Fullerene Anion Formation by Electron Transfer from Amino Donor to

J. Phys. Chem. 1995, 99, 16234-16241

16234

Fullerene Anion Formation by Electron Transfer from Amino Donor to Photoexcited c 6 0 . Electron Paramagnetic Resonance Study Vlasta Brezovh, Andrej StaSko," and Peter Rapta Faculty of Chemical Technology, Slovak Technical University, RadlinskCho 9, SK-812 37 Bratislava, Slovakia

Gunter Domschke Institute of Organic Chemistry, Technical University, Mommseizstrasse 13, D-010 62 Dresden, Germany

Anton Bart1 and Lothar Dunsch IFW, Helmholtzstrasse 20, 0 - 0 1 0 69 Dresden, Germany Received: April 5, 1995: In Final Form: July 19, 1995@

Upon UV irradiation of CMfullerene in 1: 1 toluene/(methanol, acetonitrile, or toluene) solutions the photoexcited state of c60 was quenched in the presence of Et3N, and single lines of anion radicals A and B were observed in in situ EPR measurements. The formation of A is accompanied by the appearance of a band at 1077 nm in vidnear-IR spectrum characteristic of C6O monoanion. Radical B is a consecutive product of A (probably associate of C6O'- or c6!]dianion). Monoanion A with g = 2.0000 and peak-to-peak width, pp = 0.09 m T converts to radical B with g = 2.0006 and pp = 0.035 m T increasingly after prolonged irradiation and at higher ratios of Et3N:Cbo. The shortest lifetime of C60 monoanion was found in the presence of methanol ( i l l : = 28 s), and it increased considerably upon the addition of tetrabutylammonium perchlorate (TBAP) salt (iIn2 = 210 s in 0.38 M TBAP) or if methanol was replaced by acetonitrile (t112 = 260 s). Lowering the temperature from 300 to 200 K, the linewidth of A decreases from pp(300 K) = 0.09 m T to pp(200 K) = 0.038 mT. At 100 K a broadened line spread over 3 m T was found with a narrow line superimposed on it. The ratio of the broadened to the narrow line increases with the increased time intervals and is more pronounced following prior prolonged irradiation at 300 K or upon the addition of TBAP.

resolved EPR through a wide temperature range (7-253 K)."' From the line shape analysis, pseudorotation of the Jahn-Teller Extensive research' has been focused on spectroscopic distortion was found to be important in the molecular dynamics (especially EPR) studies of fullerene anion radicals generated at low temperatures.'(Ic A narrow line with g = 2.0016"'d by various techniques: electrochemically;?-4 using alkali metobserved at 253 K in toluene was attributed to 3C6(!.20h.d.r als' ' and organometallic compounds;' photochemically in hoA significant problem intensively discussed in recent literature mogeneousx and heterogeneous system^;^,'^^ and radiolytically. I I is the interpretation and assignment of EPR spectra to the The formation of C6(j- by photoinduced electron transfer from fullerene anions. Various broadened single lines without any various electron donors to c60 in an excited triplet state (3chf~) further hyperfine structure are described; consequently even so was investigated by time-resolved EPR I ? and CIDNP,13 laser simple a species as Chi! monoanion has resisted an unambiguous flash p h o t o l y s i ~ ~and . ~ ~transient .~~ UVhisInear-IR spectrosassignment up to now. The results may be divided into two The quenching rate of photogenerated 3C60 (with groups. In the first group ChOmonoanion is described with a simultaneous generation of C60*-) using aliphatic or aromatic broad line (up to 50 G and more),2a.b.icfrequently with a amines as the donor was described by Arbogast,xd and good superimposed narrow line.hh." The generation is verified by a correlation between the measured rate constants and those quantitative yield in EPR measurements and the assignment is calculated from the Rehm and Weller equation" was obtained.8d confirmed with a characteristic vishear-IR line at 1075 nm,l.[ The excited states of fullerene were intensively studied in The narrow lines in these studies are frequently interpreted as solutions6c.'h-18as well as in polymer matnces.l9 3Ch~ is formed originating from impurities, or subsequently from the consecuby irradiation at 355 and 532 nm with the quantum yields of tive products. The main point of these investigations is that 1.O.xa.lhJ Fullerene C ~ triplet O state is long-lived with reported the EPR measurements are usually carried out after the lifetimes from 40 to 280 ps in aromatic solvent at room generation of the radicals (i.e.. not using in situ measurements) t e m p e r a t ~ r e . ~ ~ . ~ The " . ' results ~ ~ . ' ~of~ ~pulsed-EPR ~~ measureand mainly at the low In the second group ments indicate that triplet-triplet annihilation is an important C ~ O * is - described with well-defined narrow line and its triplet quenching mechanism at room temperature. The triplet formation is also confirmed by vishear-IR spectroscopy.iL1.'" quenching due to the encounters with ground state C6!] is of The main point of difference here from the first group is that minor importance.13.11"18c.' The variability in the 3C60 lifetime the EPR measurements are carried out in situ, directly in the from different laboratories may be rationally explained assuming EPR spectrometer at room temperature. the presence of various traces of oxygen which is an extremely efficient triplet state quencher.lXe The dynamic of 'Ch(J in Unquestionably, the results of both groups are correct. How isotropic and liquid crystalline matrix was studied by timethen are the agreement in the identification of Ch()monoanion by vishear-IR study presented by both groups, and their diverse % Abstract published i n A h m t r ACS Ah\frnci\ October 15. 1995 results describing the same radicals in EPR experiments with

Introduction

0022-3654/95/2099- 16234$09 OO/O

0 1995 American Chemical Society

J. Phys. Chem., Vol. 99, No. 44, 1995 16235

Fullerene Anion Formation by Electron Transfer narrow and with broadened lines, to be explained? The shape of EPR lines is strongly influenced by such parameters as electron spin-electron spin dipolar interactions, electron spin exchange, electron transfer, and others besides.22 These changes contribute essentially to the shape of the line in EPR, but probably have only a negligible influence on vishear-IR spectra, and for that reason agreement in vishear-IR studies and differences in EPR investigations were observed. EPR spectroscopy with its sensitive ability to reflect very delicate stages of species with various line forms can provide very detailed information on different forms of Cm'- not obtainable by other methods. Therefore a convincing presentation of experimental data, verifying that the narrow lines are an inseparable, integral part in the reduction process of fullerene, is an important step for focusing attention on this phenomenon. This paper provides an unambiguous assignment of a narrow line to C60 monoanion and also demonstrates that the wide lines appear subsequently to the after narrow lines. Their formation is significantly influenced by a wide range of parameters as the choice of solvents, in situhon-in situ experimental setups, the reduction time, the presence of counterions, and temperature. Additionally, the experimental procedure presented here represents a simple photochemical generation that forms the basis for a further series of experiments, namely, use of laserand time-resolved EPR spectroscopy with a resolution up to the nanosecond region. The present study may bring further new information on the primary form of fullerene anions, which can hardly be predict at the present time.

Experimental Section Materials. High-purity fullerene Cm was supplied from Hoechst AG (Germany) or from our own production (IFW in Dresden) prepared according to the procedure described in ref 23. Triethylamine, methanol, toluene, LiC104, all of analytical grade, from Lachema Bmo (Czech Republic) were used. Acetonitrile for UV spectroscopy, tetrabutylammonium perchlorate (TBAP), tetraethylammonium perchlorate (TEAP) were purchased from Fluka, as were other salts used of electrochemical grade. Generation of Radicals. The corresponding solutions of fullerene, Et3N, and salts in 1:1 toluene/(methanol, acetonitrile, or toluene) were prepared in concentrations as specified below. The samples were thoroughly purged with argon and placed in flat cells (a Varian electrolytical cell with platinum working and counter electrodes or a Bruker cell optimized for standard rectangular cavity). The measurements at various temperatures were carried out by means of liquid nitrogen using a Bruker variable-temperature unit ER 41 11 VT, in the cylindrical tubes (4 mm 0.d.) with an inserted quartz capillary (2.8 mm 0.d.) in order to maintain quality of cavity. The radicals were (photochemically or electrolytically) generated directly in the cavity of EPR spectrometer. A 250 W medium pressure mercury lamp (Applied Photophysics, England) with a photon flux of 2 x lOI5 photons s-I, determined directly in the EPR cavity by ferrioxalate a~tinometry,'~served as the radiation source. A Pyrex glass filter was used for cutting out the radiation below 300 nm. Amperostatic electrolysis at various current densities employing a Radelkis OH-405 instrument was used to generate the anion radicals. The reference experiments with photosensitized Ti02 suspensions were carried out as described in ref 10. EPR Measurements. A Bruker 200D EPR spectrometer, assembled with an Aspect 2000 computer equipped with a standard program, was used to measure and to evaluate the EPR spectra. Unless other stated all experiments were carried out

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Figure 1. EPR spectra observed during a continuous irradiation of various c 6 0 and Et3N concentrations in 1:1 toluenehethanol solutions: (a) 0 Cm, 34 mM Et3N; (b) 0.05 mM C ~ O0,Et3N; (c) 0.05 mM c60, 0.66 mM Et3N; (d) 0.05 mM Cm, 34 mM Et3N.

at room temperatures. Standard settings for series of experiments were as follows: gains (103-105), time constant 100 ms, sweep time 20 or 50 s, modulation amplitude 0.05 mT, microwave power 13 dB. To evaluate the spectral parameters, the modulation, microwave power were lowered and the sweep time prolonged. The g values were determined by means of a marker built in the spectrometer. The uncertainty in g values for the narrow line was fO.OOO1 and in peak-to-peak widths (pp) was less than f0.003 mT. Vismear-IR Measurements. The solutions were prepared as described above, placed in 1 cm cell, and irradiated, and spectra were immediately recorded by means of a Shimadzu 3100 W/vis/near-IR spectrophotometer. The shortest sweep time enabled us to record the spectra in 2 min intervals.

Results and Discussion Photochemical Properties of the System Cm, EQN in 1:l TolueneMethanol. Fundamental photochemical properties of the investigated system and its individual components with respect to the formation of free radicals and their characterization by EPR technique are summarized in Figure 1. No radical species are observable under the continuous irradiation of 34 mM Et3N in 1:l toluene/methanol solution (Figure la). Under the same conditions, irradiated 0.05 mh4 c 6 0 solution gives a single-line spectrum T with pp = 0.02 mT and g = 2.0017 as shown in Figure lb, which vanishes immediately after stopping irradiation. This behavior and the spectral parameters observed are characteristic of the c 6 0 triplet state.20b,d,eConcerning the 3C60 lifetime; from the determined photon flux of IOl5 photons/ s, and quantum yield of C a triplet generation (4 = 1)8a-'6a reported in the literature, and the obtained stationary radical concentration of line T, we estimated the average lifetime of

BrezovA et al.

16236 J. Phys. Chenz., Vol. 99, No. 44, I995 concentration (mM)

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Figure 2. EPR spectra observed after 5-s irradiation periods Of 0.03 m M Cbo and variable EtlN concentrations in 1 : 1 tolueneiacetonitrile solutions (Et;N in mM): (a) 0; ( b ) 0.66: (c) 2.1; (d) 34.

Figure 3. EPR spectra observed after 5-s irradiation periods of 2.1 mM Et3N and variable Chi)in I : 1 tolueneiacetonitrile solutions (Cnoin p M ) : (a) 0: (b) 1.6; (c) 3.2: (d) 3.5.

C60 triplet state to 200 pus. However, we are aware that such estimations are burdened with a high probability of error. If the above-described irradiation of c 6 0 is carried out in the presence of a low donor concentration (0.66 mM Et3N), the appears only at the very beginning of single-he of 3c6~ irradiation, then gradually vanishes, and is finally replaced with a new radical product A characterized by pp = 0.09 mT and g = 2.0000 (Figure IC). According to our vishear-IR investigations described below, the spectrum of radical A can be assigned to CHImonoanion. The formation of A is considerably enhanced at higher donor concentrations (34 mM Et3N), as shown in Figure Id. The radical A is relatively stable (more details on the stability will be given later). Its behavior and the observed spectral parameters are the same as we previously found in the photoreduction of C60 in Ti02 suspension.IO The formation of Cb~) monoanion in a similar photochemical system as presented here was also suggested by Arbogast et aLXa They detected C6”’- using time resolved vishear-IR spectroscopy. Since the assignment of the narrow EPR lines presented here to C6(j- remains an intensively discussed question, we investigated this system in more detail. Influence of C60 and Et3N Concentration on the Radical Products Formed. Figure 2 shows the EPR spectra of radicals generated at a constant fullerene (0.03 mM C60) concentration, and various Et3N, concefitrations in 1:1 toluene/acetonitrile solvent. A discontinuous irradiation was applied here. At the beginning of every scan the probe was irradiated in spectrometer for 5 s and the spectrum was measured with a 20 s sweep time. The integral irradiation periods are quoted in Figure 2 and also later in Figure 3. During such a discontinuous irradiation of a 0.03 mM Chn solution in 1: 1 toluene/acetonitrile, the EPR spectrum of ? C h o shown in Figure 1 was not observed in Figure

2a due to its limited ~ t a b i l i t y . ~ ~ . ’ ~ .When ’ ’ ~ . ’donor ~ , ~ ~ was ~ monoanion added to such a solution to give 0.66 mM Et3N, A with g = 2.0000 and pp = 0.09 mT was found as shown in Figure 2b. A further increase of Et3N concentration (2.1 and 34 mM) also stimulated the formation of the second radical B, characterized in ref 10 as a consecutive product of A with a single line with g = 2.0006 and pp = 0.035 mT (Figure 2c,d). The assignment of B remains an open question so far. Tentatively, the assignment most compatible with the experimental data is the formation of c 6 0 dianion. However the formation of C6o associates or consecutive products cannot be excluded. From the spectra shown in Figure 2 it can be concluded that the ratio of B to A increases at the higher ratios of Et3N:Cso and with prolonged irradiation time. The same conclusion is also implied by Figure 3. where under the conditions as specified in Figure 2, the (260 concentration was varied at a constant EtiN concentration (2.1 mM). In a 2.1 mM Et3N in 1:1 toluene/acetonitrile solution (in the absence of C60) no radical product was observable (Figure 3a). Afterward, if c 6 0 was added only up to very low concentration (1.6 p M c60), both radicals A and B were evident in Figure 3b. The ratio of B to A becomes smaller at the increasing C60 concentration (3.2 pM) as shown in Figure 3c, and only single-line of monoanion A was found (Figure 3d) after C6O concentration was increased to 4.5 pM. This demonstrates again that C ~ monoanion O is formed at high ratios of fullerene and its consecutive product B dominates at higher Et3N:Cso ratios. Stability of Radicals Generated. In further experiments the influence of solvents (methanol, toluene, acetonitrile) on the single-line spectrum A of C ~ monoanion O was investigated. After the optimal radical concentration was obtained, the irradiation was stopped and the EPR spectra were recorded in 40 s intervals.

~

Fullerene Anion Formation by Electron Transfer

J. Phys. Chem., Vol. 99, No. 44, 1995 16237 a)

mixture

’k

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D

1“ mol dm-

0 0.W

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200 250 -T/K+ 300 Figure 5. Variation of EPR spectrum of Cm monoanion A photogenerated in 0.05 mM Cm, 0.5 mM Et3N 1:1 toluenelacetonitrile solution. (a) a1 at 300 K; a2 probe a1 at 100 K; a3 probe a1 in the presence of 0.1 M TBAP; a4 probe a3 warmed to 200 K for 20 min and cooled to 100 K. (b) The dependence of peak-to peak widths (in ,UT)elucidated by the lowering the temperature from 300 to 200 K. (Sweep width 10 mT and modulation amplitude 0.5 mT for depicted spectra and 1 mT and 0.025 mT in elucidation of pp, respectively.)

The relative intensities of the corresponding spectra are quoted upon the time in Figure 4a. The highest radical concentration and the highest stability were found in the system containing acetonitrile. A stability comparable to that observed in acetonitrile was found in pure toluene solution, having a radical concentration about 2 magnitudes lower. A high radical concentration but very limited stability is characteristic for the presence of methanol (Figure 4a). Probably the protolytic character of methanol solvent is responsible for the relatively short lifetime of C ~ anions O as compared with acetonitrile and toluene. Generally, this limited stability may be also associated with an insufficient stabilization of the generated anion radicals by the counterions derived from amine donor.8a Therefore, in a manner similar to that stated in Figure 4a, we investigated the stability of C a monoanion in 0.05 mM C a , 34 mM Et3N 1:1 toluene/methanol solutions with various additions of BUqNC104. The results obtained are summarized in Figure 4b, where it is clearly evident that the stability of monoanion A increases with the addition of increasing amounts of TBAP. In similar experiments, the influence of various cations (Bu4Nf, EhN+, and Li’) on the stability of radical A was investigated. At a constant salt concentrations (0.1 M) the stabilization effect was high in the case of B@+ but no significant influence was found for EkN+ and Li+. Similar stability behavior was also found for the spectra of mixed radical products A and B. Investigations at Low Temperatures. In the laboratory of Eaton’s groupzaand other investigators,2b.4cthe cathodically and

also otherwise reduced C ~ fullerene O was nearly quantitatively converted to monoanion and then, afterward, it was characterized in EPR measurements at low temperatures as a wide-line EPR spectrum, sometimes with superimposed traces of narrow single lines.6b In our in situ experiments at 295 K we found exclusively only narrow lines as described above?a.6c.loTherefore, we also investigated the primary spectrum A of C a monoanion, with a narrow line, formed in an irradiated 0.05 mM C a , 0.5 mM Et3N in 1:l toluene/acetonitrile solution at low temperatures down to 100 K. The results obtained are summarized in Figure 5. The single-line spectrum (Figure 5al) generated at 300 K becomes narrower with decreasing temperature as shown in Figure 5b, where the peak-to-peak widths, pp, are quoted upon the corresponding temperatures. The temperature dependence of pp can be well fitted with the following relation, pp = (35.92 (1696 e~p(-874/T))~)”*,and this relation is similar to that previously obtained on the fullerene reduction in Ti02 suspension.I0 At temperatures below 180 K the solution is frozen and the single line became asymmetric. The contribution of a broadened line spread over 3 mT is evident as shown in Figure 5a2. This broadening is more pronounced after a longer irradiation and with the increasing time. Still more pronounced broadening was observed on addition of salts as shown in Figure 5a3, where the irradiation was carried out in the presence of 0.1 M TBAP and after time delay (Figure 5a4). Because of a limited conversion of fullerene to its unstable monoanion characterized with single variously broadened lines, a quantitative evaluation of the complex superimposed spectra is hardly possible. Vis/Near-IR Experiments. Since the most reliable identification of c 6 0 anions is based on the characteristic absorptions

0.24 0.12 0.0

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Figure 4. Stability of Cm monoanion A monitored by relative intensity of its photogenerated EPR spectrum after stopping irradiation: (a) In 0.05 mM Cm, 34 mM Et3N 1:l toluenelsolvent-X solution. X: (A) acetonitrile; (0)toluene; (0)methanol. (b) In 0.05 mM Cm, 34 mM Et3N 1:1 toluenelmethanol on the addition of various amounts of TBAP (in mol dm-3): (0)0; (A) 0.12; (0)0.24; (*) 0.38.

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16238 J. Phys. Chem., Vol. 99, No. 44, 19% I

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Figure 6. Time evolution of vislnear-IR spectra observed after 100 5 irradiation of a 0.05 mM C ~ O0.5 . mM EtlN 1 : I toluenelacetonitrile solution. The inset shows the time monitoring of EPR spectra under analogous experimental conditions.

'

of the individual ions in the near-infrared region,Z~.b,.5d,'2.8.~,' h.'Jh we completed our investigations here with the vishear-IR measurements. The Cho anion radicals were generated in an UV cell under conditions similar to those described in EPR experiments above. The probe thus prepared was then placed in vishear-IR spectrometer, and the time evolution of radicals generated was recorded. Figure 6 shows the time monitoring of vishear-IR spectra recorded after 100 s irradiation of 0.05 mM C ~ O0.5 . mM EtjN 1 : 1 toluene/acetonitrile solution. A characteristic spectrum of Chi) monoanion with maximal absorption at 1077 nm was observed similarly as described in refs 2a,b, 4b. Sd, 7a. 8a. 1 lb. and 14b. In EPR measurements under analogous conditions, spectrum of radical A with narrow line (pp = 0.09 mT) and g = 2.0000 was found. Its time evolution is shown in the inset of Figure 6. Its relative intensity, with time, t (in seconds), can be described by the following equation: IFF = 0.71 exp(-0.006t) 0.27. Similarly the relative changes of the absorbance at 1077 nm in the vishear-IR spectrum are described by: A = 0.42 exp(-O.O02t) 0.39. An exact quantitative comparison of radical concentrations in vishearIR and EPR experiments is difficult, due to the problems in the determination of absolute concentrations in EPR" and also the variability of molar extinction coefficients of C6,:- at 1075 nm given in the literature with E ! ( ~ : s = (1.2-3.0) x IO4 mol-' dm' cm-l ,'- 2 . ,.id .'b as well as due to differing vishear-IR and EPR cell geometries. However the approximate estimations show that the ChO.- concentrations in EPR experiments with about 0.025 mM are lower than in vishear-IR with about 0.040 mM (using E 1 0 7 5 = 2.0 x lo-' mol-' dm3 cm-')? for the first recorded spectrum after stopping irradiation. Also the time dependencies show that the corresponding formal rate constants found in EPR ( ~ E P R= 0.006 s-I) is considerably higher than those obtained in vishear-IR (k, I , / n e a r . ~ ~= 0.002 s-l). These apparently contrary EPR-vishear-IR results may be explained assuming that ChO monoanion exists in various differently stable forms (associates) represented with variously broadened lines in EPR but one characteristic absorption band in vishear-IR at 1077 nm. Thus the primary form of Ch(1'- with narrow EPR line (product A) may be converted with the prolonged time to the wide-line form, which is not observable at room temperature but increasingly found at low temperatures.'h Consequently.

eR,

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Figure 7. Visinear-IR spectra observed in a 0.05 niM Chf1.0.2 M EtTN 1 : l tolueneiacetonitrile colution after (a) 100 s irradiation and (b) 300 s irradiation. The spectra were recorded 2 min (full lines) and 4 min (dotted lines) after stopping irradiation.

longer lifetimes and higher C6,)'- concentrations are found in vishear-IR than in EPR in situ experiments at room temperature. In future investigations also the question of whether the band at 1077 nm is an unambiguous characteristic of the integral monoanion concentration can be pursued. The shoulders at the lower wavelengths may possibly represent various associates or other independent forms of Chi) monoanion. It is also possible that other unspecified species may absorb in this region. Preliminary, we found evidence for such behavior derived from two experiments on the irradiation of 0.05 m i l Chi), 0.2 M Et3N 1 : I toluene/acetonitrile solutions for 100 s (Figure 7a) and 300 s (Figure 7b), where the first vishear-IR spectra (Figure 7a.b full lines) were recorded 2 min after stopping irradiation. Then the next sweep was carried out after a further 2 min (Figure 7a.b dotted lines). In Figure 7b clear evidence is given for the rearrangement between the absorption peaks at 992 and 1077 nm. The absorption band at 992 nm a s well as at 1077 nm is in literature assigned to Ch(1monoanion,5d.?'The results obtained imply the conversion between various species of Cho anions. In further vishear-IR experiments we investigated whether the consecutive product of Chon-,radical B, found in EPR investigations, can be assigned to ChO dianion. The vishearIR and EPR spectra of both (A. B) are shown in Figure 8. Again the band at 1077 nm characteristic of Ch,)monoanion dominates here. In the first sweep. the absorbance in the region 9001000 nm. especially at 950 nm, characteristic for dianion. is slightly higher in the relation to 1077 nm bands compared to that observed in Figure 6, whereas in EPR only radical A is found. But unambiguous evidence for Chi) dianion is not

Fullerene Anion Formation by Electron Transfer

J. Phys. Chem., Vol. 99, No. 44, 1995 16239

1.2

4 1077nm

4

0.0

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wavelength/nm Figure 8. Time evolution of vishear-IR spectra observed after 100-s irradiation of a 0.05 mM Car 0.2 M Et3N in 1:l toluene/acetonitrile solution. The inset represents the time monitoring of EPR spectra under analogous experimental conditions.

obtainable from these data. The inset in Figure 8 illustrates the decay of A and B in an analogous EPR experiment. The estimated concentration of B in such superimposed EPR spectra is less than 10% and that of A is about 90% as found from an analogous simulation of EPR spectra described previously.'o If B is present as a dianion in such relatively low concentrations, this may be the reason why it is not unambiguously evident in near-IR spectra at 950 nm. Under the conditions that only single line B was seen in EPR, its concentration is also very low due to the limited stability ( t l / 2 = 40 s) and practically no absorption was found in 900- 1300 nm region. Additionally, we reinvestigated by vidnear-IR spectroscopy the stabilization effect of TBAP on monoanion A described in EPR investigations (Figure 4b). After irradiation of a 0.05 mM C a , 0.5 mM Et3N 1:l toluene/methanol solutions for 100 s, a vidnear-IR spectrum was found as shown in Figure 9a. In the second near-IR scan (after a period of 2 min) c60 monoanion almost completely vanished. The inset in Figure 9a illustrates the analogous behavior of radical A from an EPR measurement. Under the same conditions as stated for Figure 9a, the salt was added to the probe to give a 0.38 M TBAP solution. Then, after its irradiation, a series of vidnear-IR spectra was found (Figure 9b) with a considerably higher concentration and longer stability of C"-. The monitoring of EPR spectra shown in the inset of Figure 9b confirms again a similar salt stabilization effect upon c 6 0 monoanion observed in EPR and vis/near-;R experiments. One of the frequently discussed question is the influence of the generation method on the nature of fullerene anion radicals observed. Cathodic reduction is generally considered to be the cleanest and the best-defined technique in the reduction reactions. Therefore, it was also argued that the other techniques are complicated, not specific, and lead to the formation of various narrow lines that are possibly specific only for the corresponding reduction system but not characteristic for the fundamental fullerene reduction products. Now our investigations summarized in Figure 10 show that with the systematic variation of the experimental conditions such as c 6 0 concentration, reduction or irradiation time, and Et3N or Ti02 concentration, practically identical radical products as found by the most appropriate cathodic reduction (Figure lob)

'

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wavelength/nm Figure 9. Time evolution of vidnear-IR spectra observed after 100-s irradiation of a 0.05 mM C60, 0.5 mM Et3N (1:l)-toluene/methanol solution: (a) without addition of TBAP salt; (b) in 0.38 M TBAP solution. Insets show the time monitoring of EPR spectra under analogous experimental conditions.

can be also obtained by rather different reduction methods as photoexcited Ti02 suspensions (Figure 1Oa) and quenching photoexcited c 6 0 with electron donor (Figure 1Oc). Figure 10 shows also that the primary product A found here by all three techniques (Figure 10 a l , b l , c l ) is the monoanion of Cm with ~ 0.09 mT, if the fullerene concentrations gA = 2.0000 and p p = are sufficiently high and Ti02 (al), Et3N (cl) concentrations low or the reduction periods short (bl). If Ti02 and Et3N concentrations are slightly increased or the reduction period is prolonged, the second product B (gB = 2.0006, p p = ~ 0.04 mT) is indicated in low concentrations(spectra in Figure 10 a2,b2,c2). A further increase of Ti02 and Et3N concentrations or reduction period (spectra in Figure 10 a3,b3,c3) considerably increases the concentration of the consecutive product B relatively to the monoanion (A). If the concentration of fullerene is very low, then under all three techniques only consecutive product radical B dominates, as shown in Figure 10 a4,b4,c4. These results demonstrate that all three techniques are equivalent. The claim that only cathode provides a clean source of electrons without any further side reactions, can also be attributed to canying out the reduction in Ti02 suspensions, and also to the quenching of the photoexcited c 6 0 by electron donors. This is a very remarkable and important result showing that the narrow line radicals are universal radical products independent from the method of generation. An apparent anomaly in EPR studies at the present is to be found in the description of as a narrow line with low stability at room temperature on one handlo and as a wide line with high stability at the low temperature on the other hand.2a.b,3c This anomaly may be resolved assuming that the primary

16240 J. Plzxs. Chenz.. Vol. 99, No. 44, 1995

Brezovi et al.

Conclusions

Reduction of C,O ).

T i 0 , + hu

ai

b) cathodic ( + e - )

0)

Two apparently contrary types of results are reported in the literature describing C h o monoanion in EPR studies. with narrow lines (pp 0.1 mT) on one hand, and with broaden lines (pp up to 5 mT and more) on the other hand. Our systematic investigations showed that the narrow lines are characteristic for the in situ generations on a time scale of a few seconds up to a few minutes at room temperatures. The broadened lines are dominant after prolonged reduction times, especially if the EPR measurements are carried out at the low temperatures. In both cases the formation of Cf,(;- is confirmed by an independent technique (vishear-IR) with a characteristic monoanion band at 1077 nm. The results obtained imply that the in situ formed C ~ anion O undergoes consecutive transformations. Formation of its various associated forms is the most plausible alternative to explain the results obtained. The photochemical technique presented is completely equivalent with the well-established cathodic reduction and offers useful possibilities for further time resolved studies

EtjN + hu

-

bz

n

c3

Acknowledgment. We thank Prof. P. Grier for helpful discussion and Slovak Grant Agency for financial support. References and Notes

Figure 10. EPR spectra of Cbrlanion radicals generated by various techniques: (a) in photosensitized Ti02 suspension: i b ) cathodic reduction: ( c ) quenching of photoexcited C(,O by EtTN donor i n I : I toluene/methanol solutions. Individual cpectra were selected from various series of measurements R here the trend from I 2 3 4 represents the increasing reduction time. Et2N or Ti02 concentration I the increased fullerene concentrations. and from 4

- - --

-

observed anion with a narrow line converts (increasingly with increasing time and decreasing temperature) to the various associates and ion pairs such as Cho'-C(,o. C h ( l C h ~ ~ -C(,,;-. BuJN' 12.21,2h which are characterized with wide line spectra at low temperatures. This is implied also by our observations in Figure 5, where at 100 K the narrow line is superimposed with a wide one (Figure 5a2), and this widening becomes still more pronounced in the presence of TBAP (Figure 5a3) and increasing time (Figure 5a4 repeated the measurement after 30 min), where the ratio of the wide line component increased. Upon warming the probe described in Figure 5a4 from 100 to 300 K. the spectrum shown in Figure 5 a l was observed again.

Two pathways are mainly discussed in the mechanism of the photoinduced C ~ O ' formation. The primary step is certainly the excitation of C ~ to O its singlet state C~o*.x:'.'fi~i This may be quenched by electron donors I" to ChO.- monoanion (Chi)* Et3N C~O'- Et3N'-) or Chi)* converts first to its triplet state ' C ~ Oand then 3Cho is quenched to Chi).- by an electron donor ('C~O-t Et3N Ch().- Et3N"). A rapid intersystem crossing of C ~ Oto* ? C ~isOdescribed in the literature.'h".'yJwhere iCh(iis formed with a nearly 100% quantum The formation of ?Cho as primary radical species was also confirmed in our experiments (Figure 1 b,c). Its stationary concentration decreased upon the addition of Et3N and with the progressive formation of fullerene anions. This is entirely compatible with the quenching of triplet state 3Ch~) with Et;N to monoanion Chli.-.

-

+

+

+

+

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