J . Phys. Chem. 1992, 96,61286131
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Summary IR spectra of CO and NO adsorbed on Cm films at 77 K were presented. These spectra showed that two different adsorption sites were present on Cm. Relatively large spectral shifts from the gas-phase frquencies were recorded for both CO and NO, indicating strong interaction with the Cm. These results were compared to previous results obtained with the other allotropes
of carbon. The differences in behavior were related to the structure Of c60.
Refereaces
Notes
(1) Kroto, H. W.; Allaf, A. W.; Baln, S . P. Chem. Rev. 1991, 91, 1213-1235. (2) Tsidoni, E.; Kozirovski, Y.;Folman, M.;Heidberg, J. J . Electron Spectrosc. Relat. Phenom. 1987, 44, 89-98.
Paramagnetic States and Dynamics of Photoexcited C6,, Haim Levanon,*.t.t Vladimir Meiklyar: Albert MichaeliJ Shalom Michaeli: and Ayelet Reged Department of Physical Chemistry and The Farkas Center for Light- Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, and Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: April 23, 1992; In Final Form: June 9, 1992)
Photoexcited triplet and photoinduced doublet states of Cm oriented in a nematic liquid crystal (LC) and toluene, were studied by time-domain EPR. The data indicate that in both matrices, 3Cmundergoes temperature dependent molecular rotation. However, whereas in the LC the rotation rate is increased monotonously between 7 and 298 K, the dynamics in toluene depends strongly on its phase diagram. About the melting point of toluene, a polarized doublet is observed concurrently with the disappearance of the triplet. At higher temperatures, where the liquid phase prevails, the dynamics of the photoinduced doublet suggests an unpaired electron delocalized on an aggregate of CMin toluene, with relatively long-lived magnetization of -25 MS.
Introduction The first EPR study of photoexcited triplet states of CWand C70was reported by Wasielewski et al.,l followed by the EPR and ODMR work of Lane et ala2Both investigationswere carried out with toluene solutions or films of Cm cooled to low temperatures with an EPR time resolution of -200 ps. Evidently, such experimental conditions prevent monitoring any fast dynamics due to molecular motion, that may occur at early times after the light excitation. In the present study we report on the triplet and doublet states generated by photoexcitation of Ca, oriented in isotropic or anisotropic matrices. The molecular and spin dynamics were examined by time-resolved EPR detection, with time resolution of -200 ns,3 over an extended temperature range (7-300 K). In both matrices, the timeresolved EPR spectra exhibit remarkable dynamic effects, which depend on the particular host, temperature, and solvent-chromophore interactions. Experimental Results Experiments were carried out with purified Cm donated to us by Dr. D. Gruen, Dr. L. Stock, and Dr. K. Chatterjee of Argonne National Laborat~ry.~ Prior to each measurement, the optical ahsorption spectrum, in the range of 285-700 nm, was compared with spectra of purified Cm reported r e c e n t l ~ .Samples ~ were prepared by dissolving CW(-5 X lo4 M) in either toluene or E-7 liquid crystal (LC), degassed thoroughly under vacuum conditions, and were studied by laser excitation-time-resolved CW-EPR (direct detecti~n).~ It should be reemphasized6 that Cm in toluene or in E.7 is light sensitive and undergm irreversible photochemistry, thus affecting the optical absorption spectra. As a consequence, the triplet zero-field splitting (ZFS)parameters, line shape and spin dynamics may change significantly and be misinterpreted. Thus, all the EPR data described below was checked to be reproducibleover a complete cycle of temperatures. Moreover, in the case of toluene samples, their optical absorption 'The Hebrew University of Jerusalem.
* Argonne National Laboratory.
spectra were verifed to be identical, within the experimental error, before and after the experiment.' Since interpretation of the LC results is simpler than that of toluene, we present first the data obtained using the former matrix. Pbotoexcited Tripkt States in Liquid Crystals, Figure 1 shows typical triplet spectra (Figure la,b) and transient kinetics (Figure IC) at different temperatures. The spectra, at the parallel and perpendicular orientations,8 were taken from the kinetic traces, 300 ns after the laser pulse? It is evident from the spectra shown in Figure 1 that the triplet line shape undergoes dramatic changes with temperature, attributed to molecular motion of Cm in the LC. The spectrum at 7 K is spin polarized, with ZFS parameters of 0.01 14 cm-'and about 0.0007 cm-l for ID1 and lq,respectively, in agreement with those reported Even at such low temperature, line shape simulations indicate that the triplet spectrum is not completely static, implying that molecular motion of Cm is not f r o ~ e n .These ~ observations are in line with recent NMR data of solid Cm, where the chemical shift is averaged out, due to motional narrowing as a function of temperature.'O Unlike many other investigated triplet chromophores oriented in Lc's, the triplet spectra at low temperatures of this buckyball (Figure 1) do not depend upon the mutual orientation between the dinctor, L,and the external magnetic field, B.* We attribute this indifference to the nearly isotropic nature of Cm and/or its motion in the LC matrix. The fact that triplets are EPR detectable over a wide temperature range in the LC nematic phase8" extends the dynamic temperature range quite considerablyas reflected by the gradual narrowing of triplet spectral width upon increasing of temperature (Figure 1). A rough estimate of the activation energy for the molecular rotation is obtained from the reduction of the spectral width, in the high temperature region, to be 3 kcal/mol, in agreement with recent NMR data on solid C60 (1-6 kcal/ mol).'& The dynamics of the spin system as reflected by the temporal behavior of the transient magnetization, M,,(t), corresponds to the triplet line shape variations. The kinetics at 7-75 K, exhibits Torrey oscillations1 about the microwave field (transient nutations). These oscillations occur despite the appreciable rate of
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F i p e 1. (Left) Temperature dependence of direct detection time-resolved triplet EPR spectra of Csoin E-7, in the parallel (a) and perpendicular (b) orientations. All spectra were taken 300 ns after the laser pulse (A = 532 nm, 20-25 mJ/pulse), microwave power 40 mW, and concentration 5 X lo-' M. (Right) Temporal behavior of the triplet magnetization, M,(t) at the corresponding temperatures and laser and microwave powers.
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Figwe 2. (Left) Temperature dependence of direct-detection time-resolved triplet EPR spectra of Csoin toluene. All spectra were taken 300 ns after the laser pulse (A = 532 nm, 20-25 mJ/pulse), microwave power 40 mW, and concentration 5 X lo4 M. (Right) Temporal behavior of the triplet magnetization, M,(t) at the corresponding temperatures and laser and microwave powers. The single rise of M,(t) at 177 K was taken at field position a (cf. spectrum at left). The rise and decay of M,(f) was taken at field position b (cf. spectrum at left). All other kinetic traces were taken at the low-field peak positions of the corresponding triplets.
molecular rotation ( 107-108 s-l) at these low temperatures? suggesting anisotropic motion of 3C60in the LC host.I2 The disappearance of the nutations at temperatures about 75 K, corresponds to the point where rotational motion is sufficiently fast to average out the outermost broad lines of the triplet spectrum. Photoexcited Triplet and Doublet States in Toluene. The spin dynamics of Csoin toluene is more complicated and less understood. However, it is apparent that the dynamics of CMfollows the phase transitions of toluene. Therefore, analysis of the light-induced time-resolved EPR spectra (Figure 2) covers three temperature regions, according to the following toluene phase transitions:
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(a) Glass Phase. The spectrum at 7 K is almost identical with that observed in the LC matrix. Moreover, although toluene is frozen, the spectral changes between 7 and 1 1 7 K are due to molecular dynamics (motion) in the glass phase. Similar to the LC phase, the clear change in line shape suggests that, also in toluene, Cs0 undergoes molecular motion but with a different temperature dependence as compared to the LC. (b) Amorphous Phase. Above the phase transition, at 117 K, (vitrification point, Tg),I3the toluene is in its amorphous phase, or soft glass. Unlike Csoin the LC matrix, the nature of this phase is reflected by the C,-toluene interaction. Both the respective macroscopic and microscopic characterizations, namely, that of the glass matrix and of the CM triplet probe, are changed. Specifically, the toluene glass appears to be amorphous and inhomogeneous, and the original color (clear violet) is gradually
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Figure 3. Direct-detection time-resolved transient EPR spectra of photoexcited Cb0in toluene (5 X lo4 M),at (a) 300 ns and (b) 2700 ns after the laser pulse at 253 K. The temporal changes of My(‘)were taken at different microwave powers as indicated on each trace. Other experimental conditions as in Figure 1. Inset: EPR spectra iecorded with light modulation (550 Hz,X > 400 nm), field modulation (100 KHz), 40 m W microwave power, and T = 253 K. changing into an opaque brown, up to the toluene melting point at 178 K. These steps, which are probably associated with the state of aggregation and/or solvation of c60, are manifested by the triplet EPR spectra and dynamics, which differ from those in the glass phase. At this temperature range, represented by the spectra at 135 and 168 K (Figure 2), we propose that the solvation shell of toluene around CV participates also in the dynamics, thus, hindering the rate of rotahon as compared to the glass phase. This is manifested by the two cycles of triplet spectra that depend upon the temperature, Le., in the glass and amorphous phases. (c) Liquid Phase. Above the melting point of toluene (178 K), the triplet state escapes EPR detection. Nevertheless, a different starts to appear about time and temperature dependence of My(?) this temperature. The fast rotating triplet of Cmis accompanied by the formation of a narrow line at g 2.0 (Figure 2; spectrum a, taken at 177 K). The profiles of the kinetic curves associated with the triplet decay and the concurrent formation of the narrow single-line spectrum (Figure 2) suggest that the precursor of this spectrum is 3C60. Since radical dynamics could be monitored in liquid toluene, photolysis was carried out also at elevated temperatures. Kinetic data at 253 K, for different microwave powers, are presented in Figure 3, with spectra taken 300 ns (a) and 2700 ns (b) after the laser pulse. The assignment of the narrow spectrum to Cm’- (produced electrochemi~allyl~~ or photochemically in the presence of donor molecules’4b)is unlikely, since its spectral characteristics (g = 1.997, strong line width changes with temperature)I4 differ significantly from those found in this study. We, therefore, propose the following reactions to account for the observations in toluene:
A single-line spectrum is also compatible with the symmetry of Cm,in which the two available sites for the unpaired electron are identical. Such an assignment is very similar to that previously suggested by Lane et a1.* for the single-line EPR spectrum in photoexcited films of Cb0. The dependence of MJr)upon B, is typical of combined kinetics, resulting in transient nutations, due to the primary radical’s dynamics (eq 2) and convoluted with a relatively slower secondary dynamics, that stems from either an electron transfer or electron exchange.” It is the secondary process which accounts for the slow decay of My(?), with long spin lattice relaxation time (-25 ps), as shown in upper trace of Figure 3. This model is also consistent with the spectral line width behavior (Figure 3, left), Le., early broad spectrum (a) that evolves in time into a narrow spectrum (b) with g = 2.0016 (3), as determined by light and field modulation EPR method (inset, Figure 3). The broad and narrow spectra should originate from the same species, which tentatively is attributed to the delocalized electron, formed by the process described by eq 2 (trace a in Figure 3). Finally, it should be pointed out that the single-line spectrum was detected neither at low temperature nor at the liquid crystalline matrix. Hence, its appearance in liquid toluene, with the same g-factor as that observed in films? is a corroborating evidence for the unique aggregation state of C60in toluene. This work, in its preliminary stages, demonstrates that the photophysics and photochemistry of these clusters involve several consecutive steps which consist of triplet and doublet states. It is astounding how close the fullerenes are to conventional chromophores. A comprehensive study will be presented in a forthcoming paper.
Reaction 2 describes the formation of a loose ion pair within an aggregate which consist of CWclusters, i.e., [Cm+,e-],whose EPR spectrum is governed by the delocalized electron. This spectrum in absorption mode, may be formed via triplet mechanism (TM), due to a proper combination of the selective population of the triplet spin states, and the sign of the ZFS parameters.Is
Acknowledgment. We are grateful to Dr. D. Gruen, D. L. Stock, and Dr. K. Chatterjee of Argonne National Laboratory, who provided us with the sample of Cm. This work was partially supported by the US.-Israel BSF and by the U S . Department of Energy, Office of Basic Energy Sciences, Division of Chemical The Farkas Sciences, under Contract W-31-109-Eng-38 (H.L.). Research Center is supported by the Bundesministerium filr die Forschung iind Technologie and the Minerva Gesellschaft fiir die Forschung GmbH, FRG. This work is in partial fulfillment of the requirements for a Ph.D. degree (S.M.) at the Hebrew
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J. Phys. Chem. 1992, 96, 6131-6133 University of Jerusalem. The helpful discussions with Dr. J. R. Norris are highly acknowledged. References and Notes (1) Wasielewski, M. R.;ONeil, M. P.; Lykke, K. R.; PeUin, M. J.; Gruen, D. M. J. Am. Chem. SOC.1991,113, 2114. Shinar, J.; Engel, J. P.; Barton, (2) Lane, P. A.; Swanson, L. S.; Ni, 0.-X.; T. J.; Jones, L. Phys. Rev. Lett. 1992,68, 887. (3) For a full description of EPR direct-detection, spectra accumulation, line shape, and spin dynamics determination, see, e.g.: Gonen, 0.; Levanon, H. J. Chem. Phys. 1986,85,4132. (4) Parker. D. H.: Wurz.P.: Chatteriee. K.: Lvkke. K. R.: Hunt. J. E.: Peliin, M. J.; Hemminger, J. C.; Gruen, D.M.; Stock, L. M. J. Am. Chem. SOC.1991, 113, 7499. ( 5 ) (a) Hare, J. R.;Kroto, H. W.; Taylor, R. Chem. Phys. Lett. 1991, 177, 394. (b) Ebbesen. T. W.; Taniaaki, K.; Kuroshima, S. Chem. Phys. Lett. 1991,181, 501. (c) Leach, S.; V&loet, M.; DesprL, A,; Briheret, E.; Hare, P. J.; Dennis, T. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Chem. Phys. 1992, 160, 451. (6) Dimitrijevic, N. M.; Kamat, P. V. J. Phys. Chem. 1992, 96, 4811. (7) Optical absorption spectra were not taken with LC matrices due to low transmittance of E-7in the nematic phase. (8) For triplet EPR detection in liquid crystals see, e.&: (a) Levanon, H. Rev. Chem. Intermed. 1987,8,287. (b) Regev, A,; Levanon, H.; Murai, T.; Sessler, J. L. J. Chem. Phys. 1990, 92, 4718. (c) Regev, A.; Galili, T.;
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Levanon, H. J. Chem. Phys. 1991, 95, 7907. (9) Line shape simulations of triplet EPR spectra of Cm in LC and toluene indicate anisotropic rotational motion: (a) Gamliel, D.; Levanon, H. J. Chem. Phys., submitted for publication. (b) Regev, A.; Meiklyar, V.; Michaeli, S.; Levanon, H. To be published. (10) (a) Yannoni, C. S.; Johnson, R.D.; Meijer, G.;Bethune, D. S.; Salem, J. R. J. Phys. Chem. 1991,95,9. (b) Tycko, R.; Haddon, R.C.; Dabbagh, G.;Glarum, S. H.; Douglas, D. C.; Mujsce, A. M. J. Phys. Chem. 1991,95, 518. (c) Tycko, R.; Dabbagh, G.;Fleming, R. M.; Haddon, R. C.; Makhija, A. V.; Zahurak, S. M. Phys. Rev. Lett. 1991, 67, 1886. (11) Torrey, H. C. Phys. Rev. 1949, 76, 1059. (12) (a) Kim, S. S.; Tsay, F.-D.; Gupta, A. J. Phys. Chem. 1987,91,4851. (b) Fessmann, J.; Rhch, N.; Ohmes, E.; Kothe, G.Chem. Phys. Leu. 1988, 152, 491. (13) Alba, C.; Busse, L. E.; List, D. J.; Angell, C. A. J. Chem. Phys. 1990, 92, 617. (14) (a) Allemand, P.-M.; Srdanov, G.;Koch, A.; Khemani, K.; Wudl, F.; Rubin, Y.; Diederich, F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Am. Chem. SOC.1991, 113, 2780. (b) Krusic, P. J.; Wasserman, E.; Parkinson, B. A.; Malone, B.; Holler, E. R., Jr. J. Am. Chem. Soc. 1991, 113, 6274. (15) Argenhofer, A.; Toporowicz, M.; Bowman, M. K.; Norris, J. R.; Levanon, H. J. Phys. Chem. 1988,92, 7164. (16) (a) Hore, P. J.; McLauchlan, K. A. J. M a p . Res. 1979,36, 129. (b) Hore, P. J.; McLauchlan, K. A. Mol. Phys. 1981,42, 533. (c) McLauchlan, K. A. In Advanced EPR Application in Biology and Biochemistry; Hoff, A. J., Ed.; Elsevier: Amsterdam, 1989; Chapter 10.
Two New Electronic States of CH, Karl K. Irikura,? Russell D. Johnson 111, and Jeffrey W. Hudgens* Chemical Kinetics and Thermodynamics Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology,t Gaithersburg, Maryland 20899 (Received: April 24, 1992; In Final Form: June 8, 1992) The new states fi(3p) and I(4p) of methylene radical have been detected by mass-resolved, one-color 2 + 1 resonance-enhanced multiphoton ionization. Since it is a strong peak,the 3p state offers a convenient and sensitive means for detecting ground-state methylene radicals. In two photons, the 3p state lies at 31 1.80 nm (64 126 cm-I) in CH2, 31 1.84 nm in CHD, and 312.01 nm in CD2, and the 4p state lies at 269.27 nm (74 254 cm-') in CH2 and 269.36 nm in CD2.
Methylene radicals, CHI (g3BI),are often found in energetic chemical environments. For lack of a convenient detection technique, however, their roles in important processes such as combustion and diamond chemical vapor deposition remain undetermined. In a previous communication, we reported resonance-enhanced multiphoton ionization (REMPI) detection of triplet CH2 through previously known states.' Although this one-color 3 1 REMPI scheme is convenient, a 2 1 REMPI mechanism would be expected to provide superior sensitivity. We have found this to be the case and report the results herein. The apparatus and procedures used to record the one-color, mass-resolved REMPI spectra have been described in detail elsewheree2 The apparatus consists of a flow reactor, a timeof-flight mass spectrometer, and a computer data acquisition system. CH2 radicals are produced in the flow reactor by sequential reactions of fluorine atoms with methane under the conditions used previously to produce ground-state triplet CH2 (g3BI).1REMPI spectra of C, CF, CH, CH3, CHF, and CH2F are also otiser~ed,~ indicating that many chemical reactions occur in the F CH, system. CD2 and CHD radicals are generated from CD, and/or CD3H. Radicals are photoionized by the focused (focal length = 150 or 250 nm), linearly polarized, frequencydoubled output (energy = 1-5 mJ/pulse) of a tunable dye laser. Photoions are mass-analyzed by time-of-flight. The laser dyes (Exciton Chemical Co.)' used in this work are Sulforhodamine 640 and DCM (308-315 and 303-334 nm, pumped with Nd:
+
+
+
*To whom correspondence should be addressed. NIST/NRC postdoctoral associate. t Formerly called the National Bureau of Standards.
YAG) and Coumarin 540A (262-289 nm, pumped with XeCl). Spectra are not corrected for the variation in laser power that occurs over the range of each dye. Frequencies are corrected to vacuum and are calibrated to observed lines of atomic carbon.* The positions of the most prominent peaks are collected in Table I. Figure 1 shows the spectra camed by CH2+( m / z 14), CHD+ ( m / z 15), and CD2+( m / z 16) between 306 and 317 nma9 All evidence indicates that these spectra originate from REMPI of the corresponding neutral methylenes. The ion signal disappears in the absence of methane, or if the microwave discharge that generates fluorine atoms is extinguished. The isotopically labeled methanes yield similar spectra carried by ions of the appropriate masses. Finally, comparison of the spectra carried by the methylene ions with those carried by methyl and methine ions indicates that the ion signals in Figure 1 result neither from photodissociation nor from detector saturation. Since the strong band at 3 11-80nm and the much weaker band at 269.27 nm are only slightly shifted by deuterium substitution, they are a_ssigntd to the vibrational origins of the previously unknown H and I states. The adiabatic ionization energy of CH2 is IP, = 83 851 cm-'(10.396 eV).I0 The new peaks are therefore identified as two-photon transitions to 3p and 4p Rydberg states (6 = 0.64 and 0.62). For comparison, the 3p and 4p quantum defects are 6 = 0.62 and 0.61 for CH3I1and 6 = 0.66 and 0.63 for CH2F.5 The spectra of Figure 1 display no vibrational progressions. The theoretical literature on CH2+indi_catesthat no vibrational progressions should be expected. The-X 3B, ground state of CH2has a bond angle of 133.90,12and the X 2Al ground state of the CH2+ ion has a bond angle of 140.8O.I3 Since the bond angles are similar,
This article not subject to U S . Copyright. Published 1992 by the American Chemical Society
