Violet/deep-blue fluorescent organic light-emitting diode based on high-efficiency novel carbazole derivative with large torsion angle

Article abstract of DOI:10.1016/j.tetlet.2019.151340

A novel and highly efficient violet/deep-blue fluorescent carbazole and naphthalene-based compound (1) is designed and synthesized. The compound shows intensive violet/deep-blue fluorescence, high photoluminescence efficiency (0.72 in CH₂Cl₂, 0.65 in film) and narrow full width at half maximum (48 nm). The large torsion angles between carbazole and naphthalene guarantee the weak intermolecular interactions and suppress the π-π interactions in solid state, resulting in the highly efficient violet/deep-blue fluorescence. The maximum emission peak, luminance and external quantum efficiency for violet/deep-blue electroluminescence are 410 nm, 1326 cd/m² and ～2%, respectively.

Full text of DOI:10.1016/j.tetlet.2019.151340

Journal Pre-proofs
Violet/deep-blue fluorescent organic light-emitting diode based on high-effi-
ciency novel carbazole derivative with large torsion angle
Xiaoting Feng, Xiaozhen Wei, Mengna Yin, Yanqin Miao, Peng Tao
PII:
S0040-4039(19)31127-X
https://doi.org/10.1016/j.tetlet.2019.151340
TETL 151340
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
6 September 2019
30 September 2019
29 October 2019
Please cite this article as: Feng, X., Wei, X., Yin, M., Miao, Y., Tao, P., Violet/deep-blue fluorescent organic light-
emitting diode based on high-efficiency novel carbazole derivative with large torsion angle, Tetrahedron Letters
(2019), doi: https://doi.org/10.1016/j.tetlet.2019.151340
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover
page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will
undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing
this version to give early visibility of the article. Please note that, during the production process, errors may be
discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier Ltd.

Graphical Abstract
Violet/deep-blue fluorescent organic light-
emitting diode based on high-efficiency novel
carbazole derivative with large torsion angle
Xiaoting Feng a, Xiaozhen Weib, Mengna Yinb, Yanqin Miaob,*,
and Peng Taob,*
A novel and highly efficient violet/deep-blue fluorescent carbazole-based compound (1) is designed and
synthesized. The compound shows intensive violet/deep-blue fluorescence, high photoluminescence efficiency
(0.72 in CH2Cl2, 0.65 in film) and narrow full width at half maximum (48 nm). The large torsion angles
between carbazole and naphthalene guarantee the weak intermolecular interactions and suppress the π-π
interactions in solid state, resulting the highly efficient violet/deep-blue fluorescence. The maximum emission
peak, luminance and external quantum efficiency for violet/deep-blue electroluminescence are 410 nm, 1326
cd/m2 and ～2%, respectively.
Highly efficientviolet/deep-blue fluorescentmaterial
N
1.0
0.8
 High ΦPL in violet/deep-blue region;
 Large torsion angle between
0.6
carbazole and naphthalene;
 Weak intermolecular interactions
0.4
410 nm
(0.16, 0.04)
in solid;
 Realization of violet/deep-blue OLED
0.2
0.0
400
500
600
700
Wavelength (nm)

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Violet/deep-blue fluorescent organic light-emitting diode based on high-
efficiency novel carbazole derivative with large torsion angle
Xiaoting Fenga, Xiaozhen Weib, Mengna Yinb, Yanqin Miaob,*, and Peng Taob,*
a Department of Chemistry and Chemical Engineering, Lu Liang University, Lvliang, 033001, P.R. China.
bMOE Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, P.R. China.
Corresponding E-mail: miaoyanqin@tyut.edu.cn (Y. Miao); iamtaopeng@gmail.com (P. Tao).
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A novel and highly efficient violet/deep-blue fluorescent carbazole and naphthalene-based
compound (1) is designed and synthesized. The compound shows intensive violet/deep-blue
fluorescence, high photoluminescence efficiency (0.72 in CH2Cl2, 0.65 in film) and narrow full
width at half maximum (48 nm). The large torsion angles between carbazole and naphthalene
guarantee the weak intermolecular interactions and suppress the π-π interactions in solid state,
resulting in the highly efficient violet/deep-blue fluorescence. The maximum emission peak,
luminance and external quantum efficiency for violet/deep-blue electroluminescence are 410
nm, 1326 cd/m2 and ～2%, respectively.
Available online
Keywords:
Carbazole
Naphthalene
2017 Elsevier Ltd. All rights reserved.
Organic light-emitting diodes
Violet/deep-blue fluorescence
The blue emission is the essential chromaticity component for
both display and solid-state lighting.[1,2] Deep-blue emitters
greatly affect the efficiency and lifetime of organic light-emitting
diodes (OLEDs) due to their inherent defect, thus limiting
industrialization of organic electroluminescence (EL).[3,4] The
current materials system for blue emitters are the pure organic
fluorescent materials, noble metal-based phosphors and thermally
activated delayed fluorescent (TADF) materials.[5] Although the
phosphors and TADF materials could realize high peak
efficiency, the high cost and efficiency roll-off are still the main
challenge suffering from the triplet-triplet annihilation of
excitons under the high current density. In addition, it is quite
difficult to achieve pure blue emission (deep-blue emission) for
phosphors and TADF materials, and they often show broad
emission spectrum, reducing the color purity. Thus, from the
above point of view, the pure organic emitters with flexible
structural design ability have plenty room to develop pure blue
emitters. Very recently, we developed novel blue fluorescent
emitters structured by linking triphenylamine, naphthalene and
anthracene derivatives for OLEDs with external quantum
efficiency (EQE) exceeding 5%, however, the 1931 Commission
Internationale de L’Eclairage (CIE) coordinates of this device is
still not in the region of deep-blue.[2a] This encourages us to
enrich this family of blue emitters by more rational molecular
design for further realizing the deep-blue electroluminescence.
In this letter, one novel highly efficient violet/deep-blue
fluorescent emitter (1) was developed by linking carbazole and
naphthalene with large torsion angles. The large torsion angles
between carbazole and naphthalene moiety could not only give
rise to the weak intermolecular interactions and suppress the π-π
interactions in solid state, guaranteeing the highly efficient
violet/deep-blue fluorescence. The fluorescent emitter developed
shows bright violet/deep-blue fluorescence with quite high
photoluminescence efficiency (ΦPL) and narrow full width at half
maximum (FWHM) suitable for high-performance deep-blue
electroluminescence. The maximum emission peak, brightness
and external quantum efficiency for violet/deep-blue
electroluminescence are 410 nm, 1326 cd/m2 and ～ 2%,
respectively.
The highly efficient violet/deep-blue fluorescent carbazole and
naphthalene-based compound was synthesized in several steps
with high yields. As shown in Scheme 1, the target compound
was prepared in high yield of 92% by coupling reaction of
naphthalen-1-ylboronic acid with 3,6-dibromo-9-butyl-9H-
carbazole under the under catalysis of Pd(PPh3)4 in toluene at 85
oC. And 3,6-dibromo-9-butyl-9H-carbazole was obtained through
deprotonation, alkylation and bromination in one-pot process.
1.NaH, dry DMF, 0oC
H
2.1-bromobutane, 25oC
N
N
3.NBS, dry DMF, 25oC
Br
Br
HO
HO
B
N
ORTEP of 1
1
Scheme 1. Synthetic route, chemical structure and the ORTEP diagram of 1.
The compound was purified as colorless crystals and fully

2
confirmed by nuclear magnetic resonance (NMR), mass spectra,
highlighted by grey. H atoms and butyl groups were removed for clarity.
elemental analyses and single crystal X-ray diffraction
(Supporting Information). The four-carbon chain butyl group
was introduced into the carbazole for increasing the solubility in
common organic solvent. The NMR spectra were recorded in
chloroform-d. Although the much large torsion angles between
carbazole and naphthalene moiety, in the proton NMR, eight
well-resolved signals of aromatic protons were observed,
corresponding to the two equivalent part of the whole molecule,
originated from the C2 symmetry owing to the rapid rotation of
naphthalene unit in the solution state at room temperature.
Similar situation was observed in carbon NMR, there are sixteen
resonances in the aromatic region of 13C NMR spectrum
corresponding to the sixteen different aromatic carbons on the
two equivalent part of the whole molecule. Resonance signal in
4.47 ppm should be assigned to the proton attached to the carbon
bonded to the nitrogen atom of carbazole. Quite different from
the solution state, the large torsion angles between carbazole and
naphthalene moiety of compound 1 were clearly observed in the
single crystals, as shown in Figure 1, torsion angles are up to
a)
1.00
0.75
0.50
0.25
0.00
1.00
0.75
0.50
0.25
0.00
in DCM
in DCM
in film
250 300 350 400 450 500 550 600
Wavelength (nm)
b)
o
59.09 and 58.42 , respectively. Such large torsion angles indeed
induce much weaker intermolecular interactions and suppress the
π-π interactions in solid state, resulting in the highly efficient
violet/deep-blue fluorescence. In the crystals, only weak CH-π
interactions between adjacent molecules could be observed
(Figure 1). Notably, this large torsion angle design strategy
effectively reduce the intermolecular π-π interactions, which
commonly cause the red shift of emission spectrum and the
aggregation induced quenching of fluorescent materials.
HOMO
LUMO
Figure 2. (a) UV-visible absorption, photoluminescence spectra of 1 at 298 K
(in CH2Cl2 and film); (b) Calculated electron cloud distributions of
HOMO/LUMO of 1.
The photophysical properties of emitter 1 were explored in
dichloromethane, film and exhibited in Figure 2a and Table 1,
1
S1. In the solution of dichloromethane, the π→π* transition of
cloud distributions of highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) of 1 were
shown in Figure 2b.[6] The electron cloud of HOMO distribute
over almost the entire molecule, while the electron cloud of
LUMO mainly distribute in the two naphthalene moieties, which
is consistent with the fact that the electron density of carbazole is
much higher than that of naphthalene. The ΦPL of the emitter is
quite high up to 0.72 in dichloromethane, and the lifetime of the
emitter is 5.81 ns in CH2Cl2. The emission exhibited narrow
FWHM (48 nm). In the neat film, the ΦPL of the emitter is also as
high as 0.65, and the lifetime of the emitter is 6.02 ns. The
emission spectrum shows much narrower FWHM (39 nm). The
short lifetime of the excited states indicate the nature of
fluorescence. We noted that the high ΦPL of the emitter in neat
film may have positive effect on the performance of
electroluminescence induced by the less extent of aggregation
induced quenching. The cyclic voltammetry in CH2Cl2 was
carried out to investigate the electrochemical property of
compound. As shown in Figure S1, the compound shows
reversible oxidation wave and the oxidation potential was
calculated to be 0.80 V. The carbazole oxidation should be
responsible for this positive oxidation potential.[3a] The energy
levels of the highest occupied molecular orbital and the lowest
unoccupied molecular orbitals of compound based on the cyclic
voltammetry are estimated to be -5.60 and -2.42 eV, respectively.
To evaluate the electroluminescence performance, the
violet/deep-blue emitter was further used to prepare the
electroluminescent device. As depicted in Figure S2a, the non-
doped violet/deep-blue OLEDs were fabricated with the
structures of indium-tin-oxide (ITO) (180 nm)/molybdenum
trioxide (MoO3) (3 nm)/ di-(4-(N,N-di-p-tolyl-amino)-
phenyl)cyclohexane (TAPC) (40 nm)/ 4,4',4"-tris(carbazol-9-
yl)triphenylamine (TCTA) (10 nm)/emitter 1 (x nm)/ 2,2',2"-
carbazole moiety in emitter 1 corresponds to the weak absorption
band in the wavelength of 275-350 nm. The strong absorption in
the wavelength of 220-275 nm is probably attributed to the
1π→π* transitions of naphthalene unit.[2a] The emitter shows
intensive violet/deep-blue fluorescent emission with a peak of
390 nm in dichloromethane under the excitation of 305 nm. The
emission of emitter in the film was slightly red shift to 400 nm
under the same excitation wavelength. The slight red shift (20 nm)
of the emission was attributed to agglomeration of molecules in
solid states. It should be noted that the large torsion angles in the
molecule guarantee the weak intermolecular interactions and
suppress the π-π interactions in solid state, resulting in the highly
efficient violet/deep-blue fluorescence. The calculated electron
3.626 Å
59.09 o
58.42 o
Figure 1. The intermolecular packing of 1 by CH-π interactions between
adjacent molecules in crystals, and the torsion angles between carbon atoms
Table 1. Photophysical and electrochemical properties for emitter 1.

3
c)
Emissiona)
λem [nm]
Eg
HOMO/ LUMO
[eV]c)
ox b)
Eonset
Compound
[eV]
FWHM [nm]
48 (CH2Cl2)
39 (film)
τ [ns]
ΦPL
[eV]
390 (CH2Cl2)
400 (film)
5.81 (CH2Cl2)
6.02 (film)
0.72 (CH2Cl2)
0.65 (film)
1
0.80
3.18
-5.60/-2.42
a)At a concentration of 1.0 × 10-5 mol/L in CH2Cl2 or neat film, λex = 305 nm; b)In CH2Cl2; c)HOMO (eV) = -e(Eonsetox + 4.8), Eg = 1240/λ, LUMO (eV) = Eg +
HOMO.
(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) (50
nm)/ lithium fluoride (LiF) (1 nm)/ aluminum (Al) (100 nm). In
this device, ITO was used as the anode, and MoO3 was used as
hole injection layer (HIL). TAPC layer and TPBi layer were used
as hole transport layer (HTL) and electron transport layer (ETL),
respectively. TCTA was used as exciton-blocking layer, and the
non-doped emitter 1 was used as light-emitting layer (EML). The
15 nm of neat film of emitter 1 was selected to investigate the
device performance. The energy levels and the illustration of
device structure of the violet/deep-blue OLED are depicted in
Figure S2. From the energy level diagram of OLED (Figure
S2b), the HOMO/LUMO levels of the emitter match with those
of the TCTA and TPBi, indicating a good carrier trapping in this
emitter layer,[7] which is helpful for achieving high EL
performance. As depicted in Figure 2a, the emitter 1-based
OLED show excellent violet/deep-blue fluorescence with
emission peak at 410 nm with narrow FWHM of 62 nm. This is
also well verified from the 1931 Commission Internationale de
L’Eclairage (CIE) coordinates of (0.16, 0.04) corresponding to
the deep-blue region (Inset in Figure 2b). It is also found that the
normalized EL spectra are overlapped quite well at various
applying voltages from 4 V to 7 V, implying that the EL spectra
is independent of the applying voltages and the fluorescence is
only originated from the aggregation state of emitter 1.[2a]
Notably, the
EL emission is slightly red shift to that of the neat film, which is
maybe the more compact stacking under the deposition of
vacuum evaporation. Form EQE-current density (EQE-J) curve
in Figure 2b, current efficiency-luminance-power efficiency
(CE-L-PE) curves and current density-voltage-luminance (J-V-L)
curves in Figure 3, the emitter 1-based violet/deep-blue device
also realizes acceptable EL performance. For example, the turn-
on voltages is only 3.4 V, and the maximum luminance, CE, PE
and EQE are 1326 cd/m2, 0.36 cd/A, 0.27 lm/W, and 1.79%,
respectively. It is noted that the efficiency roll-off is quite small
over the whole operation progress. Although this preliminary
device performance is not high, which could be attributed to the
less efficient device structure which do not well limit electron
and hole carrier in emitting layer, inducing a low exciton
utilization rate. Compared to the previously reported deep-blue
OLEDs based on the carbazole/anthracene derivative with ethyl
group,[3c] the emission wavelength of the reported device is about
440 nm with 1931 CIE coordinates of (0.18, 0.17) and peak EQE
of ～2%. So the compound designed herein have slightly longer
alkyl chain (butyl group) have great influence on the emission
property and lead to shorter emission wavelength (violet/deep-
blue
emission).
From
the
spectral
properties
of
electroluminescence, thses indicate the emitter 1 still have huge
a)
10
CE
PE
a)
CIE
1
0.1
1.0
4 V (0.16 0.04)
5 V (0.16 0.04)
0.8
0.6
0.4
0.2
0.0
1
6 V (0.16 0.04)
7 V (0.16 0.04)
0.1
0.01
@ 6V
0.01
100
101
102
103
400
500
600
700
Luminance (cd/m2)
Wavelength (nm)
2.5
2
b)
b)
1,000
100
10
Current denisity
Luminance
600
1.5
1
450
300
150
0
1
0.5
1
0
100
200
300
400
Current denisity (mA/cm2)
4
5
6
7
8
9
10 11
Voltage (V)
Figure 2. The EL spectra (a) and EQE-J curve (b) of violet/deep-blue OLED
based on 1. Inset: EL photograph (up) and1931 CIE coordinate (down) of
device at 6 V.
Figure 3. The CE-L-PE curves (a), J-V-L curves (b) of violet/deep-blue
OLED based on 1.
Table 2. EL data for the violet/deep-blue-emitting devices.

4
x
λEL/nm
CIE (x, y) a)
(0.16, 0.04)
VONb)/V
Lmax/cd.m-2
CEmax/cd.A-1
PEmax/lm.W-1
EQE/%
FWHM/nm
62
10
410
3.4
1326
0.36
0.27
1.79
a)Current density is 100 mA.cm-2; b)Luminance is 1 cd.m-2
2017, 19, 29629; (d) E. Baranoff and B. F. E. Curchod, Dalton Trans., 2015,
44, 8318.
potential application in the development of violet/deep-blue
OLEDs.
[6] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J.
R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H.
Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G.
Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G.
A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian09, Revision
B.01; Gaussian Inc., Wallingford, CT, 2010.
In summary,
a novel highly efficient violet/deep-blue
fluorescent emitter was developed by linking carbazole and
naphthalene with large torsion angles. Owing to the large torsion
angles between carbazole and naphthalene moiety, the π-π
interactions in solid state was suppressed, ensuring the highly
efficient violet/deep-blue fluorescence. The designed emitter
shows bright violet/deep-blue emission at wavelength of 400 nm
with high ΦPL up to 0.65 in neat film, and the FWHM of emitter
is narrow (39 nm). The maximum emission peak, luminance and
EQE for violet/deep-blue electroluminescence with 1931 CIE
coordinates of (0.16, 0.04) are 410 nm, 1326 cd/m2 and ～2%,
respectively. The further exploration of this deep-blue fluorescent
material toward high-performance deep-blue electroluminescence
is going on in our laboratory.
[7] (a) Y. Miao, X. Wei, L. Gao, K. Wang, B. Zhao, Z. Wang, B. Zhao, H.
Wang, Y. Wu and B. Xu, Nanophotonics 2019,8(10), 1783; (b) Mężyk, J.;
Kalinowski, J.; Meinardi, F.; Tubino, R. Appl. Phys. Lett., 2005, 86, 111916;
(c) Y. Miao, K. Wang, B. Zhao, L. Gao, P. Tao, X. Liu, Y. Hao, H. Wang, B.
Xu and F. Zhu, Nanophotonics 2018, 7, 295.
Acknowledgments
The authors acknowledge National Natural Scientific Foundation
of China (61905120), China Postdoctoral Science Foundation
Funded Project (2018M640506), Shanxi Province Science
Foundation for Youths (201801D221110, 201801D221106),
Scientific and Technological Innovation Foundation of the
Higher Education Institutions of Shanxi Province (2019L0950,
2019L0946) and the Key Research Program of Lvliang City
(201701103) for financial support.
Highly efficientviolet/deep-blue fluorescentm
N
Supplementary data
1.0
0.8
Supplementary data (details of the compound synthesis, NMR,
and the device fabrication and testing) associated with this
article can be found in Supporting Information.


High ΦPL in violet/deep-blue
Large torsion angle betwee
carbazole and naphthalene
Weak intermolecular interac
in solid;
0.6
0.4
0.2
0.0
References and notes


410 nm
(0.16, 0.04)
[1] (a) P. Tao, Y. Miao, H. Wang, B. Xu and Q. Zhao, Chem. Rec. 2019, 19,
1531; (b) P .Tao, Y. B. Zhang, J. Wang, L. W. Wei, H. X. Li, X. L. Li, Q.
Zhao, X. W. Zhang, S. J. Liu, H. Wang, W. Huang, J. Mater. Chem. C 2017,
5, 9306; (c) Y. Miao, P. Tao, L. Gao, X. Li, L. Wei, S. Liu, H. Wang, B. Xu,
and Q. Zhao, Submitted to J. Mater. Chem. C 2018, 6, 6656.
Realization of violet/deep-bl
[2] (a) J. Zhang, Y. Zhao, H. Xu, D. Zhang, Y. Miao, R. Shinar, J. Shinar, H.
Wang, B. Xu and Y. Wu, J. Mater. Chem. C 2019, doi: 10.1039/c9tc02773d;
(b) M. Bian, Z. Chen, B. Qu and L. Xiao, Chem. Rec. 2019, 19, 1562; (c) J.
Lee, H. Chen, T. Batagoda, C. Coburn, P. Djurovich, M. Thompson and S.
Forrest, Nat. Mater. 2016, 15, 92.
400
500
600
700
Wavelength (nm)
Highlights
[3] (a) H. Xu, P. Sun, K. Wang, J. Li, F. Wang, Y. Miao, H. Wang, B. Xu and
W. Y. Wong, J. Mater. Chem. C, 2017, 5, 4455; (b) J. J. Guo, X. L. Li, H.
Nie, W. W. Luo, S. F. Gan, S. M. Hu, R. R. Hu, A. J. Qin, Z. J. Zhao, S. J. Su
and B. Z. Tang, Adv. Funct. Mater., 2017, 27, 1606458; (c) S. Kim, I. Cho, M.
Sim, S. Park, S. Park, J. Mater. Chem., 2011, 21, 9139.



Highly efficient violet/deep-blue fluorescent
carbazole derivative is designed.
The large torsion angles results in the
violet/deep-blue fluorescence.
[4] (a) S. Lee, S. O. Kim, H. Shin, H. J. Yun, K. Yang, S. K. Kwon, J. J. Kim
and Y. H. Kim, J. Am. Chem. Soc., 2013, 135, 14321; (b) G. Sarada, W. Cho,
A. Maheshwaran, V. G. Sree, H. Y. Park, Y. S. Gal, M. Song and S. H. Jin,
Adv. Funct. Mater., 2017, 27, 1701002.
Violet/deep-blue OLED with λEL of 410 nm
and 1931 CIE coordinate of (0.16, 0.04).
[5] (a) C. Lin, Y. Chang, J. Hung, C. Lin, Yun Chi, M. Chung, C. Lin, P.
Chou, G. Lee, C. Chang, W. Lin, Angew. Chem. Int. Ed. 2011, 50, 3182; (b)
H. Fu, Y.-M. Cheng, P.-T. Chou and Y. Chi, Mater. Today 2011, 14, 472; (c)
S. Urinda, G. Das, A. Pramanik and P. Sarkar, Phys. Chem. Chem. Phys.,

5