Phenylene Co-Oligomers - The

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J. Phys. Chem. C 2009, 113, 9601–9608

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Nanoaggregates from Thiophene/Phenylene Co-Oligomers Manuela Schiek,† Frank Balzer,*,† Katharina Al-Shamery,‡ Arne Lu¨tzen,§ and Horst-Gu¨nter Rubahn† Mads Clausen Institute, NanoSYD, UniVersity of Southern Denmark, Alsion 2, DK-6400 Sønderborg, Denmark, Institute of Pure and Applied Chemistry, UniVersity of Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany, and Kekule´-Institute of Organic Chemistry and Biochemistry, UniVersity of Bonn, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany ReceiVed: January 28, 2009; ReVised Manuscript ReceiVed: March 30, 2009

The growth of thiophene/phenylene cooligomers is investigated on the basal surface of muscovite mica. Both the phenyl end-capped 5,5′-di-4-biphenylyl-2,2′-bithiophene (PPTTPP) and the thienyl end-capped 4,4′-di2,2′-bithienylbiphenyl (TTPPTT) co-oligomers form fiberlike nanostructures from molecules lying on the substrate surface after vacuum deposition. The fibers are oriented either in two distinct orientations or in a range of directions between two maximal orientations. On a microscopic level, PPTTPP growth is more similar to the growth of p-phenylene oligomers on muscovite, whereas TTPPTT is more similar to the growth of R-thiophene oligomers. Introduction An important class of rodlike organic molecules for photonic and optoelectronic applications are the thiophene oligomers.1,2 This kind of organic semiconducting molecule was synthesized for the first time more than 60 years ago.3,4 Later research showed that they are promising candidates for devices, such as organic field effect transistors (OFETs)5–7 and organic light emitting diodes (OLEDs).8,9 OFETs based on oligothiophenes or their derivatives show a relatively high field-effect mobility.10 Thiophene oligomers have also been studied as model compounds to interpret chargetransfer mechanisms, since these heterocyclic materials exhibit both interesting electrical and optical properties and possess thermal and chemical stability.11Besides thiophene, also phenylene oligomers have been investigated in the past for use in optoelectronic and photonic devices.12–14 It is, however, challenging to control the emission characteristics of conjugated oligomers. Hybridizing and blending of materials are common attempts to generate and improve the properties of choice on a molecular level. Thiophene/phenylene co-oligomers, i.e., hybridized oligomers containing thiophene and phenyl moieties, have been developed and synthesized independently by different groups.15,16 Further co-oligomers and additionally perfluorophenylthiophenes have been synthesized more recently.17 One of the major advantages of this novel class of organic semiconductors is that the π-conjugation length, and hence the emission color, can be tuned as desired by changing the total number of thiophene and phenyl rings and their mutual arrangement within the molecule.16,18,19 Various molecular shapes occur, namely straight, banana, or zigzag. Besides continuous ultrathin films, the above-discussed oligomers form also discontinuous films or nanoscaled aggregates (“nanofibers”, “nanoneedles”).20–22 Obviously, such nanoaggregate formation opens up even more interesting applications for future optoelectronic devices. Structure and shape of the * To whom correspondence should be addressed. E-mail: fbalzer@ mci.sdu.dk. † University of Southern Denmark. ‡ University of Oldenburg. § University of Bonn.

aggregates and also the kind of aggregate formation dependss especially for weakly bound organic-on-dielectric systemss sensitively on crystalline and electrostatic relationships between adsorbate and substrate.23 In the case of phenylene oligomers, mutually parallel nanofiber formation on muscovite mica has been observed and intensively investigated.24–26 Similar structures have been reported for thiophene oligomers.27 On the basis of the availability of various thiophene and phenylene oligomers as well as thiophene/phenylene co-oligomers, devices with improved optoelectronic properties have been realized more recently, such as OFETs.28,29 The molecules exhibit electroluminescence in the form of ultrathin films30 and emission gain narrowing after strong optical pumping,31–33 as well as amplified spontaneous emission. Self-waveguiding,34–37 and lasing38–43 has also been reported. Motivated by the above, we explore in the present paper the surface structure formation of the two thiophene/phenylene cooligomers 5,5′-di-4-biphenylyl-2,2′-bithiophene (PPTTPP) and 4,4′-di(2,2′-bithienyl)-biphenyl (TTPPTT), which form nanostructured organic semiconductors on muscovite mica. Synthetic Approach and Experimental Setup Modification of p-phenylene oligomers with thiophene moieties leads among others to either phenyl-end-capped oligothiophenes or thienyl-end-capped oligophenylenes. However, quite a large number of thiophene/phenylene co-oligomers is accessible by changing the overall number of thiophene and phenyl rings as well as their mutual order within the molecule. Several of these oligomers have already been synthesized via Grignard, Stille coupling, or Suzuki cross-coupling reactions, and some are even commercially available.17,44–49 The thermal stability of phenyl end-capped thiophenes should be increased in comparison to oligothiophenes and thienyl-end-capped p-phenylenes. A terminal thiophene unit is susceptible to heat-induced reactions such as polymerization or decomposition because of the unsubstituted R-position. In this paper, we report on two different thiophene/phenylene co-oligomers: the phenyl-end-capped PPTTPP (sometimes called BP2T)50 and the thienyl-end-capped TTPPTT (also called

10.1021/jp9008465 CCC: $40.75  2009 American Chemical Society Published on Web 05/07/2009

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Figure 1. Synthesis of thiophene/phenylene co-oligomers via Suzuki cross-coupling reactions. The lines connecting the carbon atoms marked with asterisks represent the long molecular axis.

Figure 2. Fluorescence microscope images of domain boundaries for PPTTPP (a) and TTPPTT (b) on muscovite mica. Image sizes are 200 × 200 µm2.

Figure 3. Substrate temperature dependence of PPTTPP needle growth. Upper panel: fluorescence microscopy images (100 × 100 µm2, λexc ) 365 nm) of approximately d ) 6 nm PPTTPP on muscovite mica, deposited at different substrate temperatures. Ts ) (a) 317 K, (b) 350 K, (c) 393 K, and (d) 414 K; 25 × 25 µm2 image size. Note that the exposure times vary. Lower panel: corresponding AFM images (e) 10 × 10 µm2, (f and g) 50 × 50 µm2, and (h) 8 × 8 µm2. The white arrows define the [100] substrate directions, the other two lines emphasize the two 〈110〉 directions.

4TC2P).19 The two oligomers have been synthesized in a 2-fold Suzuki cross-coupling reaction from commercially available R-bromothiophenes and p-phenylboronic acids or esters, using 5 mol % tetrakis(triphenylphosphino)palladium as catalyst together with cesium fluoride as base in dry tetrahydrofuran. As shown in Figure 1, the synthesis of these and related oligomers has been published before,17,44 but our novel approach provides higher yields of 95% vs 77%17 and of 91% vs 57%,44

respectively, after refluxing for 50 h. The final products precipitated from the reaction mixture and have been washed with water and organic solvents repeatedly for purification. By outgassing in vacuo, residual organic solvents are removed to give the desired compounds in high purity. For both molecules, the bulk crystal structures have been determined in the past. Both crystallize in a monoclinic unit cell, forming a herringbone structure. For TTPPTT with two

Nanoaggregates from Thiophene/Phenylene

Figure 4. (a) Fluorescence microscopy image (25 × 25 µm2) and (b) AFM image (10 × 10 µm2, height scale 15 nm) of PPTTPP on muscovite mica. Dendritic islands of upright molecules are visible together with clusters and needles on both images. The dendritic islands do emit fluorescence light, but much less than the needles. The height of the dendritic islands is multiples of 2.7 or 3.2 nm, the height of the needles being around 60 nm and the height of the clusters being around 15 nm. The 1 × 1 µm2 AFM image in part c and the cross section along the black line (d) demonstrates the increased roughness for the first layer of the islands. The dashed horizontal line in part d corresponds to the height of a monolayer from upright PPTTPP molecules.

molecules per unit cell, lattice constants of a ) 5.8240 Å, b ) 7.2935 Å, and c ) 25.316 Å with an angle β ) 96.23° have been found,17 and another group found very similar values of a ) 5.816 Å, b ) 7.2527 Å, c ) 25.2863 Å, and β ) 96.265°.19 For PPTTPP with four molecules per unit cell, the lattice constants are a ) 5.7081 Å, b ) 7.6036 Å, and c ) 52.869 Å with β ) 97.147°.51 Muscovite mica (Structure Probe, Inc., West Chester, PA) is cleaved in air immediately before transferring it into the vacuum chamber with a base pressure of p ≈ 2 × 10-8 mbar. Molecules are deposited onto muscovite mica (001) by sublimation with a deposition rate of 0.1-0.2 Å/s. The nominal film thickness d is deduced with the help of a water-cooled quartz microbalance (Inficon XTC/2). Especially for higher substrate temperatures during deposition, this nominal film thickness d is larger than the “real” film thickness because of the temperature-dependent sticking coefficient. The samples are characterized in situ by

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Figure 6. Orientations of PPTTPP needles grown at Ts ) 400 K (a and b) and of TTPPTT needles grown at Ts ) 420 K (c and d). Solid lines represent Gaussian fits to the experimentally obtained distributions. The two different sets of orientations correspond to the two different domains observed on a single substrate. Dashed vertical lines represent substrate high symmetry directions [1j00], 〈110〉g, 〈110〉ng, and [100].

low-energy electron diffraction (LEED) using a sensitive multichannelplate apparatus (MCP-LEED, Omicrom) and ex situ by polarized fluorescence microscopy, UV/vis spectroscopy, and by atomic force microscopy (AFM, JPK NanoWizard) in intermittent contact mode. Results and Discussion Deposition of PPTTPP as well as TTPPTT results within a well-defined deposition temperature range in the formation of needlelike structures, aligned along specific directions on the muscovite surface. With increasing deposition temperature these structures grow longer, become more isolated, and eventually are recognizable in an optical microscope. Similar trends have been observed for p-hexaphenylene (p-6P)26 and for parafunctionalized p-quaterphenylenes,14,52–54 for quaterthiophene (R4T) and sexithiophene (R-6T),21 and for 2,5-di-4-biphenylylthiophene (PPTPP)55 on muscovite mica. UV irradiation by a highpressure mercury lamp at λexc ) 365 nm or by a HeCd-laser at λexc ) 325 nm leads to the emission of green luminescence. However, details of the growth vary considerably between the two molecules. PPTTPP grows more similar to the p-phenylenes

Figure 5. Thickness dependence of PPTTPP needle growth. Fluorescence microscopy images (200 × 200 µm2) deposited at a substrate temperature of Ts ) 393 K: d ) (a) 2 nm, (b) 4 nm, (c) 6 nm.

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Figure 7. Substrate temperature dependence of TTPPTT needle growth. Upper panel: fluorescence microscopy images (100 × 100 µm2, except part d, which is 50 × 50 µm2) of approximately 6 nm TTPPTT, deposited at different substrate temperatures on muscovite mica. Ts ) (a) 313 K, (b) 360 K, (c) 420 K, and (d) 450 K. Lower panel: corresponding AFM images for 2 nm thickness: (e and f) 20 × 20 µm2, (g) 50 × 50 µm2, and (h) 5 × 5 µm2. In part h the small clusters possess heights of 2.7 nm. White lines and arrows imply the same directions as in Figure 3.

Figure 8. (a) Fluorescence microscopy image (75 × 75 µm2) of TTPPTT on muscovite mica. The 25 × 25 µm2 AFM image with height scale of 15 nm in part b shows a similar area. Dendritic islands of upright molecules are visible together with clusters and needles on both images. The dendritic islands fluoresce only weakly and therefore appear dark in part a. The height of the dendritic islands is around 2.7 nm.

and forms straight fibers along well-defined substrate directions, whereas the growth of TTPPTT is more similar to that of the R-thiophenes with a broad range of orientations and a larger tendency to form islands from upright molecules. Both kind of fibers form two rotational domains on a muscovite substrate (see Figure 2). The domain boundaries are induced by substrate cleavage steps. This effect was observed already more than 40 years ago for copper phthalocyanine on muscovite.56 Growth of PPTTPP. A series of fluorescence microscopy images together with corresponding AFM micrographs for the growth of PPTTPP on muscovite mica at different substrate temperatures Ts during deposition is presented in Figure 3. Close to room temperature, a film of densely packed, only micrometer long fibers evolves. With optical microscopy one can barely resolve single entities, but the AFM image reveals growth along two different substrate directions within a single domain. The larger Ts, the longer the single needles become and the larger the distances between mutually parallel entities get for the same nominal thickness d. The mean length increases from 1 µm at Ts ) 317 K to 30 µm at Ts ) 393 K. This process continues until approximately Ts ≈ 410 K, where single needles only are growing from defects on the mica surface, but otherwise clusters dominate (see Figure 3d,h). These clusters with similar heights as the needles are also observed in between the needles for all the lower values of Ts. Within a few hundred nanometers around

the fibers, the number density of the clusters is diminished and a depletion zone is formed. Typical heights h and widths w of the needles at the two intermediate temperatures are in the range of h ) 30-40 nm and w ) 300-400 nm, respectively. For deposition at 317 K, the needles exhibit h ) 30-40 nm, but only w ) 150 nm. In addition to needles and clusters, flat islands are growing for all substrate temperatures, their number density being much smaller than the one for clusters. As shown in Figure 4, these islands luminesce only weakly, and in the AFM images they appear as rather flat entities with step heights of approximately 2.7 and 3.2 nm. If fibers or clusters are growing on top of these islands, their height increases considerably up to a few hundred nanometers. In Figure 4c,d, it is demonstrated that the first layer of these platelets appears rougher than the succeeding layers, having additional nanometer tall morphological structures atop, which might stem from lying molecules.57 Next to those islands a depletion area of clusters exists, too, although clusters are found even on top of the islands. As already demonstrated in Figure 2, within each domain two fiber orientations occur, forming rhombi with an acute angle of 74° ( 5° for elevated temperatures. With lower deposition temperature this angle decreases, reaching 60° for deposition at room temperature. The needles start growing as isolated entities within a bed of clusters. Increasing the nominal thickness

Nanoaggregates from Thiophene/Phenylene

Figure 9. Directions of the fluorescence maxima for the two rotational domains of PPTTPP (a and b) and TTPPTT (c and d). Solid lines represent Gaussian fits to the data, assuming the same number of components as for the distribution of orientations in Figure 6. The insets in parts a and c show Fourier plots (black high intensity, white low intensity) demonstrating the correlation between needle orientations and polarization directions. The solid horizontal and vertical lines are the 〈110〉g directions, and the dashed white lines correspond to [100] and 〈110〉ng, respectively.

Figure 10. Schematic of the orientations of a single domain of PPTTPP fibers (a) and TTPPTT fibers (b) (rectangles), together with the oligomers’ orientation within the fibers (perpendicular lines within the rectangles). The two not realized needle directions are drawn as dotted rectangles. Muscovite high symmetry directions are depicted by black arrows. The green triangles symbolize the width of the corresponding distribution of needle orientations.

of deposited material leads to an increase in length, until they eventually grow into each other and form a rhomboidal pattern such as in Figure 5. For that case, the width of the fibers is about 400 nm, whereas the height remains constant at approximately 35 nm. Increasing the film thickness in this thickness range mainly increases the length of the fibers. The same trend, i.e. an increase in width and length but not in height together with depletion zones, has also been observed for other needle-forming organic molecules such as p-6P, R-4T, R-6T, and PPTPP on mica, suggesting a similar formation mechanism.58 The fibers show no alignment along the mica high-symmetry directions such as the phenylene oligomers do.26 The white arrows in Figure 3 denote the muscovite [100] direction and the white lines the two 〈110〉 directions. In the upper panel of Figure 6, histograms of the two needle orientations for a sample

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9605 grown at Ts ) 403K are shown for the two needle domains. The dashed perpendicular lines denote the muscovite highsymmetry directions. The two needle domains are separated by an odd number of 1 nm tall muscovite cleavage steps. Muscovite is a dioctahedral mica. For the most common 2M1 polymorph, grooves are formed on the (001) face, running along one of the 〈110〉 directions, but never along [100].59 Separated by an odd number of single cleavage steps these grooves change direction by 120° back and forth. The groove and nongroove directions are denoted by the indices “g” and “ng”, i.e. 〈110〉g and 〈110〉ng. Obviously, for the two rotational domains the groove direction is the intersecting line between the two needle orientations. In Figure 6a,b, these distributions are fit by Gaussians, allowing one to determine values of ∆φ ) 38° ( 3° for the angles between the needle orientations and 〈110〉g. For lower values of Ts, 〈110〉g is still the bisecting line, but the angle ∆φ decreases accordingly. Growth of TTPPTT. For the bithienyl-end-capped biphenyl TTPPTT, the surface pattern is less well-defined as compared to PPTTPP. Instead of two distinct needle orientations, a range of orientations between two most probable values exists, as emphasized in Figure 6c,d. Again, two domains exist, with the bisecting line between the needle orientations being 〈110〉g. Now instead of two well-defined needle directions, the distribution is fit by two components with ∆φ ) (30° with respect to 〈110〉g and another two with ∆φ ) (11°. The influence of Ts on the overall morphology is similar to the case of PPTTPP and typical for needle growth, cf. the fluorescence images and the AFM micrographs in Figure 7. As demonstrated in Figure 7a,e, close to room temperature, only short fibers grow, forming a densely packed fiber film. Increasing Ts increases the length and width of the fibers from 150 nm to several tens of micrometers and from less than 150 nm to about 350 nm, respectively. The distances between fibers also increase, whereas their heights remain constant at 20-25 nm. In addition to the needles, clusters are also found in between the needles, which fluoresce under normal incidence UV irradiation; see Figures 7 and 8. Different from PPTTPP, now many very shallow clusters with heights of less than 1 nm up to 5 nm appear in addition to the ones with the needle heights. All clusters lead to fluorescence after UV-excitation, as shown in Figure 8. As a third type of aggregate, the already known islands of presumably upright standing molecules occur in between the needles. These areas can be seen by atomic force microscopy in Figure 8b as islands exhibiting steps with multiples of 2.6-3 nm; subnanometer steps are also existing, suggesting mixed islands from upright and lying molecules. The islands fluoresce only barely after UV excitation, leading to dark patterns in the fluorescence microscopy image in Figure 8a. Linear Optical Properties and Growth Model. Fluorescence spectra have been recorded after unpolarized normal incidence UV excitation of the nanoaggregates at λexc ) 365 nm. The spectra peak in the green spectral range and show wellresolved vibronic progressions with the (01) transitions being most prominent. The spectral position of the (01) transition is at 500 nm for TTPPTT and at 523 nm for PPTTPP. The emitted light is strongly polarized. An analysis of the polarization pattern by a sin2 fit according to Malus law indicates that the oligomers are oriented with their long axessdefined to lie along a connecting line between the carbon atoms at the 1-1′-positions for PPTTPP and at the 2-2′-position for TTPPTT, as shown in Figure 1salmost perpendicular ((5°) to the long fiber axes.60 This is a rather typical arrangement for organic nanofibers.58 Such a perpendicular orientation of the lying molecules with

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Figure 11. (a) LEED pattern from TTPPTT on muscovite mica, taken at an electron energy of Eel ) 40 eV. The reciprocal lattice in part b reproduces all the diffraction spots from part a. Green disks correspond to diffraction spots from muscovite and red diamonds to spots from the organic overlayer. In part c, the real space lattice from the substrate (black solid lines) together with a single domain from the overlayer including the centered molecule (red diamonds) is plotted. The green arrows represent the substrate high-symmetry directions and the red line the TTPPTT unit cell.

respect to the long needle axes can be realized by having, e.g., the (100) or (010) faces as contact faces with the substrate, an orientation found by X-ray diffraction and by electron diffraction for epitaxially grown PPTTPP fibers along the 〈110〉 directions on KCl (100).50,61 Histograms of the polarization directions from ensembles of needles are shown in Figure 9. Within (5° the distributions peak at the substrate high symmetry directions [100] and 〈110〉ng with additional two peaks in between for TTPPTT. Each fiber direction has its own polarization direction; i.e., two polarization directions exist within one domain for PPTTPP and for TTPPTT a range of polarization directions. Polarization plots58 as insets in Figure 9 demonstrate that 70 polarized microscope images have been taken, where the polarizer angle has been varied between 0° and 360° in ≈5° steps. These images are Fourier transformed, radially integrated, and that way provide a quick estimate of the fluorescence intensity and thus the long molecule axis as a function of the needle orientation. Obviously, the molecules are oriented with their long axes approximately parallel to the nongrooved high symmetry directions, not along the grooved one (Figure 10). From the similarity of the growth morphology with the p-phenylenes and with PPTPP55 (i.e., a large number of clusters and a small number of needles for small nominal thicknesses, denuded zones around the needles, and similar heights of needles and clusters), we conclude that the needle formation begins with the growth of small clusters on the surface first. Only after a certain nominal thickness is reached does needle growth by cluster agglomeration set in, leading to needles with a similar height as the preceding clusters and to denuded zones around the needles. Islands from upright molecules nucleate preferentially at already existing needles. The needles emit polarized light after excitation with UV light, revealing the orientation of the light-emitting molecules on the surface as well as within the needles. For PPTTPP, the molecules’ long axes are oriented along two of the three muscovite high-symmetry directions. The avoided direction is the grooved 〈110〉 direction, i.e. 〈110〉g, as shown in Figure 10a. For TTPPTT, a wider range of rather continuous molecule orientations is observed, but still the 〈110〉g direction is avoided; see Figure 10b. Because of an angle of approximately 90° between the molecules’ long axes and the fiber axis, this molecule orientation is reflected in the observed needle orientations. The two fiber directions of the thiophene/phenylenes are explained by the same model as previously used for p-phenylene

fibers and R-thiophene fibers:21,26 Epitaxy provides different possible needle directions, where the molecules’ long axes are approximately parallel to a high-symmetry direction, i.e., parallel to [100] or to 〈110〉. The driving force for the alignment of the molecules along only two mean directions is a maximized interaction between the electric surface fields of the muscovite mica substrate, which are approximately perpendicular to the grooved directions,62 and an an electric moment along the oligomer’s long axis.25,63 That way a molecule’s orientation with it is long axis parallel to 〈110〉g is rather unlikely; see the dotted needles in Figure 10. Wetting Layer. For the p-phenylenes on muscovite mica as well as for PPTPP, the first stage of growth is the formation of an epitaxial wetting layer from lying molecules. Such a layer has been detected by LEED25,55,64 as well as by thermal desorption spectroscopy.65 LEED experiments for either PPTTPP or TTPPTT have so far not shown evidence for an epitaxial layer of lying molecules. Instead, as presented in Figure 11 for TTPPTT, a superstructure pattern from the organic overlayer consisting of 18 weak spots is observed. Together with the six muscovite spots they create a ring of 24 spots in total. This pattern is reproduced assuming an overlayer from TTPPTT with the (001) face parallel to the muscovite substrate. For that the short unit cell axis is rotated by 22.5° with respect to the mica [100] direction and all 12 possible rotational domains are included together with glide planes along both TTPPTT unit cell vectors. For the muscovite surface unit cell (lattice constant a ) 5.2 nm, angle between unit vectors 120°) this results in the superstructure matrix

C)

(

1.28 0.50 0.21 1.50

)

suggesting a coincidence lattice.66 Obviously the pattern stems from layers of upright standing molecules, i.e. from the flat islands. Lattice points from the molecular overlayer are thus lying on lattice lines from the substrate, as demonstrated in Figure 11c. Lattice matching calculations reproduce these orientations.67 For PPTTPP, such a pattern is not observed, because the coverage of the surface by these islands is much smaller. Conclusions In this paper, it has been shown that the deposition of PPTTPP and TTPPTT thiophene/phenylene co-oligomers on muscovite

Nanoaggregates from Thiophene/Phenylene mica leads to the formation of needles with two nanometric dimensions (i.e., several tens of nanometers height, a few hundred nanometers width) on the surface. The needles are growing along distinct substrate directions. Within each direction, the needles together with the forming molecules are mutually parallelly oriented. The driving force is a combination of epitaxy and alignment of the molecules due to surface electric fields, and thus, the formation follows a rather generic mechanism for fibers from conjugated molecules. For the pphenylenes and R-thiophenes it has been shown that, with decreasing interaction of the molecules with the substrate, the number of islands from upright molecules increases.63,68 From this we conclude that for PPTTPP the interaction of the molecules with the substrate is larger as compared to TTPPTT. This leads to rather well-defined needle directions. The weaker molecule-substrate interaction for TTPPTT affects the width of the orientational distribution and leads to a large amount of islands from upright molecules.69 Due to the almost perfect perpendicular orientation of the molecules’ long axes with respect to the long fiber axes, the fibers are supposed to show improved transport properties as compared to, for example, p-6P. Experimental Section 5,5′-Di-4-biphenylyl-2,2′-bithiophene (PPTTPP). Under an argon atmosphere 5,5′-dibromo-2,2′-bithiophene (500 mg, 1.54 mmol), 4-biphenylylboronic acid (627 mg, 3.16 mmol, 2.05 equiv), tetrakis(triphenylphosphine)palladium(0) (88 mg, 0.077 mmol, 5 mol %), and cesium fluoride (702 mg, 4.6 mmol, 3 equiv) were dissolved in 60 mL of absolute tetrahydrofuran and refluxed for 50 h. After cooling to room temperature, the reaction mixture was diluted with n-hexane, and the precipitate was collected by filtration. The crude product was washed with water, ethyl acetate, and dichloromethane to give 692 mg (1.47 mmol, 95%) of a yellow-orange amorphous solid. MS (EI): m/z 470.1 (M+), 235.0 (M2+). HR-MS (EI): calcd. for C32H22S2: 470.1162, found 470.1162. UV-vis: λmax ) 393 nm. 4,4′-Di-2,2′-bithienylbiphenyl (TTPPTT). Under an argon atmosphere, 4,4′-biphenyldiboronic acid bis(neopentyl glycol)ester (719 mg, 1.9 mmol), 5-bromo-2,2′-bithiophene (1.00 g, 4.08 mmol, 2.1 equiv), tetrakis(triphenylphosphine)palladium(0) (109 mg, 0.095 mmol, 5 mol %), and cesium fluoride (1.16 g, 7.6 mmol, 4 equiv) were dissolved in 80 mL of absolute tetrahydrofuran and refluxed for 50 h. After cooling to room temperature, the reaction mixture was diluted with n-hexane, and the precipitate was collected by filtration. The crude product was washed with water, ethyl acetate, and dichloromethane to give 830 mg (1.72 mmol, 91%) of a bright yellow amorphous solid. MS (EI): m/z 482.0 (M+), 241.0 (M2+). HR-MS (EI): calcd for C28H18S4: 482.0291, found 482.0283. UV-vis: λmax ) 375 nm. Acknowledgment. The authors are grateful to the Danish research agencies FNU and FTP as well as the Danish Advanced Technologies Trust for supporting this work by various grants. A.L. thanks the German research foundation DFG for financial support. F.B. thanks the Hanse-Wissenschaftskolleg Delmenhorst for a fellowship. References and Notes (1) Fichou, D. Handbook of Oligo- and Polythiophenes; Wiley-VCH: Weinheim, 1999. (2) Ziegler, C. Thin Film Properties of Oligothiophenes. Handbook of Organic ConductiVe Molecules and Polymers. Vol. 3. ConductiVe Polymers: Spectroscopy and Physical Properties; Wiley: New York, 1997.

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