J. Phys. Chem. B 1999, 103, 9279-9284
9279
Transient Photochemistry of Diflunisal: Photoejection and Trapping of Hydrated Electrons Leading to the Formation of Phenoxy Radicals, Photostimulated Defluorination, and Cross Combination Reaction Salvatore Sortino,*,†,‡ J. C. Scaiano,† and Giuseppe Condorelli‡ Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie, Ottawa, Ontario K1 N 6N5 Canada, and Dipartimento di Scienze Chimiche, UniVersita’ di Catania, Viale Andrea Doria 8, 95125, Catania, Italy ReceiVed: March 30, 1999; In Final Form: June 19, 1999
The transient photochemistry of diflunisal has been investigated in aqueous solution. The photoreactivity of the compound is quite unusual for fluorinated derivatives. The loss of fluoride does not occur from an excited state, as expected, but is photostimulated through a reaction involving trapping by the ground state of electrons photoejected from diflunisal. Mono- and biphotonic pathways are involved in the photoionization process. A phenoxy radical generated after photoionization has been characterized as the main transient species involved in the photodegradation. A cross combination reaction between phenoxy and aryl radicals is controlled by the persistent radical effect and is responsible for the formation of the stable photoproduct.
Introduction
SCHEME 1
Drug photochemistry is a topic which has received much recent attention.1-3 Beyond the concern for both drug phototostability and phototoxicity, this subject offers opportunities to study new mechanisms. Recent time-resolved and steadystate studies on antiinflammatory,4-11 vasoregulator,12,13 and antibacterial fluoroquinolones14-19 drugs have provided very interesting examples of photochemical and photophysical deactivation pathways. Ketoprofen4-7 and tolmetin8 involve ground- and excited-state carbanions as key intermediates in the photodecomposition. Suprofen and tiaprofenic acid9,10 are characterized by a photochemistry which involves the participation of thermally populated upper triplet states. A bichromophoric photochemistry involving energy and electron-transfer processes is characteristic of nabumetone11 and naphazoline.12,13 The processes involved in the fluoroquinolone antibacterials photochemistry represent further examples of interesting photochemical mechanism. Photodefluorination reactions involving aromatic carbene carbocation intermediates have been observed for lomefloxacin, enoxacin, norfloxacin, and ofloxacin,14-16 whereas an unusual photodecarboxylation for a fluoroquinolonelike chromophore is characteristic of rufloxacin photodegradation.17 Finally, due to the presence of two different protonation sites, many fluoroquinolones are present in their cationic, zwitterionic, and anionic forms. In light of this, pH-dependent photochemistry and photophysic have been pointed out for enoxacin18 and norfloxacin.19 Diflunisal (DF), 2′,4′-difluoro-4-hydroxy-[1,1′-biphenyl]-3carboxylic acid, is a nonsteroidal antiinflammatory drug belonging to the salycilates class and known to induce photohemolysis of red blood cells under UVA irradiation.20 A study concerning its steady-state photochemistry20 has pointed out that under UVA irradiation in neutral buffer solution, DF (present in its carboxylate form) undergoes photodefluorination leading to the formation of 2′-(2′′′,4′′′-difluoro-3′′-carboxy-[1′′,1′′′-biphenyl]4′′-oxy)-4′-fluoro-4-hydroxy-[1,1′-biphenyl]-3-carboxylic acid † ‡
University of Ottawa. Universita’ di Catania.
(PhP) as the main photoproduct in the absence of oxygen (Scheme 1). This compound was found to be the main species responsible for the red blod cell photohemolysis. The mechanism which has been proposed to account for the formation of PhP involves a photodefluorination which does not occur from an excited state of the molecule but from a radical anion formed after an electron transfer between an excited and a ground-state molecule.20 Such a photodefluorination pathway is quite anomalous for aryl halides, which often dehalogenate immediately following excitation. Nevertheless, no studies on the transient intermediates produced after excitation of DF have been performed to support this mechanism. We have employed mainly laser flash photolysis techniques with the goal of characterizing the transient species involved in the photodegradation of DF, thereby contributing to the general understanding of the solution photochemistry of aryl halides as well as to the molecular basis of the phototoxic effects induced by drugs. Experimental Section Diflunisal (molecular weight 250.2) was purchased from Sigma Chemical Company (St. Louis, MO). Sorbic acid, 4-OHTEMPO (4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy, free radical), mannitol, 2-propanol, tributylgermanium hydride, tetraethylammonium bromide, tetraethylammonium chloride, and
10.1021/jp9910892 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/03/1999
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Figure 1. Absorption (s) and fluorescence (- - -) spectra of DF in phosphate buffer solution, 10-2 M, pH 7.4. Inset: fluorescence decay monitored in the region 400-440 nm.
di-tert-butyl peroxide were purchased from Aldrich. Di-tertbutyl peroxide was treated in an alumina column before use. Water was purified through a Millipore Milli-Q system. Phosphate buffer 10-2 M pH 7.4 was prepared from reagent grade products. The pH of solutions was measured with a glass electrode. Acetonitrile, EM Science, was used as received. Steady-state absorption spectra were obtained using a Cary 1E spectrophotometer. Luminescence spectra were recorded using a Perkin-Elmer LS 50 instrument. Quinine sulfate in 1 N H2SO4 was employed as a standard for the fluorescence quantum yield determination. All the transient spectra and kinetics were recorded by employing a flow system with a 7 × 7 mm2 Suprasil quartz cell with a 2 mL capacity and were purged in a storage tank with N2 for 30 min before as well as during the acquisition. The same procedure was used to prepare either oxygen or N2Osaturated solutions. A similar laser flash photolysis system has been previously described.21,22 Briefly, the samples were excited with a Lumonics EX-530 laser with Xe/HCl/Ne mixtures generating pulses at λexc ) 308 nm of ∼6 ns and e60 mJ/pulse. The experiments with λexc ) 355 nm were performed by employing the third harmonic of a Surelite ND/YAG laser generating pulses of 8 ns duration and ∼25 mJ output energy. The signals from the monochromator/photomultiplier system were initially captured by a Tektronix 2440 digitizer and transferred to a PowerMacintosh computer that controlled the experiment with software developed in the LabView 3.1.1 environment from National Instruments. Fluorescence lifetimes were obtained in the single-photon counting mode by exciting with the fourth harmonic (i.e., 266 nm) of a pulsed picosecond Nd:YAG laser and detecting the emission with a Hamamatsu C-4334 streak camera. Results and Discussion Figure 1 shows the absorption and the fluorescence spectra of DF recorded in buffer solution at pH 7.4. The absorption spectrum is characterized by three main bands centered at 210, 252, and 306 nm and by two shoulders around 220 and 278 nm. The fluorescence spectra exhibits an intense band centered at 420 nm. The fluorescence quantum yield was 0.10, similar to the value of 0.13 reported for biphenyl in polar solvents.23 A large Stokes shift observed between the absorption and emission maxima indicates a large difference between the geometry of the ground and the excited state. This shift was ca. 20 nm bigger than that observed for 4-biphenylcarboxylic acid, suggesting that
Sortino et al.
Figure 2. Transient absorption spectra observed in a DF 1.5 × 10-4 M solution in phosphate buffer, 10-2 M, pH 7.4, upon 308 nm laser excitation, 0.10 µs after the pulse: (O) N2-saturated solution; (b) N2Osaturated solution.
Figure 3. Transient absorption spectra observed in a DF 1.5 × 10-4 M N2-saturated solution in phosphate buffer 10-2 M pH 7.4 upon 308 nm laser excitation: (b) 0.7, (O) 3.1, (9) 6.7, and (0) 17.0 µs after the pulse. The inset shows a decay trace recorded at 380 nm.
the Stokes shift of DF cannot be rationalized simply on the basis of the planarity change of the two phenyl rings. The additional contribution can be attributed to a shift of the proton in the hydrogen bond toward the carbonyl oxygen (Scheme 2). This behavior is typical of salicylic acid derivatives.24,25 The fluorescence lifetime decay is monoexponential and characterized by a value of 3.3 ns (Inset, Figure 1). The transient absorption spectra obtained after 308 nm laser excitation of DF in N2- and N2O-saturated buffer solution at pH 7.4 and recorded 0.1 µs after the laser pulse are reported in Figure 2. The spectrum taken in N2 is characterized by a shoulder around 350 nm, a maximum at 380 nm, and a long absorption extending beyond 800 nm. The virtually complete elimination of the transient absorbance in the 450-800 nm region in the presence of either of N2O or 2-chloroethanol as electron scavengers26,27 leads us to conclude that a photoionization process involving the formation of hydrated electrons (e-aq)28 occurs in the DF photolysis. The time evolution of transient absorbance changes for a nitrogen-saturated solution of DF are reported in Figure 3. It should be noted that after the decay of the 380 nm band was complete two clear bands centered at 350 and 540 nm appeared. The decay at 380 nm (inset Figure 3) was monoexponential with a rate constant of k1 ) 1.6 × 105 s-1. This transient is quenched by oxygen, sorbic acid, and 4-OH-TEMPO with bimolecular rate constants of 4.6 × 109, 2.5 × 109, and 1.9 × 109 M-1 s-1, respectively. On the basis of the effect of these quenchers as well as the position of the absorption band,29 the
Transient Photochemistry of Diflunisal
J. Phys. Chem. B, Vol. 103, No. 43, 1999 9281
SCHEME 2
Figure 4. Kinetic traces relative to a DF 1.5 × 10-4 M N2-saturated solution in phosphate buffer 10-2 M pH 7.4, upon 308 nm laser excitation monitored at (O) 350 and (b) 540 nm.
380 nm transient can be safely assigned to the lowest excited triplet state of DF. The 350 and 540 nm transients are very long lived, and their decay is well described by a second-order fit (Figure 4) with rate constants of k1/1 ) 1.5 × 105 cm s-1 and k2/2 ) 4.2 × 105 cm s-1, respectively. Inasmuch as the normalization of the kinetic traces leads to the same value of the second-order rate constant, we can safely conclude that the 350 and 530 nm bands can be attributed to the same transient species. On the basis of both the usual photoprocesses involved in the photochemistry of arylhalides and on the loss of fluorine observed in steady-state experiments,20 it is conceivable that the 350-540 nm transient could be a carbocation-carbene originating from a fast heterolytic photodefluorination (homolysis of the C-F bond is not likely given that the bond dissociation energy C-F ≈ 125 kcal/mol, whereas the DF excited singlet lays at ca. 82 kcal/mol). The lack of quenching by oxygen, as well as of several nucleophiles such as OH- (up to 10-2 M), tetraethylammonium bromide, and tetraethylammonium chroride
(up to 0.1 M), led us to discard the hypothesis of the carbocation carbene. Further, the formation of the this species would not account for the main stable photoproduct formation found in the steady-state experiments.20 With the aim to determine both the nature of the photoionization process and if the 350-540 nm transient formation is related to or independent of the electron photoejection, we performed a laser intensity dependence of the initial transient absorbance at 720, 350, and 540 nm. Since the absorbance at 350 and 540 nm is strongly affected by the absorption of the triplet on short time scales, we decided to examine the laser intensity effect at these wavelengths 17 µs after the laser pulse. At this delay time, the decay of the triplet is almost complete and the decay at both 350 and 540 is negligible. Figure 5a shows that the data, relative to all the examined wavelengths, are fitted well by a second-order polynomial. This suggests that the multiphotonic photoprocesses are involved in the formation of the hydrated electrons as well as the 350-530 nm transient. In our case, the laser intensity dependence of the transients investigated is complex; however, the data reported in Figure 5b show that, in the limit of low pulse energies, the relation30
∆A/E ) a + bE holds, E being the laser intensity, a a coefficient depending on the quantum yield of the one-photon process, and b a factor depending on the extintion coefficient and yield of the intermediate steps of the consecutive two-photon process.31 The obtained results suggest that mono- and biphotonic pathways are involved in the formation of both hydrated electrons and the 350-540 nm transient and that these two species may be related. From the intercept of the plot relative to the hydrated electrons and by using K4Fe(CN)6 (Φe-aq ) 0.1 at 308 nm)32 as a standard, a one-photon quantum yield of 0.012 for e-aq production from DF was obtained.
9282 J. Phys. Chem. B, Vol. 103, No. 43, 1999
Sortino et al.
Figure 6. Transient absorption spectrum taken 30 µs after 355 nm laser excitation of a 2.5 M di-tert-butyl peroxide N2-saturated solution in CH3CN in the presence of 0.015 M DF. The inset shows the buildup recorded at 540 nm.
Figure 5. Laser power dependence of the (a) absorbance changes and of the (b) ratio between the absorbance changes and the laser intensity observed in a DF N2-saturated solution in phosphate buffer 10-2 M pH 7.4 upon 308 nm laser excitation at (b) 720 nm, 0.1 µs after pulse; (O) 380 nm, 17.0 µs after pulse; (9) 540 nm, 17.0 µs after pulse.
between the observed transient and those of the p-phenylphenoxy radical. The latter is, in fact, characterized by absorption maxima at 502 and 540 nm.44 A key experiment which demonstrated our hypothesis was performed using the di-tertbutyl peroxide as a source of tert-butoxy radicals by using 355 nm (in order to avoid a direct absorption of DF) laser excitation in 1:1 acetonitrile/di-tert-butyl peroxide. In the presence of a hydrogen donor such us the phenol group of DF, the tert-butoxy radical formed abstracts a hydrogen atom, competing with the pseudo-first-order decay of tert-butoxy radical by either reaction with solvent or β-cleavage,44 i.e., hν
t-BuOO-t-Bu 98 2-t-BuO• k2
Taking into account the molecular structure of DF, we can identify in the carboxylate and in the phenolic moiety the two main sites where the photoionization could be originating from. Photoionization of carboxylate4,5,33-35 as well as phenolic groups36-40 is, in fact, not an uncommon process. We believe that the photoionization takes place from the phenolic moiety of DF. A possible photoionization occurring from the carboxylate group would lead to the formation of an aryloxyl radical, which is expected to lose carbon dioxide.4,5,41-43 No evidence for any photodecarboxylation reactions concerning the DF photochemistry have been reported.20 On the other hand, the nature of the singlet excited state of DF suggests that the proton transfer from the phenol to the carbonylic oxygen (Scheme 2) could play a key role in the photoionization. The formation of the acid form of the carboxylate and the consequent basic form of the phenol should increase the ionization potential of the carboxylic and consequently decrease the ionization potential of the phenol. Moreover, we think that this proton transfer could favor an interaction of the phenolic oxygen with the solvent. The structure of the solvent around the OH groups is known to play a major role in the phenol photoionization process. The formation of the H-bonded supramolecular structures in which the OH group binds two water molecules by acting as both H-donor and H-acceptor is believed to play a key step in the photoionization of phenols.36-40 This structure could be favored by the aformentioned proton transfer. On the basis of these findings and by assuming that the photoionzation is the only photoprocess occurring, it would be reasonable to assign the 350-540 nm transient to the phenoxy radical formed after electron ejection from the phenol moiety. This assignment is supported by the similar spectral features
t-BuO• 98 first-order decay k3
t-BuO• + ArOH 98 ArO• + t-BuOH Figure 6 shows the spectrum obtained obtained after 355 nm laser excitation of di-tert-butyl peroxide in the presence of DF 0.015 M and taken 30 µs after the laser pulse. Even though it was impossible to detect any absorption below 360 nm, owing to the huge absorption of the ground state under the condition of the experiment, and the intensity of the signals was low, the shape of the spectrum as well the position of the maximum in the visible region resemble what is observed upon direct excitation of DF (spectra at 17 µs in Figure 3). This result confirms the assignment of the 350-540 nm transient to the phenoxy radical generated after photoionization. Experiments carried out by using different concentrations of DF supported well this hypothesis. The buildup of the signal due to the phenoxy radical (inset Figure 6) followed pseudo-first-order kinetics according to the relation
kgrowth ) k2 + k3[DF] A bimolecular quenching constant of 2.3 × 106 M-1 s-1 was obtained. Unfortunately, due to experimental reasons, we could not improve the quality of the signal and the consequent accuracy on the determination of the quenching constant. Nevertheless, the obtained value is of the same order of magnitude expected for this reaction. On the basis of literature data,44 the usual bimolecular constants for hydrogen abstraction from phenols in non-hydrogen-bonding solvents are in the range 108-109 M-1 s-1. The reactivity of the phenolic OH group is markedly reduced when it is involved in hydrogen bond with
Transient Photochemistry of Diflunisal the solvent. In these cases, the bimolecular rate constants drop to the order of 106 M-1 s-1. In our case, it is reasonable to suppose that the smaller quenching constant observed is due to an intramolecular hydrogen bond involving the phenol group and the carbonyl of the close carboxyl group (in the solvent used the carboxyl moiety is not dissociated). While hydrogen bonding could also involve the acetonitrile used as a solvent, it is likely that the intramolecular bond dominates. According to its chemical properties, this phenoxy radical was unreactive toward hydrogen donors such us mannitol, 2-propanol, and tributhyl germanium hydride. Moreover, from the intercepts of the plots relative to the laser energy effect performed at 720 and 540 nm, we can estimate an extinction coefficient of the phenoxy radical at 540 nm by taking into account that the extinction coefficient of the hydrated electrons at 720 nm is 19 000 M-1 cm-1.28 The ratio between the intercept relative to the 720 and 540 nm plot is ca 5, a value of 3800 M-1 cm-1 at 540 is therefore obtained for the extinction coefficient of the phenoxy radical. This value seems reasonable for such an intermediate. A value of ca 3000 M-1 cm-1 is in fact reported in the literature for the p-phenylphenoxy radical at the same wavelength.44 By analyzing the decay of the hydrated electrons at 720 nm, we observed that the kinetics are strongly influenced by the concentration of ground state DF. We found that this transient species is efficiently trapped with a rate constant of 7 × 109 M-1 s-1. This high value seems consistent with a reaction which involves a trapping of the hydrated electrons by the aromatic ring bearing the two fluorine atoms. Due to the presence of the two electronegative substituents, this ring should bear a partial positive charge. As a consequence, it seems to be the most suitable site for the scavenging of electrons and subsequent formation of a radical anion. Such a mechanism accounts well for the observed defluorination of DF according to literature data.20 In fact, in the stimulated SRN1 reactions of arylhalides, the substitutions occurs via a mechanism in which the key chemical step is the formation of a radical anion formed by electron transfer from an electron donor to the aryl halide followed by fast dehalogenation.45 In our case, the fast defluorination process would lead to the formation of an aryl radical (Scheme 2), which is expected to be transparent in the range of wavelengths monitored. The obtained results are in good agreement with the quantitative data concerning both the photodegradation and the fluorescence quantum yield of the starting compound. The photodegradation quantum yield obtained with steadystate irradiation was 0.02.20 This value is almost 2 times larger than the quantum yield of electron photoejection relative to the monophotonic contribution (Φe-aq ) 0.012). Given that this process is the only observable by irratiation with UV lamp, it is easy to conclude that the two values fit nicely. The higher value of the photodegradation quantum yield is in fact due the photostimulated defluorination involving the reaction between the a hydrated electron and a molecule in the ground state, leading to the degradation of both an excited molecule and a ground-state molecule. Moreover, a comparison between the fluorescence quantum yields of biphenyl (Φf ) 0.13) and DF (Φf ) 0.10) account well for the obtained value of the monophotonic photoionization quantum yield, suggesting that the electron photoejection could be the only photochemical reaction occurring from the excited singlet state. For an hypothetical more efficient photoreaction, a smaller value of the fluorescence quantum yield would have been expected.
J. Phys. Chem. B, Vol. 103, No. 43, 1999 9283 In light of this, the second-order decay observed for the phenoxy radical is in agreement with the our interpretation of the results. This decay can be attributed to a bimolecular reaction involving the combination of the phenoxy radical formed after photoionization and the aryl radical formed after defluorination (Scheme 2). This reaction accounts well for the formation of the main stable photoproduct observed in steady-state experiments.20 In systems involving a reactive and an unreactive radical, the almost exclusive formation of the cross combination product is quite common and can be explained as a result of the Fischer-Ingold persistent radical effect.46 Conclusions Our results demonstrate that the main process involved in the diflunisal photochemistry is photoionization. This reaction occurs through a complex mechanism involving a mixture of mono- and biphotonic pathways. The nature of the singlet excited state of DF involves a hydrogen transfer and plays an key role in the photoionization process. The main intermediate that originates from electron photoejection has been identified as a phenoxy radical absorbing at 350 and 540 nm. This species decays with a second-order decay. The hydrated electrons are efficiently trapped by the ground state of DF, and this process is believed to be responsible for a photostimulated defluorination reaction leading to the formation of an aryl radical. A cross combination reaction involving phenoxy and aryl radicals is controlled by the persistent-radical effect and is the pathway leading to the formation of the stable photoproduct. Acknowledgment. Financial support from MURST: Cofinanziamento di Programmi di Ricerca Di Rilevante Interesse Nazionale, and the Natural Sciences and Engineering Research Council of Canada is acknowledged. References and Notes (1) De Guidi, G.; Giuffrida, S.; Miano, P.; Condorelli, G. In Modern Topics in Photochemistry and Photobiology; Vargas, F., Ed.; Trivandrum: India, 1997; p 65. (2) Bosca´, F.; Miranda, M. A. J. Photochem. Photobiol. B: Biol. 1998, 43, 1. (3) Symposium in print on Drug: Photochemistry and Phototoxicity. Photochem. Photobiol. 1998, 68, 633. (4) Monti, S.; Sortino, S.; De Guidi, G.; Marconi, G.. J. Chem. Soc., Faraday Trans. 1997, 93, 2269. (5) Martinez, L. J.; Scaiano, J. C. J. Am. Chem. Soc. 1997, 119, 11066. (6) Monti, S.; Sortino, S.; De Guidi, G.; Marconi, G. New J. Chem. 1998, 22, 599. (7) Cosa, G.; Martinez, L.; Scaiano, J. C. Submitted. (8) Sortino, S.; Scaiano, J. C. Photochem. Photobiol. 1999, 69, 167. (9) Sortino, S.; De Guidi, G.; Marconi, G.; Monti, S. Photochem. Photobiol. 1998, 67, 603. (10) Encinas; S.; Miranda, M. A.; Marconi, G.; Monti, S. Photochem. Photobiol. 1998, 68, 633. (11) Martinez, L. J.; Scaiano, J. C. Photochem. Photobiol. 1998, 68, 646. (12) Sortino, S.; Scaiano, J. C. Photochem. Photobiol., in press. (13) Sortino, S.; Cosa, G.; Scaiano, J. C. Submitted. (14) Fasani, E.; Mella, M.; Caccia, D.; Fagnoni, M.; Albini, A. Chem Commun. 1997, 1329. (15) Martinez, L. J.; Li, G.; Chignell, C. F. Photochem. Photobiol. 1997, 65, 599. (16) Fasani, E.; Profumo, A.; Albini, A. Photochem. Photobiol. 1998, 68, 666. (17) Condorelli, G.; De Guidi, G.; Giuffrida, S.; Sortino, S.; Chillemi, R.; Sciuto, S. In press. (18) Sortino, S.; De Guidi, G.; Giuffrida, S.; Monti, S.; Velardita, A. Photochem. Photobiol. 1998, 67, 167.
9284 J. Phys. Chem. B, Vol. 103, No. 43, 1999 (19) Bliski, P.; Martinez, L. J.; Koker, E. B.; Chignell, C. F. Photochem. Photobiol. 1996, 64, 496. (20) De Guidi, G.; Chillemi, R.; Giuffrida, S.; Condorelli, G.; Cambria Fama´, M. J. Photochem. Photobiol. B: Biol. 1991, 10, 221. (21) Scaiano, J. C. J. Am. Chem. Soc. 1980, 102, 7747. (22) Scaiano, J. C.; Tanner, M.; Weir, D. J. Am. Chem. Soc. 1985, 107, 4396. (23) Wintgens, V. In Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 1, p 405. (24) Weller, A. Z. Electrochem. 1956, 60, 1144. (25) Kovi, P. J.; Miller, C. L.; Schulman, S. G. Anal. Chim. Acta 1972, 61, 7. (26) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 516. (27) Steenken, S.; Warren, C. J.; Gilbert, B. C. J. Chem. Soc., Perkin Trans. 2 1990, 335. (28) Hug, G. L. Optical Spectra of Non-Metallic Inorganic Transient Species in Aqueous Solution; Natl. Stand. Ref. Data Ser. 69; National Bureau of Standards: Washington, 1981. (29) Carmichael, I.; Hug, G. L. In Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 1, p 369. (30) Lachish, U.; Shaffermann, A.; Stein, G. J. Chem. Phys. 1976, 64, 4205. (31) Grabner, G.; Getoff, N.; Gantchev, Ts.; Angelov, D.; Shopova, M. Photochem. Photobiol. 1991, 54, 673.
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