Novel family of molecular glasses based on silicon-containing compounds

Article abstract of DOI:10.1246/cl.2005.290

Novel family of molecular glasses based on silicon-containing compounds, α,ω-dimethyloligo(diphenylsilylene)s are synthesized and characterized. Glass-forming properties of these materials tended to have even-odd effect of chain length. The morphology of

Full text of DOI:10.1246/cl.2005.290

290
Chemistry Letters Vol.34, No.3 (2005)
Novel Family of Molecular Glasses Based on Silicon-containing Compounds
Ichiro Imae and Yusuke Kawakamiꢀ
School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST),
1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292
(Received November 5, 2004; CL-041316)
Novel family of molecular glasses based on silicon-contain-
of polysilylenes, and so that it is interesting from both the view-
points of expansion of the range of family of molecular glasses
and the development of the new field of application of oligosily-
lenes, such as hole-transport, electroluminescent, and photoresist
materials. Here, we report the synthesis and glass-forming prop-
erties of novel ꢀ,!-dimethyloligo(diphenylsilylene)s (Figure 1).
Diphenyldichlorosilane and diphenyldimethylsilane (S1)
were purchased from Shin-Etsu Chemical Co., Ltd. 1,2-Dimeth-
yl-1,1,2,2-tetraphenyldisilane (S2)6 and decaphenylcyclopenta-
silane7 were synthesized according to the literatures. 1,3-Di-
methyl-1,1,2,2,3,3-hexaphenyltrisilane (S3), 1,4-dimethyl-1,1,
2,2,3,3,4,4-octaphenyltetrasilane (S4), and 1,7-dimethyl-1,1,2,2,
3,3,4,4,5,5,6,6,7,7-tetradecaphenylheptasilane (S7) were synthe-
sized by the coupling reaction of the corresponding chlorosilanes
and silyllithiums (Scheme 1). The materials were purified by
ing compounds, ꢀ,!-dimethyloligo(diphenylsilylene)s are syn-
thesized and characterized. Glass-forming properties of these
materials tended to have even-odd effect of chain length. The
morphology of glassy samples was found to be mainly amor-
phous with only two-dimensional order of hexagonal packing.
Since the photo- and electroactivity of the organic conjugat-
ed polymeric materials were found, many types of polymers,
such as poly(acetylene)s, poly(thienylene)s, poly(p-phenylene
vinylene)s, and poly(dialkylsilylene)s, were synthesized and
their optical, electrical, photoelectric, and magnetic properties
in the solid state were investigated for the application to the pho-
to- and electroactive materials. However, relationship between
the structure and properties of these conjugated polymers is
ill-established, because of their molecular weight distribution
and structural defect.
1
repetitive recrystallization, and characterized by H, 13C, 29Si
NMR, IR, and ESIMS spectroscopies and elemental analysis.
Ph
Me Si Cl
Ph
Ph
2 Me Si Cl
Ph
Contrary, low-molecular weight organic materials are good
candidates for the photo- and electroactive materials to show the
superior property to those of polymeric materials because of
their well-defined structures, although they tend to have crystal-
linity. To eliminate the problem of crystallinity of low-molecular
weight materials, Shirota et al. firstly developed the amorphous
glassy materials based on low-molecular weight compounds and
introduced the novel concept of ‘‘molecular amorphous glasses’’
into the materials science.1 After their reports, numerous molec-
ular glasses based on ꢁ-electron systems have been synthesized
and applied as photo- and electroactive materials, like electrolu-
minescent, photovoltaic, photochromic, and resist materials.
Silicon-containing polymers are widely used as the thermal-
ly stable insulator to photoconductors and photoresist, depend-
ing on the type of chemical bond.2 In particular, Si–Si catenated
polymers, polysilylenes, are interesting materials from the view-
point of photo- and electroactive materials, because of ꢂ-conju-
gation effect, such as electroluminescent and photoresist materi-
als.2,3 Very recently, Yatabe, Minami and their group reported
the liquid crystallinity4 and the hole-transport property of several
oligosilylene derivatives,5 and showed that their hole drift mobi-
lities in bulk film are higher than those of polysilylenes by one
order. However, there are no reports on the glass-forming prop-
erties of oligosilylenes. From the above-mentioned points, crea-
tion of glassy oligosilylenes will give a novel category of molec-
ular glasses having similar photo- and electroactivities to those
Ph
Ph
2 Me Si Li
Ph
Ph
Li
Me Si Me
Me Si Me
THF
THF
THF
Ph
Ph
4
3
S4
S3
Ph
2 Me Si Cl
Ph
Ph
Ph
Si
Ph
Ph
Li Si
Ph
2 Li
THF
Me Si Me
Li
5
THF
Ph
7
5
S7
Scheme 1. Synthesis of S3, S4, and S7.
Differential scanning calorimetry (DSC) thermograms of S4
and S7 are shown in Figure 2, and the data of all materials are
summarized in Table 1. While S1 is liquid at room temperature,
S2, S3, S4 and S7 are solid at room temperature and showed an
endothermic peak at 142, 103, 215, and 217 ꢁC, respectively, due
to the melting behavior in the first heating process. S2 showed
the same melting behavior in the second heating process after
the melt sample was cooled, and this behavior did not change
by changing the cooling rate from 1 to 50 degree minꢂ1. S4
showed similar melting behavior, but showed glass-transition
temperature at 31 ꢁC in the second heating process, when the
melt sample was cooled faster than 10 degree minꢂ1, followed
by crystallization at 86 ꢁC and melting at 215 ꢁC. Interestingly
enough, oligosilylenes having odd number of silylene units,
i.e., S1, S3, and S7, showed only glass-transition temperatures,
and did not crystallize in the second heating process (from the
first heating process in the case of S1). This behavior was ob-
served even when the melt sample was cooled at 1 degree minꢂ1
.
S1 : n = 1
S2 : n = 2
Especially, the glassy state of S7 is fully stable at room temper-
ature for several months. It is known that poly- and oligosily-
lenes adopt trans-zigzag or helical structures depending on the
structure of side-chains. If oligosilylenes studied in this report
adopts trans-zigzag structure, the direction of the methyl termi-
S3 : n = 3
S4 : n = 4
S7 : n = 7
H3C
Si
CH3
n
Figure 1. ꢀ,!-Dimethyloligo(diphenylsilylene)s.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.3 (2005)
291
nal group is parallel for the oligosilylenes having even number of
silylene unit, but nonparallel for the oligosilylenes having odd
number of silylene unit, and the former is symmetric but the later
is not. Thermal properties of pentamer and hexamer are not in-
vestigated yet because of the difficulty of the synthesis of them,
but this assumption is possibly reasonable to explain the even-
odd effect of chain length on the glass-forming properties. Al-
though such even-odd effect is known for the effect of alkyl
spacer length of liquid crystalline materials, there is no report
of the effect on glass-forming property.
(a)
(b)
Table 1. Thermal properties of oligosilylenesa
Tg/ꢁCb
ꢂ89
Tc/ꢁCc
Tm/ꢁCd
0
10
20
2
30
/ degree
40
50
60
θ
S1
S2
S3
S4
S7
Figure 3. X-ray diffraction patterns of (a) crystalline and (b)
glassy S7.
142
14e
(31)g
77e
103f
215f
217f
(86)g
creation of glassy low-molecular weight materials based on
silicon-containing compounds.
aScan rate: 10 degree minꢂ1. Glass-transition temperature.
b
This work was supported in part by grants for Basic Science
Research Projects from the Sumitomo Foundation and for
Scientific Research (16205016) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. The authors are
grateful to Dr. Keiko Miyabayashi (JAIST) for the ESIMS
measurement of oligosilylenes, and to Prof. Shintaro Sasaki
(JAIST) and Mr. Noritomo Kobayashi (JAIST) for the XRD
measurement of oligosilylenes.
cCrystallization temperature. dMelting temperature. eSecond
heating. First heating. Second heating after the melt was
f
g
cooled faster than 10 degree minꢂ1
.
(a)
1mW
Tg = 31 °C
Tm = 215 °C
Tc = 86 °C
References
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(b)
3
4
Tg = 77 °C
0
50
100
150
200
250
Temperature / °C
Figure 2. DSC thermograms of second heating process of (a)
S4 and (b) S7. Scan rate = 10 degree minꢂ1
5
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To obtain the information on the morphology of the glassy
samples, X-ray diffraction (XRD) was investigated (Figure 3).
While XRD of recrystallized S7 showed typical sharp peaks
due to the crystallinity of the sample, XRD of the glassy S7
ꢁ
ꢁ
ꢀ
showed two broad peaks at 2ꢃ ¼ 8:5 (d ¼ 10:4 A) and 20.6
ꢀ
(d ¼ 4:31 A). It is reported that polysilylenes having two alkyl
groups show a strong peak at 7–9ꢁ indexed as superposition of
d110 and d020 of an orthorhombic lattice with hexagonally packed
6
7
8
chains, and a broad peak centered at around 20ꢁ indexed as d013
.
8
Thus, peaks observed in glassy S7 can be assigned similarly to
the above-mentioned case of polysilylenes. From these data,
the distance between two neighbor oligosilylenes can be estimat-
ꢀ
ed as 12.0 A, which is coincident with the diameter of the oligo-
(diphenylsilylene) molecules having trans-zigzag structures. In
polarized microscopy, the image of glassy samples in cross-nicol
state is dark, and suggests to be isotropic. Thus, the arrangement
of molecules in glassy S7 might be mainly amorphous, with only
two-dimensional order of hexagonally packed structure.
To our best knowledge, our result is the first report on the
and M. Moller, J. Phys. Chem., 100, 5470 (1996).
¨
Published on the web (Advance View) January 29, 2005; DOI 10.1246/cl.2005.290