Diboron and triboron compounds based on linear and star-shaped conjugated ligands with 8-hydroxyquinolate functionality: Impact of intermolecular interaction and boron coordination on luminescence

Article abstract of DOI:10.1021/jo060841+

New 8-R-quinoline functionalized linear and star-shaped conjugated molecules have been synthesized using Suzuki-Miyaura coupling methods (R = MeO, L1-L5; R = CH₃OCH₂O, L1′-L5′). When treated with HCl, L1′-L5′ are converted readily to

Full text of DOI:10.1021/jo060841+

Diboron and Triboron Compounds Based on Linear and
Star-Shaped Conjugated Ligands with 8-Hydroxyquinolate
Functionality: Impact of Intermolecular Interaction and Boron
Coordination on Luminescence
Yi Cui and Suning Wang*
Department of Chemistry, Queen's UniVersity, Kingston, Ontario, K7L 3N6 Canada
wangs@chem.queensu.ca
ReceiVed April 21, 2006
New 8-R-quinoline functionalized linear and star-shaped conjugated molecules have been synthesized
using Suzuki-Miyaura coupling methods (R ) MeO, L1-L5; R ) CH3OCH2O, L1′-L5′). When treated
with HCl, L1′-L5′ are converted readily to the corresponding 8-hydroxyquinoline compounds L(OH)-
1-L(OH)5 which react readily with BPh3 in refluxing THF to produce the corresponding polyboron
chelate compounds B1-B5 in good yields. L1-L5 and B1-B5 display similar thermal stability with Td
at ∼300 °C. Experimental and molecular orbital calculation results showed that the chelation by boron
stabilizes the LUMO level of the ligand and narrows the HOMO-LUMO gap, resulting in the blue
emission of the ligands and the green or orange emission of the boron compounds. Crystal structures of
L1, L3, and L5 showed that these molecules have layered arrangements in the solid state with significant
intermolecular π-π interactions. The linear diboron B5 displays concentration and temperature-dependent
emission in solution, attributable to intermolecular interactions. The properties of a monoboron compound
BPh2(5-Ph-8-MeO-q) (B0) and its corresponding free ligand L0 were investigated and compared to the
closely related diboron compound B1 and the ligand L1, which revealed that the increase of the number
of chromophores linked by an aromatic group has a significant impact on thermal stability and the HOMO
and LUMO energy levels.
Introduction
derivative molecules are small aluminum chelate compounds
with the general formula of Alq′3 where q′ is a 8-hydroxyquino-
late analogue that contains various substituent groups.2-7 Other
8-hydroxyquinoline metal chelate compounds that have been
investigated include Liq,8 Mq2 where M ) Be, Mg, and Zn,9
8-Hydroxyquinoline and its derivatives have attracted much
research interest since the breakthrough report on organic light
emitting devices (OLEDs) based on a chelate complex Alq3 (q
) 8-hydroxyquinolate) by Tang and VanSlyke.1 Inspired by the
successful applications of Alq3 in OLEDs, much research efforts
have been dedicated to the development of new derivative
molecules based on 8-hydroxyquinoline. The bulk of the new
(2) (a) Jang, H.; Do, L.-M.; Kim, Y.; Gon Kim, J.; Zyung, T.; Do, Y.
Synth. Met. 2001, 121, 1669. (b) Jang, H.; Do, L.-M.; Kim, Y.; Zyung, T.;
Do, Y. Synth. Met. 2001, 121, 1667.
(3) (a) Matsumura, M.; Akai. T. Jpn. J. Appl. Phys. 1996 35, 5357. (b)
Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S. R.;
Cronin, J. A.; Thompson, M. E. J. Appl. Phys. 1996, 79, 7991.
(1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
10.1021/jo060841+ CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/26/2006
J. Org. Chem. 2006, 71, 6485-6496
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Cui and Wang
and Mq3 where M ) Ga, In, Sc.10 We reported a series of
monoboron compounds with the general formula of BPh2q′ and
BAr2q′ where q′ is either 8-hydroxyquinolate or its derivative
and Ar is an aryl group other than phenyl.11 The key feature of
the previously investigated hydroxyquinolate derivative com-
pounds is the presence of N,O-chelation with a metal ion or a
main group element because such chelation is critical for the
chromophore to achieve a high luminescent efficiency and a
high thermal stability (e.g., Alq3 versus the free 8-hydrox-
yquinoline). In addition to small molecules, a number of polymer
compounds that contain covalently attached boron 8-hydrox-
yquinolate chelate units or aluminum 8-hydroxyquinolate chelate
units have been reported with the aim to enhance the solution
processing ability and the thermal stability of the materials.12
To improve the properties of small molecule based materials,
we have extended our investigation on 8-hydroxyquinolate boron
chelate compounds to di- and triboron compounds using linear
and star-shaped ligands that contain multiple quinolate groups.
In addition to enhanced thermal stability, the new polyboron
compounds provide an unique opportunity for the investigation
of intermolecular interactions and their impact on physical and
photophysical properties because of the highly anisotropic shape
of linear and star-shaped molecules. Furthermore, the presence
of multiboron centers and multiple chromophores in the linear
and star-shaped molecules allows us to examine the possible
cooperative electronic effects. Five new linear and star-shaped
ligands and their corresponding boron chelate compounds have
been achieved. For comparison purposes, a monoboron com-
pound BPh2(5-Ph-8-MeO-q) that is closely related to one of
the linear diboron compounds has also been synthesized. The
synthetic details and the properties of these new molecules are
reported herein.
linear or star-shaped conjugated core, 4,4′-di[5′′-(8′′-methoxy-
quinoline)]biphenyl (L1), 4, 4′-di[5′′-(8′′-methoxy-2′′-meth-
ylquinoline)]biphenyl (L2), 1, 3, 5-tri[p-5′′-(8′′-methoxyquino-
line)phenyl]benzene (L3), 2, 4, 6-tri[p-5′′-(8′′-methoxyquinoline)-
phenyl]-1,3.5-triazine (L4) and 4-di(2′-thienyl-5′-(8-methoxy-
quinoline)-benzene (L5). The synthetic methods employed for
L1-L5 are Suzuki coupling reactions involving the appropriate
boronic acid of the central core and 5-bromo-8-methoxyquino-
line or 5-bromo-8-methoxy-2-methylquinoline as shown in
Scheme 1. The boronic acids for L1-L4 were synthesized from
the corresponding parent bromo compounds according to
modified known procedures.14 The key intermediates 1,3,5,-tri-
(p-bromophenyl)benzene and 1,3,5,-tri(p-bromophenyl)triazine
for L3 and L4 were prepared by trimerization of 4-bromoben-
zonitrile and 4-bromoacetophenone, respectively, using previ-
ously reported procedures.21 The synthesis of L5 involved the
preparation of the intermediate, 4-di(2-thienyl)benzene and its
conversion to the boronic acid using a literature procedure.15
In principle, the Suzuki coupling reactions for L1-L5 could
also be accomplished by the reaction of 8-methoxyquinoline-
5-boronic acid with the appropriate bromo derivative of the
central core. However, the conversion from 5-bromo-8-meth-
oxyquinoline to its boronic acid was not satisfactory and as a
result, this approach was abandoned. For comparison purposes,
the monocompound 5-phenyl-8-MeO-quinoline (Ph-8-MeO-q,
L0) was also synthesized. The L0 molecule is exactly one-half
of L1.
Attempts were made to convert the 8-methoxy group to a
8-hydroxy group in ligands L1-L5 by using BBr3 or HCl to
produce the corresponding L(OH)1-L(OH)5 ligands. However,
surprisingly, the yield of the demethylation reaction was low,
despite many trials. To improve the yield of the demethylation
reaction, we synthesized the second set of ligands L1′-L5′
where the 8-methoxy group is replaced with a CH3OCH2O
(MOMO) group to make it a better leaving group. Using the
same synthetic procedures as for ligands L1-L5, ligands L1′-
L5′ were obtained in good yields (62-71%). In contrast to L1-
L5, the MOM group in L1′-L5′ can be removed readily with
methanolic HCl to afford excellent yields (85-92%) of the
8-hydroxy quinoline ligands L(OH)1-L(OH)5. Unlike the
methyl or the MOM protected ligands L1-L5 and L1′-L5′
which are moderately soluble in solvents such as CH2Cl2 or
THF, L(OH)1-L(OH)5 have a poor solubility in CH2Cl2 or
THF and are slightly soluble in strong polar solvents such as
Results and Discussion
Syntheses
The first set of new ligands synthesized are derivatives of
8-methoxyquinoline or 8-methoxy-2-methylquinoline with a
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Polym. Chem. 2003, 41, 3006.
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Anzenbacher, P., Jr. AdV. Mater. 2004, 16, 2001. (c) Pohl, R.; Anzenbacher,
P., Jr. Org. Lett. 2003, 5, 2769.
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S.; Thin Solid Films 2005, 478, 252. (b) Jaafaria, A.; Ouzeaua, V.; Elya,
M.; Rodrigueza, F.; Yassar, A,; Aaron, J. J.; Benalloulb, P.; Barthou, C.
Synth. Met. 2004, 147, 175.
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9, 1159.
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Trecourt, F.; Mallet, M.; Mongin, F.; Queguiner, G. Tetrahedron 1995, 51,
11743. (c) Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 7411.
(d) Goldschmid, H. R.; Musgrave, O. C. J. Chem. Soc. C 1970, 488. (e)
Liu, Q. D.; Jia, W. L.; Wu, G.; Wang, S. Organometallics, 2003, 22, 3781.
(15) (a) Suzuki, A. Acc. Chem. Res. 1982, 15, 178. (b) Miyaura, N.;
Suzuki, A. Chem. ReV. 1995, 95, 2457.
(9) (a) Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.;
Riccio, K. K. C.; Fogarty, D.; Kohlmann, H.; Ferris, K. F.; Burrows, P. E.
J. Am. Chem. Soc. 2002, 124, 6119. (b) Donze´, N.; Pe´chy, P.; Gra¨tzel, M.;
Schaer, M.; Zuppiroli, L. Chem. Phys. Lett. 1999, 315, 405.
(10) Burrows, P. E.; Sapochak, L. S.; McCarty, D. M.; Forrest, S. R.;
Thompson, M. E. Appl. Phys. Lett. 1994, 64, 2718.
(11) (a) Cui, Y.; Liu, Q.-D.; Bai, D.-R.; Jia, W.-L.; Tao, Y.; Wang, S.
Inorg. Chem. 2005, 44, 601. (b).Wu, Q.; Esteghamatian, M.; Hu, N. H.;
Popovic, Z.; Enright, G.; Tao, Y.; D’Iorio, M.; Wang, S. Chem. Mater.
2000, 12, 79.
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669. (b) Meyers, A.; Weck, M. Macromolecules, 2003, 36, 1766. (c)-
Yamamoto, T.; Yamaguchi, I. Polym. Bull. 2003, 50, 55. (d) Qin, Y.; Pagba,
C.; Piotrowiak, P.; Ja¨kle, F. J. Am. Chem. Soc. 2004, 126, 7015.
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Crystallography; Kynoch Press: Birmingham, AL, 1974; Vol. 4, Table 2.2A.
(b) Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, C34. (c) Spek, A. L.
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Diboron and Triboron Compounds
SCHEME 1
DMSO, which is most likely caused by intermolecular hydrogen
bonding interactions of L(OH)1-L(OH)5 in the solid state.
Despite their poor solubility, ligands L(OH)1-L(OH)5 were
proven to be effective for chelation to a boron center. Previously
we have demonstrated that the reaction of BPh3 with 8-hydrox-
yquinoline and its derivatives (q′) can be used to synthesize
the chelate boron compound BPh2q and BPh2q′ readily.11 Using
similar procedures, BPh3 was reacted with L(OH)1-L(OH)5,
respectively, in refluxing THF, which resulted in the isolation
of the corresponding chelate boron compounds B1-B5, respec-
tively, in good yields, as shown in Scheme 2. The monoboron
compound BPh2(Ph-q) (B0) using the Ph-8-OH-q as the starting
material was also prepared.
The ligands L0, L1-L5 and the boron complexes B0, B1-
B5 were fully characterized by NMR (1H and 13C) and high-
resolution MS spectroscopic analyses. To establish the hydrogen
bonding patterns of L(OH)1-L(OH)5 in the solid state, many
attempts were made to grow their single crystals, but none were
successful. Our attempts to obtain single crystals of the boron
compounds B1-B5 only resulted in the isolation of crystals
that are too small for structural determination on our X-ray
diffractometer. Nonetheless, single crystals of L1, L3, L5, and
B0 were obtained successfully, and their structures in the solid
state were determined by single-crystal X-ray diffraction
analyses.
Crystal Structures of L1, L3, L5, and B0. The molecular
structures of L1, L3, and L5 as determined by X-ray diffraction
analyses are shown in Figures 1-5, respectively. The two linear
molecules L1 and L5 possess a crystallographically imposed
inversion center symmetry. The central cores (biphenyl, p-
dithienylphenyl, respectively) in L1 and L5 are coplanar. The
two 8-methoxyquinoline rings have an anti arrangement. The
mean plane of the 8-methoxyquinoline ring in L1 and L5 is
twisted ∼119.2° and ∼43.1°, respectively, relative to the central
conjugated core. The relatively small dihedral angle in L5 can
be attributed to the diminished nonbonding interactions between
the ortho hydrogen atoms of the central core and the hydrogen
atoms of the quinoline ring, which, in turn leads to more efficient
conjugation between the central core and the quinoline ring in
L5, as reflected by the relatively short bond length (1.463(6)
J. Org. Chem, Vol. 71, No. 17, 2006 6487

Cui and Wang
SCHEME 2
Å) of C(11)-C(5) in L5 compared to that (1.496(3) Å) in L1.
The triphenylbenzene central core in L3 is not coplanar as
indicated by the dihedral angles between the phenyl rings and
the benzene core in L3 of 39.5°, 68.6°, and 146.3°, respectively.
The 8-methoxyquinoline moieties are also not coplanar with
the adjacent phenyl rings (dihedral angles are 49.8°, 56.4°, and
60.5°, respectively).
L1, L3, and L5 have distinct molecular packing patterns in
the solid state. The linear molecules of L1 all orient parallel to
each other along the same direction with significant π-π
interactions between two adjacent 8-methoxyquinoline rings
with the shortest atomic separation distance being 3.61(2) Å,
as shown in Figure 1. The linear molecules of L5 also orient
parallel to each other in the crystal lattice. However, unlike the
molecules of L1 which have an extended linear architecture,
the molecules of L5 form a wavelike extended structure that
propagates along the c axis as shown in Figure 5. A close
examination reveals that the central cores of two adjacent
molecules of L5 are not parallel to each other but with a dihedral
angle of 57.8°. However, the 8-methoxyquinoline rings between
the two adjacent molecules in the crystal of L5 have ap-
proximately parallel π-π stacking interactions with the shortest
separation distance being 3.50(2) Å. The star-shaped molecules
of L3 form interlocked pairs in the crystal lattice as shown in
Figure 3, which further arrange into layered architecture. The
most significant π-π stacking interactions are again between
the parallel 8-methoxyquinoline groups of interdigitated pairs
with the shortest atomic separation distance being 3.71(2) Å.
The crystal structure of BPh2(Ph-q) is shown in Figure 6.
The tetrahedral coordination environment around the boron
center is similar to the previously reported BPh2q and BPh2q′
compounds.11 One important feature is that the 8-hydroxyquino-
late ring has a dihedral angle of 55.2° and 61.4° with the phenyl
ring, for the two independent molecules in the asymmetric unit,
respectively, which are similar to those observed in L1.
Physical, Electronic and Photophysical Properties. Our
investigation on physical, electronic, and photophysical proper-
ties of the new compounds focused on the 8-methoxyquinoline
series L1-L5 and their corresponding boron compounds. The
properties of L1′-L5′ closely resemble those of L1-L5, thus
they are not presented here. For the 8-hydroxyquinoline series
L(OH)1-L(OH)5, it is difficult to investigate their properties
6488 J. Org. Chem., Vol. 71, No. 17, 2006

Diboron and Triboron Compounds
FIGURE 1. A diagram showing the molecular structure of L1 with
labeling schemes (top) and the packing diagram showing intermolecular
π-π interactions (bottom).
FIGURE 3. Diagrams showing the interlocked pair of L3 (left, top
view; right, side view) (top) and the packing diagram showing
intermolecular π-π interactions (bottom).
FIGURE 4. A diagram showing the molecular structure of L5 with
labeling schemes.
any melting point. However, it undergoes complete sublimation
at ∼163°C and one atmosphere, which is clearly due to its much
smaller molecular weight, compared to L1. The boron com-
plexes B1-B5 exhibited similar thermal properties as the free
ligands with the thermal decomposition temperature in the range
of 307-346 °C. The monoboron compound B0 has a decom-
position temperature of 261 °C, significantly lower than the
diboron compound B1. This enhanced thermal/morphological
stability for the diboron compound can be attributed to the
increased molecular weight and the extended intermolecular
interactions of the linear molecule B1. TGA diagrams for all
compounds are provided in supporting materials.
Absorption Spectra. Ligands. All free ligands display
intense absorption bands in the UV region with the absorption
maxima at ∼280 to 300 and 330-340 nm as shown in Figure
7, which can be attributed to π-π* electronic transitions.
Ligands L1-L4 have no significant absorption at λ > 400 nm
while one of the absorption bands of L5 covers the 300-450
nm region, which is consistent with the fact that L1-L4 are
colorless while L5 is yellow. Using the absorption edge, the
optical energy gaps for L1-L5 were estimated to be 3.26, 3.29,
3.30, 3.08, and 2.80 eV, respectively. The star-shaped molecule
with a triazine core L4 has a smaller band gap than the benzene
core analogue L3, which is consistent with the general trend
observed previously for star-shaped molecules with triazine and
benzene cores.21 The small band gap of L5 is due to the
involvement of the thienyl groups and the greater degree of
FIGURE 2. A diagram showing the molecular structure of L3 with
labeling schemes.
because of their insolubility in common organic solvents. The
data of the ligands L1-L5 provide a good comparison to those
of the boron compounds, thus allowing us to appreciate the
impact of the chelation by the BPh2 group. One noteworthy
feature for L1-L5 and B1-B5 is that albeit being small
molecules, they can form transparent films readily on a glass
substrate by a simple spin coating process. Between the two
groups of compounds, the boron compounds form better and
more uniform films than the free ligands. To compare the
difference between the diboron compound B1 and the monobo-
ron compound B0, the properties of B0 and corresponding free
ligand L0 were also investigated.
Thermal Properties. The thermal stability of ligands L1-
L5 and the corresponding boron compounds B1-B5 were
examined by DSC and TGA. No melting points and glass
transition temperatures up to 300 °C were observed for L1-
L5 in the DSC diagrams. TGA experiments confirmed that ∼3%
weight loss occurs at the temperature range of 302-380 °C for
all the ligands, an indication that L1-L5 are thermally stable
up to ∼300 °C. Similar to L1, the molecule L0 does not display
J. Org. Chem, Vol. 71, No. 17, 2006 6489

Cui and Wang
FIGURE 5. A space-filling packing diagram showing the intermo-
lecular π-π interactions and the wavelike arrangement of molecule
L5 in the crystal lattice (top); detail of the enlarged area viewed at
∼90° angle (bottom).
FIGURE 7. The UV-vis spectra of L1-L5 in CH2Cl2 (top) and the
UV-vis spectra of B1-B5 in CH2Cl2 (bottom).
known to have a smaller band gap compared to the neutral
ligand, on the basis of similar observation of BPh2q and
BPh2q′.11 The role of the boron center is simply to stabilize the
anionic ligand. Consistent with the trend of the free ligands,
the boron compound B5 has the smallest band gap, and its
absorption band tails to 540 nm. The absorption data and optical
band gaps for L1-L5 and B1-B5 are provided in Table 1 and
Table 2.
Electrochemical Properties. Ligands. To estimate the
HOMO and LUMO energy levels and to examine the stability
of the reduced and oxidized species of the free ligands and the
boron compounds in solution, cyclic voltammetric diagrams
were recorded. Ligands L1-L2 display two similar and well
resolved irreversible oxidation peaks with the first one at 1.46
V and the second one at 1.80 and 1.83 V, respectively, which
may be attributable to the sequential oxidation of the two
quinolate rings. To determine if the two oxidation peaks
displayed by L1 are indeed due to the oxidation of the quinolate
groups, the CV diagram of the L0 molecule was recorded, which
displays a single oxidation peak at 1.70 V within the solvent
limit window (-2.0 to +2.0 V) which is at about midpoint of
the two oxidation peaks displayed by L1. This led us to believe
that the relatively low first oxidation potential and the presence
of two oxidation peaks in L1 are a consequence of electronic
communication between the two quinolate groups mediated via
the biphenyl link. Hence, increasing the number of chro-
mophores linked via aromatic groups clearly has a significant
impact on the HOMO level of the molecule. The first oxidation
potential (1.45 V) of the star-shaped L3 is nearly identical to
those of L1 and L2. However, its second oxidation potential
(1.88 V) is slightly higher than those of L1 and L2, which can
be attributed to the low degree of conjugation of the central
triphenylbenzene core in L3 as revealed by the crystal structure,
compared to the coplanar biphenyl core in L1 and L2. (Although
FIGURE 6. A diagram showing the molecular structure of BPh2(Ph-
q) with labeling schemes.
conjugation in L5, compared to other ligands. The L0 molecule
has a band gap of 3.37 eV which is significantly bigger than
that of L1.
Boron Compounds. In contrast to the free ligands, the boron
compounds are all colored: B1-B4 are yellow-green in solution
while B5 is yellow-orange, consistent with the presence of an
absorption band in the 340-550 nm region. The color change
from the free ligand to the boron compound is due to the
decrease of the HOMO-LUMO energy gap, as confirmed by
the band gap energy obtained from the absorption edge for B1-
B5 (2.58, 2.64, 2.48, 2.57, and 2.38 eV, respectively) which is
about 0.50-0.70 eV smaller than the corresponding free ligand.
The band gap of B1 is comparable to the related monoboron
compound B0 (2.54 eV). The somewhat bigger band gap of
B2, compared to that of B1, is consistent with the previous
observation that the 2-methyl group on the hydroxyquinolate
ligand leads to a small blue shift of the band gap.11a The decrease
of the HOMO-LUMO gap in the boron compounds is due to
the fact that the ligands in the complexes are anionic, which is
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Diboron and Triboron Compounds
TABLE 1. UV-Visible and Emission Data for L1-L5 and B1-B5
quantum yield/(Φem, %)
UV-visa
(λmax, nm)
emission
(λmax, nm)
b
compds
CH2Cl2
SSQEc
conditions
L0/B0
246, 322/230, 268, 414
294,330/230, 270, 304, 418
308,332/258, 306, 406
282,330/242, 270, 298, 416
340/266, 346, 420
405/530
388/525
415/537
418/536
409/529
416/513
413/533
422/531
445/528
456/508
462/594
486/593
89/13
N/A
68/10
N/A
N/A
CH2Cl2, 298 K
film, 298 K
CH2Cl2, 298 K
film, 298 K
CH2Cl2, 298 K
film, 298 K
CH2Cl2, 298 K
film, 298 K
CH2Cl2, 298 K
film, 298 K
CH2Cl2, 298 K
film, 298 K
L1/B1
L2/B2
L3/B3
L4/B4
L5/B5
23/6
20/8
21/6
10/11
8/3
79/23
61/15
63/23
20/1
340/234, 260, 366, 430(s)
a Concentration: [M] ) 5 × 10-6 M. b Using anthracene as the standard for L0 and 9,10-diphenylanthracene as the standard for the remaining compounds
in CH2Cl2 at room temperature. c Solid State Quantum Efficiency (SSQE) were measured from ligands films spin cast onto fused silica substrates from
chloroform solutions.
TABLE 2. Electrochemical Data and Experimental and Theoretical (MO) HOMO/LUMO Energy Level
L1
L2
L3
L4
1.51
-1.73
3.24
L5
E
E
ox (V)a
red (V)a
1.46, 1.80
N/A
1.46, 1.83
N/A
1.45, 1.88
N/A
1.27, 1.95
-1.33
2.60
electrochemical ∆E (eV)
N/A
N/A
N/A
optical ∆E (eV)b
3.26
4.06
-2.50/-1.58
-5.76/-5.64
3.29
4.07
-2.50/-1.49
-5.79/-5.56
3.30
4.14
-2.45/-1.58
-5.75/-5.72
3.08
3.65
-2.73/-2.17
-5.81/-5.82
2.80
3.46
-2.83/-1.89
-5.63/-5.35
MO ∆E (eV)
LUMO (eV) exptlc/MO
HOMO (eV) exptld/MO
B1
B2
B3
B4
B5
E
E
ox (V)a
red (V)a
1.45
-1.62
3.07
2.55
-3.20
-5.75
1.45
-1.80
3.25
2.64
-3.11
-5.75
1.53
-1.46
2.99
2.48
-3.28
-5.76
1.49
-1.58
3.07
2.57
-3.21
-5.78
1.10
-1.25, -1.60
2.35
2.38
-3.02
-5.40
electrochemical ∆E (eV)
optical ∆E (eV)b
LUMO (eV) exptlc
HOMO (eV) exptld
a Measured in CH2Cl2, except B3 which was measured in DMF: Bu4N[PF6] as the electrolyte; AgCl/Ag as the reference electrode. b Recorded in CH2Cl2
with a concentration ≈ 10-6 M. c From the optical band gap and the HOMO energy. d From the oxidation potential, calibrated using the oxidation potential
and the absolute HOMO energy level of FeCp2.
the oxidation may be localized on the quinoline portion, because
of its partial conjugation with the phenyl or biphenyl group of
the central core, the oxidation potential can be affected by the
degree of conjugation through the entire molecule.) For L4 the
first oxidation potential (1.51 V) is significantly higher than
those of L1-L3 and the second oxidation potential of L3, >
2.0 V, is beyond the solvent limit for measurement, which is
caused by the electronegative central triazine ring that stabilizes
the HOMO level, compared to the biphenyl or the benzene core
in L1-L3. For L5, the first oxidation potential (1.27 V) is much
lower than those of L1-L4 and can be attributed to the
oxidation of the thienyl-containing central ring. The oxidation
peak of the 8-methoxyquinoline ring in L5 is not resolved. For
compounds L1-L3, no reduction peaks within the solvent limit
(-2.0 V) were observed. For L4 a quasi-reversible reduction
peak was observed at -1.73 V, which can be attributed to the
reduction of the central triazine core. Clearly the electronegative
triazine ring lowers the LUMO level significantly, compared
to the benzene core analogue L3. For L5, an irreversible
FIGURE 8. The CV diagrams for L1-L5 recorded in CH2Cl2.
reduction peak was observed at -1.33 V, which may be due to
the reduction of the central core. Selected CV diagrams for L1-
L5 are shown in Figure 8. The electrochemical data along with
the absolute energy (eV) converted by using the ferrocene redox
couple as the standard are shown in Table 2. On the basis of
the electrochemical data, ligands L4 and L5 may be suitable as
electron transport materials because of their relatively deep
LUMO levels.
Boron Compounds. The CV diagrams for all boron com-
pounds (Figure 9) were recorded in CH2Cl2 except B3 whose
J. Org. Chem, Vol. 71, No. 17, 2006 6491

Cui and Wang
FIGURE 9. The CV diagrams for B1, B2, B4, and B5 recorded in
CH2Cl2 and for B3 in DMF.
FIGURE 10. The experimental HOMO and LUMO levels for L1-
L5 and B1-B5.
CV diagram was recorded in DMF because of its poor solubility
in CH2Cl2. Reversible or quasi-reversible oxidation peaks were
observed for the boron compounds B1-B3 and B5. The fact
that most of the boron compounds display a reversible oxidation
peak indicates that the oxidized species of the boron compounds
are more stable than those of the free ligands. The oxidation
potentials for all boron compounds were converted to HOMO
energies using ferrocene as the standard. As shown in Table 2,
the HOMO energies of the boron compounds are similar to those
of the corresponding free ligands except B5 which has a
considerably higher HOMO level than that of L5. Quasi-
reversible reduction peaks for most boron compounds were
observed. The reduction potentials of the boron compounds are
in general much less negative than those of the corresponding
free ligands. In fact the distinct difference between the free
ligands and the boron compounds is the LUMO energy level.
For all boron compounds, the LUMO level is about 0.20 to
0.70 eV lower than that of the corresponding ligand, which
indicates that the chelation by the Lewis acid boron center to
the ligands L1-L5 significantly stabilizes the LUMO level and
makes the boron compounds potentially better electron transport
materials than the free ligands. Previously we have demonstrated
that monoboron BPh2q and BPh2q′ compounds could function
effectively as electron transport materials in OLEDs while the
corresponding free ligands were not effective as electron
transport materials in OLEDs.11 The role of the boron center in
these compounds is parallel to the role of the Al(III) center in
the well-known electron transport compound Alq3sstabilizing
the anionic ligand q and lowering the LUMO level. The
reversibility of the redox waves of the boron complexes indicates
that they are electrochemically more stable than the free ligands
toward reduction, which can be again attributed to the presence
of the Lewis acid boron center in the complexes. The monobo-
ron compound B0 displays an oxidation and a reduction peak
in CH2Cl2 at 1.64 and -1.67 V, respectively. The key difference
between this molecule and the related diboron molecule B1 is
the significant decrease of the oxidation potential in the diboron
compound (corresponding to about 0.2 eV increase of the
HOMO level), which can be again contributed to the presence
of electronic communication between the two boron chelate
chromophores in the diboron compound.
center causes some degree of destabilization of the HOMO level
but dramatic stabilization of the LUMO level, and the net
decrease of the band gap, compared to those of the free ligands.
Molecular Orbital Calculations for L1-L5. To gain a
deeper understanding on the electronic properties of molecules
L1-L5, molecular orbital calculations of ligands have been
performed using Gaussian 98 (DFT), B3LYP parameters, and
the 6-311G** basis set.22 The initial geometric parameters for
L1, L3, and L5 used in the calculations were from crystal data
and were allowed for further geometry optimization. The
geometric parameters for L2 were calculated by using the
coordinates of L1 and replacing the hydrogen atoms at C-2
positions of the methoxyquinoline moieties with two CH3
groups. Similarly, geometric parameters of L4 were obtained
by replacing the three C-H in the central benzene ring of L3
with three N atoms. The final geometric parameters for L2 and
L4 were obtained by geometry optimization. As shown by the
data in Table 2, the general trend of the HOMO/LUMO energies
obtained from the calculations is in agreement with that obtained
from experimental data, although there are considerable devia-
tions between the calculated values and the observed data. The
diagrams of HOMO and LUMO orbitals of L1, L3, L4, and
L5 are shown in Figure 11. For the linear ligands L1 (L2 is
very similar to that of L1) and L5, the HOMO and the LUMO
levels involve contributions from the entire molecule. The lowest
electronic transition in these molecules can be therefore
considered as π-π* transitions. For the star-shaped molecule
of L3 with a central benzene core, the HOMO and LUMO levels
can also be considered as π and π* orbitals, respectively,
involving the entire molecule. In contrast, however, the HOMO
and the LUMO levels of L4 with a triazinie core are quite
different: the HOMO involves contributions from the three
8-methoxyquinoline legs, but little contributions from the central
triphenyltriazine core while the LUMO is dominated by the
triphenyltriazine core with little contributions from the three
8-methoxyquinoline legs. On the basis of this, the lowest
electronic transition in L4 is most likely charge-transfer between
the peripheral 8-methoxyquinoline legs and the central electro-
negative triazine ring. Although molecular orbital calculations
for the boron compounds B1-B5 were not performed because
of their large sizes, the frontier orbitals of the boron compounds
As shown by the experimental HOMO and LUMO energy
level diagram in Figure 10, for the free ligands, there is little
variation on the HOMO level but substantial decrease on the
LUMO level for L4 and L5. The coordination by the boron
(22) (a) Beck, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B: Condens. Matter Mater. Phys. 1988, 37,
785.
6492 J. Org. Chem., Vol. 71, No. 17, 2006

Diboron and Triboron Compounds
FIGURE 13. Solvent dependent emission spectra of L4 (∼10-6 M).
observed (Figure 13). For L4, this is understandable since the
lowest electronic transition is charge transfer between the
8-methoxyquinoline leg and the central triazine core. For L5 it
is not yet understood what the origin of the large red shift of
emission energy is. One possibility is the change of molecular
conformation of L5. In the solid state, as shown by the crystal
structure, the two thienyl groups in L5 have an anti arrangement.
Molecular orbital calculations also confirmed that this is indeed
the preferred conformation. In solution, the molecule likely
undergoes rapid interconversion of different molecular confor-
mations and some of the nonplanar conformations may indeed
lead to charge-transfer emission that is solvent polarity depend-
FIGURE 11. HOMO and LUMO diagrams for L1, L3, L4, and L5.
ent. In CH2Cl2, L1-L4 are efficient blue emitters with λmax
)
409-445 nm and the quantum yield being 61-79%. In contrast,
the thienyl-containing molecule L5 has a relatively low quantum
yield (20%) and the longest emission wavelength (λem ) 462
nm) among the five free ligands, which can be attributed to the
presence of the heavy sulfur atoms in L5 that can reduce the
fluorescent quantum efficiency via “heavy atom effects”. The
high rotational flexibility between the two thienyl groups may
also be responsible for the low emission quantum efficiency of
L5. A similar phenomenon has been observed in the mono-
nuclear BPh2q′ compounds where thienyl functionalized hy-
droxyquinoline compounds have a low quantum efficiency.11a
The molecule L0 emits at 405 nm in CH2Cl2, which is about
10 nm blue-shifted, compared to L1, attributable to the increased
conjugation and the presence of two quinolate chromophores
in L1.
The emission spectra of L1-L5 in the solid-state resemble
those of solutions, but are red-shifted by 10-20 nm. L5
experiences the largest red shift among all the ligands, which
can be attributed to intermolecular interactions as demonstrated
by the crystal structures of L1, L3, and L5. Further evidences
for the presence of intermolecular interactions are the significant
lower quantum yields of the ligands in the solid state (8-23%),
compared to those measured in CH2Cl2. Again, the thienyl-
containing ligand L5 has the lowest quantum yield in the solid
state. Intermolecular π-π interactions that significantly red-
shift emission energy and lower the emission quantum yield
have been frequently observed previously.23 Although molecules
L1-L4 are bright blue emitters in solution and have a high
FIGURE 12. Emission spectra of L1-L5 recorded in CH2Cl2 (∼10-6
M).
TABLE 3. Solvent Effects on Emission of L1-L5 and B1-B5
(∼10-6 M)
solvents
L1/B1
L2/B2
L3/B3
L4/B4
L5/B5
toluene
CH2Cl2
DMF
406/532
416/537
421/539
403/524
408/524
413/532
406/527
413/533
419/538
419/519
443/522
483/524
456/587
462/594
471/600
should resemble those of the corresponding ligands, on the basis
of our earlier investigation on mononuclear BPh2q and BPh2q′
compounds.11a
Luminescent Properties. Ligands. When irradiated by UV
light, the free ligands L1-L5 emit a blue light in solution and
in the solid state. The fluorescent spectra of the ligands are
shown in Figure 12. In solution, the emission energy of L1-
L3 shows a relatively small red shift with the increase of the
solvent polarity, as shown by data in Table 3. In contrast, a
dramatic red shift by L4 and L5 with the solvent polarity was
(23) (a) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi,
N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147. (b) Wang, R. Y.; Jia, W.
L.; Aziz, H.; Vamvounis, G.; Wang, S.; Hu, N. X.; Popovic´, Z. D.; Coggan,
J. A. AdV. Funct. Mater. 2005, 15, 1483.
J. Org. Chem, Vol. 71, No. 17, 2006 6493

Cui and Wang
FIGURE 14. Emission spectra of B1-B4 recorded in CH2Cl2 (∼10-6
M).
thermal stability, the strong intermolecular interactions in the
solid-state hamper their use as blue emitters in OLEDs because
of the low quantum efficiency in the solid state. On the other
hand, strong intermolecular interactions can facilitate charge
transport in OLEDs, if the molecules can undergo reversible
oxidation (hole transport) or reduction (electron transport). For
L4, the reduction peak is quasi-reversible, which along with
the relatively low LUMO level makes it a potential candidate
for electron transport. The fluorescent spectra of the free ligands
do not show significant change with concentration or temper-
ature.
Boron Compounds. In contrast to the free ligands which
emit in the blue region, the boron compounds B1-B4 emit in
the green region (λem ) 528-542 nm) in CH2Cl2 which do not
change significantly with concentration (Figure 14). For com-
pound B5, however, the emission color is dependent on the
concentration of the solution. Compared to the free ligands,
especially L4 and L5, the red-shift of the emission spectra of
the boron compounds with the increase of the solvent polarity
is much less pronounced as shown by Table 3, an indication of
a decreased polarity of the excited state in the boron compounds.
For B4, this can be explained by the coordination of the boron
center to the hydroxyquinoline chelate site which effectively
reduces the electron density on the hydroxyquinoline leg, thus
reducing the polarity of the charge transfer in excited state,
compared to that of L4. In the solid state, the emission spectra
of B1-B4 either experience no change or a small blue shift,
compared to the solution emission spectra. For compound B5,
however, a dramatic red shift was observed which will be further
discussed below. Among the five boron complexes, B4 has the
highest luminescent efficiency both in solution (23% in CH2-
Cl2) and in the solid state (11%), while B5 has the lowest
luminescent efficiency (1% in CH2Cl2 and 3% in solid state )
at room temperature. The boron compounds are in general less
efficient as emitters in solution than the corresponding ligands
as shown by data in Table 1.
FIGURE 15. Emission spectral change of B5 with concentration in
CH2Cl2: (top) the low concentration range (0.02 × 10-5 to 0.51 ×
10-5 M); (bottom) the high concentration range (0.51 × 10-5 to 11 ×
10-5 M).
5.0 × 10-5 M, the emission maximum of B5 is at 593 nm,
about a 130 nm red-shift from the low concentration emission
peak. The high concentration emission spectrum of B5 is in
fact identical to its emission spectrum in the solid state. The
phenomenon of the concentration dependent emission can be
therefore attributed to intermolecular interactions of B5 which
become significant at high concentrations. Although the crystal
structure of B5 was not determined owing to the lack of suitable
single crystals, intermolecular π-π interactions as revealed by
the crystal structure of L5 must be responsible for the
concentration dependence of the emission spectrum of B5. It is
not understood yet why the free ligand L5 does not display
similar concentration-dependent emission. It is noteworthy that
the absorption spectra of B5 are essentially identical from the
solution to the solid state (the same is also true for B1-B4 and
the free ligands), indicative of that intermolecular interactions
have a dramatic impact on the excited state of the boron
compound B5, but not on the ground state (see Supporting
Information). The concentration induced red shift of B5 is
therefore likely caused by the formation of excimers in the high
concentration range.24
Concentration-Dependent Emission of B5. The most in-
teresting observation for the boron compounds is the concentra-
tion dependent emission of B5. As shown by Figure 15 (top),
in the low concentration range (1.0 × 10-7 to 5.0 × 10-6 M),
the emission maximum of B5 is at ∼462 nm, and the emission
intensity increases linearly with the increase of concentration.
Interestingly, however, as the concentration is increased to above
5.0 × 10-6 M, the emission energy shifts gradually to a longer
wavelength, accompanied by a substantial decrease of the
emission intensity (Figure 15, bottom). At the concentration of
Temperature-Dependent Emission of the Boron Com-
pounds. Most of the boron compounds except B5 experience a
(24) (a) Sims, M.; Bradley, D. D. C.; Ariu, M.; Koeberg, M.; Asimakis,
A.; Grell M.; Lidzey, D. G. AdV. Funct. Mater. 2004, 14, 765. (b) Jaramillo-
Isaza, F.; Turner, M. L. J. Mater. Chem. 2006, 16, 83. (c) Lakowicz, J. R.
Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/
Plenum: New York, 1999.
6494 J. Org. Chem., Vol. 71, No. 17, 2006

Diboron and Triboron Compounds
Conclusions
We have demonstrated that by incorporating more than one
8-hydroxyquinolate groups into a linear or star-shaped central
core, the thermal stability of the molecule can be greatly
enhanced. The linear and star-shaped molecules of L1-L5 have
highly anisotropic layered structures in the crystal lattice as
confirmed by the crystal structures L1, L3, and L5 (The
structures of L2 and L4 are very likely to be similar to their
analogues L1 and L3, respectively.). The extended intermo-
lecular interactions as established by the crystal structures are
believed to be responsible for the enhanced thermal/morphologi-
cal stability and the significantly diminished luminescent
efficiency of these new organic molecules in the solid state.
Compounds L4 and L5 have a fairly low LUMO level, which
along with their strong intermolecular interactions makes them
potential electron transport materials. The chelation by boron
centers to molecules L1-L5 does not have a great impact on
thermal properties but dramatically decreases the LUMO levels,
thus making the boron compounds B1-B5 better candidates
as electron transport materials. Furthermore, the chelation by
the boron center substantially decreases the HOMO-LUMO
band gap and red-shifts the emission energy, compared to the
free ligands. The key difference between the polyboron com-
pounds and the monoboron B0 reported here and the monoboron
compounds reported previously is the enhanced thermal stability
and the dramatic temperature and concentration dependent
phenomena as amplified by B5, which is clearly caused by the
much enhanced intermolecular interactions in the polyboron
compounds. Furthermore, the inclusion of polychromophores
or polyboron centers that are linked together by an aromatic
group appears to have significant impact on the HOMO level
of the molecule, as demonstrated by L0 and L1 and B0 and
B1.
FIGURE 16. Temperature-dependent emission for B5 at low concen-
tration (lc) (5.0 × 10-6 M) and at high concentration (hc) (5.0 × 10-5
M) in CH2Cl2.
blue shift of the emission energy from ambient temperature to
low temperature in solution. For example, the linear molecule
B1 emits at 537 nm at ambient temperature which is shifted to
515 nm at 77 K. The blue shift of emission energy at low
temperature can be generally attributed to the increased envi-
ronmental rigidity and has been observed in other systems
previously.25 Similarly, in the solid state, the emission energy
of all boron compounds experience a blue shift when the
temperature is lowered to 77 K.
The temperature-dependent emission of B5 is influenced by
concentration. In the low concentration range (1.0 × 10-7 to
5.0 × 10-6 M), the emission maximum of B5 shifts from blue
(462 nm) at ambient temperature to yellow (564 nm) at 77 K.
In the high concentration range (>5.0 × 10 -5 M), the emission
color changes from red-orange (595 nm) at ambient temperature
to orange (579 nm), as shown in Figure 16. The red shift of the
low concentration sample with the decrease of temperature is
clearly associated with the increased intermolecular interactions
at low temperature. The blue shift of the high concentration
sample can be explained as following. As shown by Figure 14,
the intermolecular interactions of B5 reach saturation at
concentrations greater than 5.0 × 10-5 M at ambient temper-
ature, thus, further decreasing the temperature does not cause
any further red shift of the emission energy. On the other hand,
the decrease of the temperature increases the environmental
rigidity, thus causing a blue shift of the emission energy, as
observed for other boron compounds.
Experimental Section
All solvents were freshly distilled over appropriate drying agents
prior to use. Reactions that required oxygen-free conditions were
1
carried out under nitrogen using standard Schlenk techniques. H
and 13C NMR spectra were recorded on either a 300 MHz or a 400
NMR spectrometer. High-resolution mass spectra were recorded
on a mass spectrometer equipped with an electrospray source. L1-
L5 MS data were recorded using a 1:1 solvent mixture of CH2Cl2
and methanol, while B1-B5 data were recorded in CH3NO2. UV-
vis spectra and excitation and emission spectra were recorded at
room temperature. DSC measurements were carried out at a heating
rate of 10°C/min under an argon atmosphere. TGA measurements
were performed using a heating rate of 10°C/min under nitrogen.
Cyclic voltammetry was measured in CH2Cl2 or DMF using 0.10
M Bu4N[PF6] as the electrolyte. Ag/AgCl was used as the reference
electrode, and a platinum electrode was used as the working
electrode in a conventional three-compartment cell with ferrocene/
ferrocenium (FeCp2/FeCp2+) couple as the external standard. The
scans toward the anodic and cathodic directions were performed
separately at a scan rate of 100 and 500 mV/s, respectively, at room
temperature. TLC was carried out on SiO2. Column chromatography
was carried out on silica. 8-Hydroxyquinoline, 8-hydroxy-2-
methylquinoline, 1, 4-dibromobenzene, 4, 4′-dibromobiphenyl,
4-bromobenzonitrile, and 4-bromoacetophenone were purchased.
5-Bromo-8-methoxyquinoline and 5-bromo-8-methoxy-2-meth-
ylquinoline were prepared using previously reported procedures.5,
13 MOM ethers, 5-bromo-8-(methoxymethoxy)quinoline, and 5-bro-
mo-8-(methoxymethoxy)-2-methylquinoline were prepared by alky-
lation of the corresponding alkoxide anions (deprotonated via NaH
at 0 °C) with MOMCl in THF. All the boronic acids were prepared
The dramatic color change of B5 from blue to red-orange
with concentration and temperature is rare in main group
organometallic compounds, although similar phenomena have
been frequently observed in extended transition metal chain
compounds where the emission color changes with the degree
of metal-metal interactions that are dictated by concentration
and temperature.25a,26 This unique temperature-dependent prop-
erty of B5 makes it a potential candidate as a fluorescent
temperature sensor.
(25) (a) Wang, S.; Garzon, G.; King, C.; Wang, J. C.; Fackler, J. P., Jr.
Inorg. Chem. 1989, 28, 4623. (b) Lees, A. J. Chem. ReV. 1987, 87, 711 (c)
Ferraudi, G. J. Elements of Inorganic Photochemistry; John Wiley &
Sons: New York, 1988. (d) Liu, Q.-D.; Madadu, M. S.; Thummel, R.; Tao,
Y.; Wang, S. AdV. Funct. Mater. 2005, 15, 143.
(26) (a) Assefa, Z.; Omary, M. A.; McBurnett, B. G.; Mojamed, A. A.;
Patterson, H. H.; Staples, R. J.; Fackler, J. P., Jr. Inorg. Chem. 2002, 41,
6274. (b) Omary, M. A.; Patterson, H. Inorg. Chem. 1998, 37, 1060.
J. Org. Chem, Vol. 71, No. 17, 2006 6495

Cui and Wang
according to modified literature procedures.14 All the ligands L1/
L1′-L5/L5′ including intermediate 4-di(2-thienyl)benzene were
synthesized using general Suzuki coupling procedures.15
complexes, the reaction between BPh3 (123 mg, 0.51 mmol) and
L(OH)1 (106 mg, 0.24 mmol) in THF afforded the yellow-green
powder of B1 in 67% yield (123 mg, 0.16 mmol). 1H NMR (CD2-
Cl2, δ, ppm): 8.69 (1H, d, J ) 8.4 Hz), 8.67 (1H, d, J ) 4.8 Hz),
7.86 (2H, d, J ) 6.6 Hz), 7.77 (1H, d, J ) 7.8), 7.71 (1H, dd, J )
4.8, 8.4 Hz), 7.64 (2H, d, J ) 7.8 Hz), 7.47-7.45 (4H, m), 7.30-
7.27 (7H, m). 13C NMR (CD2Cl2, δ, ppm): 158.2, 139.7, 139.6,
138.0, 137.7, 137.4, 133.0, 131.9 (x2C), 130.1, 127.5 (x2C), 127.4,
126.90, 125.9, 123.2, 109.3. 11B NMR (CD2Cl2, ppm): 11.2 ppm.
HRMS: calcd for C54H38B2N2O2K [M + K]+, 807.2768; found [M
+ K]+, 807.2780.
Synthesis of 4, 4′-Di[5′′-(8′′-methoxyquinoline)]biphenyl (L1).
4,4′-Biphenyldiboronic acid (0.24 g, 1 mmol), 5-bromo-8-meth-
oxyquinoline (0.545 g, 2.3 mmol), Pd(PPh3)4 (69 mg, 0.06 mmol),
and Na2CO3 (0.50 g, 4.7 mmol) were added to a flask under
nitrogen. A solvent mixture of toluene (45 mL), water (15 mL),
and ethanol (15 mL) was degassed and added to the reaction
mixture. The resulting mixture was refluxed with vigorous stirring
for 18 h. After the mixture was cooled to room temperature, the
organic phase and the aqueous phase were separated. The aqueous
phase was extracted with CH2Cl2 (3 × 40 mL), and the extract
was combined with the organic phase. After being dried over
MgSO4, the solvents were removed under reduced pressure. The
residue was purified by column chromatography using THF/Hexane
(4/1) as the eluent, and the subsequent recrystalization from hexane/
The synthetic procedures for B2-B5 and B0 are provided in
the Supporting Information.
X-ray Crystallography Analyses. Single-crystals of L1, L3,
L5, and B0 were obtained from CH2Cl2/hexane solution and were
mounted on glass fibers in a brass pin. The data were collected on
a single-crystal X-ray diffractometer with a detector and graphite-
monochromated Mo KR radiation operating at 50 kV and 30 mA
at 25 °C, except for L5 whose data were collected at -93 °C. No
significant decay was observed during the data collection. Data were
processed using the SHELXTL software package (version 5.10).16
Neutral atom scattering factors were taken from Cromer and
Waber.17 Empirical absorption correction was applied to all crystals.
The crystals of L1 and L3 belong to the triclinic space group Ph1
while the crystals of L5 and B0 belong to the orthorhombic space
group Pbca and the monoclinic space group P21, respectively. The
structures were solved by direct methods. Disordered CHCl3 solvent
molecules were located in the crystal lattice of L3. Because of the
difficulty in modeling the disordered solvent molecules in L3, their
contributions were removed by using the SQUEEZE routine of the
Platon software suite.17b,17c All non-hydrogen atoms except some
of the disordered atoms were refined anisotropically. The positions
of hydrogen atoms were calculated, and their contributions in
structural factor calculations were included. Complete crystal-
lographic data are provided in Supporting Information.
Quantum Yield Measurements. Quantum yields of compounds
B0-B5 and L0-L5 were determined using either anthracene (Φr
) 0.25) or 9,10-diphenylanthracene as the standard in CH2Cl2 at
298 K (Φr ) 0.95).18 The absorbance of all the samples and the
standard at the excitation wavelength is approximately 0.098-0.109.
The quantum yields were calculated using previously known
procedures.19 The absolute solid-state quantum efficiencies (SSQE)
of luminescence were measured from freshly spin-coated films of
chloroform solutions using an integrating sphere in terms of the
previously reported procedures.20
1
CH2Cl2 afforded 0.37 g of L1 as a white powder (yield 79%). H
NMR (CDCl3, δ, ppm): 9.02 (1H, d, J ) 3.8 Hz), 8.38 (1H, d, J
) 8.5 Hz), 7.84 (2H, d, J ) 7.78 Hz), 7.60 (2H, d, J ) 7.8 Hz),
7.55 (1H, d, J ) 7.9 Hz), 7.48 (1H, dd, J ) 3.8 Hz, 8.5 Hz). 7.19
(1H, d, J ) 8.0 Hz), 4.19 (3H, s). 13C NMR (CDCl3, δ, ppm):
154.8, 149.0, 139.7, 138.6, 134.6, 131.8, 130.7, 127.6, 127.5, 127.2,
121.7, 114.3, 107.3, 56.1. HRMS: calcd for C32H25N2O2 [M +
H]+, 469.1916; found, 469.1906.
The synthetic procedures and analytical data for L2-L5 and L0
are provided in the Supporting Information.
Syntheses of L1′-L5′. Molecules of L1′-L5′ are analogues of
L1-L5 with the 8-methoxy group being replaced by a 8-meth-
oxymethoxy (MOMO) on the quinoline ring. The synthetic
procedures for L1′-L5′ are identical as those used for L1-L5
except that one of the starting materials, 5-bromo-8-methoxyquino-
line, is replaced by 5-bromo-8-methoxymethoxyquinoline for L1′
and L3′-L5′, and by 5-bromo-8-methoxymethoxy-2-methylquino-
line for L2′. For details, please see the Supporting Information.
General Procedures for the Syntheses of L(OH)1-L(OH)5.
L1′-L5′ were dissolved in 30 mL of mixed solvents of MeOH
and CH2Cl2 (2:1) as a suspension in a round-bottom flask equipped
with a condenser. When 1 mL of concentrated HCl was slowly
added, the reaction mixture immediately turned to a clear yellow
solution. After it was refluxed for about 24 h, the yellow mixture
became a suspension again from a clear solution. After it was cooled
to room temperature, the mixture was neutralized with saturated
NaHCO3, and yellow-green precipitation was obtained. By washing
with CH2Cl2 (3 × 20 mL), L(OH)1-L(OH)5 were collected in
excellent yields (85-92%) by filtration, which were used for
cheleation with BPh3 without further purification.
L(OH)1: Yield 91%. 1H NMR (DMSO, δ, ppm): 10.00(1H, s),
8.93 (1H, d, J ) 2.7 Hz), 8.33 (1H, d, J ) 8.0 Hz), 7.92 (2H, d,
J ) 6.4 Hz), 7.62-7.59 (3H, m), 7.50 (1H, d, J ) 8.0 Hz), 7.20-
(1H, d, J ) 8.0 Hz).
Data for L(OH)2-L(OH)5 and L(OH)0 are provided in the
Supporting Information.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial support.
We are in debt to Dr. Jian-Ping Lu at the National Research
Council for his assistance in recording the quantum yields in
the solid state.
General Procedures for the Synthesis of Boron Complexes.
THF solutions of 2 equiv of BPh3 for B1, B2, and B5 and 3 equiv
for B3 and B4 were added to a stirred suspension of the
corresponding ligands L1(OH)1-L(OH)5 in THF. Each reaction
mixture was refluxed. The initial suspension dissolved completely
after the mixture was refluxed for over 24 h. After concentrating
the solution by vacuum, the crude yellow-green powders of B1-
B4 or orange powder of B5 was isolated and was further purified
by recrystallization from CH2Cl2 and hexane.
Supporting Information Available: Detailed experimental and
synthetic procedures for all compounds reported here, complete
crystal data for L1, L3, L5, and B0 including tables of atomic
coordinates, thermal parameters, bond lengths and angles, and
hydrogen parameters, UV-vis data, NMR spectra, and TGA data,
as well as Cartesian atom coordinates and absolute energies
for ligands L1-L5 obtained from Gaussian 98 calculations.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Synthesis of 4,4′-Bis(Ph2B-8′′-hydroxyquinolate)biphenyl (B1).
Following the general procedure for the syntheses of the boron
JO060841+
6496 J. Org. Chem., Vol. 71, No. 17, 2006