Synthesis, structures, and photophysical properties of silicon and carbon-bridged ladder oligo(p-phenylenevinylene)s and related π-electron systems

Article abstract of DOI:10.1016/j.jorganchem.2005.05.019

A series of partially or fully fused ladder oligo(p-phenylenevinylene)s (LOPVs) and related π-electron systems has been synthesized. Thus, the intramolecular reductive cyclization of o-silyl-substituted bis(phenylethynyl) benzenes with lithium naphthaleni

Full text of DOI:10.1016/j.jorganchem.2005.05.019

Journal of Organometallic Chemistry 690 (2005) 5365–5377
www.elsevier.com/locate/jorganchem
Synthesis, structures, and photophysical properties of silicon
and carbon-bridged ladder oligo(p-phenylenevinylene)s and related
p-electron systems
*
Shigehiro Yamaguchi , Caihong Xu, Hiroshi Yamada, Atsushi Wakamiya
Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan
SORST, Japan Science and Technology Agency (JST), Chikusa, Nagoya 464-8602, Japan
Received 5 March 2005; received in revised form 7 May 2005; accepted 12 May 2005
Available online 18 July 2005
Abstract
A series of partially or fully fused ladder oligo(p-phenylenevinylene)s (LOPVs) and related p-electron systems has been synthe-
sized. Thus, the intramolecular reductive cyclization of o-silyl-substituted bis(phenylethynyl)benzenes with lithium naphthalenide
produces partially silicon-bridged bis(styryl)benzenes consisting of silaindene or disilaindacene skeletons. By combining this cycli-
zation with the Friedel–Crafts type electrophilic cyclization, a homologous series of the fully fused LOPVs and related compounds,
bearing silicon and carbon bridges, has been synthesized in fairly good yields. The longest example of the LOPVs is the 13-ring-fused
system that has a nearly flat p-conjugated framework with a length of 2.9 nm, as proven by X-ray crystallography. All the produced
ladder p-electron systems show intense fluorescence in the visible region with high quantum yields as well as relatively small Stokes
shifts. As the silicon contents increase or the disilaindacene skeleton is incorporated, the emission maxima shift to the longer wave-
lengths and the fluorescent quantum yields slightly decrease. These trends can be rationalized as due to the r* effect of silicon,
wherein the silicon bridges contribute to the electronic structure through r*–p* orbital interaction that cause the red-shifts in
the emission maxima and suppress the radiative decay process from the singlet excited state.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Oligo(p-phenylenevinylene)s; Ladder p-conjugated systems; Silicon; Intramolecular reductive cyclization; Fluorescence
1. Introduction
nating ladder molecules, such as ladder oligo- or poly(p-
phenylene)s with carbon [2] or heteroatom bridges [3],
polyacenes and heteroacenes [4], and related p-conju-
gated systems [5]. However, only a limited attention
has been paid to the phenylenevinylene-based ladder
molecules, probably due to their difficult synthesis [6].
For example, whereas the simplest methylene-bridged
stilbene 1 is well known to show an intense fluorescence
with the quantum yield of nearly unity [7], its synthesis
has not been improved from its original report [8] and
no related work has been reported on the synthesis of
its higher oligomers or polymers.
Ladder p-conjugated molecules are promising materi-
als for organic electronics and optoelectronics, including
electroluminescence (EL) devices, thin film transistors,
and optically pumped lasers [1]. The virtue of this class
of molecules is their rigid coplanar structures that prom-
ise enhanced p-conjugation, leading to a set of desirable
properties, such as an intense luminescence and high
carrier mobility. In the last decade, the persistent inter-
est in this field has led to the synthesis of various fasci-
*
In contrast to the carbon-bridged system, significant
progress has recently been made in the chemistry of its
Corresponding author. Tel.: +81527892291; fax: +81527895947.
E-mail address: yamaguchi@chem.nagoya-u.ac.jp (S. Yamaguchi).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.05.019

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S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
silicon analog, i.e., bis-silicon-bridged stilbene 2 [9]. In
1997, Barton and his coworkers [10] reported the first
synthesis of the bis-silicon-bridged stilbene 2 by the ele-
gant rearrangement of 5,6-disiladibenzo[c,g]cycloocty-
nes. They installed this skeleton as a blue-emitting
pendant group on polymeric systems. We also indepen-
dently reported the efficient synthesis of the silicon-
bridged stilbene derivatives based on a newly developed
intramolecular reductive cyclization, as shown in
Scheme 1 [11]. On the basis of this methodology, we pre-
pared various difunctionalized stilbene derivatives, and,
with these derivatives in hand, we succeeded in the syn-
thesis of a series of fluorescent p-conjugated polymers 3
[12]. However, as for the fully fused ladder oligo(p-
phenylenevinylene)s (LOPVs), tetrakis-silicon-bridged
bis(styryl)benzene 4 (Chart 1) prepared by our cycliza-
tion method was still the longest example [11]. There-
fore, the exploration of more efficient and general
routes to longer ladder systems has been a compelling
subject in our research. By extending the chemistry of
the reductive cyclization, we have now succeeded in
developing the first general synthetic route to the
LOPVs and related p-electron systems annelated with
silicon and carbon bridges. This methodology allows
us to synthesize a homologous series of molecules up
to a 13-ring-fused system. In this paper, we describe
the comprehensive study of silicon and carbon-bridged
LOPVs including not only the synthesis but also their
structures and properties. A part of these results has al-
ready been published as communications [13,14]. The
elucidation of the silicon effects on the photophysical
properties is of particular interest, which will be dis-
cussed in detail in this paper.
2. Results and discussion
2.1. Intramolecular reductive cyclization of mono-
(o-silyl)diphenylacetylene
Our previous intramolecular reductive cyclization
shown in Scheme 1 employs the bis(o-silyl)-substituted
diphenylacetylene 5 as the starting material. The reac-
tion of 5 with lithium naphthalenide (LiNaph) formally
undergoes a two-electron reduction at the acetylene moi-
ety to produce a dianionic intermediate, followed by the
subsequent double cyclization to give the bis-silicon-
bridged stilbene (disila-indeno[2,1-a]indene) [11]. We
now extended this reaction to the synthesis of the sila-
indene skeleton by replacing the starting material with
mono(o-silyl)-substituted diphenylacetylene 6. Thus,
when compound 6 having hydrogen as the leaving group
on the silicon atom was treated with a 2 mol amount of
LiNaph followed by quenching with water, the desired
phenylsilaindene 7 was obtained in 17% yield, together
with its dimeric product 8 in 74% yield, as shown in
Scheme 2 [15]. This result demonstrates that the present
cyclization indeed proceeds even using the mono(o-si-
lyl)-substituted derivatives, while the formation of the
latter dimeric product suggests that the two-electron
reduction proceeds in a stepwise fashion.
X
R
R
SiR2
1) 4 LiNaph
THF, r.t., 5 min
Si
2) I2
Si
R2Si
R
R
Li+
LiNaph =
X
2
5 (X = H or OEt)
2e– reduction
double
cyclization
X
Me2
Si
Notably, the present reductive cyclization more effi-
ciently proceeds when p-extended diacetylenic starting
materials are employed instead of the monoacetylenic
compound, as shown in Scheme 3. Thus, the reaction
Si
Me2
X
Scheme 1.
H
SiMe2
1) 2 LiNaph
R
R
R
R
THF, r.t., 5 min
X
Si
Si
2) H2O
Si
Si
6
Me Me
Si
X
n
R
R
R
R
3 (X = F, OMe)
1
2
Me Me
Si
Me Me
Si
+
Me Me
Si
H
Si
Me Me
Si
Si
Me Me
7 17%
Me Me
8 74%
4
Chart 1.
Scheme 2.

S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
5367
H
Pd (dba)
2 3
(0.04 mol amt.)
L (0.08 mol amt.)
SiMe2
1) 4 LiNaph
THF, r.t., 5 min
+
12d
Br
K PO
3
4
2) electrophile (E+)
4/1 toluene/H O
2
Me2Si
H
Me
9
Me
Me Me
Si
E
Si
Cy
2
P
OMe
Si
Me Me
L =
Si
electrophiles: H2O, (MeO)2SO2,
HMe2SiCl, i-PrOBpin, Br2, or C6F6.
pin = pinacolato
Me
E
Me
O
Me
10a (E = H) 52%
10b (E = Me) 83%
10c (E = SiMe2H) 87%
10d (E = Bpin) 43%
10e (E = Br) 10%
12g 30%
Scheme 4.
10f (E= C6F5) 12%
H
SiMe2
1) 4 LiNaph
THF, r.t., 5 min
tage in terms of functionalization. The present
reductive cyclization produces anionic intermediates
that can be easily transformed into various functional-
ized derivatives by treatment with appropriate electro-
philes. Thus, the reaction of 9 with a 4 mol amount of
LiNaph yielded a dianionic intermediate 15, which
was subsequently trapped with various electrophiles
to give a series of substituted bis(silaindenyl)benzenes
10b–10f having Me, SiMe2H, Bpin, Br and C6F5
groups, as shown in Scheme 3, although the yields
of 10e and 10f were rather low due to undesirable
side-reactions such as protonation. Similarly, 3,7-Bpin-
and 3,7-C6F5-substituted disilaindacenes 12d and 12f
were also synthesized. Among these functional groups,
the Bpin and Br functionalities may be useful for further
transformation through the well-established cross-
coupling methodologies. For instance, the Pd com-
plex-catalyzed coupling reaction of 12d with mesityl
bromide under the Buchwaldꢀs condition [18] pro-
duced 3,7-dimesityl derivative 12g in 30% yield, as
shown in Scheme 4. This compound has a strong fluo-
rescence not only in dilute solution but also in the so-
lid state, the details of which will be reported
elsewhere.
2) electrophile (E+)
Me2Si
H
Me
Me
E
Si
11
Si
E
Me
Me
12a (E = H) 90%
12d (E = Bpin) 59%
12f (E= C6F5) 24%
H
Me Me
Si
SiMe2
1) 4 LiNaph
THF, r.t., 5 min
Si
2) H2O
Me Me
Me2Si
H
14 35%
13
Scheme 3.
of bis(phenylethynyl)benzene 9, having two silyl groups
at the terminal phenyl rings, with a 4 mol amount of
LiNaph followed by quenching with water afforded
bis(silaindenyl)benzene 10a in 52% yield. The use of an
analogous compound 11, having two silyl groups
not at the terminal, but at the central benzene moiety,
led to a totally new disila-s-indacene derivative 12a in
90% yield. In addition, the present cyclization
also proceeded in the case of bis(o-silylphenyl)di-
acetylene 13, which produced bi(silaindene) 14 in 35%
yield.
2.3. Silicon and carbon-bridged bis(styryl)benzenes
In the synthesis of the functionalized silaindenes,
noteworthy is the treatment of the anionic cyclized inter-
mediates with ketones as the electrophile, which pro-
duces the corresponding alcohols. By combining this
transformation with the traditional Friedel–Crafts type
electrophilic cyclization, a very powerful method for
the construction of the ladder p-conjugated frameworks
[2], we next conducted the synthesis of the silicon
and carbon-mixed bridged ladder phenylenevinylene
derivatives.
+
Li
Me Me
Si
Si
Me Me
+
Li
15
2.2. Functionalization of silaindene skeleton
We first examined the reaction of 9 using benzophe-
none as the electrophile, as shown in Scheme 5. Thus,
the treatment of 9 with a 4 mol amount of LiNaph
Compared with the traditional synthesis of silaind-
enes [16,17], the present cyclization has a great advan-

5368
S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
Ph
Ph
Me Me
Si
HO
C
1) LiNaph
THF
9
Si
Me Me
2) Ph C=O
2
3) NH Cl aq.
4
C
OH
Ph
Ph
16 84%
Ph Ph
C
Me Me
Si
BF OEt
3
2
Si
Me Me
17 96%
C
Ph Ph
Me Me
Si
Ar
Ar
C
1) LiNaph
THF
HO
11
2) Ar C=O
2
OH
C
Si
3) NH Cl aq.
4
Ar
Ar
Me Me
18a (Ar = Ph) 85%
18b (Ar,Ar = 2,2'-biphenyl) 71%
Me Me
Si
Ar Ar
C
BF OEt
3
2
C
Si
Me Me
Ar Ar
19a (Ar = Ph) 75%
19b (Ar,Ar = 2,2'-biphenyl) 77%
Fig. 1. ORTEP drawings of compound 19b: (a) side view and (b) top
view. Selected bond lengths (A) and angles (ꢁ): Si1–C8, 1.870(2); Si1–
˚
Scheme 5.
C10, 1.879(2); C7–C8, 1.356(3); C7–C11, 1.468(3); C10–C11, 1.424(3);
C5–C6, 1.527(3); C6–C7, 1.532(3); C5–C9, 1.406(3); C8–C9, 1.462(3);
C8–Si1–C10, 91.29(9); Si1–C8–C7, 108.40(15); C8–C7–C11,
118.64(18); C7–C11–C10, 113.17(17); Si1–C10–C11, 108.49(14); C5–
C6–C7, 100.74(16); C6–C7–C8, 111.78(17); C7–C8–C9, 108.95(18);
C8–C9–C5, 108.81(18); C6–C5–C9, 109.71(17).
followed by trapping with excess benzophenone gave a
diol 16 in 84% yield. The next electrophilic cyclization
of 16 was carried out using BF3 ÆOEt2 as a Lewis acid.
Upon the addition of BF3 ÆOEt2 to a CH2Cl2 solution
of 16, the cyclization immediately took place as indi-
cated by the appearance of an intense fluorescence,
and the desired Si,C,C,Si-bridged bis(styryl)benzene
17 was obtained in 96% yield as a bright yellow solid.
No other isomer was detected, suggesting that the elec-
trophilic cyclization proceeded in a regio-specific man-
ner. We also employed compound 11, having two silyl
groups at the central benzene ring, as the starting
material. The successive two-step procedures afforded
the C,Si,Si,C-bridged 19a in a 64% overall yield via
the diol 18a.
One notable advantage of the present procedure is
that the substituents on the carbon bridges can be easily
modified by changing the ketones, whereas the available
ketones are limited to diarylketones at this stage [19].
For example, the use of fluorenone instead of benzophe-
none resulted in producing the fluorene-bound bis(sty-
ryl)benzene 19b in a 55% overall yield (based on 11).
This compound has an interesting molecular structure,
as shown in Fig. 1, in which the completely coplanar
bis(styryl)benzene skeleton and fluorene planes are
orthogonally arranged to each other through spiro con-
nections. The crystal data of this compound have been
reported in the previous communication [14].
2.4. General synthetic route to the extended LOPVs and
related p-electron systems
According to the results shown in Scheme 5, (o-silyl-
phenyl)acetylene 20 and 2,5-bis(silyl)-1,4-diethynylben-
zene 21 can be considered as the synthetic equivalents
of the 3-ring-fused terminating unit A and 5-ring-fused
spacer unit B, respectively, as shown in Chart 2. On
the basis of this idea, we carried out the synthesis of
the more extended ladder molecules 22–24 shown in
Chart 3. For example, the 13-ring-fused LOPV 24, con-
sisting of the two terminating units A and the spacer unit
B those of which were connected with two benzene link-
ages, was synthesized starting from 20 (R = hexyl) and
21 (R = hexyl), as shown in Scheme 6. Thus, the itera-
tive Sonogashira coupling reactions of compound 21
with p-iodobromobenzene, then with 20 produced an
oligo(phenyleneethynylene) derivative 25. The reductive
cyclization of 25 proceeded at the four acetylene moie-
ties to produce the corresponding tetraol 26 in 44%
yield, and the subsequent electrophilic cyclization gave
the fully annelated 24 as a bright orange solid in 52%

S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
5369
H
R R
H
SiR
2
SiHex
Si
2
≡
+
Br
I
2
C
Ar Ar
3-ring terminating unit A
Hex Si
2
20
H
PdCl (PPh ) , CuI
2
3 2
21
toluene/Et N
3
R
R
H
Ar Ar
C
SiR
2
Si
H
SiHex
≡
2
C
Si
Ar Ar
Br
Br
R Si
2
R
R
H
5-ring spacer unit B
Hex Si
21
2
H
88%
Chart 2.
H
SiHex
2
PdCl (PPh ) , CuI
2
3 2
toluene/Et N
3
2
Hex Hex
C
Hex Hex
Si
Hex Hex
Si
20
H
H
SiHex
SiHex
2
2
C
C
Ph Ph
Ph Ph
22
Hex Si
Hex Si
2
2
Hex Hex
H
H
Hex Hex
Si
Ph Ph
C
C
25 54%
1) LiNaph, THF 2) Ph C=O
C
2
Si
Hex Hex
23
3) NH Cl aq.
4
C
Ph Ph
Ph
C
Hex Hex
Ph
Hex Hex
Si
HO
Ph
C
Ph
Si
Ph Ph
C
Hex Hex
Si
HO
Hex Hex
Si
Ph Ph
Si
Hex Hex
Hex Hex
Si
C
OH
C
Ph
Si
Hex Hex
Ph
C
C OH
Hex Hex
Si
Hex Hex
24
Ph
C
Ph Ph
Ph
26 44%
Ph Ph
Chart 3.
•
BF OEt
3
2
yield. Similarly, the 9-ring-fused compound 22 and the
11-ring-fused system 23 were prepared from a fluo-
rene-centered or -teminated diacetylenic compounds,
27 and 28, respectively, by the combined procedure of
the reductive cyclization with the Friedel–Crafts type
cyclization, as shown in Scheme 7. The produced ladder
compounds have a substantial thermal stability. For in-
stance, the thermogravimetric analysis of 24 in a N2
atmosphere showed that the initial 5% weight loss
(Td5) occurred at 455 ꢁC.
24 52%
Scheme 6.
shown in Fig. 2, together with that of the 13-ring-fused
system 24. The crystal data of 24 have been already re-
ported in the previous communication [14]. All these
compounds have nearly flat p-conjugated frameworks.
The approximate lengths of the p-conjugated frame-
works for 22, 23, and 24 are 2.1, 2.5, and 2.9 nm, respec-
tively. The other common structural features of this
series of compounds are that the flat rigid p-conjugated
frameworks are covered by the substituents on the sili-
con and carbon bridges. Apparently, this structural
characteristic helps to prevent the formation of insoluble
aggregates and let them be soluble to some extent in
the common organic solvents, despite their long rigid
2.5. Crystal structures of the extended ladder
p-conjugated systems
The structures of the 9-ring-fused system 22 and 11-
ring-fused system 23 have been determined by the
X-ray crystallography in this study. Their crystal data
are listed in Table 1, and their ORTEP drawings are

5370
S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
Hex Hex
SiHex H
HHex Si
2
2
i, ii
22
overall 26%
27
SiHex H
2
i, ii
23
overall 16%
HHex Si
2
Hex Hex
Hex Hex
28
Reagents and conditions: i, 1) LiNaph, THF; 2) Ph C=O; 3) NH Cl aq., ii, BF OEt .
2
4
3
2
Scheme 7.
Table 1
A series of partially or fully bridged bis(styryl)benz-
Crystallographic data for compounds 22 and 23
enes, including Si,n,n,Si-bridged 10a, n,Si,Si,n-bridged
12a, Si,C,C,Si-bridged 17, C,Si,Si,C-bridged 19a, and
Si,Si,Si,Si-bridged 4, are now available, where Si, C,
and n denote SiMe2, CPh2, and non-bridged moiety,
respectively. For a comparison, the fluorescence spec-
tra for the fully bridged derivatives 17, 19a, and 4
are shown in Fig. 3. A comparison of this series of
compounds shows the effects of the silicon moieties
on their photophysical properties. The notable points
are summarized as follows. (1) Both the absorption
and emission maxima tend to shift to longer wave-
lengths as the number of the silicon bridges increases.
These red shifts can be ascribed to the contribution
of the silicon moieties to the electronic structure
through r*–p* conjugation in the silole substructure,
which decrease the LUMO energy level [11,20]. (2)
The positions of the silicon moieties also have a sub-
stantial influence on the absorption and emission max-
imum wavelengths. In particular, the central
disilaindacene skeleton more predominantly affects
the photophysical properties of the bis(styryl)benzene
chromophore than does the terminal silaindene skele-
ton. For instance, while the red-shift in the emission
maxima from 17 to 4 is 30 nm, that from 19a to 4 is
only 9 nm and their emission maxima are no longer
significantly different from each other. (3) This trend
is also observed in the fluorescence quantum yield.
The incorporation of the disilaindacene skeleton tends
to decrease the quantum yield, while the terminal sila-
indene skeleton retains the high quantum yield compa-
rable to the parent 1,4-bis(styryl)benezene 27 (kmax,em
405 nm, UF 0.77) [21].
Compound
Formula
Mol weight (g molꢀ1
Cryst dimens (mm)
Cryst syst
22
23
C110H138Si2
1516.38
0.20 · 0.20 · 0.20
Monoclinic
C2/c (No. 15)
29.430(5)
C91H110Si2
)
1259.97
0.20 · 0.20 · 0.10
Triclinic
ꢀ
Space group
P1 (No. 2)
˚
a (A)
15.8269(17)
16.6540(19)
17.274(2)
76.920(8)
68.134(7)
65.069(7)
3818.8(8)
2
1.096
1368
0.091
173(2)
˚
b (A)
15.336(3)
22.300(4)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
112.5683(8)
3
˚
V (A )
9294(3)
4
1.084
3304
0.085
173(2)
37072
10605 (I > 2r(I))
1.107
Z
Dcalcd (g cmꢀ3
F (0 0 0)
)
l (Mo Ka) (cmꢀ1
)
Temp (K)
No. of measd reflns
No. of ind reflns
Goodness-of-fit
Final R indices
R1
25841
13208 (I > 2r(I))
1.100
0.0633
0.1640
0.0670
0.1413
wR2 [I > 2r(I)]
p-conjugated frameworks. For instance, compound 24
has a good solubility in THF of 0.3 g/mL.
2.6. Photophysical properties: Silicon effects on the
fluorescence
The full photophysical data, including UV–vis
absorption spectra, fluorescence spectra, fluorescence
lifetime, and excited-state dynamics, for the synthesized
ladder molecules are listed in Table 2, together with
those of 4 and the parent non-substituted 1,4-bis(sty-
ryl)benzene 29 for comparisons (Scheme 8). All the lad-
der molecules show an intense fluorescence in the visible
region. The high quantum yields (0.40–0.86) as well as
the relatively small Stokes shifts (12–26 nm) are their
notable common features.
On the basis of the UF and the excited-state lifetime
ss, determined by the time-resolved fluorescence spec-
troscopy, we calculated the radiative (kr) and non-
radiative (knr) decay rate constants from the singlet excited
states, based on Eqs. (1) and (2). Fig. 4 shows the histo-
gram of the rate constants, which reveals a distinct

S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
5371
Fig. 2. ORTEP drawings of compounds (a) 22, (b) 23, and (c) 24 (50% probability for thermal ellipsoids). Hexyl groups on the silicon atoms and on
the saturated carbon atoms in the fluorene substructures are omitted for clarity. A part of hexyl groups and phenyl groups is disordered. For the
details, see Section 4.
trend. Thus, the incorporation of the disilaindacene
skeleton as well as the increase in the silicon content sig-
nificantly decrease the kr, whereas the knr value only
slightly increases. These results mean that the lowering
of UF by introducing the disilaindacene skeleton or by
increasing the silicon content is based not on the accel-
erated non-radiative decay pathways including the inter-
nal conversion and intersystem crossing, but on the
suppression of the radiative decay process.
pounds 170 and 190 at the TDDFT/B3LYP/6-31G(d)//
B3LYP/6-31G(d) level of theory demonstrate that the
decrease in the oscillator strength for the longest p–p*
transition amounts to 22% from 170 (f = 0.91) to
19a0 (f = 0.70), whereas the difference in m2 between 17
and 19a is only 6%. This is rationalized as due to the
more significant contribution of the silicon moieties
through the r*–p* conjugation in the disilaindacene
skeleton than in the silaindene skeleton, which results
in the decreased oscillator strength of the pertinent p–
p* transition. Fig. 5 shows the calculated LUMOs for
170 and 190 that confirms the more significant contribu-
tion of the lobes at the silicon atoms in 190 than in 170.
This is the origin of the decrease in UF by introducing
the disilaindacene skeleton or by increasing the silicon
contents.
kr ¼ UF=ss;
knr ¼ ð1 ꢀ UFÞ=ss;
ð1Þ
ð2Þ
In general, the radiative rate constant kr has linear
relationships with the square of the wavenumber of the
absorption maximum m2 and the oscillator strength f,
as shown in Eq. (3). Since the differences in m are not very
significant among the series of the bis(styryl)benezene
derivatives, the decrease in the oscillator strength f may
be responsible for the suppressed radiative decay pro-
cess. In fact, preliminary calculations of model com-
kr ꢁ m2f ;
ð3Þ
where m is the wavenumber of the absorption maximum
and f is the oscillator strength.

5372
S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
Table 2
Photophysical data for LOPVs and related p-conjugated compounds
Compound
UV–vis absorptiona
Fluorescencea
Stokes shift
Lifetime
Radiative rate
constant
Non-radiative rate
constant
knr (sꢀ1
)
c
kmax (nm)b
loge
kmax (nm)b
UF
Dk
ss (ns)
kr (sꢀ1
)
Bis(styryl)benzenes
10a
12a
17
Si,n,n,Si
397 (sh)
420 (sh)
425
438
447
350e
4.59
4.51
4.49
4.41
4.36
413
445
443
464
473
405e
0.86
0.73
0.73
0.56
0.50
0.77e
16
25
18
26
26
1.9
2.6
2.7
3.4
3.8
4.5 · 108
2.8 · 108
2.7 · 108
1.6 · 108
1.3 · 108
7.4 · 107
1.0 · 108
1.0 · 108
1.3 · 108
1.3 · 108
n,Si,Si,n
Si,C,C,Si
C,Si,Si,C
Si,Si,Si,Si
n,n,n,n
19a
4d
29e
LOPVs and related compounds
22
23
24
a
9-ring
11-ring
13-ring
433
476
502
4.80
4.72
4.88
445
499
523
0.84
0.40
0.59f
12
23
21
2.5
2.4
2.1
3.4 · 108
1.7 · 108
2.8 · 108
6.4 · 107
2.5 · 108
2.0 · 108
In THF.
b
All compounds show vibronic absorption and emission bands. Only the longest absorption maxima and the shortest emission maxima are
reported in this table.
c
Determined with perylene as a standard, unless otherwise stated. The UF is the average values of repeated measurements within 5% errors.
[11].
The data from [21].
Determined with fluorescein as a standard.
d
e
f
radiative rate constant
nonradiative rate constant
29
Scheme 8.
10a
12a
17
19a
4
Si,n,n,Si
n,Si,Si,n
Si,C,C,Si
C,Si,Si,C Si,Si,Si,Si
Fig. 4. Radiative (kr) and non-radiative (knr) decay rate constants
from the singlet excited states for the bridged bis(styryl)benzene
derivatives.
rials based on the silicon-bridged LOPV and related
materials. Beneficially, the present silaindene-containing
derivatives 10a and 17 exhibit an intense emission in the
blue region and may be promising as new blue emitting
materials. Their applications to the organic EL devices
are currently under investigation.
400
450
500
550
600
650
wavelength / nm
Fig. 3. Fluorescence spectra of fully bridged bis(styryl)benzenes in
THF: 17, solid line; 19a, broken line; 4, dotted line.
Despite these silicon effects, however, it is also note-
worthy that the incorporation of the silicon moieties
does not necessarily decrease the UF. Thus, the siliaind-
ene skeleton does not reduce the UF. This fact will be
important for the further design of highly emissive mate-
In the series of long ladder systems, the extension of
the p-conjugation from 22 to 23 to 24 effectively red-
shifts the emission maxima by about 80 nm, as shown
in Fig. 6. As a consequence, the emission colors varies
from blue 22 to green 23 to yellow 24. With regard to

S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
5373
3. Conclusion
Oligo(p-phenylenevinylene)s are an important class
of compounds not only as models of their polymeric
analogs, but also as the fundamental chromophores
or fluorophores for the creation of new functional p-
conjugated materials. We have described the first general
synthesis of the ladder type oligo(p-phenylenevinyl-
ene) LOPVs and related p-electron systems that have
partially or fully annelated p-conjugated frameworks
with silicon and carbon bridges. On the basis of the
newly developed intramolecular reductive cyclizations,
we have achieved their facile synthesis. Notably, our
synthesis is essentially a simple reduction of the pre-
cursory acetylenic compounds with lithium naphthale-
nide. However, the appropriate designs of the
acetylenic starting materials enabled us to synthesize
various types of molecules, including not only the sila-
indene and disilaindacene-based partially bridged
p-electron systems, but also the fully silicon and car-
bon-bridged LOPVs and related compounds. The syn-
thesized longest LOPV is the 13-ring-fused system 24
that has a completely flat p-conjugated framework
with the length of 2.9 nm. Unambiguously, the present
methodology will serve as a powerful protocol for the
further synthesis of new related ladder p-conjugated
materials.
The other important finding in this study is the
rationalization of the silicon effect on the photophysi-
cal properties of the produced ladder molecules. The
detailed inspection of the photophysical data for the
series of the bis(styryl)benzenes revealed the effect of
the silicon moieties. A major contribution of the silicon
bridges is the participation to the LUMO through the
r*–p* conjugation, which lowers the LUMO level
resulting in the red-shift of the absorption and emission
maxima, decreases the oscillator strength of the p–p*
transition, and thus suppresses the radiative decay
pathway from the singlet excited state. As a conse-
quence, the fluorescence quantum yield UF slightly de-
creases. This kind of effect may be rather general
phenomena for p-conjugated systems containing heavy
main group elements, and may deserve to recognize as
a new concept, ‘‘r* effect’’ of main group elements. In
the present ladder systems, however, it is also notewor-
thy that the incorporation of the silicon moiety into the
p-conjugated framework does not necessarily decrease
the UF. Thus, the silaindene skeleton does not affect
the efficiency of the fluorescence due to less significant
r* effect in this skeleton. This fact will provide us a
new design principle for the highly emissive silicon-
bridged ladder p-conjugated molecules. Further studies
on the design and synthesis of more promising ladder
materials for the applications in organic electronics
and optoelectronics are now in progress in our
laboratory.
Fig. 5. Kohn–Sham LUMOs for model compounds: (a) 170 and (b) 190
calculated at the B3LYP/6-31G(d) level of theory.
400
450
500
550
600
650
wavelength / nm
Fig. 6. Fluorescence spectra of LOPV and related p-electron systems
in THF: 22, solid line; 23, broken line; 2 4, dotted line.
the UF, compounds 23 and 24 bearing the disilaindacene
skeleton are again proved to have slightly low values,
while the silaindene-capped fluorene 22 has a rather high
quantum yield UF of 0.84.

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S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
4. Experimental
127.25, 127.85, 128.09, 140.35, 142.09, 149.74, 157.85.
HRMS (FAB): 646.3291 (M+). Anal. Calc. for
C38H48B2O4Si2: 646.3277.
Melting point (m.p.) determination was performed
with a Yanaco MP-S3 instrument. H and 13C NMR
1
spectra were measured with a JEOL A-400 or JEOL
GSX-270 spectrometer. UV–vis absorption spectra and
fluorescence spectra were measured with a Shimadzu
UV-3150 spectrometer and a F-4500 Hitachi spectrome-
ter, respectively, in degassed spectral grade THF. Time-
resolved fluorescence spectra were measured using a
Hamamatsu C4780 system equipped with a PLP-10
picosecond light pulser (LED wavelengths: 375 or
405 nm). Thin layer chromatography (TLC) was per-
formed on plates coated with 0.25 mm thick silica gel
60F-254 (Merck). Column chromatography was per-
formed using PSQ 100B (Fuji Silysia). Recycling pre-
parative gel permeation chromatography (GPC) was
performed using polystyrene gel columns (JAIGEL 1H
and 2H, Japan Analytical Industry) with chloroform
as an eluent. Medium pressure liquid chromatography
(MPLC) was carried out using a silica gel packed col-
umn (Yamazen, Ultra Pack, SI-40B). [o-(Dihexylsi-
lyl)phenyl]acetylene 20 was prepared by the lithiation
of (o-bromophenyl)trimethylsilylacetylene with n-BuLi
in ether, followed by treatment with dihexylchlorosilane
and then desilylation in the presence of K2CO3 in a
THF/EtOH mixed solvent. 2,5-Dibromo-1,4-bis(trim-
ethylsilylethynyl)benzene was prepared according to
the procedure described in the literature [22].
4.2. Cross-coupling reaction. Synthesis of compound 12g
A mixture of 12d (100 mg, 0.155 mmol), mesityl bro-
mide (62.0 mg, 0.311 mmol), Pd2(dba)3 (5.7 mg,
6.2 lmol), 2-(dicyclohexylphosphino)-20,60-dimethoxyl-
1,10-biphenyl (5.1 mg, 12.4 lmol), and K3PO4
(98.7 mg, 0.464 mmol) in a 4/1 toluene/H2O mixed sol-
vent (2.5 mL) was stirred at 100 ꢁC for 51 h. After addi-
tion of 1 N HCl aqueous solution, the reaction mixture
was extracted with CH2Cl2. The organic layer was
washed with brine, dried over MgSO4, filtered, and con-
centrated under reduced pressure. The obtained mixture
was passed through a short silica gel column using
CHCl3 as an eluent. Further purification by means of
a preparative GPC with CHCl3 as an eluent gave 12g
(29 mg, 0.046 mmol) in 30% yield as a pale yellow solid:
1
m.p. >300 ꢁC. H NMR (270 MHz, CDCl3): d 0.41 (s,
12H), 2.03 (s, 12H), 2.36 (s, 6H), 6.92 (s, 4H), 6.94 (s,
2H), 6.95–6.97 (m, 4H), 7.03–7.07 (m, 2H), 7.09–7.11
(m, 4H). 13C NMR (100.4 MHz, CDCl3): d ꢀ2.94,
20.06, 21.23, 125.81, 126.14, 127.64, 128.02, 128.30,
134.97, 135.85, 136.25, 140.07, 140.09, 141.51, 148.63,
152.12. HRMS (FAB): 630.3129 (M+). Anal. Calc. for
C44H46Si2: 630.3138.
4.3. Synthesis of 2,5-bis(dihexylsilyl)-1,4-
diethynylbenzene (21)
4.1. A typical procedure for the intramolecular reductive
cyclization followed by the functionalization. Synthesis of
compound 12d
To a solution of 2,5-dibromo-1,4-bis(trimethylsilyle-
thynyl)benzene (4.00 g, 9.34 mmol) in Et2O (140 mL)
was added a hexane solution of n-BuLi (1.60 M,
12 mL, 19.2 mmol) dropwise at ꢀ78 ꢁC. After stirred
for 1 h, dihexylchlorosilane (4.61 g, 19.6 mmol) was
added via syringe at the same temperature and the mix-
ture was allowed to warm to room temperature over 6 h
with stirring. The mixture was filtered and concentrated,
and then passed through a silica gel short column (hex-
ane, Rf = 0.88) to give 5.84 g (8.75 mmol) of 2,5-
bis(dihexylsilyl)-1,4-bis(trimethylsilylethynyl)benzene in
94% yield as pale yellow oil, which was directly used
for next desilylation reaction without further purifica-
tion: 1H NMR (400 MHz, CDCl3): d 0.26 (s, 18H),
0.83–0.98 (m, 20H), 1.24–1.32 (m, 32H), 4.28 (quin,
J = 3.6 Hz, 2H), 7.59 (s, 2H).
A solution of 2,5-bis(dihexylsilyl)-1,4-bis(trimethylsil-
ylethynyl)benzene (5.83 g, 8.73 mmol) and K2CO3
(241 mg, 1.74 mmol) in a 1/1 THF/EtOH mixed solvent
(80 mL) was stirred for 10 h at room temperature. After
concentrated under reduced pressure, hexane was added
to the mixture. The insoluble material was removed by
filtration and the filtrate was concentrated under
A mixture of granular lithium (76 mg, 10.9 mmol)
and naphthalene (1.40 g, 10.9 mmol) in THF (8 mL)
was stirred at room temperature under argon for 4 h.
To the produced solution of lithium naphthalenide
was added
a solution of compound 11 (1.00 g,
2.53 mmol) in THF (3 mL) at room temperature. After
stirred for 5 min, the resulting dark green solution was
cooled to 0 ꢁC and 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (2.00 g, 10.8 mmol) was added.
The mixture was then stirred for additional 0.5 h at
the same temperature. Water was added and the mixture
was extracted with CH2Cl2 for several times. The com-
bined organic layer was washed with brine, dried over
MgSO4, filtered, and concentrated under reduced pres-
sure. A small amount of hexane was added to the result-
ing mixture and an insoluble material was collected by
filtration, which was further washed with an additional
amount of hexane to give pure 12d (0.97 g, 1.49 mmol)
1
in 59% yield as a pale yellow solid: m.p. >300 ꢁC. H
NMR (270 MHz, CDCl3): d 0.36 (s, 12H), 1.29 (s,
24H), 7.18–7.36 (m, 10H), 7.61 (s, 2H). 13C NMR
(67.8 MHz, CDCl3): d ꢀ3.81, 24.87, 83.93, 126.33,

S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
5375
reduced pressure and subjected to column chromatogra-
phy on a silica gel (hexane, Rf = 0.83) to give 4.45 g
(8.51 mmol) of compound 21 in 97% yield as pale yellow
was stirred at room temperature under argon for 4 h.
To the produced solution of lithium naphthalenide
was added a solution of compound 25 (0.368 g,
0.29 mmol) in THF (4 mL) at room temperature. After
stirred for 5 min, a THF (3 mL) solution of benzophe-
none (0.523 g, 2.87 mmol) was added to the reaction
mixture at room temperature. The reaction mixture
was stirred for 20 min and then quenched with a satu-
rated aqueous solution of NH4Cl. The mixture was ex-
tracted with ether. The organic layer was washed with
brine, dried over MgSO4, filtered, and concentrated un-
der reduced pressure. The resulting mixture was purified
by silica gel column chromatography (10/1 hexane/
EtOAc, Rf = 0.25) to give 0.255 g (0.127 mmol) of 26
in 44% yield as a white solid: m.p. 164–166 ꢁC. 1H
NMR (400 MHz, C6D6): d 81–0.94 (m, 36H), 1.14–
1.36 (m, 68H), 2.97 (s, 2H), 3.13 (s, 2H), 6.76–6.81 (m,
8H), 6.88 (dt, J = 1.2 and 7.6 Hz, 2H), 6.94 (dt,
J = 1.2 and 7.0 Hz, 2H), 7.00–7.15 (m, 24H), 7.30 (s,
2H), 7.31 (d, J = 8.0 Hz, 2H), 7.46 (dd, J = 1.2 and
7.0 Hz, 2H), 7.55 (dd, J = 1.2 and 8.0 Hz, 8H), 7.62
(dd, J = 1.2 and 8.0 Hz, 8H). 13C NMR (100.4 MHz,
C6D6): d 12.35, 14.91, 15.01, 23.56, 23.60, 24.66, 32.36,
32.47, 34.02, 34.05, 84.22, 84.22, 126.60, 127.21,
127.44, 128.22, 128.27, 128.50, 128.74, 129.13, 129.19,
129.50, 130.10, 132.08, 132.64, 137.96, 139.08, 139.47,
139.88, 146.78, 147.00, 147.34, 147.86, 148.85, 151.76,
156.36, 157.25. HRMS (FAB): 1999.1893 (M+). Anal.
Calc. for C138H166O4Si4: 1999.1863.
1
oil: H NMR (270 MHz, CDCl3): d 0.85 (t, J = 6.9 Hz,
12H), 0.91–0.96 (m, 8H), 1.23–1.34 (m, 32H), 3.30 (s,
2H), 4.32 (quin, J = 3.6 Hz, 2H), 7.63 (s, 2H). 13C
NMR (67.8 MHz, CDCl3): d 11.51, 14.19, 22.65,
24.72, 31.53, 32.85, 81.38, 84.31, 126.94, 139.50, 140.12.
4.4. Synthesis of tetraacetylenic starting material 25
A mixture of compound 21 (2.00 g, 3.82 mmol),
p-iodobromobenzene (2.22 g, 7.69 mmol), PdCl2-
(PPh3)2 (54.8 mg, 0.077 mmol), and CuI (29.3 mmol,
0.154 mmol) in a 3/1 toluene/Et3N mixed solvent
(80 mL) was stirred for 5.5 h at room temperature. After
filtration of the reaction mixture, the filtrate was concen-
trated under reduced pressure. The mixture was sub-
jected to column chromatography on a silica gel
(hexane, Rf = 0.83) to give 2.81 g (3.37 mmol) of pure
2,5-bis(dihexylsilyl)-1,4-bis[(p-bromophenyl)ethynyl]ben-
1
zene in 88% yield as a white solid: m.p. 61–63 ꢁC. H
NMR (400 MHz, CDCl3): d 0.83 (t, J = 6.8 Hz, 12H),
0.94–1.03 (m, 8H), 1.19–1.40 (m, 32H), 4.41 (quin,
J = 3.6 Hz, 2H), 7.39 (d, J = 8.4 Hz, 4H), 7.50 (d,
J = 8.4 Hz, 4H), 7.69 (s, 2H). 13C NMR (67.8 MHz,
CDCl3): d 11.71, 14.20, 22.64, 24.79, 31.54, 32.90,
91.77, 92.43, 122.13, 122.64, 127.45, 131.62, 132.61,
138.79, 139.86. Anal. Calc. for C46H64Br2Si2: C, 66.33;
H, 7.74. Found: C, 65.98; H, 7.63%.
The mixture of 2,5-bis(dihexylsilyl)-1,4-bis[(p-bromo-
phenyl)ethynyl]benzene (1.00 g, 1.20 mmol), o-(dihexyl-
silyl)phenylacetylene (0.80 g, 2.66 mmol), PdCl2(PPh3)2
(16.8 mg, 23.9 lmol), and CuI (9.1 mg, 47.8 lmol) in
40 mL of a 3/1 toluene/Et3N mixed solvent was stirred
at 85 ꢁC for 16 h. After filtration of the mixture, the fil-
trate was concentrated under reduced pressure. The
resulting mixture was subjected to column chromatogra-
phy on a silica gel (hexane, Rf = 0.56) to give 0.82 g
(0.64 mmol) of the tetraacetylenic compound 25 in 54%
4.6. A typical procedure for the Friedel–Crafts type
cyclization to synthesize the LOPVs and related p-
systems. Synthesis of 13-ring-fused LOPV 24
To a solution of tetraol 26 (0.137 g, 0.068 mmol) in
CH2Cl2 (35 mL) was added BF3 ÆOEt2 (34 lL,
0.272 mmol) at room temperature. After stirred for
15 min, the reaction mixture was quenched with 95%
ethanol. The organic layer was washed with brine, dried
over MgSO4, filtered, and concentrated under reduced
pressure. The resulting mixture was subjected to column
chromatography on a silica gel (10/1 hexane/EtOAc,
Rf = 0.56), followed by the further purification by
means of MPLC (10/1 hexane/EtOAc) to give 0.068 g
(0.035 mmol) of 24 in 52% yield as an orange red solid:
1
yield as a pale yellow sticky oil: H NMR (400 MHz,
CDCl3): d 0.82–0.86 (m, 24H), 0.96–1.06 (m, 16 H),
1.22–1.43 (m, 64H), 4.44 (quin, J = 3.6 Hz, 4H), 7.32
(dt, J = 1.2 and 7.2 Hz, 2H), 7.37 (dt, J = 1.2 and
7.2 Hz, 2H), 7.52 (s, 8H), 7.54–7.57 (m, 4H), 7.72 (s,
2H). 13C NMR (100.4 MHz, CDCl3): d 11.61, 11.76,
14.09, 22.54, 24.71, 24.75, 31.47, 31.50, 32.83, 32.85,
91.46, 92.58, 92.60, 93.31, 123.06, 123.47, 127.60,
127.66, 128.58, 129.08, 131.32, 131.33, 132.08, 135.70,
138.98, 139.15, 139.98. Anal. Calc. for C86H126Si4: C,
81.19; H, 9.98. Found: C, 81.48; H, 10.31%.
1
m.p. 255–258 ꢁC. H NMR (400 MHz, CDCl3): d 0.73–
0.80 (m, 32H), 0.90–0.94 (m, 8H), 1.06–1.18 (m, 56H),
1.26–1.34 (m, 8H), 7.01–7.06 (m, 6H), 7.14–7.22 (m,
28H), 7.30–7.33 (m, 18H), 7.44–7.46 (m, 2H). 13C
NMR (100.4 MHz, CDCl3): d 11.63, 11.83, 14.04,
22.45, 22.47, 24.05, 24.16, 31.28, 31.35, 32.80, 32.94,
66.76, 118.16, 118.46, 123.23, 125.65, 126.46, 127.56,
127.88, 127.99, 129.07, 129.11, 132.39, 141.35, 141.98,
142.44, 142.64, 142.73, 142.96, 144.44, 145.16, 145.24,
156.96, 157.22, 170.62, 171.09. Anal. Calc. for
4.5. Synthesis of tetraol compound 26
A mixture of granular lithium (19.9 mg, 2.87 mmol)
and naphthalene (0.362 g, 2.87 mmol) in THF (4 mL)

5376
S. Yamaguchi et al. / Journal of Organometallic Chemistry 690 (2005) 5365–5377
C138H158Si4: C, 85.92; H, 8.26. Found: C, 85.41; H,
8.29%.
Acknowledgments
We thank Dr. T. Sasamori and Prof. N. Tokitoh
(Kyoto University) for their helpful discussions about
the X-ray crystallographic analysis, and Dr. H. Takagi
(Toyota Central R&D Labs.) for the TGA measure-
ment. We also acknowledge a reviewer for a valuable
suggestion with regard to the ‘‘r* effect’’ as a general
concept. This work was supported by Grants-in-Aid
(No. 12CE2005 for Elements Science and No.
15205014) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan, and SORST,
Japan Science and Technology Agency (JST).
4.7. X-ray crystallography
Single crystals of 22 and 23 suitable for X-ray crystal
analysis were obtained by recrystallization from hexane
and a 10/1 hexane/EtOAc mixed solvent, respectively.
Intensity data were collected at 173 K on a Rigaku sin-
gle crystal CCD X-ray diffractometer (Saturn 70 with
˚
MicroMax-007) with Mo Ka radiation (k = 0.71070 A)
and graphite monochromator. The crystal data for these
compounds are summarized in Table 1. Their structures
were solved by direct methods (SHELXS-97) and refined
by the full-matrix least-squares on F2 (SHELXL-97) [23].
All non-hydrogen atoms were refined anisotropically
and all hydrogen atoms were placed using AFIX
instructions.
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