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Environmental and laboratory studies of the photodegradation of the pesticide fenarimol
1. Phorochem. Phcmbiol. A: Chem,
80 (1994) 4Op-416
Environmental and laboratory the pesticide fenarimol M. Concei$io
D.A.
Mate&,
Abflio
409
studies of the photodegradation
M. Silva”
and
Hugh
of
D. Burrowsb
“UCEH, Universidude do Algawe, 8000 Far0 (Portugal) bDepmtwnento de Quimiza, Univenidade de Coimbm, 3049 Coimbra (Portugal)
Abstract The photophysics and photochemistry of the pesticide fenarimol were studied. From the absorption and fluorescence spectra, quantum yields and lifetimes, it is suggested that the lowest excited singlet state has predominantly n,rr* character, and is localized on the pyrimidine ring. Phosphorescence measurements suggest a small singlet-triplet splitting. Halide ions are found to quench fenarimol fluorescence. Photodecomposition of fenarimol in a variety of solvents was studied by gas chromatography-mass spectrometry. Although the products have not yet been characterized, degradation of this compound appears to involve cleavage of bonds to the quaternary carbon without significant dechlorination. Preliminary studies of the kinetics and relative quantum yields of photodegradation show that the reaction proceeds via a first-order process, which is independent of pH, but which involves a photoactive intermediate. Chloride and bromide ions inhibit the photolysis, whereas the triplet quencher sorbic acid has no effect.
1. Introduction Photochemical transformation of pesticides, herbicides and other xenobiotic molecules is of major importance, from both environmental and economic viewpoints [l, 21. The fungicide fenarimol (Fig. 1, 1) (( f )-2,4’-dichloro-a-(pyrimidin-Syl)benzhydryl alcohol) is widely used for spraying both fruit and ornamental plants [3]. However, although it is known that this compound is readily
[21
(31
Fig. 1. Structures of fenarimol and suggested major degradation products.
1010-6030/94/$07.00 8 1994 Elsevier Sequoia. AU rights reserved SSDI 1010-6030(94)01047-J
broken down by sunlight [3], details of the photolysis products and reaction mechanism remain obscure. We report the photophysical properties of this molecule, and a preliminary study of its photodegradation reaction in various solvents.
2. Experimental
details
Fenarimol (Lilly Pharma, pure and Riedel 99.7%) was used without further treatment. There were no differences in the behaviour of the two samples. Water, which had been deionized and then distilled, was used for the preparation of aqueous solutions. Other solvents were of high performance liquid chromatography (HPLC) grade, and were used without further treatment. UV-visible absorption spectra were run in 1 or 10 cm quartz cuvettes on a Shimadzu UV-260 spectrophotometer. Fluorescence spectra of solutions were recorded in 1 cm cells in appropriate solvents on a Jasco FP-777 spectrofluorometer in a 90” configuration. Quantum yields of fluorescence were determined by excitation, at 295 nm, of solutions with absorbances between 0.05 and 0.09. A degassed solution of 2-aminopyridine in H,SO, (0.2 M) [4] and an aerated solution of 2-methyl naphthoate in cyclohexane [5] were used as stan-
410
M. Conce&io DA
Mateus et al. f Photodegradation of fenarimo!
dards. Although differences in the measured quantum yields were observed with the two standards, the ratio of the values was constant, suggesting a systematic difference between the two quantum yield standards. Phosphorescence spectra were observed on a Spex Fluoroiog 112 (Centro de Tecnologia Quimica e Biol&ica (CTQB), Lisbon) using Aash excitation and time delay for observation of the emission. Fluorescence lifetimes were determined by time-correlated single-photon counting using a home-built apparatus at CTQB [6, 71 and an Edinburgh Analytical Instruments FL900 fluorometer with appropriate software for data analysis. For photodegradation studies, sunlight and a “merry-go-round” photoreactor, fitted with a medium-pressure mercury lamp (400 W), were used. With the photoreactor, the 313 nm mercury line was employed, using an aqueous solution of potassium chromate and sodium carbonate [8] as filter. Actinometry was performed using a solution of p-nitroacetophenone (10e5 M) and pyridine (4.2x lo-’ M) (PNAP-Pyr) for sunlight and a solution of p-nitroanisol (10m5 M) and pyridine (2.9 x 10m3 M) (PNA-Pyr) for the photoreactor studies [9]. Photolysis was followed by HPLC using a Merck-Hitachi 655A-11 liquid chromatograph with a 655A-22 UV detector, or a Shimadzu UV-visible photodiode array detector (model SPDM6A). Attempts to identify the photodegradation products were carried out by gas chromatography-mass spectrometry (GC-MS) using a Hewlett Packard 5890 series II gas chromatograph with a 5971 series mass selective detector. For solar irradiation studies, 100 ml. crystallization dishes were filled with aqueous fenarimol solution (5 mg l-l), and covered with quartz plates. Care was taken to avoid the formation of air bubbles, and samples were kept at about 20 “C for sup to 6 days. Where necessary the pH was adjusted with 0.1 M NaOH or HCl solution using a Crison pH meter micropH 2003. Every 24 h, samples of each solution and of actinometer solution were taken and kept in the dark in a refrigerator at 4 “C. At the end of the 6 days of photolysis, all the samples were analysed by HPLC, together with the blank, which had been kept for the same time in the absence of light. The blank showed no sign of degradation of the pesticide during this period. For the photoreactor studies, samples were irradiated in 1 cm quartz cells with filtered light of intensity 4.5 X 10Wh einstein s-’ 1-l (determined by a ferrioxalate actinometer) at a temperature of 25 “C, maintained by a cooling bath with external
circulation. Samples were taken every 20 min for analysis. Studies on photoproducts were carried out by solar irradiation for 11 days of samples of fenarimol (500 mg 1-l) in methanol, ethanol, acetonitrile, dichloromethane and hexane in quartz tubes.
3. Results 3.1. Photophysical s&dies Absorption spectra were taken of soIutions of fenarimol in water (5 mg l-l, in a 10 cm quartz cell), methanol-water (50%), ethanol, acetonitrile, dichloromethane and hexane (7.5 X 10m4 M, 1 cm cells). Typical spectra are shown in Fig. 2(a) and, for comparison, spectra of solutions of fenarimol, pyrimidine and chlorobenzene under similar conditions are also given (Fig. 2(b)). The long-wavelength absorption in fenarimol is very similar to the first band in the pyrimidine absorption spectrum, and strongly suggests that the first transition is localized on this group. The first absorption band in fenarimol (near 300 nm in hexane) shows a pronounced blue shift with increasing solvent polarity. The shift is particularly marked with solvents containing hydroxyl groups, and is similar to that previously observed with pyrimidine [lo]. For pyrimidine, the absorption has been assigned to an n +T? transition, and it therefore seems probable that the S, state of fenarimol also has largely n,?r* character. The extinction coefficient of the shoulder in the fenarimol spectrum at 290 nm (approximately 350 M-l cm-l) also supports a partially forbidden character to this transition. As we are primarily interested in the photodegradation of fenarimol by sunlight, we concentrate on the absorption at wavelengths greater than 290 nm. However, in Fig. 2(b), details of the shortwavelength absorption of fenarimol are also shown. Fenarimol shows a relatively weak fluorescence around 360 nm (Fig. 3(a)), which is similar to that observed with pyrimidine [lo]. The emission maxima show a slight solvent dependence. However, a more marked effect of solvent is seen in the emission quantum yield, with the smallest values observed for solvents containing hydroxyl groups (Table 1). Again, this is similar to that observed with pyrimidine [lo]. The solvent dependence of the fluorescence lifetimes (typically, about 1 ns) follows the same behaviour as the quantum yields, and provides further support to the suggestion that the lowest excited singlet state of fenarimol has n,ti character, and is localized on the pyrimidine ring. The fluorescence lifetimes
411
.-‘-. acetonibi1e
a
250
300 wavelength
I nm
-.-.
fenarimol diluted
b
300 wavelength
I nm
Fig. 2. (a) Absorption spectra of solutions of fenarimol (7.5x10-” M) in methanol-water, M), pyrimidine (b) Absorption spectra of solutions of fenarimol (1.5 X 10m3 and 0.9X10-’ (3X10e3 M) in methanol.
ethanol, (1.5~10-~
acetonitrile and hexane. M) and chlorobenzene
412
M. Conceipio D-A. Matecu et al. / Photodepadation
offenwimol
a
330
350
370
390
470
430
Wavelength, nm
b
500
wavelength
600
/ nm
Fig. 3. (a) Fluorescence spectra of solutions of fenarimol (1.SX1O-3 M) m acetonitrile, hexane and ethanol. (b) Phosphorescence emission of fenarimol in diethy ether gktss at 77 K. The x axis is different from (a) to indicate that, although phosphorescence occurs at longer wavelengths than fluorescence, there is no significant emission above 550 nm.
of fenarirnol and pyrimidine are also given in Table 1. The fluorescence decay was apparently biexponentiai, but this was thought to be due to an impurity, and the kinetics were not pursued further. Halide ions were found to quench the fluorescence of fenarimol, with quenching ability increasing in the order Cl- < Br-
Attempts were made to study the phosphorescence of fenarimol in diethyl ether glass at 77 K. A weak emission is observed at slightly longer wavelengths than the fluorescence (Fig. 3(b)). The small separation between the S1 and T, states is in complete agreement with the assignment of a largely n,+ character to the lowest excited state.
M. Concei@o DA. Mateus et aL f Photodepadation of fenarimol TABLE 1. Fluorescence quantum yields and lifetimes of fenarimol and pyrimidine in various solvents COmpound
Solvent
Fluorescence Quantum
Fenarimol
Hexane Dichlocomethane Acetonitrile Ethanol
Byrimidine
‘Standard, %tandard,
310
Acetonitrile Ethanol
yield
Lifetime
(4)
(ns)
0.0328” 0.0046b 0.0038” 0.0058b 0.0033 0.0058b < 0.001” 0.0016b
0.88
0.0083’ < 0.001’
2.3
(7)
1.37 1.36 0.23
Z-aminopyridine in H&O+ 2-methyl naphthoate in cyclohexane.
330
350
370 390 410 Wavelength, nm
430
450
Fig. 4. Fluorescence spectra of fenarimol (1.5X10w’ M) in methanol-water (50%), alone (line I) and with solutions (0.5 M) of NaCl (line 2), NaBr (fine 3) and NaI (line 4).
of the photodegradafion reaction As fenarimof is only slightly soluble in water [3], attempts to characterize the main degradation products involved the photolysis of this compound in a variety of organic solvents. With solutions in dichloromethane and hexane, small quantities of solid are precipitated on photolysis. However, it has not yet proven possible to characterize this. More valuable information comes from GC-MS. From photolysis of solutions in dichloromethane, hexane, acetonitrile, ethanol and methanol, two major peaks, with m/e of 250 and 190, are observed (Fig. 5). These show further peaks (m+Z) at 252 and 192 respectively, indicating that both of these fractions contain chlorine. These are tentatively assigned to structures 2 and 3 (Fig. 1). In alcoholic solutions, the formation of two other products in reasonable yield was observed, with m/e values of
3.2. Studies
413
314 and 278. In addition, there may have been a number of smaller photodegradation fragments or minor products. Following photolysis of solutions of fenarimol (1.5 X 10p3 M) in 50% water-methanol, a few drops of silver nitrate solution were added. A slightly turbid solution was obtained which, on standing, deposited traces of a white solid, tentatively identified as AgCl. Photolysis was accompanied by changes in the UV absorption spectrum. This shows up most clearly in the analysis of the degradation products by HPLC using a diode array detector (Fig. 6), where loss of fenarimol is seen to be accompanied by the formation of compounds with shorter retention times. Peak (1) corresponds to a mixture of low-molecular-weight compounds with relatively short-wavelength absorptions. Peak (3) has an absorption (250-300 nm) typical of aromatic compounds. Although characterization of the degradation products is still in progress, the results indicate that the primary breakdown route involves cleavage of the bond between one of the rings and the quaternary carbon atom.
3.3. Kinetics and relative quantum yields of photo&s Qualitative kinetic studies of the photodegradation of fenarimol using both sunlight and artificial light sources have been made under a variety of actual or simulated environmental conditions (these will be presented elsewhere). To try to identify the primary processes involved in the photodegradation, quantitative studies of the kinetics and relative quantum yields were made on photolysis of aqueous solutions with a variety of additives. The kinetics were studied by monitoring the loss of fenarimol using HPLC. Quantitative differences exist between the rates obtained using sunlight and the photoreactor for irradiation. However, the overall trends are identical and, in all cases, degradation follows tit-order kinetics. The rates are independent of pH. However, the formation of an intermediate was observed, the yield of which increased with pH (Fig. 7). This species was found to be photodegraded, although it has not yet been possible to characterize it. The results of the effect of additives on the photolysis are given in Table 2. Halide ions significantly slow down the photolysis, with Br - > Cl-. This parallels the fluorescence quenching effect of these anions. In contrast, sorbic acid, a strong triplet quencher, has no effect on the rate. Although it is not the only possible explanation, it is tempting
414
M. Conwig&
D.A. Mateus et al. I Photodegradation of fenarimoi
Fig. 5. Mass spectra of major components separated by GC-MS following photo&s for 11 days of a solution of fenarimol (500 mg I-‘) in methanol: (a) gas chromatogram; (b) mass spectrum of unreacted fenarimol (retention time, 34.7 min); (c), (d), mass spectra of components indicated by arrows in (a) (retention times, 13.8 and 19 min).
to attribute the behaviour from the fenarimol singlet
to photodegradation state.
4. Discussion The pesticide fenarimol undergoes photodegradation in solution in a variety of solvents. Under environmental conditions, with aqueous solutions and sunlight, decomposition occurs over a period of days. Because of the relatively low solubility of fenarimol in water, attempts to characterize the main degradation products were made by photolysis of solutions in organic solvents. Two main products were seen in al the solvents studied, and are suggested to arise from cleavage of the bonds between the quaternary carbon and either the aromatic or pyrimidine ring. Although the photofysis of chlorobenzene derivatives frequently involves dechlorination reactions [ll, 121, the presence of chiorine in both of these derivatives was shown by mass spectrometry. However, this does not preclude the involvement of a dechlorination
step in the overall mechanism, as qualitative tests show the formation of traces of chloride ion on photolysis in aqueous solutions. In addition, other products were obsenred in alcoholic solutions. Studies of the effect of additives on the rate and relative yield of photolysis of fenarimo1 show that the reaction is inhibited by Cl- and Br-, but is unaffected by the triplet quencher sorbic acid. Absorption and luminescence spectral studies of fenarimol show that the lowest excited state is localized on the pyrimidine ring, and from studies of the effect of the solvent on the absorption maxima, fluorescence quantum yields and lifetime, it is suggested that the S1 state has predominantly n,7ir character. From a comparison of the fenarimol absorption spectra with those of chtorobenzene and pyrimidine, the lowest rr,,7jl: state lies at higher energy, and is unlikely to be readily excited by sunlight. Other transitions, particularly charge transfer transitions, may be possibIe with fenarimol but we found no evidence for these in the longwavelength region of the first absorption band. Fenarimol fluorescence is quenched by halide ions,
M. Conce@io
D.A. Muteus et al. I Phoiedegrudation of fenarimol
415
-._f PO0
:
t
4
t
i
5
1
TlNECmin)
b
:: ;: sIt
Fig. 6. Study of products of photo@& (sunlight) of solutions of fenarimol in acetonitrile using HPLC with a diode array detector: (a) standard solution of fenarimol (500 mg 1-l) in acetonitriIe; (b) after 11 days of irradiation (unreacted fenarimol is indicated with an arrow; (l)-(4) indicate peaks corresponding to the main degradation products).
TABLE radation
P
‘:
2. Kinetics and relative quantum of aqueous solutions of fenarimol
yields of photodegwith sunlight”
N
m
.
::
k? /
pH5
Kp (day-‘)
ti rclb
pH 4-7 N&l (0.5 M) NaBr (0.5 M) Sorbic acid (1 mM)
0.14 (+0.01) 0.06 < 0.03 0.13
1 co.3 =0.13 =I
*Latitude 37” N; all solutions the same conditions. ‘Relative to pure water.
irradiated
I
m
/
‘W4
Additive
PHI
pH7
in winter
time under
I OH8
Fig. 7. HPLC of the products of photolysis of an aqueous solution of fenarimol as a function of pH.
and a comparison of the quenching behaviour with the effect of these anions on photolysis suggests that photodegradation may occur through this n,+ state.
We are grateful to CTQB (Lisbon) for providing facilities for the measurement of the fluorescence lifetime and phosphorescence and, in particular, to Professor A.L. Maganita and Professor E.C. Melo for their assistance and valuable suggestions. Financial support from Instituto National de
416
Investiga@o CientSica knowledged.
M. Concei@
(INK)
DA. Mazeus et al. I Phofodegmdaiion
is gratefully ac-
5 A.L. Mqanita. 6
References 8 1 G.C Choudhty and G.R.B. Webster, Residue Rev., 96 (1985) 79. 2 L. Marchetesse, G.G. Choudhry and G.R.B. Webster, Rev. Em&m. Cumum. TbxicoL, 103 (1988) 61. 3 CR. Worthing, i% Pesticide Manual, British Crop Protection Council, London, 7th edn., 1983. 4 D.F. Eaton, Pure A@. Chem., 60 (1988) 1107.
g 10 11 12
offenarimol
J. Magalh%es, A. Dias, H. Teles and E. Iglesias, J. C&-m. Sot., Faraday Trans., 86 (1990) 4011. A.L. Mafanita, F.P. Costa, S.M.B. Costa, EC. Melo and H. Santos, J. Phys. Chem, 93 (1989) 336. A. Dias, A.P. Varela, M.G. Miguel, A.L. Macanita and R.S. Becker, J. Pfiys. Gem., % (19;12) 10 290. S.L Murov, Handbook of Photo&em&y, Marcel Dekker, New York, 1973, Sections 9-14. D. Dullin and T. Mill. Environ. Sci. Technol., 16 (1982) 815. H. Baba, L. Goodman.and P.C. Valaenti, L km. dhem.‘Soc., 88 (1966) 5410. A. Tissot, P. Bouie and J. Lemaire, Chemosphere, I2 (1983) 859. G.G. Choudhry, G.R.B. Webster and 0. Hutzinger, Taricol. Envimn. C/tern., 13 (1986) 27.
80 (1994) 4Op-416
Environmental and laboratory the pesticide fenarimol M. Concei$io
D.A.
Mate&,
Abflio
409
studies of the photodegradation
M. Silva”
and
Hugh
of
D. Burrowsb
“UCEH, Universidude do Algawe, 8000 Far0 (Portugal) bDepmtwnento de Quimiza, Univenidade de Coimbm, 3049 Coimbra (Portugal)
Abstract The photophysics and photochemistry of the pesticide fenarimol were studied. From the absorption and fluorescence spectra, quantum yields and lifetimes, it is suggested that the lowest excited singlet state has predominantly n,rr* character, and is localized on the pyrimidine ring. Phosphorescence measurements suggest a small singlet-triplet splitting. Halide ions are found to quench fenarimol fluorescence. Photodecomposition of fenarimol in a variety of solvents was studied by gas chromatography-mass spectrometry. Although the products have not yet been characterized, degradation of this compound appears to involve cleavage of bonds to the quaternary carbon without significant dechlorination. Preliminary studies of the kinetics and relative quantum yields of photodegradation show that the reaction proceeds via a first-order process, which is independent of pH, but which involves a photoactive intermediate. Chloride and bromide ions inhibit the photolysis, whereas the triplet quencher sorbic acid has no effect.
1. Introduction Photochemical transformation of pesticides, herbicides and other xenobiotic molecules is of major importance, from both environmental and economic viewpoints [l, 21. The fungicide fenarimol (Fig. 1, 1) (( f )-2,4’-dichloro-a-(pyrimidin-Syl)benzhydryl alcohol) is widely used for spraying both fruit and ornamental plants [3]. However, although it is known that this compound is readily
[21
(31
Fig. 1. Structures of fenarimol and suggested major degradation products.
1010-6030/94/$07.00 8 1994 Elsevier Sequoia. AU rights reserved SSDI 1010-6030(94)01047-J
broken down by sunlight [3], details of the photolysis products and reaction mechanism remain obscure. We report the photophysical properties of this molecule, and a preliminary study of its photodegradation reaction in various solvents.
2. Experimental
details
Fenarimol (Lilly Pharma, pure and Riedel 99.7%) was used without further treatment. There were no differences in the behaviour of the two samples. Water, which had been deionized and then distilled, was used for the preparation of aqueous solutions. Other solvents were of high performance liquid chromatography (HPLC) grade, and were used without further treatment. UV-visible absorption spectra were run in 1 or 10 cm quartz cuvettes on a Shimadzu UV-260 spectrophotometer. Fluorescence spectra of solutions were recorded in 1 cm cells in appropriate solvents on a Jasco FP-777 spectrofluorometer in a 90” configuration. Quantum yields of fluorescence were determined by excitation, at 295 nm, of solutions with absorbances between 0.05 and 0.09. A degassed solution of 2-aminopyridine in H,SO, (0.2 M) [4] and an aerated solution of 2-methyl naphthoate in cyclohexane [5] were used as stan-
410
M. Conce&io DA
Mateus et al. f Photodegradation of fenarimo!
dards. Although differences in the measured quantum yields were observed with the two standards, the ratio of the values was constant, suggesting a systematic difference between the two quantum yield standards. Phosphorescence spectra were observed on a Spex Fluoroiog 112 (Centro de Tecnologia Quimica e Biol&ica (CTQB), Lisbon) using Aash excitation and time delay for observation of the emission. Fluorescence lifetimes were determined by time-correlated single-photon counting using a home-built apparatus at CTQB [6, 71 and an Edinburgh Analytical Instruments FL900 fluorometer with appropriate software for data analysis. For photodegradation studies, sunlight and a “merry-go-round” photoreactor, fitted with a medium-pressure mercury lamp (400 W), were used. With the photoreactor, the 313 nm mercury line was employed, using an aqueous solution of potassium chromate and sodium carbonate [8] as filter. Actinometry was performed using a solution of p-nitroacetophenone (10e5 M) and pyridine (4.2x lo-’ M) (PNAP-Pyr) for sunlight and a solution of p-nitroanisol (10m5 M) and pyridine (2.9 x 10m3 M) (PNA-Pyr) for the photoreactor studies [9]. Photolysis was followed by HPLC using a Merck-Hitachi 655A-11 liquid chromatograph with a 655A-22 UV detector, or a Shimadzu UV-visible photodiode array detector (model SPDM6A). Attempts to identify the photodegradation products were carried out by gas chromatography-mass spectrometry (GC-MS) using a Hewlett Packard 5890 series II gas chromatograph with a 5971 series mass selective detector. For solar irradiation studies, 100 ml. crystallization dishes were filled with aqueous fenarimol solution (5 mg l-l), and covered with quartz plates. Care was taken to avoid the formation of air bubbles, and samples were kept at about 20 “C for sup to 6 days. Where necessary the pH was adjusted with 0.1 M NaOH or HCl solution using a Crison pH meter micropH 2003. Every 24 h, samples of each solution and of actinometer solution were taken and kept in the dark in a refrigerator at 4 “C. At the end of the 6 days of photolysis, all the samples were analysed by HPLC, together with the blank, which had been kept for the same time in the absence of light. The blank showed no sign of degradation of the pesticide during this period. For the photoreactor studies, samples were irradiated in 1 cm quartz cells with filtered light of intensity 4.5 X 10Wh einstein s-’ 1-l (determined by a ferrioxalate actinometer) at a temperature of 25 “C, maintained by a cooling bath with external
circulation. Samples were taken every 20 min for analysis. Studies on photoproducts were carried out by solar irradiation for 11 days of samples of fenarimol (500 mg 1-l) in methanol, ethanol, acetonitrile, dichloromethane and hexane in quartz tubes.
3. Results 3.1. Photophysical s&dies Absorption spectra were taken of soIutions of fenarimol in water (5 mg l-l, in a 10 cm quartz cell), methanol-water (50%), ethanol, acetonitrile, dichloromethane and hexane (7.5 X 10m4 M, 1 cm cells). Typical spectra are shown in Fig. 2(a) and, for comparison, spectra of solutions of fenarimol, pyrimidine and chlorobenzene under similar conditions are also given (Fig. 2(b)). The long-wavelength absorption in fenarimol is very similar to the first band in the pyrimidine absorption spectrum, and strongly suggests that the first transition is localized on this group. The first absorption band in fenarimol (near 300 nm in hexane) shows a pronounced blue shift with increasing solvent polarity. The shift is particularly marked with solvents containing hydroxyl groups, and is similar to that previously observed with pyrimidine [lo]. For pyrimidine, the absorption has been assigned to an n +T? transition, and it therefore seems probable that the S, state of fenarimol also has largely n,?r* character. The extinction coefficient of the shoulder in the fenarimol spectrum at 290 nm (approximately 350 M-l cm-l) also supports a partially forbidden character to this transition. As we are primarily interested in the photodegradation of fenarimol by sunlight, we concentrate on the absorption at wavelengths greater than 290 nm. However, in Fig. 2(b), details of the shortwavelength absorption of fenarimol are also shown. Fenarimol shows a relatively weak fluorescence around 360 nm (Fig. 3(a)), which is similar to that observed with pyrimidine [lo]. The emission maxima show a slight solvent dependence. However, a more marked effect of solvent is seen in the emission quantum yield, with the smallest values observed for solvents containing hydroxyl groups (Table 1). Again, this is similar to that observed with pyrimidine [lo]. The solvent dependence of the fluorescence lifetimes (typically, about 1 ns) follows the same behaviour as the quantum yields, and provides further support to the suggestion that the lowest excited singlet state of fenarimol has n,ti character, and is localized on the pyrimidine ring. The fluorescence lifetimes
411
.-‘-. acetonibi1e
a
250
300 wavelength
I nm
-.-.
fenarimol diluted
b
300 wavelength
I nm
Fig. 2. (a) Absorption spectra of solutions of fenarimol (7.5x10-” M) in methanol-water, M), pyrimidine (b) Absorption spectra of solutions of fenarimol (1.5 X 10m3 and 0.9X10-’ (3X10e3 M) in methanol.
ethanol, (1.5~10-~
acetonitrile and hexane. M) and chlorobenzene
412
M. Conceipio D-A. Matecu et al. / Photodepadation
offenwimol
a
330
350
370
390
470
430
Wavelength, nm
b
500
wavelength
600
/ nm
Fig. 3. (a) Fluorescence spectra of solutions of fenarimol (1.SX1O-3 M) m acetonitrile, hexane and ethanol. (b) Phosphorescence emission of fenarimol in diethy ether gktss at 77 K. The x axis is different from (a) to indicate that, although phosphorescence occurs at longer wavelengths than fluorescence, there is no significant emission above 550 nm.
of fenarirnol and pyrimidine are also given in Table 1. The fluorescence decay was apparently biexponentiai, but this was thought to be due to an impurity, and the kinetics were not pursued further. Halide ions were found to quench the fluorescence of fenarimol, with quenching ability increasing in the order Cl- < Br-
Attempts were made to study the phosphorescence of fenarimol in diethyl ether glass at 77 K. A weak emission is observed at slightly longer wavelengths than the fluorescence (Fig. 3(b)). The small separation between the S1 and T, states is in complete agreement with the assignment of a largely n,+ character to the lowest excited state.
M. Concei@o DA. Mateus et aL f Photodepadation of fenarimol TABLE 1. Fluorescence quantum yields and lifetimes of fenarimol and pyrimidine in various solvents COmpound
Solvent
Fluorescence Quantum
Fenarimol
Hexane Dichlocomethane Acetonitrile Ethanol
Byrimidine
‘Standard, %tandard,
310
Acetonitrile Ethanol
yield
Lifetime
(4)
(ns)
0.0328” 0.0046b 0.0038” 0.0058b 0.0033 0.0058b < 0.001” 0.0016b
0.88
0.0083’ < 0.001’
2.3
(7)
1.37 1.36 0.23
Z-aminopyridine in H&O+ 2-methyl naphthoate in cyclohexane.
330
350
370 390 410 Wavelength, nm
430
450
Fig. 4. Fluorescence spectra of fenarimol (1.5X10w’ M) in methanol-water (50%), alone (line I) and with solutions (0.5 M) of NaCl (line 2), NaBr (fine 3) and NaI (line 4).
of the photodegradafion reaction As fenarimof is only slightly soluble in water [3], attempts to characterize the main degradation products involved the photolysis of this compound in a variety of organic solvents. With solutions in dichloromethane and hexane, small quantities of solid are precipitated on photolysis. However, it has not yet proven possible to characterize this. More valuable information comes from GC-MS. From photolysis of solutions in dichloromethane, hexane, acetonitrile, ethanol and methanol, two major peaks, with m/e of 250 and 190, are observed (Fig. 5). These show further peaks (m+Z) at 252 and 192 respectively, indicating that both of these fractions contain chlorine. These are tentatively assigned to structures 2 and 3 (Fig. 1). In alcoholic solutions, the formation of two other products in reasonable yield was observed, with m/e values of
3.2. Studies
413
314 and 278. In addition, there may have been a number of smaller photodegradation fragments or minor products. Following photolysis of solutions of fenarimol (1.5 X 10p3 M) in 50% water-methanol, a few drops of silver nitrate solution were added. A slightly turbid solution was obtained which, on standing, deposited traces of a white solid, tentatively identified as AgCl. Photolysis was accompanied by changes in the UV absorption spectrum. This shows up most clearly in the analysis of the degradation products by HPLC using a diode array detector (Fig. 6), where loss of fenarimol is seen to be accompanied by the formation of compounds with shorter retention times. Peak (1) corresponds to a mixture of low-molecular-weight compounds with relatively short-wavelength absorptions. Peak (3) has an absorption (250-300 nm) typical of aromatic compounds. Although characterization of the degradation products is still in progress, the results indicate that the primary breakdown route involves cleavage of the bond between one of the rings and the quaternary carbon atom.
3.3. Kinetics and relative quantum yields of photo&s Qualitative kinetic studies of the photodegradation of fenarimol using both sunlight and artificial light sources have been made under a variety of actual or simulated environmental conditions (these will be presented elsewhere). To try to identify the primary processes involved in the photodegradation, quantitative studies of the kinetics and relative quantum yields were made on photolysis of aqueous solutions with a variety of additives. The kinetics were studied by monitoring the loss of fenarimol using HPLC. Quantitative differences exist between the rates obtained using sunlight and the photoreactor for irradiation. However, the overall trends are identical and, in all cases, degradation follows tit-order kinetics. The rates are independent of pH. However, the formation of an intermediate was observed, the yield of which increased with pH (Fig. 7). This species was found to be photodegraded, although it has not yet been possible to characterize it. The results of the effect of additives on the photolysis are given in Table 2. Halide ions significantly slow down the photolysis, with Br - > Cl-. This parallels the fluorescence quenching effect of these anions. In contrast, sorbic acid, a strong triplet quencher, has no effect on the rate. Although it is not the only possible explanation, it is tempting
414
M. Conwig&
D.A. Mateus et al. I Photodegradation of fenarimoi
Fig. 5. Mass spectra of major components separated by GC-MS following photo&s for 11 days of a solution of fenarimol (500 mg I-‘) in methanol: (a) gas chromatogram; (b) mass spectrum of unreacted fenarimol (retention time, 34.7 min); (c), (d), mass spectra of components indicated by arrows in (a) (retention times, 13.8 and 19 min).
to attribute the behaviour from the fenarimol singlet
to photodegradation state.
4. Discussion The pesticide fenarimol undergoes photodegradation in solution in a variety of solvents. Under environmental conditions, with aqueous solutions and sunlight, decomposition occurs over a period of days. Because of the relatively low solubility of fenarimol in water, attempts to characterize the main degradation products were made by photolysis of solutions in organic solvents. Two main products were seen in al the solvents studied, and are suggested to arise from cleavage of the bonds between the quaternary carbon and either the aromatic or pyrimidine ring. Although the photofysis of chlorobenzene derivatives frequently involves dechlorination reactions [ll, 121, the presence of chiorine in both of these derivatives was shown by mass spectrometry. However, this does not preclude the involvement of a dechlorination
step in the overall mechanism, as qualitative tests show the formation of traces of chloride ion on photolysis in aqueous solutions. In addition, other products were obsenred in alcoholic solutions. Studies of the effect of additives on the rate and relative yield of photolysis of fenarimo1 show that the reaction is inhibited by Cl- and Br-, but is unaffected by the triplet quencher sorbic acid. Absorption and luminescence spectral studies of fenarimol show that the lowest excited state is localized on the pyrimidine ring, and from studies of the effect of the solvent on the absorption maxima, fluorescence quantum yields and lifetime, it is suggested that the S1 state has predominantly n,7ir character. From a comparison of the fenarimol absorption spectra with those of chtorobenzene and pyrimidine, the lowest rr,,7jl: state lies at higher energy, and is unlikely to be readily excited by sunlight. Other transitions, particularly charge transfer transitions, may be possibIe with fenarimol but we found no evidence for these in the longwavelength region of the first absorption band. Fenarimol fluorescence is quenched by halide ions,
M. Conce@io
D.A. Muteus et al. I Phoiedegrudation of fenarimol
415
-._f PO0
:
t
4
t
i
5
1
TlNECmin)
b
:: ;: sIt
Fig. 6. Study of products of photo@& (sunlight) of solutions of fenarimol in acetonitrile using HPLC with a diode array detector: (a) standard solution of fenarimol (500 mg 1-l) in acetonitriIe; (b) after 11 days of irradiation (unreacted fenarimol is indicated with an arrow; (l)-(4) indicate peaks corresponding to the main degradation products).
TABLE radation
P
‘:
2. Kinetics and relative quantum of aqueous solutions of fenarimol
yields of photodegwith sunlight”
N
m
.
::
k? /
pH5
Kp (day-‘)
ti rclb
pH 4-7 N&l (0.5 M) NaBr (0.5 M) Sorbic acid (1 mM)
0.14 (+0.01) 0.06 < 0.03 0.13
1 co.3 =0.13 =I
*Latitude 37” N; all solutions the same conditions. ‘Relative to pure water.
irradiated
I
m
/
‘W4
Additive
PHI
pH7
in winter
time under
I OH8
Fig. 7. HPLC of the products of photolysis of an aqueous solution of fenarimol as a function of pH.
and a comparison of the quenching behaviour with the effect of these anions on photolysis suggests that photodegradation may occur through this n,+ state.
We are grateful to CTQB (Lisbon) for providing facilities for the measurement of the fluorescence lifetime and phosphorescence and, in particular, to Professor A.L. Maganita and Professor E.C. Melo for their assistance and valuable suggestions. Financial support from Instituto National de
416
Investiga@o CientSica knowledged.
M. Concei@
(INK)
DA. Mazeus et al. I Phofodegmdaiion
is gratefully ac-
5 A.L. Mqanita. 6
References 8 1 G.C Choudhty and G.R.B. Webster, Residue Rev., 96 (1985) 79. 2 L. Marchetesse, G.G. Choudhry and G.R.B. Webster, Rev. Em&m. Cumum. TbxicoL, 103 (1988) 61. 3 CR. Worthing, i% Pesticide Manual, British Crop Protection Council, London, 7th edn., 1983. 4 D.F. Eaton, Pure A@. Chem., 60 (1988) 1107.
g 10 11 12
offenarimol
J. Magalh%es, A. Dias, H. Teles and E. Iglesias, J. C&-m. Sot., Faraday Trans., 86 (1990) 4011. A.L. Mafanita, F.P. Costa, S.M.B. Costa, EC. Melo and H. Santos, J. Phys. Chem, 93 (1989) 336. A. Dias, A.P. Varela, M.G. Miguel, A.L. Macanita and R.S. Becker, J. Pfiys. Gem., % (19;12) 10 290. S.L Murov, Handbook of Photo&em&y, Marcel Dekker, New York, 1973, Sections 9-14. D. Dullin and T. Mill. Environ. Sci. Technol., 16 (1982) 815. H. Baba, L. Goodman.and P.C. Valaenti, L km. dhem.‘Soc., 88 (1966) 5410. A. Tissot, P. Bouie and J. Lemaire, Chemosphere, I2 (1983) 859. G.G. Choudhry, G.R.B. Webster and 0. Hutzinger, Taricol. Envimn. C/tern., 13 (1986) 27.