Circular dichroism and circularly polarized luminescence triggered by self-assembly of tris(phenylisoxazolyl)benzenes possessing a perylenebisimide moiety

Article abstract of DOI:10.1039/c2cc31512b

Chiral tris(phenylisoxazolyl)benzenes possessing a perylenebisimide moiety assembled to form helical stacks. The self-assembling behavior of the helical stacks responded to changes in solvent properties, temperature, and concentration. Strong circular dichroism (CD) and circularly polarized luminescence (CPL) of their assemblies were displayed, and were controlled by external stimuli.

Full text of DOI:10.1039/c2cc31512b

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Circular dichroism and circularly polarized luminescence triggered by
self-assembly of tris(phenylisoxazolyl)benzenes possessing a
perylenebisimide moietyw
Toshiaki Ikeda,a Tetsuya Masuda,a Takehiro Hirao,a Junpei Yuasa,b Hiroyuki Tsumatori,b
Tsuyoshi Kawaib and Takeharu Haino*a
Received 29th February 2012, Accepted 24th April 2012
DOI: 10.1039/c2cc31512b
Chiral tris(phenylisoxazolyl)benzenes possessing a perylenebisimide
moiety assembled to form helical stacks. The self-assembling
behavior of the helical stacks responded to changes in solvent
properties, temperature, and concentration. Strong circular
dichroism (CD) and circularly polarized luminescence (CPL)
of their assemblies were displayed, and were controlled by
external stimuli.
Fig. 1 Structure of molecules S- and R-1 and the schematic scheme of
Circularly polarized luminescence (CPL) is the selective emis-
their chiral self-assembly and circularly polarized luminescence (CPL).
sion of right- and left-handed circularly polarized lights from
chiral molecular systems. This field has been dominated by
a well-known chromophore that creates very strong fluores-
chiral lanthanide complexes so far.1 In recent years, there has
cence with high quantum yield in the visible region.9 The
been a growing interest in developing organic molecules capable
introduction of PBI units onto the helical assemblies should
of emitting CPL towards their potential applications in opto-
arrange them in a helical fashion, capable of developing the
electronic devices.2,3 Strategies to create CPL emitting organic
CPL emission.6b,c Herein, we report novel CPL emitting
materials include attaching chiral side chains to conjugated
polymers,2,4 embedding organic chromophores in a chiral matrix,5
and developing inherently chiral conjugated molecules.6 Some
assemblies formed from chiral tris(phenylisoxazolyl)benzenes
1 possessing a perylenebisimide (PBI) moiety and its self-
assembling behaviors, providing the distinctive CD and CPL
conjugated materials in condensed states exhibit an enhanced
properties (Fig. 1).
CPL dissymmetry factor, glum = 2(IL À IR)/(IL + IR), where
The self-assembling behavior of S-1 was studied using
IL and IR, respectively, denote intensity of left and right
1H NMR and absorption spectroscopies. The signals that
circularly polarized light.2,4,6 Controlling CPL by a supra-
appeared in the aromatic region were assigned by 2D NMR
techniques (see ESIw). The 1H NMR signals of S-1 were
concentration-dependent in chloroform-d1 (Fig. 2). Most of
molecular method represents a prime scientific challenge,7
because CPL emission can be turned ‘‘on’’ and ‘‘off’’.6e However,
there are very few reports describing that CPL emission is
triggered by supramolecular assembly.
the signals shifted upfield upon increasing the concentration
from 0.2 to 20 mmol LÀ1. This indicates that S-1 forms
Recently, we have reported that tris(phenylisoxazolyl)-
benzenes form one-dimensionally stacked helical assemblies.8
The size and dimension of the helical self-assemblies are sensitive
to a solvent system; thus, the assembling behavior can be
controlled by using a certain solvent. Perylenebisimide (PBI) is
a Department of Chemistry, Graduate School of Science,
Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima,
739-8526, Japan. E-mail: haino@hiroshima-u.ac.jp;
Fax: +81 82 424 0724; Tel: +81 82 424 7427
b Graduate School of Materials Science, Nara Institute of Science and
Technology, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan
w Electronic supplementary information (ESI) available: Synthetic
procedure for S- and R-1, UV–vis absorption, fluorescence, and CD
Fig. 2 Concentration-dependent 1H NMR spectra of S-1 at 298 K in
spectra of S- and R-1 in chloroform and MCH, the van’t Hoff plot of
chloroform-d1. The concentrations of S-1 are (a) 0.50, (b) 1.0, and
(c) 5.0 mmol LÀ1. * indicate solvent peaks.
S-1 in decalin, and NMR spectra of all new compounds. Correction
for CPL is also provided. See DOI: 10.1039/c2cc31512b
c
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stacked assemblies in which the aromatic protons are placed in
the shielding regions produced by the neighboring aromatic
rings. Plotting the chemical shift changes of the protons vs.
the concentrations of S-1 produced hyperbolic curves.
Non-linear curve-fitting analysis by applying the isodesmic
model10 produced the estimated complexation-induced shifts
(Dd = À0.48, À0.56, À0.73, À1.07, À1.43, À0.45, À0.53,
À0.14, and À0.86 ppm for Ha, Hb, Hc, Hd, He, Hf, Hg, Hh,
and HPBI, respectively) (see ESIw) and the association constant
(KE = 10.0 Æ 0.9 L molÀ1). The characteristic upfield shifts of
the aromatic protons of Hc, Hd, He, and HPBI imply that S-1
stacks around the pseudo C3-axis on the tris(phenylisoxazolyl)-
benzene, and the peripheral substituent PBI creates a close
contact with the neighbors.
(1.0 Â 10À5 mol LÀ1) of S-1 displayed sharp absorption
bands at 353 K, consistent with those observed in chloro-
form (Fig. 3b). Upon cooling the solution, the absorption
bands red-shifted, and new bands appeared at 310, 498,
and 535 nm, characteristic of the J-aggregates of PBIs.9
Plots of the molar extinction coefficient at 540 nm vs.
temperatures display a clear sigmoidal shape, indicating that
the self-association of S-1 obeys an isodesmic model (Fig. S4,
ESIw).11,12
To obtain an insight into the self-association of S-1, the
thermodynamic parameters DH, DS, and the association
constant (KE) at 338 K were determined by a van’t Hoff
plot to be À39 Æ 1 kcal molÀ1, À90 Æ 3 cal molÀ1
K
À1, and
(2.6 Æ 0.8) Â 105 L molÀ1, respectively (Fig. S5 and S6, ESIw).
These values are quite large as compared to those of a simple
tris(phenylisoxazolyl)benzene reported previously.8 This ratio-
nalizes that the assembly of S-1 receives the additional gain of
the enthalpic contribution which probably resulted from the
attractive intermolecular stacking interactions of the PBI
units; but, the large enthalpic contribution reduces the free-
dom of molecular movement, leading to the large negative
entropic component, which cannot be compensated by
desolvation.
The absorption spectral changes because of the self-
assembly of S-1 depend on solvent properties. A diluted
solution (1.0 Â 10À5 mol LÀ1) of S-1 in chloroform displayed
well-defined absorption bands with the characteristic p–p*
transitions of the tris(phenylisoxazolyl)benzene unit and the
PBI unit, at 270 nm and at 450, 480, and 515 nm.9 The
absorption spectra were not temperature- and concentration-
dependent (Fig. S2a and S3, ESIw). S-1 mostly exists in a
monomeric form in chloroform at the low concentrations.
By contrast, the absorption spectra of S-1 were highly
influenced by the temperatures and concentrations of its
solution in decahydronaphthalene (decalin). A diluted solution
During the self-assembly of S- and R-1, the chiral side
chains get closer to ensure the steric communications with
their neighbors, and reduce the symmetry of the assembly,
leading to the chiral induction. This prompted us to study the
chiroptical response of S- and R-1. The CD spectra of S- and
R-1 exhibited a mirror image relationship with respect to the
line of [y] = 0 in decalin at 293 K (Fig. 4a and S10, ESIw). The
assembly of S-1 gave rise to strong Cotton effects in the visible
region, characteristic of the stacked structure of the PBI
moiety with fairly large gabs value of 0.0014 at 545 nm. The
strong CD bands were temperature-dependent. Upon warming
the solution of S-1 to 353 K, the CD turned silent (Fig. 3a).
Accordingly, the induced CD bands originated from the
assembly of S-1. A right-handed helicity of the assembly of
S-1 was confirmed by the plus-to-minus patterns observed in
ascending energy terms in the CD spectrum of S-1 at around
545 nm and 300 nm.8a,d,9b,13
Non-aggregated S-1 displayed a well-defined fluorescence
band at 539 nm with high quantum yield (f = 0.56) in
chloroform. By contrast, S-1 exhibited two characteristic
bands in decalin; the sharp emission band at 525 nm is
assigned to its monomeric form, and the broad emission band
at 650 nm with moderate quantum yield (f = 0.07) originated
from the chiral assembly of S-1 (Fig. 4b and S7, ESIw).9 This
created clear CPL at the band, assigned to its assembly, with a
dissymmetry factor glum of 0.007 in decalin, whereas no CPL
signal was observed in chloroform (Fig. 4a).14 The CPL
spectrum of R-1 displayed the mirror image of S-1. These
results clearly conclude that the CPL signals observed in
decalin are induced by the chiral assemblies of S- and R-1.
The CPL of S- and R-1 can be regulated via assembling and
disassembling of the chiral monomers in response to the
solvent properties. The present glum is rather larger than
the typical glum values of p-conjugated chiral molecules.
Generally, the glum is inversely proportional to the transition
dipole moment.1 Although the emission rate constant of
Fig. 3 Temperature-dependent (a) CD and (b) UV–vis absorption
spectra of S-1 (1.0 Â 10À5 mol LÀ1) in decalin. The temperatures of the
solution of S-1 are 353, 343, 333, 323, 313, 303, and 293 K. Arrows
indicate changes upon decreasing the temperature.
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Fig. 4 (a) CD (solid line) and CPL (dotted line) spectra of S-1 in
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concentration is 1.0 Â 10À4 mol LÀ1. lex = 488 nm.
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In summary, we have demonstrated the unique chiroptical
properties induced by the chiral self-assembly of the tris(phenyl-
isoxazolyl)benzenes possessing a PBI moiety. The self-assembling
behaviors of S- and R-1 were discussed using 1H NMR,
UV–vis absorption, and fluorescence spectroscopies. Most of
the S- and R-1 did not form aggregates in chloroform, whereas
they assembled to form the helical stacks in decalin. The CD
signals of S-1 were enhanced with decreasing temperature in
decalin, and gabs reached 0.0014 at 293 K. S- and R-1 exhibited
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12 S-1 formed aggregates with very large association constant in
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14 CPL spectra were measured with a home-made CPL measurement
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This research work was supported by Grant-in-Aid for
Scientific Research (B) (No. 21350066) and challenging
Exploratory Research (No. 23655105) of JSPS. We are grateful
to Izumi Science and Technology Foundation and Electric
Technology Research Foundation of Chugoku. TI thanks
Shorai Foundation for Science and Technology, and Tokuyama
Science Foundation.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6025–6027 6027