Brightly tricolored mechanochromic luminescence from a single-luminophore liquid crystal: Reversible writing and erasing of images

Article abstract of DOI:10.1002/anie.201100914

A glowing image: The photoluminescent colors reddish-orange, yellow, and green, are generated from a single liquid-crystalline mixture containing one luminophore (see picture). The colors are easily distinguished by the naked eye and can be reversibly written and erased. Moreover, these luminescent colors can be switched by mechanical and thermal stimuli. Copyright

Full text of DOI:10.1002/anie.201100914

Communications
DOI: 10.1002/anie.201100914
Materials Chemistry
Brightly Tricolored Mechanochromic Luminescence from a Single-
Luminophore Liquid Crystal: Reversible Writing and Erasing of
Images**
Yoshimitsu Sagara and Takashi Kato*
Luminescent materials that switch their photoluminescent
properties in response to various external stimuli have
attracted much attention for a decade because of their
potential application for memory devices, sensors, security
materials, and informational displays.[1–5] To induce change in
the luminescent colors of organic and organometallic materi-
als, one could switch the molecularly assembled structures.[4–7]
Crystals,[8–24] liquid crystals,[25–28] and polymers[29–32] have been
reported to change their luminescent colors by mechanical
and thermal stimuli. The observed phenomena are referred to
as piezo(mechano)chromic or thermochromic luminescence.
Such materials contain just one type of luminophore and they
form only two luminescent states. In the condensed states, to
date, multiemission colors that are generated from a single
luminophore and switch between their colors using external
stimuli have not been achieved for organic and organometal-
lic materials; the one exception is a crystalline compound.[16]
If such materials are prepared using liquid crystals, it leads to
new applications of stimuli-responsive luminescent materials
that are flexible, sophisticated, and highly functional.
Liquid crystals are functional soft materials that exhibit
mobile and ordered states.[33] Because of their dynamic
properties, liquid crystals are good candidates for stimuli-
responsive luminescent materials.[25–28] In our previous stud-
ies, we have prepared pyrene-, anthracene-, and naphthalene-
based liquid crystals that show piezo(mechano)chromic
luminescence and thermochromic luminescence in liquid-
crystalline (LC) states.[25,26] However, multiluminescent colors
have not been achieved for these LC materials.
Herein we report a new type of stimuli-responsive
luminescent liquid crystal that exhibits three luminescent
colors, which can be switched by mechanical and thermal
stimuli (Figure 1). The liquid crystal is composed of equimo-
Figure 1. Procedures for obtaining the three different luminescent
colors exhibited by a mixture of compounds 1 and 2. The photo-
luminescent images were taken for the mixtures between quartz
substrates under UV irradiation at 365 nm. Scale bar: 5 mm.
lar amounts of the dumbbell-shaped compound 1 and
compound 2 (Scheme 1). The three luminescent colors
observed are reddish-orange, yellow, and green, which are
easily distinguished by the naked eye. Moreover, the LC
mixture of 1 and 2 contains only one type of luminophore,
9,10-bis(phenylethynyl)anthracene,[34,35] and no additives are
required to induce the luminescent color changes. Unlike
crystalline materials, the thin-film states are easily prepared
(see below). The change in the luminescent colors and the
phase-transition behavior of the LC mixture are summarized
in Figure 1.
The mixture forms a thermotropic micellar cubic phase
upon heating from room temperature to 1468C. Recently
thermotropic cubic phases classified as micellar cubic phases
were reported.[25,36] The characterization of the LC phases is
discussed in the Supporting Information. Under UV irradi-
ation (365 nm), reddish-orange photoluminescence is
observed for the micellar cubic phase (Figure 1, top left).
We have found that mechanical shearing to the mixture in the
cubic phase at 908C triggers a change in the luminescent color
from reddish-orange to green (Figure 1, top left!top right).
The piezo(mechano)chromic luminescent behavior is accom-
panied by a shear-induced phase transition from the micellar
cubic phase to the columnar phase.[25] The shear-induced
columnar phase is stable from room temperature to 1468C
[*] Dr. Y. Sagara, Prof. T. Kato
Department of Chemistry and Biotechnology
School of Engineering, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: kato@chiral.t.u-tokyo.ac.jp
Homepage: http://kato.t.u-tokyo.ac.jp/index-e.html
[**] We thank Prof. K. Araki and Dr. T. Mutai for measurements of
emission lifetime and quantum efficiency. This work was partially
supported by the Grant-in-Aid for Scientific Research on Innovative
Areas of “Fusion Materials: Creative Development of Materials and
Exploration of Their Function through Molecular Control” (No.
2206) (T.K.) from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), and The Global COE Program (Chemistry
Innovation through Cooperation of Science and Engineering) (T.K.)
from MEXT. Y.S. is grateful for financial support from the Japan
Society for the Promotion of Science Research Fellowship for Young
Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100914.
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phase exhibiting yellow emission
(Figure 1, bottom). Upon heating
the mixture in the mesomorphic
phase, the mixture shows a transition
to the isotropic phase at 1458C. On
subsequent cooling, reddish-orange
emission is observed again from the
mixture in the micellar cubic phase.
And on heating the mesomorphic
phase from ꢀ508C, a few of exother-
mic peaks were observed on the
differential scanning calorimetry
trace (see the Supporting Informa-
tion). The results imply the meso-
morphic phase is a thermodynami-
cally metastable phase.
By following proper procedures
(see the Supporting Information),
the tricolored luminescent pattern
in Figure 2 was obtained at room
temperature. The quantum yields of
the mixture in the condensed state
are lower than that observed for
compound 1 in a chloroform solution
(Table 1). However, these values are
sufficient for various applications
such as mechano-sensors, indicators
of mechano-history, security papers, optoelectronic devices,
and data storage.
Scheme 1. Molecular structures of compounds 1 and 2.
and shows no isothermal transition back to the micellar cubic
phase. The increase of the transition enthalpy corresponding
to clearing points suggests that the shear-induced columnar
phase is more thermodynamically stable than the cubic phase
(see the Supporting Information). However, once the mixture
of compounds 1 and 2 forms the isotropic phase, the columnar
phase does not appear without mechanical shearing. We have
also found that the mixture exhibits another change in
luminescent color. When the mixtures in the micellar cubic
or columnar phases are mechanically sheared at room
temperature, the mixtures show an unidentified mesomorphic
Table 1: Emission lifetime and quantum efficiency of the mixture of
compounds 1 and 2.[a]
Lifetime [ns] Quantum efficiency
compound 1 in chloroform solution 2.0
0.80
0.05
0.13
0.15
mixture in the cubic phase
mixture in the columnar phase
1.8, 20
1.6, 4.0
mixture in the mesomorphic phase 1.4, 9.3
[a] All measurements were carried out at room temperature. The
concentration of compound 1 in chloroform is 1ꢀ10ꢀ6 m.
The spectroscopic measurements of the material were
performed to obtain insight into the change in the lumines-
cence of the mixture. The absorption spectrum of 1 in a
chloroform solution (1 ꢀ 10ꢀ5 m; Figure 3a, gray line) displays
an absorption band with vibronic structures between 400 and
500 nm (S0!S1 transition). For the mixture in the assembled
states, the absorption bands ascribed to the S0!S1 transition
broaden compared to that of 1 in the chloroform solution
(Figure 3a, green, yellow, and red lines), thus indicating that
ground-state electronic interactions between luminescent
groups occur in the assembled states. In addition, the spectral
features for the three assembled states are different from each
other. These observations suggest that the arrangements of
the luminescent groups of compound 1 change on the shear-
induced phase transitions.
Figure 2. Procedures of writing and erasing tricolored luminescent
images. Photoluminescent images were taken for the mixtures on a
glass substrate under UV irradiation at 365 nm. Scale bar: 1 cm. Top
panel shows two mechanically sheared sections a and b.
The spectral features of the emission in these states are
obviously different. In the emission spectrum of 1 in a
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As for the mixture in the mesomorphic phase, two peaks
appear in the emission spectrum (Figure 3b, yellow line). The
broad peak with lmax = 583 nm can be attributed to partial-
overlap excimers of the emission cores. A partial-overlap
excimer is an excimer with partial overlap of the aromatic
cores.[37] Compared to normal excimers, the partial-overlap
excimers exhibit emission bands at shorter wavelengths than
those observed for normal excimers. In addition, the emission
lifetime of the partial-overlap excimers is often shorter than
that of normal excimers. The partial-overlap excimers have
been observed in the highly viscous medium such as polymers
and Langmuir–Blodgett films.[38] The photophysical proper-
ties of the mixture in the mesomorphic phase support the
formation of the partial-overlap excimers. From emission
lifetime measurements, the mixture contains a lifetime
component (9.3 ns) that is shorter than that of the excimer
in the cubic phase and longer than that observed for the
columnar phase. The position of the other peak with lmax
=
541 nm (Figure 3b, yellow line) is identical to that of the peak
in the emission spectrum for the mixture in the columnar
phase (Figure 3b, green line). In addition, similar lifetime
components are detected for the mixture in the columnar and
mesomorphic phases (Table 1). Therefore, we concluded that
the yellow emission observed for the mesomorphic phase is
composed of the emissions from partial-overlap excimers and
the same emission spices as in the columnar phase. Notably,
possibility that twisting of the chromophores may have a little
influence on the change of luminescent colors cannot be ruled
out.
The self-assembled structures of compounds 1 and 2 in the
LC phases and the mesomorphic phase are proposed in
Figure 4. In the cubic phase, compounds 1 and 2 form micellar
structures (Figure 4a). Each micelle contains an equal
number of compounds 1 and 2.
Figure 3. a) Absorption spectra of 1 in chloroform (1ꢀ10ꢀ5 m; gray
line), the mixture of compounds 1 and 2 in the cubic (red line),
columnar (green line), and mesomorphic (yellow line) phases. b) Nor-
malized emission spectra of 1 in chloroform (1ꢀ10ꢀ5 m; gray line), the
mixture in the cubic (red line), columnar (green line), and mesomor-
phic (yellow line) phases. lex =440 nm. All absorption and emission
spectra were obtained at room temperature.
chloroform solution (1 ꢀ 10ꢀ5 m; Figure 3b, gray line), a well-
resolved vibronic structure indicative of the monomeric state
of 1 is observed (lmax = 497 nm). As for the cubic phase, the
emission band is significantly red-shifted and broadened
(Figure 3b, red line) compared to that of 1 in the chloroform
solution. The broad and structureless emission band at lmax
=
630 nm (red line) is attributed to the excimer formation of the
luminescent cores, 9,10-bis(phenylethynyl)anthracene moiet-
ies. The emission decay time measurements of the samples
(Table 1) support the excimer formation. Compared to the
emission lifetime for 1 in chloroform solution (2.0 ns), a
longer lifetime component of 20 ns is observed for the
mixture in the cubic phase at room temperature. 9,10-
Bis(phenylethynyl)anthracene derivatives have been
reported to exhibit excimer emission.[35] The emission spec-
trum of the mixture in the columnar phase (Figure 3b, green
line) displays an emission band (lmax = 540 nm) at shorter
wavelengths relative to that of the mixture in the cubic phase.
The large hypsochromic shift of the emission band on the
shear-induced phase transition from the cubic phase to the
columnar phase leads to apparent change in luminescent
color from reddish-orange to green (Figure 1, top left!top
right). For the columnar phase, the longer lifetime component
like that observed for the cubic phase cannot be detected
(Table 1).
Figure 4. Schematic illustration of the assembled structures of com-
pounds 1 and 2. a) Cubic phase, b) columnar phase, and c) mesomor-
phic phase. Amide groups of compound 1: blue spheres. Compound 2
and dendritic moieties of compound 1 are omitted in the detailed
illustration to the right of each structure.
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The total number of molecules forming each micelle is
approximately 32, which is calculated based on the results of
the X-ray measurements (see the Supporting Information). In
each micelle approximately 16 molecules of compound 1
form the segmented column. The segmented columnar
structure is built through both the formation of p–p stacked
structures of the emission cores and the formation of hydro-
gen bonding between the amide groups of adjacent molecules
(see the Supporting Information). The emission cores should
be arranged in a disordered stacking arrangement in the
segmented columns, because the distance between adjacent
arenes forming the p–p stacked structures is generally shorter
than the length of H-bonded amide groups.[25] The combina-
tion of the dumbbell-shaped compound 1 with compound 2
leads to the formation of stable segmented columnar assem-
bly. As a consequence, reddish-orange excimer emission
occurs in the cubic phase (Figure 1, top left). Such assembled
structures are supported by our previous results on the
assembled structures of pyrene, anthracene, and naphthalene
derivatives having similar molecular structures to compound
1.[25]
between three different luminescent colors in the condensed
state depending upon the molecularly assembled structures.
Moreover, these luminescent images are capable of being
written and erased. If a single luminophore component is
sufficient to achieve a multiluminescent color device, it can
lead to cost reduction in the production of multicolor
luminescent displays and sophisticated stimuli-responsive
luminescent materials. Our results also imply that the switch-
ing of assembled structures of luminescent groups is one of
the most promising ways to obtain external-stimuli-respon-
sive luminescent materials, which adds to the conventional
approaches of inducing change to the molecular structures
itself by light, pH, redox, and mechanical stimuli.
Received: February 5, 2011
Revised: April 16, 2011
Published online: July 27, 2011
Keywords: liquid crystals · luminescence · materials science ·
.
phase transitions · supramolecular chemistry
The molecularly assembled structures in the columnar
phase are depicted in Figure 4b. In each column, compound 1
forms columnar structures through the formation of a linear
hydrogen-bonding array that is not observed in the cubic
phase. The luminescent cores are spaced by 5 ꢁ (approx.)
intervals, thus leading to the interference with the excimer
formation of 9,10-bis(phenylethynyl)anthracene moieties.
These proposed assembled structures of compound 1 are
supported by previous reports on the crystal structures for
some arenedicarboxamides.[39] Green photoluminescence of
the mixtures in the columnar phase (Figure 1, top right) is
attributed to these depicted assembled structures. Upon
formation of the shear-induced columnar phase, compound
2 may exist among the hydrogen-bonded columns formed by
the dumbbell-shaped compound 1. The existence of 2 may
stabilize the columnar structures consisting of compound 1.
Without compound 2, compound 1 does not form a columnar
phase (see the Supporting Information).
In the mesomorphic phase (Figure 4c), it is assumed that
compound 1 forms less-ordered columnar structures, though
no clear peaks appear in the X-ray diffraction pattern (see the
Supporting Information). This assumption is based on the fact
that almost all of the amide groups of compound 1 are
involved in the formation of linear hydrogen bonds similar to
that in the columnar phase (see the Supporting Information).
In addition, a lifetime component similar to that observed in
the columnar phase was also detected in the mesomorphic
phase (Table 1). These results suggest that the luminescent
cores in the mesomorphic phase are partially arranged in a
linear arrangement similar to that in the columnar phase. In
addition, some of the cores are partially overlapped, thus
leading to the partial-overlap excimer emission (Figure 4c).
Energy migration and energy transfer may occur from non-
overlapped luminescent cores to the partial-overlap excimer
sites. Therefore, the mixture exhibits yellow emission in the
mesomorphic phase (Figure 1, bottom).
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