Electronic and electrochemical properties as well as flowerlike supramolecular assemblies of fulleropyrrolidines bearing ester substituents with different alkyl chain lengths

Article abstract of DOI:10.1039/c4ra10654g

A series of alkyl (methyl, ethyl, propyl, butyl) benzoate ester substituted fulleropyrrolidine derivatives (FP1-FP4) were synthesized and their electronic and electrochemical properties were investigated by means of absorption spectra, electronic structure calculation, and cyclic voltammetry (CV), respectively. The LUMO-HOMO energies and energy gaps of fullerene derivatives were estimated by the first reduction potential measured with CV combined with absorption spectra, which are consistent with those obtained from density functional theory (DFT) calculations. It was found that all fulleropyrrolidines showed very similar absorption spectra, orbital energies and redox behaviors, which are comparable with those of well-known phenyl-C₆₁-butyric acid methyl ester (PCBM). The flowerlike supramolecular architectures obtained from the self-assembly of FP1-FP4 in chloroform-alcohol mixture solvents were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). A lamellar structure with a d-spacing of 1.92-2.02 nm that depends on the molecular size, corresponding to the thickness of a bilayer structure, suggested a face-to-face conformation of the substituent of C₆₀ and an interdigitation of the bare C₆₀ side packing. These fulleropyrrolidines have high C₆₀ content, are energetically PCBM-like, and are capable of forming complex flowerlike architectures, which provide fundamental insights into molecular design toward advanced fullerene materials.

Full text of DOI:10.1039/c4ra10654g

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Electronic and electrochemical properties as well
as flowerlike supramolecular assemblies of
fulleropyrrolidines bearing ester substituents with
different alkyl chain lengths†
Cite this: RSC Adv., 2014, 4, 60342
*
Xuan Zhang, Xu-Dong Li, Li-Xia Ma and Bei Zhang
A series of alkyl (methyl, ethyl, propyl, butyl) benzoate ester substituted fulleropyrrolidine derivatives (FP1–
FP4) were synthesized and their electronic and electrochemical properties were investigated by means of
absorption spectra, electronic structure calculation, and cyclic voltammetry (CV), respectively. The LUMO–
HOMO energies and energy gaps of fullerene derivatives were estimated by the first reduction potential
measured with CV combined with absorption spectra, which are consistent with those obtained from
density functional theory (DFT) calculations. It was found that all fulleropyrrolidines showed very similar
absorption spectra, orbital energies and redox behaviors, which are comparable with those of well-
known phenyl-C61-butyric acid methyl ester (PCBM). The flowerlike supramolecular architectures
obtained from the self-assembly of FP1–FP4 in chloroform–alcohol mixture solvents were characterized
by scanning electron microscopy (SEM) and X-ray diffraction (XRD). A lamellar structure with a d-spacing
of 1.92–2.02 nm that depends on the molecular size, corresponding to the thickness of a bilayer
structure, suggested a face-to-face conformation of the substituent of C60 and an interdigitation of the
bare C60 side packing. These fulleropyrrolidines have high C60 content, are energetically PCBM-like, and
are capable of forming complex flowerlike architectures, which provide fundamental insights into
molecular design toward advanced fullerene materials.
Received 17th September 2014
Accepted 3rd November 2014
DOI: 10.1039/c4ra10654g
www.rsc.org/advances
solubility for C60 are very limited (usually aromatic solvents and
carbon disulde). Alternatively, molecular-tailoring of fuller-
1. Introduction
enes via attachment of proper substituent groups on periphery
of C60 cage not only enhances its solubility in common organic
solvents, which is benet for solution processing in device
fabrication, but also enables the regulation of self-assembly.29–44
For example, various superstructures such as nanobers, vesi-
cles, owerlike architectures have been achieved by full-
eropyrrolidines bearing multiple alkyl chains.41–44 However, the
attachment of bulky and insulating appends largely decreased
C60 content (usually lower than 50%), leading to these
chemically-modied fullerenes are less electronically active that
is actually undesirable for their optoelectronic applications.
Therefore, graing small substituent groups on fullerene
surface should be more promising due to high C60 content will
be retained in these fullerene derivatives.45,46 Recently, a small
pyridine group equipped fulleropyrrolidine was reported to be
capable of organizing into C60-rich and photoconductive ow-
erlike architectures, which retain high C60 content (84%) and
Fullerene (C60) and its derivatives have been employed to a
considerable extent as excellent electron-acceptor materials in
various optoelectronic devices (such as organic solar cells and
eld effect transistors).1–6 On the other hand, as typical
p-conjugated molecules, fullerenes are also important building
blocks for fabrication of functional supramolecular assem-
blies.7–10 It has been recognized that the performance of
molecular device could be improved by ne molecular-tailoring
and directed self-organization of p-conjugated units. In this
regard, many efforts have been devoted to self-assembly of
fullerenes to manipulating properties of fullerene-based mate-
rials.11–19 For example, several well-dened organized nano/
microstructures such as spheres,20 rods,21 wires,22 belts,23,24
whiskers,25,26 and sheets,27,28 have been fabricated from the
pristine C60, which retain the intrinsic optoelectronic properties
of fullerenes but the numbers of solvents owning enough
photoinduced carrier-transporting properties comparable with
College of Chemistry, Chemical Engineering & Biotechnology, Donghua University,
Shanghai 201620, China. E-mail: xzhang@dhu.edu.cn
45
pristine C60
.
Furthermore, it was also found that the
morphology of self-organized objects could also be tuned by the
position of N-atom in pyridine group. This example provides a
useful molecular design concept for construction of C60-rich
† Electronic supplementary information (ESI) available: Data of additional
computation results for C60 and PCBM at various basis sets, other molecular
properties (ionization potential, electro affinity, electronegativity, global
hardness, etc.) and MOs for fullerenes are available. See DOI: 10.1039/c4ra10654g
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Scheme 1 Chemical structures of fulleropyrrolidine derivatives FP1–FP4.
complex supramolecular assemblies via graing
a small electrode) in 0.1 M tetrabutylammonium tetrauoroborate (n-
substituent group on periphery of fullerene cage. Thus the Bu4NBF4) solution in 1,2-dichlorobenzene (o-DCB) at a potential
design and synthesis of other small substituent attached scan rate of 100 mV sꢀ1, where the reduction potentials were
fullerene derivatives to investigate their electronic properties calibrated using ferrocene/ferrocenium (Fc/Fc+) redox couple as
and self-assembly would further provide fundamental insights internal standard. The geometries of fulleropyrrolidine deriva-
into fullerene-containing materials.
tives were optimized by density functional theory (DFT) at the
In this work, by considering the molecular structure of well- B3LYP/6-31G(d) level, and the orbital energy and orbital anal-
known fullerene derivative, phenyl-C61-butyric acid methyl ester ysis were calculated at the PBEPBE/6-311G(d,p) level, with the
(PCBM), a small benzoate ester substituent group was then Gaussian 09 program package.47
graed on C60 to synthesize a series of alkyl (methyl, ethyl,
Fullerene C60 (99%) was purchased from XFNANO (Nanjing)
propyl, butyl) benzoate ester-substituted fulleropyrrolidines and other reagents as well as solvents (AR) were obtained from
(FP1–FP4, Scheme 1), and their electronic and electrochemical Sinopharm Chemical Reagents Co. (Shanghai). Alkyl (methyl,
properties as well as self-assembly were investigated by means ethyl, propyl, butyl) 4-formylbenzoates were prepared by ester-
of absorption spectra, electronic structure calculation, cyclic ication reaction from 4-formylbenzoic acid with the corre-
voltammetry (CV), scanning electron microscopy (SEM) and X- sponding alcohols in the presence of thionyl chloride.
ray diffraction (XRD) analysis, respectively. It was found that Fulleropyrrolidine derivatives (FP1–FP4, Scheme 1) were
the LUMO–HOMO levels of these fullerene derivatives esti- synthesized by reuxing the toluene solution of C60, sarcosine
mated by the experiments and theoretical calculations, are and the corresponding benzaldehyde according to the standard
energetically PCBM-like. All of these fulleropyrrolidines could Prato reaction.48
organize into owerlike architectures of lamellar structures
1, 1H NMR (400 MHz, CDCl3): d (ppm) ¼ 8.12 (d, 2H, J ¼ 8.0
with alkyl chain length dependent d-spacing. These small ester Hz), 7.94 (br, 2H), 5.07 (br, 2H), 4.33 (d, 1H, J ¼ 8.0 Hz), 3.91 (s,
substituent attached fullerenes are high C60 content (76–79%), 3H), 2.85 (s, 3H). MALDI-TOF-MS [DCTB] m/z calcd, C71H13NO2
energetically PCBM-like, and capable of forming owerlike 911.23; found, 911.12.
assemblies, which provide fundamental insights into molecular
design toward advanced fullerene materials.
2, 1H NMR (400 MHz, CDCl3): d (ppm) ¼ 8.13 (d, 2H, J ¼ 8.0
Hz), 7.96 (br, 2H), 5.11 (br, 2H), 4.40–4.35 (m, 3H), 2.88 (s, 3H),
1.39 (t, 3H, J ¼ 8.0 Hz). MALDI-TOF-MS [DCTB] m/z calcd,
C
72H15NO2 925.25; found, 925.76.
3, 1H NMR (400 MHz, CDCl3): d (ppm) ¼ 8.12 (d, 2H, J ¼ 8.0
2. Experimental details
Hz), 7.92 (br, 2H), 5.05 (br, 2H), 4.33–4.25 (m, 3H), 2.84 (s, 3H),
1.82–1.76 (m, 2H), 1.03 (t, 3H, J ¼ 8.0 Hz). MALDI-TOF-MS
[DCTB] m/z calcd, C73H17NO2 939.28; found, 939.86.
1
Absorption, H NMR and mass spectra were recorded on a TU-
1901 (Persee), Brucker Avance III (400 MHz) and MALDI-TOF-
MS (AB Sciex 4700) spectrometer, respectively. Scanning elec-
tron microscopy (SEM) and transmission electron microscopy
(TEM) were performed on a HITACHI TM-1000 and JEOL-2100
microscopes, respectively. X-Ray diffraction (XRD) patterns
were obtained using a RIGAKU D/Max-2550 PC X-ray diffrac-
tometer. Cyclic voltammetry (CV) and differential pulse vol-
tammetry (DPV) curves were measured using a CHI 660D
electrochemical workstation (Chenhua, Shanghai). The CV and
DPV curves were collected at room temperature under N2 using
4, 1H NMR (400 MHz, CDCl3): d (ppm) ¼ 8.13 (d, 2H, J ¼ 8.0 Hz),
7.97 (br, 2H), 5.12 (br, 2H), 4.37–4.30 (m, 3H), 2.88 (s, 3H), 1.79–
1.71 (m, 2H), 1.51–1.45 (m, 2H), 0.98 (t, 3H, J ¼ 8.0 Hz). MALDI-
TOF-MS [DCTB] m/z calcd, C74H19NO2 953.31; found, 953.87.
3. Results and discussion
3.1. Photophysical properties
a conventional three-electrode system (glassy carbon working The absorption spectra (400–750 nm) of fulleropyrrolidine
electrode, Pt wire counter-electrode and Ag/Ag+ quasi-reference derivatives FP1–FP4 in chloroform (1.3 ꢁ 10ꢀ4 M) were shown
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in Fig. 1, together with the spectrum of pristine C60 (6.5 ꢁ 10ꢀ5
M) for comparison. All of fulleropyrrolidine derivatives exhibit
nearly the same broad absorption bands from 400 to 750 nm
those extend to longer wavelength range than that of the pris-
tine C60 (Fig. 1). The difference of photophysical properties
between FP1–FP4 and the pristine C60 could be explained from
the symmetry breaking effect in the former that enhanced the
forbidden transitions to some extent.49 It was noted that the
fulleropyrrolidine derivatives have stronger visible light
absorption in the red-shied absorption edge above 650 nm
with the longer lonset around 724 nm (Table 1), which is similar
to that of PCBM and important to contribute the light harvest-
ing in photovoltaics.31 The similar absorption spectra observed
among fulleropyrrolidine derivatives FP1–FP4 indicated that
the alkyl chain length scarcely affects their photophysical
properties.
3.2. Electrochemical properties
It is well-known that fullerenes have rich electrochemical
property and can accept up to six electrons in solution.50,51 The
rst reduction potentials of fullerene acceptors were usually
used to estimate their lowest unoccupied molecular orbital
(LUMO) energy, which determines the open circuit voltage
performance of polymer solar cells together with the highest
occupied molecular orbital (HOMO) energy levels of electron
donor.52–54 We therefore measured the cyclic voltammograms of
FP1–FP4 and the pristine C60 for a comparison. As shown in
Fig. 2, a series of cyclic voltammograms shows that three well-
dened and reversible redox waves are retained in the func-
tionalized fullerenes ranging from 0 to ꢀ2.6 V vs. Ag/Ag+, under
room temperature and anhydrous air-free conditions, relative to
the ferrocene/ferrocenium (Fc/Fc+) internal standard. As shown
in Fig. 3, three sharp and equal intensity peaks together with the
noticeable fourth peaks were clearly observed from differential
pulse voltammetry (DPV), further revealing that FP1–FP4 are
typical redox-reversible fullerene derivatives. From the CV, it
can be noted that three reversible redox peaks of all derivatives
FP1–FP4 are all shied to negative potentials by ca. 100 mV as
compared to those of C60, an indication of the stronger electron
acceptor. The measured half-wave potentials are listed in
Table 1 together with the LUMO energy levels of the fullerene
derivatives, estimated from the their onset reduction potentials
red
red
(E
) according to the equation, LUMO (eV) ¼ ꢀe(E
+
onset
onset
red
onset
4.60).55 The E
of FP1–FP4 were ꢀ0.77, ꢀ0.78, ꢀ0.78, and
ꢀ0.78 V, corresponding to the LUMO energy levels of ꢀ3.83,
ꢀ3.82, ꢀ3.82, and ꢀ3.82 eV, respectively (Table 1), indicating
the alkyl chain length has a little effect on their electrochemical
properties. The LUMO energy levels of FP1–FP4 are ca. 0.1 eV
higher than that of C60 (ꢀ3.92 eV), which is desirable for an
electron acceptor in polymer solar cells to get higher open-
circuit voltage.52–54 The redox properties, LUMO, HOMO and
energy gap (Egap) of all derivatives are similar and also very close
to those of PCBM (Table 1),55 an indicative of FP1–FP4 are
PCBM-like electron acceptors.
Fig. 1 Absorption spectra of fulleropyrrolidines FP1–FP4 and the
pristine C60 in chloroform.
Table 1 Redox properties, energy gaps, LUMO and HOMO energy
levels of fullerenes FP1–FP4, PCBM and the pristine C60
Parameters
C60
PCBM
FP1
FP2
FP3
FP4
E1a [V]
E2a [V]
ꢀ0.75
ꢀ1.14
ꢀ1.60
ꢀ0.68
ꢀ3.92
ꢀ4.18
693
ꢀ0.90h
ꢀ1.31h
ꢀ1.85h
ꢀ0.77h
ꢀ3.83
ꢀ4.00
723
ꢀ0.84
ꢀ1.24
ꢀ1.77
ꢀ0.77
ꢀ3.83
ꢀ4.02
724
ꢀ0.85
ꢀ1.24
ꢀ1.77
ꢀ0.78
ꢀ3.82
ꢀ4.01
724
ꢀ0.85
ꢀ1.25
ꢀ1.78
ꢀ0.78
ꢀ3.82
ꢀ4.01
724
ꢀ0.83
ꢀ1.23
ꢀ1.76
ꢀ0.78
ꢀ3.82
ꢀ4.01
724
E3a [V]
red
E
b [V]
onset
LUMOc [eV]
LUMOd [eV]
e
lonset [nm]
Egapf [eV]
1.79
1.69
1.72
1.52
1.72
1.50
1.72
1.50
1.72
1.50
1.72
1.50
Egapd [eV]
HOMOg [eV]
HOMOd [eV]
ꢀ5.71
ꢀ5.55
ꢀ5.55
ꢀ5.54
ꢀ5.54
ꢀ5.54
ꢀ5.87
ꢀ5.52
ꢀ5.52
ꢀ5.51
ꢀ5.51
ꢀ5.51
a
red
onset
Half-wave potential. b Onset reduction potential. c LUMO ¼ ꢀe(E
+
d
e
4.60).
Values from the DFT calculation.
Onset absorption
f
wavelength. Band gap ¼ hc/lonset, converted [J] to [eV]; h, Planks
g
h
constant; c, speed of light. HOMO ¼ LUMO ꢀ Egap [eV]. Taken
Fig. 2 CV curves of fulleropyrrolidine derivatives FP1–FP4 and the
pristine C60 in o-DCB.
from ref. 55.
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fulleropyrrolidine derivatives FP1–FP4 have smaller hardness
values (ca. 0.75 eV) than that of C60 (ca. 0.84 eV), revealing they
are more reactive. Orbital analysis of HOMO and LUMO were
also carried out for FP1–FP4, PCBM and C60 at the PBEPBE/6-
311G(d,p) level (Fig. 5). The fulleropyrrolidine derivatives FP1–
FP4, PCBM and C60 showed very similar orbital distributions, in
which both the LUMO and HOMO are mainly located on the
fullerene cage, respectively. But it can still be noted that the
electron delocalization for fullerene derivatives are slightly
weakened relative to C60, which has been attributed to
symmetry breaking of C60 molecule by functionalization.49,56 It
should be noted that C60 has 3-fold degenerated LUMOs and 5-
fold degenerated HOMOs (Fig. S1, ESI†), but the LUMO and
HOMO of fulleropyrrolidine derivatives FP1–FP4 become non-
degenerated (Table S3 and Fig. S2, ESI†). Further orbital anal-
ysis for other occupied and vacant MOs showed that the highest
occupied and lowest vacant MOs localized on phenyl substit-
uent are energetically far from the HOMO and LUMO of FP1–
FP4 (Fig. S3–S6 and Table S3, ESI†), suggesting a little contri-
bution to the frontier MOs. These ndings suggest that the
hybridization change (from sp2 to sp3) of C60 carbon atoms
involved in connection with substituent, which modied the
localization and the nature of HOMO and LUMO, also
contribute to the observed substituent effect. The similar orbital
distribution of fullerene derivatives may explain the similar
electronic and electrochemical properties observed in absorp-
tion spectra and CV.
Fig. 3 DPV curves of fulleropyrrolidine derivatives FP1–FP4 in o-DCB.
The parameters of DPV: amplitude, 0.05 V; pulse width, 0.2 s; sample
width, 0.02 s; pulse period, 0.5 s.
3.3. DFT calculations
To further understand the substituent effect on the LUMO–
HOMO energy levels of fullerene acceptors, DFT calculations
were performed by using the Gaussian 09 program package.47
Ground state geometrical structures of four fulleropyrrolidine
derivatives FP1–FP4, PCBM and C60 were optimized at the
B3LYP/6-31G(d) level, and the molecular orbital energies were
calculated at the more reliable PBEPBE/6-311G(d,p) level based
on above optimized geometries.46,49,56,57 The calculated results
are summarized in Table 1, and the schematic diagrams of
HOMO and LUMO levels are shown in Fig. 4. It can be noted
that the HOMO–LUMO energy gaps of fulleropyrrolidine deriv-
atives FP1–FP4 reduced to 1.72 eV by comparison with 1.79 eV of
3.4. Self-assembly
To demonstrate how the small alkyl benzoate ester substituent
groups affect the packing of C60 molecules in solid state, the
self-organized objects of FP1–FP4 were further investigated by
SEM, TEM and XRD. The self-organized objects were prepared
by mixing the chloroform solution of FP1–FP4 (1 mg mlꢀ1) with
2-propanol or ethanol (10 : 1, v/v), then dropping on Si wafers
and evaporating the solvent in a capped petri dish up to 1 h
until dryness at room temperature. The assembled objects on Si
wafers were directly used for SEM or XRD characterization,
while they were scratched off and dispersed in EtOH for a TEM.
Fig. 6 shows SEM and TEM images of self-organized architec-
tures formed from FP1–FP4 derivatives. It can be noted that
owerlike objects with size distribution around 2–8 mm were
consisted of plate nanostructures (Fig. 6a–e), which was also
conrmed by TEM (Fig. 6f). To get information on molecular
C
60, while their LUMO and HOMO energies are 0.17 and 0.35 eV
higher than those of C60 (ꢀ4.18 and ꢀ5.87 eV), respectively
(Fig. 4 and Table 1), which may explain the red-shied
absorption of substituted fullerenes. The calculated orbital
energies of derivatives FP1–FP4 are nearly same as those of
PCBM and also close to the experimental values (Table 1),
conrming the PBE method a reliable in orbital energy calcu-
lation for fullerene systems. In addition, other molecular
properties, such as molecular hardness, electrophilicity, elec-
tronegativity, etc., have also been provided (Table S2, ESI†).
Considering the molecular hardness, large value means a hard
molecule with less global reactivity. It was noted that the
Fig. 4 Schematic diagrams of HOMO and LUMO energy levels of C60
,
Fig. 5 LUMO (top) and HOMO (bottom) contours for C60, PCBM, and
PCBM, and fulleropyrrolidine derivatives FP1–FP4 calculated at fulleropyrrolidine derivatives FP1–FP4 calculated at PBEPBE/6-
PBEPBE/6-311G(d,p) level.
311G(d,p) level.
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Fig. 6 SEM (a–e) and TEM (f) images of self-organized architectures of FP1–FP4 obtained from the mixture of chloroform–alcohol. (a) FP1 from
chloroform–2-propanol; (b) FP2 from chloroform–2-propanol; (c) FP3 from chloroform–2-propanol; (d) FP4 from chloroform–ethanol; (e) and
(f) FP4 from chloroform–2-propanol.
packing, the bulk owerlike objects obtained from FP1–FP4 on face-to-face conformation of the substituent of C60 and an
Si wafer were further investigated by XRD (Fig. 7). As shown in interdigitation of the bare C60 side packed as suggested
Fig. 7, one intense diffraction peak appeared at around 5ꢂ before.7,13,45,58 These observations suggest that the smaller
accompanied by several weaker peaks at higher angle region in graed group insufficiently disturbing the strong p–p interac-
FP1–FP4 derivatives, resembling those previously observed in tions between adjacent C60 molecules,13 especially the strongest
others fulleropyrrolidines.42–45,58 These peaks are therefore peaks from fcc packing were clearly noticed in the case of FP1
similarly attributed to the reections of (001), (002) and (003) (Fig. 7).
planes with a d-spacing value of 1.92, 1.95, 1.98 and 2.02 nm for
FP1, FP2, FP3, and FP4, respectively, suggesting a lamellar
structure. In addition, by comparison with the patterns
4. Conclusions
obtained from the pristine C60, the residue peaks can be readily
assigned to the reections of (111), (220), (311) and (222) planes
of a fcc structure with the average spacing of 0.8 nm (as indi-
cated by asterisks in Fig. 7), suggesting a p–p (C60/C60) inter-
In conclusion, a series of small alkyl (methyl, ethyl, propyl,
butyl) benzoate ester group substituted fulleropyrrolidine
derivatives (FP1–FP4) were synthesized and their electronic and
electrochemical properties as well as self-assembly were inves-
action in the assemblies. By taking the molecular size of 1.3–1.5
tigated by means of absorption spectra, DFT calculation, CV,
nm of FP1–FP4 obtained from the DFT calculation, the
SEM, TEM and XRD, respectively. The LUMO–HOMO energies
d-spacing of 1.92–2.02 nm that depends their molecular sizes,
of fullerene derivatives were estimated by the rst reduction
corresponds well to the thickness of a bilayer structure, where a
potential measured with CV combined with absorption spectra,
which are in consistent with those obtained from DFT calcula-
tions. All fulleropyrrolidines FP1–FP4 showed the very similar
absorption spectra, orbital energies and redox behaviors, which
are comparable with those of well-known PCBM, indicating
alkyl chain length has little effect on their electronic and elec-
trochemical properties. The SEM investigation of self-assembly
of FP1–FP4 in chloroform–alcohol solution mixture solvents
showed that owerlike supramolcular architectures can be
facilely achieved. XRD analysis revealed that these owerlike
objects were lamellar structures with the d-spacing of 1.92–2.02
nm that depends on the molecular size of FP1–FP4, corre-
sponding well to the thickness of a bilayer structure. This sug-
gested a face-to-face conformation of the substituent of C60 and
an interdigitation of the bare C60 side packing fashion. The
fulleropyrrolidines FP1–FP4 are high C60 content (76–79%),
energetically PCBM-like, and capable of forming complex
owerlike architectures, which provide fundamental insights
into molecular design toward advanced fullerene materials and
may nd potential application in optoelectronics.
Fig. 7 XRD patterns of flowerlike objects of FP1–FP4 obtained from
the mixture of chloroform–2-propanol.
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Acknowledgements
This work was nancially supported by Innovation Program of
Shanghai Municipal Education Commission (12ZZ067),
Shanghai Pujiang Program (11PJ1400200), the Research Fund
for the Doctoral Program of Higher Education of China
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60348 | RSC Adv., 2014, 4, 60342–60348
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