Two main chain polymeric metal complexes as dye sensitizers for dye-sensitized solar cells based on the coordination of the ligand containing 8-hydroxyquinoline and phenylethyl or fluorene units with Eu(III)

Article abstract of DOI:10.1002/pola.23961

Two novel main chain polymeric metal complexes containing 8-hydroxyquinoline europium complexes and phenylethyl or fluorene units: 1,4-Dioctyloxy-2,5-bis[2-(8-hydroxyquinoline)-vinyl]-benzene Eu(III) (3) and 2,7-bis[2-(8-hydroxyquinoline)vinyl]-9,9'-diocthylfluorene Eu(III) (4) with donor-acceptor-π-conjugated structure (D-π-A) have been synthesized and investigated as dye sensitizers for dye-sensitized solar cells dyes (DSSCs). They have been determined and studied by FT-IR, TGA, DSC, GPC, Elemental analysis, UV-vis absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, and application in dyesensitized solar cells (DSSCs) as dye sensitizers. On the basis of optimized dye and molecular structure, they have shown solar-to-electricity conversion efficiency 2.25% for 3 (J_sc = 4.77 mA cm^{- 2}, V_oc = 630 mV, FF = 0.75) and 3.04% for 4 (J_sc = 6.33 mA cm _-2, V_oc = 640 mV, FF = 0.75), under the illumination of AM1.5G, 100 mW/cm². The IPCE of 3 and 4 are 30% and 46% at 400 nm, respectively. Besides, they showed good stabilities with thermal decomposition temperatures at 280 °C and 225 °C, respectively, which are suitable for DSSCs.

Full text of DOI:10.1002/pola.23961

Two Main Chain Polymeric Metal Complexes as Dye Sensitizers for
Dye-Sensitized Solar Cells Based on the Coordination of the Ligand
Containing 8-Hydroxyquinoline and Phenylethyl or Fluorene
Units with Eu(III)
LIFEN XIAO, YUAN LIU, QIAN XIU, LIRONG ZHANG, LIHUI GUO, HAILIANG ZHANG, CHAOFAN ZHONG
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University,
College of Chemistry, Xiangtan, Hunan 411105, People’s Republic of China
Received 6 December 2009; accepted 27 January 2010
DOI: 10.1002/pola.23961
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Two novel main chain polymeric metal complexes
containing 8-hydroxyquinoline europium complexes and phenyl-
ethyl or fluorene units: 1,4-Dioctyloxy-2,5-bis[2-(8-hydroxyquino-
line)-vinyl]-benzene Eu(III) (3) and 2,7-bis[2-(8-hydroxyquinoline)-
vinyl]-9,90-diocthylfluorene Eu(III) (4) with donor–acceptor-p-conju-
gated structure (D-p-A) have been synthesized and investigated as
dye sensitizers for dye-sensitized solar cells dyes (DSSCs). They
have been determined and studied by FT-IR, TGA, DSC, GPC, Ele-
mental analysis, UV–vis absorption spectroscopy, photolumines-
cence spectroscopy, cyclic voltammetry, and application in dye-
sensitized solar cells (DSSCs) as dye sensitizers. On the basis of
optimized dye and molecular structure, they have shown solar-to-
electricity conversion efficiency 2.25% for 3 (Jsc ¼ 4.77 mA cmꢀ2
,
Voc ¼ 630 mV, FF ¼ 0.75) and 3.04% for 4 (Jsc ¼ 6.33 mA cmꢀ2, Voc
¼ 640 mV, FF ¼ 0.75), under the illumination of AM1.5G, 100 mW/
cm2. The IPCE of 3 and 4 are 30% and 46% at 400 nm, respectively.
Besides, they showed good stabilities with thermal decomposition
temperatures at 280 ꢁC and 225 ꢁC, respectively, which are suitable
C
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for DSSCs. 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 48: 1943–1951, 2010
KEYWORDS: 8-hydroxyquinoline; DSSCs; dyes/pigments; fluo-
rene; functionalization of polymers; metal-polymer complexes;
phenylethyl; polymeric metal complexes
INTRODUCTION Organic electron donor–acceptor-p-conju-
gated (D-p-A) compounds are of interest owing to the possi-
bility of attaining multiple redox states (p-type or n-type
doping).1–5 Importantly, donor–acceptor conjugated polymer
systems have been widely studied for their potential applica-
tions in organic electronics and optoelectronics, including
light-emitting diodes, field-effect transistors, and photovoltaic
cells.6–10 Among them, polymer solar cells (PSCs) are more
attractive because of the importance of solar energy utiliza-
tion and its advantages, such as low cost, light weight, easy
preparation, and the possibility to fabricate flexible.11,12
donor and the electron acceptor by a conjugated spacer.16
Although ruthenium complexes are appropriate as photosen-
sitizers and the high solar-to-electric power conversion effi-
ciencies more than 11% has been achieved with DSSCs,17
their extensive application would be confined by the limited
availability and environmental issues.18 Thus, the commercial
application of the organic dyes in DSSCs is promising.
Up-to-now, a profound change in philosophy and a huge exten-
sion of possible structures has taken place. Many researchers
are eager to design functional crosslinked polymer, which is
also an attractive tool to construct well-organized p-conjugated
materials and have actually been advanced to be among the
most frequently used p-conjugated materials.19 For the further
development of functional crosslinked polymers, studies on the
polymers with various electron-accepting units and electron-
donating units are helpful for a better understanding of the
relationship between the chemical structures and properties of
the polymers. The changes in physical properties of these poly-
mers strongly depended on the nature of both the p-conjuga-
tion and the type of D-A substitution.20–25 In most cases, elec-
troactive monomers, such as thiophene,26 carbazole,19,27 and
triarylamine19 are essential for electro-polymerization. Then, in
the past few years, these structures came up as highly
Currently, dye-sensitized solar cells (DSSCs) as the most
promising alternatives for the photovoltaic conversion of so-
lar energy have attracted much interesting attention because
the groundbreaking work by O’Regan and Gra¨tzel13 due to
their high efficiency, potential low-cost, and simple assemble
technology, and so forth.14 The main components of a DSSC
are a nanocrystalline oxide semiconductor electrode, a pho-
tosensitizer, a redox electrolyte, and a counter electrode.15
Among them, the photosensitizers play a crucial role for the
DSSCs to get higher solar-to-electricity conversion efficiency
(g) and have been actively researched by scientists around
the world, whose common design is to link the electron
Correspondence to: C. Zhong (E-mail: zhongcf798@yahoo.com.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1943–1951 (2010) 2010 Wiley Periodicals, Inc.
TWO MAIN CHAIN POLYMERIC METAL COMPLEXES, XIAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Molecular Structures of DB8QB-Eu(III) (3) and B8QDF-Eu(III) (4).
promising conjugated materials, because of the introducing of
the organic functional groups, which can be applied in organic
electronic devices.28
tionally, Eu(III) complexes are designed to meet two criteria:
(1) The potential of Eu3þ is over the conduction level of
TiO2 and (2) they can be used as energy acceptor units in
29
energy transfer processes.45 When compared with other
metal complexes via axial ligands, the synthesis of metal
polymeric complexes are easier with less steric barrier.46
The metal polymeric complexes containing europium could
make better electric contact to be good energy acceptor.
There is no doubt that chemical modifications between Eu3þ
and 8-HQ may satisfy application requirements as an
acceptor to make an effective way in the application of
DSSCs, consequently.
Based on the report that the complexable and p-conjugated
crosslinked polymers do not decrease, the injection into the
TiO2 electrode and Jsc is comparable with the other materi-
als. So, if the design of the crosslinked metal polymers is
possible, their applications in the DSSCs would be potentially
better than the other materials.29 To the best of our knowl-
edge, the reports on the crosslinked coordination polymers
and their electrochemical properties are still rare.
Despite recent advances in DSSCs materials, for example, Zn(II)
metal-porphyrins compounds,30,31 Ru-sensitized DSSCs exist in
which the dye is attached to the TiO2 via axial ligands32–34 or
through anchoring groups on the macrocycle periphery,35
research into ligand design for electronic materials has been
dominated by the dipyridine and azaindole moiety. Furthermore,
practical applications of these compounds in DSSCs would
require a new generation of organic ligands that are inexpensive
and robust. 8-Hydroxyquinoline (8-HQ) is a chelating ligand
with varies properties, which has been extensively exploited in
the synthesis of metal complexes. To improve the optoelectronic
output of this class of materials, studies involving either 8-HQ or
8-HQ-like species are currently under way.36–38 Especially, den-
dritic derivatives containing 8-hydroxyquinoline have been
designed and investigated so far.39
On the basis of the concept of donor-p conjugation bridge-
acceptor structure, we here designed and synthesized a kind
of new dyes. Among the electron-donating units, phenyl-
ethyl47–49 and fluorene50 units are very attractive: They not
only have low band gap but also are the p-conjugated ring
moieties. On the other hand, to synthesize Euq3-functional-
ized polymers is typical, combination of phenylethyl or fluo-
rene units to metal polymeric main chain complexes might
expand the p-conjugation system, sustain the stability of the
dye molecule and would have ability of donor and acceptor
simultaneously. The p-conjugated systems between the do-
nor (phenylethyl or fluorene group) and the acceptor (Euq3)
were systematically extended to adjust the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molec-
ular orbital (LUMO) levels of the dyes (see Scheme 1). More-
over, the solubility in common organic solvents might be
enhanced by introducing fluorene and benzene groups at the
5-position of 8-hydroxyquinoline by Wittig-reaction. Especially,
their photophysical and electrochemical properties as well as
the application in DSSCs were investigated in this article.
Recently, lanthanide complexes are under active research
because of their great potential for applications in optics and
electronics.40–42 They combine the outstanding luminescent
properties with the remarkable stability of various hosts and
these lanthanide complexes could be finely tuned by alter-
nating independently or matching the different groups of
D-p-A dyes.41 Such as Ir(III) complexes are of particular in-
terest due to their promising using as photosensitizers in
electroluminescent materials;43 Eu(III) complexes that ex-
hibit both high emission quantum yields and fast radiation
rates are desirable electrochemical materials as well.44 Addi-
EXPERIMENTAL
Materials
All materials were obtained from Shanghai chemical re-
agent (Shanghai, China) and were used without further
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INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
purification. All solvents used in this work were analytical
grade. Europium chloride (EuCl3ꢂ6H2O) was obtained by
dissolving Eu2O3 (99.99%) in hydrochloric acid followed
by successive fuming to remove excess acid and then dis-
solved in ethanol. 1,4-Bis(bromomethyl)-2,5-dioctyloxyben-
zene and 2,7-dibromomethyl-9,90-dioctylfluorene are synthe-
sized according to the methods well-described in the refs. 51
and 52, respectively.
tion in one portion under the ice-water and then stirred at
ambient temperature. After 12 h, the solution was poured
the cool water. The organic layer was collected and dried
over anhydrous MgSO4, the removal of solvent gave a yellow
1
solid compound (5.1 g, yield 98%). H NMR (CDCl3, d, ppm):
10.29 (s, 1H), 9.75 (d, 1H), 9.57–9.60 (d, 1H), 8.01–8.06 (d,
1H), 7.80–7.82 (d, 1H), 7.58–7.59 (d, 2H), 7.40–7.42 (d, 2H),
7.35–7.40 (d, 1H), 0.85–0.88 (t, 3H).
Instrument and Measurements
1,4-Dioctyloxy-2,5-bis(8-hydroxyquinoline-vinyl) benzene
(DB8QB) (1)
Nuclear magnetic resonance all 1H NMR were performed in
CDCl3 and recorded on a Bruker NMR 400 spectrometer, and
using TMS (0.00 ppm) as the internal reference. The FT-IR
spectra were obtained on a Perkin-Elmer Spectrum One Fou-
rier transform infrared spectrometer by incorporating sam-
ples in KBr disks. Thermogravimetric analyses were run on a
Shimadzu TGA-7 Instrument in nitrogen atmosphere at a heat-
1,4-Bis(triphenylphosphoniomethyl)-2,5-dioctyloxybenzene
dibromine (9.72 g, 0.01 mol) and 5-formyl-8-tosyloxyquino-
line (6.92 g, 0.02 mol) were dissolved in 100 mL of anhy-
drous enthanol. Under an ice-water bath, EtONa (2 g sodium
in 20 mL of anhydrous enthanol) was added into the solu-
tion. After 30 min, the solution was stirred for 12 h. Then
added some K2CO3 and refluxed for 8 h. The yellow solid
was collected at the end of the condensation reaction and
washed witꢁh distilled water and ethanol. After dried in vac-
uum at 45 C for 1 day, compound 1 was obtained as a pale
ꢁ
ing rate of 20 K/min from 25 to 900 C. Differential scanning
calorimetry was performed on materials using a Perkin-Elmer
DSC-7 thermal analyzer under nitrogen atmosphere at a heat-
ꢁ
ing rate of 20 K/min from 25 to 250 C. UV–vis spectra were
1
yellow solid (3 g, yield 45%). H NMR (CDCl3, d, ppm): 8.80
taken on a Lambda 25 spectrophotometer. Photoluminescent
spectra were taken on a Perkin-Elmer LS55 luminescence
spectrometer with a xenon lamp as the light source. Elemental
analysis for C, H, and N was carried out using a Perkin-Elmer
2400 II instrument. Cyclic voltammetry was conducted on a
CHI chi630c Electrochemical Workstation, in a 0.1 mol/
L[Bu4N]BF4 (Bu ¼ butyl) DMF solution at a scan rate of 50
mV/s at room temperature. The working electrode was a
glassy carbon rod, the auxiliary electrode was a Pt wire elec-
trode, and Ag/AgCl was used as reference electrode. Gel per-
meation chromatography (GPC) analyses were done on
WATER 2414 system equipped with a set of HT3, HT4, and
HT5, l-styrayel columns with DMF as an eluent (1.0 mL/min)
at 80 ꢁC, calibrated by polystyrene standard.
(s, 2H), 8.56–8.58 (d, 2H), 7.74–7.76 (d, 2H), 7.66–7.70 (d, 2H),
7.48–7.50 (d, 2H), 7.34–7.38 (d, 2H), 7.19–7.21 (d, 2H), 7.07
(d, 2H), 3.73–4.01 (t, 4H), 2.18–2.25 (d, 4H), 1.23–1.57
(m, 20H), 0.83–0.90 (m, 6H). FT-IR (KBr, cmꢀ1): 3411 (OAH),
2915, 2856 (aromatic and vinylic CAH), 1638 (C¼¼N), 1529
(C¼¼C), 962 (CH¼¼CH trans). Anal. Calcd. for [C44H52O4N2]:
C, 78.54; H, 7.79; N, 4.16; Found: C, 78.01; H, 8.08; N, 4.58.
2,7-Bis[2-(8-hydroxyquinoline)-vinyl]-9,90-diocthylfluorene
(B8QDF) (2)
2,7-bis[2-(8-hydroxyquinoline)-vinyl]-9,90-diocthylfluorene
(B8QDF) (2) was obtained by the same way with compound
1 using 2,7-bis(triphenylphosphoniomethyl)-9,90-dioctylfluor-
ene dibromine and 5-formyl-8-tosyloxyquinoline, a yellow
solid. (3.6 g, yield 49%). 1H NMR (CDCl3, d, ppm): 8.46 (s,
2H), 8.40 (d, 4H), 8.30 (d, 4H), 7.80–7.90 (m, 6H), 7.53–7.54
(m, 6H), 7.50 (d, 2H), 7.23 (d, 2H), 2.43–2.46 (t, 4H), 1.08–
1.26 (m, 24H), 0.78–0.83 (t, 6H). FT-IR (KBr, cmꢀ1): 3366
(OAH), 2928, 2864 (aromatic and vinylic CAH), 1579
(C¼¼N), 1520 (C¼¼C), 960 (CH¼¼CH trans). Anal. Calcd. for
[C51H56O2N2]: C, 84.03; H, 7.74; N, 3.84; Found: C, 83.69; H,
7.98; N, 4.03.
Synthesis
1,4-Bis(triphenylphosphoniomethyl)-2,5-dioctyloxybenzene
dibromine (a1)
A
solution of 1,4-bis(bromomethyl)-2,5-dioctyloxybenzene
(2.6 g, 0.011 mol) and triphenylphosphine (7.4 g, 0.028 mol)
in 150 mL of dried p-xylene was stirred at reflux for 12 h.
The reaction system was then allowed to cool to room tem-
perature and the white precipitate was collected by filtration,
washed with dried ether and acetone repeatedly followed by
drying. 1H NMR (400 MHz, CDCl3, d): 7.60–7.75 (m, 30H),
6.72 (s, 2H), 5.31–5.34 (d, 4H), 3.00 (t, 4H), 1.58 (d, 4H),
1.04–1.31 (m, 20H), 0.89–0.92 (t, 6H).
DB8QB-Eu(III) (3)
A ethanol solution (10 mL) of EuCl3ꢂ6H2O (0.366 g, 1 mmol)
was dropped to a mixed THF solution (20 mL) of DB8QB
(2.02 g, 3 mmol). The reaction mixture was neutralized care-
fully with 1 M aqueous sodium hydroxide until neutral to
slightly acidic pH and was refluxed overnight. Then recrystal-
lize by ethanol. Filtered, washed with ethanol and water
repeatly, the light-yellow precipitate was collected (1.8 g,
yield 83%). FT-IR (KBr, cmꢀ1): 2934, 2844 (aromatic and
vinylic CAH), 1630 (C¼¼N), 1499 (C¼¼C), 488 (NAM). Anal.
Calcd. for [C132H156O12N6Eu]: C, 73.04; H, 7.24; N, 3.87;
Found: C, 72.86; H, 7.56; N, 3.71.
2,7-Bis(triphenylphosphoniomethyl)-9,90-dioctylfluorene
dibromine (a2)
With the same manner with 1a, a white solid (1.9 g, yield
1
75%). H NMR (CDCl3, d, ppm): 7.73–7.80 (m, 24H), 7.62–7.67
(t, 6H), 7.37–7.40 (d, 2H), 7.03–7.05 (d, 2H), 7.00 (s, 2H), 5.46–
5.50 (d, 4H), 1.46 (t, 4H), 1.09–1.23 (m, 24H), 0.82–0.90 (t, 6H).
5-Formyl-8-tosyloxyquinoline (b)
5-Formyl-8-hydroxyquinoline (synthesized according to the
ref. 53) (15 mmol, 2.6 g) was dissolved in methylene chlo-
ride (80 mL), TsCl (18 mmol, 3.2 g) was added in the solu-
B8QDF-Eu(III) (4)
In the same manner as described for 3, a red solid (1.98 g,
yield 85%). FT-IR (KBr, cmꢀ1): 2922, 2850 (aromatic and
TWO MAIN CHAIN POLYMERIC METAL COMPLEXES, XIAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Synthesis of DB8QB-Eu(III) (3) and B8QDF-Eu(III) (4).
vinylic CAH), 1559 (C¼¼N), 1550 (C¼¼C), 524 (NAM). Anal.
Calcd. for [C153H162O6N6Eu]: C, 78.77; H, 7.00; N, 3.60;
Found: C, 78.56; H, 7.13; N, 3.45.
plexes 3 and 4, respectively, which are absent in the free
corresponding ligands, can be attributed to metal-nitrogen
bonds.55
Gel permeation chromatography (GPC) studies showed that
3 and 4 have weight-averaged molecular weights at 18 and
23.2 kg/mol (7 and 8 repeating units on average for 3 and
4, respectively) with a relatively narrow polydispersity index
(PDI) between 1.14 and 1.21 for 3 and 4, respectively
(shown in Table 1). As expected, 3 and 4 were soluble in or-
ganic solvents such as DMF, toluene, and DMSO, but present
poor solubility in THF, DCM, and MeOH. The elemental analy-
sis of the polymeric metal complexes 3 and 4 show that the
ratio of europium metal and DB8QB or B8QDF is 1:3. These
results, combined with elemental analysis, indicate that the
polymers are obtained.
RESULTS AND DISCUSSION
Synthesis and Characterization
Scheme 2 outlines synthetic routes of the two ligands
DB8QB and B8QDF, which were synthesized by Wittig-reac-
tion. Figure 1(a) gives the 1H NMR spectra of DB8QB (1),
the characteristic signals at around 7.36 and 7.20 ppm can
be assigned to the resonance of protons on vinylene group.
ACH2A linked to oxygen is at 3.73 ppm, the signals at 2.18–
0.83 ppm correspond to the protons of the long alkyl chain
and 7.07 ppm is attributed to the signals of protons of phe-
nyl hydrogen; 8.80 ppm, 8.56–8.58 ppm, 7.74–7.76 ppm, and
7.66–7.70 ppm are attributed to the protons of 8-hydroxy-
quinoline. Figure 1(b) shows the 1H NMR spectra of B8QDF
(2), the vinyl proton peaks appeared at around 7.50 and
7.23 ppm; 8.46 ppm, 8.40 ppm, 8.30 ppm, and 7.66–7.70
ppm are attributed to the protons of 8-hydroxyquinoline, the
protons of fluorene are observed at 7.80–7.90 ppm and
7.53–7.54 ppm. The signals at 2.46–0.78 ppm were attrib-
uted to the alkyl of fluorine. Besides, there are no detectable
signals for the OH group in CDCl3 in both of the spectra.54
Optical and Thermal Properties of the Polymers
Figure 3 presents the UV–vis absorption spectra of 3 and 4
recorded in solution using DMF as solvent and adsorbed on
TiO2 films. We consider that the spectra of complexes 3 and
4 exhibit kmax at 318 and 393 nm in DMF solution, respec-
tively. We can see both of the dyes with a strong absorption
maximum in the visible region absorbed on TiO2 films corre-
sponding to intramolecular charge transfer absorption. The
peaks are located at 436 and 448 nm for 3 and 4, respec-
tively. The absorption spectra of dyes attached to TiO2 film
are red-shifted compared with those of the corresponding
solution spectra, which is attributed to the formation of dye
aggregates on the TiO2 surface and an interaction between
the dyes and TiO2.56–58 This phenomenon would benefit the
photoelectrical conversion efficiency of the dye-sensitized
The FT-IR spectra of the polymeric metal complexes 3 and 4
are shown in Figure 2. Upon comparison of the ligands, all
the bands of the corresponding complexes are red shift,
which is attributed to the extending of p-conjugation after
coordinated with metal ions successfully. There are bands
appears at 488 cmꢀ1 and 505 cmꢀ1 in polymeric metal com-
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ARTICLE
menon ensures that all photons absorbed by the polymeric
metal complexes are useful for the photovoltaic conversion.
The thermal stability of the two polymeric metal complexes
3 and 4 were studied by means of thermo gravimetric analy-
ses (TGA) and differential scanning calorimetry (DSC) under
nitrogen atmosphere, which are shown in Table 1 and Figure
5. It is seen that 3 and 4 have glass transition temperature,
Tg of 168 ꢁC and 150 ꢁC, respectively. TGA showed that the
ꢁ
ꢁ
5 wt % loss temperature for 3 and 4 are 280 C and 225 C,
respectively, suggesting that the polymers may be served as
an advantage for the fabrication processes of DSSCs and
other optoelectronic devices.
Electrochemical Properties
The electrochemical behaviors of the obtained polymers
were investigated by cyclic voltammetry, which is an impor-
tant property for organic materials used in solar cells. Figure
6 shows the cyclic voltammetry curves of 3 and 4. The cyclic
FIGURE 1 1H NMR spectra of DB8QB (a), B8QDF (b) in CDCl3.
solar cells.59 At the same time, the spectra become broader
than that in solution, which is attributed to a greater amount
of absorbed dye on the TiO2 films, and as a result of the
increased intermolecular p–p interaction with expanding
p-conjugation.60
We also measured the PL spectra of the polymeric metal
complexes in DMF solution and the solid state shown in Fig-
ure 4. The PL spectra of the polymeric metal complexes
show an emission peak at 503 and 458 nm for 3 and 4,
respectively, in DMF solution. It can be seen that the PL peak
of 3 and 4 in the solid state is red-shifted by about 70,103
nm to 573 and 561 nm, respectively, in comparison with
that of the solution, which is assigned to that the coordina-
tion of metal ions enhances the capability of accept-electron
of ligands, decreases and steadies the electron transition
energy of the intra ligand charge-transfer and indicates that
the conformation of the two metal polymeric complexes 3
and 4 changed significantly in the solid state. This pheno-
FIGURE 2 FT-IR spectra of DB8QB, DB8QB-Eu(III) (a), B8QDF(2),
B8QDF-Eu(III) (b).
TWO MAIN CHAIN POLYMERIC METAL COMPLEXES, XIAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Molecular Weight and Thermal Properties
of DB8QB-Eu(III) and B8QDF-Eu(III)
a
b
Mn
(ꢃ103)
Mw
(ꢃ103)
Tg
Td
PDI
(ꢁC)
(ꢁC)
DB8QB-Eu(III)
B8QDF-Eu(III)
a
18
20.5
28
1.14
1.21
168
150
280
225
23.2
Glass transition temperature measured from DSC traces of the poly-
meric metal complexes.
b
The data were obtained from TGA of the polymeric metal complexes.
voltammetry of complexes were measured in DMF solution
containing [Bu4N]BF6(Bu ¼ butyl) as supporting electrolyte
and Ag/AgCl as reference electrode at a scan rate of 50 mV/
s. The HOMO and LUMO are measured by electrochemical
cyclic voltammetry (CV). When Ag/AgCl electrode is used as
the reference electrode, the correlation can be expressed as
the equation: HOMO ¼ ꢀe(Eox þ 4.40) (eV); LUMO ¼
ꢀe(Ered þ 4.40) (eV).61,62 The corresponding data are
obtained and tabulated in Table 2. The reduction and oxida-
tion potentials of 3 were measured to be Ered ¼ ꢀ1.224 V
FIGURE 4 PL spectra of DB8QB-Eu(III) (3) and B8QDF-Eu(III) (4)
in DMF solution and in the solid state.
and Eox ¼ 0.914 V, respectively. The energy band gap was
2.038 eV, the energy value of the HOMO was calculated to be
ꢀ5.314 eV and the energy value of the LUMO was calculated
to be ꢀ3.276 eV. In the case of 4, the HOMO and LUMO
energy value were calculated as ꢀ5.302 eV and ꢀ3.404 eV,
respectively, with an Eg of 1.898 eV. In the comparison of 3,
the HOMO of 4 is higher than 3 resulted from the stronger
hole-transporting capacity of 4, indicating that using strong
donors with high HOMO energy levels and strong acceptors
with low LUMOs can control the extent of the polymeric
metal complexes band gap.
Fabrication of DSSCs
Titania paste was prepared following a procedure: Fluorine-
doped SnO2 conducting glass (FTO) were cleaned and
immersed in aqueous 40 mM TiCl4 solution at 70 ꢁC for 30
ꢁ
min, then washed with water and ethanol, sintered at 450 C
for 30 min. The 20–30 nm particles sized TiO2 colloid was
coated onto the above FTO glass by sliding glass rod method
to obtain a TiO2 film of 10–15 lm thickness. After drying,
FIGURE 3 Normalized absorption spectra of DB8QB-Eu(III) (3)
and B8QDF-Eu(III) (4) in DMF solution (a) and adsorbed on
TiO2 films (b).
FIGURE 5 TGA plots of DB8QB-Eu(III) (3) and B8QDF-Eu(III) (4)
with a heating rate of 10 ꢁC/min under nitrogen atmosphere.
1948
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 6 CV curves of DB8QB-Eu(III) (3) and B8QDF-Eu(III) (4)
measured in DMF containing [Bu4N]BF6(Bu ¼ butyl) as support-
ing electrolyte at a scan rate of 50 mV/s.
FIGURE 7 J–V curves of DSSCs based on DB8QB-Eu(III) (3) and
B8QDF-Eu(III) (4) in DMF solution.
the TiO2-coated FTO glass were sintered at 450 ꢁC for 30
3.04%, which is higher that of the device based on 3
(2.25%).
ꢁ
min, then treated with TiCl4 solution and calcined at 450 C
ꢁ
for 30 min again. After cooling to 100 C, the TiO2 electrodes
were soaked in 0.5 mM the dye-sensitized samples dyes 3
and 4 in DMF solution, then kept at room temperature under
dark for 24 h. 3-Methoxypropionitrile solution containing LiI
(0.5 M), I2 (0.05 M), and 4-tert-butyl pyridine (TBP) (0.5 M)
was used as the electrolyte. A Pt foil used as counter elec-
trode was clipped onto the top of the TiO2 using as working
electrode. The dye-coated semiconductor film was illumi-
nated through a conducting glass support without a mask.
Photoelectron chemical performance of the solar cell was
measured using a Keithley 2602 Source meter controlled by
a computer. The cell parameters were obtained under an
incident light with intensity 100 mW ꢂ cmꢀ2, which was gen-
erated by a 500 W Xe lamp passing through an AM 1.5G fil-
ter with an effective area of 0.196 cm2.
The low Jsc is ascribed to the low charge separation and
transportation efficiency, which results from the long dis-
tance between donor and acceptor63 and can be seen from
the input photon to converted current efficiency (IPCE)
curves as shown in Figure 8. The maximum external quan-
tum efficiency of 3 and 4 are only 30% and 46% at 400 nm,
respectively. It should be noted that 3 and 4 still show weak
IPCE response at the range of long wavelength region. As
discussed in the UV–vis absorption spectra section, the
higher power conversion efficiency of 4 result in greater
adsorption than 3 and aggregation of the dyes on the TiO2
surface, which lead to less efficient electron injection64 to
TiO2 because of quenching of the excited state and thus may
not give a satisfactory g value. Also, we speculate 4 suppress
the charge recombination, resulting in elongation of the elec-
tron lifetime. So, to obtain a high g value, a suitable p-conju-
gated length should be adapted to the dyes.
Photocurrent-Voltage Measurements
Figure 7 shows the irradiation source for the photocurrent
density-voltage (J–V) measurement of the DSSCs devices
based on the two polymeric metal complexes 3 and 4. The
corresponding open-circuit voltage (Voc), short-circuit current
density (Jsc), fill factor (FF), and power conversion efficiency
(g) are listed in Table 3. It can be seen that The Voc values of
3 and 4 dyes are 0.63 V and 0.64 V, respectively, and the
corresponding FF values are both 0.75. However, the Jsc val-
ues increased from 4.77 mA/cm2 for 3 to 6.33 mA/cm2 for
4. The power conversion efficiency based on 4 reached
The power conversion efficiency is relatively low in the de-
vice, and further work on optimizing the device performance
is under investigation.
CONCLUSIONS
In summary, we reported the synthesis and characterization
of two novel europium complexes: 1,4-Dioctyloxy-2,5-bis[2-
(8-hydroxyquinoline)-vinyl]-benzene Eu(III) (3) and 2,7-bis[2-
(8-hydroxyquinoline)-vinyl]-9,90-diocthylfluorene Eu(III) (4)
TABLE 2 Cyclic Voltammetric Results of the Polymeric Metal
Complexes
TABLE 3 Photovoltaic Performances of DSSCs
ox
red
E
E
HOMO
(eV)
LUMO
(eV)
EEgC
Illumination Jsc
Voc
g
onset
onset
(V)
(V)
(eV)
Polymer
Solvent Time (min) (mA/cm2) (V) ff
(%)
DB8QB-Eu(III)
B8QDF-Eu(III)
0.914
0.902
ꢀ1.124
ꢀ0.996
ꢀ5.314
ꢀ5.302
ꢀ3.276
ꢀ3.404
2.038
1.898
DB8QB-Eu(III) DMF
B8QDF-Eu(III) DMF
2
2
4.77
6.33
0.63 0.75 2.25
0.64 0.75 3.04
TWO MAIN CHAIN POLYMERIC METAL COMPLEXES, XIAO ET AL.
1949

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
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However, there are many challenges to surmount, Jsc and
IPCE are very low based on both the materials, the light
absorption, electron injection efficiency, and adsorption affin-
ities on the TiO2 are the main reasons that we cannot obtain
outstanding g. One or two anchoring groups such as carbox-
ylic acid or sulfonic acid groups should be introduced in the
structure for good solubility and strong adsorption onto the
surface of TiO2.65,66 Accordingly we should find the further
structural optimization with broad spectral coverage, excel-
lent charge separation, and transportation are expected to
produce more efficient photosensitizers. We managed to
enhance the solubility of the polymeric metal complexes able
to evaluate the absorption spectra in several solvents and to
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