Influence of Halogen Substituents on the Self-Assembly of

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Influence of Halogen Substituents on the Self-Assembly of Oligothiophenes-A Combined STM and Theoretical Approach Mohamed M. S. Abdel-Mottaleb,§ Gu¨nther Go¨tz, Pinar Kilickiran,£ Peter Ba¨uerle, and Elena Mena-Osteritz* Abteilung Organische Chemie II, UniVersitat Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany ReceiVed September 21, 2005. In Final Form: NoVember 22, 2005 Iodinated quaterthiophenes 2-3 have been synthesized and their self-assembling behavior investigated at the liquid-solid interface by means of high-resolution scanning tunneling microscopy in comparison to parent oligothiophene 1. All three compounds spontaneously give well-ordered 2D crystalline monolayers at the graphite surface and order in a lamella-type arrangement of the conjugated backbones concomitant with an interlocking of the alkyl side chains. Symmetrically substituted oligothiophenes 1 and 3 without a relevant dipole moment self-assemble in a similar fashion, exhibiting comparable unit cells, whereas monoiodo derivative 2 arranges as pairs along the lamella axis due to the presence of a permanent dipole moment induced by the polarizable halogen group. Corroborated by quantum chemical calculations, novel head-to-head (iodo-iodo) intermolecular interactions were found to take place for this unsymmetrical derivative. The investigation of mixed solutions clearly reveals that at the solid-liquid interface a homogeneous layer of this compound is formed, which comprises the highest packing density leading to a separation process at the interface.

1. Introduction Self-organizing molecular systems are in the focus of nanotechnology research because of their potential use in “bottomup” approaches toward functional (supra)molecular devices.1 The design and synthesis of molecules that form self-assembled monolayers (SAMS) on substrates are fairly understood and developed. Scanning tunneling microscopy (STM) has emerged as one of the best tools to image, analyze, and control selfassembled molecular systems on substrates under various conditions (ambient, UHV, liquid-solid interface) with resolutions down to the (sub)molecular level.2 In this respect, various STM investigations have been devoted to self-organizing oligo- and polythiophenes, a class of materials which combine excellent electronic and transport properties, as well as good stability under ambient conditions, placing them at the forefront for applications in organic electronics.3 In STM images of self-assembled oligo- and polythiophene monolayers, typically the brighter parts (higher tunneling current) correspond to the conjugated π system, the darker ones to insulating alkyl side chains which support and direct the self-organization through van der Waals interactions.4-16 * To whom correspondence should be addressed. E-mail: [email protected]. § New address: Institut fu ¨ r Physik, Technische Universita¨t Chemnitz, Reichenhainerstr. 70, 09107 Chemnitz, Germany. £ New address: Materials Science Laboratories, Sony Deutschland GmbH, Hedelfinger Strasse 61, 70327 Stuttgart, Germany. (1) Maruccio, G.; Cingolani, R.; Rinaldi, R. J. Mater. Chem. 2004, 14, 542554. (2) (a) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; Wiley-VCh: New York, 1996. (b) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491-501. (3) (a) Fichou, D., Ed., Handbook of Oligo- and Polythiophenes; Wiley-VCh: New York, 1998. (b) Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998. (4) Azumi, R.; Go¨tz, G.; Ba¨uerle, P. Synth. Met. 1999, 101, 569-572. (5) (a) Soukopp, A.; Glo¨ckler, K.; Ba¨uerle, P.; Sokolowski, M.; Umbach, E. AdV. Mater. 1996, 7, 902-906. (b) Seidel, C.; Soukopp, A.; Li, R.; Ba¨uerle, P.; Umbach, E. Surf. Sci. 1997, 374, 17-30. (c) Soukopp, A.; Glo¨ckler, K.; Kraft, P.; Schmitt, S.; Sokolowski, M.; Umbach, E.; Mena-Osteritz, E.; Ba¨uerle, P.; Ha¨dicke, E. Phys. ReV. B 1998, 58, 13882-13894. (6) Stabel, A.; Rabe, J. P. Synth. Met. 1994, 67, 47-53.

β-Alkylated oligothiophenes typically adsorb well and selfassemble on the rather inert surface of highly oriented pyrolytic graphite (HOPG).6-8 In general, lamellar arrangements induced and stabilized by multiple van der Waals interactions of the alkyl side chains were found. In particular, head-to-tail coupled oligo(3-alkylthiophenes), which represent models for the very prominent regioregular poly(3-alkylthiophenes) frequently used in organic electronic devices, lead to very stable monolayers spontaneously formed on the substrate surface.8b,9 Over wide areas, these polymers, as well, show a perfect self-organization of the polymeric strands in comparable lamellar structures both at the liquid-solid interface8b,10 and in dry films.11 In addition to the rather stiff rigid rodlike polymer structure adopting a straight line in STM images, clearly resolved folding of polymer chains due to a syn conformation of consecutive thiophene rings was visualized. Very recently, similar lamellar and folded structures were found in STM measurements on a chiral regioregular poly(7) Ba¨uerle, P.; Fischer, T.; Biddlingmaier, B.; Stabel, A.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 303-307. (8) (a) Azumi, R.; Go¨tz, G.; Debaerdemaeker, T.; Ba¨uerle, P. Chem. Eur. J. 2000, 6, 735-744. (b) Mena-Osteritz, E. AdV. Mater. 2002, 14, 609-616. (9) Kirschbaum, T.; Azumi, R.; Mena-Osteritz, E.; Ba¨uerle, P. New J. Chem. 1999, 23, 241-251. (10) Mena-Osteritz, E.; Meyer, A.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2000, 39, 2679-2684. (11) (a) Gre´vin, B.; Rannou, P.; Payerne, R.; Pron, A.; Travers, J. P. AdV. Mater. 2003, 14, 881-884. (b) Gre´vin, B.; Rannou, P.; Payerne, R.; Pron, A.; Travers, J. P. J. Chem. Phys. 2003, 118, 7097-7102. (12) Koeckelberghs, G.; Samyn, C.; Miura, A.; De Feyter, S.; De Schryver, F.; Sioncke, S.; Verbiest, T.; de Schaetzen, G.; Persoons, A. AdV. Mater. 2005, 17, 708-712. (13) (a) Kro¨mer, J.; Rios-Carreras, I.; Fuhrmann, G.; Musch, C.; Wunderlin, M.; Debaerdemaeker, T.; Mena-Osteritz, E.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2000, 39, 3481-3486. (b) Mena-Osteritz, E.; Ba¨uerle, P. AdV. Mater. 2001, 13, 243-246. (14) Mu¨ller, T.; Werblowsky, T. L.; Florio, G. M.; Berne, B. J.; Flynn, G. W. Proc. Nat. Acad. Sci. 2005, 102, 5315-5322. (15) (a) Mu¨ller, H.; Petersen, J.; Strohmaier, R.; Gompf, B.; Eisenmenger, W.; Vollmer, M. S.; Effenberger, F. AdV. Mater. 1996, 8, 733-737. (b) Vollmer, M. S.; Effenberger, F.; Stecher, R.; Gompf, B.; Eisenmenger, W. Chem. Eur. J. 1999, 5, 96-101. (16) (a) Gesquie`re, A.; De Feyter, S.; De Schryver, F.; Schoonbeek, F.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Nano Lett. 2001, 1, 201-206. (b) De Feyter, S.; Larsson, M.; Schuurmans, N.; Verkuijl, B.; Zoriniants, G.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; van Esch, J.; Feringa, B. L.; van Stam, J.; De Schryver, F. Chem. Eur. J. 2003, 9, 1198-1206.

10.1021/la052566c CCC: $33.50 © 2006 American Chemical Society Published on Web 12/31/2005

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Abdel-Mottaleb et al. Scheme 1

(i) Hg(C5H11COO)2, CHCl3/HOAc (9:1), I2; (ii) 2.2 Hg(C5H11COO)2, CHCl3/HOAc (9:1), 2.2 I2.

(3-alkylthiophene) at the liquid-solid interface.12 Fully macrocyclic oligothiophenes form highly ordered 2D crystalline monolayers over large areas at the liquid-solid interface. A perfect hexagonal honeycomblike packing with an optimal packing density of the macrocycles is typically observed in submolecularly resolved STM images.8b,13 The attachment of polar end groups to the conjugated backbone influences the self-assembling behavior through dipole-dipole interactions. The self-assembling properties of 1-halohexanes on graphite have been recently analyzed.14 STM images of formylsubstituted quinquethiophenes and steroid-bridged oligothiophenes showed that one polar substituent leads to a distinct molecular packing in comparison to the symmetric parent compound and the dialdehyde.15 Bis-urea-substituted bithiophenes efficiently assemble at the liquid-HOPG interface and form extended lamellae. In these monolayers, the dominating intermolecular forces are multiple hydrogen bonds between the urea groups.16 The self-ordering behavior of regioregular quater(3hexylthiophene) carboxylic acid and the corresponding pentapeptide-oligothiophene is dominated by the mutual interplay of van der Waals forces due to the hydrophopic block and hydrogen bonding due to the hydrophilic moiety.17 Herein, we present the self-assembling behavior of β-alkylated oligothiophenes which are substituted by one or two iodine atoms at the R position. In contrast to more polar groups which form stronger hydrogen bonds, halogen atoms exert a smaller dipole moment and a higher polarizability, giving rise to distinct interactions, strongly dependent on the mono- or bifunctionalization of the oligothiophenes. Furthermore, the reactive halogen functions are interesting because iodoarenes can undergo coupling reactions on surfaces.18

2. Results and Discussion 2.1. Synthesis. The symmetrically substituted didodecylquaterthiophene 1 was readily prepared by a Ni0-catalyzed crosscoupling reaction of 3-dodecylthien-2-ylmagnesium bromide and 5,5′-dibromo-2,2′-bithiophene in 75% yield.7 5-Iodo- and 5,5′′′diiodo derivatives 2 and 3 were synthesized from 1 by an electrophilic substitution reaction with mercury hexanoate and subsequent ipso-reaction with iodine, a protocol which we recently developed for selective iodinations of oligothiophenes.19 The (17) Klok, H.-A.; Ro¨sler, A.; Go¨tz, G.; Mena-Osteritz, E.; Ba¨uerle, P. Org. Biomol. Chem. 2004, 2, 3541-3544. (18) (a) Hla, S. W.; Bartels, L.; Meyer, G.; Rieder, K.-H. Phys. ReV. Lett. 2000, 85, 2777-2780. (b) Hla, S. W.; Meyer, G.; Rieder, K.-H. ChemPhysChem. 2001, 2, 361-366. (19) Kirschbaum, T.; Briehn, C. A.; Ba¨uerle, P. J. Chem. Soc., Perkin Trans. 1 2000, 1211-1216.

use of 2.2 equiv of the reagents efficiently led to analytically pure 5,5′′′-diiodo-quaterthiophene 3 in 75% yield. However, the selectivity for a monofunctionalization is drastically reduced and in general a problem for oligothiophenes.20 Therefore, by reacting quaterthiophene 1 with 1 equiv of mercury caproate and iodine, a mixture of the desired monoiodinated, diiodinated, and unreacted quaterthiophene was obtained. Monoiodo derivative 2 was enriched by column chromatography followed by multiple recrystallizations. On a small scale, final purification was achieved either on a preparative TLC plate or by preparative HPLC, giving a yield of 40% (Scheme 1). 2.2. STM Investigations at the Liquid-Solid Interface. STM experiments with the three quaterthiophene derivatives 1-3 were performed at the solid-liquid interface using various solvents. Prior to imaging, the compounds 1-3 were dissolved either in 1-phenyloctane, 1,2,4-trichlorobenzene, or 1-octanol and a drop from such solutions was applied on a freshly cleaved surface of HOPG. In all cases, in the STM images no solvent-dependent features were observed, regardless of the compound or solvent under investigation. The STM images presented in this paper were obtained at the 1-phenyloctane-HOPG interface. The selfordering behavior of parent quaterthiophene 1 has been described previously7,8a and was reinvestigated in order to compare with the novel functionalized derivatives under identical conditions. A typical high-resolution STM image of a self-organized monolayer of 1 at the solid-liquid interface is shown in Figure 1a. As is generally found for π-electron-rich systems, the oligothiophene backbones appear as bright structures corresponding to high tunneling currents, whereas the alkyl side chains are not well resolved and occupy the dark areas (low tunneling currents).4-17 The π-conjugated oligothiophene units are arranged in a lamella-type structure with the long axis forming an angle, γ, of 30 ( 3° with respect to the lamellar axis (see also Figure 5). The different domains observed in the monolayer, related with the crystallographic axes of the underlying graphite, give clear proof of a molecule-substrate interaction.8a The unit cell parameters are determined to a ) 1.1 ( 0.1 nm, b ) 2.1 ( 0.2 nm and R ) 78 ( 2°, respectively, and are in a good agreement with the first investigations (a ) 1.2 ( 0.1 nm, b ) 2.15 ( 0.05 nm, R ) 75° ( 3°) and with the calculated values (a ) 1.08 nm, b ) 2.14 nm, R ) 76°, Figure 1b). The unit cell contains one molecule, and the main driving force for the observed molecular arrangement is due to van der Waals interactions of the interdigitating alkyl side chains. It is interesting to note that the oligothiophene moieties appear with three different contrasts (20) Ba¨uerle, P.; Wu¨rthner, F.; Go¨tz, G.; Effenberger, F. Synthesis 1993, 10991103.

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Figure 1. (a) STM image of a monolayer of oligothiophene 1 at the liquid-solid interface (10.4 × 10.4 nm2, Iset ) 0.4 nA, Vbias ) -122 mV). The unit cell is indicated. (b) Semiempirically calculated molecular model of the most stable monolayer. The highest occupied molecular orbitals, HOMOs, are shown (left, for the isolated molecule; right, for a pair of molecules). In brackets is the percentage of contribution of each molecule to the HOMO.

Figure 2. (a) STM image of a monolayer of iodo-oligothiophene 2 observed at the liquid-solid interface (11.5 × 11.5 nm2. Iset ) 0.5 nA, Vbias ) -414 mV). The colored arrows indicate the two different molecules in the pairs (yellow ellipsoids). The unit cell is indicated. (b) Semiempirically calculated molecular model of the most stable monolayer. The HOMO and HOMO-1 are shown for the paired molecules. In brackets is the percentage of contribution of each molecule to the HOMO.

(white, red, and green arrows in Figure 1a). Every third oligothiophene in a lamella appears in full extension (∼1.4 nm, white arrows), whereas the others are imaged as bright areas of around 0.7 nm in length and are visible either on the righthand-side of the lamella (red arrows) or on the left-hand-side (green arrows). The detailed analysis of the submolecularly resolved features shows in the first case eight lobes per molecule which could correspond, omitting the substrate contribution, to the local density of state (LDOS) of the HOMO level (Figure 1b). By imaging the underlying HOPG surface (not shown here), we find that the oligothiophene moieties along the lamella are not commensurate with the underlying HOPG lattice and occupy different positions by a leftward shift with respect to one of the graphite main axes. The distinct contrast observed for the molecules in a lamella can be understood with the help of theoretical calculations: the analysis of the molecular orbitals in an ensemble of molecules reflecting the arrangement in the monolayer shows that the molecules contribute inhomogeneously to the occupied molecular orbitals. Figure 1b, right, shows the HOMO of a pair of 1 in which the contribution of one molecule is larger than that of the second one (62% vs 38%). This difference will induce an anisotropic tunneling current through the molecules aligned in a lamella and explains the distinct contrast observed in the STM images. Furthermore, the incommensurability of the monolayer can intensify this effect. The calculations show that the lamellar arrangement of the backbones favors the linear alignment of the small molecular dipoles (µ ) 0.012 D) (Figure 1b, gray arrows) and maximizes

the interactions between the π systems: the close proximity of the thiophene backbones in the lamellae (0.52 nm, red circle in Figure 1b) can be explained by weak electrostatic interactions between the sulfur atoms of the terminal thiophenes and the β-hydrogen atoms of the internal thiophenes of the neighboring molecules (βH-S distance of 0.25 nm). This interaction will be designated as a tail-to-tail interaction to compare later with the head-to-head interaction of the iodine substituted part in compounds 2 and 3. The molecule-to-lamella orientation in the calculated model for molecule 1 agrees perfectly with the observed in the STM images (γ ) 32.1° vs 30° ( 3°). Iodinated quaterthiophene 2 spontaneously forms SAMs at the HOPG-liquid interface. A submolecularly resolved STM image also shows a lamella-type arrangement of the oligothiophene backbones forming an angle of 23 ( 3° with respect to the lamellar axis (Figure 2a). In comparison to parent compound 1, this angle is smaller, reflecting the influence of the iodine atoms at the R position of the oligothiophenes. The unit cell parameters a, b, and R, as indicated on the figure, were determined to be 2.9 ( 0.2 nm, 2.2 ( 0.2 nm, and 96 ( 2° respectively, and are in a good agreement with the calculated values (a ) 2.85 nm, b ) 2.13 nm, R ) 92°, γ ) 20°, Figure 2b). Two molecules of compound 2 (red and white arrows in Figure 2a) are included in the unit cell, forming pairs (yellow ellipsoids). Theoretical calculations show that the iodines are pointing outside of the pairs (Figure 2b). The distance between two paired molecules is similar to that observed for compound 1, indicating similar tail-to-tail intermolecular interactions. In the perpendicular

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Figure 3. (a) 3D energy potential surface of two molecules of the model compound 5-iodo-4-dodecyl-bithiophene (normalized to the minimum). (b) Energy diagram for the three different intermolecular interactions for compound 2 in the monolayer.

Figure 4. (a) STM image of a monolayer of diiodo-oligothiophene 3 observed at the liquid-solid interface (10.2 × 10.2 nm2, Iset ) 1 nA, Vbias ) -450 mV). The unit cell is indicated. White arrows show the molecular and alkyl chain axes (see text). (b) Semiempirically calculated molecular model of the most stable monolayer. The HOMO and HOMO-1 for the isolated molecule and the HOMO for a pair of molecules are shown. The percentage of contribution of each molecule to the HOMO is shown in brackets.

direction, the distance between two lamellae, 2.2 nm, points to an all-trans conformation of the alkyl side chains which fully interdigitate and are partially resolved. For the individual quaterthiophene moieties, eight bright features can be identified in the submolecularly resolved STM images. The calculated HOMO and HOMO-1 of molecule 2 accordingly show eight lobes with no contribution from the iodine atom (Figure 2b). The different contrast of the molecules in the pairs (Figure 2a) can be explained, as for compound 1, by analyzing the distinct contribution to the occupied molecular orbitals (56% and 44% to the HOMO of a pair of 2, Figure 2b, bottom). The calculated dipole moment of molecule 2 (1.64 D) is oriented almost parallel to the thiophene backbone axis (Figure 2b, gray arrows). The antiparallel alignment of the molecular dipoles in the lamellae is a consequence of stabilizing dipole-dipole forces, which govern the 2D structure formation (see below). “Ab initio” and semiempirical quantum chemical calculations performed on a model iodobithiophene show a total minimum for the potential surface at a distance between the iodines (d(I-I)) of 0.32 ( 0.02 nm and an angle (γ) of 131° ( 5° (Figure 3a). This value has been employed to calculate the unit cell parameters already mentioned before and agree with the data of iodo-iodo interactions in monolayers and crystals.14,21-22 Analyzing the intermolecular interactions taking place in the monolayer, the calculations show that the head-to-head molecular (21) Giancarlo, L. C.; Flynn, G. W. Annu. ReV. Phys. Chem. 1998, 49, 297336. (22) Cso¨regh, I.; Brehmer, T.; Bomicz, P.; Weber, E. Cryst. Eng. 2001, 4, 343-357.

arrangement (Figure 3b) is 26 kcal/mol (1.15 eV) more stable than the corresponding tail-to-tail fashion and 18 kcal/mol (0.79 eV) than the interlamellar molecule-molecule interaction (governed by the van der Waals forces between alkyl chains). In this way, the calculations explain the large separation between the molecules of 2 in the lamella at the iodine moiety that leads to an area per molecule in the 2D crystal which is bigger (A/M ) 3.17 nm2) in comparison to 1 (A/M ) 2.26 nm2) and 3 (A/M ) 2.10 nm2). For compound 2, the van der Waals forces between lamellae represent only the second strongest stabilization forces, whereas for 1 and 3, they are the main forces responsible for the monolayer formation. A submolecularly resolved STM image of a monolayer of diiodinated quaterthiophene 3 formed at the HOPG-solution interface after the application of a drop of a solution of 3 is shown in Figure 4a. The molecules clearly show the oligothiophene part as eight bright lobes. The oligothiophene backbones are forming angles of 44° ( 1° with respect to the lamellar axis. The unit cell parameters a, b, and R, as indicated in the figure, were determined to be 0.93 ( 0.05 nm, 2.3 ( 0.3 nm, and 78 ( 3° respectively, and are in a good agreement with the calculated values (a ) 0.98 nm, b ) 2.44 nm, R ) 79°, γ ) 47°, Figure 4b). In this case, the unit cell contains one molecule. A detailed analysis of the well-resolved image (Figure 4a) shows an angle between the molecular backbone and the apparent axis of the partly resolved alkyl chains as large as 106°. This value differs from this of the calculated model (90° in Figure 4b). The calculations establish that the molecular conformation

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Figure 5. A comparison of the packing patterns observed for compounds 1-3: left, long-range STM image of 1: 16.4 × 16.4 nm2, Iset ) 0.4 nA, and Vbias ) -143 mV; middle, long-range STM image of 2: 22.6 × 22.6 nm2, Iset ) 0.5 nA, and Vbias ) -414 mV; right, long-range STM image of 3: 19.2 × 19.2 nm2, Iset ) 1 nA, and Vbias ) -676 mV. The parameters of the unit cells of the three derivatives (a, b, and R) are provided, as well as the angle between the molecular axis and the lamellar axis (γ) and the density of molecules in the monolayer (M/nm2).

comprising a 90° angle between the backbone and the alkyl side chain is 19 kcal/mol (0.84 eV) more stable than the 106° conformer. Analyzing the molecule-molecule forces for the 106° conformer in a monolayer, less-stable interactions are obtained in all cases (intra- and interlamellar) than for the 90° conformer. The parameters of the most stable and compact monolayer formed with the 106° conformer (a ) 0.89 nm, b ) 2.73 nm, and R ) 72°) are different than those of the experimental STM images (a ) 0.93 ( 0.05 nm, b ) 2.3 ( 0.3 nm, and R ) 78.4 ( 1.2°). Moreover, the molecule density in the monolayer decreeses from 0.48 molecules/nm2 for the 90° conformer to 0.43 M/nm2 for the 106° conformer. We conclude from these results that the molecular conformation in the adsorbed monolayer corresponds to the most stable 90° conformer shown in Figure 4b and that some coupling effects of the alkyl chains with the underlying HOPG might be one cause for the mismatch between the seeming interpretation of the interlamellar areas in the STM images and the results of the quantum chemical calculations. A comparison of the packing pattern and parameters of quaterthiophenes 1-3 in the 2D crystalline monolayer clearly reveal that the symmetrically substituted compounds 1 and 3 without a relevant dipole moment arrange in a similar lamellatype fashion (Figure 5). The unit cell parameters are well comparable; however, the adopted angle between the oligothiophene moieties and the lamellar axis (γ) is larger for the iodinefunctionalized derivative 3. This allows a more densely packed arrangement and can be attributed to the presence of the iodine atoms. Monofunctionalized oligomer 2 orders differently on the substrate surface: due to the internal dipole moment, dimers are formed in a way that the dipole moments are compensated. Therefore, unit cell parameters a and R are different for monolayers of 2, whereas b is comparable because b corresponds

to the distance between two lamellae which is governed by the length of the dodecyl side chain, invariable for all three compounds. 2.3. STM Investigations of Mixtures at the Liquid-Solid Interface. STM experiments with mixtures of the three quaterthiophene derivatives 1-3 were performed at the liquidsolid interface using various ratios in casted solutions. The STM images of a mixture of compounds 1 and 2 predominantly show a monolayer of 1, even at high excesses of iodo derivative 2 [up to ratios of 1:10 (1/2)]. Only at much higher ratios of 2 are monolayers of 2 observed from such a mixture. This can be rationalized by the packing density of each derivative. While compound 1 has a packing density of 0.44 molecules/nm2, iodo derivative 2 has a lower packing density of only 0.32 M/nm2. The same finding was observed for mixtures of derivatives 2 and 3 (0.48 M/nm2), as well as for mixtures of 1 and 3. Up to ratios of 5:1 (1/3), only monolayers of the diiodo derivative 3 were observed. These results strongly suggest that from solutions containing mixtures of rather similar compounds fully homogeneous 2D crystalline layers of this compound are formed which pack with the highest density. Such separations could not be achieved by recrystallization from solution (vide infra).

3. Conclusion In conclusion, R-quaterthiophene 1 with two solubilizing dodecyl side chains in β positions has been substituted with one or two iodine atoms at the free R positions of the conjugated backbone to give iodo-quaterthiophene 2 and diiodo-quaterthiophene 3, respectively. The self-assembling behavior of the three oligothiophenes, 1-3, has been investigated at the liquidsolid interface by means of high-resolution STM. Lamella-type packing of the molecules is found in each case, whereby significant

1448 Langmuir, Vol. 22, No. 4, 2006

changes in the packing pattern are observed in particular for the monohalogenated derivative 2. While the parent and diiodinated quaterthiophene, 1 and 3, respectively, order in a quite similar lamellar pattern with higher packing density for the diiodo compound 3, monoiodo derivative 2 arranges as pairs along the lamella. These results clearly indicate that the arrangement of molecules in the monolayer is influenced by introduction of a polarizable halogen group to the conjugated backbone. The presence of a permanent dipole moment in the molecules gives rise to fundamental changes in the molecular arrangement. Novel head-to-head (iodo-iodo) intermolecular interactions were found to take place for the unsymmetrical derivative. The investigation of mixed solutions clearly reveals that at the solid-liquid interface a homogeneous layer of this compound is formed which comprises the highest packing density leading to a separation process at the interface. 4. Experimental Section Synthesis. Solvents and reagents were purified prior to use and dried by usual methods. Thin-layer chromatography (TLC) was carried out on aluminum Silica gel 60 F254 sheets from Merck. Developed sheets were dried and examined under a UV lamp. Preparative TLC was performed on PLC plates (20 × 20 cm2) Silica gel 60 F254, 2 mm. The plates were developed in n-hexane; the separated zone of interest on the silica layer was scratched off from the plate and the compound extracted with petroleum ether in a Soxhlett apparatus. Preparative column chromatography was performed on glass columns of different sizes packed with silica gel 60 (0.04-0.06 mm, Merck) and HPLC on a Hibar (250-25 mm) with LiChrospher Si 60 (5 µm) from Merck. Melting points were detected with a Bu¨chi 545 melting point unit and are not corrected. 1H NMR spectra were recorded on a Bruker AVANCE 400 (400 MHz) spectrometer (with deuterated solvent as lock-in and tetramethylsilane as internal reference). 13C NMR spectra were recorded on a Bruker AVANCE 400 (100 MHz) spectrometer. 3,3′′′-Didodecyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (1). 1 has been prepared by the nickel-catalyzed cross-coupling of the Grignard reagent of 3-dodecyl-2-bromothiophene and 5,5′-dibromo-2,2′bithiophene in 75% yield according to ref 7. 5-Iodo-3,3′′′-didodecyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (2). To a solution of oligothiophene 1 (0.67 g, 1 mmol) in 20 mL of CHCl3/HOAc (9:1) mixture was added mercury hexanoate (0.43 g, 1 mmol) with stirring at room temperature. After 2 h, a yellow precipitate formed. The heterogeneous mixture was cooled in an ice bath, and a solution of 0.25 g (1 mmol) of iodine in 12 mL of CHCl3 was added within 2 h. The cooling bath was removed, and the mixture was stirred for 4 h. The chloroform solution was washed with water, twice with saturated NaHCO3, aqueous KI and Na2S2O3 solution, and once with water. After removal of the solvent in a vacuum, the crude product was purified twice by flash chromatography on a column of silica with petroleum ether (p.e., bp 50-60 °C). In each first fraction, a total amount of 276 mg of quaterthiophene 2 was isolated. The main fractions contained 2 contaminated with small amounts of starting material 1. These impurities were finally removed either on preparative TLC plates with n-hexane on a small scale (10 mg) or on a preparative SiO2 (5 µm) HPLC column with the same solvent, yielding 0.32 g (40%) 2 as a yellow solid. mp ) 40-41

Abdel-Mottaleb et al. °C; Rf (SiO2/p.e.) ) 0.22; 1H NMR (CDCl3, 400 MHz): δ 7.17 (d, 3J ) 5.2 Hz, 1H, H5′′′), 7.11 (d, 3J ) 3.7 Hz, 1H, H3′′), 7.095 (d, 3J ) 3.7 Hz, 1H, H4′), 7.07 (s, 1H, H4), 7.01 (d, 3J ) 3.8 Hz, 1H, H4′′), 6.95 (d, 3J ) 3.8 Hz, 1H, H3′), 6.93 (d, 3J ) 5.2 Hz, 1H, H4′′′), 2.77 (t, 3J ) 7.8 Hz, 2H, -CH2-), 2.72 (t, 3J ) 7.8 Hz, 2H, -CH2), 1.64-1.57 (m, 4H, -CH2-), 1.25 (m, 36H, -CH2-), 0.87 (t, 3J ) 7 Hz, 6H, -CH3); 13C NMR (CDCl3, 100 MHz): δ 141.68 (C3), 139.97 (C3′′′), 139.82 (C4), 137.45 (C2′), 136.49, 136.36 (C5′, C2′′), 135.64 (C5′′), 133.75 (C2), 130.30 (C4′′′), 130.09 (C2′′′), 126.96 (C3′), 126.53 (C4′′), 124.04 (C4′), 123.92 (C3′′), 123.78 (C5′′′), 71.76 (C5), 31.94, 30.66, 30.58, 29.69, 29.66, 29.61, 29.57, 29.52, 29.47, 29.43, 29.42, 29.37, 29.28, 28.94, 22.70, 14.12 (aliph. C); Anal. Calcd for C40H57IS4: C, 60.58; H, 7.24; I, 16.00; S, 16.17. Found: C, 60.52; H, 7.20; I, 15.95; S, 16.45. 5,5′′′-Diiodo-3,3′′′-didodecyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (3). The preparation of quaterthiophene 3 was performed under similar conditions as for 2, starting from 1 (667 mg, 1 mmol) in 20 mL of CHCl3/HOAc (9:1), mercury hexanoate (948 mg, 2.2 mmol), and iodine (558 mg, 2.2 mMol) in 20 mL of CHCl3. The crude product was recrystallized from petroleum ether yielding 685 mg (75%) of 3 as an orange solid. mp ) 92.5-93 °C; Rf (SiO2/p.e) ) 0.28; 1H NMR (CDCl3, 400 MHz): δ 7.09 (d, 3J ) 3.8 Hz, 2H, H4′/H3′′), 7.07 (s, 2H, H4/H4′′′), 6.95 (d, 3J ) 3.8 Hz, 2H, H3′/ H4′′), 2.72 (t, 3J ) 7.8 Hz, 4H, -CH2-), 1.60 (dt, 4H, -CH2-), 1.40-1.25 (m, 36H, -CH2-), 0.87 (t, 3J ) 6.8 Hz, 6H, -CH3); 13C NMR (CDCl3, 100 MHz): δ 141.75 (C3/C3′′′), 139.84 (C4/C4′′′), 137.11 (C2′/C5′′), 136.26 (C5′/C2′′), 134.03 (C2/C2′′′), 126.97 (C3′/ C4′′), 124.00 (C4′/C3′′), 71.86 (C5/C5′′′), 31.94, 30.57, 29.69, 29.67, 29.56, 29.42, 29.41, 29.37, 28.94, 22.70, 14.12 (aliph. C); Anal. Calcd for C40H56I2S4: C, 52.28; H, 6.14; I, 27.62; S, 13.96. Found: C, 52.49; H, 6.21; I, 27.45; S, 14.21. STM. STM measurements were carried out under ambient conditions with a low-current scanning tunneling microscope (Rochester Hills, MI (RHK)) controlled by a RHK STM 1000 control system. Both mechanically cut and electrochemically etched Pt/Ir tips (80/20, diameter 0.2 mm) were used. The etched tips were prepared using a 2 N KOH/6 N NaCN solution in water. The images represented were obtained in a constant-height mode (variable-current mode). In the STM image, white (bright color) corresponds to the highest and black (dark color) to the lowest measured tunneling current. The experiments were repeated in different sessions using differrent tips to check for reproducibility and to avoid artifacts. Different settings for the tunneling current and the bias voltage were used, ranging from 0.05 to 1 nA and from -0.1 to -0.9 V, respectively. All STM images contain raw data and are not subjected to any manipulation or image processing. Calculations. Theoretical calculations were performed on a semiempirical basis with INDO methods, Austin Model 1(AM1), and the PM3 parameterization from the Hyperchem (Hypercube, Inc., FL) software package. All possible starting conformations, concerning the torsion angle between thiophenes, have been calculated in order to find the global minimum energy for the isolated molecule.

Acknowledgment. We would like to thank the European Union (Rene´ Descartes Prize 2000) and the German Research Foundation (SFB 569) for financial support. LA052566C