Conjugated polymer with benzimidazolylpyridine ligands in the side chain: Metal ion coordination and coordinative self-assembly into fluorescent ultrathin films

Article abstract of DOI:10.1021/ma200126e

The synthesis and characteristic properties of a new conjugated copolymer with poly(phenylene-alt-fluorene) main chain and 2,6-bis(1′- methylbenzimidazolyl)pyridine (bip) ligands attached to the main chain via flexible spacer groups are described. The cop

Full text of DOI:10.1021/ma200126e

ARTICLE
pubs.acs.org/Macromolecules
Conjugated Polymer with Benzimidazolylpyridine Ligands in
the Side Chain: Metal Ion Coordination and Coordinative
Self-Assembly into Fluorescent Ultrathin Films
Irina Welterlich and Bernd Tieke*
Department Chemie, Universit €a t zu K €o ln, Luxemburger Str. 116, D-50939 K €o ln, Germany
S Supporting Information
b
ABSTRACT: The synthesis and characteristic properties of a new con-
jugated copolymer with poly(phenylene-alt-fluorene) main chain and
0
2
,6-bis(1 -methylbenzimidazolyl)pyridine (bip) ligands attached to the
main chain via flexible spacer groups are described. The copolymer was
prepared upon Pd-catalyzed Suzuki coupling of the 2,7-bispinacolatoboron
ester of 9,9-dihexylfluorene and a 1,4-dibromobenzene derivative carrying
two ω-bip-substituted alkoxy groups in the 2- and 5-position. The yield was
8
8%. The polymer is readily soluble in common organic solvents and
exhibits a strong blue fluorescence with maximum at 416 nm; the
fluorescence quantum yield in toluene is 85%. UV/vis titration of the
polymer with zinc chloride in toluene/methanol (99:1 v/v) and ferrous
perchlorate in toluene/methanol (24:1 v/v) indicates formation of 2:1
ligand:metal ion (bis)complexes. Complex formation is accompanied by
ion specific color changes (ionochromism) and quenching of the ligand fluorescence. The backbone fluorescence is partially
retained. The coordinative interactions between the ligand-substituted polymer (“polytopic ligand”) and the divalent metal ions
(Zn(II), Cu(II)) can be used for layer-by-layer assembly of organized films on solid supports. Multiple sequential adsorption of
metal salts and polymer leads to coordinative supramolecular assembly of the cross-linked polymerꢀmetal ion complex on the
substrate, the thickness being controlled by the number of adsorption steps applied. Since the fluorescence of the polymer backbone
is not completely quenched by the ligandꢀmetal ion interaction, the films exhibit a bluish luminescence. The new materials might be
useful for metal ion sensing and for preparation of fluorescent coatings.
1. INTRODUCTION
crystallization takes place, and small defects occurring during film
growth can be healed. Besides the frequently used terpyridine
groups, other heteroaromatic units such as 2,6-bis(1-methylben-
In recent years the supramolecular organization of organic and
ꢀ
inorganic building blocks has proven to be useful for the
1
ꢀ3
zimidazolyl)pyridine (bip) groups are also able to coordinate
transition metal ions and form octahedral ligand:metal com-
preparation of novel functional materials.
Especially coordi-
native interactions between organic ligands and metal ions
facilitate the preparation of new metallopolymers and polymer
28
plexes with 2:1 stoichiometry. Recent studies have shown
that conjugated macromonomers containing bip units at both
ends can be successfully assembled into high-molecular-weight
3
ꢀ9
network structures.
Metallopolymers with application poten-
2
a,10ꢀ14
tial in electronic and optoelectronic devices, or as sensory
have been described. Recently studies were re-
29,30
1
5ꢀ17
metallo-supramolecular polymers.
materials
In this study, for the first time the synthesis of a polymer with
bip ligands in the side chain is described. The polymer consists of
fluorene and phenylene units in alternating fashion introducing
fluorescent properties and of bip ligands attached to the pheny-
lene units via hexamethylene spacer groups. The aliphatic spacer
groups were introduced to minimize electronic interactions
between the conjugated main chain and the metalꢀligand
ported, in which the coordinative interactions between divalent
transition metal ions and di- or multitopic ligands were applied
for the construction of organized films with precisely controlled
17ꢀ27
thickness in the nanometer range.
For the buildup of the
films a layer-by-layer adsorption strategy of alternate assembly of
metal ions and ligands was used. Self-assembled films with
21ꢀ27
functional properties
were obtained, among them being
3
1
complexes such as quenching of the fluorescence. Further-
electrochromic polymer network films consisting of zinc, cobalt,
more, the metal ion complexation of the polymers in solution is
or nickel ions and conjugated polymers with terpyridine sub-
2
5ꢀ27
stituent groups.
Especially polytopic ligands, i.e., polymers
with ligand groups attached to the main chain, have proven useful
Received: January 18, 2011
1
7,24ꢀ27
for the preparation of stable, homogeneous films.
They
Revised:
April 28, 2011
are advantageous over ditopic ligands because no desorption or
Published: May 06, 2011
r 2011 American Chemical Society
4194
dx.doi.org/10.1021/ma200126e
|Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
a
Scheme 1. Preparation of Monomer 3
a
Reagents and conditions: (i) H PO , 205 °C, 14 h, 64%; (ii) 1,6-dibromohexane, acetone, reflux, 24 h, 43%; (iii) Br , CCl , 5 °C, 2 h, 67%; (iv) KOH,
3
4
2
4
DMSO, 47 °C, 24 h, 45%.
1
Figure 1. H NMR spectrum of monomer 3 in deuterated chloroform.
described leading to metallo-supramolecular network structures
via coordinative interactions. Finally the buildup of fluorescent
ultrathin films via multiple sequential adsorption in a coordina-
tive self-assembly process has been investigated. Studies were
4
195
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
carried out using spectroscopic methods (UV/vis and fluores-
cence spectroscopy), microscopic methods (scanning electron
microscopy), EDX, and thickness measurements of the films.
hydroxide (0.37 g, 6.60 mmol) in 40 mL of DMSO was purged with
nitrogen for 30 min. To this suspension 0.94 g (2.64 mmol) of 1 was
added and stirred until the mixture turned reddish. Then 0.76 g (1.28
mmol) of 1,4-dibromo-2,5-bis(6-bromohexyloxy)benzene (2) was dis-
solved in DMSO, and the solution was added to the reaction mixture. The
mixture was stirred at 47 °C for 24 h and subsequently poured into ice
2. EXPERIMENTAL SECTION
water. The precipitate was collected by filtration and dried in vacuum at
2
.1. Materials. The starting compounds were purchased from
1
room temperature. Yield: 0.86 g (29%); mp 235 °C. H NMR (d
1
-CHCl
3
,
Aldrich, Fluka, Merck, or Acros and used without further purification.
All reactions were carried out under a nitrogen atmosphere. Some of the
3
1
4
2
1
00 MHz): δ (ppm) 1.63 (m, 8H), 1.89 (m, 8H), 3.98 (t, 4H), 4.26 (m,
6H), 7.11 (s, phenyl-H, 2H), 7.39 (m, phenyl-H, 12H), 7.88 (m, phenyl-H,
H), 7.95 (m, phenyl-H, 4H). C NMR (d -CHCl , 300 MHz): δ (ppm)
1 3
5.71, 28.83, 29.08, 32.68, 68.85, 70.10, 76.63, 77.05, 77.48, 110.09, 112.50,
18.51, 119.82, 123.33, 123.96, 136.77, 149.84, 166.80. Anal. Calcd for
0
solvents were distilled before use (e.g., toluene from sodium). 2,6-Bis(1 -
13
methylbenzimidazolyl)-4-hydroxypyridine (1), 1,4-dibromo-2,5-bis(6-
bromohexyloxy)benzene (2), and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolan-2-yl)-9,9-dihexylfluorene (4) were prepared as described in
30,32ꢀ34
60 2 10 4
C H58Br N O : C, 63.05; H, 5.11; N, 12.25. Found: C, 62.19; H,
the literature.
The syntheses of compound 3 and polymer 5 are
5.23; N, 11.19.
described below, and the syntheses of compound 1 and polymer 6 are
Polymer 5. 0.100 g (0.088 mmol) of monomer 3 was dissolved in 5 mL
described in the Supporting Information.
0
of a degassed toluene/dioxane (1:1) solvent mixture and stirred under
nitrogen atmosphere at 110 °C for 30 min. Subsequently, 0.051 g (0.088
mmol) of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexyl-
fluorene (4) in 1 mL of toluene/dioxane (1:1) was added, and the mixture
was stirred for 30 min. To the mixture were added 3 mL of aqueous 10 N
NaOH, 1 mL of a toluene/dioxane (1:1) solution containing 8 mol %
1
,4-Dibromo-2,5-bis(6-(2,6-bis(1 -methylbenzimidazolyl)pyridyloxy)
hexyloxy)benzene (3). A suspension of freshly powdered potassium
Pd(PPh
5 °C and stirred for 24 h. After cooling to room temperature, the reaction
mixture was diluted with 100 mL of toluene and extracted with brine. The
organic phase was dried over MgSO and filtered over Celite to remove
3 4
) , and 0.5 mL of 15-crown-5. The reaction mixture was heated to
9
4
residual palladium. After concentration in vacuum the residue was poured
into ether to precipitate the polymer. The polymer was filtered off and dried
1
under vacuum for 24 h. Yield: 0.099 g (88%); softening point: 110 °C. H
NMR (d -CHCl , 300 MHz): δ (ppm) 0.77 (m, 12H), 1.09 (m, 6H), 1.68
1
3
(
(
m, 12H), 4.07 (m, 4H), 4.27 (m, 16H), 7.2 (m, 2H), 7.43 (m, 14H), 7.59
m, 2H), 7.76 (m, 2H), 7.83 (m, 4H), 7.96 (m, 4H).
3
Substrates. Quartz substrates (30 ꢁ 12 ꢁ 1 mm ) were cleaned in a
fresh piranha solution (7:3 mixture of 98% H SO /30% H O ; caution:
2
4
2 2
Figure 2. UV/vis and fluorescence spectra of 3 in chloroform.
the mixture is strongly oxidizing and may detonate upon contact with organic
a
Scheme 2. Preparation of Polymer 5
a
3 4 2
Reagents and conditions: (i) Pd[(PPh ) ] (8 mol %), NaOH (10 N aqueous), 15-crown-5, toluene/dioxane, N , reflux, 24 h, 88%.
4
196
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
1
Figure 3. H NMR spectrum of polymer 5 in deuterated chloroform.
toluene/DMF/MeOH/n-hexane solvent mixture, (c) dipped into the
solution of polymer 5, and (d) washed again with the solvent mixture.
Subsequently steps aꢀd were repeated, etc. Immersion times were
1
0 min for steps aꢀd.
1
Instrumentation. H NMR spectra were recorded using a Bruker
DPX 300 spectrometer operating at 300 MHz. UV/vis absorption
spectra were recorded using a Perkin-Elmer Lambda 14 spectrometer.
Photoluminescence spectra were recorded using a Perkin-Elmer LS50B
spectrometer. For gel permeation chromatography a Waters/Millipore
UV detector 481 and an SEC column combination (Latek/Styragel 50/
1
000 nm pore size) were used. Electrochemical experiments were
performed using a PG390 potentiostat from Heka Co. (model PG
90, Heka Electronic, Lambrecht, Germany). Data acquisition and
3
potentiostat control were accomplished using the Potpulse software,
version 8.4 (Heka). All experiments were carried out in a conventional
three-electrode glass electrochemical cell at room temperature employ-
ing the polymer film on an ITO-coated glass substrate; reference and
counter electrodes were platinum. The experiments were carried out in
acetonitrile (saturated with nitrogen) containing 0.1 M tetrabutylam-
Figure 4. UV/vis and fluorescence spectra of 5 in THF.
material), washed with Milli-Q water, and successively ultrasonicated
in alkaline isopropanol and 0.1 M aqueous HCl at 60 °C for 1 h each.
After careful washing with Milli-Q water the substrates were
silanized with 3-aminopropylmethyldiethoxysilane (Fluka) in toluene
and finally coated with three polyelectrolyte layers in the sequence PSS
monium hexafluorophosphate (TBAPF ) as electrolyte salt. Scanning
6
electron microscopic (SEM) images were obtained using a Zeiss
Supra 40 VP scanning electron microscope. EDX spectra were mon-
itored using an INCA DryCool instrument coupled to the SEM (Zeiss
2
Neon 40). The accelerating voltage was 20 kV, the area was 2500 μm ,
(
polystyrenesulfonate), PEI (polyethylenimine), PSS as previously
and the time for the measurement was 250 s. Film thickness was
measured with a Dektak 3 apparatus from Vecco. The film was partially
scratched from the surface, and a height profile of the surface was
scanned. The error in the measurements was (2.5 nm.
23
described.
ITO-coated glass substrates were successively ultrasonicated in
ethanol and water at 60 °C for 30 min each. Then two polyelectrolyte
layers were deposited in the sequence PEIꢀPSS in the same way as
reported for the quartz substrates.
2
.2. Methods. Film Preparation. If not especially indicated, the
3. RESULTS AND DISCUSSION
dipping solutions of the metal hexafluorophosphates were prepared by
mixing identical volumes of a 0.02 M solution of potassium hexafluor-
ophosphate in toluene/DMF/MeOH/n-hexane (12:2:4:2 v/v) and
a 0.01 M solution of the corresponding metal chloride in the same
3
.1. Synthesis of Monomer and Polymer. At first the
conjugated fluorescent poly(phenylene-alt-fluorene) carrying
2,6-bis(1-methylbenzimidazolyl)pyridine (bip) ligands as sub-
stituent groups was synthesized. The ligands were separated from
the main chain via alkyl spacer groups in order to avoid or
minimize quenching of the fluorescence of the main chain, which
often occurs upon complex formation of the ligands with metal
ꢀ
ꢀ
4
solvent mixture. As dipping solution of the polytopic ligand, a 5 ꢁ 10
monomolar solution of polymer 5 in the same solvent mixture was
used. For film preparation, the pretreated substrates were (a) dipped
into the solution of the metal hexafluorophosphate, (b) washed with the
4
197
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
Scheme 4. Metal Ion Coordination of Monomer 3 Leading to Linear Metallopolymer Chain
a
Scheme 3. Preparation of Polymer 6
a
Reagents and conditions: (i) Pd[(PPh
3
)
4
] (3 mol %), K
2
CO
3
(2 M aqueous), toluene/dioxane, N
2
, reflux, 24 h; (ii) K
2
CO
3
, DMSO, 47 °C, 24 h, 45%.
3
2,33
ions. Bip-ligand 1 was synthesized similar to a previously
carried out as described in the literature;
and 67%, respectively.
the yields were 43
3
0
described method. Details are given in the Supporting Infor-
mation. For preparation of monomer 3 we started from hydro-
quinone. First, the OH-groups of hydroquinone were etherified
with surplus 1,6-dibromohexane, and subsequently the resulting
Monomer 3 was obtained upon reaction of 2 with bip-ligand 1
in a yield of 45%. The H NMR spectra of intermediates 1 and 2
are shown in the Supporting Information. The spectrum of
monomer 3 is shown in Figure 1. All signals could be assigned.
Signals of the aromatic protons appear between 7 and 8 ppm, and
1
1
5
,4-disubstituted phenylene was brominated in the 2- and
-position to yield compound 2 (Scheme 1). Both steps were
4
198
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
The optical properties of polymer 5 were studied using UV/vis
and fluorescence spectroscopy. As shown in Figure 4, the polymer
exhibits two absorption maxima at 318 and 360 nm originating
from πꢀπ* transitions of the ligand (318) and the backbone
chromophore (360). Fluorescence maxima appear at 365 and
4
17 nm, again originating from the ligand (365) and the backbone
chromophore (417). A picture of the fluorescing solution is also
shown in Figure 4. The band assignment is based on polymer 6,
which was obtained upon Suzuki coupling of compounds 2 and 4
as shown in Scheme 3a (for details of the synthesis of 6 see
Supporting Information). Polymer 6 contains the same backbone
chromophore as 5, but the ligand chromophore in the substituent
groups is missing. Consequently, the polymer only exhibits a single
absorption band with maximum at 355 nm and a single fluores-
cence maximum at 417 nm. Unfortunately, all attempts to convert
polymer 6 into 5 upon etherification of the ω-bromoalkyl side
chains with the phenolate of 1 (see Scheme 3b) failed.
Figure 5. UV/vis absorption spectra of monomer 3 (concentration:
ꢀ4
ꢀ1
4
.46 ꢁ 10 mol L ) in acetonitrile/chloroform (9:1 v/v) before and
after addition of 1 μL aliquots of zinc perchlorate (concentration: 1.24 ꢁ
3.2. Metal Ion Complexation in Solution. Before studying
the metal ion complexation of polymer 5, we investigated the
corresponding reaction of monomer 3. 3 not only represents a
monomer for the synthesis of conjugated polymers via palladium-
catalyzed coupling reactions but also is a ditopic ligand for the
formation of nonconjugated metallopolymers as shown in
Scheme 4. The formation of the metallopolymer was followed
upon UV/vis titration of 3 in a 9:1 (v/v) mixture of acetonitrile/
chloroform with a zinc(II) salt in the same solvent mixture. UV/vis
absorption spectra monitored after stepwise addition of small
amounts of zinc perchlorate are shown in Figure 5. 3 behaves like
other ditopic ligands with nonconjugated spacers between the bip
ꢀ
3
ꢀ1
1
0
mol L ) in acetonitrile/chloroform (9:1 v/v). The inset shows
the absorbance at 347 nm versus the molar ratio of zinc ions and 3.
30
units, which initially show an intense absorption with maxima at
3
15 and 321 nm originating from a πꢀπ* transition of the bip unit.
Uponadditionofzinc perchloratea new bandforms, whichexhibits
a maximum at 347 nm and a shoulder at 365 nm. The end point of
titration is reached, if 1 equiv of zinc ions with regard to compound
3
is added. The complex formation involves an isosbestic point at
337 nm, indicating the existence of a single equilibrium between
metal ion complexing and noncomplexing ligands.
Figure 6. UV/vis absorption spectra of polymer 5 (concentration:
Using the same solutions as used for the UV/vis titration, a
fluorescence titration of compound 3 was also carried out. The
addition of the zinc salt led to a gradual decrease of the
fluorescence with maximum at 368 nm until at a molar ratio
zinc(II)/3 of 1.0 a weak and constant fluorescence with max-
imum at 400 nm was left. Again the behavior is similar to other
ditopic bip ligands with nonconjugated spacer groups (spectra
are shown in the Supporting Information). Our experiment
shows that the zinc complex of 3 is fluorescent although the
fluorescence is weaker than for the metal-free ligand.
UV/vis titration was also used to study the complexation of
polymer 5. In this case, a toluene/methanol 99:1 (v/v) mixture
served as solvent for the polymer, and small amounts of zinc
chloride in the same solvent mixture were successively added. As
shown in Figure 6, the absorption maximum at 327 nm gradually
decreases in intensity and a new maximum at 352 nm with
shoulder at 372 nm appears. The spectral changes are accom-
panied by an isosbestic point at 350 nm. The color transition was
complete as soon as a 1:1 ratio of zinc ions and monomoles of
polymer 5 was reached in the solution. This indicates the
formation of the bis-complex consisting of one metal ion per
two ligand groups (see Scheme 5).
ꢀ
4
ꢀ1
1
.82 ꢁ 10 mol L ) in toluene/methanol (99:1 v/v) before and after
ꢀ3
addition of 1 μL aliquots of zinc chloride (concentration: 1.37 ꢁ 10
ꢀ
1
mol L ) in toluene/methanol (99:1 v/v).
signals of the methylene protons of the hexyl chain appear
3
0
between 1.5 and 2 ppm. The signals of the OꢀCH groups of
2
the alkyl chains appear at 3.9 and 4.3 ppm; the signal of the
N-methyl group of the imidazole moiety is also found at 4.3 ppm.
In chloroform solution, monomer 3 shows an intense absorption
at 310 nm and a blue fluorescence with maximum at 360 nm. The
fluorescence spectrum and a picture of the fluorescing solution
are also shown in Figure 2.
Polymer 5 was prepared upon Suzuki coupling of 3 and the
3
4
known 9,9-dihexylfluorene-2,7-bispinacolatodiboron ester 4 as
shown in Scheme 2. A light yellow powder was obtained in a yield
of 88%, which was well soluble in tetrahydrofuran (THF) and
toluene. The polymer is fluorescing in the solid state and in
1
solution. The quantum yield in toluene is 85%. The H NMR
spectrum (Figure 3) in deuterated chloroform exhibits broad
signals. Compared with the spectrum of 3, additional signals
appear between 7 and 8 ppm, which can be ascribed to the
fluorene unit. The signals of the methylene groups of the
dihexylated fluorene and the spacer units connecting the bip
ligands and the main chain are situated between 0.5 and 2.5 ppm.
The titration of the polymer was also carried out in an
acetonitrile/chloroform solvent mixture (9:1 v/v) using zinc
perchlorate for complex formation. Again, the end point of
4
199
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
Scheme 5. Metal Ion Coordination of Polymer 5 Involving Cross-Linking of Side Chains
Figure 7. UV/vis absorption spectra of polymer 5 (concentration:
Figure 8. Change of fluorescence of polymer 5 (concentration: 1.09 ꢁ
ꢀ
4
ꢀ1
7
.82 ꢁ 10 mol L ) in toluene/methanol (24:1 v/v) before and
ꢀ4
ꢀ1
10
mol L ) in acetonitrile/chloroform (9:1 v/v) before and after
after addition of 8 μL aliquots of iron perchlorate (concentration:
addition of 1 μL aliquots of zinc perchlorate (concentration: 1.24 ꢁ
5
.22 ꢁ 10ꢀ mol L ) in toluene/methanol (24:1 v/v).
4
ꢀ1
ꢀ3
ꢀ1
10
mol L ) in acetonitrile/chloroform (9:1 v/v).
titration was reached as soon as a 1:1 molar ratio of Zn(II) and
polymer 5 was present in the solution. However, in this case the
color change during titration did not involve a clear isosbestic point
with copper(II) acetate in the same solvent mixture. Again the
polymer absorption with maximum at 324 nm decreased and a
shoulder at 371 nm appeared. The complex formation involved an
isosbestic point at 344 nm (see Supporting Information). Different
from the addition of Zn(II) or Fe(II) ions, the change in absorbance
of the complex already slowed down after about 0.3 equiv of
copper(II) was added, and further salt addition had only little effect
on the absorbance. We believe the copper complexation proceeds
more rapidly so that a colloidal solution of the complex is formed in
which a further copper ion complexation is slowed down.
(
see also Supporting Information). In a further experiment, a
polymer solution in toluene/methanol (24:1 v/v) was titrated with
ferrous perchlorate. Upon addition of Fe(II) the solution attained a
deep purple color originating from a metal-to-ligand charge transfer
(MLCT). As shown in Figure 7, the color change was complete
at a 1:1 ratio of metal ions and monomoles of polymer. The
polymerꢀFe(II) complex exhibits maxima at 350, 366, and 584 nm,
the latter one originating from MLCT. The complex formation
involves an isosbestic point at 340 nm (see Figure 7). A solution of
polymer 5 in acetonitrile/chloroform (9:1 v/v) was also titrated
We also carried out a fluorescence titration of polymer 5 with
zinc perchlorate. A 9:1 (v/v) mixture of acetonitrile and chloro-
form was used as solvent for polymer and metal salt. As shown in
4
200
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
Scheme 6. Supramolecular Sequential Assembly of Coordination Polymer Films Containing Metal(II) Salts and Polymer 5
Small Counterions Are Omitted for Clarity)
(
Figure 10. Fluorescence spectra of layer-by-layer assembled films of poly-
mer 5 and different metal ions (12 dipping cycles): (a) Zn(II); (b) Cu(II).
Figure 8, the broad polymer fluorescence with shoulder at 370
and maximum at 416 nm decreases upon salt addition but does
not disappear completely. If a 1:1 ratio of Zn(II): monomoles of
5
is reached, no further changes occur. The residual fluorescence
with maximum at 410 nm can be ascribed to the backbone
chromophore, whereas the fluorescence of the bip units (the
shoulder at 370 nm) is completely quenched by the complex
formation. The same fluorescence titration was carried out with
copper acetate. Again, the fluorescence of bip was quenched, and
the polymer fluorescence decreased until a weak emission at
4
16 nm remained. As for the change in absorbance, the change in
fluorescence intensity was largely complete after addition of only
.3 equiv of copper(II), and further salt addition only had a small
Figure 9. UV/vis absorption spectra of layer-by-layer assembled films
of polymer 5 and different metal ions monitored after different numbers
n of dipping cycles: (a) Zn(II), (b) Cu(II). In (c) the effect of the last
dipping step on the spectra in shown (last dip into polymer solution:
n = 15 or 18; last dip into Cu(II) solution: n = 14.5 or 17.5).
0
effect (see Supporting Information).
3.3. Coordinative Assembly of Ultrathin Films. For the
preparation of ultrathin films we used quartz supports, which
4
201
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
Figure 11. SEM pictures of films (a) of polymer/zinc ion complex and (b) of polymer/copper ion complex. Samples were subjected to 12 dipping
cycles. The film thickness is also indicated.
were pretreated as described in the Experimental Section. The
films were built up on the substrates upon alternate dipping into
solutions of the metal salt and polymer 5 as outlined in Scheme 6.
In order to remove surplus salt or polymer and to avoid
contamination of the dipping solutions, additional washing steps
in pure solvent were applied. Films were prepared using zinc(II)
and copper(II) chloride dissolved in a mixture of toluene/DMF/
MeOH/n-hexane (12:2:4:2 v/v). Potassium hexafluoropho-
sphate was added to the solutions in order to decrease the
solubility of the polymerꢀmetal ion complexes and to favor the
adsorption on the substrate. In order to further decrease the
solubility, small amounts of the nonsolvent n-hexane were also
present in the dipping solution. Dipping times in the individual
solutions were 10 min.
The fluorescence with maximum at 426 nm can be ascribed to the
poly(phenylene-alt-fluorene) backbone chromophore. A corre-
sponding film of the copper complex also shows a weak
fluorescence with maximum at about 424 nm originating from
the backbone chromophore (Figure 10b).
3.5. Film Morphology and Composition. In further studies
morphology and metal ion content of self-assembled films of the
zinc and copper complexes of 5 were investigated. Typical SEM
pictures of the morphology are shown in Figure 11. Films of the
zinc and copper complex exhibit a rather smooth surface with
only few inhomogeneities distributed over the whole surface.
The thickness of the films prepared upon 12 dipping cycles were
55 and 14 nm for the zinc and copper complex, respectively, as
determined using profilometry. The metal ion content of the film
was analyzed using EDX. Since an ITO-coated glass substrate
was used as support, the most frequent elements were silicon,
oxygen, indium, and tin. Zinc (1.14 wt % of inorganic material)
was also found. For a corresponding film of the copper complex
of 5, a copper content of 0.59 wt % was found (for details see
Supporting Information).
3
.4. Optical Properties of Films. The buildup of the films was
followed by monitoring UV/vis absorption spectra of the coated
supports after 2, 5, 8, and 11 dipping cycles and an additional
dipping into the metal salt solution. Films of the polymerꢀzinc
ion complex are colorless exhibiting absorption maxima at 358
and 371 nm (Figure 9a). The maxima are consistent with the
maxima found for UV titration in solution. They indicate that the
zinc ion complex of 5 has formed on the substrate. As shown in
the inset, the maximum absorption increases nearly linearly with
the number of dipping cycles indicating homogeneous film
growth; i.e., in each dipping cycle the same amount of the
polymer complex is deposited. Using copper(II) instead of
zinc(II), it was also possible to build up films on the substrate.
Corresponding spectra are shown in Figure 9b. An absorption
band with maximum at 364 nm is found, which indicates the
formation of the copper complex on the substrate. From the plot
of the absorbance at 364 nm vs the number of dipping cycles n, a
linear relation is found; i.e., equal amounts of the complex are
adsorbed in each dipping cycle. It should be mentioned that
spectra of substrates last dipped into the polymer solution were
different from those last dipped into the metal salt solution. As
shown in Figure 9c, the absorption maximum is bathochromi-
cally shifted by about 50 nm after dipping into the Cu(II) salt
solution, and the shift is reverted after dipping into the polymer
solution. This indicates a partial reversibility of the complex
formation depending on whether the substrate is exposed to an
excess of metal ions or polymer in the solution. However,
prolonged exposure to the polymer salt solution for up to a
week did not lead to a dissolution of the film.
4. CONCLUSIONS
A new conjugated polymer with poly(fluorene-alt-phenylene)
0
backbone and 2,6-bis(1 -methylbenzimidazolyl)pyridine substi-
tuent groups was synthesized. The polymer is able to form
complexes with metal ions such as zinc(II) or copper(II) which
exhibit a 2:1 ligand:metal ion stoichiometry. The polymer is
ionochromic; i.e., the color changes from colorless to yellow,
orange, or even purple depending on the metal ion used for
complexation. Because of presence of an alkyl spacer between
ligand and main chain the electronic properties of the two
conjugated parts of the molecules are decoupled, and two
absorption and fluorescence maxima occur. Upon addition of
the metal ions, the fluorescence of the ligand is quenched,
whereas the backbone fluorescence is only decreased in intensity,
but not completely quenched. Elongation of the spacer unit may
help to decouple the electronic interactions more efficiently and
increase the fluorescence of the main chain. The formation of the
metal ion complex proceeds under cross-linking of the polymer,
but neither a precipitation nor a gel formation was observed.
Because of the low polymer concentration and a proper choice of
the solvent and the counterion of the metal salt, the formation of
a gel or a precipitate are prevented and a colloidal solution is
formed instead.
Since the fluorescence titration in solution (section 3.2)
indicated that the zinc ion complex of the polymer exhibits a
weak fluorescence with maximum at 414 nm, it was also of
interest to study the fluorescence of the films. Not surprisingly, a
film of the zinc ion complex of 5 was fluorescent (Figure 10a).
Our study also shows that a proper choice of the solvent (or
solvent mixture) causes that the coordination polymer network
becomes insoluble and is deposited on the substrate. Fluorescent
4
202
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203

Macromolecules
ARTICLE
films of the coordination polymer network are formed, the
thickness being controlled by the number of dipping cycles.
The supramolecular assembly is driven by coordinative interac-
tions between the ligand groups and the metal ions. Variation
of the aromatic groups in the main chain and exchange of the
bip ligands for other ligands such as bisbenzothiazolyl- or
bisbenzoxazolylpyridine will alter the ionochromic properties,
and other coordination polymer network structures may become
accessible.
(16) (a) Kimura, M.; Horai, T.; Hanabusa, K.; Shirai, H. Adv. Mater.
1998, 10, 459. (b) Wang, B.; Wasielewski, M. R. J. Am. Chem. Soc. 1997,
119, 12. (c) Zhu, S. S.; Swager, T. M. J. Am. Chem. Soc. 1997, 119, 12568.
(17) (a) Greenstein, M.; Ishay, R. B.; Maoz, B. M.; Leader, H.;
Vaskevich, H.; Rubinstein, I. Langmuir 2010, 26, 7277. (b) Rubinstein,
I.; Vaskevich, A. Isr. J. Chem. 2010, 50, 333. (c) Wanunu, M.; Vaskevich,
A.; Cohen, S. R.; Cohen, H.; Arad-Yellin, R.; Shanzer, A.; Rubinstein, I.
J. Am. Chem. Soc. 2005, 127, 17877.
(
18) Krass, H.; Papastravrou, G.; Kurth, D. G. Chem. Mater. 2003,
5, 196.
19) (a) Marcon, R. O.; Brosztain, S. Langmuir 2007, 23, 11972. (b)
1
(
’
ASSOCIATED CONTENT
Guang, C.; Hong, H.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 247. (c)
Mwaura, K.; Thomsen, D. L.; Phely-Bobin, T.; Taher, M.; Theodor-
opulos, S.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2000, 122, 2647.
S
Supporting Information. Synthesis of ligand 1 and poly-
b
1
(d) Thomsen, D. l.; Phely-Bobin, T.; Papadimitrakopoulos, F. J. Am.
mer 6; H NMR spectra of 1, 2, and 6; UV/vis titration of
polymer 5 with zinc perchlorate and copper acetate in acetoni-
trile/chloroform (9:1 v/v); EDX spectra of layer-by-layer as-
sembled films of zinc and copper complex of 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
Chem. Soc. 1998, 120, 6177. (e) Abe, M.; Michi, T.; Sato, A.; Kondo, T.;
Zhou, W.; Ye, S.; Uosaki, K.; Sasaki, Y. Angew. Chem., Int. Ed. 2003,
4
2, 2912. (f) Evans, S. D.; Ulman, A.; Goppert-Berarducci, K. E.;
Gerenser, L. J. J. Am. Chem. Soc. 1991, 113, 5866.
20) Tuccitto, N.; Delfanti, I.; Torrisi, V.; Scandola, F.; Chiorboli,
(
C.; Stepanenko, V.; W €u rthner, F.; Licciardello, A. Phys. Chem. Chem.
Phys. 2009, 11, 4033.
’
AUTHOR INFORMATION
(
21) (a) Pan, Y.; Tong, B.; Shi, J.; Zhao, W.; Shen, J.; Zhi, J.; Dong,
Corresponding Author
Y. S. J. Phys. Chem. C 2010, 114, 8040. (b) Cui, Y.; Zhang, D.-D.; Cao, J.;
Han, B.-H. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1310.
*E-mail: tieke@uni-koeln.de.
(
22) Kwok, C.-C.; Yu, S. C.; Sham, I. H. T.; Che, C. M. Chem.
Commun. 2004, 2758.
23) Pyrasch, M.; Amirbeyki, D.; Tieke Adv. Mater. 2001, 13, 1188.
’
ACKNOWLEDGMENT
(
Ruth Bruker and Prof. Klaus Meerholz (Department of
(24) Belghoul, B.; Welterlich, I.; Maier, A.; Toutianoush, A.;
Rabindranath, A. R.; Tieke, B. Langmuir 2007, 23, 5062.
Chemistry, University of Cologne) are thanked for technical
support in structural characterization of the samples. The
Deutsche Forschungsgemeinschaft is thanked for financial sup-
port (Project TI219/11-2).
(25) Maier, A.; Rabindranath, A. R.; Tieke, B. Adv. Mater. 2009,
2
2
1, 959.
(26) Maier, A.; Rabindranath, A. R.; Tieke, B. Chem. Mater. 2009,
1, 3668.
(
27) Maier, A.; Fakhrnabavi, H.; Rabindranath, A. R.; Tieke, B.
’
REFERENCES
J. Mater. Chem. 2011, 21, 5795.
(
1) (a) Lehn, J. M. Angew. Chem., Int. Ed. 1988, 27, 89. (b) Lehn,
J. M. Science 2002, 295, 2400.
2) (a) Whittell, G. R.; Hager, M. D.; Schubert, U. S.; Manners, I.
(28) (a) Petoud, S.; B €u nzli, J.-C. G.; Schenk, K. J.; Piguet, C. Inorg.
Chem. 1997, 36, 1345. (b) Piguet, C.; B €u nzli, J.-C. G. Eur. J. Solid State
Inorg. Chem. 1996, 33, 165.
(
Nature Mater. 2011, 10, 176.(b) Schubert, U. S.; Hofmeier, H.;
Newkome, G. R. Modern Terpyridine Chemistry; Wiley-VCH: Weinheim,
Germany, 2006. (c) Schubert, U. S.; Eschbaumer, C. Angew. Chem., Int.
Ed. 2002, 41, 2892.
(29) (a) Knapton, D.; Iyer, P. K.; Rowan, S. J.; Weder, C. Macro-
molecules 2006, 39, 4069. (b) Knapton, D.; Rowan, S. J.; Weder, C.
Macromolecules 2006, 39, 651. (c) Weng, W.; Beck, J. B.; Jamieson,
A. M.; Rowan, S. J. J. Am. Chem. Soc. 2006, 128, 11663. (d) Beck, J. B.;
Rowan, S. J. J. Am. Chem. Soc. 2003, 125, 13922.
(30) Rowan, S. J.; Beck, J. B. Faraday Discuss. 2005, 128, 43.
(31) Burnworth, M.; Mendez, J. D.; Schroeter, M.; Rowan, S. J.;
Weder, C. Macromolecules 2008, 41, 2157.
ꢀ
(
3) Abd-El-Aziz, A. S., Carraher, C. E., Jr., Horvey, P. D., Jr., Zeldin,
ꢀ
M., Eds. Macromolecules Containing Metal and Metal-Like Elements; John
Wiley & Sons: Hoboken, NJ, 2010; Vol. 10.
ꢀ
(
4) Nguyen, P.; Gοmes-Elipe, P.; Manners, I. Chem. Rev. 1999,
9
9, 1515.
5) (a) Rehahn, M. Acta Polym. 1998, 49, 201. (b) Velten, U.;
Rehahn, M. Macromol. Chem. Phys. 1998, 199, 127.
(32) Whiteside, M. S.; Kurrasch-Orbaugh, D.; Marona-Lewicka, D.;
Nichols, D. E.; Monte, A. Bioorg. Med. Chem. 2002, 10, 3301.
(33) Valenkamp, T.; Wegner, G. Macromol. Chem. Phys. 1994,
195, 1933.
(
(6) Kaliyappan, T.; Kannan, P. Prog. Polym. Sci. 2000, 25, 343.
(
7) Constable, E. C.; Housecroft, C. E.; Lambert, J. N.; Malarek,
(34) (a) Beyerlein, T. Dissertation, Universit €a t zu K €o ln, 2001. (b) Jo,
J.; Chi, C.; H €o ger, S.; Wegner, G.; Yoon, D. Y. Chem.—Eur. J. 2004,
10, 2681.
D. A. Chem. Commun. 2005, 3739.
(
(
8) Wang, X.; McHale, R. Macromol. Rapid Commun. 2010, 31, 331.
9) Williams, K. A.; Boydston, A. J.; Bielawski, C. W. Chem. Soc. Rev.
2
007, 36, 729.
(
10) Qiu, D.; Cheng, Y.; Wang, L. Dalton Trans. 2009, 3247.
(11) (a) Han, F. S.; Higuchi, M.; Kurth, D. G. J. Am. Chem. Soc. 2008,
1
1
30, 2073. (b) Han, F. S.; Higuchi, M.; Kurth, D. G. Adv. Mater. 2007,
9, 3928.
(
12) Chen, Y.-Y.; Tao, Y.-T.; Lin, H.-C. Macromolecules 2006,
9, 8559.
13) Dobrawa, R.; Lysetska, M.; Ballester, P.; Gr €u ne, M.; W €u rthner,
F. Macromolecules 2005, 38, 1315.
14) Yu, S.-Z.; Kwok, C.-C.; Chan, W.-K.; Che, C.-M. Adv. Mater.
003, 15, 1643.
15) Knapton, D.; Burnworth, M.; Rowan, S. J.; Weder, C. Angew.
Chem., Int. Ed. 2006, 45, 5825.
3
(
(
2
(
4
203
dx.doi.org/10.1021/ma200126e |Macromolecules 2011, 44, 4194–4203