9380
J. Phys. Chem. 1995,99, 9380-9385
Redox and Excitation Studies with Cm-Substituted Malonic Acid Diethyl Esters Dirk M. Guldi,” Hartmut Hungerbiihler, and Klaus-Dieter Asmus*9+ Hahn-Meitner-Institut Berlin, Bereich Physikalische Chemie, Glienicker Strasse 100, 14109 Berlin, Germany Received: March 23. 1995@
Flash photolytic formation of excited triplet states, their consecutive reductive quenching with diazabicyclooctane (DABCO), and pulse radiolytic formation of n-radical anions of various Cm-derivatives have been recorded. The fullerenes were functionalized via single, double, and triple cyclopropylation of CHI with bromomalonic acid diethyl ester. Flash photolytic irradiation at 308 nm of these Cm derivatives in toluene solution yielded triplet excited states which exhibited a strong blue shift of Am= by nearly 100 nm as compared to plain 3Cm. This is rationalized in terms of a gradual destruction of the fullerene’s n-system with an increasing number of bis(ethoxycarbony1)methylene groups. The blue shift coincides with a significant slow down for the rate of reductive quenching of the excited triplet states by DABCO, Le., 1.3 x lo6 M-’ s-l vs 2.5 x lo9 M-’ s-l for the quenching of eq~atorial-(~C~)[C(COOEt)~]~ and 3C60,respectively. The radical-induced reduction of functionalized c 6 0 has been studied in a toluene/acetone/2-propanolmixture by means of timeresolved pulse radiolysis with measurements being conducted in the characteristic near-IR region. An almost linear dependence is obtained between the energy of the most significant IR-n-radical anion band versus the number of bis(ethoxycarbony1)methylene groups at the fullerene core, with the respective A,, ranging from 1080 nm for Cm’- to 1015 nm for equatorial-(C60’-)[C(COOEt)~]3. A corresponding trend emerges from cyclic voltammetry measurements on the redox potential in toluene/2-propanol, They show a difference of 330 mV between the formation of Cm’- (E112 = -0.55 V vs SCE) and the first reduction of equatorial-Cm[C(COOEt)2]3(E112= -0.86 V vs SCE). It appears that all these physicochemical parameters very sensitively reflect the site and degree of functionalization of C ~ O .
Introduction Photoinduced electron transfer encompasses a wide range of processes that are of great importance in chemistry and biology.’ Recent studies regarding the chemistry of fullerenes have focused on their reaction with free radicals2-I3 and electron transfer processes both in the ground state and excited triplet ~tate.‘~-~~ An increasing field of interest is that of functionalized fullerenes. Such molecules with interesting structural and physical properties can now be synthesized via a large variety of addition reaction^.^^-^^ In general, this allows the properties of the fullerene core to be fused with those of other molecules. Lately, even some derivatives have been reported with potential biological a ~ t i v i t y , ~e.g., ~ - ~ molecular ~ recognition in the inhibition of HIV p r o t e a ~ e . ~One ~ . ~of~the versatile approaches to fullerene functionalization is cyclopropylation of Cm with bromomalonic acid diethyl The first step in the synthesis is the formation of a mono-adduct CmC(COOEt)2 which upon further addition undergoes a successive transformation to defined regioisomers of bis- and tris-adducts, (260[C(COOEt)2], (n = 2,3). Very little and practically no quantitative information is available so far regarding the redox reactions of these Cmderivatives. This statement pertains also to the electronic properties and reactivity of their excited and reduced states. Among the published work, the photosensitized generation of singlet oxygen appears to be of greater importance due to its participation in DNA cleavage.36 In this work we will report on the formation and quenching of triplet excited states and on the radical-induced and electrochemicalreduction of chemically synthesized mono-, bis-, and tris-adducts of Cm. We will also demonstrate the complementarity of radiation- and light-induced techniques for such studies on fullerene properties. + Address for correspondence: Radiation Laboratory, University of Notre Dame. Notre Dame. IN 46556. Abstract published in Advance ACS Absrracts, May 15, 1995. @
Experimental Section Cm was purchased from Hoechst AG, the Cm-cyclohexanone adduct from MER COT. (Tucson, AZ),and diazabicyclooctane (DABCO) from Aldrich. All organic solvents were redistilled before use. The various Cm-derivatives, e.g., mono-, bis-, and tris-adducts (C~OC(COOE~)Z, C~O[C(COOE~)~]Z, and c60[C(COOEt)z]3),were synthesized according to already described method^.^^.^^ The identities of the compounds have been confirmed by I3C-Nh4R spectroscopy. The synthesis of the mono-adduct leads exclusively to the formation of a single product, while a second addition of a bis(ethoxycarbony1)methylenegroup yields eight regioisomers. We isolated those bis-adducts which were formed at higher yields via silica gel chromatography. These were the transz-, trans3-, equatorial-, and, in minor yield, cis3-isomers. The nomenclature is based on the following rules:35 Considering the threedimensional geometry of the fullerene, the second addition can take place within either the same hemisphere (referred to as “cis”-isomer; Figure la), at the equator (“equatorial”-isomer; Figure lb), or within the opposite hemisphere (“trans”-isomer; Figure IC). The increasing indices at the trans-isomers, e.g. trans2 and trans3, indicate the decreasing distance of the second substituent from the equator. The all-equatorial tris-product could be obtained via addition of bromomalonic acid diethyl ester to the equatorial bis-isomer (yield a 40%). Pulse radiolysis experiments were performed by utilizing either 500-11s pulses of 1.55-MeV electrons from a Van de Graaff accelerator facility or 50-11s pulses of 15-MeV electrons from a linear electron accelerator (LINAC). Basic details of the equipment and the data analysis have been described e l s e ~ h e r e . Dosimetry ~ ~ , ~ ~ was based on the oxidation of SCNto (SCN)i-, which in aqueous, NzO-saturated solutions takes place with G a 6 ( G denotes the number of species per 100 eV, or the approximate micromolar concentration per 10 J of
0022-365419512099-9380$09.00/0 0 1995 American Chemical Society
Cm-Substituted Malonic Acid Diethyl Esters
cis
J. Phys. Chem., Vol. 99, No. 23, 1995 9381
trans
equatorial
Figure 1. Configuration of cis-, equatorial-, and trans-isomers.
absorbed energy). The radical concentration generated per pulse amounts to (1-3) x M for all the systems investigated in this study. For flash-photolysis experiments, the fullerene solutions were prepared to give an optical density of 0.25 at 308 nm, the wavelength of irradiation. The apparatus for laser-flash photolysis has been reported elsewhere.39 A XeCl excimer laser (Lambda Physics EMG 201 MSC) was used as the light source. The Xe lamp was triggered synchronously with the laser. A monochromator (CVI Instruments Digistoem 240) in combination with either a Hamamatsu R5108 photomultiplier or a fast germanium diode was employed to monitor transient absorption spectra. Cyclic voltammetry measurements were performed with a Pt or glassy carbon rotating working electrode, polished with a 1-pm diamond paste before each run, a Pt counter electrode, and an aqueous standard calomel electrode as reference. All potential values cited within this paper are versus SCE. The electrochemical unit comprised a Princeton Applied Research 273A potentiometer, a 286 AT computer with a Princeton Applied Research Model 270 electrochemical analysis software 3.0, and a rotating disk electrode Methrom 628-10 with a Pt or glassy carbon electrode tip. The scan rate was varied between 25 and 500 mV s-l. If not specifically noted, the experiments were conducted at 50 mV/s. All experiments were carried out at room temperature, 22 f 2 "C. Experimental error limits are estimated to be f 1 0 % unless otherwise noted. Results and Discussion Formation of Triplet Excited States. Laser-induced excitation of c60, dissolved in toluene, leads exclusively to the formation of the excited singlet state. The latter undergoes a rapid and quantitative intersystem crossing to the excited triplet state.I9 The low reduction potential of the triplet state permits an efficient quenching as has been demonstrated for a large variety of compounds. Among the suitable quenchers, transferring an electron to 3C60,the best investigated ones are tertiary amines. Recently we reported on the successful application of diazabicyclooctane (DABCO) in a study of reductive quenching of 3C60in various solvents and heterogeneous en~ir0nments.I~ 6 '0
3c60 +Q
- "60
[3~60*
Functionalization therefore reduces the extent of resonance stabilization within the conjugated n-system. For comparison, also another mono-adduct has been investigated, namely, the cyclohexanone adduct Cm(C4&0).33 Upon excitation of this fullerene derivative in a N2-saturated toluene 8000 6000 4000
2000 0 -2000
3c60
- - 91 = [cm'--9'+1-cm*-+ Q*+
The various mono-, bis-, and tris-adducts Cm[C(C00Et)2]1-3 exhibit similar solubility in toluene as the unmodified Cm, despite the change in polarity introduced by the functional group(s). In order to obtain a desired ground state absorbance of 0.2 at the excitation wavelength (308 nm), about 20 pmol of the respective Cm-adduct has been dissolved for each experiment. Excitation of a N2-saturated toluene solution of 2 x M mono-adduct, CmC(COOEt)2, yielded a differential spectrum which is displayed in Figure 2. It shows strong bleaching of the ground state absorption around 330 nm and a very distinct new absorption band with a maximum at 720 nm. This absorption is ascribed to the triplet excited state (3Cm)C(COOEt)2, in analogy to the other f u l l e r e n e ~ . ' ~ It . ' ~is noted, however, that the triplet absorption for this mono-adduct shows a hypsochromic shift of 27 nm (69 meV) relative to that of plain 3Cm. This blue shift is also accompanied by a comparatively lower extinction coefficient. Both observations can be rationalized considering that formation of the mono-adduct involves saturation of one of the 30 double bonds of c60.
(2)
In toluene, however, Cm*- and Q'+ could not be detected despite the fact that quenching undoubtedly occurred. Most likely this is due to the low polarity of toluene which inhibits charge separation.
11
-4000
300
i "
" ' '
400
I
'
'
'
'
'
"
'
600
"
"
'
'
'
'
'
'
'
'
'
700 800 900 Wavelength, nm Figure 2. Differential absorption spectrum obtained upon flash photolysis at 308 nm of 2.0 x lo-* M C.&(COOEt)* in a nitrogensaturated toluene solution.
500
Guldi et al.
9382 J. Phys. Chem., Vol. 99, No. 23, 1995 TABLE 1: Photophysical Data for the Excitation of Fullerene Derivatives and Rate Constants for Reductive Quenching of DABCO in Toluene (at Room Temperature) triplet
compound C60
CmC(COOEtj2
absorption (nm)
747 720
E
kq
at &,let
(M-' cm-l) (lo9M-' s-l 1 16000 2.5"
9000
0.017 (0.009)b
710 equatorial-C~[C(COOEt)2]2 710 705 trans&0[C(COOEtj~]2 690 cis3-C~[C(COOEtjz]~ trans&,~[C(COOEt)& 690 eq~atorial-C60[C(COOEt)2]3 650 a Taken from ref 14. In 1-butanol. Not Ca(C4H60)
7000 9700
0.0064
8000
0.0014
8400 8200 7500 measured.
1.9 -
F
2
0.0058
1.85 -
$-his 1cg-bis
0
.I
1.8 -
4 4-
0.0082 c
0
e-tris
1.75 -
'E
0
cyclohexanone 0
1.7 -
:5-bis
0
e-bis
mono
. C60
0.0013
solution, a differential absorption spectrum is obtained which shows a triplet band at 710 nm. Compared to the ester derivative, this triplet excitation energy is blue shifted by another 10 nm, and there is also a corresponding further decrease of the extinction coefficient. Four different bis-adducts Cm[C(COOEt)2]2 have been investigated, an equatorial-, a cis3-, and two trans-products, Le., trans2 and transj. The triplet energies show quite a noticeable dependence on the structure of the respective bis-adduct. The maxima of their absorption bands spread over a range of 20 nm, reaching from 710 nm for the equatorial-product to 690 nm in the case of the transl-bis-adduct, suggesting that the equatorial-bis-product is the mesomerically best stabilized one in this series. On the other hand, moving the second substituent closer to, or further away from, the first bis(ethoxycarbony1)methylene group (referencing to the cis and trans isomers), the electronic structure of the fullerene seems to suffer a higher degree of disturbance. Most interestingly, all this matches with the synthetic yields of the four different products where equatorial addition of the second bis(ethoxycarbony1)methylene group appears to be the dominant process, while the synthetic yields of the cis- or trans-bis-adducts are lower compared with that of the equatorial compound. Finally, the decreased extinction coefficients, determined at the maxima of the triplet absorptions, are also well in line with these features. Addition of a third bis(ethoxycarbony1)methylene group to the fullerene core leads to various tris-adducts which are also formed at very different yields. Due to synthetic limitations we had to restrict our present studies to the most stable and also statistically favored among these isomers, an all-equatorial derivative. By extrapolation from the results of the two monoand four bis-adducts, one may expect a further significant blue shift for the triplet absorption energies in the tris-adduct. Indeed, a maximum at a wavelength as low as 650 nm has been observed for the excited triplet state of (3Cm)[C(COOEt)2]3(2 x M all-equatorial tris-adduct, Nz-saturated solution in toluene). The large shift of nearly 100 nm, compared to the plain 3Cm, is again associated with a substantial decrease in extinction coefficient (see Table 1). Figure 3 displays the complete correlation of the triplet energies versus the number of bis(ethoxycarbony1)methylene groups bound to the fullerene core. The data reflect the gradual destruction of the n-system with an increasing number of lifted double bonds. The small blue shift found for the cyclohexanone adduct (710 nm) in comparison with the mono-bis(ethoxycarbony1)methylene-adduct (720 nm) may be attributed to the higher electron withdrawing effect induced by the keto group located at the cyclohexanone ring as compared to the bis(ethoxycarbony1)methylene function.
Reductive Quenching of Triplet Excited States. Left alone, the triplet excited state decays via second order triplet-triplet annihilation, resulting in the regeneration of the ground state. The kinetics change, however, to pseudo-first order when suitable quenchers are added. In an earlier work we have demonstrated that in toluene 3Cm can rapidly be quenched by DABCO, for example (see eq 3).14 This process presumably involves an electron transfer; the low polarity of the solvent prevents, however, charge separation (the latter readily occurs only in solvents of higher polarity). One of the major contributions that limits the quenching rate, is the difference in the redox potentials of the excited triplet state and of the quencher. The remarkable dependence of the triplet energies on the extent of functionalization (Figure 3), which is considered to reflect a change in electronic properties, would, on the same basis, suggest a corresponding trend in redox potentials of the triplet states and thus in the quenching rates. Reductive electron transfer from DABCO to 3Cm is one of fastest quenching processes established so far under the current conditions. The measured rate constant for this process is (2.5 & 1) x lo9 M-' SKI. In related systems, only tetramethylphenylenediamine and p-anisidine were found to be more effi~ient.'~ The use of DABCO has the advantage that neither this molecule nor its oxidized radical cation DABCO'+ add on to ground state Cm, in contrast to primary and secondary amine^.^^,^^ Oxygen has, of course, to be excluded from any solution, since jC60 is already quenched by traces of 0 2 . In our present investigations, addition of various concentrations of DABCO (1- 10 mh4) to a toluene solution of 2 x M CmC(COOEt)2 resulted in the expected accelerated decay of the excited triplet state. The observed first order rate constant (k&) was linearly dependent (Figure 4a) on the DABCO concentration, indicating the quenching reaction (3) as the rate determining step. The rate constant of 7.7 x lo7 M-I s-l (Table
(C,,,-)C(COOEt),
+ DABCO"
(3)
1) derived from these measurements is significantly lower than that for the above mentioned corresponding quenching of 3Cm. This finding is again attributed to the disturbance of the electronic resonance structure in the functionalized fullerene and the more negative reduction potential of the mono-adduct in the ground state resulting therefrom (see also electrochemical data in later section). In addition, some steric hinderance, caused
Ca-Substituted Malonic Acid Diethyl Esters
J. Phys. Chem., Vol. 99, No. 23, 1995 9383
35 r
0
1
2
1
3
4
5
[DABCO] x lo2 M
Figure 4. Linear plot of quenching of (a) 2.0 x
[DABCO] for the reductive triplet M C,&(COOEt)* and (b) 2.0 x M eq~atorial-C~[C(COOEt)2]~ in toluene solution. kobS vs
by the space filling substituent, may contribute to the slow down of the quenching rate. The electron transfer mechanism was confirmed by applying a solvent of higher polarity, namely, 1-butanol. This solvent is very suitable since it allows not only a reasonable dissolution of the fullerene but provides also an environment for a successful charge separation in the reductive quenching mechanism. In the respective experiment with CaC(COOEt)z, the electron transfer from DABCO to the triplet state of the fullerene was monitored through the formation of the characteristic (Ca*-)C(CO0Et)z radical anion absorption in the near-IR region (Amax = 1040 nm, for further details see Pulse Radiolytic Formation of n-Radical Anions), where DABCO'+ does not absorb significantly. The rate of formation of the radical anion, measured at various wavelengths in the 900- 1100-nm range, was found to be similar to the rate of decay of the (3C,3)C(COOEt)2 absorption at 650-800 nm. The derived quenching rate of 9 x lo6 M-' s-I in 1-butanol is even lower by another order of magnitude compared to toluene. The same trend, Le., decreasing quenching rate with increasing solvent polarity, has also been observed for the unmodified fullerene, Le., the 3 C a DABCO r e a ~ t i 0 n . l ~ The investigation of the other mono-adduct originating from c60, namely, the cyclohexanone adduct, Ca(C4H60), revealed a further slow down for the quenching rate by nearly 1 order of magnitude as compared to the mono-ester derivative ( k = 8.2 x lo6 vs 7.7 x lo7 M-' s-l in toluene). It follows suit with the slight blue shift in triplet energies, Le., the electronic properties of the species. Steric hindrance, on the other hand, seems to be less important in this case since the less space filling nature of the cyclohexanone adduct compared to the bis(ethoxycarbony1)methylene group would oppose this trend. Among the bis-adducts the most effective triplet state quenching by DABCO is observed for the equatorial compound,
+
well in line with the trend in triplet state absorption energies. The first order rate constant kobs, derived from the exponential decay of the triplet absorption in toluene, showed the expected linear dependence on the DABCO concentration (4-50 mM), as demonstrated in Figure 4b. It is interesting to note that in the case of the two trans-products, quenching of the transz-triplet takes place much faster than that of the trans3-triplet, opposite to the trend in triplet absorption (see Table 1). We cannot offer any conclusive rationale for this finding. It is reasonable to assume that the optical characteristics reflect predominantly the electronic parameters, while steric components may have a comparatively larger impact on the reaction kinetics. It is, therefore, conceivable that the latter play a somewhat bigger role for the bis- than for the mono-adducts. The slightly shifted site of location for the second bis(ethoxycarbony1)methylene group in the trans3-species might perhaps cause a higher degree of shielding of the fullerene core and thus complicate the DABCO approach. Finally, addition of various DABCO concentrations to a deaerated toluene solution of about 2 x M equatorialC60[C(COOEt)2]3, resulted also in a linear dependence of the triplet life time of this tris-adduct on the DABCO concentration. From the linear plot of kobs versus [DABCO], a quenching rate of 1.3 x lo6 M-' s-l has been derived (see Table 1). The equatorial tris-adduct exhibits in all terms, Le., triplet energy and quenching rate, the most dramatic changes as compared to 3C60. This is understandable on the basis of the strong effect of the triple addition of bis(ethoxycarbony1)methylene groups exerts to the fullerene core by destroying the n-character to the comparatively highest extent and presumably also by shielding it sterically against any further reaction. Pulse Radiolytic Formation of n-Radical Anions. Another possibility for generating the radical anions, besides reductive triplet quenching, is to reduce the fullerenes via radiation chemical processes, e.g., in a solvent mixture containing 80 vol % toluene, 10 vol % 2-propanol, and 10 vol % a c e t ~ n e . ~ This -~ environment has been chosen to achieve optimal solubility for Cm and the presently investigated derivatives. The reducing species, generated in this solvent mixture, is the radical formed by hydrogen abstraction from 2-propanol, Le., (CH3)2C.(OH).
The same reducing radical also derives from acetone upon electron capture and subsequent protonation. (CH,),C=O
+ esol-+ H+ -(CH,),C'(OH)
(5)
This radical is known to react quite rapidly with, for example, c 6 0 and C70, yielding the respective radical anions, e.g. Cm*and C70'-.2-4 Pulse irradiation of deaerated solutions containing 2 x M of our functionalized fullerenes resulted in the formation of distinct absorptions in the near IR. These IR bands are ascribed to the x-radical anions formed in the general reaction (CH,),C'(OH)
+ c60'Rx
(CH,),CO
+ H+ + (C60>Rx)'-
(6)
(x denotes the number of substituents R, e.g., bis(ethoxycarb0ny1)methylene groups at the fullerene core.) The differential spectrum obtained upon pulse radiolysis of CaC(COOEt)2, for example, is displayed in Figure 5. It exhibits a distinct maximum at 1040 nm. The hypsochromic shift of 40 nm (45 meV) for the (Ca*-)C(COOEt)2 radical anion as
9384 J. Phys. Chem., Vol. 99, No. 23, 1995
Guldi et al.
60
50
40 30
-
-
-
.-2
0.2
-0.2
-0.6
-1.0
0.8
0.0
-0.8
-1.6
e
10 0 750
I
I
1
850
950 Wavelength, nm
1050
1 I50
Figure 5. Transient absorption spectrum of (Ca'-)C(COOEt)z radical anion obtained upon pulse radiolysis of 2.0 x M CaC(COOEt)* in a nitrogen-saturated toluene/2-propanol/acetone (8:1 :1) mixture. 1.95 0
1.90
58
E [VI vs SCE Figure 7. Cyclic voltammograms of (a) Cm and (b) CmC(CO0Et)I in
e-tris
toluenel2-propanol (9:1) (deoxygenated solutions containing 0.1 M Bu4N+BF4-, glassy carbon electrode, sweep rate 50 mV/s, temperature 22 "C).
1.85
.e
E2 -2
-
c3-bis 1 S-bis
1.80 e-bis 0
Sa 1.75
0
6
1.70
TABLE 2: IR Absorption Bands of kRadical Anions Measured by Time-Resolved Pulse Radiolysis
$-his
mono
compound
0
L60
1.65 1.14
C60 CaC(C0OEt)z
1.16
1.18
1.2
1.22
equatorial-Ca[C(COOEt)?]2
1.24
trans3-Ca[C(COOEt)z]* cis3-C~,[C(C0OEt)?]* trans2-Cm[C(COOEt)*]* equatorial-Cm[C(COOEt)&
n-radical anion absorption [eV]
Figure 6. Plot of absorption maximum for the n-radical anion (eV) vs the absorption maximum for the excited triplet energies (eV). a
compared to Cm*- (A, = 1080 nm) is again rationalized by the destruction of the n-system with increasing addition of bis(ethoxycarbony1)methylene groups. Corresponding blue shifts have been observed for the radical anions of the bis- and tris-adducts. They resemble our earlier finding on the shift of absorption of the excited triplet states and the reductive quenching rate constants. This is quantified by the correlation showing the dependence of the energy of the IR band of the n-radical anion versus the absorption maximum of the excited triplet energies in Figure 6. The full range, starting from the plain Cm*- and ending at the equatorial-(Cm'-)[C(COOEt)2]3 product, covers 65 nm (74 meV). It is noted that the absorption of the equatorial-(Cm*-)[C(COOEt)2]2radical anion, in contrast to expectation, is slightly red shifted as compared to that of the mono-adduct (Cm'-)C(C0OEt)z radical anion. We cannot offer any rationale for this at the moment. The trans2-, transs-, or cis3-geometry systems, however, show the expected blue shifts. Furthermore, the relative trend within all the bis-adducts is the same as for the triplet absorption energies or quenching rate constants. Moving the second bis(ethoxycarbony1)methylene group away from the equatorial position (see Figure la-c) seems to add to the overall disturbance of the mcharacter. Differential absorption changes occur not only in the IR but also in the W / v i s range. The observed changes in the W / v i s are, however, either dominated by strong bleaching of the ground state absorption (in the W),or exhibit broad and not very characteristic tailing (in the vis region). Therefore, the
n-radical anion absorption (nm) 1080"
1040 1065 1035 1030 1030 1015
Taken from ref 3.
present work underlines the importance of detecting the absorption changes in the near IR which are much more specific, especially in studies involving redox processes. All optical data concerning the IR bands for the various derivatives are listed in Table 2. Cyclic Voltammetry Measurements. In order to confirm the conclusions established from the light- and radical-induced processes, some initial electrochemical experiments were conducted on the redox potentials of Cm and several C ~ O [C(COOEt)2], in toluene. Dissolution of the applied supporting electrolyte (0.1 M BW+BF4-) required addition of 10 vol % 2-propanol. Parts a and b of Figure 7, for example, display the cyclic voltammograms of two compounds, C ~ O and CmC(COOEt)2 (glassy carbon electrode and scan rate of 50 mV/s). The measured potential of -0.55 V vs SCE for Cm is in good agreement with that reported previously for the reduction of the fullerene in a toluene/acetonitrile solvent mixture.42 The respective value for the mono-adduct is -0.64 V, i.e. it is somewhat more difficult to reduce this functionalized fullerene. Furthermore, the mono-adduct shows a larger difference in peak potentials, E,, - Ered, and lower current ratios, iox/ired. than Cm (Table 3 and Figure 7). The redox potentials for the first reduction step of the respective equatorial bis- and tris-adducts, e.g., equatorial-C60[C(COOEt)~]~ and equatorial-Cm[C(C0t)~l3, confirms this finding. While for the bis-adduct a redox potential of -0.76 V vs SCE was derived, the respective value of the tris-adduct amounts to -0.86 V. The observed increase in E,,
Cm-Substituted Malonic Acid Diethyl Esters
J. Phys. Chem., Vol. 99, No. 23, 1995 9385
TABLE 3: Redox Data of Cm, C&(COOEt)*, equatorial-Cm[C(C00Et)~l~, and equatorial-Cm[C(COOEt)z]3 Obtained upon Cyclic Voltammetry in Toluene/2-F'ropanol at Room Temperature
C60 CmC(COOEt)? equatorial-Cm[C(COOEt)~]? equatorial-Cw[C(COOEt)?]3
-0.55 -0.64 -0.76 -0.86
0.42 0.61 0.53 0.93
0.92 0.43 0.3 0.23
- Ered and decrease in ioxlired may be attributed to a higher degree of irreversibility in the redox process which would also coincide with a decrease in general stability of these compounds. This interpretation of the electrochemical data may, however, not be conclusive since CV experiments in low dielectric media often show larger peak separations simply due to IR drop problems and are not directly associable with the molecule of interest.
Conclusion In conclusion, functionalization of Cm leads, as expected, to a disturbance of the fullerenes n-systems. This very sensitively shows up in many electronically related parameters, such as optical absorptions of triplet excited states and radical anions, and the entire redox properties as could be demonstrated by application of a variety of techniques. Markable effects are even noticed for fullerenes carrying the same number of functionalized groups if these are located at different sites relative to each other. In general, the degree of disturbance relative to plain Cm increases with the number of functional groups. Among the multifunctionalized fullerenes the resonance within the n-system seems least affected by equatorial positioning of the functionalized groups. Acknowledgment. The authors appreciate the cooperation of Dr. Eichmiiller and Dr. Komowski (HMI Berlin) in the use of the laser apparatus and Dr. Hirsch (University of Tiibingen) for kindly providing us with a first sample of equatorial-Cm[C(COOEt)z12. References and Notes (1) Fox, M. A.; Chanon, M. Photoinduced Electron Transfer; Elsevier: Amsterdam, 1988. (2) Guldi, D. M.; Hungerbiihler, H.; Janata, E.; Asmus, K.-D. J . Chem. Soc., Chem. Commun. 1993, 84. 131 Guldi. D. M.: Huneerbiihler. H.: Janata. E.: Asmus. K.-D. J . Phvs. Chem.' 1993, 97, 11258. " (41 Guldi. D. M.: Huneerbuhler. H.: Wilhelm. M.: Asmus. K.-D. J . Chem. Soc., Faraday Tran; 1994, 90, 1391. (5) Guldi, D. M.; Neta, P.; Asmus, K.-D. J . Phys. Chem. 1994, 98, 4617. (6) Hungerbiihler, H.; Guldi, D. M.; Asmus, K.-D. J . Am. Chem. SOC. 1993, 115, 3386.
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