Photosensitized Reactions of Psoralen and Herbicides Revealed by the Pump-Probe Method BRYANTSEVA Natalya G.1,a, TCHAIKOVSKAYA Olga N.1,b*, SULTIMOVA Natalya B.2,c, SVETLICHNYI Valery A.1,3,d, LAPIN Ivan N.1,3,e and KRAIUKHINA Vlada S.1,f 1
Tomsk State University, 36, Lenin Ave., Tomsk 634050, Russia
2
N. M. Emanuel Institute of Biochemical Physics RAS, 4, Kosygina St., Moscow 119334, Russia 3
Siberian Physical Technical Institute, 1, Novosobornaya Sq., Tomsk 634050, Russia
a
[email protected], *
[email protected],
[email protected],
[email protected], e
[email protected],
[email protected]
Keywords: 8-MOP, herbicide, pump-probe.
Abstract. Formation of triplet states in aqueous herbicide solutions excited by nanosecond radiation of 4th harmonic of Nd:YAG laser (wavelength 266 nm)is revealed by the pump-probe method. During photosensitized 8-MOP oxidation of herbicides, formation of triplet states of the photosensitizer and of the hydrated electron is observed in the process of two-step photoionization of 8-MOP. Introduction Herbicides are substances destroying undesirable plants. Herbicidal activity of substances is caused by their ability to penetrate into those or other parts of a plant, to mix inside of it, to influence on the processes of vital activity of the plant, and also to be subjected to metabolism under the influence of enzymes or other substances contained in the plant and soil with formation of less (or more) toxic products. For soil herbicides, their adsorption and desorption, motion in soil and washing away from it, decomposition under the influence of moisture, light, and soil microflora, as well as the ability to persist for a long time in soil are important [1–3]. Among numerous herbicides used in the world today, 2,4-dichlorophenoxyacetic acid (DCPA or 2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA) occupy the leading positions on the scale of their production and application [4–6]. In addition to phenoxyacetic herbicides, hormonal herbicides are actively used to control broadleaf weeds in wheat, oats, and barley. For example, a(4-chloro-2-methylphenoxy)propanoic acid (Mecoprop) and 3,6-dichloro-2-methoxybenzoic acid (Dicamba) have a system action, are adsorbed on leaves, and move to roots [3]. As is well known, strong herbicides are very badly decomposed and persist long. They are washed away by rain from soil, and fall into natural water reservoirs [3, 4]. Purification of water and soil by the conventional methods (for example, chlorination and/or UV purification) usually does not lead to complete decomposition of phenoxyacetic herbicides, and is accompanied by the formation of even more toxic products. In this regard, the photosensitized purification methods are very attractive. As a photosensitizer, well-known 8-methoxypsoralen (8-MOP) is often used [7]. Photochemistry of 8MOP has already been studied in ample detail; therefore, the compound has long and successfully been used for treatment of skin diseases, for example, psoriasis, and in photodynamic therapy [7, 8]. The present work is aimed at investigation of direct and photosensitized 8-MOP oxidation by 2,4dichlorophenoxyacetic, 2-methyl-4-chlorophenoxyacetic, a-(4-chloro-2-methylphenoxy)propionic, and 3,6-dichloro-2-methoxybenzoic acids in aqueous solutions by the method of laser photolysis. Chemically pure (99%) objects of research (Aldrich Co.) were investigated, including 2,4-dichlorophenoxyacetic (DCPA), 2-methyl-4-chlorophenoxyacetic (MCPA), a-(4-chloro-2metylphenoxy)propionic (MEC), and 3,6-dichloro-2-methoxybenzoic (DIC) acids.
Material and methods To investigate the photostability of the examined solutions, unfocused monochromatic pulsed radiation (6 ns, 15 Hz) of the 3rd (355 nm, 20 mJ) and 4th harmonics (266 nm, 20 mJ) of a solid-state Nd:YAG laser (LS-2132UTF, LOTIS TII) was used. Solutions were put into a quartz cell and were illuminated with different exposure times. The deposited energy (absorbed by the sample) was estimated as the difference between incident radiation and radiation transmitted through the cell with the examined sample. Energies (average radiation powers) before and after the cell with the sample were measured with an Ophir 12A-P power meter equipped with a calorimetric head and a Nova II display. The transient absorption spectra were investigated by the pump-probe method on the setup with a fluorescent probe [9] excited by radiation of the 3rd and 4th harmonics of the same pulsed Nd:YAG laser. The block diagram of the pump-probe setup [9] used in this work is shown in Fig. 1. Probing radiation was performed both simultaneously with pumping (variant 1) and with a 30 ns delay (variant 2). Such experimental 1 configuration allowed us to separate, if necessary, short-living (singlet-singlet with lifetime of 10-8 s and less) and long-living 4 H (triplet-triplet, long-living radicals, intermediate photoproducts, etc. with M2 K1 lifetime longer than 10-8 s). The fluorescent L2 2 probe from a mixture of well radiative and photostable organic dyes provided 5 broadband spontaneous radiation in the L3 3 spectral range 370–800 nm (analog of K2 “white” light in pure solvents with picoM1 and femtosecond pumping). L1 L4 The 4th harmonic of the Nd:YAG laser was used for direct excitation of the examined herbicides. Excitation by the 3rd PD harmonic of the Nd:YAG was used to populate excited herbicides states through Figure 1 – Block diagram of the pump-probe the corresponding states of a more longsetup comprising rotating mirrors М1 and М2; wavelength compound – substituted 8long-focus spherical and cylindrical lenses L1 methoxypsoralen. and L2, respectively; cell C1 with a fluorescent Phototransformations of samples were probe (solution of a mixture of specially investigated on a СМ2203 selected organic dyes), collimators L3 and L4, spectrofluorimeter (Closed Joint-Stock cell C2 with the examined sample, and Company SOLAR, Belarus) based on spectrometer PD based on CDD array with changes in the absorption spectra fiber optical input, compared with the spectra of the initial 1 – Nd:YAG laser (266/355 nm), 2 – Pump of compounds. a fluorescent probe, 3 – Pumping beam, 4 – Delay line (30 ns ~ 10 m), 5 – Probing beam
Results of investigations Under natural conditions herbicides being acids exist as anion species (рКа = 3 (DIC) [3], pKa = 3.68 (MEC), pKa = 2.64 (DCPA), and pKa = 3.3 (MCPA)). It is well known that the absorption spectra of neutral herbicide species coincide with the absorption spectra of their anion species and are characterized by the main absorption bands with maxima in the regions of 232 nm and 284 nm [3]. Upon laser illumination by light at 266 nm, the intensities of bands in the absorption spectra in the region of 250 nm and 310 nm increase (Fig. 2), which can be attributed to
Absorbance, rel. un.
the absorption of newly formed quinone and lactone photoproducts, respectively [10]. The newly formed photoproducts act as photoinductors for further phototransformations of herbicides rather than are accumulated in the solution [3]. The kinetics of herbicide destruction obeys the first-order law 8 2,5 with rate constants of 4.210–3 s–1 7 (MCPA), 7.710–3 s–1 (DCPA), 1.110–2 6 2,0 s–1 (MEC), and 1.710–4 s–1 (DIC), 5 respectively. 4 1,5 Fig. 3 shows the kinetics 3 of herbicide destruction with and 2 1,0 without 8-МОР depending on the 1 t irr exposure time upon exposure to light at 0,5 266 nm and 354 nm. As can be seen from figure 2, 0,0 200 250 300 350 400 450 herbicides do not absorb radiation at an , nm excitation wavelength of 355 nm; hence, upon excitation we activate only the sensitizer, and herbicides serve as Figure 2 – Electronic absorption spectra of aqueous –4 electron and-or hydrogen atom donors. DCPA solutions (510 mol/L) depending on concentrations of the exposure time by light at 266 nm: 1 – 0, 2 – 10 s, 3 – The photosensitizer and herbicide were in 20 s, 4 – 30 s, 5 – 1 min, 6 – 2 min, 7 – 4 min, 8 – the ratio 1:5. Figure 3b shows the 8 min destruction kinetics for the initial herbicides in the presence of 8-MOP versus the exposure time. Comparing the destruction kinetics of herbicides illuminated by light at 266 nm and 355 nm, we see that herbicides are actively phototransformed upon direct excitation rather than under the action of the photosensitizer. 1,00 C/C0
a) photosensitized photolysis
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Figure 3 – Kinetics of herbicide (510–4 mol/L) destruction at 230 nm registration in aqueous solutions with (a) and without 8-MOP 110 mol/L (b). Here curve 1 is for DIC, curve 2 is for DCPA, curve 3 is for MEC, and curve 4 is for MCPA As can be seen from Fig. 2, herbicides do not absorb radiation at an excitation wavelength of 355 nm; hence, upon excitation we activate only the sensitizer, and herbicides serve as electron and-or hydrogen atom donors. The concentrations of the photosensitizer and herbicide were in the ratio 1:5. Fig. 3b shows the destruction kinetics for the initial herbicides in the presence of 8-MOP versus the exposure time. Comparing the destruction kinetics of herbicides illuminated by light at 266 nm and 355 nm, we see that herbicides are actively phototransformed upon direct excitation rather than under the action of the photosensitizer.
The intermediate products formed during direct and photosensitized 8-MOP oxidation of aqueous herbicide solutions are analyzed in Fig. 4.
Figure 4 – Transient absorption spectra of intermediate DIC photolysis products without (1) and with delay by 30 ns (2) after laser pulse at λ = 266 nm (a) and λ = 355 nm (b). b – with 8-MOP It was obtained that upon excitation by light at 266 nm, intermediate products were formed that were effectively quenched by molecular oxygen and were characterized by the transient absorption spectrum with a maximum in the region of 450 nm (Fig. 4а) that can be attributed to the formation of triplet herbicide states. Upon excitation of the aqueous 8-MOP solution by light at 355 nm, intermediate products were formed with the absorption band in the region of 360–380 nm and the wide band from 600 to 800 nm (Fig. 4b). It is well known [10] that the triplet 8-MOP state is formed upon single-quantum excitation. It has the absorption band in the region of 370 nm. It seems likely that the intermediate product that absorbs in the long-wavelength range 600–800 nm can be attributed to the formation of a hydrated electron during two-step photoionization [10]. Analogous intermediate products were observed for herbicide photolysis in the presence of 8-MOP. Summary Thus, the method of lamp and laser photolysis has been used to study the photochemical transformations of some herbicides. It was revealed that toxic photoproducts were formed under long-term illumination of herbicides. By the pump-probe method it was demonstrated that upon excitation of aqueous herbicides solutions by light at 266 nm, triplet states were formed. During photosensitized 8-MOP oxidation of herbicides, triplet states were formed of the photosensitizer and of the hydrated electron that was formed in the process of two-step 8-MOP photoionization. Acknowledgement This work was carried out under the State Assignment of the Ministry of Education and Science of the Russian Federation (Project No. 1347) and was supported in part by the President of the Russian Federation under the Program of Support of Leading Scientific School (Project No. NSh512.2012.2). References [1] F. Ya. Rovinskii, ed., Unified Methods of Monitoring of Background Pollution of the Environment / Moscow: Gidrometeoizdat, 1986.
[2] N. P. Hill, A. E. McIntyre, R. Perry and J. N. Lester, Behaviour of chlorophenoxy herbicides during the activated sludge treatment of municipal waste water, Water Res. 20 (1986). 45–51. [3] P. Boule, L. Meunier, F. Bonnemoy, et al., Direct phototransformation of aromatic pesticides in aqueous solution, Int. J. Photoenergy. 4 (2002) 69–74. [4] N. B. Sultimova, P. P. Levin, O. N. Chaikovskaya, Laser photolysis study of the transient products of 4-carboxybenzophenone-sensitized photolysis of chlorophenoxyacetic acid-based herbicides in aqueous micellar solutions, High Energy Chem. (Khimiya Vysokikh Energii) 44 (2010) 393–398. [5] O. Tchaikovskaya, E. Karetnikova, I. Sokolova, et al., The phototransformation of 4-chloro-2methylphenoxyacetic acid under KrCl and XeBr excilamps irradiation in water: Biodegradability of phototreated solutions, J. Photochem. Photobiol. A: Chem. 228 (2012) 8–14. [6] O. N. Tchaikovskaya, E. A. Karetnikova, I. V. Sokolova, et al., Study of the effect of UV radiation on the decomposition of 4-chloro-2-methylphenoxyacetic acid, Russ. Phys. J. 56 (2013) 853–859. [7] J. Brownfield and S. Collins, The Luminescence Spectra of the 8-Methoxypsoralen ExcitedState Complexes and Photochemical Product in Argon, Methanol/Argon, and Water/Argon Matrices at 10 K, J. Phys. Chem. A. 104 (2000) 3759–3767. [8] R. V. Bensasson, E. J. Land, T. G. Truscott, Flash-Photolysis and Pulse Radiolysis: Contributions to the Chemistry of Biology and Medicine, Pergamon, 1983. [9] V. A. Svetlichnyi, A setup for investigation the absorption spectra of dyes in excited states by the pump-probe method utilizing a fluorescence probe, Instrum. Exp. Tech., 53 (2010) 575–580. [10] O. Tchaikovskaya, I. Sokolova, G. Mayer, et al., The role of UV-irradiation pretreatment on the degradation of 2,4-dichlorophenoxyacetic acid in water, Luminescence (Willey InterScience). 26 (2011) 156–161.
