Synthesis and characterization of reversible chemosensory polymers: Modulation of sensitivity through the attachment of novel imidazole pendants

Article abstract of DOI:10.1002/chem.201201437

Three novel electron donor-acceptor conjugated polymers (P1-P3) bearing various imidazole pendants have been synthesized. Their excellent photophysical and electrochemical properties make them suitable transduction materials for chemosensing applications. Indeed, polymers P1-P3 have been found to show remarkable sensing capabilities towards H⁺ and Fe²⁺ in semi-aqueous solutions. Upon titration with H⁺, polymers P1 and P2 showed hypsochromic shifts of their absorptions and photoluminescence (PL) maxima with enhanced fluorescence intensities. However, P3 showed diminished absorption and fluorescence intensities under similar conditions due to static quenching. The anomalous behavior of P3 compared with P1 and P2 has been clarified in terms of electronic distributions through computational analysis. Furthermore, P3 (K_SV=1.03×10⁷) showed a superior sensing ability towards Fe²⁺ compared with P1 (K_SV=2. 01×10⁶) and P2 (K_SV=4.12×10⁶) due to its improved molecular wire effect. Correspondingly, the fluorescence lifetime of P3 was greatly decreased (almost 11-fold) compared to those of polymers P1 (4.6-fold) and P2 (6.2-fold) in the presence of Fe²⁺. By means of a fluorescence on-off-on approach, chemosensing reversibilities in protonation-deprotonation and metallation-demetallation have been achieved by employing triethylamine (TEA) and the disodium salt of ethylenediaminetetraacetic acid (Na₂-EDTA)/phenanthroline, respectively, as suitable counter ligands. ¹H NMR titrations have revealed the unique behavior of P3 compared with P1 and P2. To the best of our knowledge, there have been no previous reports of Fe²⁺ sensors based on single imidazole receptors conjugated to a main-chain polymer showing such a diverse sensitivity pattern depending on their attached substituents.

Full text of DOI:10.1002/chem.201201437

FULL PAPER
DOI: 10.1002/chem.201201437
Synthesis and Characterization of Reversible Chemosensory Polymers:
Modulation of Sensitivity through the Attachment of
Novel Imidazole PenACTHNUTRGNEdNUG ants
Rudrakanta Satapathy, Harihara Padhy, Yen-Hsing Wu, and Hong-Cheu Lin*[a]
Abstract: Three novel electron donor–
acceptor conjugated polymers (P1–P3)
bearing various imidazole pendants
have been synthesized. Their excellent
photophysical and electrochemical
properties make them suitable trans-
duction materials for chemosensing ap-
plications. Indeed, polymers P1–P3
have been found to show remarkable
sensing capabilities towards H+ and
Fe2+ in semi-aqueous solutions. Upon
titration with H+, polymers P1 and P2
showed hypsochromic shifts of their
absorptions and photoluminescence
(PL) maxima with enhanced fluores-
cence intensities. However, P3 showed
diminished absorption and fluorescence
intensities under similar conditions due
to static quenching. The anomalous be-
havior of P3 compared with P1 and P2
has been clarified in terms of electronic
distributions through computational
analysis. Furthermore, P3 (KSV =1.03ꢀ
107) showed a superior sensing ability
of a fluorescence on-off-on approach,
chemosensing reversibilities in protona-
tion–deprotonation and metallation–
demetallation have been achieved by
employing triethylamine (TEA) and
towards Fe2+ compared with P1 (KSV
=
the disodium salt of ethyl
ACHTUNGTRENNUNGene-
2.01ꢀ106) and P2 (KSV =4.12ꢀ106) due
to its improved molecular wire effect.
Correspondingly, the fluorescence life-
time of P3 was greatly decreased
(almost 11-fold) compared to those of
polymers P1 (4.6-fold) and P2 (6.2-
fold) in the presence of Fe2+. By means
ACHTUNGTNERdNUGN iaminetetraacetic acid (Na2-EDTA)/
phenanthroline, respectively, as suitable
counter ligands. 1H NMR titrations
have revealed the unique behavior of
P3 compared with P1 and P2. To the
best of our knowledge, there have been
no previous reports of Fe2+ sensors
based on single imidazole receptors
conjugated to a main-chain polymer
showing such a diverse sensitivity pat-
tern depending on their attached sub-
stituents.
Keywords: computational analysis ·
donor–acceptor systems · fluores-
cence · molecular wire effect · sen-
sors · transduction material
Introduction
have enormous advantages over small molecules for sensing
applications due to facile exciton transport and energy mi-
gration along their conjugated polymer backbones, which
enhance the electronic communication between receptors
and polymers.[4] Conjugated polymers are attractive for serv-
ing as semi-conductive “molecular wires” owing to their p-
electron resonances through conjugations between donor–
acceptor repeat units. This allows such polymers to be utiliz-
ed as one of the most vital classes of sensing materials by
virtue of their high sensitivities derived from molecular wire
effects.[5] Furthermore, conjugated polymer sensors hold
great promise because their emission intensities, absorban-
ces, charge-transport properties, conductivities, and exciton
migrations can be easily perturbed by external chemical spe-
cies, such as acids, bases, and ions, leading to substantial
changes in their measurable signals.[1,6,7] The incorporation
of sensitive and selective functionalities (as probes) for both
protons and metal ions, together with well-known fluoro-
phores, into conjugated polymers has yielded new materials
suitable for chemosensory applications.[6] Thus, the synthesis
of soluble and stable fluorescent conjugated polymers has
yielded improved selective recognition and transduction ma-
The molecular design of new sensors is of prime importance
because of tremendous demands in analytical, biomedical,
biotechnological, and nanotechnological applications.[1] The
success of fluorescence-based sensors can be attributed to
their advantageous features of specific sensitivity, selectivity,
and real-time screening for various moieties.[2] Rigid and
ladder-type low band-gap conjugated polymers can display
excellent redox and optoelectronic properties, such as broad
and long-wavelength light absorption, good luminescence in-
tensity, and favorable carrier mobility, which result in poten-
tial applications in optoelectronics, microelectronics, light-
emitting diodes, photovoltaics, and field-effect transistors.[3]
Some interesting recent extensions of these materials con-
cerned their subsequent application in fluorescent and bio-
logical sensors.[4] Conjugated polymers (as molecular wires)
[a] R. Satapathy, Dr. H. Padhy, Y.-H. Wu, Prof. H.-C. Lin
Department of Materials Science and Engineering
National Chiao Tung University
Hsinchu, R.O.C. (Taiwan)
Fax : (+886)35724727
terials for chemical and biological sensing purposes.[8–10]
A
E-mail: linhc@mail.nctu.edu.tw
key feature for such sensing applications is a need for rever-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201437.
sibility within certain ranges of recognition processes.[7] Imi-
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dazole-based ligands are widely used by virtue of their re-
versible fluorescence (on-off-on) by protonation/deprotona-
tion with an acid/base as well as metallation/demetallation
in the presence of metal ions/suitable counter ligands, which
can induce a large energy perturbation.[6]
On the other hand, fluoride anion is of particular interest
among halide anions owing to its association with preventing
dental caries, as well as treatments for osteoporosis, anaes-
thetics, hypnotics, psychiatric drugs, and nerve gases. Its de-
tection is also pertinent to the analysis of drinking water
and the refinement of uranium used in nuclear weapon man-
ufacture.[12a–d] The detection of fluoride is often subject to in-
terference from other halides.[12e,f] Hence, selective fluoride
detection in the presence of other halides has also become
an objective of many researchers.[12g–j]
With these concepts in mind, we have synthesized three
novel imidazole-based conjugated fluorescent polymers
(P1–P3; Scheme 1). The photophysical and electrochemical
properties of these polymers indicate that they might serve
as excellent energy-transfer materials with remarkable sta-
bilities, making them suitable for use in chemosensors. Com-
pared with P1 and P2, polymer P3 showed a different spec-
tral change upon pH sensing, probably due to its distinct
electronic distribution. All of the polymers showed reversi-
bility in pH sensing upon the addition of TFA as a protonat-
ing agent and TEA as a counter ligand for deprotonation.
Furthermore, the quenched fluorescences of polymers P1–
P3 were restored upon the addition of counter ligands.
Hence, reversible sensing capabilities of polymers P1–P3 to-
Fe2+ is the most abundant transition metal ion in biologi-
cal systems and plays significant roles in metabolic process-
es.[7] Fe2+ ions are essential for most organisms, but both de-
ficiency and overload of Fe2+ can cause various syndromes
as a result of the disruption of iron transport, storage, and
balance.[8] For example, Fe2+ deficiency leads to anaemia
and breathing problems, while excess iron in the body
causes DNA, liver, and kidney damage (hemochromato-
sis).[9] Introduction of specific probes, such as epoxy, bipyri-
dine, terpyridine, quinoline, dipyrrolylquinoxaline, imida-
zole, and sulfate on the backbones or side chains of conju-
gated polymers has been successfully employed for metal
ion detection on the basis of characteristic metal-to-ligand
charge-transfer effects.[10] However, to the best of our
knowledge, there have been no reports of Fe2+ sensors with
a single imidazole receptor conjugated to a main-chain poly-
mer backbone. Herein, we report three imidazole-based pol-
ymers that show remarkable sensing capabilities towards
Fe2+
.
Scheme 1. Synthetic routes for the preparation of monomers and polymers.
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Imidazole-Substituted Polymers with Molecular Wire Properties
FULL PAPER
wards Fe2+ ions were achieved by the addition of suitable
counter ligands (Na2-EDTA/phenanthroline). Furthermore,
all of the monomeric precursors (1B–4B) showed distinct
sensitivities towards fluoride ions over the other halides.
Photophysical characterization: Normalized absorption and
emission spectra of the polymers in both solutions and solid
films are shown in Figure 1, and their characteristic optical
data are summarized in Table 1. All of these polymers ex-
hibited two distinct broad absorption peaks, with the high-
energy sharp peaks at 300–400 nm being attributable to d-d*
transitions, and the low-energy broad bands at 500–650 nm
being attributable to localized intramolecular charge trans-
Results and Discussion
Synthesis: Four imidazole-based monomers (M1–M4) were
synthesized according to Scheme 1. Benzo[1,2-b:4,3-b’]di-
thiophene-4,5-quinine (1-A) was prepared in two steps, that
is, initial preparation of 3,3’-bithiophene followed by acyla-
tion with oxalyl chloride in the absence of a Lewis acid. It
was found that the best route to 3,3’-bithiophene was to
treat 3-bromothiophene with nBuLi and then to perform an
oxidative coupling using commercial grade CuCl2, which
gave the product in 77.9% yield (Scheme S1 in the Support-
ing Information). Furthermore, 1,10-phenanthroline-5,6-
dione (4-A) was prepared by treating 1,10-phenanthroline
with H2SO4, HNO3, and KBr.[13a] Phenanthrene-9,10-dione
(2-A) and acenaphthylene-1,2-dione (3-A) were obtained
from commercial sources. Imidazole derivatives (1-B–4-B)
were obtained by cyclization of 3-thiophenecarboxaldehyde
with the corresponding diketo compounds (1-A–4-A) using
NH4OAc in acetic acid. All imidazole derivatives (1-B–4-B)
were directly employed without further purification. Mono-
mers (M1–M4) were prepared by N-alkylation of 1-B–4-B
using K2CO3 as base and an alkyl iodide in DMF and were
subsequently purified by column chromatography to afford
good yields. To the best of our knowledge, none of these
monomers has been synthesized previously. Finally, Suzuki
polymerization of monomers M1–M4 with the diboronic
ester of benzothiadiazole afforded the respective polymers
P1–P4 (Scheme 1). It is worthy of mention that, except for
P4, all of these polymers proved to be soluble in common
organic solvents, such as CH2Cl2, CHCl3, THF, and 1,2-di-
chlorobenzene. However, the average molecular weight of
polymer P3 (Mw=13718) was higher than those of P1
(Mw=4614) and P2 (Mw=5755) due to the good solubility
of the acenaphthylene-imidazole units in P3 compared to
the benzodithiophene units in P1 and the phenanthrene
units in P2. Further characterizations of polymers P1–P3
were carried out in semi-aqueous solutions of THF/H2O
(1:1).
Figure 1. Normalized (a) UV absorption and (b) photoluminescence spec-
tra of P1, P2, and P3 in THF solutions (sol) and solid films (film).
Table 1. Photophysical, electrochemical, and thermal properties of P1, P2, and P3.
Polymer Solution[a]
Film[b]
Energy levels
[e]
[d]
[f]
emax,abs
emax,em
(nm)
631
Stokes
emax,abs
emax,em
Stokes
eonset,abs
Eg,opt
(eV)
1.86
Eonset,ox (V)/
Eonset,red[d] (V)/
LUMO (eV)
À0.86/À3.49
Eg,el
Tg
(8C)
234
(nm)
shift[g] (nm) (nm)
(nm)
shift[g] (nm) (nm)
HOMO (eV)
1.05/À5.40
(eV)
1.91
P1
P2
P3
305,
515
301,
521
319,
470
119
119
224
325,
535
329,
539
327,
497
A
,
,
,
149
172
212
664
676
662
684
(684)[c]
711
(683)[c]
709
640
594
C
1.83
1.87
1.02/À5.37
1.06/À5.41
À0.87/À3.48
À0.86/À3.49
1.89
1.92
+
274
249
AHCTUNGTRENNUNG
+
[a] In THF solutions. [b] Spin-coated from THF solutions on a glass surface. [c] Shoulder peak. [d] EHOMO/ELUMO =[À(EonsetÀEonset (Fc/Fc
[e] Optical bandgap Eg.op =1240/ledge. [f] Electrochemical bandgap Eg.el =Eonset/ox À Eonset/red. [g] Stokes shift=lemÀlabs
)) À 4.8].
vs Ag/Ag
.
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H.-C. Lin et al.
fers between electron donors and acceptors in the polymer
backbones. Significant bathochromic shifts of the absorp-
tions as well as photoluminescence (PL) emissions of all of
the polymers in solid films could be ascribed to interchain
aggregations of rigid planar segments in the polymer back-
bones.[14a–c] The shoulders seen in the PL spectra in Fig-
ure S1(b) may be ascribed to the increased vibronic coupling
associated with molecular rigidity in solid films. The notice-
ACHTUNGTRENNUNGably larger values of the Stokes shifts of all of the polymers
signify differences in the geometrical structures in the
ground and excited states. This further indicates the occur-
rence of significant excited-state intermolecular charge
transfer in the chromophores, which is essential for chemo-
sensory applications.[14d,e] The optical bandgaps (~1.85 eV)
of these polymers (P1–P3) were found to be smaller than
that of P3HT (~1.90 eV)[14g] due to the combination of elec-
tron donor and acceptor moieties. In addition, the bandgaps
of P1–P3 were also smaller than that of the analogous poly-
mer PT2BT (1.97 eV) containing bithiophene (as donor)
and benzothiadiazole (as acceptor) units,[14h] which implies
that, compared to the bithiophene unit in PT2BT, the 3-thio-
phene-imidazole unit (with a stronger electron-donating
ability) induced more effective conjugation between the
donor and acceptor segments in polymers P1–P3. Therefore,
it could be concluded that polymers P1–P3 have good elec-
tronic communication between receptors in their backbones
through facile migration of excitons during the sensing proc-
ess.[14i]
Electrochemical characterization: Cyclic voltammograms of
the polymers in solid films are shown in Figure 2, and the re-
lated data are summarized in Table 1. All of the polymers
displayed reversible or quasi-reversible n-doping/dedoping
(reduction/reoxidation) processes at negative potentials,
which indicated good structural stability in their charged
states. Furthermore, the highest occupied molecular orbital
(HOMO) levels of polymers P1–P3 (ca. À5.4 eV) were
lower than À5.2 eV, which showed their good air stability.[15c]
Besides structural stability, these low HOMO levels suggest
that polymers P1–P3 should be capable of serving as good
electron-donor materials for sensing applications.[15]
pH sensing and reversibility: The pH-sensing properties of
these polymers were investigated by UV/Vis and PL titra-
tions. As shown in Figure 3(a), upon increasing [H+] by the
addition of trifluoroacetic acid (TFA), a hypsochromic shift
in the absorption maximum as well as the onset occurred,
with a concomitant decrease in the absorption intensity of
P1. Polymer P2 showed a similar pH-sensing response (Fig-
ure 3(b)), probably due to a similar electronic distribution.
At higher [H+], the imidazole units were protonated, which
hindered the effective charge transfer from the benzodithio-
phene and phenanthrene units to the backbones of P1 and
P2, respectively. The excited state was more strongly desta-
bilized than the ground state, due to the protonation of
donor imidazole units, and thus hypsochromic shifts of the
absorption and emission spectra were observed.[16a] The PL
Figure 2. Cyclic voltammograms of polymers (a) P1, (b) P2, and (c) P3 in
solid films obtained at a scan rate of 100 mVsÀ1
.
spectra of P1 and P2 in Figure 4(a) and (b), respectively,
show significant blue shifts with enhanced intensities upon
increasing [H+]. Photoinduced electron transport (PET)[16b]
from the imidazole units to the polymer backbones of P1
and P2 caused weak fluorescence. However, protonation of
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Imidazole-Substituted Polymers with Molecular Wire Properties
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Figure 3. UV/Vis absorption spectra of (a) P1, (b) P2, and (c) P3 in THF/H2O (1:1) (1.4ꢀ10À5 m) at various concentrations of trifluoroacetic acid (TFA)
and after final neutralizations with triethylamine (TEA).
the imidazole moieties diminished PET effects at higher
[H+], which in turn restored the fluorescence originating
from the imidazole unit and hence the fluorescence was en-
hanced.[16c,d] Due to the different electronic distribution of
the five-membered anti-aromatic (acenaphthylene) rings in
P3, this polymer showed a different pH response from P1
and P2 containing six-membered aromatic rings (benzodi-
thiophene and phenanthrene, respectively). In contrast to
P1 and P2, upon increasing [H+], the absorption intensity of
P3 (Figure 3(c)) decreased without significant blue shifts of
its absorption maxima. Sequentially, a new shoulder ap-
peared at 360–400 nm at moderate [H+] (2ꢀ10À6 À2.1ꢀ
10À3 m). Surprisingly, unlike for P1 and P2, the fluorescence
intensities of P3 decreased with increasing [H+] (Fig-
ure 4(c)), which could possibly be attributed to static quen-
ching.[16e–g] The notion of static quenching was further con-
firmed by a Stern–Volmer plot for the fluorescence quench-
ing of P3 upon titration with [H+] at various temperatures
(Figure 4(d)), which revealed that the binding constant of
the quencher H+ for P3 was reduced upon increasing the
temperature. Furthermore, each of the polymers recovered
most of its original absorption and fluorescence upon the
addition of triethylamine (TEA) due to deprotonation of
the imidazolium salt. Consequently, the modulation of both
the UV/Vis and PL spectra upon the addition of TFA and
TEA clearly indicates that all of the polymers (P1–P3) are
promising reversible pH-sensing materials in terms of both
absorption and fluorescence ratiometries.
We further investigated the sensing mechanism by theo-
retical calculations based on computational analysis (see
Figure 5). The HOMO electron clouds are mostly localized
over the benzodithiophene units of P1, while the LUMO
electron clouds are localized over the benzothiadiazole
units. A more electron-rich center is induced by the aroma-
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H.-C. Lin et al.
Figure 4. PL spectra of (a) P1, (b) P2, and (c) P3 in THF/H2O (1:1) (1.4ꢀ10À5 m) at various concentrations of trifluoroacetic acid (TFA) and after final
neutralizations with triethylamine (TEA). Insets in (a)–(c) are the respective Stern–Volmer plots. (d) Stern–Volmer plots for the fluorescence quenching
of P3 by H+ at 258C and 508C.
ticity of the benzodithiophene unit in P1, and thus the
HOMO electron cloud resides over this unit. As a highly
electron-accepting group, the benzothiadiazole unit gathers
most of the LUMO electron cloud. Thus, there is effective
intramolecular charge transfer (ICT) from the pendant ben-
zodithiophene unit to the polymer backbone through the
imidazole linkage. The aromaticity of the phenanthrene ring
in P2 makes it a strong electron-donating unit. The HOMO
electron cloud is thus mostly localized over the phenan-
threne unit. Similarly to P1, the LUMO electron cloud in
P2 is mostly localized over the benzothiadiazole unit. Thus,
there is effective ICT in the polymer backbone of P2 due to
the charge-separated states. In both P1 and P2, the nitrogen
lone pairs of the imidazole units belong to the HOMOs of
the respective polymers. Upon titration with H+, the nitro-
gen lone pairs become coordinated with the H+ ions, leading
to bonding-type interactions and thus to blocking of the
original ICT that occurs in the polymers. Furthermore, upon
complexation with H+, both frontier orbitals (HOMO and
LUMO) are located only over the polymer backbones. This
cannot give a completely charge-separated state for an ef-
fective ICT process, and thus the fluorescence is enhanced
for both P1 and P2 after their complexation with H+. The
anti-aromaticity of the acenaphthylene ring in P3 makes it a
less electron-rich center compared with the aromatic benzo-
dithiophene (in P1) and phenanthrene (in P2) units. Thus,
the HOMO electron cloud is not fully delocalized over the
acenaphthylene unit in P3. The LUMO electron cloud is lo-
calized over the polymer backbone. Unlike in P1 and P2,
the charge-separated states are not well-defined in the case
of P3. However, upon complexation with H+ ions, a well-or-
ganized charge-separated state appears in P3. Finally, this
causes an effective charge transfer in the polymer backbone.
This can be attributed to the plausible quenching mecha-
nism in P3.
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Imidazole-Substituted Polymers with Molecular Wire Properties
FULL PAPER
presence
of
10 equiv
of
Fe2+. This may have been due
to a better coordination ability
of imidazole receptors towards
Fe2+ ions, which may be attrib-
uted to energy- and/or charge-
transfer due to the stronger
ligand–metal
interaction.[16e]
The selectivity for Fe2+ was fur-
ther proven by its interference
with other background metal
ions. As illustrated in Figure 7,
the sensitivities of polymers
P1–P3 towards Fe2+ were not
significantly affected by other
competing metal ions. Howev-
er, P3 showed better quenching
efficiency compared to those of
P1 and P2. This could be attrib-
uted to the higher molecular
weight of P3, which gave rise to
a better semiconducting “mo-
lecular wire effect” than P1 and
P2. The larger number of
Figure 5. Localization of electron clouds in HOMO and LUMO of P1–P3 before (left) and after (right) com-
plexation with H+.
Figure 6. Fluorescence emission response profiles (I0/I with error bars) of
P1, P2, and P3. Polymer concentration (1.2ꢀ10À5 m), Fe2+ added=
10 equiv (1.2ꢀ10À4 m), other metal ions added=30 equiv (3.6ꢀ10À4 m)
Figure 7. Fluorescence emission response profiles (I/I0 with error bars) of
P1, P2, and P3. Polymer concentration (1.2ꢀ10À5 m), [Fe2+]=1.2ꢀ10À4
(10 equiv) and in the presence of other metal ions at 1.2ꢀ10À4
(10 equiv) (dual-metal system).
m
m
(single-metal system).
Selectivity, sensitivity, and reversibility in metal ion sensing:
The chemosensing properties of the fluorescent polymers
were investigated for a variety of metal ions, such as Li+,
repeat units in P3 provided more binding sites and thus
caused complete fluorescence quenching.
Na+, K+, Ca2+, Ba2+, Mg2+, Zn2+, Co2+, Ni2+, Cd2+, Fe2+
,
The PL quenching efficiencies (PLQE) and static Stern–
Volmer quenching constants (KSV) are listed in Table 2. Due
to the larger molecular wire effect, KSV of P3 is about 5.1
and 2.5 times higher than those of P1 and P2, respectively.
As depicted in Figure 8, sharp decreases in the fluorescence
intensities of P1–P3 were observed upon the addition of
Ag+, Mn2+, and Cu2+ (Figure 6). The fluorescence intensities
of P1–P3 were only slightly altered upon the addition of up
to 30 equiv of Li+, Na+, K+, Ca2+, Ba2+, Ni2+, Zn2+, Ag+,
Co2+, Mn2+, and Cu2+. However, the fluorescence intensities
of polymers P1–P3 were decreased significantly in the
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H.-C. Lin et al.
Table 2. Photoluminescence quenching efficiency (PLQE) and Stern–
observed over six successive cycles (Figure 10). Thus, a re-
markable fluorescence on-off-on behavior was achieved for
the polymer solutions for the sensing of Fe2+ based on rever-
sible binding to the imidazole receptors and dissociation
upon the addition of appropriate counter ligands (such as
EDTA and phenanthroline) to form more stable complexes.
To the best of our knowledge, this is the first report of inter-
vention by the phenanthroline ligand eliciting efficient fluo-
rescence recovery of imidazole receptors in the presence of
metal ions.
Volmer quenching constants (KSV) of polymers upon the addition of
Fe2+
.
[b]
Metal Ion
Polymers
PLQE (%)[a]
KSV
P1
P2
P3
78.22
81.57
96.94
2.01ꢀ106
4.12ꢀ106
1.03ꢀ107
Fe2+
[a] PLQE=A0ÀA/A0; where A0 =area under the PL curve without metal
ion, A=area under the PL curve in presence of 10 equiv of metal ions.
[b] Stern–Volmer quenching constants can be evaluated by the static
Stern–Volmer equation I0/I=1 + KSV [Q], where I0 is the PL intensity of
the polymer (1.2 ꢀ10À5 m) in the absence of the quencher, I is the PL in-
tensity in the presence of each quencher, KSV is the Stern–Volmer
quenching constant, and [Q] is the quencher concentration.
1
To further elucidate the binding mode, H NMR titrations
were conducted by the addition of Fe2+ (in D2O) to M1–M3
(binding probes of P1–P3, respectively) in [D8]THF (see
Figure 11). In the case of M1, the signals of the aromatic
protons were gradually shifted upfield upon the sequential
addition of Fe2+. The peak corresponding to the dibromo-
thiophene unit was shifted upfield by 0.1 ppm (from d=7.55
to 7.45 ppm). Likewise, upon the addition of Fe2+, a notable
shift was observed in the signals of the aromatic protons cor-
responding to M2. That of the proton corresponding to di-
bromothiophene was shifted upfield by 0.13 ppm (from d=
7.58 to 7.45 ppm). Unlike for M1 and M2, the proton peak
corresponding to the dibromothiophene unit in M3 was dra-
matically downfield shifted. The signal of the dibromothio-
phene protons in M3, originally at d=7.42 ppm, was shifted
downfield by 0.16 ppm to d=7.58 ppm. This was probably
due to the different electronic distribution in M3 due to the
presence of the anti-aromatic acenaphthylene moiety (as
discussed previously in relation to the computational analy-
sis).
Table 3. Photoluminescence properties of P1, P2, and P3 upon titration
with Fe2+
Sample
.
[b]
lem(nm)[a]
Fem
t[c]
P1
633
628
640
632
661
630
0.82
0.15
0.84
0.11
0.89
0.05
0.97
0.21
1.12
0.18
1.14
0.10
P1+Fe2+
P2
P2+Fe2+
P3
P3+Fe2+
Fe2+ up to one molar equivalent. The fluorescence quantum
yields and lifetimes of polymers P1–P3 before and after the
addition of Fe2+ are summarized in Table 3. Upon the addi-
tion of Fe2+, the fluorescence lifetimes of the polymers de-
creased significantly (see Figure 9).[17] In the presence of
Fe2+, the fluorescence lifetimes of P1 and P2 were de-
creased almost 4.6- and 6.2-fold, respectively. The larger de-
crease in the fluorescence lifetime of polymer P3 (11.4-fold)
further confirmed its superior sensing ability due to its
stronger molecular wire effect. Stern–Volmer plots for P1–
P3 in the presence of Fe2+ indicated 1:1 stoichiometries
(Figure S1). Fluorescence recovery tests were carried out
with two suitable counter ligands, namely the disodium salt
of ethylenediaminetetraacetic acid (Na2-EDTA) and phen-
FÀ sensing of 1-B, 2-B, 3-B, and 4-B: The precursors of mon-
omers M1–M4 (i.e., imidazole derivatives 1-B, 2-B, 3-B, and
4-B with free NH groups) were investigated for their halide
ion sensing capabilities. As shown in Figure S2 (a)–(d), all of
these imidazole derivatives were capable of sensing FÀ effec-
tively over other halides (ClÀ, BrÀ, and IÀ). Upon the addi-
tion of FÀ to 1-B, the PL peak at 500 nm decreased gradual-
ly (Figure S1(e)). Likewise, upon the addition of FÀ (ap-
proaching 1 equiv) to 2-B, the PL peak at 325 nm increased
and a new peak simultaneously developed at 430 nm (Fig-
ure S1(f)). For 3-B, the PL peaks at 325, 525, and 580 nm de-
creased upon the addition of FÀ (Figure S1(g)). However, 4-
B showed the best sensing behavior towards FÀ ions. The PL
peak of 4-B at 420 nm decreased gradually and a new peak
developed at 504 nm with the addition of FÀ (Figure S2(h)).
ACHTUNGTRENNUNGanthroline. As illustrated in Figure 8, upon the addition of
Na2-EDTA or phenanthroline in THF to the quenched Fe2+
-polymer (P1–P3) solutions, the fluorescence intensities
were about 65–85% recovered. This indicated the reversibil-
ity of the association of Fe2+ with imidazole units, and the
preferential formation of a stable Fe-EDTA complex. Simi-
larly, upon the addition of phenanthroline to the quenched
Fe2+-polymer (P1–P3) solutions, the fluorescence intensities
were almost 90% recovered. This suggested that Fe2+ coor-
dinates strongly through a double coordinating bond with
phenanthroline, thereby releasing Fe2+ from the polymers.
Thus, for the fully quenched Fe2+-polymer (P1–P3) solu-
tions, the phenanthroline-mediated fluorescence recovery
was more prominent than that with EDTA. Photographs of
the quenching and recovery of the fluorescences of each of
the polymers (P1–P3) are also shown in the insets in
Figure 8. Moreover, the fluorescence on-off-on behavior was
Conclusion
Three novel imidazole-based low-bandgap polymers (P1–
P3) have been synthesized. Due to their donor–acceptor
conjugations, these polymers show excellent photophysical
and electrochemical properties (such as brilliant fluorescen-
ces, low bandgaps, large Stokes shifts, and low HOMO
levels). As a result, they constitute structurally stable, semi-
conducting molecular wires capable of acting as promising
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Chem. Eur. J. 2012, 18, 16061 – 16072

Imidazole-Substituted Polymers with Molecular Wire Properties
FULL PAPER
Figure 8. Sequential PL quenching of P1 (a, left), P2 (b, left), and P3 (c, left) (1.2ꢀ10À5 m) in THF/H2O (1:1) acquired by the addition of 0–10 equiv of
Fe2+ and the recovery of fluorescence by the addition of Na2-EDTA or phenanthroline. Lower insets: photographs of fluorescence quenching in the poly-
mer solutions upon addition of Fe2+ and restoration of the original fluorescence upon addition of phenanthroline or Na2-EDTA. PL quenching of P1 (a,
right), P2 (b, right), and P3 (c, right) as a function of 0–10 equiv [Fe2+].
Chem. Eur. J. 2012, 18, 16061 – 16072
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www.chemeurj.org
16069

H.-C. Lin et al.
Figure 9. Time-resolved fluorescence of polymers P1 (a), P2 (b), and P3
(c), before (empty circles) and after (solid circles) the addition of Fe2+
Figure 10. On-off-on switching of the fluorescence spectra of P1 (a), P2
(b), and P3 (c,) over seven successive cycles (with error bars) upon the
addition of Fe2+ and phenanthroline.
.
transduction materials for chemosensory applications. These
imidazole-based polymers showed remarkable sensing capa-
bilities towards H+ and Fe2+ in semi-aqueous solutions.
However, polymer P3 showed a distinct sensitivity response
compared with polymers P1 and P2. Upon titration with
H+, polymer P3 showed reduced absorption and fluores-
cence intensities due to static quenching. Polymers P1 and
P2 showed hypsochromic shifts of their absorption and PL
maxima, with enhanced fluorescence intensities under simi-
lar conditions. Compared with P1 and P2, the anomalous
behavior of P3 was proven by computational analyses of the
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Imidazole-Substituted Polymers with Molecular Wire Properties
FULL PAPER
Measurements and characterizations: 1H and 13C NMR spectra were re-
corded on a Bruker DX-300 spectrometer (300 MHz for 1H, 75 MHz for
13C) using CDCl3 and [D6]DMSO as solvents. Elemental analyses were
performed on a HERAEUS CHN-OS RAPID elemental analyzer. Ther-
mogravimetric analyses (TGA) were conducted with a TA Instrument
Q500 at a heating rate of 108CminÀ1 under nitrogen. The molecular
weights of the polymers were measured by gel permeation chromatogra-
phy (GPC) using a Waters 1515 separation module, employing poly-
AHCTUNGERTGsNNUN tyrene standards, and eluting with tetrahydrofuran (THF). UV/Vis ab-
sorption spectra were recorded on an HP G1103A spectrophotometer
from solutions in dilute solutions (10À5 m) in THF or as solid films (spin-
coated on glass substrates from THF solutions with a concentration of
10 mgmLÀ1). Cyclic voltammetry (CV) measurements were performed
using a BAS 100 electrochemical analyzer with a standard three-electrode
electrochemical cell in a 0.1m solution of tetrabutylammonium hexafluor-
ophosphate, (TBA)PF6, in acetonitrile at room temperature at a scanning
rate of 100 mVsÀ1. Prior to the CV measurements, the solutions were
purged with nitrogen for 30 s. In each case, a carbon working electrode
coated with a thin layer of the polymer, a platinum wire as the counter
electrode, and a silver wire as the quasi-reference electrode were used,
and an Ag/AgCl (3m KCl) electrode served as a reference electrode for
all potentials quoted herein. The redox couple of ferrocene/ferrocenium
ion (Fc/Fc+) was used as an external standard. The corresponding
HOMO and LUMO levels were calculated using Eox/onset and Ered/onset
from experiments on solid polymer films of similar thickness, which were
deposited by drop-casting from solutions in chloroform (ca. 5 mgmLÀ1).
The onset potentials were determined from the intersections of two tan-
gents drawn at the rising and background currents of the CV measure-
ments. The pH-sensing properties were assessed in THF/H2O (1:1) solu-
tions by the addition of TFA as a proton source and TEA for reversibili-
ty tests. Similarly, the metal-sensing properties were assessed by the addi-
tion of metal ions to the polymer solutions in THF/H2O (1:1). Reversibil-
ity in metal ion sensing was assessed using the disodium salt of
ethylenediaminetetraacetic acid (Na2-EDTA) or phenanthroline.
Figure 11. 1H NMR titrations of monomers M1 (a), M2 (b), and M3 (c)
in [D8]THF upon the sequential addition of 0–1 equiv of Fe2+ (in D2O).
(Arrow marks show the shifts of the peak corresponding to the dibromo-
thiophene units).
Acknowledgements
electron densities in the HOMOs and LUMOs of P1–P3
before and after complexation with H+. Recoveries of the
original fluorescences and absorptions were achieved by
adding TEA. Furthermore, P3 showed the best sensing abili-
ty for Fe2+ among P1–P3 due to its stronger molecular wire
effect. Correspondingly, in the presence of Fe2+, the fluores-
cence lifetime of P3 was extensively decreased (almost 11-
fold) compared with those of polymers P1 (4.6-fold) and P2
Financial support of this project has been provided by the National Sci-
ence Council of Taiwan (ROC) through NSC 99–2113-M-009–006-MY2
and NSC 99–2221-E-009–008-MY2, and National Chiao Tung University
through 97W807.
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Experimental Section
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Received: August 31, 2012
Revised: September 5, 2012
Published online: October 30, 2012
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Chem. Eur. J. 2012, 18, 16061 – 16072