Poly(p-phenylenevinylene) derivatives containing electron-transporting aromatic triazole or oxadiazole segments

Article abstract of DOI:10.1021/ma048990m

We report the synthesis, optical and electrochemical details, and properties of copolymers P1-P3 consisting of alternate hole-transporting 1,4-bis(hexyloxy)-2,5-distyrylbenzene (HDB) and electron-transporting 4-(4-(hexyloxy)phenyl)-3,5-diphenyl- 4H-1,2,4-

Full text of DOI:10.1021/ma048990m

Macromolecules 2005, 38, 53-60
53
Poly(p-phenylenevinylene) Derivatives Containing
Electron-Transporting Aromatic Triazole or Oxadiazole Segments
Shinn-Horng Chen and Yun Chen*
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan,
Republic of China
Received May 21, 2004; Revised Manuscript Received October 8, 2004
ABSTRACT: We report the synthesis, optical and electrochemical details, and properties of copolymers
P1-P3 consisting of alternate hole-transporting 1,4-bis(hexyloxy)-2,5-distyrylbenzene (HDB) and electron-
transporting 4-(4-(hexyloxy)phenyl)-3,5-diphenyl- 4H-1,2,4-triazole (EDT) or 2,5-diphenyl-1,3,4-oxadiazole
(EDO) segments linked via an ether spacer or a twisted σ-bond (biphenyl). These copolymers are soluble
in common organic solvents such as chloroform, NMP, and 1,1,2,2-tetrachloroethane and exhibit good
thermal stability with decomposition temperatures higher than 375 °C. P1-P3 show efficient energy
transfer from EDT or EDO to EDO fluorophores when photoexcited. Optical and electrochemical properties
of P1-P3 are also investigated in detail by comparing with P4 and P5 containing similar chromophores.
From the cyclic voltammograms the onset oxidation and reduction potentials for isolated P1 and conjugated
P2 are comparable, indicating that the effect of the twisted σ-bond in P2 is similar to that of the ether
spacer in P1. The optimized geometries of P2 and P3 show that the torsion angle between HDB and
EDT or EDO are 83.6° or 89.6°, respectively, based on MNDO semiempirical calculations. The large torsion
angle in P2 and P3 significantly limits delocalization of charges between hole- and electron-transporting
segments. Accordingly, in P2 and P3 the oxidation and reduction starts at the hole- and the electron-
transporting, respectively, like those in isolated P1. The HOMO and LUMO energy levels of P1, P2, and
P3, estimated from electrochemical data, are -5.16, -5.12, and -5.19 eV and -3.35, -3.38, and -3.23
eV, respectively. Single-layer light-emitting diodes (Al/P1-P3/ITO) have been successfully fabricated,
and they reveal blue or yellow electroluminescence.
Introduction
In this work three copolymers P1-P3 (Scheme 2),
consisting of alternate isolated or covalently attached
hole-transporting HDB and electron-transporting
EDT14-17 or EDO segments, have been synthesized and
characterized. Furthermore, their optical and electro-
chemical properties are investigated by comparing with
polymers P4 and P5 (as shown in Scheme 3) possessing
similar structures.18,19 The connection between hole- and
electron-transporting segments of P1-P5 can be divided
into three types: (1) an ether spacer for P1 and P4, (2)
a single bond for P2 and P3, and (3) a vinylene bond
for P5. We studied the optical properties of P1-P4 to
elucidate the influence of conjugated and isolated
structures. Furthermore, electrochemical properties of
these polymers are also investigated in detail to evalu-
ate the influence of geometric structures in P2 and P3.
Electroluminescent (EL) polymeric materials are gen-
erally categorized into fully conjugated polymers, main-
chain or side-chain polymers with isolated chromophores,
and polymeric blends.1-3 Among these materials π-con-
jugated polymers with donor-acceptor architectures are
currently of interest because the electron or hole affini-
ties can be enhanced4 and the intramolecular charge
transfer can lead to small band gap semiconducting
copolymers for red emission.5
In our previous research6-8 isolated copoly(aryl ether)s
consisting of defined hole-transporting (donor) or electron-
transporting (acceptor) segments did enhance electron
and hole affinities simultaneously, among which the
nonconjugated spacers conduced toward large band gap
for blue emission. However, the characteristics, which
normally occur in isolated polymers, are barely observed
in conjugated polymers.
Experimental Section
Materials. The synthetic procedures of 16, 17 (Scheme 2),
M2, M3, P4, and P5 (Scheme 3) have been described in the
literature.18,19 N-Methyl-2-pyrrolidone (NMP, Riedel-Dehaen
Co.), N-cyclohexylpyrrolidone (CHP, Janssen Chimica Co.),
chloroform (CHCl3, Tedia Co.), tetrahydrofuran (THF, Tedia
Co.), and other solvents were HPLC-grade reagents. All
reagents were used without any further purification.
Instrumentation. All new compounds were identified by
1H NMR, FT-IR, and mass or elemental analysis (EA). The
1H NMR spectra were recorded on a Bruker AMX-400 MHz
FT-NMR, and chemical shifts are reported in ppm using
tetramethylsilane (TMS) as an internal standard. The FT-IR
spectra were measured as a KBr disk on a Fourier transform
infrared spectrometer, model Valor III from Jasco. Nominal
and high-resolution mass spectra were recorded using an
analytical mass spectrometer, model 70-250S, from VG. El-
emental analysis was carried out on a Heraus CHN-Rapid
elemental analyzer. Thermogravimetric analysis (TGA) of the
In the past the torsion effect on optical properties of
conjugated polymers has been discussed frequently. The
greater the torsion angle, the more diminished the
effective conjugation length or reduced interchain
interaction.9-11 However, Bard et al. recently investi-
gated electrogenerated chemiluminescence (ECL) and
verified that the twisted structure between donor and
acceptor groups in conjugated molecules leads to dimin-
ished electron delocalization and generation of localized
radical cations and ions.12,13 It seems that this specific
characteristic in twisted structure can be incorporated
into conjugated EL polymers with donor and acceptor
groups.
* To whom correspondence should be addressed. E-mail:
yunchen@mail.ncku.edu.tw.
10.1021/ma048990m CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/13/2004

54 Chen and Chen
Macromolecules, Vol. 38, No. 1, 2005
Scheme 1
Scheme 3
using the ferrocence (FOC) value of -4.8 eV with respect to
vacuum level, which is defined as zero.20 Single-layer light-
emitting diodes with a configuration of Al/P1-P3/ITO were
fabricated by spin coating polymer layer. The film thickness
was about 60-100 nm as measured by atomic force microscopy
(AFM). The thin aluminum layer was deposited as the cathode
by thermal evaporation under a vacuum about 10-5 Torr. The
optoelectronic characteristics of these devices were measured
by a Keithley power supply (model 2400), and the EL spectra
of the device were also measured by an Ocean Optics usb2000
fluorescence spectrophotometer. Fabrication and characteriza-
tion of the devices were under ambient environment.
Synthesis of Monomers (Scheme 1). Synthesis of Com-
pounds 9-11. To a two-necked flask were added with 4-bro-
mobenzoyl chloride (1; 1.32 g, 6 mmol) hydrazine monohydrate
(3; 0.15 g, 3 mmol), and 15 mL of NMP. The mixture was
allowed to react at room temperature for 5 h. After reaction
was completely promoted, it was precipitated from distilled
water. The appearing products were collected by filtration,
washed with ethyl acetate, and dried in a vacuum oven to
provide 4 (77%).
Scheme 2
The mixture 4 (0.84 g, 2.1 mmol) and phosphorus pen-
tachloride (0.96 g, 4.62 mmol) was dissolved in 10 mL of
toluene and stirred at 120 °C for 3 h in a nitrogen atmosphere.
Toluene was stripped off under vacuum, and the residue was
washed with distilled water. The appearing solids were col-
lected by filtration, dried, and recrystallized from ethanol and
dichloromethane to afford yellow solids of 6 (85%).
The mixture of 6 (0.86 g, 2 mmol), 4-hexoxylaniline (0.39 g,
2 mmol), and 15 mL of N,N-dimethylaniline was stirred at 135
°C in a nitrogen atmosphere for 12 h. After reaction was
completely promoted, 30 mL of 2 N HCl(aq) was added and the
mixture was stirred for another 0.5 h. The solids were filtered,
dried, and then purified by column chromatography on silica
gel using ethyl acetate/n-hexane as the eluent. Evaporation
of the eluent afforded 9 (40%). The procedures for compounds
5 (68%), 7 (91%), and 10 (48%) were analogous to those for 4,
6, and 9, respectively. The ring closure of 6 in the presence of
POCl3 at the reflux temperature was carried out to yield
compound 11 (41%).
4: mp > 200 °C. 1H NMR (DMSO-d6, ppm): δ 10.62 (s, 2H,
-NH-), 7.85 and 7.83 (d, 4H, Ar H), 7.75 and 7.73 (d, 4H, Ar
H). FT-IR (film, cm-1): 3191 (-CONH-), 3016, 2839, 2558,
1684, 1609, 1495, 1468, 1322, 1072 (C-Br), 1011, 931, 840.
Anal. Calcd for C14H10Br2N2O2: C, 42.24; H, 2.53; N, 7.04.
Found: C, 42.28; H, 2.64; N, 7.03.
polymers was performed under nitrogen atmosphere at a
heating rate of 20 °C/min using a Perkin-Elmer TGA-7 thermal
analyzer. Thermal properties of the polymers were measured
using a differential scanning calorimeter (DSC), Perkin-Elmer
DSC 7, under nitrogen atmosphere at a 20 °C/min heating rate.
UV-vis spectra were measured on a Jasco V-550 spectropho-
tometer, and the photoluminescence (PL) spectra were ob-
tained using a Hitachi F-4500 fluorescence spectrophotometer.
The diagrammatic curves of cyclic voltammetry were measured
on a BAS CV-50W at room temperature under nitrogen
atmosphere using ITO glass as the working electrode, a Ag/
AgCl electrode as the reference electrode, and a platinum wire
electrode as the auxiliary electrode supported in 0.1 M
(n-Bu)4NClO4 in acetonitrile. The energy levels were calculated
5: mp > 200 °C. 1H NMR (DMSO-d6, ppm): δ 10.53 (s, 2H,
-NH-), 7.99-7.95 (m, 4H, Ar H), 7.37-7.33 (m, 4H, Ar H).
FT-IR (film, cm-1): 3209 (-CONH-), 3012, 2844, 2558, 1609,
1579, 1461, 1222, 1100 (C-F), 1012, 857. Anal. Calcd for
C14H10F2N2O2: C, 60.87; H, 3.65; N, 10.14. Found: C, 60.82;
H, 3.79; N, 10.11.
6: mp ) 144-145 °C. 1H NMR (DMSO-d6, ppm): δ 7.99
and 7.97 (d, 4H, Ar H), 7.82 and 7.79 (d, 4H, Ar H). FT-IR

Macromolecules, Vol. 38, No. 1, 2005
Poly(p-phenylenevinylene) Derivatives 55
(film, cm-1): 3171, 3080, 2804, 2297, 1910, 1784, 1579 (-Nd
N-), 1483, 1393, 1222, 1066 (C-Br), 1011, 920, 825. Anal.
Calcd for C14H8Br2Cl2N2: C, 38.66; H, 1.85; N, 6.44. Found:
C, 38.67; H, 1.93; N, 6.45.
Table 1. Polymerization Results and Characterization of
P1∼P3
Mna ( 104)
Mwa ( 104)
PDIa
Tdb (°C)
P1
P2
P3
2.05
0.40
0.37
5.04
0.51
0.50
2.45
1.25
1.51
377
408
403
1
7: mp ) 133-134 °C. H NMR (DMSO-d6, ppm): δ 8.14-
8.10 (m, 4H, Ar H), 7.46-7.40 (m, 4H, Ar H). FT-IR (film,
cm-1): 3109, 3071, 1903, 1605, 1575 (-NdN-), 1504, 1402,
1297, 1230, 1155, 1096 (C-F), 928, 839. Anal. Calcd for
C14H8F2Cl2N2: C, 53.70; H, 2.58; N, 8.95. Found: C, 53.72; H,
2.78; N, 8.931.
a Mn, Mw, and PDI of the polymers were determined by gel
permeation chromatography using polystyrene standards in CHCl3.
b The 5% weight-loss temperatures.
9: mp ) 179-180 °C. 1H NMR (acetone-d, ppm): δ 7.57
and 7.56 (d, 4H, Ar H), 7.54-7.31 (m, 6H, Ar H), 7.06 and
7.04 (d, 2H, Ar H), 4.07-4.02 (m, 2H -CH2-), 1.81-1.74 (m,
2H, -CH2-), 1.50-1.38 (m, 2H, -CH2-), 1.37-1.33 (m, 2H,
-CH2-), 0.92-0.88 (m, 3H, -CH3). FT-IR (film, cm-1): 2930,
2864, 1599, 1513 (CdN), 1468, 1257 (-C-O-C-), 1066 (C-
Br), 1007, 1840. Anal. Calcd for C26H25Br2N3O: C, 56.24; H,
4.54; N, 7.57. Found: C, 56.02; H, 4.61; N, 7.58.
M1: mp ) 161-162 °C. 1H NMR (acetone-d, ppm): δ 7.45-
7.42 (m, 8H, Ar H), 7.32-7.29 (m, 2H, Ar H), 7.04 and 7.01 (d,
2H, Ar H), 6.98 and 6.97 (d, 4H, Ar H), 6.89 and 6.87 (d, 4H,
Ar H), 4.04-3.99 (m, 2H -CH2-), 1.79-1.74 (m, 2H, -CH2-
), 1.51-1.43 (m, 2H, -CH2-), 1.35-1.30 (m, 4H, -CH2-),
1.27-1.24 (m, 18H, -CH3), 0.92-0.87 (m, 3H, -CH3). FT-IR
(film, cm-1): 2958, 2869, 1600, 1508 (CdN), 1474, 1247 (-C-
O-C-), 1171, 1108, 1012, 831. Anal. Calcd for C46H51N3O3:
C, 79.62; H, 7.41; N, 6.06. Found: C, 79.53; H, 7.47; N, 6.05.
Synthesis of Polymers P1-P3 (Scheme 2). P1-P3 were
prepared by the nucleophilic displacement reaction or Horner-
Wadsworth-Emmons reaction described as follows. To a two-
necked 25-mL glass reactor 10 (0.22 g, 0.50 mmol), 16 (0.26 g,
0.50 mmol), 10 mL of toluene, 5 mL of solvent mixture of NMP/
CHP (v/v ) 1/1), and an excess of K2CO3 (0.166 g, 1.20 mmol)
were added. The reaction mixture was then heated to 170 °C
and reacted for 80 h. The toluene was removed by condensing
in the Dean-Stark trap. The mixture was dropped into 150
mL of a methanol/distilled water (v/v ) 2/1) mixture. The P1
precipitates were collected by filtration and further purified
by Soxhlet extraction for 24 h using isopropyl alcohol as solvent
(yield 51%).
10: mp ) 173-174 °C. 1H NMR (acetone-d, ppm): δ 7.53-
7.48 (m, 4H, Ar H), 7.33-7.31 (m, 2H, Ar H), 7.17-7.02 (m,
6H, Ar H), 4.06-4.01 (m, 2H -CH2-), 1.83-1.74 (m, 2H,
-CH2-), 1.53-1.33 (m, 6H, -CH2-), 0.92-0.87 (m, 3H,
-CH3). FT-IR (film, cm-1): 3063, 2940, 2874, 1609, 1511 (Cd
N), 1479, 1256 (-C-O-C-), 1222, 1100 (C-F), 1024, 843.
Anal. Calcd for C26H25F2N3O: C, 72.04; H, 5.81; N, 9.69.
Found: C, 72.03; H, 5.83; N, 9.59.
11: mp > 200 °C. 1H NMR (DMSO-d6, ppm): δ 8.08 and
8.05 (d, 4H, Ar H), 7.85 and 7.83 (d, 4H, Ar H). FT-IR (film,
cm-1): 3080, 2930, 2362, 1930, 1604, 1543, 1478, 1393, 1272
(C-O-C), 1077 (C-Br), 1006, 835. Anal. Calcd for C14H8-
Br2O: C, 44.25; H, 2.12; N, 7.37. Found: C, 44.29; H, 2.14; N,
7.37.
Synthesis of Compounds 13 and 14. Compounds 13 and 14
were synthesized by Suzuki biaryl coupling reaction as follows.
To a two-necked 10-mL glass reactor 9 (1.11 g, 2 mmol),
4-formylbenzeneboronic acid 12 (0.76 g, 4.4 mmol), Pd(Ph3)4
(0.36 g, 0.31 mmol), 20 mL of toluene, 20 mL of ethanol (99%),
and 10 mL of 2 M Na2CO3(aq) were added under nitrogen
atmosphere. The mixture was allowed to react at 80 °C for 12
h. After reaction was completely promoted, it was poured into
distilled water and extracted with chloroform. The organic
layer was dried with anhydrous magnesium sulfate and
vacuum concentrated. The crude product was purified by
washing with n-hexane and ethyl acetate and then dried in a
vacuum to afford compound 13 (64%). The synthetic procedure
of 14 was analogous to 13 with a yield of 40%.
A two-necked flask 25-mL glass reactor was charged with
13 (0.31 g, 0.5 mmol), 17 (0.28 g, 0.5 mmol), 6 mL of NMP,
and solid potassium tert-butoxide. The mixture was stirred at
ambient temperature for 24 h and then precipitated in a large
amount of methanol to isolate solid products. The P2 was
purified by Soxhlet extraction for 24 h using acetone as solvent
(yield 61%). The synthesis procedure of P3 was analogous to
P2 with a yield of 66%.
P1: 1H NMR (CDCl3, ppm) δ 7.49-7.43 (m, 12H, Ar H),
7.10-6.92 (m, 14H, Ar H), 4.05-3.97 (m, 6H -CH2-), 1.87-
1.46 (m, 6H, -CH2-), 1.35-1.19 (m, 18H, -CH2-), 0.97 (s,
9H, -CH3). FT-IR (film, cm-1): 2936, 2855, 1707, 1595 (Cd
N), 1506, 1471, 1241 (-C-O-C-), 1167, 1008, 841. Anal.
Calcd for C60H65N3O5: C, 79.26; H, 7.32; N, 4.62. Found: C,
76.60; H, 6.57; N, 4.76.
13: mp > 200 °C. 1H NMR (acetone-d, ppm): δ 10.04 (s,
2H, -CHO), 8.03 and 8.00 (d, 4H, Ar H), 7.93 and 7.91 (d, 4H,
Ar H), 7.79 and 7.76 (d, 4H, Ar H), 7.65-7.62 (d, 4H, Ar H),
7.45 and 7.42 (d, 2H, Ar H), 7.12-7.09 (d, 2H, Ar H), 4.06 (s,
2H -CH2-), 1.83-1.74 (m, 2H, -CH2-), 1.43-1.32 (m, 6H,
-CH2-), 0.88 (s, 3H, -CH3). FT-IR (film, cm-1): 2933, 2853,
1701 (-CHO), 1609, 1511 (CdN), 1479, 1247 (-C-O-C-),
1003, 843. HR-MS (EI with DCI probe) m/z (M+) calcd for
C40H35N3O3: 605.2678; obsd 605.2676.
P2: 1H NMR (CDCl3, ppm) δ 7.60-7.26 (m, 16H, Ar H),
7.03-6.82 (m, 8H, Ar H), 4.24-4.02 (m, 6H -CH2-), 1.64-
1.22 (m, 24H, -CH2-), 0.94-0.92 (m, 6H, -CH3). FT-IR (film,
cm-1): 2947, 2865, 1689, 1603 (CdN), 1509, 1471, 1245 (-C-
O-C-), 1203, 962, 845. Anal. Calcd for C60H65N3O3: C, 82.25;
H, 7.48; N, 4.80. Found: C, 80.78; H, 7.36; N, 4.94.
P3: 1H NMR (CDCl3, ppm) δ 8.26-8.23 (m, 4H, Ar H),
7.81-7.55 (m, 12H, Ar H), 6.93-6.82 (m, 4H, Ar H), 4.12-
4.08 (m, 4H -CH2-), 1.89-1.43 (m, 16H, -CH2-), 0.95-0.75
(m, 6H, -CH3). FT-IR (film, cm-1): 2927, 2857, 1703, 1607
(CdN), 1486, 1420, 1203 (-C-O-C-), 1003, 962, 810. Anal.
Calcd for C48H48N2O3: C, 82.55; H, 6.90; N, 4.00. Found: C,
79.60; H, 6.67; N, 3.93.
14: mp > 200 °C. 1H NMR (DMSO-d6, ppm): δ 10.08 (s,
2H, -CHO), 8.30 and 8.28 (d, 4H, Ar H), 8.07-8.04 (m, 12H,
Ar H). FT-IR (film, cm-1): 3075, 2830, 1694 (-CHO), 1604,
1483, 1378, 1237 (C-O-C), 1201, 1006, 835. HR-MS (EI with
DCI probe) m/z (M+) calcd for C28H18N2O3: 430.1317; obsd
430.1316.
Synthesis of Model Compound M1 (Scheme 1). To a two-
necked 25-mL glass reactor was charged with 10 (0.22 g, 0.5
mmol), 15 (0.18 g, 1.2 mmol), 5 mL of solvent mixture of NMP/
CHP (v/v ) 1/1), and an excess amount of K2CO3 (0.166 g, 1.20
mmol). The reaction mixture was then heated to 170 °C and
reacted for 48 h. After the reaction was completely promoted,
the reaction mixture was poured into distilled water. The
mixture was extracted with chloroform, and the extract was
washed with distilled water, dried with anhydrous magnesium
sulfate, and then concentrated under reduced pressure. The
crude products were purified by column chromatography using
ethyl acetate/n-hexane (v/v ) 1/3) as eluent. Evaporation of
the eluent afford white solid of M1 (35%).
Results and Discussion
Synthesis and Characterization of Copolymers.
The number-average (Mn) and the weight-average mo-
lecular weights (Mw) of P1-P3, determined by gel
permeation chromatography using polystyrene as stan-
dard, are listed in Table 1. All the synthesized polymers
were soluble in common organic solvents such as
chloroform, NMP, and 1,1,2,2-tetrachloroethane. Ther-
mal properties of the synthesized polymers were evalu-
ated by TGA under nitrogen atmosphere. The weight
losses were less than 5% on heating to 375 °C for each

56 Chen and Chen
Macromolecules, Vol. 38, No. 1, 2005
Figure 1. Photoluminescence (excitation ) 285 nm for M1
and 397 nm for M2) and UV-vis absorption spectra of model
compound M1 (- -) and M2 (s) in CHCl3 solutions of 1 × 10-5
M.
polymer. No melting temperature and obvious glass-
transition temperature were observed below 300 °C in
the DSC thermogram. The polymerization and charac-
terization results of P1-P3 are tabulated in Table 1.
Optical Properties. Figure 1 illustrates the absorp-
tion and photoluminescence spectra (in CHCl3) of model
compounds M1 and M2, which, respectively, simulate
the isolated EDT and HDB fluorophores of P1. The
emission peaks of M1 and M2 are located at 356 and
449 nm, respectively. For isolated copolymers P1 it is
reasonable that its emissions originate from the isolated
fluorophores, i.e., electron-transporting EDT and hole-
transporting segment HDB segments. As shown in
Figure 2, however, the PL spectra of P1 (excitation at
388 nm) are mainly dominated by HDB fluorophores
with longer emissive wavelength and show a maximum
peak at 453 nm. This indicates that efficient excitation
energy transfer from EDT fluorophores to HDB fluoro-
phores has occurred in P1. The emission spectra of M1
overlap greatly with the absorption of M2 (Figure 1),
suggesting that excitation energy might transfer from
M1 (energy donor) to M2 (energy acceptor) if the
distance between M1 and M2 is close enough. To
ascertain this presumption, PL spectral transforms of
M1 and M2 mixture in CHCl3 upon increasing concen-
tration are traced. As shown in Figure 3 the relative
intensity of M2 (λmax ) 450 nm), as compared to that of
M1 (λmax ) 356 nm), significantly enhances when the
concentration was increased from 5 × 10-6 to 5 × 10-5
M. The transformation of the PL spectra (excitation at
285 nm) with concentration would be caused by in-
creased reabsorption and excitation energy transfer due
to reduced intermolecular distance, which is mainly
controlled by concentration in solution. Accordingly, P1
shows only one peak at 453 nm that must be due to
efficient intrachain energy transfer because EDT (en-
ergy donor) and HDB (energy acceptor) segments are
connected only via an ether spacer. Similar efficient
energy transfer has also been observed in polymers P2-
P4 (Figure 2). The relative phenomenon has been
discussed in our previous work.19
Figure 2. Photoluminescence and UV-vis absorption spectra
of P1-P4 in CHCl3 with a concentration of 1 × 10-5 M (- - -)
and in films coated on a quartz plate (s). Excitation ) 388,
422, 421, and 400 nm for P1, P2, P3, and P4, respectively.
conjugation, which is interrupted by ether spacers.
Moreover, the PL spectral peaks of P2 and P3 shift
greatly from 476 and 487 nm in solution to 538 and 535
nm in films coated onto quartz plate, respectively,
suggesting the formation of intermolecular excimers20
in the film state. A similar phenomenon was also
observed in P5 (Scheme 3).21 However, the red shifts of
P1 and P4 in the film state are only 11 and 13 nm,
respectively, as compared with those in chloroform
solution. The results suggest that formation of inter-
chain interaction can probably be reduced by limiting
the effective conjugation length. The optical properties
of the polymers (P1-P5) and model compounds (M1,
M2) are summarized in Table 2. As shown in Table 2,
the relative PL quantum yields (ΦPL) of P1 (0.46) and
P2 (0.39) containing triazole segments are lower than
those of P3 (0.54) and P4 (0.58) possessing oxadiazole
segments. Furthermore, the ΦPL of isolated P4 is higher
than fully conjugated P5 (0.40), and the ΦPL of the
twisted P3 is located between those of P4 and P5.
Therefore, confinement of conjugation, whether via
The PL spectra of P1, P2, P3, and P4 in CHCl3
solution exhibit peaks at 453, 476, 487, and 452 nm,
respectively, as shown in Figure 2. The PL spectral
peaks of isolated P1 and P4 reveal a blue shift of 23
and 35 nm, respectively, as compared with those of
conjugated P2 and P3. This is due to diminished

Macromolecules, Vol. 38, No. 1, 2005
Poly(p-phenylenevinylene) Derivatives 57
Figure 3. Photoluminescence (excitation ) 285 nm) spectra
of P1 (1 × 10-5 M) and a mixture of M1 and M2 5 × 10-6 (s)
and 5 × 10-5 M (- ‚ -) in CHCl3.
Table 2. Optical Properties of Model Compounds and
Polymers
UV-vis
UV-vis
λmax
a
a
λmax
PL λmax
PL λmax
film
(nm)
b
solution
(nm)
film
solution
(nm)
ΦPL
(nm)
solution
P1 287, 388
P2 348, 422
P3 353, 421
P4 330, 400
293, 394
426
372, 427
328, 401
453, 478s 464
476, 506s 538
487, 512s 535
452, 475s 465, 486s
0.46
0.39
0.54
0.58
0.40
Figure 4. Cyclic voltammograms of P1 (s), P2 (- -), P3 (- -
), and P5 (- ‚ -) (100 mV/s) in film coated on ITO glass electrode
in a CH3CN solution of Bu4NClO4 (0.1 M).
P5 362s, 442 382s, 459 515, 547s 550
M1 285
M2 333, 397
356
449, 466s
a 1 × 10-5 M in CHCl3. s means the wavelength of the shoulder.
b These values were estimated using quinine sulfate (dissolved in
1 N H2SO4(aq) with a concentration of 10-5 M, assuming ΦPL of
0.55) as a standard. The excitation wavelengths were 350 nm for
P1-P5 solutions (10-6 M repeating unit at which their absorbance
was less than 0.05).
energy via semiempirical MNDO calculations in the gas
state.22 Each HDB group in P2 or P3 is twisted from
the EDO or EDT segment at 83.6° or 89.6°, respectively.
However, if the HDB is connected with EDO via a
vinylene bond like in P5, the torsion is decreased
substantially to ca. 1°. Therefore, it is suggested that
the twisted structure between hole- (HDB) and electron-
transporting segments (EDT or EDO) in P2 and P3
effectively limits delocalization of electronic charges as
that of the ether spacer does in P1 and P4. Therefore,
P2 and P3 show similar onset oxidation and reduction
potentials to those of P1 and P4, respectively. Further-
more, the oxidation and reduction of P2 and P3 would
also selectively start at the hole- and electron-transport-
ing segments, respectively. However, as observed in
Figure 2, the PL spectral peaks of P2 and P3 reveal a
red shift of 23 and 35 nm compared to P1 and P4,
respectively, suggesting that the torsion in P2 or P3
does not completely break conjugation length in poly-
meric chain.
As shown in Figure 4b the onset oxidation and
reduction potentials of P3 are 0.39 and -1.96 V,
respectively, whereas those of P5 are 0.34 and -2.31
V. This result indicates that P3 is much more readily
reduced than P5, but the oxidation tendency is just the
opposite. P3 and P5 contain the same HDB (hole-
transporting) and EDT (electron-transporting) seg-
ments, but the linkages between HDB and EDT are the
σ-bond and vinylene bond, respectively. Although the
conjugated length of P3 and P5 is slightly different,
clearly the torsion in P3 also plays an important role
in determining its electrochemical property.
ether spacer or twisted biphenyl linkages, seems to be
an effective way to enhance PL quantum yield.
Electrochemical Properties. In our previous stud-
ies on copoly(aryl ether)s with isolated alternate hole-
and electron-transporting segments we verified that
oxidation and reduction start at the former and latter
segments, respectively.6-8 This characteristic is nor-
mally found in isolated copolymers and confirms that
it can enhance electron and hole affinities simulta-
neously if proper electron- and hole-transporting seg-
ments are incorporated. Isolated P1 and P4 are typical
copolymers with alternate hole- and electron-transport-
ing segments.
Figure 4a shows the cyclic voltammograms (CVs) of
P1 and P2 films coated on ITO glass. The onset
oxidation and reduction potentials of P1 are observed
at 0.36 and -1.42 V, respectively. It is noteworthy that
P2 shows almost the same potentials (at 0.32 and -1.45
V) as those of P1, suggesting that conjugated P2 and
isolated P1 possess similar electrochemical properties
regardless of their structural difference. Accordingly, the
biphenyl linkages (between HDB and EDT segments)
in P2 exert a similar effect on the electrochemical
property as the ether spacer in P1. Bard et al. observed
similar results in low molecular weight compounds with
a twisted structure between donor and acceptor seg-
ments.12,13
Optimized molecular geometries for repeat units of
P2, P3, and P5 (Figure 5) were obtained by minimizing
Furthermore, in anodic scan P1-P4 exhibit almost
the same onset oxidation potentials around 0.36-0.39

58 Chen and Chen
Macromolecules, Vol. 38, No. 1, 2005
Figure 6. P1 (s), P2 (- - -), P3 (- -), and P4 (- ‚ -) (100 mV/
s) in film coated on ITO glass electrode in a CH3CN solution
of Bu4NClO4 (0.1 M).
Figure 7. Energy band diagram of P1-P3 (hole-transporting
segments, s; electron-transporting segments, - - -) and P5.
P2 and P3 can be estimated similarly as those for P1.
The HOMO and LUMO energy levels of P1, P2, and
P3 are -5.16, -5.12, and -5.19 eV and -3.35, -3.38,
and -3.23 eV, respectively. The band gaps (Egopt) for
M1, M2, and M3, calculated from their onset of absorp-
tion, were 3.71, 2.79, and 2.53 eV. The energy band
diagrams of P1-P3 and P5 are proposed in Figure 7.
For P1-P3 the LUMO levels of hole-transporting
segments (top horizontal line of the broken rectangle)
can be estimated from the HOMO levels of the polymers
by adding Egopt of the corresponding model compounds.
The HOMO levels of the electron-transporting segments
(bottom horizontal line of the solid rectangle) can be
estimated similarly. The electrochemically determined
band diagrams of P1-P3 are the overlapped portions
of the corresponding composing model compounds.
Therefore, hole and electron affinities of P1-P3 can be
promoted simultaneously. However, as compared with
the LUMO level of conjugated P5 (-2.83 eV), the
twisted P3 (-3.23 eV) exhibits a lower barrier of
electron injection, which is probably attributable to
electron-withdrawing aromatic oxadiazole chromophores.
The HOMO energy levels calculated from the semiem-
pirical MNDO method (for instance, -8.50 eV for
repeating unit of P2) are much lower than those
obtained from cyclic voltammograms. However, this
large deviation in HOMO energy levels might result
from disordered arrangement in polymers, solvation
effects, or ion-pair interaction.13 Therefore, we compare
Figure 5. Optimized geometries obtained from semiempirical
MNDO calculation for P2 (a), P3 (b), and P5 (c).
V, as shown in Figure 6. This is attributable to the fact
that they all contain the same hole-transporting seg-
ments (HDBs), regardless of whether the HDBs are
linked to electron-transporting segment (EDT or EDO)
via ether spacer or σ-bond with torsion. Therefore, it
can be concluded that the electron and hole affinities of
isolated copoly(aryl ether) (P1) or twisted conjugated
polymers (P2 and P3) can be engineered similarly by
selecting proper hole- and electron-transporting seg-
ments.
The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels of
P1 can be calculated by comparing with the ferrocence
value of -4.8 eV below the vacuum level.23 For P2 and
P3, oxidation and reduction would selectively start at
the hole- and electron-transporting segments, respec-
tively. Thus, the HOMO and LUMO energy levels for

Macromolecules, Vol. 38, No. 1, 2005
Poly(p-phenylenevinylene) Derivatives 59
of electrons and holes in isolated polymers.18 This
drawback is presumably alleviated in P3 because its
conjugation is not broken absolutely. As shown in Figure
9, the maximum brightness of P3 (43 cd m-2) has
improved as compared with P1 (10 cd m-2). However,
its brightness is still lower than P5 (ca. 1800 cd m-2),21
which can be attributed to the excimer formation or
twisted structure.
Conclusion
Three polymers (P1-P3) consisting of alternate hole-
transporting (HDB) and electron-transporting segments
(EDT or EDO) have been synthesized and characterized.
Their optical and electrochemical properties have also
been studied by comparing with those of polymers P4
and P5 containing similar chromophores. From PL
spectral investigation it is found that P1-P4 efficiently
show energy transfer as compared with a mixture of the
corresponding model compounds. Moreover, P2 and P3
exhibit interchain interaction (excimer) in the film state.
From the cyclic voltammograms the onset oxidation and
reduction potentials for P1 and P2 are similar. This can
be explained by the optimized geometry of P2 and P3,
determined from semiempirical MNDO calculations,
which shows that HDB and EDT or EDO are twisted
83.6° and 89.6°, respectively. The torsions in P2 and
P3 significantly limit the delocalization of charge be-
tween HDB and EDT or EDO segments. Therefore, the
oxidation and reduction would start at the hole- (HDB)
and electron-transporting (EDT or EDO) segments,
respectively. The HOMO and LUMO energy levels of
P1, P2, and P3, estimated from electrochemical data,
are -5.16, -5.12, and -5.19 eV and -3.35, -3.38, and
-3.23 eV, respectively. The twisted σ-bond in P2 or P3
shows a similar effect to the ether spacer in P1 in
interrupting conjugation range. The EL spectra of these
copolymers are very similar to their PL spectra in the
film state and reveal blue or yellow electroluminescence.
The turn-on electric fields of P3 and P4 are analogous
and lower than that in P5, but the brightness of the
P3 device is much higher than that of P4.
Figure 8. Electroluminescent spectra of P1-P5.
Figure 9. Current density-electric field and brightness-
electric field characteristics of the EL devices (ITO/polymer/
Al).
energy levels of the repeating unit of P3 in a coplanar
geometry (angle ) 0° between each HDB and EDO
segments) with those in a twisted geometry (optimized
structure of HDB and EDO groups). It is noteworthy
that the HOMO and LUMO levels of P3 in twisted
conformation are about 1.24 and 0.72 eV, respectively,
higher than those in the coplanar one. This result
suggests that the twisted geometry reduces π-electron
delocalization and makes it hard to oxidize and reduce.13
Device Properties. Figure 8 shows electrolumines-
cence (EL) spectra of single-layer PLED devices (Al/P1-
P5/ITO). The EL spectra are very similar to their PL
spectra in films and exhibit blue or yellow electrolumi-
nescence. Therefore, similar energy transfer still occurs
in isolated copolymers (P1, P4) during device operation,
whereas excimer forms in twisted (P2, P3) and fully
conjugated copolymers (P5). The energy transfer of P1
and P4 leads to a reduced emission range to improve
light purity. It is noteworthy that the current density
versus electric field characteristic of P3 is analogous to
that of P4 and their turn-on electric fields are lower
than that of P5 as shown in Figure 9. This result is
coincident with the previous conclusion of comparable
electrochemical properties between P3 (consisting of
HDB and EDO via a single bond) and P4 (consisting of
HDB and EDO via ether spacers).
Acknowledgment. We thank the National Science
Council of the Republic of China for financial aid
through project NSC 93-2216-E-006-006. We also thank
Dr. Shyh-Gang Su, Associate Professor in the Depart-
ment of Chemistry, National Cheng Kung University
of Republic of China, for aid in semiempirical NMDO
calculation.
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