W

324 325 326 mT

Fig. 16.16. EPR spectra of (a) KjC^ at room temperature (RT) and at 5 K and (b) Rb3C60 at RT and 4.3 K [16.66].

324 325 326 mT

Fig. 16.16. EPR spectra of (a) KjC^ at room temperature (RT) and at 5 K and (b) Rb3C60 at RT and 4.3 K [16.66].

g-value of 2.0002 appears to be characteristic of all Qo radical anions, we can conclude that the spins on K3C60 are located on the C^ ions [16.17, 71, 84]. The EPR lines have a relatively large intensity and their linewidths do not vary with magnetic field, from which it has been concluded that the EPR linewidth is related to the intrinsic lifetime of the conduction electrons [16.33],

16.2.3. Time-Resolved EPR Study of Triplet State of Fullerenes

Time-resolved EPR techniques have been applied to study the excited triplet state of C60 [16.85-94] and of C70 [16.89,90,93], The experiments are typically done using a laser pulse to excite the triplet state, and

Fig. 16.17. Observed temperature dependence of the EPR linewidth of M^C«, fullerides. For RbjQo only the wide component is found; for K3CH, EPR spectra show both a wide component and a narrow component of 2ABu ~ 0.1 mT whose width does not depend on temperature [16.66].

Fig. 16.17. Observed temperature dependence of the EPR linewidth of M^C«, fullerides. For RbjQo only the wide component is found; for K3CH, EPR spectra show both a wide component and a narrow component of 2ABu ~ 0.1 mT whose width does not depend on temperature [16.66].

time-resolved EPR techniques are used to measure the EPR signal while it decays. Pulse-echo experiments are especially useful for lifetime measurements, and many of the studies have been made as a function of temperature.

Observation of zero-field splittings in the excited triplet state show that the fullerene molecules become distorted in the excited state (due to the Jahn-Teller effect), with static distortions occurring at low T (e.g., 3-10 K) and dynamic distortions at higher T (e.g., 77 K) [16.90], If there were no distortions of the fullerene molecules, there would be no time-resolved EPR signal. These zero-field splittings affect the frequency for transitions with Ams = ±1 between the three levels ms = —1,0,1 of the lowest triplet excited state, giving rise to an angular dependence

w(0, <}>) = ù)0 + -D(3 cos2 d - 1) + -E sin2 0(cos2 </> - sin2 4>) (16.8)

where w0 is the Larmor frequency, (0, <£) are the Euler angles, and (D,E) are fine structure parameters, where D is associated with an axial elongation of the molecule and E with nonaxial distortions. A negative value for D, corresponding to a prolate spheroidal shape for distorted C60 in the excited triplet state, is verified by comparisons of the zero-field splittings for C60 with those for C70, which is known to be a prolate spheroid from x-ray studies (see §3.2) [16.90],

Most of the time-resolved EPR experiments have been done in solution, or in frozen solution going down to very low-temperature. In solution, the time-resolved EPR spectrum shows a g-value of 2.00135 and a very narrow line (only 0.14 G linewidth) [16.89], due to motional narrowing which averages out the contributions of D and E in Eq. (16.8). The identification of the motional narrowing mechanism is supported by the much larger linewidth (6.6 G) of C70 in solution, due to the prolate spheroidal shape of the C70 molecule [16.89]. In studies of the pulsed EPR signal of C60 in solid benzonitrile as a function of temperature, it is found that the intersystem crossing produces about equal populations for the | ± 1) triplet states, but an underpopulation of the |0) triplet state (for D < 0) [16.87]. The microwave resonance serves to change these relative populations, and these population changes are then probed by the EPR measurement. The triplet decay process is strongly temperature dependent and includes contributions from both spin-lattice relaxation (7^), which is relatively more important at higher temperatures, and triplet decay, which is relatively more effective at low-temperatures. The observation of significant spin-lattice relaxation (7i ~ 5 /as) at low-temperatures (8 K) gives evidence for molecular motion down to very low-temperatures [16.87,89]. The experimental time-resolved EPR spectra also indicate the presence of two different triplet states with different D and E values, one corresponding to a delocalized triplet state over the whole fullerene molecule where D — 117 x 10~4 cm-1 and E = 6.9 x 10"4 cm-1, and a second more localized triplet state with smaller D and E values (D = 47 x 10~4 cm'1 and E ~ 0) [16.87]; evidence for more than one triplet state is also provided by the optically detected magnetic resonance (ODMR) technique (see §13.2.4).

16.2.4. EPR Studies of Strong Paramagnets

Electron paramagnetic resonance experiments provide four important quantities— (1) absorption intensity, (2) g-factors, (3) lineshape, and (4) linewidth—which are of use for the study of paramagnetic systems and systems undergoing a transition to a magnetically ordered state. From this standpoint, EPR experiments have been carried out on TDAE-C„c when nc = 60,70,84,90,96 [16.45,95-99],

Shown in Fig. 16.18 are the EPR spectra for TDAE-C60 as a function of temperature. The EPR spectra show a Lorentzian lineshape at room temperature and a narrowing of the linewidth as the temperature is lowered, and this narrowing is attributed to the increased exchange interaction between spins [16.95], particularly as the molecules become orientationally frozen below T0l ~ 170 K [16.99], As TDAE-C60 undergoes a magnetic phase transition, a broadening of the EPR line occurs, and is attributed to

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