In the Laboratory
Photochemistry without Light: Oxidation of Rubrene in a Microemulsion with a Chemical Source of Singlet Molecular Oxygen (1O2, 1Dg) Véronique Nardello, Marie-Josée Marti, Christel Pierlot, and Jean-Marie Aubry* Equipe de Recherches “Oxydation et Formulation”, ENSCL, BP 108, F-59652 Villeneuve d’Ascq cedex, France; *
[email protected] In freshman chemistry courses, molecular oxygen is often used as an example to illustrate the validity of molecular orbital (MO) theory. According to Hund’s rule, the two π* electrons split, with parallel spins, into the two degenerate orbitals of the highest occupied molecular orbital (HOMO) (Fig. 1). Hence, MO theory predicts that ground state oxygen, 3 O2 ( 3 Σg{), is a triplet state which should be paramagnetic. This is demonstrated by the fact that liquid or gaseous oxygen is drawn toward a magnet (1). The instructor often explains that a simple rearrangement in the distribution of these two electrons within the degenerate π* orbitals leads to an excited electronic state. Thus, the lowestlying excited state is a singlet state, 1O2 ( 1∆g), with two electrons having opposite spins in the same orbital (2). However, the chemical consequences of the electronic structures of these two species are usually overlooked. Actually, 3O2 behaves as a diradical species with a strong tendency to react through monoelectronic processes. It reacts rapidly with free radicals and triplet excited states (2) but is relatively unreactive toward ground state organic molecules in spite of its high redox potential (E ° = 1.23 V). In contrast, 1O2 exhibits a reactivity diametrically opposed to that of ground state oxygen. This species is a powerful and highly selective bielectronic oxidant, which has found considerable synthetic utility (3). It can be generated very easily by photosensitization or by a variety of chemical reactions, since the chemiexcitation process is not very energy-demanding (22.53 kcal/mol). Finally, its lifetime is relatively long (45 min in vacuum and 4.4 µs in water) because its spontaneous decay to ground state oxygen is highly forbidden. The laboratory experiment reported here uses the catalytic system hydrogen peroxide–sodium molybdate to generate 1O2 chemically. This excited species is then used to oxidize a polycyclic aromatic hydrocarbon, rubrene 1, in a microheterogeneous medium. This experiment may be of interest for students at several levels. It can be used in a freshman course to illustrate the theory of molecular orbitals. It can also be used in a photochemistry course to demonstrate a chemiexcitation process and the reactivity of a short-lived excited species. Finally, it can be used in more advanced courses (such as colloid chemistry) to illustrate the formulation of a microemulsion and its ability to solubilize inorganic watersoluble reactants as well as hydrophobic organic substrates. Experimental Procedure
Reagents Sodium molybdate dihydrate (99%) and rubrene (5,6,11,12-tetraphenylnaphthacene, 98%) were purchased from Sigma Aldrich Chemie and were used as received. Sodium dodecylsulfate (SDS, 98%), 1-butanol (Normapur), meth-
2p σu*
2p πg* 2p
2p π u
2p
2p σg 2s σu * 2s
2s 2s σg
O
O2
O
Figure 1. The molecular orbital energy-level description for O2.
ylene chloride (Normapur) and hydrogen peroxide 30% (110 volumes, ≈ 10 M) were obtained from Prolabo. CAUTION: Safety goggles and gloves are required to protect the students from the burning effects of H2O2. Unused H2O2 must not be put back into the bottle or placed in the solvent waste container because impurities could catalyze its explosive disproportionation. Waste H2O2 should be diluted with a large excess of water and discarded down the sink. Rubrene is photosensitive and must be protected from light (aluminum foil).
Preparation of the Microemulsion These amounts are adequate for five experiments. The microemulsion is prepared at room temperature by adding an aqueous solution of 0.2 M Na2MoO4?2H2O (290.4 mg in 6 mL of water) dropwise to a magnetically stirred slurry of SDS (4.7 g), 1-butanol (9.4 g), and methylene chloride (60 mL). After a few minutes, the turbid suspension is converted into a mobile and transparent liquid. Rubrene Oxidation One-tenth gram (0.2 mmol) of rubrene 1 is introduced into a small Erlenmeyer flask followed by 15 mL of the above microemulsion. The medium is magnetically stirred for 10 min in dimmed light to prevent the autosensitized photooxidation of rubrene. Then, 100 µL of 30% H2O2 (1 mmol) is added to the red solution and the reaction medium is stirred at room temperature. The reaction is complete after about 45 min, as indicated by the fading of the solution to colorless. Monitoring of the Reaction by UV–Visible Spectroscopy The oxidation of rubrene 1 is monitored by visible spectroscopy at 522 nm by diluting 100 µL of the reaction
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In the Laboratory
medium into 10 mL of absolute ethanol. This solution is then tenfold diluted and analyzed by UV spectroscopy. The reference cell is filled with the microemulsion described above diluted in the same way. The formation of the endoperoxide 2 is characterized by the appearance of a growing peak at 246 nm with an isobestic point at 280 nm (see Fig. 3).
Treatment of the Reaction Medium and Isolation of the Oxidation Product The solvents (methylene chloride, water, and 1-butanol) are removed (40 °C) using a rotary evaporator. The pasty residue that remains after 30 min is stirred vigorously with 20 mL of methylene chloride. The resulting suspension is then filtered by suction through a sintered-glass funnel to recover the solid sodium molybdate and SDS. The filtrate is concentrated using a rotary evaporator at 20 °C. The colorless solid is washed successively with 5 mL of methanol and 5 mL of ether to remove traces of SDS and 1-butanol. The insoluble endoperoxide 2 is recovered either by centrifugation or by filtration. The colorless powder is then dried using a rotary evaporator (to eliminate traces of ether) giving 110.5 mg (98%) of pure endoperoxide 2 as a crystalline colorless solid. Evidence for the Formation of the Endoperoxide Within the framework of a teaching laboratory, thin layer chromatography (TLC) is a simple and striking method to confirm the formation of the endoperoxide 2 and its ability to give back rubrene by warming. Three spots are deposited on an aluminum sheet coated with fluorescent silica gel (type 60F254). The first one corresponds to the starting material, rubrene 1 (in solution in methylene chloride). The second spot is the oxidation product. The third is the red product obtained by briefly warming a small sample (5 mg) of the endoperoxide 2—contained in a small Pyrex tube—to melting, with a flame. The TLC plate is then dried with a hair-drier and eluted with a mixture of cyclohexane–methylene chloride (50:50). After elution, spots 1 and 3 appear red, with an R f of .85, corresponding to rubrene 1. Under an UV lamp (365 nm), these two spots exhibit an orange fluorescence. Spot 2, which corresponds to the endoperoxide 2 (R f = .44), appears dark. Other minor spots may appear, due either to the impurities contained in the starting rubrene or to other minor products resulting from the thermal decomposition of the endoperoxide (4 ). 13C
NMR Data for 2 NOTE: Only the main peaks of the tetracenic core are given. δ ppm/CDCl3: 84.7 (C-5 and C-12), 124.7 (C-1 and C-4), 126.1 (C-8 and C-9), 126.8 (C-7 and C-10), 134.6 (C-2 and C-3), 140.4 (C-6 and C-11). Results and Discussion
Chemical Sources of Singlet Oxygen 1 O2 is typically generated by photosensitization, which involves the irradiation of a solution containing a photosensitizer such as methylene blue or rose Bengal in the presence of oxygen (eq 1): 3
hν
1 O2 → O2 sensitizer
(1)
On the other hand, many chemical reactions can be used 1286
to generate 1O2 in the absence of light (5). The most celebrated one involves the oxidation of H2O2 by ClO{, which produces 1O2 quickly and quantitatively (eq 2) (6, 7 ): water
H2O2 + ClO{ → H2O + Cl{ + 1O2 (100%) (2) This chemical source of 1O2 has received little attention in organic synthesis because the high oxidizing power of ClO{ often leads to side reactions. This reaction, however, has found application as a generator of gaseous 1O2 required by the powerful chemical iodine lasers (8). These lasers are transportable and were studied as antimissile arms during the «Star Wars Program» initiated by former president of the United States Ronald Reagan. A milder chemical generator of 1O2 has recently been discovered. It involves the disproportionation of hydrogen peroxide catalyzed by molybdate ions (9–13). This reaction is very efficient, it occurs at room temperature, and all the oxygen evolved is in the singlet excited state (eq 3): MoO42{
2H2O2 → 2H2O + 1O2 (100%) water
(3)
Unfortunately, this reaction proceeds smoothly only in pure water, and therefore only organic substrates with significant hydrophilic character can be oxidized on a preparative scale with this system (14, 15). The standard approach to overcome this problem involves using a biphasic solvent system (water– immiscible organic solvent) maintained in emulsion by vigorous stirring (16 ). However, this approach is very inefficient because the short lifetime of 1O2 in water (4.4 µs) precludes its diffusing out of the aqueous phase into the organic solvent. Actually, the typical size of the aqueous droplets (>10 µm) is much larger than the mean travel distance of 1O2 (≈200 nm). Thus, most of the 1O2 is wasted through deactivation by water molecules before it can reach the organic phase, which contains the substrate.
Microemulsions ( 17–22) In contrast, microemulsions are quite suitable for sustaining the chemical formation of 1O2 and the oxidation of highly hydrophobic substrates, on the preparative scale, with a minimal loss of 1O2 (23). A microemulsion, which consists of water (W), organic solvent (O), surfactant, and, in most cases, cosurfactant, is defined as a transparent or translucent, thermodynamically stable isotropic dispersion of two immiscible liquids with microdomains of one or both liquids stabilized by an interfacial film of surface-active molecules (20). Depending on the proportions of the constituents, three main types of structures can be distinguished: reverse micelles (W/O), direct micelles (O/W), and bicontinuous structures. The W/O microemulsion used in the present work is described as roughly spherical water microdroplets coated by an interfacial film of SDS (white circle and broken line) and 1-butanol (gray circle and straight line) dispersed in a continuous phase of methylene chloride (Fig. 2). One important feature of these media is their ability to dissolve simultaneously considerable amounts of hydrophilic compounds confined in aqueous droplets and hydrophobic organic molecules localized in the continuous organic phase. Thus, hydrogen peroxide and sodium molybdate are compartmentalized in an aqueous microreactor, where they generate 1 O2 quantitatively (eq 3). Since 1O2 is a small and uncharged
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
In the Laboratory
to illustrate this type of reaction because it exhibits two useful properties: 1. It is red, whereas the endoperoxide 2 is colorless. The fading that occurs as 1 is oxidized to 2 can be easily followed by visible spectroscopy at 529 nm and the end of the reaction can be detected by the complete fading of the solution. 2. Rubrene traps 1O2 reversibly. In 1926, Dufraisse observed that the melting (280 °C) of the endoperoxide of rubrene, 2, led to the release of oxygen and the reappearance of the red color of rubrene (26). This regeneration of the starting substrate occurs in good yield (ca. 80%) and is convincing evidence for the formation of the endoperoxide (eq 4). Figure 2. Schematic representation of the microemulsion used to oxidize rubrene 1 to endoperoxide 2 using H2O 2/MoO42{ to generate singlet oxygen. At the interface, white and gray circles symbolize the hydrophilic heads (–SO4{, –OH) of SDS and butanol, respectively, whereas broken and straight lines represent the C12 and C4 hydrophobic chains, respectively. 1/1000 dilution
1/100 dilution
1.5
Absorbance
1.0
0.5
0.0 210
260
310
360
410
460
510
560
610
Wavelength / nm Figure 3. Evolution of the UV–vis spectra during the chemical oxidation of rubrene 1 with 1O2 generated from the system H2O2/MoO42{.
molecule, it can diffuse freely through the charged (coming from the anionic surfactant SDS) interfacial region. Moreover, the typical size of the microdroplets (ca. 10–50 nm in diameter) is much smaller than the mean travel distance of 1O2 in water (≈200 nm). Therefore, despite the short lifetime of 1O2 in water (4.4 µs), this species can diffuse, before deactivation, from the aqueous droplets to the methylene chloride phase, in which it can react with the substrate.
Oxidation of Rubrene The oxidizing properties of singlet oxygen, 1O2 (1∆g), are very different from those of the other activated forms of oxygen. It can actually oxidize selectively various electron-rich compounds such as olefins, phenols, sulfides or heterocycles (3, 24, 25). However, the most characteristic reaction is the cycloaddition [4 + 2] of 1O2 with conjugated cyclic dienes and polycyclic aromatic hydrocarbons. Rubrene 1 was chosen
(4)
The structure of the endoperoxide 2 can also be established using spectroscopic methods. Unfortunately, the IR and 1 H NMR spectra are intricate and provide little structural information. The 13C NMR spectrum is simpler and shows only 19 aromatic carbons on account of the symmetry of the endoperoxide. Among these peaks, the peak at 84.7 ppm is characteristic of carbons C-5 and C-12 bound to the peroxide bridge. The presence of only one peak in this area means that a fixation of oxygen atoms occurs, which keeps a plane of symmetry in the oxidation product (27 ). Rubrene 1 and its endoperoxide 2 can also easily be characterized by UV–vis spectroscopy at 522 nm (ε522 nm = 11000 L mol{1 cm{1) and 246 nm (ε246 nm = 67900 L mol{1 cm{1), respectively. This technique allows monitoring the disappearance of rubrene 1 in the visible range and the formation of the endoperoxide 2 in the UV range where the appearance of an isobestic point at 280 nm may be observed (Fig. 3). Conclusion In the ’70s and ’80s, microemulsions were studied primarily for use in assisted petroleum recovery (28). More recently, organic chemists have recognized their exceptional solubilizing properties. Microemulsions are able to simultaneously dissolve large amounts of purely ionic inorganic compounds such as sodium molybdate as well as typical organic compounds such as rubrene 1. The reaction described herein takes advantage of the microheterogeneity of these media by permitting the generation of an uncharged, short-lived species, singlet oxygen, in the aqueous droplets and its diffusion, before deactivation, into the organic continuous phase where it can react with the substrate. Literature Cited 1. Shimada, H.; Yasuoka, T.; Mitsuzawa, S. J. Chem. Educ. 1990, 67, 63. 2. Kasha, M. In Singlet Oxygen, Vol. I; Frimer, A. A. Ed.; CRC: Boca Raton, FL, 1985; pp 1–12. 3. Wasserman, H. H.; Ives, J. L. Tetrahedron 1981, 37, 1825. 4. Schmidt, R.; Brauer, H.-D. J. Photochem. 1986, 34, 1. 5. Aubry, J. M. In Membrane Lipid Oxidation, Vol. II; Vigo-Pelfrey,
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In the Laboratory C., Ed.; CRC: Boca Raton, FL, 1991; pp 65–102. 6. Held, A. M.; Halko, D. J.; Hurst, J. K. J. Am. Chem. Soc. 1978, 100, 5732. 7. Shakhashiri, B. Z.; Williams, L. G. J. Chem. Educ. 1976, 53, 358. 8. McDermott, W. E.; Pchelkin, N. R.; Bernard, D. J.; Bousek, R. R. Appl. Phys. Lett. 1978, 32, 469. 9. Aubry, J. M. J. Am. Chem. Soc. 1985, 107, 5844. 10. Aubry, J. M.; Cazin, B. Inorg. Chem. 1988, 27, 2013. 11. Böhme, K.; Brauer, H. D. Inorg. Chem. 1992, 31, 3468. 12. Niu, Q. J.; Foote, C. S. Inorg. Chem. 1992, 31, 3472. 13. Nardello, V.; Marko, J.; Vermeersch, G.; Aubry, J. M. Inorg. Chem. 1995, 34, 4950. 14. Nardello, V.; Bouttemy, S.; Aubry, J. M. J. Mol. Catal. 1997, 117, 439. 15. Aubry, J. M.; Cazin, B.; Duprat, F. J. Org. Chem. 1989, 54, 726. 16. McKeown, E.; Waters, W. A. J. Chem. Soc. B. 1966, 1040. 17. Shulman, J. H.; Stoeckenius, W.; Prince, L. M. J. Phys. Chem.
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1959, 63, 1677. 18. Friberg, S. E. J. Chem. Educ. 1979, 56, 553. 19. Fargues-Sakellariou, R.; Rivière, M.; Lattes, A. New J. Chem. 1985, 9(2), 95. 20. Friberg, S. E.; Bothorel, P. Microemulsions: Structure and Dynamics; CRC: Boca Raton, FL, 1987. 21. Schomäcker, R. J. Chem. Res., Miniprint 1991, 810. 22. Casado, J.; Izquierdo, C.; Fuentes, S. Moya, M. L. J. Chem. Educ. 1994, 71, 446. 23. Aubry, J. M.; Bouttemy, S. J. Am. Chem. Soc. 1997, 119, 5290. 24. Gollnick, K. Chim. Ind. 1982, 64, 156. 25. Prein, M.; Adam, W. Angew. Chem., Int. Ed. Engl. 1996, 35, 447. 26. Moureu, C.; Dufraisse, C.; Dean, P. M. C. R. Hebd. Seaces Acad. Sci. 1926, 182, 1440. 27. Gobert, F.; Altenburger-Combrisson, S.; Albouy, J. P. Org. Magn. Reson. 1979, 12, 202. 28. Healy, R. N.; Reed, R. L. Soc. Petr. Eng. J. 1974, 14, 491.
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