INVITED ARTICLE
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Self-assembly of 1-D organic semiconductor nanostructures Thuc-Quyen Nguyen,*a Richard Martel,b Mark Bushey,c Phaedon Avouris,d Autumn Carlsen,e Colin Nuckollsf and Louis Brusf Received 13th July 2006, Accepted 29th November 2006 First published as an Advance Article on the web 4th January 2007 DOI: 10.1039/b609956d This review focuses on the molecular design and self-assembly of a new class of crowded aromatics that form 1-D nanostructures via hydrogen bonding and p–p interactions. These molecules have a permanent dipole moment that sums as the subunits self assemble into molecular stacks. The assembly of these molecular stacks can be directed with electric fields. Depending on the nature of the side-chains, molecules can obtain the face-on or edge-on orientation upon the deposition onto a surface via spin cast technique. Site-selective steady state fluorescence, time-resolved fluorescence, and various types of scanning probe microscopy measurements detail the intermolecular interactions that drive the aromatic molecules to selfassemble in solution to form well-ordered columnar stacks. These nanostructures, formed in solution, vary in their number, size, and structure depending on the functional groups, solvent, and concentration used. Thus, the substituents/side-groups and the proper choice of the solvent can be used to tune the intermolecular interactions. The 1-D stacks and their aggregates can be easily transferred by solution casting, thus allowing a simple preparation of molecular nanostructures on different surfaces.
1. Introduction The self-organization of small molecules into larger functional nanostructures is a cornerstone of biological systems and is a powerful tool to create novel materials with emergent or amplified properties.1 Recently, there is increasing interest in using elementary building blocks such as atoms and molecules to form molecular wires or 1-D nanostructures in a controlled and predictable fashion.2 Discotic liquid crystals,3 discovered in 1977 by Chandrasekar and coworkers,4 are examples of such systems. This class of materials self-assembles to form arrays of columnar stacks. Individual stacks have been compared to molecular wires because the column’s interior consists of co-facially aromatic cores (conducting cores) while its exterior is surrounded by a hydrocarbon chain (an insulating layer).5 This arrangement of the aromatic cores yields useful and interesting electronic and optic properties.6 For traditional discotics, the intermolecular interactions between the subunits are weak due to the poor electrostatic attraction between the electron rich p-surfaces.7 Several approaches have been used to increase the affinity between the molecules within the columnar stack including metal–ligand interactions,8 recognition of polymer strands,9 electrostatic complimentarity between p-faces,10 and hydrogen bonds.11 Understanding self-assembly processes and the preferred molecular arrangement of functional aromatic compounds a
Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106. E-mail:
[email protected] b De´partement de Chimie, Universite´ de Montre´al, Montre´al, Que´bec, Canada c Scripps Research Institute, La Jolla, CA, USA d IBM Watson Research Center, Yorktown Heights, NY, USA e Physics Department, Albany University, Albany, NY, USA f Chemistry Department, Columbia University, New York, NY, USA
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are prerequisites for achieving predictive models that take into consideration the composition of the molecular components. As an example, organic and polymeric semiconductors having better order and packing density usually exhibit better electrical conduction.12 A systematic investigation on the local order properties of a given molecular design is therefore important for making and improving organic semiconducting assemblies, but a detailed investigation of the resulting structure of ordered assemblies remains challenging experimentally. Scanning probe microscopy is now among the most powerful techniques to probe the arrangement at a surface with subnanometer resolution. Scanning tunneling spectroscopy (STM) has been used widely to study the self-assembled processes of aromatic molecules and alkyl thiols on metal surfaces13 and molecules at the solid-liquid interface on highly oriented pyrolytic graphite (HOPG) substrates.14 Atomic force microscopy (AFM) is commonly used to probe surface topography. Electrostatic force microscopy (EFM) is another form of scanning probe microscopy that allows the simultaneous mapping of surface topography and electrostatic field gradients. EFM has been employed to study trapped charge in SiO2 layers15 and surface charge in semiconductors16 and organic materials.17 A combination of proximal probe techniques is however required in order to gain insights on the local structure of the assembly and on the specific interactions driving the assembly process. This review discusses the assembly characteristics in solution and thin film of a new class of columnar discotic liquid crystal materials that is held together with hydrogen bonds and p–p interactions.18 These aromatic molecules are composed of a 1-D stack of an aromatic core surrounded by a hydrocarbon sheet. The core is a hexa-substituted aromatic (1 and 2 in Fig. 1) consisting of three meta-disposed amides that Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1515
Fig. 1 Crowded aromatics and their energy minimized molecular models. Side-chains and hydrogens have been removed to clarify the view (in the model: the R 0 and R00 have been removed and red = oxygen; blue = nitrogen; grey = carbon).
are flanked by substituents other than hydrogen at each of the remaining positions. Unlike traditional discotics, these molecules stack to form molecular fibers due to a synergy between p-stacking and hydrogen bonding to produce a relatively strong association (molecule-to–molecule cohesion) in the stacking direction but a comparatively weaker interaction between fibers (Fig. 1A).18 The substituent, R 0 (R00 for 2a), is a long alkyl chain to ensure the solubility in common organic solvents whereas R 0 contains an amide group, which provides a dipole moment and hydrogen bonding that contributes to enhance the molecular cohesion in the stacking direction (Fig. 1). The substituents in the side groups (R 0 and R00 ) can be used to tailor the orientation of molecules on a surface and the intermolecular distance within a fiber through steric interactions.18c Monitoring the assembly of these mesogens in monolayer films by scanning probe microscopy has yielded films with two orientations. In one surface conformation a twodimensional sheet results that is macroscopically polar (faceon orientation, Fig. 2A). In the other orientation on the surface, 1-D p-stacks result that are only a few molecules wide but microns in length (edge-on orientation, Fig. 2B). The length of the fibers formed on surfaces depends on the solvent, the concentration as well as the underlying substrate. In the
bulk, these materials form highly regular and well-organized columnar assemblies.18g In solution, fluorescence spectroscopy gave clear indication that the underlying self-assembly process produces 1-D stacks. The length of the fibers and their number can be varied dramatically using different solvent and concentration. In ultra-thin films, 1b, 1c, 2a, and 2b assembles into dipolar columns that have their long axes and dipole moments parallel to the surface19 that can be directed with the electric field. The self-assembly of molecules 1 and 2 on graphite was examined by AFM, EFM, and ultrahigh vacuum (UHV)STM. There are six sections below describing the self-assembly characteristics of 1 and 2: (1) the molecular design of this new class of discotic crystals, (2) controlling the self-assembly by functional groups, (3) the self-assembly in solution, (4) the effects of solvents on the aggregation/self-assembly, (5) the effects of concentration and temperature on the self-assembly, and (6) the effects of the surface type on the self-assembly.
2. Molecular design Although there are other examples of benzene rings that are held cofacially by hydrogen bonds,20 the highly substituted nature of these subunits gives rise to new nanostructures,
Fig. 2 Schematic drawings of the face-on top view (A) and edge-on side view (B) orientation of hexa-substituted aromatics on graphite substrates. Reprinted with permission from ref. 18c. Copyright 2004 American Chemical Society.
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unique polar properties, and unusual phase behavior. For the molecules in Fig. 1, the design principle explored was how to use the flanking alkoxy groups for 1 and alkynyl substituents for 2 to force the amides out of the plane of the central aromatic ring and into a conformation that is predisposed to form three intermolecular hydrogen bonds. Fig. 1 shows the energy-minimized dimeric models for both 1 and 2.18 The flanking alkoxyl groups for 1 and alkynl groups for 2 force the amides out the plane of the central aromatic ring and into a conformation that allows the formation of three intermolecular hydrogen bonds. The size of the functional groups determines the angle of twist for the amide out of the aromatic ring plane and consequently modulates the distance between adjacent benzene rings. From models, the center-to-center distance between the benzene rings is ca. 3.8 A˚ for 1 and ca. 3.6 A˚ for 2 reflecting the relative size of the alkoxyl and the alkynyl groups. Additionally, each of the subunits has a permanent dipole moment that is perpendicular to the aromatic ring plane. The dipoles could sum as the molecules stack yielding columns that have a macroscopic dipole moment, similar to the moment that is seen for some metallomesogens and conical liquid crystals.21 These polar columns could be used as model systems to understanding how polar properties emerge on the nanoscale as well as how charges transport in 1-D nanostructures. Because this class of discotic molecules was unknown before the studies below were initiated, a large number of derivatives were synthesized to establish the structure/property relationships. The synthetic procedures were developed by the Nuckolls group. Bulk self-assemblies of 1 and 2 were studied by X-ray diffraction. X-Ray diffraction studies of 1a show two crystalline phases below 85 1C. Above 85 1C, a fluid phase develops and at 120 1C, the diffraction pattern has reflections that index to a rectangular lattice with parameters a = 38 A˚ and b = 22 A˚. Rectangular packing has been observed in other columnar liquid crystals and results from a distortion of the lattice. This distortion could arise because the side groups are mismatched in size, which frustrates hexagonal packing, or are bulky, which tilts or offsets the subunits.18a The diffraction pattern of 1b at 200 1C is dominated by a single sharp peak at low angle, signature of columnar assemblies. Up to fifth order diffraction peak is seen that can be indexed to a hexagonal lattice. The lateral core-to-core separation is 21 A˚ as expected for columns with noninterdigitating, extended side chains.18a There is no X-ray diffraction data available for 1c. Besides X-ray diffraction experiments, the assembly in bulk of 1 is deduced from a combination of experiments including polarized light microscopy, infrared spectroscopy, and differential scanning calorimetry.18 The results from these experiments show that the assembly process is dominated by the size and polarity of the amide side groups. Solution and thin film studies via steady state and time-resolved spectroscopy and scanning probe microscopy also support this conclusion. For example when the amide substituents are relatively small and flat, such as the phenyl substituents of 1b, the material assembles into regular cylinders that are hexagonally-packed into millimeter-scale domains. When the phenethyl side-chain is exchanged for the t-butylester of glycine (1a), the material is no longer able to stack into perfect cylinders and compensates This journal is
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by positioning the subunits either canted or offset. These misalignments produce a distorted hexagonal lattice for 1a. Further magnifying this trend, when the amide substituent is now changed to the t-butylester of D-alanine (now made even bulkier!), there is no discernable mesomorphism in bulk samples. Although there is less data, it appears that the substituents on the side-chains of 2 have less of an influence on the mesomorphism. The X-ray diffraction patterns for bulk samples of 2a and 2b show a columnar assembly whose primary reflection is 18.1 and 18.7 A˚, and the core-to-core distances are 2.14 and 2.09 A˚, respectively. For both cases, higher-order reflections allow the lattices to be indexed to a 2-D hexagonal arrangement of columns.18e The difference in stacking propensity between 1 and 2 is likely the result of the gear-like arrangement of side-chains in 1 that is lacking in 2 due to the linearity of the alkynl groups. Compounds 1, 2 and other derivatives are soluble in common organic solvents such as methylene chloride and chloroform. Typically, thin films are formed by spin-coating a solution between 1000–1500 rpm onto graphite substrates.
3. Controlling the self-assembly by functional groups The general hypothesis about the molecular design in discotic systems is the following: larger p-core and more compact side groups enhance p-stacking and reduce steric interactions, thereby reducing the intermolecular distance between molecules within the stack. The studies in this section test this assumption using a series of different molecule design (Fig. 1) and show that the packing order and the spacing between molecules within a columnar stack depend strongly on the functional groups. In principle, a better molecular control on the assembly could favorably influence charge transport along the columnar stack. Charge transport along the columnar stack depends strongly on the degree of p-orbital overlap, which is influenced by the intermolecular distance and the size of the conducting core. The former is supported by recent simulations, which show that a molecule-to–molecule distance of less than 4 A˚ could lead to band-like transport as oppose to thermal activated hopping.22 The latter has been shown experimentally by measuring the bulk charge mobilities of six different discotic liquid crystalline materials as a function of the core size.23 When these molecules self-assemble on a surface, they can either adopt a face-on or edge-on orientations depending on the functional groups. Fig. 2 shows the schematic drawing of the two orientations. The face-on orientation leads to the formation of patches or islands on surface (Fig. 2A) whereas the edge-on orientation results in the formation of columnar stacks (Fig. 2B).18 If the molecules obtain the edge-on orientation on the surface, the height of these fibers/stacks should be the same or within the range of the molecules’ diameters. One feature of the assembly of derivatives of 1 and 2 that is potentially useful has to do with their polar properties and how they develop on the nanoscale. To investigate the assembly and polarity of 1 and 2 on these length scales, conditions were found to produce films that have less than a monolayer of Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1517
coverage on highly ordered pyrolitic graphite (HOPG) substrates through spin casting. The topography, polarity/molecular orientation, and molecular packing of these films can be measured using AFM, EFM, and STM, respectively. 3.1.
Polar monolayers
When a bulky side group R00 is used such as the t-butylester of glycine (1a), the molecules obtain the face-on orientation. This functional group can rotate in and out of the aromatic plane. The length of this group measured from the amide unit is B7.9 A˚. If this side group rotates out of the aromatic plane, it acts as a spacer between the subunits, and hence, reducing the intermolecular interactions between molecules substantially. The energy minimized molecular model of 1a shows that two out of three t-butylester groups point out of the aromatic plane (point in the plan of the page) whereas the third one (blue circle) is almost in the same plane with the aromatic ring (Fig. 3, right). The model of 1b is also shown in Fig. 3 for a comparison. The phenyl groups are aligned with the aromatic ring. The height of the overlayer shown in Fig. 3 is ca. 0.5 nm. This height is consistent with the distance through the aromatic plane (ca. 0.4 nm) but much too small for the diameter of the molecules measured in molecular models and bulk to be ca. 1.2 nm. Also, the monolayer height of B0.5 nm is consistent with the functional group dimension because when deposited onto a surface, the surface restricts the rotation of the functional group. Thus, the functional group is aligned in the same plane as the aromatic ring. We infer from this data that the bulky glycine ester sidechains of 1a decrease the intermolecular attraction relative to the attraction with the graphite so that the molecules form 2-D, monolayer sheets
with aromatic cores parallel to the substrate. Thus, the permanent dipole moment is perpendicular to the sample surface. To further confirm the orientation of the molecules on the surface, EFM technique was used to measure the dipole moment (surface charge density) of the molecules caused by the change in the columnar orientation with respect to the surface. EFM probes the long-range electrostatic attraction between a conductive AFM cantilever and a conductive substrate.24 A schematic of the EFM setup is shown along with the results in Fig. 4. Surface charges (Q) and permanent dipoles (P) generate interactions with the total charge on the EFM tip through a Coulombic interactions. The attraction between the cantilever and the substrate is proportional to the square of the voltage difference between them. Thus, a sinusoidal voltage V = Vdc + Vac(sin ot) applied to the tip yields components of the attractive force at zero, o and 2o frequencies. The force at o gives a local measure of Q and P, and the force at 2o provides information about the material dielectric constant. The details of EFM experiment setup and theory are described elsewhere.25 EFM is extremely sensitive being able to measure a net charge of about 0.1 electron at a distance of 10 nm.24a Therefore, EFM could potentially detect the difference in dipole moment from columns of opposite polarity. Due to the face-on orientation with the permanent dipole moment is perpendicular to the sample surface, a large EFM signal is expected compared to molecules that self-assemble with edge-on orientation. The important finding shown in Fig. 4A and B is that the AFM and EFM images look essentially the same and the film has a universal net negative charge. The EFM signal is a convolution of the molecular charge due to charge transfer from the substrate and/or a perpendicular, permanent dipole moment. The molecules have a net dipole
Fig. 3 Left: topographic AFM image (10 10 mm) and the cross section of compound 1a spin-cast from methylene chloride on a HOPG substrate. Right: energy minimized molecular models of compounds 1a and 1b. AFM image is reprinted with permission from ref. 18c. Copyright 2004 American Chemical Society.
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Fig. 4 (A) 2.2 2.2 mm AFM image (data scale: 0–3 nm), (B) 1o EFM image of the same film (data scale: 0–400 Hz), (C) Dipole and electron transfer in thin films of 1a, and (D) EFM set-up for measuring surface charges and permanent dipoles in self-assembled columnar films. Reprinted with permission from ref. 18c. Copyright 2004 American Chemical Society.
perpendicular to the surface when the amides are forced out of the aromatic plane. Because of the partial positive charged on the amide N–H coupled with its out of plane conformation, a N–H/p interaction with the graphite substrate as shown in Fig. 4C could result. Also, the reason that 1a does not adsorb with the dipole pointed toward the surface due to electron repulsion between the electron rich surface (graphite surface) and the partial negative charged on the amide CQO. It is more stable for the partial positive charged on the amide N–H pointed toward the graphite substrate. In fact, similar interactions have already been observed between amines and carbon nanotube surfaces.26 Effectively, the surface could serve to orient the amide substituents and thereby direct the molecular This journal is
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dipoles.27 Electron transfer from the graphite to the monolayers could also be a contributor to the EFM signal. 3.2.
Isolated stacks
Remarkably, the opposite surface orientation, where the columns align parallel to the surface (edge-on orientation, Fig. 2B), is adopted for molecules 1b, 1c, 2a, and 2b shown in Fig. 1 and others tested that form hexagonal arrangements in bulk. Fig. 5 shows the topography AFM images and the cross sections of 1b, 1c, 2a, and 2b films spin-cast from methylene chloride solution. The four molecules form long molecular fibers on graphite at low concentration. The length Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1519
Fig. 5 Topographic AFM images and cross sections of films spin-cast from methylene chloride on HOPG substrates for: (A) compound 1c (1.54 1.54 mm), (B) compound 1b (2 2 mm), (C) compound 2a (3 3 mm), and (D) compound 2b (2 2 mm).
of these molecular fibers is concentration dependent (see Section 6 below). Fig. 5A shows the topographic image and the cross section of molecule 1c with the height of the fiber height at 0.581 0.183 nm from the surface. The height of fibers formed by molecule 1b, 2a, and 2b are 1.51 0.215 nm (Fig. 5B), 2.17 0.152 nm (Fig. 5C), and 1.91 0.178 nm (Fig. 5D), respectively. It is important to note that the height of molecule 1c is much smaller than its diameter (ca. 1.2 nm from the molecular model) but the fact that they form wire structure rules out the possible face-on orientation. We speculate that the molecules still achieve the edge-on orientation but they are tilted relative to the surface at a small angle. This is probably induced by the steric hindrance of the bulky side group with free rotation as discussed above leading to weaker interactions between adjacent molecules within the columnar stack and stronger interactions with the surface. From the cross sections of Fig. 5B, C, 1520 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532
and 5D, 1b, 2a, and 2b clearly stack with an edge-on orientation because the heights of the fibers are about the diameters of the molecules. Both the aromatic backbone and the sidegroups contribute to the height value but the aromatic backbone plays a dominant role due to its rigidity whereas the side groups are some what flexible. The height of 2b is slightly larger than 1b and this is possibly due to the substitution of the ether oxygen atom in R00 position for a carbon atom to form a triple bond leading to a larger p-core in 2b. These values correlate well with the 1.8 nm column spacing measured from bulk synchrotron X-ray diffraction.18e For 2, substitution of the alkyl chain by the phenyl group in R 0 position increases the diameter of the fiber further. The nanostructures observed in the AFM images are not individual fibers. There are several fibers packed next to each other (a sheet of fibers) due to strong van der Waals interactions between the fibers. The samples often comprise of individual fibers and sheet of fibers This journal is
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Fig. 6 Topographic (top) and 1 o EFM (bottom) images of compound 1c (A) (AFM data scale: 0–4 nm and EFM data scale: 0–15 Hz) and compound 1b (B) (data scale: 0–2 nm for AFM and 0–200 Hz for EFM). All images are 2 2 mm. AFM and EFM images are reprinted with permission from ref. 18c. Copyright 2004 American Chemical Society.
with various widths. The width of the nanostructures depends on the spin-speed and the molecule concentration: the higher the spin-speed and concentration, the thinner the width. The EFM images of compounds 1b and 1c shown in Fig. 6 (bottom) at sub-monolayer coverage on graphite trace the same morphology as the topographic image (top). Similar to molecule 1a, these films have a universal net negative charge. Electron transfer from the graphite to the molecular column can also contribute to the EFM signal in compound 1c (shown in Fig. 6A), but it is unlikely the main contribution to the signal because the hydrocarbon exterior chains minimize charge transfer with the substrate. Overall, this EFM result together with the AFM data confirms the orientation of compound 1c on the surface: the molecules achieve the edgeon orientation but they are tilted at a small angle relative to the surface. This tilt results in a significant dipole moment perpendicular to the stacking direction and also efficient charge transfer can take place. EFM measurements performed on columnar stacks of compound 1b reveals that they have essentially very weak or no measurable dipole or charge, which is consistent with the column axial dipole moment being parallel to the surface (Fig. 1B). Moreover, the hydrocarbon exterior insulates these stacks against charge transfer from the substrate. This is quite different compared to typically discotic liquid crystals, which do not form isolated 1-D structures but rather 2-D sheets with face-on orientation because molecular This journal is
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cohesion from the weak p-stacking forces holding the column together are nearly equal to the van der Waals intermolecular forces between alkyl side chains. For 1b, 2b, and 2a, the selfassociation in the stacking direction outweighs these van der Waals interactions leading to isolated stacks with edge-on orientation. From the topographic AFM images, all three molecules selfassemble to form fibers on graphite. However, we cannot determine exactly how the molecules pack together at molecular level. To investigate further the molecular packing in these fibers, we use high-resolution UHV scanning tunneling microscopy (STM) images such as those in Fig. 7. STM images of molecule 1c (Fig. 7A), molecule 1b (Fig. 7B), and molecule 2b (Fig. 7C) have been acquired with atomic and molecular resolution. These images clearly show that the three molecules self-assemble to form very different structures at the molecular level. The different structures result from a subtle change in molecule–molecule and molecule–surface interactions due to the functional groups. STM images of molecule 1c are shown in Fig. 7A. The images of the assembly are consistent with a packing of molecules bonded together through hydrogen bonding to form long molecular wires with the long alkyl chains (R00 ) anchored down to the graphite lattice. The orientation of the alkyl chains is clearly visible in Fig. 7A. The affinity of the molecules toward the graphite surface imposes a preferential orientation Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1521
Fig. 7 STM images of: (A) compound 1c (left image: 25 25 nm, right image: 10 10 nm) (B) compound 1b (left image: 70 70 nm, right image: 10 10 nm), and (C) compound 2b (left image: 15 15 nm, right image: 5 5 nm). Individual bright spots are monomers.
of the wire relative to the lattice direction of graphite. The detailed structure of the wires is somewhat intriguing. The long fiber is composed of a periodic pattern made of large bright spots alternated by small dark regions. Although these images clearly resolve the lattice of the graphite and the molecular arrangement of the molecules, they do not provide enough contrast to provide an unambiguous structure to the molecular packing. Details from the STM images allow us however to add important observations about the packing structure of molecule 1c. By counting the number of alkyl chains in these images, we note that there are about 108 side chains for twelve bright features (or clusters) in the left image (Fig. 7A). Therefore, each cluster has a total of 9 alkyl chains located on both sides from their center. Since each molecule has three alkyl chains, each cluster units has therefore to be 1522 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532
composed of three molecules. Due to bulky side-groups, the molecules cannot stack closely to each other as a result of a steric hindrance; thus, they pack as a group of three with a dislocation in between the groups of three molecules. By changing the side groups with a compact unit (molecule 1b and 2b), we expect that the interactions among molecules will be enhanced, thereby reducing the spacing between molecules within the stack. Fig. 7B shows the STM images of molecule 1b. Surprisingly, all the fibers show kinks in the same direction and no alkyl chain perpendicular to the stacking direction. The width of the fiber is too large (B4 nm) to be a diameter of a single molecule. Therefore, it is possible that three fibers bundle to form helix, and the kinks are helical pitches. If this is the case, molecule 1b would form a sheet-like conformation similar to b-sheet. The actual molecular packing This journal is
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motif is rather compact and seems too complicated to interpret from the STM image. It is clear that modeling will be needed in this case. For molecule 2b, we substitute the oxygen atom in R00 by an alkyne to create a larger p-core with the expectation that the larger p-core would strengthen p–p interactions within a columnar stack. Inspection of Fig. 7C shows that molecule 2b attains straight individual fibers with similar intermolecular distances. This result supports our hypothesis that the larger p-core enhances p–p interactions and reduces the spacing between molecules. Using the graphite lattice as a reference,28 we estimate the spacing between molecules is ca. 4.3 A˚. Although most molecules stack to form fibers, there are still some individual molecules (monomers) present on the surface. The monomers appear as isolated bright spots as seen in some areas of the film. The diameter of a molecule is ca. 2.0 nm in agreement with the value obtained by AFM studies. Although the exact mechanisms of the molecular packing motifs are not conclusive, the STM results reveal that functional groups influence the molecular packing motif and the self-assembly process. The results in this section show that it is possible to use the functional side groups of the molecules as a tool to control the molecular packing, orientation, and intermolecular spacing of overcrowded aromatics in thin films. By further tuning the property of this class of material, these nanostructures could be used as model systems to study electronic transport properties in 1-D organic semiconductors.
4. Evidence of self-assembly in solution This section focuses on the self-assembly characteristics of hydrogen-bond enforced, crowded aromatics into 1-D p-stacks in solution. It is not only important to master the intermolecular assembly but also to understand how to form and interface these assemblies with useful substrates.8,29 The question asked is do whether the molecules self-assemble in solution prior to deposition onto a surface. The assembly can be monitored with optical spectroscopy due to ground and excited states being electronically delocalized upon assembly. In this work we utilize site-selective wavelength dependent fluorescence spectroscopy––a valuable technique to understand the self-organization in solution for molecular systems such as proteins, peptides, and membrane-bound probes30—to determine whether 2a and 2b self-assemble into columnar structures in solution. Our approach is based on the different fluorescence response from individual molecules, which we call ‘‘monomers,’’ compared to molecules stacked to form columns, which we refer to ‘‘aggregates’’ (see Fig. 8B for the schematic drawing). The terms ‘‘fibers’’ or ‘‘aggregates’’ will be used interchangeably to refer to the 1-D nanostructures when visualized by microscopy. Furthermore, these fibers/ aggregates form species with ground and excited electronic properties distinct from that of the monomer. We selectively excite the monomers or the aggregates to obtain information about the assembly processes in solution. The excited state of the aggregate is significantly longer lived than from the isolated molecules and easily observed in time-resolved photoluminescence experiments. Aggregates for both 2a and 2b This journal is
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Fig. 8 Schematic representation of possible molecular arrangements of compound 2: (A) monomer; (B) aggregates/fibers of different sizes; (C) a bundle of aggregates/fibers; (D) a complex, ill-defined packing structure. Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
could be detected in methylene chloride solutions as low as 107 M. Spectroscopy and microscopy of films cast from solutions of 2a and 2b provide strong evidence that the structural integrity of the aggregates in solution is preserved during the transfer process onto the substrate.31 Thus, we can use the solvent and concentration to control the film morphology. Further organization of these aggregates into films with higher order structures is determined by whether the surface is hydrophilic or hydrophobic. In these discotic materials, a balance must be met between the subunits affinity for itself and for its solvent medium in a solution and for its substrate in a film. On the one hand, when the stacking forces between molecules are great the assemblies grow too large to be effectively solvated and precipitate into ill-defined superstructures. On the other hand, if the association between the subunits is too slight (i.e. due to the steric of the bulky side group), the p–p overlap between conjugated cores will be diminish and interactions with the surface will dominate resulting in a face-on orientation on the surface. The steric bulk of the side-chains on this crowded core is one of the determining factors of how well these molecules self-assemble. Besides the side group and its interplay with the core size, the solvent is also a crucial factor in the self-assembly process. This process can be an entropy driven process when the solvent molecules are released upon the self-assembly formation,32 but, if the solvent has very strong interaction with the molecules, the aggregate cannot be formed. Below we monitor the assembly of 2a and 2b, that have different groups on the alkynes flanking their amides (either hydrocarbon for 2b or phenyl for 2a), and probe how this assembly is crucially affected by its environment (solvent, temperature, and concentration). These solution-phase aggregates pre-determine Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1523
Fig. 9 Normalized (at the monomer peak) absorption and PL spectra of 2a and 2b in methylene chloride (concentration = 105 M, l = (excitation) = 290 nm for 2b and 320 nm for 2a). Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
(both in terms of quality and quantity) the type of nanostructured morphology manifest in thin films. Here we show that the self-assembly process exists in solution prior to the deposition of the supramolecular assembly on the surface. Fig. 9 shows the normalized absorption and photoluminescence (PL) of 2a and 2b in methylene chloride. Several observations can be made from this simple comparison. First, for 2a, the substitution of central ring with the phenylethynyl groups at R00 substituent causes a red shifting of ca. 40 nm in the absorption and ca. 20 nm in the emission compared to 2b with an alkyl group substituting the alkyne on the central aromatic ring at the R00 position. The red shifting is due to the larger conjugated core of compound 2a compared to 2b. Second, for two compounds, the PL spectra have a bifurcated peak structure. As explained below, one peak is from the monomers and the peak appeared at a longer wavelength is from the aggregates. Third, the ratio of the monomer/aggregate peak of 2a is larger, meaning that the concentration of monomers in the solution is higher in 2a than in 2b. Last, while the monomer emission is similarly shifted compared to that of the absorption spectra, the aggregate emission from compound 2a is blue shifted relative to that from the aggregate peak of 2b. One explanation for this blueshift in the aggregates with the larger aromatic core is that the freely rotating phenyl group on the ethynl-substituents causes problems in the packing. The PL spectrum of 1c is also included for a comparison. Similar to 2, the PL spectrum of 1c shows two peaks but both peaks are blue-shifted compared to 2. The monomer peak is blue-shifted B50 nm from the monomer peak of 2b, a result of having smaller conjugated core. The aggregate peak is also blue-shifted B25 nm compared to 2b. Additionally, this peak does not shift to longer wavelength with increasing the excitation wavelength as in the case of 2. Therefore, we conclude that 1c could form dimmer and trimer in solution. Compound 1c does not form large aggregates as in 2a and 2b because of the weak intermolecular interactions, a result of bulky functional groups as discussed in the previous section. The emission spectra can be understood by considering the fact that the molecules 2a and 2b stack in solution into columnar aggregates. These supramolecular structures form 1524 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532
helical stacks with an intermolecular distance for the cofacially arranged aromatic rings to be within their van der Waals radii. In PL, the excited state of the monomers is localized on a single molecule and the emission wavelength depends mostly on the size of the conjugated core. In contrast, the emission of the columnar structure is red-shifted relative to the monomer, because it has an excited state that is delocalized over several subunits/molecules within the stack. The delocalization of the excited state wave function across several molecules lowers the energy relative to the localized excited state wave function of a monomer resulting in a red-shifted luminescence from the aggregate. Of course, if the aggregate has poor overlap between the molecular subunits, the excitation will be more localized and therefore at a wavelength closer to the monomer emission. Thus, the position of the emission peak provides a good characterization of the intermolecular interaction in the aggregates; i.e., aggregates with stronger interactions present emission spectra that are red shifted compared to weakly interacting aggregates.33 The monomer peak at about 370–390 nm is clearly resolved from the broad structure in the emission at wavelengths between 400 nm and 600 nm, which originates from the aggregates. The broad emission spectrum of the aggregates suggests a wide distribution of sizes or the presence of different forms of aggregates. This point will be discussed further below. Fig. 10 shows the PLE (Fig. 10A) and PL (Fig. 10B) spectra of 2b in methylene chloride collected as the excitation and emission are moved to longer wavelengths. Because both 2a and 2b show similar results, only the results of 2b are presented below. A large red shift is observed in both PLE and PL spectra as the emission and excitation wavelengths increase. Generally, this red shift is either due to the strong electronic interactions among the chromophores or to the motion of chromophores in a restricted condensed media such as in a very viscous solution, in a membrane or in a micelle. 34 For the experiments performed here, the solvents used, i.e. methylene chloride and methanol, have similar viscosity (0.413 vs. 0.544 centipoice at 25 1C) while the viscosity of dodecane is three times larger (1.383 centipoice at 25 1C). However, there is no shift in the PL spectra for dodecane with increased excitation wavelength (see Section 5 for the effect of solvent).35 We can rule out the contribution of the red-shifted PL due to the increase in the solution viscosity. In addition, there are many cofacially stacked p-systems that show red-shifted absorbance and emission upon aggregation.36 PLE spectroscopy has no interfering background so it is very sensitive at low concentration. This is why the red-shifted absorption/aggregate bands in Fig. 10A at low concentration are observed only in the PLE spectra and not in the UV-visible absorption spectra. As seen in Fig. 10, the emission from the aggregates can be enhanced by preferentially exciting toward longer wavelengths (400 nm–500 nm) of PLE band maxima. Fig. 10B plots the normalized PL spectra of 2b excited at 290 nm (the peak of monomer exciton absorption) and at longer wavelength, 360 nm–480 nm, (aggregate band). The fluorescence spectra show that the aggregate emission shifts continuously to the red with the excitation wavelength. This can be explained in terms of a wide distribution in the aggregate length and number. From the PL spectra, it is This journal is
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Fig. 11 The fluorescence decays of compound 2b in methanol: the monomer was excited at 320 nm, and its emission was monitored at 380 nm (opened circles); the aggregates were excited at 420 nm and their emission were monitored at 480 nm (crosses); the fit to each curve is shown as solid line. Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
Fig. 10 (A) PLE and (B) normalized PL spectra of compound 2b in methylene chloride collected at different emission and excitation wavelengths. Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
difficult to estimate the size or number of the aggregates because their absorption spectra consist of a superposition of states with transition energies varying continuously. Moreover, their absorption cross-sections are unknown. Nevertheless, the results presented here provide direct evidence for aggregate formation in solution. The aggregate and its size can be visualized and estimated from the AFM images in Section 6 below. To further characterize the aggregates in solution, we measure the lifetimes of the excited states at different excitation and emission wavelengths. Because of the difference in the excited states between the aggregates and the monomers, the lifetimes are expected to be quite different. Our ability to excite selectively one or the other allows us to test this idea; i.e. the aggregate excited state would have a longer lifetime due to the delocalization through several p-stacked monomer units. With the correct choice of excitation and emission wavelengths, the excited state dynamics of the monomer and the aggregate are measured independently. From the PLE results in Fig. 10, we select the excitation for the aggregate in the red portion of the main absorption band ( Z 400 nm), while the monomer is obtained with excitation in the blue portion (r350 nm). Fig. 11 shows the fluorescent decay dynamics of compound 2b in methanol. Methanol was used because there was a substantial amount of the monomer present. In this experiment, the This journal is
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monomer is excited at 320 nm and the emission is collected at 380 nm. Since the aggregate emits much further to the red, we set the excitation to 420 nm and the emission was collected at 480 nm. The decay curve at 320 nm excitation in Fig. 11 was fitted to a single exponential with the lifetime of 1.72 ns— presumably dominated by the monomer lifetime. The decay at 420 nm excitation, however, fits to two lifetimes, one at 5.72 ns and one at 1.46 ns. The longer lifetime, 5.72 ns, is clearly the decay dynamic of the aggregates. The difference in the shorter lifetimes (1.72 ns versus 1.46 ns) is likely due to the error in assuming that the decay curve is comprised of two distinct lifetimes when in reality it is likely a composite of aggregates of different length and therefore different lifetimes. In methylene chloride 2b also shows two distinct lifetimes—one at 7.08 ns and the other at 1.70 ns.
5. The effects of the solvents on the aggregation In this section, we examine the effects of solvents on the molecular packing of molecule 2b. The aggregation process is a result of more favorable molecule–molecule interactions over molecule–solvent interactions.37 If the interaction between solutes and solvent molecules is stronger than molecule– molecule and solvent–solvent interactions, there would be very few aggregates in the solution and mostly monomers. On the contrary, if the molecule-solvent interaction is weak, there would be very few monomers present in solution. Fig. 12 shows the PLE and PL spectra of 2b (normalized at the monomer emission peak) in methylene chloride, methanol, a mixture of methylene chloride and methanol (4 : 1 by volume, referred to as ‘‘mix’’ in Fig. 12 and 13), and in dodecane at the same concentration (104 M). The aggregate absorption and emission peak positions and intensities are used to estimate their numbers and sizes. From Fig. 6, the aggregate peak is red-shifted and increases in intensity as it goes from dodecane, pure methanol, mixed solvent, to methylene chloride. This Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1525
Fig. 12 (A) PLE and (B) PL spectra of compound 2b in different solvents: methylene chloride (MeCl2, solid curve), mixture of methylene chloride and methanol (4 : 1 by volume) (Mix, dashed curve), methanol (MeOH, dashed-dot curve), and dodecane (long and two short dashed curve). They are normalized at the monomer peak (310 nm for PLE and 360 nm for PL). The excitation was at 320 nm and the emission was collected at 360 nm. Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
implies that there is a higher degree of aggregation in pure methylene chloride than in the other solvents tested. In methanol, however, the number of monomers increases due to the additional competition for the hydrogen bonds between the solvent and the molecules. As we go from methylene chloride to mixed solvents and then to pure methanol, the monomer emission increases smoothly with the methanol concentration probably due to the competition for the hydrogen bonding with the solvent. This effect is clearly seen from the spectra of the mixed solvent. However, there is still a measurable number of aggregates in these two solutions indicating that hydrogen bonding is not the only force that holds the molecules together in the aggregates. For example, p–p interactions and solvophobic effects between the aromatic cores can also play an important role in holding molecules together. As discussed below, methanol takes an active part in the formation of a new complex aggregate structure for 1. Surprisingly, the aggregate emission in dodecane is very weak and most, if not all, of the PL emission is from the monomer. This effect is likely due to the strong solvophobic interactions between the solvent and the long alkyl chains 1526 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532
attached to the monomer. The solvent stabilizes the monomer in the solution and therefore interferes with the packing process. This result is unexpected and coupled with the results from the studies with methanol indicate that not only hydrogen bonds but also solvophobic forces from the sidechains and the core facilitate the assembly. Fig. 13 presents the AFM topology of films made with compound 2b (B104 M) on graphite obtained by spin casting from methylene chloride, methanol, a mixture of methylene chloride/methanol, and dodecane at room temperature. The film in Fig. 13A from methylene chloride is composed of short fibers arranged in a multi-layered film ordered over small domains of about 0.1 0.1 mm. In the same conditions, the film cast obtained with the mixed solvent (methylene chloride/ methanol) in Fig. 13B has a much lower number of fibers. Thus, the AFM results agree very well with our interpretation of the PL data for both solutions. The electronic structure of the films evidenced by the long wavelength emission survives the casting process. The films obtained from the methanol solution contain large, tangled bundles with complex packing structures (Fig. 13C). For this solution, the PL spectra show a lower concentration of aggregates relative to the monomer, probably resulting from the strong competition for hydrogen bonding with the solvent. This competitive action prevents the formation of fibers with isolated columns (with a diameter of 1 B 1.9 nm) in favor of larger physical aggregates (with the diameter of ca. 30–50 nm) that have no influence on the optical properties of 2b in methanol. Presumably, the aggregate size in this bundle is smaller and there are fewer numbers of aggregates in methanol-cast film than for methylene chloride-cast film. This is in agreement with the blue shifted peaks of the aggregate emission in the PL emission when compared to the fibers emission observed in methylene chloride. These observations are consistent with poor molecular packing of units that interact weakly with each other. Thus, it is likely that the solvent participates in this packing structure by competing for hydrogen bonding. The case of the dodecane solution is rather simple. It forms a film on graphite (see Fig. 13D) with only a few aggregates/ fibers. The material deposited is mostly embedded in a featureless layer. In dodecane, although hydrogen bonds can be formed, this system does not favor the assembly and yields poor results. This is again consistent with the PL results described above. As we spin cast the solutions of 2a and 2b in methylene chloride onto graphite (B104 M), high-aspect ratio aggregates can be seen in the film morphology shown in Fig. 14. The topographic images of 2b (Fig. 14A) and 2a (Fig. 14B) are of short fibers packed closely together in random directions. The fibers of 2b are straight and packed closely to from ordered sheets/layers. However, this is not the case for 2a. The fibers of compound 2a have the tendency to tangle together and form poorly defined layers. We speculate that this disorder is related to the freely rotating phenyl groups on the triple bonds that introduce steric interaction in the assembled structure. The emission spectra of these fibrous films can be correlated with the type of solvent that is used for the casting. The important point is not that the exact morphology of a film is present in solution, but rather that the size and number of the This journal is
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Fig. 13 Topographic images of compound 2b in (A) methylene chloride (scan size: 1 1 mm), (B) mix (scan size: 1 1 mm), (C) methanol (scan size: 1.5 1.5 mm), and (D) dodecane (scan size: 1 1 mm) spin-cast on graphite. Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
underlying aggregates in a thin film is a consequence of concentration and the type of solvent used. Fig. 15 displays the PLE and PL of compound 2b spin cast from methylene chloride and methanol solutions. When the films are spin-cast from methylene chloride the aggregate absorption and emission bands are relatively high in intensity (vide infra). For methanol, the long wavelength aggregate emission is substantially attenuated (Fig. 15) implying that the delocalized aggregates are less in films from methanol. Again, this is qualitatively the same behavior observed in solution.
6. The effects of concentration and temperature To understand how temperature effected the aggregation of 2b, we collected the PL of 2b in methylene chloride at room
temperature, at 50 1C, at 80 1C, at 2 1C, and at 40 1C. We observed no apparent change in the PL intensity or the emission peak in the PLE and PL spectra as the solution was cooled down. At high temperature (50 1C and 80 1C), the PL intensity decreases slightly as excited at the monomer band, probably, due to the monomer PL quantum yield is lower at high temperature. There is no apparent change both in the PL intensity and the emission peak as we excited the aggregate band at 80 1C. Films cast from this solution show both fibers and some monomers. Heating these films at 80 1C does not change the orientation of the molecules with respect to the surface (face-on or edge-on orientations) or the number of the monomers/aggregates. We observe only phase segregation between the monomers and the aggregates. Heating the films spin-coated from 1b and 2b solutions in methylene
Fig. 14 Topographic images from methylene chloride spin-cast on graphite for: (A) compound 2b (scan size: 1.5 1.5 mm) and (B) compound 2a (scan size: 2 2 mm). Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
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Fig. 15 PLE and PL of compound 2b in methylene chloride (MeCl2, solid curve) and in methanol (MeOH, dashed curve) spin-cast on quartz substrates. The spectra are normalized at the monomer peak. The PLE spectra were collected at 360 nm and the PL spectra were excited at 320 nm. Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
chloride at 245 1C for 1 min and cooling down at 5 1C min1, only aggregates/fibers are observed as shown in Fig. 16. These fibers are packed closely together with the same height as observed in individual fibers before annealing. Next, we investigate the effects of concentration on the way the molecules pack in solution and on the film morphology. Fig. 17 presents the PLE and PL spectra (normalized at the monomer emission peak) of 2b in methylene chloride at different concentrations, B104 M, B105 M, B106 M. As the concentration increases, the aggregate band is redshifted and increases in the intensity. This peak is at about 340 nm at low concentration and shifted to 365 nm at higher concentration, and it is related to the fibrous structures seen with the AFM (Fig. 5C, 13, 16, and 18). In addition, the number of fibers/aggregates increases with concentration, as seen in the enhancement of the red-shifted emission (aggregate emission at 450 nm). The PL spectra are also red shifted with an increase in concentration, which is a signature of the formation of aggregates in solution. This can also be seen in the film morphology as shown in Fig. 18. At high concentration, the fibers formed are short and pack closely together. They also form a multilayered film on the graphite substrate (Fig. 18A and Fig. 15A). As the concentration of the solution decreases, the fiber length increases and creates isolated stacks of fibers, oriented according to the graphite lattice (Fig. 18B). This can be explained using basic concepts of crystal growth, but in this case, it is a 1-D growth process. There are several factors that influence the numbers of fibers formed and their length. These are the nucleation sites, the nucleation rate, and the growth rate. Generally, the nucleation sites, which determines the number of fibers formed, increase with the concentration, and this is the same for the nucleation rate.38 The growth rate is controlled by the diffusion process, and hence, at higher concentration or higher temperature, the probability that the molecules encounter a nucleation site increases.38 Consequently, the monomer depletion rate is much higher for concentrated solution than for dilute solution. Thus, there 1528 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532
Fig. 16 AFM images of 2b (A, scan size: 3 3 mm) and 1b (B, scan size: 2 2 mm) films annealing at 245 1C for 1 min and the cross section of 1b. The height of the fiber is 1.558 nm.
are more fibers in concentrated solution, but they are much shorter compared to the low concentration. Therefore, concentration can be used to control the length and number of the fibers formed. This journal is
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Fig. 17 (A) PLE and (B) PL spectra of compound 2b in methylene chloride at different concentration: 9.45 104 M (dot-dashed curve), 9.45 105 M (dashed curve), 4.74 105 M (dot curve), and 4.74 106 M (solid curve). The PLE (collected at 420 nm) and PL (excited at 270 nm) spectra are normalized at the monomer peak. Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
7. The effects of the surface type on the self-assembly In this section, we examine how the surface polarity influences the self-assembled nanostructures. Spin-casting 1b from CH2Cl2 solution (B106 M) onto the basal plane of highly ordered pyrolytic graphite (HOPG) or
onto a silicon wafer having a 200 nm thermally grown silicon oxide layer produces very thin, elongated nanostructures. A typical atomic force micrograph (AFM) is shown in Fig. 19A on HOPG. The fibers are only one-molecule high, a few molecules in width, but microns in length. On graphite, the molecules form straight fibers in registry with the graphite lattice. When using a cleaned silicon wafer (200 nm silicon oxide) as the substrate, the fibers were much longer and tended to bundle together to form ropes. This aggregation may arise from the mismatch between the polar and hydrophilic silicon oxide and the hydrophobic columns trying to minimize contact. Nonetheless, we found conditions to form very long ropes of columns that are about 50 nm in diameter. They are shown in the micrograph Fig. 19B where each fiber is about 25 molecules wide. In contrast to the regular arrangement of fibers on HOPG that arrange along the graphite lattice, on silicon oxide these ropes orient randomly on the glassy, amorphous silicon oxide layer. The interaction between the molecule and the surface is important for guiding the patterns formed by the film, such as the orientation of the fibers and their size or number on the surface. For example, compound 1b deposited on graphite at low concentration (B106 M) gives fibers that are spread out to form a monolayer on graphite, due to the strong van der Waals interaction between graphite and the molecules. The fibers are straight, packed parallel and in registry with the graphite lattice at either 601 or 1201 angles. On Si/SiO2 (see Fig. 19B), the fibers have the tendency to form bundles (7–50 nm in diameter) and orient randomly. This is a result of a less favorable interaction between this hydrophilic substrate and the hydrophobic aggregates. Thus, the fibers bundle up together to minimize the interactions with the surface and optimize the van der Waals interaction among the fibers. It is possible to use the electric field between vertical (between two ITO-coated glass plates) and lateral electrodes (between two gold electrodes patterned on silicon wafers) to direct the self-assembly of these 1-D nanostructures. For the lateral electrodes, there are several physical parameters influenced the alignment of the molecules under an external electric field: the width of the electrodes, the height/thickness of the electrode, the gap size, the strength of the electric field, and the number of molecules between the gap area.
Fig. 18 AFM images of compound 2b in methylene chloride at different concentration spin-cast on graphite: (A) 9.45 105 M (scan size: 1.5 1.5 mm) and (B) 9.45 106 M (scan size: 3 3 mm). Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
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Fig. 19 Topographic images of 1b on HOPG (A, scan size: 2 2 mm) and on a cleaned silicon wafer with 200 nm of thermally grown silicon dioxide (B, scan size: 3 3 mm).
8. Summary and future work This review details the design, synthesis and the self-assembly in solution and in thin film of hexasubstituted aromatics that form columnar nanostructures via hydrogen bonds and p–p interactions. The structures of 1 and 2 are unique synthetic targets due to the highly substituted central benzene ring. The functional side groups can be used as a tool to control the molecular packing, orientation, and intermolecular spacing of overcrowded aromatics in thin films. When using compact side groups as in molecule 1b, the molecules stack to form fiber and these fibers bundle together to form helices. Using compact side groups and a larger p-core as in 2b leads to the formation of straight and isolated fibers with a reducing of the intermolecular spacing within a columnar stack. The assembly of these subunits produces polar stacks in solution and can be transferred onto surfaces. Spin casting films of 1a produces polar monolayers (face-on orientation) whereas in 1b, 1c, 2a, and 2b, molecules self-assemble forming long fibers that can be visualized with AFM, EFM, and STM. The exact mechanisms of molecular packing motifs in 1b and 1c are unclear at the moment and appears to be quite complex. Thus, modeling is needed to further understand these molecular packing motifs. The processing conditions such as solvent, concentration, and type of substrate used provide control on the size, orientation and number of the aggregates formed. Finally, it is possible to align 1-D organic nanostructures using DC voltage, and that the alignment depends on the strength of the electric field, the thickness of the electrodes, the gap size, and the number of molecules within the gap area. Using this method with more electroactive molecules represents an untested method to assemble and wire 1-D organic semiconductors in devices. By the proper tuning of the chemical functionality, these columnar structures can be used as model systems for investigating the charge transport in 1D organic semiconducting nanostructures.
Acknowledgements This work was supported by the Office of Basic Energy Sciences, US D.O.E. (#DE-FG02-01ER15264) and US National Science Foundation CAREER award (#DMR-021530 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532
37860). TQN thanks the UCSB setup fund for financial support. We thank Dr Arnold Tamayo for the energy minimized molecular models of compounds 1a and 1b shown in Fig. 3.
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