10158
J. Phys. Chem. B 1998, 102, 10158-10164
Aggregation of Fullerene, C60, in Benzonitrile Sukhendu Nath, Haridas Pal, Dipak K. Palit, Avinash V. Sapre,* and Jai P. Mittal† Chemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India ReceiVed: June 1, 1998; In Final Form: September 9, 1998
C60 solutions in benzonitrile have been found to show concentration dependent optical absorption behaviour. At lower concentrations (100 µM), however, the C60 solutions in benzonitrile show very broad absorption tail, extending beyond 900 nm. At higher concentrations the solutions are also visually opaque. From picosecond laser flash photolysis experiments it is seen that the triplet quantum yield of C60 in benzonitrile at higher concentrations (∼400 µM) is much less than unity and increases with the dilution while in decalin and benzene it is always close to unity and independent of the C60 concentration. Dynamic light scattering experiments indicate the presence of particles of mean size of about 250 nm in C60 solutions in benzonitrile with concentration >100 µM, while in 100 µM and that the aggregated and the monomeric form of C60 are in equilibrium.
1. Introduction Unusual color changes of the fullerene C60 and C70 solutions in certain solvent mixtures are very recent interesting observations. Sun and Bunker1 have shown that C70 solutions in toluene (TL) and acetonitrile (AN) mixtures show large changes in the optical absorption spectra and display unusual change in color from violet to pinkish purple when the volume percentage of AN is increased above 60%. Such changes have been attributed to the aggregation of C70 molecules in the solvent mixtures. The aggregation behavior of both C60 and C70 have extensively been investigated in our laboratory in a variety of solvent mixtures using optical absorption, fluorescence, and dynamic light scattering (DLS) techniques.2 It has been seen that the size of the aggregates varies from about 100 nm to about 1000 nm depending on the fullerene concentration. The aggregation process has also been observed to be reversible in nature.2 The only report on the aggregation of the fullerene in a neat solvent is the observation of Ying et al.3,4 Using both dynamic and static light scattering measurements the authors have shown slow aggregation of C60 in neat benzene (BZ) on long keeping (more than a month) at concentrations g1 mM at room temperature, to give weakly bound clusters of a mean diameter of about 16 nm. The aggregates observed by them are, however, very unstable, even to the mechanical shaking. Martin et al.5 have also reported clusters of C60 in the solid state with aggregation number up to 55. Recently Ahn et al.6 have seen the aggregation of C60 near the freezing temperature of a number of organic solvents by photoluminescence spectroscopy. The authors have also reported the dependence of the aggregation on C60 concentration in frozen toluene solutions. Recently Fujitsuka et al.7 have recorded the ground state absorption spectra of C60 fine particles in carbondisulphide (CS2)-ethanol (EtOH) medium. The absorption spectra of the fine particles * Correspondence author. Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India. Fax: 91-22-5515151. † Also affiliated with Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India.
is quite broad with a long absorption tail on the red edge of the spectrum and the absorption peak is slightly blue shifted as compared to the spectrum of free C60 molecules in CS2. They have also determined particles of ∼270 nm size by DLS. Temperature dependence of the solubility of C60 in different solvents has been found to follow an unusual trend.8,9 It is seen that the solubility of C60 in a given solvent first increases reaches a maximum value and then reduces as the temperature is progressively increased in the range 270-330 K. Bezmelnitsyn et al.10 have proposed that such unusual temperature effect on the C60 solubility is due to the aggregation of the fullerene in solution and have theoretically modeled the solubility which reproduces the experimental observations quite nicely. During our earlier studies on the interaction of the fullerenes with amine donors in different solvents we observed some unusual behavior of C60 in neat benzonitrile (BZN). It was noticed that when the C60 concentration in BZN was fairly high, the color of the solution was quite different from the usual purple color of C60. The absorption spectra of such C60 solutions in BZN were also seen to be drastically different from those with the lower concentrations of C60 in the same solvent. The gross observations with moderately concentrated C60 solution in BZN were very similar to those observed by Sun and Bunker1 and Ghosh et al.2 from the aggregation of C70 in mixed TL-AN solvents and those observed by the Fujitsuka et al.7 for the C60 fine particles in CS2-EtOH medium. In the present study the concentration dependent behavior of C60 in neat BZN has been investigated using ground state absorption, picosecond laser flash photolysis (ps-LFP), DLS, and scanning electron microscopy (SEM) techniques. All these results support the aggregation of C60 in neat BZN at higher concentrations. To the best of our knowledge this is the first report on the formation of stable and visually recognizable aggregates of a fullerene in a neat solvent. The results indicate that the photophysical proprerties of the aggregated C60 are drastically different as compared to those of the free C60 in a solution.
10.1021/jp9824149 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/24/1998
Aggregation of C60 in Benzonitrile
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10159
2. Material and Methods C60 was obtained from SES Corporation, U.S.A., and was used without further purification. All the experiments were carried out with two different lots of C60 to check the effect of impurity, if any. Both the lots gave very similar results. All the solvents used were of spectroscopic grade obtained from Spectrochem, India, and used as received. C60 solutions were prepared by ultrasonication followed by centrifugation and decantation. In some cases the solutions were also prepared by keeping the solid C60 in the solvent overnight and collecting the supernatent solution after centrifugation and decantation. Ground state optical absorption measurements were made with a Shimadzu UV-160A spectrophotometer. DLS studies were carried out with a Brookhaven Instruments model BI-90 particle size analyzer, functioning on the principle of quasielastic laser light scattering. A vertically polarized light (λ ) 632.8 nm) from a He-Ne laser is focused onto a thermostated liquid cell, and the amount of light scattered at a right angle to the direction of the laser beam was determined using a photomultiplier tube. Minimum particle sizes that can be detected with this instrument are about 10 nm. Image of the aggregated C60 was recorded using a scanning electron microscope (JEOL-JSMT330A). Microfilm of the C60 aggregates for SEM studies was prepared by putting a drop of concentrated C60 solutions (∼400 µM) in BZN on a brass stub and then drying off the solvent by evaporation. Picosecond LFP experiments were carried out using a pumpprobe spectrometer, described elsewhere.11 Briefly, the second harmonic output (532 nm, 4 mJ, 35 ps) of an active passively mode locked Nd:YAG laser (Continuum, USA, model 501-C10) was used for the excitation of the samples. The transients produced in the irradiated sample were detected in terms of their time resolved optical absorption spectra. A white light continuum (∼400-950 nm) produced by focusing the residual fundamental (1064 nm) of the Nd:YAG laser onto a 10 cm length quartz cell containing a 50:50 (v/v) H2O-D2O mixture was used as the monitoring light source. The probe light was passed through a variable optical delay line (1 m long) and then splitted into two parts using a 50:50 beam splitter. One part of the monitoring light was used as the reference beam and the other was used as the analyzing beam (passing through the irradiated sample). Both the reference and the analyzing beams were dispersed through a spectrograph and monitored using a dual diode array based optical multichannel analyzer which is interfaced to a personal computer to process the data and to obtain the optical absorption spectra of the transients at different time delays between the excitation and the analyzing beams. 3. Results 3.1. Ground State Absorption Study. Figure 1a and b shows the ground state absorption spectra of C60 at different concentrarations in neat BZN and decalin (DL). The spectra in Figure 1a and b are plotted after normalization for the highest concentration of C60 used in each of the solvents. In the present study, DL is chosen as a representative nonaromatic solvent where C60 has a good solubility but does not have any specific interactions, especially the π-π interactions, as reported for some aromatic molecules with the fullerenes.12-14 It is seen from Figure 1 that while the spectra of C60 in DL (Figure 1a) are independent of the C60 concentration, those in BZN are not (Figure 1b). Thus, as the concentration of C60 in BZN is increased, a long absorption tail extending beyond 900 nm gradually develops and the absorption peak at 535 nm gradually shifts to the shorter wavelength. The depth of the characteristic
Figure 1. Absorption spectra of C60 (a) in decalin, (b) in benzonitrile at different C60 concentrations. All the absorption spectra are normalized to the highest concentration used in each solvents. (c) Comparison of the absorption spectra of C60 in decalin, benzene, and benzonitrile. The spectra in benzonitrile was taken with very low concentration (∼70 µM) of C60 and represented by the lower curve A. The same spectra is also shown after normalization to those in benzene/decalin.
Figure 2. Lambert-Beer (L-B) plots for C60 ground state optical absorption in benzonitrile (O) and decalin (9). Solid line represents the initial slope of the benzonitrile data. Dashed line is the linear fit for the decalin data.
valley at ∼440 nm for the C60 absorption also gradually reduces with C60 concentration in BZN. Absorption characteristics of C60 with different concentrations were also investigated in detail in BZ and observed to be very similar to those in DL. It is thus evident that the C60 solutions in BZN behave differently as compared to those in other solvents like BZ and DL. Figure 2 shows the Lambert-Beer (L-B) plots for the C60 absorbances in BZN and DL at 535 nm. It is seen that while in DL the L-B plot is at least linear up to ∼500 µM, the same in BZN deviates largely from linearity beyond ∼100 µM. For BZ, too the L-B plot (not shown in the figure) was seen to be linear at least up to ∼500 µM of C60. The molar absorption coefficients () for C60 in DL and BZ at the absorption peaks (λmax) were estimated from the slope of the L-B plots and are given in Table 1. Dilute solutions (50 % BZN-BZ or BZN-DL mixed solvents showing absorption behavior similar to the C60 solutions in neat BZN clearly indicates that the behavior is due to the bulk solvent properties than to any specific interaction. In most of the cases we prepared the solutions by using sonication method and since Beck et al.15 had indicated that sonication may result in the formation of colloids, we checked whether the spectral changes in BZN at higher C60 concentrations are related to the sonication process. For this purpose we prepared C60 solutions (∼400 µM) by two different methods. In one method the solution was prepared by sonication followed by centrifugation and decantation. In the other, an excess amount of solid C60 was left overnight in BZN and the solution was then collected after centrifugation and decantation. In both the cases the solutions show exactly similar absorption characteristics. 3.2. Picosecond Laser Flash Photolysis Study. C60 in the excited singlet state undergoes fast intersystem crossing (ISC) to give C60 triplet with almost unit yield in solvents like BZ and TL.16-19 To know about the ISC and the triplet quantum yields (φT) in concentrated C60 solutions in BZN, ps-LFP experiments were carried out on C60 solutions in BZN, DL, and
Figure 3. Transient absorption spectra of C60 in benzonitrile at different time delays between pump and probe pulses. The C60 concentration used was 350 µM. Time delays for the spectra 1-6 are 0, 0.25, 0.46, 1.06, 2.25, and 3.7 ns respectively. Inset: Comparison of the triplet absorption spectra of C60 in decalin, benzene and benzonitrile.
BZ using 35 ps 532 nm laser pulses as the excitation source. In all the cases it was seen that immediately after the excitation pulse, a broad transient absorption spectra appears in the 820930 nm region. The broad spectra have been assigned to the S1 f Sn absorption of C60.20,21 As the time delay between the pump and the probe pulses increases, the absorption due to S1 f Sn transition decreases and concomitantly an absorption band at around 740 nm which is due to C60 triplet,22-26 increases. Typical such results are shown in Figure 3 for 350 µM C60 solution in BZN. Since the T1 state is populated via the ISC process from S1 state, the lifetime of the S1 state was estimated following the growth rate of the triplet absorption at 740 nm. Lifetimes of the S1 state of C60 in DL, BZ and BZN are thus estimated to be 1.14, 1.17, and 1.16 ns, respectively. These values are in good agreement with the reported S1 state lifetimes of C60.19-22,27 These results further indicate that the S1 state lifetime of the C60 does not depend on the solvent polarities or polarizibilities. The yield of C60 triplet (φT) is reported to be close to unity in BZ.19 Since the φT of C60 in DL solutions is not reported in the literature, we estimated the φT of C60 in DL by comparing the absorbances at 740 nm in this solvent at very long delay (∼6 ns) to those in BZ solutions. Taking the same OD’s at the excitation wavelength (532 nm) of the C60 solutions in both the solvents and assuming φT for C60 in BZ as unity, the φT for C60 in DL is also estimated to be unity within experimental error and is independent of the C60 concentration. In these estimations it was assumed that for the C60 triplet absorption (T) at 740 nm in DL is equal to that in BZ. This assumption does not introduce any appreciable error in the φT estimation because the shape as well as the peak position of the T1 f Tn absorptions in DL and BZ are very similar, as shown in the inset of Figure 3. Interesting results were obtained in the ps-LFP of C60 solutions in BZN. It was seen that though the T1 f Tn absorption spectrum of C60 in BZN was quite similar to those in BZ and DL solutions (Inset of Figure 3), the triplet signal in the BZN was weaker in comparison to those in the latter two solvents. It must be mentioned here that in these comparisons, the ground state absorbance of C60 solutions at the excitation wavelength (532 nm) were kept the same for all the three solvents. The low C60 triplet signal in BZN in comparison to those in DL and BZ could be due to the following reasons.
Aggregation of C60 in Benzonitrile
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10161
Figure 4. Plot of triplet quantum yield (φT) vs the concentration of C60 in BZN (b) and decalin-benzonitrile (50:50, v/v) mixed solvent (9).
Firstly, it could be possible that T in BZN is much lower than those in DL and BZ. Secondly it could be possible that the φT of C60 in BZN itself is very low. To distinguish between these possibilities we carried out the ps-LFP experiments on C60 solutions in BZN with different C60 concentrations. For comparison similar experiments in DL-BZN mixed solvent systems (50:50 v/v) were also carried out where no unusual spectral changes were seen (see section 3.1). In the 50:50 (v/v) DL-BZN mixed solvent systems the transient ODs at 740 nm at 6 ns delay for a number of C60 concentrations were measured and compared with those obtained in neat DL. The ground state ODs’ of C60 in both the solvent systems were kept the same at the excitation wavelength (532 nm). It was seen that irrespective of the C60 concentrations, the ratio of the triplet signal in DL-BZN mixed solvent system to the corresponding signal in neat DL (i.e., ODT (DL-BZN)/ ODT(DL)) was unity within experimental error. Further the shape and the peak position of the T1 f Tn absorption spectra were seen to be very similar for both DL and DL-BZN solvents. The S1 state lifetimes of C60 were also similar in both DL and DL-BZN mixed solvents. Thus the T in DL-BZN mixed solvent can be taken equal to that in neat DL. On the basis of this assumption, the φT values of C60 in DL-BZN mixed solvent were estimated at different C60 concentrations. The φT values are close to unity and are independent of C60 concentrations as shown in Figure 4. Following the above results in DL-BZN mixed solvent, it is assumed that in neat BZN the T will be similar to those in DL and BZ. It is thus expected that the low triplet absorption signals in BZN in comparison to those in DL and BZ are due to the low φT in neat BZN. To investigate further we estimated the φT values of C60 in BZN for a number of C60 concentrations and using C60 triplet yield in DL as the reference (φT ) 1). The result are shown in the Figure 4. It is seen from this figure that φT in BZN is strongly dependent on the C60 concentration, and at ∼400 µM the φT value is much less than unity (∼0.3). As the concentration of C60 in BZN is reduced, the φT value gradually increases and at ∼120 µM of C60 the φT is as high as 0.6. It is to be mentioned that for 100 µM C60 which show solvatochromic changes give strong light scattering signals. Analysis of the DLS data indicate that these solutions contain particles of mean size of about 250 nm, with a particle size distribution of about 100 to 500 nm. It was observed that the mean particle size as well as the size distribution are more or less independent of the C60 concentrations. These results differ from those observed by Ghosh et al.2 for the C70 aggregation in TL-AN mixed solvent systems, where the particle size had been seen to increase largely with the fullerene concentration. A typical histogram obtained from the DLS experiment of a concentrated C60 solution in BZN is shown in Figure 5. The above observations indicate that the results of DLS experiments agree well with the results from ground state absorption studies as well as from the picosecond laser flash photolysis studies. A point to be mentioned here that, for the concentrated C60 solutions in BZN (>100 µM), although the DLS results show that the particle size and the size distribution does not change much, the intensity of the light scattered by these solutions gradually increases as the C60 concentration was increased. Hence, the addition of more C60 to the BZN solutions increases the concentration of the particles, without affecting the particle size much. The results of SEM measurements as shown in the inset of the Figure 5, directly indicate the presence of particles with average size of ∼250 nm which match with DLS measurements. 4. Discussion From the ground state absorption measurements it is clear that the absorption characteristics of the dilute solutions of C60 in BZN (100 µM is attributed to the aggregation of C60. Similar solvatochromic changes have also been observed by Sun and Bunker1 for C70 in TN-AN mixture. Ghosh et al.2 have observed similar solvatochromic changes attributable to aggregation of both C60 and C70 solutions in TN-AN and several other solvent mixtures. Recently Fujitsuka et al.7 made similar spectral observations on C60 fine particles in CS2ethanol mixtures. Formation of aggregates of C60 in the present systems are confirmed by the DLS and SEM experiments. From our results given above, following points can be noted. The observed positive deviations from the linearity of the L-B plot in Figure 2 indicate that the of the C60 aggregates are much higher than those of the monomers in the spectral range of 400-700 nm. However, some contribution from the scattering cannot be neglected. The increase in the value of on aggregation can be explained by the following arguments. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the C60 molecule has hu and t1u symmetry, respectively.28 Since both the states are having the same parity, an electric dipole transition between these two states involving the absorption of a single photon is forbidden by the parity consideration. However, because of the “Herzberg-Teller” (H-T) coupling,29 a type of symmetry breaking mechanism by electron-vibration (vibronic) interaction, the above transition becomes partially allowed. Because of the high symmetry of the C60, both HOMO and LUMO are highly degenerate. As in the aggregates every C60 molecule is surrounded by many other C60 molecules, the local symmetry of the molecule will be lowered and thus the high degeneracy of the intramolecular modes will be lifted. Decrease in the degeneracy will lead to higher H-T coupling, enhancing the electronic transition. In BZN solutions it is found that the depth of the characteristic valley at ∼440 nm reduces as C60 concentration is increased. Catalan et al.30,31 have made similar observation for some aromatic solvents including BZN and attributed it to some specific solute-solvent interactions. We have excluded this possibility on the following basis. The absorption spectra of dilute solution of C60 in BZN was compared with those in BZ and DL after normalization (Figure 1c). It is clear from this figure that there is no difference in the spectra in these three solvents. If any specific interaction between solute and solvent exists in BZN, it should be present at any C60 concentration and thus one should expect spectral changes even in the dilute solutions, which were not observed. This clearly indicates that the spectral changes in concentrated C60 solutions in BZN is not due to specific interaction between C60 and BZN. Catalan et al.30 have excluded the possibility of the aggregation on the basis of the observation that the C60 solutions in those solvents where there were an enhanced absorption in the 440 nm region, did not change the shape of the spectra on filtration. The pore size of the filter papers used by them was about 0.45 mm. From DLS experiments we observed that the mean size of the aggregated particles in BZN is about 250 nm with a size distribution of about 100-500 nm. It is thus quite expected that the aggregated particles of these sizes will pass through the filter paper of pore size of about 0.45 mm. We had infact repeated the similar absorption spectral studies using filtration (Filter paper Whatman 542) of the aggregated C60 solutions in BZN. Contrary to Catalan et al.30 it was seen that on filtration the absorption characteristics slightly changed, though the gross picture of the absorption spectra remains same. It was seen that, after filtration for a number of times, the depth of the 440 nm valley increases and the long absorption tail reduces substantially for the concentrated C60 solution in BZN. We
Nath et al. explain these observations as due to the adsorption of some of the aggregated C60 on the cellulose materials of the filter papers. That the C60 aggregates largely gets adsorbed on the filter paper is also visually indicated from the blackish appearance of the filter paper after filtration. However, according to our results, since the behavior of C60 at concentrations
