7914
J. Phys. Chem. B 2000, 104, 7914-7918
Photophysics of Open C60 Derivatives Robert Stackow, Georg Schick, Thibaut Jarrosson, Yves Rubin, and Christopher S. Foote* Department of Chemistry & Biochemistry, UniVersity of California, Los Angeles, California 90095-1569 ReceiVed: April 7, 2000; In Final Form: June 12, 2000
In this report we examine the photophysical properties of two novel C60 derivatives. These compounds consist of a ring-opened C60 derivative (2) and its dihydro precursor (1), both of which include an orifice. The groundstate absorption spectra and the triplet-triplet absorption spectra were recorded for both derivatives. Extinction coefficients for the ground-state absorption () and the triplet excited state (∆T-T) were calculated. The derivatives have triplet energies (ET) lower than that of C60, and the triplet quantum yields (φT) are somewhat lower than that of C60. These compounds remain good photosensitizers for the formation of singlet oxygen, with high quantum yields (φ∆). Thus, in many respects the photophysics of these buckyballs with an orifice still resemble pristine fullerenes.
Introduction Since the initial discovery of fullerenes in 1985,1 this class of compounds has fascinated and excited chemists. It was quickly discovered that atoms could be inserted into the inner cavity of the fullerenes.2 However, insertion has been limited to various metal3,4 and noble gas atoms5 with very low yields. The ability to synthetically modify these highly symmetrical carbon clusters has provided a possible route to the production of endohedral fullerenes. By opening a sizable orifice on the C60 surface, via chemical modification, a very important step in this process has been achieved.6 The photophysical properties of fullerenes, especially C60, have been well characterized. C60 is known to be an excellent photosensitizer for the production of singlet oxygen, with a quantum yield near unity.7,8 These properties arise from the extended conjugated π-network covering the entire fullerene surface. Disturbing this core chromophore via chemical derivatization has been shown to alter the photophysical properties of the parent fullerene in a fairly systematic way.9-17 In this article the effect of opening up the fullerene structure significantly, to the dihydro bis-lactam 1 and its aromatized derivative 2, is studied by photophysical methods. Experimental Section Materials. Samples of the ring-opened C60 derivative 2 and its dihydro precursor 1 were synthesized as reported elsewhere.6 C60 (purity >99.5%) was obtained from MER Corp.18 Rubrene (98%), tetraphenylporphine (TPP, >99%), and toluene (Optima grade) were purchased from Aldrich Chemical Company. Benzo[a]pyrene (98%) was obtained from Fluka. Perylene (98%) was obtained from Sigma Chemical Company. transAzobenzene (>95%) was obtained from Eastman Organic Chemicals. Photosensitizer-grade ferrocene was obtained from J. T. Baker Chemical Company.19 All chemicals were used as received, with the exception of dicyanoanthracene (DCA), which was recrystallized prior to use. Measurements. UV-vis absorption spectra were recorded on Beckman DU-650 or Hewlett-Packard 8453 spectrophotometers. The ground-state extinction coefficients of the open
derivatives were calculated by measuring the absorbance of each derivative at concentrations ranging from 1 to 7 × 10-5 M, in toluene. Triplet-triplet spectra, triplet-triplet extinction coefficients, triplet quantum yields, and triplet energies (using energy transfer) were determined using a transient absorption setup described previously.20 The excitation wavelength for all triplettriplet transient absorption experiments was either 355 or 532 nm. Samples with A ≈ 0.1-0.4 at the excitation wavelength, in optical-grade toluene, were placed in a 1 cm quartz cuvette and purged with argon for 30 min. Energy transfer experiments to observe quenching of the triplet state of the open fullerene derivatives were accomplished using the transient absorption methods described above. The selected quencher was added to toluene solutions of the derivatives and then purged with argon. The open adducts were selectively excited at 355 nm for energy transfer to rubrene, benzo[a]pyrene, and ferrocene, and at 532 nm for transfer to trans-azobenzene, perylene, and DCA. The triplet decay was recorded at one of the local maxima of the triplet-triplet spectrum for the derivatives, 630 nm, and the rate constant of quenching was extracted using curve-fitting analysis. For the quenching of 3TPP by the open derivatives, TPP was selectively excited at 532 nm and its decay observed at 465 nm.21 The decay traces were analyzed in the same manner as above. Singlet oxygen quantum yields for the open derivatives were determined using a liquid nitrogen cooled germanium photodiode to observe the 1268 nm luminescence of 1O2.22 Airsaturated samples dissolved in toluene were excited at 355 nm. A silicon cutoff filter at 1100 nm and a 1270 nm interference filter were used to eliminate ambient and scattered laser light. Signal collection was performed at a 90° angle to the laser beam, and the intensity was enhanced by addition of a parabolic mirror at a 270° angle. The data were recorded and analyzed as described above. Results and Discussion The ring-opened bis-lactam C60 derivatives examined are shown in Scheme 1. The endo isomer of the dihydro precursor 1 was chosen for examination because of its stability under ambient conditions compared to the exo isomer.6 This stability
10.1021/jp001358k CCC: $19.00 © 2000 American Chemical Society Published on Web 07/27/2000
Photophysics of Open C60 Derivatives
J. Phys. Chem. B, Vol. 104, No. 33, 2000 7915
SCHEME 1
Figure 2. Triplet-triplet absorption spectra of (a) open dihydro precursor, 1, and (b) open derivative, 2, uncorrected for ground-state absorption. The spectra were recorded immediately after exciting degassed samples with A ≈ 0.1-0.4, in optical-grade toluene, at 355 nm. (O, PMT data; b, near-IR spectrometer data).
Figure 1. UV-vis absorption spectra of (a) open dihydro precursor, 1, (b) open derivative, 2, and (c) C60 recorded in toluene. Molar extinction coefficients are listed with the wavelength at which they were recorded in parentheses. Dashed lines ×10, except C60 which is ×15.
is presumably due to the fact that the protons of the endo isomer point into the orifice and are more protected from dehydrogenation than exo protons. The lactam moieties in 1 and 2 are in conjugation with the remainder of the fullerene core. An additional result of functionalization is the increased solubility
of 1 and 2, compared to C60, for all solvents used during these experiments. UV-vis Spectroscopy. As can be seen from the UV-vis absorption spectra in Figure 1, the ground-state absorbance of both open fullerene derivatives is different from C60. The peak observed for C60 at 329 nm is nearly lost upon functionalization, and the valley at 440 nm is virtually nonexistent for 1 and 2 due to the appearance of a new absorption centered at 460 nm. Both of the open derivatives maintain the long-wavelength bands, and in fact exceed the absorbance of C60 in the red portion of the visible spectrum. These long-wavelength absorbances give the derivatives a golden color in solution. Triplet-Triplet Spectra. The triplet-triplet transient absorption spectra of the open fullerene derivatives, recorded in degassed toluene, are shown in Figure 2. The spectra show that the open derivatives still maintain the long-wavelength triplet
7916 J. Phys. Chem. B, Vol. 104, No. 33, 2000
Stackow et al.
TABLE 1: Photophysical Properties of C60, 1 and 2 ∆T-T, M-1 cm-1 C60 open dihydro precursor, 1 open derivative, 2
comparative method
energy transfer
λmax,T-T, nm
φTa
φ∆
20 200b 9 942d 9 293d
20 200b 9 074 9 278
740 365, 415, 640, 780e 350,420, 630, 810e
1c 0.51 0.57
1c 0.47 0.57
a The values of φ for the open derivatives were experimentally calculated using the values of ∆ 21 b See T T-T from the energy transfer method. ref 23. c See ref 7. d The value of ∆T-T was calculated by observing the intensity at 630 nm for 1 and 2 and 740 nm for C60. e Italics denote the largest absorptions in the triplet-triplet spectra.
absorbance associated with 3C608 and other substituted C60 derivatives9 around 700 nm, but it is blue-shifted to about 635 nm for both compounds. In addition, 1 shows an additional band at ca. 780 nm, with a shoulder at 810 nm. This band is not observed in other C60 derivatives. Compounds 1 and 2 also exhibit a generally featureless decrease in triplet absorbance further into the near-infrared portion of the triplet-triplet spectra, as is seen with other fullerene derivatives. The most striking result of functionalization on the triplettriplet spectra is the appearance of two very strong and welldefined absorbances in the blue and UV regions. In the spectra of 1, these high-energy bands are comparable in intensity to the common fullerene absorbance observed around 700 nm, and in 2 they dominate the spectra. Bands in the blue and UV regions are not uncommon among fullerenes and fullerene derivative triplets,9-14,17 but they are seldom have the intensity and definition observed for these open derivatives, especially compound 2. Triplet Extinction Coefficients and Triplet Quantum Yields. The extinction coefficients (∆T-T) for the absorbance of the triplet states of the derivatives were calculated to be somewhat lower than that observed for C60.23 By assuming the triplet quantum yield (φT) to be equal to the singlet oxygen quantum yield (φ∆), vide infra, we can estimate the extinction coefficients using the comparative method shown in eq 1.15
φT,sub∆T-T,sub ∆ODsub ) φT,ref∆T-T,ref ∆ODref
(1)
When comparing the intensity of the triplet absorbances of optically matched samples of the substrate (sub) and a reference compound (ref) with a known φΤ and ∆T-T, we used the relationship in eq 1 to calculate ∆T-T of the substrate. In this case, C60 was chosen as the reference because its φT is known to be unity7 and ∆T-T to be 20 200 M-1 cm-1,23 and its structure closely resembles that of the derivatives. As can be seen in Table 1, the calculated extinction coefficients for the triplet states of the derivatives are roughly half the value for C60. In addition, the coefficients for the derivatives are nearly identical, probably reflecting their structural similarity. The extinction coefficients of the open derivatives were also calculated by using the energy transfer method.21 By selectively exciting the open derivatives, rubrene (ET ) 26 kcal/mol,24 ∆T-T(460) ) 26 000 M-1 cm-1 21,25) can be used as an energy acceptor (A) via the pathway shown in eq 2, where S denotes the open derivative substrate and the rate constant of energy transfer is referred to as ket. ket
S* + 1A0 98 1S0 + 3A*
3
(2)
The quenching of the triplet states of 1 and 2 and the rise and decay of triplet state of rubrene were followed using flash photolysis. An example is shown in Figure 3. The calculated
Figure 3. Example of Stern-Volmer analysis applied to the quenching of the triplet state of the open dihydro precursor, 1, by rubrene. Insert: decay traces observed for (a) the rubrene triplet, measured at 465 nm,26 and (b) the open dihydro precursor triplet, 1, measured at 630 nm, a local maximum of the triplet-triplet spectra.
value of ∆T-T was corrected for incomplete energy transfer using eq 3.21
Ptr ) ket[ref]/(ket[ref] + kd)
(3)
With this equation, the probability of energy transfer (Ptr) from the fullerene derivative triplet to rubrene (ref) can be calculated. In this instance, ket is the rate constant of energy transfer from the fullerene derivative triplet to ground-state rubrene, and kd is the natural decay rate constant of the fullerene derivative triplet state. The final ∆T-T value was then calculated using eq 4.21
∆T-T,derv ) ∆T-T,ref
(
)( )
∆ODderv 1 ∆ODref Ptr
(4)
In this equation ∆T-T,ref is the extinction coefficient of the reference compound at the wavelength studied, and ∆OD is the intensity of the signal, corrected for detector response. The final ∆T-T values calculated using this alternative method agree surprisingly well with the values calculated using the φ∆ assumption method (Table 1). Although there is an inherently large error associated with the comparative energy transfer method, it lends support to the singlet oxygen assumption used above. The results of this additional ∆T-T calculation could then be used to reapply the comparative method15 (eq 1) to experimentally calculate the triplet quantum yield (φT) for the open derivatives. The results of both the ∆T-T energy transfer method and the φT calculation are shown in Table 1. The good agreement observed between the two methods of calculating ∆T-T indicates that the triplet quantum yield is very close to the value calculated for the singlet oxygen quantum yield (vide infra). This is a characteristic of fullerenes that is apparently conserved in the open derivatives.
Photophysics of Open C60 Derivatives
J. Phys. Chem. B, Vol. 104, No. 33, 2000 7917
TABLE 2: Quenching Rate Constants (kq) of the Triplet States of Open Derivatives by Various Quenchers quencher O2 rubrene trans-azobenzene perylene ferrocene DCA benzo[a]pyrene
kq (×10-10 M s)
ET (kcal/mol)24 Open Derivative, 2 22 26 29 35 38 42 42
=1 0.13 ∼0.001 0.9a
Open Dihydro Precursor, 1 O2 22 rubrene 26 trans-azobenzene 29 a
=1 0.21
kq probably contains a large contribution from electron transfer.
TABLE 3: Observed Rate of Quenching of 3TPP by 1 and 2 kq (×10-8 M s)a open dihydro precursor, 1 open derivative, 2 a 3TTP
7.64 20.4
was observed at 465 nm.21,26
Triplet Energies. The triplet energies of the open derivatives were calculated by performing additional quenching experiments using flash photolysis. Quenchers of known triplet energy were used to quench the selectively excited triplet states of the open derivatives. This method can be used to bracket the triplet energies of the open derivatives. The results of this experiment are shown in Table 2. This series of quenching experiments shows that the open fullerene derivatives have very similar triplet energies, with compound 2 being slightly lower in energy. Both of the derivatives have triplet energies of ca. 29 kcal/mol, lying substantially below the 35 kcal/mol triplet energy observed for the parent C60 molecule.27 The anomalous quenching result observed for ferrocene in Table 2 can be rationalized by a large contribution to kobs by electron transfer from ferrocene to the triplet excited state of the open derivative 2.16 To confirm the triplet energies inferred from this series of quenching experiments, we used tetraphenylporphine, ET ) 33 kcal/mol,24 to perform a set of reverse quenching experiments. In this case, TPP was selectively excited at 532 nm and the open derivatives were used to quench the triplet excited state of TPP. The results of these experiments, seen in Table 3, further indicate that the triplet energies of 1 and 2 fall below 33 kcal/ mol.28 From our bracketing of the triplet energies we conclude that the triplet energies of 1 and 2 are ca. 29 kcal/mol. This is lower than the triplet energy of C60 (35 kcal/mol)27 and may be explained, in part, by the lactam addends which are in conjugation with the remainder of the original conjugated π-system of the C60 molecule, lowering the total energy of the molecule. Singlet Oxygen Quantum Yield. C60 is well-known to be an efficient and rugged photosensitizer, with a singlet oxygen quantum yield (φ∆) of nearly 1.7,8 The effect of opening an orifice in this unique photosensitizer was further explored by determining the singlet oxygen quantum yield for 1 and 2. This was accomplished by observing the decay of 1O2 luminescence at 1268 nm, upon exciting air-saturated samples of 1 and 2, using a liquid nitrogen cooled germanium photodiode. Both compounds give high singlet oxygen quantum yields, as seen in Table 1.29
Compound 2 was observed to have a higher φ∆ than compound 1, possibly because of its slightly larger conjugated area after aromatization of the dihydro precursor 1.9 The φ∆ values calculated agree well with the experimentally determined values for φT (Table 1). The relatively high φ∆, the resistance to irradiation, and the correlation between φ∆ and φT show that these molecules behave much like pristine fullerenes. Conclusions The triplet-state photophysical properties of a unique pair of fullerene derivatives containing an orifice in the carbon cage have been studied. The observed fullerene-like bands in the triplet-triplet spectra, the close relationship between φT and φ∆, and the high values of φ∆ clearly show that even after extensive modification these molecules retain many of the attributes associated with fullerenes. The ring-opened C60 derivative, 2, and its dihydro precursor, 1, are important stepping stones to a synthetic pathway to endohedral fullerenes. In the future, it will be important to note how the insertion of exogenous atoms or molecules into the cavities of these derivatives affects the observed photophysical characteristics. The importance of the photophysical parameters of these molecules will become apparent as work toward zipping up the buckyball ensues. Acknowledgment. We thank Dr. Marcia Levitus and Professor Miguel Garcia-Garibay for their advice and aid in the attempted acquisition of luminescence data. We are grateful to the National Science Foundation (CHE97-03086) for support. References and Notes (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162-163. (2) Heath, J. R. O’Brien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Tittel, F. K.; Smalley, R. E. J. Am. Chem. Soc. 1985, 107, 77797780. (3) Nagase, S.; Kobayashi, K.; Akasaka, T. Bull. Chem. Soc. Jpn. 1996, 69, 2131-2142. (4) Bethune, D. S.; Johnson, R. D.; Salem, J. R.; de Vries, M. S.; Yannoni, C. S. Nature 1993, 366, 123-128. (5) Saunders: M.; Cross, R. J.; Jime´nez-Va´zquez, H. A.; Shimshi, R.; Khong, A. Science 1996, 271, 1693-1697. (6) Schick, G.; Jarrosson, T.; Rubin, Y. Angew. Chem., Int. Ed. Engl. 1999, 38, 2360-2363. (7) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem. 1991, 95, 11-12. (8) Terazima, M.; Noburo, H.; Shinohara, H.; Saito, Y. J. Phys. Chem. 1991, 95, 9080-9085. (9) Prat, F.; Stackow, R.; Bernstein, R.; Qian, W. Y.; Rubin, Y.; Foote, C. S. J. Phys. Chem. A 1999, 103, 7230-7235. (10) Anderson, J. L.; An, Y. Z.; Rubin, Y.; Foote, C. S. J. Am. Chem. Soc. 1994, 116, 9763-9764. (11) Williams, R. M.; Koeberg, M.; Lawson, J. M.; An, Y.-Z.; Rubin, Y.; Paddon-Row, M. N.; Verhoeven, J. W. J. Org. Chem. 1996, 61, 50555062. (12) Guldi, D. M.; Hungerbu¨hler, H.; Asmus, K. D. J. Phys. Chem. 1995, 99, 9380-9385. (13) Nakamura, Y.; Minowa, T.; Hayashida, Y.; Tobita, S.; Shizuka, H.; Nishimura, J. J. Chem. Soc., Faraday Trans. 1996, 92, 377-382. (14) Nakamura, Y.; Taki, M.; Tobita, S.; Shizuka, H.; Yokoi, H.; Ishiguro, K.; Sawaki, Y.; Nishimura, J. J. Chem. Soc., Perkin Trans. 2 1999, 127-130. (15) Bensasson, R. V.; Bienvenue, E.; Janot, J. M.; Leach, S.; Seta, P.; Schuster, D. I.; Wilson, S. R.; Zhao, H. Chem. Phys. Lett. 1995, 245, 566570. (16) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1997, 119, 974-980. (17) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 4093-4099. (18) The MER Corp., 7960 S. Kolb Road, Tucson, AZ 85706.
7918 J. Phys. Chem. B, Vol. 104, No. 33, 2000 (19) J. T. Baker Chemical Co., 222 Red School Lane, Phillipsburg, NJ 08865. (20) Arbogast, J. W.; Foote, C. S. J. Am. Chem. Soc. 1991, 113, 88868889. (21) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1-250. (22) Applied Detector Corp., 2325 McKinley Ave., Fresno, CA 93703. (23) Bensasson, R. V.; Hill, T.; Lambert, C.; Land, E. J.; Leach, S.; Truscott, T. G. Chem. Phys. Lett. 1993, 201, 326-335. (24) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry; Marcel Dekker: New York, 1993. (25) Malkin, Y. N.; Pirogov, O.; Kuzmin, V. A. J. Photochem. 1984, 26, 193-202.
Stackow et al. (26) 3Rubrene and 3TTP were observed at 465 nm because this corresponds to the minima in the triplet-triplet absorption spectra of the ring-opened fullerene derivatives. (27) Zeng, Y.; Biczok, L.; Linschitz, H. J. Phys. Chem. 1992, 96, 52375239. (28) Unfortunately, despite repeated attempts, no luminescence from these ring-opened fullerene derivatives could be detected under any experimental conditions to further confirm the triplet energy assignment. The calculated emission wavelengths are outside the sensitive region of the photomultiplier in the phosphorimeter. (29) No change in the UV-vis spectra of the derivatives was noted after laser photolysis.
