Synthesis and photophysical study of some new highly thermostable blue fluorescent poly(1,3,4-oxadiazole-imide)s containing dimethylamine groups

Article abstract of DOI:10.1016/j.reactfunctpolym.2016.03.022

New poly(1,3,4-oxadiazole-imide)s were synthesized from a diamine containing 1,3,4-oxadiazole ring, 4,4′-diamino-4″-[(2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole]triphenylmethane, and different aromatic dianhydrides: 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 9,9-bis[(3,4-dicarboxyphenoxy)phenyl]fluorene dianhydride, 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) and perylene-3,4,9,10-tetracarboxylic dianhydride. The polymers were easily soluble in polar organic solvents and exhibited high thermal stability, having 5% weight loss in the range of 445-470 °C and glass transition temperature in the domain of 201-244 °C. The optical properties were investigated in different media. The absorption characteristics (intensity, absorption maximum and shape) of investigated compounds depended on their chemical structures and the nature of environment. The emission of these samples was represented by one broadened and blue emission band. The largest and solvent dependent Stokes shifts values were obtained. The fluorescence quantum yield (QY) values of polymer with hexafluoroisopropylidene groups, in tetrahydrofuran solution, depended on the wavelength used for excitation. This is an exception from Kasha's rule and Vavilov's law, which was attributed either to photodecomposition or to the enhancement of intramolecular energy transfer process competing with internal conversion between singlet states. High QY values were obtained for polymers with isopropylidene or fluorene units in dimethyl sulfoxide (polar solvent), which may be due to the strong overlap of electronic transitions of species from system.

Full text of DOI:10.1016/j.reactfunctpolym.2016.03.022

Reactive and Functional Polymers 103 (2016) 17–25
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis and photophysical study of some new highly thermostable
blue fluorescent poly(1,3,4-oxadiazole-imide)s containing
dimethylamine groups
⁎
Corneliu Hamciuc , Mihaela Homocianu, Elena Hamciuc, Ionela-Daniela Carja
“Petru Poni” Institute of Macromolecular Chemistry, 41A Gr. Ghica Voda Alley, Iasi, 700487, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
New poly(1,3,4-oxadiazole-imide)s were synthesized from a diamine containing 1,3,4-oxadiazole ring,
4,4′-diamino-4″-[(2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole]triphenylmethane, and
different aromatic dianhydrides: 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 9,9-bis[(3,4-
dicarboxyphenoxy)phenyl]fluorene dianhydride, 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride)
and perylene-3,4,9,10-tetracarboxylic dianhydride. The polymers were easily soluble in polar organic solvents
and exhibited high thermal stability, having 5% weight loss in the range of 445–470 °C and glass transition tem-
perature in the domain of 201–244 °C. The optical properties were investigated in different media. The absorp-
tion characteristics (intensity, absorption maximum and shape) of investigated compounds depended on their
chemical structures and the nature of environment. The emission of these samples was represented by one
broadened and blue emission band. The largest and solvent dependent Stokes shifts values were obtained. The
fluorescence quantum yield (QY) values of polymer with hexafluoroisopropylidene groups, in tetrahydrofuran
solution, depended on the wavelength used for excitation. This is an exception from Kasha's rule and Vavilov's
law, which was attributed either to photodecomposition or to the enhancement of intramolecular energy trans-
fer process competing with internal conversion between singlet states. High QY values were obtained for poly-
mers with isopropylidene or fluorene units in dimethyl sulfoxide (polar solvent), which may be due to the
strong overlap of electronic transitions of species from system.
Received 5 December 2015
Received in revised form 24 March 2016
Accepted 30 March 2016
Available online 2 April 2016
Keywords:
Polyimide
1,3,4-Oxadiazole ring
Dimethylamine groups, Thermal stability
Blue fluorescence
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
aromatic main chains of the polyimides lead to lower glass transition
temperature as well as significant improvement of solubility [5–7].
Aromatic polyimides are a class of polymers known for their high
thermal stability, high glass transition, low thermal expansion coef-
ficient, high radiation resistance, low dielectric constant, excellent
mechanical properties, and good chemical resistance. The excellent
physical properties result from rigid polymer structures and strong
intermolecular interactions [1,2]. These polymers have been widely
used in aerospace and automotive industries, and as interlayer di-
electric insulators in microelectronics. They were studied for appli-
cations in advanced technologies as gas separation membranes,
polymer matrix for composites and nanocomposites, fuel cells, pho-
tovoltaics, and materials having electroluminescent characteristics
[3,4]. However, the applications of most polyimides are limited due
to their insolubility in conventional solvents and extremely high
glass transition temperature which make their processing very diffi-
cult. Various efforts have been made on the synthesis of soluble and
processable polyimides without much sacrifice of their excellent
thermostability. The aromatic ether linkages inserted into the
Also, the incorporation of hexafluoroisopropylidene (6F) groups
into polymer backbones enhances the polymer solubility without
sacrificing thermal stability [8–10]. Therefore, polyetherimides and
fluorinated polyimides were studied as high performance materials
having both high performance characteristics and good
processability.
The presence of 1,3,4-oxadiazole rings in the macromolecular chains
of polymers improves their properties such as thermo-oxidative and
chemical stability, strength, stiffness, that makes them good candidates
for various applications. Particularly interesting are properties deter-
mined by their electronic structure because they are characterized by
high electron affinity. Their electron-withdrawing character facilitates
the injection and transport of electrons being thus used as material for
light-emitting diodes [11–12]. Poly(1,3,4-oxadiazole imide)s can be pre-
pared by using two methods. The first method implies two steps: the re-
action of an aromatic dianhydride with a diamine containing preformed
1,3,4-oxadiazole ring resulting
a poly(amic acid) that can be
cyclodehydrated in the second step to the corresponding imide structure
[13]. The other method consists in the cyclodehydration at elevated tem-
perature of poly(hydrazide imide)s [14].
⁎
Corresponding author.
E-mail address: chamciuc@icmpp.ro (C. Hamciuc).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.03.022
1381-5148/© 2016 Elsevier B.V. All rights reserved.

18
C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
Scheme 1. Preparation of model compound 3.
Nowadays, novel fluorescent materials have been developed for
2. Experimental
applications in light-emitting diodes. Much research has been directed
towards studying blue light-emitting polymer materials with high
thermal stability and high emission efficiency. The glass transition
temperature of polymers is also an important property for these
applications [15]. One approach to develop such structures is the
introduction of blue light-emitting chromophore segments in the
main or side chains of polymers. Heterocyclic thermostable poly-
mers are studied as potential luminescent polymers due to their
high glass transition temperature and thermal and chemical stabili-
ty, essential characteristics required in many applications as emis-
sive materials [16]. In our laboratory different classes of blue
fluorescent polymers containing 1,3,4-oxadiazole groups were de-
veloped. Thus, highly fluorinated thermostable poly(1,3,4-
oxadiazole-ether)s containing bis(p-phenyl)-1,3,4-oxadiazole seg-
ments and 3-trifluoromethylphenyl-2,2,2-trifluoroethane or 3,5-
ditrifluoromethylphenyl-2,2,2-trifluoroethane units were prepared
[17]. They exhibited fluorescence in the blue region when excited with
UV light of 300–340 nm. Their fluorescence properties were influenced
by the solvent nature, excitation wavelength and protonation. The
solvatochromic properties in the binary solvent environments were pre-
dominantly influenced by the acidity and basicity of the solvent systems
[18]. It is known that 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 [19–21]. The substituted diphenyl-1,3,4-oxadiazole
units are able to transport holes as well as electrons and show an intense
fluorescence [22]. Therefore, we investigated polyamide and poly(amide
imide)s having 2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-
oxadiazole pendant groups which showed excellent fluorescence both
in solution and in solid state. Solutions of the polymers exhibit fluores-
cence in the blue region, with high quantum yield in the range of 38.5–
58.1%, and large Stokes shift values (110–120 nm). Protonation with
HCl as a dopant caused a significant decrease of fluorescence intensity.
Fluorescence quenching in the presence of 2,5-dinitrophenol was ana-
lyzed using Stern-Volmer equation [23].
2.1. Materials
4,4′-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4′-
(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) (6HDA) and
perylene-3,4,9,10-tetracarboxylic dianhydride (PEDA) were pro-
vided from Aldrich. All other reagents were used as received
from commercial sources or were purified by standard methods.
2,5-Bis(p-aminophenyl)-1,3,4-oxadiazole was prepared by the reaction
of p-aminobenzoic acid with hydrazine hydrate, in polyphosphoric acid
[24].
In our continuous efforts to obtain new polyimides with
enhanced solubility and a good combination of properties for future
advanced applications as potential luminescent polymers, we consid-
ered that it would be interesting to introduce 2-(4-phenoxy)-5-(4-
dimethylaminophenyl)-1,3,4-oxadiazole groups in the side chain in
order to improve the optical properties of the polymers maintaining
their thermal stability. Thus, this article presents the synthesis and
characterization of novel blue fluorescent polyimides based on a
diamine containing 1,3,4-oxadiazole ring and various aromatic
dianhydrides exhibiting different degrees of flexibility. The aim of this
work is to establish new correlations between the structure and physi-
cal properties of the polymers. The basic properties such as solubility,
molecular weight, thermal stability and glass transition temperature
were investigated. A study on the photo-optical properties was
performed with the purpose of establishing structure–property rela-
tionships. The modification of the optical properties by doping with
HCl was also described.
Fig. 1. 1H NMR spectrum of model compound 3 (400 MHz, CDCl3).

C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
19
Fig. 2. Chemical structures of polymers 4.
9,9-Bis[4-(3,4-dicarboxyphenoxy)phenyl]fluorene dianhydride
(FLDA) was synthesized according to the literature using a three-
step synthetic procedure starting from the nitro-displacement reaction
of 4-nitrophthalonitrile with 9,9-bis(4-hydroxyphenyl)fluorene, in dry
N,N-dimethylformamide (DMF), in the presence of potassium carbon-
ate, at room temperature, followed by alkaline hydrolysis of the
tetranitrile intermediate and dehydration with acetic anhydride of the
resulting tetracarboxylic acid [25,26]. M.p.: 252–254 °C. FTIR (KBr,
cm−1): 1840, 1775, 1600, 1248. 1H NMR (400 MHz, DMSO-d6, δ): 7.11
(4H, m), 7.28 (4H, m), 7.45 (4H, m), 7.52 (4H, m), 7.75 (2H, d), 7.97
(2H, m), 8.02 (2H, d).
4,4′-Diamino-4″-[2-(4-phenoxy)-5-(4-dimethylaminophenyl)-
1,3,4-oxadiazole)]triphenylmethane (1) was prepared by nucleo-
philic substitution reaction of a fluorinated compound, namely 2-
(4-dimethylaminophenyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole, with
4,4′-diamino-4″-hydroxytriphenylmethane, as it was previously re-
ported [23]. The fluorinated compound was obtained by reacting 4-
dimethylamino-benzhydrazide with p-fluorobenzoyl chloride, and
cyclodehydration of the resulting product with POCl3. M.p.: 205–
207 °C. FTIR (KBr, cm−1): 3436, 3350, 3030, 2890, 1611, 1508, 1241,
1013, 956. 1H NMR (400 MHz, DMSO-d6, δ): 3.00 (6H, s), 4.93 (4H,
bs), 5.72 (1H, s), 6.52 (4H, d), 6.78 (4H, d), 6.83 (2H, d), 7.03 (2H, d),
7.14 (4H, d), 7.87 (2H, d), 8.05 (2H, d).
2.2. Synthesis of the model compound (3)
Diamine 1 (0.553 g, 1 mmol), phthalic anhydride (2) (0.296 g,
Fig. 3. FTIR spectra of polymers 4.
2 mmol) and glacial acetic acid (6 mL) were introduced into a flask

20
C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
Fig. 4. 1H NMR spectrum of polymer 4e.
and heated at reflux temperature (120 °C) for 8 h. The reaction mixture
was cooled to room temperature and the solid was filtered, washed
with water and methanol and dried at 80 °C for 10 h. Yield: 90%. FTIR
0.7 mmol) and N-methyl-2-pyrrolidinone (NMP) (6 mL). The mixture
was stirred under nitrogen to the complete dissolution. The dianhydride
FLDA (0.45 g, 0.7 mmol) was added to the resulting solution and stirring
was continued for 10 h. Then 3 mL pyridine and 3 mL acetic anhydride
were added into the flask, and the solution was heated at 100 °C for 4 h.
The flask was cooled-down to room temperature and the solution was
poured into water to precipitate the polymer. The solid product was fil-
tered, washed three times with water under stirring and dried at 100 °C.
Polymer 4d was prepared using another procedure. 1 (0.553 g,
1 mmol), 6FDA (0.377 g, 0.85 mmol), PEDA (0.058 g, 0.15 mmol), NMP
(14 mL) and LiCl (0.525 g) were introduced into a flask and the mixture
was heated under stirring and inert atmosphere at 180–200 °C, for 6 h.
The flask was cooled-down to room temperature and the polymer was
isolated by precipitation with water. It was filtered, washed three
times with water under stirring, and then it was dried at 100 °C.
(KBr, cm−1): 3060, 1770, 1723, 1601, 1370, 1241, 1014, 981, 720. 1
H
NMR (400 MHz, DMSO-d6, δ): 8.12 (2H, s), 7.92 (8H, s), 7.78 (4H, d),
7.45 (4H, d), 7.35 (4H, d), 7.24 (2H, d), 7.12 (2H, d), 7.05 (2H, d), 6.85
(2H, d), 5.70 (1H, s), 3.1 (6H, s).
2.3. Preparation of the polymers (4)
The polymers 4a, 4b and 4c were prepared by a two-step polycon-
densation reaction of aromatic diamine containing 1,3,4-oxadiazole
ring, 1, with different aromatic dianhydrides, such as 6FDA, 6HDA, and
FLDA, respectively. Polyimide 4d was synthesized starting from diamine
1 and a mixture of dianhydrides 6FDA/PEDA (0.85/0.15 M ratio), while
polyimide 4e was obtained by reacting 6FDA with an equimolecular
amount of 1 and 2,5-bis(p-aminophenyl)-1,3,4-oxadiazole. A typical
polycondensation reaction was run as shown in the following example
for 4c: In a 100 mL three-necked flask equipped with mechanical stirrer
and nitrogen-inlet and outlet were introduced diamine 1 (0.387 g,
2.4. Measurements
Melting points of the monomers and intermediates were measured
on a Melt-Temp II (Laboratory Devices).
Table 1
GPC results and thermal characteristics of polymers 4.
Sample
Mn (g mol−1
)
Mw (g mol−1
)
Mw/Mn
Tg (°C)
T5 (°C)
T10 (°C)
Tmax1 (°C)
Tmax2 (°C)
Char yield
at 700 °C (%)
4a
4b
4c
4d
4e
16,200
22,800
18,500
–
20,800
32,100
25,700
–
1.28
1.41
1.39
–
–
460
470
465
445
460
508
520
494
480
489
554
536
504
553
543
623
590
577
–
61.2
48.7
61.7
50.4
56.8
201
244
–
18,500
25,300
1.37
220
620

C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
21
Differential Scanning Calorimetry (DSC) was performed on a Mettler
Toledo DSC822e. Approximately 10 mg of sample were tested applying
a heating rate of 10 °C min−1 from 20 to 200 °C, in air (50 mL min−1).
Heat flow vs. temperature scans from the second heating run were plot-
ted and used for reporting the glass transition temperature. The mid-
point of the inflexion curve resulting from the typical second heating
was assigned as the glass transition temperature of the respective
polymers.
Thermal stability was analyzed by means of a Mettler Toledo TGA-
SDTA851e derivatograph. The thermogravimetric (TG) and derivative
thermogravimetric (DTG) curves were recorded in nitrogen, with a
flow of 20 mL min−1 in the temperature interval 25–700 °C and with
heating rate of 10 °C min−1. Constant operational parameters were
preserved for all the samples, which had the mass ranged between 1.8
and 4 mg, so as to obtain comparable data.
The UV\\VIS electronic absorption spectra of 3 and of the polymers
were measured with a Shimadzu UV 3600 absorption spectrometer, in
tetrahydrofuran (THF), dichloromethane (DCM), NMP, and dimethyl
sulfoxide (DMSO) solutions. The fluorescence excitation and emission
spectra were obtained using a LS 55 PerkinElmer spectrometer. All
used solvents were spectroscopic grade.
Fig. 5. TG curves of polymers 4.
The fluorescence quantum yields, QY, were determined using FLS
980 apparatus equipped with a Quanta-phi integrating sphere accesso-
ry, using solutions with absorbance lower than 0.1. Both the sample and
reference were excited at the same wavelength. All the measurements
were performed at room temperature.
The infrared spectra were recorded on a FTIR Bruker Vertex 70 Spec-
trophotometer at frequencies ranging from 4000 to 400 cm−1. Samples
were mixed with KBr and pressed into pellets.
1H NMR (400 MHz) spectra were obtained at room temperature on a
Bruker Advance DRX spectrometer, using CDCl3 as solvent.
The molecular weights and their distribution were determined by
gel permeation chromatography (GPC) with a PL-EMD 950 evaporative
mass detector instrument. Two poly(styrene-co-divinylbenzene) gel
columns (PLgel 5 μm Mixed-D and PLgel 5 μm Mixed-C) were used as
stationary phase while DMF was the mobile phase. The eluent flow
rate was 1.0 mL min−1. Polystyrene standards of known molecular
weight were used for calibration.
3. Results and discussion
3.1. General characterization of model compound (3) and
poly(1,3,4-oxadiazole-imide)s (4)
The model compound 3 was prepared starting from 1 and phthalic
anhydride (2), in glacial acetic acid, as shown in Scheme 1. The structure
of 3 was identified by FTIR and 1H NMR spectroscopy. Fig. 1 presents the
1H NMR spectrum of 3 with the assignment for all the protons. The
protons in ortho position to 1,3,4-oxadiazole or imide rings appeared
at the highest ppm values due to the electron withdrawing characteris-
tic of these heterocycles. Aliphatic protons of N(CH3)2 groups appeared
at around 3.1 ppm, while aliphatic proton of CH group appeared at
5.70 ppm.
Table 2
Positions of absorption wavelengths, λmaxabs (nm), excitation wavelengths, λmaxex(nm),
emission wavelengths, λmaxem(nm) maxima, Stokes shifts (SS, nm), emission quantum
yields (QY,%) values for investigated samples in different solvents (c = 1.57 × 10−5 mol
L−1 (for absorption measurements) and c = 2.59 × 10−5 mol L−1 (for emission
measurements)).
The polymers 4a, 4b and 4c were synthesized by a two-step polycon-
densation reaction. In the first step the intermediates poly(amic acid)s
were synthesized by the polycondensation reaction of equimolar
amounts of 1 and dianhydrides 6FDA, 6HDA or FLDA, at room tempera-
ture, in NMP as solvent. The concentration of the reaction mixture was
adjusted to around 10–12% w/w. In the second step acetic anhydride
and pyridine were added to these solutions and the resulting mixtures
were heated at 100 °C in order to perform chemical cyclization of
poly(amic acid)s to fully cyclized polyimides. Polymer 4e was prepared
by using the same two-steps polycondensation reaction starting from
6FDA and equimolar amounts of 1 and 2,5-bis(p-aminophenyl)-1,3,4-
oxadiazole. Due to the low reactivity and insolubility of PEDA in organic
solvents, polymer 4d was prepared in one step, at high temperature,
using as solvent NMP containing 3.5% LiCl, following a method previous-
ly reported [27]. The chemical structures of the polymers were illustrat-
ed in Fig. 2.
ex
Sample Medium λmaxabs, nm
λmax
nm
,
λmaxem, nm
SS, QY, %
nm
3
THF
333
–
417
84
25.36
DCM
NMP
DMSO
THF
DCM
NMP
DMSO
THF
DCM
NMP
DMSO
THF
DCM
NMP
DMSO
THF
DCM
NMP
DMSO
THF
DCM
NMP
DMSO
334
341
340
268, 334 414
274, 345 452
276, 344 463
273, 335 420
276, 340 419
276, 345 453
276, 345 463
272, 335 419
275, 335 417
279, 346 454
277, 346 465
273, 335 419
274, 335 414
276, 346 454
276, 346 469
80
111
–
–
123 28.29
4a
4b
4c
4d
4e
255a, 336
255a, 338
341
84
81
8.73
–
112
–
344
115 8.24
247, 331
247, 336
339
88
17.72
81
115
–
–
338
127 22.92
234, 309, 333
244, 309, 336
309, 339
310, 338
257, 334
86
17.61
78
115
–
–
126 23.46
The structure of polymers was confirmed by FTIR and 1H NMR
spectroscopy. The FTIR spectra of polymers 4 are shown in Fig. 3. The
complete disappearance of the absorption bands of 1660–1670 and
2500–3000 cm−1 characteristic for o-carboxy-amide groups of
poly(amic-acid)s confirms the complete cyclization of these com-
pounds to fully cyclized polyimide structure. In all spectra characteristic
absorption bands appeared at around 1240 cm−1 due to aromatic
\\C\\O\\C\\ linkages. The absorption bands appearing at around
1020 and 980 cm−1 were assigned to oxadiazole rings while the
–
417, 527b,566b 83
13.34
266a, 334,483b, 518b 273, 334 413, 532b, 570b 79
–
–
341, 484b, 523b
276, 346 451, 534a
274, 346 462
273, 335 417
273, 342 413
275, 346 450
277, 345 456
110
341, 490b, 525b
121 8.85
102 5.39
315
334
312
340
79
138
–
–
116 5.78
a
Shoulder.
Bands with a weak intensity.
b

22
C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
bands appearing at 2920 and 2860 cm−1 are characteristic to aliphatic
groups. Absorptions bands of imide rings appeared at 1770–1780 and
1720–1730 cm−1 (C_O of imide asymmetric and symmetric stretching
vibrations), 1360–1370 cm−1 (C\\N stretching vibrations in imide ring)
and 730–740 cm−1 (C_O bending of imide ring). Aromatic C\\H ab-
sorptions were found at 3070 cm−1 while aromatic C_C absorptions
were found at 1600 and 1500 cm−1. In the FTIR spectra of 4a, 4d and
4e the absorption bands appearing at 1210 and 1180 cm−1 were
assigned to hexafluoroisopropylidene groups. In the case of 4d, a charac-
teristic absorption band was observed at 1665 cm−1 due to symmetric
stretching of C_O in the six member of imide ring.
of the polymer 4d was not determined by GPC due to its low solubility
in organic solvents.
3.2. Thermal properties
The glass transition temperature (Tg) of the present polymers, eval-
uated from DSC curves, was in the range of 201–244 °C (Table 1). The
DSC measurements showed no evidence of crystallisation or melting
which proves an amorphous morphology. Generally, Tg of polymers
depends on chain rigidity. The polymer 4a did not show a Tg in the inter-
val of measurements. This is due to the presence of 6F groups and
pendant 2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole
units which increased the rigidity of the macromolecules. Also, 4d did
not exhibit a Tg because this polymer contained additionally rigid
segments derived from perylene dianhydride. In the case of 4e, the re-
duction of pendant 2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-
oxadiazole content due to the introduction of 2,5-bis(p-aminophenyl)-
1,3,4-oxadiazole as a comonomer led to a polymer having a Tg value of
220 °C. The presence of aromatic ether linkage in the chemical structure
decreased the Tg values. Thus, the lower Tg value (201 °C) exhibited the
polymer 4b containing both ether and isopropylidene flexible groups.
Polymer 4c exhibited a higher Tg value (244 °C) because it contained
additionally bulky fluorene groups which decreased the flexibility of
the macromolecular chains.
The thermal stability was investigated by TGA effectuated under
inert atmosphere. Fig. 5 illustrates the TG curves of polymers 4.
All the polymers exhibited high thermal stability, with less signifi-
cant weight loss up to 440 °C. They lost 5% weight in the range of
445–470 °C, the temperature of 10% weight loss (T10) being in the
range of 480–520 °C (Table 1). As can be seen from differential thermo-
gravimetric (DTG) curves, the degradation process exhibited two
maxima of decomposition. The first one (Tmax1) was in the range of
The 1H NMR spectra of the polymers showed characteristic peaks for
aliphatic protons of diamine segments at 5.72 ppm (CH of triphenyl-
methane groups) and at 3.00 ppm (N(CH3)2 groups). Fig. 4 presents
the 1H NMR spectrum of copolymer 4e with the assignment for all the
protons, according to the numbering scheme presented in the interior
figure. The structure of 4e was determined by calculating the integral
values of protons 9 and 14 and the ratio between these two values. It
was found that the composition of the polymer is similar with the com-
position of the reactants used for the synthesis. The structure of 4d was
not confirmed by this method due to the low solubility of this polymer
in organic solvents.
The model compound 3 and the polymers 4b, 4c and 4e were easily
soluble in organic solvents such as NMP, DMF and chloroform. Polymer
4a was soluble in NMP and DMF but it was less soluble in chloroform,
probably due to a higher rigidity of the macromolecular chains. Polymer
4d exhibited less solubility in organic solvents. Thus, this polymer was
soluble in NMP, partially soluble in DMF and insoluble in chloroform,
probably due to the presence in the chemical structure of the units
derived from perylene dianhydride.
The molecular weight of the polymers was determined by GPC. The
values of weight-average molecular weight (Mw) were in the range of
20,800–32,100 g mol−1, the number-average molecular weight (Mn)
were in the range of 16,200–22,800 g mol−1 and the polydispersity
Mw/Mn was in the range of 1.28–1.41 (Table 1). The molecular weight
503–554 °C and the second maximum of the decomposition (Tmax2
)
was in the range of 577–623 °C (Table 1). The polyimide 4d exhibited
Fig. 6. UV–vis absorption spectra of studied samples: 3 (■), 4a (▼), 4b (♦), 4c (▲), 4d (★) and 4e (●), in DCM solutions. Inset shows zoomed region of 400–550 nm, where appear two
weak absorption peaks at 483 and 518 nm originating from perylene moieties.

C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
23
only one maximum of decomposition at 553 °C. The char yields at 700 °C
were in the range of 48.7–61.7%.
3.3. Optical properties
The optical properties of the studied polyimides were investigated in
different solvents (in tetrahydrofuran (THF), dichloromethane (DCM),
N-methyl-2-pyrrolidinone (NMP), and dimethyl sulfoxide (DMSO) so-
lutions) to analyze the effect of environment. The obtained absorption
and emission spectral characteristics are listed in Table 2. Fig. 6 depicts
UV–vis absorption spectra of all studied samples in DCM solution. It can
be observed that the shape of the absorption spectra of polymers, 4a, 4b
and 4c as well as of the copolymers, 4d and 4e, was comparable to that
of the corresponding model compound, 3.
These spectra displayed one or two absorption bands depending
on the molecular structure of samples and the transparency of the
used solvents. Each solvent is transparent (non-absorbing) in a well-
defined region of the UV–vis spectrum. For example, in tetrahydrofuran
the absorption data were collected in spectral range of 230 to 600 nm
and for investigated samples were observed two absorption bands (at
cca. 255 nm and 336 nm), instead in NMP solution (where data were
collected in spectral range of 270 to 600 nm) one absorption band
was obtained. The band at 230–270 nm originates from the aromatic
rings inclusive phenylene rings (reported in Table 2 and Fig. 6) in THF
and DCM solutions, while the absorption maxima located around
336 nm are attributed to the π–π* transitions involving the phenyl-
1,3,4-oxadiazole π-conjugated system. The sample 4c exhibited
Fig. 8. Emission spectra of polymer 4d in different media (tetrahydrofuran (THF),
dichloromethane (DCM), N-methyl-2-pyrrolidinone (NMP), and dimethyl sulfoxide
(DMSO) solutions).
additionally a small absorption peak at 309 nm due to the presence of
fluorene moiety. Instead, the copolymer 4d showed additionally two
weak absorption peaks situated at 483 and 518 nm in DCM solution,
originating from perylene moieties (see inset of Fig. 6).
Fig. 7 shows the excitation and emission spectra of the studied sam-
ples in DMSO solution. Excitation spectra were recorded by using the
maximum emission wavelength value corresponding to the sample
under study. The fluorescence emission spectra of the all studied poly-
imide derivatives exhibited a broad emission band, excepting for the
copolymer 4d, where additionally two weak emission bands appeared
at around 530 and 570 nm (attributed to perylene moieties, see Fig. 8
and Table 2) and for copolymer 4e, which showed one poorly resolved
shoulder (375 nm), only in DMSO solution (see Fig. 7b). From Fig. 8
we can observe that these two weak emission bands were much better
resolved in non-polar solvents (THF and DCM) compared with those
Fig. 7. Excitation (a) and emission (b) spectra of studied samples in dimethyl sulfoxide
solution, at room temperature.
Fig. 9. CIE diagram of polymer 4c in different media.

24
C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
Table 3
The fluorescence quantum yield (QY, %) values for polymer 4a, in tetrahydrofuran solution, under excitation with different wavelength.
Sample
4a
QY, % at λex = 320 nm
QY, % at λex = 334 nm
QY, % at λex = 350 nm
QY, % at λex = 360 nm
QY, % at λex = 365 nm
QY, % at λex = 375 nm
8.98
8.73
7.43
7.91
6.45
4.23
from NMP and DMSO solvents. Also, from spectral data (Table 2) we
observed that the wavelengths of maximum emission of all samples
were red shifted depending on the solvent characteristics. The solvent
polarity exerted strongly effect (emission spectra showed a remarkable
bathochromic shift with increasing solvent polarity, Δλmax = 1–55 nm)
on its excited species compared to its ground-state electronic transition
(only minor shifts of the absorption band of these compounds occurred
from changing the solvent polarity, Δλmax = 2–9 nm). Hence,
solvatochromism was more pronounced in the fluorescence spectra.
The remarkable shifts of the emission maxima with increasing of the
medium polarity reflect the fact that the excited state of investigated
molecules is more polar than its ground state and also suggest an
increase in dipole moment of excited state compared to ground state.
Thus, the solvent (medium) with higher polarity will stabilize the excited
state more preferably with respect to the ground state [28].
To obtain detailed information about optical characteristics of
studied compounds their fluorescence excitation spectra were mea-
sured. Fig. 7 exemplified the excitation spectra of all analyzed samples
in DMSO solution. These spectra were characterized by two main
bands in the 250–400 nm region. The position of the excitation maxima
wavelength of the Band I of the studied samples in all used solvents ap-
peared at around 272 nm. These excitation maxima were shifted to the
long wavelength (~272 nm) compared to the first peak from the
absorption spectra (~247 nm, see Table 2) and indicates that the ab-
sorbing species of Band I give a weak fluorescence signal. Instead, the
second excitation maxima for all samples occurred at wavelength
values very close (the small red-shifts to the absorption maxima,
Δλmax b 6 nm) to those of the corresponding absorption maxima ones,
indicating that the excited emission species can be considered as iden-
tical to the absorbing ones [29].
Also, for polyimide 4c, the Fig. 9 illustrates the CIE-1931 chromaticity
diagram of this compound in different media. The values of the chroma-
ticity coordinates were represented functions of solvent media. Thus,
the emission color changes from THF (x — 0.1523; y — 0.0514,
λem = 419 nm), DCM (x — 0.1530; y — 0.398, λem = 414 nm),
NMP (x — 0.1536; y — 0.1752, λem = 454 nm) to DMSO (x —
0.1735; y — 0.2420, λem = 469 nm).
The calculated Stokes shift values were relatively large, ranging from
80 to 123 nm (3), 81–115 nm (4a), 81–127 nm (4b), 78–126 nm (4c),
79–121 nm (4d) and 79–138 nm (4e) (see Table 2).
Fluorescence quantum yields (QY) of the investigated samples have
also been measured in THF and DMSO solutions and the obtained values
were summarized in Table 2. The QY values were sensitive to solvent
polarity and to excitation wavelength values (Table 3). Thus, the quan-
tum yields of sample 4a in THF decreased with increasing excitation
wavelength from 320 to 375 nm (see Table 3). This is an exception
from Kasha's rule and Vavilov's law, which was attributed either to pho-
todecomposition or to the enhancement of intramolecular energy trans-
fer process competing with internal conversion between singlet states.
Also, it can be observed that for samples 4b and 4c high quantum
yield values were obtained in DMSO (polar solvent) which may be
due to the strong overlap of the electronic transitions of species from
system [28].
The pH effect on fluorescence characteristics of 3 and 4a samples in
THF was investigated by adding various amounts of HCl 1N (Fig. 10). For
compound 3 (see Fig. 10a) the maximum of emission intensity from
416 nm was gradually reduced and red shifted to 452 nm after 500 μL
HCl 1 N were added. This red-shift is attributable to protonation of nitro-
gen atoms from molecular structure of compound 3. In the case of sam-
ple 4a, when low amounts of HCl (1–10 μL, see Fig. 10b) were added, the
emission intensity of system was slightly enhanced, probably in this
context the effect of heavy-atom was reduced [30], instead further in-
creased HCl concentrations induce a gradual emission quenching and
maxima of emission intensity were found red-shifted by ≈15 nm
upon increasing the concentration of HCl. These changes in emission
characteristics upon adding acid are due to protonation of the nitrogen
atoms from investigated molecular structure.
4. Conclusions
The
incorporation
of
pendant
2-(4-phenoxy)-5-(4-
dimethylaminophenyl)-1,3,4-oxadiazole units in the macromolecular
chains of some aromatic polyimides containing flexible aromatic
ether, isopropylidene, hexafluoroisopropylidene or fluorene groups
leads to polymers having high thermal stability and good solubility in
organic solvents. The optical properties including UV–visible absorption
spectra, fluorescence excitation and emission spectra, Stokes shift
values and the relative fluorescence quantum yield were measured in
four different solvents. The fluorescence characteristics consist in a
blue emission band for all samples, large and solvent dependent Stokes
shifts values, as well as solvent dependent quantum yields values.
Namely, the fluorescence quantum yield of 4a in THF decreases as the
Fig. 10. Fluorescence spectra of model compound 3 (a) and polymer 4a (b) in
tetrahydrofuran solution, before and after the addition of various amounts of HCl.

C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
25
[14] C. Hamciuc, B. Schulz, M. Bruma, Poly(oxadiazole-imide)s containing
hexafluoroisopropylidene units, Macromol. Mater. Eng. 235 (1996) 111–120.
[15] S.C. Hsu, W.T. Whang, C.S. Chao, Electroluminescence and electron transport charac-
teristics of aromatic polyimides containing 1,3,4-oxadiazole moiety, Thin Solid Films
515 (2007) 6943–6948.
excitation wavelength changes from 320 nm to 375 nm. The emission
intensity of samples 3 and 4a in THF solution was quenched and
red-shifted in the presence of a protonic acid (HCl 1N).
[16] E. Grabiec, M. Kurcok, E. Schab-Balcerzak, Poly(amide imides) and poly(ether im-
ides) containing 1,3,4-oxadiazole or pyridine rings: Characterizations and optical
properties, J. Phys. Chem. A 113 (2009) 1481–1488.
Acknowledgements
[17] A.M. Ipate, C. Hamciuc, M. Homocianu, V.E. Musteata, A. Nicolescu, M. Bruma, N.
Belomoina, Highly fluorinated poly(1,3,4-oxadiazole-ether)s. Structural, optical
and dielectric characteristics, J. Polym. Res. 22 (2015) 95.
[18] M. Homocianu, A.M. Ipate, C. Hamciuc, A. Airinei, Environment effects on the optical
properties of some fluorinated poly(oxadiazole-ether)s in binary solvent mixtures,
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[19] E. Hamciuc, C. Hamciuc, M. Cazacu, Poly(1,3,4-oxadiazole-ether-imide)s and their
polydimethylsiloxane-containing copolymers, Eur. Polym. J. 43 (2007) 4739–4749.
[20] E. Hamciuc, C. Hamciuc, M. Bruma, B. Schulz, Poly(1,3,4-oxadiazole-imide)s contain-
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The financial support of MEN-UEFISCDI through PN-II-PT-PCCA-
2013-4-1861 project, Partnership in S&T priority domains, contract no.
272/2014, is gratefully acknowledged.
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