Mechanistic insights into the interface-directed transformation of thiols into disulfides and molecular hydrogen by visible-light irradiation of quantum dots

Article abstract of DOI:10.1002/anie.201310249

Quantum dots (QDs) offer new and versatile ways to harvest light energy. However, there are few examples involving the utilization of QDs in organic synthesis. Visible-light irradiation of CdSe QDs was found to result in virtually quantitative coupling of a variety of thiols to give disulfides and H₂ without the need for sacrificial reagents or external oxidants. The addition of small amounts of nickel(II) salts dramatically improved the efficiency and conversion through facilitating the formation of hydrogen atoms, thereby leading to faster regeneration of the ground-state QDs. Mechanistic studies reveal that the coupling reaction occurs on the QD surfaces rather than in solution and offer a blueprint for how these QDs may be used in other photocatalytic applications. Because no sacrificial agent or oxidant is necessary and the catalyst is reusable, this method may be useful for the formation of disulfide bonds in proteins as well as in other systems sensitive to the presence of oxidants. Copyright

Full text of DOI:10.1002/anie.201310249

Angewandte
Chemie
DOI: 10.1002/anie.201310249
Photocatalysis
Mechanistic Insights into the Interface-Directed Transformation of
Thiols into Disulfides and Molecular Hydrogen by Visible-Light
Irradiation of Quantum Dots**
Xu-Bing Li, Zhi-Jun Li, Yu-Ji Gao, Qing-Yuan Meng, Shan Yu, Richard G. Weiss,
Chen-Ho Tung, and Li-Zhu Wu*
Abstract: Quantum dots (QDs) offer new and versatile ways to
harvest light energy. However, there are few examples involv-
ing the utilization of QDs in organic synthesis. Visible-light
irradiation of CdSe QDs was found to result in virtually
quantitative coupling of a variety of thiols to give disulfides and
H2 without the need for sacrificial reagents or external oxidants.
The addition of small amounts of nickel(II) salts dramatically
improved the efficiency and conversion through facilitating the
formation of hydrogen atoms, thereby leading to faster
regeneration of the ground-state QDs. Mechanistic studies
reveal that the coupling reaction occurs on the QD surfaces
rather than in solution and offer a blueprint for how these QDs
may be used in other photocatalytic applications. Because no
sacrificial agent or oxidant is necessary and the catalyst is
reusable, this method may be useful for the formation of
disulfide bonds in proteins as well as in other systems sensitive
to the presence of oxidants.
concerning how QDs function in specific processes are
unclear. Miguel and co-workers, for example, applied CdS
and CdSe QDs in the photocatalytic reduction of aromatic
azides to amines.[4] Niemeyer and Ipe employed QD/enzyme
nanohybrids for photocatalyzing the transformation of my-
ristic acid to a- and b-hydroxymyristic acid.[5] Most recently,
Ford and co-workers found that 1,1-dithiooxalate bound to
the surfaces of CdSe QDs can be converted into carbon
disulfide under aerobic conditions through oxidative cleav-
age.[6] Herein, we report that visible-light irradiation of CdSe
QDs leads to virtually quantitative coupling of a variety of
thiols to give disulfides and molecular hydrogen without the
need for sacrificial reagents or external oxidants and with
relatively high turnover numbers (TONs). The mechanistic
insights presented offer a blueprint for how these QDs may be
used in other applications.
Coupling reactions are useful in the synthesis of new
molecules, as well as for various medical, biological, materials,
and nanotechnological applications. For this reason, extensive
efforts have been made over the years to find efficient
methods for coupling the atoms of two unlinked molecular
species.[7–11] Among these is the selective coupling of thiols to
produce disulfides, a seemingly simple synthetic procedure
that is, in fact, not simple at all. Such reactions play an
important role in the tertiary and quaternary structure and
the function of proteins, and as in drug delivery systems and
smart materials.[12–15] Disulfides are also of interest as
protecting groups in synthetic applications and as vulcanizing
agents for rubber. A broad range of reagents (e.g., molecular
oxygen, metal complexes, peroxides, halogens, and organic
oxidants) has thus been employed to synthesize disulfides by
the coupling of thiols in both heterogeneous and homoge-
neous systems.[16–23] However, most of these procedures suffer
from low selectivity and overoxidation of the thiols to give
undesired products such as sulfoxides and sulfones (Path I of
Scheme 1) and/or involve very expensive reagents.[24–26]
C
olloidal semiconductor nanocrystals (quantum dots; QDs)
have attracted widespread interest because their physical
properties are quite different from those of bulk materials.[1–3]
Owing to their quantum confinement effects, rich surface
binding properties, high surface-to-volume ratios, broad and
intense absorption spectra in the visible region, and low cost,
QDs offer new and versatile ways to harvest light energy.
However, few examples involving the utilization of QDs in
organic synthesis have been reported, and many of the details
[*] X.-B. Li, Z.-J. Li, Y.-J. Gao, Q.-Y. Meng, S. Yu, Prof. Dr. C.-H. Tung,
Prof. Dr. L.-Z. Wu
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry & University
of Chinese Academy of Sciences, The Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: lzwu@mail.ipc.ac.cn
Prof. Dr. R. G. Weiss
Department of Chemistry, Georgetown University
Washington, DC 20057-1227 (USA)
[**] We are grateful for financial support from the Ministry of Science
and Technology of China (2013CB834804, 2013CB834505 and
2014CB239402), the National Science Foundation of China
(21390404, 21090343 and 91027041), the Solar Energy Initiative of
the Knowledge Innovation Program of the Chinese Academy of
Sciences, and the Chinese Academy of Sciences. R.G.W. acknowl-
edges the Chinese Academy of Sciences for a fellowship and the US
National Science Foundation (Grant CHE-1147353) for its support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310249.
Scheme 1. Two general approaches for the conversion of thiols into
disulfides.
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In 2010, Yoon and co-workers introduced a photooxida-
tive coupling method that is not catalytic and is applicable to
a limited number of aromatic thiols.[27] Later, Ando and co-
workers reported the aerobic photocoupling of thiols to give
disulfides by using rather toxic diaryltellurides as catalysts.[28]
Notably, Fan and co-workers were able to convert thiols into
disulfides and molecular hydrogen for the first time by using
a [Mn(CO)5Br] complex as the photosensitizer, but the
reported TON of the coupling reaction is less than 20, and
UV irradiation and molecular oxygen are required.[29] Herein,
we report a simple, clean, recyclable, and efficient photo-
catalytic (l > 400 nm) method for the virtually quantitative
and selective conversion a variety of thiols into disulfides and
molecular hydrogen at room temperature by visible-light
irradiation of CdSe-QD/thiolate conjugates (Path II of
Scheme 1). Given that no sacrificial agent or oxidant is
necessary and that the catalyst is reusable, this method may be
attractive for the formation of disulfide bonds in proteins, as
well as in other systems sensitive to the presence of oxidants.
Typically, an aqueous stock solution of water-soluble
CdSe QDs (see Figures S1 and S2 in the Supporting
Information) was treated with hydrochloric acid (0.1m) to
aggregate and precipitate the QDs (Figure S3), which were
isolated by centrifugation and washed with water to remove
any residual ligands not bound to the surface of the QDs. The
precipitate was then redispersed in water and a thiol, such as
3-mercaptopropionic acid (MPA), was added. After the pH
value was adjusted to be near the pKa value of the thiol (9.0 in
the case of MPA), the reaction mixture was deaerated by
bubbling with nitrogen and then irradiated (l > 400 nm) at
room temperature for a specified period either in the air or
under a nitrogen atmosphere. Significantly, the disulfide and
H2 were the only products detected; the yields in Table 1 are
based on the amount of thiol reacted. As expected, the
conversion and yield increased when the irradiation time was
increased (Table 1, entries 1,2). However, the conversion of
MPA was lower and slower when the pH value was lower or
higher than 9.0 (Table 1, entries 3–7). Disulfide production
was not observed in the absence of irradiation or the CdSe
QDs (Table 1, entry 8).
Because the reaction proceeded smoothly without added
oxidants and H2 was evolved in a virtually quantitative yield,
we expected (and found) that coupling could be facilitated by
the presence of a cocatalyst for H2 evolution. Upon the
addition of a small amount of a nickel(II) salt such as NiCl2,
Ni(NO3)2, or Ni(OAc)2 (which were recently reported to be
exceptional photocatalysts for H2 evolution[30,31]), both the
conversion of thiols into disulfides and the rate of H2
evolution improved dramatically. Within 1.5 h, the conversion
approached 99% and the yields of the disulfide and H2
photoproducts were near 100% (Table 1, entry 9). Even
under aerobic condition, 92% and 95% yields of the
disulfides and H2, respectively, could be achieved (Table 1,
entry 10). Clearly, the combination of CdSe QDs and nick-
el(II) ions under visible-light irradiation is a promising
approach for the conversion of thiols into disulfides.
As shown in Table 2, the protocol can be used to convert
a broad range of thiols into their corresponding disulfides
while leaving carboxyl, amino, methoxy, and halide substitu-
Table 2: Substrate scope and TONs for the conversion of thiols into
disulfides and H2.[a]
entry
Substrate
Yield [%]
disulfide[b]
TON[d]
[c]
H2
1
2
3
4
5
6
7
8
HOOCCH2SH
HOOCCH2CH2SH
C6H5SH
4-NH2C6H4SH
4-MeC6H4SH
4-(CH3)3CC6H4SH
4-MeOC6H4SH
4-ClC6H4SH
4-CF3C6H4SH
3-MeOC6H4SH
2-MeOC6H4SH
2-ClC6H4SH
99
99
99
83
99
99
88
95
96
98
99
86
97
96
91
96
98
2500
2500
2500
2075
2500
2500
2150
2375
2380
<100
0
Table 1: Variation of the conditions for the coupling of MPA to give 3,3’-
dithiodipropanoic acid and H2 in water.[a]
9
96
Conv.[b]
Yield [%]
disulfide[c]
TON[e]
10
11
12
ND[e]
NR[f]
NR[f]
trace
0
0
Entry
pH value
t [h]
[d]
H2
0
1
2
3
4
5
6
7
9.0
9.0
3.0
5.0
7.0
10.0
12.0
9.0
9.0
9.0
5.0
8.0
5.0
5.0
5.0
5.0
5.0
5.0
1.5
5.0
86
91
15
50
73
83
34
0
83
88
12
44
70
79
30
0
86
91
15
50
73
83
34
0
990
1040
170
580
836
950
390
0
[a] Conditions: thiol (0.5 mmol), NiCl2 (0.1 mm) and QDs (0.02 mm) at
a pH value near the pKa value of the corresponding thiol in 10 mL of
water or a mixture of water and THF, irradiated for 4.0 h (l>400 nm).
[b] Yield of isolated product. [c] Determined by GC. [d] TON based on the
product yields. [e] Not determined owing to extremely low conversion.
[f] No reaction.
8[f]
9[g]
10[h]
99
95
95
92
99
95
1150
1092
ents unaffected. Aliphatic thiols bearing a carboxyl group
were converted into the corresponding disulfides in excellent
yields (99%; Table 2, entries 1 and 2). Varying the length of
the alkyl chain on the thiols did not perceptibly affect the
coupling reaction. For substrates that are not soluble in water,
such as thiophenol, the reaction was carried out in a 1:1 (v/v)
mixture of water and tetrahydrofuran (THF); again, the
corresponding products were obtained in virtually quantita-
[a] Conditions: MPA (0.23 mmol) and acid treated QDs (0.02 mm) in
10 mL H2O under a 500 W high-pressure mercury lamp with a 400 nm
cutoff filter. [b] Based on the starting amount of MPA. [c] Determined by
1H NMR spectroscopy with diphenylacetonitrile as an internal standard.
[d] Determined by GC with CH4 as an internal standard. [e] TON: mmol
substrate converted/mmol CdSe QDs. [f] Without irradiation or the QDs.
[g] In the presence of 0.1 mm NiCl2. [h] Under aerobic conditions in the
presence of 0.1 mm NiCl2.
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tive yields (Table 2, entry 3). Thiophenols with electron-
donating groups (Table 2, entries 4–7) or electron-withdraw-
ing groups (Table 2, entries 8,9) at the para-position were
converted into the corresponding disulfides in excellent
yields. Despite the absence of a clear electronic-substituent
effect among these groups, there is a strong steric effect.
When the methoxy group was moved from the para to the
ortho or meta positions of thiophenol, the chemical yields
decreased dramatically (Table 2, entries 10,11) and like the
ortho-chloro derivative(Table 2, entry 12), these compounds
were nearly unreactive.
Information). Given that the TONs of the QDs are more
than 103, each cadmium ion on a QD surface was able to
convert at least 20 thiolates into disulfides.
The photoinduced formation of sulfenyl radicals was
confirmed by electron paramagnetic resonance (EPR) spec-
troscopy. Although an attempt to directly observe sulfur-
centered radicals failed, the spin adducts of sulfenyl radicals
with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were suc-
cessfully detected. Before irradiation, no spectral signal was
observed (Figure 2a). After irradiation (l > 400 nm) for 10 s,
To assess the economic and environmental attractiveness
of the method, the same batch of QD/Ni2+ catalyst was used to
convert four aliquots of MPA sequentially. After each run, the
catalyst was precipitated and separated from the liquid by
centrifugation. A loss of activity of only approximately 5%
between the first and last runs, based on the yields of both
products (Figure 1), was observed and can be attributed to
mild photocorrosion (Figure S4) and/or some loss of catalyst
during physical transfers.
Figure 2. EPR signals from an aqueous solution containing MPA
(0.023m), QDs (0.02 mm), and DMPO (0.02m) at pH 9.0 before (a)
and after (b) irradiation at l>400 nm for 10 s.
however, the spectrum contained spin-adduct spectral fea-
tures for DMPO (labeled with *) which are typical for the
trapping of a sulfenyl radical (RSC; aN = 15.2 G, aH = 17.0 G).
Simultaneously, another set of signals appeared (marked by
~
gray ), which are ascribed to an adduct of hydrogen radicals
(HC) with DMPO (aN = 16.6 G, aH = 22.6 G) (Figure 2b). The
coupling constants of both set of signals are consistent with
reported values.[38,39] When a small amount of nickel(II) ions
were present, identical signals with relatively stronger inten-
sities were observed (Figure S7).
Figure 1. Recycling efficiency of the CdSe-QD catalyst for the coupling
of MPA as measured by yields of disulfide (left-hand columns) and H2
(right-hand columns). Each run was conducted with 0.23 mmol of
MPA under the conditions of Entry 9 in Table 1.
In addition, evidence for the homocoupling of thiols to
give disulfides and H2 (i.e., coupling on a single QD) was
found. CdSe QDs with adsorbed MPA or p-toluenethiol were
prepared separately. After removal of the solvent by rotary
evaporation under vacuum, the two QD samples were
dispersed together in a mixture of H2O and THF (1:1, v/v)
and then irradiated until no more H2 was evolved. No cross-
coupled disulfides of the two thiols were detected (Figure S8).
The photogenerated sulfenyl radicals thus couple to form
disulfide products on their QD surface of origin before being
able to diffuse into the solution. The high efficiency of the
catalytic homocoupling also implies that the desorption of
thiolates from the surface is very slow and is more difficult
than the desorption of disulfides. This assumption is further
supported by the fact that the coupling reaction of MPA was
nearly absent when CdSe QDs were not prepared with acid
treatment (Figure S9). All of these observations suggest that
the catalytic reaction takes place on the surface of the QDs
rather than in the solvent.
In order to elucidate the reaction mechanism, the
interaction between the thiols and the CdSe QDs was studied
in some detail. Samples of CdSe-QD solutions with different
amounts of MPA at pH 9.0 were prepared and placed in the
dark for sufficient time to allow maximal complexation before
steady-state and dynamic-decay emission data were collected.
The band-edge emission of the QDs was significantly
diminished and the rate of emission decay was accelerated
(Figure S5) by the MPA, perhaps as a result of hole transfer
from excited QDs to bound thiolates (thus leading to the
formation of surface-bound sulfenyl radicals or radical-like
species; see below).[32] In solutions at pH values near the pKa
value of the thiol groups (4–7 for aromatic thiols and 9–11 for
aliphatic thiols),[33,34] thiolates, which can coordinate easily to
cadmium(II) ions at the QD surfaces,[35,36] are present.
Since the size of the CdSe QDs is about 1.9 nm, as
determined by high resolution TEM, and the estimated area
per surface cadmium site is about 0.2 nm2,[37] the average
number of thiolate binding sites on the surface of each CdSe
QD after acid treatment is approximately 57. This number is
very close to the experimentally determined number of
thiolates bound per QD (Figure S6 in the Supporting
When the reaction was conducted in D2O (instead of
H2O), D2 was generated (instead of H2) as the only byproduct,
and with no alteration in the reaction efficiency. Furthermore,
only a trace amount of H2 was evolved when the reaction was
carried out in neat [D6]acetone (Figure S10). These results
indicate that the source of the H atoms in the product H2 is the
aqueous solvent.
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Based on the accumulated experimental evidence, the
mechanism in Scheme 2 is proposed for the photocatalytic
transformation of thiols into disulfides and molecular hydro-
gen on the QD surfaces. When the water-soluble CdSe QDs
In conclusion, we have established an attractive and clean
catalytic approach for the selective oxidative coupling of
thiols to give disulfides; the ꢀreduction productꢁ is molecular
hydrogen. The protocol does not require sacrificial agents or
oxidants and proceeds efficiently under visible light irradi-
ation at room temperature. In fact, some reactions have been
conducted successfully by exposing the solutions to sunlight
under aerobic conditions. Upon the addition of nickel(II) salts
to the system, the efficiency and rate of conversion dramat-
ically improve because the formation of hydrogen atoms is
facilitated, thereby leading to faster regeneration of the
ground-state QDs. Because of these attributes, this method
offers an attractive alternative to existing procedures for
synthesizing disulfides from thiols. The mechanistic informa-
tion obtained, which demonstrates that the CdSe QDs have
many active surface sites ready to induce photochemical
transformations, will facilitate the exploration and exploita-
tion other potential photocatalytic applications of QDs in the
future.
Scheme 2. Proposed mechanism for the photocatalytic conversion of
thiols into disulfides and molecular hydrogen.
Received: November 26, 2013
Published online: January 27, 2014
are treated with acid, ꢀvacant sitesꢁ are created on the surface
of the QDs. Deprotonated thiols bind to the surface through
cadmium–sulfur bonds to form QD/thiolate conjugates.
Visible-light excitation of the CdSe QDs results in the
promotion of electrons to the conduction band (CB), thereby
leaving holes in the valence band (VB; Figure S11). The
excited states of the QDs are subsequently reductively
quenched by the bound thiolates, thereby producing sulfur-
centered radicals (RSC) or radical-like species, which couple to
form the corresponding disulfides on the surface of the QDs.
The disulfides are released into the solvent and then addi-
tional thiolates from the solution bind to the vacant sites and
complete the reaction cycle. At the same time, the CB
potential of CdSe (> 2.0 V) is sufficiently negative to reduce
H+ to HC radicals.[40] We hypothesize that the HC radicals are
stabilized on the QD surfaces as well, thereby allowing two
putative HC radicals to combine and generate one molecule of
H2. Nickel(II) ions on the QD surface provide even more
efficient sites for proton reduction.
Keywords: disulfides · photocatalysis · quantum dots ·
radical reactions · thiols
.
[1] A. M. Smith, S. Nie, Acc. Chem. Res. 2010, 43, 190 – 200.
[2] P. Zrazhevskiy, M. Sena, X. Gao, Chem. Soc. Rev. 2010, 39,
4326 – 4354.
[3] P. V. Kamat, K. Tvrdy, D. R. Baker, J. G. Radich, Chem. Rev.
2010, 110, 6664 – 6688.
[4] M. Warrier, M. K. F. Lo, H. Monbouquette, M. A. Garcia-
Garibay, Photochem. Photobiol. Sci. 2004, 3, 859 – 863.
[5] B. I. Ipe, C. M. Niemeyer, Angew. Chem. 2006, 118, 519 – 522;
Angew. Chem. Int. Ed. 2006, 45, 504 – 507.
[6] C. M. Bernt, P. T. Burks, A. W. DeMartino, A. E. Pierri, E. S.
Levy, D. F. Zigler, P. C. Ford, J. Am. Chem. Soc. 2013, DOI:
10.1021/ja4083599.
[7] J. K. Stille, Angew. Chem. 1986, 98, 504 – 519; Angew. Chem. Int.
Ed. Engl. 1986, 25, 508 – 524.
[8] A. F. Littke, G. C. Fu, Angew. Chem. 2002, 114, 4350 – 4386;
Angew. Chem. Int. Ed. 2002, 41, 4176 – 4211.
The coupling of thiols in this system is a surface-directed
process. As a result, it possesses several interesting and
synthetically advantageous characteristics: 1) The reaction
occurs on the QD surfaces, rather than by diffusive coupling
of the sulfenyl radicals in solution. Acid treatment of the QDs
improves the yields of the disulfides and their rates of
formation by providing more vacant surface sites for the
adsorption of thiolates. 2) When the pH value of the solution
is near the pKa value of the thiol, the thiolate population is
large and the thiolates are easily adsorbed onto the QD
surfaces. 3) Because the disulfides are bound much more
weakly on the QD surfaces than the corresponding thiolates,
overoxidation of the initially produced disulfides is avoided;
the disulfides become unbound from the surface before
additional reaction can occur. 4) The simple catalyst, which is
generated in situ, can be reused several times with very little
loss of activity.
[9] D. Zhong, J.-H. Franke, S. K. Podiyanachari, T. Blçmker, H.
Zhang, G. Kehr, G. Erker, H. Fuchs, L. Chi, Science 2011, 334,
213 – 216.
[10] F. Su, S. C. Mathew, L. Mçhlmann, M. Antonietti, X. Wang, S.
Blechert, Angew. Chem. 2011, 123, 683 – 686; Angew. Chem. Int.
Ed. 2011, 50, 657 – 660.
[11] Q.-Y. Meng, J.-J. Zhong, Q. Liu, X.-W. Gao, H.-H. Zhang, T. Lei,
Z.-J. Li, K. Feng, B. Chen, C.-H. Tung, L.-Z. Wu, J. Am. Chem.
Soc. 2013, 135, 19052 – 19055.
[12] M.-J. Gething, J. Sambrook, Nature 1992, 355, 33 – 45.
[13] A. M. Almeida, R. Li, S. H. Gellman, J. Am. Chem. Soc. 2012,
134, 75 – 78.
[14] J.-H. Ryu, R. T. Chacko, S. Jiwpanich, S. Bickerton, R. P. Babu,
S. Thayumanavan, J. Am. Chem. Soc. 2010, 132, 17227 – 17235.
[15] J. A. Yoon, J. Kamada, K. Koynov, J. Mohin, R. Nicolaꢂ, Y.
Zhang, A. C. Balazs, T. Kowalewski, K. Matyjaszewski, Macro-
molecules 2011, 44, 142 – 149.
[16] D. P. Gamblin, P. Garnier, S. van Kasteren, N. J. Oldham, A. J.
Fairbanks, B. G. Davis, Angew. Chem. 2004, 116, 846 – 851;
Angew. Chem. Int. Ed. 2004, 43, 828 – 833.
2088 www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 2085 –2089

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Chemie
[17] R. Priefer, Y. J. Lee, F. Barrios, J. H. Wosnick, A.-M. Lebuis,
P. G. Farrell, D. N. Harpp, A. Sun, S. Wu, J. P. Snyder, J. Am.
Chem. Soc. 2002, 124, 5626 – 5627.
[18] J. L. Garcꢃa Ruano, A. Parra, J. Aleman, Green Chem. 2008, 10,
706 – 711.
[19] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chem. Commun.
2010, 46, 6476 – 6478.
[20] A. Corma, T. Rodenas, M. J. Sabater, Chem. Sci. 2012, 3, 398 –
404.
[29] K. Y. D. Tan, G. F. Teng, W. Y. Fan, Organometallics 2011, 30,
4136 – 4143.
[30] Z.-J. Li, J.-J. Wang, X.-B. Li, X.-B. Fan, Q.-Y. Meng, K. Feng, B.
Chen, C.-H. Tung, L.-Z. Wu, Adv. Mater. 2013, 25, 6613 – 6618.
[31] Z. Han, F. Qiu, R. Eisenberg, P. L. Holland, T. D. Krauss, Science
2012, 338, 1321 – 1324.
[32] K. P. Acharya, R. S. Khnayzer, T. OꢁConnor, G. Diederich, M.
Kirsanova, A. Klinkova, D. Roth, E. Kinder, M. Imboden, M.
Zamkov, Nano Lett. 2011, 11, 2919 – 2926.
[21] A. Dhakshinamoorthy, S. Navalon, D. Sempere, M. Alvaro, H.
Garcia, ChemCatChem 2013, 5, 241 – 246.
[33] J. D. Gough, J. M. Gargano, A. E. Donofrio, W. J. Lees, Bio-
chem. 2003, 42, 11787 – 11797.
[22] D. R. Dreyer, H.-P. Jia, A. D. Todd, J. Geng, C. W. Bielawski,
Org. Biomol. Chem. 2011, 9, 7292 – 7295.
[23] R. Kumar, N. Sharma, U. K. Sharma, A. Shard, A. K. Sinha, Adv.
Synth. Catal. 2012, 354, 2107 – 2112.
[24] J. B. Arterburn, M. C. Perry, S. L. Nelson, B. R. Dible, M. S.
Holguin, J. Am. Chem. Soc. 1997, 119, 9309 – 9310.
[25] A. R. Hajipour, S. E. Mallakpour, H. Adibi, J. Org. Chem. 2002,
67, 8666 – 8668.
[26] J. K. Vandavasi, W.-P. Hu, C.-Y. Chen, J.-J. Wang, Tetrahedron
2011, 67, 8895 – 8901.
[34] T. V. DeCollo, W. J. Lees, J. Org. Chem. 2001, 66, 4244 – 4249.
[35] P. Schapotschnikow, B. Hommersom, T. J. H. Vlugt, J. Phys.
Chem. C 2009, 113, 12690 – 12698.
[36] J. Aldana, N. Lavelle, Y. Wang, X. Peng, J. Am. Chem. Soc. 2005,
127, 2496 – 2504.
[37] R. Koole, P. Schapotschnikow, C. de Mello Donegꢄ, T. J. H.
Vlugt, A. Meijerink, ACS Nano 2008, 2, 1703 – 1714.
[38] K. Huvaere, M. L. Andersen, M. Storme, J. Van Bocxlaer, L. H.
Skibsted, D. De Keukeleire, Photochem. Photobiol. Sci. 2006, 5,
961 – 969.
[27] H. J. Kim, J. H. Yoon, S. Yoon, J. Phys. Chem. A 2010, 114,
12010 – 12015.
[39] M. Tada, M. Kohno, S. Kasai, Y. Niwano, J. Clin. Biochem. Nutr.
2010, 47, 162 – 166.
[28] M. Oba, K. Tanaka, K. Nishiyama, W. Ando, J. Org. Chem. 2011,
76, 4173 – 4177.
[40] J. Zhao, M. A. Holmes, F. E. Osterloh, ACS Nano 2013, 7, 4316 –
4325.
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