FRET function of polymeric ionic liquid film containing rhodamine moieties for exhibiting emissions by excitation at wide wavelength areas

Article abstract of DOI:10.1016/j.jphotochem.2011.06.009

This paper reports synthesis of an imidazolium-type fluorescent polymeric ionic liquid film containing rhodamine moieties, which shows fluorescence resonance-energy-transfer (FRET) property, resulting in emissions by excitation at wide wavelength areas. First, radical copolymerization of two polymerizable ionic liquid, which had one and two polymerizable groups, respectively, was carried out with AIBN as an initiator to confirm whether a transparent polymeric ionic liquid film could be produced. Then, radical terpolymerization of the two ionic liquids with a rhodamine derivative having a polymerizable group was performed under the similar conditions as that for the above copolymerization to give the fluorescent polymeric ionic liquid film containing rhodamine moieties. The fluorescence spectra of the resulting film exhibited emissions due to the rhodamine group by excitation at wide wavelength areas. This result indicated occurrence of FRET from the polymeric ionic liquid units to the rhodamine moieties in the film because the excitation-wavelength-dependent emissions due to the polymeric ionic liquid were partially overlapped with an absorption peak due to the rhodamine group.

Full text of DOI:10.1016/j.jphotochem.2011.06.009

Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 283–287
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
FRET function of polymeric ionic liquid film containing rhodamine moieties for
exhibiting emissions by excitation at wide wavelength areas
Satoshi Wakizono, Kazuya Yamamoto, Jun-ichi Kadokawa∗
Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper reports synthesis of an imidazolium-type fluorescent polymeric ionic liquid film containing
rhodamine moieties, which shows fluorescence resonance-energy-transfer (FRET) property, resulting in
emissions by excitation at wide wavelength areas. First, radical copolymerization of two polymerizable
ionic liquid, which had one and two polymerizable groups, respectively, was carried out with AIBN as an
initiator to confirm whether a transparent polymeric ionic liquid film could be produced. Then, radical
terpolymerization of the two ionic liquids with a rhodamine derivative having a polymerizable group was
performed under the similar conditions as that for the above copolymerization to give the fluorescent
polymeric ionic liquid film containing rhodamine moieties. The fluorescence spectra of the resulting film
exhibited emissions due to the rhodamine group by excitation at wide wavelength areas. This result
indicated occurrence of FRET from the polymeric ionic liquid units to the rhodamine moieties in the film
because the excitation-wavelength-dependent emissions due to the polymeric ionic liquid were partially
overlapped with an absorption peak due to the rhodamine group.
Received 30 January 2011
Received in revised form 19 April 2011
Accepted 13 June 2011
Available online 23 June 2011
Keywords:
Fluorescence resonance-energy-transfer
Polymeric ionic liquid
Rhodamine
Excitation-wavelength-dependent emission
Film
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
rescent substrates, in which excitation energy is transferred from
a donor to an acceptor without emission of a photon [5].
In recent years, considerable effort has been devoted to develop-
ing new fluorescent dye-based materials such as probes as excellent
sensors for biomolecules [1]. To develop the novel polymeric
fluorescent materials, furthermore, fluorescent dye-containing
polymers, exampling polymethacrylate, polyacrylamide, and con-
jugated polymer bearing covalently linked rhodamine groups, have
been synthesized [2]. As appeared in these studies, rhodamines are
one of the most common fluorescent dyes for the red region of
the visible spectrum and have been widely used in various appli-
cations owing to their excellent spectroscopic properties such as
large molar extinction coefficient and high fluorescence quantum
yields [3].
Besides exhibiting emission excited at the characteristic wave-
length of each fluorescent dye, the fluorescent materials in the
practical applications are sometimes required to exhibit fluores-
cent emissions by excitation at wide wavelength areas. For the
purpose to develop such dye materials, we have noted a fluores-
cence resonance-energy-transfer (FRET) technique, which has been
used in designed fluorescent materials to obtain a large shift of the
excitation wavelength from that the dyes natively show [4]. FRET
is an interaction between the electronic excited states of two fluo-
On the other hand, ionic liquids, which are low-melting point
salts and form liquids at temperatures below the boiling point of
water, have been used as green solvents for environmental pro-
tection because of their negligible vapor pressure and excellent
thermal stability [6]. Beyond such traditional properties, recently,
ionic liquids have increasingly attracted much interesting atten-
tion as designer substrates with controllable physical and chemical
properties or even specific functions [7], so-called ‘Task-Specific
Ionic Liquids [8].’ As one of unique and specific properties of
the ionic liquids, it has been reported that imidazolium-type
ionic liquids exhibit excitation-wavelength-dependent fluorescent
behavior due to the presence of energetically different associated
species [9].
On the basis of the above viewpoints, we successfully pro-
vided
a unique FRET system using a rhodamine derivative
(rhodamine 6G) and an imidazolium-type ionic liquid (1-butyl-
3-methylimidazolium chloride, BMIMCl) [10]. In the study, we
found that FRET from BMIMCl as a donor to rhodamine 6G
as an acceptor occurred in
a solution of rhodamine 6G in
BMIMCl by excitation at wide wavelength areas because the spe-
cific excitation-wavelength-dependent fluorescent emissions of
BMIMCl excited at each wavelength were overlapped with an
absorption of rhodamine 6G. Consequently, the emissions due to
rhodamine 6G were appeared by excitation at the wide wavelength
areas in the solution. Furthermore, we prepared the gel material
∗
Corresponding author. Tel.: +81 99 285 7743; fax: +81 99 285 3253.
E-mail address: kadokawa@eng.kagoshima-u.ac.jp (J.-i. Kadokawa).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.06.009

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S. Wakizono et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 283–287
O
AIBN
+
N
N
O
N
N
O
O
n
Br
Cl
2
1
N
N
N
N
Cl
m
Br
3
Scheme 1. Synthesis of 3 by radical copolymerization of 1 with 2 initiated with AIBN.
exhibiting the function of such FRET system by means of a gelling
procedure of BMIMCl using guar gum of a natural polysaccharide;
the gelling system had been reported in our previous studies [11].
In this study, we attempt to incorporate such unique FRET func-
tion into a film material, which exhibits emissions due to the
fluorescent dye by excitation at wide wavelength areas. For this
purpose, a film form of the imidazolium-type ionic liquid, i.e., the
polymeric ionic liquid film [12], was first attempted to synthesize
by radical polymerization of ionic liquids having polymerizable
groups, so-called the polymerizable ionic liquids. Consequently,
we found a good polymeric ionic liquid film was obtained by
the radical copolymerization of two imidazolium-type polymer-
izable ionic liquids 1 having a vinylbenzyl polymerizable group
and 2 having vinyl and methacrylate polymerizable groups initi-
ated with AIBN. Then, the rhodamine moieties as the fluorescent
dye group were incorporated into the film by terpolymerization
of the polymerizable ionic liquids with a rhodamine derivative
having a polymerizable group. Consequently, we found that FRET
from the polymeric ionic liquid units to the rhodamine moi-
eties in the film occurred by excitation at wide wavelength areas
to exhibit the emissions due to the rhodamine group at each
wavelength.
was sandwiched between two glass plates and heated at 65 ◦C for
30 min. Further heating it at 75 ◦C for 2 h gave a film, which was
then removed from the glass plates. After the resulting film was
soaked in ethanol, it was dried under reduced pressure to give a
film of 3.
2.1.2. Synthesis of rhodamine derivative 4 having a methacrylate
group
Under argon, a mixture of rhodamine B (0.719 g, 1.50 mmol),
1,3-dicyclohexylcarbodiimide (DCC, 0.619 g, 3.00 mmol), 4-
dimethylaminopyridine (DMAP, 0.0475 g, 2.96 mmol), and
2-hydroxyethyl methacrylate (HEMA, 0.360 mL, 2.96 mmol)
in 1,2-dichloroethane (10 mL) was stirred at room temperature for
32 h. To the mixture, DCC (0.310 g, 1.50 mmol) was additionally
supplied and the reaction was further performed for 40 h. After a
small amount of hydroquinone as a radical inhibitor was added to
the reaction mixture, it was evaporated and dried under reduced
pressure. Then, water was added to the resulting residue and the
insoluble material was removed by filtration. After the filtrate was
concentrated and dried under reduced pressure, the residue was
dissolved in a small amount of methanol. The solution was poured
into diethyl ether to precipitate the product, which was isolated by
decantation and dried under reduced pressure to give 4 (0.701 g,
1.19 mmol) in 79.0% yield based on rhodamine B. 1H NMR (CDCl3)
ı 8.30 (d, J = 6.9 Hz), 7.85 (t, J = 6.9 Hz), 7.75 (t, J = 7.1 Hz), 7.35 (d,
J = 7.8 Hz) (benzoate, 4H), 7.08 (d, J = 9.2 Hz, 1,8-XanH, 2H), 6.94
(dd, J = 9.6, 2.3 Hz, 2,7-XanH, 2H), 6.83 (d, J = 2.3 Hz, 4,5-XanH, 2H),
6.03 (s, CH2, (trans to –CH3), 1H), 5.56 (s, CH2, (cis to –CH3),
1H), 4.31 (dd, J = 5.7, 3.4 Hz, –OCH2–, 2H), 4.19 (dd, J = 5.5, 3.7 Hz,
–OCH2–, 2H), 3.66 (q, J = 7.2 Hz, –CH2N–, 8H), 1.90 (s, C–CH3, 3H),
1.34 (t, J = 7.1 Hz, –CH2–CH3, 12H).
2. Experimental
2.1. Materials
Polymerizable ionic liquids 1 and 2 were prepared according
to the literature procedures [13]. Methanol was purified by dis-
tillation. Other reagents were commercially available and used as
received.
2.1.1. Preparation of polymeric ionic liquid film 3
2.1.3. Preparation of polymeric ionic liquid film 5 containing
rhodamine moieties
The two polymerizable ionic liquid 1 (0.402 g, 1.71 mmol) and
2 (0.0501 g, 0.173 mmol), 4 (1.00 mg, 0.00169 mmol), and AIBN
The polymerizable ionic liquid 1 (0.400 g, 1.70 mmol) and 2
(0.0511 g, 0.177 mmol) and AIBN (3.00 mg, 0.0182 mmol) were dis-
solved in methanol (0.40 mL) with stirring. The resulting solution
Fig. 1. Photographs of film of 3 (a) and it under UV light irradiation at 365 nm (b).

S. Wakizono et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 283–287
285
O
O
O
O
O
O
O
O
AIBN
n
1
2
+
+
Cl
CH2CH3
CH2CH3
N
O
N
CH3CH2
CH3CH2
CH2CH3
CH2CH3
N
O
N
O
N
N
O
N
N
Cl
4
Cl
m
Br
CH2CH3
CH2CH3
5
Scheme 2. Synthesis of 5 by radical terpolymerization of 1, 2, with 4 initiated with AIBN.
ture of the product was confirmed by the 1H NMR spectrum as
described in the experimental section. The radical terpolymeriza-
tion of 1, 2, with 4 was performed by the similar procedure as that
for 3 (Scheme 2). The molar ratio of 4 to 1 + 2 was optimized to
(3.00 mg, 0.0182 mmol) were dissolved in methanol (0.40 mL) with
stirring. The resulting solution was sandwiched between two glass
plates and heated at 65 ◦C for 30 min. Further heating it at 75 ◦C
for 2 h gave a film, which was then removed from the glass plates.
After the resulting film was soaked in ethanol, it was dried under
reduced pressure to give a film of 5.
2.2. Measurements
1H NMR spectra were recorded on a JEOL ECX400 spectrometer.
UV–vis and fluorescence spectra were recorded on a Jasco V-650Q1
and FP-6300Q3, respectively.
3. Results and discussion
3.1. Preparation of film of polymeric ionic liquid 3
To synthesize a cross-linked polymeric ionic liquid for the
formation of a film, we employed the two imidazolium-type poly-
merizable ionic liquids 1 and 2; the former has a vinylbenzyl
polymerizable group and the latter contains vinyl and methacry-
late polymerizable groups. Because 2 has the two polymerizable
groups, radical copolymerization of 1 with 2 gives the cross-linked
polymeric ionic liquid 3 (Scheme 1). For the preparation of the film
form of 3, a solution of 1 and 2 (10:1), and AIBN as a radical initiator
(1 mol% for 1 + 2) was sandwiched between two glass plates. Then,
the system was heated at 65 ◦C for 30 min and subsequently at 75 ◦C
for 2 h to occur the copolymerization with simultaneously evapo-
rating methanol. The resulting material was removed from the glass
plates, washed with ethanol to remove the unreacted 1 and 2, and
dried under reduced pressure to give a transparent film of 3 (Fig. 1a).
The UV–vis spectrum of the film (Fig. 2a) showed small absorp-
tions at 280–550 nm, which were probably related to the fluores-
cent emissions of the imidazolium-type ionic liquids, besides large
absorptions at the wavelengths below 280 nm. The fluorescence
spectra of the film (Fig. 2b) exhibited excitation-wavelength-
dependence fluorescent emission maxima at around 430–530 nm
by excitation at 260–480 nm in addition to scattering peaks of exci-
tation lights. As shown in a picture (Fig. 1b), the film exhibited
blue fluorescent emission by UV light irradiation at 365 nm. The
fluorescent behavior of the film was similar as that of the general
imidazolium-type ionic liquid such as BMIMCl, indicating a possi-
bility for occurrence of FRET from the polymeric ionic liquid units
to the rhodamine group if it would be incorporated into the film.
3.1.1. Preparation and fluorescent behavior of film of polymeric
ionic liquid 5 containing rhodamine moieties
The rhodamine derivative 4 having a polymerizable group was
synthesized by the esterification of rhodamine B with HEMA using
DCC/DMAP as a condensing agent in 1,2-dichloroethane. The struc-
Fig. 2. UV–vis spectrum (a) and fluorescence spectra excited at 340–480 nm (b) of
film of 3.

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S. Wakizono et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 283–287
Fig. 3. Photographs of film of 5 (a) and it under UV–vis light irradiations at 254 and 365 nm ((b)–(c)).
be 1 mol% for the following photochemical examinations on the
basis of the results for the terpolymerization in various feed ratios
of the monomers. After the resulting film was soaked and washed
with ethanol for the purification, the ethanol fractions were char-
acterized by the 1H NMR analysis. Consequently, the rhodamine
monomer 4 was hardly present in the solution, indicating that the
monomer 4 used was quantitatively copolymerized and incorpo-
rated in the polymeric ionic liquid 5. The resulting film was also
transparency as shown in Fig. 3a, but showed red color attributed
to the rhodamine moieties.
When the fluorescence spectra of the film were measured by
excitation at 260–480 nm, emissions at ca. 620 nm due to the rho-
damine group in addition to scattering peaks of excitation lights
were observed in all the spectra (Fig. 4a). On the other hand, flu-
orescent emissions at around 430–550 nm due to the units 1 and
2, which had already been shown in Fig. 2b, did not appear. These
results suggested occurrence of FRET from the units 1 and 2 to the
rhodamine group in the film. In deed, all the fluorescent emissions
(colored lines) of the film of 3 excited at various wavelengths were
partially overlapped with an absorption peak (black line) of the
film of 5 at wavelength areas of around 450–600 nm as shown in
Fig. 4b. Furthermore, the connection of the rhodamine group with
the units 1 and 2 by covalent linkages in a polymeric chain results
in their short distance to probably contribute to the occurrence of
the efficient FRET in the film. Actually, the film of 5 showed the red
fluorescent emissions by the UV–vis light irradiations at 254 and
365 nm (Fig. 3b–c).
4. Conclusion
In this paper, we reported the synthesis of the film of the
polymeric ionic liquid film 5 containing the rhodamine moieties
and its FRET function for exhibiting the rhodamine emissions
by excitation at wide wavelength areas. The radical copolymer-
ization of the two polymerizable ionic liquids 1 and 2 gave
the transparent film of 3. The resulting film exhibited the spe-
cific excitation-wavelength-dependent fluorescent behavior as the
imidazolium-type ionic liquids had shown. Then, the radical ter-
polymerization of 1, 2, with the rhodamine derivative 4 having
a polymerizable group was performed to give the film of 5 con-
taining the rhodamine moieties. The fluorescence spectra of the
film exhibited the emissions due to the rhodamine group at ca.
620 nm by excitation at 260–480 nm, whereas the fluorescent emis-
sions due to the units 1 and 2 did not appear. These data indicated
occurrence of FRET from the units 1 and 2 to the rhodamine group
in the film because all the fluorescent emissions of the film of 3
excited at various wavelengths were partially overlapped with an
absorption peak of the film of 5 at wavelength areas of around
450–600 nm.
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