Development of a general route to periphery-functionalized azaborines and ladder-type azaborines by using common intermediates

Article abstract of DOI:10.1039/b706418g

Azaborines and ladder-type azaborines bearing various functional groups can be synthesized starting from common dibromo derivative intermediates, and among several substituents, the carbazol-9-yl group was shown to enhance the photo-luminescence quantum y

Full text of DOI:10.1039/b706418g

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Development of a general route to periphery-functionalized azaborines
and ladder-type azaborines by using common intermediates{
Tomohiro Agou, Junji Kobayashi and Takayuki Kawashima*
Received (in Cambridge, UK) 27th April 2007, Accepted 26th June 2007
First published as an Advance Article on the web 5th July 2007
DOI: 10.1039/b706418g
However, to tune the electronic and optical properties of
azaborines using this previous synthetic method, the synthesis of
different precursors for each new azaborine is necessary, and such
shortcomings in synthetic generalities have hampered the screening
of azaborine-based optical materials, and an alternative diversity-
oriented synthetic strategy needs to be developed. In this
communication, we describe a general synthetic method for the
preparation of various azaborines from common starting materi-
als. This new strategy was successfully applied to the synthesis of
periphery-functionalized azaborines and ladder-type azaborines. In
particular, amino-functionalized azaborines exhibited red-shifted
absorption and emission spectra, and carbazol-9-yl substituted
azaborines showed an increased photo-luminescence yield that was
nearly equal to a value of unity.
Azaborines and ladder-type azaborines bearing various func-
tional groups can be synthesized starting from common
dibromo derivative intermediates, and among several substi-
tuents, the carbazol-9-yl group was shown to enhance the
photo-luminescence quantum yield of the azaborines up to a
value of unity.
Recently, p-conjugated molecules bearing main group elements
have attracted much attention, because electronic interactions
between the p-orbitals and main group elements can change the
electronic state of the parent p-systems by raising the HOMO or
lowering the LUMO,1 and these molecules can have various
interesting and useful properties, such as electro-luminescence.2
Among the many main group elements that can be used, boron
occupies a special position,3 because the electronic interaction
between the vacant 2p orbital of boron and the p* orbital is
favorable due to the efficient overlapping of these orbitals, and
Bromo derivatives show potential as common intermediates for
functionalized azaborines due to their diverse reactivity in many
types of reaction. Direct bromination of azaborines is difficult due
to the low tolerance of the boron moiety against oxidation.
Alternatively, the dibromo derivatives 3a and 3b were prepared
from bis(2,4-dibromophenyl)hexylamine (1) and ArB(OMe)2 (Ar =
Mes (2a) or Tip (2b)) taking advantage of the ortho-selective
dilithiation of 1 (Scheme 1).9
boron exhibits
a strong electron-withdrawing characteristic,
substantially lowering the energy level of a LUMO.
The construction of a widespread library of emissive materials
covering the full color range is an important issue for their
application in devices, such as electro-luminescence devices. The
introduction of electron donors or acceptors to a p-conjugated
framework is an efficient way to modulate the electronic and
optical properties of organic materials, such as the HOMO–
LUMO energy gap and emission color, and by following such a
strategy, p-conjugated molecules exhibiting various properties can
be obtained easily when starting from a common synthetic
intermediate. Because boranes are potentially strong electron
acceptors, several donor–acceptor or acceptor–acceptor hybrid
molecules using the borane functionality as an acceptor moiety
have been investigated.2a,2b,3,4
We first investigated palladium-catalyzed coupling reactions
utilizing the bromo functionality, selecting the Hartwig–Buchwald
amination, because amino groups can work as efficient donor
groups and can decrease the HOMO–LUMO energy gap
(Scheme 2).10 After screening various amines, HexnNH2 and
Ph2NH were coupled with 3a or 3b using a standard amination
condition (Pd(dba)2–BINAP) without affecting the boron centers.
Carbazole, a less reactive amine, could be also introduced by
taking advantage of a more powerful catalyst system (Pd(dba)2–
PBut2(2-Biph)). In contrast, coupling with cyclic dialkylamines
(pyrrole, morpholine and piperazine), which have been reported to
be efficient coupling partners for various aryl halides,11 gave
complex mixtures. Although the reason for this is unclear, much
more basic and nucleophilic dialkylamines can attack boron
centers and can degrade an azaborine framework.
Recently, we reported on the synthesis and properties of
heteraborins, which are p-conjugated molecules bearing two main
group elements in a 9,10-dihydroanthracene framework.5,6 The
structure and properties of heteraborins varied according to the
main group element.7 We have also synthesized ladder-type
molecules based on an azaborine functionality (ladder-type
azaborines), which exhibited reduced HOMO–LUMO energy
gaps and had strong fluorescence emissions.8
To examine the generality of the coupling reactions, the
introduction of carbon-functional groups was investigated, and
Department of Chemistry, Graduate School of Science, The University
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: takayuki@chem.s.u-tokyo.ac.jp; Fax: +81-3-5800-6899;
Tel: +81-3-5841-4338
{ Electronic supplementary information (ESI) available: Synthesis of the
compounds and details of the theoretical calculations. See DOI: 10.1039/
b706418g
Scheme 1
3204 | Chem. Commun., 2007, 3204–3206
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Scheme 2
Scheme 4
Scheme 3
Table 1 Optical data of the azaborines
lmaxa/nm (log e)
lema/nm (W)
DEb/cm21
lemc/nm
the preliminary results are shown in Scheme 3. The Sonogashira
coupling with phenyl acetylene proceeded well, indicating that
alkyne substituted azaborine oligomers or azaborine–alkyne
co-polymers can be obtained using the Sonogashira coupling
protocol. In addition, halogen–lithium exchange reaction could be
carried out without affecting the triarylborane moiety,12 and the
generated di-lithio derivative reacted with MeI to give dimethyl
derivative 3g quantitatively. Such a result shows that other
functional groups, including B(OR)2 and SnR3 groups, can be
introduced into the azaborine framework.
3c
3d
3e
3h
a
453 (3.77)
466 (3.80)
400 (3.86)
405 (4.02)
501 (0.51)e
494 (0.49)e
443 (1.00)d
421 (0.48)d
1500
1800
2400
940
c
530
519
448
456
b
In cyclohexane at rt. DE = 1/lmax 2 1/lem
.
In the solid state.
d
Determined by using 9,10-diphenylanthracene in cyclohexane (W
1.00) as a standard. Determined by using fluorescein in 0.1 M aq.
NaOH (W 0.85) as a standard.
e
Electronic and fluorescence spectra of the substituted azaborines
were recorded in cyclohexane at room temperature, and the optical
data are summarized in Table 1. The absorption wavelength of 3e
was close to that of the reference compound, dimethyl derivative
3h (Fig. 1), indicating a weak electronic effect of the carbazolyl
group. On the other hand, azaborines bearing more electron-
donating amino groups (Ph2N or HexnNH) showed red-shifted
absorptions. Such a difference in absorption wavelength indicated
that electron-donating groups at the para-positions to the nitrogen
Because the azaborine functionality was shown to tolerate
coupling conditions, the expansion of this protocol to more
extended molecules, such as ladder-type azaborines, was then
investigated. The common precursor, dibromo-derivative 5a, was
prepared from hexabromide 4. Surprisingly, the halogen–lithium
exchange of 4 proceeded in an ortho-selective fashion, similar to
the case of 1, and the generated tetralithio derivative reacted with
MesB(OMe)2 to give an orange precipitate that was insoluble in
ordinary solvents. Although the crude product contained 5a as the
1
major component, judging from the H NMR spectrum and the
FAB-MS spectrum, the isolation of 5a was difficult due to its low
solubility. We tried amination reactions using the crude material
without further purification. Amino-substituted pentacene-type
azaborines 5b–d could be obtained following the optimized
conditions for the amination of 3a and 3b (Scheme 4). These
extended azaborines had a high solubility in ordinary organic
solvents, and could be purified using GPC and recrystallization.13
These results show that the common precursor strategy using
dibromo derivatives is applicable in general for various azaborine-
based materials.
Fig. 1 The structures of reference azaborine and ladder-type azaborines.
Chem. Commun., 2007, 3204–3206 | 3205
This journal is ß The Royal Society of Chemistry 2007

View Article Online
yellow to red, in accordance with the absorption and emission
wavelength in solution.
Table 2 Optical data of the ladder-type azaborines
lmaxa/nm (log e)
lema,b/nm (W)
DEc/cm21
lemd/nm
In summary, we have developed a new and general synthetic
method to introduce various functional groups on the periphery of
azaborines and ladder-type azaborines. This methodology will
enable the construction of organic functional materials, such as
luminescent polymers. In addition, by screening the electronic
effect of amino groups, the carbazol-9-yl group was revealed to
enhance the photo-luminescence quantum yield of azaborines and
ladder-type azaborines substantially without affecting the
HOMO–LUMO energy gap, and thus, these amino groups can
work as pure photo-luminescence enhancers.
5b
5c
5d
5e
6
580 (4.14)
568 (3.99)
529 (4.14)
523 (4.23)
609 (4.28)
603 (0.12)
592 (0.20)
546 (0.98)
534 (0.69)
626 (0.55)
660
620
590
390
450
620
636
570
599
683
a
b
In cyclohexane at rt. Determined by using rhodamine B in EtOH
c
(W 0.61) as a standard. DE = 1/lmax 2 1/lem
d
.
In the solid state.
atom of azaborine decreased the HOMO–LUMO energy gap by
elevating the HOMO energy level. Theoretical calculations on
model azaborines support this conclusion, with the electron-
donating groups elevating the HOMO, while the energy level of
the LUMO does not change much on substitution.14
This work was supported by Grants-in-Aid for The 21st COE
Program for Frontiers in Fundamental Chemistry and for
Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan (T.K.) and Research
Fellowships of the Japan Society for the Promotion of Science for
Young Scientists (T.A.). We thank Tosoh Finechem Corp. for the
generous gift of alkyllithiums.
The azaborines exhibited strong fluorescence emissions in
solution. In particular, the introduction of a carbazol-9-yl group
enhanced the fluorescence quantum yield of azaborines dramati-
cally, probably due to the molecular rigidity of both the azaborine
and carbazole frameworks. In addition, irradiation of the
absorption bands of carbazole (3e) or Ph2N (3d) groups (ca.
300 nm) resulted in an emission from the azaborine moiety with
the same quantum yield, indicating an efficient energy transfer
from the amino groups. The emission wavelengths of the
azaborines in the solid state are also summarized in Table 1.
Irradiation by UV-lamp (254 nm or 366 nm) of these compounds
showed a strong emission that varied from blue (3e and 3h) to
green (3c and 3d), which was probably due to the prevention of
intermolecular interactions by the bulky Mes or Tip groups on the
boron atoms.
Notes and references
1 (a) S. Yamaguchi and K. Tamao, Chem. Lett., 2005, 34, 2; (b)
M. Hissler, P. W. Dyer and R. Re´au, Top. Curr. Chem., 2005, 250, 127.
2 (a) W. L. Jia, M. J. Moran, Y.-Y. Yuan, Z. H. Lu and S. Wang,
J. Mater. Chem., 2005, 15, 3326; (b) M. Kinoshita, H. Kita and
Y. Shirota, Adv. Funct. Mater., 2002, 11–12, 780; (c) H.-C. Su,
O. Fadhel, C.-J. Yang, T.-Y. Cho, C. Fave, M. Hissler, C.-C. Wu and
R. Re´au, J. Am. Chem. Soc., 2006, 128, 983.
3 (a) C. D. Entwistle and T. B. Marder, Chem. Mater., 2004, 16, 4574; (b)
S. Yamaguchi and A. Wakamiya, Pure Appl. Chem., 2006, 78, 1413.
4 Strong solid-state emissive molecules bearing a Mes2B group as an
acceptor and an Ar2N group as a donor have been reported recently.
C.-H. Zhao, A. Wakamiya, Y. Inukai and S. Yamaguchi, J. Am. Chem.
Soc., 2006, 126, 15934.
5 (a) P. M. Maitlis, J. Chem. Soc., 1961, 425; (b) M. Kranz, F. Hampel
and T. Clark, J. Chem. Soc., Chem. Commun., 1992, 1247; (c) M. B.
Ryzhikov, A. N. Rodionov, V. V. Nekrasov and D. N. Shigorin, Zh.
Fiz. Khim., 1988, 62, 2491; (d) M. B. Ryzhikov, A. N. Rodionov, O. V.
Nesterova and D. N. Shigorin, Zh. Fiz. Khim., 1988, 62, 1097.
6 (a) T. Agou, J. Kobayashi and T. Kawashima, Org. Lett., 2005, 7, 4373;
(b) T. Agou, J. Kobayashi and T. Kawashima, Inorg. Chem., 2006, 45,
9137; (c) M. H. Lee, T. Agou, J. Kobayashi, T. Kawashima and
F. P. Gabba¨ı, Chem. Commun., 2007, 1133.
7 Oxaborin and thiaborin frameworks were used in bidentate Lewis acids.
(a) S. So´le and F. P. Gabba¨ı, Chem. Commun., 2004, 1284; (b)
M. Mela¨ımi, S. So´le, C.-W. Chiu, H. Wang and F. P. Gabba¨ı, Inorg.
Chem., 2006, 45, 8136.
8 T. Agou, J. Kobayashi and T. Kawashima, Org. Lett., 2006, 8, 241.
9 L. Fan, L. Yang, C. Guo, B. M. Foxman and O. V. Ozerov,
Organometallics, 2004, 23, 4778.
10 (a) J. F. Hartwig, Acc. Chem. Res., 1998, 31, 852; (b) J. F. Hartwig,
Angew. Chem., Int. Ed., 1998, 37, 2046.
11 J. P. Wolfe and S. L. Buchwald, J. Org. Chem., 2000, 65, 1144.
12 If BunLi was used instead of ButLi, 3b was recovered in 65% yield.
Coordination of a butyl group to the boron center may prevent the
halogen–lithium exchange reaction.
13 When Ph2NH or carbazole was used as a coupling partner instead of
Ar2NH or di-But-carbazole, respectively, the generated amino-substi-
tuted ladder-type azaborine could not be isolated due to low solubility.
14 The results of the theoretical calculations are included in Electronic
Supplementary Information.
The optical data of ladder-type azaborines in cyclohexane are
summarized in Table 2. Similar to the azaborines, the absorption
maxima of the ladder-type azaborines were shifted to longer
wavelengths in accordance with the increase in the electron-
donating capability of the amino group, i.e., carbazolyl derivative
5d showed an absorption maximum that was close to that of the
reference compound, butyl derivative 5e, but Ar2N- or HexnNH-
substituted molecules 5b or 5c exhibited absorption spectra
that were more red-shifted, reflecting their enhanced donating
capability.
Fluorescence spectra of the ladder-type azaborines were
recorded in cyclohexane at room temperature. The emission color
of 5d (green) was almost the same as that of 5e, but the
fluorescence quantum yield of 5d increased up to a value close to
unity, like 3e, and thus, the carbazol-9-yl group was shown to
work as an enhancer of the photo-luminescence emission of
azaborines without affecting the HOMO–LUMO energy gap.
Although the fluorescence quantum yield was not very high, 5b
and 5c exhibited a red emission, as heptacene-type azaborine 6
does.8 The Stokes shift of these ladder-type molecules was smaller
than that of the azaborines, probably due to the rigid framework.
On irradiation using a UV-lamp (254 or 366 nm), the ladder-type
molecules showed photo-luminescence in the solid-state that could
be observed by the naked eye. The luminescence color varied from
3206 | Chem. Commun., 2007, 3204–3206
This journal is ß The Royal Society of Chemistry 2007