Mesomorphic properties of oligosilanes with phenyl end groups

Article abstract of DOI:10.1246/cl.2000.742

Oligosilanes with phenyl end groups, Ph(SiMe₂)_nPh where n = 10, 11, 12 and 13, were synthesized and their thermal and structural properties were studied. Dodecamer and tridecamer possessed mesophases showing the interdigitated structure in the range of 126-139 and 127-155 °C, respectively. In the mesophase, the silicon chains adopted the all-trans conformation.

Full text of DOI:10.1246/cl.2000.742

742
Chemistry Letters 2000
Mesomorphic Properties of Oligosilanes with Phenyl End Groups
Tetsuo Yatabe,* Nobutsugu Minami, Hajime Okumoto, and Katsuhiko Ueno
National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565
(Received April 3, 2000; CL-000305)
Oligosilanes with phenyl end groups, Ph(SiMe2)nPh where
groups might facilitate intermolecular carrier hopping. Herein
n = 10, 11, 12 and 13, were synthesized and their thermal and
structural properties were studied. Dodecamer and tridecamer
possessed mesophases showing the interdigitated structure in
the range of 126–139 and 127–155 °C, respectively. In the
mesophase, the silicon chains adopted the all-trans conforma-
tion.
we report the synthesis and the thermal and structural properties
of oligosilanes with phenyl end groups, Ph(SiMe2)nPh where n
= 10 (1), 11 (2), 12 (3), and 13 (4).
Oligosilanes 1–4 were synthesized according to Scheme 1.
1,6-Dichloropermethylhexasilane7 was treated with 1 equiv of
PhMgBr to give hexasilane 5. Heptasilane 6 was prepared by
treatment of hexasilane 5 with PhMe2SiLi. Treatment of hep-
tasilane 6 with 2 equiv of CF3SO3H, followed by addition of 2
equiv of PhMe2SiLi gave nonasilane 7. Undeca- and trideca-
silanes 2 and 4 were prepared by repeating this reaction. Deca-
and dodecasilanes 1 and 3 were obtained from octasilane 83 in a
similar manner. The products were purified by silica gel chro-
matography and subsequently by HPLC.8
The thermal properties of oligosilanes 1–4 were studied
using polarizing microscopy and DSC. The transition tempera-
tures and the associated enthalpies are summarized in Table 1.
Deca- and undecasilanes 1 and 2 did not show any mesophases
unlike decasilanes with alkyl end groups. In contrast, oligo-
silanes 3 and 4 showed enantiotropic mesophases; a texture
similar to a lancet texture typical of a smectic B phase9 was
observed both on heating and on cooling. For 3 the mesomor-
phic state occurred in the range of 126–139 °C on heating and
138–122 °C on cooling. For 4 it was 127–155 °C on heating
Linear oligosilanes, R(SiR2)nR where R = organic groups,
are a new class of interesting organic optoelectronic materials
because of their mesomorphism1–3 and photoconductive proper-
ties.4,5 We found that a series of permethyloligosilanes,
Me(SiMe2)nMe where n = 9, 10, 11, and 12, possess smectic B
phase, where the molecular axes are perpendicular to the layer
plane and hexagonally packed within each layer.2,6 Reflecting
such an ordered structure, Me(SiMe2)10Me demonstrates high
charge carrier mobility exceeding 1 × 10–3 cm2/Vs in its crys-
talline phase that is formed by annealing via the mesophase.5
Moreover, our series of works established that their mesomor-
phic properties can be tuned by manipulating the molecular
structure. The mesomorphic transition temperature becomes
higher for longer Si chains. The replacement of the Me end
groups of permethyldecasilane with longer alkyls (Et, Pr, and
Bu) results in the occurrence of the interdigitated B phase,
where the oligosilane molecules are hexagonally ordered and
the alkyl chains are interdigitated between the neighboring lay-
ers.3 In addition, the use of Pr and Bu end groups here consid-
erably lowers the mesomorphic transition temperature (Me:
82–114; Pr: 22–87; Bu: 23–69 °C). On the other hand, from the
viewpoint of optoelectronic applications, the use of phenyl end
groups is intriguing because the π-interaction between the end
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
743
conformation: dSiSi = 1.99; dSiC1 = 1.52; dC1C2 = dC3C4 = 0.61;
dC2C3 = 1.15 Å, as estimated from a molecular model shown in
Figure 2. These results indicate that the silicon chains adopted
the all-trans conformation and were perpendicular to the layer
plane and hexagonally ordered within the layer. Furthermore,
the phenyls at both ends of a silicon chain were shown to be
interdigitated between the neighboring layers, as illustrated in
Figure 3. Such interdigitated B phases of 3 and 4 are similar to
those of the decasilanes with alkyl chains (Et, Pr, and Bu) and
mark a difference from permethyldodecasilane 9.
References and Notes
1
T. Yatabe, Y. Sasanuma, A. Kaito, and Y. Tanabe, Chem. Lett.,
1997, 135.
2
3
T. Yatabe, A. Kaito, and Y. Tanabe, Chem. Lett., 1997, 799.
T. Yatabe, T. Kanaiwa, H. Sakurai, H. Okumoto, A. Kaito, and
Y. Tanabe, Chem. Lett., 1998, 345.
4
5
6
H. Okumoto, T. Yatabe, M. Shimomura, A. Kaito, N. Minami,
and Y. Tanabe, Synth. Metals, 101, 461 (1999).
H. Okumoto, T. Yatabe, M. Shimomura, A. Kaito, N. Minami,
and Y. Tanabe, Synth. Metals, in press.
Permethyloligosilanes, Me(SiMe2)nMe where n =2–12, were
synthesized in 1960’s. M. Kumada and M. Ishikawa, J.
Organomet. Chem., 1, 153 (1963); M. Kumada, M. Ishikawa,
and S. Maeda, J. Organomet. Chem., 5, 120 (1966).
H. Gilman and S. Inoue, J. Org. Chem., 29, 3418 (1964).
and 155–123 °C on cooling. Compared with permethyldode-
casilane 9 (smectic B: 95–147 °C),2 3 exhibited the lower clear-
ing point and the narrower mesomorphic range.
7
8
1
1: H MNR (CDCl3, δ) 0.08 (12 H, s), 0.12 (12 H, s), 0.14 (12
Intermolecular and interlayer distances for 3 and 4 in the
mesophase were obtained by X-ray diffractometry. The d-
spacings are listed in Table 2. For 3, a intense peak at 6.81 Å
(d100) and a very weak peak at 3.97 Å (d110) are attributed to the
reflections from the hexagonal lattice with a = b = 7.86 Å, γ =
120° (Figure 1). The lattice constant a corresponds to the inter-
molecular distance between the nearest molecules within the
layer. Its value was almost the same as that of permethylde-
casilane 9 (7.92 Å).2 Two peaks at 28.0 Å (d002) and 14.0 Å
(d004) are attributed to the reflections from the smectic layer.3,10
It should be noted that the interlayer distance (28.0 Å) was
shorter by 3.4 Å than the extended molecular length estimated
form a molecular model; this is different from 9, for which they
coincide. The interlayer distance for 4 (30.0 Å) was longer by
2.0 Å than that for 3. These distances are in good agreement
H, s), 0.16 (12 H, s), 0.38 (12 H, s), 7.27–7.35 (6 H, m),
7.39–7.47 (4 H, m); 13C NMR (CDCl3, δ) −5.28, −4.38, −4.08,
1
−4.03, −2.92, 127.69, 128.30, 133.74, 139.90. 2: H MNR
(CDCl3, δ) 0.07 (12 H, s), 0.12 (12 H, s), 0.14 (12 H, s), 0.16
(12 H, s), 0.17 (6 H, s), 0.37 (12 H, s), 7.28–7.34 (6 H, m),
7.39–7.46 (4 H, m); 13C NMR (CDCl3, δ) −5.29, −4.38, −4.09,
1
−4.02, −3.99, −2.93, 127.68, 128.29, 133.73, 139.90. 3: H
MNR (CDCl3, δ) 0.07 (12 H, s), 0.12 (12 H, s), 0.14 (12 H, s),
0.16 (12 H, s), 0.17 (12 H, s), 0.37 (12 H, s), 7.26–7.35 (6 H,
m), 7.38–7.46 (4 H, m); 13C NMR (CDCl3, δ) −5.29, −4.39, −4.09,
1
−4.02, −3.99, −2.93, 127.68, 128.29, 133.73, 139.90. 4: H
MNR (CDCl3, δ) 0.08 (12 H, s), 0.12 (12 H, s), 0.14 (12 H, s),
0.17 (12 H, s), 0.18 (18 H, s), 0.38 (12 H, s), 7.28–7.34 (6 H,
m), 7.39–7.46 (4 H, m); 13C NMR (CDCl3, δ) −5.29, −4.38, −4.08,
−4.00, −3.97 (× 2), −2.93, 127.68, 128.29, 133.73, 139.90.
D. Demus and L. Richter, in “Textures of Liquid Crystals,”
Verlag Chemie, New York (1978), Chap. 6, p. 164.
9
with the values calculated by the use of an equation: (n-1)dSiSi
+
10 The peaks assigned to d001 and d003 were not observed because
of the extinction rule of reflection. M. Kakudo and N. Kasai, in
“X-Ray Diffraction for Polymer,” Maruzen, Tokyo (1968),
Chap. 11, p. 207.
2dSiC1 + 2dC1C2 + dC2C3 + dC3C4, where n is the number of sili-
con atoms and dXY is the spacing between atoms X and Y pro-
jected onto the axis of the silicon chain having the all-trans