Highly fluorescent solid-state asymmetric spirosilabifluorene derivatives

Article abstract of DOI:10.1021/ja042762q

A series of four asymmetrically aryl-substituted 9,9′-spiro-9- silabifluorene (SSF) derivatives, 2,2′-di-tert-butyl-7,7′-diphenyl- 9,9′-spiro-9-silabifluorene (PhSSF), 2,2′-di-tert-butyl-7,7′- dipyridin-2-yl-9,9′-spiro-9-silabifluorene (PySSF), 2,2′-di-tert- butyl-7,7′-dibiphenyl-4-yl-9,9′-spiro-9-silabifluorene (BPhSSF), and 2,2′-di-tert-butyl-7,7′-bis(2′,2″-bipyridin-6-yl)-9, 9′-spiro-9-silabifluorene (BPySSF) are prepared through the cyclization of the corresponding 2,2′-dilithiobiphenyls with silicon tetrachloride. These novel spiro-linked silacyclopentadienes (siloles) form transparent and stable amorphous films with relatively high glass transition temperatures (T_g = 203-228°C). The absorbance spectrum of each compound shows a significant bathochromic shift relative to that of the corresponding carbon analogue as a result of the effective σ*-π* conjugation between the σ* orbital of the exocyclic Si-C bond and the π* orbital of the oligoarylene fragment. Solid-state films exhibit intense violet-blue emission (λ_PL = 398-415 nm) with high absolute photoluminescence quantum yields (Φ_PL = 30-55%).

Full text of DOI:10.1021/ja042762q

Published on Web 06/07/2005
Highly Fluorescent Solid-State Asymmetric
Spirosilabifluorene Derivatives
Sang Ho Lee,*,† Bo-Bin Jang, and Zakya H. Kafafi*
Contribution from the Optical Sciences DiVision, U.S. NaVal Research Laboratory,
Washington, D.C. 20375
Received December 1, 2004; E-mail: sangho@ccs.nrl.navy.mil; kafafi@nrl.navy.mil
Abstract: A series of four asymmetrically aryl-substituted 9,9′-spiro-9-silabifluorene (SSF) derivatives, 2,2′-
di-tert-butyl-7,7′-diphenyl-9,9′-spiro-9-silabifluorene (PhSSF), 2,2′-di-tert-butyl-7,7′-dipyridin-2-yl-9,9′-spiro-
9-silabifluorene (PySSF), 2,2′-di-tert-butyl-7,7′-dibiphenyl-4-yl-9,9′-spiro-9-silabifluorene (BPhSSF), and 2,2′-
di-tert-butyl-7,7′-bis(2′,2′′-bipyridin-6-yl)-9,9′-spiro-9-silabifluorene (BPySSF) are prepared through the
cyclization of the corresponding 2,2′-dilithiobiphenyls with silicon tetrachloride. These novel spiro-linked
silacyclopentadienes (siloles) form transparent and stable amorphous films with relatively high glass transition
temperatures (Tg ) 203-228 °C). The absorbance spectrum of each compound shows a significant
bathochromic shift relative to that of the corresponding carbon analogue as a result of the effective σ*-π*
conjugation between the σ* orbital of the exocyclic Si-C bond and the π* orbital of the oligoarylene fragment.
Solid-state films exhibit intense violet-blue emission (λPL ) 398-415 nm) with high absolute photolumi-
nescence quantum yields (ΦPL ) 30-55%).
Introduction
mophores in the solid state. The spiro-linked molecules com-
pared to the corresponding non-spiro-linked parent compounds
Spirobifluorenes have been developed and used in organic
light-emitting diodes (OLEDs),1 organic photovoltaic cells
(OPVs),2 optically pumped solid-state lasers,3 and organic
phototransistors4 and as nonlinear optical (NLO)5 and photo-
chromic materials.6 The introduction of a “spiro” linkage into
low molecular weight organic compounds results in many
advantageous properties, such as high thermal and morphologi-
cal stabilities, facile processability, and high luminescence
quantum efficiencies. In general, spiro-linked molecules consist
of two identical (symmetric) or different (asymmetric) molecular
units which are orthogonally arranged and connected through
a central atom (spiro center).7 This molecular structure deter-
mines their electronic and optical properties. Steric factors can
lead to an enhanced rigidity in the spiro center, thereby
preventing rotation of the adjacent aryl groups, which reduces
close packing and intermolecular interaction between chro-
exhibit greater morphological stability and more intense fluo-
rescence. These enhanced properties occur without a significant
change in their absorption and fluorescence spectra. For
example, 2,2,7,7-tetrakis(diphenylamino)-9,9-spirobifluorene (a
spiro-TPD analogue for hole transport) shows a high glass
transition temperature (Tg ) 133 °C), more than double that of
N,N′-diphenyl-N,N′-(m-tolyl)benzidine (TPD) (Tg ) 60 °C),
while similar electronic and optical properties are maintained.8
Silicon-containing π-conjugated compounds, especially si-
lacyclopentadienes (siloles), have emerged as a new class of
electroactive materials with intense solid-state fluorescence
and/or good electron transport properties in OLEDs.9-17
Siloles have a relatively low-lying lowest unoccupied mo-
lecular orbital (LUMO) level due to the σ*-π* conjugation
between the σ* orbital of the exocyclic Si-C bond and the π*
orbital of the butadiene fragment, resulting in a high electron
affinity.9-11 Recently, a silole derivative, namely, 2,5-bis(2′,2′′-
† Also at SFA Inc., Largo, MD 20774.
(1) (a) Salbeck, J.; Yu, N.; Bauer, J.; Weisso¨rtel, F.; Bestgen, H. Synth. Met.
1997, 91, 209. (b) Steuber, F.; Staudigel, J.; Sto¨ssel, M.; Simmerer, J.;
Winnacker, A.; Spreitzer, H.; Weisso¨rtel, F.; Salbeck, J. AdV. Mater. 2000,
12, 130. (c) Wong, K.-T.; Chien, Y.-Y.; Chen, R.-T.; Wang, C.-F.; Lin,
Y.-T.; Chiang, H.-H.; Hsieh, P.-Y.; Wu, C.-C.; Chou, C. H.; Su, Y. O.;
Lee, G.-H.; Peng, S.-M. J. Am. Chem. Soc. 2002, 124, 11576. (d) Wu, C.
C.; Lin, Y. T.; Chiang, H. H.; Cho, T. Y.; Chen, C. W.; Wong, K. T.;
Liao, Y. L.; Lee, G. H.; Peng, S. M. Appl. Phys. Lett. 2002, 81, 577.
(2) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso¨rtel, F.; Salbeck, J.;
Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583.
(3) (a) Johansson, N.; Salbeck, J.; Bauer, J.; Weisso¨rtel, F.; Bro¨ms, P.;
Andersson, A.; Salaneck, W. R. AdV. Mater. 1998, 10, 1136. (b) Salbeck,
J.; Scho¨rner, M.; Fuhrmann, T. Thin Solid Films 2002, 417, 20.
(4) Saragi, T. P. I.; Pudzich, R.; Fuhrmann, T.; Salbeck, J. Appl. Phys. Lett.
2004, 84, 2334.
(5) Kim, S. Y.; Lee, M.; Boo, B. H. J. Chem. Phys. 1998, 109, 2593.
(6) Tian, H.; Chen, B.; Liu, P.-H. Chem. Lett. 2001, 990.
(8) Salbeck, J.; Weisso¨rtel, F. Macromol. Symp. 1997, 125, 121.
(9) Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi, S. J.
Am. Chem. Soc. 1996, 118, 11974.
(10) Yamaguchi, S.; Endo, T.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Tamao,
K. Chem. Eur. J. 2000, 6, 1683.
(11) Watkins, N. J.; Ma¨kinen, A. J.; Gao, Y.; Uchida, M.; Kafafi, Z. H. Proc.
SPIE Int. Soc. Opt. Engr. 2004, 5214, 368.
(12) Palilis, L. C.; Ma¨kinen, A. J.; Uchida, M.; Kafafi, Z. H. Appl. Phys. Lett.
2003, 82, 2209.
(13) Palilis, L. C.; Murata, H.; Uchida, M.; Kafafi, Z. H. Org. Electron. 2003,
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Furukawa, K. Chem. Mater. 2001, 13, 2680.
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Phys. Lett. 2001, 339, 161.
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9
10.1021/ja042762q CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 9071-9078
9071

A R T I C L E S
Lee et al.
Elemental analyses were performed by Quantitative Technologies Inc.
(QTI).
bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene
(PyPySPyPy), was shown to exhibit nondispersive and air-stable
electron transport with a time-of-flight mobility (2.0 × 10-4
cm2/V s at E ) 0.64 MV/cm) more than 2 orders of magnitude
higher than that of tris(8-hydroxyquinolinato)aluminum(III)
(Alq3), the most commonly used electron-transporting material
in OLEDs.15 In addition, PyPySPyPy formed Ohmic contacts
with CsF/Al cathodes, which led to efficient electron injection
and space-charged transport in single-layer “electron-only”
devices.17 Theoretical calculations of the electronic and molec-
ular structures of PyPySPyPy revealed a very small dihedral
angle (32°) between the silole ring and the bipyridyl moiety,
which should give rise to tight packing and strong intermolecular
interactions in the solid state.18 This may facilitate electron
hopping between adjacent molecules and explain the superior
electron transport properties of PyPySPyPy.
Materials. All commercially available chemicals, reagents, and
solvents were used as received without further purification, unless
otherwise stated. Tetrahydrofuran (THF) and diethyl ether were distilled
over sodium/benzophenone under dry nitrogen. Toluene was distilled
over P2O5 prior to use. Benzyltrimethylammonium dichloroiodonate
(BTMAICl2),20 tetra-n-butylammonium tribromide (Bu4NBr3),21 and
6-(trimethylstannyl)-2,2′-bipyridine22 were prepared according to pub-
lished procedures.
3-Bromo-4-iodoaniline (1). To a solution of 3-bromoaniline (2.39
g, 13.9 mmol) in dichloromethane (200 mL) and methanol (80 mL)
were added BTMAICl2 (4.84 g, 13.9 mmol) and CaCO3 (1.81 g, 18.1
mmol). The mixture was stirred for 1 h at room temperature until the
solution color changed from yellow to light brown. The excess CaCO3
was filtered, and the filtrates were concentrated. The residue was poured
into an aqueous solution of NaHSO3 (5%, 70 mL). The mixture was
extracted with 50 mL of ethyl acetate, dried over MgSO4, and
concentrated to yield a dark brown residue. The crude product was
purified by column chromatography (silica gel, hexane/ethyl acetate
) 8/1) to afford 1 as a pale yellow oil (3.97 g) in 96% yield. 1H NMR
(CDCl3): δ 7.52 (d, J ) 8.51 Hz, 1H), 6.99 (s, 1H), 6.37-6.33 (dd, J
) 8.68, 2.66 Hz, 1H), 3.73 (s, 1H). 13C NMR (CDCl3): δ 147.5, 140.1,
129.8, 118.9, 115.8.
High solid-state photoluminescence quantum yields (ΦPL
)
30-100%) with a high degree of color tunability have been
reported upon changing substituents at the 2- and 5-positions
on the silole ring.10,16 The unusually high PL quantum yield of
siloles in the solid state has been attributed to aggregation-
induced emission (AIE).19 AIE-active molecules such as the
siloles are excellent candidates as light-emitting materials in
OLEDs and solid-state lasers. OLEDs based on siloles exhibit
excellent performance with external electroluminescence quan-
tum efficiencies (ηEL ∼4.8% at 100 A/m2) close to the
theoretical limit for a fluorescent material and very high
brightness (luminance over 10 000 cd/m2).12,16 Unfortunately,
these siloles easily crystallize due to their low glass transition
temperatures. This strong tendency for crystallization contributes
to device degradation when these materials are incorporated in
OLED structures.
N-(3-Bromo-4-iodophenyl)acetamide (2). To a solution of 3-bromo-
4-iodoaniline (3.50 g, 11.7 mmol) in acetic anhydride (12 mL) was
added a catalytic amount of H2SO4. The mixture was stirred for
20 min at room temperature. The solution was neutralized with 100
mL of saturated aqueous sodium carbonate and extracted with 150
mL of ethyl acetate. The organic layer was washed with water (100
mL) and brine (70 mL), dried over MgSO4, and concentrated. The
crude solid was recrystalized from chloroform to afford 2 as a white
1
solid (3.95 g). H NMR (CDCl3): δ 7.88 (s, 1H), 7.73 (d, J ) 8.66
Hz, 1H), 7.60 (s, 1H), 7.19-7.16 (dd, J ) 8.54, 2.36 Hz, 1H), 2.17 (s,
3H). 13C NMR (CDCl3): δ 169.0, 140.1, 138.9, 129.8, 123.8, 120.0,
94.3, 24.5.
To circumvent this problem, we synthesized a novel class of
9,9′-spiro-9-silabifluorenes (SSFs), which are expected to exhibit
high glass transition temperatures and solid-state PL quantum
yields. We investigate the effect of adding electron-withdrawing
or -donating substituents on the optical and electronic properties
of this series of asymmetrically substituted SSFs.
2-Bromo-4-tert-butylaniline (3). To a solution of 4-tert-butylaniline
(4.07 g, 27.0 mmol) in 70 mL of DMF:H2O [7:3 (v/v)] was added
slowly Bu4NBr3 (13.2 g, 27.0 mmol) in 10 mL of DMF at 0 °C. The
mixture was stirred for 10 min at a temperature of 0-5 °C. The residue
was poured into an aqueous solution of NaHSO3 (5%, 200 mL). The
mixture was extracted with 70 mL of ethyl acetate, dried over MgSO4,
and concentrated to yield a dark brown oil. The crude product was
purified by column chromatography (silica gel, hexane) to afford 3 as
a pale yellow oil (4.00 g) in 64% yield. 1H NMR (CDCl3): δ 7.40 (s,
1H), 7.13-7.10 (dd, J ) 8.45, 2.20 Hz, 1H), 6.69 (d, J ) 8.36 Hz,
1H), 3.85 (s, 2H), 1.25 (s, 9H).
2-Bromo-4-tert-butyl-1-iodobenzene (4). To a solution of 2-bromo-
4-tert-butylaniline (10.6 g, 46.5 mmol) in concentrated H2SO4 (25 mL)
and H2O (230 mL) was slowly added sodium nitrite (4.17 g, 60.4 mmol)
in H2O (10 mL) at 5 °C. The resulting solution was stirred for 2 h at
10 °C and then slowly added into a vigorously stirred solution of KI
(61.9 g, 0.37 mol) in 150 mL of H2O at 5 °C. The mixture was stirred
at room temperature for 6 h, followed by reflux for 20 min. The residue
was poured into an aqueous solution of NaHSO3 (5%, 300 mL) and
then stirred for 20 min at room temperature. The mixture was extracted
with 250 mL of ethyl acetate, dried over MgSO4, and concentrated to
yield a brown oil. The crude product was purified by column
chromatography (silica gel, hexane) to afford 4 as a colorless oil (12.5
g) in 79% yield. 1H NMR (CDCl3): δ 7.71 (d, J ) 8.32 Hz, 1H), 7.61
(s, 1H), 7.00-6.96 (dd, J ) 8.43, 2.36 Hz, 1H), 1.25 (s, 9H). 13C NMR
(CDCl3): δ 153.3, 139.7, 130.0, 129.5, 125.9, 97.1, 34.6, 31.0.
Experimental Section
Measurements. 1H and 13C NMR spectra were recorded on a Bruker
300 MHz NMR spectrometer using tetramethylsilane (TMS) as the
internal reference (0.00 ppm). Column chromatography was performed
with 32-63 mesh silica gel. Differential scanning calorimetry (DSC)
was carried out on a TA Instruments DSC 2920 differential scanning
calorimeter with a scan rate of 20 °C/min. UV/vis spectra were
measured on a Hewlett-Packard 8453 spectrophotometer. Fluorescence
spectra were obtained with an ISA Fluorolog-3 (JY Horiba) spectro-
fluorimeter with excitation and emission slit widths set at 1.0 nm. Cyclic
voltammetry was performed on a Bioanalytical Systems Inc. model
CV-50W potentiostat in a three-electrode cell with a Pt counter
electrode, a Ag/AgCl reference electrode, and a glassy carbon working
electrode at a scan rate of 100 mV/s with 0.1 M tetrabutylammonium
perchlorate as the supporting electrolyte in degassed CH3CN:THF [9:1
(v/v)] solution purged with argon. At the end of each set of voltammetric
experiments, ferrocene (Fc) was added to the solution in order to correct
the observed potential with a Fc/Fc+ reference potential (0.1 V). Mass
spectra were recorded on a Finnigan TSQ-70 mass spectrometer.
(18) Risko, C.; Kushto, G. P.; Kafafi, Z. H.; Bre´das, J. L. J. Chem. Phys. 2004,
121, 9031; ibid, 2005, 122, 099901.
(19) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S. M. F.; Williams,
I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535.
(20) Kajigaeshi, S.; Kakinami, T.; Yamasaki, H.; Fujisaki, S.; Okamoto, T. Bull.
Chem. Soc. Jpn. 1988, 61, 600.
(21) Tidwell, J. H.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 11797.
(22) Cardenas, D. J.; Sauvage, J. P. Synlett 1996, 9, 916.
9
9072 J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005

Solid-State Asymmetric Spirosilabifluorenes
A R T I C L E S
2-(2-Bromo-4-tert-butylphenyl)-4,4,5,5-tetramethyl-1,3,2-di-
oxaborolane (5). To a solution of 2-bromo-4-tert-butyl-1-iodobenzene
(7.87 g, 23.0 mmol) in 150 mL of THF was slowly added i-PrMgBr
(11.6 mL of a 2 M solution in THF, 23.0 mmol) at -40 °C. The
resulting solution was stirred for 2 h at -40 °C. Subsequently,
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6.98 g, 37.0
mmol) was added rapidly to the resulting intermediate at -40 °C. The
mixture was allowed to warm up to room temperature slowly and then
further stirred for 5 h. The organic mixture was extracted with 200
mL of ethyl acetate, washed with 150 mL of H2O, dried over MgSO4,
and concentrated to yield a colorless oil. The crude product was purified
by column chromatography (silica gel, hexane/chloroform ) 5/1) to
afford 5 as a colorless oil (5.58 g) in 71% yield. 1H NMR (CDCl3): δ
7.58-7.55 (m, 2H), 7.31-7.27 (dd, J ) 7.83, 1.67 Hz, 1H), 1.36 (s,
12H), 1.29 (s, 9H). 13C NMR (CDCl3): δ 155.7, 136.4, 129.8, 123.4,
84.0, 34.8, 30.9, 24.7.
2-(2,2′-Dibromo-4′-tert-butylbiphenyl-4-yl)pyridine (8b). To a
solution of 2,2′-dibromo-4-tert-butyl-4′-iodobiphenyl (1.24 g, 2.51
mmol) in toluene (50 mL) were added 2-tributylstannylpyridine (1.02
g, 2.76 mmol) and Pd(PPh3)4 (96.0 mg, 0.08 mmol). The reaction
mixture was refluxed for 12 h and then diluted with ethyl acetate (100
mL). The organic mixture was washed with brine, dried over MgSO4,
and concentrated to yield a crude solid. The crude product was puri-
fied by column chromatography (silica gel, hexane/ethyl acetate )
1
7/1) to afford 8b as a white solid (1.06 g) in 95% yield. H NMR
(CDCl3): δ 8.68 (d, J ) 4.93 Hz, 1H), 8.34 (s, 1H), 7.97-7.94 (dd, J
) 8.18, 1.73 Hz, 1H), 7.72-7.67 (m, 3H), 7.39-7.36 (dd, J ) 8.18,
2.03 Hz, 1H), 7.32 (d, J ) 7.88 Hz, 1H), 7.23-7.17 (m, 2H), 1.34 (s,
9H). 13C NMR (CDCl3): δ 155.3, 152.8, 149.6, 142.2, 140.3, 138.5,
136.8, 131.4, 130.8, 130.4, 129.4, 125.3, 124.1, 123.0, 122.6, 120.4,
34.6, 31.1.
2,2′-Dibromo-4-tert-butyl-[1,1′;4′,1′′;4′′,1′′′]quaterphenyl (8c). To
a solution of 2,2′-dibromo-4-tert-butyl-4′-iodobiphenyl (1.10 g, 2.23
mmol) in toluene (50 mL) and aqueous saturated Ba(OH)2 (30 mL)
were added 2-biphenylboronic acid (0.46 g, 2.23 mmol) and Pd(PPh3)4
(26.0 mg, 0.02 mmol). The reaction mixture was refluxed for 12 h and
then diluted with ethyl acetate (100 mL). The organic mixture was
washed with brine, dried over MgSO4, and concentrated to yield a crude
solid. The crude product was purified by column chromatography (silica
gel, petroleum ether) to afford 8c as a white solid (0.91 g) in 79%
N-(2,2′-Dibromo-4′-tert-butylbiphenyl-4-yl)acetamide (6). 2-(2-
Bromo-4-tert-butylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (10.0
g, 29.5 mmol), N-(3-bromo-4-iodophenyl)acetamide (13.1 g, 38.5
mmol), Pd(PPh3)4 (0.89 g, 0.77 mmol), and CsF (4.50 g, 29.6 mmol)
were added to a round-bottom flask equipped with a stir bar. The round-
bottom flask was capped with a septum and then evacuated. To the
reaction mixture was added 170 mL of THF via cannula and refluxed
for 24 h. The reaction mixture was then diluted with ethyl acetate (300
mL), washed with brine (200 mL), dried over MgSO4, and concentrated
to yield a tan solid. The crude product was purified by column
chromatography (silica gel, hexane/ethyl acetate ) 4/1) to afford 6 as
a white solid (10.8 g) in 86% yield. 1H NMR (CDCl3): δ 7.89 (s, 1H),
7.64 (s, 1H), 7.62 (s, 1H), 7.53-7.50 (dd, J ) 8.29, 2.06 Hz, 1H),
7.38-7.34 (dd, J ) 8.15, 2.06 Hz, 1H), 7.17 (d, J ) 8.36 Hz, 1H),
7.14 (d, J ) 8.07, 1H), 2.21 (s, 3H), 1.35 (s, 9H). 13C NMR (CDCl3):
δ 167.2, 150.9, 138.4, 136.9, 134.5, 129.5, 129.3, 127.5, 122.7, 121.7,
121.4, 121.0, 116.4, 33.0, 29.5, 22.6. Anal. Calcd for C18H19Br2NO:
C, 50.85; H, 4.50; N, 3.29. Found: C, 50.82; H, 4.23; N, 3.24.
1
yield. H NMR (CDCl3): δ 7.95 (s, 1H), 7.70-7.62 (m, 8H), 7.50-
7.37 (m, 4H), 7.33 (d, J ) 7.89 Hz, 1H), 7.22 (d, J ) 8.00 Hz, 1H),
1.37 (s, 9H). 13C NMR (CDCl3): δ 152.8, 141.7, 140.7, 140.6, 140.2,
138.6, 137.9, 131.4, 130.7, 130.6, 129.5, 128.8, 127.5, 127.4, 127.3,
126.9, 125.5, 124.2, 124.1, 123.2, 34.6, 31.1.
6-(2,2′-Dibromo-4′-tert-butylbiphenyl-4-yl)[2,2′]bipyridinyl (8d).
To a solution of 2,2′-dibromo-4-tert-butyl-4′-iodobiphenyl (2.40 g, 4.86
mmol) in toluene (120 mL) were added 6-(trimethylstannyl)-2,2′-
bipyridine (2.01 g, 6.30 mmol) and Pd(PPh3)4 (0.19 g, 0.16 mmol).
The reaction mixture was refluxed overnight and then diluted with ethyl
acetate (150 mL). The organic mixture was washed with brine, dried
over MgSO4, and concentrated to yield a crude solid. The crude product
was purified by column chromatography (silica gel, hexane/ethyl acetate
2,2′-Dibromo-4-tert-butyl-4′-iodobiphenyl (7). A solution of N-(2,2′-
dibromo-4′-tert-butylbiphenyl-4-yl)acetamide (2.87 g, 6.80 mmol) in
ethanol (10 mL) and concentrated HCl (10 mL) was refluxed for 1 h
and then evaporated in vacuo. To an amine solution in 2 N HCl (50
mL) and THF (5 mL) was slowly added sodium nitrite (0.61 g, 8.80
mmol) in H2O (3 mL) at 5 °C. The resulting solution was stirred for 2
h at 10 °C and then slowly added into a vigorously stirred solution of
KI (8.90 g, 53.6 mmol) in 100 mL of H2O at 5 °C. The mixture was
stirred at room temperature for 6 h, followed by reflux for 20 min.
The residue was poured into an aqueous solution of NaHSO3 (5%, 150
mL) and then stirred for 20 min at room temperature. The mixture
was extracted with 200 mL of ethyl acetate, dried over MgSO4, and
concentrated to yield a dark brown residue. The crude product was
purified by column chromatography (silica gel, hexane) to afford 7 as
1
) 7/1) to afford 8d as a pale yellow solid (1.73 g) in 68% yield. H
NMR (CDCl3): δ 8.71-8.69 (m, 1H), 8.65 (d, J ) 7.83 Hz, 1H), 8.49
(s, 1H), 8.42 (d, J ) 7.49 Hz, 1H), 8.14-8.11 (dd, J ) 8.11, 1.70 Hz,
1H), 7.95-7.79 (m, 3H), 7.69 (s, 1H), 7.43-7.32 (m, 3H), 7.22 (d, J
) 8.10 Hz, 1H), 1.38 (s, 9H). 13C NMR (CDCl3): δ 155.9, 155.8,
154.4, 152.9, 149.0, 142.4, 140.3, 138.7, 137.8, 136.8, 131.4, 130.9,
130.5, 129.6, 129.5, 125.4, 124.2, 123.8, 123.1, 121.2, 120.2, 119.8,
34.7, 31.2.
2,2′-Di-tert-butyl-7,7′-diphenyl-9,9′-spiro-9-silabifluorene (9a). To
a solution of 2,2′-dibromo-4-tert-butyl-[1,1′;4′,1′′]terphenyl (3.92 g, 8.82
mmol) in diethyl ether (200 mL) at -78 °C was added n-BuLi (11.0
mL of 1.6 M in hexane, 17.6 mmol). The reaction solution was allowed
to warm to room temperature and stirred for 6 h. Silicon tetrachloride
(0.72 g, 4.23 mmol) in 10 mL of diethyl ether was added to the resulting
solution at room temperature. The reaction mixture was stirred at room
temperature for 1.5 h and refluxed for 3 h. A yellow solution and a
white precipitate were obtained. The residue was washed with H2O,
dried over MgSO4, and concentrated to yield a crude solid. The crude
product was purified by column chromatography (silica gel, hexane/
ethyl acetate ) 27/1) to afford 9a as a white solid (1.92 g) in 73%
1
a colorless oil (2.41 g) in 72% yield. H NMR (CDCl3): δ 8.01 (s,
1H), 7.69-7.64 (m, 2H), 7.38-7.35 (dd, J ) 8.06, 1.85 Hz, 1H), 7.12
(d, J ) 8.09 Hz, 1H), 6.96 (d, J ) 8.04 Hz, 1H), 1.34 (s, 9H). 13C
NMR (CDCl3): δ 153.2, 141.6, 140.5, 138.0, 136.1, 132.5, 130.3, 129.6,
124.6, 124.3, 122.9, 93.5, 34.7, 31.1. Anal. Calcd for C16H15Br2I: C,
38.90; H, 3.06. Found: C, 38.82; H, 2.93.
2,2′-Dibromo-4-tert-butyl-[1,1′;4′,1′′]terphenyl (8a). To a solution
of 2,2′-dibromo-4-tert-butyl-4′-iodobiphenyl (0.92 g, 1.86 mmol) in
toluene (50 mL) and aqueous saturated Ba(OH)2 (30 mL) were added
2-phenylboronic acid (0.27 g, 2.23 mmol) and Pd(PPh3)4 (21.5 mg,
0.02 mmol). The reaction mixture was refluxed for 12 h and then diluted
with ethyl acetate (100 mL). The organic mixture was washed with
brine, dried over MgSO4, and concentrated to yield a crude solid. The
crude product was purified by column chromatography (silica gel,
1
yield. H NMR (CD2Cl2): δ 8.00 (d, J ) 8.12 Hz, 1H), 7.93 (d, J )
8.21 Hz, 1H), 7.77-7.74 (dd, J ) 8.09, 1.88 Hz, 1H), 7.64 (s, 1H),
7.61-7.58 (dd, J ) 8.18, 1.98 Hz, 1H), 7.56-7.50 (m, 3H), 7.37-
7.23 (m, 3H), 1.27 (s, 9H). 13C NMR (CD2Cl2): δ 151.5, 149.5, 147.6,
140.9, 140.5, 134.3, 133.1, 133.0, 131.4, 130.4, 129.2, 129.0, 127.6,
127.1, 121.6, 121.3, 35.1, 31.5. Anal. Calcd for C44H40Si: C, 88.54;
H, 6.75. Found: C, 88.23; H, 6.58.
1
hexane) to afford 8a as a white solid (0.68 g) in 83% yield. H NMR
(CD2Cl2): δ 7.92 (s, 1H), 7.71 (s, 1H), 7.67-7.61 (m, 3H), 7.51-
7.41 (m, 4H), 7.32 (d, J ) 8.03 Hz, 1H), 7.22 (d, J ) 7.90, 1H), 1.37
(s, 9H).
2,2′-Di-tert-butyl-7,7′-dipyridin-2-yl-9,9′-spiro-9-silabifluorene (9b).
9b was synthesized using the same procedure described above for
9
J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005 9073

A R T I C L E S
Lee et al.
Scheme 1. Synthesis of Asymmetrically Substituted Spirosilabifluorenes
9a. A pale yellow solid was obtained in 68% yield. 1H NMR
(CDCl3): δ 8.60-8.58 (m, 1H), 8.17-8.13 (dd, J ) 8.26, 1.58 Hz,
1H), 8.03-8.00 (m, 2H), 7.94 (d, J ) 8.23 Hz, 1H), 7.68-7.64 (m,
2H), 7.61-7.58 (dd, J ) 8.26, 2.08 Hz, 1H), 7.51 (s, 1H), 7.16-7.11
(m, 1H), 1.27 (s, 9H). 13C NMR (CDCl3): δ 157.1, 151.2, 150.7, 149.5,
147.1, 138.3, 136.6, 133.6, 133.0, 132.7, 131.3, 130.0, 128.7, 121.9,
121.1, 121.0, 120.5, 34.9, 31.3. Anal. Calcd for C42H38N2Si: C, 84.24;
H, 6.40; N, 4.68. Found: C, 83.84; H, 6.17; N, 4.71.
Results and Discussion
Synthesis. Aryl-substituted asymmetric spirosilabifluorene
(SSF) derivatives were synthesized using the method developed
by Gilman:24 lithiation of substituted 2,2′-dibromobiphenyls 8,
followed by spirocyclization with silicon tetrachloride (Scheme
1). Functionalized 2,2′-dibromobiphenyls 8 were obtained from
a single key building block, asymmetric 2,2′-dibromobiphenyl
amide 6. The tert-butyl group was introduced to ensure the
solubility of the resulting spiro-linked siloles and to provide
steric hindrance leading to solid-state amorphous films. We have
employed two different methods for the synthesis of asymmetric
biphenyl 6. The first method (not shown) involved conventional
Sandmeyer reaction25 from 2,2′-diaminobiphenyl to 2,2′-dibro-
mobiphenyl, which resulted in low chemical yields (5-7%) due
to the poor solubility of the products in acidic media. The second
route was based on palladium-catalyzed Suzuki cross-coupling
as depicted in Scheme 1. The synthesis of 1-bromo-2-iodoben-
zene amide 2 was a straightforward (two steps) procedure using
readily available starting material, 3-bromoaniline, with a high
chemical yield (95%). Iodination of 3-bromoaniline was ac-
complished by treatment with benzyltrimethylammonium dichlor-
oiodate (BTMAICl2) followed by acetylation with acetic
anhydride. The other precursor for the Suzuki coupling was pre-
pared by bromination of 4-tert-butylaniline with tetrabutylam-
monium tribromide (Bu4NBr3) in DMF:H2O [7:3 (v/v)] to yield
2-bromo-4-tert-butylaniline 3, which was then converted to
2,2′-Di-tert-butyl-7,7′-dibiphenyl-4-yl-9,9′-spiro-9-silabifluorene
(9c). 9c was synthesized using the same procedure described above
1
for 9a. A white solid was obtained in 65% yield. H NMR (CD2Cl2):
δ 8.03 (d, J ) 8.15, 1H), 7.95 (d, J ) 8.17 Hz, 1H), 7.84-7.81 (dd,
J ) 8.18, 2.10 Hz, 1H), 7.72 (s, 1H), 7.67-7.59 (m, 7H), 7.52 (s, 1H),
7.44-7.39 (m, 2H), 7.35-7.32 (m, 1H), 1.30 (s, 9H). 13C NMR
(CD2Cl2): δ 150.9, 149.0, 147.0, 140.3, 139.7, 139.3, 133.7, 132.5,
132.4, 131.0, 129.6, 128.6, 128.5, 127.1, 126.9, 126.7, 121.0, 120.6,
34.6, 31.0. Anal. Calcd for C56H48Si: C, 89.79; H, 6.46. Found: C,
89.22; H, 6.42.
2,2′-Di-tert-butyl-7,7′-bis(2′,2′′-bipyridin-6-yl)-9,9′-spiro-9-silabi-
fluorene (9d).23 9d was synthesized using the same procedure de-
scribed above for 9a. A pale yellow solid was obtained in 1% yield.
1H NMR (CD2Cl2): δ 8.64-8.62 (m, 1H), 8.55-8.52 (m, 1H),
8.43-8.40 (dd, J ) 8.23, 2.03 Hz, 1H), 8.32-8.29 (dd, J ) 7.69,
1.02 Hz, 1H), 8.14-8.10 (m, 2H), 8.01 (d, J ) 8.29, 1H), 7.83-7.71
(m, 3H), 7.65-7.62 (dd, J ) 8.15, 1.96 Hz, 1H), 7.53 (s, 1H), 7.32-
7.27 (m, 1H), 1.28 (s, 9H). 13C NMR (CDCl3): δ 156.3, 156.2, 155.6,
151.3, 150.8, 148.9, 147.2, 138.5, 137.6, 136.9, 133.4, 133.2, 132.7,
131.3, 130.4, 128.8, 123.7, 121.4, 121.2, 121.0, 120.5, 119.2, 34.9,
31.3.
(24) Gilman, H.; Gorsich, R. D. J. Am. Chem. Soc. 1958, 80, 1883.
(25) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s
Textbook of Practical Organic Chemistry, 5th ed.; John Wiley and Sons:
New York, 1989.
(23) Due to the extremely low product yield (∼1%) of BPySSF 9d, we could
not collect enough highly pure BPySSF 9d (purified by column chroma-
tography, followed by sequential train sublimation).
9
9074 J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005

Solid-State Asymmetric Spirosilabifluorenes
A R T I C L E S
2-bromo-4-tert-butyl-1-iodobenzene 4 via Sandmeyer reaction.
Chemoselective I-Mg exchange26 of 2-bromo-4-tert-butyl-1-
iodobenzene 4 with a Grignard reagent, i-PrMgBr, proceeded
smoothly under mild reaction conditions. Subsequent reaction
with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane gave
2-bromobenzene boronate 5 in good yield (71%).
We initially used a general palladium-catalyzed Suzuki cross-
coupling of iodobenzene 2 with boronate 5 using a mixture of
basic aqueous solution (e.g., aq. Na2CO3 or aq. K2CO3) and
organic solvents (THF or toluene) to make 2,2-dibromobiphenyl
6. Although the isolated product could be obtained in high
purity, the low product yield and difficulty of purification
prompted us to consider alternative synthetic routes. We have
found that palladium-catalyzed coupling of 2-bromobenzene
boronate 5 is in competition with the base-catalyzed protode-
boronation27 of the boronic entity in aqueous solution. To
address this problem, we selected nonaqueous Suzuki cross-
coupling28 using CsF as a base, which is soluble in organic
solvents (e.g. THF, toluene or dioxane) and provides relatively
weak basicity and poor nucleophilicity. This approach proved
useful in synthesizing asymmetric 2,2′-dibromobiphenyl 6,
because the reaction proceeded selectively, and could be used
with a variety of functional groups and led to reasonably high
chemical yields (86%).
Figure 1. Differential scanning calorimetric (DSC) thermograms of tBSSF,
PhSSF, and BPhSSF measured in a dry nitrogen atmosphere at a heating
rate of 20 °C/min.
Table 1. Thermal Properties of Asymmetric Spirosilabifluorenes
compd
Tm
(°
C)
Tg
(°
C)
compd
Tm
(°
C)
Tg (°C)
PhSSF
PySSF
314
334
203
203
BPhSSF
tBSSF
382
248
228
141
temperature. This suggests that the introduction of both the tert-
butyl and phenyl groups on the SSF molecule gives rise to an
increase in the number of conformers and steric hindrance. The
asymmetric substitution coupled with the nonplanar molecular
structure leads to the dramatic increase in morphological
stability. In addition, this value is 125 °C higher than that of
the air-stable electron transporter PyPySPyPy (Tg ) 77 °C),14
demonstrating the superior morphological stability of asym-
metric PhSSF 9a, which we attribute to the spiro-linked bi-
fluorene moiety. Further heating above the glass transition tem-
perature resulted in the appearance of exothermic peaks due to
crystallization at 239 °C. This phenomenon is commonly
observed in organic materials.
Asymmetric PySSF 9b exhibited a melting point of 334 °C
and a Tg of 203 °C, respectively, which are similar to those for
PhSSF 9a (Table 1). Asymmetric BPhSSF 9c exhibited a quite
high Tg of 228 °C, which is 25 °C higher than those of
asymmetric PhSSF 9a and PySSF 9b, and 87 °C higher than
that of symmetric tBSSF (Tg ) 141 °C). This result suggests
that the improved morphological stability of asymmetric
BPhSSF 9c results from an increase in molecular weight and
volume. The Tgs of asymmetric SSFs increase linearly with the
molecular weight of substituent incorporated into SSF moiety,
as illustrated in Figure 1. This relationship thus may be useful
in estimating the Tgs of asymmetric SSFs. As a result, we predict
that BPySSF 9d has a Tg higher than that of PySSF 9b. It should
be also noted that amorphous neat films (∼300 nm) of asym-
metric SSFs prepared by vacuum vapor deposition onto quartz
do not show any sign of crystallization in ambient atmosphere.
Optical Properties. The optical properties of asymmetric
SSFs were measured both in chloroform solutions and in neat
solid-state films.
Biphenyl 6 was converted into 4-amino-4′-tert-butyl-2,2′-
dibromobiphenyl via hydrolysis, followed by Sandmeyer reac-
tion to afford 4-tert-butyl-4′-iodo-2,2′-dibromobiphenyl 7 in
good yield (73%). Subsequent selective palladium-catalyzed
Suzuki or Stille coupling of 2,2′-dibromo-4-iodobiphenyl 7 with
aryl boronic acids or aryl tin derivatives successfully pro-
vided functionalized 2,2′-dibromobiphenyls 8 in high yields
(68-85%). The series of asymmetric SSFs was purified by
column chromatography followed by sequential train sublima-
tion under high vacuum (∼10-6 Torr). The purity and chemical
1
structures of these compounds were confirmed by H NMR,
13C NMR, and elemental analysis.
Thermal Properties. The glass transition temperatures (Tg)
of asymmetric SSFs were determined by differential scanning
calorimetry (DSC) in a nitrogen atmosphere. A heating rate of
20 °C/min was used after first melting the compound and
subsequent rapid cooling to room temperature under ambient
conditions. The Tg of symmetric SSF, 2,2′,7,7′-tetra-tert-butyl-
9,9′-spiro-9-silabifluorene (tBSSF), was also measured and
compared to those of asymmetric SSFs in order to investigate
the effect of introducing an asymmetric functionality on the SSF
ring, in particular with regard to its thermal and morphological
stability. The crystalline sample of asymmetric PhSSF 9a,
purified by vacuum sublimation, melted at 314 °C to give
isotropic liquids upon heating during the first scan and then
changed into a glassy state upon cooling. As shown in Figure
1, when the amorphous glassy sample was heated again, a quite
high Tg of 203 °C was observed. In contrast, no glass transition
temperature was determined for the unmodified symmetric 9,9′-
spiro-9-silabifluorene,29 which is easily crystallized at room
(a) In Chloroform Solutions. The normalized absorbance and
photoluminescence (PL) spectra of asymmetric SSFs in chlo-
roform solutions measured at room temperature are shown in
Figure 2. Their photophysical properties are summarized in
Table 2. The absorbance spectrum of asymmetric PhSSF 9a is
characterized by a strong absorption peak centered around 307
nm with a weak shoulder at 347 nm. This spectrum should be
similar to that of 2-tert-butyl-7-phenylsilafluorene due to the
(26) Poirier, M.; Chen, F.; Bernard, C.; Wong, Y.-S.; Wu, G. G. Org. Lett.
2001, 3, 3795.
(27) (a) Kuivila, H. G.; Reuwer, J. F.; Mangravite J. A. J. Am. Chem. Soc.
1964, 86, 2666. (b) Muller, D.; Fleury, J.-P. Tetrahedron Lett. 1991, 32,
2229.
(28) Wright, S. W.; Hageman, D. L.; McClure, L. D. J. Org. Chem. 1994, 59,
6095.
(29) Lee, S. H.; Kafafi, Z. H. Am. Chem. Soc. Org. Prepr. 2002, 224, 490.
9
J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005 9075

A R T I C L E S
Lee et al.
Figure 2. Normalized absorbance and photoluminescence spectra of PhSSF
(open circles), BPhSSF (open triangles), and PySSF (closed circles) in
chloroform solutions.
Figure 3. Normalized absorbance and photoluminescence spectra of neat
solid-state films of PhSSF (open circles), BPhSSF (open triangles), and
PySSF (closed circles).
Table 2. Photophysical Properties of Asymmetric
Spirosilabifluorenes
which is significantly red-shifted relative to that of 2-phen-
ylfluorene (λem ) 343 and 329 nm),30 reflecting the important
role played by silicon in the modified silacyclopentadiene ring.
A larger bathochromic shift can be also obtained by lengthening
the conjugated system with the addition of a phenyl group to
the 9-silafluorene ring. A bathochromic shift of 10 nm was
indeed observed for BPhSSF 9c compared to that for PhSSF
9a, yielding an intense violet-blue emission with a maximum
centered at 393 nm. On the other hand, the electron-accepting
pyridyl groups in PySSF 9b showed no significant influence
on its PL spectrum, suggesting that the inductive effect of the
pyridyl groups may be counterbalanced by the extended
π-conjugation due to the reduced ortho-ortho hydrogen interac-
tion between the silole core and the pyridine unit. Considering
that the PL spectra of previously studied 2,5-diaryl-3,4-
diphenylsiloles strongly depend on the nature of aryl groups at
the 2- and 5-positions,10 all three asymmetric SSFs do not show
significant spectral changes because the electronic interaction
is minimal between the silicon atom and the substituents on
the 9-silafluorene ring. The observed Stokes shifts were
measured to be 6464 cm-1 (0.80 eV) for PhSSF 9a, 5368 cm-1
(0.67 eV) for PySSF 9b, and 6301 cm-1 (0.78 eV) for BPhSSF
9c. In asymmetric SSFs, the aryl-substituted 9-silafluorene may
be considered as a modified p-oligophenylene in which the
single bonds between the phenyl rings have double bond
character, i.e. more planar configuration in the excited state.32
Nonplanar configuration of aryl-substituted 9-silafluorene in the
ground state may differ from its excited-state geometry (more
planar configuration), leading to the large Stokes shifts between
the absorption and emission spectra.
λ
abs (nm)
λPL (nm)
a
compd
CHCl3
filmb
CHCl3
filmb
ΦPL,film
Egopt (eV)c
PhSSF
PySSF
BPhSSF
307
317
315
304
313
318
383
382
393
398
415
410
0.44
0.31
0.54
3.38
3.39
3.29
a Determined using an integrating sphere. b Neat films prepared by
vacuum deposition on quartz substrates (thickness ∼300 nm). c Estimated
from the tail of the absorption spectra.
tetrahedral bonding arrangement around the silicon atom and
the lack of any π-conjugation. The latter should be identical to
that of 2-phenylsilafluorene as a result of the weak inductive
effect of the tert-butyl group. Similarly, the optical properties
of 2,2′-diphenylspirobifluorene, the carbon analogue of asym-
metric PhSSF 9a, are expected to be analogous to those of
2-phenylfluorene. Hence, one may compare the optical spectrum
of PhSSF 9a to that of 2-phenylfluorene. The maximum peak
observed for asymmetric PhSSF 9a exhibits a significant red
shift relative to that of 2-phenylfluorene (λmax ) 287 nm),30
suggesting strong σ*-π* conjugation between the σ* orbital
of the exocyclic Si-C bond and the π* orbital of the p-terphenyl
moiety in asymmetric PhSSF 9a. This observation is consistent
with a recent study on the distinct photophysical properties
observed for 7-sila-7,7-dimethyl-7H-dibenzo[c,g]fluorene (e.g.,
bathochromic shifts in both absorbance and fluorescence spectra)
relative to those of the corresponding acyclic analogues.31 When
compared to that of tBSSF (λabs ) 287 and 337 nm),29 the optical
absorption of PhSSF 9a is bathochromically shifted by 10-20
nm, indicating that the phenyl substituent plays a role in
determining the gap between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular or-
bital (LUMO). Comparing the absorbance spectra of PySSF 9b
and BPhSSF 9c to that of PhSSF 9a, the former showed a
slight red shift (8-10 nm) in the lowest energy electronic
transition.
(b) Neat Solid-State Films. As shown in Figure 3, the
absorbance spectra of neat solid-state films of asymmetric SSFs
are very similar to those measured in chloroform solutions,
suggesting that these are true molecular solids with minimal
intermolecular interactions in the solid state. The optical band
gaps (Egopt) of the asymmetric SSFs determined from the onset
of the solid-state absorbance spectra are summarized in Table
2. All three asymmetric SSFs have optical band gaps in the
range 3.29 to 3.39 eV. The PL spectra of the three SSFs in
Upon excitation at 320 nm, asymmetric PhSSF 9a showed
an intense violet-blue emission centered at 383 nm (Figure 2),
(30) Berlman, I. B. J. Chem. Phys. 1970, 52, 5616.
(31) Hoshi, T.; Tomonobu, N.; Suzuki, T.; Ando, M.; Hagiwara, H. Organo-
metallics 2000, 19, 4483.
(32) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules,
2nd ed.; Academic Press: New York, 1971.
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9076 J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005

Solid-State Asymmetric Spirosilabifluorenes
A R T I C L E S
Table 3. Electrochemical Properties of Asymmetric
Spirosilabifluorenes
compd
Eonsetox (V)a
Eonsetred (V)a
EHOMO (eV)b
ELUMO (eV)b
Eg (eV)
PhSSF
PySSF
BPhSSF
Alq3
1.09
1.17
1.11
0.73c
-
-5.79
-5.87
-5.81
-5.53
-
-
-2.24
-2.42
-2.30c
-2.46
-2.28
-2.50
3.41
3.53
3.03
a Determined from cyclic voltammetry in CH3CN:THF [9:1 (v/v)] using
Ag/AgCl (0.01 M) as a reference electrode at a scan rate of 100 mV/s.
b HOMO and LUMO levels were determined using the following equations:
ox
red
EHOMO (eV) ) -e(Eonset + 4.8), ELUMO (eV) ) -e(Eonset + 4.8).
c Reference 36.
The electrochemical reduction process of PhSSF 9a was not
observed. Upon changing the substituents on the 9-silafluorene
ring, all three asymmetric SSFs had similar anodic peak
potentials (Epa) of around 1.30 V, suggesting that the HOMO
is primarily localized on the silafluorene and its energy level is
not affected by the aryl substituents. The HOMO and LUMO
levels for asymmetric SSFs were estimated on the basis of an
oxidation potential of -4.8 eV (below the vacuum level) for
the ferrocene/ferrocenium (Fc/Fc+).35 The onset potentials of
oxidation and reduction for BPhSSF 9c were determined to be
1.11 and -2.42 V, respectively (vs Ag/AgCl), corresponding
to 1.01 and -2.52 V (vs Fc/Fc+). Thus, the HOMO and LUMO
levels for BPhSSF 9c are estimated to be -5.81 and -2.28
eV, respectively. Similarly, the HOMO and LUMO levels for
PySSF 9b are -5.87 and -2.46 eV, respectively. In Figure 4,
the reduction CV curve of PySSF 9b shows a relatively low
(i.e. large electronic affinity) onset potential (-2.34 V vs
Fc/Fc+), maintaining a wide (HOMO-LUMO) electrochem-
ical band gap (3.41 eV). Hence PySSF 9b may be a good
candidate as an electron transporter as well as an efficient
hole blocker. Its reduction potential is nearly identical to that
of Alq3 (ca. -2.30 V vs Fc/Fc+),36 the most commonly used
electron-transporting material in OLEDs but considerably
larger than that of the air-stable electron-transporting material
PyPySPyPy (-2.05 V vs Fc/Fc+).37 The estimated electro-
chemical band gaps of asymmetric SSFs (3.4-3.5 eV) are
consistent with the measured optical band gaps (3.3-3.4 eV)
listed in Table 3.
Figure 4. Cyclic voltammograms of BPhSSF (dashed line) and PySSF
(solid line) measured in CH3CN:THF [9:1 (v/v)] at a scan rate of
100 mV/s.
solution are almost identical, whereas a significant broadening
in the solid-state PL spectrum of PySSF 9b is observed. This
behavior suggests that the excited-state geometry of PySSF 9b
may favor an effective coplanar configuration in the solid state,
which is a result of the small dihedral angle18 between the ortho-
hydrogen-free pyridine and the 9-silafluorene ring, allowing
better π orbital overlap. However, no fluorescence due to
excimers was detected, indicating that the rigidity of the spiro-
linkage prevents strong intermolecular interactions. The absolute
photoluminescence quantum yields (ΦPL) of the neat films of
asymmetric SSFs were measured using an integrating sphere33
and are given in Table 2. The absolute PL quantum yields were
found to be quite high between 0.31 and 0.54 (λex ) 325 nm),
suggesting that asymmetric SSFs show an aggregation-induced
emission (AIE) in the solid state. The enhancement of the
absolute PL quantum yield (ΦPL ) 0.54) for asymmetric
BPhSSF 9c is a direct result of both the unusually high PL
quantum yield of the silole as an AIE active material and the
incorporation of the highly fluorescent biphenyl moiety on the
silole ring.
Electrochemical Properties. The electrochemical properties
of asymmetric SSFs were characterized using cyclic voltam-
metry (CV) versus Ag/AgCl with a ferrocene/ferrocenium
internal standard. An estimate of their HOMO and LUMO
energy levels was deduced from their redox potentials. Figure
4 shows cyclic voltammograms of the three asymmetric SSFs
that undergo single reduction and single oxidation; both
processes are chemically irreversible. Irreversible reduction may
be due to the extremely short lifetime (on the order of several
nanoseconds) of the corresponding radical anions that cannot
be detected on the CV time scale.34 Similar irreversible redox
processes have been observed in many silole derivatives,
suggesting that the redox properties of 9-silafluorenes are
governed by the modified silole ring of the asymmetric SSFs.
When sweeping cathodically, BPhSSF 9c displayed a cathodic
peak potential (Epc) of -2.70 V, whereas PySSF 9b showed a
smaller reduction peak potential of -2.45 V, reflecting the
electron-accepting nature of the pyridyl group in PySSF 9b.
Conclusion
In summary, we have described a facile route for the synthesis
of highly fluorescent asymmetrically aryl-substituted spirosi-
labifluorenes. The materials exhibited excellent thermal and
morphological stability. The measured high glass transition
temperatures (Tg ) 203-228 °C) were attributed to the increased
rigidity of the spiro-linked orthogonal bifluorene moieties. The
absorbance spectra of the asymmetric spirosilabifluorenes
showed significant bathochromic shifts relative to those of their
carbon analogues as a result of the effective σ*-π* conjugation
between the σ* orbital of the exocyclic Si-C bond and the π*
orbital of the oligoarylene fragment. All of the asymmetric
spirosilabifluorenes were found to exhibit intense violet-blue
(35) Lee, S. H.; Jang, B.-B.; Tsutsui, T. Macromolecules 2002, 35, 1356.
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(34) Moiseeva, A. A.; Rakhimov, R. D.; Beloglazkina, E. K.; Butin, K. P.;
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(37) Uchida, M. Private communication.
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A R T I C L E S
Lee et al.
emission (λPL ) 398-415 nm) with high solid-state photolu-
minescence quantum yields (30-55%), suggesting that they are
excellent candidates as emissive materials in OLEDs and solid-
state lasers. PySSF has a low-lying LUMO energy level (-2.46
eV) and a relatively wide electrochemical (HOMO-LUMO)
band gap (3.41 eV). Further studies on the asymmetric SSFs
and their incorporation as electroactive materials (violet-blue
emitters and/or electron transporters) in OLEDs are in progress.
Acknowledgment. This work was financially supported by
the Office of Naval Research.
Note Added after ASAP Publication: There were two
typographical errors, one in ref 23 and one in footnote b of
Table 3, in the version published on the Web June 7, 2005.
The version published June 9, 2005 and the print version are
correct.
JA042762Q
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