J. Phys. Chem. 1994,98, 1444-7441
7444
Photoinduced Electron Transfer from Triplet Fullerene, 3 Cto ~Tetracyanoethylene. Fourier Transform Electron Paramagnetic Resonance Study S. Michaeli,? V. Meiklyar,? M. Schulz,* K. Miibius,* and H. Levanon**t Department of Physical Chemistry and The Farkas Center for Light- Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, and Institute of Molecular Physics, Free University of Berlin, I41 95 Berlin, Germany Received: April 19, I994O
Fourier transform EPR spectroscopy was employed in studying the electron transfer (ET) and the quenching mechanisms of the photoexcited triplet state of C, (electron donor) in the presence of the electron acceptor tetracyanoethylene (TCNE) in a benzonitrile solution. The ET reaction product, which is the stable anion radical T C N P - , interacts with 3Cm (both detected by EPR in the liquid phase), leading to chemically induced dynamic electron polarization of TCNE-, via triplet4oublet mixing mechanism.
Introduction The dynamics of photoexcited fullerenes are subjected to extensive studies by different optical and magnetic resonance spectroscopies.1-11 Particular attentioninourlaboratoryisfocused on the photoexcited triplet-state dynamics of Cm and C ~ when O dissolved in isotropic and anisotropicsolvents.~3J2Because these carbon clusters behave in many respects like normal chromophores, it is of interest to investigate fullerenes in conjunction with photoinduced electron-transfer (ET) reactions. Indeed, recent studies show that fullerenes are good electron acceptors in ET reactions, deduced by opticalsll and EPR investigations.6Qe However, data concerning the participation of Cm as an electron donor in ET reactionsare scarce, although the oxidation potential values of C a (1 -76 V, SCE in benzonitrile),13 its lowest triplet energy (1.57 eV),14 should lead to ET reactions with well-known acceptors such as tetracyanoethylene (TCNE) (E1p = 0.24 V, SCE).15 Recently, Nadtochenko et aL16 reported on the quenching of 3Cm by TCNE in benzonitrile, employing laser-flash photolysis. In that study they showed that the reaction scheme results in the formation of an exciplex,
hv
c,, 3c, 3C,
+ TCNE-
[C,'+***TCNE'-]
(2)
and a back ET process (-25 ps half-lifetime) occurs within the cage, whose constituents diffuse apart to the separate neutral species: [C6;+.-TCNE'-]
+
C,
+ TCNE
(3)
The optical results described above are based upon the triplet decay of 3Cm and the formation and decay of Cm*+. Since the transient absorptionsof 3Cm and TCNE'overlap, no direct kinetic data are presented in the optical study.16 TCNP- is a wellstudied radical with characteristic EPR features.l7-lq Thus, it is natural to have an independent verification of the optical findings by following the ET reaction by means of EPR spectroscopy, where the paramagnetic species in this reaction can be identified unambiguously. In this study we report on the reaction between 3Cm and TCNE by employing time-resolved pulsed-EPR spectroscopy (Fourier t The Hebrew
University of Jerusalem.
t Free University of Berlin.
*Abstract published in Aduance ACS Abstracts, July 15, 1994.
0022-3654/94/2098-7444S04.50/0
transform, FT). Monitoring the photoexcited triplet, 3Ca, and its quenching by an electron acceptor should provide valuable data on the ET products as well as information about the mechanisms of the electron spin polarization (ESP) and the quenching of 3 C ~ This . type of data is not attainable by the optical methods. Moreover, unlike the optical results,I6 timeresolved EPR shows unambiguously, and as expected, that the radical anion of TCNE is very long-lived. To the best of our knowledge, this is the first study by EPR spectroscopy where the photoexcited triplet of C a participates as an electron donor.
Experimental Section FT-EPR experimentswere carried out with a commercialpulsed EPR spectrometer (Bruker ESP 380) interfaced to a pulsed Nd: YAG laser (Continuum, Model 661-2D) providing pulses (A = 532 nm) at a repetition rate of 20 Hz with energy of 3-6 mJ/ pulse. The pulsed EPR experiments (X-band, 24-11s microwave pulses) were carried out at a fixed temperature of 283 K. Free induction decays (FIDs) were detected at different delay times ( ~ d )between the microwave and laser pulses, varying between 10 ns and 100 ps. The FID dead time is 130 ns, but by application of the linear prediction singular value decomposition (LPSVD) routine,20the FIDs within the dead time could be reconstructed. TCNE (98% purity) and benzonitrile (HPLC grade), both from Aldrich,were used without further purification. The preparation of Cm was carried out as described earlier,21 and its final purification was carried out by column chromatographyon A1203. EPR samples of pure Cm (5 X 10-5- 5 X 1W M) and the mixture of Cm (2 X 1W M) with TCNE (10-4- 5 X 1 6 3 M) were prepared in glass tubes, degassed by freeze-pumpthaw cycles and sealed under vacuum. N
Results and Discussion It is well-accepted that the triplet state of Cm dissolved in isotropic and anisotropic liquids is easily detectable over a wide temperature range by time-domain EPR spectroscopy.1-5v7 In these experiments,a single sharp Lorentzian line shape (g factor 2.0012), in absorption mode, is detected with a solventdependent line width (0.14-0.5 G). The narrow line width observed at high temperatures was attributed to the Brownian rotational diffusion proce~s,~J~.22 combined with Jahn-Teller jumps.l.5 Both processes are sufficiently fast to average out the zero-field splitting tensor. In Figure 1 we show the time-evolved EPR spectra of a photoexcited solution of a mixture of Cm and TCNE in benzonitrile. Each of the FT-EPR spectra shown in Figure Ib-d N
0 1994 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 1445 1.2l
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Since photoexcitation forms the stable radical anion, TCNP-, it accumulates during a train of light pulses into a steady-state concentration. The 3Cm can be quenched by TCNP-, C&, and also neutral TCNE (in the latter case, not leading to an effective ET reaction). Finally, the triplet decay rate of Cm, in the absence of a quencher, should also be taken into account. The set of reactions are summarized below: 3C,
-25 -20 -15
Time (ps)
Figure 2. Temporal dependences of the 3Cm FT-EPR signal intensity: (a) pure Cm (2 X lo-' M); (b, c) mixtures of C a (2 X lo-' M) with TCNE ( 5 X 10-4 and lo-' M, respectively).
J
1
1
0.1
+ TCNE*-
kl
C,
+ TCNE'-
(5)
25
MHZ
Figure 1. FT-EPR spectra of 'Cm ([Cmlo = 2 X 10-4 M) and TCNE([TCNEIo = 5 X 10-4 M) in benzonitrile: (a) stable radical obtained after sample irradiation outside the cavity for 10 min; (b, c, d) solution of the stable radical exposed to photoexcitation in the EPR cavity at different delay times (indicated on the traces). Top scale denotes the nuclear quantum numbers of the TCNE' hyperfine lines. All spectra are distorted by an artifact at zero frequency, whose resonance position is independent of the magnetic field.
-
consists of two signals, namely, a single relatively broad line and a narrow-line multiplet. Upon cessation of the photoexcitation, the broad single line in the spectrum disappears and the multiplet persists over a time period of several hours (Figure la). In view of reactions 1 and 2, the broad line at --5.9 MHz may be attributed to either 3Cm0rC"+. However, its transient behavior and magnetic parameters (g = 2.0012 with a line width of -0.85 MHz or -0.3 G) are the same as those found for 3C60in benzonitrile6 and verified in the present study. We therefore assign this line to 3C60. Evidently, the triplets are formed instantaneously within the duration of the laser pulse. Due to selective intersystem crossing the initial signal intensity of 3Cm is polarized, indicating deviation from Boltzmann spin population. After spin-lattice relaxation has been established, only triplets with Boltzmann spin population of the levels are observed.3 Regarding the multiplet, its magnetic parameters correspond to those of TCNP-, reported over three decades ago by Weissman and co-workers." The T C N F - radical is characterized by a g factor of 2.0026 and nine 14N hyperfine lines with an hyperfine constant of a = 1.56 G.23 We, therefore, suggest the following ET reaction to occur:
+
3C60 TCNE
ka
C 6 t + + TCNE'-
(4)
Unfortunately, all attempts to detect a signal attributed to Cm*+ under a variety of experimental conditions (temperature change, different delay times between excitation and detection) failed. This may be due to inhomogeneous line broadening and short spin relaxation,which is within the dead time of the EPR detection.
k3
3c, + c, 3C,0
+ TCNE
k4
C,
2c,
(7)
+ TCNE
(8)
(9) In Figure 2 we show the temporal dependence of the 3Cm signal intensity as a function of various TCNE concentrations, demonstrating clearly the quenching effect. The intensity buildup of the triplet signal (Figure 2) is characterized by the formation of thermal triplets with the rate constant l/T,, leading to the timedependent signal intensity given by6 ~ ( t= ) z ~ { ( P 1) - exp(-t/T,)
+ I}[~c,]
(10)
where IO and P a r e the normalization and the initial polarization coefficients, respectively. The decay part consists of the ET reaction pathway (eq 4) and the quenching processes (eqs 5-9). The concentration of 3Cm as a function of time was determined by solvingthe differential equation associated with reactions 4-9, resulting in [3c60i
= a [ 3 C 6 0 1 ~ / ~ ( b ~+ 3 Ca)~ exp(at) l~ - b[3C6010} (11)
+
+
+
where a = ks + k3[Cm] (k2 kl)[TCNP-] + (k4 ka)[TCNE]; b = 4 3 ; [ ' C ~ ) Oand [TCNEIo are the initial concentrations of triplets and acceptor molecules in solution, respectively; [Cm] = [Cm]o - [Cm*+];and [TCNE] = [TCNEIo - [TCNE*-1. Substituting for in eq 10
Letters
7446 The Journal of Physical Chemistry, Vol. 98, No. 31, 199'4
TABLE 1: Magnetic Parameters and Quenching Rates of F a in Benzonitrile Darameter CSO C a + TCNE P 0.21(*0.02) 0.2 1(*0.02) Tia
kl + k2b ket + hb k3' 1/ksa kexb a
In ~
0.45(*0.03) 2.5(*0.3) 40(&5)
1
0.45(t0.03) 30(*5) 3 .O( h0.3) 2.5(*0.3) 40(&5) 2.0(&0.2)
-
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-1
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1
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3
4
a
s b ,In lo* M-I s-I.
Simulations of the curves shown in Figure 2 were carried out using eq 12 from which we calculated the sum (kl + k2) of the two rates of the triplet-doublet quenching processes, as well as the sum (k4 k,) (Table 1). It should be pointed out that the rates of the 3Cm quenching by the ground-state molecules are slower than those of the quenching rates by radicals. This difference can be attributed to an increase of the reaction radius in the latter case, due to the solvation of charged molecules in the relatively high polar benzonitrile. This effective solvation should account for the higher triplet-doublet quenching rates, k1,2(a4rDr),where D is the diffusion coefficient and r is the effective reaction radii of the triplet and the solvated doublets, T C N P - and c60'+. Direct evidence of triplet4oublet quenching (eqs 5 and 6) is gained by the noticeable chemically induced dynamic electron polarization (CIDEP) effects associated with the time-dependent spectra of both the triplet and doublet species, 3Cm and T C N P (Figure 1). Radical-triplet polarization mechanism (RTPM) can be manifested by multiplet and/or net effects of the experimental spectra.2ba The sign rules are summarized in recent publi~ations.~~-~g From the spectra shown in Figure 1 the net effect is not apparent, and the spectra are governed mainly by the multiplet effect. Thus, assuming a negative exchangeintegral, J , the observed absorption/emission (A/E, in frequency units) pattern is in line with a triplet precursor (in-cage quartet). Because of the short singlet lifetime of Cm,we rule out the other possibility of a singlet precursor (in-cage doublet) combined with J > 0. Similar to singletriplet, the expression for the doublet-triplet mixing rate, wu, is given by27
+
where the indices T and R stand for triplet and radical, respectively; a R is the hyperfine constant, and MR is the radical's nuclear quantum number. In our case, A g r = ~ -0.0014, a = 1.56 G, and MRi = 4 to -4. Thus, since wu changes its sign, the direction of polarization is changed as well. In Figure lb-d both 3Cm and T C N P - are clearly observed, and from the time-evolved spectra the multiplet effect can be easily demonstrated. Each spectrum of T C N P - consists of two contributions, i.e., a thermal and a polarized one. In the time interval of 10 ps, the intensity of each hyperfine component is determined by its resonance position relative to that of 3Cmand by thedelay time, rd. In fact, the intensities of the high-frequency hyperfine lines, with respect to 3Cao,decrease upon increasing the delay time, Td, while the intensities of the low-frequency lines increase. In Figure 3a we show the transient spectrum of T C N P after the thermal spectrum (light off) was subtracted from the experimental spectrum taken at 2 after the laser pulse. Such a subtraction leads to the unambiguous A/E polarization pattern (or E/A in magnetic field units). At Td > 20 ps the transient part of the spectrum disappeared. Moreover, Figure 3a demonstrates clearly that the sign of polarization changes through the resonance position of 3C60. Also, similar to the traditional radical pair mechanism (RPM) the polarization depends on OH; lines close to 3Caoresonance are less affected. At this stage of investigation
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Figure 3. (a) FT-EPR spectrum obtained by subtracting spectrum a from spectrum c shown in Figure 1. Spectrum a is due to the stable TCNE*-in thermal equilibrium,while spectrum c includes polarization. (b) FT-EPR spectrum of the mixture of Ca (2 X l e M) and relatively high concentration of TCNE ( 5 X M) in benzonitrile after laser irradiation for 10 min. Notice that both spectra are distorted by the
-
artifact at zero frequency.
no evidence for the net polarization is deduced from the spectral behavior of TCNP-. Unfortunately, the participation of Ca*+ (eq 6) in the quenching of 3Cacannot be observed directly, since the corresponding EPR signal escapes detection. In addition to the triplet dynamics, the stable TCNE radical anion can react with neutral TCNE through the exchange reactionl7J8 TCNE'-
-
+ TCNE ka T C N E + TCNE'-
(1 4)
resulting in line broadening as depicted in Figure 3b. The rate constant calculated for this exchange was carried out by following the procedure describe by Ward and W e i s ~ m a nresulting ,~~ in k,, given in Table 1. In conclusion, this first EPR investigation employs the photoexcited triplet of Cm in ET reaction as an electron donor that interacts with the reaction product TCNP-, leading to CIDEP through triplet-doublet spin mixing process. Acknowledgment. We are grateful to Mrs. A. Berman, Mr. G. Zilber, and Dr. V. Rozenshtein for their kind help. The financial assistance of the Wolfson Foundation is gratefully acknowledged. This work was partially supported by the US.Israel Binational Science Foundation, the Deutsche Forschungsgemeinschaft (Sfb 337), and the 1992 Max-Planck Research Award (H.L. and K.M.). The Farkas Research Center is supported by the Bundesministerium fiir die Forschung und Technologie and the Minerva Gesellschaft fiir Forschung GmbH, FRG. This work is in partial fulfillment of the requirements for a Ph.D. degree (S.M.) at the Hebrew University of Jerusalem and a Diploma degree (M.S.) at the Free University of Berlin. References and Notes (1) Closs, G. L.; Gautam, P.;Zhang, D.; Krusic, P. J.; Hill, S. A.; Wasserman,E. J. Phys. Chem. 1992, 96, 5228. ( 2 ) Levanon, H.; Meiklyar, V.;Michaeli, A,; Michaeli, S.; Regev, A. J . Phys. Chem.1992,96,6128.
Letters (3) Regev, A.; Gamliel, D.; Meiklyar, V.; Michaeli, S.;Levanon, H. J. Phys. Chem. 1993,97,3671. (4) Ruebsam, M.; Dinse, K. P.; Plueschau, M.; Fink, J.; Kritschmer, W.; Fostiropoulos, K.; Taliani, C. J. Am. Chem. Soc. 1992, 114, 10059. ( 5 ) Zhang, D.; Norris, J. R.; Krusic, P. J.; Wasserman, E.; Chen, C. C.; Leiber, C. M. J. Phys. Chem. 1993, 97,5886. (6) Steren,C. A,; Levstein,P. R.;Van Willigen, H.;Linschitz, H.; Biczok, L. Chem. Phys. Lett. 1993, 204, 23. (7) Goudsmit, G.-H.; Paul, H. Chem. Phys. Lett. 1993, 208, 73. (8) Bennati, M.; Grupp, A.; Biuerle, P.; Mehring, M. Preprint, 1993. (9) Arbogast, J. W.; Foote,C. S.;Kao,M. J. Am. Chem.Soc. 1992,114, 2217. (10) Biczok, L.; Linschitz, H.; Walter, R. Chem. Phys. Lett. 1992, 195, 339. (11) Samanta, A,; Kamat, P. V. Chem. Phys. Lett. 1992, 199, 635. (12) Levanon, H.; Meiklyar, V.; Michaeli, S.;Gamliel, D. J. Am. Chem. Soc. 1993. 115. 8722. (13) Noneli. S.;Arbogast, J. W.; Foote, C. S.J. Am. Chem. SOC.1992, 96, 4169. (14) Zeng, Y.;Biczok, L.; Linschitz, H. J. Phys. Chem. 1992, 96,5237. Kardos, A. M.; (15) Meites, L.; Zuman, P.;Scott, W. J.; Campbell, B. H.; Fenner, T. L.; Rupp, E. B.; Lampugani, L.; Zuman, R. In Handbook Series in Organic Electrochemistry; Meites, L., Zuman, P., Eds.; CRC Press: Cleveland, 1976; Vol. I, p 254. (16) Nadtochenko, V. A.; Denisov, N. N.; Rubtsov, I. V.; Lobach, A. S.;
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7447 Moravskii, A. P. Chem. Phys. Lett. 1993, 208, 431. (17) Philip, W. D.; Rowell, J. C.; Weissman, S.I. J. Chem. Phys. 1960, 33, 626. (18) Eastman, M. P.; Kooser, R. G.; Das, M. R.; Freed, J. H. J. Chem. Phys. 1969, 51, 2690. (19) Rieger, P. H.; Bernal, I.; Fraenkel, G. K. J. Am. Chem. Soc. 1961, 83, 3918. (20) Barkhuysen, H.; de Beer, R.; Bovk, W. M. H. J.; van Ormond, D. J. Magn. Reson. 1986, 67, 371. (21) Parker, D. H.; Wurz, P.; Chatterjee, K.; Lykke, K. R.; Hunt, J. E.; Pellm, M. J.; Hemminger, J. C.; Gruen, D. M.; Stock, L. M. J. Am. Chem. SOC.1991, 113, 7499. (22) Gamliel, D.; Levanon, H. J. Chem. Phys. 1992, 97, 7140. (23) The 20 low-intensity EPR lines due to the two equivalent groups of I3C nuclei with hyperfine constants a1 = 9.541 G and 02 = 2.203 G were not observed in the mesent s w t r u m . (24) Blittlei C.; PaG, H. Res. Chem. Intermed. 1991, 16, 201. (25) Shushin, A. I. Chem. Phys. Lett. 1993, 208, 173. (26) Goudsmit, G.-H.; Paul, H.; Shushin, A. I. J. Phys. Chem. 1993,97, 13243. (27) Kawai. A.; Okutsu, T.; Obi, K. J. Phys. Chem. 1991, 95, 9130. (28) Kawai, A.; Obi, K. J. Phys. Chem. 1992, 96, 52. (29) Kawai, A,; Obi, K. Res. Chem. Intermed. 1993, 19, 865. (30) Ward, R. L.; Weissman, S.I. J. Am. Chem. SOC.1957, 79, 2086. I-
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