J. Phys. Chem. 1996, 100, 9439-9443
9439
Aggregation of C70 in Solvent Mixtures Hirendra N. Ghosh, Avinash V. Sapre, and Jai P. Mittal*,† Chemistry DiVision, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India ReceiVed: NoVember 22, 1995; In Final Form: March 11, 1996X
The unusual solvatochromism of C70 is investigated in a variety of solvent mixtures by optical absorption and fluorescence techniques. Distinct reversible color change from pink to purple is seen in the solvent mixtures studied. Such changes are seen also for C60 solutions in some solvent mixtures. Formation of clusters is found to be responsible for the observed optical changes. Light scattering studies are carried out to confirm the presence of clusters which show that the particle size varies from ∼100 to ∼1000 nm depending on the concentration of the fullerene. It is found that the solubility of the fullerene in the solubilizing solvent and that in the solvent mixtures are the major factors governing the aggregation behavior of the fullerene. Polarity of the solvent plays a minor role in the formation of aggregates.
Introduction Photophysical properties of fullerenes C60 and C70 in solution phase have attracted considerable attention.1-7 Optical absorption spectra of the ground state fullerene molecules in the UVvisible range have been well studied.1,8,9a-c Fluorescence properties of these molecules have also been investigated in solutions and in frozen matrices at 77 K.2,9-13 Although, the major features of the weak fluorescence of both the C60 and C70 molecules have been understood, discrepancies exist between the results of several groups.3,8-10 Two-dimensional luminescence spectra have been recorded in glassy, polycrystalline, and Shpolskii matrices at 77 K and a pronounced solvent effect has been noticed.2 Recently, it has been reported that C70 exhibits unusual solvatochromism in toluene-acetonitrile mixtures at room temperature.14 The authors find that as the volume fraction of acetonitrile exceeds 70%, dramatic color change from reddish orange to pinkish purple takes place. The absorption spectra become broader and intense in the region 550-800 nm. The usual structural features of the spectra in the region of 300400 nm are lost. The spectral changes are found to be reversible and are attributed to the formation of new microscopic cluster type of species. If cluster formation is responsible for the absorption changes, it should get reflected in the fluorescence properties. Hence, we have carried out steady state and time-resolved fluorescence measurements to gain insight into the unusual solvatochromism observed for C70 in a variety of solvent mixtures. Particle size measurements have been carried out to confirm the formation of clusters. Some experiments are also carried out on C60 in typical solvents mixtures to observe cluster formation. Experimental Section C60 and C70 from SES Corp., USA, were used after checking their purity by absorption spectroscopy. All the solvents used were of spectroscopic or reagent grade. Absorption spectra were recorded with Shimadzu Model 160A spectrophotometer. Steady state and time-resolved fluorescence measurements were carried out with Hitachi Model F-4010 and Edinburgh Instruments Model EI-199 spectrophotofluorimeters respectively. * Author to whom correspondence should be addressed. † Also associated with Jawaharlal Nehru Center for Advanced Scientific Research, Bangalore, India. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
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Solutions are carefully purged with high-purity N2 before taking fluorescence measurements. Particle size studies have been carried out with Brookhaven Instruments Model BI-90 particle size analyzer functioning on the principle of quasielastic laser light scattering. A vertically polarized light from He-Ne laser (5 mW) is focused on a thermostated liquid cell, and the scattered light at 90° to the incident beam is measured using a photomultiplier tube, and the fluctuations about the mean value are correlated to give particle size. Results and Discussion 1. Absorption Spectra. (a) Studies on C70 Solutions. To start with, the dramatic changes in the absorption spectra of C70 on increasing the volume fraction of acetonitrile (AN) in toluene (TN) have been confirmed14 and are given in Figure 1A. The absorption maxima show red shift and broadening as the AN% exceeds 70%. Other solvent mixtures namely (1) benzene (BZ)-AN, (2) BZ-methanol (MEOH), (3) BZ-hexane (HX), (4) benzonitrile (BZN)-HX, (5) BZN-AN, and (6) o-dichlorobenzene (DCB)-AN have also been examined for the presence of the unusual absorption behavior of C70 in these solvent mixtures. The solvents are chosen so that suitable combinations of aromatic, aliphatic, polar, and nonpolar solvents could be obtained. The concentration of C70 is also varied in these mixtures to see the onset of cluster formation. Figure 1A shows the optical absorption behavior of C70 ([C70] ) 3.5 × 10-6 mol‚dm-3) in toluene-AN solvent mixtures.14 Figure 1, B and C, shows the absorption behavior of C70 in BZN-AN ([C70] ) 4.5 × 10-6 mol‚dm-3) and BZ-MEOH ([C70] ) 6 × 10-6 mol‚dm-3) solvent mixtures, respectively; in which a shoulder appears ∼590 nm. C70 in BZ-HX and BZN-HX mixtures does not display the color changes. It is clear from Figure 1 that the absorption changes in C70 are not specific to toluene-AN mixtures but do occur in many other solvent mixtures and are reversible. Such absorption changes observed in toluene-AN mixtures have been attributed to the formation of C70 aggregates or clusters above 70% AN (v/v).14 On increasing C70 concentration at the same solvent composition, a prominent new band due to clusters appears and cluster formation is found to be depend on the concentration of C70. Absorption spectra observed for C70 in BZN-AN and BZMEOH solvent mixtures (Figure 1B,C) are different and could arise due to different size distribution of the clusters in these solvents. It is also observed from Figure 1 (spectra A3, A4, B2, B3, C2, and C3) that in different solvent mixtures the © 1996 American Chemical Society
9440 J. Phys. Chem., Vol. 100, No. 22, 1996
Figure 1. Optical absorption spectra of C70: (A) AN-toluene mixtures (v/v): (1) no AN, (2) 50% AN, (3) 70% AN, (4) 90% AN, [C70] ) 3.5 × 10-6 mol‚dm-3. (B) BZN-AN mixtures (v/v): (1) no AN, (2) 70% AN, (c) 90% AN, [C70] ) 4.5 × 10-6 mol‚dm-3. (C) BZ-MEOH mixtures (v/v): (1) no MEOH, (2) 70% MEOH, (3) 90% MEOH, [C70] ) 6 × 10-6 mol‚dm-3. (D) Optical absorption spectra of C60 in ANtoluene mixtures (v/v): (1) no AN, (2) 70% AN, [C60] ) 3.5 × 10-6 mol‚dm-3.
absorption spectrum of the aggregated C70 molecules is different. Solubility difference of C70 in the corresponding components of the solvent mixtures and the concentration of C70 could be responsible for such behavior. Experiments have been also carried out under two different conditions: (1) the C70 solutions are cooled to 77 K and then brought back to normal temperature, (2) the solutions are heated upto 330 K and then allowed to cool to room temperature. The absorption spectra are taken before and after heating/cooling in both the above cases and do not change appreciably. It shows that heating/cooling does not induce precipitation or coagulation of the clusters in the solutions studied. The color of the solutions at low C70 concentrations does not change for a few days. However, the OD of the solution somewhat decreases in a couple of days. The OD of these solutions do not change for at least 3 h even after continuous shaking, agitation, and other mechanical handling. In toluene, benzene, and benzonitrile, C70 has almost similar solubility, and hence the cluster formation has been observed in the same composition range of the solvents. In o-dichlorobenzene (DCB) the solubility of C70 is ∼25 times higher than that of benzene and toluene. Although in AN and toluene mixtures (70:30 v/v) a C70 concentration of 1.5 × 10-6 mol‚dm-3 is enough to give clusters, in the case of AN and DCB solvent mixtures (70:30 v/v) 36 × 10-6 mol‚dm-3 C70 is required (∼25 times that in toluene) to see the cluster formation. The color of the solution containing clusters in AN/DCB mixtures is blackish. C70 concentration and its relative solubility in the solubilizing solvent, e.g. TN, BZ, BZN, and DCB, seem to be important parameters governing the aggregation.
Ghosh et al. (b) Studies on C60 Solution. C60 and C70 are both structurally similar, and therefore aggregation is expected in C60 solutions also. Further, solubilities of both C60 and C70 molecules in different solvents are similar.22,23 Hence, absorption spectra have been taken in the above-mentioned solvent mixtures to see the aggregation of C60. Experiments are carried out at different compositions of acetonitrile and toluene. In >70% AN (v/v) solutions, C60 ([C60] ) 3.5 × 10-6 mol‚dm-3) is found to give color change from purple to reddish, although the change is less marked in comparison with C70 solutions. The absorption spectrum on aggregation extends further into the red region as compared to the C60 solutions in neat toluene (Figure 1D). Concomitantly, ODs of the peaks in the UV region reduce. The structural feature at 404 nm becomes less prominent and that at 600 nm is lost. Similar behavior is seen for other solvent mixtures studied. 2. Steady State Emission Spectra. Both steady state and time-resolved fluorescence experiments are carried out to see whether the formation of clusters is reflected in such studies. Both the emission and excitation spectra (resolution ∼ 3 nm) shown are uncorrected for the instrument response. The fluorescence emission from C70 in toluene solutions ([C70] ) 3.5 × 10-6 mol‚dm-3) is weak (quantum yield Φ ) 5 × 10-4).11,16 The excitation and emission spectra of C70 (3.5 × 10-6 mol‚dm-3) in toluene and toluene-AN (30:70) mixtures are displayed in Figure 2, A and B, respectively. In neat toluene, the emission spectrum shows a maximum at 664 nm (Figure 2Ba) corresponding to the fluorescence of C70.16 The other peak at 552 nm is due to the scattering (Figure 2Ba). Although the excitation spectra (Figure 2Aa) and absorption spectra (Figure 1A.1) for C70 in toluene do not match perfectly, the major peak at 480 nm and structure at 350 nm are reproduced. Catalan and Elguero9 have reported similar observation. Our spectra are somewhat broad as compared to those of Catalan and Elguero9 due to lesser spectral resolution. The fluorescence peak at 664 nm decreases considerably on aggregation in toluene-AN (30:70 v/v) mixtures and the scattering peak at 552 nm (λex ) 550 nm) increases in intensity by ∼100 times as compared to the scattering peak in neat toluene solution. Two additional emission peaks appear at 595 and 638 nm (Figure 2Bb). The corresponding excitation spectrum (Figure 2Ab) does not show any resemblance with the absorption spectrum of C70 (Figure 1A.3). The emission peak is found to shift when excitation wavelength is changed from 400 to 650 nm. The excitation peak is also found to be shifted when the emission wavelength is changed. Hence, it is concluded that in the toluene-AN (30:70 v/v) mixtures, the emission arises due to scattering and the fluorescence of C70 is diminished considerably. Figure 2C shows the emission spectra of C70 (3.6 × 10-6 mol‚dm-3) in toluene, toluene-AN (50:50 v/v), and tolueneAN (10:90 v/v) mixtures taken at higher gain as compared to Figure 2B. It is seen from Figure 2Cb that in 50:50 (v/v) ANtoluene solution, the fluorescence of C70 decreases and a shoulder appears at 630 nm. In 90% AN solutions the fluorescence due to C70 diminishes drastically and the emission at 638 nm dominates. The spectra in Figure 2C are normalized to their maximum intensity. The nature of the emission spectra does not change in solvent mixtures containing high AN% (7095%). These observations show that even at 50% AN, changes in fluorescence behavior have been observed which could not be seen in the absorption spectra at this solvent composition. Similar studies are also carried out using BZN-AN, BZMEOH, and DCB-AN solvent mixtures. In these solutions
Aggregation of C70 in Solvent Mixtures
J. Phys. Chem., Vol. 100, No. 22, 1996 9441
Figure 2. (A) Excitation spectra: (a) 100% toluene (λem ) 664 nm), (b) 30% toluene (λem ) 638 nm). (B) Emission spectra of C70 in AN-toluene solvent mixtures (v/v) λex ) 550 nm: (a) no AN, (b) 70% AN. (C) Emission spectra of C70 in AN-toluene solvent mixtures (v/v) λex ) 550 nm: (a) no AN, (b) 50% AN, (c) 90% AN, [C70] ) 3.5 × 10-6 mol‚dm-3. Spectra are normalized for respective maximum emission intensities.
also it is observed that on aggregation fluorescence intensity decreases and scattering increases. For solvent mixtures like BZ-HX and BZN-HX no major changes have been observed both in the optical absorption and emission spectra and there is no evidence for cluster formation. However, in BZN-HX mixtures C70 fluorescence spectrum shows vibronic structure changes due to reverse Ham effects as a function of solvent polarity similar to those observed by Sun and Bunker.21 3. Fluorescence Polarization Studies. Fluorescence polarization experiments were carried out on C70 solutions (3.5 × 10-6 mol‚dm-3) in neat toluene and toluene-AN (30:70 v/v) mixtures (λex ) 550 nm). Figure 3A shows the plot of fluorescence intensity vs wavelength in both parallel (I|) and perpendicular (I⊥) orientations (Figure 3, Aa and Ab, respectively) for C70 in toluene. Figure 3, Ba and Bb, shows similar results for C70 in toluene-AN (30:70 v/v) mixtures (λex ) 550 nm). The depolarization ratio (d) defined as I|/I⊥ was calculated. The parameter d changes from 0.18 to 0.05 at 552 nm for neat toluene to toluene-AN solution of C70 and d for the C70 fluorescence peak at 664 nm is 0.53. This clearly reflects the change in scattering behavior on aggregation. For the C70 aggregates in toleuene-AN mixtures, the ratio d at 552 and 638 nm is similar, i.e., 0.05 and 0.06, showing that the origin of the two peaks are similar and the emission probably arises due to similar scattering processes. The d values are in agreement with the notion that scattered light is strongly polarized. The scattering peak at 552 nm (λex ) 550 nm) due to Rayleigh scattering (particle size
