Synthesis and photophysical properties of asymmetric substituted silafluorenes

Article abstract of DOI:10.1021/om300891n

Several 1,3-diphenyl-substituted silafluorene compounds were synthesized and characterized as potential fluorescent materials for OLED fabrication and bioimaging. Introducing phenyl groups into the silafluorene ring at the 1- and 3-positions led to a red shift in the emission, resulting in blue light emitting compounds (λ_max 368-375 nm in solution; λ _max 362-371 and 482 nm in the solid state), and improved the quantum yield efficiency both in solution and as solids. Aggregation enhanced emission of the silafluorenes (AEE) was also investigated. Theoretical MO calculations were carried out to aid in understanding the optical properties of these molecules. Since these compounds might be useful in bioimaging, their toxicity was also investigated in skin fibroblast cells. All compounds were found to be nontoxic to the investigated cell cultures.

Full text of DOI:10.1021/om300891n

Article
pubs.acs.org/Organometallics
Synthesis and Photophysical Properties of Asymmetric Substituted
Silafluorenes
Erika Pusztai,† Irina S. Toulokhonova,† Nicole Temple,† Haley Albright,† Uzma I. Zakai,† Song Guo,‡
Ilia A. Guzei,† Rongrong Hu,§ and Robert West†,*
†Organosilicon Research Center, Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison,
Wisconsin 53706, United States
‡Carbone Cancer Center, Wisconsin Institutes for Medical Research, 1111 Highland Avenue, Madison, Wisconsin 53705, United
States
§Department of Chemistry and State Key Laboratory of Molecular Neuroscience, Institute of Molecular Functional Materials, The
Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
S
* Supporting Information
ABSTRACT: Several 1,3-diphenyl-substituted silafluorene
compounds were synthesized and characterized as potential
fluorescent materials for OLED fabrication and bioimaging.
Introducing phenyl groups into the silafluorene ring at the 1-
and 3-positions led to a red shift in the emission, resulting in
blue light emitting compounds (λmax 368−375 nm in solution;
λmax 362−371 and 482 nm in the solid state), and improved
the quantum yield efficiency both in solution and as solids.
Aggregation enhanced emission of the silafluorenes (AEE) was
also investigated. Theoretical MO calculations were carried out
to aid in understanding the optical properties of these molecules. Since these compounds might be useful in bioimaging, their
toxicity was also investigated in skin fibroblast cells. All compounds were found to be nontoxic to the investigated cell cultures.
We were interested in synthesizing new blue light emitting
silafluorenes. With molecular modification, the electronic and
optical properties of π-conjugated compounds can be tuned;2
thus, attaching conjugating substituents to the symmetric
silafluorene ring can lead to red shifting, driving the emission
maximum from the UV into the blue region.3 Attaching
different groups to the silicon atom in the silafluorene ring also
influences the fluorescence quantum yield efficiency.18
In this paper we provide a new route for synthesizing 1,3-
diphenyl-9-silafluorene derivatives. We report X-ray structural
analysis and photophysical properties, including AEE character-
istics and solid-state fluorescence, for both previously
synthesized and novel silafluorene derivatives. Also included
are the cell toxicity studies of compounds 3, 4, 6, and 6′.
INTRODUCTION
■
Dibenzoannulated analogues of silacyclopentadienes, or sila-
fluorenes, have high electron affinities1−3 and are promising
potential materials as electron transporters and emitters for
fabrication of organic light emitting diodes (OLEDs)4 and
photovoltaic cells.5−9 Silafluorene conjugates are also antici-
pated to be useful alternatives for expanding the repertoire of
traditional fluorescent dyes in many biological assays and
fluorescent imaging techniques.10 Important features of useful
fluorophores for such applications include high absorption, high
quantum yield, high stability with respect to photobleaching,
and compatibility with biological systems.
Silafluorenes having 2-fold symmetric structures can be
prepared by coupling of o,o′-dilithiobiphenyl with chlorosi-
lanes11 and by thermolysis of phenylchlorosilanes.12 Substituted
silafluorenes have been synthesized by [2 + 2 + 2]
cycloaddition of Si-bridged 1,6-diynes with alkynes in the
presence of an Ir(I)−phosphine catalyst,13 by intramolecular
sila-Friedel−Crafts cyclization,14 by addition of a silyl group to
an alkyne,15 and by the cross-coupling reaction of silicon-
bridged biaryls.16 Asymmetric silafluorenes are of interest, as
asymmetrically aryl-substituted 9,9-spiro-9-silabifluorene (SSF)
derivatives prepared through the cyclization of the correspond-
ing 2,2-dilithiobiphenyls with silicon tetrachloride have
demonstrated remarkably high absolute photoluminescence
quantum yields (1/4PL): 30−55%.17
RESULTS AND DISCUSSION
■
2,2′-Dilithio-3,5-diphenylbiphenyl (1) was synthesized by the
reaction of 1-bromo-2,4,6-triphenylbenzene with n-butyllithium
in diethyl ether.19,20 This reaction gave the desired product in
over 90% yield in diethyl ether, but in THF or in hexane only 1-
lithio-2,4,6-triphenylbenzene was obtained. Thus, it appears
that the Li-coordinating diethyl ether molecules21 are crucial for
Received: November 12, 2012
© XXXX American Chemical Society
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the second lithiation step to occur on a neighboring phenyl ring
at the ortho position.
9,9-Dichloro-1,3-diphenyl-9-silafluorene (2) and 1,1′,3,3′-
tetraphenyl-9-silaspirofluorene (3) were synthesized by the
reaction of 1 with silicon tetrachloride, similarly to the
procedure for their unsubstituted analogues22 (Scheme 1).
The photophysical properties of 9,9-spirosilafluorene (7) and
9,9-diphenylsilafluorene (8) (Figure 1) were compared with
those of 3, 4, 6, and 6′ in order to investigate the effect of
introducing phenyl substituents at positions 2 and 4 on the
silafluorene moiety. 7 and 8 were synthesized by following
known procedures.22
Phenyl substituents can affect the optical properties of
silafluorenes in several ways. They can enhance the free
mobility of π electrons and the charge transportation in excited
states, increasing the quantum yield efficiency. However, phenyl
substituents can also provide nonradiative extinction pathways
and thereby reduce quantum yield efficiency in solution.
Finally, in the solid state angular phenyl rings may prevent
aggregation, increasing the photoefficiency.
Scheme 1. Synthesis of Silafluorene 2
All investigated silafluorenes give blue emission. In solution
the emission ranges lie between 340 and 450 nm (Figures
2−4), while in the solid state the emission ranges are broader,
and for 6 and 6′ additional peaks appear, resulting a broad
emission from 320 to 500−550 nm (see the Supporting
Information).
The highest molar extinction was observed for the new
asymmetric spirosilafluorene 3, while the highest quantum yield
efficiency in organic solvent as well as in the solid state was
measured for compound 4. The solid-state quantum yield
efficiency of 4 (0.42) approaches that of the more complex
silafluorenes of Lee et al.17 (0.31−0.54), obtained via a much
longer synthesis.
When 1 was treated with an 8-fold excess of SiCl4 at −95 °C,
the yield of 2 was almost 90%. However, the yield substantially
decreased when a 2−4-fold excess of SiCl4 was used, and
spirosilafluorene 3 was formed in 50% yield. When 1 molar
equiv of silicon tetrachloride was used, the yield of 3 reached
90%. In the reaction of 1 with dichlorodiphenylsilane the
diphenylsilafluorene 4 was obtained in high yield (Scheme 2).
Scheme 2. Synthesis of Diphenylsilafluorene 4
On comparison of the UV spectrum of 3 to that of 7 and the
spectrum of 4 to that of 8, the absorption below 300 nm is
more intense for 3 and 4 due to the additional phenyl groups.
The broad absorption bands at 300−340 nm are also more
intense in the case of 3 and 4, as these bands belong to the
silafluorene ring, and the absorption can be enhanced by the
conjugating phenyl substituents (see Theoretical Calculations).
The fluorescence emission maxima for silafluorenes 3, 4, 6,
and 6′ were red-shifted by about 20 nm in comparison to the
maxima of unsubstituted silafluorenes 7 and 8. Fluorescence
spectra recorded in dichloromethane show that the intensity of
emission of the new phenyl-substituted silafluorenes is greater
than that of the unsubstituted analogues.
Different substituents on the silicon atom of the silafluorene
ring do not influence significantly the energy of the emission
but do affect the intensity. As shown in Table 1, electron-
donating substituents on the silicon atom enhance the
fluorescence quantum yield efficiency, while electron-with-
drawing groups decrease it. Furthermore, 3, 4, 7, and 8 have
more extended conjugated systems than 6 and 6′, which also
influences the intensity.
Metalation of Dichlorosilafluorene 2. Reaction of
dichlorosilafluorene 2 with excess metallic Li in THF at room
temperature gives the 9,9-dilithiosilafluorene 5.22,23 The
formation of dianion 5 was confirmed by the treatment of
the mixture with Me3SiCl, upon which 9,9-bis(trimethylsilyl)-
1,3-diphenyl-9-silafluorene (6) was formed (Scheme 3).
Scheme 3. Metalation of Silafluorene 2
Although some symmetric silafluorenes were already
prepared in the 1950s, aggregation induced emission (AIE)
or aggregation enhanced emission (AEE) of the reported
silafluorenes was usually not investigated,25,26 and AEE was
observed only for some ring-fused dibenzosiloles.27 Com-
pounds showing AEE characteristics are beneficial for sensitive
sensing processes (with detection limits down to 0.1 ppm) and
selectivity, enabling the development of biological probes to
detect proteins, DNA, and RNA.28
Aggregation enhanced emission (AEE) for our compounds
was examined in THF−water solutions (see the Supporting
Information). Compounds 4 and 7 show no significant increase
in fluorescence intensity when aggregated, but they do not
undergo quenching either. Compounds 6, 6′, and 8 show
typical AEE characteristics. Their fluorescence intensity
Silafluorene 6 was obtained previously by a radical intra-
molecular ring closure reaction.24 9,9-Bis(phenyldimethylsilyl)-
1,3-diphenyl-9-silafluorene (6′) was synthesized similarly in the
reaction of 5 and phenyl(dimethyl)chlorosilane.
1
The structures of 2, 3, 4, 6, and 6′ were confirmed by H,
13C, and 29Si NMR and X-ray structure analysis, as presented in
the Supporting Information.
Photophysical Properties. The photophysical properties
of silafluorenes 3, 4, 6, and 6′ are shown in Table 1.
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a
Table 1. Photophysical Properties of Silafluorenes 3, 4, 6, 6′, 7, and 8
UV−vis
λex, nm
fluorescence in CH2Cl2
solid state fluorescence
λmax, nm
silafluorene
λmax, nm
log ε (at λex)
λmax, nm
Φfl
λex, nm
Φfl
3
265
265
260
260
280
281
330
330
330
330
320
310
3.49
3.03
3.16
3.34
3.23
2.97
375
368
368
368
358
348
0.13
0.17
0.09
0.07
0.10
0.10
325
325
325
325
325
325
371
0.15
0.42
0.12
0.13
0.21
0.16
4
361
6
365; 482
362; 482
368
6′
7
b
8
340; 355
a
Definitions: λmax, absorption or fluorescence maximum; λex, excitation wavelength; ε, molar absorption coefficient; Φfl, fluorescence quantum yield
b
efficiency, estimated for CH2Cl2 solutions relative to 2-aminopyridine standard in 0.1 M H2SO4 aqueous solution. Also reported by Yabusaki et al.
(with excitation wavelength 250 nm.18
Figure 1. Structures of compounds 7 and 8.
Figure 4. UV absorption and fluorescence emission of compounds 7
and 8 in CH2Cl2.
Figure 2. UV absorption and fluorescence emission of compounds 3
and 4 in CH2Cl2.
Figure 5. AEE characteristics of 3 in a THF−water mixture.
and the solid -state emission verifies this presumption.
Accordingly, near the maximum the nanoaggregated particles
probably exhibit a crystalline structure, and then with increasing
water content, and presumably with increasing aggregate size,
amorphous particles are formed.29
In compounds 3, 4, 6, 6′, and 8 there are two or four phenyl
groups that provide a nonradiative extinction pathway by their
free rotation. In the case of usual organic fluorescent molecules
Figure 3. UV absorption and fluorescence emission of compounds 6
π−π stacking can occur in nanoaggregates, resulting in
quenching upon aggregation.30 In the nanoaggregates of the
investigated silafluorenes the π−π stacking is partially hindered
by the asymmetric substitution or by the nonplanar structure of
the silafluorene frame. Restriction of intramolecular rotation
(RIR) of the phenyl groups probably plays an important role in
AEE as well, since the quantum yield efficiency increases with
the hindrance of rotation of the phenyl groups, indicating the
disappearance of nonradiative extinction pathways. RIR was
and 6′ in CH2Cl2.
increases rapidly with aggregation. Compound 3 has a peculiar
maximum in fluorescence intensity when the water content of
its THF solution is increased (Figure 5).
This phenomenon suggests that for 3 there is a structural
change in the aggregates as the water/THF ratio changes.
Crystallization enhanced emission (CEE) may be occurring,
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investigated in ethylene glycol; details are described in the
Supporting Information.
quantum efficiency 0.7%, and the current efficiency 1.08 cd/A
at 27.6 mA/cm2 current density. (For further details see the
Supporting Information.) The results above suggest that
compound 4 is the most promising candidate for the
manufacture of electroluminescence devices.
Theoretical Calculations. Molecular orbital calculations
were performed with the TD-DFT method at the B3LYP/6-
31G* level using the Gaussian 09 package. The calculated
energies from HOMO-3 to LUMO+3 are shown in Figure 7.
The new phenyl-substituted silafluorene compounds were
compared to their unsubstituted versions and to their carbon
analogues.
On comparison of the silafluorenes with their carbon
analogues, all energy levels show a decrease, but this is greater
for the LUMO levels than the HOMO levels; thus, the
HOMO−LUMO gap is decreased. Compound 7 shows the
greatest decline of 0.23 eV. On comparison of the phenyl-
substituted silafluorenes with their unsubstituted analogues (3
and 4 with 7 and 8), the HOMO levels are raised slightly and
the LUMO levels are lowered, narrowing the HOMO−LUMO
gap by 0.22 and 0.19 eV, respectively. This is in good
agreement with the UV spectra, where 7 and 8 have absorption
at around 280 nm while the silafluorene ring of 3 and 4 absorbs
at 298 nm, which shows up as a shoulder because of the very
intense peak of the substituent phenyl groups.
In the solid state, on comparison of 7 to 3, the difference in
the emission maximum decreases as well as the quantum yield
efficiency. In contrast, on comparison of 4 to 8, phenyl
substitution causes a red shift in the emission maximum and an
increase in the quantum yield efficiency, prominently in the
solid state. Compounds 6 and 6′ exhibit behavior different from
that of the other compounds. Si−Si bonds have a strong effect
on the optical properties. This is especially true for 6′, but both
compounds show intense aggregation enhanced emission and
their quantum yield efficiency is also increased in the solid state.
6 and 6′ have a second emission peak around 482 nm in the
solid state. These emission bands can be assigned to the
conjugation of the Si−Si bond and the silafluorene (and
phenyl) ring.31
When the solid compounds are observed visually, the
brightest compounds are 6 and 6′ (Figure 6). The reason for
Due to the decrease of the HOMO−LUMO gap in the
phenyl-substituted silafluorenes, significant changes were
expected in the emission spectra as well. The measured results
show that the originally UV-emitting compounds can be tuned
with phenyl substitution and with significant red shift turned
into blue-emitting compounds.
Molecular orbital calculations were carried out also for
electronically advantageous conformations, where the phenyl
groups are in the same plane with the silafluorene ring and the
aromatic rings can conjugate. These conformations are at the
rotation barrier maxima; thus, the probability of these
conformations is low, but according to the calculated excited
states they are responsible for the broad absorption bands in
the UV spectra at around 330 nm (see the Supporting
Information). The conjugating phenyl groups do not cause a
large change in the HOMO levels but lower the LUMO levels
and decrease the HOMO−LUMO gap by 0.23−0.35 eV
(Figure 8).
Figure 6. Fluorescence of crystals under UV lamp (from left: 3, 4, 6,
6′, 7, and 8, respectively).
this might be that only 6 and 6′ have their major emission in
the region of blue and cyan (or sky blue) colors, while all the
other compounds emit mostly violet light.
Finally, light-emitting devices with the structure ITO/m-
MTDATA (40 nm)/NPB (20 nm)/4 (20 nm)/TPBi (40 nm)/
LiF (1 nm)/Al (100 nm) and ITO/m-MTDATA (40 nm)/
NPB (20 nm)/6 (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100
nm) were fabricated. At 15 V this device emitted blue light at
448 nm with a maximum luminance of 2890 cd/m2. Its onset
voltage was 4.4 V, the peak external quantum efficiency 1.7%,
and the current efficiency 2.11 cd/A at 5.36 mA/cm2 current
density. The second device containing compound 6 also
emitted blue light at 464 nm with a maximum luminance of 962
cd/m2 at 20 V. Its onset voltage was 8.6 V, the peak external
Figure 7. Calculated molecular orbital levels at optimized geometries in the energy minimum.
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solid state, enhanced emission was observed. All compounds
showed increased quantum yield efficiency in the solid state: up
to 42% for compound 4.
The effect of different substituents on the silicon atom was
also studied. Substitution on the silicon atom did not
significantly influence the emission maxima or range in
solution. Phenyl groups were found to be the most beneficial,
as they increased the fluorescence emission intensity, while
organosilyl groups caused decreased the fluorescence emission
intensity and thus decreased the fluorescence quantum yield
efficiency.
Si−Si bond containing compounds 6 and 6′ exhibit
characteristics different from those of the other silafluorenes.
They showed the most intense AEE characteristics, and both
compounds have a second emission maximum at 482 nm in the
solid state.
All compounds were found to be potential blue light emitting
OLED materials; from the electroluminescence results
compound 4 is an especially promising candidate. According
to toxicity assays on human skin fibroblast cells, all compounds
are potential candidates also in bioimaging.
Figure 8. Calculated molecular orbital levels for phenyl-substituted
silafluorenes and their carbon analogues at rotational maxima of the
phenyl groups.
Toxicity Assays. The silafluorenes in this study emit blue
light, which is not ideal for in vivo imaging but is suitable for
biological research. As an initial exploration of their potential
utility, a selection of the compounds reported in Table 1 was
assayed for cytotoxicity in Normal Adult Human Primacy
Dermal Fibroblasts cell line (NHDF from ATCC cat. #PCS-
201-012). Cells were treated for 72 h with each test compound,
and then the amount of live cells was quantified using an ATP
determining assay reagent, Cell Titer Glo (Promega, cat.
#G7572). ATP measurement is becoming more widely
accepted as an indicator of the number of viable cells present.
When cell viability is lost and membrane integrity is
compromised, there is a rapid drop in the level of ATP
present, resulting from the combination of a loss of the cell’s
ability to synthesize more ATP and the removal of any
remaining ATP by the action of ATPases.
EXPERIMENTAL SECTION
■
General Synthetic Procedure, Reagents, and Solvents.
Triphenylbenzene, bromine, n-BuLi (1.6 M solution in hexane),
silicon tetrachloride, and carbon disulfide were purchased from Aldrich
and used without further purification. All solvents were distilled from
sodium benzophenone ketyl in standard stills. Procedures involving
air- and moisture-sensitive materials were carried out using standard a
Schlenk line under a nitrogen atmosphere.
FT-IR spectra were obtained on a Mattson Polaris FT-IR
1
spectrometer (ATR). H, 13C, and 29Si NMR spectra were recorded
in CDCl3. 29Si NMR was recorded on a Varian INOVA-500
1
1
spectrometer at 500 MHz for H (99.38 MHz for 29Si). H NMR
and 13C NMR spectra were recorded on a Varian MercuryPlus 300
spectrometer at 300 MHz for H and 75.40 MHz for 13C nuclei. The
1
crystal structure evaluations for 2, 3, 4, 6, and 6′ data collections were
performed on a Bruker Quazar SMART APEXII diffractometer with
Mo Kα (λ = 0.71073 Å) radiation or on a Bruker SMART APEXII
diffractometer with Cu Kα (λ = 1.54178 Å) radiation, depending on
the parameters of the single crystal. Mass spectra were recorded with a
Bruker REFLEX II spectrometer using the MALDI technique and
anthracene matrix. UV and fluorescence spectra were recorded on a
HP 8452 UV−visible spectrophotometer and F-4500 fluorescence
spectrophotometer. Solid-state fluorescence measurements were
carried out according to the method described by de Mello et al.,32
using 325 nm CW light from a He−Cd laser for excitation and an
Ocean Optics USB2000 miniature fiber optics spectrometer.
Fluorescence quantum yield efficiency was estimated relative to a 2-
aminopyridine standard in 0.1 M H2SO4 aqueous solution using the
equation
The IC50 values were determined using curve-fitting
software (xlfit 5.0 (IDBS)) to find the concentration at which
point 50% cytotoxicity is observable. Cell viability and
proliferation remained intact for 3, 4, 6, and 6′ in normal
fibroblast cells below concentrations of 100 μM, and the IC50
values appear to be above 100 μM for each of the studied
compounds. The lack of cytotoxicity of these molecules
presents them as suitable agents for bioimaging purposes.
CONCLUSIONS
■
9,9-Dichloro-1,3-diphenylsilafluorene and its four derivatives
were synthesized in only three- or five-step reactions. All
compounds were found to be stable, crystalline, blue-emitting
compounds. The optical properties were investigated in
dichloromethane solution, in aggregate form, and in the solid
state as well. Furthermore, compounds 4 and 6 were tested in
electroluminescence devices (see the Supporting Information).
Theoretical calculations showed that phenyl substitution on
the silafluorene ring leads to a decrease in the HOMO−LUMO
gap. In good agreement with this result, the new asymmetric
phenyl-substituted compounds show significant red shifts in the
UV and emission spectra in comparison to their unsubstituted
analogues. Due to the fact that these additional phenyl groups
can conjugate with the silafluorene ring, they also increase the
fluorescence emission intensity. The observed intensity increase
in solution was less than that expected, probably because the
rotating phenyl groups also provide a nonradiative extinction
pathway. As this rotation was hindered in aggregates and in the
2
(Φ )x = (Φ ) (Gradx/Gradst)(η 2/η )
fl
fl st
x
st
where (Φfl)x is the quantum efficiency of an unknown compound,
(Φfl)st is the quantum efficiency of the standard, Grad is the slope of
the plot of integrated fluorescence intensity vs absorbance, and η is the
refractive index of the solvent (x and st refer to the unknown
compound and the standard, respectively).33
Bromotriphenylbenzene was obtained as described by Kohler and
Blanchard34 in the reaction of triphenylbenzene with bromine in
carbon disulfide. 1-Bromo-2,4,6-triphenylbenzene can be obtained
after recrystallization from ethanol with minimum 90% yield in high
35
1
purity. (The purity was verified by H and 13C NMR. )
2,2′-Dilithio-3,5-diphenylbiphenyl (1). This compound was
synthesized in the reaction of bromotriphenylbenzene with 2 equiv of
n-BuLi in a hexane−diethyl ether mixture. A 5.0 g (13 mmol) portion
of bromotriphenylbenzene was dissolved in 40 mL of diethyl ether and
E
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10 mL of hexane and cooled to −78 °C, and 17 mL (27 mmol) of n-
butyllithium in 1.6 M solution in hexane was added dropwise to the
suspension within 1 h. The solution was stirred for a further 3 h at low
temperature and then 4 h at room temperature. After the reaction was
completed, the volatile compounds were evaporated from the solvents
under reduced pressure.
bis(trimethylsilyl) derivative 6 was obtained: mp 143 °C. 1H, 13C, and
1
29Si NMR of 6 were similar to that of prepared in ref 14. H NMR
(300.133 MHz, CDCl3): δ −0.03 (CH3Si), 7.3−7.8 (aromatic). 13C
NMR (75.403 MHz, CDCl3): δ 150.4 (Cq), 148.7 (Cq), 148.3 (Cq),
145.7 (Cq), 142.1 (Cq), 141.5 (Cq), 141.2 (Cq), 137.8 (Cq), 134.0
(CH), 129.0 (CH), 128.9 (CH), 128.5 (CH), 128.4 (CH), 127.7
(CH), 127.6 (CH), 127.4 (CH),126.7 (CH), 126.4 (CH), 121.5
(CH), 119.0 (CH), −0.5 (SiCH3). 29Si NMR (99.314 MHz, CDCl3):
δ −13.6 (SiCH3), −40.3(Si-SiCH3). IR: 625.46, 693.83, 758.45,
829.97, 1028.95, 1242.82, 1493.06, 1590.58, 2891.10, 2945.37,
3039.92, 3059.24 cm−1. MS (MALDI) m/z (%): 480.7 (62), 481.7
(23), 482.7 (15).
9,9-Bis(phenyldimethylsilyl)-1,3-diphenyl-9-silafluorene
(6′). This compound was prepared very similarly to 6, but 5 was
treated with phenyl(dimethyl)chlorosilane: mp 129 °C. 1H NMR
(300.133 MHz, CDCl3): δ 0.11 (CH3Si) 7.1−8.1 (aromatic). 13C
NMR (75.403 MHz, CDCl3): δ 150.4 (Cq), 148.6 (Cq), 148.3 (Cq),
145.4 (Cq), 142.2 (Cq), 141.4 (Cq), 140.1 (Cq), 138.5 (Cq), 137.2
(Cq), 134.7 (CH), 134.2 (CH), 129.0 (CH), 128.9 (CH), 128.8 (CH),
128.6 (CH), 128.4 (CH), 127.8 (CH), 127.6 (CH), 127.5 (CH),
127.4 (CH), 126.6 (CH), 126.5 (CH), 121.3 (CH), 119.0 (CH), −2.6
(SiCH3). 29Si NMR (99.314 MHz, CDCl3): δ −16.5 (SiCH3), −41.6
(Si-SiCH3). IR: 645.61, 693.73, 730.52, 758.61, 801.75, 832.42, 877.53,
1106.78, 1249.12, 1425.81, 1500.43, 1592.29, 2955.58, 3051.63 cm−1.
MS (MALDI) m/z (%): 605.0 (58), 606.0 (28), 606.9 (14).
9,9-Dichloro-1,3-diphenyl-9-silafluorene (2). A solution of 1
(13 mmol in 50 mL of Et2O) was added dropwise to a solution of
SiCl4 (33 mL, 129 mmol) in Et2O (160 mL) at −95 °C. The reaction
mixture was stirred at −95 °C and then warmed to room temperature
by spontaneous evaporation of the cooling bath overnight. A yellow
solution and a white precipitate were obtained. The white precipitate,
mostly LiCl, was removed by filtration, and excess SiCl4 and solvents
were removed under vacuum. The residue was dissolved in Et2O. After
crystallization at −20 °C from Et2O, 4.66 g (89%) of 2 was obtained.
Selected data for 2 are as follows. 1H NMR (300.133 MHz, CDCl3): δ
7.3−8.1 (aromatic). 13C NMR (75.403 MHz, C6D6): δ 149.7 (Cq),
147.3 (Cq), 146.8 (Cq), 145.1 (Cq), 142.0 (Cq), 140.6 (Cq), 133.2
(CH), 132.6 (CH), 132.1 (Cq), 130.2 (CH), 129.4 (CH), 129.2 (CH),
128.9 (CH), 128.8 (CH), 128.7 (CH), 128.5 (CH), 128.3 (CH),
127.6 (CH), 121.5 (CH), 119.0 (CH), 0.2 (SiCH3). 29Si NMR
(99.314 MHz, C6D6): δ +5.8 ppm.
1,1′,3,3′-Tetraphenyl-9-silaspirofluorene (3). To 13 mmol of 1
in 75 mL of a hexane/Et2O solution was added 6.5 mmol (1 equiv) of
SiCl4 at −78 °C. The temperature was raised to room temperature,
and the reaction mixture was stirred overnight. After the reaction was
complete, the solvents and excess SiCl4 were removed by pumping and
Et2O was added to dissolve the solid residue. After crystallization at
ASSOCIATED CONTENT
* Supporting Information
■
S
1
−20 °C from Et2O, 3.93 g (91%) of 3 was isolated: mp 255 °C. H
Text, figures, tables, and CIF files giving NMR, IR, UV, and
fluorescence spectra, crystallographic data, and detailed results
of the toxicity assays for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
The crystal structures are also available free of charge from the
Cambridge Crystallographic Data Center; the CCDC codes are
890700, 890701, 890702, 890703, 890704, 890705, and 890706
for compounds 7, 3, 8, 4, 6, 2, and 6′, respectively.
NMR (300.133 MHz, CDCl3): δ 6.763−7.895 (aromatic) and 6.496−
7.688 ppm. 13C NMR (75.403 MHz, CDCl3): δ 150.9 (Cq), 149.6
(Cq), 149.3 (Cq), 144.4 (Cq), 142.6 (Cq), 141.5 (Cq), 135.5 (Cq),
133.8 (CH), 131.1 (Cq), 131.0 (CH), 129.0 (CH), 128.0 (CH), 127.8
(CH), 127.7 (CH), 127.5 (CH), 127.4 (CH), 126.8 (CH), 126.6
(CH), 121.3 (CH), 118.7 (CH). 29Si NMR (99.314 MHz, CDCl3): δ
−6.8 ppm. IR: 628.34, 694.51, 733.33, 753.63, 837.22, 881.05, 1026.99,
1068.40, 1129.87, 1255.32, 1389.83, 1442.45, 1492.46, 1545.74,
1591.50, 3027.69, 3049.51 cm−1. MS (MALDI) m/z (%): 635.3
(25), 636.2 (40), 637.2 (26), 638.2 (8), 639.2 (2).
AUTHOR INFORMATION
Corresponding Author
*E-mail for R.W.: west@chem.wisc.edu.
1,3,9,9-Tetraphenyl-9-silafluorene (4). To 13 mmol of 1 in 75
mL of a hexane/Et2O solution was added 13.0 mmol (3.3 g) of
Ph2SiCl2 in 20 mL of hexane at −78 °C. The temperature was raised
to room temperature, and the reaction mixture was stirred overnight.
After the reaction was complete, the solvents were removed by
pumping and Et2O was added to dissolve the solid residue. Following
crystallization at −20 °C from diethyl ether 5.6 g (90%) of 4 was
■
Notes
The authors declare no competing financial interest.
1
ACKNOWLEDGMENTS
obtained: mp 185 °C. H NMR (300.133 MHz, CDCl3): δ 7.113−
■
8.094 ppm (aromatics). 13C NMR (75.403 MHz, CDCl3): δ 148.5
(Cq), 144.4 (Cq), 144.2 (Cq), 142.5 (Cq), 141.3 (Cq), 142.2 (Cq),
137.4 (Cq), 135.9 (CH), 134.2 (CH), 133.2 (Cq), 130.8 (CH), 130.0
(CH), 129.3 (CH), 129.0 (CH), 128.4 (CH), 128.3 (CH), 128.0
(CH), 127.9 (CH), 127.7 (CH), 127.5 (CH), 125.3 (CH), 121.4
(CH), 119.0 (CH). 29Si NMR (99.314 MHz, CDCl3): δ −10.3 ppm.
IR: 652.47, 695.32, 758.03, 880.16, 1026.95, 1114.90, 1387.66,
1429.90, 1494.83, 1589.17, 3025.82, 3056.90 cm−1. MS (MALDI)
m/z (%): 485.1 (5), 486.1 (53), 487.1 (28), 488.1 (9), 489.1 (5).
9,9-Dilithio-1,3-diphenyl-9-silafluorene (5). A solution of 9,9-
dichloro-1-silafluorene 2 (5.0 g, 13 mmol) in THF (180 mL) was
stirred with lithium metal (1.0 g, 130 mmol) at 0 °C for 2 h. After
stirring for 10 min the reaction mixture turned red. The color of the
solution then changed to a deep purple. (Unfortunately, we were not
able to record 29Si NMR signal of the dianion because of the presence
of a paramagnetic species formed as a result of over-reduction of the
dilithio-dianion, as has been described previously.22) The reaction
mixture was treated with excess Me3SiCl at 0 °C to give 9,9-
bis(trimethylsilyl)-1,3-diphenyl-9-silafluorene (6). The volatiles were
removed under reduced pressure, and the residue was extracted with
hexane (250 mL). The hexane solution was washed with distilled
water, dried over MgSO4, and filtered. Upon crystallization from the
residue at room temperature, 3.84 g (98.6%) of white crystals of
We thank the Rosztoczy Foundation and the Organosilicon
Research Center for providing us the opportunity of working
on this project. We acknowledge Seunghyun Jang and Terri Lin
(University of WisconsinMadison) for synthesizing the
reference compounds. We thank Ben Zhong Tang (Hong
Kong University of Science & Technology) for the solid-state
fluorescence measurements. NMR spectra were financed by
NSF CHE-9208463 and NSF CHE-9629688. Mass spectrom-
etry was partially funded by NSF Award 9520868 to the
Department of Chemistry. Theoretical calculations were carried
out thanks to NSF Grant CHE-0840494.
REFERENCES
■
(1) Hong, J. H.; Boudjouk, P.; Stoenescu, I. Organometallics 1996, 15,
2179.
(2) Wong, W. W. H.; Hooper, J. F.; Holmes, A. B. Aust. J. Chem.
2009, 62, 393.
(3) Agou, T.; Hossain, M. D.; Kawashima, T. Chem. Eur. J. 2010, 16,
368.
(4) Zhang, S.; Chen, R.; Yin, J.; Liu, F.; Jiang, H.; Shi, N.; An, Z.; Ma,
C.; Liu, B.; Huang, W. Org. Lett. 2010, 12, 3438.
F
dx.doi.org/10.1021/om300891n | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(5) Lu, G.; Usta, H.; Risko, C.; Wang, L.; Facchetti, A.; Ratner, M. A.;
Marks, T. J. J. Am. Chem. Soc. 2008, 130, 7670.
(6) Boudreault, P.-L. T.; Michaud, A.; Leclerc, M. Macromol. Rapid
(34) Kohler, E. P.; Blanchard, L. W., Jr. J. Am. Chem. Soc. 1935, 57
(2), 367−371.
(35) Hintermann, L.; Dang, T. T.; Labonne, A.; Kribber, T.; Xiao, L.;
Naumov, P. Chem. Eur. J. 2009, 15, 7167.
Commun. 2007, 28, 2176.
(7) Chan, K. L.; McKiernan, M. J.; Towns, C. R.; Holmes, A. B. J.
Am. Chem. Soc. 2005, 127, 7662.
(8) Jin, J. K.; Choi, J. K.; Kim, B. J.; Kang, H. B.; Yoon, S. C.; You, H.;
Jung, H. T. Macromolecules 2011, 44, 502.
(9) Duan, C.; Cai, W.; Huang, F.; Zhang, J.; Wang, M.; Yang, T.;
Zhong, C.; Gong, X.; Cao, Y. Macromolecules 2010, 43, 5262.
(10) Wang, E.; Li, C.; Zhuang, W.; Peng, J.; Cao, Y. J. Mater. Chem.
2008, 18, 797−801.
(11) (a) Gilman, H.; Gorsich, R. D. J. Am. Chem. Soc. 1955, 77, 6380.
(b) Gilman, H.; Gorsich, R. D. J. Am. Chem. Soc. 1958, 80, 1883.
(c) Gilman, H.; Gorsich, R. D. J. Am. Chem. Soc. 1958, 80, 3243.
(12) (a) Chernyshev, E. A.; Ryzhova, O. G.; Molchanov, B. V.;
Mosin, A. M.; Krasnova, T. L.; Kisin, A. V.; Rogachevskii, V. L. Zh.
Obshch. Khim. 1979, 49, 139. (b) Chernyshev, E. A.; Komalenkova, N.
G.; Elagina, O. V.; Rogachevskii, V. L.; Bashkirova, S. A.; Dunaeva, L.
V. Zh. Obshch. Khim. 1985, 55, 2314.
(13) Matsuda, T.; Kadowaki, S.; Goya, T.; Murakami, M. Org. Lett.
2007, 9, 133.
(14) (a) Furukawa, S.; Kobayashi, J.; Kawashima, T. J. Am. Chem. Soc.
2009, 131, 14192. (b) Furukawa, S.; Kobayashi, J.; Kawashima, T.
Dalton Trans. 2010, 39, 9329.
(15) (a) Ilies, L.; Tsuji, H.; Sato, Y.; Nakamura, E. J. Am. Chem. Soc.
2008, 130, 4240. (b) Yamaguchi, S.; Xu, C. H.; Tamao, K. J. Am.
Chem. Soc. 2003, 125, 13662. (c) Xu, C. H.; Wakamiya, A.; Yamaguchi,
S. Org. Lett. 2004, 6, 3707. (d) Yamaguchi, S.; Xu, C. H.; Yamada, H.;
Wakamiya, A. J. Organomet. Chem. 2005, 690, 5365. (e) Li, C. H.; Qiu,
H. Y.; Xu, C. H. Chin. Chem. Lett. 2007, 18, 1436.
(16) Shimizu, M.; Mochida, K.; Hiyama, T. Angew. Chem., Int. Ed.
2008, 47, 9760.
(17) Lee, S. H.; Jang, B. B.; Kafafi, Z. H. J. Am. Chem. Soc. 2005, 127,
9071.
(18) Yabusaki, Y.; Ohshima, N.; Kondo, H.; Kusamoto, T.; Yamanoi,
Y.; Nishihara, H. Chem. Eur. J. 2010, 16, 5581.
(19) Hardman, N. J.; Twamley, B.; Stender, M.; Baldwin, R.; Hino,
S.; Schiemenz, B.; Kauzlarich, S. M.; Power, P. P. J. Organomet. Chem.
2002, 643, 461.
(20) Jenkins, H. A.; Abeysekera, D.; Dickie, D. A.; Clyburne, J. A. C.
Dalton Trans. 2002, 3919.
(21) Olmstead, M. M.; Power, P. P. J. Organomet. Chem. 1991, 408,
1.
(22) Liu, Y. X.; Stringfellow, T. C.; Ballweg, D.; Guzei, I. A.; West, R.
J. Am. Chem. Soc. 2002, 124, 49.
(23) Choi, S. B.; Boudjouk, P. Tetrahedron Lett. 2000, 41, 6685.
(24) Millevolte, A. J.; Winkel, Y.; Powell, D. R.; West, R.
Organometallics 1997, 16, 5375.
(25) Janzen, E. G.; Harrison, W. B.; Pickett, J. B. J. Organomet. Chem.
1969, 20, P13.
(26) Shuvaev, A. T.; Zemlyano, A. P.; Kolodyaz, Y. V.; Osipov, O. A.;
Tatevosy, M. M.; Eliseev, V. N.; Morgunov, M. M. Izv. Akad. Nauk
SSSR Ser. Fiz. 1974, 38, 541.
(27) Shimizu, M.; Mochida, K.; Katoh, M.; Hiyama, T. Sci. China-
Chem. 2011, 54, 1937.
(28) Li, Z.; Dong, Y. Q.; Lam, J. W. Y.; Sun, J.; Qin, A.; Hauβler, M.;
̈
Dong, Y. P.; Sung, H. H. Y.; Williams, I. D.; Kwok, H. S.; Tang, B. Z.
Adv. Funct. Mater. 2009, 19, 905−917.
(29) Dong, Z. Y.; Lam, J. W. Y.; Qin, A.; Sun, J.; Liu, J.; Li, Z.; Sun, J.;
Sung, H. H. J.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. Chem.
Commun. 2007, 3255.
(30) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009,
4332.
(31) Nespurek, S.; Schauer, F.; Kadashchuk, A. Monatsh. Chem. 2001,
132, 159−168.
(32) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. Adv. Mater.
1997, 9, 230−232.
(33) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107.
G
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