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J. Phys. Chem. A 2011, 115, 280–285
Direct Photooxidation and Xanthene-Sensitized Oxidation of Naphthols: Quantum Yields and Mechanism Michael Oelgemo¨ller,† Jochen Mattay,*,‡ and Helmut Go¨rner§ School of Pharmacy and Molecular Sciences, James Cook UniVersity, TownsVille, QLD 4811, Australia, Organische Chemie I, Fakulta¨t fu¨r Chemie, UniVersita¨t Bielefeld, D-33501 Bielefeld, Germany, and Max-Planck-Institut fu¨r Bioanorganische Chemie, D-45413 Mu¨lheim an der Ruhr, Germany ReceiVed: September 16, 2010; ReVised Manuscript ReceiVed: NoVember 26, 2010
The photoinduced oxidation of 1-naphthol to 1,4-naphthoquinone and of 5-hydroxy-1-naphthol to 5-hydroxy1,4-naphthoquinone was studied by steady-state and time-resolved techniques. The direct photooxidation of naphthols in methanol or water takes place by reaction of the naphoxyl radical (•ONaph) with the superoxide ion radical (O2•-), the latter of which results from the reaction of the solvated electron with oxygen after photoionization. The sensitized oxidation takes place by energy transfer from the xanthene triplet state to oxygen. From the two oxygen atoms, which are consumed, one is incorporated into the naphthol molecule giving naphthoquinone and the second gives rise to water. The effects of eosin, erythrosin, and rose bengal in aqueous solution, pH, and the oxygen and naphthol concentrations were studied. The quantum yield of the photosensitized transformation was determined, which increases with the naphthol concentration and is largest at pH > 10. The quantum yield of oxygen uptake is similar. The pathway involving singlet molecular oxygen is suggested to operate for the three sensitizers. The alternative pathway via electron transfer from the naphthol to the xanthene triplet state and subsequent reaction of •ONaph with O2•-, the latter of which is formed by scavenging of the xanthene radical anion by oxygen, does also contribute. Introduction Singlet molecular oxygen, O2(1∆g), is a key intermediate in the photosensitized oxidation of biomolecules1 and has been frequently used to initiate photooxidation of phenols.2-14 In various studies, ketones,13 quinones,10 tris(2,2′-bipyridine)ruthenium salts or a Cr(phen)3 complex,8,9 methylene blue,11 porphyrins,11 riboflavin,14 eosin,3,4 and rose bengal5-7 served as sensitizers. The photoinduced key step is energy transfer from the triplet excited sensitizer to triplet oxygen. The related photooxidation of monohydroxy- and dihydroxy-substituted naphthalenes to naphthoquinones has been investigated and has found widespread applications.15-25 In particular, the photooxidation of 1-naphthol to 1,4-naphthoquinone (NQ) and of 5-hydroxy-1-naphthol (1,5-dihydroxynaphthalene) to 5-hydroxy1,4-naphthoquinone (5-HONQ: Juglone) has been intensively studied.17-25 Juglone serves as a valuable building block for the synthesis of biologically active quinonoid compounds. As sensitizers, eosin and rose bengal have frequently been employed. In a typical experiment a sensitizer (S) is excited and after intersystem crossing (isc) O2(1∆g) is formed from the xanthene triplet state (3*S), reaction 1 in Scheme 1. O2(1∆g) reacts with the naphthol molecule (HONaph), step 3, or with the solvent, step 2. Generally, the quantum yield of O2(1∆g) formation (Φ∆) is only slightly smaller than that of intersystem crossing (Φisc).26 The rate constant k3 of reaction (3) of singlet molecular oxygen with (hydroxy)naphthols as quenchers depends strongly on the applied conditions. For the 2-acetonaphthone/5-hydroxy-1-naphthol system in 1:1 dichloromethane/ acetonitrile k3 ) 0.7 × 107 M-1 s-1 has been reported.19 The * To whom correspondence should be addressed. † James Cook University. ‡ Universita¨t Bielefeld. § Max-Planck-Institut fu¨r Bioanorganische Chemie.
quantum yield (ΦS) of the photosensitized transformation of 1-naphthol to parent NQ in air-saturated aqueous solution is known to increase strongly with increasing pH.19,21 ΦS is likewise much larger for the phenolate.6 These effects are mainly due to the much larger rate constant k3 for the phenolate/napththolate.19-21 The photophysical and photochemical properties of phenols27-32 and 1- and 2-naphthol33-36 in organic solvents and aqueous solution have been intensively examined. The photooxidation of phenol in water has been studied by electron paramagnetic resonance and nuclear spin polarization.27 The direct photooxidation of 1-naphthol and derivatives to NQs, that is, overall eq 1, has been studied by Richard and co-workers.34,35 The quantum yield (ΦQ) of the direct photochemical transformation of 1-naphthol to parent NQ in deaerated and oxygen-saturated aqueous solution is ΦQ ) 0.003 and 0.03, respectively.35 Naphthalenes are also generators of O2(1∆g), but Φisc of naphthols (HONaph) is relatively low.19,34-36
HONaph + O2 + hV f NQ + H2O
(1)
Triplet quenching by oxygen can also yield the hydroperoxyl/ superoxide ion radicals (HO2•/O2•-), the properties of which are well documented,37,38 but its role in the photooxidation of naphthols has been considered to be minor.15-21 5-Hydroxy-1-naphthol readily gives 4-20% of 5-HONQ when directly irradiated in the presence of oxygen.24 Higher yields have been obtained in acetone when UV-irradiated in a quartz vessel.22,24 No time-resolved measurements are available for acetone, in contrast to the benzophenone case.39 Electron transfer rather than triplet energy transfer has been reported to be operating, based on laser flash photolysis studies of the interaction between triplet benzophenone and the phenolate or naphtholate anion.39-41 The photoreaction of thioxanthone with
10.1021/jp108832x 2011 American Chemical Society Published on Web 12/17/2010
Photooxidation and Oxidation of Naphthols
J. Phys. Chem. A, Vol. 115, No. 3, 2011 281
SCHEME 1
phenolic derivatives of biological relevance has been studied using the magnetic field effect.41 To account for the role of acetone as solvent and/or sensitizer, several mechanisms can be considered. One is energy transfer from the sensitizer to oxygen, thereby generating O2(1∆g), Scheme 1. Another pathway is electron transfer, reactions (4 + 5 or 5′ in Scheme 2), thereby free radicals are expected to be involved. In principle, the reactive oxygen could be generated by electron transfer.42-44 The photooxygenation generally refers to electron or H-atom transfer to the sensitizer (type I) and energy transfer (type II), yielding HO2•/O2•- and O2(1∆g), respectively.1 The reactive oxygen species in the ketone-sensitized conversion to NQs is open to question. In this study, we aim at a better understanding of the mechanism of photooxidation of 1-naphthol and 1,5-dihydroxynaphthalene. The quantum yields in the absence ΦQ and presence of a sensitizer ΦS were measured under various conditions. For the latter purpose, we employed the three most common xanthene dyes: eosin, erythrosin, and rose bengal, where Φisc and Φ∆ are 0.5-0.8.1,26,42-44 The reactive oxygen species in the direct photoconversion of naphthols to NQs is suggested to be O2•-. Electron transfer from HONaph to the xanthene triplet state 3*S in oxygen- or air-saturated aqueous methanol could be proposed to initiate a subsequent reaction of the naphthoxyl radical with O2•-. This has to be excluded for the xanthene-sensitized oxidation of phenols in solution. Energy transfer from the 3*S state to oxygen may successfully compete and has to be considered as alternative pathway for the present cases. The following questions were addressed: (i) Which intermediates can be characterized by flash photolysis of the two naphthols? (ii) What is the mechanism of sensitized photooxidation? (iii) Which are the major factors influencing the quantum yields ΦQ and ΦS? Furthermore, we would like to understand (iv) the role of pH in water on the xanthenesensitized oxidation processes. Materials and Methods The compounds (EGA, Sigma) and solvents (Merck, Fluka) were checked for impurities and used as received except for 5-hydroxy-1-naphthol which was purified by repeated sublimation. Water was from a Millipore Milli-Q system. The absorption spectra were monitored on a UV/vis diode array spectrophotometer (HP, 8453). The molar absorption coefficient of 5-hydroxy-1-naphthol is ε320 ) 3600 M-1 cm-1.22-24 For 1-naphthol ε300 ) 5000 M-1 cm-1 was taken and for NQ ε360 ) 5000 M-1 cm-1. For photoconversion λirr ) 313 nm was used either from a 1000 W Hg-Xe lamp and a monochromator or a 250 W Hg lamp and a suitable filter. The slope of the linear part of Aobs as a function of irradiation time is a measure of the quantum yield ΦQ. As reference for λirr ) 308/313 nm, ΦQ ) 0.03 for 1-naphthol in oxygen-saturated aqueous solution was taken.35 For actinometry with λirr ) 500-560 nm, a system containing rose bengal and 0.2 mM 5-hydroxy-1-naphthol in air-saturated methanol was taken: ΦS ) 0.003.18 In the case of air saturation, a homogeneous solution was achieved by stirring.
The oxygen uptake was measured using a Clark electrode cell.43 The oxygen concentration (under air atmosphere) was found to remain essentially constant in water, 0.27 mM, when the light was removed. The oxygen concentration decreases with irradiation time and the slope of the initial linear dependence can be taken as relative quantum yield of the transformation of the naphthol to NQ. The sensitized oxidation was measured analogously. The absolute quantum yields for direct (Φ′Q) and sensitized oxidation (Φ′S) were obtained by calibration. The flash photolysis was done using an excimer laser (Lambda Physik, EMG 200, pulse width of 20 ns and energy
