2-Bromoanthraquinone as a highly efficient photocatalyst for the oxidation ofsec-aromatic alcohols: experimental and DFT study

Article abstract of DOI:10.1039/d0ra06414a

Anthraquinones are recognized as high efficiency photocatalysts which can perform various redox reactions in aqueous or organic phases. We have experimentally proven that 2-BrAQ can undergo hydrogen transfer with an alpha-aromatic alcohol under light conditions, thereby efficiently oxidizing the aromatic alcohol to the corresponding product. The yield of 1-phenethanol to acetophenone can reach more than 96%. In subsequent catalyst screening experiments, it was found that the electronegativity of the substituent at the 2 position of the anthraquinone ring and the acidity of the solvent affect the photocatalytic activity of anthraquinones. After using various aromatic alcohol substrates, 2-BrAQ showed good conversion and selectivity for most aromatic alcohols, but showed C-C bond cleavage and low selectivity with non-α-position aromatic alcohols. In order to explore the mechanism of the redox reaction of 2-BrAQ in acetonitrile solution, the corresponding free radical reaction pathway was proposed and verified by density functional theory (DFT). Focusing on calculations for 2-BrAQ during the reaction and the first-step hydrogen transfer reaction between the 2-BrAQ triplet molecule and the 1-phenylethanol molecule, we recognized the changes that occurred during the reaction and thus have a deeper understanding of the redox reaction of anthraquinone compounds in organic systems.

Full text of DOI:10.1039/d0ra06414a

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2-Bromoanthraquinone as a highly efficient
photocatalyst for the oxidation of sec-aromatic
alcohols: experimental and DFT study†
Cite this: RSC Adv., 2020, 10, 37014
ab
a
a
ab
c
Shengfu Liao,
and Longlong Ma*
Jianguo Liu, Long Yan, Qiying Liu,
Guanghui Chen
ab
Anthraquinones are recognized as high efficiency photocatalysts which can perform various redox
reactions in aqueous or organic phases. We have experimentally proven that 2-BrAQ can undergo
hydrogen transfer with an alpha-aromatic alcohol under light conditions, thereby efficiently oxidizing the
aromatic alcohol to the corresponding product. The yield of 1-phenethanol to acetophenone can reach
more than 96%. In subsequent catalyst screening experiments, it was found that the electronegativity of
the substituent at the 2 position of the anthraquinone ring and the acidity of the solvent affect the
photocatalytic activity of anthraquinones. After using various aromatic alcohol substrates, 2-BrAQ
showed good conversion and selectivity for most aromatic alcohols, but showed C–C bond cleavage
and low selectivity with non-a-position aromatic alcohols. In order to explore the mechanism of the
redox reaction of 2-BrAQ in acetonitrile solution, the corresponding free radical reaction pathway was
proposed and verified by density functional theory (DFT). Focusing on calculations for 2-BrAQ during the
reaction and the first-step hydrogen transfer reaction between the 2-BrAQ triplet molecule and the 1-
phenylethanol molecule, we recognized the changes that occurred during the reaction and thus have
a deeper understanding of the redox reaction of anthraquinone compounds in organic systems.
Received 23rd July 2020
Accepted 18th August 2020
DOI: 10.1039/d0ra06414a
rsc.li/rsc-advances
formed for alcohol oxidation, but homogeneous catalysis is
recognized for its higher activity and dispersion. The most used
1
Introduction
Aromatic alcohol oxidation has been one of the most widely catalysts in homogeneous photocatalysis include ruthenium tri(-
1–3
7
used reactions in organic chemistry. 1-Phenylethanol has the bipyridine), iridium tris(phenylpyridine), eosin, and rhodamine-
typical monomer structure of a secondary aromatic alcohol in 6G. However, the expense of noble metals, undesired over-
the lignin structure and its efficient oxidation to acetophenone oxidation, use of co-catalysts (such as TEMPO) and cumbersome
4,5
is the research goal of many scholars. By oxidizing the syntheses remain large obstacles for the practical application of
secondary alcohol hydroxyl group in the lignin b-O-4 linkage to these approaches in a cost-effective fashion. Consequently,
the corresponding ketocarbonyl group, the bond energy of the choosing a highly selective and non-precious photocatalyst is
lignin b-O-4 bond can be effectively reduced and the depoly- a general trend for catalysis applications.
4
merization of lignin can be accelerated. Therefore, study of the
Anthraquinone (AQ) and its derivatives are well-recognized for
8,9
oxidation of secondary aromatic alcohols is very useful for their exceptional redox abilities in the chemical industry.
development in the chemical industry.
Photocatalytic oxidation is a mild and green oxidation method ical industries, in peroxide production, and in anti-cancer, anti-
Anthraquinone is widely used in the pharmaceutical and chem-
10–13
compared to traditional thermal catalysis and has been popular in inammatory, antinociceptive activities, among others.
In
the oxidation of aromatic compounds since it was discovered by addition, AQs are the most abundant group of dyes apart from azo
6
Masamichi Fujihira. Many photocatalysis systems have been dyes. As one of the cheapest organic dyes, AQs have been devel-
14
oped very quickly during the past decades for their wide range of
colors. A typical example of their application in redox reactions is
Biomass Catalytic Conversion Laboratory, Guangzhou Institute of Energy, Chinese 2-ethylanthraquinone (EAQ) catalyzing hydrogen peroxide (H O )
2 2
a
Academy of Sciences, Guangzhou, Guangdong 510640, China. E-mail: mall@ms. generation from hydrogen (H
) and oxygen (O ) on an industrial
2 2
giec.ac.cn; Fax: +86-20-87057673; Tel: +86-20-87057673
15
scale. In this process, the EAQ was rst reduced to 2-ethyl-
b
University of Chinese Academy of Sciences, Beijing 100049, China
anthrahydroquinone (EAQH ) by H . EAQH possesses moderate
2
2
2
c
Department of Chemistry, Shantou University, Shantou 515063, Guangdong, PR
16,17
reduction potentials
and is reduced to EAQ by O in the next
2
China
18,19
step. At the same time, H O was produced.
However, in many
2
2
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra06414a
photocatalytic oxidation studies, anthraquinone, as an organic
1
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20,21
dye, is oen degraded.
In recent years, with the application of 2.2 Photocatalytic oxidation of alcohols
anthraquinone in photocatalytic bromination, iodination, and
2
.2.1 Photocatalytic alcohol oxidation. Photocatalytic
22–24
hydrogen peroxide production,
the excellent photocatalytic
alcohol oxidation was carried out in a self-made photoreactor.
In a typical reaction, the reaction mixture (substrate 0.1 mmol,
nitrobenzene 0.1 mmol, 2-Br-AQ 0.01 mmol, acetonitrile 3 mL)
was added to a reaction tube (25 mL). Then, the reaction tube
was sealed and the gas in the tube was changed to argon
through three freeze–pump–thaw cycles. Aer ve minutes of
performance of anthraquinone has begun to receive close
attention.
In this work, we used 2-bromoanthraquinone as a photo-
catalyst to promote the transfer of protons from alcohols to the
2
hydrogen acceptor (nitrobenzene). As is well known, oxygen (O )
is the most used oxidant due to its strong oxidizing properties
and easy availability. However, due to its free radical reaction
characteristics, the oxidation process is uncontrollable and
ꢀ
sonication, the reaction mixture was placed in a 30 C water
bath and illuminated by a 30 W purple LED (about 400 nm).
Aer the reaction, the solution was diluted 3 times and the
internal standard (ethyl levulinate) for gas chromatography
analysis was added.
25,26
over-oxidation occurs.
a hydrogen acceptor. This process is divided into three steps:
rst, anthraquinone molecules were excited by light to generate
Therefore, we chose nitrobenzene as

2
.2.2 Gas chromatography (GC) analysis. For quantitative
analysis, the samples were analyzed using a GC-2014C (Shi-
madzu, Japan) equipped with capillary column (HP-
excited singlet states. The singlet molecules were unstable and
rapidly formed triplet molecules through intersystem crossing
a
(ISC) in a solvent. Second, the triplet anthraquinone molecules
INNOWAX, 30 m  0.25 mm  0.25 mm) and ame ionization
were reduced by alcohol to generate hydroanthraquinone
molecules and the hydroxyl group was simultaneously con-
verted to the corresponding aldehyde or ketone group. Thirdly,
hydroanthraquinone molecules were oxidized by nitrobenzene,
generating anthraquinone molecules and aniline.
ꢀ
detector. The injector and detector temperatures were 240 C
ꢀ
and 280 C, respectively. The temperature program was set as
follows: hold at 80 C for 2 min, rise from 80 C to 270 C at
a rate of 10 C min , and hold at 270 C for 2 min.
ꢀ
ꢀ
ꢀ
ꢀ
À1
ꢀ
1
2.2.3 H NMR test. The NMR tests were conducted on
In recent years, in order to explore the photoredox mechanism
of anthraquinone catalysts, many theoretical and experimental
studies have been published. Density functional theory is one of
the most popular theoretical research methods and is favored by
many scholars. The B3LYP hybrid functional research method has
been the most widely used in quantum chemistry since it was
proposed in 1994, due to its universal applicability and high cost
performance. In 2012, David Lee Phillips and colleagues dis-
cussed the redox reaction of 2-hydroxyethylanthraquinone in
a JNM-ECA600 NMR spectroscope. p-Dimethoxybenzene was
used as the internal standard to quantitatively study the reac-
tion products. In a typical process, 0.1 mmol p-dimethox-
ybenzene was added to the reaction mixture aer the reaction
was completed. Then, 1.2 mL of the reaction mixture was placed
into a 2 mL centrifuge tube and the solvent was blown dry with
nitrogen. Finally, 0.6 mL of deuterated DMSO was added and
the mixture was injected into the NMR tube for analysis.
27
aqueous solutions using the B3LYP method. They also used
time-resolved spectroscopy and density functional theory to
2.3 Density functional theory (DFT) calculation
unravel the photodeprotection mechanism of anthraquinon-2- Density function theory (DFT) calculations were performed in
28
ylmethoxycarbonyl-caged alcohols. In both studies, anthraqui- the Gaussian 09W D01 program package. In all calculations,
none molecules formed a hydroanthraquinone structure, proving acetonitrile was used as the default solvent model. In the
that this is a key intermediate in the anthraquinone molecule's geometric optimization calculations, Becke's three-parameter
redox reaction. Therefore, this study will also explore the key
intermediates of anthraquinone molecules in organic solvents to
reveal the anthraquinone redox cycle under free radical reaction
conditions.
2
Experimental
2.1 Materials
30 W purple LED lights (wavelength range around 400 nm) were
purchased from Shenzhen Leixi Lighting Co. Ltd. 2-Bromoan-
thraquinone, 2-chloroanthraquinone, 2-methylanthraquinone,
anthraquinone, and anthraquinone-2-carboxylic acid were ob-
tained from Sarn Chemical Technology Co., Ltd. 1-Phenyl-
ethanol was purchased from Adamas Reagent Co., Ltd.
Acetonitrile was purchased from Shanghai Macklin Biochem-
ical Technology Co., Ltd. Nitrobenzene, benzyl alcohol and
other alcohols were obtained from Shanghai Aladdin Bio-Chem
Technology Co., Ltd. All chemicals were used without further
purication.
Fig. 1 Molar extinction coefficient of 2-BrAQ photocatalyst. Condi-
tions: 0.5 g L 2-bromoanthraquinone in acetonitrile solution, optical
path length 1 cm.
À1
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hybrid method with Lee–Yang–Parr correlation function from Fig. 1 that the absorption spectrum of anthraquinone
approximation with D3 dispersion correction (B3LYP-D3BJ) compounds is concentrated in the wavelength band before
with a 6-311G* basis set was employed. Calculation of the 420 nm. This shows that, in order to achieve good photo-
excited state of 2-BrAQ was carried out under the time- catalytic effect, the light source must also have a band before
dependent (TD) UB3LYP-D3BJ and 6-311+G* basis set. TD-DFT 420 nm. Therefore, we chose a 30 W purple LED lamp with
methodology was also used to compute the low-lying excited a wavelength of about 400 nm.
states of transient species of interest. The relative energy in the
2
-BrAQ photoredox cycle was calculated using the M06-2X
3.2 Photocatalytic oxidation of aromatic alcohols
density functional with the def2-TZVP basis set. The calcula-
In order to test the reactivity of different anthraquinone cata-
lysts, we selected several anthraquinones with different
substituents. It can be seen from Table 1 that when the
hydrogen atom at position 2 in anthraquinone is replaced by
a halogen atom, the reactivity is increased and that the bromine
substitution is stronger than the chlorine substitution. This can
tion results were viewed with GaussView 5.0 soware.
3
Result and discussion
3.1 Molar extinction coefficient of anthraquinones
The molar extinction coefficient of anthraquinones was be explained by the anthraquinone photocatalytic reaction
measured using a UV-visible spectrophotometer. It can be seen pathway. In the photophysical process of anthraquinone
a
Table 1 Oxidation activities of various photocatalysts
b
Conversion (%)
Yield (%)
Entry
Catalyst
Reaction time (h)
1
2
3
4
1
2
3
4
5
6
7
8
9
2-Cl-AQ
2-Br-AQ
2-Me-AQ
5
5
5
5
5
5
5
5
5
70
76
49
5
57
64
0
31
29
25
0
14
30
0
70
76
48
5
56
62
0
20
20
9
0
9
9
0
0
0
2-NH
2
-AQ
2-COOH-AQ
AQ
1,4-2NH
Eosin Y
2
-AQ
10
0
0
0
10
0
Rhodamine 6G
a
ꢀ
General conditions: 0.1 mmol 1-phenylethanol, 0.1 mmol nitrobenzene, 0.01 mmol catalyst, 3 mL acetonitrile, 30 W LED illuminated for 5 h, 30 C
b
water bath. Yield was determined by gas chromatography.
a
Table 2 Effects of solvents on oxidation of 1-phenylethanol
b
Conversion (%)
Yield (%)
Entry
Solvent
Reaction time (h)
1
2
3
4
1
2
3
4
5
6
7
8
9
Acetonitrile
Acetone
Ethyl acetate
DCM
Ethanol
Toluene
THF
DMF
Acetonitrile
Acetone
5
5
5
5
5
5
5
5
76
75
63
64
8
0
4
1
90
88
29
37
37
28
100
87
100
91
76
75
63
61
8
0
4
1
90
88
20
14
15
7
0
27
12
26
15
11
20
20
38
45
1
0
a
General conditions: 0.1 mmol 1-phenylethanol, 0.1 mmol nitrobenzene, 0.01 mmol 2-Br-AQ, 3 mL solvent, 30 W LED illuminated for specied
ꢀ
b
time, 30 C water bath. Yield was determined by gas chromatography.
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molecules, an excited anthraquinone molecule excited by light
of a specic wavelength undergoes an intersystem crossing
process and becomes a triplet molecule. The triplet anthraqui-
none molecule is considered to have a strong intramolecular
electron transfer ability with electron enrichment on the
carbonyl oxygen atom and is an active molecule in the redox
2
9,30
reaction.
substituted by a halogen atom, the quantum-yield ratio of
When a hydrogen atom in the molecule is
31,32
phosphorescence to uorescence increases.
This indicates
an increase of the intersystem crossing coefficient, leading to
a relatively large number of triplet molecules and resulting in
enhanced photocatalytic reactivity. The larger the molecular
weight of the substituted halogen atom, the stronger this effect.
It may explain why 2-bromoanthraquinone is the most reactive
photocatalyst among these catalysts. When anthraquinone has
amino substitution, there is no reactivity. This is because the
excited 2-aminoanthraquinone molecule has a large deactiva- Fig. 2 Time course plots of each main reactant and product. Black, 1-
3
0
phenylethanol and acetophenone; red, nitrobenzene; blue, aniline.
General conditions: 0.1 mmol 1-phenylethanol, 0.1 mmol nitroben-
zene, 0.01 mmol 2-Br-AQ, 3 mL acetonitrile, 30 W LED illuminated for
tion constant in acetonitrile, leading to the decrease in the
reactivity of the molecules. In theory, one molecule of nitro-
benzene completely reduced to aniline can accept six hydrogen
atoms and one molecule of 1-phenylethanol converted to ace-
tophenone can provide two hydrogen atoms. Therefore, the
conversion ratio of 1-phenylethanol to nitrobenzene should be
ꢀ
specified time, 30 C water bath.
a
Table 3 1-Phenylethanol oxidation controlled variable experiment
3
/1. However, according to entry 2 of Table 1, the conversion
ratio of 1-phenylethanol to nitrobenzene in the experiment was
.6/1, which indicates that nitrobenzene was consumed in
Catalyst amount Nitrobenzene
Entry (equiv.) amount (equiv.) alcohol (%)
Conversion of Yield of
ketone (%)
2
excess in this process and not completely reduced to aniline.
This is why the yield of aniline is lower than the conversion rate
of nitrobenzene. In fact, there are many paths in the reduction
process of nitrobenzene in which some incomplete reduction
1
2
3
4
5
6
7
0.1
0.1
0
0.1
0.05
0.1
0.1
1
1
1
0
1
0.5
2
76
0
76
0
5
28
63
65
79
b
5
41
65
66
79
33
products (nitrosobenzene, N-phenylhydroxylamine) or poly-
34
merization products will be produced.
Next, we investigated the effects of solvents on the oxidation
of 1-phenylethanol. As shown in Table 2, acetonitrile is
considered the best solvent for this reaction. According to
entries 1, 2, 9 and 10, there is no large difference between
acetone and acetonitrile. We chose acetonitrile as the standard
solvent for subsequent experiments. The conversions of 1-phe-
nylethanol using other solvents such as EtOH, toluene, THF and
a
General conditions: 0.1 mmol 1-phenylethanol, nitrobenzene, and 2-
ꢀ
Br-AQ, 3 mL of solvent, 30 W LED illuminated for 5 h, 30 C water
bath. Reaction proceeded in the dark.
b
while there is no reactivity when it is carried out without light.
DMF (entries 5–8) are very low, but the corresponding conver- When there is no catalyst, the reaction rarely gave conversion.
sion of nitrobenzene was very high. This is because these When nitrobenzene was missing, the reaction conversion and
solvents could reduce nitrobenzene, thereby preventing the
oxidation of 1-phenylethanol. The GC spectra aer the reaction
are shown in ESI Fig. S9–S12.† When 1-phenylethanol is
oxidized to acetophenone, nitrobenzene is also partially
reduced to aniline. This is probably because the protons on the
alcohol are transferred to the nitrobenzene, accompanied by the
reduction of nitrobenzene.
selectivity were reduced. When the amount of catalyst or
nitrobenzene was reduced, the reaction conversion rate
decreased about 10%. When the molar ratio of nitrobenzene to
1-phenylethanol was 2 : 1, the 1-phenylethanol conversion and
acetophenone yield only increased by 3%. Therefore, using
a catalyst equivalent of 0.1 and a nitrobenzene equivalent of 1
achieves a reasonable yield.
Then, we explored the effect of reaction time on the reaction.
It can be seen from Fig. 2 that when the reaction proceeds to
Under standard reaction conditions, we tested the conversion
and selectivity of various alcohol substrates. For pyridine aromatic
alcohols, the oxidation results are not satisfactory, which may be
caused by the electron-deciency of the pyridine ring. For entries
20 h, the acetophenone yield reaches more than 90%. The
reaction proceeded very rapidly in the rst 8 hours, but aer 8
hours, became slow and the various reactants and products
tended to plateau.
4, 8, 10, 11, 15, 22, 23 and 24 in Table 4, the reactions all showed
rather high conversion and selectivity, which may be due to the
relatively stable ketone structure formed. For entries 5, 7, 9, 13, 16,
To prove that this reaction is light-driven, we performed
a series of control experiments. From entries 1 and 2 in Table 3, 17, 19 and 20 in Table 4, the reaction showed rather high
we can see that this reaction gave 76% conversion using light
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a
Table 4 Photocatalytic oxidation of different aromatic alcohols
Table 4 (Contd.)
b
b
Entry Substrate
Conversion (%) Yield (%)
Entry Substrate
Conversion (%) Yield (%)
1
71
1
1
1
5
6
7
96
2
0
0
100
99
c
3
4
5
6
7
0
100
44
6
c
c
c
1
1
2
8
9
0
66
96
100
0
c
2
2
1
2
100
87
48
c
8
9
86
65
c
2
2
3
4
90
c
100
1
1
0
1
92
100
c
25
67
1
2
12
a
General conditions: 0.1 mmol substrate, 0.1 mmol nitrobenzene,
.01 mmol 2-Br-AQ, 3 mL solvent, 30 W LED illuminated for 20 h,
0
3
1
1
3
4
71
0
ꢀ
b
1
0 C water bath. Yield was determined by H NMR using 1,4-
c
dimethoxybenzene as internal standard. Yield was determined by
gas chromatography.
0
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conversion but low selectivity. This may be due to the unstable
products produced or unnecessary side reactions in the system. It
can be seen from Table S1 and Fig. S1† that when a product
containing an aldehyde group is formed, dehydration condensa-
tion occurs with the generated aniline, which has been conrmed
35
by Itoh et al. If the alcoholic hydroxyl group and benzene ring in
the substrate are not on the same carbon atom, such as in entries
18 and 25 in Table 4, C–C bond cleavage occurs aer oxidation
and the selectivity is low. This indicates that 2-bromoan-
thraquinone has a strong regioselectivity for the oxidation of a-
position aromatic alcohols.
Scheme 1 Possible reaction mechanism of 1-phenylethanol photo-
catalytic oxidation.
3
.3 Possible reaction mechanism of 1-phenylethanol

rst, the 2-BrAQ molecule is excited by light to form an excited
photocatalytic oxidation on 2-BrAQ
singlet state. The excited singlet state is unstable and has a low
lifetime. It quickly becomes ground or triplet molecules
through internal conversion, uorescence emission, and inter-
system crossing. The triplet molecule is then excited to collide
with the 1-phenylethanol molecule to generate a 2-BrAQH
radical and a 1-phenylethanol radical. 1-Phenylethanol radicals
continue to react with the 2-BrAQ molecule, generating 2-
BrAQH molecules and acetophenone. Finally, the 2-BrAQH
molecule is oxidized by the nitrobenzene to 2-BrAQ and the
nitrobenzene is simultaneously reduced to aniline.
In order to experimentally verify the radical process of the
reaction, a series of radical capture experiments was carried out.
We used butylated hydroxytoluene (BHT) as a free radical
scavenger to scavenge free radicals generated during the reac-
tion. The results are shown in Fig. 4. When 1 equivalent of BHT
was used, the reaction conversion and yield were reduced to
In order to understand the reaction mechanism in depth, we

rst calculated the surface charges of the catalyst and the 1-
phenylethanol molecule using Gaussian 09W. The results are
shown in Fig. 3. As can be seen from Fig. 3, the negative charge
of the anthraquinone molecule is concentrated at two oxygen
atoms and the negative charge of the upper oxygen atom is
slightly higher than the other oxygen atom. The electron density
of the oxygen atom on the excited 2-BrAQ molecule is slightly
lower than that on the ground state molecule. This may be due
to the relative dispersion of the charge caused by the electron
transition from the HOMO energy level to the LUMO energy
level. The positive charge on the 1-phenylethanol molecule is
concentrated on the hydrogen atom on the hydroxyl group. This
shows that the rst step of the reaction is likely to be the
transfer of a hydrogen atom from the 1-phenylethanol hydroxyl
group to an oxygen atom from 2-BrAQ. As shown in Scheme 1,
Fig. 3 Catalyst and reactant molecular charge distributions using B3LYP/6-311+G*. (a) 2-BrAQ ground state, (b) 2-BrAQ excited singlet state, (c)
-BrAQ excited triplet state and (d) 1-phenylethanol.
2
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Fig. 4 Radical capture experiments of 2-BrAQ photocatalytic oxida-
tion. General conditions: 0.1 mmol 1-phenylethanol, 0.1 mmol nitro-
benzene, 0.01 mmol 2-Br-AQ, butylated hydroxytoluene (BHT), 3 mL
ꢀ
acetonitrile, 30 W LED illuminated for specified time, 30 C water bath.
1
2%. This indicates that the use of BHT inhibits the progress of Scheme 2 Diagram of 2-BrAQ orbital transition. Calculated by TD-
B3LYP/6-311G*-D3(BJ).
the reaction by scavenging free radicals in the reaction. When
the amount of BHT was increased to 5 equivalents, the 1-phe-
nylethanol in this system did not undergo any oxidation. In triplet molecules through photoexcitation and intersystem
addition, experiments using N-tert-butyl-a-phenylnitrone (PBN) migration. Then, the triplet 2-HEAQ molecules undergo
as a free radical scavenger (Fig. S1†) show that when free radi- protonation and isomerization in an acidic aqueous solution
cals in the reaction are trapped, 1-phenylethanol oxidation and become hydrogenation intermediates. Finally, the hydro-
proceeds with difficulty. Through these experimental results, genation intermediate is oxidized to form the original 2-HEAQ
the free radical path of the reaction was veried.
molecule, completing a redox cycle. The anthraquinone mole-
cules follow the ionic reaction mechanism in acidic aqueous
solution. The high hydrogen ion concentration in acidic
4
DFT and TD-DFT calculation
36
aqueous solution has a quenching effect on the triplet state.
4
.1 Calculation of 2-BrAQ excited state
However, in neutral acetonitrile, the levels of various ions are
very low, so anthraquinone molecules should follow the
mechanism of the free radical reaction.
In order to explore the spectral characteristics and energy data
of 2-BrAQ molecules from a theoretical perspective, this study
uses TD-DFT calculations to calculate the excited state of 2-
BrAQ molecules. Based on the calculation results, we have
drawn a graph of the energy of 2-BrAQ in the reaction; see
Scheme 2 for details. The 2-BrAQ molecule needs 3.026 eV to
transition from the ground state (S
state (S ), corresponding to an excitation wavelength of 410 nm.
This transition has a 96.3% chance of the electron moving from
the electron-occupied orbit 70 to LUMO orbit 72. The corre-
sponding electron transition energies of S , S , S and S are
Aer undergoing processes such as photoexcitation and
intersystem crossing, the triplet 2-BrAQ molecule is a good
hydrogen atom acceptor. Note that photohydration takes
2
7
place at both the ortho and meta positions. According to
Gaussian 09W structure optimization and single-point energy
0
) to the rst excited singlet
calculations, the ortho-hydrogenated 2-BrAQ molecule
1
À1
(
generate HT1) showed an energy about 1.4 kcal mol lower
than that of meta-hydrogenated 2-BrAQ molecule (generate
3
6,37
HT2). This would yield a transient with a longer lifetime.
2
3
4
5
Therefore, ortho-hydrogenated 2-BrAQ (HT1) was the most
likely hydrogenation intermediate in this reaction. Based on
this, we optimized the most likely structure of 2-BrAQ in the
reaction. Aer the 2-BrAQ molecule was excited by light, as
shown in Scheme 4, the molecular unsaturation and structure
were changed due to the electrons in the occupied molecular
orbital transitioning to the LUMO orbital. During this process,
the C]O bond length and molecular dipole moment were
3
.309, 3.318, 3.672 and 3.750 eV, respectively. The energy cor-
responding to the T state is 2.620 eV. More detailed calculation
data are listed in the ESI (Table S4†).
1
4
.2 DFT calculation for reaction process analysis
In order to better understand the photo-redox reaction of 2-
BrAQ in acetonitrile solution, DFT calculations were used to
check the intermediates and activation energy that the reaction
may produce. The reaction history of anthraquinone photo-
catalyst in acid aqueous solution is well understood. As can be
seen in Scheme 3, rst, 2-HEAQ molecules become excited
1
increased. In addition, the triplet T molecule has a relatively
long life, which allows it to easily undergo a hydrogen transfer
reaction with 1-phenylethanol molecules to generate hydro
anthraquinone transition state (HT) molecules. The electrons
27
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Scheme 3 Proposed photohydration reaction mechanism for 2-HEAQ in acidic aqueous solution, from David Lee Phillips and co-workers.
in the HT molecules were rearranged, resulting in lower
energy hydro anthraquinone ground state (HS) molecules.
Finally, the HS molecule was oxidized by the oxidant, lost the
hydrogen atom and became the ground state (S
ending the redox cycle.
0
) molecule,
Fig. 5 shows the relative energy curve of the 2-bromoan-
thraquinone redox cycle. Inspection of Fig. 5 indicates that the
initial excitation of 2-bromoanthraquinone requires
À1
6
3.6 kcal mol of energy. When the triplet state T is reached,
1
À1
the relative energy is reduced by about 9 kcal mol . When the
ortho-hydrogenated 2-BrAQ molecule transition state (HT1) is
formed through the hydrogen transfer process, the relative
energy is reduced to À317.3 kcal mol , which indicates that
À1
Fig. 5 Reactive energy profile obtained from (U)M06-2X/def2-TZVP
calculations after optimization for the photo redox reaction of 2-BrAQ.
À1
2
-bromoanthraquinone transfers up to 371.6 kcal mol of
energy through the collision reaction with 1-phenylethanol to
À1
activate 1-phenylethanol. About 48.7 kcal mol of energy is
dissipated in the isomerization of hydro anthraquinone
molecules.
5
Conclusion
In summary, we have found a new strategy to efficiently
oxidize a-position aromatic alcohols. In the presence of
nitrobenzene, under specic light conditions, the use of
hydrogen transfer between 2-bromoanthraquinone and an a-
position aromatic alcohol can oxidize the a-position aromatic
Scheme 4 Redox cycle of 2-BrAQ molecule in acetonitrile solution.
Optimization calculated by (TD)B3LYP/6-311G*-D3(BJ).
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alcohol to the corresponding ketone with high selectivity. By
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1
1
1
Conflicts of interest
There are no conicts to declare.
9 E. Santacesaria, M. Di Serio, A. Russo, U. Leone and
R. Velotti, Chem. Eng. Sci., 1999, 54, 2799–2806.
Acknowledgements
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G. Alhakimi and I. Rajab, Res. Chem. Intermed., 1997, 23,
This work is nancially supported by the National Key R&D
Program of China (2018YFB1501402), the Natural Science Foun-
dation of Guangdong Province (2017A030308010), the National
Natural Science Foundation of China (51576199, 51976225 and
247–274.
2
2
2
2
2
2
2
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630.
DNL201916), the “Transformational Technologies for Clean
Energy and Demonstration”, Strategic Priority Research Program
of the Chinese Academy of Sciences (No. XDA21060102), and the
Local Innovative and Research Teams Project of Guangdong Pearl
River Talents Program (2017BT01N092). In addition, special
thanks to Department of Chemistry of Shantou University for
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