Synthesis of fully-fused bisboron azomethine complexes and their conjugated polymers with solid-state near-infrared emission

Article abstract of DOI:10.1039/d0cc02301a

We describe herein a robust π-conjugated molecules with solid-state emission in the near-infrared (NIR) region (ΦF = 0.03-0.06). Initially, the diastereomers of bisboron azomethine complexes having phenyl groups in the same and opposite directions to the π-plane were synthesized. These diastereomers showed emission properties with larger red-shifts (>200 nm) and 10 times larger emission efficiencies than those of the mononuclear complex. Theoretical calculation data indicate that superior optical properties of the bisboron complexes should be attributable to efficient expansion of the π-conjugated system. In addition, the bisboron compounds and their conjugated polymers exhibited intense NIR emissions even in the solid state. This journal is

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Synthesis of fully-fused bisboron azomethine
complexes and their conjugated polymers with
Cite this:DOI: 10.1039/d0cc02301a
solid-state near-infrared emission†
Received 30th March 2020,
Accepted 2nd May 2020
Shunsuke Ohtani,
Yoshiki Chujo
Masashi Nakamura, Masayuki Gon,
Kazuo Tanaka * and
DOI: 10.1039/d0cc02301a
rsc.li/chemcomm
We describe herein a robust p-conjugated molecules with solid-state p-conjugated systems facilitate consecutive p–p stacking and
emission in the near-infrared (NIR) region (UF = 0.03–0.06). Initially, other electronic interactions (e.g., energy transfer, inter-, or intra-
the diastereomers of bisboron azomethine complexes having phenyl molecular charge transfer, and excited state reactions).7 As
groups in the same and opposite directions to the p-plane were a result, these materials suffer from the aggregation caused
synthesized. These diastereomers showed emission properties with quenching (ACQ) effect.8
larger red-shifts (4200 nm) and 10 times larger emission efficiencies
Recently, we proposed the concept of ‘‘element-blocks’’,
than those of the mononuclear complex. Theoretical calculation data which are minimum functional units containing heteroatoms,
indicate that superior optical properties of the bisboron complexes to create functional materials according to preprogrammed
should be attributable to efficient expansion of the p-conjugated designs.9 On the basis of this strategy, it has been revealed
system. In addition, the bisboron compounds and their conjugated that introduction of heteroatoms into organic scaffolds is one
polymers exhibited intense NIR emissions even in the solid state.
of the facile and effective methods to expand p-conjugated
systems. In particular, boron-containing ‘‘element-blocks’’ are
Near-infrared (NIR) light has various advantageous features promising building blocks for developing luminescent materials
such as low photodamage to biological samples, deep tissue by constructing planar and rigid conjugated systems.10 Further-
penetration and minimum interference from background auto- more, ladder-type p-conjugated multi-boron complexes, posses-
fluorescence by biomolecules in the living systems.1 In addi- sing fused polycyclic skeletons with four coordinated boron
tion, if dyes have solid-state luminescence properties, these atoms, have ideal emissive properties since their effectively-
dyes can be a key unit in advanced optical devices, such as extended p-conjugated systems can contribute to the formation
telecommunication devices,2 laser amplifiers3 and organic of the planar and rigid molecular skeletons.11 In a recent report,
light-emitting diodes (OLEDs).4 One of the strategies for obtain- we reported the synthesis and unique solid-state luminescence
ing NIR-emissive dyes is to reduce the energy band gap (Eg) properties of a boron-fused azomethine complex modified with a
between HOMO (highest occupied molecular orbital) and diethylamino group. It was observed that this complex is able to
LUMO (lowest unoccupied molecular orbital) levels by using avoid ACQ and shows intense emission in crystals larger than
extended p-conjugated systems.5 As is often the case with those in amorphous and diluted solution states (crystallization-
polymer syntheses, another strategy is to construct an alternating induced emission enhancement, CIEE).12,13 Furthermore, the
array of electron-donor and acceptor moieties. Because the HOMO donor–acceptor conjugated copolymers involving the boron-
and LUMO levels can be elevated and lowered by electron donors fused azomethine acceptor unit exhibited intense emission even
and acceptors, respectively, intense emission in longer wavelength in thin films.14 These results suggest that p-conjugated systems
regions can be obtained.6 Although there are these established should be extensively developed using the boron-fused azo-
design strategies, there are still a limited number of emissive methine unit. From these studies, we presumed that further
organic dyes having NIR and especially solid-state luminescence extension of p-conjugated systems involving the boron-fused
properties. In the solid state, organic chromophores having large azomethine complexes might be a potential design strategy for
obtaining a solid-state emissive material in the NIR region.
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto
Herein, we report bisboron azomethine complexes fused by
phenyl-substituted boron atoms. Two kinds of diastereomers
were successfully isolated, and it was observed that the bis-
University Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
E-mail: tanaka@poly.synchem.kyoto-u.ac.jp
† Electronic supplementary information (ESI) available. CCDC 1993544. For ESI
boron complexes exhibited NIR emission not only in solution
but also in the solid state because of steric hindrance of the
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0cc02301a
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Fig. 1 Chemical structures of monoBAm, syn-, anti-bisBAm, p-bisBAm-F
and p-bisBAm-T.
Fig. 3 (a and c) UV-vis absorption and (b and d) photoluminescence
spectra of monoBAm, syn-bisBAm, anti-bisBAm, p-bisBAm-F and
p-bisBAm-T in solution (CHCl3, 1.0 ꢁ 10ꢀ5 M) and crystalline states.
phenyl substituents. In addition, NIR-luminescent film materi-
als can be obtained from the alternating copolymers containing
the bisboron complex in the main chains owing to good film-
formability.
(Fig. 2 and Table S1, ESI†). There seems to be subtle interfacial
p–p interaction in the crystal packing of anti-bisBAm because the
large degree of steric hindrance of the six phenyl groups prevented
the overlap among the p-conjugated skeletons. From these results,
the remaining diastereomer A was assigned as syn-bisBAm
where the phenyl groups on the boron atoms were on the same
side of the molecular plane.
In the UV-vis absorption spectra (Fig. 3a) of the bisboron
complexes syn- and anti-bisBAm in diluted CHCl3 solution, the
longest absorption bands showed a remarkable red-shift from
that of monoBAm (Table 1). This result suggested that the
conjugated systems in the ground state should have been
efficiently extended by the bisboron complexation. Addition-
ally, from the TD-DFT (time-dependent density functional
theory) calculations at the B3LYP/6-311G(d,p) level (see ESI†),
it was found that the molecular orbitals of both the HOMO and
LUMO were more delocalized over the entire molecules in syn-
and anti-bisBAm, compared to that of monoBAm (Fig. S34,
ESI†). Corresponding to the molecular orbital spread, the Eg
between the HOMO and LUMO of syn- and anti-bisBAm
became narrower than that of monoBAm. The energy levels
were determined by calculation with the optical band gap
The bisboron complex bisBAm containing two boron-fused
structures was synthesized from the azomethine tridentate ligand
3 (Fig. 1 and Schemes S1–S3, Fig. S1–S9, ESI†). To evaluate the
effect on the expansion of the p-conjugated system by bisboron
complexation, we also synthesized the mononuclear boron
complex monoBAm (Fig. 1 and Schemes S4, S5, Fig. S10–S14,
ESI†). The 1H NMR spectrum of bisBAm showed two singlet peaks
at d = 8.04 and 8.05 ppm, which corresponded to the azomethine
protons, indicating that bisBAm is obtained as a diastereomeric
mixture (syn- and anti-bisBAm) due to the stereogenic boron
centers (Fig. S28, ESI†). To examine the optical properties of these
diastereomers individually, we tried separating these diastereo-
mers by a reprecipitation method. After pouring Et2O (poor
solvent) into the diastereomeric mixture solution in CHCl3 (good
solvent) and stirring for 1 h at room temperature, precipitation
was generated. After filtration, the solvent was removed from the
filtrate to afford diastereomer A as a deep blue powder. We also
successfully obtained diastereomer B as a deep blue powder by
purifying the precipitate. Fortunately, single crystal pieces of
diastereomer B with sufficient size for single-crystal X-ray analysis
were obtained by a vapor diffusion method from the CHCl3
solution under a Et2O atmosphere. According to the data obtained
at 106 K, diastereomer B was assigned as anti-bisBAm in which
the two phenyl moieties protruding from the four-coordinated
boron atoms were on opposite sides of the molecular framework
(DEg,opt) obtained from the UV-vis absorption spectra (EHOMO
=
ELUMO ꢀ DEg,opt) and the data from cyclic voltammetry (CV)
(Fig. S36, ESI†). Accordingly, bisboron complexes showed narrower
HOMO–LUMO gaps than that of monoBAm originating from
increases and decreases in the HOMO and LUMO levels,
respectively. It should be emphasized that the decreases in
the LUMO levels were much larger than the increases in the
HOMO levels, suggesting that the electron-accepting ability was
enhanced by introducing the second boron-fused moieties.
The photoluminescence (PL) spectra of syn- and anti-bisBAm
show dramatically red-shifted emission bands by more than
200 nm compared to that of monoBAm and their emission
maxima reached the NIR region (Fig. 3b and Fig. S31, ESI†). These
exceptionally-large red-shifts compared to the corresponding mono-
nuclear boron complex have never been reported in previous
literature regarding p-conjugated multi-boron compounds.11e,f,h,i
Fig. 2 Thermal ellipsoid (probability level 50%) (left) and packing structure
(right) of anti-bisBAm. Hydrogen atoms were omitted for clarity.
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Table 1 Optical properties of synthesized compounds in the solution
quenching was hardly observed in both syn-bisBAm and anti-
bisBAm crystalline samples. This is probably because the
protruding phenyl groups on boron atoms prevented p–p
stacking in the crystal. As a result, the NIR emissive properties
in solution can be preserved even in the crystal. Notably, it was
found that the PL spectrum of syn-bisBAm was observed in
the longer wavelength region compared to that of anti-bisBAm
(syn-bisBAm: lem = 757 nm; anti-bisBAm: lem = 745 nm) despite
the fact that almost the same emission properties were
obtained in solution and in the films. This is likely because
syn-bisBAm tends to form stronger intermolecular interaction
between adjacent molecules than anti-bisBAm. This result
might be responsible for the structural feature of syn-bisBAm,
such as a vacant molecular surface resulting from the phenyl
statea
b
c
f
lmax,abs lem
(nm)
te
krf
knr
d
(nm) FF
(ns) (ꢁ 108 sꢀ1
)
(ꢁ 108 sꢀ1
)
monoBAm
419
522
731
729
742
742
0.003
—
—
—
syn-bisBAm 599
anti-bisBAm 595
p-bisBAm-F 610
p-bisBAm-T 614
0.035 1.5 0.23
0.038 1.5 0.25
0.060 1.4 0.47
0.062 1.3 0.50
6.2
6.3
6.6
7.3
a
CHCl3 solution, 1.0 ꢁ 10ꢀ5 M (per repeating units in the case of
b
c
polymer samples). The longest absorption maximum. Fluorescence
maximum with the excitation at the longest absorption maximum.
d
f
e
Absolute fluorescence quantum yield. Emission lifetime at lem
.
kr = FF/t, and knr = (1 ꢀ FF)/t.
These NIR luminescence properties likely originated from the groups on boron atoms orienting in the same direction.
robust conjugation involving the linearly-connected tandem azo- To probe the applicability of bisboron complexes as a
methine units assisted by the fully-fused borons, realizing a planar monomer for constructing conjugated polymers, we actually
and rigid molecular framework. Furthermore, their absolute synthesized the donor–acceptor type conjugated copolymers
fluorescence quantum yields (FFs) were about 10 times higher p-bisBAm-F and p-bisBAm-T from the dibrominated bisBAmBr
than that of monoBAm.
and fluorene, and bithiophene comonomers (Fig. 1 and
To investigate the influence of molecular motion on the Schemes S6–S9, Fig. S15–S27, ESI†). For this experiment, the
luminescence properties, emission yields were monitored with monomer bisBAmBr was used as a diastereomeric mixture
polystyrene films including 1 wt% monoBAm or bisBAm (Table S3 for polymerization due to the difficulty in separation of the
and Fig. S29, S32, ESI†). The FF of monoBAm in the film state diastereomers. Polymerizations were accomplished by the
increased to 0.013. In addition, smaller Stokes shifts were palladium-catalyzed Migita–Kosugi–Stille coupling reaction15
observed in the film state than those in the solution state, clearly with fluorene (F) or bithiophene derivatives (T) in toluene in
indicating that the excited state of monoBAm is deactivated via a the presence of conventional coupling reagents. Size-exclusion
non-radiative process in solution, and the decay process can chromatography using CHCl3 as an eluent with the polystyrene
be disturbed by the restriction of the intramolecular motion standards clearly indicated the generation of polymerization
(Tables S2 and S3, ESI†). In contrast, in the case of bisBAm, the products (Table S6, ESI†). In the absorption spectra of the
FF value in the film state was hardly changed compared to that in polymer solutions, the absorption bands were observed in a
the solution state, suggesting that bisboron complexation much longer wavelength region with larger molar extinction
enhances the rigidity of the molecular framework.
coefficients than those of the monomeric bisBAm diastereo-
To get further insight about the kinetics, the radiative (kr) mers, clearly proving that robust conjugation extended through
and non-radiative rate constants (knr) were determined from PL the polymer main chains (Table 1 and Fig. 3c). From the
lifetime measurements under various conditions (Table 1 and TD-DFT calculations with model compounds p-bisBAm-F0 and
Fig. S33, ESI†). In the dispersed films of monoBAm and bisBAm p-bisBAm-T0, delocalization of the HOMOs was observed,
compounds, it was found that the kr of monoBAm was much suggesting that extension of the conjugated systems through
smaller than those of the bisBAm compounds, suggesting that the main polymer chains plays a significant role in narrowing
the enhancement of FF by bisboron complexation originated the HOMO–LUMO energy band gaps and improving the oscil-
from the large oscillator strength of the S1 - S0 transition lator strengths of the S0 - S1 transition (Fig. S35, ESI†).
owing to the extension of the p-conjugated system. There was Although the HOMO orbitals of p-bisBAm-F0 and p-bisBAm-T0
less significant difference between syn- and anti-bisBAm in the were more delocalized than that of bisBAm, there was hardly
absorption and PL properties of the solution states. This fact any significant difference between the HOMO levels of fluorene
suggests that the electronic structures of the isolated bisBAm and bithiophene. This theoretical result was consistent with the
molecules are almost identical.
experimentally obtained results from the optical measure-
Next, the emissive properties of syn- and anti-bisBAm in ments. We assume that the delocalization of the HOMO orbi-
crystalline states were examined (Fig. 3b and Table S4, ESI†). tals might be mostly limited within bisBAm scaffolds. As a
The NIR emissions of bisBAm can be detected even in the result, the fluorene and bithiophene moieties did not influence
solid state with FF values obtained as follows: syn-bisBAm: the HOMO energy levels. Furthermore, the FFs of p-bisBAm-F,
FF = 0.035; anti-bisBAm: FF = 0.052. These values are relatively -T were determined as 0.066 and 0.064, respectively, which were
high for NIR emitters because, in general, the FF in the NIR almost twice those of the monomeric bisBAm compounds
region is significantly lower than that in the visible region due (Table 1 and Fig. 3d). According to the kinetic data (Table S2,
to the energy-gap law. In general, consecutive p–p stacking ESI†), larger krs were observed from the polymers. This result
leads to emission quenching in solids through migration of proves that the fluorescence enhancement should be ascribed
excited energy. On the other hand, significant emission to the acceleration of radiation processes caused by extended
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3 X. Wang, Z.-Z. Li, S.-F. Li, H. Li, J. Chen, Y. Wu and H. Fu, Adv. Opt.
Mater., 2017, 5, 1700027.
4 A. Zampetti, A. Minotto and F. Cacialli, Adv. Funct. Mater., 2019,
29, 1807623.
5 (a) M. Adachi and Y. Nagao, Chem. Mater., 2001, 13, 662–669;
(b) K. Sezukuri, M. Suzuki, H. Hayashi, D. Kuzuhara, N. Aratani
and H. Yamada, Chem. Commun., 2016, 52, 4872–4875.
6 (a) S. A. Jenekhe, L. Lu and M. M. Alam, Macromolecules, 2001, 34,
7315–7324; (b) H. Van Mullekom, J. Vekemans, E. Havinga and E. Meijer,
Mater. Sci. Eng., R, 2001, 32, 1–40; (c) Q. T. Zhang and J. M. Tour, J. Am.
Chem. Soc., 1998, 120, 5355–5362; (d) Y. Adachi, Y. Ooyama, Y. Ren,
p-conjugation. Finally, it was demonstrated that further
red-shifted emission bands were obtained from the film
samples without critical losses of FFs (p-bisBAm-F: FF = 0.049,
lem = 751 nm; p-bisBAm-T: FF = 0.042, lem = 751 nm, Table S5
and Fig. S30, ESI†). Owing to the film-formability and superior
luminescence properties, NIR-emissive films, which are a key
material in advanced low-cost and flexible OLEDs, can be
readily obtained.
In conclusion, robust p-conjugation systems can be con-
structed with boron-fused azomethine units owing to the rigid
and planar skeleton. Consequently, NIR emission can be
obtained from the bisboron complexes and conjugated poly-
mers. Meanwhile, despite the molecular planarity, ACQ can be
suppressed in the condensed state. According to the X-ray
analysis, it is proposed that the substituent at the boron atom
could play a critical role in disturbing intermolecular inter-
action followed by efficient solid-state emission. Our finding
might be widely applicable for the design of advanced NIR-
emissive materials both in biotechnology and materials
science.
¨
X. Yin, F. Jakle and J. Ohshita, Polym. Chem., 2018, 9, 291–299.
`
7 (a) J. Massin, W. Dayoub, J.-C. Mulatier, C. Aronica, Y. Bretonniere
and C. Andraud, Chem. Mater., 2011, 23, 862–873; (b) X. Cheng,
K. Wang, S. Huang, H. Zhang, H. Zhang and Y. Wang, Angew. Chem.,
Int. Ed., 2015, 54, 8369–8373.
8 (a) R. T. K. Kwok, C. W. T. Leung, J. W. Y. Lam and B. Z. Tang, Chem.
Soc. Rev., 2015, 44, 4228–4238; (b) J. Mei, N. L. C. Leung, R. T. K. Kwok,
J. W. Y. Lam and B. Z. Tang, Chem. Rev., 2015, 115, 11718–11940.
9 (a) Y. Chujo and K. Tanaka, Bull. Chem. Soc. Jpn., 2015, 88, 633–643;
(b) M. Gon, K. Tanaka and Y. Chujo, Polym. J., 2017, 50, 109–126;
(c) A. Lik, L. Fritze, L. Mu¨ller and H. Helten, J. Am. Chem. Soc., 2017,
139, 5692–5695; (d) Y. Matsumura, M. Ishidoshiro, Y. Irie, H. Imoto,
K. Naka, K. Tanaka, S. Inagi and I. Tomita, Angew. Chem., Int. Ed.,
2016, 55, 15040–15043; (e) K. Tanaka and Y. Chujo, Polym. J., 2020,
DOI: 10.1038/s41428-020-0316-y.
´
10 (a) J. S. Dhindsa, R. R. Maar, S. M. Barbon, M. Olivia Aviles,
This work was partially supported by a The Kyoto University
Foundation grant (for M. G.), a Grant-in-Aid for Young Scien-
tists (for M. G.) (JSPS KAKENHI Grant number 18K14275),
Grants-in-Aid for ScientificResearch (B) (JSPS KAKENHI Grant
number 17H03067) and (A) (JSPS KAKENHI Grant number
17H01220), a Grant-in-Aid for Scientific Research on Innovative
Areas ‘‘New Polymeric Materials Based on Element-Blocks
(No. 2401)’’ (JSPS KAKENHI Grant Number 24102013) and a
Challenging Research grant (Pioneering) (JSPS KAKENHI Grant
Number 18H05356).
´
Z. K. Powell, F. Lagugne-Labarthet and J. B. Gilroy, Chem. Commun.,
2018, 54, 6899–6902; (b) A. Wakamiya, T. Taniguchi and S. Yamaguchi,
Angew. Chem., Int. Ed., 2006, 45, 3170–3173; (c) M. Gon, K. Tanaka and
Y. Chujo, Angew. Chem., Int. Ed., 2018, 57, 6546–6551; (d) M. Gon,
K. Tanaka and Y. Chujo, Bull. Chem. Soc. Jpn., 2019, 92, 7–18.
11 (a) D. Li, H. Zhang and Y. Wang, Chem. Soc. Rev., 2013, 42, 8416–8433;
´
(b) M. Urban, K. Durka, P. Jankowski, J. Serwatowski and S. Lulinski,
J. Org. Chem., 2017, 82, 8234–8241; (c) T. Nakamura, S. Furukawa and
E. Nakamura, Chem. – Asian J., 2016, 11, 2016–2020; (d) Y. Kubota,
K. Kasatani, T. Niwa, H. Sato, K. Funabiki and M. Matsui, Chem. – Eur. J.,
2016, 22, 1816–1824; (e) J. Wang, Q. Wu, C. Yu, Y. Wei, X. Mu, E. Hao
and L. Jiao, J. Org. Chem., 2016, 81, 11316–11323; ( f ) Z. Zhang, H. Bi,
Y. Zhang, D. Yao, H. Gao, Y. Fan, H. Zhang, Y. Wang, Y. Wang, Z. Chen
and D. Ma, Inorg. Chem., 2009, 48, 7230–7236; (g) D. Li, Z. Zhang,
S. Zhao, Y. Wang and H. Zhang, Dalton Trans., 2011, 40, 1279–1285;
(h) D. Li, K. Wang, S. Huang, S. Qu, X. Liu, Q. Zhu, H. Zhang and
Y. Wang, J. Mater. Chem., 2011, 21, 15298–15304; (i) A. Wakamiya,
T. Taniguchi and S. Yamaguchi, Angew. Chem., Int. Ed., 2006, 45,
3170–3173; ( j) M. J. Calhorda, D. Suresh, P. T. Gomes, R. E. Di Paolo
and A. L. Maçanita, Dalton Trans., 2012, 41, 13210–13217.
Conflicts of interest
There are no conflicts to declare.
12 S. Ohtani, M. Gon, K. Tanaka and Y. Chujo, Chem. – Eur. J., 2017, 23,
11827–11833.
Notes and references
1 (a) J. Fabian, H. Nakazumi and M. Matsuoka, Chem. Rev., 1992, 92, 13 (a) Y. Dong, J. W. Y. Lam, A. Qin, Z. Li, J. Sun, H. H.-Y. Sung,
1197–1226; (b) G. Yang, J. Liu, Y. Wu, L. Feng and Z. Liu, Coord.
Chem. Rev., 2016, 320–321, 100–117; (c) K. Kiyose, H. Kojima and
T. Nagano, Chem. – Asian J., 2008, 3, 506–515; (d) Z. Cheng, Y. Wu,
I. D. Williams and B. Z. Tang, Chem. Commun., 2007, 40–42;
ˇ
ˇ
(b) P. Galer, R. C. Korosec, M. Vidmar and B. Sket, J. Am. Chem.
Soc., 2014, 136, 7383–7394.
Z. Xiong, S. S. Gambhir and X. Chen, Bioconjugate Chem., 2005, 16, 14 S. Ohtani, M. Gon, K. Tanaka and Y. Chujo, Macromolecules, 2019,
1433–1441. 52, 3387–3393.
2 (a) H. Suzuki, A. Yokoo and M. Notomi, Polym. Adv. Technol., 2004, 15 (a) M. Kosugi, K. Sasazawa, Y. Shimizu and T. Migita, Chem. Lett.,
15, 75–80; (b) N. Tessler, V. Medvedev, M. Kazes, S. H. Kan and
U. Banin, Science, 2002, 295, 1506–1508.
1977, 301–302; (b) D. Milstein and J. K. Stille, J. Am. Chem. Soc., 1978,
100, 3636–3638.
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