5228
J . Phys. Chem. 1992, 96, 5228-5231
(14) (a) EerNisse, E. P. J . Appl. Phys. 1973, 44, 4482-4485. (b) EerNisse, E. P. J . Appl. Phys. 1972,43, 1330-1337. (c) Cheek, G. T.; OGrady, W. E. J . Elecrroanal. Chem. 1990, 277, 341. (15) Impedance analysis was performed with a Hewlett-Packard 4194A impedancc/gain-phase analyzer capable of performing measurements over a frequency range of 100 Hz-40 MHz in the impedance mode. Data collection was accomplishedvia an HPIB interface with a Macintosh personal computer. (16) Colacicco, G.;Buckelew, A. R., Jr.; Scarpelli, E. M. J . Colloid Interface Sci. 1974, 46, 147. (17) Colacicco, G.; Basu, M. K.; Littman, J.; Scarpelli, E. M. Adu. Chem. Ser. 1975, No. 144, 239. (18) (a) Kanazawa, K. K.; Gordon, J. G.,I1 Anal. Chem. 1985, 57, 1770-1771. (b) Kanazawa, K. K.; Gordon, J. G., I1 Anal. Chim. Acta 1985, 175, 99-105. (19) (a) Thompson, M.; Arthur, C. L.; Dhaliwal, G.K. Anal. Chem. 1986, 58, 1206-1209. (b) Rajakovic, L. V.; Cavic-Vlasak, B. A.; Ghaemmaghami, V.; Kallury, K. M. R.; Kipling, A. L.; Thompson, M. Anal. Chem. 1991.63, 615-621. (c) Kipling, A . L.; Thompson, M. Anal. Chem. 1990, 62, 1514-1519. (d) Khurana, A. Phys. Today 1988,41, :7. (e) Krim, J.; Widom, A. Phys. Rev. 1988, 838, 12184.
(20) 555. (21) (22) (23)
Deamer, D. W.; Cornwell, D. G. Biochim. Biophys. Acra 1966,116,
Derjaguin, B. V.; Green-Kelly, D. Trans. Faraday Soc. 1%4,60,449, Krim, J.; Watts, E. T.; Digel, J. J. Vac. Sei. Technol. 1990, A8,3417. McCafferty, E.; Pravdic, V.; Zcttlemoyer, A. C. Trans. Faraday Soc. 1970. 66. 1720. (24) Kiseleva, 0. A.; Sobolev, V. D.; Starov, V. M.; Churaev, N. V. Kolloidn. Zh. 1979, 41, 245. (25) Palmer, L. S.; Cunliffe, A.; Houeh, J. M. Nature 1952. 1970. 796. (26) (a) Israelachvilli, J. Acc. Chem. R&. 1987, 20,415. (b) Israelachvilli, J. N.; McGuiggan, P. M.; Homola, A. M. Science 1988, 240, 189. (c) Israelachvilli, J. N.; Kott, S. J. J . Colloid Interface Sci. 1989, 129, 461. (27) (a) Ulman, A. Ado. Mater. 1991, 3,298. (b) Hautman, J.; Bareman, J. P.; Mar, W.; Klein, M. L. J . Chem. Soc., Faraday Trans. 1991,87,2031. (c) Barton, S.W.; Goudot, A.; Bouloussa, 0.;Rondelez, F.; Lin. B.; Novak, F.; Acero, A,; Rice, S. J . Chem. Phys. 1992, 96, 1343. (d) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zshack, P.; Dutta, P. J . Chem. Phys. 1992, 96, 1556. (28) Okahata, Y.; Kimura, K.; Ariga, K. J . Am. Chem. SOC.1989, 1 1 1 , 9190. ~
Steady-State and Time-Resolved Direct Detection EPR Spectra of Fullerene Triplets in Liquid Solution and Glassy Matrices. Evidence for a Dynamic Jahn-Teller Effect in Triplet Ceot Gerhard L. Gloss,*** Pennathur Gautam,*+sDaisy Zhang,* Department of Chemistry, The University of Chicago, Chicago, Illinois 60637
Paul J. Krusic,* Steven A. Hill, and Edel Wasserman* Central Research and Development, E . I. du Pont de Nemours & Co., Wilmington, Delaware 19 90-0328 (Received: April 7, 1992; In Final Form: May 4, 1992) UV irradiation of methylcyclohexane solutions of C, produces a very narrow, transient EPR absorption which is assigned to the first excited triplet state of Cm The line width of only 0.14 G, uncommon for motionally narrowed triplet EPR spectra, is attributed to a very rapid interchange of the magnetic axes by pseudorotation converting the degenerate Jahn-Teller states into each other. Time-resolved, direct-absorption EPR measurements with a time resolution of 0.5 ps support this conclusion. They indicate that in solution the triplet EPR absorption decays at rates comparable with those obtained by optical methods for )Cb0. A relaxation time TIof 8 ps was obtained from the oscillations observed in the early stages of the decay curve and s, too following laser excitation. This T , , and the line width in solution, require correlation times between short for rotation. Polarized, partially averaged powder triplet spectra were also observed in methylcyclohexane glasses at low temperatures. The pseudorotation proposal is supported by the distinctly different behavior of C70.
Among the many spectroscopic measurements made recently on CU and other fullerenes,l there has been the detection of the EPR spectra of the lowest triplet states of C , (3C,) and c70 (3C70) in rigid matrices at 5 K.2 The spectrum of 3C, had nonvanishing zero-field splitting parameters indicating the loss of spherical symmetry in the triplet state as is expected from the Jahn-Teller distortions in the excited states of C,.2c In this communication we wish to report the CW and time-resolved EPR spectra of the triplet states of CU and CT0in liquid solution and in glasses at different temperatures. When a degassed and saturated solution of C60in methylcyclohexane is irradiated inside the cavity of an EPR spectrometer with a xenon arc lamp a t temperatures between 300 and 180 K, a very sharp (0.14 G) line is observed at g = 2.001 35 (Figure 1A). As shown in the inset, the line width does not change appreciably from room temperature to 200 K. Below 180 K the line begins to broaden and can no longer be detected in a conventional EPR experiment below 145 K. This signal decays rapidly
'du Pont Contribution No. 6222. 'Deceased, May 24, 1992. f Present address: Center for Fast Kinetics Research, University of Texas at Austin, Austin, TX 78712.
when the light is extinguished and can be observed repeatedly without loss of intensity, indicating the absence of efficient photochemical changes. A possible candidate for the carrier of the spectrum is the lowest triplet state of Cm. Optical studies reported by Foote and collaborators3and corroborated by others4 have determined lifetimes of 40 ps and longer for the triplet state. To obtain evidence that the EPR signal originates from the triplet state, time-resolved EPR experiments were carried out using the direct detection method with a time resolution of 0.5 ps.5 In these experiments the carrier of the EPR signal is generated by pulses from an excimer laser with a wavelength of 308 nm and width of 12 ns fwhm. The laser repetition rate is set at 80 Hz, and the magnetic field is swept a t 5 G/min. Using a boxcar integrator with a 100-ns gate width and a 5-ps delay between the laser pulse and the sampling gate, an absorption spectrum is obtained and is shown in Figure 1B. The line width and its gvalue are the same as in the steady-state experiment, assuring that the carrier is the same in the two different experiments. By changing the delay between the laser pulse and the sampling gate, it is possible to obtain the decay kinetics of the signal. They are displayed in Figure 2 and show complex behavior a t short times and an exponential decay after 20 ps. The time evolution of the EPR signal can be simulated reasonably well by solving the Bloch equations to which a damping term has been added to account for the slower
0022-3654/92/2096-5228%03.00/0 0 1992 American Chemical Society I
,
The Journal of Physical Chemistry, Vol. 96, NO. 13, 1992 5229
Letters
TABLE I: Lifetime T (in ps) of C , Triplet State as a Function of Temperature and Laser Power temperature, K 181 193 203 213 228 243 253 EPR 41 24 opticala 62 56 50 41 20 opticalb 125 111 105 91 80 71 opticalC 125 111 105 95 91 88 a Flash photolysis method with laser power at 20 mJ/pulse. with laser power at 0.7 mJ/pulse.
0
258
273
293
59 80
50 71
40 61
10
Flash photolysis method with laser power at 2.6 mJ/pulse.
Flash photolysis method
t
+
Figure 3. Direct detection EPR signals measured 1 ps after the laser flash for Cm in methylcyclohexane glass at the temperatures indicated. Total sweep width is 400 G. Figure 1. (A) Field-modulated CW EPR signal of Cmin methylcyclohexane irradiated with a xenon lamp. (B) Direct detection signal 5 ps after the laser flash (308 nm) of the same solution used in (A). Sweep width in both experiments is 10 G. Inset: line width as a function of temperature.
-3c
I 2600
i
7 .
1400 1100
1
time (us)
Figure 2. Decay of the direct detection signal as function of time. Solid line is the best fit for the experimental data to the equation y = Ae-&‘+ Be-‘/Tlsin (?HI - 8) where A = 2916,B = 1000,I/& = 41 ps, TI= 8 ps, yHI = 9.0X lo5 radls, and 8 = T .
decay of the carrier.6 The fit is shown in Figure 2 and yields a T I of 8 MS and 41 ps for the slow decay constant. The oscillation frequency and amplitude are power dependent as expected. For a comparison with the optical measurements, the flash photolysis experiments of Foote and *workers were repeated with a different and more favorable wavelength (745 nm) for observation of the triplet-triplet absorption. The time constants obtained by flash photolysis at different temperatures are shown together with the slow decay constants obtained by EPR in Table I. As reported by others,4b it was found that the lifetime is strongly dependent on laser power and on the temperature. This is best explained by triplet-triplet annihilation that becomes less
important at lower triplet concentration and with the slower diffusion rates at lower temperatures. Unfortunately, the minimum laser power required in the EPR experiments for acceptable signal-to-noise ratios is higher than that in flash photolysis. Accepting this fact, there is reasonable agreement between the two types of experiments. To make the connection with the reported EPR spectrum of 3C60, the sample was cooled to the glass transition point of methylcyclohexane and below. As can be seen in Figure 3, new features begin to appear that spread out as the temperature is lowered. Qualitatively, these changes are understandable if axis averaging motions are slowed down to frequencies approaching the width of the powder spectrum of immobilized 3C60 (244 G,” 4 X lo9 rad/s). It should be noted that the spectra are strongly polarized as observed previously.” Only at 8 K, the spectrum becomes a broadened version of the reported spectrum with similar zero-field splitting parameters. With the evidence presented above, there can be little doubt In the that the carrier of the spectra at all temperatures is 3C60. liquid phase the dipolar interactions are averaged by very rapid motion. It appears, but it is not certain, that the signal a short time after laser excitation contains some positive spin polarization. The observation of a single sharp line in the EPR spectrum of a photoexcited triplet state is rare. The few cases where sharp spectra for triplets in liquids have been observed previously are characterized by very small zero-field splittings. The requirements for observing narrow spectra in solution are (1) small zero-field splittings and (2) fast orientational averaging of the magnetic axes. This averaging can be accomplished by fast rotation of the molecule or by internal interchange of the magnetic axes by pseudorotation associated with interconverting degenerate JahnTeller states. The fit of the fast decay pattern of the solution spectrum requires a T I relaxation time of 8 MS. This, together with the experimentally measured zero-field splitting parameters, yields a correlation time T for motional averaging of no longer than lo-” s as calculated by the expression for the dipoldipole
5230 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 100 K
145 K
3 140
160
180
T(K)
Figure 4. CW EPR spectra obtained upon UV irradiation of a methylcyclohexane solution of C70at the indicated temperatures. The sharp line at the center of the spectrum a t 110 K may be originating from freely rotating molecules in cavities of the matrix. Spectral integration indicates of the total intensity. Inset: the line width of the that it represents -3% motionally narrowed spectrum of T7,,as a function of the temperature above 140 K.
induced relaxation rate (eqs 1 and 1/Tl
(2/15)*D27((1
*D = (D2 + 3E2)lI2(rad/s), 1/T2 = (1/15)*D27{3
+
T
U2T2)-l
+ 4(1 + 4WZT2)-') (1)
w = spectrometer frequency (rad/s)
+ 5(1 +
for
Even the observed line
2).798
+ 2(1 + 4 ~ ~ 7 ~ )(2) -')
w%~)-I
