Visible-Light-Triggered Quantitative Oxidation of 9,10-Dihydroanthracene to Anthraquinone by O2 under Mild Conditions

Article abstract of DOI:10.1002/cssc.201903549

The development of mild and efficient processes for the selective oxygenation of organic compounds by molecular oxygen (O₂) is key for the synthesis of oxygenates. This paper discloses an atom-efficient synthesis protocol for the photo-oxygenation of 9,10-dihydroanthracene (DHA) by O₂ to anthraquinone (AQ), which could achieve quantitative AQ yield (100 %) without any extra catalysts or additives under ambient temperature and pressure. A yield of 86.4 % AQ was obtained even in an air atmosphere. Furthermore, this protocol showed good compatibility for the photo-oxidation of several other compounds with similar structures to DHA. From a series of control experiments, free-radical quenching, and electron paramagnetic resonance spin-trapping results, the photo-oxygenation of DHA was probably initiated by its photoexcited state DHA*, and the latter could activate O₂ to a superoxide anion radical (O₂^.?) through the transfer of its electron. Subsequently, this photo-oxidation was gradually dominated by the oxygenated product AQ as an active photocatalyst obtained from the oxidation of DHA by O₂^.?, and was accelerated with the rapid accumulation of AQ. The present photo-oxidation protocol is a good example of selective oxygenation based on the photoexcited substrate self-activated O₂, which complies well with green chemistry ideals.

Full text of DOI:10.1002/cssc.201903549

Accepted Article
Title: Visible Light-directly Triggered Quantitative Oxidation of 9,10-
dihydroanthracene to Anthraquinone by Molecular Oxygen under
Normal Temperature and Pressure
Authors: Dabo Jiang, Wenwei Hu, Mengke Chen, Zaihui Fu, Anqun Su,
Bo Yang, Feng Mao, Chao Zhang, Yachun Liu, and Dulin Yin
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201903549
Link to VoR: http://dx.doi.org/10.1002/cssc.201903549
A Journal of
www.chemsuschem.org

ChemSusChem
10.1002/cssc.201903549
Visible Light-directly Triggered Quantitative Oxidation of 9,10-dihydroanthracene to
Anthraquinone by Molecular Oxygen under Normal Temperature and Pressure
[a]
[a]
[a]
[a]
[a]
[a]
[a]
[a]
Dabo Jiang, Wenwei Hu, Mengke Chen, Zaihui Fu, * Anqun Su, Bo Yang, Feng Mao, Chao Zhang,
[a]
[a]
Yachun Liu and Dulin Yin
a
National and Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of
Resources, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha Hunan 410081,
China.
Abstract: The development of mild and efficient process for the selective oxygenation of organic compounds by
molecular oxygen (O
2
) can be one of the key technologies for synthesizing oxygenates. This paper discloses an
to
atom-efficient synthesis protocol for the photo-oxygenation of 9,10-dihydroanthracene (DHA) by O
2
anthraquinone (AQ), which can achieve quantitative AQ yield (100%) without any extra catalysts and additives
under normal temperature and pressure. Also, 86.4% AQ yield is obtained even in air atmosphere. Furthermore,
this protocol has a good compatibility for the photo-oxidation of several other compounds with a similar structure
to DHA. Based on a series of control experiments and free radical quenching and EPR spin trapping results. The
photo-oxygenation of DHA is likely initiated by its photo-excited state DHA* and the latter can activate O
superoxide anion radical (O
-) via a transfer of its electron. Subsequently, this photo-oxidation is gradually
dominated by the oxygenated product AQ as an active photo-catalyst, obtained from the oxidation of DHA by O
-,
and it is accelerated with the rapid accumulation of AQ. The present photo-oxidation protocol gives a good
2
to a
2
2
2
example for the selective oxygenation based on the photo-excited substrate self-activated O , which compliances
well with ‘green’ chemistry ideals.
Keywords: 9,10-dihydroanthracene; Anthraquinone; Molecular oxygen; Photo-oxidation; Superoxide free
radical
1
. Introduction
Anthraquinone (AQ) is one of important fine chemicals and chemical intermediates. It is often used to produce the
anthraquinone dye [1,2] and hydrogen peroxide [3-7] and to enhance the Kraft process for paper production.[8] It is
also employed in the isomerization of vegetable oils, in nickel electro-deposition, seed and grain protection and in
the production of stabilizing additives. [9] Intense interest has been focused on the synthesis of anthraquinone
Corresponding author. Tel:+86 731 88872576; Fax:+86 731 88872531.
E-mail address: fzhhnnu@126.com
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
derivatives due to their potential utility in cancer chemotherapy as antioxidants and antitumor agents. [10,11] AQ is
commercially produced in several ways, including oxidation of anthracene (AN) with chromic acid [12] or nitric
acid [13] and vapor-phase oxidation of AN by air at 390 C over a supported iron vanadate potassium catalyst.
o
[12]
In
addition, AQ and its derivatives can be obtained more conveniently by Diels-Alder condensation reaction of
benzene or its derivatives with phthalic anhydride. [8] These catalysis oxidative systems, however, suffer from
several drawbacks such as corrosion of equipment, higher energy consumption and high toxicity. [14] It is, therefore,
of great practical importance to develop a cost-effective and environmentally friendly method for the synthesis of
AQ. Recently, a series of works have been reported to replace the conventional oxidative systems of AQ synthesis,
such as metal (Fe, Cu, Co, Ru, Mn) complexes, [15-25] Ru(tmp)(O) O, [26] air/oxygen/nitric acid, [13]
/N
2 2
[
27]
and molecular sieves catalyst [38-30] etc.. The use of dinuclear CuII complexes of pyrrolic
5 2 2
H PV Mo10O40/O
II
Schiff-base macrocycles in combination with FeCl
3
(Cu -FeCl
3
) as an effective catalyst for the oxidation of
9
,10-dihydroanthracene (DHA) to AQ was reported by Pankhurst et. al., giving a higher DHA conversion (100%)
and 87.0% AQ yield, but high cost severely restricted its application for the synthesis of AQ. [18] Selvaraj et. al.
reported that CrSBA-15 was also an efficient solid catalyst for liquid-phase oxidation of AN to AQ and achieved
9
0.6% AN conversion and 100% AQ selectivity. [28] Unfortunately, not only the synthesis routes of the catalyst are
complex, but also it is very expensive to use a large amount of catalyst (100 wt.%) and t-butyl hydroperoxide
TBHP) as an oxidant. 9-Mesityl-10-methylacridinium ion, as an effective electron-transfer photo-catalyst for
(
oxidation of AN to AQ, was reported by Kotani et. al., [31] in this case, a maximum AQ yield of 75.0% was
obtained. However, this catalyst is not readily available and its photocatalytic efficiency still lacks competitiveness.
Hawthorne et. al. [32] reported that pyridine as a solvent, and benzyltrimethylammonium hydroxide as a catalyst,
9
2
0.0% AQ yield was obtained by oxidation of DHA with O at mild conditions (temperature, 70°C; time, 2 h). The
solvent and catalyst used in the reaction system, however, result in a serious impact on the environment, which
runs in the opposite direction the idea of ‘green chemistry’.
In recent years, light-triggered chemical reaction has emerged as a powerful synthesis technique for the
production of fine chemicals at mild reaction conditions [33-35]. In particular, the selective oxidation of organic
2
compounds by dioxygen (O ) or air under visible light is believed to be user- and environmentally friendly and
thus of great academic and application significance [36,37]. Herein, we would report an atom-efficient and
eco-friendly photo-oxidation protocol for the synthesis of AQ from DHA without any extra catalysts and additives
under normal temperature and pressure, which can fully meet the requirements of the green chemistry principles.
This protocol can achieve quantitative conversion of DHA to AQ (100% yield) under 10 h of visible light
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
2 2
irradiation and pure O atmosphere and even shortening the illumination time to 5 h or using air instead of pure O
can also achieve AQ yields of 99.4 and 86.4%, respectively, which has a remarkably comprehensive advantage
over the systems previously reported in Table 1. Furthermore, it has a good adaptability for the photo-oxidation of
several other compounds with a similar structure to DHA. The present photo-oxidation protocol opens up a new
2
voyage for the selective oxygenation based on the photo-excited substrate self-activated O .
Table 1. Comparison of the reported oxidation systems for the synthesis of AQ from DHA or AN
o
Catalyst
Substrate
(mmol)
Atm.
(atm)
Solvent
(mL)
T ( C),
AQ yield
(%)
Entry
Ref.
/
Oxidant
None
t (h)
a
1
DHA, 0.1
DHA, 0.1
DHA, 0.15
DHA, 0.15
DHA, 50
DHA
O
2
(1)
Acetone (5)
Acetone (5)
MeCN
rt., 5 or 10
rt.,10
99.3 or 100
86.4
This work
a
2
None
Air (1)
-
This work
II
b
c
3
4
5
6
Cu -FeCl
3
/TBHP
rt., 16
200, 20
70, 2
87.0
18
d
Ru(tmp)O
2
N
2
O (10)
Benzene (14)
Pyridine (50)
Benzene
89.1
26
e
Triton B
O
2
(1)
72.0
32
f
[TptBu,Me]Ni(O
2
)
-
40, 120
rt. 10
95.0
38
a
7
None
AN, 0.1
O
2
(1)
Acetone (5)
MeCN (5)
92.4
This work
g
c
8
9
NBPBFTC /TBHP
AN, 0.28
AN, 0.9
-
50, 1
99.3
39
30
30
40
40
41
42
c
Al-MCM-41/TBHP
-
-
-
-
-
-
Benzene (10)
Benzene (10)
Benzene (3)
Benzene (3)
HAc (20)
75, 20
75, 20
80, 12
80, 12
100, 0.5
80, 2
21.5
c
1
0
1
Ti-HMS/TBHP
AN, 0.9
90.9
h
c
1
V2PA /TBHP
AN, 56
41.8
i
c
1
1
1
2
3
4
SBAV2(30) /TBHP
AN, 56
60.0
j
RuCl
3
/AHPO
AN, 0.28
AN, 0.25
96.0
k
j
Cat. /AHPO
DMC(3), MeCN(3)
91.1
a
b
II
II
3
Visible light irradiation; Cu -FeCl , dinuclear Cu complexes of pyrrolic Schiff-base macrocycles in combination
c
d
e
with FeCl
3
; TBHP, tert-butylhydroperoxide; tmp: 5,10,15,20-tetramesitylporphyrin; Triton B, benzyltrimethyl
ammonium hydroxide; [TptBu,Me]Ni(O
f
), superoxo-nickel(II) trispyrazolylborate complexes; NBPBFTC, [N-butyl
g
2
pyridine]+ [FeBrCl
] ; V2PA, H
-
h
PV
Mo10
O4032.5 H
O; SBAV2(30)=30 wt% V2PA loaded on NH
i
+-SBA-15;
3
5
2
2
3
j
Aqueous hydrogen peroxide; Tp*,BrCu(NCMe), Tp denotes hydrotris (3,5-dimethylpyrazolylborate).
k
*
2
. Experimental
2.1. Reagents and Materials
Materials and reagents used in this study were 9,10-dihydroanthracene (DHA), xanthene, fluorene,
diphenylmethane, 9,10-dihydroacridine, anthracene (AN), anthrone (AO), 1,2,3,4-tetrahydronaphthalene (THN),
xanthone, fluorenone, acetone, acetonitrile (MeCN), anthraquinone (AQ), isopropanol (IPA), benzoquinone (BQ),
2
,2,6,6-Tetramethylpiperidinooxy (TEMPO), 5,5-dimethyl pyrroline N-oxide (DMPO), all of which were of
analytical grade.
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
2.2. Characterization of samples
UV-Vis spectra of liquid samples of DHA and AQ in pure acetone solution were measured from 200 to 800
nm on UV-2450 spectro-photometer (Shimadzu, Japan). H and 13C NMR spectra of the isolated products were
1
measured on a DRX-400 (Bruker) spectrometer with CDCl and DMSO-d6 as the solvent in the presence of SiMe
3
4
as an internal standard. EPR spectra of the samples were measured in 50 μL quartz tubes on a Bruker
A300-10/12/S at room temperature. The spectrometer at 9.8 GHz, modulation frequency 100 kHz, modulation
amplitude 3 G. EPR spectra were simulated using an extended version of the program Bruker Win EPR.
2.3. Visible-light-catalytic oxidation experiments
2
The visible light-driven oxygenation of DHA by O was performed in a self-assembly common glass
o
photo-reactor (See Figure S1) equipped with an ethanol (0-5 C)-cooled condenser and an oxygen storage vessel (1
atm). [43] A 35 W tungsten–bromine lamp purchased from NVC lighting corporation (With an Osram brand UV
2
light filter; Light intensity, 535 mW/cm ; Operating voltage, 12 V) was used as a visible light source and its
emission spectrum could be found in Figure S2. This lamp was built in a quartz glass tube and then immersed in
acetone solution containing DHA (0.1 mmol). The reaction mixture was stirred magnetically (800 rpm) under pure
oxygen (1 atm) or air atmosphere and sustained visible light irradiation, and the temperature of reaction solution
o
increased to about 25 C because of the heating effect of light irradiation. After the desired irradiation time had
elapsed, the reaction was stopped and the oxygenated products were determined on an Agilent 1260 HPLC
(Agilent, USA) with an external standard method and the specific operation conditions were as follows: 4.6×250
mm of Agilent SB-C 18 chromatographic column, acetonitrile/water 65:35 (v/v) of mobile phase, 0.5 ml/min of
o
flow rate, the UV detector 260 nm, column temperature 30 C, injection volume 20 μL. The yield of AQ was
calculated by dividing the actually produced molar mass of AQ by the theoretical molar mass of AQ. Each
photoreaction in this work was repeated in duplicate and the obtained data between two parallel experiments was
lower than ca.1% and the average of the two experimental data is used in the charts. The isolated goal products
were identified by H and 13C NMR spectra.
1
3
. Results and discussion
3.1. The oxidation of DHA under different conditions
Table 2 lists the data for the oxidation of DHA under various reaction conditions. Under the best standard
conditions, with acetone solvent, 1 atm pure O and 10 h of sustained visible light irradiation, DHA can be
2
quantitatively oxidized to the target product AQ with a 100% yield (Entry 1), representing an atom-economic
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
character of this oxidation protocol. Entries 2 and 3 show that even if shortening the irradiation time to 5 h or using
air instead of pure O , this protocol still exhibits a high efficiency, affording AQ yields of 99.3 and 86.4%,
2
respectively. Among the other three solvents examined, MeCN is also a good solvent for this photo-oxidation,
providing 90.7% AQ yield (Entry 4). One possible reason why MeCN is slightly inferior to acetone in AQ yield is
because the former has a lower solubility for AQ than the latter, thus leading to a decrease of light absorption
efficiency in the gradually turbid MeCN solution. Another two solvents DMF and DMA are almost or completely
invalid to the photo-oxidation of DHA to AQ although they have a better solubility to DHA and AQ than acetone
o
(
Entries 5 and 6). Notably, the elevating photoreaction temperature to 80 C can significantly enhance AQ yield to
7
5
0.8% in DMF but is still low efficient to improve AQ yield (only 18.7%) in DMA (data in parentheses of entries
and 6). In addition, Entries 7 and 8 show that adding 0.2 mmol DMF or DMA in the standard reaction solution
results in about 8% or 71% decrease in AQ yield, respectively. Photoluminescence spectral results of DHA shown
in Figure S3 illuminates that the stability (or lifetime) of the photo-excited state of DHA is influenced by the above
solvents, presenting an increasing sequence from acetone, MeCN, DMA to DMF. These findings support that the
favorable solvation effect of MeCN and especially acetone should be due to that they may stabilize the
photo-excited state of DHA but hardly affect the activity of the superoxide free radical (O
2
-) generated in
photoreaction. While DMF and especially DMA, as very inert solvents of DHA photo-oxidation, should be partly
due to their weaker stabilizing effect on the excited state of DHA. And another important reason may be that the

-
O
2
species is captured by these two solvents, leading to a perceptible decrease of its activity in DMF, even almost
inactive in DMA (Here, DMA exerts free radical quenching). Entries 9-10 showcase that this photo-oxidation can
also occur under an external radiation mode of visible or UV light, but the external radiation mode of visible light
exhibits a much lower utilization efficiency than its internal irradiation mode and its excitation efficiency is
obviously lower than that of UV light, which is consistent with an UV-vis absorption spectral curve of DHA
(
Figure S4). In order to make clear the specific work wavelength of DHA, the photo-oxidation of DHA was further
studied with a monochromatic visible light as an external illumination (The reaction device see Figure S5) and the
results indicated that this oxidation can be triggered by a violet light of 400 nm but not by a blue light of 500 nm
(Entries 11 and 12). Notably, when a catalytic amount of AQ (1% mol relative to DHA) was pre-added into the
photooxidation system of Entry 12, the photo-oxidation of DHA was accelerated significantly and the obtained AQ
yield (88.6%, Entry 13) was much higher than that obtained in Entry 12 (44.2%), supporting that the target product
2
AQ is an efficient catalyst for the photo-oxidation of DHA with O and can function well under violet light. Two
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
blank experiments conducted in the absence of O
irradiation are necessary for the oxidation of DHA (Entries 14 and 15).
Table 2 The oxidation of DHA to AQ under different conditions
2
or without light irradiation confirmed that O
2
and light
b
Yield of products (%)
Entry
Change from standard conditionsa
AO
AQ
100
AN
0
1
2
Standarda
0
Visible light illumination 5 h
0
99.3
86.4
90.7
8.1 (70.8)c
0.1 (18.7)c
91.9
29.0
76.8
20.7
0
0
3
Air instead of O
2
1.3
0
4
MeCN instead of acetone
DMF instead of acetone
DMA instead of acetone
Adding 0.2 mmol DMF
Adding 0.2 mmol DMA
28 W UV light, 10 h
0
0
5
0 (8.2)c
0
6
0 (0.5)c
0
7
0
3.2
3.5
6.8
0
0
8
0
9d,e
0.8
1.4
0
1
0e
35 W visible light, 10 h
Blue light, 10 h
1
1e,f
2 e,g
3 e,g
4h
1
1
1
Violet light, 10 h
1.8
0
44.2
88.6
0
0
Violet light, 10 h, 1 mol% AQ
Without light illumination
0
0
0
1
5
N
2
instead of O
2
0
3.4
0.3
a
2
Standard reaction conditions: DHA, 0.1 mmol (0.018 g); Acetone, 5 mL; Pure O , 1 atm; 35 W tungsten-bromine
b
lamp as a built-in light source, illumination time, 10 h; AO, AQ and AN denoted anthrone, anthraquinone and
c
o
d
anthracene, respectively; Photoreaction was carried out at 80 C; 28 W UV light with a center wavelength of 254
e
f
nm; By use of an external radiation; Using a monochromatic green light with a center wavelength of 500 nm
g
obtained by a 300 W xenon lamp equipped with a light filter of 500 nm; DHA, 0.4 mmol; Acetone, 20 mL; Using
a monochromatic violet light with a center wavelength of 400 nm obtained by a 300 W xenon lamp equipped with
h
o
a light filter of 400 nm; DHA, 0.4 mmol; Acetone, 20 mL; 35 C, in the dark.
3.2. Photooxidation of various substrates
In order to verity the adaptability of the above photooxidation protocol, the photooxidation of several other
compounds with a similar structure to DHA was also checked by use of the above standard reaction conditions. As
shown in Table 3, the photo-oxidations of most substrates can occur efficiently, among them, anthrone (AO)
shows a slightly higher reactivity than DHA (Entries 2 vs 1), affording a 99.9% AQ yield at illumination 5 h.
Anthracene (AN) also has an excellent suitability to this photo-oxidation protocol and its AQ yield is up to 92.4%
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
after light illumination time 12 h (Entry 3). In addition, by the use of this photooxidation protocol, two hetero-atom
skeleton substituted-DHA derivatives 9,10-dihydroacridine and especially xanthene can be also smoothly
converted to the corresponding ketones, achieving 62.2 and 90.8% yields, respectively (Entries 4 and 5). However,
this protocol is low efficient for the photo-oxidation of fluorene (Entry 6, 10.8% yield) and especially
diphenylmethane (Entry 7, 3.4% yield) and even invalid to the photo-oxidation of 1,2,3,4-tetrahydronaphthalene
(THN) (Entry 8, 0.1% yield) and cyclohexane (Entry 9, no product). It is worth noting that the photo-oxidation
efficiency of four substrates in entries 6-9 can be drastically improved in the case of using 5 mol% DHA (relative
to the substrate) as the initiator and the yields of the corresponding target products fluorenone, benzophenone,
1
-naphthone and cyclohexanone are significantly increased to 86.0, 97.5, 90.2 and 16.8%, respectively. According
to the oxidizable C-H bond dissociation energies (BDEs) of these substrates listed in Table 3, it is reasonable that
the automatic photo-oxidation efficiency of DHA, AO and AN to provide the same product AQ is dependent on
the reactivity of their C-H bonds. However, considering only the reactivity of the C-H bond can’t explain why
fluorine, diphenylmethane and especially THN have an almost equivalent C-H bond energy to DHA, showing a
very low or no photo-reactivity.
In order to explore whether the target product contributes to the photooxidation, three representative target
products AQ, xanthone, and fluorenone were used to photo-catalyze the oxidation of DHA, xanthene and fluorine
under the standard conditions and the obtained results after light irradiation 2 h are listed in Table 4. Among these
three photocatalysts, AQ exhibits the best activity and the yields of the target products in the photo-oxidation of
DHA, xanthene and fluorine catalyzed by AQ, are 68.0, 88.6 and 16.4%, respectively. Xanthone also has activity
for the photo-oxidation of xanthene and especially DHA, yielding the target products of 15.7 and 43.8%,
respectively. But, for the photo-oxidation of DHA and especially fluorine, the catalytic activity of fluorenone is
almost negligible. These findings strongly support that the photo-oxidation efficiency of the substrate is mainly
dominated by the photo-catalytic activity of its target product.
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
Table 3. Data obtained from photo-oxidation of various substrates by O
2
under visible lighta
Entry
Substrate
Productb
BDE (C-H) (kcalmol-1)
Time (h)
5
Yield (%)
c
d
1
2
3
4
5
6
7
8
9
78.0g
99.3 (99)
70.6e
∼111f
73.7i
75.5g
80.1g
82.0g
85.0i
99.0j
5
99.9c
12
12
12
12
12
12
12
92.4c
62.2c
90.8c
c
h
10.8 (86.0)
c
h
3.4 (97.5)
0.1 (90.2)h
0 (16.8)h
a
Reaction condition: Substrate, 0.1 mmol; Gas atmosphere, O
2
; Acetone, 5 mL; b 1H and 13C NMR spectra of the
isolated products could be found in Supporting Information; c Yield of the products were determined by high
performance liquid chromatography (HPLC) analysis using external standard method; d The values in the
parentheses corresponds to the isolated yield of product directly obtained by evaporating the solvent, and the liquid
e
chromatogram of the isolated product AQ is shown in Figure S6; DFT calculation; Basis set, b3lyp/6-311+g (d,
p). f Reference [24]; Reference [25]; The values in the parentheses corresponds to the yield when using 5 mol%
g
h
i
j
DHA as an initiator; Reference [44]; Reference [20].
Table 4. Comparison of photocatalytic performance of three target products of DHA, xanthene and fluorenea
Entry
Substrate
DHA
Catalyst
AQ
Yield of target product (%)
1
2
3
4
5
6
7
68.0b
88.6
16.4
43.8
15.7b
3.0
Xanthene
Fluorene
DHA
AQ
AQ
Xanthone
Xanthone
Fluorenone
Fluorenone
Xanthene
DHA
Fluorene
0.9b
aReaction condition: Substrate, 0.1 mmol, catalyst, 0.001 mmol, light, gas atmosphere, O
, acetone, 5 mL,
2
b
Reaction time, 2 h; When catalyst itself is the target oxidation product, its added amount was deducted from
calculating product yield.
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
3.3. Effect of substrate dosage and illumination time on the photooxidation of DHA
The effect of substrate dosage and illumination time on the photooxidation of DHA was also checked under
standard reaction conditions and the results are as shown in Figures 1 and 2, respectively. Figure 1a illuminates
that DHA conversion is not influenced by its dosage and can attain 100% under standard conditions. However, the
yield of AQ is gradually and markedly reduced from 100 to 72.9% with increasing DHA dosage from 0.1 to 1.0
mmol, along with the appearance of small amount of the intermediate product AO (ca. 3%) and other unknown
products. AN, an oxidative dehydrogenation product, is not detected out. A main reason to result in a decrease in
the photo-oxidation efficiency of high concentration of DHA is likely because a large number of precipitated AQ
solid generated during reaction seriously affects the transmittance of visible light (Figure S7). In order to confirm
this speculation and to improve the efficiency of DHA photooxidation at high concentration, the current photo-
oxidation was performed using two 35 W bromine-tungsten lamps as a built-in light source. As shown in Figure 1b,
when the amount of DHA does not exceed 0.5 mmol, DHA can be almost quantitatively photo-oxidized to AQ at
twice the light intensity. Even a further increase in DHA amount to 1 mmol would only result in a slight 3.8%
reduction in AQ yield. Also, the supplementary experiment shown in Figure S8 shows that after filtrating to
remove the yellow solid product AQ produced in the middle stage of photo-oxidation (4 h), the subsequent
photo-oxidation of the resulting filtrate for another 6 h can proceed more efficiently at high dosage of DHA, thus
leading to an obvious increase in AQ yield.
Figure 1. Effect of DHA dosage on its photo-oxidation under the standard conditions in Table 2 (a) using a 35 W
bromine-tungsten lamp and (b) using two 35 W bromine-tungsten lamps
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
Figure 2 gives the effect of irradiation time on the photo-oxidation of DHA in the absence or presence of AQ. As
shown in Figure 2a, this photo-oxidation without pre-addition of AQ proceeds very slow at the beginning of the
reaction (15 min), providing AO as a main oxygenated product. When the time is prolonged from 15 to 180 min,
the conversion rapidly increases from 1.9 to 93.3% and the yield of AO is quickly increased from 1.5 to 36.5%,
along with a gradually increase of AQ yield from 0.01 and 20.9%. At the same time, the yield of AN also
undergoes a change that increases first and then decreases. After that, AQ yield rapidly increases to 99.3% with
further prolonging the time to 5 h, along with the complete consumption of DHA, AO and AN. The time
dependence of the above-mentioned AQ yield curve shows an abnormal dynamic behavior and clearly depicts the
evolutionary process from the automatic photooxidation of DHA to its photocatalytic oxidation, as supported by
the UV-Vis spectrum analysis of the reaction solution obtained at different illumination time (see Figure S9). This
is mainly because the automatic photooxidation of DHA proceeds very slowly in the early reaction period, only
providing its initial oxidation product AO. In the mid-reaction, its photo-oxidation is accelerated extremely owing
to the photo-catalysis of the in situ generated AQ, the obtained main products are AO, AQ and trace amount of AN
and anthracen-9-ol are also detected out in this reaction stage, anthracen-9-ol has been identified by LC-MS
analysis shown in Figure S10. In the late reaction, the highly accumulated AQ, as an active photo-catalyst, is able
to achieve the complete conversion of DHA and various intermediate products in the reaction system, finally
affording an almost quantitative transformation DHA to AQ at 5 h. Notably, the abnormal dynamic behavior in
Figure 2a can be corrected when a catalytic amount of the target product AQ (1% mol relative to DHA) was
pre-added into the above photo-oxidation system. As shown in Figure 2b, the transformation of DHA in the
presence of AQ is accelerated markedly in the early stage of the reaction, and the yield of its intermediate products
AO and AN undergoes a change that increases first and then decreases in the whole period of reaction, along with
a rapid increase in the yield of the target product AQ. The time dependence of AQ yield curve in Figure 2b clearly
reflects a normal photocatalytic dynamic behavior.
Figure 2. Effect of reaction time on the oxidation of DHA in the absence (a) or presence (b) of 0.001 mmol AQ
(Note: Using the standard conditions in Table 2. Deducting the added AQ when calculating the DHA-derived AQ
yield)
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
3.4. Proposed photo-oxidation mechanism for synthesis of AQ from DHA
Quenching experiments of active oxygen species: Hydroxyl (OH) and superoxide (O -

) free radicals as well as
2
+
hole (h ) are generally recognized as three active species in the O
2
-based selective oxidation, and the quenching
experiments for these three active oxygen species were conducted by adding 0.2 mM TEMPO (a quencher of OH
2 2 2 2 4
and O ) and Na C O (a quencher of hole
), isopropanol (a quencher of OH), BQ (a quencher of O -
-
h+)respectively. [45] As shown in Figure 3, the photo-oxidation of DHA to AQ is almost completely quenched in
the presence of TEMPO and decelerated extremely in the presence of BQ, but it is almost unaffected by the
-
quencher isopropanol and Na
2 2 4 2
C O . The quenching experiments undoubtedly suggest that O is the main active
species for the current photo-oxidation protocol.
Figure 3. The effect of various scavengers on the photo-oxidation of DHA (Reaction conditions: 0.1 mmol DHA,
mL acetone, 0.2 mmol quencher, 1atm O , illumination 4 h)
5
2
EPR spin trapping: In order to further support the above free radical quenching experiments, a superoxide radical
anion capture test was performed using the oxidation of DHA (0.01 g) by O in acetone with DMPO (50 mM)
2
under visible light illumination or without light. After 5 min of reaction, the sample was immediately transferred to
a test tube and its EPR signal measured under normal conditions. As shown in Figure 4, no any EPR signal was
•
-
observed when the experiment was performed in the dark. However, the DMPO-O
2
EPR signal could be found
clearly under visible light illumination, providing a direct proof that O
2
•-, as the most important active oxygen
species, exists in the photo-oxidation of DHA, which is accordant with the results reported by other authors. [46-49]
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
2
Figure 4. The EPR spectrum of DHA/O after 5 min visible light irradiation and in the dark
3.5. Proposed photo-oxidation mechanism
According to the results presented here, as well as those in previous studies, [19,21,32,40,50,51] it is apparent that the
above data are better explained according to the following reaction pathways (Scheme 1). DHA is converted to its
excited state (DHA*, A) under visible light illumination. Then, DHA* may transfer its one electron to O
2
to give a
+
-
+
+
-
DHA cation and a superoxide radical anion (O ), and DHA can further transfer its one H to O to yield a
2
2
carbon-centered free radical (B) and a hydroperoxyl radical. Followed by that the radical B combines with oxygen
to form its peroxy radical (C) and the latter begins the propagation of the radical chains as follows (i.e., an
auto-oxidation pathway): The C abstracts one H atom on the 9 or 10 carbon sites of DHA to give its hydroperoxide
(
D) and another radical B. Then the species D is converted to anthracen-9-ol (E) by eliminating one molecule of
water, and the species E can further turn to its stable keto isomer AO (F). In the same way, AO is eventually
oxidized by O to the target products AQ under visible light through undergoing the formation and transformation
2
of the intermediates G to H. In addition, the radical B may be converted to a diradical I when one hydrogen atom
on its para position is deprived by a hydroperoxyl radical, the diradical I can rapidly decay to form AN and the
latter can also transform to AQ by the above oxidation pathway. The proposed mechanism is supported by
analyzing the products and UV-Vis spectra of DHA oxidation solution obtained at different illumination time (see
Figure S9 and S11).
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
According the present results and the literature reports, [52-56] a mechanism of AQ-photocatalyzed DHA
oxidation with O is proposed as follows (Scheme 2): Based on the UV-Vis spectrum of high concentrations of
2
AQ in Figure S12, AQ can absorb a visible light more efficiently than DHA to form its excited state AQ*. Then,
this AQ*, which is likely more active than DHA*, directly deprives one H atom on the 9 or 10 carbon sites of
DHA to yield its reduced species (K) and the above radical B. Then, the radical B combines with oxygen to form
its peroxy radical (C) and the latter is more likely to deprive one H atom of the reduced species K to yield its
hydroperoxide (D) and regenerate AQ (photocatalysis cycling pathway). [43] The species D is then converted to AO
2
(F) by the transformation routes described in scheme 1 and the latter is eventually oxidized by O to the target
product AQ by the photo-catalysis pathway described above. Of course, the auto-oxidation route described in
scheme 1, as a minor pathway, may exist in the present photo-catalytic reaction. In this case, the regeneration of
2
catalyst AQ can be achieved by the re-oxidation of its reduced species K by O .
Scheme 1. Possible pathways of photo-oxidation of DHA under visible light irradiation
Scheme 2. Possible pathways for the product AQ-photo-catalyzed DHA oxidation under visible light irradiation
4
. Conclusion
In summary, an atom-efficient photo-oxidation protocol has been developed to synthesize AQ from DHA and its
outstanding advantages, which can fully reflect the principle of green chemistry, are summarized as follows: 1)
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
2
Using inexpensive, readily available visible light and O /air as an illumination source and a green oxidant,
respectively; 2) Achieving a quantitative transformation without any extra catalysts and additives; 3) Mild reaction
conditions, easy to operate; 4) A high purity of the product AQ is obtained directly by evaporating the solvent;
And 5) Having a good adaptability for the oxidation of other compounds with a similar structure to DHA. With
this atom-efficient photo-oxidation protocol in hand, we are interested in further exploring the use of its in
-
situ-generated active oxidant O
are more inert than DHA.
2
and photo-catalyst AQ to achieve the photo-oxidation of other hydrocarbons that
Acknowledgements
We acknowledge the financial support for this work by the National Natural Science Fund of China (21676079,
2
2
1546010), the Natural Science Fund of Hunan Province (2018JJ3335, 14JJ2148, 11JJ6008, 10JJ2007), Hunan
011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity and
Effective Resource Utilization.
References
[
[
[
[
[
1] D. J. Lv, J. Cui, Y. F. Wang, G. H. Zhu, M. J. Zhang, X. J. Li, Rsc Adv. 2017, 7, 33494-33501.
2] D. K. Hobson, D. S. Wales, J. Soc. of Dyers and Colour. 1998, 114, 42-44.
3] Q. L. Chen, Chem. Eng. Process. 2008, 47, 787-792.
4] T. Nishimi, T. Kamachi, K. Kato, T. Kato, K. Yoshizawa, Eur. J. Org. Chem. 2011, 22, 4113-4120.
5] T. P. Sterenchuk, L. B. Belykh, N. I. Skripov, S. B. Sanzhieva, K. L. Gvozdovskaya, F. K. Schmidt, Kinet. and
Catal. 2018, 59, 585-592.
[6] H. Kim, Y. Choi, S. Hu, W. Y. Choi, J. H. Kim, Appl. Catal. B: Environ. 2018, 229, 1-31.
[7] H. B. Li, B. Zheng, Z. Y. Pan, B. N. Zong, M. H. Qiao, Front. Chem. Sci. Eng. 2018, 12, 124-131.
[8] B. E. Butterworth, O. B. Mathre, K. Ballinger, Mutagenesis. 2001, 16, 169-177.
[9] T. Hatano, H. Uebayashi, H. Ito, S. Shiota, T. Tsuchiya, T. Yoshida, Chem. Pharm. Bull. 1999, 47, 1121-1127.
[10] N. B. R. Vittar, L. Comini, I. M. Fernadez, E. Agostini, S. N. Montoy, J. L. Cabrera, V. A. Rivarol, Photodiagn.
Photodyn. Ther. 2014, 11, 182-192.
[
[
[
[
11] A. S. Tikhomirov, A. A. Shtil, A. E. Shchekotikhin, Recent Pat. Anticancer Drug Discov. 2018, 13, 159-183.
12] A. Bordoloi, F. Lefebvre, S. B. Halligudi, J. Catal. 2007, 247, 166-175.
13] C. K. Das, N. S. Das, J. Chem. Tech. Bioiechnol. 1982, 32, 643-649.
14] B. R. Madje, K. F. Shelke, S. B. Sapkal, G. K. Kakade, M. S. Shingare, Green Chem. Lett. Rev. 2010, 3,
2
69-273.
[
[
[
[
15] A. J. Johansson, M. R. A. Blomberg, P. E. M. Siegbahn, J. Phys. Chem. C 2007, 111, 12397-12406.
16] G. Xue, A. Pokutsa, L. Que, J. Am. Chem. Soc. 2011, 133, 16657-16667.
17] D. Wang, M. Zhang, P. Buhlmann, L. Q, J. Am. Chem. Soc. 2010, 132, 7638-7644.
18] J. R. Pankhurst, M. Curcio, S. Sproules, G.C.L. Jones, J.B. Love, Inorg. Chem. 2018, 57, 5915-5928.
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
[
[
[
[
[
19] A. I. Nguyen, R. G. Hadt, E. I. Solomon, T. D. Tilley, Chem. Sci. 2014, 5, 2874-2878.
20] D. Dhar, W. B. Tolman, J. Am. Chem. Soc. 2015, 137, 1322-1329.
21] D. Mallick, S. Shaik, ACS Catal. 2016, 6, 2877-2888.
22] S. Lindsay, S. L. Mader, V.R.I. Kaila, C.R. Hess, Chem. Sel. 2018, 3, 1602-1608.
23] S. Kundu, J. V. K. Thompson, L. Q. Shen, M. R. Mills, E. L. Bominaar, A. D. Ryabov, T. J. Collins, Chem.
Eur. J. 2015, 21, 1803-1810.
[
[
[
[
[
[
[
24] J. R. Bryant, J. M. Mayer, J. Am. Chem. Soc. 2003, 125, 10351-10361.
25] W. L. Man, W. W. Y. Lam, H. K. Kwong, S. M. Peng, W. T. Wong, T. Lau, Inorg. Chem. 2010, 49, 73-81.
26] K. Hashimoto, H. Tanaka, T. Ikeno, T. Yamada, Chem. Lett. 2002, 31, 582-583.
27] G. Maayan, B. Ganchegui, W. Leitner, R. Neumann, Chem. Commun. 2006, 21, 2230-2232.
28] M. Selvaraj, S. Kawi, Microporous Mesoporous Mater. 2007, 101, 240-249.
29] N. Srinivas, V. R. Rani, S. J. Kulkarni, K.V. Raghavan, J. Mol. Catal. A: Chem. 2002, 179, 221-231.
30] R. S. Araújo, D. C. S. Azevedo, E. R. Castellón, A. J. López, C. L. Cavalcante Jr., J. Mol. Catal. A: Chem.
2
008, 281, 154-163.
[
[
[
31] H. Kotani, K. Ohkubo, S. Fukuzumi, J. Am. Chem. Soc. 2004, 126, 15999-16006.
32] J. O. Hawthorne, K. A. Schowalter, A. W. Simon, M. S. Morgan, Adv. Chem. 1968, 75, 203-215.
33] S. P. Tang, J. L. She, Z. H. Fu, S. Y. Zhang, Z. Y. Tang, C. Zhang, Y. C. Liu, D. L. Yin, J. W. Li, Appl. Catal.
B-Environ. 2017, 214, 89-99.
[
34] S. Tang, W. Wu, Z. Fu, S. Zou, Y. Liu, H. Zhao, S. R. Kirk and D. Yin, ChemCatChem, 2015, 7, 2637-2645.
35] B. Yang, Z. Fu, A. Su, J. She, M. Chen, S. Tang, W. Hu, C. Zhang and Y. Liu, Appl. Catal. B-Environ.l, 2019,
[
2
42, 249-257.
[
[
[
[
36] X. C. Ji, M. Y. Tan, M. Fu, G. J. Deng, H. W. Huang, Org. Biomol. Chem. 2019, 17, 1-6.
37] Z. Z. Wang, X. C. Ji, J. W. Zhao, H. W. Huang, Green Chem., 2019, 21, 5512-5516.
38] W. L. Green, University of Delaware, 2016, 1-271
39] Y. G. Wang, X. Y. Wei, S. K. Wang, R. L. Xie, P. Li, F. J. Liu, Z. M. Zong, Fuel Process. Technol. 2015, 135,
1
57-161.
[
[
[
[
40] A. Bordoloi, F. Lefebvre, S. B. Halligudi, J. Catal. 2007, 247, 166-175.
41] P. K. Tandon, R. Baboo, A. K. Singh, Gayatri, Appl. Organometal. Chem. 2006, 20, 20-23.
42] A. Conde, M. M. D. Requejo, P. J. Pérez, Chem. Commun. 2011, 47, 8154-8156.
43] W. F. Wu, X. L. He, Z. H. Fu, Y. C. Liu, Y. L. Wang, X. L. Gong, X. L. Deng, H. T. Wu, Y. H. Zou, N. Y. Yu,
D. L. Yin, J. Catal. 2012, 286, 6-12.
[44] T. Matsumoto, K. Ohkubo, K. Honda, A. Yazawa, H. Furutachi, S. Fujinami, S. Fukuzumi, M. Suzuki, J. Am.
Chem. Soc. 2009, 131, 26, 9258–9267.
[45] J. P. Zou, D. D. Wu, J. M. Luo, Q. J. Xing, X. B. Luo, W. H. Dong, S. L. Luo, H. M. Du, S. L. Suib, Acs Catal.
2
016, 6, 6861-6867.
[
46] Z. Nie, Q. Tian, Y. P. Liu, Y. Liu, Magn. Reson. Chem. 2006, 44, 38-44.
47] C. E. D. Uribe, M. C. Daza, F. Martínez, E. A. P. Mozo, C. L. B. Guedes, E. D. Mauro, J. Photochem.
Photobiol. A Chem. 2010, 215, 172-178.
[
[48] K. C. Das, H. P. Misra, Mol. Cell. Biochem. 2004, 262, 127-133.
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201903549
[49] Z. Barbieriková, M. Bella, J. Kučerák, V. Milata, S. Jantová, D. Dvoranová, M. Veselá, A. Staško, V. Brezová,
Photochem. Photobiol. 2011, 87, 32-44.
[50] P. R. Herrington, Energy Fuels 2001, 15, 444-448.
[
51] V. D. Taran, M. A. Posthumus, S. Boeren, M. G. Boersma, C. J. Teunis, I. M. C. M. Rietjens, C. Veeger, Eur. J.
Biochem. 1998, 253, 659-668.
[
[
[
[
[
52] N. A. Romero, D. A. Nicewicz, Chem. Rev. 2016, 116, 10075-10166.
53] D. Ravelli, M. Fagnoni, A. Albini, Chem. Soc. Rev. 2013, 42, 97-113.
54] D. Petzolda, B. König, Adv. Synth. Catal. 2017, 4, 1-8
55] W. Y. Zhang, J. Gacs, I. W. C. E. Arends, F. Hollmann, Chemcatchem. 2017, 9, 3821-3826.
56] S. Lerch, L. N. Unkel, M. Brasholz, Angew. Chemie. 2015, 45, 6558-6562.
This article is protected by copyright. All rights reserved.