Azahelicene-Fused BODIPY Analogues Showing Circularly Polarized Luminescence

Article abstract of DOI:10.1002/anie.202001186

Helical carbazole-based BODIPY analogues were readily synthesized via aza[7]helicenes. The structures of azahelicene-incorporated BF₂ dyes were elucidated by x-ray diffraction analysis. DFT calculations revealed that the π-conjugated system exp

Full text of DOI:10.1002/anie.202001186

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Accepted Article
Title: Azahelicene-Fused BODIPY Analogs Showing Circularly
Polarized Luminescence
Authors: Chihiro Maeda, Keiji Nagahata, Takuma Shirakawa, and
Tadashi Ema
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10.1002/anie.202001186
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Azahelicene-Fused BODIPY Analogs Showing Circularly
Polarized Luminescence
Chihiro Maeda,* Keiji Nagahata, Takuma Shirakawa, and Tadashi Ema*[a]
Abstract: Helical carbazole-based BODIPY analogs were readily
synthesized via aza[7]helicenes. The structures of azahelicene-
incorporated BF2 dyes were elucidated by X-ray diffraction analysis.
This work
Previous work
DFT calculations revealed that the p-conjugated system expanded
from the helicene moiety to the BODIPY framework. The azahelicene-
fused boron complexes showed the Cotton effects and the circularly
polarized luminescence (CPL) in the visible region. Furthermore, an
axially chiral binaphthyl group was attached to the helically chiral dyes,
which enhanced the chiroptical properties.
R
X
X
R
N
N
N
N
B
B
Y
Y
Y
Y
X = O, S
Y = F, 1,1'-binaphthyl-2,2'-dioxy
helicene + BODIPY framework
Helicenes are interesting and challenging molecules, and
chiroptical properties of helicenes have been actively studied.[1–4]
Circularly polarized luminescence (CPL) dyes attract
considerable attentions since they have potential applications to
chiroptical devices,[3–8] and various helicenes showing CPL have
been reported.[3,4] However, most helicenes reported so far
showed CPL in a blue region partly because functionalization of
helicenes to tune the CPL wavelength is difficult. Some nitrogen-
containing helicenes show CPL in a visible light region via
coordination, protonation, or donor–acceptor interactions.[3d–f,4f–g]
BODIPY derivatives and congeners have been extensively
studied because of the excellent fluorescent properties.[9]
Although BODIPY itself is an achiral dye, CD and CPL can be
induced by the incorporation of a chiral unit. Indeed, chiral
BODIPYs have been recognized as promising CPL materials.[8]
We have investigated the synthesis and fluorescence properties
of carbazole-based BODIPY analogs,[10] including chiral ones
showing CPL (Figure 1).[10e,f] In this study, we used
azahelicenes[2,3] to design novel chiral carbazole-based boron
complexes as color-tunable CPL dyes. In particular, we aimed for
NIR-responsive CPL dyes since such red-shifted CPL dyes have
scarcely been reported.[7g] We considered that aza[7]helicenes
such as 7 and 10 are good candidates for chiral carbazole-based
dyes (Scheme 1). 7 was prepared via the reported method,[2g]
while we developed p-extended aza[7]helicene 10 for the first time.
These aza[7]helicenes were further converted into the
corresponding chiral carbazole-based BODIPY analogs 1–6.
These azahelicene-incorporated boron complexes showed red-
shifted emission and CPL.
Figure 1. Molecular designs for the chiral carbazole-based boron complexes.
X
(a)
1.
N
Pd(OAc)2, PPh3
Cu(OAc)2•H2O
K2CO3, toluene
reflux
1. (Bpin)2, dtbpy
[Ir(OMe)(COD)]2
1,4-dioxane
reflux
Br
NH
X
NH
N
N
2. BF3•OEt2
iPr2NEt
toluene
2. CuBr2
THF/MeOH/H2O
90 °C
B
F
(rac)-1: X = O (11% from 7)
(rac)-2: X = S (11% from 7)
F
7
8
100 °C
(b)
B(OH)2
Pd(PPh3)4
K2CO3
Br
Br
FeCl3
CH2Cl2
MeNO2
rt
toluene
EtOH
H2O
N
H
N
H
N
H
9 (99%)
10 (91%)
100 °C
X
N
1.
Pd(OAc)2, PPh3
Cu(OAc)2•H2O
K2CO3, toluene
reflux
X
NBS
CHCl3
rt
2. BF3•OEt2
iPr2NEt
toluene
N
N
N
H
B
Br
F
F
100 °C
(rac)-3: X = O (20% from 10)
(rac)-4: X = S (25% from 10)
BF3•OEt2
iPr2NEt
X
(P)-3: X = O (86%)
(P)-4: X = S (97%)
(R,P)-5 (X = O)
(R,P)-6 (X = S)
toluene
100 °C
N
N
B
(R)-BINOL
Et2AlCl
O
chiral column
O
CH2Cl2
rt
BF3•OEt2
iPr2NEt
(M)-3: X = O (90%)
(M)-4: X = S (96%)
(R,M)-5 (X = O)
(R,M)-6 (X = S)
toluene
100 °C
(dia)-5: X = O (84%)
(dia)-6: X = S (91%)
Scheme 1. Synthesis of helical dyes (a) 1–2 and (b) 3–6.
First, helical carbazole-based BF2 dyes 1 and 2 were
synthesized as follows (Scheme 1a). The Ir-catalyzed borylation
of aza[7]helicene 7 and the subsequent bromination with CuBr2
gave 8. Direct arylation of 8 with benzoxazole and benzothiazole,
followed by the BF2 complexation afforded (rac)-1 and (rac)-2,
respectively. Single crystals of 2 suitable for X-ray diffraction
analysis were obtained by slow vapor diffusion of ethanol into a
dichloromethane solution of 2. X-ray crystal structure of 2
unambiguously elucidated the structure of the azahelicene-
incorporated BF2 dye (Figure 2a).[11a] In the crystal packing, the
[a]
Dr. C. Maeda, K. Nagahata, T. Shirakawa, Prof. T. Ema
Division of Applied Chemistry, Graduate School of Natural Science
and Technology, Okayama University,
Tsushima, Okayama 700-8530, Japan
E-mail: cmaeda@okayama-u.ac.jp
ema@cc.okayama-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.202001186
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enantiomers (P)-2 and (M)-2 stack alternately to form a 1-D
column (Figure S1 in SI). Optical resolution of both racemates
using Chiralpak IA gave two fractions which showed mirror-image
CD spectra (Figure 3 and S2 in SI). The absolute configurations
were determined by TD-DFT calculations of (P)-1 and (P)-2, and
the calculated rotatory strengths were in good agreement with the
CD spectra of the first fractions in both cases.
of the low solubility. Therefore, we converted 3 and 4 into (dia)-5
and (dia)-6, respectively, with binaphthyl units via the Et2AlCl-
mediated reaction. The diastereomeric mixtures were
successfully separated by chiral HPLC (SI). The optically pure 5
and 6 were then converted back into enantiomerically pure 3 and
4, respectively, under the standard BF2 complexation conditions.
The absolute configurations were determined by TD-DFT
calculations of (P)-3–6 (Figure 3b and 3c).
(a)
(b)
200
(a)
2
(M)-1
(M)-2
glum = +1.9 x 10–3
glum = +1.6 x 10–3
100
0
1
0
/
Δε
I/
Δ
-1
-2
-100
glum = –1.7 x 10–3
glum = –1.3 x 10–3
(P)-1
(P)-2
-200
4
3
2
1
ε
0
(c)
500
550
600
650
700
750
800
200
300
400
500
600
wavelength / nm
wavelength / nm
40
glum = +1.0 x 10–3
glum = +8.0 x 10–4
(M)-3
(M)-4
(b)
1.0
0.5
0.0
20
0
/
I/
Δε
Δ
-0.5
-1.0
-20
glum = –8.7 x 10–4
glum = –7.0 x 10–4
(P)-3
(P)-4
-40
4
3
2
1
ε
0
200
300
400
500
600
500
550
600
650
700
750
800
wavelength / nm
wavelength / nm
2
1
0
(c)200
(R,M)-5
(R,M)-6
glum = +1.2 x 10–3
glum = +8.8 x 10–4
100
Figure 2. X-ray crystal structures of (a) 2, (b) 10, and (c) 3. Hydrogen atoms
except for NH proton and solvent molecules are omitted for clarity. The thermal
ellipsoids are drawn at the 50% probability level.
/
0
Δ
I/
Δ
-100
-1
-2
glum = –1.5 x 10–3
glum = –1.2 x 10–3
(R,P)-5
(R,P)-6
-200
8
6
4
We then synthesized another type of helical carbazole-based
BF2 dyes 3 and 4 with triphenylene units (Scheme 1b). The
Suzuki–Miyaura coupling of 3,6-dibromocarbazole with 2-
biphenylboronic acid gave 3,6-bis[(1,1’-biphenyl)-2-yl]carbazole
(9). Oxidative fusion reaction with FeCl3 afforded p-extended
aza[7]helicene 10 completely regioselectively. This regiospecific
reaction at 4- and 5-positions is surprising because 10 is the most
strained regioisomer of the three possible regioisomers.[12] The
structure of helical p-extended carbazole 10 was confirmed by X-
ray diffraction analysis (Figure 2b).[11b] Treatment of 10 with NBS
gave monobrominated compound 11. Direct arylation of 11 with
benzoxazole and benzothiazole, followed by the BF2
complexation afforded (rac)-3 and (rac)-4, respectively. The
structure of 3 was determined by X-ray diffraction analysis to
show the p-extended helical carbazole-based BF2 dye (Figure
2c).[11c] The aza[7]helicene moiety of 3 is quite similar to the
structure of 10. We then attempted the optical resolution of 3 and
4 using a chiral HPLC column, which resulted in failure because
2
ε
0
200
300
400
500
600
500
550
600
650
700
750
800
wavelength / nm
wavelength / nm
(d)
7
1
2
10
3
4
5
6
Figure 3. CD, UV/vis, CPL, FL spectra of (a) 1 and 2, (b) 3 and 4, and (c) 5 and
6 in 1,4-dioxane. Rotatory strengths of (P)-enantiomers/diastereomers are
inserted in the spectra. (d) Photographs of 1,4-dioxane solutions of 1–7 and 10
under black light (l = 365 nm).
The chiroptical properties of optically pure azahelicene-based
dyes 1–6 were investigated (Figure 3 and Table 1). The UV-vis
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10.1002/anie.202001186
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absorption spectra showed bands at 200–400 and 400–550 nm,
which are more red-shifted as compared to the parent
aza[7]helicenes such as 10 (411 nm). The CD spectra of
enantiomers of 1–4 showed mirror images. On the other hand, the
diastereomers of 5 and 6 showed negative split CD around 230
nm for the (R)-binaphthyl and nearly mirror images at 350–550
nm. The |gabs| values are 1.1 ×10–3–3.1 ×10–3 at the longest
absorption maxima (ca. 500 nm). They showed fluorescence in
the visible region with the quantum yields of 0.20–0.36.[13] In
DFT calculations were performed to investigate the molecular
orbitals (Figure 4 and S8 in SI).[15] HOMO and LUMO of 1–4 have
electronic coefficients mainly at the aza[7]helicene moieties and
benzoazole moieties, respectively. This trend is also seen for 5
and 6, while small contributions at the binaphthyl moieties were
observed for the HOMO. These results suggest intramolecular
charge transfer (ICT) from the azahelicene moiety to the BODIPY
framework, which is responsible for the CD and CPL in the visible
region.[4f,8m] In addition, ICT from the chiral binaphthyl units may
contribute to the enhancement of CPL of 5 and 6.[16] Thus the
results of DFT calculations are consistent with the experimental
chiroptical properties.
addition, CPL was also observed in the same region with the |glum
|
values of 7.0 × 10–4–1.9 × 10–3. The higher |glum| values in
comparison to the previously reported nonhelicenic dyes with a
binaphthyl unit (3.0 ×10–4–5.2 ×10–4) are probably ascribed to
the CPL ability of the [7]helicene framework superior to that of the
binaphthyl group.[10e,14] Interestingly, the chiroptical properties of
helical chirality were enhanced by the attachment of axial chirality.
Specifically, 5 and 6 showed more intense Cotton effect and CPL
than 3 and 4, respectively. In addition, (R,P)-5 and (R,P)-6
showed more intense Cotton effect and CPL than (R,M)-5 and
(R,M)-6, which suggests that the (P)-aza[7]helicene moiety and
(R)-binaphthyl group enhanced the effect of chirality. We then
investigated the chiroptical properties of 10 to evaluate the gabs
and glum values of 3–6. Optical resolution of 10 was achieved by
the diastereomer method (SI), and CD and CPL of
enantiomerically pure 10 were measured. The gabs and glum values
of (P)-10 are –1.8 ×10–3 at 411 nm and –1.9 ×10–3 at 428 nm,
respectively, which are comparable to those of 3–6. This result
indicates that 3–6 have effective p-extension without significant
loss of chirality of the azahelicene moieties, which allows the
distinct CPL reaching the NIR region. Considering the facile
synthesis of 3–6 and the chiroptical properties comparable to 1
and 2, triphenylene-based aza[7]helicene 10 is a useful platform
for helical CPL dyes including 3–6.
(a)
(b)
(c)
Figure 4. HOMO (bottom) and LUMO (top) of (a) (P)-1, (b) (P)-3, and (c) (P)-5
calculated at the B3LYP/6-31G* level.
In summary, we have synthesized helical carbazole-based
BODIPY analogs 1–6 via aza[7]helicenes 7 and 10. p-Extended
azahelicene 10 was readily synthesized for the first time in 90%
yield in
2
steps from commercially available 3,6-
Table 1. Chiroptical properties in 1,4-dioxane
dibromocarbazole. The novel azahelicene 10 was characterized
by X-ray diffraction analysis, and 10 showed CPL in a blue region
with glum of –1.9 ×10–3. 7 and 10 were further converted into
helical carbazole-based boron complexes 1–6. X-ray crystal
structures of 2 and 3 revealed the structures of azahelicene-
[a]
[b]
compd
(P)-1
l
A [nm]
gabs
l
F [nm]
FF
glum
495
508
508
524
508
509
530
532
411
–2.7 ´ 10–3
–3.1 ´ 10–3
–1.2 ´ 10–3
–1.1 ´ 10–3
–2.3 ´ 10–3
+1.5 ´ 10–3
–1.8 ´ 10–3
+1.5 ´ 10–3
–1.8 ´ 10–3
568[c]
594[d]
566[d]
592[e]
576[d]
571[d]
605[e]
602[e]
428[f]
0.218
0.200
0.330
0.210
0.296
0.364
0.204
0.265
0.310
–1.7 ´ 10–3[g]
–1.3 ´ 10–3[g]
–8.7 ´ 10–4[g]
–7.0 ´ 10–4[h]
–1.5 ´ 10–3[g]
+1.2 ´ 10–3[g]
–1.2 ´ 10–3[h]
+8.8 ´ 10–4[h]
–1.9 ´ 10–3[f]
(P)-2
incorporated BF2 dyes. As
a result of p-extended chiral
conjugated system, 1–6 showed red-shifted CPL. Attachment of
the axially chiral binaphthyl to helically chiral 3 and 4 improved the
chiroptical properties of 5 and 6. Further development of chiral
carbazole-based boron complexes and azahelicene-based chiral
dyes are currently underway in our laboratory.
(P)-3
(P)-4
(R,P)-5
(R,M)-5
(R,P)-6
(R,M)-6
(P)-10
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
18K05083, Tokuyama Science Foundation, and Wesco Scientific
Promotion Foundation. We thank Prof. A. Osuka and Dr. T.
Tanaka (Kyoto University) for X-ray diffraction analysis,
fluorescence measurements and optical resolution, and Prof. H.
Yorimitsu (Kyoto University) for mass measurements.
[a] gabs = De/e at lA. [b] glum = 2(IL–IR)/(IL+IR). Average value in the range of
F ± 10 nm. [c] Excited at l = 460 nm. [d] Excited at l = 480 nm. [e] Excited
l
at l = 500 nm. [f] Excited at l = 320 nm. [g] Excited at l = 360 nm. [h] Excited
at l = 400 nm.
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Keywords: carbazoles • helicenes • circularly polarized
luminescence • BODIPY • dyes/pigments
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[11] a) Crystal data for 2: Formula: C35H19N2SBF2, Mw = 548.39, monoclinic ,
space group C 2/c, a = 28.920(8), b = 10.056(3), c = 20.328(8) Å, b =
92.522(9)°, V = 5906(3) Å3, Z = 8, rcalcd = 1.234, T = –180 °C, 35078
measured reflection, 4751 unique reflections (Rint = 0.1197), R1 = 0.0984,
wR2 = 0.2905 (all data), GOF = 1.038; b) Crystal data for 10: Formula:
C36H21N, Mw = 467.54, monoclinic , space group P 21/a, a = 12.156(2), b
= 13.5034(15), c = 13.738(2) Å, b = 98.885(4)°, V = 2227.9(5) Å3, Z = 4,
rcalcd = 1.394, T = –180 °C, 30306 measured reflection, 4439 unique
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10.1002/anie.202001186
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reflections (Rint = 0.0223), R1 = 0.0390, wR2 = 0.1182 (all data), GOF =
G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J.
J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R.
Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S.
Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, GAUSSIAN 09 (Revision
C.01), Gaussian, Inc., Wallingford CT, 2010.
1.024; c) Crystal data for 3: Formula: C43H23N2OBF2·1.5(C6H5Cl), Mw
=
801.27, triclinic, space group P–1, a = 8.8018(15), b = 13.706(2), c =
16.431(5) Å, a = 78.63(2), b = 89.34(3), g = 73.00(2)°, V = 1856.1(8) Å3,
Z = 2, rcalcd = 1.434, T = –180 °C, 21023 measured reflection, 5692
unique reflections (Rint = 0.0438), R1 = 0.0963, wR2 = 0.2594 (all data),
GOF = 1.032. CCDC 1976458 (2), 1976459 (10), and 1976460 (3)
contains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystal Data
Centre via www.ccdc.cam.au.uk/data_request/cif.
[12] The HOMO of 9 exhibited large electronic coefficients at the 4- and 5-
positions of carbazole but no coefficients at the 2- and 7-positions, which
supports the selective synthesis of 10 (Figure S7 in SI).
[16] The aza[7]helicene moieties of the optimized structures of (P)-3 and
(R,P)-5 are quite similar, and ICT is more plausible than a simple chiral
perturbation exerted by the BINOL moiety on the chiral helicenic
chromophore.
[13] We investigated the solvent effect on the fluorescence quantum yield of
3 (Figure S4 in SI).
[14] Related [7]helicenes also showed comparable glum values.[4e]
[15] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson,
H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino,
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Helical carbazole-based boron
complexes were readily synthesized
C. Maeda,* K. Nagahata, T. Shirakawa,
T. Ema*
CPL
via aza[7]helicenes. The p-conjugated
system expanded from the helicene
moiety to the BODIPY framework, and
the azahelicene-fused boron
complexes showed the Cotton effects
and the distinct circularly polarized
luminescence (CPL) in the visible
Page No. – Page No.
Azahelicene-Fused BODIPY Analogs
Showing Circularly Polarized
Luminescence
region.
HELICENE + BODIPY framework
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