Oxidation of phenol by singlet oxygen photosensitized by the tris(2,2′-bipyridine)ruthenium(II) ion

Article abstract of DOI:10.1021/jp9937104

The photochemical excitation of aqueous solutions of Ru(bpy)₃²⁺ and phenol (PhOH) in the presence of air produces 1,4-benzoquinone (BQ) as the only organic product. In this study, we examined the *Ru(bpy)₃²⁺/ PhOH/O₂ system in terms of the dependence of the quantum yield of BQ formation (φ_BQ) as a function of [O₂], [PhOH], temperature, pH, and the composition of the solvent. The increase in φ_BQ from its low value in acidic solution to its maximum at pH ～ 9.4 is attributed to the increasingly competitive quenching of *Ru(bpy)₃²⁺ by PhO^- and O₂. A maximum in φ_BQ is also observed at ～45 °C, an effect caused by the variation in the solubility of O₂ in solution as well as the activation energies of the competing steps in the mechanism. The proposed mechanism of the reaction, which could involve the formation of an endoperoxide intermediate from the reaction of ¹O₂ by PhOH, is consistent with the observed decrease of φ_BQ as a function of the mole fraction of H₂O in CH₃CN-H₂O solvent mixtures.

Full text of DOI:10.1021/jp9937104

5998
J. Phys. Chem. A 2000, 104, 5998-6002
Oxidation of Phenol by Singlet Oxygen Photosensitized by the
Tris(2,2′-bipyridine)ruthenium(II) Ion
Cang Li and Morton Z. Hoffman*
Department of Chemistry, Boston UniVersity, Boston, Massachusetts 02215
ReceiVed: October 18, 1999; In Final Form: April 12, 2000
2
+
The photochemical excitation of aqueous solutions of Ru(bpy)
produces 1,4-benzoquinone (BQ) as the only organic product. In this study, we examined the *Ru(bpy)
PhOH/O system in terms of the dependence of the quantum yield of BQ formation (ΦBQ) as a function of
], [PhOH], temperature, pH, and the composition of the solvent. The increase in ΦBQ from its low value
in acidic solution to its maximum at pH ∼ 9.4 is attributed to the increasingly competitive quenching of
3
and phenol (PhOH) in the presence of air
2+
3
/
2
[O
2
2+
-
*
Ru(bpy)
variation in the solubility of O
mechanism. The proposed mechanism of the reaction, which could involve the formation of an endoperoxide
3
by PhO and O
2
. A maximum in ΦBQ is also observed at ∼45 °C, an effect caused by the
2
in solution as well as the activation energies of the competing steps in the
1
intermediate from the reaction of O
of the mole fraction of H O in CH
2
by PhOH, is consistent with the observed decrease of ΦBQ as a function
CN-H O solvent mixtures.
2
3
2
Introduction
In a previous publication,52 we reported that the photochemi-
2
+
cal excitation of aqueous solutions of Ru(bpy)3 and PhOH in
the presence of air produced 1,4-benzoquinone (BQ) as the only
organic product; in the absence of O2, no BQ or any other
product was formed. Because the quantum yield of BQ
formation (ΦBQ) was significantly higher in D2O, it was clear
The photooxidation of phenols is an important degradation
mode of these pollutants in contaminated water. Previous studies
1
-8
have examined their direct excitation (λ ∼ 250 nm), organic
dye,9
and metal complex photosensitization, radiolyti-
cally induced decomposition, photocatalytic oxidation on TiO2
-21
22-24
25
1
that the photooxidation mechanism involved O2, which has a
2
6-33
surfaces,
and catalytic oxidation by transition metal com-
Organic and inorganic dye-sensitized solar photo-
3
8
longer lifetime in D2O than in H2O. On the basis of the
plexes.3
4,35
2
+
knowledge of the rate constant for the quenching of *Ru(bpy)3
oxidation is of particular interest in wastewater treatment,
especially for phenols that are resistant to biodegradation;
oxidation in the presence of a sensitizer, visible light, and air
can lead to photoproducts that are easily biodegradable, low
molecular weight, oxygenated compounds.9,36,37
Energy transfer from an excited photosensitizer to ground-
state dioxygen, O2, produces singlet oxygen, O2( ∆g), which
5
3
1
54
by O2, the rate constant for the quenching of O2 by PhOH,
1
2+
and the quantum yield of formation of O2 from *Ru(bpy)3
5
5
1
(
φ∆ ) 0.5), we concluded that the reaction between O2 and
PhOH to form BQ occurs with approximately unitary efficiency.
2
+
1
If the Ru(bpy)3 -photosensitized oxidation of PhOH by O2
occurs as envisioned, ΦBQ should be a function of the various
experimental parameters in a predictable way. In this paper,
we examine the *Ru(bpy)3 /PhOH/O2 system in detail in order
to test the mechanism and its accompanying kinetics. We report
here on the dependence of ΦBQ as a function of [O2], [PhOH],
temperature, pH, and the composition of CH3CN-H2O solvent
mixtures.
1
-
1
lies 94 kJ mol above the ground state; the physical and
chemical properties of O2 have been very well documented.
2+
1
38-42
1
Phenols can quench O2, leading to the formation of quinones
and coupling products.41 The reductive quenching of an excited
photosensitizer by phenols can also occur if the electron-transfer
energetics are suitable and the reaction can kinetically compete
with the natural decay of the excited species. The oxidation of
phenols or phenolate ions results in the formation of phenoxyl
radicals, which undergo very rapid dimerization in solution,
Experimental Methods
Materials. [Ru(bpy)3]Cl2‚6H2O (GFS Chemicals) was re-
crystallized from water and dried over silica gel. Phenol was
purchased from Aldrich and was purified by sublimation.
Potassium ferrioxalate was prepared by the literature method.56
Distilled water was further purified by passage through a
Millipore purification train. Acetonitrile (Baker HPLC grade)
was purified and dried as described in the literature.57 Buffer
solutions (pH 2-13) were prepared by literature methods.58
Continuous Photolysis. A 1000 W Xe lamp (Oriel) and
monochromator (Bausch & Lomb) were used; the light intensity
could be adjusted with the lamp power supply or with the
monochomator slit. In a typical experiment, 4 mL of a
magnetically stirred solution in a 1 × 1 cm cuvette within a
43
especially if they are unsubstituted; for the phenoxyl radical
from phenol (PhOH; pKa 9.98),44 2k ) 2.6 × 10 M s .
The archtypical metal complex photosensitizer, especially for
aqueous solutions, is Ru(bpy)32+ (bpy ) 2,2′-bipyridine). With
an excited-state standard reduction potential of 0.93 V (vs
9
-1 -1 45-47
4
8
2+
NHE), *Ru(bpy)3 is reductively quenched by phenolate ion
-
o
49
(
PhO ) in alkaline solution (Eox ) -0.86 V vs NHE), yielding
+
• 50,51
Ru(bpy)3 and PhO ;
however, it is not quenched by PhOH
o
in acidic and neutral solution (Eox ) -0.86 - 0.0582(pKa -
pH) V vs NHE at 20 °C).49
*
Corresponding author. Tel: (617)353-2494. Fax: (617)353-6466.
E-mail: hoffman@chem.bu.edu.
1
0.1021/jp9937104 CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/06/2000

Oxidation of Phenol by Singlet Oxygen
J. Phys. Chem. A, Vol. 104, No. 25, 2000 5999
Figure 2. ΦBQ as a function of [PhOH] for air-saturated aqueous
Figure 1. ΦBQ as a function of [O
2
] for aqueous solutions of 40 µM
and 4.0 mM PhOH at 20 °C and pH 6.0. Error bars are
10%. The solid curve is a fit of eq 6.
2+
solutions of 40 µM Ru(bpy)
3
at 20 °C and pH 6.0. Error bars are
2
+
Ru(bpy)
3
(10%. The solid curve is a fit of eq 6.
(
thermostated cell holder was irradiated at 450 nm. The light
-
5
intensity incident on the cell (Io ) (1.06 ( 0.05) × 10 ein
-1 -1
L
s ) was monitored with a Rhodamine 6G photon counter,
which was routinely calibrated against the potassium ferrioxalate
chemical actinometer.
HPLC Analyses. A Waters HPLC system (C-18 reversed-
phase column (3.9 × 300 mm), UV-vis photodiode array
detector, and Millennium 2010 software) was used to identify
quantitatively species from the continuous photolyses. Concen-
trations of possible photoproducts could be detected at concen-
trations of g0.01 µM.
Procedures. After the irradiation of solutions of Ru(bpy)32
and PhOH, a 20 µL sample of the photolyte was injected into
the HPLC port; a 3:1 (v/v) H2O/CH3CN solution was used as
the eluent with a flow rate of 1.0 mL/min, and the photodiode
array detector was set at 254 nm. The photoproduct and starting
materials were identified by comparison of their retention times
+
Figure 3. ΦBQ as a function of pH for air-saturated aqueous solutions
2+
of 80 µM Ru(bpy)
3
and 2.0 mM PhOH at 20 °C. Error bars are (10%.
and absorption spectra with those of authentic samples: Ru-
The solid curve is a fit of eq 16.
+
(
bpy)32 , 1.5 min; BQ, 5.8 min; PhOH, 10.3 min.
Results
The continuous photolysis (λex ) 450 nm) of air-saturated
aqueous solutions of Ru(bpy)32 and PhOH yielded BQ as the
only detectable photoproduct in the pH range 2.9-12.5; no
hydroquinone (HQ) or 4,4′-biphenol were ever found. The slope
of linear plots of the yield of BQ as a function of photolysis
time (an example is shown in the Supporting Information) leads
to values of ΦBQ, which were reproducible to better than (10%
+
-
3
variation. For example, ΦBQ ) 8.6 × 10 for solutions of
.04-0.2 mM Ru(bpy)32 and 4.0 mM PhOH at 20 °C and pH
+
0
6
.0.
Values of ΦBQ, determined as a function of [O2], [PhOH],
pH, and temperature are shown in Figures 1-4, respectively;
the data are given in the Supporting Information.
The value of ΦBQ decreased with increasing mole fraction
of acetonitrile (øAN) in CH3CN-H2O mixtures; no BQ was
detected in neat CH3CN. Figure 5 shows the dependence of
ΦBQ on øAN; the data are given in the Supporting Information.
Figure 4. ΦBQ as a function of temperature for air-saturated aqueous
2+
solutions of 80 µM Ru(bpy)
bars are (10%.
3
and 3.2 mM PhOH at pH 6.0. Error
Discussion
2
+
followed by the decay of *Ru(bpy)3 (reaction 2; ko ) 1.6 ×
6
-1 53
Reaction Mechanism. In its simplest terms, the reaction
mechanism in acidic and neutral solution can be viewed as
occurring via the excitation of the photosensitizer (reaction 1)
10 s ) in competition with its energy-transfer quenching by
9
-1 -1 55
1
O2 (reaction 3; kq ) 3.4 × 10 M s ); the resultant O2
decays in H2O (reaction 4; kde ) 2.7 × 10 s ) and reacts
5
-1 38

6000 J. Phys. Chem. A, Vol. 104, No. 25, 2000
Li and Hoffman
(reaction 11) would remove these radicals from any involvement
in the production of BQ.
-
+
PhOH a PhO + H
(7)
(8)
2
+
-
+
•
*
Ru(bpy)3 + PhO f Ru(bpy) + PhO
3
Ru(bpy)3+ + O f Ru(bpy) + O2
2+
•-
(9)
(10)
(11)
2
3
Ru(bpy)3+ + PhO f Ru(bpy) + PhO
•
2+
-
3
•
•-
-
PhO + O f PhO + O
2
2
1
It is known from other studies that the interaction of O2 with
PhOH changes as a function of pH. Okamoto et al. found that
1
the efficiency of photodegradation of PhOH by O2 generated
by rose bengal increased in the pH range 9.0-10.7, an effect
Figure 5. ΦBQ as a function of øAN for air-saturated CH
3
CN-H
2
O
2
+
attributed to a faster rate of reaction 12 compared to reaction
solutions of 40 µM Ru(bpy)
bars are (10%.
3
and 4.0 mM PhOH at 20 °C. Error
9
8
-1 -1 54
5; Tratnyek and Hoign e´ determined krx′ ) 1.6 × 10 M s .
1
-
O + PhO f f BQ
(12)
6
-1 -1 54
2
with PhOH (reaction 5; krx ) 2.6 × 10 M s ) to yield
BQ ultimately with presumably unitary efficiency.
If the reaction mechanism is modified to include reactions 8
•
•-
and 12, and it is assumed that PhO and O2 do not ultimately
hν
Ru(bpy)3 +
2
9
8 *Ru(bpy)3
2+
(1)
(2)
(3)
(4)
(5)
give rise to BQ, ΦBQ can be written in terms of eq 13 through
the application of the steady-state approximation for O2 as
1
2
before.
*
Ru(bpy)3 + f Ru(bpy)32+ + hν′
ΦBQ
)
Ru(bpy)3 + + O f Ru(bpy)3
2
2
2+
1
+ O2
*
-
φ k [O ](k [PhOH] + k ′[PhO ])
∆
q
2
rx
rx
-
-
1
(
k + k [O ] + k ′[PhO ])(k + k [PhOH] + k ′[PhO ])
O f O + hν′
o q 2 q de rx rx
2
2
(13)
1
O + PhOH f f BQ
-
2
The [PhOH] and [PhO ] terms are related to the initial
concentration of phenol, [PhOH]0, through reaction 7 and its
Ka.
For continuous irradiation at a known [Ru(bpy)32+] and
absorbance at 450 nm, application of the steady-state ap-
proximation for O2 yields eq 6; use of the known values of k
1
H+
[
PhOH] ) [PhOH]0
(14)
(15)
and φ∆ leads to an excellent fit of the data in Figures 1 and 2.
+
(
)
H + Ka
d[BQ]
Ia dt
φ
∆
k
q
k
rx[O
2
][PhOH]
ΦBQ
)
)
(6)
Ka
-
(k + k [O ])(k + k [PhOH])
[PhO ] ) [PhOH]
o
q
2
de
rx
0
+
(
)
H + Ka
Equation 6 also predicts that a plot of 1/ΦBQ vs 1/[PhOH]
should be linear at constant [O2] at low [PhOH] where kde .
krx[PhOH]. When the data from Figure 2 are treated in that way,
an excellent linear plot (correlation coefficient 0.999) results
By substituting eqs 14 and 15 into eq 13, an expression for
ΦBQ as a function of pH (eq 16) is obtained, where A ≡
φ∆kq′krx′Ka[O2][PhOH]0; B ≡ φ∆kq′krx[O2][PhOH]0; C ≡ kqKa-
[
PhOH]0 + kqKa[O2] + kq′Ka[PhOH]0; D ≡ ko + kq[O2]; E ≡
(Supporting Information).
kde; F ≡ krx′Ka[PhOH]0; G ≡ krx[PhOH]0; H ≡ Ka.
Effect of pH. The deprotonation of PhOH (reaction 7)
changes the dynamics of the photosensitization. In the first place,
-
pH
2
+
-
A + B × 10
the quenching of *Ru(bpy)3 by PhO (reaction 8; kq′ ) 1.1 ×
ΦBQ
)
(16)
-
pH
8
-1 -1 50
1
0 M s ), which will compete with reaction 3, must be
included; the yield of the geminate redox pair, Ru(bpy)3 and
PhO , into bulk solution upon escape from the quenching
cage (ηce) has been determined to be 0.40. Because of the
-pH
F + G × 10
(
C + D × 10 ) E +
+
(
-pH
)
H + 10
•
59
Under the experimental conditions of Figure 3, where
[PhOH]0 and [O2] are constant quantities, the data can be fitted
to eq 16 quite satisfactorily, using the known values of the rate
and equilibrium constants. In the pH 7-9 range, reaction 12
begins to compete more with reaction 4 than does reaction 5;
the dramatic decrease in ΦBQ beyond its maximum around pH
9.4 occurs as reaction 8 overwhelms reaction 3. The shallow
upward slope of the low pH region of Figure 3 is undoubtedly
due to contributions of reactions 8 and 12, which would compete
+
rapid electron-transfer reaction of Ru(bpy)3 with O2 (reaction
9
-1 -1 48
9
; ket ) 4.3 × 10 M s ), the back electron-transfer reaction
between Ru(bpy)3 and PhO (reaction 10; kbt ) 5.3 × 10 M
s ) does not have a chance to compete; O2 undergoes
+
•
9
-1
-1 59
•-
60
•
disproportionation with its conjugate acid. PhO reacts slowly,
if at all, with O2; there does not seem to be any report of a
measured rate constant, which is probably <10 M s .
Although PhO undergoes dimerization to form dihydroxy-
5
-1 -1
•
diphenyls,4
5-47
the anticipated rapid electron transfer with O2
•-
somewhat with reactions 3 and 5 even at low [PhO ].
-

Oxidation of Phenol by Singlet Oxygen
J. Phys. Chem. A, Vol. 104, No. 25, 2000 6001
However, an alternative explanation of the decrease in ΦBQ
in alkaline solution must be considered. Lipczynska-Kochany
and Bolton reported that the yield of BQ as the principal primary
product from the flash photolysis of PhOH in aqueous solution
SCHEME 1
in the presence of air was constant at pH 3-8, increased at pH
4
8
-10, and decreased at pH 10-11. In addition, they observed
secondary products, such as HQ and 2-hydroxy-p-benzoquinone,
at pH >8. Their results were explained in terms of a lower
stability of BQ at high pH. Indeed, it is known that BQ forms
-
1
-
reversible adducts (K ) 8.3 M ) with OH in alkaline
solution;61 however, we demonstrated that the HPLC analysis
for BQ was not affected by this interaction, leading us to believe
that the values of ΦBQ are not distorted by the analytical
procedure.
Effect of Temperature. Okamoto et al. examined the yield
of BQ as a function of temperature in the range 5-40 °C with
rose bengal as the photosensitizer and found an activation energy
cally, proton transfer is required for the formation of BQ or its
quinoid precursors. A possible mechanism, admittedly specula-
tive, is shown in Scheme 1, in which the first step involves
1,4-cycloaddition of O to the aromatic ring of PhOH to yield
endoperoxide I; this process has previously been postulated
-
1 9
for ΦBQ of 17 kJ mol ; a plot of ln ΦBQ vs 1/T for that
temperature range with Ru(bpy)32 as the photosensitizer yields
+
1
2
-
1
an activation energy of 31 kJ mol . The difference in the
temperature dependencies can be attributed in the first instance
to the expected differences in the activation parameters of the
photophysical and quenching processes of the excited states of
the two photosensitizers. However, as Figure 4 shows for pH
1
2
for the reaction of O with para-substituted 2,6-di-tert-butyl-
1
5
phenols and could be operative here as well. In the second
step, the peroxide-oxygen atom of I could undergo intra-
molecular H-abstraction to give hydroperoxycyclohexadienone
II (pathway a) in competition with solvolysis (pathway b) to
yield an unstable hydroperoxide, which could also lead to II
through the loss of H2O; similar pathways have been proposed
6
.0, ΦBQ rises to a maximum at ∼45 °C that is about a factor
of 5 higher than at its lower and higher temperature limits.
This complex dependence undoubtedly arises from the wide
range of temperature dependencies of all the rate and equilibrium
constants in eq 16, and of the solubility of O2, which, in air-
saturated aqueous solution, can be approximated by [O2] (in
units of molarity) ) 0.025 exp(-0.015T).58 Clearly, without
having all the information for all the activation parameters in
eq 16, it is not possible to model the data in Figure 4
quantitatively. Nevertheless, it is probably safe to say that at
low temperatures (5-40 °C), the generally positive temperature
dependencies of the rate constants dominate, and ΦBQ increases
with increasing temperature; at high temperatures (50-80 °C),
the negative temperature dependence of the solubility of O2
dominates, and ΦBQ decreases with increasing temperature.
1
earlier for the reaction of O2 with 2,4-di-tert-butylresorcinol
1
3
in CH3OH.
On the basis of the observed effect of the solution medium
on ΦBQ, we suggest that pathway b is the dominant process
here; the reaction would be rapid in aqueous solution but would
become slower as øAN is increased. Loss of H2O from II would
yield BQ. In neat CH3CN, I could form, but pathway b would
be inoperative; pathway a might be rendered slow due to the
formation of a dipolar transition state. In this case, I could
6
4,65
dissociate thermally to the original reactants
ground-state O2.
or directly to
Summary
Effect of Solution Medium. As øAN is increased at 20 °C,
the lifetime of *Ru(bpy)32 (τo ) 1/ko) increases from 0.60 µs
+
Knowledge of the parameters in eqs 6 and 16 for the Ru-
62
2+
in H2O to 0.97 µs in neat CH3CN. Similarly, the lifetime of
(bpy)3 /PhOH/O system dictates that the following conditions
2
1
O2 (τde ) 1/kde) increases from 3.7 µs in H2O to 58 µs in neat
should be employed in order to maximize ΦBQ in the photo-
sensitized oxidation of phenol to BQ: aqueous solution,
CH3CN.38 In addition, the solubility of O2 in organic solvents
63
is generally greater than it is in H2O. If all the other parameters
in eq 6 remained the same as the solution medium was changed,
ΦBQ would be expected to increase as øAN is increased.
However, Figure 5 shows that not to be the case.
preferably with D O as the solvent; T ∼ 45 °C; pH ∼9.4;
2
solution O -saturated, preferably at g1 atm pressure. The value
2
of φ could be increased to ∼1 by the use of a different Ru(II)
∆
2
+
photosensitizer, such as Ru(bpz)
however, PhOH as well as PhO quench *Ru(bpz)3 , yielding
Ru(bpz)3 and PhO , which would compete with the generation
of O2.
3
(bpz ) 2,2′-bipyrazine);
-
2+
Clearly, reactions 5 and 12 are complex, and previous
investigators have not reached complete agreement about their
mechanisms or even stoichiometries. For example, Okamoto et
al. found BQ to be the primary product from the reaction of
+
• 52
1
1
-
Acknowledgment. This work was supported by the Division
of Chemical Sciences, U.S. Department of Energy.
O2 with PhO in alkaline aqueous solution photogenerated from
9
rose bengal. However, Steenken et al. found HQ as the primary
product from the reaction of O2, generated photolytically from
1
Supporting Information Available: Tables of ΦBQ as a
function of [O2], [PhOH], temperature, pH, composition of CH3-
CN-H2O mixtures; plot of yield of BQ as a function of
photolysis time; plot of 1/ΦBQ vs 1/[PhOH]. This material is
available free of charge via the Internet at http://pubs.acs.org.
methylene blue or thermally from the decomposition of an
1
9
endoperoxide, with PhOH in neutral aqueous solution; in a
secondary reaction, HQ was converted to BQ. In general, the
1
evidence appears quite strong that the reaction of O2 with
aromatic compounds mainly involves cycloaddition to give
initially an endoperoxide, which is transformed in a series of
secondary steps into the final products.41
References and Notes
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The conclusion must be reached from the results obtained
here that H2O is required for the formation of BQ; mechanisti-
Helz, G. R., Zepp, R. G., Crosby, D. G., Eds.; Lewis Publishers: Boca
Raton, FL, 1994; pp 467-490.

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