4169
J . Phys. Chem. 1992, 96, 4169-4170
Production of C6,, Radical Cation by Photosensitized Electron Transfer
Santiago Nonell? James W. Arbogast, and Christopher S. Foote* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1569 (Received: March 9, 1992; In Final Form: April 2, 1992)
The radical cation of c60 is prepared by electron transfer to singlet N-methylacridinium hexafluorophosphate. It has A,, near 980 nm. The radical cation can also be prepared in higher yield by cosensitization,using high concentrations of biphenyl to form biphenyl radical cation which is the ultimate oxidant for c60.
Since their discovery,l the fullerenes have caused much excitement; substantial quantities of Cso and C7,, can now be readily prepared.24 The photophysical properties have been studied,5-'2 but the chemical reactivity of these new compounds is just beginning to be explored. Although Cso is easy to reduce and reacts readily with nucleophiles, it is very difficult to oxidize.I3-l7 Recently, photochemical electron transfer has been used to prepare c 6 0 radical anion.lsJ9 The radical cation (c60'+) has been produced by y-irradiation at 77 K in a glass and has an absorption maximum at 980 nm.20 We wished to generate Cm'+ by electron transfer to a photoexcited acceptor.2'.22 Since the only definitive electrochemical oxidation of c 6 0 has been reported to occur at a potential of +1.76 V vs SCE in ben~onitrile,'~ we used singlet excited N-methylacridinium hexafluorophosphate (MA+), which has a reduction potential of 2.31 V, sufficient to oxidize Cso.23924
'MA+
+ c60
-
MA'
+ c60'+
We modified our previously described transient absorption spectrometer5to operate in the near-IR region.25 Irradiation of a solution containing MA+ (2 X M) and c 6 0 (2 X lo4 M, the highest soluble concentration) produced weak transient absorption with a rise time of 270 ns, close to the detector time constant. Less than 10%of the absorbed photons were absorbed by Cm. The decay of the transient was monoexponential with a lifetime of 9 f 1 w. The transient absorption spectrum (recorded immediately after the laser pulse) is shown in Figure 1 and has a maximum at 980 nm. It is assigned to the c 6 0 radical cation because of its similarity to the spectrum reported by Kat0 et and because of the experiments described below. From the rise time of the signal and the Cso concentration, the lower limit for the rate constant for electron transfer is 2 X 1O1O M-' s-I, essentially diffusion-controlled. Stern-Volmer quenching of MA+ fluorescence was difficult to measure accurately because of the low saturation concentration of Cso. The quenching rate constant is -4 X 1O'O M-' s-l, assuming a MA+ singlet lifetime of 36 ns.23 The near identity of the fluorescence quenching rate and the rise time of the transient confirms that the transient is formed by quenching singlet MA+. The short 'MA+ lifetime, combined with the low concentration of CW at saturation, limits the amount of Cm*+formed by direct electron traqsfer. We used c o s e n s i t i ~ a t i o n to ~ ~increase * ~ ~ ~ ~the ~ quantum yield of Ca*+ formation. In this process, the acceptor (MA+) is excited and abstracts an electron from a donor, producing a cation radical with a longer lifetime and a sufficiently high oxidation potential to cause secondary electron transfer from 'On leave from the CETS Institut Quimic de Sarria, 08017 Barcelona, Catalonia, Spain.
0.4
p
-
0.3:
e
e
-
2
: 0.2-
-
5
10
15
20
25
35x1C
1. SBC
a
0.0 I 900
I
*
I
I
,
,
,
950
,
I
- 1050
1000
Wavelength,
1100
nm
Figure 1. Transient absorption spectrum from MA' and C60(2 X M, A,, = 420 nm) in CH2C12. Inset: transient decay (hh= 990 nm, average of 10 shots). c60. Biphenyl (BP, E (BP+/BP) = 1.96 V)22 was used as cosensitizer.
'MA+ + BP
-
MA'
+ BP'+
(1) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R. 1985, 318, 162-163.
E. Nature
(2) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354-358. (3) Kritschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167-170. (4) Ajie, H.; Alvarez, M. M.; Anz,
S. A.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kratschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L. J . Phys. Chem. 1990, 94, 8630-8633. (5) Arbogast, J. W.; Darmanyan, A. 0.;Foote, C. S.;Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S.J.; Whetten, R. L. J . Phys. Chem. 1991, 95, 11-12. (6) Arbogast, J.; Foote, C. S. J . Am. Chem. SOC.1991,113, 8886-8889. (7) Kajii, Y.; Nakagawa, T.; Suzuki, S.; Achiba, Y . ;Obi, K.; Shibuya, K. Chem. Phys. Lett. 1991, 181, 100-104. (8) Hung, R. R.; Grabowski, J. J. J . Phys. Chem. 1991,95,6073-6075. (9) Haufler, R. E.; Wang, L A . ;Chibante, L. P. F.; Changming, J.; Conceicao, J. J.; Chai, Y.; Smalley, R. E. Chem. Phys. Lett. 1991, 179, 449-454. (10) Terazima, M.; Hirota, N.; Shinohara, H.; Saito, Y. J . Phys. Chem. 1991. 95. 6490-6495. (1'1) Ebbesen, T. W.; Tanigaki, K.; Kuroshima, S. Chem. Phys. Lett. 1991, 181, 501-504. (12) Tanigaki, K.; Ebbesen, T. W.; Kuroshima, S . Chem. Phys. Lett. 1991, 185, 189-192.
0022-365419212096-4169%03.00/00 1992 American Chemical Societv
4170
J . Phys. Chem. 1992, 96, 4170-4174
The near-IR transient absorption spectrum of a solution containing 0.2 M BP and MA+ and c 6 0 (both 2 X lo4 M) is shown in Figure 2. The shape of the spectrum is very similar to that in the absence of BP (Figure I), but about 10 times as strong. The rise time of the signal was 270 ns, which also suggests a diffusionantrolled quenching of BP" by Cm The decay kinetics X
(13) Allemand, P.-M.; Srdanov, G.; Koch, A.; Khemani, K.; Wudl, F. J . Am. Chem. SOC.1991, 113, 2780-2781. (14) Allemand, P.-M.; Koch, A.; Wudl, F.; Rubin, Y.; Diederich, F.; Alvarez, M. M.; Anz, S. J.; Whetten. R. L. J . Am. Chem. SOC.1991, 113, 1050-1051. (15) Dubois, D.; Kadish, K. M.; Flanagan, S.; Wilson, L. J. J . Am. Chem. SOC.1991, 113,1773-7774. (16) Dubois, D.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante, L. P. F.; Wilson, L. J. J. Am. Chem. SOC.1991, 113, 4364-4366. (17) Jehoulet, C.; Bard, A. J. J . Am. Chem. SOC.1991,113,5456-5457. (18) Arbogast, J. W.; Kao, M.; Foote, C. S. J . Am. Chem. Soc. 1992,114, 2271-2218. (19) Sension, R. J.; Szarka, A. Z.; Smith, G. R.; Hochstrasser, R. M. Chem. Phys. Lett. 1991, 185, 179-183. (20) Kato, T.; Kodama, T.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromaru, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Left. 1991, 180, 446-450. (21) Eriksen, J.; Foote, C. S. J . Am. Chem. SOC.1980, 102, 6083-6088. (22) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J . Am. Chem. Soc. 1990, 1 12, 4290-4301. (23) Gould, I. R.; Moser, J. E.; Armitage, B.; Farid, S.; Goodman, J. L.; Herman, M. S.J. Am. Chem. SOC.1989, 1 1 1 , 1917-1919. (24) Todd, W. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R. J . Am. Chem. SOC.1991, 113, 3601-3602. (25) Solutions in a I-cm quartz fluorescence cuvette were irradiated with 420-nm output of a pulsed dye laser (Quanta Ray PDL-2, Exciton Stilbene 420, ca. 10 mJ per pulse), pumped by the third harmonic of a Nd:YAG laser (Quanta Ray DCR). Transient absorption was monitored at right-angles using a CW 75 W Xe lamp filtered with 3 cm of water and an 850-nm cutoff filter (Schott RG 850). The light transmitted by the solution was passed through a 1/4-m monochromator (Jarrell-Ash 82-410) with a grating blazed at loo0 nm,and 500-pm slits, and detected with a liquid N2 cooled germanium diode (North Coast E0817 P). To prevent saturation of the detector, a Uniblitz shutter was opened 1 ms before the laser pulse and closed after 9 ms. Linearity of the detector and analysis system was checked by varying the lamp intensity with calibrated neutral-density filters. The output of the detector was acquired with a Lecroy 9410 transient recorder, and analyzed using a Macintosh IIci computer with Labview and Igor software. N-Methylacridinium hexafluorophosphate (MA+) was synthesized as described in ref 24 and references therein. Biphenyl (BP, Aldrich) was recrystallized from methanol. Cm was prepared by Ms. Jennifer Cho. Solutions in reagent-grade dichloromethane (Fisher) were purged with argon for 30 min.
40
o b , , 900
,
,
I
950
,
,
,
.
I
,
1000 Wavelength, nm
,
,
,
I
120x10"
,
,
,
1050
,
li 1100
Figure 2. Transient absorption spectrum from MA+ and Cm (2 X M, A,, = 420 nm) containing biphenyl (0.2 M) in CH2C12(A);C60 absent (0). Inset: transient decay (Aob = 980 nm, average of 10 shots).
of the transient were complex and deviated substantially from monoexponential behavior (see inset, Figure 2). We have not yet investigated the details of the decay kinetics. In the absence of Cso,a broad, structureless, and much weaker absorption between 900-1 100 nm was observed (Figure 2). Its decay was monoexponentialwith a lifetime of 50 f 2 ps. We made no attempt to assign this absorption. In conclusion, the radical cation of c 6 0 is formed by photoinduced electron transfer and detected by near-IR transient absorption with an absorption maximum at 980 nm. Cosensitization allows production of this ion with improved efficiency. We are currently investigating the yield and energetics of this process and its subsequent chemistry. Acknowledgment. S.N. thanks the Max Planck Society for an Otto-Hahn Fellowship. Supported by N S F grant No. CHE8911916 and N I H grant No. GM-20080. (26) Schaap, A. P.; Lopez,L.; Anderson, S. D.; Gagnon, S. D. Tetrahedron Lett. 1982, 23, 5493-5496. (27) Spada, L. T.; Foote, C. S. J. Am. Chem. SOC.1980, 102, 391-393.
Monte Carlo Simulation of the Peptide Condensing System 0.5 M CuC12/5 M NaCI/H,O Bernd M. Rode Institut fur Anorganische und Analytische Chemie, Universitdt Innsbruck, Innrain 52 a, A-6020 Innsbruck, Austria (Received: October 29, 1991; In Final Form: March 25, 1992) Monte Carlo simulations have been performed for the system 0.5 M CuCIz/5 M NaC1/HzO which has been found to induce condensation of amino acids to peptides. Two water potentials (CF and MCY) have been used and compared. Methodical aspects and comparison with experimental data are in favor of the results obtained with the CF potential. A large number of solvate species are found to coexist in the system, and the results of the simulations allow one to conclude which species should be most active in the reaction mechanism of the peptide formation. CuCl+(H,O), can be assumed to be the main species for amino acid complexation, whereas sufficient Nat(H20), ions with incomplete hydration shell are found in order to provide a strong source of water removal in the condensation process.
Introduction It has been reported recently14 that amino ah& a n easily form oligopeptides in aqueous solution of high sodium chloride antent (I) Schwendinger, M. G.; Rode, B. M. Anal. Sci. 1989,5, 41 1 . (2) Rode, B. M.; Schwendinger, M. G. Orig. Lqe 1990, 20,401. (3) Schwendinger, M. G.; Rode, B. M. tnorg, Chim, Acta 1991,186, 247, (4) Schwendinger, M. G.;Mer, A. H.; Sa&, s.; Rode, B. M. Submitted for publication.
in the presence of Cu(I1) ions. Within the context of prebiotic evolution, this is SO far the simplest known mechanism enabling peptide formation on the primordial earth, and it displays characteristics very much in favor of a realistic role in the actual chemical evolution under such conditions: the availability of all components needed under primitive earth conditions,* the preference of a-over @-amino acid^,^,^ and very slow racemization.4 Some indications toward the possible reaction mechanism have been obtained from electrochemical s t ~ d i e sthe ; ~ detailed com-
0022-3654/92/2096-4170$03.00/0 0 1992 American Chemical Society
