Syntheses and emission properties of novel violet-blue emissive aromatic bis(diazaborole)s

Article abstract of DOI:10.1039/b203485a

Violet-blue emission from novel aromatic bis(diazaborole)s, which were synthesized by the reaction of 2,5-bis(hexyloxy)-1,4-phenylenediboronic acid and 1,2-phenylenediamine derivatives or diaminonaphthalene derivatives, is described. White-blue emission front a 4-methoxy-1,2-phenylenediamine-derived molecule was observed in DMF using 340 nm as the excitation wavelength.

Full text of DOI:10.1039/b203485a

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Syntheses and emission properties of novel violet-blue emissive
aromatic bis(diazaborole)s{
Sumio Maruyama*a and Yuji Kawanishib
aJapan Society for the Promotion of Science (JSPS), c/o Nanotechnology Research Institute,
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan.
E-mail: maruyama-s@aist.go.jp; Fax: 181-298-61-6296; Tel: 181-298-61-6299
bNanotechnology Research Institute, National Institute of Advanced Industrial Science and
Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565,
Japan
Received 9th April 2002, Accepted 1st May 2002
First published as an Advance Article on the web 13th June 2002
Violet-blue emission from novel aromatic bis(diazaborole)s, which were synthesized by the reaction of 2,5-
bis(hexyloxy)-1,4-phenylenediboronic acid and 1,2-phenylenediamine derivatives or diaminonaphthalene
derivatives, is described. White-blue emission from a 4-methoxy-1,2-phenylenediamine-derived molecule was
observed in DMF using 340 nm as the excitation wavelength.
properties. We also synthesized other molecules derived from
Introduction
2,3-diaminonaphthalene and 1,8-diaminonaphthalene. More-
over, these boronic acid-derived molecules would also expected
to be stable due to the presence of two boron–nitrogen bonds,
which would act like chelate effects. The emission properties of
these aromatic bis(diazaborole)s are also described.
Recently, the synthesis and development of p-conjugated
oligomers have become important in organic materials science.
Poly(aryleneethynylene)s, poly(arylenevinylene)s, and oligo-
arylenes are useful for new organic materials such as light-
emitting diodes, emissive materials, photovoltaic devices,
nonlinear optics, two-photon dyes, molecular switches, and
molecular wires.1–4 In the area of emissive materials, blue
emissive materials have been reported such as poly(fluorene),
poly(p-phenylene) (PPP), and so on. Among them, attention
has recently been paid to boron containing molecules.5–10
Boron acts as an acceptor due to its strong electron affinity, so
that their band gaps are increased, resulting in blue emission.
Moreover, the boron atom makes a bond with carbon and/or
a hetero atom using an sp2 orbital, which results in longer
conjugation in the system. However, the main problem is that
in general organic boron containing molecules are unstable to
air and moisture. In order to obtain stable boron containing
molecules, one method is to use bulky groups such as tert-butyl
and mesityl groups which cause steric hindrance of the boron
atom.
Exceptionally, boronic acids are well-known to be stable
towards air and moisture. Boronic acids are also well used for
molecular recognition of saccharide11 and as leaving groups for
the Suzuki cross-coupling reaction.12 Herein we will report on
the syntheses of novel boron containing molecules derived from
boronic acid. Boronic acids are well known to be able to form
covalent linkages with diols or 1,2-phenylenediamine.11 In
order to study the usefulness of blue emissive materials derived
from boronic acids, we chose 2,5-alkoxy-1,4-phenylenediboro-
nic acid as a key precursor. 2,5-Alkoxy-1,4-phenylenediboronic
acids are often used as precursors to, for instance, alkoxy-PPP
due to their increased solubility in organic solvents and to
insert a twisted angle to shorten the conjugation length of mole-
cules, resulting in a blue emission. 1,2-Phenylenediamine and
its derivatives with methoxy and nitrile groups were chosen as
other key precursors in order to investigate the effect of donor
and/or acceptor groups which might affect the emission
Results and discussion
Syntheses of molecules
The desired molecules (1a–1e) were synthesized according to
Schemes 1 and 2. 1,4-Hydroquinone 2 was reacted with 1-hexyl
bromide in DMF at 80 uC in the presence of K2CO3 to give
1,4-bis(hexyloxy)benzene 3 in 67.2% yield. 1,4-Dibromo-2,5-
bis(hexyloxy)benzene 4 was obtained by reaction of 3 and
bromine in 92.8% yield. The key precursor, 2,5-bis(hexyloxy)-
1,4-phenylenediboronic acid 5, was synthesized by the lithia-
tion of 4 followed by treatment with trimethoxyborane, then
hydrolysis, in 65.8% yield according to a similar procedure in
ref. 13. The other key precursors, 1,2-phenylenediamine deriva-
tives, 6 and 7 were commercially available, and 9 was obtained
by reduction of 8 in 89.6% yield.14 Finally, 5 and 1,2-
phenylenediamine derivatives were refluxed in toluene (for
1a) or toluene–triethylamine (for 1b) or toluene–DMF (for 1c)
to give the desired molecules 1a–1c in 65.8–78.1% yields,
respectively. 1d and 1f were also synthesized from 5 and 2,3-
diaminonaphthalene 10 or 1,8-diaminonaphthalene 11 in 71.3
and 54.0% yield, respectively. In order to study the effect of
boron, we also synthesized 1f as a reference compound by
normal Suzuki cross-coupling reaction12 in 72.7% yield as
shown in Scheme 3.
Absorption and emission properties of molecules
The molecules 1a, 1b, and 1f are easily dissolved in dichloro-
methane, acetone, THF, DMSO, and DMF. On the other
hand, 1c, 1d, and 1e hardly dissolved in dichloromethane, and
dissolved in DMSO and DMF. Therefore we used DMF for the
optical measurements. Absorption and emission spectra of
1a–1f in DMF are shown in Fig. 1 and 2. The optical data are
summarized in Table 1. The molecules 1a–1c showed absorp-
tion peaks around 350 nm, which was not observed for 1f,
{Electronic supplementary information (ESI) available: pictures of 1a–
1e in DMF using 366 nm as excitation wavelength. See http://
www.rsc.org/suppdata/jm/b2/b203485a/
DOI: 10.1039/b203485a
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This journal is # The Royal Society of Chemistry 2002
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Scheme 1 Reagents and conditions: i) hexyl bromide, K2CO3, DMF, 80 uC, 67.2%; ii) Br2, CCl4, v5 uC, 92.8%; iii) BuLi–hexane, Et2O, rt; iv)
B(OCH3)3, 240 uC; v) HCl, rt, 70.7%; vi) 6 or 7 or 9, toluene (for 1a) or toluene–NEt3 (for 1b) or toluene–DMF (for 1c), reflux, 78.1% (1a), 65.8%
(1b), 75.4% (1c); vii) hydrazine monohydrate, ruthenium (5 wt%) on carbon, ethanol, reflux, 89.6%.
indicating that their conjugation lengths are increased com-
pared with that of 1f. The molecule 1a showed main two peaks
around 315 and 342 nm, respectively. The molecule 1b showed
a main absorption peak around 351 nm, which was red-shifted
compared with that of 1a. This phenomenon would be
attributed to the effect of the methoxy group which acts as a
donor, resulting in a red-shift of the spectrum. The molecule 1c
showed similar absorption to 1a. The molecule 1d showed
absorption maxima at 279, 291, 353, and 366 nm. On the other
hand, 1e showed a broad absorption peak around 344 nm. The
Scheme 2 Reagents and conditions: i) 10, toluene, reflux, 71.3%; ii) 11, toluene, reflux, 54.0%.
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Fig. 2 Emission spectra of 1a–1f in DMF at rt: [1a–1f] ~ 1.0 6
1026 mol dm23; lex ~ 340 nm.
by charge transfer between the methoxy group and the boron
atom. 1d showed emission peaks at 376, 395, and 417 nm.
However, the emission peak of 1e, a positional isomer of 1d,
could not be observed. From Fig. 2, the emission properties of
1a–1e except for 1b and 1e, would be dependent on the boron
atom. The relative emission quantum yields of 1a–1f were
measured using p-terphenyl as a standard (0.92 in cyclohexane
at 303 nm as excitation wavelength15). The relative emission
quantum yield of 1e could not be determined due to the weak
emission intensity. The emission quantum yields of 1a–1d were
higher than that of 1f, indicating that the boron atom will be
useful for emissive molecules. The pictures of 1a–1e in DMF
using 366 nm as an excitation wavelength (UV-lamp) are
shown in the Electronic Supplementary Information{ (ESI; 1a,
1b, 1c, 1d, and 1e from left to right, respectively). Unfortu-
nately, when we tried to measure the emission spectra of these
molecules in the solid state, a spin-coated film could not be
prepared due to deposition of the samples on the film.
Scheme 3 Reagents and condition: i) iodobenzene, Pd(PPh3)4, toluene–
methanol–2 M Na2CO3 aq., reflux, 72.7%.
The cyclic voltammograms (CV) of 1a–1f were measured in
DMF and the data are summarized in Table 2 (vs. Fc/Fc1 in
DMF at 298 K16). The reduction peaks of 1a–1f were irrever-
sible and too weak probably due to decomposition, so that we
could not obtain the reduction potential values. In 1a–1c, the
order of the oxidation potential values was 1b w 1a w 1c.
Fig. 1 Absorption spectra of 1a–1f in DMF at rt: [1a–1f] ~ 1.0 6
1025 mol dm23
.
Table 2 The electrochemical data for 1a–1f in DMF at 298Ka
emission spectra of 1a–1f were measured using 340 nm as an
excitation wavelength. The molecules 1a and 1c exhibited violet
emission. The emission maxima of 1a and 1c were similar to
that of 1f although the emission peaks were split. Interestingly,
the emission maxima of 1b are different from those of 1a and
1c, and observed around 438 nm which corresponds to the blue
region. The reason why a strong red-shift of 1b, compared with
1a and 1c, was observed is unknown so far; it might be due to
increase of the conjugation length, and/or might be produced
Compound
Ered/V
Eox/V
1a
1b
1c
1d
1e
1f
NDb
NDb
NDb
NDb
NDb
NDb
0.460
0.180
0.756
0.500
0.200
0.740
aFc/Fc1 in DMF at 298K;16 bnot observed.
Table 1 The optical data for 1a–1f in DMF at rt
Compound
l
abs/nma(log e/dm3 mol21 cm21
)
l
emis/nmb,c
we,f
1a
1b
1c
1d
1e
1f
303 (4.37), 315 (4.46), 342 (4.52), 356 (4.47)
309 (4.21), 323 (4.38), 351 (4.60), 364 (4.51)
306 (4.39), 318 (4.61), 347 (4.55), 357 (4.51)
279 (4.66), 291 (4.62), 353 (4.71), 366 (4.76)
370, 389, 405
438
378, 389
0.99 ¡ 0.01
0.78 ¡ 0.03
0.88 ¡ 0.01
0.85 ¡ 0.04
NDg
376, 395, 417
344 (4.61)
323 (4.05)
c
NDd
392
e
0.66 ¡ 0.01
a[1a–1f] ~ 1.0 6 1025 mol dm23
.
b[1a–1f] ~ 1.0 6 1026 mol dm23. lex ~ 340 nm. dVery weak. Based on p-terphenyl in cyclohexane (0.92
at 303 nm as the excitation wavelength). The errors reported are based on the standard deviation values measured in duplicate runs. gNot
determined.
f
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These phenomena were related to the substituent groups. The
oxidation potential of 1f was larger than those of 1a and 1d,
indicating that the nitrogen atom would be more easily
oxidized compared with the boron atom.
was recrystallized from dichloromethane–ethanol to give 4
(24.750 g, 92.8%) as crystals.
1H NMR (400 MHz, CDCl3, TMS): (ppm) 0.91 (t, J 7.0 Hz,
6H, CH3), 1.32–1.52 (m, 12H, CH2), 1.76–1.83 (m, 4H, CH2),
3.94 (t, J 6.5 Hz, 4H, ArOCH2), 7.08 (s, 2H, arom. H).
Conclusion
2,5-Bis(hexyloxy)-1,4-phenylenediboronic acid 5
In conclusion, we synthesized organic boron containing violet-
blue emissive molecules derived from boronic acid. These
molecules would be useful for emissive materials such as EL
materials, molecular wires, and so on. Further researches on
the application of devices is now under way.
1,4-Dibromo-2,5-bis(hexyloxy)benzene 4 (5.008 g, 11.480 mmol)
was stirred in Et2O (30 ml) at rt under nitrogen. To the solution
was added a 1.58 M hexane solution of BuLi (20 ml), and the
resulting solution was stirred at rt overnight. Trimethoxybor-
ane (5.6 ml, 50.389 mmol) was added to the solution at 240 uC
and stirred overnight from 240 uC to rt. The reaction was
quenched by the addition of HCl aq., and the resulting
precipitate was collected, washed with water, Et2O, and dried
to give 5 (2.971 g, 70.7%) as a white solid.
Experimental
Materials
1H NMR (400 MHz, DMSO-d6): (ppm) 0.88 (t, J 7.0 Hz, 6H,
CH3), 1.29–1.43 (m, 12H, CH2), 1.69–1.745 (m, 4H, CH2), 3.99 (t,
J 6.5 Hz, 4H, ArOCH2), 7.19 (s, 2H, arom. H), 7.80 (s, 4H, OH).
All chemical reagents and dehydrated solvents were commer-
cially available and used without further purification. Analy-
tical thin layer chromatography was performed on commercial
Merck plates coated with silica gel 60F254. Silica gel used for
chromatography was Wakogel C300 and Merck silica gel 60.
DMF and cyclohexane for fluorimetry were degassed, and
nitrogen was bubbled into the solvents for 1 h before use.
4-Cyano-1,2-phenylenediamine 9
4-Amino-3-nitrobenzonitrile 8 (3.006 g, 18.426 mmol) was
reduced in the presence of hydrazine hydrate (4.0 ml, 82.300
mmol) and activated charcoal (0.800 g) in ethanol (60 ml) at
reflux until the foam of the reaction mixture became colorless.
After being cooled, the reaction mixture was filtered, and the
filtrate was evaporated to dryness, giving a white powder.
1H NMR (400 MHz, DMSO-d6): (ppm) 4.86 (s, 2H, NH2),
5.44 (s, 2H, NH2), 6.55 (d, J 8.0 Hz, 1H, arom. H), 6.76 (d, J
1.8 Hz, 1H, arom. H), 6.80 (dd, J 8.0 and 1.9 Hz, 1H, arom. H).
Instrumentation
1H NMR spectra were recorded in CDCl3 and/or DMSO-d6
solutions on a Bruker ARX400 spectrometer (400 MHz).
Chemical shifts in CDCl3 are reported in ppm downfield from
TMS, and coupling constants are in hertz. FAB-MS spectra
were recorded on a JEOL JMS-DX303 mass spectrometer
using 3-nitrobenzyl alcohol as a matrix. Elemental analyses
were performed at Chemical Analysis Center of AIST. Absorp-
tion spectra were recorded on a Jasco V-560. Emission spectra
were recorded on a SPEX Fluorolog. The relative emission
quantum yields were determined based on p-terphenyl in
cyclohexane15 according to the literature procedures.17 Cyclic
voltammetry measurements were performed using BAS100BW
under the following conditions: internal standard, ferrocene/
ferrocenium; working electrode, Pt disk (25 mm diameter);
auxiliary electrode, Pt wire; reference electrode, Ag/AgNO3;
solvent, DMF; electrolyte, 0.1 M tetra(n-butyl)ammonium
hexafluorophosphate; temperature, 298 K.
Syntheses of aromatic bis(diazaborole)s
1a. A mixture of 2,5-bis(hexyloxy)-1,4-phenylenediboronic
acid 5 (0.703 g, 1.920 mmol) and 1,2-phenylenediamine 6
(0.447 g, 4.015 mmol) was refluxed in toluene (20 ml) under
nitrogen for 1 day. After cooling to rt, the resulting precipitate
was collected and recrystallized from toluene twice to give
product 1a (0.765 g; 78.1%) as needle-like crystals.
1H NMR (400 MHz, DMSO-d6): (ppm) 0.89 (t, J 7.1 Hz, 6H,
CH3), 1.31–1.525 (m, 12H, CH2), 1.88–1.95 (m, 4H, CH2), 4.15
(t, J 6.7 Hz, 4H, ArOCH2), 6.82–6.86 (m, 4H, arom. H), 7.11–
7.155 (m, 4H, arom. H), 7.51 (s, 2H, arom. H), 8.62 (s, 4H,
NH); FAB-MS: m/z ~ 510 (M1); Calcd. for C30H40B2N4O2: C,
70.61; H, 7.90; N, 10.98; Found. C, 70.31; H, 7.90; N, 10.83%.
1,4-Bis(hexyloxy)benzene 3
A mixture of hydroquinone 2 (10.008 g, 63.572 mol) and
1-hexyl bromide (29.0 ml, 0.207 mol) was stirred in DMF
(60 ml) in the presence of K2CO3 (26.351 g, 0.191 mol) at 80 uC
under nitrogen for 1 day. The solution was poured into 300 ml
of water, and extracted with chloroform (150 ml) twice. The
organic layer was dried with MgSO4, and filtered. The solvent
was removed in vacuo, and the residue was passed through
silica gel using chloroform as eluent. Finally, recrystallization
from methanol gave 3 (17.001 g, 67.2%) as crystals.
1b. 4-Methoxy-1,2-phenylenediamine dihydrochloride
7
(1.560 g, 7.390 mmol) was stirred in toluene (25 ml) in the
presence of triethylamine (2.2 ml) at rt under nitrogen for 2 h to
hydrolysis. To the solution was added 2,5-bis(hexyloxy)-1,4-
phenylenediboronic acid 5 (0.900 g, 2.459 mmol), and the
resulting solution was refluxed for 2 days. After cooling to rt,
200 ml of water were added, and the resulting precipitate
was collected, washed with water, and dried. Finally, it was
recrystallized from toluene–acetone twice to give product 1b
(0.923 g; 65.8%) as fluffy crystals like cotton wool.
1H NMR (400 MHz, DMSO-d6): (ppm) 0.89 (t, J 7.1 Hz, 6H,
CH3), 1.315–1.52 (m, 12H, CH2), 1.87–1.94 (m, 4H, CH2), 3.73
(s, 6H, OCH3), 4.125 (t, J 6.7 Hz, 4H, ArOCH2), 6.45 (dd, J 8.4
and 2.5 Hz, 2H, arom. H), 6.77 (d, J 2.4 Hz, 2H, arom. H), 6.99
(d, J 8.4 Hz, 2H, arom. H), 7.465 (s, 2H, arom. H), 8.43 (s, 2H,
NH), 8.54 (s, 2H, NH); FAB-MS: m/z ~ 570 (M1); Calcd. for
C32H44B2N4O2: C, 67.39; H, 7.78; N, 9.82; Found. C, 67.44; H,
7.76; N, 9.73%.
1H NMR (400 MHz, CDCl3, TMS): (ppm) 0.90 (t, J 7.0 Hz,
6H, CH3), 1.31–1.46 (m, 12H, CH2), 1.46–1.765 (m, 4H, CH2),
3.895 (t, J 6.6 Hz, 4H, ArOCH2), 6.82 (s, 4H, arom. H).
1,4-Dibromo-2,5-bis(hexyloxy)benzene 4
1,4-Bis(hexyloxy)benzene 3 (17.016 g, 61.113 mol) was dis-
solved in CCl4 (75 ml) and stirred below 5 uC. To the solution
was added dropwise 8.0 ml (0.155 mol) of bromine over the
course of 1 h. The resulting solution was stirred overnight. The
remaining bromine was quenched by the addition of Na2SO3
aq., then the organic layer was extracted with dichloromethane
(50 ml) twice, washed with water (150 ml), dried with MgSO4,
and filtered. The solvent was removed in vacuo, and the residue
1c. A mixture of 2,5-bis(hexyloxy)-1,4-phenylenediboronic
acid 5 (0.901 g, 2.461 mmol) and 4-cyano-1,2-phenylenedia-
mine 9 (0.720 g, 5.409 mmol) was refluxed in toluene (30 ml)
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and DMF (10 ml) under nitrogen for 2 days. After cooling to rt,
the resulting precipitate was collected and reprecipitated from
DMF–toluene twice to give product 1c (0.765 g; 78.1%) as a
white solid.
1H NMR (400 MHz, DMSO-d6): (ppm) 0.85 (t, J 6.7 Hz, 6H,
CH3), 1.26–1.49 (m, 12H, CH2), 1.87–1.935 (m, 4H, CH2), 4.15
(t, J 6.7 Hz, 4H, ArOCH2), 7.295–7.32 (m, 4H, arom. H), 7.51
(s, 2H, arom. H), 7.52 (s, 2H, arom. H), 9.05 (s, 2H, NH), 9.31
(s, 2H, NH); FAB-MS: m/z ~ 560 (M1); Calcd. for C32H38-
B2N6O2: C, 68.60; H, 6.84; N, 15.00; Found. C, 68.20; H, 6.86;
N, 14.78%.
evaporated in vacuo, and the residue was purified by column
chromatography on silica gel using chloroform–hexane (1 : 2
v/v) as eluent. Finally, recrystallization from methanol gave
product 1f (0.770 g; 72.7%) as a white solid.
1f: 1H NMR (400 MHz, CDCl3, TMS): (ppm) 0.86 (t, J
6.9 Hz, 6H, CH3), 1.20–1.39 (m, 12H, CH2), 1.64–1.71 (m, 4H,
CH2), 3.90 (t, J 6.5 Hz, 4H, ArOCH2), 6.985 (s, 2H, arom. H),
7.305–7.345 (m, 2H, arom. H), 7.39–7.43 (m, 4H, arom. H),
7.59–7.615 (m, 4H, arom. H); FAB-MS: m/z ~ 430 (M1);
Calcd. for C30H38O2: C, 83.68; H, 8.89; Found. C, 83.75; H,
8.92%.
1d. A mixture of 2,5-bis(hexyloxy)-1,4-phenylenediboronic
acid 5 (0.554 g; 1.513 mmol) and 2,3-diaminonaphthalene 10
(0.500 g; 3.160 mmol) was refluxed in toluene (25 ml) under
nitrogen for 2 days. After cooling to rt, the resulting precipitate
was collected and reprecipitated from DMF–toluene twice to
give product 1d (0.659 g; 71.3%) as a white solid.
1H NMR (400 MHz, DMSO-d6): (ppm) 0.89 (t, J 7.1 Hz, 6H,
CH3), 1.32–1.56 (m, 12H, CH2), 1.92–1.99 (m, 4H, CH2), 4.20
(t, J 6.8 Hz, 4H, ArOCH2), 7.23–7.26 (m, 4H, arom. H), 7.52
(s, 4H, arom. H), 7.60 (s, 2H, arom. H), 7.78–7.80 (m, 4H,
arom. H), 8.83 (s, 4H, NH); FAB-MS: m/z ~ 610 (M1); Calcd.
for C38H44B2N4O2: C, 74.77; H, 7.27; N, 9.18; Found. C, 74.86;
H, 7.34; N, 9.13%.
Acknowledgement
We thank Professor Dr Xu-tang Tao, Dr Takashi Okubo, and
Professor Dr Hiroyuki Sasabe for helpful discussions. One of
the authors (S. M.) expresses his gratitude to Japan Science and
Technology Corporation (JST) and JSPS for supporting him as
a Domestic Research Fellow.
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1e. A mixture of 2,5-bis(hexyloxy)-1,4-phenylenediboronic
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2,5-Bis(hexyloxy)-1,4-diphenylbenzene 1f. A mixture of 2,5-
bis(hexyloxy)-1,4-phenylenediboronic acid 5 (0.901 g, 2.461
mmol), iodobenzene (0.70 ml, 6.308 mmol), and Pd(PPh3)4
(0.145 g, 0.125 mmol) was refluxed in toluene (20 ml)–methanol
(10 ml)–2 M Na2CO3 aq. (10 ml) under nitrogen for 1 day.
After cooling to rt, the organic layer was extracted from
toluene (20 ml) three times, washed with water (50 ml), dried
with MgSO4 (anhydrous), and filtered. The solvent was
J. Mater. Chem., 2002, 12, 2245–2249
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