An alternately introduced heterometallo-supramolecular polymer: Synthesis and solid-state emission switching by electrochemical redox

Article abstract of DOI:10.1039/c3cc41176a

By utilizing different coordination properties of a lanthanide ion and a transition metal ion, a heterometallo-supramolecular polymer with Eu(iii) and Fe(ii) ions introduced alternately (polyEuFe) was precisely prepared via stepwise complexation of the metal ions to a new unsymmetrical ligand with both a dicarboxylate-substituted terpyridine and an unsubstituted terpyridine. The polymer showed unique, reversible on-off switching of the Eu(iii) luminescence by electrochemical redox of the Fe ions in the solid-state. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc41176a

Showcasing research from Takashi Sato and
Masayoshi Higuchi at the National Institute for
Materials Science, Tsukuba, Japan
As featured in:
An alternately introduced heterometallo-supramolecular
polymer: synthesis and solid-state emission switching by
electrochemical redox
A heterometallo-supramolecular polymer with Eu(III) and Fe(II)
ions introduced alternately was prepared by utilizing different
coordination properties of the metal ions. Reversible
“on-off”switching of the luminescence in the solid-state film was
triggered by electrochemical redox.
See Masayoshi Higuchi,
Chem. Commun., 2013, 49, 5256.
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An alternately introduced heterometallo-supramolecular
polymer: synthesis and solid-state emission switching by
electrochemical redox†
Cite this: Chem. Commun., 2013,
49, 5256
Received 13th February 2013,
Accepted 21st March 2013
Takashi Satoa and Masayoshi Higuchi*ab
DOI: 10.1039/c3cc41176a
www.rsc.org/chemcomm
By utilizing different coordination properties of a lanthanide ion
Metallo-supramolecular polymers were obtained via com-
and a transition metal ion, a heterometallo-supramolecular poly- plexation of metal ions with organic ligands bearing two
mer with Eu(III) and Fe(II) ions introduced alternately (polyEuFe) coordination sites.5 We have already reported the electro-
was precisely prepared via stepwise complexation of the metal chromic property of metallo-supramolecular polymers pre-
ions to a new unsymmetrical ligand with both a dicarboxylate- pared from Fe(II) ions and bis(terpyridyl)benzene derivatives.6
substituted terpyridine and an unsubstituted terpyridine. The These polymers exhibited the metal-to-ligand charge transfer
polymer showed unique, reversible ‘‘on–off’’ switching of the (MLCT) absorption at around 570 nm and the absorption
Eu(III) luminescence by electrochemical redox of the Fe ions in disappeared upon the electrochemical oxidation of Fe(II) to
the solid-state.
Fe(III).6,7 In addition, we recently succeeded in synthesizing a
Eu(III)-based metallo-supramolecular polymer by complexation
Lanthanide metal complexes have unique fluorescent proper- of Eu(III) ions with a bis(dicarboxylate-substituted terpyridyl)-
ties including high emission quantum efficiency, a long-lived benzene derivative which was prepared as a new ligand.8 This
fluorescence lifetime, and a large Stokes shift.1 Stimuli-responsive Eu-based polymer showed red emission at 613 nm based on
reversible ‘‘on–off’’ switching of the fluorescence is an attrac- an f–f transition in the Eu ions. We chose Eu(III) and Fe(II)
tive research target due to its potential applications such as ions as the fluorescent and redox sites, respectively, in a new
sensors and displays.2 The luminescence switching triggered by heterometallo-supramolecular polymer and intended to intro-
electrochemical redox has been already demonstrated in duce them alternately into the polymer chain. An alternate
solution,3 but there are no reports on the switching in the alignment of two metal ion species in a metallo-supramolecular
solid-state, as far as we know, despite the fact that it is essential polymer is quite difficult when symmetrical ligands are used.9
to device application. One of the problems is difficulty in the Newkome et al. reported an elegant higher-ordered macro-
film formation of the metal complexes on an electrode, because molecular architecture using terpyridine derivatives and different
of their high crystallinity. So, introduction of amorphous metallic ions such as Fe and Ru,9c,e but heterometallo-
polymeric structures into the complexes is considered to supramolecular polymers incorporating lanthanide ions are
become a practical solution to obtain a uniform film.4 Further- not well known.
more, if the fluorescent polymer complex has a redox-active site
We synthesized an unsymmetrical bisterpyridine with both
which quenches the fluorescence, a smart system of fluores- terpyridine and dicarboxylate-substituted terpyridine moieties
cence switching driven by electrochemical redox will be con- (L1, Scheme 1) as a new ligand for alternate introduction of
structed. We herein report alternately precise introduction Eu(III) and Fe(II) ions into a metallo-supramolecular polymer
of two metal ion species (Eu(III) and Fe(II)) into a metallo- structure. L1 was prepared by Suzuki-coupling reaction of a
supramolecular polymer structure and the reversible ‘‘on–off’’ dicarbonyl ester group-containing terpyridine8 with an unsubsti-
switching of luminescence of Eu(III) triggered by redox of Fe(II) tuted terpyridine10 using a Pd catalyst, followed by hydrolysis.
1
in a film state.
The structure of L1 was confirmed by H-, 13C-NMR and HRMS
spectroscopy (Fig. S1 and S2, ESI†).
a Electronic Functional Materials Group, Polymer Materials Unit,
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044,
Japan. E-mail: HIGUCHI.Masayoshi@nims.go.jp; Fax: +81 29-860-4721
b JST-CREST, Japan
Complexation behavior of L1 with Eu(NO3)3 and Fe(BF4)2
was investigated in detail by UV-vis spectroscopic measure-
ments (Fig. 1). The spectrum of a methanol solution of L1
(20 mM), which was deprotonated with two equivalents of
tridodecylamine, changed with addition of Eu(NO3)3 up to a
molar ratio of [Eu(NO3)3]/[L1] = 0.5 (Fig. 1a): the L1 absorbance
† Electronic supplementary information (ESI) available: Experimental proce-
dures, synthesis of the polymer, UV-vis and photoluminescence spectra. See
DOI: 10.1039/c3cc41176a
c
5256 Chem. Commun., 2013, 49, 5256--5258
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Upon addition of only Fe(BF4)2 to an L1 solution, the MLCT
absorption increased until the ratio of [Fe(BF4)2]/[L1] = 1.5
(Fig. S6, ESI†). Fe(II) is first complexed with tpy-COOÀ, probably
because the binding constant of Fe(II) with tpy-COOÀ is higher
À
than that with tpy due to the counter anion exchange from BF4
to carboxylate groups. However, the bulky dicarboxylate sub-
stituents in tpy-COOÀ prevent the second complexation of FeL1
with another tpy-COOÀ. Therefore, this complexation behavior
indicates that the molar ratio of Fe(II) ions and tpy-COOÀ in the
complex is not 1 : 2 but 1 : 1 (Fig. S3c, ESI†). In fact, UV-vis
titration of L1 with the addition of Fe(BF4)2 and the second
addition of Eu(NO3)3 showed no spectral change, indicating
that polyEuFe was not obtained (Fig. S7, ESI†). Therefore, the
order to add metallic ions is important for the synthesis of
polyEuFe.
Scheme 1 Synthesis of an unsymmetrical ligand (L1) and the heterometallo-
supramolecular polymer with Eu(III) and Fe(II) ions introduced alternately
(polyEuFe).
PolyEuFe was obtained in a one-pot synthesis by complexa-
tion of L1 with the addition of 0.5 equiv. of Eu(NO3)3 and the
further addition of 0.5 equiv. of Fe(BF4)2 in the presence of
tridodecylamine (Scheme 1) as a violet solid in a 36% yield.
It was soluble in alcohols such as methanol and ethylene glycol
or polar solvents such as DMSO and DMF, but insoluble in
the common organic solvents such as n-hexane, chloroform,
dichloromethane, and tetrahydrofuran. Molecular weight of
polyEuFe was determined by the SEC-Viscometry/RALLS method
using poly(ethylene glycol) as standard (Mw = 1.7 Â 104,
Mw/Mn = 1.3) (Fig. S8, ESI†),8,11 which strongly indicated that
polyEuFe forms a polymer structure. Polydispersity of metallo-
supramolecular polymers is generally large.14 Analyzing the
structure of polyEuFe by NMR spectroscopy was difficult
because of paramagnetic Eu(III) ions, but it was fully identified
by the emission spectrum and cyclic voltammogram. The
Fig. 1 (a) UV-vis spectral change of a methanol solution of L1 in the presence of
2 equivalents of tridodecylamine during the titration of 0–0.5 equiv. of Eu(NO3)3
at room temperature. (b) UV-vis spectral change of the solution during the
addition of 0–1.5 equiv. of Fe(BF4)2. (c) The normalized absorption at 345 nm as a
function of the molar ratio of Eu(NO3)3/L1 in Fig. 1a. (d) The normalized
absorption at 570 nm as a function of the molar ration of Fe(BF4)2/L1 in Fig. 1b.
at around 345 nm increased and the one at 303 nm decreased emission spectrum in ethylene glycol dilute solution exhibited
5
7
(Fig. 1a). The absorbance at 345 nm and the molar ratio of characteristic peaks corresponding to the D0 - F1–4 transi-
added Eu(III) ions to L1 showed a linear relationship (Fig. 1c). tions of Eu(III) ions between the 550 and 720 nm range, which is
Then Fe(BF4)2 was added to the solution (Eu(L1)2) up to a molar close to the wavelength of the MLCT band of the Fe(II)–tpy
ratio of [Fe(BF4)2]/[L1] = 1.5 (Fig. 1b). A new absorption at complex (Fig. S9, ESI†). The intensity of emission peaks was
570 nm based on the MLCT band appeared during the very low (absolute quantum yield (F): 0.07) compared with that
titration.11 The MLCT absorption was clearly saturated at the of polyEu (Fig. S10, ESI†) because of the energy transfer from
ratio of [Fe(BF4)2]/[L1] = 0.5 (Fig. 1d). These spectral changes Eu(III) to the Fe(II)-tpy moiety. The intensity was also lower than
suggest the formation of
a heterometallo-supramolecular that of the mixture of polyEu and metallo-supramolecular
polymer with Eu(III) and Fe(II) ions introduced alternately polymer including Fe(II) ions (polyFe)7 (F = 0.12) (Table S1,
(polyEuFe), according to a stepwise complexation behavior ESI†), which indicated that the alternate complexation of Eu(III)
shown in Fig. S3a of ESI.† The spectrum didn’t change upon and Fe(II) ions promoted the effective quenching.
the further addition of Eu(NO3)3, indicating that polyEuFe was
stable in solution (Fig. S4, ESI†).
Cyclic voltammogram of polyEuFe exhibited a reversible
redox wave of the Fe(II)/Fe(III) couple (E1/2 = 0.8 V) (Fig. 2a).
The proposed stepwise complexation behavior was also The redox potential is similar to that of polyFe (0.78 V),7 which
supported by the following additional experiments (Fig. S3a, supports the observation that Fe(II) ions form complexes with
ESI†). When Eu(NO3)3 was added to the L1 solution at a molar the tpy moiety of L1 in the polymer. A film of polyEuFe can be
ratio of [Eu(NO3)3]/[L1] greater than 0.5, the spectral change prepared by many methods such as solvent-casting, spin-coating,
continued up to the ratio of [Eu(NO3)3]/[L1] = 0.66 (Fig. S5, dip and spray-coating. Among them, a thin film of polyEuFe
ESI†), which means that Eu(III) was also complexed with the was prepared on an ITO glass substrate by a solvent-casting
unsubstituted terpyridine moiety (tpy) of L1 with a molar ratio method from an ethylene glycol solution (1 mg mLÀ1) and dried
of Eu(III) ions : tpy = 1 : 3 after finishing 1 : 2 complexation of in vacuo. The UV-vis spectra of the polyEuFe film in an electro-
Eu(III) ions and a dicarboxylate-substituted terpyridine moiety lyte solution of n-Bu4NClO4 (0.10 M) showed a reversible
(tpy-COOÀ) of L1, because Eu(III) ions can form a 1 : 3 ratio electrochromic behavior by the electrochemical redox of Fe
complex with tpy as shown in Fig. S3b of ESI,†12 though the ions (Fig. 2b): the MLCT absorption at 570 nm of the Fe(II)–
binding constant is lower than that with tpy-COOÀ.13
tpy complex disappeared upon the oxidation of Fe(II) to Fe(III)
c
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introduced alternately (polyEuFe) was precisely prepared via
stepwise complexation of the metal ions to a new unsymme-
trical ligand with both a dicarboxylate-substituted terpyridine
and an unsubstituted terpyridine. PolyEuFe showed reversible
‘‘on–off’’ switching of the Eu(III) luminescence by the electro-
chemical redox of the Fe(II) ions in the solid-state. This strategy
utilizing different coordination properties among metal species
in the synthesis of heterometallo-supramolecular polymers will
open the door toward designing new soft materials and exploiting
smart functions to lead to applications.
Notes and references
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Fig. 2 (a) A cyclic voltammogram of a polyEuFe film coated on a glassy carbon
electrode as a working electrode at room temperature (an electrolyte solution:
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À
À
from BF4 to ClO4 during redox reaction was indicated by the
change in the solubility of polyEuFe after redox reaction, but the
electrochromic and emission properties were not changed during
the redox switching. This means that the counter anions and
electrolyte do not affect the photophysical properties.
In summary, based on the different coordination properties
of a lanthanide metal ion and a transition metal ion, a hetero-
metallo-supramolecular polymer with Eu(III) and Fe(II) ions
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c
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This journal is The Royal Society of Chemistry 2013