A mechano- and thermoresponsive luminescent cyclophane

Article abstract of DOI:10.1039/c6cc01614f

The first fluorescent cyclophane with mechano- and thermoresponsive solid-state fluorescence characteristics is reported. The new cyclophane comprises two 9,10-bis(phenylethynyl)anthracene moieties that are bridged by tetraethylene glycol spacers. The stimuli-responsiveness is based on molecular assembly changes.

Full text of DOI:10.1039/c6cc01614f

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A Mechano- and Thermoresponsive Luminescent Cyclophane
*
a,b
b,c
a
*b
Yoshimitsu Sagara, Yoan C. Simon, Nobuyuki Tamaoki and Christoph Weder
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first fluorescent cyclophane with mechano- and thermo- which they adopt different conformations and/or intermolecular
6
g
responsive solid-state fluorescence characteristics is reported. The interactions. Organic molecules of various shapes – from
new cyclophane is comprised of two 9,10-bis(phenylethynyl)- simple rods to fan or dumbbell shapes – have been demonstraꢀ
2
,6ꢀ8
ted to form assemblies with MRL properties.
Here we report
anthracene moieties that are bridged by tetraethylene glycol
spacers. It is shown that the solid-state emission color of the
compound can be influenced by mechanical and thermal
treatments. This stimuli-responsive behavior is based on changes
of the molecular assembly.
the first fluorescent cyclophane with mechanoꢀ and thermoresꢀ
ponsive solidꢀstate fluorescence characteristics. While the
properties of cyclic compounds have been widely studied in
9
solution, their stimuliꢀresponsive photophysical properties in
the solid state were little explored. On a more fundamental
level, our study demonstrates that simple cyclic structures are
useful to induce several thermodynamically (meta)stable states,
presumably on account of spatial restrictions that prevent the
luminophores to assemble in thermodynamically most stable
closedꢀpacked structures. We expect that the findings can be
generalized and that the MRL feature may be readily imparted to
The last decade has witnessed a continuously increasing number of
studies on materials in which the application of mechanical force
1
,2
3
leads to the controlled cleavage of stable or dynamic covalent
4
bonds, or the dissociation of supramolecular motifs. This general
mechanism has been shown to be useful for the design of
stimuliꢀresponsive materials, whose properties can be altered
upon application of mechanical forces. If the forceꢀinduced
separation involves mechanically active motifs (referred to as
mechanophores) with extended πꢀconjugated groups, changes
of the material’s absorption and/or photoluminescence
other luminescent motifs by converting into cyclic structures
.
Fig.1 Molecular structures of cyclophane 1 and the linear reference compound 2.
1
,2
properties occur. Especially, the latter can be easily detected,
making mechanically responsive (luminescent) colorꢀchanging
materials potentially useful, for example in the in situ
5
monitoring of material failure due to stress fracture or fatigue.
However, the number of luminophores whose photophysical
properties can be altered by mere dissociation of covalent or
nonꢀcovalent bonds is still limited and few generally applicable
1
bꢀd,h,2
design principles have been established.
Mechanical forces can also be exploited to induce changes of
molecular arrangements, and a large number of mechanoꢀ
responsive luminescent (MRL) materials have been reported,
which change their emission characteristics upon mechanical
2
,6ꢀ8
stimulation.
A most effective strategy to create MRL
materials is to design luminescent molecules so they can
Cyclophane 1 (Fig. 1) was designed to feature two 9,10ꢀ
assemble in different thermodynamically (meta)stable states, in bis(phenylethynyl)anthracene moieties, as these chromophores (in
their nonꢀconnected state) are well known to exhibit strong green
1
0
fluorescence in good solvents. Flexible tetraethylene glycol linkers
were chosen to connect the peripheral phenyl rings of two 9,10ꢀ
bis(phenylethynyl)anthracene moieties to serve as bridge and to
enhance the solubility. Because of its cyclic structure, compound 1
was expected to show intramolecular excimer formation and/or exciꢀ
ton coupling between the two luminescent cores in the molecularly
isolated state. In the solid state, intraꢀ and intermolecular interactions
between the luminophores were expected to occur, which should inꢀ
dependently affect the photophysical properties of the resulting moꢀ
lecular material. The linear monoꢀ9,10ꢀbis(phenylethynyl)anthracene
2
was also prepared and used as a reference compound.
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Despite the tetraethylene glycol linkers, cyclophane
1 shows an extra vibronic band at higher energy (459 nm) and a proꢀ
a low solubility in common organic solvents, presumably due to nounced shoulder that stretches far into the red. The latter is
1
1
the rigid structure and strong
πꢀπ interactions between indicative of intramolecular excimer formation, which has
molecules. NMR spectroscopic measurements were, however, also been observed in the case of pyreneꢀ or peryleneꢀbased
1
possible in hot chloroform. A comparison of the H NMR cyclophanes, i.e., in cases where the planar luminophores can
1
2,13
spectra of
the resonances of the luminescent cores for cyclophane
and 0.49 ppm for the outer and inner protons of the anthracene characteristics of
groups, respectively), indicative of the formation of a cyclic state lifetimes ( ) and photoluminescence quantum yields (
structure, which changes the circular current. Together with the in chloroform (Table S1, ESI†). Fig. S2 (ESI†) shows timeꢀ
mass spectroscopy data, the large difference clearly confirms resolved emission decay profiles for both and . In the case of
the success of the cyclization reaction (Scheme S1, ESI†). the linear reference compound , the decay curve is well fitted
by a single exponential decay function with a lifetime of 2.9 ns
Fig. S2a, ESI†), which is characteristic of fluorescence from
wellꢀindividualized chromophores. The emission decay of
1
and
2
(Fig. S1, ESI†) reveals an upꢀfield shift of form faceꢀtoꢀface stacked structures.
(0.46 To gain further insight into the difference of the emission
and in solution, we measured the excited
1
1
2
τ
PL
Φ )
1
2
2
(
1
was measured at 495 and 600 nm (Fig. S2b, ESI†). In both
cases, multiꢀexponential decay functions fitted the data better
than a single exponential decay function. The decay curve
collected at 600 nm comprises a minor component with a
lifetime of 8.8 ns that is ascribed to intramolecular excimers.
By contrast, the longest emission lifetime observed at 495 nm
was 5.8 ns, which can be attributed to monomer emission after
dissociation of intramolecular excimers. At both wavelengths,
the emission lifetime related to monomer (2.6 ns) is shorter
than in the case of
2, on account of the formation of
intramolecular excimers. The fastest decay process with
emission lifetimes of 0.3 and 0.2 ns at 495 and 600 nm
respectively is likely associated with exciton coupling between
adjacent luminophores in the cyclophane. Indeed, as shown in
Fig. 2a, the molar absorption coefficient at 446 nm is larger
than that at 473 nm, which is indicative of exciton coupling
with Hꢀtype geometry, although the precise conformation of
in chloroform is unclear. Aside from minor internal
reabsorption features, the emission and absorption spectra of
remained unchanged when the concentration in chloroform was
1
1
–
5
ꢀ5
ꢀ6
ꢀ7
Fig. 2 Absorption (a) and emission (b) spectra of dilute (1·10 M) chloroform solutions
of cyclophane 1 (orange line) and the linear reference compound 2 (green line). All
spectra were measured at room temperature. The emission spectra were recorded
reduced from 10 to 10 (absorption) or 10 M (emission)
ESI†, Fig. S3), confirming that the observed exciton coupling
is an intramolecular process and does not arise from the
aggregation of cyclophane . This interpretation is further
(
with λex = 400 nm and intensities were normalized.
1
supported by the fact that previously reported peryleneꢀbased
Spectroscopic measurements of cyclophane
compound
chloroform solutions (Fig. 2). The absorption bands between emission peak at 459 nm (vide supra). While radiative
00 and 500 nm (Fig. 2a) show similar features, but upon closer relaxation from the higher energy level resulting from the
inspection subtle differences can be discerned. While the splitting of the lowest unoccupied molecular orbital of the
spectrum of cyclophane
displays two peaks at 446 and 473 monomer species is forbidden in an ideal Hꢀaggregate geometry,
nm, these are observed at 450 and 471 nm in the case of the the twisted arrangement of transition dipole moments allows
1
and model cyclophanes also exhibit absorption bands that correspond to Hꢀ
–
5
13
2
were first carried out in dilute (1·10 M) type exciton coupling,
and the additional highꢀenergy
4
1
linear reference compound
two transitions is almost identical in
2
. Moreover, the magnitude of the the direct decay process from the higher and forbidden energy
4 –1 –
14
2
(
ε
= 3.7·10 L·mol ·cm
the molar absorption coefficient at 446 nm (ε =
4
level to some extent. Our interpretation of the emission decay
profiles is further supported by the results of
photoluminescence quantum yield measurements. Indeed, the
1
)
5
, whereas in
1
4
–1
–1
.7·10 L·mol ·cm ) is larger than that at 473 nm (ε = 4.7·10
–1 –1
L·mol ·cm ). The difference in spectral shape seen in the photoluminescence quantum yield of
absorption spectra of and
and the fact that the extinction 0.07) is one order of magnitude lower than that of reference
are only slightly (and not two times) higher compound PL = 0.90), on account of intramolecular
are the first indication of intramolecular, excimer formation and exciton coupling in the cyclophane.
The thermoꢀ and mechanoresponsive luminescent behavior
of cyclophane is summarized in Fig. 3. Upon precipitating
In dilute (1·10 M) chloroform solution, the linear reference cyclophane from a hot concentrated chloroform solution into
compound
is highly emissive and the emission spectrum hexane, a yellowꢀlight emitting crystalline powder (YCRꢀform)
PL
1 in chloroform (Φ =
1
2
coefficients of
than those of
1
2
2 (Φ
groundꢀstate electronic interactions between two 9,10ꢀbisꢀ
phenylethynyl)anthracene moieties in cyclophane
(
1.
1
–
5
1
2
reveals a vibronic structure with maxima at 492 and 522 nm is formed, while other solvent combinations typically afforded
and a weak shoulder around 560 nm (Fig. 2b). These features orangeꢀyellow emissive powders (Fig. S4, ESI†). Surprisingly,
are characteristic of wellꢀindividualized 9,10ꢀbis(phenylꢀ we found that a transition to another crystalline form that
1
0a,c
ethynyl)anthracene molecules in a good solvent.
lower emission intensity was observed when cyclophane
excited under identical conditions (note that the intensities in experiments involve heating the YCRꢀform for 10 min at 250
Fig. 2b are normalized). The emission spectrum (Fig. 2b) shows While cyclophane
displayed thermoresponsive luminescent
A much exhibits reddishꢀorange photoluminescence (ROCRꢀform) can
was be achieved through thermal treatment; the hereꢀreported
C.
1
°
1
2
| J. Name., 2012, 00, 1-3
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characteristics, this behavior was not observed for linear reddishꢀorange ROCRꢀform. The phase transition was confirmed
reference compound in the solid state. Moreover, cyclophane by differential scanning calorimetry (DSC). The DSC trace of
2
–
1
also shows mechanochromic luminescence behavior. Grinꢀ the YCRꢀform displays an endothermic peak (
ding of the ROCRꢀform leads to an amorphous state showing
ꢀ
H
= 4.4 kJ·mol
1
) at 221 °C (Fig. S6a, ESI†), which we associate with the tranꢀ
yellow photoluminescence (YAM1ꢀform). Subsequent annealing sition to the ROCRꢀform. By contrast, the DSC curve measured
at 250 C for 10 min largely restored the reddishꢀorange for the ROCRꢀform (Fig. S6a, ESI†) is void of this and any other
emission color (ROLOꢀform), although the molecular packing characteristic signals. Thermogravimetric analysis experiments
appears to be lessꢀordered (LO) than in the ROCRꢀform. of
(Fig. S7, ESI†) show no significant weight loss below 300
Grinding the YCRꢀform leads to another amorphous state (YAM2
C and confirm that the transition from the YCRꢀform to the
form), whose emission spectrum is slightly different from that ROCRꢀform is not due to the release of trapped solvent. The
of the YAM1ꢀform (vide infra). However, subsequent annealing linear reference compound
could not be converted into a
also caused the conversion to ROLOꢀform. While various moleꢀ reddishꢀorange emissive state, indicating that cyclization is the
cular structures have been found to show thermoꢀ or mechanoꢀ key to the stimuliꢀresponsive behavior of cyclophane
°
1
ꢀ
°
2
1
.
2
,6ꢀ8,15
responsive luminescence,
compound
1
, to our knowledge,
As discussed above, grinding transforms the YCRꢀform and
ROCRꢀform into forms that emit yellow light (YAM1ꢀform and
is the first reported cyclophane exhibiting such behavior.
Y
AM2ꢀform, respectively). The powder XRD patterns of both
amorphous forms are void of any diffraction peaks (Fig. S5c
and e, ESI†), suggesting that grinding induces conversions from
crystalline to amorphous states. Annealing of both amorphous
forms for 10 min at 250 °C caused restauration of some of the
peaks in the XRD (ROLOꢀform, Fig. S5d and f, ESI†), which
appear at similar spacings as observed for the ROCRꢀform.
Taking into account the fact that the emission spectrum is
similar to that of the ROCRꢀform (vide infra), we conclude that
the thermal treatment restores a molecular assembly that is
similar to the ROCRꢀform. However, the intensities of the peaks
observed in the diffractogram of the ROLOꢀform are much
lower than observed for ROCRꢀform, and the DSC trace
acquired upon heating the YAM1ꢀform is featureless (Fig. S6a,
ESI†), indicating that annealing of the amorphous form of
leads to assembled structures with limited degree of order.
1
The photophysical properties of
1 were also examined in the
solid state and the influence of thermal and mechanical
treatments were probed. Fig. 4 shows the steadyꢀstate emission
spectra of cyclophane 1. The spectra correspond well with the
Fig. 3 Thermo- and mechanoresponsive luminescence behavior of cyclophane
1
emission color observed by the unassisted eye (Fig. 3). The
YCRꢀform and ROCRꢀform display broad emission bands with
maxima around 555 and 614 nm, respectively. In contrast to the
with pictures showing the emission color in each state. All pictures were taken
under UV irradiation ( ex = 365 nm) on quartz substrates at room temperature.
λ
emission spectrum of cyclophane
1 in chloroform, which shows
wellꢀresolved phonon modes, the solidꢀstate spectra are broad
and featureless. The emission band observed for the ROCRꢀform
is ascribed to static excimer formation of the 9,10ꢀbis(phenylꢀ
ethynyl)anthracene cores. Indeed,
πꢀstacked 9,10ꢀbis(phenylꢀ
ethynyl)anthracene moieties have been reported to exhibit a
similar emission band with maximum around 630 nm, which
8
c
was ascribed to the formation of static excimers. Gratifyingly,
emission lifetime measurements support the formation of static
excimers in the ROCRꢀform. Fig. S8 (ESI†) shows timeꢀ
resolved fluorescent decays measured for YCRꢀform and ROCR
form at 600 nm. The data are fitted well by a double
exponential function, resulting in a long emission lifetime of
ꢀ
Fig. 4 Emission spectra of the YCR-form (yellow solid line), ROCR-form (orange solid line),
AM1-form (orange dotted line), YAM2-form (yellow dotted line), ROLO-form obtained
from the YAM1-form (orange dash-dotted line), and ROLO-form obtained from the YAM2
form (yellow dash-dotted line) of cyclophane 1. All spectra were measured at room
temperature with ex = 400 nm.
Y
-
1
Y
0
2.0 ns (Table S1, ESI†). By contrast, the decay trace of the
CRꢀform is dominated by a component with a short lifetime of
.8 ns, which can be attributed to exciton coupling. In
λ
comparison to the ROCRꢀform, the YCRꢀform features a much
smaller proportion of excimer emission. The two crystalline
states have a similar quantum yield of 0.4 (Table S1, ESI†).
The emission spectra of the amorphous YAM1ꢀform and
Powder Xꢀray diffraction (XRD) patterns of compound
Fig. S5, ESI†) clearly reveal that the photoluminescence color
1
(
changes induced by thermal or mechanical treatment result
from alterations of the molecular packing. The XRD pattern
obtained for the YCRꢀform (Fig. S5a, ESI†) displays many
diffraction peaks that reflect the crystalline nature of this form.
The diffractogram changes substantially upon annealing the
YAM2ꢀform of cyclophane
1 (Fig. 4, orange and yellow dotted
lines) show maxima that are situated between those of the YCR
ꢀ
form and the ROCRꢀform. The small difference between the
emission spectra of the two amorphous forms may be ascribed
to a small amount of residual crystalline particles after grinding.
In the case of the YAM1ꢀform, this would enable energy transfer
to excimer sites from the parent ROCRꢀform, leading to the
observed slight redꢀshift of the emission band compared to that
sample for 10 min at 250 °C (Fig. S5b, ESI†). Thus, the thermal
treatment causes a solidꢀsolid phase transition between two
different crystal structures, which is responsible for the
emission color change from the yellow YCRꢀform to the
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1
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,
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the optical properties change on account of alterations of both
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was obtained by annealing the YAM1ꢀform (Fig. 4, orange dashꢀ
dotted line), is very similar to that of ROCRꢀform, although a
small shoulder appears around 540 nm. The emission spectrum
of the ROLOꢀform obtained from YAM2ꢀform also shows the
same spectral feature (Fig. 4, yellow dashꢀdotted line). This
behavior is consistent with the results obtained from powder
XRD measurements and confirms that the annealing procedure
does not lead to the complete recovery of the wellꢀordered
3
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In summary, the first mechanoꢀ and thermoresponsive
luminescent cyclophane has been reported. The compound’s
optical properties in the solid state depend strongly on the
morphology, which in turn can be influenced by mechanical
and/or thermal treatment. We showed that switching between
three discrete morphologies is possible. Interpreting the results
broadly, our study demonstrates that the integration of a
fluorescent motif into a cyclic structure is a promising approach
to design stimuliꢀresponsive luminescent molecular materials.
We expect our findings to be general and applicable to
cyclophanes comprising other luminescent cores and/or spacers.
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We thank Prof. T. Nakano for optical and thermal
measurements, and Prof. Y. Urano for emission lifetime and
quantum yield experiments. Y.S. is grateful for financial
support from JSPS Postdoctoral Fellowships for Research
Abroad. C.W. acknowledges support from the National Center
of Competence in Research (NCCR) BioꢀInspired Materials, a
research instrument of the Swiss National Science Foundation,
the European Research Council (ERCꢀ2011ꢀAdG 291490ꢀ
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DOI: 10.1039/C6CC01614F
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A Mechano- and Thermoresponsive Luminescent Cyclophane
The first mechanoꢀ and thermoresponsive luminescent
cyclophane is described in this report.
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