Dye-sensitized hematite compososite photocatalyst and its photocatalytic performance of aerobic annulation

Article abstract of DOI:10.1016/j.jphotochem.2018.04.037

A visible light initiated catalyst (DP-pfa@Fe₂O₃) is synthesized and used as a photocatalyst for the synthesis of pyrazoles under mild reaction conditions. Holes (h⁺) and [rad] OH are the essential for the light-induced th

Full text of DOI:10.1016/j.jphotochem.2018.04.037

JournalofPhotochemistry&PhotobiologyA:Chemistry360(2018)132–136
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Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Full Length Article
Dye-sensitized hematite compososite photocatalyst and its photocatalytic
performance of aerobic annulation
Dong-Liang Huang, Peng-Li Zhang, Ling-Ling Qu⁎, Qiu-Yun Chen⁎
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photocatalyst
Hematite
A visible light initiated catalyst (DP-pfa@Fe2O3) is synthesized and used as a photocatalyst for the synthesis of
pyrazoles under mild reaction conditions. Holes (h+) and % OH are the essential for the light-induced the pyr-
azole annulation reaction using water as oxygen source. Results demonstrate that photocatalyzed reaction of
methyl hydrazine and 2-benzylidenemalononitrile produced 2-((2-methylhydrazinyl)(phenyl)methylene)mal-
ononitile, which can transfer into pyrazoles in the presence of ∙ OH. Our results provide a new catalyst and an
economic synthetic method for the synthesis of pyrazoles through aerobic annulation.
Chemical synthesis
1. Introduction
methyl)phenyl)-5,5-difluoro-1,3,7,9-tetra-
methyl-5H-44λ4,5λ4-di-
pyrrolo[1,2-c-2′1′f][1,3,2]diazaborinine (DP) is used as a sensitizer to
absorb visible light due to its excellent photophysical properties, such
as long-wavelength absorption, high quantum yields, photochemical
stabilities and fluorescence emission in the visible region [11]. After
being sensitized by DP, the absorption intensity of hematite is greatly
improved. The photocatalytic results demonstrate that DP-pfa@Fe2O3 is
a promising visible-light-driven photocatalyst for the synthesis of pyr-
azoles by aerobic annulation, which will provide a mild synthetic
method of pyrazoles.
Photocatalysis, which is an environmental-friendly technology to
convert solar energy into chemical energy, has been attracting much
research interest [1]. There are many materials which show high
photocatalytic activity irradiated with light [2]. Hematite (α-Fe2O3) is
one of the promising photocatalysts due to its high chemical stability,
high efficiency and low-cost [3]. It has been reported that increasing
the absorption of light and decreasing the recombination rate of the
photogenerated charge carriers can enhance the photocatalytic activity
[4]. Researchers have tried to use many methods to improve photo-
catalytic properties of hematite, such as doping with other materials,
modification of surface structure, formation of composites and hetero-
structures, etc [5]. Among of these methods, dye-sensitization is an
innovative method to improve photo-responsibility of photocatalyst.
Some dyes have been studied with semiconductor for the degradation of
pollutants, for example, porphyrin, rhodamine and methylene blue [6].
On the other hand, photocatalytic organic reactions especial pho-
toredox reactions have emerged as a “green” tool for chemical trans-
formation [7]. Studies show that photocatalyst can lower the reaction
temperature, reduce side reactions and speed up chemical reaction by
promoting to generate reactive intermediates [8]. Current photoredox
reactions research is mainly focused on the oxidation of alcohols and
amines, CeH activation reaction and classic cross-coupling reactions, so
on [9]. In our previous work, RuII(bpy)3Cl2·6H2O has been successfully
used to photocatalyze aerobic annulation [10]. Considering the high
cost of RuII(bpy)3Cl2·6H2O, we design and synthesis a novel dye-sensi-
tized hematite composite (DP-pfa@Fe2O3) as a photocatalyst of aerobic
annulation. In this paper, 10-(4-((3,4-dihydroquinolin-1(2H)-yl)
2. Experimental
2.1. Materials
All chemicals were analytical grade and used as received without
further purification. Pyrrole-2-carboxylic acid, polyvinylpyrrolidone
(PVP k30), 2,4-dimethylpyrrole, p-chloromethylbenzoyl chloride,
1,2,3,4-tetrahydroisoquinoline, triethylamine, boron trifluoride ether,
cyanide, benzaldehyde and methyl hydrazine (40% aqueous solution)
were purchased from Aladdin Reagent Co. Ltd., Shanghai. Ferric
chloride hexahydrate (FeCl3•6H2O), sodium hydroxide (NaOH) and
concentrated hydrochloric acid (HCl) were supplied by Sinopharm
Chemical Reagent Co. Ltd, China. Deionized water was obtained from a
Millipore Milli-Q system.
2.2. Characterization
X-ray powder diffraction (XRD) measurements of samples were
⁎
Corresponding authors.
E-mail addresses: llqu@ujs.edu.cn (L.-L. Qu), chenqy@ujs.edu.cn (Q.-Y. Chen).
https://doi.org/10.1016/j.jphotochem.2018.04.037
Received 19 February 2018; Received in revised form 13 April 2018; Accepted 18 April 2018
Availableonline22April2018
1010-6030/©2018ElsevierB.V.Allrightsreserved.

D.-L. Huang et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry360(2018)132–136
performed on a Bruker D8 diffractometer operated at 40 kV and 40 mA
using Cu-Ka radiation (l = 1.54056 A). Infrared spectrawere recorded
on a Nicolet-470 spectrophotometer in the
wavenumber range of 4000–400 cm−1 using KBr pellets. Electronic
absorption spectra were recorded in the 900–190 nm a Varian CARY 50-
BIO UV–VIS spectrophotometer. Fluorescence measurements were
performed on a fluorescence spectrofluorometer Model CARY Eclipse
(VARIAN, USA),
a
1.0 cm quartz cell (ex = 325 nm, slit
width = 10 nm). TEM was performed at roomtemperature on a JEOL
JEM-200CX transmission electron microscope using an accelerating
voltage of 200 kV. The EPR spectra of complexes in CH3CN were ac-
quired at 298 K with a 0.201 mW power, 0.5 G modulation amplitude,
and 100 kHz modulation frequency. The volume of O2 was measured by
direct methods, via connecting of the reaction vessel with a U-tube to a
calibrated microburet with collection of the gas released. Column
chromatography was generally performed on silica gel (200–300 mesh)
and reactions were monitored by thin layer chromatography (TLC)
using UV light to visualize the course of the reactions. The 1H
(400 MHz) and 13C NMR (100 MHz) data were recorded on Bruker
AVANCE II 400 MHz spectrometer using CDCl3 as solvent. The chemical
shifts (δ) are reported in ppm and coupling constants (J) in Hz. 1H NMR
spectra was recorded with tetramethylsilane (δ = 0.00 ppm) as internal
reference; 13C NMR spectra was recorded with CDCl3 (δ = 77.00 ppm)
as internal reference.
Scheme 1. Preparation of DP-pfa@Fe2O3.
Scheme 2. Photocatalytic reaction of CH3NHNH2 and 2-benzylidenemalono-
nitrile derivatives.
methyl hydrazine aqueous solution, 0.5 mmol of 2-benzylidenemalo-
nonitrile or its derivatives were added to 4 mL of CH3CN/H2O mixture
(1:1, v/v). After purging with N2 for about 10 min, the mixture was
illuminated by the LED lamp (450–550 nm, 4W) for 4 h. Then the cat-
alyst was collected by filtration and the product was isolated by silica
chromatography after extraction and dryness (Scheme 2).
2.3. Synthesis of pfa@Fe2O3
Black pfa@Fe2O3 is prepared by adding FeCl3•6H2O (0.25 M 20 mL)
and PVP (100 mg) into the aqueous of pyrrole-2-carboxylic acid (0.5 M
20 mL). After stirring about 15 min, the mixture is heated at 160 °C for
6 h in a 50 mL autoclave. The products are obtained by filtration, wa-
shed three times with ethanol and dried in vacuum.
5-amino-1-methyl-3-phenyl-1H-pyrazole-4-carbonitrile (a). 1H NMR
(400 MHz, CDCl3) & 7.90–7.88 (m, 2H), 7.44–7.39 (m, 3H), 4.54 (s,
2H), 3.68 (s, 3H). 13C NMR (100 MHz, CDCl3) & 150.36, 150.21,
131.11, 129.02, 128.83, 126.23, 115.80, 73.23, 34.76.
5-amino-1-methyl-3-(p-tolyl)-1H-pyrazole-4-carbonitrile (b). 1H
NMR (400 MHz, CDCl3) & 7.79 (d, J = 8.2 Hz, 2H), 7.26 (d, J = 8 Hz,
2H), 4.34 (s, 2H), 3.70 (s, 3H), 2.40(s, 3H). 13C NMR (100 MHz, CDCl3)
& 151.77, 150.47, 139.04, 129.44, 128.47, 126.11, 115.89, 77.14,
34.70, 21.36.
2.4. Synthesis of 10-(4-((3,4-dihydroquinolin-1(2H)-yl)methyl)phenyl)-
5,5- difluoro-1,3,7,9-tetramethyl-5H-44λ4,5λ4-dipyrrolo[1,2-c-2′1′-f]
[1,3,2]diazaborinine (DP)
5-amino-3-(4-methoxyphenyl)-1-methyl-1H-pyrazole-4-carbonitrile
(c). 1H NMR (400 MHz, (CD3)2SO) & 7.72 (d, J = 8.8 Hz, 2H), 7.01 (d,
J = 8.8 Hz, 2H), 6.63 (s, 2H), 3.79 (s, 3H), 3.57 (s, 3H). 13C NMR
(100 MHz, (CD3)2SO) & 160.00, 153.56, 149.01, 127.42, 124.73,
116.70, 114.59, 70.05, 55.66, 35.09.
DP is synthesized according to the reported method [12]. Typically,
8-(4-Chlorobenzyl)-4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-
diaza-s-indacene (137 mg, 0.37 mmol) and 3,4-dihydro-2H-1λ-quino-
line (72.8 mg, 0.550 mmol) were dissolved in 100 mL acetonitrile, fol-
lowed by addition of triethylamine (74 mg, 0.74 mmol). The resulting
mixture was stirred for 24 h at room temperature. Next, the solution
was concentrated and the residues were redissolved in DCM and wa-
shed with water. The organic phase was dried over anhydrous Na 2SO4.
The residue was purified using silica gel column chromatography
(petroleum ether–ethyl acetate (3: 1) to afford green oil (160 mg, 92%).
(Found: C,74.52; H,6.31; N, 8.90. Calcd for C29H30 BF2N5: C, 74.21; H,
5-amino-3-(4-fluorophenyl)-1-methyl-1H-pyrazole-4-carbonitrile
(d). 1H NMR (400 MHz, CDCl3)
& 7.89–7.86 (m, 2H), 7.11 (t,
J = 8.8 Hz, 2H), 4.55 (s, 2H), 3.68 (s, 3H). 13C NMR (100 MHz,
(CD3)2SO) & 162.67 (d, J = 244.5 Hz), 153.71, 148.22, 128.68, 128.12
(d, J = 8.4 Hz), 116.44, 116.05 (d, J = 21.4 Hz), 70.34, 35.19.
5-amino-3-(4-chlorophenyl)-1-methyl-1H-pyrazole-4-carbonitrile
(e). 1H NMR (400 MHz, CDCl3) & 7.84 (d, J = 8.8 Hz, 2H), 7.40 (d,
J = 8.4 Hz, 2H), 4.46 (s, 2H), 3.69 (s, 3H). 13C NMR (100 MHz, CDCl3)
& 151.55, 149.19, 134.92, 129.74, 128.96, 127.46, 115.33, 73.62,
34.81.
6.44; N, 8.95%.); 1H NMR (400 MHz, CDCl3)
δ = 7.55–7.53 (d,
J = 8 Hz, 2H), 7.26–7.25 (d, J = 4 Hz, 2H), 7.14 (m, 3H), 6.98–6.96 (d,
J = 8 Hz,1H), 5.99 (s, 2H), 3.80 (s, 2H), 3.65 (s, 2H), 2.96–2.93 (t,
J = 6 Hz, 2H), 2.82–2.79 (t, J = 6 Hz, 2H), 2.56 (s, 6H), 1.41 (s, 6H).
MS (ESI) Calcd for C29H30 BF2N5: ([M+H+]) = 470.36, found: m/
z = 470.54, (100%). UV–vis(CH2Cl2/nm) (ε × 104, dm3 mol−1 cm−1):
230 (3.15), 337 (0.8), 501 (8.3).
5-amino-3-(4-bromophenyl)-1-methyl-1H-pyrazole-4-carbonitrile
(f), 1H NMR (400 MHz, CDCl3) & 7.78 (d, J = 8.8 Hz, 2H), 7.23 (d,
J = 8.8 Hz, 2H), 4.39 (s, 2H), 3.71 (s, 3H). 13C NMR (100 MHz, CDCl3)
& 151.46, 149.22, 131.92, 130.18, 127.71, 123.21, 115.24, 73.72,
34.83.
2.5. Synthesis of DP-pfa@Fe2O3
Pfa@Fe2O3 (30 mg) and DP (2.5 mg) were suspended in di-
chloromethane (5 mL), the suspension is stirring for 4 h at room tem-
perature. After centrifugal separation, the products are washed with
dichloromethane for several times and dried in vacuum (Scheme 1).
3. Results and discussion
The XRD patterns of DP-pfa@Fe2O3 and pfa@Fe2O3 are shown in
Fig. 1. From Fig. 1, we can find DP-pfa@Fe2O3 and pfa@Fe2O3 have
similar XRD patterns which correspond to the cubic phase hematite
(JCPDS no. 73–0603). The broad peak between 20° and 30° are assigned
to amorphous PVP. No other diffraction peaks can be detected, in-
dicating that the introduction of pyrrole-2-carboxylic acid and DP have
2.6. Photocatalytic annulation reaction
In photocatalytic reactor, DP-pfa@Fe2O3 (10 mg), 0.6 mmol of
133

D.-L. Huang et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry360(2018)132–136
Fig. 1. XRD patterns of DP-pfa@Fe2O3 and pfa@Fe2O3.
Fig. 3. Solid-UV spectra of DP- pfa@Fe2O3 and pfa@Fe2O3.
Considering that the construction of pyrazole rings is usually in-
itiated under severe conditions, we use DP-pfa@Fe2O3 as photocatalyst
to develop a green photocatalytic system of the pyrazole annulation
reaction. At first, the annulation between methyl hydrazine (R1) and 2-
benzylidenemalononitrile (R2) is used as a model reaction in the pre-
sence of DP-pfa@Fe2O3 as the photocatalyst to deduce the optimized
conditions. The resulted data are shown in Table 1. From Table 1, we
can conclude that DP-pfa@Fe2O3 is an efficient photocatalyst for the
pyrazole annulation reaction irritated by LED lamp. The influence of
the catalyst amount, raw materials molar ratio, system solvent and
reaction temperature on the yield of reaction has been examined
carefully. The photocatalytic results show that the yield of product(5-
amino-1-methyl-3-phenyl-1H-pyrazole-4-carbonitrile) can reach 85%
under the conditions of the amount of DP-pfa@Fe2O3 10mmg, the
molar ratio of R1 to R2 1:1.5, the CH3CN/H2O (1:1) solvent, reaction
time 24 h and reaction temperature 25 °C(entry 5), which provide a new
photocatalyst and new green ways for annulation reaction. Further-
more, no reaction occurs between R1 and R2 in the absence of H2O
(entry 9) and illumination (entry 15). The facts indicate that H2O and
illumination may play the important roles.
Fig. 2. FT-IR spectrum of DP, DP-pfa@Fe2O3 and pfa@Fe2O3.
no effect on the crystal structure of Fe2O3. Moreover, we have per-
formed the FT-IR spectroscopy to identify the functional groups of DP-
pfa@Fe2O3 and pfa@Fe2O3. As shown in Fig. 2, the wider band at
3250–3405 cm−1 indicated the presence of (DP)F-H-N (pyrrole) hy-
drogen bonds in DP-pfa@Fe2O3. It confirms the hydrogen interaction
between DP and pfa@Fe2O3·The stretching frequency at 1709 cm−1 in
the IR spectrum of pfa@Fe2O3 indicated the existence of the PVP’s
eC]O and the peak at 1664 cm−1 come from a pyrrol-conjugated
carboxyl group. The wider absorption at 510 nm in the solid UV ab-
sorption spectra of DP-pfa@Fe2O3 further confirms the existence of DP
in DP-pfa@Fe2O3.
The solid UV absorption spectra of DP-pfa@Fe2O3 further confirm
the existence of DP in DP-pfa@Fe2O3. From Fig. 3 we can observe both
DP- pfa@Fe2O3 and pfa@Fe2O3 show absorption in the visible-light
region. DP- pfa@Fe2O3 possess stronger ability of absorb visible-light
than pfa@Fe2O3. So DP- pfa@Fe2O3 can be a visible light driven pho-
tocatalyst. The band at 510 nm of DP-pfa@Fe2O3 can be attributed to
the π—π* transition of DP, which indicates the existence of DP in DP-
pfa@Fe2O3.
Fig. 4 shows the TEM of DP-pfa@Fe2O3 and pfa@Fe2O3. It is clearly
seen that both DP-pfa@Fe2O3 and pfa@Fe2O3 display perfect spherical
core-shell structure with the diameter of 55 nm. There are no any ob-
vious changes in diameter between them, which indicate that the
content of DP in DP-pfa@Fe2O3 is small. For DP-pfa@Fe2O3 and pfa@
Fe2O3 the differences still exist between them. The surface of pfa@
Fe2O3 is smooth while DP-pfa@Fe2O3 owns rough surface due to the
modification of DP.
To explore the efficacy and general applicability of DP-pfa@Fe2O3,
the pyrazole annulation reaction of methyl hydrazine (R1) and a series
of the para-substituted olefins bearing two nitriles are performed under
the optimal conditions. As shown in Fig. 5, DP-pfa@Fe2O3 shows
photoactivity for the pyrazole annulation reaction of different sub-
strates to afford desirable products. However, the introduction of sub-
stituent (electron-donating or electron-withdrawing substituent) can
decrease the yield more or less. 2-benzylidenemalononitrile has higher
yield than the other derivatives because H has minimum steric hin-
drance in comparison with the other substituents.
To understand the specific role of water for the pyrazole annulation
reaction and photocatalytic mechanism of DP-pfa@Fe2O3, we have
studied photocatalytic activity of water oxidation catalyzed by DP-pfa@
Fe2O3 and carried out trapping experiments by using proper scavengers.
We find that DP-pfa@Fe2O3 is a promising light-driven photocatalyst
for water oxidation and DP-pfa@Fe2O3 catalyzes O2 evolution with a
turnover frequency (TOF) of 0.04 min−1 and a turnover number (TON)
of 6.51 mol/min in water. We deduce that water is an oxygen source for
the pyrazole annulation reaction. As we know, EDTA is an effective
quenching agent for holes (h+) and tert-butyl alcohol (TBA) is widely
used as a scavenger of hydroxyl radicals (OH %). After EDTA (entry 16)
and TBA (entry 17) are added to the model reaction mixture, respec-
tively, we find no product (5-amino-1-methyl-3-phenyl-1H-pyrazole-4-
carbonitrile) produce. These reveal that holes (h + ) and OH % are the
essential for the light-induced the pyrazole annulation reaction using
water as oxygen source. Furthermore, we have explored the monitor
experiment to gain better insights into the reaction mechanism. The
134

D.-L. Huang et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry360(2018)132–136
Fig. 4. TEM images of pfa@Fe2O3(a) and DP-pfa@Fe2O3(b).
quantitative change of 2-((2-methylhydrazinyl)(phenyl)methylene)
malononitrile (M) is observed when the reaction is in process. There are
little M in the system after 5 h and the number of M increase as the
reaction time goes on. But after 12 h the number of M begin to drop
slowly until there are the trace M after 24 h.
On the base of the results of the control experiments, water oxida-
tion and trapping experiments, we propose a possible reaction me-
chanism as shown in Fig. 6. Upon light irradiation, DP-pfa@Fe2O3
generates electro-hole pairs. The photo-generated holes oxidize water
to OH% radicals and electrons which reduce Fe3+ to Fe2+. The con-
version of Fe(II) and Fe(III) result in the most efficient separation of
electron-hole pairs. In the presence of OH% radicals, 2-((2-methylhy-
drazinyl)(phenyl)methylene)malononitrile (M) convert to the product
1-methyl-3-phenyl-1H-pyrazole-4,5-dicarbonitrile.
4. Conclusions
In summary, a dye loaded hematite DP-pfa@Fe2O3 is synthesized
and used as a photocatalyst for the synthesis of pyrazoles through
aerobic annulation. The results demonstrate that DP-pfa@Fe2O3 is a
promising visible-light-driven photocatalyst for the synthesis of pyr-
azoles. Holes (h+) and OH % are the essential for the light-induced
pyrazole annulation reaction using water as oxygen source. Steric
hindrance has some effect on the synthesis of pyrazoles catalyzed by the
DP-pfa@Fe2O3. Moreover, light driven reaction of R1 and R2 produces
2-((2-methylhydrazinyl)(phenyl)methylene)-malononitile, which can
convert into pyrazoles in the presence of catalyst and water. Our results
provide a new catalyst and an economic synthetic method for the
synthesis of pyrazoles through aerobic annulation.
Table 1
Photocatalytic annulation reaction of methyl hydrazine (R1) and 2-benzylidenemalononitrile (R2) in the presence of DP-pfa@Fe2O3.a
Entry
Catalyst (mg)
Solvent (v/v)
R1/R2 (mol)
Time (h)
Temperature (°C)
Yield (%)b
0
1
2
3
4
5
6
7
0
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
MeCN
(DMSO:H2O)1:1
(EtOH: H2O)1:1
(MeOH: H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
(MeCN:H2O)1:1
1:1
1:1
1:1
1:1
1:1
1:1.5
1:2
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
24
24
24
24
24
24
24
12
48
24
24
24
24
24
24
24
24
24
25
25
25
25
25
25
25
25
25
25
25
25
25
50
75
25
25
25
0
10
20
30
10
10
10
10
10
10
10
10
10
10
10
10
10
10
71
73
75
74
85
81
50
90
0
45
71
68
83
82
0
8
9
10
11
12
13
14
15c
16d
17e
0
0
a
Reaction conditions: methyl hydrazine(R1, 0.5 mmol), 4 mL solvent and LED light (λ≥420 nm,5 W) illumination.
b
c
Isolated yield.
Reaction conditions: no illumination, DP-pfa@Fe2O3(10 mg), methyl hydrazine(R1, 0.5 mmol), 2-benzylidenemalononitrile(R2, 0.75 mmol), CH3CN/H2O(2 mL/
2 mL), 24 h, 25 °C.
d
Reaction conditions: scavenger(EDTA, 1 mmol) DP-pfa@Fe2O3(10 mg), methyl hydrazine(R1, 0.5 mmol), 2-benzylidenemalononitrile(R2, 0.75 mmol), CH3CN/
H2O(2 mL/2 mL), LED light(λ≥420 nm,5 W), 24 h, 25 °C.
e
Reaction conditions: scavenger(TBA, 1 mmol) DP-pfa@Fe2O3(10 mg), methyl hydrazine(R1, 0.5 mmol), 2-benzylidenemalononitrile(R2, 0.75 mmol), CH3CN/
H2O(2 mL/2 mL), LED light(λ≥420 nm,5 W), 24 h, 25 °C.
135

D.-L. Huang et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry360(2018)132–136
Acknowledgements
We thank the National Science Foundation of China for financial
support (No. 21571085 and 21701056), Highly Qualified Professional
Initial Funding of Jiangsu University (12JDG052).
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Fig. 6. Proposed mechanistic pathway for photocatalytic annulation reaction.
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