Intramolecular hydrogen-bond-assisted planarization of asymmetrically functionalized alternating phenylene-pyridinylene copolymers

Article abstract of DOI:10.1002/chem.200500078

We report on the synthesis and characterization of a series of asymmetrically functionalized amphiphilic polymers with alternating π-donor units (e.g., substituted benzene) and π-acceptor units (e.g., pyridine) along the polymer backbone. The purpose of our present work involves incorporation of functional groups along the main chain to form intrachain hydrogen bonds, which promote planarization of the polymer backbone, and to fine-tune the optical properties. The structure-property relationship of polymers P1-P6 was investigated by means of analytical methods, such as FTIR spectroscopy, ¹H and ¹³C NMR spectroscopy, UV/Vis spectroscopy, fluorescence spectroscopy, gel permeation chromatography, thermogravimetric analysis, cyclic voltammetry, and X-ray powder diffraction. All polymers were soluble in common organic solvents, and the optical and fluorescence spectra of the polymers showed significant changes according to the formation (P4, P5) or absence (P6) of intramolecular hydrogen bonding along the polymer backbone. Moreover, the 2,6-or 3,5-linkage of the pyridine rings in P5 and P6, respectively, reduced the conjugation along the polymer backbone and this is reflected in their optical properties. The optical properties of the polymers were influenced by the addition of acid (P1-P6), base (P4-P6), and metal ions (e.g., Cu²⁺, Fe³⁺, Ag⁺, Ni²⁺, Pd²⁺, Mn²⁺, Zn²⁺, Mg²⁺, and Pr ³⁺). Such polymers could be used in various applications, including sensors and stimuli-responsive displays.

Full text of DOI:10.1002/chem.200500078

FULL PAPER
DOI: 10.1002/chem.200500078
Intramolecular Hydrogen-Bond-Assisted Planarization of Asymmetrically
Functionalized Alternating Phenylene–Pyridinylene Copolymers
Muthalagu Vetrichelvan[a] and Suresh Valiyaveettil*[a, b]
Abstract: We report on the synthesis
and characterization of series of
spectroscopy, 1H and 13C NMR spec-
troscopy, UV/Vis spectroscopy, fluores-
cence spectroscopy, gel permeation
lecular hydrogen bonding along the
polymer backbone.Moreover, the 2,6-
or 3,5-linkage of the pyridine rings in
P5 and P6, respectively, reduced the
conjugation along the polymer back-
bone and this is reflected in their opti-
cal properties.The optical properties of
the polymers were influenced by the
addition of acid (P1–P6), base (P4–
a
asymmetrically functionalized amphi-
philic polymers with alternating p-
donor units (e.g., substituted benzene)
and p-acceptor units (e.g., pyridine)
along the polymer backbone.The pur-
pose of our present work involves in-
corporation of functional groups along
the main chain to form intrachain hy-
drogen bonds, which promote planari-
zation of the polymer backbone, and to
fine-tune the optical properties.The
structure–property relationship of poly-
mers P1–P6 was investigated by means
of analytical methods, such as FTIR
chromatography,
thermogravimetric
analysis, cyclic voltammetry, and X-ray
powder diffraction.All polymers were
soluble in common organic solvents,
and the optical and fluorescence spec-
tra of the polymers showed significant
changes according to the formation
(P4, P5) or absence (P6) of intramo-
P6), and metal ions (e.g., Cu2+, Fe3+
Ag+, Ni2+, Pd2+, Mn2+, Zn2+, Mg2+
and Pr3+).Such polymers could be
,
,
used in various applications, including
sensors and stimuli-responsive displays.
Keywords: optical tuning · planari-
zation · poly(paraphenylene) · pyri-
dine · sensors · transition metals
Introduction
cepting ability, 2) N-protonation, 3) N-oxidation, 4) N-alky-
lation, and 5) metal complexation.[15–29] p-Conjugated poly-
mers that incorporate aryl heterocyclic units, such as pyri-
dine, pyrrole, and thiophene rings, have been synthesized
and employed in industrial applications.[30–34] The ladder-
type polymers incorporated with pyrazines,[35] benzothiad-
azoles,[36]pyrroloimides,[37] pyridobisimidazoles,[38] and benzo-
thiazole[39] that contain intramolecular hydrogen bonds
along the polymer backbone are also known in the litera-
ture.One of the most important features of the pyridine-in-
corporated polymers is the high electron affinity of pyridine
compared to phenylene-based polymers.[40] Furthermore, the
use of a donor–acceptor system[41] and planarization of the
polymer backbone by means of weak interactions (e.g., in-
trachain hydrogen bonds) allow us to fine-tune the optical
and conducting properties of the polymers.[21–22]
Recently, a few reports have focused on the incorporation
of pyridine into a conjugated polymer backbone.[27–29,42–46]
Chemical sensors based on conjugated polymers are inter-
esting owing to their high sensitivity.[8] The synthesis of con-
jugated polymers with bipyridine/phenanthroline moieties
or crown ethers or a combination of both groups have al-
ready been reported in the literature.[8,15,46] Our research ef-
Conjugated polymers have received much attention in the
past two decades owing to their potential applications in
light-emitting diodes,[1–7] thin-film transistors,[1–2] chemical
sensors,[8] as well as electronic and photonic devices.[9–11] The
optoelectronic properties of conjugated polymers vary sig-
nificantly depending on the degree of extended conjugation
between the consecutive repeating units and the inherent
electron densities on the polymer backbone.[12–14] Among
them, conjugated pyridine-incorporated polymers have at-
tracted much attention owing to the possibility of fine-
tuning the optical properties by means of 1) the electron-ac-
[a] Dr.M.Vetrichelvan, Prof.S.Valiyaveettil
Singapore-MIT Alliance, 4 Engineering Drive 3
National University of Singapore—117 576 (Singapore)
E-mail: chmsv@nus.edu.sg
[b] Prof.S.Valiyaveettil
Department of Chemistry, 3 Science Drive 3
National University of Singapore—117 543 (Singapore)
Fax : (+65)6779-1691
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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forts are focused on the synthesis and structure–property in-
vestigations of asymmetrically functionalized planar poly-
(paraphenylene)s (PPPs).[47–49] The target PPPs contain alter-
nating functionalized phenylene rings and 2,5-, 2,6-, or 3,5-
substituted pyridine rings on the polymer backbone.
which results in planarization of the molecule.Even though
it is difficult to see a one-to-one structural correlation be-
tween oligomers and polymers, we incorporated such func-
tional groups along the main chain of our target polymers to
facilitate intramolecular hydrogen-bond-assisted planariza-
tion of the polymer backbone.
Dodecyloxy groups are incorporated to increase the solu-
The general synthetic route
to the target polymers (P1 and
P4) is outlined in Scheme 1.
The monomer 1-benzyloxy-4-
dodecyloxyphenyl-2,5-bis(bor-
onic acid) (5) was synthesized
from hydroquinone following a
reported procedure.[42] 2,5-Di-
bromohydroquinone (2) was
obtained by bromination of hy-
droquinone with bromine in
acetic acid.[51] Monoalkylation
of 2 with dodecyl bromide in
the presence of NaOH/EtOH
at 45–508C gave compound 3,
which was benzylated to afford
4.Compound
4 was reacted
with n-butyllithium followed by
quenching with triisopropyl borate and hydrolysis with hy-
drochloric acid to afford bis(boronic acid) 5.All polymeri-
zations were carried out by means of Suzuki polycondensa-
tion reactions[42–43,46] in a mixture (3:2 v/v) of toluene and
aqueous potassium carbonate solution (2m) containing 3.0
mol% [Pd(PPh3)4] with vigorous stirring at 85–908C for 72 h
under a nitrogen atmosphere.After completion of the reac-
tion, the polymer was precipitated from methanol.The
target polymers, P4–P6, were prepared by debenzylation of
a solution of P1–P3 in CHCl3/THF/EtOH under a hydrogen
atmosphere with 10% Pd/C as the catalyst.The resulting so-
lution was filtered (silica gel/celite), concentrated, and refil-
tered.The residue was washed with methanol and dried to
yield polymers P4 and P5.In the case of polymer P6, the
filtrate was concentrated to afford a solid polymer.
bility and ordered packing through alkyl-chain crystalliza-
tion.The intramolecular hydrogen bonds between the adja-
cent phenolic OH groups in P4 and P5 and the nitrogen
atom of the pyridine ring help to planarize the polymer
backbone.However, in polymer P6, no such intramolecular
hydrogen bonds are expected.The optical properties of the
polymer can be fine-tuned by protonation or alkylation of
the nitrogen atom, deprotonation of the OH groups, and
metal-ion complexation.
Results and Discussion
Synthesis and characterization: Recently, a simple oligomer
of the target polymer was synthesized by our group.It dem-
onstrated planarization of the rings owing to the formation
of intramolecular hydrogen bonds.[50] The crystal structure
of the oligomer (Figure 1) revealed the presence of hydro-
gen bonds between the hydroxyl groups and the pyridine ni-
trogen atoms.Interestingly, the torsion angle between the
hydroquinol ring and the pyridine ring is reduced to 4.78,
The molecular weights of the polymers were measured by
using gel permeation chromatography (GPC) with THF as
the eluant and polystyrene as the standard (Table 1).The
observed values are relatively low with a polydispersity
index of 1.1–1.4. This may be attributable to the fractiona-
tion of soluble low-molecular-weight polymer in solution.
Figure 1.Molecular structure of the oligomer (left), and its crystal structure: top view (middle) and side view (right).Hydrogen bonds are marked as
dotted lines.
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Phenylene–Pyridinylene Copolymers
FULL PAPER
rings were observed at dꢀ9.0,
8.2, and 8.1 ppm. Those of the
phenylene ring protons ap-
peared at d=7.9 and 7.4 ppm.
The -CH2- signal of the benzyl
group appeared at 5.2 ppm and
-OCH2- at 4.1 ppm. The re-
maining peaks at d=1.8, 1.3,
and 0.9 ppm were assigned to
the hydrogen atoms on the ali-
phatic dodecyloxy groups.Data
for polymers P2–P3 were also
in good agreement with the
proposed structure.The NMR
signals of the target polymers
P4–P6 appeared as broad sig-
nals, and it is expected that the
polymer may contain head-to-
head and head-to-tail units
along the backbone.[42]
Thermal properties: The ther-
mal stability of the polymers
was evaluated by TGA with a
heating rate of 108CminÀ1
under a nitrogen atmosphere.
Benzylated parent polymers
P1–P3 showed good thermal
stability, and the onset degrada-
tion temperatures were in the
Scheme 1.Synthesis of polymers P1 and P4: i) Br2/AcOH, 80%; ii) NaOH/EtOH, CH3(CH2)11Br, 45–508C,
10 h, 65%; iii) K2CO3, C6H5CH2Br, 508C, 10 h, 90%; iv) nBuLi, THF, À788C, triisopropyl borate, RT, 10 h,
70%; v) K2CO3 (2m), toluene, [Pd(PPh3)4] (3.0 mol%), reflux, 3 days; vi) H2, 10% Pd/C, CHCl3/EtOH/THF.
range of 290–3108C (Figure 2).This decomposition corre-
sponds to the degradation of the benzyl groups on the poly-
mer backbone.The second onset temperature at 390–520 8C
corresponds to the decomposition of dodecyloxy groups and
the polymer backbone.Thus, two-step degradations were
observed for polymers P1–P3.For the target polymers P4–
P6, decomposition temperatures were observed at 80 and
2208C; the first decomposition is attributed to evaporation
Table 1.Molecular weight and polydispersity index (PDI) of polymers
P1–P6 at room temperature.
[a]
[b]
Polymer
Color
Mn
Mw
PDI
P1
P2
P3
P4
P5
P6
bright yellow
yellow
light brown
dark brown
dark brown
dark brown
3265
4354
3659
2234
2642
2357
3883
6421
5284
2389
3292
2591
1.1
1.4
1.4
1.1
1.3
1.1
[a] Number-average molecular weight.[b] Weight-average molecular
weight.
According to Müllen et al., the GPC results for rigid rod
polymers, particularly polymers with polar groups, are not
completely reliable with polystyrene standards, and the mea-
sured value is often lower than the actual molecular
weight.[52] Moreover, the structure and properties of the
low-molecular-weight PPPs may be similar to the oligomer
discussed above rather than to the high-molecular-weight
polymers.The obtained polymers P1–P6 are soluble in
common organic solvents, such as THF, chloroform, toluene,
and trifluoroacetic acid.
All polymers (P1–P6) were characterized by means of
1
FTIR, H NMR, 13C NMR, GPC, thermogravimetric analy-
sis (TGA), and X-ray diffraction.The assignments of the 1H
and 13C NMR peaks are given in the Experimental Section.
For polymer P1, peaks assigned to protons on the pyridine
Figure 2.TGA traces of precursor polymers P1–P3.TGA data of P4–P6
are given in the Supporting Information.
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5891

S.Valiyaveettil and M.Vetrichelvan
of the entrapped solvents from the polymer lattice.The
second decomposition above 3708C corresponds to the loss
of dodecyloxy groups and the polymer backbone.
lishing the electron-transport properties.Additional oxida-
tion peaks were observed at À0.1 and À0.4 V for P1 and
P2, respectively.The oxidation potential ( Eox) of P1 is
higher (1.52 V) than that of P2 (1.32 V), which indicates
that P2 can be easily oxidized or p doped compared with
P1.Consequently, HOMO and LUMO energy levels of P2
were lower than those of P1.[53,54] For target polymers P4
and P6, there was a slight difference in the oxidation poten-
tial compared with the precursor polymers P1 and P2.Poly-
mer P4 exhibited an oxidation peak at +1.51 V and a small
peak at À0.3 V. The strong interaction of the -OC12H25
groups with the counteranion of the supporting electrolyte
(perchlorate) may be one of the reasons for the observed ir-
reversibility.[46] Only P1 showed a reduction peak at À2.1 V.
The observed band gaps for the phenylene–pyridinylene
copolymers are generally 3.0–3.5 eV.[54] The calculated band
gap for our polymers P1–P6 are 3.0, 2.9, 3.3, 2.5, 2.9, and
3.2 eV, respectively.[55] The values of the band gap for the
benzylated precursor polymers (P1–P3) are higher than
those of the corresponding debenzylated target polymers
(P4–P6), and the values for benzylated polymers, P1–P3,
are comparable to the symmetrically functionalized PPP co-
polymers.[54] However, in the case of the debenzylated poly-
mers (P4–P6), the band gaps are lower than 3.0 eV, except
for P6, which contains meta-linked pyridine linkages on the
polymer backbone.Thus, the functionalization and intramo-
lecular hydrogen bonds facilitated a reduction in the size of
the band gap to 2.5 and 2.8 eV for P4 and P5 polymers, re-
spectively.
Electrochemical properties: The electrochemical behavior
of polymers P1–P6 was investigated by means of cyclic vol-
tammetry (CV).The experiments were performed in a 01.
m
solution of Bu4NClO4 in acetonitrile, with a scan rate of
50 mVsÀ1 at room temperature under a nitrogen atmos-
phere.A glassy carbon electrode was coated with a thin
polymer film and was used as the working electrode.Cyclic
voltammograms of polymers P1 and P4 are shown in
Figure 3, and the electrochemical data of the polymers P1–
P6 are summarized in Table 2.
Figure 3.Cyclic voltammogram of a thin film of polymers P1 and P4 re-
corded with a glassy carbon electrode under a nitrogen atmosphere.CV
traces of the other polymers are given in the Supporting Information.
Optical properties
Absorption and emission spectroscopy: The absorption and
emission properties of the polymers (P1–P5 in chloroform
and P6 in methanol) are summarized in Table 3.The optical
properties of the polymers in chloroform are dependent on
the linkage (2,5-, 2,6-, or 3,5-) of the pyridyl rings and plana-
rization of the polymer backbone that results from hydrogen
bonding.From the structure of the oligomer (Figure 1), it is
clear that intramolecular hydrogen bonding facilitates plana-
rization of the molecule.The absorption spectra of the poly-
mers showed two maxima located in the range lmax =270–
300 nm and one above 300 nm.For polymers ( P1 and P4)
with 2,5-substituted pyridine rings, the absorption maxima
were observed at 384 and 425 nm, respectively, (Figure 4).
For P2 and P5 with 2,6-substitution, the lmax were blueshift-
Table 2.Electrochemical potentials and energy levels of polymers P1–
P6 at room temperature.[a]
Polymer Band Eox (onset) Ered (onset) HOMO Calcd LUMO
gap[b]
[V][c]
[V][c]
[eV][d]
[eV][e]
P1
P2
P3
P4
P5
P6
3.0
3.1
3.3
2.5
2.9
3.2
1.24
0.96
–
0.9
–
À1.85
5.68
5.40
–
5.34
–
À2.68
À2.27
–
–
–
–
–
–
À2.84
–
1.00
5.44
À2.22
[a] Electrochemical data are not available for P3 and P5.[b] Estimated
from the onset wavelengths of the polymer in solution.[c] Estimated
from cyclic voltammetry.[d] Calculated from the onset oxidation poten-
tial.[e] Because no reduction potentials were observed, they had to be
derived from the band gap and the HOMO.
Table 3.Absorption and emission wavelengths for the polymers P1–P6
in chloroform.
The appearance of multiple oxidation and reduction
peaks is common to polymers containing electroactive
groups such as pyridine rings.In the oxidation region, a
peak was observed at ꢀ+1.3–1.5 V with reference to (Ag+/
Ag) and was assigned to the oxidation of the phenylene
group (main polymer chain).Such oxidation potentials have
been observed for other pyridine-containing copoly-
mers,[16,42,46] and these values may be useful towards estab-
lmax [nm]/E [eV]
emax
lemiss [nm]/E [eV]
P1
P2
P3
P4
P5
P6[a]
384/3.2
359/3.5
331/3.7
425/2.9
362/3.4
302/4.1
9.15”104
13.04”104
9.89”104
4.83”104
5.28”104
1.50”104
431/2.9
402/3.1
393/3.2
503/2.5
420/3.0
392/3.2
[a] Methanol or tetrahydrofuran.
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Phenylene–Pyridinylene Copolymers
FULL PAPER
Figure 4.Absorption (left) and emission (right) spectra of polymers P1–P3 in chloroform at room temperature.Absorbance and intensity have arbitrary
units.
ed to 359 and 362 nm, respectively (Table 3).The latter is at-
tributed to the low p-conjugation length on the polymer
backbone because of the meta-linked pyridine rings.
ular hydrogen bonding in the planarization of the polymer
backbone.Also, the lmax value of linear P1 or P4 is higher
than that of the bent polymers P2 and P3 and the corre-
sponding debenzylated polymers P5 and P6.The FTIR
spectra of the target polymers showed a sharp absorption in
the region of 3437–3406 cmÀ1, which indicates hydrogen-
bond formation inside the polymer lattice.The value of lmax
for polymer P4 in a polar solvent, such as tetrahydrofuran
(THF, weak intramolecular hydrogen bond) is smaller
(383 nm) than in the nonpolar solvent, chloroform (425 nm).
This is only applicable for P4, and in all other cases there
were no changes in the values of lmax in different solvents.
This may be attributable to the formation of strong intramo-
lecular hydrogen bonds in nonpolar chloroform leading to
high planarity and conjugation length.[37]
The emission spectra of the polymers (P1–P6) were re-
corded by exciting them at the maximum absorption wave-
length observed from the corresponding UV-visible spectra
of the polymers.All our polymers ( P1–P6), except P4, emit
in the blue region.However, polymer P4 emits in the green
region (Figure 5, Table 3).Absorption and emission spectra
of P1–P3 and P4–P6 are given in Figures 4 and 5, respec-
The absorption maxima (lmax) for polymers P3 and P6
with 3,5-substituted pyridine rings were blueshifted to 331
and 302 nm, respectively, implying a minimum p-conjuga-
tion length.It is also interesting to note that the lmax for P3
is higher than that for P6, which is presumably caused by in-
termolecular hydrogen-bond formation in the P6 polymer
lattice.This reduces the planarity and effective conjugation
length along the polymer backbone in P6.
The target polymers (P4–P6) have higher lmax values
than the corresponding precursor polymers (P1–P3) with a
Dlmax of 4–45 nm.The shift in the absorption maximum of
precursor polymer P1 to the target polymer P4 was
ꢀ41 nm.This large shift may be attributable to planariza-
tion of the polymer backbone, which is caused by the forma-
tion of intramolecular hydrogen bonds between the adjacent
phenolic OH groups and the pyridine nitrogen atoms as
seen in the crystal lattice of the oligomer (Figure 1).More-
over, the lmax value of P6 is significantly smaller than that
observed for P4.This again confirms the role of intramolec-
Figure 5.Absorption (left) and emission (right) spectra of polymers P4–P6 in chloroform at room temperature.The observed blueshift in the maxima of
P6 and P5 indicates less planarization for the polymer backbone.Absorbance and intensity have arbitrary units.
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5893

S.Valiyaveettil and M.Vetrichelvan
tively.Polymer P4 emits in the green region (lmax =503 nm)
with a large Stokes shift of ꢀ72 nm.This may be attributa-
ble to the possibility of hydrogen-bond-assisted excited-state
intramolecular proton transfer (ESIPT), as observed for
other hydrogen-bonded oligomers and polymers.[57–61] Simi-
lar Stokes shifts were observed for P5 and P6 (Table 2).
However, for the 2,6-substituted (P2 and P5) and 3,5-substi-
tuted (P3 and P6) polymers, the differences in lemiss are 18
and 1 nm, respectively.The difference in lemiss between P2
and P5 can also be explained by ESIPT, as described for
2,5-substituted polymers, (P1 and P4) owing to the forma-
tion of hydrogen bonds between the phenolic OH group
and the pyridine nitrogen atom.However, the reduction in
the value may be caused by the meta-linked pyridine rings
along the polymer chain.In the case of polymers P3 and
P6, intrachain hydrogen bonds cannot be formed owing to
the 3,5-linkage of pyridine rings along the polymer chain
and this may explain the insignificant difference in lemiss
values.However, strong intermolecular hydrogen bonding is
possible in the lattice of P6, which is insoluble in chloro-
form, but soluble in more polar solvents, such as methanol.
The intermolecular hydrogen bonds also reduce the planari-
ty of the polymer chain, as seen in the values of lmax
(Table 3).Polymer P4 emits in the green region (503 nm) in
chloroform and in the blue region (432 nm) in THF at the
excitation wavelength of 365 nm.The other polymers did
not show such solvatochromism.This again indicates the for-
mation of strong intramolecular hydrogen bonds along the
polymer chain in chloroform and weak interactions in polar
solvents, such as THF.
Figure 6.Possible structure of the protonated benzylated polymer (left)
and the target polymer (right).
The difference in lmax of the target polymers (debenzy-
lated polymer) upon addition of acid is smaller compared
with the precursor polymers (benzylated polymer).It indi-
cates the presence of a strong interaction between the ether
oxygen and the protonated pyridine nitrogen in the case of
precursor polymers and large conformational changes and
planarization caused by protonation.Such a significant red-
shift was also observed for other polymers (P2–P6).
Figure 7 shows the influence of an acid on polymer P1 and
the influences of acid and base on polymer P4.Figure 8 de-
picts the changes in the emission spectra of polymer P1
with increasing concentrations of CF3COOH (0 to 400 mm).
The emission intensities diminished rapidly with CF3COOH
concentration and all other polymers (P1–P3, P5–P6)
showed similar changes in the absorption and emission
properties.
In addition, the target polymers P4–P6 are sensitive to
sodium hydroxide on account of the presence of acidic hy-
droxyl groups on the polymer backbone.As seen in
Figure 7, there is a significant blueshift (Dlmax of 45 nm) in
the absorption maximum in the presence of base (20 mm
NaOH) in a solution of P4 in chloroform (Table 4).This
was expected owing to the formation of electron-rich pheno-
late anions along the polymer backbone and the loss of pla-
narization as a result of hydrogen bonding.Similar influen-
ces on the optical properties upon addition of acids were re-
ported for pyridine-incorporated copolymers.[43] For the
polymers P4 to P6, the optical properties were different
owing to hydrogen bonding between phenolic groups and
the pyridine nitrogen atom as well as to the different substi-
tution pattern of the pyridine ring.Moreover, all polymers
are sensitive to the H+ and OHÀ ion concentrations.Thus
the absorption and emission properties of the polymers can
be fine-tuned over a wide range by varying the quantity of
acid or base added.
Influence of acid and base—Protonation and deprotonation
of polymers: The precursor polymers P1–P3 and the target
polymers P4–P6 were expected to react with protic acids
and metal ions.The influence of acids and bases on the opti-
cal properties of the polymers is summarized in Table 4.
In the case of benzylated polymer P1, protonation in
chloroform leads to a redshift of lmax from 384 to 413 nm
(Dlmax =29 nm), and for the debenzylated polymer P4,
Dlmax =10 nm was observed on the addition of 20 mm
CF3COOH.This redshift may be attributable to planariza-
tion through hydrogen bonding (Figure 6) and charge trans-
fer from the electron-rich phenyl ring to the electron-defi-
cient pyridine moiety, which is enhanced by the protonation
of nitrogen atoms of the pyridine ring.[29] Moreover, the for-
+
À
mation of O···H N hydrogen bonds also enhances planari-
Ionochromic effects of polymers: The metal-induced iono-
chromic effects of the bipyridine- or phenanthroline-incor-
porated conducting polymers
zation, which is more evident in P1 than in P4 (Figure 6).
have already been reported by
a
few research groups.[15,62–64]
[a]
Table 4.Influence of acid and base on the absorption/emission spectra of the polymers in chloroform.
Owing to the good complexa-
tion ability of the pyridine ni-
trogen atom and adjacent phe-
nolic OH groups, it was expect-
ed that polymers P4–P6 would
show significant responses to
various metal ions.The iono-
Polymer
Precursor polymers
Target polymers
P1
P2
P3
P4
P5
P6
lmax/lemiss [nm]
protonation, Dl [nm]
deprotonation, Dl [nm]
384/431
359/402
331/393
425/503
+10/+13
À45/À70
362/420
+5/+6
À10/À8
302/392
+6/+5
À6/À4
+29/+70
+8/+95
+12/+100
[b]
[b]
[b]
[a] + and À indicate red- and blueshifts, respectively.[b] No changes were expected owing to the absence of
phenolic groups.
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Chem. Eur. J. 2005, 11, 5889 – 5898

Phenylene–Pyridinylene Copolymers
FULL PAPER
Figure 7.Absorption spectra in neutral pH for polymer P1 (a) and P4 (c), in the presence of H+ for P1 (b) and P4 (d), in the presence of OHÀ for poly-
mer P4 (e) in chloroform at room temperature.Concentrations of P1 and P4 were 3.06”10À5 m and 3.52”10À5 m, respectively.Note the redshift in lmax
caused by protonation and the blueshift caused by deprotonation.Absorbance has arbitrary units.
served redshifts in the lmax of the polymers were attributed
to the conjugation enhancement along the polymer back-
bone induced by the coordination of metal ions with the
pyridine nitrogen and the phenolic group.[15]
For different metal ions, the changes in lmax may be attrib-
uted to the binding ability of the metal ions with the poly-
mer backbone.Based on the spectral response, the precur-
sor polymers (P1–P3) were only able to complex selected
transition-metal ions, particularly Cu2+ and Fe3+ ions.In the
case of polymer P1, the redshifts in the absorption were
ꢀ35 nm and 146 nm upon the addition of Cu2+ and Fe3+
ion solutions, respectively.Similar influences of these metal
ions were observed for the other two precursor polymers P2
and P3.Thus, addition of metal salts completely quenched
Figure 8.Changes in the emission spectra of P1 in chloroform at differ-
ent concentrations (in mm) of CF3COOH: 1) 0, 2) 4, 3) 8, 4) 12, 5) 16,
the fluorescence of polymers.This indicates strong coordina-
tion between the metal ions and the polymer.
6) 20, 7) 24, 8) 28, 9) 40, 10) 60, 11) 80, 12) 120, 13) 240, 14) 320, and
The target polymers P4–P6 showed significant changes in
lmax in the presence of Cu2+, Fe3+, Co2+, and Ag+ metal
ions (see Table 5).Polymer P4 showed more sensitivity to-
wards metal ions, which may be caused by the adjacent posi-
tion of the pyridine nitrogen atom and the phenolic group
on the polymer backbone.Possible geometries of the metal
complexes for P1 and P4 are given in Figure 9 (left).“Ln”
represents other ligands or complexation sites from the adja-
cent polymer chains, as shown in Figure 9 (right).
Addition of a Cu2+ or Fe3+ salt solution to a solution of
P4 in THF resulted in a redshift (Dlmax) of 44 and 144 nm,
respectively.This is caused by the formation of metal com-
plexes.In a few cases, the metal
15) 400 mm in methanol.The concentration of P1 was 3.06”10À5 m.Data
of other polymers are given in the Supporting Information.Intensity has
arbitrary units.
chromic effects of the polymers P1–P6 were studied in
THF with aliquots of solutions of the metal salts in metha-
nol (Table 5).The colors vary from originally colorless/
yellow to pink, green, purple, reddish brown, and so forth,
depending on the metal ions and polymers used for analyses.
The differences in the absorption (Dlmax) of the polymers
upon addition of metal ions are given in Table 5.The ob-
complexes showed a blueshift
in their lmax indicating that dif-
Table 5.Absorption responses of P1–P6 upon addition of metal ions in THF.
Polymer
lmax [nm]
of free ion
Dlmax [nm] in the presence of metal ions[a]
ferent metal ions demand dif-
ferent geometries in order to be
able to form interchain com-
plexes from adjacent polymer
chains.This imposes considera-
ble strain on the conformation
of the polymer backbone
(Figure 9).It is more pro-
Cu2+
Fe3+
Co2+
[b]
Ni2+
[b]
Pd2+
[b]
Mn2+
[b]
Zn2+
[b]
Ag+
Mg2+
[b]
Pr3+
[b]
P1
P2
P3
P4
P5
P6
384
362
331
383
360
302
+35
+38
+64
+44
+49
+56
+146
+72
+70
+144
+114
+20
À4
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
+15
+4
+2
À6
À18
À5
À4
À13
À9
À3
+8
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
[b]
À3
[a] The + and À are red- and blueshifts, respectively.[b] No significant influence.
Chem. Eur. J. 2005, 11, 5889 – 5898
ꢀ 2005 Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim
www.chemeurj.org
5895

S.Valiyaveettil and M.Vetrichelvan
Figure 9.Representation of a possible structure of metal complexes of benzylated polymer P1 and target polymer P4 (left), and a sketch representing a
possible structure of the polymer–metal complex inside the polymer lattice (right).
in the presence of other transition-metal ions, such as Co2+
(Dlmax =15 nm), Ni2+ (Dlmax =6 nm), Pd2+ (Dlmax =18 nm),
Ag+ (Dlmax =11 nm), Mn2+ (Dlmax =5 nm), and Zn2+
(Dlmax =4 nm) (Table 4).There were no significant changes
in lmax in the presence of alkali, alkaline earth metals, or
rare-earth metal ions.However, there was a slight change in
the absorption maximum in the presence of Mg2+ (Dlmax
3 nm) and Pr3+ (Dlmax =8 nm) ions.
=
X-ray diffraction (XRD) studies: The XRD patterns of the
powdered samples of polymers P1–P6 were recorded to un-
derstand their self-assembly in the solid state.The typical X-
ray patterns for polymers P1 and P4 are given in Figure 12.
Precursor polymers P1–P3 exhibited two peaks: a small
peak in the low-angle region (2q=5.50–4.13 (d=15.0–
21.0 Š)) which correspond to a distance between the poly-
Figure 10.Changes in the emission spectra of P4 in THF at different con-
centrations (in mm) of Cu2+ (methanolic solution): 1) 0, 2) 0.8, 3) 1.6,
4) 2.4, 5) 3.2, 6) 4.0, 7) 4.8, 8) 5.6, 9) 6.4, 10) 7.2, 11) 8.0 mm.The concen-
tration of P4 was 3.52”10À5 m.The spectral data of the other polymers
are given in the Supporting Information.Intensity has arbitrary units.
nounced in the case of P4, in which two complexation sites
from the adjacent rings participate in the complexation.
Figure 10 depicts the emission spectra of polymer P4 in
THF in the presence of Cu2+ and the corresponding changes
in fluorescence intensity with other transition-metal ions are
shown in Figure 11.
A maximum quenching was observed in the presence of a
solution of ~6 mm Cu2+ and ~1.6 mm Fe3+ ions.Polymer
P4 also showed significant changes in its optical properties
Figure 12.XRD patterns of polymers P1 and P4.Data for the other
polymers are given in the Supporting Information.
mer chains separated by the long alkoxy groups, and a
broad peak in the wide-angle region of 2q values at 21.02–
21.80 (d=4.0–4.2 Š), which may be attributable to the side-
to-side distance between loosely packed alkyl chains.[20,46]
Thus, in the solid lattice, polymers are considered to be or-
ganized because the long alkoxy groups are able to crystal-
lize.For the target polymers P4–P6, a broad peak at the
wide-angle region of 2q values at 20.80–23.128 was observed
at d=3.8–4.2 Š.
~
!
&
*
^
)
Figure 11.Titration traces of metal ions (Cu , Fe , Ag , Ni , Pd
added to polymer P4 in THF (concentration of P4 is 3.52”10À5 m).In-
tensity has arbitrary units.
5896
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Chem. Eur. J. 2005, 11, 5889 – 5898

Phenylene–Pyridinylene Copolymers
FULL PAPER
oxy-4-dodecyloxyphenyl-2,5-bis(boronic acid) (5) were synthesized by
Conclusion
means of the standard procedure reported in the literature.[47,51]
General procedure for the polymerization of precursor polymers P1–P3:
Compound 5 (4.11 g, 9 mmol) and dibromopyridine (2.14 g, 9 mmol)
were dissolved in degassed toluene (90 mL) under a nitrogen atmosphere.
After the addition of an aqueous solution of K2CO3 (2m, 60 mL), the cat-
alyst tetrakis(triphenylphosphino)palladium(0) (3 mol%, 0.52 g) was
added, and the mixture was stirred vigorously for 72 h at 85–908C,
cooled to room temperature, and poured into methanol (1 L).The yellow
solid was filtered, washed with 1) water, 2) methanol (2”25 mL), and
3) acetone (3”25 mL).The crude product was purified by repeated pre-
cipitation from methanol.The observed low molecular weight may have
been caused by fractionation of soluble components during GPC.
A series of luminescent conjugated polymers with alternat-
ing amphiphilic phenylene and pyridine units was synthe-
sized by the Suzuki polycondensation method.All polymers
exhibited good thermal properties, strong blue emissions in
solution, and optimum solubility in common organic sol-
vents, such as chloroform, THF, toluene, and trifluoroacetic
acid.Significant changes in the optical properties were ob-
served, depending upon the substitution of the pyridine ring
(2,5-, 2,6-, or 3,5-) and the formation of intramolecular hy-
drogen bonds.The observed blueshift in the absorption and
emission maxima observed for polymers P4–P6 implies a
reduction in planarization along the polymer backbone.The
large Stokes shift in the values of lmax and lemiss also con-
firmed the possibility of excited-state intramolecular proton
transfer (ESIPT) for P4 and P5 that was facilitated by intra-
molecular hydrogen bonding.The optical properties of the
polymers can be fine-tuned to a large extent by protonation,
deprotonation, and the addition of metal ions.Addition of
acid to the polymer solution gave a bathochromic shift and
the presence of base showed a hypsochromic effect.In all
the polymers, lmax and lemiss exhibited metal-ion dependence.
Therefore, the derived target polymers can be used for
metal-ion sensors.
Polymer P1: Bright yellow powder; yield 83%; 1H NMR (CDCl3): d=
8.99 (d, 1H; py-H), 8.18 (m, 1H; py-H), 8.03 (m, 1H; py-H), 7.90 (b, 2H;
Ar-H), 7.48 (b, 5H; Ar-H), 5.24 (b, 2H; PhOCH2Ph), 4.10 (b, 2H;
PhOCH2CH2-), 1.80 (b, 2H; -OCH2CH2-), 1.30 (b, 18H; -CH2-
(CH2)9CH3), 0.90 ppm (b, 3H; -CH3); 13C NMR (CDCl3): d=149.5, 138.1,
136.4, 128.4, 127.8, 127.4, 127.2, 71.8, 69.4, 31.8, 29.6, 26.0, 22.6, 14.0 ppm;
FTIR (KBr): n˜ =3062, 2922, 2852, 1590, 1504, 1459, 1420, 1359, 1230,
1197, 1024, 843, 732, 694 cmÀ1
.
Polymer P2: Yellow solid; yield 86%; 1H NMR (CDCl3): d=8.04 (b,
1H; py-H), 7.90 (m, 2H; py-H), 7.71–7.30 (b, 7H; Ar-H), 5.21 (b, 2H;
PhOCH2Ph), 4.14 (b, 2H; PhOCH2CH2-), 1.81 (b, 2H; -OCH2CH2-), 1.30
(b, 18H; -CH2(CH2)9CH3), 0.86 ppm (b, 3H; -CH3); 13C NMR (CDCl3):
d=159.7, 154.4, 151.5, 128.3, 127.5, 123.6, 71.6, 31.8, 29.5, 26.3, 22.6,
14.0 ppm; FTIR (KBr): n˜ =3061, 2923, 2852, 1567, 1503, 1451, 1379, 1196,
1022, 814, 734, 694 cmÀ1
.
Polymer P3: Light brown solid; yield 73%; 1H NMR (CDCl3): d=8.85
(b, 2H; py-H), 8.17 (b, 1H; py-H), 7.70–7.10 (m, 6H; Ar-H), 5.10 (b, 2H;
PhOCH2Ph), 3.94 (b, 2H; PhOCH2CH2-), 1.77 (b, 2H; -OCH2CH2-), 1.22
(b, 18H; -CH2(CH2)9CH3), 0.87 ppm (b, 3H; -CH3); 13C NMR (CDCl3):
d=150.9, 149.9, 148.7, 137.5, 136.7, 133.1, 128.5, 127.8, 127.1, 116.8, 115.5,
71.8, 69.5, 31.8, 29.6, 26.0, 22.6, 14.0 ppm; FTIR (KBr): n˜ =3032, 2923,
2852, 1946, 1744, 1589, 1507, 1436, 1378, 1267, 1201, 1022, 863, 729 cmÀ1
.
Experimental Section
General syntheses of polymers P4–P6: Precursor polymer P1 (2.80 g)
was dissolved in a mixture of CHCl3/THF/EtOH (100:50:30 mL) at room
temperature.10% Pd/C (50. g) and three drops of concentrated HCl
were added, and the mixture was flushed with nitrogen gas.The flask was
fitted with a hydrogen gas balloon and the mixture was stirred at room
temperature for about 2–3 days and then filtered through silica gel/celite.
The filtrate was concentrated under reduced pressure.The concentrated
solute was precipitated from methanol, filtered, and washed with
1) methanol, 2) acetone.
Chemicals and instrumentation: All reactions were carried out under an
inert atmosphere (N2 or argon), unless specified otherwise.All reagents
were purchased from Aldrich, Fluka, or Merck and were used without
further purification, unless otherwise stated.All reactions were carried
out with freshly distilled anhydrous solvents under an inert atmosphere.
THF was purified by distillation over sodium under a nitrogen atmos-
phere.The 1H and 13C NMR spectra were collected on a Bruker ACF300
spectrometer operating at 300.135 and 75.469 MHz, respectively, in
[D]chloroform or [D4]methanol.Tetramethylsilane was used as an inter-
nal standard.FTIR spectra were recorded on a Bio-Rad FTS165 spectro-
photometer in a KBr matrix.UV-visible spectra were recorded on a Shi-
madzu3101PC spectrophotometer and fluorescence measurements were
carried out on a Shimadzu RF-5301PC spectrofluorophotometer.Spectra
were recorded in chloroform for polymers P1–P5 and in methanol for
P6.For metal complexation studies, the metal salt solution (in methanol
or water) was mixed with polymer solution in THF.Thermogravimetric
analyses were performed on TA Instruments SDT2960 with a heating
rate of 108CminÀ1 under a nitrogen atmosphere.Gel permeation chro-
matography (GPC) was used to obtain the molecular weight of the poly-
mers with reference to polystyrene standards with THF as the eluant.X-
ray powder diffraction patterns were obtained on a SiemensD5005 X-ray
diffractometer with CuKa (1.54 Š) radiation (40 kV, 40 mA). Samples
were mounted on a sample holder and scanned between 2q=2–408 with
a step size of 2q=0.018.Cyclic voltammetry (CV) was performed on an
EG&G Princeton model 273 A potentiostat/galvanostat system with a
three-electrode cell in a solution of Bu4NClO4 (0.10m) in acetonitrile as
the electrolyte, with an Ag/AgCl reference electrode, a platinum wire as
the counterelectrode, and a glassy carbon electrode as the working elec-
Polymer P4: Dark brown solid; yield 82%; 1H NMR (CDCl3/
CF3COOD): d=9.02 (b, 1H; py-H), 8.82 (b, 1H; py-H), 8.35 (b, 1H; py-
H), 7.66 (b, 1H; Ar-H), 7.30 (b, 1H; Ar-H), 4.17 (b, 2H; PhOCH2CH2-),
1.91 (b, 2H; -OCH2CH2-), 1.31 (b, 18H; -CH2(CH2)9CH3), 0.92 ppm (b,
3H; -CH3); 13C NMR (CDCl3): d=190.7, 149.8, 136.8, 131.8, 128.5, 128.4,
127.8, 127.2, 71.8, 31.8, 29.6, 26.0, 22.6, 14.0, 6.6 ppm; FTIR (KBr): n˜ =
3408, 3061, 2923, 2852, 2037, 1945, 1591, 1545, 1505, 1462, 1417, 1219,
1017, 844, 734, 696 cmÀ1
.
Polymer P5: Dark brown solid; yield 85%; 1H NMR (CDCl3): d=9.80
(s, 1H; py-H), 8.00 (b, 1H; py-H), 7.87 (b, 1H; py-H), 7.71 (b, 1H; Ar-
H), 7.46 (s, 1H; Ar-H), 4.14 (b, 2H; PhOCH2CH2-), 1.78 (b, 2H; -
OCH2CH2-), 1.26 (b, 18H; -CH2(CH2)9CH3), 0.86 ppm (b, 1H; -CH3);
13C NMR (CDCl3): d=191.0, 190.0, 159.5, 134.5, 128.4, 128.3, 127.4,
125.4, 63.2, 31.7, 29.5, 26.1, 22.5, 13.9, 6.9 ppm; FTIR (KBr): n˜ =3437,
3059, 2923, 2852, 1567, 1505, 1451, 1375, 1198, 1020, 812, 733, 695 cmÀ1
.
Polymer P6: Dark brown solid; yield 76%; 1H NMR ([D7]DMF/
CF3COOD): d=10.35 (b, 1H; py-H), 10.24 (b, 1H; py-H), 9.10 (b, 1H;
py-H), 7.36 (b, 1H; Ar-H), 6.95 (b, 1H; Ar-H), 4.31 (s, 2H;
PhOCH2CH2-), 1.82 (b, 2H; -OCH2CH2-), 1.25 (b, 18H; -CH2-
(CH2)9CH3), 0.86 ppm (b, 1H; -CH3); 13C NMR (CD3OD): d=163.8,
151.4, 151.1, 147.9, 130.4, 129.3, 128.2, 128.0, 126.1, 117.3, 70.3, 62.0, 35.4,
31.3, 23.7, 14.5 ppm; FTIR (KBr): n˜ =3406, 2925, 2854, 1638, 1507, 1455,
trode at a scan rate of 50 mVsÀ1
.
Synthesis: The synthetic strategy for polymers P1–P6 is shown in
Scheme 1.2,5-Dibromo hydroquinone ( 2), 2,5-dibromo-4-dodecyloxyphe-
nol (3), 2,5-dibromo-1-benzyloxy-4-dodecyloxybenzene (4), and 1-benzyl-
1383, 1204, 1057, 1022, 868, 737, 698 cmÀ1
.
Chem. Eur. J. 2005, 11, 5889 – 5898
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S.Valiyaveettil and M.Vetrichelvan
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