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J. Phys. Chem. C 2007, 111, 17720-17724
Photophysical Properties of C84 Major Isomers† Eric C. Booth,‡,| Sergei M. Bachilo,‡ Mito Kanai,§ T. John S. Dennis,§ and R. Bruce Weisman*,‡ Department of Chemistry, MS-60, Rice UniVersity, 6100 Main Street, Houston, Texas 77005, and Department of Physics, Queen Mary, UniVersity of London, Mile End Road, London E1 4NS, United Kingdom ReceiVed: February 20, 2007; In Final Form: May 4, 2007
Photophysical properties of C84 isomers have been measured separately and compared. Intrinsic triplet state lifetimes at room temperature are found to differ dramatically: 4.5 µs for the D2d(II) isomer, 125 µs for Cs(a), and 640 µs for D2(IV). The triplet lifetime of D2(IV) C84 represents the second longest found to date among fullerenes. Measurements between 77 and 320 K reveal that the triplet decay kinetics of D2d(II) is largely temperature-independent, whereas thermally activated channels dominate triplet decay in the D2(IV) isomer. Oxygen-quenching studies suggest that the T1 energy of D2d(II) lies near 7600 cm-1 but that of D2(IV) exceeds 8000 cm-1. These results highlight the importance of isomeric separation in exploring the rich photophysics of higher fullerenes.
Introduction Since the discovery of fullerenes in 1985,1 their photophysical properties have been the subject of extensive scientific scrutiny. One interesting phenomenon is the tendency of these carbon allotropes to populate long-lived triplet excited states following optical excitation.2 This has suggested several different applications, including the use of fullerenes and their derivatives as model photosynthetic systems,3,4 energy acceptors in molecular solar cell designs,5,6 saturable absorbers for optical limiting,7-9 and generators of singlet oxygen for photodynamic cancer therapy.10 However, the photophysical properties of “higher fullerenes” (i.e., those larger than C76) remain poorly understood, largely because of the challenge posed by multiple isomeric forms of these compounds.11 It is of interest to study the photophysics of higher fullerenes to uncover trends as a function of fullerene size and to compare properties among structural isomers. C84 is the most abundant of the higher fullerenes. It has been calculated that C84 can assume 24 distinct isomeric structures that satisfy the isolated pentagon rule.12 These isomers represent a number of different symmetry species; in fact, multiple C84 structures can exist within the same point group. For this reason, the isomers are labeled by their point group symbol followed by a parenthesized Roman numeral index.12 Experimentally, seven C84 isomers are found in standard Huffman-Kraetschmer soot.13 Of these, the two most abundant isomers are D2(IV) (ca. 50%) and D2d(II) (ca. 25%). Among the minor species is Cs(a) (ca. 5%). The similar high-performance liquid chromatography (HPLC) retention properties of C84 isomers make separation a very challenging task that has been accomplished only rather recently.13-15 Consequently, prior photophysical studies of C84 were hampered by the use of unresolved isomeric mixtures.16,17 †
Part of the special issue “Richard E. Smalley Memorial Issue”. * Corresponding author. E-mail:
[email protected]. Rice University. § University of London. | Current address: Department of Chemistry and Physics, Southeastern Louisiana University SLU-10878, Hammond, LA 70402. ‡
In this report we describe the first isomerically resolved photophysical study of C84. Experimental Methods Separation of the major isomers was performed by means of recycling-HPLC on a JAI system, using isocratic CS2 elution from a preparative Cosmosil 5PYE column. After sample reconstitution in toluene, ground state UV-vis-NIR absorption spectra were acquired on a Cary 5000 spectrophotometer. Triplet state measurements were made with a home-built transient absorption spectrometer.18 In this instrument, samples, dissolved either in toluene or in poly(methyl methacrylate) (PMMA) films, were excited with 532 nm laser pulses from a frequency-doubled Q-switched Nd:YAG laser (New Wave Research MiniLase II) or from a pyridine-1-charged YAG-pumped dye laser (Lumonics HD-500). The resulting induced absorption was probed in a near-coaxial geometry with a beam from a current-stabilized tungsten-halogen lamp. Probe wavelengths of interest were selected with a J-Y Triax 180 monochromator coupled to a Kaiser holographic notch filter8 used to reject residual 532 nm laser light. The monochromated probe beam was detected by an amplified silicon photodiode. For measurements of induced spectra and kinetics, film samples were held in a sealed cell that was degassed to remove O2 and then back-filled with N2. Variable temperature kinetics were measured on film samples mounted in an optical cryostat (Oxford Instruments OptistatDN). We determined ground state molar absorptivities using a quenching-kinetics technique developed by Bachilo.19 In this method, palladium-octaethylporphyrin (Pd-OEP) is dissolved in a solution of the fullerene analyte. After freeze-pump-thaw degassing, the rate of the porphyrin’s triplet decay is measured. Since this decay rate is diffusion-limited by encounters with fullerene molecules, kinetic analysis gives the absolute concentration of the fullerene quencher and allows calibration of the fullerene’s ground state absorbance spectrum in molar absorptivity units.
10.1021/jp071447y CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007
Photophysical Properties of C84 Major Isomers
Figure 1. Normalized transient absorbance kinetics, monitored at 525 nm, of deoxygenated PMMA films enriched in the D2(IV) and D2d(II) isomers. Note the differing magnitudes of faster and slower components.
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Figure 2. UV-vis ground state absorptivity spectrum of C84 {D2(IV)} in toluene at 298 K.
Results and Discussion Triplet State Kinetics. Preparation of isomerically pure C84 samples is known to be very challenging.13,15 If photophysical measurements are made on isomerically mixed samples in fluid solution, kinetic results can be greatly complicated by energy transfer between species.2 However, the use of samples immobilized in rigid films prevents such complications. Using recycling HPLC, we obtained a fraction heavily enriched in the two major C84 isomers, D2(IV) and D2d(II). PMMA was dissolved into this fraction, and a film was formed by evaporation of the toluene solvent. Transient absorption measurements on the film revealed double-exponential decay kinetics with well-separated decay components having lifetimes of ca. 640 and 4 µs. We attribute these components to the triplet state decays of the D2(IV) and D2d(II) isomers. However, assignment of kinetic components to specific isomers required studies on more highly separated samples. Recycling HPLC methods, along with more specialized columns, were used to obtain nominally pure samples of D2(IV) and D2d(II). After preparing PMMA films from these fractions, we measured their transient absorption kinetics. Figure 1 shows early delay portions of the kinetic data obtained with 525 nm probing. The film containing the D2(IV) sample displayed both kinetic components described above, indicating that isomeric separation was incomplete. Furthermore, the kinetics from the D2d(II) sample film displayed four distinct components. Assignment of decay components to the major isomers therefore remained unclear. Investigation revealed that one of the four components in the D2d(II)-enriched sample had a multimillisecond lifetime matching that of C70 and thus most likely arose from C70 contamination. To minimize this component’s contribution to our kinetic data, we set the probing wavelength to 570 nm where C70 is known to show near-zero induced absorption. We also excited the sample at 705 nm, a wavelength beyond the ground state absorption range of C70. Then the residual C70 component was subtracted from the kinetic data after it was quantified by setting the probing wavelength to the C70 transient peak at 980 nm and measuring kinetics extending to delays of many milliseconds. After this, three kinetic components remained from the D2d(II)-enriched film. One had a lifetime of ca. 125 µs. We assigned this component to the Cs(a) isomer for several reasons. First, it is known to elute after D2d(II) in recycling HPLC13 and is therefore more likely to appear as an impurity in D2d(II) fractions. In addition, our D2d(II)-enriched sample displayed a ground state absorption shoulder near 1100 nm, which is consistent with Cs(a) but not with either of the major isomers.
Figure 3. Transient spectrum of deoxygenated C84 {D2(IV)} in PMMA at 298 K. The spectrum was measured at a delay of 37 µs after 532 nm excitation.
Finally, the 125 µs component was not observed in samples containing only D2(IV) and D2d(II). After accounting for the two extra kinetic components in the D2d(II) film, we were able to assign the remaining components by correlating their relative amplitudes with the predominant major isomers in the two film samples. We find that the T1 lifetime of the D2(IV) isomer at room temperature is 640 ( 10 µs and that of the D2d(II) isomer is 4.5 ( 1 µs. C84 {D2(IV)} Ground State Absorption. Figure 2 shows the absorption spectrum of our D2(IV) sample in room temperature toluene. We find the absorption onset of this isomer at 1050 nm, consistent with a prior report. Absorption maxima are observed at 396, 569, and 900 nm with molar absorptivity values of ∼28 000, 4650, and 1500 M-1 cm-1. Shoulders appear at 314, 430, 475, 669, and 739 nm, and minima occur at 363, 548, and 876 nm. C84 {D2(IV)} Transient Absorption. The induced absorption spectrum of D2(IV) in a PMMA film is plotted in Figure 3. Such spectra contain positive contributions from Tn r T1 absorption superimposed with negative contributions from ground state depletion (bleaching). Because the minima observed near 470 and 565 nm correspond to bleached ground state absorption peaks, the maxima near 520 and 605 nm are likely not true Tn r T1 peaks. However, we identify the 740 nm maximum as a Tn r T1 peak. Although this 740 nm wavelength matches that of a strong and well-characterized absorption peak of triplet state C60,20,21 the ca. 140 µs lifetime of T1 C60 is far shorter than the lifetime of the transient found in our D2(IV) sample. We therefore assign the observed 740 nm Tn r T1 feature in Figure 3 to the D2(IV) isomer of C84 rather than to a C60 impurity. Figure 4 shows transient absorption kinetic data measured at 740 nm for a PMMA film of D2(IV). By setting the probing wavelength to a peak in the D2(IV) transient spectrum, we
17722 J. Phys. Chem. C, Vol. 111, No. 48, 2007
Figure 4. Transient absorption kinetics measured at 740 nm for deoxygenated C84 {D2(IV)} in PMMA at 298 K. Symbols show data points, and the solid curve is a kinetic simulation computed for firstorder decay with an exponential lifetime of 643 µs.
Figure 5. Emission spectrum from C84 {D2(IV)} in air-saturated toluene solution at 298 K after correction for toluene overtone absorption. (Two artifacts are visible: irregularities between 1100 and 1200 nm arise from the correction for solvent overtone absorption, and the rising signal below 910 nm is from a filter.)
Figure 6. Overlaid normalized fluorescence and absorbance spectra of C84 {D2(IV)} near the S1 origin.
minimized the contribution of isomeric impurities. The solid curve in Figure 4 shows an accurate kinetic simulation based on a 643 µs single exponential decay. We note that this is a remarkably long intrinsic triplet lifetime for a fullerene; only C70 is known to have a more persistent triplet state. C84 {D2(IV)} Fluorescence and Singlet Oxygen Luminescence. The emission spectrum of D2(IV) in an air-saturated toluene solution is plotted in Figure 5. It shows a broad peak at 1000 nm and a smaller, sharper peak around 1277 nm. The latter is assigned to emission from 1∆g molecular oxygen, which must be formed through energy transfer from triplet state C84. This indicates that the energy of the D2(IV) T1 state lies higher than 7830 cm-1. Figure 6 shows the superimposed normalized fluorescence and ground state absorption spectra. Analysis using the mirror-symmetry relationship of 0-0 bands22 gives an S1 origin energy for D2(IV) of ∼10 600 cm-1.
Booth et al.
Figure 7. T1 decay rate of C84 {D2(IV)} in deoxygenated PMMA vs temperature. Open circles show experimental data. The lower solid curve and dashed curve are from models that include components with activation energies of 2360 and 300 cm-1, respectively, plus one (temperature-independent) tunneling component. The upper solid curve shows a model fit based on two activation energies (2360 and 300 cm-1) plus tunneling.
C84 {D2(IV)} Variable Temperature Kinetics. To explore the temperature dependence of triplet state relaxation in the D2(IV) isomer, a sample film was mounted in our optical cryostat and used for transient absorption studies. We measured decay kinetics at 20 K intervals from 80 to 320 K using probe wavelengths of 525 and 565 nm. Because the former wavelength corresponds to a maximum of D2(IV)’s transient spectrum and the latter to a minimum with near-zero ∆A, comparison allowed effective separation of the D2(IV) signal from that of D2d(II) impurity at early delays when both excited species are present. We also performed double-exponential kinetic fits at each temperature point to further isolate each isomer’s contribution. The temperature-dependent rate coefficient for D2(IV) triplet state decay is shown as open symbols in Figure 7. These data could not be fit by a simple Arrhenius expression. As was found previously for lower fullerenes and their derivatives,23,24 the temperature dependence requires a three-component model for adequate fitting. One component is a constant term representing T1 f S0 relaxation through temperature-independent tunneling. The other two are thermally activated components with activation energies deduced to be 300 and 2360 cm-1. The 300 cm-1 term probably reflects vibrationally induced T1 f S0 crossing. By contrast, the 2360 cm-1 activation energy exceeds any vibrational frequency of the molecule and instead suggests a process of T1 decay via thermal repopulation of the S1 state. C84 {D2d(II)} Ground State Absorption. As discussed earlier, our D2d(II)-enriched samples also contained a significant amount of Cs(a). It was therefore not possible to observe the D2d(II) long-wavelength absorption onset directly, or to check for consistency with the 1050 nm absorption origin reported earlier.15 However, other absorption features in toluene solution agreed with the earlier report. As shown in Figure 8, we found maxima at 400 and 622 nm with absorptivities of 38 400 and 5660 M-1 cm-1, respectively. Minima were observed at 369 and 544 nm; their respective absorptivities are 31 700 and 4500 M-1 cm-1. Shoulders were also seen near 465, 500, 581, 663, 739, and 904 nm. C84 {D2d(II)} Transient Absorption: Spectrum and Kinetics. The sample used for measuring D2d(II) triplet state spectroscopy contained an appreciable concentration of D2(IV) impurity, so transient spectra included contributions from both species. To separate these spectral components, kinetic data at each probe wavelength were fit to a double-exponential decay
Photophysical Properties of C84 Major Isomers
Figure 8. UV-vis ground state absorptivity spectrum of C84 {D2d(II)} in toluene at 298 K. The shoulder near 1100 nm suggests minor contamination by the Cs(a) isomer.
Figure 9. Induced absorbance spectrum deduced for C84 {D2d(II)} in PMMA at 297 K.
Figure 10. Normalized fluorescence emission spectrum of C84 {D2d(II)} in aerated toluene at 297 K after correction for solvent absorption. No significant emission from 1∆g O2 is present.
model. The short-lived kinetic amplitude found at each wavelength was then replotted to obtain the induced absorption spectrum of the D2d(II) isomer, as shown in Figure 9. In this spectrum, induced absorbance was positive at all probed wavelengths, indicating that the triplet state’s absorptivity exceeds that of the ground state. Maxima were seen at 540, 870, and 980 nm with a minimum located near 820 nm. We found the intrinsic triplet decay lifetime for D2d(II) in a degassed PMMA film to be 4.5 ( 1 µs at room temperature. The possibility that the 980 nm peak arose from C70 impurity was excluded by several experimental checks. C84 {D2d(II)} Fluorescence and Singlet Oxygen Luminescence. The emission spectrum of the D2d(II)-enriched sample, shown in Figure 10, has a maximum at 1000 nm similar to that from the D2(IV)-enriched sample. This may be attributed to cross-contamination in the two fractions. However, the two emission spectra do show notable differences. First, the D2d(II) sample gives a more pronounced shoulder near 1100 nm.
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Figure 11. T1 decay rate of C84 {D2d(II)} in deoxygenated PMMA versus temperature. Open circles show experimental data and the solid curve is the 2-component fit described in the text.
Second, there is virtually no evidence of singlet oxygen emission. This latter finding is consistent with oxygen-sensitization experiments in which Pd-OEP was added as the sensitizer. The resulting 1∆g luminescence decay was more rapid than in the absence of the C84 isomer (6.5 versus 30 µs). This indicates that D2d(II) is quenching the singlet oxygen with a rate constant of ca. 4.5 × 109 M-1 s-1, or about half the diffusion limit. We therefore conclude that the T1 energy of the D2d(II) isomer lies below but within approximately 1 kT of the 1∆g O2 level. This gives an estimate of ca. 7600 cm-1 for the D2d(II) T1 energy. Assignment of the S1 energy for the D2d(II) isomer is more challenging than for D2(IV) because of indistinct structure of the origin absorption band. However, we were able to estimate an S1 origin value of approximately 10 600 cm-1, which is equal within experimental uncertainty to the value deduced for the D2(IV) isomer. The D2d(II) S1-T1 gap appears to be near 3000 cm-1. C84 {D2d(II)} Variable Temperature Kinetics. Triplet decay kinetics of the D2d(II) isomer were extracted from data measured on a deoxygenated film sample held at temperatures between 77 and 320 K. These decay constant data are plotted in Figure 11. There are two obvious differences from the D2(IV) temperature dependence shown in Figure 7. First, the triplet decay is more than 2 orders of magnitude faster for D2d(II). Second, the relative dependence of decay constant on temperature is much weaker for D2d(II) than for D2(IV). Modeling of the data suggests the presence of two decay processes: rapid, temperature-independent tunneling, plus vibrationally accelerated intersystem crossing with a 650 cm-1 activation parameter. We suggest that this behavior may arise from the proximity effect,25 in which strong vibronic interaction between T1 and a nearby electronic surface (perhaps T2) accelerates T1 nonradiative decay. This proposal is consistent with both the rapid decay of D2d(II) and the minimal influence of temperature on its kinetics. Conclusions We believe that this work provides the first experimental comparison of photophysical properties among fullerene structural isomers. The lowest triplet states of the D2(IV) and D2d(II) major isomers of C84 differ significantly in energy with the D2d(II) T1 level estimated at 7600 cm-1 and the corresponding D2(IV) state apparently lying above 8000 cm-1. This difference allows the triplet state of D2(IV) to be quenched by molecular oxygen while D2d(II) is not. The triplet-induced absorption spectra of the two species show substantial differences. Furthermore, the intrinsic lifetimes of C84 isomers against T1 f S1 nonradiative decay span more than 2 orders of magnitude. The D2(IV) isomer’s triplet lifetime is approximately
17724 J. Phys. Chem. C, Vol. 111, No. 48, 2007 640 µs, which makes it the second longest-lived fullerene triplet species yet reported. The less abundant Cs(a) shows a triplet decay lifetime of ca. 125 µs. Finally, triplet decay of the D2d(II) major isomer is far faster with an intrinsic lifetime of only 4.5 µs. Remarkably, this decay shows only weak temperature dependence and remains rapid even at cryogenic temperatures. Our findings illustrate that even structurally similar isomers of higher fullerenes can display dramatically different photophysics. This emphasizes the need for isomeric resolution when exploring the physical properties of higher fullerenes. Acknowledgment. This research was supported by the Welch Foundation (grant C-0807) and the National Science Foundation (grant CHE-0314270). E.C.B. thanks Professors Vicki Colvin, John Hutchinson, and Mason Tomson for their expert guidance and expresses gratitude to the late Professor John Margrave, whose personal and professional contributions will be greatly missed. References and Notes (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162-3. (2) Fraelich, M. R.; Weisman, R. B. J. Phys. Chem. 1993, 97, 1114511147. (3) Carbonera, D.; Di Valentin, M.; Corvaja, C.; Agostini, G.; Giacometti, G.; Liddell, P. A.; Kuciauskas, D.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1998, 120, 4398-4405. (4) Imahori, H. Org. Biomol. Chem. 2004, 2, 1425-1433. (5) Kamat, J. P.; Haria, M.; Hotchandani, S. J. Phys. Chem. B 2004, 108, 5166-5170. (6) Mozer, A. J.; Denk, P.; Schwarber, M. C.; Neugebauer, H.; Sariciftci, N. S.; Wagner, P.; Lutsen, L.; Vanderzande, D. J. Phys. Chem. B 2004, 108, 5235-5242.
Booth et al. (7) Riggs, J. E.; Sun, Y.-P. J. Chem. Phys. 2000, 112, 4221-4230. (8) Sun, Y.-P.; Riggs, J. E.; Liu, B. Chem. Mater. 1997, 9, 12681272. (9) Kost, A.; Jensen, J. E.; Loufty, R. O.; Withers, J. C. Appl. Phys. B 2005, 80, 281-283. (10) Da Ros, T.; Prato, M. Chem. Commun. 1999, 663-669. (11) Diederich, F.; Whetten, R. L. Acc. Chem. Res. 1992, 25, 119126. (12) Manolopoulos, D. E.; Fowler, P. W. J. Chem. Phys. 1992, 96, 7603-7614. (13) Dennis, T. J. S.; Kai, T.; Asato, K.; Tomiyama, T.; Shinohara, H.; Yoshida, T.; Kobayashi, Y.; Ishiwatari, H.; Miyake, Y.; Kikuchi, K.; Achiba, Y. J. Phys. Chem. A 1999, 103, 8747-8752. (14) Avent, A. G.; Dubois, D.; Penicaud, A.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1997, 1907-1910. (15) Dennis, T. J. S.; Kai, T.; Tomiyama, T.; Shinohara, H. Chem. Commun. 1998, 1998, 619-620. (16) Kamat, P. V.; Sauve, G. Proc. - Electrochem. Soc. 1995, 95-10, 431-440. (17) Terazima, M.; Hirota, N.; Shinohara, H.; Asato, K. Proc. Electrochem. Soc. 1995, 95-10, 267-273. (18) Bachilo, S. M.; Benedetto, A. F.; Weisman, R. B.; Nossal, J. R.; Billups, W. E. J. Phys. Chem. A 2000, 104, 11265-11269. (19) Bachilo, S. M., to be submitted for publication. (20) Samuels, D. A.; Weisman, R. B. Chem. Phys. Lett. 1998, 295, 105112. (21) Ausman, K. D.; Weisman, R. B. Res. Chem. Intermed. 1997, 6, 431-451. (22) Harris, D. C.; Bertolucci, M. D. Electronic Spectroscopy. In Symmetry and Spectroscopy: An Introduction To Vibrational and Electronic Spectroscopy; Dover: New York, 1989; Chapter 5, pp 307-419. (23) Benedetto, A. F.; Weisman, R. B. Chem. Phys. Lett. 1999, 310, 25-30. (24) Bachilo, S. M.; Benedetto, A. F.; Weisman, R. B. Proc. Electrochem. Soc. 2000, 2000-10, 208-215. (25) Lim, E. C. J. Phys. Chem. 1986, 6770-6777.
