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Langmuir 2006, 22, 5366-5373
Synthesis and Characterization of Monolayers and Langmuir-Blodgett Films of an Amphiphilic Oligo(ethylene glycol)-C60-hexadecaaniline Conjugate Zhexiong Tang,† Prashant A. Padmawar,§ Taizoon Canteenwala,§ Yuan Gao,† Erik Watkins,‡ Jaroslaw Majewski,‡ Long Y. Chiang,*,§ and Hsing-Lin Wang*,† MSJ586, C-PCS, Chemistry DiVision and Los Alamos Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Department of Chemistry, UniVersity of Massachusettss Lowell, Lowell, Massachusetts 01854 ReceiVed January 9, 2006. In Final Form: March 22, 2006 A novel amphiphilic oligo(ethylene glycol)-C60-hexadecaaniline (A16) tricomponent conjugate, C60>(A16-EG43), possessing a well-defined number of repeating aniline donor units and a hydrophilic ethylene glycol oligomer chain was synthesized. The compound is composed of a covalently bound donor-acceptor chromophore structure. Molecular self-assembly of C60>(A16-EG43) at the air-water interface formed a densely packed Langmuir monolayer with all highly hydrophobic fullerene cages located above the liquid interface. The monolayer can then be transferred onto a glass substrate via Langmuir-Blodgett (LB) deposition. LB multilayered thin films formed by multiple deposition of the monolayer yielded broadened optical absorption peaks extending beyond 600 nm into the 950 nm region, suggesting strong intermolecular interactions among the C60 cages and the A16 moieties. An X-ray reflectometry study clearly reveals that the Langmuir film at the air-water interface consists of a C60 top layer and a bottom layer containing hexcadecaaniline and oligo(ethylene glycol) with gradually decreasing electron density over a distance of approximately 130 Å above bulk water. The pressure isotherm shows that the packing density of the C60>(A16-EG43) monolayer, corresponding to a molecular area of ∼95 Å2/molecule, is similar to that of the surface area of the C60 monolayer. This result suggests that C60 packing plays a dominant role in guiding the formation of the monolayer structure. Further photoexcitation of hexadecaaniline moieties of aligned (C60>)-A16 layers by a flash light source induces cross linking between adjacent A16 segments forming an interlinked A16 array. Our results have demonstrated a unique fabrication method for preparing the aligned donor-acceptor array using strong intermolecular interactions between fullerenes as the molecular orientation guiding force in the Langmuir-Blodgett technique.
Introduction Since the discovery of polyacetylene and its subsequent doping to achieve metallic conductivity nearly three decades ago, conducting polymers have shown tremendous potential for commercial applications of optical and electronic devices such as PLED1 and photovoltaics.2,3 Recent advances in the synthesis and processing of functionalized conjugated polymers have improved their potential for uses in the fields of chemical sensors4,5 and biosensors.6 Among all conducting polymers, polyanilines (PANI) are promising materials because of their low cost, facile synthesis, and environmental stability. Characteristic electrochromic,7 anticorrosion,8,9 and antistatic10 properties make PANI attractive for commercial development. Significantly improved * Corresponding author. E-mail:
[email protected]. † Chemistry Division, Los Alamos National Laboratory. ‡ Los Alamos Neutron Scattering Center, Los Alamos National Laboratory. § University of MassachusettssLowell. (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539-541. (2) Pei, Q. B.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086-1088. (3) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789-1791. (4) Mcquade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537-2574. (5) Wosnick, J. H.; Swager, T. M. Curr. Opin. Chem. Biol. 2000, 4, 715-720. (6) Chen, L. H.; Mcbranch, D. W.; Wang, H. L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287-12292. (7) Gao, J. B.; Liu, D. G.; Sansinena, J. M.; Wang, H. L. AdV. Funct. Mater. 2004, 14, 537. (8) Camalet, J. L.; Lacroix, J. C.; Aeiyach, S.; Lacaze, P. C. J. Electroanal. Chem. 1998, 445, 117-124. (9) Li, P.; Tan, T. C.; Lee, J. Y. Synth. Met. 1997, 88, 237-42. (10) Yam, P. Sci. Am. 1995, 273, 82-87.
sensitivity toward the detection of certain gas molecules was revealed in a recent investigation of ultrathin PANI films.11 However, PANI suffers from intractability that is mainly due to the existence of intrachain and interchain H-bonding interactions arising from the N-quinonoid and anilinoid moieties. To mitigate this problem, dopant-induced processibility has shown success in dissolving PANI in organic solvents with limited solubility.12 In our previous studies, primary, secondary, and tertiary amines as gel inhibitors in stoichiometric amounts with respect to N-quinonoid moieties were used to enable the formation of highly concentrated PANI solutions (>20 wt %) that remain stable for more than a few hours.13 Another way to avoid this problem is to develop a synthesis for aniline oligomers. Unlike PANI, oligomers can be prepared by controllable syntheses to provide the advantage of well-defined molecular structures, end groups, chain length, and intrinsic redox states.7,14 As the size of the oligomer reaches more than eight aniline repeat units, aniline oligomers may exhibit electronic and optical properties that closely resemble those of high-molecular-weight PANI.15-18 Accordingly, aniline oligomers were also used in the fabrication (11) Xie, D.; Jiang, Y. D.; Pan, W.; Li, D.; Wu, Z. M.; Li, Y. R. Sens. Actuators, B 2001, 81, 158. (12) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1993, 57, 3514. (13) Mattes, B. R.; Wang, H. L.; Yang, D.; Zhu, Y. T.; Blumenthal, W. R.; Hundley, M. F. Synth. Met. 1997, 84, 45. (14) Mccoy, C. H.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6934. (15) Singer, R. A.; Sadighi, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1998, 120, 213. (16) Rebourt, E.; Joule, J. A.; Monkman, A. P. Synth. Met. 1997, 84, 65. (17) Kwon, O.; Mckee, M. L. J. Phys. Chem. B 2000, 104, 1686. (18) Hartmann, V.; Losche, M.; Mello, S. V.; Oliveira, O. N., Jr. Mater. Sci. Eng., C 1999, C8/C9, 425.
10.1021/la060083i CCC: $33.50 © 2006 American Chemical Society Published on Web 04/26/2006
Oligo(ethylene glycol)-C60-hexadecaaniline Conjugate
of chemical sensors that exhibit high sensitivity toward specific gases.19,20 Oligomers possessing a lower oxidation potential than that of aniline can be applied to initiate and catalyze the polymerization reaction of aniline. Our recent study has demonstrated the use of aniline oligomers to catalyze the growth of chiral PANI nanofibers with very high optical activity.21 In view of their versatile chemical reactivity and redox properties, aniline oligomers may be considered to be a novel class of functional materials with the potential of fabricating robust thin film devices. In these practices, the Langmuir-Blodgett (LB) method was used to prepare PANI nanostructures by organizing aniline amphiphiles at the air-water interface, followed by consequent oxidative polymerization at a pH value lower than 3.22,23 An alternative method of generating PANI-based monolayer thin films involved first spreading PANI derivatives on the water surface, compressing the resulting self-oriented polymer nanostructures at the air-water interface, and then transferring the monolayer onto a substrate to create Langmuir-Schaefer (LS) films.24 These LS thin films of PANI were used as prototype gas sensors. Because our aim was to create derivatized PANI nanoassemblies with improved functionality and stability, we have designed a novel, amphiphilic oligo(ethylene glycol)-C60hexadecaaniline conjugate, C60>(A16-EG43), that will allow us to construct a hexadecaanilinated C60 monolayer structure at the air-water interface using the LB technique. In the design of this molecular system, strong hydrophobic π-conjugated aromatic intermolecular interactions among fullerene moieties lead to the preferred location of all C60 cages above the air-water interface that, in turn, bring covalently linked hexadecaaniline (A16) moieties into close contact with each other. This intimate contact among oligomeric segments of hexadecaaniline facilitates photoinduced cross linking between oligoaniline segments, forming robust monolayer structures, where interaction among the C60 cages guides the formation of Langmuir-Blodgett thin films. Here, we report the fabrication and characterization of these Langmuir-Blodgett films in terms of optical and electronic properties. Despite a number of physical measurements showing an applicable conductivity of 10-4 S/cm on C60-doped PANI25-27 due to effective charge transfer from PANI to C60, a wide range of lower conductivities were also reported to be on the order of 10-7-10-8 S/cm.28,29 The latter is consistent with our measurements based on compressed amorphous solids in a disordered form revealing only signs of weak charge transfer and inefficient charge transport between conjugated C60 and hexadecaaniline (A16) linked by covalent bonding. As C60-A16 conjugate molecules are assembled into an ordered array in the form of an LB film, a progressive increase in electronic coupling between (C60>)A16 molecules was observed as compared with that in the solution state. This enhanced coupling in LB films may lead to an (19) Feng, J.; Macdiarmid, A. G. Synth. Met. 1999, 102, 1304. (20) Yuan, J.; El-Sherif, M. A.; Macdiarmid, A. G.; Jones, W. E. Proc. SPIE Int. Soc. Opt. Eng. 2001, 4205, 170. (21) Li, W. G.; Wang, H. L. J. Am. Chem. Soc. 2004, 126, 2278-2279. (22) Sagisaka, S.; Yoshida, S.; Ando, M.; Iyoda, T.; Shimidzu, T. Thin Solid Films 1995, 271, 138. (23) Kloeppner, L. J.; Batten, J. H.; Duran, R. S. Macromolecules 2000, 33, 8006. (24) Ram, M. K.; Adami, M.; Sartore, M.; Salerno, M.; Paddeu, S.; Nicolini, C. Synth. Met. 1999, 100, 249. (25) Wei, Y.; Tian, J.; Macdiarmid, A. G.; Masters, J. G.; Smith, A. L.; Li, D. Y. J. Chem. Soc., Chem. Commun. 1993, 603. (26) Lim, H. Y.; Jeong, S. K.; Suh, J. S.; Oh, E. J.; Park, Y. W.; Ryu, K. S.; Yo, C. H. Synth. Met. 1995, 70, 1463. (27) Sapurina, I.; Mokeev, M.; Lavrentev, V.; Zgonnik, V.; Trchova, M.; Hlavata, D.; Stejskal, J. Eur. Polym. J. 2000, 36, 2321. (28) Tanaka, K.; Matsuura, Y.; Oshima, Y.; Yamabe, T. Synth. Met. 1994, 66, 193. (29) Matsuura, Y.; Oshima, Y.; Tanaka, K.; Yamabe, T. Synth. Met. 1996, 79, 7.
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improvement in the degree of electron transfer and charge transport between headecaaniline and C60 moieties. With A16 oligomer moieties being assembled in close proximity to each other and forming a well-aligned thin film, it becomes plausible to carry out a subsequent cross-linking reaction among the oligomers upon photoexcitation. In this article, we demonstrate an approach to ultrathin PANI film fabrication with largely enhanced morphology stability and electronic properties via the rational design of molecular building blocks and self-assembly assisted by intermolecular interaction. Experimental Section General. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) was purchased from Aldrich Chemicals. Oligo(ethylene glycol) was purchased from Acros Ltd. A C60 sample with a purity of 99.5% was purchased from the NeoTech Product Company and used as received. Further purification of C60 was made by thin-layer chromatography (TLC, SiO2, toluene). Toluene and benzene were dried and distilled over sodium. Hexadecaaniline was prepared by the oxidation of tetraaniline.30-32 1H and 13C NMR spectra were recorded on either a Bruker Spectrospin-400 or a Bruker AC-300 spectrometer. Infrared spectra were recorded for KBr pellets on a Nicolet 750 series FTIR spectrometer. UV-vis spectra were recorded on a Hitachi U-3410 UV spectrometer. X-ray Reflectometry at the Air-Water Interface. Synchrotron X-ray reflectometry (XR) experiments on oligo-C60-hexadecaaniline monolayers at the air-water interface were carried out at the BW1 (undulator) beam line at the HASYLAB synchrotron source33 (Hamburg, Germany) using a dedicated liquid surface diffractometer with an incident X-ray wavelength of λ ≈ 1.3 Å. A thermostated trough equipped with a Wilhelmy balance for measuring the surface pressure and a barrier for manipulating the surface area was mounted on the diffractometer. XR was carried out to obtain an in-plane averaged electron density distribution perpendicular to the air-water interface. The absolute reflectivity data was derived by subtracting the measured background and normalizing to the incident flux. As a precaution against beam damage, the sample was completely renewed by repeated translation by the full 2 mm width of the beam. Remeasuring part of the reflectivity curve before and after translation afforded a check of the reproducibility. The intensity of the specular reflectivity and the real-space electron density profile are related by an inverse transformation. Because phase information is lost when measuring specular reflectivity and because of the nonlinear nature of the inverse transformation itself, a unique solution to the problem cannot be obtained analytically. To obtain an electron density profile, the reflectivity data was first analyzed by the model-independent B-spline method.34 Starting with a random B-spline curve representing the electron density profile, a fitting procedure is applied to obtain profiles that reproduce the measured reflectivity data. The fitting routine requires input parameters corresponding to the electron density of the subphase, the number of B splines, a dampening factor, and an approximate monolayer thickness.34,35 Curves with physical relevance to the system are chosen and refined by varying these parameters to minimize χ2. Using B-spline analysis as a guideline, box model analysis was performed to provide physically significant parameters to describe the electron density profile. Our philosophy was to use the simplest possible model with physical significance. The electron density was described by a stack of layers (boxes) of different electron densities, (30) Wang, L. Y.; Anantharaj, V.; Ashok, K.; Chiang, L. Y. Synth. Met. 1999, 103, 2350-2353. (31) Zhang, W. J.; Feng, J.; Macdiarmid, A. G.; Epstein, A. J. Synth. Met.′ 1997, 84, 119-120. (32) Anantharaj, V.; Wang, L. Y.; Canteenwala, T.; Chiang, L. Y. Jo. Chem. Soc., Perkin Trans. 1 1999, 3357-3366. (33) Majewski, J.; Popovitzbiro, R.; Bouwman, W. G.; Kjaer, K.; Alsnielsen, J.; Lahav, M.; Leiserowitz, L. Chem.sEur. J. 1995, 1, 304-311. (34) Pedersen, J. S.; Hamley, I. W. J. Appl. Crystallogr. 1994, 27, 36-49. (35) Pedersen, J. S.; Hamley, I. W. Physica B 1994, 198, 16-23.
5368 Langmuir, Vol. 22, No. 12, 2006 and the interfacial roughness between adjacent layers was described using an error function centered at the interface. The thickness, average electron density, and FWHM (full width half maximum) of the error function were used as the model parameters. Reflectivities were calculated from the model using the Parratt formalism, and the parameters were refined to minimize χ2.36 Measured data and results of our calculations are presented in the form divided by the Fresnel function (reflectivity from the infinitely sharp, steplike interface). This acounts for sharply decreasing reflectivities and enhances the visibility of the interference fringes. Synthesis of Oligo(ethylene glycol) Ethylmalonate (1). A solution of oligo(ethylene glycol) (PEG, Mn ) 2000, 10.6 g, 5.3 mmol) and pyridine (0.54 mL, 6.6 mmol) in dry methylene dichloride (500 mL) was cooled in an ice bath. To this, ethylmalonyl chloride (0.85 mL, 6.0 mmol) was added dropwise over a period of 30 min and was stirred for 2.0 h. The ice bath was removed, and the mixture was then stirred for an additional 20 h at room temperature. At the end of the reaction, the mixture was quenched with an aqueous HCl (1.0 N) solution. The organic layer was separated, washed twice with water, dried over magnesium sulfate, concentrated, and dried in vacuum to give oligo(ethylene glycol) ethylmalonate (1) (10.4 g) as semisolids. Spectroscopic data of 1: 1H NMR (250 MHz, CDCl3): δ 4.30 (t, 2H, -COO-CH2-PEG), 4.20 (q, J ) 8 Hz, 2H, Me-CH2O-CO), 3.64 (broad, s, methylene protons of PEG), 3.41 (s, 2H, -CO-CH2-CO-), 3.38 (s, 3H, -OCH3), 1.28 (t, J ) 8 Hz, 3H, CH3 of ethyl ester). Synthesis of 1,2-Dihydro-1,2-methanofullerene[60]-61-(carbonyloxyethane)-carbonyloligo-(ethylene glycol), C60>(C2-EG43) (2). C60 (1.0 g, 1.4 mmol) and carbon tetrabromide (458 mg, 1.4 mmol) were dissolved in dry toluene (50 mL). In a separate flask, 1 (2.9 g, 1.4 mmol) was dissolved in dry toluene (10 mL), and the solution transferred into the fullerene reaction mixture via a cannula with stirring. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU, 0.4 mL, 2.7 mmol) was added dropwise. The mixture was stirred for 48 h at ambient temperature, and the solution turned brown. The solution was concentrated to 30% of the original volume, yielding a darkbrown solution. Brown solids were precipitated by the addition of hexane (100 mL). The solids were isolated by centrifugation to leave a purple toluene-hexane supernatant. Further purification was performed by dissolving the solids in toluene, followed by reprecipitation upon addition of hexane. This procedure was repeated until a colorless supernatant was observed. The product 1,2-dihydro1,2-methanofullerene[60]-61-(carbonyloxyethane)-carbonyloligo(ethylene glycol), C60>(C2-EG43) (2) was obtained (∼3.4 g) as brown solids in 85% yield. We use C60>(C2-EG43) to symbolize compound 2, where C60> represents the malonate ester fused to the C60 sphere through a cyclopropyl ring. EG43 is the ethylene oligomer with 43 repeat units connected to one of the ester groups, and C2 is the ethyl group connected to another ester group. Spectroscopic data of 2: UV-vis (CHCl3, nm ()): λmax 235 (1.07 × 105), 253.50 (1.00 × 105), and 319.80 (3.7 × 104 L/mol‚ cm). ATR-IR (thin film, cm-1): υmax 2880 (s), 1743 (m), 1644 (w), 1466 (m), 1342 (s), 1279 (m), 1240 (m), 1146 (w), 1103 (vs), 1060 (m), 962 (s), and 841 (s). 1H NMR (250 MHz, CDCl3): δ 4.62 (t, 2H, -COO-CH2-PEG), 4.53 (q, 2H, Me-CH2-O-CO), 3.953.4 (broad, s, methylene protons of PEG), 3.35 (s, 3H, -OCH3), and 1.47 (t, 3H, CH3 of ethyl ester). Synthesis of 1,2-Dihydro-1,2-methanofullerene[60]-61-(carbonylhexadecaaniline)-carbonyloligo(ethylene glycol), C60>(A16EG43) (3). A sample of 1,2-dihydro-1,2-methanofullerene[60]-61(carbonyloxyethane)-carbonyloligo(ethylene glycol), C60>(C2-EG43) (2) (100 mg, 0.035 mmol) was dissolved in anhydrous DMSO under a nitrogen atmosphere. In a separate flask, hexadecaaniline (56 mg, 0.038 mmol) dissolved in dry DMSO as a dark-blue solution was transferred via cannula into the C60>(C2-EG43) solution. A catalytic amount of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 20 µL) was added with stirring. The reaction mixture was then stirred at 90100 °C for an additional 48 h under a nitrogen atmosphere. It was (36) Parratt, L. G. Phys. ReV. 1954, 95, 359-369.
Tang et al. allowed to cool to room temperature. By the addition of acetonitrile (8 parts) to this solution, dark-blue solids were precipitated with a blue DMSO-acetonitrile supernatant. The solids were isolated by centrifugation, redissolved in DMSO, and reprecipitated by the addition of actonitrile. They were washed repeatedly with acetonitrile, a mixture of acetonitrile-THF, and acetone in sequence to give crude solid products (135 mg). Further purification was carried out by flash chromatography (silica gel) using the DMSO solution of the solids to afford a dark-blue DMSO solution, from which darkblue solids of 1,2-dihydro-1,2-methanofullerene[60]-61-(carbonylhexadecaaniline)-carbonyloligo(ethylene glycol), C60>(A16-EG43) (3) were obtained by precipitation upon the addition of acetonitrile. Compound 3 was air dried and kept under THF in a wet condition. Spectroscopic data of 3: ATR-IR (thin film, cm-1): υmax 3304 (br), 3026 (w), 2864 (m), 1651 (s), 1594 (s), 1495 (vs), 1288 (s), 1243 (w), 1096 (s), 1030 (s), 947 (m), 814 (s), 746 (m), and 691 (m). 1H NMR (250 MHz, DMSO-d6): δ 7.45-6.4 (br, aromatic proton of hexadecaaniline), 3.8-3.25 (broad, s, methylene protons of PEG), and 3.23 (s, terminal -OCH3). Preparation of Langmuir Films. A sample of 3 (2.0 × 10-3 mmol) was dissolved in a solvent mixture of DMSO-chloroform (2:8, 10 mL), giving a 0.2 mM solution. In general, approximately 500 µL of this solution was spread on the top of the water surface. Measurements of the surface pressure-area (Π-A) isotherms were carried out using a KSV 2000 standard LB trough (KSV Instrument, Helsinki, Finland) equipped with a Wilhelmy-type balance, a Teflon trough, and symmetrical hydrophilic Delrin barriers. The system was controlled by a computer. The trough was set in an enclosure to be protected from dust and drafts with the temperature controlled to within 0.5 °C. Ultrapure water (resistivity greater than 18 MΩ/ cm) from a Barnstead e-pure system was used as the subphase. In a typical experiment, the monolayer was formed by spreading C60>(A16-EG43) solution in chloroform onto an ultrapure water subphase at 21 °C. A period of 15 min was necessary to allow solvent evaporation. It was followed by the isothermal data collection by gradually reducing the surface area of the deposited layer at a rate of 10 cm2/s. During the film deposition on the substrate surface, the transfer surface pressure was fixed at 25 mN m-1. All solid substrates for the LB film deposition were cleaned by immersion in a 7:3 mixture of concentrated sulfuric acid/30% H2O2 at 80 °C for 1 h (Piranha etch treatment). Prior to use, substrates were treated with octadecyltrichlorosilane (OTS, 1% in hexadecane solution) at 40 °C for 45 min and then washed thoroughly with CHCl3 to generate a fresh hydrophobic surface.
Results and Discussions Synthesis of the Amphiphilic Oligo(ethylene glycol)-C60hexadecaaniline Conjugate. Hexadecaaniline (A16) possessing 16 repeating aniline units is a rigid rodlike oligomer molecule exhibiting rich reversible electrochemical redox and chemical protonation-deprotonation behavior that closely resembles that of polyanilines (PANI). Air-oxidized hexadecaaniline emeraldine, containing 4 quinonoid subunits with the rest of the aniline segments in a benzenoid structure, is the most stable form. Its synthesis involved the treatment of tetraaniline, in its leucoemeraldine reduced form, with (NH4)2S2O8 in aqueous HCl solution at ambient temperature for 2.0 h, according to the modified literature procedure.30-32 Characterizations of A16 products were made by various spectroscopic methods, including the confirmation of hexadecaaniline molecular weight by the direct detection of a group of its molecular ion mass peaks at m/z 1450-1458 in the negative ion desorption chemical ionization mass spectrum (DCI--MS).32 The nucleophilic substitution reactivity of one terminal amino (-NH2) group of hexadecaaniline was found to be higher than that of the in-chain secondary anilino groups. As a macromolecule, the activity of A16 was also observed to be substantially higher on the ester moiety of ethylmalonated
Oligo(ethylene glycol)-C60-hexadecaaniline Conjugate
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Scheme 1. Synthesis Procedure for C60>(C2-EG43) 2 and C60>(A16-EG43) 3a
a Reagent and conditions: (i) pyridine, r.t., 22 h; (ii) C , CBr , 60 4 DBU, r.t., 48 h; (iii) hexadecaaniline emeraldine, DBU (cat.), 90100 °C, 48 h.
C60 in replacing its -OEt group under basic conditions than the addition activity on the C60 cage.37 This difference in reactivity minimized the possible multihexadecaanilinated C60 byproducts. Accordingly, we applied similar nucleophilic reactivity to the synthesis of 3 as C60>(A16-EG43) from the corresponding carbonyloxyethane analogue precursor molecule C60>(C2-EG43) 2, as shown in Scheme 1. Synthetically, the water-soluble oligo(ethylene glycol) segment was incorporated into the C60 cage by the cyclopropanation reaction of C60 with 1 in the presence of carbon tetrabromide and a bases1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).38 The reaction gave the product 2 as brown solids in 85% yield. The structure of C60>(C2-EG43) was confirmed by the disappearance of a singlet proton peak at δ 3.41 in the 1H NMR spectrum of 2, corresponding to the chemical shift of two R protons of the starting material 1. That indicated the attachment of 1 onto the C60 cage occurring at the R carbon of the malonate moiety. The UV-vis spectrum of 2 provided consistent results showing two bands centered at 254 ( ) 1.0 × 105) and 320 nm ( ) 3.7 × 104 L/mol‚cm) corresponding to optical absorptions of the fullerene moiety. The subsequent reaction of C60>(C2-EG43) with hexadecaaniline emeraldine was carried out in anhydrous DMSO in the presence of a catalytic amount of DBU (cat.) at 90 °C under a nitrogen atmosphere. The formation of the amide linkage between the malonate moiety and A16 was confirmed by a new strong band centered at 1650 cm-1 in the ATR-IR spectrum of 3, corresponding to the optical absorption of the amide carbonyl group. The replacement of the ethyl group by A16 was evident in the 1H NMR spectrum of 3, which displayed large proton signals of the ethylene oxide moiety with no ethyl proton peaks, which consist of a triplet methyl proton signal at δ 1.47 and a quartet methylene proton signal at δ 4.53. New aromatic proton peaks appearing as a broad band at δ 6.4-7.5 are characteristic chemical shifts of hexadecaanilinyl protons, consistent with the overall structure of C60>(A16-EG43). Steady-State Optical Spectra of the Oligo(ethylene glycol)C60-hexadecaaniline Conjugate. Optical absorption properties (37) Anantharaj, V.; Patil, S. V.; Canteenwala, T.; Wang, L. Y.; Chiang, L. Y. Synth. Met. 2001, 121, 1123-1124. (38) Bingel, C. Chem. Ber./Recl. 1993, 126, 1957-1959.
Figure 1. FTIR spectra of C60>(A16-EG43), C60>(C2-EG43), and hexadecaaniline.
of C60>(A16-EG43) were applied to the new compound characterization. The IR spectrum of hexadecaaniline displayed four strong bands centered at 1598, 1506, 1300, and 820 cm-1 and one weak band at 1156 cm-1 (Figure 1a) corresponding to the optical absorption of A16 segments in an emeraldine base form. Similar absorption bands recorded for C60>(A16-EG43) are shown in Figure 1c, indicating the successful incorporation of a hexadecaaniline unit into its structure. The attachment of A16 to the malonate ester moiety of 2 resulted in the formation of a new amide functional group that effectively shifts the carbonyl absorption band of the ester group from 1710 (Figure 1b) to 1640 cm-1 (Figure 1a), which is assigned to the carbonyl group in the amide linkage. One new peak centered at 1167 cm-1 was assigned to the combined C-C(CdO)-O and O-C-C stretch absorption bands of the ester and oligo(ethylene glycol) moieties, respectively, consistent with those of C60>(C2-EG43), as shown in Figure 1b. The nature of the other new peak centered at 1019 cm-1 is not fully understood. However, its relative peak intensity was too strong to be accounted for by one C-C(CdO)-N amide group per molecule 3, using the relative intensity of the carbonyl group at 1640 cm-1 as the reference. This peak was also absent in the IR spectrum of A16 (Figure 1a) and therefore cannot be attributed to the absorption of C-C-N moieties of oligoaniline. Perhaps it might be associated with the positively charged hexadecaaniline moieties arising from partial electron transfer of its benzenoid subunits to the C60 cage.25,39 A model compound 1,2-dihydro-1,2-methanofullerene[60]61-di(carbonyloxyethane), C60>(CO2Et)2 (4), was prepared for spectral comparison. UV absorption of the fullerene moiety can be correlated to C60 monoadduct 4 showing a strong peak with a maximum at ∼270 nm (Figure 2b), whereas the UV absorbance of oligo(ethylene glycolated) hexadacaaniline, (A16-EG43) (5), appears at ∼300 nm (Figure 2a). That allowed us to clearly assign two peaks at 250 and 300 nm in the C60>(A16-EG43) spectrum (Figure 2c) to fullerene and A16 moieties, respectively. In the visible region, very weak absorbance of 4 at wavelengths (39) Baibarac, M.; Baltog, I.; Lefrant, S.; Mevellec, J. Y.; Chauvet, O. Chem. Mater. 2003, 15), 4149-4156.
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Figure 3. Pressure-area (Π-A) isotherm of a C60>(A16-EG43) 3 monolayer. Figure 2. UV-vis spectra of (a) A16-EG43 5, (b) C60>(CO2Et)2 4, and (c) C60>(A16-EG43) 3.
between 500-600 nm coupled with the broad absorbance band of A16-EG43 centered at 590 nm constituted the main absorption component of the peak centered at ∼625 nm in the spectrum of 3. The overall spectrum of C60>(A16-EG43) displayed the main features of the aniline oligomer and the monofunctionalized C60 cage. Additional absorbance at the shoulder of this broad band extending into 900 nm, as compared with Figure 2a, may reveal either intermolecular segment aggregation of A16 moieties to give extended π-electron conjugative interactions or the involvement of weak partial electron transfer from the A16 moiety to the fullerene cage leading to the formation of a partially positively charged oligoaniline. This hypothesis is consistent with our previous study where reduced hexa(hexaadecaanilino)[60]fullerenes, consisting of six A16 oligomers covalently linked on one C60, showed an large increase in absorbance above 600 nm upon photoexcitation with a significant decrease in intensity of the intrinsic fullerene peak at 300 nm.40 The phenomena were attributed to the occurrence of photoinduced electron transfer from reduced A16 moieties to the fullerene cage. The possibility of electron transfer between C60 and PANI leading to the formation of polarons on fullerene-doped polyanilines was reported previously, showing a broad shoulder absorption band at roughly 820 nm.26 Pressure-Area Isotherm Characteristics of the C60>(A16EG43) Monolayer at the Air-Water Interface. A diluted sample solution of 3 in a solvent mixture of DMSO-chloroform was spread on top of the water surface to form a thin layer at the air-water interface. This layer was compressed into a densely packed monolayer. The following sections describe the preparation and characterization of C60>(A16-EG43)-derived LangmuirBlodgett thin films and photoactivated cross-linking processes of the film into ultrathin, robust PANI layers with well-defined C60-A16 molecular orientation. The Π-A isotherm of the C60>(A16-EG43) layer is shown in Figure 3. The isotherm exhibited liquid expanded (LE) phase formation over the range of 0.0 mN/m < Π < 16 mN/m. In the higher-pressure region of 16 mN/m < Π < 50 mN/m, a phase transition to a liquid condensed (LC) phase was evident, prior to collapse of the film at pressures above 50 mN/m. From the linear part of the LC phase, the molecular area extrapolated at zero pressure (A0) is ∼95 Å2/molecule. This surface area approximately matches the size of C60 (82 Å2/molecule). The result suggested the formation of a rather densely packed monomolecular layer of 3 at the air/ water interface. Furthermore, a small amount of hysteresis in the isotherm for 3 (Figure 4) implied a high reversibility of the (40) Canteenwala, T.; Anantharaj, V.; Patil, S. V.; Halder, M.; Chiang, L. Y. J. Macromol. Sci., Pure Appl. Chem. 2002, A39, 1069.
Figure 4. Three successive compression and recompression cycles on a C60>(A16-EG43)-derived monolayer at the air-water interface.
pressure cycle if the surface pressure was kept below the collapse value. However, a slight shift of the pressure isotherm in the LE region after each cycle (Figure 4) indicated the gradual occurrence of irreversible 3-D aggregation at the air-water interface. It is worthwhile to note that the area/molecule estimated from the liquid condensed state of the pressure isotherm in the repeated compression/decompression cycles was essentially the same. The main difference among pressure isotherms of three compression and decompression cycles was in the liquid expanded state in which the slope became progressively steeper, accompanied by a smaller onset area, as the number of compression/decompression cycle increased. We suggest this could be due to a better alignment between hexadecaanilines that required little rearrangement between the onset and collapse pressures. The area/molecules (intercept of the tangent line for the liquid condensed state with the x axis) in a densely packed monolayer of ∼95 Å2/molecule is clearly defined by the intermolecular interaction between C60 cages, with the size of the bare C60 estimated to be ∼82 Å2. These results are consistent with our previous study that demonstrated the high tendency of amphiphilic C60 derivatives to self-assemble into a densely packed monolayer at the airwater interface and, upon repeated compression and decompression cycles, to form irreversible aggregates because of their strong intermolecular interactions. Fabrication of Multilayer LB Films on the Substrate. After establishing the formation of a densely packed monolayer at the air-water interface, we proceeded to transfer the C60>(A16EG43) monolayer onto a quartz substrate at a surface pressure of 25 mN/m. The layer-by-layer deposition process was monitored by UV-vis spectroscopy. The transfer ratio of LB films for C60>(A16-EG43) was about 0.90 for the up cycles and almost 0 for the down cycles. The very low transfer ratio during the down cycles indicated weak interactions between the fullerene spheres where the adhesion strength was insufficient to assist a deposition,
Oligo(ethylene glycol)-C60-hexadecaaniline Conjugate
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Figure 5. UV-vis spectra of C60>(A16-EG43)-derived LB films of up to 10 layers.
Figure 7. UV-vis spectra of a C60>(A16-EG43)-derived LB thin film before and after flash light exposure.
Figure 6. UV-vis spectra of C60>(A16-EG43) in either DMSO or LB films.
thus the multilayer thin films are likely to be Z type. This observation contradicts our understanding of strong interactions between fullerene spheres. We speculated that the spherical feature causes the C60 cages to slide away from each other and overcome the strong intermolecular interaction between fullerene cage moieties because the pulling rate was not slow enough to allow C60 spheres to reestablish interactions. UV-vis spectra of C60>(A16-EG43)-derived multilayer LB thin films are shown in Figure 5. A linear increase in the UV absorbance clearly indicated a reproducible deposition of C60>(A16-EG43) layers for each deposition cycle. Comparison of UV-Vis Spectra of C60>(A16-EG43) in Solution and in LB Films. Absorption spectra of C60>(A16EG43) in DMSO and in the LB films are shown in Figure 6. The solution absorbance spectrum displayed one sharp, strong band centered at 326 nm and a broad band centered at 650 nm. Presumably, it represents the absorption features of the individual C60>(A16-EG43) molecule; therefore, we comfortably assigned the peak at 326 nm to the absorption of the monofunctionalized C60 moiety with certain contributions from the oligoaniline moiety. The broad peak at 650 nm is attributed to the exciton band of hexadecaaniline with a rather minor absorption contribution from the C60 cage, as indicated in Figure 2b. The shoulder band extended beyond the 600 nm region to 950 nm. Covalently linking hexadecaaniline to C60 in close proximity exhibits weak donoracceptor-type electronic interactions with no significant doping characteristics. In the case of the 10-layer LB films of C60>(A16EG43), the spectrum showed a major feature band centered at 310 nm and a broad peak centered around 600 nm. Two main differences between the absorbance spectra of the LB films and the solution spectrum are (1) a slight blue shift of the exciton peak and (2) a shoulder band stronger in intensity over 800 nm with a higher degree of absorption peak broadening. Observed broadening of the optical absorption in LB films clearly indicates
Figure 8. Fresnel divided X-ray reflectivity data and reflectivities calculated from the B-spline and box model fits. All reflectivities are divided by the reflectivity of a steplike interface (Fresnel reflectivity) to show the closeness of fit better. The solid line corresponds to the fit obtained using B splines, and the dashed line corresponds to the box model fit.
increasing electronic interactions among C60 and oligoaniline moieties of adjacent molecules. Cross Linking of Hexadecaaniline Segments in the LB Film upon Camera Flash Photoexcitation. Tricomponent 3 consists of a highly water-soluble oligo(ethylene glycol) (EG43, MW 2000) component, a highly hydrophobic C60 cage, and a polar hexadecaaniline (A16). When it is deposited onto the aqueous surface, molecular self-assembly of C60>(A16-EG43) at the airwater interface leads to a balance between the high tendency of EG43 components to stretch into the water phase and the tendency of fullerene components to stay above the liquid interface. This results in polar A16 components right at the interfacial area, as depicted in Figure 9. Maintaining this preference of molecular alignment and orientation with minimum disturbance, the monolayer can be transferred to a glass substrate, giving an approximately aligned, tilted rigid-rodlike C60-A16 layer on the surface. Subsequent photoexcitation of hexadecaaniline by camera flash induced cross linking between adjacent A16 segments to form a robust interlinked A16 array. Kaner et al. reported the first photowelding of PANI nanofibers to form a PANI thin film by
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Tang et al. Table 1. Parameters for the Box Model Analysis of XR from the C60-Hexadecaaniline Monolayer at the Air-Water Interfacea
C60 A16
H2O
thickness (Å)
electron density (e-/Å3)
roughness (Å)
15.5 14.8 13.6 15.7 18.3 29.8 37.9
0.21 0.18 0.23 0.24 0.28 0.31 0.32 0.33
4.1 3.0 7.9 6.4 11.2 19.4 6.9 13.6
a
Figure 9. B-spline and box model electron density profiles. The electron density profile for the box model fit is shown both unsmeared and smeared by interfacial roughness. The bulk water interface is placed at Z ) 0 Å. An elevated electron density region is observed near Z ) 137 Å corresponding to the location of the C60 units. Below this is a low electron density region where water is excluded, followed by a graded interface approaching the electron density of bulk water (left). The corresponding monolayer structure indicates the origin of electron density. The water molecules solvating the PEO chains along with the extended aniline oligomer form a columnlike structure (right).
using a camera flash light source.41 Energy absorbed by PANI was unable to dissipate effectively, thus generating the heat that led to melting and cross linking of the polymer chain. This is consistent with previous studies that demonstrated cross-linking reactions between PANI chains upon exposure to elevated temperatures (greater than 150 °C).42-45 Using a similar approach, we observed evidence of the cross-linking reaction between aligned hexadecaaniline chains in the LB thin films. Figure 7 shows UV-vis spectra of C60>(A16-EG43)-derived thin films before and after photoexcitation using a flash light. Prior to exposure to the flash light, the hexadecaaniline moiety of C60>(A16-EG43) in the thin film can easily be doped and undoped by an acid and by a base, respectively. Doped C60>(A16-EG43) exhibited a large red shift of the optical absorbance to λmax at 785 nm. This peak is attributed to the absorbance of the localized polaron band in a fully doped polyaniline.46 A pronounced difference was observed for the photo-cross-linked LB sample, which became fairly inert upon immersion in acid solution. The corresponding absorbance spectrum of the phototreated film showed only a slight change in λmax from 585 (undoped) to 610 nm, suggesting a minimum doping level of the materials. Similarly, IR spectra of C60>(A16-EG43) in the LB film indicated essentially no change in the absorption bands before and after photoexcitation. The IR results suggested no major structural changes upon phototreatment. A plausible explanation is the small number of photo-cross-linking sites among hexadecaaniline chains, which are presumably due to the photoinduced addition reaction of the anilinoid -NH- functional group on the N-quinonoid ring of the adjacent A16 chain resembling Michael addition. Each cross-linking site effectively disrupts the full conjugation length of two A16 chains, which significantly reduces the overal doping capability ofl A16 segments without having many cross-linking sites. (41) Virji, S.; Huang, J. X.; Kaner, R. B.; Weiller, B. H. Nano Lett. 2004, 4, 491-496. (42) Ding, L. L.; Wang, X. W.; Gregory, R. V. Synth. Met. 1999, 104, 73-78. (43) Pandey, S. S.; Gerard, M.; Sharma, A. L.; Malhotra, B. D. J. Appl. Polym. Sci. 2000, 75, 149-155. (44) Wei, Y.; Hsueh, K. F. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 4351-4363. (45) Mathew, R.; Mattes, B. R.; Espe, M. P. Synth. Met. 2002, 131, 141-147. (46) Zheng, W.; Min, Y.; Macdiarmid, A. G.; Angelopoulos, M.; Liao, Y. H.; Epstein, A. J. Synth. Met. 1997, 84, 109.
The top row describes the region containing the C60 components followed by a row describing a layer of hexadecaaniline components. The next five rows represent decreasing volume fractions of hexadecaaniline and increasing volume fractions of H2O. The final row represents bulk water.
XR Characterization of Oligo-C60-hexadecaaniline Monolayers at the Air-Water Interface. To understand the packing behavior of C60>(A16-EG43) in LB films better, X-ray reflectometry and grazing incidence X-ray diffraction were used to probe the monolayer structure at the air-water interface prior to LB deposition. For these experiments, C60>(A16-EG43) molecules were spread on a water subphase and compressed to 10 mN/m, which is at the transition between the LE and LC phases. By elastically scattering a beam of X-rays from the airwater interface, an interference pattern was measured and analyzed (Figure 8) to acquire an electron density profile perpendicular to the water (Figure 9, Table 1). Obtained using modelindependent B-spline fitting, the electron density profile provides information about the position of the C60>(A16-EG43) molecules relative to the water surface and the orientation of the molecules within the monolayer (Figure 9). The box model fit is similar to the B-spline fit and provides more quantitative information about the location and packing of the C60 units (Figure 9). Bulk water has an electron density value of 0.334 e-/Å3 as shown at a Z position of 0 in Figure 9. From bulk water, the electron density gradually decreases over a distance of approximately 130 Å. After this decrease, there is a region of higher electron density corresponding to the location of the C60 units. Box models lacking such a region of higher electron density yielded calculated reflectivities incompatible with the measurement. Furthermore, model-independent B-spline fits provided corroborative evidence of this feature. Therefore, we have significant confidence in the identification of a localized C60 region. Using the box model parameters, this region had an electron density of 0.21 e-/Å3 and a thickness of 15.5 Å. Assuming this layer contained only C60 units, our model predicted a packing density of 111 Å2/molecule, which is very similar to the 115 Å2/molecule observed in pressure-area isotherm experiments conducted at 10 mN/m. Above the C60 layer, the electron density profile decayed rapidly to zero. This indicated that little to no material resided on top of C60 and that both the hexadecaaniline and ethylene glycol components were extended toward the water subphase. The majority of the decreasing electron density region below the C60 layer was attributed to a mixture of hexadecaaniline, PEG, and water. Because of the graded nature of the electron density profile, it was difficult to define the exact location of the air-water interface. Instead, the decreasing electron density profile can be interpreted as an increasing volume fraction of hexadecaaniline components and a gradual decrease in the volume fraction occupied by H2O with increasing distance from the subphase. Because the calculated lengths of a fully extended A16 oligomer and fully stretched EG43 are approximately 100 and
Oligo(ethylene glycol)-C60-hexadecaaniline Conjugate
160 Å, respectively, the 30 Å decreasing electron density region adjacent to the bulk water was attributed to a mixture of only PEG and H2O. At a bulk mass density of 1 g/cm3, PEG polymer has an electron density of 0.329 e-/Å3, and any mixture of PEG and H2O will yield an electron density slightly below that of bulk water. The decreased electron density attributed to the mixture of PEG and H2O was observed from 100 to 130 Å below the C60 layer. It is possible that the length of 130 Å corresponds to the full extent of the PEG polymer due to coiling. Another possible scenario is that the PEG molecules extend further into the subphase but could not be observed because of limited contrast with bulk water. In summary, the X-ray reflectivity data suggest that the profile consists of a C60 layer on top of a mixed hexadecaaniline and an ethylene glycol underlayer. The longer PEG molecules extend beyond the hexadecaaniline into the water subphase. This threecomponent molecule interacts with the water subphase to form a columnlike structure that facilitates packing at the air-water interface and allows a rationalization of the steep pressure isotherm from the liquid expanded state to the liquid condensed state. The electron density profile derived from the X-ray reflectivity data is consistent with our spectroscopy data and also provides unique insight into the structure and properties of nanoassemblies consisting of complex multicomponent molecules. Together with the reflectometry, we have also performed grazing incidence X-ray diffraction measurements33 in order to address the in-plane structure of the C60-hexadecaaniline monolayer at the air-water interface. The data (not shown) did not yield any in-plane diffraction peaks, suggesting no coherency in packing C60-hexadecaaniline molecules along the air-water interface.
Conclusions We have demonstrated the synthesis of a novel oligoanilineC60 conjugate C60>(A16-EG43) possessing a well-defined number of repeating aniline donor units and a hydrophilic oligo(ethylene glycol) chain. The molecular design enhances its amphiphilic characteristics while composing a covalently bound donor-
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acceptor chromophore structure. This novel amphiphile facilitated its molecular self-assembly capability at the air-water interface into a densely packed monolayer. The monolayer was transferable onto a glass substrate. Multideposition of the layer formed Langmuir-Blodgett multilayered thin films. Broadening of optical absorption peaks of C60>(A16-EG43)-derived LB films suggested strong intermolecular interactions among the C60 cages and the A16 oligomers. X-ray scattering results revealed that the molecular structure of the C60>(A16-EG43) Langmuir film at the air-water interface has a top C60 layer, hexadecaanilines at the interface, and EG43 chains stretched into the subphase. This molecular diad forms a columnlike structure that facilitates packing at the air-water interface and allows a rationalization of the steep pressure isotherm from the liquid expanded state to the liquid condensed state. Our in-plane grazing incidence X-ray diffraction did not reveal any in-plane registry between C60>(A16EG43) molecules, suggesting their disorder along the water interface. Further photoexcitation of C60-A16 layers on the substrate surface by a camera flash light source induced cross linking between adjacent A16 segments and formed an interlinked A16 array. This cross linking between A16 moieties stabilizes the charge-transfer nanoassemblies where C60 (electron acceptor) and hexadecaaniline (electron donor) layers are organized by way of self-assembly. Furthermore, we demonstrated a method of preparation of the aligned donor-acceptor array using strong intermolecular interactions between fullerenes as the molecular orientation guiding force in the Langmuir-Blodgett technique. Acknowledgment. We acknowledge financial support from the Laboratory Directed Research and Development (LDRD) fund and the Office of Science (DOE). Partial support from the Cross Enterprise Technology Development Program of the National Aeronautics and Space Administration (NASA) and the Asian Office of Aerospace Research and Development under contract no. FA5209-04-P-0540 is also greatly appreciated. We thank Dr. Kristian Kjaer from the Risoe National Laboratory in Denmark for collaboration on the GIXD and XR measurements. LA060083I
