Blue light-emitting polyamide and poly(amide-imide)s containing 1,3,4-oxadiazole ring in the side chain

Article abstract of DOI:10.1016/j.dyepig.2014.10.018

A new diamine monomer containing a 1,3,4-oxadiazole ring, 4,4′-diamino-4″-[2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole]triphenylmethane, was synthesized and characterized. The diamine is used to prepare novel polyamide and poly(amide-imide)s

Full text of DOI:10.1016/j.dyepig.2014.10.018

Accepted Manuscript
Blue light-emitting polyamide and poly(amide-imide)s containing 1,3,4-oxadiazole ring
in the side chain
Corneliu Hamciuc , Elena Hamciuc , Mihaela Homocianu , Alina Nicolescu , Ionela-
Daniela Carja
PII:
S0143-7208(14)00415-X
10.1016/j.dyepig.2014.10.018
DYPI 4569
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 26 August 2014
Revised Date: 24 October 2014
Accepted Date: 25 October 2014
Please cite this article as: Hamciuc C, Hamciuc E, Homocianu M, Nicolescu A, Carja I-D, Blue light-
emitting polyamide and poly(amide-imide)s containing 1,3,4-oxadiazole ring in the side chain, Dyes and
Pigments (2014), doi: 10.1016/j.dyepig.2014.10.018.
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Highlights
•
•
A new blue fluorescent diamine having a diphenyloxadiazole group was synthesized.
Blue fluorescent polyamide and poly(amide-imide)s with pendant oxadiazole groups were
prepared.
•
•
•
High quantum yields and large Stokes shifts of the polymer solutions.
Fluorescence characteristics were studied in the presence of hydrochloric acid.
Fluorescence quenching process was observed in the presence of 2,5-dinitrophenol.

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Blue light-emitting polyamide and poly(amide-imide)s
containing 1,3,4-oxadiazole ring in the side chain
Corneliu Hamciuc1, Elena Hamciuc, Mihaela Homocianu, Alina Nicolescu, Ionela-Daniela Carja
“Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda, 41A, Iasi, 700487,
Romania
ABSTRACT
A new diamine monomer containing a 1,3,4-oxadiazole ring, 4,4′-diamino-4″-[(2-(4-phenoxy)-5-(4-
dimethylaminophenyl)-1,3,4-oxadiazole]-triphenylmethane, was synthesized and characterized. The
diamine is used to prepare novel polyamide and poly(amide-imide)s via a polycondensation
reaction with various diacid chlorides in N-methyl-2-pyrrolidone. The structures of the monomers
1
and polymers are characterized by means of FTIR, H-, 13C- and 15N NMR spectroscopy. The
polymers result in high yields, and exhibit film forming ability and good solubility in many organic
solvents. They show high thermal stability, with decomposition temperature being above 420 °C.
The optical properties, absorption and fluorescence characteristics, in solution and solid state, are
investigated. Solutions of the polymers exhibit fluorescence in the blue region, having high
quantum yield in the range of 38.5-58.1%, and large Stokes shift values (110-120 nm). The
introduction of hydrochloric acid into the polymer solutions has a significant effect upon the optical
properties. Fluorescence quenching in the presence of 2,5-dinitrophenol was analysed using Stern-
Volmer equation.
1
Correspondence to: Corneliu Hamciuc, “Petru Poni” Institute of Macromolecular Chemistry,
Aleea Gr. Ghica Voda, 41A, Iasi, 700487, Romania (E-mail: chamciuc@icmpp.ro)

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Keywords:
polyamides; 1,3,4-oxadiazole; high thermostability; blue fluorescence; high quantum yield;
fluorescence quenching
1. Introduction
Aromatic polyamides are among the oldest members of the class of thermally stable polymers.
They have received considerable attention with regard to the production of high performance
materials, due to their outstanding thermal stability, chemical resistance, and electrical and
mechanical properties [1,2]. However, their high glass transition temperature and limited solubility
in organic solvents make their processability quite difficult. To overcome these drawbacks to some
degree an attractive synthetic approach is the incorporation of bulky substituents such as pendant
groups along the polymer backbone [3]. In many cases, the introduction into the macromolecular
chains of polyamides of various heterocycles including imide and 1,3,4-oxadiazole rings together
with flexible groups led to polymers exhibiting a combination of remarkable properties and
processability, particularly in thin films and coatings [4,5].
Heterocyclic polymers containing 1,3,4-oxadiazole rings in the main chain or in the side chain
are well known for their high thermal resistance in oxidative atmosphere, good hydrolytic stability,
low dielectric constant and tough mechanical properties that make them good candidates for various
applications such as high temperature fibers, reinforced materials and membranes for gas separation
[6-10]. Particularly interesting are the properties determined by the electronic structure of the 1,3,4-
oxadiazole ring because it is characterized by high electron affinity. Due to their electron-
withdrawing character which facilitates the injection and transport of electrons, such polymers can
be used as promising electron-transporting materials in light-emitting and electronic memory
devices [11-15]. The external quantum efficiency in electroluminescence devices with p-conjugated

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polymers, polymers with isolated chromophores and dye-dispersed polymer films can be improved
by the introduction of oxadiazole derivative layers between an emissive layer and the cathode of
organic light-emitting diodes [16].
Much research is made in developing blue light-emitting polymer materials with high thermal
stability and high emission efficiency. A blue light-emitting device can generate all colors by using
the appropriate dyes, in comparison with green or red which cannot emit blue light in this way [17].
One approach to develop such structures is the introduction in the main chain or in the side chain of
the chromophore segments which emit blue light. Polymers with a well defined conjugation length
can be obtained, which is highly desired, taking into account that a larger distribution of
conjugation length leads to a larger broadness of the emission bands [17-21].
Among the efficient organic luminophors 2,5-diphenyl-substituted-1,3,4-oxadiazoles are known
to emit light in the blue region. This can be explained by the effect of the 1,3,4-oxadiazole ring
which is close to that of p-phenylene group in a conjugated aromatic system [22]. 2,5-Diaryl-1,3,4-
oxadiazoles are similar to polyphenyl hydrocarbons with the corresponding number of aromatic
nuclei. Electron donor substituents like p-dimethylamino group can significantly raise the
fluorescence intensity in comparison with electron acceptor substituents. Also, the incorporation of
electron donor substituents in the structure of 2,5-diaryl-substituted oxadiazoles gives large
absorption shifts and photoluminescence spectra toward longer wavelength, as well as a greater
Stokes shift [23].
From previous investigations it was concluded that certain polymers containing dimethylamino
substituents in the para-position of the pendant chromophoric 2,5-diphenyl-1,3,4-oxadiazole unit
are promising candidates for emissive materials in light-emitting devices [24]. The substituted
diphenyl-1,3,4-oxadiazole units are able to transport holes as well as electrons and show an intense
fluorescence. Thus, in our laboratory were developed poly(1,3,4-oxadiazole-ether-imide)s [25,26],
poly(1,3,4-oxadiazole-imide)s [27] and poly(1,3,4-oxadiazole-ester-imides) [28] which were

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synthesized by a polycondensation reaction of 2-(4-dimethylaminophenyl)-5-(3,5-diaminophenyl)-
1,3,4-oxadiazole with certain aromatic dianhydrides, such as 1,1,1,3,3,3-hexafluoro-2,2-bis-[(3,4-
dicarboxyphenoxy)phenyl]propane dianhydride, 2,2′-bis-[(3,4-dicarboxyphenoxy)phenyl]-1,4-
phenylenediisopropylidene dianhydride, bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride, and
1,4-[2-(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)]-naphthalene-bis(trimellitate)
dianhydride. The obtained polymers exhibited fluorescence in the blue region when excited with
UV light. The presence of the heteroatom with a free electron pair, in this case the nitrogen atoms in
the oxadiazole rings and dimethylamino groups, gives the opportunity for protonation with
hydrochloric acid (HCl) that may change the optical properties [29].
In this paper we describe the synthesis and characterization of a new diamine containing 1,3,4-
oxadiazole ring, 4,4′-diamino-4″-[(2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,5-oxadiazole]-
triphenylmethane. Novel polyamide and poly(amide-imide)s were prepared by polycondensation
reaction of the new diamine with various diacid chlorides. The properties of the resulting polymers,
such as solubility, thermal and optical characteristics were investigated. The polymers exhibited
high thermal stability and pure blue light emission.
2. Experimental
2.1. Materials
4-Dimethylaminobenzhydrazide, 1, 4-fluorobenzoyl chloride, 2, 4-hydroxybenzaldehyde,
terephthalic acid, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 4-aminobenzoic acid, 4-
bromo-o-xylene, dimethyldichlorosilane, aniline, aniline hydrochloride, phosphorus oxychloride
(POCl3), potassium carbonate (K2CO3), thionyl chloride, N-methyl-2-pyrrolidone (NMP), N,N-

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dimethylacetamide (DMAc), dimethylsulfoxide (DMSO) and pyridine were provided from Aldrich
and used as received.
2.2. Characterization
Melting points of the monomers and intermediates were measured on a Melt-Temp II
(Laboratory Devices). The inherent viscosity (ηinh) of the polymers was determined with an
Ubbelohde viscometer, by using polymer solutions in NMP, at 20 °C, at a concentration of 0.5 g
dL-1. The molecular weight was determined by gel permeation chromatography (GPC) using UV
and SLS detectors and PLgel 5micro Mixed C Agilent and PLgel 5micro Mixed D Agilent
Columns. Measurements were carried out with polymer solutions in DMF by using DMF as eluent.
Polystyrene standards of known molecular weight were used for calibration. FTIR spectra were
recorded on a Bruker Vertex 70 at frequencies ranging from 400 to 4000 cm-1 using KBr pellets or
films. The NMR spectra were recorded on a Bruker Avance III 400 spectrometer operating at 400.1,
1
100.6 and 40.6 MHz for H, 13C, and 15N nuclei, respectively. Samples were recorded with either a
5 mm multinuclear inverse detection z-gradient probe (1H spectra and all H-C/H-N 2D experiments)
or with a 5 mm four nuclei direct detection z-gradient probe (13C spectra). All the experiments were
recorded using standard pulse sequences, in the version with z-gradients for 2D spectra, as
delivered by Bruker with TopSpin 2.1 PL6 spectrometer control and processing software. Chemical
shifts are reported in δ units (ppm) and were referenced to the internal deuterated solvent for 1H and
1
13C chemical shifts (DMSO referenced at H: 2.51 ppm and 13C: 39.47 ppm) and referenced to
liquid ammonia (0.0 ppm) using nitromethane (380.2 ppm) as external standard for 15N chemical
shifts. Atomic Force Microscopy (AFM) images were taken in air, on a SPM SOLVER Pro-M
instrument. A NSG10/Au Silicon tip with a 35 nm radius of curvature and 255 kHz oscillation mean
frequency was used. The apparatus was operated in semi-contact mode, over a 2x2 ꢀm2 scan area,

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256x256 scan point size images being thus obtained. Thermogravimetric analysis (TGA) was
carried out in nitrogen, at a heating rate of 10 °C min-1, using a Mettler Toledo TGA/SDTA 851e
balance. The heating scans were performed on 2 to 5 mg of sample in the temperature range of 25-
900 °C. Alumina crucible (70 µL) is used as sample holder. The initial decomposition temperature
is characterized as the temperature at which the sample achieves 5% weight loss. The temperature
of 10% weight loss (T10) and the temperature of maximum decomposition rate which is the
maximum signal in differential thermogravimetry (DTG) curves were also recorded. Differential
scanning calorimetry (DSC) analysis was performed using a Mettler Toledo DSC 1 (Mettler Toledo,
Switzerland) operating with version 9.1 of Stare software. The samples were encapsulated in
aluminum pans having pierced lids to allow escape of volatiles. The heating rate of 10 °C min-1 and
nitrogen purge at 120 mL min-1 was employed. Heat flow versus temperature scans from the second
heating run were plotted and used for reporting the glass transition temperature. UV-Vis absorption
and photoluminescence (PL) spectra were recorded with a Analytik Jena - Specord 200
spectrophotometer and a Perkin-Elmer LS55 luminescence spectrofluorimeter, respectively. All
solvents used in the present paper were spectrophotometric grade. Quenching studies were carried
out by adding small amounts of the quencher (3,5-dinitrophenol) to the studied polymer in DMAc
solution. PL spectra were recorded after each addition. All measurements were performed at room
temperature. For PL measurements polymer films were prepared by using 1% polymer solutions in
NMP, which were spin-coated on quartz slides at about 1000 rpm, for 30 s, at room temperature.
2.3. Synthesis
2.3.1. N-(4-dimethylaminobenzoyl)-N′-(4-fluorobenzoyl)hydrazine (3)
4-Dimethylaminobenzhydrazide (8.95 g, 0.05 mol), NMP (65 mL) and pyridine (4.0 mL) were
placed in a 100 mL three-necked flask equipped with mechanical stirrer, and the mixture was stirred

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until complete dissolution was achieved. The solution was cooled to 0 °C, and 4-fluorobenzoyl
chloride (7.925 g, 0.05 mol) was added under stirring. The flask content was maintained at 0 °C for
1 h and at room temperature for 6 h. The reaction mixture was poured into water while stirring, and
the resulting precipitate was filtered, washed with water and dried.
3
2
8
9
H3C
H3C
6
5
7
10
4
1
F
N
COHN NHOC
Yield: 13.50 g (89.7 %). Mp: 238-240°C. FTIR (KBr, cm-1): 3321 (NH), 2895 (C-H aromatic),
1680 and 1636 (C=O), 1604, 1507 (C-C aromatic). 1H NMR (400 MHz, DMSO-d6, δ): 2.99 (6H, s,
CH3), 6.75 (2H, d, 8.8 Hz, H-2), 7.37 (2H, t, 8.8 Hz, H-9), 7.82 (2H, d, 8.4 Hz, H-3), 8.01 (2H, dd,
5.6, 8.0 Hz, H-8), 10.15 (1H, s, NH-4), 10.40 (1H, s, NH-6). 13C NMR (100 MHz, DMSO-d6, δ):
2
39.65 (2C, CH3), 110.80 (2C, C-2), 115.46 (2C, d, JC-F = 22 Hz, C-9), 118.86 (1C, C-4), 128.87
4
3
(2C, C-3), 129.25 (1C, d, JC-F = 3 Hz, C-7), 130.1 (2C, d, JC-F = 9 Hz, C-8), 152.45 (1C, C-1),
15
164.13 (1C, d, 1JC-F = 249 Hz, C-10), 164.91 (1C, CO-6), 165.75 (1C, CO-5). N NMR (40 MHz,
DMSO-d6, δ): 52.3 (N), 118.8 (NH-5), 122.4 (NH-6). Anal. calcd. for C16H16O2N3F1: C 63.78, H
5.31, N 13.55; found: C 63.49, H 5.62, N 13.38.
2.3.2. 2-(4-Dimethylaminophenyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole (4)
N-(4-dimethylaminobenzoyl)-N′-(4-fluorobenzoyl)hydrazine, 3, (12 g) and POCl3 (150 mL)
were introduced into a flask equipped with magnetic stirrer, and refluxed for 6 h. After cooling to
room temperature, the reaction mixture was poured into an ice-water mixture, and the resulting
precipitate was filtered, washed several times with water and dried.

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3
9
2
8
N
N
H3C
H3C
10
1
7
4
F
N
6
5
O
Yield: 10 g (88.3 %). Mp: 161-165°C. FTIR (KBr, cm-1): 3063 (C-H aromatic), 2890, 2812 (C-H
1
aliphatic), 1611, 1493 (-C-C- aromatic), 1010, 989 (1,3,4-oxadiazole ring). H NMR (400 MHz,
DMSO-d6, δ): 3.03 (6H, s, CH3), 6.86 (2H, d, 9.2 Hz, H-2), 7.47 (2H, t, 8.8 Hz, H-9), 7.91 (2H, d,
8.8 Hz, H-3), 8.15 (2H, dd, 5.4, 8.8 Hz, H-8). 13C NMR (100 MHz, DMSO-d6, δ): 39.60 (2C, CH3),
109.56 (1C, C-4), 111.70 (2C, C-2), 116.55 (2C, d, 2JC-F = 22 Hz, C-9), 120.36 (1C, d, 4JC-F = 3 Hz,
C-7), 127.91 (2C, C-3), 129.0 (2C, d, 3JC-F = 9 Hz, C-8), 152.31 (1C, C-1), 162.04 (1C, C-6), 163.86
1
(1C, d, JC-F = 249 Hz, C-10), 164.66 (1C, C-5). 15N NMR (40 MHz, DMSO-d6, δ): 54.8. Anal.
calcd. for C16H14O1N3F1: C 67.64, H 4.94, N 14.84; found: C 67.35, H 5.12, N 14.58.
2.3.3. 4,4′-Diamino-4″-hydroxy-triphenylmethane (5)
4-Hydroxybenzaldehyde reacted with aniline in the presence of an acid catalyst (aniline
hydrochloride) to give diamine 5, according to a published procedure [30].
Yield: 31%. Mp: 202-203 °C. FTIR (KBr, cm-1): 3382 and 3321 (NH-amine), 2855 (C-H aliphatic).
1H-NMR (400 MHz, DMSO-d6, δ): 4.86 (4H, bs, NH2), 5.05 (1H, s, CH), 6.45 (4H, d, 8.4 Hz), 6.64
(2H, d, 8.4 Hz), 6.70 (4H, d, 8.4 Hz), 6.84 (2H, d, 8.4 Hz), 9.14 (1H, s, OH).
2.3.4.
4,4′-Diamino-4″-[(2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,5-oxadiazole]-
triphenylmethane (6)
The diamine containing 1,3,4-oxadiazole ring 6 was prepared by the reaction of 2-(4-
dimethylaminophenyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole, 4, with 4,4′-diamino-4″-hydroxy-
triphenylmethane, 5, in the presence of K2CO3, in NMP as solvent. In a 100 mL round-bottom flask
fitted with magnetic stirrer, Dean-Stark trap, and nitrogen inlet and outlet, were placed 4 (2.83 g,
0.01 mol), 5 (2.90 g, 0.01 mol), anhydrous K2CO3 (1.52 g, 0.011 mol), NMP (23 mL), and toluene

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(15 mL). The mixture was heated at 140 °C for 3 h, and then the toluene was distilled off under
vacuum. The reaction mixture was further heated at 140 °C for 17 h, with stirring, under nitrogen. It
was cooled to room temperature, poured into water, and the resulting solid was filtered, washed
several times with water and dried. The product was purified by treatment with 35% HCl until a
clear solution was obtained, which was filtered and further precipitated with 10% NaOH solution.
The resulting diamine 6 was filtered, washed with water and dried.
3
2
5
4
1
H2N
NH2
6
7
8
18
17
N
N
CH3
CH3
9
15
16
14
13
N
O
19
10
O
11 12
Yield: 3.9 g (70.5 %). Mp: 205-207 °C. FTIR (KBr, cm-1): 3436 (NH2 asymmetric stretching), 3350
(NH2 symmetric stretching), 3030 (C-H aromatic), 2890, 2890 (C-H aliphatic), 1611, 1508 (C-C in
1
aromatic rings), 1241 (C-O-C), 1013, 956 (1,3,4-oxadiazole ring). H NMR (400 MHz, DMSO-d6,
δ): 3.00 (6H, s, CH3), 4.93 (4H, bs, NH2), 5.22 (1H, s, H-5), 6.52 (4H, d, 8 Hz, H-2), 6.78 (4H, d, 8
Hz, H-3), 6.83 (2H, d, 8.8 Hz, H-18), 7.03 (2H, d, 8 Hz, H-8), 7.14 (4H, d, 8 Hz, H-7 and H-11),
7.87 (2H, d, 8.8 Hz, H-17), 8.05 (2H, d, 8.4 Hz, H-12). 13C NMR (100 MHz, DMSO-d6, δ): 39.56
(2C, CH3), 53.92 (1C, C-5), 109.71 (1C, C-16), 111.68 (2C, C-18), 113.76 (4C, C-2), 118.07 (2C,
C-11), 118.11 (1C, C-13), 119.29 (2C, C-8), 127.79 (2C, C-17), 128.45 (2C, C-12), 129.3 (4C, C-
3), 130.49 (2C, C-7), 131.75 (2C, C-4), 142.07 (1C, C-6), 146.62 (2C, C-1), 152.20 (1C, C-19),
152.94 (1C, C-9), 159.91 (1C, C-10), 162.43 (1C, C-14), 164.35 (1C, C-15). 15N NMR (40 MHz,
DMSO-d6, δ): 54.9 (N), 57.15 (NH2). Anal. calcd. for C35H31O2N5: C 75.54, H 5.60, N 12.65;
found: C 75.38, H 5.53, N, 13.31.
2.3.5. Terephthaloyl chloride (7a)

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Terephthaloyl chloride was prepared by refluxing the terephthalic acid with excess of thionyl
chloride. It was purified by recrystallization from chloroform. Mp: 79-80°C
2.3.6. 2,2-Bis[N-chloroformylphenyl)phthalimidyl]hexafluoropropane (7b) and 2,2-bis[N-
chloroformylphenyl)phthalimidyl]dimethylsilane (7c)
These compounds were obtained by treating with thionyl chloride the corresponding
dicarboxylic acids resulting from the condensation reaction of p-aminobenzoic acid with 4,4′-
(hexafluoroisopropylidene)diphthalic anhydride and bis(3,4-dicarboxyphenyl)dimethylsilane
dianhydride, respectively, in glacial acetic acid as a solvent and dehydrating reagent, at reflux
temperature [31,32]. Bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride was prepared by a
multistep reaction, following a published method, starting from the reaction of 4-bromo-o-xylene
with dimethyldichlorosilane to produce bis(3,4-dimethylphenyl)dimethylsilane; the further
oxidation with potassium permanganate resulted in bis(3,4-dicarboxyphenyl)dimethylsilane which
by dehydration in refluxing acetic anhydride gave the corresponding dianhydride [33].
7b. Mp: 311-313 °C [31]. FTIR (KBr, cm-1): 1780 (C=O of imide ring and COCl), 1720 (C=O of
1
imide ring), 1600 (aromatic), 1390 (C-N), 1210 and 1180 (C-F), 1100 and 720 (imide ring). H
NMR (400 MHz, DMSO-d6, δ): 7.61 (4H, d, 8.8 Hz), 7.77 (2H, bs), 7.99 (2H, d, 8 Hz), 8.10 (4H, d,
8.4 Hz), 8.22 (2H, d, 8 Hz).
7c. Mp: 218-222 °C [32]. FTIR (KBr, cm-1): 1780 (C=O of imide ring and COCl); 1730 (C=O of
1
imide ring); 1375 (C-N); 740 (imide ring) and 1220 and 820 (methyl-silane bonds). H NMR (400
MHz, DMSO-d6, δ): 0.79 (6H, s, CH3), 7.60 (4H, d, 8.4 Hz), 8.00 (2H, d, 7.2 Hz), 8.09 (4H, d, 8.8
Hz), 8.13 (2H, d, 7.6 Hz), 8.15 (2H, bs).
2.3.7. Model compound (8)

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Synthesis of model compound 8 was performed by solution condensation reaction of diamine 6
with benzoyl chloride, in the presence of pyridine as acid acceptor. In a 50 mL three-necked flask
equipped with mechanical stirrer, and nitrogen inlet and outlet, were placed diamine 6 (0.94 g, 1.7
mmol), NMP (7 mL) and pyridine (0.27 mL) and the mixture was stirred under nitrogen until
complete dissolution. The solution was cooled to -10 °C and benzoyl chloride (0.478 g, 3.4 mmol)
was added with rapid stirring. The flask content was stirred below 0 °C for 20 min and at room
temperature for 6h. Then the solution was precipitated by pouring into water. The precipitated
product was filtered, washed with water and twice with ethanol under stirring, and dried under
vacuum at 110 °C for 4 h.
18 17
23 22
20
O
19
16 15
14
24
N
NHOC
21
H
13
12
2
3
8
9
N
N
CH3
CH3
11
1
4
6
N
O
5
7
10
O
Yield: 1.2 g (93 %). Mp: 172-174 °C. FTIR (KBr, cm-1): 3300 (NH stretching), 3030 (aromatic C-
H), 2887, 2808 (aliphatic C-H), 1655 (C=O of amide), 1612, 1489 (aromatic -C-C-), 1243 (aromatic
ether), 1015 and 945 (1,3,4-oxadiazole ring). 1H NMR (400 MHz, DMSO-d6, δ): 3.00 (6H, s, CH3),
5.62 (1H, bs,H-15), 6.84 (2H, d, 7.9 Hz, H-2), 7.10-7.24 (10H, m, H-9, H-12, H-13, H-17), 7.53-
7.59 (6H, m, H-23 and H-24), 7.78 (4H, d, 7.60 Hz, H-18), 7.89 (2H, d, 8.00 Hz, H-3), 7.97 (4H, d,
6.60 Hz, H-22), 8.08 (2H, d, 7.9 Hz, H-8), 10.28 (2H, bs, NH). 13C NMR (400 MHz, DMSO-d6, δ):
39.56 (2C, CH3), 53.18 (1C, C-5), 109.72 (1C, C-4), 111.68 (2C, C-2), 118.26 (2C, C-9), 119.56
(2C, C-12), 120.40 (4C, C-18), 127.60 (4C, C-22), 127.80 (2C, C-3), 128.31 (4C, C-23), 128.49
(2C, C-8), 129.08 (2C, C-17), 130.67 (2C, C-13), 131.47 (2C, C-24), 134.93 (2C, C-21), 137.41
(2C, C-19), 139.14 (2C, C-16), 140.30 (1C, C-14), 152.21 (1C, C-1), 153.48 (1C, C-11), 159.74

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(1C, C-10), 162.42 (1C, C-6), 164.38 (1C, C-5), 165.44 (2C, CO). 15N NMR (400 MHz, DMSO-d6,
δ): 56.1 (N), 129.0 (NH).
2.3.8. Synthesis of the polymers (9)
The polymers 9 were synthesized by a solution polycondensation reaction of equimolecular
amounts of diamine 6 with diacid chlorides 7, in NMP as a solvent and in the presence of pyridine
as acid acceptor. In a typical experiment, polymer 9b was prepared as follows: in a 100 mL three-
necked flask equipped with mechanical stirrer, and nitrogen inlet and outlet, were placed diamine 6
(1.106 g, 0.002 mol), NMP (20 mL) and pyridine (0.32 mL) and the mixture was stirred under
nitrogen until complete dissolution. The solution was cooled to -10 °C and diacid chloride 7b
(1.438 g, 0.002 mol) 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
temperature after which it was stirred for another 4 h to yield a viscous polymer solution. The
solution was diluted to about 5% by addition of more NMP and the polymer was precipitated by
pouring into water. The precipitated product was filtered, washed twice with ethanol under stirring,
and dried under vacuum at 110 °C for 5 h to give the polymer 9b.
Flexible transparent free standing films were obtained by casting onto glass plates 10% polymer
solutions of 9a, 9b and 9c followed by heating at 100, 130, 160, 190 and 220 °C, each for 30 min,
to evaporate the solvent. The films were stripped off the plate by immersion in hot water for 2 h.
Films prepared from polymer 9a were brittle.
Polymer 9a
18 17
23 22
17' 18'
20
O
16'
19
16 15
19'
HO
N
N
n H
24
21
H
H
14
O
13
12
2
3
8
9
N
N
CH3
CH3
11
1
4
6
N
O
5
7
10
O

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Yield: 91%. FTIR (KBr, cm-1): 3334 (NH stretching), 3032 (aromatic C-H), 2919, 2855, 2804
(aliphatic C-H), 1664 (C=O of amide), 1611, 1508 (aromatic -C-C-) 1240 (aromatic ether), 1016
and 944 (1,3,4-oxadiazole ring). 1H NMR (400 MHz, DMSO-d6, δ): 3.02 (6H, s, CH3), 5.64 (1H, s,
H-15), 6.84 (2H, bs, H-2), 7.12-7.23 (10H, m, H-9, H-12, H-13, H-17), 7.77 (4H, bs, H-18), 7.88
(2H, bs, H-3), 8.07-8.09 (6H, m, H-8, H-22, H-23), 10.42 (2H, bs, NH). 13C NMR (100 MHz,
DMSO-d6, δ): 39.58 (2C, CH3), 54.16 (1C, C-15), 109.70 (1C, C-4), 111.70 (2C, C-2), 118.28 (2C,
C-9), 119.58 (2C, C-12), 120.50 (4C, C-18), 127.65 (4C, C-22 and C-23), 127.81 (2C, C-3), 128.51
(2C, C-8), 129.13 (4C, C-17), 130.68 (2C, C-13), 137.20 (2C, C-19), 137.37 (1C, C-24), 139.37
(2C, C-16), 140.24 (1C, C-14), 152.23 (1C, C-1), 153.30 (1C, C-11), 159.73 (1C, C-10), 162.42
(1C, C-6), 162.42 (1C, C-5), 164.68 (2C, C-20). 15N NMR (40 MHz, DMSO-d6, δ): 54.9 (N), 128.3
(CONH).
Polymer 9b
34
25
CF3
28
31
17
18
22
17' 18'
23
27
32
CO
OC
OC
19'
20
16'
16 15
19
29
CF3
33
N
N
N
H
HO
N
n
24
21
H
H
14
CO
26
O
O
30
13
12
2
3
8
9
N
N
CH3
CH3
11
1
4
6
N
O
5
10
7
O
Yield: 94%. FTIR (KBr, cm-1): 3340 (NH stretching), 3065 (aromatic C-H), 2887, 2808 (aliphatic
C-H), 1785 and 1724 (C=O of imide ring), 1673 (C=O of amide), 1610, 1507 (aromatic -C-C-),
1367 (C-N stretching of imide) 1242 (aromatic ether), 1017 and 962 (1,3,4-oxadiazole ring), 718
(C=O bending of imide ring). 1H NMR (400 MHz, DMSO-d6, δ): 3.01 (6H, s, CH3), 5.64 (1H, s, H-
15), 6.84 (2H, bs, H-2), 7.13-7.22 (10H, m, H-9, H-12, H-13 and H-17), 7.62 (4H, bs, H-23), 7.78
(6H, bs, H-18 and H-28), 7.87 (2H, bs, H-3), 8.00 (2H, bs, H-30), 8.08 (6H, bs, H-8 and H-22), 8.21
(2H, bs, H-31), 10.38 (2H, bs, NH). 13C NMR (100 MHz, DMSO-d6, δ): 39.8 (2C, CH3), 54.14 (1C,

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C-15), 64.34 (1C, C-33), 109.71 (1C, C-4), 111.73 (2C, C-2), 118.29 (2C, C-9), 119.61 (2C, C-12),
120.33 (4C, C-18), 123.71 (2C, C-28), 124.49 (2C, C-31), 127.03 (4C, C-23), 127.83 (2C, C-3),
128.29 (4C, C-22), 128.54 (2C, C-8), 129.17 (4C, C-17), 130.71 (2C, C-13), 132.57 (2C, C-32),
132.99 (2C, C-27), 134.32 (2C, C-24), 134.72 (2C, C-21), 135.91 (2C, C-30), 137.38 (4C, C-19 and
C-29), 139.29 (2C, C-16), 140.31 (1C, C-14), 152.27 (1C, C-1), 153.51 (1C, C-11), 159.78 (1C, C-
10), 162.46 (1C, C-6), 164.40 (1C, C-5), 164.91 (2C, C-20), 165.74, 165.88 (4C, C-25 and C-26).
15N NMR (40 MHz, DMSO-d6, δ): 55.6 (N), 127.3 (CONH).
Polymer 9c
33
25
CH3
Si
28
31
17
18
22
17' 18'
23
27
32
CO
OC
OC
19'
20
16'
16 15
19
29
N
N
N
H
HO
N
n
24
21
CH3
H
H
14
CO
26
O
O
30
13
12
2
3
8
9
N
N
CH3
CH3
11
1
4
6
N
O
5
10
7
O
Yield: 92%. FTIR (KBr, cm-1): 3354 (NH stretching), 3052 (aromatic C-H), 2880, 2808 (aliphatic
C-H), 1773 and 1720 (C=O of imide ring), 1670 (C=O of amide), 1610, 1507 (aromatic -C-C-),
1366 (C-N stretching of imide) 1240 (aromatic ether), 1017 and 960 (1,3,4-oxadiazole ring), 810
1
(methylsilane), 718 (C=O bending of imide ring). H NMR (400 MHz, DMSO-d6, δ): 0.79 (6H, s,
CH3-Si), 3.01 (6H, s, CH3), 5.64 (1H, s, H-15), 6.84 (2H, bs, H-2), 7.13-7.22 (10H, m, H-9, H-12,
H-13 and H-17), 7.61 (4H, bs, H-23), 7.77 (4H, bs, H-18), 7.87 (2H, bs, H-3), 8.00 (2H, bs, H-31),
8.06-8.14 (12H, m, H-8, H-22, H-28 and H-30), 10.35 (2H, bs, NH). 13C NMR (100 MHz, DMSO-
d6, δ): -3.39 (2C, CH3-Si), 40.11 (2C, CH3), 54.11 (1C, C-15), 109.69 (1C, C-4), 111.67 (2C, C-2),
118.25 (3C, C-7 and C-9), 119.50 (2C, C-12), 120.28 (4C, C-18), 122.67 (2C, C-31), 126.82 (2C,
C-23), 127.75 (2C, C-3), 128.11 (6C, C-8 and C-22), 128.34 (2C, C-28), 129.07 (4C, C-17), 130.62
(2C, C-13), 130.73 (2C, C-27), 132.45 (2C, C-32), 134.34 (2C, C-21), 134.49 (2C, C-24), 137.28
(2C, C-19), 139.19 (2C, C-16), 140.24 (3C, C-14 and C-30), 145.63 (2C, C-29), 152.21 (1C, C-1),

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153.47 (1C, C-11), 159.70 (1C, C-10), 162.38 (1C, C-6), 164.32 (1C, C-5), 164.81 (2C, C-20),
166.60 (2C, C-26), 166.83 (2C, C-25). 15N NMR (40 MHz, DMSO-d6, δ): 55.9 (N), 127.7 (CONH).
3. Results and discussion
3.1. Synthesis of diamine 6
A new diamine 6 was prepared through a sequence of reactions as shown in scheme 1. 4-
Dimethylaminobenzhydrazide, 1, reacted with p-fluorobenzoyl chloride, 2, in NMP as solvent and
pyridine as catalyst to give N-(4-dimethylaminobenzoyl)-N′-(4-fluorobenzoyl)hydrazine, 3. The
cyclodehydration of 3 with POCl3 gave the corresponding 1,3,4-oxadiazole derivative 2-(4-
dimethylaminophenyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole, 4. The diamine 6 was prepared by
nucleophilic substitution reaction of fluorinated compound
4
with 4,4′-diamino-4″-
hydroxytriphenylmethane, 5, in NMP, in the presence of K2CO3 as catalyst, at high temperature.
This method is based on the nucleophilic displacement of the activated fluorine substituent by
potassium phenoxide, in 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
fluorine group through a Meinsenheimer complex, analogous to conventional activating group such
as ketone or sulfone [34].
Scheme 1
The structure of intermediate compounds 3, 4 and 5 and of diamine 6 was confirmed by FTIR
1
and H-, 13C- and 15N NMR spectroscopy. All the spectra were in good agreement with the
proposed molecular structures. In FTIR spectrum of diamine 6 characteristic absorption bands of
the functional groups appeared at 3430 and 3334 (NH asymmetric and symmetric stretching), 1241

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and 1165 cm-1 (-C-O-C stretching), 1012 and 960 cm-1 (=C-O-C= stretching in oxadiazole ring)
confirming the presence of primary amine groups, aromatic ether linkage and 1,3,4-oxadiazole ring.
The chemical structure of the synthesized compounds was also confirmed by NMR
1
spectroscopy. Unambiguous assignments for H-, 13C- and 15N chemical shifts were based on two-
1
dimensional homo- and heteronuclear correlations like H,1H-COSY (Correlation Spectroscopy),
1H,13C and 1H,15N HSQC (Heteronuclear Single Quantum Coherence) as well as HMBC
(Heteronuclear Multiple Bond Coherence). The 1H-, 13C- and 1H,13C-HSQC NMR spectra
1
corresponding to diamine 6 are presented in Fig. 1. The assignments of the H and 13C chemical
shifts were made according to the numbering presented in experimental part. The J-coupled protons
1
were assigned from the H,1H-COSY spectrum (not shown) while the carbons from CH groups
1
were assigned from the H,13C-HSQC spectrum. The quaternary carbons were identified from the
1H,13C-HMBC spectrum (not shown). The 15N chemical shifts were obtained as projections from
1
1
the 2D H,15N-HSQC and H,15N-HMBC spectra. The chemical shifts of the nitrogen atoms from
the oxadiazole ring could not be determined because there were no protons within two or three bond
distances. In the 1H NMR spectrum of 6 the primary aromatic amine protons appeared at 4.93 ppm
as a singlet and the protons in ortho position to the amine groups H-2 appeared as doublet at 6.52
ppm. The 13C NMR spectrum of 6 showed signals which resonated in the region of 39.6-164.4 ppm.
In the 15N NMR spectrum of 6 characteristic peaks appeared at 54.9 ppm due to the presence of
dimethylamino groups and at 57.15 ppm due to the presence of amino groups. All these results
demonstrated that the diamine monomer 6 was successfully prepared.
Figure 1
3.2. Synthesis and general characterization of model compound 8 and polymers 9

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The model compound 8 was synthesized by condensation reaction of diamine 6 with benzoyl
chloride. The polyamide 9a was prepared by low-temperature solution polycondensation reaction of
diamine 6 with terephthaloyl chloride, 7a, in NMP as solvent and with pyridine as the acid acceptor.
Poly(amide-imide)s 9b and 9c were prepared by the same method starting from diamine 6 and two
diacid chlorides containing preformed imide rings 7b and 7c (Scheme 2). The polycondensation
proceeded homogenously during the reaction and afforded clear, viscous polymer solutions.
Scheme 2
The structures of model compound 8 and polymers 9 were identified by FTIR and NMR
spectroscopy. In the FTIR spectra of 8 and 9 characteristic absorption band appeared at around 1670
cm-1 due to carbonyl stretching vibration of amide groups. The band at about 1240 cm-1 appeared
because of the absorption of the aromatic ether linkage of the pendant segments coming from
diamine 6. The absorption bands appearing at around 1017 and 940 cm-1 were assigned to
oxadiazole rings while the bands appearing at 2919 and 2855 cm-1 were characteristic of aliphatic
groups. Characteristic bands at 3070 and 1602 cm-1 were due to aromatic C-H stretching and
aromatic C-C stretching, respectively. In the FTIR spectra of 9b and 9c the strong bands appearing
at 1785 and 1725 cm-1 were attributed to asymmetrical and symmetrical stretching vibrations of
carbonyl groups of imide rings while the characteristic absorption band at 740 cm-1 was due to
imide ring deformation. Polymer 9b exhibited absorption bands at 1180 and 1210 cm-1 due to the
presence of hexafluoroisopropylidene groups (C-F stretching). In the case of polymer 9c containing
dimethylsilane groups a characteristic absorption band appeared at 810 cm-1 (Si-C stretching).
Fig. 2 illustrates the 1H- and 13C NMR spectra of polyamide 9a. The assignments of the 1H and
13C chemical shifts were made according to the numbering presented in experimental part and they
were based on 2D NMR experiments. The NMR data provided evidence for the formation of the
1
amide group. In H NMR spectrum of 9a, the peak corresponding to amide protons is shifted
downfield as compared to the peak corresponding to amine protons from the diamine 6. In 15N

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NMR spectrum of 9a characteristic peak for amide groups appeared at 128.3 ppm confirming the
formation of polyamide during polycondensation reaction.
Figure 2
The solubility of the polymers 9 was tested qualitatively in various organic solvents. All the
polymers were easily soluble at room temperature in polar aprotic solvents, such as NMP, DMAc,
N,N-dimethylformamide and DMSO. The good solubility of polyamide 9a was due to the presence
of voluminous pendant segments 2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole
which prevented the macromolecular chains from packing into tight structure through hydrogen
bonding between amide groups and led to decreased chain-chain interactions, thus allowing the
solvent to penetrate among polymer chains. In the case of poly(amide-imide)s 9b and 9c the
presence of hexafluoroisopropylidene or dimethylsilane units contributed even more to an increase
of the solubility. The inherent viscosity values of polymers 9 were in the range of 0.26–0.53 dL g-1
(Table 1).
The molecular weight of polymers was determined by GPC. The data of molecular weights are
collected in table 1. The number-average molecular weight (Mn) was in the range of 16100-32100 g
mol-1, the weight-average molecular weight (Mw) was in the range of 20800-42300 g mol-1, and the
polydispersity (Mw/Mn) was in the range of 1.29-1.44.
Table 1
The polymers 9 showed film forming ability. Their solutions in NMP having a concentration of
about 10% were cast onto glass substrates and dried to yield films having a thickness of 20-50 ꢀm.
Free standing flexible films were obtained only in the case of poly(amide-imide)s 9b and 9c. The
films of 9a were brittle, probably due to a higher rigidity and lower molecular weight of the
polymer. Very thin films were prepared from diluted NMP solutions (concentration 2%) by spin
coating onto silicon wafers. The quality of such films as deposited on substrates was studied by

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atomic force microscopy (AFM). 2D and 3D AFM images of polymer film surface 9a are shown in
Fig. 3.
Figure 3
It can be seen that the film exhibited compact and homogeneous surface, without pinholes or
cracks. The scanning rate of 2x2 ꢀm was used to evaluate the root mean square (RMS) roughness.
The RMS of the films was in the range of 4-6 Å being of the same order of magnitude as that of
neat highly polished silicon wafers which were used as substrates. It means that the present films
are very smooth, practically defectless. These qualities are required when such films are used in
microelectronic devices.
A glass transition temperature was not observed from DSC measurements up to 350 °C. The
presence of voluminous pendant segments of (2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,5-
oxadiazole) and other aromatic moieties in the main chain hindered the segmental movement of the
polymer chain which resulted in a glass transition temperature higher than the limit of DSC
measurements.
The thermal stability was investigated by thermogravimetric analysis (TGA). Fig. 4 illustrates
the TG and DTG curves of polymers 9. As expected, these polymers showed excellent thermal
stability, with insignificant weight loss up to 400 °C. The temperatures of 5% (T5) and 10% (T10)
weight losses were in the range of 423-433 °C and 455-473 °C, respectively. The polymers showed
high char yield at 700 °C, in the range of 50.4-53.7%. It can be seen that the values of T5 and T10 of
poly(amide-imide)s 9b and 9c were similar to those of polyamide 9a suggesting that the
introduction of imide rings did not improve to a great extent the thermal stability. This can be
explained by the presence in the polymer structure of amide linkages which decompose at lower
temperature and can determine the initial decomposition temperature [35]. As it can be seen from
DTG curves the degradation process of 9b and 9c exhibited two maxima of decomposition. The
first (Tmax1) was in the range of 494-538 °C and was due to the destruction of amide groups more

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sensitive to thermal degradation, oxadiazole rings and ether linkages. The second maximum of
decomposition (Tmax2) was in the range of 556-609 °C and was due to the degradation of polymer
chain itself. Polymer 9a without imide rings exhibited only one maxima of decomposition at 511 °C
(Table 1).
Figure 4
3.3. Optical properties of 6, 8 and 9
The optical properties of diamine 6, model compound 8 and polymers 9 were analysed by UV-
vis and PL spectroscopy measurements. The UV-vis spectra of these compounds in NMP solution
are illustrated in Fig. 5a. The absorption spectra of 6 showed only one absorption band centred at
343 nm. The model compound 8 having two benzoyl units in the backbone exhibited two absorption
bands: one intense at low energy between 260-310 nm, and the other located around 310-370 nm.
The electronic properties of the polymers 9 depended on the electronic character and volumes of the
substituent units present in the backbone.
Figure 5
In the absorption spectra of polymers 9 appeared two absorption bands more less evidentiated
and blue shifted. This is due to a decrease of electron delocalization that occured in the case of the
polymers. For polymers 9b and 9c containing hexafluoroisopropylidene and dimethylsilane bulky
units, the absorption band from high energy was more intense comparatively with absorption
characteristics of polymer 9a. The peak at 330-343 nm could be attributed to the absorption of
π
electronic system delocalized along the 2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-
oxadiazole. The presence of electron donating dimethylamino group and aromatic ether linkage
resulted in a higher absorption value due to
π-π
* transition of oxadiazole group. Instead, the intense
band at λ<300 nm resulted from transitions mainly centred on the phenyl groups introduced at

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backbone of compound 6. The values of the maximum absorption bands of the studied compunds
are shown in Table 2.
Table 2
The energy band gap (Eg) could be estimated from the following equation:
Eg = hc/λedge
(1)
where h is the Planck constant, c is the light velocity and λedge is the wavelength of the absorption
edge. Table 2 shows the Eg values of the studied compounds. In NMP solution the Eg values were in
the range of 3.11-3.16 eV and were not influnced to a great extent by the chemical structure.
Fig. 5b shows the emission spectra of the studied compound in dilute NMP solution. All the
polymer solutions had emission maxima around 454 nm, under excitation wavelength λex of 330
nm. At studied concentration (10-5M) the solutions showed one emission band, without other
shoulders, as a conseqence of the presence in their structure of only one kind of emission
chromophore
(2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole.
The
main
photophysical properties (maximum emission wavelength, λem, full with at half maximum, fwhm,
and Stokes shift) of the studied compounds are summarized in Table 3. The values of fwhm of the
emision spectra were in the usual range (78-82 nm) and did not change significantly with the
chemical structure. The Stokes shift is the difference between fluorescence and UV–vis absorption
peaks. If the Stoke Shift is too small, the emittance and absorption spectra will overlap more and
then the emitting light will be self-absorbed and the luminescent efficiency will decrease in the
devices [36]. The polymers exhibited high Stokes shift values, in the range of 110-120 nm,
indicating that self-absorption was reduced as a result of little overlap of the absorption and
emitting spectra.
Table 3
The PL spectra of polymers 9 in solid state are shown in Fig. 6. The emission band was broad
and structureless, having fwhm in the range of 109-164 nm. A blue shift of λem (by around 30 nm,

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Table 3) in the solid state can be observed when compared with λem in NMP solutions of the
corresponding compounds. This blue shift can be explained as due to reduced rotational freedom of
the polymer chains in the solid state. They tend to exhibit excimeric emissions due to the stacking
of individual chromophores.
Figure 6
To obtain further information about the fluorescence properties the emission spectra were
measured for polymer 9a, at different excitation wavelengths (λex), in DMAc. To investigate the
change of fluorescence properties after excitation with different excitation wavelengths, the
emission spectra were monitored using λex = 320, 340, 360 and 380 nm (Fig. 7). Comparing these
spectra the maximum of fluorescence intensity was obtained under excitation with 360 nm.
Figure 7
The fluorescence performance of the studied compounds (ability to emit light) was evaluated by
their quantum yield values. The fluorescence quantum yield of 8 and 9 in dilute DMAc solution was
measured using integrating sphere. The absolute values of fluorescence quantum yield reach 87.6%
for 8, 38.53% for 9a, 52.76% for 9b and 58.12% for 9c, suggesting that these compounds exhibited
high fluorescence. For studied compounds comparable emission characteristics (intensive blue
emission, Stokes shifts values, high fluorescence quantum yield, full with at half maximum) with
those of other polyamides with side oxadiazole rings [24, 37] were found.
In the color perception study, one of the first mathematically defined color spaces is the CIE-
1931, XYZ color space [38]. The chromatic sensation of human eyes to a specific optical spectrum
is usually characterized by a chromatic diagram. According to the available CIE standard, the
chromaticity coordinates for 9a solution in DMAc were x = 0.159 and y = 0.152. As shown in Fig.
8 they were located in the blue region, which means that this polymer emits pure blue light.
Figure 8

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Spectral properties before and after protonation with HCl were monitored in NMP solution. Fig.
9a presents UV-vis absorption spectra for unprotonated and protonated polymer 9a with HCl in
NMP solution. It can be seen that by protonation the maximum absorption band from 335 nm was
blue shifted. Thus, in the case of UV-vis absorption spectra of 9a in the presence of 0.5M and 2M
HCl the maximum absorption band appeared around 317 nm. UV-vis spectrum of 9a-HCl (2M)
showed an additional shoulder at 365 nm. The PL spectra of polymer 9a in NMP solution, without
and in presence of HCl, were presented in Fig. 9b. Protonation of the polymer determined a blue
shift in the emission band in comparison with the unprotonated polymer solution (Table 4). It can
be noticed that by increasing the HCl concentration a decrease of fluorescence intensity appeared.
The fluorescence disappeared for a concentration of 2M HCl. As it can be seen from Table 4, the
Stokes shift increased in the case of a concentration of 0.5 M HCl. The changes in the spectral
properties (absorption and emission) after protonation can be attributed to the modification of the
electronic structure of the polymer chains due to the interaction of oxadiazole rings and
dimethylamino groups (nitrogen atom) with the proton of HCl. Namely, it is expected that the lone
electron pair of nitrogen of 1,3,4-oxadiazole rings and dimethylamino groups to be protonated by a
protic acid and the changes in optical properties to be induced [29,39].
Figure 9
Table 4
3.4. Fluorescence behavior of model compound 8 and polymer 9a in the presence of 3,5-
dinitrophenol
Studied compounds with above mentioned fluorescence characteristics (narrow and symmetric
emission band, high quantum yield) attracted considerable attention for investigations of their
interactions with some substances (e. g. nitroaromatic compounds) that would change their physical

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properties. Generally, the fluorescence intensity of fluorescent compounds decreases progressively
with increasing concentrations of the nitroaromatic compounds. This intensity decrease of the
emission (fluorescence quenching) occurs as a result of the molecular interactions that can lead to
energy transfer, molecular rearrangement, ground-state complex and exciplex-formation, excited-
state reactions etc [40,41]. In the fluorescence studies nitroaromatic derivatives can be used as
active compounds, especially as fluorescence quenchers. The fluorescence quenching is a
significant method of nitroaromatic compounds detection due to the sensitive and cost effective
nature. The fluorescence quenching can take place by two different mechanisms, static and
dynamic. Two studied compounds, 8 and 9a, with high fluorescence signal, were chosen for
investigation in the presence of 3,5-dinitrophenol (DNF).
The Stern-Volmer equation is usually used to investigate the fluorescence quenching process
[39]:
I0/I = 1 + KSV[Q]
(2)
where I0 and I are the fluorescence intensities of the solute without and with quencher (DNF),
respectively, KSV is the Stern-Volmer (S-V) quenching constant and [Q] is the quencher
concentration. The constant KSV provides a direct measure of the quenching sensitivity.
Fig. 10a and 10b show the characteristic changes in excited state photophysical properties
during the interaction of DNF with compound 8 and polymer 9a in DMAc solution. On gradual
addition of DNF, for both samples appreciable decreases in fluorescence intensity, without a shift in
the emission wavelength, were observed indicating a strong excited state interaction of DNF with
analysed samples. The Stern-Volmer plots I0/I, at 454 nm, versus quencher concentration Q were
linear at low concentrations (inset plots from Figs. 10a and 10b) but showed upward curvature at
higher concentrations of the quencher. From the slopes of the initial linear parts of Stern-Volmer
curves, the S-V constants were evaluated. It can be noticed that the samples 8 and 9a exhibited
closer KSV values (1.54x10-3 and 1.79x10-3 molL-1, respectively) indicating that the close quencher

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concentration was needed to quench the fluorescence. The upward curvature of the Stern-Volmer
plot at higher concentrations suggested that the quenching process is not purely dynamic and may
be due to a static contribution.
Figure 10
4. Conclusions
The incorporation of 2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,5-oxadiazole pendant
segments into the macromolecular chain of polyamide or poly(amide-imide)s resulted in soluble
polymers that retain a high thermal stability. The polymers can be processed into very thin films
having the thickness in the range of tens of nanometres by using spin-coating technique. The films
were smooth, homogeneous, practically without defects, when studied by atomic force microscopy.
The poly(amide-imide)s can be processed from solution into thin flexible films having the thickness
in the range of tens of micrometers by using casting technique. The polymers showed blue
fluorescence in solution or in solid state. The presence of oxadiazole rings and dimethylamino
groups containing nitrogen atoms available for interaction with Bronsted acids gave the opportunity
for modification of polymer optical properties. Protonation with HCl as a dopant caused a
significant decrease of fluorescence intensity. The Stern-Volmer equation used to investigate the
fluorescence quenching process in the presence of DNF, showed that quenching process is not
purely dynamic and may be due to a static contribution. All these properties, associated with an
easy processability make the present polymers potential candidates for high-performance
applications.
Acknoledgements

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Paper dedicated to the 65th anniversary of "Petru Poni" Institute of Macromolecular Chemistry of
Romanian Academy, Iasi, Romania. The financial support of UEFISCDI through PN-II-PT-PCCA-
2013-4 project, contract no. 43/2014, funded under Joint Applied Research Projects - Parteneriate
2013 is gratefully acknowledged.
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