Electronic and optical properties of ladder-type heteraborins

Article abstract of DOI:10.1002/chem.200700622

Ladder-type π-conjugated molecules bearing heteraborin (azaborine or thiaborin) units were synthesized, and X-ray crystallographic analysis of pentacene-type molecules showed that these molecules have rigid and planar structures. UV-visible spectroscopy a

Full text of DOI:10.1002/chem.200700622

FULL PAPER
DOI: 10.1002/chem.200700622
Electronic and Optical Properties of Ladder-Type Heteraborins
Tomohiro Agou, Junji Kobayashi, and Takayuki Kawashima*[a]
Abstract: Ladder-type p-conjugated
molecules bearing heteraborin (azabor-
ine or thiaborin) units were synthe-
sized, and X-ray crystallographic analy-
sis of pentacene-type molecules
showed that these molecules have rigid
and planar structures. UV-visible spec-
troscopy and theoretical calculations
revealed the enhancement of electronic
interaction between heteraborin units,
the decrease in HOMO–LUMO
energy gaps, and the strong effect of
the bridging main-group elements (ni-
trogen or sulfur) on the electronic
states. The ladder-type molecules emit-
ted strong fluorescence both in solution
and in the solid state, and the emis-
sion-band shapes were different from
each other, indicating the existence of
intermolecular interactions in the solid
state. Complex formation of the ladder
molecules with fluoride ion was moni-
tored by UV-visible spectroscopy,
which revealed that the Lewis acidity
of these molecules can be controlled by
the elongation of p systems and the
change of the bridging main-group ele-
ments.
Keywords: conjugation · heterocy-
cles · optical properties · pi interac-
tions · X-ray crystallography
Introduction
properties due to their weak aromaticity and negative hyper-
conjugation derived from their low-lying s* orbitals (s* or-
ꢀ
ꢀ
p-Conjugated molecules bearing main-group elements
(hetero-p-conjugated molecules) have attracted much atten-
tion in various research fields such as fundamental chemis-
try, supramolecular chemistry, material chemistry, and indus-
trial chemistry. This is because atomic orbitals of main-
group elements can interact with p orbitals on parent hydro-
carbons and differentiate the electronic structures of the
molecules substantially, and hence novel properties are ex-
pected to appear in such molecules. Hetero-p-conjugated
molecules bearing nitrogen, oxygen, or sulfur have been in-
vestigated over recent decades;indeed, some of these mole-
cules have been applied to, for example, organic electrolu-
minescence devices. However, the compounds with other
main-group elements have not been used so much, in spite
of their valuable electronic properties.[1] Among such main-
group element compounds, silicon- or phosphorus-contain-
ing p-conjugated molecules, especially heteracyclopenta-
dienes (heteroles), have recently been investigated because
they exhibit anomalous and fruitful electronic and optical
bitals of Si R or P R bonds). In fact, these molecules have
a decreased HOMO–LUMO energy gap relative to the
parent cyclopentadiene, and the weak aromaticity enhances
delocalization of p electrons over the butadiene moiety.[2,3]
On the one hand, a boron atom is very attractive when it
is introduced to a hetero-p-conjugated system, such as triar-
ylborane-based p-conjugated molecules.[4] A tricoordinated
boron atom has a vacant 2p orbital and acts as a strong elec-
tron acceptor through its efficient overlap of p orbitals. An-
other interesting feature of boranes is reversible structural
and electronic transformation upon complexation with
Lewis bases. When boranes form complexes with Lewis
bases, the resulting tetracoordinated boron complexes no
longer exhibit the conjugated electronic properties that tri-
coordinated boron compounds do. Thus, both the electronic
and optical properties of borane-based p-conjugated mole-
cules can be controlled by external stimuli, leading to the
application to chemical sensors. Such properties allow triar-
ylboranes to be good candidates for many applications, such
as luminescence polymers,[5] nonlinear optics,[6] electrolumi-
nescence,[7] electron-carrier materials,[8] and chemical sensors
for Lewis bases.[9] The structural diversity of hetero-p-conju-
gated systems, however, is still insufficient to construct the
various types of functional materials desired. For example,
many boron-containing p-conjugated molecules must have a
Mes2B unit for effective steric protection around boron cen-
ters, and the freedom of molecular design is limited.
[a] Dr. T. Agou, Dr. J. Kobayashi, Prof. T. Kawashima
Department of Chemistry, Graduate School of Science
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku
Tokyo 113–0033 (Japan)
Fax : (+81)3-5800-6899
E-mail: takayuki@chem.s.u-tokyo.ac.jp
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2007, 13, 8051 – 8060
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8051

On the other hand, molecular planarity is known to be an
important parameter for enhancement of p conjugation be-
cause interactions between conjugated p orbitals and atomic
orbitals on main-group elements are maximized in planar
framework. Fixation of a p-conjugated system into a rigid
and planar structure not only enhances the p delocalization
but also intensifies luminescence emission because such a
structure can prevent thermal degradation from the excited
states. A ladder-type molecule is an efficient motif for con-
struction of a planar and rigid p-conjugated system.[10] From
such a view point, hybrid systems of ladder-type p-conjugat-
ed molecules and hetero-p-conjugated molecules have been
recently reported. For example, ladder-type molecules bear-
ing silicon[11] or chalcogen (sulfur, selenium, or tellurium)[12]
in the conjugated backbones have exhibited improved opti-
cal or electronic properties, such as strong blue lumines-
cence or good organic-field effect transistor property.
Dibenzoheteraborin is one of the hetero-p-conjugated
molecules based on a dihydroanthracene framework bearing
a boron atom and another main-group element.[13,14] In this
rigid and planar molecule, the synergistic electronic interac-
tions were expanded into three factors, that is, a boron 2p
orbital, a lone pair of the main-group element, and p orbi-
tals of the conjugation system, and photoluminescence effi-
ciency will be increased. Besides the enhancement of the
photoluminescence property, the rigid structure furnishes
enough stability to the triarylborane moiety. Thus, dibenzo-
heteraborin is expected to be a good candidate for organic
electronic and optical materials. However, the reported di-
benzoheteraborins are limited to only four molecules;dibor-
in, azaborin, oxaborin, and thiaborin, and the electronic and
optical properties of these heteraborins have not been inves-
tigated precisely.[15]
Scheme 1. Ladder-type heteraborins and parent dibenzoheteraborins.
procedure (Scheme 2);the tetrabromide was lithiated with
nBuLi in Et2O, and the generated tetralithio derivative was
treated with MesB(OMe)2 to give pentacene-type thiaborin
A
2 as yellow solids.
Recently, we reported the electronic and optical proper-
ties of new hetero-p-conjugated molecules based on diben-
zoheteraborin units.[16] In the previous communication, we
reported the synthesis and preliminary study on optical
properties of ladder-type azaborines.[17] UV-visible spectra
of these molecules showed red shift of absorption maxima
in accordance with extension of the p systems, indicating en-
hancement of p conjugation through the ladder-type molec-
ular framework. Here we report the synthesis of the ladder-
type heteraborins containing nitrogen or sulfur atoms as
bridging main-group elements and the systematic study of
their optical properties, as well as their complexation abili-
ties against fluoride ion.[18,19]
Scheme 2. Synthesis of ladder-type thiaborin 2.
It is important to add MesB(OMe)2 at room temperature
A
(around 300 K) as slowly as possible to obtain reproducible
results. Otherwise, many unidentified products were gener-
ated, and, especially in the synthesis of 5, the yield of the
ladder-type molecules decreased drastically by rapid addi-
tion. The syntheses of ladder-type thiaborins 4 and 6 as well
as parent dibenzothiaborin 8 were achieved in the similar
manner (see Experimental Section). The ladder-type hetera-
borins were obtained in good to moderate yields, and all the
compounds were stable against air and moisture.
Results and Discussion
Synthesis of ladder-type heteraborins: The synthesized
ladder-type molecules have two or three heteraborin units
connected by phenylene linkages (Scheme 1). The syntheses
of pentacene-type azaborines 1 and 3 as well as heptacene-
type azaborine 5 have been already reported.[17]
X-ray crystallographic analysis of pentacene-type thiaborin
2: A single crystal of 2 was obtained by recrystallization
from CHCl3/EtOH, and the molecular structure of 2 is
shown in Figure 1. The crystal of 2 contains two independent
molecules in the unit cell, and one of the two molecules is
They were synthesized from the corresponding poly-
bromides, and the synthesis of 2 is shown as a representative
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Ladder-Type Heteraborins
FULL PAPER
marized in Table 1. The absorption maxima of the ladder-
type heteraborins with para-type substitution mode were
red shifted according to the elongation of p conjugation
Table 1. Optical data of the ladder-type heteraborins in cyclohexane at
298 K.
lmax [nm][a] (log e)
lem [nm][b]
DE[c] [cmꢀ1
]
t [ns]
1
2
3
4
5
6
7
8
523 (4.23)
499 (4.04)
415 (3.94)
399 (3.81)
609 (4.28)
558 (3.95)
405 (4.02)
387 (3.94)
534 (0.69)
517 (0.72)
428 (0.21)
420 (0.06)
626 (0.55)
580 (0.61)
421 (0.48)
403 (0.39)
3.9”102
7.0”102
7.3”102
1.3”103
4.5”102
6.8”102
9.4”102
1.0”103
18
–
–
–
33
–
9.3
–
[a] Absorption maxima in UV-visible spectra and absorption coefficiency
in parenthesis. [b] Emission maxima of fluorescence spectra and quantum
Figure 1. ORTEP drawings of pentacene-type thiaborin 2 (50% probabil-
ity). Hydrogen atoms are omitted for clarify. a) Top view. b) Side view.
yield in parenthesis. [c] DE=1/lmaxꢀ1/lem
.
shown here. The molecular structures of 1 and 2 are nearly
the same,[17] and both molecules have a planar and rigid
framework as expected, indicating that the ladder frame-
work is effective for extension of p conjugation to improve
the photoluminescence properties (see below). Intermolecu-
lar interactions, such as p–p stacking of the pentacene
moiety or CH–p interaction, are not observed in the packing
diagrams of 1 and 2, probably due to the steric congestion
of Mes groups.
from the anthracene-type (7 or 8) to the heptacene-type
molecules (5 or 6), whereas meta-type substitution mole-
cules 3 and 4 showed almost the same absorption maxima as
those of 7 and 8.
The HOMO–LUMO energy gaps of these para-substitut-
ed molecules are decreased in accordance with the elonga-
tion of p-conjugation systems, indicating that the electronic
communication between the heteraborin units and delocali-
zation of p electrons are more effectively operated in the
para-substitution mode than in the meta-substitution mode.
Thus, the para-type substitution mode provides a better con-
tribution to the extension of p conjugation than the meta-
type substitution mode.
UV-visible spectra of ladder-type heteraborins: UV-visible
absorption spectra of the ladder-type heteraborins in cyclo-
hexane are shown in Figure 2, and the optical data are sum-
Comparing the spectra of the azaborines with those of the
thiaborins, the former series always showed more red shifted
absorption bands and larger absorption coefficients than the
latter series. The longest absorption bands of the ladder-
type heteraborins arise from intramolecular charge transfer
from bridging main-group elements (nitrogen or sulfur) to
boron atoms, and the donor–acceptor interaction between
nitrogen and boron atoms is stronger than that between
sulfur and boron atoms. The stronger donor–acceptor inter-
action of the azaborines than that of the thiaborins can be
interpreted to provide the azaborines with more decreased
HOMO–LUMO energy gaps as well as more enhanced os-
cillator strength of optical absorption.
Evaluation of effective conjugation length of ladder-type he-
teraborins: It is well known that transition energy (E) of p-
conjugated oligomers or polymers can be expressed empiri-
cally as a linear function of a reciprocal number of repeating
units (n) [Eq. (1)].[20]
E ¼ E1 þ DE Â ð1=nÞ
ð1Þ
E1 is the infinitive value of E when n!1, and thus this
value indicates the effective conjugation length of the p-con-
jugated oligomers or polymers. These parameters can be ob-
Figure 2. UV-visible spectra of the ladder-type heteraborins in cyclohex-
ane at 298 K. a) Azaborines. b) Thiaborins.
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T. Kawashima et al.
tained simply by plotting the observed transition energy
against the number of repeating units and by using the least-
square fitting method.
This method was applied to the ladder-type azaborines
and thiaborins, and the results are shown in Figure 3. By
Theoretical calculations on frontier orbitals of the ladder-
type heteraborins:[22,23] Electronic structures of the ladder-
type heteraborins were investigated by using the DFT
method (B3LYP/6–31G(d)), and the energy levels of the
frontier orbitals are summarized in Table 2. Theoretical cal-
culations were carried out on model ladder-type heterabor-
ins bearing hydrogen atoms instead of alkyl and mesityl
groups in the real systems in order to reduce the computa-
tion burden.
Table 2. Calculated frontier orbital energy and energy gaps.[a]
HOMO [eV]
LUMO [eV]
DE [eV]
1
2
3
4
5
6
7
8
ꢀ5.33
ꢀ5.75
ꢀ5.71
ꢀ6.03
ꢀ5.10
ꢀ5.61
ꢀ5.98
ꢀ6.03
ꢀ2.34
ꢀ2.71
ꢀ2.03
ꢀ2.31
ꢀ2.61
ꢀ2.95
ꢀ1.71
ꢀ2.12
2.99
3.04
3.68
3.72
2.49
2.66
4.27
3.91
Figure 3. Plot of transition energy (E [cmꢀ1]) obtained from UV-visible
spectra versus reciprocal number of heteraborin units (n) of the ladder-
&
type hetetaborins. Least-square fitting lines are also shown. : azaborines,
[a] Level of calculations: B3LYP/6–31G(d)//B3LYP/6–31G(d).
*
: thiaborins.
using the least-square fitting method, we found the follow-
ing regression Equations (2) and (3) for the azaborines and
thiaborins, respectively (n: number of heteraborin units per
molecule):
As indicated by the UV-visible spectra in the previous
section, elongation of the molecular framework showed the
decrease in HOMO–LUMO energy gaps due to effective p
conjugation in the para-type substitution, but meta-substitut-
ed molecules 3 and 4 did not show the effective extension of
p conjugation. The ladder-type azaborines have higher-
energy frontier orbitals because the molecular orbitals of
the azaborines should move to higher energies due to the
strong electron donation from the nitrogen atoms. In con-
trast, the frontier orbitals of the ladder-type thiaborins lie at
lower energy levels, reflecting the weaker electron donation
from sulfur compared with that of nitrogen. As a result, the
HOMO–LUMO energy gaps did not differ so much be-
tween the azaborines and the thiaborins. In addition, a plot
of energy gaps (DE) against the reciprocal number of re-
peating units (n) (Figure 5) gave linear regression Equa-
tions (4) and (5) for the azaborines and the thiaborins with
para-substitution mode, respectively.
E Â 10ꢀ4 cmꢀ1 ¼ 1:22 Â ð1=nÞ þ 1:26 ðR2 ¼ 0:9935Þ
E Â 10ꢀ4 cmꢀ1 ¼ 1:20 Â ð1=nÞ þ 1:40 ðR2 ¼ 0:9997Þ
ð2Þ
ð3Þ
E1 values of the ladder-type azaborines and thiaborins
were estimated to be 1.26”104 cmꢀ1 (1.3 eV) and 1.40”
104 cmꢀ1 (1.5 eV), respectively. These values are comparable
with the theoretical limits of transition energy of several
conjugated molecules based on fully p-conjugated carbon
frameworks (Figure 4).[21] The E1 values of the ladder-type
heteraborins are larger than that of polyacene that has isoe-
lectronic structure with the ladder-type heteraborins, and
thus delocalization of p electrons over the ladder-type heter-
aborins is weaker and p electrons are relatively localized on
the benzene rings and the bridging main-group elements (ni-
trogen or sulfur).
Figure 5. Theoretical energy gap (DE [eV]) of the model ladder-type he-
Figure 4. Theoretical limits of transition energy (E1) of several p-conju-
gated polymers obtained by extrapolation of experimental data.
teraborins against the reciprocal number of heteraborin units per mole-
&
*
cule (n). : ladder-type azaborines, : ladder-type thiaborins.
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Ladder-Type Heteraborins
FULL PAPER
DE ½eVꢁ ¼ 2:13 Â ð1=nÞ þ 1:87
DE ½eVꢁ ¼ 1:85 Â ð1=nÞ þ 2:08
ð4Þ
ð5Þ
contained at least three components, indicating the genera-
tion of other photoexcited states than the lowest excited
state. For the para-substituted ladder-type azaborines, time-
resolved fluorescence spectra were also measured in cyclo-
hexane at 298 K, and the fluorescence lifetimes (t/ns) are
shown in Table 1. The fluorescence lifetime increased in ac-
cordance with the extension of the p-conjugated framework.
Investigations of these two interesting phenomena are now
under way.
These equations show that the theoretical limits of the
energy gaps of the ladder-type azaborines and thiaborins are
1.9 and 2.1 eV, respectively. The existence of limitation in
HOMO–LUMO energy gaps is in marked contrast to the
case of polyacenes because infinite polyacene is revealed to
have no energy gap and becomes true organic metal or sem-
imetal.[21d,e] Such a difference is considered to arise from the
localization of p electrons in the ladder-type heteraborins.
Solid-state fluorescence spectra of the ladder-type hetera-
borins: Solid-state fluorescence spectra of the ladder-type
heteraborins are shown in Figure 7, and the spectral data
Fluorescence spectra of the ladder-type heteraborins:
Steady-state fluorescence spectra of the heteraborins were
recorded in cyclohexane at 298 K (Figure 6), and the optical
Figure 7. Solid-state fluorescence spectra of the ladder-type heteraborins
(powder sample). a) Azaborines. b) Thiaborins.
Figure 6. Steady-state fluorescence spectra of the ladder-type heterabor-
ins in cyclohexane at 298 K. a) Azaborines. b) Thiaborins.
are summarized in Table 3. The photoimages of the powder
samples are available in the Supporting Information. These
molecules showed intense photoluminescence in the solid
state as well as in solution, except for heptacene-type mole-
cules 5 and 6, probably because the bulky Mes groups pro-
hibit aggregation that results in degradation from the excit-
properties of these molecules are summarized in Table 1.
Reflecting the red shift of the absorption maxima in the
UV-visible spectra, emission maxima of the ladder-type he-
teraborins also showed red shift, going from 7 and 8 to 5
and 6, respectively. Stokes shifts of the ladder-type hetera-
borins are still small (102–103 cmꢀ1), indicating that the struc-
tural relaxation is very small in these ladder-type molecules
due to the molecular rigidity. Fluorescence quantum yields
of the ladder-type heteraborins increased relative to those
of the parent dibenzoheteraborins 7 and 8, except for the
meta-type substituted molecules 3 and 4. These photolumi-
nescence properties imply that the para-substituted ladder-
type molecules are expected to be used for optical devices.
In addition, the fluorescence of the ladder-type molecule 5
Table 3. Optical data of the ladder-type heteraborins in the solid state.
[b]
lmax[a] [nm]
lem [nm]
DE[c] [cmꢀ1
]
1
2
5
6
7
8
556
519
645
590
430
405
599
550
683
613
456
432
1.3”103
1.1”103
8.6”102
6.4”102
1.3”103
1.5”103
[a] Excitation spectrum maxima in the solid state. [b] Emission maxima
of fluorescence spectra in the solid state. [c] DE=1/lmaxꢀ1/lem
.
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T. Kawashima et al.
ed state through nonradiation processes. The photolumines-
cence intensities of 5 and 6 in the solid state were moderate
because substantial overlap of the absorption and emission
bands of these molecules may result in self quenching of the
fluorescence emission. The wavelengths of emission
maxima, however, are shifted relative to those of the fluo-
rescence in solution, and the shifts of the ladder-type aza-
borines 1 and 5 increased by almost twice that of parent di-
benzoazaborine 7, indicating the existence of intermolecular
dipole–dipole interactions in the excited states. Although
direct p–p stacking is ruled out by steric repulsion, it is
likely that the long-range interaction between excited di-
poles may influence fluorescence emission in the solid state.
In addition, the solid-state fluorescence spectrum of parent
dibenzothiaborin 8 exhibited two emission maxima at 432
and 540 nm. The latter emission peak may originate from
excimer formation in the solid state because the single crys-
tals of this molecule contained p–p dimeric structure, shown
by X-ray crystallographic analysis (Figure 8).
Figure 8. ORTEP drawing of dibenzothiaborin 8 (50% probability). Hy-
drogen atoms were omitted for clarify. Shortest contacts between two
molecules in the dimeric structure are indicated by dashed lines.
Table 4. Complex-formation constants of the ladder-type heteraborins
with fluoride ion.
K1 [mꢀ1
]
K2 [mꢀ1
]
K3 [mꢀ1
]
Complex formation with fluoride ion: As the ladder-type he-
teraborins have several boron atoms in one molecule, these
molecules are expected to function as a multistep fluoride
ion sensor.[9d,24] Upon the complex formation, the electronic
state of the ladder-type heteraborins is switched, and it will
cause multistep colorimetric change that can be monitored
by UV-visible spectroscopy. The complex formation was car-
ried out in THF by using (nBu)4NF (TBAF) as a fluoride
ion source. The spectral changes of the heptacene-type he-
teraborins 5 and 6 are shown in Figure 9, and the complex
formation constants are summarized in Table 4. Heptacene-
type azaborine 5 formed monofluoroborate upon the addi-
1
2
5
6
7(1)”104
3.2(7)”105
2.8(8)”105
8(4)”106
1.1(1)”102
2.0(1)”103
5.5(5)”102
4(3)”105
–
–
–
[a]
2.3(1)”103
[a] Complex formation was not observed.
tion of three equivalents of TBAF with color change from
red to orange (Figure 9a), but further reaction hardly oc-
curred in spite of the addition of a large excess of TBAF
(Figure 9b). Heptacene-type thiaborin 6, however, reacted
with up to three equivalents of TBAF quantitatively to form
Figure 9. UV-visible spectral change upon addition of TBAF in THF. (a) and (b) 5. (c) and (d) 6.
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Ladder-Type Heteraborins
FULL PAPER
Bromo-4-butylaniline,[17] 1,4-dibromo-2,5-diiodobenzene and 1,5-dibro-
mo-2,4-diiodobenzene,[25] and bis(2-bromophenyl) sulfide[26] were synthe-
sized according to literature procedures.
mono-, di-, and trifluoroborates in a stepwise fashion, fol-
lowed by color change from pink to colorless via yellow, in-
dicating the increased Lewis acidity of boron centers (Figur-
es 9c and d). Comparing the formation constants of the fluo-
roborates, extension of p systems or change of the bridging
main-group elements from nitrogen to sulfur seems to in-
crease the Lewis acidity. The explanations for such phenom-
ena are as follows: 1) extension of p systems decreased the
LUMO energy level, 2) the thiaborins have lower LUMO
energy levels than those of the azaborines due to a weaker
donating ability of a sulfur atom.
10-Mesityl-10H-9-thia-10-boraanthracene (8): To a Et2O (100 mL) solu-
tion of bis(2-bromophenyl)sulfide (0.50 g, 1.5 mmol) was added tBuLi
(2.2m in pentane, 3.4 mL, 7.5 mmol) at ꢀ758C, and the mixture was
stirred for 30 min at this temperature and then for 30 min at room tem-
perature. To this mixture was added MesB(OMe)2 (0.34 mL, 1.6 mmol),
A
and the mixture was refluxed for 4 h. The precipitates were filtered off,
and the crude products were subjected to GPLC. After the solvents were
removed under reduced pressure, vacuum drying gave 8 as pale-yellow
solids (0.25 g, 53%). 1H NMR (500 MHz, CDCl3): d=1.93 (s, 6H), 2.40
(s, 3H), 6.94 (s, 2H), 7.30 (td, J=7.4, 1.0 Hz, 2H), 7.62 (td, J=7.6,
1.5 Hz, 2H), 7.78 (d, J=8.1 Hz, 2H), 7.87 ppm (dd, J=7.7, 1.3 Hz, 2H);
13C NMR (126 MHz, CDCl3): d=21.3, 22.9, 124.6, 125.2, 126.9, 132.2,
136.8, 138.7, 139.6, 144.3 ppm (two signals corresponding to the ipso-car-
bons of the boron atom could not be observed due to broadening by a
quadrupole moment of 11B nuclei); 11B NMR (128 MHz, CDCl3): d=
58.6 ppm;UV/Vis (cyclohexane): lmax (log e)=387 nm (3.94);HRMS
(FAB+): m/z: calcd for C21H1932S10B: 313.1337;found: 313.1304;elemen-
tal analysis calcd (%) for C21H19BS: C 80.26, H 6.09;found: C 80.04, H
6.28.
Conclusion
We synthesized the ladder-type p-conjugated molecules
based on heteraborins (azaborine or thiaborin) units. These
molecules were shown to have a rigid and planar frame-
work, judging from X-ray crystallographic analysis and theo-
retical calculations. UV-visible and fluorescence spectrosco-
py as well as theoretical calculations proved that the ladder-
type structures are effective for the improvement of elec-
tronic communication between the heteraborin units and de-
localization of p electrons. The ladder-type azaborines emit-
ted intense fluorescence both in solution and in the solid
state because of the rigid framework and inhibition of
strong intermolecular interactions such as p–p stacking.
Lewis acidity of these molecules can be controlled by the
extension of the p system or the change of bridging main-
group elements, as demonstrated in the complex formation
with fluoride ion.
2-Bromo-4-butylbenzenethiol: To a suspension of 2-bromo-4-butylaniline
(50 g, 0.22 mol) in aq. HCl (6m, 400 mL) at ꢀ58C was added dropwise a
solution of NaNO2 (23 g, 0.33 mol) in H2O (100 mL), and the mixture
was stirred for 1 h. To the mixture was added a solution of EtOCS2K
(60 g, 0.38 mol) in H2O (100 mL), and the solution was stirred at 808C
for 4 h. The aqueous layer was extracted with Et2O, and the organic layer
was dried over Na2SO4. After the evaporation of the solvents, the crude
products were dissolved in anhydrous THF (100 mL), and the mixture
was slowly added to the suspension of LiAlH4 (20 g, 0.53 mol) in THF
(200 mL) at 08C. After stirring at RT for 10 h, the mixture was poured
into a mixture of concd HCl (100 mL) and crushed ice (caution: strong
stench!). The aqueous layer was extracted with Et2O, and the organic
layer was dried over Na2SO4. The solvents were evaporated, and the
crude material was purified by distillation (3 mmHg, 1308C) to give the
title compound as colorless oil (38 g, 71%). B.p. 1308C (3 mmHg);
1H NMR (400 MHz, CDCl3): d=0.91 (t, J=7.3 Hz, 3H), 1.32 (sext, J=
7.4 Hz, 2H), 1.55 (q, J=7.6 Hz, 2H), 2.52 (t, J=7.7 Hz, 2H), 3.98 (s,
1H), 6.98 (dd, J=7.5, 1.6 Hz, 1H), 7.24 (d, J=7.5 Hz, 1H), 7.35 ppm (d,
J=1.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=13.9, 22.3, 33.4, 34.7,
122.2, 128.1, 129.5, 130.1, 132.8, 142.0 ppm;HRMS (FAB +): m/z: calcd
for C10H1379BrS: 243.9922;found: 243.9938.
Experimental Section
General procedure: General chemicals were used as received. Cyclohex-
ane and THF for spectrochemical or fluorometric grade (Dojindo) were
used for optical measurements. All manipulations were carried out using
Schlenk techniques under an argon atmosphere. Solvents were purified
by using the MBRAUN MB-SPS system. Wet column chromatography
(WCC) was performed by using Kanto Silica Gel 60N. Gel permeation
liquid chromatography (GPLC) was performed using LC-918 or LC-908
with JAIGEL 1H+2H columns (Japan Analytical Industry) by using
chloroform or toluene as solvents, respectively. NMR spectra were re-
1,4-Dibromo-2,5-bis(2-bromo-4-butylphenylsulfanyl)benzene (9):[27]
mixture of 2-bromo-4-butylbenzenethiol (2.9 g, 12 mmol), 1,4-dibromo-
2,5-diiodobenzene (2.9 g, 5.9 mmol), [Pd(dba)2] (69 mg, 0.12 mmol)
A
AHCTREUNG
(dba=(E,E)-dibenzylideneacetone), DPPF (66 mg, 0.12 mmol) (DPPF=
1,1’-bis(diphenylphosphino)ferrocene), NaOtBu (1.7 g, 18 mmol), and tol-
uene (50 mL) was stirred at 808C for 1 d. The reaction was quenched by
the addition of aqueous NH4Cl, and the aqueous layer was extracted
with C6H6. The organic layer was dried with Na2SO4, and the solvents
were evaporated. The crude product was separated by column chroma-
tography (SiO2, eluent: hexane) to give 9 as colorless solid (2.7 g, 64%).
M.p. 126–1288C; 1H NMR (500 MHz, CDCl3): d=0.91 (t, J=8.6 Hz,
6H), 1.35 (sext, J=7.4 Hz, 4H), 1.60 (q, J=7.6 Hz, 4H), 2.60 (t, J=
7.8 Hz, 4H), 7.04 (s, 2H), 7.12 (dd, J=7.3, 1.8 Hz, 2H), 7.24 (d, J=
7.3 Hz, 2H), 7.52 ppm (d, J=1.8 Hz, 2H); 13C NMR (126 MHz, CDCl3):
d=13.9, 22.3, 33.1, 34.9, 122.8, 127.5, 128.8, 130.1, 133.4, 133.8, 134.3,
136.9, 145.9 ppm;LRMS (FAB +): m/z: 718 [M+];elemental analysis
calcd for (%) C26H26Br4S2: C 43.24, H 3.63;found: C 43.14, H 3.64.
corded by using
a
JEOL AL-400 spectrometer (1H, 400 MHz; 13C,
100 MHz; 11B, 128 MHz), a JEOL ECX-400 spectrometer (1H, 400 MHz;
13C, 100 MHz), a Bruker DRX-500 spectrometer (1H, 500 MHz; 13C,
126 MHz), or
a
Bruker AV-500 spectrometer (1H, 500 MHz; 13C,
126 MHz). Chemical shifts are reported in d. 1H NMR spectra are refer-
enced to residual protons in deuterated solvent; 13C NMR spectra are ref-
erenced to carbon-13 in the deuterated solvent; 11B NMR spectra are ref-
erenced to an external standard of BF3·Et2O. Low-resolution mass spec-
tra were obtained with a JEOL JMS-700P by using m-nitrobenzyl alcohol
(mNBA) as a matrix. High-resolution mass spectra were recorded by
using a JEOL JMS-700P with PEG400, PEG600, or Ultramark as inter-
nal standards. UV/Vis spectra were recorded by using a JASCO V-530
spectrophotometer. Fluorescence spectra were recorded by using a HI-
TACHI F4500 fluorescence spectrophotometer. All melting points were
measured by using a Yanaco MP-S3 and are uncorrected. Elemental
analyses were performed by the Microanalytical Laboratory of the De-
partment of Chemistry, Faculty of Science, The University of Tokyo. 2-
1,5-Dibromo-2,4-bis(2-bromo-4-butylphenylsulfanyl)benzene (10): A mix-
ture of 2-bromo-4-butylbenzenethiol (2.4 g, 9.8 mmol), 1,5-dibromo-2,4-
diiodobenzene (2.4 g, 4.9 mmol), [Pd(dba)2] (0.28 g, 0.48 mmol), DPPF
G
(0.27 g, 0.49 mmol), NaOtBu (1.4 g, 15 mmol), and toluene (50 mL) was
stirred at 808C for 2 d. The reaction was quenched by the addition of
aqueous NH4Cl, and the aqueous layer was extracted with C6H6. The or-
ganic layer was dried with Na2SO4, and the solvents were evaporated. To
the crude mixture was added hexane, and the mixture was triturated to
Chem. Eur. J. 2007, 13, 8051 – 8060
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8057

T. Kawashima et al.
give 10 as colorless solids. Recrystallization from EtOH/CHCl3/hexane
gave pure 10 (2.1 g, 60%). M.p. 110–1128C; H NMR (500 MHz, CDCl3):
pared from TIPS-SH (0.44 mL, 2.1 mmol) (TIPS-SH=triisopropylsilane-
thiol) and nBuLi (1.6m in hexane, 1.2 mL, 1.9 mmol) in THF (10 mL),
and the mixture was stirred at 808C for 12 h. The reaction was quenched
by the addition of aqueous NH4Cl, and the aqueous layer was extracted
with Et2O. The combined organic layer was dried over Na2SO4, and the
solvents were removed under reduced pressure. The crude product was
subjected to GPLC to give 12 as colorless solids (0.50 g, 60%). M.p. 152–
1548C; 1H NMR (500 MHz, CDCl3): d=0.93 (t, J=7.3 Hz, 6H), 1.35
(sext, J=7.4 Hz, 4H), 1.61 (quint, J=7.7 Hz, 4H), 2.62 (t, J=7.7 Hz,
4H), 6.97 (s, 2H), 7.16 (dd, J=7.9, 1.9 Hz, 2H), 7.28 (s, 2H), 7.37 (d, J=
7.9 Hz, 2H), 7.55 ppm (d, J=1.9 Hz, 2H); 13C NMR (126 MHz, CDCl3):
d=13.89, 22.28, 33.05, 34.99, 121.81, 124.93, 128.53, 138.97, 129.10,
1
d=0.95 (t, J=7.5 Hz, 6H), 1.36 (sext, J=7.4 Hz, 4H), 1.59 (q, J=7.6 Hz,
4H), 2.58 (t, J=7.9 Hz, 4H), 6.21 (s, 1H), 7.01 (dd, J=8.0, 2.0 Hz, 2H),
7.20 (d, J=8.0 Hz, 2H), 7.33 (d, J=2.0 Hz, 2H), 7.74 ppm (s, 1H);
13C NMR (126 MHz, CDCl3): d=13.9, 22.4, 33.1, 35.1, 119.9, 128.5, 129.0,
+
129.6, 133.6, 135.3, 136.1, 137.8, 145.8, 148.7 ppm;HRMS (FAB
,
mNBA, PEG600): m/z: calcd for C26H2679Br432S2: 717.8210;found:
717.8212;elemental analysis calcd (%) for C 26H26Br4S2: C 43.24, H 3.63;
found: C 43.45, H 3.65.
2,9-Dibutyl-7,14-dimesityl-7,14-dihydro-5,12-dithia-7,14-diborapentacene
(2): To a Et2O solution (500 mL) of 9 (0.50 g, 0.69 mmol) at 08C was
added nBuLi (1.6m in hexane, 2.2 mL, 3.5 mmol), and the mixture was
+
132.61, 133.62, 134.02, 135.43, 135.48, 139.65, 146.63 ppm;LRMS (FAB
): m/z: 982 [M+];elemental analysis calcd (%) for C 32H28Br6S3: C 38.89,
stirred for 20 min at 08C. To this mixture was added MesB(OMe)2
A
H 2.86;found: C 38.76, H 2.95.
(0.36 mL, 1.7 mmol). The resulting fluorescent green suspension was re-
fluxed for 1 h, and the solvents were removed under reduced pressure.
The resulting red solids were extracted with CHCl3, and the solvent was
evaporated. The crude product was subjected to GPLC to give 2 as
yellow solids (0.21 g, 45%). M.p. 278–2808C (dec.); 1H NMR (500 MHz,
CDCl3): d=0.87 (t, J=7.4 Hz, 6H), 1.29 (sext, J=7.4 Hz, 4H), 1.52 (q,
J=7.7 Hz, 4H), 1.95 (s, 12H), 2.45 (s, 6H), 2.59 (t, J=7.7 Hz, 4H), 6.98
(s, 4H), 7.46 (dd, J=8.2, 1.8 Hz, 2H), 7.66 (d, J=8.2 Hz, 2H), 7.68 (d,
J=1.8 Hz, 2H), 8.23 ppm (s, 2H); 13C NMR (126 MHz, CDCl3): d=13.9,
21.4, 22.2, 23.0, 33.9, 35.2, 125.3, 127.1, 131.5, 133.4, 133.6, 136.1, 136.9,
138.7, 139.2, 139.4, 139.8, 142.2 ppm; 11B NMR (128 MHz, CDCl3): d=
58.6 ppm (h1/2 =1900 Hz);UV/Vis (cyclohexane): lmax (log e)=499 (4.04),
475 (3.80), 331 nm (4.18);HRMS (FAB +): m/z: calcd for C44H48S210B11B:
661.3420;found: 661.3377;elemental analysis calcd (%) for
C44H48B2S2·H2O: C 77.65, H 7.40;found: C 77.81, H 7.37.
2,12-Dibutyl-7,14,18-trimesityl-7,18-dihydro-14H-5,9,16-trithia-7,14,18-tri-
boraheptacene (6): To a Et2O solution (300 mL) of 12 (0.20 g, 0.20 mmol)
at ꢀ758C was added tBuLi (2.2m in pentane, 1.2 mL, 2.6 mmol), and the
mixture was stirred for 20 min. To this mixture was added MesB(OMe)2
G
(0.19 mL, 0.92 mmol) at ꢀ758C. The resulting mixture was refluxed for
1 h, and the solvents were removed under reduced pressure. The resulting
solids were extracted with C6H6, and the solution was filtered over Celite
pad, then the solvents were evaporated. The crude product was subjected
to GPLC to give 6 as red solids (63 mg, 35%) and 13 (see below) as
yellow solids (41 mg, 22%).
6: Red solids. Decomposed above 2508C. 1H NMR (500 MHz, CDCl3):
d=0.88 (t, J=7.4 Hz, 6H), 1.29 (sext, J=7.4 Hz, 4H), 1.53 (quint, J=
7.6 Hz, 4H), 1.93 (s, 12H), 2.00 (s, 6H), 2.43 (s, 6H), 2.49 (s, 3H), 2.60 (t,
J=7.7 Hz, 4H), 6.94 (s, 4H), 7.03 (s, 2H), 7.47 (dd, J=8.3, 1.2 Hz, 2H),
7.67 (d, J=8.3 Hz, 2H), 7.74 (d, J=1.2 Hz, 2H), 8.15 (s, 2H), 8.30 ppm
(s, 2H); 13C NMR (126 MHz, CDCl3): d=13.89, 21.31, 21.38, 22.22, 22.98,
23.18, 33.91, 35.16, 125.35, 127.15, 127.37, 129.07, 133.38, 133.69, 136.12,
136.51, 137.00, 137.25, 137.40, 138.57, 138.72, 138.88, 139.01, 139.35,
2,10-Dibutyl-12,14-dimesityl-12,14-dihydro-5,7-dithia-12,14-diborapenta-
cene (4): To a Et2O solution (300 mL) of 10 (0.20 g, 0.28 mmol) at ꢀ758C
was added tBuLi (2.2m in pentane, 1.1 mL, 2.4 mmol), and the mixture
was stirred for 20 min at ꢀ758C. To this mixture was added MesB
A
139.51, 142.16 ppm; 11B NMR (128 MHz, CDCl3): d=56.2 ppm (h1/2
=
(0.14 mL, 0.68 mmol). This mixture was refluxed for 10 h, and the sol-
vents were removed under reduced pressure. The resulting red solids
were extracted with CHCl3, and the solvent was evaporated. The crude
product was subjected to GPLC to give 4 as yellow solids (88 mg, 48%).
M.p. 210–2128C; 1H NMR (400 MHz, CDCl3): d=0.86 (t, J=7.4 Hz,
6H), 1.29 (sext, J=7.4 Hz, 4H), 1.52 (q, J=7.7 Hz, 4H), 1.95 (s, 12H),
2.45 (2, 6H), 2.58 (t, J=7.8 Hz, 4H), 6.71 (s, 4H), 7.46 (dd, J=8.2,
2.0 Hz, 2H), 7.65 (d, J=8.2 Hz, 2H), 7.71 (d, J=2.0 Hz, 2H), 7.98 (s,
1H), 8.05 ppm (s, 1H); 13C NMR (100 MHz, CDCl3): d=13.9, 21.4, 22.3,
22.7, 34.0, 35.2, 119.4, 125.3, 126.5, 128.8, 129.9, 130.9, 133.2, 136.3, 138.2,
139.3, 139.9, 140.2, 147.8, 153.7 ppm; 11B NMR (128 MHz, CDCl3): d=
57.5 ppm (h1/2 =1900 Hz);UV/Vis (cyclohexane): lmax (log e): 399 (3.81),
384 (3.81), 339 (4.60), 266 nm (4.58);HRMS (FAB +, mNBA, PEG600):
3700 Hz);HRMS (FAB +): m/z: calcd for C59H61S310B11B2: 897.4250;
found: 897.4247;elemental analysis calcd (%) for C 59H61H3S3·H2O: C
77.30, H 6.93;found: C 77.77, H 6.97.
12
m/z: calcd for C44H4832S210B11B: 661.3419;found: 661.3434;elemental
13: Yellow solids. M.p. 257–2598C; 1H NMR (500 MHz, CDCl3): d=0.88
(t, J=7.4 Hz, 6H), 1.30 (sext, J=7.4 Hz, 4H), 1.54 (quint, J=7.6 Hz,
4H), 1.95 (s, 12H), 2.45 (s, 6H), 2.61 (t, J=7.7 Hz, 4H), 6.98 (s, 4H),
7.50 (dd, J=8.2, 1.2 Hz, 2H), 7.68 (d, J=1.2 Hz, 2H), 7.76 (d, J=8.2 Hz,
2H), 8.31 (s, 2H), 8.66 ppm (s, 2H); 13C NMR (126 MHz, CDCl3): d=
13.91, 21.36, 22.21, 22.97, 33.90, 35.18, 118.71, 125.10, 127,12, 133.31,
133.38, 133.80, 133.94, 136.87, 137.63, 137.77, 138.82, 139.23, 139.36,
139.82, 140.18, 141.42 ppm;HRMS (FAB +): m/z: calcd for C50H50S311B2:
768.3261;found: 768.3308.
analysis calcd (%) for C44H48B2S2·2.2H2O: C 75.26, H 7.52;found: C
75.06, H 7.49.
1,4-Dibromo-2-(2-bromo-4-butylphenylsulfanyl)-5-iodobenzene (11):
mixture of 2-bromo-4-butylbenzenethiol (11 g, 45 mmol), 1,4-dibromo-
2,5-diiodobenzene (22 g, 45 mmol), NaOtBu (6.5 g, 68 mmol), [Pd(dba)2]
A
AHCTREUNG
(1.3 g, 2.3 mmol), DPPF (1.3 g, 2.3 mmol), and toluene (200 mL) was
stirred at 1008C for 14 h. The reaction was quenched by the addition of
aqueous NH4Cl, and the aqueous layer was extracted with Et2O. The or-
ganic layer was washed with water and dried with Na2SO4, and the sol-
vents were evaporated. The crude product was separated by column
chromatography (SiO2, eluent: hexane) to give 11 as colorless oil (9.1 g,
33%) and 9 (5.3 g, 33%). 1H NMR (500 MHz, CDCl3): d=0.93 (t, J=
7.4 Hz, 3H), 1.36 (sext, J=7.4 Hz, 2H), 1.61 (quint, J=7.6 Hz, 2H), 2.62
(t, J=7.7 Hz, 2H), 6.90 (s, 1H), 7.15 (dd, J=7.9, 1.6 Hz, 1H), 7.35 (d, J=
7.9 Hz, 1H), 7.55 (d, J=1.6 Hz, 1H), 7.98 ppm (s, 1H); 13C NMR
(126 MHz, CDCl3): d=13.90, 22.26, 33.00, 34.96, 97.70, 121.20, 128.69,
128.86, 128.92, 129.19, 131.37, 133.96, 135.54, 140.11, 142.79, 146.63 ppm;
HRMS (FAB+): m/z: calcd for C16H1479Br3SI: 601.7411;found: 601.7420.
X-ray crystallographic analyses of 2 and 8: Crystallographic data for 2:
C44H48B2S2, yellow plate, monoclinic, space group P21/n, a=15.995(5) b=
10.582(3) c=22.316(7) Š, b=106.6961(13)8, V=3617.9(18) Š3, Z=4, F-
A
27500 reflections were collected, of which 8299 were independent (Rint
=
0.0382) and employed for refinement: 441 parameters, 0 restraints, Good-
ness of fit on F2 =0.982, R1 [I>2s(I)]=0.0549, wR2 (all data)=0.1653.
Crystallographic data for 8: C21H19BS, pale-yellow cube, monoclinic,
space group P21/n, a=9.0261(11) b=21.984(3) c=9.2912(11) Š, b=
113.4148(15)8, V=1691.8(4) Š3, Z=4, F
(000)=664, crystal size 0.20”
R
Bis[2,5-dibromo-4-(2-bromo-4-butylphenylsulfanyl)phenyl]
(12):[28] To a mixture of 11 (1.0 g, 1.7 mmol), [Pd
(dba)2] (50 mg, 87 mmol),
DPPF (50 mg, 87 mmol), and toluene (20 mL) was added TIPS-SLi pre-
sulfide
0.20”0.20 mm3, 6.04=2V=54.94. In total, 12779 reflections were collect-
ed, of which 3835 were independent (Rint =0.0275) and employed for re-
finement: 221 parameters, 0 restraints, Goodness of fit on F2 =1.060, R1
AHCTREUNG
8058
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 8051 – 8060

Ladder-Type Heteraborins
FULL PAPER
[I>2s(I)]=0.0638, wR2 (all data)=0.1366. The intensities of reflections
were collected at 120 K by using a RIGAKU MSC Mercury CCD diffrac-
tometer with a graphite-monochromated MoKa radiation (l=0.71070 Š)
by using CrystalClear (Rigaku Corp.). The structure was solved by direct
methods (SHELXS) and expanded by using Fourier techniques. The
structure was refined by full-matrix least-squares methods on F2
(SHELXL-97).[29] All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were assigned idealized positions and were included in
structure-factor calculations. CCDC-299490 and CCDC-635514 contain
the supplementary crystallographic data for 2 and 8, respectively. These
data can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data/request/cif.
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Acknowledgements
This work was supported in part by the 21st Century COE Program
(T.K.) and Scientific Research (T.K.) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan. T.A. acknowledges the
financial support by Research Fellowships of the Japan Society for the
Promotion of Science for Young Scientists. We thank Prof. Dr. Hiroshi
Nishihara, Dr. Masaki Murata, Dr. Shoko Kume, Prof. Dr. Hiro-o Hama-
guchi, and Dr. Hideaki Kano (the University of Tokyo) for their valuable
advice on optical measurements. We thank Prof. Kei Ohkubo (Osaka
University, SORST) and Prof. Shunichi Fukuzumi (Osaka University),
and Horiba Ltd. for measurements of time-resolved fluorescence spectra.
We also thank Tosoh Finechem Corp. for the generous gifts of alkyllithi-
ums.
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8060
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 8051 – 8060