Influence of Different Substitution Patterns on the 2D Crystalline Aggregation of Small Molecules on HOPG Surfaces

A series of benzoic and terephthalic acid derivatives and their corresponding ethyl esters have been synthesized. These are decorated with dendritic units carrying alkoxy chains that differ in the number of alkoxy groups and/or their substitution pattern. These molecules were deposited from solution on highly oriented pyrolytic graphite, and the resulting supramolecular surface patterns have been studied by scanning tunneling microscopy at the solid/liquid interface. Thus, we were able to perform a systematic study on the influence of orthogonal functional groups governing the supramolecular assembly, that is, the alkoxy chains of dendritic units and the carboxylic acid (ester) functions, on the intermolecular interactions, that is, van der Waals interactions and hydrogen bonding between the adsorbed species that drive the pattern formation.

Enantiomer surface chemistry: conglomerate versus racemate formation on surfaces

Research on surface chirality is motivated by the need to develop functional chiral surfaces for enantiospecific applications. While molecular chirality in 3D has been the subject of study for almost two centuries, many aspects of 2D chiral surface chemistry have yet to be addressed. In 3D, racemic mixtures of chiral molecules tend to aggregate into racemate (molecularly heterochiral) crystals much more frequently than conglomerate (molecularly homochiral) crystals. Whether chiral adsorbates on surfaces preferentially aggregate into heterochiral rather than homochiral domains (2D crystals or clusters) is not known. In this review, we have made the first attempt to answer the following question based on available data: in 2D racemic mixtures adsorbed on surfaces, is there a clear preference for homochiral or heterochiral aggregation? The current hypothesis is that homochiral packing is preferred on surfaces; in contrast to 3D where heterochiral packing is more common. In this review, we present a simple hierarchical scheme to categorize the chirality of adsorbate-surface systems. We then review the body of work using scanning tunneling microscopy predominantly to study aggregation of racemic adsorbates. Our analysis of the existing literature suggests that there is no clear evidence of any preference for either homochiral or heterochiral aggregation at the molecular level by chiral and prochiral adsorbates on surfaces.

Revealing the structural detail of individual polymers using a combination of electrospray deposition and UHV-STM

The structure of a PPV based polymer was characterised at the nanoscale using a combination of electrospray deposition and UHV-STM.

Self-assembly of conjugated oligomers and polymers at the interface: structure and properties

In this review, we give a brief account on the recent scanning tunneling microscopy investigation of interfacial structures and properties of π-conjugated semiconducting oligomers and polymers, either at the solid-air (including solid-vacuum) or at the solid-liquid interface. The structural aspects of the self-assembly of both oligomers and polymers are highlighted. Conjugated oligomers can form well ordered supramolecular assemblies either at the air-solid or liquid-solid interface, thanks to the relatively high mobility and structural uniformity in comparison with polymers. The backbone structure, substitution of side chains and functional groups can affect the assembling behavior significantly, which offers the opportunity to tune the supramolecular structure of these conjugated oligomers at the interface. For conjugated polymers, the large molecular weight limits the mobility on the surface and the distribution in size also prevents the formation of long range ordered supramolecular assembly. The submolecular resolution obtained on the assembling monolayers enables a detailed investigation of the chain folding at the interface, both the structural details and the effect on electronic properties. Besides the ability in studying the assembling structures at the interfaces, STM also provides a reasonable way to evaluate the distribution of the molecular weight of conjugated polymers by statistic of the contour length of the adsorbed polymer chains. Both conjugated oligomers and polymers can form composite assemblies with other materials. The ordered assembly of oligomers can act as a template to controllably disperse other molecules such as coronene or fullerene. These investigations open a new avenue to fine tune the assembling structure at the interface and in turn the properties of the composite materials. To summarize scanning tunneling microscopy has demonstrated its surprising ability in the investigation of the assembling structures and properties of conjugated oligomers and polymers. The information obtained could benefit the understanding of the elements affecting the film morphology and helps the optimization of device performance.

Two-dimensional self-assembly of linear molecular rods at the liquid/solid interface

We report on the synthesis and scanning tunneling microscopy (STM) studies of a series of linear molecular rods (1-5) comprising different numbers and/or spatial arrangements of perfluorinated benzene and benzene subunits interlinked with diacetylenes in the para position and decorated with or without terminal dodecyl chains. The molecules organize themselves into well-ordered 2D crystal structures at the liquid/solid interface through intermolecular and molecule-substrate interactions. Whereas the molecules substituted by dodecyl chains form the lamellar structures with alternating rigid core rows and alkyl chain rows, the unsubstituted ones change the orientation of the rigid backbones with respect to the lamellar axis. The molecular arrangement is not influenced by fluoro substituents on any phenyl ring of the backbone, which suggests that the interactions between the π-conjugated backbones are dominated by close packing rather than by the dipole moments of the rods or fluorine-based intermolecular interactions.

Star-shaped oligofluorenes end-capped with carboxylic groups: syntheses and self-assembly at the liquid-solid interface

A series of star-shaped oligofluorenes end-capped with carboxylic acid groups were synthesized. Different numbers of carboxyl groups that can form hydrogen bonds, and long alkane chains that have stabilizing effects, were intentionally introduced. The resulting molecular architectures of the so-prepared star-shaped oligofluorenes at the liquid-solid interface were investigated by scanning tunneling microscopy. It is found that the number of hydrogen-bonding groups and the symmetry of the target molecules have crucial influences on the structures of the ordered assemblies.

Influence of Supramolecular Organization on Energy Transfer Properties in Chiral Oligo(p-phenylene vinylene) Porphyrin Assemblies

A comparative study on oligo(p-phenylene vinylene) (OPV)-appended porphyrins containing all trans-vinylene (either hydrophilic or lipophilic) or amide linkages (lipophilic) is presented. The type of supramolecular arrangement obtained in organic solvents proves to be strongly dependent on the nature of the covalent connection. In the case of all trans-vinylene linkages, a J-type intermolecular packing is obtained and the assemblies are only of moderate stability. Conversely, the supramolecular structures obtained from the amide-linked system display an H-type stacking arrangement of enhanced stability and chirality as a consequence of intermolecular hydrogen bonding along the stack direction, favorably interlocking the stacked building blocks. Interestingly, the observed differences in stability and organization are qualitatively illustrated by monitoring the sequential energy transfer process in both types of assemblies. Efficient intramolecular energy transfer from the OPVs (donors) to the respective porphyrin cores is followed by energy transfer from Zn-porphyrin (donor) to free-base porphyrin (acceptor) in both systems. However, the improved intermolecular organization for the amide-linked system increases the energy transfer efficiency along the stack direction. In addition, the water-soluble (OPV)-appended porphyrin system forms highly stable assemblies in an aqueous environment. Nevertheless, the poor energy transfer efficiency along the stack direction reveals a relative lack of organization in these assemblies.

Hydrogen-Bonded Oligo(p-phenylenevinylene) Functionalized with Perylene Bisimide: Self-Assembly and Energy Transfer

We describe the synthesis, supramolecular ordering on surfaces and in solution, and photophysical characterization of OPV4UT-PERY, an oligo(p-phenylenevinylene) (OPV) with a covalently attached perylene bisimide moiety. In chloroform, the molecule forms dimers through quadruple hydrogen bonding of the ureido-s-triazine array. This is supported by scanning tunneling microscopy (STM) studies, which reveal dimer formation at the liquid (1,2,4-trichlorobenzene)/solid (graphite) interface. Moreover, contrast reversal in bias-dependent STM imaging provides information on the ordering and different electronic properties of the oligo(p-phenylenevinylene) and perylene bisimide moieties. In dodecane, the molecule self-assembles into H-type aggregates that are still soluble as a result of the hydrophobic shell formed by the dodecyloxy wedges. The donor-acceptor molecule is characterized by efficient energy transfer from the photoexcited OPV to the perylene bisimide. Mixed assemblies with analogous OPVs lacking the perylene bisimide unit have been prepared in dodecane solution and energy transfer to the incorporated perylene bisimides has been studied by fluorescence spectroscopy.

The Chemistry of Organic Nanomaterials

The development of nanotechnology using organic materials is one of the most intellectually and commercially exciting stories of our times. Advances in synthetic chemistry and in methods for the investigation and manipulation of individual molecules and small ensembles of molecules have produced major advances in the field of organic nanomaterials. The new insights into the optical and electronic properties of molecules obtained by means of single-molecule spectroscopy and scanning probe microscopy have spurred chemists to conceive and make novel molecular and supramolecular designs. Methods have also been sought to exploit the properties of these materials in optoelectronic devices, and prototypes and models for new nanoscale devices have been demonstrated. This Review aims to show how the interaction between synthetic chemistry and spectroscopy has driven the field of organic nanomaterials forward towards the ultimate goal of new technology.

Pyrazolino[60]fullerene-Oligophenylenevinylene Dumbbell-Shaped Arrays: Synthesis, Electrochemistry, Photophysics, and Self-Assembly on Surfaces

Symmetrically substituted oligophenylenevinylene (OPV) derivatives bearing terminal p-nitrophenylhydrazone groups have been prepared and used for the synthesis of dumbbell-shaped bis(pyrazolino[60]fullerene)-OPV systems. In these triad arrays, the OPV-type fluorescence is dramatically quenched as a consequence of ultrafast OPV-->C60 singlet energy transfer. In its turn the fullerene singlet state is quenched by pyrazoline-->C60 electron transfer, in line with the behavior of the corresponding reference fullerene molecule. The occurrence of electron transfer in the multicomponent arrays is evidenced by recovery of fullerene fluorescence at 77 K in CH2Cl2 and in toluene at 298 K. Under these conditions the OPV-->C60 energy transfer is unaffected. The rate of this process turns out to be higher for the OPV trimer than for the corresponding pentameric OPV arrays, in agreement with energy-transfer theory expectations. Scanning tunneling microscopy (STM) and scanning force microscopy (SFM) revealed that the bis(pyrazolino[60]fullerene)-OPV can self-assemble into ordered layered crystalline architectures on the basal plane of highly oriented pyrolitic graphite.

Self-Assembly at the Liquid/Solid Interface: STM Reveals

The liquid/solid interface provides an ideal environment to investigate self-assembly phenomena, and scanning tunneling microscopy (STM) is the preferred methodology to probe the structure and the properties of physisorbed monolayers on the nanoscale. Physisorbed monolayers are of relevance in areas such as lubrication, patterning of surfaces on the nanoscale, and thin film based organic electronic devices, to name a few. It's important to gain insight in the factors which control the ordering of molecules at the liquid/solid interface in view of the targeted properties. STM provides detailed insight into the importance of molecule-substrate (epitaxy) and molecule-molecule interactions (hydrogen bonding, metal complexation, and fluorophobic/fluorophilic interactions) to direct the ordering of both achiral and chiral molecules on the atomically flat surface. By controlling the location and orientation of functional groups, chemical reactions can be induced at the liquid/solid interface, via external stimuli, such as light, or by controlled manipulation with the STM tip. The electronic properties of the self-assembled physisorbed molecules can be probed by taking advantage of the operation principle of STM, revealing spatially resolved intramolecular differences within these physisorbed molecules.

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.

Helically Annelated and Cross-Conjugated Oligothiophenes: Asymmetric Synthesis, Resolution, and Characterization of a Carbon−Sulfur [7]Helicene

The synthesis and characterization of a novel oligothiophene, in which the thiophene rings are annelated into a [7]helicene with cross-conjugated pi-system, are described. Such [7]helicenes may be viewed as fragments of the unprecedented carbon-sulfur (C(2)S)(n)() helix, possessing sulfur-rich molecular periphery. Racemic synthesis of [7]helicene is based upon iterative alternation of two steps: C-C bond homocouplings between the beta-positions of thiophenes and annelation between the alpha-positions of thiophenes. Asymmetric synthesis is carried out using (-)-sparteine-mediated annelation of the axially chiral bis(aryllithium) with electrophilic sulfur equivalent. Alternatively, enantiomers of the [7]helicene are obtained via resolution using menthol-based chiral siloxanes. Racemic [7]helicene and four other macrocyclic products of the annelation are characterized by X-ray crystallography. One of the solvent polymorphs of the [7]helicene possesses pi-stacked columns of opposite enantiomers and multiple short intermolecular contacts, including both homochiral and heterochiral short S...S contacts, suggesting an effective intermolecular electronic coupling in two-dimensions. The [7]helicene is configurationally stable at room temperature and racemizes at 199 degrees C with a half-time of about 11 h. Selected physicochemical studies (UV-vis absorption, CD, optical rotation, and cyclic voltammetry) of the [7]helicene are described.

A nanoscale view of supramolecular stereochemistry in self-assembled monolayers of enantiomers and racemates

The effect that molecular chirality has on the formation of monolayer structures by pure enantiomers and their racemates at the liquid/solid interface has been investigated for two chiral compounds (1 and 2) which differ from each other by the presence or absence of an ester function in their respective molecular structures. 1 shows pseudoracemate formation when the achiral graphite support is exposed to a solution containing a racemate while 2 shows racemic conglomerate formation. This difference is rationalized in terms of the orientation of the pure enantiomers with respect to the graphite substrate and highlights the importance of molecular structure and its influence on balancing the interplay between molecular conformation and molecular packing on the surface. For 1, nonstoichiometric mixtures of both enantiomers have been investigated, and the results are discussed in the framework of the sergeant and soldiers principle. These results are important for the understanding and prediction of spontaneous resolution in monolayer systems.

Expression of molecular chirality and two-dimensional supramolecular self-assembly of chiral, racemic, and achiral monodendrons at the liquid-solid interface

We have investigated the two-dimensional ordering of chiral and achiral monodendrons at the liquid-solid interface. The chiral molecules self-assemble into extended arrays of dimers. As expected, the R enantiomer forms the mirror image type pattern of the chiral two-dimensional structure formed by the S enantiomer. A racemic mixture applied from solution onto the substrate undergoes spontaneous segregation: the enantiomers separate on the surface and appear in different domains. In contrast to the chiral molecules, the achiral analogue self-assembles into cyclic tetramers. Moreover, the pattern formed by the achiral molecule strongly depends on the solvent used. In the case of 1-phenyloctane, solvent molecules are coadsorbed in a 2:1 (dendron:solvent) ratio whereas in 1-octanol, no solvent molecules are coadsorbed. By the appropriate solvent choice, the distance between the potential "supramolecular containers" can be influenced.

Bias-Dependent Visualization of Electron Donor (D) and Electron Acceptor (A) Moieties in a Chiral DAD Triad Molecule

The 2D crystal lattice structure and bias-dependent contrast of a chiral electron donor-acceptor-donor triad system, composed of two oligo(p-phenylene vinylene) electron donors and a perylenediimide electron acceptor (OPV4-PDI-OPV4), have been studied by means of scanning tunneling microscopy (STM) at the liquid-graphite interface. OPV4-PDI-OPV4 is ordered in rows and forms a well-ordered 2D crystal lattice structure. The electrical properties of the donor and acceptor parts are distinguished by the contrast in bias-dependent STM imaging.