Syntheses, optical properties, and theoretical investigation of silafluorenes and spirobisilafluorenes bearing electron-donating aminostyryl arms around a silafluorene core

Article abstract of DOI:10.1002/chem.200901481

π-Extended silafluorenes and spirobisilafluorenes bearing electrondonating aminostyryl substituents at the 2,7- or 3,6-positions were synthesized by a Horner-Wadsworth-Emmons reaction. The electronic influence of spirocyclic structure and substitution mode of the aminostyryl substituents was investigated by UV/Vis spectroscopy, which indicated the existence of a spiroconjugation effect in the 3,6-substituted spirobisilafluorene. They exhibited moderate to strong fluorescence emission, and the fluorescence properties were compatible with the UV/Vis absorption characteristics, except for the 3,6-substituted spirobisilafluorene, which showed relatively large enhancement of fluorescence quantum yield and Stokes shift. The influence of the spirocyclic structure and substitution mode on the photophysical properties of the silicon compounds was investigated by DFT calculations.

Full text of DOI:10.1002/chem.200901481

DOI: 10.1002/chem.200901481
Syntheses, Optical Properties, and Theoretical Investigation of Silafluorenes
and Spirobisilafluorenes Bearing Electron-Donating Aminostyryl Arms
around a Silafluorene Core
Tomohiro Agou,[a, b] Md. Delwar Hossain,[a, c] and Takayuki Kawashima*[a]
Abstract: p-Extended silafluorenes and
spirobisilafluorenes bearing electron-
donating aminostyryl substituents at
the 2,7- or 3,6-positions were synthe-
tence of a spiroconjugation effect in
the 3,6-substituted spirobisilafluorene.
They exhibited moderate to strong
fluorescence emission, and the fluores-
cence properties were compatible with
the UV/Vis absorption characteristics,
except for the 3,6-substituted spirobi-
AHCTUNGTRENNsGUN ilafluorene, which showed relatively
large enhancement of fluorescence
quantum yield and Stokes shift. The in-
fluence of the spirocyclic structure and
substitution mode on the photophysical
properties of the silicon compounds
was investigated by DFT calculations.
sized
by
a
Horner–Wadsworth–
Emmons reaction. The electronic influ-
ence of spirocyclic structure and substi-
tution mode of the aminostyryl sub-
stituents was investigated by UV/Vis
spectroscopy, which indicated the exis-
Keywords: conjugation
functional calculations
·
·
density
fluores-
cence · silicon · spiro compounds
Introduction
toelectronic properties, such as strong light absorption in the
longer wavelength region, intense photoluminescence emis-
sion, and electron or hole transport.
p-Conjugated molecules containing main group elements
(hetero-p-conjugated molecules) are expected to be power-
ful building blocks for optoelectronic materials.[1–3] Electron-
ic interaction between main group elements and p orbitals
on hydrocarbon frameworks raises the HOMO energy level
or lowers the LUMO energy level and decreases HOMO–
LUMO energy gaps.[4] The small HOMO–LUMO energy
gaps provide hetero-p-conjugated molecules with useful op-
Silicon has a special position as a component of organic
functional materials, because incorporation of silicon atoms
into p-conjugated systems can decrease LUMO energy
ꢀ
levels substantially thanks to a strong s*ACTHUNRTGNEUNG(Si C)–p* hyper-
conjugation effect.[5] Its high abundance in the earthꢀs crust
is also advantageous from the point of view of applica-
tions.[6] Especially, silole[1b,5,7] and its benzo-fused derivatives
(benzosilole[8] and dibenzosilole (silafluorene)[9]), are the
most important silicon-containing p-conjugated systems, be-
ꢀ
cause of their improved optoelectronic properties. The Si C
bonds of the silole derivatives are fixed perpendicular to p
[a] Dr. T. Agou, Dr. M. D. Hossain, Prof. Dr. T. Kawashima
Department of Chemistry
ꢀ
orbitals on the ring, and thus s*
(Si C)–p* hyperconjugation
Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5800-6899
E-mail: takayuki@chem.s.u-tokyo.ac.jp
is effective. In addition, silafluorenes have further advantag-
es over the other silole derivatives, including the rigidity and
planarity of the silafluorene rings, synthetic versatility, and
chemical stability, which have stimulated a number of stud-
ies on applying silafluorene to functional materials. The var-
ious silafluorene derivatives that have been synthesized in-
clude ladder-type silafluorenes[9e,10] and polymeric silafluo-
[b] Dr. T. Agou
Present address: Kyoto University Pioneering Research Unit for Next
Generation
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
AHCTUNGTRENNUNG
renes,[9a,d] and their optical and electronic properties have
[c] Dr. M. D. Hossain
been revealed. However, spirobisilafluorene, a three-dimen-
sionally expanded silafluorene derivative, has not attracted
much attention as a component of organic optoelectronic
materials until recently,[11] despite its interesting structural
and electronic characteristics.
Present address: Department of Chemistry and Applied Chemistry
Graduate School of Science and Engineering, Saga University
1 Honjo-machi, Saga-city, Saga 840-8502 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901481.
368
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Chem. Eur. J. 2010, 16, 368 – 375

FULL PAPER
Spirobisilafluorene can be recognized as a hybrid of sila-
fluorene and spirobifluorene.[12] Spirobifluorene has a small
HOMO–LUMO energy gap and good photoluminescence
properties, owing to a spiroconjugation effect that raises its
HOMO and delocalizes p electrons over the two biphen-
pounds 3 and 4. We selected electron-donating aminostyryl
groups as p-conjugated substituents on the silafluorene
rings, because donor–acceptor interactions between them
and the silafluorene cores should provide improved UV/Vis
absorption and fluorescence properties.
ACHTUNGTRENNUNGylene units. With regard to its solid-state properties, its rigid
and pseudotetrahedral structure enhances amorphousness in
the solid state, an important feature for organic thin-film de-
Results and Discussion
ꢀ
vices. The combination of these properties with the s
G
C)*–p* hyperconjugation effect of silafluorene is expected
to yield excellent materials with decreased HOMO–LUMO
energy gaps, and spirobisilafluorene-based conjugated mole-
cules are therefore candidates for organic optoelectronic
materials. For this purpose, the effects of the spiro structure
and the substitution pattern of the p-conjugated systems on
the optoelectronic properties of spirobisilafluorene must be
studied systematically, because this information is essential
for molecular design of spirobisilafluorene derivatives with
desired properties. However, the optical properties of sila-
fluorenes and the corresponding spirobisilafluorenes have
not been directly compared. In addition, almost all silafluo-
Syntheses: 3,6-Substituted silafluorene 1 and spirobisila-
fluorene 2 were synthesized by using the Horner–Wads-
worth–Emmons (HWE) reaction as a key step (Scheme 2).
ACHTUNGTRENNUNGrenes and spirobisilafluorenes have p-conjugated systems at
their 2,7-positions, and thus the effect of the substitution
pattern can not be elucidated.[13] Here we report an investi-
gation of the influence of spirocyclic framework and substi-
tution pattern on the photophysical properties of spirobisila-
fluorene derivatives, by using optical spectroscopy and theo-
retical calculations.
Scheme 2. Synthesis of 3,6-substituted silafluorene 1 and spirobisilafluo-
AHCTUNGTRENGNrUN ene 2.
Our silafluorenes and spirobisilafluorenes designed for
systematically studying the optical properties of spirobisila-
fluorenes are shown in Scheme 1. By comparison of the op-
tical properties of silafluorenes 1 and 3 with those of spiro-
bisilafluorenes 2 and 4, respectively, the effect of the spiro-
cyclic framework on their photophysical properties can be
revealed. The electronic effect of the substitution pattern of
the p systems can be clarified by comparisons between 3,6-
substituted compounds 1 and 2 and 2,7-substituted com-
Starting from 2,2’-dibromo-5,5’-diiodobiphenyl (5), iodine–
magnesium exchange reaction followed by treatment with
DMF gave 2,2’-dibromo-5,5’-diformylbiphenyl (6).[14] The
HWE reaction of 6 with phosphonate 7 in THF afforded 8,
a precursor of the 3,6-substituted compounds, as a cis/trans
1
mixture (>95% trans, determined by H NMR spectrosco-
py), and iodine-catalyzed isomerization of the crude materi-
al in CH2Cl2 provided pure 8 (all-trans form) in 49% yield.
Treatment of 8 with nBuLi and Me2SiCl2 in THF produced
silafluorene 1 in 50% yield as a light yellow-green solid
after column chromatography on silica gel. Spirobisilafluo-
AHCTUNGTREGrNNUN ene 2 was obtained from 8 under the same conditions.
After addition of SiCl4 to a yellow solution of the dilithio
derivative of 8, the mixture was heated to reflux to give 2 as
a yellow solid (60%). Compounds 1 and 2 are highly soluble
in common organic solvents, and 2 even dissolves in hexane.
Its spirocyclic structure may inhibit intermolecular interac-
tions in the solid state and enhance amorphousness and sol-
ubility.
2,7-Substituted silafluorene 3 and spirobisilafluorene 4
were synthesized by a similar procedure (Scheme 3). Treat-
ment of 12 with nBuLi followed by reaction with Me2SiCl2
and SiCl4 gave silafluorene 3 and spirobisilafluorene 4, re-
spectively. Both silicon compounds were obtained as deep
yellow solids, and their solubility was lower than those of 1
and 2, in spite of the longer alkyl chains on the nitrogen
atoms.
Scheme 1. 3,6-Substituted silafluorene 1, 3,6-substituted spirobisilafluo-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Chem. Eur. J. 2010, 16, 368 – 375
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
369

T. Kawashima et al.
Scheme 3. Synthesis of 2,7-substituted silafluorene 3 and spirobisilafluo-
rene 4.
ACHTUNGTRENNUNG
UV/Vis absorption characteristics: UV/Vis absorption spec-
tra of silicon compounds 1–4 were measured in cyclohexane
or CH2Cl2. The spectra are shown in Figure 1, and the opti-
cal data are summarized in Table 1.
In cyclohexane (Figure 1a), spirobisilafluorenes 2 and 4
exhibit a small bathochromic shift of the longest wavelength
absorption peaks compared to those of silafluorenes 1 and
3, respectively, that is, the spirobisilafluorenes have smaller
HOMO–LUMO energy gaps.
Figure 1. UV/Vis absorption spectra of 1–4 (black line: 1, double line: 2,
gray line: 3, dashed line: 4). a) In cyclohexane. b) In CH2Cl2.
This may be ascribed to two ef-
fects: 1) The different types of
Table 1. Optical data of 1–4.
In cyclohexane
In CH2Cl2
lem
substituents on the silicon
atoms of the silafluorenes (one
biarylene ligand and two
methyl groups) and the spirobi-
silafluorenes (two biarylene li-
ꢀ
lmax
e
lem
F[a] Stokes shift[b] lmax
[103 cmꢀ1 [nm] [105 mꢀ1 cmꢀ1
e
F[a] Stokes shift[b]
[103 cmꢀ1
[nm] [105 mꢀ1 cmꢀ1
]
[nm]
G
]
]
[nm]
N
]
1
2
3
4
365
380
410
416
0.63
1.43
0.98
1.74
406, 428 0.16 2.8
441 0.21 3.6
450, 476 0.74 2.2
458, 485 0.74 2.2
376
395
421
431
0.76
2.37
0.92
2.56
464
492
503
523
0.16 5.0
0.28 6.1
0.76 3.8
0.76 4.0
gands) influences the sACTHNUTRGNEU(GN Si C)*–
[a] Absolute quantum yields were recorded on a Hamamatsu Photonics Absolute Quantum Yield Determina-
p* hyperconjugation effect and
thus changes the LUMO
energy levels. 2) Spiroconjuga-
tion between the two biarylene
tion System C9920-02. [b] Stokes shift=1/lmaxꢀ1/lem
.
ligands of the spirobisilafluorenes elevates the HOMO
energy levels. Both effects would contribute to smaller
HOMO–LUMO energy gaps of spirobisilafluorene 2 or 4
ꢀ
tion maxima of the 2,7-substituted compounds (3 and 4) ex-
hibited large bathochromic shifts indicating decreased
HOMO–LUMO energy gaps of the latter compounds. In
the case of the 2,7-substituted compounds, the p orbitals on
the biarylene ligands are more efficiently delocalized over
the aminostyryl substituents and the central biphenyl moiet-
ies, because the 4,4’-biphenylene linkage in 3 and 4 is a
better p connecting unit than the 3,3’-biphenylene linkage in
1 and 2.
compared with 1 or 3, respectively. Therefore, sACTHNUTRGNEUG(N Si C)*–p*
hyperconjugation and spiroconjugation effects are thought
to work synergistically and reduce the HOMO–LUMO
energy gap, as anticipated in the Introduction, and this was
further investigated theoretically (vide infra).
Changing the solvent from cyclohexane to CH2Cl2 had
little effect on the absorption wavelengths (Figure 1b,
Table 1). Such small solvatochromic shifts indicate that the
absorptions can be attributed to normal p–p*-type excita-
tions rather than intramolecular charge transfer from the pe-
ripheral nitrogen atoms to the silafluorene cores.
Fluorescence properties: Fluorescence spectra of 1–4 were
recorded in cyclohexane or CH2Cl2 (Figure 2), and the data
are included in Table 1.
In cyclohexane, the fluorescence quantum yields of 3,6-
substituted compounds 1 and 2 are smaller than those of
2,7-substituted compounds 3 and 4. The larger Stokes shifts
As for the influence of the substitution pattern, compared
with the 3,6-substituted compounds (1 and 2), the absorp-
370
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Chem. Eur. J. 2010, 16, 368 – 375

Silafluorenes and Spirobisilafluorenes
FULL PAPER
fluorene 1 in addition to the change in fluorescence band
shape.
In CH2Cl2, the fluorescence spectra of 1–4 completely lost
their vibrational structure, and the emission maxima were
redshifted compared to fluorescence in cyclohexane. This in-
dicates a more polar nature of the emissive states compared
to the Frank–Condon states. The emission maxima of 3,6-
substituted compounds 1 and 2 showed larger solvatochro-
mic shifts than those of 3 and 4, and thus 1 and 2 have much
more polarized emissive states. The fluorescence quantum
yields of compounds 1–4 in CH2Cl2, however, were almost
the same as those in cyclohexane.
Theoretical calculations: To gain further insight into the op-
toelectronic properties of the silicon compounds, DFT calcu-
lations were performed at the B3LYP/6-31G(d) level of
theory.[16] All calculations were carried out with the Gaussi-
an 03 program package,[17] and Kohn–Sham orbitals were
visACHTNUTGRNEUNG
ualized with GaussView 4.1.[18] To reduce the computa-
tional burden, model compounds 1’–4’ (Scheme 4), which
Figure 2. Fluorescence spectra of 1–4 (black line: 1, double line: 2, gray
line: 3, dashed line: 4). a) In cyclohexane. b) In CH2Cl2.
in 1 and 2 mean that the structural change from the Frank–
Condon states to the emissive states is considerable in 1 and
2. Because their aminostyryl substituents are not strongly
fixed in planarity due to the weak p conjugation through
the 3,3’-biarylene linkage, conformational change in the ex-
cited states can easily occur. The many degrees of freedom
of internal rotations and vibrations also dissipate the excita-
tion energy via nonradiative paths, and this results in low
fluorescence quantum yields.
Scheme 4. Model compounds 1’–4’ employed in theoretical calculations.
On the other hand, the improved fluorescence quantum
yields in 3 and 4 may be the result of expansion of p conju-
gation over the biarylene ligands, confirmed by UV/Vis
spectroscopy. In general, broadening of p-conjugated sys-
tems is related to an increase in the transition dipole
moment between the ground state and the excited states,
which results in enhancement of fluorescence quantum
yield.[15] Extension of the p systems also improves the mo-
lecular planarity in both ground and excited states and pro-
hibits structural relaxation to some extent, and thus a de-
crease in the Stokes shifts results.
In the case of the 2,7-substituted compounds, the effect of
the spirocyclic structure on the fluorescence emission is very
small, since the fluorescence properties of 3 and 4 are very
similar, except for the small redshift of the emission maxima
of 4. On the other hand, 3,6-substituted spirobisilafluorene 2
exhibited a slight improvement in fluorescence quantum
yield and larger Stokes shift compared to corresponding sila-
have NH2 groups instead of the NR2 groups in the real mol-
ecules, were employed, and molecular symmetry was im-
posed in all calculations (1’: C2, 2’: D2, 3’: C2, 4’: S4).
Energy diagrams of frontier orbitals are shown in
Figure 3. For silafluorenes 1’ and 3’, only HOMO and
LUMO orbital energy levels are shown. For spirobisilafluo-
A
also shown in addition to those of the HOMO and LUMO.
In the case of 2’, LUMO and LUMO+1 are degenerate,
but the energy gap between HOMO and HOMOꢀ1 is small
(DE=0.11 eV). On the contrary, not only LUMOs but also
the HOMOs of 4’ are degenerate. The origin of this differ-
ence is discussed below.
Corresponding to the bathochromic shift of the absorp-
tion maximum, the HOMO–LUMO energy gap of spirobi-
AHCTUNGTRENNUNG
Chem. Eur. J. 2010, 16, 368 – 375
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371

T. Kawashima et al.
lower LUMOs, compatible with the observed bathochromic
shifts in the UV/Vis spectra. This is due to the difference in
p-orbital delocalization between the 3,6- and 2,7-substituted
biarylene ligands. The latter system enhances p conjugation,
since the 4,4’-biarylene linkage is a better p spacer than the
3,3’-biarlylene spacer. On the other hand, considering the
difference in electronic structure of 3’ and 4’, 4’ hardly bene-
fits from its spirocyclic structure, unlike 2’. The HOMO and
HOMOꢀ1 of 4’ are degenerate, unlike those of 2’, that is,
spiroconjugation is not effective in the 2,7-substituted spiro-
bisilafluorene. Because the optimized geometries around the
central silicon atoms of 2’ and 4’ are almost the same, geo-
metrical reasons, such as the distance between the two biar-
ylene ligands, can not explain the weak spiroconjugation in
4’. Therefore, the difference in the electronic structure
should be the main reason. The HOMO of 1’ (a model for
half of 2’) is fairly distributed over carbon atoms C8a and
C9a. Accordingly, in 2’, the spiroconjugation is effective due
to efficient orbital overlap of the two p orbitals on the biar-
ylene ligands. In contrast, the HOMO of 3’ (a model for half
of 4’) has smaller lobes on C8a and C9a, which result in de-
creased spiroconjugation efficiency in 4’.
Figure 3. Energy diagram of frontier orbitals of 1’-4’.
the LUMO may be due to the different substituents on the
silicon atoms of 1’ and 2’, which affect the extent of s
C)*–p* hyperconjugation. The rise of the HOMO of 2’ may
originate from spiroconjugation between the highest occu-
pied p orbitals on the two biarylene ligands, as revealed by
the plots of the HOMO and HOMOꢀ1 of 2’ (Figure 4). In
ꢀ
G
The results of the TDDFT calculations are summarized in
Table 2. The calculated absorption wavelengths agreed mod-
erately well with the experimental ones (Table 1). Because
Table 2. Results of the TDDFT calculations.[a]
l [nm]
f[b]
Excited state
!
1’
2’
388
372
0.4595
0.1877
LUMO HOMO
!
LUMO HOMOꢀ1,
!
LUMO+1 HOMO
!
408
408
391
391
387
0.2361
0.2385
0.2369
0.2342
1.1453
LUMO HOMO
!
LUMO+1 HOMO
!
LUMO HOMOꢀ1
!
LUMO+1 HOMOꢀ1
!
Figure 4. Kohn–Sham orbitals of 2’. a) HOMO. b) HOMOꢀ1. Hydrogen
LUMO+1 HOMOꢀ2,
!
atoms are not shown.
LUMO HOMOꢀ3
!
3’
4’
437
453
2.0684
0.0892
LUMO HOMO
!
LUMO HOMOꢀ1,
!
LUMO+1 HOMOꢀ1,
the HOMO, two p orbitals on the two biarylene ligands in-
teract with each other in an antibonding fashion, that is, the
phases of the two p orbitals are not matched. On the other
hand, the two p orbitals interact with each other in a bond-
ing fashion in the HOMOꢀ1, judging from the phases of the
two orbitals. These findings mean that the p orbitals on the
two biarylene ligands are mixed to form two delocalized p
orbitals, HOMO and HOMOꢀ1. Although the splitting be-
tween HOMO and HOMOꢀ1 is not so large (DE=0.11 eV)
as that of spirobifluorene (DE=0.27 eV, calculated at the
B3LYP/6-31G(d) level of theory), such orbital interaction
clearly indicates the existence of spiroconjugation in 2’,
which results in an increase in the HOMO energy level com-
pared with 1’. The weak spiroconjugation in 2’ should origi-
nate from long bonds between silicon and carbon atoms, be-
cause the overlap of the two p orbitals is affected by the dis-
tance between them.
!
(2 excitations)
443
LUMO HOMO,
!
LUMO+1 HOMO
!
1.7616
LUMO HOMOꢀ1,
!
LUMO+1 HOMOꢀ1,
!
(2 excitations)
LUMO HOMO,
!
LUMO+1 HOMO
[a] The TDDFT calculations were performed at the B3LYP/6-31G(d)
level of theory. Only singlet excited states were calculated. [b] Oscillator
strength.
4’ has degenerate HOMOs and LUMOs, the TDDFT calcu-
lation predicted two sets of degenerate excitations. All cal-
culated excitation energies were smaller than the experi-
mental values by about 103 cmꢀ1, probably because the cal-
culations were performed on the optimized geometries with
completely planar p systems, and hence p conjugation was
overestimated. The calculations reproduced the redshifts of
the longest absorption maxima of 2,7-substituted compounds
3 and 4 compared to 3,6-subsituted compounds 1 and 2.
Compared with 3,6-substituted compounds 1’ and 2’, 2,7-
substituted counterparts 3’ and 4’ have higher HOMOs and
372
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Chem. Eur. J. 2010, 16, 368 – 375

Silafluorenes and Spirobisilafluorenes
FULL PAPER
ular sieve columns before use, and other solvents were purified by
common methods. Column chromatography was carried out with Kanto
Silica Gel 60N. Gel permeation liquid chromatography (GPLC) was per-
formed on Japan Analytical Industry LC-918 and LC-908 with JAIGEL
1H+2H columns and chloroform as a solvent. 1H and 13C NMR spectra
were recorded on a Bruker DRX-500 spectrometer, and 29Si NMR spec-
tra were recorded on a JEOL AL-400 spectrometer. Tetramethylsilane
was used as external standard for 1H, 13C, and 29Si NMR spectroscopy.
Low-resolution mass spectra were measured with a JEOL JMS-700P
mass spectrometer (FAB positive) with m-nitrobenzyl alcohol or 2-nitro-
phenyl octyl ether as matrix. High-resolution mass spectra were recorded
on a JEOL JMS-700P mass spectrometer (FAB positive) by using m-ni-
trobenzyl alcohol or 2-nitrophenyl octyl ether as matrix and PEG-600 or
Ultramark as external standard. UV/Vis spectra were recorded on a
JASCO V-670 spectrophotometer, and fluorescence spectra were mea-
sured with a JASCO FP-6500 fluorescence spectrophotometer. Fluores-
cence quantum yield was determined by a conventional comparison
method with a JASCO FP-6500 fluorescence spectrophotometer or an
absolute method with a Hamamatsu Photonics Absolute Quantum Yield
Measurement System C9920-02 equipped with an integral sphere. All
melting points were determined on a Yanaco MP-S3 micro melting point
apparatus and are uncorrected. Elemental analyses were performed by
the Microanalytical Laboratory of Department of Chemistry, Faculty of
Science, the University of Tokyo. 2,2’-Dibromo-5,5’-diiodobiphenyl (5),[13]
diethyl 4-(diethylamino)benzylphosphonate (7),[19] diethyl 4-(di-n-hexyla-
mino)benzylphosphonate (11),[20] and 2,2’-dibromo-4,4’-diiodobiphenyl
(9)[21] were prepared according to literature procedures.
More importantly, the calculations confirmed that spirobi-
ACHTUNGTRENNUNGsilafluorenes 2’ and 4’ have longer absorption wavelengths
(2’: 408 nm, 4’: 453 nm) than silafluorenes 1’ and 3’ (1’:
388 nm, 3’: 437 nm), respectively, which reveals the influence
of the spirocyclic structure on the UV/Vis absorption prop-
erties. The strongest optical transitions of 2,7-substituted
compounds 3’ (437 nm) and 4’ (443 nm) have higher oscilla-
tor strengths than those of 3,6-substituted compounds 1’
(388 nm) and 2’ (408 nm), in accordance with the higher
fluorescence quantum yields of the former pair. Especially,
4’ has several allowed optical transitions around 450 nm,
and this may be the reason for its highly intense light ab-
sorption.
In summary, 2,7-substituted compounds 3’ and 4’ have
smaller HOMO–LUMO energy gaps and redshifted absorp-
tion maxima compared to 3,6-substituted compounds 1’ and
2’, which is comprehensible by considering the difference in
p-conjugation efficiency between the 4,4’-biarylene units of
3’ and 4’ and the 3,3’-biarylene units of 1’ and 2’. However,
the theoretical calculations also showed that spiroconjuga-
tion does not contribute to the electronic properties of 4’
very much, unlike 2’, and the two biaryl units of 4’ may be
described as independent of each other rather than electron-
ically coupled. On the other hand, 2’ was shown to have spi-
roconjugation between the two biaryl units in addition to
ꢀ
2,2’-Dibromo-5,5’-diformylbiphenyl (6): At 08C, iPrMgCl (2.0m in THF,
32 mL, 64 mmol) was added to a solution of 5 (15 g, 26 mmol) in THF
(400 mL). After the mixture had been stirred for 20 min, DMF (5.0 mL,
65 mmol) was added slowly to the solution, and the mixture was allowed
to warm to room temperature. After stirring for 12 h, the reaction was
quenched with aqueous NH4Cl. The mixture was extracted with dichloro-
methane, and the organic layer was dried over anhydrous MgSO4. After
filtration, the solvents were removed under reduced pressure. The crude
product was recrystallized from ethanol to give 7 as a colorless solid
(5.7 g, 59%). M.p. 151–1528C; 1H NMR (500 MHz, CDCl3, 258C, TMS):
d=10.04 (s, 2H), 7.88 (d, J=8.2 Hz, 2H), 7.80 (dd, J=8.2, 2.0 Hz, 2H),
7.75 ppm (d, J=2.0 Hz, 2H); 13C NMR (126 MHz, CDCl3, 258C, TMS):
d=190.7, 141.8, 135.4, 133.8, 131.7, 130.5, 130.3 ppm; elemental analysis
calcd (%) for C14H8Br2O2: C 45.69, H 2.19; found: C 45.46, H 2.29.
s*ACHTUNGTRENNUNG(Si C)–p* hyperconjugation, and thus this substitution
pattern can be useful for three-dimensional delocalization of
p electrons through these two conjugation effects.
Conclusions
We have synthesized silafluorenes and spirobisilafluorenes
bearing electron-donating aminostyryl groups at the periph-
ery using the Horner–Wadsworth–Emmons reaction. Incor-
poration of the spirocyclic structure bathochromically shift-
ed the UV/Vis absorption maxima because of the synergetic
ꢀ
2,2’-Dibromo-5,5’-bis[4-(diethylamino)styryl]biphenyl (8): At ꢀ788C,
tBuOK (5.4 g, 48 mmol) was slowly added to a mixture of 6 (3.4 g,
9.3 mmol), 7 (5.6 g, 19 mmol), and THF (300 mL), and the mixture was
stirred for 12 h at room temperature. The solvent was removed under re-
duced pressure, and water was added to the residue. The mixture was ex-
tracted with dichloromethane, and the organic layer was dried over anhy-
drous MgSO4 and filtered. After removal of the solvent under reduced
pressure, the crude mixture was subjected to column chromatography (di-
chloromethane/hexane=1/1) to give a light greenish solid containing a
trace amount of cis isomers (confirmed by 1H NMR spectroscopy). This
mixture was stirred with iodine (0.35 g, 1.4 mmol) in toluene (20 mL) and
dichloromethane (20 mL) at room temperature for 24 h. The reaction
was quenched with aqueous Na2SO3. The aqueous layer was extracted
with dichloromethane, and the combined organic layer was dried over
anhydrous MgSO4 and filtered. The solvents were removed under re-
duced pressure to give 8 as a yellow solid (3.0 g, 49%). M.p. 184–1858C;
1H NMR (500 MHz, CDCl3, 258C, TMS): d=7.60 (d, J=8.3 Hz, 2H),
7.35 (m, 8H), 7.05 (d, J=16.2 Hz, 2H), 6.84 (d, J=16.2 Hz, 2H), 6.65 (d,
J=8.7 Hz, 4H), 3.40 (q, J=6.9 Hz, 8H), 1.20 ppm (t, J=6.9 Hz, 12H);
13C NMR (126 MHz, CDCl3, 258C, TMS): d=147.6, 142.2, 137.6, 132.6,
130.0, 128.2, 128.0, 126.6, 124.2, 122.0, 120.9, 111.6, 44.4, 12.6 ppm; ele-
mental analysis calcd (%) for C36H38Br2N2: C 65.66, H 5.82, N 4.25;
found: C 65.39, H 5.85, N, 3.99.
effect of sACHTUNGTRENNUNG(Si C)*–p* hyperconjugation and spiroconjuga-
tion, confirmed by theoretical calculations. The substitution
mode of the aminostyryl groups also affects optical proper-
ties, reflecting the efficiency of p conjugation. The silicon
compounds exhibit moderate to strong fluorescence in the
blue to green region, and the fluorescence properties were
also affected by both the spirocyclic structure and the substi-
tution pattern of the aminostyryl groups. Theoretical calcu-
lations on the silicon compounds revealed the influence of
spiroconjugation around the central silicon atoms on their
electronic states and photoexcitation behavior.
Experimental Section
General remarks: Commercial chemicals were used as received. Spectro-
chemical-grade dichloromethane (Dojindo) was used for optical measure-
ments. All manipulations were performed under argon atmosphere using
standard Schlenck techniques. Dehydrated solvents (THF and Et2O)
were purchased from Kanto Chemicals and further purified with an
MBRAUN MB-SPS system equipped with activated alumina and molec-
2,2’-Dibromo-4,4’-diformylbiphenyl (10): At 08C, iPrMgCl (2.0m in THF,
8.5 mL, 17 mmol) was added to a solution of 9 (4.6 g, 8.2 mmol) in THF
(100 mL). After the mixture had been stirred for 1 h, DMF (1.5 mL,
20 mmol) was added slowly to the reaction mixture. The solution was
Chem. Eur. J. 2010, 16, 368 – 375
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
373

T. Kawashima et al.
gradually warmed to room temperature, and after stirring for 1 h, the re-
action was quenched with aqueous NH4Cl. The mixture was extracted
with dichloromethane, and the organic layer was dried over anhydrous
MgSO4 and filtered. The solvents were removed under reduced pressure
to give 10 as a colorless solid (3.0 g, 100%), and the crude material was
used for the next step directly. M.p. 171–1738C; 1H NMR (500 MHz,
CDCl3, 258C, TMS): d = 10.03 (s, 2H), 8.19 (d, J=1.5 Hz, 2H), 7.91 (dd,
J=7.8, 1.5 Hz, 2H), 7.40 ppm (d, J=7.8 Hz, 2H); 13C NMR (126 MHz,
CDCl3, 258C, TMS): d=190.2, 146.6, 137.5, 133.8, 131.2, 128.3,
123.9 ppm; HRMS (FAB+): m/z calcd for C14H879Br81BrO2: 367.8871,
found: 367.8850.
lution of 12 (0.12 g, 0.13 mmol) in THF (30 mL). The resulting mixture
was stirred at this temperature for 30 min, and a solution of Me2SiCl2
(20 mL, 0.14 mmol) in THF (10 mL) was slowly added to the resulting
mixture. The mixture was allowed to warm to room temperature and
stirred for 20 h. The reaction was quenched with water. The aqueous
layer was extracted with dichloromethane, and the organic layer was
dried over anhydrous Mg2SO4 and filtered. The solvents were removed
under reduced pressure, and the residue subjected to column chromatog-
raphy (dichloromethane/hexane=3/7) to give 3 as yellow-green solid
(80 mg, 77%). M.p. 112–1138C; 1H NMR (500 MHz, CDCl3, 258C,
TMS): d=7.74 (d, J=7.5 Hz, 2H), 7.73 (s, 2H), 7.52 (d, J=7.5 Hz, 2H),
7.39 (d, J=8.6 Hz, 4H), 7.07 (d, J=16.2 Hz, 2H), 6.91 (d, J=16.2 Hz,
2H), 6.62 (d, J=8.6 Hz, 4H), 3.28 (t, J=7.5 Hz, 8H), 1.58 (br, 8H), 1.32
(br, 24H), 0.89 (t, J=6.9 Hz, 12H), 0.46 ppm (s, 6H); 13C NMR
(126 MHz, CDCl3, 258C, TMS): d=147.7, 146.2, 139.3, 137.0, 130.1,
128.3, 128.0, 127.7, 124.6, 123.6, 120.9, 111.5, 51.0, 31.7, 27.3, 26.8, 22.7,
14.1, ꢀ3.1 ppm; HRMS (FAB+) m/z calcd for C54H76N2Si 780.5778,
found: 780.5761.
2,2’-Dibromo-4,4’-bis[4-(di-n-hexylamino)styryl]biphenyl (12): At ꢀ788C,
tBuOK (2.7 g, 24 mmol) was slowly added to a mixture of 10 (3.0 g,
8.2 mmol), 11 (7.0 g, 17 mmol), and THF (100 mL), and the mixture was
warmed to room temperature and stirred for 48 h. The solvent was re-
moved under reduced pressure, and water added to the residue. The mix-
ture was extracted with dichloromethane, and the organic layer dried
over anhydrous MgSO4 and filtered. After removal of the solvent under
reduced pressure, the crude mixture was recrystallized from hexane to
give 12 as a yellow-green solid (5.6 g, 78%). M.p. 71–728C; 1H NMR
(500 MHz, CDCl3, 258C, TMS): d=7.83 (d, J=1.4 Hz, 2H), 7.50 (d, J=
8.0 Hz, 2H), 7.45 (d, J=8.8 Hz, 4H), 7.26 (dd, J=8.0, 1.4 Hz, 2H), 7.14
(d, J=16.7 Hz, 2H), 6.89 (d, J=16.7 Hz, 2H), 6.69 (d, J=8.8 Hz, 4H),
3.35 (t, J=7.4 Hz, 8H), 1.66 (br, 8H), 1.39 (br, 24H), 0.97 ppm (t, J=
6.8 Hz, 12H); 13C NMR (126 MHz, CDCl3, 258C, TMS): d=148.0, 139.9,
139.6, 131.2, 130.6, 129.5, 128.0, 124.5, 123.9, 123.8, 121.4, 111.5, 51.1,
31.7, 27.3, 26.8, 22.7, 14.1 ppm; HRMS (FAB+): m/z calcd for
C52H7079Br81BrN2 882.3885, found: 882.3862.
9,9’-Spirobi
(2,7-bis{4-[di
E
(4):
At
ꢀ788C, nBuLi (1.57m in hexane, 0.5 mL, 0.79 mmol) was added dropwise
to a solution of 12 (0.35 g, 0.39 mmol) in Et2O (50 mL). The solution was
allowed to warm to room temperature and stirred for 6 h. A solution of
SiCl4 (23 mL, 0.20 mmol) in Et2O (10 mL) was added slowly to the result-
ing solution at room temperature. The reaction mixture was stirred at
room temperature for 1.5 h and heated to reflux for 24 h. The reaction
was quenched with water, and the aqueous layer was extracted with di-
chloromethane. The organic layer was dried over anhydrous MgSO4 and
filtered, and the organic solvents were removed under reduced pressure.
The crude products were subjected to GPLC to give 4 as a yellow solid
3,6-Bis[4-(diethylamino)styryl]-9,9-dimethylsilafluorene
(1):
nBuLi
1
(0.21 g, 72%). M.p. 102–1038C; H NMR (500 MHz, CDCl3, 258C, TMS):
(1.53m in hexane, 0.34 mL, 0.52 mmol) was added at ꢀ788C to a solution
of 8 (0.17 g, 0.26 mmol) in THF (20 mL), and the resulting solution was
stirred for 30 min at room temperature. A solution of Me2SiCl2 (33 mL,
0.27 mmol) in THF (10 mL) was added to the solution, and the mixture
was stirred for 20 h. The reaction was quenched with water. The aqueous
layer was extracted with dichloromethane, and the organic layer dried
over anhydrous MgSO4 and filtered. After removal of the solvents under
reduced pressure, the residue was separated by column chromatography
(hexane/EtOAc=10/1) to give 1 as a light yellow-green solid (85 mg,
50%). M.p. 101–1028C; 1H NMR (500 MHz, CDCl3, 258C, TMS): d=
8.00 (s, 2H), 7.59 (d, J=7.3 Hz, 2H), 7.47 (d, J=8.5 Hz, 4H), 7.42 (d, J=
7.3 Hz, 2H), 7.23 (d, J=16.1 Hz, 2H), 7.04 (d, J=16.1 Hz, 2H), 6.73 (d,
J=8.5 Hz, 4H), 3.44 (q, J=6.6 Hz, 8H), 1.23 (t, J=6.6 Hz, 12H),
0.46 ppm (s, 6H); 13C NMR (126 MHz, CDCl3, 258C, TMS): d=148.2,
147.4, 140.2, 137.5, 132.8, 129.3, 127.9, 125.1, 124.7, 124.0, 118.3, 111.7,
44.4, 12.6, ꢀ2.7 ppm; HRMS (FAB+): m/z calcd for C38H44N2Si:
556.3274, found: 556.3251; elemental analysis calcd (%) for C38H44N2Si:
C 81.96, H 7.96, N 5.03; found: C 81.76, H 7.97, N 4.84.
d=7.88 (d, J=8.1 Hz, 4H), 7.61 (s, 4H), 7.58 (d, J=8.1 Hz, 4H), 7.28 (d,
J=8.8 Hz, 8H), 6.97 (d, J=16.2 Hz, 4H), 6.79 (d, J=16.2 Hz, 4H), 6.55
(d, J=8.8 Hz, 8H) 3.23 (t, J=7.2 Hz, 16H), 1.54 (br, 16H), 1.29 (br,
48H), 0.87 ppm (t, J=6.4 Hz, 24H); 13C NMR (126 MHz, CDCl3, 258C,
TMS): d=148.2, 147.6, 137.7, 133.3, 131.6, 129.3, 128.8, 127.7, 124.5,
123.0, 121.1, 111.4, 51.0, 31.7, 27.2, 26.8, 22.7, 14.0 ppm; HRMS (FAB+)
m/z calcd for C104H140N4Si: 1474.0879, found: 1474.0802.
Acknowledgements
This work was partially supported by a Grants-in-Aid for the Global
COE Program for Chemistry Innovation and Scientific Researches from
MEXT and JSPS. We thank Tosoh Finechem Corp. and Shin-etsu Chemi-
cal Co. Ltd. for generous gifts of alkyllithiums and silicon reagents, re-
spectively.
9,9’-SpirobiACHTUNGTRENNUNG{3,6-bis[4-(diethylamino)styryl]silafluorene} (2): nBuLi (1.53m
in hexane, 0.68 mL, 1.0 mmol) was added to a suspension of 8 (0.34 g,
0.52 mmol) in Et2O (20 mL) at ꢀ788C, and the mixture was stirred for
6 h at room temperature. A solution of SiCl4 (30 mL, 0.26 mmol) in Et2O
(10 mL) was added to the solution, and the reaction mixture was stirred
for 1.5 h at room temperature and then heated to reflux for 12 h. The re-
action was quenched with water. The aqueous layer was extracted with
dichloromethane, and the organic layer was dried over anhydrous MgSO4
and filtered. The solvents were removed under reduced pressure, and the
crude material was separated by column chromatography (hexane/
EtOAc=10/1) to give 2 as a light yellow solid (0.16 g, 60%). M.p. 192–
1938C; 1H NMR (500 MHz, CDCl3, 258C, TMS): d=8.15 (s, 4H), 7.25–
7.50 (m, 20H), 7.02 (d, J=16.3 Hz, 4H), 6.70 (d, J=8.6 Hz, 8H), 3.40 (q,
J=6.6 Hz, 16H), 1.20 ppm (t, J=6.6 Hz, 24H); 13C NMR (126 MHz,
CDCl3, 258C, TMS): d=150.3, 148.0, 141.3, 134.2, 131.4, 130.0, 128.0,
126.0, 124.1, 123.2, 118.1, 118.5, 44.3, 12.4 ppm; 29Si NMR (99 MHz,
CDCl3, 258C, TMS): d=ꢀ8.34 ppm; HRMS (FAB+) m/z calcd for
C72H76N4Si: 1024.5839, found: 1024.5870; elemental analysis calcd (%)
for C72H76SiN4: C 84.33, H 7.47, N 5.46; found: C 83.69, H 7.73, N, 5.33.
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374
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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