7858
J. Phys. Chem. B 1997, 101, 7858-7862
Photoinduced Electron Transfer between Carotenoids and Solvent Molecules Tatyana A. Konovalova† and Lowell D. Kispert* Department of Chemistry, P.O. Box 870336, UniVersity of Alabama, Tuscaloosa, Alabama 35487
Valery V. Konovalov‡ Institute of Chemical Kinetics and Combustion SB RAS, 630090 NoVosibirsk, Russia ReceiVed: March 10, 1997; In Final Form: July 8, 1997X
EPR evidence for photoinduced electron transfer from β-carotene and canthaxanthin to solvent molecules under 308-578 nm photolysis of frozen carotenoid solutions in CCl4, CHCl3, CH2Cl2, and CS2 has been obtained. The paramagnetic species are stable at 77 K and are attributed to carotenoid radical cations (Car•+) and solvent derived radicals. For a CS2 solution, solvent radicals are identified as CS2•- radical anions, and for chlorinated solvents, they can be assigned to either a predissociation species (R‚‚‚Cl)•- or radical products R•. The proposed reaction mechanism includes energy and electron transfer from the S1 and S2 excited singlet states of carotenoid molecules to the solvent to form an intermediate donor-acceptor complex 1[Car‚‚‚Sol]* which decays yielding solvent-separated radical-ion pairs (Car•+‚‚‚Sol‚‚‚Sol•-).
Introduction Carotenoids (Car) are important components of the photosynthetic apparatus carrying out light-harvesting and photoprotective functions.1-3 They absorb solar energy in the spectral region 420-550 nm where chlorophylls are not effective absorbers, and this energy is subsequently transferred to chlorophylls. Carotenoids are also involved in charge-transfer reactions4 in photosynthetic reaction centers where carotenoid radical cations (Car•+) are formed.5 Various photosynthetic model systems have been devised in recent years, which include carotenoids as effective electron donors and quinones,6,7 fullerenes,8 or imides9 as primary electron acceptors (A). Carotenoids have been directly attached to acceptors to form dyads (Car-A)9,10 or linked via porphyrin (P) to form triads (Car-P-A).6-9 Photoinduced electron transfer in both systems leads to the formation of charge-separated states (Car•+-A•-) or (Car•+-P-A•-) which have been detected by transient optical absorption techniques. Light absorption of carotenoids in the region 420-550 nm is governed by the S0(1Ag) f S2(1Bu) transition where the second excited singlet state S2 converts to the lower-lying dipoleforbidden S1(2Ag) state within 200 fs.11 Despite the fact that the S1 state is much longer lived (8-11 ps11,12) than the S2 state, the roles of both singlet levels in the mechanism of energy transfer from carotenoids to chlorophylls are not well established, and the possibility of a contribution from S2 has been demonstrated.3,11 In redox reactions the excess energy of the S2 state (5500-6500 cm-1 above S1) could be in some cases a sufficient driving force for electron transfer from this state to acceptor molecules. To date, the reactivities of the carotenoid excited singlet states (Car* Sn, n ) 1, 2, ...) remain unknown. It is known that the reactions of Car* Sn depend strongly on the conditions, such as solvent and temperature.13-15 Good electron acceptor solvents, such as carbon tetrachloride, are expected to be more efficient in the generation of the carotenoid radical cation Car•+ as a * To whom correspondence should be addressed. † On leave from the Institute of Catalysis SB RAS, 630090 Novosibirsk, Russia. ‡ Present address: Department of Chemistry, The University of Alabama. X Abstract published in AdVance ACS Abstracts, September 1, 1997.
S1089-5647(97)00876-6 CCC: $14.00
primary photoproduct of Car* Sn decay. The species quenching Car* Sn in these systems are unknown, but participation of solvent molecules has been proposed. Photoexcitation products generated by laser pulse photolysis (XeCl laser, 308 nm) of carotenoids in liquid CCl4 at room temperature have been studied by X- (9 GHz) and Q-band (35 GHz) time-resolved electron paramagnetic resonance (EPR) spectroscopy.15 Spinpolarized EPR spectra were attributed to a radical anion of the solvent CCl4•- and the carotenoid radical cation Car•+. Although the EPR signal characteristic for the CCl3• radical that is formed upon dissociation of CCl4•- was not observed, formation of solvent-separated radical-ion pairs as a result of electron transfer from the carotenoid excited singlet state to the solvent has been suggested. Participation of solvent molecules in the photoinduced electron-transfer reaction in the chlorophyllquinone system has been demonstrated by EPR measurements at low temperature.16 It has been shown that chlorophyll sensitizes the electron transfer from ethanol to quinones producing alcohol radical cations and quinone radical anions as primary photoproducts. To better understand the roles of carotenoid excited states in the electron-transfer reactions and the influence of solvent on the efficiency of this process, we studied the low-temperature photolysis of β-carotene and canthaxanthin in different solvents. Experimental Section β-Carotene was supplied by Sigma and canthaxanthin and tetrabutylammonium hexafluorophosphate (TBAHFP) by Fluka. The carotenoids were stored in the dark at -14 °C in a desiccator containing activated CaSO4 and were allowed to warm to room temperature just before use. Purity of the samples was checked by 1H NMR (360 MHz, CDCl3) and TLC analyses. The solvents CCl4, CHCl3 (Aldrich, HPLC grade), and CH2Cl2 (Aldrich, anhydrous) were used as received; CS2 (Sigma, HPLC grade) was distilled from P2O5 under a N2 atmosphere. The photochemical experiments were carried out in quartz EPR tubes. Sample solutions (ca. 10-3 M) were degassed by four freeze-pump-thaw cycles or oxygenated by passing a stream of oxygen through the solutions before freezing. They were then irradiated for 5-20 min at 77 K with a Questek XeCl laser (308 nm, 40 Hz, 10 mJ/pulse) or a 1 kW Xe/Hg lamp © 1997 American Chemical Society
Photoinduced Electron Transfer
Figure 1. EPR spectra of degassed canthaxanthin solution in CH2Cl2 at 77 K: (a) irradiation by Xe/Hg lamp (578 nm) at 77 K; (b) (solid line) difference spectrum after subtraction of Car•+ signal, (dotted line) electrochemically generated at 298 K, then cooled to 77 K and measured at that temperature.
J. Phys. Chem. B, Vol. 101, No. 39, 1997 7859
Figure 2. (a) EPR spectrum obtained upon irradiation (578 nm) of degassed β-carotene solution in CHCl3 at 77 K, (b) difference spectrum after subtraction of Car•+ signal.
equipped with a Kratos monochromator (365-578 nm, 5-30 mW). EPR spectra of both photo- and electrochemically generated radicals were recorded at 77 K with an X-band (9.0908 GHz) Varian E-12 EPR spectrometer, equipped with a rectangular cavity. Canthaxanthin radical cation was also generated electrochemically at room temperature in a small EPR cell.17 Electrolysis was controlled by an electrochemical analyzer BAS 100W. Carotenoid solutions in methylene chloride (+0.1 M TBAHFP) were deaerated with N2, electrolyzed at a potential 0.1 V higher than the first oxidation peak for 1 min, sealed, and immediately cooled to 77 K. Results and Discussion Photolysis of β-carotene and canthaxanthin frozen (77 K) solutions in CCl4, CHCl3, CH2Cl2, and CS2 by either a XeCl laser (308 nm) or a Xe/Hg lamp (365-578 nm) results in the formation of stable (for several days at 77 K) paramagnetic species with estimated quantum yields of 10-3-10-4. Representative EPR spectra of these radicals are shown in Figures 1a-4a. It was found that EPR signals are similar for both β-carotene and canthaxanthin carotenoid solutions. Warming of the samples to 250-270 K, even for only a second, caused immediate disappearance of all EPR signals, although radical cations of β-carotene and canthaxanthin generated electrochemically in CH2Cl2 are known to be stable at room temperature for several minutes. The warming effect can be explained by recombination of the photogenerated radical pairs, which may be the carotenoid radical cations Car•+ and solvent counter radical anions Sol•- or their reaction products. Photolysis of neat solvents under the same conditions did not produce any EPR signals. Since interpretation of solid-state EPR spectra is complicated by considerable line broadening, a microwave power study was carried out. Decreasing the microwave power enhances the central part of the spectrum, whereas the outer lines become less intense. Thus there are two different types of radicals. Lines with g ) 2.0028 ( 0.0001 and ∆Hpp ) 13.2 ( 0.2 G can be attributed to the carotenoid radical cation Car•+. These data are consistent with parameters of canthaxanthin radical cations electrochemically generated in CH2Cl2 at 298 K and then recorded at 77 K (Figure 1b, dotted line), and they are nearly the same as the known literature data.15,18 Since the signals of β-carotene and canthaxanthin radical cations have equal line widths and g values,15 we subtracted its signal formed electrochemically from the EPR spectra of the
Figure 3. (a) (solid line) EPR spectrum obtained upon XeCl laser irradiation (308 nm) of degassed β-carotene solution in CCl4 at 77 K, (dotted line) simulated spectrum, (b) (solid line) difference spectrum after subtraction of Car•+ signal (dotted line) obtained electrochemically.
Figure 4. EPR spectra obtained from degassed β-carotene solution in CS2 at 77 K: (a) irradiation (365 nm) at 77 K, (b) difference spectrum after subtraction of Car•+ signal from spectrum (a).
irradiated β-carotene solutions. The difference spectra (Figures 1b-4b) are very similar to the spectra of the respective neat solvents γ-irradiated at low temperatures19-23 and can be attributed to anion radicals (RCl•- and CS2•-) or their decay radical products (CCl3•, CHCl2•, or CH2Cl•). For a CS2 solution at 77 K (Figure 4b) the features of the difference spectrum are in good agreement with the EPR parameters of the radical anion CS2•- (g1 ) 2.0080) prepared by γ-radiolysis22,23 and the dimer radical anion (CS2)2•- (g2 ) 2.024).23 After keeping the sample for 2 days at 77 K or warming it for 10 min to 180 K, the intensity of the central EPR signals decreased, but the relative intensity of dimer feature with g2 increased.
7860 J. Phys. Chem. B, Vol. 101, No. 39, 1997
Figure 5. EPR spectra of oxygen-saturated β-carotene solution in CCl4: (a) upon irradiation (365 nm) at 77 K, (b) after warming the same sample to 150 K then cooling to 77 K and measuring at that temperature.
The difference spectra obtained for the CH2Cl2, CHCl3, and CCl4 solutions after subtracting the central Car•+ line (Figures 1b-3b) are typical for anisotropic, randomly oriented chlorinecontaining radicals at low temperatures (35Cl coupling)20,24-26 with an axially symmetric g-tensor, where g⊥ is close to 2.015. The difference spectrum of irradiated β-carotene solution in CCl4 at 77 K (Figure 3b) shows that the average separation between the lines in the wings of the spectrum is 18.7 ( 0.2 G, which is consistent with the value for the hyperfine splitting by 35Cl in the CCl3• radical formed chemically in argon and xenon matrices (20 K),20 by γ-radiolysis of CCl4 in tetramethylsilane (TMS) matrices (77 K)26 and by irradiation of trichloroacetamide single crystals (77 K).24 Of the solvents used in this paper, CCl4 is the most extensively studied and documented. Optical and EPR techniques indicated formation of the primary CCl4•- radical anions under radiolysis of neat CCl419,27 and hydrocarbon matrices containing CCl4.26,28-30 At room temperature, CCl4•- dissociates on the subnanosecond time scale to give the CCl3• radical and the chloride ion Cl-.31-33 γ-Radiolysis of CCl4 in TMS matrices at 4 or 77 K produced a predissociation molecular structure (CCl3‚‚‚Cl)•- with C3V symmetry, which converted to CCl3• radical upon warming to 150 K.26 In our previous CIDEP studies,15 upon photolysis of a carotenoid solution in CCl4 at room temperature a resolved EPR spectrum due to CCl3• formation34 was not observed. One reason for the failure to observe the CCl3• spectrum is that the lifetime of the CCl3• radical is on the order of milliseconds, a time period during which CIDEP effects are not observed. Also, the possibility that CCl3• radicals might be rapidly removed by reaction with carotenoids cannot be excluded. The experimental spectrum in CCl4 was simulated35 with reasonable accuracy (Figure 3a, dotted line) by using literature EPR parameters for two possible cases: as a sum of the anisotropic CCl3• radical signal (g| ) 2.0073, g⊥ ) 2.0125; a| 35Cl ) 19.1 G, a 35Cl ) 2 G)20,26 and the isotropic Car•+ line ⊥ (g ) 2.0028, a ) 7 G); and as a sum of the anisotropic CCl4•radical anion signal (g| ) 2.0040, g⊥ ) 2.0150; a| ) 9 G, a⊥ ) 2 G)26 and the isotropic Car•+ line. Although the CCl3• radical simulation more closely resembles the experimental spectrum, the presence of (CCl3‚‚‚Cl)•- cannot be excluded. Additional support for CCl3• radical formation in CCl4 matrices was obtained in oxygen-saturated β-carotene solutions where new paramagnetic species were detected after warming the samples initially irradiated at 77 K. Figure 5a shows that the EPR spectrum of this system after irradiation (365 nm) at 77 K differs from the spectrum (Figure 3a) obtained in the absence of oxygen. The line broadening of the signal can be explained by the presence of molecular oxygen. After warming the sample to 150 K to allow diffusion and then cooling again to 77 K, a new EPR signal appeared (Figure 5b). It displays
Konovalova et al. substantial g-tensor anisotropy with g| ) 2.035 and g⊥ ) 2.0015 consistent with the literature data (g| ) 2.037, g⊥ ) 2.003) for CCl3O2• peroxyl radicals obtained by γ-radiolysis of aerated tetrachloromethane solutions.27,36 The peroxyl radicals RO2• are much more reactive than CCl3• and can form additional radical adducts with carotenoids, (CarRO2•),37-39 or even oxidize them to produce radical cation, Car•+. It has been shown that reactions of CCl3O2• radicals with several carotenoids in Triton-X100 micelles result in the formation of two absorption bands in the near-infrared region, which were assigned to carotenoid radical cations and adducts of the carotenoid with CCl3O2• radicals.36 In our EPR spectra in oxygenated solutions we cannot observe the EPR line of Car•+, nevertheless there is a good indirect evidence that Car•+ is present. In Figure 3a, in the absence of oxygen, the EPR spectrum shows that Car•+ has been formed. It should be noted that formation of long-lived radical species in CS2 occurs only when the irradiating light has wavelengths less than 365 nm, while in chlorinated solvents radical products are already observed at 578 nm. The influence of the solvent on this threshold shift can be explained as a result of either polarity or donor-acceptor properties of the solvents. Since we used only nonpolar or weakly polar solvents, it seems likely that the different energy requirements (and, hence, the activation energy of this process) are determined by the acceptor properties of the solvents. These properties depend on the electron affinity of the molecules and differ considerably for CS2 (Ae ) 0.51 eV) and CCl4 (Ae ) 2.06 eV).40 Correlation between the acceptor strength and energy characteristics of the process assumes formation of intermediate donor-acceptor complexes. Photoexcitation of the donor (Car) produces its excited singlet states Car*Sn with subsequent quenching by a solvent molecule to give the donor-acceptor complex [1Car‚‚‚Sol]*, which is then converted to a primary radical ion pair [Car•+‚‚‚Sol•-]. To avoid recombination and to stabilize the charge-separation state, this primary pair requires a spacer between the donor and acceptor units. The following mechanism of the photoinduced electron transfer from carotenoids to solvent molecules is consistent with our results: 1. photoexcitation of carotenoid to excited singlet states (So f Sn, n ) 2, ...) which are converted to the lowest excited singlet state S1, hν
Car 98 (Car*Sn) f Car* S2 τ2 ) 200 fs
Car* S2 98 Car* S1 τ1 ) 10 ps
Car*S1 98 Car
(1)
(2)
(2a)
2. formation of donor-acceptor complex, kn
1 Car* Sn + Sol 98 [Car‚‚‚Sol]*, (n ) 1, 2, ...)
(3)
3. decay of donor-acceptor complex through electron transfer to solvent molecules yielding primary radical-ion pairs, 1
[Car‚‚‚Sol]* f (Car•+‚‚‚Sol•-)
(4)
4. formation of solvent-separated radical-ion pairs:
(Car•+‚‚‚Sol•-) + Sol f (Car•+‚‚‚Sol‚‚‚Sol•-)
(5)
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(a) Sol ) CS2,
(Car•+‚‚‚CS2•-) + CS2 f (Car•+‚‚‚CS2‚‚‚CS2•-) (5a) CS2•- + CS2 f (CS2)2•-
(6)
(b) Sol ) RCl (chloroalkanes),
(Car•+‚‚‚RCl•-) + RCl f [Car•+‚‚‚RCl‚‚‚(R‚‚‚Cl)•-] (5b) (Car•+‚‚‚RCl•-) f Car•+ + R• + Cl-
(5c)
(R‚‚‚Cl)•- f R• + Cl-
(7)
R• + Car f Car-R•
(8)
(c) O2 containing systems,
(R‚‚‚Cl)•- + O2 f RO2• + Cl-
(9)
R• + O2 f RO2•
(9a)
RO2• + Car f Car•+ + RO2-
(10)
RO2• + Car f Car-RO2•
(11)
In chloroalkanes the solvent radical products can result from either solvent separated radical pairs 5b with subsequent decay of predissociated radical anion after warming 7 or via direct decay of the primary ion pair 5c. Our experimental data fit both of these reaction pathways. The quantum yield of radical formation was estimated as φ ≈ 10-3 in chlorinated solvents and as about 10-4 in CS2. These values allow estimation of the rate constants k1(S1) and k2(S2) of the S1 and S2 excited singlet state reactions with solvent molecules 3 from known lifetimes of these states. It follows from the equation φ1,2 ) (τ1,2k1,2Csol)-1, where Csol is the solvent concentration, that in chlorinated solvents k1 ≈ 107 mol-1 s-1 and k2 ≈ 5 × 108 mol-1 s-1. These values greatly exceed the expected diffusional rate constants at 77 K and indicate that a donor-acceptor complex [1Car‚‚‚Sol]* is formed from the excited singlet states of carotenoids and solvent molecules. Thus, despite the very short lifetimes of carotenoid excited singlet states, the electron transfer to strong electron acceptors can compete with the decay of excited states. Further experiments in inert matrices with different concentrations of acceptors would answer the question whether tight association of solvent molecules and carotenoids is responsible for the phototransfer reaction. The estimated value of k2 demonstrates also that a direct electron transfer from the S2 state (as well as from higher Sn states) to solvent cannot be ruled out. The excess energy of the S2 state (5500-6500 cm-1 above S1) could play a role in the electron transfer to different electron acceptors. The yield φ in chlorinated solvents is about 10 times higher than in CS2 which correlates with the lower electron affinity of the latter. Conclusion Photolysis (308-578 nm) of β-carotene and canthaxanthin frozen solutions in CCl4, CHCl3, CH2Cl2, and CS2 produces paramagnetic species that are stable for several days at 77 K. EPR spectra of irradiated carotenoid solutions with resolved hyperfine structure are reported for the first time. These signals consist of the lines due to carotenoid radical cations (Car•+) and solvent derived radicals. In the case of CS2 solutions the CS2•- radical anions are observed at 77 K; at higher tempera-
tures (above 180 K) these radicals are converted to the dimer radical anions (CS2)2•-. For chlorinated solvents the radicals can be assigned to either a predissociation complex (R‚‚‚Cl)•or radical products R•. The experimental spectrum in CCl4 was simulated with literature parameters for CCl4•- radical anion and CCl3• radical in TMS matrix at 77 K.26 Simulated spectra based on either (CCl3‚‚‚Cl)•- or CCl3• are in close agreement with the experimental spectrum, although the consistency is better when parameters for the CCl3• radical were used. Since photolysis of frozen oxygen-saturated carotenoid samples in CCl4 with subsequent warming results in the formation of Cl3CO2• peroxyl radicals, generation of the precursor CCl3• radicals is confirmed. EPR evidence for photoinduced electron transfer reactions from the carotenoid excited singlet states to solvent molecules resulting in the formation of long-lived solvent-separated radical ion pairs was obtained. The yield of this process in chlorinated solvents is considerably (about 10 times) higher than in CS2. Further investigations of solvent influence on the efficiency of electron transfer involving other carotenoids, solvents, and additional electron acceptors are in progress. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Grant DE-FG05-86ER13465. We thank Dr. Elli Hand for helpful discussions and Dr. Alexander Shubin for providing the ESR-1 simulation program. References and Notes (1) Frank, H. A.; Violette, C. A.; Trautman, J. K.; Shreve, A. P.; Owens, T. G.; Albrecht, A. C. Pure Appl. Chem. 1991, 63, 109. (2) Koyama, Y. J. Photochem. Photobiol., B 1991, 9, 265. (3) Bensasson, R. V.; Land, E. J.; Truscott, T. G. Excited States and Free Radicals in Biology and Medicine; Oxford University Press: Oxford, 1993. (4) Grant, J. L.; Krammer, V. J.; Ding, R.-S.; Kispert, L. D. J. Am. Chem. Soc. 1988, 110, 2151. (5) Telfer, A.; De Las Rivas, J.; Barber, J. Biochim. Biophys. Acta 1991, 1060, 106. (6) (a) Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J.-C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A.; Lehman, W. R.; Nemeth, G. A.; Moore, A. L. Nature 1984, 307, 630. (b) Land, E. J.; Lexa, D.; Bensasson, R. V.; Gust, D.; Moore, T. A.; Moore, A. L.; Liddell, P. A.; Nemeth, G. A. J. Phys. Chem. 1987, 91, 4831. (c) Kuciauskas, D.; Liddell, P. A.; Hung, S. C.; Lin, S.; Stone, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. B 1997, 101, 429. (7) Osuka, A.; Yamada, H.; Shinoda, S.; Nozaki, K.; Ohno, T. Chem. Phys. Lett. 1995, 238, 37. (8) Liddell, P. A.; Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1997, 119, 1400. (9) Osuka, A.; Yamada, H.; Maruyama, K.; Mataga, N.; Asahi, T.; Ohkouchi, M.; Okada, T.; Yamazaki, I.; Nishimura, Y. J. Am. Chem. Soc. 1993, 115, 9439. (10) Imahori, H.; Cardoso, S.; Tatman, D.; Lin, S.; MacPherson, A. N.; Noss, L.; Seely, G. R.; Sereno, L.; Chessa de Silber, J.; Moore, T. A.; Moore, A. L.; Gust, D. Photochem. Photobiol. 1995, 62, 1009. (11) Shreve, A. P.; Trautman, J. K.; Owens, T. G.; Albrecht, A. C. Chem. Phys. Lett. 1991, 178, 89. (12) (a) Wasielewski, M. R.; Kispert, L. D. Chem. Phys. Lett. 1986, 128, 238. (b) Wasielewski, M. R.; Liddell, P. A.; Barrett, D.; Moore, T. A.; Gust, D. Nature 1986, 322, 570. (13) O’Neil, M. P.; Wasielewski, M. R.; Khaled, M. M.; Kispert, L. D. J. Chem. Phys. 1991, 95, 7212. (14) Berman, A.; Izrael, E. S.; Levanon, H.; Wang, B.; Sessler, J. L. J. Am. Chem. Soc. 1995, 117, 8252. (15) (a) Jeevarajan, A. S.; Khaled, M; Forbes, M. D. E.; Kispert, L. D. Z. Phys. Chem. 1993, 82, 51. (b) Jeevarajan, A. S.; Kispert, L. D.; Avdievich, N. I.; Forbes, M. D. E. J. Phys. Chem. 1996, 100, 669. (16) Harbour, J. R.; Tollin, G. Photochem. Photobiol. 1974, 19, 147. (17) Fiedler, D. A.; Koppenol, M.; Bond, A. M. J. Electrochem. Soc. 1995, 142, 862. (18) Jeevarajan, A. S.; Khaled, M.; Kispert, L. D. J. Phys. Chem. 1994, 98, 7777. (19) Alger, R. S.; Anderson, T. H.; Webb, L. A. J. Chem. Phys. 1959, 30, 695.
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