Structure and Dynamics in Solvent-Polarity-Induced Aggregates from

Langmuir 2005, 21, 12139-12145

12139

Structure and Dynamics in Solvent-Polarity-Induced Aggregates from a C60 Fullerene-Based Dyad S. Shankara Gayathri,† Amit K. Agarwal,‡ K. A. Suresh,‡ and Archita Patnaik*,† Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India, and Liquid Crystals Group, Raman Research Institute, Bangalore 560080, India Received August 24, 2005. In Final Form: September 23, 2005 A novel methanofullerene dyad based on a hydrophobic (acceptor C60 moiety)-hydrophilic (bridge with benzene and ester functionalities)-hydrophobic (donor didodecyloxybenzene) network is designed and synthesized. Electronic absorption spectral features revealed the molecule to exhibit a strong tendency to self-aggregate in binary solvent mixtures at room temperature, where the dielectric constant exceeds a critical value, ∼30. The dynamic structure factors of these spherical aggregates revealed stretched exponential decay with sizes varying between 110 and 250 nm with an increasing concentration, estimated from the dynamic light scattering experiments. However, a loss of shape selectivity of these aggregates was noted at lower water volume fractions in the binary solvent mixtures. The water-extracted spherical clusters were identified to be fractals with a dimension of 1.85, leading to diffusion-limited cluster aggregation as the mechanistic route for clusterization.

Introduction A promising route to materials synthesis is the formation and self-assembly of colloidal particles, which gives the opportunity to create highly ordered structures on length scales from nano- to micrometers.1-6 In this context, Fullerene (C60) has attracted a great interest not only for its promising applications but also for its unusual molecular structure and truncated icosahedral symmetry.7-9 Aggregation of C60 has caused significant changes in its photochemical and photophysical properties, as compared to isolated molecules in solution;10-12 one such example played a crucial role in the preparation of photovoltaic cells.13 Thus, controlling the aggregation of C60 in solution is of great importance for the development of functional materials.8,14 It has been shown that there is no single solvent parameter that predicts in general the solubility of C60 and that it usually dissolves in a solvent that has a large * To whom correspondence should be addressed. Telephone: +9144-2257-4217. Fax: +91-44-2257-4202. E-mail: archita59@ yahoo.com. † Indian Institute of Technology Madras. ‡ Raman Research Institute. (1) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Langmuir 2003, 19, 4798-4803. (2) Zhang, P.; Li, J.; Liu, D.; Qin, Y.; Guo, Z.; Zhu, D. Langmuir 2004, 20, 1466-1472. (3) Alargova, R. G.; Deguchi, S.; Tsuji, K. J. Am. Chem. Soc. 2001, 123, 10460-10467. (4) Deguchi, S.; Alargova, R. G.; Tsuji, K. Langmuir 2001, 17, 60136017. (5) Angelini, G.; De Maria, P.; Fontana, A.; Pierini, M.; Maggini, M.; Gasparrini, F.; Zappia, G. Langmuir 2001, 17, 6404-6407. (6) Martin, N.; Guldi, D. M. J. Mater. Chem. 2002, 12, 1978-1992. (7) Taylor, R.; Walton, D. R. M. Nature 1993, 363, 685-693. (8) Prato, M. J. Mater. Chem. 1997, 7, 1097-1109. (9) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (10) Mohan, H.; Palit, D. K.; Mittal, J. P.; Chiang, L. Y.; Asmus, K.; Guldi, D. M. Faraday Trans. 1998, 94, 359-363. (11) Yevlampieva, N. P.; Biryulin, Y. F.; Melenevskaja, E. Y.; Zgonnik, V. N.; Rjumtsev, E. I. Colloids Surf., A 2002, 209, 167-171. (12) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695-703. (13) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15-26. (14) Prato, M. Top. Curr. Chem. 1999, 199, 173-188.

refractive index, a dielectric constant around 3-4, a large molecular volume, and a tendency to act as a moderate nucleophile.9,15 C60 aggregates have been extensively characterized in neat polar solvents and binary solvent mixtures, and the aggregation process has been largely determined by the polarity of the medium.16 It is however difficult to control its structure because of the tendency of random aggregation in these solvents.15 Thus, alternative routes have been adopted to realize the aggregation of C60. Clusterization of C60 has been observed by incorporating them into heterogeneous media such as micelles,17 liposomes,18 and vesicles.19 Modification of the fullerene to be amphiphilic is one of the steps toward controlled aggregation through self-organization. Further, interesting biological activities of water-soluble fullerene derivatives have been discovered with an increasing interest in the preparation of aqueous solutions of fullerenes.17,20-24 Several groups have reported the aggregation properties of the water-soluble C60 derivatives, such as poly(ethylene oxide) (PEO),1 acetyl carnitine,5 bola amphiphiles,25 etc.2,21,26,27 Recently, our group has reported details of the aggregate structure and mechanism of (15) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993, 97, 3379-3383. (16) Nath, S.; Pal, H.; Palit, D. K.; Sapre, A. V.; Mittal, J. P. J. Phys. Chem. B 1998, 102, 10158-10164. (17) Hungerbuhler, H.; Guldi, D. M.; Asmus, K. J. Am. Chem. Soc. 1993, 115, 3386-3387. (18) Bensasson, R. V.; Bienvenue, E.; Dellinger, M.; Seta, S. J. J. Phys. Chem. 1994, 98, 3492-3500. (19) Janot, J. M.; Bienvenue, E.; Seta, P.; Bensasson, R. V.; Tome, A. C.; Enes, R. F.; Cavaleiro, J. A. S.; Camps, S. X.; Hirsch, A. Perkin Trans. 2000, 22, 301-306. (20) Isobe, H.; Nakamura, E. Acc. Chem. Res. 2003, 36, 807-815. (21) Samal, S.; Choi, B.; Geckeler, K. E. Chem. Commun. 2000, 13731374. (22) Da Ros, T.; Prato, M. Chem. Commun. 1999, 663-669. (23) Dugan, L. L.; Turesky, D. M.; Du, C.; Lobner, D.; Wheeler, M.; Almli, C. R.; Shen, C. K. F.; Luh, T.; Choi, D. W.; Lin, T. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 9434-9439. (24) Sayes, C. M.; Fortner, J. D.; Guo, W.; Lyon, D.; Boyd, A. M.; Ausman, K. D.; Tao, Y. J.; Sitharaman, B.; Wilson, L. J.; Hughes, J. B.; West, J. L.; Colvin, V. L. Nanolett. 2004, 4, 1881-1887. (25) Sano, M.; Oishi, K.; Ishi-i, T.; Shinkai, S. Langmuir 2000, 16, 3773-3776. (26) Tan, C. T.; Ravi, P.; Dai, S.; Tam, K. C.; Gan, L. H. Langmuir 2004, 20, 9882-9884.

10.1021/la052313j CCC: $30.25 © 2005 American Chemical Society Published on Web 11/03/2005

12140

Langmuir, Vol. 21, No. 26, 2005

Gayathri et al. Scheme 1a

a (a) C12H25Br, K2CO3, 18-crown-6, butanone, 24 h, reflux. (b) NaOH, H2O/C2H5OH (1:4), 8 h, reflux. (c) SOCl2, 3 h, reflux; hydroquinone, CH2Cl2, pyridine, 6 h, reflux. (d) DCC, o-DCB, 12 h, 0-25 °C.

formation from pristine C60 in different solvent media.28-30 Thus, a good understanding on the aggregation properties of pristine C60, its intermolecular interactions, and thermodynamic conditions necessary for successful assembly of an amphiphilic molecule led us to design a C60based dyad with a hydrophobic-hydrophilic-hydrophobic network. The work reported in this paper is aimed at studying the formation and stability of colloidal dispersions of this dyad in binary solvent mixtures by increasing the composition of polar organic solvents that are poor solvents for this molecule. A combination of the solvents is intended to increase the polarity of the medium. Further, extracting the dyad molecules into the more polar solvent water, by evaporation of the solvent under nitrogen purging, extends the above study. These aggregates and their stability were studied through UV-vis absorption spectroscopy, and the hydrodynamic radii and diffusion coefficients were obtained from dynamic light-scattering experiments. Experimental Procedures Materials. The C60-based dyad was synthesized as previously reported, according to Scheme 1.31 The solvents tetrahydrofuran (THF), acetonitrile (ACN), dichloromethane (DCM), and toluene (27) Georgakilas, V.; Pellarini, F.; Prato, M.; Guldi, D. M.; MelleFranco, M.; Zerbetto, F. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 50755080. (28) Bokare, A.; Patnaik, A. J. Phys. Chem. B 2005, 109, 87-92. (29) Bokare, A.; Patnaik, A. J. Chem. Phys. 2003, 119, 4529-4538. (30) Bokare, A.; Patnaik, A. J. Phys. Chem. B 2003, 107, 6079-6086. (31) Gayathri, S. S.; Patnaik, A. Chem. Phys. Lett. 2005, 414, 198.

were HPLC-grade, obtained from Merck India Ltd., and used without further purification. The ultrapure water produced by a Millipore Elix A3-MilliQ system (MilliQ, Germany) was used in the experiments. Preparation of Dyad Aggregates. Two pathways were adopted to study the aggregation behavior of the dyad. (1) In Binary Solvent Mixtures. A THF solution of the dyad of 0.01 mM concentration was prepared. To this water was added as various volume fractions from 10 to 100%. These solutions were used for UV-vis and dynamic light-scattering measurements. Similar concentrations were used for the THF-ACN, DCM-ACN, and solvent mixtures. (2) In a Water-Extracted Phase. A 1 mL THF solution of the dyad was injected into deionized water (5 mL) in a wide-mouthed standard flask with vigorous stirring. High pure nitrogen (99.99%) was continuously purged through the system during the process to remove THF. Immersing the system in a water bath compensated the evaporation-induced cooling during extraction, and the cycle was repeated 5 times. The dyad aggregates were formed after the complete removal of THF. Finally, water was added up to the original volume to compensate for the evaporated water during the nitrogen purge. 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 of the Stable Colloids. The electronic absorption spectra were recorded on a Varian Cary 5E double-beam spectrophotometer using a 1 cm pathlength 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. Dynamic Light Scattering (DLS) from the Stable Colloids. The particle dimensions were measured from the DLS experiments. Measurements were done on a Malvern 4700C, U.K., equipped with an Ar+ laser operating at 25 mW power and

Aggregates from a C60 Fullerene-Based Dyad

Langmuir, Vol. 21, No. 26, 2005 12141

Figure 1. UV-vis absorption spectra of the dyad in THF. (Inset) Beer-Lambert plot for the absorbance versus the dyad concentration in THF at 431 and 495 nm. 488 nm wavelength. The output signals were fed into a 256 channel digital correlator, which facilitated in computing the intensity-intensity autocorrelation of the scattered light. 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 done in a special dust-free lightscattering cell. About 2 mL of the dyad solution was transferred into the cell for the experiment, and the temperature of the solution was maintained at 25 °C. Transmission Electron Microscopy. Transmission electron microscopic (TEM) imaging was performed on carbon-coated copper grids using a Philips CM12 transmission electron microscope, equipped with a 100 kV electron gun.

Results and Discussion Effect of Solvent Polarity on Dyad Aggregates. The dyad with a hydrophobic (acceptor C60 moiety)hydrophilic (bridge with benzene and ester functionalities)-hydrophobic (donor didodecyloxybenzene) structure was found to be readily soluble in a wide range of solvents: 31 cyclohexane, chlorinated organic solvents (o-dichlorobenzene, chlorobenzene, dichloromethane, carbontetrachloride, and chloroform), polar cyclic organic solvents (tetrahydrofuran and dioxan), aromatic solvents (benzene, toluene, and benzonitrile), ethyl acetate, and carbon disulfide up to ∼7 mg/mL and was sparingly soluble in ethanol, dimethylsulfoxide, etc. The UV-vis spectra of the dyad in organic solvents of varying polarity showed absorption maxima at 259, 328, and 431 nm,31 in agreement with the reported32,33 spectroscopic characteristics of monomeric monofunctionalized fullerenes. Figure 1 illustrates the absorption characteristics of the dyad in THF over a concentration range of 10-3 to 1 mM. The Beer-Lambert (BL) linearity in the inset over the given concentration range implies the absence of a probable molecular association in the single-component THF solvent. 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. To explore the role of solvent polarity34 toward the structure aggregation/self-assembly, the dielectric con(32) Bensasson, R. V.; Bienvenne, E.; Dellinger, M.; Leach,; Seta, P. J. Phys. Chem. 1994, 98, 3492-3500. (33) Guldi, D. M. J. Phys. Chem. A 1997, 101, 3895-3900.

Figure 2. (a) UV-vis absorption spectra of the dyad in THFwater mixtures in the range of 10-100% (v/v) THF. (Inset) Expanded visible region with structureless absorption indicating the presence of aggregates. (b) Aggregation curve as a function of the solvent composition in the binary THF-water mixture. (c) Plot of absorbance versus  values of the THFwater mixture.

stant () for the binary solvent combinations was calculated using the equation34,35

mix ) fAA + fBB

(1)

where the suffixes A, B, and mix represent the solvents A, B, and mixed solvent, respectively, and f represents the volume fraction of the solvent. Figure 2a shows the UV-vis spectra of 0.01 mM dyad in THF/water mixtures in the range of 100-10% (v/v) of THF with a visible color change of the solution from maroon to yellow. A progressive loss in the resolution of the spectrum is observed because of a decrease in absorbance at lower wavelengths (259 and 328 nm) and an increase in absorbance at 431 nm. A notable feature is the red shift and broadening of the 259 nm absorption maximum to 273 nm with its onset at 70:30% THF/water, further extending to a higher H2O content. The absorbance at 431 nm disappears, and the visible spectrum becomes structureless beyond the above composition range. This behavior is attributed to the formation of aggregates/ colloids with a drastic electronic structure change2,5 in reference to the monomeric dyad. Molecular interaction types responsible for the aggregation mechanism could be attributed to a cumulative effect of the hydrophobic/ van der Waals interaction between the core C60 molecules and the alkyl chains, π-stacking, dipolar interaction between water molecules and the polarization interaction between the C60 molecule and water; the latter more specifically has been referred to as a charge-transfer interaction between the water oxygen lone pair and the (34) Nath, S.; Pal, H.; Sapre, A. V. Chem. Phys. Lett. 2000, 327, 143148. (35) Hirata, Y.; Kanda, Y.; Mataga, N. J. J. Phys. Chem. 1983, 87, 1659-1662.

12142

Langmuir, Vol. 21, No. 26, 2005

Gayathri et al.

Figure 3. Variation of absorbance as a function of  for binary solvent mixtures, prepared from a 0.01 mM dyad in the less polar solvent. Table 1. Critical E Values for Dyad Aggregation in Various Binary Solvent Mixtures at 25 °C solvent mixture

critical  values for aggregation

THF-water DCM-ACN THF-ACN

∼30 ∼29.5 ∼29.5

π* orbital of C60.5,32,36 Time-dependent absorption spectral characteristics implied the stability of the clusters to span over weeks. The subsequent section discusses the structure and dynamics in such colloids. A further investigation into the absorption characteristics based on the empirical relationship5

∆Abs(259-431)norm ) ∆AbsTHF {int} - ∆AbsTHF min (2) 100 ∆AbsTHF pure - ∆AbsTHF min

[

]

with the subscripts THF int, THF min, THF pure referring to the differences in the absorbance of the dyad in binary solvent mixtures at intermediate, minimum, and maximum (100%) percentages of THF, respectively, resulting in an aggregation curve (Figure 2b) as a function of the solvent composition. Nonlinearity of the aggregation process is revealed, implying a possible phase change of the dyad aggregate. Changes in the absorbance at 431 nm with  as calculated from eq 1 in Figure 2c for the THF-water mixture indicate the onset of aggregation at  ∼ 30. Similar values have also been obtained for the solvent mixtures THF-ACN and DCM-ACN (Figure 3) and are tabulated in Table 1. Thus, the onset of aggregation at a critical  value of ∼30 against an  ∼ 13 for pristine C6034 indicates that beyond this  the dyad molecule always exists in the aggregate form only. This change in the onset  could probably be related to the total dipole moments associated with these systems and makes it a necessary condition for aggregation to occur. Water-Extracted Dyad Aggregates. When a faint maroon solution of the dyad was injected into water, a yellow and visually homogeneous solution was formed. The color remained stable even after the removal of THF. Evidence for aggregation was obtained from the UV-vis spectra of the water-extracted dyad aggregates as shown in the Figure 4. The absorption peaks of the dyad are (36) Prilutski, Y. I.; Shapovalov, G. G. Phys. Status Solidi 1997, 201, 361-370.

Figure 4. Visual and spectral analysis of the dyad in (a) (i) 0.2 mM dyad dissolved in THF, (ii) water added to the dyad/ THF solution resulting in a yellow colloidal solution, (iii) resultant aqueous solution after the evaporation of THF, and (iv) dyad aggregates back extracted into toluene. (b) UV-vis absorption spectra of 0.2 mM dyad in THF (s) and its aggregates in water (- - -).

broad and red-shifted from 259 to 273 nm and from 328 to 344 nm with a decrease in intensity and a featureless absorption in the visible region. The influence of the concentration on the formation of dyad aggregates was studied with 0.01, 0.04, 0.2, and 1 mM THF solutions. The extracted aggregates did show variation in size with respect to the concentration as obtained from the light-scattering experiments and will be discussed in the next section. The water-extracted colloidal solutions were stable for several weeks. Figure 5 illustrates the absorption behavior of the dyad aggregates in the presence of 1% NaCl. The absence of an appreciable peak shift and a negligible absorption at longer wavelengths with the disappearance of color implied disruption of the aggregate structure, resulting in sedimentation. These sediments could be extracted back into toluene and were found to retain most of the spectral characteristics of the dyad, as shown in Figure 5. However, in the absence of NaCl, such a back extraction into toluene was not feasible. This order of stability can be attributed to the negative charge associated with these aggregate surfaces.4 Deguchi et al. explained this negative charge to have come from the adsorption of the hydroxyl ions from water because of the formation of clathrate

Aggregates from a C60 Fullerene-Based Dyad

Langmuir, Vol. 21, No. 26, 2005 12143

Figure 5. UV-vis absorption spectra of the dyad immediately after extraction into water (s) and after addition of 1% NaCl (- - -). (Inset) UV-vis absorption spectra of the toluene solution of the dyad back extracted from the aqueous dyad aggregates in the presence of NaCl.

crystals.4,37 However, the absorption edge analysis shows the absorption of dyad in water to start at 1.65 eV, which is equivalent to the absorption of the pristine dyad (1.70 eV), implying the formation of hydrated crystals38 instead of clathrate crystals37 in water. Further, ONIOM (MP2: PM3) model calculations39 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 C60,39 with the C60 hexagon center-oxygen distance ranging between 3.19 and 3.09 Å. Estimation of Aggregate Dimension. DLS. The Brownian movement of a light-scattering particle gives rise to temporal fluctuations in the scattered light, measured by DLS. 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.1 The intensity of the scattered light in the present experiments was analyzed by the photon correlation spectroscopy (PCS), wherein the normalized autocorrelation function of the intensity was achieved by the stretched exponential method41 and is described by the expression

g(2)(t,τ) ) Ae-2Dq τ - 1 2

(3)

where A is the fitting parameter, D stands for the diffusion coefficient, and τ stands for the relaxation time with an average decay rate of Dq2; q is the scattering vector, given by the expression3

q)

sin(θ/2) [4πn λ ]

(4)

where n stands for the refractive index of the medium, λ is the wavelength of the incident light, and θ is the (37) Leach, S.; Vervloet, M.; Despres, A.; Breheret, E.; Hare, J. P.; Dennis, T. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Chem. Phys. 1992, 160, 451-466. (38) Prilutski, Y.; Durov, S.; Bulavin, L.; Valerji, P.; Astashkin, Y.; Yashchuk, V.; Ogul’Chanski, T.; Buzaneva, E.; Andrievsky, G. Mol. Cryst. Liq. Cryst. 1998, 324, 65-70. (39) Fomina, L.; Reyes, A.; Guadarrama, P.; Fomine, S. Int. J. Quantum Chem. 2004, 97, 679-687. (40) Yoshida, Z.; Takekuma, H.; Takekuma, S.; Matsubara, Y. Angew. Chem., Int. Ed. Engl. 1994, 33, 1597-1599. (41) Gotter, R.; Goldmann, W. H.; Isenberg, G. FEBS Lett. 1995, 359, 220-222.

Figure 6. Dynamic structure factors for dyad aggregates (a) water extracted, 0.2 mM, q ) 8.8, 14.4, and 19.6 µm-1 and (b) 10% THF-90% water binary solvent mixture, q ) 8.9, 14.6, and 19.8 µm-1 for scattering angles, 30°, 50°, and 70°.

scattering angle. Thus,

D)

1 τq2

(5)

In the stretched exponential method, the temporal autocorrelation function is fitted to a second-order polynomial from where a plot of q2 versus 1/τ yields a direct measure of the diffusion coefficient. A plot of delay time versus the normalized autocorrelation function for a few of the measured scattering angles is shown in Figure 6. The decay time distribution for each exponential is seen to be dependent upon the scattering angle, and the plateau height decreases markedly with an increasing q. A stretched exponential decay42 in the dynamic behavior of the dyad clusters is evidenced from the shifts in the decay time to lower values with an increasing scattering angle. The variation of decay rates with the square of the scattering vector (q2) exhibits a linear relationship with the fitting line passing through the (0, 0) point (Figure 7), indicating the decay to be due to translational diffusion of the dyad aggregates in solution.1 For the translational diffusion mode, the hydrodynamic diameter is given by the Stokes-Einstein equation

dh )

kBT 3πηD

(6)

with kBT as the thermal energy and η as the viscosity of

12144

Langmuir, Vol. 21, No. 26, 2005

Gayathri et al.

Figure 8. Size distribution of the dyad aggregates in THFwater binary mixtures.

Figure 7. Decay rate versus q2 for the dyad aggregates (a) water extraction, 0.2 mM and (b) 10% THF-90% water binary solvent mixture. Table 2. Characteristics of the Water-Extracted Dyad Aggregates from THF Solution concentration of the dyad extracted in water (mM)

diffusion coefficient, D (cm2s-1)

hydrodynamic diameter, dh (nm)

0.01 0.04 0.2 C6043 (0.06)

4.2 × 10-8 2.7 × 10-8 1.9 × 10-8 2.2 × 10-6

116 176 253 90

the medium. The apparent hydrodynamic diameters calculated for the water-extracted dyad aggregates range from 110 to 250 nm as a function of the concentration and are summarized in Table 2 along with the D values. A two-order magnitude lower D value in comparison to that of the pristine C6043 aggregate indicates larger aggregate size in the case of the dyad. In THF-H2O binary solvent mixtures, the particle size is overestimated as compared to H2O-extracted dispersions as shown in Figure 8 and increases abruptly at 30% THF and thereafter. This abrupt rise in particle size is thought to have arisen from a deviation from the ideal spherical colloid geometry, assumed in the framework of DLS. However, with an increasing water content, as in the water-extracted dispersions, the system behaved ideal. Mechanism of the Dyad Cluster Growth: The Fractal Concept. In the framework of the diffusionlimited cluster aggregation (DLCA) model,44 fractal struc(42) Weitz, D. A.; Krall, A. H. Phys. Rev. Lett. 1998, 80, 778-781. (43) Bokare, A.; Patnaik, A. Carbon 2003, 41, 2643-2651. (44) Bezmel’nitsyn, V. N.; Eletskii, A. V.; Okun’, M. V. Phys.-Usp. 1998, 41, 1091-1114.

Figure 9. (a) TEM image of 1 mM water-extracted dyad aggregate. (b) Determination of fractal dimension of the dyad aggregates. The plots 1 and 2 refer to two cluster points at which the box counting was done on a 1 mM water-extracted dyad concentration.

tures are formed as a result of irreversible aggregation of small particle-like clusters with diffusion as the ratelimiting step during aggregation. Here, aggregation is a fast process and is limited by the time taken for the particles to encounter each other by diffusion. A correlation between their mass and size can be attributed to their

Aggregates from a C60 Fullerene-Based Dyad

Langmuir, Vol. 21, No. 26, 2005 12145

fractal structure as

R ) RN1/DF

(7)

where R is the cluster size, N is the number of primary particles in the aggregate, R is the lacunarity constant, and DF is the fractal dimension, which is a measure of the density correlation.30,44 The infinitely large clusters or fractals result from the diffusion of various particles toward a seed particle, giving rise to dilation symmetry. In the present experiments, the water-extracted ∼250 nm dyad aggregates formed fractals at 1 mM concentration (Figure 9a) as revealed by transmission electron microscopy.45 The fractal dimension was estimated by adopting to Hausdorf box counting method30 as

DF ) lim sf0

log N(s) log A ≈ log(1/s) log rg

(8)

where A is the area coverage of the cluster aggregate and rg is the radius of gyration. Figure 9b shows linear plots with DF ) 1.85, estimated from the slopes and in compliance30 with the cluster growth process governed by the DLCA mechanism. (45) Gayathri, S. S.; Patnaik, A. Unpublished work.

Conclusion The aggregation behavior of a C60-based dyad has been studied in binary solvent mixtures and in water extractions. Beyond a critical dielectric constant of ∼30, the dyad exists in the form of aggregates. The aggregates formed in binary water mixtures were less stable than those extracted into water, as revealed by absorption spectroscopy. The stability of the water-extracted aggregates was attributed to a charge-transfer interaction between the oxygen lone pair of water and the π* orbitals of C60. The absorption edge analysis proved that the dyad forms hydrated crystals instead of clathrate crystals. DLS experiments showed the dyad aggregates to undergo stretched exponential decay, and their size increased with an increasing concentration. In binary THF-water mixtures, the shape selectivity was lost and the aggregates were nonspherical beyond a 70% THF-30% water mixture. Fractalization in the water-extracted dyad aggregate at 1 mM concentration was explained using the diffusionlimited cluster aggregation mechanism leading to a dimension of 1.85. Acknowledgment. This work was supported by the Department of Science and Technology (DST), Government of India, under Grant number SP/SI/H-37/2001. LA052313J