Preparation of a spontaneous resolution chiral fluorescent system using 2-anthracenecarboxylic acid

Article abstract of DOI:10.1039/b808242a

A spontaneous resolution chiral fluorescent system is prepared by using 2-anthracenecarboxylic acid, and racemic (rac)-1-phenylethylamine or rac-1-cyclohexylethylamine. Although chiral functional organic materials are usually prepared using chiral molecules, in this system, a chiral organic fluorophore, which exhibits fluorescence in the solid state, is prepared from achiral and rac molecules.

Full text of DOI:10.1039/b808242a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Preparation of a spontaneous resolution chiral fluorescent system using
2-anthracenecarboxylic acid†
Yoshitane Imai,*a Kensaku Kamon,a Katuzo Murata,a Takunori Harada,b Yoko Nakano,c Tomohiro Sato,b
Michiya Fujiki,c Reiko Kurodab,d and Yoshio Matsubara*a
Received 15th May 2008, Accepted 6th June 2008
First published as an Advance Article on the web 10th July 2008
DOI: 10.1039/b808242a
A spontaneous resolution chiral fluorescent system is prepared by using 2-anthracenecarboxylic acid,
and racemic (rac)-1-phenylethylamine or rac-1-cyclohexylethylamine. Although chiral functional
organic materials are usually prepared using chiral molecules, in this system, a chiral organic
fluorophore, which exhibits fluorescence in the solid state, is prepared from achiral and rac molecules.
In this paper, we report the formation of a novel chiral
supramolecular organic fluorophore and its optical properties;
the chiral fluorophore is prepared by employing achiral or rac
supramolecular building blocks. 1 is used as one of the acid
building blocks. Achiral and rac amine molecules, i.e., 4-tert-
butylbenzylamine (2), rac-1-phenylethylamine (rac-3), and rac-1-
cyclohexylethylamine (rac-4) are used as the other base building
blocks.
Introduction
The potential application of solid-state fluorescence techniques to
organic electroluminescence (EL) devices, optoelectronic devices,
etc. has attracted considerable attention.1 In particular, the
demand for a chiral fluorophore exhibiting circularly polarized
luminescence (CPL) has increased. Previously reported solid-state
organic fluorophores mostly comprise a single molecule,2 and there
are very few reports on supramolecular organic fluorophores con-
taining two or more organic molecules.3 Most of these supramolec-
ular complexes, however, do not exhibit chirality. Recently, we
have succeeded in preparing a solid-state chiral supramolec-
ular organic fluorophore composed of two types of organic
molecules–fluorescent 2-anthracenecarboxylic acid (1), which is
one of the most famous and important fluorescent units, and
chiral 2-amino-1,2-diphenylethanol.4 Although chiral functional
organic materials are usually prepared using chiral molecules, in
this system the chiral supramolecular organic fluorophore has
been obtained using achiral 1 and racemic (rac)-2-amino-1,2-
diphenylethanol—an equimolar mixture of (1R,2S)–and (1S,2R)-
2-amino-1,2-diphenylethanol. That is, this chiral fluorophore is a
Results and discussion
spontaneous resolution fluorescent system. Generally, most chiral
molecules are not easily available and are more expensive than
First, the complexation behavior of the 1–2 system is studied. The
achiral or rac molecules. Therefore, chiral supramolecular organic
chiral supramolecular complex is prepared via crystallization from
fluorophores prepared using achiral or rac molecules can be
an ethanol (EtOH) solution. A mixture of 1 and 2 is dissolved in the
used for the industrial production of chiral fluorophores without
EtOH solution and allowed to stand at room temperature. After
the use of any chiral auxiliaries, by a preferential crystallization
a few days, a large number of crystals of complex I, composed
of 1 and 2, are obtained. The X-ray crystal structure analysis
of complex I is shown in Fig. 1†. Complex I is a rac complex.
method. As previously reported, the fluorescent 1–2-amino-1,2-
diphenylethanol system is useful for the preparation of novel chiral
fluorophores. However, it is not easy to predict the formation of
spontaneous resolution systems for new component molecules.
That is, the stoichiometry of complex I is 1 : 2 = 1 : 1, and its
space group is P21/c. This complex has a characteristic pseudo-
21-helical columnar network structure along the b-axis (Fig. 1a).
This column is mainly formed by the carboxylate oxygens of the
carboxylic acid anions (indicated by the molecules in blue in Fig. 1)
and the ammonium hydrogen of the protonated amines (indicated
by the molecules in green). Although complex I is formed by
the assembly of the pseudo-21-columns, obvious intercolumnar
interactions are not present (Fig. 1b).5
A useful feature of the supramolecular system is that the
structure of the supramolecular complex can be easily controlled
by changing the component molecules. Thus the rac amine
molecules (3 and 4) can be used instead of 2. Next, the 1–rac-3
aDepartment of Applied Chemistry, Faculty of Science and Engineering,
Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan.
E-mail: y-imai@apch.kindai.ac.jp, y-matsu@apch.kindai.ac.jp.
bJST ERATO-SORST Kuroda Chiromorphology Team, 4-7-6, Komaba,
Meguro-ku, Tokyo 153-0041, Japan
cGraduate School of Materials Science, Nara Institute of School and
Technology, Takayama, Ikoma, Nara 630-0192, Japan
dDepartment of Life Sciences, Graduate School of Arts and Sciences, The
University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
† CCDC reference numbers 681595–681597. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b808242a
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Fig. 1 Crystal structure of complex I. Molecules of 1 and 2 are indicated
by the molecules in blue and green, respectively. (a) Pseudo-21-helical
columnar hydrogen- and ionic-bonded network parallel to the b-axis.
(b) Packing structure observed along the b-axis.
Fig. 2 Crystal structure of complex II. Molecules of 1 and 3 are
indicated by the molecules in blue and green, respectively. (a) 21-Helical
columnar network structure along the b-axis. (b) Packing structure
observed along the b-axis. The red, blue, and purple arrows indicate the
anthracene–anthracene edge-to-face, benzene–anthracene edge-to-face,
and anthracene–benzene edge-to-face interactions, respectively.
system is studied. Similarly to the preparation of the 1–2 system,
the chiral fluorophore 1–3 system is prepared via crystallization
from an EtOH solution. A mixture of 1 and rac-3 is dissolved
in the EtOH solution and allowed to stand at room temperature.
After a few days, colorless crystals of complex II, consisting of
1 and 3, are obtained. The X-ray crystal structure analysis of
complex II is shown in Fig. 2†. Interestingly, this complex exhibits
chirality and is the same as that obtained from the 1–(R)-3 system.
The stoichiometry of complex II is 1 : (R)-3 = 1 : 1, and its
space group is P21. This difference between complex II and the
previously reported 1–2-amino-1,2-diphenylethanol system is that
the former does not contain EtOH molecules. This complex has
a characteristic 21-helical columnar network structure along the
b-axis (Fig. 2a). This column is mainly formed by the carboxylate
oxygen of the carboxylic acid anions (Fig. 2, indicated by the
molecules in blue) and the ammonium hydrogen of the protonated
amines (Fig. 2, indicated by the molecules in green). Complex II is
formed by the self-assembly of the 21-columns by two anthracene–
by the red arrows in Fig. 2b), two benzene–anthracene edge-to-
˚
face interactions (2.74 and 2.97 A, indicated by the blue arrows),
˚
and one anthracene–benzene edge-to-face interaction (2.92 A,
indicated by the purple arrow).
Subsequently, rac-4 with a cyclohexane ring is used instead of
rac-3. Using the same procedure, a chiral supramolecular organic
fluorophore is prepared by crystallization from the same EtOH
solution. As a result, complex III, containing 1 and 4 without any
EtOH molecules, is obtained. The crystal structure of complex
III is shown in Fig. 3†. Interestingly, complex III also exhibits
chirality and is the same as that obtained from the 1–(S)-4 system.
The stoichiometry of complex III is 1 : (S)-4 = 1 : 1, and its space
group is the same as that of crystal II, i.e., P21. This complex also
has a characteristic 21-helical columnar network structure along
˚
anthracene edge-to-face interactions (2.68 and 2.85 A, indicated
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appropriate combination from the three molecules [1, (R)-3 (or
-4), and (S)-3 (or -4)]. In the second step, on the basis of the
packing structure, suitable chiral crystals are formed by varying
the 21-helical column packing, i.e., by the self-assembly of the 21-
helical columns with the same chirality. Moreover, by comparing
the crystal structures of the chiral complexes II and III with that
of the previously studied 1–2-amino-1,2-diphenylethanol system,
it appears that 1, along with the amine molecules, easily forms the
21- or pseudo-21-helical columnar network structure comprising
two 1 and two amine molecules when observed along the column.
Therefore, this system may relatively easily form chiral complexes
because chiral complexes can be formed only if columns with the
same chirality are assembled.6
In order to study the asymmetric environments of complexes II
and III, their solid-state circular dichroism (CD) spectra have been
measured using a KBr pellet. The solid-state CD and absorption
spectra of complex II are shown in Fig. 4 (indicated by the
blue line). The peaks in the CD spectrum originating from the
anthracene ring are observed at 358, 375, 397, and 404 nm. The
circular anisotropy (gCD = DOD/OD) factor of the last Cotton
effect (kCD = 404 nm) is approximately −0.6 × 10−3. In order
to check if the crystal has caused any artifact in the spectrum,
complex IIꢀ, composed of (S)-1-phenylethylamine [(S)-3] instead
of (R)-3 is prepared, and the CD and absorption spectra of
complex IIꢀ are measured (Fig. 4, indicated by the green line). This
CD spectrum is found to be a mirror image of the CD spectrum
of complex II.
Fig. 3 Crystal structure of complex III. Molecules of 1 and 4 are
indicated by the molecules in blue and green, respectively. (a) 21-Helical
columnar network structure along the b-axis. (b) Packing structure
observed along the b-axis. The red and blue arrows indicate the CH–p
and anthracene–anthracene edge-to-face interactions, respectively.
Fig. 4 CD and absorption spectra of complexes II (blue line) and IIꢀ
(green line) in the solid state (KBr pellets).
the b-axis, formed by the carboxylate oxygen of the carboxylic acid
anions and the ammonium hydrogen of the protonated amines
(Fig. 3a). In contrast with complex II, complex III is formed by the
self-assembly of the 21-columns by one CH–p (3.00 A, indicated
by the red arrow in Fig. 3b) and three anthracene–anthracene
Similarly, the solid-state CD and absorption spectra of complex
III are shown in Fig. 5 (indicated by the green line). The features in
the CD spectrum originating from an anthracene ring are observed
at 376, 396, and 407 nm. The gCD factor of the last Cotton effect
(kCD = 407 nm) is approximately 0.6 × 10−3. On the other hand, the
CD spectrum of complex IIIꢀ, composed of (R)-4, is found to be a
mirror image of the CD spectrum of complex III (indicated by the
blue line in Fig. 5). These results show that an effective chirality
transfer from 3 or 4 to 1 takes place through complexation. When
the CD spectra of the 1–3 system are compared to those of the 1–4
system, the two spectra are found to be similar, particularly the
wavelengths and signs of the Cotton effect, and the gCD factors are
almost the same. This similarity is caused by the relative positions
˚
˚
edge-to-face interactions (2.74, 2.79, and 2.84 A, indicated by the
blue arrows). In both 1–rac-3 and 1–rac-4 systems, chiral crystals
IIꢀ, composed of 1 and (S)-3, and chiral crystals IIIꢀ, composed of
1 and (R)-4, were also obtained in each batch, respectively. This
shows that these systems are spontaneous resolution systems.
From the X-ray crystallographic analysis, it is confirmed that
these spontaneous resolution systems form the chiral crystals in
two steps. In the first step, a chiral 21-helical columnar structure
suitable for the 1D network structure is formed by selecting the
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Table 1 Crystal shape and solid-state fluorescence spectral data of
complexes II and III and fluorescent molecule 1
˚
˚
(C, Fig. 2b and 3b) decreases (15.35 A and 14.65 A for complexes
II and III, respectively). It is thought that the difference in kem of
complexes II and III is due to the difference in the excited singlet
state, mainly caused by small changes in the relative arrangements
of the fluorescent anthracene rings.
Crystal shape
kex/nm
kem/nm
UF
Complex II
Complex III
1
Colorless needle
Colorless needle
Yellow powder
371
372
408
430
440
464
0.20
0.24
0.04
Conclusions
A novel chiral supramolecular organic fluorophore has been
successfully prepared by using 1 and rac-3 (or -4). That is, a novel
spontaneous resolution chiral fluorescence system has been found.
Since it is not easy to predict the structure of supramolecular
organic complexes for new component molecules, it is expected
that this study will provide useful information on the formation of
novel spontaneous resolution systems consisting of supramolec-
ular complexes. Moreover, although many organic fluorophores
lose their fluorescence in the solid state, this chiral supramolecular
fluorophore shows stronger fluorescence on complexation as
compared to 1 in the solid state. This further enhances the potential
of this fluorescence system, enabling its application in the design
of novel solid-state chiral supramolecular organic fluorophores
exhibiting spontaneous resolution.
Experimental
Fig. 5 CD and absorption spectra of complexes III (green line) and IIIꢀ
General methods
(blue line) in the solid state (KBr pellets).
All reagents were used directly as obtained commercially. Compo-
nent molecules 1–4 were purchased from Tokyo Kasei Kogyo Co.
EtOH was purchased from Wako Pure Chemical Industry.
of the anthracene rings in complexes II and III being almost the
same, and does not depend on the type of amine molecule used.
Since this spontaneous resolution system comprises 1, which
is fluorescent, the obtained chiral complexes may exhibit fluores-
cence. In order to study the solid-state optical properties of this
system, the solid-state fluorescence spectra of complexes II and III
have been measured (Table 1).
Formation of the complex by crystallization from an
EtOH solution
1 (10 mg, 0.045 mmol) and an amine (0.045 mmol) were dissolved
in EtOH (2 mL). After 3–5 days, a large number of crystals [crystals
of complex I for the 1–2 system (6 mg), a mixture of crystals of
complexes II and IIꢀ for the 1–3 system (5 mg), and a mixture of
crystals of complexes III and IIIꢀ for the 1–4 system (6 mg)] were
obtained. The weight of the crystals is the total weight of crystals
obtained in one batch.
Although many organic fluorophores lose their fluorescence in
the solid state, complexes II and III show fluorescence even in the
solid state. The solid-state fluorescence maxima (kem) of complexes
II and III are 430 and 440 nm, respectively, and hypsochromic
shifts (34 nm for complex II and 24 nm for complex III) are
observed relative to the kem of 1. The absolute values of the
photoluminescence quantum yield (UF) of complexes II and III
increase from 0.04 to 0.20 and from 0.04 to 0.24, respectively,
relative to that of 1. A comparison of the structures of complex
II (or III) and 1 reveals that although 1 may exist as a dimer
due to hydrogen bonding in the solid state, 1 in complexes II
and III comprises a strong hydrogen- and ionic-bonded columnar
network that suppresses the concomitant nonradiative processes.
It is inferred that one of the reasons for the increase in UF after
complexation is the change in the bonding style of 1. On the other
hand, when comparing complexes II and III, although UF and
kem in these two complexes are different, significant differences
are not observed despite using the cyclohexane ring instead of
the benzene ring in complex III. From the detailed structural
analyses, comparision of complex II with complex III shows that
the distance between the anthracene rings along the column is
X-ray crystallographic study of the complexes
X-ray diffraction data for single crystals were collected using
BRUKER APEX. The crystal structures were solved by the
direct method7 and refined by full-matrix least-squares using
SHELX97.8 The diagrams were prepared using PLATON.9
Absorption corrections were performed using SADABS.10 Non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters, and hydrogen atoms were included in the models in
their calculated positions in the riding model approximation.
Crystallographic data for I: C11H17N·C15H10O2, M = 385.49,
monoclinic, space group P21/c, a = 18.2494(14), b = 6.5375(5),
◦
3
˚
˚
c = 17.7953(13) A, b = 99.5320(10) , V = 2093.8(3) A , Z = 4,
Dc = 1.223 g cm−3, l(Mo–Ka) = 0.077 mm−1, 12454 reflections
measured, 4745 unique, final R(F2) = 0.0674 using 3578 reflections
with I > 2.0r(I), R(all data) = 0.0940, T = 100(2) K. CCDC
681597. Crystallographic data for II: C8H11N·C15H10O2, M =
343.41, monoclinic, space group P21, a = 9.319(3), b = 6.1500(17),
˚
˚
found to slightly decrease from 6.15 A to 6.11 A (A, Fig. 2a and 3a).
Although the distance between the 21-columns (B, Fig. 2b and 3b)
˚
increases from 9.32 to 10.91 A, the distance between the columns
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◦
3
˚
˚
Notes and references
c = 15.347(4) A, b = 90.088(5) , V = 879.6(4) A , Z = 2, Dc =
1.297 g cm−3, l(Mo–Ka) = 0.082 mm−1, 7730 reflections measured,
3934 unique, final R(F2) = 0.0518 using 3257 reflections with I >
2.0r(I), R(all data) = 0.0663, T = 120(2) K. CCDC 681595.
Crystallographic data for III: C8H17N·C15H10O2, M = 349.46,
monoclinic, space group P21, a = 10.9057(9), b = 6.1084(5),
1 (a) J. Shinar, Organic Light-Emitting Devices, Springer, New York,
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and N. Tohnai, Bull. Chem. Soc. Jpn., 2007, 80, 1162; (g) Y. Imai,
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◦
3
˚
˚
c = 14.6448(12) A, b = 102.4310(10) , V = 952.71(14) A ,
Z = 2, Dc = 1.218 g cm−3, l(Mo–Ka) = 0.077 mm−1, 8407
reflections measured, 4179 unique, final R(F2) = 0.0453 using
3527 reflections with I > 2.0r(I), R(all data) = 0.0559, T =
135(2) K. CCDC 681596. Crystallographic data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge, CB2 1EZ, UK; fax: (+44)1223-336-033;
deposit@ccdc.cam.ac.uk)†.
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.11
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5 This was determined by PLATON geometry calculations.
6 K. Kinbara, Y. Tagawa and K. Saigo, Tetrahedron: Asymmetry, 2001,
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7 G. M. Sheldrick, SHELX97, Program for the solution of crystal
strctures, University of Goettingen, Germany, 1997.
8 G. M. Sheldrick, SHELX97, Program for the refinement of crystal
strctures, University of Goettingen, Germany, 1997.
Measurement of solid-state fluorescence spectra
Solid-state fluorescence spectra and absolute photo-luminescence
quantum yields were measured by an absolute PL quantum yield
measurement system (C9920-02, HAMAMATSU PHOTONICS
K. K.) under an air atmosphere at room temperature.
9 A. L. Spek, PLATON, Molecular geometry and graphics program,
University of Utrecht, The Netherlands, 1999.
10 G. M. Sheldrick, SADABS, Program for Empirical Absorption Cor-
rection of Area Detector Data, University of Gottingen, Germany,
1996.
Acknowledgements
This work was supported by the Kansai Research Foundation for
technology promotion.
11 R. Kuroda and Y. Saito, Bull. Chem. Soc. Jpn., 1976, 49,
433.
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