Dynamics of Positive Charge Carriers on Si Chains of Polysilanes

Article abstract of DOI:10.1021/ja039840e

The transient absorption of radical cations of a variety of substituted polysilanes is discussed quantitatively in terms of the molar extinction coefficient and oscillator strength by nanosecond pulse radiolysis. Oxygen-saturated polysilane solutions in benzene exhibit a strong transient absorption band ascribed to the polysilane radical cation. The transient species react with N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) to produce TMPD radical cations. On the basis of the molar extinction coefficient of the TMPD radical cation, the molar extinction coefficients for the radical cations of polysilanes are found to increase in the range 3.3 × 10⁴ to 2.0 × 10⁵ M^-1 cm ^-1 with increasing polymer segment length. The stepwise increase in the total oscillator strength with an increase in the number of phenyl rings directly bonded to the Si skeleton suggests the delocalization of the positive polaron state and/or the SOMO state over the phenyl rings, indicating the importance of phenyl rings in intermolecular hole transfer processes.

Full text of DOI:10.1021/ja039840e

Published on Web 02/25/2004
Dynamics of Positive Charge Carriers on Si Chains of
Polysilanes
Shu Seki,* Yoshiko Koizumi, Tomoyo Kawaguchi, Hidefumi Habara, and
Seiichi Tagawa*
Contribution from the Institute of Scientific and Industrial Research, Osaka UniVersity,
8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Received November 28, 2003; E-mail: seki@sanken.osaka-u.ac.jp; tagawa@sanken.osaka-u.ac.jp
Abstract: The transient absorption of radical cations of a variety of substituted polysilanes is discussed
quantitatively in terms of the molar extinction coefficient and oscillator strength by nanosecond pulse
radiolysis. Oxygen-saturated polysilane solutions in benzene exhibit a strong transient absorption band
ascribed to the polysilane radical cation. The transient species react with N,N,N′,N′-tetramethyl-p-phenylene-
diamine (TMPD) to produce TMPD radical cations. On the basis of the molar extinction coefficient of the
TMPD radical cation, the molar extinction coefficients for the radical cations of polysilanes are found to
increase in the range 3.3 × 104 to 2.0 × 105 M-1 cm-1 with increasing polymer segment length. The stepwise
increase in the total oscillator strength with an increase in the number of phenyl rings directly bonded to
the Si skeleton suggests the delocalization of the positive polaron state and/or the SOMO state over the
phenyl rings, indicating the importance of phenyl rings in intermolecular hole transfer processes.
Introduction
chromophores, indicating the highly ordered backbone confor-
mations of these polymers.11
Polysilanes bearing saturated Si backbones are of current
interest because of their characteristic electron delocalization
along the Si chain (σ-conjugated system1) and associated near-
UV absorption,2 photodecomposition,3 and nonlinear optical
properties.4 On the basis of their utility as positive charge
conductors, the dynamics of excess electrons and holes on the
Si skeletons have been investigated vigorously in view of their
potential application in electroluminescent diodes and as pho-
toconductors.5,6 The dominant process of charge carrier transport
in polysilanes is the hopping of holes between localized states
originating from domain-like subunits along the Si chain.7,8 The
mean length of the segments is controlled by steric hindrance
of the side chains, thermal molecular motion, or both. Several
groups have reported the synthesis of a series of polysilanes
with backbone conformations varying from random coil to stiff
and rodlike by changing the polymer substitution pattern.9
Recently, Fujiki developed optically active polysilanes with
tightly locked rodlike helical Si backbones achieved by the
incorporation of chiral substituents.10 The screw sense of the
Si backbone was subsequently inverted by thermal treatment
in diarylpolysilanes with chiroptical switching of the main chain
Despite positive charge being the dominant charge carriers
in the transport process, there have been few dynamics or
quantitative analyses on the electronic state of positive charges
on the Si chain other than by transient spectroscopy12,13 or
electron spin resonance.14,15 Localization of the charge carriers
was revealed to be suppressed in the polysilanes bearing bulky
pendant groups, suggesting not only that the localization in
typical dialkyl polysilanes arises from the flexibility of Si
catenation, but also that delocalization occurs in polysilanes with
stiff or rodlike Si skeletons. However, the quantitative correla-
tion between the molecular stiffness and the degree of positive
charge delocalization had not been elucidated to date.
The present paper reports on the direct observation of
polysilane radical cations by pulse radiolysis. Pulse-radiolysis
transient absorption spectroscopy (PR-TAS) is a very powerful
and useful technique for achieving the selective formation of
radical ions in matrixes (solvents) and tracing reaction kinetics.
To date, quantitative tracing of polysilane radical cations has
been very difficult because of their very low ionization potentials
(<6 eV)16,17 and complicating side reactions. Recent improve-
ments in our transient spectroscopy system have made it possible
(1) Rice, M. J. Phys., Lett. 1979, 71A, 152.
(2) West, R. J. Organomet. Chem. 1986, 300, 327.
(3) Trefonas, P.; West, R.; Miller, R. D. J. Am. Chem. Soc. 1985, 107, 2737.
(4) Kajzar, F.; Messier, J.; Rosilio, C. J. Appl. Phys. 1986, 60, 3040.
(5) Kepler, R. G.; Zeigler, J. M.; Harrah, L. A.; Kurtz, S. R. Phys. ReV. 1987,
B35, 2818.
(11) Koe, J. R.; Fujiki, M.; Nakashima, H.; Motonaga, M. Chem Commun. 2000,
389.
(12) Ban, H.; Sukegawa, K.; Tagawa, S. Macromolecules 1987, 20, 1775.
(13) Ban, H.; Sukegawa, K.; Tagawa, S. Macromolecules 1988, 21, 45.
(14) Kumagai, J.; Yoshida, H.; Ichikawa, T. J. Phys. Chem. 1995, 99, 7965.
(15) Seki, S.; Cromack, K. R.; Trifunac, A. D.; Yoshida, Y.; Tagawa, S.; Asai,
K.; Ishigure, K. J. Phys. Chem. B 1998, 102, 8367.
(16) Seki, K.; Mori, T.; Inokuchi, H.; Murano, K. Bull. Chem. Soc. Jpn. 1988,
61, 351.
(17) Ishii, H.; Yuyama, A.; Norioka, S.; Seki, K.; Hasegawa, S.; Fujino, M.;
Isaka, H.; Fujiki, M.; Matsumoto, N. Synth. Met. 1995, 69, 595.
(6) Suzuki, H.; Meyer, H.; Hoshino, S.; Haarer, D. J. Lumin. 1996, 66-67,
423.
(7) Fujino, M. Chem. Phys. Lett. 1987, 136, 451.
(8) Abkowitz, M.; Knier, F. E.; Yuh H. J.; Weagley, R. J.; Stolka, M. Solid
State Commun. 1987, 62, 547.
(9) Fujiki, M. J. Am. Chem. Soc. 1996, 118, 7424.
(10) Fujiki, M. J. Am. Chem. Soc. 1994, 116, 6017.
9
10.1021/ja039840e CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 3521-3528
3521

A R T I C L E S
Seki et al.
Table 1. Substitution Patterns and Molecular Weights of Polysilanes
entry
R1
R2
M a
M b
entry
R1
R2
M a
M b
w
n
w
n
PH1
PH2
PH3
PH4
PH5
PH6
PH7
PH8
PH10
PH12
PD4
PD5
PD6
PD7
CH3
C2H5
n-C3H7
n-C4H9
n-C5H11
C6H13
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
n-C4H9
n-C5H11
n-C6H13
n-C7H15
5.5 × 104
3.4 × 104
5.2 × 104
2.0 × 104
1.7 × 106
4.6 × 106
2.1 × 105
5.2 × 106
8.8 × 105
4.2 × 105
7.8 × 105
1.1 × 106
1.1 × 106
1.9 × 106
4.8 × 104
1.5 × 104
2.4 × 104
1.1 × 104
8.0 × 105
1.1 × 106
8.9 × 104
1.8 × 106
4.0 × 105
1.4 × 105
2.8 × 105
4.1 × 104
5.1 × 105
9.4 × 105
PD8
n-C8H17
n-C10H21
n-C12H25
C3H7
C4H9
C5H11
C6H13
C8H17
C10H21
C12H25
n-C8H17
n-C10H21
n-C12H25
CH3
CH3
CH3
CH3
CH3
CH3
CH3
1.4 × 106
6.1 × 105
1.4 × 106
0.91 × 105
1.0 × 105
1.0 × 105
0.85 × 105
5.5 × 105
1.9 × 105
0.87 × 105
4.9 × 105
3.2 × 105
6.1 × 105
6.8 × 104
1.7 × 105
5.2 × 105
1.5 × 104
0.96 × 104
1.6 × 104
1.0 × 104
3.5 × 104
2.8 × 104
2.1 × 104
1.8 × 105
1.1 × 105
1.5 × 105
PD10
PD12
ME3
ME4
ME5
ME6
ME8
ME10
ME12
PCHMS
DPA
C7H15
C8H17
n-C10H21
n-C12H25
n-C4H9
n-C5H11
n-C6H13
n-C7H15
C6H11
t-C4H9-C6H5
n-C4H9-C6H5
CH3
t-C4H9-C6H5
n-C4H9-C6H5
DPB
a Weight-average molecular weight. b Number-average molecular weight (determined relative to polystyrene).
to probe dilute solutions of transient species with a very high
signal-to-noise (S/N) ratio.18 This paper reports the molar
extinction coefficient and oscillator strength of polysilane radical
cations determined by both spectroscopic techniques and an
efficient charge-transfer reaction between polysilane radical
cations and N,N,N′,N′-tetramethyl-p-phenylene-diamine (TMPD).
The degree of positive charge delocalization on Si chains is
discussed in terms of the molecular stiffness of polysilanes, and
the role of pendant phenyl rings in the charge carrier transport
processes in polymer materials is addressed.
Experimental Section
General. Reagents and chemicals were purchased from Wako
Chemical Co. unless otherwise stated. Nuclear magnetic resonance
(NMR) spectra were collected using a JEOL EX-270 spectrometer for
1H and 13C spectra and using a Bruker ARX-400 spectrometer for 29Si
spectra, with chloroform-d as a solvent and tetramethylsilane as an
internal standard. The molecular weight distribution of the obtained
polymer was measured by gel permeation chromatography (GPC) using
a Shimadzu LC-10Avp liquid chromatography system with tetrahy-
drofuran (THF) as an eluent and polystyrene calibration standards. The
ultraviolet-visible (UV-vis) absorption spectra of the polysilanes in
THF were recorded using a JASCO V-570 and a Shimadzu UV-
3100PC. The photoluminescence spectra of Ar-saturated solutions were
measured using a Perkin-Elmer LS-50B spectrophotometer.
Synthesis. All the monomers except for the compounds described
below were purchased from Shin-Etsu Chemical Co. Ltd. and distilled
twice prior to use. Polymerization was carried out by the Kipping
method using R1-R2-dichlorosilanes with sodium metal in dry toluene
for 5-20 h at 110 °C.19 The polymer solution was precipitated in
Figure 1. Structures of the series of polysilanes employed.
isopropyl alcohol (IPA) after filtration through a 0.45-mm PTFE filter
to eliminate NaCl, and the precipitates were dried in a vacuum. The
toluene solutions of the polymers were transferred into a separatory
funnel, washed with water to eliminate remaining NaCl, and precipitated
in IPA. The polymers were vacuum-dried at 80 °C and precipitated
again in THF-methanol. The polymers were fractionally precipitated
again to eliminate the lower molecular weight fraction of the bimodal
distribution. The synthesized polymer entries and corresponding
molecular weights are listed in Table 1, and the structures are
schematized in Figure 1. The synthesis of poly[bis(p-t-butyl-phenyl)-
silane] (DPA) and poly[bis(p-n-butyl-phenyl)silane] (DPB) are de-
scribed elsewhere.18
n-propylbromide with magnesium metal was cooled to -78 °C. Doubly
distilled phenyltrichlorosilane purchased from Shin-Etsu Chemical Co.
Ltd. was then added rapidly into the solution (1.0 equiv) and stirred
for 2 h. The solution was slowly warmed to room temperature and
refluxed for another 1 h. After rough filtration to remove MgBrCl, the
solution was concentrated and distilled three times. The boiling points
(bp) of P2 and P3 were 70 °C/5 Torr and 90 °C/5 Torr, respectively
(yield: 52-60%). The polymerization of P2 and P3 gave PH2 and
PH3 (yield: 32 and 28%). P2: 1H NMR (270 MHz, CDCl3), δ 1.14 (t,
3H, CH3), 1.33 (q, 2H, CH2), 7.41-7.54, (m, 3H, ArH), 7.70 (t, 2H,
ArH). P3: 1H NMR (270 MHz, CDCl3), δ 1.02 (t, 3H, CH3), 1.31-
1.37 (m, 2H, CH2-CH3), 1.52-1.67 (m, 2H, Si-CH2), 7.41-7.54 (m,
3H, ArH), 7.70-7.73 (m, 2H, ArH).
Ethylphenyldichlorosilane (P2) and Propylphenyldichlorosilane
(P3). A solution of ethylmagnesium bromide or n-propylmagnesium
bromide in diethyl ether obtained by the reaction of ethylbromide or
n-Butyl (P4), n-Pentyl (P5), n-Hexyl (P6), n-Heptyl (P7), n-Octyl
(P8), n-Decyl (P10), and Dodecyl (P12) Phenyldichlorosilanes.
Solutions of n-alkylmagnesium bromide in THF were obtained by
reaction of the corresponding n-alkylbromide with magnesium metal.
(18) Seki, S.; Matsui, Y.; Yoshida, Y.; Tagawa, S.; Koe, J. R.; Fujiki, M. J.
Phys. Chem. B 2002, 106, 6849.
(19) Kipping, F. S. J. Chem. Soc. 1924, 125, 2291.
9
3522 J. AM. CHEM. SOC. VOL. 126, NO. 11, 2004

Positive Charge Carriers on Si Chains of Polysilanes
A R T I C L E S
Phenyltrichlorosilane (1.2 equiv) was then added to the solution and
stirred for 5 h. The solution was refluxed for another 2 h at 68 °C.
After filtration, the solution was concentrated and distilled three times.
The bp values were 104 °C/5 Torr (P4), 108 °C/5 Torr (P5), 120 °C/5
Torr (P6), 128 °C/5 Torr (P7), 136 °C/5 Torr (P8), 164 °C/5 Torr (P10),
and 189 °C/5 Torr (P12) (yield: 65-70%). The polymerization of P4
and P12 gave PH4 and PH12 (yield: 7-22%). P4: 1H NMR (270
MHz, CDCl3), δ 0.90 (t, 3H, CH3), 1.31-1.44 (m, 4H, CH2), 1.47-
1.59 (m, 2H, Si-CH2), 7.42-7.55 (m, 3H, ArH), 7.70-7.74 (m, 2H,
ArH). P5: 1H NMR (270 MHz, CDCl3), δ 0.86 (t, 3H, CH3), 1.27-
1.42 (m, 6H, CH2), 1.45-1.58 (m, 2H, Si-CH2), 7.42-7.54 (m, 3H,
ArH), 7.70-7.75 (m, 2H, ArH). P6: 1H NMR (270 MHz, CDCl3), δ
0.87 (t, 3H, CH3), 1.24-1.45 (m, 8H, CH2), 1.49-1.58 (m, 2H, Si-
CH2), 7.39-7.51 (m, 3H, ArH), 7.69-7.73 (m, 2H, ArH). P7: 1H NMR
(270 MHz, CDCl3), δ 0.87 (t, 3H, CH3), 1.25-1.45 (m, 10H, CH2),
1.49-1.60, (m, 2H, Si-CH2), 7.41-7.53 (m, 3H, ArH), 7.70-7.73,
(m, 2H, ArH). 13C NMR (270 MHz, CDCl3): 14.20 (CH3), 20.80 (CR,
1JSiC ) 71 Hz), 22.56, 22.75, 28.86, 31.73, 32.47, 128.19, 131.40,
133.19, 133.23 ppm. 29Si NMR (400 MHz, CDCl3): 19.2 ppm. P8:
1H NMR (270 MHz, CDCl3), δ 0.87 (t, 3H, CH3), 1.25-1.39 (m, 12H,
CH2), 1.48-1.57, (m, 2H, Si-CH2), 7.41-7.53 (m, 3H, ArH), 7.70-
7.73 (m, 2H, ArH). P10: 1H NMR (270 MHz, CDCl3), δ 0.86 (t,3 H,
CH3), 1.23-1.38 (m, 16H, CH2), 1.47-1.59 (m, 2H, Si-CH2), 7.40-
7.53 (m, 3H, ArH), 7.69-7.72 (m, 2H, ArH). 13C NMR (270 MHz,
1
CDCl3) 14.25 (CH3), 20.79 (CR, JSiC ) 71 Hz), 22.55, 22.80, 29.18,
29.41, 29.54, 29.68, 29.83, 31.99, 32.51, 128.19, 131.40, 132.63, 133.22
ppm. 29Si NMR (400 MHz, CDCl3): 19.2 ppm. P12: 1H NMR (270
MHz, CDCl3), δ 0.88 (t, 3H, CH3), 1.25-1.35 (m, 20H, CH2), 1.49-
1.54 (m, 2H, Si-CH2), 7.37-7.50 (m, 3H, ArH), 7.68-7.72 (m, 2H,
1
ArH). 13C NMR (270 MHz, CDCl3): 14.28 (CH3), 20.81 (CR, JSiC
)
71 Hz), 22.58, 22.84, 29.20, 29.43, 29.58, 29.70, 29.85, 29.90, 29.93,
31.90, 32.54, 128.19, 131.40, 132.63, 133.22 ppm. 29Si NMR (400 MHz,
CDCl3): 19.2 ppm.
Figure 2. UV-vis absorption spectra of (a) poly(n-alkylphenylsilane)s,
(b) poly(di-n-alkylsilane)s, and (c) poly(methyl-n-alkylsilane)s. Spectra were
recorded at 25 °C for solutions of polysilanes in THF at 1.0 × 10-4 mol
dm-3 (base mol unit).
Di-n-heptyl (D7), Di-n-octyl (D8), Di-n-decyl (D10), and Di-n-
dodecyl (D12) Dichlorosilane. The corresponding n-alkyl-trichloro-
silane (1.2 equiv) was rapidly added to the solutions of n-alkylmag-
nesium bromide in diethyl ether at 0 °C, allowed to warm to room
temperature, and stirred for 12 h. The solution was refluxed for another
2 h. After filtration, the solution was concentrated and distilled three
times. The bp were 142 °C/3 Torr (D7), 165 °C/3 Torr (D8), 190 °C/1
Torr (D10), and 195 °C/0.5 Torr (P12) (yield: 40-55%). The freezing
points of D10 and D12 were 7 and 12 °C, respectively. The
polymerization of D7 and D12 gave PD7 and PD12 (yield: 11-24%).
Results and Discussion
The UV-vis absorption spectra of three series of polysilanes
(n-alkylphenyl, di-n-alkyl, and methyl-n-alkyl substituted) are
shown in Figure 2. The intense UV absorption band observed
for steady-state polysilane solutions is ascribed to the transition
between the valence band (VB) and the lowest excitonic states
(ES) of the Si backbones.22 The molar extinction coefficient
(ꢀabs) and oscillator strength (fVB-ES) of the absorption are listed
in Table 2. The values of both ꢀabs and fVB-ES depend strongly
on the substitution patterns, with a considerable increase
accompanying the change from asymmetric substitution (ME3-
ME12) to symmetric (PD4-PD12). The values also increase
dramatically with elongation of n-alkyl substituents from methyl
to n-octyl in the case of poly(n-alkylphenylsilane)s, with only
a slight drop with further elongation beyond n-octyl. Recently,
Fujiki reported an empirical relationship between ꢀabs and the
viscosity index R, reflecting the geometric structure of the
Pulse Radiolysis. All polysilanes were dissolved at 5 mM (base
mol unit) in benzene (Bz) (spectroscopic grade from Dojin Chemical
Co. Ltd.) or distilled 1-chlorobutane. O2 was bubbled through the
solutions for at least 5 min. After recrystallization in Bz, TMPD was
added to the benzene solutions at concentrations of 0.098-0.37 mM.20
The solutions were place into Suprasil quartz cells with a 2-cm optical
path and irradiated with a single 8-ns electron pulse at room temperature.
An Xe flash lamp was used as the source of light for analysis, with a
continuous spectrum from 300 to 1600 nm. The analyzing light was
monitored with a Ritsu MC-10N monochromator and detected by a
PIN Si or InGaAs photodiode. The pulse radiolysis measurement was
performed using an L-band electron linear accelerator at the Radiation
Laboratory of the Institute of Scientific and Industrial Research, Osaka
University. Dose per pulse was determined based on the Gꢀ value of
(SCN)2- as 2.23 × 10-2 dm2 J-1 for the irradiation of KSCN aqueous
solution at 5.0 × 10-3 mol dm-3 concentrated, giving the value of dose
per pulse in the present apparatus as 200-220 Gy. Other details of the
apparatus are described elsewhere.21 The typical instrument function
was ca. 8 ns.
polymer main chain. The following empirical formula was
9
obtained for the relationship between R and ꢀabs
:
ꢀabs ) 1130‚e2.9R
(1)
The relationship between gyration length (Rg) and R is as
follows:
(20) Stegman, J.; Cronkright, W. J. Am. Chem. Soc. 1970, 92, 6736.
(21) Seki, S.; Yoshida, Y.; Tagawa, S.; Asai, K. Macromolecules 1999, 32, 1080.
(22) Tachibana, H.; Kawabata, Y.; Koshihara, S.; Tokura, Y. Solid State
Commun. 1990, 75, 5.
9
J. AM. CHEM. SOC. VOL. 126, NO. 11, 2004 3523

A R T I C L E S
Seki et al.
Table 2. UV Absorption, Fluorescence, and Optical Properties of Cation Radicals of Poly(n-alkylphenylsilane)s
abs
max
fl
max
•+
abs
max
fl
max
•+
λ
λ
λ
λ
λ
λ
max
max
entry
(nm)
ꢀabs
fVB-ES
(nm)
(nm)
ꢀ•+ c
f•+
entry
PD8
PD10
PD12
ME3
ME4
ME5
ME6
ME8
ME10
ME12
PCHMS
DPA
(nm)
ꢀabs
fVB-ES
(nm)
(nm)
ꢀ•+ c
5.2
f•+
a
b
a
b
PH1
PH2
PH3
PH4
PH5
PH6
PH7
PH8
PH10
PH12
PD4
PD5
PD6
PD7
339
331
336
341
342
347
348
348
348
348
317
318
319
321
7900
4300
5700
8000
9900
11 700
14 300
17 500
16 600
12 800
10 400
10 400
10 600
10 900
0.091
0.0547
0.0642
0.0774
0.0914
0.11
365
366
366
366
367
369
369
370
369
369
346
347
347
348
365
360
363
365
369
372
372
372
370
370
346
346
344
340
9.4
6.9
7.1
9.8
12
15
16
18
17
0.49
0.38
0.42
0.51
0.59
0.65
0.59
0.63
0.66
0.60
0.31
0.32
0.37
0.38
321
324
325
307
306
307
307
309
310
311
326
377
393
11 600
11 700
10 600
5200
6400
6100
5600
5500
5300
4700
0.057
0.057
0.050
0.043
0.045
0.045
0.043
0.041
0.040
0.034
0.054
0.0750
0.11
348
349
350
338
336
336
336
336
337
336
347
342
342
342
348
342
346
344
338
338
0.39
0.41
0.46
0.24
0.25
0.28
0.33
0.32
0.26
0.24
0.33
0.65d
>0.80d
5.5
5.5
3.8
4.1
4.5
5.3
4.5
3.3
3.5
4.9
0.11
0.13
0.13
0.110
0.053
0.056
0.056
0.057
16
342
4.5
4.7
5.1
5.2
7320
7600
13 300
355
392d
<405d
17d
>20d
DPB
a Molar extinction coefficient per Si unit, in mol-1 dm3 cm -1 b Oscillator strength obtained by numerical integration. c Molar extinction coefficient per
.
radical cation at the transient absorption maximum, in 104 mol-1 dm3 cm-1
.
d Data quoted from ref 18.
all of the polysilanes in the present study, flexible Kuhn chains
are acceptable as the first approximation model of their chain
configurations on the basis of R and molecular weight. The
gyration length of the flexible Kuhn chain is given by
1/2
r2
0
R )
(3)
(
)
g
6
where r2 0 is the mean-square end-to-end distance observed in
θ solutions. The value of q at the chosen molecular weight is
derived from the mean-square end-to-end distance ( r2 ) as
follows:
ML
q ) r2
(4)
(
)
Figure 3. Viscosity index (R) of poly(n-alkylphenylsilane)s, poly(di-n-
alkylsilane)s, and poly(methyl-n-alkylsilane)s as a function of length of
2M
n-alkyl substituents. Values of a are estimated from eq 1 based on ꢀabs
.
where ML is the mass per unit length. The excluded volume
parameter a is defined as a2 ≡ r2 / r2 0 and has been reported
to be less than 1.2 for DPB at 25 °C in toluene, which is a
better solvent than THF.24 The low second virial coefficient of
PD6 in THF (1.14 × 10-4 mL mol/g2) also supports the small
contributions of excluded volume effects on the conformations
of polysilanes with longer segment lengths than PD6.23,24 Thus,
the assumption of a ) 1 gives an appropriate estimate of the
segment length for DPB, PH5-PH12, and PD6-PD12, although
the segment length of polysilanes with highly flexible backbones
may be underestimated. On the basis of the Si-Si bond length
(0.2414 nm) and Si-Si-Si bond angle (114.4°),27 the estimated
Kuhn segment length varies from 0.55 to 8.6 nm (PH2-PH8)
for the poly(n-alkylphenylsilane)s, 0.48-1.1 nm (ME3-ME10)
for the poly(methyl-n-alkylsilane)s, and 2.4-6.3 nm (PD4-
PD12) for the poly(di-n-alkylsilane)s.
Rg ) κMν
(2)
where κ is a constant, M is the molecular weight of the polymer,
and ν ) (R + 1)/3. The estimated values of R are shown in
Figure 3; they vary from 0.46 for PH2 to 0.94 for PH8.
Assuming a constant polymerization degree for the polysilanes,
Rg is expected to increase with ꢀabs. The actual Kuhn segment
lengths (q) of the polymers were found to be q ) 1.1 nm for
PH1,18 3.0 nm for PD6,23,24 and 4.5 nm for DPB determined
by light-scattering experiments.18,25 All the polysilanes prepared
in the present study have high molecular weights (>104), low
polydispersities (<4), and monomodal distributions similar to
the Schulz-Zimm distribution. The polysilanes with high values
of R such as DPB or PH5-PH12, expected to exhibit longer
segment length, have molecular weights higher than 105. A
flexible wormlike chain model with a persistence length of ∼10
nm successfully reproduced the global dimension of the Si chain
in DPB with a molecular weight of ∼105.26 Thus, for almost
The segment length also reflects the degree of delocalization
of the lowest excitonic state along the Si chains and is described
by the following empirical relationship:9,18,28
ꢀabs ) 330‚L
(5)
(23) Cotts, P. M.; Ferline, S.; Dagli, G.; Pearson, D. S. Macromolecules 1991,
24, 6730.
(24) Shukla, P.; Cotts, P. M.; Miller, R. D.; Russel, T. P.; Smith, B. A.; Wallraff,
G. M.; Baier, M.; Thiyagarajan, P. Macromolecules 1991, 24, 5606.
(25) Koe, J. R.; Fujiki, M.; Motonaga, M.; Nakashima, H. Macromolecules 2001,
34, 1082.
(26) Cotts, P.; Miller, R. D.; Sooriyakumaran, R. In Silicon-Based Polymer
Science: A ComprehensiVe Resource; American Chemical Society: Wash-
ington, DC, 1990; Chapter 23, p 397.
where L is the segment length in Si repeating numbers. Equation
3 gives a shortest value of L = 13 for PH2 and a longest value
(27) Koe, J. R.; Powell, D. R.; Buffy, J. J.; Hayase, S.; West, R. Angew. Chem.,
Int. Ed. 1998, 37, 1441.
(28) Schreiber, M.; Abe, S. Synth. Met. 1993, 55-57, 50.
9
3524 J. AM. CHEM. SOC. VOL. 126, NO. 11, 2004

Positive Charge Carriers on Si Chains of Polysilanes
A R T I C L E S
of L = ∼53 for PH8 in the present study, consistent with the
changes in fVB-ES
.
In the pulse radiolysis experiment, the incident electron pulse
in the solution produces radical cations (Bz•+), excited states
(Bz*), and electrons as follows:
Bz f Bz•+, Bz*, e-
(6)
The excited states and excess electrons are rapidly scavenged
in oxygen-saturated solutions ([O2] ) 11.9 mM at 1 atm,
25 °C), giving singlet oxygen molecules (1O2) and oxygen
anions (O2-).29-31 Polysilanes (PS) in the solution react with
Bz•+ via Bz2•+, yielding polymer radical cations (PS•+):
Bz•+ + PS
9
k 8 Bz + PS•+
(7)
1
We have already discussed the transient spectra of radical
ions of polysilanes, including PH1, PD6, etc.12,18,21,29,32,33 PS•+
was found to exhibit two absorption bands in the near-UV
(∼350-400 nm) and infrared (IR; >1600 nm) regions. The
transient spectra of PS•+ suggest the presence of an interband
level occupied by a hole (IBL+). The UV and IR bands in PS•+
are due to transition from IBL+ to the conduction band (CB)
and from the valence band (VB) to IBL+, respectively. The
presence of IBL+ is well interpreted as a delocalized positive
polaron state21,34-36 and/or Anderson localization state on an
Si segment.37,38 Figures 4-6 show a series of transient absorp-
tion spectra of radical cations observed in poly(n-alkylphenyl-
silane)s, poly(methyl-n-alkylsilane)s, and poly(di-n-alkylsilane)s,
respectively. The spectra observed in Figures 5 and 6 are almost
identical, with only a small dependence on the chain length of
n-alkyl substituents. In contrast, poly(n-alkylphenylsilane)s in
Figure 4 exhibit dramatic changes in the IR band with a
considerable red-shift upon elongation of the n-alkyl chains.
The peak energy of the IR band in relation to the binding energy
of positive polarons on the Si chains21,39 is given by:
2
∆V
2A úp
a
δꢀ ≈
(8)
(
)( )
where δꢀ denotes the binding energy of a polaron, a denotes a
lattice unit of a trans-chain segment, and êp is the polaron width.
V is the matrix element describing the interaction between two
atomic orbitals consisting of a covalent bond, and ∆ denotes
the matrix element between two atomic orbitals of an Si atom.
The parameter A is given by
Figure 4. Transient absorption spectra of cation radicals of (a) PH1, (b)
PH2, (c) PH5, (d) PH6, (e) PH8, (f) PH10, and (g) PH12 in Bz at 5.0 ×
10-3 mol dm-3 (base mol unit). Spectra were observed for O2-saturated
solutions at room temperature, recorded 100 ns after electron pulse
irradiation.
(29) Kawaguchi, T.; Seki, S.; Okamoto, K.; Saeki, A.; Yoshida, Y.; Tagawa, S.
Chem. Phys. Lett. 2003, 374, 353.
(30) Candeias, L. P.; Wildeman, J.; Hadziioannou, G.; Warman, J. M. J. Phys.
Chem. B 2000, 104, 8366.
(31) Grozema, F. C.; Siebbeles, L. D. A.; Warman, J. M.; Seki, S.; Tagawa, S.;
Scherf, U. AdV. Mater. 2002, 14, 228.
(32) Seki, S.; Yoshida, Y.; Tagawa, S. Radiat. Phys. Chem. 2001, 60, 411.
(33) Seki, S.; Kunimi, Y.; Nishida, K.; Yoshida, Y.; Tagawa, S. J. Phys. Chem.
2001, B105, 900.
(34) Rice, M. J.; Phillpot, S. R. Bull. Am. Phys. Soc. 1987, 58, 937.
(35) Abkowitz, M. A.; Rice, M. J.; Stolka, M. Philos. Mag. 1990, B61, 25.
(36) Seki, S.; Yoshida, Y.; Tagawa, S.; Asai, K.; Ishigure, K.; Furukawa, K.;
Fujiki, M.; Matsumoto, N. Philos. Mag. 1999, B79, 1631.
(37) Ichikawa, T.; Yamada, Y.; Kumagai, J.; Fujiki, M. Chem. Phys. Lett. 1999,
306, 275.
(38) Ichikawa, T.; Sumita, M.; Kumagai, J. Chem. Phys. Lett. 1999, 307, 81.
(39) Pitt, C. G. In Homoatomic Rings, Chains, and Macromolecules of Main
Group Elements; Rheingold, A. L., Ed.; Elsevier: Amsterdam, 1977.
A ≡ [V(l) - ∆]
a
sin θ
l )
(9)
where 2θ is the tetrahedral bond angle. Thus, ∆ is a parameter
specifying the degree of delocalization of σ-electrons on a
σ-conjugated segment, while V describes the localization of a
pair of electrons in a local bond. The relative polaron width on
9
J. AM. CHEM. SOC. VOL. 126, NO. 11, 2004 3525

A R T I C L E S
Seki et al.
Figure 5. Transient absorption spectra of cation radicals of (a) ME3, (b)
ME4, (c) ME5, (d) ME6, (e) ME8, (f) ME10, and (g) ME12 in Bz at
5.0 × 10-3 mol dm-3 (base mol unit). Spectra were observed for
O2-saturated solutions at room temperature, recorded 100 ns after electron
pulse irradiation.
Figure 6. Transient absorption spectra of cation radicals of (a) PD4, (b)
PD5, (c) PD6, (d) PD7, (e) PD8, (f) PD10, and (g) PD12 in Bz at 5.0 ×
10-3 mol dm-3 (base mol unit). Spectra were observed for O2-saturated
solutions at room temperature, recorded 100 ns after electron pulse
irradiation.
the polymers can be estimated based on eq 8. With the
previously reported values of ∆ in poly(dimethylsilane)40,41 and
V in poly(dichlorosilane),27,41 the value of (∆V/2Α) can be
estimated to be ca. 1.6 eV. The transition energy of the IR band
observed for PH2 is 0.77 eV, the highest value for all the
polysilanes, yielding êp ) ∼2 Si repeating units as the minimum
value. The considerable red-shift of the IR band observed for
poly(n-alkylphenylsilane)s induces a dramatic increase in the
polaron width and, hence, a highly delocalized positive charge
state. Figure 7 shows the transient UV band observed for the
ME3 solution. The spectra appear to suggest the simultaneous
formation of the radical anion and cation in the Ar-saturated
(40) Kumada, M.; Tamao, K. AdV. Organomet. Chem. 1968, 6, 80.
(41) Pollard, W. B.; Lucovsky, G. Phys. ReV. B 1982, 26, 3172.
9
3526 J. AM. CHEM. SOC. VOL. 126, NO. 11, 2004

Positive Charge Carriers on Si Chains of Polysilanes
A R T I C L E S
Figure 8. Kinetic traces of ME3 cation radicals in O2-saturated Bz at
5.0 × 10-3 mol dm-3 (base mol unit). Traces are observed at 348 nm
and 570 nm (dotted: ME3 with TMPD at 0.19 × 10-3 mol dm-3).
Figure 7. UV band in the transient absorption spectra of ME3 in Bz at
5.0 × 10-3 mol dm-3 (base mol unit). Spectra were recorded 100 ns after
electron pulse irradiation. Solid (0), dashed (]), and dot-dashed (O) lines
are spectra observed in O2-saturated and Ar-saturated with TEA at 2.0 ×
10-3 mol dm-3 and Ar-saturated solutions, respectively.
(solid: ME3 only, dashed: ME3 with TMPD at 0.19 × 10-3 mol dm-3
)
In the quantitative elucidation of the degree of charge
delocalization on the Si skeleton, we have already reported the
degree of charge delocalization (ndel) determined through
simultaneous observation of the transient bleaching of the lowest
excitonic backbone peak (∆ODBl) and the formation of the UV
bands (∆OD•(), as given by18,32
solution, leading to overlap of the two transient absorption bands
at 358 and 348 nm, which have been determined independently
in previous work.29,32,33 In contrast to the Ar-saturated solution,
only the radical anion or the radical cation is selectively formed
in the Ar-saturated solution with 20 mM triethylamine (TEA)
or the O2-saturated solution. The kinetic trace is recorded over
a wide range of observation time from a few nanoseconds to
microseconds. The extinction coefficient of the radical cations
can be determined by the charge-transfer reaction between PS•+
and TMPD as follows:
Bl
∆
∆
OD‚ꢀ•(
ndel
)
(12)
•(
OD
‚ꢀES
The empirical relationship between the apparent oscillator
strength of the UV band (f •() and the value of ndel has also
been obtained, expressed as:18,33
PS•+ + TMPD
9
k 8 TMPD•- + PS
(8)
3
ndel f •(
(13)
(14)
However, TMPD•+ is also formed by direct reaction with Bz•+,
i.e.:
f
•( ) 4.32 × 10-9
ꢀ
•( dν
∫
Bz•+ + TMPD
9
k 8 Bz + TMPD•+
(9)
The values of f •+ can be obtained by numerical integration
of the Gaussian fit to the IBL+-CB transitions. The obtained
values of f •+ are also summarized in Table 2. The half-Gaussian
(high-energy side), half-Lorentzian (low-energy side) fit gives
lower values of f •+, but the difference between the two fittings
is less than 20% for all spectra. The values of f •+ for a series
of poly(dialkylsilane)s, poly(n-alkylphenylsilane)s, and poly-
(arylsilane)s are plotted as a function of R in Figure 9. The
values of ꢀ•+ principally reflect the values of R and exhibit a
similar dependence on the degree of delocalization of excitonic
states on the Si backbone. However, ꢀ•+ and f •+ values observed
for the phenyl-substituted polysilanes are clearly higher than
those for the dialkyl-substituted polysilanes.
The values of both q and L derived from eqs 4 and 5 clearly
indicate that the physical segmentation length in Si backbones
of the ground-state or excited-state polysilanes is well interpreted
in terms of molecular stiffness R. In the case of radical cations,
the dependence of f •+, or ndel, on molecular stiffness can be
apparently categorized into three series: polysilanes bearing no,
one, or two phenyl rings. It should be noted that the transient
spectra in the present study were recorded after intrachain
charge-transfer processes, which were expected to occur within
a few nanoseconds;43 thus, the spectra are mainly due to
2
ME3•+ and TMPD•+ decay primarily via second-order reactions
-
with O2 as follows:
PS•+ + O2- 98 PS, O2, oxidized polymers
(10)
(11)
k4
TMPD•+ + O2- 98 TMPD, O2, products
k5
The formation process of TMPD•+ can be observed as an
increasing process in the transient absorption at 570 nm, at which
TMPD•+ in Bz peaks with an extinction coefficient of 1.2 ×
104 M-1 cm-1 20
Figure 8 shows the formation kinetics of
.
TMPD•+ at 570 nm and the decay process of ME3•+ at 348 nm
by the addition of TMPD. The values of k1, k2, and k3 are
estimated to be 5.6 × 109, 3.2 × 109, and 1.8 × 1010 M-1 s-1
for ME3. The kinetic trace observed for ME3•+ in the presence
of TMPD is well fitted by a simulation on the basis of these k1,
k2, and k3 values and the assumption of k4-5 ≈ 105 M-2 s-1
,
with only minor disagreement in the longer time region. The
fitting of the trace based on the extinction coefficient of TMPD•+
gives PS•+ extinction coefficients (ꢀ•+) of 3.3 × 104 to 2.0 ×
105 M-1 cm-1, as listed in Table 2. The initial free ion yield in
benzene (G(Bz+)) is estimated to be G(Bz+) ) 0.050 ( 0.004,
which is in good agreement with the previously reported value.42
(42) Gee, N.; Freeman, G. R. Can. J. Chem. 1992, 70, 1618.
9
J. AM. CHEM. SOC. VOL. 126, NO. 11, 2004 3527

A R T I C L E S
Seki et al.
the phenyl ring is a good explanation for the increase in f •+ for
poly(n-alkylphenylsilane). This explanation is supported by
models of the spread of the singly occupied molecular orbital
(SOMO) state into substituents, as calculated theoretically for
polysilane radical cations.14,37 Therefore, the UV band includes
both the transition from IBL+ to the CB and the transition from
IBL+ to pseudo-π at an energy of ∼0.2 eV below the CB.
Conclusion
The molar extinction coefficients of radical cations were
examined quantitatively for a variety of substituted polysilanes.
On the basis of a charge-transfer reaction between PS•+ and
TMPD, the extinction coefficients of PS•+ (ꢀ•+) were estimated
to be 3.3 × 104 to >2.0 × 105 M-1 cm-1. The values of ꢀ•+
principally reflect the values of R and exhibit a similar tendency
to the degree of delocalization of the excitonic state on the Si
backbone. The observed values of ꢀ•+ and f •+ for phenyl-
substituted polysilanes were significantly higher than those for
dialkyl-substituted polysilanes. A contribution by σ-π mixing
effects in polysilanes bearing phenyl rings readily explains the
higher oscillator strength in poly(n-alkylphenylsilane)s and poly-
(diarylsilane)s.
Figure 9. Oscillator strength (f •+) of UV band vs viscosity index (R).
energetically favorable segments in polysilane backbones.
Despite the fact that PH2 exhibit almost identical Stokes shifts
to those of any poly(dialkylsilanes), a relatively high value of
f •+ is observed for PH2. This is strongly suggestive of a crucial
contribution from the phenyl rings in the delocalization of
positive charges. Takeda et al. suggested that, on the basis of
theoretical calculations, the valence and excitonic states of
steady-state polysilanes bearing phenyl rings are considerably
affected by σ-π mixing.44 The present results also indicate that
this is the case, giving rise to IBL+. A delocalized IBL+ over
Acknowledgment. We acknowledge Prof. M. Fujiki of Nara
Institute of Science and Technology (NAIST), Graduate School
of Material Science, and Prof. J. R. Koe of Department of
Chemistry, International Christian University for supplying DPA
and DPB. We thank Dr. Y. Kunimi, Mr. Suemine, and Mr. T.
Yamamoto of the ISIR, Osaka University for experimental
support. This work was supported in part by a grant-in-aid for
scientific research from the Japan Society for the Promotion of
Science.
(43) Matsui, Y.; Nishida, K.; Seki, S.; Yoshida, Y.; Tagawa, S.; Yamada, K.;
Imahori, H.; Sakata, A. Organometallics 2002, 21, 5144.
(44) Matsui, Y.; Seki, S.; Tagawa, S. Chem. Phys. Lett. 2002, 357, 346.
(45) Takeda, K.; Teramae, H.; Matsumoto, N. J. Am. Chem. Soc. 1986, 108,
8186.
JA039840E
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3528 J. AM. CHEM. SOC. VOL. 126, NO. 11, 2004