Novel porphyrin-phthalocyanine heterodimers and heteropentamers: Synthesis, characterization and application in organic solar cells

Article abstract of DOI:10.1039/c3ra41058g

Two novel types of covalently ether-linked porphyrin-phthalocyanine (Por-Pc), heterodimer and heteropentamer, connected through the meso phenyl group of the porphyrin have been synthesized for the first time. The Por-Pc heterodimer and heteropentamer have more delocalized π-electrons and therefore their conjugated degree is higher. In addition, these compounds possess better solubility and a wider light absorption than typical single porphyrins and phthalocyanines. Solar cell devices based on as-synthesized porphyrin-phthalocyanine heterodimers and heteropentamers as donor materials and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor material are prepared and show a noticeably better performance. The suitable ZnPor-(O-ZnPc)₄:PCBM weight ratio for photovoltaic devices is 4:1 and the highest photovoltaic properties are achieved based on ZnPor-(O-ZnPc) ₄:PCBM (4:1) annealed at 140°C in a vacuum for 20 min. The highest PCE is 2.08.

Full text of DOI:10.1039/c3ra41058g

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Novel porphyrin–phthalocyanine heterodimers and
heteropentamers: synthesis, characterization and
application in organic solar cells3
Cite this: DOI: 10.1039/c3ra41058g
ab
a
a
a
a
Tianhui Zhang, Min Liu, Qingdao Zeng, Zhijiao Wu, Lingyu Piao*
b
and Suling Zhao*
Two novel types of covalently ether-linked porphyrin–phthalocyanine (Por–Pc), heterodimer and
heteropentamer, connected through the meso phenyl group of the porphyrin have been synthesized
for the first time. The Por–Pc heterodimer and heteropentamer have more delocalized p-electrons and
therefore their conjugated degree is higher. In addition, these compounds possess better solubility and a
wider light absorption than typical single porphyrins and phthalocyanines. Solar cell devices based on as-
synthesized porphyrin–phthalocyanine heterodimers and heteropentamers as donor materials and [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor material are prepared and show a noticeably
Received 4th March 2013,
Accepted 9th May 2013
4
better performance. The suitable ZnPor–(O–ZnPc) : PCBM weight ratio for photovoltaic devices is 4 : 1
DOI: 10.1039/c3ra41058g
4
and the highest photovoltaic properties are achieved based on ZnPor–(O–ZnPc) : PCBM (4 : 1) annealed
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at 140 uC in a vacuum for 20 min. The highest PCE is 2.08.
Organic solar cells have attracted wide interests due to their
potential properties of light-weight, flexibility, low-cost and
coatings applicable to large areas. In organic solar cells,
require expensive equipments, which result in difficulties for
large scale applications. As another kind of typical molecule
used in organic solar cell, porphyrins (Pors) have a good
absorption in the region of 400–600 nm and better dissolubi-
lity in organic solvents. Considering the respective advantages
of these two kinds of materials, we have deduced that the
conjugates of Pors and Pcs will have potential applications as
donor materials in the field of organic photovoltaics. These
conjugates should promote absorptions in the visible light
region and solubility in organic solvents. Moreover, since the
Q-band absorption of Pcs almost overlaps with the fluores-
cence wavelength of Pors, the excitation and the singlet-state
intramolecular energy transfer from Pors to Pcs can be
suppressed greatly in Por–Pc dimers. So, these conjugates will
become a good candidate in molecular photonics, catalysis
1
polymer solar cells still suffer from some shortcomings, such
as batch-to-batch variation, indefinite molecular weight,
polydispersity and impurity. In contrast, small-molecule
materials intrinsically do not have such drawbacks.
Furthermore, small molecule organic materials could exhibit
a higher charge carrier mobility, as well as tuning more easily
their band structure to absorb sunlight efficiently. Notably,
phthalocyanines (Pcs), as a typical small organic material
molecule used in organic solar cells, have attracted extensive
attention in recent years. Pcs have highly delocalized
p-electron systems and show unique electronic and optical
2
–4
properties.
Consequently, they are very important in the
9
field of photovoltaics and molecular photonics as the p-type
and light harvesting architectures. However, few studies
about the synthesis and application in organic solar cells of
Por–Pc compounds have been reported until now.
5
–8
photoconductors.
However, Pcs just have the Soret-band
(y340 nm) and Q-band (y670 nm). The light absorption of
Pcs in the range of 400–600 nm, corresponding to the
strongest intensity in the sunlight, is very weak. The
dissolubility of films of unsubstituted Pcs is relatively lower.
Further, the film fabrication processes are complicated and
In this paper, we have designed and synthesized two novel
Por–Pc heterodimer and heteropentamer compounds. The
obtained novel Por–Pc compounds have more delocalized
p-electrons. The properties of the solar cells based on these
two novel compounds indicate that these novel compounds
have a noticeably better performance than single porphyrin
and phthalocyanine.
a
National Center for Nanoscience and Technology, Beijing, 100190, China.
E-mail: piaoly@nanoctr.cn; Fax: 010 8254 5653; Tel: 010 8254 5653
b
Key Laboratory of Luminescence and Optical Information, Ministry of Education,
Two different synthetic approaches have been followed for
the synthesis of the dimer (ZnPor–(O–ZnPc)) and pentamer
Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing, 100044,
China. E-mail: slzhao@bjtu.edu.cn
(ZnPor–(O–ZnPc)
4
). The synthetic routes for the dimer (ZnPor–
3
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3ra41058g
(O–ZnPc)) and pentamer (ZnPor–(O–ZnPc)
4
) are shown in
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Under
2 4
N protection, pentamer (ZnPor–(O–ZnPc) ) is
synthesized through the condensation of the excess
O-benzenedinitrile and TDPP. O-Dichlorobenzene and
N,N-dimethylaminoethanol (DMAE) are the solvents and zinc
chloride is the template in this reaction. After a 24 h reflux
reaction and purification by column chromatography, the
pentamer (ZnPor–(O–ZnPc)
4
) is primarily obtained with a
10.8% yield. The low yield is partly due to wastage in the
purification procedure. The mass spectrum revealed a mole-
+
cular ion peak at 3045.86 m/z [M] . The calculated molecular
mass of the pentamer (ZnPor–(O–ZnPc)
4
) should be 3045.79 g
mol . The result indicates that the obtained (ZnPor–(O–
ZnPc) ) sample has the same molecular mass as the calculated
2
1
4
molecular mass. All the analysis data of the (ZnPor–(O–ZnPc)4)
sample, such as IR, UV-vis absorption spectrum, HNMR and
3
the mass spectrum can be seen in the ESI, Fig. S6–S9. The
1
results that come from IR, UV-vis absorption, HNMR and the
mass spectrum confirm that the ZnPor–(O–ZnPc) has the
4
expected molecular structure. These two novel compounds are
the covalently ether-linked Por–Pc heterodimer and hetero-
pentamer (see Fig. S1b and S1c, ESI ). We have confirmed that
3
the two novel compounds are synthesized and that they have
the structures as we designed them.
The ball-and-stick models of ZnPor–(O–ZnPc) and ZnPor–
4
(O–ZnPc) can be seen in Fig. 1a and 1b. Two macrocycles of
the heterodimers are linked through an ether bond. The ends
of the ether bond are the meso-phenyl groups of the Por and
phenyl groups of the Pc, respectively. The heteropentamer is
composed of four ZnPc molecules and one ZnPor molecule.
The ZnPor molecule is linked with four ZnPc molecules by four
ether bonds and located in the centre of these four ZnPc
molecules. The oxygen atom, which links two macrocycles,
plays the role of ‘‘active’’ spacer.
Scheme 1 Syntheses of the Por–Pcs heterodimer and heteropentamer.
Scheme 1. Initially, meso-tetra(4-hydroxylphenl) porphyrin
1
0
(
THPP) is prepared as indicated by the literature. The
meso-tetra [4-(3, 4-dicyanophenoxy) phenyl]-porphyrin (TDPP)
is synthesized by a nitro-group displacement reaction (mole-
Fig. 1c shows the UV-vis absorption spectrum of dimer
cular structure of TDPP is shown in the ESI,
3
Fig. S1a). The
4
(ZnPor–(O–ZnPc)) and pentamer (ZnPor–(O–ZnPc) ) in DMF
yield of the reaction is about 92.8%. Most of the reagent THPP
is converted to product and the lost yield is attributed to
wastage in the purification procedure. The high yield may
benefit from the increased nucleophilic nature of the attacking
oxygen on the Por species.
compared with those of zinc meso-tetraphenylporphyrin
(ZnTPP) and ZnPc. The strong absorptions at 415 nm and
670 nm are assigned to the Soret band of Por and Q band of
1
0,11
Pc, respectively.
The absorption spectrum of the ZnPor–
show that they have the
(O–ZnPc) and ZnPor–(O–ZnPc)
4
The synthesis of dimer (ZnPor–(O–ZnPc)) is through the
direct condensation of TDPP with subphthalocyanine (SubPc)
characteristic absorptions of the individual two macrocyclic
components at the same time. Compared with the absorption
of ZnTPP and ZnPc, it is known that the absorptions around
(Scheme 1). The condensation reaction is carried out under the
conditions of N protection and zinc chloride as the template.
340 nm and 430 nm of ZnPor–(O–ZnPc) and ZnPor–(O–ZnPc)
4
2
The yield of the reaction is around 33.4%. The mass spectrum
reveals a molecular ion peak at 1697.34 m/z [M] . The
come from the Soret band of Por and Pc respectively, and the
absorptions about 600 nm and 670 nm come from the Q band
of Pc. The absorption at 638 nm for ZnPor–(O–ZnPc) and
+
calculated molecular mass of the dimer (ZnPor–(O–ZnPc))
2
1
should be 1697.38 g mol . The result indicates that the
obtained (ZnPor–(O–ZnPc)) sample has the same molecular
mass as the calculated molecular mass. All analysis data of the
ZnPor–(O–ZnPc) is a new absorption peak compared with that
4
of Por and Pc. The UV-vis absorption spectrum confirms that
the formation of a Pc ring according to the appearance of a Pc
Q-band (670 nm). But for ZnPor–(O–ZnPc) and ZnPor–(O–
(
ZnPor–(O–ZnPc)) sample such as IR, UV-vis absorption
1
spectrum, HNMR and mass spectrum can be seen in the
ESI, Fig. S2–S5. The results that come from IR, UV-vis
absorption, HNMR and the mass spectrum confirm that the
ZnPor–(O–ZnPc)) has the expected molecular structure. The
ZnPc)
from 415 nm to 426 nm of ZnPor–(O–ZnPc) and 415 nm to 428
nm for ZnPor–(O–ZnPc) are detected, respectively. The red
shifts of the Soret bands in ZnPor–(O–ZnPc) and ZnPor–(O–
4
, the remarkable red shifts of the Soret band absorption
3
1
4
(
detailed analysis of these results can be seen in the ESI.
3
ZnPc)
4
suggest that the Pc substituent possesses an electron-
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Fig. 1 (a, b) Ball-and-stick models of the Por–Pc ZnPor–(O–ZnPc) and ZnPor–(O–ZnPc)
4
, (c) UV-vis absorption spectrum of ZnPor–(O–ZnPc) and ZnPor–(O–ZnPc)
4
in
DMF, ZnTPP and ZnPc, (d) UV-vis absorption spectra of PCBM, ZnPor–(O–ZnPc), ZnPor–(O–ZnPc)
).
4
, ZnPor–(O–ZnPc) : PCBM (film 1) and ZnPor–(O–ZnPc) : PCBM (film
4
2
donor character and help to enhance the absorption in the
visible light region. This results in a reduction of their HOMO–
LUMO gap. As a consequence, the electron-donating periph-
eral substituent on the Por complexes gives rise to the red
shifts in the Q absorption. The relative intensity of the Q
band absorption relative to that of the Soret band is higher for
ZnPc moieties compared with ZnPor–(O–ZnPc) and ZnPor–(O–
4
PCBM molecules. Therefore ZnPor–(O–ZnPc) and ZnPor–(O–
ZnPc) can be used as electron donors and PCBM as an electron
acceptor to fabricate solar cells. Fig. 1d shows the UV-vis
absorption spectra of the films of ZnPor–(O–ZnPc), ZnPor–(O–
ZnPc) , PCBM and the active films of ZnPor–(O–ZnPc) : PCBM
1
2
4
4
(film 1) and ZnPor–(O–ZnPc) : PCBM (film 2). It is found that
the absorption of active films is the liner superposition of the
absorption of the donor and acceptor. This demonstrates that
there is no interaction between the ground state of the donor
molecules and acceptor molecules. Compared with the
ZnPc)
4
. There is almost no shift in the Q-band position (670
. This
nm) for ZnPc, ZnPor–(O–ZnPc) and ZnPor–(O–ZnPc)
4
result excludes the possibility of a charge transfer interaction
in the ground states of ZnPor–(O–ZnPc) and ZnPor–(O–
absorption spectra of ZnPor–(O–ZnPc) and ZnPor–(O–ZnPc)
4
1
3
ZnPc)
4
.
The results of the UV-vis spectra indicate that
in solution, the UV absorption of the films has a red shift
which indicates a strong interaction between molecules in the
solid state. Fig. 1d also shows that the absorption range of the
ZnPor–(O–ZnPc) and ZnPor–(O–ZnPc) have a broader spectral
4
coverage in the visible region, as we expected.
The energy levels of the pentamer (ZnPor–(O–ZnPc) ) and
ZnPor–(O–ZnPc) : PCBM film is wider than that of ZnPor–(O–
4
4
dimer (ZnPor–(O–ZnPc)) were evaluated by cyclic voltammetric
studies. The value of EHOMO, ELUMO and Eg are 25.42 eV,
ZnPc) : PCBM, as well as the intensity being stronger.
Consequently more excitons will occur in ZnPor–(O–
2
3.71 eV and 1.72 eV, respectively for ZnPor–(O–ZnPc)
4
, 25.38
ZnPc)
photocurrent of the solar cells should be improved. Devices
based on ZnPor–(O–ZnPc), ZnPor–(O–ZnPc) and PCBM were
prepared by the spin-coating method. The compared devices
based on porphyrin (ZnTPP) : PCBM and phthalocyanine
(ZnTCPc) : PCBM were prepared at the same time. The
structures of the four devices are:
4
: PCBM under sunlight illumination, and thus, the
eV, 23.62 eV, and 1.7 eV, respectively for ZnPor–(O–ZnPc).
According to the reference, the HOMO and LUMO of PCBM, a
typical electron acceptor of organic solar cells, are 26.0 eV and
4
1
4
4.3 eV, respectively. The LUMOs of ZnPor–(O–ZnPc)4 and
ZnPor–(O–ZnPc) are clearly higher than that of PCBM.
Consequently, it is energetically favorable for photoexcited
ZnPor–(O–ZnPc)
4
and ZnPor–(O–ZnPc) to transfer electrons to
Device 1: ITO/PEDOT : PSS/ZnPor–(O–ZnPc) : PCBM/Al.
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ZnPc)
4
as the electron donor, possess similar VOC. The JSC of
device 2 is improved compared to device 1, which is attributed
to the enhanced sunlight adsorption of device 2 compared
with that of device 1, according to the results shown in Fig. 1d.
More excitons are formed correspondingly and the current
density of device 2 is improved. These results primarily reveal
that ZnPor–(O–ZnPc)
4
has shown the best photovoltaic
property performance among these materials.
Then we fabricated solar cell devices based on ZnPor–(O–
ZnPc)
acceptor with different weight ratios. The device structure is:
ITO/PEDOT : PSS/ZnPor–(O–ZnPc) : PCBM/LiF/Al.
4
as the electron donor and PCBM as the electron
4
The weight ratio of ZnPor–(O–ZnPc)4 : PCBM is 1 : 1, 2 : 1,
: 1, 4 : 1, 1 : 2 and 1 : 3 respectively. The photovoltaic
3
properties were detected under the same conditions. The
results are shown in Fig. 3a and the parameters are listed in
Table 2. It is found that VOC is almost the same for different
devices but the JSC is not. In organic photovoltaic devices with
the same structure, VOC depends mainly on the energy
difference between the HOMO energy level of the donor and
the LUMO of the acceptor. So VOC is almost the same for all
devices with different weight ratios of ZnPor–(O–
Fig. 2 I–V characteristics of devices based on porphyrin and phthalocyanine
compounds.
Device 2: ITO/PEDOT : PSS ZnPor–(O–ZnPc)4 : PCBM/Al.
Device 3: ITO/PEDOT : PSS/ZnTPP : PCBM/Al.
Device 4: ITO/PEDOT : PSS/ZnTCPc/PCBM/Al.
4
ZnPc) : PCBM. When the weight ratio of the donor and
The weight ratio of the donor and acceptor is 1 : 1.
PEDOT : PSS was prepared by the spin-coating method to
modify the anode. 80 nm Al was prepared by the thermal
evaporation method. The thickness of PEDOT : PSS is 40 nm.
The thickness of the active layer in four devices is the same as
acceptor is different, JSC increases with the increasing donor
concentration. JSC achieved a maximum value when the weight
ratio was 4 : 1. This means that 4 : 1 is the suitable weight
ratio to achieve the best photovoltaic performance.
Table 2 shows that the suitable ratio of ZnPor–(O–
80 nm. Other parameters are listed in the ESI.
3
4 4
ZnPc) : PCBM is 4 : 1. A decrease of the ZnPor–(O–ZnPc)
In order to investigate the effect of the ratio of donor and
acceptor, different devices were prepared with different weight
ratios of ZnPor–(O–ZnPc) : PCBM, for example 1 : 3, 1 : 2,
ratio will result in weak sunlight absorption. Correspondingly,
the number of formed excitons will decrease. The molecule
weight of ZnPor–(O–ZnPc)
larger than that of PCBM. When the weight ratio of ZnPor–(O–
ZnPc) : PCBM is 4 : 1, the mole ratio is close to 1 : 1, which is
4
is about 3000, almost 4 times
1
: 1, 2 : 1, 3 : 1 and 4 : 1, respectively. In addition, the
annealing temperature effect was investigated subsequently.
LiF was used to modify the cathode in these devices.
According to the above results, we can confirm that ZnPor–
4
helpful to form uniform domains of phase separation for
exciton dissociation. When the ratio of PCBM increases or
(
O–ZnPc) and ZnPor–(O–ZnPc) possess relatively good photo-
4
ZnPor–(O–ZnPc) decreases, the excess PCBM aggregates and
4
voltaic properties, as we expected. Fig. 2 shows the I–V
forms a big phase domain. The result is that the interface of
characteristics for these devices under an AM 1.5G 100 mW
exciton dissociation decreases and the photovoltaic perfor-
mance of the device lowers correspondingly. Even though to
increase the weight ratio of ZnPor–(O–ZnPc)4 can help to
increase the absorption of sunlight, the recombination of
formed excitons will increase too. According to the experiment
result the suitable weight ratio is 4 : 1.
2
2
cm
illumination. Photovoltaic parameters of these cells,
such as open circuit voltage (VOC), short circuit current (JSC),
filling factor (FF) and power conversion efficiency (g) were
calculated from the analysis of the J–V characteristics (seen in
Fig. 2), as shown in Table 1.
The properties of device 1 and device 2 are superior to tose
of device 3 and device 4. The results indicate that device 1 and
device 2, respectively based on ZnPor–(O–ZnPc) and ZnPor–(O–
Annealing can improve the crystallization of the photo
active layer. The better crystallization of the photo active layer
is beneficial for carrier transfer in addition to the formation of
Table 1 Photovoltaic parameters of solar cells based on different porphyrin phthalocyanine compounds
2
2
Device
Solar cell
VOC (V)
JSC (m cm
)
FF (%)
g (%)
1
2
3
4
ITO/PEDOT : PSS/ZnPor–(O–ZnPc) : PCBM/Al
ITO/PEDOT : PSS/ZnPor–(O–ZnPc) : PCBM/Al
4
ITO/PEDOT : PSS/ZnTPP : PCBM/Al
ITO/PEDOT : PSS/ZnTCPc/PCBM/Al
0.58
0.59
0.55
0.54
2.38
2.94
1.27
1.04
37.5
39.2
32.3
28.3
0.52
0.68
0.23
0.16
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Fig. 3 I–V characteristics of devices with different ratios of ZnPor–(O–ZnPc)
4
: PCBM (a) and devices with ZnPor–(O–ZnPc)
4
: PCBM = 4 : 1 annealed at different
temperatures (b).
Table 2 Photovoltaic parameters of devices with different ratios of ZnPor–(O–
ZnPc) : PCBM
our experiment, the photo active layer was prepared by the
solution method by using N,N-dimethylformamide (DMF) as
the solvent. DMF is a high boiling point solvent (152.8 uC). Its
volatilizing is very slow during the spincoating process.
Therefore, it is necessary to anneal the photo active layer to
eliminate the residual DMF. 140 uC is close to the boiling point
of DMF. Annealing under 140 uC allows DMF to volatilize
slowly. The crystallization of the photo active layer will be
improved. The lower annealing temperature is not helpful for
DMF volatilizing and residual DMF will affect the carrier
4
2
2
Weight Ratio
VOC(V)
JSC (mA cm
)
FF (%)
g (%)
1
1
1
2
3
4
: 1
: 2
: 3
: 1
: 1
: 1
0.58
0.59
0.6
0.59
0.59
0.61
3.30
3.00
2.72
3.50
3.72
3.99
39.6
38.5
39.9
39.2
39.9
39.7
0.76
0.68
0.65
0.81
0.86
0.97
uniform domains. Therefore, the photoactive layer was
annealed at different temperatures in vacuum, 100 uC, 120
uC, 140 uC and 160 uC, respectively, for 20 min. Fig. 3b is the
curve of the current density versus the voltage. It shows that
the current density and filled factor increase along with the
increasing of the annealing temperature. The best perfor-
mance is achieved below 140 uC annealing. The current
2
2
22
density increases from 3.99 mA cm to 4.97 mA cm , and
the filled factor increases from 39.7% to 57.5% (shown in
Table 3). Combined with the results of AFM shown in Fig. 4, it
is concluded that best and uniform phase separation is
achieved in the photo active layer annealed at 140 uC. The
phase separation implies that an interpenetrating network
structure is formed in the photo active layer, which is
beneficial for exciton dissociation and charge transfer. In
Table 3 Photovoltaic parameters of devices with different annealing tempera-
tures
2
2
Annealing T/uC
VOC (V)
JSC (mA cm
)
FF (%)
g (%)
1
1
1
1
00
20
40
60
0.62
0.61
0.63
0.63
3.82
4.19
4.97
4.34
55.2
55.8
57.5
57.8
1.31
1.43
2.08
1.58
4
Fig. 4 AFM images of the thin films of ZnPor–(O–ZnPc) : PCBM with different
annealing treatments, (a) annealing at 100 uC, (b) annealing at 120 uC, (c)
annealing at 140 uC and (d) annealing at 160 uC.
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transfer and collection. But at higher annealing temperatures
DMF volatilizes faster, which will result in a roughness of the
photo active layer and affect the ageing of the photo active
layer.
Notes and references
1
2
3
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Conclusions
We have designed and synthesized two novel compounds
4
5
6
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(Por–Pc conjugates) in this work. The Por–Pc heterodimer and
heteropentamer have more delocalized p-electrons and there-
fore their conjugation degree is higher. These compounds
possess a better solubility and wider light absorption. Por–Pc
conjugates that are linked through covalent linkers are a kind
of effective light harvesters. They have a broader spectral
coverage in the visible region. These two kinds of ZnPor–ZnPc
can be used as electron donor materials in solar cells. The
properties of the solar cells based on them have been
investigated. The results confirmed that the device properties
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of ZnPor–(O–ZnPc) : PCBM and ZnPor–(O–ZnPc)
superior to single Por and Pc for solar cell applications. The
suitable weight ratio of ZnPor–(O–ZnPc) : PCBM is 4 : 1. The
4
: PCBM are
1
1
1
1
1
4
highest photovoltaic properties were achieved with ZnPor–(O–
ZnPc) : PCBM (4 : 1) annealed at 140 uC in a vacuum for 20
4
min. The highest power conversion efficiency (PCE) is 2.08.
The Por–Pc conjugates possess the structure and properties as
we expected. Our work has remarkably promoted the
performance of the active layer materials of organic solar
cells. It can provide a new strategy for the improvement of the
power conversion efficiency of organic solar cells.
2
3, 1065–1071.
3 Z. X. Zhao, T. Nyokong and M. D. Maree, Dalton Trans.,
005, 23, 3732–3737.
2
4 F. Padinger, R. S. Rittberger and N. S. Sariciftci, Adv. Funct.
Mater., 2003, 13, 85–88.
RSC Adv.
This journal is ß The Royal Society of Chemistry 2013