Click polymerization: Synthesis of novel σ-π Conjugated organosilicon polymers

Article abstract of DOI:10.1016/j.jorganchem.2011.05.013

Poly[silylene-1,4-phenylene-(1,2,3-triazol-4-yl)-1,4-phenylene]s were prepared via step-growth Click coupling polymerization of bis(p-ethynylphenyl) silanes and 1,4-diazidobenzene. The organosilicon units in the backbones may contribute to an electronic c

Full text of DOI:10.1016/j.jorganchem.2011.05.013

Journal of Organometallic Chemistry 696 (2011) 3000e3005
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Click polymerization: Synthesis of novel
s-p
conjugated organosilicon polymers
,
Yike Wang a, Dengxu Wang a, Caihong Xu b, Rui Wang b, Jianjun Han a, Shengyu Feng a
*
a Key Laboratory of Special Functional Aggregated Materials, Ministry of Education; School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, P. R. China
b Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Poly[silylene-1,4-phenylene-(1,2,3-triazol-4-yl)-1,4-phenylene]s were prepared via step-growth Click
coupling polymerization of bis(p-ethynylphenyl)silanes and 1,4-diazidobenzene. The organosilicon units
Received 5 April 2011
Received in revised form
11 May 2011
in the backbones may contribute to an electronic communication among the
p-conjugated units and
improve the solubility of the polymers. Fourier transform infrared spectroscopy (FT-IR) and nuclear
magnetic resonance spectroscopy (NMR) were used to confirm the polymers structure. The UVevis
absorption wavelength of silicon-containing compounds red-shifted ca. 15 nm when compared with the
compounds without silicon atoms. The fluorescence emission of polymers in CHCl3 solutions was
Accepted 22 May 2011
Keywords:
Click chemistry
s-p interaction
Organosilicon
Fluorescence
observed in visible blue region (ca. 440 nm), and the intensity and quantum yield (
F) were enhanced due
to the influence of interaction. The results of the absorption and emission supported the weak s-p
s
-
p
conjugation between the silylene and aromatic segments, and these polymers can be potentially applied
in organic light emitting devices (OLEDs).
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
However, the p-conjugated polymers are always insoluble due to
the rigid backbone structures, and the method of the above pub-
lished papers is to introduce long alkyl or other flexible groups as
side chain. As we know, incorporating silicon atoms into the
The Cu(I)-Catalyzed Azides and Alkynes Cycloaddition (CuAAC),
which is defined as “Click Chemistry”, has been intensively applied
in polymer synthesis since its discovery by Sharpless and Meldal
groups in 2002 [1,2]. This kind of reactions exhibit a lot of advan-
tages, including benign reaction conditions, simple workup, high
reaction rate, good solvent tolerance and excellent yield. So the
powerful and efficient synthetic method has gained popularity in
polymer synthesis and material science over the last decade [3e6].
p-conjugated polymer backbone is another choice to effectively
improve the flexibility, solubility, and processability [15,16]. As
a Group 14 element, the silicon atoms have unique d-orbital and
exhibit electron-donating properties to the adjacent
organic structure, and the interaction between the -orbital and the
-orbital is named conjugation, can influence the electron
delocalization between the organosilicon and -conjugated units.
The conjugated polymers mean the polymers composed of
silylene or oligosilylene group as units and arylene, ethynylene,
thienylene, vinylene or other aromatic heterocycle groups as units,
and the optical properties of the polymers will be influenced by the
electron delocalization. Therefore, the conjugated organosilicon
p-conjugated
s
p
s-p
Recently, many works have been done to prepare the
p
-conju-
p
gated polymers by the Click reaction [7e11]. Meudtner’s group
report the synthesis of Poly[(1,2,3-triazol-4-yl-1,3-pyridine)-alt-
(1,2,3-triazol-1-yl-1,3-phenylene)]s via the Click reaction of
2,6-diethynylpyridines and 3,5-diazidobenzoate monomers. In
addition, the heterocycles in the polymer backbones can serve as
ligands to coordinate with Zn2þ, Fe2þ and Eu3þ ions. Thus metal-
losupramolecular gels are obtained and can be potentially used as
s-p
s
p
s-p
polymers can be potentially applied in organic light emitting devices
(OLEDs), electroluminescent display, semiconductors, optical sensor
magnetic and emissive materials [12]. Another
p
-conjugated Click
and ceramic precursors [17e21]. But most of the s-p conjugated
polymer is synthesized from diethynyl- and diazido-based fluorene
monomers. The dye-sensitized solar cell (DSSC) fabricated from the
fluorene-based Click polymers shows high power conversion effi-
ciency of 4.62% and can be used as electronic materials in future
[13,14].
polymers are synthesized via Sonogashira reaction, Heck reaction,
Stille reaction, Suzuki reaction, Hydrosilylation and so on. Expensive
metal catalysts (Palladium, Rhodium or Platinum complexes) and
extreme conditions (isolation of humidity and oxygen) are required,
which limit the development of the s-p conjugated polymers. So we
attempt to introduce the Click polymerization to synthesize the
conjugated organosilicon polymers.
s-p
We prepared Poly[silylene-1,4-phenylene-(1,2,3-triazol-4-yl)-
1,4-phenylene]s (P1, P2) by step-growth Click polymerization of
* Corresponding author. Tel.: þ86 531 8836 4866; fax: þ86 531 8856 4464.
E-mail address: fsy@sdu.edu.cn (S. Feng).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.05.013

Y. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 3000e3005
3001
bis(p-ethynylphenyl)silanes and 1,4-diazidobenzene (Scheme 1).
The Click coupling reaction was carried out in DMF/pyridine
mixture using CuI as a catalyst. The obtained conjugated
s-p
organosilicon polymers in dilute CHCl3 solutions exhibited emis-
sion in visible blue region (ca. 440 nm) with moderate quantum
yields (F＞0.4). So these polymers may be potentially applied as
emission materials in OLEDs.
2. Results and discussion
2.1. Polymer synthesis
Poly[silylene-1,4-phenylene-(1,2,3-triazol-4-yl)-1,4-phenylene]s
(P1, P2) were prepared from bis(p-ethynylphenyl)silanes and 1,4-
diazidobenzene under typical Click coupling reaction. The struc-
tures were characterized by FT-IR and NMR spectra. In Fig. 1, the
peaks of corresponding to ethynyl (3270 cmꢀ1
) and azide
(2125 cmꢀ1) groups disappear after the reaction, and a new reso-
nance (3128 cmꢀ1) belongs to CeH stretching vibration of triazole
group is observed in the spectra. The 1H NMR spectrum of the
polymers shows a characteristic signal of triazole group at about
9.40 ppm (C^CeH), and the peak of phenyl attached to triazole
shift to 7.97 ppm (Fig. 2). Fig. 3 shows the 13C NMR spectra of P1,
two vinyl carbons from triazole unit are clearly shown at 149.2 and
140.5 ppm. The peaks around 120e140 ppm are due to the benzene
rings in the polymer backbone. These results confirm the Click
reaction has occurred.
Fig. 1. FT-IR spectra of diazide (a), dialkyne (b) and polymer (c).
summarized. The UVevis spectra were shown in Fig. 4. All the
compounds exhibit a similar absorption maximum at around
230 nm due to p-p
* transition of triazole groups [22]. In addition,
The average molecular weight of polymers was evaluated using
GPC and the results were summarized in Table 1. The average
molecular weights of P1 and P2 were 6200 and 7150, respectively.
The molecular weights were only low to moderate due to the rigid
structure in the main chains. The degrees of polymerization
calculated from the number average molecular weights were over
15, which indicate that the polymers were prepared successfully via
the Click polymerization.
the phenyl unit in M1 shows absorption at 246 nm, while the
absorption of the phenyl unit in M2 is around 261 nm, which is ca.
15 nm red-shift compared to that of M1. As we know, silicon atom
has empty 3d-orbitals which can conjugate with adjacent
p-conjugated systems, so the red-shift can be ascribed to the p-to-s
charge transition between phenyl and silylene units. The phenyl
units in P1 and P2 exhibit similar absorption at 262 and 259 nm,
respectively. No obvious wavelength shift is found between the
polymers and M2. The above results illuminate the silicon atom can
2.2. Optical properties
influence the electron delocalization of the
but the electronic transitions is not strong enough to connect the
adjacent -conjugated units, and only weak conjugation
occurred along the main chain.
p-electron segments,
In order to understand the nature of the polymers, two model
compounds (M1, M2) were synthesized (Scheme 2). The M2, whose
structure was similar with one repeating unit of the polymers, was
prepared to verify whether the silylene can contribute electronic
communication or insulation in the backbones. On the other hand,
the structure of M1 was similar with M2 but without organosilicon
units and we attempt to find out the effect of organosilicon units
through comparing the optical properties of M1 and M2.
p
s-p
In Table 2 the UVevis absorption and emission data of the
polymers and model compounds in CHCl3 solution were
a
a
Fig. 2. 1H NMR spectra of P1 DMSO-d6 as solvent as solvent, solvent peaks are
Scheme 1. Synthesis of P1, P2.
marked with asterisks.

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Y. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 3000e3005
Table 2
UVevis and emission results of M1, M2, P1 and P2.
lab [nm]
lab 2nd[nm]
lem [nm]
F a
condition (298K)
P1
P2
231
262
439
455
440
457
461
445
0.45
CHCl3, lex ¼ 350 nm
solid state, lex ¼ 300 nm
CHCl3, lex ¼ 350 nm
solid state, lex ¼ 300 nm
CHCl3, lex ¼ 350 nm
CHCl3, lex ¼ 350 nm
229
259
0.47
M1
M2
229
228
246
261
0.13
0.19
a
Quantum yields are based on quinine sulfate as standard.
and 0.47, higher than that of M1 and M2. Thus, the results suggest
that the
s-p conjugation can effectively enhance the emission
efficiency.
In addition, the emission spectra of polymers in different
solvents and solid state were shown in Fig. 6(a) and (b), respec-
tively. The solubility of polymers was better in polar solvents, such
as DMSO and DMF, may be ascribe to the polar structure in the main
chain. The Fig. 6(a) shows that the emission region in different
solvents are near 400e435 nm, insensitive to the solvent polarity.
The emission of P1 in solid state is observed around 445e460 nm,
which is considerably red-shifted compared with that in solution,
a
a
Fig. 3. 13C NMR spectra of P1 DMSO-d6 as solvent as solvent, solvent peaks are
marked with asterisks.
presumbly due to the p-p stacking interactions of the fluorophores.
In summary, the results of both UVevis and PL measurements
revealed that the silicon atoms have a little electronic communi-
Table 1
cation with the adjacent
p-conjugated units. Although the s-p
The yields and GPC results of P1, P2.
conjugation of the polymers was too weak to influence the emis-
sion wavelength, the emission and quantum yields were still
Yield (%)a
Mn (g/mol)b
Mw (g/mol)b
PDIb
DPb
P1
P2
67
62
6200
7150
8620
10500
1.39
1.47
15
17
enhanced under the
s-p conjugation. Finally, the similar spectra
suggest that the side groups on the silicon atom had little effect on
the photoproperties of the s-p conjugated organosilicon polymers.
a
Isolated yield after reprecipitation into petroleum ether.
Estimated by GPC, THF as eluent, relative to polystyrene standards.
b
2.3. Thermal properties
The polymers were further investigated by photoluminescence
(PL) spectrum (Fig. 5). The emission maximum of the polymers and
M2 in CHCl3 solutions are observed at around 440 nm excited at
350 nm, while the emission maximum of M1 is 461 nm. All the
compounds show fluorescence emission in visible blue region, and
the emission maximum wavelength of silicon-containing
compounds is about 20 nm blue-shifted compared with that of
M1, which can be attributed to charge transition between the
silicon atoms and aromatic rings. Similar results were described by
Suzuki et al. [23]. The authors have synthesized polymers with
terfluorene and organosilicon groups in the backbones, and the
emission maximum wavelengths of the polymers are ca. 400 nm
while polyfluorene is 418 nm, the blue-shift reveals that the sily-
lene units contribute an electronic communication between con-
nected fluorenes. However, no obvious emission wavelength
difference between M2 and the polymers was observed, which
illuminate the charge communication is not strong enough to
The thermal stability of the polymers with heating rate of 10 ꢁC/
min under N2 atmosphere was shown in Fig. 7. The polymers
exhibit good heat-resistant properties, the 5 and 10% weight loss
temperatures (Td5 and Td10) are almost the same for P1 (322,
350 ꢁC) and P2 (325, 370 ꢁC). The TGA profiles of P1 and P2 are
composed of two similar decomposed stages. The first degradation
(ca. 260e400 ꢁC) is due to the thermal decomposition of SieCH3
groups. Then the backbone begins to break down when the
temperature is higher than 400 ꢁC. The residue after 800 ꢁC
pyrolysis of P2 is 59.8%, slightly higher than that of P1 (52.1%). This
is probably due to the crosslinking reaction between vinyl groups in
P2. The DSC measurement had also been done, but no obvious glass
transition temperature (Tg) was found for any polymer.
3. Conclusion
connect the whole backbones of the polymer and only partial s-p
conjugation occurred along the main chain. The relative intensities
of the polymers were stronger than that of M1 and M2 due to the
polymeric structure. The quantum yields (F) of P1 and P2 were 0.45
Poly[silylene-1,4-phenylene-(1,2,3-triazol-4-yl)-1,4-phenylene]s
were successfully synthesized from bis(p-ethynylphenyl)silanes
and 1,4-diazidobenzene via Click step-growth polymerization. The
Scheme 2. Synthesis of M1, M2.

Y. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 3000e3005
3003
Fig. 4. UVevis absorption spectra of M1, M2, P1 and P2 (CHCl3, 10ꢀ5 M).
polymers showed unique UVevis and PL properties, which indi-
cated that the weak conjugation occurred between the silylene
and aromatic segments. The present results provided a reliable and
cheap method to modify the solubility, processability and optical
mass spectra were obtained using positive mode on Aglient Tech-
nologies Q-TOF 6510. Elemental analysis (C, H, N) was measured on
a Vario EI Ⅲ elemental analyzer. The average molecular weight of
the polymers was determined in THF (1 ml/min) at 40 ꢁC by
waters515 liquid chromatograph equipped with the refractive-
index detector and three Styragel columns, and using the stan-
dard polystyrene (PS) for calibration.
s-p
properties of the p-conjugated polymers. The obtained novel s-p
conjugated organosilicon polymers were observed in visible blue
region (ca. 440 nm) in the fluorescence emission spectrum. The
emission intensity and quantum yield were also enhanced due to
the
polymers could be used as potential optical materials in OLEDs.
Other conjugated organosilicon polymers with different orga-
nosilicon or aromatic segments in backbone are currently investi-
gated in progress.
s-p interaction of the polymer main chains. Therefore, these
4.2. Preparation of 1,4-diazidobenzene
s-p
The mixture of 1,4-diiodobenzene (3.30 g, 10.0 mmol), NaN3
(1.56 g, 24.0 mmol), CuSO4 (0.16 g, 1.0 mmol), ascorbate acid (0.35 g,
2.0 mmol), Na2CO3 (0.21 g, 2.0 mmol), L-proline (0.23 g, 2.0 mmol),
DMSO (9 ml) and H2O (1 ml) was stirred in dark for 24 h at 65 ꢁC.
Then the mixture was extracted with EtOAc (200 ml), washed with
H2O (3 ꢂ 100 ml), dried over MgSO4, filtered and distilled under
reduced pressure. The crude product was purified by a silica gel
4. Experimental
4.1. General
Anhydrous tetrahydrofuran (THF) and diethyl ether (Et2O) were
freshly distilled over sodium and benzophenone before use.
n-Butyllithium (n-BuLi, 2.5 mol/L, in hexane) was purchased from
Mayer. 1,4-diazidobenzene was prepared by modifying the
reported procedure [24,25]. 1-bromo-4-trimethylsilylethyny-
lbenzene was obtained according to the published literatures
[16,20,26]. Petroleum ether (PE), ethyl acetate (EtOAc), dimethyl
sulfoxide (DMSO), dimethylformamide (DMF) and other chemicals
were purchased from Aldrich and used as received unless other-
wise noted.
FT-IR spectra were recorded with a Bruker Tensor27 spectro-
photometer. 1H NMR and 13C NMR spectra were measured using
CDCl3 and DMSO-d6 as solvent on Bruker AVANCE-300 NMR
Spectrometer. UVevis absorption and fluorescence spectra were
analyzed with UV-7502PC and ISS K2-Digital spectrophotometer,
respectively. Fluorescence quantum yields were measured using
quinine sulfate in 0.1 N H2SO4
(
F
¼ 54.6%) as standard. TGA was
carried out under nitrogen flow at a heating rate of 10 ꢁC/min on
a MettlerToledo SDTA-854 TGA system. DSC measurements were
performed with MettlerToledo DSC822 series. High-resolution
Fig. 5. Emission spectra of M1, M2, P1 and P2 (CHCl3, 10ꢀ5 M).

3004
Y. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 3000e3005
Fig. 6. Emission spectra of P1. (a) in different solution (10ꢀ5 M, lex ¼ 350 nm); (b) in the solid state (lex ¼ 300 nm).
column (PE as eluent) to give 1,4-diazidobenzene as a light yellow
crystalline, which is light-sensitive (turn to brownish when
exposed to light) and should be stored in the dark (Yield: 56%).
IR (KBr plate) 3300, 2109, 1250 cmꢀ1; HRMS (m/z): calcd for
C11H14Si 174.3177, found 174.3165.
1H NMR ( in CDCl3) 7.01 (s); 13C NMR (
d d in CDCl3) 120.4,
136.8 ppm; FT-IR (KBr plate) 2125 cmꢀ1(eN3); HRMS (m/z): calcd
4.4. Preparation of bis(p-ethynylphenyl)silanes
for C6H4N6 160.1382, found 160.1374.
The bis(p-ethynylphenyl)silanes were prepared from dichloro-
dimethylsilane and dichloromethylvinylsilane similar to p-(trime-
thylsilyl) phenylacetylene.
4.3. Preparation of p-(trimethylsilyl) phenylacetylene
Data for bis(p-ethynylphenyl)dimethylsilane: colorless solid
(Yield: 70%);
The mixture of 1-bromo-4-trimethylsilylethynylbenzene (2.52 g,
10 mmol) and dry Et2O (100 ml) was cooled at ꢀ78 ꢁC under argon
atmosphere, and n-BuLi solution (4.8 ml, 12.0 mmol) was dropped
slowly and kept for 2 h. Then trimethylchlorosilane (1.09 g,10 mmol)
was added in 10 min. The mixture was allowed to warm to room
temperature overnight. After quenching with NH4Cl solution, the
mixture was extracted with Et2O (100 ml) and washed with H2O
(3 ꢂ 50 ml), dried over MgSO4, filtered and distilled under reduced
pressure. Then the residue and K2CO3 (0.14 g, 1 mmol) were dis-
solved in THF/MeOH (50/50 ml) mixture, and stirred for 12 h at room
temperature. Then solvent was distilled and the residue was purified
by a silica gel column (PE as eluent) to give p-(trimethylsilyl) phe-
nylacetylene 1.34 g (Yield: 77%) as colorless liquid.
1H NMR (
d
in CDCl3) 7.47 (s, 8H), 3.10 (s, 2H), 0.55 (s, 6H); 13C
NMR (
d
in CDCl3) 138.8, 133.6, 131.4, 123.4, 84.0, 79.2, ꢀ4.0 ppm; FT-
IR (KBr plate) 3270, 2113, 1255 cmꢀ1; HRMS (m/z): calcd for
C18H16Si 260.4105, found 260.4119.
Data for bis(p-ethynylphenyl)methylvinylsilane: colorless liquid
(Yield: 68%);
1H NMR (
d
in CDCl3) 7.49 (s, 8H), 6.46 (m, 1H), 6.25 (m, 1H), 5.82
(m, 1H) 3.13 (s, 2H), 0.65 (s, 3H); 13C NMR (
d
in CDCl3) 137.4, 136.2,
135.3, 135.1, 131.9, 123.7, 84.3, 78.5, ꢀ3.9 ppm; FT-IR (KBr plate)
3296, 2108, 1253 cmꢀ1; HRMS (m/z): calcd for C19H16Si 272.4215,
found 272.4207.
1H NMR ( in CDCl3) 7.49 (s, 4H), 3.10 (s, 1H), 0.28 (s, 9H); 13C
d
NMR (
d
in CDCl3) 138.5, 133.7, 131.7, 122.9, 84.3, 77.2, ꢀ3.8 ppm; FT-
4.5. Preparation of model compound (M1, M2)
Phenylacetylene (0.20 g, 2 mmol), 1,4-diazidobenzene (0.16 g,
1 mmol), copper iodide (0.02 g, 5 mol %), pyridine (1 ml) and DMF
(4 ml) was stirred for 24 h at 60 ꢁC under dark. Then the mixture
was poured into EtOAc (100 ml) and washed with H2O (3 ꢂ 50 ml),
dried over MgSO4, filtered and distilled under reduced pressure.
The crude product was purified by a silica gel column (PE:
EtOAc ¼ 3:2) to give M1 as a pale yellow solid (Yield: 53%).
1H NMR (
7.45 (m, 6H); 13C NMR (
d
in DMSO-d6) 9.44 (s, 2H), 7.97 (s, 4H), 7.72 (d, 4H),
in DMSO-d6) 149.3, 140.6, 137.2, 135.6,
d
130.1, 129.3, 122.5, 120.7 ppm; FT-IR (KBr plate) 3125, 3051, 1508,
847, 823 cmꢀ1; Anal. Calc. for C22H16N6: C, 72.53; H, 4.40; N, 23.07.
Found: C, 72.77; H, 4.55; N, 22.68%.
The M2 were prepared from p-(trimethylsilyl) phenylacetylene
in a similar procedure.
Data for M2: pale yellow solid (Yield: 56%);
1H NMR (
d
in DMSO-d6) 9.47 (s, 2H), 7.95 (s, 4H), 7.74 (d, 4H),
7.35 (m, 6H), 0.28 (s, 18H); 13C NMR (
d
in DMSO-d6) 149.6, 140.2,
137.8, 134.3, 130.2, 128.5, 122.5, 120.7, ꢀ3.5 ppm; FT-IR (KBr plate)
3127, 3069,1511,1253, 843, 820 cmꢀ1; Anal. Calc. for C28H32N6Si2: C,
66.14; H, 6.30; N, 16.54. Found: C, 65.65; H, 6.53; N, 16.71%.
Fig. 7. TGA profile of P1, P2 (10 ꢁC/min, N2).

Y. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 3000e3005
3005
4.6. Preparation of polymers (P1, P2)
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1H NMR (
7.49 (m, 4H), 0.63 (m, 6H); 13C NMR (
d
in DMSO-d6) 9.40 (s, 2H), 7.97 (m, 4H), 7.56 (m, 4H),
in DMSO-d6) 149.2, 140.5,
d
138.4, 135.3, 129.7, 128.1, 121.5, 120.9, ꢀ4.5 ppm; FT-IR (KBr plate)
3128, 3065, 1521, 1251, 845, 819 cmꢀ1; Anal. Calc. for (C24H20N6Si)n:
C, 68.57; H, 4.76; N, 20.00. Found: C, 67.42; H, 4.67; N, 19.20%.
The P2 were prepared from bis(p-ethynylphenyl)methyl-
vinylsilane in a similar procedure.
Data for P2: beige solid (Yield: 62%);
1H NMR (
d
in DMSO-d6) 9.47 (s, 2H), 7.98 (m, 4H), 7.66 (m, 4H),
7.51 (m, 4H), 6.53 (m, 1H), 6.25 (m, 1H), 5.80 (m, 1H), 0.65 (m, 3H);
13C NMR (
in DMSO-d6) 149.7, 144.3, 138.4, 136.7, 135.8, 135.2,
d
129.7, 129.1, 121.5, 120.9, ꢀ4.3 ppm; FT-IR (KBr plate) 3129, 3051,
1517, 1252, 840, 817 cmꢀ1; Anal. Calc. for (C25H20N6Si)n: C, 69.44; H,
4.63; N, 19.44. Found: C, 69.13; H, 4.75; N, 19.65%.
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Acknowledgment
The authors gratefully thank National Natural Science Founda-
tion of China (NSFC, Grant No. 20874057 and 20574043) for
financial support.
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