Synthesis and characterization of new self-assembled metallo-polymers containing electron-withdrawing and electron-donating bis(terpyridine) zinc(II) moieties

Article abstract of DOI:10.1021/ma902110v

A series of rigid π-conjugated bis(terpyridines) (M1-M7) bearing electron-acceptor spacer units in 4′-position was synthesized in moderate to high yields by Pd°-catalyzed Sonogashira cross-coupling reactions. The compounds were fully characterized by NMR

Full text of DOI:10.1021/ma902110v

Macromolecules 2010, 43, 2759–2771 2759
DOI: 10.1021/ma902110v
Synthesis and Characterization of New Self-Assembled Metallo-Polymers
Containing Electron-Withdrawing and Electron-Donating
Bis(terpyridine) Zinc(II) Moieties
†
Florian Schlutter, Andreas Wild,†,‡ Andreas Winter,‡,§ Martin D. Hager,†,‡
€
Anja Baumgaertel,†,‡ Christian Friebe,†,‡ and Ulrich S. Schubert*,†,‡,§
†Laboratory of Organic and Macromolecular Chemistry, Friedrich-Schiller-University Jena, Humboldtstr.
10, 07743 Jena, Germany, ‡Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven,
§
The Netherlands, and Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of
Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Received September 26, 2009; Revised Manuscript Received January 17, 2010
ABSTRACT: A series of rigid π-conjugated bis(terpyridines) (M1-M7) bearing electron-acceptor spacer
units in 40-position was synthesized in moderate to high yields by Pd0-catalyzed Sonogashira cross-coupling
reactions. The compounds were fully characterized by NMR spectroscopy, MALDI-TOF mass spectro-
metry, elemental analysis and their photophysical properties were discussed in detail. These new bis-
(terpyridines) were applied for the self-assembly reaction with ZnII ions to form metallo-homo polymers
(P1-P7). Broadened NMR signals and UV-vis titration experiments confirmed the successful polymeri-
zation. The electro-optical properties of the materials were investigated in detail. Band gaps up to 2.08 eV and
bright blue to orange photoluminescence with quantum yields of 18 to 66% were observed strongly
depending on the nature of the π-conjugated bis(terpyridine) system. In combination with electron-donor
ditopic terpyridine ligands (MD1 and MD2), two ZnII random copolymers (R1 and R2) were synthesized.
These materials were investigated by UV-vis absorption and photoluminescence experiments in dilute
solution and in the solid state, prepared by spin-coating from DMF solutions. Thereby, random copolymer
R2 featured an energy transfer from the donor to the acceptor part in dilute solution.
Introduction
Such metal complexes have found numerous applications as
luminescent sensors in molecular biology and medical diagno-
In the extensive search for new materials for optoelectronic
applications, metal-ligand coordination has gained much inter-
est in the last decades.1-3 By varying the metal-ligand combina-
tion, the properties of such supramolecular assemblies can be
tuned, not only with respect to their binding strength, reversibility
and solubility, but also with respect to their optoelectronic
properties.4-10
stics, as photocatalysts, effective materials in self-assembled
molecular devices as well as molecular wires.21,22 The combina-
tion of the properties of [M(tpy)2]2þ complexes (i.e., optical,
electrochemical, magnetic properties) with the versatile proper-
ties of organic polymers (i.e., mechanical properties, solubility,
processability) leads to fascinating possibilities.2,23-25 The group
of Constable and Thompson developed an approach for the
utilization of ditopic [2,20:60,200]-terpyridines as building blocks
for the self-assembly with transition metal ions to metallo-
polymers and -oligomers.26 Beside highly stable RuII metallo-
polymers, ZnII terpyridine metallo-polymers gained recently
interest due to their well-defined structures and enhanced photo-
and electroluminescent properties at room temperature.23,27,28
Therefore, ZnII containing metallo-polymers are promising ma-
terials for potential applications, e.g., in organic light-emitting
diodes (OLED). Such systems provide large equilibrium con-
stants, which are essential for the thermodynamic driven polymer
formation.9,10,29 In previous work in this field, the group of
Therefore, several N-heterocyclic ligands, e.g., 2,20-bipyridine,
1,10-phenanthroline, and, in particular, [2,20:60,200]-terpyridine,
attracted much interest as supramolecular templates due to their
high binding affinity towards many transition metal ions in low
oxidation states, through dπ-pπ* bonding and the prevention of
Δ/Λ-chirality compared to 2,20-bipyridine metal complexes.11,12
This work focuses on [2,20:60,200]-terpyridines bearing π-conju-
gated substituents in 40-position,8,12-15 which are showing inter-
esting photophysical as well as electrochemical properties.
Furthermore, bis(terpyridines) allow the electronic communica-
tion between the metal-complexed terpyridine units pointing out
their potential in the design of functional materials.9,10,16-19 In
combination with transition metal ions [2,20:60,200]-terpyridines
form distorted octahedral complexes showing different stabilities
from kinetically inert (e.g., RuII and NiII) to labile (e.g., ZnII).10,20
With respect to this, the availability and the low costs of zinc(II)
ions show a considerable advantage compared with the other
potential metals.
€
Wurthner showed the metal-directed self-assembly of highly
fluorescent [2,20:60,200]-terpyridine bearing perylene bisimdide in
40-position to photoactive metallo-polymers and presented their
spectroscopic and structural properties.9,19 Lin et al. applied
fluorene substituted ditopic [2,20:60,200]-terpyridines for the syn-
thesis of ZnII containing metallo-homopolymers and metallo-
alt-copolymers.23,28 According to Che and Cao, the incor-
poration of [2,20:60,200]-terpyridine ZnII moieties into different
main-chain structures leads to a variety from violet to yellow
colors with high PL quantum yields.30,31 Furthermore, we
*Author for correspondence. Telephone: þ49(0) 3641 948200. Fax:
þ49(0) 3641 948202. E-mail: ulrich.schubert@uni-jena.de.
r 2010 American Chemical Society
Published on Web 02/17/2010
pubs.acs.org/Macromolecules

€
Schlutter et al.
2760 Macromolecules, Vol. 43, No. 6, 2010
showed several types of bis(terpyridines) bearing electron-donat-
ing π-conjugated spacer units with remarkable photophysical
properties and used them as building blocks for the construction
of ZnII containing metallo-polymers.24,32,33 We could improve
the film-forming ability of such systems by incorporating poly-
mer side-chains to π-conjugated bis(terpyridines).16
synthetic procedures and characterization). The synthesis of
40-(4-((1E,7E)-4-(2-(4-(2-(4-((E)-4-([2,20:60,200]terpyridine-40-yl)-
styryl)phenyl)ethynyl)-2,5-bis(2-ethylhexyloxy)phenyl)ethynyl)-
styryl)-phenyl)-[2,20:60,200]terpyridine (MD1) and 2,7-bis((4-([2,20:
60,200]terpyridine-40-yl)phenyl)ethynyl)-9,9-bis(octyloxy)-9H-fluo-
rene (MD2) as well as their ZnII metallo-homo polymers is reported
elsewhere.50
In continuation of this work, we focused on the synthesis of
bis(terpyridine) building blocks with electron-accepting π-con-
jugated spacer units, which should significantly influence the
optical properties and, lead in combination with the established
electron-donor units, towards supramolecular low band gap
materials.34 Potential photovoltaic materials, e.g., π-conjugated
systems, have to cover the red and near-infrared ranges of
the terrestrial solar spectrum, based on the maximum photon
flux density of the sunlight, located at approximately 700 nm
(bandgap, Eg ≈ 1.77 eV). Hence, it is desirable to develop
π-conjugated systems with broader absorptions by reducing their
Eg values.35 The combination of electron-acceptors with electron-
donors leads to a significant decrease of the distance (bandgap,
Eg) between the highest occupied molecular orbital (HOMO) of
the donor and the lowest unoccupied molecular orbital (LUMO)
of the acceptor, which results in a bathochromic shift of the
absorption.34,35 Therefore, we attached well-known strong elec-
tron-acceptors, e.g., [2,1,3]benzothiadiazole, terephthalates,
thieno[3,4-b]pyrazine, quinoxaline as well as a nitrile-groups
containing system to the [2,20:60,200]-terpyridine in 40-position
using the Pd0-catalyzed Sonogashira cross-coupling reaction.
These ditopic ligands were polymerized via the coordination of
ZnII ions into the corresponding metallo-homo polymers.
Furthermore, two statistical metallo-random polymers were
synthesized in combination with ditopic terpyridine ligands
containing π-conjugated electron-donating spacers. This combi-
nation should lead to a transfer of energy from the excited donor
part to the acceptor moiety, which can consequently relax under
emission. The investigation of energy as well as electron transfer
processes is crucial for the understanding of the photosynthetic
processes and the design of artificial light-to-energy conversion
systems.36,37 Besides that, an efficient energy transfer process
allows long-range information transmission, which could be
applied in the development of molecular devices and machines
for information technology.38
Instrumentation. 1D (1H, 13C) and 2D (1H-1H gCOSY, HSQC,
HMBC) nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker Cryomagnet BZH 400 (400 MHz), Bruker AC 300
(300 MHz) or Bruker AC 250 (250 MHz) instrument at 298 K.
Chemical shifts are reported in parts per million (ppm, δ scale)
relative to the residual signal of the deuterated solvent. Coupling
constants are given in Hz. UV-vis absorption and photolumine-
scence (PL) emission spectroscopy were performed on an Analytik
Jena SPECORD 250 and Jasco FP-6500 spectrometer, respectively,
at 298 K. Absolute photoluminescence quantum yields were evalu-
ated at 298 K on a Hamamatsu photonic multi-channel analyzer C
10027. For these techniques 10-6-10-5 M solutions in chloroform
or N,N-dimethylformamide (DMF) were used. Matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF) mass
spectra were obtained from an Ultraflex III TOF/TOF mass
spectrometer with dithranol as matrix in reflector as well as linear
mode. Elemental analyses were carried out on a CHN-932 Automat
Leco instrument. Cyclic voltammetry (CV) measurements were
performed using a PA4 polarographic analyzer (Laboratory instru-
ments, Prague, Czech Republic) with a three electrode cell. Platinum
(Pt) wire electrodes were used as both working and counter electro-
des and a nonaqueous Ag/AgI electrode (Ag in 0.1 M AgNO3
solution) was used as reference electrode. Thin films coated onto Pt
electrodes from acetonitrile solutions containing 0.1 M tetra-
n-butylammonium hexafluorophosphate ((TBA)PF6) were used
for these measurements. All measurements and film preparations
were performed under nitrogen atmosphere within a glovebox.
Polymer thin films were prepared by spin-coating polymer solutions
from DMF (20 mg/mL) using a spin-coating rate of 1000 rpm for
100 s onto silica fused substrates for optical studies. The glass
slides were ultrasonicated in 2% Helmanex-solution (Helma) and
2-propanol for 20 min, in between washed 12 times with distilled
water, and finally placed into an ethanol solution and dried using
nitrogen. UV-vis absorption and PL emission spectra of these thin
films were measured with a Hitachi F-4500 fluorescence spectro-
photometer. Layer thicknesses were measured outside the glovebox
using a KLA-Tencor P-10 profilometer.
General Procedure for the Synthesis of the Bis(terpyridine)
Monomers (M1-M7). To an argon-degassed mixture of 40-(4-
ethynylphenyl)-[2,20:60,200]terpyridine (A, 0.50 mmol) and an
aromatic bromide (1-7, 0.25 mmol) in dry THF (30 mL) and
dry triethylamine or diisopropylamine (10 mL) were added
tetrakis(triphenylphospine)palladium(0) (10 mol %) and cop-
per(I) iodide (10 mol %), and the reaction mixture was refluxed
until TLC indicated complete conversion (5 to 48 h). After the
reaction had cooled to room temperature, the precipitated
ammonia salt was filtered off and washed intensively with
THF. Subsequently, dichloromethane was added and the
solution was washed with saturated aqueous NH4Cl/EDTA
solution and dried over anhydrous MgSO4. After removal of
the solvents, the product was precipitated from methanol.
Further purification was achieved by column chromatography
(aluminum oxide, CH2Cl2 as eluent).
The photophysical and electrochemical properties of the new
type of bis(terpyridines) and their corresponding metallo-poly-
meric materials are discussed in detail.
Experimental Section
Materials and General Experimental Details. All chemicals
were purchased from Aldrich, Acros Organics and Alfa Aesar
and were of reagent grade and used as received, unless otherwise
specified. The solvents were purchased from Biosolve, Aldrich
and Acros Organics and were dried and distilled according to
standard procedures. Chromatographic separation was per-
formed with standardized silica gel 60 (Merck) and aluminum
oxide 90 neutral (Molekula). The reaction progress was con-
trolled by thin layer chromatography (TLC) using aluminum
sheets precoated with silica gel 60 F254 (Merck) and aluminum
oxide 60 F254 neutral (Macherey-Nagel).
40-(4-Ethynylphenyl)-[2,20:60,200]terpyridine (A),39-41 4,7-di-
bromo-[2,1,3]benzothiadiazole (1),42 dioctyl 2,5-dibromo-ter-
ephthalate (2),43 5,8-dibromo-2,3-bis(2-ethylhexyl)quinoxaline
(3),44,45 (2Z,20Z)-3,30-(2,5-dibromo-1,4-phenylene)bis(2-(4-(2-ethyl-
hexyloxy)phenyl)acrylonitrile) (4),46,47 5,7-dibromo-2,3-bis(4-(octyl-
oxy)phenyl)thieno[3,4-b]pyrazine (5),48,49 tetraoctyl 5,50-([2,1,3]ben-
zothiadiazole-4,7-diylbis(ethyne-2,1-diyl))bis(2-bromoterephthalate)
(6), and tetraoctyl 5,50-(2,5-bis(octyloxy)-1,4-phenylene)bis(ethyne-
2,1-diyl)bis(2-bromoterephthalate) (7) were prepared according to
the literature (see the Supporting Information for the detailed
4,7-Bis((4-([2,20:60,200]terpyridine-40-yl)phenyl)ethynyl)[2,1,3]-
benzothiadiazole (M1). According to the above-mentioned gen-
eral procedure, M1 was obtained after filtration from the
reaction mixture, intensive washing with water and repeated
recrystallization from large amounts of chloroform as orange
1
solid (169 mg, 85%). H and 13C NMR spectra could not be
obtained due to the very low solubility of M1 in organic solvents.
MALDI-TOF MS (dithranol): m/z = 799.25 (100%, [M þ
H]þ). Anal. Calcd for C52H30N8S: C, 78.18; H, 3.78; N, 14.03;
S, 4.01. Found: C, 77.98; H, 4.02; N, 14.21; S, 3.76.

Article
Dioctyl 2,5-Bis((4-([2,20:60,200]terpyridine-40-yl)phenyl)ethynyl)-
Macromolecules, Vol. 43, No. 6, 2010 2761
procedure, M6 was obtained after two times column chromato-
graphic purification and three times precipitation from metha-
nol as an orange solid (121 mg, 30%).51 1H NMR (CDCl3, 300
MHz, δ): 0.86 (mc, 12H, CH3), 1.19-1.52 (m, 40H, CH2),
1.77-1.89 (m, 8H,00OCH2-CH2), 4.40-4.49 (m, 8H, O-CH2),
terephthalate (M2). According to the above-mentioned general
procedure, M2 was obtained as a yellow solid (184 mg, 70%). 1H
NMR (CDCl3, 300 MHz, δ): 0.84 (m, 6H, CH3), 1.19-1.51 (m,
20H, CH2), 1.78-1.88 (m, 004H, OCH2-CH2), 4.44 (mc, 4H,
O-CH2), 7.37 (mc, 4H, H5,5 ), 7.73 (d, 3J = 7.5 Hz, 4H, Ha,b),
3
7.38 (mc, 4H, H5,5 ), 7.56 (d, J = 8.7 Hz, 4H, Ha,b), 7.75 (d,
00
00
7.88 (mc, 8H, H4,4 , Ha,b), 7.95 (d, 3J = 070 .5 Hz, 4H, Ha,b), 8.26
(s, 2H, HA), 8.69 (d, 3J = 7.08 Hz, 4H, H3,3 ), 8.75 (d, 3J = 3.9 Hz,
3J = 8.4 Hz, 4H, Ha,b), 7.95 (mc, 6H, H4,4 , HC), 7.97 (d,
3J = 9.0 Hz, 4H, Ha,b), 8.32 (s, 2H, HB), 8.40 (s, 2H, HA), 8.70 (d,
00
0
00
00
0
0
4H, H6,6 ), 8.76 (s, 4H, H3 ,5 ). 13C NMR (CDCl3, 75 MHz, δ):
14.1 (CH3), 22.6, 26.1, 28.7, 29.2, 29.3, 31.8 (CH2), 66.1(O-CH2),
89.0 (CtC), 96.4 (CtC), 118.7 (tC-C), 121.4, 122.9, 123.6,
123.9, 127.3, 132.4, 134.5, 136.1, 136.9, 138.8, 149.2, 149.3, 156.1,
156.1 (Caryl), 165.1 (CdO). MALDI-TOF MS (dithranol):
m/z = 1053.51 (100%, [M þ H]þ). Anal. Calcd for C70H64N6-
O4: C, 79.82; H, 6.12; N, 7.98. Found: C, 79.45; H, 6.46; N, 7.79.
2,3-Bis(2-ethylhexyl)-5,8-bis((4-([2,20:60,200]terpyridine-40-yl)-
phenyl)ethynyl)quinoxaline (M3). According to the above-men-
tioned general procedure, M3 was obtained as a yellow-orange
solid (164 mg, 65%). 1H NMR (CDCl3, 300 MHz, δ): 0.87 (mc,
6H, CH3), 1.00 (mc, 6H, CH3), 1.23-1.58 (m, 16H, CH2), 2.31
3J = 7.8 Hz, 4H, H3,3 ), 8.76 (mc, 4H, H6,6 ), 8.78 (s, 4H, H3 ,5 ).
13C NMR (CDCl3, 75 MHz, δ): 14.1 (CH3), 22.6, 26.1, 28.6,
29.1, 29.2, 29.3, 31.8 (CH2), 66.2 (O-CH2), 88.9 (CtC), 95.1
(CtC), 116.0, 118.7, 118.8, 121.4, 123.2, 124.0, 127.3, 127.4,
132.4, 139.0, 139.3, 142.2, 146.9, 149.2, 154.3, (Caryl), 156.0,
156.1 (CdO). MALDI-TOF MS (dithranol): m/z = 1623.85
(100%, [M þ H]þ). Anal. Calcd for C104H102N8O8S: C, 76.91%;
H, 6.33%; N, 6.90%; S, 1.97%. Found: C, 76.65%; H, 5.98%;
N, 6.61%; S 1.80%.
Tetraoctyl 5,50-(2,5-bis(octyloxy)-1,4-phenylene)bis(ethyne-
2,1-diyl)bis(2-((4-([2,20:60,200]-terpyridine-40-yl)phenyl)ethynyl)-
terephthalate) (M7). According to the above-mentioned gene-
ral procedure, M7 was obtained after two times column
chromatographic purification and three times precipitation
from methanol as an orange solid (141 mg, 31%).51 1H NMR
(CDCl3, 300 MHz, δ): 0.82-1.00 (m, 18H, CH3), 1.21-1.65
(m, 60H, CH2), 1.76-1.93 (m, 12H, OCH2-CH2), 3.99-4.09
t
3
(mc, 2H, C-H), 3.05 (d, J = 6.9 Hz, 4H, NdC-CH2), 7.38
00
(mc,004H, H5,5 ), 7.81 (d, 3J = 8.4 Hz, 4H, Ha,b), 7.91 (mc, 10H,
H4,4 , Ha,b, HA), 7.98 (d, 3J = 8.4 Hz, 4H, Ha,b), 8.70 (d, 3J = 8.1
00
00
3
Hz, 4H, H3,3 ), 8.76 (d, J = 4.2 Hz, 4H, H6,6 ), 8.79 (s, 4H,
H
0
0
3 ,5 ). 13C NMR (CDCl3, 75 MHz, δ): 11.0, 14.1 (CH3), 23.1,
26.0, 28.9, 32.9 (CH2), 38.1 (tCH), 39.1 (NdC-C), 88.5 (CtC),
96.7 (CtC), 118.7 (tC-C), 121.4, 123.1, 123.9, 124.3, 127.3,
131.8, 132.4, 136.9, 138.4, 140.9, 149.2, 149.4, 156.1, 156.2, 157.4
(Caryl). MALDI-TOF MS (dithranol): m/z = 1017.53 (100%,
[M þ H]þ). Anal. Calcd for C70H64N8: C, 82.64; H, 6.34;
N, 11.01. Found: C, 82.85; H, 5.97; N, 11.01.
(m, 4H, O-CH2), 4.34-4.54 (m, 8H, O-CH2), 7.05 (s, 2H,
00
HC), 7.39 (mc, 4H, H5,5 ), 7.74 (d, 3J = 8.1 Hz, 4H, Ha,b), 7.94
00
(mc, 6H, H4,4 , Ha,b),080 .25 (s, 2H, HB), 8.26 (s, 2H, HA), 8.70 (d,
00
0
0
3J = 7.8 Hz, 4H, H3,3 ), 8.76 (mc, 4H, H6,6 ), 8.78 (s, 4H, H3 ,5 ).
13C NMR (CDCl3, 75 MHz, δ): 14.1 (CH3), 22.6, 26.1, 28.6,
29.1, 29.2, 29.3, 31.8 (CH2), 66.2 (O-CH2), 88.9 (CtC),
95.1 (CtC), 116.0, 118.6, 118. 8, 121.4, 123.2, 124.0, 127.3,
127.4, 132.4, 139.0, 139.3, 142.2, 146.9, 149.2, 154.3,
(Caryl), 156.0, 156.1 (CdO). MALDI-TOF MS (dithranol):
3-(4-(1-Cyano-2-(4-(2-ethylhexyloxy)phenyl)vinyl)-2,5-bis-
((4-([2,20:60,200]terpyridine-40-yl) phenyl)ethynyl)phenyl)-2-(4-(2-
ethylhexyloxy)phenyl)acrylonitrile (M4). According to the
above-mentioned general procedure, M4 was obtained as
a yellow-orange solid (187 mg, 72%). 1H NMR (CDCl3, 300
MHz, δ): 0.95 (mc, 12H, CH3), 1.22-1.58 (m, 16H, CH2), 1.75
m/z
=
1822.32 (100%, [M
C120H136N6O10: C, 79.09; H, 7.52; N, 4.61. Found: C, 78.71;
H, 7.58; N, 4.32.
þ
H]þ). Anal. Calcd for
(mc, 2H, tC-H), 3.94 (mc, 4H, O-CH2, EþZ isomers),
General Procedure for the Metallo-Polymerization. To the
bis(terpyridine) monomers (M1-M7, 0.025 mmol) in N-methyl-
pyrrolidone (NMP, 5 mL) was added zinc(II) acetate (0.025
mmol) in NMP (1 mL). The resulting solution was stirred at
105 °C under argon atmosphere for 24 h. An excess of NH4PF6
(50 mg) was added to the hot solution and stirring was continued
for 1 h. The solution was poured into methanol (50 mL), and the
resulting metallo-polymer was filtered off and washed with
methanol (10 mL). Further purification was achieved by re-
peated dissolving of the metallo-polymer in NMP (2 mL) and
precipitation from diethyl ether. Finally, the products were
dried under vacuum at 40 °C for 24 h.
00
6.88-7.06 (d, 3J = 8.4 Hz, 4H, Ha,b), 7.37 (mc, 8H, H5,5
,
,
00
3
H
H
a,b), 7.73 (d, J = 5.7 Hz, 4H, Ha,b), 7.90 (mc, 8H, H4,4
a,b), 8.06 (s, 2H, HA), 8.46-8.54 (s,002H, CdC-HB, EþZ
00
iso0m0ers), 8.69 (d, 3J = 8.4 Hz, 4H, H3,3 ), 8.77 (mc, 8H, H6,6
,
H
3 ,5 ). 13C NMR (CDCl3, 75 MHz, δ): 11.1, 14.1 (CH3), 23.0,
23.8, 29.1, 30.5 (CH2), 39.4 (tCH), 70.8 (O-CH2), 88.0 (CtC),
97.9 (CtC), 114.2 (tC-C), 115.1, 115.2, 117.5, 118.7, 118.8,
121.4, 123.1, 123.9, 126.3, 127.5, 127.6, 131.0, 132.1, 132.3,
132.5, 135.7, 136.3, 137.0, 139.0, 149.1, 156.1 (Caryl), 160.9
(O-Cd). MALDI-TOF MS (dithranol): m/z = 1251.69
(100%, [M þ H]þ). Anal. Calcd for C86H74N8O2: C, 82.53;
H, 5.96; N, 8.95. Found: C, 82.18; H, 5.58; N, 8.56.
Metallo-Homo Polymer P1: {[Zn(M1)](PF6)2}n. According to
the above-mentioned procedure, homo polymer P1 was ob-
tained as a yellow solid (22 mg, 84%). 1H NMR (DMSO-d6, 300
2,3-Bis(2-ethylhexyl)-5,7-bis((4-([2,20:60,200]terpyridine-40-yl)-
phenyl)ethynyl)thieno[3,4-b]pyrazine (M5). According to the
above-mentioned general procedure, M5 was obtained as a
red solid (225 mg, 75%). 1H NMR (CDCl3, 300 MHz, δ): 0.90
(mc, 6H, CH3), 1.23-1.31 (m, 20H, CH2), 1.78-1.88 (m, 4H,
00
00
MHz, δ):007.50 (mc, H5,5 ), 7.98 (mc, H6,6 ), 8.32 (mc, Haryl), 8.56
00
0
0
(mc, H4,4 ), 8.76 (mc, Haryl), 9.17 (mc, H3,3 ), 9.46 (mc, H3 ,5 ).
Anal. Calcd for C52H30F12N8P2SZn: C, 54.11; H, 2.62; N, 9.71;
S, 2.78. Found: C, 54.61; H, 2.34; N, 8.98; S, 2.12.
OCH2-CH2), 4.00 (mc, 4H, O-CH2), 6.88 (d, 3J = 9.0 Hz, 4H,
00
Ha,b) 7.40 (mc, 4H, H5,5 ), 7.56 (d, 3J = 8.7 Hz, 4H, Ha,b), 7.78
Metallo-Homo Polymer P2: {[Zn(M2)](PF6)2}n. According
to the above-mentioned procedure, homo polymer P2 was
obtained as yellow solid (29 mg, 82%).1H NMR (DMSO-d6,
300 MHz, δ): 0.72 (mc, CH3), 1.09-1.22 (m, CH2), 4.44 (mc,
00
(d, 3J = 8.4 Hz, 4H, Ha,b), 7.90 (mc, 4H, H4,4 ),007.97 (d, 3J = 8.4
3
Hz, 040 H, Ha0 ,b), 8.70 (d, J = 7.8 Hz, 4H, H3,3 ), 8.77 (mc, 8H,
0
H6,6 , H3 ,5 ). 13C NMR (CDCl3, 75 MHz, δ): 14.1 (CH3), 22.7,
26.0, 29.2, 29.4, 29.5, 31.8 (CH2), 68.1 (O-CH2), 88.9 (CtC),
96.5 (CtC), 114.2, 115.2, 118.7, 121.4, 123.6, 123.9, 127.3,
131.2, 131.6, 132.3, 136.9, 138.6, 142.8, 149.2, 149.3, 156.1,
156.2 (Caryl), 160.3 (CdO). MALDI-TOF MS (dithranol): m/
z = 1207.61 (100%, [M þ H]þ). Anal. Calcd for C80H70N8O2S:
C, 79.57; H, 5.84; N, 9.28; S, 2.66. Found: C, 79.41; H, 5.99;
N, 9.08; S, 2.92.
00
00
O-CH2), 7.52 (mc, H5,5 ), 7.97 (mc, H6,6 ), 8.30 (mc, Haryl), 8.58
00
00
0
0
(mc, H4,4 ), 8.78 (mc, Haryl), 9.18 (mc, H3,3 ), 9.46 (mc, H3 ,5 ).
Anal. Calcd for C70H64F12N6O4P2Zn: C, 59.69; H, 4.58; N, 5.97.
Found: C, 58.91; H, 4.23; N, 4.98.
Metallo-Homo Polymer P3: {[Zn(M3)](PF6)2}n. According to
the above-mentioned procedure, homo polymer P3 was ob-
tained as yellow solid (27 mg, 78%). 1H NMR (DMSO-d6, 300
MHz, δ): 0.82 (mc, CH3), 1.01 (mc, CH3), 1.18-1.58 (mc, CH2),
Tetraoctyl 5,50-([2,1,3]benzothiadiazole-4,7-diylbis(ethyne-
2,1-diyl))bis(2-((4-([2,20:60,200]-terpyridine-40-yl)phenyl)ethynyl)-
terephthalate) (M6). According to the above-mentioned general
00
00
3.09 (mc, NdC-CH2), 7.54 (mc, H5,5 ), 7.99 (mc, H6,6 ), 8.32
00
00
(mc, Haryl), 8.60 (mc, H4,4 ), 8.79 (mc, Haryl), 9.18 (mc, H3,3 ), 9.48

€
Schlutter et al.
2762 Macromolecules, Vol. 43, No. 6, 2010
Scheme 1. Schematic Representation of the Synthesis of Bis(terpyridines) M1-M7 Using Pd0-Catalyzed Sonogashira Cross-Coupling Reactions
0
0
00
(mc, H3 ,5 ). Anal. Calcd for C70H64F12N8P2Zn: C, 61.25; H, 4.70;
N, 8.16. Found: C, 60.71; H, 4.13; N, 7.58.
8.08 (mc, Haryl), 8.31 (mc, H4,4 ), 8.69 0(mc, Haryl), 8.77 (mc,
00
0
Haryl), 9.16 (mc, H3,3 ), 9.45 (mc, H3 ,5 ). Anal. Calcd for
C120H136F12N6O10P2Zn: C, 66.18; H, 6.29; N, 3.86. Found:
C, 65.35; H, 4.23; N, 3.37.
Metallo-Random Copolymer R1: {[Zn(M1)](PF6)2}n{[Zn-
(MD1)](PF6)2}m. According to the above-mentioned procedure,
random copolymer R1 was obtained as yellow solid (42 mg,
68%). 1H NMR (DMSO-d6, 300 MHz, δ): 0.90 (mc, CH3), 0.99
Metallo-Homo Polymer P4: {[Zn(M4)](PF6)2}n. According to
the above-mentioned procedure, homo polymer P4 was ob-
tained after three times precipitation as yellow solid (8 mg,
20%).51 1H NMR (DMSO-d6, 300 MHz, δ): 0.85 (mc, CH3),
1.04-1.54 (m, CH2), 3.08 (mc, O-CH2), 7.05 (mc, Haryl), 7.52
00
00
00
(mc, H5,5 ), 7.93 (mc, H6,6 ), 8.30 (mc, Haryl), 08.60 (mc, H4,4 ),
00
0
8.79 (mc, Haryl), 9.13 (mc, H3,3 ), 9.34 (mc, H3 ,5 ). Anal. Calcd
for C86H74F12N8O2P2Zn: C, 64.28; H, 4.64; N, 6.97. Found:
C, 63.78; H, 4.04; N, 6.77.
(mc,00CH3), 1.26-1.80 (m, CH2), 4.01 (mc, O-CH2), 7.56 (mc,
00
H5,5 ), 7.78 (mc, Haryl), 7.99 (mc, H6,6 ), 8.13 (mc, Haryl), 8.32
00
(mc, H4,4 ), 8.58 (mc, Haryl) 8.74 (mc, Haryl), 8.78 (mc, Haryl), 9.15
00
0
0
Metallo-Homo Polymer P5: {[Zn(M5)](PF6)2}n. According to
the above-mentioned procedure, homo polymer P5 was ob-
tained as orange solid (21 mg, 54%). 1H NMR (DMSO-d6, 300
MHz, δ): 0.84 (mc, CH3), 0.99-1.47 (m, CH2), 3.99 (mc,
(mc, H3,3 ), 9.41 (mc, H3 ,5 ).
Metallo-Random Copolymer R2: {[Zn(M3)](PF6)2}n{[Zn-
(MD2)](PF6)2}m. According to the above-mentioned procedure,
random copolymer R2 was obtained as green-yellow solid
00
00
O-CH2), 7.07 (mc, Haryl), 7.52 (mc, H5,5 ), 7.97 (mc, H6,6 ),
1
(48 mg, 70%). H NMR (DMSO-d6, 300 MHz, δ): 0.80 (mc,
00
00
8.03 (mc, Haryl), 8.30 (mc, H4,4 ), 8.76 (mc, Haryl), 9.19 (mc,
CH3), 0.94-1.57 (mc, CH2), 3.14 (mc, O-CH2), 7.54 (mc, H5,5 ),
3,300
30,50
), 9.48 (mc, H ). Anal. Calcd for C80H70F12N8O2P2SZn:
6,600
7.66 (mc, Haryl), 7.78 (mc, Haryl), 7.99 (mc, H ), 8.06 (mc, Haryl),
H
00
C, 61.48; H, 4.51; N, 7.17; S, 2.05. Found: C, 60.63; H, 3.58;
N, 6.64; S, 1.65.
8.31 (mc, H4,4 ), 8.58 (mc, Haryl), 8.69 (mc, Haryl), 8.77 (mc, Haryl),
00
0
0
9.18 (mc, H3,3 ), 9.47 (mc, H3 ,5 ).
Metallo-homo polymer P6: {[Zn(M6)](PF6)2}n. According to
the above-mentioned procedure, homo polymer P6 was ob-
tained after three times precipitation as yellow solid (23 mg,
Results and Discussion
48%).51 1H NMR (DMSO-d6, 300 MHz, δ): 0.75 (mc, CH3),
Synthesis and Characterization of Monomers M1-M7.
The synthetic route towards the ditopic bis(terpyridine)
monomers M1-M7 is illustrated in Scheme 1. The accep-
tor-type spacer units (1-7) were synthesized in multistep
procedures, starting from commercial available compounds
(see the Supporting Information for details). According
to Scheme 1, the aromatic dibromides 1-7 were reacted
with two equivalents of 40-(4-ethynylphenyl)-[2,20:60,200]ter-
pyridine (A) under Pd0-catalyzed Sonogashira cross-cou-
pling conditions. After precipitation from methanol and
column chromatographic purification, the bis(terpyridines)
00
0.98-1.48 (m, CH2), 4.40 (mc, O-CH2), 7.52 (mc, H5,5 ), 8.06
00
00
(mc, H6,6 ), 8.08 (mc, Haryl), 8.30 (mc, H4,4 ), 80.68 (mc, Haryl), 8.77
00
0
(mc, Haryl), 9.15 (mc, H3,3 ), 9.41 (mc, H3 ,5 ). Anal. Calcd for
104H102F12N8O8P2SZn: C, 63.11; H, 5.19; N, 5.66; S, 1.62.
Found: C, 62.47; H, 4.78; N, 5.37; S, 1.22.
C
Metallo-Homo Polymer P7: {[Zn(M7)](PF6)2}n. According to
the above-mentioned procedure, homo polymer P7 was ob-
tained as yellow solid (44 mg, 82%). 1H NMR (DMSO-d6, 300
MHz, δ): 0.76 (mc, CH3), 0.97-1.53 (m, CH2), 4.18 (mc,
00
00
O-CH2), 4.40 (mc, O-CH2), 7.55 (mc, H5,5 ), 8.04 (mc, H6,6 ),

Article
Macromolecules, Vol. 43, No. 6, 2010 2763
Figure 1. UV-vis absorption (solid lines) and normalized photoluminescence spectra (dashed line) of selected bis(terpyridines). For all spectra:
10-6 M in CHCl3, room temperature.
Table 1. Selected Photophysical Properties of Bis(terpyridines) M1-M7
ε ꢀ 104 a,b (M-1 cm-1
)
ΦPL
Stokes shift (cm-1
)
Egopt d (eV)
a,c
bis(terpyridine)
λabs,maxa (nm)
λPL,maxa (nm)
3
M1
M2
M3
M4
M5
M6
M7
423, 365, 316, 282
373, 325, 279, 254
395, 317, 267, 252
401(s), 354, 283, 253
498, 440, 346, 281, 255
433, 361, 322, 279, 255
427, 361, 328, 283
503
424
443
484, 509(s)
587
498
2.07
5.93
3.01
8.80
2.02
3.23
2.39
0.79
3760
3225
2743
4277
3045
3014
3011
2.62
3.00
2.83
2.74
2.17
2.58
2.56
0.63
0.68
0.37
0.69
0.65
0.57
490
a For all spectra: 10-6 M in chloroform; (s) = shoulder. b Extinction coefficients of the lowest-energy absorption band. c Absolute quantum yields,
uncorrected with respect to reabsorption. d Egopt = h c/λ0.1max
.
3
M1-M7 were obtained in moderate to good yields and fully
characterized by NMR, UV-vis absorption, and photolu-
minescence spectroscopy as well as MALDI-TOF mass
spectrometry and elemental analysis. The photophysi-
cal properties of M1-M7 are summarized in Figure 1 and
Table 1.
HOMO energy location with a simultaneous stabilization of
the LUMO energy location, which induces a significantly
lowered energy band gap and a strong red-shift of the
absorption, as observed. The dimension of this bathochro-
mic shift correlates well with the intensity of the intramole-
cular charge transfer.34 The π-extended systems M6 and M7
also exhibit a bathochromic shift as well as a lowered energy
band gap, in comparison to their analogues with smaller
spacer units (M1 and M2). In particular, the comparison of
M6 and M1 reveals that the influence of the π-extension is
partially compensated by a steric hindrance due to the alkyl
chains, which results only in a small red-shift of the absorp-
tion maximum of about 10 nm. The increase of the extinction
coefficient of M6 relative to M1 can be ascribed to an
increasing absorption cross section of the central chromo-
phores within the bis(terpyridines).33
In order to investigate the influence of the electron-accep-
tor spacers and the length of the π-conjugated system on the
photophysical properties of bis(terpyridines) M1-M7,
UV-vis absorption and photoluminescence spectra were
measured. In accordance with the literature, the absorption
spectra of the ditopic ligands M1-M7 featured two intense
band regions.2,9,10,32,33,52 The bands at about 250 to 350 nm
were assigned to the characteristic π-π* transitions of the
terpyridine moieties and show high extinction coefficients (ε)
up to 95,000 M-1 cm-1 23,28,33,52
Furthermore, the spectra
.
3
are characterized by an intensive band in the visible region
(λabs = 370-498 nm) corresponding to π-π* transitions of
the overall π-conjugated system with extinction coefficients
The photoluminescence spectra and absolute quantum
yields were obtained by excitation at the lowest-energy
absorption maxima of the bis(terpyridine) systems. In dilute
solution, emission maxima in the range of 392 to 587 nm,
up to 88 000 M-1 cm-1. However, in comparison to all other
3
synthesized systems, bis(terpyridine) M5 showed the most
pronounced bathochromic shift of the absorption maximum
and, thereby, the smallest optical energy band gap, although
M5 bears n-octyl chains which are known to disturbe rigidity
and planarity.48 This indicates a strong electronic interaction
of the lateral electron-rich p-dialkoxy-benzene substituents
with the electron-poor thieno[3,4-b]pyrazine unit, resulting
in a push-pull effect. In consequence of this interaction,
the quinoide contribution to the electronic structure of the
ground state is increased, resulting in a destabilization of the
corresponding to Stokes shifts between 2700 and 4500 cm-1
,
were observed. The large Stokes shift of bis(terpyridine)
M4 (4300 cm-1) can be explained by a more extended
conjugation in the excited state in comparison to the other
systems.33,50 Theoretical calculations on these systems
would help to investigate this behavior further.
In general, the photoluminescence properties are equally
influenced by the electronic structure as previously discussed
for the UV-vis absorption behavior. Again, bis(terpyridine)
M5 showed the most distinctive bathochromic shift of the

€
Schlutter et al.
2764 Macromolecules, Vol. 43, No. 6, 2010
Scheme 2. Schematic Representation of the Metallo-Polymerization of M1-M7 to the Homo Polymers P1-P7 (Top) and Random Copolymers R1
and R2 (Bottom)
emission maximum, thus the effects mentioned above have
to be considered again. As a consequence of its Stokes shift
(3000 cm-1), M5 emits with an intensive orange-red color
(λPL,max = 587 nm).
By incorporation of electron-rich dialkoxy-benzene sub-
units into the spacer (push-pull effect) and the resulting
elongation of the π-conjugation, the emission maximum of
M7 (bright green emission) is notably red-shifted in compari-
son to the structurally related bis(terpyridine) M2 (blue
emission).
random copolymers R1 and R2 (yields: 75% and 70%, res-
pectively). Because of the reactivity of ZnII ions and the
stability of the bis(terpyridine)-ZnII moieties, the self-assem-
bly process occurs under comparably mild conditions.23,24
In comparison to the monomers M2-M7 (M1 was hardly
soluble itself), all metallo-polymers showed a significantly
decreased solubility in common organic solvents. This behav-
ior is mainly caused by the highly linear-rigid structure of
the polymer backbone as well as by the involved charged
metal ions attached leading to solubility only in highly
polar aprotic solvents, e.g., DMSO, DMF, NMP, or acet-
onitrile.9,23,24
The molar mass of the resulting polymers could not yet be
determined because both size exclusion chromatography
(SEC) and MALDI-TOF mass spectrometry were not
usable for the characterization of ZnII-based metallo-poly-
mers. In contrast to related ruthenium(II)- or nickel(II)-
containing metallo-polymers,53 ZnII-based systems are not
stable under the measurement conditions owing to a con-
siderably weaker binding strength of the terpyridine ligand
to the metal ion.2
All ditopic ligands featured high quantum yields (ΦPL) in
the range of 40 to 80%. The highest quantum yields were
obtained for bis(terpyridines) M1 and M5.
Synthesis of the Zinc(II)-Containing Metallo-Polymers.
The synthesis of the ZnII-based metallo-polymers is depicted
in Scheme 2. The metallo-polymerization by self-assembly
was carried out according to methods described in litera-
ture.9,16,23 Bis(terpyridines) M1-M7 were heated with
zinc(II) acetate at an exact stoichiometric ratio of 1:1 in
N-methylpyrrolidone (NMP), followed by subsequent anion
exchange with ammonium hexafluorophosphate to yield the
metallo-homo polymers P1-P7. In the case of the random
copolymers (R1 and R2) donor-type bis(terpyridines)50 with
a dialkoxy-benzene (MD1) and a fluorene spacer unit
(MD2), respectively, were utilized in combination with the
acceptor-type bis(terpyridines) (M1 and M3). Here, both
ditopic ligands were used in a 1:1 ratio in order to obtain
metallo-copolymers with a 50% content of each monomer.
Since, no signals are separated in the NMR spectra of R1 and
R2, we could not apply a selective integration in order to get
information about the arrangement of the monomers
(alternating/random) along the polymer chains. For this
purpose, we assume a random distribution in both copoly-
mers (R1 and R2).
However, characterization by 1H NMR spectroscopy
indicated the formation of the desired metallo-polymers by
broadened signals of the terpyridine ligand, which is a
typically feature of polymeric materials, as well as by the
absence of the signals from the uncomplexed terpyridine
unit (Figure 2). Additionally, this behavior proves that the
polymeric structure remains unaffected in polar solvents.
Furthermore, and in accordance with the literature, a clear
downfield shift of the (5,500)-, (4,400)-, (3,300)- and (30,50)-
terpyridine signals, upon coordination to the ZnII ions, was
observed.9,19,23,24 The (6,600)-terpyridine signals are signifi-
cantly upfield shifted due to the location above the ring plane
of the adjacent ligand. These assignments are based on the
comparison of the spectra to those reported for related
homoleptic ZnII model complexes.9,24 A calculation of the
molar mass of the metallo-polymers via the integration of
The resulting polymers were purified by repeated precipi-
tation from NMP in diethyl ether and dried in vacuum,
leading to homo polymers P1-P7 (yield: 20-84%) and

Article
Macromolecules, Vol. 43, No. 6, 2010 2765
end group-signals was not possible due to the absence of
any end group-signals. However, considering the typical
limit of the NMR spectroscopy (≈ 5%), the synthesized
metallo-polymers should consist of more than 30 repeat-
ing units [for comparison, see ref 60]. Hence, the molar
mass of P1-P7 was estimated to be not less than 25 000 to
50 000 g/mol.
Additionally, elemental analysis of the obtained metallo-
polymers provides an indication that the relative composi-
tion is close to the expected stoichiometric ratio of 1:1 for the
ZnII ions and bis(terpyridine) ligands.
For further characterization of the homo polymers
P1-P7, UV-vis titration experiments were carried out to
confirm their supramolecular structure (Figure 3). It is
known from the literature that the formation of linear
metallo-polymers can be controlled by the exact stoichio-
metric ratio of metal ion to ditopic ligand.9,24,28 Upon
stepwise addition of FeII to a solution of bis(terpyridine)
M2, the absorption spectra revealed the appearance of a
metal-to-ligand charge-transfer (MLCT) band at 583 nm,
characteristic for iron(II)-based supramolecular assemblies.
Furthermore, a shift of three other absorption bands at 373,
333, and 260 nm could be observed. The intensity of the
MLCT band is increasing linearly up to a stoichiometric
ratio of 1:1, which indicates the formation of a metallo-
polymer.
The absence of any shoulder at the π-π* transition of
the whole π-conjugated system at 373 nm with increasing
addition of FeII ions accommodates to the fact that
an electron-withdrawing spacer unit is attached to the
terpyridine. Usually this circumstance is attributed to a
charge-transfer occurring between electron-rich spacer units
and the electron deficient metal-coordinated terpyridine
moiety.23,28
The titration experiments clearly revealed that at a 1:1
ratio of metal ion to bis(terpyridine) ligand a supramolecular
assembly is formed. Depolymerization beyond the point of
equivalence23,24 has not been observed in the present case
due to the stable FeII(tpy)2 moiety.
Electrochemical Properties. Applying cyclic voltammetry,
the energy levels of the HOMO, corresponding to the
ionization potential, and of the LUMO, corresponding to
the electron affinity, can be estimated. The electrochemical
properties of the homo polymers (P1-P7) and random
copolymers (R1 and R2) were obtained from thin films
of the materials coated on Pt wires and are summarized in
Table 2 and Figure 4.
All homo polymers (P1-P7) and both random copoly-
mers (R1 and R2) exhibited quasi-reversible reduction peaks
between -1.29 and -1.65 V at scans up to -2.5 V. In
agreement with the literature, these waves were attributed
to the reduction of the terpyridine moiety and the attached
π-conjugated spacer unit inside the polymers.23,24,28 Further-
more, most of the metallo-polymers featured distinct reduc-
tion waves at more negative potentials. As expected, no
significant oxidation processes were observed up to 1.5 V.
Because of the stable d10 electron configuration of the ZnII
metal centers, oxidation is difficult to observe.23,24,28 The
estimated LUMO levels are based on the first reduction wave
Figure 2. 1H NMR spectra (aromatic region) of bis(terpyridine) M3
(top, CDCl3), homo polymer P3 (middle, DMSO-d6) and random
copolymer R2 (bottom, DMSO-d6). The signals belonging to the
terpyridine moiety are assigned; the other signals correspond to the
π-conjugated spacer. For all spectra: 300 MHz, room temperature.
Figure 3. UV-vis absorption spectra acquired upon the titration of monomer M2 (10-5 M in chloroform) with FeCl2 (10-4 M in methanol). The
absorption at the MLCT band (583 nm) as a function of added FeCl2 solution is shown as an inset.

€
Schlutter et al.
2766 Macromolecules, Vol. 43, No. 6, 2010
Table 2. Electrochemical Properties of the Homo Polymers P1-P7 and the Random Copolymers R1 and R2, respectively
polymer
E1/2red a (V)
Eonsetred a (V)
LUMOb (eV)
HOMOc (eV)
Egopt d (eV)
P1
P2
P3
P4
P5
P6
P7
R1
R2
-1.44, -1.81, -2.05
-1.29, -1.60
-1.29, -1.62
-1.61, -2.16
-1.65, -2.19
-1.61, -2.01
-1.65
-1.69
-1.49
-1.49
-1.68
-1.63
-1.62
-1.45
-1.73
-1.54
-3.16
-3.36
-3.36
-3.17
-3.22
-3.23
-3.40
-3.12
-3.31
-5.75
-6.21
-6.14
-5.90
-5.30
-5.56
-5.91
-5.88
-6.22
2.59
2.85
2.78
2.73
2.08
2.33
2.51
2.76
2.91
-1.54, -1.87
-1.29, -1.66, -1.95
a Half-wave potential of the reduction waves (films (0.1 M (TBA)PF6 in CH3CN) coated on Pt wires, scan rate 50 mV s-1); potentials are reported
3
versus a Ag/AgI electrode (Ag in 0.1 M AgNO3 solution). b LUMO levels were calculated from the measured first reduction potential versus Fc/Fcþ
according to eq 1. c HOMO levels were calculated from the optical band gap Egopt and the respective LUMO levels. d Egopt = h c/λ0.1max
.
3
M1-M7 ; characteristic absorptions between 250 and
480 nm, where the transitions at higher energy correspond
to the terpyridine motive itself and the lower energy transi-
tions to absorptions of the whole π-conjugated system. As
expected, the ZnII cores do not participate in the transitions,
due to the filled d10 electron shell. Furthermore, a mostly
hypsochromic shift of the π-π* transition of 5 to 25 nm
compared to the free bis(terpyridines) was observed. As
reported for ZnII-based metallo-polymers with electron-
donating spacer units, a charge-transfer (CT) between
electron-rich central units and the metal-coordinated
electron-deficient terpyridine unit causes a red-shift in
absorption.55 In the present cases, the central chromophores
consist of electron-withdrawing groups which are not able to
undergo such CT processes with the terpyridine units. In-
ductive effects, which are caused by the complexed metal ion,
do not compensate the missing CT and, therefore, result in a
small blue-shift of the absorption bands. Additionally, the
UV-vis absorption data of the metallo-polymers revealed a
considerable decrease of the values of the extinction coeffi-
cients (Tables 1 and 2), which might be caused by a parallel
arrangement of the dipole moments of the bis(terpyridine)
units within the polymer backbone.24
Figure 4. Cyclic voltammograms of selected metallo-polymers
(cathodic scans are shown). For all measurements: films (0.1 M
(TBA)PF6 in CH3CN,) coated on Pt wires, scan rate 50 mV/s.
and on the reference energy level of ferrocene, according
to eq 1.54
Homo polymer P5 resembled this behavior impressively
with a decrease in the extinction coefficient of about one
order of magnitude. It simultaneously exhibits the highest
absorption wavelength (λabs = 485 nm) and, therefore, the
smallest distance between the HOMO and the LUMO
energy level. On the other hand, homo polymer P2 possesses
the lowest absorption wavelength (λabs = 362 nm), equiva-
lent with the largest HOMO-LUMO energy band gap. The
reference
ELUMO ¼ ½ -ðEonsetred -Eonset
Þ -4:8ꢁ eV
ð1Þ
The LUMO levels range between -3.12 and -3.40 eV.
Since no oxidation potentials could be determined, the
corresponding estimation of the ionization potential (i.e.,
the HOMO energy level) and the electrochemical band gap
could not be derived. Therefore, the optical energy band gaps
of the metallo-polymers were estimated from the UV-vis
absorption spectra by extrapolating to the 0-0 transition of
the longest wavelength absorption band (at 10% of the
absorption maximum). They ranged from 2.08 to 2.85 eV
and were strongly influenced by the nature of the attached
π-conjugated spacer unit. In order to obtain any information
about the HOMO levels of the ZnII metallo-polymers, the
opt
values for Eg were obtained by extrapolating the tails of
the lowest energy absorption edge to the 0-0 transition as
mentioned above.
The homo polymers (P1-P7) covered a wide range of PL
emission maxima from 424 to 586 nm (Figure 5, dashed line),
which strongly depends on the effective π-conjugation length
and push-pull effects in the case of homo polymer P5 (see
above). Also absolute photoluminescence quantum yields
(ΦPL) of the materials were determined. In the case of
the homo polymers, a decrease of ΦPL in comparison
to the ditopic terpyridine ligands M1-M7 upon complexa-
tion was observed (M1-M7, ΦPL = 0.37-0.79; P1-P7,
opt
calculated optical band gaps Eg and LUMO levels were
used for the estimation. HOMO levels from -5.30 to -6.22
eV were derived. Taking this into account, the photophysical
and electrochemical properties of the metallo-polymers can
be tuned by variation of the chromophore, which was shown
representatively for the small energy band gap of homo
polymer P5.
Photophysical Properties. Further investigation on the
photophysical properties of homo polymers P1-P7 and, in
particular, of the random copolymers R1 and R2, was carried
out by UV-vis absorption and photoluminescence spectro-
scopy. The spectroscopic data are summarized in Table 3
and Figures 5-7. In general, all metallo-polymers showed -
in agreement with the observations for the bis(terpyridines)
Φ
PL = 0.18-0.66). Whereas P7 showed only a third of the
value measured for the corresponding bis(terpyridine) M7,
only a small decrease of 15% was detected for P3 relative
to M3.
In comparison to conventional π-conjugated polymers
bearing similar backbone units, the metallo-polymers exhi-
bited higher PL quantum yields. The group of Klemm
obtained for similar thieno[3,4-b]pyrazine-bearing π-conju-
gated polymers ΦPL values in solution of about 0.15, which
correlates with a 3-fold larger value for P5.49,56 In addition,

Article
Macromolecules, Vol. 43, No. 6, 2010 2767
Table 3. Selected Photophysical Properties of the Homo Polymers P1-P7 and the Random Copolymers R1 and R2, respectively
ε ꢀ 104 a,b (M-1 cm-1
)
ΦPL
Stokes shift (cm-1
)
Egopt d (eV)
a,c
polymer
λabs,maxa (nm)
λPL,maxa (nm)
3
P1
P2
P3
P4
P5
P6
P7
R1
R2
415, 359, 321, 284
362, 325, 279
392, 322, 288
397 (s), 362, 331, 284
485 (s), 400 324 (s), 288
405, 317, 284
411, 326, 284
404, 337, 284
369, 320, 281
513
424
443
446
586
511
518
453, 470 (s)
409 (s), 432
0.99
2.74
1.75
2.35
0.33
2.69
2.24
9.07
11.25
0.66
4603
4039
2937
2767
3554
5122
5026
2677
3952
2.59
2.85
2.78
2.73
2.08
2.33
2.51
2.76
2.91
0.43
0.53
0.26
0.42
0.31
0.18
0.90
0.81
a For all spectra: 10-6 M in DMF; (s) = shoulder. b Extinction coefficients at the lowest-energy absorption band. c Absolute quantum yields,
uncorrected with respect to reabsorption. d Egopt = hc/λ0.1max
.
Figure 5. UV-vis absorption (solid line) and normalized photoluminescence spectra (dashed line) of selected homo polymers. For all spectra: 10-6
in DMF, room temperature.
M
Chen et al. reported ΦPL values of [2,1,3]benzothiadiazole-
and quinoxaline-bearing π-conjugated polymers of about
0.19 and 0.22, respectively.57 Compared to these π-conju-
gated polymers, metallo-polymers P1 and P3 (ΦPL = 0.66 to
0.53) emit much more efficiently.
Moreover, the random copolymers R1 and R2 were
characterized by UV-vis absorption and PL spectroscopy
in order to study the influence of a donor- and acceptor-
bridged bis(terpyridine) within one ZnII metallo-polymer
(Figures 6 and 7).
tion of the random copolymer R1 is predominated by the
donor-part. The same applies also for the PL emission of R1.
Excitation at the lowest energy absorption band of random
copolymer R1 (λex = 404 nm) prefers considerably the PL
emission of the electron-donating part. By means of varie-
gating the excitation wavelengths to smaller extinction dif-
ferences, e.g., λex = 440 nm (Figure 6, bottom), and by
measuring the PL emission of different molar ratios of the
ditopic monomers (MD1 and M1, see Figure 6, middle left),
the emission behavior of the random copolymer R1 could be
attributed mainly to the large extinction coefficient. At
wavelengths with almost equivalent extinction coefficients
(e.g., λabs = 440 nm) the PL emission of the acceptor is
dominating. This leads to the assumption that the very high
quantum yield of R1 (ΦPL = 0.90) is caused by both parts
and that an energy transfer from the donor to the acceptor
moiety within the random copolymer R1 is not favored.
Consequently, a second donor-acceptor pair was chosen
for random copolymer R2, which exhibits a preferably large
overlap of acceptor absorption with the emission of the
donor unit, in order to achieve similar energies for several
vibronic transitions of the donor and the acceptor, leading to
Therefore, the UV-vis absorption spectra of the indivi-
dual donor- and acceptor-based homo polymers (P858 and
P1, respectively) were compared to that of random copoly-
mer R1 (Figure 6, top). Additionally, the spectra of both
homo polymers were numerically summed up. The resulting
spectrum showed good conformity of the appearing bands
with the experimental spectrum of R1, but less accordance
with the measured extinction coefficients. The latter fact
might be a hint for a deviation from the exact 1:1 ratio of
the monomeric bis(terpyridines) (M1 and MD1) within the
present random copolymer R1, attributed to a small excess of
the MD1. The overall UV-vis absorption spectrum of R1 is
dominated by three bands between 250 to 420 nm, corre-
sponding to the aforementioned characteristic transitions.
Because of the large difference in the extinction coefficient
between donor-type (P8) and acceptor-type homo poly-
a resonant coupling. These are requirements to obtain an
€
36,37
energy transfer, in particular of the Forster-type (FRET).
From the materials at hand, the acceptor-based bis-
(terpyridine) M3 with a quinoxaline spacer-unit and a donor-
based bis(terpyridine) bearing a fluorene-moiety (MD2,58
see Scheme 2) provide these conditions. The position of the
mer (P1) (ε = 5.3 ꢀ 104 M-1 cm-1 and ε = 0.99 ꢀ 104
3
M-1 cm-1, respectively), the extinction of the π-π* transi-
3

€
Schlutter et al.
2768 Macromolecules, Vol. 43, No. 6, 2010
Figure 6. Top: UV-vis absorption spectra of random copolymer R1 compared to the homo polymers of the acceptor-based bis(terpyridine) (P1) and
the donor-based bis(terpyridine) (P8).58 Middle left: Normalized PL spectra obtained at different acceptor (M1) to donor (MD1) ratios. Middle right:
Normalized PL spectra of R1 compared to the homo polymers of the acceptor-based (P1) and the donor-based bis(terpyridine) (P8). Bottom:
Normalized PL spectra of R1 at different excitation wavelengths. For all spectra: (10-6 M in DMF).
PL emission maximum of the donor bis(terpyridine) (λPL
=
The resulting spectrum is almost identical to the measured
UV-vis absorption spectrum of random copolymer R2,
both in band positions and in the extinction coefficients.
This is an indication for an almost 1:1 ratio of the donor
and acceptor within the synthesized copolymer. As for R1,
the typical π-π* transitions of the terpyridine and the whole
π-conjugated system were obtained for R2. The longest
403 nm) is almost at the same wavelength as the π-π*
transition of the π-conjugated system of the acceptor M3
(λabs = 396 nm). Additionally, the extinction coefficients are
in a comparable order of magnitude.
Therefore, the UV-vis absorption spectra of both mono-
mers were measured and numerically added (Figure 7, top).

Article
Macromolecules, Vol. 43, No. 6, 2010 2769
Figure 7. Top:UV-vis absorption spectra ofR2compared to the acceptor-based bis(terpyridine) (M3) and the donor-based bis(terpyridine) (MD2).58
Middle left: Normalized PL spectra obtained at different acceptor (M3) to donor (MD2) ratios. Middle right: Normalized PL spectra of R2 at different
excitation wavelengths compared to the monomers M3 and MD2. Bottom: UV-vis absorption and photoluminescence (PL) spectra of random
copolymer R2 in the solid state (80 nm thickness). For all spectra: 10-6 M in DMF, respectively, spin coated from DMF solution (20 mg/mL, 1000 rpm,
100 s).
wavelength absorption band of random copolymer R2 is
located at 370 nm with a shoulder at 411 nm, induced by the
long wavelength flank of the acceptor system.
In order to gain further insight into the behavior of
random copolymer R2 at the excited state, the PL properties
were investigated. In a first experiment the donor- and
acceptor-monomers (MD2 and M3) were mixed in different
molar ratios (Figure 7, middle left) and excited at their
longest wavelength absorption maximum. At a 1:1 ratio
the emission originating from the donor bis(terpyridine)
(MD2) is slightly dominating due to the larger extinction
coefficient at 370 nm, which inverses with increasing accep-
tor portion. Consequently, the PL behavior of the mixtures
of MD2 and M3 is only an addition of both monomers and

€
Schlutter et al.
2770 Macromolecules, Vol. 43, No. 6, 2010
no energy transfer was observed. Nevertheless, excitation at
the longest absorption band wavelength of the random
copolymer R2 resulted in a completely different PL emission
spectrum (Figure 7, middle right). In this case, the emission
attributed to the acceptor-unit, though slightly red-shifted,
was clearly dominating. Only a shoulder confirms the emis-
sion originating from the donor-part. Both, decreasing as
well as increasing the excitation wavelength, lead to a further
reduction of the donor-based emission. At λex = 370 nm the
emission ratio of the acceptor- to the donor-unit is 1:0.85,
although the ratio of the extinction coefficients at this
particular wavelength (1:1.66) indicates a larger absorption
of the donor.
This circumstance can be explained by a radiationless
transfer of energy from the excited donor moieties in random
copolymer R2 to the electron-accepting units.36,37,59 This
leads to the excitation of the acceptor-parts, which are now
able to deactivate under emission of the transferred energy.36
The fact that a simple mixture of the involved monomers
(MD2 and M3) revealed no indication for an energy transfer
whereas the corresponding ZnII-based random copolymer is
able to do, shows that the central ZnII-terpyridine moieties
play a crucial role for the mediation of such a transfer.
In order to obtain information about the applicability of
ZnII metallo-polymers as OLED devices, the solid-state
properties are crucial. Therefore, the photophyiscal pro-
perties of random copolymer R2 in the solid state were
additionally investigated. Low solubility of all synthesized
metallo-polymers in common organic solvents was observed.
Spin-coating from DMF solution (20 mg/mL, 1000 rpm,
100 s) was applied, resulting in a homogeneous film (80 nm
thickness) of random copolymer R2 which was used for the
UV-vis absorption and photoluminescence measurements
(Figure 7, bottom). The spectra reveal a slight hypsochromic
shift (10 nm) of the absorption maximum in comparison to
those observed in solution. However, the PL maximum
shows a bathochromic shift of 90 nm in comparison to the
solution of random copolymer R2. In accordance with the
literature, this large Stokes shift of about 7000 cm-1 can
be attributed to strong intermolecular π-π-interactions of
the metallo-polymeric chains. This behavior results from a
very close orientation in the solid state, which prefers a dis-
turbance among themselves and destabilizes occupied orbi-
tals.24,28,30
obtain information about the potential applicability of ZnII
metallo-polymers as OLED devices.
The herein synthesized metallo-polymers (homo and random)
show promising properties with respect to potential opto-electro-
nic applications in OLED or PLED devices.
Acknowledgment. The authors acknowledge the financial
support from the Dutch Polymer Institute (DPI), the Neder-
landse Organisatie voor Wetenschappelijk Onderzoek (NWO,
VICI award for U.S.S.), the Fonds der Chemischen Industrie and
€
ꢀ
the Freistaat Thuringen. We also thank Vera Cimrova (cyclic
voltammetry: Institute of Macromolecular Chemistry, Academy
of Science of the Czech Republic, Prague, Czech Republic) and
Beate Lentvogt (elemental analysis) for the help with the respec-
tive measurements.
Supporting Information Available: Text giving full experi-
mental details and characterization of the aromatic dibromides
(1-7), bis(terpyridines) (M1-M7) and metallo-polymers
(P1-P7, R1, R2) including reaction schemes and structures,
and figures showing NMR and MALDI-TOF mass spectra of
bis(terpyridines) (M1-M7). This material is available free of
charge via the Internet at http://pubs.acs.org.
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