A Typical Metal-Ion-Responsive Color-Tunable Emitting Insulated π-Conjugated Polymer Film

Article abstract of DOI:10.1002/anie.201603160

We report the synthesis of an insulated π-conjugated polymer containing 2,2′-bipyridine moieties as metal coordination sites. Metal coordination to the polymer enabled easy and reversible tuning of the luminescent color without changes to the main chain skeleton. The permethylated α-cyclodextrin (PM α-CD)-based insulation structure allowed the metalated polymers to demonstrate efficient emission even in the solid state, with identical spectral shapes to the dilute solutions. In addition, the coordination ability of the metal-free polymer was maintained in the solid state, resulting in reversible changes in the luminescent color in response to the metal ions. The synthesized polymer is expected to be suitable for application in recyclable luminescent sensors to distinguish different metal ions.

Full text of DOI:10.1002/anie.201603160

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201603160
Luminescent Polymer Materials Very Important Paper German Edition:
DOI: 10.1002/ange.201603160
A Typical Metal Ion-Responsive Color-Tunable Emitting Insulated p-
Conjugated Polymer Film
Takuro Hosomi, Hiroshi Masai, Tetsuaki Fujihara, Yasushi Tsuji, and Jun Terao*
[
9]
Abstract: We report the synthesis of an insulated p-conjugated
polymer containing 2,2’-bipyridine moieties as metal coordi-
nation sites. Metal coordination to the polymer enabled easy
and reversible tuning of the luminescent color without changes
to the main chain skeleton. The permethylated a-cyclodextrin
in such a condensed phase. It is difficult to control such
strong and random interactions in the solid state.
To suppress interpolymer p–p interactions, insulated p-
conjugated polymers (ICPs) have received growing attention
in recent years. ICPs possess bulky side chains or macro-
cycles that cover p-conjugated chains to suppress interpoly-
mer interactions, and such systems have been reported to
show efficient emission even in the solid state. For example,
[10]
(
PM a-CD)-based insulation structure allowed the metalated
polymers to demonstrate efficient emission even in the solid
state, with identical spectral shapes to the dilute solutions. In
addition, the coordination ability of the metal-free polymer was
maintained in the solid state, resulting in reversible changes in
the luminescent color in response to the metal ions. The
synthesized polymer is expected to be suitable for application
in recyclable luminescent sensors to distinguish different metal
ions.
[11]
a high fluorescent quantum yield (F = 0.62),
full-color
have been
F
[12]
[13e]
emission,
and phosphorescent emission
reported in insulated solid-state polymers. We previously
developed ICPs with polyrotaxane structures using perme-
thylated a-cyclodextrins (PM a-CDs) as organic soluble
[13]
macrocycles covalently linked to the main chains. These
ICPs exhibited superior optical properties, owing to the
linked rotaxane structure, thus enabling well-defined and
efficient insulation. We herein report the design and synthesis
of a PM a-CD-based ICP bearing 2,2’-bipyridine units as
metal coordination sites (Figure 1). The PM a-CD positions
are fixed by linkages to provide a size-limited cavity around
the metal coordination sites. This structure allows metal ions
to access the coordination sites, simultaneously preventing
interactions between the metallopolymers.
D
ue to their superior luminescent properties and process-
ability p-conjugated polymers (CPs) are promising materials
[
1]
for use in optical devices. For such applications, it must be
possible to tune the luminescent colors of the polymers
according to the desired practical use. In general, the
luminescent colors of CPs are altered by modifying the
conjugated backbones, which often involves reconstruction of
the corresponding monomers. In addition, control of a poly-
merꢀs luminescent colors may be possible through non-
[2,3]
covalent bonding with external stimulating agents,
and
allows easy, direct, and reversible control of a polymerꢀs
luminescent color without changes to the main chain skeleton.
In terms of external stimulating agents, metal-salts have been
extensively researched due to their facile coordination with
basic polymers and resulting redistribution of the electron
[
4–7]
cloud of the conjugated chains.
It has been reported that
tuning of the luminescent wavelength of p-conjugated
Figure 1. Design of the color-tunable ICP.
molecules was possible by varying the metal or anionic
[5d–f,8]
species employed.
Application of these functions to
solid-state materials is therefore expected to yield metal ion
sensors. However, despite many successful examples in the
Scheme 1 shows the synthetic routes to polymer 3 and the
uninsulated reference 3’. The key step in this process is the
quantitative self-inclusion of 1’, bearing two PM a-CDs linked
to the p-conjugated chain, into the insulated molecule 1, by
[4–7]
solution state,
no examples of reversible luminescent color
change in neat CP films by metal ions have been reported,
although stimulating agents such as protons or boranes have
dissolving in a hydrophilic solvent mixture (MeOH/H2O = 2/
[
3]
[13b]
been shown to be effective. This is due to strong interpo-
1).
Under the same solvent conditions, 1 was elongated via
lymer p-p interactions causing luminescence self-quenching
an iodo-selective Sonogashira–Hagihara cross-coupling with
-bromo-5-iodopyridine using a water-soluble TPPTS (tris(3-
2
sulfophenyl)phosphine trisodium salt) as the ligand in
[
*] T. Hosomi, Dr. H. Masai, Prof. Dr. T. Fujihara, Prof. Dr. Y. Tsuji,
Prof. Dr. J. Terao
Department of Energy and Hydrocarbon Chemistry, Graduate School
of Engineering, Kyoto University
Kyoto 615-8510 (Japan)
MeOH/H O = 2/1 (v/v) to afford 2. Insulation of 2 was
2
maintained under various solvent conditions as the linkage
structures prevent the PM a-CDs threading out from the
elongated p-conjugated chains. The insulated structure of 2
was confirmed by observing the nuclear Overhauser effect
E-mail: terao@scl.kyoto-u.ac.jp
(
NOE) between the inner PM a-CD protons and aryl protons
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603160.
1
1
in the H– H ROESY NMR spectra (see Figure S1 in the
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Scheme 1. Synthetic routes to polymers 3 and 3’.
Supporting Information). 2’ was selectively synthesized from
precursor 1’ by simply changing solvents from a hydrophilic
i
solvent (MeOH/H2O) to an organic solvent ( Pr NH). Com-
2
pounds 2 and 2’ were then polymerized with equivalent
n
amounts of Sn Bu via sequential stannylation and Migita–
2
6
[
14]
Kosugi–Stille cross-coupling to give polymer 3. Insulated
polymer 3 was highly soluble in a wide range of solvents (e.g.,
toluene, chloroform, tetrahydrofuran, acetonitrile, methanol,
and N,N-dimethylformamide), reflecting the insulating effect
of the PM a-CDs. The number average molecular weight (M )
n
4
of insulated polymer 3 was determined to be 4.7 ꢁ 10 by size
exclusion chromatography (SEC; Figure S2). Uninsulated
reference 3’ was also synthesized by a similar procedure, and
4
gave a lower M value (2.7 ꢁ 10 ) due to its lower solubility
n
(
1
Figure S3). The high molecular weight fractions (M > 4.9 ꢁ
0 ) of polymers 3 and 3’ were fractionated by preparative
n
4
SEC and used for optical analyses described below.
The emission properties of these polymers were compared
in the various solvents and in the solid state (Table 1). Both 3
and 3’ showed blue emission with vibrational bands in CHCl3
Figure 2. Emission spectra (excitation: 380 nm) of a) polymer 3 and
b) 3’ in CHCl (blue), 90% MeCN (green), 90% MeOH (yellow), or the
3
(
Figure 2, blue line). The Stokes shift of 3 was significantly
solid-state film (red).
smaller than that of 3’ as the linked PM a-CDs restricted
rotation of the aryl rings in phenylene-ethynylene, thus fixing
[
13d]
their dihedral angles at a slightly twisted position.
As
stant emission maximum even in the solid state (Figure 2a),
although the relative intensity of the 0–0 transition band
decreased. This clearly differed from uninsulated 3’, where
a 100 nm redshift of the emission maxima was observed
(Figure 2b). The weakening of the 0-0 transition band of 3 is
likely the result of the small Stokes shift (re-absorption
effect), which also explains the lower FPL values of 3
compared with metalated 3-M in the solid state (vide infra).
p-Conjugated polymers bearing metal coordination sites
(3 and 3’) were then coordinated with various metals (3-M, 3’-
a result, non-radiative decay of 3 was suppressed to give
a higher quantum yield (FPL = 0.58) than 3’ (FPL = 0.28) in
CHCl . The value of l and FPL of insulated 3 were much
insensitive to the polarity of solvents (CHCl , MeCN or
MeOH) than those of 3’, reflecting the insulating effect
3
PL
3
(
Figure 2). Moreover, insulated polymer 3 exhibited a con-
Table 1: Emission properties of polymer 3 and 3’.
M) by the addition of a range of salts (ZnCl , CdCl , GaCl ,
3
lAbs
3’
lAbs
2
2
3
[
a]
[b]
[c]
[a]
[b]
[c]
Solvent
CHCl3
lPL
FPL
lPL
FPL
InCl , or SnCl ), resulting in a redshift of the emission
3 4
wavelength (Table 2). The degree of shifting tended to
increase with an increase in valence of the metallic species.
3-M gave comparable or greater FPL values than those of 3. In
contrast, the FPL value decreased for uninsulated polymer 3’
upon coordination to high-valence metals. These results
demonstrate the desired adjustability of the fluorescence
color for insulated polymer 3. Previously, Ma and co-workers
396
394
393
–
417, 438 0.58
416, 439 0.33
416, 438 0.25
417, 439 0.16
408
409
415
–
444
465
475
540
0.28
0.05
0.09
0.10
d
9
9
0% MeCN
0% MeOH
d
Film
[a] Unit: nm. [b] Unit: nm, excited at 380 nm. [c] Excited wavelengths
were scanned at 350–400 nm. F were measured using an integrating
PL
[
15]
sphere. [d] 10% CHCl was added to dissolve uninsulated 3’.
3
2
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Table 2: Optical properties of 3-M and 3’-M in CHCl3.
lated chromophores took place. This sensitivity renders the
CPs particularly suitable for use in metal-sensing applica-
tions.
[
a]
[b]
Compounds
lAbs/nm
lPL/nm
FPL
[
18]
II
3
-
Zn
Cd
405
398
410
404
414
423
414
438
457
451
489
475
506
551
506
N.D.
550
N.D.
0.68
0.58
0.61
0.62
0.47
0.20
0.32
N.D.
0.09
N.D.
II
The 3-M films were then fabricated by casting their CHCl3
III
solutions on SiO substrates. As stated above, p-conjugated
2
Ga
III
polymers generally show significant fluorescence spectral
changes and decreased emission efficiency upon film forma-
In
Sn
IV
II
[9]
3’-
Zn
Cd
tion. However, 3-M exhibited strong emission even in the
II
solid state. The emission spectra of the films (Figure 4, solid
lines) were comparable to those of dilute solutions (Figure 4,
III
Ga
III
In
Sn
426
449, 389
IV
[a] Excited at 380 nm. [b] Excited wavelengths were scanned at 350–
4
00 nm. The FPL values were measured using an integrating sphere.
II
[16]
examined the origin of the redshift of Zn -coordinated CPs.
II
They found that Zn orbitals did not participate in any
II
frontier orbital interactions, but that Zn acted as a positive
point charge, causing a decrease in the energy level of
adjacent molecular orbitals. Consequently, the lowest unoc-
cupied molecular orbital (LUMO) localized around the
bipyridine moiety and the emission took on the character of
a charge-transfer transition. This calculated result agrees with
our experimental result, where an increase in metal valence
caused an increase in the redshift. Furthermore, the metal-
induced emission redshift of polymer 3 was significantly
suppressed compared to that of 3’ (Table 2). It is suggested
that the insulated structure of 3 suppresses large structural
changes in the excited states accompanying charge-transfer
transition (the possibility that 3’ formed a 2:1 complex was
II
Figure 4. Emission spectra (excitation: 380 nm) of 3-Cd (light blue),
3
III
IV
-In (green), and 3-Sn (yellow). Dashed lines indicate spectra
measured in CHCl . Solid lines indicate spectra measured from solid
3
state films.
[5g,17]
II
eliminated by Scatchart plot; Figure S16).
dashed line), with the exception of 3-Zn , which showed
To gain a closer insight into the effect of metal ions,
fluorescence titrations with SnCl4 were conducted using
polymer 3 (Figure 3). The relative emission intensity at each
wavelength varied in a nonlinear fashion and was saturated by
a redshift of about 20 nm from the solution value (Table 3).
Furthermore, insulated metallopolymers 3-M maintained
efficient emission (FPL = 0.16–0.46; Table 3) in contrast to
the corresponding uninsulated metallopolymers 3’-M (FPL
IV
the addition of 0.5 equiv. of SnCl . This differed from the
< 0.19, Figure S9). In particular, the quantum yield of 3-Sn
4
absorption trace of the excited wavelength (380 nm), which
showed a much more gradual decrease upon the addition of
(FPL = 0.46) was significantly high among conjugated metal-
lopolymer films. These results demonstrate the superior
insulating ability of the PM a-CD-based linked rotaxane
structure, as generally, the suppression of interactions
between p-conjugated metallopolymers (including ionic
interactions and p- p interactions) is challenging.
up to 1.0 equiv. SnCl (Figure S14). This behavior suggests
4
that energy transfer from metal-free chromophores to meta-
Finally, the coordination ability and reversibility of the
metal-free polymer film 3 were examined. The blue-emitting
film of 3 (Figure 5A) was dipped into a Et O solution
2
containing 0.3 mm SnCl at room temperature for 5 s. The
4
emission color of the dipped moieties immediately changed
into the representative colors for SnCl (Figure 5B), and the
4
emission spectra of the dipped regions were identical to those
IV
of the films formed by casting 3-Sn solution (Figure S10).
The larger area of the film was subsequently dipped into
Table 3: Emission properties of polymer 3-M in the solid state.
II
II
III
III
IV
3-
Zn
Cd
Ga
In
Sn
l /nm
FPL
477
0.35
451
0.16
486
0.31
480
0.39
507
0.46
PL
Figure 3. Changes in the emission spectra of polymer 3 upon SnCl4
titration (excitation: 380 nm). The inset indicates emission intensity
[a]
traces at 418 nm (derived from the metal-free chromophore) and
[a] Excited wavelengths were scanned at 350–400 nm. FPL were mea-
sured using an integrating sphere.
IV
5
05 nm (derived from the Sn -coordinated chromophore).
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Figure 5. Change in emission color of polymer 3 film with exposure to metal ions. Photographic images were taken under UV irradiation at
65 nm. Double-headed arrows indicate the dipped regions of the films. These steps were conducted in order A-B-C-D.
3
a 3 mm InCl Et O solution for 20 s, and the non-metalated
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2
region changed into the representative colors for InCl3
IV
(
Figure 5C). Interestingly, the Sn -coordinated region did
not exhibit a luminescent color change upon contact with the
III
IV
In solution due to the strong Sn coordination. Further-
more, the above luminescent color changes were easily erased
through removal of the coordinated metals by immersion in
an aqueous NH3 solution (Figure 5D). The recycled film
exhibited the original blue emission, and once again demon-
strated a coordination ability to the various metals, thus
confirming the reversibility of this process. This polymer is
therefore expected to be applicable in recyclable sensor
materials to distinguish metal ions. The ICP synthesized in
this work is an example of a novel polymer design, which
enables metallopolymers to extend their unique properties to
solid-state materials.
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Acknowledgements
This research was supported by JSPS KAKENHI grant
numbers JP16H00834, JP16H00965, and JP26288090. This
research was also supported by Tokuyama Science Founda-
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2014, 136, 14714.
Published online: && &&, &&&&
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Communications
Luminescent Polymer Materials
T. Hosomi, H. Masai, T. Fujihara, Y. Tsuji,
J. Terao*
&&&& — &&&&
A Typical Metal Ion-Responsive Color-
Tunable Emitting Insulated p-Conjugated
Polymer Film
Metal-ion sensor: An insulated p-conju-
gated polymer containing metal-binding
sites was synthesized. The coordination
of metal ions to these sites alters the
electronic properties of the polymer con-
siderably. As a consequence the emission
wavelength of the polymer varies and the
polymer acts as a sensor for metal ions.
6
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