Photophysical and photochemical studies of thifensulfuron-methyl herbicide in aqueous solution

Article abstract of DOI:10.1016/j.jphotochem.2009.11.017

The photophysical and photochemical studies of a sulfonylurea herbicide, thifensulfuron-methyl (THM), have been investigated in a buffered aqueous solution. In the first part, the influence of pH on the spectroscopic properties was studied. This allowed the determination of the ground and excited state acidity constants, pK_a=4 and 4.4, respectively, thus exhibiting the potential existence of a photoinduced protonation in the singlet state. In the second part, the photolysis kinetics was studied at different pH and varying oxygen concentrations, using an HPK 125W lamp and followed up by the identification of photoproducts formed under continuous photo-irradiation. The kinetics results suggest that the photolysis process is faster in acidic (k=3×10^-4s^-1) than in basic medium (k=9.8×10^-5s^-1). The photolysis products were identified by high performance liquid chromatography HPLC-DAD, HPLC-MS and HPLC-MS-MS. In order to obtain a better understanding of the photodegradation mechanism, a laser flash photolysis study was performed. By comparing the quenching rate constant (k_q=9.64×10⁸mol^-1ls^-1) obtained from triplet state quenching by molecular oxygen and from the Stern-Volmer relation (k_q=0.41×10⁸mol^-1ls^-1), the role of the singlet state in the photodegradation process was demonstrated. The photoproducts originating from both singlet and triplet excited states have been identified and hypothetical photodegradation pathways of the thifensulfuron-methyl in aqueous solution are proposed.

Full text of DOI:10.1016/j.jphotochem.2009.11.017

Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photophysical and photochemical studies of thifensulfuron-methyl
herbicide in aqueous solution
Saâdia Aziza,b, Stéphane Dumasa,∗, Mohammed El Azzouzib, Mohamed Sarakhac, Jean-Marc Chovelona
a Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR-CNRS 5256, Université Lyon 1, 2, avenue Albert Einstein, 69626, Villeurbanne Cedex, France
b Université Mohammed V Agdal, Faculté des sciences, Rabat, Morocco
c Université Blaise Pascal, Laboratoire de Photochimie Moléculaire et Macromoléculaire, UMR CNRS 6505, F-63177, Aubière Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2009
Received in revised form
19 November 2009
Accepted 24 November 2009
Available online 29 November 2009
The photophysical and photochemical studies of a sulfonylurea herbicide, thifensulfuron-methyl (THM),
have been investigated in a buffered aqueous solution. In the first part, the influence of pH on the spec-
troscopic properties was studied. This allowed the determination of the ground and excited state acidity
constants, pKa = 4 and 4.4, respectively, thus exhibiting the potential existence of a photoinduced protona-
tion in the singlet state. In the second part, the photolysis kinetics was studied at different pH and varying
oxygen concentrations, using an HPK 125 W lamp and followed up by the identification of photoproducts
formed under continuous photo-irradiation. The kinetics results suggest that the photolysis process is
faster in acidic (k = 3 × 10−4 s−1) than in basic medium (k = 9.8 × 10−5 s−1). The photolysis products were
identified by high performance liquid chromatography HPLC-DAD, HPLC–MS and HPLC–MS–MS. In order
to obtain a better understanding of the photodegradation mechanism, a laser flash photolysis study was
performed. By comparing the quenching rate constant (kq = 9.64 × 108 mol−1 l s−1) obtained from triplet
state quenching by molecular oxygen and from the Stern–Volmer relation (kq = 0.41 × 108 mol−1 l s−1), the
role of the singlet state in the photodegradation process was demonstrated. The photoproducts originat-
ing from both singlet and triplet excited states have been identified and hypothetical photodegradation
pathways of the thifensulfuron-methyl in aqueous solution are proposed.
Keywords:
Photodegradation
Sulfonylurea
Thifensulfuron-methyl
Laser flash photolysis
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
rate is pH dependent and follows a pseudo first-order kinetics.
At high and low pH values the hydrolysis is faster with half-lives
A new class of agrochemical herbicides named sulfonylurea has
been developed since the middle of the seventies [1]. The sul-
fonylurea herbicide structure consists of two chromophore groups
connected by a sulfonylurea bridge. One group (R1) is constituted
by an aliphatic, aromatic, or heterocyclic molecule while the other
(R2) is principally a triazine, pyrimidine, or triazole [2,3]. Sul-
fonylureas are efficient at low doses (10–15 g ha−1) [4] and the
mode of action of these herbicides consists of inhibiting acetolac-
tate synthase (ALS) which is a key enzyme in the biosynthesis of
branched amino-acids (valine, leucine, and isoleucine) [5]. Because
of their effectiveness, sulfonylureas have been widely used in agri-
culture but their fate in soil and water remain a major problem.
Indeed three processes can occur concurrently in water or soil:
chemical hydrolysis, microbial and photolysis degradation [6]. For
example Cambon and Bastide [7] studied the hydrolysis kinetics
of thifensulfuron-methyl in aqueous solutions buffered at differ-
ent pH values (pH = 4, 5, 9, and 10), showing that the hydrolysis
of 28.8 and 6.0 h at pH 4 and 10, respectively [7]. Concerning
the degradation mechanism, in acidic solutions, two parallel reac-
tions occurred, cleavage of the sulfonylurea bridge and opening
of the triazine ring, whereas at alkaline pH, thifensulfuron-methyl
hydrolyzed to thifensulfuron, which was then slowly transformed
by cleavage of the sulfonylurea bridge and O-demethylation
[7,8].
Although it is claimed that chemical hydrolysis and micro-
bial degradation represent the main degradation pathways of
sulfonylureas, several papers indicate that at neutral pH, photol-
ysis could be an alternative pathway for the photodegradation of
thifensulfuron-methyl [9–11]. This is the reason we have focused
our attention on the photodegradation of sulfonylurea.
Whereas the photodegradation of sulfonylurea herbicides has
been studied in the last decade [9–12] mainly from an analytical
point of view, few studies refer to their photophysical and pho-
tochemical properties in solutions. This paper reports on the latter
aspects, with reference to thifensulfuron-methyl herbicide in aque-
ous solution. A laser flash photolysis approach has been employed
so as to better understand the role played by the excited (singlet
or triplet) states. Following degradation, the photoproducts were
∗
Corresponding author. Tel.: +33 0472432783.
E-mail address: stephane.dumas@ircelyon.univ-lyon1.fr (S. Dumas).
1010-6030/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2009.11.017

S. Aziz et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
211
Scheme 1. Chemical structure of thifensulfuron-methyl as a weak acid, and its conjugate base.
actinometer [13] and a value of 1.63 × 1016 photon cm−2 s−1 was
analysed using LC/DAD (liquid chromatography diode array detec-
tor) and LC/MS (liquid chromatography, mass spectrometry).
found.
2. Materials and methods
2.4. HPLC analysis
2.1. Reactants
2.4.1. HPLC/DAD analysis
The concentration profile of THM during irradiation and the
formation of intermediates were performed using a Shimadzu
VP series high-pressure liquid chromatography (HPLC) system
equipped with a photodiode array detector (HPLC-DAD). Analyt-
ical separation was performed using a column Hypersil BDS C18
(4 ␮m, 125 mm × 4 mm i.d.), flow rate of 1 ml min−1, injection vol-
ume of 20 ␮l, and mobile phase 55:45 (v/v) methanol/water (pH set
to 2.6 using formic acid). The detection wavelength was 245 nm.
For photoproducts formation, the following gradient was used:
10% (methanol) at t = 0 min during 13 min, then increase to 60% for
20 min then 100% during 10 min.
[Thifensulfuron-methyl,3-[[[(4-methoxy-6-methyl-
1,3,5-triazine-2-yl)amino]carbonyl]amino]sulfonyl]-2-
thiophenecarboxylate (Scheme 1) (98% purity) acquired from
Riedel-de Haen was used without further purification. This sul-
fonylurea is characterized by a high solubility in water which is
2400 mg l−1 at 25 ◦C at pH 6 [10]. 2-Amino-4-methoxy-6-methyl-
1,3,5-triazine was purchased from Sigma–Aldrich (purity > 97%). It
was used without further purification.
For photodegradation experiments, thifensulfuron-methyl
(THM) was solubilized in phosphate buffers (H3PO4, KH2PO4, and
K2HPO4). The water used for buffer preparation was purified with
a Millipore Waters Milli-Q water system (MilliQ-50 18 Mꢀ). As the
herbicide was not easy to be dissolved in acidic medium, a small
quantity of ethanol 5% was added and this for all pH.
2.4.2. LC/MS and LC/MS/MS analysis
The LC–MS and LC–MS–MS identification of photoproducts of
the aerated solution was carried out with a Waters (Alliance
2695) HPLC system coupled to a Quattro LC triple quadrupole
mass spectrometer (Micromass, Manchester, UK) equipped with
a pneumatically assisted electrospray ionization source (ESI). Data
acquisition and processing were performed by MassLynx NT 3.5
system. The system is also coupled with a UV–vis spectropho-
tometer. Analytical separation was performed: using a Nucleodur
2.2. UV absorption, fluorescence emission and time resolved
transient spectra
The UV absorption spectra were recorded with a double beam
UVIKON 930 spectrophotometer (KONTRON INSTRUMENT), while
fluorescence spectra were measured with a FP-6500 spectrofluo-
rometer (JASCO).
column C18 (5 ␮m, 250 mm × 4.6 mm i.d.), flow rate of 1 ml min−1
,
Transient absorption experiments in the 20 ns–400 ␮s time
scale were carried out on a nanosecond laser flash photoly-
sis spectrometer from Applied Photophysics (LKS.60). Excitation
(ꢁ = 266 nm) was from the fourth harmonic of a Quanta Ray GCR
130-01 Nd:YAG laser (pulse width ≈ 9 ns), and was used in a right-
angle geometry with respect to the monitoring light beam. The
transient absorbance at preselected wavelength was monitored
by a detection system consisting of a pulsed xenon lamp (150 W),
monochromator, and a 1P28 photomultiplier. The signal from the
photomultiplier was digitized by a programmable digital oscillo-
scope (HP54522A). A 32 bit RISC-processor kinetic spectrometer
workstation was used to analyse the digitized signal. A 3 cm3
volume of solution was used in a quartz vessel, and was stirred
after each flash irradiation. Deoxygenation of the solutions was
accomplished by nitrogen bubbling. The sample temperature was
297 K.
wavelength for detection of 254 nm, mobile phase (buffered at
pH 3 with sodium phosphate buffer) with the following gradient
methanol/water: 40% (methanol) at t = 0 min, then increase to 70%
for 3 min then decrease to 40% until 12 min.
The LC/MS identification of photoproducts of the deaerated
solution was performed on a HP 1100 LC system (Hewlett-Packard
GmbH) consisting of quaternary pump, degasser, autosampler,
column oven, and DAD UV detector, interfaced with a HP 1100
MSD quadrupole mass spectrometer equipped with an electro-
spray ionization source. The chromatographic conditions were
similar to those used previously for monitoring the evolution
of intermediates by HPLC/DAD. The injection volume was 20 ␮l.
The optimized electrospray parameters were: spray voltage, 4 kV;
cone voltage, 80 V; drying gas (nitrogen), 12 l min−1; nebulizer
pressure, 55 psig; source temperature, 350 ◦C. Mass spectra were
acquired in both positive and negative ion modes in a mass range
of m/z 80–1000. The photoproducts were identified by mass spec-
troscopy.
2.3. Irradiation experiments
The irradiation experiments were carried out in an open borosil-
icate (Pyrex) glass cell (cut-off at 295 nm, 50 ml volume), open to
air with an optical window of 11 cm2 area, equipped with a mag-
netic stirring bar and water circulating jacket. The light source was
a HPK 125 W Philips mercury lamp with main emission wavelength
at 365 nm, cooled with a water circulation maintained at 20 ◦C. Vol-
umes of 25 ml of the aqueous solution of thifensulfuron-methyl
(2.5 × 10−5 M) were irradiated. The radiant flux entering the irra-
diation system was measured using Uranyle oxalate from Fluka as
2.5. Calculation of the quantum efficiencies
The quantum efficiencies were calculated using custom appli-
cation software “photon” [11], and their value have been defined
using the equation:
dn/dt
ꢀ
ꢂ =
ꢁ
2 Iabs(ꢁ) ꢃꢁ
ꢁ
1

212
S. Aziz et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
Fig. 2. Absorption spectra of thifensulfuron-methyl in different solvents: water (+),
ethanol (ꢀ), methanol (ꢁ), acetonitrile (×), acid aqueous medium (♦), and hexane
(ꢀ).
other sulfonylureas containing a triazinic chromophore, a char-
acteristic intense band close to 232 nm is observed.
An intense band centred on 246 nm. Comparison of the UV spec-
•
tra of the pesticide in water with that of thiophene or triazine
shows that this band is characteristic of the thiophene part of
the herbicide. Considering its high molar extinction coefficient
ε2 = 27 300 l mol−1 cm−1, it can again be ascribed to a ␲–␲* tran-
sition while the relatively weak shoulder centred on 280 nm is
characteristic of the n–␲* transition implying the non-binding
electrons from the sulfur atom.
Fig. 1. Absorption and emission spectra (a) in aqueous solution in neutral medium
and (b) in acid aqueous medium.
The fluorescence spectrum of THM in water Fig. 1a is character-
ized by the main band centred on 343 nm (ꢁexc = 279 nm). Calcu-
lating the quantum efficiencies of fluorescence of thifensulfuron-
methyl (given using naphthalene as reference [15]) yields a value of
0.0019 2 × 10−4. For the fluorescence emission spectrum of thio-
phene, one band centred on 330 nm with a quantum efficiency of
fluorescence ϕF = 0.0021 2 × 10−4 was found, which is very sim-
ilar to the result obtained with THM. Since no fluorescence for
triazine is observed, the result shows that the fluorescence of the
THM is probably due to the thiophene part of the molecule. The
deactivation of the excited singlet state by radiative way of THM as
thiophene remains very low.
From Fig. 1a we can conclude that the absorption spectrum of
THM represents the sum of the absorption spectra of thiophene
derivatives and of the triazinic group while the emission spectrum
of thiophene is not modified by the presence of the triazinic group.
The absence of new specific bands and the small spectral shifts
observed allow us to suppose that only weak interactions take place
between the two chromophores and the sulfonylurea connecting
bridge.
where dn/dt denotes the variation of the concentration of the sub-
stance for time t and Iabs is the absorbed light for the same time in
the wavelength range studied.
2.6. Sample preparation and pre-concentration
During the irradiation, solution of thifensulfuron-methyl was
collected at 3 and 7 h. As photoproducts were obtained at very
low concentration, a pre-concentration step was dispensable to
perform HPLC/MS analysis. Each sample was extracted on a
solid phase extraction (SPE) cartridge isolute ENV+ (50 mg, 6 ml).
The solid phase was first conditioned with 3 ml of methanol
and then 6 ml of deionised water. The solvent for elution is
methanol.
3. Results and discussions
3.1. Photophysical properties of thifensulfuron-methyl (THM) in
water
3.2. Photophysical properties of thifensulfuron-methyl in
different media
Fig. 1a presents the UV–vis absorption and fluorescence emis-
sion spectra of THM, thiophene and triazine in water at neutral pH.
The UV–vis THM absorption spectrum shows several characteristic
bands:
The absorption spectra of thifensulfuron-methyl in different sol-
vents were compared (Fig. 2) to obtain more information on the
type of transition. The influence of solvent on the spectroscopic
and photophysics properties of THM was interpreted using the
solvatochromic Taft parameters [16]. The analysis is based on 3
parameters (see Table 1):
•
An intense band centred on 232 nm. The very high molar extinc-
tion coefficient ε1 = 29 600 l mol−1 cm−1 indicates that this band
is mainly a ␲–␲* transition. The literature [14] shows that for

S. Aziz et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
213
Table 1
Solvatochromic parameters.
Solvents
ꢄ*
0.54
0.60
1.09
0.75
−0.08
˛
ˇ
Ethanol
Methanol
Water
Acetonitrile
Hexane/heptane
0.83
0.93
1.17
0.19
0.00
0.77
0.62
–
0.31
0.00
(1) The ꢄ* parameter is an index of solvent polarity and polariz-
ability.
(2) The ˛ parameter describes the aptitude of solvent to donate a
proton.
(3) The ˇ parameter represents the acceptor character of solvent
proton.
No correlation between the maximum absorption wavelength of
THM and ˛ and ꢄ* parameters was noticed. On the other hand a
correlation between the ˇ parameter and the maximum absorption
wavelength is observed and indeed a blue shift is observed when
the ˇ parameter decreases. These results can be explained by the
existence of the pesticide in two forms, viz.
Fig. 3. (a) Absorption spectra of thifensulfuron-methyl vs wavelength: effect of pH.
Insert: (b) variation of absorbance as a function of pH (ꢁ = 230 nm).
•
A molecular form: this form is present in solvents with weak pro-
ton acceptor character (low ˇ parameter). This is confirmed by
the similarity of the absorption spectrum in acidified water.
An anionic form: this form is present in solvents with a high pro-
ton acceptor character (high ˇ parameter), i.e. solvents which
can remove a proton from the molecule thus stabilizing it in the
anionic form. The absorption spectrum in this case is identical to
that obtained in aqueous medium for which the anionic form is
predominant.
where C0 is the initial concentration of thifensulfuron-methyl, εSH
(21 900 l mol−1 cm−1) and εS− (31 300 l mol−1 cm−1) are the molar
extinction coefficients of the molecular and anionic forms respec-
tively at 230 nm and ABS represents the absorbance at ꢁ = 230 nm
of a solution of thifensulfuron-methyl for each pH. The pKa value
of acid–base equilibrium (Scheme 1) determined in this condition
is 4.0. Similar values were obtained for other sulfonylureas [5].
Although the curves exhibit two isosbestic points, only one pKa
value has been determined meaning that the deprotonation of the
second proton does not occur within the pH range studied (pH
2–10).
The variation of the fluorescence signal as a function of pH was
also studied. From Fig. 4a, it is shown that the emission intensity
decreases when the pH increases. The evolution of the emission
intensity as a function of pH allows us to determine the value of the
acid–base equilibrium constant (cf. Fig. 4b). From this figure we can
see that a similar value is obtained compared to the first method
which leads us to suppose that the deactivation of the singlet state
(kd) occurs faster than the deprotonation (kꢂ1) (see Scheme 2).
•
3.3. Effect of pH on the thifensulfuron-methyl spectroscopic
properties
Taking into account the results concerning the solvent effect on
the photophysics properties and the presence of a labile proton
in the molecule, a pH effect on the spectroscopic properties was
investigated.
Fig. 1b presents the UV–vis absorption and fluorescence emis-
sion spectra of THM, thiophene and triazine in acid aqueous
solution. In the absorption spectrum of THM, the band centred on
280 nm is better defined in acidic than in neutral medium (Fig. 1a)
and a hypsochromic shift of the band centred on 222 nm is observed
when the pH decreases.
For the fluorescence spectra, a shift toward higher wave-
lengths (bathochromic shift) of 4 nm for thiophene, and 10 nm
for thifensulfuron-methyl was observed in acidic when com-
pared to neutral medium. Moreover, the quantum efficiency of
thifensulfuron-methyl fluorescence is higher in acid (ϕF = 0.011)
than in neutral medium (ϕF = 0.0021) and this is also the case for
thiophene-2-carboxylic acid.
The absorption spectrum of THM as a function of pH is shown
in Fig. 3a. Due to the presence of two isosbestic points (225 and
279 nm), it is possible to follow the evolution of THM absorbance
as a function of pH at wavelength of 230 nm.
The variation of the absorbance spectra as a function of pH
(ꢁ = 230 nm) makes it possible to determine the value of the appar-
ent acid–base equilibrium constant in the ground state Fig. 3b.
Adjustment of the curve by the following equation gives the pKa
value [17]:
(εSH − ε − )C0 × 10−pK
a
Fig. 4. (a) Emission spectra of thifensulfuron-methyl vs wavelength: effect of pH.
Insert: (b) variation of fluorescence intensity as a function of pH (ꢁexc = 279 nm;
ꢁem = 347 nm).
S
ABS =
+ εSHC0
10−pK + 10−pH
a

214
S. Aziz et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
Scheme 2. Schematic representation of proton transfer for both the electronic
ground and excited (S1) states in aqueous solution.
Fig. 7. Transient absorption spectra obtained by laser flash photolysis of
thifensulfuron-methyl at pH 7 in deaerated solution.
ES− = 388.4 kJ mol−1 respectively for molecular and anionic form).
In this condition a pKa∗ value of 4.4 was calculated meaning that
thifensulfuron-methyl becomes less acid in the excited singlet
state.
3.4. Kinetic studies of the photodegradation
3.4.1. Influence of pH
The disappearance of THM obeys an apparent first-order
kinetics. Fig. 5 shows the variation of the rate constant k of
thifensulfuron-methyl as a function of pH. From this figure it
appears that the photolysis rate constant is faster for the low-
est pH when THM is in its neutral form. This result might be
explained by a better wavelength match between the absorption
and lamp emission spectra for the molecular form than for the
anionic form.
The quantum efficiencies of photodegradation according to the
pH were also plotted. Here too (Fig. 6), the results clearly show a
better efficiency of the degradation at acidic pH since the quantum
efficiency of the molecular form is at least three times higher than
Fig. 5. Variation of photolysis rate constant k (s−1) of thifensulfuron-methyl vs pH.
In this case the acid–base equilibrium constant of the excited
singlet state was calculated using the Forster cycle [18]:
NAhc
pKa∗ = pKa +
(¯S− − ¯SH
)
2.303RT
where T is the absolute temperature, h is Planck’s constant, NA is
Avogadro’s number, c is the light speed, and v¯SH and ¯S− are the
frequency of neutral and anionic species respectively (cm−1) (v¯SH
31 645, 56 cm−1 and ¯S− = 31 948, 88 cm−1).
=
Because of the absence of mirror-image symmetry between
the absorption and emission spectra, the energy of the excited
singlet state was given by the intersection between the emis-
sion and standardized absorption spectra (ESH = 382.8 kJ mol−1 and
Fig. 8. Decay of transient absorption monitored at 320 nm of thifensulfuron-methyl
at pH 7 as a function of oxygen concentration.
Fig. 6. Variation of quantum efficiency of thifensulfuron-methyl vs pH.

S. Aziz et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
215
Table 2
Photodegradation kinetic parameters: rate constant k (s−1), half-life t1/2 (h−1), observed rate constant kobs (␮s−1) and lifetimes of triplet state ꢅ (␮s) of thifensulfuron-methyl
under different oxygen concentration.
Medium
k (s−1
)
t1/2 (h)
R2
ꢂ
kobs (␮s−1
)
ꢅ = 1/kobs (␮s)
Deaerated
Aerated
Oxygen saturated
1.48 × 10−4
1.13 × 10−4
0.60 × 10−4
1.3
1.7
3.2
0.9788
0.9734
0.9407
1.06 × 10−3
6.24 × 10−4
4.56 × 10−4
0.0413
0.469
1.43
24.2
2.13
0.69
for the anionic form. These results show also that the photodegra-
dation of the molecular form is pH dependent unlike in the case of
the ionic form.
As shown in Fig. 7, the transient absorption spectrum obtained
in deaerated aqueous solution presents a large band with a
maximum at 320 nm and a shoulder at 370 nm. The intensity
of this band decreases uniformly with time without formation
of any significant band. Thus the transient absorption spectrum
can be attributed to the T–T absorption without any transient
species.
3.4.2. Laser flash photolysis studies
To establish the mechanism and kinetic details of THM degrada-
tion, a nanosecond laser flash photolysis technique was employed.
Table 3
HPLC retention times (Rt), m/z values from mass spectrometry, and UV data of thifensulfuron-methyl photoproducts.
Compound
Rt (min)
Mass (m/z) ES
−
UV data, ꢁmax (nm)
Medium
3.8
139 (M−H), 141 (M+H)
195 nm219 nm245 nm (shoulder)
Deaerated/aerated
7.5
184 (M+H), 167 and 141
195 nm223 nm245 nm (shoulder)
Deaerated/aerated
Deaerated
No identified
T3
10.2
18.3
410 (M+Na), 388 (M+H), 205, 184, 167 and 141
220 nm 232 nm 245 nm 280 nm
Deaerated/aerated
21.8/23.2
281 (M+H), 249, 141
315 nm
Deaerated
22.4
1.7
346 (M+Na), 324 (M+H), 292 (M–OMe)
300 nm
Deaerated
Aerated
221 (M−H), 163 (M–CO2Me), 141 (M–SO3H)
230 nm
230 nm
260 nm
270 nm
9.0
162 (M−H)
Aerated

216
S. Aziz et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
Scheme 3. Main likely mass fragmentations of both photoproducts T5 and T6.
Laser flash photolysis experiments were carried out in deoxy-
constant for the quenching process. By plotting the triplet rate con-
stant observed kobs (Table 2) as a function of oxygen concentration,
the quenching rate constant k2 is deduced from the slope of the
curve. A quenching process by molecular oxygen was observed as
genated, aerated and oxygen saturated conditions (see Fig. 8). The
oxygen quenching rate constant kinetic of triplet state was deter-
mined with the Stern–Volmer equation [19].
shown by the value of the rate constant k2 = 9.64 × 108 mol−1 l s−1
,
d[T]
dt
indicating that the quenching rate is diffusion-controlled. The effect
of oxygen on the kinetic disappearance of the transient confirms
the formation of a triplet state. The deactivation curve of the triplet
state fitted well with exponential kinetics with a lifetime of 0.69 ␮s
−
= (k1 + k2[O2])[T] = kobs[T]
where k1 and k2 represent respectively the rate constant for the
triplet decay in the absence of oxygen and the second order rate
Scheme 4. Hypothetical photodegradation pathway of thifensulfuron-methyl in aqueous solution (ISC: InterSystem Crossing).

S. Aziz et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
217
in oxygen saturated conditions and 2.13 ␮s in aerated solution but
3.6.1. Deaerated medium
no other transient species were detected in these experimental
conditions. This result leads us to assume that the energy trans-
fer between the triplet excited state of thifensulfuron-methyl and
molecular oxygen is highly efficient. This energy transfer could gen-
erate reactive species such as singlet oxygen and superoxide anions
which could be involved in the photodegradation processes.
Consequently, the photoproducts quantitatively formed in
deaerated medium could be produced from the triplet excited state.
Photoproduct T1: This product quickly eluted shows a UV spec-
trum typical of a triazine derivative. Mass spectroscopy revealed
quasi-molecular ions at m/z = 140.9 (M+H)+. The product was iden-
tified by direct comparison of its UV, MS and chromatographic data
with those of standard product 2-amino-4-methoxy-6-methyl-
1,3,5-triazine (AMMT).
Photoproduct T2: The UV spectrum shows that this prod-
uct also contains a triazine derivative and this conclusion was
corroborated by the mass spectrometric data which revealed
characteristic fragments of the triazine derivative at m/z = 167
and 140.9 as reported [9,11,20]. Moreover, quasi-molecular
ions in positive mode at m/z = 184 (M+H)+ as well as the lit-
3.5. Effect of oxygen concentration
The kinetic rate constant was studied as a function of oxygen
concentration in aqueous solution in order to obtain a bet-
ter understanding of the thifensulfuron-methyl photodegradation
mechanism.
The observed rate constants and half-lives, calculated by ln 2/k,
in aqueous solution, are given in Table 2.
The study of the effect of oxygen concentration showed that
the initial rate of thifensulfuron-methyl disappearance decreased
with increasing oxygen concentration thus showing that the pho-
todegradation of thifensulfuron-methyl is inhibited by oxygen.
As a result, the triplet state plays the main role in the pho-
todegradation of thifensulfuron-methyl.
In order to determine whether the triplet state plays an impor-
tant role in the photodegradation, the Stern–Volmer equation [19]
was employed:
erature data allowed us to propose
Table 3).
It has been found that T1 and T2 are stable with regard to oxygen
concentration and it is likely that the mechanism involves the first
excited singlet state.
Product T4: T4 was identified as being THM using the
molecule as standard. The LC–MS/MS main data in positive
mode of THM are m/z = 410, 388, 205, 167, 141 corresponding
to (M+Na), (M+H), thiophene and triazine derivatives respec-
tively.
Photoproduct T5: The mass spectrum in positive mode of this
product presents three characteristics peaks, the first is the quasi-
molecular ion peak at m/z = 281 (M+H), the second is a fragment
ion peak at m/z = 249 corresponding to (M–OMe) while the third
one, is a fragment ion peak at m/z = 141 corresponding to triazine
derivative.
a structure for T2 (cf.
ꢂ0
The main fragment ions of this photoproduct is shown in
Scheme 3.
= k2ꢅ0[O2] + 1
ꢂ
A similar compound was also found by Schneiders et al. [21,22].
It is the major hydrolysis product of rimsulfuron at pH 7 and at pH
9.
where ꢂ0 and ꢂ represent, respectively, the quantum yield in the
absence and in the presence of oxygen, and ꢅ0 is the lifetime of
the triplet state in the absence of oxygen. By plotting the ꢂ0/ꢂ
vs oxygen concentration, we obtain a straight line, the value of
k2ꢅ0 = 1011.1 mol−1 l is deduced from the slope of the curve. ꢅ0
has been determined by laser flash photolysis (ꢅ0 = 24.2 ␮s). Under
these conditions we determined that the quenching rate constant
k2 is equal to 0.41 × 108 mol−1 l s−1. Since this value is lower than
the calculated one (k2 = 9.64 × 108 mol−1 l s−1), we can conclude
that in addition to the triplet state, the singlet excited state also
plays a role in the photodegradation mechanism of the pesticide.
Photoproduct T6: The mass spectrum of APCI in positive mode
of T6 showed an abundant peak at m/z = 346 (M+Na), a quasi-
molecular ion peak at m/z = 324 (M+H), and a fragment ion peak
at m/z = 292 corresponding to (M–OMe). The absorption UV spec-
trum of this product presents a large band at 300 nm due to an
extended conjugated system and a greater dipole moment between
the triazine and thiophene parts, that could explain the high
absorption observed at 300 nm. We have also observed other frag-
ment ion peaks with mass–mass spectrometric in positive mode
at m/z = 281, 167, 158, 141 and 126. The main fragmentation of T6
is shown in Scheme 3. This process involves concerted elimina-
tion of SO2 through contraction of sulfonylurea bridge to generate
T6.
3.6. Identification of the photoproducts
In order to avoid hydrolysis, degradation of THM and identifi-
cation of photoproducts were carried out at pH 7. Using LC/DAD,
two kinds of photoproducts were detected, viz. those containing
the triazinic group for which the maximum of absorbance have
wavelengths lower than 250 nm and those containing the thio-
phene group which absorb at wavelengths higher than 250 nm. The
formation of these photoproducts can be explained as a result of the
cleavage within the sulfonylurea bridge.
A similar product was identified as primary hydrolysis product
of rimsulfuron at pH 5 by Schneiders et al. [21,22]. Elimination of
SO2 was also observed as photoproduct of the tribenuron-methyl
by Bhattacharjee and Dureja [23] and as photolysis product of the
triflusulfuron-methyl by Chafik [24].
Photoproduct T7: The mass spectrum of APCI in positive mode
of this product, was similar to that of T5: an abundant peak at
m/z = 281 (M+H) and a fragment ion peak at m/z = 249 correspond-
ing to (M–OMe). The absorption UV spectrum of this product
presents a high absorption at about 315 nm. As a conclusion, T7
and T5 could be isomeric enol.
The use of LC/DAD and LC/MS, as well as LC/MS/MS in negative
and positive ions allowed us to suggest different photoproducts
(Table 3).

218
S. Aziz et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 210–218
Since photoproducts T5, T6 and T7 were quantitatively generated in
deaerated medium we can conclude that they are produced from
triplet excited state.
[5] E. Vulliet, C. Emmelin, J.M. Chovelon, Influence of pH and irradiation wave-
length on the photochemical degradation of sulfonylureas, J. Photochem.
Photobiol. A: Chem. 163 (2004) 69–75.
[6] J.P. Cambon, J. Bastide, Chemical or microbiological degradation of sulfony-
lurea herbicides in soil. III. Cas du thifensulfuron methyl, Weed Res. 32 (1992)
357–362.
[7] J.P. Cambon, J. Bastide, Hydrolysis kinetics of thifensulfuron methyl in aqueous
buffer solutions, J. Agric. Food Chem. 44 (1996) 333–337.
[8] J.P. Cambon, J. Bastide, D. Vega, Mechanism of thifensulfuron-methyl transfor-
mation in soil, J. Agric. Food Chem. 46 (1998) 1210–1216.
[9] L. Scrano, S.A. Bufo, P. Perucci, Photolysis and hydrolysis of rimsulfuron, Pestic.
Sci. 55 (1999) 955–961.
[10] J. Headly, J.L. Du, X. Xu, K. Peru, Kinetics of photodegradation of thifensulfuron-
methyl in aqueous solutions, Commun. Soil Sci. Plant Anal. 33 (2002)
3287–3302.
[11] E. Vulliet, C. Emmelin, M.F. Grenier-Loustallot, J.M. Chovelon, Simulated
sunlight-induced photodegradation of triasulfuron and cinosulfuron in aque-
ous solutions, J. Agric. Food Chem. 50 (2002) 1081–1088.
[12] A. Pusino, M.G. Fiori, I. Braschi, C. Gessa, Adsorption and desorption of triasul-
furon by soil, J. Agric. Food Chem. 51 (2003) 5350–5354.
3.6.2. Aerated medium
Other photoproducts in aerated medium were identified with
mass–mass spectrometry in negative mode.
Photoproduct T8: The mass–mass spectrum of T8 presents a
quasi-molecular ion peak at m/z = 221 (M−H), and a fragment
ion peaks at m/z = 163 and 141 corresponding to (M–CO2Me)
and (M–SO3H) respectively. It was tentatively identified as being
C6H6S2O5.
Photoproduct T9: The mass–mass spectrum of T9 presents a
quasi-molecular ion peak at m/z = 162 (M−H) which was tentatively
identified as being sulfonamid thiophene.
Scheme 4 represents photoproducts obtained from the singlet
and triplet excited states and summarizes the hypothetical pho-
todegradation pathways of THF in aqueous solution.
Since photoproducts T8 and T9 were quantitatively generated
in aerated medium, we can suppose that the mechanism required
simultaneously both triplet excited state and oxygen molecules.
[13] H. Barkani, Etude de la dégradation photochimique des herbicides imidazolines
en milieu aqueux: Imazaquin et imazethapyr, Thesis, Rabat Agdal University,
Faculty of Sciences, Rabat, Morocco, 2005.
[14] M. Brigante, C. Emmelin, J.M. Chovelon, Abiotic degradation of iodosulfuron-
methyl-ester in aqueous solution, J. Agric. Food Chem. 53 (2005) 5347–
5352.
[15] S.L. Murov, I. Carmichael, G.L. Hug, Handbook of Photochemistry, 2nd ed.,
Revised and Expanded, Marcel Dekker, New York, 1993.
[16] M.J. Kamlet, Linear solvation energy relationships. 23. A comprehensive col-
lection of the solvatochromic parameters, ꢄ*, ˛, ˇ, and some methods for
simplifying the generalized solvatochromic equation, J. Org. Chem. 48 (1983)
2877–2887.
[17] S. Dumas, Réactivité des états photoexcités de l’hypéricine. Etude comparative
avec des dérivés analogues, Thesis, Joseph Fourier University, Grenoble, France,
2000.
[18] A.M. Halpern, G. McBane, Photophysical and molecular spectroscopy, in: Exper-
imental Physical Chemistry: A Laboratory Textbook, 2nd ed., W.H. Freeman and
Company, England, 2006.
[19] K. Zgolak, A. Jeszka, T. Kabzinski, The study of the interactions between keto-
profen and n-acetyl-tyrosine ethyl ester, J. Food. Chem. Biotechnol. 71 (2007)
49–58.
4. Conclusion
In this work the photophysical properties and photodegrada-
tion of thifensulfuron-methyl were investigated in different media.
The results clearly show that the photodegradation process is more
efficient at pH < pKa(S0) and that the protonated form is more eas-
ily photodegraded than the ionic form. The photolysis mechanism
involves both first excited singlet and first triplet states but the
latter appears to be more widely implicated in the photodegrada-
tion mechanism. The main photoproducts obtained derive from the
elimination of SO2 and the contraction of the sulfonylurea bridge.
[20] M. Sleiman, P. Conchon, C. Ferronato, J.-M. Chovelon, Iodosulfuron degradation
by TiO2 photocatalysis: kinetic and reactional pathway investigations, Appl.
Catal. B: Environ. 71 (2006) 279–290.
[21] G.E. Schneiders, M.K. Koeppe, Fate of rimsulfuron in the environment, J. Agric.
Food Chem. 41 (1993) 2404–2410.
References
[22] M.K. Koeppe, C.M. Hirata, H.M. Brown, W.H. Kenyon, D.P. O’Keefe, S.C. Lau, W.T.
Zimmerman, J.M. Green, Basis of selectivity of the herbicide rimsulfuron in
maize, Pestic. Biochem. Physiol. 66 (2000) 170–181.
[23] A.K. Bhattacharjee, P. Dureja, Light induced transformation of tribenuron-
methyl, Chemosphere 38 (1999) 741–749.
[1] A. Coly, J.-J. Aaron, Photochemically-induced fluorescence determination of
sulfonylurea herbicides using micellar media, Talanta 49 (1999) 107–117.
[2] A.K. Sarmah, J. Sabadie, Hydrolysis of sulfonylurea herbicides in soils and aque-
ous solutions, J. Agric. Food Chem. 50 (2002) 6253–6265.
[24] N. Chafik, Contribution à l’étude du comportement de l’herbicide triflusulfuron-
méthyle dans le sol et dans les milieux aquatiques, Thesis, Université Hassan II
Ain-Chock, Faculté des Sciences-Casablanca, 2002.
[3] P. Degelmann, J. Wenger, R. Niessner, Development of a class-specific ELISA for
sulfonylurea herbicides, Environ. Sci. Technol. 38 (2004) 6795–6802.
[4] T.K. James, P.T. Holland, A. Rahman, R.Y. Lu, Degradation of the sulfonylurea
herbicides chlorsulfuron and triasulfuron in a high-organic matter volcanic soil,
Weed Res. 39 (1999) 137–147.