A solid-state chiral fluorophore obtained from achiral fluorescent biphenyl-4-carboxylic acid and rac-phenylmethylamine derivatives

Article abstract of DOI:10.1246/cl.2008.726

A chiral supramolecular organic fluorophore is developed by using achiral biphenyl-4-carboxylic acid and rac-1-phenyl-ethylamine. This fluorophore is a spontaneously resolved chiral fluorescence system. Copyright

Full text of DOI:10.1246/cl.2008.726

726
Chemistry Letters Vol.37, No.7 (2008)
A Solid-state Chiral Fluorophore Obtained from Achiral Fluorescent Biphenyl-4-carboxylic Acid
and rac-Phenylmethylamine Derivatives
Yoshitane Imai,ꢀ1 Katuzo Murata,1 Hideki Matsuno,1 Kensaku Kamon,1 Tomohiro Sato,2
Reiko Kuroda,2;3 and Yoshio Matsubaraꢀ1
1Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University,
3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502
2JST ERATO-SORST Kuroda Chiromorphology Team, 4-7-6 Komaba, Meguro-ku, Tokyo 153-0041
3Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo,
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902
(Received April 15, 2008; CL-080390; E-mail: y-imai@apch.kindai.ac.jp, y-matsu@apch.kindai.ac.jp)
A chiral supramolecular organic fluorophore is developed
by using achiral biphenyl-4-carboxylic acid and rac-1-phenyl-
ethylamine. This fluorophore is a spontaneously resolved chiral
fluorescence system.
rac complex. The stoichiometry of complex I was molecule 1:
molecule 2 = 1:1, and its space group was P21=c. This crystal
had a characteristic one-dimensional (1D) columnar network
structure along the b axis (Figure ESI-1a). The column was
mainly formed by the carboxylate oxygen of a carboxylic acid
anion (Figure ESI-1, indicated by the molecules in blue) and
the ammonium hydrogen of a protonated amine (Figure ESI-1,
indicated by the molecules in green). Complex I was formed
by the self-assembly of the column (Figure ESI-1b). The column
interacted via benzene–benzene edge-to-face interaction
The potential application of solid-state fluorescence tech-
niques to organic electroluminescence (EL) devices, optodevice
materials, etc. has attracted considerable attention.1 To date,
solid-state organic fluorophores have mostly comprised a single
molecule,2 and there have been only a few reports concerning
supramolecular organic fluorescent complexes containing two
or more organic molecules.3 Almost all these reported com-
plexes, however, have no chirality. Recently, we have succeeded
in developing a chiral supramolecular organic fluorophore that
shows circularly polarized luminescence (CPL) in the solid state
by combining two types of organic molecules—fluorescent
biphenyl-4-carboxylic acid (1) (Chart 1) and chiral (1R,2R)-
1,2-diphenylethylenediamine.4 However, if a chiral supramolec-
ular organic fluorophore is formed by using achiral or racemic
(rac) component molecules instead of chiral component mole-
cules, it is very useful from the viewpoint of industrialization.
In this paper, we report the formation of a chiral supramo-
lecular organic fluorophore and its optical property; the chiral
fluorophore was prepared by employing achiral or rac supramo-
lecular building blocks. An acid molecule, achiral fluorescent
biphenyl-4-carboxylic acid (1), was used as one of the building
blocks. Achiral and rac base amine molecules, i.e., benzylamine
(2) and rac-1-phenylethylamine (rac-3), respectively, were used
as the other building blocks.
˚
(3.74 A, indicated by a red arrow in Figure ESI-1b) and CH–ꢀ
7
˚
interaction (3.57 A, indicated by a blue arrow in Figure ESI-1b).
A useful feature of these supramolecular systems is that the
structure of the complex can be easily controlled by changing the
component molecules.3a,8 Subsequently, rac-3 is used instead of
2. Using the same procedure, we attempt the formation of a
chiral supramolecular organic fluorophore by crystallization
from the MeOH and EtOH solutions. As a result, different
complexes II and III are obtained, respectively.6 The crystal
The complexation behavior of the 1/2 system was studied.
The formation of a chiral supramolecular organic fluorophore
was attempted via crystallization from MeOH and EtOH
solutions. A mixture of 1 (10 mg, 0.05 mmol) and 2 (6 mg,
0.06 mmol) was dissolved in each alcohol solution (2 mL)
and left to stand at room temperature. After a few days, a large
number of identical crystals I (7 mg)5 were obtained from both
solutions. The X-ray crystal structure analysis of complex I is
shown in Figure ESI-1.6 Unfortunately, this complex is a
Figure 1. Crystal structures of complexes II and III. Molecules
of 1 and 3 are indicated as blue and green molecules, respective-
ly. MeOH and EtOH molecules are indicated as red space-filling
molecules. (a) View along c axis. 1D columnar network structure
is parallel to b axis. (b) Packing structure observed along b axis.
Red and blue arrows indicate benzene–benzene edge-to-face
and CH–ꢀ interactions, respectively.
H
CH3
CH2NH2
COOH
NH2
1
2
rac-3
Chart 1.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.7 (2008)
727
Table 1. Crystal forms and solid-state fluorescence spectral
data of complexes I–III and fluorescent molecule 1
containing MeOH and EtOH are similar, a comparison of com-
plex II (or III) and complex I reveals that the relative arrange-
ment of the biphenyl unit is different. In other words, the stack-
ing between the neighboring biphenyl units in complexes II and
III is smaller than that in complex I. Moreover, although the
bonding style of 1 is almost the same in these three complexes,
the torsion angle between benzene rings of 1 in complexes II and
III (39.2ꢁ for II and, 35.3ꢁ and 36.0ꢁ for III) is smaller than that
in complex I (49.2ꢁ). It is believed that the increase in the photo-
luminescence quantum yields of complexes II and III is mainly
caused by these structural changes, especially, the increase of
planality of biphenyl unit of 1. The fluorescent complexes II
and III have chirality, and therefore, they show CPL. However,
when KBr pellets were prepared for measuring the CPL spectra,
all the alcohol molecules were released from the complexes.
Therefore, the CPL spectra of these chiral complexes could
not be measured.
In conclusion, a novel chiral supramolecular organic fluoro-
phore was successfully prepared by using achiral 1 and rac-3.
In other words, a spontaneously resolved chiral fluorescence sys-
tem was found. Although many organic fluorophores lose the
fluorescence property in the solid state, this chiral supramolecu-
lar fluorophore showed the fluorescence property in the solid
state. This further enhanced the capability of the fluorescence
system, enabling its application in the design of novel solid-state
chiral supramolecular organic fluorophores.
Complex
Crystal form
ꢁex/nm
ꢁem/nm ꢂF
I
II
III
colorless needle
colorless needle
colorless needle
324
321
327
312
395
385
387
393
0.47
0.63
0.61
0.38
Compound 1 colorless needle
structures of complexes II and III are shown in Figure 1.6
Interestingly, both these complexes are chiral and are the
same as those obtained from the 1/(R)-3 system.9 The stoichio-
metries of complexes II and III are 1:(R)-3:MeOH (or
EtOH) = 1:1:0.5, and their space groups are C2. These com-
plexes have characteristic 21-helical columnar hydrogen- and
ion-bonded network structures along the b axis, similar to com-
plex I (Figure 1a). Both complexes are formed by the self-as-
sembly of these 21-columns, which gives rise to the channel-like
cavities (Figure 1b). In complex II, these 21-columns interact by
three types of benzene–benzene edge-to-face interactions (3.59,
˚
3.64, and 3.70 A; indicated by the red arrows in Figure 1b) and
˚
CH–ꢀ interaction (3.61 A, indicated by a blue arrow in
Figure 1b).7 On the other hand, complex III has four types of
benzene–benzene edge-to-face interactions (3.53, 3.62, 3.63,
˚
and 3.66 A, indicated by the red arrows in Figure 1b) apart from
˚
CH–ꢀ interaction (3.57 and 3.80 A, indicated by the blue arrows
in Figure 1b).7 Although the alcohol molecules (Figure 1, indi-
cated as red space-filling molecules) are disordered, they are
trapped one-dimensionally along the direction of the cavity.
In one batch, chiral crystals composed of 1 and (S)-3 were
also obtained, respectively.9 This shows that this system is a
spontaneous resolution system.
Since this system comprises 1, which is fluorescent, the
obtained complexes may also exhibit fluorescence. In order to
study the solid-state fluorescence property of this system, the
solid-state fluorescence spectra of these complexes have been
determined (Table 1).6
Although the most serious problem encountered in a solid-
state organic fluorophore is fluorescence quenching in the
crystalline state, this is not observed in any of the complexes ob-
tained. A comparison of complex I and the fluorescent molecule
1 reveals that their solid-state fluorescence maxima (ꢁem) are
similar. However, the absolute value of the photoluminescence
quantum yield (ꢂF) increases from 0.38 to 0.47. On the other
hand, the solid-state fluorescence maxima (ꢁem) of complexes
II and III are similar (385 and 387 nm, respectively), and
small hypsochromic shifts (8 and 6 nm) are observed in these
complexes relative to the hypsochromic shift of 1. Moreover,
the absolute photoluminescence quantum yields in complexes
II and III (ꢂF ¼ 0:63 and 0.61) are observed to be higher than
the yield in 1 (ꢂF ¼ 0:38).
This work was supported by the Kansai Research Founda-
tion for technology promotion.
References and Notes
1
Organic Light-Emitting Devices, ed. by J. Shinar, Springer-
Verlag, New York, Berlin, Heidelberg, 2004.
2
a) K. Yoshida, Y. Ooyama, S. Tanikawa, S. Watanabe, J.
Chem. Soc., Perkin Trans. 2 2002, 708. b) Z. Fei, N. Kocher,
C. J. Mohrschladt, H. Ihmels, D. Stalke, Angew. Chem., Int.
Ed. 2003, 42, 783. c) J. L. Scott, T. Yamada, K. Tanaka, Bull.
Chem. Soc. Jpn. 2004, 77, 1697. d) J. L. Scott, T. Yamada,
K. Tanaka, New J. Chem. 2004, 28, 447. e) Y. Ooyama, K.
Yoshida, New J. Chem. 2005, 29, 1204, and references cited
therein.
3
4
a) Y. Mizobe, N. Tohnai, M. Miyata, Y. Hasegawa, Chem.
Commun. 2005, 1839. b) Y. Mizobe, H. Ito, I. Hisaki, M.
Miyata, Y. Hasegawa, N. Tohnai, Chem. Commun. 2006,
2126. c) S. Oshita, A. Matsumoto, Langmuir 2006, 22,
1943. d) Y. Mizobe, T. Hinoue, M. Miyata, I. Hisaki, Y.
Hasegawa, N. Tohnai, Bull. Chem. Soc. Jpn. 2007, 80, 1162.
Y. Imai, K. Kawaguchi, T. Harada, T. Sato, M. Ishikawa,
M. Fujiki, R. Kuroda, Y. Matsubara, Tetrahedron Lett.
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5
6
This weight is the total crop of obtained crystals in one batch.
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
A comparison of the crystal structures of complex I and the
fluorescent molecule 16 reveals that the relative arrangement of
the biphenyl unit is similar in both. Although 1 exists as a dimer
in the crystalline state, 1 in complex I comprises a strong ionic-
bonded columnar network that suppresses the concomitant non-
radiative processes. It can be inferred that the main reason for an
increase in the photoluminescence quantum yield after complex-
ation is the change in the bonding style of the fluorescent mole-
cule 1. Although the crystal structures of complexes II and III
7
8
Distance between carbon and the center of benzene ring.
a) Y. Imai, T. Sato, R. Kuroda, Chem. Commun. 2005, 3289.
b) Y. Imai, K. Kawaguchi, T. Sato, R. Kuroda, Y. Matsubara,
Tetrahedron Lett. 2006, 47, 7885.
9
The absolute configuration of 3 was determined by HPLC
analysis.
Published on the web (Advance View) June 7, 2008; doi:10.1246/cl.2008.726