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A mechano- and thermoresponsive luminescent
cyclophane†
Yoshimitsu Sagara,*ab Yoan C. Simon,bc Nobuyuki Tamaokia and Christoph Weder*b

Received 22nd February 2016,
Accepted 18th March 2016
DOI: 10.1039/c6cc01614f
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The first fluorescent cyclophane with mechano- and thermoresponsive
solid-state fluorescence characteristics is reported. The new cyclophane comprises two 9,10-bis(phenylethynyl)anthracene moietiesthat are bridged by tetraethylene glycol spacers. The stimuliresponsiveness is based on molecular assembly changes.

The last decade has witnessed a continuously increasing number ofstudies on materials where mechanical force leads to the controlledcleavage of stable1,2 or dynamic3 covalent bonds, or the dissociationof supramolecular motifs.4 This general mechanism is useful forthe design of stimuli-responsive materials, whose properties canbe altered upon application of mechanical forces. If the forceinduced separation involves mechanically active motifs (referred
to as mechanophores) with extended p-conjugated groups,changes of the material’s absorption and/or photoluminescence
properties occur.1,2 Especially, the latter can be easily detected,
making mechanically responsive (luminescent) color-changing
materials potentially useful, for example in the in situ monitoringof material failure due to stress fracture or fatigue.5 However, thenumber of luminophores whose photophysical properties can bealtered by mere dissociation of covalent or non-covalent bonds isstill limited and few generally applicable design principles havebeen established.1b–e,2Mechanical forces can also be exploited to induce changesof molecular arrangements, and a large number of mechanoresponsive luminescent (MRL) materials have been reported, whichchange their emission characteristics upon mechanical stimulation.2,6–8 A most effective strategy to create MRL materials is to design

luminescent molecules so they can assemble in different thermodynamically (meta)stable states, in which they adopt different conformations and/or intermolecular interactions.6d Organic moleculesof various shapes – from rods to fan or dumbbell shapes – have been
demonstrated to form assemblies with MRL properties.2,6–8 Here we
report the first fluorescent cyclophane with mechano- and thermoresponsive solid-state fluorescence characteristics. While the properties of cyclic compounds have been widely studied in solution,9 theirstimuli-responsive photophysical properties in the solid state were
little explored. Our study demonstrates that simple cyclic structures
are useful to induce several thermodynamically (meta)stable states,
presumably on account of spatial restrictions that prevent theluminophores to assemble in thermodynamically most stableclosed-packed structures. We expect that the findings can begeneralized and that the MRL feature may be readily imparted
to other luminescent motifs by converting into cyclic structures.
Cyclophane 1 (Fig. 1) was designed to feature two 9,10-bis(phenylethynyl)anthracene moieties, as these chromophores (in their nonconnected state) are known to exhibit strong green fluorescence ingood solvents.10 Flexible tetraethylene glycol linkers were chosen toconnect the peripheral phenyl rings of two 9,10-bis(phenylethynyl)anthracene moieties to serve as bridge. Because of its cyclicaResearch Institute for Electronic Science, Hokkaido University, N20, W10, Kita-Ku,
Sapporo 001-0020, Japan. E-mail: sagara@es.hokudai.ac.jpb
Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, CH-1700
Fribourg, Switzerland. E-mail: christoph.weder@unifr.chcSchool of Polymers and High Performance Materials, the University of Southern
Mississippi, 118 College Dr. #5050, Hattiesburg, MS, 39406, USA† Electronic supplementary information (ESI) available: Synthesis and characterization data of compounds 1 and 2, NMR, DSC, TG, XRD, grinding procedure,
precipitation of 1 from other solvent combinations, emission lifetime measurements, absorption and emission spectra of 1 and 2 under various conditions. See
DOI: 10.1039/c6cc01614f

5694 | Chem. Commun., 2016, 52, 5694--5697

Fig. 1 Molecular structures of cyclophane 1 and the linear referencecompound 2.

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structure, compound 1 was expected to show intramolecular
excimer formation and/or exciton coupling between the twoluminescent cores in the molecularly isolated state. In the solidstate, intra- and intermolecular interactions between the luminophores were expected to occur, which should independently aﬀect
the photophysical properties of the resulting molecular material.
The linear mono-9,10-bis(phenylethynyl)anthracene 2 was also
prepared and used as a reference compound.
Despite the tetraethylene glycol linkers, cyclophane 1 shows a low
solubility in common organic solvents, presumably due to the rigid
structure and strong p–p interactions between molecules. NMR
spectroscopic measurements were, however, possible after dissolution
in hot chloroform. A comparison of the 1H NMR spectra of 1 and 2(Fig. S1, ESI†) reveals an up-field shift of the resonances of theluminescent cores for cyclophane 1 (0.46 and 0.49 ppm for the outerand inner protons of the anthracene groups, respectively), indicative of
the formation of a cyclic structure, which changes the circular current.
Together with the mass spectroscopy data, the large difference clearly
confirms the success of the cyclization reaction (Scheme S1, ESI†).
Spectroscopic measurements of 1 and 2 were first carried out indilute (1  10 5 M) chloroform solutions (Fig. 2). The absorption
bands between 400 and 500 nm (Fig. 2a) show similar features, but
upon closer inspection subtle diﬀerences can be discerned. While
the spectrum of 1 displays two peaks at 446 and 473 nm, theseare observed at 450 and 471 nm in the case of 2. Moreover, themagnitude of the two transitions is almost identical in 2 (e = 3.7 
104 L mol 1 cm 1), whereas in 1 the molar absorption coeﬃcientat 446 nm (e = 5.7  104 L mol 1 cm 1) is larger than that at 473 nm(e = 4.7  104 L mol 1 cm 1). The diﬀerence in spectral shape seenin the absorption spectra of 1 and 2 and the fact that the extinction

Fig. 2 Absorption (a) and emission (b) spectra of dilute (1  10 5 M)
chloroform solutions of cyclophane 1 (orange line) and the linear referencecompound 2 (green line). All spectra were measured at room temperature.
The emission spectra were recorded with lex = 400 nm and intensities werenormalized.

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coeﬃcients of 1 are only slightly (and not two times) higher thanthose of 2 are the first indication of intramolecular, ground-stateelectronic interactions between two luminophores in 1.
In dilute (1  10 5 M) chloroform solution, 2 is highly emissive
and the emission spectrum reveals a vibronic structure with maxima
at 492 and 522 nm (Fig. 2b). These features are characteristic of wellindividualized 9,10-bis(phenylethynyl)anthracene molecules in agood solvent.10a,c A much lower emission intensity was observed
when 1 was excited under identical conditions. The emission
spectrum (Fig. 2b) shows an extra vibronic band at higher energy(459 nm) and a pronounced shoulder that stretches far into the red.
The latter is indicative of intramolecular excimer formation,11 which
has also been observed in the case of pyrene- or perylene-based
cyclophanes, i.e., in cases where the planar luminophores can form
face-to-face stacked structures.12,13
To unravel the emission characteristics of 1 and 2 in solution, we
measured the excited state lifetimes (t) and photoluminescencequantum yields (FPL) in chloroform (Table S1, ESI†). Fig. S2 (ESI†)shows time-resolved emission decay profiles for both 1 and 2. In thecase of 2, the decay curve is well fitted by a single exponential decayfunction with a lifetime of 2.9 ns (Fig. S2a, ESI†), characteristic of wellindividualized chromophores. The emission decay of 1 was measured
at 495 and 600 nm (Fig. S2b, ESI†). In both cases, multi-exponential
decay functions fitted the data. 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 tomonomer 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 indicativeof exciton coupling with H-type geometry, although the precise
conformation of 1 in chloroform is unclear. Aside from minor internalreabsorption features, the emission and absorption spectra of 1
remained unchanged when the concentration in chloroform
was reduced from 10 5 to 10 6 (absorption) or 10 7 M (emission)(Fig. S3, ESI†), confirming that the observed exciton coupling is an
intramolecular process and does not arise from the aggregation of
cyclophane 1. This interpretation is further supported by the fact that
previously reported perylene-based cyclophanes also exhibit absorption bands that correspond to H-type exciton coupling,13 and the
additional high-energy emission peak at 459 nm (vide supra). While
radiative relaxation from the higher energy level resulting from thesplitting of the lowest unoccupied molecular orbital of the monomer
species is forbidden in an ideal H-aggregate geometry, the twisted
arrangement of transition dipole moments allows the direct decay
process from the higher and forbidden energy level to some extent.14
Our interpretation of the emission decay profiles is further supported
by quantum yield measurements. Indeed, the photoluminescencequantum yield of 1 in chloroform (FPL = 0.07) is much lower than thatof 2 (FPL = 0.90), on account of intramolecular excimer formation andexciton coupling in the cyclophane.

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Fig. 3 Thermo- and mechanoresponsive luminescence behavior of 1
with pictures showing the emission color in each state. All pictures weretaken under UV irradiation (lex = 365 nm) at room temperature.

The thermo- and mechanoresponsive luminescent behaviorof 1 is summarized in Fig. 3. Upon precipitating 1 from a hot
concentrated chloroform solution into hexane, a yellow-lightemitting crystalline powder (YCR-form) is formed, while othersolvent combinations typically aﬀorded orange-yellow emissive
powders (Fig. S4, ESI†). We found that a transition to anothercrystalline form that exhibits reddish-orange photoluminescence (ROCR-form) can be achieved by annealing the YCR-form
for 10 min at 250 1C. While 1 displayed thermoresponsive
luminescent characteristics, this behavior was not observedfor 2 in the solid state. Moreover, 1 shows mechanochromic
luminescence behavior. Grinding of the ROCR-form leads to an
amorphous state showing yellow photoluminescence (YAM1-form).
Subsequent annealing at 250 1C for 10 min largely restored thereddish-orange emission color (ROLO-form), although the molecular
packing appears to be less-ordered (LO) than in the ROCR-form.
Grinding the YCR-form leads to another amorphous state (YAM2form), whose emission spectrum is slightly diﬀerent from that of the
YAM1-form (vide infra). However, subsequent annealing also caused
the conversion to ROLO-form. While various molecular structures
have been found to show thermo- or mechanoresponsive luminescence,2,6–8,15 compound 1, to our knowledge, is the first reported
cyclophane exhibiting such behavior.
Powder X-ray diﬀraction (XRD) patterns of 1 (Fig. S5, ESI†) clearly
reveal that the emission color changes induced by thermal ormechanical treatment result from alterations of the molecular packing. The XRD pattern obtained for the YCR-form (Fig. S5a, ESI†)
displays many diﬀraction peaks that reflect the crystalline nature ofthis form. The diﬀractogram changes substantially upon annealing
the sample for 10 min at 250 1C (Fig. S5b, ESI†). Thus, the thermal
treatment causes a solid–solid phase transition between two different
crystal structures, which is responsible for the emission color change
from the YCR-form to the ROCR-form. The phase transition was
confirmed by differential scanning calorimetry (DSC). The DSC traceof the YCR-form displays an endothermic peak (DH = 4.4 kJ mol 1) at
221 1C (Fig. S6a, ESI†), which we associate with the transition to the
ROCR-form. By contrast, the DSC curve measured for the ROCR-form

5696 | Chem. Commun., 2016, 52, 5694--5697

Communication(Fig. S6a, ESI†) is void of this and any other characteristic signals.
Thermogravimetric analysis experiments of 1 (Fig. S7, ESI†) show no
significant weight loss below 300 1C and confirm that the transition
from the YCR-form to the ROCR-form is not due to the release oftrapped solvent. Compound 2 could not be converted into a reddishorange emissive state, indicating that cyclization is the key to thestimuli-responsive behavior of 1.
The powder XRD patterns of both grinding-induced, amorphous forms (YAm1-form and YAM2-form) are void of any diﬀraction peaks (Fig. S5c and e, ESI†), suggesting that grinding induces
conversions from crystalline to amorphous states. Annealing ofboth amorphous forms for 10 min at 250 1C caused restoration ofsome of the peaks in the XRD (ROLO-form, Fig. S5d and f, ESI†),
which appear at similar spacings as observed for the ROCR-form.
Taking into account the fact that the emission spectrum is similar
to that of the ROCR-form (vide infra), we conclude that the thermal
treatment restores a molecular assembly that is similar to the
ROCR-form. However, the intensities of the peaks observed in thediﬀractogram of the ROLO-form are much lower than observedfor ROCR-form, and the DSC trace acquired upon heating the
YAM1-form is featureless (Fig. S6a, ESI†), indicating that annealingof the amorphous form of 1 leads to assembled structures withlimited degree of order.
The photophysical properties of 1 were also examined in thesolid state. Fig. 4 shows the steady-state emission spectra of 1.
The spectra correspond well with the emission color observed by
the unassisted eye (Fig. 3). The YCR-form and ROCR-form display
broad emission bands with maxima around 555 and 614 nm,
respectively. In contrast to the emission spectrum of 1 in chloroform, the solid-state spectra are broad and featureless. The
emission band observed for the ROCR-form is ascribed to static
excimer formation of the 9,10-bis(phenylethynyl)anthracene
cores. Gratifyingly, emission lifetime measurements support
the formation of static excimers in the ROCR-form. Fig. S8 (ESI†)shows time-resolved fluorescent decays measured for YCR-form
and ROCR-form at 600 nm. The data are fitted well by a double
exponential function, resulting in a long emission lifetimeof 12.0 ns (Table S1, ESI†). By contrast, the decay trace of the
YCR-form is dominated by a component with a short lifetimeof 0.8 ns, which can be attributed to exciton coupling.

Fig. 4 Emission spectra of the YCR-form (yellow solid line), ROCR-form(orange solid line), YAM1-form (orange dotted line), YAM2-form (yellow dottedline), ROLO-form obtained from the YAM1-form (orange dash-dotted line),and ROLO-form obtained from the YAM2-form (yellow dash-dotted line) of 1
measured at room temperature with lex = 400 nm.

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In comparison to the ROCR-form, the YCR-form features a muchsmaller 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 YAM1-form andYAM2-form of cyclophane 1 (Fig. 4) show maxima that aresituated between those of the YCR-form and the ROCR-form.
The 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 YAM1-form,
this would enable energy transfer to excimer sites from theparent ROCR-form, leading to a slight red-shift of the emission
band compared to that of the YAM2-form. While it is, unfortunately, impossible to distinguish if the grinding processchanges inter- or intramolecular arrangements of the emissive
cores of 1, one can speculate that probably a broad range of
species with diﬀerent conformation is present in the two
amorphous forms and that the optical properties change on
account of alterations of both these factors. The emission
spectrum of the ROLO-form, which was obtained by annealing
the YAM1-form (Fig. 4), is very similar to that of ROCRform, although a small shoulder appears around 540 nm. The
emission spectrum of the ROLO-form obtained from YAM2-form
also shows the same spectral feature (Fig. 4). 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 well-ordered molecularassembled structures that were observed for ROCR-form.
However, it appears that in the ROLO-form, energy transferfrom excited states with higher energy to excimer sites is atplay, so that the relatively poorly ordered material displays
emission characteristics that are reminiscent of the highlyordered ROCR-form. Compound 2 also shows a mechanicallyinduced shift of the emission band (Fig. S9, ESI†), which is
however much less pronounced than the one observed in thecase of cyclophane 1. The comparison clearly shows the benefit
of integrating the luminescent motif in a cyclic structure.
In summary, the first mechano- and thermoresponsive
luminescent cyclophane has been reported. The compound’s
optical properties in the solid state depend strongly on themorphology, which in turn can be influenced by mechanicaland/or thermal treatment. Interpreting the results broadly, ourstudy demonstrates that the integration of a fluorescent motif
into a cyclic structure is a promising approach to design
stimuli-responsive luminescent molecular materials. We expect
our findings to be general and applicable to cyclophanes
comprising other luminescent cores and/or spacers.
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 Competencein Research (NCCR) Bio-Inspired Materials, a research instrument of the Swiss National Science Foundation, the European
Research Council (ERC-2011-AdG 291490-MERESPO), and the
Adolphe Merkle Foundation.

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