Photooxidative degradation of QTX (a thioxanthone derivative)

J. Braz. Chem. Soc., Vol. 22, No. 2, 217-222, 2011. Printed in Brazil - ©2011 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Article Photooxidative Degradation of QTX (a Thioxanthone Derivative) Miguel G. Neumann,*,a Carla C. Schmitt,a Alessandra L. Poli,a José Carlos Netto-Ferreirab and Josy A. Osajimaa Instituto de Química de São Carlos, Universidade de São Paulo, CP 780, 13560-970 São Carlos-SP, Brazil a Departamento de Química, Instituto de Ciências Exatas, Universidade Federal Rural do Rio de Janeiro, 23851-970 Seropédica-RJ, Brazil b Foi estudada a degradação fotooxidativa de QTX (cloreto de 2-hidroxi-3-(3,4-dimetil-9-oxo9H-tioxanton-2-iloxi)-N,N,N-trimetil-1-propano) em solução aquosa. Os produtos resultantes foram caracterizados por HPLC/ESI-MS e a fotodegradação do corante foi acompanhada por espectroscopia de absorção. O corante apresenta duas bandas em 273 e 402 nm, que decrescem durante a irradiação, enquanto que aparece uma banda na região de 310-350 nm, típica do QTX oxidado. Novos picos devidos aos fotoprodutos da degradação de QTX foram observados por cromatografia e identificados por espectrometria de massa. Os picos em m/z 388, 420 e 270 correspondem a fotoprodutos, e o de m/z 372 é atribuído ao QTX. O pico de m/z 388 corresponde à formação de um grupo sulfóxido e o de m/z 420 sugere a formação de uma sulfona com uma hidroxila no grupo metila. O pico de m/z 270 sugere que o oxigênio reage com QTX abstraindo um hidrogênio do carbono ligado à hidroxila, simultaneamente à eliminação de um grupo amino. The photooxidative degradation of QTX (2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthen2-yloxy)-N,N,N-trimethyl-1-propanium chloride) has been investigated and photoproducts from QTX degradation in aqueous solution were characterized using HPLC/ESI-MS. The photooxidation of the dye was monitored by electronic absorption spectroscopy. The spectrum of the dye shows bands at 273 and 402 nm. During irradiation, the intensity of these bands decreases together with the appearance of a band at 310-340 nm, typical of oxidized QTX. The chromatograms show the presence of new peaks due to photoproducts of QTX degradation that were identified by mass spectroscopy. The peaks at m/z 388, 420 and 270 were attributed to photoproducts, whereas the peak at m/z 372 is due to QTX. The peak at m/z 388 corresponds to the formation of a sulfoxide group and that at m/z 420 suggested the formation of a sulfone with a hydroxyl on the methyl group. The peak at m/z 270 suggests that oxygen reacts with QTX abstracting a hydrogen atom from the carbon bonded the hydroxyl, together with the release of the amino group. Keywords: substituted thioxanthones, photodegradation, QTX, photopolymerization, photosensitizers, photoinitiators Introduction Compounds based on thioxanthones are among the most used photoinitiators for free radical polymerization.1,2 The photophysics and the interaction of the excited states of substituted thioxanthone were studied to predict their behaviour as photoinitiators in photopolymerization systems.3 The triplet-triplet and transient absorption of non-substituted and substituted thioxanthones were studied in different solvents and it was found that the amount of *e-mail: triplet species and radicals depended on the solvent as well as on the substituents on the aromatic ring.4 It is well known that the activity as photoinitiators of these compounds is based on radical formation through the interaction of the thioxanthone (TX) triplet excited state with tertiary amines.3,5 QTX is a useful photoinitiator of polymerization in aqueous solution and can be used in various applications, as for example, in dental resins.6 Various organic dyes are used as photoinitiators for photopolymerization as well as sensitizers for the photodegradation of polymers.7-9 In a previous work, thioxanthone was used as photosensitizer for the degradation A 218 Photooxidative Degradation of QTX (a Thioxanthone Derivative) of poly(alkyl methacrylate) films. The polymer degradation mechanism involved the initial formation of macroradicals originated by the abstraction of hydrogen from the polymer chain by the triplet state of thioxanthone.10 However, there are few reports about the oxidation of thioxanthones themselves.11 The irradiation of organic dyes leads to reactive electronic excited states. In the presence of oxygen, these transients will react to form reactive oxygen species, that initiate the photodegradation chain mechanism which may involve the participation of singlet oxygen and superoxide anion as the most reactive oxygen species.12 Recently, photocatalytic and photolytic degradation of various dye solutions have been suggested in the literature.13-15 The photooxidative degradation of QTX (2-hydroxy-3(3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy)-N,N,Ntrimethyl-1-propanium chloride, Scheme 1) in aqueous solution was studied in this work, and the degradation photoproducts were analyzed and characterized by HPLC/ESI-MS. Scheme 1. QTX. Experimental To determine the molar extinction coefficient for ground state QTX, spectra were collected in the concentration range of 3.0 × 10-5 to 1.0 × 10-4 mol L-1, from which a e-value of 4.77 × 103 L mol-1 cm-1 at 402 nm, in aqueous solution, was obtained. Sample preparation Solutions containing QTX 3 × 10-5, 5 × 10-5, 7 × 10-5 and 1 × 10-4 mol L-1 were prepared in Milli Q water. The solutions were placed in Pyrex test tubes and the photodegradation process took place in an irradiation chamber using 16 6-W fluorescent lamps (see emission spectra of lamp together with absorption of QTX in Figure 1). The temperature was kept at 25 °C to avoid thermal degradation. For the identification and quantification of the generated products, J. Braz. Chem. Soc. Figure 1. Emission spectrum of the lamp and absorption spectrum of QTX in aqueous solution. aliquots were withdrawn from the reaction flasks at different times and analysed. High performance liquid chromatography (HPLC) After 160 h of irradiation, the QTX samples were preconcentrated by vacuum evaporation in a Savant/Speedvac system. Degradation products were determined injecting 20 mL samples in a Waters 2695 HPLC system with a Waters 2996 photodiode array detector. A C-18 3.5 mm 4.6 × 75 mm column (Symmetry) was used at 30 °C, using methanol:water (70:30) as the mobile phase at a flow rate of 0.5 mL min-1. The HPLC system was coupled to an ESI-MS (electrospray ionization mass spectrometer, Waters ZQ 2000). The operating conditions were optimised in order to achieve maximum sensitivity values: i.e., capillary voltage of 3 kV, the source temperature 100 °C, the desolvation temperature 350 °C, and desolvation gas flow 350 L h-1. Results and Discussion Figure 2A shows the spectra of an irradiated QTX 1.0 × 10-4 mol L-1 aqueous solution. Before irradiation the dye presented bands at 273 and 402 nm. There is an initial increase of the absorption of these bands up to the first 10 h wh (...truncated)