Visible-Light Photoredox-Catalyzed Decarboxylative Alkylation of Heteroarenes Using Carboxylic Acids with Hydrogen Release

Article abstract of DOI:10.1021/acs.orglett.9b02539

Herein, we have developed visible-light photoredox-catalyzed decarboxylating carboxylic acids for alkylation of heteroarenes under mild conditions. The transformation occurred smoothly without the requirement of stoichiometric oxidants in the presence of

Full text of DOI:10.1021/acs.orglett.9b02539

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Visible-Light Photoredox-Catalyzed Decarboxylative Alkylation of
Heteroarenes Using Carboxylic Acids with Hydrogen Release
†
,‡,§
†,§
†
†
,†
Wan-Fa Tian,
Chun-Hong Hu, Ke-Han He, Xiao-Ya He, and Yang Li*
†Center for Organic Chemistry, Frontier Institute of Science and Technology and State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, P. R. China
‡
Institute of Organic Chemistry, Jiangxi Science & Technology Normal University, Key Laboratory of Organic Chemistry,
Nanchang, Jiangxi 330013, P. R. China
*
S Supporting Information
ABSTRACT: Herein, we have developed visible-light photo-
redox-catalyzed decarboxylating carboxylic acids for alkylation of
heteroarenes under mild conditions. The transformation occurred
smoothly without the requirement of stoichiometric oxidants in
the presence of 0.3 equiv of base, which benefited from the release
of hydrogen (H ) and carbon dioxide (CO ). Various substrates
2
2
and functional groups were tolerated. Primary mechanistic studies
suggest that an oxidative quenching pathway and a reductive quenching pathway are both possible in the catalytic cycle.
arboxylic acids are comparatively stable, nontoxic, and
readily available from natural resources. Utilization of
decarboxylative alkylation of N-heteroarenes by applying
comparatively complex redox-active carboxylic esters, such as
N-hydroxyphthalimide esters, as starting materials was also
C
carboxylic acids as starting materials for decarboxylative
1
6d,7
transformations has been booming in recent decades.
developed (Scheme 1, eq 2).
Moreover, an additional step
2
Meanwhile, Minisci reactions have attracted wide attention
was required for transformation of the carboxylic acids to the
corresponding carboxylic esters.
for functionalization of heterocycles, which are prevalent
structural motifs in bioactive natural products and synthetic
H release coupling reactions demonstrate a nearly 100%
2
3
pharmaceuticals. Decarboxylative alkylation of heteroarenes
atom economy for constructing various chemical bonds
with carboxylic acids was developed in the 1970s by Minisci
8
without stoichiometric oxidants. Very recently, Ackermann
4
(
Scheme 1, eq 1). As visible-light photoredox catalysis has
and co-workers achieved decarboxylative C−H functionaliza-
tion of azoles under oxidant-free conditions by visible-light
photoredox catalysis with H2 release (Scheme 1, eq 3).
However, with 3 equiv of K HPO as a base, only adamantane
Scheme 1. Decarboxylative Minisci C−H Alkylation of
Heteroarenes
2
4
9
carboxylic acid was reported. We are interested in H release
2
10
transformations, including H release with CO release by
2
2
11
photoredox catalysis as stoichiometric oxidants, and bases
could be avoided. Meanwhile, the carboxylic acid group could
be utilized as a regioselective group. Until now, employing this
strategy for decarboxylative cross-coupling reactions has been
1u
rare. With respect to Minisci-type reactions, a pioneering
work of trifluoromethylation of (hetero)arenes with TFA
12
under UV light irradiation was reported by Su and Li. Later,
Zeng reported an elegant electrocatalytic decarboxylative
13
Minisci acylation with α-keto acids. Herein, we report
5
been developed as a powerful tool in organic transformations,
decarboxylating carboxylic acids for alkylation of heteroarenes
visible-light photoredox-catalyzed decarboxylating carboxylic
acids for alkylation of heteroarenes was recently reported by
with H release by visible-light photoredox catalysis under mild
2
4c,6
conditions. More than stoichiometric oxidants were avoided.
Diverse alkylations of various heteroarenes were achieved with
the groups of Glorius and others (Scheme 1, eq 1).
However, these elegant approaches required more than
stoichiometric oxidants, such as persulfates. Furthermore,
more than the stoichiometries of additives, such as TFA,
Na HPO ·12H O, or Cs CO , were usually needed. Thus,
0
.3 equiv of n-Bu NOAc by taking advantage of CO release
4
2
(
Scheme 1, eq 4).
2
4
2
2
3
comparatively lower functional group tolerance and vast wastes
were caused. In addition, visible-light photoredox-catalyzed
Received: July 21, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02539
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
The designed decarboxylative alkylation of heteroarenes
would initiate generation of an alkyl carboxylic radical via
visible-light redox catalysis with a catalytic base, which is
followed by decarboxylation to generate an alkyl radical. The
resulting alkyl radical nucleophilically attacks the heteroarene
Bu OAc (0.3 equiv) as a base under blue LED irradiation,
4
various solvents were investigated (Table 1, entries 1−5). To
our delight, the desired product 3aa was obtained in 64% yield
in ethyl acetate (EtOAc) (Table 1, entry 5). No product or
inferior yield was observed in other solvents of CH CN,
3
to afford the desired product by H release. One of the
MeOH, DCM, and 1,4-dioxane (Table 1, entries 1−4).
Subsequently, the effect of other cobalt catalysts was tested
2
competitive pathways is hydrodecarboxylation by the gen-
1k
erated alkyl radical. Meanwhile, the presence of cobalt
complexes as the H2 release catalysts would induce
dehydrogenative decarboxyolefination to form alkenes as
(
Table 1, entries 6−8). [Co(dmgH) Py ]PF induced higher
2
2
6
reaction efficiency (Table 1, entry 8). The influence of other
bases of CsOAc, CsF, and DABCO was also examined (Table
1v,z,aa
another competitive pathway (Scheme 2).
1
, entries 9−11). n-Bu NOAc displayed the higher yield, which
4
should be attributed to its better solubility. Other photo-
sensitizers of Ru(bpy) Cl (E *
Acr -Mes ClO (E *
PC /PC−
5a
Scheme 2. Designed and Competitive Pathways
= +0.77 V vs SCE),
3
2
+
PC /PC−
16
= +2.06 V vs SCE), and 4CzlPN
= +1.35 V vs SCE) showed no catalytic activities,
4
PC /PC−
17
(
E *
although there are possibilities of oxidating the adamantyl
−
•
anion (AdCO ) to an adamantyl radical (AdCO ) based on
2
2
AdCOO•/AdCOO−
the redox potential analysis (E 1
= +0.79 V vs
8
SCE) (Table 1, entries 12−14). Reducing the amounts of
the photosensitizer and the cobalt catalyst gave lower yields
(
Table 1, entries 15 and 16). Finally, 92% isolated yield was
obtained by prolongation of the reaction time to 30 h (Table 1,
entry 18). Control experiments proved that a photosensitizer, a
cobalt complex, a base, and visible-light irradiation were
essential (Table 1, entries 20−23). The yields of H and CO
With these considerations in mind, we commenced our
study by selecting benzothiazole (1a) and 1-adamantane
carboxylic acid (2a, AdCO H) as the model substrates (Table
2
14
6
1
). By applying Ir[dF(CF )ppy](dtbbpy)PF
as a photo-
3
2
2
15
sensitizer, Co(dmgH) PyCl as a H release catalyst, and n-
were consistent with that of the products in most cases. In the
2
2
a
Table 1. Optimization of the Reaction Conditions
a
entry
photosensitizer (PS)
[Co]
solvent
H (%)
2
CO (%)
3aa (%)
2
1
2
3
4
5
6
7
8
Ir[dF(CF )ppy] dtbbpy
Co(dmgH) PyCl
CH CN
MeOH
DCM
―
―
<5
54
68
56
57
69
56
―
―
<5
59
63
50
59
72
57
―
―
<5
49
64
49
50
70
50
3
2
2
3
Ir[dF(CF )ppy] dtbbpy
Co(dmgH) PyCl
3
2
2
Ir[dF(CF )ppy] dtbbpy
Co(dmgH) PyCl
3
2
2
Ir[dF(CF )ppy] dtbbpy
Co(dmgH) PyCl
1,4-dioxane
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
3
2
2
Ir[dF(CF )ppy] dtbbpy
Co(dmgH) PyCl
3
2
2
Ir[dF(CF )ppy] dtbbpy
Co(dmgH) (4-NMe Py)Cl
3
2
2
2
Ir[dF(CF )ppy] dtbbpy
Co(dmgH) (4-CO NMe Py)Cl
3
2
2
2
2
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
3
2
2
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
b
9
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
3
2
2
2
c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
49
58
54
3
2
2
2
d
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
―
―
―
―
59
60
77
94
44
―
―
―
<5
―
―
―
―
75
―
―
―
―
58
59
80
94(92)
43
―
―
―
<5
3
2
2
2
Ru(bpy) Cl
Acr -Mes ClO
4CzlPN
[Co(dmgH) Py ]PF
2 2
3
2
+
−
[Co(dmgH) Py ]PF
2 2
4
[Co(dmgH) Py ]PF
2
2
e
f
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
2 2
3
2
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
69
3
2
2
2
g
h
h
l
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
84
103
3
2
2
2
i
j
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
3
2
2
2
,
k
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
EtOAc/CH CN
49
3
2
2
2
3
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
EtOAc
EtOAc
EtOAc
EtOAc
―
―
―
<5
3
2
2
2
―
[Co(dmgH) Py ]PF
2 2
Ir[dF(CF )ppy] dtbbpy
―
3
2
m
Ir[dF(CF )ppy] dtbbpy
[Co(dmgH) Py ]PF
3
2
2 2
6
a
Reaction condtions: 1a (0.20 mmol), 2a (0.40 mmol), PS (1.0 mol %), [Co] (3.0 mol %), n-Bu NOAc (30 mol %), solvent (1.0 mL), irradiation
4
1
with blue LEDs for 12 h. H and CO were detected by GC by applying CH as an internal standard. H NMR yield of product 3aa was reported by
2
2
4
b
c
d
e
applying Cl CHCHCl as an internal standard. CsOAc was used as a base. CsF was used as a base. DABCO was used as a base. PS (0.5 mol %).
2
2
f
g
h
i
j
[
Co] (1 mol %). 20 h. 30 h. As the hydrodecarboxylation as the competitive pathway, more than 100% CO yield is possible. Isolated yield.
2
l m
k
Mixed solvent was conducted as the redox potentials were measured in the mixed solvent. In the dark. Without base.
B
DOI: 10.1021/acs.orglett.9b02539
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
cases of the slightly higher yields of CO than the yields of H
the C4 selectivity. As mentioned above, in the cases of the
slightly higher yields of CO than the yields of H and the
products, it should be attributed to the hydrodecarboxylation.
Next, the scope of carboxylic acids was explored (Scheme
4). Gratifyingly, the 1-adamantane and bicyclo[2.2.2]octane-
2
2
and 3aa (Table 1, entries 10, 15−18), it should be attributed
2
2
1k
to hydrodecarboxylation as the competitive pathway. This
deduction is further confirmed by observation of adamantane
by MS under the standard conditions (see Figures S1−S2).
With the optimized conditions in hand, the substrate scope
was explored (Scheme 3). First, benzothiazoles with 6-
a
Scheme 4. Substrate Scope of Carboxylic Acids
a
Scheme 3. Substrate Scope of Heteroarenes
a
Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), optimized
conditions, 30 h. Isolated yields were reported. H and CO were
2
2
detected by GC by applying CH as an internal standard.
4
b
c
d
Co(dmgH) (DMAP)CI as cobalt catalyst. i-PrOH as solvent. As
2
the dehydrogenative decarboxyolefination as the competitive pathway,
e
more than 100% yield of H is possible. As the hydrodecarboxylation
2
as the competitive pathway, more than 100% CO yield is possible.
2
f
g
h
CuCl ·2H O (10 mol %) as additive. 2(0.6 mmol). 2 (0.8 mmol),
a
2
2
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), optimized
2
2 h.
conditions, 30 h. Isolated yields were reported, and H and CO were
2
2
b
detected by GC by applying CH as an internal standard. As the
4
hydrodecarboxylation as the competitive pathway, more than 100%
related tertiary carboxylic acids reacted smoothly with 1a to
afford the C2-alkylated benzothiazoles in high yields (3ab−
c
CO yield is possible. 42 h.
2
3
ad, 69%−88%). Notably, the groups of hydroxyl, ketone, and
methoxyl, 6-bromo, and 5-bromo afforded the products
ester were well tolerated. In the cases of pivalic acid and 1-
methylcyclohexane-1-carboxylic acid, the competitive pathways
of hydrodecarboxylation and dehydrogenative decarboxyolefi-
3
ba−3da in 85−90% yields. In addition, 4,5-dimethylthiazole
resulted in 53% yield (3ea), which should be caused by the
lower electrophilicity toward the adamantyl radical. The
benzoxazoles bearing various electron-donating groups and
electron-withdrawing groups were studied. Methoxy-, methyl-,
fluoro-, chloro-substituted benzoxazoles and benzoxazole lead
to excellent reaction efficiencies (3fa−3ja, 86−94%). The
oxazole with phenyl group on the C4 position afforded product
nation occurred based on the obviously higher yields of H and
2
1k,v,z,aa
CO2.
Thus, the corresponding products were obtained in
acceptable yields (3ae, 37%; 3af, 42%). Notably, applying
Co(dmgH) (DMAP)Cl as the catalyst in i-PrOH increased the
2
yield of 3af to 42% in comparison with 22% under standard
conditions (see Table S1). The secondary alkyl acid, such as
Boc-protected piperidine-4-carboxylic acid, resulted in product
3ag in 30% yield. In the cases of secondary alkyl acids and
primary alkyl acid with heteroatoms, such as O and N at the β-
position, the desired products were obtained in 57−62% yields
(3ah−3ak) after slight optimization. In the case of 2-
3
ka in 81% yield. In the case of the oxazole with an ester group
on the C5 position, the products of monofunctionalization on
the C2 position and difunctionalization on C2 with C4
positions were obtained in 64% and 25% yields (3la, 3la′),
respectively. Protected benzoimidazoles with Cbz- and Boc-
groups gave products 3ma and 3na both in 63% yields. In
comparison with benzoimidazoles, Boc-protected imidazole
resulted in moderate yield (3oa, 41%), which should also be
caused by its lower electrophilicity. Benzofuran gave product
tetrahydrofuroic acid, applying Co(dmgH) (DMAP)Cl as
2
catalyst with addition of 10 mol % of CuCl ·2H O, the yield
2
2
of 3ah was increased from 34% under standard conditions to
61% (see Table S2). The copper salt may act as a Lewis acid to
activate the benzothiazole.
3
pa in 62% yield with the C2 selectivity. Bromo and aldehyde
groups on the 3-position of the benzothiophene could be
tolerated with products 3qa in 77% yield and 3ra in 56% yield,
respectively. 3-Substituted thiophenes bearing a cyan group
and aldehyde group resulted in products 3sa in 91% yield and
Applying Co(dmgH) (DMAP)Cl as the catalyst with 3
2
equiv of the carboxylic acids, dimethylglycine, pyrrolidin-1-yl-
acetic acid, and piperidin-1-yl-acetic acid afforded the products
3ai−3ak in moderate to good yields. 3-Phenylpropionic acid,
an example of typical primary alkyl acids without the
heteroatom at the β-position, did not show reaction efficiency
with the observation of a trace amount of styrene (Figure S3).
3
ta in 74% yield, respectively. Quinazolin-4(3H)-one and
phenanthridine displayed high reaction efficiency (3ua, 71%;
va, 82%). Quinazoline gave product 3wa in 69% yield with
3
C
DOI: 10.1021/acs.orglett.9b02539
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Furthermore, 3-hydroxyadamantane-1-carboxylic acid, 2-
adamantone-5-carboxylic acid, and 2-tetrahydrofuroic acid
demonstrated high efficiencies for the functionalization of
benzoxazole, lepidine, and quinoline, respectively (3al−3an,
mixed solvent of CH CN/EtOAc (1:1) as the good solubility
3
to the supporting electrolytes n-Bu NPF and good ionic
4
6
conductivity. Meanwhile, the standard reaction displayed
reactivity in CH CN/EtOAc (Table 1, entry 19). The
3
IV/III
III/II
6
8−83%). In the case of quinoline, monofunctionalization
products on the C2 and C4 positions were obtained (3ao,
0%; 3ao′, 40%) accompanied by a difunctionalization product
3ao″, 4%). Notably, for preparation of 3ae−3ah, 3an, and
ao−3ao″, hydrodecarboxylation and dehydrogenative decar-
boxyolefination should occur based on the obviously higher
ground-state potentials E
and E
of Ir[dF(CF )ppy]-
3
(dtbbpy)PF in CH CN/EtOAc were determined to be +1.74
6
3
1
(
3
V vs SCE and −1.33 V vs SCE, respectively. The excited-state
energy E of Ir[dF(CF )ppy](dtbbpy)PF was read from the
0
0
3
6
cross-point of the UV−vis absorption and luminescence
spectra at 439 nm as 2.82 eV (Figure S3). Therefore, the
1k,v,z,aa
yields of H and CO .
oxidation potential and reduction potential of *Ir[dF(CF )-
2
2
3
Furthermore, a gram-scale reaction gave 3aa (2.18 g) in 81%
yield (Scheme S1).
ppy](dtbbpy)PF were calculated to be −1.08 V vs SCE
6
IV/III
IV/III
2
III /II
III/II
(E
0
* = E
− E ) and +1.49 V vs SCE (E * = E
+
0
0
0
Subsequently, a series of experiments were conducted to
give insight into the reaction mechanism. First, the UV−vis
absorption spectrum of each component and the reaction
mixture confirmed that Ir[dF(CF )ppy](dtbbpy)PF acts as a
E ) (see SI). Obviously, the *Ir(III) was capable of
0
−
AdCOO‑/AdCOO•
abstracting electrons from AdCO2 [E
= +0.79
V vs SCE in CH CN/EtOAc (1:1)] or transferring electron to
3
Co(III)/Co(II)
the cobaloxime complex [E
= −0.29 V vs SCE in
3
6
visible-light photosensitizer (Figure 1, in the left). When the
CH CN/EtOAc (1:1)] (see SI).
3
Based on the above results, two reaction pathways are
III
proposed (Scheme 6). The photosensitizer Ir complex is
Scheme 6. Proposed Mechanism
Figure 1. UV−vis absorption spectrum (left) and emission-quenching
plots (right).
radical scavenger of TEMPO or BHT was added under the
standard reaction conditions, respectively, the transformation
was completely suppressed (Scheme 5, eq 1). In addition, 1a
Scheme 5. Mechanistic Studies
III
initially irradiated by blue LEDs to its exited state *Ir . The
III
−
*
Ir is reductively quenched by AdCO via a SET to afford
2
II
•
the Ir species and AdCO , which undergoes a fast
decarboxylation to give Ad . Ad nucleophilically attacks 1a
to afford 3aa by release of an electron (e ) and a proton (H )
via the intermediate 7. The Ir transfers an electron to the Co
to regenerate the Ir and produce the Co species. The Co is
further reduced to the Co by accepting one released electron.
The Co reacts with a proton to form a Co −H hydride,
which reacts with another H to release H and close the cobalt
2
•
•
−
+
II
III
III
II
II
I
with 2-cyclopropylacetic acid gave ring-opened product 3ap
I
III
(Scheme 5, eq 2). These experiments indicate that the
+
transformation involves a radical pathway. Besides, a quantum
yield value of ϕ = 0.24 was determined (see SI), which is
inconclusive on whether the reaction proceeded via a
2
III
II
cycle (Path A). Reduction of the Co −H to the Co −H is
followed by protonation to produce H and the Co or the
II
2
19
III
photoredox-catalytic pathway or a radical-chain pathway.
homolytic cleavage involving two Co −H to generate H , and
2
II
15b,c
Next, the luminescent quenching experiments were con-
the Co could not be excluded.
Alternatively (path B), the
III
III
ducted (Figure 1, in the right). Benzothiazole 1a and AdCO H
*Ir is oxidatively quenched by the Co via a SET to form the
2
IV
II
IV
2
a showed only slight quenching phenomena, but AdCO n-
Ir and the Co . The Ir subsequently abstracts an electron
2
−
III
Bu N displayed an obvious quenching phenomenon. Mean-
from AdCO2 to give AdCO2• and regenerate the Ir to close
4
•
II
while, [Co(dmgH) Py ]PF showed a similar quenching rate
the catalytic cycle. Then, AdCO2 and the Co follow the
similar processes as described in path A to afford 3aa with H2
release.
2
2
6
with AdCO n-Bu N. These experiments suggest that single
2
4
electron transfer (SET) of the exited photocatalyst *Ir(III)
dominantly occurs with the 1-adamantane carboxylate anion or
the Co(III).
The thermodynamic feasibility of the photoinduced SET
was analyzed by the oxidation−reduction potentials in the
In summary, we have developed a synthetically valuable
decarboxylating carboxylic acid for alkylation of heteroarenes
under mild conditions. The transformation occurred smoothly
under blue LED irradiation at ambient temperature without
D
DOI: 10.1021/acs.orglett.9b02539
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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which benefited from the release of H and CO . This method
2
2
displays good tolerance of various heteroarenes, carboxylic
acids, and functional groups. High yield was achieved for a
gram-scale reaction.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02539.
Experimental procedure, product characterization, and
spectral data (PDF)
(ab) Manley, O. M.; Fan, R.; Guo, Y.; Makris, T. M. J. Am. Chem. Soc.
AUTHOR INFORMATION
Corresponding Author
■
2019, 141, 8684−8688. For selected examples on α-oxocarboxylic
acids and vinyl carboxylic acids, see: (ac) Fang, P.; Li, M.; Ge, H. J.
Am. Chem. Soc. 2010, 132, 11898−11899. (ad) Liu, J.; Liu, Q.; Yi, H.;
Qin, C.; Bai, R.; Qi, X.; Lan, Y.; Lei, A. Angew. Chem., Int. Ed. 2014,
*E-mail: liyang79@mail.xjtu.edu.cn
ORCID
5
3, 502−506. (ae) Huang, H.; Jia, K.; Chen, Y. Angew. Chem., Int. Ed.
Yang Li: 0000-0003-4724-4785
Author Contributions
§W.-F.T. and C.-H.H. contributed equally to this work.
Notes
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The authors declare no competing financial interest.
̈
ACKNOWLEDGMENTS
This work was supported by the NSFC (Nos. 21472145) and
the Beijing National Laboratory for Molecular Sciences
(
■
A.; Gianatassio, R.; Fujiwara, Y.; Sobel, A. L.; Baran, P. S. J. Am. Chem.
Soc. 2010, 132, 13194−13196. (b) Fujiwara, Y.; Dixon, J. A.; O’Hara,
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(
BNLMS, BNLMS20160111). We thank Prof. Gang He,
Xi’an Jiaotong University, for the support on cyclic
voltammetry experiments and emission-quenching experi-
ments. We also thank the Instrument Analysis Center of
Xi’an Jiaotong University for the support on HRMS measure-
ment.
4
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