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Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70
17 July 2002
Chemical Physics Letters 360 (2002) 422–428 www.elsevier.com/locate/cplett
Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70 Sukhendu Nath, Haridas Pal 1, Avinash V. Sapre
*,1
Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received 5 February 2002
Abstract Effect of solvent polarity on the aggregation behaviour of C70 has been investigated in several mixed solvents using optical absorption, fluorescence, dynamic light scattering and scanning electron microscopic measurements and compared with those observed for the other fullerene analogue, C60 . It is seen that similar to C60 , aggregation of C70 also requires the solvent polarity to exceed some critical value. In terms of solvent dielectric constant the critical solvent polarity, required for C70 aggregation is found to be in the range of 27–31, which is much higher than that required for C60 aggregation (12–14). The large difference in the critical solvent polarity required for C60 and C70 aggregation has been rationalized on the basis of the molecular shapes and the polarizabilities of two fullerene molecules. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Fullerenes, the all carbon cage molecules, C60 and C70 , display anomalous behaviour in solution due to the formation of aggregates [1–9]. Since the specific surface energies of interactions of fullerene molecules with each other and with the solvent molecules are very close in magnitude [1], fullerenes in solution often tend to form aggregates. Since the first observation of C70 aggregation by Sun and Bunker [2] in toluene (TL)–acetonitrile (AN) solvent mixtures, the aggregation behaviour of fullerenes has been inves-
*
1
Corresponding author. Fax: +91-22-550-5151. E-mail address: [email protected] (S. Nath). Also corresponding author.
tigated quite extensively in different solvents and solvent mixtures in the last few years [3–14]. Ying et al. [6,7] has reported that on long keeping C60 forms aggregates even in benzene solution. However, such aggregates are very unstable even to the mechanical shaking. We have recently reported the formation of stable C60 aggregates in aromatic solvents, like, benzonitrile (BZN) and benzyl alcohol (BZA) [8,9]. There are reports that the fullerenes undergo aggregation in different neat solvents at low temperature [10]. It is also reported that due to aggregation in solution, the solubility of fullerenes in many solvents displays non-monotonic temperature dependence [15–17]. Besides in homogeneous solvents, the aggregation of fullerenes has also been observed in many heterogeneous media like, micelles, liposome and vesicles [12,18,19].
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 7 8 0 - 7
S. Nath et al. / Chemical Physics Letters 360 (2002) 422–428
In our recent work it has been shown that the aggregation of C60 in solution is largely determined by the polarity of the medium [9]. Thus, C60 undergoes aggregation only in polar aromatic solvents like, BZN and BZA, but not in relatively non-polar solvents like, benzene (BZ), TL, chlorobenzene (CBZ), etc. A detailed study on the effect of solvent polarity on the C60 aggregation has shown that for the aggregation to take place the solvent polarity must exceed some critical value, which appears to be 12–14 in terms of the solvent dielectric constant values. For C70 aggregation, no such systematic study on the effect of solvent polarity has been reported so far. In the present work we have investigated in detail the effect of solvent polarity on the aggregation of C70 with the aim to see if there is any characteristic difference in the aggregation behaviour of the two fullerene analogues, C60 and C70 .
423
cence measurements were carried out with a Hitachi F-4010 spectrofluorimeter. Dynamic light scattering (DLS) studies were carried out with a Brookhaven Instruments model (BI-90) particle size analyzer, works on the principle of photon correlation spectroscopy of quasi-elastic light scattering (PCS-QELS). A He–Ne laser (k ¼ 632:5 nm) is used as the excitation light source and the light scattered from the sample at right angle to the excitation light is detected by a PMT. Scanning electron microscopic (SEM) measurements were carried out with a JEOL electron microscope (model JSM-T330A). Microfilms for the SEM studies were prepared by putting few drops of the C70 solutions of interest on a brass stub and then drying off the solvents by evaporation at ambient temperature.
3. Results 2. Materials and methods
3.1. Ground state optical absorption studies
C70 (>99.5%), obtained from SES Corporation USA, was used without further purification. All the experiments were carried out with two different lots of C70 to check the reproducibility of the results. All the solvents were of spectroscopic grade from Spectrochem, India or E. Merck, Germany. Stock solutions of C70 in suitable solvent where C70 has fairly good solubility were prepared by ultrasonication followed by centrifugation and decantation. A Toshniwal, India, made ultrasonicator (model SW 4.5) was used for sonication. Stock solutions of C70 thus prepared were diluted appropriately by other cosolvents to make the desired mixed solvent compositions with suitable fullerene concentrations. For mixed solvents the dielectric constant (eMS ) values were calculated using the following relation [20,21]
Ground state absorption spectra of C70 have been recorded in different solvents and solvent mixtures. Fig. 1a shows the absorption spectra of C70 solutions in BZN. The spectra show a broad absorption band in the visible region with absorption maxima at 470 nm with a small shoulder at 550 nm and a strong narrow absorption peak at 380 nm. All these features are the characteristics of the C70 monomer absorption [22,23]. It is observed that even upto the saturation limit of C70 there is no change in the absorption characteristics in BZN, indicating that for all the concentrations, the C70 remains as the monomer in BZN solution. In our earlier work it was observed that contrary to C70 , the C60 forms aggregate in BZN solution when the latter’s concentration exceeds 100 lM. That C70 remains as monomer in BZN even upto its saturation limit indicates that the polarity of BZN might not be sufficient to initiate aggregation of C70 in solution. This is further supported by the fact that in all other aromatic solvents investigated, e.g., BZ, TL, CBZ, etc., the C70 solutions show the absorption characteristics of its monomer, irrespective of its concentrations. Since in highly polar non-aromatic
eMS ¼ fA eA þ fB eB ;
ð1Þ
where the suffix A, B and MS represent the solvents A, B and mixed solvent, respectively, and f represents the volume fraction of the cosolvent. Ground state optical absorption measurements were carried out with a Shimadzu spectrophotometer (model- UV-160A). Steady-state fluores-
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Fig. 1. Ground state absorption spectra of C70 solution in (a) BZN and (b) BZ–AN solvent mixtures at different AN compositions: (1) 0, (2) 30, (3) 50, (4) 65, (5) 70, (6) 72, (7) 75 and (8) 85% of AN in BZ.
solvents the solubility of C70 is very poor [24], the solvent polarity effect on C70 aggregation could not be investigated in neat polar solvents. To see if the solvent polarity plays any role in C70 aggregation, the absorption characteristics of C70 solutions have thus been investigated in different solvent mixtures, by judicially choosing a cosolvent in which C70 solubility is reasonably high. Different mixed solvent combinations investigated in the present work are given in Table 1. Table 1 Critical eMS values for C70 aggregation in different solvent mixtures Solvent mixture
Critical eMS
BZ–AN TL–AN CBZ–AN DCBZa –AN BZN–MeOH BZN–AN
27 27 30 31 30 31
a
DCBZ – 1,2-dichlorobenzene.
Fig. 1b shows the absorption spectra of C70 in BZ–AN solvent mixtures with different cosolvent compositions but with the same C70 concentration (9 lM). From Fig. 1b it is evident that as long as the composition of the polar cosolvent AN remains below 70%, the solutions show the absorption characteristics for the monomeric C70 (cf. Figs. 1a and b). However, on increasing the AN percentage beyond 70%, the absorption characteristics of the solutions display appreciable changes. Thus at these high AN compositions the absorption spectra become quite broad, undergo a gradual red shift and develop a longer wavelength absorption tail, which extends even beyond 900 nm. Similar spectral changes have already been reported in the literature and are attributed to the aggregation of C70 [2,3]. Present results thus indicate that in AN–BZ solvent mixtures the C70 undergoes aggregation when the composition of the polar cosolvent AN exceeds 70%. It may be mentioned that the aggregation of C70 in these solutions can also observed visually. Thus, below 70% of AN, the C70 solutions show the characteristic reddish orange colour of C70 monomer. The colour changes to light brown on increasing the AN composition >70% due to aggregation. It is seen that the change in the absorption spectra of C70 solutions due to aggregation is reversible in nature. Thus, if excess BZ is added to the C70 solution in BZ–AN solvent mixture containing the aggregated C70 the absorption spectra reverts back to that of the C70 monomer. Similarly the colour of the solution also changes from light brown to reddish orange colour on adding excess BZ to the solution. Exactly similar changes in the absorption characteristics of C70 have also been observed in other solvent mixtures indicated in Table 1. In all these cases, it is seen that C70 starts the aggregation process only when the composition of the polar cosolvent exceeds a certain value. The critical eMS values required for C70 aggregation for different solvent mixtures were estimated using Eq. (1) and are given in Table 1, which fall within a narrow range of 27–31. Fig. 2 shows the changes in the absorbance of C70 solution in different mixed solvents measured at 550 nm with the dielectric constant (eMS ) of the solvents used. It is seen from this figure that in all
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3.2. Steady-state fluorescence studies
Fig. 2. Plot of absorbance at 550 nm vs. the dielectric constant of the medium for C70 solutions in different solvent mixtures: (a) BZ–AN, (b) CBZ–AN and (c) BZN–MeOH.
the solvent mixtures studied the absorbances at 550 nm do not change significantly as long as the eMS is less than 27–31. However, as the eMS value exceeds this limit, the absorbance gradually increases, showing a distinct change over in the absorption characteristics of the solutions. Present results clearly indicate that C70 remains as monomer in the solution as long as the eMS is lower than the critical value of 27–31 and starts aggregating as the eMS exceeds this critical limit. It is thus apparent that just like C60 , the C70 also needs a critical solvent polarity to initiate its aggregation process. On comparing the critical eMS values required for C60 and C70 aggregation (12–14 for C60 and 27–31 for C70 ) it is, however, evident that C70 requires much more polar medium than C60 for the onset of the aggregation process.
The aggregation process of C70 has also been investigated by the steady-state fluorescence measurements in different solvent mixtures. Fig. 3 shows typical fluorescence results obtained for the C70 solutions in different TL–AN mixtures. On excitation at 500 nm, the C70 solution in TL shows an emission spectrum, characteristics of the monomeric C70 fluorescence, with a maximum at 670 nm and a shoulder at 690 nm [25,26]. On the addition of AN to this solution, the nature of emission spectra remains the same as long as the AN percentage remains below 70. However, on further increase in the AN percentage, the emission intensity is found to decrease suddenly with a concomitant red shift (15 nm) and broadening of the emission spectra compared to that of the monomeric emission. Similar changes in the fluorescence spectra were also observed earlier and assigned to the formation of C70 aggregates [3]. Thus, the fluorescence results support the inferences drawn from the absorption measurements. It is important to mention here that along with the above changes in the emission characteristic as the AN% exceed 70, a new strong emission band is also observed at 590 nm with 500 nm excitation, which is found to be strongly dependent on the excitation wavelength. Following Ghosh et al. [3],
Fig. 3. The steady-state fluorescence spectra (kex ¼ 500 nm) of C70 solution (12.6 lM) in TL–AN solvent mixtures at different solvent composition: ð–––Þ 0, ð Þ 70 and ð- - - - - -Þ 78%. Intensity of all these spectra has been normalized for comparison.
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this new emission band has been assigned to the Raman scattering peak. The intensity of the Raman peak is seen to increase with the AN composition beyond 70%, indicating the increase in the number of aggregates in the solution with an increase in the AN% beyond 70. 3.3. Dynamic light scattering and scanning electron microscopic measurements Dynamic light scattering (DLS) studies have been carried out for C70 solutions in different solvents to substantiate the results obtained by optical absorption and fluorescence measurements. In neat solvents like BZ, TL, BZN, etc., the DLS measurements show the absence of any detectable particle in the solution even for the saturated solutions of C70 . These results clearly indicate that no aggregates are formed in these solvents. Similarly, the C70 solutions in mixed solvents do not show any significant scattered light intensity as long as the percentage of the polar cosolvent remains below that required for critical eMS limit, indicated from absorption and fluorescence studies. When the composition of the polar cosolvent is, however, made beyond the critical limit, the DLS measurements show a considerable amount of scattered light. It is seen that with an increase in the composition of the polar cosolvent beyond the critical limit, the scattered light intensity gradually increases. Analysis of the DLS signals following PCS-QELS theory and assuming the aggregates are of effective spherical shape results in the estimation of the mean size of particles of 230–250 nm. The particle size is seen to increase only marginally on increasing the solvent polarity for any particular solvent mixture. Fig. 4 shows the variation in the count rate of the scattered light intensity with the eMS of the medium for BZN– MeOH solvent mixtures. Inset of Fig. 4 shows a typical histogram obtained from the DLS measurement of C70 solution in BZN (25%)–MeOH (75%) solvent mixture. It is evident from the DLS results that the aggregation sets on only after crossing the critical eMS value for the solvents used. The DLS results thus substantiate the results obtained from the absorption and fluorescence measurements, indicating that aggregation of C70
Fig. 4. Variation of the intensity of the scattered light from C70 solution (14 lM) in BZN–MeOH solvent mixtures with the solvent polarity. Inset: Typical histogram obtained by DLS experiment for C70 solution in BZN (25%)–MeOH (75%) solvent mixtures.
starts only when the solvent polarity exceeds a critical value. To obtain a direct evidence for the presence of aggregates in the present systems, we have also carried out the SEM measurements. The SEM films prepared from C70 solutions in solvents like BZ, TL, BZN, etc., do not show the presence of any particle. Similarly for C70 solutions in mixed solvents no particles could be detected in SEM when the dielectric constant of the solvent medium is less than the critical eMS value indicated from the absorption and fluorescence studies. However, the films prepared from C70 solutions in mixed solvents with eMS higher than the critical eMS value show the presence of small particles. Fig. 5 shows a
Fig. 5. Typical SEM micrograph corresponding to 12.6 lM solution of C70 in AN–BZN solvent mixture with 70% of AN.
S. Nath et al. / Chemical Physics Letters 360 (2002) 422–428
typical SEM micrograph obtained for C70 aggregates prepared in AN (70%)–BZN (30%) mixture. It is important to mention here that the particle density observed in the SEM measurement is very low, as compared to that observed for C60 aggregates [8,9]. This is due to very low concentration (10 lM) of the C70 solution used in the present study, as compared to 350 lM concentration used for C60 solutions. In the case of C70 aggregation, since a large volume percentage of the polar cosolvent is required in which C70 is almost insoluble [24], a very high concentration of C70 could not be achieved in the present studies.
4. Discussion The present study clearly indicates that similar to C60 , aggregation of C70 also requires a critical solvent polarity to be reached to set in the aggregation process. It is interesting to note, however, that the critical solvent polarity required for C70 aggregation is much higher (eMS ¼ 27–31) in comparison to that required for C60 aggregation (eMS ¼ 12–14). Since both C60 and C70 are nonpolar molecules, such a large difference in the required critical solvent polarity for the aggregation to set in for the two fullerenes appears to be very unusual. We feel that such a difference in critical eMS for C60 and C70 aggregation arises due to the differences in the polarizability of two fullerene molecules. The polarizability values for C60 and C70 are 6:4 1023 and 8 1023 cm3 , respectively [27]. Thus, compared to spherical C60 , the ellipsoidal C70 [28] has higher polarizability. As fullerenes are non-polar molecules, their aggregation in solution must be determined by the relative magnitudes of the different dispersive forces operating between solute–solute and solute–solvent molecules, in which the polarizability of the fullerene molecules plays a direct role. In less polar solvents fullerene monomers easily remain solvated due to fullerene–solvent dispersion forces. A small stabilization for the monomer solubility in solution may also come from the specific p–p interaction between the fullerene and the aromatic solvent molecules, where latter is one of the cosolvent [29–31]. As the polarity of the medium is
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increased, the fullerene–solvent interactions gradually become less dominant as the solute molecules are non-polar. Thus, with increasing solvent polarity, the fullerene–fullerene interaction gradually starts dominating over the fullerene–solvent interactions. As a result, beyond a critical composition of the polar cosolvent, where fullerene solubility is very poor, the fullerene–fullerene interaction will cause the aggregation to set in. As the polarizability of C70 is higher compared to that of C60 , in the former case the fullerene–solvent dispersive interaction makes these molecules to remain dissolved in the solution in their monomeric forms upto a reasonably higher solvent polarity as compared to that happens for less polarizable C60 molecules. Thus, it is expected that critical eMS for C70 aggregation will be much higher than that of C60 , as observed in the present study. At this point it is interested to point out another distinct difference observed between C60 and C70 aggregates in relation to their absorption spectra. For C60 aggregation, a blue shift is observed in the absorption maxima in comparison to its monomer spectra [8,9] where as for C70 the aggregation causes a red shift in the absorption maxima. These observations indicate that the mode of packing of the fullerene molecules in the aggregates could be different for two fullerene analogues. The blue shift in the absorption maxima for C60 indicates that the stacking of the monomers in the aggregates results in the formation of H-type of aggregation where as a red shift for C70 aggregation indicates that the monomers stacking results in the J-type of aggregation [32,33]. The reasons for such differences in the type of aggregation for C60 and C70 are not very clear to us. We, however, feel that it could be due to the difference in the shape of two fullerene molecules, spherical for C60 and ellipsoidal for C70 [28] and that leads to different type of stacking during the aggregation.
5. Conclusion Similar to C60 , the C70 in solution also requires a minimum solvent polarity to be reached before it can undergo the aggregation process. In terms of the solvent dielectric constant, the critical solvent
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polarity required for C70 aggregation is, however, found to be much higher, in the range 27–31, than that required for C60 aggregation (12–14). Such a large difference in the critical solvent polarity necessary for the aggregation of C60 and C70 has been rationalized on the basis of the polarizability of these two fullerene molecules. The shifts in the absorption spectra due to aggregation indicate the H-type of aggregation for C60 and J-type of aggregation for C70 . The shapes of the fullerene molecules seem to determine the nature of stacking of these molecules in the aggregates.
Acknowledgements We are grateful to Mr. K.K. Kutty for his kind help in the SEM measurements.
References [1] V.N. Bezmel’nitsyn, A.V. Eletskii, M.V. Okun, Phys. Uspekhi 41 (1998) 1091. [2] Y.P. Sun, C.E. Bunker, Nature 365 (1993) 398. [3] H.N. Ghosh, A.V. Sapre, J.P. Mittal, J. Phys. Chem. 100 (1996) 9439. [4] M. Fujitsuka, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, A. Watanabe, O. Ito, Chem. Lett. (1997) 1211. [5] T. Rudalevige, A.H. Francis, R. Zand, J. Phys. Chem. A. 102 (1998) 9797. [6] Q. Ying, J. Marecek, B. Chu, J. Chem. Phys. 101 (1994) 2665. [7] Q. Ying, J. Marecek, B. Chu, Chem. Phys. Lett. 219 (1994) 214. [8] S. Nath, H. Pal, D.K. Palit, A.V. Sapre, J.P. Mittal, J. Phys. Chem. B 102 (1998) 10158. [9] S. Nath, H. Pal, A.V. Sapre, Chem. Phys. Lett. 327 (2000) 143. [10] J.S. Ahn, K. Suzuki, Y. Iwasa, N. Otsuka, T. Mitani, J. Lumin. 76&77 (1998) 201. [11] F. Diederich, J. Effing, V. Jonas, L. Jullien, T. Plesnivy, H. Ringsdorf, C. Thilgen, D. Weinstiein, Angew. Chem. Int. Ed. Engl. 31 (1992) 1599.
[12] H. Hungerbuhler, D.M. Guldi, K.D. Asmus, J. Am. Chem. Soc. 115 (1993) 3386. [13] Y.M. Wang, P.V. Kamat, L.K. Patterson, J. Phys. Chem. 97 (1993) 8793. [14] P.V. Kamat, S. Barazzouk, K.G. Thomas, S. Hotchandani, J. Phys. Chem. B 104 (2000) 4014. [15] R.S. Ruoff, R. Malhotra, D.L. Huestis, D.S. Tse, D.C. Lorents, Nature 361 (1993) 140. [16] X. Zhou, J. Liu, Z. Jin, Z. Gu, Y. Wu, Y. Sun, Full. Sci. Tech. 5 (1997) 285. [17] V.N. Bezmelnitsyn, A.V. Eletskii, E.V. Stepanov, J. Phys. Chem. 98 (1994) 6665. [18] R.V. Bensasson, E. Bienvenue, M. Dellinger, S. Leach, P. Seta, J. Phys. Chem. 98 (1994) 3492. [19] J.M. Janot, E. Bienvenue, P. Seta, R.V. Bensasson, A.C. Tome, R.F. Enes, J.A.S. Cavaleiro, S. Leach, X. Camps, A. Hirsh, J. Chem. Soc. Perkin Trans. 22 (2000) 301. [20] Y. Hirata, Y. Kanda, N. Mataga, J. Phys. Chem. 87 (1983) 1662. [21] H. Masuhara, N. Mataga, Acc. Chem. Res. 14 (1981) 312. [22] J.P. Hare, H.W. Kroto, R. Taylor, Chem. Phys. Lett. 177 (1991) 394. [23] H. Ajie, M.M. Alvarez, S.J. Anz, R.D. Beck, F. Diederich, K. Fostiropoulos, D.R. Huffman, W. Kratschmer, Y. Rubin, K.E. Schriver, D. Sensharma, R.L. Whetten, J. Phys. Chem. 94 (1990) 8630. [24] N. Sivaraman, R. Dhamodaran, I. Kaliappan, T.G. Srinivasan, P.R.V. Rao, C.K. Mathews, Full. Sci. Tech. 2 (1994) 233. [25] J.W. Arbogast, C.S. Foote, J. Am. Chem. Soc. 113 (1991) 8886. [26] J. Catalan, J. Elguero, J. Am. Chem. Soc. 115 (1993) 9249. [27] N. Matsuzawa, D.A. Dixon, J. Phys. Chem. 96 (1992) 6241. [28] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, in: Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1996, p. 60. [29] C.N.R. Rao, R. Seshadri, A. Govindaraj, J.P. Mittal, H. Pal, T. Mukherjee, J. Mol. Struct. 300 (1993) 289. [30] P.R. Ogilby, D.S. Scurlock, J. Photochem. Photobiol. A 91 (1995) 21. [31] S.P. Sibley, Y.T. Nguven, L. Campbell, H.B. Silber, Spectrochim. Acta, Part A 53 (1997) 684. [32] G. Scheibe, in: W. Foerst (Ed.), Optische Anregungen Organischer System, Verlag Chemie, Weinheim, 1996, p. 109. [33] A.H. Herz, Adv. Coll. Interface Sci. 8 (1977) 237.
Chemical Physics Letters 360 (2002) 422–428 www.elsevier.com/locate/cplett
Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70 Sukhendu Nath, Haridas Pal 1, Avinash V. Sapre
*,1
Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received 5 February 2002
Abstract Effect of solvent polarity on the aggregation behaviour of C70 has been investigated in several mixed solvents using optical absorption, fluorescence, dynamic light scattering and scanning electron microscopic measurements and compared with those observed for the other fullerene analogue, C60 . It is seen that similar to C60 , aggregation of C70 also requires the solvent polarity to exceed some critical value. In terms of solvent dielectric constant the critical solvent polarity, required for C70 aggregation is found to be in the range of 27–31, which is much higher than that required for C60 aggregation (12–14). The large difference in the critical solvent polarity required for C60 and C70 aggregation has been rationalized on the basis of the molecular shapes and the polarizabilities of two fullerene molecules. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Fullerenes, the all carbon cage molecules, C60 and C70 , display anomalous behaviour in solution due to the formation of aggregates [1–9]. Since the specific surface energies of interactions of fullerene molecules with each other and with the solvent molecules are very close in magnitude [1], fullerenes in solution often tend to form aggregates. Since the first observation of C70 aggregation by Sun and Bunker [2] in toluene (TL)–acetonitrile (AN) solvent mixtures, the aggregation behaviour of fullerenes has been inves-
*
1
Corresponding author. Fax: +91-22-550-5151. E-mail address: [email protected] (S. Nath). Also corresponding author.
tigated quite extensively in different solvents and solvent mixtures in the last few years [3–14]. Ying et al. [6,7] has reported that on long keeping C60 forms aggregates even in benzene solution. However, such aggregates are very unstable even to the mechanical shaking. We have recently reported the formation of stable C60 aggregates in aromatic solvents, like, benzonitrile (BZN) and benzyl alcohol (BZA) [8,9]. There are reports that the fullerenes undergo aggregation in different neat solvents at low temperature [10]. It is also reported that due to aggregation in solution, the solubility of fullerenes in many solvents displays non-monotonic temperature dependence [15–17]. Besides in homogeneous solvents, the aggregation of fullerenes has also been observed in many heterogeneous media like, micelles, liposome and vesicles [12,18,19].
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 7 8 0 - 7
S. Nath et al. / Chemical Physics Letters 360 (2002) 422–428
In our recent work it has been shown that the aggregation of C60 in solution is largely determined by the polarity of the medium [9]. Thus, C60 undergoes aggregation only in polar aromatic solvents like, BZN and BZA, but not in relatively non-polar solvents like, benzene (BZ), TL, chlorobenzene (CBZ), etc. A detailed study on the effect of solvent polarity on the C60 aggregation has shown that for the aggregation to take place the solvent polarity must exceed some critical value, which appears to be 12–14 in terms of the solvent dielectric constant values. For C70 aggregation, no such systematic study on the effect of solvent polarity has been reported so far. In the present work we have investigated in detail the effect of solvent polarity on the aggregation of C70 with the aim to see if there is any characteristic difference in the aggregation behaviour of the two fullerene analogues, C60 and C70 .
423
cence measurements were carried out with a Hitachi F-4010 spectrofluorimeter. Dynamic light scattering (DLS) studies were carried out with a Brookhaven Instruments model (BI-90) particle size analyzer, works on the principle of photon correlation spectroscopy of quasi-elastic light scattering (PCS-QELS). A He–Ne laser (k ¼ 632:5 nm) is used as the excitation light source and the light scattered from the sample at right angle to the excitation light is detected by a PMT. Scanning electron microscopic (SEM) measurements were carried out with a JEOL electron microscope (model JSM-T330A). Microfilms for the SEM studies were prepared by putting few drops of the C70 solutions of interest on a brass stub and then drying off the solvents by evaporation at ambient temperature.
3. Results 2. Materials and methods
3.1. Ground state optical absorption studies
C70 (>99.5%), obtained from SES Corporation USA, was used without further purification. All the experiments were carried out with two different lots of C70 to check the reproducibility of the results. All the solvents were of spectroscopic grade from Spectrochem, India or E. Merck, Germany. Stock solutions of C70 in suitable solvent where C70 has fairly good solubility were prepared by ultrasonication followed by centrifugation and decantation. A Toshniwal, India, made ultrasonicator (model SW 4.5) was used for sonication. Stock solutions of C70 thus prepared were diluted appropriately by other cosolvents to make the desired mixed solvent compositions with suitable fullerene concentrations. For mixed solvents the dielectric constant (eMS ) values were calculated using the following relation [20,21]
Ground state absorption spectra of C70 have been recorded in different solvents and solvent mixtures. Fig. 1a shows the absorption spectra of C70 solutions in BZN. The spectra show a broad absorption band in the visible region with absorption maxima at 470 nm with a small shoulder at 550 nm and a strong narrow absorption peak at 380 nm. All these features are the characteristics of the C70 monomer absorption [22,23]. It is observed that even upto the saturation limit of C70 there is no change in the absorption characteristics in BZN, indicating that for all the concentrations, the C70 remains as the monomer in BZN solution. In our earlier work it was observed that contrary to C70 , the C60 forms aggregate in BZN solution when the latter’s concentration exceeds 100 lM. That C70 remains as monomer in BZN even upto its saturation limit indicates that the polarity of BZN might not be sufficient to initiate aggregation of C70 in solution. This is further supported by the fact that in all other aromatic solvents investigated, e.g., BZ, TL, CBZ, etc., the C70 solutions show the absorption characteristics of its monomer, irrespective of its concentrations. Since in highly polar non-aromatic
eMS ¼ fA eA þ fB eB ;
ð1Þ
where the suffix A, B and MS represent the solvents A, B and mixed solvent, respectively, and f represents the volume fraction of the cosolvent. Ground state optical absorption measurements were carried out with a Shimadzu spectrophotometer (model- UV-160A). Steady-state fluores-
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Fig. 1. Ground state absorption spectra of C70 solution in (a) BZN and (b) BZ–AN solvent mixtures at different AN compositions: (1) 0, (2) 30, (3) 50, (4) 65, (5) 70, (6) 72, (7) 75 and (8) 85% of AN in BZ.
solvents the solubility of C70 is very poor [24], the solvent polarity effect on C70 aggregation could not be investigated in neat polar solvents. To see if the solvent polarity plays any role in C70 aggregation, the absorption characteristics of C70 solutions have thus been investigated in different solvent mixtures, by judicially choosing a cosolvent in which C70 solubility is reasonably high. Different mixed solvent combinations investigated in the present work are given in Table 1. Table 1 Critical eMS values for C70 aggregation in different solvent mixtures Solvent mixture
Critical eMS
BZ–AN TL–AN CBZ–AN DCBZa –AN BZN–MeOH BZN–AN
27 27 30 31 30 31
a
DCBZ – 1,2-dichlorobenzene.
Fig. 1b shows the absorption spectra of C70 in BZ–AN solvent mixtures with different cosolvent compositions but with the same C70 concentration (9 lM). From Fig. 1b it is evident that as long as the composition of the polar cosolvent AN remains below 70%, the solutions show the absorption characteristics for the monomeric C70 (cf. Figs. 1a and b). However, on increasing the AN percentage beyond 70%, the absorption characteristics of the solutions display appreciable changes. Thus at these high AN compositions the absorption spectra become quite broad, undergo a gradual red shift and develop a longer wavelength absorption tail, which extends even beyond 900 nm. Similar spectral changes have already been reported in the literature and are attributed to the aggregation of C70 [2,3]. Present results thus indicate that in AN–BZ solvent mixtures the C70 undergoes aggregation when the composition of the polar cosolvent AN exceeds 70%. It may be mentioned that the aggregation of C70 in these solutions can also observed visually. Thus, below 70% of AN, the C70 solutions show the characteristic reddish orange colour of C70 monomer. The colour changes to light brown on increasing the AN composition >70% due to aggregation. It is seen that the change in the absorption spectra of C70 solutions due to aggregation is reversible in nature. Thus, if excess BZ is added to the C70 solution in BZ–AN solvent mixture containing the aggregated C70 the absorption spectra reverts back to that of the C70 monomer. Similarly the colour of the solution also changes from light brown to reddish orange colour on adding excess BZ to the solution. Exactly similar changes in the absorption characteristics of C70 have also been observed in other solvent mixtures indicated in Table 1. In all these cases, it is seen that C70 starts the aggregation process only when the composition of the polar cosolvent exceeds a certain value. The critical eMS values required for C70 aggregation for different solvent mixtures were estimated using Eq. (1) and are given in Table 1, which fall within a narrow range of 27–31. Fig. 2 shows the changes in the absorbance of C70 solution in different mixed solvents measured at 550 nm with the dielectric constant (eMS ) of the solvents used. It is seen from this figure that in all
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3.2. Steady-state fluorescence studies
Fig. 2. Plot of absorbance at 550 nm vs. the dielectric constant of the medium for C70 solutions in different solvent mixtures: (a) BZ–AN, (b) CBZ–AN and (c) BZN–MeOH.
the solvent mixtures studied the absorbances at 550 nm do not change significantly as long as the eMS is less than 27–31. However, as the eMS value exceeds this limit, the absorbance gradually increases, showing a distinct change over in the absorption characteristics of the solutions. Present results clearly indicate that C70 remains as monomer in the solution as long as the eMS is lower than the critical value of 27–31 and starts aggregating as the eMS exceeds this critical limit. It is thus apparent that just like C60 , the C70 also needs a critical solvent polarity to initiate its aggregation process. On comparing the critical eMS values required for C60 and C70 aggregation (12–14 for C60 and 27–31 for C70 ) it is, however, evident that C70 requires much more polar medium than C60 for the onset of the aggregation process.
The aggregation process of C70 has also been investigated by the steady-state fluorescence measurements in different solvent mixtures. Fig. 3 shows typical fluorescence results obtained for the C70 solutions in different TL–AN mixtures. On excitation at 500 nm, the C70 solution in TL shows an emission spectrum, characteristics of the monomeric C70 fluorescence, with a maximum at 670 nm and a shoulder at 690 nm [25,26]. On the addition of AN to this solution, the nature of emission spectra remains the same as long as the AN percentage remains below 70. However, on further increase in the AN percentage, the emission intensity is found to decrease suddenly with a concomitant red shift (15 nm) and broadening of the emission spectra compared to that of the monomeric emission. Similar changes in the fluorescence spectra were also observed earlier and assigned to the formation of C70 aggregates [3]. Thus, the fluorescence results support the inferences drawn from the absorption measurements. It is important to mention here that along with the above changes in the emission characteristic as the AN% exceed 70, a new strong emission band is also observed at 590 nm with 500 nm excitation, which is found to be strongly dependent on the excitation wavelength. Following Ghosh et al. [3],
Fig. 3. The steady-state fluorescence spectra (kex ¼ 500 nm) of C70 solution (12.6 lM) in TL–AN solvent mixtures at different solvent composition: ð–––Þ 0, ð Þ 70 and ð- - - - - -Þ 78%. Intensity of all these spectra has been normalized for comparison.
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this new emission band has been assigned to the Raman scattering peak. The intensity of the Raman peak is seen to increase with the AN composition beyond 70%, indicating the increase in the number of aggregates in the solution with an increase in the AN% beyond 70. 3.3. Dynamic light scattering and scanning electron microscopic measurements Dynamic light scattering (DLS) studies have been carried out for C70 solutions in different solvents to substantiate the results obtained by optical absorption and fluorescence measurements. In neat solvents like BZ, TL, BZN, etc., the DLS measurements show the absence of any detectable particle in the solution even for the saturated solutions of C70 . These results clearly indicate that no aggregates are formed in these solvents. Similarly, the C70 solutions in mixed solvents do not show any significant scattered light intensity as long as the percentage of the polar cosolvent remains below that required for critical eMS limit, indicated from absorption and fluorescence studies. When the composition of the polar cosolvent is, however, made beyond the critical limit, the DLS measurements show a considerable amount of scattered light. It is seen that with an increase in the composition of the polar cosolvent beyond the critical limit, the scattered light intensity gradually increases. Analysis of the DLS signals following PCS-QELS theory and assuming the aggregates are of effective spherical shape results in the estimation of the mean size of particles of 230–250 nm. The particle size is seen to increase only marginally on increasing the solvent polarity for any particular solvent mixture. Fig. 4 shows the variation in the count rate of the scattered light intensity with the eMS of the medium for BZN– MeOH solvent mixtures. Inset of Fig. 4 shows a typical histogram obtained from the DLS measurement of C70 solution in BZN (25%)–MeOH (75%) solvent mixture. It is evident from the DLS results that the aggregation sets on only after crossing the critical eMS value for the solvents used. The DLS results thus substantiate the results obtained from the absorption and fluorescence measurements, indicating that aggregation of C70
Fig. 4. Variation of the intensity of the scattered light from C70 solution (14 lM) in BZN–MeOH solvent mixtures with the solvent polarity. Inset: Typical histogram obtained by DLS experiment for C70 solution in BZN (25%)–MeOH (75%) solvent mixtures.
starts only when the solvent polarity exceeds a critical value. To obtain a direct evidence for the presence of aggregates in the present systems, we have also carried out the SEM measurements. The SEM films prepared from C70 solutions in solvents like BZ, TL, BZN, etc., do not show the presence of any particle. Similarly for C70 solutions in mixed solvents no particles could be detected in SEM when the dielectric constant of the solvent medium is less than the critical eMS value indicated from the absorption and fluorescence studies. However, the films prepared from C70 solutions in mixed solvents with eMS higher than the critical eMS value show the presence of small particles. Fig. 5 shows a
Fig. 5. Typical SEM micrograph corresponding to 12.6 lM solution of C70 in AN–BZN solvent mixture with 70% of AN.
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typical SEM micrograph obtained for C70 aggregates prepared in AN (70%)–BZN (30%) mixture. It is important to mention here that the particle density observed in the SEM measurement is very low, as compared to that observed for C60 aggregates [8,9]. This is due to very low concentration (10 lM) of the C70 solution used in the present study, as compared to 350 lM concentration used for C60 solutions. In the case of C70 aggregation, since a large volume percentage of the polar cosolvent is required in which C70 is almost insoluble [24], a very high concentration of C70 could not be achieved in the present studies.
4. Discussion The present study clearly indicates that similar to C60 , aggregation of C70 also requires a critical solvent polarity to be reached to set in the aggregation process. It is interesting to note, however, that the critical solvent polarity required for C70 aggregation is much higher (eMS ¼ 27–31) in comparison to that required for C60 aggregation (eMS ¼ 12–14). Since both C60 and C70 are nonpolar molecules, such a large difference in the required critical solvent polarity for the aggregation to set in for the two fullerenes appears to be very unusual. We feel that such a difference in critical eMS for C60 and C70 aggregation arises due to the differences in the polarizability of two fullerene molecules. The polarizability values for C60 and C70 are 6:4 1023 and 8 1023 cm3 , respectively [27]. Thus, compared to spherical C60 , the ellipsoidal C70 [28] has higher polarizability. As fullerenes are non-polar molecules, their aggregation in solution must be determined by the relative magnitudes of the different dispersive forces operating between solute–solute and solute–solvent molecules, in which the polarizability of the fullerene molecules plays a direct role. In less polar solvents fullerene monomers easily remain solvated due to fullerene–solvent dispersion forces. A small stabilization for the monomer solubility in solution may also come from the specific p–p interaction between the fullerene and the aromatic solvent molecules, where latter is one of the cosolvent [29–31]. As the polarity of the medium is
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increased, the fullerene–solvent interactions gradually become less dominant as the solute molecules are non-polar. Thus, with increasing solvent polarity, the fullerene–fullerene interaction gradually starts dominating over the fullerene–solvent interactions. As a result, beyond a critical composition of the polar cosolvent, where fullerene solubility is very poor, the fullerene–fullerene interaction will cause the aggregation to set in. As the polarizability of C70 is higher compared to that of C60 , in the former case the fullerene–solvent dispersive interaction makes these molecules to remain dissolved in the solution in their monomeric forms upto a reasonably higher solvent polarity as compared to that happens for less polarizable C60 molecules. Thus, it is expected that critical eMS for C70 aggregation will be much higher than that of C60 , as observed in the present study. At this point it is interested to point out another distinct difference observed between C60 and C70 aggregates in relation to their absorption spectra. For C60 aggregation, a blue shift is observed in the absorption maxima in comparison to its monomer spectra [8,9] where as for C70 the aggregation causes a red shift in the absorption maxima. These observations indicate that the mode of packing of the fullerene molecules in the aggregates could be different for two fullerene analogues. The blue shift in the absorption maxima for C60 indicates that the stacking of the monomers in the aggregates results in the formation of H-type of aggregation where as a red shift for C70 aggregation indicates that the monomers stacking results in the J-type of aggregation [32,33]. The reasons for such differences in the type of aggregation for C60 and C70 are not very clear to us. We, however, feel that it could be due to the difference in the shape of two fullerene molecules, spherical for C60 and ellipsoidal for C70 [28] and that leads to different type of stacking during the aggregation.
5. Conclusion Similar to C60 , the C70 in solution also requires a minimum solvent polarity to be reached before it can undergo the aggregation process. In terms of the solvent dielectric constant, the critical solvent
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polarity required for C70 aggregation is, however, found to be much higher, in the range 27–31, than that required for C60 aggregation (12–14). Such a large difference in the critical solvent polarity necessary for the aggregation of C60 and C70 has been rationalized on the basis of the polarizability of these two fullerene molecules. The shifts in the absorption spectra due to aggregation indicate the H-type of aggregation for C60 and J-type of aggregation for C70 . The shapes of the fullerene molecules seem to determine the nature of stacking of these molecules in the aggregates.
Acknowledgements We are grateful to Mr. K.K. Kutty for his kind help in the SEM measurements.
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