Journal ArticleWe have used a variety of cw optical and optically detected magnetic-resonance techniques to characterize  long-lived photoexeitations in methanofullerene (C6 1) as a dispersion in polystyrene glass, at temperatures  below 10 K. We identify triplet excitons with zero-field-splitting (ZFS) parameters D = 105 X 10-4 cm -1 and E = 9X10-4 cm-1 . The triplet excitons are characterized by two photoinduced absorption (PA) bands in the triplet manifold at 1.5 and 1.7 eV, both with a lifetime of 160 µsec, and a phosphorescence emission (PL) band with onset at 1.85 eV and a lifetime of 10 µsec. The  difference in lifetimes between PA and PL is explained as being due to different recombination kinetics  of the triplet sublevels. This and the finite axial symmetry ZFS parameter E=0 compared to E=0 for  triplets in C 6 0 show the lower symmetry of triplets in C6 1 , consistent with its postulated distortions from  icosahedral symmetry in the ground-state structure

Leng, J. M.

Vardeny, Zeev Valentine

English

The University of Utah: J. Willard Marriott Digital Library

PHYSICAL REVIEW B VOLUME 48, NUMBER 24 15 DECEMBER 1993-11 Magneto-optical studies of photoexcitations in C61 J. M. Leng, X. Wei, and Z. V. Vardeny Department of Physics, University of Utah, Salt Lake City, Utah 84112 K. C. Khemani, D. Moses, and F. Wudl Institute of Polymers and Organic Solids and Department of Physics and Department of Chemistry, University of California, Santa Barbara, California 93106 (Received 24 May 1993) We have used a variety of cw optical and optically detected magnetic-resonance techniques to charac-terize long-lived photo excitations in methanofullerene (C61 ) as a dispersion in polystyrene glass, at tem-peratures beIow 10 K. We identify triplet excitons with zero-fieId-splitting (ZFS) parameters D=IOSXIO-4 cm- 1 and E=9XIO- 4 em-I. The triplet excitons are characterized by two photoin-duced absorption (PA) bands in the triplet manifold at I.S and 1.7 eV, both with a lifetime of 160 f1.sec, and a phosphorescence emission (PL) band with onset at 1.85 eV and a lifetime of 10 f1.sec. The difference in lifetimes between P A and PL is explained as being due to different recombination kinetics of the triplet sublevels. This and the finite axial symmetry ZFS parameter E=I=O compared to E =0 for triplets in C60 show the lower symmetry of triplets in C6b consistent with its postulated distortions from icosahedral symmetry in the ground-state structure. The recent synthesis of macroscopic amounts of the all-carbon fullerenes C60 and C701 has stimulated a great deal of experimental work. Most dramatically, C60 films show superconductivity at Tc as high as 33 K,2,3 when doped with alkali metals. The icosahedral (Ih) symmetry of C60, the prolate spheroid with D 5h symmetry of C70, and their electronic energy bands, have been measured with good agreement between experiment and theory.4,5 Recent optical studies of C60 and C 70 include optical ab-sorption of the ground6- 8 and excited states,9-12 photo-luminescence, \3-17 light-induced ESR,16 and optically detected magnetic resonance of solutions and glasses, 15 where the role of triplet excitons has been emphasized. eV, respectively, and a phosphorescence emlSSlOn band with onset at 1.8S eV, followed by several phonon side-bands down to 1.3 eV. One manifestation of the expected symmetry breaking in 3Ct! is that at 9 K the triplet sub-level lifetimes are 10 and 160 /-Lsec, respectively, for the fastest and slowest sublevels. Another is that the nonaxi-al symmetric zero-field parameter E=I=O. In this work we extend our recent magneto-optical studies of photoexcitations in fullerenes l8 to another member of the family, namely the diphenyl methano ful-lerene (C61 ). The chemical structure of this fullerene is shown in the inset of Fig. 1. 19 Of all the 61 carbon (C) atoms, S8 of them are in nearly SP2 molecular orbitals forming the regular icosahedral structure, and three of them have an SP3 molecular orbital, of which one sticks out of the C60 sphere to bridge the C60 and two other phenyl rings. Success in synthesizing this methanoful-lerene not only adds to the diversity of the materials in this family, but also opens a way to synthesize the C60 po-lymer. 19 Based on the schematic C 61 structure shown in Fig. 1, we expect that a reduction of the icosahedral C60 symmetry occurs in C61 , probably into the symmetry group C 2v ' The consequent effect of such a symmetry reduction on the properties of C61 excited states, com-pared to those of C60, should be interesting. Similar to C60 and C70, 18 we found that the long-lived photoexcitations in C61 molecules are triplet excitons (3C6\). These are characterized by two photoinduced ab-sorption (P A) bands in the triplet manifold at 1.S and 1.7 0163-1829/93/48(24)/18250(4)/$06.00 48 In our work we study photo excitations in C6! as a dispersion in toluene/polystyrene (PS) glass using a variety of cw optical measurement techniques. This in-cludes l8 ,20 photomodulation (PM), photoluminescence (PL), and their respective versions of optically detected magnetic resonance (ODMR): absorption-detected mag-netic resonance (ADMR) (Ref. 20) and the PL-detected ~ h ~ ;::; 4 20 '" .... . ~GO I'l --:. :::l "l 2 10 ..Q C) ... .... ~ ~ -y c.. 010 0 c.. 1.2 14 1.6 1.8 2.0 2.2 0 h ;::; -1 GO "'--:. S; -2 ~ ~ 10 12 14 16 18 20 2.2 C) -.: Photon Energy (eV) FIG. 1. PM, PL, and two P-ADMR spectra of C61 :PS glass at 4 K. The inset shows the proposed C61 molecular structure (Ref. 19). 18250 @) 1993 The American Physical Society 48 MAGNETO-OPTICAL STUDIES OF PHOTOEXCITATIONS IN C6l 18251 magnetic resonance (PDMR). IS The PM and PL spectra have been excited by an Ar+ laser beam at 458 nm (pump) with intensity IL of 200 mW cm -2, which was modulated at frequency f ranging from 20 Hz to 50 kHz by an acousto-optical modulator. The PL from the sam-ple and the transmission of a cw lamp (probe beam) were dispersed by a t-m monochromator and measured by various solid-state detectors in the spectral range of 0.3-2.7 eV. For the ADMR measurements the pump and probe beams constantly illuminate the sample mounted at 3 GHz in a high-Q microwave cavity equipped with optical windows and a superconducting magnet producing a field H up to 3 T. Microwave (,i wave) resonant absorption, modulated at a frequency be-tween 20 Hz and 4 KHz, leads to small changes 8 T in the transmission T proportional to 8n, the change in pho-toexcitation density n produced by the pump beam. 8n is induced by transitions in the f.-L-wave range that change spin-dependent recombination rates. With suitable signal averaging, the system 8T /T sensitivity was about 10-7 . In PDMR we measure changes 8L in the PL intensity L with sensitivity 8L/L = 10- 5 . Two types of ADMR (PDMR) spectra were obtained: the H-ADMR spec-trum, in which 8T (8L) is measured at a fixed probe wavelength A while sweeping H, and the P-ADMR spec-trum in which 8T is measured at a constant H, in reso-nance, while A (probe) is varied. The details of synthesizing C61 has been reported in Ref. 19. In short, a reaction of C60 with more than one equivalent of diphenyldiazomethane in toluene at room temperature for - 1 h produced a monoadduct diphenyl methanofullerene C61 • For our magneto-optical studies the C61 powder was mixed in a solution of degassed toluene/polystyrene (PS), which was subsequently evaporated to form C61 :PS glass. The optical and magneto-optical measurements of C 61 :PS glass at 4 K are shown in Figs. 1 and 2. The PM spectrum (Fig. 1) contains two PA bands at 1.5 and 1.7 eV, respectively. We note that in C60 similar PA bands were observed at 1.65 and 1.8 eV, respectively. 18 The changes in the PA-band energies demonstrate the sensi-tivity of the excited energy levels to small variation of the fullerene structure. Both P A bands increase linearly with I L' showing a monomolecular recombination kinetics for the photoexcitations. The H-ADMR spectrum at 1.7 eV (Fig. 2) shows a 200-G-wide triplet powder pattern at "full field" (FF) (Am s = ± 1) around H 0 = 1071 G with 8n/n=-5XIO- 3, and a narrow (5 G) signal at "half field" (Am s=±2) peaked at 533 G. To correlate the ADMR and PM spectra, we measured the P-ADMR spectra at 1027 G (FF) and 533 G (half field), respective-ly, as shown in Fig. 1. Since the P-ADMR and PM spec-tra in Fig. 1 are identical, we conclude that the two P A bands in the PM spectrum are due to triplet transitions of photogenerated triplet excitons JC61 . The PL spectrum of C61 :PS is also shown in Fig. 1. It contains several maxima at 1.78, 1.68, 1.5, 1.4, and 1.3 eV, which we consider to be phonon sidebands of a weak 0-0 transition at about 1.85 eV, the onset of the PL band. In a recent experimental work where the detailed C60 ab-4 ....... til PDMR .... .... !:I ;:l ..Q 2 .... ro "-....:j G() 0 ....... ADMR til .... .... UeV ~ ;:l ~ 10 ~m=2 ro ~m=l "-h G() 20400 600 800 1000 1200 1400 Magnetic Field (G) FIG. 2. The H-ADMR and H-PDMR spectra of 3Ctl at 4 K. The half-field (llm = ±2) and full-field (llm = ± 1) powder pat-terns are assigned. sorption bands were unraveled, the elusive 1.85-eV ener-gy level was identified21 as due to a 3T2g -1 Ag weak transi-tion, involving triplet-singlet intersystem crossing caused by the spin-orbit interaction. Therefore, we tentatively assign the PL band as due to phosphorescence coming from a triplet level at 1.85 eV. The H-PDMR spectrum shown in Fig. 2 and the PL lifetime determined from the excitation modulation frequency dependence shown in Fig. 3 are in agreement with our model. The H-PDMR spectrum (Fig. 2) of the full PL band (Fig. 2), which shows both Ams = ± 1 and Ams = ±2 powder pattern resonances, is identical to the H-ADMR spectrum of 3C61 at 1.7 eV (also shown in Fig. 2) except the sign; 8L > 0, whereas An < O. The similar magnetic-resonance spectra indicate that the same photo excited states, namely triplets, are involved in the P A and PL bands. To check this assumption we measured the laser excitation modulation frequency dependence of the PL and PA bands as shown in Figs. 3 and 4, respectively. The photoexcitation lifetime r can be determined for a monomolecular recombination kinetics22 by measuring the in-phase and out-of-phase signals as a function of the laser modulation frequency f( =W/21T). In such a case the in-phase signal follows a (1 +W2~)-1 functional dependence, whereas the out-of-phase response gets a maximum at wr= 1, at half the value of the in-phase sig-nal at w--*O. Figures 3 (inset) and 4 show that the PL and P A dynamics at 9 K indeed follow the predicted monomolecular frequency dependence given above. From Figs. 3 and 4 we therefore determine the PL and PA lifetimes to be 10 and 160 f.-Ls, respectively. Moreover, the similar in-phase and out-of-phase PL spectra at 100 Hz shown in Fig. 3 indicate that a single excited electron-ic band is involved in the PL emission. The seemingly apparent difficulty of different PL and PA lifetimes (Figs. 3 and 4) but otherwise identical triplet magnetic-resonance spectra (Fig. 2) can be self-18252 LENG, WEI, VARDENY, KHEMANI, MOSES, AND WUDL 48 20 ~-\; 10 co " -20 2 ::. -c:i ~ ~ -.J ----Q... rn ..... 10 ...... i:1 1 10 2 10 3 104 10 5 ;:j ..Q FrequencylHz) Io-t to 0 ---->-1 P.. OUT OF PHASE -10 Photon FIG. 3. The in-phase and out-of-phase PL spectra of 3Ctl at 9-K and lOO-Hz modulation. The inset shows the in-phase and out-of-phase PL signals vs modulation frequency. consistently explained using the fact that different triplet sublevels are involved in the PL and P A responses. The phosphorescence emission predominantly comes from the triplet sublevel which is the most strongly coupled (by the spin-orbit interaction) to the singlet ground state and therefore has a relatively short lifetime, consistent with h <J , 20 5 2 / / / / / / / / /-9 K in phase 9 K out of phase Frequency FIG. 4. The 3Ctl in-phase and out-of-phase PM signals at 1.7 eV vs modulation frequency at 9 K. the shorter 'T (PL). On the other hand, the P A intensity is proportional to the total triplet population, which in turn is dominated by the slowest triplet sublevel. This sublev-el is only weakly coupled to the ground state, and is therefore dominated by a nonradiative recombination. This is consistent with the longer 'T (P A). Then the iden-tical ADMR and PDMR spectra can readily be explained since J.1,-wave transitions from the slowest to the fastest triplet sublevels decrease the overall triplet population causing 8n < 0, but at the same time increase the triplet radiative recombination channel, resulting in 8L > 0, as in Fig. 2. This model has been further substantiated by detailed solutions of the respective PL and P A rate equa-tions under magnetic-resonance conditions. 22 Our measurements of the 3C61 lifetime are similar to those of 3C60 measured in C60/PS by transient-light-induced ESR. 16 It was found that 'Tx =410 J.1,s and 'Tz =290 J.1,s, and these values compare favorably with 'Tx ='Ty = 160 J.1,S that we measured for 3e61 . The deep valley at H in between the two central peaks in the FF triplet powder pattern shown in Fig. 2 does not agree with the triplet powder pattern of thermalized spins having sublevel populations with a Boltzmann distribu-tion. 15 We note that triplet powder patterns in ODMR spectroscopy at low temperatures are in general not due to thermalized spins. 23 Instead the triplet sublevel popu-lations n i are determined by their respective generation rates Gi and recombination rates Ri to the ground state,23 where ni=GiIRi . As in C60,24 we could fit the FF powder pattern of 3C61 to unequal steady-state popu-lations nx ' ny, and nz of triplet sublevels T x , Ty, and T z , respectively, as follows. The triplet Hamiltonian H in a magnetic field H is given by3 H=g{3S'H+D[S;-tS(S+l)]+E(S;-S;) , (1) where g and {3 are the gyromagnetic constant and Bohr --'" ..... .....>:t ;:l ..ci ... a ~ 0:: ~ Q 0.. 1300 Magnetic Field (G) FIG. 5. The H-PDMR FF powder pattern of 3Ctl (a) and its fit (b); the parameters used in the fit are given in the text. 48 MAGNETO-OPTICAL STUDIES OF PHOTOEXCITATIONS IN C61 18253 magneton, respectively, and D and E are the triplet zero-field-splitting (ZFS) parameters, given by averages in the triplet excited states; D ~ < (r 2 - 3z 2) / r 5) and E~«y2-x2)/r5). The triplet FF powder pattern con-tains, in the general case,23 six singularities at ±D, ±(D +3E)/2, and (D -3E)/2, around H o. From Fig. 2 it seems that the FF powder pattern for 3C6't contains fewer singularities. This happens since 7 x =Ty »7z • We could satisfactorily fit the H-PDMR FF powder pattern of 3C6't as shown in Fig. 5 with the following parameters: D=105XlO- 4 cm-I, E=9XlO- 4 cm- I, spin-spin re-laxation time T2 =22 ns, relative generation rates Gx =0.1 and Gy =Gz =0.4, respectively, and relative recombination rates R x =Ry =O.1 and R z =1.6, respec-tively, corresponding to their respective measured life-times: 7 x =Ty = 160 j-£S, and T z = 10 j-£s. We could previously fit the FF powder pattern of 3C60 at 2 K with an axial symmetric ZFS tensor (E =0) so that it contains four main singularities at ±D and ±D /2, respectively, around Ho, with D = 115 X 10-4 em -1.22,24 Assuming that D is determined by the bipolar interaction between spin-t electron (e) and hole (h) on the fullerene, from our measured D we obtain an e-h distance of 6.3 A for 3C61 compared to 6.1 A for 3C60. 24 This indicates that a larger Jahn-Teller (JT) distortion exists in the C61 excit-ed state,25 in agreement with the reduced symmetry pos-tulated for its ground state. 19 The fact that for 3C61 the IW. Kratschrner, L. D. Lamb, K. Fostiropoulos, and D. R. Huffman, Nature 347, 354 (1990). 2A. F. Hebard, M. J. Rosseinsky, R. C. Haddon, D. W. Mur-phy, S. H. Glaurn, T. T. M. Pastra, A. P. Ramirez, and A. R. Kortan, Nature 350, 600 (1991). 3A.F. Hebard, Phys. Today 45, (11),26 (1992), and references therein. 4H. W. Kroto, A. W. Allaf, and S. P. Balm, Chern. 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Friend, W. J. nonaxial symmetric ZFS parameter E=I=O, whereas E =0 for 3C60,24 shows that 3C61 and its associated JT distor-tions25 has a lower symmetry than 3C60 and its IT distor-tions. 25 - 27 In summary, using a variety of optical and magneto-optical techniques, we have measured the characteristic properties of 3C61 . From the onset of the phosphores-cence emission band we have determined that the 3C61 en-ergy level is at about 1.85 eV from the ground state. The PM spectrum shows two triplet-triplet transitions at 1.5 and 1.7 eV, respectively. We have also found a pro-nounced asymmetry between the 3C61 sublevels, with ra-diative and nonradiative lifetimes of 10 and 160 j-£s, re-spectively, for the respective fastest and slowest sublevels. We also determined the ZFS parameters from the fit to the FF powder pattern; we found D = 105 X 10-4 cm- I and E =9X 10-4 em-I. These ZFS parameters show that 3C61 has a lower symmetry than 3C60, in agreement with its distorted ground state with lower symmetry than I h • The work at the University of Utah was supported in part by BP America, the DOE Grant No. DE-FG 03-93 ER 45490 and by ONR Grant No. NOOOI4-91-C-0104 at the Utah Laser Institute. The work at UCSB was sup-ported by DMR 88-20933 and NSF Grant No. CHE 89-08323 and DMR 91-22536. Romanow, J. P. McCauley, Jr., N. Coastel, J. E. Fishcer, and A. B. Smith III, J. Phys. C 3, 9259 (1991). 15p. A. Lane, L. S. Swanson, Q. X. Ni, J. Shinar, J. P. Engel, T. J. Barton, and L. Jones, Phys. Rev. Lett. 68, 887 (1992). 16M. R. Wasielewski, M. P. O'Neal, K. R. Lykke, M. J. Pellin, and D. M. Gruen, Am. Chern. Soc. 113,2774 (1991). 17S. P. Sibley, S. M. Argentine, and A. H. Francis, Chern. Phys. Lett. 188, 187 (1992). 18X. Wei, S. Jeglinski, O. Paredes, Z. V. Vardeny, D. Moses, V. I. Srdanov, G. D. Stucky, K. C. Khernani, and F. Wudl, Solid State Cornrnun. 85, 455 (1993). 19T. Suzuki, Q. Li, K. C. Khernani, F. Wudl, and O. Alrnarsson, Science 254, 1186 (1991). 20X. Wei, B. Hess, Z. V. Vardeny, and F. Wudl, Phys. Rev. Lett. 68,666 (1992), 21S. Leach, M. Vervloet, A. Despres, E. Breheret, J. P. Hare, T. John Dennis, H. W. Kroto, R. Taylor, and D. R. M. Walton, Chern. Phys. 160,451 (1992). 22X. Wei, Ph.D. thesis, University of Utah (1992). 23H. C. Brenner, in Triplet State ODMR Spectroscopy, edited by R. H. Clarke (Wiley, New York, 1982). 24X. Wei, Z. V. Vardeny, K. C. Khernani, D. Moses, and F. Wudl, Synth. Met. 54,549 (1993). 25N. Koga and K. Morokurna, Chern. Phys. Lett. 196, 191 (1992). 26G. L. Closs, P. Gautarn, D. Zhang, P. J. Krusic, S. A. Hill, and E. Wasserman, J. Phys. Chern. 96, 5228 (1992). 27H. Levanon, V. Meiklyar, Albert Michaeli, Shalom MichaeIi, and A. Regev, J. Phys. Chern. 96, 6128 (1992). 

Magneto-optical studies of photoexcitations in C61

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Magneto-optical studies of photoexcitations in C61

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