Structure of Self-Assembled Multilayers Prepared from Water-Soluble

Jun 8, 2006 - (GSMR), Turku; and Department of Physical Chemistry, Åbo Akademi UniVersity,. FIN-20050 Turku (Åbo), Finland. ReceiVed February 23 ...
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Langmuir 2006, 22, 6078-6086

Structure of Self-Assembled Multilayers Prepared from Water-Soluble Polythiophenes Antti Viinikanoja,*,†,‡ Sami Areva,‡,§ Natalia Kocharova,† Timo A ¨ a¨ritalo,† Maarit Vuorinen,† † † Arto Savunen, Jouko Kankare, and Jukka Lukkari*,† Department of Chemistry, UniVersity of Turku, FIN-20014 Turku; Graduate School of Materials Research (GSMR), Turku; and Department of Physical Chemistry, Åbo Akademi UniVersity, FIN-20050 Turku (Åbo), Finland ReceiVed February 23, 2006. In Final Form: May 3, 2006 We have studied the structure and morphology of self-assembled polyelectrolyte multilayers prepared using poly(styrenesulfonate) (PSS) and four different cationic poly(alkoxythiophene) derivatives bearing methylimidazoliumterminated ionic side chain at the 3-position of the thiophene ring: poly(1-methyl-3-[3-[3-thienyloxy]-propyl]-1Himidazolium) (P3TOPIM), poly(1-methyl-3-[6-[3-thienyloxy]-hexyl]-1H-imidazolium) (P3TOHIM), poly(1-methyl3-[2-[(4-methyl-3-thienyl)oxy]-ethyl]-1H-imidazolium ) (P4Me-3TOEIM), and poly(1-methyl-3-[6-[(4-methyl-3thienyl)oxy]-hexyl]-1H-imidazolium ) (P4Me-3TOHIM). All the multilayers exhibited regular growth. The thickness of the multilayers was measured with ellipsometry, their layer-by-layer growth was followed by polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and ellipsometry, and the morphology of the films was studied by atomic force microscopy (AFM). The length of the methylimidazolium-terminated side chain (Cn, n ) 2, 3, 6) and the substituent (H or Me) at the 4-position of the thiophene ring were varied. All multilayers were inhomogeneous in the sub-micrometer scale and contained aggregates of two kinds. The large ones with a low and constant surface number density were attributed to PSS, whereas the small aggregates were polythiophene-based. The surface density of these organic semiconducting nanoparticles greatly depended on the structure of polythiophene, being favored by polymer regioregularity and the length of the side chain. The side chains remained disordered in all the multilayers, but with polythiophenes having hexyl chains both the imidazolium and thiophene rings tended to orient themselves more perpendicular to the surface than in films containing shorter chains (C2 or C3). The relative water content of the multilayers (at 7.1% relative humidity) did not depend on the film thickness and was the lowest for P4Me-3TOHIM. As the number of bilayers increased the methylimidazolium-sulfonate ion pairs gradually weakened and became more individually hydrated.

Introduction The adjustable electrical and optical properties of polythiophenes (PTs) can be harnessed in electronic devices, e.g. light-emitting diodes (LEDs), sensors, and field-effect transistors (FETs), and their processability and solubility can be modified by side chain substitution.1 Solubility in water is important for biocompatibility and for the detection of biological compounds in aqueous media. Water solubility of conducting polymers has been accomplished through the introduction of cationic,2 anionic,3 * Corresponding authors. Telephone: +358-2-333 6714 (A.V.); +3582-333 6712 (J.L.). Fax: +358-2-333 6700 (A.V.); +358-2-333 6700 (J.L.). E-mail: [email protected] (A.V.); [email protected] (J.L.). † Department of Chemistry, University of Turku. ‡ GSMR, University of Turku. § Åbo Akademi University. (1) Diaz, A. F.; Nguyen, M. T.; Leclerc, M. In Physical Electrochemistry: Principles, Methods and Applications; Rubinstein, I., Ed.; Dekker: New York, 1995; p 555. (2) (a) Lukkari, J.; Saloma¨ki, M.; Viinikanoja, A.; A ¨ a¨ritalo, T.; Paukkunen, J.; Kocharova, N.; Kankare, J. J. Am. Chem. Soc. 2001, 123, 6083. (b) Zhai, L.; McCullough, R. D. AdV. Mater. 2002, 14, 901. (c) Li, C.; Numata, M.; Bae, A.-H.; Sakurai, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4548. (d) Takeoka, Y.; Iguchi, Y.; Rikukawa, M.; Sanui, K. Synth. Met. 2005, 154, 109. (3) (a) Patil, A. O.; Ikenoue, Y.; Wudl, F.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 1858. (b) Arroy-Villan, M. I.; Diaz-Quijada, G. A.; Abdou, M. S. A.; Holdcroft, S. Macromolecules 1995, 28, 975. (c) Chayer, M.; Faı¨d, K.; Leclerc, M. Chem. Mater. 1997, 9, 2902. (d) Leclerc, M.; Faı¨d, K. AdV. Mater. 1997, 9, 1087. (e) Kim, J.; Tripathy, S. K.; Kumar, J.; Chittibabu, K. G. Mater. Sci. Eng., C 1999, 7, 11. (f) Kim, J.; Wang, H.-C.; Kumar, J.; Tripathy, S. K.; Chittibabu, K. G.; Cazeca, M. J.; Kim, W. Chem. Mater. 1999, 11, 2250. (g) Viinikanoja, A.; Lukkari, J.; A ¨ a¨ritalo, T.; Laiho, T.; Kankare, J. Langmuir 2003, 19, 2768 (h) Allard, D.; Allard, S.; Brehmer, M.; Conrad, L.; Zentel, R.; Stromberg, C.; Schultze, J. W. Electrochim. Acta 2003, 48, 3137.

or zwitterionic4 groups in the hydrophobic backbone. One example of tethered cationic groups is the alkyl-substituted imidazolyl moiety, a derivative of the so-called ionic liquids, which renders the PT chain water-soluble and displays recognition properties toward anions, proteins, and nucleic acids.5 The application of these materials for sensing purposes takes advantage of the cooperative tendency of individual water-soluble conjugated polymer chains to self-assemble into supramolecular structures, which manifests itself as a change in the optical properties (light absorption or fluorescence).6 Various parameters, such as the substitution at the conjugated backbone, the nature of the solvent, or the temperature, are known to govern this process. The understanding and control of this aggregation behavior, both in solution and at the interface, is important for the development of new sensor materials for biological media. In addition to water-solubility, the use of pendant ionic groups in the PT backbone opens a way for the film preparation by the electrostatic layer-by-layer (LbL) self-assembly.7 The ionic LbL self-assembly is a versatile method to adsorb durable ultrathin (4) (a) Nilsson, K. P. R.; Ingana¨s, O. Nat. Mater. 2003, 2, 419. (b) Nilsson, K. P. R.; Ingana¨s, O. Macromolecules 2004, 37, 9109. (5) (a) Ho, H.-A.; Boissinot, M.; Bergeron, M. G.; Corbeil, G.; Dore´, K.; Boudreau, D.; Leclerc, M. Angew. Chem. Int. Ed. 2002, 41, 1548. (b) Ho, H.-A.; Leclerc, M. J. Am. Chem. Soc. 2003, 125, 4412. (c) Ho, H.-A.; Leclerc, M. J. Am. Chem. Soc. 2004, 126, 1384. (d) Dore´, K.; Dubus, S.; Ho, H.-A.; Le´vesque, I.; Brunette, M.; Corbeil, G.; Boissinot, M.; Boivin, G.; Bergeron, M. G.; Boudreau, D.; Leclerc, M. J. Am. Chem. Soc. 2004, 126, 4240. (6) Liu, B.; Bazan, G. C. Chem. Mater. 2004, 16, 4467. (7) (a) Decher, G. In ComprehensiVe Supramolecular Chemistry; Sauvage, J. P., Hosseini, M. W., Eds.; Pergamon: Oxford, U.K., 1996; p 507. (b) Decher, G. Science 1997, 277, 1232. (c) Decher, G., Schlenoff, J. B., Eds. Multilayer Thin Films; Wiley-VCH: Weinheim, Germany, 2003.

10.1021/la060519u CCC: $33.50 © 2006 American Chemical Society Published on Web 06/08/2006

Self-Assembled Polythiophene Multilayers

films on arbitrarily shaped or sized substrates, the only requirement being that the surfaces are charged. The very small thickness of the film is advantageous in chemical sensors because it improves the response time and sensitivity. Rubner et al. were the first to describe the formation of multilayers incorporating conjugated, electrically conducting, or luminescent polymers.8 During the self-assembly process the conjugated polymers can remain molecularly dispersed or form aggregates, either in solution or on the film surface, with profound effects on the film structure and properties. We have previously used UV-vis-near-IR spectroscopy to follow the multilayer formation with PT-based polyelectrolytes.2a,3g,9 This technique gives quantitative information about the amount of PTs adsorbed during the layer-by-layer deposition, but it lacks molecular level information. Surface-sensitive IR techniques, including polarization modulation infrared reflectionabsorption spectroscopy (PM-IRRAS), are able to probe the structure, organization, chemical composition, and interactions within these thin films based on changes in the position and intensity of specific vibrations. A number of studies which utilize IRRAS methods to characterize thin polymer films have recently appeared. The effect of solvent,10 deposition time,11 and irradiation12 on the chemical composition and orientation of poly(alkylthiophene) films has been investigated. Poly(styrenesulfonate), PSS, is one of the most common polyanions used in the polyelectrolyte multilayers, and surface-sensitive IR methods have been used to study the growth of the layers,13 the presence of the active components,14 or the effect of added electrolyte on the orientation of polyions.15 In this work, to gain insight into the local environment and average orientation of the polymers, we have chosen vibrational spectroscopy to study the LbL selfassembly of poly(alkoxythiophene)/poly(styrenesulfonate) (PT/ PSS) multilayers. Using PM-IRRAS, we can gain information about the interactions between the cationic imidazolium and anionic sulfonate groups, the order of the alkoxy side chain, and the film orientation. We have previously shown that the mechanism of the layerby-layer assembly of rigid-rod polyelectrolytes differs from that of conventional flexible polyelectrolytes.9c In aqueous solutions, ionically substituted rigid PTs tend to form aggregates, a consequence of the poor solubility of the polymer backbone, and this property can be regulated by adjusting the length of the side chain and the polymer backbone regioregularity. The control of the structure of the molecular assembly at the nanoscale is important because the macroscale properties are dependent on the supramolecular structure. However, infrared spectroscopy is a laterally averaging technique that cannot distinguish between the two distinct processes possible during the film growth: the formation and growth of aggregated and ordered domains in the medium of disordered regions, on one hand, and the gradual ordering of a laterally homogeneous nonaggregated film, on the (8) (a) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985. (b) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. (c) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115. (9) (a) Lukkari, J.; Viinikanoja, A.; Paukkunen, J.; Saloma¨ki, M.; Janhonen, M.; A ¨ a¨ritalo, T.; Kankare, J. J. Chem. Soc., Chem. Commun. 2000, 571. (b) Lukkari, J.; Viinikanoja, A.; Saloma¨ki, M.; A ¨ a¨ritalo, T.; Kankare, J. Synth. Met. 2001, 121, 1403. (c) Lukkari, J.; Saloma¨ki, M.; A ¨ a¨ritalo, T.; Loikas, K.; Laiho, T.; Kankare, J. Langmuir 2002, 18, 8496. (10) Xu, G.; Bao, Z.; Groves, J. T. Langmuir 2000, 16, 1834. (11) (a) Matsuura, T.; Sakaguchi, H.; Shimoyama, Y. J. Mater. Sci.: Mater. Electron. 2003, 14, 353. (b) Shimoyama, Y. Thin Solid Films 2004, 464-465, 403. (12) Cumpston, B. H.; Jensen, K. F. Synth. Met. 1995, 73, 195. (13) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (14) Caruso, F.; Furlong, D. N.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 4559. (15) Gregoriou, V. G.; Hapanowicz, R.; Clark, S. L.; Hammond, P. T. Appl. Spectrosc. 1997, 51, 470.

Langmuir, Vol. 22, No. 14, 2006 6079 Scheme 1. Structures of the Polyelectrolytes Used

other hand. Atomic force microscopy (AFM) has been generally used as a tool for the characterization of the morphology in thin PT16 and thiophene oligomer films.17 In this work we apply both the surface-IR and AFM techniques for studying the LbL selfassembly process of rigid rod polymers. We have varied the molecular structure of PTs at two critical positions (Scheme 1). Substitution of the thiophene ring at the 4-position by a methyl group strongly favors the head-to-tail coupling during oxidative polymerization and increases the regioregularity of the polymer.18,19 On the other hand, the length of the side chain is an important factor in the supramolecular organization of PTs.20 Using PSS with different cationic PTs, we focus on the effects of the side chain length and the polymer regioregularity on the structure and morphology of the multilayers. Experimental Section Materials. Sodium fluoride (NaF, J. T.Baker), 1-decanesulfonic acid, sodium salt (DS, Aldrich), 2-mercaptoethanesulfonic acid, sodium salt (MESA, Aldrich), and poly(sodium 4-styrenesulfonate) (PSS), Mw ) 70 kDa (Aldrich), were used as received. Monomers. 3-(3-Bromo)propoxythiophene was synthesized from 3-methoxythiophene and 3-bromo-1-propanol using conditions reported by Chayer et al.3c The product was refluxed with 1-methylimidazole in acetonitrile and evaporated to dryness, and the crude product was washed with ethyl acetate and diethyl ether.5a The white solid obtained was further dissolved in water, and bromide counteranion was exchanged to chloride anion by elution through an ion-exchange resin. Evaporation of water produced a colorless oil of 1-methyl-3-[3-[3-thienyloxy]-propyl]-1H-imidazolium chloride (3TOPIM). 1-methyl-3-[6-[3-thienyloxy]-hexyl]-1H-imidazolium iodide (3TOHIM) was also prepared with this method using 6-iodohexanol (prepared by refluxing 6-chlorohexanol with NaI) and 3-methoxythiophene as starting reagents. Using similar procedure 1-methyl-3-[2-[(4-methyl-3-thienyl)oxy]-ethyl]-1H-imidazolium bromide (4Me-3TOEIM) and 1-methyl-3-[6-[(4-methyl-3-thienyl)oxy])hexyl]-1H-imidazolium bromide (4Me-3TOHIM) were prepared starting from 3-bromo-4-methylthiophene21 that was subsequently (16) Kiriy, N.; Ja¨hne, E.; Adler, H.-J.; Schneider, M.; Kiriy, A.; Gorodyska, G.; Minko, S.; Jehnichen, D.; Simon, P.; Fokin, A. A.; Stamm, M. Nano Lett. 2003, 3, 707. (17) Lecle`re, P.; Surin, M.; Viville, P.; Lazzaroni, R.; Kilbinger, A. F. M.; Henze, O.; Feast, W. J.; Cavallini, M.; Biscarini, F.; Schenning, A. P. H. J.; Meijer, E. W. Chem. Mater. 2004, 16, 4452. (18) Le´vesque, I.; Leclerc, M. J. Chem. Soc., Chem. Commun. 1995, 2293. (19) McCullough, R. D. AdV. Mater. 1998, 10, 93. (20) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L. J. Org. Chem. 1993, 58, 904. (21) Daoust, G.; Leclerc, M. Macromolecules 1991, 24, 455.

6080 Langmuir, Vol. 22, No. 14, 2006 converted to 3-methoxy-4-methylthiophene. This product was reacted either with 2-bromo-1-ethanol or 6-bromo-1-ethanol resulting in 3-(2-bromo(ethoxy-4-methylthiophene) (1) or 3-(6-bromo(hexoxy4-methylthiophene) (2), respectively, as described by Chayer et al.3c Refluxing compounds 1 and 2 with 1-methylimidazole in acetonitrile (vide supra) gave monomers 4Me-3TOEIM and 4Me-3TOHIM, respectively, as white solids. Yields: 75% (3TOPIM), 31% (3TOHIM), 95% (4Me-3TOEIM), 45% (4Me-3TOHIM) (see Supporting Information for the 1H and 13C NMR spectra). Polymerization. The polymerizations were carried out with anhydrous FeCl3 (mole ratio ca. 4:1) in dry CHCl3.3c,5a The mixture was stirred at room temperature in the dark under Ar atmosphere for a period of 48-96 h. After evaporation of the reaction mixture, the crude product was dissolved in an excess of acetone. An equal amount of water was added into the mixture before dialysis against water for 1-2 days (using a 3500 nominal molecular weight cutoff membrane). Dialysis was continued against 0.1 M NaF and 0.1 mM NaF solutions overnight to make sure that all the anions were changed to F- and the residual oxidant was removed. After dialysis the polymer solutions were freezed and lyophilized (Christ Alpha 2-4 freezedryer). Solid polymers (P3TOPIM, P3TOHIM, P4Me-3TOEIM, and P4Me-3TOHIM; Scheme 1) were used without further purification for the solution and surface studies. Polymer Characterization. The UV-vis spectra were recorded using a Hewlett-Packard 8453 spectrophotometer. Fourier transform infrared (FTIR) spectra of the bulk PTs and PSS were measured using a dry-air-purged Nexus 870 FTIR spectrometer (Nicolet) equipped with a DTGS detector in the transmission mode. The bulk polymers were mixed with KBr, and each spectrum was taken by collecting 256 scans at a resolution of 4 cm-1 (see Supporting Information for the IR spectra). The molecular weight determination of cationic PTs is notoriously difficult and was not attempted in this work.2a,5a,b Pretreatment of Substrates. Microscope slides were first cleaned in a fresh piranha solution (concentrated H2SO4/30% H2O2 (3:1)s Warning! Piranha solution is Very corrosiVe and must be treated with extreme caution; it reacts Violently with organic material and must not be stored in tightly closed Vessels), rinsed thoroughly with water (distilled twice in quartz containers), and dried. Then they were silanized with (3-aminopropyl)triethoxysilane in dry toluene solution (1% (v/v)) for 4 min at 60 °C,22 and a thin layer of gold (∼100 nm) was thermally evaporated on top of the glass slides using an Edwards E306A coating system. Negative charge on substrates, required for the polyelectrolyte adsorption, was accomplished by immersing the gold-plated substrates in an aqueous 1 mM solution of mercaptoethanesulfonic acid (MESA). Preparation of Multilayers. The mono- and multilayer depositions were made on evaporated gold substrates primed with MESA. For this purpose 1 mM (with reference to the monomer) polymer solutions were made by diluting each PT derivative (P3TOPIM, P4Me-3TOEIM, P3TOHIM, and P4Me-3TOHIM) with quartz distilled water. Also an aqueous 10 mM solution of PSS was prepared. Ionic strength of the solutions was adjusted to 0.1 M with NaF, which did not cause any spectral changes in the polymer solutions. Alternate adsorption of the PT and PSS solutions afforded the multilayer growth on negatively charged substrates. The adsorption time was 30 min for PTs and 15 min for PSS. Between every deposition step the substrates were rinsed for 3 × 1 min with quartz distilled water and dried with N2 stream. For the AFM studies the solution used for depositions was filtered with 0.25 µm filters, and multilayers were constructed in a class 100 clean room. Characterization of Multilayers. PM-IRRAS. Reflectionabsorption spectra were collected on the spectrometer (vide supra) equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT-A) detector and a PEM-90 photoelastic modulator (HINDS Instruments) with 80° angle of incidence relative to the surface normal. The effective modulation frequency was 74 kHz. Signals from each polarization (Rp and Rs) were detected simultaneously by (22) Siqueira Petri, D. F.; Wenz, G.; Schunk, P.; Schimmel, T. Langmuir 1999, 15, 4520.

Viinikanoja et al. a lock-in amplifier and were used for the calculation of the differential reflectivity ∆R/R ) (Rp - Rs)/(Rp + Rs). For each spectrum, 2048 scans at 4 cm-1 spectral resolution were collected. To eliminate the effects of optical components and substrate, the experimental PMIRRAS spectrum of the film on the substrate was divided by the PM-IRRAS spectrum of the bare substrate, recorded under the same experimental conditions.23 Atomic Force Microscopy Measurements. Surface morphology of PT/PSS multilayers was investigated using a Nanoscope II (Digital Instruments, Inc., Santa Barbara, CA) atomic force microscope under ambient conditions in the tapping mode. The ultrasharp silicon cantilevers used were 125 µm in length and had a resonance frequency of approximately 325 kHz. The tip height was 15-20 µm and the nominal radius of curvature less than 10 nm. For the images shown in this paper, the raw AFM data obtained were processed only by flattening and plane fitting. The Scanning Probe Image Processor (SPIP, Image Metrology, Denmark) software was used for the roughness analysis of the images. Ellipsometric Film-Thickness Measurements. The thicknesses of the films were determined with ellipsometry. A Nanofilm EP3 SE spectroscopic imaging ellipsometer (Nanofilm Technologie GmbH, Go¨ttingen, Germany), with spectroscopic light source (Xenon arc lamp), incident upon the sample at 64°, was used to measure the analyzer and polarizer angles of the Au substrate in the wavelength range of 710-1000 nm (in which the polymers do not absorb). The complex refractive indexes for the substrates were calculated. After the film formation, the samples were again analyzed and the thicknesses of the films were determined from a three-phase model using a real refractive index of 1.47 for multilayers2a,24 and the previously measured complex refractive indexes for the substrate. At least three different sampling points were considered to get the averaged thickness value.

Results and Discussion Atomic Force Microscopy of the Multilayers. Each multilayer with 10 bilayers on a gold substrate was imaged using a tapping mode AFM in ambient atmosphere. The AFM images in Figure 1a-d show that the adsorption of polyelectrolyte films leads to inhomogeneous film structure on the length scale of hundreds of nanometers. A globular structure was observed for all (PT/PSS)10 films, the overall surface root-mean-square (rms) roughness (measured on 10 × 10 µm2 scanning areas) increasing in the order [P4Me-3TOEIM/PSS]10 (10.3 ( 1.1 nm) ≈ [P3TOHIM/PSS]10 (10.8 ( 0.7 nm) < [P4Me-3TOHIM]10 (12.2 ( 2.6 nm) < [P3TOPIM/PSS]10 (14.4 ( 1.5 nm). Similar surface morphology has previously been found with multilayers containing PSS or PVS (poly(vinylsulfonic acid)).9c,25 Most of the roughness in each film arises from large aggregates that have a broad size distribution in the range of 200-600 nm and the height of 70-160 nm (depending on the size). The size distribution of these large aggregates varied only slightly between the samples. In multilayers containing P3TOHIM or P4Me-3TOHIM ca. 80% of the aggregates were within 200-400 nm, whereas in P3TOPIM/PSS and P4Me-3TOEIM/PSS multilayers the average size was larger, 80% of the aggregates being in the range of 300-500 nm. Poly(styrenesulfonate) is known to form particles in the range of ca. 300-750 nm by self-aggregation.26 Therefore, we ascribe these large features in the multilayers to PSS aggregates, due to their similarity with those previously observed in PSS solutions (we have also observed that in order to remove (23) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectrosc. 1991, 45, 380. (24) (a) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552. (b) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (25) (a) Kim, D. K.; Han, S. W.; Kim, C. H.; Hong, J. D.; Kim, K. Thin Solid Films 1999, 350, 153. (b) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655. (26) (a) Tanahatoe, J. J.; Kuil, M. E. J. Phys. Chem. B 1997, 101, 5905. (b) Tanahatoe, J. J.; Kuil, M. E. J. Phys. Chem. B 1997, 101, 9233. (c) Tanahatoe, J. J.; Kuil, M. E. J. Phys. Chem. B 1997, 101, 10839.

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Figure 1. Representative AFM images of (a) (P3TOPIM/PSS)10, (b) (P4Me-3TOEIM/PSS)10, (c) (P3TOHIM/PSS)10, and (d) (P4Me3TOHIM/PSS)10 films deposited onto the evaporated gold substrate. Size distribution of (e) PT-based and (f) PSS aggregates.

all particles from PSS solutions, they have to be filtered through 0.1 µm filters; however, new aggregated particles form in approximately 30 min) and PSS/PDADMA multilayers. In addition, the density of these large aggregates on the surface is independent of the PT used in the multilayer, being always ca. 2-3 particles/µm2. In the area between the large aggregates, the surface roughness varies between 4.5 and 5.5 nm (average of several 2 × 2 µm2 squares), except in case of (P4Me-3TOHIM/PSS)10 films, where the roughness is 7-8 nm. Smaller aggregates were observed in each of the PT/PSS multilayers, and their density clearly depends on the type of the multilayer (Figure 1). Most of them have a diameter of 150-175 nm and height of ca. 20-30 nm. The relative size distribution of the smaller aggregates did not depend on the film (except for P3TOPIM), but their density varied markedly between 26 and 125 particles/µm2, being the highest in the case of P4Me-3TOHIM/PSS multilayers (see Supporting Information). These structures closely resemble PT aggregates reported in the literature, both in size and form.16,27 Recent studies on the aggregation of amphiphilic regioregular PTs suggest that

the regioregularity of the polymer is a prerequisite for the formation of crystalline domains.27c In regioirregular PTs, the nonequidistant packing of the alkyl chains prevented the ordering. The highest density and largest average size of the aggregates were found in multilayers of P4Me-3TOHIM with 4-methyl substitution (implying regioregularity) and the longest alkoxy side chain. Therefore, we attribute these smaller aggregates to PT particles or polyelectrolyte complexes between PTs and PSS, a conclusion also supported by spectroscopic investigations (vide infra). PT Aggregates in Water. To clarify the formation and origin of the smaller aggregates in the PT/PSS multilayers, we have carried out spectroscopic studies in aqueous solution. Soluble PTs typically exhibit red shift in their absorbance spectrum in (27) (a) Liu, G.; Qiao, L.; Guo, A. Macromolecules 1996, 29, 5508. (b) Yamamoto, T.; Komarudin, D.; Arai, M.; Lee, B.-L.; Suganuma, H.; Asakawa, N.; Inoue, Y.; Kubota, K.; Sasaki, S.; Fukuda, T.; Matsuda, H. J. Am. Chem. Soc. 1998, 120, 2047. (c) Reitzel, N.; Greve, D. R.; Kjaer, K.; Howes, P. B.; Jayaraman, M.; Savoy, S.; McCullough, R. D.; McDevitt, J. T.; Bjørnholm, T. J. Am. Chem. Soc. 2000, 122, 5788. (d) Ong. B. S.; Wu, Y.; Liu, P.; Gardner, S. AdV. Mater. 2005, 17, 1141.

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Figure 2. UV-vis spectra of (a) P3TOPIM, (b) P4Me-3TOEIM, (c) P3TOHIM, and (d) P4Me-3TOHIM solutions (0.1 mM with respect to monomer in 0.1 M NaF(aq)) (___), and after the addition of equimolar amounts of DS (- - -) or PSS (‚ ‚ ‚).

poor solvents, relative to good solvents. This solvatochromism has been attributed to changes in the chain conformation, which eventually leads to aggregate formation.28 We have studied the efficiency of two sulfonates, PSS and the surfactant decyl sulfonate (DS), to form complexes with PTs. Here DS is used as a sensitive tool to detect aggregate formation, whereas PSS reveals whether polyelectrolyte complexes can be expected in the multilayers studied. All PT derivatives (P3TOPIM, P4Me-3TOEIM, P3TOHIM, and P4Me-3TOHIM) exhibit a broad absorption at 400-500 nm (Figure 2), in accordance with similar poly(alkoxythiophene)s.5a,b Small shoulders on the low-energy side of the maximum in some of the spectra imply the presence of a small amount of an ordered solid-state phase, i.e., aggregate formation. The PTs behave quite differently upon addition of DS. With regioirregular P3TOPIM and P3TOHIM the absorption maxima (at 483 and 401 nm in 0.1 M NaF, respectively) did not change upon addition of an equimolar concentration (with respect to monomer units) of DS. For P4Me-3TOEIM, the absorption maximum blue-shifts from 385 to 373 nm with a concomitant decrease in absorbance between 500 and 600 nm. However, with P4Me-3TOHIM the spectrum experienced a red shift from 370 to 384 nm and three sharp peaks appeared at 500, 539, and 584 nm. The energy difference between the peaks (0.18 eV) is ca. 1450 cm-1, which coincides with the symmetric CdC vibration of the thiophene ring21 (vide infra, Supporting Information). The red shift and the appearance of the vibronic fine structure are characteristic of the formation of an ordered solid-state PT aggregate.29 The efficient π-stacking of polymer backbone in alkyl-substituted PTs is strongly dependent on the regioregularity of the backbone, both in solution and in thin films.27c The addition of CH3OH to the CHCl3 solution of head-to-tail (HT) substituted poly(hexylthiophene) leads to a similar bathochromic shift and the splitting of the π-π* absorption band as observed here in the case of P4Me3TOHIM.27b Decyl sulfonate does not form micelles at the concentrations used in our experiments (critical micelle con(28) (a) Brustolin, F.; Goldoni, F.; Meijer, E. W.; Sommerdijk, N. A. J. M. Macromolecules 2002, 35, 1054. (b) Fraleoni-Morgera, A.; Marazzita, S.; Frascaro, D.; Setti, L. Synth. Met. 2004, 147, 149. (c) Wang, Y.; Euler, W. B.; Lucht, B. L. J. Chem. Soc., Chem. Commun. 2004, 686. (29) (a) Koren, A. B.; Curtis, M. D.; Kampf, J. W. Chem. Mater. 2000, 12, 1519. (b) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. J. Mol. Struct. 2000, 521, 285.

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centration (cmc) ) 25.0-30.3 mM).30 This suggests that charge compensation by the surfactant molecules decreases the electrostatic repulsion between the polymers and leads to π-stacking of the polymer backbones. Especially, the regioregularity of the 4-methyl-substituted PTs enables high coplanarity of the backbones and effective intermolecular stacking, facilitating the formation of lamellar supramolecular assemblies or well-ordered rodlike structures. The aggregation of 1-alkyl-3-methylimidazolium cations in aqueous solutions is known to depend on the length of the alkyl chain,31 and we assume that in the case of P4Me-3TOEIM the short ethoxy side chains cannot stabilize the aggregates. When an equivalent amount (with respect to monomer units) of PSS was added to the PT solutions, the spectral behavior was qualitatively rather similar to that upon addition of DS. The spectral changes were negligible with the regioirregular P3TOPIM and P3TOHIM, whereas P4Me-3TOEIM and P4Me-3TOHIM exhibited red shifts to 407 and 479 nm, respectively. However, in PSS/PT complexes the mismatch between the charged groups leads to less efficient packing, and only a broad red-shifted absorbance band is seen without any fine structure. A similar spectral behavior has been observed during the aggregation of PTs with reduced regioregularity.27b These results show that the aggregate formation induced by PSS strongly correlates with the observed surface density of the smaller particles on the multilayers. Therefore, the spectral observations strongly support their assignment as PT-based aggregates. PM-IRRAS Spectroscopy. The PM-IRRAS method has been used successfully to elucidate the molecular orientation of oligothiophenes and PTs on metal surfaces11 and at air-solution interfaces.27c In surface IR, vibrational modes having a component of the dipole moment change normal to the surface are enhanced most and a strong correlation between the IR intensity and orientation of a corresponding functional group is observed.32 In this work, the PM-IRRAS spectra of thin films were measured after the deposition of the first and second layers and, thereafter, upon the completion of the fourth, seventh, and tenth bilayers. All multilayers share some common spectral features: (1) vibrations due to OH groups (ca. 3400-3600 cm-1) showing the presence of water; (2) C-H stretching vibrations of aromatic rings and alkyl side chains at 2800-3200 cm-1; (3) multiple in-plane stretching vibrations of the aromatic rings and the alkyl chain deformation vibrations at 1300-1600 cm-1; (4) very strong bands due to asymmetric and symmetric stretching vibrations of the sulfonic acid group at ca. 1200 and 1036 cm-1. In every case, the observed band intensity depends both on the surface concentration of the molecules and the orientation of the transition moments. Build-up of the Polyelectrolyte Layers. The integrated intensity of the in-plane C-H and CdC stretching vibrations of the imidazolium moiety (between 3134-3194 and 1549-1591 cm-1, respectively) were used to follow the sequential build-up of PT/ PSS multilayers on gold. Both spectral regions reveal a basically similar behavior (Figure 3). After the adsorption of the first PSS layer the intensity of these vibrations clearly decreases in all the films, but after four bilayers the linear increase suggests a regular growth of the polyelectrolyte film. The films composed of PTs with short alkoxy chain (P3TOPIM and P4Me-3TOEIM) exhibit consistently higher intensity of the in-plane vibrations than those containing the hexoxy chain. The smallest intensity and slope in the case of P3TOHIM implies a smaller amount of adsorbed (30) Nakamura, H.; Sano, A.; Matsuura, K. Anal. Sci. 1998, 14, 379. (31) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C.; Heenan, R. K. Langmuir 2004, 20, 2191. (32) Greenler, R. G. J. Chem. Phys. 1966, 44, 310.

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Figure 4. Dependence of integrated intensity of the O-H stretching vibrations of H2O (3200-3690 cm-1) on the number of (0) P3TOPIM/PSS, (O) P4Me-3TOEIM/PSS, (4) P3TOHIM/PSS, and (3) P4Me-3TOHIM/PSS bilayers. Inset: the ratio of the intensities of ν(OH) and νas(SO3) vibrations.

Figure 3. Dependence of the integrated intensity of (a) the C-H and (b) CdC in-plane stretching vibrations of the imidazolium ring (3134-3194 and 1549-1591 cm-1, respectively) on the number of bilayers: (0) P3TOPIM/PSS, (O) P4Me-3TOEIM/PSS, (4) P3TOHIM/PSS, and (3) P4Me-3TOHIM/PSS. Filled symbols and the dashed line in a refer to ellipsometric thickness.

polymer or a more planar ring plane orientation. The P4Me3TOHIM/PSS multilayers exhibit a more complex behavior. Up to the fourth bilayer the film build-up resembles that of multilayers containing P3TOHIM but after that the slope increases and the curve becomes parallel with those of P3TOPIM/PSS and P4Me3TOEIM/PSS multilayers (a smaller change in slope can be seen with P3TOHIM, too). The thickness of the films composed of PSS and different polypeptides, including poly-L-histidine, has been known to vary with the peptides, and the differences observed were attributed to the influence of the hydrophobic and van der Waals interactions in the LbL assembly process.33,34 Similarly, with P3TOHIM and P4Me-3TOHIM, the space-demanding long hydrophobic side chains can result in the adsorption of less material. In films of regioregular P4Me-3TOHIM the AFM and spectral measurements imply that the aggregate formation is favored, which can lead to a change in the slope of the intensity curve, either due to different orientation or more dense packing of the chains in the aggregates. Although changes in the orientation of the groups can affect the evolution of the integrated surfaceIR intensities, the ellipsometric thickness of the multilayers closely (33) Barreira, S. V. P.; Garcı´a-Morales, V.; Pereira, C. M.; Manzanares, J. A.; Silva, F. J. Phys. Chem. B 2004, 108, 17973. (34) Kotov, N. A. Nanostruct. Mater. 1999, 12, 789.

follows the behavior of the IR bands, indicating that the amount of adsorbed material is the major factor contributing to the observed behavior in Figure 3, except in very thin films. The measured thicknesses are clearly dependent on the PT used, increasing in the order (for 10 bilayers) P3TOHIM (15.2 ( 2.1 nm) < P4Me-3TOHIM (18.7 ( 3.2 nm) < P4Me-3TOEIM (25.5 ( 0.8 nm) < P3TOPIM (26.3 ( 0.2 nm), the order coinciding with the intensity of the IR stretching bands in the same figure. Especially, the ellipsometric thickness increases linearly with the number of bilayers for the film prepared from P3TOHIM and PSS (ca. 1.6 nm per bilayer), which shows that the PT/PSS multilayers exhibit a regular growth. Water Content. Water is present in all PT/PSS multilayers measured in air at a constant relative humidity of 7.1%. In the first bilayer the weak signal from water originates mainly from free OH groups (3620 cm-1) belonging to substrate-bound water, whereas in thicker multilayers the main signal comes from water molecules forming hydrogen bridges (3450 cm-1) within the films.35 Generally, the nature of the water environment within the film depends on whether the outer layer is polycation or polyanion, and PSS-capped films are known to associate strongly with water.36 The amount of water in the PT/PSS multilayers was dependent on the type of PT. However, in all the films the water content constantly increased with the number of layers without leveling (Figure 4), unlike in PSS/PDADMA films, where it stabilized after four bilayers.37 In the (P3TOPIM/PSS)n and (P4Me-3TOEIM/PSS)n multilayers the water content was substantially higher than in multilayers containing P3TOHIM or P4Me-3TOHIM. In PSS/PDADMA and PSS/PAH films exposed to ambient atmosphere, six to eight water molecules per ion pair are found, depending on the hydrophobicity of the film constituents.36b,37 In Figure 4 we have also plotted the ratio of water content and the integrated intensity of the asymmetric sulfonate stretching vibration of PSS (1150-1270 cm-1), which reflects the number of ion pairs in the multilayer. This ratio is similar and remains practically independent of film thickness in all multilayers except the P4Me-3TOHIM/PSS films, in which the relative water content is clearly lower, although constant. (35) Zundel, G. Hydration and Intermolecular Interaction; Academic Press: New York, 1969. (36) (a) Schwarz, B.; Scho¨nhoff, M. Langmuir 2002, 18, 2964. (b) McCormick, M.; Smith, R. N.; Graf, R.; Barrett, C. J.; Reven, L.; Spiess, H. W. Macromolecules 2003, 36, 3616. (c) Smith, R. N.; McCormick, M.; Barrett, C. J.; Reven, L.; Spiess, H. W. Macromolecules 2004, 37, 4830. (37) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621.

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Figure 5. Evolution of the C-H stretching peaks of the (a) P3TOPIM/PSS, (b) P4Me-3TOEIM/PSS, (c) P3TOHIM/PSS, and (d) P4Me-3TOHIM/PSS multilayers (on MESA-primed gold) as a function of the number of bilayers. The spectra are normalized with respect to the νa(CH2) peak at ca. 2925 cm-1. The arrows indicate specified vibrations given in a. Dotted lines show the νa(CH2) band position in the PT monolayers.

Therefore, the water content in the multilayers does not reflect the hydrophobicity of the side chain but can be related to the supramolecular packing of the polymer chains in the films. The P4Me-3TOHIM/PSS multilayers exhibit clearly the highest number of aggregates which possess hydrophobic, relatively water-free interiors. Side Chain Order. The positions of the asymmetric and symmetric methylene vibrations, νa(CH2) and νs(CH2), are known to be sensitive to changes in the number of methylene units, conformational order, and packing of the alkyl chains.38 In crystalline all-trans polyalkanes these modes are found at 2915 and 2846 cm-1 with half-widths of 10-13 and 7-10 cm-1 for the νa(CH2) and νs(CH2) modes, respectively. The all-trans character is, however, critically dependent on the chain length, and for chain lengths less than 12 the chains possess an increasing population of disordered methylenes.39 In more disordered “liquidlike” layers the asymmetric vibrations shift up to 2924 cm-1 and the peaks broaden.40 The PM-IRRAS results of the MESA-primed gold substrate show that the asymmetric and symmetric methylene stretching vibrations appear at 2932 and 2859 cm-1, respectively (Figure 5a). With P3TOPIM, after the adsorption of the first PT layer the bands shift to 2935 and 2856 cm-1. Upon completion of the first bilayer with PSS the relative intensity and width increase and the bands shift to 2928 and 2858 cm-1. Peak deconvolution (Supporting Information) reveals that both methylene stretches are composed of two closely spaced bands; one pair for P3TOPIM and another for PSS. In addition, the appearance of a new asymmetric vibration as a shoulder around 2915 cm-1 suggests the formation of ordered “all-trans” structures, which could be tentatively ascribed to ordered structure within aggregates. Except for the small blue shift in the asymmetric methylene stretching mode with longer alkoxy chains, the behavior of the asymmetric and symmetric methylene stretches is similar in all the PT/PSS films (Figure 5b-d). The νa(CH2) mode peak position reaches minimum after the deposition of four bilayers and the positions (38) (a) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (b) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (39) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (40) Byrd, H.; Pike, J. K.; Talham, D. R. Chem. Mater. 1993, 5, 709.

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Figure 6. Relative intensity of νs(CH) of the methyl imidazolium group (compared to total intensity of the spectrum in the C-H stretching region (2800-3200 cm-1)), ICH,Im/ICH,tot., in (0) P3TOPIM/ PSS, (O) P4Me-3TOEIM/PSS, (4) P3TOHIM/PSS, and (3) P4Me3TOHIM/PSS multilayers versus the number of bilayers. The relative intensities are normalized with respect to the ratio in bulk PT shown by horizontal line. The insets show the relative intensities of inplane C-C stretching vibrations of methyl imidazolium and thiophene (TP) rings with respect to the in-plane C-C stretching vibration of benzene (Bz) ring in PSS, ICC,Im/ICC,Bz and ICC,TP/ICC,Bz, respectively.

of symmetric methylene modes remain constant. Therefore, although PSS appears to induce a very small ordering effect, the alkyl chain conformation remains practically disordered in all the multilayers studied. This is not unexpected, given the short length of the alkoxy side chain and the large size of the imidazolium group, which prevents dense packing of the alkyl chains. Similar disordered structures have been observed in spincast films of regioregular poly(3-hexylthiophene).10 C-H Stretching Vibrations in the Imidazolium and Thiophene Rings: Orientational Effects. The in-plane C-H stretching vibrations of the imidazolium and thiophene rings can be used to examine the relative orientation of the PT chains in the films. Imidazolium compounds are known to have several C-H vibrations in the range of 3000-3200 cm-1, which overlap with the thiophene C-H stretching modes (present only in P3TOPIM and P3TOHIM). The two bands in Figure 5 at 3130-3180 cm-1 were assigned to imidazolium C-4 and C-5 hydrogen stretching, and the band at 3109 cm-1 (resolved after peak fitting) was assigned to the C-2 hydrogen stretching. The aforementioned bands will split and shift to lower energy by ∼100 cm-1 in the case of strong hydrogen bonding with anions.41 In addition, the substituent methyl groups have modes that can help to verify whether the ring plane adopts a standing-up or flat-lying configuration with respect to the surface. The transition dipole moment of the methyl symmetric mode, νs(CH3), (≈2876 cm-1) lies in the plane of the ring and one asymmetric stretch, νa(CH3), (ca. 2965 cm-1) is polarized perpendicular to the symmetric mode. These vibration modes have been used to follow the orientational changes of alkyl side chains in poly(3-dodecylthiophene) monolayers.11,42 In our case, the situation is slightly more complicated, because P4Me-3TOEIM and P4Me-3TOHIM contain methyl groups in both the imidazolium and thiophene rings, with overlapping vibrations. In Figure 6 we have plotted the ratio of the integrated intensity of the in-plane C-H vibration modes of the methyl imidazolium ring and the total integrated intensity in the C-H stretching (41) (a) Tait, S.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 4352. (b) Dieter, K. M.; Dymek, C. J, Jr.; Heimer, N. E.; Rovang, J. W.; Wilkes, J. S. J. Am. Chem. Soc. 1988, 110, 2722. (c) Elaiwi, A.; Hitchcock, P. B.; Seddon, K. R.; Srinivasan, N.; Tan, Y.-M.; Welton, T.; Zora, J. A. J. Chem. Soc., Dalton Trans. 1995, 3467. (d) Ko¨lle, P.; Dronskowski, R. Inorg. Chem. 2004, 43, 2803. (42) Gao, Z.; Siow, K. S.; Chan, H. S. O. Synth. Met. 1995, 75, 5.

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Figure 7. Evolution of the low-frequency peaks of (a) P3TOPIM/PSS, (b) P4Me-3TOEIM/PSS, (c) P3TOHIM/PSS, and (d) P4Me3TOHIM/PSS multilayers (on MESA-primed gold, number of bilayers indicated in figures). The spectra are normalized with respect to the most intense peak. The arrows indicate specified vibrations given in a. Benzene ring skeleton stretching vibrations are marked with an asterisk. The close-up shows the evolution of the νs(SO3) intensity and position during the film growth. Lower: MESA (only in a), PT and PT/PSS; upper: 4, 7, and 10 PT/PSS bilayers (multiplied by 2 in c). The spectra of PT monolayers and films with four bilayers are in red for clarity.

region (2800-3200 cm-1), ICH,Im/ICH,tot.. The denominator contains contributions also from PSS, alkyl side chains and the thiophene 4-position, including their orientation. The alkyl side chains remain practically randomly oriented (as also in bulk) and, therefore, do not affect changes in the intensity ratio. On the other hand, the small contribution from substituent at the thiophene 4-position can be ignored in the qualitative inspection of the results. In the PT monolayers the imidazolium in-plane C-H vibration (3150 cm-1) intensity is lower than in the bulk (except with P3TOHIM), implying that the rings in these monolayers tend to favor orientations parallel to the surface. Upon adsorption of the first PSS layer, the relative intensity of the above-mentioned stretching modes decreases, and additional bands due to the benzene ring in-plane C-H stretching vibrations of PSS appear at ca. 3030 cm-1 (see Figure 5.). As the denominator now contains a contribution from PSS (absent in bulk PT), an exact conclusion cannot be drawn. However, the in-plane CdC and CdN stretching vibrations of the imidazolium ring (1573 and 1564 cm-1, respectively) and the four in-plane vibrations of the thiophene ring (in the region of 1400-1550 cm-1) in Figure 7 also decrease upon adsorption of the first PSS layer. On the other hand, ellipsometry shows (Figure 4a) that the film thickness grows linearly implying that desorption of material is not the main factor behind the observed spectral changes. Instead, they suggest a substantial reorientation upon which the methyl imidazolium units become more parallel to the surface. However, in thicker multilayers the imidazolium orientation randomizes again, and in the multilayers containing P3TOPIM

or P4Me-3TOEIM the relative intensity stabilizes at the value close to that of bulk PT, growing further in two other films. Assuming a 1:1 ratio between the polycation (PT) and polyanion (PSS), supported by the regular growth of the film, the amount of PSS does not affect the changes in the intensity ratio after the first bilayer. Therefore, with P3TOHIM and P4Me-3TOHIM the imidazolium ring plane tends to become more perpendicular to the surface while their average orientation is close to random (or probably somewhat more vertical) in the two other multilayers. Qualitatively similar relative behavior can be seen (insets of Figure 6) in the intensity ratio of the imidazolium or thiophene ring skeleton stretching vibrations (ca. 1573 or 1453 cm-1, respectively) and the corresponding modes of the PSS benzene ring (1601, 1495, and 1413 cm-1, marked with asterisks (/) in Figure 7.). As a function of multilayers thickness both ratios follow the same general trend as observed in the imidazolium C-H modes, and increase in the order P4Me-3TOEIM (almost constant) < P3TOPIM e P3TOHIM < P4Me-3TOHIM. Therefore, the average orientations of the imidazolium and thiophene rings generally become more perpendicular to the surface, and the observed differences in the orientation of different PTs can be partly attributed to the amount of ordered aggregate formation in the multilayers. Sulfonate Groups in PSS and MESA. For the MESA layer, only the 1228 and 1052 cm-1 bands of νa(SO3) and νs(SO3) vibrations, respectively, were distinguishable from noise (Figure 7a.) and the comparison with the bulk spectrum indicates a decrease of the relative intensity of the asymmetric mode, in

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accordance with the tilted orientation of the MESA layer.43 Upon adsorption of the first PT layer, the symmetric SO3 vibration of MESA downshifts from 1052 to 1040-1042 cm-1. Similar shifts in the symmetric sulfonate band after the adsorption of a cationic poly(allylamine) layer have been observed previously and attributed to the replacement of the small countercations by the cationic polymer.43a Upon adsorption of the first PSS layer the νs(SO3) band in P3TOPIM/PSS and P4Me-3TOEIM/PSS bilayers shifts to higher frequency and splits to two components at 1055 and 1048 cm-1, whereas in P3TOHIM/PSS and P4Me-3TOHIM/ PSS bilayers the peaks only broaden and slightly shift to 1041 (P3TOHIM) or 1039 cm-1 (P4Me-3TOHIM). Similar splitting of the νs(SO3) mode has been observed upon adsorption of PSS or PVS on PAH.43a In thicker PT/PSS multilayers the symmetric stretching mode of the sulfonate group remains split to at least two components and the relative intensity of the components depends on the number of layers and the structure of PT (see Figure 7). The band at lower frequency dominates in the (P3TOPIM/PSS)4 and (P4Me-3TOEIM/PSS)4 films, whereas in the (P3TOHIM/PSS)4 and (P4Me-3TOHIM/PSS)4 multilayers it is present as a shoulder. Changes in the intensities of the components are quite similar in all multilayers as the film thickness is further increased (to 7-10 bilayers). The relative intensity of the low-frequency band at 1036 cm-1 grows continuously and is the main component of the sulfonate symmetric stretching after 10 bilayers in each case. Consequently, the high-frequency component (at ca. 1047-1057 cm-1) weakens in multilayers containing P3TOPIM, P4Me-3TOEIM, or P3TOHIM and almost disappears in the (P4Me-3TOHIM/PSS) films. The weakening of the anion-cation interaction, e.g. due to ion hydration by water molecules, is known to shift the sulfonate symmetric stretching vibration toward lower frequency.35 Lowry and Mauritz have noted similar behavior for Nafion sulfonate membranes and interpreted it as a shift from an ion pair within a solvent cage toward cations and anions separated by individual hydration shells.44 Additionally, the νs(SO3) band is known to split into two components depending whether they interact directly with cations or not, the latter resulting in a band at lower frequency.45 Therefore, the behavior of the νs(SO3) band in the PT/PSS multilayers implies gradual weakening of the polyion pairs with increasing number of bilayers. The water content in all multilayers increased almost linearly with the film thickness but the number of water molecules per ion pair remained practically constant. We suggest that the changes in the shape and intensity of the sulfonate vibrations reflect the development of the different zones in polyelectrolyte multilayers.46 The substrate/film and film/ambient interfaces constitute zones I and III, respectively, while zone II refers to the bulk of the film. During the multilayer build-up the combined zones I and III form first and gradually become separated by the bulk phase as the film thickness increases (above approximately 10 bilayers). The IR spectra suggest that in the first bilayer part of the sulfonate groups form ion pairs with the polycation and part are hydrated, resulting in two sulfonate vibrations of approximately equal (43) (a) Bonazzola, C.; Calvo, E. J.; Nart, F. C. Langmuir 2003, 19, 5279. (b) Zotti, G.; Zecchin, S.; Schiavon, G.; Vercelli, B.; Berlin, A.; Porzio, W. Chem. Mater. 2004, 16, 2091. (44) Lowry, S. R.; Mauritz, K. A. J. Am. Chem. Soc. 1980, 102, 4665. (45) Kudelski, A. Langmuir 2002, 18, 4741. (46) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249.

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intensity. As the number of bilayers increases the relative intensity of the lower frequency component, attributed to sulfonate groups not involved in ion pair formation, also increases. However, if the number of water molecules encaging a large ion pair or hydrating individual cations and anions is approximately similar, the relative water content in the film remains practically constant. In the thin multilayers studied in this work, zone II is not properly formed and the hydrated outer zone III dominates the film properties.9c In this outer zone the intrinsic charge compensation is not complete, and as the thickness of the zone increases the amount of free hydrated sulfonate groups also increases. These measurements were carried out with dried films at low relative humidity, but this behavior should be even more pronounced in solution.

Conclusions We have studied the effect of the structure of ionically substituted PTs on the polyelectrolyte multilayers, self-assembled using the layer-by-layer technique with poly(styrenesulfonate), by varying thiophene substitution at the 4-position (H or Me) and the length of the methylimidazolium-terminated side chain at the 3-position. The multilayers are inhomogeneous in the submicrometer scale and contain large PSS (diameter 200-800 nm) and smaller (100-200 nm) PT-based aggregates. The number density of the latter is critically dependent on the PT structure, being favored by the regioregularity of the backbone and the length of the side chain. The growth of the PT-based multilayers is, however, regular as revealed by ellipsometry and IR spectroscopy. The thickness of the multilayers is higher for PTs with short imidazoliumterminated side chains. The structural order of the PT chains varies with the polymer structure. The side chains remain disordered in all the multilayers, but, on the other hand, in multilayers with PTs having longer side chains (C6) both the imidazolium and thiophene rings tend to orient themselves more perpendicular to the surface than in films containing thiophenes with shorter chains (C2 or C3). The IR spectra indicated that the methylimidazoliumssulfonate ion pairs gradually weaken as the hydrated part of the multilayer, the so-called zone III, grows with increasing film thickness. This is in accordance with the partly extrinsic charge compensation in this outer layer. In all multilayers the absolute water content is proportional to the film thickness but is lowest with P4Me-3TOHIM, which shows the highest number of ordered aggregates. Multilayers containing ordered semiconducting nanoparticles can be important for applications in sensors and solar energy conversion, and we are currently investigating these possibilities. Acknowledgment. The financial aid from the Academy of Finland (Grants No. 102279 and 106215), the Emil Aaltonen Foundation, the Turku University Foundation, and the Graduate School of Materials Research (GSMR, Turku) is gratefully acknowledged. Supporting Information Available: IR spectra of P3TOPIM, P3TOHIM, P4Me-P3TOEIM, and P4Me-P3TOHIM in KBr pellet, 400 MHz 1H NMR and 13C NMR spectra of thiophene monomers, and surface density of PT-based aggregates in multilayers. This material is available free of charge via the Internet at http://pubs.acs.org. LA060519U