- Email: [email protected]
The triplet state of C70. A zero-field study
4 September 1998
Chemical Physics Letters 293 Ž1998. 528–534
The triplet state of C 70 . A zero-field study M.V. Bronsveld, X.L.R. Dauw, E.J.J. Groenen
)
Centre for the Study of Excited States of Molecules, Huygens Laboratory, UniÕersity of Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands Received 25 June 1998
Abstract We report the optical detection in zero field of the three microwave transitions between the triplet sublevels of C 70 . In n-pentane the bandwidth is 4 MHz Ž30 MHz in decalinrcyclohexane. and the fine-structure parameters, particularly E, have been found to be site-dependent. The intrinsic population and decay characteristics of the sublevels have been determined. A common decay of the Tz l Tx and Tz l Ty transitions has been observed owing to pseudo-rotation within the C 70 molecule at 1.2 K. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction After optical excitation of C 70 virtually all molecules decay back to the ground state through the metastable triplet state. Most molecules decay radiationless and the phosphorescence is weak. For C 70 in toluene w1x and in n-pentane w2x well-resolved phosphorescence spectra have been observed and the resolution in n-pentane allowed a vibronic analysis of the spectrum. These data and quantum-chemical calculations w3x have revealed that the lowest triplet state T0 of C 70 has AX 2 orbital character Žin terms of the D5h symmetry of the ground state.. To further characterise the triplet state T0 of C 70 use has been made of the paramagnetic nature of this state. Electron paramagnetic resonance ŽEPR. studies were performed at 9 GHz where the magnetic field is so large that the Zeeman splitting outweighs the zero-field splitting between the triplet sublevels. )
Corresponding author. E-mail: [email protected]
From such studies values of the fine-structure parameters D and E were derived. Wasielewski et al. reported < D < s 156 MHz and < E < s 21 MHz for C 70 in toluene w4x and subsequently similar values have been found in various matrices w5–7x. The small, non-zero value of E indicates a slight distortion from D5h symmetry in T0 . Time-resolved EPR at 9 GHz has shown that above liquid-helium temperatures spin–lattice relaxation, most probably reflecting pseudo-rotation, governs the decay of the EPR signal w8x. At lower temperatures results are confusing and have led to contradictory conclusions as regards the intrinsic population and decay properties of the substates of T0 . Intersystem crossing leading to predominant population of Tz w6x Ž z is the long molecular axis. or Ty w7x has been reported as well as the virtual absence of selectivity w5x. Sublevel decay rates of 1.7, 1.6 and 1.4 msy1 have been deduced corresponding to an average triplet lifetime of 0.6 ms w5x while phosphorescence-decay experiments at 77 K point to a lifetime of 53 ms w4x.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 1 2 - 4
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
Our recent study of the triplet state of C 70 in decalinrcyclohexane and in toluene by electron– spin–echo spectroscopy at 95 GHz and 1.2 K has removed the discrepancies w9x. The data were consistent with known estimates of D and E and revealed that the D parameter is positive, i.e., the sublevel Tz is the lowest in energy. From the decay of the echo intensity as a function of the time after the laser excitation the relative populating probabilities and the decay rates of the triplet substates were determined. Both populating and decay were found to be substate selective, the Tz state being preferentially populated and decaying fastest. In the analysis of the kinetic data we had to make two assumptions. First, the substates Tx and Ty have equal populating probabilities. This would be a priori true for a C 70 molecule that conserves its D5h symmetry upon excitation into the triplet state, a symmetry that implies a zero value of the fine-structure parameter E. The actual value of E differs only slightly from zero Ž< E < f 0.13 < D <.. Secondly, we assumed a simple relation between the populating probabilities and decay rates of the sublevels in magnetic field and in zero field which only holds true if the decay channels do not interfere w10x. A more straightforward and accurate approach, if possible, would be a study of T0 in zero field. Results of an attempt to optically detect the transitions between the triplet substates of C 70 in toluene in zero field were reported by Saal et al. w11x. These authors were not successful using the microwave-induced delayed phosphorescence ŽMIDP. technique, but observed variations in the phosphorescence decay upon application of a laser pulse immediately followed by a microwave pulse. These variations as a function of microwave frequency were interpreted in terms of three broad zero-field transitions Ž40–100 MHz FWHM. between the triplet substates and absolute values of D and E were extracted. The value of D was in agreement with that derived from EPR studies but the value of E came out twice as large. The signals were weak and precluded a study of the selectivity in the population and decay of the triplet substates. From studies of the phosphorescence as a function of temperature these authors concluded to equal decay rates of the three substates which is at variance with the recent results from electron–spin– echo spectroscopy. Here we report on a zero-field optically detected
529
magnetic-resonance investigation of C 70 in decalinrcyclohexane and n-pentane at 1.2 K. The microwave transitions between the triplet sublevels have been detected as changes in the phosphorescence intensity. The population and decay of the substates have been investigated by MIDP and phosphorescence-decay experiments and are found to be selective. Nevertheless, even at 1.2 K, the Tz l Tx and Tz l Ty transitions show a common decay which is discussed in terms of pseudo-rotation of the molecule around its long Ž z . axis.
2. Experimental Highly purified C 70 Ž) 99.9%. was dissolved in either decalinrcyclohexane Ž3:1 vrv. or n-pentane and samples were cooled down in a bath cryostat to superfluid helium temperatures. For optical excitation an Arq laser ŽSpectra Physics 2017. or a dye laser ŽSpectra Physics 375 B. pumped by the same Arq laser was used. The laser fluence was kept low Ž; 0.5 mWrmm2 . and plasma lines were suppressed by a 545 nm LP filter. For the microwave-induced delayed phosphorescence ŽMIDP. experiments pulsed-laser excitation was achieved using an acousto-optical modulator. After passing a 1 m monochromator ŽSPEX 1704. or two 780 nm HP filters, the phosphorescence emitted by the sample was detected by a liquid-nitrogen cooled germanium detector ŽNorth Coast E0-817L.. The output voltage was measured by a digital oscilloscope ŽLe Croy 9310. or lock-in amplifier ŽOrtec Brookdeal 9503.. Microwaves were generated by a sweep oscillator ŽHewlett-Packard 8690B. in combination with various plug-in units. In the low-frequency range a 40 or 70 MHz LP filter was used to eliminate higher harmonics. For the phosphorescence-detected magnetic resonance ŽPDMR. experiments, microwaves were applied through a closed loop around the sample tube and amplitude modulated at ; 11 Hz. For the MIDP experiments an LC resonant circuit with a tuning range between 110 and 180 MHz was used and the onroff periods of the microwaves were of various lengths.
530
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
3. Results and discussion Transitions between the substates of the triplet state T0 have been detected as microwave-induced changes in the phosphorescence intensity. For C 70 in decalinrcyclohexane the PDMR spectra represented in Fig. 1 were obtained by detection on the total phosphorescence. Identical spectra were observed by detection on specific vibronic bands. The zero level corresponds to the steady-state phosphorescence intensity under continuous illumination. The two microwave transitions between 100 and 220 MHz ŽFig. 1a. show up as a decrease of the phosphorescence intensity and maximum effects of ; 17% were observed. Measurements as a function of microwave power showed that saturation is not yet reached and the linewidth of ; 30 MHz does not result from
Fig. 2. PDMR spectra detected on the total phosphorescence of C 70 in n-pentane at 1.2 K. Excitation at 514 nm Ža. and site-selective excitation at 632.6 nm Žb., and at 631.6 nm Žc.. The zero level corresponds to the steady-state phosphorescence in the absence of microwaves. For comparison the spectrum in decalinrcyclohexane Žcf. Fig. 1. is represented by the dotted line in Ža..
Fig. 1. PDMR spectra detected on the total phosphorescence of C 70 in decalinrcyclohexane at 1.2 K under optical excitation at 514 nm. The zero level corresponds to the steady-state phosphorescence in the absence of microwaves. The spectra in Žb. are obtained with Žlower trace. and without Župper trace. continuous irradiation of microwaves at 130 MHz.
power broadening. A double-resonance technique is found to be required to reveal the third transition expected for a triplet state. A small change in the phosphorescence intensity is observed around 38 MHz when besides modulated low-frequency microwaves continuous microwaves at 130 or 170 MHz are applied ŽFig. 1b.. Similar experiments have been performed for samples of C 70 in n-pentane, quickly cooled to helium temperatures. As Fig. 2a shows, microwaveinduced changes in the phosphorescence intensity are observed in the same frequency ranges as for decalinrcyclohexane. Instead of the broad unstructured bands in the latter glass, the bands in n-pentane seem to be composed of a distinct number of lines with a width of ; 4 MHz. The PDMR spectrum in
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
Fig. 2a was measured with optical excitation at 514 nm. When the excitation is shifted to the red, only the transitions at 129 and 172 MHz remain for excitation at 15807 cmy1 Ž632.6 nm, Fig. 2b., while the transitions at 121 and 181 MHz are most prominent for excitation at 15833 cmy1 Ž631.6 nm, Fig. 2c.. B o th th e b ro a d b a n d s o b ta in e d in decalinrcyclohexane and the narrower bands obtained in n-pentane are inhomogeneously broadened. Comparison with optical experiments provides insight in the observed linewidths. As shown in Fig. 3a, the phosphorescence spectrum of quickly cooled samples of C 70 in n-pentane consists at 1.2 K of multiplets of relatively narrow vibronic bands instead of the broad bands for C 7 0 in decalinrcyclohexane. The individual 3 cmy1 wide lines within each multiplet ŽFig. 3b. derive from C 70
Fig. 3. Phosphorescence spectra of C 70 in n-pentane at 1.2 K for excitation at 514 nm Ža., partly represented on an enlarged scale in Žb., and for site-selective excitation at 632.6 nm Žc.. In the latter spectrum the most intense line is clipped at 2r3 of its real intensity and a cluster of Raman lines is indicated by ‘R’.
531
molecules in distinct sites and upon shifting the excitation to the red edge of the absorption spectrum one of these sites is selected. As Fig. 3c shows, only phosphorescence from molecules in this site is detected. The microwave transitions in Figs. 1 and 2 show similar characteristics. The large width of the zero-field transitions in decalinrcyclohexane derives from many overlapping transitions corresponding to molecules in a continuous distribution of sites in the glassy matrix. A distinct number of sites remains in the polycrystalline n-pentane matrix, which causes the structure in the PDMR spectrum Žcf. Fig. 2a.. Phosphorescence-excitation experiments suggest that the optical transitions in Fig. 3c still result from the overlap of lines corresponding to C 70 molecules in different, albeit similar environments w12x. Consequently, the linewidth of 4 MHz of the magnetic-resonance transitions in n-pentane probably is still determined by inhomogeneous broadening. Site selection occurs upon excitation at 15807 cmy1 and from the many lines in Fig. 2a only two show up in Fig. 2b corresponding to the Ty l Tz transition at 129 MHz and the Tx l Tz transition at 172 MHz. Because for C 70 the Tz sublevel is lowest in energy w9x, the energy of the triplet sublevels of the molecules in this site becomes Xrh s 71.7 MHz, Yrh s 28.7 MHz and Zrh s y100.3 MHz Ži.e., D s 150.5 MHz and E s y21.5 MHz.. Slightly more to the blue, at 15833 cmy1 , excitation is again selective, albeit a little less, and the PDMR spectrum in Fig. 2c is dominated by transitions of molecules in another site. For these molecules the Ty l Tz and Tx l Tz transitions occur at 121 and 181 MHz, respectively, corresponding to sublevel energies of Xrh s 80.3 MHz, Yrh s 20.3 MHz and Zrh s y100.7 MHz Ži.e., D s 151 MHz and E s y30 MHz.. The two sets of zero-field transitions thus refer to C 70 molecules that are characterised by virtually identical values of D while E differs significantly. Two observations suggest that molecules having different values of D exist as well. First, the absence of mirror symmetry in the PDMR spectrum in n-pentane ŽFig. 2a. as regards the parts below and above 150 MHz and, secondly, the fact that the low-frequency transition in decalinrcyclohexane is more or less symmetric around 38 MHz while the maxima of the two high-frequency transitions are separated by 30 MHz. The non-zero value of E
532
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
reflects the loss of axial symmetry of C 70 upon triplet excitation. The variation of D and E reveals changes in the fine-structure tensor which seems to indicate that differently distorted C 70 molecules are involved. To be more precise, the experiments do not distinguish between intra- and intermolecular effects, i.e., the word ‘‘site’’ and the corresponding zero-field tensor refer to a supermolecule consisting of a Ždistorted. C 70 molecule and the surrounding matrix molecules. The precise values of the fine-structure parameters obtained in this study are in line with those extrapolated previously from experiments in magnetic fields w4–7x. The value of E does not agree with that from an earlier zero-field study w11x. Our
Table 1 Time constants Ž t1 , t 2 . and amplitudes Ž A1 , A 2 . of the Žbi.exponential fits to the curves in Fig. 4b and Fig. 5
Values between square brackets have been kept fixed in the fit procedure.
Fig. 4. Ža. A 10 ms time window of the phosphorescence decay of C 70 in decalinrcyclohexane at 1.2 K upon excitation at 514 nm with and without a 1 ms microwave pulse at time td s 210 ms after the laser pulse. Žb. The MIDP signal D I Žcf. a. as a function of td for microwave frequencies of 136 or 166 MHz. Bottom and left-hand axes refer to the experiment at 136 MHz, top and right-hand axes refer to the experiment at 166 MHz.
observation of a low-frequency transition around 38 MHz raises doubts about the results of that study, in particular the interpretation of a feature around 106 MHz in terms of overlapping transitions. In order to unravel the decay characteristics of the sublevels of T0 , MIDP experiments have been performed for C 70 in decalinrcyclohexane. A laser pulse of 400 ms was applied, long enough to build up a steady-state population in T0 . At a delay time td after the laser was switched off, a microwave pulse of 1 ms was applied. As an example, Fig. 4a shows 10 ms time intervals of the phosphorescence decay without microwaves and with a microwave pulse applied at td s 210 ms. Measurements were performed with various delay times between 0 and 400 ms, for microwave frequencies of 136 and 166 MHz corresponding to the maxima of the zero-field transitions in Fig. 1a. The difference D I in the intensity of the phosphorescence at Ž td q 1. ms with and without microwaves is represented in Fig. 4b as a function of td . For both microwave frequencies, a decrease of the phosphorescence is observed for delay times shorter than 63 ms and an increase for longer delay times. A mono-exponential fit to the decays between 150 and 400 ms followed by a bi-exponential fit to the whole
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
curves while keeping the long-time constant fixed, results for both the Tz l Tx and the Tz l Ty transition in the time constants of 82 and 26 ms Žcf. Table 1.. Comparison of the two decay constants, 82 and 26 ms, with the results of electron–spin–echo experiments in magnetic field show that the shorter time reflects the faster decay of the Tz sublevel w9x. Because of the small non-zero value of the finestructure parameter E one would expect a Žslight. difference between the decay rates of Tx and Ty . The observed equivalence suggests that the slower decay corresponds to the average of k x and k y . This interpretation is consistent with pseudo-rotation of the C 70 molecule around its z-axis, i.e., the rotation of the distortion Žand so the magnetic x- and y-axis. within the C 70 molecule. From the analysis of the spin–lattice relaxation previously seen in 95 GHz electron–spin–echo experiments, the characteristic tim e of pseudo-rotation for C 7 0 in a decalinrcyclohexane matrix at 1.2 K was found to be of the order of a microsecond w9x. Being fast compared with the decay, the pseudo-rotation around z averages x and y, and Tx and Ty seem to decay with the same time constant. While fast compared with the decay of the triplet, pseudo-rotation is still in the slow-motion regime and the Tz l Tx and Tz l Ty transitions show up as two resolved lines in the PDMR experiments whose width is not determined by pseudo-rotation but by inhomogeneity of the molecular environments. The relative populating probabilities pu Ž u s x, y, z . of the triplet sublevels have been determined from the phosphorescence decay following laser pulses of various lengths. Fig. 5a shows the decay after a laser pulse long enough to reach steady state. The decay may be described by the sum of two exponential functions but to fit their contributions additional information is needed. The pre-exponential factors given in Table 1, which are in the ratio of 3.7 to 1, have been obtained by setting the long-time constant at 82 ms, the value derived from the MIDP experiments. For shorter laser pulses, varying between 4 and 32 ms, the ingrowth and decay of the phosphorescence are represented in Fig. 5b. The decays have been simultaneously fit to bi-exponential functions, while the two time constants were required to be the same for all pulse lengths. The
533
Fig. 5. Phosphorescence decay of C 70 in decalinrcyclohexane at 1.2 K after a 514 nm laser pulse of 750 ms Ža. and after laser pulses of 4, 8, 16 and 32 ms Žb.. The phosphorescence intensity at t s 0 corresponds to steady state for Ža. and zero for Žb..
long-time constant was kept fixed at 82 ms. Pre-exponential factors on average in the ratio of 9.2 to 1 are obtained Žcf. Table 1.. The decay of the phosphorescence after irradiation of the Tz l Tx transition does not differ from that after irradiation of the Tz l Ty transition, from which we conclude that besides the decay rates of Tx and Ty also their populating probabilities cannot be distinguished. Under the assumption that the ratio of the radiative and the total decay rate is independent of the sublevel Ž k ur rk u is not a function of u., the pre-exponential factors of the components in the phosphorescence decay following a long laser pulse are proportional to pu . From the long-pulse data in Table 1 we find p x : p y : pz s 1:1:7.4. On the other hand, the pre-exponential factors of the components in the decay following a short laser pulse are proportional to k ur pu . With k u instead of k ur , in line with the above assumption, the observed decays after the short pulses lead to p x : p y : pz s 1:1:5.8. The error in the latter ratio is larger than in that derived from the
534
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
decay after steady-state because the amplitude of the long-time component in the short-pulse experiment is smaller than in the long-pulse experiment. We conclude to a preferential intersystem crossing into Tz compared to Tx ŽTy . by a factor of 7. Both the decay rates and the relative populating probabilities obtained in this zero-field study are compatible with those extrapolated from the highfield electron–spin–echo experiments previously w9x. The present values are more accurate, in particular that of k x Ž k y .. The values of pu and k u make up for a population distribution under steady-state illumination of Nx : Ny : Nz s 1:1:2.2. Irradiation of microwaves of the proper frequency leads to the transfer of population from Tz to either Tx or Ty , which means that the phosphorescence intensity will decrease as seen in the PDMR experiment. In passing we note that we have performed microwave-induced phosphorescence and saturation-recovery experiments as well. The observed transients do not conform to standard theory, most probably because of ground-state depletion at the laser powers we had to use.
4. Conclusions Optical detection has allowed the observation in zero field of the transitions between the triplet sublevels of C 70 . For C 70 in decalinrcyclohexane a bandwidth of 30 MHz has been observed, for C 70 in n-pentane of 4 MHz. Site-selective optical excitation in n-pentane enabled the determination of the finestructure parameters of triplet C 70 in two distinct sites. Values of 150.5 and 151 MHz for D and y21.5 and y30 MHz for E have been found. A population and decay analysis of the triplet state has been performed by means of MIDP and phosphorescence-decay experiments, which shows that the Tz is preferentially populated by approximately a factor of 7 and decays about 3 times faster than Tx and Ty
Ž26 vs. 82 ms.. At 1.2 K pseudo-rotation is in the slow-motion regime and the width of the zero-field transitions is determined by inhomogeneous broadening. On the other hand, pseudo-rotation is fast compared to the lifetimes of the substates and the Tz l Tx and Tz l Ty transitions show identical decay. Acknowledgements We gratefully acknowledge G. Meijer ŽNijmegen. with whom we have a standing co-operation in the research on fullerenes. This work forms part of the research program of the ‘‘Stichting voor Fundamenteel Onderzoek der Materie’’ ŽFOM. and has been made possible by the financial support from the ‘‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek’’ ŽNWO.. References w1x B.S. Razbirin, A.N. Starukhin, A.V. Chugreev, Y.S. Grushko, S.N. Kolesnik, JETP Lett. 60 Ž1994. 451. w2x J.B.M. Warntjes, I. Holleman, G. Meijer, E.J.J. Groenen, Chem. Phys. Lett. 261 Ž1996. 495. w3x F. Negri, G. Orlandi, J. Phys. B. 29 Ž1996. 5077. w4x M.R. Wasielewski, M.P. O’Neil, K.R. Lykke, M.J. Pellin, D.M. Gruen, J. Am. Chem. Soc. 113 Ž1991. 2774. w5x M. Terazima, N. Hirota, H. Shinohara, Y. Saito, Chem. Phys. Lett. 195 Ž1992. 333. w6x G. Agostini, C. Corvaja, L. Pasimeni, Chem. Phys. 202 Ž1996. 349. w7x H. Levanon, V. Meiklyar, S. Michaeli, D. Gamliel, J. Am. Chem. Soc. 115 Ž1993. 8722. w8x G.L. Closs, P. Gautam, D. Zhang, P.J. Krusic, S.A. Hill, E. Wasserman, J. Phys. Chem. 96 Ž1992. 5228. w9x X.L.R. Dauw, O.G. Poluektov, J.B.M. Warntjes, M.V. Bronsveld, E.J.J. Groenen, J. Phys. Chem. A 102 Ž1998. 3078. w10x R.A. Schadee, J. Schmidt, J.H. van der Waals, Chem. Phys. Lett. 41 Ž1976. 435. w11x C. Saal, N. Weiden, K.-P. Dinse, Appl. Magn. Reson. 11 Ž1996. 335. w12x X.L.R. Dauw, M.V. Bronsveld, A. Kruger, J.B.M. Warntjes, ¨ M.R. Witjes, E.J.J. Groenen, J. Chem. Phys. Ž1998. submitted.
Chemical Physics Letters 293 Ž1998. 528–534
The triplet state of C 70 . A zero-field study M.V. Bronsveld, X.L.R. Dauw, E.J.J. Groenen
)
Centre for the Study of Excited States of Molecules, Huygens Laboratory, UniÕersity of Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands Received 25 June 1998
Abstract We report the optical detection in zero field of the three microwave transitions between the triplet sublevels of C 70 . In n-pentane the bandwidth is 4 MHz Ž30 MHz in decalinrcyclohexane. and the fine-structure parameters, particularly E, have been found to be site-dependent. The intrinsic population and decay characteristics of the sublevels have been determined. A common decay of the Tz l Tx and Tz l Ty transitions has been observed owing to pseudo-rotation within the C 70 molecule at 1.2 K. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction After optical excitation of C 70 virtually all molecules decay back to the ground state through the metastable triplet state. Most molecules decay radiationless and the phosphorescence is weak. For C 70 in toluene w1x and in n-pentane w2x well-resolved phosphorescence spectra have been observed and the resolution in n-pentane allowed a vibronic analysis of the spectrum. These data and quantum-chemical calculations w3x have revealed that the lowest triplet state T0 of C 70 has AX 2 orbital character Žin terms of the D5h symmetry of the ground state.. To further characterise the triplet state T0 of C 70 use has been made of the paramagnetic nature of this state. Electron paramagnetic resonance ŽEPR. studies were performed at 9 GHz where the magnetic field is so large that the Zeeman splitting outweighs the zero-field splitting between the triplet sublevels. )
Corresponding author. E-mail: [email protected]
From such studies values of the fine-structure parameters D and E were derived. Wasielewski et al. reported < D < s 156 MHz and < E < s 21 MHz for C 70 in toluene w4x and subsequently similar values have been found in various matrices w5–7x. The small, non-zero value of E indicates a slight distortion from D5h symmetry in T0 . Time-resolved EPR at 9 GHz has shown that above liquid-helium temperatures spin–lattice relaxation, most probably reflecting pseudo-rotation, governs the decay of the EPR signal w8x. At lower temperatures results are confusing and have led to contradictory conclusions as regards the intrinsic population and decay properties of the substates of T0 . Intersystem crossing leading to predominant population of Tz w6x Ž z is the long molecular axis. or Ty w7x has been reported as well as the virtual absence of selectivity w5x. Sublevel decay rates of 1.7, 1.6 and 1.4 msy1 have been deduced corresponding to an average triplet lifetime of 0.6 ms w5x while phosphorescence-decay experiments at 77 K point to a lifetime of 53 ms w4x.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 1 2 - 4
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
Our recent study of the triplet state of C 70 in decalinrcyclohexane and in toluene by electron– spin–echo spectroscopy at 95 GHz and 1.2 K has removed the discrepancies w9x. The data were consistent with known estimates of D and E and revealed that the D parameter is positive, i.e., the sublevel Tz is the lowest in energy. From the decay of the echo intensity as a function of the time after the laser excitation the relative populating probabilities and the decay rates of the triplet substates were determined. Both populating and decay were found to be substate selective, the Tz state being preferentially populated and decaying fastest. In the analysis of the kinetic data we had to make two assumptions. First, the substates Tx and Ty have equal populating probabilities. This would be a priori true for a C 70 molecule that conserves its D5h symmetry upon excitation into the triplet state, a symmetry that implies a zero value of the fine-structure parameter E. The actual value of E differs only slightly from zero Ž< E < f 0.13 < D <.. Secondly, we assumed a simple relation between the populating probabilities and decay rates of the sublevels in magnetic field and in zero field which only holds true if the decay channels do not interfere w10x. A more straightforward and accurate approach, if possible, would be a study of T0 in zero field. Results of an attempt to optically detect the transitions between the triplet substates of C 70 in toluene in zero field were reported by Saal et al. w11x. These authors were not successful using the microwave-induced delayed phosphorescence ŽMIDP. technique, but observed variations in the phosphorescence decay upon application of a laser pulse immediately followed by a microwave pulse. These variations as a function of microwave frequency were interpreted in terms of three broad zero-field transitions Ž40–100 MHz FWHM. between the triplet substates and absolute values of D and E were extracted. The value of D was in agreement with that derived from EPR studies but the value of E came out twice as large. The signals were weak and precluded a study of the selectivity in the population and decay of the triplet substates. From studies of the phosphorescence as a function of temperature these authors concluded to equal decay rates of the three substates which is at variance with the recent results from electron–spin– echo spectroscopy. Here we report on a zero-field optically detected
529
magnetic-resonance investigation of C 70 in decalinrcyclohexane and n-pentane at 1.2 K. The microwave transitions between the triplet sublevels have been detected as changes in the phosphorescence intensity. The population and decay of the substates have been investigated by MIDP and phosphorescence-decay experiments and are found to be selective. Nevertheless, even at 1.2 K, the Tz l Tx and Tz l Ty transitions show a common decay which is discussed in terms of pseudo-rotation of the molecule around its long Ž z . axis.
2. Experimental Highly purified C 70 Ž) 99.9%. was dissolved in either decalinrcyclohexane Ž3:1 vrv. or n-pentane and samples were cooled down in a bath cryostat to superfluid helium temperatures. For optical excitation an Arq laser ŽSpectra Physics 2017. or a dye laser ŽSpectra Physics 375 B. pumped by the same Arq laser was used. The laser fluence was kept low Ž; 0.5 mWrmm2 . and plasma lines were suppressed by a 545 nm LP filter. For the microwave-induced delayed phosphorescence ŽMIDP. experiments pulsed-laser excitation was achieved using an acousto-optical modulator. After passing a 1 m monochromator ŽSPEX 1704. or two 780 nm HP filters, the phosphorescence emitted by the sample was detected by a liquid-nitrogen cooled germanium detector ŽNorth Coast E0-817L.. The output voltage was measured by a digital oscilloscope ŽLe Croy 9310. or lock-in amplifier ŽOrtec Brookdeal 9503.. Microwaves were generated by a sweep oscillator ŽHewlett-Packard 8690B. in combination with various plug-in units. In the low-frequency range a 40 or 70 MHz LP filter was used to eliminate higher harmonics. For the phosphorescence-detected magnetic resonance ŽPDMR. experiments, microwaves were applied through a closed loop around the sample tube and amplitude modulated at ; 11 Hz. For the MIDP experiments an LC resonant circuit with a tuning range between 110 and 180 MHz was used and the onroff periods of the microwaves were of various lengths.
530
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
3. Results and discussion Transitions between the substates of the triplet state T0 have been detected as microwave-induced changes in the phosphorescence intensity. For C 70 in decalinrcyclohexane the PDMR spectra represented in Fig. 1 were obtained by detection on the total phosphorescence. Identical spectra were observed by detection on specific vibronic bands. The zero level corresponds to the steady-state phosphorescence intensity under continuous illumination. The two microwave transitions between 100 and 220 MHz ŽFig. 1a. show up as a decrease of the phosphorescence intensity and maximum effects of ; 17% were observed. Measurements as a function of microwave power showed that saturation is not yet reached and the linewidth of ; 30 MHz does not result from
Fig. 2. PDMR spectra detected on the total phosphorescence of C 70 in n-pentane at 1.2 K. Excitation at 514 nm Ža. and site-selective excitation at 632.6 nm Žb., and at 631.6 nm Žc.. The zero level corresponds to the steady-state phosphorescence in the absence of microwaves. For comparison the spectrum in decalinrcyclohexane Žcf. Fig. 1. is represented by the dotted line in Ža..
Fig. 1. PDMR spectra detected on the total phosphorescence of C 70 in decalinrcyclohexane at 1.2 K under optical excitation at 514 nm. The zero level corresponds to the steady-state phosphorescence in the absence of microwaves. The spectra in Žb. are obtained with Žlower trace. and without Župper trace. continuous irradiation of microwaves at 130 MHz.
power broadening. A double-resonance technique is found to be required to reveal the third transition expected for a triplet state. A small change in the phosphorescence intensity is observed around 38 MHz when besides modulated low-frequency microwaves continuous microwaves at 130 or 170 MHz are applied ŽFig. 1b.. Similar experiments have been performed for samples of C 70 in n-pentane, quickly cooled to helium temperatures. As Fig. 2a shows, microwaveinduced changes in the phosphorescence intensity are observed in the same frequency ranges as for decalinrcyclohexane. Instead of the broad unstructured bands in the latter glass, the bands in n-pentane seem to be composed of a distinct number of lines with a width of ; 4 MHz. The PDMR spectrum in
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
Fig. 2a was measured with optical excitation at 514 nm. When the excitation is shifted to the red, only the transitions at 129 and 172 MHz remain for excitation at 15807 cmy1 Ž632.6 nm, Fig. 2b., while the transitions at 121 and 181 MHz are most prominent for excitation at 15833 cmy1 Ž631.6 nm, Fig. 2c.. B o th th e b ro a d b a n d s o b ta in e d in decalinrcyclohexane and the narrower bands obtained in n-pentane are inhomogeneously broadened. Comparison with optical experiments provides insight in the observed linewidths. As shown in Fig. 3a, the phosphorescence spectrum of quickly cooled samples of C 70 in n-pentane consists at 1.2 K of multiplets of relatively narrow vibronic bands instead of the broad bands for C 7 0 in decalinrcyclohexane. The individual 3 cmy1 wide lines within each multiplet ŽFig. 3b. derive from C 70
Fig. 3. Phosphorescence spectra of C 70 in n-pentane at 1.2 K for excitation at 514 nm Ža., partly represented on an enlarged scale in Žb., and for site-selective excitation at 632.6 nm Žc.. In the latter spectrum the most intense line is clipped at 2r3 of its real intensity and a cluster of Raman lines is indicated by ‘R’.
531
molecules in distinct sites and upon shifting the excitation to the red edge of the absorption spectrum one of these sites is selected. As Fig. 3c shows, only phosphorescence from molecules in this site is detected. The microwave transitions in Figs. 1 and 2 show similar characteristics. The large width of the zero-field transitions in decalinrcyclohexane derives from many overlapping transitions corresponding to molecules in a continuous distribution of sites in the glassy matrix. A distinct number of sites remains in the polycrystalline n-pentane matrix, which causes the structure in the PDMR spectrum Žcf. Fig. 2a.. Phosphorescence-excitation experiments suggest that the optical transitions in Fig. 3c still result from the overlap of lines corresponding to C 70 molecules in different, albeit similar environments w12x. Consequently, the linewidth of 4 MHz of the magnetic-resonance transitions in n-pentane probably is still determined by inhomogeneous broadening. Site selection occurs upon excitation at 15807 cmy1 and from the many lines in Fig. 2a only two show up in Fig. 2b corresponding to the Ty l Tz transition at 129 MHz and the Tx l Tz transition at 172 MHz. Because for C 70 the Tz sublevel is lowest in energy w9x, the energy of the triplet sublevels of the molecules in this site becomes Xrh s 71.7 MHz, Yrh s 28.7 MHz and Zrh s y100.3 MHz Ži.e., D s 150.5 MHz and E s y21.5 MHz.. Slightly more to the blue, at 15833 cmy1 , excitation is again selective, albeit a little less, and the PDMR spectrum in Fig. 2c is dominated by transitions of molecules in another site. For these molecules the Ty l Tz and Tx l Tz transitions occur at 121 and 181 MHz, respectively, corresponding to sublevel energies of Xrh s 80.3 MHz, Yrh s 20.3 MHz and Zrh s y100.7 MHz Ži.e., D s 151 MHz and E s y30 MHz.. The two sets of zero-field transitions thus refer to C 70 molecules that are characterised by virtually identical values of D while E differs significantly. Two observations suggest that molecules having different values of D exist as well. First, the absence of mirror symmetry in the PDMR spectrum in n-pentane ŽFig. 2a. as regards the parts below and above 150 MHz and, secondly, the fact that the low-frequency transition in decalinrcyclohexane is more or less symmetric around 38 MHz while the maxima of the two high-frequency transitions are separated by 30 MHz. The non-zero value of E
532
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
reflects the loss of axial symmetry of C 70 upon triplet excitation. The variation of D and E reveals changes in the fine-structure tensor which seems to indicate that differently distorted C 70 molecules are involved. To be more precise, the experiments do not distinguish between intra- and intermolecular effects, i.e., the word ‘‘site’’ and the corresponding zero-field tensor refer to a supermolecule consisting of a Ždistorted. C 70 molecule and the surrounding matrix molecules. The precise values of the fine-structure parameters obtained in this study are in line with those extrapolated previously from experiments in magnetic fields w4–7x. The value of E does not agree with that from an earlier zero-field study w11x. Our
Table 1 Time constants Ž t1 , t 2 . and amplitudes Ž A1 , A 2 . of the Žbi.exponential fits to the curves in Fig. 4b and Fig. 5
Values between square brackets have been kept fixed in the fit procedure.
Fig. 4. Ža. A 10 ms time window of the phosphorescence decay of C 70 in decalinrcyclohexane at 1.2 K upon excitation at 514 nm with and without a 1 ms microwave pulse at time td s 210 ms after the laser pulse. Žb. The MIDP signal D I Žcf. a. as a function of td for microwave frequencies of 136 or 166 MHz. Bottom and left-hand axes refer to the experiment at 136 MHz, top and right-hand axes refer to the experiment at 166 MHz.
observation of a low-frequency transition around 38 MHz raises doubts about the results of that study, in particular the interpretation of a feature around 106 MHz in terms of overlapping transitions. In order to unravel the decay characteristics of the sublevels of T0 , MIDP experiments have been performed for C 70 in decalinrcyclohexane. A laser pulse of 400 ms was applied, long enough to build up a steady-state population in T0 . At a delay time td after the laser was switched off, a microwave pulse of 1 ms was applied. As an example, Fig. 4a shows 10 ms time intervals of the phosphorescence decay without microwaves and with a microwave pulse applied at td s 210 ms. Measurements were performed with various delay times between 0 and 400 ms, for microwave frequencies of 136 and 166 MHz corresponding to the maxima of the zero-field transitions in Fig. 1a. The difference D I in the intensity of the phosphorescence at Ž td q 1. ms with and without microwaves is represented in Fig. 4b as a function of td . For both microwave frequencies, a decrease of the phosphorescence is observed for delay times shorter than 63 ms and an increase for longer delay times. A mono-exponential fit to the decays between 150 and 400 ms followed by a bi-exponential fit to the whole
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
curves while keeping the long-time constant fixed, results for both the Tz l Tx and the Tz l Ty transition in the time constants of 82 and 26 ms Žcf. Table 1.. Comparison of the two decay constants, 82 and 26 ms, with the results of electron–spin–echo experiments in magnetic field show that the shorter time reflects the faster decay of the Tz sublevel w9x. Because of the small non-zero value of the finestructure parameter E one would expect a Žslight. difference between the decay rates of Tx and Ty . The observed equivalence suggests that the slower decay corresponds to the average of k x and k y . This interpretation is consistent with pseudo-rotation of the C 70 molecule around its z-axis, i.e., the rotation of the distortion Žand so the magnetic x- and y-axis. within the C 70 molecule. From the analysis of the spin–lattice relaxation previously seen in 95 GHz electron–spin–echo experiments, the characteristic tim e of pseudo-rotation for C 7 0 in a decalinrcyclohexane matrix at 1.2 K was found to be of the order of a microsecond w9x. Being fast compared with the decay, the pseudo-rotation around z averages x and y, and Tx and Ty seem to decay with the same time constant. While fast compared with the decay of the triplet, pseudo-rotation is still in the slow-motion regime and the Tz l Tx and Tz l Ty transitions show up as two resolved lines in the PDMR experiments whose width is not determined by pseudo-rotation but by inhomogeneity of the molecular environments. The relative populating probabilities pu Ž u s x, y, z . of the triplet sublevels have been determined from the phosphorescence decay following laser pulses of various lengths. Fig. 5a shows the decay after a laser pulse long enough to reach steady state. The decay may be described by the sum of two exponential functions but to fit their contributions additional information is needed. The pre-exponential factors given in Table 1, which are in the ratio of 3.7 to 1, have been obtained by setting the long-time constant at 82 ms, the value derived from the MIDP experiments. For shorter laser pulses, varying between 4 and 32 ms, the ingrowth and decay of the phosphorescence are represented in Fig. 5b. The decays have been simultaneously fit to bi-exponential functions, while the two time constants were required to be the same for all pulse lengths. The
533
Fig. 5. Phosphorescence decay of C 70 in decalinrcyclohexane at 1.2 K after a 514 nm laser pulse of 750 ms Ža. and after laser pulses of 4, 8, 16 and 32 ms Žb.. The phosphorescence intensity at t s 0 corresponds to steady state for Ža. and zero for Žb..
long-time constant was kept fixed at 82 ms. Pre-exponential factors on average in the ratio of 9.2 to 1 are obtained Žcf. Table 1.. The decay of the phosphorescence after irradiation of the Tz l Tx transition does not differ from that after irradiation of the Tz l Ty transition, from which we conclude that besides the decay rates of Tx and Ty also their populating probabilities cannot be distinguished. Under the assumption that the ratio of the radiative and the total decay rate is independent of the sublevel Ž k ur rk u is not a function of u., the pre-exponential factors of the components in the phosphorescence decay following a long laser pulse are proportional to pu . From the long-pulse data in Table 1 we find p x : p y : pz s 1:1:7.4. On the other hand, the pre-exponential factors of the components in the decay following a short laser pulse are proportional to k ur pu . With k u instead of k ur , in line with the above assumption, the observed decays after the short pulses lead to p x : p y : pz s 1:1:5.8. The error in the latter ratio is larger than in that derived from the
534
M.V. BronsÕeld et al.r Chemical Physics Letters 293 (1998) 528–534
decay after steady-state because the amplitude of the long-time component in the short-pulse experiment is smaller than in the long-pulse experiment. We conclude to a preferential intersystem crossing into Tz compared to Tx ŽTy . by a factor of 7. Both the decay rates and the relative populating probabilities obtained in this zero-field study are compatible with those extrapolated from the highfield electron–spin–echo experiments previously w9x. The present values are more accurate, in particular that of k x Ž k y .. The values of pu and k u make up for a population distribution under steady-state illumination of Nx : Ny : Nz s 1:1:2.2. Irradiation of microwaves of the proper frequency leads to the transfer of population from Tz to either Tx or Ty , which means that the phosphorescence intensity will decrease as seen in the PDMR experiment. In passing we note that we have performed microwave-induced phosphorescence and saturation-recovery experiments as well. The observed transients do not conform to standard theory, most probably because of ground-state depletion at the laser powers we had to use.
4. Conclusions Optical detection has allowed the observation in zero field of the transitions between the triplet sublevels of C 70 . For C 70 in decalinrcyclohexane a bandwidth of 30 MHz has been observed, for C 70 in n-pentane of 4 MHz. Site-selective optical excitation in n-pentane enabled the determination of the finestructure parameters of triplet C 70 in two distinct sites. Values of 150.5 and 151 MHz for D and y21.5 and y30 MHz for E have been found. A population and decay analysis of the triplet state has been performed by means of MIDP and phosphorescence-decay experiments, which shows that the Tz is preferentially populated by approximately a factor of 7 and decays about 3 times faster than Tx and Ty
Ž26 vs. 82 ms.. At 1.2 K pseudo-rotation is in the slow-motion regime and the width of the zero-field transitions is determined by inhomogeneous broadening. On the other hand, pseudo-rotation is fast compared to the lifetimes of the substates and the Tz l Tx and Tz l Ty transitions show identical decay. Acknowledgements We gratefully acknowledge G. Meijer ŽNijmegen. with whom we have a standing co-operation in the research on fullerenes. This work forms part of the research program of the ‘‘Stichting voor Fundamenteel Onderzoek der Materie’’ ŽFOM. and has been made possible by the financial support from the ‘‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek’’ ŽNWO.. References w1x B.S. Razbirin, A.N. Starukhin, A.V. Chugreev, Y.S. Grushko, S.N. Kolesnik, JETP Lett. 60 Ž1994. 451. w2x J.B.M. Warntjes, I. Holleman, G. Meijer, E.J.J. Groenen, Chem. Phys. Lett. 261 Ž1996. 495. w3x F. Negri, G. Orlandi, J. Phys. B. 29 Ž1996. 5077. w4x M.R. Wasielewski, M.P. O’Neil, K.R. Lykke, M.J. Pellin, D.M. Gruen, J. Am. Chem. Soc. 113 Ž1991. 2774. w5x M. Terazima, N. Hirota, H. Shinohara, Y. Saito, Chem. Phys. Lett. 195 Ž1992. 333. w6x G. Agostini, C. Corvaja, L. Pasimeni, Chem. Phys. 202 Ž1996. 349. w7x H. Levanon, V. Meiklyar, S. Michaeli, D. Gamliel, J. Am. Chem. Soc. 115 Ž1993. 8722. w8x G.L. Closs, P. Gautam, D. Zhang, P.J. Krusic, S.A. Hill, E. Wasserman, J. Phys. Chem. 96 Ž1992. 5228. w9x X.L.R. Dauw, O.G. Poluektov, J.B.M. Warntjes, M.V. Bronsveld, E.J.J. Groenen, J. Phys. Chem. A 102 Ž1998. 3078. w10x R.A. Schadee, J. Schmidt, J.H. van der Waals, Chem. Phys. Lett. 41 Ž1976. 435. w11x C. Saal, N. Weiden, K.-P. Dinse, Appl. Magn. Reson. 11 Ž1996. 335. w12x X.L.R. Dauw, M.V. Bronsveld, A. Kruger, J.B.M. Warntjes, ¨ M.R. Witjes, E.J.J. Groenen, J. Chem. Phys. Ž1998. submitted.