Synthesis and applications of main-chain Ru(ii) metallo-polymers containing bis-terpyridyl ligands with various benzodiazole cores for solar cells

Article abstract of DOI:10.1039/c0jm02532a

A series of π-conjugated bis-terpyridyl ligands (M1-M3) bearing various benzodiazole cores and their corresponding main-chain Ru(ii) metallo-polymers were designed and synthesized. The formation of metallo-polymers were confirmed by NMR, relative viscosity, and UV-visible titration measurements. The effects of electron donor and acceptor interactions on their thermal, optical, electrochemical, and photovoltaic properties were investigated. Due to the strong intramolecular charge transfer (ICT) interaction and metal to ligand charge transfer (MLCT) in Ru(ii)-containing polymers, the absorption spectra covered a broad range of 260-750 nm with the optical band gaps of 1.77-1.63 eV. In addition, due to the broad sensitization areas of the metallo-polymers, their bulk heterojunction (BHJ) solar cell devices containing [6,6]-phenyl-C ₆₁-butyric acid methyl ester (PCBM) as an electron acceptor exhibited a high short-circuit current (J_sc). An optimum PVC device based on the blended polymer P1:PCBM = 1:1 (w/w) achieved the maximum power conversion efficiency (PCE) value up to 0.45%, with V_oc = 0.61 V, J_sc = 2.18 mA cm^-2, and FF = 34.1% (under AM 1.5 G 100 mW cm ^-2), which demonstrated a novel family of conjugated polyelectrolytes with the highest PCE value comparable with BHJ solar cells fabricated from ionic polythiophene and C₆₀.

Full text of DOI:10.1039/c0jm02532a

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www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and applications of main-chain Ru(II) metallo-polymers containing
bis-terpyridyl ligands with various benzodiazole cores for solar cells†
Harihara Padhy,a Duryodhan Sahu,a I-Hung Chiang,a Dhananjaya Patra,a Dhananjay Kekuda,b
a
Chih-Wei Chubc and Hong-Cheu Lin
*
Received 4th August 2010, Accepted 7th October 2010
DOI: 10.1039/c0jm02532a
A series of p-conjugated bis-terpyridyl ligands (M1–M3) bearing various benzodiazole cores and their
corresponding main-chain Ru(II) metallo-polymers were designed and synthesized. The formation of
metallo-polymers were confirmed by NMR, relative viscosity, and UV-visible titration measurements.
The effects of electron donor and acceptor interactions on their thermal, optical, electrochemical, and
photovoltaic properties were investigated. Due to the strong intramolecular charge transfer (ICT)
interaction and metal to ligand charge transfer (MLCT) in Ru(II)-containing polymers, the absorption
spectra covered a broad range of 260–750 nm with the optical band gaps of 1.77–1.63 eV. In addition,
due to the broad sensitization areas of the metallo-polymers, their bulk heterojunction (BHJ) solar cell
devices containing [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor exhibited
a high short-circuit current (Jsc). An optimum PVC device based on the blended polymer P1 : PCBM ¼
1 : 1 (w/w) achieved the maximum power conversion efficiency (PCE) value up to 0.45%, with Voc
¼
0.61 V, Jsc ¼ 2.18 mA cmꢀ2, and FF ¼ 34.1% (under AM 1.5 G 100 mW cmꢀ2), which demonstrated
a novel family of conjugated polyelectrolytes with the highest PCE value comparable with BHJ solar
cells fabricated from ionic polythiophene and C60.
two terpy units are linked back-to-back with a covalent bond
Introduction
through a spacer, are able to form stable complexes with a large
variety of transition-metal ions for the design of functional
materials.9 Three chelating pyridyl units in terpy ligands offer
higher binding constants and the formation of distorted octa-
hedral 2 : 1 ligand–metal complexes. In particular, by the proper
selection of metal–ligand combinations, it is possible to realize
the formation of ligand–metal complexes from kinetically labile
to inert but nevertheless thermodynamically stable bonds.
Constable et al. have developed the concept concerning the
utilization of ditopic bis-terpyridyl ligands as building blocks for
coordination oligomers and polymers.10 By coordinating to
suitable metal ions, the electronic properties of these materials
can be tuned and that highlights their potential in the applica-
tions of new functional materials.11,12 In the past years, a large
number of terpy complexes containing heavy transition metal
ions, such as ruthenium(II), iridium(III), or osmium(II), as pho-
toactive species, and also coordination polymers built up from
terpy ligands have been introduced by several research groups.9,13
Zinc(II) ions have also recently attracted much interest as novel
templates for the fabrication of structurally well-defined photo-
luminescent and electroluminescent supramolecular metallo-
cycles and metallo-polymers.14,15 In our previous reports,15
a series of terpyridyl Zn(II) metallo-polymers containing poly-
fluorene backbones as emitters were fabricated into PLED
devices with multilayer structures.
In the extensive search of new materials for optoelectronics
applications, supramolecular metallo-polymers have gained
much interest in last decades.1–3 The self-recognition and self-
assembly of functional structures into supramolecular metallo-
architectures through the interactions of transition metal ions
and appropriate chelating ligands has afforded many intriguing
architectures that have attained much attention in modern
supramolecular chemistry.4,5 Polypyridyl ligands, especially
2,20:60,200-terpyridine (terpy) derivatives, which have high
binding affinities towards transition-metal ions due to dp–pp*
back-bonding of metal ions to pyridine rings have been utilized
recently for multinuclear supramolecular interactions.6 Terpy
derivatives bearing p-conjugated substituents at the 40-position
possess fascinating photophysical, electrochemical, catalytic, and
magnetic properties with promising applications in the field of
self-assembled molecular devices and photoactive molecular
wires,7 and their properties can be tuned by varying substituents
at the 40-position of the terpy moieties.8 Furthermore, chemically
and thermally stable ditopic bis-terpyridine derivatives, where
aDepartment of Materials Science and Engineering, National Chiao Tung
University,Hsinchu, Taiwan, ROC. E-mail: linhc@cc.nctu.edu.tw; Fax:
+8863-5724727; Tel: +8863-5712121 ext.55305
bResearch Center for Applied Sciences, Academia Sinica, Taipei, Taiwan,
ROC
cDepartment of Photonics, National Chiao Tung University, Hsinchu,
Taiwan, ROC
Nowadays, some terpyridyl Ru(II) complexes have attracted
researchers to use them in the application of photovoltaic cells
(PVC).16,17 The insertion of ruthenium metals into conjugated
backbones has several advantages, such as facilitating the charge
generation by extending its absorption range due to its charac-
teristic long-lived metal to ligand charge transfer (MLCT)
† Electronic Supplementary Information (ESI) available: Experimental
details and characterization of aromatic dibromides 5a–5c,
bis-terpyridyl ligands M1–M3 and metallo-polymers P1–P3, including
reaction schemes, structures, and figures showing NMR, UV-Visible
titrations. See DOI: 10.1039/c0jm02532a
1196 | J. Mater. Chem., 2011, 21, 1196–1205
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transition9d and to exhibit a reversible Ru(II,III) redox process
along with some ligand-centered redox processes. Motivations
for examining the potential incorporation of such conjugated
polyelectrolytes into solar cell development include the easy
processability, layer-by-layer (LBL) processing capability, and
the fact that their applications in a variety of chemical and
sensory schemes have shown that they are efficiently quenched by
electron acceptors. Dye-sensitized solar cells (DSSCs) based on
such complexes have also been fabricated and the power
conversion efficiency (PCE) values in excess of 10% have been
achieved when liquid electrolytes were used.18 However, the
presence of liquid electrolytes inside DSSCs seriously limits the
industrial development of the technology due to the poor long-
term stabilities of liquid cells. Recently, terpyridyl Ru(II)
complexes have been utilized in bulk heterojunction (BHJ) PVC
devices by using LBL self-assembled techniques. Chan et al.
reported that the use of a terpyridyl Ru(II) complex containing
conjugated polymer (Ru–PPV) and sulfonated polyaniline
(SPAN) for the fabrication of different multilayer PVCs by the
LBL deposition method. The short-circuit currents in the PVC
devices were measured ca. 7.5–32.9 mA cmꢀ2 and the PCE values
were in the range of 0.95–4.4 ꢁ 10ꢀ3%.17a They also synthesized
conjugated polymers with pendant Ru(II) terpyridine trithio-
cyanato complexes and applied them in bulk heterojunction
photovoltaic cells with high PCE values ca. 0.12%.16a Similarly,
Mikroyannidis et al. used metallo-polymers in bulk hetero-
junction photovoltaic cells as a buffer layer along with an active
layer of polymer blend P3HT : PCBM (1 : 1 w/w) and found the
maximum power conversion efficiency value of 0.71% among the
four metallo-polymers studied.16c In all cases, the PCE values
were limited either by the low open-circuit voltage (Voc) or low
short circuit current (Jsc). Due to relatively high HOMO levels
and reduced sensitization ranges in all the reported polymers,
inefficient photocurrents were generated which probably affected
their PCE values.
also investigated. In addition, the PVC devices fabricated by
these bis-terpyridyl ligands (M1–M3) and metallo-polymers
(P1–P3) with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)
inserted between a transparent anode (ITO/PEDOT:PSS) and
a cathode (Ca) were explored, and all photovoltaic parameters
obtained are also comparable with the BHJ solar cells fabricated
from ionic polythiophene and C60.19
Experimental
Materials
All chemicals and solvents were reagent grade and purchased
from Aldrich, ACROS, Fluka, TCI, TEDIA, and Lancaster
Chemical Co. Toluene, tetrahydrofuran, and diethyl ether were
distilled over sodium/benzophenone to keep them anhydrous
before use. Chloroform (CHCl3) was purified by refluxing with
calcium hydride and then distilled. If not otherwise specified, the
other solvents were degassed by nitrogen 1 h prior to use.
Measurements and Characterization
1H NMR spectra were recorded on a Varian Unity 300 MHz
spectrometer using CDCl3 and DMSO-d6 solvents. Elemental
analyses were performed on a HERAEUS CHN-OS RAPID
elemental analyzer. Thermogravimetric analyses (TGA) were
conducted with a TA Instruments Q500 at a heating rate of 10 ꢂC
minꢀ1 under nitrogen. Viscosity measurements were conducted
using 10% weight of metallo-polymer solutions (in NMP) and
compared to measurements made on bis-terpyridyl ligand solu-
tions under the same condition (with viscosity h ¼ 6 cp) on
a BROOKFILEL DV-III+ RHEOMETER system at 25 ꢂC
(100 RPM, Spindle number 4). UV-visible absorption spectra
were recorded in dilute chloroform (for M1–M3) and DMF
(for P1–P3) solutions (10ꢀ6 M) on a HP G1103A spectropho-
tometer. The solid films for the solid-state UV-vis measurements
were spin-coated on quartz substrates from chloroform and
DMF solutions with a concentration of 10 mg mLꢀ1 of the bis-
terpyridyl ligands (M1–M3) and polymers (P1–P3), respectively.
UV-vis titrations were performed by taking 10ꢀ5 M of bis-ter-
pyridyl ligands (M1–M3) in the co-solvent of chloro-
fom : acetonitrile (8 : 2 v/v), and titrated with 50 mL aliquots of
3.9 ꢁ 10ꢀ4 M solutions containing metal salts Zn(OAc)2 in
EtOH. Cyclic voltammetry (CV) measurements were performed
using a BAS 100 electrochemical analyzer with a standard three-
electrode electrochemical cell in a 0.1 M tetrabutylammonium
hexafluorophosphate (Bu4NPF6) solution (in acetonitrile) at
room temperature with a scanning rate of 100 mV sꢀ1. During the
CV measurements, the solutions were purged with nitrogen for
30 s. In each case, a carbon working electrode coated with a thin
layer of monomers or polymers, a platinum wire as the counter
electrode, and a silver wire as the quasi-reference electrode were
used, and Ag/AgCl (3 M KCl) electrode was served as a reference
electrode for all potentials quoted herein. The redox couple of
ferrocene/ferrocenium ion (Fc/Fc+) was used as an external
standard. The corresponding HOMO and LUMO levels were
calculated using Eox/onset and Ered/onset for experiments in solid
films, which were performed by drop-casting films with similar
thicknesses from THF or DMF solutions (ca. 5 mg mLꢀ1).
One of the feasible solutions to conquer these problems, i.e. to
get a higher Voc value and a more favorable overlap of the
absorption spectra from both active layer and solar emission, is
to introduce electron donor–acceptor structures to the cores of
bis-terpyridyl ligands. By choosing different electron donors and
acceptors, their absorption edges can be extended due to the
intramolecular charge transfer (ICT) between them and also the
energy band structures, i.e., highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
levels, of these materials can be tuned. The incorporation of the
thiophene donor units with benzodiazole acceptor units at the
cores of bis-terpyridyl ligands in Ru(II)-containing metallo-
polymers to have better photophysical, electrochemical, and
photovoltaic properties are very intriguing and thus to motivate
this study. Here, we report the design, synthesis, properties, and
device applications of Ru(II)-containg metallo-polymers (P1–P3)
containing donor–acceptor (D–A) bis-terpyridyl ligands bearing
different benzodiazole acceptors, including benzothiadiazole,
benzoselenodiazole, and benzoxadiazole cores sandwiched
between symmetrical thiophene and terpyridyl units. By intro-
ducing such D–A structures, broad absorption ranges ca.
300–750 nm and ideal HOMO/LUMO levels of the metallo-
polymers were obtained. The effects of their donor–acceptor
strengths on the electronic and optoelectronic properties were
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Fabrication of PVC device
124.16, 121.54, 117.84, 116.94, 31.96, 30.69, 29.98, 29.55, 22.90,
14.36. MS (FAB): m/z [M+] 1096; calcd m/z [M+] 1095.49.
Element anal. calcd for C64H54N8S5: C, 70.17; H, 4.97; N, 10.23;
found: C, 69.51; H, 5.26; N, 10.15.
The photovoltaic (PV) cells in this study were composed of an
active layer of blended bis-terpyridyl ligands (M1–M3) or met-
allo-polymers (P1–P3) with [6,6]-phenyl-C61-butyric acid methyl
ester (PCBM) in solid films, which were sandwiched between
a transparent indium tin oxide (ITO) anode and a metal cathode.
Prior to the device fabrication, ITO-coated glass substrates
(1.5 ꢁ 1.5 cm) were ultrasonically cleaned in detergent, deionized
water, acetone, and isopropyl alcohol sequentially. After routine
solvent cleaning, the substrates were treated with UV ozone for
15 min. Then, a modified ITO surface was obtained by spin-
M2. According to the above-mentioned general procedure,
M2 was obtained as a black solid (yield: 63%). 1H NMR (CDCl3,
300 MHz, d): 8.74 (d, J ¼ 4.2 Hz, 4H), 8.61 (m, 8H), 7.85 (dd, J ¼
7.8 Hz, J ¼ 1.8 Hz, 4H), 7.78 (s, 2H), 7.67 (d, J ¼ 3.9 Hz, 2H),
7.62 (s, 2H), 7.35 (dd, J ¼ 7.8 Hz, J ¼ 1.8 Hz, 4H), 7.19 (d, J ¼
3.6 Hz, 2H), 2.84 (t, J ¼ 6.9 Hz, 4H), 1.74 (m, 4H), 1.40–1.29
(m, 12H), 0.93 (t, J ¼ 5.4 Hz, 6H). 13C NMR (CDCl3, 75 MHz,
d): 158.41, 156.22, 149.31, 143.22, 141.15, 140.79, 138.44, 137.83,
137.05, 132.93, 130.51, 127.98, 126.54, 125.44, 124.11, 121.67,
117.84, 116.88, 31.99, 30.63, 29.98, 29.61, 22.93, 14.39. MS
(FAB): m/z [M+] 1143; calcd m/z [M+] 1142.39. Element anal.
calcd for C64H54N8S4Se: C, 67.29; H, 4.76; N, 9.81; found:
C, 66.77; H, 5.30; N, 9.63.
coating
a layer of poly(ethylene dioxythiophene): poly-
styrenesulfonate (PEDOT:PSS) (ꢃ30 nm thick). After baking at
130 ꢂC for one hour, the substrates were transferred to
a nitrogen-filled glove box. The PVC devices were fabricated by
spin-coating solutions of blended bis-terpyridyl ligands
(M1–M3) or metallo-polymers (P1–P3):PCBM (1 : 1 w/w) onto
the PEDOT:PSS modified substrates at 1500 rpm for 60 s
(ca. 100 nm), and placed in a covered glass Petri dish. Initially,
the blended solutions were prepared by dissolving both bis-ter-
pyridyl ligands (M1–M3) and PCBM (1 : 1 w/w) in chloroform
(20 mg mLꢀ1) and both metallo-polymers (P1–P3) and PCBM
(1 : 1 w/w) in DMF (20 mg mLꢀ1), and followed by continuous
M3. According to the above-mentioned general procedure,
1
M3 was obtained as a dark purple solid (yield: 76%). H NMR
(CDCl3, 300 MHz, d): 8.75 (d, J ¼ 4.2 Hz, 4H), 8.69 (m, 8H), 7.96
(s, 2H), 7.87 (dd, J ¼ 7.2 Hz, J ¼ 1.8 Hz, 4H), 7.73 (d, J ¼ 3.6 Hz,
2H), 7.50 (s, 2H), 7.37 (dd, J ¼ 6.9 Hz, J ¼ 1.2 Hz, 4H),
7.25 (d, J ¼ 3.9 Hz, 2H), 2.88 (t, J ¼ 7.2 Hz, 4H), 1.78 (m, 4H),
1.40–1.29 (m, 12H), 0.93 (t, J ¼ 6.9 Hz, 6H). 13C NMR (CDCl3,
75 MHz, d): 155.72, 148.96, 148.35, 143.10, 141.16, 138.10,
137.36, 133.05, 132.09, 130.68, 129.41, 128.63, 126.85, 125.28,
122.94, 121.61, 117.69, 116.97, 31.98, 30.60, 30.03, 29.60, 22.94,
14.38. MS (FAB): m/z [M+] 1078; calcd m/z [M+] 1078.33.
Element anal. calcd for C64H54N8S4O: C, 71.21; H, 5.04; N,
10.38; found: C, 70.75; H, 5.21; N, 10.11.
ꢂ
stirring for 12 h at 50 C. Finally, a calcium layer (30 nm) and
a subsequent aluminium layer (100 nm) were thermally evapo-
rated through a shadow mask at a pressure below 6 ꢁ 10ꢀ6 Torr,
where the active area of the device was 0.12 cm2. All PVC devices
were prepared and measured under ambient conditions. The
solar cell testing was done inside a glove box under simulated
AM 1.5G irradiation (100 mW cmꢀ2) using a Xenon lamp based
solar simulator (Thermal Oriel 1000W). The light source was
a 450 W Xe lamp (Oriel Instrument, model 6266) equipped with
a water-based IR filter (Oriel Instrument, model 6123NS). The
light output from the monochromator (Oriel Instrument, model
74100) was focused onto the photovoltaic cell under test.
General synthetic procedure for metallo-polymers (P1–P3)
In a flame dried flask, a mixture of RuCl3$3H2O (0.11 mmol) and
AgBF4 (0.38 mmol) was refluxed for 2 h in acetone (15 mL).
After cooling to room temperature, the precipitated AgCl was
filtered off, and the obtained solution was evaporated to dryness.
The remaining solid was redissolved in n-butanol (15 mL), and to
this solution, bis-terpyridyl ligand M1, M2, or M3 (0.1 mmol)
was added, and the resulting solution was refluxed for 5 days. As
soon as the precipitation of the formed polymer was observed,
a small portion of DMA was added to the mixture (S z 20 mL)
to redissolve the product. Finally, an excess of NH4PF6 (50 mg in
20 mL DMA) was added to the hot solution and stirring was
continued for 1 h. The resulting solution was poured dropwise
into methanol (200 mL). The precipitated metallo-polymer
product was filtered off and washed with methanol (200 mL).
Further purification was achieved by repetitively dissolving the
metallo-polymer in NMP (2 mL) followewd by precipitation
from diethyl ether. Finally, the products were dried under
vacuum at 40 ꢂC for 24 h.
General synthetic procedure for bis-terpyridyl ligands (M1–M3)
M1–M3 were prepared via a Stille coupling reaction using tet-
rakis(triphenylphosphine)palladium as a catalyst. In a flame
dried two-neck flask, 1.00 eq. of dibromo compounds (5a–5c)
and 2.50 eq. of compound 2 in toluene were degassed with
Argon. Then, 0.03 eq. Pd(PPh3)4 was added and refluxed for
2 days. The reaction mixtures were cooled to room temperature,
and the solvents were removed by reduced pressure. After
removal of the solvents, the product was precipitated from
methanol. Further purification was achieved by column chro-
matography on alumina with chloroform as an eluant to give the
products.
M1. According to the above-mentioned general procedure,
M1 was obtained as a purple solid (yield: 68%). 1H NMR
(CDCl3, 300 MHz, d): 8.77 (d, J ¼ 4.2 Hz, 4H), 8.63 (m, 8H), 7.91
(s, 2H), 7.85 (dd, J ¼ 7.8 Hz, J ¼ 1.8 Hz, 4H), 7.72 (S, 2H), 7.69
(d, J ¼ 3.9 Hz, 2H), 7.35 (dd, J ¼ 7.8 Hz, J ¼ 1.8 Hz, 4H), 7.22
(d, J ¼ 3.9 Hz, 2H), 2.87(t, J ¼ 6.9 Hz, 4H), 1.75 (m, 4H), 1.40–
1.29 (m, 12H), 0.93 (t, J ¼ 5.4 Hz, 6H). 13C NMR (CDCl3,
75 MHz, d): 156.29, 149.43, 149.35, 143.29, 141.28, 138.26,
137.10, 132.95, 132.09, 130.91, 129.51, 128.83, 126.60, 125.47,
P1. According to the above-mentioned procedure, metallo-
polymer P1 was obtained as a dark solid (yield: 66%). 1H NMR
(DMSO-d6, 300 MHz, d): 9.36 (br, 4H), 9.12 (br, 4H), 8.52
(br, 2H), 8.24 (br, 8H), 7.67 (br, 6H), 7.31 (br, 4H), 3.05 (br, 4H),
1.86 (br, 4H), 1.25–1.40 (br, 12H), 0.92 (br, 6H).
P2. According to the above-mentioned procedure, metallo-
polymer P2 was obtained as a dark solid (yield: 58%). 1H NMR
(DMSO-d6, 300 MHz, d): 9.36 (br, 4H), 9.12 (br, 4H),
1198 | J. Mater. Chem., 2011, 21, 1196–1205
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8.48 (br, 2H), 8.14 (br, 8H), 7.68 (br, 6H), 7.30 (br, 4H), 3.08 (br,
4H), 1.83 (br, 4H), 1.20–1.53 (br, 12H), 0.91 (br, 6H).
(M1–M3) were obtained in moderate to good yields and fully
1
characterized by H NMR, 13C NMR, MS spectroscopy, and
P3. According to the above-mentioned procedure, metallo-
polymer P3 was obtained as a dark solid (yield: 77%). 1H NMR
(DMSO-d6, 300 MHz, d): 9.39 (br, 4H), 9.13 (br, 4H), 8.54 (br,
2H), 8.10 (br, 8H), 7.65 (br, 6H), 7.33 (br, 4H), 2.94 (br, 4H),
1.83 (br, 4H), 1.20–1.53 (br, 12H), 0.89 (br, 6H).
elemental analysis.
The synthesis of Ru(II)-based metallo-polymers (P1–P3) is also
depicted in Scheme 1. The metallo-polymerization by self-
assembly was carried out according to the methods described in
the literature.13e In a typical polymerization process, an appro-
priate quantity of ruthenium trichloride was activated by
de-chlorinating with AgBF4 in acetone. The resulting hexa-
acetone Ru(III) complex was reacted with exactly 1 equiv. of bis-
terpyridyl ligands M1–M3 in n-butanol/DMA for 5 days, which
involved a reduction of Ru(III) to Ru(II) with the chain growth
process using the n-butanol solvent itself as a reducing agent. The
resulting metallo-polymers (P1–P3) were purified by repetitive
precipitation from NMP in diethyl ether and dried in vacuum,
leading to homopolymers P1–P3 (yield: 58–78%). The resulting
highly linear rigid polymer containing charged metal ions
exhibited reduced solubility in common organic solvents as
compared with the terpyridyl ligands M1–M3, but were soluble
in highly polar aprotic solvents, e.g., DMSO, DMF, NMP, or
DMA.
Fig. 1 shows 1H NMR spectra in the aromatic regions of
ligands and metallo-polymers. As a result of metallo-polymeri-
zation, broadened signals of bis-terpyridyl ligands, as well as the
absence of the signals from the uncomplexed terpyridyl units
were observed. Furthermore, clear and dramatic downfield shifts
of the (5,500)-, (4,400)-, (3,300)-, and (30,50)-terpyridyl signals and
upfield shifts of the (6,600)- signals were observed upon poly-
merization, which are consistent with the reported literature.20
The assignments of terpyridyl signals are shown in Fig. 1 based
on those reported for Ru(II) model complexes.13 Due to the lack
of end group-signals in the 1H NMR spectra and also considering
the limit of NMR spectroscopy, the formation of cyclic oligo-
mers can be discarded and the synthesized metallo-polymers
should consist of more than 30 repeating units.13e Hence, the
molar masses of P1–P3 were estimated to be higher than 30000 g
molꢀ1. To further confirm the formation of supramolecular
metallo-polymers, the relative viscosity measurements of
Results and discussion
Synthesis and structural characterization
The general synthetic routes of ditopic bis-terpyridyl ligands
(M1–M3) and metallo-polymers (P1–P3) are shown in Scheme 1.
The ditopic bis-terpyridyl monomers in which, acceptor spacer
units sandwiched in between two hexyl thiophene units were
synthesized in multistep procedures. Detail synthesis and char-
acterization are described in the supporting information.† 40-(2-
Bromo-5-thienyl)-2,20,60,200-terpyridine (1) was prepared by the
modified method described in the literature,7f then tributyltin
group was introduced using a palladium(0) catalyzed reaction of
compound 1 with excess bis-(tributyltin). Hexyl-thiophene units
were added to both sides of each acceptor units through the
Suzuki coupling reaction between 2-(4-hexylthiophen-2-yl)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2) and dibromo arenes
(3a–3c) in the presence of a catalyst Pd(PPh3)4. Then these
compounds were brominated with NBS to produce compounds
5a–5c. The aromatic dibromides 5a–5c were reacted with two
equivalents
of
40-(5-tributylstannanyl-thiophen-2-yl)-
[2,20;60,200]terpyridine (2) under Pd0-catalyzed Stille cross-
coupling conditions to form ditopic bis-terpyridyl ligands
(M1–M3). After precipitation from methanol and column
chromatographic purification, the bis-terpyridyl ligands
Fig. 1 1H NMR spectra (aromatic region) of bis-terpyridyl ligands
M1–M3 (in CDCl3) and metallo-polymers P1–P3 (in DMSO-d6). The
signals belonging to the terpyridyl moiety are assigned in the represen-
tative ligand M1 and polymer P1, and the other signals correspond to the
p-conjugated spacer. For all spectra: 300 MHz, room temperature.
Scheme 1 Synthetic route for bis-terpyridyl ligands (M1–M3) and
Ru(II)-containing metallo-polymers (P1–P3).
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 1196–1205 | 1199