Cationic Polycarbazole Networks as Visible-Light Heterogeneous Photocatalysts for Oxidative Organic Transformations

Article abstract of DOI:10.1021/acscatal.7b04494

Photoredox catalysis has aroused great interest from chemists, as it offers a powerful tool for organic synthesis. Cationic polycarbazole networks (CPOP-28 and CPOP-29) were prepared via simple oxidative coupling reaction and applied as heterogeneous photocatalysts for a wide range of oxidative organic transformations, including oxidation of sulfides, hydroxylation of arylboronic acids, and cross-dehydrogenative coupling reactions, in the presence of visible light and air. Remarkably, photocatalytic activities are enhanced by ingenious introduction of trifluoromethyl groups to the polymeric network CPOP-29. The effects of the trifluoromethyl group on photocatalytic activities were elucidated in terms of photophysical and electrochemical properties. The appealing photocatalytic performance of the trifluoromethylated polymer is ascribed to superior light-absorption ability, longer fluorescence lifetime, and stronger oxidative capability. In addition, the photocatalysts showed good recyclability and could be reused after a simple separation workup.

Full text of DOI:10.1021/acscatal.7b04494

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Article
Cationic Polycarbazole Networks as Visible-Light Heterogeneous
Photocatalysts for Oxidative Organic Transformations
Haipeng Liang, Qi Chen, and Bao-Hang Han
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04494 • Publication Date (Web): 30 Apr 2018
Downloaded from http://pubs.acs.org on May 1, 2018
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Cationic Polycarbazole Networks as Visible-Light
Heterogeneous Photocatalysts for Oxidative
Organic Transformations
a,b
,a
,a,b
HaiꢀPeng Liang, Qi Chen,* BaoꢀHang Han*
a
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS
Center for Excellence in Nanoscience, National Center for Nanoscience and
Technology, Beijing 100190, China
b
Sino-Danish Center for Education and Research, University of Chinese
Academy of Sciences, Beijing 100190, China
Tel: +86 10 8254 5576; Email: hanbh@nanoctr.cn
Tel: +86 10 8254 5708; Email: chenq@nanoctr.cn
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ABSTRACT: Photoredox catalysis has aroused great interest of chemists as it offers a
powerful tool to organic synthesis. Cationic polycarbazole networks (CPOP-28 and
CPOP-29) were prepared via simple oxidative coupling reactions and applied as
heterogeneous photocatalysts for a wide range of oxidative organic transformations
including oxidation of sulfides, hydroxylation of arylboronic acids, and
crossꢀdehydrogenative coupling reactions in the presence of visible light and air.
Remarkably, photocatalytic activities are enhanced by ingenious introduction of
trifluoromethyl group to the polymeric network. The effects of trifluoromethyl group on
photocatalytic activities were elucidated in terms of photophysical and electrochemical
properties. The appealing photocatalytic performance of trifluoromethylated polymer is
ascribed to superior lightꢀabsorption capability, longer fluorescence lifetime, and more
oxidative photoexcited state. In addition, the photocatalysts showed good recyclability
and could be reused after simple separation workup.
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KEYWORDS: cationic, polycarbazole, trifluoromethyl group, heterogeneous catalyst,
photocatalyst
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ꢀ INTRODUCTION
In face of everꢀincreasing global demands on fossil fuels, solar energy as an
abundant and renewable resource from nature has been exploited for supply of chemical
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energies and to actuate artificial photosynthesis. The use of light to promote organic
transformations has attracted great attention over the past decades. As yet, a diversity of
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photocatalysts including inorganic semiconductors, organic dyes, and transitionꢀmetal
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complexes have been developed. Semiconductorꢀtype photocatalysts such as TiO and
2
ZnO typically require UVꢀirradiation owing to their large band gaps. Elementꢀdoping,
dye sensitization, or construction of heterojunction is frequently adopted to raise the
visibleꢀlight absorption ability and photocatalytic activity, however such intricate
7
modifications limit their practical use. With respect to organic dyes, they have been
employed as photoredox catalysts for organic transformations since seminal contributions
8
by Neumann and coworkers, whereas the issue of photobleaching is known to have
detrimental impacts on photocatalytic system.
Recently, transitionꢀmetalꢀbased photocatalysts such as ruthenium and iridium
complexes have been well investigated and developed on grounds of their wide
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absorption range in visible light region and superior excitedꢀstate lifetimes. In
comparison with ruthenium complexes, a library of ligands with disparate electronic
properties are available for iridium to tailor the chemical structures and redox properties,
which renders iridiumꢀbased photocatalysts designability and tunability. Iridium
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complexes have been exploited as catalysts for water photolysis
and CO2
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photoreduction. Outstanding efficiencies of iridiumꢀbased photocatalysts were also
demonstrated in a wide range of useful chemical transformations, for example,
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cycloaddition reactions,
trifluoromethylation of arenes, and αꢀC–H arylation of
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amine. The photoredox nature of iridium complexes renders them dual power for
oxidation or reduction of substrates. Given the versatility and good efficiency for
photocatalysis, iridiumꢀbased photocatalysts are emerging as promising candidates to
achieve the goals of green chemistry and atom economy.
Albeit remarkable achievements in developing novel iridiumꢀbased photocatalysts
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6,17,18
have been made, effective approach for heterogenization is scarcely reported.
From
a sustainable and environmentalꢀfriendly standpoint, immobilization of precious metal
catalysts on heterogeneous supports for the purpose of facile separation and easy
recycling drastically lowers the cost. Historically, mesoporous materials and polymeric
resins were frequently employed as solid supports for immobilization of organometallic
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catalyst via covalent linkage or physical adsorption.
The postꢀmodification strategy
was also found applicable to the stateꢀofꢀtheꢀart porous materials, as exemplified by
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porous organic polymers
and metal–organic frameworks.
Alternatively,
copolymerization strategy by which catalytic functionalities are introduced as integral
parts of polymer backbone has been considered as a potent tool against leaching of active
1
7, 24
species.
Polycarbazole networks, prepared by simple and costꢀeffective
FeCl ꢀpromoted coupling reactions, have been explored as heterogeneous photocatalysts
3
for various organic transformations, in which cases they functioned as organic
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semiconductors.
With enhanced visibleꢀlight absorption and reduced likelihood of
exciton recombination, these polymers proved highly active in catalyzing both oxidative
and reductive reactions.
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Very recently, our group devised a novel photocatalytic system based on
polycarbazole containing facꢀIr(ppy) (ppy = phenylpyridine) moieties, which showed
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outstanding catalytic performance toward azaꢀHenry reaction.
With the aim of
developing versatile catalysts coping with multiple tasks, herein we present cationic
polycarbazole networks (CPOP-28 and CPOP-29, where CPOPs denote a series of
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8,29,30,31,32,33,34
carbazoleꢀbased porous organic polymers developed by our group
) based
on heteroleptic iridium complexes containing carbazolyl moieties, and apply them as
heterogeneous photocatalysts for a wide range of aerobic organic transformations, i.e.
selective oxidation of sulfides, hydroxylation of arylboronic acids, and
crossꢀdehydrogenative reactions. The polymeric networks function well in benign
conditions and can be recycled after simple separation workup. The influence of
trifluoromethyl substituent on electronic structure, photophysical property, and catalytic
performance was investigated. Enhanced photocatalytic efficiencies for the
trifluoromethylated polymer were demonstrated in oxidative organic transformations.
Furthermore, the mechanisms for photocatalysis were studied.
ꢀ
EXPERIMENTAL SECTION
Materials and Characterization.
Iridium(III)
chloride
hydrate
and
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tetrakis(triphenylphosphine) palladium(0) were purchased from Energy Chemical Ltd.
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ꢀBromoꢀ5ꢀ(trifluoromethyl)pyridine,
1,4ꢀdibromobenzene,
tetrahydroisoquinoline,
sulfides, arylboronic acids, phosphite ester, nitromethane, and nitroethane were purchased
from J&K Scientific Ltd. All other chemicals and reagents were purchased from Aladdin
reagent Co., Ltd. (4ꢀ(9HꢀCarbazolꢀ9ꢀyl)phenyl)boronic acid and cyclometalating ligand
9ꢀ(4ꢀpyridinꢀ2ꢀylꢀphenyl)ꢀ9Hꢀcarbazole (PPC) were synthesized following the literature
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procedures. Unless otherwise stated, all chemicals and reagents were used as received
without further purification.
The characterization methods are available in the Supporting Information.
Synthesis
of
Cyclometalating
Ligand
9-(4-(5-(Trifluoromethyl)pyridine-2-yl)-phenyl)-9H-carbazole
(TfPPC).
An
ovenꢀdried Schlenk flask was charged with 4ꢀ(9ꢀcarbazolyl)phenylboronic acid (3.0 g,
0.5 mmol), 2ꢀbromoꢀ5ꢀ(trifluoromethyl)pyridine (5.92 g, 26.2 mmol), and DMF (50
1
mL). After two freeze–pump–thaw degassing cycles, aqueous solution of potassium
carbonate (2.0 M, 20 mL) and tetrakis(triphenylphosphine) palladium(0) (190 mg, 164
ꢁmol) were added. Then another two degassing cycles were applied and the reaction
mixture was heated at 120 °C for 2 d under magnetic stirring. After cooling to room
temperature, the resulting mixture was poured into 200 mL water. The organic layer was
collected by separatory funnel, and the aqueous layer was extracted by ethyl acetate (30
mL × 3). The combined organic phase was dried over Na SO , and subsequently
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separated from the insoluble inorganic impurities with a Büchner funnel. The solution
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was removed under reduced pressure and the crude product was rinsed with a large
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quantity of hexane to offer pure product as white solid (2.77 g, 78%). H NMR (400 MHz,
CDCl ): δ 9.00 (s, 1H), 8.26 (d, J = 8.4 Hz, 2H), 8.15 (d, J = 7.7 Hz, 2H), 8.02 (dd, J =
3
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8.3, 2.0, 1H), 7.90 (d, J = 8.3 Hz, 1H), 7.72 (d, J = 8.4 Hz, 4H), 7.52−7.39 (m, 4H), 7.31
13
(
t, J = 4 Hz, 3H). C NMR (100 MHz, CDCl ) δ 159.7, 146.8, 140.6, 139.5, 136.7, 134.2,
3
128.8, 127.3, 126.1, 124.9, 123.7, 120.4, 120.3, 119.9, 109.8. MS (MALDIꢀTOF) m/z:
calculated for C H F N : 388.4 [M]; found: 388.2 [M].
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Synthesis of Chloro-bridged Iridium Dimers [Ir(PPC) Cl] and [Ir(TfPPC) Cl] . A
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mixture of PPC (1.0 g, 3.12 mmol), iridium(III) chloride hydrate (396 mg, 1.25 mmol),
ꢀethoxyethanol (21 mL), and water (7 mL) was heated at 120 °C for 1 d under nitrogen.
2
After cooling to room temperature, the reaction mixture was poured into 100 mL water to
precipitate the crude product that was then collected by filtration and washed with water
(100 mL), methanol (100 mL), and diethyl ether (100 mL), and dried in vacuo.
[Ir(TfPPC) Cl] was synthesized according to a similar synthetic procedure for
2
2
[
Ir(PPC) Cl] except for replacing PPC with TfPPC. [Ir(PPC) Cl] (1.01 g, 75%) and
2 2 2 2
[Ir(TfPPC) Cl] (1.09 g, 70%) were recrystallized in hexane to afford pure products. For
2
2
1
[
Ir(PPC) Cl] : H NMR (400 MHz, DMSOꢀd ) δ 9.78 (d, J = 5.9 Hz, 2H), 9.57 (d, J = 5.8
2
2
6
Hz, 2H), 8.46 (d, J = 8.2 Hz, 2H), 8.38 (d, J = 8.2 Hz, 2H), 8.26 (d, J = 8.3 Hz, 2H), 8.17
1
3
(d, J = 7.9 Hz, 10H), 7.95 (ddd, J = 9.9, 5.4, 2.5 Hz, 4H), 7.57−7.04 (m, 36H). C NMR
(
100 MHz, DMSOꢀd ) δ 166.9, 166.5, 154.4, 152.7, 151.1, 147.6, 142.9, 142.4, 139.8,
6
139.5, 138.9, 138.2, 137.4, 128.2, 126.9, 126.5, 125.7, 124.2, 123.6, 123.3, 121.1, 120.9,
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120.8, 120.5, 120.0, 119.6, 110.4. For [Ir(TfPPC) Cl] : H NMR (400 MHz, DMSOꢀd ) δ
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0.52 (s, 2H), 9.93 (s, 2H), 8.60 (dd, J = 22.4, 8.7 Hz, 4H), 8.38 (dd, J = 16.5, 8.5 Hz,
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H), 8.30 (d, J = 8.4 Hz, 2H), 8.19 (d, J = 7.8 Hz, 8H), 7.50−7.17 (m, 32H). C NMR
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(100 MHz, DMSOꢀd ) δ 170.7, 170.5, 155.0, 149.4, 148.7, 147.5, 141.1, 140.6, 139.8,
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39.3, 138.9, 137.4, 136.4, 128.8, 128.0, 127.8, 126.6, 125.2, 124.8, 124.7, 124.4, 124.1,
123.6, 123.5, 121.9, 121.7, 121.0, 120.9, 120.6, 120.1, 110.4, 110.2.
Synthesis of Heteroleptic Iridium Complexes [Ir(PPC) (bpy)]Cl and
2
[
Ir(TfPPC) (bpy)]Cl. A mixture of [Ir(PPC) Cl] (1.0 g, 0.58 mmol) and 2,2′ꢀbipyridine
2 2 2
(225 mg, 1.44 mmol) in DCM/MeOH (30 mL, V:V = 1:1) was refluxed at 60 °C for 24 h.
After cooling to room temperature, the solution was removed under reduced pressure.
The resulting crude product was redissolved in CH CN (20 mL) and the leftover iridium
3
dimer was removed by filtration. The filtrate was concentrated to 5 mL under reduced
pressure and the desired product was precipitated by addition of hexane (100 mL). The
yellow solid [Ir(PPC) (bpy)]Cl (973 mg, 82%) was collected by filtration after cooling
2
the suspension at 0 °C for 3 h. Similar synthetic procedure except for replacing
[Ir(PPC) Cl] with [Ir(TfPPC) Cl] was performed to obtain [Ir(TfPPC) (bpy)]Cl as
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2
2
2
2
1
yellow solid (1.06 g, 79%). For [Ir(PPC) (bpy)]Cl: H NMR (400 MHz, DMSOꢀd ) δ
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.98 (d, J = 8.2 Hz, 2H), 8.46 (d, J = 8.1 Hz, 2H), 8.41−8.30 (m, 4H), 8.27−8.14 (m, 6H),
7.86 (t, J = 7.8 Hz, 2H), 7.79 (t, J = 6.8 Hz, 2H), 7.71 (d, J = 5.6 Hz, 2H), 7.49−7.37 (m,
13
6
H), 7.36−7.20 (m, 8H), 7.07 (t, J = 6.6 Hz, 2H), 6.45 (d, J = 1.8 Hz, 2H). C NMR (100
MHz, DMSOꢀd ) δ 166.4, 155.9, 152.5, 151.0, 149.6, 143.0, 140.4, 139.6, 138.5, 129.5,
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127.9, 126.9, 126.5, 125.8, 124.6, 123.4, 121.1, 121.0, 120.8, 119.9, 110.5. MS
+
+
(
MALDIꢀTOF) m/z: calculated for [C H N Ir] [(M−Cl) ]: 987.2; found: 987.6. For
56 38 6
1
[
Ir(TfPPC) (bpy)]Cl: H NMR (400 MHz, DMSOꢀd ) δ 9.01 (d, J = 8.2 Hz, 2H), 8.66 (d,
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2
3
4
5
6
7
8
9
0
J = 8.7 Hz, 2H), 8.49 (d, J = 8.5 Hz, 2H), 8.42 (t, J = 7.9 Hz, 2H), 8.37−8.25 (m, 4H),
.20 (d, J = 7.7 Hz, 4H), 7.87 (t, J = 6.8, 2H), 7.70 (s, 2H), 7.51 (dd, J = 8.4, 1.8 Hz, 2H),
8
1
3
7.45−7.20 (m, 12H), 6.50 (d, J = 1.8 Hz, 2H). C NMR (100 MHz, CD OD) δ 170.4,
3
1
1
56.0, 152.5, 150.8, 145.0, 140.6, 140.5, 140.4, 139.6, 136.0, 128.9, 128.0, 127.9, 125.6,
25.3, 123.7, 120.4, 120.2, 119.9, 109.5. MS (MALDIꢀTOF) m/z: calculated for
+
+
[C H F N Ir] [(M−Cl) ]: 1123.2; found: 1123.4.
5
8
36
6
6
Synthesis of CPOP-28 and CPOP-29. To a suspension of FeCl (0.54 g, 3.3 mmol) in
3
anhydrous dichloromethane (20 mL), a dichloromethane solution (30 mL) of
[
Ir(PPC) (bpy)]Cl (200 mg, 0.195 mmol) or [Ir(TfPPC) (bpy)]Cl (200 mg, 0.173 mmol)
2 2
was added dropwise via a dropping funnel under nitrogen. The reaction mixture was kept
stirring at room temperature for 24 h to complete the polymerization. Then, the mixture
was stirred in methanol (60 mL) for 1 h. The precipitate was collected by filtration and
washed with methanol, tetrahydrofuran, and dichloromethane, respectively. The
asꢀprepared polymers were further purified with methanol for 24 h and tetrahydrofuran
for another 24 h in a Soxhlet extractor, and dried in vacuo at 70 °C for 12 h. CPOP-28
(yield: 93%) and CPOP-29 (yield: 90%) were obtained as yellow and orange solid,
respectively.
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9
1
1
1
1
1
1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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5
5
5
6
ꢀ RESULTS AND DISCUSSION
Scheme 1. Schematic Synthesis of Heteroleptic Iridium Complexes and Their
Corresponding Polymers
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
N
N
N
N
N
IrCl3—H2O
Clꢀ
Ir+
2
ꢀethoxyethanol/H2O
bipyridine
Cl
Cl
Ir
Ir
120
C
DCM/MeOH, 55 °C
N
N
N
N
N
N
N
R
2
2
R
R
R
R
R=H
PPC
R=CF3 TfPPC
[
Ir(PPC)2Cl]2
[Ir(TfPPC)2Cl]2
[Ir(PPC)2(bpy)]Cl
[Ir(TfPPC)2(bpy)]Cl
FeCl3, CH2Cl2
R.T.
R
CPOPꢀ28 (R=H)
R
N
N
R
N
R
N
N
N
N
N
Ir+
Clꢀ
Ir+
Clꢀ
N
N
N
N
N
CPOPꢀ29 (R=CF3)
N
Clꢀ
Ir+
N
N
N
N
R
R
The electronic effects of substituents have been studied extensively in
35 , 36 , 37
lightꢀemitting materials containing iridium phosphors.
Electronꢀwithdrawing
fluorous substitutes are of substantial use to construct blue emitters inasmuch as
3
8,39
fluorination can effectively shift the frontier orbital energies.
This prompts us to
investigate the photophysical properties and photocatalytic activities of iridium catalyst
bearing trifluoromethyl group. As shown in Scheme 1, for comparison, two distinct
iridium complexes and their corresponding polymeric networks were designed and
synthesized. To ingeniously introduce trifluoromethyl group, cyclometalating ligand
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ACS Catalysis
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2
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8
9
TfPPC
ꢀbromoꢀ5ꢀ(trifluoromethyl)pyridine via a Suzuki reaction. The chloroꢀbridged iridium
dimer [Ir(TfPPC) Cl] was synthesized through a Nonoyama reaction in which TfPPC
was
synthesized
using
4ꢀ(9ꢀcarbazolyl)phenylboronic
acid
and
2
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
and iridium trichloride hydrate were refluxing in a mixture of water and 2ꢀethoxyethane.
Heteroleptic iridium complex [Ir(TfPPC) (bpy)]Cl was obtained by substituting the dimer
2
with ancillary ligand 2,2′ꢀbipyridine. [Ir(PPC) (bpy)]Cl was synthesized following a
2
similar synthetic procedure for [Ir(TfPPC) (bpy)]Cl. CPOP-28 and CPOP-29 were
2
prepared in high yield through FeCl ꢀpromoted coupling polymerization at room
3
temperature under nitrogen.
As probed by scanning electron microscopy (SEM) (Figure S1, Supporting
Information) and transmission electron microscopy (TEM) (Figure S2, Supporting
Information), both CPOP-28 and CPOP-29 feature aggregates comprising
interconnected nanoparticles with sizes of 100−200 nm. The asꢀprepared crossꢀlinked
polymers are chemically stable and insoluble in water and common organic solvents such
as alcohol, DCM, THF, and DMF. The thermal stabilities of the polymers were examined
by thermogravimetric analysis (TGA) under nitrogen (Figure S3, Supporting
Information). Approximatively 20% weight loss was observed when the temperature was
raised to 450 °C, which was ascribed to the dissociation of ancillary ligands.
1
3
19
Structural details of the polymers were elaborated by solidꢀstate C and F NMR
spectroscopy. As shown in Figure 1a, the two polymers show similar resonance peaks
ranging from 175−105 ppm. The peaks centered at 170 ppm for CPOP-29 and 164 ppm
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
for CPOP-28 are assigned to the carbons bonding to pyridinyl nitrogen in
cyclometalating ligands. The broad peak at 159−146 ppm belongs to carbons neighboring
to nitrogen in bipyridine ligand. The peak at about 140 ppm corresponds to the carbons
bonding to nitrogen in carbazole moiety. The peak at about 125 ppm results from the
residual carbons in pyridinyl ring and substituted phenyl carbons. The signals for
unsubstituted phenyl carbons are found at 119 and 110 ppm. The successful introduction
of trifluoromethyl group is substantiated by the fluorine signal at −63.3 ppm (Figure 1b).
a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
125
140
119
110
1
70 153
CPOPꢀ29
124
139
119
152
110
164
CPOPꢀ28
2
20 200 180 160 140 120 100
Chemical shift (ppm)
80
60
b
ꢀ63.3
*
*
ꢀ
30
ꢀ40
ꢀ50
ꢀ60
ꢀ70
ꢀ80
ꢀ90
ꢀ100
Chemical shift (ppm)
1
3
Figure 1. (a) Solidꢀstate C CP/MAS NMR spectra of CPOP-28 and CPOP-29. (b)
19
Solidꢀstate F NMR of CPOP-29 (asterisks denote spinning sidebands).
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ACS Catalysis
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8
9
a
C1s
F1s
O1s
N1s
Ir4f
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Cl2p
CPOPꢀ29
CPOPꢀ28
8
00 700 600 500 400 300 200 100
Binding evergy (eV)
0
Ir 4f7/2
b
III
Ir
2.24
6
Ir 4f5/2
IrꢀO
61.00
6
9
68 67 66 65 64 63 62 61 60 59 58
Binding energy (eV)
Ir 4f7/2
c
III
Ir
2.54
6
Ir 4f5/2
IrꢀO
1.63
6
6
9
68 67 66 65 64 63 62 61 60 59 58
Binding energy (eV)
Figure 2. (a) XPS survey spectra. (b) Ir 4f of CPOP-28. (c) Ir 4f of CPOP-29.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Elemental compositions of the polymers were surveyed by Xꢀray photoelectron
spectroscopy (XPS) and energy dispersive Xꢀray spectroscopy (EDS). The overall
elements (688.6 eV for F 1s, 400.6 eV for N 1s, 285.6 eV for C 1s, 198.6 eV for Cl 2p,
and 62.6 eV for Ir 4f) were unambiguously confirmed in XPS spectra (Figure 2a) and
EDS spectra (Figure S4, Supporting Information). The oxygen peak is ascribed to partial
oxidation of iridium during oxidative polymerization and the coordinated solvent
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
0
molecules. An inꢀdepth analysis of Ir 4f region (Figures 2b and 2c) indicates +3
4
1
III
oxidation state of Ir species. The binding energy of Ir species in CPOP-29 was found
0.3 eV higher than that of CPOP-28 due to the electronic effect induced by
trifluoromethyl group. As disclosed by EDS, the Ir contents of CPOP-28 and CPOP-29
are 19.7 wt% and 15.2 wt%, respectively, which are consistent with the theoretical values
(18.8 wt% for CPOP-28 and 16.6 wt% for CPOP-29).
The permanent porosities of CPOPs were studied by nitrogen sorption analysis at
77 K (Figure S5, Supporting Information). The Brunauer−Emmett−Teller (BET) surface
2
−1
area of CPOP-28 and CPOP-29 are 97 and 76 m g , respectively, which are
significantly lower than the values of previouslyꢀreported polycarbazole
2
8,29,30,31,32
networks.
The lack of rigidity in coordination structures, inclusion of
counterions, and the stoichiometric amounts of heavy iridium atoms in the polymeric
, ,
42 43,44 45
network may account for the low surface area.
A close inspection on optoelectronic properties of iridium complexes and their
corresponding polymers was carried out by UV–Vis and steady/transient fluorescence
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ACS Catalysis
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9
spectroscopy (Figure 3). As recorded in CH CN at 298 K, fine electronic structures are
3
reflected in the absorption spectra of [Ir(PPC) (bpy)]Cl and [Ir(TfPPC) (bpy)]Cl. The
2
2
predominant bands with higher energies in the range of 200−300 nm arise from
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
spinꢀallowed singlet ligandꢀcentered (LC) π−π transitions of cyclometalating and
ancillary ligands. The lowerꢀenergy bands with weaker intensity between 300−450 nm
1
1
are assigned to spinꢀallowed singlet MLCT, ligand−ligand charge transfer ( LLCT) and
3
3
3
spinꢀforbidden triplet transitions such as MLCT, LLCT and LC π−π. It’s noticeable that
introduction of trifluoromethyl group results in distinct bathochromic shift of about 30
nm compared to the unsubstituted counterpart. Similar phenomena were also observed in
38,46
some other fluorinated iridium complexes, mainly due to the lower LUMO level.
The absorption spectra of polymers span the whole UV−Vis region as a
consequence of extended πꢀconjugated structures. Specifically, UV−Vis diffuse
reflectance spectra of CPOP-29 are significantly red shifted for ca. 50 nm with respect to
CPOP-28, which is in good conformity with the color change of solid samples, from
yellow to orange for CPOP-28 and CPOP-29, respectively. Upon excitation at 420 nm,
[Ir(PPC) (bpy)]Cl and [Ir(TfPPC) (bpy)]Cl emit greenishꢀyellow and bluishꢀgreen color,
2
2
respectively, in CH CN at 298 K. A broad and featureless emission peak centered at 568
3
3
nm is observed for [Ir(PPC) (bpy)]Cl indicating the main contribution from MLCT.
2
[Ir(TfPPC) (bpy)]Cl manifests vibrational structure with an emission maximum at 507
2
3
nm and a shoulder peak at 543 nm, which suggests a larger portion of LC π−π character
3
47
than MLCT in excited state. Broad peaks with similar emission maxima were recorded
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
when the solid samples of polymers were excited at 430 nm.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
a
[Ir(TfPPC) (bpy)
]Cl
[Ir(PPC) (bpy)]Cl
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
200 250 300 350 400 450 500 550 600 650 700 750
Wavelength (nm)
1
.2
.0
.8
.6
.4
.2
1.2
b
1
0
0
0
0
0
1.0
0.8
0.6
0.4
0.2
0.0
CPOPꢀ28
CPOPꢀ29
.0
3
00 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Figure 3. Normalized UV−Vis absorption and emission spectra of iridium complexes in
acetonitrile (a) and polymers in solid state (b) at 298 K.
The excitedꢀstate lifetimes of complexes in solution and polymers in solid state
were measured by timeꢀresolved fluorescence spectroscopy in air at 298 K (Figure S6,
Supporting Information). The polymers have shorter lifetimes than their corresponding
monomers owing to the interaction between excited and nonꢀexcited molecules in high
concentration. Moreover, [Ir(TfPPC) (bpy)]Cl and CPOP-29 that bear trifluoromethyl
2
group have much longer decay lifetimes than their nonꢀsubstituted counterparts, i.e.
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ACS Catalysis
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9
3
[Ir(PPC) (bpy)]Cl and CPOP-28, which can be ascribed to the greater LC π−π
2
4
8,49
character.
a
Table 1. Photocatalytic Selective Oxidation of Sulfides Using CPOPs
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
b
c
Entry Aryl
Catalyst
Time (h)
10
Conv. (%)
79
Select. (A:B)
98:2
1
2
3
4
5
6
7
8
9
Ph
Ph
CPOP-28
CPOP-29
CPOP-28
CPOP-29
CPOP-28
CPOP-29
CPOP-29
CPOP-29
10
>99
71
99:1
4ꢀCH −Ph
10
98:2
3
4ꢀCH −Ph
10
>99
74
97:3
3
4ꢀBr−Ph
4ꢀBr−Ph
4ꢀCl−Ph
4ꢀF−Ph
15
98:2
15
>99
>99
>99
96
96:4
16
98:2
13
98:2
4ꢀMeO−Ph CPOP-29
10
96:4
a
As standard conditions for oxidation of sulfides were sulfides (0.5 mmol), CPOPs (5
mol, 1 mol% based on monomer), MeOH (0.5 mL), air, 23 W white LED lamp and
ꢁ
b
1
c
room temperature. Conversions were determined by H NMR. Selectivities of A:B
1
were determined by H NMR.
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
00
1
st
2nd
3rd
4th
5th
8
6
4
2
0
0
0
0
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
10
20
30
Time (h)
40
50
Figure 4. Recyclability of CPOP-29 in oxidation of methyl phenyl sulfide within five
cycles. The catalyst was recycled from reaction mixture by centrifugation, washed with
MeOH and DCM, dried, and reused in fresh reaction solution.
Given the remarkable light absorption ability in visible region and long
excitedꢀstate lifetimes, the photocatalytic activities of CPOPs as heterogeneous catalysts
for aerobic oxidation reactions were systematically investigated. As a model reaction, the
selective oxidation of sulfide was performed (Table 1). The oxidation reactions of sulfide
to sulfoxide were catalyzed by CPOPs in air with irradiation of a 23 W white LED lamp.
Light irradiation, air, and photocatalyst are indispensable to promote the oxidation
reactions (Table S1, Entries 1−3, Supporting Information). All sulfides were transformed
to corresponding sulfoxides in moderate to high conversions. Superior efficiencies are
more readily to achieve with CPOP-29 than CPOP-28 under the same conditions (Table
1
, Entries 1−6). Furthermore, various substrates bearing electronꢀdonating groups (EDG,
Table 1, Entries 4 and 9) and electronꢀwithdrawing groups (EWG, Table 1, Entries 6−8)
were converted quantitatively with the aid of CPOP-29. In consistence with previous
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5,26
literatures,
sulfides with electronꢀwithdrawing substituents (−F, −Cl and −Br) were
found having much slower reaction kinetics than those with electronꢀdonating
substituents (−CH and −OCH ). It should be noted that high selectivities of sulfides
3
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
against sulfones were manifested in all cases. CPOP-29 was recycled from reaction
mixture of oxidation of methyl phenyl sulfide, 90% conversion remained after five cycles
(Figure 4).
Photooxygenation of sulfides involves either electronꢀtransfer process in which
•−
superoxide radical (O ) plays a crucial role as electron mediator in photoredox cycle or
2
energyꢀtransfer process in which singlet oxygen reacts with sulfides to afford the
50, 51
sulfoxides.
To get a better understanding of the reaction mechanism, electron
paramagnetic resonance (EPR) measurements were performed to provide direct evidence.
1
No EPR signal of O was detected when 2,2,6,6ꢀtetramethylꢀ1ꢀpiperidine (TEMP) was
2
•−
used as trapping agent. Instead, characteristic quartet peak of O2 was unequivocally
detected when a solution of CPOP-29 and 5,5′ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) in
MeOH was irradiated (Figure S7, Supporting Information). When hole scavenger KI was
added to the reaction mixture, the conversion declined to 16%. Using pꢀbenzoquinone as
•−
O2 scavenger, only 7% conversion was observed (Table S1, Entries 4−7, Supporting
Information). Based on these findings, a plausible reaction mechanism for oxidation of
sulfides was proposed in Scheme S1 (Supporting Information). The excited photocatalyst
*
CPOP undergoes oxidative quenching process by transferring electrons to oxygen
+
molecules. Through a redox reaction, the substrate sulfides are oxidized by CPOP with
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
simultaneous regeneration of neutral CPOP and formation of sulfide radical cation which
•−
is further oxidized by O2 to afford the desired products. Meanwhile, sulfides are
oxidized to sulfoxides by oxygen intermediates that are generated by holeꢀinduced water
oxidation.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
Table 2. Photocatalytic Hydroxylation of Arylboronic Acids Using CPOPs
b
Entry
Aryl
Ph
Catalyst
CPOP-28
CPOP-29
CPOP-28
CPOP-29
CPOP-28
CPOP-29
CPOP-29
CPOP-29
CPOP-29
CPOP-29
CPOP-29
CPOP-29
Yield (%)
76
1
2
3
4
5
6
7
8
9
Ph
98
4ꢀCHO−Ph
4ꢀCHO−Ph
4ꢀCOOMe−Ph
4ꢀCOOMe−Ph
4ꢀOMe−Ph
4ꢀCN−Ph
86
99
89
96
82
99
3ꢀCN−Ph
83
1
0
2ꢀCN−Ph
67
11
4ꢀCOOH−Ph
4ꢀMe−Ph
99
1
2
85
a
As standard conditions for hydroxylation of arylboronic acids were arylboronic acids
(
0.5 mmol), CPOPs (10 ꢁmol, 2 mol% based on monomer), iPr NEt (1 mmol), DMF (0.5
2
b
mL), air, 48 h, 23 W white LED lamp and room temperature. Yields were determined by
1
H NMR.
Phenols as important intermediates in pharmaceutical and chemical industries have
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2
been conventionally prepared through hydroxylation of arylboronic acids. Nevertheless,
5
3,54
strong oxidants such as H O and mꢀchloroperoxybenzoic acid are usually required.
2
2
In view of environmental concerns, synthesis of arylboronic acids in mildly oxidizing
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
5,56
manners has been the aim of chemists.
Next, we applied CPOPs to photocatalytic
hydroxylation of arylboronic acids (Table 2). The catalysis reactions were performed
using oxygen air as oxidizing agent and N,Nꢀdiisopropylethylamine (iPr NEt) as
2
sacrificial electron donor. Under the same reaction condition, higher yields are achieved
in the reactions catalyzed by CPOP-29 (Table 2, Entries 1−6,). A wide scope of
substrates bearing electronꢀneutral substitute (Table 2, Entry 1), EWG (Table 2, Entries
3
−6 and 8−11), and EDG (Table 2, Entries 7 and 12,) were studied. Most of the substrates
were quantitatively depleted and transformed into corresponding phenols within 48 h. In
general, substrates with EWG (−COOMe, −COOH, −CHO, and −CN) have better
reaction efficiencies than those with EDG (−Me and −OMe). In addition, the reaction
efficiencies are correlated with the positions of substitute (4ꢀCN > 3ꢀCN > 2ꢀCN).
Employment of KI as hole scavenger, the conversion decreased significantly to 23%. By
•−
using pꢀbenzoquinone as O2 scavenger, a low conversion of 12% was obtained (Table
S2, Entries 4−7, Supporting Information).
C–H bond activation provides powerful synthetic tools for synthesis of natural
5
7
products and meets the demands of green chemistry. Regarded as one of the most
promising C−H activation methodologies, crossꢀdehydrogenative coupling (CDC)
reactions have come into focus for the reason that chemical bonds (e.g., C−C, C−P, C−O)
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3
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7
8
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
5
8,59,60
can be constructed directly without preꢀactivation of substrate molecules.
To
demonstrate the versatility, CPOPs were employed to catalyze CDC reactions under
aerobic conditions (Table 3). Prior to the test, CPOPs were subject to a solution of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
N,N,N′,N′ꢀtetramethylꢀpꢀphenylenediamine (TMPD) under aerobic and irradiation
61,62
conditions to verify the ability to mediate an electronꢀtransfer process.
The color
change of solution from colorless to blue and new absorption bands at 564 and 613 nm
clearly indicate the generation of cationic radical of TMPD, which in turn proves that
CPOPs has mediated the electron transfer from TMPD to dioxygen (Figure 5). The more
intense absorption band for the solution containing CPOP-29 suggests stronger electron
mediation ability. The substrates were converted smoothly in both phosphonation
reactions (Table 3, Entries 1−5) and azaꢀHenry reaction (Table 3, Entries 6−8) in good
yield (80−95%). As in the foregoing examples, CPOP-29 was found to be a more
effective catalyst. The scope of substrates (Nꢀarylꢀtetrahydroisoquinoline) and
nucleophiles (nitroalkanes and phosphite esters) were examined to establish applicability
of the catalyst. It was shown that EWG on Nꢀaryl ring gave higher yield than EDG. It is
noted that the species of phosphite and nitroalkane do not exhibit marked difference on
the reaction efficiencies.
The charged polycarbazole networks (CPOP-28 and CPOP-29) proved highly
active for oxidative organic transformations despite their low surface area. Indeed, high
42,43,44
catalytic efficiencies have been observed for polymers with lowꢀsurface area
or
4
5,63
even nonꢀporousity.
This can be reasonably explained by the swelling of crossꢀlinked
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polymers in solvents, in which cases the distended skeletons enrich accessible catalytic
sites and expedite the mass transport process.
a
Table 3. Photocatalytic Cross-Dehydrogenative Coupling Reactions Using CPOPs
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
b
Entry
R
H
H
NuH
Catalyst
Product
Yield (%)
1
2
CPOP-28
CPOP-29
83
92
N
P
3
4
5
Br
OMe
H
CPOP-29
CPOP-29
CPOP-29
89
80
94
EtO
O
Br
EtO
6
7
H
H
CPOP-28
CPOP-29
81
96
CH NO
3
2
8
H
C H NO
2
CPOP-29
95
2
5
a
As
standard
condition
for
phosphonylation
of
amines
were
Nꢀarylꢀtetrahydroisoquinolines (0.5 mmol), phosphite ester (0.5 mmol), CPOPs (5 ꢁmol,
mol% based on monomer), MeOH (0.5 mL), air, 24 h, 23 W white LED, room
1
temperature. AzaꢀHenry reactions were conducted in similar conditions except 1 mL
b
nitromethane or nitroethane was used instead of phosphite ester and MeOH. Yields were
1
determined by H NMR.
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
1
1
.0
.5
.0
No cat. CPOPꢀ29
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
CPOPꢀ29
0.5
CPOPꢀ28
No catalyst
600
0.0
4
00
450
500
550
650
700
Wavelength (nm)
Figure 5. UV−Vis absorption spectra and photograph of the cationic radical species of
N,N,N′,N′ꢀtetramethylꢀpꢀphenylenediamine generated by CPOPs in the presence of
oxygen and irradiation of white LED for 10 min.
ꢀ2.0
CPOPꢀ28
CPOPꢀ29
ꢀ
1.5
1.0
ꢀ0.5
E
(M/M‾)
1/2
–1.40
–0.95
–
1.28
ꢀ
+
*
E
(M /M )
–0.88
1/2
0
0
1
1
.0
.5
.0
.5
*
E
( M/M‾)
1/2
0.85
1.30
1
.40
+
E
(M /M)
1/2
1
2.0
Figure 6. Redox potentials of CPOP-28 and CPOP-29. M = photocatalyst, * = excited
state.
To rationalize the superior efficiency of trifluoromethylated polymer CPOP-29 for
photocatalytic hydroxylation of arylboronic acids and CDC reactions, electrochemical
measurements and fluorescence quenching experiments were performed. Reversible
III IV
oxidation waves that are attributed to oxidation of Ir /Ir redox couple are shown in
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anodic scans for both polymers. Additionally, an irreversible and a quasiꢀreversible
reduction wave are observed for CPOP-28 and CPOP-29, respectively, upon cathodic
scans. The reduction waves are assigned to the reduction of ancillary ligand bipyridine as
well as cyclometalating ligand (Figure S8, Supporting Information). The excitedꢀstate
potentials that are essential for photocatalytic chemical transformations can be simply
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0–0 64,65
approximated on the basis of groundꢀstate potentials and E .
The groundꢀstate and
excitedꢀstate potentials of the polymers are summarized in Table S3 (Supporting
3
8, 66 , 67
Information). In keeping with previous observations,
introduction of
trifluoromethyl group markedly shifts both oxidation and reduction potential to more
positive values (Figure 6). Indeed, the excited photocatalyst can be quenched by both
reductive quencher iPr EtN and oxidative quencher O considering the potentials of
2
2
excited state. More effective fluorescence quenching was observed when iPr EtN and
2
Nꢀphenylꢀtetrahydroisoquinoline (NꢀphenylꢀTHIQ) were used as quencher than oxygen,
which feasibly supported a reductive quenching process in the catalysis reactions
(Figures S9−11, Supporting Information). This is reasonable considering the fact that the
+
65
oxidation potential of iPr EtN (E (M /M) = 0.90 V vs SCE)
and NꢀphenylꢀTHIQ
2
1/2
+
68
(
E (M /M) = 0.83 V vs SCE)
is more negative than the reduction potential of excited
1/2
*
−
photocatalyst. The more positive value of E ( M/M ) for CPOP-29 that contributes to
1/2
thermodynamic driving force in reductive quenching, stronger visibleꢀlight absorption
ability, and longer excitedꢀstate lifetime are probably the main reasons for better
photocatalytic efficiencies.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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5
6
The proposed mechanisms for photocatalytic hydroxylation of arylboronic acids
and CDC reactions are illustrated in Schemes S2 and S3 (Supporting Information). The
*
excited photocatalyst CPOP is first reductively quenched by amines (iPr EtN or
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−
NꢀarylꢀTHIQ) to produce amine radical cations and CPOP . Some amines are oxidized by
•−
photogenerated holes to give radical cations. O2 radicals are generated through electron
−
transfer processes with CPOP and photogenerated electrons. The arylboronic acids react
•−
with O2 to produce anion intermediates that further transform into phenols via the
•−
electronꢀmediating and hydrolysis processes. As for CDC reactions, O2 attracts a
hydrogen from the amine radical cation to form iminium ion that is subsequently attacked
•−
by nucleophile to provide the desired product. The direct evidence for generation of O2
was supported by EPR experiments (Figures S12−13, Supporting Information). The EPR
•−
signals of O2 was evidently detected when CPOP-29 was irradiated in DMF and MeOH
1
•−
solution of DMPO, yet no O species was captured by TEMP. The intensity of O
2
2
signal was significantly increased upon addition of iPr EtN, which was ascribed to the
2
−
rapid formation of reduced photocatalyst (CPOP ) and thus generation of superoxide
radicals. Upon addition of NꢀphenylꢀTHIQ, the EPR signal attenuates due to the
•−
consumption of O2 by imine radical cation.
ꢀ CONCLUSIONS
Cationic polycarbazole networks (CPOP-28 and CPOP-29) were prepared via
simple FeCl ꢀpromoted oxidative coupling reactions. By introducing trifluoromethyl
3
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groups (–CF ) into the polymeric backbone, CPOP-29 displayed greater visibleꢀlight
3
absorption ability, longer excitedꢀstate lifetime, and more oxidative photoexcited state as
compared to CPOP-28. The polymers were employed as heterogeneous photocatalysts in
farꢀranging aerobic organic transformations including selective oxidation of sulfides,
hydroxylation of arylboronic acids, and crossꢀdehydrogenative coupling reactions under
aerobic and white LED irradiation conditions. In spite of low surface area, CPOP-28 and
CPOP-29 are highly active in photocatalytic synthesis. Taking advantages of the superior
photophysical and oxidative properties, CPOP-29 manifests greater catalytic efficiency
in oxidative transformation reactions. Moreover, good catalytic efficiency was retained
after multiꢀround use. This work highlights the potential of obtaining highly efficient
photocatalyst through simply modulating the substitute of skeleton. More research studies
concerning tuning the photocatalytic properties by regulating the electronic structures of
CPOPs and expanding the scope of organic transformations are underway.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢀ
AUTHOR INFORMATION
Corresponding Authors
*
*
Eꢀmail: hanbh@nanoctr.cn. Phone: +86 10 8254 5576.
Eꢀmail: chenq@nanoctr.cn. Phone: +86 10 8254 5708.
ORCID
BaoꢀHang Han: 0000ꢀ0003ꢀ1116ꢀ1259
Notes
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The authors declare no competing financial interest.
ꢀ
ASSOCIATED CONTENT
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Supporting Information
Instrumentations and characterization methods, general procedures for photocatalysis,
1
13
additional tables, catalysis mechanisms, additional figures, H NMR, C NMR and mass
spectra.
This material is available free of charge via the Internet at http://pubs.acs.org.
ꢀ
ACKNOWLEDGEMENTS
The financial support of the National Natural Science Foundation of China (Grants
1574031, 21574032, and 21302150) and the SinoꢀGerman Center for Research
Promotion (Grant GZ1286) is acknowledged.
2
ꢀ
REFERENCES
1] Kannan, N.; Vakeesan, D. Solar Energy for Future World: ꢀ A Review. Renew. Sustain.
Energy Rev. 2016, 62, 1092–1105.
2] Berardi, S.; Drouet, S.; Francàs, L.; GimbertꢀSuriñach, C.; Guttentag, M.; Richmond,
C.; Stoll, T.; Llobet, A. Molecular Artificial Photosynthesis. Chem. Soc. Rev. 2014, 43,
501–7519.
[
[
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3] Yang, M.ꢀQ.; Xu, Y.ꢀJ. Selective Photoredox Using GrapheneꢀBased Composite
Photocatalysts. Phys. Chem. Chem. Phys. 2013, 15, 19102–19118.
4] Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X.
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