Qenching of fullerene triplets by stable nitroxide radicals

Volume 199,number 6

CHEMICALPHYSICS LETTERS

20 November 1992

Qenching of fullerene triplets by stable nitroxide radicals A. Samanta l and P.V. Kamat Radiation Laboratory, Universityof NotreDame, NotreDame, IN 46556, USA Received 21 July 1992;in iinal form 4 September 1992

The quenching behaviour of the triplets of C, and Cl0 towards various nitroxide radicals has been investigated by the flashphotolysis technique. The quenching rate constants lie between I .9 x lo9 and 3.7 x lo9 M- ’ s- ‘. Possible quenching mechanisms are discussed.

1. Introdnctlon The simple method of production of fullerenessuch as CsOand Co in high yields [l-S ] and their availability from commercial sources have prompted many chemists to enter the field of fullererie chemistry [ 6-241. One of the long-term goals of this research is the preparation and characterization of derivatives of fullerenes which may have interesting properties. Any attempt in this direction should naturally make use of low reduction potentials of these compounds [ 10,12,131. The first reduction of Cd0is observed at -0.42 V (versus SCE in acetonitrile) [ 121. Thus, reduction of fullerenes should be possible with potential electron donors with low ionization potentials. The radical anion of fullerene, generated from such an electron-transfer reaction, is a reactive intermediate and the choice of suitable reagents could trap this intermediate in the form of the desired product. However, very often generation of CG’ and CG’ may not be possible owing to unfavourable thermodynamic conditions. In such situations, the photoexcitation of fullerenes may provide the energy necessary for the electron-transfer reaction. Recently, there have been a number of studies which attempt to explore the electron accepting ca-

Correspondenceto: A. Samanta, Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA. ’ Present address: School of Chemistry, University of Hyderabad, Hyde&ad 500134,India. Elsevier Science Publishers B.V.

pability of excited fullerenes [ 14-l 7 1. Observation of donor-acceptor complexes of fullerene with amines [ 14,171 prompted us to investigate the behaviour of ground and excited fullerenes towards another class of potentially rich electron donors such as stable nitroxide radicals. We wished to find out whether the stable radicals (which serve as a spin trap in ESR experiments) attach themselves to fullerenes giving rise to intermediates of interest. It is likely that Csowith 30 carbon-carbon double bonds could easily add on to the free radicals. Recent evidence indicates that Cm can behave as a radical sponge [ 18,191. Such an intermediate is also postulated between t-butyl radical and CsD [ 201. This study .focuses on the interaction between fullerenes and some stable radicals.

2. Experimental The mixed fullerenes were received from Aldrich. Csoand CT0were separated by following procedures outlined in refs. [l-S]. For flash-photolysis experiments, the fullerene solutions were prepared in methylene chloride so as to give an optical density (OD) of 0.25 (in 2 mm) at 337 nm. The structure of the radicals used in this study are shown in scheme 1. Di-tert-butyl nitroxide (DTBN, I) was purchased from Eastman and used as received. 2,2,6,6tetramethyl-piperidinyl- 1-oxy (TEMPO, II) and 4-hydroxy -2,2,6,6-tetramethyl-piperidinyl- 1-oxy (HTEMPO, III) were purchased from Aldrich. 635

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20 November 1992

III

Scheme 1.

HTEMPO was recrystallized twice from cyclohexane before experiment. Steady-state absorption spectra were recorded on a Perk&Elmer 3840 diode array spectrophotometer. The apparatus for nanosecond time-resolvedlaser flash photolysis experiments has been discussedelsewhere [25]. The quenching experiments were performed by addition of a few microlitres of 50 mM stock solution of stable radicals to 1 ml of degassed solutions of fullerenes. To avoid quenching due to oxygen no more than 10 pl of quencher solution was added The OD at the excitation wavelength remained unchanged before and after the addition of quencher, indicating that the stable radicals were not excited.

3. Resultsand discussion The absorption spectra of fullerenes in methylene chloride (micromolar concentrations) in the presence of 6x 10m4M TEMPO are shown in fig. 1. As the spectra are identical to those reported for fullerenes, ground-state complex formation with TEMPO can be ruled out. Similar behaviour is noticed for the other two radicals. It is to be noted that complexation between CsOand dimethyl aniline has recently been observed at a much higher concentration of amine [ 141. However, the nitroxide radicals used in this study do absorb in the visible region. The typical absorption spectra of the radicals are shown in the inset of fg 1 to illustrate this point. Thus, it was not possible to increase the radical concentration beyond a certain point. The spectra of fullerenes shown in fig. 1 were recorded with the maximum concentration of TEMPO used for flash-photolysisstudies. The fuherene triplets were generated by 337.1 nm laser excitation and were monitored at 420 nm. The choice of excitation wavelength was guided by the 636

Wavrlrngth, nm Fig. 1. Absorption spectra of CW (dashed) and CT0(solid) in metbylene chloride in the presence of 6 x low4M TEMPO. The concentrations of fidlerenes were maintained at 1-2.pm. The inset shows the absorption spectra of I (-), II (**a) and III (- - -) in methylene chloride.

fact that the radicals do not contribute at this wavelength. To avoid triplet-triplet annihilation a neutral density filter was employed in the laser path. The triplet lifetimes of &, and C,,, in the absence of quenchers were measured to be 17.5 and 16 ps respectively in methylene chloride. Addition of the radicals at subrnillimolar concentration resulted in progressive decrease of triplet lifetimes of Cm and Go. However, the triplet yields remained more or less constant. That these paramagneticspeciesdo not affect the triplet yield is in agreement with the reported #is value of fullerenes which is almost unity [21,22]. The first-order rate constants (Ic,& for the decay of fullerene triplets were measured in the presence of varying concentrations of the stable radicals and the bimolecular quenching rate constants ($) were obtained from the plots based on the relationship kobs= rf ’ + kg [Q], where [Q] is the concentration of the quencher and rT is the triplet lifetime of &,, or C,, in the absence of quencher. A typical decay profile for the Go triplet at 420 nm and the bleaching of the singlet at 330 nm are shown in fig. 2. Fig. 3 shows the experimental data for Cso,illustrating the linear dependence of/c,,,,*as a function of [Q 1. With Co, excellent linear dependence was observed. The observed k, data re summarized in table 1. As seen from table 1, quenching rate constants range between 1.9x lo9 and 3.7x lo9 M-’ s-l. The

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CHEMICALPHYSICSLETTERS

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-4

2000

4000

6000

0

8000

4

2

6

8

[Q]/lo-!M

Time, ns Fig. 2. Decay traces of methylene chloride solution of Cw in the presence of 6x 1 Od4 M HTEMPO. The concentrations were adjusted to give an OD of 0.2-0.3 (in 2 mm) at 337.1 nm. Hollow circles represent the triplet decay monitored at 420 nm and the solid circles represent the recovery of bleaching monitored at 330 nm.

Fig. 3. Plot of observed rate constant (&,) of triplet decay of C, versus the quencher concentrations ( (0 ) DTBN, (0) TEMPO, and (A ) HTEMPO).

Table 1 Quenching rates of Csaand &, triplets by stable &oxide radicals Quencher

Cm

CSO AG I) (kcai mol- ’ )

kg (lO’M-‘s-l)

AG ‘) (kcal mol- ’ )

TEMPO

-13.3

3.3

-11.1

3.7

HTEMPO DTBN

-12.1 - 14.9

1.9 3.0

-9.9 - 12.7

2.6 3.3

kg (109W’s-1)

‘) Based on Erz = -0.42 V [ 31and ER2 ofTEMPO,HTEMPOand DTBNarerespectively0.63,0.68and 0.56V (seetext). A.J& was takenas37.5kcalmol” [24]. ‘) Basedon the similar reduction potential [ 131and AE,, valueof 35.3kealmol-’ [ 25] forC,,,. transient absorption spectra, recorded on the nano-

second and microsecond time scales, do not indicate the presence of any other transient than CW or CT0 triplet. Also, it is seen that the bleaching at 330 nm (for C,) and 470 nm (C,,) recovered completely, indicating that no product is formed between fullerenes and the stable radicals (see fig. 2 ) . The lack of observation of transients other than the triplet makes the elucidation of the quenching mechanism difficult. The stable &oxide radicals can quench an electronically excited state by a number of processes as follows: 3P*+2R’~[3P*...2R’]+1P+2R* (or’R*),

(i)

(W +‘P+*R’

,

(iii)

In the present case, the energy transfer from the fidlerene triplets to the nitroxide radicals (process (i) ) can be ruled out on energetic grounds. The triplets of C, and CT,,,located at 37.5 kcal/mol [23] and 35.3 kcal/mol [24], respectively, are considerably lower than the doublet energies of the radicals ( %47 kcal/mol) estimated from the onset of the first absorption bands (see fig. 1). 637

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The enhanced intersystem crossing in the collision complex (process (ii)) can also be ruled out as a possible mechanism. Porter and co-workers have concluded from a study of quenching of triplets of aromatic hydrocarbons by NO and DTBN that this mechanism may be an important factor for triplets with energy lower than 42 kcal/mol [ 261. As a consequence of the energy gap law, as the triplet energy is lowered the quenching rate constant is increased. Porter’s work was subsequently verified and extended to other radicals by Kuzmin et al. [ 271 and Scaiano [ 281. It should be noted that the rate constant for quenching due to this mechanism is of the order of only 10’. The maximum rate constant (3.5 x lo7 M-l s-l) was found for naphthacene triplet with energy 29 kcal/mol [26]. Thus, one should expect a rate constant similar to or lower than that observed for naphthacene triplet and DTBN. As the measured rate constant in the present case is two orders of magnitude higher than expected from exchange interaction in the collision complex, the quenching must be due to some other mechanism. We suggestthat the quenching of ftierene triplets is due to a charge-transfermechanism (process (iii) ) . To find out whether such a process is thermodynamically feasible in the excited state, the Weller equation can be used: AG=23.06 (lZ$ -Erz)-AEo,o. Based on this relationship, using the oxidation potential data of the nitroxide radicals from ref. [29 1, AG values of several pairs are calculated and listed in table 1. As seen from this table, the electron transfer is highly exothermic; therefore, this can account for almost diffusion-controlled rate constants observed in this case. In both cases, the rate constants are found to be the smallest for HTEMPO which could be due to low AG value. We conclude that charge-transfer interaction is responsible for quenching of fullerene triplets by stable nitroxide radicals. The lack of observation of radical anions of fullerenes could be due to efficient backelectron transfer which is quite likely in solvent with a low dielectric constant such as methylene chloride. The poor solubility of Cso or C70in polar solvents, such as acetonitrile, did not allow us to examine the electron transfer derived products. Also it should be noted that the radical anions of Cd0or C79have strong 638

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absorption around 1100 nm [ 151. As the sensitivity of our flash-photolysis apparatus falls rapidly beyond 900 nm, such an experiment cannot be conducted. Further studies involving the trapping of fullerenes are in progress.

Acknowledgement The research described herein was supported by the OffIce of Basic Energy Sciences of the Department of Energy.This is contribution No. NDRL35 15 from the Notre Dame Radiation Laboratory. We thank Dr. PK. Das for useful discussions.

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