Organometallic networks based on 2,2′-bipyridine-containing poly(p-phenylene ethynylene)s

Article abstract of DOI:10.1021/ma0502147

Conjugated polymers that comprise 2,2′-bipyridine moieties as part of the macromolecular backbone represent versatile precursors for the formation of conjugated metallo-supramolecular networks, which are readily accessible via ligand-exchange reactions. Poly{2,2′-bipyridine-5,5′- diylethynylene[2,5-bis(2-ethylhexyl)oxy-1,4-phenylene]ethynylene} (BipyPPE ₁) and a statistical copolymer comprising 5,5′-diethynyl-2, 2′-bipyridine and 1,4-diethynyl-2,5-bis(alkyloxy)benzene moieties (BipyPPE₂) were synthesized via the Pd⁰-catalyzed cross-coupling reaction of 1,4-diethynyl-2,5-bis(octyloxy)benzene, 1,4-bis[(2-ethyl-hexyl)oxy]-2,5-diiodobenzene, and 5,5′-diethynyl-2, 2′-bipyridine. Complexation studies involving these polymers and a variety of transition metals suggest that ligand exchange leads to three-dimensional networks, which feature BipyPPE-metal-BipyPPE cross-links and display interesting optoelectronic properties. It is found that complexes with group 12 d¹⁰ ions (Zn²⁺ and Cd²⁺) are emissive, while other transition metals such as Cu⁺, Co²⁺, and Ni ²⁺ form nonradiative metal-to-ligand charge-transfer complexes with the polymers.

Full text of DOI:10.1021/ma0502147

3800
Macromolecules 2005, 38, 3800-3807
Organometallic Networks Based on 2,2′-Bipyridine-Containing
Poly(p-phenylene ethynylene)s
Akshay Kokil, Peter Yao, and Christoph Weder*
Department of Macromolecular Science and Engineering, Case Western Reserve University,
2100 Adelbert Road, Cleveland, Ohio 44106-7202
Received February 1, 2005; Revised Manuscript Received February 25, 2005
ABSTRACT: Conjugated polymers that comprise 2,2′-bipyridine moieties as part of the macromolecular
backbone represent versatile precursors for the formation of conjugated metallo-supramolecular networks,
which are readily accessible via ligand-exchange reactions. Poly{2,2′-bipyridine-5,5′-diylethynylene[2,5-
bis(2-ethylhexyl)oxy-1,4-phenylene]ethynylene} (BipyPPE1) and a statistical copolymer comprising 5,5′-
diethynyl-2,2′-bipyridine and 1,4-diethynyl-2,5-bis(alkyloxy)benzene moieties (BipyPPE2) were synthesized
via the Pd0-catalyzed cross-coupling reaction of 1,4-diethynyl-2,5-bis(octyloxy)benzene, 1,4-bis[(2-ethyl-
hexyl)oxy]-2,5-diiodobenzene, and 5,5′-diethynyl-2,2′-bipyridine. Complexation studies involving these
polymers and a variety of transition metals suggest that ligand exchange leads to three-dimensional
networks, which feature BipyPPE-metal-BipyPPE cross-links and display interesting optoelectronic
properties. It is found that complexes with group 12 d10 ions (Zn2+ and Cd2+) are emissive, while other
transition metals such as Cu+, Co2+, and Ni2+ form nonradiative metal-to-ligand charge-transfer complexes
with the polymers.
Introduction
exhibit substantially better charge transport charac-
teristics than the linear parent polymers6 and can, at
least in part, overcome the problems associated with
interchain charge transfer between individual macro-
molecules.13 Our previous work in this arena was based
on the exploitation of the ethynyl moieties comprised
in the PPE backbone as ligand sites,5-7 which can form
η2-alkyne complexes with a number of metals. While bis-
(η2-diphenylethynylene)Pt0 complexes14 have been tre-
mendously useful for our work, other examples of
suitable bis(η2-diphenylethynylene)metal complexes,
which could provide electronic conjugation between
chains, are rare.15 We therefore considered the intro-
duction of auxiliary ligands into the polymer backbone
and opted to incorporate 2,2′-bipyridine (Bipy) moieties.
This ligand has extensively been used as a chelating
ligand for a broad variety of metals and was recently
heralded as the “most widely used ligand”.16 Indeed, the
Bipy moiety has already been introduced into a plethora
of macromolecules that form the basis of metallo-
supramolecular systems.17 Pioneering work on PPEs
with Bipy groups in the polymer backbone and linear
metal complexes of these polymers has been carried out
by the groups of Schanze18 and Klemm.19 Interestingly,
the metal-complexed PPEs investigated in these studies
were virtually exclusively prepared by polymerizing
metal-complexed monomers, rather than by complex-
ation of the Bipy-containing polymer with metals.20 We
show here that the latter framework, whichsmainly
with sensor applications in mind and not under consid-
eration of potential network formationshas been ap-
plied by a number of groups for a variety of other
conjugated polymer platforms,21-26 is formidably suited
to prepare metallo-supramolecular PPE networks.
Conjugated semiconducting polymers are attracting
significant interest, since these materials may combine
the processability and mechanical properties of poly-
mers with the readily tailored optoelectronic properties
of organic molecules.1 Especially their (potential) use
in light-emitting diodes,2 field-effect transistors,3 pho-
tovoltaic cells,4 and other devices has propelled the
development of conjugated polymers with unique prop-
erty profiles. Our group has recently embarked to
explore well-defined conjugated polymer networks.5-7
Interestingly, materials with this structural motif have
in the past received little attention,8 possibly due to the
difficulty of introducing conjugated cross-links and
retaining adequate processability. Using poly(p-phe-
nylene ethynylene) (PPE) derivatives9 as an example,
we have demonstrated that this problem can be over-
come by synthesizing such polymers in the form of
spherical particles, which can be processed from (aque-
ous) dispersions.10,11 The size of the resulting particles
can be readily tuned over a wide range (mm to nm) via
the detailed reaction conditions, and it appears that the
approach is universally applicable for many polymer
systems. We have also conducted studies addressing
organometallic polymer networks based on PPE deriva-
tives and metallic cross-links. These materials are
accessible through ligand-exchange reactions between
the linear PPE and low-molecular-weight metal com-
plexes. This synthetic framework relies on the formation
of noncovalent bonds between the ligand sites comprised
in the polymer and the metal, which are governed by
the equilibrium constant of the metal-ligand interac-
tions andsdepending on the kinetic stability of the
metal-ligand interactionsmay be of reversible “dy-
namic” nature.12 As a result, the formation of metallo-
supramolecular polymer networks becomes controllable
and allows for straightforward processing of these
materials.5-7 We demonstrated that such networks may
Experimental Section
Materials. 1,4-Bis[(2-ethylhexyl)oxy]-2,5-diiodobenzene (1),27
1,4-diethynyl-2,5-bis(octyloxy) benzene27 (2), and 5,5′-dibromo-
2,2′-bipyridine28 were prepared as described before. All other
solvents, reagents, metal complexes, and catalysts were pur-
* Corresponding author. E-mail: Christoph.Weder@case.edu.
10.1021/ma0502147 CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/25/2005

Macromolecules, Vol. 38, No. 9, 2005
Organometallic Networks 3801
chased from Aldrich Chemical Co., Fisher Scientific, Strem
Chemicals, or Fluka (analytical grade quality) and were used
without further purification. Unless otherwise stated, spec-
troscopic grade CHCl3 (Aldrich) was employed for all optical
absorption and emission experiments and was passed through
a plug of neutral Alox before use. Anhydrous toluene and
anhydrous diisopropylamine used for the polymerization reac-
tions were purchased from Aldrich Chemical Co. and were
deoxygenated by sparging with Ar for at least 1 h prior to use.
Methods. All cross-coupling reactions were carried out
under a dry Ar atmosphere using standard vacuum-line and
glovebox techniques. Unless otherwise stated, 1H NMR spectra
were recorded in CDCl3 at 22 °C on a Varian XL (300 MHz) or
a Varian Inova (600 MHz) spectrometer, and chemical shifts
are expressed in ppm relative to an internal TMS standard.
Elemental analyses were obtained from Prevalere Life Sciences
Inc. and Galbraith Laboratories Inc. UV-vis absorption
spectra were recorded on a Perkin-Elmer Lambda 800 spec-
trometer. Photoluminescence (PL) spectra were measured
under excitation at 410 nm on a SPEX Fluorolog 3 (model FL3-
12) spectrometer; corrections for the spectral dispersion of the
Xe lamp, the instrument throughput, and the detector re-
sponse were applied.
dried in a vacuum. The polymer was redissolved in CHCl3 (50
mL) and washed with saturated aqueous EDTA solution (3 ×
50 mL). The organic layer was separated off, and the aqueous
phase was reextracted with CHCl3 (50 mL). The combined
organic layers were washed with deionized water (50 mL),
concentrated in a vacuum, and introduced dropwise to vigor-
ously stirred methanol. The solid thus precipitated was filtered
off, washed with hot hexanes (50 mL), and dried in a vacuum
to yield BipyPPE1 as a red solid (227 mg, 62.4%). 1H NMR (600
MHz): δ 8.82 (s, Bipy, 2H), 8.43 (s, Bipy, 2H), 7.92 (s, Bipy,
2H), 7.46, 7.17 (dd, 4H, ar/end groups), 7.34, 6.94 (2 × s, 2H,
ar/end groups), 7.07 (s, 2H, ar), 3.96 (m, 4H, OCH2), 2.39 (s,
6H CH3/end groups), 1.81 (s, 6H, 2 × CH2 + 2 × CH), 1.58 (m,
16H, CH2), 1.24 (s, 20H, CH2), 0.89 (d, 6H, CH3); Xn ) 11.
Synthesis of BipyPPE2. 1,4-Bis[(2-ethylhexyl)oxy]-2,5-
diiodobenzene (1, 548 mg, 0.934 mmol), 1,4-diethynyl-2,5-bis-
(octyloxy)benzene (2, 285 mg, 0.743 mmol), 5,5′-diethynyl-2,2′-
bipyridine (50.0 mg, 0.244 mmol), p-iodotoluene (41.3 mg, 0.189
mmol), Pd(PPh3)2 (47.6 mg, 0.0412 mmol), and CuI (8.2 mg,
0.043 mmol) were combined in a mixture of toluene (30 mL)
and diisopropylamine (12 mL), and the reaction mixture was
stirred under Ar at 70 °C. The formation of what appeared to
be ammonium iodide salts was observed immediately after the
start of the reaction, and at the same time the mixture became
highly fluorescent. After 24 h the reaction was stopped by
adding the mixture to a saturated aqueous EDTA solution (50
mL). After stirring for 2 h the aqueous phase had turned
turquoise. The organic phase was collected, washed with
deionized water (2 × 50 mL), and added dropwise to vigorously
stirred methanol. The reddish polymer thus precipitated was
filtered off and dried in a vacuum. The polymer was redissolved
in CHCl3 (50 mL) and washed with saturated aqueous EDTA
solution (3 × 50 mL). The organic layer was separated off, and
the aqueous phase was reextracted with CHCl3 (50 mL). The
combined organic layers were washed with deionized water
(50 mL), concentrated in a vacuum, and introduced dropwise
to vigorously stirred methanol. The solid thus precipitated was
filtered off, washed with hot hexanes (50 mL), and dried in a
Synthesis of 5,5′-Bis((trimethylsilyl)ethynyl)-2,2′-
bipyridine.12e,29 5,5′-Dibromo-2,2′-bipyridine (1.00 g, 3.18
mmol), Pd(P(Ph3)3)2Cl2 (178 mg, 0.25 mmol), and CuI (48 mg,
0.25 mmol) were dissolved in a mixture of triethylamine (20
mL) and DMF (16 mL) at 55 °C, and trimethylsilylacetylene
(625 mg, 6.36 mmol) was added over a period of 1 h under
vigorous stirring. After the addition of trimethylsilylacetylene
was completed, the reaction mixture was stirred at reflux for
1 h. The resulting suspension was cooled to RT, CHCl3 (100
mL) was added, and the now clear solution was washed with
a saturated aqueous EDTA solution (100 mL) for 1 h. The
organic phase was separated off, and the aqueous layer, which
had turned turquoise, was reextracted with CHCl3 (100 mL).
The combined organic layers were washed with deionized
water (200 mL) and dried over MgSO4, and the solvent was
evaporated in a vacuum. The solid residue was purified by
column chromatography (silica gel; hexanes/ethyl acetate 4:1)
1
vacuum to yield BipyPPE2 as a red solid (486 mg, 72.7%). H
NMR (600 MHz, 42 °C): δ 8.82 (s, Bipy, 2H), 8.43 (s, Bipy,
2H), 7.92 (s, Bipy, 2H), 7.42, 7.12 (dd, 4H, ar/end groups), 7.29,
6.91 (s, 2H, ar/end groups), 7.07 (s, 2H, ar), 3.96 (m, 4H, OCH2),
2.39 (s, 6H CH3/end groups), 1.81 (m, 6H, 2 × CH2 + 2 × CH),
1.58 (m, 16H, CH2), 1.24 (m, 20H, CH2), 0.89 (dd, 6H, CH3);
Xn ) 16.
Preparative Synthesis of Bis(2,2′-bipyridine)Cu(I)
Hexafluorophosphate (4). 2,2′-Bipyridine (502.7 mg, 3.218
mmol) and tetrakis(acetonitrile)Cu(I) hexafluorophosphate
(599.7 mg, 1.609 mmol) were separately dissolved in anhydrous
CHCl3 (5 mL) and CH3CN (6 mL), respectively. Over a period
of 20 min the colorless tetrakis(acetonitrile)Cu(I) hexafluoro-
phosphate solution was added dropwise to the stirred colorless
2,2′-bipyridine solution, and the mixture immediately turned
deep red. The solvents were evaporated in a vacuum to yield
4 as a red solid (829 mg, 98.7%). UV-vis (CHCl3): λmax ) 433
and 523 nm (shoulder).
In-Situ Synthesis of Bis(2,2′-bipyridine)Cu(I) Hexa-
fluorophosphate (4). An aliquot of a solution of tetrakis-
(acetonitrile)Cu(I) hexafluorophosphate in CH3CN (0.2 mL,
2.42 × 10-4 M) was added to a solution of 2,2′-bipyridine in
CHCl3 (3 mL, 3.23 × 10-5 M). The mixture was allowed to
stand for 15 min before an optical absorption spectrum was
recorded. UV-vis (CHCl3:CH3CN ) 15:1 v/v): λmax ) 348, 433,
and 523 nm (shoulder).
Optical Measurements of Polymer-Metal Complexes.
The polymers were dissolved in CHCl3 at a concentration of
1.93 × 10-5 M (BipyPPE1) and 3.23 × 10-5 M (BipyPPE2). (All
concentrations are quoted with respect to the Bipy ligand
comprised in the polymers.) The metal complexes (tetrakis-
(acetonitrile)Cu(I) hexafluorophosphate, cobalt tetrafluorobo-
rate hexahydrate, nickel perchlorate hexahydrate, zinc per-
chlorate hexahydrate, and cadmium perchlorate hydrate were
dissolved in spectroscopic grade CH3CN (concentration range
2.16 × 10-5-5.55 × 10-4 M). Polymer-metal complexes were
produced by adding aliquots of a solution of the selected metal
1
to yield white crystals (800 mg, 72.1%). H NMR (300 MHz):
δ 8.72 (d, 2H, ar), 8.34 (d, 2H, ar), 7.84 (dd, 2H, ar), 0.28 (s,
18H, SiCH3).
Synthesis of 5,5′-Diethynyl-2,2′-bipyridine (3). Metha-
nol (19 mL) and aqueous KOH (1.3 mL, 20%) were added to a
stirred solution of 5,5′-bis((trimethylsilyl)ethynyl)-2,2′-bipyri-
dine (750 mg, 2.15 mmol) in THF (38 mL), and the mixture
was stirred at RT for 2 h. The reaction mixture was filtered,
washed twice with deionized water (50 mL), and dried over
MgSO4. The organic phase was separated off, and the solvents
were evaporated in a vacuum to obtain an off-white crystalline
solid. The product was purified by column chromatography
(silica gel, hexanes/ethyl acetate 10:1) to afford 3 in the form
of off-white crystals (324 mg, 73.7%), which were stored in the
1
dark under Ar. H NMR (300 MHz): δ 8.77 (d, 2H, ar), 8.38
(d, 2H, ar), 7.88 (dd, 2H, d), 3.30 (s, 2H, CtH). Anal. Calcd
for C14N2H8: C, 82.33%; N, 13.71%; H, 3.94%. Found: C,
80.23%; N, 13.17%; H, 3.75%.
Synthesis of Poly{2,2′-bipyridine-5,5′diylethynylene-
[2,5-bis(2-ethylhexyl)oxy-1,4-phenylene]ethynylene} (Bi-
pyPPE1). 1,4-Bis[(2-ethylhexyl)oxy]-2,5-diiodobenzene (1) (362
mg, 0.617 mmol), 5,5′-diethynyl-2,2′-bipyridine (3, 150 mg,
0.734 mmol), p-iodotoluene (51.2 mg, 0.234 mmol), Pd(PPh3)2
(34.1 mg, 0.0295 mmol), and CuI (6.0 mg, 0.032 mmol) were
combined in a mixture of toluene (23 mL) and diisopropy-
lamine (10 mL), and the reaction mixture was stirred under
Ar at 70 °C. The formation of what appeared to be ammonium
iodide salts was observed immediately after the start of the
reaction, and at the same time the mixture became highly
fluorescent. After 24 h the reaction was stopped by adding the
mixture to a saturated aqueous EDTA solution (50 mL). After
stirring for 2 h the aqueous phase had turned turquoise. The
organic phase was collected, washed with deionized water (2
× 50 mL), and added dropwise to vigorously stirred methanol.
The reddish polymer thus precipitated was filtered off and

3802 Kokil et al.
Macromolecules, Vol. 38, No. 9, 2005
Table 1. Optical Absorption and PL Emission Data of
BipyPPE1 and BipyPPE2 after Complexation with
Different Metal Ionsa
Scheme 1. Synthesis and Molecular Structure of the
2,2′-Bipypridine-Containing
Poly(2,5-dialkyloxy-p-phenylene ethynylene)s
BipyPPE1 and BipyPPE2
BipyPPE1
BipyPPE2
absorption
λmax (nm)
emission
absorption
λmax (nm)
emission
λmaxb (nm)
λmaxb (nm)
ion free
Cu+
423
452
458
458
464
468
459
c
440
447
450
450
450
448
482
482
Co2+
Ni2+
c
482
482
482, 646
482, 646
c
619
591
Zn2+
Cd2+
a All spectra were recorded in CHCl3:CH3CN (15:1 v/v) at a
concentration of polymer-bound Bipy of 1.93 × 10-5 M (BipyPPE1)
or 3.23 × 10-5 M (BipyPPE2) and an ion concentration of 2.16 ×
10-5-5.55 × 10-4 M. b Excitation at 410 nm. c Emission is fully
quenched.
complex in CH3CN (0.2 mL) to a solution of BipyPPE1 or
BipyPPE2 in CHCl3 (3 mL); the concentration of the metal
complex solution was varied to produce solutions of different
[metal]:[Bipy] ratios but identical solvent composition of
CHCl3:CH3CN ) 15:1 v/v. The mixtures were allowed to stand
for 15 min to 4 h after addition of the metal salt to the polymer
solutions, before conducting optical studies on these systems.
Film Preparation. The polymers (BipyPPE1 or BipyPPE2)
were dissolved in CHCl3 at a concentration of ∼2.5% w/v. Metal
complexes (see above) were dissolved in CH3CN at appropriate
concentrations, and aliquots of metal solutions (0.1 mL) and
polymer solution (0.2 mL) were mixed and allowed to stand
for 15-20 min before thin films were produced by spin-coating
onto glass substrates.
Results and Discussion
The conjugated polymers employed in the present
study, poly{2,2′-bipyridine-5,5′diylethynylene[2,5-bis(2-
ethylhexyl)oxy-1,4-phenylene]ethynylene} (BipyPPE1)
and a statistical copolymer comprising 5,5′-diethynyl-
2,2′-bipyridine and 1,4-diethynyl-2,5-bis(alkyloxy)ben-
zene moieties (BipyPPE2), were designed to comprise
different fractions of the Bipy moiety (BipyPPE1: x:y )
1:0; BipyPPE2: x:y ) 1:2.5; see Scheme 1) in order to
explore the influence of the latter on the electronic
properties of the polymer (vide infra). These polymers
were synthesized by the Pd0-catalyzed cross-coupling
reaction of 1,4-bis[(2-ethylhexyl)oxy]-2,5-diiodobenzene
(1),27 1,4-diethynyl-2,5-bis(octyloxy)benzene (2),27,30 and
5,5′-diethynyl-2,2′-bipyridine (3)12e,28,29 (Scheme 1). Mono-
mers 1-3 were synthesized according to established
routes.27-30 The polymerizations were conducted in
analogy to Klemm’s protocol19a with monomer concen-
trations that were slightly lower than routinely em-
ployed.27,30,31 If the reactions were conducted at higher
monomer concentrations, the mixtures gelled; in view
of the facts that the molecular weight was deliberately
limited by offsetting the monomer stoichiometry and
using p-iodotoluene as an end-capper27 and that the
isolated polymers were soluble, this finding is indicative
of the formation of network structures due to complexes
between the Bipy moieties and the copper cocatalyst
employed for the polymerization reaction. To remove the
catalyst from the final product, the workup of the
polymers included rigorous extraction with aqueous
ethylenediamine tetraacetate (see Experimental Section
for details).12e The isolated polymers were soluble in
concentrations of at least 1% w/v in solvents such as
CHCl3 and toluene. 1H NMR spectra confirm the chemi-
cal structure of the polymers, reveal 4-toluene and 2,5-
Figure 1. UV-vis absorption (a) and PL emission (b) spectra
acquired upon addition of tetrakis(acetonitrile)Cu(I) hexafluo-
rophosphate to BipyPPE1 (concentration of polymer-bound
Bipy ) 1.93 × 10-5 M) in CHCl3:CH3CN (15:1 v/v). Shown are
spectra at selected [Cu+]:[Bipy] ratios of 0 (solid line), 0.09
(filled squares), 0.19 (filled circles), 0.28 (filled triangles), 0.38
(filled inverted triangles), 0.48 (filled rhombus), 0.57 (empty
squares), 0.76 (empty circles), 0.96 (empty triangles), and 1.92
(empty inverted triangles). The insets show the absorption at
452 nm (a) and the emission at 459 nm (b) as a function of
[Cu+]:[Bipy] ratio.
bis[(2-ethylhexyl)oxy]-4-iodobenzene end groups, and
put the number-average molecular weights to ca. 3100
for BipyPPE1 (number-average degree of polymeriza-
tion, Xn, ) 11) and 6100 for BipyPPE2 (Xn ) 16),
respectively.
The photophysical characteristics of the uncomplexed
polymers were investigated in dilute solutions. A solvent
mixture of CHCl3:CH3CN (15:1 v/v) was found suitable
for the ligand-exchange experiments conducted (vide
infra) and was therefore employed for all optical mea-
surements conducted in this study. The results are
summarized in Table 1; UV-vis absorption and photo-
luminescence (PL) emission spectra are shown in Fig-
ures 1 and 2 for BipyPPE1 and BipyPPE2, respectively.
BipyPPE1 exhibits essentially identical absorption and

Macromolecules, Vol. 38, No. 9, 2005
Organometallic Networks 3803
appear to efficiently “isolate” individual chromophores
(vide infra).19
[CuI(Bipy)2]X complexes represent a class of carefully
studied, kinetically stable bis(bipyridine) complexes,33
and we therefore elected to conduct a systematic com-
plexation study involving BipyPPE1 or BipyPPE2 on one
hand and Cu+ on the other. Adopting the general
protocol employed for the preparation of similar com-
plexes by Mu¨ller et al.,34 we first conducted a series of
reference experiments and prepared the known
[CuI(Bipy)2]PF6 (4).35 This complex was synthesized by
combining a solution of free (i.e., not polymer-bound)
2,2′-bipyridine in anhydrous CHCl3 with a solution of
[CuI(CH3CN)4]PF6 in CH3CN, using 2 equiv of ligand
with respect to the metal ion. The rather concentrated
mixture (∼0.29 M Bipy) immediately adopted the char-
acteristic deep red color of [CuI(Bipy)2]+. The complex
could be quantitatively isolated as the hexafluorophos-
phate by evaporation of the solvents. We also conducted
this ligand-exchange reaction under more dilute condi-
tions (∼3.2 × 10-5 M Bipy) in a 15:1 v/v CHCl3:CH3CN
mixture and characterized 4 in situ by means of UV-
vis spectroscopy (see Supporting Information for spec-
tra). The absorption spectrum of the mixture displays
a maximum at 433 nm, a shoulder at 523 nm, and a
feature at higher energy (348 nm) and matches the one
of the isolated compound and the one published for [CuI-
(Bipy)2]ClO4 elsewhere,34 suggesting that under the
selected conditions the ligand exchange proceeds
smoothly and predominantly leads to the formation of
4. The main absorption band centered around 433 nm
is characteristic of a Cu+-diimine system and is at-
tributed to a metal-to-ligand charge transfer (MLCT)
from the filled Cu 3d orbitals (3d10) to the lowest empty
π* orbitals (LUMO) of the 2,2′-bipyridyl ligand.34,36 As
is the case for many other Cu+-diimine systems,
[CuI(Bipy)2]PF6 does not display any appreciable fluo-
rescence.
We observed that a similar exchange reaction
takes place upon addition of CH3CN solutions of
[CuI(CH3CN)4]PF6 to CHCl3 solutions of BipyPPE1 or
BipyPPE2. If the polymer concentration was of the order
of 1% w/v, the mixtures gelled (BipyPPE2) or formed a
gellous precipitate (BipyPPE1), indicating the formation
of metallo-supramolecular networks with Bipy-metal-
Bipy cross-links. Gelation and/or formation of visible
precipitation could be circumvented by reducing the
absolute concentrations of the polymers (and concomi-
tantly [CuI(CH3CN)4]PF6) to a level of Bipy (as com-
prised in the polymer) of between 1.93 × 10-5 and 3.23
× 10-5 M. In this case, the color of the polymer solutions
changed instantaneously from a pale to a deep yellow
upon addition of Cu+. With the goal of attaining insight
into the formation of the polymer-metal complexes
and to elucidate their optical properties, we titrated
[CuI(CH3CN)4]PF6 to BipyPPE1.37 UV-vis absorption
and PL emission spectra acquired in CHCl3:CH3CN are
shown in Figure 1 for selected [Cu+]:[Bipy] ratios. As
expected, the absorption spectrum of BipyPPE1 changed
significantly upon addition of Cu+. The intensity of the
characteristic π-π* transition associated with the con-
jugated polymer backbone around 423 nm systemati-
cally weakened when [CuI(CH3CN)4]PF6 was added, and
a new band centered around 452 nm developed (Figure
1a). As can be seen from the inset in Figure 1a, the
intensity of the new absorption band at 452 nm steadily
intensified with increasing [Cu+]:[Bipy] ratio, before
Figure 2. UV-vis absorption (a) and PL emission (b) spectra
acquired upon addition of tetrakis(acetonitrile)Cu(I) hexafluo-
rophosphate to BipyPPE2 (concentration of polymer-bound
Bipy ) 3.23 × 10-5 M) in CHCl3:CH3CN (15:1 v/v). Shown are
spectra at selected [Cu+]:[Bipy] ratios of 0 (solid line), 0.1 (filled
squares), 0.2 (filled circles), 0.3 (filled triangles), 0.4 (filled
inverted triangles), 0.5 (filled rhombus), 0.6 (empty squares),
0.8 (empty circles), 1.0 (empty triangles), and 2.0 (empty
inverted triangles). The insets show the absorption at 440 nm
(a) and the emission at 482 nm (b) as a function of [Cu+]:[Bipy]
ratio.
emission characteristics as a polymer of the same
structure investigated in detail before.19d The absorption
spectrum features a strong band with λmax ) 423 nm
(Figure 1a), which in line with previous studies18,19,27,30
is assigned to the π-π* transition of the polymer
backbone. Optical excitation of BipyPPE1 solutions at
wavelengths that match this transition cause intense
PL with λmax ) 459 nm and features that are consistent
with π-π* fluorescence of the polymer (Figure 1b). The
absorption and emission spectra of BipyPPE1 are sig-
nificantly blue-shifted compared to poly(2,5-dialkoxy-
1,4-phenylene ethynylene)s such as poly[2,5-dioctyloxy-
1,4-diethynyl-phenylene-alt-2,5-bis(2′-ethylhexyloxy)-
1,4-phenylene] (λmax ≈ 436 and 474 nm for absorption
and emission, respectively).27,30 It was suggested earlier
that this blue shift is related to low torsional barriers
for motions of the aromatic chain segments19d and the
nonplanar nature of the bipyridine moiety,32 which
significantly limits the conjugation.22 BipyPPE2, which
comprises fewer Bipy units than BipyPPE1, displays
essentially identical absorption (λmax ) 440 nm) and
emission (λmax ) 482 nm) characteristics (Figure 2) as
poly(2,5-dialkoxy-1,4-phenylene ethynylene)s that com-
prise no Bipy.27,30 This is consistent with the formation
of sufficiently long oligo(p-phenylene ethynylene) seg-
ments, which exhibit optical absorption and PL emission
properties that have converged to those of macro-
molecular chains. These segments (presumably of sta-
tistical length) are linked by Bipy moieties, which (in
their uncomplexed state) appear to exert little influence
on the optical properties of the PPE segments but

3804 Kokil et al.
Scheme 2. Schematic Representation of the
Macromolecules, Vol. 38, No. 9, 2005
Formation of Metallo-Supramolecular Networks via
the Formation of Metal-Bis(ligand) Complexes with
2,2′-Bipypridine-Containing
Poly(2,5-dialkyloxy-p-phenylene ethynylene)s
BipyPPE1 and BipyPPE2
leveling off at a [Cu+]:[Bipy] ratio of about 0.5. Similarly,
the polymer’s PL was gradually quenched upon addition
of [CuI(CH3CN)4]PF6 (Figure 1b). Also, this effect scaled
with the [Cu+]:[Bipy] ratio, and the PL was completely
suppressed beyond a [Cu+]:[Bipy] ratio of about 0.5.
Scatchart plots38 of the data presented in Figure 1 (see
Supporting Information) are curved and characteristic
of positive cooperative binding.39 Control experiments
in which [CuI(CH3CN)4]PF6) was added to poly[2,5-
dioctyloxy-1,4-diethynyl-phenylene-alt-2,5-bis(2′-ethyl-
hexyloxy)-1,4-phenylene] (EHO-OPPE,27 a PPE deriva-
tive lacking the Bipy recognition site) under similar
conditions showed no optical changes. Thus, these
results are consistent with the formation of BipyPPE1-
Cu+-BipyPPE1 cross-links between the conjugated mac-
romolecules (Scheme 2) and point to relatively large
binding constants. In view of the similarities to the
optical characteristics of the low-molecular-weight model
compound 4, we tentatively interpret the optical proper-
ties of the Cu+-BipyPPE1 networks with the formation
of a nonradiative metal-to-ligand charge-transfer com-
plex between the metal’s 3d and the BipyPPE1 π*
orbitals. The absorption maximum of the polymeric
metal complex is red-shifted by 19 nm when compared
to 4, presumably as a result of the extended conjugation
of the polymeric ligand. The fact that the partially
metalated BipyPPE1 retained a significant extent of PL
emission (Figure 1b) is indicative of limited exciton
migration along the polymer to the nonradiative low-
band-gap sites. This unusual feature was reported
before18,19d and appears to be related to the “deconju-
gated” nature of uncomplexed, twisted Bipy moieties.
These groups cause efficient “optical insulation”, which
allows the coexistence of multiple chromophores on the
same macromolecule. The fact that they display weak
electronic coupling is in marked contrast to the PPE-
based polymer systems reported by Swager and co-
workers, which act as “molecular wires” and display
energy migration over up to ∼50 repeat units.30,40
The titration of [CuI(CH3CN)4]PF6 to BipyPPE2 in
CHCl3:CH3CN caused optical changes (Figure 2) that
were qualitatively similar to those observed for the
Figure 3. UV-vis absorption (a) and PL emission (b) spectra
acquired upon addition of free 2,2′-bipyridine to a mixture of
BipyPPE1 (concentration of polymer-bound Bipy ) 1.93 × 10-5
M) and tetrakis(acetonitrile)Cu(I) hexafluorophosphate (ratio
of [Cu+]:[Bipy comprised in the polymer] ) 1:0.96) in CHCl3:
CH3CN (15:1 v/v). Shown are spectra for ratios of [free Bipy]:
[polymer-bound Bipy] of 0:1 (dashed line), 1:1 (filled squares),
2:1 (filled circles), 5:1 (filled triangles), and 10:1 (filled
rhombus). The spectra of neat BipyPPE1 (solid line) are
included for reference.
BipyPPE2’s luminescence was not fully extinguished
upon addition of more than stoichiometric amounts of
Cu+ (Figure 2b). This feature reflects the absence of
dominant intermolecular energy transfer processes and
significant internal absorption and is consistent with
the statistical product distribution dictated by the
nature of the polymerization reaction; the number-
average degree of polymerization and the Bipy concen-
tration in the polymer are both low, and due to the
statistical incorporation of the Bipy monomer, a small
fraction of the macromolecules does not comprise any
Bipy moieties and therefore remains emissive upon
addition of [CuI(CH3CN)4]PF6. Similar to BipyPPE1, the
optical changes caused by titration of BipyPPE2 with
Cu+ leveled off at a [Cu+]:[Bipy] ratio of about 0.5, but
minor spectral changes can be discerned upon further
addition of Cu+ (Figure 2). Scatchart plots of the data
presented in Figure 2 (see Supporting Information) are
almost linear and indicate the absence of cooperative
binding. Thus, while these results also point to the
formation of BipyPPE2-Cu+-BipyPPE2 cross-links, the
driving force for their formation appears to be less
pronounced than in the case of BipyPPE1. This may
reflect the fact that in contrast to BipyPPE1, which
features a regular structure of alternating 1,4-diethynyl-
Bipy and 2,5-bis(2-ethylhexyl)oxy-1,4-phenylene moi-
eties, the Bipy moieties comprised in BipyPPE2 are
statistically distributed, stifling the formation of Bi-
pyPPE2-Cu+-BipyPPE2 moieties at higher Cu+ con-
centrations due to geometric constraints and limitations
with respect to chain mobility.
+
Cu -BipyPPE1 complexes. The shift of the main ab-
sorption band from 440 to 447 nm (Figure 2a) was less
pronounced than in the latter case (∆λmax ) 29 nm) due
to the smaller band-gap of the uncomplexed BipyPPE2.
In contrast to the Cu+-BipyPPE1 complexes, the
To investigate the reversibility of the complexation
of the Bipy-containing PPEs with Cu+, we have titrated

Macromolecules, Vol. 38, No. 9, 2005
Organometallic Networks 3805
Figure 5. UV-vis absorption (a) and PL emission (b) spectra
acquired upon addition of cobalt(II) tetrafluoroborate hexa-
hydrate (filled squares), nickel(II) perchlorate hexahydrate
(empty circles), zinc(II) perchlorate hexahydrate (filled tri-
angles), and cadmium(II) perchlorate hydrate (filled circles)
to BipyPPE2 (concentration of polymer-bound Bipy ) 3.23 ×
10-5 M) in CHCl3:CH3CN (15:1 v/v). Shown are spectra for
[M2+]:[Bipy] ratios of 0.5:1. The spectra of neat BipyPPE2 (solid
line) are included for reference.
Figure 4. UV-vis absorption (a) and PL emission (b) spectra
acquired upon addition of cobalt(II) tetrafluoroborate hexa-
hydrate (filled squares), nickel(II) perchlorate hexahydrate
(empty circles), zinc(II) perchlorate hexahydrate (filled tri-
angles), and cadmium(II) perchlorate hydrate (filled circles)
to BipyPPE1 (concentration of Bipy comprised in the polymer
in solution ) 1.93 × 10-5 M) in CHCl3:CH3CN (15:1 v/v).
Shown are spectra for [M2+]:[Bipy] ratios of 0.5:1. The spectra
of neat BipyPPE1 (solid line) are included for reference.
while BipyPPE2 displayed the weak residual emission
(Figure 5b) that appears to be associated with the Bipy-
free fraction of macromolecules (vide supra). These
a CHCl3:CH3CN solution comprising equimolar amounts
of BipyPPE1 and [CuI(CH3CN)4]PF6 with free (i.e., not
polymer-bound) 2,2′-bipyridine. Gratifyingly, upon ad-
dition of Bipy, the UV-vis absorption (Figure 3a) and
PL emission (Figure 3b) spectra converged back to those
of the uncomplexed polymer. At a [free Bipy]:[polymer-
bound Bipy] ratio of 10:1 the optical properties of
BipyPPE1 were fully restored. Thus, the experiment
unequivocally demonstrates that decomplexation of the
polymer-metal system occurs upon addition of a com-
peting ligand and that the ligand exchange reaction is
fully reversible.
With the objective to explore the influence of the
metallic cross-linker on the optical properties of the
polymers at hand, we have conducted complexation
experiments involving BipyPPE1 or BipyPPE2 and per-
chlorates of Co2+, Ni2+, Zn2+, or Cd2+. With reference
to the above-summarized complexation study with Cu+
and with the objective to form [M(II)(Bipy)2]2+ com-
plexes that function as cross-links between the macro-
molecules, the [M2+]:[Bipy] ratio was kept constant at
a level of 0.5:1. UV-vis absorption and PL emission
spectra acquired in CHCl3:CH3CN are shown for
BipyPPE1 and BipyPPE2 in Figures 4 and 5, and the
results are summarized in Table 1. The addition of
Co(ClO4)2 or Ni(ClO4)2 led to very similar optical changes
as were observed before for [CuI(CH3CN)4]PF6 at the
same [M2+]:[Bipy] ratio. Complexation with Co2+ or Ni2+
shifted the maximum of the absorption band of
BipyPPE1 (Figure 4a) from 423 to 458 nm (Cu+: 452
nm) and that of BipyPPE2 (Figure 5a) from 440 to 450
nm (Cu+: 447 nm). The PL of BipyPPE1 was completely
quenched upon addition of Co2+ or Ni2+ (Figure 5a),
results are consistent with the formation of Bipy-M2+
-
Bipy moieties and reflect the fact that also Co2+ and
Ni2+ both exhibit a strong tendency for the formation
of metal-to-ligand charge-transfer complexes with imine
ligands.41 Interestingly, the addition of Zn(ClO4)2 or Cd-
(ClO4)2 caused a somewhat different response. Com-
plexation with these metals shifted the maximum of the
absorption band of BipyPPE1 from 423 to 464 (Zn2+) and
468 nm (Cd2+) (Figure 4a) and that of BipyPPE2 from
440 to 450 (Zn2+) and 448 nm (Cd2+) (Figure 5a). Thus,
the changes were somewhat more pronounced than in
the case of the other metals investigated here. The
characteristic PL band of BipyPPE1 was completely
quenched upon addition of Zn2+ or Cd2+, while BipyPPE2
displayed the weak residual emission associated with
the Bipy-free macromolecules (vide infra). In the case
of both metals, however, new, broad, structureless
emission bands centered at 619 (BipyPPE1-Zn2+), 591
(BipyPPE1-Cd2+), and 646 nm (BipyPPE2-Zn2+ and
BipyPPE2-Cd2+) developed (Figures 4b and 5b). These
results reflect the fact that Zn2+ and Cd2+ both exhibit
a fully occupied d-orbital (Zn2+: 3d10; Cd2+: 4d10), which
frequently displays a weak tendency for the formation
of metal-to-ligand charge transfer.42 Hence, the com-
plexation of these metals with Bipy-containing polymers
does not usually lead to MLCT complexes.25 Rather, the
optical changes appear to be related to a significant
reduction of the polymers’ π-π* transition on account
of a planarization of the Bipy moiety22,23 as well as
electron density variation upon complexation with the
electron-poor metals.25 In view of the fully occupied
d-orbital of the metal, the observed emission cannot be

3806 Kokil et al.
Macromolecules, Vol. 38, No. 9, 2005
Figure 6. (a) UV-vis absorption and (b) PL emission spectra
of spin-coated films of complexes produced by ligand-exchange
reactions between BipyPPE1 and zinc perchlorate hexahydrate
(filled rhombus) or cadmium perchlorate hydrate (filled squares).
Shown are spectra for [metal]:[Bipy] ratios of 0.5:1. The spectra
of neat BipyPPE1 (solid line) are included for reference.
Figure 7. (a) UV-vis absorption and (b) PL emission spectra
of spin-coated films of complexes produced by ligand-exchange
reactions between BipyPPE2 and zinc perchlorate hexahydrate
(filled rhombus) or cadmium perchlorate hydrate (filled squares).
Shown are spectra for [metal]:[Bipy] ratios of 0.5:1. The spectra
of neat BipyPPE2 (solid line) are included for reference.
related to a d-d transition but appears to be caused by
intraligand π-π* transitions.
PPE networks that are accessible via ligand-exchange
reactions. The resulting three-dimensionally cross-
linked, conjugated BipyPPE1-metal and BipyPPE2-
metal networks display interesting optoelectronic prop-
erties. As expected, the coordination of the metal
markedly influences the photophysical characteristics
of the polymer. It is obvious that the approach is broadly
applicable to other conjugated polymer platforms and
also to other ligands. We are aware of the fact that the
(pseudo)-tetrahedral coordination of four-coordinated
metals (as in the case of some of the complexes explored
here) might ultimately not represent the most desirable
geometry from a charge-transfer point of view and that
the ionic nature of the ions used here might lead to
charge trapping.6,7 After having employed these com-
plexes in the present study to demonstrate the possibil-
ity to prepare a new family of π-conjugated organo-
metallic polymer hybrid systems, we envision to extend
the present concepts to the use of square-planar com-
plexes and conduct experiments related to the charge-
carrier transport in such systems.
Homogeneous thin films of good optical quality could
be produced by spin-coating solutions of the polymer-
metal complexes (see Experimental Section for details).
The films thus produced were insoluble in CHCl3, which
is a good solvent for the uncomplexed polymers, con-
sistent with the formation of cross-linked BipyPPE1-
metal and BipyPPE2-metal networks. On the other
hand, if free (i.e., not polymer-bound) 2,2′-bipyridine was
added to the solvent, the films readily redissolved,
demonstrating again that the complexation reactions
are reversible. The metal-complexed films show similar
absorption spectra as in solution, with absorption
maxima that range from 420 nm (uncomplexed film) to
454 nm (Zn2+) in the case of BipyPPE1 (Figure 6a) and
from 427 nm (uncomplexed film) to 462 nm (Zn2+) in
the case of BipyPPE2 (Figure 7a). The emissive char-
acteristics of the Zn(II) and Cd(II) complexes of
BipyPPE1 and BipyPPE2 were retained after processing
these materials into thin films (Figures 6b and 7b), and
these films displayed orange-red emission with maxima
at 612 and 609 nm, respectively. While the development
of red solid-state emitters was clearly beyond the
original focus of our work, the relatively intense emis-
sion, in particular of the Zn-coordinated PPEs, appears
to represent an interesting byproduct of our work.
Acknowledgment. We are grateful for financial
support from NSF (DMR-0215342), the Petroleum Re-
search Fund (ACS-PRF 38525-AC), and DuPont (Young
Professor Grant to C.W.) and thank Prof. Stuart J.
Rowan, J. Benjamin Beck, Prof. Parameswar K. Iyer,
and Dr. Daniel Knapton for many stimulating discus-
sions.
Conclusions
In summary, we have presented a new synthetic
framework that allows for the synthesis and processing
of conjugated organometallic polymer networks. We
have shown at the example of poly(p-phenylene eth-
ynylene) derivatives that the introduction of 2,2′-bi-
pyridine moieties leads to conjugated polymers, which
are formidable precursors for metallo-supramolecular
Supporting Information Available: UV-vis absorption
spectra of model compound 4 and Scatchard plots of the data
presented in Figures 1 and 2. This material is available free
of charge via the Internet at http://pubs.acs.org.

Macromolecules, Vol. 38, No. 9, 2005
Organometallic Networks 3807
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