4800
Langmuir 2007, 23, 4800-4808
Aggregation of a C60-Didodecyloxybenzene Dyad: Structure, Dynamics, and Mechanism of Vesicle Growth S. Shankara Gayathri and Archita Patnaik* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India ReceiVed September 14, 2006. In Final Form: February 2, 2007 The mechanism of formation and the stability of spontaneously formed vesicles upon self-assembly of a partially ground-state charge-separated, nonpolar-polar-nonpolar fullerene(C60)-didodecyloxybenzene (DDB) dyad in binary solvent mixtures requiring a critical dielectric constant of ∼30 are reported. Molecular interactions giving rise to defined vesicles with in-plane bilayer packing are detailed from the predominant van der Waals and electrostatic interactions existing on the dyad’s framework. The vesicles are formed with a large bending rigidity of 18kBT, which on further extraction into a polar water medium resulted in uniform spheres that corroborated well with the theoretical predictions. Furthermore, the water-extracted spherical dyad aggregates at an increased dyad concentration, leading to the formation of giant micrometer-sized fractals following diffusion-limited cluster aggregation. These dyad aggregates act as efficient quenchers of fluorescent dyes with a quenching rate of 4.6 × 1013 M-1 s-1.
Introduction The rational design of functional nanoscopic architectures requires a detailed understanding of the interactions between individual nanosized carbon building blocks. Precise control of the geometry of the self-assembled structures allows for the fine tuning of the functional properties of these materials toward building nanoelectronics.1,2 Fullerenes as nanoscopic carbon cages are proposed as promising materials for quantum computing nanodevices.3,4 Furthermore, the aggregation state of fullerenes can significantly change the physicochemical properties of fullerenes and thus have a profound influence on the applications of fullerene-based materials.5,6 Examples of fullerene-based systems played a crucial role in the preparation of photovoltaic cells.7-9 The association behavior of the substituted fullerenes in solution has been extensively studied, including a variety of structural morphologies such as spheres,10-14 nanorods,14,15 nanotubules,14 fibers,16,17disks,16 stars,18 fractals,19,20 nanowhiskers,21ves* Corresponding author. E-mail:
[email protected]. Tel: +91-442257-4217. Fax: +91-44-2257-4202. (1) Nirengarten, J. F. New J. Chem. 2004, 28, 1177. (2) Guldi, D. M.; Zerbetto, F.; Georgakilas, V.; Prato, M. Acc. Chem. Res. 2005, 38, 38. (3) Meyer, C.; Harneit, W.; Weidinger, A.; Lips, K. Phys. Status Solidi B 2002, 233, 462. (4) Harneit, W.; Meyer, C.; Weidinger, A.; Suter, D.; Twamley, J. Phys. Status Solidi B 2002, 233, 453. (5) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695. (6) Innocenzi, P.; Brusatin, G. Chem. Mater. 2001, 13, 3126. (7) Tanigaki, K. Optoelectron.-DeVices Technol. 1995, 10, 231. (8) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15-26. (9) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481. (10) Biju, V.; Sudeep, P. K.; Thomas, K. G.; George, M. V.; Barazzouk, S.; Kamat, P. V. Langmuir 2002, 18, 1831. (11) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Langmuir 2003, 19, 4798. (12) Wang, C.; Ravi, P.; Tam, K. C. Langmuir 2006, 22, 2927. (13) Zhou, S.; Ouyang, J.; Golas, P.; Wang, F.; Pan, Y. J. Phys. Chem. B 2005, 109, 19741. (14) Georgakilas, V.; Pellarini, F.; Prato, M.; Guldi, D. M.; Melle-Franco, M.; Zerbetto, F. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5075. (15) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Angew. Chem., Int. Ed. 1999, 38, 2403. (16) Nakashima, N.; Ishii, T.; Shirakusa, M.; Nakanishi, T.; Murakami, H.; Sagara, T. Chem.sEur. J. 2001, 7, 1766.
icles22,23 and so forth24,25 In particular, C60-based molecules have been well studied for their biological and electronic applications. For example, the self-assembly of the potassium salt of pentaphenyl fullerenes produced spherical bilayer nanovesicles in water.22 Vesicles were found in the aqueous solutions of a bola-amphiphilic fullerene, 1,9-dihydro-bis(11-bromo triammoniumundecanoyloxyl)-1,9-(methanobenzenomethano)fullerene, at concentrations above 3.0 mM.23 The size of spherical vesicles from the self-assembly of oligo(ethylene glycolated)diphenylaminofluorene in water increases with an increase in concentration.26 Vesicle formation was also observed in a dendritic methano[60]fullerene octadeca-acid in aqueous solutions of pH 7.4.27 Highly stable spherical microvesicles have also been reported from the self-assembly of a π-electronic amphiphile consisting of a Zn-porphyrin-fullerene dyad, appended with triethylene glycol chains in aqueous media.28 The primary interaction type responsible for the molecular assembly of fullerene derivatives in polar media is hydrophobic interactions leading to such self-organized structures ranging from vesicles to spheres to rods. An onion phase (multilamellar vesicular phase or LR phase) was prepared from salt-free zero-charged cationic and anionic (catanionic) surfactant mixtures of tetradecyltrimethylammonium hydroxide (TTAOH)/lauric acid (LA)/H2O.29 The concentration dependence of the aggregate formation, micellar sizes, and structure of the polymer-substituted fullerenes (17) Wolffs, M.; Hoeben, F. J. M.; Beckers, E. H. A.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 13484. (18) Bokare, A. D.; Patnaik, A. J. Phys. Chem. B 2005, 109, 87. (19) Ying, Q.; Zhang, J.; Liang, D.; Nakaishi, W.; Isobe, H.; Nakaura, E.; Chu, B. Langmuir 2005, 21, 9824. (20) Bokare, A. D.; Patnaik, A. J. Chem. Phys. 2003, 119, 4529. (21) Miyazawa, K.; Kuwasaki, Y.; Obayashi, A.; Kuwabara, M. J. Mater. Res. 2002, 17, 83. (22) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944. (23) Sano, M.; Oishi, K.; Ishii, T.; Shinkai, S. Langmuir 2000, 16, 3773. (24) Nakanishi, T.; Schmitt, W.; Michinobu, T.; Kurth, D. G.; Ariga, K. Chem. Commun. 2005, 5982. (25) Sawamura, M.; Kawai, K.; Matsuo, K.; Kato, T.; Nakamura, E. Nature 2002, 419, 702. (26) Verma, S.; Hauck, T.; El-Khouly, M. E.; Padmawar, P. A.; Canteenwala, T.; Pritzker, K.; Ito, O.; Chiyang, L. Y. Langmuir 2005, 21, 3267. (27) Hao, J.; Li, H.; Liu, W.; Hirsch, A. Chem. Commun. 2004, 602. (28) Charvet, R.; Jiang, D-L.; Aida, T. Chem. Commun. 2004, 2664. (29) Li, H; Jia, X.; Li, Y.; Shi, X.; Hao, J. J. Phys. Chem. B 2006, 110, 68.
10.1021/la0626961 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/28/2007
Aggregation of a C60-Based Dyad
Figure 1. (a) Chemical structure of the dyad. (b) Ab initio (B3LYP/ 3-21G*) geometry-optimized structure of the dyad showing the bridge and donor benzene rings to be planar. Color code: dark gray, carbon; light gray, hydrogen; black, oxygen.
has been explained recently.30 However, reports on such vesicleforming systems are still limited to a phenomenological description. In this report, we describe the structural aspects and the mechanistic route to the formation of vesicles from a truncatedconical C60-based dyad monomer in binary THF-water mixtures. On increasing the polarity of the medium and upon extraction into the single polar-phase H2O, a cone-to-sphere structural transformation was noted, which finally culminated in spherical fractals with increasing concentration of the dyad monomer. The quenching behavior of these aggregates has been further explored. Materials and Methods Synthesis of the Dyad. The fullerene(C60)-didodecyloxybenzene (DDB) dyad in Figure 1a was synthesized as previously reported31,33 as a pure[6,6]isomer with 99.8% purity as confirmed from HPLC and were characterized by 13C NMR, UV-vis, FTIR, MALDITOF, and CHN analysis techniques. Preparation of Dyad Aggregates. Two pathways were adopted to study the dyad’s aggregation behavior. 1. In Binary SolVent Mixtures. A stock solution of the dyad in THF (1 mM) was prepared. This was diluted to a total concentration 0.01 mM in 10 mL in each of the binary solvent mixture ranging from 10-100% THF by the addition of varying water volume fractions to the stock. Color changes from colorless to yellow were indicative of the aggregation of the dyad in these binary solvent mixtures. These solutions were used for UV-vis and dynamic light scattering measurements. Similar concentrations were used for the THF-ACN (tetrahydrofuran-acetonitrile), DCM-ACN (dichloromethane-acetonitrile), and BZN-ACN (benzonitrile-acetonitrile) solvent mixtures. 2. In a Water-Extracted Phase. A THF solution (1 mL) of the dyad was injected into deionized water (5 mL) in a wide-mouthed standard flask with vigorous stirring. High-purity nitrogen (99.99%) was continuously purged through the system during the process to remove THF. Immersing the system in a water bath compensated for the evaporation-induced cooling during extraction, and the cycle was repeated five times. Finally, water was added up to the original (30) Liao, Q.; Qu, X.; Chen, L.; Jin, X. J. Phys. Chem. B 2006, 110, 7153. (31) Gayathri, S. S.; Patnaik, A. Chem. Phys. Lett. 2005, 414, 198. (32) Guldi, D. M. J. Phys. Chem. A 1997, 101, 3895. (33) Gayathri, S. S.; Agarwal, A. K.; Suresh, K. A.; Patnaik, A. Langmuir 2005, 21, 12139.
Langmuir, Vol. 23, No. 9, 2007 4801 volume to compensate for the evaporated water during the nitrogen purge. A clear yellow solution of dyad aggregates was formed, and the complete removal of THF was checked with the GC-MS technique. Dyad solutions of concentrations 0.01, 0.04, 0.2, and 1 mM in THF were used for the preparation of water-extracted aggregates. UV-Vis Absorption Spectroscopy. The electronic absorption spectra were recorded on a Varian Cary 5E double-beam spectrophotometer using a 1 cm path length Infrasil cuvette. During the experiment, about 4 mL of the solution was placed in a 5 mL quartz cell, and the temperature of the solution was kept constant at 25 °C. Fluorescence Spectroscopy. The emission spectra were acquired on Jobin Vyon fluorolog-3-11 spectrofluorimeter with a resolution of 0.2 nm. The spectra were acquired at 90° sample geometry with respect to the source and the detector. To obtain the Stern-Volmer quenching plot, water-extracted dyad aggregates of concentrations 0.001-1 mM were used as quenchers of rhodamine B, whose concentration was fixed at 0.01 mM. Dynamic Light Scattering (DLS). The particle dimensions were measured from the DLS experiments. Measurements were made on a Malvern 4700C (England) equipped with an Ar+ laser operating at 25 mW power and a 488 nm wavelength. The system allowed DLS measurements at various scattering angles between 30 and 120°. The varying refractive indices of the binary solvent combinations were measured using an Abbe refractometer. The measurements were made in a special dust-free light-scattering cell at 25 °C. Transmission Electron Microscopy. Transmission electron microscopy (TEM) imaging was performed on carbon-coated copper grids using a Philips CM12 transmission electron microscope equipped with a 100 kV electron gun. Microfilms for TEM studies were prepared by placing a drop of the aggregated solution on a carbon-coated copper grid, and the solvent was evaporated at ambient temperature. Thus, all of the experiments followed natural drying, and the aggregate structure in the solution was replicated on the TEM grid. Samples were then transferred to the microscope in a special vacuum-transfer sample holder. Differential Scanning Calorimetry and Optical Polarizing Microscopy. The differential scanning calorimetry measurements were made on a Netzsch STA 409C thermal analyzer (GmbH, Germany) under a nitrogen atmosphere at a scan rate of 1 K/min. The optical polarizing microscopy images were taken on a Euromex microscope. Cyclic Voltammtery. The cyclic voltammetric measurements for the spin-coated film electrodes of water-extracted dyad aggregates were made on a CH 608B electrochemical workstation with a threeelectrode setup. The ITO substrates with a potential window of -1.5 to +1.0 V acted as a working electrode, the counter electrode was a platinum wire, and the pseudo-reference electrode was a silver wire with Fc/Fc+ as an internal standard. The supporting electrolyte used was tetrabutylammonium hexafluorophosphate (TBAPF6) with a concentration that was 100 times that of the analyte for the experiments being done strictly under an argon atmosphere.
Results and Discussion Ground-State Electronic Structure of the Dyad. The chemical structure and the ab initio geometry-optimized dyad, 1-(3-carboxy)-(3,4-di(dodecyloxy)benzoicacid-4-carboxy phenylester) propyl-1-phenyl[6,6]-C61 with a hydrophobic-hydrophilic-hydrophobic (C60-bridge-didodecyloxybenzene) network, is depicted in Figure 1a. The geometry optimization and singlepoint energy calculations gave the conformation shown in Figure 1b, leading to a total dipole moment of 8.9 D with the bridging benzene and the donor benzene being planar with respect to each other.41 The binding energy was 221.784 kcal mol-1, and (34) Fomina, L.; Reyes, A.; Guadarrama, P.; Fomine, S. Int. J. Quantum Chem. 2004, 97, 679. (35) Thomas, K. G.; Biju, V.; Guldi, D. M.; Kamat, P. V.; George, M. V. J. Phys. Chem. B 1999, 103, 8864. (36) Schobel, U.; Coille, I.; Brecht, A; Steinwand, M.; Gauglitz, G. Anal. Chem. 2001, 73, 5172.
4802 Langmuir, Vol. 23, No. 9, 2007
Gayathri and Patnaik
Table 1. Critical E Values for Dyad Aggregation in Various Binary Solvent Mixtures at 298 K solvent mixture
critical
THF-water DCM-ACN THF-ACN BZN-ACN
30.0 29.5 29.5 30.0
the heat of formation was 571.647 kcal mol-1, with the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) amplitudes localized on the donor and acceptor moieties, respectively. The HOMO-LUMO gap, estimated to be 1.6 eV, compares reasonably well with the overestimated calculated value of 2.3 eV from DFT B3LYP/ 3-21G*. It was fairly well established that the molecule forms a ground-state intramolecular charge-transfer complex from the absorption, emission, and electrochemical studies.31 Absorption Spectral Features of the Dyad Aggregates: Effect of Solvent Polarity. The dyad was found to be readily soluble in a wide range of solvents: cyclohexane, chlorinated organic solvents (o-dichlorobenzene, chlorobenzene, dichloromethane, carbontetrachloride, and chloroform), polar cyclic organic solvents (tetrahydrofuran (THF) and dioxan), aromatic solvents (benzene, toluene, and benzonitrile), ethyl acetate, and carbondisulfide up to ∼7 mg/mL and was sparingly soluble in ethanol and dimethylsulfoxide. The UV-vis spectra of the dyad in organic solvents of varying polarity showed absorption maxima at 259, 328, 431, and 495 nm, in agreement with the reported spectroscopic characteristics of monomeric monofunctionalized fullerenes.31,32 The absorption characteristics of the dyad in THF, dichloromethane, and benzonitrile revealed Beer-Lambert (BL) linearity in the inset over a concentration range of 10-3 to 1 mM, implying the absence of a probable molecular association in the single-component solvents. It is worth mentioning here that the absence of any solvatochromic shifts discards the formation of ordered aggregates of the dyad in this one-component solvent medium. Furthermore, the absorption spectral characteristics, indicative of the aggregation of the dyad in binary THF-water mixtures and the water extractions, were well established by us in our recent study.33 The role of solvent polarity toward the structural organization/self-assembly was further explored quantitatively, and a critical dielectric constant of g30 is required for the dyad to aggregate in the polar solvent mixtures; these data are summarized in Table 1. The water-extracted colloidal solutions were stable for several weeks. The absorption peaks of the dyad were broad and redshifted from 259 to 273 nm and from 328 to 344 nm with a decrease in intensity and featureless absorption in the visible region. However, the absorption edge analysis revealed the absorption of the dyad in water to start at 1.65 eV, which is equivalent to the absorption of the pristine dyad (1.6 eV), implying the formation of hydrated crystals instead of clathrate crystals in water.33 Furthermore, ONIOM (MP2:PM3) model calculations34 have shown the formation of C60-water complexes from the charge-transfer interaction between the oxygen lone pair of water and the π* orbitals of C60, with the C60 hexagon centeroxygen distance ranging from 3.19 to 3.09 Å. Emission Spectra of the Dyad Aggregates: Fluorescence Quenching. Figure 2 illustrates the emission spectral features (37) Feng, J.; Mack, S.; Shan, G.; Gee, S.; Hammock, B. D.; Kennedy, I. M. Proc. SPIE 2003, 4967, 156. (38) Mohan, H.; Iyer, R. M. Faraday Trans. 1992, 88, 41. (39) Chen, L. Shen, H.; Eisenberg, A. J. Phys. Chem. B 1999, 103, 9488. (40) Vinson, P. K.; Talmon, Y.; Walter, A. Biophys. J. 1989, 56, 669. (41) Laughlin, R. G. Colloids Surf., A 1997, 128, 27.
Figure 2. Emission spectra of the dyad in BZN-ACN mixtures in the range of 10-100% (v/v) THF. (Inset) Expanded visible region with increased/broadened emission indicating the presence of aggregates.
of the dyad with decreasing percentage of BZN in BZN-ACN binary solvent mixtures. As can be visualized, the spectrum reveals three emission peaks at 479, 498, and 704 nm. With decreasing percentage of BZN, a progressive loss of fluorescence intensity of the peaks at 479 and 498 nm is seen along with an increase in intensity, broadening, and a red shift of the 704 nm band to 713 nm. Similar results were obtained with the other binary solvent mixtures used in this study. The above results corroborate well with the UV-vis data and with emission spectral results of the dyad aggregates.35 Fluorescence quenching has been widely used as a method of detection in biological assays where the fluorescence resonance energy transfer (FRET) plays a very important role in biological analysis.36 Water-soluble fullerene clusters display advantages over normal fluorescent quenchers and FRET systems in biological applications with low intrinsic fluorescent emission and the ability to quench a very wide range of fluorescent dyes.37 Figure 3a shows the effect of water-extracted dyad aggregates on rhodamine B fluorescence with an excitation wavelength of 531 nm and emission at 575 nm. The reduction in emission intensity is due to fluorescence quenching, and the data can be plotted in Stern-Volmer form according to eq 1 as
Io ) 1 + KSV[Q]... I
(1)
where Io and I are the fluorescence intensities in the presence and absence of the quencher [Q], respectively. KSV is the SternVolmer quenching constant, which can be given as KSV ) kqτo where kq is the rate of quenching and τo is the lifetime of the fluorophore in the absence of a quencher.37 The SV plot is shown in Figure 3b, from which the SV quenching constant was found to be 1.85 × 105 M-1 against a value of 1.98 × 105 M-1 for C60 clusters in water.37 The lifetime (τo) of a rhodamine molecule38 is approximately 4 × 10-9 s, and thus the rate of quenching (kq) is estimated to be 4.6 × 1013 M-1s-1. It is well established that the dyad clusters are negatively charged,33 and hence it is very likely that the charge on the clusters promotes interactions over a sphere of action that is effectively greater than the physical size of the cluster.37 Thus the electrostatic interactions may account for the highly effective nature of the quenching by the dyad clusters. Dynamic Light Scattering: Estimation of Aggregate Size. The Brownian movement of a light-scattering particle gives rise
Aggregation of a C60-Based Dyad
Langmuir, Vol. 23, No. 9, 2007 4803
Figure 3. (a) Quenching of the rhodamine B (0.01 mM) fluorescence by water-extracted dyad aggregates. (b) Stern-Volmer quenching plot leading to KSV ) 1.85 × 105 M-1. Table 2. Characteristics of the Water-Extracted Dyad Aggregates from THF Solution conc of the dyad extracted in water (mM)
diffusion coefficient, D (cm2s-1)
hydrodynamic radius, rh (nm)
aggregate sizes from TEM (nm)
0.01 0.04 0.2 C60 (0.08)20
4.2 × 10-8 2.7 × 10-8 1.9 × 10-8 2.2 × 10-6
58 88 127 45
60 82 120
to temporal fluctuations in the scattered light, as measured by dynamic light scattering. This temporal variation of the scattered radiation yields the Doppler shift, and the broadening of the central Rayleigh line is used to determine the dynamic properties of the system.11 The intensity of the scattered light in the present experiments was analyzed by photon correlation spectroscopy (PCS), wherein the normalized autocorrelation intensity function was achieved by the stretched exponential method and the size distributions have been well established according to our previous studies.33 The size distributions are summarized in Table 2 and Figure 4, corroborating well with the TEM measurements. Transmission Electron Microscopy (TEM) of the Dyad Aggregates. TEM images formed upon 10-90% water addition to the dyad in THF as described in Materials and Methods are shown in Figures 5 and 6. No geometrical/self-assembled structures were seen with a water content varying between 1030%, indicating that the system is only a molecular solution at this stage. The image in Figure 5ii reveals the self-assembly of the dyad molecule into spherical vesicles at 50% water content, below which micrometer-sized rod/sheetlike aggregates existed (cf. Figure 5i). A rod-to-vesicle transition has been reported to be associated with a relaxation time of 300-5000 s in polystyrene310-b-poly(acrylicacid)52 diblock copolymers in dioxane/water mixtures.39 At larger water contents (Figure 6), such as 60 and 90%, the vesicles seem to fuse with each other, where the initial interaction is assumed to be the contact and adhesion of two vesicles, followed by coalescence leading to varied structures. Closed spherical structures are not the only particle geometries that are assumed by vesicle-forming surfactants. Nonspherical closed structures, 3D open structures, and flat sheets have also been reported ,suggesting that the vesicular structure is not a fundamental property of the vesicle-forming molecules.40,41 In the present work, the TEM images at higher water contents fall into this category. Vesicle enlargements in response to water addition have been reported to be associated with a relaxation time of ∼10-50 s.42 Thus, the phase behavior in selfassembly is influenced not only by the molecular structure/
geometry but also by the magnitudes of the system’s variables, such as the solvent composition. Furthermore, these vesicles serve as precursors toward the formation of water-extracted spherical dyad aggregates as shown in the TEM images (cf. Figure 7) under highly polar conditions. The selected-area diffraction patterns taken from these aggregates were indexed to a simple cubic lattice, and the parameters are tabulated in Table 3 and are shown in Figures 5 and 6. The water-extracted aggregates showed a spherical structure at 0.01 mM concentration, which on increasing the concentration to 1 mM formed giant fractals as shown in Figure 7. Mechanism of Bilayer Vesicle Growth. Amphiphilic molecules with a hydrophobic part and a hydrophilic head group assemble in aqueous solution into a variety of morphologically different structures.43 External stimuli such as the concentration, pH, temperature, solvent types, and solvent composition can control the sizes and shape of the aggregates. The molecular organization/self-assembly depends upon a number of internal factors such as the structure of the molecule, competing intramolecular forces, the flexibility and length of the hydrocarbon chains, and the intermolecular forces. The relative magnitudes of the attractive hydrophobic forces between the tails, the repulsive electrostatic forces between the head groups, and the hydration effects also influence the aggregate structure and stability,44 leading to a variety of aggregate types. Vesicles are hollow, lamellar spherical/ellipsoidal structures, the dimensions of which range from nanometers to hundreds of micrometers, and their types vary depending upon the chemical constitution and size of the molecule, preparation method, and environmental factors. Owing to their structural specificity, vesicles are fascinating systems for both theoretical and experimental studies.45 The actual form assumed by an aggregate depends largely on the molecular constitution of the amphiphile and is explained by geometric considerations. In a first-order approximation, the geometry of an amphiphile is described by the critical packing parameter46 p, defined as the ratio of the hydrophobic volume (υs) to the product of the head group area (as) and the chain length (ls) as p ) υs/asls. The critical packing (42) Choucair, A. A.; Kycia, A. H.; Eisenberg, A. Langmuir 2003, 19, 1001. (43) Laughlin, R. G. The Aqueous Phase BehaVior of Surfactants; Academic Press: London, 1994. (44) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley-Interscience: New York, 1980. (45) Kita-Tokarczyk, K.; Grumelard, J.; Haefele, T.; Meier, W. Polymer 2005, 46, 3540. (46) Pashley, R. M.; Karaman, M. E. Applied Colloid and Surface Chemistry; John Wiley and Sons: Hoboken, NJ, 2004.
4804 Langmuir, Vol. 23, No. 9, 2007
Gayathri and Patnaik
Figure 4. Size distributions from DLS of the dyad aggregates in (a) 0.01 and (b) 0.2 mM water extractions and (c) 10:90 and (d) 20:80 THF-water binary solvent mixtures.
Figure 5. Transmission electron micrographs of the dyad aggregates in THF-water binary solvent mixtures and their corresponding selected-area diffraction patterns. (i) THF/water 60:40 solvent mixture and (ii) THF/water 50:50 solvent mixture; the inset illustrates the vesicle size distribution with a mean radius of 45 nm.
parameter determines the preferred curvature of the aggregates formed.46 Fullerene[60]-based surfactants with a rigid hydrophobic sphere and flexible hydrocarbon tails have intrinsic geometric constraints leading to the formation of vesicular membranes. The attachment of a single hydrophilic addend to functionalized fullerenes promoted water solubility, but controlling the aggregated structure required larger hydrophobicity. Because of the larger size of the C60 fullerene moiety compared to that of the single-chain tail, the intermolecular packing obtained was poor.2,16 Subsequently, multichain frameworks10,14,24,47 are better suited to self-organization and packing with stronger hydrophobicity and steric repulsion. The presence of hydrophilic ester groups along with a calculated dipole ground-state moment of 8.9 D indicates that the molecule is sufficiently polar. Thus, the dyad molecule with its dimensions as shown in Figure 6 has a critical packing (47) Zhang, P.; Li, J.; Liu, D.; Qin, Y.; Guo, Z.; Zhu, D. Langmuir 2004, 20, 1466.
Figure 6. Transmission electron micrographs of the dyad aggregates in THF-water binary solvent mixtures and their corresponding selected-area diffraction patterns. (i) THF/water 40:60 solvent mixture and (ii) THF/water 10:90 solvent mixture.
parameter of ∼0.52 and forms a truncated cone, which upon self-assembly has given rise to spherical vesicles. The vesicles are likely to be made of bilayers with a C60 fullerene head-tohead conformation. By appropriate substitution, C60 fullerenes have been transformed into water-soluble stabilized anions, giving rise to large aggregated structures. A laser light scattering study of the association behavior of the potassium salt of pentaphenyl and pentamethyl fullerenes (Ph5C60K, Me5C60K) in water revealed that their hydrocarbon anions associate into bilayers, forming stable spherical vesicles with an average hydrodynamic radius of 17 nm for Ph5C60- and 26.8 nm for Me5C60-. 22 An amphiphilic fullerene C60 derivative with two ammonium headgroups upon self-organization in water formed spherical vesicles ranging from nano- to micrometers.23 It has been well established that bilayers can exist in several equilibrium phases such as a gel phase, ripple phase, and fluid phase.48 In particular, ripple phases have been found to exist in phospholipids where bilayer-bilayer interactions have played
Aggregation of a C60-Based Dyad
Langmuir, Vol. 23, No. 9, 2007 4805
Figure 8. Dyad molecule with a critical packing parameter of ∼0.52 forming a truncated cone. The molecular dimensions were obtained from the CPK model upon geometry optimization.
Figure 7. Transmission electron micrographs of the water-extracted dyad aggregates and their corresponding selected-area diffraction patterns: (a) extraction from 0.1 mM THF solution and (b) extraction from 1 mM THF solution. Table 3. Unit Cell Parameters Obtained from the Selected Area Diffraction Patterns of the Dyad Aggregates in Binary Solvent Mixtures and in Water Extractionsa sample C60 (JCPDS) dyad aggregates in a 60:40 THF/water solution C60 (JCPDS) dyad aggregates in a 50:50 THF/water solution C60 (JCPDS) dyad aggregates in a 40:60 THF/water solution C60 (JCPDS) dyad aggregates in a 10:90 THF/water solution C60 (JCPDS) water-extracted dyad aggregates (0.01 mM)
d (Å)
hkl
d (Å)
1.7551 (800) 1.1952 1.7600 (800) 1.1894
hkl
d (Å)
hkl
(875) (875)
1.3150 (864) 1.0924 (10 8 2) 1.1952 (875) 1.3188 (864) 1.0960 (864) 1.1936 (875) 2.0266 (444) 1.7551 2.0315 (444) 1.7565
(800) 1.2362 (881) (800) 1.2376 (881)
1.9856 (345) 1.9913 (345) 3.3094 (411) 1.7283 3.3120 (411) 1.7338
(741) (741)
a
Comparison with JCPDS data for the simple cubic C60 (JCPDS no. 79-1715) system is included.
an important role in ripple formation.49,50 Ripple phases in a bilayer can arise from the interplay of a variety of factors: (i) bilayer-bilayer interaction,49 (ii) bilayer-substrate interaction,51 and (iii) electrostatic coupling between water dipoles and the dipolar head groups of the amphiphile.52 In the present study, we observed the formation of a ripple phase as seen in Figure (48) Alaouie, A. M.; Smirnov, A. I. Langmuir 2006, 22, 5563. (49) Fang, Y.; Yang, J. J. Phys. Chem. 1996, 100, 15614. (50) Cevc, G.; Zeks, B.; Podgornik, R. Chem. Phys. Lett. 1981, 84, 209. (51) Mou, J.; Yang, J.; Shao, Z. Biochemistry 1995, 33, 4459. (52) Doniach, S. J. Chem. Phys. 1979, 70, 4587.
Figure 9. Transmission electron micrographs of the dyad aggregates in a 50:50 THF/water binary solvent mixture. (a) High-resolution TEM. (b) Embossed view of one of the vesicles showing a bilayer ripple pattern as indicated by arrows. (c) Polarizing optical microscopic image of the donor addend (3,4-di(dodecyloxy)benzoic acid-4-hydroxy-phenyl ester) at its melting transition temperature.
9b as a consequence of in-plane bilayer packing. However, the ripple structure need not be a true thermal equilibrium state of a bilayer at room temperature.49 The existence of the ripple phase generally depends on the hydration level. Thus, it can be seen that these ripples are formed only for a water content of 50% in binary water mixtures where these ripples not being a stable phase leads to the bending of the bilayer to form vesicles as predicted by the packing parameter values discussed above. Recent studies on dimyristoylphosphatidylcholime (DMPC) indicate that ripple phases disappear at a humidity value of less than 93%48 (i.e., the bilayer ripple phase disappeared with increasing dimethylsulfoxide (DMSO) solvent content53). As pointed out by one of the reviewers of this article, we were (53) Tristran-Nagle, S.; Moore, T.; Petrache, H. I.; Nagle, J. F. Biochim. Biophys. Acta 1998, 1369, 19.
4806 Langmuir, Vol. 23, No. 9, 2007
unable to conclude whether the hydrocarbon chains were in the crystalline state. However, DSC studies on individual donor addend 3,4-di(dodecyloxy)benzoic acid-4-hydroxy-phenyl ester31 indicated a melting temperature of 97 °C and the formation of fanlike structures under a polarizing microscope at this temperature as seen in Figure 9c. Therefore, we expected the dyad to be birefringent. In contrast to the observation on the donor addend, the dyad melted at around 101 °C, leading to a very viscous liquid state as observed by optical polarizing microscopy. Thus, the dyad did not show any typical biefringent texture, which made it difficult to conclude the nonexistence of a mesophase in this system. Such a situation has also been encountered with other fullerene derivatives.54,55 However, the diffraction patterns revealed by the aggregates clearly indicate the presence of crystallinity in the system. Furthermore, Carson and Sethna56 have shown through their calculations that mesomorphic substances are capable of forming bilayer ripples. Several other theoretical models have also been put forward to understand the mechanism of ripple formation.52,57 With the above experimental evidence, we conclude that a bilayer has been formed in the present study, which further bends to a membrane with in-plane bilayer packing depending on various other factors as discussed below. The stability of a bilayer membrane upon spherical deformation, in particular whether and why a curved bilayer will be favored over a flat one, has been concluded from the free-energy calculations for diblock copolymer systems. Whenever the composition of the diblocks is sufficiently asymmetric with longer hydrophobic blocks, the constituent monolayers will have a strong tendency to curve away from the aqueous phase.45 Thus in the present case, the dyad with a nonpolar-polar-nonpolar network leading to asymmetry results in flat bilayer instability with respect to spherical deformation; spherical vesicles become favored over the flat bilayers that curve away from the aqueous phase in the THF-water mixture to form a spherical vesicle as seen in Figure 9a,b. According to Kita-Tokarczyk et al., the lower free energy of vesicles as compared to that of flat bilayers is explained as follows: when each monolayer has a tendency to curve away from the solvent, this new geometry decreases the free energy of the outer layer, whereas the free energy of the inner membrane increases. When the number of molecules per unit area decreases for the inner leaflet, the inner membrane free-energy increase can be partially diminished. Moreover, because there are more molecules in the outer monolayer, the free-energy decrease in the outer shell compensates for the free-energy increase in the inner layer to a greater extent.45 A few other interaction types such as electrostatic interactions, hydrogen bonding, and donoracceptor interactions have also been found to facilitate vesicle formation. For example, the above-mentioned interactions led to vesicle formation in58 poly(1,2-butadiene)216-block-poly(cesium methacrylate)29 and polystyrene211-blockpoly(1-methyl4-vinylpyridinium iodide)33. Furthermore, from the fluorescence quenching experiments, a predominant electrostatic interaction in the dyad aggregates has been established. Scheme 1 illustrates the various stages in the formation of vesiclular membrane with in-plane bilayer packing upon selfassembly of the dyad in a 50:50 (v/v) THF-water mixture. Each monolayer in the planar bilayer ribbon is characterized by (54) Ravaine, S.; Faye, V.; Nguyen, H. T.; Delhaes, P. J. Phys. Chem. Solids 1997, 11, 1753. (55) Chuard, T.; Deschenaux, R. HelV. Chim. Acta 1996, 79, 736. (56) Carlson, I. M.; Sethna, I. P. Phys. ReV. A 1987, 36, 3359. (57) Kranenburg, M.; Smit, M. J. Phys. Chem. B 2005, 109, 6553. (58) Schrage, S.; Sigel, R.; Schlaad, H. Macromolecules 2003, 36, 1417.
Gayathri and Patnaik Scheme 1. Schematic Representation of the Truncated Cone-Shaped Dyad Molecule Forming a Spherical Bilayer Vesicle (A) According to the Predominant Hydrophobic Effects with a Bending Elastic Modulus of K ) 18 kBT and (B) a Probable Electrostatic Contribution from the Ground-State Charge-Separated Dδ+-B-Aδ- Dyad
spontaneous curvature that, in general, is nonzero. The sign of the spontaneous curvature and its magnitude are governed by the balance of the moments associated with the lateral forces acting in the monolayer. These are the steric and/or electrostatic repulsive forces between the C60-C60 heads, the CH2-CH2 chain hydrophobic attraction, and the attractive C60-C60 hydrophobic/ van der Waals/π-π interactions and C60-(CH2) chain van der Waals interactions. In view of the ground-state charge-separated Dδ+-B-Aδ- dyad,31 the curvatures of the two identical monolayers comprising a bilayer as shown in Scheme 1B are opposite in sign and equal in magnitude. Thus, the formation of the bilayer vesicle involves a minimum free energy associated with this difference and is met from the mechanical energy supplied during the solvent mixing. Spontaneous vesicle formation has been experimentally observed in several systems. Mohanty and Dey59 reported aggregation leading to vesicle formation in sodium N-(4-dodecyloxybenzoyl)-L-valinate in water. Kaler et al. have reported vesicle formation in a mixture of surfactants with oppositely charged head groups but similar hydrocarbon tails.60 The sheetlike aggregates are formed in solution as shown in Figure 5i, grow in size, and lose energy owing to surface tension, causing aggregate closure into the vesicular form with a bending energy Ebend. The vesicles are thus stabilized by one of the two distinct mechanisms61 depending on the bending constant, κ. (i) When κ ≈ kBT, the interbilayer potential as a result of bilayer (59) Mohanty, A.; Dey, J. Langmuir 2004, 20, 8452. (60) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. Science 1989, 245, 1371.
Aggregation of a C60-Based Dyad
Langmuir, Vol. 23, No. 9, 2007 4807
fluctuations becomes repulsive. In this case, the repulsive bilayer potential exceeds the van der Waals attraction between the bilayers, resulting in unilamellar vesicles.62 (ii) When κ . kBT, the vesicles are stabilized by spontaneous curvature that picks out a particular vesicle radius, whereas other radii are disfavored energetically.61 For a bilayer membrane bent in vertical and horizontal directions, two different radii R1 and R2 arise, giving rise to a bending energy (Ebend) according to Israelachvili63 in eq 2 of the form
(
)
Ebend κ 1 1 2 ) + ... A 2 R1 R2
(2)
where the mean curvature is equal to (1/R1 + 1/R2) and A is the area of the vesicle given by 4πR2. In the case of spherical vesicle formation from bilayer bending, R1 ) R2 ) R. Thus, the above equation yields Ebend ) 8πκ, where the bending energy of the spherical vesicle is independent of its radius. In the present work, first the bending constant was estimated from the experimentally accessible values of the mean vesicle radius, R, and the standard deviation, σ, of the vesicle radius using Helfrich notation64 as in eq 3 below.
κ)
kBT R 2 ... 16π σ
()
Figure 10. Space-filling model of a 90-molecule dyad cluster tending toward spherical geometry.
(3)
A mean vesicle radius of 45 nm was deduced from the distribution function shown in the inset of Figure 5ii with a standard deviation of 1.5 nm. Accordingly, a calculated bending elasticity of 7.36 × 10-20 J ≈ 18 kBT was achieved, implying a much larger bending rigidity of the dyad vesicle as compared to fluid lipid bilayers.63 In compliance with κ, a large bending energy of ∼450 kBT was estimated, attributing stability to the spontaneously formed vesicle. These vesicular aggregates with an increasing percentage of water coalesce and tend to spherical geometry when extracted completely in a polar water medium as shown in Figures 6 and 7a. These spherical aggregates, with increasing concentration of the dyad, undergo fractal formation (Figure 7b) under the purview of diffusion-limited cluster aggregation with a fractal dimension of df ) 1.89.33 The molecular dynamics simulation of these water-extracted dyad aggregates showed an initial π-π interaction between C60 and the bridging benzene to be predominant with negligible C60-C60 van der Waals interactions.65 However, with an increasing number of dyad molecules tending to cluster, the hydrophobic C60-C60 and C60-(CH2)/chain interactions became prevalent, and the computed energies of the clusters signified the lowest-energy configurations for aggregation numbers 15, 41, and 90, with the latter tending toward a spherical cluster as shown in Figure 10. These calculations corroborated well with our recent proposition of an octadecahedral model.65 Redox Behavior of the Dyad Aggregates. Figure 11 illustrates the redox behavior of the spin-coated water-extracted dyad aggregates from a 0.01 mM THF solution on an indium tin oxide (ITO) electrode with 0.5 cm2 surface area. Potential cycling caused the irreversible appearance of C601- and C602- ions within the potential window of the ITO electrode at half-wave potentials (61) Jung, H. T.; Coldren, B.; Zasadzinski, J. A.; Iampietro, D. J.; Kaler, E. W. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 1353. (62) Herve, P.; Roux, D.; Bellocq, A. M.; Nallet, F.; Gullik-Krzywicki, T. J. Phys. II 1993, 3, 1255. (63) Israelachvili, J. N. Intermolecular and Surface Forces: With Applications to Colloidal and Biological Systems, 2nd ed.; Academic Press: New York, 1992. (64) Denkov, N. D.; Yoshimura, H.; Kouyama, T.; Walz, J.; Nagayama, K. Biophys. J. 1998, 74, 1409. (65) Gayathri, S. S.; Patnaik, A. J. Chem. Phys. 2006, 124, 131104.
Figure 11. Electrochemical behavior of the spun-cast film of the dyad aggregates on an ITO electrode in ACN. CE, Pt; RE, Ag/Ag+; and SE, tetrabutyl ammonium hexafluorophosphate (0.1 M). The inset shows the reversible waves of the C61 acceptor on a GC electrode.
of -0.83 and -1.19 V. The peaks are seen to be anodically shifted with respect to the C61 acceptor at -1.03 and -1.48 V, respectively. Similar redox behavior was evidenced in all of the concentrations of water-extracted dyad aggregates. Literature reports on the redox properties of aggregates of C60 and its derivatives are scarce. However, the cyclic voltammetric behavior of a supramolecular host-guest complex, C60-γ-CD in water, showed one-electron reduction waves corresponding to -0.62 and -1.03 V with the second reduction being irreversible. Such electrochemical behavior was speculated to have arisen from a chemical reaction.66 In corroboration with the above results, EPR spectroscopy confirmed the presence of the C611- radical anion at g ) 1.986 with a line width of 0.5 mT.65 The irreversible nature of the cyclic voltammogram implies the probable rectification behavior of the dyad aggregate.
Conclusions The electronic absorption spectral features of the nonpolarpolar-nonpolar dyad revealed the molecule to aggregate in binary solvent mixtures with a critical dielectric constant of ∼30. The molecular structure, density, solvent polarity, and mode of preparation strictly governed the aggregate geometry and size; from a truncated cone-shaped unit structure with a packing parameter of ∼0.52, geometrical packing dictated the bilayer formation and experimentally confimed the formation of the ripple phase. Along with the predominant hydrophobic attractive (66) Boulas, P.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Phys. Chem. 1994, 98, 1282.
4808 Langmuir, Vol. 23, No. 9, 2007
interactions and electrostatics, a model is proposed for the formation and bending of the planar bilayers toward the formation of spherical bilayer vesicles in a sufficiently polar medium. The large magnitude of the bilayer bending energy of ∼450kBT signified the rigidity of the vesicle membrane. On further extraction into water, these aggregates formed spheres, which corroborated well with the molecular dynamics calculations and the octadecahedral model and acted as efficient quenchers of visible dye, rhodamine B, with a Stern Volmer quenching constant of 1.85 × 105 M-1. The water-dispersed spherical aggregates at
Gayathri and Patnaik
a larger dyad concentration were identified to be fractals with a dimension of 1.89, following diffusion-limited cluster growth. The irreversible nature of the cyclic voltammogram entitles the aggregates to be probable rectifiers. Acknowledgment. This work was supported by the Department of Science and Technology (DST), Government of India, under grant number SP/SI/H-37/2001. LA0626961
