Metallo-homopolymer and metallo-copolymers containing
light-emitting poly(fluorene/ethynylene/(terpyridyl)zinc(II))
backbones and 1,3,4-oxadiazole (OXD) pendants
Yi-Yu Chen, Hong-Cheu Lin
*
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan ROC
Received 8 May 2007; received in revised form 15 June 2007; accepted 19 June 2007 Available online 23 June 2007
Abstract
A series of novel metallo-polymers containing light-emitting poly(fluorene/ethynylene/(terpyridyl)zinc(II)) backbones and electron-transporting 1,3,4-oxadiazole (OXD) pendants (attached to the C-9 position of fluorene by long alkyl spacers) were synthesized by self-assembled reactions. The integrated ratios of1H NMR spectra reveal a facile result to distinguish the well-defined main-chain metallo-polymeric structures which were constructed by different monomer ligand systems (i.e. single, double, and triple monomer ligands with various pendants). Further-more, UVevis and photoluminescence (PL) spectral titration experiments were carried out to verify the metallo-polymeric structures by varying the molar ratios of zinc(II) ions to monomers. As a result, the enhancement of thermal stability (Td) and quantum yields were introduced by the
metallo-polymerization, and their physical properties were mainly affected by the nature of the pendants. The photophysical properties of these metallo-polymers exhibited blue PL emissions (around 418 nm) with quantum yields of 34e53% (in DMF). In contrast to metallo-polymers containing alkyl pendants, the quantum yields were greatly enhanced by introducing 1,3,4-OXD pendants but reduced by carbazole (CAZ) pendants. Moreover, electroluminescent (EL) devices with these light-emitting metallo-polymers as emitters showed green EL emissions (around 550 nm) with turn-on voltages of 6.0e6.5 V, maximum efficiencies of 1.05e1.35 cd A1(at 100 mA/cm2), and maximum luminances of 2313e3550 cd/m2(around 15 V), respectively.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Metallo-polymers; 1,3,4-Oxadiazole; Carbazole
1. Introduction
Recently, the use of transition or rare earth metal com-plexes to build up polymeric light-emitting diode (PLED) devices has attracted much attention because of the
enhance-ment in EL efficiency [1e5]. Chan and co-workers
demon-strated that the construction of conjugated polymers made of ruthenium bipyridyl complexes can enhance the light-emitting performance by utilizing energy transfer from the triplet
excited state [6e8]. A whole set of coordination polymers
consisting of ditopic electro- and photo-active terpyridyl ligands complexed with zinc ions were recently published by
Che and co-workers[9]. These coordination polymers exhibit
different emission wavelengths ranging from violet to yellow colors with high PL quantum yields, and these polymers were successfully applied to PLED devices. Hence, tuning electroluminescent (EL) properties could be achieved through the incorporation of different transition metal complexes into
polymer main chains [6e14]. Moreover, it is confirmed that
due to the d10 zinc(II) species the phenomena of intraligand
charge transfers (ILCTs) happen between terpyridine/zinc(II) complexes and chromophores even in fully conjugated
metallo-polymers [9e12]. Therefore, the incorporation of
terpyridine/zinc(II) moieties into metallo-polymers with
fine-tuned chromophores can provide good quantum yields * Corresponding author. Tel.: þ886 3 5712121x55305; fax: þ886 3
5724727.
E-mail address:[email protected](H.-C. Lin).
0032-3861/$ - see front matterÓ 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2007.06.040
and thermal stabilities, and thus to have the potential to be-come high-performance emissive or host materials in PLED
applications[15e17].
However, some important and fundamental challenges re-main to solve, including the maximization of luminescence and power efficiency, the designs and syntheses of new mate-rials for purer colors, and the modes of addressing devices for full-color displays with optimized resolutions. A major factor responsible for poor device performance is that the charge in-jection and transportation in emissive materials are generally unbalanced. This imbalance arises because the energy barrier between the indium tin oxide (ITO) anode and the highest occupied molecular orbital (HOMO) level of an emissive material is different from that between the metal cathode and
its lowest unoccupied molecular orbital (LUMO) level [18].
In previous studies, molecular and polymeric 1,3,4-oxadiazole (OXD) derivatives are one of the most widely studied classes of electron injection and/or hole-blocking materials, mainly because of their electron deficiencies, high photoluminescence
quantum yields, and chemical stabilities[18e22]. Therefore,
the introduction of electron-deficient OXD groups into the C-9 position of the fluorene units increase the electron affinities of resulting polymers and lead to more balanced charge injec-tion and transporting properties as well as better recombinainjec-tion
behavior[23e25].
In this context, various electron- and hole-transporting sub-stituents, i.e. 1,3,4-oxadiazole (OXD) and carbazole (CAZ) pendants, were incorporated into poly(fluorene/ethynylene/ (terpyridyl)zinc(II))-based metallo-copolymers (as shown in
Scheme 1). In addition, the 1H NMR, thermal, photophysical, and electrochemical properties were investigated as well. Furthermore, the PLED applications of metallo-copolymers as emitters in multilayer EL devices with two different heterojunction configurations of ITO/PEDOT:PSS/polymer/
TPBI[2,20,200-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole]/
LiF/Al and ITO/PEDOT:PSS/polymer/BCP(2,9-dimethyl-4,7- diphenyl-1,10-phenanthroline)/ALQ(tris(8-hydroxyquinoline)-aluminium)/LiF/Al were studied.
2. Experimental 2.1. Measurements
1
H NMR spectra were recorded on a Varian Unity 300 MHz
spectrometer using CDCl3and DMSO-d6solvents. Elemental
analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Phase transition temperatures were deter-mined by differential scanning calorimetry (DSC, model:
Per-kin Elmer Diamond) with a heating and cooling rate of 10C/
min. Thermogravimetric analysis (TGA) was conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950
thermogravimetric analyzer at a heating rate of 10C/min
under nitrogen. Melting points were determined by a Buchi SMP-20 capillary melting point apparatus. Viscosity measure-ments were proceeded by 10% weight of polymer solutions (in NMP) in contrast to those proceeded by the same
condi-tion of monomer solucondi-tions (with viscosity h¼ 6 cP) on
a BROOKFILEL DV-IIIþRHEOMETER system (100 rpm,
spindle number: 4) at 25C. UVevisible (UVevis) absorption
spectra were recorded in dilute DMF solutions (105M) on
a HP G1103A spectrophotometer, and fluorescence spectra were obtained on a Hitachi F-4500 spectrophotometer. Fluo-rescence quantum yields were determined by comparing the integrated photoluminescence (PL) intensity of coumarin-1
in ethanol with a known quantum yield (ca. 5 106M,
quan-tum yield¼ 0.73). Cyclic voltammetry (CV) was performed at
a scanning rate of 100 mV/s on a BAS 100 B/W electrochem-ical analyzer, which was equipped with a three-electrode cell. Pt wire was used as a counter electrode, and an Ag/AgCl elec-trode was used as a reference in the CV measurements. The CV experiments were performed by solid samples immersed into electrochemical cell containing 0.1 M
tetrabutylammo-nium hexafluorophosphate (Bu4NPF6) solutions (in DMF)
with a scanning rate of 100 mV/s at room temperature under nitrogen. UVevis and PL titrations were performed by the
1.0 105M of monomer solutions in the solvent of
CH3CN/CHCl3(2/8 in vol.) were titrated with 50 ml aliquots
of 3.9 104M of Zn(OAc)2 solutions in the same solvent
composition as described. The addition was done stepwise and the formation of Zn(II)-coordination polymers was moni-tored by UVevis spectroscopy. Polymer thin solid films in UVevis and PL measurements were prepared by spin-coating polymers on quartz substrates from DMF solutions with a concentration of 10 mg/ml. A series of EL devices with two device configurations of
ITO/PEDOT:PSS/polymer/TPBI-(2,20,200-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole])//
LiF/Al and ITO/PEDOT:PSS/polymer/BCP(2,9-dimethyl-4,7- diphenyl-1,10-phenanthroline)/AlQ(tris(8-hydroxyquinoline)-aluminium)/LiF/Al were made, where TPBI and AlQ were used as electron-transporting layers, and BCP was used as a hole-blocking layer collocated to AlQ. ITO substrates were routinely cleaned by ultrasonic treatments in detergent solutions and diluted water, followed by rinsing with acetone and then with ethanol. After drying, ITO substrates were kept in oxygen plasma for 4 min before being loaded into the vacuum chamber. The solutions (10 mg/ml) of light-emitting materials in DMF were spin-coated on glass slides precoated with indium tin oxide (ITO) having a sheet resistance of w20 U/square and an
effec-tive individual device area of 3.14 mm2. The spin coating rate
was 6000 rpm for 60 s with PEDOT:PPS, 4000 rpm for 60 s with resulting polymers, and the thicknesses of PEDOT:PPS and polymers were measured with an Alfa Step 500 Surface Profiler (Tencor). TPBI, BCP, and AlQ were thermally
depos-ited at a rate of 1e2 A˚ /s under a pressure of w2 105Torr
in an Ulvac Cryogenic deposition system. Under the same deposition conditions and systems, one layer of LiF was
ther-mally deposited as a cathode at a rate of 0.1e0.2 A˚ /s, which
was followed by capping with aluminum. 2.2. Materials
Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Solvents were purified and dried according to standard
procedures. Chromatography was performed with Merck silica gel (mesh 70e230) and basic aluminum oxide, which was
deactivated with 4 wt% of water. 40
-[[(Trifuroromethyl)sulfo-nyl]oxy]-2,20:60,600-terpyridines and compounds 1a, 4b (M2),
and 4c (M3) were prepared and purified according to literature
procedures[11,12,25e27]. The synthetic routes of monomers
4ae4c (M1eM3) and metallo-polymers P1eP4 are
illus-trated inSchemes 1 and 2.
2.3. Synthetic procedures of monomers 2.3.1. Compound 2a
To a solution of compoumd (1a) (28 mmol) in 60 mL of THF/Et3N (1/1), 3-methyl-1-butyn-3-ol (84 mmol) was added. After the solution was degassed with nitrogen for 30 min,
Pd(PPh3)2Cl2(0.28 mol), PPh3(11 mol), and CuI (2.8 mmol)
were added. The reaction was then refluxed at 70C under
N2for 12 h. The solvent was removed under reduced pressure.
The resulting solid was extracted with CH2Cl2/H2O then dried
over MgSO4. The crude product was purified by column
chro-matography (silica gel, hexane/ethyl acetate¼ 4/1) to afford
a white solid; mp 76e77C. 1H NMR (300 MHz, CDCl3):
d 8.01e8.05 (m, 8H), 7.60 (d, J¼ 7.8 Hz, 2H), 7.39e7.42
(m, 4H), 7.32 (d, J¼ 8.4 Hz, 4H), 6.94 (d, J ¼ 9 Hz, 4H),
3.90 (t,J¼ 6.3 Hz, 4H), 2.44 (s, 6H), 2.04 (s, 2H), 1.95 (br,
4H), 1.59e1.66 (m, 16H), 1.13e1.18 (m, 12H). Yield: 77%.
FABMS: m/e 996; C65H66N4O6requiresm/e 998.50.
2.3.2. Compound 3a
A mixture of 2a (1.63 mmol) and KOH (6.5 mmol) in
60 mL of 2-propanol was heated to reflux under N2 with a
vigorous stirring for 3 h. The solvent was then removed and crude product was purified by column chromatography (silica
gel, hexane) to afford a white solid; mp 82e83C.1H NMR
(300 MHz, CDCl3): d 7.99e8.04 (m, 8H), 7.64 (d, J¼
7.8 Hz, 2H), 7.47e7.50 (m, 4H), 7.32 (d, J¼ 8.4 Hz, 4H),
6.95 (d,J¼ 9 Hz, 4H), 3.91 (t, J ¼ 6.3 Hz, 4H), 3.16 (s, 2H),
2.44 (s, 6H), 1.97 (br, 4H), 1.59e1.66 (m, 4H), 1.13e1.20
(m, 12H). Yield: 77%. FABMS: m/e 882; C59H54N4O4
requiresm/e 882.41.
2.3.3. Monomer 4a (M1)
Compound 3a (0.5 mmol) and 40
-[[(trifuroromethyl)sulfo-nyl]oxy]-2,20:60,600-terpyidine (1.1 mmol) were dissolved in
nitrogen-degassed benzene, then [Pd0(PPh3)4] (70 mg,
0.06 mmol) was added followed by the addition of 20 ml of
nitrogen-degassed iPr2NH. The solution was then heated to
70C. After complete consumption of starting materials, the
solvent was evaporated and the product was purified by
column chromatography (alumina, hexane/dichloromethane¼
10/1 in vol.) to afford a white solid; mp 115e116C.1H NMR
(300 MHz, CDCl3): d 8.7 (d,J¼ 5.1 Hz, 4H), 8.60e8.64 (m,
8H), 7.93e7.97 (m, 8H), 7.86 (t, J¼ 7.8 Hz, 4H), 7.75 (d,
J¼ 8.7 Hz, 2H), 7.58e7.61 (m, 4H), 7.33e7.37 (m, 4H),
7.30 (d,J¼ 8.1 Hz, 4H), 6.93 (d, J ¼ 8.7 Hz, 4H), 3.92 (t, J ¼
6.6 Hz, 4H), 2.43(s, 6H), 2.08 (br, 4H), 1.64 (br, 4H), 1.22 (br,
12H). Yield: 75%. FABMS:m/e 1343; C89H72N10O4requires
m/e 1344.57. Anal. Calcd for C89H72N10O4: C, 79.44; H,
5.39; N, 10.42; O, 4.76. Found: C, 80.15; H, 5.77; N, 10.22; O, 4.41.
2.4. Synthetic procedures of metallo-polymers 2.4.1. Metallo-homopolymer P1
To monomer 4a (M1) (0.52 mmol) in 30 ml of NMP solu-tion, zinc acetate (0.52 mmol) in NMP (10 ml) was added Scheme 1. Synthetic routes of monomers 4ae4c (M1eM3).
dropwise. The resulting solution was heated at 105C under
nitrogen atmosphere. After stirring for 24 h, excess KPF6
(1.2 mmol) was added into the hot solution. The resulting solution was poured into methanol and the precipitate obtained was purified by repeated precipitations using NMP and ether.
The polymers were dried under vacuum at 60C for 24 h and
collected as yellow solids. Yields: 78e82%. 2.4.2. Metallo-alt-copolymer P2
To zinc acetate (1.25 mmol) in 20 ml of NMP (N-methyl-pyrrolidinone) solution, monomer 4b (M2) (0.61 mmol) in NMP (20 ml) was added dropwise. After stirring at r.t. for 2 h, monomer 4a (M1) (0.64 mmol) was also added dropwise.
The resulting solution was heated at 105C under a nitrogen
atmosphere. After stirring for 24 h, excess KPF6 (2.6 mmol)
was added into the hot solution. The resulting solution was poured into methanol and the precipitate obtained was purified by repeated precipitations using NMP and ether. The polymers
were dried under vacuum at 80C for 24 h and collected as
yellow solids. Yields: 74e80%. 2.4.3. Metallo-alt-copolymer P3
The procedure is analogous to that described for P2. Yield: 80e84%.
2.4.4. Metallo-copolymer P4
To zinc acetate (0.92 mmol) in 20 ml of NMP (N-methyl-pyrrolidinone) solution, monomer 4b (M2) (0.45 mmol) in NMP (20 ml) was added dropwise. After stirring at r.t. for 2 h, mixture monomers 4a (M1) and 4c (M3) (0.42 mmol,
4a(M1):4c (M3)¼ 1:1) was also added dropwise. The
result-ing solution was heated at 105C under nitrogen atmosphere.
After stirring for 24 h, excess KPF6was added into the hot
so-lution. The resulting solution was poured into methanol and the precipitate obtained was purified by repeated precipitations using NMP and ether. The polymer was dried under vacuum at
80C for 24 h and collected as a yellow solid. Yield: 80%.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic routes of monomers 4a, metallo-homopolymer
P1and metallo-copolymers P2eP4 are illustrated inSchemes
1 and 2. Metallo-homopolymer P1 was obtained by refluxing
monomer 4a (M1) with Zn(OAc)2 at a ratio of 1:1 in NMP
solution and followed by subsequent anion exchange [11,12].
The key steps in the syntheses of metallo-alt-copolymers were first to functionalize two end terpyridyl units of Scheme 2. Synthetic routes of metallo-polymers P1eP4.
monomers 4b (M2) and 4c (M3) with Zn(OAc)2at a ratio of 1:2 to afford complexes 5b and 5c, respectively. Then, com-plexes 5b and 5c as initiators were coordinated with monomer
4a (M1) (see Scheme 2) at a ratio of 1:1 (as a
sequential-coupling method), respectively, to obtain metallo-alt-copolymers
P2 and P3 [11]. Moreover, metallo-copolymer P4 was
obtained by reacting the mixture of monomers 4a (M1) and
4c (M3) (at a ratio of 1:1) with complex 5b (monomer
mixture:complex 5b¼ 1:1). In contrast to other
polymeriza-tion methods, i.e. the Witting or Heck coupling reacpolymeriza-tions, there are three points worthy to be noted. First, the reactive lability of zinc(II) ions and the stability of six-coordinate bis-terpyri-dine zinc(II) moieties allow self-assembled reactions to take
place under refluxing conditions[3e5,9]. Second, the present
procedure does not need any catalysts. Third, the chemical structures of metallo-copolymers can be controlled by proper
stoichiometries of metals and monomers [11,12].
3.2. Structural characterization with1H NMR
1H NMR spectra of monomers 4ae4c (M1eM3),
com-plexes 5b and 5c and metallo-polymers P1eP4 were recorded
in DMSO-d6 as shown in Fig. 1. Compared with 1H peaks
of monomers 4b (M2) and 4c (M3) in our previous study
[11], those in terpyridyl units of complexes 5b and 5c show
the same downfield shift effect in proton peaks of (6,600)-,
(5,500)-, (4,400)-, (30,50)-, and (3,300)-H. Furthermore, proton
peaks of (4,400)-H in terpyridyl units of complexes 5b and 5c
overlap with1H peaks of fluorene units. It appears that the
for-mation of complexes 5b and 5c can be, respectively, proven by
the disappearance of original1H peaks in terpyridyl units of
monomers 4a and 4b (M1 and M2) in the mixture of zinc
ions and monomers at a ratio of 2:1 [11]. The formation of
homopolymer P1 is clearly indicated by the appearance of
a new set of1H peaks and the absence of the original1H peaks
in terpyridyl units, which belong to the uncomplexed
mono-mer 4a (M1). (The assignments of1H peaks of the terpyridyl
units for all polymers are made by asterisks with respect to
4-chloro-terpyridine Zn2þcomplexes). In terms of1H peaks of
1,3,4-OXD and carbazole (CAZ) pendants, there are no obvi-ous changes in chemical shifts among monomers 4a (M1), 4c
(M3) and polymers P3, P4. Therefore, the most up-shifted1H
peaks in the terpyridyl units of polymers P3 and P4 could be
overlapped with the1H peaks of these pendants. To distinguish
the structural differences among these main-chain metallo-polymers P1eP4, it is feasible to compare the relative
inte-grated ratios of the1H peaks. As a result, the integrated ratios
of the1H peaks in the terpyridyl units (*Afor P1,*A1for P2,
*A2 for P3, and *A3 for P4) and the 1H peaks of the alkyl
chains (spacer eCH2e) attached to 1,3,4-OXD (B for P1,
B1 for P2, B2 for P3, and B3 for P4) and CAZ units (C2
for P3 and C3 for P4) of these polymers are compared. It
reveals that the relative integrated ratios are *A/B¼ 0.5,
*A1/B1¼ 1, *A2/B2¼ 1, and *A3/B3¼ 2 for
metallo-polymers P1, P2, P3, and P4, respectively, which suggests that the integrated ratios of polymers were consistent with the
monomer amounts containing pendant 1,3,4-OXD units [11].
In contrast to the relative integrated ratio of B2/C2 (¼1.1) in polymer P3, that of B3/C3 was 1 for polymer P4. Accord-ing to these results, the input ratios (molecular ratios) of monomers for polymerization were very similar to the output
ratios (the relative integrated ratios of1H NMR) of the
met-allo-polymers. Consequently, the amounts of monomer ligands incorporated in the monomer ligand-based metallo-polymers
can be confirmed by1H NMR[28e30].
3.3. Thermal and viscosity properties
The thermal and viscosity properties of monomers 4ae4c (M1eM3) and metallo-polymers P1eP4 were studied by thermogravimetric analysis (TGA) and rheometry as
summa-rized in Table 1. The decomposition temperatures (Td) (5%
Fig. 1.1H NMR spectra of monomers 4ae4c (M1eM3), complexes 5a and 5b, and metallo-polymers P1eP4 in DMSO-d6.
weight loss measured by TGA) of monomers under nitrogen
atmosphere were ranged from 221 to 351C, and those of
polymers were ranged from 355 to 389C. In contrast to
monomers, polymers exhibited slightly enhanced thermal sta-bility due to the increased rigidity of the main-chain structures
[8]. As the bulky OXD and CAZ pendants are attached to the
backbones of the metallo-polymers, it leads to reduced rigidity
of the polymers[31,32]. Hence, compared with polymers P1,
P3, and P4 containing more bulky OXD and CAZ pendants,
P2shows the highestTdvalue among these metallo-polymers
due to its higher molar ratio of less bulky alkyl pendants. This
behavior was also confirmed by that theTdvalue (422C) of
metallo-homopolymer containing M2 (with alkyl pendants) is
larger than that (399C) of metallo-homopolymer containing
M3 (with CAZ pendants) in our previous report, [11] and
bothTdvalues are larger than that (355C as shown inTable 1)
of metallo-homopolymer P1 containing M1 (with OXD pendants). Moreover, the same trend of the pendant size effect
on theTgvalues were also observed in the monomers, i.e., 4b
(M2 with alkyl pendant)¼ 103C > 4c (M3 with CAZ
pendant)¼ 92C > 4a (M1 with OXD pendant)¼ 67C, so
molecular structures with larger pendant groups have lower
Tg values. This obviously indicates that the presence of
OXD and CAZ pendants in these metallo-polymers suppresses the crystallinity (and chain aggregation) of the polymers effec-tively. Similar results were also observed in poly(fluorene)-based copolymers containing various 1,3,4-oxadiazole dendritic
pendants [25]. However, all metallo-polymers P1eP4 did
not show any phase transition temperatures, including Tg.
It suggested that the fully aromatic conjugated main-chain structures may induce the aggregation in metallo-polymers
P1eP4by pep stacking and thus to reduce the phase
transi-tion behavior[11]. In conclusion, the thermal properties were
truly affected by the nature of different pendants incorporated into the polymers.
To further confirm the structures of metallo-polymers, mo-lecular weights of these metallo-polymers should be investi-gated. However, these polymers showed poor solubilities in
THF, CH3Cl, and alcoholic solvents, so standard
measure-ments of molecular weights by GPC (gel permeation chroma-tography) could not be proceeded. Therefore, the relative
viscosities of polymers to monomers were carried out to sup-port the formation of polymeric structures. Solutions of mono-mers 4a, 4b (M1, M2) (10% weight ratio) in NMP with
viscosities h¼ 6e7 cP at 25C were used as references to
de-termine the viscosities of the respective polymers. In compar-ison with the viscosities of monomers 4ae4c (M1eM3), those
of metallo-polymers P1eP4 exhibit increased viscosities (h¼
9e11 cP) by adding Zn2þ ions, and the ratios of relatively
increased viscosities of polymers to those of monomers were in the range of 1.50e1.66. Similar phenomena were also
reported in our previous study [11].
3.4. Electrochemical properties
The electrochemical behavior of polymers were studied by cyclic voltammetry (CV), and the electrochemical properties
are summarized inTable 1. The lowest unoccupied molecular
orbital (LUMO) energy levels were estimated from reduction potentials by the reference energy level of ferrocene (4.8 eV below the vacuum level) according to the following equation:
ELUMO¼ [(Eonset 0.45) 4.8] eV [26,27]. However, the
oxidation potentials of all metallo-polymers were not detect-able, so the highest occupied molecular orbital (HOMO) en-ergy levels can be estimated by the sums of LUMO enen-ergy levels and optical band gaps. All metallo-polymers exhibit
re-versible reduction peaks around1.54 V in cathodic scans (up
to2.5 V). These peaks are attributed to the reduction of
ter-pyridyl-based moieties[9,11]. The absence of oxidation peaks
in the anodic scans (up to 1 V) of these polymers, are due to
the metal oxidation of d10zinc(II) ion species being extremely
difficult to be observed[33e35]. The optical band gaps were
estimated from absorption spectra in DMF solutions by extrap-olating the tails of the lowest energy peaks, and the optical band gaps of these polymers were ranged from 3.15 to 3.16 eV. Since metallo-polymers P1eP4 possess similar back-bone structures, there are no obvious differences in the optical band gaps. The electrochemical results indicate that the incor-poration of 1,3,4-OXD pendant groups into the backbones of metallo-polymers will efficiently reduce the LUMO energy levels, and thus to reduce the electron injection barrier
be-tween the cathode and the emitters [36,37].
Table 1
Physical properties of metallo-polymers P1eP4
Polymer Td(C)a h (cP)b,c Ered/peak(V)d ELUMO(eV)e EHOMO(eV)f Band gap (eV)g
P1 355 11 1.54 (r) 3.10 6.26 3.16
P2 389 10 1.54 (r) 3.04 6.20 3.16
P3 366 10 1.54 (r) 2.88 6.03 3.15
P4 356 9 1.55 (r) 2.87 6.02 3.15
a
The decomposition temperatures (Td) (5% weight loss) were determined by TGA with heating rates of 20C/min1under N2atmosphere. TheTdvalues were 221C for 4a (M1), 351C for 4b (M2), and 354C for 4c (M3), respectively.
b
The viscosities of metallo-polymers (10% in weight) in NMP solutions at 25C (100 rpm, spindle number: 4) were determined by rheometer system. c
Solutions of monomers 4ae4c (M1eM3) (10% in weight) in NMP (with viscosities h¼ 6e7 cP, 25C) were used as references to determine the viscosities of metallo-polymers.
d
Reduction peaks in N2-purged DMF, r in parentheses means reversible. e
LUMO energy levels were calculated from the measured reduction potentials versus the ferrocene/ferrocenium couple in DMF solutions. f
HOMO energy levels were estimated from the measured optical band gaps and LUMO energy levels. g
3.5. UVevis and photoluminescence titration
To characterize the formation of metallo-hompolymer P1 and complexes 5a and 5b, the UVevisible (UVevis) and photoluminescence (PL) titration experiments are clear ways
to investigate their stepwise changes. Fig. 2 described that
upon addition of Zn2þ ions to monomer 4a (M1) reaching a
ratio of Znþ2:4a (M1)¼ 1:1, the spectra revealed clear shifts
of three absorption bands at 364, 379, and 405 nm along with one isosbestic point, which suggests that an equilibrium occurred between a finite number of spectrorscopically
dis-tinct species. The titration curve (in the inset ofFig. 3) showed
a linear increase and a sharp end point at a ratio of Znþ2:4a
(M1)¼ 1:1, indicating the formation of metallo-homopolymer
P1. The syntheses and characterization of
metallo-homopoly-mers by adding Zn2þions to monomers 4b (M2) or 4c (M3) to
the ratio of Znþ2:4b (M2)¼ 1:1 or Znþ2:4c (M3)¼ 1:1 have
also been investigated [11]. Beyond this point (Figs. 3 and
4), the subsequent addition of Zn2þ ions induced new peaks
at 279, 285, 348, and 394 nm for complex 5b (332, 348, and 393 nm for 5c) as well as new isosbestic points to form, which points out that an equilibration arose between
spectroscopi-cally distinct species. Thus, Figs. 5 and 6 depicted that the
ratios of Zn2þ ions to monomers 4b (M2) or 4c (M3) are
above 1:1, and the depolymerization is driven by the formation
of chain-terminating complexes 5b or 5c [11,38]. All of the
UVevis titration spectra showed the lowest absorption around
labs¼ 394e404 nm, which corresponds to a charge transfer
occurring between the electron-rich central fluorenyl
300 400 500 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 1.00 1.04 1.08 1.12 1.16 Absorption (a.u.) Equivalents of Zn(OAc)2 @ 380 nm 4a (M1) to P1 Absorption (a.u.) Wavelength (nm)
Fig. 2. UVevis spectra acquired in the process of 4a (M1) to P1 upon the titration of monomer 4a (M1) in CH3CN/CHCl3(2/8 in vol.) with Zn(OAc)2. The spectra are shown at selected ranges of Znþ2:4a (M1)¼ 0e1. The inset shows the normalized absorption at 380 nm as a function of Znþ2:4a (M1) ratio. 300 400 500 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 2.5 1.44 1.46 1.48 1.50 1.52 Absorption (a.u.) Equivalents of Zn(OAc)2 @ 274 nm Absorption (a.u.) Wavelength (nm) 4b (M2) to 5b
Fig. 3. UVevis spectra acquired (in the process of 4b (M2) to 5b) upon the titration of monomer 4b (M2) in CH3CN/CHCl3(2/8 in vol.) with Zn(OAc)2. The spectra are shown at selected ranges of Znþ2:4b (M2)¼ 0e2. The inset shows the normalized absorption at 274 nm as a function of Znþ2:4b (M2) ratio. 300 400 500 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 2.5 0.60 0.64 0.68 0.72 0.76 0.80 Absorption (a.u.) Equivalents of Zn(OAc)2 @ 315 nm Absorption (a.u.) Wavelength (nm) 4c (M3) to 5c
Fig. 4. UVevis spectra acquired (in the process of 4c (M3) to 5c) upon the titration of monomer 4c (M3) in CH3CN/CHCl3(2/8 in vol.) with Zn(OAc)2. The spectra are shown at selected ranges of Znþ2:4c (M3)¼ 0e2. The inset shows the normalized absorption at 315 nm as a function of Znþ2:4c (M3) ratio. 400 500 600 700 0 1000 2000 3000 4000 5000 6000 7000 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 Quantum yields Equivalents of Zn(OAc2) 4a (M1) to P1 4a (M1) to P1 PL Intensity (a.u.) Wavelength (nm)
Fig. 5. PL spectra acquired upon the titration of monomer 4a (M1) in CH3CN/ CHCl3(2/8 in vol.) with Zn(OAc)2. The spectra are shown at selected ranges of Znþ2:4a (M1)¼ 0e1. The inset shows the quantum yields as a function of Znþ2:4a (M1) ratio.
components and the electron-deficient metal-coordinated
ter-pyridyl moieties[38,39]. InFig. 5, monomer 4a (M1) showed
an emission band around 429 nm. As the ratio of Zn2þ:4a
(M1) reached 1:1, a new emission band at 456 nm was in-duced. The PL quantum yields of medium complexes, i.e.
the ratio of Znþ2:4a (M1) gradually approached 1:1 in the
inset of Fig. 5, scarcely increased followed by increasing
molar ratio of Znþ2 ions. Therefore, the PL quantum yields
of metallo-homopolymer P1 can be marginally enhanced by attaching 1,3,4-OXD pendant groups to the polymer back-bones. Furthermore, the monomer 4b (M2) exhibited a similar PL result by the formation of metallo-homopolymer reaching
the ratio of Zn2þ:4b (M2)¼ 1:1, except for the
metallo-homopolymer synthesized from monomer 4c (M3) with CAZ pendants possessing a much lower quantum yield (in solution)
than its monomer 4c [11]. In titration experiments, not only
different complexed situations could be confirmed by various stoichiometries, but also the alteration of photophysical prop-erties between the monomers and polymers were also observed during the coordination processes.
3.6. Photophysical properties
The photophysical measurements of monomers 4ae4c (M1eM3) and metallo-polymers P1eP4 were carried out
by UVevis absorption and photoluminescence (PL) experi-ments in both dilute DMF (N,N-dimethylformamide) solutions (with some solubility problems) and solid films, and their
photophysical properties are presented in Table 2. In Fig. 6,
similar absorption features of polymers P1eP4 were observed at 286, 320, and 377 nm. In general, other absorption peaks
can be assigned to pendant groups (i.e. labs¼ 295 and
300 nm for CAZ and OXD pendants, respectively) and these absorption peaks attributed to pendant groups also can assist to confirm the metallo-copolymer structures. PL emissions of all monomers and polymers are assigned to intraligand (p*ep) fluorescence. They showed purple-blue emission colors in DMF solutions, where the values of PL emission peaks (lmax,PL) were around 414 nm (in DMF) with quantum yields (F) of 20e33% for monomers 4ae4c (M1eM3) and lmax,PLw 417 nm (in DMF) with quantum yields (F) of 34e53% for metallo-polymers P1eP4. The enhancement of the quantum yields in metallo-polymer was attributed to the polymerization procedure and similar results were reported
in literature [9,11,12]. Nevertheless, PL emissions from the
OXD pendants were not observed, even when metallo-polymers were excited at the absorption peaks of the OXD pendants. This indicates the existence of efficient energy
trans-fer from the OXD pendants to the polymer backbones [26].
Comparing PL quantum yields of metallo-copolymers P2e
P4with the same molar ratio (50%) of OXD pendants, the
or-der of PL quantum yields (in solutions) is P2 > P4 > P3, which is reverse to the order of the molar ratio of CAZ pendants in metallo-polymers P2eP4, i.e. P2 (0% CAZ) < P4 (25% CAZ) < P3 (50% CAZ). It suggests that of the metallo-polymers containing M2 (with alkyl pendants) have larger PL quantum yields than those containing M3 (with CAZ pendants). This behavior fits well with the PL quantum yield (53%) of metallo-homopolymer P1 containing M1
(with OXD pendants) being larger than that (23%) [11] of
metallo-homopolymer containing M2 (with alkyl pendants),
and also larger than that (11%)[11]of metallo-homopolymer
containing M3 (with CAZ pendants). Overall, in contrast to metallo-polymers containing alkyl pendants, the quantum yields were greatly enhanced by introducing 1,3,4-OXD pendants but reduced by carbazole (CAZ) pendants. Hence, PL quantum yields of metallo-polymers were truly affected by incorporating different monomer ligands into main-chain polymeric structures. 300 350 400 450 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Absorption (a.u.) Wavelength (nm) P1 P2 P3 P4
Fig. 6. Normalized UVevis spectra of metallo-polymers P1eP4 in DMF solutions.
Table 2
Photophysical properties of monomers 4ae4c (M1eM3) and metallo-polymers P1eP4
Compound labs, sol(nm)a lmax, PL, sol/FPL, sol(nm)a,b,c labs, film(nm) lmax, PL, film(nm)
4a(M1) 292, 363, 378 414/0.33 e e 4b(M2) 285, 319, 363, 373 415/0.20 e e 4c(M3) 287, 294, 319, 347, 364, 374 414/0.25 e e P1 288, 327, 361, 377 418/0.53 289, 298, 337, 399 500 P2 287, 317, 361, 378 416/0.51 299, 354, 413 502 P3 287, 294, 318, 361, 377 416/0.34 289, 354, 408 502 (472, 533) P4 288, 295, 316, 348, 361, 378 416/0.35 (401) 297, 353, 410 500 (473) a Concentration of 1 106M in DMF. b
Coumarin-1 in ethanol (ca. 5 106M, quantum yield¼ 0.73) used as a reference to determine the quantum yields of PL in solutions. c
Regarding solid films of these metallo-polymers, they
emit-ted blue to green colors with PL values of lmax,PLin the range
509e520 nm (Fig. 7). According to PL emissions of polymer
films, all polymers showed large Stokes shifts (ca. 83e 104 nm) in contrast to their PL emissions in solutions, which were attributed to the excimer formation resulting from pe
p stacking of aromatic interaction in solid films [40,41].
When the bulky pedants (i.e. CAZ) were attached to the C-9 position of fluorene units, they were able to suppress the
exci-mer formation as shown in our previous result [9]. However,
metallo-copolymers P3 and P4 only showed small shoulders
around 472 nm (seeFig. 7), which correspond to the emissions
of polymer backbones, and metallo-polymers P1 and P2 even with dominant excimer emissions around 500 nm. Never-theless, compared with our previous result of metallo-homopolymers containing CAZ pendants, OXD pendants in all metallo-polymers P1eP4 did not suppress the excimer formation efficiently, which might be due to more flexible
pendant configurations (with lower Tg values) of
metallo-polymers containing 1,3,4-OXD pendants in comparison with those containing CAZ pedants.
3.7. Electroluminescence (EL) properties
Two device configurations were applied in the EL measure-ments, where the metallo-polymers were used as an emission layer and PEDOT:PSS was utilized as a hole-transporting layer. Either TPBI or BCP/ALQ was used as an electron-transporting layer, and the results are separately discussed as follows:
(a) ITO/PEDOT:PSS/polymer(P1eP2)/TPBI/LiF/Al devices. Good photoluminescence properties exhibited by solutions of metallo-polymers P1 and P2 suggest both polymers are suitable candidates for fabrication of PLED devices, and devices with configurations of ITO/PEDOT:PSS (40e 55 nm)/polymer(P1eP2) (50e65 nm)/TPBI (40 nm)/LiF
(1 nm)/Al (150 nm) were fabricated.Fig. 8shows
normal-ized EL spectra of metallo-polymers P1 and P2, and an
emission band around 570 nm and a small shoulder around 400 nm originated from TPBI emissions were observed
[42,43]. It is worthy of noting that the EL spectra of PLED devices do not resemble their corresponding PL
spectra in solid films (as shown inFig. 7). This is
presum-ably due to the EL and PL emissions originating from
different excited states and/or ground states [11,44]. In
comparison with the HOMO values of polymers P1 (6.26 eV) and P2 (6.20 eV), TPBI (6.20 eV) layer could not block holes from emitters sufficiently for
polymers P1 and P2 in the PLED devices [42,43], so
they exhibited very similar results to each other. Hence, no further investigation has been done in the other TBPI-based PLED devices regarding polymers P3 and P4.
(b) ITO/PEDOT:PSS/polymer/BCP/ALQ/LiF/Al devices.
Green EL emissions were obtained for all metallo-polymers with configurations of ITO/PEDOT:PSS (40e 55 nm)/polymer(P1eP4) (50e65 nm)/BCP (10 nm)/ALQ (30 nm)/LiF (1 nm)/Al (150 nm), and the
electrolumines-cence properties are listed in Table 3. Fig. 9 illustrates
the normalization EL spectra of metallo-polymers P1e
P4 with emission bands around 550 nm. Similarly, the
EL spectra of BCP/ALQ devices do not resemble their corresponding PL spectra in solid films due to the same reason as explained in the TPBI-based PLED devices.
At a bias voltage of 10 V, all PLED devices in Fig. 9
show green emissions with lmax, EL values around
550 nm and their EL intensities were enhanced by increas-ing the bias voltages. The turn-on voltages of all PLED devices based on BCP/ALQ were approximately 6.0e 6.5 V. The power efficiency and maximum luminance of PLED devices for all polymers were ranged from 1.05
to 1.35 cd A1 (at 100 mA/cm2) and 2313e3550 cd/m2
(at 14e15 V), respectively. The current densityevoltage (IeV) and luminanceevoltage (LeV) characteristic curves of PLED devices containing metallo-polymers P1eP4 are
shown inFigs. 10 and 11, and similar turn-on voltages for
both current density and luminance demonstrate that a matched balance of both injection and transportation
300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 PL Intensity (a.u.) Wavelength (nm) P1 (solution) P2 (solution) P3 (solution) P4 (solution) P1 (films) P2 (films) P3 (films) P4 (films)
Fig. 7. Normalized PL spectra of metallo-polymers P1eP4 in solutions and solid films. 400 500 600 700 800 900 0.0 0.2 0.4 0.6 0.8 1.0 EL Intensity (a.u.) Wavelength (nm) P1 P2 TPBI
Fig. 8. Normalized EL spectra of PLED devices with configurations of ITO/ PEDOT:PSS/polymer (P1eP2)/TPBI/LiF/Al at 10 V.
in charges were achieved [26], which were much im-proved in contrast to TPBI-based PLED devices. The
BCP layer (HOMO¼ 6.70 eV, LUMO ¼ 3.20 eV)
can offer a large hole barrier between BCP and the emitter but shows no influence on electron-transporting behavior
of ALQ (HOMO¼ 6.00 eV, LUMO ¼ 3.30 eV) [45].
As shown in Fig. 11, it indicates that no ALQ emission
band was observed. The EL spectra of BCP/ALQ-based devices (polymers P1 and P2) revealed less Stocke shifts
(ca. 20 nm) than those of TPBI-based devices. The dif-ferent timing for the charge arriving at the emitter region may be related to different electron mobilities for these
two transporting materials[43,45,46]. From this result, it
can be concluded that the incorporation of electron-transporting (OXD) or hole-electron-transporting (CAZ) pendants into polymer backbones can improve the EL performance
of the PLED devices[47].
4. Conclusions
In summary, a series of novel poly(fluorene/ethynylene/ (terpyridyl)zinc(II))-based metallo-polymers containing 1,3,4-OXD pedants were obtained by different stoichiometric ratios of complexes. Furthermore, the thermal, photophysical, and electroluminescence properties are greatly affected by the nature of the pendant groups in the C-9 position of fluorene via long alkyl spacers. The incorporation of various pendants into polymer backbones will change the rigidities and the
ther-mal stability (as well as theTdandTgvalues) of the
metallo-polymers. In comparison with metallo-polymers containing alkyl pendants, the quantum yields were greatly enhanced by introducing 1,3,4-OXD pendants but reduced by carbazole (CAZ) pendants. The enhancement of PL quantum yields by the introduction of 1,3,4-OXD pendants into metallo-polymers is due to the energy transfer happened between pedants and polymer backbones. By utilization of these metallo-polymers Table 3
Electroluminescence (EL) properties of PLED devicesacontaining a layer of emitting metallo-polymers P1eP4
Polymer lmax, EL(nm) Von(V)b Max. luminescence (cd/m2) (V) Power efficiency (cd A1)c CIE coordinates (x and y)
P1 549 6.5 2913 (14.5) 1.12 (0.41, 0.52)
P2 549 6.5 3550 (15) 1.35 (0.41, 0.52)
P3 551 6.0 3320 (15) 1.25 (0.41, 0.55)
P4 550 6.5 2313 (14.5) 1.05 (0.41, 0.52)
a
Device structure: ITO/PEDOT:PSS/polymer(P1eP4)/BCP/AlQ/LiF/Al, where the metallo-polymer (P1eP4) is an emitting layer. b
Vonis the turn-on voltage. c
Power efficiencies were obtained at 100 mA/cm2.
400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 EL Intensity (a.u.) Wavelength (nm) P1 P2 P3 P4
Fig. 9. Normalized EL spectra of PLED devices with configurations of ITO/ PEDOT:PSS/polymer (P1eP4)/BCP/ALQ/LiF/Al at 10 V. 0 2 4 6 8 10 12 14 16 0 100 200 300 400 500 600 700
Current Density (mA/cm
2) Voltage (V) P1 P2 P3 P4
Fig. 10. Current densityevoltage (IeV) curves of PLED devices with config-urations of ITO/PEDOT:PSS/polymer (P1eP4)/BCP/ALQ/LiF/Al.
0 2 4 6 8 10 12 14 16 0 500 1000 1500 2000 2500 3000 3500 4000 Luminescence (cd/m 2) Voltage (V) P1 P2 P3 P4
Fig. 11. Luminanceevoltage (LeV) curves of PLED devices with configura-tions of ITO/PEDOT:PSS/polymer (P1eP4)/BCP/ALQ/LiF/Al.
as emitting materials to fabricate PLED devices, green EL emissions and high EL performance can be obtained in the double-heterojunction device structures with PEDOT as the hole-transporting material and either TPBI or BCP/ALQ as the electron-transporting material.
Acknowledgements
We thank the financial support from Chung-Shan Institute of Science and Technology (in Taiwan) and the National Science Council of Taiwan (ROC) through NSC 94-2113-M-009-005, and the instrumental support provided by Prof. Yu-Tai Tao (vacuum deposition) at Institute of Chemistry, Academia Sinica and Prof. Ching-Fong Shu (CV measure-ments) at Dept. of Applied Chemistry, National Chiao Tung Univ. (in Taiwan).
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