Experimental
Three types of multiwalled CNTs were individually incorporated into PS
matrix, i.e., a) arc-made CNTs [19], b) arc-made BCNTs [11,20], and c) Fe-filled
CNTs made by pyrolysis of ferrocene [21]. The degree of graphitization for arcmade CNTs and BCNTs is similar and the boron concentration in the CNTs is
ca. 1 %. The Fe-filled CNTs are less graphitic, compared with the two arc-generated materials. Note: The extent of defects in CNTs made by pyrolysis of hydrocarbons is strongly determined by the temperature, type of metal catalysts, and
precursors. For example, pyrolysis of acetylene (or camphor) at 600 C and
900 C using Fe powder as a catalytic agent will produce a different degree of defects in the CNTs. For the sake of uniformity, here we use ferrocene (self-catalytic) as precursor and the pyrolytic temperature is 1370 K, as previously reported
[21]. The preparation of CNT±PS composite films follows a similar procedure to
that previously described [22] and the CNT-to-PS ratio varies from 1:4 to 1:8.
As-cast films were hot-pressed [23] to yield a uniform thickness (2.5 mm). Scanning electron microscopy (SEM) investigations show that the CNTs are in chaotic arrangements in the PS matrix, similar to a previous report [23]. Films were
carefully machined to fit the waveguide sample holders, which minimizes the reflection loss via the gap between the film and waveguide sample holders. The
complex permittivity measurements were carried out in waveguide 16, using a
HP 8510C network analyzer and the through-reflect-line (TRL) calibration procedure [24]. A pure PS film was used as reference material.
±
[1]
[2]
[3]
[4]
[5]
[6]
[7]
Received: October 4, 2002
Final version: December 16, 2002
M. F. Lin, D. S. Chuu, K. W. K. Shung, Phys. Rev. B 1997, 56, 1430.
C. A. Grimes, C. Mungle, D. Kouzoudis, S. Fang, P. C. Eklund, Chem.
Phys. Lett. 2000, 319, 460.
V. V. Belavin, A. V. Okoyrub, L. G. Bulusheva, Phys. Solid State 2002, 44,
663.
M. Hughes, G. Z. Chen, M. Shaffer, D. J. Fray, A. H. Windle, Chem.
Mater. 2002, 14, 1610.
P. M. Ajayan, O. Stephan, C. Colliex, D. Trauth, Science 1994, 265, 1212.
L. S. Schadler, S. C. Giannaris, P. M. Ajayan, Appl. Phys. Lett. 1998, 73,
3842.
H. G. Dai, E. W. Wong, C. M. Lieber, Science 1996, 272, 523.
Adv. Mater. 2003, 15, No. 7±8, April 17
DOI: 10.1002/adma.200304813
[8] J. Nygard, D. H. Cobden, P. E. Lindelof, Nature 2000, 408, 342.
[9] J. Kong, E. Yenilmez, T. W. Tombler, W. Kim, H. G. Dai, R. B. Laughlin,
L. Liu, C. S. Jayanthi, S. Y. Wu, Phys. Rev. Lett. 2001, 87, 106 801.
[10] M. Terrones, W. K. Hsu, S. Ramos, R. Castillo, H. Terrones, Fullerene Sci.
Technol. 1998, 6, 787.
[11] W. K. Hsu, S. Y. Chu, E. Munoz-Picone, J. L. Boldu, S. Firth, P. Franchi,
B. P. Roberts, A. Schilder, H. Terrones, N. Grobert, Y. Q. Zhu, M. Terrones, M. E. McHenry, H. W. Kroto, D. R. M. Walton, Chem. Phys. Lett.
2000, 323, 572.
[12] W. K. Hsu, T. Nakalima, Carbon 2002, 40, 462.
[13] N. Grobert, W. K. Hsu, Y. Q. Zhu, J. P. Hare, H. W. Kroto, D. R. M. Walton, M. Terrones, H. Terrones, P. Redlich, M. Rühle, R. Escudero, F. Morales, Appl. Phys. Lett. 1999, 75, 3363.
[14] F. Beuneu, C. I. Huillier, J. P. Salvetat, J. M. Bonard, L. Forro, Phys. Rev.
B 1999, 59, 5945.
[15] A. P. Ramirez, R. C. Haddon, O. Zhou, R. M. Fleming, J. Zhang, S. M.
McClure, R. E. Smalley, Science 1994, 265, 84.
[16] P. C. P. Watts, W. K. Hsu, unpublished.
[17] D. L. Carroll, P. Redlich, X. BlasØ, J. C. Charlier, S. Curran, P. M. Ajayan,
S. Roth, M. Rühle, Phys. Rev. Lett. 1998, 81, 2332.
[18] C. A. Grimes, E. C. Dickey, C. Mungle, K. G. Ong, D. Qian, J. Appl. Phys.
2001, 90, 4134.
[19] S. Iijima, Nature 1991, 354, 56.
[20] W. K. Hsu, S. Firth, P. Redlich, M. Terrones, H. Terrones, Y. Q. Zhu,
R. J. H. Clark, H. W. Kroto, D. R. M. Walton, J. Mater. Chem. 2000, 10,
1425.
[21] C. N. R. Rao, R. Sen, B. C. Satishkumar, A. Govindaraj, Chem. Commun.
1998, 1525.
[22] P. C. P. Watts, W. K. Hsu, G. Z. Chen, D. J. Fray, H. W. Kroto, D. R. M.
Walton, J. Mater. Chem. 2001, 11, 2482.
[23] C. H. Poa, S. R. P. Silva, P. C. P. Watts, W. K. Hsu, H. W. Kroto, D. R. M.
Walton, Appl. Phys. Lett. 2002, 80, 3189.
[24] Hewlett Packard, Product Note 8510±3, August 1985.
Focused Microwave-Assisted Synthesis
of 2,5-Dihydrofuran Derivatives as Electron
Acceptors for Highly Efficient Nonlinear Optical
Chromophores**
By Sen Liu, Marnie A. Haller, Hong Ma, Larry R. Dalton,
Sei-Hum Jang, and Alex K.-Y. Jen*
Electro-optic (EO) materials have shown great potential
for applications in high-speed photonic devices due to their
large EO coefficients (r33), ultra-fast response time, and ease
of integration.[1] In general, the macroscopic EO response of a
polymer is induced by aligning the dipole of nonlinear optical
(NLO) chromophores via a high external electric field to create non-centrosymmetry.[2] In the past decade, the search for
EO polymeric materials with very large r33 values has led to
extensive efforts in exploring ªpush-pullº type chromophores
with high dipole moment (l) and molecular nonlinearity (b).
Among three molecular building blocks commonly used for
±
[*] Prof. A. K.-Y. Jen, S. Liu, M. A. Haller, Dr. H. Ma, Dr. S.-H. Jang
Department of Materials Science and Engineering
University of Washington
PO Box 352120, Seattle, WA 98195-2120 (USA)
E-mail: [email protected]
Prof. L. R. Dalton
Department of Chemistry, University of Washington
Seattle, WA 98195 (USA)
[**] We thank NSF-NIRT, AFOSR (Polymer Smart Skins), Boeing-Johnson
Foundation, and the Center for Nanotechnology at the University of
Washington for financial support. We also thank P. Bedworth and S. Ermer
for very useful discussions.
Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
603
COMMUNICATIONS
no dangling bonds are present in BC3-type defects, whereas
an increase in spin±orbit paramagnetic coupling is apparent
for defective CNTs.[14] The above description explains why
Fe-filled CNTs exhibit a higher e¢ value than the BCNTs.
Meanwhile, the presence of a high e¢ value in defective CNTs
can also explain the supercapacitor structure made by incorporating defective CNTs into polypyrrole via the electrochemical procedure previously reported by the Cambridge
group.[4] They believe that the acquired supercapacitors are
mainly due to the presence of porous structures and the micro-bilayer nature in the composite films. However, one
should not overlook the fact that the guest CNTs, embedded
in host polymer, also play a crucial role in changing the overall dielectric constant.
At room temperature, the localized defects do not significantly alter the electronic properties of the CNTs, but they act
as polarized centers, which accounts for the presence of higher
e¢ values in defective CNTs than in defect-free CNTs. The nature of defects in BCNTs is different from that in Fe-filled
CNTs, which explains the presence of a higher e² value in
BCNTs than in Fe-filled CNTs. The results presented in this
paper are also similar to a previous report by Grimes et al., in
which graphitized CNTs were shown to exhibit lower permittivity values than as-grown materials.[18] The defective CNTs
are excellent dielectric components for high-energy storage
systems, and also possibly for RAMs and EMIS.
COMMUNICATIONS
NLO chromophores, the development of electron donors and
conjugating bridges is already quite mature that they can meet
most of the synthetic and material requirements.[3] Therefore,
the major task for optimizing the lb value of a chromophore
is the development of novel electron acceptors.
Recently, several electron-deficient heterocyclic compounds
have emerged as strong electron acceptors for nonlinear optics.[4] Among them, an unconventional 2-dicyanomethylen-3cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF)-based chromophore has been reported that possesses an exceptionally large
lb value.[5] In light of this development, modifications of 2,5dihydrofuran derived structures have been extensively studied
to further enhance their electron-accepting strength for NLO
chromophores. However, the synthetic methodology commonly adopted to prepare TCF acceptors is a one step process
in which it has been impossible up to now to isolate the key
intermediate, a cyclized imine, 2-imino-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran.[6] This is because the a-hydroxyketone
starting material condenses with malononitrile initially to
form a cyclized imine, which itself is quite reactive toward
further condensation with malononitrile to form the final
product. If this imine could be isolated as an intermediate, it
can be used to further condense with activated methylene
compounds to create a broad variation of 2,5-dihydrofuranderived electron acceptors. Numerous reaction conditions
have been attempted; however, they failed to yield the imine
but rather afforded a quite inert lactone by-product.[6] Further
efforts to activate this lactone by TiCl4 and piperidine also
failed.
Fortunately, a synthetic procedure using focused microwave-assisted chemistry was reported recently that can be
used to successfully isolate this active imine.[7] Although this
paper provided a detailed description of how to generate the
imine, there are no further experiments reported concerning
its further derivatization. By applying the same concept, we
have employed the focused single-mode microwave methodology to control the step-wise synthesis of a series of 2,5-dihydrofuran derivatives and adjust their electron-accepting
strength. Among them, an exemplary electron acceptor, 2-dicyanomethylene-3-cyano-4,5-dimethyl-5-trifluoromethyl-2,5dihydrofuran (CF3-TCF)[8] and several chromophores derived
from this acceptor were prepared. By incorporating a CF3TCF-containing chromophore 7b into a high-temperature
polymer polyquinoline (Tg = 265 C) as guest (25 wt.-%), we
have demonstrated a very large EO coefficient
(r33 = 116 pm V±1 at 1.3 lm) and excellent long-term alignment stability at 85 C for more than 500 h.
Because the focused microwave methodology provides
heating at local reaction sites very efficiently, it enhances the
possibility of isolating the cyclized imines 1 and 5 before they
condense further with another malononitrile molecule. Subsequently, various activated methylene compounds, such as
ethyl cyanoacetate, nitro acetonitrile, or even the sterically
hindered thiobarbituric acid, can be condensed with these imines to afford 2,5-dihydrofuran derivatives, 2, 3, 4, and 6, under microwave irradiation (Scheme 1). Thus, by using differ-
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Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
A
O
OH +
CN
EtONa/EtOH
CN
CN
Microwave 20 W
O
NH
1
2: A1 = CN, A2 = COOEt
A1
CN
A1
A2
O
EtONa/EtOH
Microwave 20 W
3: A1 = CN, A2 = NO2
A2
2-4
4: A1 , A2 =
O
O Et
N
N S
Et
B
O
OH +
CN
N
EtONa/EtOH
Microwave 20 W
N
O
5
NH
CN
CN
N
CN
EtONa/EtOH
O
Microwave 20 W
CN
6
C
Li
O
O
OEt
CF3
2
OH
EtO
CF3
CN
Microwave 20 W
OH
CF3
CN
CN
CN
EtONa/EtOH
dilute HCl
F3C O
CN
CF3-TCF
Scheme 1. A series of diversified 2,5-dihydrofuran electron acceptors synthesized via focused microwave condensation. Type A are molecules with adjustable electron-withdrawing groups at the 2-position; type B are molecules with
an electron-deficient heterocyclic group at the 3-position; type C are molecules
with a fluorinated group at the 5-position.
ent combinations of imines and activated methylene
molecules, it should be possible to tune the resulting 2,5-dihydrofuran derivatives to balance b, absorption at the near-IR
region, and thermal, chemical, and photochemical stabilities
in the resulting NLO chromophores.
Through the focused microwave-assisted synthesis, these
2,5-dihydrofuran derivatives can be easily coupled with aromatic, heteroaromatic, and polyene conjugating bridges
through their acidic methyl terminals at C4. The properties of
the resulting chromophores can be tuned in a variety of ways,
for example, two very efficient chromophores (7a, 7b) containing CF3-TCF as the acceptor can be obtained under this
condition (Scheme 2). From the molecular structure point of
view, CF3-TCF can be regarded as a CF3-modified TCF variant. Surprisingly, the electronic absorption maxima (kmax) of
http://www.advmat.de
Adv. Mater. 2003, 15, No. 7±8, April 17
NC
neglect of differential overlap) calculations on one of the
shorter conjugation length analogs (7c) indicate that the electronic density change from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital
(LUMO) at the C5 atom is much larger in the CF3-TCF-based
electronic acceptor than in the TCF-based one. As a result,
the CF3 ªislandº could merge with the main conjugation path
to function as an additional electron-withdrawing moiety,
along with the three CN groups on the conjugating bridge.
This leads to a significant red-shift of kmax in the absorption
spectrum and a significant enhancement of molecular nonlinearity in CF3-TCF derived chromophores than in those of the
TCF analogs.
Following an early procedure to evaluate EO polymers,[10]
7a was doped at 23 wt.-% in poly(methyl methacrylate)
(PMMA) (Mw = 15 000). Thin-film samples of this guest/host
polymer were poled under nitrogen using a parallel electrode
Adv. Mater. 2003, 15, No. 7±8, April 17
http://www.advmat.de
NC
NC
CN
140
120
100
E-O c o e ffic ie n t (p m/V)
CN
80
60
40
20
0
0
100
200
300
400
500
Time (hour)
Fig. 1. Temporal stability of r33 value in the poled film of the 7b/PQ-100 guest/
host system.
Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
605
COMMUNICATIONS
configuration at a temperature close to its
Tg (85 C) for five minutes with a DC elecO
tric field of 1.0 MV cm±1. A very large r33
S
O
O
S
value of 128 pm V±1 (1.3 lm) was obtained
R'
R'
Bu
N
Bu
using the simple reflection technique that
R
Bu
N
Microwave 20 W
R
was reported by Teng and Man.[11] This is
Bu
7a: R=H, R'=CF3; 7b: R=CH2OTBDMS, R'=CF3
the highest value ever reported at 1.3 lm in
8a: R=H, R'=CH3; 8b: R=CH2OTBDMS, R'=CH3
a guest/host polymer system. Moreover,
Scheme 2. Syntheses of highly efficient NLO chromophores.
since the film possessed very little absorbance at the measurement wavelength,
there should be only a negligible contribution from the ab7a and 7b in dioxane were significantly red-shifted (62 nm)
sorption-induced resonant enhancement. To prove that the
compared to their TCF-based analogs (8a, 8b), respectively
CF3-TCF group is indeed a superior acceptor than the regular
(Table 1). The unanticipated shift of their p±p* charge transTCF, a PMMA guest/host system containing the reference
fer bands is quite interesting. Since the inductive effect alone
chromophore 8a was also poled under the same conditions. A
much smaller r33 value (68 pm V±1) was obtained in this sysTable 1. Comparison of maximum absorption wavelengths of CF3-TCF- and
TCF-based NLO chromophores.
tem. This result further confirms that CF3-TCF is an excellent
electron acceptor for NLO chromophores.
Entry
kmax [a]
Entry
kmax [a]
Entry [b]
kmax [a]
Although a large EO coefficient has been demonstrated in
7a
696
7b
706
7c
594
the PMMA system, the low Tg (~ 85 C) of the doped PMMA
8a
634
8b
644
8c
539
film prohibits its use in practical devices. In order to alleviate
[a] In dioxane. [b] The structures of 7c and 8c are shown below.
this problem, a high-temperature polyquinoline[12] (PQ-100)
(Tg = 265 C) was employed to improve the thermal stability
NC
NC
of the poling-induced alignment. On the other hand, the polNC
NC
ing efficiency and compatibility between the highly nonlinear
CN
CN
chromophore and the aromatic polymer can be improved
O
O
N
N
by attaching a bulky side chain onto the chromophore, such
CF3
8c
7c
as the compound 7b, with a tert-butyl-dimethylsiloxy
(TBDMSO) group to modify its shape. This structural modification was quite effective in our previous studies at shielding
from the electron-deficient CF3 group should not be strong
intermolecular electrostatic interactions among large lb chroenough to provide such a large contribution to enhance the
mophores, especially when higher concentrations were used.
electron-withdrawing power of the CF3-TCF because the CF3
Using a similar procedure, the EO polymer thin films were
group is not even on the main conjugation path, it is possible
prepared by doping 25 wt.-% of 7b into PQ-100. An r33 value
that the electronic effect from the hyperconjugation plays a
of
116 pm V±1 (1.3 lm) was obtained under the same poling
more significant role in enhancing the strength of the accepcondition. Subsequently, the temporal stability of EO signal in
tor.[9] In fact, by rotating the CF3 group, electrons from the
this polymer was monitored at 85 C under vacuum. It reC±F single bond can communicate with the lone pair electrons
tained over 85 % of its original value (98 pm V±1) after it was
on the O atom and the p orbital on the C4 atom to allow for
isothermally heated for more than 500 h (Fig. 1). This is a
hyperconjugation. ZINDO (Zerner-modified intermediate
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very exciting result for EO material development since it is
the first system that demonstrates both ultrahigh EO activity
and very good thermal stability.
In conclusion, a very diversified family of 2,5-dihydrofuran
derivatives has been synthesized as a new class of tunable
electron acceptors using single-mode focused microwave irradiation. A high poling efficiency and very large r33 values (128
and 116 pm V±1 at 1.3 lm) have been demonstrated using one
of the CF3-TCF-based chromophores in PMMA and a hightemperature polyquinoline (PQ-100). An excellent long-term
temporal stability has also been demonstrated in the PQ
guest/host system.
Experimental
All chemical reagents were purchased from Aldrich and were used as
received. Nitroacetonitrile [13], 2-dicyanomethylene-3-cyano-4,5,5-trimethyl2,5-dihydrofuran [6], N,N-dibutyl-4-[(1E,3E)-4-(5-formyl-thien-2-yl)-1,3-butadienyl]aniline [12], and N,N-dibutyl-4-[(1E,3E)-4-(3-tert-butyldimethylsiloxy-5formyl-thien-2-yl)-1,3-butadienyl] aniline [12] were prepared according to
methods described in the literature. All reactions were carried out under inert
nitrogen atmosphere unless otherwise specified. 1H NMR spectra (200 MHz)
were taken on a Bruker-200 FT NMR spectrometer, all spectra were obtained
in CDCl3 (unless otherwise noted) at 18 C. UV-vis spectra were obtained on a
Perkin-Elmer Lambda-9 spectrophotometer with a 4.5 cm3 quartz cell. Elemental analysis was carried out at QTI (Whitehouse, NJ). ESI-MS spectra were recorded on a Bruker Daltonics Esquire ion trap mass spectrometer. Microwave
irradiations were carried out on a CEM Discovery focused microwave system.
2-Imino-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (1): A mixture of 3-hydroxy-3-trimethyl-butan-2-one (3 mmol, 306 mg), malononitrile (3 mmol,
198 mg), and sodium ethoxide (0.3 mmol), prepared from sodium (0.3 mmol,
6.9 mg), in ethanol (0.3 mL) was irradiated under focused microwaves at 20 W
for 8 min. Solvent was removed by rotary evaporation and the residue was purified via flash chromatography on silica gel with a gradient eluent of CH2Cl2 to
2 % EtOH in CH2Cl2 to afford 200 mg of product as a pale solid (yield, 44 %).
1
H NMR (d in CDCl3): 7.06 (br., 1H), 2.18 (s, 1H), 1.45 (s, 1H). Anal. Calcd. for
C8H10N2O: C, 63.98; H, 6.71; N, 18.65. Found: C, 63.98; H, 6.92, N, 18.36. ESIMS (m/z): Calcd. 150.1; Found: 150.0.
2-(1¢-Ethoxycarbonyl-1¢-cyano)methylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (2): To a mixture of 2-imino-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (0.5 mmol, 75 mg) and ethyl cyanoacetate (0.6 mmol, 68 mg) in ethanol
(1 mL) was added sodium (0.1 mmol, 2.3 mg). After the sodium was completely dissolved in the solution, the mixture was irradiated under focused microwaves at 20 W for 8 min. Solvent was removed by rotary evaporation and
the residue was purified via flash chromatography on silica gel with a gradient
eluent of CH2Cl2 to 0.25 % EtOH in CH2Cl2 to afford 72 mg of product as a
white solid (yield, 58 %). 1H NMR (d in CDCl3): 4.31 (q, J = 7.0 Hz, 2H), 2.33
(s, 3H), 1.56 (s, 6H), 1.33 (t, J = 7.0 Hz, 3H). Anal. Calcd. for C13H14N2O3: C,
63.40; H, 5.73. Found: C, 63.36; H, 5.94. ESI-MS (m/z): Calcd. 246.1; Found:
246.1.
2-(1¢-Cyano-1¢-nitro)methylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (3),
5-(3-Cyano-4,5,5-trimethyl-2,5,-dihydro-2-furanyl)-1,3,-diethyl-2-thiobarbituric
acid (4): These compounds were prepared in a procedure similar to that for 2.
3: (yield, 41 %). 1H NMR (d in CDCl3): 2.54 (s, 3H), 1.78 (s, 3H), 1.77 (s, 3H).
Anal. Calcd. for C10H9N3O3: C, 54.79; H, 4.14; N, 19.17. Found: C, 54.21; H,
4.32, N, 18.96. ESI-MS (m/z): Calcd. 219.1; Found, 219.1. 4: (yield, 10 %). 1H
NMR (d in CDCl3): 4.55 (q, J = 7.0 Hz, 4H), 2.41 (s, 3H), 1.64 (s, 6H), 1.28 (t,
J = 6.4 Hz, 6H). Anal. Calcd. for C16H19N3O3S: C, 57.64; H, 5.74; N, 12.60.
Found: C, 57.32; H, 5.38, N, 12.49. ESI-MS (m/z): Calcd. 333.1; Found: 333.2.
2-Imino-3-(2¢-pyridyl)-4,5,5-trimethyl-2,5-dihydrofuran (5): This compound
was prepared in a procedure similar to that for 1. Quantitative yield. 1H NMR
(d in CDCl3): 8.63±8.67 (m, 1H), 7.71±7.75 (m, 2H), 7.18±7.26 (m, 1H), 2.14 (s,
3H), 1.47 (s, 6H). Anal. Calcd. for C12H14N2O: C, 71.26; H, 6.98; N, 13.85.
Found: C, 70.87; H, 7.13, N, 13.99. ESI-MS (m/z): Calcd. 202.1; Found, 202.2.
2-Dicyanomethylene-3-(2¢-pyridyl)-4,5,5-trimethyl-2,5,-dihydrofuran (6): This
compound was prepared using a procedure similar to that for 2. Yield: 79 %. 1H
NMR (d in CDCl3): 8.71±8.75 (m, 1H), 7.76±7.85 (m, 1H), 7.24±7.42 (m, 2H),
1.97 (s, 3H), 1.62 (s, 6H). Anal. Calcd. for C15H13N3O: C, 71.70; H, 5.21; N,
16.72. Found: C, 70.92; H, 5.29, N, 16.17. ESI-MS (m/z): Calcd. 251.1; Found,
251.1.
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Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3-Hydroxy-3-trifluoromethyl-butan-2-one: To a solution of ethyl vinyl ether
(10.8 g, 150 mmol) in 50 mL of tetrahydrofuran (THF) was added 58.8 mL of
tert-butyl lithium (1.7 M, 100 mmol) dropwise at ±78 C. The solution was
stirred for 15 min at ±78 C, warmed with an ice bath, stirred for another
30 min, and then re-cooled to ±78 C. To this lithiated enol ether was added a
solution of trifluoroacetone (5.6 g 50 mmol) in 10 mL of THF. The resulting
mixture was stirred for 30 min at ±78 C and then for 1 h at 0 C and warmed to
room temperature. The reaction was then quenched with 70 mL of a saturated
NH4Cl aqueous solution. The organic layer was separated and the aqueous
layer was extracted with 3 ” 100 mL of diethyl ether. The combined organic
layer was dried over Na2SO4 and concentrated via rotary evaporation to afford
9.2 g of 2-ethoxy-3-hydroxy-3-trifluoromethyl-butene as a pale yellow oil. The
crude product was used in the subsequent reaction without further purification.
1
H NMR (d in CDCl3): 4.38 (d, J = 3.0 Hz, 1H), 4.22 (d, J = 3.6 Hz, 1H), 3.81(q,
J = 5.7 Hz, 2H), 3.44 (s, 1H), 1.51 (s, 3H), 1.32 (t, J = 7.8 Hz, 3H).
To a solution of 2-ethoxy-3-hydroxy-3-trifluoromethyl-butene (9.2 g,
50 mmol) in 25 mL of methanol was added dropwise 55 mL of HCl(aq) (1 M,
55 mmol) at room temperature. The exothermal reaction was cooled using a
water bath and monitored by thin layer chromatography (TLC). After stirring
for 2 h, the resulting mixture was neutralized with NaHCO3, concentrated via
rotary evaporation, and extracted with 3 ” 120 mL of dichloromethane. The
combined organic layer was dried over Na2SO4 and concentrated to afford
5.5 g of product as a pale oil (yield, 70 %). 1H NMR (d in CDCl3): 2.39 (s, 1H),
2.18 (s, 3H), 1.56 (s, 3H). Anal. Calcd. for C5H7F3O2: C, 38.47; H, 4.52. Found:
C, 38.71; H, 4.39. ESI-MS (m/z): Calcd. 156.0; Found, 156.0.
2-Dicyanomethylene-3-cyano-4,5-dimethyl-5-trifluoromethyl-2,5-dihydrofuran (CF3-TCF): A mixture of 3-hydroxy-3-trifluromethylbutan-2-one (3 mmol,
470 mg), malononitrile (6 mmol, 396 mg), and sodium ethoxide (0.45 mmol),
in-situ prepared from sodium (0.45 mmol, 10.4 mg), in ethanol (0.45 mL) was
irradiated under focused microwaves at 20 W for 20 min. Solvent was removed
by rotary evaporation and the residue was purified via flash chromatography
on silica gel with a gradient eluent of CH2Cl2 to 2 % EtOH in CH2Cl2 to afford
418 mg product (yield, 55 %). 1H NMR (d in CDCl3): 2.44 (s, 3H), 1.83 (s, 3H).
Anal. Calcd. for C11H6F3N3O: C, 52.18; H, 2.39; N, 16.60. Found: C, 52.35; H,
2.24, N, 16.28. ESI-MS (m/z): Calcd. 253,0; Found, 253.1.
Chromophore 7a: A mixture of N,N-dibutyl-4-[(1E,3E)-4-(5-formyl-thien-2yl)-1,3-butadienyl]aniline (73.5 mg, 0.2 mmol) and 2-dicyanomethylene-3-cyano-4,5-dimethyl-5-trifluoromethyl-2,5-dihydrofuran (51 mg, 0.2 mmol) in
1 mL of ethanol was irradiated under microwaves (20 W) for 8 min. The resulting mixture was concentrated and purified through flash chromatography on silica gel with a gradient eluent of hexanes/ethyl acetate (20:1±3:1) to afford
80 mg of 7a as a dark solid (yield, 66 %). 1H NMR (d in CDCl3): 8.11 (d,
J = 15 Hz, 1H), 7.42 (d, J = 3.8 Hz, 1H), 7.32 (d, J = 9.2 Hz, 2H), 7.01 (d,
J = 4.0 Hz, 1H), 6.9±7.1 (m, 1H), 6.65±6.75 (m, 3H), 6.59 (d, J = 9.0 Hz, 2H), 6.46
(d, J = 15.4 Hz, 1H), 3.29 (t, J = 7.6 Hz, 4H), 1.89 (s, 3H), 1.57 (quin., J = 8.8 Hz,
4H), 1.35 (sex., J = 6.8 Hz, 4H), 0.95 (t, J = 7.2 Hz, 6H). Anal. Calcd. for
C34H33F3N4OS: C, 67.75; H, 5.52. Found: C, 67.31; H, 5.77. ESI-MS (m/z):
Calcd. 602.2; Found, 602.4.
Chromophore 7b: Following the same method used to prepare 7a (yield,
57 %). 1H NMR (d in CDCl3): 8.07 (d, J = 15 Hz, 1H), 7.42 (s, 1H), 7.32 (d,
J = 9.2 Hz, 2H), 6.95±7.09 (m, 1H), 6.65±6.76 (m, 3H), 6.59 (d, J = 9.0 Hz, 2H),
6.45 (d, J = 15.2 Hz, 1H), 4.69 (s, 2H), 3.30 (t, J = 7.4 Hz, 4H), 1.89 (s, 3H), 1.46±
1.62 (m, 4H), 1.35 (sex., J = 6.8 Hz, 4H), 0.95 (t, J = 7.4 Hz, 6H), 0.92 (s, 6H),
0.11 (s, 9H). Anal. Calcd. for C41H49F3N4O2SSi: C, 65.92; H, 6.61; N, 7.50.
Found: C, 65.98; H, 6.67, N, 7.25. ESI-MS (m/z): Calcd. 746.3; Found, 746.5.
Chromophore 8a: Following the same method used to prepare 7a (yield,
80 %). 1H NMR (d in CDCl3): 7.74 (d, J = 16.0 Hz, 1H), 7.33 (d, J = 2.6 Hz, 1H),
7.30 (d, J = 7.4 Hz, 2H), 6.91±7.08 (m, 2H), 6.70 (m, 2H), 6.69 (d, J = 3.0 Hz,
1H), 6.58 (d, J = 7.4 Hz, 2H), 6.53 (d, J = 14.0 Hz, 1H), 3.29 (t, J = 7.0 Hz, 4H),
1.72 (s, 6H),1.49±1.66 (br., 4H), 1.34 (sex., J = 7.6 Hz, 4H), 0.94 (t,
J = 7.2 Hz,6H). Anal. Calcd. for C34H36N4OS: C, 74.42; H, 6.61; N, 10.21. Found:
C, 74.10; H, 6.31, N, 10.45. ESI-MS (m/z): Calcd. 548.3; Found: 548.5.
Chromophore 8b: Following the same method used to prepare 7a (yield,
74 %). 1H NMR (d in CDCl3): 7.68 (d, J = 15.9 Hz, 2H), 7.34 (s, 1H), 7.31 (d,
J = 8.4 Hz, 2H), 6.63±6.73 (m, 2H), 6.58 (d, J = 8.7 Hz, 2H), 6.52 (d, J = 15.9 Hz,
2H), 4.59 (s, 2H), 3.29(t, J = 6.3 Hz, 4H), 1.73 (s, 6H), 1.56 (br. m, 8H), 1.34
(sex., J = 6.9 Hz, 4H), 0.94 (t, J = 6.9 Hz, 6H), 0.92 (s, 9H), 0.10 (s, 6H). Anal.
Calcd. for C41H52N4O2SSi: C, 71.06; H, 7.56; N, 8.08. Found: C, 71.38; H, 7.51,
N, 7.92. ESI-MS (m/z): Calcd. 692.4; Found: 692.6.
±
[1]
Received: December 31, 2002
Final version: February 10, 2003
a) M. Lee, H. E. Katz, C. Erben, D. M. Gill, P. Gopalan, J. D. Heber, D. J.
McGee, Science 2002, 298, 1401. b) Y. Shi, C. Zhang, H. Zhang, J. H.
Betchel, L. R. Dalton, B. H. Robinson, W. H. Steier, Science 2000, 288,
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http://www.advmat.de
Adv. Mater. 2003, 15, No. 7±8, April 17
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[6]
[7]
[8]
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[12]
[13]
Growth Direction and Cross-Sectional Study
of Silicon Nanowires**
By Chi-Pui Li, Chun-Sing Lee, Xiu-Liang Ma, Ning Wang,
Rui-Qin Zhang, and Shuit-Tong Lee*
Silicon nanowires (SiNWs) have attracted intense interest
in recent years[1] because they are not only important for fundamental studies on various size-dependent phenomena, but
they are also potential building blocks for nanoscale electronic and optoelectronic devices.[1±2] Various growth methods, including laser ablation,[3±6] thermal evaporation,[6±9] and chemical vapor deposition,[10±11] have been used to synthesize
SiNWs. One important target of the synthetic approaches is to
achieve a high degree of control of the diameter, growth orientation, and morphology of the SiNWs. A good understanding of the growth mechanism is obviously the key to obtaining
SiNWs with desired properties required for various novel ap-
plications. While recent efforts have substantially advanced
our understanding on the growth mechanisms of SiNWs,[6]
many fundamental issues remain unclear. For example, it is
not clear why the growth directions of SiNWs prepared by the
oxide-assisted growth (OAG) are mainly along the [112] and
[110] directions,[6] while those grown by the metal-catalyst vapor±liquid±solid (VLS) method are mainly along [111].[3] Although SiO2-clad SiNWs appear generally circular in crosssection as shown by transmission electron microscopy (TEM)
images, the cross-section of the Si crystalline core has been
neither observed nor investigated. The cross-sectional study
of SiNWs is expected to provide structural information about
the boundary between the crystalline core and the oxide
sheath, which, in turn, may shed light on the growth mechanism.
In the present work, we have developed a unique samplepreparation technique to prepare the cross-sectional samples
of SiNWs for TEM examinations. Using this technique, we
successfully prepared and studied the cross-sections of SiNWs,
which revealed that the Si core is bounded by well-defined
facets of low-index crystallographic planes. We also obtained
statistical data to elucidate the growth directions of the
SiNWs synthesized by the OAG approach. We point out that
the developed technique should be generally applicable to
preparing the cross-sectional TEM samples of one-dimensional nanomaterials including nanowires and nanotubes.
The as-grown SiNWs are randomly oriented with a length
of more than 10 lm and a typical diameter of about 20 nm, as
shown in Figure 1a. It is clearly revealed that SiNWs have different shapes. Some have a uniform size along the length of
a
b
±
[*] Prof. S.-T. Lee, C.-P. Li, Dr. C.-S. Lee, Dr. X.-L. Ma, Dr. R.-Q. Zhang
Department of Physics and Materials Science
Center of Super-Diamond and Advanced Films (COSDAF)
The City University of Hong Kong
Hong Kong SAR (China)
E-mail: [email protected]
Dr. N. Wang
Department of Physics
The Hong Kong University of Science and Technology
Hong Kong SAR (China)
[**] This work was supported in part by a Central Allocation grant (Project
No. CityU 3/01C (8730016)) and a CERG grant (Project No. CityU 1063/
01P (9040637)) of the Research Grants Council of Hong Kong SAR.
Adv. Mater. 2003, 15, No. 7±8, April 17
DOI: 10.1002/adma.200304409
Fig. 1. a) TEM image of SiNWs grown by thermal evaporation of SiO powder.
b) Cross-sectional TEM image of SiNWs.
Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
607
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