J. Phys. Chem. 1996, 100, 501-506
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Identity of Transients Formed from Chlorinated Fullerenes in Organic Solvents: A Pulse Radiolysis Study K. I. Priyadarsini,† H. Mohan,† P. R. Birkett,‡ and J. P. Mittal*,†,§ Chemistry DiVision, Bhabha Atomic Research Centre, Trombay, Bombay-400085, India, and The School of Chemistry and Molecular Sciences, UniVersity of Sussex, Brighton BN1 9QJ, U.K. ReceiVed: June 16, 1995; In Final Form: October 10, 1995X
The UV-vis absorption spectra of chlorinated fullerenes (C60Cl6, C60Cl12) in cyclohexane exhibit bands with λmax ) 210 and 255 and shoulders at 275 and 385 nm. The sharpness and molar absorptivity decrease with increase in the number of chlorine atoms. Pulse radiolysis studies of these chlorinated fullerenes were carried out in benzene, 1,2-dichloroethane, and cyclohexane. In benzene, the triplets of chlorinated fullerenes were observed. The radical cations of chlorinated fullerenes, generated in 1,2-dichloroethane, exhibit absorption bands with λmax ) 400 nm for C60Cl12 and 380 nm for C60Cl6. In cyclohexane, a slow forming transient at λmax ) 430 nm for C60Cl12 and 370 nm for C60Cl6 was generated. These bands appear to be different from the triplet and parent radical cation.
Introduction The recent success in the preparation of macroscopic quantities of fullerenes in high purity has led to extensive studies of their physical and chemical properties.1,2 New derivatives of C60 are constantly being prepared with varied applications.3,4 Due to the presence of a number of π bonds, C60 can undergo addition reactions with many electrophiles and nucleophiles.5,6 One of the recent interests in C60 has been to produce halogenated derivatives.6-10 C60 has high affinity toward halogens, and it can accommodate as many as 24 chlorine and bromine atoms.10 Recently, Birkett et al. have reported the formation of C60Cl6 by the controlled addition of chlorine.8 Although the photophysical, photo, and radiation chemical properties of C60 are well documented in both organic and aqueous media,3,11-16 such studies with derivatives of C60 have not been carried out. Here we report some of our recent studies on transient species formed in the pulse radiolysis of chlorinated fullerene in organic solvents. Experimental Section Spectrograde benzene, cyclohexane, and 1,2-dichloroethane were obtained from M/S Spectro Chem. India and were used without any further purification. Iolar grade N2, N2O, and O2 gases were obtained from M/S Indian Oxygen Ltd. C60 was obtained from SES Research Chemicals. Iodine monochloride (98%) was obtained from Aldrich Chemicals and used without any further purification. The optical absorption studies were carried out with a Hitachi 330 spectrophotometer. Pulse Techniques. Pulse Radiolysis. For pulse radiolysis studies, 7 MeV electron pulses from a linear accelerator with a pulse width of 50 ns and 2 µs were used.17 The transient species were detected by kinetic spectrophotometry. The absorbed dose for a 50 ns pulse is ∼10 Gy, whereas it is 4-5 times higher for 2 µs pulses. As far as possible, the dose absorbed was kept to the minimum, and fresh samples were used for each experiment to avoid secondary reactions. The absorbed dose was deter†
Bhabha Atomic Research Centre. University of Sussex. § Also associated with the Jawahar Lal Nehru Centre for Advanced Scientific Research, Bangalore, India. X Abstract published in AdVance ACS Abstracts, December 15, 1995. ‡
0022-3654/96/20100-0501$12.00/0
mined by using aerated KSCN solutions as the dosimeter using G ) 21 520 dm3 mol-1 cm-1 for 100 eV of absorbed dose (the G value is the number of radicals or molecules produced for 100 eV of absorbed energy, and is the molar absorptivity of (SCN)2•- at 500 nm). Laser Flash Photolysis. Nanosecond laser flash photolysis experiments were performed with a computerized laser kinetic spectrometer (Applied Photophysics Ltd., U.K.) with a KrF excimer laser (248 nm, fwhm ∼12 ns, 70 mJ) as excitation source.18 Each signal was stored after averaging over four laser shots. Picosecond laser photolysis experiments were carried out with a pulsed, mode-locked Nd:YAG laser (Continuum, Model 501-C-10) generating pulses of 35 ps.19 It was frequency doubled, tripled, and quadrupled to give wavelengths of 532, 355, and 266 nm, respectively. Laser pulses of 355 nm were used in the present experiments. The transient species were probed by white analyzing light (400-900 nm) produced by focusing the fundamental frequency in a H2O-D2O cell (50: 50). The arrival of the probe pulse was delayed from 0 ps to 6 ns using a delay rail assembly. Preparation of Chlorinated Fullerenes. Preparation of C60Cl12. C60Cl12 was prepared by treating 15 cm3 of benzene solution of C60 (2.0 × 10-4 mol dm-3) with iodine monochloride (0.5 cm3). The solution was stored in the dark for 3 days. The solvent, an excess of ICl, and liberated I2 were removed under nitrogen atmosphere at 45 °C. Orange crystals of chlorinated fullerene were obtained after complete dryness. The crystals readily dissolved in benzene, cyclohexane, and other nonpolar solvents, and they have no solubility in polar solvents. Any trace of free I2 left in the sample solution was tested spectrophotometrically by converting I2 to I3- on extracting the solution with aqueous potassium iodide solution. The result showed an absence of any free I2. Preparation of C60Cl6. C60Cl6 was prepared at The School of Chemistry and Molecular Sciences, University of Sussex, U.K., according to the procedure given in the literature.8 Estimation of Cl in the Chlorinated Fullerene. The number of chlorines in the chlorinated fullerene was determined by the well-known sodium fusion method.20 A known amount of chlorinated fullerene was fused with an excess of sodium and dissolved in a known amount of water. The Cl- formed was estimated spectrophotometrically by the Hg(CNS)2-Fe(ClO4)3 method after calibration with a standard NaCl solution.21 The © 1996 American Chemical Society
502 J. Phys. Chem., Vol. 100, No. 2, 1996
Figure 1. Ground state optical absorption spectrum of (a) C60Cl12 (5.2 × 10-5 mol dm-3), (b) C60Cl6 (2.9 × 10-5 mol dm-3), and (c) C60 (1.4 × 10-5 mol dm-3) in cyclohexane.
absorbance of the complex was followed at 450 nm. The percent of chlorine in C60Cl12 was determined to be 38 ( 5. This corresponds to a number of chlorine atoms in the prepared sample of C60Cl12 equal to 12-14. The sample of C60Cl6 was also analyzed for chlorine content, and the number of chlorine atoms in the sample was determined to be equal to 6, demonstrating the accuracy of the sodium fusion method.8,20 The number of chlorine atoms present in the chlorinated fullerene may be 6, 12, or 18 depending upon the conditions of the experiment.22 To compare the results, C60Cl6 prepared by Birkett et al.8 at the University of Sussex and C60Cl12 prepared at BARC (Bhabha Atomic Research Centre) were used. All the studies were carried out with both these samples. Results and Discussion Optical Absorption Studies. Figure 1a shows the optical absorption spectrum of C60Cl12 in cyclohexane ((5.2 ( 1.5) × 10-5 mol dm-3). It exhibits absorption bands at 210, 255, 275, and 385 nm. The molar absorptivity at 255 nm was determined to be 31 730 ( 9000 dm3 mol-1 cm-1. The absorption spectrum of C60Cl12 (prepared at BARC) was compared with that of C60Cl6 (prepared at the University of Sussex). The position of the absorption bands of C60Cl6 in cyclohexane (Figure 1b) were similar to those of C60Cl12, but the bands are sharper and have higher molar absorptivity (255 ) 75 860 ( 12 300 dm3 mol-1 cm-1). C60 in cyclohexane (1.44 × 10-5 mol dm-3) has absorption bands at 213, 260, and 330 nm (Figure 1c). The absorption bands of C60 in cyclohexane are very sharp and have a much higher molar absorptivity (260 ) 1 75 000 dm3 mol-1 cm-1).1 From these studies, it appears that the presence of chlorine in C60 lowers the molar absorptivity and broadens the absorption bands. Formation of Excited States. The radiolysis of benzene produces excited species (singlet and triplet states) due to fast geminate recombination processes. The G value for the triplet excited state of benzene is 4.2.23 Its energy level is 84 kcal mol-1. These excited species of benzene undergo energy transfer to solute molecules, and thus pulse radiolysis of benzene solutions containing chlorinated fullerenes can help in generating
Priyadarsini et al.
Figure 2. Transient optical absorption spectrum obtained on pulse radiolysis (5 µs after the pulse) of N2-saturated solutions of (a) C60Cl12 in benzene (2.0 × 10-4 mol dm-3, dose ) 54 Gy). Inset shows the absorption-time signal at 380 nm. (b) Transient spectrum of C60Cl6 (1.2 × 10-4 mol dm-3, dose ) 59 Gy) in benzene.
triplet excited states. The overall scheme is represented below:
BZ Df BZ+, e-, BZ*
(1)
BZ+ + e- f 1BZ*, 3BZ*
(2)
S + e- f S-
(3)
BZ+ + S f BZ + S+
(4)
BZ+ + S- f BZ + 3S*, or 3BZ* + S
(5)
S+ + S- f 1S* + 3S*
(6)
BZ* + S f 3S* + BZ
(7)
3
Here BZ is benzene and S is the solute. (a) Pulse Radiolysis of C60Cl12 in Benzene. Pulse radiolysis of N2-saturated solutions of C60Cl12 in benzene showed formation of transient species absorbing from 340 to 600 nm (Figure 2a). The transients exhibit two distinct regions of absorption, one with an absorption maximum at 380 nm and another broad absorption band in the region 440-540 nm. The transient in both the regions decayed by good first-order kinetics with k ) 7.1 × 103 s-1 (inset of Figure 2). In the presence of oxygen both the yield and the decay rate of the species changed. By following the decay of the transient in the presence of different known concentrations of oxygen, the rate constant for its reaction with oxygen was determined to be 1.2 × 107 dm3 mol-1 s-1 (Table 1). (b) Pulse Radiolysis of C60Cl6 in Benzene. Pulse radiolysis studies of N2-saturated solutions of C60Cl6 in benzene (1.2 × 10-4 mol dm-3) showed the formation of a transient with an absorption maximum at 370 nm and a broad absorption in the region 440-530 nm (Figure 2b). It decayed by first-order kinetics with k ) 1.1 × 104 s-1. The decay and the yield of this band were affected by oxygen, as in the case of C60Cl12, and the rate constant for its reaction with oxygen was determined to be 3.3 × 107 dm3 mol-1 s-1 (Table 1). (c) Laser Flash Photolysis Studies. The triplets of chlorinated fullerenes can be generated by direct excitation by laser flash photolysis. During photolysis under monophotonic conditions,
Identity of Transients Formed from Chlorinated Fullerenes TABLE 1: Kinetic Parameters for the Transients Formed on Pulse Radiolysis of Chlorinated Fullerenesa system
λmax (nm)
C60Cl12/Bz C60Cl6/Bz C60Cl12/DCE C60Cl6/DCE C60Cl12/CyH C60Cl6/CyH
380, 440-540 370, 440-530 400,
