Electron donating group stimulated aggregation induced emission enhancement of oligophenylenevinylene-cored luminogens

Article abstract of DOI:10.1039/c4ra08663e

Three new oligophenylenevinylene (OligoPV)-cored luminogens were synthesized by utilizing the Suzuki coupling/Buchwald-Hartwig amination reaction and their aggregation-induced emission properties have been studied. Out of three, two are aggregation-induce

Full text of DOI:10.1039/c4ra08663e

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Electron donating group stimulated aggregation
induced emission enhancement of
Cite this: RSC Adv., 2014, 4, 62684
oligophenylenevinylene-cored luminogens†
a
b
a
a
Received 14th August 2014
Accepted 12th November 2014
*
Priyabrata Roy, Debabrata Jana, Ashis Kundu and Animesh Pramanik
DOI: 10.1039/c4ra08663e
www.rsc.org/advances
Three new oligophenylenevinylene (OligoPV)-cored luminogens were caused quenching (ACQ) of conventional chromophores. Now,
synthesized by utilizing the Suzuki coupling/Buchwald–Hartwig ami- researchers have greatly expanded their search for AIEE lumi-
nation reaction and their aggregation-induced emission properties nogens that show specic applications in materials science7 and
have been studied. Out of three, two are aggregation-induced emis- biological technology.8 For example, tetraphenylethene (TPE) is
sion enhancement (AIEE)-active molecules: weakly fluorescent in a representative AIEE-active luminophore and TPE-derivatives
solution, while highly emissive in the aggregation state. The presence have been widely used as uorescent probes for the detection
of the OligoPV core has turned an aggregation-caused quenching of ions, biomolecules and explosives.9 Recently, Tang et al.
(ACQ) luminophore into an AIEE one. They show excellent solubility in shown that tetraphenylethene (TPE) could be used as a modier
common organic solvents, emit light in the green region (500–516 to turn an ACQ luminophore to an AIEE one.4,10 These successes
nm) and have high thermal stability.
triggered our interest to conjecture whether oligophenylenevi-
nylen (OligoPV) free of ACQ effect and promising emitters for
organic light-emitting diodes (OLEDs)? Are they serves as a core
and conventional chromophores as peripheries? To answer
these questions, herein, we elaborate three luminogens by
decorating a p-phenylenevinylene core with four electron
donating ACQ luminophores [triphenylamine (TPA), thiophene
(T) and carbazole (C)] at para positions. Generally electron
donating groups have quite a different effect on p-delocaliza-
tion of conjugate systems, especially when attached at the para
position giving a highest uorescence quantum yield compared
to ortho- and meta-substitution.11
Oligophenylenevinylene (OligoPV) based core unit is acces-
sible from the general synthesis route given in Scheme 1, with
the key building blocks being a diphosphonate 3 and 4,4'-
dibromobenzophenone. 1,4-Bisdodecyloxybenzene (1)12 was
bisbromomethylated in glacial acetic acid solution with para-
formaldehyde and hydrogen bromide to yield dibromo deriva-
tive 2.13 This dibromide 2 is then treated with triethyl phosphite
in an Arbuzov reaction to give diphosphonate 3. A Horner–
Wadsworth–Emmons (HWE) reaction between diphosphonate
3 and 4,4'-dibromobenzophenone yields the OligoPV based core
unit 4 (OligoPV-4Br).
Fluorescent molecular materials have been intensively studied
as active materials for organic light-emitting diodes (OLEDs),
lasers, and sensor applications.1,2 Many of the uorescent
molecules have efficient emission in solution, however, the
emission tends to vanish in the solid-state due to aggregation-
caused quenching (ACQ).3 The best approach to this notorious
problem is to develop new luminophoric materials whose
aggregates can emit more efficiently than their solutions. Along
this line of approach, two novel photoluminescence (PL)
processes have been identied: one is the “aggregation-induced
emission enhancement (AIEE)”4 and another is the
“aggregation-induced emission (AIE)”.5,6 These phenomenon
are mainly ascribed to the restriction of intramolecular rota-
tions (RIR) process. For example, the nonluminescence of silole
in solution is attributed to the active intramolecular rotations of
multiple phenyl rings on its periphery, while the strong emis-
sion in the solid state is caused by the restriction of intra-
molecular rotations of those phenyl rings. This intriguing
nding paves a new avenue to tackle the notorious aggregation-
Final construction of the targeted molecule is illustrated in
Scheme 2. To achieve the synthesis of these oligomers Suzuki-
type coupling reactions have been explored. The synthesis of
the tetra-substituted OligoPVs 5/6 was achieved by using Suzuki-
type cross-coupling reaction between 2-thienylboronic acid/4-
(diphenylamino)phenylboronic acid and bromo derivative 4.
aDepartment of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata 700009,
India. E-mail: animesh_in2001@yahoo.co.in; Fax: +91-33-23519755; Tel: +91-33-
24841647
bDepartment of Chemistry, Indian Institute of Engineering Science and Technology,
Shibpur, Howrah 711103, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra08663e
62684 | RSC Adv., 2014, 4, 62684–62688
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
Scheme 1 Reagents and conditions: (a) paraformaldehyde, 50% HBr in acetic acid, AcOH, 80 ꢀC, 5 h; (b) triethyl phosphite, 150 ꢀC, 12 h; (c) 4,4'-
dibromobenzophenone, t-BuOK, THF, 20 h, rt.
Scheme 2 Reagents and conditions: (a) 2-thienylboronic acid, Pd(PPh3)4, K2CO3, toluene, H2O, aliquat 336, 90 ꢀC, 16 h; (b) 4-(diphenylamino)
phenylboronic acid, Pd(PPh3)4, K2CO3, toluene, H2O, aliquat 336, 90 ꢀC, 16 h; (c) carbazole, Pd(OAc)2, P(t-Bu)3, NaOt-Bu, toluene, 100 ꢀC, 3 days.
Products OligoPV–T 5 and OligoPV–TPA 6 were obtained in
We measured the UV/Vis absorption and uorescence spec-
good yields ranging from 74% to 71%. OligoPV–C 7 was troscopic data of these synthesized compounds in toluene and
obtained from tetrabrominated OligoPV based core 4 with tetrahydrofuran solutions and the results are presented in
carbazole14 via fourfold Pd-catalyzed Buchwald-Hartwig ami- Table 1. The UV/Vis absorption spectra of the luminogens in
nation in 49% yield. These oligomers are readily soluble in THF are shown in Fig. 1a. The absorption spectra of three
common organic solvents, such as CHCl3, CH2Cl2, THF and compounds have two absorption bands in the 312–360 nm and
toluene but insoluble in methanol and water. They were puri- 388–420 nm regions. The rst absorption band in the short
ed conveniently by column chromatography rather than wavelength region around 312–360 nm was attributed to the
vacuum sublimation usually used for the purication of insol- central phenylenevinylene segment based on the presence of
uble organic semiconductors. 1H and 13C NMR spectra and absorption band at 343 nm for compound 4 in THF. The second
MALDI-TOF MS associated with elemental analysis were absorption band at 388–420 nm is due to p–p* transition from
employed to conrm the structure and purity of all new between central phenylenevinylene unit and peripheral electron
compounds.
donating unit. Particularly for compound OligoPV–TPA, the rst
Table 1 Photophysical properties of derivatives OligoPV–TPA, OligoPV–C and OligoPV–T
Stokes shisc
a
3max
(104 molꢁ1 cmꢁ1
UV-vis (labs, nm)
)
PL (lmax, nm, 4
)
(103 cmꢁ1
)
DEd (eV)
b
Compound
Tol
THF
Tol
THF
Tol
THF
Solid
Tol
THF
Tol
THF
Td (ꢀC)
OligoPV–TPA
OligoPV–C
OligoPV–T
356, 418
320, 411
307, 388
360, 420
324, 413
312, 388
9.16
6.84
6.15
8.12
5.46
5.16
510(0.194)
500(0.230)
509(0.221)
515(0.186)
504(0.225)
516(0.213)
516
534
526
0.85
1.10
0.61
0.84
1.12
0.64
3.48
3.87
3.19
3.44
3.82
3.19
340
390
352
a
3max is the extinction coefficient at lmax of absorption. b Fluorescence quantum yields, measured in solution using Rhodamine B (F ¼ 0.43, in
acidic methanol) as standard, excited at 551 nm. c Stokes shi ¼ (1/labs ꢁ 1/lem). Determined from UV-vis absorption maximum.
d
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 62684–62688 | 62685

View Article Online
RSC Advances
Communication
Fig. 1 (a) Absorption spectra of OligoPV-cored luminogens (C ¼ 1 ꢂ 10ꢁ6 M) in THF. (b) Photoluminescence spectra of OligoPV-cored lumi-
nogens (C ¼ 1 ꢂ 10ꢁ7 M) in THF. Inset: fluorescence emission image in THF. (c) Corresponding emission spectra of compound OligoPV–TPA in
aqueous THF with different water–THF ratios at 10ꢁ7 M. Inset: fluorescence emission image of OligoPV–TPA (fw ¼ 0, 50 and 90%). (d) Changes of
peak intensity with water fraction of compound OligoPV–TPA. (e) Corresponding emission spectra of compound OligoPV–T in aqueous THF
with different water–THF ratios at 10ꢁ7 M. Inset: fluorescence emission image of OligoPV–T (fw ¼ 0, 50 and 90%). (f) Normalized fluorescence
(FL) spectra in powdered state. Inset: photographs in powder form. All photos are taken under UV illumination. Excitation wavelength: 365 nm.
absorption peak (360 nm) is slightly red shied compare to that (excited at 324 nm) and OligoPV–T (excited at 388 nm) in THF
of OligoPV–C (324 nm) and OligoPV–T (312 nm). The red shi resulted in an emission at 504 nm and 516 nm respectively. All
we ascribe to the increased p-electron density on the central absorption and emission spectra are blue-shied in less polar
phenylenevinylene segment, arising from the electron-donating solvent (toluene) compare to THF. The emission peak of Oli-
nature of the triphenylamine moiety. Optical absorption spec- goPV–TPA was blue shied about 5 nm in toluene than that of
trum of OligoPV–C was slightly red shied by ca. 12 nm in THF, which showed that there was an obvious much more p–p*
comparison to OligoPV–T which could be attributed to the delocalization with the introduction of the triphenylamine
carbazole group having the high electron-donating ability moiety. Same type of blue shi was also observed in OligoPV–C
compare to the thiophene group. The uorescence emission (4 nm) and OligoPV–T (7 nm). The high molar extinction coef-
bands of OligoPV–TPA, OligoPV–C and OligoPV–T were almost cient values of these compounds OligoPV–TPA, OligoPV–C and
identical (Fig. 1b), only one emission band was observed in the OligoPV–T (51 600–91 696 L molꢁ1 cmꢁ1) compared to TPA, C
green region. The uorescence spectrum of OligoPV–TPA and T (12–3493 L molꢁ1 cmꢁ1) can be reasoned as a combined
(exited at 360 nm) in THF showed a sharp emission band at effect of high molecular weights and of the presence of multi
515 nm. Similarly, the uorescence spectrum of OligoPV–C conjugated OligoPV–core in each molecule. The Stokes' shis of
62686 | RSC Adv., 2014, 4, 62684–62688
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
OligoPV–TPA, OligoPV–C, and OligoPV–T were calculated to be maximum uorescence intensity has been recorded, and the
0.84, 1.12, and 0.64 nm in THF and 0.85, 1.10, and 0.61 nm in corresponding amplication factor aAIEE (aAIEE ¼ Iaggr/Isoln) is
toluene respectively (Table 1). The negligible Stokes' shis 8.4. Aer reaching a maximum, PL intensities decreased with
indicate that the intramolecular charge transfer (ICT) properties higher water content. This phenomenon was oen observed in
for these luminogens are very weak. The uorescence quantum some compounds with AIEE properties, but the reason remains
yields (F) of OligoPV–TPA, OligoPV–C and OligoPV–T are unclear.15 One possible explanation is that in a mixture with a
measured in different solvents, by using Rhodamine B as a “low” water fraction, solute molecules steadily assemble in an
standard, presented in Table 1. We found the F values of ordered pattern to form more emissive, crystalline aggregates.
OligoPV–TPA, OligoPV–C and OligoPV–T to be in the range of In a mixture with “high” water content, solute molecules
0.18–0.23. It was found that the F value of OligoPV–C is higher quickly agglomerate in a random way to form less emissive,
than that of OligoPV–TPA and OligoPV–T in diluted solution. amorphous counterparts.16
This may be due to the slight rotor nature of compound Oli-
Similar enhancement was observed for oligomer OligoPV–T.
goPV–TPA and OligoPV–T i.e. triphenylamine/thiophene moiety When the water fraction was increased from 0 to 90%, the
undergoes active intramolecular rotations in the solution which uorescence intensities of OligoPV–T were enhanced 5.8-fold
effectively quenching the light emission. The HOMO–LUMO (Fig. S2, ESI†). The measured PL intensity shows no regularity in
gaps (DE) of luminogens OligoPV–TPA, OligoPV–C and Oli- high water content.17 The results indicate that the AIEE effect of
goPV–T are 3.48, 3.87 and 3.19 eV in toluene and 3.44, 3.82 and the compound OligoPV–TPA is higher than that of OligoPV–T.
3.19 eV in THF respectively. It is worth noting that the change of But, compound OligoPV–C exhibited different phenomenon in
the substituent groups from triphenylamine, carbazole and aggregation study. In aggregate state, the emission behavior is
thiophene moieties causes a little inuence on band gap of quenched for OligoPV–C i.e., when the water fraction was
compound. The emission spectra are also measured in the increased from 0 to 90%, the uorescence intensity of OligoPV–
powdered state of the compounds (Fig. 1). Oligomer OligoPV–C C was correspondingly decreased (3.6-fold). This decrease in
exhibited red shied (ꢃ30 nm) emission in the powdered state uorescence intensity may be due the aggregation-caused
compared to those observed in THF or toluene solution (Table quenching (ACQ) effect. Probably the planar architecture of
1). Moreover, this derivative shows red shied broad peak in the OligoPV–C facilitates the ACQ in highly concentrated solution.
powdered state at around 534 nm indicates molecular aggre-
The thermal properties of the oligomers were analyzed by
gation or excimer formation in the solid state. The emission thermogravimetric analysis (TGA) and are presented in Table 1.
spectra of Oligomers OligoPV–TPA and OligoPV–T in powdered These compounds exhibited good thermal stabilities. The
state displayed slightly red shied peak about 7 nm and 17 nm decomposition temperatures with 5% weight loss under N2
respectively compare to solution state in toluene.
atmosphere (Td) for OligoPV–TPA, OligoPV–C and OligoPV–T
For determining the aggregation behavior of synthesized were 340, 390 and 352 ꢀC, respectively (Fig. 1). The Td values
luminogens, the uorescence behaviour of their diluted decreased with the increasing rotor nature of oligomers.
mixtures was studied in a mixture of water/THF under different
water fractions.
In summary, three new thermally stable OligoPV-cored lumi-
nogens were synthesized and characterized. Out of three, two are
Since the compounds were insoluble in water, increasing the AIEE-active molecules: weakly uorescent in solutions, while
water fraction in the mixed solvent could change their existing highly emissive in the aggregation state. The presence of the
forms from a solution or well-dispersed state in THF to the OligoPV core has endowed to turn an ACQ luminophore to an
aggregated particles in the aq. THF with high water content. AIEE one. These properties suggest that p-phenylenevinylene-
Changes in the PL peak intensities vs. water fraction of the cored luminogens can be employed in light-emitting application.
mixture for compound OligoPV–TPA were plotted, as shown in
Fig. 1 as an example.
Acknowledgements
In THF–water mixtures with low water fraction (fw # 40%),
the uorescence intensity remain unchanged. When fw is higher
P.R. thanks the University Grants Commission (UGC), New
than 40%, the solutions emit bright yellowish green light with
Delhi, India for the award of Dr D. S. Kothari Postdoctoral
lem at 519 nm upon photoexcitation. The pronounced uores-
Fellowship [no. F.13-755/2012(BSR)].
cence enhancement phenomenon can be interpreted as the
aggregation of OligoPV–TPA molecules at higher fw values.
According to the above-mentioned RIR mechanism, when the
OligoPV–TPA is dissolved in THF or THF–water mixtures with fw
Notes and references
# 40%, the active intramolecular rotations of multiple phenyl
rings exhaust the energy of the excited states and result in
uorescence quenching. When more water, a nonsolvent for
OligoPV–TPA, is added into the solution, the molecules tend to
form aggregates, thus the intramolecular rotations of the
phenyl rings are restricted and the excited states decay to the
corresponding ground states through irradiative channels
thereby strong emission is recorded. At the fw of 70%, a
1 (a) J. H. Burroughes, D. D. C. Bradley, A. R. Brown,
R. N. Marks, K. Mackay, R. H. Friend, P. L. Nurns and
´
A. B. Holmes, Nature, 1991, 347, 539; (b) J.-L. Bredas,
D. Beljonne, V. Coropceanu and J. Cornil, Chem. Rev., 2004,
104, 4971.
2 (a) S. Kim, T. Ohulchanskyy, H. Pudavar, R. Pandey and
P. Prasad, J. Am. Chem. Soc., 2007, 129, 2669; (b) G. Zhang,
J. Lu, M. Sabat and C. Fraser, J. Am. Chem. Soc., 2010, 132,
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 62684–62688 | 62687

View Article Online
RSC Advances
Communication
2160; (c) C. Dou, D. Chen, J. Iqbal, Y. Yuan, H. Zhang and
Y. Wang, Langmuir, 2011, 27, 6323.
3 A. C. Grimsdale, K. Leok Chan, R. E. Martin, P. G. Jokisz and
A. B. Holmes, Chem. Rev., 2009, 109, 897.
4 (a) R. Deans, J. Kim, M. R. Machacek and T. M. Swager, J. Am.
Chem. Soc., 2000, 122, 8565; (b) B. K. An, S. K. Kwon,
S. D. Jung and S. Y. Park, J. Am. Chem. Soc., 2002, 124, 14410.
5 (a) J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang and
B. Z. Tang, Adv. Mater., 2014, 26, 5429; (b) R. Hu,
N. L. C. Leungb and B. Z. Tang, Chem. Soc. Rev., 2014, 43,
4494.
Y. Hong, Y. Dong, M. Haussler, J. W. Y. Lam, Z. Li, Z. Guo
and B. Z. Tang, Chem. Commun., 2006, 3705; (d) M. Wang,
D. Zhang, G. Zhang, Y. Tang, S. Wang and D. Zhu, Anal.
Chem., 2008, 80, 6443; (e) Y. Liu, C. M. Deng, L. Tang,
A. Qin, R. R. Hu, J. Z. Sun and B. Z. Tang, J. Am. Chem.
Soc., 2011, 133, 660; (f) Y. Liu, A. J. Qin, X. J. Chen,
X. Y. Shen, L. Tong, R. R. Hu, J. Z. Sun and B. Z. Tang,
Chem.–Eur. J., 2011, 17, 14736; (g) J. Wang, J. Mei,
W. Z. Yuan, P. Lu, A. Qin, J. Z. Sun, Y. G. Ma and
B. Z. Tang, J. Mater. Chem., 2011, 21, 4056.
10 Z. Zhao, S. Chen, J. W. Y. Lam, P. Lu, Y. Zhong, K. S. Wong,
H. S. Kwoka and B. Z. Tang, Chem. Commun., 2010, 46, 2221.
6 (a) Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun.,
ˇ
ˇ
2009, 4332; (b) Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. 11 P. Galer, R. C. Korosec, M. Vidmar and B. Sket, J. Am. Chem.
Soc. Rev., 2011, 40, 5361. Soc., 2014, 136, 7383.
7 (a) J. Huang, X. Yang, X. Li, P. Chen, R. Tang, F. Li, P. Lu, 12 1,4-bis(dodecyloxy)benzene was prepared by alkylation of
Y. Ma, L. Wang, J. Qin, Q. Li and Z. Li, Chem. Commun.,
2012, 48, 9586; (b) J. Huang, N. Sun, J. Yang, R. Tang,
Q. Li, D. Ma, J. Qin and Z. Li, J. Mater. Chem., 2012, 22,
12001; (c) Z. Zhao, J. Geng, Z. Chang, S. Chen, C. Deng,
T. Jiang, W. Qin, J. W. Y. Lam, H. S. Kwok, H. Qiu, B. Liu
and B. Z. Tang, J. Mater. Chem., 2012, 22, 11018; (d) X. Du,
1,2-dihydroquinone with 1-bromododecane in the presence
of K2CO3 and DMF, according to the literature procedure.
A. Llanes-Pallas, K. Yoosaf, H. Traboulsi, J. Mohanraj,
T. Seldrum, J. Dumont, A. Minoia, R. Lazzaroni,
N. Armaroli and D. Bonifazi, J. Am. Chem. Soc., 2011, 133,
15412.
J. Qi, Z. Zhang, D. Ma and Z. Y. Wang, Chem. Mater., 2012, 13 This compound was prepared using a slight modication of
24, 2178; (e) B. Xu, Z. Chi, J. Zhang, X. Zhang, H. Li, X. Li,
S. Liu, Y. Zhang and J. Xu, Chem.–Asian J., 2011, 6, 1470; (f)
C. Y. K. Chan, Z. Zhao, J. W. Y. Lam, J. Liu, S. Chen, P. Lu,
the reported literature procedure. B. C. Thompson,
Y.-G. Kim, T. D. McCarley and J. R. Reynolds, J. Am. Chem.
Soc., 2006, 128, 12714.
M. Faisal, X. Chen, H. H. Y. Sung, H. S. Kwok, Y. Ma, 14 Carbazole
was
prepared
through
intramolecular
I. D. Williams, K. S. Wong and B. Z. Tang, Adv. Funct.
Mater., 2012, 22, 378; (g) Y. Sun, X. Liang, S. Wei, J. Fan
and X. Yang, Spectrochim. Acta, Part A, 2012, 97, 352.
palladium(II)-catalyzed oxidative carbon–carbon bond
´
formation of diphenylamine, see: B. Liegault, D. Lee,
M. P. Huestis, D. R. Stuart and K. Fagnou, J. Org. Chem.,
2008, 73, 5022.
8 (a) W. Qin, D. Ding, J. Liu, W. Z. Yuan, Y. Hu, B. Liu and
B. Z. Tang, Adv. Funct. Mater., 2012, 22, 771; (b) Y. Hong, 15 (a) Y. Dong, J. W. Y. Lam, A. Qin, Z. Li, J. Sun, H. H. Y. Sung,
L. Meng, S. Chen, C. W. T. Leung, L.-T. Da, M. Faisal,
D.-A. Silva, J. Liu, J. W. Y. Lam, X. Huang and B. Z. Tang, J.
Am. Chem. Soc., 2012, 134, 1680; (c) Y. Hong, C. Feng,
Y. Yu, J. Liu, J. W. Y. Lam, K. Q. Luo and B. Z. Tang, Anal.
Chem., 2010, 82, 7035.
I. D. Williams and B. Tang, Chem. Commun., 2007, 40; (b)
¨
H. Tong, Y. Hong, Y. Dong, Y. Ren, M. Haubler,
J. W. Y. Lam, K. S. Wong and B. Tang, J. Phys. Chem. B,
2007, 111, 2000; (c) J. Liu, J. W. Y. Lam and B. Tang, J.
Inorg. Organomet. Polym. Mater., 2009, 19, 249.
9 (a) Y. Hong, M. Haussler, J. W. Y. Lam, Z. Li, K. K. Sin, 16 Y. Dong, J. W. Y. Lam, A. Qin, J. Sun, J. Liu, Z. Li, J. Sun,
Y. Dong, H. Tong, J. Liu, A. Qin, R. Renneberg and
H. H. Y. Sung, I. D. Williams, H. S. Kwok and B. Z. Tang,
B. Z. Tang, Chem.–Eur. J., 2008, 14, 6428; (b) Y. Dong,
Chem. Commun., 2007, 3255.
J. W. Y. Lam, A. Qin, Z. Li, J. Liu, J. Sun, Y. Dong and 17 X. Zhang, Z. Chi, B. Xu, C. Chen, X. Zhou, Y. Zhang, S. Liua
B. Z. Tang, Chem. Phys. Lett., 2007, 446, 124; (c) H. Tong, and J. Xu, J. Mater. Chem., 2012, 22, 18505.
62688 | RSC Adv., 2014, 4, 62684–62688
This journal is © The Royal Society of Chemistry 2014