A key role of orbital interaction in the main group element-containing π-electron systems

Article abstract of DOI:10.1246/cl.2005.2

Various types of main group element-containing π-electron systems have been synthesized using silole, dibenzoborole, or bis-silicon-bridged stilbene as the key building units. In these π-electron systems, the orbital interaction between the main group element moiety and the π-conjugated framework, such as p_π-π* and σ*-π* conjugation, plays a crucial role in determining their characteristic electronic structures and makes them promising materials for applications in organic electronics and optoelectronics. Copyright

Full text of DOI:10.1246/cl.2005.2

2
Chemistry Letters Vol.34, No.1 (2005)
Highlight Review
A Key Role of Orbital Interaction in the Main Group
Element-containing ꢀ-Electron Systems
Shigehiro Yamaguchiꢀ and Kohei Tamao
(Received July 20, 2004; CL-048010)
Abstract
(a)
Various types of main group element-containing ꢀ-electron sys-
tems have been synthesized using silole, dibenzoborole, or bis-
silicon-bridged stilbene as the key building units. In these ꢀ-
electron systems, the orbital interaction between the main group
R
B
pπ
Borole
π
*
LUMO of Borole
element moiety and the ꢀ-conjugated framework, such as p –ꢀ*
ꢀ
and ꢁ*–ꢀ* conjugation, plays a crucial role in determining their
characteristic electronic structures and makes them promising
materials for applications in organic electronics and optoelec-
tronics.
(b)
R
R
Si
π
*
Silole
σ*
LUMO of Silole
Ç
Introduction
Figure 1. Schematic and graphical drawings of the orbital
interaction in the LUMOs of (a) borole and (b) silole rings.
The graphical drawings are based on the calculations at the
HF/6-31G(d) level of theory.
The progress in organic electronics and optoelectronics,
such as organic thin film transistors and light emitting diodes
(LEDs), highly relies on the development of new ꢀ-conjugated
oligomeric and polymeric materials. One crucial issue in their
molecular designs is how to tune their electronic structures in or-
der to produce desirable photophysical and electronic properties.
In this regard, increasing attention has recently been paid to the
main group element-containing ꢀ-electron systems,1 wherein
the orbital interaction between the main group element moiety
with the ꢀ-conjugated framework plays a crucial role in produc-
ing varied electronic structures.2–5 For example, as shown in
Figure 1, in the boron-containing 5-membered cyclic diene,
the borole skeleton, the orbital interaction between the vacant
p orbital of the boron atom with the ꢀ* orbital of the butadiene
moiety effectively occurs, giving rise to a significant low-lying
LUMO. Similarly, in its silicon analogue, the silole skeleton,
the ꢁ*–ꢀ* conjugation between the ꢁ* orbital of the exocyclic
two ꢁ bonds on the silicon atom with the ꢀ* orbital of the buta-
diene moiety decreases its LUMO energy level.2 These element
effects can also be seen in the extended ꢀ-conjugated systems.
Figure 2 shows the calculation results, reported by Saltzner
and co-workers, about polyheteroles having various main group
elements.3 The calculations suggest that their electronic struc-
tures are highly dependent on the main group element to be in-
corporated. In particular, polyborole and polysilole have signifi-
cantly low-lying LUMOs. Thus, these electronic perturbations
by the main group elements are a very powerful way to modulate
the nature of the ꢀ-conjugated systems. Exploiting these orbital
-1
-2
-3
-4
-5
-6
-7
LUMO (eV)
−1.61
−2.97
−3.2
−2.63
−3.58
−3.63
−3.73
−3.75
−4.83
−4.95
−4.61
−4.77
−5.14
−5.23
−5.31
−5.5
−5.52
−5.64
HOMO (eV)
SiH
BH
CH
NH
E
O
PH
S
Se
Te
2
2
E
E
E
E
E
polyheterole
Figure 2. Calculated electronic structures of parent polyheter-
oles: the data taken from U. Salzner, J. B. Lagowski, P. G.
Pickup, and R. A. Poirier, Synth. Met., 96, 177 (1998).
interactions as the guiding principles in the molecular designs,
we have synthesized various types of main group element-
containing ꢀ-conjugated systems. This short account describes
our recent progress in this field, focusing attention on the
development of new synthetic methodologies for them as well
Prof. Shigehiro Yamaguchiꢀ
Department of Chemistry, Graduate School of Science, Nagoya University, and PRESTO, Japan Science and Tech-
nology Agency (JST), Chikusa, Nagoya 464-8602, Japan
E-mail: yamaguchi@chem.nagoya-u.ac.jp
Prof. Kohei Tamao
International Research Center for Elements Science, Institute for Chemical Research, Kyoto University, Uji, Kyoto
611-0011, Japan
E-mail: tamao@scl.kyoto-u.ac.jp
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.1 (2005)
3
as the effects of the main group elements on their photophyiscal
properties.
1) LiNaph
2) ZnCl
2
Li
Li
cross-coupling
intramolecular
reductive
cyclization
Si
Si
Ç
Key Molecular Designs
R
R
R
R
A key point of our molecular design is to employ the main
1
group element-containing 5-membered cyclic diene (heterole) as
the basic skeleton. In the LUMO of the ring skeleton, the orbital
of the main group element moiety, such as the above mentioned
vacant p orbital of the boron atom or ꢁ* orbital of the silicon
moiety, is fixed parallel to the ꢀ* orbital of the butadiene frame-
work with orbital symmetry in-phase to each other.7 This perfect
structural demand causes an effective interaction between each
other, resulting in a decrease of the LUMO level. Notably, the
tuning of the LUMO level is a currently important issue in the
designs of electron-accepting n-type organic semiconductors.6
On the basis of this consideration, we employed three types of
5-membered ring frameworks as the building units. They include
the heterole itself, dibenzoheterole, and element-bridged stil-
bene, as shown in Figure 3.
3) ArBr
Pd. cat.
ClZn
ZnCl
Ar
Si
Ar
Si
R
R
R
R
2
42-97 %
Scheme 1.
N
N
N
N
N
N
Si
Si
Me Me
Me Me
3
4
Figure 4. Silole-based excellent electron-transporting materi-
als.
E
E
E
E
synthesis of new silole derivatives16–19 as well as their unique
photophysical properties such as strong fluorescence in the solid
state.20
element-bridged
stilbene
heterole
dibenzoheterole
Figure 3. Key building units consisting of 5-membered cyclic
diene skeletons.
Ç
Dibenzoborole-containing ꢀ-Electron
Systems
Ç
Silole-based ꢀ-Electron Systems
Among the dibenzoheterole ring systems, we focused our at-
tention on the dibenzoborole skeleton, which is a group 13 boron
analogue of fluorene and carbazole. While both the fluorene and
carbazole currently represent core building units for highly fluo-
rescent emitting materials21 and hole-transporting materials,22
respectively, there has been no example to employ the dibenzo-
borole skeleton as a building unit for the extended ꢀ-electron
systems, when we started this chemsitry. From the viewpoint
of application of the dibenzoborole skeleton in organic electron-
ics, worthy of note is its charactristic electronic structure, which
is quite different from those of fluorene and carbazole. Our pre-
liminary calculations (Figure 5) suggest that, while the HOMO
level of the dibenzoborole is comparable to that of fluorene or
slightly lower than carbazole, its LUMO level is significantly
lower than those of fluorene and carbazole by more than
1.2 eV. This is apparently attributable to the contribution of
As the heterole ring system, we have so far devoted consid-
erable efforts to the synthesis of various types of silole-based ꢀ-
electron systems, represented by the 2,5-diarylsiloles,7 oligo-
and poly(2,5-silole)s,8 silole-containing cooligomers, and co-
polymers with other types of monomer units,9 and silicon-cate-
nating poly(1,1-silole)s.10 While the details of this chemistry
have been covered in previously published reviews,11 it may
be emphasized here that the synthesis of all these systems has
been achieved based on an intramolecular reductive cyclization
of the bis(phenylethynyl)silanes 1,8a as shown in Scheme 1. For
example, the combination of the cyclization with the cross-cou-
pling reaction allows us to synthesize a series of 2,5-diarylsilole
derivatives 2 in one pot from 1.7
The silole-based ꢀ-electron systems have currently attracted
significant attention as new materials for the organic LEDs. In
1996, we showed the utilization of 2,5-diarylsiloles in organic
LEDs for the first time.12 While several silole derivatives were
examined, 2,5-dipyridylsilole 3 (Figure 4) exhibited an extreme-
ly high performance as an electron-transporting material. Its per-
formance exceeds that of the aluminum-quinorinol complex
Alq3, which is one of the most widely used electron-transporting
materials.12 We further optimized the structure and found that its
bipyridyl derivative 4 has a superior performance to that of 3 in
terms of both the electron-transporting ability and the stability of
the device.13 Recently, Murata and co-workers indeed demon-
the p –ꢀ* conjugation in the dibenzoborole skeleton between
ꢀ
the vacant p orbital of the boron atom and the ꢀ* orbital of
the biphenyl moiety. This unique electronic structure of the di-
benzoborole skeleton makes it a promising candidate as a new
building unit for electron-transporting materials. A recent calcu-
`
ˆ ´
lation of the polydibenzoborole, reported by Briere and Cote, al-
so suggests an interesting electronic structure such as a low-lying
LUMO energy level and thus a low bandgap for the polymeric
conjugated systems.23
Experimentally, we have recently achieved the first synthe-
sis of a series of dibenzoborole-based ꢀ-conjugated systems us-
ing 3,7-dihalogenated dibenzoboroles 6 as the key precursor.24
Our molecular design relied on the following two points. First,
in order to overcome the inherent instability of the boron com-
strated its high electron mobility of 2:0 ꢁ 10ꢂ4 cm2Vꢂ1sꢂ1
,
which is two orders of magnitude higher than that of Alq3.14
The elucidation of the origin of the high performance of 4 is
still a continuous topic in this chemistry,15 together with the
Published on the web (Advance View) November 27, 2004; DOI 10.1246/cl.2005.2

4
Chemistry Letters Vol.34, No.1 (2005)
E / eV
−0.62
spectrum and exhibits a green fluorescence at 514 nm with a low
quantum yield of 0.09. The large Stokes shift (ꢃ100 nm) and the
significantly longer emission maximum compared to those of
fluorene (ꢂem 314 nm) and carbazole (ꢂem 349 nm)25 are partic-
ularly noteworthy, suggesting the significant contribution of the
boron vacant p orbital to the electronic structure. Actually, the
shoulder absorption band of 10 can be ascribed to the transition
from the HOMO delocalized over the biphenyl moiety to the
LUMO delocalized over the dibenzoborole skeleton through
−0.74
−2.00
LUMO
HOMO
−5.32
−5.73
−5.78
the p –ꢀ* conjugation.
ꢀ
E =
BMe
CMe2
NMe
On the basis of the unique nature of the dibenzoborole skel-
eton, the extended systems 7–9 also show characteristic absorp-
tion and emission spectra (Figure 6a). In THF, while these com-
pounds have weak absorption bands around 480–505 nm, they
show yellow to orange emissions around 550–575 nm with low
quantum yields of 0.02–0.04. The large red shifts in their absorp-
tion and emission maxima relative to that of 10 suggest the effec-
tive extension of the ꢀ-conjugation.
E
Figure 5. Kohn–Sham HOMO and LUMO energy levels of the
dibenzoheteroles calculated at the B3LYP/6-31G(d) level of
theory.
Notably, the fluorescence properties of these compounds are
highly dependent on the solvent. Thus, in donor solvents such as
DMF, the emission bands observed in THF completely disappear
and new intense blue emission bands appear around 420–
480 nm, whose quantum yields are dramatically increased to
about 0.5–0.9. Thus, about 100–140 nm blue shifts and 20–30
fold increments in the quantum yields are observed by changing
the solvent from THF to DMF. Figure 6b shows the visual
changes in their fluorescence depending on the solvents. This
significant solvatochromism can be rationalized owing to the co-
ordination of the donor solvent to the boron atom, which turns
MeO
X
OMe
X
MeO
Br
OMe
1) n-BuLi, THF
2) MgBr
2
Br
B
3) TipB(OMe)
2
i-Pr
i-Pr
I
I
5
i-Pr
6a (X = Br) 38%
6b (X = Î) 47%
1) t-BuLi
2) ICH CH I
2
2
MeO
Ar
OMe
Ar
from 6b
ArSnBu
7
8
9
Ar =
NPh2 43%
30%
3
B
off the p –ꢀ* conjugation in the LUMO and increases the HO-
Ar =
Ar =
ꢀ
Pd (dba)
4P(furyl)
THF
2
3
S
S
i-Pr
i-Pr
3
MO–LUMO gap, resulting in the blue shift of the emission. On
the basis of these observations, we have applied this class of
compounds as new chemosensors for the fluoride ion, wherein
the binding event of the fluoride ion by the formation of the cor-
responding borate complex induces a similar change in the fluo-
rescence spectra.26
27%
S
i-Pr
Scheme 2.
pounds, we introduced a bulky 2,4,6-triisopropylphenyl (Tip)
group onto the boron atom. Second, we incorporated methoxy
groups at the 2,8 positions of the skeleton for the facile introduc-
tion of functional groups onto the 3,7 positions. As shown in
Scheme 2, compound 6a was synthesized by the dimetalation
of 4,40-dibromo-2,20-diiodobiphenyl 5, prepared by the bromina-
tion of the corresponding 2,20-diiodobiphenyl, followed by the
reaction with TipB(OMe)2. After the transformation from 6a
to 6b via ortho-lithiation with t-BuLi, the Kosugi–Migita–Stille
coupling of 6b with various arylstannanes produced a series of
extended dibenzoborole ꢀ-electron systems 7–9. All the pro-
duced dibenzoborole derivatives have a substantial stability in
air due to the kinetic stabilization by the bulky Tip group and
can be handled without special care.
(a)
7 (DMF)
8 (DMF)
9 (DMF)
7 (THF, x10 scale)
8(THF, x10 scale)
9 (THF, x10 scale)
400
450
500
550
600
650
wavelength / nm
(b)
B
i-Pr
i-Pr
i-Pr
10
The elucidation of the fundamental properties of the parent
dibenzoborole is important to understand the intrinsic nature of
this skeleton. In THF, compound 10 shows a characteristic weak
shoulder absorption band at 410 nm (log ", 2.39) in the UV–vis
Figure 6. Solvent-dependent fluorescence of 7–9: (a) fluores-
cence spectra and (b) a photo of their solutions under irradiation
of 365-nm black light: from left to right, 7 in THF and DMF, 8 in
THF and DMF, 9, in THF and DMF (ca. 1 mM).

Chemistry Letters Vol.34, No.1 (2005)
5
The fact that the donor solvent or the fluoride ion can coor-
dinate to the boron atom may suggest the incomplete steric pro-
tection of the ring boron atom by the 2,4,6-triisopropylphenyl
group. However, it is important to note that this steric protection
is effective enough to stabilize the reduced state. Thus, in the cy-
clic voltammetry measurements, we found that compound 10
showed a nice reversible redox process with a reduction poten-
tial at ꢂ2:10 V (vs Ferrocene/Ferroceneþ). The high stability
of the reduced state as well as the low reduction potential are
the primary requisites for the electron-transporting materials.
The present preliminary result indicates the significant potential
of the dibenzoborole skeleton as the key building unit for elec-
tron-transporting materials. The application of the series of di-
benzoborole compounds to organic LEDs is now under investi-
gation.
X
Me Me
Si
SiMe2
1) 4 LiNaph
THF, rt, 5 min
2) I
2
Si
Me2Si
Li+
Me Me
LiNaph =
X
13
11
X = H or OEt
X = H, 70%
X = OEt, 65%
double
cyclization
–
2e reduction
X
Me Me
Me2
Si
Li
Si
Si
Li
Si
Me2
X
Me Me
14
15
Scheme 3.
Barton and co-workers on the basis of the thermal or photo rear-
rangement of the 5,6-disiladibenzo[c,g]cyclooctynes.29 They al-
so synthesized some attractive polymeric systems bearing this
skeleton as a pendant group.30 We independently investigated
the synthetic method of this skeleton and have recently devel-
oped a new efficient intramolecular reductive double cyclization,
as shown in Scheme 3.31
Ep = −2.23
E1/2 = −2.10
Our reaction is based on a very simple procedure, that is, the
treatment of bis(o-silylphenyl)acetylene derivatives 13 with a re-
ducing agent such as lithium naphthalenide (LiNaph). Under this
condition, the two-electron reduction on the acetylene moiety
formally produces a dianionic intermediate 14, which undergoes
a subsequent double cyclization to give the desired product 11.
The key point of this procedure is the quenching of the reaction
mixture with iodine. The over-reduction of the produced stilbene
derivatives 11 is an inevitable side reaction for the present reac-
tion, because the stilbene derivatives have lower reduction po-
tentials than those of the starting acetylene compounds. In this
regard, the use of iodine efficiently reoxidizes the over-reduced
dilithiated product 15 to regenerate the desired stilbene products.
The choice of the leaving group on the silicon atom is another
important point in this reaction. The Si–H or Si–OR functional-
ities are our choice, because these groups are generally inert to-
ward LiNaph which we used as a reducing agent.
+0.5 +0.0 −0.5 −1.0 −1.5 −2.0 −2.5 −3.0
Potential / V vs Fc/Fc+
Figure 7. Cyclic voltammogram of compound 10 (1 mM) in
ꢂ
THF containing Bu4NþClO4 (0.1 M) with scan rate of
100 mV sꢂ1
.
Ç
Bis-silicon-bridged Stilbenes
Our third key building unit is the bis-silicon-bridged stilbene
(5,10-dihydro-5,10-disilaindeno[2,1-a]indene) skeleton 11,
which is a silicon analogue of the methylene-bridged stilbene
12. In general, the annelation of the ꢀ-conjugated framework
with certain bridging moieties is an effective way to enhance
the ꢀ-conjugation, which provides a rigid planar structure with-
out conformational disorder, leading to a set of desirable proper-
ties such as a strong luminescent property and a high charge ca-
reer mobility. One typical example is that of 12. While trans-stil-
bene only shows a weak fluorescence (ꢀF ꢃ 0:05) at ambient
temperature, the fluorescence quantum yield of 12 (R ¼ H) ap-
proaches almost unity.27 In the bis-silicon-bridged stilbene, it
is expected that the silicon-bridges would not only stiffen the co-
planar structure but also contribute to the electronic structure
through an orbital interaction, as seen in the silole chemistry.
Notably, the present cyclization is more effective for the
preparation of the extended homologue 17, as shown in
Scheme 4. Thus, upon treatment of the diacetylenic compound
16 with excess LiNaph at room temperature, the intramolecular
reductive double cyclization proceeded at the two acetylene
moieties to afford the tetrakis-silicon-bridged derivative 17 in
EtO
EtO
SiMe2
SiMe2
1) 8 LiNaph
THF, rt, 5 min
R
R
R
R
2) I quench
2
C
Si
Me2Si
Me2Si
OEt
OEt
C
Si
16
Me Me
Si
R
R
R
R
Me Me
Si
11
12
One of the challenges in this chemistry is how to efficiently
produce this skeleton. While the carbon analogues 12 are known
compounds, the lack of generality in their conventional synthesis
limits their utility as a building unit.28 For the silicon analogue,
the first synthesis of compound 11 (R ¼ Me) was achieved by
Si
Me Me
Si
Me Me
17 91%
Scheme 4.

6
Chemistry Letters Vol.34, No.1 (2005)
Hex Hex
Si
23
OMe
M
= 7,000, n ≈ 11
n
λ
420 nm
max,abs
λ
F
476 nm
max,em
Φ 0.36
Si
MeO
greenish-blue emission
n
Hex Hex
Hex Hex
Si
Hex Hex
Si
24
F
OMe
F
M
= 23,000, n ≈ 19
n
λ
418 nm
max,abs
λ
F
467 nm
max,em
Si
Si
Φ 0.51
blue emission
MeO
Hex Hex
Hex
Hex
n
Figure 8. ORTEP drawing of 17 (50% probability for thermal
ellipsoids).
Me Me
Si
25
OR
OMe
M
= 37,500, n ≈ 51
n
λ
482 nm
max,abs
λ
F
506 nm
max,em
Hex Hex
Si
Hex Hex
Si
Φ
0.15
Si
Me Me
OMe
Y
OMe
RO
1) 4 s-BuLi
MeO
green emission
n
THF, −78 °C
Y
2) I or i-PrOB(pin)
2
Si
Si
MeO
MeO
Hex Hex
Si
26
Hex Hex
Hex Hex
OMe
M
= 12,000, n ≈ 15
n
20 (Y = I) 31%
21 (Y = B(pin)) 29%
18
S
λ
494 nm
547 nm
max,abs
λ
F
max,em
Φ
S
Si
Hex Hex
0.025
MeO
orange emission
n
Hex Hex
Si
Hex Hex
Si
F
I
t
F
1) 4 Li Bu ZnTMP
2
THF, rt
Figure 9. The structures of the synthesized bis-silicon-bridged
stilbene-containing homopolymers and copolymers and their
photopysical properties in THF.
I
2) I
2
Si
Si
F
F
Hex Hex
22 86%
Hex Hex
19
Scheme 5.
Me Me
E
91% yield as a bright yellow solid. The X-ray crystallographic
analysis of 17 proved its nearly coplanar structure due to the tet-
rakis-silicon bridges, as shown in Figure 8.
E
Me Me
In order to employ the bis-silicon-bridged stilbene as a
building unit, it is essential to introduce certain functional
groups such as halogens or metal functionalities required for
the further cross-coupling reactions. By incorporation of ortho-
directing groups, such as OMe or F, on the 3,8 positions of the
skeleton, a variety of functional groups can be introduced onto
the 2,7 positions via the ortho-metalation.32 Advantageously,
the present cyclization readily affords the 3,8-dimethoxy or di-
fluoro derivatives, 18 and 19, respectively. After the screening
of bases, we found that the ortho-metalation of 18 with sec-Bu-
Li, or of 19 with LitBu2ZnTMP (TMP: tetramethylpiperidine),33
followed by treatment with appropriate electrophiles successful-
ly gave the corresponding diiodides and diboronic esters 20–22,
as shown in Scheme 5. With these functionalized derivatives in
hand, various types of polymeric materials, including the stil-
bene homopolymer 23 and copolymers 24–26, have been synthe-
sized by the cross-coupling methodologies (Figure 9).32
Figure 10. Photo of the THF solutions of 11 (left) and 12 (right)
under the irradiation of black light (365 nm).
lower-lying LUMO of the silicon analogue. Thus, 11 has a
0.55 eV lower LUMO than that of the carbon analogue, and
the orbital interaction with the silicon moieties and stilbene
framework plays an important role to lower the LUMO level.
The ꢀ-conjugated polymers listed in Figure 9 also shows
moderate to intense fluorescence in the visible region. Their
emission colors vary from blue to orange, dependent on the na-
ture of the ꢀ-conjugated frameworks, suggesting the readily ac-
cessible color tuning over a wide range.
In the photophysical properties, a comparison between the
bis-silicon-bridged stilbene 11 (R ¼ Me) and its carbon ana-
logue 12 (R ¼ Me) shows a clear-cut effect of the silicon
bridges. Thus, in THF, the silicon-bridged one 11 has its absorp-
tion maximum at 360 nm and emission maximum at 428 nm,
which are longer by 38 and 64 nm, respectively, than those of
the carbon analogue 12 (ꢂmax,abs 322 nm, ꢂmax,em 367 nm), al-
though the fluorescence quantum yield decreases from 0.92 for
12 to 0.58 for 11. As the consequence of this substantial red shift,
the silicon analogue shows an intense blue emission, whereas 12
does not show a fluorescence in the visible region, as shown in
Figure 10. The molecular orbital calculations of these two com-
pounds proved that the origin of their difference is due to the
Ç
Summary and Outlook
Exploiting the orbital interaction between the main group el-
ement moiety and the ꢀ-conjugated framework in the heterole-
based ring systems as a guiding principle in the molecular de-
signs, we have developed a variety of new ꢀ-electron systems
containing silole, dibenzoborole, and bis-silicon-bridged stil-
bene as the key building units. It is particularly noteworthy that
some of the silole derivatives indeed show considerably high
performances as electron-transporting materials for organic
LEDs. In the dibenzoborole systems, not only the significant
contribution of the p –ꢀ* conjugation to the electronic structure
ꢀ
but also their great potential as the building unit for the electron-

Chemistry Letters Vol.34, No.1 (2005)
7
transporting materials have been experimentally demonstrated.
In the chemistry of the bis-silicon-bridged stilbene systems,
the synthetic methodology for them has been well established
based on the newly developed intramolecular reductive cycliza-
tion. Although we have not yet fully investigated their applica-
bility to organic electronics, the fundamental studies of their
photophysical properties reveal the substantial silicon effects
on the electronic structures. All these results demonstrate the ef-
fectiveness of the present approach in the designs of new ꢀ-elec-
tron systems. We believe that further studies along this line will
lead to the creation of truly excellent functional materials.
14 a) H. Murata, G. G. Malliaras, M. Uchida, Y. Shen, and
Z. H. Kafafi, Chem. Phys. Lett., 339, 161 (2001). b) H.
Murata, Z. H. Kafafi, and M. Uchida, Appl. Phys. Lett., 80,
189 (2002).
15 a) L. C. Palilis, A. J. Makinen, M. Uchida, and Z. H. Kafafi,
Appl. Phys. Lett., 82, 2209 (2003). b) A. J. Makinen, M.
Uchida, and Z. H. Kafafi, Appl. Phys. Lett., 82, 3889
(2003). c) A. J. Makinen, M. Uchida, and Z. H. Kafafi,
J. Appl. Phys., 95, 2832 (2004).
16 J. Lee, Q.-D. Liu, M. Motala, J. Dane, J. Gao, Y. Kang,
and S. Wang, Chem. Mater., 16, 1869 (2004).
17 N. Roques, P. Gerbier, J.-P. Sutter, P. Guionneau, D. Luneau,
´
References and Notes
and G. Guerin, Organometallics, 22, 4833 (2003).
1
For recent reviews, see: a) C. D. Entwistle and T. B. Marder,
Angew. Chem., Int. Ed., 41, 2927 (2002). b) S. Yamaguchi
and K. Tamao, in ‘‘The Chemistry of Organic Silicon
Compounds,’’ ed. by Z. Rappoport and Y. Apeloig, John
Wiley & Sons, Chichester (2001), Vol. 3, Chap. 11,
18 A. J. Boydston, Y. Yin, and B. L. Pagenkopf, J. Am. Chem.
Soc., 126, 3724 (2004).
19 Y. Lee, S. Sadki, B. Tsuie, P. Schottland, and J. R. Reynolds,
Synth. Met., 119, 77 (2001).
20 B. Z. Tang, X. Zhan, G. Yu, P. P. S. Lee, Y. Liu, and D. Zhu,
J. Mater. Chem., 11, 2974 (2001).
´
pp 641–694. c) M. Hissler, P. W. Dyer, and R. Reau, Coord.
Chem. Rev., 244, 1 (2003).
S. Yamaguchi and K. Tamao, Bull. Chem. Soc. Jpn., 69,
2327 (1996).
U. Salzner, J. B. Lagowski, P. G. Pickup, and R. A. Poirier,
Synth. Met., 96, 177 (1998).
21 M. R. Craig, M. M. de Kok, J. W. Hofstraat, A. P. H. J.
Schenning, and E. W. Meijer, J. Mater. Chem., 13, 2861
(2003) and references therein.
22 J. F. Morin and M. Leclerc, Macromolecules, 34, 4680
(2001) and references therein.
2
3
`
ˆ `
4
5
J. Ma, S. Li, and Y. Jiang, Macromolecules, 35, 1109 (2002).
A. Alparone, A. Millefiori, and S. Millefiori, Chem. Phys.,
298, 75 (2004).
23 J.-F. Briere and M. Cote, J. Phys. Chem. B, 108, 3123 (2004).
24 S. Yamaguchi, T. Shirasaka, S. Akiyama, and K. Tamao,
J. Am. Chem. Soc., 124, 8816 (2002).
6
7
8
C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater.,
14, 99 (2002).
25 I. B. Berlman, ‘‘Handbook of Fluorescence Spectra of
Aromatic Molecules,’’ Academic Press, New York (1971).
26 Recently, a similar phenomenon was observed for the dithie-
noborole-based compounds: S. Kin, K.-H. Song, S. O. Kang,
and J. Ko, Chem. Commun., 2004, 68.
27 a) J. Saltiel, O. C. Zafiriou, E. D. Megarity, and A. A.
Lamola, J. Am. Chem. Soc., 90, 4759 (1968). b) J. Saltiel,
A. Marinari, D. W. L. Chang, J. C. Mitchener, and E. D.
Megarity, J. Am. Chem. Soc., 101, 2982 (1979).
S. Yamaguchi, T. Endo, M. Uchida, T. Izumizawa, K.
Furukawa, and K. Tamao, Chem.—Eur. J., 6, 1684 (2000).
a) K. Tamao, S. Yamaguchi, and M. Shiro, J. Am. Chem.
Soc., 116, 11715 (1994). b) S. Yamaguchi, R.-Z. Jin, Y.
Itami, T. Goto, and K. Tamao, J. Am. Chem. Soc., 121,
10420 (1999).
a) K. Tamao, S. Yamaguchi, M. Shiozaki, Y. Nakagawa,
and Y. Ito, J. Am. Chem. Soc., 114, 5867 (1992). b) K.
Tamao, S. Ohno, and S. Yamaguchi, Chem. Commun.,
1996, 1873. c) S. Yamaguchi, T. Goto, and K. Tamao,
Angew. Chem., Int. Ed., 39, 1695 (2000).
9
28 D. Hellwinkel, H.-J. Hasselbach, and F. Lammerzahl,
¨
Angew. Chem., Int. Ed. Engl., 23, 705 (1984).
29 M. Serby, S. Ijadi-Maghsoodi, and T. J. Barton, XXXIIIrd
Symposium on Organosilicon Chemistry, Saginaw,
Michigan, U.S.A., 2000, Abstr., No. PA-35.
30 Z. Ma, S. Ijadi-Maghsoodi, and T. J. Barton, Polym. Prepr.,
38, 249 (1997).
10 S. Yamaguchi, R.-Z. Jin, and K. Tamao, J. Am. Chem. Soc.,
121, 2937 (1999).
11 a) S. Yamaguchi and K. Tamao, J. Chem. Soc., Dalton
Trans., 1998, 3693. b) S. Yamaguchi and K. Tamao, J.
Organomet. Chem., 653, 223 (2002).
31 S. Yamaguchi, C. Xu, and K. Tamao, J. Am. Chem. Soc.,
125, 13662 (2003).
12 K. Tamao, M. Uchida, T. Izumizawa, K. Furukawa, and
S. Yamaguchi, J. Am. Chem. Soc., 118, 11974 (1996).
13 M. Uchida, T. Izumizawa, T. Nakano, S. Yamaguchi,
K. Tamao, and K. Furukawa, Chem. Mater., 13, 268 (2001).
32 C. Xu, H. Yamada, A. Wakamiya, S. Yamaguchi, and
K. Tamao, Macromolecules, in press.
33 Y. Kondo, M. Shilai, M. Uchiyama, and T. Sakamoto,
J. Am. Chem. Soc., 121, 3539 (1999).