Planarized: B, N -phenylated dibenzoazaborine with a carbazole substructure: Electronic impact of the structural constraint

Article abstract of DOI:10.1039/c9ob00934e

A B,N-diphenyl-5,10-dihydro-dibenzo-1,4-azaborine, in which both phenyl groups on the boron and nitrogen atoms are planarized to generate a carbazole substructure, was synthesized. The structral constraint around the boron and nitrogen atoms alters the π-conjugation mode and thus the photophysical and electrochemical properties. Specifically, this structurally constrained dibenzoazaborine showed an intense blue emission with a narrow full width at half maximum. One of its derivatives exhibited near infrared absorption in the one-electron-oxidized state.

Full text of DOI:10.1039/c9ob00934e

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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Planarized B,N-Phenylated Dibenzoazaborine with a Carbazole
Substructure: Electronic Impact of the Structural Constraint
Mikinori Ando,a Mika Sakai,a Naoki Ando,a Masato Hirai,b and Shigehiro Yamaguchi*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A B,N-diphenyl-5,10-dihydro-dibenzo-1,4-azaborine, in which both
phenyl groups on the boron and nitrogen atoms are planarized to
generate a carbazole substructure, was synthesized. The structral
constraint around the boron and nitrogen atoms alters the -
conjugation mode and thus the photophysical and electrochemical
properties.
Specifically,
this
structurally
constrained
dibenzoazaborine showed an intense blue emission with a narrow
full width at half maximum. One of its derivatives exhibited near
infrared absorption in the one-electron-oxidized state.
Embedding heteroatoms into polycyclic aromatic
hydrocarbons (PAHs) is an effective way to alter their structural
and electronic properties by capitalizing on the size effects, the
inductive effects, and the orbital interactions of the
heteroatoms.1 Based on this design strategy, a variety of
fascinating -electron materials have been synthesized.2 PAHs
containing multiple heteroatoms are particularly important as
synergistic effects of the heteroatoms can give rise to intriguing
electronic properties. For example, B,N-containing PAHs have
attracted significant attention as the boron and nitrogen atoms
have complementary electron-accepting and -donating
properties.3
One way of embedding boron and nitrogen atoms is to use B-
N bonds as an isostere of the C=C double bond, and several
PAHs containing B-N bonds have been developed and used in
organic electronic devices including organic field-effect
transistors (OFETs).4 Another strategy is to embed boron and
nitrogen atoms in remote positions with respect to each other,
Figure 1. (a) Representative examples of previously reported dibenzoazaborine-based -
conjugated compounds and (b) planarized B,N-phenylated dibenzoazaborine 1 together
with reference compounds 2–4.
acene-like -extended dibenzoazaborines such as comopund B
that show intriguing photophysical properties.5 Recently,
Hatakeyama and co-workers have synthesized a series of PAHs
that contain multiple 1,4-azaborine rings, and some of these
PAHs represented by compound C showed excellent thermally
activated delayed fluorescence (TADF) with pure-blue colors of
a narrow full width at half maximum (FWHM). Some of these
PAHs have subsequently been employed in organic light-
emitting diodes (OLEDs).6
The aforementioned results demonstrate the promising
potential of the dibenzoazaborine skeleton as a building block
for -electron materials. However, further advancements in this
class of materials should require a more detailed understanding
of the relationship between their structure and their electronic
structure, and hence their photophysical and electrochemical
properties. In this context, we are interested in the correlation
between the ring-fusion mode and the degree of structural
constraint within the dibenzoazaborine skeleton. The structural
constraint into a planar fashion is an effective strategy to
expand the -conjugation and to increase the chemical
stability.2c We have already demonstrated that the fixation of
three aryl groups of triarylboranes into a planar structure
effectively stabilizes such compounds, even in the absence of
steric protection on the boron center.7 On the other hand,
as exemplified by
a B,N-diphenyl-5,10-dihydro-dibenzo-1,4-
azaborine A (hereafter abbreviated as dibenzoazaborine) (Figure
1a). Kawashima and co-workers have synthesized a series of
a. Department of Chemistry, Graduate School of Science, Nagoya University, Furo,
Chikusa, Nagoya, 464-8602, Japan.
b. Integrated Research Consortium on Chemical Sciences (IRCCS), Nagoya University,
Furo, Chikusa, Nagoya, 464-8602, Japan. Institute of Transformative Bio-
Molecules (WPI-ITbM), Nagoya University, Furo, Chikusa, Nagoya, 464-8602,
Japan. E-mail: yamaguchi@chem.nagoya-u.ac.jp.
†Electronic Supplementary Information (ESI) available: Experimental procedures,
spectral data, details of the computational studies, and crystallographic data for 1
and 4. CCDC 1911122-1911123. See DOI: 10.1039/x0xx00000x
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planarization of the triphenylamine skeleton to form a bond lengths [B1–C1: 1.512(2) Å; B1–C17: 1.493(2) Å; B1–C19:
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DOI: 10.1039/C9OB00934E
carbazole substructure decreases the electron-donating ability 1.507(2) Å] are shorter than those of a previously reported
of the amine moiety.8 These considerations encouraged us to dibenzoazaborine derivative with a mesityl group on the boron
modify the B- and N-phenyl groups in the dibenzoazaborine in a atom [B–C: 1.522(3)–1.584(3) Å].9 This difference should be
planar fashion to generate a new family of planarized attributed to the planarization of the B-phenyl group, as 4 also
triarylboranes with a carbazole substructure (1; Figure 1b). To exhibits B–C bond lengths that are comparable to those of 1
better understand the impact of the structural constraint on the (Figure S2 and Table S1). The contracted B–C bonds, resulting
electronic structure and the photophysical and electrochemical from the structural constraint around the boron atom, imply
properties of 1, compounds 2–4 were synthesized as reference that effective electron donation occurs from the benzene rings
compounds for comparison.
to a vacant p orbital of the boron atom,7a,10 which probably
Compound 1 was prepared in three steps from silicon- contributes to the high chemical stability of 1.
bridged N-phenylcarbazole 5 (Scheme 1). The silicon-boron
exchange reaction of 5 with BBr3 proceeded smoothly and a
subsequent treatment of
yl)phenyllithium provided
6
with 2,6-di(propen-2-
C7
7
in 75% yield. Two-fold
N1
C6
intramolecular Friedel–Crafts cyclization of 7 in the presence of
Sc(OTf)3 furnished 1 in 84% yield.7a Compound 1 was sufficiently
stable to be purified by standard column chromatography on
silica gel. Moreover, the boron moiety in 1 showed desirable
robustness against electrophiles and nucleophiles, which
allowed further functionalization of the carbazole moiety in 1
by conventional methods. Specifically, the bromination of 1
with N-bromosuccinimide occurred at the benzene ring of the
carbazole moiety to furnish bromo-substituted 8. Subsequent
C18
C1
C17
B1
C19
Figure 2. Crystal structure of 1 (thermal ellipsoids at 50% probability; hydrogen atoms
are omitted for clarity). Selected bond lengths (Å) and angles (°): B1–C1 = 1.512(2), B1–
C17 = 1.493(2), B1–C19 = 1.507(2), C1–B1–C17 = 118.0(1), C1–B1–C19 = 122.9(1), C17–
cross-coupling reactions of
8 successfully afforded the B1–C19 = 119.0(1), C6–N1–C7 = 131.6(1), C6–N1–C18 = 121.7(1), C7–N1–C18 = 106.6(1).
corresponding -expanded derivatives. For instance, the
Suzuki-Miyaura coupling of 8 with 4-t-butylphenylboronic acid
under basic conditions produced 9 in 93% yield. For comparison,
3 was synthesized by a procedure similar to that of 1, while 2
and 4 were prepared via literature procedures (Schemes S1 and
S2).5a
Subsequently, we carried out
a comparison of the
photophysical properties of 1 and 2–4 (Figure 3 and Table 1).
The UV-vis absorption spectrum of 2 in THF exhibited
absorption band maxima (abs) at 389 and 371 nm, while that of
N-phenyl-fused 3 showed one maximum at abs = 400 nm.
Comparable abs values were also observed in the B-phenyl-
planarized derivatives, i.e., 4 showed absorption bands with a
vibronic structure at abs = 404 and abs = 387 nm, while N-
phenyl-fused 1 exhibited an intense absorption band at abs
=
402 nm.
The fluorescence spectra of 1–4 in THF exhibited an intense
blue emission with a narrow FWHM. Specifically, 1 showed an
intense deep blue emission (em = 416 nm; F = 0.82) with a
FWHM of 27 nm, which is most likely due to the rigid structure
of 1 that leads to a loss of the vibronic structure.
(a)
(b)
Scheme 1 Synthesis of 1 and -expanded derivative 9. Reagents and conditions: (a) BBr3,
CH2Cl2, r.t.; (b) 2,6-di(propen-2-yl)phenyllithium, toluene, 50 °C; (c) Sc(OTf)3, 1,2-
dichloroethane, 60 °C; (d) NBS, CH2Cl2, r.t.; (e) 4-t-butylphenylboronic acid, Pd(PPh3)4,
K2CO3, toluene/H2O, reflux.
The solid-state structure of 1 was determined unambiguously
by a single-crystal X-ray diffraction analysis (Figure 2), which
revealed a highly planar structure for 1, albeit that the N-
phenylcarbazole substructure is slightly distorted due to the
steric repulsion between the carbazole moiety and the N-
phenyl group. Both the boron and nitrogen atoms adopt a
trigonal planar geometry with bond-angle sums of 359.9°
around the B1 and N1 atoms, indicative of effective -
interaction around the boron and nitrogen centers. The B–C
Figure 3. (a) UV-vis absorption and (b) fluorescence spectra of 1 (red), 2 (blue), 3 (green)
and 4 (orange) in THF.
To understand the effects of the structural constraint on the
electronic properties, DFT calculations were carried out for 1–4
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at the B3LYP/6-31G(d) level of theory (Figure 4). The optimized in Figure S5. In general, the BNC4 ring of the dibenzoazaborine
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DOI: 10.1039/C9OB00934E
structure for 1 at this level of theory is consistent with the exhibits weak aromatic character as exemplified by the NICS(1)
experimentally determined crystal structure. In 2 and 4, in value of the BNC4 ring in 2 (–4.4 ppm). In contrast, the BNC4 ring
which the N-phenyl groups are orthogonally aligned, both the in 1 exhibits a nonaromatic character with a positively shifted
highest occupied molecular orbitals (HOMOs) and the lowest NICS(1) value (–1.8 ppm). In contrast, the NC4 ring in 1 exhibits
unoccupied molecular orbitals (LUMOs) are delocalized over a relatively negative NICS(1) value (–6.5 ppm), indicative of
the dibenzoazaborine framework. In contrast, the HOMOs of N- maintaining an aromatic character on the NC4 ring. These
phenyl-fused dibenzoazaborines 1 and 3 are delocalized over results suggest that 1 contains a more dominant contribution
both the dibenzoazaborine and the carbazole moieties, while from the N-phenylcarbazole substructure rather than the
the LUMOs contain a large contribution from the boron atom. azaborine substructure.
These results indicate that the structural constraint significantly
affects the -conjugation mode of the dibenzoazaborine
framework.
A close inspection of the energy levels of the HOMO and
LUMO provides an in-depth understanding on the impact of the
structural constraint on the electronic structure. Upon
planarization of the B-phenyl group with two methylene tethers
(2  4), the energy level of the HOMO increases, mainly due to
the electron-donating effect of the alkyl groups, whereas the
LUMO level remains unchanged (Figure S9).11 In contrast, the
fusion of the N-phenyl group (4  1) decreases the HOMO and
LUMO levels. To obtain further insight into the effect of the ring-
fusion, we also used model compound 4', which has a coplanar
alignment of the N-phenyl group relative to the
dibenzoazaborine framework, for a theoretical comparison.12
Upon changing the alignment of the N-phenyl group from an
orthogonal to a coplanar conformation (4  4'), the energy
Figure 4. Energy diagram and Kohn–Sham HOMOs and LUMOs for 1–4 together with
levels of the HOMO and LUMO decreased by 0.16 eV and 0.26
those for model compound 4', calculated at the B3LYP/6-31G(d) level of theory. The
results of TD-DFT calculations are also shown.
eV, respectively. These energy levels further decreased by ca.
0.1 eV upon formation of the carbazole substructure in 1.
Overall, in planarized B,N-phenylated 1, the energy level of the
LUMO is lower than that of 2, while the energy level of the
HOMO is comparable. These electronic effects are responsible
for the red-shifted absorption of 1 compared to that of 2.
Time dependent (TD)-DFT calculations at the same level of
theory showed that the oscillator strength (f) also increases
upon restraining the substituents on the boron and nitrogen
atoms, resulting in the largest molar absorption coefficient ()
value for 1 among all the relevant compounds. Thus, the
The electrochemical properties of 1–4 were examined by
cyclic voltammetry (CV) to experimentally confirm the
electronic features imposed by the structural constraint (Figure
S3 and Table 1). The cyclic voltammograms of 1–4 in THF
revealed one reversible reduction wave irrespective of the
structural constraint around the boron and nitrogen atoms. The
half reduction potentials are positively shifted in the order 2 (–
2.79 V) ≈ 4 (–2.75 V) < 1 (–2.47 V) ≈ 3 (–2.41 V) (vs Fc/Fc+),
demonstrating an impact of the formation of the carbazole
substructure on the energy level of the LUMOs.
planarization and the ring-fusion induces
a red-shifted
absorption and an increased  value.
The oxidation processes, which were measured in CH2Cl2,
revealed a reversible oxidation wave for 4 with a half oxidation
potential of +0.73 V (vs Fc/Fc+), whereas irreversible oxidation
waves were observed for 1–3. This difference is probably due to
The effect of the structural constraint on the aromaticity was
assessed by carrying out nucleus-independent chemical shifts
(NICS) calculations at the B3LYP/6-31G(d) level of theory.13 The
NICS(1) values for the optimized structures of 1–4 are illustrated
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Table 1 Photophysical and Electrochemical Properties of 1–4
Absorption a
Fluorescence a
Oxidation potential d,e
Reduction potential a,d
b
f
Compd
abs
[nm]
402
389
400
404

[104 M–1 cm–1]
3.42
em
[nm]
416
400
420
415
F
FWHM c
[nm]
27
36
32
Epa
[V]
+1.06
+1.22
+1.02
+0.79
E1/2
[V]
–
E1/2
[V]
–2.47
–2.79
–2.41
–2.75
1
2
3
4
0.82
0.96
0.72
0.95
1.45
2.19
2.45
–
–
28
+0.73
a In THF. b Absolute fluorescence quantum yields were determined by a calibrated integrating sphere system within an error of ± 3%. c Full width at half maximum. d
e
f
Determined by cyclic voltammetry (supporting electrolyte: 0.1 M nBu4N+PF6–; scan rate: 100 mV s–1; internal standard: ferrocene). In CH2Cl2. Peak oxidation
potential.
concomitant with a weak and broad absorption band at ~1500
nm. The new absorption band in the near-infrared (NIR) region
should be attributed to radical cation 9·+, which is supported by
the TD-DFT calculations in CH2Cl2 at the UB3LYP/6-31+G(d) level
of theory employing a PCM model (Figure S8 and Table S3). The
HOMO and SOMO are delocalized over the t-butylphenyl-
substituted N-phenylcarbazole substructure with a negligible
contribution from the boron atom, while the LUMO is
delocalized over the N-phenylcarbazole moiety including the
boron atom. The longest absorption band should be attributed
in essence to the HOMO()–SOMO() transition, which exhibits
a small oscillator strength (f = 0.0463) and a small transition
energy (0.69 eV), consistent with the NIR absorption (1793 nm)
In summary, we have successfully synthesized a series of
Figure 5. UV-vis-NIR absorption spectra of 9 and in-situ-generated radical cation 9·+,
dibenzoazaborine derivatives 1–4, and investigated the impact
which was obtained in-situ from treatment of 9 with an excess of NO+SbF6– in CH2Cl2 (2.2
of the structural constraint on their electronic structure and
hence the photophysical and electrochemical properties. A
comparison of 1–4 revealed that the planarization of the N-
phenyl group to form a carbazole substructure has a more
profound effect on the electronic structure than the
planarization of the B-phenyl group. Specifically, the
planarization of the N-phenyl group converted the acene-like -
conjugation on the dibenzoazaborine skeleton into a boron-
bridged N-phenylcarbazole-like -conjugation, whereas the
planarization of the B-phenyl group resulted mainly in an
expansion of the -conjugation. Notably, among the
× 10–4 M).
the suppression of the dimerization of the in-situ-generated
radical cation 4·+ by the dimethylmethylene bridges. In general,
radical cations of triphenylamine derivatives readily dimerize to
form the corresponding tetraphenylbenzimides.14 However, the
para-position relative to the nitrogen atom in the
dibenzoazaborine skeleton of 4 is likely protected by the steric
hindrance of the dimethylmethylene bridges, resulting in an
improved electrochemical stability of the dibenzoazaborine
skeleton. In contrast, the planarization of N-phenyl group
decreased the electrochemical stability of 1, as the spin is
delocalized onto the N-phenyl group. Indeed, a high spin density
was theoretically confirmed at the para-position relative to the
nitrogen atom in the carbazole substructure for the radical
cation 1·+ (Figure S7). Given this result, we assumed that in 1,
the introduction of substituents at the para-position relative to
the nitrogen atom at the carbazole substructure could induce
superior electrochemical stability. Based on this hypothesis, we
synthesized 4-t-butylphenyl-substituted 9, which indeed
exhibited reversible redox waves with a half oxidation potential
of +0.84 V (vs Fc/Fc+).
compounds investigated,
1 exhibited an intense blue
fluorescence with a narrow full width at half maximum (27 nm).
Moreover, the radical cation of -expanded derivative 9
showed a characteristic absorption in the near infrared region.
These results demonstrate the potential utility of B,N-
embedded compounds in which the B and N atoms are spatially
separated as building blocks for -conjugated materials, and
provide important guidelines for the molecular design of
structurally constrained -electron systems.
This work is partly supported by JSPS KAKENHI grant 18H03909
and 18H05261. M. S. thanks Graduate Program of
Transformative Chem-Bio Research in Nagoya University,
(MEXT, WISE program). ITbM is supported by the World Premier
International Research Center (WPI) Initiative, Japan.
Encouraged by this result, the chemical oxidation of 9 was
investigated by in-situ UV-vis-NIR absorption spectroscopy. In
CH2Cl2, 9 showed an absorption band at abs = 408 nm ( = 3.60
× 104 M–1cm–1). Upon treatment of 9 with an excess of the
oxidant NO+SbF6 , the absorption band of 9 diminished and an
–
intense absorption band at abs
= 1085 nm emerged
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Conflicts of interest
There are no conflicts of interest to declare.
Notes and references
1
(a) A. Fukazawa and S. Yamaguchi, Chem. Asian. J., 2009, 4,
1386; (b) X. He and T. Baumgartner, RSC Adv., 2013, 3, 11334;
(c) S. M. Prake, M. P. Boone and E. Rivard, Chem. Commun.,
2016, 52, 9485.
2
Recent reviews: (a) A. Narita, X.-Y. Wang, X. Feng, and K.
Müllen, Chem. Soc. Rev., 2015, 44, 6616; (b) M. Stępien, E.
Gonka, M. Zyła and N. Sprutta, Chem. Rev., 2017, 117, 3479;
(c) M. Hirai, N. Tanaka, M. Sakai and S. Yamaguchi, Chem. Rev.,
DOI: 10.1021/acs.chemrev.8b00637.
For reviews on B,N-embedded PAHs, see: (a) Z. Liu and T. B.
Marder, Angew. Chem. Int. Ed., 2008, 47, 242; (b) M. J. D.
Bosdet and W. E. Piers, Can. J. Chem., 2009, 87, 29; (c) P. G.
Cambell, A. J. V. Marwitz and S.-Y. Liu, Angew. Chem. Int. Ed.,
2012, 51, 6074; (d) X.-Y. Wang, J.-Y. Wang and J. Pei, Chem.
Eur. J., 2015, 21, 3528; (e) J.-Y. Wang and J. Pei, Chin. Chem.
Lett., 2016, 27, 1139; (f) J. Huang and Y. Li, Front. Chem., 2018,
6, 341.
3
4
5
6
(a) T. Hatakeyama, S. Hashimoto, T. Oda and M. Nakamura, J.
Am. Chem. Soc., 2012, 134, 19600; (b) X.-Y. Wang, F.-D.
Zhuang, R.-B. Wang, X.-C. Wang, X.-Y. Cao, J.-Y. Wang and J.
Pei, J. Am. Chem. Soc., 2014, 136, 3764.
(a) T. Agou, J. Kobayashi and T. Kawashima, Org. Lett., 2006,
8, 2241; (b) T. Agou, J. Kobayashi and T. Kawashima, Chem.
Commun., 2007, 3204; (c) T. Agou, H. Arai and T. Kawashima,
Chem. Lett., 2010, 39, 612.
(a) T. Hatakeyama, K. Shiren, K. Nakajima, S. Nomura, S.
Nakatsuka, K. Kinoshita, J. Ni, Y. Ono and T. Ikuta, Adv. Mater.,
2016, 28, 2781; (b) K. Matsui, S. Oda, K. Yoshiura, K. Nakajima,
N. Yasuda and T. Hatakeyama, J. Am. Chem. Soc., 2018, 140,
1195.
(a) Z. Zhou, A. Wakamiya, T. Kushida and S. Yamaguchi, J. Am.
Chem. Soc., 2012, 134, 4529; (b) S. Saito, K. Matsuo and S.
Yamaguchi, J. Am. Chem. Soc., 2012, 134, 9130; (c) C. Dou, S.
Saito, K. Matsuo, I. Hisaki and S. Yamaguchi, Angew. Chem. Int.
Ed., 2012, 51, 12206.
7
8
9
B. E. Koene, D. E. Loy and M. E. Thompson, Chem. Mater.,
1998, 10, 2235.
M. Kranz, F. Hampel and T. Clark, J. Chem. Soc., Chem.
Commun., 1992, 1247.
10 Y. Kitamoto, T. Suzuki, Y. Miyata, H. Kita, K. Funaki and S. Oi,
Chem. Commun., 2016, 52, 7098.
11 (a) T. Kushida, Z. Zhou, A. Wakamiya and S. Yamaguchi, Chem.
Commun., 2012, 48, 10715; (b) H. Iwahara, T. Kushida and S.
Yamaguchi, Chem. Commun., 2015, 52, 1124; (c). N. Ando, T.
Kushida and S. Yamaguchi, Chem. Commun., 2018, 54, 5213.
12 A single-point calculation of 4' was carried out using the
optimized structure of 4, except for the alignment of the N-
phenyl group.
13 (a) P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao and N. J.
R. v. E. Hommes, J. Am. Chem. Soc., 1996, 118, 6317; (b) C.
Chen, C. S. Wannere, C. Corminboeuf, R. Puchta and P. v. R.
Schleyer, Chem. Rev., 2005, 105, 3842.
14 (a) E. T. Seo, R. F. Nelson, J. M. Fritsch, L. S. Marcoux, D. W.
Leedy and R. N. Adams, J. Am. Chem. Soc., 1996, 88, 3498; (b)
L. S. Marcoux, R. N. Adams, and S. W. Feldberg, J. Phys. Chem.,
1969, 73, 2611.
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