Synthesis, structure, and physical properties of 5,7,14,16-tetraphenyl-8:9, 12:13-bisbenzo-hexatwistacene

Article abstract of DOI:10.1002/asia.201100733

A novel compound, 5,7,14,16-tetraphenyl-8:9,12:13-bisbenzo-hexatwistacene (TBH), has been successfully synthesized through a retro-Diels-Alder reaction. Single-crystal structure analysis indicated that TBH has a twisted configuration with a torsion angle

Full text of DOI:10.1002/asia.201100733

DOI: 10.1002/asia.201100733
Synthesis, Structure, and Physical Properties of 5,7,14,16-Tetraphenyl-
8:9,12:13-bisbenzo-hexatwistacene
Jinchong Xiao,[a, b] Shuwei Liu,[c] Yi Liu,[a] Li Ji,[d] Xuewei Liu,[d] Hua Zhang,[a]
Xiaowei Sun,*[c] and Qichun Zhang*[a]
Abstract:
A
novel
compound,
the difference between the half-wave
redox potentials (E1/2ox =+0.40 eV and
E1/2red =À1.78 eV) is 2.18 eV, which is
in good agreement with the band gap
(2.19 eV) derived from the UV/Vis ab-
sorption data. In addition, organic
light-emitting devices using TBH as
emitter have been fabricated. The re-
sults revealed that TBH is a promising
red light-emitting candidate for appli-
cations in organic light-emitting diodes.
5,7,14,16-tetraphenyl-8:9,12:13-bisben-
zo-hexatwistacene (TBH), has been
successfully synthesized through
retro-Diels–Alder reaction. Single-crys-
tal structure analysis indicated that
TBH has a twisted configuration with a
torsion angle of 27.348. The HOMO–
LUMO gap of TBH calculated from
a
Keywords: OLED
tors structure elucidation
synthetic methods · twistacene
· semiconduc-
·
·
Introduction
through molecular designs by introducing the twist concept
into p systems to enlarge the distance between neighboring
molecules and to reduce p-stacking interactions.[4] Because
polycyclic aromatic hydrocarbons (PAHs)[5] have been dem-
onstrated to be a promising system for OLEDs and their
properties can be tuned through the precise design of mole-
cules, it should be very interesting to integrate the twist con-
cept into PAHs.[6,7] Such integration could produce a novel
type of compounds, twistacenes. The twisted p-system in
such compounds should have a negligible effect on spectro-
scopic characteristics and electronic properties but reduce
the p-stacking interactions. Practically, a twistacene can be
realized by introducing bulky substituents on a conjugated
framework to create a more sterically hindered p-system.
Recently, an efficient white light-emitting diode based on a
novel “twistacene” has been demonstrated by Wudl and
Yang.[8]
During the past decades, organic semiconductors have at-
tracted considerable attention due to fundamental and tech-
nological interests.[1] Compared to inorganic semiconductors,
the possibility of flexible, low-cost, and large-area processing
makes organic materials more suitable for many applications
of devices such as light-emitting diodes (LEDs), field-effect
transistors (FETs), biosensors, lasers, memory devices, and
photovoltaic cells.[2] Recently, several promising results have
been reported in developing novel organic materials for
OLEDs.[3] However, many OLEDs suffer from the lower
emission intensity of organic materials caused by an aggre-
gation quenching effect. This problem could be solved
In our continued work,[7] we focus on extending twista-
cenes that contain only one pyrene unit at the end. Here, we
report the synthesis, structure, and physical properties of a
novel compound, 5,7,14,16-tetraphenyl-8:9,12:13-bisbenzo-
hexatwistacene (TBH, Scheme 1), and its application in
OLEDs.
[a] Dr. J. Xiao, Y. Liu, Prof. H. Zhang, Prof. Q. Zhang
School of Materials Science and Engineering
Nanyang Technological University
50 Nanyang Avenue, Singapore 639798 (Singapore)
Fax : (+65)67909081
E-mail: qczhang@ntu.edu.sg
[b] Dr. J. Xiao
Key Laboratory of Chemical Biology of Hebei Province
College of Chemistry and Environment Science
Hebei University
Baoding 071002 (P. R. China)
[c] S. Liu, Prof. X. Sun
School of Electrical and Electronic Engineering
Nanyang Technological University
50 Nanyang Avenue, Singapore 639798 (Singapore)
[d] L. Ji, Prof. X. Liu
School of Physical and Mathematical Sciences
Nanyang Technological University
1 Nanyang Walk, Singapore 637616 (Singapore)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100733.
Scheme 1. Molecular structure of 5,7,14,16-tetraphenyl-8:9,12:13-bisben-
zo-hexatwistacene (TBH).
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Results and Discussion
The synthetic route of TBH is presented in Scheme 2. The
precursors 1 and 2 were prepared according to reported
methods.[7a,b,9,10] The intermediate bridge compound 3 was
Scheme 2. Route for the synthesis of TBH.
obtained in 54% yield by injecting a solution of 1 in di-
chloroethane (DCE) and isoamyl nitrile alternately into a
solution of 2 in DCE under nitrogen atmosphere at reflux
temperature. Compound 3 was then heated at 2408C under
vacuum for 4 h to afford a red product (TBH), which was
1
characterized by FT-IR, H NMR, 13C NMR, MALDI-TOF
and HRMS spectroscopies (see the Supporting Informa-
tion). The synthesized twistacene TBH is soluble in
common organic solvents such as methylene chloride,
chloroform, tetrahydrofuran, and N,N-dimethylformamide.
A single crystal of twistacene TBH was grown by slowly
evaporating a solution of TBH in methylene chloride/metha-
nol (1:1) under nitrogen atmosphere at room temperature
(Figure 1). The twistacene TBH adopts a triclinic unit cell,
space group P1 (CCDC number: 841858). The parameter
data are: a=7.7160 ꢁ, b=11.1718 ꢁ, c=22.5538 ꢁ, a=
92.1108, b=99.1018, and g=102.1278. As shown in Fig-
ure 1a, the crystal structure displayed molecules with a
twisted topology with a torsion angle of 27.348. Figure 1b
shows the packing diagram along the a axis. Only the
pyrene units are involved in p-stacking interactions; the tet-
racene moieties do not show any p–p interactions. Com-
pounds with such a twisted structure could be very interest-
ing as materials in OLEDs.
The photophysical properties of compound TBH were
studied by UV/Vis and fluorescence spectroscopy. In meth-
ylene chloride, TBH presents three low-energy absorption
peaks at 491, 527, and 567 nm (Figure 2a), a typical charac-
teristic of polycyclic aromatic hydrocarbons.[4,7] TBH emits
red light, and the maximum emission peak is at 577 nm with
a shoulder at 623 nm under excitation at 491 nm (Figure 2a).
The quantum yield (Ff) of TBH in methylene chloride was
recorded as 0.02 using rhodamine 6G (ethanol solution, Ff =
0.95) as the reference at room temperature.[12] It should be
noted that the Stokes shift was very small (10 nm), which in-
dicates that the excited molecules have a small structural de-
formation. In the film, the absorbance of TBH was red-shift-
ed by 5–10 nm (peaks at 496, 537, and 575 nm, see Figure S1
in the Supporting Information). The emission spectrum of
Figure 1. a) Single-crystal structure of TBH and b) molecule stacking
along the a axis. Carbon atoms are shown as spheres and hydrogen atoms
are omitted.
[11]
ꢀ
Figure 2. a) UV/Vis and fluorescence spectra of TBH in CH2Cl2.
b) Cyclic voltammogram of TBH in TBAP/ODCB at a scan rate of
100 mVsÀ1
.
TBH in the thin film shows two peaks at 589 and 632 nm,
which are slightly red-shifted compared to those in solution.
The bathochromic effect might result from a higher percent-
age of aggregation in the solid state compared to that in
dilute solution.
The electrochemical behavior of TBH was measured at
room temperature in dry ortho-dichlorobenzene (ODCB)
containing 0.1m tetrabutylammonium perchlorate (TBAP)
as the supporting electrolyte. As shown in Figure 2b, the ox-
idation and reduction processes were found to be chemically
and electrochemically reversible. The HOMO–LUMO gap
of TBH calculated from cyclic voltammetry measurements
was 2.18 eV, a value very similar to the optical band gap
(2.19 eV) determined from the UV/Vis data. The thermal
gravimetric analysis shows that TBH is stable up to 3908C
(Figure 3).
562
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Chem. Asian J. 2012, 7, 561 – 564

5,7,14,16-Tetraphenyl-8:9,12:13-bisbenzo-hexatwistacene
Figure 3. Thermal gravimetric analysis (TGA) of TBH under nitrogen at-
mosphere at a heating rate of 108CminÀ1
.
Considering that the twisted structure could decrease the
p-stacking interactions, the OLED performance using TBH
molecules as emitting material was investigated. The devices
were fabricated by the physical vapor deposition method
(PVD), and the configurations of the OLED devices are as
follows: ITO/TBH (50 nm)/Alq3 (50 nm)/LiF (0.6 nm)/Al
(100 nm) (OLED-1) and ITO/TBH (50 nm)/BCP (10 nm)/
Alq3 (50 nm)/LiF (0.6 nm)/Al (100 nm) (OLED-2), in which
ITO is indium tin oxide, Alq3 is tris(8-hydroxyquinolinato)a-
luminum, and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phe-
nanthroline.
Figure 4. Characteristics of OLED-1 and OLED-2. a) EL spectra. b) V-L
and I-L curves. c) EQE-L and CE-L curves.
TBH is a promising red light-emitting candidate for such ap-
plications.
The only difference between OLED-1 and OLED-2 is the
BCP layer at the TBH/Alq3 interface. BCP and Alq3 have a
LUMO value of approximately 2.9 eV and 3.2 eV, respec-
tively. As a result, the additional barrier for electron injec-
tion is smaller in OLED-2. One could see that the current
efficiency of OLED-2 (0.04 cdAÀ1) was smaller than that of
OLED-1 (0.06 cdAÀ1), which suggested that TBH had a
very low quantum yield, consistent with the results of the
fluorescence test experiments.
Experimental Section
Materials
All chemical reagents were purchased from Alfa Aesar (VWR Singapore
Pte. Ltd., Singapore) or Sigma–Aldrich (Singapore) and used without
any further purification. The solvents were purified by standard methods.
Instrumentation
FT-IR spectra were measured on a Perkin–Elmer Spectrum GX spectro-
photometer using KBr pellets of compounds. 1H NMR and 13C NMR
spectra were recorded on a Bruker 300 MHz spectrometer. MALDI-
TOF mass spectrometric measurements were performed on a Shimadzu
Biotech AXIMA-TOF2TM instrument. UV/Vis and fluorescence spectra
of TBH in CH2Cl2 and in thin film were recorded on an UV-2501 and an
RF-5301 spectrophotometer (Shimadzu), respectively. X-ray crystallo-
graphic data were collected at 293 K using a Bruker APEX II CCD dif-
fractometer and graphite-monochromatic MoKa radiation (l=0.71073 ꢁ).
Empirical absorption was done, and the structure was solved by the
direct method and refined with the aid of the SHELX-TL program pack-
age. All hydrogen atoms were calculated and refined with a riding
model.
The electroluminescence (EL) spectra of both OLEDs
displayed peaks at 596 and 655 nm; OLED-1 exhibited an
additional peak at 524 nm (Figure 4a). The latter peak be-
longs to Alq3, thereby further confirming that the electrolu-
minescence results from the TBH layer in OLED-2 and that
the higher efficiency of OLED-1 is due to light emission
from the Alq3 side. The current efficiency (CEmax) and exter-
nal quantum efficiency (EQE) of device OLED-2 were
0.04 cdAÀ1 and 0.07%, respectively, at a current density of
20 mAcmÀ2. The maximum luminance (Lmax
)
was
Synthesis of 3
262 cdmÀ2. These experimental results also indicate that the
performance of the device was moderate; thus, an optimiza-
tion of the device fabrication is needed.
2-Methyl-1,4-diphenylisoquinolin-3-one (2, 170 mg, 0.54 mmol) was pre-
heated at 908C in degassed 1,2-dichloroethane (DCE). Subsequently, a
solution of compound 1 (281 mg, 0.54 mmol) in DCE and isoamyl nitrile
(1 mL) were alternately added during 1 h under a nitrogen atmosphere.
The mixture was kept overnight at reflux temperature. After that, the re-
action mixture was cooled to room temperature, washed with water, and
extracted with methylene chloride (40 mLꢂ3). The organic phase was
concentrated under reduced pressure, and the crude product was purified
by column chromatography (silica gel) with methylene chloride/petrole-
um ether (5:4, v/v) to afford the intermediate 3 as a light-yellow solid
(225 mg, 54%). FT-IR (KBr): n˜ =3056, 2926, 2871, 1689, 1455, 1440,
Conclusions
In summary, a novel conjugated molecule, 5,7,14,16-tetra-
phenyl-8:9,12:13-bisbenzo-hexatwistacene (TBH), has been
successfully synthesized and characterized. The reduced p–p
interactions caused by the twisted structure makes TBH an
interesting compound for applications in OLEDs. The char-
acteristics of the as-prepared OLED devices suggest that
1311, 1184, 828, 751, 704, 539 cmÀ1 1H NMR (300 MHz, CDCl3, 298 K):
;
d=8.23–8.20 (m, 2H), 7.92 (s, 1H), 7.88–7.83 (m, 6H), 7.78–7.76 (m,
2H), 7.70 (s, 1H), 7.57–7.29 (m, 16H), 7.11–7.08 (m, 2H), 6.97 (m, 1H),
6.75 (m, 1H), 2.84 ppm (s, 3H); 13C NMR (75 MHz, CDCl3, 298 K): d=
Chem. Asian J. 2012, 7, 561 – 564
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X. Sun, Q. Zhang et al.
FULL PAPERS
178.24, 143.59, 142.20, 141.83, 141.68, 139.22, 138.96, 136.85, 136.65,
134.42, 134.04, 131.94, 131.90, 131.69, 131.50, 130.71, 130.67, 130.48,
130.17, 130.12, 130.08, 129.69, 129.56, 129.23, 129.03, 128.98, 128.73,
128.58, 128.48, 127.97, 127.87, 127.36, 127.31, 127.22, 126.98, 126.87,
126.81, 126.32, 125.86, 125.80, 125.74, 125.44, 124.69, 124.65, 124.49,
123.95, 121.35, 70.92, 61.75, 35.65 ppm; MALDI-TOF MS: calcd for
g) L. Zayat, M. Salierno, R. Etchenique, Inorg. Chem. 2006, 45,
1728.
[3] a) J. E. Anthony, Chem. Rev. 2006, 106, 5028; b) T. Yang, F. Wudl,
Adv. Mater. 2009, 21, 1401; c) S.-J. Su, E. Gonmori, H. Sasabe, J.
Kido, Adv. Mater. 2008, 20, 4189; d) P. Sonar, M. S. Soh, Y. H.
Cheng, J. T. Henssler, A. Sellinger, Org. Lett. 2010, 12, 3292; e) J. N.
Moorthy, P. Natarajan, P. Venkatakrishnan, D. F. Huang, T. J. Chow,
Org. Lett. 2007, 9, 5215; f) Q. Niu, Y. Zhou, L. Wang, J. Peng, J.
Wang, J. Pei, Y. Cao, Adv. Mater. 2008, 20, 964; g) G. M. Farinola, R.
Ragni, Chem. Soc. Rev. 2011, 40, 3467; h) L. Xiao, Z. Chen, B. Qu,
J. Luo, S. Kong, Q. Gong, J. Kido, Adv. Mater. 2011, 23, 926.
[4] a) J. Lu, D. M. Ho, N. J. Vogelaar, C. M. Kraml, S. Bernhard, N.
Byrne, L. R. Kim, R. A. Pascal Jr, J. Am. Chem. Soc. 2006, 128,
17043; b) R. S. Walters, C. M. Kraml, N. Byrne, D. M. Ho, Q. Qin,
F. J. Coughlin, S. Bernhard, R. A. Pascal Jr, J. Am. Chem. Soc. 2008,
130, 16435.
[5] a) E. Clar, Polycyclic Hydrocarbons, Academic Press, London, 1964;
b) C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater.
2001, 11, 15; c) C. D. Dimitrakopoulos, A. R. Brown, A. Pomp, J.
Appl. Phys. 1996, 80, 2501; d) L. B. Roberson, J. Kowalik, L. M. Tol-
bert, C. Kloc, R. Zeis, X. Chi, R. Flemming, C. Wilkings, J. Am.
Chem. Soc. 2005, 127, 3069.
À
C58H37NO [M]: 763.29, found [M-(Me N=C=O)] 706.06. HR-MS: calcd
for C58H37NO [M+1], 764.2953; found, 764.2961.
5,7,14,16-tetraphenyl-8:9,12:13-bisbenzo-hexatwistacene (TBH)
The intermediate 3 (120 mg, 0.16 mmol) was purged three times with ni-
trogen gas and then gradually heated at 2408C for 4 h to form a red
powder. The crude product was washed thrice with ether, affording TBH
as a red solid (87 mg, 78%). FT-IR (KBr): n˜ =3056, 3022, 1600, 1483,
1435, 1394, 1305, 1168, 1072, 1024, 825, 756, 701, 543 cmÀ1 1H NMR
;
(300 MHz, CDCl3, 298 K): d=8.36 (s, 2H), 7.82–7.74 (m, 9H), 7.51–
7.23 ppm (m, 23H); 13C NMR (75 MHz, CDCl3, 298 K): d=141.77,
138.82, 137.34, 136.44, 132.12, 131.25, 130.76, 130.72, 130.63, 129.45,
129.26, 128.88, 128.56, 128.27, 127.21, 127.14, 126.72, 126.25, 126.11,
125.83, 124.91, 124.79 ppm; MALDI-TOF MS: calcd for C56H34 [M],
706.27; found [M], 706.17. HR-MS: calcd for C56H34 [M+1], 707.2739;
found, 707.2742.
[6] a) R. A. Pascal Jr, Chem. Rev. 2006, 106, 4809; b) H. M. Duong, M.
Bendikov, D. Steiger, Q. C. Zhang, G. Sonmez, J. Yamada, F. Wudl,
Org. Lett. 2003, 5, 4433; c) B. Purushothaman, M. Bruzek, S. R.
Parkin, A. F. Miller, J. E. Anthony, Angew. Chem. 2011, 123, 7151;
Angew. Chem. Int. Ed. 2011, 50, 7013; d) M. M. Payne, S. R. Parkin,
J. E. Anthony, J. Am. Chem. Soc. 2005, 127, 8028; e) D. Chun, Y.
Cheng, F. Wudl, Angew. Chem. 2008, 120, 8508; Angew. Chem. Int.
Ed. 2008, 47, 8380; f) I. Kaur, M. Jazdzyk, N. N. Stein, P. Prusevich,
G. P. Miller, J. Am. Chem. Soc. 2010, 132, 1261; g) S. S. Zade, M.
Bendikov, Angew. Chem. 2010, 122, 4104; Angew. Chem. Int. Ed.
2010, 49, 4012.
Device Fabrication
First, the ITO glass substrates were treated using routine cleaning proce-
dures, such as washing in detergent and subsequent ultrasonication in
acetone, isopropyl alcohol, and deionized water, respectively. After
drying in an oven, the ITO substrates were treated using O2 plasma to
remove possible organic residues. Subsequently, the substrates were
loaded into the deposition chamber where all materials were deposited
using a thermal evaporation method; details were described previously.[13]
[7] a) J. C. Xiao, Y. Divayana, Q. C. Zhang, H. M. Doung, H. Zhang, F.
Boey, X. W. Sun, F. Wudl, J. Mater. Chem. 2010, 20, 8167; b) Q. C.
Zhang, Y. Divayana, J. C. Xiao, Z. J. Wang, E. R. T. Tiekink, H. M.
Doung, H. Zhang, F. Boey, X. W. Sun, F. Wudl, Chem. Eur. J. 2010,
16, 7422; c) Q. C. Zhang, J. C. Xiao, Z. Y. Yin, H. M. Duong, F.
Qiao, F. Boey, X. Hu, H. Zhang, F. Wudl, Chem. Asian J. 2011, 6,
856; d) J. Xiao, B. Yang, J. L. Wong, Y. Liu, F. Wei, K. Tan, X. Teng,
Y. Wu, L. Huang, C. Kloc, F. Boey, J. Ma, H. Zhang, H. Yang, Q.
Zhang, Org. Lett. 2011, 13, 3004; e) J. Xiao, H. Yang, Z. Yin, J. Guo,
F. Boey, H. Zhang, Q. Zhang, J. Mater. Chem. 2011, 21, 1423.
[8] Q. Xu, H. M. Duong, F. Wudl, Y. Yang, Appl. Phys. Lett. 2004, 85,
3357.
Acknowledgements
Q.Z. acknowledges financial support from Nanyang Technological Uni-
versity (start-up grant) and the AcRF Tier 1 (RG 18/09) from MOE. We
thank Prof. Fred Wudl for his important input.
[1] a) Q. Meng, H. Dong, W. Hu, D. Zhu, J. Mater. Chem. 2011, 21,
11708; b) S. Sanvito, Chem. Soc. Rev. 2011, 40, 3336; c) H. Minema-
wari, T. Yamada, H. Matsui, J. Y. Tsutsumi, S. Haas, R. Chiba, R.
Kumai, T. Hasegawa, Nature 2011, 475, 364; d) H. Kobayashi, A.
Kobayashi, H. Tajima, Chem. Asian J. 2011, 6, 1688; e) X. Zhao, X.
Zhan, Chem. Soc. Rev. 2011, 40, 3728; f) G. Horowitz, Eur. Phys. J.
Appl. Phys. 2011, 53, 33602/1; g) P. Sonar, J. P. F. Lim, K. L. Chan,
Energy Environ. Sci. 2011, 4, 1558; h) H. Usta, A. Facchetti, T. J.
Marks, Acc. Chem. Res. 2011, 44, 501; i) T. Sekitani, Mol. Electron.
Bioelectron. 2010, 21, 215; j) W. Chen, D. C. Qi, H. Huang, X. Gao,
A. T. S. Wee, Adv. Funct. Mater. 2011, 21, 410.
[2] a) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, A. P. H. J. Schenning,
Chem. Rev. 2005, 105, 1491; b) T. Zhai, L. Li, Y. Ma, M. Liao, X.
Wang, X. Fang, J. Yao, Y. Bando, D. Golberg, Chem. Soc. Rev. 2011,
40, 2986; c) X. Duan, L. Liu, F. Feng, S. Wang, Acc. Chem. Res.
2010, 43, 260; d) A. Pron, P. Gawrys, M. Zagorska, D. Djurado, R.
Demadrille, Chem. Soc. Rev. 2010, 39, 2577; e) W. Rieb, H. Riel,
P. F. Seidler, H. Vestweber, Synth. Met. 1999, 99, 213; f) A. Nadara-
jah, R. C. Word, J. Meiss, R. Kçnenkamp, Nano Lett. 2008, 8, 534;
[9] R. A. Pascal Jr, W. D. McMillan, D. Van Engen, R. G. Eason, J. Am.
Chem. Soc. 1987, 109, 4660.
[10] H. M. Duong, Ph. D. Dissertation 2003, UCLA.
[11] Crystallographic data for TBH (CCDC number: 841858): C56H34
;
ꢀ
Mr =706.83; triclinic; space group P1; a=7.7160(3) ꢁ, b=
11.1718(5) ꢁ, c=22.5538(10) ꢁ; a=92.110(2)8, b=99.101(2)8, g=
102.127(2)8, V=1871.91(14) ꢁ3, Z=2, 1calcd =1.254 g·cmÀ3, F
ACHTUNGTRENNUNG(000)=
740, GOF=0.986, a total of 11845 reflections were collected, of
which 4818 were unique. R1 =0.1316 and wR2 =0.1852 for all data,
R1 =0.0604 and wR2 =0.1809 for data with I>2s(I).
[12] R. F. Kubin, A. N. Fletcher, J. Lumin. 1982, 27, 455.
[13] S. W. Liu, Y. Divayana, X. W. Sun, Y. Wang, K. S. Leck, H. V.
Demir, Opt. Express 2011, 19, 4513.
Received: August 29, 2011
Published online: December 23, 2011
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Chem. Asian J. 2012, 7, 561 – 564