Syntheses, structure, and optical properties of ladder-type fused azaborines

Article abstract of DOI:10.1021/ol060539n

Ladder-type fused azaborines were synthesized. X-ray crystallographic analysis of a pentacene-type molecule shown here revealed the planarity of a fused azaborine. It was revealed by UV-vis and fluorescence spectra that such fused molecular structures are

Full text of DOI:10.1021/ol060539n

ORGANIC
LETTERS
2006
Vol. 8, No. 11
2241-2244
Syntheses, Structure, and Optical
Properties of Ladder-Type Fused
Azaborines
Tomohiro Agou, Junji Kobayashi, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science,
The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
takayuki@chem.s.u-tokyo.ac.jp
Received March 4, 2006
ABSTRACT
Ladder-type fused azaborines were synthesized. X-ray crystallographic analysis of a pentacene-type molecule shown here revealed the planarity
of a fused azaborine. It was revealed by UV vis and fluorescence spectra that such fused molecular structures are efficient for the extension
of -conjugated systems containing main group elements.
−
π
Organic light-emitting devices (OLEDs) have been studied
extensively as alternative devices to conventional cathode
ray tube or liquid crystalline displays. They have many
advantages over the conventional devices, such as improved
brightness and color purity, as well as lower drive voltages.1
π-Conjugated small molecules and polymers are used for
the constituents of OLEDs, and π-conjugated molecules
bearing main group elements are one of the latest targets in
this research area.
In these molecules, the main framework consists of a
π-conjugated hydrocarbon backbone and main group ele-
ments because the electronic interactions between the
π-orbitals and the donor/acceptor orbitals can substantially
decrease the HOMO-LUMO energy gap. Group 15 and 16
elements, which have lone pairs, usually act as electron
donors, and group 13 and 14 elements have acceptor orbitals
and can behave as acceptors. Many main group element
compounds have been investigated as electron acceptors in
π-conjugated systems, and extensive research on heteracy-
clopentadienes (heteroles) has revealed that π-conjugated
molecules substituted by boron, silicon, and phosphorus show
good optical and electronic properties.2
Boron is a unique element among such electron acceptors.3
A vacant 2p orbital provides the electron-withdrawing
property, and the acceptor property can be switched revers-
(2) (a) Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R. A. Synth.
Met. 1998, 96, 177. (b) Yamaguchi, S.; Tamao, K. In The Chemistry of
Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley:
Chichester, 2001; Vol. 3, p 641. (c) Yamaguchi, S.; Tamao, K. Chem. Lett.
2005, 34, 2. (d) Hissler, M.; Lescop, C.; Re´au, R. Pure Appl. Chem. 2005,
77, 2099. (e) Hissler, M.; Dyer, P. W.; Re´au, R. Coord. Chem. ReV. 2003,
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10.1021/ol060539n CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/28/2006

ibly when Lewis bases coordinate to the vacant orbital and
diminish acceptor ability. Boron-containing π-systems pos-
sess two interesting properties: (1) small HOMO-LUMO
gaps due to the strong acceptor property; and (2) dynamic
control of the electronic state and molecular structure by
external stimuli (Lewis bases). From such a viewpoint, much
attention has been paid to boron-containing π-conjugated
molecules.
respectively. The ladder-type molecules showed decreased
HOMO-LUMO gaps, because of the two main group
elements, as well as efficient π-conjugation because of the
molecular planarity, judging from X-ray crystallographic
analysis, UV-vis spectroscopy, and fluorescence spectro-
scopy. The ladder molecules emitted strong fluorescence with
a small Stokes shift because of their rigid molecular
structures.
Dibenzoheteraborins are π-conjugated molecules bearing
one or more main group elements with boron atoms on the
framework, and the electronic state can be tuned by the
exchange of the main group elements. Optical properties and
some reactivities of dibenzoazaborine including a nitrogen
atom have been reported by several researchers.4 We also
reported the syntheses of dibenzophosphaborins, π-conju-
gated molecules containing phosphorus atoms as electron
donors.5 Although dibenzophosphaborins showed UV-vis
absorption similar to that of dibenzoazaborine, their photo-
luminescence properties are inferior in terms of broadening
of emission peaks and quantum yields.
p-Phenylenediamine 1 and m-phenylenediamine 2 were
synthesized from 2-bromo-4-butylaniline and 1,4-dibromo-
2,5-diiodobenzene or 1,3-dibromo-4,6-diiodobenzene via
palladium-catalyzed amination.8 Pentacene-type ladder mol-
ecules 3 and 4 were synthesized by the reaction of MesB-
(OMe)2 with tetralithio derivatives prepared from 1 and 2
under reflux conditions (Scheme 1). Para-type compound 3
Scheme 1. Syntheses of Ladder Molecules 3 and 4
Ladder-type and planar π-conjugated molecular architec-
tures bearing main group elements on main chains or the
periphery are current topics in material chemistry because
the rigid and planar framework of ladder molecules is
expected to improve several of the properties important for
applications, including the degree of π-conjugation as well
as photo- and electroluminescence quantum yields. For
example, silicon-bridged phenylene-vinylene oligomers
have been synthesized via an efficient intramolecular cy-
clization, and these compounds exhibited good optical
properties (small HOMO-LUMO gaps and strong light
emission).6
If π-conjugated oligomers or polymers bearing boron
atoms in the main chains are fixed into planar ladder-type
structures, such molecules will be among the best candidates
for OLEDs because of the substantially decreased HOMO-
LUMO energy gaps and strong emissive properties of
triarylboranes.7 The boron-containing ladder molecules,
however, have not been reported yet, despite their desirable
properties.
In this paper, we report the syntheses of new planar
π-conjugated molecules having a fused azaborine framework.
These ladder-type molecules have boron and nitrogen atoms
in the main chains as electron acceptors and donors,
was obtained as a red solid, and meta-type compound 4 was
obtained as a yellow solid. Both compounds are stable against
air and moisture because of steric protection of the boron
atoms by the bulky mesityl groups and rigid structure.
To synthesize a precursor for an extended ladder molecule,
a sequential palladium-catalyzed amination protocol was
employed. Benzophenone imine, an ammonia synthon, was
coupled with iodide 5, and subsequent deprotection gave
amine 6.9 The precursor 7 for a longer ladder molecule was
obtained by coupling of 5 and 6 followed by methylation at
the central nitrogen atom. Heptacene-type ladder molecule
8 was synthesized from 7 and MesB(OMe)2 by the same
procedure described above (Scheme 2). 8 was obtained as a
violet solid that was stable against air and moisture.
(4) (a) Maitlis, P. M. J. Chem. Soc. 1961, 425. (b) Kuznetsov, V. A.;
Gamzatov, A. Z.; Barabonin, S. Y.; Reikhsfel’d, V. O.; Nesterova, S. V.
Zh. Obshch. Khim. 1990, 60, 2505. (c) Ryzhikov, M. B.; Rodionov, A. N.;
Nesterova, O. V.; Shigorin, D. N. Zh. Fiz. Khim. 1988, 62, 1097. (d)
Ryzhikov, M. B.; Rodionov, A. N.; Nekrasov, V. V.; Shigorin, D. N. Zh.
Fiz. Khim. 1988, 62, 2491. (e) Ryzhikov, M. B.; Rodionov, A. N.; Godik,
V. A.; Shigorin, D. N. Zh. Fiz. Khim. 1989, 63, 638. (f) Nesterova, S. V.;
Gamzatov, A. Z.; Kuznetsov, V. A.; Reikhsfel’d, V. O. Zh. Obshch. Khim.
1988, 58, 1296. (g) Ryzhikov, M. B.; Rodionov, A. N.; Serov, S. A.;
Shigorin, D. N. Khim. Geterotsikl. Soedin. 1989, 1676. (h) Kranz, M.;
Hampel, F.; Clark, T. Chem. Commun. 1992, 1247.
(5) Agou, T.; Kobayashi, J.; Kawashima, T. Org. Lett. 2005, 7, 4373.
(6) (a) Yamaguchi, S.; Xu, C.; Yamada, H.; Wakamiya, A. J. Organomet.
Chem. 2005, 690, 5365. (b) Yamaguchi, S.; Xu, C. J. Synth. Org. Chem.,
Jpn. 2005, 63, 1115. (c) Xu, C.; Wakamiya, A.; Yamaguchi, S. J. Am. Chem.
Soc. 2005, 127, 1638.
(8) (a) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852. (b) Hartwig, J. F.
Angew. Chem. Int. Ed. 1998, 37, 2046.
(9) Wolfe, J. P.; Ahman, J.; Sadighi, J. P.; Singer, R. A.; Buchwald, S.
L. Tetrahedron Lett. 1997, 38, 6367.
(7) (a) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
10776. (b) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998,
120, 5112.
2242
Org. Lett., Vol. 8, No. 11, 2006

Scheme 2. Synthesis of Ladder Molecule 8
Figure 1. ORTEP drawings of pentacene-type ladder molecule 3
with a thermal ellipsoid plot (50% probability). H atoms are omitted
for clarity. (a) Top view. (b) Side view. Mesityl, n-butyl, and methyl
groups are not shown in the side-view drawing.
absorption maxima compared to that of 9, exhibiting
extended π-conjugation by ladder-type frameworks. The
longest absorption band of meta-type ladder molecule 4,
however, stays at almost the same wavelength as 9, indicating
weak or no π-conjugation between the two neighboring
azaborine units in the meta-type connection mode.
The steady-state fluorescence spectra of the ladder mol-
ecules and 9 are shown in Figure 3, and the photophysical
data are listed in Table 1.
These compounds exhibited small Stokes shifts (∆λ < 20
nm) because the rigid framework restricts structural deforma-
tion upon photoexcitation. Para-type ladder molecules 3 and
8 showed higher quantum yields than 9, but the meta-type
ladder molecule 4 gave only a low quantum yield. The energy
transfer may occur between two neighboring mesityl groups
Single crystals of 3 suitable for X-ray crystallographic
analysis were obtained by recrystallization from CHCl3/EtOH
solution (Figure 1).10,11 The diazadiborapentacene framework
is nearly planar, judging from the dihedral angles between
the peripheral benzene rings and the azaborine rings. 3 does
not show any significant intermolecular interactions such as
π-π stacking or CH-π interaction in the crystalline state,
despite its extended planar π-conjugated systems. Such
intermolecular interaction might be prevented by the bulky
substituents on the boron atoms.
The UV-vis spectra of the ladder molecules, as well as
those of parent dibenzoazaborine 9, are shown in Figure 2.
Para-type ladder molecules 3 and 8 showed red-shifted
(10) Crystallographic data for 3: C46H54B2N2, red cube, monoclinic,
space group P21/n, a ) 11.654(3) Å, b ) 10.754(3) Å, c ) 15.597(4) Å, â
) 107.3673(11)°, V ) 1865.6(9) Å3, Z ) 2, F(000) ) 708, crystal size
0.20 × 0.20 × 0.20 mm3, 6.42 e 2Θ e 55.00. In total, 10052 reflections
were collected, of which 2844 were independent (Rint ) 0.0168) and
employed for refinement: 231 parameters, 0 restraints, R1 (I > 2σ(I)) )
0.0377, wR2 (all data) ) 0.0961. The intensities of reflections were collected
at 120 K on a RIGAKU MSC Mercury CCD diffractometer with graphite-
monochromated Mo KR radiation (λ ) 0.71070 Å) using CrystalClear
(Rigaku Corp.). The structure was solved by direct methods (SHELXS)
and expanded using Fourier techniques. The structure was refined by full-
matrix least-squares methods on F2 (SHELXL-97). All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were assigned to idealized
positions and were included in structure factor calculations. CCDC-298596
contains the supplementary crystallographic data for this paper. These data
can be obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ,
U.K.; fax: (+44) 1223-336-033 or deposit@ccdc.cam.ac.uk).
Figure 2. UV-vis spectra of the ladder molecules in cyclohexane
at 298 K. Green: 3. Red: 4. Blue: 8. Black: 9.
(11) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Org. Lett., Vol. 8, No. 11, 2006
2243

608 nm is similar to those of the other ladder compounds,
but the other emission was observed around 580 nm. The
origin of this peak remains unclear, and further investigation
on this fluorescence is now underway.
In summary, three new π-conjugated ladder-type mol-
ecules featuring azaborine have been synthesized via pal-
ladium-catalyzed amination reactions as key steps. These are
the first examples for boron-containing π-conjugated ladder
molecules. The X-ray crystallographic analysis revealed the
planar molecular structure that is important for the extension
of π-conjugation. The para-type ladder molecules showed
red-shifted absorption maxima and fluorescence according
to the molecular length. These results indicate that para-type
ladder molecules are good candidates for novel π-conjugated
molecules, taking advantage of electronic characteristics of
main group elements.
Figure 3. Fluorescence spectra of the ladder molecules in
cyclohexane at 298 K. Green: 3. Red: 4. Blue: 8. Black: 9.
Table 1. Photophysical Data of the Ladder Compoundsa
Acknowledgment. This work was supported by Grants-
in-Aid for The 21st Century COE Program for Frontiers in
Fundamental Chemistry (T.K.) and for Scientific Research
(T.K.) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We thank Prof. Dr. Hiroshi
Nishihara, Dr. Masaki Murata, and Dr. Shoko Kume (The
University of Tokyo) for their valuable advice for the optical
measurements. We also thank Tosoh Finechem Corp. for the
generous gifts of alkyllithiums.
λmax/nm (log ꢀ)
λem/nm
Φ
3
4
8
9
523 (4.23)
415 (3.94)
608 (4.28)
405 (4.02)
534
428
625
421
0.69
0.21
0.55
0.48
a The UV-vis absorption spectra and the fluorescence spectra were
recorded in cyclohexane at 298 K. The fluorescence quatum yields were
determined using anthracene in EtOH (Φ 0.27), fluorescein in 0.1 M aqueous
NaOH (Φ 0.85), or rhodamine B in EtOH (Φ 0.71) as standards.
Supporting Information Available: Experimental pro-
cedures for the syntheses of all the compounds and the X-ray
crystallographic data in CIF format for 3. This material is
available free of charge via the Internet at http://pubs.acs.org.
in the excited state of 4. These data suggest that para-type
connections between main group elements in the ladder
molecules are efficient for light-emitting devices.
Longer ladder molecule 8 showed two emission maxima,
unlike the other compounds. The peak shape observed around
OL060539N
2244
Org. Lett., Vol. 8, No. 11, 2006