Effects of oxygen atom in the side chain on physical and optical properties of dodecapentoxypentasilane

Article abstract of DOI:10.1016/S0009-2614(03)00078-2

The effects of the alkoxy side chain on the conformation of oligosilane have been studied for perpentoxypentasilane. The UV-absorption maxima shifted extremely to a shorter wavelength than that of peralkyloligosilane. Molecular dynamics and ab initio MO c

Full text of DOI:10.1016/S0009-2614(03)00078-2

Chemical Physics Letters 370 (2003) 154–160
www.elsevier.com/locate/cplett
Effects of oxygen atom in the side chain on physical and
optical properties of dodecapentoxypentasilane
a
Haruhisa Kato , Takashi Karatsu , Akira Kaito , Kayori Shimada ,
a,
b
b
*
a,1
Akihide Kitamura
a
Department of Materials Technology, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
b
National Institute of Advanced Industrial Science and Technology (AIST), 1-1 Higashi, Tsukuba, Ibaragi 305-8565, Japan
Received 21 October 2002; in final form 2 December 2002
Abstract
The effects of the alkoxy side chain on the conformation of oligosilane have been studied for perpentoxypentasilane.
The UV-absorption maxima shifted extremely to a shorter wavelength than that of peralkyloligosilane. Molecular
dynamics and ab initio MO calculations showed that the excitation energy of the twisted structure for peroxyloligo-
silane (a model of perpentoxypentasilane) was smaller than that of permethyloligosilane, because of the interaction
between the n orbital of the oxygen atom and the r orbital of the Si–Si bond. The Coulomb repulsion energies between
the oxygen atoms greatly affect the stability of the twisted conformation of the silicon backbone.
Ó 2003 Elsevier Science B.V. All rights reserved.
1
. Introduction
stituent as a side chain [6–10]. In addition, positive
trials have been carried out for the polysilanes
containing a heteroatom in the side chain, to dis-
solve in water [11–13], to introduce oxygen or ni-
trogen atom into the side chain showing unique
solvatochromism [14–17], to introduce a chirality
and a heteroatom in the side chain [18].
Then a question arises regarding whether the
oxygen atom affects the structure of the main chain
by electrostatic effects between the oxygen atoms
or the interaction between the n orbital of oxygen
and r orbital of the Si–Si bonds. Unfortunately,
only poor discussions on this matter are available
at present. To understand the effect of oxygen in-
troduced in the side chain on the properties of an
oligosilane, we prepared dodecapentoxypentasi-
Because of the r electron delocalization on the
main chain, polysilane is known as a polymer with
unique electronic and optical properties. It has
been confirmed that various physical properties of
this polymer greatly depend on the main chain
structures [1,2], and that it is possible to control
the main chain structure by the side chain [3–5]. In
order to control the structure of the Si-skeleton,
the polysilanes, which have a selective spiral
structure, were made by introducing a chiral sub-
*
Corresponding author. Fax: +81-43-290-3039.
E-mail address: karatsu@xtal.tf.chiba-u.ac.jp (T. Karatsu).
Also corresponding author.
1
0
009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00078-2

H. Kato et al. / Chemical Physics Letters 370 (2003) 154–160
155
lane (POS5) and examined its physical properties
by experimental techniques, molecular orbital
the model compounds have many rotationable
dihedral angles. In this simulation, long-range
ꢀ
non-bonded forces were cut off between 9 A. MD
(
MO) calculations, and molecular dynamics (MD)
simulations. These approaches clarify the rela-
tionship between the physical properties and the
structure.
calculations were carried out using the PCFF force
field, which was optimized by Sun et al. [22] for
organosilicon compounds. The calculations were
performed for decapentoxytetrasilane (POS4),
POS5 and decahexyltetrasilane (HS4), respec-
tively. The atomic partial charges were calculated
at every 10 fs by the QEQ method [23]. In the
calculations, the central Si–Si bonds were set at
given dihedral angles, at every 5° or 10°. In the
simulation, side chains are changed their confor-
mation freely. To determinate the initial structures
for the simulation, the geometric parameters were
optimized by force field calculations. The initial
temperature was set at 600 K for 1 ps and then
decreased to 298.15 K. After 30 ps, the system was
assumed to have reached equilibrium. The total
energies, recorded for 100 ps at 20 fs intervals,
were averaged. For pentasilanes, near the energy
minimum, the dihedral angle was changed at
each 5°.
Ab initio MO calculations were carried out
using the GAUSSIAN 98 program on IBM-RS6000.
Because POS5 was too large to perform the cal-
culations, peroxyloligosilanes (decahydroxyltetr-
asilane, OHS4 and dodecahydroxylpentasilane,
OHS5) were used as model compounds. In order
to discuss the effects of oxygen atoms on the silicon
chain, permethyloligosilanes were also calculated.
The geometries were fully optimized at the
B3LYP/6-31G** level. Using the geometries as
such determined, CIS calculations were performed
at the CIS/6-31G* level. In these calculations, the
dihedral angles of the central Si–Si bonds were set
at given dihedral angles at every 10° or 30°.
2
. Experimental
2
(
.1. Preparation of dodecapentoxypentasilane
POS5)
Decaphenylcyclopentasilane prepared from
dichlorodiphenylsilane as reported [19,20] was
refluxed in tetrachloroethane for 40-min to give 1,
5
-dichlorodecaphenylpentasilane [21]. The dichlo-
ropentasilane in dry benzene was introduced HCl
gas for 7-h in the presence of AlCl . The solution
3
was carefully introduced into 10 ml of n-pentanol
under nitrogen, and allowed to stand for 5-h at
RT. The solvent was then removed in vacuo. The
residue was washed with methanol, filtered, and a
viscous colorless residue was obtained. Matrix-
assisted laser desorption/ionization time-of-flight
mass (MALDI-TOF MS) spectral measurement
was performed by a Shimadzu/Kratos Kompact-
MALDI III with 3-indoleacetic acid used as a
matrices. After several times reprecipitation
(
3
CHCl -methanol), the obtained mass number of
the product was appeared as a single peak at 1184,
which was assigned to POS5. The NMRspectra
3
were measured in CDCl in the presence of TMS
as reference by a JEOL JNM-GSX600 spectro-
1
meter (600 MHz). H-NMR d 3.6–3.8(m, 2H,
O–CH
2
–), 1.5–1.7(m, 2H, O–CH
.5(m, 4H, CH –CH –CH ), 0.8–1.1(m, 3H, CH
C-NMR d 14.0, 14.1(CH ), 22.8, 22.9(–CH
CH ), 29.0, 29.1, 29.2(–CH –CH –CH ), 37.5,
2
2
–CH –), 1.3–
1
3
3
).
2
2
13
3
2
–
3
2
2
3
3
5.8, 35.9(–O–CH –CH –), 65.65, 65.7, 65.8(–O–
2
3. Results and discussion
2
2
CH –).
3.1. Thermochromic properties of POS5
2
.2. MD and MO calculations
Fig. 1 shows the UV absorption spectra of
All calculations were performed by a general
POS5 in 3-methylpentane in the range between 298
and 80 K. These spectra have three remarkable
characteristics. First, the band maxima in this
temperature range were observed at shorter
molecular mechanics program Cerius 2 (Accelrys
Inc.). In this study, MD calculation was chosen as
the method of conformational analysis, because of

156
H. Kato et al. / Chemical Physics Letters 370 (2003) 154–160
3.2. Result of MD simulation
The calculated potential energy distribution
curves as a function of the dihedral angles of each
tetrasilane; HS5 and POS5, are shown in Figs. 2a
and b, respectively. The curve of the solid line is
the potential energies obtained from MD calcula-
tions. Another two curves, the dotted line and the
broken line, are extracted from the potential en-
ergies to the Coulomb energies and van der Waals
energies, respectively. It was observed that the
energy distribution curves of two kinds of tetra-
silanes had two similar energy minima; trans
(around 170°), and ortho (85°), respectively.
However, there are remarkable differences in that
HS5 is stabilized at a trans conformation, while
POS5 has an energy minimum in the ortho con-
formation. The reason for these phenomena is
explained by the Coulomb and the van der Waals
energies for each conformer. The Coulomb energy
is a dominant factor in determining the potential
energy of POS5. However, for HS5, the two energy
Fig. 1. UV absorption spectra of POS5 in 3-methylpentane as a
function of temperature.
wavelengths compared to those of peralkylpenta-
silanes. This characteristic will be discussed after
this session. Second, it was shown that the ab-
sorption spectra were broad, having a shoulder up
to 290 nm. Third, the change in the band maxima
on decreasing the temperature was very small.
These tendencies were also observed for HS5.
Fig. 2. Calculated ground state conformational energies of HS4 (a) and POS4 (b) as a function of /1 and / . Potential energies
2
(squares), Coulomb energies (triangles), and van der Waals energies (circles). (c) Potential energies (kcal/mol, relative to the lowest
minimum) of POS5 as a function of / and / .
3
4

H. Kato et al. / Chemical Physics Letters 370 (2003) 154–160
157
distribution curves are quite similar to the poten-
tial energy curve. Columbic repulsion between
oxygen atoms makes the conformation twisted,
and the population of the twisted conformation is
larger than that of HS5.
responding oscillator strengths (f) at the same level
calculations are shown in Fig. 3b. Because of a
large number of transitional states, the results of
only the four lowest excited states are shown in the
figure. Four states are called A and B due to the
symmetry of the orbital and numbered 1 and 2
from the low-lying excited singlet states, respec-
tively. These results are similar to the previously
The potential energy map of POS5 is shown in
Fig. 2c as functions of the dihedral angles / and
3
/4
. The complete ranges of possible dihedral an-
gles are covered in this diagram. The lowest
energy minimum of the energy surface is Ôortho–
orthoÕ conformation; therefore, this figure also
proves that the twisted conformation is the most
stable.
reported results for Si
tetrasilane (MS4) [24–26].
4
H10 [24] and decamethyl-
As seen in Fig. 3b, the f to the states, 1B and 2B
are calculated to be very strong and dependent on
the /, while 1A and 2A are very weak in all ranges
of the angles. These phenomena are reasonable,
because the transitions to the B state mostly con-
sist of symmetry-allowed transitions. It is also
confirmed that the fÕs of 2B were higher than those
of 1B when dihedral angles were <120°, however,
their orders were changed at 120°. This angle
agreed with the results for MS4, but not for
3
.3. Result of ab initio MO simulation
Fig. 3a shows the results of CIS/6-31G* calcu-
lations of singlet excitation energies (E
S
) as a
function of dihedral angles / for OHS4. The cor-
4
Si H10. This showed that there are no effects of the
substituents, even the oxyl or methyl groups, on
the change in f due to the conformation of the Si-
skeleton. The two transitions to 1B and 2B are
dependent on the angle, and they have large f in
the trans and cis conformations, respectively. The
S
E at those conformations are determined to be 6.7
and 7.7 eV, respectively. We have done also the ab
initio MO calculations for MS4 on an equal basis
S
in order to compare the equal order of the E with
OHS4. It was calculated that the excitation ener-
gies of MS4 needed 6.7 and 8.0 eV in the trans and
S
cis conformations, respectively. The E were dif-
ferent between the oligosilanes with hydroxyl and
methyl groups, especially in the region near cis
conformations. It can also be predicted that the E
in the twisted structure of OHS4 has a lower value
than that of MS4. The difference in the E is ex-
S
S
plained by the molecular orbitals. For MS5, the
MO coefficients of the silicons are large and the
connecting carbons have the opposite phase;
therefore, they are localized in each atom in
HOMO. In LUMO, the coefficients of silicons are
delocalized along the silicon chain. For POS5,
molecular orbitals have a similar character to
MS5, except that oxygen atoms have large n-or-
bital coefficients that the carbons do not have in
HOMO. However, the difference is more remark-
Fig. 3. Calculated (CIS/6-31G*) E
function of /. 1A (circles), 1B (squares), 2A (triangles), 2B
rhombus), see in text.
S
(a) and f (b) of OHS4 as a
(

158
H. Kato et al. / Chemical Physics Letters 370 (2003) 154–160
able in LUMO, where the molecular orbital con-
sists of vacant silicon and oxygen orbitals, and the
coefficients of the silicons are delocalized along the
silicon chain and also delocalized on the oxygen
atoms. Therefore, the coefficients did not change
due to the rotation of the dihedral angle in
HOMO; however, the coefficients on the oxygen
atoms became larger by twisting in LUMO. This
difference in the molecular orbital affected the en-
ergy of LUMO and thus the transition energies of
these oligosilanes. This result is fully consistent
with the explanation of the long wavelength shift
of the absorption spectrum of the polysilane hav-
ing oxygen in the side chain [17]. The twisting of
silicon main chain is not significantly influenced to
the delocalozation in LUMO, therefore, reported
alkoxypolysilanes have absorption maxima at
longer wavelength than those of simple alkyl-
polysilanes even they take both twisted and ex-
tended conformations in random coil structure in
solution. In addition, their small temperature de-
pendence and shift to the shorter wavelengths by
decreasing temperature are reasonably matched
above discussions.
We also carried out CIS/6-31G* calculations of
E as a function of the dihedral angles (/ and / )
S
6 7
of OHS5. Calculated E
S
and the oscillator
strengths of the four lowest singlet excitation states
are shown in Figs. 4a–c. At this time, we assumed
three conformational models. First, extended
states is consist of conformers which have dihedral
angles between 180° and 150°, and twisted states
consist of those which have dihedral angles
between 90° and 60°. The three conformers of
extended–extended, extended–twisted, and twis-
ted–twisted are defined as follows, extended–
extended correspond to T
–D ; extended–twisted correspond to T
T –G , D –O , and D –G ; twisted–twisted
ꢀ
–T
ꢀ
, T
ꢀ
–D
ꢀ
, and
D
ꢀ
ꢀ
ꢀ
ꢀ
–O
ꢀ
,
Fig. 4. Calculated (CIS/6-31G*) wavelength (scaled by 0.78)
29] for OHS5 of three conformational models, (a) extended–
[
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
extended, (b) extended–twisted, and (c) twisted–twisted. Spectra
estimated by (a)–(c) and Boltzmann distribution for POS5 (d).
correspond to O –O , O –G , and G –G ,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
where T, D, O, and G indicate transoid, deviant,
ortho, and gauche conformation, respectively, as
shown by Michl and West [27]. The obtained
spectra have been resolved into these three con-
formers as shown in Figs. 4a–c. The theoretically
obtained absorption spectra composed of the three
conformers are similar to those of MS5 calculated
by us and MS4 reported [25].
3.4. Comparison between calculated data and
observed spectra
By combining the data of Figs. 2c and 3a–c, it is
possible to predict the spectrum in the gas state.
Assuming that a Boltzmann distribution is estab-

H. Kato et al. / Chemical Physics Letters 370 (2003) 154–160
159
lished, the abundance ratio of each conformation,
p, is obtained according to Eqs. (1)–(3)
ꢀ
ꢁ
0
E
¼ exp À E
=RT ;
/1/2
ð1Þ
ð2Þ
ð3Þ
/
1/2
Z ¼
X E0
1/2;
/
p/1/2 ¼ E/0 1/2=Z:
In the equation, R is a gas constant and T is the
temperature in equation.
The apparent oscillator strength was synthe-
sized by the superposition of the oscillator strength
obtained for each conformer using its population
as in Eq. (4)
f
k
¼
X f
k;/1/2
p
/1/2
:
ð4Þ
The obtained spectrum is shown in Fig. 4d. This
agrees well with the experimentally obtained
spectrum of POS5 in 3-methylpentane as shown in
Fig. 1. This indicates that the calculated results
(
oscillator strength and potential energy) are re-
producible of the experiment, i.e., the result that
the twisted conformation is stabilized by the
Coulomb repulsion between the oxygen atoms for
POS5 is reasonable. In the spectra in Fig. 1, the
2
00–240 nm region was strongly temperature-de-
pendent, however the 260–280 nm region was
weakly temperature-dependent. This is explained
by the potential energy surface in Fig. 2c. The
equilibrium shifted from unstable twisted–extend-
ed conformer to the most stable twisted–twisted
conformer, but increase of intermediate extended–
extended conformer was small.
Fig. 5. (a) UV absorption spectra of POS5 in different solvents.
(b) Observed UV spectrum resolved into three model con-
formers. (c) Relative population of conformers.
3
.5. Solvent effect on the conformation of POS5
Fig. 5a shows the UV absorption spectra in
acetonitrile/alcohol mixtures. The spectral was
changed with increase of shoulder at 230 nm and
decrease at 210 nm according to the increase in the
polarity of the solvent having an isosbestic point
at 220 nm. The samples were not changed after the
measurements, therefore, three spectral changes
were not due to the exchange alkoxy side chains by
the solvent. This is explained by the simple ex-
tended–twisted model. At first, the observed spec-
trum was separated into three Gaussian functions
as shown in Fig. 5b. Those have a maximum at 210,
230, and 260 nm, and their half-widths at half-
height were 3410, 4100, and 7470 cm , respec-
tively. These functions were tentatively assigned
to twisted–twisted, extended–twisted, and ex-
tended–extended conformers, respectively. This
assumption is reasonable according to the above
discussions. The increase in solvent polarity led
decrease of the twisted–twisted and increase of the
À1

160
H. Kato et al. / Chemical Physics Letters 370 (2003) 154–160
extended–twisted form as shown in Fig. 5c. This
spectral change must be explained as follows. An
alcoholic solvent stabilizes the conformer by hy-
drogen bonding or by protonation when the acidity
of the solvent is high. In the former case, it also
contains the steric effect of solvation [13], and/or
the molecular structure is changed due to the po-
larity of the solvent atmosphere [28,29]. In addi-
tion, the molecular dipole moment of the
extended–twisted conformer (5.4 D) is higher than
those of the other conformers. The direction of the
pentoxy substituent is isotropic; therefore, the di-
pole moments compensate each other in the cases
of extended–extended (2.3 D) and twisted–twisted
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