Blue fluorescent polyamides containing naphthalene and oxadiazole rings

Article abstract of DOI:10.1002/pola.24500

New polyamides containing 1,3,4-oxadiazole and naphthalene rings were prepared by low-temperature solution polycondensation reaction of a new diamine containing preformed oxadiazole ring with various aromatic diacid chlorides. Elemental analysis, mass, infrared, and nuclear magnetic resonance spectroscopy were used to confirm the structure of the monomers and corresponding polymers. The thermal stability and glass transition temperatures of these poly(oxadiazole-amide)s were measured and compared with those of related polymers. Their good solubility allows them to be processed in very thin films with smooth surfaces, without pinholes or cracks, when studied by atomic force microscopy. Upon irradiation with UV light the polymers showed photoluminescence maxima in the blue spectral range, both in solution and in solid state. Cyclic voltammetry (CV) was performed in order to obtain information about the electrochemical stability and reversibility of the redox processes of these polyamides. The highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels, and electrochemical and optical band gap values were calculated by using the results of CV and UV/vis, respectively, showing very good electron and hole injection and transport characteristics. These properties make the present polymers suitable for application in electroluminescent devices.

Full text of DOI:10.1002/pola.24500

Blue Fluorescent Polyamides Containing Naphthalene
and Oxadiazole Rings
MARIANA-DANA DAMACEANU, RADU-DAN RUSU, ALINA NICOLESCU, MARIA BRUMA
‘‘Petru Poni’’ Institute of Macromolecular Chemistry, Polycondensation and Physical Characterization of Polymers Departments,
Aleea Grigore Ghica Voda 41A, Iasi-700487, Romania
Received 10 November 2010; accepted 15 November 2010
DOI: 10.1002/pola.24500
Published online 14 December 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: New polyamides containing 1,3,4-oxadiazole and
naphthalene rings were prepared by low-temperature solution
polycondensation reaction of a new diamine containing pre-
formed oxadiazole ring with various aromatic diacid chlorides.
Elemental analysis, mass, infrared, and nuclear magnetic reso-
nance spectroscopy were used to confirm the structure of the
monomers and corresponding polymers. The thermal stability
and glass transition temperatures of these poly(oxadiazole-
amide)s were measured and compared with those of related
polymers. Their good solubility allows them to be processed in
very thin films with smooth surfaces, without pinholes or
cracks, when studied by atomic force microscopy. Upon irradi-
ation with UV light the polymers showed photoluminescence
maxima in the blue spectral range, both in solution and in
solid state. Cyclic voltammetry (CV) was performed in order to
obtain information about the electrochemical stability and re-
versibility of the redox processes of these polyamides. The
highest occupied molecular orbital and the lowest unoccupied
molecular orbital energy levels, and electrochemical and opti-
cal band gap values were calculated by using the results of CV
and UV/vis, respectively, showing very good electron and hole
injection and transport characteristics. These properties make
the present polymers suitable for application in electrolumines-
C
V
cent devices. 2010 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 893–906, 2011
KEYWORDS: electrochemistry; fluorescence; new oxadiazole
monomer; polyamides; smooth thin films
INTRODUCTION Aromatic polyamides are considered to be
high-performance materials due to their superior thermal
and mechanical properties, which make them useful for
advanced technologies.1 There is an increasing demand for
use as advantageous replacements for metals or ceramics in
currently used goods, or even as new materials in novel
technological applications: advanced fabrics, coatings, and
fillers, advanced composites in the aerospace and armament
industry, asbestos substitutes, electrical insulation, bullet-
proof body armor, industrial filters, and protective and sport
clothing, among others.2–4 However, the extremely high tran-
sition temperatures of the commercial aramids, which lie
above their decomposition temperatures, and their poor sol-
ubility in common organic solvents give rise to processing
difficulties and limit their applications.5,6 As a consequence,
recent basic and applied research has focused on enhancing
their processability and solubility to broaden the scope of
the technological applications of these materials. Attempts to
increase the solubility of aromatic polyamides employed a
variety of methods to chemically modify their structure, such
as through the introduction of bulky, packing-disruptive
groups in the polymer chain or as side groups,7–10 the incor-
poration of flexible chains into the polyamide backbone,11–13
or the use of meta-oriented or asymmetrically substituted
monomers.14–17 Conventionally, the modified polyamides and
polyimides have been prepared by using diamine monomers
containing flexible groups.18–21 Also, it has been demon-
strated that incorporation of both ether and naphthyl units
into the polymer backbones may enhance the solubility and
processability of thermally stable polymers including aro-
matic polyamides and polyimides without any significant
reduction in thermal stability. In continuing the preparation
of new high-performance, processable polyamides, it is of in-
terest to synthesize processable polyamides with high tem-
perature resistance.
Fully aromatic poly(1,3,4-oxadiazole)s form another class of
high performance polymers that are designated for use in
small quantities, but with very high end-value.22 High per-
formances include high thermal resistance in oxidative
atmosphere, good hydrolytic stability, tough mechanical
properties, optical properties, highly ordered systems, and
other special properties determined by the electronic struc-
ture of this particular heterocycle.23–26 There is currently
much interest in high-brightness blue light-emitting diodes
to be used in full colour displays, full colour indicators, and
light sources for lamps, which have high efficiency and high
reliability as characteristics. For such a purpose polyoxadia-
zoles are very attractive because, due to the electron-with-
drawing character of the 1,3,4-oxadiazole rings, they can
Correspondence to: M.-D. Damaceanu (E-mail: damaceanu@icmpp.ro)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 893–906 (2011) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
facilitate electron injection and transport. But the applicabil-
ity of these polymers is limited because they have rigid mol-
ecules due to the delocalization of p-electrons along the
polymer chain, which makes them insoluble in organic sol-
vents and free of or with too high glass transition tempera-
ture, and therefore their processing is very difficult.27
Research of poly(1,3,4-oxadiazole)s has considerably
extended in the last years as can be seen from the large
number of publications and patents in this field. Most of this
research aims to improve solubility and processability of aro-
matic polyoxadiazoles by introduction of certain substituents
on aromatic rings, flexible bridges in the polymer chain or
voluminous units pendant to the chain, provided that the
conjugation is not disturbed and the high-performance prop-
erties are not affected.28,29
SCHEME 1 Synthesis of the diamino-oxadiazole M1.
The combination of these structural modifications, that is,
the incorporation of 1,3,4-oxadiazole rings, naphthalene units
and flexible groups in the polymer backbones, minimizes the
trade-off between the processability and properties of wholly
aromatic polyamides. In our continuing efforts to develop
processable high-performance polymers with potential appli-
cations in electroluminescent devices, this work describes
the successful synthesis of a series of poly(oxadiazole-
amide)s based on a new monomer containing 1,3,4-oxadia-
zole ring and both ether and naphthalene moieties and three
different diacid chlorides containing solubility improving
flexible groups. The properties of these poly(oxadiazole-
amide)s such as solubility, molecular weights, thermal stabil-
ity, glass transition temperature, photo-optical, and electro-
chemical properties, as well as the quality of the thin films
obtained from these polymers were investigated.
2,5-Bis-[4-(5-amino-naphthalene-1-yloxy)-
phenyl]-1,3,4-oxadiazole (M1)
2,5-Bis(p-fluorophenyl)-1,3,4-oxadiazole (7.74 g, 30 mmol),
which was prepared from 4-fluorobenzoic acid and hydra-
zine hydrate by a previously reported procedure,30 5-amino-
naphthol (10.1 g, 63 mmol) and potassium carbonate (13.8
g, 100 mmol) were added, sequentially, while maintaining
good stirring, to a 250 mL three-necked round-bottomed
flask, containing 80 mL of 1-methyl-2-pyrrolidinone (NMP),
equipped with a mechanical stirrer, nitrogen inlet and outlet
and a Dean-Stark trap under a reflux coꢁndenser. The reaction
mixture was stirred and heated to 180 C, and maintained at
this temperature, under a gentle nitrogen flow, for 40 h for
complete conversion. During this time, progress of the reac-
tion was monitored by thin-layer chromatography (TLC). The
reaction mixture was gradually cooled to room temperature
and poured into water in order to precipitate the solid com-
pound. The dark violet monomer was washed with plenty of
water and then purified by recrystallization using coprecipi-
tation in N,N-dimethylformamide and water in order to
remove the unreacted starting compounds and the high
boiling solvent. Finally, 2,5-bis-[4-(5-amino-naphthalene-1-
yloxy)-phenyl]-1,3,4-oxadiazole (M1) was obtained as dark
violet crystals after drying in an oven, under vacuum, at 100
ꢁC for 6 h. Yield: 10.13 g (63%). mp: 183–185 ꢁC. 1H NMR
(DMSO, 25 ꢁC), d (ppm): 5.89 (4H, s,NH2), 6.73 (2H, d, 7.6
Hz, H-8,), 7.08 (2H, d, 8.4 Hz, H-10), 7.12 (4H, d, 8.8 Hz, H-
12), 7.18 (2H, d, 6.8 Hz, H-3), 7.21 (2H, t, 8 Hz, H-9), 7.40
(2H, t, 7.6 Hz, H-2), 8.01 (2H, d, 8.4 Hz, H-1), 8.07 (4H, d,
8.8 Hz, H-13). 13C NMR (DMSO, 25 ꢁC), d (ppm): 108.17 (C-
8), 108.34 (C-10), 116.18 (C-3), 117.27 (C-12), 117.47 (C-
14), 119.84 (C-1), 123.44 (C-2), 124.27 (C-6), 127.49 (C-9),
129.59 (C-5), 128.79 (C-13), 145.31 (C-7), 150.21 (C-4),
161.24 (C-11), 163.41 (C-15). 15N NMR (DMSO, 25 ꢁC), d
(ppm): 60.2 (NH2). FTIR (KBr, powder, cmꢂ1): 3443, 3381,
(NH stretch), 3063 (ArAH stretch), 1612 (NH stretch), 1488
(aromatic C¼¼C stretch), 1275 (CAN stretch), 1246, 1165
(ACAOACA stretch), 1009, 960 (oxadiazole stretch), 780
(NH2 wag). MS (ESI, m/z): calcd for C34H24N4O3, 536.5794;
found, 537.4023 [M þ H]. Anal calcd for C34H24N4O3: C,
EXPERIMENTAL
Starting Materials
5-Amino-naphthol (97%), 4-fluorobenzoic acid (98%), di-
phenyl-dichlorosilane (ꢀ98%; GC), 4,40-(hexafluoroisopropy-
lidene)bis(benzoic acid) (98%), p-bromotoluene (98%), 4,40-
oxybis(benzoic acid) (99%), chromium oxide (98%), hydra-
zine hydrate (reagent grade), 1-methyl-2-pyrrolidinone
(HPLC grade; NMP), N,N-dimethylacetamide (HPLC grade;
DMAc), dimethylsulfoxide (99.7%; DMSO), anhydrous N,N-
dimethylformamide (99.8%; DMF), anhydrous tetrahydrofu-
ran (99%), n-hexane (99.5%), lithium (dispersion in mineral
oil, 99.95%) and thionyl chloride (99%) were purchased
from Sigma-Aldrich (Taufkirchen, Germany). Acetic anhydride
(98%; GC) and pyridine (99.5%; GC) were purchased from
Merck (Darmstadt, Germany). Chloroform (95%) and potas-
sium carbonate (reagent grade) were purchased from Chimo-
par (Bucharest, Romania). Sulfuric acid (98%) and ethanol
(96%) were purchased from Chemical Company (Iasi, Roma-
nia). All reagents were used without further purification. Sol-
vents were purified by standard procedures and handled in
a moisture-free atmosphere.
Synthesis of Monomers
The new oxadiazole-containing monomer and polymers were
synthesized as shown in Schemes 1 and 2, respectively. The
detailed synthetic procedures are as follows.
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SCHEME 2 Synthesis of poly(ox-
adiazole-amide)s.
76.09; H, 4.52; N, 10.44; O, 8.95. Found: C, 76.31; H, 4.35; N,
10.58; O, 8.76.
fied by recrystallization from n-hexane. Yield: 80%. mp: 93
ꢁC (lit. mp: 94 ꢁC). 1H NMR (DMSO, 25 ꢁC), d (ppm): 7.17
(4H, d, 8.8 Hz), 8.00 (4H, d, 8.8 Hz). C NMR (DMSO, 25 C),
13
ꢁ
4,40-(Hexafluoroisopropylidene)bis(benzoyl chloride) (M2)
The fluorine-containing diacid chloride M2 was prepared by
treating the corresponding 4,40-(hexafluoroisopropylidene)-
bis(benzoic acid) with excess thionyl chloride at reflux.31 It
d (ppm): 118.71, 126.43, 131.81, 159.49, 166.63.
Synthesis of Polymers
Poly(1,3,4-oxadiazole-amide)s P1–P3 have been synthesized
by low-temperature solution polycondensation reaction of
equimolar amounts of the aromatic diamine containing oxa-
diazole and naphthalene rings, M1, with diacid chlorides
M2–M4 having hexafluoroisopropylidene, silicon, or ether
groups, respectively, using NMP as a solvent and with pyri-
dine as an acid acceptor. The relative amounts of monomers
and NMP were adjusted so as to have a solid content of 10–
15%.
was purified by recrystallization from n-hexane. Yield: 74%.
1
ꢁ
ꢁ
ꢁ
mp: 93–94 C (lit. mp: 93-94 C). H NMR (DMSO, 25 C), d
(ppm): 7.46 (4H, d, 8.4 Hz), 8.04 (4 H, d, 8.4 Hz). 13C NMR
(DMSO, 25 C), d (ppm): 64.21 (septet, 25 Hz), 123.73 (quar-
ꢁ
tet, 285 Hz), 129.72, 130.05, 132.05, 136.23, 166.46.
Bis(p-chlorocarbonyl-phenylene)-diphenylsilane (M3)
The silicon-containing diacid chloride M3 was prepared by a
multi-step reaction.32 First, p-bromotoluene reacted with
lithium to give p-tolyl-lithium which further reacted with di-
phenyl-dichlorosilane to produce bis(p-tolyl)-diphenylsilane.
The later was oxidized with chromium oxide in a mixture of
sulfuric acid and acetic anhydride and gave the correspond-
ing bis(p-carboxyphenylene)-diphenysilane which was fur-
ther treated with excess thionyl chloride at reflux to result
in bis(p-chlorocarbonyl-phenylene)-diphenylsilane. The final
product was purified by recrystallization from ligroin. Yield:
71%. mp: 181–183 ꢁC (lit. mp: 184–185 ꢁC). 1H NMR
A typical reaction was carried out as follows. Diamine M1
(2.68 g, 5 mmol), 28 mL NMP and 0.5 mL of pyridine, were
placed in a 100 mL three-necked flask equipped with a me-
chanical stirrer and a nitrogen inlet and outlet and the mix-
ture was stirred under nitrogen until complete dissolution.
ꢁ
The solution was cooled to ꢂ10 C and hexafluoroisopropyli-
dene-containing diacid chloride M2 (2.145 g, 5 mmol) was
added with rapid stirring. The content of the flask was kept
below 0 ꢁC for 15 min. The cooling bath was then removed
and the reaction mixture was allowed to reach room temper-
ature after which it was stirred for additional 6 h at this
temperature. The resulting dark violet solution of poly(1,3,4-
oxadiazole-amide), P1, was poured into water to precipitate
the solid polymer. The dark violet polymer was washed with
plenty of water and finally treated with ethanol in a Soxhlet
apparatus for 1 day to remove the unreacted monomers and
oligomers and the high boiling solvent. Finally, poly(1,3,4-
oxadiazole-amide) P1 was obtained as a lightꢁviolet powder
after drying in an oven under vacuum, at 100 C for 6 h.
ꢁ
(DMSO, 25 C), d (ppm): 7.43 (4H, t, 7.9 Hz), 7.47 (4H, d, 7.6
Hz), 7.49 (2H, t, 7.6 Hz), 7.61 (4H, d, 8.4 Hz), 7.99 (4H, d,
8.0 Hz). 13C NMR (DMSO, 25 ꢁC), d (ppm): 128.44, 128.78,
130.30, 132.23, 132.34, 135.89, 136.07, 138.89, 167.24.
4,40-Oxybis(benzoyl chloride) (M4)
The ether bridge-containing diacid chloride M4 was pre-
pared by treating the corresponding 4,40-oxybis(benzoic
acid) with excess thionyl chloride at reflux, in the presence
of N,N-dimethylformamide as reaction catalyst.33 It was puri-
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Polymer P1
recorded using standard pulse sequences in the version with
z-gradients, as delivered by Bruker with TopSpin 2.1 PL6
operating software. The 15N chemical shifts were obtained as
projections from the 2D indirectly detected H,N-HMQC spec-
tra, using standard pulse sequences in the version with z-
gradients as delivered by Bruker and are referred to external
liquid ammonia (0.0 ppm) using as external standard nitro-
methane (380.2 ppm).
Yield: 3.05 g (68%). 1H NMR (DMSO, 25 ꢁC), d (ppm): 7.23
(4H, d, 8.0 Hz, H-12), 7.30 (2H, d, 6.8 Hz, H-3), 7.53–7.64
(8H, m, H-2 þ H-9 þ H19), 7.73 (2H, d, 6.4 Hz, H-8), 7.90–
8.00 (4H, m, H-1 þ H-10), 8.13 (4H, d, 7.6 Hz, H-13), 8.26
(4H, d, 8.0 ꢁHz, H-18), 10.69 (2H, s, amideANH). 13C NMR
(DMSO, 25 C), d (ppm): 64.11 (septet, 25 Hz, C-21), 116.04
(C-3), 117.82 (C-12), 118.02 (C-14), 119.71 (C-10), 120.43
(C-1), 123.78 (quartet, 285 Hz, C-22), 124.78 (C-8), 126.18,
126.26 (C-2 þ C-9), 127.09 (C-5), 128.28 (C-18), 128.90 (C-
13), 129.83 (C-19), 130.83 (C-6), 134.07 (C-7), 135.12 (C-
20), 135.54 (C-17), 150.74 (C-4), 160.72 (C-11), 163.40 (C-
Mass spectral (MS) data were collected on an Agilent 6520
Accurate Mass ESI Q-ToF Liquid Chromatography/Mass Spec-
trometer. Elemental analysis was performed on an Elemental
Analyzer CHNS 2400 II Perkin Elmer.
15
ꢁ
15), 165.37 (C-16). N NMR (DMSO, 25 C), d (ppm): 121.4
(amide NH). Anal calcd for C51H30F6N4O5: C, 68.60; H, 3.39;
F, 12.77; N, 6.28; O, 8.96. Found: C, 68.73; H, 3.45; F, 12.63;
N, 6.39; O, 8.80.
The inherent viscosities of the polymers were determined at
20 ꢁC, by using NMP-polymer solutions of 0.5 g/dL concen-
tration, with an Ubbelohde viscometer. The infrared (FTIR)
spectra were recorded on FT-IR Bruker Vertex 70 Spectro-
photometer in transmission mode, by using KBr pellets.
Polymer P2
1
ꢁ
Yield: 2.66 g (72%). H NMR (DMSO, 25 C), d (ppm): 7.05–
7.26 (4H, bs), 7.26–7.35 (2H, bs), 7.40–7.57 (10H, m), 7.57–
7.65 (4H, bs), 7.65–7.80 (6H, bs), 7.85–8.00 (4H, m), 8.13
(4H, d, 8.4Hz), 8.19 (4H, d, 7.2 Hz), 10.6 (2H, bs, amide-NH).
13C NMR (DMSO, 25 ꢁC), d (ppm): 116.02, 117.78, 117.99,
119.57, 120.42, 124.70, 126.12, 126.24, 127.08, 127.36,
128.32, 128.87, 130.17, 130.84, 132.54, 134.21, 135.82,
135.92, 137.50, 150.71, 160.72, 163.38, 166.15. 15N NMR
(DMSO, 25 ꢁC), d (ppm): 121.3 (amide NH). Anal calcd for
Average-molecular weights were measured by means of gel
permeation chromatography (GPC) using a PL-EMD 950
evaporative mass detector instrument. Polystyrene standards
of known molecular weight were used for calibration and
dimethylformamide as the mobile phase.
The thermal stability of the polymers was investigated by
thermogravimetric analysis (TGA) using a STA 449F1 Jupiter
derivatograph (Netzsch), operating at a heating rate of 10
ꢁC/min, in nitrogen atmosphere, from room temperature to
C60H40N4O5Si: C, 77.89; H, 4.37; N, 6.06; O, 8.64; Si, 3.04.
Found: C, 78.06; H, 4.48; N, 6.13; O, 8.48; Si, 2.85.
ꢁ
700 C. The onset on the TG curve was considered to be the
Polymer P3
beginning of decomposition or the initial decomposition tem-
perature (IDT). The temperature of maximum rate of decom-
position which is the maximum signal in differential ther-
mogravimetry (DTG) curves was also recorded.
Yield: 2.43 g (80%). 1H NMR (DMSO, 25 ꢁC), d (ppm): 7.24
(4H, d, 8.8 Hz), 7.28–7. 33 (6H, m), 7.62 (4H, t, 7.6 Hz), 7.72
(2H, d, 7.2 Hz), 7.96 (4H, d, 8 Hz), 8.15 (4H, d, 8.4 Hz), 8.23
(4H, d, 8.4 Hz), 10.54 (2H, bs, amide-NH). 13C NMR (DMSO,
25 ꢁC), d (ppm): 116.12, 117.81, 118.01, 118.57, 119.58,
120.57, 124.93, 126.17, 126.33, 127.11, 128.95, 129.80,
130.28, 131.02, 134.40, 150.70, 158.85, 160.80, 163.43,
165.38. 15N NMR (DMSO, 25 ꢁC), d (ppm): 120.1 (amide
NH). Anal calcd for C48H30N4O6: C, 75.97; H, 3.99; N, 7.39; O,
12.65. Found: C, 76.12; H, 3.95; N, 7.50; O, 12.43.
The glass transition temperature (Tg) of the precipitated
polymers was determined by using a Pyris Diamond DSC
Perkin Elmer calorimeter. Approximately 3 to 8 mg of each
polymer were crimped in aluminum pans and run in nitro-
gen with a heat-cool-heat profile from room temperature to
380 ꢁC at 10 ꢁC/min. The midpoint temperature of the
change in slope of the DSC signal of the second heating cycle
was used to determine the glass transition temperature val-
ues of the polymers.
Preparation of Polymer Films
Very diluted polymer solutions in NMP with concentration of
1% were used to obtain very thin films having the thickness
in the range of nanometers onto silicon wafers, by spin-coat-
ing technique, at a speed of 5000 rpm. These films, as de-
posited, were gradually heated from room temperature up to
180 ꢁC, and kept at 180 ꢁC for 1 h to remove the residual
solvent and were used for atomic force microscopy (AFM)
investigations.
The quality of very thin films as-deposited on silicon plates
was investigated by atomic force microscopy (AFM) using a
Scanning Probe Microscopy Solver PRO-M, NT-MDT equip-
ment made in Russia, in semi-contact mode, semi-contact to-
pography technique.
The UV–vis absorption and photoluminescence spectra of
polyamides were registered with Specord M42 apparatus
and Perkin Elmer LS 55 apparatus, respectively, by using
very diluted polymer solutions (ꢃ10ꢂ5 M) or very thin films
cast from NMP solutions.
Measurements
The NMR spectra have been recorded on a Bruker Advance
III 400 spectrometer, equipped with a 5 mm multinuclear
inverse detection probe, operating at 400.1, 100.6, and 40.6
MHz for 1H, 13C, and 15N nuclei, respectively. 1H and 13C
chemical shifts are reported in d units (ppm) relative to the
Cyclic voltammetry (CV) was performed on a Bioanalytical
System, Potentiostat-Galvanostat (BAS 100B/W). The electro-
chemical cell was equipped with three-electrodes: a working
electrode (ITO-coated glass with 2.5 ꢄ 2.5 cm2 area covered
residual peak of solvent (ref: DMSO, H, 2.51 ppm; 13C, 39.47
ppm). H,H-COSY, H,C-HSQC, and H,C-HMBC experiments were
1
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ARTICLE
FIGURE 1 NMR spectra of monomer M1 in DMSO-d6: (a) numbering scheme; (b) 1H NMR spectrum; (c) 13C NMR spectrum; (d)
H,H-COSY spectrum; (e) H,C-HSQC spectrum.
with the polymer films), an auxiliary electrode (platinum
wire) and a reference electrode (silver wire coated with
AgCl). All potentials were reported with respect to Ag/AgCl
electrode at room temperature under a nitrogen atmosphere.
Ferrocene was used as an external reference for calibration
(þ0.425 vs. Ag/AgCl). Tetrabutylammonium perchlorate
(TBAP)/acetonitrile (CH3CN) was used as supporting
electrolyte.
polar aprotic solvents such as DMAc or NMP. The oxadiazole
moiety can accept a negative charge and lower the activation
energy for the displacement of the p-substituted fluoro
group through a Meisenheimer complex, analogous to con-
ventional activating groups such as ketone or sulfone.
Basic Characterization
This new compound was characterized by elemental analysis,
MS, IR, 1H and 13C NMR spectroscopy, thereby fully proving
the chemical structure of this monomer. In IR spectra M1
revealed characteristic absorption bands of the functional
groups at 3443 and 3381 (NH stretching), 3063 (aromatic
CAH stretching), 1612 (NH stretching), 1488 (aromatic C¼¼C
stretching), 1275 (CAN stretching), 1246 and 1165
(ACAOACA stretching), 1009 and 960 (¼¼CAOAC¼¼ stretch-
ing in oxadiazole rings), 780 cmꢂ1 (NH2 wagging), confirm-
ing the presence of 1,3,4-oxadiazole ring, ether linkage and
primary amine groups in the product. Figure 1(b,c) illus-
trates the 1H NMR and 13C NMR spectra corresponding to
the new oxadiazole compound (M1). The assignments for
RESULTS AND DISCUSSION
A new oxadiazole-containing aromatic diamine, namely 2,5-
bis-[4-(5-amino-naphthalene-1-yloxy)-phenyl]-1,3,4-oxadiazole
(M1), was successfully prepared in one step by aromatic
nucleophilic substitution reaction of 2,5-bis(p-fluorophenyl)-
1,3,4-oxadiazole, which was synthesized by a previously
reported procedure,30 with two-fold molar amounts of 5-
amino-1-naphthol, in the presence of K2CO3 (Scheme 1).
This method is based on the nucleophilic displacement of
the activated fluoro-substituents by potassium phenoxide, in
NEW BLUE FLUORESCENT POLY(OXADIAZOLE-AMIDE)S, DAMACEANU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 FTIR (KBr Pellets, cm21) Data of the Synthesized
range of 1020–1010 cmꢂ1 and 970–950 cmꢂ1 (¼¼CAOAC¼¼
stretching). CAH and C¼¼C linkages in aromatic rings showed
Polyamides
absorption peaks at 3100–3000 and 1490–1480 cmꢂ1
,
Band Assign
P1
P2
P3
respectively. The strong absorption bands in the range of
1250–1240 and 1170–1160 cmꢂ1, more intense in the case
of polymer P3, were linked to the presence of aromatic ether
stretchings. Also, polymer P1 displayed CAF linkage charac-
NAHastr
3434
1668
1529
969
3424
1667
1524
959
3424
1664
1528
947
Amide I, C¼Oastr.
Amide II, NAHastr.
Oxadiazole, CAOACastr.
Oxadiazole, CAOACastr.
bAr. CAHastr.
teristic absorption peaks in the range of 1100–1300 cmꢂ1
,
while polymer P2 exhibited phenyl-Si typical absorption
bands at 1420, 1105, and 700 cmꢂ1
1015
3070
1489
1240
1167
1014
3067
1488
1239
1165
1011
3066
1489
1240
1166
.
Figure 2(b,c) illustrate the 1H NMR and 13C NMR spectra of
polyamide P1. The assignments of the 1H and 13C chemical
shifts were made according to the numbering presented in
Figure 2(a) and they were based on 2D NMR experiments.
bAr. C¼Castr.
bAr. CAOACcasym.
bAr. CAOACdsym.
a
str: Stretching.
The NMR data provided evidence for the formation of the
amide group. In 1H NMR spectrum of polymer P1, the peak
corresponding to amide protons is shifted downfield as com-
pared to the peak corresponding to amine protons from the
monomer M1. The strongest evidence is provided by H,N-
HMQC spectra of the polymers, presented in Figure 3. In
these spectra, the 15N chemical shift values are in the inter-
val characteristic to secondary amides.
b
Ar: Aromatic.
c
asym: Asymmetric stretching.
d
sym: Symmetric stretching.
the 1H and 13C chemical shifts were made according to the
numbering scheme presented in Figure 1(a). These assign-
ments are based on 2D NMR homo- and heteronuclear corre-
lations. Thus, the J-coupled protons were assigned from H,H-
COSY (Correlation Spectroscopy) spectrum presented in Fig-
ure 1(d). The assignments of the carbons from CH groups
were based on H,C-HSQC (Heteronuclear Single Quantum Co-
herence) spectrum presented in Figure 1(e). The quaternary
carbons were identified from H,C-HMBC (Heteronuclear Mul-
tiple Bond Coherence) spectrum.
The solubility behavior of these polymers was tested qualita-
tively in various organic solvents, and the results are summar-
ized in Table 2. All poly(oxadiazole-amide)s were easily solu-
ble at room temperature in polar aprotic solvents (as NMP,
DMAc, DMF, and DMSO) and, except for polymer P3, even in
less polar solvents such as tetrahydrofuran. The improved sol-
ubility of these polymers compared with conventional poly
(arylene-oxadiazole)s, which are only soluble in strong acids
such as H2SO4 and methansulfonic acid, and with other poly
(oxadiazole-amide)s, which are little soluble in NMP and only
in the presence of lithium chloride, can be explained by the
presence in the macromolecular chain of flexible ether link-
ages, voluminous hexafluoroisopropylidene groups or phenyl
substituents and the bending of the chains at silicon atoms.
These groups prevent the macromolecular chains from pack-
ing into tight structures through hydrogen bonds between
amide groups and lead to a decreased chain–chain interaction,
thus allowing the solvent to penetrate among the polymer
chains. Therefore, their excellent solubility makes the present
poly(oxadiazole-amide)s potential candidates for practical
applications in spin-coating and casting processes.
Low-temperature solution polycondensation reaction of
aromatic diamine, 2,5-bis-[4-(5-amino-naphthalene-1-yloxy)-
phenyl)-1,3,4-oxadiazole, M1, with 4,40-(hexafluoroisopropyli-
dene)bis(benzoyl chloride), M2, bis(p-chlorocarbonyl-phenyl-
ene)-diphenylsilane, M3, or 4,40-oxybis(benzoyl chloride),
M4, gave aromatic poly(oxadiazole-amide)s containing hexa-
fluoroisopropylidene groups, P1, diphenylsilane units, P2, or
ether bridges, P3, respectively, as seen in Scheme 2. This
simple, well-known method allows obtaining highly pure
materials required in optoelectronic applications without the
use of any difficult to remove catalyst. The polymerization
proceeded homogeneously throughout the reaction and
afforded clear, highly viscous polymer solutions. The poly(ox-
adiazole-amide)s precipitated in a tough, fiber-like form
when the resulting polymer solutions were slowly poured
into water.
The molecular weights of the polymers were measured by
gel permeation chromatography (GPC), by using polystyrene
standards. The molecular weight values, Mn, are in the range
of 34,500–53,400 Dalton, Mw in the range of 51,800–95,000
Dalton, and polydispersity Mw/Mn in the domain of 1.47–
1.78, a range expected for condensation polymers. The inher-
ent viscosity values of polyamides P1–P3 were in the range
of 0.4–0.5 dL/g (Table 3). As can be seen in Table 3, the
present polymers have fairly high values of molecular weight
and narrow molecular weight distribution. It should be
noted that gel permeation chromatography measurements by
using polystyrene standards, which have a completely differ-
ent structure as compared with the present polyamides,
The structures of the polymers were identified by means of
elemental analysis, IR, 1H NMR, and 13C NMR spectroscopy.
The FTIR spectra of the synthesized poly(oxadiazole-amide)s
provided evidence that all of the naphthalene units, oxadia-
zole rings and amide groups were successfully incorporated
into the polymer chain, as listed in Table 1. All the polymers
showed characteristic amide group absorptions in the range
of 3450–3200 cmꢂ1 (NH stretching, wide bands), 1670–
1660 cmꢂ1 (carbonyl stretching, amide I), and 1530–1520
cmꢂ1 (NH deformation, amide II). Characteristic absorption
peaks of 1,3,4-oxadiazole ring were also very evident in the
898
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FIGURE 2 NMR spectra of poly-
amide P1 in DMSO-d6: (a) num-
bering scheme; (b) 1H NMR
spectrum; (c) 13C NMR spectrum.
[Color figure can be viewed in the
online issue, which is available at
wileyonlinelibrary.com.]
provide only a crude estimate of molecular weights and not
an accurate evaluation.
data of the polymers are summarized in Table 4. Representa-
tive TGA curves are shown in Figure 4.
The polyamides showed Tg values between 248 and 279 ꢁC.
As expected, Tg was dependent on the structure of the diacid
chloride component and decreased with increasing flexibility
of the polymer backbones. The lowest Tg value of polymer
P3 can be explained in terms of flexibility and low rotation
barrier of its diacid chloride segment.
Thermal Stability
The thermal properties of the poly(oxadiazole-amide)s were
evaluated by means of DSC and TGA. The thermal behavior
The polymers showed excellent thermal stability, as expected
in case of aromatic polyamides. The initial decomposition
TABLE 2 Solubility of Polyamides P1–P3 at Room Temperature
Solvents
Polymer
DMF
DMAc
DMSO
NMP
THF
a
P1
P2
P3
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
ꢂ
b
a
þ: Soluble.
b
FIGURE 3 H,N-HMQC spectrum of polyamide P1 in DMSO-d6.
ꢂ: Insoluble.
NEW BLUE FLUORESCENT POLY(OXADIAZOLE-AMIDE)S, DAMACEANU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 Average Molecular Weights of the Polyamides
Inherent
Polymer
Mn (Da)
Mw (Da)
PDI
Viscosity (dL/g)
P1
P2
P3
53,400
49,600
34,500
95,000
73,300
51,800
1.78
1.47
1.50
0.47
0.46
0.40
temperatures (IDT) of the poly(oxadiazole-amide)s were
ꢁ
about 395–425 C and the temperatures of 10% gravimetric
loss (T10%), that are important criteria for evaluation of ther-
mal stability, were in the range of 417–467 ꢁC, indicating a
high thermal stability. The decomposition of the poly(oxadia-
zole-amide)s takes place in two steps as shown by DTG
curves and by the values of the maximum decomposition
temperatures calculated from these curves: the first step
FIGURE 4 TGA (heavy line) and DTG (dashed line) curves of
polyamides P1–P3. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
ꢁ
occurs in the range 435–495 C and the second in the range
510–555 ꢁC. The first maximum may be attributed to the
degradation of ether linkages nearby the naphthalene units
and the second one corresponds to the degradation of oxa-
diazole rings in the polymer chain. These values are compa-
rable with those of related polyoxadiazoles without any flexi-
ble groups,23 which means that on introducing ether bridges,
silicon or hexafluoroisopropylidene units into the main chain
of poly(oxadiazole-amide)s the thermal stability was not
affected, while the solubility was very much improved. The
polymers displayed similar thermal properties with related
poly(oxadiazole-amide)s based on the same diacid chlorides
and similar diamines, but without naphthalene units.8 The
large interval between decomposition and glass transition of
polymers may be useful for the processing of these polymers
by thermoforming techniques. This is an important advant-
age when comparing with related poly(oxadiazole-amide)s,
previously reported, which do not have any glass
transition.32
exhibited compact, homogeneous surface, without pinholes
or cracks, when studied by atomic force microscopy (AFM;
Fig. 5). Their root mean square roughness (RMS) is in the
range of 4–6 Å being of the same order of magnitude as that
of neat, highly polished silicon wafers which have been used
as substrates. It means that the present polymer films are
very smooth, practically defectless. The films had strong ad-
hesion to silicon wafers: only by scratching with a sharp ra-
zor blade small film pieces could be taken off.
Photo-Optical Properties
There is currently much research directed toward the prepa-
ration of blue light-emitting polymers, because blue light can
be converted to green or red using the proper dyes, which
means a blue light-emitting device alone is capable of gener-
ating all colors, while green or red cannot emit blue light by
this method. Moreover, blue light is difficult to be attained
with the already known inorganic electroluminescent materi-
als. In this regard, the 1,3,4-oxadiazole rings are promising
chemical moieties for introduction into the polymer back-
bone or in the pendant structure.33 In addition, the use of
electroluminescent thin films made from highly thermostable
polymers would avoid the thermal degradation in the final
device while in service at elevated temperatures. Since 1,3,4-
oxadiazole rings and naphthalene units are known as light-
emissive,34 the light-emitting properties of polyamides P1-
P3 containing these chromophores have been investigated.
The light-emitting ability was assessed on the basis of the
photoluminescence (PL) spectra which were recorded for
both polymer solutions in NMP and polymer films cast from
NMP solutions, after excitation with UV light of different
wavelengths. The UV–vis and PL spectra of poly(oxadiazole-
amide)s P1–P3 are displayed in Figures 6 and 7.
Film Quality
All these polymers possess film forming ability. The polymer
solutions (10%) in NMP were processed into thin films by
casting onto glass plates. The free-standing films having a
thickness in the range of 20–50 lm were flexible, tough and
creasable, and maintained their integrity after repeated
bendings. Very thin films having a thickness in the range of
20–50 nm, which have been deposited onto silicon wafers
TABLE 4 Thermal Properties of the Polymers
aTg
(ꢁC)
bIDT
(ꢁC)
cT10%
(ꢁC)
dTmax1
(ꢁC)
dTmax2
(ꢁC)
Polymer
P1
P2
P3
a
279
275
248
400
425
395
443
467
417
476
435
495
545
510
555
The prominent feature in these materials is their extended
conjugated p-system; such extension is mirrored by the elec-
tronic absorption spectrum. All the polyamides displayed
one strong UV absorption maximum in NMP solution, at
about 312 nm, their spectra being quite identical (Fig. 6).
Tg: Glass transition temperature.
b
c
IDT: Onset on the TG curve.
T
10%: Temperature of 10% weight loss on the TG curve.
d
T
max: Temperature of maximum rate of decomposition.
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ARTICLE
FIGURE 5 AFM images of polyamide P3 film on silicon wafer (a, top view; b, side view; c, histogram; d, section).
The absorption maximum of these polymers is mainly deter-
mined by the diphenyl-1,3,4-oxadiazole unit, because the
unsubstituted diphenyl-1,3,4-oxadiazole in hexane shows its
absorption maximum at 284 nm.35 Due to the substitution of
the phenylene rings in para-position with oxygen, the
absorption is slightly red shifted to about 310 nm. In the
ground state the oxygen behaves just like a weak conjugation
bridge because the difference in absorption, which depends
on the substitution position of the nearby naphthalene at the
oxygen, is very weak. In the excited state, the oxygen acts as
FIGURE 6 UV–vis and PL spectra of polyamides in NMP
solutions.
FIGURE 7 UV–vis and PL spectra of polyamides in solid state.
NEW BLUE FLUORESCENT POLY(OXADIAZOLE-AMIDE)S, DAMACEANU ET AL.
901

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 5 Optical Properties of Poly(oxadiazole-amide)s in Solution and Solid State
UV, sol
UV, sol
edge (nm)
UV, film
UV, film
Eg,sol
Eg,film
Polymer
kabs (nm)
k
kabs (nm)
kedge (nm)
(eV)
(eV)
P1
P2
P3
312
312
314
363
361
362
343
346
350
378
380
371
3.42
3.43
3.43
3.28
3.26
3.34
k
k
abs: Wavelength of the maximum absorption peak.
edge: Wavelength of the absorption edges (onset) of the optical absorption spectra.
Eg: Energy band gap.
s: Shoulder.
a better bridge for the delocalization of the charge, because
the position of the naphthalene ring next to the oxygen
determines the photoluminescence more significantly.
UV light, with the highest intensity of luminescence in solu-
tion corresponding to polymer P3. The emission maximum
was centered on 440 nm (Fig. 6) and it was determined by
the nature of the new monomer, particularly by the oxadia-
zole and naphthalene chromophores. All the polymers
showed one shoulder at ꢃ520 nm due to negligible intermo-
lecular interactions of the conjugated parts of the polymer
chains. Maximum emission wavelength along with Stokes
shift values of these polymers are collected in Table 6. The
introduction of ether bridges, silicon or hexafluoroisopropyli-
dene units into the main chain of poly(oxadiazole-amide)s
did not affect the wavelength of the PL emission in solution,
while it had an important influence on the solubility and
processability of the polymers.
In films cast from NMP solution all polyamides presented
one absorption maxima (shoulder) in the range of 343–350
nm (Fig. 7) that can be attributed to p-p* transitions involv-
ing oxadiazole-naphthalene framework. The absorbance spec-
tra of solutions and thin films allow us to compare the posi-
tion of absorption bands kabs, as seen in Table 5. The red-
shift (31–38 nm) of the absorption maxima in solid state
with respect to solution could be explained by the fact that
the surrounding chromophore polarity became weaker, and
the intermolecular interaction of the polymer backbones
became stronger. The absorption edge, being a long-wave-
length wing of the longest-wavelength band of the spectrum,
moves to longer wavelength (smaller energy) for the poly-
mer film when compared with the solution of the same poly-
mer. This means that conformation of polymer chain can be
different depending on the surroundings.36
The PL spectra of the films cast from NMP solutions of these
poly(oxadiazole-amide)s containing oxadiazole and naphtha-
lene chromophores showed the main maxima in the blue
spectral range, at 413–420 nm and 445–465 nm, and small
peaks or shoulders at 433 nm and 498 nm in the case of
polymers P1 and P3, respectively (Fig. 7; Table 6). The maxi-
mum from 413–420 nm can be attributed to phenyl-oxadia-
zole emitter while the maximum from 445–465 can be due
to the naphthalene-containing chain segments. The small
peak at 433 nm and the shoulder at 498 nm in the PL spec-
tra of the polyamide films P1 and P3, respectively, are due
to the various degrees of aggregation and the excimer forma-
tion in the films, leading to photoluminescence quenching,
emission band broadening, and bathochromic shift.37 Inter-
estingly, while most conjugated polymers exhibit bathochro-
mic shifts from solution to solid state, the emission spectra
of polymers P1–P3 exhibit hypsochromic shifts in thin films,
The energy band gap (Eg) could be estimated from the fol-
lowing equation:
Eg ¼ h ꢄ c=kedge
where h is the Planck constant, c is the light velocity, and
kedge is the wavelength of the absorption edges of the optical
absorption spectra. This simple method allows estimating
and then comparing the energy gaps for the investigated
polymers in the form of solution and thin film, as it is gath-
ered in Table 5. The energy gap of poly(oxadiazole-amide)s
P1–P3 varies from 3.42 to 3.43 eV for polymer solutions
and from 3.26 to 3.34 eV for polymer films. The Eg values in
solution do not seem to be influenced by the nature of the
diacid chloride segment of the polymer chain. The Eg values
of the polyamide films are smaller than the Eg values of the
corresponding polymer solutions, due to the more aggre-
gated conformation of polymer main chains in solid state.
For the polymer films, the highest energy bandgap was
found for P3 containing ether bridges in the diacid chloride
segment and the smallest energy bandgap was found for P2
incorporating silicon atom in the macromolecule, which do
not disturb the conjugation along the main chain.
TABLE 6 PL Properties of Poly(oxadiazole-amide)s P1–P3
Solution
PL kem
Films
Stokes
Shift
PL kem
Stokes
Shift
Polymer
(nm)
(nm)
P1
P2
P3
444, 522s
440, 521s
441, 523s
132
128
127
413, 433, 464
420, 465
420, 445, 498s
70
74
70
The most striking feature was that all these polymers in so-
lution emitted an intense blue color upon irradiation with
k
em: Wavelength of the maximum PL emission peak.
s: Shoulder.
902
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
rate of 100 mV/s are shown in Figure 8 (a: anodic scan; b:
cathodic scan).
The CV diagrams indicate three irreversible oxidation proc-
ox
esses at anodic peak potentials (Epa) of 0.85 V (Eonset
¼
0.53 V), 1.03 V and 1.61 V, respectively, and one reversible
red
reduction redox couple at cathodic peak potentials Epc
of
red
ꢂ1.78 V (Eonset ¼ ꢂ1.33 V) and Epcox of ꢂ1.88 V. The elec-
tron removal in the first two stages of oxidation process for
polyamide P1 is assumed to occur at the N atom surrounded
by naphthalene and phenyl groups due to the electron-
donating amide linkages in the main chain, leading to the
formation of radical cations. The third peak, at 1.61 V, can be
ascribed to oxidation of phenoxy groups, causing radical
recombination and formation of a dication having a quinoid
structure. The third oxidation step is stable since no change
in the position of the anodic peak and only the current in-
tensity increase was observed (Fig. 9). However, the other
oxidation steps and reduction processes seemed to be not
very stable as that of the first one. As shown in Figure 9, the
redox waves of polymer P3 showed a slight shift of the
FIGURE 8 The first CV scan of the cast film of polyamide P1 on
ITO coated glass substrate (a, anodic scan; b, cathodic scan).
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
which is indicative of a solid-state organization that disrupts
the extent of conjugation.38 The Stokes shift (the difference
between the main fluorescence and UV–vis peaks) are in the
range of 127–132 nm for polymer solutions and 70–74 for
polymer films. It is known that if the Stokes shift is too
small, the emission and absorption spectra will overlap
more, and the emitted light will be self-absorbed and the lu-
minescence efficiency will decrease in the device. All these
poly(oxadiazole-amide)s display enough high values of the
Stokes shift for good luminescence efficiency. Therefore these
polymer films are promising candidates for application as
emitting materials in blue light-emitting devices.
Electrochemical Characterization
The electrochemical properties of poly(oxadiazole-amide)s
P1–P3 were investigated by CV for cast films on ITO-coated
glass substrates as a working electrode in dry acetonitrile
(MeCN) containing 0.1 M of tetrabuthylamonium perchlorate
(TBAP) as electrolyte salt, under nitrogen atmosphere. The
cyclic voltammograms of polyamide P1 recorded at a scan
FIGURE 9 Repetitive CV scan of the polyamide P3 film on an
ITO electrode in 0.1 M TBAP/MeCN solution (6 cycles; a, anodic
scan; b, cathodic scan). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
NEW BLUE FLUORESCENT POLY(OXADIAZOLE-AMIDE)S, DAMACEANU ET AL.
903

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
showed similar oxidation redox waves, due to the high elec-
tronegative surroundings of the amide groups (ether or hex-
afluoroisopropylidene), the polyamide P2 displayed only two
well defined oxidation peaks at 0.79 and 1.68 eV due to the
lower electronegativity of diphenylsilane unit that limits the
formation of electrochemical species. Also, it can be observed
a large current intensity increase in the oxidation CV waves
of polyamide P2 as compared with P1 and P3. In contrast
with poly(oxadiazole-amide)s P1 and P3 that exhibit an
interruption of electron conjugation along the macromole-
cule, the silicon atom incorporated into the main chain of
polyamide P2 does not disturb the conjugation and supports
the transport of electronic charges.39 The cathodic peaks at
ꢂ1.78 V (vs. Ag/AgCl) and ꢂ1.88 V (vs. Ag/AgCl) for poly-
amide P1 can be attributed to the reduction and oxidation,
respectively, of oxadiazole rings. Polyamides P2 and P3 did
not show the redox peaks corresponding to the oxidation of
oxadiazole ring.
The energy levels of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of
the investigated polyamides can be determined from the oxi-
dation onset potentials (Eonsetox) and reduction onset poten-
tials (Eonsetred) versus Ag/AgCl. The onset potentials were
determined from the intersection of the two tangents drawn
at the rising current and baseline charging current of the CV
curves. The external ferrocene/ferrocenium (Fc/Fcþ) redox
standard Eonset is 0.425 V versus Ag/AgCl in MeCN. Assum-
ing that the HOMO energy for the Fc/Fcþ standard was 4.80
eV with respect to the zero vacuum level, we can estimate
the HOMO energy levels of polyamides P1-P3 (in electron
volts, eV) according to the equation reported by Li et al.40
FIGURE 10 Cyclic voltammograms of the cast films of polya-
mides P1–P3 on ITO-coated glass substrate in 0.1 M TBAP/
MeCN solution at a scan rate of 100 mV/s (a, anodic scan; b,
cathodic scan). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
red
ox
Thus, ELUMO ¼ Eonset þ 4.37 and EHOMO ¼ Eonset þ 4.37
and the value of energy gap, Eg ¼ EHOMO – ELUMO. The energy
level values and the energy gap characteristics of polyamides
containing oxadiazole rings in the main chain, P1–P3, are
summarized in Table 7.
The HOMO and LUMO energy levels of the materials are
very crucial parameters for electroluminescence device con-
figuration. The LUMO energy values of these poly(oxadia-
zole-amide)s was thus determined to be in the range of
3.04–3.27 eV. These values are almost the same as that of
cyano-polyphenylenevinylene (CN-PPV) (3.02 eV) and other
aromatic polyoxadiazoles (2.8–3.2 eV), which all exhibit good
electron-injection properties.34 The HOMO energy level was
anodic/cathodic peaks and a slight increase/decrease in cur-
rent density after six cyclic scans in a potential range of
ꢂ2.5 up to þ2.5 V at the scan rate of 100 mV/s. The oxida-
tion of those polyamide films is accompanied by no colour
changes.
Figure 10 compares the CV curves of structurally similar pol-
yamides P1, P2, and P3. While polyamides P1 and P3
TABLE 7 Electrochemical Properties of Poly(oxadiazole-amide)s
ox
1
2
3
red
red
ox
Eonset
Epa
Epa
Epa
Eonset
Epc
Epc
EHOMO
ELUMO
Eg
Polymer
(V)
(V)
(V)
(V)
(V)
(V)
(V)
(eV)
(eV)
(eV)
P1
P2
P3
0.53
0.42
0.30
0.85
0.79
0.68
1.03
1.68
0.90
1.61
ꢂ
ꢂ1.33
ꢂ1.10
ꢂ1.32
ꢂ1.78
ꢂ1.86
ꢂ2.02
ꢂ1.88
ꢂ
ꢂ
4.9
3.04
3.27
3.05
1.86
1.52
1.62
4.79
4.67
1.60
Eonsetox: Oxidation onset potential.
Eonsetred: Reduction onset potential.
E
E
HOMO: The HOMO energy levels.
LUMO: The LUMO energy levels.
E
E
pa: Anodic peak potential.
pc: Cathodic peak potentiall.
Eg: Energy gap measured from electrochemical data according to the
Eonset
.
904
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ARTICLE
TABLE 8 The Barrier Energies of P1–P3 and PPV
estimated to be in the range of 4.67-4.9 eV. These values are
comparable with that of methoxy-ethyl-hexyloxy-polyphenyl-
enevinylene (MEH-PPV) (4.87 eV),41 that display a very good
hole injection ability. The energy barriers between the emit-
ting polymers and electrodes can be estimated by comparing
the work functions of electrodes with the highest occupied
molecular orbital (HOMO) or the lowest unoccupied molecu-
lar orbital (LUMO) of emitting polymers. Thus, the hole injec-
tion barrier is DEh ¼ EHOMO – 4.8 (eV) (4.8 eV is the work
function of the ITO anode), whereas the electron injection
barrier is DEe ¼ 4.3 – ELUMO (eV), where 4.3 eV is the work
function of aluminum cathode. The difference between the
electron and hole injection barriers (DEe – DEh) is a useful
parameter for evaluating the balance in the two injections.
Lower (DEe – DEh) values mean improved injection balance
of electrons and holes from the cathode and anode, respec-
tively. The band structures of P1–P3 and poly(p-phenylene-
vynilene) (PPV) are depicted together in Figure 11 for com-
parison, and the barrier energies are listed in Table 8.
Polymer
DEh
DEe
DEe – DEh
P1
0.1
1.26
1.03
1.25
1.70
1.16
1.02
1.12
1.40
P2
0.01
0.13
0.30
P3
PPVa
DEh: Barrier energy for hole injection from ITO to polymer.
DEe: Barrier energy for electron injection from Al to polymer .
DEe – DEh: The difference of barrier energy between electron injection
and hole injection from electrodes to polymer.
a
From ref. 42.
CONCLUSIONS
Design and synthesis of a new aromatic diamine containing
1,3,4-oxadiazole and naphthalene rings and of new thermally
stable polyamides based on it with improved solubility and
interesting optical properties was the main objective of this
work. A novel diamine, namely 2,5-bis-[4-(5-amino-naphtha-
lene-1-yloxy)-phenyl]-1,3,4-oxadiazole, was successfully pre-
pared by aromatic nucleophilic substitution reaction of 2,5-
bis(p-fluorophenyl)-1,3,4-oxadiazole with 5-amino-1-naph-
thol, in the presence of K2CO3. This compound was charac-
It is evident that the presence of 1,3,4-oxadiazole rings in all
these polymers decrease the energy barrier for electron
injection from Al electrode to the polymer layer. In the same
time, the amide groups surrounded by phenyl and naphtha-
lene units decrease the energy barrier for hole injections in
the present poly(oxadiazole-amide)s. The DEe, DEh, and (DEe
– DEh) of P2 are lower than those of P1 and P3 due to the
presence of silicon atom in the polymer chains that do not
disturb the conjugation. However, the barrier energy differ-
ences (DEe – DEh) of the poly(oxadiazole-amide)s P1, P2,
and P3 are lower than those of PPV by values of 0.24, 0.36,
and 0.28 eV, respectively. As a general conclusion, our poly-
mers are excellent candidates for electroluminescence devi-
ces (LEDs), presenting very good electron and hole injection
properties and very low barrier energy between electron
injection and hole injection from electrodes to polymer, the
poly(oxadiazole-amide) P2 being the best one.
1
terized by elemental analysis, MS, IR, H NMR, and 13C NMR
spectroscopy, fully confirming the chemical structure of the
monomer. Low-temperature solution polycondensation reac-
tions of this diamine with aromatic diacid chlorides resulted
in preparation of different poly(oxadiazole-amide)s with a
nice balance of properties, thermal stability versus solubility.
All polyamides were easily soluble at room temperature in
polar aprotic solvents and, except for one of them, they were
soluble even in less polar solvents such as tetrahydrofuran.
The polymers showed excellent thermal stability, up to
395 ꢁC, and displayed glass transition in the range of 248–
279 ꢁC, with a large interval between decomposition and
glass transition temperatures that make these polyamides
appropriate for processing by a thermoforming technique, as
well. All the polymers, both in solution and in solid state,
emitted an intense blue color upon irradiation with UV light.
These polymers exhibit very good electron and hole injection
properties and very low barrier energy between electron
injection and hole injection from electrodes to polymer, thus
being promising for application in electroluminescent de-
vices. Further research is necessary to study their use and
performance in blue-light-emitting devices. Therefore, the new
diamine containing oxadiazole and naphthalene rings pro-
vides an important guideline for the design of new alternat-
ing copolymers that can be used to fabricate light-emitting
materials.
One of the authors (M. D. Damaceanu) acknowledges the fi-
nancial support of European Social Fund ‘‘Cristofor I. Simio-
nescu’’ Postdoctoral Fellowship Programme (ID POSDRU/89/
1.5/S/55216), Sectoral Operational Programme Human
Resources Development 2007-2013. The authors are grateful
to Dr. Loredana Vacareanu from ‘‘Petru Poni’’’ Institute of
FIGURE 11 Band diagrams of P1–P3 and PPV as determined
from electrochemical data.
NEW BLUE FLUORESCENT POLY(OXADIAZOLE-AMIDE)S, DAMACEANU ET AL.
905

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