Dibenzosiloles and 12H-indololo[3,2-d] naphtho[1,2-b][1]siloles: Exploration of organic chromophores exhibiting efficient solid-state fluorescence

Article abstract of DOI:10.1002/tcr.201402084

The construction of a diorganosilylene bridge over a biaryl moiety at the 2,2?- positions is a versatile strategy for fine-tuning its HOMO-LUMO energy gap, which is closely linked to the electronic and optical properties of the compounds. Therefore, there

Full text of DOI:10.1002/tcr.201402084

Personal Account
DOI: 10.1002/tcr.201402084
Dibenzosiloles and 12H-Indololo[3,2-d]
naphtho[1,2-b][1]siloles: Exploration of
Organic Chromophores Exhibiting
Efficient Solid-State Fluorescence
T H E
C H E M I C A L
R E C O R D
Masaki Shimizu[a]
[a]Department of Biomolecular Engineering, Graduate School of Science and Technology, Kyoto
Institute of Technology, 1 Hashikami-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan),
E-mail: mshimizu@kit.ac.jp
Received: September 2, 2014
Published online: December 11, 2014
Chem. Rec. 2015, 15, 73–85
© 2014 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library 73

P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
ABSTRACT: The construction of a diorganosilylene bridge over a biaryl moiety at the 2,2′-
positions is a versatile strategy for fine-tuning its HOMO–LUMO energy gap, which is closely
linked to the electronic and optical properties of the compounds. Therefore, there is growing
interest in the use of silicon-bridged biaryl motifs as key cores of various types of advanced
functional materials, such as light-emitting, semiconducting, photovoltaic, and sensing materials.
To accelerate the advances of materials based on silicon-bridged biaryls, it is essential to create new
classes of biaryls and explore their functions and properties. This Personal Account describes recent
research on the development of organic chromophores based on functionalized dibenzosiloles and
12H-indololo[3,2-d]naphtho[1,2-b][1]siloles as solid-state emitters.
Keywords: chromophores, fluorescence, polycycles, silicon, structure–activity relationships
1. Introduction
Hex Hex
Si
Over the last decade, silicon-bridged biaryl frameworks, such
as dibenzosiloles and dithienosiloles, have emerged as versatile
cores of organic materials in optoelectronic devices.[1] This is
because the silicon bridge between the two aromatic rings
stabilizes the π-conjugated systems by imparting co-planarity
to the rotatable skeleton and lowering the LUMO through σ*–
π* conjugation between the σ* orbitals of the exocyclic Si–C
bonds and the π* orbital of the biaryl moiety.[2] The report by
Matsushita and Uchida in 2003, in which 5,5’-
spirobi(dibenzo[b,d]silole) (1) was used as a hole-blocking
material in green phosphorescent electroluminescent (EL)
devices, appears to have instigated the evolution (Figure 1).[3]
In 2004, Thompson and co-workers disclosed that 1 also acted
as an efficient host material in phosphorescent EL devices.[4] In
2005, there were significant breakthroughs in the development
of organic functional materials based on silicon-bridged biaryl
motifs. Holmes and co-workers demonstrated that poly(3,7-
dibenzo[b,d]silole) (2) served as a stable and efficient blue
emitter for an EL device.[5] They also showed that poly(2,8-
dibenzo[b,d]silole) (3) had a sufficiently high triplet energy to
act as a host material for green EL devices.[6] Ultraviolet fluo-
rescence of poly(5,5-dialkyl-2,8-dibenzo[b,d]silole) was also
reported by Cao and co-workers.[7] Lee et al. reported
that unsymmetrical spirobi(dibenzo[b,d]silole) 4 exhibited
intense violet–blue fluorescence with good quantum yields in
the solid state.[8] Alternating copolymer 5, consisting of
spirobi(dibenzo[b,d]silole) and triphenylamine moieties, was
revealed by Tian and co-workers to possess hole-transporting
ability.[9] These landmark achievements were followed by the
emergence of dibenzosilole–thiophene and dithienosilole–
thiophene copolymers 6 and 7 as semiconducting materials,
which were designed by Facchetti and co-workers in 2006.[10,11]
Around this time, we were studying the synthesis and
photophysical properties of 1,9-dimethylsilylene- and 1,9-
tetramethyldisilylene-bridged dibenzosiloles.[12] During this
study, we found that 5,5-dimethyl-3,7-diphenyldibenzosilole
exhibited violet fluorescence in the solid state with a good
quantum yield of 0.65. This finding prompted our interest in
Si
Ph
Ph
Ph
n
1
2
Oct Oct
Si
Si
n
Ph
Ph
Ph
3
4
Ph
N
Si
n
5
Hex Hex
Si
Oct Oct
Si
S
S
S
S
n
n
6
7
Fig. 1. Examples of dibenzosiloles- and dithienosilole-containing materials.
functionalized dibenzosiloles as solid-state emitters, which
are essential for the advance of optoelectronic devices,
such as organic light-emitting diodes,[13] light-emitting field-
effect transistors,[14] and solid-state lasers.[15] However, at
the time, the practical synthesis of dibenzosiloles was limited
to a dilithiation–silylation sequence starting from 2,2′-
dibromobiaryls 8 with alkyllithiums and dichlorosilanes
(Scheme 1).[16] Hence, the repertoire of accessible dibenzo-
siloles 9 was mostly confined to symmetrical ones with
a narrow functional group tolerance due to the formid-
able preparation of functionalized and unsymmetrical
Chem. Rec. 2015, 15, 73–85
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
R1 R2
Si
R' R'
Si
R' R'
Si
R1
R2
R1
R2
R
R
iPr iPr
R
R
Si
CN
N
N
NC
Si
iPr iPr
R' R'
Si
R
N
Scheme 1. Conventional approach and our approaches to silicon-bridged
biaryls. Cy = cyclohexyl, DMA = N,N-dimethylacetamide, dppe = 1,2-
bis(diphenylphosphino)ethane.
Me
Fig. 2. Molecular structures of dibenzosiloles and 12H-indololo[3,2-
d]naphtho[1,2-b][1]siloles presented herein.
2,2′-dibromobiaryls and the high reactivity of alkyllithiums.
Then, we developed a palladium-catalyzed intramolecular cou-
pling of 2-(arylsilyl)aryl triflates 10 as a novel approach to
silicon-bridged biaryls 11.[17] This novel cyclization allowed us
to synthesize symmetrical and unsymmetrical dibenzosiloles,
and also various heteroaromatic ring-fused benzosiloles, in
good to excellent yields, most of which were difficult to obtain
through conventional methods.[18] In addition, we found that,
when 2-(2-indolylsilyl)aryl triflates 12 were subjected to
similar reaction conditions, intramolecular direct arylation
proceeded with accompanying 1,2-migration of a silylene
tether, giving rise to [1]benzosilolo[3,2-b][1]indoles 13 that
were intensely blue emissive in the solid state.[19] Herein,
we focus on the functionalized dibenzosiloles and 12H-
indololo[3,2-d]naphtho[1,2-b][1]siloles shown in Figure 2,
which were available through the new cyclization method, and
provide an overview of their potential as solid-state emitters.
2. Organic Chromophores Based
on Dibenzosiloles
2.1. Photophysical Properties of
5,5-Diorgano-5H-dibenzo[b,d]siloles: Effect of Silicon
Substituents
Absorption and fluorescence properties of 5,5-diorgano-5H-
dibenzo[b,d]siloles 14 with no substituents on the biphenyl
moiety were investigated to disclose the effect that carbona-
ceous substitution on silicon had on their photophysical prop-
erties (Figure 3). The data are summarized in Table 1.[20] The
structures of the absorption spectra and the highest absorption
maxima were similar, irrespective of the substituents on silicon,
while the absorption edges of 14d and 14e were slightly red-
shifted compared with those of 14a–14c (Figure 4). These
results suggest that the presence of one or two phenyl groups at
Masaki Shimizu was born in 1965 in
Tokyo, Japan. He received his Ph.D.
from the Tokyo Institute of Technology
in 1994 under the supervision of Profes-
sors Takeshi Nakai and Koichi Mikami.
After working at Mitsubishi Chemical
Co. Ltd., he joined Professor Tamejiro
Hiyama’s group at the Research Labora-
(1999–2000) at the Massachusetts Institute of Technology
as a Postdoctoral Fellow under the supervision of Professor
Stephen L. Buchwald. After promotion to Associate Profes-
sor at Kyoto University in 2003, he became Professor at
Kyoto Institute of Technology in 2012. His research interest
focuses on the design and invention of functional
π-conjugated molecules as well as the development of syn-
thetic reactions, reagents, and methodologies directed
toward functional organic materials and biologically active
substances.
tory of Resources Utilization, Tokyo
Institute of Technology, as an Assistant Professor in 1995.
In 1998, he moved to Kyoto University to continue his
collaboration with Professor Hiyama. He spent one year
Chem. Rec. 2015, 15, 73–85
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
14a (R1, R2 = Me, Me)
14b (R1, R2 = Et, Et)
14c (R1, R2 = iPr, iPr)
14d (R1, R2 = tBu, Ph)
14e (R1, R2 = Ph, Ph)
R1 R2
Si
Fig. 3. Molecular structures of 14.
Table 1. Photophysical properties of 14.
Absorption
Fluorescence
Microcrystals
In cyclohexane
In cyclohexane
λ
max [nm][a]/ϵ [b]
λ
max [nm]/Φ
λmax [nm]/Φ
14a
14b
14c
14d
14e
276/11000
277/12600
277/13200
278/11500
279/12300
338/0.16
337/0.15
338/0.17
339/0.16
344/0.16
349/0.11
347/0.18
347/0.25
345/0.33
351/0.18
Fig. 5. HOMO and LUMO drawings of 14d and 14e.
[a]Wavelength of the highest absorption maximum. [b]ϵ: molar absorption coef-
ficient (M–1 cm–1).
Fig. 6. Fluorescence spectra of 14 in cyclohexane.
presumably due to the longer effective conjugation length
resulting from the participation of a diphenylsilylene moiety.
The reason why such a bathochromic shift was not observed
with tert-butylphenylsilylene derivative 14d is unclear. The
fluorescence quantum yields of all derivatives of 14 in cyclo-
hexane were within the range of 0.15–0.17; this indicated that
the emission efficiency in solution was not related to the nature
of the substituents on silicon. In addition, microcrystals of all
derivatives of 14 showed distinct spectra, which were redshifted
by 6 to 11 nm compared with those in cyclohexane (Figure 7).
As shown in Table 1, the quantum yields of the crystals varied
significantly depending on the substituents on silicon; the
highest quantum yield (Φ = 0.33) was observed with the
diisopropylsilylene derivative (14c). The bulky silylene moiety
was also beneficial for obtaining designed dibenzosiloles using
Fig. 4. Absorption spectra of 14 in cyclohexane.
silicon induces a minute decrease of the HOMO–LUMO gap
due to σ*–π* conjugation. Indeed, the LUMOs of 14d and
14e, calculated by DFT calculations (B3LYP/6–31G*//
B3LYP/6–31G* level), were confirmed to develop over both
the biphenyl skeleton and the phenyl ring(s), while there was
no distribution of the HOMO lobes on either the tert-
butylphenylsilylene or diphenylsilylene moieties (Figure 5).
The normalized fluorescence spectra of 14 in cyclohexane
are shown in Figure 6. The emission maxima of 14a–14d were
almost the same (ca. 338 nm) and the spectra in the wave-
length region longer than 330 nm completely overlapped. On
the other hand, the maxima of 14e appeared at 344 nm and no
overlap of its spectrum with others was observed. As observed
for the absorption edges, the redshift of the spectrum of 14e is
palladium-catalyzed cyclization. Therefore, we chose
a
diisopropylsilylene moiety as a silicon bridge in the following
studies.
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
Table 2. Photophysical properties of 15.
Absorption
Fluorescence
In cyclohexane
In cyclohexane
In PMMA film[a]
λmax[b] [nm]/ϵ
λmax [nm]/Φ
λmax [nm]/Φ
15a
15b
15c
15d
15e
15f
297/17000
323/17100
357/28700
391/29300
403/35700
484/44600
361/0.25
397/0.26
410/0.60
413/0.64
425/0.50
515/0.46
364/0.38
404/0.45
434/0.66
446/0.82
469/0.80
594/0.80
[a]PMMA = poly(methyl methacrylate). [b]Absorption maximum at the longest
wavelength.
Fig. 7. Fluorescence spectra of 14 in microcrystals.
15a (R1 = OMe, R2 = H)
iPr iPr
15b (R1 = NMe2, R2 = H)
Si
15c (R1 = NMe2, R2 = CN)
15d (R1 = NPh2, R2 = CN)
15e (R1 = NPh2, R2 = CHO)
15f (R1 = NPh2, R2 = CH=C(CN)2)
R1
R2
15
Fig. 8. Molecular structures of 15.
2.2. Photophysical Properties of 3-Mono- and
3,7-Disubstituted 5,5-Diisopropyl-5H-dibenzo[b,d]siloles
Incorporation of electron-donating (D) and -accepting (A)
groups at the terminals of a π-conjugated system is an effective
strategy for tuning electronic properties in the ground and
excited states of the π system as well as the HOMO–LUMO
energy gap, which is closely related to the electronic and optical
properties of the system.[21] Based on this approach, various
2-D-7-A-substituted fluorenes were developed, which exhib-
ited efficient solid-state emission, two-photon absorption, or
nonlinear optical polarizability.[22] In contrast, 3-D-7-A-
substituted dibenzosiloles, which could be regarded as the
silicon analogues of the D-π-A-type fluorenes, remained
unexplored, probably due to the lack of facile synthetic
methods for such unsymmetrical dibenzosiloles. However, the
development of the palladium-catalyzed cyclization of 2-[3-
(diorganoamino)phenylsilyl]-4-cyanophenyl triflates, which
gave 15c and 15d in excellent yields as single regioisomers,
enabled access to the D-π-A-type dibenzosiloles 15c–15f, and
allowed us to explore the potential of 15 as functional materials
(Figure 8).[18a] The formyl and dicyanoethenyl derivatives 15e
and 15f were easily obtained by diisobutylaluminum hydride
reduction of 15d and Knoevenagel condensation of 15e with
CH2(CN)2.
Fig. 9. Absorption spectra of 15 in cyclohexane.
3-dimethylamino-substituted dibenzosiloles 15a and 15b.[18a]
As seen in Figure 9, the absorption maxima redshifted signifi-
cantly when D was switched from methoxy to dimethylamino
(compare 15a and 15b) and from dimethylamino to
diphenylamino (compare 15c and 15d), and the electron-
withdrawing nature of A increased (compare 15d, 15e, and
15f). Thus, as expected, the HOMO–LUMO gap of 15 can be
altered drastically by changing the combination of D and A
groups.
The normalized fluorescence spectra of 15 in cyclohexane
and in a thin film of PMMA are shown in Figures 10 and 11.
The stronger the electron-accepting character of A, the longer
the wavelength of the emission maximum, as seen in the
absorption spectra. The use of a more polar solvent than cyclo-
hexane also induced a redshift of the spectra and the degree of
the shift became larger as the polarity of the solvent increased
(data not shown). This solvatochromic behavior indicates that
the excited state of 15 consists of a charge-separated electronic
structure. For each derivative of 15, the wavelength of the
emission maximum in the doped PMMA film was longer than
that in cyclohexane, which suggested that the excited state of
The photophysical properties of 15c–15f are summarized
in Table 2, along with those of 3-methoxy- and
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
Fig. 12. Fluorescence spectra of 15e in polymer films.
Fig. 10. Fluorescence spectra of 15 in cyclohexane.
Fig. 13. Fluorescence spectra of a mixture of 15e and 15f dispersed in PMMA
film.
Fig. 11. Fluorescence spectra of 15 dispersed in PMMA film.
15 was more stabilized in the PMMA film than in cyclohexane.
The quantum yields of 15 dispersed in the PMMA film were
higher than those in cyclohexane. This is presumably because
intramolecular rotation that leads to a loss of the excited energy
is efficiently suppressed by the polar polymer matrix.
The dependence of the fluorescence on the polymer
matrix was also examined with polystyrene (PS), PMMA, poly-
(acrylonitrile) (PAN), and poly(ethylene glycol) (PEG). As a
representative example, the spectra of 15e doped in these
polymer films are shown in Figure 12. The spectra exhibited a
bathochromic shift as the polarity of the matrix polymer
increased. Thus, the emission color of 15e varied from blue to
green by simply changing the polymer host. In the case of 15f,
the color changed from orange to red, depending on the
polymer.
were mixed in a ratio of 19:1 in the PMMA film, the polymer
film exhibited an intense white emission (CIE coordinates:
x = 0.30, y = 0.34) with a quantum yield of 0.81 upon UV
irradiation at 390 nm (Figure 13). Although further optimiza-
tion of the ratio is necessary to attain pure white light (CIE
coordinates: x = 0.33, y = 0.33), it is remarkable that energy
transfer occurs very efficiently from the excited state of 15e to
15f in the ground state, with an excellent quantum yield.
2.3. Photophysical Properties of 2-Mono- and
2,8-Disubstituted 5,5-Diisopropyl-5H-dibenzo[b,d]siloles
The effect of substitution at the 2- and 8-positions of
dibenzosilole on the photophysical properties was scrutinized
for 16a–16d (Figure 14). Table 3 provides a summary of the
photophysical properties.[20] The absorption maxima of 16
were in the range of 262–285 nm, which were similar or much
closer to that of 14c (277 nm) than those of 15b and 15a, in
Highly efficient blue and orange emission of 15e and 15f
in the PMMA film can be applied to the realization of white
luminescence with high efficiency. Thus, when 15e and 15f
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
iPr iPr
16a (R1 = NMe2, R2 = H)
16b (R1 = OMe, R2 = H)
16c (R1 = OMe, R2 = OMe)
16d (R1 = Ph, R2 = Ph)
Si
R1
R2
16
Fig. 14. Molecular structures of 16.
Table 3. Photophysical properties of 16.
Absorption
Fluorescence
In cyclohexane
max [nm]/ϵ
In cyclohexane
In PMMA film
Compd.
λ
λmax [nm]/Φ
λmax [nm]/Φ
Fig. 16. Fluorescence spectra of 16 in cyclohexane.
16a
16b
16c
16d
269/33200
280/10200
285/12000
262/76100
382/0.24
333/0.28
340/0.30
362/0.21
395/0.28
336/0.22
342/0.27
358/0.27
Fig. 17. Fluorescence spectra of 16 dispersed in PMMA film.
2.4. Application of Orange-Emissive
Dibenzo[b,d]silole-Substituted Fumaronitrile
to a White-Light-Emitting Polymer
Fig. 15. Absorption spectra of 16 in cyclohexane.
which dimethylamino or methoxy groups were introduced at
the 3-position (Figure 15). Thus, incorporation of an amino or
alkoxy group at the 2-position did not induce significant elon-
gation of the effective conjugation length. This would not be
unexpected considering that this position corresponds to the
meta position of the other phenyl group.
Figures 16 and 17 show normalized fluorescence spectra
for 16 in cyclohexane and in doped PMMA film, respectively.
The emission maxima of 16 were in the UV and violet regions,
and redshifted in the order of 16b<16c<16d<16a in both
media, corresponding to the absorption edge order. The
quantum yields were from 0.21 to 0.30. Hence, molecular
modification of dibenzosilole at the 2- and 8-positions appears
to be inappropriate for the development of efficient visible-
light emitters.
White-light emission of organic chromophores in the solid
state is essential for the realization of solid-state lighting.[23] We
have demonstrated that doping of blue and orange D-π-A-type
dibenzosiloles 15e and 15f in PMMA film is effective for
attaining white luminescence in the solid state (Section
2.2).[18a] From a practical viewpoint for application to solution
processing, the development of a white-light-emitting polymer
is also crucial.[24] Thus, we questioned whether a dibenzosilole
moiety could be used as a component of white-light-emitting
polymers.
A
star-like polymer consisting of 4,7-[4-
(diphenylamino)phenyl]-2,1,3-benzothiadiazole as an orange-
emissive core with four polyfluorene arms as the blue-emissive
portions, was demonstrated by Wang and co-workers to serve
as an efficient white-light emitter in EL devices.[25] With refer-
ence to the molecular design, we modified orange-emissive 15f
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
R' R'
R
R' R'
iPr iPr
R
R
O
O
O
O
Si
CN
B
B
Br
Br
+
N
N
(R' = C8H17
)
(R' = C8H17)
NC
Si
18
19
(0.1 mmol)
(0.1 mmol)
R
iPr iPr
+
Br
Br
R' R'
iPr iPr
Br
Br
Si
CN
17a (R = H), 17b (R = Br), 17c (R =
)
N
N
n
(R' = C8H17
)
NC
Si
Fig. 18. Structures of dibenzo[b,d]silole-substituted fumaronitriles 17.
iPr iPr
17b
(x eq)
iPr
iPr
Pd(PPh3)4 (2 mol%)
K2CO3 (10 eq)
Aliquat 336 (0.5 eq)
Si
toluene/H2O
90 °C, 24 h
a, b, c
d
15e
17a
N
69%
(3 steps)
73%
CN
19
(0.2 eq)
90 °C, 6 h
90 °C, 8 h
a) NaBH4, THF, 0 °C; b) PBr3, CH2Cl2, 0 °C to r.t.; c) NaCN, DMSO, 90 °C
d) I2, NaOMe, THF/EtOH, –78 °C
Ph–Br
(2 eq)
Scheme 2. Synthesis of 17a.
17c
to give fumaronitrile 17a as the core to which the four arms
could be attached (Figure 8).[26]
Scheme 3. Synthesis of 17c.
Fumaronitrile 17a was prepared as shown in Scheme 2.[20]
Formyl-substituted dibenzosilole 15e was reduced with NaBH4
to provide the corresponding benzylic alcohol. Bromination of
the alcohol with PBr3 followed by substitution of the resulting
benzylic bromide functionality with a cyanide ion gave the
corresponding cyanomethyl derivative in 69% yield in three
steps. Treatment of the cyanomethyl derivative with NaOMe in
the presence of I2 produced 17a in 73% yield.
Fumaronitrile 17a showed brilliant orange emission
(λmax = 597 nm, Φ = 0.83) upon irradiation at 430 nm in
cyclohexane.[20] As shown in Figure 19, the absorption spec-
trum had a large, broad band in the blue-to-green region,
which was beneficial for energy acceptance from the blue-light-
emitting segment. In addition, there was almost no overlap
between the absorption and fluorescence spectra, suggesting
that Förster-type energy loss from the excited molecule to one
in the ground state occurs with difficulty. These photophysical
properties validate molecular modification to 17a.
Polymer 17c was synthesized by the Suzuki–Miyaura
cross-coupling reaction of 9,9-dioctylfluorene (18) and 2,7-
diboryl-9,9-dioctylfluorene (19) in the presence of a small
amount of 17b, prepared from 17a by tetrabromination using
N-bromosuccinimide (NBS) followed by end-capping with
Fig. 19. Absorption and fluorescence spectra of 17a in cyclohexane.
phenyl bromide (Scheme 3). To tune the emission color, the
amount of 17b used was varied, as listed in Table 4.[20]
Figure 20 shows the normalized fluorescence spectra of 17c in
a spin-coated film as well as that of polyfluorene (20). As the
proportion of the orange-emissive segment in 17c increased,
the intensity of the emission peaks at 427 and 446 nm
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
Table 4. Yields, number-average molecular weights (Mn), polydispersity indexes (Mw/Mn), quantum yields (Φ), and CIE coordinates (x, y) of
17c and polyfluorene 20.
Yield [%][a]
Mn
Mw/Mn
Φ (film)
CIE (x,y)
[b]
17b [equiv]
17c-1
17c-2
17c-3
17c-4
20
3.0 × 10–3
1.4 × 10–3
1.0 × 10–3
3.6 × 10–4
0
87
74
94
89
99
14100
10200
14300
12900
16300
2.56
2.32
2.44
2.69
2.68
0.59
0.28
0.53
0.27
0.28
(0.47, 0.37)
(0.40, 0.32)
(0.37, 0.29)
(0.27, 0.22)
(0.17, 0.11)
[a]Yield of product isolated. [b]Number-average molecular weight was estimated by gel permeation chromatography using PS as standards.
Fig. 20. Fluorescence spectra of spin-coated films of 17c and 20.
Fig. 21. Fluorescence spectra of 17c-3 in toluene and in PMMA films.
decreased and the peak intensity at 595 nm increased. The CIE
coordinates (0.37, 0.29) of 17c-3 were the closest of the poly-
mers we prepared to those of ideal white light (0.33, 0.33).
When we annealed the film of 17c-3, prepared by spin-coating,
at 100°C, the CIE coordinates changed to (0.28, 0.23), which
corresponded to the decreased intensity of the broad band in
the orange region (Figure 21).[27] In addition, compound
17c-3 in toluene emitted blue light with CIE coordinates of
(0.16, 0.04) and a quantum yield of 0.84, indicating that the
excited energy did not efficiently transfer from the polyfluorene
arms to the dibenzosilole-substituted fumaronitrile core in
solution. These results suggest that the fluorescent properties of
17c in the film are dependent on the entanglement, distance,
and/or morphology of the polymer chains.
R' R'
Si
R' R'
Si
R4
R
N
N
R1
R
Me
21
22
Fig. 22. Molecular structures of [1]benzosilolo[3,2-b][1]indoles 21 and 12H-
indololo[3,2-d]naphtho[1,2-b][1]siloles 22.
2-(arylsilyl)aryl triflates, we found that the reaction using
2-(2-indolylsilyl)aryl triflates served as a versatile synthetic
method for [1]benzosilolo[3,2-b][1]indoles 21 (Figure 22).[19]
Gratifyingly, indoles 21 were a novel class of compounds
and were intensely blue emissive, particularly in the solid
state.[28] Subsequently, we studied extensively the crystal
structures, photophysical properties, redox properties, and
molecular orbitals of 21, and concluded that 3,2′-
diorganosilylene-bridged 2-arylindoles, including naphthyl
derivatives 22, have great potential as blue emitters for appli-
cation to EL devices.[29] Since the chemistry of 21 was previ-
ously reported,[30] this section focuses on the properties and
applications of 22.
3. Organic Chromophores Based on
12H-Indololo[3,2-d]naphtho[1,2-b][1]siloles
Blue-emissive materials are essential for the realization of solid-
state light-emitting devices applicable to full-color displays and
white lighting. Hence, the creation of blue-emitting materials
with high performances is an area of intense research. During
our study on the palladium-catalyzed cyclizations of
Chem. Rec. 2015, 15, 73–85
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T H E C H E M I C A L R E C O R D
Fig. 23. HOMO and LUMO drawings of 22a (R = H; R’ = iPr).
3.1. Preparation of
12H-Indololo[3,2-d]naphtho[1,2-b][1]siloles
Figure 23 shows the HOMO and LUMO diagrams of the
parent compound 22a (R = H; R’ = iPr), which were obtained
using DFT calculations. With regard to the naphthyl moiety,
there was no HOMO lobe at the 3-, 5-, and 7-positions, nor a
LUMO lobe at the 3- and 7-positions. Hence, the introduction
of substituents at these positions appeared ineffective for
tuning the energy levels of the HOMO and LUMO. In other
words, we considered that molecular modification of the naph-
thyl moiety at some or all of the 4-, 5-, and 6-positions would
be effective for band gap control. Meanwhile, our synthetic
route to 22 required a starting material or intermediate that
possessed a 1-bromo-2-naphthol functionality. Bromide can be
easily introduced at the 1-position by bromination of
2-naphthol. Thus, 2-naphthol derivatives that possess a
halogen at some or all of the 4-, 5-, and 6-positions are attrac-
tive candidates for starting materials. Taking this into consid-
eration, we chose commercially available 6-bromo-2-naphthol
(23) as the starting material for the preparation of silylene-
bridged 2-napthylindoles of type 22, in which the 6-position of
the naphthyl ring was arylated.
Scheme 4. Synthesis of 22.
Table 5. Fluorescent properties of 22.
Emission maxima [nm]/quantum yields (Φ)
Compd.
In toluene
In PMMA film[b]
Powder
22a
22b
22c
22d
22e
456/0.49[a]
478/0.55
466/0.68
467/0.72
472/0.50
459/0.69
469/0.58
468/0.70
469/0.80
477/0.67
476/0.70
498/0.72
504/0.61
489/0.85
477/0.50
Scheme 4 shows the synthesis of 22a–22e.[20] Nonarylated
22a and 22b and arylated 22c–22e were prepared from com-
mercially available 1-bromo-2-naphthol (25; R = H) and 23,
respectively. Suzuki–Miyaura coupling of 23 with arylboronic
acids followed by bromination with NBS gave 6-aryl-1-bromo-
2-naphthols 25c–25e. Silylation of 25 with chloro(2-
indolyl)diisopropylsilane or -diphenylsilane (26) provided silyl
ether 27. Retro-Brook rearrangement of 27 with tBuLi and
subsequent triflation of the resulting naphthol with PhNTf2
gave triflates 28. Finally, palladium-catalyzed cyclization of 28
afforded the target 22. Gram-scale preparation of 22 is possible
using this synthetic scheme.
[a]Measured in cyclohexane. [b]Prepared by spin-coating.
blue fluorescence with emission maxima ranging from 456 to
478 nm in toluene and in PMMA films. Excluding 22b, the
spectra in toluene and PMMA films for each of 22 almost
overlapped (the spectra of 22a are shown in Figure 26 as a
representative example), which suggested that 22 was truly
dispersed in PMMA film and did not form aggregates. The
spectra of powder 22, excluding 22e, generally exhibited sig-
nificantly redshifted emission relative to those in toluene and in
PMMA film. The quantum yields in PMMA films and as
powders were good to high (0.50–0.85), which suggested that
22 could potentially be used as an emitting host as well as a
dopant in EL devices.
3.2. Photophysical Properties of
12H-indololo[3,2-d]naphtho[1,2-b][1]siloles
Figure 26 shows absorption and fluorescence spectra of
22a in cyclohexane and the fluorescence spectra in a PMMA
film and as a powder. It was confirmed that there were few
overlaps between the absorption and fluorescence spectra of
The fluorescence data for 22 are summarized in Table 5, and
the spectra as PMMA films and powders are shown in
Figures 24 and 25, respectively.[31] Compounds 22 emitted
Chem. Rec. 2015, 15, 73–85
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
Fig. 26. Absorption and fluorescence spectra of 22a.
Fig. 24. Fluorescence spectra of 22 in doped PMMA films.
Fig. 27. Device structures. ITO = indium tin oxide; ETM-033 = an electron-
transporting material that is commercially available from Merck & Co.,
Inc;
NPB = 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl;
SH03:
a host material that is commercially available from SFC Co., Ltd; TBADN:
2-tert-butyl-9,10-di(naphthalen-2-yl)anthracene, MADN: 2-methyl-9,10-
di(naphthalen-2-yl)anthracene.
Fig. 25. Fluorescence spectra of 22 as powders.
22a in cyclohexane, which indicated that the excited energy
of 22a was unlikely to transfer to the ground-state molecule of
22a through a Förster mechanism. This characteristic of the
electronic structures of 22 may be one of the reasons for its
high efficiency in solid-state luminescence.
respectively.[29] Device I emitted blue light with a maximum at
471 nm and CIE coordinates of (0.152, 0.094). The emission
maximum was almost identical to that of 22a in the powder
(476 nm), which implied that the observed blue light was
indeed irradiated from 22a and not from a material consisting
of an electron- or hole-transporting layer. It was noteworthy
that the CIE coordinates were very close to the ideal blue
coordinates (0.14, 0.08). The turn-on voltages of devices I–IV
were very low (2.5–3.0 V), which indicated that electron and
hole injections into the charge-transporting layers from the
electrodes were very smooth. The emission maxima of devices
II–IV ranged from 458 to 466 nm, which were slightly blue-
shifted compared with that of device I, as observed in the
photoluminescence spectra of 22 in the powder and doped
film. The EQE value was dependent on the devices; the highest
was 4.49 for device II. Since the theoretical EQE limit in
conventional fluorescent EL devices is postulated to be 5.0, the
3.3. EL Performances of
12H-Indololo[3,2-d]naphtho[1,2-b][1]silole
Because 22 exhibited high efficiency of fluorescence as a
powder and in a doped PMMA film, we examined the perfor-
mances of EL devices in which 22a was used as an emitting
layer (EML; device I) and a dopant in a host material (devices
II–IV).[29] The structures of the devices are shown in Figure 27.
The EL data of the devices with a current density of
10 mA cm–2 are summarized in Table 6, and normalized
electroluminescence spectra and current efficiency–current
density plots of the devices are shown in Figures 28 and 29,
Chem. Rec. 2015, 15, 73–85
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
Table 6. EL data for devices I–IV.
Luminance
efficiency
Power
λmax
efficiency EQE[b]
Device[a] [nm]
CIE (x,y)
[cd A−1][c] [lm w−1][c] [%][c]
I
471 0.152, 0.094
466 0.153, 0.158
460 0.148, 0.118
458 0.149, 0.115
3.80
6.68
3.19
4.20
3.64
5.58
3.20
3.82
0.73
4.49
2.86
3.76
II
III
IV
[a]evice structures: device I: ITO (1500 Å)/MoO3:NPB (600 Å)/NPB (200 Å)/
22a (50 Å)/ETM-033 (600 Å)/LiF (10 Å)/Al (1500 Å); device II: ITO
(1500 Å)/MoO3:NPB (600 Å)/NPB (200 Å)/SH03:22a (7 wt%) (200 Å)/
ETM-033 (400 Å)/LiF (10 Å)/Al (1500 Å); device III: ITO (1500 Å)/
MoO3:NPB (600 Å)/NPB (200 Å)/TBADN:22a (7 wt%) (200 Å)/ETM-033
(400 Å)/LiF (10 Å)/Al (1500 Å); device IV: ITO (1500 Å)/MoO3:NPB
(600 Å)/NPB (200 Å)/MADN:22a (7 wt%) (200 Å)/ETM-033 (400 Å)/LiF
(10 Å)/Al (1500 Å). [b]External quantum efficiency. [c]Values for a current
density of 10 mA cm–2.
Fig. 29. Current efficiency–current density plots of devices I–IV.
Acknowledgements
I would like to thank Prof. T. Hiyama and Prof. Y. Nakao for
their valuable advice and suggestions as well as my students
(Dr. K. Mochida, M. Kato, Y. Asai, R. Kaki, A. Yamatani, Y.
Inamoto, and H. Fukui) for their pure devotion to this
research. Collaboration with N. Nagai, Dr. H. Yamagishi, Dr.
H. Furutani, Dr. K. Inoue, Dr. T. Otsuka, and M. Nishida is
also gratefully acknowledged. The works presented herein were
supported financially by Grants-in-Aid from the Ministry of
Education, Culture, Sports, Science and Technology
(Kakenhi), and a Grant-in-Aid for the Adaptable and Seamless
Technology Transfer Program through target-driven R&D (no.
AS2414069M) from the Japan Science and Technology
Agency.
Fig. 28. Normalized electroluminescence spectra of devices I–IV.
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Chem. Rec. 2015, 15, 73–85
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