Structure-activated copper photosensitisers for photocatalytic water reduction

Article abstract of DOI:10.1002/chem.201602598

A series of phenanthroline-based ligands have been synthesised and their influence as bidentate nitrogen ligands in heteroleptic [Cu(P^P)(N^N)]⁺ photosensitisers in light-driven water reduction has been studied. In this noblemetal- free Cu–Fe-based photocatalytic water reduction system, the structural effects of the nitrogen ligands have been explored, including the steric and electronic effects of substituents at the 2,9- and 4,7-positions of phenanthroline. Ligands were prepared that led to increased hydrogen generation, with turnover numbers (TON_Cu) of up to 1388 being observed. All the new complexes were electrochemically and photophysically characterised. We demonstrate for the first time that the presence of fluorine in nitrogen ligands increases the efficacy of copper complexes in photocatalytic hydrogen production.

Full text of DOI:10.1002/chem.201602598

A Journal of
Accepted Article
Title: Structure activated copper photosensitizers for photocatalytic
water reduction
Authors: Shu-Ping Luo, Nan-Yu Chen, Yuan-Yuan Sun, Alastair J.
J. Lennox, Liang-Min Xia, Hao Chen, Hai-Ming Jin, Henrik
Junge, Qin-An Wu, Jian-Hong Jia, and Matthias Beller
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Structure activated copper photosensitizers for photocatalytic
water reduction
Nan-Yu Chen,[a]† Liang-Min Xia,[a]† Alastair J. J. Lennox,[b]† Yuan-Yuan Sun,[a] Hao Chen,[a] Hai-Ming
Jin,[a] Henrik Junge,[b] Qin-An Wu,[a] Jian-Hong Jia,[a] Matthias Beller,*[b] and Shu-Ping Luo*[a]
Abstract: A series of phenanthroline-based ligands have been
synthesized and their influence as bidentate N ligands in heteroleptic
[Cu(P^P)(N^N)]+ photosensitizers in light driven water reduction has
been studied. In this noble-metal free Cu-Fe-based photocatalytic
water reduction system, the structural effects of these N ligands have
been explored, including the steric and electronic effects in the 2, 9
and 4, 7 substituent positions of phenanthroline. Ligands were
prepared that showed an increase in hydrogen generation, up to 1388
turnover numbers (TONCu) was observed. All new complexes were
electrochemically and photophysically characterized. This report
provides the first demonstration that the fluorine effect in N ligand
increased the photochemical transfer ability of copper complexes.
organic dyes[7], biomimetic zinc[8] and magnesium porphyrins
(chlorophyll A)[9] are comparatively scarce due to their low
efficiencies and fast photodegradation.
Previously, we reported the successful application of a series of
heteroleptic copper (I) complexes possessing the general formula
[Cu(N^N)(P^P)]PF6, as photosensitizers.[10] Our recent
mechanistic studies demonstrated that the dominant species
observed in solution was the heteroleptic complex
[Cu(P^P)(N^N)]PF6.[11] Indeed, this was found to be the active PS
species, rather than the homoleptic species, [Cu(N^N)2]+ (or
[Cu(P^P)2]+), that was only formed initially in very low quantities
and, its growth over time was correlated to the reaction rate decay.
During optimization studies, the necessity of appropriate ligands
in the 2, 9 and 4, 7 positions of the 1, 10-phenanthroline ligand
(N^N) for a successful H2 generation became obvious. Especially,
substitution in the 2, 9-position was found to have a high impact
on rates and catalyst longevities. For example, by switching from
n-butyl to sec-butyl and to isobutyl, the PS activity was strongly
influenced and TON´s (Cu) of 954, 1330 and 224, respectively,
were obtained.[10b] Thus, the importance and the influence of the
steric hindrance were preliminary demonstrated. Moreover, it was
established that without substituents in the 4, 7 positions no H2
generation was detected. However, besides phenyl groups no
other groups had tested so far. Considering the importance of
these key positions in the CuPS, further investigations have now
been performed.
Introduction
The development of technologies for the application of
sustainable and abundant energy sources remains one of the
most important issues of our modern society.[1] In this respect, the
use of sunlight for the production of hydrogen from water is a
highly desirable option. As the resulting H2 is not carbon based, it
could indeed constitute an environmentally benign source of
chemical energy.[2] Commonly, approaches focus on the
hydrogen half reaction in a first step applying three component
photocatalytic systems, consisting of a suitable photosensitizer
(PS), a water reduction catalyst (WRC) and a sacrificial electron
donor (SR). A key component constitutes the PS; it´s electron
transfer efficiency and lifetime of the excited state are crucial
regarding costs, productivity and feasibility of hydrogen
generation. Since the 1970´s, noble-metal-based complexes
including ruthenium,[3] platinum,[4] iridium[5] and rhenium[6] have
been successfully employed as PS. However, the high price and
toxicity of a number of these metals limit their potential application.
In contrast, reports on noble metal-free photosensitizers, e.g.,
A variety of phenanthroline derivatives containing different
substituents in their 2, 9 and 4, 7 positions were synthesized.
Using these ligands, the in situ generated heteroleptic copper
complexes [Cu(P^P)(N^N)]PF6 (P^P
= xantphos (I) and
thixantphos (II)) have been tested in the photocatalytic water
reduction (Figure 1).[11] In addition, the resulting PS have been
photophysically characterized.
[a]
N. Chen, L. Xia, Y. Sun, H. Chen, H. Jin, Prof. Dr. Q. Wu, Prof. Dr.
J. Jia, Prof. Dr. S. Luo
State Key Laboratory Breeding Base of Green Chemistry-Synthesis
Technology
Zhejiang University of Technology
Chaowang Road 18, 310014 Hangzhou (China)
E-mail: luoshuping@zjut.edu.cn
[b]
[†]
Dr. A. J. J. Lennox, Dr. H. Junge, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse an der Universität Rostock e.V.
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
Figure 1. Photocatalytic proton reduction applying in situ generated heteroleptic
copper PS and triirondodecacarbonyl as WRC.
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Firstly, the application of thiaxantphos (II) resulted in higher or at
least equal productivity compared to the respective xantphos
containing complexes (Entries 1- 4, 9 and 10). This effect can be
attributed to the large planar structure at phosphorous in
thixantphos and therefore less steric hindrance to the N^N ligand.
In contrast, xantphos (I) contains rotatable phenyl groups which
can interact with substituents in the 2,9-positions of the
phenanthroline derivatives. For this reason the thixantphos
derivative II was preferably employed for the subsequent test
reactions.
Secondly, comparing the influence of substituents in the 2,9
positions of the N^N ligand an increased bulkiness is beneficial
for productivity. This was exemplarily shown for methyl and iso-
propyl groups (Entries 2, 4, and 6). On the other hand, a decrease
occurred switching from s-Bu to phenyl (Entries 6 and 8). In
general, the steric bulk of different substituents can be
represented by Charton steric parameters. The relationship
between the productivity and bulkiness of substituents in the
CuPS is provided by diagram in Figure 3. The volcano plot shows
that there is an optimal steric parameter for the CuPS in
photocatalytic proton reduction. Furthermore, aldehyde or
carboxylic acid moieties in the 2,9-positions completely deactivate
the photosensitizer (entries 11 and 12). This may be due to an
inhibition of the original coordination of the metal centre to the
N^N ligand by competition with oxygen containing functionalities.
In contrast, introduction of closely related acetal group resulted in
almost the same productivity as the methyl group (Entry 14).
Figure 2. Synthesized 1, 10-phenanthroline derivatives with variations in 2, 9
and 4, 7 positions.
Results and Discussion
In addition to sp3 substituents in the 2,9 positions, e.g., Me, i-Pr,
s-Bu and t-Bu (ligands A-C, E), we were also interested in the
potential effect of sp2 carbon centers. Thus, we synthesized the
respective N^N ligands containing phenyl (D), aldehyde (F) and
carboxyl (G) groups, as well as a range of aryl vinyl moieties
(ligands I-O) in 2,9 positions with Ph in the 4, 7 positions.
Alternatively, ligands bearing different aromatic moieties in the 4,
7 positions (P-T) with i-Pr groups in the 2, 9 positions were
prepared. The synthesis methods were varied depending on the
desired structures. Ligands B-E were prepared by reaction of 4,7-
Thirdly, the results applying ligands I-O (Entries 15-22) illustrate
a fine balance between electronic and steric effects is essential.
In order to get a rational understanding of electronic effects, the
groups attached to the trans olefin was varied (ligands I-O). This
variation is far enough away from the metal centre to not
drastically alter the steric environment and so only electronic
effects can be measured. By substitution of the phenyl group by
thiophene an increase of TON was observed (Entries 15-17). A
further increase in the electron withdrawing was achieved by the
introduction of a CF3 group and perfluorinated phenyl groups
(Entries 19, 21). However, the introduction of two CF3 groups in
meta positions lead to decreased TON (Entry 20). In this case, a
steric effect likely hampers the PS activity. In contrast, the
introduction of electron donating substituents like Me and OMe in
the para position resulted either in almost unchanged or lower
productivities, respectively (Entries 18 and 22). The influence of
various substituents, on the productivity is summarized in Figure
4. The advantages of strongly electron withdrawing groups can
be attributed to a stable excited state, which improves the electron
transport efficiency to the water reduction catalyst.
diphenyl-1,10-phenanthroline
with
the
corresponding
organolithium reagent,[12] while ligands F and G were obtained by
oxidation of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (A).[13]
Further, ligand H was prepared starting from F by reaction with
triethylorthoacetate.[14] The aryl vinyl phenanthrolines I-O were
synthesized by aldol condensation of 2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline and aromatic aldehydes catalyzed by acetic
anhydride.[15] Ligands P and Q were synthesized by reaction of
4,7-bis(4-bromophenyl)-1,10-phenanthroline
and
4,7-bis(5-
bromothiophen-2-yl)-1,10-phenanthroline, respectively, with
isopropyl lithium and subsequent quenching by CO2.[16] Ligand R
was prepared by reaction of 4,7-di ([1, 1'-biphenyl]-4-yl)-1, 10-
phenanthroline with isopropyl lithium. Finally, ligands S and T
were
synthesized
by
N
alkylation
catalyzed
by
Pd2(dba)3/xantphos between 4,7-bis (4-bromophenyl)-1,10-
phenanthroline and piperidine or bis (4-(tert-butyl)phenyl)amine
and subsequent treatment with isopropyl lithium.[12, 17]-See SI for
full details.
Fourthly, substituents in 4,7 positions also have a significant
influence. So far only CuPS containing phenyl groups in these
positions were active in the photocatalytic proton reduction. The
introduction of a carboxylic acid group into the para position of
phenyl had almost no influence. Changing the substituents in the
para-position of the phenyl groups from carboxylic acid to phenyl
resulted in a slightly decreased TON (Entry 28). However, in this
position, piperidine and bis(4-(tert-butyl)phenyl)amine, which
were often used as electron donor group in organic dye, lead to a
significant decrease of the productivity (Entries 30 and 32). Even
more surprising was the substitution of phenyl by thiophene,
leading to deactivation of the PS (Entries 23-26). The most
probable reason for the latter might be the destabilization of the
excited state of CuPS by the good electronic conductivity of the
thiophene group.
Photocatalytic hydrogen production
The heteroleptic copper (I) complexes were obtained by a
previously developed in situ method.[11] This allows for the
application of optimized ratios of each ligand and copper
precursor. The photocatalytic production of H2 was performed for
each PS with Fe3(CO)12 as WRC and triethylamine as SR in a
4:3:1 (v:v:v) mixture of THF:Et3N:water (Table 1). Noteworthy, the
in situ generated complexes showed the same activity as the
molecularly-defined isolated CuPS.[10] Comparing the different
CuPS structures and their productivities in the photocatalytic
proton reduction the following was established:
Additionally, for ligands P to T, the same trend achieving higher
TONs by application of thiaxantphos (II) compared to xantphos (I)
has been obtained (Entries 23, 25, 27, 29, 31).
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(4:3:1), 25 °C, Xe-light irradiation (output 1.5 W), without light filter, gas evolution
quantitatively measured via manual gas burettes, gas analysis via GC. All given
values are the averages of at least two experiments. The results differ between
1 and 18%. [b] See figure 1, I = xantphos, II = thixantphos derivative. [c] Time
until no further H2 evolution was observed. [d] TONCu = nH/nCu rounded to
nearest 10.
Table 1. Photocatalytic H2 production with copper-based photosensitizers by in-
situ method in the presence of [Fe3(CO)12] as WRC and TEA as SR.a
N^N
Vol.H2
(mL)
P^P
d
Entry
Time(h)c
TONCu
ligandb
Ligand
1600
I
II
1
2
A
A
B
B
C
C
D
D
E
E
F
I
II
I
27
6
33.0
40.7
54.0
59.5
54.3
56.2
2.7
770
950
1260
1388
1267
1312
63
1200
800
400
0
B
C
3
17
11
24
11
7
4
II
I
A
5
6
II
I
E
D
7
0.5
0.75
1
1.25
1.5
1.75
8
II
I
6
4.6
107
229
131
-
Charton parameter
Figure 3. Charton parameters and productivities of CuPS containing ligands A-
E. Charton parameters taken from ref 18a.
9
10
7
9.8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
II
I
5.6
4
0.2
G
H
H
I
I
4
0.1
-
I
19
7
24.3
40.0
5.6
567
934
90
II
I
7
I
II
II
II
II
II
II
II
I
12
11
11
11
7
20.3
29.0
19.4
40.6
13.4
27.8
15.7
43.8
57.0
0.8
330
470
320
660
220
500
257
1020
1330
-
J
K
L
Figure 4. Hammett plot for the productivity of CuPS containing ligands I-O
(Entries 16-22) Hammett values taken from ref 18b. Estimated values used for
M (3,5-CF3, m doubled), J (2-thiophenyl, as p) and N (C6F5, as p).
M
N
O
P
P
Q
Q
R
R
S
S
T
11
9
Photophysical studies
20
9
For a deeper insight of the effect of structure on the productivity
of the CuPS, UV-vis absorption and photoluminescence spectra
were recorded. For these measurements, ligands B, H, I, J, L, O,
P, Q, R, S and phosphine II were selected.
II
I
4
The copper photosensitizers were self-assembled by our in situ
method.[11] The corresponding data are summarized in Table 2.
All complexes showed absorption in the UV region with a typical
absorption band around 380 nm (Figure 5) contributing to a MLCT
transition from a copper d-orbital to the π* of the N^N ligands.[19]
A small band in the proximity of 450 nm was found, which
probably arises from the formation of the homoleptic complex,
[Cu(N^N)2]PF6 due to the ligand redistribution in solution that has
been previously described for these kind of systems.[20]
Compared to the other complexes, Cu(J)(II) complex containing
thiophene showed a more efficient absorption peak from 350nm
to 450nm, as thiophene is a good chromogenesis group.[21] In
addition, CuPS containing ligands I, J, L and O possessed a
significantly stronger absorption ability (Figure 5). This might be
due to the elongated conjugated π-system formed by the aryl or
thiophene, the vinyl group and the phenanthroline derivative. The
II
I
4
0.8
-
43
6
38.9
44.3
15.1
17.1
16.2
18.1
908
1034
352
399
378
423
II
I
20
9
II
I
10
7
T
II
[a] Reaction conditions: N^N ligand (3.5 μmol), P^P ligand (3.5 μmol),
Cu(MeCN)4PF6 (3.5 μmol), [Fe3(CO)12] (5 μmol) (Among them ligand G-M (5.0
μmol ), P^P ligand (5 μmol), Cu(MeCN)4PF6 (5 μmol)), 10 mL THF/TEA/H2O
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comparison of the absorption maxima wavelengths revealed that
increasing the degree of conjugated π-system of the N^N scaffold
resulted either in a blue- (ligands L, O, R; table 2, entries 5, 6, 8),
red-(ligand P, Entry 7) or no significant shift (Ligand J, Entry 4) of
the absorption bands of the respective CuPS compared to the
ligands B and H (Entries 1 and 2).
Cu(I)(II)PF6
Cu(J)(II)PF6
Cu(L)(II)PF6
Cu(O)(II)PF6
Cu(I)(II)PF6
Cu(J)(II)PF6
Cu(L)(II)PF6
Cu(O)(II)PF6
The emission spectra of selected photosensitizers are shown in
Figure 6. The maximum emission wavelengths are located
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength [nm]
Wavelength [nm]
around 450 nm. A comparison of the emission maxima (λmax
)
Figure 5. Normalized absorption and Emission spectra for Cu (I)(II)PF6,
Cu(J)(II)PF6, Cu(L)(II)PF6, Cu(O)(II)PF6. Measurements made in CH2Cl2 at
room temperature.
reveals the same systematic trend upon changing the aryl or
thiophene substituents at the 2,9-positions of the N^N ligands in
both series of complexes I-O. Electron withdrawing groups at the
2,9-positions of the N^N scaffold results in a blue-shift of the
emission bands (∆J-L=28 nm), which means higher energy
emissive states and a smaller energy difference between the
absorption and emission maxima. The photoluminescence
intensity of copper complex containing ligand L is much higher
than those containing ligands I and J. This can be attributed to the
effect of fluorine, being a strong electron withdrawing group which
can enhance the photoluminescence intensity.[22] Compared to
the other complexes, carboxylic acid containing ligand P showed
a significant red-shift of the emission band. This is probably a
result of the strong electron withdrawing property of the carboxylic
acid group. Although CuPS bearing ligand Q showed more visible
light absorption compared to ligand P, its luminescence is almost
totally lost. A transient fluorescence test confirmed that by
applying ligand Q, the lifetime of CuPS is decreased to 200 ns,
which is much less than 2 μs for Cu(P)(II) (see supporting
information).
Cu(P)(II)PF6
Cu(P)(II)PF6
Cu(Q)(II)PF6
Cu(Q)(II)PF6
400 450 500 550 600 650 700 750 800
400
500
600
700
800
Wavelength [nm]
Wavelength [nm]
Figure 6. Normalized absorption and Emission spectra for Cu(P)(II)PF6,
Cu(Q)(II)PF6. Measurements made in CH2Cl2 at room temperature.
Electrochemical studies
Cyclic voltamograms of copper complexes containing
phenanthroline ligands I, J, L, B, O, R and phosphine ligand II
were recorded in THF solution (Figure 7). Upon reduction, all of
the complexes display comparable behaviour, with a reversible
single-electron-reduction couple, which is associated with a
reduction of the phenanthroline ligand, thus forming [CuII(N^N)•-
(P^P)]+. This behaviour is known from previous studies of related
[Cu(N^N)2]+ complexes.[23] Comparing the reduction potentials of
complexes I (-1.63V) and J (-1.74V versus Fc/Fc+) or L (-1.60V)
and O (-1.70V versus Fc/Fc+), a shift of about 90 mV or 100 mV
is observed, thus indicating an influence of the substitution pattern
at the 2,9-positions of the phenanthroline scaffold that correlates
with Hammett parameters. Complexes B and R also indicate an
influence of the substitution pattern at the 4,7-positions. They all
appear at more negative potentials than the reduction peak of the
WRC. However, so far no correlation between CV results and the
productivity in proton reduction could be detected.
Table 2. Electronic absorption and luminescence data for different
[Cu(N^N)(P^P)]PF6 complexes in degassed CH2Cl2 at room temperature.a
Abs
Em
Abs
ɛ(M-1cm-1)
Cu(N^N)
(P^P)
Entry
Φ
λmax
(nm)
λmax
(nm)
1
2
Cu(B)(II)
Cu(H)(II)
Cu(I)(II)
389
387
-
1628.6
2571.4
-
446
414
440
462
434
452
550
525
467
455
0.13
0.07
-
3
4
Cu(J)(II)
Cu(L)(II)
Cu(O)(II)
Cu(P)(II)
Cu(Q)(II)
Cu(R)(II)
Cu(S)(II)
388
371
376
394
392
379
-
27414.3
30085.7
16828.6
6142.9
4514.3
4257.1
-
0.02
0.27
0.08
0.07
0.01
0.37
-
5
6
7
Cu(B)(II)PF6
Cu(R)(II)PF6
8
9
Cu(I)(II)PF6
Cu(L)(II)PF6
Cu(O)(II)PF6
Cu(J)(II)PF6
10
[a] conditions: N^N ligand (1.75 μmol), P^P ligand (1.75 μmol), Cu(MeCN)4PF6
(1.75μmol) formed copper photosensitizers by in-situ method in CH2Cl2 , then
move to 50mL Volumetric flask.
-3
-2
-1
0
Potential vs. Fc/Fc+ [V]
Figure 7. Cyclic voltammograms of complexes Cu(I)(II)PF6, Cu(J)(II) PF6,
Cu(L)(II) PF6, Cu(B)(II) PF6, Cu(O)(II) PF6, Cu(R)(II) PF6 in THF solution. All
potentials are referenced to the Fc/Fc+ couple.
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temperature for 12 h. The reaction was quenched with water (10
mL) Then, the mixture was extracted with EtOAc (4×10 mL) and
dried over MgSO4.. Filtration, concentration and purification of the
orange residue by flash column chromatography (1:4 EA/PE)
gave a pale yellow solid, yield 56 %.[12]
Conclusions
In summary, we have synthesized a range of new copper
complexes containing several new phenanthroline-based ligands
by an in situ method. The complexes have been tested as
photosensitizers in light driven proton reduction from water and
Synthesis of ligand S. Step 1: A stirred solution of 4,7-bis(4-
bromophenyl)-1,10-phenanthroline (0.3 g, 0.61 mmol), Pd2(dba)3
(52.8 mg, 0,06 mmol), xantphos (54 mg, 0.12 mmol), t-BuOK
(0.32 g, 3.66 mmol) and piperidine (0.24 g, 3.66 mmol) in 10 mL
of toluene (dry) were refluxed at 120 °C for 24 h. Filtration,
concentratio and purification of the orange residue by flash
column chromatography (EA) gave an orange solid (4,7-bis(4-
(piperidin-1-yl)phenyl)-1,10-phenanthroline), yield 82%. Step 2: A
mixture of 4,7-bis(4-(piperidin-1-yl)phenyl)-1,10-phenanthroline
(0.2 g, 0.4 mmol) and 5 mL of dry ether were stirred in a sealed
tube under an argon atmosphere and isopropyl lithium (1.2 mL,
2.0mmol) was added at 0 °C. The resulting suspension was
stirred for 30 min at 0 °C and for 12 h at room temperature. The
reaction was quenched with water (10 mL) and the mixture was
extracted with EtOAc (4×10 mL) and dried over MgSO4. Filtration,
concentration and purification of the orange residue by flash
column chromatography (1:4 EA/PE) gave an orange solid. The
yields were 42 % for ligand S. [12, 17]
characterized
by
spectroscopic,
photo-physical
and
electrochemical techniques. The effect of the steric features of the
N^N ligands were investigated by varying substituents in the 2,9
and 4,7 positions. Interestingly, the 2,9 positions are very
sensitive to steric bulk, and electron withdrawing groups in this
position increased the productivity. For 4,7-positions phenyl
groups are of fundamental importance. Analysis of the photo-
physical properties revealed a strong emission in solution. The
copper PS exhibited a significant hydrogen generation ability if
applied together with an iron-based WRC. A maximum turnover
number (TONCu) of 1388 was achieved in this noble metal free
system.
Experimental Section
Materials and Synthesis. All reagents and solvents were
commercially available and used as received without further
purification.
Synthesis of ligand T. Using the same method of synthesis of
ligand S, the yield of ligand T was 34 %.
Photocatalytic proton
reduction.
A
double-walled
Synthesis of ligands B-E. N^N ligands B-E were synthesized
according to a literature procedure.[12]
thermostatically controlled reaction vessel is evacuated and
purged with argon. The N^N ligands, P^P ligands, Cu(MeCN)4PF6
and [Fe3(CO)12] are added as solids. The corresponding solvent
mixture (THF/TEA/H2O, 4:3:1) is added and the system is taken
to 25 °C before switching on the light source. It is then stirred at
25 °C until no further gas evolution is observed. All given values
are the averages of at least two experiments. The results differ
between 1 and 19 % except for volumes < 10 mL (up to 45 %).
Synthesis of ligand F. A stirred solution containing 2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (0.5 g, 1.4
mmol) and selenium dioxide (0.375 g, 3.4 mmol) in 10 mL of
dioxane was heated at 80 °C in a sealed tube under an argon
atmosphere for 2 h. After the reaction was finished the, mixture
turned dark orange. Filtration, concentration and purification of
the orange organic phase by crystallization at 4 °C gave yellow
solid, yield 85 %.[13]
Synthesis of ligand G. A mixture containing ligand F (0.2 g,
0.52mmol) and 10 mL of hydrogen peroxide was stirred at 80 °C
for 8 h. Filtration, concentration and purification of the orange
organic phase by crystallization gave yellow solid, yield 80 %.[13]
Photoluminescence Spectroscopy. General Methods.
Absorption spectra were recorded using a 50 Conc (VARIAN) UV-
vis spectrometer. Luminescence spectra were measured with a
spectrofluorometer
(F-4600,
5J2-004,
HITACHI).
The
Synthesis of ligand H. A stirred mixture of ligand F (0.2 g,
0.52mmol), p-toluene sulfonic acid, 10 mL ethanol and 15 mL the
triethylorthoacetate were refluxed at 100 °C for 8 h under an argon
atmosphere. Filtration, concentration and purification of the
orange residue by flash column chromatography (1:4 EA/PE) give
light yellow solid, yield 87 %.[14]
luminescence quantum yields were determined relative to quinine
bisulfate (Aladdin. Co) in 0.1 M sulfuric acid (Aladdin. Co) as
quantum yield standard (Φ = 0.53).
Electrochemical Measurements. General Methods. All
studies were performed at room temperature in dried THF p. A.
(VWR) under Argon atmosphere with 0.1 M Tetrabutylammonium
hexafluorophosphate ([Bu4N]PF6) as conducting salt using an
Autolab (PGSTAT 128N, Metrohm). Working electrode was a
glassy carbon disk electrode (d=2 mm), the counter electrode a
Pt-electrode and the reference electrode an Ag/AgNO3 sat. All the
potentials were measured with regard to this reference system
and were checked by using the ferrocen/ferrocenium internal
reference system. The cyclic voltammetry scans were done two
times at a scan rate of 100 mVs-1. Concentrations of 0.1 mM
analyte were used for the measurements.
Synthesis of ligand I-O. The N^N ligands (I-O) were
synthesized according to a literature procedure. [15] A mixture of
BCP and arylcarboxaldehyde or thiophene-2-carbaldehyde (6
equiv) in acetic anhydride was refluxed for 5-16 h. The cooled
reaction mixture was poured into menthol/water to give a solid.
The solid precipitate was filtered off, solved in dichloromenthane
and extracted with water in a separating funnel. The organic
phase was washed with deionized water, dried over anhydrous
magnesium sulphate and evaporated. The resulting yellow and
brown solids were found to be stable under air and moisture and
were obtained in moderate yields (50-68 %).
Synthesis of ligand P and ligand Q. The N^N ligands P and Q
were synthesized according to a literature procedure.[16]
Synthesis of ligand R. A mixture of 4,7-di([1,1'-biphenyl]-4-yl)-
1,10-phenanthroline (0.2 g, 0.41 mmol) and 5 mL of dry ether was
stirred in a sealed tube under an argon atmosphere, and isopropyl
lithium (1.3 mL, 2.05 mmol) was added at 0 °C. The resulting
suspension was further stirred at o °C for 30 min then at room
ACKNOWLEDGMENT
The authors would like to thank Prof. Shan-Zhong Luo, Prof. Ai-
Min Ren and Prof. Lu-Yi Zou for helpful discussions, and Prof.
Mei-Chao Li for help with electrochemical measurements. The
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10.1002/chem.201602598
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[11] A. J. J. Lennox, S. Fischer, M. Jurrat, S. P. Luo, N. Rockstroh, H. Junge,
R. Ludwig, M. Beller. Chem. Eur. J. 2016, 22(4), 1233-1238.
authors acknowledge the National Nature Science Foundation of
China (No.21376222, No.21361130021 and No.51201152) for
financial support, and A.J.J.L. would like the thank the Alexander
von Humboldt foundation for generous funding.
[12] M. Schmittel, C. Michel, S. X. Liu, D. Schildbach, D. Fenske. Eur. J. Inorg.
Chem. 2001, 1155-1166.
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6661-6670.
Keywords：water reduction • photosensitizers • copper • 1,10-
phenanthroline derivatives • Hydrogen production
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10.1002/chem.201602598
Chemistry - A European Journal
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Layout 2:
FULL PAPER
NanYu Chen,[a] Liang-Min Xia,[a] Alastair
J. J. Lennox,[b] Yuan-Yuan Sun,[a] Hao
Chen,[a] Hai-Ming Jin,[a] Henrik Junge,[b]
Qin-An Wu,[a] Jian-Hong Jia,[a] Matthias
Beller,*[b] and Shu-Ping Luo*[a]
Page No. – Page No.
A series of phenanthroline-based ligands have been synthesized and their heteroleptic
[Cu(P^P)(N^N)]+ photosensitizers in light driven Cu-Fe-based water reduction has been
studied. In this noble-metal free system, the structural effects of these N ligands have
been explored, including the steric and electronic effects in the 2, 9 and 4, 7 substituent
positions of phenanthroline. This report provides the first demonstration that the fluorine
effect in N ligand increased the copper. photocatalytic hydrogen product ability.
Structure activated copper
photosensitizers for photocatalytic
water reduction
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