Tuning the electronic properties of conjugated polymer by tethering low-bandgap rhenium(I) complex on the main chain

Article abstract of DOI:10.1002/pola.23996

Low-bandgap rhenium(I) complex with absorption onset at 795 nm in solution was tethered onto π-conjugated polymer. The conjugated copolymer provides solution processability of the metallopolymer, and the pendant allows the low energy-absorbing Re(I) compl

Full text of DOI:10.1002/pola.23996

Tuning the Electronic Properties of Conjugated Polymer by Tethering
Low-Bandgap Rhenium(I) Complex on the Main Chain
CHRIS S. K. MAK, QING YUN LEUNG, CHI HO LI, WAI KIN CHAN
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
Received 21 December 2009; accepted 26 February 2010
DOI: 10.1002/pola.23996
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Low-bandgap rhenium(I) complex with absorption
onset at 795 nm in solution was tethered onto p-conjugated
polymer. The conjugated copolymer provides solution process-
ability of the metallopolymer, and the pendant allows the low
energy-absorbing Re(I) complex units to be evenly distributed
on the thin film. The copolymer tethered with low-bandgap
rhenium complex was synthesized by Suzuki cross-coupling
reaction. The metal-free polymer (poly-1) tethered with function-
alized intramolecular charge transfer dye, 2-phenyl-3-pyridin-2-
yl-5,7-di-2-thienylthieno[3,4-b]pyrazine, exhibited high molecu-
lar weight, good film-forming properties, and excellent solution
processability. The pendants of the conjugated polymer possess
donor–acceptor characters and broaden the absorption band.
These pendants can function as bidentate ligands for metal che-
lation. The solubilizing groups on the monomers provide good
solubility to the polymer even with high content of metal chela-
tion. Upon the complexation with rhenium(I) pentacarbonyl
chloride, the absorption spectrum of the resulting metallopoly-
mer was further extended toward the near-infrared region. Pho-
tovoltaic performances based on this metallopolymer have
been studied. The design approach of these metallopolymers
provides synthetic feasibility for coordinating wide range of
metal ions on the pendant, and the resulting low-bandgap poly-
mer can be a potential candidate for light harvesting material in
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solar cell applications. 2010 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 48: 2311–2319, 2010
KEYWORDS: charge transfer; conjugated polymers; low
bandgap; metal-containing polymers; metal–polymer com-
plexes; pendant; photophysics; rhenium(I)
INTRODUCTION The research and development on metal-
containing polymers, or metallopolymers, are widely studied
stemming from their intriguing properties. This kind of
materials combines the merits of the tunability of the redox
and photophysical properties of metal complex and the pro-
cessability of polymer, which is particularly important for
the low-cost large area device manufacturing in the field of
optoelectronics. Electrophosphorescent conjugated polymers
containing iridium(III)1–7 or ruthenium(II)8–11 metal centers
have been extensively studied for OLED applications.
Recently, metallopolymers were also used in photovoltaic
applications. For example, solution processable platinum(II)
polyynes have demonstrated impressive power conversion
efficiency in polymer solar cells.12–16 Besides, the intrinsic
high extinction coefficient of the metal-to-ligand charge
transfer (MLCT) transition of Ru(II) polypyridyl made it a
suitable candidate as photovoltaic material. Photovoltaic per-
formances based on the polymers with Ru(II) polypyridyl ei-
ther on the main chain17,18 or as pendant19 have also investi-
gated. Our previous works have demonstrated that the metal
complexes of the metallopolymer were capable of facilitating
the charge carrier mobility of the polymers by functioning as
extra charge carriers.20,21 Nevertheless, most of the metal-
containing polymers for optoelectronic applications in the
literature are based on Ir(III), Ru(II), and Pt(II) complexes;
less attention has been paid to the use of rhenium com-
pounds as photosensitizer, and report on low band-gap Re(I)
complex containing polymer is sparse.
Chlorotricarbonyl rhenium(I) diimine [Re(N^N)(CO)3Cl] com-
plexes are of particular interest because of their well-known
coordination chemistry, good electrochemistry, and unique
photophysical properties such as long excited-state lifetime
and high luminescent quantum yield.22 The simple coordina-
tion geometry of Re(N^N)(CO)3Cl allows the energy of the
MLCT transition to be easily fine tuned by modifying the di-
imine ligand via facile synthesis. The emissive nature of
3MLCT excited state and the reasonable luminescence quan-
tum yield of Re(N^N)(CO)3Cl (where N^N is 2,20-bipyridine
or 1,10-phenanthroline) allow it to be suitable emitter for
both small molecule and polymer OLED applications. Numer-
ous phosphorescent OLEDs based on Re(I) metal complexes
as emissive dopant have been reported.23–25 A conjugated
bipyridyl fluorene copolymer complexed with rhenium,
which exhibited electrophosphorescence in the red region,
was used for polymer OLED.26,27 Incorporation of fac-Re(N^N)
(CO)3Cl onto varies types of p-conjugated polymer backbone
such as polyfluorene,26,27 poly(p-phenyleneethynylene),28–30
and poly(phenylenevinylene)9,31 for the photophysical and
Additional Supporting Information may be found in the online version of this article. Correspondence to: W. K. Chan (E-mail: waichan@hku.hk)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2311–2319 (2010) 2010 Wiley Periodicals, Inc.
TETHERING LOW-BANDGAP RHENIUM(I) COMPLEX, MAK ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic route to the low-bandgap Re(I) model complex. Reagents and conditions: (i) EtOH, N2, RT, 3 days, 84%; (ii)
rhenium(I) pentacarbonyl chloride, toluene, reflux, 16 h, 74%.
electroluminescent studies has been reported. On the con-
trary, the properties for the photovoltaic material are essen-
tially opposite to that of the OLED material. Materials with
high extinction coefficient, board absorption throughout the
visible and near-infrared region, nonemissive excited state,
and short excited-state lifetime for efficient charge separa-
tion are desired for efficient photovoltaic cells. The well-
known emissive Re(I)-containing polymers reported in the
literature do not fulfill the requirements. In this article, we
aim to explore new type of low-bandgap metallopolymer
based on Re(I) complex. The synthesis and characterizations
of a conjugated copolymer tethered with a novel low-
bandgap Re(I) complex are reported. The incorporation of
change-transfer ligand with low absorption energy to the
Re(I) leading to a low-bandgap and nonemissive complex.
Detailed electrochemical and photophysical properties have
been studied on the model complex and the metallopolymer.
Theoretical calculation has been performed on the model
complex to have a better understand on the electron distri-
bution and transition origins. We also anticipate the opportu-
nity of the use of this polymer as light harvesting material in
photovoltaic devices.
NaHCO3/MgSO4 buffer solution at room temperature. It is
suggested that the acid formation is more favored because
the diketone intermediate undergoes oxidative cleavage
faster than its formation. Besides, the better solubility of the
diketone intermediate in polar solvent than that of the par-
ent alkyne leads to the diketone intermediate have a better
contact with the oxidant.33 Condensation of the resulting dis-
symmetric diketone with 30,40-diamino-2,20:50,200-terthio-
phene in ethanol at room temperature for 3 days gave a pur-
ple solid of the cyclized product 3 in an excellent yield of
93%. The nitrogen atoms on the pyridyl ring and the pyraz-
ine core can function as the coordination sites for metal
ions. Alkylation of 2,5-dibromo-4-octyloxyphenol with
3
under basic condition afforded monomer 4 in 90% yield.
Demetallation was observed by the color change of the reac-
tion mixture from dark green to purple when using com-
plexed monomer 4 in the present of base during polymeriza-
tion. Therefore, polymerization was carried out before the
complexation. Polymerization of 4 with fluorenyl diboronic
ester via Suzuki cross coupling afforded a purple fibrous
polymer, poly-1. The Mw was determined by gel permeation
chromatography (GPC) in THF at 35 ꢀC and found to be
81,500 g molꢁ1 with a polydispersity index (PDI) of 2.6
against polystyrene standards. The degree of polymerization
(DP) estimated from Mn was 26. Complexation of poly-1
with Re(CO)5Cl in refluxing toluene provided a green poly-
mer, poly-2. The solubility of the resulting metal-containing
polymer is enhanced by the long aliphatic chains on both
monomers, and it is readily soluble in common organic sol-
vents such as CHCl3, 1,1,2,2-tetrachloroethane, THF, and
chlorobenzene.
RESULTS AND DISCUSSION
Synthesis and Characterization
Our synthetic routes toward the low-bandgap rhenium(I)
model complex, the tethered intramolecular charge transfer
(ICT) monomer, and the corresponding metal-containing
polymer are presented in Schemes 1 and 2. The synthesis of
1 was carried out by the condensation of 2-phenyl-3-pyri-
din-2-yl-5,7-di-2-thienylthieno[3,4-b]pyrazine with 30,40-di-
amino-2,20:50,200-terthiophene in ethanol at room temperature
The structure and the degree of complexation of the Re(I)
metal ion with the functionalized poly-1 were analyzed by
proton NMR and FTIR spectroscopies. The NMR spectrum of
poly-1 shows a characteristic proton signal in the aromatic
region at d 8.47 ppm, which originates from the deshielded
proton sits next to the nitrogen atom on the pyridyl ring.
The incorporation of Re(CO)3Cl in poly-2 is evidenced by
the shift of this proton absorption from d 8.47 to 9.04 ppm
as the pyridyl ring becomes more electron deficient after the
metal coordination. The peak assignment of the polymers
was confirmed by comparing the chemical shifts of the free
ligand (8.45 ppm) and the model complex (9.09 ppm) (Sup-
porting Information). A slight excess of Re(I) precursor was
used for the complexation with poly-2 to ensure all the
available coordination sites on the polymer are occupied.
for
3 days. Dark purple needle-shaped crystals were
obtained as product by recrystallization. Treatment of 1 with
Re(CO)5Cl in refluxing toluene afforded the dark green rhe-
nium model complex Re-1 with an overall yield higher than
60%.
The synthesis of monomer 4 bearing ICT pendant was
started with the oxidation of 1-(10-bromodecyloxy)-4-(2-pyr-
idylethynyl)benzene with potassium permanganate to the
dissymmetric diketone 2 according to the modified literature
method.32 The low product yield (50%) was due to the oxi-
dative cleavage of the diketone product to their correspond-
ing carboxylic acids by potassium permanganate, even with
the careful pH control of the reaction mixture by using
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SCHEME 2 Synthetic route to the
conjugated polymer with low-
bandgap Re(I) complex as pend-
ant. Reagents and conditions: (i)
KMnO4, NaHCO3, MgSO4, ace-
tone, RT, 50%; (ii) 30,40-diamino-
2,20:50,200-terthiophene, EtOH, N2,
RT, 3 days, 93%; (iii) 2,5-dibromo-
4-octyloxyphenol,
K2CO3,
KI,
DMF, reflux, 16 h, 90%; (iv) 1: 2,7-
di(4,4,5,5-tetramethyl-1,3,2-dioxa-
boralane)-9,9-diethylhexylfluorene,
Pd(PPh3)4, TEAOH, toluene, 90 ꢀC,
16 h, 2: bromobenzene, 90 ꢀC, 2 h,
3: phenylboronic acid, 90 ꢀC, 2 h,
70%; (v) rhenium(I) pentacarbonyl
chloride, toluene, reflux, 16 h,
74%.
The disappearance of the proton signal at d 8.47 ppm indi-
cated that the reaction was complete and the Re(I)-ICT unit
complexation was highly efficient. Elemental analysis on
poly-2 showed that the percentage of rhenium coordination
(anal. calc. for Re, 12.19; found Re, 10.49) is >98%. The
coordination of Re(CO)3Cl on the polymer was further con-
firmed by FTIR spectroscopy. Three prominent bands (1904–
2021 cmꢁ1) correspond to the CO stretching of the rhenium
carbonyl moieties in the facial configuration were observed
in the infrared spectrum of poly-2, while these were not
shown in poly-1.
cupied molecular orbital (LUMO) and the bandgap of these
compounds can be selectively fine tuned by varying the pe-
ripheral substituents on the acceptor unit. Thus, the elec-
tronic properties of the ICT unit can be determined by the
electron density of the acceptor, which can be perturbed by
the peripheral substituents with electron-donating or elec-
tron-withdrawing nature.34 The charge transfer (CT) absorp-
tion energy of poly-1 is the same as that of 1; this observa-
tion reveals that the p system of the conjugated polymer
does not influence the CT state of the pendant unit. In con-
trast, the complexation of Re(CO)3Cl with the ICT unit has
greatly affected the electronic structure of the thieno[3,4-
b]pyrazine ligand as observed in the absorption spectra of
Re-1 and poly-2. The ICT transition band exhibited a signifi-
cant red shift from 555 to 633 nm in chloroform upon metal
chelation. The shift is attributed to the increase in electron
delocalization from the thienopyrazine unit to the pyridine
unit, which is forced to be coplanar with the thienopyrazine
with metal coordination. This observation is consistent with
electron density plot from the DFT calculation. A new
absorption peak originated from MLCT (5dp(Re)-p*(ICT))
transition was observed at ꢂ490 nm. The MLCT absorption
band was overwhelmed by the leading edge of the p–p* tran-
sition of the conjugated polymer backbone in poly-2.
Optical and Electrochemical Properties
The absorption spectra of 1, poly-1, Re-1, and poly-2 are
illustrated in Figure 1. For the metal-free materials, 1 and
poly-1, two distinct chromophoric units were observed. The
strong absorption peak at around 350 nm originates from
the p–p* transition of the conjugated polymer backbone and
the ligand centered (LC, p–p*) transition of the pendant,
whereas the broad absorption band in the visible region is
ascribed to the ICT transition from the terthiophene donor
to the thienopyrazine acceptor. The tunability of photophysi-
cal and electrochemical properties of a series of analog com-
pound of 1 was reported.34 The energy of the lowest unoc-
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Theoretical calculation was performed on the ground-state
geometrical structures of the model compounds to under-
stand the electronic transition properties of the low-bandgap
material, which is influenced mainly by the ground-state
electronic structure. As the optical properties of poly-1 and
poly-2 in the visible region were determined by the low-
bandgap pendant, and the coordination geometry of Re(I)
metal center on the polymer pendant and the model complex
is the same, therefore, the calculation was performed on the
free ligand (1) and model metal complex (Re-1) for simplic-
ity. The single ground-state (S0) geometrical structures of 1
and Re-1 were optimized at the level of density functional
theory (DFT) using the B3LYP functional and a LANL2DZ ba-
sis set, which is known to yield reliable ground state geome-
tries [Fig. 2(a)].35 The pyridyl ring is twisted (28.8ꢀ) on the
free ligand and is forced to be coplanar with the pyrazine
unit upon the coordination of Re(I) metal center. The
ground-state coordination geometry of Re-1 is a distorted
octahedral geometry. One of the thiophene rings on the ter-
thiophene of the ICT ligand located near the Re(I) metal cen-
ter is twisted out of plane of the ligand with an angle 60ꢀ
due to the steric hindrance of the metal coordination.
FIGURE 1 Absorption spectra of 1 (solid line), Re-1 (dashed
line), poly-1 (dotted line), and poly-2 (dash-dotted line) in
CHCl3. The solution color of poly-1 and poly-2 is displayed in
the inset photographs.
FIGURE 2 (a) Optimized ground-
state geometrical structures of
1
and Re-1 at the B3LYP/LANL2DZ
level. (b) Molecular orbital energy
level diagram accompanied with
the electron density plots of 1 and
Re-1 calculated at the B3LYP/
LANL2DZ level. The energies of the
highest occupied molecular orbital
(HOMO), the lowest unoccupied
molecular orbital (LUMO), HOMO – 1,
and LUMO þ 1 were corresponding to
theenergyscalebarontheleft.
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ARTICLE
ferent polarity, indicating its CT nature (Fig. 3). The ICT
absorption peak of Re-1 spanned from 609 nm in methanol
to 633 nm in chloroform. This small shift shows that the dif-
ference of the dipole moment between the ground state (lg)
and the excited state (le) is small. The hypsochromic shift
with increasing solvent polarity reveals that the ground state
is more polar than the excited state, that is, lg > le (nega-
tive solvatochromism).37,38 The MLCT transition band
showed similar shift as the ICT transition. The MLCT absorp-
tion maxima were blue shifted from 490 nm in chloroform
to 468 nm in methanol. The plot of absorption maxima (nm)
for both transitions versus Reichard’s ET(30) solvent polarity
parameter is shown in the inset of Figure 3. There is a linear
correlation among all the solvents with the absorption wave-
length. The slope indicates the sensitivity of the transition
toward the solvent polarity, and both transitions exhibited
similar sensitivity to solvent polarity.
FIGURE 3 Electronic absorption spectra of Re-1 in various or-
ganic solvents. The inset shows the plot of absorption maxima
of the intramolecular charge transfer (square) and metal-to-
ligand charge transfer (triangle) transitions of Re-1 versus em-
pirical solvent polarity scale ET(30).
Cyclic voltammetry was used to examine the electrochemical
behavior and electronic properties of the polymers. Thin film
of poly-1 or poly-2 was coated on glassy carbon working
electrode, and the cyclic voltammograms are illustrated in
Figure 4. Both polymer films exhibited distinct oxidation and
reduction processes. An irreversible oxidation peak Eoxid
occurs at þ0.99 V versus ferrocene/ferrocenium (Fc/Fcþ) re-
dox couple for poly-1 during anodic scanning, which corre-
sponds to the oxidation of the p-system of the polymer back-
bone. A reversible reduction couple was observed at ꢁ1.55
V versus Fc/Fcþ, which is originated from the ICT pendant.
The tethered Re(I) complex of poly-2 showed an irreversible
oxidation at þ0.79 V versus Fc/Fcþ and one quasi-reversible
reduction at ꢁ1.26 V versus Fc/Fcþ. The HOMO and LUMO
were determined by the onset potential of the oxidation and
reduction peaks, respectively. The HOMO level energy for
poly-1 is ꢁ5.39 eV, which is similar to that of poly-2 at
ꢁ5.25 eV. The LUMO energy of poly-2 (–3.64 eV) is lower
than that of the poly-1 (–3.25 eV) with the coordination of
The electron density plots and the energy levels of the high-
est occupied molecular orbital (HOMO) – 1, HOMO, LUMO,
LUMO þ 1 of 1 and Re-1 are shown in Figure 2(b). For the
free ligand 1, the electron density of the HOMO level is
mainly localized on the terthiophene donor, and the electron
density of the LUMO level is on the pyrazine unit. This calcu-
lated electron distribution reveals that the lowest energy
transition is the ICT transition from the terthiophene donor
(HOMO) to the pyrazine acceptor (LUMO). The CT nature of
the HOMO–LUMO transition is supported by the solvatochro-
mic shifts of the transition band at 550 nm (Supporting In-
formation). Similar electron density distribution of the
HOMO and LUMO, which is mainly contributed by the ICT
ligand, was observed for Re-1. The HOMO and LUMO ener-
gies of Re-1 are lower than that of the 1 because of the sta-
bilization by the metal coordination. The HOMO–LUMO levels
of the ICT ligand were perturbed by the Re(I) metal center
with the electron density contributed from the Re(CO)3Cl in
the model complex [Fig. 2(b)]. The electron density distribu-
tion of the HOMO ꢁ 1 level is exclusively on the Re(CO)3Cl,
whereas the LUMO þ 1 is mainly localized on the ligand.
The MLCT transition from the Re dp orbital to the ligand p*
therefore has higher energy than the ICT transition. The sin-
glet ground-state DFT calculation has a good agreement with
the observation in the optical absorption spectrum.
As the solubility of poly-2 in polar solvents such as metha-
nol, acetone, and DMF was limited by the conjugated poly-
mer backbone, the solvatochromic effect was studied by
using the model complex Re-1, which showed similar optical
properties as the Re(I) complex attached as the pendant of
poly-2. Solvent effect on the spectral shift of the absorption
bands in the UV–vis spectra was studied with the use of em-
pirical solvent polarity scale ET(30).36 From the absorption
spectra of Re-1, both transition bands in the visible region
exhibited solvatochromic shifts in various solvents with dif-
FIGURE 4 Cyclic voltammograms of poly-1 (solid line) and
poly-2 (dashed line) thin films with scan rate of 100 mV sꢁ1
(vs. Fc/Fcþ) in acetonitrile.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
layer. Tapping-mode atomic force microscopy (AMF) was
used to study the thin film morphology. As the exciton diffu-
sion length of organic materials is generally around 10 nm,
therefore, a good interpenetrating network of the donor and
acceptor is essential to ensure charge separation can take
place at the interfaces. However, the AFM image of the bulk-
heterojunction thin film (Fig. 6) revealed a coarse phase sep-
aration and large aggregates (<200 nm). Poor mixing of the
polymer with the PCBM was observed, which may account
for the low current density of the devices.
EXPERIMENTAL
Measurements
1H and 13C NMR spectra were recorded on Bruker DPX-300
(300 and 75 MHz, respectively) and Bruker DRX-400 (400
and 100 MHz, respectively) NMR spectrometers. Positive-ion
electron impact (EI) mass spectra were recorded on a Finni-
gan MAT 95 mass spectrometer. Cyclic voltammetry was car-
ried out using a eDaq potentiostat and recorded by e-corder.
A Teflon shrouded 3 mm glassy carbon electrode was used
as working electrode, and platinum foil and Ag/AgCl elec-
trode were used as counter electrode and pseudo reference
electrode, respectively. The reference potential was calibrated
internally with ferrocene (Fc/Fcþ). The solvent was anhy-
drous and deoxygenated with Ar before the measurement. A
total of 0.1 M of tetrabutylammonium tetrafluoroborate was
used as supporting electrolyte, and the scan rate of all the
measurements was 100 mV sꢁ1. Solution and thin-film UV–
vis spectra were obtained using a Varian Cary 50 Bio spec-
trophotometer. The pure polymer thin films were prepared
by drop-casting the polymer solution (CHCl3) on the clean
ITO glass substrates. The number-average (Mn) and weight-
average (Mw) molecular weights were determined by GPC at
FIGURE 5 Current density–voltage characteristics of the photo-
voltaic cells with device structure ITO/PEDOT:PSS/poly-2:PCBM
[1:4 (solid line) or 1:2 (dashed line)]/Al under AM1.5 solar simu-
lation and in dark (dotted line).
Re(I) onto the ICT ligand. The electrochemical bandgaps of
poly-1 and poly-2 are 2.14 and 1.61 eV, respectively, which
is in good agreement with the optical bandgap for poly-1
(1.82 eV) and poly-2 thin films (1.49 eV) determined by UV–
vis spectroscopy (Supporting Information, Fig. S9).
Photovoltaic Properties
Bulk heterojunction solar cells based on the poly-2 and
[6,6]-phenyl-C61-butyic acid methyl ester (PCBM) were fabri-
cated, and the photovoltaic performances were examined
under simulated AM 1.5 solar illumination (100 mW cmꢁ2
)
and in the dark. Poly-2 and PCBM were used as electron do-
nor and electron acceptor, respectively. A layer of PEDOT:PSS
was spin-coated on the ITO to facilitate the hole transport.
Thin films (80 nm) of the poly-2/PCBM bulk-heterojunction
layer were prepared from chlorobenzene solution. The
4
ꢀ
35 C against the polystyrene standard with ISCO V absorb-
ance detector using THF as eluent. The AFM measurements
were performed under ambient condition using an Asylum
Research Institute MFP 3D instrument. The amplitude and
ꢀ
resulting films were annealed at 80 C under vacuum for 30
min. The current density–voltage (JꢁV) characteristics were
measured at ambient condition in a device configuration of
ITO/PEDOT:PSS/poly-2:PCBM (1:4 or 1:2)/Al, and the plot
of the J–V curves is shown in Figure 5. The fill factor (FF)
and open-circuit voltage (Voc) of the devices with two differ-
ent blending ratios are similar. Under AM 1.5 illumination (1
Sun), device with poly-2:PCBM (1:2) exhibits short circuit
current density (Jsc) ¼ 80 lA cmꢁ2, Voc ¼ 920 mV, and FF ¼
0.23, resulting in a PCE of 0.02%. The Jsc was increased by
78% to 0.13 mA cmꢁ2 with the PCE of 0.03% with the
blending ratio of poly-2:PCBM increased to 1:4. The large
increased in Jsc suggested that the increased amount of
PCBM facilitated the charge transport within the thin film.
The low PCE may due to the relatively low absorptivity of
the CT transition in the visible range compared with the fully
allowed p–p* transition in the ultraviolet region (Supporting
Information Fig. S8). Thus, the inadequate light harvesting
limits the charge generation and hence lowers the power
conversion efficiency. Besides, the surface morphology may
also account for the poor charge transport within the active
FIGURE 6 Tapping-mode AFM image of poly-2:PCBM (1:4) thin
film. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
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phase of the oscillating cantilever operating in tapping mode
were simultaneously monitored. The scanning rate and the
drive frequency were 1–2 Hz and ꢂ650 kHz, respectively.
mg, 2.42 mmol) dissolved in water (27 mL) were added to
the reaction mixture. Potassium permanganate (770 mg, 4.83
mmol) was added in one portion, and the reaction was
monitored by silica TLC to ensure the completeness. Mini-
mum amount of sodium hydrogen sulfite was added until
complete reduction of the excess potassium permanganate.
The crude was extracted with DCM (3 ꢃ 10 mL), and the
separated organic layer was washed with brine (3 ꢃ 30 mL).
The combined organic layer was dried over MgSO4. The sol-
vent was removed under vacuum, and the crude product
was purified by column chromatography on silica gel using
hexane/ethyl acetate (3:1) as eluent and afforded desired
product (270 mg, 50%) as white solid.
Syntheses
All reagents were purchased from Aldrich, Strem, and Lan-
caster and used as received. All reactions were preformed
under a nitrogen atmosphere unless specified otherwise.
HPLC grade acetonitrile (Fluka) was used, and dichlorome-
thane and hexane were dried from alumina columns. Column
chromatography was performed using Merck 9385 silica gel
60 (0.040–0.063 mm).
Synthesis of 1
30,40-Diamino-2,20:50,200-terthiophene (0.28 g, 1.10 mmol) and
1-phenyl-2-pyridin-2-yl-ethane-1,2-dione (0.21 g, 0.99 mmol)
were dissolved in ethanol (80 mL). The mixture was stirred
at room temperature under N2 for 3 days. The solvent was
removed under reduced pressure to give a purple solid. The
crude product was then purified by silica column chromatog-
raphy with dichloromethane/hexane (1:1) as eluent and fol-
lowed by recrystallization from toluene/MeOH to afford a
purple needle product (0.38 g, 84%).
1H NMR (300 MHz, CDCl3): d 1.15–1.50 (m, 12H), 1.70–1.90
(m, 4H), 3.40 (t, 2H, J ¼ 6.8), 4.02 (t, 2H, J ¼ 6.5 Hz), 6.95 (d,
2H, ArH, J ¼ 8.8 Hz), 7.51 (ddd, 1H, ArH, J ¼ 7.1, 4.8, 1.1 Hz),
7.86–7.97 (m, 3H, ArH), 8.19 (d, 1H, ArH, J ¼ 7.8 Hz), 8.68
(d, 1H, ArH, J ¼ 4.7 Hz). 13C NMR (100 MHz, CDCl3): d 25.9,
28.1, 28.7, 29.0, 29.2, 29.3, 29.4, 32.8, 34.1, 68.4, 114.7,
123.2, 126.1, 128.0, 132.1, 137.2, 149.9, 151.9, 164.5. HRMS-
EI (m/z): Mþ calcd for C23H28O3N1Br1, 445.1247; found,
445.1242. MS-EI m/z (% relative intensity): 445 (3), 339
(28), 149 (68), 121 (100).
1H NMR (400 MHz, CDCl3): d 7.11 (dd, 1H, ArH, J ¼ 1.3, 4.2
Hz), 7.12 (dd, 1H, ArH, J ¼ 1.3, 3.7 Hz), 7.30–7.34 (m, 4H,
ArH), 7.38 (dd, 1H, ArH, J ¼ 1.1, 4.0 Hz), 7.40 (dd, 1H, ArH,
J ¼ 1.1, 4.0 Hz), 7.53–7.55 (m, 2H, ArH), 7.68–7.70 (m, 2H,
ArH), 7.84 (td, 1H, ArH, J ¼ 1.8, 7.8 Hz), 8.01 (dt, 1H, ArH,
J ¼ 1.0, 7.8 Hz), 8.45 (dq, 1H, ArH, J ¼ 0.9, 4.8 Hz). 13C NMR
(75 MHz, CDCl3): d 123.8, 124.9, 125.2, 125.4, 125.5, 126.0,
127.15, 127.18, 127.7, 127.8, 128.2, 128.3, 129.1, 130.0,
134.9, 135.0, 137.1, 137.4, 138.1, 139.5, 149.1, 151.9, 153.5,
157.9. MS-EI m/z (% relative intensity): 453 (100%) [Mþ],
279 (24%). Anal. Calcd. for [C25H15N3S3]: C, 66.20; H, 3.33;
N, 9.26; found: C, 65.85; H, 3.35; N, 9.27.
Synthesis of Compound 3
30,40-Diamino-2,20:50,200-terthiophene (33 mg, 0.13 mmol) and
2 (47 mg, 0.11 mmol) were dissolved in ethanol (40 mL).
The mixture was stirred at room temperature under N2 for 3
days. The solid was filtered and washed with MeOH. The
crude product was then purified by silica column chromatog-
raphy with DCM/hexane (1:1) afforded a sticky purple solid
(67 mg, 93%).
1H NMR (300 MHz, CDCl3): d 1.34–1.48 (m, 12H, 6 ꢃ CH2),
1.76–1.93 (m, 4H, 2 ꢃ CH2), 3.43 (t, 2H, CH2, J ¼ 6.9 Hz),
4.00 (t, 2H, CH2, J ¼ 6.5 Hz), 6.85 (d, 2H, ArH, J ¼ 8.8 Hz),
7.10–7.13 (m, 2H, ArH), 7.33 (dd, 1H, ArH, J ¼ 6.1, 7.5 Hz),
7.41 (dt, 2H, ArH, J ¼ 0.9, 4.9 Hz), 7.50 (d, 2H, ArH, J ¼ 8.8
Hz), 7.70 (dt, 2H, ArH, J ¼ 0.9, 3.7 Hz), 7.88 (dt, 1H, ArH, J ¼
1.8, 7.7 Hz), 8.00 (d, 1H, ArH, J ¼ 7.8 Hz), 8.54 (d, 1H, ArH,
J ¼ 4.8 Hz). 13C NMR (75 MHz, CDCl3): d 26.4, 28.6, 29.1,
29.6, 29.7, 29.75, 29.8, 33.2, 34.4, 68.4, 114.4, 118.6, 123.8,
124.9, 125.3, 125.9, 127.01, 127.05, 127.6, 127.8, 131.6,
132.0, 135.0, 135.1, 137.1, 137.3, 138.2, 149.2, 151.9, 152.9,
158.2, 160.3, 226.9. HRMS-EI (m/z): Mþ calcd for
Synthesis of Re-1
Rhenium pentacarbonyl chloride (176 mg, 0.49 mmol) and
L-1 (227 mg, 0.49 mmol) were dissolved in toluene (50 mL)
and heated to reflux under N2 overnight. The dark green
mixture was cooled down to room temperature. The solvent
was removed under reduced pressure. The crude product
was washed with MeOH and diethyl ether and then purified
by recrystallized from DCM/MeOH to afford a dark green
solid (336 mg, 74%).
1H NMR (400 MHz, CDCl3): d 7.14 (dd, 1H, ArH, J ¼ 3.7, 5.1
Hz), 7.22 (dd, 1H, ArH, J ¼ 3.6, 5.2 Hz), 7.37 (dt, 1H, ArH, J
¼ 0.9, 7.8 Hz), 7.41–7.54 (m, 6H, ArH), 7.57 (dd, 1H, ArH, J
¼ 1.1, 5.2 Hz), 7.57 (dd, 1H, ArH, J ¼ 1.1, 5.2 Hz), 7.67 (td,
1H, ArH, J ¼ 1.5, 8.0 Hz), 7.72 (dd, 1H, ArH, J ¼ 1.1, 5.2 Hz),
7.84–7.86 (m, 2H, ArH), 9.09 (dq, 1H, ArH, J ¼ 0.7, 3.9 Hz).
MS-FAB m/z (%): 759 [M]þ, 723 [M-Cl]þ, 640 [M-3CO-Cl]þ,
306 [M-ICT]þ. Anal. Calcd. for [C28H15ClN3O3ReS3]: C, 44.29;
H, 1.99; N, 5.53; found: C, 44.36; H, 2.04; N, 5.63.
C
35H34O3N3S3Br1, 687.1042; found, 687.1024. MS-EI m/z (%
relative intensity): 690 (30%) [Mþ], 608 (100%) [M-Br]þ,
468 (58%) [M-(CH2)10Br]þ. Anal. Calcd. for [C35H34
-
O3N3S3Br1]: C, 61.03; H, 4.98; N, 6.10; found: C, 61.34; H,
5.03; N, 6.14.
Synthesis of Compound 4
2-[4-(10-Bromo-decyloxy)-phenyl]-3-pyridin-2-yl-5,7-dithio-
phen-2-yl-thieno[3,4-b]pyrazine 3 (67 mg, 0.10 mmol), 2,5-
dibromo-4-octyloxy-phenol (37 mg, 0.10 mmol), potassium
carbonate (54 mg, 0.39 mmol), and catalytic amount of po-
tassium iodide were added to DMF (5 mL). The resulting
suspension was heated to reflux under N2 overnight. The
purple mixture was cooled to RT. Distilled water was added
Synthesis of Compound 2
1-(10-Bromodecyloxy)-4-(2-pyridylethynyl)benzene (500 mg,
1.21 mmol) was dissolved in acetone (45 mL). Sodium
hydrogen carbonate (61 mg, 0.72 mmol) and MgSO4 (595
TETHERING LOW-BANDGAP RHENIUM(I) COMPLEX, MAK ET AL.
2317

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
(50 mL), and the suspension was acidified with dilute HCl.
The mixture was extracted with DCM (3 ꢃ 50 mL). The or-
ganic extracts were dried over MgSO4, and the solvent was
removed under reduced pressure. The crude product was
purified by silica column chromatography with DCM as elu-
ent to give a sticky purple solid (87 mg, 90.3%).
gen overnight. The dark green solution was cooled down to
room temperature and poured into distilled water (50 mL).
The product was extracted with DCM (3 ꢃ 50 mL). The or-
ganic extracts were dried over MgSO4, and the solvent was
removed under vacuum. The dark green solid was redis-
solved in minimum amount of CHCl3, and the polymer was
precipitated from MeOH. The reprecipitation was repeated
twice and afforded the product as green solid. (52 mg, 74%).
1H NMR (300 MHz, CDCl3): d 0.86–0.91 (m, 3H, CH3), 1.26–
1.48 (m, 22H, 11 ꢃ CH2), 1.78–1.82 (m, 6H, 3 ꢃ CH2), 3.92–
3.99 (m, 6H, 3 ꢃ CH2), 6.82 (d, 2H, ArH, J ¼ 8.8 Hz), 7.08
(s, 2H, ArH), 7.10–7.13 (m, 2H, ArH), 7.28–7.32 (m, 1H,
ArH), 7.38 (t, 2H, ArH, J ¼ 4.6 Hz), 7.48 (d, 2H, ArH, J ¼ 8.8
Hz), 7.68 (t, 2H, ArH, J ¼ 3.1 Hz), 7.85 (dt, 1H, ArH, J ¼ 1.7,
7.7 Hz), 7.96 (d, 1H, ArH, J ¼ 7.8 Hz), 8.50 (d, 1H, ArH, J ¼
4.3 Hz). 13C NMR (75 MHz, CDCl3): d 14.5, 23.1, 26.3, 26.4,
29.5, 29.6, 29.7, 29.8, 29.9, 30.1, 32.2, 68.4, 70.7, 111.6,
114.4, 118.6, 118.9, 123.7, 124.9, 125.3, 125.9, 127.0, 127.6,
127.8, 131.6, 135.0, 135.1, 137.1, 137.3, 138.2, 149.2, 150.5,
151.9, 152.9, 158.3, 160.3, 226.9. Anal. Calcd. for
[C49H53Br2N3O3S3ꢄhexane DCM]: C, 58.03; H, 6.00; N, 3.63;
found: C, 58.11; H, 5.54; N, 3.55.
1H NMR (300 MHz, CDCl3): d 0.66 (bm, CH3), 0.88 (bm,
CH2), 1.26–1.37 (bm, CH2), 1.49–1.55 (bm, CH2), 1.71 (bm,
CH2), 2.07 (bm, CH2), 3.93 (bs, CH2), 6.91 (bm, 2 ꢃ ArH),
7.07 (bm, 2 ꢃ ArH), 7.26–7.43 (bm, 4 ꢃ ArH), 7.45 (bm, 2
ꢃ ArH), 7.66 (bm, 5 ꢃ ArH), 7.75 (bm, 3 ꢃ ArH), 9.04 (bm,
1 ꢃ ArH).
Computational Details
The geometry optimization of the model compounds was
performed by using DFT calculation with the Becke three-pa-
rameter exchange function (B3) and the Lee–Yang–Parr (LYP)
correlation functional and was fully optimized using the
LANL2DZ basis set. The calculation was performed with the
Gaussian98 program.
Synthesis of Poly-1
To a dried Schlenk tube, monomer 4 (87 mg, 0.09 mmol),
2,7-di(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)-9,9-diethyl-
hexylfluorene (56 mg, 0.09 mmol), and tetrakis(triphenyl-
phosphine) palladium(0) (5 mg, 4 lmol) were degassed, and
1.5 mL of distilled toluene was added. The mixture was
heated at 90 ꢀC under nitrogen for 10 min before the
degassed tetraethylammonium hydroxide (0.6 mL, 0.9 mmol)
was added, and the resulting mixture was stirred at 90 ꢀC
under nitrogen overnight. The viscous mixture was added
phenyl boronic acid (11 mg, 0.09 mmol) and stirred for 2 h.
Bromobenzene (0.01 mL, 0.09 mmol) was then added to the
mixture and stirred for another 2 h to end cap the polymer.
The resulting purple mixture was cooled down to RT and
poured into distilled water (50 mL). The product was
extracted with DCM (3 ꢃ 50 mL). The organic extracts were
dried over MgSO4, and the solvent was removed under vac-
uum. The purple solid was redissolved in minimum amount
of CHCl3, and the polymer was precipitated from MeOH. The
purple fiber was filtered and redissolved in CHCl3. The cata-
lyst was removed by SiO2 column chromatography with
CHCl3 as an eluent. The volume of the polymer solution was
reduced, and the polymer was reprecipitated from MeOH.
The reprecipitation was repeated twice and afforded the
product as purple fiber (74 mg, 70%).
Device Fabrication
Prepatterned ITO glass substrates (15 X sqꢁ1) were cleaned
by sonication in detergent, distilled water, ethanol, acetone
and dried in an oven. The substrates were then cleaned with
UV ozone for 300 s. A thin layer of PEDOT:PSS (ꢂ50 nm)
was spin coated on the cleaned ITO as hole-conducting layer
before spin coating the polymer layer. Aluminum cathode
(800 Å) was evaporated through a shadow mask to define
the active area of the devices (2-mm diameter circles). The
overall active layer thickness was measured by Veeco Dektak
3ST surface profilometer. The current–voltage characteristics
were measured using a Keithley 2400 sourcemeter. For
white light efficiency measurements (at 100 mW cmꢁ2), Abet
solar simulator with AM1.5 filter was used. The light inten-
sity was calibrated with a standard silicon solar cell certified
by National Renewable Energy Laboratory.
CONCLUSIONS
p-Conjugated polymers tethered with metal-containing low-
bandgap units were synthesized by Suzuki cross-coupling
reaction and reported. The metal-free conjugated polymer
(poly-1) bearing functionalized ICT dye exhibited high mo-
lecular weight, good film-forming properties, and excellent
solution processability. Complexation of rhenium(I) pentacar-
bonyl chloride with poly-1 leads to a dark green polymer
poly-2. Both polymers were characterized by the NMR, FTIR,
CV, and optical spectroscopy. Poly-1 and poly-2 showed two
prominent absorption bands in the UV and visible region,
which corresponds to the p–p* transition of the conjugated
polymer backbone and the CT transitions of the pendant.
The CT transitions exhibited negative solvatochromism in
different organic solvents. Poly-1 and poly-2 exhibited low
energy absorption with an optical band edge at 1.82 and
1.49 eV, respectively. The photovoltaic response has been
observed based on this low-bandgap metallopolymer with
1H NMR (300 MHz, CDCl3): d 0.60–0.65 (bm, CH3), 0.87 (bm,
CH2), 1.29 (bm, CH2), 1.37 (bm, CH2), 1.71 (bm, CH2), 2.07
(bm, CH2), 3.94 (bs, CH2), 6.80 (bm, 2 ꢃ ArH), 7.08 (bm, 2
ꢃ ArH), 7.35 (bm, 2 ꢃ ArH), 7.45 (bm, 2 ꢃ ArH), 7.65 (bm,
5 ꢃ ArH), 7.74 (bm, 3 ꢃ ArH), 7.92 (bm, 1 ꢃ ArH), 8.47
(bm, 1 ꢃ ArH). GPC (THF, polystyrene standards): Mn
31,500, Mw ¼ 81500, PDI ¼ 2.6.
¼
Synthesis of Poly-2
Poly-1 (55 mg, 0.06 mmol) was dissolved in toluene (30
mL) and rhenium pentacarbonyl chloride (22 mg, 0.06) was
added. The purple mixture was heated to reflux under nitro-
2318
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
13 Liu, L.; Ho, C.-L.; Wong, W.-Y.; Cheung, K.-Y.; Fung, M.-K.;
Lam, W.-T. Djurisˇic´, A. B.; Chan, W. K. Adv Funct Mater 2008,
18, 2824–2833.
PCBM as the electron acceptor in the bulk-heterojunction so-
lar cells. The low photovoltaic performance may be attrib-
uted to the relatively low absorptivity of the CT transition in
the visible compared with the fully allowed p–p* transition
in the ultraviolet region. Thus, the inadequate light harvest-
ing limits the power conversion efficiency. Besides, the AFM
image of the bulk-heterojunction thin film revealed a coarse
phase separation. Poor mixing of the polymer with the PCBM
was observed, which may also account for the low current
density of the devices. The results suggested that the metal
complex pendant on the conjugated polymer chain would
influence the packing and mixing geometry of the active
layer. Hence, it would be interesting to investigate and com-
pare the photophysical properties and morphology of the
metal-containing polymer in the presence of CT metal com-
plex on the main chain. In addition, coordination of different
metal ions/complexes onto the functionalized ICT pendant is
anticipated to fine tune the optical and electronic properties
of the metal-containing polymers in the future studies.
14 Wu, P.-T.; Bull, T.; Kim, F. S.; Luscombe, C. K.; Jenekhe, S.
A. Macromolecules 2009, 42, 671–681.
15 Mei, J.; Ogawa, K.; Kim, Y.-G.; Heston, N. C.; Arenas, D. J.;
Nasrollahi, Z.; McCarley, T. D.; Tanner, D. B.; Reynolds, J. R.;
Schanze, K. S. ACS Appl Mater Interfaces 2009, 1, 150–161.
16 Baek, N. S.; Hau, S. K.; Yip, H.-L.; Acton, O.; Chen, K.-S.;
Jen, A. K.-Y. Chem Mater 2008, 20, 5734–5736.
17 Man, K. Y. K.; Wong, H. L.; Chan, W. K.; Kwong, C. Y.;
Djurisˇic´, A. B. Chem Mater 2004, 16, 365–367.
18 Man, K. Y. K.; Wong, H. L.; Chan, W. K.; Djurisˇic´, A. B.;
Beach, E.; Rozeveld, S. Langmuir 2006, 22, 3368–3375.
19 Cheng, K. W.; Mak, C. S. K.; Chan, W. K.; Ng, A. M.-C.; Djurisˇic´,
A. B. J Polym Sci Part A: Polym Chem 2008, 46, 1305–1317.
20 Chan, W. K.; Ng, P. K.; Gong, X.; Hou, S. J Mater Chem
1999, 9, 2103–2108.
The project was supported by the Research Grants Council of
the Hong Kong Special Administrative Region, China (project
nos. HKU 7008/07P and 7005/08P). Financial support from
the University Development Fund, Strategic Research Theme,
and Small Project Fund (administrated by The University of
Hong Kong) is also acknowledged.
21 Tse, C. W.; Lam, L. S. M.; Man, K. Y.; Wong, W. T.; Chan,
W. K. J Polym Sci Part A: Polym Chem 2005, 43, 1292–1308.
22 Wolcan, E.; Fe´liz, M. R. Photochem Photobiol Sci 2003, 2,
412–417.
23 Ranjan, S.; Lin, S.-Y.; Hwang, K.-C.; Chi, Y.; Ching, W.-L.;
Liu, C.-S. Inorg Chem 2003, 42, 1248–1255.
24 Li, F.; Zhang, M.; Cheng, G.; Feng, J.; Zhao, Y.; Ma, Y.; Liu,
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