Chiral supramolecular thiophene fluorophore consisting of thiophenecarboxylic acid derivatives

Article abstract of DOI:10.1016/j.tet.2011.07.086

Solid-state chiral supramolecular thiophene fluorophore has been successfully prepared by using chiral (R)-1-(2-naphthyl)ethylamine and 5-bromothiophene-2-carboxylic acid. This chiral supramolecular thiophene fluorophore is formed by assembling chiral 2₁-helical columnar network structures composed of (R)-1-(2-naphthyl)ethylamine and 5-bromothiophene-2-carboxylic acid. This supramolecular organic fluorophore exhibits circularly polarized luminescence (CPL) even in the solid state.

Full text of DOI:10.1016/j.tet.2011.07.086

Tetrahedron 67 (2011) 7775e7779
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Chiral supramolecular thiophene fluorophore consisting of thiophenecarboxylic
acid derivatives
Takaya Kimoto a, Naoki Shiota a, Takafumi Kinuta a, Tomohiro Sato a, Nobuo Tajima b, Hayato Tokutome c,
,
Reiko Kuroda c, Michiya Fujiki d, Yoshio Matsubara a, Yoshitane Imai a
*
a Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
b Computational Materials Science Center, National Institute for Materials Science 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
c Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
d Graduate School of Materials Science, Nara Institute of School and Technology, Takayama, Ikoma, Nara 630-0192, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 June 2011
Received in revised form 27 July 2011
Accepted 27 July 2011
Available online 2 August 2011
Solid-state chiral supramolecular thiophene fluorophore has been successfully prepared by using chiral
(R)-1-(2-naphthyl)ethylamine and 5-bromothiophene-2-carboxylic acid. This chiral supramolecular
thiophene fluorophore is formed by assembling chiral 21-helical columnar network structures composed
of (R)-1-(2-naphthyl)ethylamine and 5-bromothiophene-2-carboxylic acid. This supramolecular organic
fluorophore exhibits circularly polarized luminescence (CPL) even in the solid state.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Chiral
Circularly polarized luminescence (CPL)
Fluorescence
Supramolecule
Thiophene
1. Introduction
is composed of a 21-helical columnar hydrogen- and ionic-bonded
network structure formed by the association of (R)-1 and 2-
The solid-state optical property of organic compounds finds
remarkable application in the development of new functional or-
ganic materials. The solid-state organic fluorescence property, in
particular, has attracted considerable attention in the field of op-
toelectronics owing to its application in organic electrolumines-
cence (EL) devices and optical sensors.1 Therefore, many solid-state
organic fluorophores composed of a single organic molecule have
been reported, of which two-component supramolecular organic
fluorophores have recently attracted greater attention.2 However,
most of the previously reported supramolecular organic fluo-
rophores lack chirality and hence do not possess solid-state chiral
optical properties, such as solid-state circular dichroism (CD) or
circularly polarized luminescence (CPL) properties.
anthracenecarboxylic acid. The advantage of these supramolecu-
lar systems is that their physical and chemical properties can be
altered by changing the component molecules.
Recently, a large number of organic functional materials con-
sisting of a polythiophene backbone have been reported.4 These
polythiophene compounds show various electrochemical and
photochemical properties. Therefore, it is expected that novel solid-
state chiral supramolecular fluorophores may be prepared by using
thiophene derivatives as one of the component molecules in su-
pramolecular complexes.
In this study, we prepare a novel solid-state chiral supramo-
lecular thiophene fluorophore with (R)-1, and study its solid-state
optical properties and crystal structures. The following three ba-
sic thiophene acid derivatives are used for the preparation: 2-
thiophenecarboxylic acid (2a), 5-chlorothiophene-2-carboxylic
acid (2b), and 5-bromothiophene-2-carboxylic acid (2c).
We recently developed a solid-state chiral supramolecular or-
ganic fluorophore composed of (R)-1-(2-naphthyl)ethylamine [(R)-
1] as the chiral component molecule and 2-anthracenecarboxylic
acid as the fluorescent component molecule.3 The structural char-
acteristic of this chiral supramolecular organic fluorophore is that it
* Corresponding author. Tel.: þ81 06 6730 5880x5241; fax: þ81 06 6727 2024;
e-mail address: y-imai@apch.kindai.ac.jp (Y. Imai).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.07.086

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T. Kimoto et al. / Tetrahedron 67 (2011) 7775e7779
2. Results and discussion
The (R)-1/2a chiral supramolecular thiophene fluorophore was
first prepared via crystallization from a methanol (MeOH) solution.
(R)-1 and 2a were dissolved in the MeOH solution and left to stand
at room temperature. After a week, a large number of crystals were
obtained. However, because good-quality crystals could not be
obtained, the X-ray analysis was unsatisfactory. Therefore, X-ray
powder diffraction (XRPD) analysis of these obtained crystals was
carried out, and the results were compared to those of the XRPD
analysis of component molecules (R)-1 and 2a (Fig. 1).
Fig. 1. X-ray powder diffraction (XRPD) patterns of (a) complex I, (b) component
molecule (R)-1, and (c) component molecule 2a.
Fig. 2. CD and absorption spectra of complexes (a) II and (b) III (black lines) and
complexes II0 and III0 (gray lines) in the solid state (using KBr pellets).
It was found that the peaks observed in molecules (R)-1 and 2a
disappeared completely and a new set of peaks had appeared. This
XRPD and 1H NMR5 analyses revealed that novel chiral supramo-
lecular thiophene complex I composed of (R)-1 and 2a was
obtained without the crystallization solvent MeOH.
and 289 nm for III) for II and III are approximately 6.8ꢀ10ꢁ4 and
1.4ꢀ10ꢁ3, respectively. In order to check whether the complexes II
and III introduced any artifacts in the spectrum, complexes II0 and
III0 were prepared, wherein (R)-1 was replaced with (S)-(ꢁ)-1-(2-
naphthyl)ethylamine [(S)-1]. Interestingly, when the solid-state
CD spectra of complexes II0 and III0 were recorded (indicated by
gray lines in Fig. 2), they were found to be mirror images of the CD
spectra of complexes II and III. These results indicate that effective
chiral transfer occurs from chiral molecule 1 to complexes II and III
through complexation.
Consequently, the solid-state CPL spectra of fluorescent com-
plexes III and III0 (that show the solid-state fluorescence property)
were recorded using KBr pellets. The CPL spectra were successfully
observed in the solid state. The solid-state CPL and fluorescence
spectra of complexes III and III0 are shown in Fig. 3. Interestingly,
the fluorescence spectra of these complexes recorded using KBr
pellets are similar to that obtained without using KBr matrices. This
result shows that the KBr matrix does not play any role in obtaining
the spectrum.
A negative solid-state CPL spectrum was obtained for complex
III whose circular anisotropy factor [jgem¼2(ILꢁIR)/(ILþIR)j] is ap-
proximately 2.6ꢀ10ꢁ3. This shows that complex III exhibits CPL in
the solid state. To the best of our knowledge, this is the first in-
stance of solid-state CPL being observed in a chiral supramolecular
thiophene organic fluorophore.
To study the crystal structures of complex II and fluorescent
complex III, X-ray crystallographic analyses of complexes II and III
were carried out, and the crystal structures obtained were com-
pared. The crystal structures of complex II are shown in Fig. 4. The
stoichiometry of II is (R)-1/2b¼1:1, and its space group is P212121.
This complex has a characteristic 21-helical columnar network
structure along the a-axis (Fig. 4a). This column is mainly composed
of the carboxylate oxygen of the carboxylic acid anions and the
ammonium hydrogen of the protonated amine. Complex II is
formed by the assembly of these 21-helical columns (indicated in
The most serious complication associated with solid-state or-
ganic fluorophores is the occurrence of fluorescence quenching in
the crystalline state. In order to study the solid-state optical
properties of the obtained complex I, it is important to record the
solid-state fluorescence spectrum. However, in the present case,
the solid-state fluorescence of complex I was hardly observed.
This prompted us to attempt the preparation of a chiral
substituted supramolecular thiophene complex. The (R)-1/2b,c
chiral supramolecular thiophene fluorophores were prepared via
crystallization from a MeOH solution in the same manner as that
used for the preparation of (R)-1/2 supramolecular complex I. (R)-1
and 2b (or 2c) were dissolved in the MeOH solution and left to
stand at room temperature. After a week, a large number of crystals
of complex II composed of (R)-1 and 2b, or crystals of complex III
composed of (R)-1 and 2c were obtained.
To first study the solid-state optical properties of the obtained
supramolecular thiophene complexes II and III, the solid-state
fluorescence spectra of these complexes were recorded. The
solid-state fluorescence property of complex II was also hardly
observed (similar to complex I). Interestingly, complex III exhibited
fluorescence even in the solid state. In the solid state, fluorescence
maximum (lem) for complex III is observed at 412 nm, and the
absolute value of the photoluminescence quantum yield (FF) for
complex III is 0.38. This fluorescence of complex III is peculiar to
the solid state. The fluorescence property of mixture of (R)-1 and 3
in MeOH solution was hardly observed.
In order to study the trends in the solid-state chiral optical
property of supramolecular thiophene fluorophore, the solid-state
CD spectra of complex II and fluorescent complex III were recor-
ded using KBr pellets. The solid-state CD and absorption spectra of
complexes II and III (indicated by black lines) are shown in Fig. 2.
The shapes of the CD spectra are similar. The circular anisotropy
factors (jgCD
¼
D
OD/ODj) of the last Cotton effect (lCD¼297 nm for II

T. Kimoto et al. / Tetrahedron 67 (2011) 7775e7779
7777
Fig. 3. CPL and fluorescence spectra of complexes III (black lines) and III0(gray lines) in
the solid state (using KBr pellets).
Fig. 5. (a) Crystal structures of complex III: (a) 21-helical columnar hydrogen- and
ionic-bonded network along a-axis and (b) packing structure observed along a-axis.
The solid red arrow
A and blue arrow B indicate intercolumnar naph-
thaleneenaphthalene edge-to-face interactions. The dotted red circle indicates 21-
helical column.
blue arrow B in Fig. 4b, respectively) between naphthalene rings in
(R)-1 (Fig. 5b).6
We would like to mention here that the fluorescent property of
the complex is increased as the substituent in the thiophene ring
changed from chlorine (Cl) to bromine (Br). First, to study the origin
of the solid-state fluorescence of complex III, the solid-state fluo-
rescence spectra of component molecules (R)-1 and 2c were
recorded. Both component molecules showed fluorescence in the
solid state. The solid-state lem of (R)-1 and 2c is observed at 340 and
428 nm, respectively, and the FF of (R)-1 and 2c is 0.08 and 0.06 in
the solid state, respectively. This result suggests that the origin of
solid-state fluorescence in complex III is the thiophene unit in
complex. Next, to study the origin of the difference of solid-state
fluorescence between complexes II and III, the crystal structures
of II and III were compared in detail. Even though the crystal
structures of complexes II and III are nearly identical, the circum-
stances around Cl and Br in the crystal structure have a small dif-
ference. Br atom possesses a near contact toward carbon atom of
Fig. 4. Crystal structures of complex II: (a) 21-helical columnar hydrogen- and ionic-
bonded network along a-axis and (b) packing structure observed along a-axis. The
solid red arrow A and blue arrow B indicate intercolumnar naphthaleneenaphthalene
and naphthaleneethiophene edge-to-face interactions, respectively. The dotted red
circle indicates 21-helical column.
Fig. 4b by dotted red circle) by one intercolumnar naph-
ꢀ
thaleneenaphthalene edge-to-face interaction (2.81 A; indicated
ꢀ
by solid red arrow A in Fig. 4b) between naphthalene rings in (R)-1
the carboxylic moiety (3.55 A), whereas the distance between Cl
ꢀ
ꢀ
and naphthaleneethiophene edge-to-face interaction (3.00 A; in-
atom and carbon atom of the carboxylic moiety is 3.61 A. Such in-
dicated by solid blue arrow B in Fig. 4b) between the hydrogen
atom of the naphthalene ring in (R)-1 and the thiophene ring of 2b
(Fig. 4b).6
teraction may reduce the spin-orbit interaction in Br atom, and
decrease the nonradiative decay by enhancing the rigidity of the
structure. In addition, the solid-state fluorescence spectrum of
component molecule 2b was also measured. This component
molecule also showed fluorescence in the solid state. The solid-
state lem and FF is observed at 426 nm and 0.09, respectively.
This may mean that the Cl/Br group over molecule 2 affects the
excited state structures of complexes II and III rather than the
ground state structures as observed in the X-ray crystal structure.
The observed increase in fluorescence of complex III may be also
due to the increased stabilization of its excited singlet state due to
the presence of Br as compared to the Cl in complex II.
The crystal structures of fluorescent complex III are shown in
Fig. 5. The stoichiometry of III is (R)-1/2c¼1:1, and its space group is
P212121. Interestingly, in II and III, the structures of 21-helical col-
umns and their packing arrangements are almost identical. This
complex also has a shared 21-helical columnar network structure
along a-axis, as observed in complex II (Fig. 5a). III is formed by
assembling of these 21-helical columns (indicated in Fig. 5b by
dotted red circle) by two naphthaleneenaphthalene edge-to-face
ꢀ
interactions (2.79 and 2.98 A; indicated by solid red arrow A and

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T. Kimoto et al. / Tetrahedron 67 (2011) 7775e7779
3. Conclusion
4.4. Measurement of 1H NMR spectrum
A chiral supramolecular thiophene fluorophore has been suc-
cessfully prepared by combining chiral (R)-1-(2-naphthyl)ethyl-
amine and 5-bromothiophene-2-carboxylic acid. This thiophene
complex is found to be composed of chiral 21-helical columnar
network structure and shows CPL even in the solid state. This fur-
ther enhances the potential of this fluorescence system, enabling its
application in the design of novel solid-state chiral supramolecular
thiophene fluorophores. A study of the complexation behavior and
optical properties of the obtained complexes is expected to provide
useful information for designing novel supramolecular fluorophore
with thiophene units.
1H NMR spectrum was recorded with a JNM-AL400 spectrom-
eter in dimethyl sulfoxide(DMSO)-d6 (400 MHz).
4.5. Measurement of XRD spectra
X-ray powder patterns were recorded on a Rigaku RINT2500
using graphite-monochromated Cu K
The spectra were measured at room temperature between 3ꢂ and
35ꢂ in the 2 scan mode with steps 0.02ꢂ in 2 and 3ꢂ/min.
a radiation (40 kV, 80 mA).
q
q
4.6. Measurement of solid-state fluorescence spectra
solid-state fluorescence spectra and absolute photo-
A
4. Experimental
luminescence quantum yield were measured by Absolute PL
Quantum Yield Measurement System (C9920-02, HAMAMATSU
PHOTONICS K. K.) under air atmosphere at room temperature. The
excited wavelength is 358 nm for complex III. The excited wave-
length of (R)-1, 2b, and 2c is 258, 367, and 373 nm, respectively.
4.1. General methods
Component molecules (R)-1 and (S)-1 were purchased from Alfa
Aesar. Component molecule 2a was purchased from Sigmae
Aldrich. Component molecules 2b and 2c were purchased from
Tokyo Chemical Industry Co., Ltd. Crystallization solvent MeOH was
purchased from Wako Pure Chemical Industry. All reagents were
used directly as obtained commercially.
4.7. Measurement of solid-state CD and absorption spectra
The CD and absorption spectra were measured using a Jasco J-
800KCM spectrophotometer. The solid-state samples were pre-
pared according to the standard procedure for obtaining glassy KBr
matrices.10
4.2. Formation of complex by crystallization from MeOH
solution
4.8. Measurement of solid-state CPL spectrum
(R)-1 (8.6 mg, 5.02ꢀ10ꢁ5 mol) and each thiophenecarboxylic
acid derivative (5.04ꢀ10ꢁ5 mol) were dissolved in MeOH (3 mL)
under heat and left to stand at room temperature. After one
week, a large number of crystals [complex I for the (R)-1/2a
system (6 mg), complex II for the (R)-1/2b system (9 mg), and
complex III for the (R)-1/2c system (8 mg)] were obtained. The
weight of crystals reported here is the total weight obtained in
one batch.
The CPL spectrum was measured using a Jasco CPL-200 spec-
trophotometer. The excited wavelength is 350 nm. The solid-state
samples were prepared according to the standard procedure for
obtaining glassy KBr matrices. The power of an incident beam of the
CPL spectrometer is 8.0 m
W/0.04 cm2 at the installation position of
sample. The CPL spectrum is approached by Simple Moving Aver-
age (SMA).
Acknowledgements
4.3. X-ray crystallographic study of crystal
This study was supported by a Grant-in-Aid for Scientific Re-
search (No. 22750133 and 23111720) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan, and a research
grant from TEPCO Research Foundation.
The X-ray diffraction data for single crystal of complex II were
collected using RIGAKU SATURN 70R. The X-ray diffraction data for
single crystal of complex III were collected using Bruker Apex. The
crystal structures were solved by the direct method7 and refined by
full-matrix least-squares using SHELXL97.7 The diagrams were
drawn using PLATON.8 The absorption corrections were performed
using SADABS.9 The nonhydrogen atoms were refined with aniso-
tropic displacement parameters, and the hydrogen atoms were
included in the models at their calculated positions in the
riding-model approximation. Crystallographic data of II: C12H13N$
C5H3O2SCl, M¼333.82, Orthorhombic, space group P212121,
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2011.07.086.
References and notes
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