2,1-b]quinazolin-6,12-dione (tryptanthrin) at the solution–graphite interface. The monolayers were formed via physisorption from a saturated solution in 1-phenyloctane. Sub-molecular resolution was achieved allowing elucidation of the primary electronic features of the molecule. The adsorbed molecules form rows that are differentiable via the apparent heights of portions of each molecule, from row to row. The variation between the rows is attributed to adsorption-induced stereoisomerization of individual molecules, with each row comprised of one stereoisomer.

Electronic properties of a π-stacked pyrene derivative at a liquid–solid interface studied with scanning tunneling spectroscopy

Two kinds of self-assembled structures of 1-pyrenehexadecanoic acid at a solution-graphite interface were investigated using scanning tunneling microscopy. The pyrenyl groups of 1-pyrenehexadecanoic acid formed a π-stacked assembly when it was co-adsorbed with 4,4′-dipyridyl although the pyrenyl groups remained isolated in the one-component self-assembled structure. On the basis of the Davidov splitting model, the shift of the pyrenyl group’s electronic levels, upon formation of the π-stacked assembly, was estimated from the observed structures. The energy shift was found to be consistent with the observed change in the electron-tunneling spectra between these two different structures.

2D Self‐Assembly of Oligo(p‐phenylene vinylene) Derivatives: From Dimers to Chiral Rosettes

Enantiomerically pure oligo(p-phenylene vinylene) diaminotriazine derivatives and a short structurally related achiral diaminotriazine derivative, all having a rigid backbone in common, are studied to self-assemble at the solution-graphite interface by scanning tunneling microscopy. As a function of the length of the backbone, different two-dimensional motifs are formed (dimers and rosettes) that are rationalized in terms of the balance between different intermolecular interactions, in this case, intermolecular hydrogen bonding and the packing requirements of the alkyl chains on a graphite surface. In addition, the effect of molecular chirality on monolayer chirality is investigated, revealing molecular size-dependent expressions of the monolayer chirality.

Self-assembly of electron donor-acceptor dyads into ordered architectures in two and three dimensions: surface patterning and columnar "double cables".

We report the synthesis and characterization of covalent dyads and multiads of electron acceptors (A) and donors (D), with the purpose of exploiting their nanophase separation behavior toward (a) two-dimensional (2D) surface patterning with well-defined integrated arrays of dissimilar molecular electronic features and (b) bulk self-assembly to noncovalent columnar versions of the so-called "double cable" systems, the likes of which could eventually provide side-by-side percolation pathways for electrons and holes in solar cells. Soluble, alkylated hexa-peri-hexabenzocoronenes (HBCs) bearing tethered anthraquinones (AQs) are shown by scanning tunneling microscopy (STM) to self-assemble at the solution-graphite interface into either defect-rich polycrystalline monolayers or extended 2D crystalline domains, depending on the number of tethered AQs. In the bulk, the thermal stability of the room-temperature HBC columnar phase is increased, which is attributed to the desired nanotriphase separation of HBC columns, insulating alkyl sheaths, and AQ units. Homeotropic alignment (columns normal to surfaces), predicted to be ideal for potential exploitation of such "double cables" in photovoltaic devices, is demonstrated.

Packing of HO(CH2)14COOH and HO(CH2)15COOH on Graphite at the Liquid−Solid Interface Observed by Scanning Tunneling Microscopy: Methylene Unit Direction of Self-Assembly Structures

Self-assembled monolayers (SAMs) composed of 15-hydroxypentadecanoic acid and 16-hydroxyhexadecanoic acid are investigated with scanning tunneling microscopy (STM) at the solution-graphite interface. The observed packing structures of these two molecules on the surface are dramatically different and reveal different intermolecular bonding arrangements exhibiting a new type of odd/even chain-length effect. In addition, solvent coadsorption has been found to occur in the case of 15-hydroxypentadecanoic acid, and an unusual surface pattern in which the molecular axes in neighboring lamellae are completely aligned with one another is observed for 16-hydroxyhexadecanoic acid.

Self-Assembly of Alkyl Substituted Schiff Base and Its Cu(II) Complex Observed on Solution–Graphite Interface by Scanning Tunneling Microscopy

Abstract Self-assembled monolayer of Schiff base ligand, N-octadecylsalicylaldimine (C18Sal) has been observed by scanning tunneling microscope on the solution–graphite interface and the molecular geometry has been assigned. The molecular pattern of the Schiff base immediately changed upon addition of Cu(II) associated with in situ complexation and formation of a new molecular framework.

STM observation of 12-hydroxyoctadecanoic acid and its 4,4′-bipyridinium salt self-assembled on a graphite surface†

Self-assembled monolayers of the gelling agent, 12-hydroxyoctadecanoic acid and its 4,4′-bipyridinium salt have been observed on the solution graphite interface using scanning tunnelling microscopy (STM) and the molecular patterns are discussed wth respect to the intermolecular hydrogen bond network.