Photophysical properties of heteroaromatic ring-fused (di)benzosiloles

Article abstract of DOI:10.1007/s11426-011-4396-6

Benzosiloles fused to heterocycles such as thiophene, benzothiophene, and benzofuran, and indole- and benzosilole-fused dibenzosiloles were prepared by palladium-catalyzed intramolecular coupling of the corresponding 2-(arylsilyl)aryl triflates in good to high yields. Molecular and crystal structures of 5,7-dihydro-5,5,7,7-tetrakis(1-methylethyl)bis[1]benzosilolo-[2,3- b:3′, 2′-d]thiophene, 6-methyl-12,12-diisopropyl-12H-indololo[3,2-b] [1]silafluorene, and 5,5,11,11-tetraisopropyl-5,11H-benzosilolo[3,2-c] silafluorene were determined by X-ray diffraction analysis. The UV absorption spectra of the (di)benzosilole derivatives in cyclohexane red-shifted when compared to 1,1-diisopropyldibenzosilole, indicating that replacing a benzene ring of dibenzosilole by the heterocycles as well as fusion of indole and benzosilole moieties onto dibenzosilole narrowed the HOMO-LUMO gaps of the π-conjugation system. The thiophene-fused benzosiloles were faintly fluorescent in solution and in the solid state, whereas the dibenzosiloles exhibited luminescence with moderate and high quantum yields in cyclohexane and in microcrystals, respectively. In other words, aggregation-induced emission was observed for the dibenzosiloles. Notably, 5,5,11,11-tetraisopropyl-5,11H- benzosilolo[3,2-c]silafluorene in microcrystals exhibited violet fluorescence (λ _max = 396 nm) with a quantum yield of 0.70. Density functional theory (DFT) calculations of the prepared (di)benzosiloles were also performed.

Full text of DOI:10.1007/s11426-011-4396-6

SCIENCE CHINA
Chemistry
December 2011 Vol.54 No.12: 1937–1947
doi: 10.1007/s11426-011-4396-6
• ARTICLES •
Photophysical properties of heteroaromatic ring-fused
(di)benzosiloles
SHIMIZU Masaki1*, MOCHIDA Kenji1, KATOH Masaki1 & HIYAMA Tamejiro2
1Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto University Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
2Current address: Research & Development Initiative, Chuo University, 1-13-27, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
Received July 22, 2011; accepted August 12, 2011; published online November 16, 2011
Benzosiloles fused to heterocycles such as thiophene, benzothiophene, and benzofuran, and indole- and benzosilole-fused
dibenzosiloles were prepared by palladium-catalyzed intramolecular coupling of the corresponding 2-(arylsilyl)aryl triflates in
good to high yields. Molecular and crystal structures of 5,7-dihydro-5,5,7,7-tetrakis(1-methylethyl)bis[1]benzosilolo-[2,3-b:3',
2'-d]thiophene, 6-methyl-12,12-diisopropyl-12H-indololo[3,2-b][1]silafluorene, and 5,5,11,11-tetraisopropyl-5,11H-benzosi-
lolo[3,2-c]silafluorene were determined by X-ray diffraction analysis. The UV absorption spectra of the (di)benzosilole deriva-
tives in cyclohexane red-shifted when compared to 1,1-diisopropyldibenzosilole, indicating that replacing a benzene ring of
dibenzosilole by the heterocycles as well as fusion of indole and benzosilole moieties onto dibenzosilole narrowed the HOMO-
LUMO gaps of the -conjugation system. The thiophene-fused benzosiloles were faintly fluorescent in solution and in the sol-
id state, whereas the dibenzosiloles exhibited luminescence with moderate and high quantum yields in cyclohexane and in mi-
crocrystals, respectively. In other words, aggregation-induced emission was observed for the dibenzosiloles. Notably, 5,5,11,11-
tetraisopropyl-5,11H-benzosilolo[3,2-c]silafluorene in microcrystals exhibited violet fluorescence (max = 396 nm) with a
quantum yield of 0.70. Density functional theory (DFT) calculations of the prepared (di)benzosiloles were also performed.
absorption, benzosilole, fluorescence, silicon, solid-state emission
[44–46]. Electronic and optical properties of -conjugated
compounds can be tuned by molecular modification such as
incorporation of electron-donating/accepting groups, fusion
of heteroaromatic rings, and/or creating a heteroatom bridge
onto the parent molecular framework. Hence, preparation
and characterization of silole derivatives fused to two dif-
ferent aromatic rings are of particular interest, with the aim
being development of novel -conjugated modules for func-
tional organic materials.
1 Introduction
Silicon-bridged biaryls have attracted much attention as key
components of functional organic materials for optoelec-
tronic devices such as organic light-emitting devices
(OLEDs), field-effect transistors (OFETs), and photovolta-
ics (OPV) [1–40]. Silylene-bridged biphenyls (diben-
zosiloles) 1 and 2,2-bithiophenes (dithienosiloles) 2 are
representative compouds, in which identical aromatic rings
are bridged by a silylene moiety (Figure 1). However,
silicon-bridged biaryls consisting of different aromatic rings
such as thienobenzosilole 3 remain unexplored [41–43],
presumably due to the lack of facile synthetic methods
*Corresponding author (email: m.shimizu@hs2.ecs.kyoto-u.ac.jp)
Figure 1 Examples of silylene-bridged biaryls.
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com www.springerlink.com

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Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
We recently developed palladium-catalyzed intramolec-
(10 mL). Saturated aq. NH4Cl (5 mL) was added to the
solution, and the aqueous layer was extracted with hexane
(20 mL × 3). The combined organic layer was washed with
H2O (15 mL × 3), sat. aq. NaCl (15 mL), dried over anhy-
drous MgSO4, and concentrated by rotary evaporation. The
residue was purified by column chromatography on silica
gel to give 1–8. Characterization data of 1–5 and 8 as well
as their precursors is available in the supporting information
of the literature we reported [47].
ular coupling of 2-(arylsilyl)aryl triflates as a new synthetic
route to silicon-bridged biaryls (eq. (1)) [47]. The modular
approach allowed us to conveniently synthesize not only
functionalized dibenzosiloles but also benzosiloles fused to
a heteroaromatic ring. Compounds of this type are difficult to
synthesize via conventional synthetic strategies for silicon-
bridged biaryls. Thus, the palladium-catalyzed intramolecular
coupling allowed us to explore silole derivatives fused to two
different kinds of aromatic rings as novel -conjugated mod-
ules. Reported herein are the preparation, structures, photo-
physical properties, and theoretical calculations of heteroar-
omatic ring-fused (di)benzosiloles 1–7 (Figure 2).
Preparation of 6
An oven-dried 80-mL Schlenk tube equipped with a mag-
netic stir bar and a rubber septum was charged with
3-bromo-2-methoxy-9-methylcarbazole (4.81 g, 16.6 mmol),
NaSEt (3.93 g, 46.6 mmol), and DMF (30 mL). The result-
ing mixture was stirred at 130 °C for 12 h before quenching
with sat. aq. NaHCO3 (20 mL). The aqueous layer was ex-
tracted with hexane (20 mL × 3). The combined organic
layer was washed with sat. aq. NaCl (15 mL), dried over
anhydrous MgSO4, and concentrated by rotary evaporation.
The residue was purified by column chromatography on
silica gel (hexane/AcOEt 3:2) to give 3-bromo-2-hydroxy-
9-methylcarbazole (3.33 g, 73%) as a colorless solid. An
oven-dried 80-mL Schlenk tube equipped with a magnetic
stir bar and a rubber septum was charged with 3-bromo-2-
hydroxy-9-methylcarbazole (1.11 g, 4.0 mmol), imidazole
(0.27 g, 4.0 mmol), and dichloromethane (20 mL). To the
solution was added (i-Pr)2SiPhCl (1.45 g, 4.0 mmol) and the
resulting solution was stirred at 40 °C for 21 h before
quenching with sat. aq. NH4Cl (20 mL). The aqueous layer
was extracted with CH2Cl2 (20 mL × 3). The combined or-
ganic layer was washed with sat. aq. NaCl (15 mL), dried
over anhydrous MgSO4, and concentrated by rotary evapo-
ration. The crude product was purified by column chroma-
tography on silica gel (hexane/AcOEt 50:1) to give
3-bromo-2-diisopropylphenylsilyloxy-9-methylcarbazole
(1.19 g, 64%). An oven-dried 80 mL Schlenk tube equipped
with a magnetic stir bar and a rubber septum was charged
with 3-bromo-2-diisopropylphenylsilyloxy-9-methylcarba-
zole (1.19 g, 2.6 mmol) and THF (10 mL). To the solution
was added t-butyllithium (1.59 M in pentane, 3.2 mL, 5.1
mmol) dropwise at –78 °C via syringe over 10 min. The
resulting solution was allowed to warm to room temperature
and then stirred for 10 h before quenching with sat. aq.
NH4Cl (20 mL). The aqueous layer was extracted with
hexane (20 mL × 3). The combined organic layer was
washed with sat. aq. NaCl (15 mL), dried over anhydrous
MgSO4, and concentrated by rotary evaporation. The resi-
due was purified by column chromatography on silica gel
(hexane/AcOEt 3:2) to give 2-hydroxy-3-diisopropylphen-
ylsilyl-9-methylcarbazole (0.66 g, 67%) as a colorless solid.
An oven-dried 20-mL Schlenk tube equipped with a mag-
netic stir bar and a rubber septum was charged with NaH
(36 mg, 1.5 mmol) and DMF (7.5 mL). To the solution was
added 2-hydroxy-3-diisopropylphenylsilyl-9-methylcarba-
2 Experimental
2.1 Synthesis of heteroaromatic ring-fused (di)ben-
zosiloles 1–7 and dibenzosilole 8
General procedure for the Pd-catalyzed intramolecular
coupling
An oven-dried 3-mL vial equipped with a magnetic stir bar
was charged with 2-(arylsilyl)aryl triflate (1.0 mmol) and
N,N-dimethylacetamide (DMA, 1.0 mL). To the solution
were added a solution of Pd(OAc)2 (11 mg, 0.05 mmol) and
PCy3 (28 mg, 0.1 mmol) in DMA (1.0 mL), and then Et2NH
(200 L, 2.0 mmol). The reaction mixture was stirred at
100 °C until Pd-black precipitated. The resulting mixture
was cooled to room temperature and diluted with CH2Cl2
Figure 2 Structures of heteroaromatic ring-fused (di)benzosiloles 1–7
and dibenzosilole 8.

Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
1939
zole (0.58 g, 1.5 mmol) and PhNTf2 (0.54 g, 1.5 mmol) in
this order at room temperature. The resulting solution was
stirred at room temperature for 14 h before quenching with
sat. aq. NH4Cl (20 mL). The aqueous layer was extracted
with hexane (20 mL × 3). The combined organic layer was
washed with sat. aq. NaCl (15 mL), dried over anhydrous
MgSO4, and concentrated by rotary evaporation. The resi-
due was purified by column chromatography (hex-
ane/AcOEt 10:1) on silica gel to give 9-methyl-3-(diisopro-
pylphenylsilyl)carbazo-2-yl trifluoromethanesulfonate (0.61
g, 78 %) as a colorless solid. Mp: 122.0–122.8 °C. TLC: Rf
rotary evaporation to give 1,4-bis(chlorodiisopropylsilyl)
benzene (3.85 g, quant.) as a colorless liquid. An oven-dried
80 mL Schlenk tube equipped with a magnetic stir bar and a
rubber septum was charged with 2-bromophenol (2.32 mL,
20 mmol), imidazole (2.04 g, 30 mmol), 1,4-bis(chloro-
diisopropylsilyl)benzene (3.85 g, 10 mmol), and CH2Cl2 (50
mL). The resulting solution was stirred at 40 °C for 12 h.
The solution was then cooled to room temperature before
quenching with sat. aq. NH4Cl (20 mL). The aqueous layer
was extracted with CH2Cl2 (20 mL × 3). The combined or-
ganic layer was washed with sat. aq. NaCl (15 mL), dried
over anhydrous MgSO4, and concentrated by rotary evapo-
ration. The residue was purified by column chromatography
on silica gel (hexane/AcOEt 30:1) to give 1,4-bis
[(2-bromophenoxy)diisopropylsilyl]benzene (4.62 g, 67%)
as a colorless liquid. An oven-dried 80-mL Schlenk tube
equipped with a magnetic stir bar and a rubber septum was
charged with 1,4-bis[(2-bromophenoxy)diisopropylsilyl]
benzene (4.62 g, 6.7 mmol) and THF (30 mL). To the solu-
tion cooled to –78 °C was added t-butyllithium (1.59 M in
pentane, 17.0 mL, 28 mmol) dropwise via syringe over 10
min. The resulting solution was allowed to warm to room
temperature and then stirred for 10 h before quenching with
sat. aq. NH4Cl (20 mL). The aqueous layer was extracted
with hexane (20 mL × 3). The combined organic layer was
washed with sat. aq. NaCl (15 mL), dried over anhydrous
MgSO4, and concentrated by rotary evaporation. The resi-
due was purified by column chromatography on silica gel
(hexane/AcOEt 3:2) to give 1,4-bis[(2-hydroxyphenyl)
diisopropylsilyl]benzene (1.68 g, 51%) as a colorless solid.
An oven-dried 20-mL Schlenk tube equipped with a mag-
netic stir bar and a rubber septum was charged with
1,4-bis[(2-hydroxyphenyl)diisopropylsilyl]benzene (1.68 g,
3.4 mmol) and Et2O (10 mL). To the solution cooled to 0 °C
was added butyllithium (1.59 M in hexane, 4.3 mL, 6.9
mmol) dropwise via syringe over 10 min. The solution was
stirred at 0 °C for 1 h before addition of Tf2O (1.7 mL, 6.9
mmol). The resulting solution was allowed to warm to room
temperature and then stirred for 12 h before quenching with
sat. aq. NH4Cl (20 mL). The aqueous layer was extracted
with hexane (20 mL × 3). The combined organic layer was
washed with sat. aq. NaCl (15 mL), dried over anhydrous
MgSO4, and concentrated by rotary evaporation. The resi-
due was purified by column chromatography on silica gel
(hexane/AcOEt 10:1) to give the corresponding ditriflate
(1.82 g, 71%) as a colorless solid. Mp: 164.8–165.7 °C.
1
0.45 (hexane/AcOEt 10:1). H NMR (400 MHz, CDCl3): 
1.02 (d, J = 7.3 Hz, 6H), 1.09 (d, J = 7.3 Hz, 6H), 1.82 (qq,
J = 7.3, 7.3 Hz, 2H), 3.87 (s, 3H), 7.25 (dd, J = 7.7, 7.1 Hz,
1H), 7.38–7.45 (m, 4H), 7.47 (s, 1H), 7.49–7.53 (m, 1H),
7.56–7.58 (m, 2H), 7.99 (dd, J = 7.8, 0.6 Hz, 1H) 8.14 (d, J
= 0.7 Hz, 1H); 13C NMR (100 MHz, CDCl3):  10.8, 18.0,
18.1, 29.4, 99.2, 108.6, 113.9, 118.4 (q, J = 319.7 Hz),
119.8, 120.4, 121.5, 121.7, 126.3, 127.5, 129.0, 130.5,
133.1, 135.7, 141.8, 142.0, 154.6; 19F NMR (282 MHz,
CDCl3):  –74.7. IR (KBr):  2949, 2864, 1630, 1597, 1474,
1456, 1422, 1242, 1215, 1206, 1142, 1111, 961, 864, 748,
704, 667, 604, 515 cm–1. MS (FAB) m/z: 520 (34, M + H+),
476 (100), 442 (7). HRMS calcd for C26H28F3N2O3SSi:
519.1511 (M+). Found: 519.1505.
6-Methyl-12,12-diisopropyl-12H-indololo[3,2-b][1]silafl-
uorene (6): Purified by silica gel column chromatography
(hexane/AcOEt 50:1). Yield: 33%, a colorless solid. Mp:
1
148.4–149.6 °C. TLC: Rf 0.48 (hexane/AcOEt 10:1). H
NMR (400 MHz, CDCl3):  1.08 (d, J = 7.5 Hz, 6H), 1.10
(d, J = 7.3 Hz, 6H), 1.47 (qq, J = 7.5, 7.3 Hz, 2H), 3.92 (s,
3H), 7.23–7.29 (m, 2H), 7.40 (d, J = 8.2 Hz, 1H), 7.44–7.49
(m, 2H), 7.64 (d, J = 7.0 Hz, 1H), 7.86 (s, 1H), 7.99 (d, J =
7.9 Hz, 1H), 8.11 (d, J = 7.7 Hz, 1H), 8.29 (d, J = 0.5 Hz,
1H); 13C NMR (100 MHz, CDCl3):  11.5, 18.4, 29.1, 101.1,
108.3, 119.0, 119.9, 120.5, 122.6, 125.2, 125.36, 125.43,
126.7, 129.7, 133.5, 137.2, 141.3, 142.9, 146.8, 149.4. IR
(KBr):  2951, 2938, 2861, 1595, 1476, 1458, 1327, 1246,
1123, 1074, 882, 766, 752, 706, 675, 656, 486, 421 cm–1.
MS (FAB) m/z: 369 (100, M+), 326 (71), 284 (25). HRMS
Calcd for C25H27NSi: 369.1913 (M+). Found: 369.1904.
Preparation of 7
An oven-dried 250 mL Schlenk tube equipped with a mag-
netic stir bar and a rubber septum was charged with
1,4-dibromobenzene (2.36 g, 10 mmol) and THF (80 mL).
To the solution cooled to –78 °C was added t-butyllithium
(1.59 M in pentane, 25.2 mL, 40 mmol) dropwise via sy-
ringe over 20 min. The resulting solution was stirred at
–78 °C for 1 h before addition of (i-Pr)2SiCl2 (3.61 mL, 20
mmol). The resulting solution was warmed to room temper-
ature, then stirred for 12 h and then diluted with hexane (50
mL). The mixture was passed through a short pad of Celite
to remove precipitates. The filtrate was concentrated by
1
TLC: Rf 0.45 (hexane/AcOEt 10:1). H NMR (400 MHz,
CDCl3):  0.98 (d, J = 7.3 Hz, 12H), 1.06 (d, J = 7.3 Hz,
12H), 1.73 (qq, J = 7.3, 7.3 Hz, 4H), 7.31 (dt, J = 7.3, 1.1
Hz, 2H), 7.41–7.43 (m, 2H), 7.46 (s, 4H), 7.47–7.52 (m,
4H); 13C NMR (100 MHz, CDCl3):  10.6, 17.8, 17.9, 118.2
(q, J = 319.0 Hz), 118.62, 118.65, 125.8, 126.5, 131.3,
133.4, 134.6, 139.3, 156.1; 19F NMR (282 MHz, CDCl3): 
–74.9. IR (KBr):  2953, 2870, 1418, 1246, 1206, 1146,

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Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
1057, 912, 745, 635, 599, 538 cm–1. MS (FAB) m/z: 754 (2,
M+), 711 (100), 685 (7). HRMS calcd for C32H40F6O6S2Si2:
754.1709 (M+). Found: 754.1682.
5,5,11,11-Tetraisopropyl-5,11H-benzosilolo[3,2-c]silaflu-
orene (7): Purified by silica gel column chromatography
(hexane/AcOEt 10:1) followed by HPLC (hexane/AcOEt
100:1). Yield: 77 %, a colorless solid. Mp: 203.0–203.8 °C.
1
TLC: Rf 0.58 (hexane/AcOEt 10:1). H NMR (400 MHz,
CDCl3):  1.09 (d, J = 7.3 Hz, 12H), 1.10 (d, J = 7.5 Hz,
12H), 1.44 (qq, J = 7.5, 7.3 Hz, 4H), 7.25 (dt, J = 7.1, 0.7 Hz,
2H), 7.44 (dt, J = 7.5, 1.3 Hz, 2H), 7.62 (d, J = 7.0 Hz, 2H),
7.89 (d, J = 7.9 Hz, 2H), 8.04 (s, 2H); 13C NMR (100 MHz,
CDCl3):  11.3, 18.3, 18.4, 120.5, 125.4, 126.6, 129.8, 133.5,
136.2, 138.3, 147.4, 148.9. IR (KBr):  2938, 2861, 1559,
1506, 1506, 1458, 1362, 1128, 1082, 990, 882, 767, 662, 642
cm–1. MS (FAB) m/z: 454 (100, M+), 411 (27), 369 (13).
HRMS Calcd for C30H38Si2: 454.2512 (M+). Found: 454.2501.
2.2 UV-vis absorption and fluorescent measurement
The spectroscopic-grade solvents for UV-vis absorption and
fluorescence measurements were purchased from Kanto
Chemical Co., Inc. and degassed with argon before use.
UV-vis absorption spectra were measured with a Shimadzu
UV-2550 spectrometer. Fluorescence spectra and absolute
quantum yields were recorded and calculated, respectively,
by Hamamatsu Photonics C9920-02 Absolute PL Quantum
Yield Measurement System.
Scheme 1 Synthesis of 1–7
zosiloles 1–5, and indole- and benzosilole-fused diben-
zosiloles 6 and 7 were prepared by palladium-catalyzed
intramolecular coupling reaction of 2-(arylsilyl)aryl triflates
[47]. In the case of 7, the two-fold coupling was sluggish
under standard conditions. To overcome this, Cy2NH was
employed in place of Et2NH and the reaction was carried
out at 140 °C, enabling the double cyclization to proceed
smoothly. All the products except for 6 were isolated in
high to excellent yields. The molecular structures of 5, 6,
and 7 were unambiguously confirmed by X-ray crystallo-
graphic analysis (vide infra).
2.3 Preparation of doped polymer film
In a tube glass, each sample was dissolved in a benzene sat.
Benzene solution of poly(methyl methacrylate) (PMMA)
with concentration of 0.1 mg mL1. The resulting solution
was dropped onto a quartz plate (10 mm × 10 mm) and
spin-coated at 100 r min–1 for 20 s and at 1000 r min–1 over
a period of 100 s. The deposited film was dried under re-
duced pressure at 50 °C for 1 h.
2.4 Density functional theory (DFT) calculation
3.2 Molecular and crystal structures of 5–7
All calculations were carried out using the Gaussian 03
program [48]. In the cases of 5–7, the energy calculations
were carried out with the molecular structures determined
by X-ray diffraction analysis of their single crystals. In the
cases of 1–4, their initial geometries were built up with the
structures of crystals 5–7 as a reference, and the geometries
were optimized at the B3LYP/6-31G* level. Energy levels of
molecular orbitals were calculated by the gauge-including
atomic orbital (GIAO) method at the B3LYP/6–31G* level.
Single crystals of 5 suitable for X-ray diffraction analysis
were obtained by recrystallization from a hexane–methanol
mixture. Compound 5 crystallizes in the triclinic space
group P–1 and the asymmetric unit contains two molecules
[49]. Since the structural characteristics of the two mole-
cules are quite similar, only one of the two is shown, for
clarity, in Figure 3. As reported for silafluorenes [22, 37],
the geometry of the silicon atoms of 5 is significantly de-
formed from that of the standard sp3-hybridized silicon at-
om; the bond angles of C(1)–Si(1)–C(9), C(1)–Si(1)–C(10),
C(5)–Si(1)–C(9), C(5)–Si(1)–C(10), and C(9)–Si(1)–C(10)
are widened to 113.1°, 113.6°, 115.4°, 111.6°, and 112.2°,
respectively, while that of C(1)–Si(1)–C(5) in the five-
membered ring was substantially narrowed to 89.1°. The
lengths of the carbon–silicon bonds range from 1.852 Å to
3 Results and discussion
3.1 Synthesis of heteroaromatic ring-fused (di)ben-
zosiloles
As illustrated in Scheme 1, heteroaromatic ring-fused ben-

Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
1941
isopropyl groups are oriented almost perpendicular to the
molecular plane; the silylene bridge adopts an ideal con-
formation for efficient *–* conjugation. The pentacyclic
molecular framework is twisted like helicene, presumably
due to the steric repulsion between hydrogens attached to
C(2) and C(8); the dihedral angles of C(2)–C(3)–C(4)–C(6)
and C(4)–C(6)–C(7)–C(8) are 12.6° and 11.9°, respectively.
Figure 4 shows the packing diagram of 5. No - stacking
can be seen between the adjacent two molecules; this is
most likely due to the bulkier silylene bridge and the helical
molecular framework.
Figure 5 displays the molecular structure of 6, which re-
fined in the orthorhombic space group Pbca [49]. The pen-
tacyclic skeleton is almost flat and the isopropyl groups are
perpendicularly positioned above and below the plane, as
already described for 5. The methyl group bonded to nitro-
gen is located in the -plane, indicating that the lone pair of
the nitrogen effectively participates in the -conjugation.
The terphenyl linkage is slightly bent, presumably because
the length of a C–N bond is shorter than that of a C–Si bond;
the bond angle of C(3)–C(9)–C(5) is 9.8°. The inner angles
of the silole ring are as follows: 91.3° for C(1)–Si(1)–C(4),
115.5° for C(1)–Si(1)–C(11), 111.0° for C(1)–Si(1)–C(10),
111.9° for C(4)–Si(1)–C(11), 115.7° for C(4)–Si(1)–C(10),
109.5° for Si(1)–C(1)–C(2), 115.3° for C(1)–C(2)–C(3),
114.7° for C(2)–C(3)–C(4), and 109.1° for C(3)–C(4)–Si(1).
Thus, the silicon atom widely deviates from the typical tet-
rahedral configuration, again mirroring the findings de-
scribed for 5. The bond angles of the N-methylpyrrole moi-
ety are 108.9° for C(6)–N(1)–C(8), 108.5° for C(5)–C(6)–
N(1), 109.3° for N(1)–C(8)–C(9), 107.0° for C(6)–C(5)–
C(9), 106.3° for C(5)–C(9)–C(8), 125.0° for C(6)–N(1)–
C(7), and 125.8° for C(7)–N(1)–C(8). Hence, no significant
strain is seen in the nitrogen-containing five-membered ring.
Figure 6 illustrates the crystal packing of 6. The unit cell (a
= 7.7183(6) Å, b = 14.9962(12) Å, c = 36.785(3) Å) con-
tains eight molecules and there is no close contact between
Figure 3 Molecular structure of 5. Hydrogen atoms are omitted for clarity.
1.868 Å; these are slightly shorter than the standard Si–C
distance of 1.88 Å in neutral four-coordinated silanes. Four
Figure 4 Packing diagram of 5: (a) view from a axis, (b) view from b
axis; (c) view from c axis. Hydrogen atoms are omitted for clarity.
Figure 5 Molecular structure of 6. Hydrogen atoms are omitted for clarity.

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Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
Figure 7 Molecular structure of 7. Hydrogen atoms are omitted for clarity.
Figure 6 Packing diagram of 6: (a) view from a axis; (b) view from b
axis; (c) slant view down bc plane. Hydrogen atoms are omitted for clarity.
the pentacyclic -planes of two adjacent molecules.
Compound 7 crystallizes in the centrosymmetric mono-
clinic space group P21/c with the center of symmetry locat-
ed at the centroid of the molecule [49]. The pentacyclic
molecular framework is completely flat with an orthogonal
arrangement of four isopropyl groups to the plane, an ideal
conformation for *–* conjugation (Figure 7). The angles
between the carbon–silicon bonds and the inner angles of
the silole ring are 91.0° for C(1)–Si(1)–C(4), 115.9° for
C(1)–Si(1)–C(5), 111.4° for C(1)–Si(1)–C(6), 115.6° for
C(4)–Si(1)–C(5), 112.0° for C(4)–Si(1)–C(6), 109.8° for
Si(1)–C(1)–C(2), 114.9° for C(1)–C(2)–C(3), 115.1° for
C(2)–C(3)–C(4), and 109.2° for C(3)–C(4)–Si(1). Thus, the
deformation of the silicon atom and the structural aspect of
the fused silole moiety are similar to those of 5 and 6, re-
spectively. The packing diagram shown in Figure 8 indi-
cates that the molecules in the crystal are well separated
from one another and no – stacking is evident in the
crystalline state.
3.3 Photophysical properties
Figure 8 Packing diagram of 7: (a) view from a axis; (b) view from b
axis; (c) view from c axis. Hydrogen atoms are omitted for clarity.
UV-vis absorption spectra of 1–7 recorded in cyclohexane
are shown in Figure 9 along with that of 1,1-diiso-
propyldibenzosilole (8) for comparison. Compared with 8,
the spectrum of 1 red-shifted, indicating that replacing the
biphenyl skeleton of 8 by a 2-phenylthiophene moiety in-
duced narrowing of the HOMO-LUMO gap (see molecular

Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
Table 1 Fluorescence data of 1–8a)
1943

max ()b) (nm)
Compound
Cyclohexanec)
PMMA filmd)
367 (0.11)
354 (0.04)
389 (0.06)
383 (0.16)
369 (0.02)
391 (0.20)
382 (0.38)
338 (0.20)
Microcrystals
1
2
3
4
5
6
7
8
366 (0.10)
356 (0.03)
386 (0.06)
379 (0.16)
372 (0.002)
386 (0.48)
382 (0.56)
336 (0.17)
366 (0.08)
329 (0.04)
395 (0.07)e)
390 (0.12)e)
362 (0.02)
395 (0.51)
396 (0.70)
348 (0.25)
Figure 9 Absorption spectra of 1–8 in cyclohexane.
a) Excitation was effected using UV irradiation at 290 nm for 1–5 and 8,
and at 320 nm for 6 and 7; b) absolute quantum yield (f) was determined
using calibrated integrating sphere system; c) measured at 1  10–5 mol L–1
in cyclohexane; d) dispersed in PMMA; e) neat liquid.
orbital calculations). The absorption edge of the spectrum of
2 also exhibited a bathochromic shift compared to 8. The
absorption band of 2 at 260–350 nm was weaker than those
of 1 and 8, most likely due to the cross-conjugated structure
of 2. The absorption edge of 3 at 366 nm was longer than
those of 2 (345 nm) and 4 (351 nm). No significant red-shift
of the absorption edge was observed for 5 in comparison to
2. This may be due to the cross-conjugated electronic struc-
ture and helical (non-planar) molecular structure of 5, as
revealed by X-ray analysis. In contrast with 1–5 and 8,
dibenzosiloles 6 and 7 exhibited strong absorption bands at
324 and 330 nm, respectively, which were attributed to the
high planarity of the doubly bridged terphenyl skeletons.
Fluorescence spectra of 1–8 were recorded in cyclohex-
ane (Figure 10). The emission maxima and quantum yields
(f) are summarized in Table 1. Thiophene-fused ben-
zosiloles 1–3, and 5 were faintly fluorescent. The emission
maxima of 1–3, and 5 (max = 356–386 nm) appeared at
wavelengths longer than that of 8 (max = 336 nm), as ex-
pected from the absorption spectra, while the quantum
yields ranged from 0.002 to 0.10, which were lower than
that of 8. The fluorescence quenching may be attributed to
the heavy atom effect of sulfur; the sulfur atom is likely to
enhance the probability of intersystem crossing from S1 to
T1 by spin–orbit coupling [50]. Indeed, when photolumi-
nescence spectra of 5 were recorded in strictly degassed
2-methyltetrahydrofuran both at room temperature and 77 K,
intense green phosphorescence was observed at 77 K, as
shown in Figure 11. Considering that the spin-orbit cou-
pling constant of oxygen is smaller than that of sulfur [50],
it is reasonable that the quantum yield (0.16) of 4, an oxy-
gen analog of 3, was higher than that (0.06) of 3. Mean-
while, 6 and 7 showed violet fluorescence (max = 386 and
382 nm), with vibrational structures that were similar in
shape to N-methylcarbazole [51] and 8. The quantum yields
of 6 and 7 were 0.48 and 0.56, respectively, which were
much higher than that of 8 (0.17). This can be rationalized
in terms of the -electron systems of 6 and 7, which are
effectively extended by fusion of indole and benzosilole
moieties, respectively, onto 8, with rigid molecular frame-
works. The fluorescence quantum yield of the
bis(dimethylsilylene) analog of 7 in dichloromethane has
been reported as 0.45 [22]. Hence, the luminescence effi-
ciency of bis(silylene-bridged) terphenyls such as 7 is
somewhat dependent on the alkyl groups of the silylene
bridges.
Fluorescence of 1–8 in doped PMMA thin films and in
microcrystals were determined, and the results are
summarized in Table 1. The spectra of 1, 4, 6, 7 and 8 in the
solid states are shown in Figures 12 and 13. As observed for
fluorescence in cyclohexane, thiophene-fused benzosi-
loles 1–3 and 5 in the solid states were faintly emissive
upon UV irradiation. PMMA films doped with 6 and 7 were
Figure 10 Fluorescence spectra of 1, 4, 6, 7 and 8 in cyclohexane. The
spectra of 2, 3, and 5 are not shown due to the low signal-to-noise (S/N)
ratios originating from the low quantum yields.
Figure 11 Photoluminescence spectra of 5 in 2-methyltetrahydrofuran at
room temperature and 77 K.

1944
Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
Figure 14 HOMO and LUMO energies of 1–8.
Figure 12 Fluorescence spectra of 1, 4, 6, 7 and 8 in PMMA thin film.
The spectra of 2, 3 and 5 are not shown due to the low S/N ratios
originating from the low quantum yields.
paring 1–4 with 8, replacement of a benzene ring in 8 by a
thiophene, benzothiophene, or benzofuran ring raised the
energies of HOMOs and lowered the energies of LUMOs.
Thus, the HOMO-LUMO gaps of 1 (4.41 eV), 2 (4.62 eV),
3 (4.20 eV), and 4 (4.36 eV) were narrower than that of 8
(4.86 eV), and the degree of the gaps was consistent with
the trend observed for the absorption edges. The
HOMO–LUMO gap of 5 was 4.45 eV, which was smaller
than that of 2, but higher than that of 3. These results imply
that fusion of a benzosilole ring onto 2 effectively lengthens
the conjugation via cross-conjugation, but the degree of the
extension is smaller than that resulting from a benzene fu-
sion. The HOMO and LUMO levels of 6 were –5.21 eV and
–0.90 eV, respectively. Hence, fusion of an indole moiety
onto 8 has the effect of significantly raising the HOMO en-
ergy level. The energy levels of the HOMO and LUMO of 7
were –5.31 and –1.11, respectively, which were higher and
lower in comparison to 8; benzosilole fusion of 8 raised the
HOMO and lowered the LUMO.
Figure 13 Fluorescence spectra of 4, 6, 7 and 8 in microcrystals. The
spectra of 1, 2, 3 and 5 are not shown due to the low S/N ratios originating
from the low quantum yields. The photograph is the luminescence image of
7 in microcrystals.
fluorescent with emission maxima similar to those in
cyclohexane, but the quantum yields of the films dropped to
0.20 and 0.38, respectively. In sharp contrast, 6 and 7 in
microcrystals exhibited briliant fluorescence with higher
quantum yields than those in cyclohexane. Thus,
aggregation-induced emission (AIE) was observed with 6
and 7 [52, 53]. Organic solid chromophores whose emission
maxima appear below 400 nm are highly desirable as
emitters for UV OLEDs as well as host materials for triplet
dopant emitters. For example, Wu, Wong and co-workers
[54, 55] developed bi(9,9-diarylfluorene)s as wide-gap
materials, which exhibited efficient fluorescence in a thin
film with emission maxima at 392 nm and high quantum
yields ranging from 0.57 to 0.66. Therefore, it is remarkable
that the absolute quantum yields of 6 and 7 in microcrystals
reached 0.51 with an emission maximum of 395 nm and
0.70 with 396 nm, respectively.
Figure 15 depicts the HOMOs and LUMOs of 1–8. In
general, the LUMOs are developed over the aromatic rings
and the silylene bridge. The silylene bridge participates in the
LUMOs via *–* conjugation, as observed for silylene-
3.4 Molecular orbital calculations
Molecular orbital calculations of 1–8 were performed by the
DFT method at the B3LYP/6-31G*//B3LYP/6-31G* level
using the Gaussian 03 package [48]. The calculated energies
of the HOMOs and LUMOs are shown in Figure 14. Com-
Figure 15 HOMO and LUMO diagrams of 1–8.

Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
1945
127: 7662–7663
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This implies that the electronic structure of 6 is governed by
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bridge does not participate in the extension of the conjuga-
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This work was supported by Grants-in-Aid for Creative Research
(16GS0209) and Scientific Research (22350081), from the Ministry of
Education, Culture, Sports, Science and Technology, Japan. The authors
thank Professor Shinzaburo Ito and Professor Hideo Ohkita of Kyoto Uni-
versity for recording phosphorescence spectra.
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Shimizu M, et al. Sci China Chem December (2011) Vol.54 No.12
1947
SHIMIZU Masaki was born in 1965 in Tokyo, Japan. He received his Ph.D. from the Tokyo In-
stitute of Technology in 1994 under the supervision of Professors Takeshi Nakai and Koichi Mikami.
After working at Mitsubishi Chemical Co. Ltd. as a research associate, he joined the Research Labor-
atory of Resources Utilization, Tokyo Institute of Technology as an Assistant Professor in 1995 to
work with Professor Tamejiro Hiyama. In 1998, he moved to Kyoto University as an Assistant Pro-
fessor to continue collaborations with Professor Hiyama. He spent one year (1999–2000) at Massa-
chusetts Institute of Technology as a postdoctoral fellow (Professor Stephen L. Buchwald) and was
promoted to Associate Professor of Kyoto University in 2003. His research interest focuses on design
and invention of functional -conjugated molecules as well as the development of synthetic method-
ologies directed toward functional organic materials and novel synthetic reagents/reactions/methods
for organofluorine compounds.
HIYAMATamejiro was born in 1946, and started his academic career in 1972 as an Assistant
Professor at Kyoto University (1972–1981); Postdoctoral Research Fellow at Harvard University
(1975–1976); Research Fellow & Group Leader, Senior Research Fellow & Group Leader, and Exec-
utive Research Fellow & Group Leader at Sagami Chemical Research Center (1981–1992); Professor
at Research Laboratory of Resources Utilization, Tokyo Institute of Technology (1992–1998); Pro-
fessor of Organic Materials at Kyoto University (1997–2010). He has received Awards of Jpn. Liq.
Cryst. Soc., 2005, Soc. Synth. Org. Chem. Jpn., 2007, and Chem. Soc. Jpn., 2008. His research inter-
est extends from the invention of novel synthetic methods with organometallic reagents, to organoflu-
orine and organosilicon chemistry, and to the synthesis of biologically active substances as well as
design and synthesis of novel functionality molecules and materials. In particular, Hiyama Coupling, Nozaki-Hiyama-Kishi
Reaction, and Hiyama Reaction are textbook-listed reactions that bear his name. At present, he is serving as the Edi-
tor-in-Chief of Chemistry Letters and as the President of the Japanese Liquid Crystal Society.