Synthesis and optical properties of a bis(benzothiaborino)carbazole, a thiaborin-carbazole mixed ladder-type π-conjugated molecule

Article abstract of DOI:10.1080/10426501003769736

A ladder-type π-conjugated molecule containing both thiaborin and carbazole units exhibited strong light absorption and emission like other ladder-type π-conjugated molecules based on heteraborins. Copyright Taylor & Francis Group.

Full text of DOI:10.1080/10426501003769736

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Phosphorus, Sulfur, and Silicon and the
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Synthesis and Optical Properties of
a Bis(Benzothiaborino)Carbazole, a
Thiaborin-Carbazole Mixed Ladder-Type
π-Conjugated Molecule
Tomohiro Agou a , Junji Kobayashi a & Takayuki Kawashima a
a Department of Chemistry , Graduate School of Science, The
University of Tokyo , Tokyo, Japan
Published online: 27 May 2010.
To cite this article: Tomohiro Agou , Junji Kobayashi & Takayuki Kawashima (2010) Synthesis and
Optical Properties of a Bis(Benzothiaborino)Carbazole, a Thiaborin-Carbazole Mixed Ladder-Type π-
Conjugated Molecule, Phosphorus, Sulfur, and Silicon and the Related Elements, 185:5-6, 947-951,
DOI: 10.1080/10426501003769736
To link to this article: http://dx.doi.org/10.1080/10426501003769736
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Phosphorus, Sulfur, and Silicon, 185:947–951, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003769736
SYNTHESIS AND OPTICAL PROPERTIES
OF A BIS(BENZOTHIABORINO)CARBAZOLE,
A THIABORIN-CARBAZOLE MIXED LADDER-TYPE
π-CONJUGATED MOLECULE
Tomohiro Agou, Junji Kobayashi, and Takayuki Kawashima
Department of Chemistry, Graduate School of Science, The University of Tokyo,
Tokyo, Japan
A ladder-type π-conjugated molecule containing both thiaborin and carbazole units ex-
hibited strong light absorption and emission like other ladder-type π-conjugated molecules
based on heteraborins.
Keywords Carbazole; ladder-type π-conjugated molecule; optical properties; thiaborin
INTRODUCTION
Ladder-type π-conjugated compounds are promising candidates for organic func-
tional materials, including electron- or hole-transporting materials and light emitting
materials. Their planar structures can enhance delocalization of π-orbitals and decrease
HOMO–LUMO energy gaps, resulting in high electron or hole affinity. In addition, as
the π-conjugated structures are fixed tightly by the rigid framework, intramolecular mo-
tions that are related to nonradiation decay of the excited state are suppressed, and thus
photoluminescence properties of the ladder-type compounds are improved.1 Recently, the
introduction of various main group elements into ladder-type conjugated molecules proved
to be a very effective way to develop new characteristics of π-conjugated molecules.2
In previous reports, we have described the optical properties of ladder-type azaborines
and thiaborins.3 However, their analogues containing both azaborine and thiaborin units
have not been synthesized yet, and thus the effect of the mixing of these two different heter-
aborins in one molecule is still unclear. In this article, we report the attempted synthesis of a
mixed-type, ladder-type heteraborin bearing azaborine and thiaborin units in a hexahydro-
heptacene framework, which resulted in the formation of a bis(benzothiaborino)carbazole
instead of the expected ladder-type heteraborin (Figure 1).
Received 9 November 2008; accepted 20 December 2008.
Dedicated to Professor Naomichi Furukawa on the occasion of his 70th birthday.
This work was partly supported by Grants-in-Aid for The Global COE program for Chemical Innovation
(T.K.) and for Scientific Research (T.K.) from MEXT. We also thank Tosoh Finechem Corporation for the generous
gifts of alkyllithium compounds.
Address correspondence to Takayuki Kawashima, Department of Chemistry, Graduate School of Sci-
ence, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: takayuki@chem.s.u-
tokyo.ac.jp
947

948
T. AGOU ET AL.
Figure 1 Ladder-type mixed heteraborin 1 and bis(benzothiaborino)carbazole 2.
RESULTS AND DISCUSSION
Synthesis
The synthesis of hexabromide 6 was accomplished by taking advantage of
Buchwald–Hartwig amination protocols (Scheme 1).4,5
The synthesis of mixed ladder-type heteraborin 1 was attempted. However, instead of
1, bis(benzothiaborino)carbazole 2 was obtained. A similar reaction using the corresponding
sulfide gave bis(benzothiaborino)dibenzothiophene 7, a sulfur analogue of 2, together with
the desired 8 (Figure 2).3a On the other hand, in the case of the synthesis of ladder-type
azaborines, carbazole derivative was not obtained at all.3b,3c Therefore, terminal thiaborins
formed by a twofold ring closure should be essential for these oxidative C–C coupling
reactions. In fact, an electron transfer from lithio-derivatives to the thiaborin moiety may
occur to give 2 or 7, because the thiaborin has a LUMO of lower energy compared to that
of the azaborine, as revealed by theoretical calculations on heteraborins by us,3a and seems
to act as a one-electron oxidant (Scheme 2).6
Optical Properties
The optical data for 2 as well as for other thiaborins are summarized in Table I.
The absorption maximum at longest wavelength of 2 lies between those of 7 and 8.
The electronic effect of the insertion of a carbazole moiety is weaker as compared with that
of the insertion of a dibenzothiaborin ring. Compared to 7, compound 2 showed red-shifted
absorption maxima, probably due to the higher electron-donating ability of the nitrogen
atom.
Scheme 1 The synthesis of hexabromide 6.

BIS(BENZOTHIABORINO)CARBAZOLE
949
Scheme 2 The formation of bis(benzothiaborino)carbazole 2.
Compound 2 exhibited green-colored photoluminescence with small Stokes shift,
indicating that it maintains a planar and rigid framework upon photo-excitation, just like
the other heteraborin-based ladder-type molecules. In addition, compound 2 can emit a
green-colored luminescence also in the solid state due to the inhibition of intermolecular
interaction by bulky mesityl groups, and thus a carbazole-thiaborin mixed ladder-type
structure will be useful to develop strong light-emissive materials.
EXPERIMENTAL
Generally chemicals were used as received. For optical measurements, commercial
spectrochemical- or fluorometric-grade solvents were used as received. All manipulations
were performed under argon atmosphere using standard Schlenck technique. Dry solvents
(THF, Et2O, hexane, and toluene) were purchased from Kanto Chemicals and further pu-
rified by an MBRAUN MB-SPS system equipped with activated alumina and molecular
sieve columns before use. Column chromatography was carried out with Kanto Silica Gel
60N. Gel permeation liquid chromatography was performed using Japan Analytical Indus-
1
try LC-918 and LC-908 with JAIGEL 1H+2H columns and chloroform as a solvent. H
and 13C NMR spectra were recorded with a Bruker DRX-500 spectrometer. Low resolution
mass spectra were measured with a JEOL JMS-700P (FAB). High resolution mass spectra
were obtained with a JEOL JMS-700P. UV-vis spectra were recorded with a JASCO V-670
Figure 2 Thiaborin and ladder-type thiaborins.

950
T. AGOU ET AL.
Table I Optical properties of bis(benzothiaborino)carbazole 2 and ladder-type thiaborins 7–10 in cyclohexane
λmax/nm (log ε)
λem/nm
Stokes shiftc/cm−1
2
521 (3.79)
482 (3.60)
558 (3.95)
387 (3.94)
499 (4.28)
540
495
580
403
517
6.8 × 102
5.4 × 102
6.8 × 102
1.0 × 103
7.0 × 102
7b
8a
9a
10a
aRef. 1.
bUnpublished result.
cStokes shift = 1/λmax—1/λem
.
spectrophotometer, and fluorescence spectra were measured with a JASCO FP-6500 fluo-
rescence spectrophotometer. All melting points were determined on a Yanaco MP-S3 micro
melting point apparatus and are uncorrected. Elemental analyses were performed by the
Microanalytical Laboratory of the Department of Chemistry, Faculty of Science, University
of Tokyo. The preparation of (2-bromo-4-butylphenyl) (2,5-dibromo-4-iodophenyl) sulfide
(3) was already reported.3a
Synthesis of 2,5-Dibromo-4-(2-bromo-4-butylphenylthio)aniline (4)
A mixture of 3 (2.0 g, 3.3 mmol), Ph2C = NH (0.61 mL, 3.6 mmol), Pd(dba)2 (0.10 g,
0.17 mmol), 1,1ꢀ-bis(diphenylphophino)ferrocene (DPPF) (0.10 g, 0.17 mmol), t-BuONa
(0.50 g, 5.2 mmol), and toluene (50 mL) was stirred at 100◦C for 45 h.4 To the mixture, aq.
NH4Cl was added, and the mixture was extracted with Et2O. The organic layer was dried
over Na2SO4, and the solvents were removed under reduced pressure. The crude material
was dissolved in THF (10 mL) and MeOH (40 mL), and to this mixture NH2OH·HCl (5.0
g, 72 mmol) and AcONa (9.0 g, 0.11 mol) were added. The mixture was stirred at room
temperature for 19 h, and the solvent was removed under reduced pressure. The resulting
material was extracted with Et2O and the organic layer was dried over Na2SO4. The solvent
was evaporated, and the crude material was chromatographed (Al2O3, CHCl3) and further
purified by GPLC to give 4 as colorless viscous oil (1.3 g, 82%). 4: colorless oil. 1H NMR
(500 MHz, CDCl3): δ = 0.90 (t, J = 7.3 Hz, 3H), 1.32 (sext, J = 7.5 Hz, 2H), 1.55 (quint,
J = 7.7 Hz, 2H), 2.53 (t, J = 7.7 Hz, 2H), 6.72 (d, J = 8.1 Hz, 1H), 6.98 (dd, J = 8.1,
1.6 Hz, 1H), 7.18 (s, 1H), 7.38 (d, J = 1.6 Hz, 1H), 7.49 (s, 1H); 13C NMR (126 MHz,
CDCl3): δ = 13.9, 22.2, 33.2, 34.7, 119.3, 121.5, 122.0, 128.1, 129.4, 129.5, 129.7, 132.9,
134.8, 139.5, 142.4, 145.8; HRMS (FAB+) m/z calcd. for C16H16N79Br3S: 490.8553; found:
490.8534.
Synthesis of 2,5-Dibromo-4-(2-bromo-4-butylphenylthio)-N-[2,5-
dibromo-4-(2-bromo-4-butylphenylthio)phenyl]-N-methylaniline (6)
A mixture of 4 (1.3 g, 2.7 mmol), 3 (1.8 g, 3.0 mmol), Pd(dba)2 (0.10 g, 0.17 mmol),
DPPF (0.10 g, 0.17 mmol), t-BuONa (0.43 g, 4.5 mmol), and toluene (30 mL) was stirred
at 100◦C for 13 h, and the reaction was quenched by the addition of aq. NH4Cl. The mix-
ture was extracted with Et2O, and the organic layer was dried over Na2SO4. The solvents
were removed under reduced pressure, and the crude material was chromatographed (SiO2,
CHCl3) to give bis(2,5-dibromo-4-(2-bromo-4-butylphenyl-thio)phenyl)amine (5) as col-
orless solid. This material was dissolved in THF (20 mL), and to this solution MeI (0.16

BIS(BENZOTHIABORINO)CARBAZOLE
951
mL, 2.6 mmol), KOH (1.1 g, 20 mmol), and Bu4NI (0.18 g, 0.49 mmol) were added. The
mixture was stirred at 40◦C for 10.5 h, and the reaction was quenched by aq. NH4Cl. The
mixture was extracted with Et2O and the organic layer dried over Na2SO4. The solvents
were evaporated, and the crude material was subjected to column chromatography (SiO2,
hexane/CHCl3 = 4:1) to give 6 as colorless solid (1.7 g, 64%). 6: colorless solid, mp
140–142◦C. 1H NMR (500 MHz, CDCl3): δ = 0.92 (t, J = 7.3 Hz, 6H), 1.34 (sext, J = 7.4
Hz, 4H), 1.59 (quint, J = 7.6 Hz, 4H), 2.58 (t, J = 7.7 Hz, 4H), 3.20 (s, 3H), 7.08–7.13 (m,
4H), 7.20 (s, 2H), 7.22 (s, 2H), 7.48 (d, J = 1.4 Hz, 2H); 13C NMR (126 MHz, CDCl3): δ =
13.9, 22.3, 33.1, 34.9, 41.5, 119.2, 124.4, 126.3, 128.0, 128.6, 130.0, 131.3, 133.0, 133.6,
136.4, 145.1, 147.4; LRMS (FAB+) m/z 983 (M+). Anal. Calcd. for C33H31Br6NS2: C,
40.23; H, 3.17; N, 2.85. Found: C, 40.45; H, 3.34; N, 2.57.
Synthesis of Bis(benzothiaborino)carbazole (2)
To an Et2O (500 mL) solution of 6 (0.50 g, 0.51 mmol), t-BuLi (2.2 M in pentane,
3.0 mL, 6.6 mmol) was added at –75◦C, and the mixture was stirred for 20 min. To this
mixture, MesB(OMe)2 (0.37 mL, 1.8 mmol) was added, and the mixture was refluxed for
2 h. The solvent was evaporated, and the crude material was suspended in benzene. The
suspension was filtered, and the solvent was removed under reduced pressure. CHCl3 was
added until all of the solid was dissolved, and the solution was concentrated to give a red
precipitate. The precipitate was subjected to GPLC to give 2 as an orange solid (0.12 g,
1
31%). 2: orange solid, mp 160–161◦C (dec.). H NMR (500 MHz, CDCl3): δ = 0.88 (t,
J = 7.3 Hz, 6H), 1.30 (sext, J = 7.5 Hz, 4H), 1.54 (quint, J = 7.6 Hz, 4H), 1.96 (s, 12H),
2.46 (s, 6H), 2.60 (t, J = 7.7 Hz, 4H), 3.95 (s, 3H), 6.98 (s, 4H), 7.47 (dd, J = 8.2, 2.0 Hz,
2H), 7.63 (d, J = 2.0 Hz, 2H), 7.75 (d, J = 8.2 Hz, 2H), 7.84 (s, 2H), 8.60 (s, 2H); 13C
NMR (126 MHz, CDCl3): δ = 13.9, 21.4, 22.2, 23.0, 29.5, 33.9, 35.2, 117.3, 118.1, 124.9,
126.0, 127.0, 132.9, 133.1, 133.3, 133.6, 136.5, 138.7, 138.8, 139.0, 140.7, 140.9, 141.9;
HRMS (FAB+) m/z calcd for C51H5311B2N32S2: 765.3806; found: 765.3799.
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