Electron-transfer oxidation of chlorophenols by uranyl ion excited state in aqueous solution. Steady-state and Nanosecond flash photolysis studies

Article abstract of DOI:10.1021/jp993676t

The oxidation of chlorophenols by photoexcited uranyl ion was studied in aqueous solution at concentrations where the ground-state interactions were negligible. Nanosecond flash photolysis showed that a clean electron-transfer process from the chlorophenols to the excited uranyl ion is involved. This is suggested to lead to the formation of a U(V)/chlorophenoxyl radical pair complex. The efficiency of this charge-transfer process is unity for the three chlorophenols. However, low product yields suggest that in the absence of oxygen, back electron transfer, both within the radical pair and from separated uranium(V) to phenoxyl radicals, appears to be the major reaction pathway. In the presence of oxygen the quantum yields of disappearance of chlorophenol and of photoproduct formation increased. This leads to the conclusion that oxygen favors reaction with uranium(V) and/or the uranium(V) - phenoxyl radical pair, leading to the formation of the superoxide anion and its conjugate acid, HO₂^*, which then regenerate UO₂²⁺. Based on this, a catalytic cycle for chlorophenol photooxidation involving uranyl ion and molecular oxygen is proposed.

Full text of DOI:10.1021/jp993676t

3142
J. Phys. Chem. A 2000, 104, 3142-3149
Electron-Transfer Oxidation of Chlorophenols by Uranyl Ion Excited State in Aqueous
Solution. Steady-State and Nanosecond Flash Photolysis Studies
Mohamed Sarakha* and Miche`le Bolte
Laboratoire de Photochimie U.M.R. CNRS 6505, F-63177 Aubie`re Cedex, France
Hugh D. Burrows
Departamento de Qu´ımica, UniVersidade de Coimbra, 3049 Coimbra, Portugal
ReceiVed: October 14, 1999; In Final Form: January 20, 2000
The oxidation of chlorophenols by photoexcited uranyl ion was studied in aqueous solution at concentrations
where the ground-state interactions were negligible. Nanosecond flash photolysis showed that a clean electron-
transfer process from the chlorophenols to the excited uranyl ion is involved. This is suggested to lead to the
formation of a U(V)/chlorophenoxyl radical pair complex. The efficiency of this charge-transfer process is
unity for the three chlorophenols. However, low product yields suggest that in the absence of oxygen, back
electron transfer, both within the radical pair and from separated uranium(V) to phenoxyl radicals, appears
to be the major reaction pathway. In the presence of oxygen the quantum yields of disappearance of
chlorophenol and of photoproduct formation increased. This leads to the conclusion that oxygen favors reaction
with uranium(V) and/or the uranium(V)-phenoxyl radical pair, leading to the formation of the superoxide
•
anion and its conjugate acid, HO2 , which then regenerate UO22+. Based on this, a catalytic cycle for chloro-
phenol photooxidation involving uranyl ion and molecular oxygen is proposed.
Introduction
400 µs time scale were carried out on a nanosecond laser flash
photolysis spectrometer from Applied Photophysics (LKS.60).
Excitation (355 nm) was from the third harmonic of a Quanta
Ray GCR 130-01 Nd:YAG laser (pulse width ≈ 5 ns), and was
used in a right-angle geometry with respect to the monitoring
light beam. A 3 cm3 volume of solution was used in a quartz
cuvette, and was stirred after each flash irradiation. Individual
cuvette samples were used for a maximum of 5 consecutive
experiments. The transient absorbances at preselected wave-
lengths were monitored by a detection system consisting of a
pulsed xenon lamp (150 W), monochromator, and a 1P28
photomultiplier. A spectrometer control unit was used for
synchronizing the pulsed light source and programmable shutters
with the laser output. This also housed the high-voltage power
supply for the photomultiplier. The signal from the photomul-
tiplier was digitized by a programmable digital oscilloscope
(HP54522A). A 32 bit RISC-processor kinetic spectrometer
workstation was used to analyze the digitized signal. Emission
decay measurements used the same system without the analyzing
light source. Some steady-state luminescence quenching mea-
surements were also carried out on a Spex Fluorolog model
111 spectrometer, as described in a previous publication.38
Steady-State Photolysis and Analytical Procedures. For
steady-state irradiations either a high-pressure mercury lamp
(Osram HBO type 125 W) with a Bausch and Lomb grating
monochromator or a medium-pressure mercury lamp with
appropriate filters was used for irradiation. General details have
been given previously.38-41 The quantum yields of degradation
of the chlorophenols were determined by high-performance
liquid chromatography (HPLC) using a Waters 540 liquid
chromatography system equipped with a diode array UV-visible
detector (Waters 990), and a reverse phase Merck column
(Lichrospher 100RP-18 5 µm-250 mm × 4 mm). Photoproducts
were separated using a Gilson preparative HPLC apparatus
equipped with Waters model 490 detector and model 201
fraction collector.
Chlorophenols are widely used as fungicides, herbicides, etc.1
They have applications in wood protection2 and are produced
in the Kraft bleaching of paper pulp.3 However, the increasing
awareness of the possible environmental effects of these
compounds has led to the demand for limiting their usage and
for the development of new methods of treating contaminated
waters in which they are present. Particular emphasis has been
given to advanced oxidation processes (AOPs)4 using photo-
chemical methods with either direct or sensitized photolysis.
The general aspects of photooxidative degradation of chlo-
rophenols have recently been extensively reviewed.5
Excited uranyl ion, *UO22+, is a strongly oxidizing species
(E° ) +2.6 ( 0.1 V),6-8 and has been shown to be capable of
oxidizing a variety of substrates. The electronic structure9 and
photochemistry10-12 of uranyl ion have recently been reviewed.
Photooxidation of substrates by *UO22+ is thought to occur by
both atom13-22 and electron-transfer18,20,23-38 processes. With
phenols, reaction has been shown to occur by a dynamic
process25 involving intermediate phenoxyl radical-uranium(V)
radical pairs.30,35 Although many of the early studies on
photooxidation of phenols by uranyl ion concentrated on the
kinetics of these processes,25 we have recently reported results
of a combined study of the kinetics and product characterization
on the uranyl-ion-sensitized photolysis of 2,6-dimethylphenol
and monophenylphenols in aqueous solution.38 In this paper we
extend these studies to the photooxidation of selected chlo-
rophenols by uranyl ion.
Experimental Section
Time-Resolved Transient Absorption and Emission Mea-
surements. Transient absorption experiments in the 20 ns to
* Author to whom correspondence should be addressed at Laboratoire
de Photochimie U.M.R. 6505, F-63177 Aubie`re Cedex, France. Fax: (33)
4 73 40 77 00. E-mail: Mohamed. SARAKHA@univ-bpclermont.fr.
10.1021/jp993676t CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/21/2000

Electron-Transfer Oxidation of Chlorophenols
J. Phys. Chem. A, Vol. 104, No. 14, 2000 3143
CHART 1
Proton nuclear magnetic resonance (1H NMR) spectra were
recorded at ambient temperature on a Bruker AC400 (Fourier
Transform) spectrometer in deuterated methanol or chloroform
1
(Aldrich). Chart I shows the H NMR features of the isolated
photoproducts.
GC mass spectral analyses were performed on a Hewlett-
Packard 5985 model apparatus equipped with a capillary column
Supelco (30 m length, i.d. 0.25 mm, and a film thickness of 25
µm). The injection and detector temperatures were 250 and 200
°C, respectively. A high grade helium was used as a carrier
gas.
Materials
All reagents were of the purest grade commercially available,
and were used without further purification. Solutions were
prepared in doubly distilled water, in equilibrium with air,
saturated with oxygen or deaerated by bubbling with argon for
30 min at 22 °C. For some luminescence quenching measure-
ments, pH was adjusted by addition of dilute nitric acid. The
ionic strength was not controlled.
Figure 1. UV-visible absorption spectra of uranyl alone and uranyl/
4-chlorophenol. (a) UO22+, 1.0 × 10-2 M; (b) UO22+, 1.0 × 10-2 M/4-
chlorophenol, 5.0 × 10-3 M; (c) UO22+, 1.0 × 10-2 M/4-chlorophenol,
1.0 × 10-2 M; (d) UO22+, 1.0 × 10-2 M/4-chlorophenol, 2.0 × 10-2
M; (e) UO22+, 1.0 × 10-2 M/4-chlorophenol, 5.0 × 10-2 M.
Results
At room temperature, the absorption spectra of mixtures of
UO22+ ([UO22+] g 1.0 × 10-2 M) with the three chlorophenols
used in the present work ([chlorophenol] < 5.0 × 10-3 M) were
shown to be equal to the sum of the component spectra, showing
that under the conditions of our experiments no significant
ground-state interaction between uranyl ion and chlorophenols
was present. Luminescence quenching and flash photolysis data,
which will be presented later, support this. However, it should
be noted that under conditions where the concentration of
chlorophenol was higher than that of uranyl ion (i.e., [chlo-
rophenol]/[UO22+] >1), an increase in absorbance over the
whole absorption spectrum was observed (Figure 1). This
Steady-State Studies. The photolysis at 334 nm of aqueous
solutions of 4-chlorophenol (1.0 × 10-3 M) in the presence of
UO2 (5.0 × 10-2 M) led to a continuous increase of the
2+
absorbance over the whole UV-visible absorption spectrum.
The HPLC analysis of the irradiated solution gave evidence for
the formation of three main photoproducts (Figure 2c). The first,
benzoquinone (E), was easily identified by comparison of its
retention time and UV-visible spectrum with a reference
sample. The photoproducts F and G were isolated from the
irradiated solution and identified by 1H NMR as 2,4′-dihydroxy-
5-chlorobiphenyl (λmax ) 257 and 295 nm), and 2,2′-dihyro-
doxy-5,5′-dichlorobiphenyl (λmax ) 280 nm) respectively (see
Experimental Section for NMR spectral features). The evolution
of these products as a function of irradiation time clearly showed
phenomenon can be related to that previously reported for
2+ 42
complexing of the phenolic hydroxyl group with UO2
.

3144 J. Phys. Chem. A, Vol. 104, No. 14, 2000
Sarakha et al.
Figure 3. Effect of the oxygen concentration on the quantum yield of
monochlorophenol disappearance.
Figure 2. High-performance liquid chromatograms observed for the
2+
products of photolysis of aerated aqueous solutions of UO2 (5.0 ×
Although only three oxygen concentrations were studied, the
quantum yield for disappearance is clearly not a linear function
of oxygen concentration for the three compounds (Figure 3).
10-2 M) and monochlorophenol (1.0 × 10-3 M): 2-chlorophenol (a);
3-chlorophenol (b); 4-chlorophenol (c). Detection wavelength ) 280
nm. A: UO22+; B: starting material; C: 2,4′-dihydroxy-3,3′-dichloro-
biphenyl; D: 4,4′-dihydroxy-3,3′-dichlorobiphenyl; E: benzoquinone;
F: 2,2′-dihyrodoxy-5,5′-dichlorobiphenyl; G: 2,4′-dihydroxy-5-chloro-
biphenyl.
Transient Absorption and Fluorescence Quenching Stud-
ies. To establish the mechanism and kinetic details of the
chlorophenol degradation photoinduced by uranyl ions, we
employed a nanosecond laser flash photolysis technique. When
that they are primary photoproducts. However, upon prolonged
irradiation a secondary photoproduct with longer retention time
was also formed, although it has not yet proved possible to
accurately assign it. In the case of 2-chlorophenol, the irradiation
2+
UO2 (0.05 M) in argon-purged aqueous solutions at pH 2.3
is flash photolyzed, a transient absorption appears with absorp-
tion maximum at approximately 580 nm (Figure 4a). The
spectrum is identical to that reported in the literature and
assigned to the excited UO22+*.17,25,44 The amount of formation
of uranyl ion excited state, as monitored by its initial absorption
at 580 nm, was linearly dependent upon the initial light intensity
within the 1-10 mJ laser energy range, clearly showing that
we are dealing only with a monophotonic process. Under our
experimental conditions the decay of UO22+* followed good
first-order kinetics at all wavelengths of the absorption spectrum,
with ko ) 7.0 × 105 s-1. This is close to what has previously
been reported for the decay of unhydrolyzed excited uranyl ion,
*[UO2(H2O)5]2+, in acidic media.17,32,45 The decay of the uranyl
luminescence at 520 nm following excitation with the Nd:YAG
laser was similar to that of the 580 nm absorption, although
under certain conditions a better fit was obtained to a biexpo-
nential decay for the luminescence data. Biexponential decay
of uranyl luminescence has previously been reported for aqueous
solutions at the natural pH,26,46-52 and the current consensus is
that this is due to emission from various hydrolyzed species,
produced in both ground and excited states.46,50,52 The fact that
we observe only a single component in the decay of the excited-
state absorption may reflect a higher molar absorption coefficient
for *[UO2(H2O)5]2+ than for any hydrolyzed uranium species
at the analyzing wavelength. As previously reported,46,53 ko is
unaffected by the presence of oxygen.
2+
at 334 nm in the presence of UO2 led to the formation of
chlorobenzoquinone, the ortho-para and para-para coupling
dimers (2,4′-dihydroxy 3,3′-dichlorobiphenyl and 4,4′-dihydroxy
3,3′-dichlorobiphenyl) as the main detectable photoproducts. In
the case of 3-chlorophenol, although disappearance of the phenol
was observed, the amounts of the generated photoproducts were
too small to enable their separation and identification from the
irradiated mixture, even on prolonged irradiation. HPLC data
on the products of photolysis of all three chlorophenols in the
presence of uranyl ion are illustrated in Figure 2.
Quantum yields were determined for photolysis of 4-chlo-
rophenol in the presence of uranyl ion in aerated, deaerated,
and oxygenated solutions, and are presented in Table 1.
These results clearly show that the disappearance of 4-chlo-
rophenol is favored by the presence of oxygen. Benzoquinone
accounts for roughly 15% and 25% of the conversion in aerated
and oxygenated solutions, respectively. Similar effects were
observed for the formation of 2,4′-dihydroxy-5-chlorobiphenyl
and 2,2′-dihydroxy-5,5′-dichlorobiphenyl. No formation of U(V)
was detected by UV-visible absorption spectroscopy under our
experimental conditions.
Similar behavior was observed for the disappearance of
2-chlorophenol and 3-chlorophenol photoinduced by UO2
2+
(Table 1).
TABLE 1: Effect of Oxygen Concentration on the Quantum Yield of 4-Chlorophenol Disappearance and Benzoquinone
Formation, and of 2-Chlorophenol and 3-Chlorophenol Disappearance ([UO22+] ) 5.0 × 10-2 M and [Chlorophenol] ) 1.0 ×
10-3 M, λexcitation ) 334 nm)
4-chlorophenol
oxygen
2-chlorophenol
3-chlorophenol
concentration, M
Φdisappearance
Φbenzoquinone
Φdisappearance
Φdisappearance
<5 × 10-6
5.9 × 10-3
3.5 × 10-2
4.5 × 10-2
6.1 × 10-2
traces
1.0 × 10-2
4.1 × 10-2
6.0 × 10-2
7.0 × 10-2
4.0 × 10-3
2.0 × 10-2
3.4 × 10-2
5.4 × 10-2
2.6 × 10-4 (ref 43)
6.0 × 10-4 Ma
1.2 × 10-3
5.0 × 10-3
9.6 × 10-3
1.5 × 10-2
a Obtained by bubbling a 1:1 mixture O2/N2.

Electron-Transfer Oxidation of Chlorophenols
J. Phys. Chem. A, Vol. 104, No. 14, 2000 3145
Figure 4. Transient absorption spectra for uranyl ion alone (1.0 × 10-2 M) (a) and uranyl (1.0 × 10-2 M) plus 4-chlorophenol (2.0 × 10-3) (b);
3-chlorophenol (2.0 × 10-3) (c) and 2-chlorophenol (2.0 × 10-3) (d).
TABLE 2: Bimolecular Rate Constants and Steady-State
When the monochlorophenols were present in the solutions
2+
Stern-Volmer Constants for the Quenching of *UO2 by
at concentrations up to 2.3 × 10-3 M, the initial *UO2
2+
Monochlorophenols in Aqueous Solutions
absorbance decays more rapidly, following the overall rate law:
a
a
k0 /105
kq /109
KSVb/103
KSV/τ/109
s-1
M-1 s-1
M-1
M-1 s-1
2+
d[*UO2
]
5.61c 10.05d
4.95c 10.78d
5.94c 10.05d
1.7c 4.1d
1.5c 4.4d
1.8c 4.1d
2+
2-chlorophenol
3-chlorophenol
4-chlorophenol
6.5
7.2
7.5
1.4
1.1
1.7
-
) (ko + kq[chlorophenol])[*UO2 ]
dt
2+
a Values obtained by using linear regression of pseudo-first-order
rate constants for decay of *UO22* absorption at 580 nm or uranyl
fluorescence at 520 nm as functions of monochlorophenol concentration;
see text for meaning of symbols. b Determined with λexcitation ) 350
nm. c For pH 0.8, using τ ) 3.3 µs. d For pH 2.3 using τ ) 2.45 µs.
where ko and kq represent the rate constant for *UO2 decay
in the absence of the quencher and the second-order rate constant
for the quenching process, respectively.
For all three monochlorophenols, the buildup of a new
absorbance was observed in the region 350-420 nm on time
scales identical to the decay of the excited uranyl ion decay.
Typical transient absorption spectra for the three chlorophenols
are illustrated in Figure 4, while kinetic traces for decay of
Both steady-state and dynamic quenching of uranyl lumi-
nescence by the monochlorophenols was also studied. The
uranyl luminescence decays were linear functions of substrate
concentration for all three chlorophenols, and followed kinetic
similar behavior to the decay of the uranyl excited-state
absorption at 580 nm. In steady-state measurements, the
luminescence between 480 and 650 nm was quenched by all
three chlorophenols, and Stern-Volmer plots were found to be
linear up to millimolar concentrations. From the slopes of these
and the excited uranyl ion lifetime, values of kq were calculated
at pH 1 and 2.3, and are also given in Table 2. The excellent
agreement between the steady-state values for kq at pH 1 and
those obtained from flash photolysis supports the earlier sug-
gestion that quenching of uranyl excited-state involves a dy-
namic process with the chlorophenols leading to charge transfer.
Taking the values of the molar absorption coefficients
reported in the literature:55 ꢀ395 nm ) 1800 M-1 cm-1 for the
2+
*UO2 absorption and grow-in of transient absorption at 400
nm for the case of 4-chlorophenol are shown in Figure 5a, b.
Comparison of these new spectra with those observed upon pulse
radiolysis of aqueous solutions of chlorophenols in the presence
of the one-electron oxidant azide radical54-56 indicates that they
are due to the monochlorophenoxyl radicals species. These
results, and the observation of reasonable isosbestic points,
indicate a clean oxidation, which we suggest involve an electron-
transfer process from the monochlorophenol to excited uranyl
ion. The growth of the monochlorophenoxyl radical obeyed a
2+
pseudo-first-order rate law, similar to that obtained for *UO2
decay in the presence of the monochlorophenols. In Table 2
we report average values, with standard deviations, for ko and
kq obtained from the linear dependence of the pseudo-first-order
rate constant for decay of the *UO22+ species or grow-in of the
monochlorophenoxyl radical upon monochlorophenol concen-
tration. The values of ko are in all cases identical to that observed
in the absence of chlorophenol. The values of kq are close to
but slightly lower than those for a diffusion-controlled process.
2-chlorophenoxyl radical, ꢀ415
3-chlorophenoxyl radical, ꢀ420
) 2220 M-1 cm-1 for the
) 5100 M-1 cm-1 for the
nm
nm
4-chlorophenoxyl radical,43,44 and ꢀ580
) 5800 M-1 cm-1
nm
2+
for the excited *UO2 species,44 the efficiency, ê, of the
electron-transfer process was calculated according to

3146 J. Phys. Chem. A, Vol. 104, No. 14, 2000
Sarakha et al.
the rest is suggested to involve physical deactivation via an
exciplex.37 With alkylbenzenes, quenching of uranyl excited
state has been suggested to occur purely through the physical
process.14 However, it should be noticed that a small fraction
(ca. 1%) of quenching of excited uranyl ion by toluene involves
hydrogen atom abstraction.61 Despite the high oxidation potential
of excited uranyl ion, some additional factor is necessary to
favor electron transfer over physical quenching. In studies on
electron transfer from polychlorophenolate anions to excited
[Ru(bpy)3]2+ in various water-methanol mixtures a marked
dependence of the yield of cage escape on the percentage of
methanol was observed.58,59 For the studies of both quenching
of uranyl ion luminescence by substituted benzenes14 and
oxidation of naphthalene by excited uranyl ion,37 it was
necessary to use mixtures of water and organic solvents to
dissolve the aromatic compound, and it is possible that a similar
dependence of the yield of electron transfer on solvent polarity
occurs. However, as with uranium(V)-halogen atom radical
pairs, energetic factors are also expected to be important.32
In the absence of oxygen, two different pathways can be
suggested for the disappearance of the uranium(V)-phenoxyl
radical pair:
Figure 5. Kinetic traces for uranyl/4-chlorophenol (1.0 × 10-2 M/
2.0 × 10-3) mixture. (a) Decay of excited uranyl ion absorption at 580
nm; (b) grow-in of 4-chlorophenoxyl radical absorption at 400 nm; (c)
decay of 4-chlorophenoxyl radical absorption at 400 nm.
Ao(*UO22+) ꢀmax radical
ê )
ꢀ580
A∞(radical)
where Ao(*UO22+) is the absorbance at 580 nm of the excited
2+
*UO2 species observed immediately after the flash, and A∞
(radical) is the absorbance of the monochlorophenoxyl radical
determined after completion of its grow-in at the maximum of
its absorption spectrum.
The values of the efficiency associated with the electron-
transfer process (ê2-chlorophenol ) 0.97; ê3-chlorophenol ) 1.0;
ê4-chlorophenol ) 0.99) are within the experimental errors, equal
to unity. These appear to be unaffected by the presence of
oxygen. These results can be contrasted with the quenching of
excited uranyl ion by alkylbenzenes, which is suggested to
proceed purely by a physical process involving exciplex
formation, with no overall electron transfer.14 Electron transfer
2+
has, however, been demonstrated in the quenching of *UO2
by naphthalene37 and in the reaction of photoexcited [Ru-
(bpy)3]2+ with chlorophenolate anions.57-59
The decays of the chlorophenoxyl radical absorbances were
examined in the presence and in the absence of oxygen. In both
cases the absorbance disappeared to the baseline via a second-
order process (Figure 5 c). Using values for the molar extinction
coefficients from ref 56, second-order rate constants (k) for
decay of chlorophenoxyl radicals at 400 nm of 4.5 ((0.3) ×
109 M-1 s-1 (2-chloro-), 9.6 ((0.7) × 109 M-1 s-1 (3-chloro-),
and 6.1 ((0.2) × 109 M-1 s-1 (4-chloro-) were determined.
The nature of this decay process will be considered in the
Discussion section.
(a) back electron transfer leading to the formation of uranyl
ion in its ground state and monochlorophenol, and (b) cage
escape leading to the formation of phenoxyl radical and U(V).
Good evidence for back electron transfer within uranium-
(V)-radical pairs has been presented for a system with halogen
atoms,32 and the fact that the quantum yield of disappearance
of chlorophenol in deaerated solutions was very low (<6 ×
10-3), even though the efficiency of electron transfer was
roughly equal to unity, may suggest that this is an important
process here. However, such a decay should follow first-order
kinetics, while the observed decay of phenoxyl radicals is
second-order. We feel therefore, in agreement with the sugges-
tion of Mao and Bakac for the uranyl-naphthalene system,37
that much of the uranium(V)-chlorophenoxyl radical pair
decays by cage escape followed by oxidation of uranium(V)
by the phenoxyl radical.
Discussion
The transient absorption and fluorescence quenching observa-
tions in this study clearly show that excited uranyl ion is capable
of rapidly oxidizing monochlorophenols. Although both electron
and hydrogen atom transfer are possible, the high bond
dissociation energy for the phenolic hydrogen (≈90 kcal
mol-1 5,60
makes atom transfer unlikely, and we suggest that
)
the mechanism is similar to that previously proposed for
2+ 30,38
oxidation of phenols by *UO2
.
The initial step in this
2+
mechanism is an electron transfer from *UO2 species to the
chlorophenol derivative leading to the formation of a uranium-
(V)-chlorophenol radical cation complex. The highly acidic
phenol radical cation (pKa -10 to -13)60 is then expected to
rapidly deprotonate within the radical pair to give the phenoxyl
radical. The efficiency of this overall electron transfer between
chlorophenols and excited uranyl ion was found to be close to
100%. For quenching of excited uranyl ion by naphthalene,
about 30% of the reaction proceeds by electron transfer while
UO2+ + ClPhO• f UO22+ + ClPhO-
2+/+
This reaction is thermodynamically favorable (E° UO2
0.16V,62 4-ClPhO•/- + 0.80 V 54), and expected to be fast.
+
In the presence of oxygen, a third pathway must be involved
since the quantum yields of monochlorophenol disappearance
and of photoproduct formation increase, and show a dependence

Electron-Transfer Oxidation of Chlorophenols
J. Phys. Chem. A, Vol. 104, No. 14, 2000 3147
upon the concentration of oxygen. Neither excited uranyl ion46,53
nor phenoxyl radicals5,63,64 react with oxygen. In addition, while
uranium(V) is oxidized to UO2 by molecular oxygen, the
The formation of benzoquinone in the case of 4-chlorophenol
may involve reaction involving the hydroperoxyl radical, HO2
as follows:
•
2+
reaction is relatively slow.65,66 However, autoxidation of ura-
nium(V) probably involves the formation of the superoxoura-
nium(VI).65 This can protonate to produce the previously
characterized uranyl-hydroperoxyl radical adduct,67 which can
dissociate to yield free hydroperoxyl radical. Although our
results do not allow us to distinguish between formation of such
a species by reaction of the uranium(V)-chlorophenoxyl radical
pair with oxygen and autoxidation of free uranium(V), we feel
it likely from the observed oxygen dependence of the quantum
yield of chlorophenol disappearance that at least some of the
oxidation occurs in the radical pair. The hydroperoxyl radical,
•
HO2 , has been shown to oxidize a variety of organic substrates68
and may also disproportionate to produce hydrogen peroxide
and oxygen either directly,68 or catalyzed by uranyl ion.67,69
Hydrogen peroxide may then react with substrates, or dissociate
to give the hydroxyl radical, ‚OH. The involvement of hydroxyl
radical in the photooxidation of various aromatic substrates by
uranyl ion has been reported.37,61,70 In addition, the reactions
of hydroxyl radicals with chlorophenols have been reviewed.5
However, the products are rather different from those observed
in the present study. For example, the reaction of hydroxyl
radical with 4-chlorophenol yields mainly 4-chlorocatechol,71,72
while the present studies indicate that the principal products of
uranyl photooxidation are benzoquinone and dimers. We,
therefore, feel that in our system reactions of hydroperoxyl
radicals or other transient species are more important than those
of hydroxyl radicals.
with the intermediacy of the benzoquinone oxide, (I). This
species has been proposed by Grabner et al. in their flash
photolysis study of the phototransformation of 4-chlorophenol.73
This species was suggested to be formed by reaction of oxygen
with the carbene 4-oxocyclohexa-2,5-dienylidene (II). The
transient (II) is also suggested to react with 4-chlorophenol to
give 2,4′-dihydroxy 5- chloro biphenyl,73 one of the products
of uranyl-sensitized photooxidation of 4-chlorophenol, by the
reaction
The evidence for the involvement of oxygen in the process
of chlorophenol disappearance highlights the fact that uranyl
ion acts as a photocatalyst in the process. As the solution is
continuously irradiated, the uranyl ion regenerated may be
excited again, reduced by chlorophenol, and again oxidized by
oxygen leading to a catalytic cycle. Similar catalytic cycles have
previously been reported for the photooxidation by uranyl ion
of toluene,61 naphthalene,37 and various other hydrocarbons.36
Following formation of the uranium(V)-chlorophenoxyl radical
pair, the subsequent steps of the catalytic cycle is suggested to
involve the formation of hydroperoxyl radicals.
However, the carbene (II), which absorbs at 370 and 384 nm,
was not detected in our study, and although it may be formed
and present at low concentrations, an alternative rout to form
(I) involves addition of hydroperoxyl radical to the 4-chlo-
rophenoxyl radical followed by elimination of HCl.
The formation of 2,4′-dihydroxy 5-chlorobiphenyl may be
explained by a reaction of the phenoxyl radical with the starting
4-chlorophenol.
The formation of the different photoproducts can be explained
through the reactivity of the phenoxyl radical, in its different
mesomeric structures, with the different species present in the
solution.

3148 J. Phys. Chem. A, Vol. 104, No. 14, 2000
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between Clermont-Ferrand and Coimbra by JNICT and CNRS
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