Visible-Light-Induced Photoredox Catalysis of Dye-Sensitized Titanium Dioxide: Selective Aerobic Oxidation of Organic Sulfides

Article abstract of DOI:10.1002/anie.201600405

TiO₂ photoredox catalysis has recently attracted much interest for use in performing challenging organic transformations under mild reaction conditions. However, the reaction scheme is hampered by the fact that TiO₂ can only be excit

Full text of DOI:10.1002/anie.201600405

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201600405
German Edition: DOI: 10.1002/ange.201600405
Photoredox Catalysis
Visible-Light-Induced Photoredox Catalysis of Dye-Sensitized
Titanium Dioxide: Selective Aerobic Oxidation of Organic Sulfides
Xianjun Lang, Jincai Zhao, and Xiaodong Chen*
Abstract: TiO2 photoredox catalysis has recently attracted
much interest for use in performing challenging organic
transformations under mild reaction conditions. However, the
reaction scheme is hampered by the fact that TiO2 can only be
excited by UV light of wavelengths l shorter than 385 nm. One
promising strategy to overcome this issue is to anchor an
organic, preferably metal-free dye onto the surface of TiO2.
Importantly, we observed that the introduction of a catalytic
amount of the redox mediator TEMPO [(2,2,6,6-tetramethyl-
piperidin-1-yl)oxyl] ensured the stability of the anchored dye,
alizarin red S, thereby resulting in the selective oxidation of
organic sulfides with O2. This result affirms the essential role of
the redox mediator in enabling the organic transformations by
visible-light photoredox catalysis.
more, the absorption edge of the substrates or the formed
products lies in the near-UV region, which could cause
undesired photochemical side reactions, leading to low
selectivity of the oxidation products. To avoid these draw-
backs, visible-light photoredox catalysis based on TiO2 should
be developed. We previously achieved the visible-light-
induced selective oxidation of an organic sulfide with O2 on
TiO2, prompted by synergistic oxidation with benzyl amines[6]
or the presence of a catalytic amount of tertiary amines.[7] We
concluded that adding a redox mediator is important in
bestowing the smooth formation of the sulfoxide product by
TiO2 photoredox catalysis.[6,7] In these reaction schemes, the
visible-light absorption was caused by the surface complex of
the Lewis base–acid interaction between an amine and the
surface Ti sites of TiO2; only a very narrow band of visible
light at approximately 400 nm was harvested.[8] Even further
improvement is required to capture more abundant visible
light.
V
isible light or sunlight has been foreseen as a potential
driving force for organic reactions for more than one hundred
years and has recently attracted renewed interest.[1] Most of
the commonly used organic substrates are colorless; such
substances cannot be directly excited by visible light. Thus,
efficient photoredox catalysts, such as Ru or Ir complexes and
some metal-free organic dyes, have been widely adopted to
drive selective redox reactions under very mild conditions.[2]
However, such catalysts are seldom exploited under oxidative
conditions with O2 as the terminal oxidant[3] because doing so
requires that they can activate O2 under visible-light irradi-
ation, which, in turn, might result in the degradation of the
more fragile chromophore of the photoredox catalyst rather
than the aerobic oxidative transformation of the substrate.
Thus, much more stable metal-oxide semiconductors, with
TiO2 as a notable example, are better suited to performing
aerobic oxidation reactions.[4]
Consequently, we propose to anchor an organic dye onto
TiO2 to extend its visible-light absorption range. If this design
is used directly for the oxidation of organic molecules, then
the same challenge of the instability of the chromophore must
be overcome. Some organic dyes, which are otherwise stable
in the presence of sunlight and O2, could be degraded easily
on TiO2 under visible-light irradiation.[9] However, a constant
redox mediator can expedite hole transfer from the excited
dye to the target molecule, preventing the accumulation of
oxidative pressure on the anchored dyes and accordingly
preserving their stability under aerobic conditions. In fact, this
strategy allows for the success of dye-sensitized solar cells
À
(DSSCs) and natural photosynthesis, in which I3 /I2 or a Mn-
Ca cluster act as the redox mediator for photogenerated
oxidative processes to release electric current or O2.[10]
TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] has been
applied as the redox mediator in some DSSCs.[11] Meanwhile,
its application in organic oxidative transformations has
increased substantially in recent years.[12]
Therefore, TEMPO is adopted as the redox mediator for
the selective oxidation of organic sulfides with O2 under dye-
sensitized TiO2 visible-light photoredox catalysis. TEMPO is
a commercially available stable free radical with particular
application in the selective oxidation of alcohols.[13] To the
best of our knowledge, only one example of the oxidation of
sulfides with TEMPO as a co-catalyst has been reported, and
the reaction was conducted under relative harsh conditions of
acetic acid solvent and MnII–CoII dual metal catalysts.[14]
Herein, we report the visible-light-induced aerobic oxidation
of sulfides to sulfoxides with TEMPO that can ensure the
stability of the organic dye anchored on TiO2 and mediate the
selective oxidation of sulfide. Moreover, the role of TEMPO
Indeed, TiO2 has been widely adopted in a wide range of
oxidation reactions with O2, including both non-selective and
selective reactions under UV irradiation.[5] Because TiO2 can
only be excited by UV light of wavelengths l shorter than
385 nm, this reaction scheme is severely limited because of
the harvesting of only a small portion of sunlight. Further-
[*] Dr. X. J. Lang, Prof. X. D. Chen
School of Materials Science and Engineering
Nanyang Technological University
50 Nanyang Avenue, Singapore 639798 (Singapore)
E-mail: chenxd@ntu.edu.sg
Prof. J. C. Zhao
Key Laboratory of Photochemistry
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
Supporting information for this article can be found under http://dx.
doi.org/10.1002/anie.201600405.
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for the oxidation of alcohols is still debated, even after
intensive research.[15] Hence, this system might have implica-
tions for the elucidation of TEMPO in mediating oxidation of
different organic molecules.
We chose anatase TiO2 (BET specific surface area of
275 m2 gÀ1, see Figures S1 and S2 for characterizations in the
Supporting Information) and alizarin red S (ARS) to produce
a dye-sensitized photoredox catalysis for the selective oxida-
tion of an organic sulfide. The high surface area of TiO2 is
beneficial to the generation of photocurrent in DSSCs that
operates on similar principles. The ARS-sensitized TiO2 was
fashioned in a very facile manner. Both TiO2 and ARS are
insoluble in ethanol (see the Supporting Information for more
details). However, after ultrasonication in ethanol, a uniform
visible-light photoredox catalyst, ARS-TiO2, was obtained
(BET specific surface area of 213 m2 gÀ1); the sample is stable
in air for months in the absence of light irradiation and is
suitable for further investigation for photoredox catalytic
aerobic oxidation.
The precise adsorption model of the organic dye could
substantially influence the eventual visible-light photochem-
ical activity, which should be discerned first. There are two
possible models for the adsorption of ARS onto the surface of
TiO2 based on the molecular structure of ARS (Figure 1). In
low-pH aqueous solution, the sulfate group has been assigned
as the docking site (Figure 1a). However, this process is quite
unlikely for the exchange of sodium cation and the surface
hydroxy groups of TiO2 in ethanol, a condition we designed
for the photoredox catalyst. The catechol group of ARS acts
as the docking site by binding with the surface hydroxy groups
of TiO2 (Figure 1b). This notion is further confirmed by the
observation that the adsorption spectrum is red-shifted
(Figures S3 and S4) after the binding of ARS and TiO2,
suggesting the chromophore group, that is, the catechol group,
interacts with the surface of TiO2.
Figure 2. Photograph of the visible-light photoredox catalytic systems
based on ARS-TiO2. I) Before visible-light reaction. II) After visible-light
irradiation for 5 minutes without TEMPO. III) After visible-light irradi-
ation for 1.5 h with TEMPO (2 mol%) redox mediator. Reaction
conditions: see the footnotes of Table 1.
systems based on dye-sensitized TiO2 for selective aerobic
oxidation. However, with the addition of a catalytic amount
(2 mol%) of TEMPO to the reaction medium, the stability of
dispersed ARS-TiO2 could be substantially increased by the
rapid shuttling of the photogenerated holes of the anchored
ARS, which releases the oxidative pressure placed upon it.
After confirming that TEMPO can substantially improve
the stability of ARS-TiO2, we next considered the details of
the photoredox catalytic activity under different conditions,
which is important for understanding the reaction mecha-
nism. In addition, thioanisole is a substrate that can be
oxidized by either the electron-transfer or the energy-transfer
pathways.[16] These two pathways can also be distinguished by
control experiments. The results for these control experi-
ments are listed in Table 1. Under standard reaction con-
ditions, the selective oxidation of thioanisole proceeds
smoothly, affording methyl phenyl sulfoxide in high selectiv-
ity (entry 1, Table 1). In the absence of TEMPO, ARS-TiO2
was decolored quickly, delivering only a slight transformation
of thioanisole (entry 2, Table 1). Anatase TiO2 exhibited no
activity, suggesting the reaction results were not caused by the
interaction between thioanisole and TiO2 (entry 3, Table 1).
This type of interaction cannot be excited by visible light of
Table 1: Control experiment for the selective aerobic oxidation of
thioanisole on ARS-sensitized TiO2 with TEMPO as the redox mediator.
Figure 1. The possible adsorption models of ARS on the surface of
anatase TiO2.
Entry
Conditions
Conv. [mol%][b]
Select. [mol%][b]
1[c]
2
3
4
5
6
7
8
9
Standard
No TEMPO
TiO2
TEMPO
ARS
0.1 MPa air
No O2
l>400 nm
l>490 nm
l>510 nm
42
9
0
0
2
33
0
50
35
31
96
99
–
After understanding the key features for the adsorption of
ARS onto the surface of anatase TiO2, we next attempted to
deduce the importance of the TEMPO redox mediator in
enabling the stability of anchored ARS under visible-light
irradiation, a characteristic required before the examination
of the photoredox catalytic activity. The comparison results
are shown in Figure 2 (diffuse reflectance UV/Vis spectros-
copy; Figure S5). In the presence of a substrate of thioanisole
and an oxidant of O2, the decoloration of the dispersed ARS-
TiO2 in CH3OH occurs only after minutes of visible-light
irradiation because of the oxidative pressure it endures,
leading to the oxidation of ARS itself rather than to oxidation
of thioanisole. This unintended oxidation of anchored dye
likely explains why the literature contains so few accounts of
–
99
97
–
95
96
97
10
[a] Reaction conditions: 0.3 mmol of thioanisole, 0.006 mmol of
TEMPO, 9.6 mg of ARS-TiO2 (0.002 mmol of ARS), 300 W Xe lamp, 5 mL
of CH3OH, l>450 nm, 0.1 MPa of O2, 1.5 h. [b] Determined by GC-FID
using chlorobenzene as the internal standard, conversion of thioanisole,
selectivity of methyl phenyl sulfoxide. [c] Errors for the conversion
numbers are Æ3 mol%.
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wavelengths longer than 450 nm. TEMPO, a red-orange solid,
process. Further experimental and theoretical work will be
possesses slight solvatochromism, appearing colorless in
CH3OH solution, thus triggering no reaction under visible-
light irradiation (entry 4, Table 1). If the reaction was initiated
by the energy transfer from the excited dye to O2, then only
ARS could perform the reaction. However, almost no
reaction occurs with ARS as the sensitizer (entry 5,
Table 1), indicating that the reaction proceeds through the
electron-transfer pathway. The reaction can also be per-
formed with oxygen in 0.1 MPa of air as the oxidant (entry 6,
Table 1). In the absence of O2, the reaction completely
stopped (entry 7, Table 1). Moreover, the reaction shows
light-wavelength-dependent behavior (entries 8–10, Table 1).
Thus, a wide arrange of visible-light spectra can be used for
this reaction, representing a giant step forward compared to
the system investigated in our previous reports, where only
visible light of approximately 400 nm could be used.[6,7]
With the results of the control experiment in hand, in
conjunction with our and othersꢀ previous findings, the
tentative mechanism of visible-light photoredox catalysis by
ARS-sensitized TiO2 with TEMPO as a redox mediator is
rationalized in Scheme 1. ARS can be anchored on TiO2 with
done in the future to present a clearer mechanistic insight.
After the formulation of the tentative reaction mecha-
nism, we finally seek to establish the scope of the substrates
for this visible-light photoredox catalysis, which is important
for demonstrating the generality for this reaction scheme. The
reaction results are summarized in Table 2. The stability of the
anchored ARS remains a concern. Thus, with the increase of
the amount of thioanisole to 5 times the standard conditions,
comparable conversion was achieved with only 3.33 times the
reaction time (entry 1, Table 2). A turnover number (TON) of
607 in terms of ARS for the aerobic oxidation of thioanisole
was achieved, demonstrating the stability of the anchored
ARS. This result also categorically reveals heterogeneous
photoredox catalysis can be performed under the condition of
a very high concentration of substrate. An amount of 0.155 g
of methyl phenyl sulfoxide was obtained as the product,
suggestive of its usefulness in preparative organic synthesis.
Table 2: Visible-light-induced selective oxidation of sulfides into sulf-
oxides with O2 on ARS-sensitized TiO2 with TEMPO as a redox
mediator.[a]
Scheme 1. Proposed mechanism for the visible-light-induced oxidation
of thioanisole with O2 on ARS-sensitized TiO2 with TEMPO as a redox
mediator.
a catechol group to form a stable complex, ARS-TiO2, which
absorbs a wide range of visible light. Under visible-light
irradiation, ARS is excited to ARS*. Then charge transfer
from ARS* to TiO2 occurs, leading to the injection of electron
+
to the conduction band of TiO2 and the formation of ARSC .
Next, the electron travels through the conduction band of
+
TiO2 to react with O2, thereby forming O2CÀ. ARSC drives the
oxidation of the redox mediator, TEMPO, to afford
TEMPO+, thereby quickly restoring the ground state of
ARS to maintain its stability in the presence of different
oxidative species. TEMPO+ interacts with thioanisole by
single-electron transfer to form an S-centered free-radical
cation. Simultaneously, TEMPO is regenerated from
TEMPO+. The S-centered free-radical cation prefers to
react with O2CÀ formed at the conduction band of TiO2,
allowing for the evolution of sulfide peroxide. In the protic
solvent CH3OH, the final methyl phenyl sulfoxide is formed,
accompanied by the side product of HCHO and H2O. At this
stage, this proposed mechanism lacks direct evidence because
of the elusive transition states in the photoinduced radical
[a] Reaction conditions: 0.3 mmol of sulfide, 0.006 mmol of TEMPO,
9.6 mg of ARS-TiO2 (0.002 mmol of ARS), 300 W Xe lamp, 5 mL of
CH3OH, l>450 nm, 0.1 MPa of O2. [b] Determined by GC-FID using
chlorobenzene as the internal standard, conversion of sulfide, and
selectivity of the corresponding sulfoxide. [c] 1.5 mmol of thioanisole,
0.03 mmol of TEMPO. [d] Errors for the conversion numbers are
Æ3 mol%.
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Thioanisole and its derivatives were all good substrates
subjected to the standard reaction conditions (entries 2–9,
Table 2). Some general patterns can be observed for the
oxidation of this type of substrate. With comparable con-
version of the sulfide substrate, thioanisoles with an electron-
donating group (entries 3 and 6, Table 2) typically require
shorter reaction times than thioanisole; conversely, thioani-
soles with electron-withdrawing groups (entries 7–9, Table 2)
commonly require longer reaction times than thioanisole
(entry 2, Table 2).
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(entry 4, Table 2) with a negligible electronic effect, has
a similar reaction time as thioanisole. Nevertheless, thioani-
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(entry 11, Table 2). When a phenyl group was substituted
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In summary, we designed a system for the visible-light-
induced selective aerobic oxidation of organic sulfides, with
the core principles being similar to that of the DSSCs.
TEMPO, a redox mediator, plays a pivotal role in facilitating
the reaction to ensure the stability of the anchored dye and
the selectivity of the desired oxidation product. Adding
a redox mediator might be especially useful for photoredox
catalysts that exhibit excellent photochemical activity but are
not stable under aerobic oxidation conditions. Accounting for
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Acknowledgements
Finical support from the Singapore National Research
Foundation (CREATE Programme of Nanomaterials for
Energy and Water Management and Singapore-Berkeley
Research Initiative for Sustainable Energy (SinBeRISE)) and
MOE Tier 1 (grant number RG130/14) is gratefully acknowl-
edged. X.L. also acknowledges the financial support from the
National Natural Science Foundation of China (grant number
21503086).
Keywords: aerobic oxidation · photoredox catalysis ·
redox meditor · sulfoxides · visible light
Received: January 14, 2016
Published online: March 10, 2016
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