Impact of the linker on the electronic and luminescent properties of diboryl compounds: Molecules with two BMes2 Groups and the peculiar behavior of l,6-(BMes2)2pyrene

Article abstract of DOI:10.1021/om800856g

To investigate the impact of the linker on the electronic and photophysical properties of diboryl compounds, three new diboryl compounds that contain two BMes₂ groups (Mes = mesityl) have been synthesized, including a planar, 1,6-(BMes₂)₂pyrene (1), a V-shaped bis(p-BMes ₂phenyl)diphenylsilane (4), and a U-shaped l,8-bis(p-BMes ₂phenyl)naphthalene (5). For comparison, two previously known compounds, p-(BMes₂)₂benzene (3) and l,8-bis(p-BMes ₂-biphenyl)naphthalene (6), were also investigated. The aromatic linkers in these molecules have been found to have a dramatic impact on the electron-accepting ability and Lewis acidity of the diboryl compounds through their distinct steric and electronic properties. Compound 1 has the most positive reduction potential (E_1/2^redl = -1.81 V, relative to FeCp₂^0/+), while 5 has the most negative reduction potential (E_1/2^redl = -2.34 V). All compounds are blue emitters with considerable variation of emission energy and efficiencies (e.g., λ_em = 446, 402, 395 nm, = ～1.0, 0.17, ～1.0 for 1, 4, and 5, respectively), and each displays a distinct and selective response toward fluoride ions. Upon addition of fluoride ions, compound 1 displays an unusual red shift and an on-off response in both absorption and fluorescent spectra. By comparing the behavior of 1 to that of the monoboryl compound l-BMes₂pyrene (2) and 3, and with TD-DFT computations on 1 and its fluoride adducts 1F and 1F₂, it has been found that the peculiar response of 1 toward fluoride ions is caused by the dominance of pyrene π orbitals at the HOMO level of 1F and the relatively low-energy charge transfer from the pyrene ring to the three-coordinate boron center in 1F. The crystal structures of 2, 4, 1F₂, and 5F₂ were determined by X-ray diffraction analyses. The potential use of compound 1 as either a blue emitter or a bifunctional emitter in OLEDs has been demonstrated by the successful fabrication of double- and triple-layer electroluminescent devices.

Full text of DOI:10.1021/om800856g

6446
Organometallics 2008, 27, 6446–6456
Impact of the Linker on the Electronic and Luminescent Properties
of Diboryl Compounds: Molecules with Two BMes2 Groups and the
Peculiar Behavior of 1,6-(BMes2)2pyrene
Shu-Bin Zhao,† Philipp Wucher,† Zachary M. Hudson,† Theresa M. McCormick,†
Xiang-Yang Liu,† Suning Wang,*,† Xiao-Dong Feng,‡ and Zheng-Hong Lu‡
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, K7L 3N6, Canada, and Department of
Materials Sciences and Engineering, UniVersity of Toronto, Toronto, Ontario, M5S 3E5, Canada
ReceiVed September 3, 2008
To investigate the impact of the linker on the electronic and photophysical properties of diboryl
compounds, three new diboryl compounds that contain two BMes2 groups (Mes ) mesityl) have been
synthesized, including a planar 1,6-(BMes2)2pyrene (1), a V-shaped bis(p-BMes2phenyl)diphenylsilane
(4), and a U-shaped 1,8-bis(p-BMes2phenyl)naphthalene (5). For comparison, two previously known
compounds, p-(BMes2)2benzene (3) and 1,8-bis(p-BMes2-biphenyl)naphthalene (6), were also investigated.
The aromatic linkers in these molecules have been found to have a dramatic impact on the electron-
accepting ability and Lewis acidity of the diboryl compounds through their distinct steric and electronic
red1
properties. Compound 1 has the most positive reduction potential (E1/2
) -1.81 V, relative to
red1
FeCp20/+), while 5 has the most negative reduction potential (E1/2
) -2.34 V). All compounds are
blue emitters with considerable variation of emission energy and efficiencies (e.g., λem ) 446, 402, 395
nm, Φ ) ∼1.0, 0.17, ∼1.0 for 1, 4, and 5, respectively), and each displays a distinct and selective
response toward fluoride ions. Upon addition of fluoride ions, compound 1 displays an unusual red shift
and an on-off response in both absorption and fluorescent spectra. By comparing the behavior of 1 to
that of the monoboryl compound 1-BMes2pyrene (2) and 3, and with TD-DFT computations on 1 and its
fluoride adducts 1F and 1F2, it has been found that the peculiar response of 1 toward fluoride ions is
caused by the dominance of pyrene π orbitals at the HOMO level of 1F and the relatively low-energy
charge transfer from the pyrene ring to the three-coordinate boron center in 1F. The crystal structures of
2, 4, 1F2, and 5F2 were determined by X-ray diffraction analyses. The potential use of compound 1 as
either a blue emitter or a bifunctional emitter in OLEDs has been demonstrated by the successful fabrication
of double- and triple-layer electroluminescent devices.
U-shaped diboryl molecule, 1,8-di(p-dimesitylborylbiphenyl)-
naphthalene, binds to fluoride with a binding constant similar
to those of the corresponding monoboron compounds.22 How-
ever, Gabbai and co-workers have shown that when two
triarylboron centers are attached directly to the 1,8-positions of
naphthalene, the two boron centers can “chelate” to fluoride
ions, giving a molecule with an exceptionally high affinity
toward fluoride.10 Most recently, Mu¨llen and co-workers have
observed an unusual spectral red shift in a large conjugated
diboryl system on addition of fluoride ions,24 differing from
Introduction
Conjugated triarylboron systems are well known to be
effective as fluorescent emitters, charge transport materials in
organic light emitting diodes,1-7 and highly selective sensors
for fluoride ions.8-23 These promising applications have recently
generated interest in molecules with multiple boron centers that
may serve as better anion sensors or emitters/electron transport
materials in OLEDs. Electronic effects are known to play a key
role in the chemistry of triarylboron centers; for example, we
have recently reported a 2,2′-bipy-linked diboryl molecule, 5,5′-
bis(dimesitylboron)-2,2′-bipy (B2bpy), that displays greatly
enhanced Lewis acidity due to the electronegative pyridine
linker.21 In addition, the geometry of the linker is of key
importance. For example, we have recently observed that a
(7) Wakamiya, A.; Mori, K.; Yamaguchi, S. Angew. Chem., Int. Ed.
2007, 46, 4237.
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123, 11372.
* Corresponding author. E-mail: wangs@chem.queensu.ca.
† Queen’s University.
(11) Melaimi, M.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2005, 127, 9680.
(e) Chiu, C. W.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2006, 128, 14248.
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‡ University of Toronto.
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10.1021/om800856g CCC: $40.75
2008 American Chemical Society
Publication on Web 11/19/2008

Electronic Properties of Diboryl Compounds
Organometallics, Vol. 27, No. 24, 2008 6447
Chart 1
the blue shift that is typically observed.21-23 Clearly, the
electronic properties and the geometry of the linker have a
significant impact on the Lewis acidity and the fluorescent
response of polyboron compounds toward anions such as
fluorides. To have a better understanding of the impact of the
linker on the electronic and photophysical properties of diboryl
molecules, we have extended our investigation to a series of
new diboryl molecules with various linkers and distinct mo-
lecular shapes, as shown in Chart 1. As examples of linear or
planar conjugated diboryl molecules, we examined 1,6-bis(di-
mesitylboron)pyrene (B2pyrene, 1) and compared this to the
monoboron-substitutedpyrene,1-(dimesitylboron)pyrene(B1pyrene,
2), and the previously known p-(BMes2)2benzene,28 3. Pyrene
is known to be a highly efficient blue fluorescent emitter in
solution, but not suitable for applications in OLEDs due to its
tendency to produce excimer emission.25-27 Decoration of
pyrene by sterically bulky groups has been shown to be highly
effective in reducing excimer emission,27 making 1 a potentially
attractive candidate for use in OLEDs. Furthermore, in contrast
to the electronegative 2,2′-bipy linker, pyrene is a large electron-
rich conjugated π system, which may lead to interesting
properties in its diboryl derivative. We have also investigated
a series of compounds in which the two boron centers do not
lie in direct conjugation: the flexible diphenylsilane derivative
4 and the naphthyl-linked compounds 5 and 6. Our investiga-
tions have produced some interesting and unexpected findings,
the details of which are presented herein.
Experimental Section
All reactions were performed under N2 with standard Schlenk
techniques unless otherwise noted. All starting materials were
purchased from Aldrich Chemical Co. and used without further
purification. DMF, THF, Et2O, and hexanes were purified using
an Innovation Technology Co. solvent purification system. CH2Cl2
was freshly distilled over P2O5 prior to use. Deuterated solvents
were purchased from Cambridge Isotopes and were used as received
without further drying. NMR spectra were recorded on Bruker
Avance 400 or 500 MHz spectrometers. High-resolution mass
spectra were obtained from a Waters/Micromass GC-TOF EI-MS
spectrometer, which was internally calibrated before use. 1,6-
dibromopyrene,29 1,4-bis(dimesitylboryl)benzene (3)28 p-bromophe-
nyldimesitylborane,5 and 1,8-bis(4-dimesitylborylbiphen-4′-yl)naph-
thalene22 (6) were prepared according to previously reported
procedures.
Cyclic voltammetry was performed using a BAS CV-50W
analyzer with a scan rate of 500 to 4 V/s and a typical concentration
of 5 mg of analyte in 3.0 mL of DMF using 0.10 M tetrabutylam-
monium hexafluorophosphate (TBAP) as the supporting electrolyte.
A conventional three-compartment electrolytic cell consisting of a
Pt working electrode, a Pt auxiliary electrode, and a Ag/AgCl
reference electrode was employed using the ferrocene/ferrocenium
couple as the internal standard (E1/2 ) 0.55 V). UV-vis spectra
were recorded on an Ocean Optics UV-visible spectrometer.
Excitation and emission spectra were recorded on a Photon
Technologies International QuantaMaster model C-60 spectrometer.
Emission lifetimes were measured on a Photon Technologies
International Phosphorescent spectrometer (Time-Master C-631F)
equipped with a xenon flash lamp and digital emission photon
multiplier tube using a band pathway of 5 nm for excitation and 2
nm for emission.
(18) Sundararaman, A.; Venkatasubbaiah, K.; Victor, M.; Zakharov,
L. N.; Rheingold, A. L.; Ja¨kle, F. J. Am. Chem. Soc. 2006, 128, 16554.
(19) Parab, K.; Venkatasubbaiah, K.; Ja¨kle, F. J. Am. Chem. Soc. 2006,
128, 12879.
(20) Ja¨kle, F. Coord. Chem. ReV. 2006, 250, 1107.
(21) Sun, Y.; Ross, N.; Zhao, S.-B.; Huszarik, K.; Jia, W. L.; Wang,
R. Y.; Wang, S. J. Am. Chem. Soc. 2007, 129, 7510.
(22) Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem., Int. Ed. 2006, 45,
5475.
(23) Bai, D. R.; Liu, X. Y.; Wang, S. Chem.-Eur. J. 2007, 13, 5713.
(24) Zhou, G.; Baumgarten, M.; Mu¨llen, K. J. Am. Chem. Soc. ASAP
article, published online (Aug 20, 2008).
(28) (a) Kaim, W.; Schulz, A. Angew. Chem., Int. Ed. Engl. 1984, 23,
615. (b) Schulz, A.; Kaim, W. Chem. Ber. 1989, 122, 1863.
(25) Mikroyannidis, J. A.; Fenenko, L.; Adachi, C. J. Phys. Chem. B
2006, 110, 20317.
Synthesis of 1,6-bis(dimesitylboryl)pyrene, 1. To a stirred THF
(40 mL) solution of 1,6-dibromopyrene (0.40 g, 1.1 mmol) at -78
°C was added dropwise, via syringe, a n-BuLi solution (1.6 M)
(1.44 mL, 2.3 mmol) over 10 min. The resulting light yellow
solution was stirred for 1 h at -78 °C, and an Et2O (15 mL) solution
of dimesitylboron fluoride (0.72 g, 90%, 2.4 mmol) was then slowly
added. After stirring at -78 °C for 1 h, the reaction mixture was
warmed to ambient temperature and stirred overnight, affording a
(26) Kim, Y. H.; Yoon, D. K.; Lee, E. H.; Ko, Y. K.; Jung, H.-T. J.
Phys. Chem. B 2006, 110, 20836.
(27) Wu, K.-C.; Ku, P.-J.; Lin, C.-S.; Shih, H.-T.; Wu, F.-I.; Huang,
M.-J.; Lin, J.-J. AdV. Funct. Mater. 2008, 18, 67.
(29) (a) Suenaga, H.; Nakashima, K.; Mizuno, T.; Takeuchi, M.;
Hamachi, I.; Shinkai, S. J. Chem. Soc., Perkin Trans.1 1998, 1263. (b)
Grimshaw, J.; Trocha-Grimshaw, J. J. Chem. Soc., Perkin Trans. 1 1972,
1622.

6448 Organometallics, Vol. 27, No. 24, 2008
Zhao et al.
yellow suspension. An aqueous solution (20 mL) of NH4Cl (0.26
g) was added and stirred for 30 min. After separation of the aqueous
layer, the organic suspension was concentrated and the residue was
purified by flash chromatography on silica gel using CHCl3 as the
eluent to afford compound 1 as a yellow powder, which was further
ZnCl2 solid (0.24 g, 1.8 mmol) was added. After 1 h at -78 °C,
the reaction mixture was warmed to ambient temperature. 1,8-
diiodonaphthalene (0.30 g, 0.8 mmol) and Pd(PPh3)4 (0.13 g, 0.12
mmol) were then added successively to this solution, and the
mixture was stirred at ambient temperature for 2 days. After the
removal of the solvent, the residue was dissolved in CH2Cl2 (50
mL) and an aqueous Na4EDTA solution (50 mL, 0.20 M, prepared
from EDTA with 5 equiv of Na2CO3). The mixture was stirred for
30 min, and the aqueous layer was then separated and extracted
with CH2Cl2 (2 × 50 mL). The organic layer was dried with Na2SO4
and concentrated, and the residue purified by flash chromatography
on silica gel using hexanes/CH2Cl2 (1:5) as the eluent to afford
1
purified by recrystallization with acetone (0.41 g, 53% yield). H
3
NMR (500 MHz, CD2Cl2): δ 8.21 (d; J ) 9 Hz; 2H, pyrene),
8.09 (d; 3J ) 8 Hz; 2H, pyrene), 7.95 (d; 3J ) 8 Hz; 2H, pyrene),
3
7.87 (d; J ) 9 Hz; 2H, pyrene), 6.87 (s, 8H, B(Mes)2), 2.35 (s,
12H, p-CH3 of B(Mes)2), 1.56 (s, 24H, o-CH3 of B(Mes)2), ppm.
13C NMR could not be obtained due to the poor solubility of 1.
HRMS: calcd for C52H52B2Na [M + Na]+ m/z 721.4152, found
721.4153. Anal. Calcd for C52H52B2: C 89.40, H 7.50. Found: C
88.78, H 7.40.
1
compound 3 as a white solid (0.36 g, 58% yield). H NMR (400
3
MHz, CDCl3): δ 7.91 (dd; J ) 8.0 Hz; 2H, naphthalene), 7.54
3
(dd; J ) 8.4 Hz; 2H, naphthalene), 7.35 (m; 2H, naphthalene),
Synthesis of 1-(Dimesitylboryl)pyrene, 2. To a stirred THF (40
mL) solution of 1-bromopyrene (0.40 g, 1.43 mmol) at -78 °C
was added dropwise, via syringe, a n-BuLi solution (1.6 M) (0.94
mL, 1.5 mmol) over 10 min. The resulting light yellow solution
was stirred for 1 h at -78 °C, and an Et2O (10 mL) solution of
dimesitylboron fluoride (0.48 g, 90%, 1.6 mmol) was then slowly
added. After stirring at -78 °C for 1 h, the reaction mixture was
warmed to ambient temperature and stirred overnight, affording a
yellow solution. After the removal of the solvent, the residue was
purified by flash chromatography on silica gel using hexanes as
the eluent to afford compound 2 as a yellow solid, which was then
recrystallized by slow evaporation of its CH2Cl2/hexanes (1:1)
solution to give yellow crystals (0.39 g, 61% yield). 1H NMR (500
3
3
7.25 (d; J ) 8.0 Hz; 4H, bridging -Ph-), 7.07 (d; J ) 8.0 Hz;
4H, bridging -Ph-), 6.80 (s; 8H, B(Mes)2), 2.32 (s; 12H p-CH3
of B(Mes)2), 1.92 (s; 24H, o-CH3 of B(Mes)2). 13C NMR: δ 147.93,
141.53, 141.27, 140.73, 140.70, 138.31, 137.24, 135.51, 131.11,
129.51, 128.77, 128.20, 128.09, 125.34, 23.80, 21.22. HRMS: calcd
for C58H58B2 [M]+ m/z 776.4725, found 776.4763. Anal. Calcd for
C58H58B2 C 89.69; H 7.53. Found: C 90.04; H 7.75.
Fabrication of Electroluminescent Devices. Two different EL
devices have been produced using a K. J. Lesker OLED cluster
tool with six high-vacuum process chambers: (A) ITO-CuPc (25
nm)/NPB (45 nm)/1 (40 nm)/LiF(1 nm)-Al and (B) ITO-CuPc (25
nm)/NPB (45 nm)/1 (40 nm)/TPBi (10 nm)/LiF (1 nm)-Al. All
materials were deposited by vacuum on 2 in. × 2 in. ITO-coated
glass substrates. The patterned ITO surface was sequentially cleaned
in acetone, methanol, and deionized water, and with UV ozone
treatment. All the testing devices have an active area of 2 × 1
mm2. The base pressures of the organic and metalization chambers
are 4 × 10-8 and 1.9 × 10-6 Torr, respectively. The pressures
during the deposition process in the two chambers are lower than
4.6 × 10-6 Torr. The growth rates are ∼1.5 Å s-1 for organic
materials, 0.1 Å s-1 for LiF, and ∼1.5 Å s-1 for aluminum.
Luminance-current density-voltage (L-J-V) characteristics were
determined in ambient atmosphere using a HP 4140B pA meter
and a Minolta LS-110 m. The dwell time for each testing point is
2 s. EL spectra were recorded using an USB2000-UV-vis miniature
fiber optic spectrometer.
Molecular Orbital Calculations. The Gaussian suite of pro-
grams (Gaussian 03)30 employing density functional theory (DFT)
includingBecke’sthree-parameterhybridmethodswithLee-Yang-Par
correlation functions (B3LYP) was used for all calculations. Crystal
structures were used as the starting point for geometry optimizations
where possible. For compounds where no crystal structure was
available, the starting geometries were prepared by modifying
optimized structures of similar compounds using Gaussview
software. All compounds were fully optimized at the B3LYP/6-
311G(d) level of theory. For compounds 1, 1F, 1F2, 2, and 2F, the
six lowest singlet and triplet transition energies were calculated
using time-dependent DFT calculations on the optimized structures,
at the B3LYP/6-311G(d) level of theory.
3
MHz, CDCl3, 25 °C): δ 8.21 (d; J ) 8.0 Hz; 1H, pyrene), 8.19
3
3
(d; J ) 9.5 Hz; 1H, pyrene), 8.16 (d; J ) 7.0 Hz; 1H, pyrene),
8.15 (d; 3J ) 9.0 Hz; 1H, pyrene), 8.14 (d; 3J ) 7.5 Hz; 1H, pyrene),
8.09 (d; 3J ) 9.0 Hz; 1H, pyrene), 8.03 (d; 3J ) 8.0 Hz; 1H, pyrene),
3
8.01 (m; 1H, pyrene), 7.90 (d; J ) 9.5 Hz; 1H, pyrene), 6.85 (s;
4H, B(Mes)2), 2.36 (s; 6H, para-CH3 of B(Mes)2), 1.99 (s; 12H,
orth-CH3 of B(Mes)2) ppm. 13C NMR: δ 144.95 (br), 144.41(br),
141.06, 139.53, 135.22, 133.92, 133.39, 131.71, 131.34, 129.16,
128.91, 128.00, 127.98, 127.75, 126.20, 125.83, 125.72, 125.35,
125.06, 124.82, 23.08, 21.72 ppm. Anal. Calcd for C34H31B: C
90.66, H 6.94. Found: C 90.83, H 6.67.
Synthesis of Di(p-dimesitylborylphenyl)diphenylsilane, 4. To
a stirred THF (30 mL) solution of di(p-bromophenyl)diphenylsilane
(0.320 g, 0.65 mmol) at -78 °C was added dropwise, via syringe,
a n-BuLi solution (1.6 M in hexanes) (0.90 mL, 1.44 mmol) over
5 min. The resulting light yellow solution was stirred for 1 h at
-78 °C, and a THF (20 mL) solution of dimesitylboron fluoride
(0.46 g, 90%, 1.55 mmol) was then slowly added. After stirring at
-78 °C for 1 h, the reaction mixture was allowed to warm slowly
to room temperature and stirred overnight, giving a yellow
transparent solution. The solvent was then evaporated and the
residue partitioned between CH2Cl2 (50 mL) and H2O (50 mL).
After separation, the aqueous layer was extracted with CH2Cl2 (30
mL × 3), and the combined organic layers then washed with
saturated aqueous NaCl (50 mL). The organic layer was then dried
(MgSO4) and concentrated. The residue was purified by flash
chromatography on silica gel using hexanes as the eluent to afford
compound 4 as a white powder (0.29 g, 55% yield). 1H NMR (500
X-ray Crystallographic Analysis. Single crystals of 2 and 4
were obtained from the slow evaporation of solvents (CH2Cl2/
hexanes). Single crystals of 1F2 and 5F2 were obtained from slow
evaporation of the CH2Cl2 solutions of 1 and 5, respectively, in
the presence of excess N(n-Bu)4F. The crystals were mounted on
glass fibers for data collection. Data were collected at either 180 K
or ambient temperature on a Bruker Apex II single-crystal X-ray
diffractometer with graphite-monochromated Mo KR radiation,
operating at 50 kV and 30 mA. The crystals of 2 and 4 belong to
3
MHz, CDCl3): δ 7.51-7.60 (m; 8H), 7.48 (d; J ) 7.9 Hz; 4H),
3
7.41 (t; J ) 7.3 Hz; 2H, phenyl), 7.33-7.39 (m; 4H, phenyl),
6.81 (s; 8H, B(Mes)2), 2.30 (s; 12H, para-CH3 of B(Mes)2), 2.01
(s, 24H, ortho-CH3 of B(Mes)2) ppm. 13C NMR: δ 147.1, 141.7,
140.8, 138.7, 138.3, 136.3, 135.8, 135.0, 133.8, 129.7, 128.2, 128.1,
127.9, 23.4, 21.2 ppm. Anal. Calcd for C60H62B2Si: C 86.53, H
7.50. Found: C 86.63, H 7.67.
j
Synthesis of 1,8-Di(p-dimesitylborylphenyl)naphthalene, 5. To
a THF solution (30 mL) of p-bromophenyldimesitylborane (0.65
g, 1.6 mmol) at -78 °C was added dropwise, via syringe, a n-BuLi
solution (1.6 M) (1.05 mL, 1.7 mmol). The resulting light yellow
solution was stirred for 1 h at -78 °C, at which point anhydrous
the triclinic space group P1, while the crystals of 1F2 and 5F2 belong
to the orthorhombic space group Pbca and Pbcn, respectively. No
(30) Frisch, M. J.; et al. Gaussian 98 (Revision A.6); Gaussian, Inc.:
Pittsburgh, PA, 1998.

Electronic Properties of Diboryl Compounds
Organometallics, Vol. 27, No. 24, 2008 6449
Table 1. Crystal Data
Table 2. Selected Bond Lengths (Å) and Angles (deg)
2
4
1F2
5F2
Compound 2
formula
fw
space group
a, Å
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
C34H31B
450.40
P1
C60H62B2Si
832.81
P1
C84H124N2B2F2 C45H63NFB
B(1)-C(17)
B(1)-C(1)
B(1)-C(26)
1.571(3)
1.572(3)
1.581(3)
C(17)-B(1)-C(1)
C(17)-B(1)-C(26)
C(1)-B(1)-C(26)
119.82(17)
120.41(17)
119.76(17)
1221.47
647.77
j
j
Pbca
Pbcn
8.5670(9) 13.038(1)
11.2119(12) 14.1669(11) 19.7648(17) 23.0165(15)
13.7498(15) 14.6777(11) 22.7120(19) 19.0849(13)
98.642(1) 67.266(1)
103.914(1) 87.495(1)
98.317(1) 80.888(1)
1244.7(2) 2468.6(3)
19.6359(16) 18.9303(13)
Compound 4
Si(1)-C(19)
Si(1)-C(1)
Si(1)-C(7)
Si(1)-C(13)
B(1)-C(16)
B(1)-C(34)
B(2)-C(22)
B(1)-C(25)
B(2)-C(52)
B(2)-C(43)
1.8692(19)
1.873(2)
1.873(2)
1.8861(19)
1.572(3)
1.577(3)
1.565(3)
1.579(3)
1.573(3)
1.580(3)
C(19)-Si(1)-C(1)
C(19)-Si(1)-C(7)
C(1)-Si(1)-C(7)
C(19)-Si(1)-C(13)
C(1)-Si(1)-C(13)
C(7)-Si(1)-C(13)
C(16)-B(1)-C(34)
C(16)-B(1)-C(25)
C(34)-B(1)-C(25)
C(22)-B(2)-C(52)
C(22)-B(2)-C(43)
C(52)-B(2)-C(43)
108.65(9)
110.93(9)
110.01(9)
106.48(8)
108.55(8)
112.09(9)
119.86(17)
117.26(17)
122.88(16)
121.86(17)
117.09(17)
121.05(17)
90
90
90
90
90
90
8814.5(13)
4
8315.5(10)
8
Z
2
2
Dcalc, g · cm-3
T, K
1.202
180
0.067
54.30
14 107
1.120
180
0.085
54.44
28 166
0.920
180
1.035
298
µ, mm-1
2θmax, deg
reflns measd
0.054
54.44
57 503
0.061
54.70
31 592
reflns used (Rint) 5405(0.034) 10 804(0.040) 9727(0.062) 2570(0.044)
Compound 1F2
parameters
322
580
435
470
R [I > 2σ(I)]: 0.0538
0.0522
0.1222
0.0789
0.2236
0.0465
0.1228
F(1)-B(1)
B(1)-C(19)
B(1)-C(10)
B(1)-C(1)
1.475(4)
1.656(4)
1.659(5)
1.672(5)
F(1)-B(1)-C(19)
F(1)-B(1)-C(10)
C(19)-B(1)-C(10)
F(1)-B(1)-C(1)
C(19)-B(1)-C(1)
C(10)-B(1)-C(1)
103.2(2)
110.0(2)
109.4(3)
104.3(2)
118.4(2)
111.0(2)
a
R1
0.1346
b
wR2
R (all data):
0.0949
0.1552
0.0897
0.1410
0.2211
0.2839
0.0583
0.1337
a
R1
wR2
b
GOF on F2
1.034
1.030
0.873
1.033
Compound 5F2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. wR2 ) [∑w[(Fo - Fc2)2]/∑[w(Fo )2]]1/2, w
b
2
2
B(1)-C(13)
B(1)-C(22)
B(1)-F(1)
B(1)-C(10)
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(4)-C(7)
C(5)-C(6)
1.640(6)
1.657(7)
1.494(5)
1.638(6)
1.345(6)
1.401(6)
1.391(6)
1.373(6)
1.436(5)
1.502(6)
1.444(8)
F(1)-B(1)-C(10)
F(1)-B(1)-C(13)
C(10)-B(1)-C(13)
F(1)-B(1)-C(22)
C(10)-B(1)-C(22)
C(13)-B(1)-C(22)
C(3)-C(4)-C(7)
C(5)-C(4)-C(7)
C(4′)-C(5)-C(4)
C(4)-C(5)-C(6)
C(1′)-C(6)-C(1)
C(1)-C(6)-C(5)
103.6(3)
108.3(4)
109.0(4)
99.6(3)
116.9(4)
117.7(4)
112.6(5)
128.5(6)
128.0(8)
116.0(4)
117.0(9)
121.5(5)
2
2
) 1/[σ2(Fo ) + (0.075P)2], where P ) [Max(Fo ,0) + 2Fc2]/3.
significant decay was observed for all crystals. Data were processed
using the Bruker SHELXTL software package (version 5.10) and
are corrected for absorption effects. The structures were solved by
direct methods. One of the n-butyl groups of the N(n-Bu)4+ cation
in 5F2 is disordered over two sites with an occupancy factor of
∼50% for each site, which was modeled and refined successfully.
The n-butyl groups in 1F2 all display some degree of disordering;
two of the disordered groups in 1F2 were modeled and refined
successfully. There are also disordered CH2Cl2 solvent molecules
in the crystal lattice of 1F2, whose contributions to the structural
data were removed using the Platon Squeeze algorithm.31 All
sufficiently ordered non-hydrogen atoms were refined anisotropi-
cally. The crystal data and selected bond lengths and angles are
provided in Tables 1 and 2, respectively. Complete crystal data
can be found in the Supporting Information.
As shown in the crystal structure for compound 2, the steric
interactions between the H atom on C(13) of the pyrene moiety
and the mesityl group on the boron center cause the noncopla-
narity between the trigonal BC3 plane (defined by B(1), C(1),
C(17), and C(26) atoms) and the plane of the pyrene linker
(dihedral angle ) 43.7°). In addition, the C(17) phenyl ring
and the C(26) phenyl rings are nearly perpendicular to the pyrene
plane, as evidenced by the large dihedral angles of 90.0° and
80.3°, respectively. Hindered rotation around the B-C bond
by the mesityl groups was confirmed by variable-temperature
1H NMR spectra of 2, as shown in Figure 2. At 220 K, all
meta-H atoms and all methyl groups of the BMes2 display
distinct chemical shifts. The rotation barriers of the B-C(mesityl)
bond for the two mesityl groups in 2 were determined to be
11.6 and 12.2 kcal mol-1, respectively. These are notably greater
than that observed in the previously reported BNPB system,33
in which the BMes2 group is attached to a phenyl ring (10.3
kcal mol-1), consistent with the greater steric interactions
imposed by the pyrene ring in 2. In the crystal lattice of 2, the
pyrene portion of the molecule stacks with extended π-π
interactions, as shown in Figure 1. Although the crystal structure
of 1 was not determined by X-ray diffraction, based on the
structure of 1F2 and molecular modeling, π-π stacking interac-
tions between the pyrene rings are unlikely in 1 due to steric
blocking by the two BMes2 groups. The separation distance
Results and Discussion
Syntheses and Characterizations. The syntheses of three
new diboryl compounds 1, 4, and 5 were accomplished using
reactions shown in Scheme 1. For comparative study, the
monoboron compound 2 was also synthesized. The syntheses
of 1, 2, and 4 are straightforward and can be achieved via
lithiation of the corresponding dibromo compound, followed
by the addition of BMes2F as depicted in Scheme 1. The
synthesis of 5 is quite challenging due to steric effects. Standard
Suzuki-Miyaura coupling methods did not produce compound
5 in good yield, though the Negishi method32 employing an
organozinc(II) intermediate and Pd(PPh3)4 as the catalyst
produced compound 5 in modest yield. All four compounds were
fully characterized by 1H and 13C NMR and elemental analyses.
Crystals suitable for X-ray diffraction analyses were obtained
for compounds 2 and 4, and their crystal structures are shown
in Figures 1 and 3, respectively.
(31) (a) Spek, A. L. Acta Crystallogr. 1990, A46, C34. (b) Spek, A. L.
PLATON, A Multipurpose Crystallographic Tool; Ultrecht University:
Ultrecht, The Netherlands, 2006.
(32) King, A. O.; Okukado, N.; Negishi, E.-i. Chem. Commun. 1977,
683.
(33) Jia, W. L.; Feng, X. D.; Bai, D. R.; Lu, Z. H.; Wang, S.; Vamvounis,
G. Chem. Mater. 2005, 17, 164.
(34) Fiedler, J.; Zalisˇ, S.; Klein, A.; Hornung, F. M.; Kaim, W. Inorg.
Chem. 1996, 35, 3039.
(35) Okada, K.; Kawata, T.; Oda, M. Chem. Commun. 1995, 233.

6450 Organometallics, Vol. 27, No. 24, 2008
Zhao et al.
Scheme 1
between the two boron centers in 1 is ∼9.06 Å, based on the
structure optimized by DFT calculations.
two BMes2 groups are likely present in 5. Nonetheless, 1H NMR
data support that the rotation barrier around the B-C(mesityl)
bond in 5 is similar to that in 4 and 6.
The Si(1) atom in compound 4 has a typical tetrahedral
geometry with normal Si-C bond distances, and the two boron
centers are 9.97 Å apart. The two trigonal BC3 planes are much
closer to coplanarity with the phenyl linkers than those in
compound 1 with pyrene, showing dihedral angles of 22.6° for
B(1) and 28.6° for B(2) between the BC3 plane and the phenyl
linker. The dihedral angles between the mesityl rings and the
phenyl linker are 69.6°, 67.3°, 73.8°, and 72.5°, respectively,
much smaller than those in 2, but similar to previously reported
compounds where the BMes2 group is attached to a phenyl
ring.21,23,24,33 This is consistent with the decreased steric
congestion in 4. 1H NMR data also indicate that 4 has a
rotational barrier around the B-C(mesityl) bond similar to that
of BNPB.
Electrochemical Properties. To examine the impact of the
linker on the electron-accepting ability of these boron com-
pounds, we recorded the CV diagrams of compounds 1-5.
Compound 1 has a very poor solubility in polar solvents. As a
result, the intensity of the CV peak of 1 is barely above the
background level. Nonetheless, as shown in Figure 4, two
reduction peaks are clearly visible and reproducible for com-
pound 1. The reduction potentials of compounds 1-5 along with
compound 6 are listed in Table 3. Among the linearly conjugated
molecules, molecule 1 has the most positive E1/2red1 at -1.81 V
red1
(vs FeCp2+/0), while 2 has the most negative E1/2
at -2.03
V. It is noteworthy that both these potentials are considerably
more positive than those of previously reported monoboron
compounds attached to either a benzene or biphenyl (typically
E1/2red1 e -2.20 V),23,33-35 though not as positive as in B2bpy
For compound 5, molecular modeling shows that the two
boron centers are ∼6.5 Å apart, much closer than those in 4
and 6 (∼10 Å); hence, greater steric interactions between the
red1
red2
(E1/2
) -1.69 V, E1/2
) -2.07 V, ∆E1/2 ) 0.38 V).20
Figure 1. Left: Diagram showing the structure of 2 with 50% thermal ellipsoids and labeling schemes. Right: Diagram showing intermolecular
π stacking of the pyrene rings in the crystal lattice of 2.

Electronic Properties of Diboryl Compounds
Organometallics, Vol. 27, No. 24, 2008 6451
1
Figure 2. Variable-temperature H NMR spectra of 2 showing the chemical shifts of the meta-H atoms and methyl protons of the mesityl
groups (CD2Cl2). The H2O impurity is marked by “X”.
Table 3. Reduction Potentialsa of Compounds 1-6
1
2
3
4
5
6
red1
red2
E1/2
E1/2
-1.81 -2.03
-1.90 -2.28 (shoulder) -2.34 -2.28
-2.16 -2.66 ∼-2.28 -2.43
-2.63 -2.68
a
0/+
ox
Measured in DMF, relative to FeCp2 (E1/2 ) 0.55 V).
peak at -2.66 V was observed that can be attributed to the
reduction of the pyrene ring (nonsubstituted pyrene has a
reduction peak at -2.58 V in DMF). For 3, because the two
boron centers are much closer to each other (∼6.0 Å) than those
in 1 (∼9.1 Å), a greater electronic communication between the
two boron centers is expected. However, molecule 3 has an E1/
2
red1 at -1.90 V, which is between those of 1 and 2, with ∆E1/2
(0.33 V) being slightly less than that of 1 (0.35 V). Furthermore,
both reduction peaks displayed by 3 have a poor reversibility,
an indication that 3 is much less stable toward reduction
compared to 1 and 2. Thus, the pyrene linker is more effective
in enhancing the electron-accepting ability and electronic
communication of the boron centers than the phenyl linker.
The V-shaped diboryl molecule 4 displays a reduction peak
at -2.43 V and a shoulder peak at -2.28 V, as shown in Figure
4, presumably due to the successive reduction of the two boron
centers. The presence of two reduction peaks, albeit not well
resolved, supports the presence of a weak electronic com-
Figure 3. Diagram showing the structure of 4 with 50% thermal
ellipsoids and labeling scheme.
Thus, it can be concluded that the electron-accepting ability of
1 is enhanced by the presence of both a second boron center
and the large conjugate ring of the pyrene group. The fact that
the ∆E1/2 of 0.35 V in 1 is similar to that of B2bpy supports
that the pyrene ring promotes a strong electronic communication
between the two boron centers as the bpy does. For the
monoboron pyrene molecule 2, a second reversible reduction
Figure 4. CV diagrams of 1-3 (left) and 4-6 (right) recorded in DMF with scan rates of 1 or 2 V.

6452 Organometallics, Vol. 27, No. 24, 2008
Zhao et al.
Table 4. Absorption and Emission Data for 1, 2, 4, and 5
emission, CH2Cl2, 298 K
compound
absorption,c nm (ꢀ, M-1 cm-1), CH2Cl2
λ
max, nm
ΦPL
1
2
426 (59400), 401 (36500), 342 (15900), 301 (32000), 246 (77600)
398 (33800), 385 (30200), 325 (17300), 310 (17700), 294 (25000), 245
(75300)
318 (27700), 272 (24400), 230 (49600)
338 (48100), 278 (20500), 232 (65600)
446
427
∼1.0a
∼1.0a
4
5
402
395
0.17b
∼1.0b
a Using 9,10-diphenylanthracene as the standard. b Using anthracene as the standard. 1.0 × 10-5 M, 298 K.
c
Table 5. Luminescence Data for Compound 1, 2, 4, and 5 in Different Solventsa
excitation wavelength (λex, nm) emission wavelength (λem, nm)
hexanes
toluene
CH2Cl2
DMF
hexanes
toluene
CH2Cl2
DMF
1
2
4
5
427
399
332
343
428
400
332
351
420
399
331
349
426
399
332
351
432
401
359
380
436
413
373
388
446
427
402
395
439
424
406
409
a [M] ) 1.0 × 10-5 mol/L, 298 K.
munication between the two boron centers in 4. The naphthyl-
linked diboryl molecule 5 has two reduction peaks at -2.34
and -2.63 V, respectively, attributable to the reduction of the
two boron centers as well. Although the boron separation
the emission spectral red shift with solvent polarity by the
molecules 1, 2, 4, and 5 is not as dramatic as that displayed by
the previously reported donor-acceptor type of boron mol-
ecules, they do suggest that the excited state of these mol-
ecules22,23,33 is likely polarized. In the case of the diborylpyrene
molecule 1, no excimer emission typical of pyrene was observed
over a concentration range of 10-4 to 10-7 M, which can be
credited to the two BMes2 groups that block intermolecular
interactions between two pyrene units. For 2, excimer emission
appears to be evident in a high concentration solution (∼10-3
M; see Supporting Information), consistent with the π-stacking
interactions among pyrene rings revealed by the crystal structure
of 2. With concentrations e ∼10-4 M (higher concentration is
not possible due to poor solubility), the emission spectrum of 2
is dominated by the πfπ* transition band. The behavior of
compound 5 is similar to that of 6, though the emission
maximum of 6 is red-shifted (λmax ) 414 nm in CH2Cl2) and
has a greater solvent dependence (hexane to CH2Cl2, 20 nm
red shift). In fact, DFT calculations for 6 established that the
HOMO level is dominated by the naphthyl ring and the LUMO
by the empty p orbitals of the two boron centers.22 Hence, the
solvent-dependent fluorescence of 5 and 6 can be explained by
a polarized electronic transition from the naphthyl to the boron
center. Due to the longer biphenyl group in 6, this molecule
will be more polarized than 5 in the excited state, and thus more
responsive to changes in solvent polarity. DFT calculations for
molecule 4 revealed that the HOMO level consists of two
degenerate orbitals, each with contributions primarily from the
mesityl groups on one boron leg only, while each of the doubly
degenerate LUMOs possesses large contributions from both
red1
red2
distances and the E1/2 values in 4 and 6 are similar, the E1/2
value of 6 is much more negative than that of 4, indicative of
the presence of a greater electronic communication between the
boron centers in 6. This is likely due to the rigid naphthyl linker
in 6, which forces strong π-π stacking interactions of the two
biphenyl groups and imposes a greater electrostatic repulsive
force between the two boron anions. Such “negative cooperat-
ivity” is related to the phenomenon observed by Ja¨kle and co-
workers in anion binding of conjugated diboryl systems,18,36
but is previously unknown in nonconjugated systems.
Compared to the conjugated molecules 1-3, the nonconju-
gated molecules 4-6 are clearly weaker electron acceptors with
red1
much more negative E1/2 values. π-Conjugation with the
BMes2 group is therefore most effective in enhancing the
electron-accepting ability of the B center. Among the directly
conjugated molecules reported here, the diborylpyrene molecule
is the strongest electron acceptor.
Absorption and Luminescent Properties. Compounds 1-5
have intense and distinct absorption bands in the 230-450 nm
region, as shown in Figure 5. Most notable is compound 1,
which has a strong absorption band in the visible region (λmax
) 426 nm), while the same band is absent in the spectrum of
2. Hence, this low-energy absorption band in 1 is clearly
associated with the presence of the two conjugated BMes2
groups, and because of it, compound 1 has a bright yellow color
in solution, while all other compounds are either colorless or
light yellow (compound 2).
Compounds 1, 2, 4, and 5 are all fluorescent with λmax
)
446, 427, 402, and 395 nm in CH2Cl2, respectively. With the
exception of the V-shaped molecule 4, which has an emission
quantum efficiency of 17%, all the new molecules have a
quantum efficiency of nearly 100%, measured using either 9,10-
diphenylanthracene or anthracene as the standard (Table 4). The
fluorescent spectra of all molecules show some degree of
dependence on the solvent polarity, with the V-shaped molecule
4 displaying the largest shift (hexane to CH2Cl2, 43 nm red shift)
and the diboryl pyrene molecule 1 the smallest shift (hexane to
CH2Cl2, 14 nm red shift), while their absorption spectra show
little dependence on solvents, as shown in Table 5. Although
(36) Sundararaman, A.; Victor, M.; Varughese, R.; Ja¨kle, F. J. Am.
Chem. Soc. 2005, 127, 13478.
Figure 5. UV-vis spectra of 1-5 in CH2Cl2 (1.0 × 10-5 M).

Electronic Properties of Diboryl Compounds
Organometallics, Vol. 27, No. 24, 2008 6453
Figure 6. Diagrams showing the HOMO and LUMO levels of 4 calculated by DFT plotted with an isocontour value of 0.03 au.
boron centers (Figure 6). Hence the lowest electronic transition
in 4 can be considered as a charge transfer from the mesityl
groups on one leg to the boron centers on both, creating a highly
polarized excited state. To further understand the photophysical
properties and the Lewis acidity of molecules 1, 2, 4, and 5,
we examined the interaction of fluoride anions with these
molecules.
absorption and emission modes. The addition of fluorides to 2
and 3 causes a blue shift of both absorption and emission spectra
as well quenching of fluorescent emission, which is typical for
linearly conjugated triarylboron molecules.21-23 The UV-vis
and fluorescent titration diagrams of 2 are shown in Figure 7.
For 3, these diagrams are provided in the Supporting Informa-
tion. The Stern-Volmer plots shown in Figure 8 for both 2
and 3 are nearly identical, indicating that both molecules have
a similar affinity toward fluoride.
Interactions with Fluoride Anions. The response of the
boron compounds toward fluoride ions was examined in both
Figure 7. UV-vis (left) and fluorescent titration diagrams of 1 (top) and 2 (bottom) in CH2Cl2 (1.0 × 10-5 M) by NBu4F. Top, inset:
photographs taken under ambient light and UV light showing the effect of the addition of 1.65 equiv of F- to the solution of 1 at 1.0 ×
10-5 M.

6454 Organometallics, Vol. 27, No. 24, 2008
Zhao et al.
described as sequential turn on and off. The Stern-Volmer plot
of 1 for the quenching of the emission peak at 445 nm (Figure
8) shows that the binding strength of 1 with the first fluoride
ion is somewhat similar to that of 2 (<2 equiv of F- is needed
to completely quench the initial emission peak). However, the
further binding of 1F to form 1F2 is much weaker than that of
2 to 2F, as evidenced by the slow quenching of the peak at 483
nm of 1 shown in Figure 8, which again can be attributed to
the negative cooperativity caused by electrostatic repulsion
between two BMes2F centers. It is noteworthy that after the
addition of excessive fluoride ions the UV-vis titration spectral
change of 1F to 1F2 and 2 to 2F becomes similar, supporting
that 1F and 2 share a common electronic structure. Nonetheless,
the absorption and emission spectra of 1F are considerably red-
shifted, compared to those of 2. The ability of molecule 1 to
bind to two fluoride ions is confirmed by the isolation of the
difluoride adduct 1F2 and its crystal structure shown in Figure
9, where the central pyrene ring is well protected by the
sterically bulky BMes2F groups. Since the diborylbenzene 3 does
not show a similar red shift with the addition of fluoride, the
pyrene linker clearly plays a key role in the peculiar response
of 1 toward fluoride.
Figure 8. Stern-Volmer plots of 1-5 titration by NBu4F.
The response of the diboryl pyrene molecule 1 toward fluoride
ions is both complex and surprising, as shown in Figure 7. In
both UV-vis absorption spectra and the emission spectra, the
addition of F- causes a distinct red shift. The absorption at 426
nm and emission at 445 nm lose intensity until about 1.7 equiv
of F- is added, as a new absorption at 436 nm and emission at
483 nm appear. Beyond this point, the new absorption and
emission peaks lose intensity with increasing concentration of
fluorides, and the saturation point is reached after ∼30 equiv
of TBAF is added. These spectral red shifts with <1.7 equiv of
fluorides cause the solution of 1 to change color from yellow
to bright yellow-green, as shown in Figure 7, and the emission
color to change from dark blue to sky blue. Further titration to
the saturation point causes a near-complete loss in emission
color. These unusual color changes and the complex behavior
of 1 toward fluoride ions may be attributed to the sequential
binding of fluoride ions to the first and second boron centers,
forming 1F and 1F2, respectively, and the coexistence of 1, 1F,
and 1F2 in solution. Hence, the response of 1 toward F- can be
Both the V-shaped 4 and the U-shaped 5 undergo fluorescent
quenching with the addition of fluoride ions. The data for
compound 4 are shown in Figure 10, and the data for 5 are
provided in the Supporting Information. The key difference
between these two molecules is that molecule 4 has a much
greater affinity toward fluoride ions than does 5, as shown by
the fluorescent Stern-Volmer plots in Figure 8. UV-vis
titrations of 4 and 5 by fluorides show a spectral blue shift and
similar Stern-Volmer plots. The binding strength of 4 with
fluoride ions is in fact on the same order of magnitude as that
of 2 and 3. In contrast, molecule 5 binds much more weakly
toward fluoride due to the strong steric interactions between
Figure 9. Diagrams showing the structure of 1F2 with labeling schemes, top view (left) and side view (right). The cations are omitted.
Figure 10. UV-vis (left) and fluorescent titration diagrams of 4 in CH2Cl2 (1.0 × 10-5 M) by NBu4F.

Electronic Properties of Diboryl Compounds
Organometallics, Vol. 27, No. 24, 2008 6455
Table 6. HOMO-LUMO Enery Gaps for 1 and 2 and Their F-
Adducts
MO HOMO-
LUMO gap
(eV)
optical energy
gap (eV),
CH2Cl2
compd
transition
1
2.81
πpyr fπpyr
*
2.79
(with B contributions)
1F
2.50
πpyr fpπ
(B) (with pyrene contributions)
2.61
1F2
2
3.41
3.03
πpyr fπpyr
*
*
3.14
2.95
πpyr fπpyr
(with B contributions)
2F
3.41
πpyr fπpyr
*
3.18
monofluoride and difluoride adducts. For comparison, we also
performed calculations on the monoboron compound 2 and its
monofluoride adduct. The calculation results show that the
lowest energy transition is dominated by the HOMO-LUMO
transition for all compounds. The calculated HOMO-LUMO
energy gaps along with the optical energy gaps obtained
experimentally from UV-vis spectra in CH2Cl2 are provided
in Table 6. The computational results confirm that the
HOMO-LUMO gap of 1F is smaller than that of 1 and 1F2,
supporting that the absorption and emission spectral red shift
of 1 after the addition of <1.7 equiv of F- is indeed due to the
formation of the monofluoride adduct. The MO diagrams for 1
shown in Figure 12 illustrate that the pyrene ring plays a key
role in the electronic transitions of 1, 1F, and 1F2. For 1, both
the HOMO and LUMO are π orbitals of the pyrene ring with
significant contributions from the B centers at the LUMO level.
This is in sharp contrast to the diborylbenzene molecule 3, in
which the HOMO level has contributions predominantly from
Figure 11. Diagram showing the structure of 5F2 with labeling
schemes. The cations are omitted.
the two boron centers, as confirmed by the crystal structure of
the difluoride adduct 5F2, shown in Figure 11. The two boron
centers in 5F2 are 7.36 Å apart, compared to 6.5 Å in 5. Thus,
the change of the boron center geometry from trigonal planar
to tetrahedral noticeably increases congestion in this rigid
molecule.
TD-DFT Calculations for 1, 2, and Their Fluoride
Adducts. To further understand the peculiar UV-vis and
fluorescent response of compound 1 toward fluoride ions, we
carried out TD-DFT calculations on compound 1 and its
Figure 12. Diagrams showing the HOMO and LUMO orbitals of 1 and its fluoride adducts calculated by TD-DFT, plotted with an isocontour
value of 0.03 au.
Figure 13. Left: The J-L-V plots. Right: The EL and PL plots.

6456 Organometallics, Vol. 27, No. 24, 2008
Zhao et al.
the mesityl groups. For 1F, the HOMO is again a pyrene π
orbital, but the LUMO level is dominated by the pπ orbital of
the trigonal boron center with significant contributions from the
pyrene π system. Therefore, the lowest electronic transition in
1F is charge transfer between pyrene and the trigonal boron
center, which is lower in energy than the πfπ* transition in 1.
The low HOMO-LUMO gap of the pyrene ring and its
relatively high HOMO level are clearly responsible for such a
low-energy charge transfer. In the case of 1F2, the HOMO and
LUMO involve the pyrene ring only, and as a result, this
molecule has the largest HOMO-LUMO gap. The HOMO and
LUMO diagrams of 2 and 2F resemble the corresponding ones
in 1F and 1F2, consistent with the similar UV-vis and emission
spectral change of 1 and 2 when excess fluorides are added.
The HOMO energy level of 1F is however much higher than
that of 2 due to the destabilization by the BMes2F group, thus
causing 1F to have a much smaller HOMO-LUMO gap than
2. Hence, the peculiar response of 1 toward fluoride ions can
be explained by the involvement of the pyrene ring that leads
to the low-energy charge transfer transition of 1F. The spectral
red shift of 1 to 1F resembles the behavior of the BPPB
molecule reported by Mu¨llen et al., in which two dimesitylboron
groups are connected by a long linear pentaphenyl linker and
the extended π-conjugation is re-enforced by two spiro rings,24
leading to a low HOMO-LUMO gap and a high HOMO level,
similar to that observed in 1.
Electroluminescent Devices of 1. Between the conjugated
molecules 1 and 2, compound 1 is the most promising for use
in OLEDs because of its blue emission with a high emission
quantum efficiency, its relatively high electron affinity (LUMO
≈ -2.9 V), and its lack of excimer emission. We therefore
decided to evaluate the performance of 1 as either an emitter
or a bifunctional electron transport emitter in electroluminescent
devices. Two types of EL devices [A (ITO-CuPc/NPB/1/LiF-
Al) and B (ITO-CuPc/NPB/1/TPBi/LiF-Al)] were fabricated,
where CuPc and NPB (N,N′-diphenyl-N,N′-bis(1-naphthalenyl)-
1,1′-biphenyl-4,4′-diamine) are employed as the hole injection
and hole transport layers, respectively. In device B, the well-
known electron transport material37 TPBi (1,3,5-tris(N-phenyl-
benzimidazol-2-yl)benzene) was used. Both devices emit a sky
blue light with a turn-on voltage of ∼4 V and similar EL spectra,
as shown in Figure 13. The EL spectra from both devices are
very broad with three peaks, at 460, 490 and 535 nm,
respectively, covering nearly the entire 400-700 nm region.
The EL spectrum matches reasonably well with the PL spectrum
of 1 in the solid state, although it is much broader with well-
resolved peaks. It is possible that exciplex emission may be
produced between the layers of NPB and compound 1,
contributing to the long-wavelength EL emission band. As
shown by the J-V and L-V diagrams in Figure 13, the triple-
layer device B is brighter and much more efficient than device
A, with A and B having a maximum brightness of 571 and
1523 cd/m2, respectively. The current efficiency at 100 cd/m2
is ∼0.75 cd/A for B, which is modest for a blue EL device.
Therefore, compound 1 can be used as a bifunctional material
in OLEDs, as demonstrated by the double-layer device A,
though an additional electron transport layer is necessary in order
to achieve a bright EL device, as illustrated by device B.
Conclusions
On the basis of the results of this investigation and our earlier
investigation on B2bpy, we can conclude the following for
diboryl systems where two BMes2 groups are linked together
by an aromatic linker. (1) Electron-accepting ability: Linearly
conjugated diboryl molecules are most favorable in achieving
high electron affinity due to the direct participation of the empty
pπ orbital of the boron center in the extended conjugate system
that lowers the energy of the LUMO level. Extended large π
systems such as pyrene or electronegative π systems such as
bipyridine are most effective in stabilizing the radical anions
of the reduced diboryl molecules and are thus most promising
as electron transport materials. (2) Lewis acidity and fluoride
binding: Although Lewis acidity is clearly related to the electron
affinity of the boron center, our study has shown that steric
interactions can also play a key role in a diboryl compound’s
ability to bind to fluoride ions. The linearly conjugated
red1
molecules 1 and 2 have a much more positive E1/2 than the
V-shaped molecule 4, yet all three molecules have a similar
binding strength with fluoride ions. The steric interactions
between the protons of the mesityls and the pyrene ring in 1
and 2 clearly hinder the boron binding of fluoride ions.
Compound 5 has a reduction potential similar to that of 4, but
displays the lowest affinity to fluoride ions due to unfavorable
steric interactions between the two boron legs in 5. Hence, to
achieve a strong acceptor for fluoride ions, linear linkers such
as bipy, which are strong electron acceptors, promote strong π
conjugation with boron centers, and do not impose great steric
barriers to fluoride binding should be used. (3) Luminescence:
Because of the conjugation of the boron center with the aromatic
linker, the linker group has a great impact on the emission
efficiency of the diboryl molecules. Among the linear molecules,
pyrene derivatives 1 and 2 are the brightest blue emitters, much
brighter than b2py and 3, a feature clearly caused by the pyrene
ring. Similarly, the naphthyl-linked molecules 5 and 6 also
display nearly unit emission quantum efficiencies, while the
V-shaped molecule 4 displays a much weaker emission quantum
efficiency due to the sp3-hybridized linker. Electronic interac-
tions between the two aryl-boron legs in 5 and 6 may also play
a role in their highly efficient emission.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial
support.
Supporting Information Available: Stern-Volmer plots of
UV-vis titrations of 1-5 with fluoride ions, UV-vis and fluo-
rescent titration diagrams of 3 and 5, UV-vis and fluorescent
spectra of 1, 2, 4, and 5 in various solvents, fluorescent spectra of
compounds 1 and 2 in CH2Cl2 at various concentrations, complete
variable-temperature 1H NMR spectra of 2, diagrams of the HOMO
and LUMO of 2, 2F, and 3, and complete crystal data for 2, 4,
1F2, and 5F2. This material is available free of charge via the
Internet at http://pubs.acs.org.
(37) Chen, C. H.; Shi, J. Coord. Chem. ReV. 1998, 171, 161.
OM800856G