Photoinduced catalytic reaction by a fluorescent active cryptand containing an anthracene fragment

Article abstract of DOI:10.1002/anie.201004928

Tripping the light fantastic: A fluorescently active cryptand containing an anthracene fragment catalyzes the photoinduced reduction of Cu^II to Cu^I with simultaneous oxidation of water to oxygen (see scheme).

Full text of DOI:10.1002/anie.201004928

Communications
DOI: 10.1002/anie.201004928
Photoreduction
Photoinduced Catalytic Reaction by a Fluorescent Active Cryptand
Containing an Anthracene Fragment**
Hong-Guo Hao, Xiao-Dan Zheng, and Tong-Bu Lu*
The use of sunlight to split water into H2 and O2 attracts great
scientific interest[1] for two reasons: the urgent need for clean
and renewable fuel to avoid greenhouse gas emissions and for
artificial photosynthesis. For achieving such a purpose, a good
catalyst capable of driving photoinduced electron transfer
(PET) and converting solar energy into chemical energy is
essential. Although many fluorescent active compounds
exhibit PET, reports in which PET is followed by chemical
transformation are much less common,[2] and most of the
photocatalytic systems employ noble-metal complexes as
catalyst and chromophore, and amine such as triethylamine
(TEA) or triethanol amine (TEOA) as a sacrificial reductant.
Many organic ligands containing an anthracene fragment as a
chromophore have been synthesized and used for ion
recognition[3] and fluorescent switches,[4] based on the fluo-
rescent and PET properties of the anthracene fragment. For
example, Fabbrizzi and co-workers found[4e,f] that alternate
oxidation and reduction at a copper center resulted in turning
the light emission of a anthracene fragment off and on in a
complex in which the anthracene fragment is covalently
linked to a copper center with a [14]ane-S4 macrocyclic
receptor. The switching off was caused by electron transfer
from the *AnCT excited state (An = anthracene moiety; CT=
charge transfer) to the CuII center. However, to the best of our
knowledge, the use of organic ligands containing an anthra-
cene fragment as a chromophore to catalyze a photoinduced
chemical reaction has not been reported so far.
irradiation time, sacrificial electron donor, and water content
in the reaction system were investigated. The most surprising
discovery was that water in the reaction system served as a
sacrificial electron donor and was oxidized to oxygen. The
reaction kinetics and potential mechanism are given based on
the experimental results.
The cryptand L was first designed by Fabbrizzi et al. for
anion sensing in aqueous solution.[5] We previously demon-
strated[6] that the C C bond of nitriles can be cleaved by
ꢀ
dinuclear metal cryptates [M2L1](ClO4)4 (M = CuII, ZnII) at
room temperature through an SN2 pathway to generate
cyanido-bridged complexes [M2L1(CN)](ClO4)3 and alcohol.
Following up our previous studies on the recognition and
activation of small guest molecules by cryptates and proton-
ated cryptands,[6,7] we tried to synthesize the dinuclear CuII
cryptate [Cu2L](ClO4)4 by the reaction of cryptand L with
Cu(ClO4)2·6H2O in acetonitrile under ambient condition.
Unexpectedly, crystals of [Cu(MeCN)4](ClO4) (1) were
obtained.
The result of X-ray crystallographic analysis reveals that
the structure of 1 is similar to the reported structure of
[Cu(MeCN)4](CF3SO3),[8] in which the tetrahedral CuI center
is coordinated to four acetonitrile molecules (Figure 1a). The
ꢀ
Cu N distances are close to the reported values for [Cu-
(MeCN)4](CF3SO3). However, when triethylamine was added
to the above acetonitrile solution containing cryptand L and
Cu(ClO4)2·6H2O, only the green crystals of [Cu2L(OH)-
(H2O)](ClO4)3·MeCN (2) were obtained, and no crystals of 1
were isolated from the solution. The result of XPS measure-
ment indicates that all the copper present in the solution
containing triethylamine is CuII, and no CuI formed (Fig-
ure S1, Supporting Information). In 2, the cryptand L
encapsulates two CuII ions at its two poles, and the axial
positions of the two CuII centers are occupied by one OHꢀ ion
and one water molecule, respectively (Figure 1b). Further
experimental results demonstrate that compounds 1 and 2 can
only be obtained under acidic and basic conditions, respec-
tively, and the acid in the mixture of cryptand L and
Cu(ClO4)2·6H2O in acetonitrile comes from the excess of
HClO4 in Cu(ClO4)2·6H2O. Moreover, compound 1 cannot be
obtained in the dark even under the acidic conditions, and
compound 1 cannot be obtained either when cryptand L1 was
treated with Cu(ClO4)2·6H2O under acidic conditions. The
above experimental results clearly demonstrate that the
reduction from CuII to CuI in acetonitrile is a photoinduced
catalytic reaction associated to anthracene fragment in L, and
the reduction can only be achieved under acidic conditions.
As observed by Fabbrizzi et al.,[5] the fluorescence inten-
sities of L is pH-dependent (Figure S2), whereby full anthra-
cene fluorescence is observed at low pH values, and the
We herein report a novel photocatalytic reductive reac-
tion of CuII to CuI in acetonitrile by a fluorescently active
cryptand L containing an anthracene fragment. Parameters
that affect the photocatalytic reductive reaction, such as
[*] H.-G. Hao, X.-D. Zheng, Prof. T.-B. Lu
MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key
Laboratory of Optoelectronic Materials and Technologies, School of
Chemistry & Chemical Engineering, Sun Yat-Sen University,
Guangzhou 510275 (China)
Fax: (+86)20-8411-2921
E-mail: lutongbu@mail.sysu.edu.cn
[**] This work was supported by the NSFC (Nos. 20625103, 20831005,
and 20821001), and the 973 Program of China (2007CB815305).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004928.
8148
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Figure 2. UV/Vis spectra of H6L(ClO4)6 and Cu(ClO4)2·6H2O (1:2) in
acetonitrile as a function of irradiation time; irradiation was performed
with a high-pressure mercury lamp (inset: plot of absorbance at
l=775 nm versus irradiation time).
ure S4), further demonstrating that the reduction from CuII to
CuI is a photoinduced catalytic reaction. Indeed, one mole of
H6L(ClO4)6 can photocatalyze over 70 equivalents of Cu-
(ClO4)2·6H2O to CuI, as monitored by the electronic absorp-
tion spectra.
The photoreduction of CuII to CuI requires a sacrificial
reductant, and the absence of a deliberately added sacrificial
reductant in the above reaction system was unexpected.
Given the composition of the reaction system, a possible
sacrificial reductant is acetonitrile, the hexaprotonated crypt-
and [H6L]6+, or water present in the acetonitrile. The
oxidation of acetonitrile was ruled out owing to its well-
known chemical stability with a potential window in excess of
6 V.[9] It seems the hexaprotonated cryptand [H6L]6+ may
serve as a sacrificial reductant. However, [H6L]6+ survives the
photoinduced catalytic reaction of H6L(ClO4)6 and Cu-
(ClO4)2·6H2O (1:70) in acetonitrile, as demonstrated by
ESIMS measurements (Figure S5), and one mole of [H6L]6+
cannot reduce 70 equivalents of CuII if [H6L]6+ served as a
sacrificial reductant. Thus, the oxidation of H6L(ClO4)6 is also
highly unlikely.
Figure 1. a) Structure of [Cu(MeCN)4]+ cation in 1. b) Structure of
[Cu2L(OH)(H2O)]3+ cation in 2.
fluorescence intensities decrease in the pH range 4–8 and then
reduces to almost zero at high pH values. The reducing
property is attributed to electron transfer from the neutral
amine groups to the proximate excited anthracene fragment
*An, thereby quenching its fluorescence.[5] Thus, the photo-
reduction of CuII to CuI can only be achieved under acidic
conditions, and dinuclear CuII compound of 2 was formed
under basic conditions. Although the coordination of CuII to
the nitrogen atoms of L prevent the electron transfer from
nitrogen to *An, the bonded CuII center in 2 cannot be
photoreduced either, as the encapsulated CuII can also quench
its fluorescence through an electron transfer mechanism.[5]
However, under the acidic conditions, the CuII center does not
directly bond to the protonated cryptand [H6L]6+, thus the
anthracene fluorescence is not quenched (Figure S2).
After ruling out the species mentioned above, we
considered that the sacrificial reductant in the reaction
system is mostly water, and the photoreduction may follow
the pathway given in Equations (1)–(3).
To monitor the photoreduction process of CuII, a solution
containing H6L(ClO4)6 (0.01 molLꢀ1) and two equivalents of
Cu(ClO4)2·6H2O in acetonitrile was irradiated with a high-
pressure mercury lamp (450 W) at 308C, and electronic
absorption spectra were recorded every 15 min. As shown in
Figure 2, the intensities of initial peak at l = 775 nm assigned
to the d–d transfer of CuII decreased along with increasing
irradiation time, and the peak disappeared after 6 h irradi-
ation, and the color of solution changed from green to yellow
after irradiation. The resulting yellow acetonitrile solution
was dried under vacuum to give a yellow powder, and the
results of XPS measurements indicate that all the copper in
the yellow powder is CuI, demonstrating all CuII centers in
acetonitrile were photoreduced to CuI (Figure S3). However,
the intensities of the peak at l = 775 nm remained almost
constant when the solution was left in dark for 16 h (Fig-
hn
*
An
An
ð1Þ
ð2Þ
ð3Þ
ƒ!
CuII
þ
An ! Cu þ An
I
þ
*
Anþ þ = H2O ! An þ = O2 þ Hþ
1
1
2
4
In this process, an electron is firstly exited from the
ground state of the An fragment in [H6L]6+ to its exited state
*An under the irradiation with light [Eq. (1)]. The electron in
the exited-state *An moiety is then transferred to CuII to
generate CuI and a An+ cation [Eq. (2)]. Finally, the An+
cation gains an electron from water to form An, and water is
oxidized to oxygen [Eq. (3)]. The overall photoinduced
catalytic reaction is shown in Equation (4).
Angew. Chem. Int. Ed. 2010, 49, 8148 –8151
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8149

Communications
hn
1
1
CuII þ = H O
CuI þ = O þ Hþ
Table 1: Observed rate constants (kobs) at various concentrations of
water.
ð4Þ
ƒ!
2
2
4
2
An
[H2O] [molLꢀ1
obs [ꢀ103]
]
1.11 1.67 2.22 3.33 4.44 5.56 6.67 7.78
8.6 9.1 10.1 10.8 11.8 12.8 14.0 14.8
If the above mechanism is correct, the reaction rate should
be dependent on the concentration of water. Indeed, the
reaction became much faster when water was added to the
reaction system. The observed rate constants at different
concentrations of water were determined. The rate law can be
described as ln(A0/At) = kobs t, where A0 and At are the
absorbance values of CuII at l = 775 nm at the irradiation
time of zero and t min, respectively, and kobs = k2[H2O]a. The
plot of ln(A0/At) versus time reveals a line with a slope of kobs
(Figure 3a). The values of kobs at different concentrations of
water are given in Table 1, and a plot of logkobs versus
log[H2O] is shown in Figure 3b. From Table 1, it can be seen
k
from those in the presence of water (Figure S6), in which the
peak around l = 736 nm (assigned to the d–d transfer of
anhydrous CuII) slowly disappeared and a new peak around
l = 495 nm appeared, and its intensity increased with increas-
ing irradiation time. After irradiation for 6 h, acetonitrile in
the purple solution was removed under vacuum to give a
purple powder, XPS measurement on which indicate that
84% of copper present in the sample is still CuII (Figure S7a);
the reduction of 16% CuII to CuI is attributed to failure to
remove water completely. When water was added to the
purple solution and it was irradiated again, the color changed
from purple to yellow, and the results of XPS measurements
indicate that all CuII in the solution was reduced to CuI after
the addition of water (Figure S7b). The above experimental
results clearly demonstrate that the sacrificial reductant in the
reaction system is water.
To detect the oxygen molecules generated during the
reaction, a solution (5 mL) containing H6L(ClO4)6 (0.5 mmol)
and Cu(CF3SO3)2 (2.5 mmol) in acetonitrile was degassed
with nitrogen for 5 min, and 0.1 mL of H218O was added. The
resulting solution was irradiated with a high-pressure mercury
lamp (450 W) at 308C. The generated 18O2 was detected by
GC–MS analysis. As shown in Figure S8 in the Supporting
Information, the ratio of 18O2/16O2 was 0.005, 0.211, and 21
after irradiation for 10 min, 1.5 h, and 4 h, respectively. This
result clearly demonstrates that 18O2 was generated during the
irradiation reaction.
It is interesting to note that the photoinduced catalytic
reaction did not occur when H6L(ClO4)6 was replaced by
anthracene or 9,10-bis(chloromethyl)anthracene. We con-
sider that H6L6+ may capture water molecules into its cavity,
and that this arrangement is beneficial for the oxidation of
water into oxygen. The detail mechanism is being investi-
gated.
In conclusion, we demonstrated herein for the first time a
photoinduced reduction of CuII to CuI by a fluorescently
active cryptand containing an anthracene fragment. The most
striking feature of the system is that the water in the reaction
system serves as a sacrificial electron donor and was oxidized
to oxygen. Usually, CuI is easy to oxidize by oxygen in the
open air; thus, the photoinduced reduction of CuII by water to
produce CuI and oxygen is unique. The fluorescently active
cryptand [H6L]6+ containing an anthracene fragment is
potentially useful as a new generation of catalyst to split
water to produce hydrogen as a cheap energy source, while
avoiding greenhouse gas emissions. Research to this end is
continuing.
Figure 3. a) Determination of the observed rate constant for the
reaction of H6L(ClO4)6 and two equivalents of Cu(ClO4)2·6H2O.
b) Determination of the second-order rate constants (k2) for different
concentrations of water.
that the observed rate constants become larger with increas-
ing concentration of water. The calculated second-order rate
constant (k2) and a value are 7.98 ꢀ 10ꢀ3 mꢀ1 sꢀ1 and 0.289,
respectively.
In contrast, the above photoinduced reduction should not
take place in the absence of water. Thus, an anhydrous
solution of acetonitrile containing H6L(ClO4)6 and anhydrous
Cu(CF3SO3)2 (1:2) was prepared and irradiated with a high-
pressure mercury lamp (450 W). The color of the solution
slowly changed from yellow-green to purple rather than
yellow with increasing irradiation time. The electronic
absorption spectra in the absence of water are different
Experimental Section
The cryptand L was prepared according to the literature method.[5]
H6L(ClO4)6 was obtained by adding the cryptand L into a solution
containing excess of HClO4 in acetone. Yield: 86%; elemental
analysis: calcd for C44H64N8Cl6O24 (L·6HClO4): C 40.60, H 4.96, N
8150
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8.61%; found: C 40.30, H 5.02, N 8.56%. MS (ESI): m/z 350.7
Keywords: copper · cryptands · photochemistry · reduction ·
water splitting
.
([H2L]2+); 700 ([HL]+); 799 (([HL(ClO4)]+).
Synthesis of [Cu(CH3CN)4](ClO4) (1):
A solution of Cu-
(ClO4)2·6H2O (0.074 g, 0.2 mmol) in acetonitrile (5 mL) was added
dropwise to a solution of H6L(ClO4)6 (0.130 g, 0.1 mmol) in acetoni-
trile (8 mL). The resulting green solution was stirred at room
temperature for 0.5 h, then filtered off and evaporated slowly at
room temperature in the open air. Two weeks later, colorless crystals
of 1 were obtained. Yield: 80%; elemental analysis: calcd for
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C8H12N4CuClO4: C 29.40, H 3.70, N 17.12%; found: C 29.10, H
ꢀ1
~
3.74, N 17.20%. IR (KBr): n = 3290 (s), 1423 (s), 1084 (s), 627 cm
(s).
Synthesis of [Cu2L(OH)(H2O)](ClO4)3 (2): A solution of Cu-
(ClO4)2·6H2O (0.078 g, 0.21 mmol) in acetonitrile (5 mL) was added
dropwise to a solution of L (0.070 g, 0.1 mmol) and triethylamine
(0.1 mL) in acetonitrile (8 mL). The resulting green solution was
stirred at room temperature for 0.5 h, then filtered off and evaporated
slowly at room temperature in the open air to give green crystals of
2·CH3CN·0.5H2O. Yield: 90%; elemental analysis: calcd for
C44H62N8Cu2Cl3O14.5 (2·0.5H2O): C 45.23, H 5.35, N 9.59%; found:
~
C 45.02, H 5.30, N 9.49%. IR (KBr, ): n = 3509 (s), 3725 (m), 2930 (w),
2880 (w), 1594 (m), 1448 (m), 1285 (w), 933 (w), 811 (w), 768 (w),
696(w), 624 cmꢀ1 (m).
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Single-crystal X-ray crystallography: The single-crystal data of 1
and 2 were collected at 153 and 123 K respectively on a Oxford
Gemini S Ultra diffractometer with CuKa radiation (l = 1.54178 ꢁ).
The structures were solved by direct methods and refined by full-
matrix least-squares analysis on F2 using the SHELX97 program
package. All non-hydrogen atoms were refined anisotropically. The
hydrogen atoms in disordered acetonitrile and water molecules in 2
were not included.
Crystal data of 1: C8H12N4CuClO4, Mr = 327.22, orthorhombic,
space group Pna2(1), a = 23.9402(3), b = 8.3507(1), c = 20.4229(3) ꢁ,
V= 4082.89(9) ꢁ3, Z = 12, 1calcd = 1.597 Mgmꢀ3, m = 4.216 mmꢀ1, F-
(000) = 1992. A total of 13804 reflections collected, 5672 independent
reflections (Rint = 0.0216) with 5259 [I > 2s(I)] observed data, 500
parameters, R1 = 0.0267, wR2 = 0.0688 [I > 2s(I)] and R1 = 0.0293,
wR2 = 0.0701 (all data) with S = 1.021. 2·CH3CN·0.5H2O:
C46H65N9Cu2Cl3O14.5, Mr = 1209.51, monoclinic, space group P21/n,
a = 13.7294(9), b = 23.3674(16), c = 16.1515(11) ꢁ, b = 91.842(6)8,
, , F-
V= 5179.1(6) ꢁ3, Z = 4, 1calcd = 1.551 Mgmꢀ3 m = 3.074 mmꢀ1
(000) = 2516. A total of 15418 reflections collected, 8092 independent
reflections (Rint = 0.0345) with 5007 [I > 2s(I)] observed data, 704
parameters, R1 = 0.0587, wR2 = 0.1517 [I > 2s(I)] and R1 = 0.0926,
wR2 = 0.1659 (all data) with S = 0.915. CCDC 772755 (1) and 772756
(2) contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
UV/Vis spectra were recorded on Shimadzu UV-3150 and
Cary 50 spectrophotometers. The XPS data were collected on an
ESCA Lab250 instrument. Fluorescent spectra were measured using
an Edinburgh FLS920 fluorescence spectrometer. ESIMS analysis
was performed on a Thermo Finigan LCQDECA XP HPLC-MSn
mass spectrometer. Elemental analyses were determined using an
Elementar Vario EL elemental analyzer. The IR spectra were
recorded in the 4000–400 cmꢀ1 region in the form of KBr pellets
using a Bruker EQUINOX 55 spectrometer. The detection of the
oxygen molecule generated during the irradiation was performed
using an Agilent 7890 A (GC)/5975 (MS) instrument with a DB-5MS
column under a He atmosphere.
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Received: August 7, 2010
Published online: September 20, 2010
Angew. Chem. Int. Ed. 2010, 49, 8148 –8151
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8151