Semiconducting Nanowires from Hairpin-Shaped Self-Assembling

Aug 10, 2010 - sources (Sigma-Aldrich, Alfa Aesar, TCI, or Advanced. ChemTech) and used ..... Figure 5d).75 After aging, the solution shows positive C...
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J. Phys. Chem. B 2010, 114, 14778–14786

Semiconducting Nanowires from Hairpin-Shaped Self-Assembling Sexithiophenes† Wei-Wen Tsai,‡ Ian D. Tevis,‡ Alok S. Tayi,§ Honggang Cui,§ and Samuel I. Stupp*,‡,§,⊥,¶ Department of Chemistry, Department of Materials Science and Engineering, Department of Medicine, Institute for BioNanotechnology in Medicine, Northwestern UniVersity, 2220 Campus DriVe, EVanston, Illinois 60208 ReceiVed: June 7, 2010; ReVised Manuscript ReceiVed: July 12, 2010

Conjugated organic molecules can be designed to self-assemble from solution into nanostructures for functions such as charge transport, light emission, or light harvesting. We report here the design and synthesis of a novel hairpin-shaped self-assembling molecule containing electronically active sexithiophene moieties. In several nonpolar organic solvents, such as toluene or chlorocyclohexane, this compound was found to form organogels composed of nanofibers with uniform diameters of 3.0 ((0.3) nm. NMR analysis and spectroscopic measurements revealed that the self-assembly is driven by π-π interactions of the sexithiophene moieties and hydrogen bonding among the amide groups at the head of the hairpin. Field effect transistors built with this molecule revealed p-type semiconducting behavior and higher hole mobilities when films were cast from solvents that promote self-assembly. We propose that hydrogen bonding and π-π stacking act synergistically to create ordered stacking of sexithiophene moieties, thus providing an efficient pathway for charge carriers within the nanowires. The nanostructures formed exhibit unusually broad absorbance in their UV-vis spectrum, which we attribute to the coexistence of both H and J aggregates from face-to-face π-π stacking of sexithiophene moieties and hierarchical bundling of the nanowires. The large absorption range associated with self-assembly of the hairpin molecules makes them potentially useful in light harvesting for energy applications. Introduction Over the past few decades, organic semiconductors have been of interest as materials for organic field-effect transistors (OFET), light emitting diodes, and photovoltaics because of their great molecular tailorability, low density, and low cost processing.1-4 Although inorganic field effect transistors still hold much higher charge mobilities than organics (>102 cm2 V-1 s-1 for crystalline silicon or germanium), various organic semiconductors based on conjugated polymers or small molecules exhibit high field-effect mobilities for both holes and electrons.5,6 For instance, organic oligomers such as crystalline acenes have reached hole mobilities over 5.0 cm2 V-1 s-1 with optimized film morphology, surpassing the best amorphous silicon with a value of 1 cm2 V-1 s-1.7,8 Hybrids of organics and zinc oxide reported earlier by our laboratory also have photoconductive gains comparable to amorphous silicon.9 Tailored electronic or optoelectronic properties such as ambipolar semiconductivity,10-12 broad light absorption,13 and covalently linked donor-acceptor junctions14-17 have been created by chemical modification, offering variability that is not accessible in conventional inorganic-based devices. Organic semiconductors are also interesting because of their enhanced solubility in organic solvents, which opens up opportunities for deposition techniques such as spin-coating, inkjet printing, stamping, or spraying in device fabrication.18 These techniques offer large area coverage of substrates in simple steps as compared with expensive vacuum depositions †

Part of the “Michael R. Wasielewski Festschrift”. * Corresponding author. Phone: +1 (847) 491-3002. Fax: +1 (847) 4913010. E-mail: [email protected]. ‡ Department of Chemistry. § Department of Materials Science and Engineering. ⊥ Department of Medicine. ¶ Institute for BioNanotechnology in Medicine.

or crystallization processes. In addition, low-temperature solution-based deposition is compatible with plastic substrates, allowing reel-to-reel production of flexible thin-film devices, which is otherwise difficult to achieve with inorganic semiconductors because of a temperature requirement over 350 °C during processing. Charge transport in organic materials is very different from their inorganic counterparts, primarily through a hopping mechanism of polarons among neighboring localized states assisted by lattice vibration.19 Charge mobility of crystalline organic semiconductors is much higher than disordered organic materials because of reduced amount of grain boundaries and minimized lattice defects that trap charge carriers.20 Within crystalline materials, large spatial overlap of π orbitals between adjacent conjugated oligomers generates large bandwidths for valence or conduction bands, assisting charge hopping among molecules by shortened intermolecular distances. This principle is manifested by the observation of anisotropic carrier mobilities along different directions of crystallographic axes that have different degrees of orbital overlaps in organic semiconductors.21-28 For example, Sundar et al. showed that the direction of maximum mobility of a rubrene crystal coincided with the crystal b axis, where the strongest π-π interactions are expected as determined by X-ray crystallography.21 Furthermore, Sirringhaus et al. compared OFET charge mobilities of poly(3-hexylthiophene) and revealed anisotropic hole mobilities of different crystalline orientations, in which the direction of π-π stacked polythiophenes parallel to the source-drain current flow exhibited much higher carrier mobility by 3 orders of magnitude (0.1 versus 0.0002 cm2 V-1 s-1).25 As discussed above, charge mobility in organic semiconductors is strongly increased when molecular packing leads to long range face-to-face stacking of the conjugated oligomers. How-

10.1021/jp105227p  2010 American Chemical Society Published on Web 08/10/2010

Semiconducting Nanowires from Sexithiophenes

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Figure 1. Molecular design of the hairpin-shaped sexithiophene molecule 1. DACH motifs are shown in blue, semiconducting moieties are shown in red, and solubilizing hexyl groups are shown in green.

ever, strong π-π interactions among conjugated molecules in crystalline domains lead to very poor solubility, while solutionprocessable conjugated molecules tend to form disordered film morphologies after deposition from solution. In this context, an interesting strategy to integrate high performance and processability is to design molecules for nanoscale self-assembly from solution into structures that allow efficient charge hopping by minimizing defect sites. Advances in supramolecular chemistry have led to soluble molecules that form nanowire architectures with π-stacked molecules from solution. By modifying chemical structures or environments (solvents, temperature, or the use of templates), molecules have been developed that spontaneously self-assemble in solution into a variety of one-dimensional (1D) nanostructures, such as cylindrical nanowires,29 nanoribbons,30 or nanotubes31 with small dimensions on the order of 5-20 nm.29-40 Because self-assembly is usually driven by weak intermolecular forces such as hydrogen bonding, π-π stacking, or van der Waals forces, π-conjugated segments can be rationally designed to have their strongest π-π interactions along the principal axis of wire-like assemblies.41 We previously demonstrated that films formed through the 1D self-assembly of quarterthiophene derivatives into nanoribbons can have conductivities that are 3 orders of magnitude higher than films cast from the same molecules but from solvents that do not promote self-assembly with optimized π stacking.30 These supramolecular polymers maintain structural regularity after transferring onto various surfaces, allowing efficient device fabrication and opportunities for nanoscale alignment, patterning, or manipulation of single nanowires.42 Although conceptually interesting, reports on solvent-induced self-assembled nanowires based on highly conjugated molecules that possess significant hole or electron mobilities remain rare in the literature.30,43,44 In this work, we study self-assembly strategies to construct semiconducting nanowires using the hairpin-shaped molecule 1 containing sexithiophene moieties as the charge-conducting segments and hydrogen bonding, forming segments to promote self-assembly. Oligothiophenes are attractive semiconductor candidates because of their excellent ability to form π-stacks as conductive pathways and their versatility in terms of chemical modification.45 Polycrystalline films of oligothiophene OFETs have reached hole46,47 or electron48,49 mobilities between 0.01 to 1 cm2 V-1 s-1. The chemical structure of 1 is shown in Figure 1.

General. All chemicals were purchased from commercial sources (Sigma-Aldrich, Alfa Aesar, TCI, or Advanced ChemTech) and used without further purification unless otherwise noted. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. All NMR spectra were taken with compound concentrations of 1-5 mM in CDCl3 or C6D6. 1H NMR and 13C NMR were recorded on a Varian 400 or 500 MHz NMR spectrometer at 295 K. The molecular weight was analyzed by using a PE Voyager DE-Pro MALDI-TOF-MS instrument. High-resolution mass spectra were recorded on an Agilent 6210 LC-TOF spectrometer with Agilent 1200 HPLC introduction. IR samples were sandwiched between NaCl plates. FT-IR data was obtained using a Fourier transform IR spectrometer in transition mode (Thermo Nicolet, Nexus 870 with the Tabletop optics module). Circular dichroism (CD) spectra were recorded on a Jasco model J-715 circular dichoism spectrometer operating between 250 and 700 nm using a quartz cell of 0.01 mm path length. UV-vis spectra were collected on a Varian Cary 500 UV/vis/NIR spectrometer using a quartz cell (Fisher Scientific, Inc.) of 0.1 cm path length. Fluorescence spectra were performed on an ISS PC1 photon counting fluorometer using the same quartz cells, in which all emission spectra were collected at a 90° angle at room temperature. The molecular length was estimated by molecular mechanics calculation performed with MM2 force field as implemented in the software Chem3D Pro 8.0 (CambridgeSoft Corporation). Gelation. In a small vial, 5-10 mg of the gelator was mixed with solvents promoting self-assembly, such as chlorocyclohexane or toluene, to give a concentration of 1.0 wt %. The solution was kept in a tightly capped vial, heated up to dissolve the solid as a homogeneous solution, and cooled down to room temperature. A self-supporting gel in chlorocyclohexane was formed in 1-2 h, whereas over 12 h was needed for a selfsupporting gel to form in toluene. Transmission Electron Microscope (TEM). To prepare TEM samples, a small volume (∼5 µL) of the solution was deposited onto a carbon-coated copper grid. Excess solution was quickly wicked away by a piece of filter paper, and the samples were subsequently dried in air. Once dried, the samples were negatively stained by placing a drop of 2 wt % uranyl acetate aqueous solution on the grid. A thin layer of uranyl acetate was left on the grid after blotting away excessive staining solution. The samples were again dried in ambient conditions. Brightfield TEM imaging of the assembled nanostructures was performed on a JEOL 1230 transmission electron microscope operating at 100 kV. Atomic Force Microscopy (AFM). For sample preparation, silicon wafers with a (111) surface were used as substrates (Silicon Quest International, Inc.). The surface was cleaned with ultrasonication in deionized water, acetone, and 2-propanol for 15 min each and subsequently dried under nitrogen flow. AFM experiments were performed immediately following sample preparation. All AFM images were recorded under ambient conditions on a multimode scanning probe microscope from Digital Instruments. Tapping-mode AFM technique was chosen to determine the morphology of the self-assembled nanofibers to minimize shear forces that cause distortion of the nanostructures and inaccuracy of the topology. Silicon cantilevers purchased from Asylum Research (AC240TS) were used in all AFM experiments. Electrochemical Measurements. Electrochemical measurements were done by cyclic voltammetry on a three-electrode setup (EG&G Princeton Applied Research potentiostat model

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SCHEME 1: Synthesis of 1 (R ) hexyl)a

a Reagents and conditions: (i) Pd(PPh3)4, DMF, 80 °C, 24 h, 89%. (ii) Vilsmeier reagent (POCl3 added into dried DMF, 40 °C for 10 min), 24 h, CH2Cl2, 30%. (iii) Lithium aluminum hydride (LAH), dried THF, 0 °C, 15 min, 100%. (iv) Succinic anhydride, 4-dimethylaminopyridine (DMAP), CH2Cl2, room temp, 80%. (v) N-Ethyl-N′-(dimethylaminopropyl)carbodiimide (EDC), 4-(dimethylamino)pyridinium-4-toluenesulfonate (DPTS), CH2Cl2, room temp, 24 h, 71%.

263A). Gold working electrode, platinum counter electrode, and Ag/AgCl in 3 M NaCl(aq) reference electrode (BAS Inc. model RE-5B) were used. A 0.1 M TBAPF6 dichloromethane solution was prepared as the electrolyte. The measurements were carried out in 250 µL of dichloromethane solution. Various concentrations were tested, ranging from 2 to 0.05 mM, before determining the optimal concentration to be 0.1 mM. Oxidative and reductive potentials were applied with a sweeping rate of 20 or 50 mV/s. Organic Field-Effect Transistor (OFET) Fabrication. The SiO2/Si substrates (2 cm ×2 cm) were cleaned by sonication in water for 15 min, acetone for 15 min, hexane for 15 min, and isopropyl alcohol for 15 min, followed by drying under nitrogen gas flow. Thin films of materials were then deposited from 0.5 wt % solution by spin-casting (1000 rpm) in a glovebox. The films were dried in the glovebox for 30 min and subsequently annealed at 90 °C for 2 h. Substrates were placed under vacuum overnight to remove residual solvent, and the films were then masked using transmission electron microscopy copper grids (Ted Pella, Quad Variable Cu grids). Gold (60 nm) was thermally evaporated onto the masked substrates to form top contacts at the rate of 0.1 Å/s. All OFET devices have channel widths of 5 µm determined by optical and scanning electron microscopy. Current-voltage measurements were performed using a Keithely 2400 SMU in conjunction with a four-probe micromanipulator setup (Cascade Microtech). Results and Discussion To design a novel molecule that self-assembles into 1D nanowires containing electronically active π-conjugated segments of sexithiophenes, we chose a chiral trans-1,2-diamidocyclohexane (DACH) group as the self-assembling motif in 1 (Figure 1). Similar cyclohexane-based derivatives with amides and hydrophobic tails are known to induce self-assembly by intermolecular hydrogen bonding and by regular packing of cyclohexane rings in organic solvents.50-60 For example, DACH derivatives containing aromatic groups, such as pyridinium groups56,57 or mesogenic cyanobiphenyl groups,58 were shown to self-assemble into fibrous morphologies, whereas DACH derivatives with larger conjugated groups, such as porphrins59

or phthalocyanines,60 coassemble with alkyl-tail substituted derivatives to form fibrous nanostructures. On the other hand, oligothiophenes from monothiophene to quarterthiophene have been attached to diamino- or diiminocyclohexane headgroups with a methylene or methyne spacer and utilized as ligands in palladium complexes for asymmetric catalysis.61,62 Although the side arms of oligothiophenes in these compounds display P or M helicity, the diamino- or diiminocyclohexane derivatives showed no tendency to self-assemble into distinct onedimensional nanostructures. We utilize the DACH motif and attach two sexithiophenes on each arm of molecule 1. Aggregation of the hairpin-shaped molecules of 1 by π stacking would form electron donor nanowires with pockets between the arms of the hairpin that would allow them to either interdigitate or harbor small molecules, for example, electron acceptors for photovoltaic functions. This would be the supramolecular rendition of the concept in recent work of tweezers-like molecules for binding fullerenes.63 Because hydrogen bonding is typically stronger than π-π interactions of oligothiophenes, the orientation of hydrogen bonds among amides with the stacking of cyclohexane rings is likely to define the π stacking direction as the principal axis of self-assembly, thus generating semiconducting nanowires. A linker between the headgroups and sexithiophenes moieties was designed to promote better packing of sexithiophenes into faceto-face π-π stacks by allowing rotational freedom. In principle, large spatial overlap among π orbitals as well as a high ratio of semiconducting moieties to alkyl tails should lead to better charge mobility.20 Since stacking allows the sexithiophene moieties to be exposed on the periphery of nanofibers, these nanowires would be expected to have better intermolecular charge transfer without being obstructed by insulating layers. Our design included the substitution of the thiophene rings with n-hexyl groups for two purposes: (1) to increase the solubility in nonpolar organic solvents and (2) to improve the cofacial π-π stacking of the conjugated segments. The propensity of the n-hexyl side chains to crystallize could promote lamellar face-to-face stacking of sexithiophenes instead of herringbone stacks as evidenced by crystal structures of similar oligothiophenes.64

Semiconducting Nanowires from Sexithiophenes The synthetic route to 1 is described in Scheme 1. Compound 5,5′-bromo-4,4′-hexyl-2,2′-bithiophene (2) was synthesized by homocoupling of 2-bromo-3-hexylthiophene using 1 mol % PdCl2(PhCN)2 as catalyst and 2 equiv of AgF/KF as additive in DMSO to selectively activate the C-H bond at the 5-position on the thiophene rings.65 Sexithiophene derivative 4 is obtained in 89% yield after Stille coupling between 2 and 5-(tributylstannyl)-2,2-bithiophene (3)66 using Pd(PPh3)4 as catalyst. Asymmetric formylation of 4 was done by using Vilsmeier reagent to obtain monofunctionalized sexithiophene aldehyde 5. The formylation led to a low yield of 30% because of undesired diformylation of sexithiophenes as side products. The complete synthesis of 1 is obtained in six steps in 15% overall yield. Gelation experiments, a good macroscopic indication of the existence of one-dimensional nanostructures from association of small molecules, were conducted to evaluate the self-assembly of 1 in organic solvents. The process of gelling includes bundling and entanglement of high-aspect-ratio nanofibers in solution into three-dimensional networks, leading to a loss of fluidity of the solution.41,67 Molecule 1 was found to induce gelation in organic solvents, including toluene, chlorocyclohexane, and cyclohexane at concentrations as low as 1.0 wt % (6.0 × 10-3 M). The formation of a self-supporting gel in toluene occurred within 12 h after cooling, whereas in chlorocyclohexane or cyclohexane, a self-supporting gel was observed within 1-2 h after cooling The gelation process is a thermally reversible, fingerprinting noncovalent self-assembly of 1. Polar aprotic solvents such as dichloromethane, tetrahydrofuran, or chlorobenzene are good solvents for 1, and 1 is insoluble in polar solvents such as ethanol or acetonitrile. A complete list of solvents tested is summarized in Table S1 of the Supporting Information. The nanostructures formed in the gelling solvents were characterized by atomic force microscopy (AFM). AFM samples were prepared by drop-casting a small amount (∼0.5 µL) of 0.2 wt % toluene solution of 1 onto a silicon surface that is subsequently air-dried. We observed networks composed of bundles of intertwined one-dimensional nanofibers on the silicon substrates, as shown in Figure 2a and 2b. In some regions, single nanofibers were seen with persistence lengths ranging from 100 to 400 nm (Figure 2c, 2d). Cross-sectional analysis of the AFM height profiles shows that single nanofibers have average heights of 3.0 ((0.3) nm, whereas overlap of two nanofibers have heights of 5.1 ((0.4) nm. These heights are roughly comparable to the dimensions of the hairpin arms (calculated to be 3.4 nm from molecular modeling), a strong indication that the nanofibers formed as a result of π-π stacked molecules of 1. The smaller nanofiber diameter is probably due to the densely packing of flexible side substituents. Transmission electron microscopy (TEM) images of 1 also revealed nanofibers with similar morphologies (see Figure 3). Without staining, we observed nanofibers with uniform diameters of 3-4 nm, consistent with our results by AFM. TEM samples negatively stained with uranyl acetate further revealed hierarchical structures formed from bundling of single nanofibers. These larger bundles of nanofibers have much longer persistent lengths of a few micrometers relative to isolated nanofibers (Figure 3d, 3e). Films cast from solutions of 1 at the same concentration in dichloromethane, a nonassembling solvent, did not show any distinct supramolecular nanostructures in either AFM or TEM. To evaluate molecular interactions in solution, FT-IR spectra obtained from toluene gel of 1 reveal an N-H stretching band at 3285 cm-1 and a CdO stretching band at 1645 cm-1,

J. Phys. Chem. B, Vol. 114, No. 45, 2010 14781

Figure 2. AFM images of 1 in toluene drop-cast on a silicon surface. Color scale: 0-8 nm. (a) A network of nanofibers formed by selfassembly of conjugated hairpin molecules. (b) Bundling of nanofibers revealing hierarchical structures varying in width. (c) Single nanofibers with an average height of 3.0 ( 0.3 nm (the inset represents crosssectional analysis of several single fibers on the substrate). (d) Nanofibers with short persistent length from 100 to 400 nm. Scale bars: 500, 200, 500, and 200 nm, respectively.

Figure 3. TEM of 1 in toluene. (a) Nanofibers with short persistent lengths. (b, c) TEM images negatively stained with uranyl acetate showing bundling of single nanofibers into an entangled network. (d, e) Hierarchical organization of single nanofibers within a bundle. Scale bars: 200, 200, 200, 50, and 100 nm, respectively.

supporting the presence of hydrogen bonds in the self-assembled nanostructures in the gels.50 1H NMR spectra of 1 were obtained in both a self-assembling solvent d-toluene and a nonassembling solvent d-chloroform (Figure 4). The enantiotopic protons on methylenes of the side chain (Figure 4a, HA and HA′ (9), HB and H′B (b), and HC and HC′ (2)) display different resonance

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Figure 4. (a) 1H NMR spectrum of 1 in d-toluene at 20 °C. (b) 1H NMR spectrum of 1 solvated in d-chloroform at 20 °C. Colored markers on the graphs correspond to the three methylenes on the side chain defined as HA and HA′ (9), HB and HB′ (b), and HC and HC′ (2). In the d-toluene solution, the enantiotopic protons on the methylenes become chemically inequivalent due to the self-assembly, which spatially prevents the molecules from free rotation. (c) 1H DOSY spectrum of 1 in d-toluene. The translational diffusion coefficient Dtol is calculated to be 2.38 × 10-10 m2 s-1. (d) 1 H DOSY spectrum of 1 in d-chloroform showing a slower translational diffusion rate (DCh ) 4.50 × 10-10 m2 s-1) than in d-toluene. The diffusion coefficients are plotted with a logarithmic scale against the chemical shift (the concentration of 1 is 1.0 wt % (6.0 × 10-3 M) in all cases).

frequencies, ν, in d-toluene at 20 °C, whereas HA and HA′ or HB and HB′ display coalescence of the peaks in d-chloroform at the same temperature (Figure 4b). The nonequivalence of protons from -CH2- groups is a marker of magnetically distinct environments. The very large ∆ν for HA, HB, and Hc in d-toluene (∆νA ) 132 Hz, ∆νB ) 101 Hz, and ∆νC ) 35 Hz) indicates a slow exchange rate of methylene protons because of restrained C-C bond rotation. The linkers between the cyclohexyl headgroups and sexithiophenes have reduced rotational freedom because molecule 1 is spatially confined when self-assemble through intermolecular interactions. In d-chloroform, the exchange rates of protons are much faster because 1 is fully solvated, allowing free side chain tumbling in solution leading to averaged 1H NMR chemical shifts. Using a variable-temperature 1H NMR technique on the d-toluene solution of 1, we observed an upfield shift of the N-H proton from 6.12 to 5.69 ppm upon heating from 20 to 80 °C, indicating the existence of hydrogen bonding at lower temperatures (Figure S1; see the Supporting Information). In a chemical environment where amides are hydrogen bonded, N-H protons are deshielded due to electron donation toward another nitrogen atom so that electron density around the protons is decreased. Furthermore, the difference in chemical shifts of the methylene protons HA/H′A, HB/H′B, and Hc/H′C was gradually reduced upon heating because of faster exchange rates. It is intriguing, however, that we did not observe complete line broadening of proton signals at 20 °C in d-toluene. Line broadening of proton signals in solution 1H NMR spectra has been shown in several self-assembly systems as a consequence of faster relaxation due to intermolecular or intramolecular dipole-dipole coupling among multiple spins.30,68 Our results suggest that molecule 1 self-assembles into highly solvated supramolecular nanofibers, thus maintaining the molecular tumbling freedom that generates slower relaxation at room temperature. We carried out 1H diffusion-ordered NMR spectroscopy (DOSY) experiments to investigate the translational motion of

molecule 1 in solution. Diffusion of 1 in solution should depend on physical parameters such as aggregate size, the hydrodynamic radius of the molecule, temperature, and viscosity of the solvent. The diffusion coefficient, D, of a spherical molecule can be described by the StokessEinstein equation

D ) kT/6πηRH where k is Boltzmann’s constant, T is the absolute temperature, η is the viscosity of solvent, and RH is the hydrodynamic radius of the molecule or supramolecular aggregates. Translational diffusion of rodlike supramolecular aggregates in a gel medium requires further correction to the StokessEinstein equation, taking into account the effect of short-range repulsions among nanostructures, which reduces the translational rate.69,70 1H DOSY experiments were performed in 1.0 wt % solutions of 1 in d-chloroform and d-toluene because both solvents have similar viscosity at 20 °C (0.57 × 10-3 Pa · s for chloroform and 0.59 × 10-3 Pa · s for toluene). The translational diffusion coefficient D of 1 in d-toluene and d-chloroform are determined by 1H DOSY to be 2.38 × 10-10 and 4.50 × 10-10 m2 s-1, respectively (Figure 4c, d). We believe that the translational movement of 1 in d-toluene is slower due to the formation of large self-assembled aggregates and also the effect of shortrange repulsions among nanofibers in the gel medium. To gain insight into the relative orientation of conjugated sexithiophene moieties in the self-assembled nanostructures, we examined the optical properties of 1 by UV-vis, fluorescence, and circular dichroism (CD) spectroscopy. The UV-vis spectrum obtained immediately after 1 was dissolved in toluene at a concentration of 1.2 × 10-3 M exhibited an absorption band at λmax ) 423 nm. The fluorescence spectrum revealed emission maxima at λmax ) 520 and 554 nm, with an additional shoulder at 600 nm (Figure 5a). These values correspond to characteristic π-π* transitions of sexithiophene derivatives in their mono-

Semiconducting Nanowires from Sexithiophenes

Figure 5. (a) UV-vis absorbance and fluorescence spectra of freshly prepared toluene solutions of 1 (1.2 × 10-3 M). An absorption maximum, λmax, is observed at 423 nm, and emission maxima are observed at 520 and 554 nm. (b) UV-vis absorbance and fluorescence spectra of toluene solution of 1 aged for 48 h. (c) Gaussian deconvolution of the UV-vis spectrum of aged toluene solution of 1 showing blue- (320 nm) and red-shifted (500 nm) absorption bands. The confidence interval R2 of the fitted curve is 0.9988. (d) Circular dichroism spectrum of a toluene solution of 1 showing signals corresponding to both H and J aggregates of sexithiophenes.

meric states.71 In fact, a solution of 1 and molecule 6 (the sexithiophene methyl alcohol precursor to molecule 1) in dichloromethane exhibit very similar absorption and emission spectra, strengthening the assumption that molecule 1 was molecularly dissolved in toluene (Figure S2; see the Supporting Information). The CD spectrum of molecule 1 also lacked signals over the range from 700 to 300 nm. From these results, it is clear that the two sexithiophene units on 1 are randomly arranged in an achiral geometry in the dissolved state without chirality transfer from the DACH headgroup to the sexithiophene moieties, presumably because of the long four-carbon linker between DACH and sexithiophenes which provides rotational freedom to the chemical bonds. We estimated the optical band gap of 1 to be 2.54 eV from the intercept of the normalized UV-vis and fluorescence spectra. As expected from the FrancksCondon principle, the absorbance and emission spectra are symmetric to each other while the transition between lowest vibrational levels, the 0-0 transition, coincides in both spectra at the same energy.72 Interestingly, the same toluene solution of 1 discussed above shows strikingly different absorption properties in the UV-vis spectrum 48 h later. In addition to the original absorption peak at 423 nm, a red-shifted band appears at 500 nm, and the absorption maximum at 313 nm becomes more prominent (Figure 5b). A Gaussian deconvolution of the UV-vis absorbance reveals a red-shifted band at 500 nm and a blue-shifted band at 320 nm (Figure 5c). According to exciton coupling theory, the transition dipoles of sexithiophenes in close proximity will generate two split excitonic states through dipole-dipole coupling based on the relative orientations of the chromophores.73 In our case, the appearance of a red-shifted peak at 500 nm over time suggested formation of J aggregates (edgeto-edge sexithiophene stacks in the interdigitated hairpins) in the toluene solution, whereas the blue-shifted peak at 320 nm suggests the presence of H aggregates (face-to-face sexithiophene stacks).

J. Phys. Chem. B, Vol. 114, No. 45, 2010 14783 On the basis of this observation, we believe that both H and J aggregates coexist during the self-assembly of 1 in toluene. This unique stacking of sexithiophene moieties is supported by the fluorescence spectrum. The emission intensity of aged toluene solutions of 1 is 59% of the original intensity (Figure 5b). Chromophores forming H aggregates are known to have very low fluorescence quantum yields because the excitation to the higher excitonic state usually leads to energy loss through nonradiative relaxations.74 Thus, we attributed the partial attenuation of fluorescence to the formation of H aggregates of the sexithiophene moieties. Futhermore, CD spectroscopy reveals an efficient chirality transfer from the chiral DACH headgroup to the achiral sexithiophenes upon self-assembly (see Figure 5d).75 After aging, the solution shows positive Cotton effects around the absorption maxima of 320 and 510 nm that correspond to H and J aggregates, respectively. The spectral behavior supports the simultaneous formation of edge-to-edge J aggregates and π-π stacked H aggregates during selfassembly of the nanofibers. The induced chirality is from the supramolecular assembly of sexithiophenes in which the transition dipoles are oriented in a chiral geometry. The other two CD signals at 350 and 478 nm may be due to different orientations of sexithiophene transition dipoles resulting in “offcenter” stacking, although the exact origin still needs to be determined.76 We proposed that self-assembly of molecule 1 into nanofibers is governed by hydrogen bonding among the trans-1,2-diamidocyclohexyl groups and π-π interactions among sexithiophene moieties (Figure 6). H aggregates are commonly observed from the stacking of conjugated oligomers, and large overlap of π orbitals generates a large bandwidth for charge transfer. J aggregates of chromophores such as cyanine dyes,77-79 porphyrins,80,81 or perylenes bisimides37,38,82 have been extensively studied, but formation of J aggregates in oligothiophenes remains rare and usually requires alignment or thin film formation.83 Our observation of both H and J aggregates in the selfassembling system studied here leads us to hypothesize that J aggregates of the sexithiophenes moieties are formed through nanofiber bundling interactions and are therefore linked to hierarchical structures. Because the transition dipoles of sexithiophenes are oriented along the long molecular axes, onedimensional stacking of molecule 1 leads to the parallel alignment of sexithiophenes into H aggregates during the initial self-assembly. After individual nanofiber strands bundle into the larger-diameter hierarchical structures observed, exciton coupling of sexithiophenes on different strands would occur, forming J aggregates with a characteristic edge-to-edge orientation (Figure 6b). The coexistence of H and J aggregates has been demonstrated in several conjugated oligomers but not in oligothiophenes.38,84-86 As far as we know, this is the first example in sexithiophene oligomers. Interestingly, in the photosynthetic machinery of plants, protein complexes such as photosystems I and II form supramolecular assemblies of H or J aggregates on cell membranes to achieve a broader absorbance range and, thus, more efficient light harvesting.87 Our findings on these highly bundled nanofibers are therefore potentially interesting for materials design in solar energy applications. The electrochemical properties of 1 were investigated by cyclic voltammetry (CV) to determine its absolute energy levels. CV was performed in 0.1 M of TBAPF6 in dichloromethane as the electrolyte with a scan rate or 20 mV/s. The reference electrode of Ag/AgCl in 3 M of NaCl(aq) was calibrated with standard ferrocene Fc/Fc+ redox couple and used to determine

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Figure 6. Schematic representation of the self-assembly process of molecule 1. (a) Hydrogen bonding among the trans-1,2-diamidocyclohexyl groups. The molecular model is approximated by a methylated derivative showing two hydrogen bonds between each molecule. (b) Self-assembly of 1 by hydrogen bonding of the headgroups (in blue) and H aggregates of the sexithiophenes (in green) leading to nanofibers of the width about the length of one arm. Bundling of the nanofibers facilitates the formation of J aggregates in the hierarchical structures shown in the TEM image.

TABLE 1: Electrochemical Properties of 1a oxidation cycle 1st oxidation potential (V) Ea1

Ec1

0.83

0.67

1/2

E1

(ox)

0.75

reduction cycle

2nd oxidation potential (V) Ea2

Ec2

1.22

0.98

1/2

E2

(ox)

1.1

1st reduction potential (V) 1/2

Ea1

Ec1

E1

-1.78

-1.90

(red)

-1.84

band gap

2nd reduction potential (V) Ea2

Ec2

-1.99

N/A

CV (eV) optical (eV)

1/2

Eelg

Eop g

N/A

2.59

2.54

E2

(ox)

Solutions of 0.1 mM in dichloromethane were used as samples. Measurements were performed at 20 °C at a sweeping rate of 20 mV/s. All data were calibrated with standard ferrocene Fc/Fc+ redox potential. Reference electrode: Ag/AgCl 3 M NaCl(aq), +0.209 V to NHE. a

SCHEME 2: Representation of the Reactive Conjugated Molecular Structures of the Sexithiophene Polaron (top) and the Dication (bottom)

the energy levels. The experimental results are summarized in Table 1, and representative voltammograms are shown in Figure S3 of the Supporting Information. The voltammograms display quasi-reversible, one-electron oxidation and reduction processes during the first several cycles. We determined the formal potential (E1/2) using the midpoints between anodic and cathodic peaks on the forward and reverse scans. It is noteworthy that two oxidation potentials (E1(ox)1/2 and E2(ox)1/2) of 1 are observed at 0.75 and 1.10 V, respectively, but only the first reduction potential, E1(red)1/2 ) -1.84 V could be determined due to the highly reactive nature of sexithiophene radical anions. The oxidation of sexithiophenes forms stable polarons during the first oxidation, and forms less stable dications upon further oxidation (see Scheme 2).88 The oligothiophene polarons and dications are stabilized by delocalization among the conjugated sexithiophenes.89-91 The HOMO and LUMO levels are estimated from the first oxidation and the first reduction potentials to be 5.36 and 2.77 eV, yielding an absolute band gap of 2.59 eV. This value is very close to the solution band gap (2.54 eV) determined by optical spectroscopy. Continuous oxidative cycles of solution 1 produce dark blue precipitates around the working electrode. The oxidative potentials eventually lose their characteristic shape because of blockage of electrode contact (Figure S4; see the Supporting Information). Because the 2′ position of sexithiophene moieties on 1 is not substituted with stabilizing groups, the dark blue precipitate is possibly dimers, oligomers,

or polymers generated by polarons reacting with neutral monomers or with other polarons. The H aggregation of 1 and the bundling morphology of the nanofibers they form should provide an efficient pathway for charge carriers. To determine the charge mobility of these nanostructures, OFETs were fabricated from thin films using an assembling solvent (toluene) or nonassembling solvents (chlorobenzene or o-dichlorobenzene) to establish charge mobility in a device configuration. Thin films of 40-50 nm in thickness were deposited by spin coating of a 0.5 wt % solution onto a SiO2/Si substrate, followed by annealing at 90 °C for 2 h. Gold electrodes were thermally evaporated onto the film to create top contacts. All OFET devices of 1 showed increasing current with increasing negative source-drain bias as shown in the I-V plot (Figure 7), which is the characteristic behavior of p-type semiconductors. This finding is expected, since oligothiophenes without electron withdrawing substituents are known to have good hole mobilities.45 Because the π-π stacking direction coincides with the nanofiber long axes in the selfassembly process, we believe nanofibers of 1 should have high conductivity as thin films between source and drain electrodes in the OFET geometry. Indeed, films of 1 deposited from toluene solution that promotes self-assembly achieved hole mobilities,

Figure 7. OFET characteristics of films of 1 deposited from toluene. The inset shows the device configuration of a top-contact OFET. The channel length and width dimensions are 210 and 5 µm, respectively. (a) IDS-VDS responses at different gate biases. (b) Transduction characteristics of the corresponding device.

Semiconducting Nanowires from Sexithiophenes µ, of 3.46 × 10-6 cm2 V-1 s-1, roughly 1 order of magnitude higher than films cast from the nonassembling solvents chlorobenzene (µ ) 9.42 × 10-7 cm2 V-1 s-1) or o-chlorobenzene (µ ) 1.79 × 10-7 cm2 V-1 s-1). This difference is a strong indication that self-assembly of the one-dimensional nanostructures formed by 1 improve hole mobility through π orbital overlap of the sexithiophene moieties, creating better charge transporting pathways.30,43,44 These mobility values could be optimized with strategies to achieve greater persistence length in the nanofibers, long-term orientation of the bundled nanostructures, or the use of channels shorter than 5 µm. Interfacial charge trapping caused by the absence of optimized charge motion pathways can also inhibit proper gating in the devices fabricated. Conclusions We have demonstrated an approach to promote self-assembly of sexithiophene nanofibers from solution using a hairpin molecular architecture and hydrogen bonding motifs to promote long-range charge mobility. The molecule studied is one of the few examples known of highly conjugated semiconducting oligomers that self-assemble into discrete nanofibers with uniform diameters. In addition, the hairpin cross section of the π stacked assemblies was found to give rise to hierarchical bundles of nanofibers promoting the coexistence of H and J aggregates, thus broadening the wavelength range of optical absorption and the potential of these systems for efficient light harvesting in photovoltaics. Charge mobility in these p-type semiconducting systems could be further optimized through strategies that improve long-range orientation of the onedimensional nanostructures. Acknowledgment. This research was supported by a grant from the U.S. Department of Energy, Basic Energy Sciences (DE-FG02-00ER45810). The authors are grateful for the use of the following shared facilities at Northwestern University: IMSERC, BIF, Keck Biophysics, and the NUANCE Center (EPIC, NIFTI, Keck-II facility). The NUANCE Center is supported by the NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University. Supporting Information Available: Details of synthesis, gelation test, NMR, UV-vis spectra, and voltammograms of 1 are included. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121–128. (2) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581–2590. (3) Peet, J.; Heeger, A. J.; Bazan, G. C. Acc. Chem. Res. 2009, 42, 1700–1708. (4) Katz, H. E.; Bao, Z. N.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359–369. (5) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070–4098. (6) Murphy, A. R.; Frechet, J. M. J. Chem. ReV. 2007, 107, 1066– 1096. (7) Kelley, T. W.; Muyres, D. V.; Baude, P. F.; Smith, T. P.; Jones, T. D. Mater. Res. Soc. Symp. Proc. 2003, 771, 169–179. (8) Podzorov, V.; Sysoev, S. E.; Loginova, E.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett. 2003, 83, 3504–3506. (9) Sofos, M.; Goldberger, J.; Stone, D. A.; Allen, J. E.; Ma, Q.; Herman, D. J.; Tsai, W. W.; Lauhon, L. J.; Stupp, S. I. Nat. Mater. 2009, 8, 68–75. (10) Tang, M. L.; Reichardt, A. D.; Miyaki, N.; Stoltenberg, R. M.; Bao, Z. J. Am. Chem. Soc. 2008, 130, 6064–6065. (11) Zaumseil, J.; Sirringhaus, H. Chem. ReV. 2007, 107, 1296–1323.

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