Kinetically stabilized dibenzoborole as an electron-accepting building unit

Article abstract of DOI:10.1039/b716107g

A synthetic route to a dibenzoborole, kinetically stabilized by the 2,4,6-tri-tert-butylphenyl group, was developed, and a series of π-extended derivatives were synthesized, which show orange-red emissions and stable electrochemical redox properties. The

Full text of DOI:10.1039/b716107g

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Kinetically stabilized dibenzoborole as an electron-accepting building
unit{{
Atsushi Wakamiya, Kotaro Mishima, Kanako Ekawa and Shigehiro Yamaguchi*
Received (in Cambridge, UK) 18th October 2007, Accepted 16th November 2007
First published as an Advance Article on the web 23rd November 2007
DOI: 10.1039/b716107g
Mes*BF2 in place of TipB(OMe)2, all the attempts resulted in
forming a monoborylbiphenyl derivative. Therefore, we adopted
an alternative route through B-halodibenzoborole as a key
precursor, as shown in Scheme 1. Thus, bromodibenzoborole 2
was obtained in 92% yield by the treatment of dimethyldibenzo-
A synthetic route to a dibenzoborole, kinetically stabilized by
the 2,4,6-tri-tert-butylphenyl group, was developed, and a series
of p-extended derivatives were synthesized, which show orange-
red emissions and stable electrochemical redox properties.
8
In boron-containing p-conjugated molecules, the pp–p* conjuga-
tion through a vacant p orbital of the boron atom endows them
with intriguing photophysical and electronic properties, and makes
them attractive materials for organic (opto)electronics.1,2 As a
representative core skeleton, dibenzoborole (9-borafluorene) is
of particular interest. While its carbon and nitrogen congeners,
fluorene and carbazole, have been widely used as the core skeleton
for emissive and hole-transporting materials, respectively, the
dibenzoborole would have significant potential as an electron-
accepting building unit, since the perpendicular arrangement of the
vacant p orbital of the boron to the biphenyl plane in this skeleton
promises an effective pp–p* conjugation.3 However, this chemistry
is still in its infancy, mainly due to the inaccessibility of a
kinetically well-stabilized dibenzoborole skeleton. For example, we
previously reported the synthesis of dibenzoborole derivatives
having a 2,4,6-triisopropylphenyl (Tip) group on the boron atom,
which showed dramatic photoluminescence changes upon treat-
ment with DMF or fluoride ions.4 These changes resulted from the
coordination of the donor species to the boron atom, causing
interruption of the pp–p* conjugation.4,5 While these results
suggest their potential use as sensory materials for anionic
analytes, from another viewpoint, the steric protection by Tip
is still not sufficient for kinetic stabilization. To utilize the
dibenzoborole skeleton for electronic device applications, the first
issue to overcome is to establish a synthetic route for introducing a
bulkier group on the boron atom. In this context, we now report
the synthesis of dibenzoborole-based p-electron systems having a
2,4,6-tri-tert-butylphenyl (supermesityl, Mes*) group on the boron
atom. Their photophysical and electrochemical properties were
also studied to unveil the potential of the dibenzoborole skeleton
as a p-electron-accepting building unit.
silole 1 with BBr3 without solvent at 50 uC, followed by
distillation. Compound 2 was further reacted with Mes*Li in
toluene to produce the Mes*-substituted dibenzoborole 3 in 64%
yield. For this transformation, toluene is crucial. The use of ether
or THF as solvents only resulted in forming complex mixtures.
The produced 3 was stable enough in air and water to purify by
column chromatography on silica gel.
A straightforward synthetic route to the extended p-conjugated
systems is to use 3,7-dihalodibenzoborole as the precursor.
However, all attempts of the direct halogenation of 3 with several
kinds of reagents such as Br2, NBS, and ICl resulted in no reaction
or the formation of complex mixtures, most likely due to
deactivation by the boryl group toward the aromatic electrophilic
substitution reactions. Therefore, we chose another route that was
the preparation of 3,7-diiododibenzoborole 8 from 4,49-diamino-
2,29-dibromobiphenyl 4 in 4 steps, as shown in Scheme 2. Thus, 4
was first converted into the bis(triazenyl) derivative 5 and its
dilithiation with t-BuLi followed by reaction with Bu2SnCl2 gave
3,7-bis(triazenyl)dibenzostannole 6. Diiodination of 6 was con-
ducted with MeI at 100 uC to give the diiododibenzostannole 7
in 45% yield.9 The transformation from dibenzostannole to
dibenzoborole 8 was accomplished by treatment with BCl3
followed by reaction with Mes*Li in toluene in 62% yield. In
this sequence of reactions, when Me2SiCl2 was employed in
place of Bu2SnCl2, the transmetalation from the corresponding
dibenzosilole with BBr3 did not give 8, due to the low reactivity of
the diiodo derivative.
With diiodide 8 in hand, further cross-coupling reactions can
produce the extended p-conjugated compounds. Thus, the reaction
of 8 with bithienylzinc chloride or 4-(diphenylamino)phenylzinc
chloride in the presence of a Pd2(dba)3?CHCl3–P(furyl)3 catalyst
system gave the corresponding p-extended dibenzoboroles 9 and
10 as orange solids in 60% and 77% yields, respectively. These
In a previous study, the Tip-substituted dibenzoborole deriva-
tives were prepared by the reaction of TipB(OMe)2 with
2,29-dimetalated biphenyls.4,6,7 In this study, whereas we first used
Department of Chemistry, Graduate School of Science, Nagoya
University, and SORST, Japan Science and Technology Agency,
Chikusa, Nagoya 464-8602, Japan.
E-mail: yamaguchi@chem.nagoya-u.ac.jp; Fax: +81-52-789-5947;
Tel: +81-52-789-2291
{ Dedicated to the late Prof. Yoshihiko Ito.
{ Electronic supplementary information (ESI) available: Synthetic proce-
dures and analytical data, X-ray analysis, UV-vis and FL, and theoretical
calculations. See DOI: 10.1039/b716107g
Scheme 1 Reagents and conditions: (i) BBr3, neat, 50 uC, 48 h; (ii)
Mes*Li, toluene, rt, 22 h. Mes* 5 2,4,6-tri-tert-butylphenyl.
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Fig. 2 Absorption and fluorescence spectra of 9: red line in THF; blue
line in DMF; dashed line in THF (8.1 6 1026 M) with CsF (0.01 M).
Scheme 2 Reagents and conditions: (i) (1) t-BuONO, BF3?OEt2, THF,
210 uC, (2) Et2NH, K2CO3, CH2Cl2, 0 uC; (ii) (1) n-BuLi, THF, 278 uC,
(2) Bu2SnCl2; (iii) MeI, neat, 100 uC; (iv) (1) BCl3, toluene, 278 uC to rt,
(2) Mes*Li, rt; (v) ArZnCl (Ar 5 bithienyl (9), p-(Ph2N)C6H4– (10)),
Pd2(dba)3?CHCl3–4P(furyl)3, THF, 40 uC, 12 h.
particularly interested in how the solvent or presence of fluoride
ions affects the fluorescence, in order to prove the effect of the
steric protection by the Mes* group.
In the UV-vis absorption spectra, 3 showed a weak absorption
band at 397 nm. The extended compounds 9 and 10 showed
intense absorption bands around 366–390 nm and weak absorp-
tion bands around 453–470 nm (Table 1 and Fig. 2). According to
calculations on 9 and 10 at the B3LYP/6-31G(d) level of theory,
in both cases, while the HOMO is spread over the entire
p-conjugated framework, the LUMO is mainly localized on the
dibenzoborole moiety with a large contribution from the vacant p
orbital of the boron atom, and the LUMO+1 is the p* orbital
delocalized over the whole p-conjugated skeleton. The TD-DFT
calculations (B3LYP/6-31+G(d)) indicate that the absorption
bands observed at the shorter and longer wavelengths can be
assigned to the HOMO–LUMO+1 (p–p*) transition and the
HOMO–LUMO (p–(pp–p*)) transition, respectively (see ESI{).
In the fluorescence spectra in THF, 3 showed a blue-green
emission at 501 nm, while 9 and 10 showed orange-red emissions
with the maxima around 600 nm. The notable common feature is
their much higher fluorescence quantum yields compared to those
of the previously reported Tip-substituted derivatives.4a This
difference is most likely attributed to the restricted rotation of
the Mes* group due to steric hindrance. When the nonpolar
solvent cyclohexane (3: 495 nm, 9: 590 nm, 10: 569 nm) was
replaced with the polar DMF (3: 505 nm, 9: 622 nm, 10: 634 nm),
the emission bands shifted to a longer wavelength, while only
subtle changes were observed in the absorption spectra. The degree
of the red shift in the fluorescence increases in the order of 3, 9,
and 10, which corresponds to the order of the p-donating ability.
These results confirm that the dibenzoborole framework acts
as a p-electron accepting unit only in the excited state, and the
enhancement in the charge transfer character in the excited state
results in the large red shift in the emission. More importantly,
upon addition of an excess amount of fluoride ions (CsF or
n-Bu4NF) in the THF solutions of 9 and 10, the emission bands
did not change at all. These results clearly indicate that the Mes*
group effectively protects the boron atom and prevents the
coordination of DMF or a fluoride ion. This is in sharp contrast to
the result obtained for the Tip-substituted dibenzoborole, which
showed significant blue shifts in emission due to the coordination
of DMF or a fluoride ion to the boron center.4
Fig. 1 Crystal structure of 9: (a) ORTEP drawing (50% probability for
thermal ellipsoids), (b) space filling model, and (c) packing structure.
compounds 9 and 10 could be purified by column chromato-
graphy on silica gel.
The X-ray crystallographic analysis of 9 revealed that the boron
is sufficiently covered with the bulky Mes* group, as shown in
Fig. 1.§ The p-conjugated framework takes a banana-shaped
planar conformation. The dihedral angles between the dibenzo-
borole plane and inner thiophene ring and between the terminal
and inner thiophene rings are 5.4u, 31.5u and 16.5u, 17.8u, respec-
tively. In the packing structure, molecules are arranged in an offset
face-to-face p-stacking array with intermolecular distances of ca.
˚
3.3 A (Fig. 1c, see ESI{).
The photophysical properties for compound 3 and extended
compounds 9 and 10 were measured (Table 1). We were
Table 1 Photophysical data for p-extended dibenzoborole derivatives
Absorption lmax/nm
(log e)
Fluorescencea
l
max/nm (WF)b
Compd
Solvent
3
Cyclohexane
THF
DMF
Cyclohexane
THF
DMF
Cyclohexane
THF
DMF
331 (2.79), 397 (2.41)
333 (2.85), 397 (2.42)
333 (2.89), 397 (2.39)
383 (4.66), 466 (3.88)
388 (4.75), 470 (4.06)
390 (4.79), 470 (4.09)
366 (4.60), 453 (3.65)
366 (4.75), 457 (3.80)
368 (4.73), 460 (3.90)
495 (0.54)
501 (0.35)
505 (0.29)
590 (0.26)
608 (0.24)
622 (0.23)
569 (0.65)
600 (0.48)
634 (0.25)
9
10
a
b
Excited at 360 nm. Absolute quantum yield determined by a
calibrated integrating sphere system.
In order to obtain insights into the electrochemical properties of
the dibenzoborole systems, the cyclic voltammograms of 3, 9, and
580 | Chem. Commun., 2008, 579–581
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and 0.31 mT for two equivalent protons (see ESI{) indicates
that the spin density on boron is estimated to be 0.18. The decrease
2
2
?
?
in the spin density on the boron atom in 9 (0.18) from 3
(0.21) suggests delocalization of the spin density to the bithiophene
skeleton.
In summary, dibenzoborole-based p-conjugated systems with
the bulky Mes* group on the boron atoms were synthesized using
3,7-diiododibenzoborole 8 as a key precursor. The Mes* group is
bulky enough to protect the boron atom and hence the molecules
showed identical fluorescence spectra even upon addition of
fluoride ions and showed stable electrochemical redox properties.
In addition, a study on the radical anions 3 and 9 2, produced
by chemical reduction, suggests spin distribution over the p
framework, which may be informative for considering their poten-
tial use as electron-transporting materials. Further study along this
line is now in progress in our laboratory.
2
?
?
Fig. 3 Cyclic voltammograms of 3, 9, and 10. Measurement conditions:
sample 1 mM in THF with n-Bu4NClO4 (0.1 M); scan rate 0.10 V s21
.
We thank Professor Kunio Awaga and Dr. Hirofumi
Yoshikawa of Nagoya University for use of the ESR spectrometer.
This work was supported by a Grant-in-Aid (No. 19685004)
from the Ministry of Education, Culture, Sports, Science, and
Technology, Japan, and SORST, Japan Science and Technology
Agency.
Notes and references
2
+
Fig. 4 ESR spectra of (a) 3 ?K and (b) 9 2?K+ obtained in THF.
?
?
§ Crystal data for 9 (from CH2Cl2–CH3CN): C46H45BS4?0.5(CH2Cl2)?
¯
0.5(CH3CN), M 5 799.86, triclinic P1 (no. 2), a 5 9.353(2), b 5 13.047(2),
˚
c 5 18.255(4) A, a 5 69.849(6), b 5 87.420(8), c 5 81.615(7)u, V 5
10 were measured in THF (Fig. 3). Dibenzoborole 3 exhibited a
reversible reduction wave at 22.28 V vs. Fc/Fc+, demonstrating the
high electron affinity of the dibenzoborole framework. The second
reduction wave was not observed even when swept to 23.2 V.
The extension of p-conjugation by two bithiophene units in 9
not only shifted the first reduction wave to a less negative potential
(E1/2 5 22.04 V) by 0.24 V compared with 3, but also exhibited
a reversible second reduction wave at 22.70 V (vs. Fc/Fc+),
indicative of the generation of the stable radical anion and dianion
under these conditions. On the other hand, the electron-donating
(diphenylamino)phenyl-substituted 10 exhibited two reversible
oxidation waves at +0.29 V and +0.51 V (vs. Fc/Fc+), in addition
to two reduction waves at E1/2 5 22.19 V and Epc 5 23.00 V,
which indicates the potential use of 10 as an ambipolar carrier
transporting material (see ESI{).
2068.9(7) A , T 5 100 K, Z 5 2, Dc 5 1.284 g cm21, 13 893 reflections
3
˚
measured, 7146 unique (Rint 5 0.0231). R1 5 0.0613, wR2 5 0.1479,
GOF 5 1.065 (I . 2s(I)). CCDC 665414. For crystallographic data in CIF
or other electronic format, see DOI: 10.1039/b716107g
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While the dianions of the dibenzoborole derivatives have been
reported by several groups,10 the properties of the radical anion are
still not well understood. We conducted a chemical reduction of
the dibenzoboroles. The reaction of 3 with potassium in THF
under vacuum at room temperature gave a dark red solution. The
2
?
ESR measurement of the resulting solution of 3 exhibited an
eleven-line signal (g 5 2.002), as shown in Fig. 4a, which
is assignable to the coupling with boron (11B: I 5 3/2, 80.20%;
10B: I 5 3, 19.80%) and proton. According to the simulated ESR
signal using the coupling constants of 0.55 mT for boron, 0.35 mT
for two equivalent protons and 0.21 mT for two equivalent
protons (see ESI{), the spin density on boron is estimated as 0.21.
In a similar manner, the reduction of 9 with potassium was
conducted in THF. The ESR spectrum of the resulting dark green
solution exhibited a six-line signal (g 5 2.002), as shown in Fig. 4b.
The simulation using the coupling constants of 0.45 mT for boron
10 (a) W. J. Grigsby and P. P. Power, J. Am. Chem. Soc., 1996, 118, 7981;
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S. A. Decker, D. Chau, T. K. Woo and M. Parvez, Organometallics,
2003, 22, 1266.
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