Generation of the C60 radical cation and the radical adduct of trichloromethyl radical to C60. Pulse radiolysis and laser photolysis of C60 in CCl4

9 June 1995

ELSEVIER

CHEMICAL PHYSICS LETTERS Chemical Physics Letters 239 (1995) 112-116

Generation of the C60 radical cation and the radical adduct of trichloromethyl radical to C60. Pulse radiolysis and laser photolysis of C60 in CC14 Si-de Yao a, Zhi-rui Lian a, Wen-feng Wang a Jia-shan Zhang a, Nian-yun Lin a, Hui-qi Hou b, Zhen-man Zhang h, Qi-zong Qin b a Laboratory of Radiation Chemistry, Academia Sinica, Shanghai Institute of Nuclear Research, P.O. Box 800-204, Shanghai 201800, People's Republic of China b Institute of Laser Chemistry, Fudan University, Shanghai 200433, People's Republic of China

Received 15 March 1994; in final form 14 March 1995

Abstract The radical cation of C6o is generated either via hole transfer from the radical cation of CCI 4 to C6o or via electron transfer from triplet C6o to CC14. The radical adduct of "CC13 to C6o, [C6oCC13]", was also produced via two different mechanisms, namely the radical addition reaction and the reaction between triplet C6o and CCl4, respectively.

1. Introduction The discovery of the facile laboratory synthesis and characterization of C6o [1] have triggered an intensive interest in exploring its basic properties. The excited state [2], cation radical [3,4], and anion radicals [5] of C6o have been studied. Electron transfer processes involving C60 both in the ground and in excited states [4,6-8] have also been reported. In our previous work [9], we have reported the generation of the radical cation of C6o (C~0") via an electron transfer reaction from the triplet excited state of C6o (3C60) to CCI 4 or CCIaCF 3 for the first time and observed a transient absorption peak of C6+0" at 650 nm. However, most of the research works were carried out in non-halogenated hydrocarbon solvents. Recently, the radical adducts of C6o following pulse radiolysis of C6o in CC14 were studied by Dimitrijivic and the radiation-induced addition of the

trichloromethyl radical ("CC13) or the trichloromethylperoxyl radical (CC13OO ") to C60 were reported [10]. In this Letter, the transient absorption spectra of radiation- and laser-induced transient species of C60 in CCI 4 have been studied synergistically and it was found that the characteristic absorption peak of C6+0 at 650 nm was unambiguously different from that of the radical adduct of "CC13 to C60 ([C60CC13]') at around 430 nm. A hole transfer reaction for production of C/,0 from pulse radiolysis of C60 in CCI 4 has also been suggested. In addition two different mechanisms of addition of "CCI 3 to C60 have been proposed.

2. Experimental C60 was prepared and characterized according to a method similar to that discovered by Kr~tschmer et

0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 0 9 - 2 6 1 4 ( 9 5 ) 0 0 4 1 3 - 0

S. Yao et al. / Chemical Physics Letters 239 (1995) 112-116

al. as described previously [9]. EEl 4 (analytical grade) was commercially available and distilled before use. The pulse radiolysis experiment was carried out using a 10 MeV linear accelerator which delivered an electron pulse with a duration of 8 ns. The maximum dose of a single pulse is 15 Gy determined by thiocyanate dosimetry and corrected for the electron density of the organic solvents employed. The details of the setup and operation conditions of the equipment were described elsewhere [9]. The laser flash photolysis experiments were carried out using a home-built KrF excimer laser which provided a 248 nm laser pulse with a duration of 15 ns. The maximum energy is about 50 mJ per pulse. A detailed technical description of this facility has been given elsewhere [11,12].

113

0.012

0.012 08 ~"

00,o oo0o

o ~ 0.006

o.o /o .x

\~

II /}-o0oo

J

65Onto /

/ ........

~

0 20 40 60 80 Time(us)

m <

0.004 o

0

0 o o Q o

0.002 0.000

......... 550

~. . . . . . . . . , ......... 650 750 Wavelength(rim)

850

Fig. 1. Transient absorption spectra of deaerated CCl4 containing 0.1 mM C~0 at different times followingthe electron pulse. (0) 1 Ixs; ( • ) 40 Ixs. The inset shows the absorption-time profile at 650 nm.

3. Results and discussion

3.1. Pulse radiolysis o f C6o in

CCI 4

Carbon tetrachloride has been commonly used as an efficient solvent to oxidize the organic solutes due to its high ionization potential (11.47 eV) [13]. Although the radiolysis of CC14 has been studied extensively, the variety of radiolytically generated intermediates is still under discussion as briefly reviewed by Neta et al. [14]. Nevertheless, despite these disagreements, it is generally accepted that the primary cation and radical species are CCI~', "CC13, and other species, E E l 4 -'-)

CCI~-',

"CC13,

CCI~',

El-,

:CC12,

C1.

(1) By monitoring species at different time regions, one can selectively studied the reaction of CCI~" or 'CCI 3 with solute molecules. Washio et al. have studied the pulse radiolysis of some aromatic molecules including benzene, pyrene and naphthalene in CC14 solution [15] and found that different transient species of cation, dimer cation radical or CT complex with C1 atom (M~+CI~-) were produced somewhat relating to the ionization potential (IP) of solute. C60 usually behaved as an aromatic compound and its IP (7.6 eV [16]) is similar to that of pyrene

(7.5 eV [17]) although it has no hydrogen atom. Therefore, it can be expected that radiolysis of C6o in CCI 4 solution should also produce a radical cation. Fig. 1 shows the transient absorption spectra recorded after the pulse radiolysis of C60 in CC14. The characteristic spectra of 3C60 did not exist as in the case of pulse radiolysis of C60 in benzene solution [3]. However an obvious absorption peak appeared at 650 nm as shown in Fig. 1 instead, which is in good accord with that of C~o" either predicted by C N D O / S calculation [4] or assigned from our previous pulse radiolysis of C60 in benzene containing electrophilic agents [9]. So the transient absorption peak at 650 nm from pulse radiolysis of C60 in CC14 should also be +. assigned to C60. However, it is generated via a hole transfer mechanism, C60 -I- CCI~-" ~ C6+0' -~ CC1 a .

(2)

CCI~-' derived from dissociation of CCI~-" should give less contribution to the production of C~o" because the ionization potential of "CCI 3 (8.78 eV) [16] is near to that of C60 (7.8 eV). The absorption-time profile of the absorption peak at 650 nm is shown in the inset of Fig. 1. At the concentration of C6o (0.1 mM) and close (about 15 Gy) employed in our experiment, the rising time of C~-0" is within 1 ~s, and its life time is around 40 txs.

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s. Yao et al. / Chemical Physics Letters 239 (1995) 112-116

From the rising trace at 650 nm the generation rate constant of C~0" following reaction (2) is estimated to be about 3.5 × 10 ]° mo1-1 dm 3 s -1. One may argue that this peak around 650 nm may stem from the dimer cation radical of C6o, (C60)~-', or CT complex (C~0+CI8-) as in the case of aromatic molecules described above [15]. However, by carefully examining the experimental conditions and data obtained, this possibility can be excluded based on the following points. (1) The rising time of this transient species is about 1 I~s which is much longer than the usual rising time of a CT complex of about several ten ns [15]. (2) Its life time is about 40 txs which is much longer than that of a CT complex which is usually shorter than 1 p.s [15]. (3) Under the conditions employed in our experiment, the concentration of C60 (0.1 raM) is much lower than that used in the study of radiolysis of aromatic molecules (i.e. several tens mM), so there is no sufficient amount of C6o to react with C~-0" for production of (C60)f'. (4) As shown in our previous pulse radiolysis of C60 in benzene with 3.5 mM GEl 4 additive [9], this should produce a much lower concentration of CI- than that from pulse radiolysis of C6o in neat CCI 4 in this experiment, so in that case the concentration of C1is too low to produce C~o+ C1~-, but the peak at 650 nm still exists. This fact indicates that the peak at 650 nm should not be assigned to C6~0+ CI 8-. 0.10

0.08

-~~ .'-:

0.10

0.08

0.06 O

40.04

0

0

0.02

0.00

111,,,,,,11

400

500

,,~ , , l , ~ l

,,,,,,JJ,l,,,,,

600 700 Wavelength(rim)

,*, ,

800

Fig. 3. Transientabsorptionspectra of deaeratedCCI 4 containing 0.1 mM C6o at differenttimes followingthe laser pulse. (©) 2 ixs; (1:]) 10 p.s; (0) 200 p.s.

In a longer time region of about several hundred Ixs after pulse radiolysis of the same solution of C6o (0.1 mM) in CC14, a broad absorption peak around 430 nm was observed (Fig. 2), which has a long growth time of around 100 p,s and a very long life time ( > 1000 IXS) as shown in the inset of Fig. 2. The characteristic absorption maximum and its life time are in good accord with that from the radical adduct of "CC13 to C60 ([C60CC13] ") generated via the radical addition mechanism proposed by Dimitrijivic [10], "CCl 3 +

430nm

0

C6o ~

[C6oCCl 3 ] ' .

(3)

0) a~

0.06

"¢~o " O0

-~ 0.04

l

0.02

I

o

~

200

3.2. Laser flash photolysis o f C6o in C C l 4

400

time(~s)

~

i~1

°'°°350 ....... 4~b ....... 5~b ......

i

650

Wavelength(rim)

i

i

i

i

,

i

r ~

I

750

Fig. 2. Transient absorptionspectra of deaerated CC14 containing 0.1 mM C6o at differenttimes followingthe electronpulse. (11) 2 p,s; (O) 10 ~s; (zx) 200 Us. The inset shows the absorption-time profile at 430 nm.

Fig. 3 shows the transient absorption spectrum recorded after 248 nm laser flash photolysis of 0.1 mM C60 in CCI 4 solution. At 1 p,s after the laser pulse, the transient spectrum recorded was in good accord with the authentic spectrum of 3C~o [2,9,18]. The UV spectrum of ground state C6o shows a very strong absorption at the wavelength of the excitation laser at 248 nm [1]. So 3C~o was produced by direct excitation of C60 and the following intersystem crossing process: C60

hv(248 nm)

1C60

isc ) 3 E g o .

(4)

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S. Yao et al. / Chemical Physics Letters 239 (1995) 112-116

Then, in a longer time region along with the decay of 3C60 at 745 nm, a new absorption peak around 650 nm appeared which is very similar to the absorption peak of C6+0"arising from the pulse radiolysis of C6o in benzene containing C C I 4 as additive [9] and the pulse radiolysis of C6o in E E l 4 as discussed in Section 3.1 of this Letter. In addition the growth trace of C6+o at 650 nm, obtained by subtracting the contribution from 3C60 using the same subtraction method as in our previous studies [9,11], is also synchronous with the decay of 3C60 at 740 nm. Therefore, the generation of C~o" here can be described according to electron transfer via an exciplex mechanism as proposed in our previous paper on the pulse radiolysis of C6o in benzene containing CC14 or C C I 3 C F 3 [9], kq 3C6o + G E l 4 .

• [ C 6 o . . " G E l 4]*

" [C~o+... CCl~-] ~' C6;' -[- C C 1 ; ' .

(5)

Also in Fig. 3, at 200 Ixs after the laser pulse when the 3C60 decayed completely, a new absorption band at around 430 nm is deafly present. Its absorption maximum and shape are very similar to the absorption band o f [C60CC13]" from pulse radiolysis of C6o in CCI 4 as confirmed in Section 3.1. So this absorption band should also be assigned to [C60CC13]'. Using the same subtraction method as above the growth trace at 440 nm as shown in Fig. 4c can be obtained by subtracting the contribution from 3C60. Fig. 4 shows that the decay of 3C60, Fig. 4b, is synchronous with the growth of [C60CC13]'. It can be seen from Fig. 4c that the growth time of [C60CC13]" is about 10 Ixs, which is much shorter than that of [C60CC13]' arising from the pulse radiolysis of the same solution discussed above (reaction (3)). The observed growth rate constant Kobs was estimated from the rising part of Fig. 4c to be 6.4 × 105 s -1, which is also much larger than that in the radical addition mechanism (reaction (3)), i.e. 8.0 × 10 4 s -1 Moreover, by carefully comparing Figs. 2 and 3, it was found that the ratio of the absorption maximum at 440 nm to that at 650 nm, A44o/A65o, of Fig. 2 (i.e. 10/1) is different from that of Fig. 3 (i.e. 3/1). So, it should be concluded that the generation mechanism of [C60CC13]' from laser

flash photolysis of C60 in C C I 4 is different from the radical addition mechanism (reaction (3)) in the case of pulse radiolysis. In addition, it has been found that the transient absorption can be diminished greatly by dissolved 0 2 which indicated that 3C60 must be a precursor of [C6oCC13]" in the photolysis of C60 in CC14 solution. Based on all the points above, we proposed the following mechanism as a novel path for the generation of [C60CC13]" in the laser flash photolysis of C6o in CC14: kq

]*

3C60 + E E l 4 .

• [C60 .. . CC14 exciplex

, [C6oCC13]" + C1.

(6)

So reactions (5) and (6), both via an exciplex between C6o and CC14, should be competing reactions and the bimolecular reaction rate constant of quenching of 3C~o by CC14 w a s determined to be 6.4 x 10 9 M - 1 s- 1 Based on the above mechanism and the reported absorption coefficients of 3C60 and [C6oCC13]" , the conversion yield of [C60CCl3]" f r o m 3C6o, denoted as 4', can be deduced from (~A74°(3C60) I t= 0 ~740(3C~0)

A 4 4 ° ( [ C 6 0 C C 1 3 ] ") I t=o =

~440([C60CC13 ] . )

(7)

Dimitrijivic [10] has given the G e 44° value of [C60CC13]' f r o m pulse radiolysis of C60 in CC14 as

0.15

~

0

.

I

0

.=

b

0 r~

"

0.05

a

" .. ,.. -,

o.oo

0

.

.,...q

_ ......

20

,.

,-',", Z"2: iii ;','", 40 60 Time(us)

..

80

i00

Fig. 4. The absorption-time profiles of deaerated CCI 4 containing 0.1 mM C60 following the laser pulse. Traces (a) and (b) were obtained at 440 and 740 nm, respectively. Trace (c) corresponding to the absorbance of the adduct radical of CC13 to C60 at 440 nm was obtained from traces (a) and (b) by the subtraction method (see Ref. [11]).

116

S. Yao et al. / Chemical Physics Letters 239 (1995) 112-116

20000, but did not give the corresponding G value. In order to obtain e44°([C60CC13]') , we should estimate the G value of [C6oCC13]" from pulse radiolysis of C60 in CC14. Since the G value of "CCl 3 from the pulse radiolysis of CCI 4 w a s 3.5 [14], the G value o f [C60CC13]" from pulse radiolysis of C60 in E E l 4 should be lower than 3.5. Therefore the ,~44°([C60CC13]') w a s deduced from the G g 440 value to be >15714 M -1 cm -1. ~74°(3C60 ) is taken as 16000 M -1 cm -1 [19]. A74°(3C60)[t=0 and A44°([C60CC13]')[t=0 were obtained by extrapolating Fig. 4b and 4c to t = 0 as A74°(3C~0) [ t= 0 ~ 0 . 2 , A44°([C60CC13]" ) [ ,=0 = 0 . 0 5 .

Therefore, the conversion yield ~b was deduced from Eq. (7) to be ~<0.7. Since "CCI 3 c a n be produced from laser flash [ghotolysis of CC14 [16], direct addition of " E E l 3 to C60 is also possible. According to this reaction mechanism, the generation of [C6oCC13]" from laser photolysis of C60 in CCI 4 should follow second-order kinetics. However, trace analysis indicated that the growth trace of [C60 CC13]' at 440 nm followed the pseudo-first-order kinetics. Hence the direct addition mechanism is less possible and should be excluded accordingly.

4. Conclusion

C6+0" is generated via hole-transfer and electrontransfer via an exciplex mechanism arising from pulse radiolysis and laser photolysis, respectively. The radical adduct of "CC13 to C60 , [C60CC13]', was also produced via two different mechanisms including a radical addition mechanism and a novel mechanism via an exciplex between C6o and CCI 4. In the later mechanism, the yield of [C60CC13]" from 3C60 was estimated to be ~<0.7.

Acknowledgement This project was supported by the National Natural Science Foundation of China.

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