J. Phys. Chem. 1996, 100, 9403-9406
9403
Control of a Photoreaction by ESR Transition of the Intermediate Radical Pair As Evaluated by Liquid Chromatography Masaharu Okazaki,* Yoshifumi Tanimoto,† Yoshinari Konishi, and Kazumi Toriyama National Industrial Research Institute of Nagoya, Hirate, Kita, Nagoya 462, Japan, and Department of Chemistry, Faculty of Science, Hiroshima UniVersity, Higashihiroshima 724, Japan ReceiVed: February 12, 1996; In Final Form: March 21, 1996X
The photochemical reduction of xanthone (XO; X ) C13H8O) by xanthene (XH2) in a sodium dodecyl sulfate micelle was controlled by a resonant microwave pulse in a magnetic field. The product yields were measured by high performance liquid chromatography. At a magnetic field of about 335 mT, a microwave pulse (9380 MHz; 1.0 kW) applied immediately after the laser pulse for 6.0 µs decreased the escape products, XOHXOH and XH-XH, to less than one-third of those without the microwave pulse. A microwave pulse much later than the laser pulse or off-resonant to the radical pair had no effect on the product yield. On the other hand, a reverse effect of the microwave pulse was observed on the yield of the cage product, XOH-XH. The yields as a function of the magnetic field correspond to the ESR spectrum of the intermediate radical pair.
1. Introduction It is well-known that a chemical reaction involving a “radical pair” as an intermediate species sometimes depends on the external magnetic field.1-4 The fate of the radical pair is dependent on its spin state: recombination occurs from only the singlet state whereas dissociation occurs from both the singlet and the triplet states. At a high magnetic field the T(1 states are energetically separated due to the Zeeman interaction and are isolated from the kinetics of the rest (T0 and S). On the other hand, at a field less than or of the same order of the hyperfine couplings (hfc), which cause mixing among these spin states, the radical pair in any triplet sublevel will gain singlet character in a short period which increases radical recombination. If the radical pair is initially produced in the triplet state, the yield of recombination reaction of the radical pair (cage product) decreases whereas that from the dissociated radicals (escape products) increases with an increase in the magnetic field strength. This is the “radical pair model” for the magneticfield dependence of chemical reactions.1-4 If the system is irradiated with a microwave field which induces the electron spin resonance (ESR) of the radical pair, a transition between the spin states of T(1 and the rest, T0 and S, is invoked even under a high magnetic field. In fact, the ESR spectra of the intermediate radical pairs have been obtained in those reactions by taking advantage of this microwave effect.5 For example, the ESR spectrum of an ion pair produced by laser, X-ray, or other radiations can be detected by monitoring the fluorescence6-10 that is emitted from an excited molecule, produced by charge recombination, upon returning to the original ground state. Nuclear spin polarization is observed for the reaction product when an ESR transition of the intermediate radical pair is induced.11 In these experiments, however, the observed quantities are not directly related to the yield of the final reaction product, and besides, the quantity modified by the ESR transition is much smaller than that detectable by the product analysis. In previous papers,12 we showed that the product yield of a “spin trapping” reaction13 in the photoreduction of a quinone in a sodium dodecyl sulfate (SDS) micelle is † X
Hiroshima University. Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00424-8 CCC: $12.00
modified by the ESR transition of the radical pair. Although we had to introduce an additional reaction step, spin trapping, as a “probe reaction”, this experiment was successful in changing the final product of a chemical reaction by the ESR transition of the intermediate radical pair. The present paper reports the first experiment in which a chemical reaction is controlled by ESR without addition of any probe reactions. High performance liquid chromatography (HPLC) was used to analyze the reaction products. The reaction system is the photoreduction of xanthone by xanthene in a SDS micelle, whose magnetic field effect was reported by Tanimoto et al.14 The reaction proceeds as in Scheme 1, where chemical structures of the products are also given. Xanthone (XO) is promoted to a singlet excited state upon absorbing a photon, which is then converted to the lowest excited triplet state. The triplet xanthone abstracts a hydrogen atom from a hydrogen donor, xanthene (XH2) in the present case, to produce a triplet radical pair (XOH‚‚XH). In the SDS micelle, the lifetime of the radical pair is lengthened15,16 and a large part of it reacts to produce a coupling product (XOH-XH) Via triplet-singlet conversion. The two unpaired electrons in the triplet radical pair do not make a chemical bond due to the spin conservation rule. Thus it dissociates as “escaped radicals”, which may react with other escaped radicals of a different origin. Therefore, XOH-XH is produced as both a cage product and an escaped product but xanthene dimer (XH-XH) as well as hydroxyxanthene dimer (XOH-XOH) are produced only as escape products. 2. Experimental Section Xanthene and xanthone of guaranteed grade were recrystallized from ethanol. SDS was from Nakarai Chemicals (Kyoto) as the purest grade and was used as supplied. Concentrations of xanthene and xanthone in a 0.4 M aqueous SDS solution were 3.0 and 1.2 mM, respectively. After deaeration with Argas bubbling, the sample solution was flowed into a flat cell in an ESR cavity (Q ) 1600) at a rate of 0.5 mL/min, where it was irradiated with a laser pulse (λ ) 355 nm, 20 mJ/pulse; 10 Hz).17 The microwave field (9380 MHz, max 1.0 kW) was applied immediately after the laser pulse for 6.0 µs. The duty cycle of the microwave pulse was kept low at 0.006% to avoid © 1996 American Chemical Society
9404 J. Phys. Chem., Vol. 100, No. 22, 1996
Okazaki et al.
SCHEME 1
heating of the sample. The sample temperature was kept at 303 K by flowing N2 gas through the ESR cavity. The product solution was frozen immediately by liquid N2 and was thawed a few minutes before chromatographic analysis, which was made by using a HPLC unit (Waters 302) equipped with a diode array detector (Waters 906) and a reversed phase column (Millipore, Pursil C-18). A mixture of methanol (90%) and water (10%) was employed as the eluent. Peak assignment was made by comparing the chromatogram (two dimensional) with those for authentic samples, which were synthesized independently.14 Concentrations of the products were determined from the peak heights in the chromatogram by comparing with those for the 1.0 mM solutions of standard authentic samples. The microwave field taken from an ESR unit (JEOL RE1X) was pulsed with a PIN switch and amplified with a solid state (Avantek APT 10566) and a TWT amplifier (Keltec Florida, XR-640). The sequence for the laser and the microwave pulses was generated with a pulse programmer (Iwatsu SY8220).
Figure 1. Experimental setup (A) and HPLC of the reaction products (B) for the photoreduction of xanthone by xanthene in a SDS micellar solution. The reactant solution flows through a quartz flat cell in an ESR cavity, where it is irradiated with a laser pulse and a microwave pulse under the magnetic field. The microwave field was applied immediately after the laser pulse for 6.0 µs. The sample solution was collected at the end of the flow apparatus. The chromatograms for the reaction products at the magnetic fields of residual (ca. 2 mT), 300, and 335 mT are obtained from the absorbance at λ ) 210 nm. Peaks a-e correspond to XOH-XOH, XO, XOH-XH, XH2, and XH-XH. Here XO and XH2 represent xanthone and xanthene, respectively; thus X represents C13H8O. The asterisk indicates that the peak is an unidentified signal which is dependent on neither the magnetic nor the microwave fields.
3. Results Figure 1 shows the experimental setup (A) and HPLC diagrams for the photoreduction of xanthone by xanthene in a SDS micellar solution (B). The sample solution flowing continuously in the ESR cavity was irradiated with both a laser pulse and an X-band microwave field simultaneously under the magnetic field.17 The product solution was collected at the end of the flow system and kept frozen until the analysis. Peaks a-e in the chromatograms (B) were assigned to XOH-XOH, XO, XOH-XH, XH2, and XH-XH, respectively, by comparing the retention times and the optical absorption spectra with those of authentic samples. A small peak around 6.0 min labeled with an asterisk is an unidentified peak which is dependent on neither the magnetic nor the microwave fields. With an increase of the magnetic field from the residual value (ca. 2 mT) to 300 mT, the yields of XOH-XOH (peak a) and XH-XH (peak e) increase considerably. On the other hand, the coupling product (XOH-XH) decreases by about 15% (see Figure 3). At 335 mT, which is the resonance field for an electron spin with the g factor of 2.00, those yields return almost to the original values at the residual field. To confirm that these changes around 335 mT are due to the ESR of the intermediate radical pair, we obtained the chromatograms for the reaction products at various magnetic fields. Figure 2 shows the yields of the main products: XOH-XH (as cage and escape product), XH-XH and XOH-XOH (escape products) as functions of the magnetic field for the reaction in
Figure 2. Reaction yields of the main products: XOH-XH (9, cage product), XH-XH (O, escape product), and XOH-XOH (b, escape products); and the sum of these (]) as functions of the external magnetic field. The reaction system was irradiated with an X-band microwave at the output power of 1.0 kW immediately after the laser pulse for 6.0 µs.
the presence of an X-band microwave field at 1.0 kW. For a radical pair initially in the triplet state, the radical pair model predicts that the cage product decreases while the escaped product increases with an increase of the magnetic field. Therefore, classification of the products either to cage product or to escape product is made straightforwardly from the magnetic field effects. It is noticed that (1) the yield as well as its magnetic field dependence for XH-XH is almost the same
Control of a Photoreaction by ESR
Figure 3. Reaction yield of XH-XH (xanthene dimer) as functions of the external magnetic field in the presence of an X-band microwave. (Upper) full range of the magnetic field; (lower) expanded view around 335 mT. Open and closed circles represent the results of two consecutive runs at the microwave power of 1.0 kW; diamonds in the lower diagram represent the results at a microwave power of 35 W.
as those for XOH-XOH; (2) the microwave effect for XOHXH is about 2 times as large as those for XH-XH and XOHXOH; and (3) the sum of these yields shown with diamonds in the figure remains constant throughout the magnetic field range. Figure 3 shows the magnetic field dependence of the yield of XH-XH in the presence of an X-band microwave field. The upper diagram shows the results displayed in the full magneticfield range up to 360 mT, and the lower gives those expanded with respect to the abscissa around 335 mT. Open and closed circles represent the results for two consecutive runs at a microwave power of 1.0 kW, and diamonds in the lower diagram show those at 35 W. From the residual field to 300 mT the magnetic field dependence is similar to those observed previously for the photoreaction of carbonyl compounds in micellar solutions.18-20 The dip observed at around 335 mT was reproducible and the cause of this can be attributed to the ESR transition of the intermediate radical pair. The width of the dip (at half-height) is about 5 mT at a microwave power of 35 W and is about 20 mT at 1.0 kW. An increase in the spectral width with an increase in the microwave power is a characteristic of the ESR spectrum.21 We call these curves “product-yielddetected ESR” (PYESR) spectra. This term has been used for the ESR spectrum detected by the yield of a spin-trapping reaction in similar reaction systems.16,21 4. Discussion Magnetic and Microwave Effects on the Reaction Yields. It is noticed that the yields and their magnetic and microwave effects of the two escape products are approximately the same and the magnetic and the microwave modulation are about onehalf of those of XOH-XH. If the diffusion behaviors of the two escaped radicals, XOH‚ and XH‚, are equal and the steric factors of the coupling reactions are the same for the three combinations, the yield of XH-XH is equal to that of XOHXOH and one-half of the yield of XOH-XH as the escape product. Since the consumption of the starting materials was almost constant (Figure 2), a decrease in the escape products should be compensated by an increase in the cage product, and Vice Versa. Therefore, the fact that the sum of the yield of XOH-XH, produced as both a cage product and an escape
J. Phys. Chem., Vol. 100, No. 22, 1996 9405 product and those of XH-XH and XOH-XOH appear independent of the magnetic field indicates that XOH-XH is the only cage product. For this stoichiometry, the magnetic field effect for XOH-XH should be deconvoluted into two curves: the magnetic and microwave dependence for XOH-XH as the cage product and that for XOH-XH as the escape product. These two changes are opposite to each other. A much lower yield of the escaped products than that of the cage product indicates that the micelle confines the radical pair very efficiently.21,22 Compared with the high-quality data for XH-XH as can be seen in Figure 3, the yields of both XOH-XOH and XOHXH fluctuate considerably. We consider that the relative instability of the latter two compounds may be related with the quality of the data. Several experimental operations are considered which may cause fluctuation in the liquid chromatography data, for example, freezing after reaction and thawing it before HPLC analysis in the presence of atmospheric oxygen. In addition, a small part of XOH-XOH is formed by the photoreduction of XO by SDS. On the other hand, XH-XH is chemically stable and is formed as the escape product only from the radical pair of (XOH‚‚XH). Product-Yield-Detected ESR Spectrum. It should be noticed that in the lower spectrum of Figure 3 a small peak appears at the center of the broad dip obtained with the highest microwave power of 1.0 kW. We assign the cause of this peak to spin locking of the intermediate radical pair, which was originally observed in the fluorescence-detected ESR experiment in an irradiated system23 and was also confirmed in the PYESR experiment by using the spin trapping technique.21 When B1 (microwave field) is much larger than the internal magnetic fields, the radical pair stays in the original spin state for a relatively long time. This phenomenon is called “spin locking” of radical pair. This is because the two electron spins rotate along B1 in the rotating frame while keeping their relative phases. On the other hand when B1 is much smaller, the microwave becomes resonant to only one of the two spins and changes the spin state from the triplet to the singlet and Vice Versa.24 Thus, this microwave effect may be called “spin inversion”. The locking peak in Figure 3 is rather small compared with that observed in the PYESR spectrum for the photoreaction of anthraquinone in a perdeuterated SDS micelle by employing the spin trapping method.21 One reason for this smaller spin-locking effect may be that the hfc of xanthene radical (>1.2 mT) is considerably larger than the hfc’s of the perdeuterated SDS radical (0.38 mT). Since spin locking is only possible for a short period, long life of the radical pair may be another reason. In the present case no spin trap is added in the system and thus the lifetime of the radical pair is considerably longer than that in the previous study.22 For the case of the radical pair composed of anthrasemiquinone and the SDS radical in the photoreduction of anthraquinone in a SDS micellar solution,16,22 the observed lifetime of 2.0 µs would have been lengthened to more than 3.3 µs if the spin trap were not added. Since hydroxyxanthene radical is less soluble than SDS radical in the aqueous phase, the lifetime of the radical pair in the present case would still be longer than 3.3 µs.25 5. Concluding Remark In the present study we showed that a photoreaction can be controlled on a synthetic scale by the ESR transition of the intermediate radical pair without adding any probe reactants, e.g., spin traps, to the reaction system.12,16,21,22 This technique can be readily applied to any reaction which involves a radical pair having a lifetime of several hundreds of nanoseconds or
9406 J. Phys. Chem., Vol. 100, No. 22, 1996 more and a small exchange interaction between the two spins. Lengthening of the lifetime of the radical pair is possible by using a micelle or other medium which provides a nanometersized environment. If we apply a much higher microwave power to a radical pair with a shorter lifetime, spin locking of the radical pair becomes easier. In this case, efficient reaction control becomes possible by a “spin manipulation technique”: 26 one reaction path is selected from the two, the escape process and the cage process, by either “spin locking” or “spin inversion” of the spin pair. References and Notes (1) Turro, N. J.; Kraeutler, B. Acc. Chem. Res. 1980, 13, 369. (2) Salikhov, K. M.; Molin, Yu. N.; Sagdeev, R. Z.; Buchachenko, A. L. Spin Polarization and Magnetic Effect in Radical Reactions; Elsevier: Amsterdam, 1984. (3) Hayashi, H. In Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1990; Vol. 1, pp 59-136. (4) Steiner, E. U.; Wolff, H.-J. In Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1991; Vol. 4, pp 1-130. (5) Lersch, W.; Michel-Beyerle, M. E. In AdVanced EPR; Hoff, A. J., Ed.; Elsevier: Amsterdam, 1989; Chapter 19. (6) Cavenett, B. C. AdV. Phys. 1981, 30, 475. (7) Frankevich, E. L.; Pristupa, A. I.; Lesin, V. I. Chem. Phys. Lett. 1977, 47, 304. (8) Molin, Y. N.; Anisimov, O. A.; Grigoryants, V. M.; Molchanov, V. K.; Salikhov, K. M. J. Phys. Chem. 1980, 84, 1853. (9) Smith, J. P.; Trifunac, A. D. J. Phys. Chem. 1981, 85, 1645.
Okazaki et al. (10) McLauchlan, K. A.; Nattrass, S. R. Mol. Phys. 1988, 65, 1483. (11) Bagryanskaya, E. G.; Grishin, Yu. A.; Avdievitch, N. I.; Sagdeev, R. Z.; Molin, Yu. N. Chem. Phys. Lett. 1986, 128, 162. (12) Okazaki, M.; Shiga, T. Nature 1986, 323, 240. Okazaki, M.; Sakata, S.; Konaka, R.; Shiga, T. J. Chem. Phys. 1987, 86, 6792. (13) Janzen, E. G. Acc. Chem. Soc. 1971, 4, 31. (14) Tanimoto, Y.; Takashima, M.; Itoh, M. J. Phys. Chem. 1984, 88, 6053. (15) Turro, N. J.; Kraeutler, B. J. Am. Chem. Soc. 1980, 100, 7432. (16) Polyakov, N. E.; Konishi, Y.; Okazaki, M.; Toriyama, K. J. Phys. Chem. 1994, 98, 10563. (17) Okazaki, M.; Konishi, Y.; Toriyama, K. Chem. Lett. 1994, 737. (18) Scaiano, J. C.; Abuin, E. B.; Stewart, L. C. J. Am. Chem. Soc. 1982, 104, 5673. (19) Turro, N. J.; Weed, G. C. J. Am. Chem. Soc. 1983, 105, 1861. (20) Okazaki, M.; Toriyama, K.; Nunome, K.; Muto, H.; Shiga, T. Can. J. Chem. 1988, 66, 1832. (21) Okazaki, M.; Toriyama, K. J. Phys. Chem. 1995, 99, 17244. (22) Okazaki, M.; Polyakov, N. E.; Konishi, Y.; Toriyama, K. Appl. Magn. Reson. 1994, 7, 149. (23) Koptyug, A. V.; Saik, V. O.; Anisimov, O. A.; Molin, Yu. N. Chem. Phys. 1989, 138, 173. (24) The spin eigenfunctions of the radical pair with hyperfine couplings larger than the spin exchange interaction are |RR〉|Rβ〉|βR〉|ββ〉. A microwave field with an amplitude much less than the hyperfine interaction induces the ESR transition such as: |RR〉 f |Rβ〉 or |βR〉. These are expressed by the other set of eigen functions as: T1 f (T0 ( S)x2. (25) Konishi, Y.; Okazaki, M.; Toriyama, K. J. Phys. Chem. 1995, 99, 12540. (26) Okazaki, M.; Toriyama, K. J. Phys. Chem. 1995, 99, 489.
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