High-efficiency photo-oxidation of thioethers over C60?PCN-222 under air

Article abstract of DOI:10.1039/c9ta07965c

The selective oxidation of primary thioethers to sulfoxides is an important reaction in the production of pharmaceuticals, agrochemicals, and other valuable fine chemicals. However, the achievement of high conversion and selectivity towards sulfoxide duri

Full text of DOI:10.1039/c9ta07965c

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High efficiency photo-oxidation of thioethers over C60@PCN-222
under air
Deng-Yue Zheng, En-Xuan Chen, Chun-Rong Ye, and Xiao-Chun Huang*
Received 00th January 20xx,
Accepted 00th January 20xx
Selective oxidation of primary thioethers to sulfoxides is an important reaction in the production of pharmaceuticals,
agrochemicals, and other valuable fine chemicals. However, the high conversion and selectivity toward sulfoxide in the
thioether oxidation under mild conditions still remain a great challenge. In this work, we found a new method to rise to the
challenge by applying C60@PCN-222 composite as the catalyst in the photo-oxidation of thioethers. The C60@PCN-222
composite is the integration of guest C60 molecules and a porous porphyrin-based metal-organic framework (MOF), PCN-
222. We propose that the photogenerated electron–hole separation of PCN-222 could be greatly enhanced through
encapsulation of C60 molecules with highly delocalized large π conjugation, which makes great contribution to the high
conversion and selectivity of this type of reaction. Under air and low optical power (50 mW/cm2) at room-temperature, the
composite exhibited excellent catalytic performance. We further explored the underlying mechanism for this photocatalyst
by recognizing reactive oxygen species via ESR spectra analysis and experimental conditions adjustment. It has been revealed
that the superoxide radicals, generated by electron transfer from a photoexcited MOF to C60 molecules, are served as the
main active species for the oxidations.
DOI: 10.1039/x0xx00000x
desired for the photocatalytic sulfoxides production. Fullerene
(C60) with precisely controllable size (0.7 nm) and structure has
unique properties owing to its special delocalized conjugated
Introduction
Sulfoxides are valuable synthetic reagents for the production of
a variety of significantly chemical and biological molecules.1
Thioethers oxidation is a common method to prepare the
corresponding series of sulfoxides. This process is widely used
in the industry of medicine, precombustion desulfurization of
fuels, innocent treatment of wastewater and chemical warfare
agents, etc.2 Traditionally, the reaction can be achieved through
thermal oxidation process by the use of several kinds of
oxidants, such as trifluoro peracetic acid, hydrogen peroxide,
tert-butyl hydroperoxide, and iodobenzene diacetate.3
Extensive efforts have been devoted to developing new
catalytic systems using environmentally benign reagents such
as O2 even air. photocatalysis is one of the most energy-efficient
method for organic transformation under mild conditions.4 In
this regard, photocatalysts including Pt, Ru, Au, and other noble
metal-based nanoparticles under O2 atmosphere have been
developed as effective catalysts,5 however, low selectivity and
catalytic activity still restrict their potential application in
industry. Therefore, the design and preparation of new type of
photocatalyst that combines the broad range of light absorption
with efficient electron (e-)–hole (h+) separation and high
reactive oxygen species (ROS) generation would be highly
structure which consists of 30 bonding molecular orbitals with
60 π electrons and is a potent generator of reactive oxygen
species.6 It is also well-known for its excellent electron affinities,
which could efficiently cause rapid photoinduced charge
separation and relatively slow charge recombination.7 It can be
expected that C60 is a potential excellent visible light
photocatalyst for thioethers oxidation while making up its weak
absorption. We believe that combining C60 with a suitable
semiconductor will improve the photoinduced charge
separation and transfer, which will overcome the disadvantage
of different single components and highly enhance the
photocatalytic activity.
Serving as a very promising catalytic platform, Metal–organic
frameworks (MOFs) are able to organize different components
to achieve photoresponse, which have attracted increasing
research interest in the application of artificial photosynthesis.8
Particularly, the most important advantage of MOFs with
respect to other catalysts is their diversified and tailorable
structures, which means that it is possible to tune the
performance of MOF photocatalysts by the selection of
appropriate building blocks and/or particular functional groups
for target structures. Recently, porphyrin-based MOFs are very
active in the research field of optoelectronic functional
materials due to the special photoelectric properties of
porphyrin molecules.9 Some porphyrin-based MOFs have been
reported to be effective photosensitizers for strong absorption
in the region of 200–800 nm and be consistent with the typical
n-type semiconductors.10 Several porphyrin-based MOFs have
Department of Chemistry and Key Laboratory for Preparation and Application of
Ordered Structural Materials of Guangdong Province, Shantou University,
Guangdong, 515063 P. R. China
Guangdong Provincial Laboratory of Chemistry and Fine Chemical Engineering,
Guangdong, 515063 P.R. China.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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was measured by BET method. High resolution VtierwanArstmicleisOsniloinne
DOI: 10.1039/C9TA07965C
electron microscope (HRTEM) images were obtained from FEI
Tecnai F20 electron microscope operated at 200 kV. UV-vis
absorption spectra were recorded on a Lambda 950
spectrophotometer in the wavelength range of 200-800 nm.
Photocurrent measurements and electrochemical impedance
spectroscopy (EIS) measurements were performed on a
CHI660E electrochemical workstation (Shanghai Chenhua
Apparatus, China) in a conventional three-electrode cell, while
saturated calomel electrode (SCE) was used as the reference
electrode and a Pt plate was used as the counter electrode.
Electron spin resonance spectra (ESR) were recorded on JES-
FA300 electron paramagnetic resonance spectrometer under
visible-light irradiation (λ > 400 nm). Scanning electron
microscopy (SEM) measurements were taken on a Gemini 300
with an acceleration voltage of 5 kV. The photoluminescence
(PL) spectra were measured on PTI QM-TM fluorescence
spectrometers. After the reaction, products were analyzed by
gas chromatography (GC, Agilent 7980B) with flame ionization
detector using HP-5 capillary column (30 m length, 0.32 ID and
0.25 μm film thickness) and confirmed by gas chromatography-
mass spectrometer (GC-MS, Shimadzu QP-2010 Ultra System).
Preparation of the porphyrin ligand (H2TCPP). The tetrakis(4-
carboxyphenyl)porphyrin (H2TCPP) ligand was synthesized
according to the previous reports.16
Scheme
1 Schematic illustration of the assembly of C60@PCN-222 for
photocatalytic oxidation of thioethers.
shown promise as photocatalytic agents facilitating the
oxidation of organics.10 Besides, it has been known that strong
π-π interactions between the curved π-surfaces of fullerenes
and the flat π-planes of porphyrins exist in both solution and
crystalline state.11,
12
Based on the above analysis, the
combination of porphyrins (electron donor) and C60 molecules
(electron acceptor) should be a good candidate for efficient
photoinduced charge separation because of the small
reorganization energy in electron transfer reactions.13,
14
Therefore, the integration of C60 molecules with
semiconductor-like porphyrin-based MOFs as ROS generator
may achieve efficient catalysis for thioether oxidation while the
synergistic effect occurs.
Synthesis of PCN-222, Zr6(μ3-OH)8(OH)8(TCPP)2
Herein, we rationally designed and prepared a new type of
photocatalyst, a composite C60@PCN-222 in which C60
molecules were stabilized by porphyrin-based MOF PCN-222.15
The optimized 3%-C60@PCN-222 exhibit excellent catalytic
activity and selectivity in the oxidation of primary thioethers to
sulfoxides in air by using visible-light irradiation (Scheme 1)
while integrating the advantages of both C60 molecules and
PCN-222, lying in their optical properties and ROS production
ability. To the best of our knowledge, this is the first catalyst to
present high conversion and selectivity for visible-light
photocatalytic oxidation of thioethers in air.
The MOF was synthesized based on the previous report with
minor modifications.15 Zirconium (IV) chloride (ZrCl4, 0.15 g),
H2TCPP (0.1 g) and benzoic acid (5.4 g) in 20 mL of N, N-
diethylformamide (DEF) were ultrasonically mixed in a 48 mL
thick wall pressure bottle. The bottle was heated in oil bath at
120 °C under stirring with a stir bar inside for 24 h. After cooling
down to room temperature, purple needle shaped crystals were
harvested by filtration and washed with N, N-
dimethylformamide (DMF). Prior to use, the solid was soaked in
200 mL acetone for 24 h to exchange DMF, and then filtered and
dried under vacuum.
Fabrication of C60@PCN-222 composites via one-pot approach
ZrCl4 (0.15 g), H2TCPP (0.1 g) and benzoic acid (5.4 g) were
added into 20 mL of DEF, and then 20 mL toluene solution of C60
(0.5 mg/mL) was added to it and stirred for 30 min. The mixture
was transferred into a 100 mL thick wall pressure bottle. The
bottle was heated in oil bath at 120 °C under stirring with a stir
bar inside for 24 h. The product was collected by filtration,
washed with DMF until the filtrate became colorless to give 3%-
C60@PCN-222. The 5%/8%-C60@PCN-222 (determined by UV-vis
analysis, Fig. S1, S2) were obtained through a similar procedure
by changing the amount of C60 solution to 30 and 40 mL,
respectively. Prior to use, the solid was soaked in 200 mL
acetone for 24 h to exchange DMF, and then filtered and dried
in a vacuum.
Experimental
Materials
Zirconium (IV) chloride (ZrCl4, Inochem, 98%), benzoic acid
(Aladdin, 99%), N,N-diethylformamide (DEF, Aladdin, 98%), N,
N-dimethylformamide (DMF, Xilong, > 99.9%), toluene (Xilong,
> 99.5%), fullerene (C60, Tanfeng Tech. Inc., 99.9%), methanol
(Aladdin, 99%), β-carotene (Alfa Aesar, 99%), p-benzoquinone
(Aladdin, 97%), 5,5-dimethyl-1-pyrroline N-Oxide (DMPO,
Sigma, 97%), 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO,
Tci, 99%). All materials were used as received without further
purification.
Instrumentation
Powdered X-ray diffraction (PXRD) patterns of all samples were
recorded using Rigaku Ultima-IV X-ray diffractometer equipped
with graphite-monochromatic Cu Kα radiation (λ = 1.54178 Å),
tube voltage 40 kV and current 40 mA. N2 adsorption-
desorption isotherms were measured by Micromeritics ASAP
2020 surface and porosity analyzer at 77 K. Specific surface area
Photocurrent measurement
Photocurrent measurements were performed on a CHI660E
electrochemical workstation in a standard three-electrode
system with the photocatalyst-coated FTO as the working
electrode, Pt plate as the counter electrode, and saturated
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calomel electrode (SCE) as the reference electrode. Fibre-optic
lights (50 mW/cm2) with a visible-light cut off filter (> 400 nm)
was used as the light source. Aqueous solution of Na2SO4 (0.2
mol/L) was used as electrolyte. The as-synthesized PCN-222 and
C60@PCN-222 composites (5 mg) were added into a mixed
solution with 2 μL of Nafion and 1 mL of ethanol, and the
working electrodes were prepared by dropping the suspension
(20 μL) onto the surface of FTO plate. The working electrodes
were dried at room temperature, and the photoresponsive
signals of the samples were recorded under chopped light at 0.6
V.
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DOI: 10.1039/C9TA07965C
Electrochemical impedance spectroscopy (EIS) measurement
EIS measurements of photocatalysts were measured on an
electrochemical workstation in a standard three-electrode
system with the photocatalyst-coated electrode as the working
electrode, Pt plate as the counter electrode, and SCE as the
reference electrode with a visible-light (50 mW/cm2, λ > 400
nm). The preparation procedure of the working electrode is
similar to that for the photocurrent measurement described
above.
ESR trapping studies
Photocatalysts (0.5 mg/mL) were mixed with 1 mL of methanol.
Then 100 mM DMPO was added into the solution to capture
superoxide radicals. In addition, 100 mM TEMPO was added
into another sample to capture singlet oxygen. The
measurement was conducted at room temperature upon
incident light irradiation with a 300 W xenon lamp (λ > 400 nm)
under open air conditions in a quartz cube.
Thioether oxidation reactions conducted
In general, a 20 mg portion of dried photocatalyst was dispersed
in 8 mL of methanol containing 0.2 mmol of thioether
compounds, and the mixture was sonicated to be homogeneous
placed in a quartz tube (100 mL). The catalytic oxidation
reaction was initiated at ambient temperature (∼ 20 °C) with air
Fig. 1 (a) Powder X-ray diffraction patterns and (b) N2 adsorption isotherms at 77 K of as-
synthesized PCN-222 and a series of C60@PCN-222 catalysts before reaction. Filled and
open symbols represent adsorption and desorption branches, respectively.
flow under visible-light irradiation (fibre-optic lights, 50 fully incorporated into the MOF pores during the PCN-222 self-
mW/cm2, λ > 400 nm, FX300, Beijing Perfectlight Technology assembly process.
Co., China). After the reaction, the catalyst was separated by
PXRD would provide the crystal lattice information and phase
centrifugation, thoroughly washed with methanol, and then purity of crystalline materials. PXRD patterns of PCN-222,
reused in subsequent runs. The yield of the product was C60@PCN-222 were shown in Fig. 1a. There was no apparent
analyzed by GC with n-dodecane as the internal standard.
loss of crystallinity and structure based on PXRD patterns for
PCN-222 after loading C60 molecules. Moreover, the PXRD
patterns of C60@PCN-222 did not exhibit the identifiable
diffraction peaks for C60, implying that there was no C60 crystal
on the surface of PCN-222.
The BET surface areas of as-synthesized PCN-222, 3%/5%/8%-
C60@CN-222 were 2171, 1639, 1261, and 656 m2g-1,
respectively (Fig. 1b). Two types of pores with diameters of 1.3
and 3.2 nm were evaluated by density functional theory (DFT)
simulation from the N2 sorption curves (Fig. S3). With the
increase of C60 content, the peaks for mesopores of 3.2 nm were
weakened. The results implied that the mesopores in the
framework could be possibly occupied by C60 molecules.
Fig. 2 were SEM image of as-synthesized PCN-222 and TEM
ones of C60@PCN-222 composites. The crystal sizes for the as-
synthesized MOFs decreased gradually with the increase of C60
content from 10 μm to 100 nm.
Results and discussion
In this work, PCN-222, one of the representative MOFs15 was
employed as a host of composite photocatalyst. PCN-222
possessing a 3D structure with channels larger than 3 nm and
high physicochemical stability as well as a large surface area
(BET, over 2000 m2g-1), is very suitable for encapsulating
functional guest molecules and hosting catalytic reactions.
A one-pot synthetic approach was used to introduce C60
molecules into PCN-222 pore channels to afford C60@PCN-222
in order to prevent the attachment of functional molecules on
the external surface of MOF, thus causing their aggregation.
Given the strong π-π interactions between the fullerenes and
the porphyrins, the quantitative C60 toluene solution should be
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DOI: 10.1039/C9TA07965C
Fig. 2 SEM images for PCN-222 (a) and TEM images for 3%-C60@PCN-222 (b), 5%-
C60@PCN-222 (c) and 8%-C60@PCN-222 (d).
The UV-vis spectrum of PCN-222 showed strong adsorption
in the range of 200-800 nm (Fig. S4), representing the photon
absorption and electron-hole separation ability upon visible-
light irradiation. According to the UV-vis absorption spectra of
C60@PCN-222 composites, the Soret absorption bands of the
porphyrin ligand were observed to shift bathochromically,
suggesting an electronic interaction of frameworks with C60
molecules.17 In addition, this was further verified by steady-
state PL measurement, which provided useful hints for the
photoexcited charge transfer and recombination. For the
pristine H2TCPP, the PL spectrum emerges two strong emission
peaks at 668 and 723 nm, which were originated from the band-
to-band recombination of electrons and holes. The intensity of
this emission peak decreased dramatically when the C60
molecules were encapsulated, which may mean that the band-
to-band recombination of electrons and holes was effectively
inhibited (Fig. S5).
Furthermore, the transient photocurrent response of pure
PCN-222 and C60@PCN-222 samples were tested to investigate
the excitation and transfer of photogenerated charge carriers
under visible light irradiation. To unveil the charge-separation
efficiency, photocurrent measurements had been carried out
and the results showed that the photocurrents of C60-decorated
PCN-222 were enhanced when compared to the pristine PCN-
222 (Fig. 3a), revealing that the formation of C60-MOF
composite facilitate the separation of the photogenerated
electron-hole pairs. The 8%-C60@PCN-222 displayed the
strongest photocurrent response than that of 5% and 3%
samples, suggesting more content of C60 molecules, higher
efficiency of charge transfer of the composites. This speculation
was also supported by the electrochemical impedance
spectroscopy (EIS) results (Fig. 3b), in which C60@PCN-222
Fig. 3 (a) Transient photocurrent response and (b) EIS Nyquist plots for PCN-222,
3%/5%/8%-C60@PCN-222 under visible-light (50 mW/cm2, λ > 400 nm) irradiation.
samples exhibited a smaller radius, indicative of a lower charge-
transfer resistance. Both results suggested that the content of
C60 played a crucial role in the separation of photogenerated
electron–hole pairs of the C60@PCN-222 photocatalysts.
Encouraged by the remarkable photocatalytic performance,
we explored the photo-oxidation of thioethers to produce
sulfoxides under air over C60@PCN-222 or related catalysts
under visible-light irradiation. Thioanisole was selected as a
model substrate in methanol under air at ambient temperature
(~ 20 °C). The data for catalytic oxidation of thioanisole in Table
1 demonstrated that all the C60@PCN-222 composites exhibited
the highest catalytic activity with 100% conversion and
selectivity for sulfoxide production (Table 1). Considering all
indexes, the catalysts in this work are undoubtedly the best
photocatalyst among all the reported ones including air and
pure O2 as oxidants (Table S1).18, 19
With the increase of C60 content (Table 1, entries 1-3), the
reaction time decreased from 3 h to 2 h when the conversion
and selectivity reached 100%, however, the TOF values based
on moles of C60 were 80, 71.4 and 45.5 h-1, respectively. These
results indicated that C60 molecules plays a key role in improving
the activity of catalysts, but the increase of C60 content may also
lead to blockage of pore channels in PCN-222 (Fig. S3) which
suppressed the diffusion of substrate molecules, thus reducing
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Table 1 Photocatalytic oxidation of thioanisole (1a)a
increased (Fig. S6). No intermediates or other bypVroiewduArctitcslewOnelirnee
observed during the reaction process. After light irradiation for
DOI: 10.1039/C9TA07965C
O
S
O
O
S
S
Table 2 Photo-oxidation of various aromatic thioethers over 3%-C60@PCN-222a
Catalytic
Air/RT
1c
1b
1a
Selectivity (%)
Conversion
(%)
Selectivity
(%)
Conversion（
%）
Entry
1
Substrate
Time
3h
Entry
Catalytic
1b
100
100
100
100
--
1c
S
> 99
> 99
100
100
1a
1
2b
3b
4
3%-C60@PCN-222
5%-C60@PCN-222
8%-C60@PCN-222
PCN-222
> 99
> 99
> 99
25
0
0
0
0
--
0
0
0
S
2
3
4
5
6
3h
3h
5h
3h
4h
Br
2a
S
> 99
44
100
100
100
100
Cl
3a
c
S
5
C60
< 5
22
O2N
HO
6c
7
C60+PCN-222
No Catalyst
100
0
4a
S
0
> 99
56
5a
8d
3%-C60@PCN-222
0
0
S
3%-C60@PCN-222
+ p-
H2N
Cl
9
3
100
100
0
0
6a
benzoquinone
3%-C60@PCN-222
+ β-carotene
10
89
S
7
4h
95
100
aReaction conditions: 1a (0.2 mmol), catalyst (20 mg), CH3OH (5 mL), room
temperature, air flow, LED lamp (50 mW/cm2, λ > 400 nm) irradiation for 3 h;
bReaction time: 2 h; cThe content of C60 is the same as those in 3%-C60@PCN-
222; dIn the dark. All products were determined by GC.
7a
S
8
9
6h
6h
> 99
53
94
the catalytic efficiency. A series of contrasted experiments were
designed and carried out under the same reaction conditions.
Without C60, PCN-222 only achieved 25% conversion, whereas
unique C60 exhibited almost no catalytic activity (Table 1, entry
4-5) because C60 has almost no adsorption in the visible region
(Fig. S1). Physical mixing of C60 and PCN-222 only gave 22% yield
(Table 1, entry 6), which should be ascribed to the absence of
interaction between separated C60 molecules and the PCN-222
framework. These results suggested that there existed
synergetic effect in the system of as-synthesized composites for
photocatalytic oxidation of 1a. Of course, both light and catalyst
were necessary (Table 1, entries 7-8). The time course of the
product distribution was recorded for the photocatalytic
oxidation of 1a in methanol over the 3%-C60@PCN-222 catalyst.
During the reaction process, the conversion of 1a gradually
8a
S
100
9a
S
10
6h
90
92
10a
aReaction conditions: photocatalyst (20 mg), thioethers (0.2 mmol), CH3OH (5
mL), room temperature, air flow, LED lamp (50 mW/cm2, λ > 400 nm) irradiation
for 3 h.
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Fig. 4 Recycling experiments of the 3%-C60@PCN-222 photocatalyst.
Fig. 5 ESR spectra for systems containing DMPO or TEMPO and C60@PCN-222
samples in methanol upon visible-light irradiation for 10 min (λ > 400 nm). (a)
DMPO–O•-2 without substrate, (b) DMPO–O•-2 with substrate (thioanisole) and
(c) TEMPO–1O2 without substrate, (d) TEMPO–1O2 with substrate (thioanisole).
3 h, 1a was quantitatively converted into the target product of
1b.
•
was performed in methanol which would quench OH,18,24 1O2
The recyclability of a heterogeneous catalyst is of great
importance for its practical application. Delightedly, the
catalytic activity of 3%-C60@PCN-222 was well retained even
after five runs of thioanisole oxidation without any treatment
or activation (Fig. 4). No significant loss of crystallinity for 3%-
C60@PCN-222 was observed from the PXRD patterns (Fig. S7),
suggesting excellent recyclability and stability of 3%-C60@PCN-
222.
and O•-2 were the possible ROS involved in the reaction system
as shown in Scheme S1. Then, we explored the underlying
mechanism for this new type of photocatalyst by recognizing
reactive oxygen species via the ESR investigation and controlled
experiments. To recognize reactive oxygen species in photo-
oxidation of thioanisole, a series of ESR measurements were
conducted. DMPO and TEMPO were employed to probe the
ROS generated upon visible-light irradiation.
After irradiation, the characteristic signals of the DMPO–O•-
2 and TEMPO–1O2 adducts were clearly observed by ESR spectra
(Fig. 5). For comparison, these signals were not observed in the
dark, which further confirmed that these oxidative species were
generated by light irradiation in the presence of C60@PCN-222
photocatalysts. The DMPO solution gave specific DMPO–O•-2
(1:1:1:1 quartet) and TEMPO solution gave specific TEMPO–1O2
(1:1:1 triplet) signals with or without substrate (thioanisole).20-
In order to further explore the universality of this new type of
photocatalyst, 3%-C60@PCN-222 was chosen to test for the
photocatalytic oxidation of a series of thioethers under the
previously optimized conditions, and the results were
summarized in Table 2. Most of diverse substituted thioethers
with different R1 and R2 groups were highly efficiently oxidized
in different reactions time. According to the previous reports,20,
21
thioanisole bearing electron-donating groups as substrate
exhibited higher yield than those bearing electron-withdrawing
groups under the same reaction condition. However, the
reaction time remained unchanged after substitution of the
electron-withdrawing group (-Cl and -Br, Table 2, entries 2-3)
implying excellent catalytic activity of this catalyst. With
stronger electron-withdrawing group substituted (-NO2, Table
2, entry 4), the conversion did decrease to 44%. As electron-
donating groups -OH and -NH2 (Table 2, entries 4-5), the
conversion of substrates substituted by the latter was only 56%
after 4 h oxidation. The reason could be speculated that the
basic group -NH2 on the substrate interacted with the ligand
porphyrin resulting in a decrease in catalyst activity. Except for
the electronic effect of substituted groups, steric hindrance also
affected the reaction efficiency (Table 2, entries 3 vs. 7, para-Cl,
100% conversion in 3 h, ortho-Cl, 95% conversion in 4 h). The
size of substrates was another important influence factor as the
larger the thioether molecule, the lower the reaction efficiency
(Table 2, entries 8-10), due to the slow transport of larger
molecules through the channels.
23
Moreover, with the increasing of C60 content in MOF, the
signal intensity of O•-2 and 1O2 were also increased. However,
the addition of substrate would increase the yield of O•-2 (Fig.
5a, b) and decrease the yield of O2 (Fig. 5c, d), indicating that
1
the main species of ROS in the reaction is O•-2. According to
similar reports,6a upon photoirradiation, C60 was known to form
a long-lived triplet state (3C60*), which could react with electron
donor (thioanisole), and then 3C60* was reduced to C•-60 which
subsequently reduced O2 to O•-2
To further prove the role of reactive oxygen species in the
reaction system, specific scavengers that would inhibit the
1
generation of O2 or O•-2, were introduced into the reaction
system. The conversion of 1a dropped from 100% to 3% in the
presence of p-benzoquinone (Table 1, entry 9) suggesting that
O•-2 was served as a main oxidative species in the reaction
process. Differently, the conversion dropped from 100% to 89%
with β-carotene as a scavenger to capture 1O2 (Table 1, entry
10). Such results clearly indicated that O•-2 was the major
oxidative species for the photocatalytic oxidation of thioethers,
which are consistent with the ESR test data.
Superoxide radicals (O•-2), singlet oxygen (1O2) and hydroxyl
radicals (•OH) were reported to be the active species for photo-
oxidation of thioethers.19-23 Since the photocatalytic reaction
From the results described above, a possible mechanism was
proposed for the photocatalytic oxidation of thioethers. As
6 | J. Name., 2012, 00, 1-3
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shown in Scheme S1A, photoinduced holes (h+) in the valence
band (VB) and electrons (e-) in the conduction band (CB) were
generated from PCN-222 under light irradiation, so PCN-222
was excited to PCN-222*. The generated electrons were then
transferred from PCN-222* to C60 due to the π structure and
excellent electron acceptability of this molecule which caused
the separation of h+ and e- more efficiently and the formation
of PCN-222+. Upon photoexcitation, C60 was known to form a
long-lived triplet state (3C60*) which could react with electron
donors and was reduced to C•-60. This species then
subsequently reduced O2 to O•-2.6a On the other hand, PCN-
222+ interacted with thioanisole by single-electron transfer to
form an S-centered free-radical cation which was also produced
by the reaction of thioanisole with 3C60*.6a Simultaneously, PCN-
222 was regenerated from PCN-222+. The S-centered free-
radical cation prefered to react with O•-2 formed at the
conduction band of C60, allowing for the evolution of sulfide
peroxide. In the protic solvent CH3OH, the final methyl phenyl
sulfoxide is formed. At this stage, this proposed mechanism still
lacks direct evidence because of the elusive transition states in
the photoinduced radical. Further experimental and theoretical
work will be done in the future to present a clearer mechanistic
insight.
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Conclusion
6
We have developed a novel heterogeneous catalytic system can
photochemically generate O•-2 as a mild oxidant for the
selective photo-oxidation of thioethers to sulfoxides. The
reactions could proceed with excellent selectivity and yield
within short reaction time under ambient conditions involving
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Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
We gratefully acknowledge financial support from NSFC (No.
21571122).
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Page 9 of 9
View Article Online
DOI: 10.1039/C9TA07965C
For the first time, fullerene (C60) was used to enhance photogenerated electron–hole separation of MOFs as for catalyst
and showed high catalytic activity in the photocatalytical oxidation of thioether in air.
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