Stimuli-responsive luminescent liquid crystals: Change of photoluminescent colors triggered by a shear-induced phase transition

Article abstract of DOI:10.1002/anie.200800164

(Chemical Presented) Shear brilliance: The photoluminescent color of a pyrene-based liquid crystal changes from yellow to blue-green upon mechanical shearing due to a phase transition from a micellar cubic to a columnar phase (see picture).

Full text of DOI:10.1002/anie.200800164

Angewandte
Chemie
DOI: 10.1002/anie.200800164
Luminescent Liquid Crystals
Stimuli-Responsive Luminescent Liquid Crystals: Change of
Photoluminescent Colors Triggered by a Shear-Induced Phase
Transition**
Yoshimitsu Sagara and Takashi Kato*
Soft materials formed by molecular self-assembly have
attracted much attention recently.[1] Liquid crystals are one
such soft material that combines ordered and dynamic
states.[2] These properties can be used to manufacture
stimuli-responsive materials. The incorporation of photo-,[3]
electro-,[4] and ion-functional[5] moieties into liquid crystals
leads to dynamic functional materials, the functions of which
can be tuned by stimuli-induced structural changes. We are
aiming to develop a new class of photo-functional nano-
structured liquid crystals that respond to mechanical stimuli,
and herein we report on a shear-induced liquid-crystalline
(LC) phase transition of pyrene derivative 1 which induces a
change of photoluminescent color. To date, only a limited
number of LC materials have been reported to show shear-
induced phase transitions,[6] and to the best of our knowledge
no photo-functional liquid crystals that show a shear-induced
LC phase transition associated with a change of their
photoluminescent color have been prepared. Pyrene is a
widely used probe due to its high fluorescent efficiency and
excimer formation,[7] and a number of pyrene derivatives have
recently been reported to show LC behavior.[8] We intend to
use the LC order-order phase transitions of pyrene-based
liquid crystals to control the luminescent properties of the
ordered bulk materials. If the assembled structures of
luminescent groups are changed by the phase transition, the
photoluminescent properties of the materials could be tuned
by external stimuli.[9]
We expected that compound 1 should exhibit both
columnar and cubic phases, although it is not easy to design
for the induction of a cubic phase rationally.[2d] These
nanostructured liquid crystals are suitable for the arrange-
ment of fluorophores. In compound 1, two dendritic groups
are linked to a central pyrene moiety by amide bonds. A
variety of dendritic groups have been examined as LC
molecular components,[10–15] and the dendron having a
methyl group at the focal point, which has a similar structure
to that of the dendron introduced into 1, has been reported to
show a columnar phase.[12] We have reported previously that
dendritic H-bonded molecules with bulky substituents exhibit
columnar–cubic phase transitions.[16] The presence of hydro-
gen bonding in the ordered phases of 1 is expected to lead to
formation of a one-dimensional chain structure.[17]
Compound 1 was synthesized from 1,6-diethynylpyrene
and the fan-shaped dendron by a Sonogashira coupling, which
yielded a, transparent, orange viscous liquid that exhibits
yellow photoluminescence. Compound 1 shows a thermo-
tropic LC cubic phase that is optically isotropic from ꢀ358C
to 1758C on simple heating (Figure 1a). The X-ray diffraction
pattern of 1 at 1608C (Figure 2a) shows peaks at 53.8, 48.0,
[*] Y. Sagara, Prof. T. Kato
Department of Chemistry and Biotechnology
School of Engineering
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-8661
E-mail: kato@chiral.t.u-tokyo.ac.jp
Homepage: http://chem-web.chem.t.u-tokyo.ac.jp/chembio/labs/
kato/index-e.html
44.1, and 31.1 Š with a reciprocal d-spacing ratio of
pffiffi pffiffi pffiffiffiffiffi
2: 5: 6: 12 that correspond to (200), (210), (211), and
(222) reflections, respectively, and are therefore indicative of
a Pm3n cubic phase (a = 107.6 Š).[16] Compound 1 exhibits
yellow photoluminescence in the cubic phase (Figure 1a),
although the photoluminescent color changes from yellow to
blue-green after the cubic–isotropic phase transition at 1758C
(Figure 1a!b). The change of photoluminescent color from
yellow to blue-green is triggered by mechanical shearing for 1
in the cubic phase (Figure 1a!c). This change by the
mechanical shearing is accompanied by the phase transition
from the optically isotropic cubic phase to the birefringent
phase.
[**] This work was supported by a Grant-in-Aid for Creative Scientific
Research of “Invention of Conjugated Electronic Structures and
Novel Functions” (grant no. 16GS0209) (T.K.) fromthe Japan
Society for the Promotion of Science (JSPS) and the Global COE
Programfor Chemistry Innovation (T.K. and Y.S.) fromthe Ministry
of Education, Culture, Sports, Science and Technology. We thank
Prof. K. Araki and Dr. T. Mutai for measuring the emission lifetimes.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
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stable than the Pm3n cubic phase. These results imply that the
cubic phase exhibited by 1 is a metastable LC phase.
The optical properties of 1 in the cubic (1608C), shear-
induced columnar (1608C), and isotropic (2008C) phases
were examined by absorption and emission spectroscopy
(Figure 3). The absorption bands for compound 1 in the shear-
induced and cubic LC phases are observed in the 350–470 nm
Figure 1. The change of photoluminescent color exhibited by 1 and the
respective polarized microscopic images: a) in the cubic phase at
1608C, b) in the isotropic phase at 2008C, and c) in the shear-induced
columnar phase at 1608C. Scale bar: 100 mm. The photoluminescent
images of 1 sandwiched between quartz substrates under UV irradi-
ation at 365 nmobserved in each condition are shown in the inset of
each polarized microscopic image. Ti: LC–isotropic phase-transition
temperature.
Figure 3. a) Absorption and b) emission spectra of 1 in the cubic
phase at 1608C (orange line), in the shear-induced columnar phase at
1608C (green line), and in the isotropic phase at 2008C (gray line).
region (Figure 3a). These bands are similar except for the
additional peak observed at 440 nm for the shear-induced
columnar phase. The band for the shear-induced columnar
phase (lmax = 496 nm) in the emission spectrum of 1 (Fig-
ure 3b) is blue-shifted with respect to that of 1 in the cubic
phase at 1608C (lmax = 579 nm). This spectral shift corre-
sponds to the change of photoluminescent color from yellow
to blue-green (Figure 1a!c). The emission bands of 1 in the
shear-induced columnar and isotropic phases are observed in
the same region (450–700 nm). These spectral features may be
indicative of the monomeric emission of 1 in the shear-
induced columnar phase. The emission band observed in the
cubic phase is broader and less structured than that for the
shear-induced columnar phase, which is attributed to excimer
formation of the pyrene moieties in the cubic phase. This
excimer formation is confirmed by the fact that the emission
lifetime for the cubic phase is longer than that for the shear-
induced columnar phase (see the Supporting Information).
This observation shows that the emitting species for the cubic
and shear-induced columnar phases are different. The exci-
mer formation of compound 1 is also supported by the results
of previous studies on the excimer emission of several 1,6-
diethynylpyrene derivatives.[18] The spectroscopic measure-
ments for 1 therefore show that mechanical shearing disturbs
the excimer formation of the 1,6-diethynylpyrene moieties of
1 and leads to a blue-green photoluminescence for the shear-
induced columnar phase.
Figure 2. X-ray diffraction patterns of 1: a) in the cubic phase at 1608C
and b) in the shear-induced columnar phase at 1608C.
X-ray diffraction measurements performed at 1608C after
the shearing (Figure 2b) indicate that a shear-induced cubic–
columnar phase transition has occurred. The diffractions
observed for 1 in the small angle region after the shearing can
be ascribed to the (11), (02), (22), and (41) reflections of a P2/
a rectangular columnar phase (a = 96.7, b = 71.2 Š) which is
stable and does not exhibit an isothermal transition back to
the cubic phase. Further heating of 1 leads to a columnar–
isotropic phase transition at 1928C with no formation of a
cubic phase (Figure 1c!b). Once compound 1 has formed
the isotropic state it does not return to the columnar phase on
simple cooling without mechanical shearing. The observed
increase of LC-Iso transition temperature and the transition
enthalpy (see the Supporting Information) suggest that the
shear-induced columnar phase is thermodynamically more
The self-assembled structures in the cubic and shear-
induced columnar phases were also studied by IR spectros-
copy (Figure 4). The IR spectra for 1 in both the cubic and
shear-induced columnar phases show that the amide groups
are involved in the formation of hydrogen bonds and no free
=
ꢀ
=
amide C O or N H groups are present (Figure 4b,c). The C
O stretching band observed for 1 in the cubic phase shifts
from 1655 to 1645 cmꢀ1 upon shear-induced cubic–columnar
phase transition. This shift indicates that the weaker H-
bonded state in the cubic phase changes to a stronger H-
bonded state in the shear-induced columnar phase. In
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Chemie
disordered stacks in these segmented columns (Figure 5a,
middle and right). This proposed self-assembled structure is
supported by the fact that the length between the H-bonded
amide groups is generally longer than the distance between
the adjacent arenes that form p-stacked structures.[20]
The shear-induced cubic–columnar phase transition of 1 at
1608C results in the formation of non-segmented columnar
structures (Figure 5b, left). The stronger H-bonded states
(Figure 4c) that are not observed in the cubic phase are
formed in these columnar structures (Figure 5b, middle and
right). Each molecule in these structures is expected to form
hydrogen bonds to adjacent molecules with a distance of
around 5 Š. This proposed assembled structure of 1 (Fig-
ure 5b) is supported by previous reports on the crystal
structures of some arenedicarboxamides,[21] where the dia-
mides that pack in one-dimensional tapes are translationally
related and each molecule is H-bonded to two adjacent
molecules with a separation of 5 Š. Moreover, the arenes of
these compounds are not overlapping.[21] In light of these
crystal structures, compound 1 with a similar structure may
assemble into the linear H-bonded structures shown in
Figure 5b, which should result in interference with the
excimer formation of the 1,6-diethynylpyrene moieties of
compound 1. The blue-green photoluminescence for the
shear-induced columnar phase is observed because of these
self-assembled structures.
Figure 4. IR spectra of 1 in different phases: a) isotropic (2108C);
b) cubic (1608C); c) shear-induced columnar (1608C).
=
ꢀ
addition, the peaks corresponding to the C O and N H
stretches observed in the shear-induced columnar phase
become sharper than those observed in the cubic phase
(Figure 4b,c). These observations also suggest that mechan-
ical shearing induces a change in the self-assembled structures
of 1 at a molecular level.
Proposed structures for the self-assembled cubic and
shear-induced columnar phases of 1 are shown in Figure 5.
Compound 1 forms micellar structures in the Pm3n cubic
In summary, we have found that compound 1 shows a
shear-induced LC order-order transition from a micellar cubic
to a columnar phase; this transition is accompanied by a
change of the luminescent color. The micellar cubic phase
exhibited by 1 is a metastable LC phase. This isothermal
transition from metastable to stable phases may be useful for
dynamically changing the properties of functional liquid
crystals. The concept of metastable liquid crystals is very
useful for the development of a variety of stimuli-responsive
materials.
Received: January 12, 2008
Revised: March 19, 2008
Published online: June 2, 2008
Figure 5. Illustrations of the assembly of 1 in the cubic phase (a) and
in the shear-induced columnar phase (b). The dendritic side-chains of
1 are omitted in the middle and right-hand images of the assembled
luminescent cores.
Keywords: liquid crystals · nanostructures ·
photoluminescence · self-assembly · supramolecular chemistry
.
phase (Figure 5a, left); a number of LC compounds have
been reported to exhibit micellar cubic phases.[12,16,19] Each
micelle in the micellar cubic phase of 1 consists of a
segmented column formed by approximately 20 individual
molecules. The number of molecules forming each micelle
was calculated based on the results of the X-ray measure-
ments (see the Supporting Information). Similar segmented
columnar structures in a micellar cubic phase have been
reported previously.[16,19] The segmented columnar structures
in the cubic phase of 1 result from hydrogen bonding between
the amide groups of adjacent molecules and the formation of
p-stacked structures of the emission cores (Figure 5a, middle
and right). This type of self-assembly for 1 leads to yellow
excimer emission. Compound 1 should be arranged in
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