8
0
A. Le Person et al. / Journal of Photochemistry and Photobiology A: Chemistry 315 (2016) 76–86
Table 1
Quantum yields (
in the presence of Cu ions (R = 20) (in bracket).
Nine coordination sites were taken into consideration but only
f
) at pH 6ꢆ5 of mesotrione photo-degradation in the absence and
five of them gave stable structures after structural optimization.
The numbering of these five sites is represented on Fig. 4. I, IV and V
are monodentate binding while II and III are chelates.
The four tries that have not been stabilized were the
monodentate binding on O7, the bidentate binding between
O1 and O3 and the bidentate binding on either the nitro and/or the
sulfonyl group. After these structural optimizations, the electronic
transitions were computed and compared with the experimental
spectrum. These results are reported on Fig. 4.
The comparison between the experimental findings directly
eliminates the hypotheses III–V as incompatible with the
experimental spectrum due to the presence of low-energy lying
transitions (above 350 nm) with significant oscillator strengths, in
disagreement with the zero value of the absorbance in this domain.
The distinction between I and II is tougher but the important
number of transitions computed for I in the 280–330 nm domain is
quite difficult to assign to the single band observed. On the
contrary, the transitions computed for II are slightly red-shifted
II
Wavelength
(10ꢁ4)
l
(nm)
280
300
310
320
f
1.46 (1.62)
1.58 (1.57)
1.64 (4.12)
1.08 (4.32)
for the four main transitions of the complex (Fig. 2, SI). Three of
these transitions appear to be mostly ligand-to-ligand electron
transfer while the lowest-energy one (317 nm) involves possibly a
ligand-to-metal charge transfer (LMCT). To have a more convincing
representation and quantification of this transfer, the charge
transfer excitation length (DCT) diagnostic tool was used, allowing a
more explicit picture of the phenomenon by the use of equivalent
ellipsoids for the density variation, as depicted on Fig. 5 [42]. It
illustrates without ambiguity an electron transfer from the ligand
to the copper cation, characterized by DCT = 1.56 Å. This LMCT
should possibly be linked to a reduction of the cation upon
irradiation.
(
about 20 nm, 0.3 eV in this wavelength domain that is compatible
2
.3. Photo-degradation of mesotrione and of the complex
with the accuracy of TD-DFT) but once this shift added, the general
shape of the spectra is well reproduced. With this computation
combined with the fact that bidentate coordination mode is
generally favored compared with monodentate mode, it appears
reasonable to propose II as the actual structure for mesotrione-Cu
complex.
The absorption spectra of mesotrione and its complex present a
significant overlap with that of the solar emission between 300 and
400 nm (Fig. 3b) leading to a possible photochemical transforma-
tion under these irradiation wavelengths.
II
Then, the structure of II was investigated and compared with
the free mesotrione one. It should be first remarked that the
2
.3.1. Effect of the irradiation wavelength on the photo-degradation
quantum yield
dihedral angle between the two
p
systems presented on
ꢃ
The quantum yields of mesotrione photo-degradation were
determined at different wavelengths in the absence and in the
presence of Cu ions (Table 1).
mesotrione is still high, a value of ꢁ102.8 being computed for
C2 ꢁꢁ C7 ꢁꢁ C8 ꢁꢁ C9. For most geometrical parameters (Table 3, SI),
the values computed for II are quite close to those computed for Ei,
II
Within the studied range of wavelengths (l= 280–320 nm), no
except for those describing the
b-diketo site: this is consistent
significant effect of on the quantum yield of mesotrione photo-
degradation was observed. This let us suggest that the same
process occurs at all these wavelengths.
l
with the coordinated copper that plays a role similar to proton in
the enol compounds. The spin density was also computed and is
localized on the metal center. So, in the ground state, no reduction
of copper cation by mesotrione should be evidenced. The nature of
the transitions was also studied. Numerous orbital pairs are
involved in each transition so the transition density was depicted
II
In the presence of Cu ions, the quantum yield is of the same
II
order of magnitude as that obtained without Cu for wavelengths
ꢅ 300 nm. On the contrary, the irradiation at wavelengths ꢀ 310
nm results in higher yields of factor around 3. This would imply
that two different processes related to the wavelength are involved
in the case of the complex. The lowest-energy transition (317 nm)
has been attributed to a LMCT whereas the other transitions are
mostly ligand-to-ligand electron transfer (Section 2.2). Thus the
excitation in the LMCT band probably leading to an electron
transfer between copper and mesotrione gives rise to a pathway
which promotes the degradation of mesotrione and which is
probably associated with a reduction of the cation whereas the
excitation in ligand–ligand transitions leads to
a different
photochemical reaction.
2
.3.2. Effect of Oxygen on the photo-degradation rate
The effect of oxygen on mesotrione photo-degradation was
studied by bubbling either oxygen or nitrogen in the free and
complexed mesotrione (40 M) solution. The solutions were
irradiated using the polychromatic source that presents
maximum emission at 313 nm. The disappearance of mesotrione
Fig. 6a) and also of complex (Fig. 6b), determined by liquid
m
a
(
chromatography, obeys a pseudo first-order kinetics in the first 3 h
of the reaction (Fig. 6c). The retention time of the complex is the
same as that of free mesotrione indicating that decomplexation of
II
Cu occurs in the chromatographic system. Several observations
Fig. 5. Ellipsoid-equivalent representation of the transition densities computed for
the transition at 317 nm of the b-keto complex [42]. A cut-off value of 0.0015 au was
can be done from Fig. 6.
applied. Blue reflects an increase of the electron density while red indicates a
depletion. The two points at the center of the ellipsoids are the barycenters of
density increase and depletion respectively.
i) The results in the absence of CuII ions show that the rate of
consumption of mesotrione decreases with oxygen