The substituent effect of bridged triarylamine helicenes on light-emitting and charge transfer properties

Article abstract of DOI:10.1246/cl.200089

Methoxy-(7a), bromo-(7b), CF₃-(7c) and hydroxyl-(7d) substituted bridged triarylamine helicenes were synthesized under mild conditions in 6598% yield. Their luminescence red-shifted with π-donating substituents and blue-shifted with the electro

Full text of DOI:10.1246/cl.200089

Advance Publication Cover Page
The Substituent Effect of Bridged Triarylamine Helicenes on Light-emitting and
Charge Transfer Properties
Chenting Yan, Rong Shang,* Masaaki Nakamoto, Yohsuke Yamamoto,* and Yohei Adachi
Advance Publication on the web February 27, 2020
doi:10.1246/cl.200089
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2020 The Chemical Society of Japan
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1
The Substituent Effect of Bridged Triarylamine Helicenes on Light-emitting and Charge Transfer
Properties
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Chenting Yan, Rong Shang, * Masaaki Nakamoto, Yohsuke Yamamoto, * Yohei Adachi
Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526
1
2
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima
39-8526
7
E-mail: yohsuke@hiroshima-u.ac.jp
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9
0
The methoxy-(7a), bromo-(7b), CF -(7c) and hydroxyl-
3
(7d) substituted bridged triarylamine helicenes were
synthesized under mild conditions in 65-98% yield. Their
luminescence red-shifted with p-donating substituents and
N
3
blue-shifted with the electron-withdrawing CF -substituent.
O
O
The absorption and emission of the hydroxyl-substituted
derivative (7d) was found to be pH-dependent, particularly
under alkaline conditions. Moreover, all derivatives of 7
showed additional reversible negative redox potential, which
may be useful as charge transfer materials.
t-Bu
1
Bridged Triarylamine R
Venkataraman, 2003
4
4
4
7
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9
1
1
1
2
Keywords: bridged triarylamine helicenes, charge transfer,
pH response
Figure 1. The molecule structure of bridged triarylamine R.
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Triarylamine helicenes 7 a-c were synthesized in 5 steps
from 1. The hydroxyl-derivative 7d were synthesized from
the 5a in two steps (Scheme 1). Starting from the
commercially available 5-tert-butyl-m-xylene 1 via 2-bromo-
5-tert-butyl-1,3-dimethylbenzene 2 and 2-bromo-5-tert-
butyl-isophthalic acid 3, the bromo substituted isophthalate 4
was readily accessible. Diamino isophthalate derivatives 5
were subsequently obtained by Cu-catalyzed Ullmann
coupling amination of 4 with methoxy-, trifluoromethyl- and
bromo- substituted diarylamines, respectively. It should be
noticed that these aminations cannot be catalyzed by
palladium (0) catalysts. Compound 5 can be hydrolyzed to
the corresponding triarylamine diacids in nearly quantitative
yield. Without further purification, the diacids were treated
with concentrated sulfuric acid, following in situ cyclization
to generate 7a-c in 65-98% yield. Unlike another reported
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Triarylamine compounds featuring a planar geometry
backbone with electron-rich p-conjugation are often colored
luminescent molecules that can be easily oxidized. They have
attracted considerable attention owing to their potential in
growing number of applications, such as organic light-
emitting diodes (OLED), solar cells and organic photovoltaic
1
, 2
devices.
Recently, the research on the hetero-helicenes
have been expanding rapidly due to their properties not only
3
4
on optical and electronic chemistry but also on bioorganic
5
6
chemistry and asymmetric synthesis .
Closely related to the luminescence and charge transfer
properties of triarylamine compounds is the energy gap of the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO). The HOMO and
LUMO energies can be effectively adjusted by i) introducing
electron-donating or withdrawing groups on the aryl groups
and/or ii) extending the p-system by annulation. Therefore,
detailed substituent effects and simple synthetic methods to
annulate of phenyl groups for p-extension are highly
8
method, this cyclization can be achieved by concentrated
sulfuric acid as the only reaction reagent at room temperature
under air. In addition, 2-(bis (4-hydroxyphenyl) amino)-5-
(tert-butyl) isophthalic acid 6 was synthesized by one pot
reaction of 5a with 1-dodecanethiol and potassium tert-
butoxide. Using the same method, the hydroxy substituted 7d
was generated and isolated.
2
, 7
desirable for tuning both the charge transfer properties and
light emitting properties.
In 2003, Venkataraman reported a bridged triarylamine
helicene R, in which the triarylamine skeleton forced into a
8
helical structure by four o,o’-carbonyl bridged bonds. R is a
highly luminescent chromophore with a high one-electron
oxidation potential (E1/2 = 1.57 V). However, the substituents
effect of this molecule has not been reported. Herein we
report a new and easy synthetic route to synthesize methoxy-
3
(7a), bromo-(7b), CF -(7c) and hydroxyl-(7d) substituted
bridged triarylamine helicene derivatives. The compounds 7
were characterized by optical and electronic spectra analysis,
the solid structures of 7a-c were characterized by singlet X-
ray crystallographic.

2
Br2, Fe,
CHCl3
KMnO4
t-BuOH / H O
Br
O
Br
O
4
4
2
3
Furthermore, adding 0.5 mL of different standard pH aqueous
MeOH, H2SO4
95 ºC, 24 h
2
HO
OH
into the different solvent (5 mL) which 7d was totally
r.t.,3 h
95 ºC, 24 h
t-Bu
-4
t-Bu
t-Bu
y.97%
44 dissolved (10 M) shows the pH response in the range of pH
45 8 to 13 in different solvent (Figure S1-S11).In particular, 7d
1
2
y.90%
3
4
4
6
7
in ethanol changed from yellow to violet and greyish blue as
pH changed from 8 to 10 and 13 respectively. A similar trend
X
X
X
X
O
Br
O
(X-C6H5)2NH
Cu
K2CO3
1) NaOH
N
48 was also observed in acetonitrile solutions (Figure 5),
suggesting 7d may be used as a pH probe in alkaline solutions
50 of ethanol and acetonitrile.
MeO
OMe
O
N
O
EtOH/H2O
O
O
4
9
MeO
OMe
n-Bu2O
t-Bu
2) conc.H2SO4
r.t.
1
80°C, 90 h
t-Bu
Ullmann reaction
t-Bu
(X=OMe) y.32%
5b (X=Br) y. 56%
5
1
5
a
7a (X=OMe) y.65%
7b (X=Br) y. 98%
7c (X=CF3) y.90%
4
y.82%
5
c
(X=CF3) y.48%
MeO
MeO
OMe
HO OH
HO
OH
t-BuOK, n-C12H25SH
conc.H SO
4
N
O
N
O
O
N
O
2
O
O
OMe
DMF
HO
OH
r.t
1
80°C, 48 h
t-Bu
7d y. 44%
t-Bu
t-Bu
6 y.55%
1
2
3
5
a
Scheme 1. Total synthetic steps to the triarylamine helicenes 7.
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0
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2
3
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5
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7
8
9
0
1
Triarylamine helicenes 7a, 7b and 7c are yellow solids
2 2
Figure 2. Solution (CH Cl ) and solid color of 7a-c under visible light
that are fluorescent in solution (Figure 2). Their absorption
spectra obtained in CH Cl at room temperature showed
(upper), solution (CH
365 nm (below).
2
Cl
2
) and solid color of 7a-c under ultraviolet light
2
2
maximum absorptions at 480 nm, 446 nm and 426 nm,
corresponding to n-π* (HOMO-LUMO) transitions (Figure
3). The p-donating substituents in 7a and 7b result in a red-
1
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4
4
shift compared with the parent bridged triarylamine R (lmax.=
8
442). In contrast, the electron-withdrawing trifluoromethyl
substituents results in a blue-shift in comparison with R. This
blue-shift trend starting from 7a to 7c indicates the prominent
electronic effect of substituents, which could be reproduced
by theoretical calculations using time-dependent density
functional theory (TDDFT) methods. The results using
B3PW91/6-31G(d) basis set corroborate well with the crystal
structures of 7a-c (Figure S14 and Table S2). The HOMO-
LUMO energy gaps are smaller in 7a (DEHOMO-LUMO= 3.243
eV) and 7b (DEHOMO-LUMO= 3.480 eV) and larger in 7c
(DEHOMO-LUMO= 3.673 eV) in comparison to the parent
triarylamine helicene R (DEHOMO-LUMO= 3.598 eV).
58
-
5
6
6
9
0
1
Figure 3. Absorption spectra (solid) and emission spectra (dashed) of 10
M 7a, 7b and 7c in CH Cl at 25°C.
2 2
Moreover, the fluorescence emission of 7a, 7b and 7c in
CH Cl were observed at 521 nm, 466 nm and 438 nm,
2 2
5
respectively. This is consistent with the trend in absorption,
showing a significant substituent-dependence (Figure 3). The
9
small Stokes shifts (41nm for 7a, 20 nm for 7b, 12 nm for
7c) were due to the lack of energy loss accompany with
conformational changes during the excitation-emission
process, implying their backbone rigidity. The quantum
yields and lifetime of 7a, 7b and 7c is 38.22 %, 9.17 %,
20.01 %and 13.98 ns, 1.77 ns, 2.46 ns, respectively.
In attempt to obtain a solvent and/or pH-dependent
phosphor, the hydroxy-substituted derivative 7d was isolated
2 2
as a red solid. Owing to the low solubility in CH Cl , the UV-
Vis measurement of 7d was performed by using more polar
solvents. Its absorption spectra in different solvent showed a
strong solvent effect (Figure 4 and Figure S12). The
maximum absorption in ethyl acetate, THF, acetone, ethanol,
6
6
2
3
-4
Figure 4. Absorption spectra of 10 M 7d in different solvent at 25°C.
3
DMF, CH CN, DMSO were observed at 481 nm, 486 nm,
484 nm, 494 nm, 491 nm, 484 nm and 495 nm, respectively.

3
C-C
1.4705(19) (a)
1.4698(18) (b)
1.472(3) (a)
1.467(3) (b)
1.469(3) (a)
1.463(3) (b)
359.91
1.470(2) (a)
1.467(2) (b)
1.471(2) (a)
1.465(2) (b)
359.40
1.449(11) (a)
1.450(12) (b)
1.449(11) (a)
1.450(12) (b)
360.01
(
& )
a b
1
.4691(18) (a)
1
.4705(17) (b)
359.99
N
S a
F
C4C2C3C5
41.04
40.58
40.81
45.47
2
2
2
2
2
2
2
2
1
2
In all structures, the central nitrogen atoms are
3 essentially planar, with the sum of the angels around
4 nitrogen (SNa) being close to 360°. Furthermore, their
5 terminal ring B and C are twist, the torsion angles
6 (FC4C2C3C5) of 7a-c are slightly smaller than the bridged
7 triarylamine R, indicating the trivial electronic effect from
8 the para-substituent, which perturbs the electron density
1
2
3
-
5
3
Figure 5. The solution color of 9.1´10 M 7d in ethanol or CH CN in
different pH.
4
5
6
7
8
9
0
Single crystals of 7a-c suitable for X-ray
crystallographic studies were obtained by vapor diffusion of
CH Cl into a hexane solution at room temperature. Their
2 2
solid-state molecular structures and selected bonding
parameters shown in Figure 6.
29 over p conjugation in the molecule. In addition, the
30 significant shorter N-C1 distances in comparison to N-
31 C2/C3 distances in all structures, suggesting the more
32 effective delocalization to ring A (Table 1). These
33 comparative results are consistent with the corresponding
34 theoretical data (Table S3).
1
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0
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7
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9
0
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2
3
4
5
6
7
Cyclic voltammetry (CV) of the 7a, 7b and 7c in CH
2 2
Cl
and 7d in DMF at room temperature with TBAPF as a
6
supporting electrolyte are shown in Figure 7. 7a and 7b show
a reversible redox wave at E1/2 = – 1.37 V and E1/2 = – 1.25
V, respectively. In addition, a quasireversible wave of 7a and
7b was observed at E1/2 = 1.38 V and E1/2 = 1.77 V,
respectively, which is similar to the reported parent bridged
8
triarylamine (E1/2 = 1.57 V). However, 7c and 7d only shows
a reversible one-electron oxidation wave at E1/2 = – 1.21 V
and E1/2 = – 1.00 V, respectively.
The p donation effect of methoxy groups is prominent
shown in the lower oxidation potential observed in 7a, due to
1
1
1
2
an increased p-electron density. In contrast, the
s
withdrawing effect of trifluoromethyl groups is clearly
reflected in the higher oxidation potential of 7c. Because of
the negative redox potentials, triarylamine helicenes 7 can be
expected to as electron-donors to form CT complexes with
suitable electron-acceptors. To verify our speculation, 7b was
1
1
1
1
1
3
4
5
6
7
Figure 6. Solid-state molecular structures of 7a-c. Thermal ellipsoids are
set at 50% probability. Ellipsoids of periphery atoms, hydrogen atoms
10
are omitted for clarity.
Table 1. Selected experimental bond lengths (Å), angles and dihedral
angles [°] for 7a-c and bridged triarylamine R.
treated
with
one
equivalent
amount
of
1
8
tetracyanoquinodimethane (TCNQ), which is an electron
acceptor. The solution color changed to green, and yellow
solid precipitated from the solution, indicating the generation
X
X
C4
C2
C5
C3
B
C
N
N
!
!
58 of CT complexes.
59
C1
A
O
O
O
"
"
O
Front view
t-Bu
t-Bu
1
2
9
0
R
7
a
7b
7c
R
N-C1
N-C2
N-C3
C=O
1.3951(16)
1.4174(16)
1.4236(16)
1.2285(16)
1.397(3)
1.421(3)
1.412(3)
1.226(3)
1.222(3)
1.3983(19)
1.4166(19)
1.416(2)
1.388(14)
1.425(9)
1.425(9)
1.232(9)
1.232(9)
1.228(2)
1
.2319(15)
1.226(2)

4
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8 References and Notes
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1
Y. Shirota, H. Kageyama, Chem. Rev. 2007, 107, 953; T. D. Selby,
K.-Y. Kim, S. C. Blackstock, Chem. Mater. 2002, 14, 1685; M.
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Yun, M. H. Park, Y. Do, S. Yoo, M. H. Lee, Macromolecules 2009,
42, 6840; T. Okamoto, E. Terada, M. Kozaki, M. Uchida, S.
Kikukawa, K. Okada, Org. Lett. 2003, 5, 373; E. Bellmann, S. E.
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T. J. Katz, Angew. Chem. Int. Ed. 2000, 39, 1921; T. J.
Wigglesworth, D. Sud, T. B. Norsten, V. S. Lekhi, N. R. Branda,
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Y. Xu, Y. X. Zhang, H. Sugiyama, T. Umano, H. Osuga, K.
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Shibata, K. Soai, Angew. Chem. Int. Ed. 2001, 40, 1096; S. D.
Dreher, T. J. Katz, K.-C. Lam, A. L. Rheingold, J. Org. Chem.
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Diederich, J. Org. Chem. 1991, 56, 6787.
2
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5
6
1
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3
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5
6
7
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9
0
1
Figure 7. Cyclic voltammetry measurement of 7a-c (1.0 mM) was
performed by using an ALS 600D potentiostat / galvanostat in CH Cl or
DMF (7d) solution containing 100 mM of [nBu N] [PF ] with a scan rate
of 100 mV/s in ambient atmosphere at room temperature. A three-
electrode cell, which was equipped with a glassy carbon disk working
electrode, a Pt wire counter electrode, and SCE reference electrode, was
used. The half wave potentials of 7 was compensated with that of
ferrocene/ferrocenium redox cycle, which is +0.46 V (vs. SCE).
2
2
4
6
7
8
N. Islam, A. H. Pandith, J. Mol. Model. 2014, 20, 2535.
J. E. Field, T. J. Hill, D. Venkataraman, J. Org. Chem. 2003, 68,
6071.
J. R. Albani, Structure and dynamics of macromolecules:
absorption and fluorescence studies, Elsevier, 2011.
CCDC numbers: 7a, 1981425; 7b, 1981427; 7c, 1981426.
9
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10
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9
0
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2
3
4
5
6
7
8
9
0
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2
3
4
5
6
7
In conclusion, we provide a new and mild synthetic
route to synthesize and isolate the bridged triarylamine
helicenes 7a-d. The UV-Vis spectra of natural compounds
show a blue-shift starting from 7a to 7c, indicating the
electron-withdrawing group increases the HOMO-LUMO
gap whereas the electron-donating group decreases the
HOMO-LUMO gap. These results are consistent with the
theoretical studies. 7a shows higher quantum yields and
longer lifetime in comparison with 7b and 7c. The hydroxy-
substituted 7d shows solvent- and pH-dependence
luminescence in alkaline conditions. These suggests their
potential application in pH probing. In addition, the CV
voltammograms of 7 reveals that these bridged triarylamine
helicenes have very low (in the negative region) oxidation
potentials, which demonstrates their potential as hole
transport materials.
The authors thank Prof. Joji Ohshita for the use of their
instruments and discussion.
This work was supported by JSPS KAKENHI Grant
(24109002) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Supporting
Information
is
available
on
http://dx.doi.org/10.1246/cl.******.