Reaction of phenol with singlet oxygen

Article abstract of DOI:10.1039/c8cp04852e

Photo-degradation of organic pollutants plays an important role in their removal from the environment. This study provides an experimental and theoretical account of the reaction of singlet oxygen O₂(¹Δ_g) with the biodegradable-resistant species of phenol in an aqueous medium. The experiments combine customised LED-photoreactors, high-performance liquid chromatography (HPLC), and electron paramagnetic resonance (EPR) imaging, employing rose bengal as a sensitiser. Guided by density functional theory (DFT) calculations at the M062X level, we report the mechanism of the reaction and its kinetic model. Addition of O₂(¹Δ_g) to the phenol molecule branches into two competitive 1,4-cycloaddition and ortho ene-type routes, yielding 2,3-dioxabicyclo[2.2.2]octa-5,7-dien-1-ol (i.e., 1,4-endoperoxide 1-hydroxy-2,5-cyclohexadiene) and 2-hydroperoxycyclohexa-3,5-dien-1-one, respectively. Unimolecular rearrangements of the 1,4-endoperoxide proceed in a facile exothermic reaction to form the only experimentally detected product, para-benzoquinone. EPR revealed the nature of the oxidation intermediates and corroborated the appearance of O₂(¹Δ_g) as the only active radical participating in the photosensitised reaction. Additional experiments excluded the formation of hydroxyl (HO), hydroperoxyl (HO₂), and phenoxy intermediates. We detected for the first time the para-semibenzoquinone anion (PSBQ), supporting the reaction pathway leading to the formation of para-benzoquinone. Our experiments and the water-solvation model result in the overall reaction rates of k_r-solvation = 1.21 × 10⁴ M^-1 s^-1 and k_r = 1.14 × 10⁴ M^-1 s^-1, respectively. These results have practical application to quantify the degradation of phenol in wastewater treatment.

Full text of DOI:10.1039/c8cp04852e

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Reaction of phenol with singlet oxygen†
a
a
a
Jomana Al-Nu’airat,
Nassim Zeinali,
Bogdan Z. Dlugogorski, * Xiangpeng Gao,
Cite this: Phys.Chem.Chem.Phys.,
a
a
b
a
Jakub Skut, Phillip R. Westmoreland,
Mohammednoor Altarawneh
Ibukun Oluwoye and
2
019, 21, 171
a
Photo-degradation of organic pollutants plays an important role in their removal from the environment.
1
This study provides an experimental and theoretical account of the reaction of singlet oxygen O
2
( D
g
)
with the biodegradable-resistant species of phenol in an aqueous medium. The experiments combine
customised LED-photoreactors, high-performance liquid chromatography (HPLC), and electron
paramagnetic resonance (EPR) imaging, employing rose bengal as a sensitiser. Guided by density
functional theory (DFT) calculations at the M062X level, we report the mechanism of the reaction and its
1
kinetic model. Addition of O
2
( D
g
) to the phenol molecule branches into two competitive 1,4-cycloaddition
and ortho ene-type routes, yielding 2,3-dioxabicyclo[2.2.2]octa-5,7-dien-1-ol (i.e., 1,4-endoperoxide
-hydroxy-2,5-cyclohexadiene) and 2-hydroperoxycyclohexa-3,5-dien-1-one, respectively. Unimolecular
1
rearrangements of the 1,4-endoperoxide proceed in a facile exothermic reaction to form the only
experimentally detected product, para-benzoquinone. EPR revealed the nature of the oxidation intermediates
1
and corroborated the appearance of O
2
( D
g
) as the only active radical participating in the photosensitised
ꢀ
ꢀ
Received 31st July 2018,
reaction. Additional experiments excluded the formation of hydroxyl (HO ), hydroperoxyl (HO
2
), and phenoxy
Accepted 21st November 2018
intermediates. We detected for the first time the para-semibenzoquinone anion (PSBQ), supporting the reaction
pathway leading to the formation of para-benzoquinone. Our experiments and the water-solvation model
DOI: 10.1039/c8cp04852e
4
ꢂ1 ꢂ1
4
ꢂ1 ꢂ1
result in the overall reaction rates of kr-solvation = 1.21 ꢁ 10
M
s
and k
r
= 1.14 ꢁ 10
M s , respectively.
rsc.li/pccp
These results have practical application to quantify the degradation of phenol in wastewater treatment.
generate reactive oxygen species (ROS), mainly singlet oxygen
1
. Introduction
1
2–4
O ( D ), to initiate the photo-degradation process.
2
g
1
Even though light-induced reactions remain one of the less explored
Dye-sensitised photo systems generate singlet oxygen O ( D )
2 g
5,6
areas of green chemistry, many researchers have attempted to use efficiently, resulting in a wide range of applications of such
7
8
9
10–12
sunlight in wastewater-detoxification processes, particularly in those processes in chemical, medical, biological, and wastewater
1
1
involving biodegradation-resistant materials such as phenol. An treatments. Singlet oxygen O
2
( D
g
) represents one of the most
ꢂ1
0
effective photosensitisation process relies on efficient light reactive ROS, residing 94 kJ mol (D
G
H
298) above the ground state
3
ꢂ
13,14
absorption. Unfortunately, the transparency of phenol mandates triplet oxygen (O
2
( S
g
)).
One of the prominent effects of singlet
15–17
the use of an intermediate (dye) to absorb visible light in order to oxygen stems from its role in chain initiation and propagation
with aromatic organic compounds such as phenol. Reactions of
1
a
O ( D ) include [p +p ] 1,2-cycloaddition to isolated CQC bonds,
[
2
g
2
2
School of Engineering and Information Technology, Murdoch University,
0 South Street, Murdoch, WA 6150, Australia.
p +p ] 1,4-cycloaddition to conjugated double bonds either in
2 4
9
E-mail: B.Dlugogorski@murdoch.edu.au; Tel: +61 8 9360 6770
Department of Chemical and Biomolecular Engineering, North Carolina State
University, Raleigh, NC 27695-7905, USA
alkenes or aromatics, and the ene (Schenck) reaction normally
involving the transfer of an allylic hydrogen from the carbon atom
to the oxygen molecule.
Removing phenol and its derivatives, by reacting them with
the ROS such as singlet oxygen, represents an attractive pathway
b
18,19
†
Electronic supplementary information (ESI) available: Fig. S1. Bleaching test of
rose bengal in buffered solution (pH = 6), Fig. S2. HPLC spectrum of (35 mM RB,
mM phenol) illuminated sample in buffered solution, Fig. S3. UV-vis absorption
1
spectra for benzoquinone and hydroquinone, Fig. S4. Calibration curve for for decreasing the toxicity of contaminated water. Phenols pollute
ferrioxalate actinometer, Fig. S5. Optimised structures of transition states of
surface water through industrial effluents in areas such as
85
phenol oxidation by singlet oxygen, Table S1. POLYMATH ordinary differential
equations (ODEs) and parameters for the estimated singlet oxygen concentration
ꢂ1
ꢂ1
refineries (6–500 mg L ) and coal processing (9–6800 mg L
)
85
plants, severely affecting aquatic life, even at concentrations
profile, Table S2. POLYMATH ordinary differential equations (ODEs) and
parameters for the solvation model, and detailed description of the actinometry
experiments. See DOI: 10.1039/c8cp04852e
4
,20
as low as 1 ppb.
On the other hand, the steady-state con-
centration of singlet oxygen that exists in natural water varies
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ꢂ14
21
within a range of (6–28) ꢁ 10
M. In natural waters, singlet (tert-butyl alcohol, 499%; sodium formate, 99.998%). 2-Propanol
2
2–24
oxygen forms with a quantum yield of 1 to 3%,
its likelihood to coexist with phenol.
indicating (499%) was from Ajax Finechem. TEMP (2,2,6,6-tetramethyl-4-
piperidone 99.9%), which functions as an EPR spin trap for
Previous investigations on the photooxidation of phenol singlet O , and TEMPO (2,2,6,6-tetramethyl-4-piperidone-N-oxyl,
2
with singlet oxygen reported different products, such as para- 97%), a standard product of the reaction of TEMP with singlet
1
0–12,25
benzoquinone, hydroquinone, and hydroxybiphenyls.
oxygen, were from Merck. Actinometry reagents (iron chloride,
However, the underlining reaction mechanism remains poorly 499%; potassium oxalate monohydrate, 98.5%; sodium acetate
understood. Both the product distribution and photochemical anhydrous, 99%; and 1,10-phenanthroline monohydrate, 99.5%)
efficiency strongly depend, in most cases, on the concentration were from Chem Supply, and iron sulfate heptahydrate and
of oxygen, a selected sensitiser and the pH of the reaction. For sulfuric acid (98%) were from Fisher Scientific. All aqueous
instance, alkaline conditions prompt phenol to deprotonate solutions were prepared for EPR, UV-vis, and HPLC standards and
ꢂ
into phenoxide ion (aka phenolate, PhO , pK = 9.8) leading to the mobile phases using ultrapure water (resistivity 418 MO cm),
a
formation of 2-hydroxy-1,4-benzoquinone, 2-hydroxyhydroquinone, tapped from a Sartorius Arium Pro UV/DI water purification system.
2
,5-dihydroxy-1,4-benzoquinone and others, rather than the
4
12
25 2.2. Photooxidation of phenol with singlet oxygen
usual para-benzoquinone. Briviba et al. and Pizzocaro et al.
demonstrated that reaction of O ( D ) with the phenol molecule Generally, polychromatic (lamp) and monochromatic (laser)
2 g
1
29
30
leads selectively to the formation of para-benzoquinone. The absence lights generate the electromagnetic energy in the UV and visible
of catechol and ortho-benzoquinone suggested the formation of range for photoreactions. The selection criteria depend on the
a 1,4-endoperoxide (2,3-dioxabicyclo[2.2.2]octa-5,7-dien-1-ol) as environmental conditions and the required efficiency. For example,
an initial intermediate via a 1,4-cycloaddition mechanism, with laser sources illuminate living cells in biological systems, while
subsequent steps proposed to involve unimolecular elimination monochromatic light such as UV radiation covers larger areas; e.g.,
2
9
of water. Other experimental studies attributed the formation in wastewater treatment. Examples of monochromatic lamps
of para-benzoquinone-type derivatives to the abstraction of include light-emitting diodes (LEDs), xenon arcs, and medium-
1
phenolic hydrogen by O
2
( D
g
), followed by reactions with the pressure Hg lamps selected by their relative photon intensity
3
ꢂ
26,27
ground-state triplet oxygen (O
2
( S
g
)).
Hence, the overall and efficiency. Among them, LEDs exhibit the most effective
oxidation pathways of phenol by singlet oxygen still remain operation, with the lighting efficiency of photo-oxygenated
3
1
largely speculative. Moreover, experiments appear insufficient system attaining 70%. Hence, the present study deployed LEDs
for conclusive mechanistic and kinetic insights because of as a light source. Fig. 1 illustrates the photoreactor. We employed
uncertainties relating to the very short lifetime of singlet molecular 24 green LEDs (4.5 ꢁ 10 ꢁ 500 mm), operated between 7 and
ꢂ4 14
oxygen (B10 s) and the possible parallel reactions involving 24 V. The LEDs form a 10 cm-long illuminating zone, assembled
3
ꢂ
ꢀ
28
triplet oxygen O
experimental investigation of the reaction of O
must be coupled with a sophisticated theoretical analysis.
2 g
( S ), and hydroxyl radical (HO ). Consequently, from eight strips, each consisting of three units. We uniformly
1
2
( D
g
) with phenol mounted the strips onto the inside of a white PVC tube casing
that surrounds the reactor. The dead-zone thickness between the
To this end, the present contribution develops an experi- LED units in a single strip amounted to 0.68 cm, based on a 1201
1
mental apparatus to react phenol with O ( D ) in a semi-batch beam angle, prompting us to centre the quartz reactor 1 cm away
2
g
1
reactor with known initial concentrations of phenol and O ( D ) from the PVC holder.
2
g
3
ꢂ
produced continuously in situ from bubbled O ( S ). We identify
Fig. S1 in ESI† shows that, rose bengal (RB) displays a strong
2
g
and quantify the intermediate and product species as a function absorption in the green band with a maximum around 510 nm
5
ꢂ1
ꢂ1 32,33
of time using UV-vis and high-performance liquid chromatography and molar absorption coefficient as high as 10 M cm
.
(
HPLC). Reported findings map out the governing mechanism for RB constitutes an efficient photosensitiser with a quantum
3
4
the reaction of phenol with singlet oxygen, where the appearance of yield of (0.76–0.85). To ensure a clean and pure source of
radical species is confirmed by electron paramagnetic resonance singlet oxygen, the energy difference between the sensitiser and
(
EPR) measurements. These results, supplemented by the quantum its first triplet excited state should be equal or greater than the
ꢂ1
chemical calculations, afford formulation of a kinetic model that singlet oxygen energy level of about 94 kJ mol (i.e. 0.98 eV)
offers practical applications to water treatment.
3
5
above the ground-state triplet oxygen. The energy difference
between RB singlet ground and its first triplet excited state
ꢂ1
36
corresponds to 166 kJ mol (1.72 eV). Another reason for using
RB lies in its stability over the time scale of our experiments (i.e.,
around 1 h). We conducted the bleaching test of RB by illuminat-
2
2
. Materials and methodology
.1. Materials
We obtained all chemicals with the highest commercially ing a 35 mM of the sensitiser-buffered solution for 0–24 h, followed
available purity. The sensitiser (rose bengal, 85%, Acros Organics), by testing the absorbance (abs) in a range of 200–800 nm using a
reactant (phenol, 499%, Sigma Aldrich) and expected products (1,4- UV-vis spectrophotometer. The bleaching rate of rose bengal
benzoquinone, 99%; hydroquinone, 98%; catechol, 499%) were (r ), calculated from the rate of change of absorbance ((abs ꢂ
B
ꢂ1
ꢂ15
ꢂ1
sourced from Sigma Aldrich, as were scavengers of the singlet oxygen abs1) ꢁ t (s) ), amounts to 3.5 ꢁ 10
M s .
(
sodium azide, 499.99%; glutathione, 498%; DL-dithiothreitol,
We prepared buffered solutions (pH = 6 or pD = 6.29,
499%; all Sigma Aldrich) and scavengers of hydroxyl radical confirmed by a Hach pH-meter) of 1 mM phenol and 35 mM
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of phenol and the yields of the reaction products from the LED
photoreactor. The analytical method involved injections of samples
of 10 mL in volume and maintaining the oven temperature at 30 1C.
ꢂ1
The mobile phase comprised a solution of 0.01 mol L phosphate
buffer (pH 4.5), methanol, and tetrahydrofuran (THF) in the ratio of
ꢂ1
9
0:5:5, v/v, flowing at a rate of 1 mL min , under low-pressure
conditions. The analyte elutes into a UV-vis detector operating at
54 nm using a reversed phase elution mode. Under these
2
conditions, phenol eluted at 25.5 min and para-benzoquinone
at 4 min (see Fig. S2, ESI†). Furthermore, we obtained the
absorption spectra of RB (Fig. S1, ESI†), phenol (not shown),
hydroquinone (Fig. S3, ESI†), para-benzoquinones (Fig. S3,
ESI†), and complex of ferrous ion and 1,10-phenanthroline
(actinometry experiment, Fig. S4, ESI†) in a quartz cell (1 cm
pathlength) using Agilent Carry 5000 UV-vis-NIR spectrophoto-
meter operated at a resolution of 1 nm. Based on the spectra in
Fig. S1, S3 and S4 (ESI†), we were able to confirm 254 nm as an
optimum wavelength for HPLC analyses.
An electron paramagnetic resonance (EPR) spectrometer
served to quantitate the concentration of spin-trapped singlet
oxygen, as well as to monitor in situ the appearance of radical
species. Chemical spin traps represent highly reactive species
that capture singlet oxygen by forming a stable compound
(
endoperoxide), detectable by the EPR spectrometry. We used
the most common singlet oxygen spin trap, 2,2,6,6-tetramethyl-
-piperidone (TEMP), because it reacts rapidly and selectively
4
1
with O ( D ) to form 2,2,6,6-tetramethyl-4-piperidone-N-oxyl
2
g
(
TEMPO), a distinct and stable endoperoxide, with no side
3
9
products. EPR spectroscopy readily detects TEMPO. TEMP
demonstrates high solubility in water and transparency to a
range of excitation wavelengths.
Fig. 1 Schematic diagram of photochemical reactor shown in side and
2
cross-sections. 1: O /He (3%) valve, 2: mass flow controller, 3: inlet gas, 4:
4
0,41
We did not select other
pyrex reactor, 5: phenol dyed solution (pH = 6), 6: LED strip (7 V), 7: PVC
holder, 8: off-gas outlet, 9: exhaust.
common spin traps (tetrapotassium rubrene-2,3,8,9-tetracarb-
oxylate (RTC) and disodium 9,10-anthracenedipro panoate
4
2
(ADP)) because of their reduced transparency.
2 2
rose bengal (5, Fig. 1) freshly in D O or H O in a temperature-
The EMXPlus 10/2.7 EPR spectrometer, fitted with an ER
102ST general-purpose rectangular X-band cavity and cooling
side plates, averaged 5 scans at a resonant frequency of 9.76
GHz. We placed the samples in a 10.5 mm flat cell (WG-812-Q),
and bubbled a mixture of 3% oxygen in He carrier gas for a least
controlled laboratory maintained at 25 1C. The illumination
experiments lasted 1 to 50 min and involved 5 mL aliquots,
sourced from the stock solution and charged into the photo-
reactor. During the photo-induced reaction, a calibrated mass
4
flow controller (Brooks 4800 series) fed a 3% O
continuously at a constant rate of 500 mL min . The helium
carrier gas (instead of the generally applied N
2
(in He balance)
2
0 min. Fig. 2 illustrates an LED light, fixed on a wooden holder
ꢂ1
to prevent metallic interference with the magnetic field, that
irradiated the sample in situ (i.e., inside the EPR resonator
cavity). The spectrometer functioned at microwave power of
2
medium)
reduced the intermolecular interaction between the excited-state
37
molecules and the inert gas. Okamoto et al. reported no effect of
superficial gas velocity (i.e., no mass transfer limitation) on the
2
0 mW, modulation frequency of 100 kHz, modulation amplitude
of 1 G, sweep width of 100 G, and with the time constant set to
.01 ms, conversion time to 50 ms, sweep time to 100 s, receiver
ꢂ1
phenol reaction rate at above 0.35 cm s , prompting us to adopt
0
ꢂ1
5
0
.44 cm s as the superficial gas velocity in our experiments.
gain to 2 ꢁ 10 , number of data points to 10 000, and the centre
We estimated the concentration of oxygen in the aqueous phase
field to 3487.00 G.
ꢂ5
to reach 3.9 ꢁ 10 M based on Henry’s law (PO2 = K
H
ꢀ
[O
in He
2
],
ꢂ1 38
2.4. Estimated LED light intensity
where K
H
= 769.23 atm M
)
and 3% concentration of O
2
4
3
carrier gas (P = 1 atm).
A potassium ferrioxalate actinometer was used to obtain light
intensity based on the Hatchard–Parker approach for reactors in
2.3. Analytical techniques
44,45
Fig. 1 and 2.
This method provides reliable results between
A Shimadzu HPLC instrument, equipped with a C18 analytical 253 and 577 nm, based on established quantum yields for
4
3
column (4.6 mm diameter and 150 mm length) and controlled potassium ferrioxalate. ESI† provides a more detailed descrip-
by data acquisition software, served to quantify the conversion tion of the experimental procedure; Fig. S4 (ESI†) illustrates the
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2+
Fig. 3 Photogenerated ferrous ions (Fe ) using 24 green LEDs at 0, 15,
0, and 50 min irradiation times.
3
ꢂ8
ꢂ1
After acquiring Dn/Dt = 5.96 ꢁ 10 mol min (the slope of
the plot in Fig. 3) for the photoreactor (Fig. 1, LED operated at
ꢂ9
ꢂ1 ꢂ1
7
V), the light intensity = 1.157 ꢁ 10 E L
s . On the other
hand, for the in situ EPR reactor (Fig. 2), the light intensity
ꢂ5
ꢂ1 ꢂ1
amounts to 32.75 ꢁ 10 E L
s , reflecting the higher voltage
of 24 V of the LED light.
2.5. Computational details
4
6
The Gaussian 09 suite of programs was used for all energy
‡
0
0
‡
0
0
Fig. 2 In situ EPR photoreactor schematic; 1: EPR resonator, 2: inlet to the
quartz flat cell (Wilmad WG-812-Q), 3: collet nut, 4: dyed sample, 5:
modulation cable, 6: green LED (24 V), 7: wooden support.
calculations (D H , D H , D G , and D G ) and structural
2
98
r
298
298
r 298
optimisations. Possible biradical character of the phenol +
1
2 g
O ( D ) system required determining the spin state of inter-
mediate and transition structures along the entire potential energy
surface. Severe spin contaminations required energy refinements
based on the approximated spin-projection scheme (AP) pioneered
measurement of the absorbance of the ferrioxalate solution by
UV-vis spectroscopy.
4
7
by the Yamagushi group, in which the located transition
structures and intermediates converge on a pure low-spin solution
with zero-spin densities. Structural optimisations that involve
fixing the frontier molecular orbitals (i.e., the broken symmetry
solution) allow assessing a plausible spin contamination by
examining a value of the total spin angular momentum operator
Eqn (1)–(3) yield the number of absorbed photons:
Dn
Iv ¼
(1)
(2)
(3)
ꢂ3
1
0
0
ꢃ F ꢃ V1 ꢃ Dt
ꢂ3
1
ꢃ V1 ꢃ V3 ꢃ Ct
V2
Dn ¼
2 48
S . A value of this operator of near unity for high spin
configuration indicates a significant spin contamination. We
found that expected values of the spin contamination operator
of all of the present intermediates and transition structures
abs:
Ct ¼
e ꢃ l
1
clustered close to zero. This means that the phenol + O ( D )
2
g
where Dn denotes change of the photo-generated ferrous iron system exists in a genuine singlet state, eliminating the need for
1
(
M), F the ferrioxalate quantum yield at the operated wave- further energy refinement. However, the energy of the O
2 g
( D )
4
3
length (0.86, 510 nm ), Dt irradiation time (s), V
volume (mL), and V the volume taken from the irradiated the value of the spin contamination operator S of approximately
samples (mL), V the diluted volume (mL), C the concentration 1. As such, we applied the AP approach to refine the energy of the
of ferrous iron after dilution (M), abs. the absorbance at 510 nm singlet oxygen molecule.
which has been corrected by the background of a blank sample, Cremer and co-workers showed that unrestricted density
l equals cm (the light path of the quartz cell) and e represents functional methods (UDFT) perform well in comparison to
1
irradiated suffers from contamination by the triplet state, as evident from
2
2
3
t
4
9
ꢂ1
ꢂ1
the molar absorptivity in M
cm
and the slope of the multireference methods for deriving energies and geometries
standard curve (e = 11 073, Fig. S4, ESI†). Our measurement of singlet biradicals. Employing hybrid DFT functionals further
of e concurs well with the recently reported values of 11 050 and enhances the accuracy of the UDFT in describing biradical
ꢂ1
ꢂ1 43
50
1
1 100 M cm
.
systems. Accordingly, we deployed the hybrid DFT functional
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Table 1 Arrhenius rate parameters fitted in the temperature range of 300–400 K and 1 M for the phenol reaction channels with singlet oxygen; Asolvation
3
ꢂ1 ꢂ1
ꢂ1
ꢂ1
is cm molecule
s
or s for unimolecular reaction and Ea-solvation in kJ mol
ꢂ1
3
ꢂ1 ꢂ1
ꢂ1
a-solvation (kJ mol
ꢂ1 ꢂ1 ꢂ1
A
solvation (s , cm molecule
s
)
E
)
k (M
s , s )
Reaction
300–400 K
300–400 K
298.15 K
1
ꢂ15
4
Phenol + O
Phenol + O
Phenol + O
Phenol + O
Phenol + O
M6 - M7
M7 - M8
2
2
2
2
2
( D
g
) - M1
) - M2
) - M3
) - M4
) - M5
8.24 ꢁ 10
1.03 ꢁ 10
9.75 ꢁ 10
1.76 ꢁ 10
1.37 ꢁ 10
1.57 ꢁ 10
1.31 ꢁ 10
2.07 ꢁ 10
15
41
37
111
116
57
134
130
1.21 ꢁ 10
4.07 ꢁ 10
1.93 ꢁ 10
3.78 ꢁ 10
3.91 ꢁ 10
1.62 ꢁ 10
2.91 ꢁ 10
3.47 ꢁ 10
1
ꢂ14
ꢂ15
ꢂ15
ꢂ13
8
ꢂ1
0
( D
g
g
g
g
1
( D
1
ꢂ14
ꢂ13
ꢂ2
ꢂ3
ꢂ2
( D
1
( D
2
1
1
2
2
M8 - para-BQ + H O
ꢅ
kisc
k
ꢅ
5
1
52
1
ꢄ
3
ꢄ
1
9
ꢂ1
3
ꢂ1
of M062X with the extended 6-311+G(3df,2p) basis set. The
M062X functional has been parameterised to provide accurate
and cost-effective performance for general application in
RB ƒ! RB ƒ! RB kisc ¼ 1 ꢁ 10 s ; k ¼ 6:5 ꢁ 10 s
(6)
(7)
(8)
5
3–55
organic chemistry.
phase by deploying the polarisable continuum model (PCM)
with a dielectric constant (e ) equal to 78.39 at 298.15 K.
Attempts to investigate the catalytic effect of discrete water
molecules, rather than applying PCM, did not yield viable
pathways. All transition structures contain one and only one
imaginary frequency. We have confirmed the identity of each
We modelled reactions in the aqueous
kt
3
ꢄ
3
1
1
9
ꢂ1 ꢂ1
RB þ O2 ƒ! RB þ O2 kt ¼ 1:2 ꢁ 10 M
s
c
kd
1
3
5
ꢂ1
O2 þ H2O ƒ! H2O þ O2 kd ¼ 5 ꢁ 10 s
kp
1
3
O2 ƒ! O2 þ hn
(9)
transition structure by performing intrinsic reaction coordinate The lifetime of singlet oxygen changes dramatically in different
IRC) calculation. In the next section, we quote changes in enthalpy solvents depending on the energy-transfer efficiency from
6
3
(
and Gibbs energy calculated at 298.15 K. Finally, the KiSThelP electronic to intramolecular vibrational states. The closer the
56
code was used for computation of the reaction rate constants in vibrational mode of the solvent to that of singlet oxygen, the
the temperature range of 300–400 K, as included in Table 1.
higher the effectiveness of the deactivating process. For
instance, O–D and O–H bonds absorb photons with energy
corresponding to 2550 and 3500 cm , respectively, whereas
ꢂ1
3. Results and discussion
singlet oxygen emits photons characterised by energy matching
3.1. Generation of singlet oxygen
ꢂ1
3
286 cm ; hence, it quenches far faster in water than deuterium
6
4
65
Eqn (4)–(9) describe Type II dye-photoreaction mechanism for oxide (heavy water). Snowden estimated the natural life-
5
7
rose bengal. Starting from a sensitiser molecule in its ground time of singlet oxygen to be thirteen times higher in D
2
O
1
state, RB absorbs green photon to form a singlet excited dianion than in H
2
O.
1
ꢂ1 58
(
RB*, eqn (4)) with singlet state energy of E = 213 kJ mol
.
The strictly hindered amine (TEMP) reacts selectively with
singlet oxygen to form nitroxide radical (TEMPO) that displays a
S
1
3
T
Subsequently, the RB* spin-flips to the triplet state ( RB*, E =
ꢂ1
59
1
66 kJ mol , reaction (6)) through intersystem crossing (ISC).
3
RB* may deactivate by phosphorescence or non-radiative
6
0
1
3
ꢂ1 57
processes to RB with a rate constant of k1 = 6.5 ꢁ 10 s
,
1
ꢂ15
ꢂ1
and RB bleaches with the calculated rate of r
B
= 3.5 ꢁ 10
M s
;
see Section 2.2. While the photosensitiser molecule relaxes to its
ground state, it transfers the excitation energy to a molecule of
triplet oxygen O , forming singlet state oxygen O (reaction (7),
3
1
2
2
9
ꢂ1 ꢂ1 57
k
t
= 1.2 ꢁ 10
M
s
). The vibronic coupling with water
quenches singlet oxygen with a pseudo-first-order rate constant
of k
singlet oxygen (k
contribution to the decay process in solutions. The product of
5
ꢂ1 61
d
= 5 ꢁ 10 s
.
We do not account for the radiative decay of
p
, reaction (9)) as this pathway makes negligible
6
2
hn in reaction (4) represents the light intensity absorbed by RB,
1
calculated earlier using the actinometry technique, and RBb
denotes the rose bengal bleaching product.
kv
1
1
ꢄ
10
ꢂ1
RB þ hn ƒ! RB kv ¼ 1 ꢁ 10 L E
(4)
(5)
Fig. 4 Double integrated area of the time course TEMPO signals in H O
2
(
2
red circles) and D O (black squares) (a), TEMPO spectra recorded by in situ
kb
2
EPR after illuminating TEMP sample in dyed D O solution at pD = 6.35,
1
RB þ hn ƒ! RBb kb ¼ 0:856 L Eꢂ1
1
g = 2.00062, and centre field = 3487.00 G (b).
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software (Table S2, ESI†). The measured trend correlates well with
the kinetic approximation, indicating an expected concentration
ꢂ6
of singlet oxygen between (3–4) ꢁ 10 M in response to 3 h
illumination by 24 V LED light.
3.2. Reaction of singlet oxygen with phenol
The photoreaction of phenol with singlet oxygen follows
eqn (10) and (11). Eqn (10) illustrates the physical quenching
(
1
spin orbital coupling), with no oxygen consumption (k
q
= 8 ꢁ
6
ꢂ1 ꢂ1 68
0 M
s ), whereas, eqn (11) describes the overall formation
of para-benzoquinone (BQ), written here as a global step and
broken down into elementary steps in the next section.
kq
1
3
O2 þ Phenol ƒ! Phenol þ O2
(10)
(11)
2
Fig. 5 EPR concentration profile of singlet oxygen generated in H O
kr
1
using 24 V LED. The dash-dotted line represents the corresponding values
obtained from POLYMATH kinetic solution detailed in Table S1 (ESI†).
O2 þ Phenol ƒ! para-BQ
3
.2.1. Mechanistic and kinetic analysis. As pictured in
1
Fig. 6, the reaction of phenol with O ( D ) branches into five
2
g
6
6,67
characteristic EPR signal.
signals in 50% (MeOH, D
double-integrating the spectral areas. The yield of TEMPO is on (Fig. S5 in ESI† depicts geometries of the located transition
average 8.45 times higher in D O than H O, corroborating a structures of all considered reactions). Fig. 6 portrays two
O. Fig. 4b depicts the in situ [p +p ] 1,4-cycloaddition reactions producing intermediates
spectral profiles of TEMPO after the reaction of TEMP with the M1 and M2 (1,4-peroxide structures). The concerted 1,4-cyclo-
In Fig. 4, we compare the TEMPO initial channels with Gibbs free energies of activation spanning
ꢂ1
ꢂ1
2
2
O) and in 50% (MeOH, H O) by a relatively wide range between 59 kJ mol and 168 kJ mol
2
2
1
longer half-life of O
2
( D
g
) in D
2
2
4
1
photogenerated singlet oxygen, as recorded for the 35 mM RB addition of O
( D ) at ipso C(OH) and a para positions requires
g
2
ꢂ1
sensitiser present in 50% (by volume) solution of MeOH in D O. a 17–22 kJ mol
lower energy barrier than corresponding
The signal increased steadily over the analysis time interval of addition at ortho and meta sites. This observation could easily
5 min. Fig. 5 contrasts the experimental and modelled profiles be rationalised based on the charge distribution in the phenol
2
3
of concentration of singlet oxygen in water, the former obtained molecule. The hydroxyl group donates charge to ortho and para
from the EPR spin-trap technique and the latter from the carbon atoms, leaving C(OH) as an electron-deficient site.
solution of the coupled ordinary differential equations that Calculated Mulliken charges at para C, meta (C), ortho (C) and
describe the kinetics of the process, as coded in the POLYMATH ipso C sites amount to ꢂ0.194 eV, ꢂ0.133 eV, ꢂ0.143 eV,
1
Fig. 6 Potential energy map for the initial channels in the reaction of O
2
( D
g
) with phenol. Values in bold and italic denote Gibbs energy of reaction and
‡
0
0
ꢂ1
activation (D G298 and D
r
G
298) calculated at 298.15 K, respectively in kJ mol . The TS1 (green) line represents the preferred pathway, subsequently
illustrated in Fig. 7.
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0
.354 eV, respectively. It follows that the para carbon appears reaction at the ortho position as a main reaction channel in the
1
1
2 g 2 g
relatively more susceptible to O ( D ) 1,4-cycloaddition than the system of phenol and O ( D ). In contrast, we predict that
ortho C atom, and consistently we find a lower energy barrier the commonly discussed route affording the M4 intermediate
for the formation of M1 in comparison to the emergence of M2 constitutes an unlikely pathway (cf. Table 1).
ꢂ1
ꢂ1
(
59 kJ mol versus 81 kJ mol ).
The formation of phenoxy radical via direct abstraction
Two ene-type reactions lead to the formation of the two could be ruled out in view of the high Gibbs free energy barrier
hydroperoxide ketones M3 and M4. Transition structures TS3 embedded in TS5. Moreover, our in situ EPR results revealed no
and TS4 are concerted abstractions of the phenolic H simultaneously formation of phenoxy radicals. This absence could potentially
1
with the addition of the O
2
( D
g
) at an ortho or para positions, be explained easily through the ionisation of the hydroxyl group
represents the
respectively. TS3 incurs a significantly lower energy barrier than (i.e., deprotonation) as in eqn (15), where K
TS4 (76 kJ mol versus 168 kJ mol ) and contrasts with direct acidity constant of the phenol group –OH. We calculated the
a
ꢂ1
ꢂ1
H-abstraction reaction, which is associated with a sizable barrier deprotonation degree of the phenolic –OH group at pH = 6
ꢂ1
ꢂ4
165 kJ mol
.
as a = 1.6 ꢁ 10 . Therefore, the ak in eqn (14) remains
PO
ꢂ
70
Table 1 lists the reaction rate constants for the five initial negligible with akPO E 0.
channels fitted in the low to intermediate temperature window
of 300–400 K. Inspection of values in Table 1 reveals that,
the formation of M1 intermediate holds importance through-
out the considered temperature range with a rather negligible
contribution from the other four channels.
kr
POH
-
1
1
O2 þ P-OH ƒ ƒƒ ! para-BQ
(12)
kPOꢂ
O2 þ POꢂ ƒƒ ! by-product
(13)
(14)
6
9
By applying the Curtin–Hammond principle, the ratio of
kr ¼ ð1 ꢂ aÞkr-POH þ akPOꢂ
para–ipso and meta–ortho products ([Ppara]/[Pmeta]) is dictated
0
P–M
by the free energy gap (D
G
G298 ) between the para and meta
1
transition states (TS1 and TS2) rather than the stabilities of the
a ¼ 1
(15)
pKa
pH
-
þ 10
0
P–M
ꢂ1
two intermediates (M1 and M2); i.e., D
G
G
298 = 21 kJ mol
.
Our ratio of [Ppara]/[Pmeta] at 298.15 K corresponds to B9000 : 1. Next, we turn our attention to the selective formation of para-
1
This finding indicates an expected dominance of the para isomer benzoquinone from reaction of phenol and O ( D ). Any major
2 g
and a negligible abundance of the meta isomer, excluding the product flux toward para-benzoquinone follows the two major
meta-channel from further mechanistic analysis. The prominence initial pathways, producing M1 and M3 intermediates. Herein,
of the channel leading to the formation of M1 concurs with the we show that an unimolecular isomerisation of the M1 moiety
experimental detection of the 1,4-peroxide structures during UV produces para-benzoquinone via a facile mechanism. Fig. 7
7
radiation of several phenolic compounds. Consistently with the depicts the potential energy surface for the formation of
results of the present calculations, literature suggests no ene-type para-benzoquinone from oxidation of phenol by singlet oxygen.
Fig. 7 Enthalpy (bold non-italic green) and Gibbs free energy (italic red) map for the formation of para-BQ from the reaction of singlet oxygen with
ꢂ1
phenol using M062X (solvation) model. All values are in kJ mol at 298.15 K.
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ꢂ1
The M1 intermediate incurs endothermicity of 25 kJ mol
(
ꢂ1
with exergicity of 16 kJ mol ) relative to the separated reactants,
and it could further stabilise into the three-member-ring
structure M6 without passing through an intrinsic reaction
barrier. A 1,2-hydrogen transfer via TS6 proceeds with an
ꢂ1
enthalpic barrier of 53 kJ mol and yields the M7 intermediate.
In the subsequent step, intramolecular hydrogen transfer from the
hydroxyl group through TS7 forms the significantly de-energised
ꢂ1
M8 intermediate through an enthalpic barrier of 140 kJ mol
.
Thus, the M7 - M8 step represents one of the ‘‘bottlenecks’’ of
the mechanism shown in Fig. 7. Finally, elimination of water from
intermediate M8 yields para-benzoquinone. The enthalpy of the
ꢂ1
final products falls 360 kJ mol below the separated reactants.
3
.2.2. Formation of product species. HPLC data provides
1
Fig. 9 Concentration profiles for phenol depletion via our experimental
2 g
major insights into the O ( D ) interaction with phenol. The
photolysis of phenol-RB buffered solution ultimately yielded
para-benzoquinone with no other products detected (Fig. 8).
2 2
results in H O (red circles) and D O (blue triangles), versus water-solvation
model (green line). The overall mechanism for the water solvation model is
discussed in Section 3.2.1 and cast into a set of ordinary differential
This high selectivity towards the formation of para-adduct equations that are listed in the ESI.†
product was also observed in the case of singlet-oxygen reaction
7
1
with salicylic acid. In Fig. 9, singlet oxygen degrades phenol
4
ꢂ1 ꢂ1
with a second-order rate constant of k = 1.14 ꢁ 10 M
s
in
r
2
H O. In addition, in deuterium oxide, phenol reacts by approxi-
mately 7.5-fold faster than in water, supporting the involvement
of singlet oxygen. We detected no trace of para-benzoquinone
when executing the photoreaction of phenol in the absence of
rose bengal or oxygen.
Illumination of phenol in RB-buffered solution (50% MOH,
pH = 6.5) gives rise to the spectrum illustrated in Fig. 10,
showing a well-defined 1 : 4 : 6 : 4 : 1 stick configuration that
represents the interaction of the radical with four identical
protons. The five superhyperfine lines in Fig. 11 show splitting
72–74
of para-semibenzoquinone anion (PSBQ) signal.
In addition
to the PSBQ signal, we observed the appearance of a semi-
reduced anion spectrum of RB (i.e., the spectrum of an RB
trianion radical), as shown in Fig. 12, and described in eqn (16)
and (17). The pattern of RB trianion radical illustrates an
Fig. 10 In situ EPR spectrum of 0.5 mM para-benzoquinone and 35 mM RB
in 50% buffered methanol solution (pH = 6.5) after illumination for 1 min.
Fig. 8 Concentration profiles for the formation of para-benzoquinone via
unpaired electron interacting with two equivalent protons
the water-solvation model (green line) and the published experimental
12
(
known as position 1 and 8 in the RB structure). Even though
measurements of Briviba et al. (blue triangles, methylene blue) versus our
7
5
present results (red circles, rose bengal).
benzoquinone, as reported by Foote et al., does not interact
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Fig. 12 EPR signal of (a) RB trianion radical formed after illuminating a
sample containing 1 mM phenol, 35 mM rose bengal in 50% methanol
solution, at pH = 6 for 1 min, (b) simulated EPR spectrum and (c) the
subtract. Simulation parameters: g = 2.00439, HFS = 1.14224 ꢆ 0.00005.
Fig. 11 Experimental (a), simulated (b), residual (c) and time-course
(
d) EPR spectra obtained upon continuous in situ irradiation of 35 mM RB
and 1 mM phenol in 50% buffered methanol solution (pH = 6.5) for 3 h.
Simulation parameters: RB trianion; g = 2.00439, hyperfine splitting
HFS = 1.14224 ꢆ 0.00005, para-semibenzoquinone anion; g = 2.00446,
HFS = 2.36955 ꢆ 0.00005.
Fig. 13 Spins-course EPR spectra obtained upon continuous irradiation
in situ of 35 mM RB and 1 mM phenol in 50% buffered methanol solution
with singlet oxygen, it quenches the triplet state of rose bengal.
Fig. 13 graphs the spin counts profiles of both signals.
(
pH = 6.5) for 2.6 h. Simulation parameters: RB trianion (red circles); g =
2
.00439, HFS = 1.14224 ꢆ 0.00005, para-semibenzoquinone anion
kredox1
(PSBQ, black squares); g = 2.00446, HFS = 2.36955 ꢆ 0.00005.
3
RB þ RB ƒƒ! RB þ RBꢀꢂ
ꢄ
3
ꢄ
ꢀþ
(16)
5
8
8
ꢂ1 ꢂ1
s
kredox1 ¼ 5:39 ꢆ 1:43 ꢁ 10 M
77
without appearance of the meta-product. We tested the effect of
radical scavengers for both singlet oxygen and hydroxyl radical
kredox2
1
3
RB þ RB ƒƒ! RB þ RBꢀꢂ
ꢄ
ꢀþ
ꢀ
(O
2
( D
g
), HO ) in order to verify their existence in our system. As
(17)
ꢀ
noted in Table 2, the addition of the HO scavengers (tert-butyl
5
8
8
ꢂ1 ꢂ1
s
kredox2 ¼ 1:73 ꢆ 0:43 ꢁ 10 M
78
79,80
alcohol, 2-propanol, and sodium formate
) seemed to have
no obvious effect on the benzoquinone production. In contrast,
ꢀ
ꢂ
ꢀ
ꢂ
1
61,81
82
RB + BQ ! RB + BQ
(18)
(19)
O ( D ) scavengers (sodium azide,
glutathione ) suppress the formation of para-benzoquinone
dithiothreitol,
and
2
g
83
ꢀ
ꢂ
ꢀ
+
BQ + RB - RB + BQ
ꢀ
(
e.g., sodium azide o0.01, Table 2). The failure of the HO
In several mechanisms suggested for the photosensitised reac- scavengers to block the formation of benzoquinone demonstrates
tion of phenol, researchers argued the effect of hydroxyl radical the absence of HO radical in this reaction, whereas a significant
ꢀ
HO ) on the product formation and selectivity. Reaction of effect of O ( D ) scavengers further supports the involvement of
2 g
ꢀ
1
(
1
phenol with hydroxyl radical proceeds via ortho, meta, and para singlet oxygen O ( D ). Moreover, the presence of only the para-
2
g
76
1
routes. Even though the meta-product was not detected in some product in the reaction of phenol with O
2
( D
g
) indicates no
ꢀ
scenarios, the ortho analogue was always present. For example, involvement of HO in the oxidation process.
ꢀ
when HO is generated using Fe(II)–EDTA/H
2
O
2
, hydroquinone
Applying the values of the kinetic parameters for solution
(para) and catechol (ortho) are produced in equivalent quantities from Table 2 and converting the units to L and mol, our calculated
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1
ꢀ
Table 2 Effect of O
( D
2 g
) and HO scavengers on the formation of Conflicts of interest
2
benzoquinone after 30 min of illumination (pH = 6, H O) at 298.15 K
The authors declare that they have no conflict of interest.
Scavengers
Benzoquinone (14 mM)
1
O
2
( D
g
)
Sodium azide (10 mM)
Dithiothreitol (10 mM)
Glutathione (10 mM)
o0.01
Acknowledgements
scavengers
1.05 ꢆ 0.05
1.43 ꢆ 0.17
We gratefully acknowledge grants of computing time from the
National Computational Infrastructure (NCI) and from the Pawsey
Supercomputing Centre, Australia as well as the financial supports
from the Australian Research Council (ARC). J. N. thanks Murdoch
University for the award of a MUSS postgraduate scholarship.
Our thanks are due to Dr Juita and Dr Marc Hampton for their
valuable help.
ꢀ
HO scavengers Sodium formate (10 mM)
13.55 ꢆ 0.51
tert-Butyl alchohol (100 mM) 13.02 ꢆ 0.32
2-Propanol (100 mM)
14.30 ꢆ 0.28
rate constant of photooxidation of phenol with singlet delta
4
ꢂ1 ꢂ1
oxygen at 298.15 K is kr-solvation = 1.21 ꢁ 10 M
s , which is
consistent with the present experimental measurement of
4
ꢂ1 ꢂ1
k
2
r
= 1.14 ꢁ 10 M
s
r
as well as the literature value of k =
4
ꢂ1 ꢂ1 12
84
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