Linearly polarized photoluminescence from an asymmetric cyclophane showing thermo- and mechanoresponsive luminescence

Article abstract of DOI:10.1039/c8tc02919a

The first cyclophane to exhibit linearly polarized photoluminescence in the liquid-crystalline and crystalline states is described. An asymmetric cyclophane featuring a 4,7-bis(phenylethynyl)-2,1,3-benzothiadiazole group forms a thermodynamically metastab

Full text of DOI:10.1039/c8tc02919a

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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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DOI: 10.1039/C8TC02919A
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Linearly Polarized Photoluminescence from an Asymmetric
Cyclophane Showing Thermo- and Mechanoresponsive
Luminescence
Received 00th January 20xx,
Accepted 00th January 20xx
Yoshimitsu Sagara,*a,b Atsushi Seki,a Yuna Kima and Nobuyuki Tamaoki*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first cyclophane to exhibit linearly polarized photoluminescence in the liquid-crystallin and crystalline states is described.
An asymmetric cyclophane featuring 4,7-bis(phenylethynyl)-2,1,3-benzothiadiazole group forms a thermodynamically
metastable nematic liquid-crystalline phase at room temperature. The compound sandwiched between two glass substrates
coated with rubbed polyimide thin layers exhibits homogeneous alignment after shearing. The transition dipole moments
of the luminophores align along the rubbing direction. As a result, linearly polarized photoluminescence is achieved at room
temperature without any other host materials. Furthermore, the polarized emission is retained after transition to the
crystalline phase, which is induced by annealing at 60 C for 1 h. The cyclophane also shows a thermal and mechanical
stimuli-induced change in the photoluminescence colors.
liquid-crystalline states. Luminescent cyclophanes are intriguing
compounds as they may exhibit unconventional photophysical
functions that have never been achieved with the acyclic
Introduction
A large number of cyclophanes have been investigated to date
since [2.2]paracyclophane was reported in 1949.1 The
symmetric and strained character make them fascinating
targets in synthetic chemistry and much effort has been
devoted to establishing reliable ways to prepare them.2 The
cyclic structures have also attracted much attention from the
viewpoint of supramolecular chemistry because the inherent
cavity can function as hosts forming inclusive complexes with
guest molecules.2b,2c,3 This association behaviour has been used
to achieve various interlocked molecules such as catenanes and
rotaxanes.4 When photoluminescent aromatic groups are
incorporated into the cyclic structures, the resultant
luminescent cyclophanes exhibit photophysical properties that
are different from those of the monomer analogues.3e,5-12
Furthermore, the photoluminescence colour and/or intensities
commonly change upon the intercalation of matching guest
molecules or ions. The quantitative modulation of the
photoluminescence properties makes the luminescent
cyclophanes promising fluorescent probes to evaluate the
concentration of guest molecules or ions.
compounds.10-12 The cyclic structure disturbs the ordered and
close-molecular packing, which results in the formation of
liquid-crystalline (LC) phases over wide temperature ranges.
Even though a number of cyclophanes have been reported to
show LC phases,13,14 their luminescent properties remain
unexplored. As LC molecules change the molecular
arrangements on LC-LC or LC-crystalline phase transitions, the
photoluminescence properties change on the phase transition
process.15 This is because the photoluminescence properties of
molecular materials depend on the arrangement of the
luminophores inside the material.15-18 Thus, if external stimuli,
such as temperature changes and mechanical forces, can induce
phase transitions of luminescent cyclophanes, the
photoluminescence properties can be also controlled. Indeed,
luminescent
cyclophanes
having
a
1,6-
bis(phenylethynyl)pyrene or 9,10-bis(phenylethynyl)anthra-
cene group have been found to exhibit clear
photoluminescence colour changes from the supercooled
nematic phases to the crystalline phases.11a,12 Furthermore, the
latter cyclophane has been used as an energy acceptor in a
triplet–triplet annihilation-based photon upconversion system
showing reversible photoluminescence colour changes with
thermal stimuli.12
Most of the literature described above has focused on
luminescent cyclophanes in solution states where each
cyclophane is molecularly dispersed. In contrast, little attention
has been paid to luminescent cyclophanes in the solid and/or
Here, we report another attractive feature of luminescent
cyclophanes that show LC behaviour as well as external stimuli-
responsive luminescence. We demonstrate that an asymmetric
a. Research Institute for Electronic Science, Hokkaido University,
N20, W10, Kita-Ku, Sapporo 001-0020, Japan.
b. JST-PRESTO
Honcho 4-1-8, Kawaguchi, Saitama 332-0012, Japan.
*Email - sagara@es.hokudai.ac.jp
cyclophane
having
a
4,7-bis(phenylethynyl)-2,1,3-
benzothiadiazole moiety (Scheme 1) shows a nematic LC phase
at room temperature and exhibits linearly polarized light
Electronic Supplementary Information (ESI) available: Synthesis of compounds
1 and
2, additional DSC curves, emission decay profiles, XRD patterns and emission
images. See DOI: 10.1039/x0xx00000x
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emission after control of the molecular alignment on the glass
substrates with rubbed polyimide layers without any host liquid
crystals. Organic materials that exhibit linearly polarized
emission have been an intriguing target because of their
potential applications such as sophisticated displays19 and
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DOI: 10.1039/C8TC02919A
polarised
organic
light-emitting
diodes.20
Blending
luminophores with liquid-crystalline hosts is one of the best way
to achieve organic materials with linearly polarized emission
character. However, miscibility of the two components
sometimes becomes critical issues. To the best of our
Figure 1. POM images of cyclophane
1
(a) in the nematic phase at 120

C, (b) in the nematic phase at room temperature, and (c) in the
crystalline phase at room temperature. The POM images were taken in
the absence of a coverslip. Directions of A: analyzer; P: polarizer. Scale
bar: 40 m. The photoluminescence images of 1 on a quartz substrate
are shown in the inset of each POM image. The photoluminescence
images were taken under an excitation light of 365 nm in the dark.
knowledge, compound
cyclophane that exhibits polarized light emission in the absence
of any host liquid crystals. Cyclophane shows a nematic-
crystalline phase transition accompanied by
1 is the first example of a luminescent
1
a
photoluminescence colour change from yellow to green
through thermal treatment. Moreover, the latter colour returns
to yellow when a mechanical stimulus is applied to the green-
emissive crystalline phase. External stimuli-responsive
luminescence, as well as stabilization of the disordered LC
phases, can be ascribed to the cyclic molecular structure.
Scheme 1. Molecular structures of cyclophane
compound
1 and the linear reference
2
.
Figure 2. DSC traces of (a) compound
isotropic state and (b) compound on the first heating from the glassy
state. Scanning rate: 5
C min-1.
1 on the first cooling from the
1

Results and discussion
Molecular designs
wide temperature range on cooling from the isotropic phase
(Figure 1, Figure 2a). The polarized optical microscopic (POM)
images display a characteristic schlieren texture of a nematic
A
4,7-bis(phenylethynyl)-2,1,3-benzothiadiazole group was
selected as the luminescent core of cyclophane because of its
relatively high quantum yield.21 Though some rod-shaped
compounds with 4,7-bis(phenylethynyl)-2,1,3-
1
phase both at 120
1b). Further cooling results in a glass transition at 8.2
any crystallization (Figure 2a). However, when cyclophane
heated from the glassy phase, a nematic-crystalline phase
transition is observed at 66.5 (Figure 2b). As the

C and at room temperature (Figure 1a and
C without
is

a
1
benzothiadiazole group have been found to show nematic and
smectic phases, these acyclic compounds form only crystalline
phases at room temperature, which make it difficult for them to
achieve a uniaxial alignment of molecules in a large area at
room temperature.22 One naphthalene group was also

C
corresponding peak in the DSC trace is exothermic, the nematic
phase observed below this temperature is a thermodynamically
metastable state. Indeed, after annealing at 60 C for 1 h, the
introduced to the cyclic structure of
1 and the two aromatic
schlieren texture disappears from the POM observation (Figure
1c), which results from a phase transition from the metastable
nematic to the thermodynamically stable crystalline phase. The
thermal stimuli-induced crystalline phase exhibits green
photoluminescence and displays a phase transition to the
moieties were bridged by flexible pentaethyleneglycol chains.
The asymmetric cyclic structure was expected to decrease the
phase transition temperature from the nematic phase to the
highly ordered phases. An acyclic compound 2 bearing the same
luminophore was also synthesized as a reference.
nematic phase at 99.3
further heating (Figure 2b). When compound
emissive crystalline state is heated from −40

C and a clearing point at 178.5
in the green
C, only two
C upon
1
Phase transition behaviour
Compound 1, as expected, shows a nematic phase exhibiting
yellow photoluminescence under excitation light of 365 nm in a

endothermic peaks, which are ascribed to the transition from
2 | J. Name., 2012, 00, 1-3
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crystalline to nematic and nematic to isotropic, are observed on and gives two emission lifetimes of 0.5 and 4.6 ns. Furthermore,
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DOI: 10.1039/C8TC02919A
the DSC trace (Figure S1a). In keen contrast to
compound forms a nematic phase in a very narrow much lower than that of linear compound
temperature range from 86.6 to 82.7 C on the first cooling
(Figure S1b). Furthermore, no nematic phase appeared on the
DSC trace of on the second heating. This comparison
1, acyclic the emission quantum yield of 1 in chloroform (Φ = 0.15) is
2
2 (Φ = 0.99).

Thermoresponsive luminescence
2
Next, we also examined the thermal stimuli-responsive
luminescence behaviour of cyclophane 1 in the condensed state. As
shown in Figure 6, the photoluminescence spectrum of compound 1
in the nematic phase at room temperature shows a maximum at 565
nm. Because the emission band of the luminophore strongly
confirmed that the introduction of cyclic structures is a reliable
methodology to enhance liquid-crystallinity.
X-ray diffraction (XRD) patterns (Figure 3) clarified the
thermal stimuli-induced phase transition from the
thermodynamically metastable nematic phase to the stable
crystalline phase. The pattern obtained from cyclophane
1 in
the nematic phase at room temperature displays no clear peaks
(Figure 3a). In contrast, the crystalline state accessed through
annealing at 60 C for 1 h gives many sharp peaks in the XRD
pattern, which reflects the well-ordered crystalline nature
(Figure 3b).
Figure 4. (a) Absorption and (b) photoluminescence spectra of the chloroform
solutions of cyclophane 1 (black line) and the reference compound 2 (gray
line) (c = 1.0  10-5 M). All spectra were recorded at room temperature.
Photoluminescence spectra were obtained with an excitation light of 400 nm.
Figure 3. XRD patterns of 1 in the (a) nematic phase and (b) crystalline phase
after annealing the nematic phase at 60 C for 1 h. The patterns were
recorded at room temperature.
Photophysical properties in solution
Before investigating the linearly polarized light emission
properties, we conducted spectroscopic measurements for the
chloroform solution of
1 to obtain the fundamental
photophysical properties of an individual molecule. The
absorption and photoluminescence spectra of the chloroform
Figure 5. Emission decay profiles of cyclophane 1 (black line) and compound
2 (gray line) in chloroform. The profiles were monitored at 550 nm at room
temperature with excitation light of 405 nm.
solution are shown in Figure 4. A chloroform solution of
1 (c =
1.0

10–5 M) presents one absorption band between 350 and
500 nm and another absorption band between 270 and 350 nm.
The peak of the former band appears at 437 nm with a molar
depends on environmental polarity,23 we were unable to
consider the emission from the nematic phase as monomer
emission based on the fact that the emission band appears in a
similar wavelength region to that of the chloroform solution.
extinction coefficient of 2.7
band of linear compound
compound . As shown in Figure 4b, the chloroform solution of
displays a broad emission band with the maximum at 557 nm.
The emission band of is slightly blue-shifted compared with
that of , which results from non-negligible electronic ground-
state interactions between two aromatic groups in cyclophane
. This is supported by the emission lifetime measurement
(Figure 5). The emission decay profile recorded for the
chloroform solution of could be fitted to a single exponential
decay function and only one emission lifetime of 6.0 ns is
obtained, while cyclophane in chloroform gives an emission

104 L mol-1 cm-1. The absorption
(gray line) is very similar to that of
2
1
The spectrum of cyclophane
1 shows a significant blue shift
1
after annealing at 60 C for 1 h (λem,max = 536 nm), which

2
coincides with the color change shown in Figure 1. The emission
quantum yield also changes from 0.28 to 0.32 on the nematic to
crystalline phase transition. The quantum yields are higher than
that of solution because the non-radiative decay pathway is
restricted owing to the suppression of the intramolecular
motion at the luminophore in the condensed states. Emission
lifetime measurements were also performed for both phases to
obtain more insight into the thermoresponsive luminescent
behaviour (Figure 7). The emission decay profiles recorded for
1
1
2
1
decay profile that is fitted to a bi-exponential decay function
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the nematic and crystalline phases could be fitted to tri- are 0.9 and 0.55 at 437 nm, respectively. TVhieew AortbicsleeOrvnleinde
DOI: 10.1039/C8TC02919A
exponential and bi-exponential decay functions, respectively. A difference in absorbance is smaller than that of the reported
relatively longer emission lifetime of 7.9 ns is observed for the nematic liquid crystal doped with a rod-shaped compound
nematic phase, whereas such longer-lived emissive species are having the same luminophore (0.25 mol%).24 This is ascribed to
not observed for
Because the latter emission lifetimes are similar to those of the compound can easily align in the nematic LC host. Whereas in
monomeric state of in chloroform, we proposed that the the case of compound , the cyclic structure disturbed the well-
1 in the crystalline phase (1.9 and 4.1 ns). the cyclic molecular structure of compound 1. The rod-shaped
1
1
luminophores in the crystalline state are in a monomer-like ordered molecular assemblies leading to a larger distribution of
environment. The emissive species with a long lifetime directors, and consequently the smaller absorbance difference.
observed for the nematic phase could be ascribed to the As shown in Figure 8c, the aligned cyclophane
1 in the nematic
associated state of the luminophores owing to the low-ordered phase clearly shows linearly polarized photoluminescence
nature of the nematic liquid crystals.
under an excitation light of 365 nm. This is the first example of
cyclophanes that show a linearly polarized light emission. The
ratio of the photoluminescence intensity is 2.9 at the peak of
564 nm in the direction parallel and perpendicular to the
rubbing direction, and the ratio does not significantly change
between 530 and 650 nm (Figure S2a). After the thermal
stimuli-induced phase transition to the crystalline state, the
emission band showed
a blue shift and the polarized
photoluminescence was still observed (Figure 8d). However, the
ratio described above decreases to 1.7 and this value is
maintained over the wavelength range from 530 to 650 nm
(Figure S2b). This implies that the crystallization process
disturbed the well-aligned structures which are readily achieved
in the nematic phase.
Figure 6. Photoluminescence spectra of 1 in the nematic phase (orange line)
and the crystalline state that was accessed thorough annealing (green line).
All spectra were recorded at room temperature with an excitation light of 400
nm.
Figure 7. Emission decay profiles of cyclophane 1 in the nematic phase
(orange line) and in the crytalline phase (green line). The profiles were
monitored at 550 nm at room temperature with excitation light of 405 nm.
Linearly polarized emission
As cyclophane 1 was found to show the nematic phase at room
temperature, we investigated the uniaxial alignment control
and linearly polarized light emission in the nematic phase of
compound
the nematic phase was sandwiched between two glass
substrates with rubbed polyimide surfaces. After compound
between the substrates were sheared along the rubbing
direction at 120 and subsequent cooled to room
temperature, a homogeneous texture of the nematic phase was
observed on the POM observation (Figure 8a). An absorption
spectroscopic measurement clarified the transition dipole
moment, which is parallel to the long axis of the luminophore
1 at room temperature (Figure 8). Compound 1 in
Figure 8. (a) POM image of cyclophane
1
sandwiched between glass
is in the
substrates with rubbed polyimide thin layers. Cyclophane
1
1
nematic phase. Directions of A: analyzer; P: polarizer; R: rubbing. (b)
Absorption spectra recorded in the direction parallel (solid line) and
perpendicular (dotted line) to the rubbing direction. (c,d) Polarized

C
photoluminescence spectra of
1 in the nematic phase (c) and the
crystalline phase accessed through an annealing process (d). The
polarized emission spectra were recorded in the direction parallel (solid
line) and perpendicular (dotted line) to the rubbing direction with an
excitation light of 365 nm. All spectra were measured at room
temperature.
of 1 24
, aligns along the rubbing direction (Figure 8b). Although
the scattering increases the baselines, the polarized
absorbances parallel and perpendicular to the rubbing direction
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in our previous study.11a We assume that the recovery process
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would be ascribed to remaining original crystals and the flexible
Mechanoresponsive Luminescence
It is noteworthy that the mechanical stimulus also changes the molecular structure of cyclophane
photoluminescence color of in the crystalline state that was emitting state still contains a tiny amount of original crystals.
obtained after annealing at 60 C for 1 h to the nematic phase. The remaining crystals could function as seed crystals and
When a mechanical stimulus is applied to the crystal of immediately induce the crystallization of the mechanical
compound
at room temperature, the photoluminescence stimuli-induced low-ordered state.25 The phase transition
color changes from green to yellow (Figure 9). The rapidly spreads over the sample due to the higher mobility of
1. After grinding, the yellow
1

1
photoluminescence spectrum showed a red shift and an the cyclophane
emission peak was observed at 571 nm (Figure 10), which is
1
that has flexible linkers.26
similar to that of the nematic phase. The emission decay profile
Conclusions
(Figure S3) is also similar to that of
1 in the nematic phase. The
obtained curve could be fitted to a tri-exponential decay
function and gave an emission lifetime of 6.8 ns. This is ascribed
to luminophores in the associated state as observed in the
nematic phase. These data suggest that mechanical stimuli lead
to a low-ordered state compared with the crystalline states,
though XRD measurements were not able to be performed
owing to the rapid recovery to the initial crystalline states. After
In conclusion, we demonstrate that an asymmetric
benzothiadiazole-based cyclophane exhibits linearly polarized
photoluminescence as well as thermo- and mechano-
responsive luminescence in the condensed state. The cyclic
character disturbs the ordered molecular packing, which leads
to the nematic LC phase at room temperature. Since compound
1
1
shows a phase transition to the crystalline phase through
the recovery, the compound
1 gave several X-ray diffraction
thermal treatment, the nematic phase is a thermodynamically
metastable state. A homogeneous alignment is obtained when
the cyclophane is sandwiched between glass substrates with
rubbed polyimide thin layers. The transition dipoles are aligned
parallel to the rubbing direction. Consequently, linearly
polarized emission is achieved and remains after a transition to
the crystalline states. To the best of our knowledge, this is the
first example of cyclophanes showing linearly polarized
emission. The molecular design strategy adopted in this study
may be general and result in a number of less-ordered
molecular assemblies that can provide unconventional
photophysical functions and stimuli-responsive behaviour.
peaks at the same positions as those for the initial crystalline
phase (Figure S4). Furthermore, this recovery process is highly
temperature dependent. As shown in Figure S5, mechanical
stimuli-induced yellow emissive part still can be seen after 5 min
when the ground sample was kept at 15
a ground sample on a hot stage kept at 40

C. In contrast, putting
C results in

immediate recovery of the initial green emission (Figure S5f).
The temperature dependent character observed here was
similar to that found for a pyrene-based asymmetric cyclophane
Experimental Section
General methods and materials
Figure 9. Images documenting the mehcanical stimuli-induced change
All reagents and solvents were purchased from Tokyo Kasei,
Wako, and Aldrich. All reactions were carried out under
nitrogen atmosphere. Silica gel column chromatography was
carried out with a Biotage Isolera Flash system. 1H NMR spectra
were measured with a JEOL JNM-ECX 400 spectrometer and all
chemical shifts are quoted in ppm relative to the signal of
tetramethylsilane (δ = 0.00) as an internal standard. 13C NMR
spectra were measured with a JEOL JNM-ECX 400 spectrometer
and all chemical shifts (δ) are quoted in ppm using residual
solvents of CDCl3 (δ = 77.16) and DMSO (δ = 39.52) as the
internal standard. Coupling constants (J) are reported in Hz and
relative intensities are also shown. Matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) mass spectra
were recorded on an AB SCIEX TOF/TOF 5800. High resolution
electrospray ionization (ESI) mass spectra were obtained on a
Thermo Scientific Exactive. Elemental analysis was conducted
with an Exeter Analytical CE440 Elemental Analyzer. The DSC
measurements were carried out using a Hitachi DSC 7020 with
a heating/cooling rate of 5 ˚C/min under nitrogen atmosphere.
Powder X-ray diffraction measurements were conducted with a
in photoluminescence color of
1
at room temperature. (a) Compound
1
in the crystalline phase before grinding. (b) Compound
1
after grinding
at room temperature. The images were taken on quartz substrates
under excitation light at 365 nm.
Figure 10. Photoluminescence spectra of 1 in the crystalline state (green
solid line) and in the ground state (orange dotted line). All spectra were
measured at room temperature (λex = 400 nm).
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JASCO V-550. Steady-state fluorescence spectra were
measured on a JASCO FP-6500. Time-resolved fluorescence
measurements were conducted with a Hamamatsu Photonics
Quantaurus-Tau. Quantum efficiencies of compounds in
solution were calculated using Rhodamine 6G in EtOH (Φ = 0.95)
as the reference. Quantum efficiencies for solid samples were
3
4
measured with
a Hamamatsu Photonics Quantaurus-QY.
Polarized optical microscopic observations were carried out
with an Olympus BX-60 optical microscope equipped with a
Sony DXC-950 3CCD camera. An AS ONE hot plate NA-1 was
used for the thermal treatment. Linear Polarizer Film was
purchased from MeCan Imaging Inc.. Emission images were
taken with Canon EOS 9000D.
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Linearly polarized photoluminescence measurements
For the linearly polarized photoluminescence measurements,
mechanically rubbed polyimide (PI)-coated glass substrates
were used. The glass substrates were washed with methanol
and dried before preparing the alignment layer. The PI
precursor solutions (SE-150) and thinner solvent (26 thinner)
were purchased from Nissan Chemical Industries, Ltd. A
solution of PI varnish was mixed with the thinner at a ratio of
1/2 (wt/wt). The mixture of the PI precursor was spun on the
glass substrates at the rate of 1500 rpm for 50 s. After the
spincoated glass substrates were prebaked on the hot plate for
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The prepared PI surface was rubbed in one direction by using a
nonwoven fabric.
6
7
The linearly polarized photoluminescence measurement
was carried out by using an Ocean Optics QEPro equipped with
a R400-7-VIS-NIR reflectance probe as emission light detector.
The excitation light source was a Handy UV lamp AS ONE SLUV-
4 (excitation wavelength: 365 nm). The sample LC cell and the
light source were set on the stage. The incident angle of the
excitation light to the sample was tuned to approximately 45
degrees. The detector was placed in the appropriate position
where reflection of the excitation UV light was not detected.
8
9
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Conflicts of interest
There are no conflicts to declare.
7
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Acknowledgements
We thank Prof. Y. Urano for emission quantum yield
measurements. This work was supported by JSPS KAKENHI
Grants JP16H00818 and JP18H02024, the Futaba Electronics
Memorial Foundation, the Kurita Water and Environment
Foundation, and the Foundation Advanced Technology
Institute.
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TOC entry
The first cyclophane to exhibit linearly polarized photoluminescence in the liquid-crystallin and
crystalline states is described.