A vapoluminescent Eu-based metallo-supramolecular polymer

Article abstract of DOI:10.1039/c2cc30972f

An Eu-based metallo-supramolecular polymer (polyEu) was prepared by self-assembly coordination polymerization. Unique vapoluminescence property of polyEu triggered by acid-base vapor was found and a photoluminescence display in switchable imaging by acid-base vapor was fabricated. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc30972f

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Showcasing research from Takashi Sato and
Masayoshi Higuchi at the National Institute for
Materials Science, Tsukuba, Japan
A vapoluminescent Eu-based metallo-supramolecular polymer
An Eu-based metallo-supramolecular polymer (polyEu) was
prepared by self-assembly coordination polymerization. Unique
vapoluminescence property of polyEu triggered by acid/base vapor
was found and a photoluminescence display in switchable imaging
by acid/base vapor was fabricated.
See T. Sato and M. Higuchi,
Chem. Commun., 2012, 48, 4947.
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COMMUNICATION
A vapoluminescent Eu-based metallo-supramolecular polymerw
Takashi Satoa and Masayoshi Higuchi*ab
Received 10th February 2012, Accepted 20th March 2012
DOI: 10.1039/c2cc30972f
An Eu-based metallo-supramolecular polymer (polyEu) was
prepared by self-assembly coordination polymerization. Unique
vapoluminescence property of polyEu triggered by acid–base
vapor was found and a photoluminescence display in switchable
imaging by acid–base vapor was fabricated.
complexes can act as antenna molecules.9 However, gels may
not always give rise to uniformly dispersed compositions even
in solution. Therefore, materials based on ligand exchange
methods are unsuitable for display devices, which require a quick
response and good reversibility. A more effective approach for
advanced materials is the synthesis of one-dimensional metallo-
supramolecular polymers having a rigid spacer group that undergo
luminescence switching without ligand exchange. This route
suggests a number of advantages in comparison with metallo-
supramolecular gel films. The first is quick-response luminescence
switching without the time losses associated with ligand exchange.
The second is the potential for reversible luminescence switching
without depolymerization by ligand exchange. Further, this route
does not suffer from aggregation and phase separation problems
during processing. Finally, such a luminescence switching system
could be applied to a broad array of systems via acid–base
reactions. Furthermore, conjugated ligand systems are expected
to offer more effective energy transfer from ligand to metal.10 The
key factors in terms of device applications are quick response and
good reversibility. However, the performance of vapoluminescent
metallo-supramolecular polymers in regard to these factors
has not been investigated in depth. Therefore, our work has
investigated the response and reversibility of acid–base vapor-
responsive metallo-supramolecular polymers for display devices.
Herein, we describe the preparation of a novel europium-based
metallo-supramolecular polymer by coordination polymerization
of the europium (Eu) ion and a bis(dicarboxylic acid-containing
terpyridine) ligand. Interestingly, we found that reversible
photoluminescence switching of the metallo-supramolecular
polymer was triggered by exposure to acid–base vapors, and
also demonstrated the vapoluminescent switching of an image
on a fabricated device.
On–off switching of solid-state luminescence by external stimuli
has become of increasing importance in a variety of applications.1
In particular, acid–base gas-responsive luminescent materials
have potential application in sensors, biochemical assays and
display devices.2 Acid–base reactions in solution are the most
typical chemical reactions for switching systems as demonstrated
for logic gates, molecular motors, and machines.3 However, there
are only a few reports describing vapoluminescent switching of
materials triggered by the presence of acid–base vapors in the
solid state.4 Most work uses Pt complexes, which are difficult to
apply as films for further applications. From the viewpoint of
device application, soft materials such as polymers are desirable
as they have attractive advantages including flexibility, ease of
fabrication, mechanical strength, and low cost as compared with
inorganic complexes.
Recently, metallo-supramolecular polymers, prepared by
the complexation of ligands and metals, have attracted much
interest because of their potential application as electrochromic
materials, stimuli-responsive gels, emissive materials, information
storage devices, sensors, photovoltaics, and self-healing materials.5
Lanthanide-based metallo-supramolecular polymers are of
particular interest as display materials owing to their specific
luminescence properties.6 In particular, europium complexes are
the excellent luminescent materials.7 Although phosphate-vapor
responsive metallo-supramolecular polymer gel films have been
reported by Rowan et al,6d the luminescence switching of these gel
films takes a long time via ligand exchange. Also, these gel films
did not exhibit completely reversible luminescence switching.
Luminescence quenching occurred via a complete ligand exchange
pathway from an aromatic ligand to an aliphatic ligand.6d,8
Luminescence switching by ligand exchange is a general concept,
and it is well-known that aromatic ligands within lanthanide
We synthesized a novel bis(dicarboxylic acid-containing
terpyridine) ligand, ligand, for binding to the Eu ion (Scheme S2,
ESIw). We first investigated the ability of ligand to form a 1 : 1
complex with Eu(NO2)3 by UV-vis spectroscopic measurements
(Fig. 1). Increasing amounts of Eu(NO2)3 were added to solutions
of ligand (10 mM), deprotonated with a stoichiometric quantity of
KOH, up to a ratio of [Eu(NO2)3]/[ligand] = 3. Complexation of
ligand and the Eu ions resulted in an increase in the ligand
absorbance at around 380 nm and a decrease in the absorbance
at 260 nm. Upon stepwise addition of Eu(NO2)3, these spectra
revealed an isosbestic point at 293 nm after the addition of
0.5 equiv. of Eu(NO2)3 (Fig. 1(a)). The isosbestic point was then
shifted to 303 nm at between 0.6 and 1.0 equiv. of Eu(NO2)3
added (Fig. 1(b)). These two isosbestic points suggest that two
a Electronic Functional Materials Group, Polymer Materials Unit,
National Institute for Materials Science (NIMS), 1-1 Namiki,
Tsukuba, Ibaraki 305-0044, Japan.
E-mail: HIGUCHI.Masayoshi@nims.go.jp; Fax: +81 29-860-4721
b JST-CREST, Japan
w Electronic supplementary information (ESI) available: Experimental
procedures, synthesis of polymers, UV-vis and fluorescence spectra.
See DOI: 10.1039/c2cc30972f
c
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Chem. Commun., 2012, 48, 4947–4949 4947

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Fig. 1 UV-vis spectra of ligand complexed with Eu(NO2)3. (a) Spectral
changes observed during the addition of 0–0.5 equiv. of Eu(NO2)3
(isosbestic point at 293 nm). (b) Spectral changes observed during the
addition of 0.6–1.0 equiv. of Eu(NO2)3 (isosbestic point at 303 nm).
The inset shows the normalized absorption at 378 nm as a function of
the Eu(NO2)3 : ligand ratio.
Fig. 2 Acid–base vapor-induced luminescence switching of a polyEu
film. (a) Photograph of a polyEu self-standing film during the exposure
to HCl–Et3N vapor. (b) Spectral changes before and after exposure to
HCl–Et3N vapor in luminescence spectra of the polyEu film. Original
polyEu (red line), after exposure to HCl gas (black line). (c) Responses
of luminescent intensity at 613 nm of polyEu during acid–base vapor
exposure cycles.
different complexes were formed upon addition of Eu(NO2)3,
indicating that the polymerization process progresses in a two-step
fashion from mono-complex formation to the supramolecular
polymer. Furthermore, plotting of the absorbance at 378 nm vs.
the ratio of metal to ligand showed a linear increase and a sharp
end point at the ratio of 1 : 1 (Ligand :Eu(NO2)3), indicating that
the metallo-supramolecular polymer was successfully formed.
Evidently, the presence of PEG chains at the pendant does not
affect the binding ratio between the ligand and the Eu ions.
A Eu-based metallo-supramolecular polymer (polyEu) was
prepared by the procedure described in Scheme 1. Coordination
polymerization was performed by refluxing an equimolar amount
of ligand with Eu(NO2)3 in methanol. PolyEu was isolated as a
yellow solid in 93% yield. PolyEu was dissolved in methanol but
insoluble in common organic solvents such as hexane, chloroform,
and THF. The molecular weight of polyEu was confirmed by
the SEC-Viscometry/RALLS method (Mn = 7.8 Â 104,
Mw/Mn = 1.7) (Fig. S1, ESIw).11 PolyEu formed a free-standing
film by a solvent-casting method. This film showed bright-red
emission under a UV lamp (lex = 365 nm). Also, the powder
X-ray diffraction analysis of polyEu showed the amorphous
state (see Fig. S2, ESIw).
bands, and the ligand emission from the organic backbone
completely disappeared, probably suggesting more efficient
energy transfer. Based on measurements of the PL quantum
yield (QY) in the solid state, a QY value of polyEu (FFL = 0.24)
was obtained, which was over ten times higher than that of the
ligand (FFL = 0.021) (see Table S1, ESIw).
Interestingly, we found that the photoluminescence of the
polyEu film could be switched off/on by exposure to acid–base
vapor (Fig. 2). Upon selective excitation at 365 nm, a free-standing
film of polyEu emitted a red luminescence corresponding to the
Eu metal (Fig. 2(a)). When this film was exposed to a HCl-gas-
enriched environment for 5 seconds, the red luminescence
disappeared to the naked eye. After subsequent exposure of
the non-luminescent film to triethylamine (Et3N) for 5 seconds, the
red luminescence was recovered as seen by the naked eye. In
photoluminescence (PL) spectra of the polyEu film (thickness:
290 mm), clear spectral changes occurred between 550 and
630 nm upon exposure to the HCl gas, and a notable change
in intensity was observed at 613 nm, corresponding to the
5D0 - 7F2 transitions of the Eu ion (Fig. 2(b)). When the film
was subsequently exposed to Et3N vapor, the red luminescence
at 613 nm corresponding to these transitions reappeared. This
The photophysical properties of polyEu in a film state and
ligand in a methanol solution were studied under a variety of
conditions (Fig. S3, ESIw). The polyEu thin film was prepared
on a glass substrate by spin-casting (20 mM in methanol). The
absorption spectrum of ligand showed a broad band with some
vibronic structures from 200 to 400 nm. Furthermore, the
emission and excitation spectra of ligand were consistent with
fluorescence emission from the direct population of the singlet
excited state. Upon excitation at 365 nm, the emission spectrum
of the polyEu film indicated the characteristic sharp peaks
5
7
difference in the peak profile of the D0 - F2 transitions was
considered to be a result of the counter-cation exchange pathway
from a potassium salt to a triethylammonium salt during the
process. Although we investigated this switching mechanism by
1H-NMR spectroscopy, the spectrum of polyEu could not be
analyzed because of its low solubility and broad peaks.
From the viewpoint of device applications, the most important
factors are reversibility, memory effect and response time. To
examine the reversibility, the spectral changes at 613 nm of a
polyEu-coated glass substrate were monitored upon alternate
exposure to HCl gas and Et3N vapor over several HCl–Et3N
cycles (Fig. 2(c), see Fig. S5 (ESIw)). The polyEu substrate
exhibited good reversibility at least 8 times, suggesting that
luminescence switching occurs without ligand exchange because
1D metallo-supramolecular polymer films do not show reversible
luminescence by ligand exchange. In addition, reversible
luminescence switching triggered by HCl–Et3N exposure
was exhibited for polyEu in methanol over at least 40 cycles
5
7
corresponding to the D0 - F1-4 transitions of the Eu metal
5
7
between 550 and 720 nm, and the hypersensitive D0 - F2
transitions dominated the spectrum.12 Most notably, polyEu
showed only the Eu(III)-centered 5D0 - 7Fj (j = 1–4) emission
Scheme 1 Synthesis of polyEu.
c
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(see Fig. S6 and S7, ESIw). The luminescence switching of
polyEu in methanol solutions was carried out by the stepwise
addition of 600 equiv. of HCl aq. and Et3N, respectively. This
luminescence switching was shown even when organic acids
such as trifluoroacetic acid were used. In addition, the memory
effect of polyEu was examined (see Fig. S8, ESIw). Separate
polyEu films exposed to HCl and Et3N vapor were kept in
a polyethylene bag at room temperature. The polyEu film
exposed to Et3N vapor showed stable red emission after at
least 100 hours. In contrast, the polyEu film exposed to HCl
gas showed the resumption of red luminescence after 48 hours,
suggesting that the HCl-protonated form had disappeared
from the polyEu film. In terms of the response time, we
examined the polyEu film upon exposure to HCl gas and
Et3N vapor, but the response time could not be observed
using our instruments because the spectral changes were too
rapid. This also suggests that the luminescence switching took
place without ligand exchange.
In summary, we have described the vapoluminescent switching
of a novel metallo-supramolecular polymer upon exposure to
acid–base vapor. The polymer was prepared by the coordina-
tion polymerization of europium ion and bis(dicarboxylic
acid-containing terpyridine), and demonstrated fast, stable,
reversible and sensitive switching behavior upon exposure to
acid–base vapor. This luminescence switching system does not
undergo depolymerization and provides access to display
materials with a quick response and good reversibility. There-
fore, our strategy represents a new development in the field of
supramolecular polymer chemistry associated with display and
sensor materials. Further investigations into these materials
and other candidates are ongoing.
Notes and references
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To confirm the reversible photoluminescence switching for
polyEu upon exposure to acid–base, the UV-vis spectrum of a
polyEu film was monitored after exposure to HCl gas. The
spectrum afforded a new transition with a shoulder peak at
around 500 nm, indicating that the structure of polyEu might
have changed upon protonation (see Fig. S9, ESIw). However,
the characteristic band at 500 nm completely disappeared after
the exposure to Et3N vapor. Thus, red luminescence was
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polyEu at around 420 nm, assignable to the n - p and p - p*
transitions of the ligand, also showed a significant hyperchromic
shift due to the protonation of polyEu. Upon addition of Et3N to
the acidified solution, the same spectroscopic signatures under-
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solution showed the lowest absorption peak at 390 nm after the
addition of HCl, which indicates protonation of the pyridine ring
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and S6 (ESIw), the PL intensity of a polyEu film and solution after
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upon addition of HCl and the luminescence switching is driven by
the protonation of polyEu.
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¨
A photoluminescent display based on the acid–base vapor
sensitive polyEu was fabricated by the following method (see
Fig. S12(a), ESIw). The image as indicated in Fig. S12(b) (ESIw)
was printed onto a glass substrate using a methanol solution
of polyEu by a stamp and dried in vacuo. As shown in
Fig. S12(b) (ESIw), luminescent red emission was obtained
under irradiation by a UV lamp (lex = 365 nm). Furthermore,
the image maintained its luminescence for at least 4 weeks without
any degradation. After exposure to HCl gas for 5 seconds, the
image disappeared completely. After exposure to a Et3N-enriched
environment for 5 seconds, the image reappeared.
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Chem. Commun., 2012, 48, 4947–4949 4949