The paper describes the synthesis and characterization of three copolymers containing perylenediimide 
units and other chromophores in the main chain, as well as various flexible groups as siloxane, hexafluoroisopropylidene or ether, with emphasis on their thermal and photo-optical properties. Very thin copolymer films were obtained from these polymers with smooth and homogeneous surfaces that are self-organized into vertically segregated structures. Upon irradiation with light of different wavelengths the 
polymers in solution showed photoluminescence maxima in the UV, blue or green-yellow spectral range. In 
solid state only a weak emission in the UV domain occurred due to aggregation of perylenediimide cromophores which leds to fluorescence quenching. The FRET phenomenon between oxadiazole to  perylene-diimide chromophores was observed to occur for which the oxygen bridge appear to be responsible.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3507

Bruma, M.

Chisca, S.

Constantin, P.C.

Damaceanu, M.D.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 1, 01NDLCN09(4pp) (2012) 
 
 
2304-1862/2012/1(1)01NDLCN09(4) 01NDLCN09-1  2012 Sumy State University 
High Performance Perylenediimide-Based Copolymers and Thin Films Made Therefrom 
 
M.D. Damaceanu*, S. Chisca†, P.C. Constantin‡, M. Bruma§ 
 
 "Petru Poni" Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, Iasi-700487, Romania 
 
(Received 20 June 2012; published online 24 August 2012) 
 
The paper describes the synthesis and characterization of three copolymers containing perylenediimide 
units and other chromophores in the main chain, as well as various flexible groups as siloxane, hex-
afluoroisopropylidene or ether, with emphasis on their thermal and photo-optical properties. Very thin co-
polymer films were obtained from these polymers with smooth and homogeneous surfaces that are self-
organized into vertically segregated structures. Upon irradiation with light of different wavelengths the 
polymers in solution showed photoluminescence maxima in the UV, blue or green-yellow spectral range. In 
solid state only a weak emission in the UV domain occurred due to aggregation of perylenediimide cromo-
phores which leds to fluorescence quenching. The FRET phenomenon between oxadiazole to perylene-
diimide chromophores was observed to occur for which the oxygen bridge appear to be responsible. 
 
Keywords: Poly(peryleneimide)s, Oxadiazole, Thin films, Optical properties, Photoluminescence. 
 
 PACS numbers: 68.60.Wm, 68.90. +g 
 
 
                                                          
* damaceanu@icmpp.ro 
† stefan.chisca@icmpp.ro 
‡ paulconstanyin@icmpp.ro 
§ mbruma@icmpp.ro 
1. INTRODUCTION 
 
Polymeric materials are distinguished by low densi-
ty, flexibility, versatile electronic properties, chemical 
inertness and low cost. As a result, they have been ex-
tensively used in fields ranging from aerospace to mi-
croelectronics, optoelectronics, composites and fiber 
optics. The growth of modern technology has posed a 
constantly increasing need for materials that can per-
form well under harsh conditions, such as elevated 
temperatures. Polyimides have various outstanding 
characteristics such as good adhesion to metal, high 
thermal stability, low thermal expansion, excellent 
film-forming properties, and especially a high Tg, of 
above 200 °C, which make them potentially useful as 
stable materials for electronic applications [1-3]. 
Among the polyimides, those containing perylene 
units appear to possess one of the highest thermal sta-
bility because of the condensed aromatic perylene ring. 
Perylene derivatives, with their promising electroactive 
and photoactive properties, have attracted a special 
interest for potential applications in organic molecular 
electronics. Perylene bisimides combine a strong ab-
sorption in the visible region with a fluorescence quan-
tum yields near unity and a high stability towards pho-
tooxidation. Furthermore, perylene bisimides feature a 
relatively low reduction potential, which enables their 
use as an n-type semiconductor and as an electron ac-
ceptor in photoinduced charge transfer reactions. Be-
cause of their appealing properties, perylene mo-
noimide and perylene bisimide derivatives have been 
utilized in various electronic and optical applications 
such as field-effect transistors, fluorescent solar collec-
tors, electrophotographic devices, dye lasers, photovol-
taic cells, and light-emitting diodes [4-8].  
However, research using polyimides containing 
perylene in optoelectronic fields can rarely be found so 
far which may be due to the poor solubility of perylene-
diimide polymers in common organic solvents. There-
fore, the design and preparation of soluble 
poly(peryleneimide)s, to give satisfactory processability 
of products without a perceptible loss of favorable 
properties, is an important and urgent problem in the 
chemistry of high performance polymers. 
Here we report a study on the synthesis and proper-
ties of new processable copolymers containing perylene, 
oxadiazole and various flexible or voluminous units in 
the main chain. Such polymers are expected to have a 
useful balance of material properties and processing 
capability, particularly for casting into very thin films 
for various applications in optoelectronics. The proper-
ties of these copolyimides such as solubility, thermal 
stability, glass transition temperature, photo-optical as 
well as the morphology and quality of nanometric films 
made therefrom were investigated. 
 
2. METHODS  
 
2.1 Monomers 
 
Perylene-3,4,9,10-tetracarboxylic dianhydride 
(PTCDA) and hexafluoroisopropylidene-dianhydride 
(6FDA) have been purchased from Aldrich and used as 
received.  
2,5-bis[4-(p-aminophenoxy)phenylene]-1,3,4-oxadi-
azole, 1,3-bis(4-phthalic anhydride) tetramethyl-
disiloxane and 1,3-bis(4-phthalic anhydride)-9,9-bis(p-
hydroxyphenyl)fluorene were prepared following pub-
lished procedures [9-11]. 
 
2.2 Polymers 
 
Copolyimides PF, PSi and PFl have been prepared 
by one-step polycondensation reaction of the diamino-
oxadiazole with a mixture of perylene-3,4,9,10-
tetracarboxylic dianhydride and hexafluoroisopropy-
lidene-, tetramethyl disiloxane- or fluorene-containing 
 M.D. DAMACEANU, S. CHISCA, P.C. CONSTANTIN, M. BRUMA PROC. NAP 1, 01NDLCN09 (2012) 
 
 
01NDLCN09-2 
dianhydride, in N-methylpyrrolidinone (NMP) and lith-
ium chloride (LiCl) as solvent, at a concentration of 7 % 
total solids, under nitrogen stream, at high tempera-
tures. The monomer ratio was adjusted up to soluble 
and film-forming polymers were obtained. 
In case of the two dianhydrides with different reac-
tivity, the less active dianhydride monomer PTCDA 
was added in the first stage of reaction to the solution 
of diamine in NMP, and then the more active dianhy-
dride monomer 6FDA, tetramethyl disiloxane- or fluo-
rene-containing dianhydride was added to achieve a 
more even random incorporation of the two monomer 
units in the polymer chain. Owing to the low solubility 
of PTCDA in NMP  LiCl at room temperature, higher 
reaction temperature is necessary to make better solu-
bility. The mixture was heated to 180 – 200 C under 
nitrogen for 6 h and then cooled to room temperature. 
The resulting dark red solution was poured into water, 
washed with plenty of water and finally treated with 
ethanol in a Soxhlet apparatus for 1 day in order to get 
rid of the unreacted monomers and high boiling sol-
vent, followed by drying in an oven, under vacuum, at 
100 C. The structure of copolyimides PF, PSi and PFl 
are shown in Fig. 1.  
 
N
O
O
O
O
N N
O
O
C
CF3
CF3 O
O
N
nxy z
x : y : z = 2 : 1 : 1
O
NN
O
O
 
PF (x : y : z = 3 : 1 : 2) 
N
O
O
O
O
N
xy
O
NN
O
O N
O
O
N
O
O
Si O
CH3
CH3
Si
CH3
CH3
nz  
PSi (x : y : z = 3 : 1 : 2) 
N
O
O
O
O
N
n
xy
z
O
NN
O
O N
O
O
N
O
O
O O
 
PFl (x : y : z =2 : 1 : 1) 
 
Fig. 1 – Structure of copolyimides PF, PSi and PFl  
 
2.3 Preparation of polymer films 
 
Very diluted polymer solutions in NMP with con-
centration of 0.5 – 1 % were used to obtain very thin 
films having the thickness in the range of nanometers 
onto silicon wafers, glass or quartz supports by drop-
casting technique. These films, as deposited, were 
gradually heated from room temperature up to 200 C 
to remove the residual solvent and were used for atom-
ic force microscopy (AFM) and photo-optical behaviour 
investigations. 
 
2.4 Measurements 
 
The infrared spectra of the polymers were recorded 
on FT-IR Bruker Vertex 70 Spectrophotometer in 
transmission mode, by using KBr pellets. 
The quality of very thin films as-deposited on silicon 
wafers was investigated by atomic force microscopy 
(AFM) using a Scanning Probe Microscopy Solver PRO-
M, NT-MDT equipment made in Russia, in the semi-
contact mode, semi-contact topography technique.  
The thermal stability of the polymers was investi-
gated by thermogravimetric analysis (TGA) using a 
STA 449 F1 Jupiter device (Netzsch, Germany) operat-
ing at a heating rate of 10 K/min, in nitrogen, from 
30 C to 900 C. The onset on the TG curve was consid-
ered to be the beginning of decomposition or the initial 
decomposition temperature (IDT). The temperature of 
maximum rate of decomposition which is the maximum 
signal in differential thermogravimetry (DTG) curves 
was also recorded. 
The glass transition temperature (Tg) of the precipi-
tated polymers was determined by using a DSC 200 F3 
Maia (Netzsch, Germany) equipment. Approximately 4 
to 6 mg of each polymer were crimped in aluminium 
pans and run in nitrogen atmosphere with a heat-cool-
heat profile from 30 C to 200 – 250 C at 10 K/min. The 
mid-point temperature of the change in slope of the 
DSC signal of the second heating cycle was used to de-
termine the glass transition temperature values of the 
polymers.  
The UV-Vis absorption and photoluminescence 
spectra of polyimides were registered with Specord 
M42 apparatus and Perkin Elmer LS 55 apparatus, 
respectively, by using very diluted polymer solutions 
(aprox. 10 – 5 M) and very thin films.  
 
3. RESULTS AND DISCUSSIONS  
 
New oxadiazole-containing copolyperyleneimides, 
PF, PSi and PFl  containing various rigidity disturb-
ing units as hexafluoroisopropylidene, siloxane or ether 
have been successfully prepared.  
The structures of the present polymers were identi-
fied by infrared spectra. In the IR spectra of all the 
polyimides characteristic imide ring absorption bands 
appeared at 1770 – 1780 cm-1 (asymmetrical C=O imide 
stretching), 1720 – 1730 cm-1 (symmetrical C=O imide 
stretching) and 720 – 730 cm-1 (imide ring defor-
mation). Characteristic absorption peaks of 1,3,4-
oxadiazole ring appeared at 960 – 970 cm-1 and 1012 –
 1020 cm-1 (=C–O–C= stretching).  C-H linkage in aro-
matic rings showed absorption peak at 3100-3120 cm-1 
while the C-F linkage showed absorption peak in the 
range 1100 – 1300 cm-1. The incorporation of tetrame-
thyldisiloxane units in the polymer chain was evi-
denced by the clear absorption peaks in the range of 
2960 – 2950 cm-1 (aliphatic C–H stretching, CH3 
groups), 1250 – 1240 cm-1 (Si–CH3 deformation), 1120 –
 1110 cm-1 (silicon–phenyl bond), 1070 – 1060 cm-1 (Si–
O–Si asymmetric stretching) and 800 – 790 cm-1 (Si–
CH3 stretching). Strong absorption bands in the range 
of 1250 – 1240 cm-1 in the FTIR spectra of copolyimides 
were assigned to the aromatic ether stretching.  
It is known that conventional aromatic 
poly(peryleneimide)s are insoluble in easily accessible 
and safe organic solvents, being soluble only in m-
cresol, due to the rigid nature of perylenediimide unit 
which dictates the overall shape of the corresponding 
macromolecules and thus facilitates the strong inter-
chain interactions and due to the compact aggregation 
of the polymer chains which occurs during imidization 
that is carried out at high temperatures. Our copo-
ly(peryleneimide)s are soluble in a convenient aprotic 
amidic solvent which is NMP, at a concentration of 
 HIGH PERFORMANCE PERYLENEDIIMIDE-BASED COPOLYMERS… PROC. NAP 1, 01NDLCN09 (2012) 
 
 
01NDLCN09-3 
0.5 – 1% due to the presence of the flexible 6F or silox-
ane groups, or of voluminous fluorene units in the main 
chains. 
All these copolyimides are highly thermostable, as 
evaluated by thermogravimetric analysis, their initial 
decomposition temperature being above 475 C (Ta-
ble 1). The temperature of 10% weight loss ranges from 
482 C to 505 C. The temperature of maximum rate of 
decomposition was found in the domain of 520 C –
 557 C. These data demonstrate that all the polyimides 
have high thermal stability, which is not affected by 
the introduction of the flexible 6F, siloxane or ether 
units in the main chain, while the solubility, hence the 
processability of the resulting copolymers are much 
improved due to the presence of these groups. Repre-
sentative TG and DTG curves of these polymers are 
shown in Fig. 1.  
 
Table 1 - Thermal properties of the polymers 
 
Polymer 
 
Tg ( C) IDT( C) T10%( C) Tmax( C) 
PF 267 475 482 530 
PSi 187 488 501 520 
PFl 304 483 505 557 
 
Tg – glass transition temperature 
IDT – temperature of 5% weight loss 
T10% – temperature of 10% weight loss 
Tmax – temperature of maximum rate of decomposition 
 
0 100 200 300 400 500 600 700 800 900
40
60
80
100
Temperature (
o
C)
M
a
s
s
/%
488
o
C
520
o
C
-6
-4
-2
0
D
T
G
/(%
/m
in
)
 
 
Fig. 1 – TG and DTG curves of PSi 
 
The glass transition temperature of the polyimides 
was evaluated by differential scanning calorimetry 
(DSC). All these polymers exhibit glass transition in 
the range of 187 – 304 C (Table 1). It can be seen that 
there is a large influence of the structure of the five-
member dianhydride on the glass transition tempera-
ture, the lowest being obtained in the case of tetrame-
thyldisiloxane-containing dianhydride and the highest 
in the case of fluorene-containing dianhydride.   
Very thin films having the thickness in the na-
nometer range were deposited by drop-casting tech-
nique onto silicon plates, by using diluted solutions of 
polymers  (concentration of 1%). The morphology of 
these films as-deposited onto silicon plates was studied 
by atomic force microscopy (AFM). The copolymer films 
are smooth and homogeneous and are self-organized 
into vertically segregated structures. The root mean 
square roughness (RMS) is in the range of 2 – 5 nm 
over a scanning area of 5  5 µm. A typical AFM image 
is shown in Fig. 2. 
 
 
 
Figю 2 – AFM images of copolyimide PF (top: 2D image, 
down: 3D image) 
 
Since 1,3,4-oxadiazole ring and perylene-diimide 
unit are known as light-emissive [8,12], the light-
emitting properties of these polymers incorporating 
both perylenediimide and oxadiazole fluorophores were 
investigated. The light-emitting ability was assessed on 
the basis of the UV-Vis and photoluminescence spectra 
which were recorded for both polymer solutions in 
NMP and polymer films cast from NMP solutions, after 
excitation with light of different wavelengths.  
Those copolymers in solutions show one strong UV 
absorption maximum in the range of 302 – 307 nm due 
to diphenyl-1,3,4-oxadiazole chromophore, and in the 
case of PFl there are three additional small peaks like 
fingers at 275, 286 and 298 nm specific to fluorene unit 
(Fig. 3). 
 
300 350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
A
b
s
o
rb
a
n
c
e
 (
a
.u
.)
Wavelength (nm)
 PF
 PSi
 PFl
 
 
Fig. 3 – UV-Vis absorption spectra of colpolymers in solution 
 
Three peaks at 452 – 462, 488 – 492 and 521 –
 528 nm due to the absorption of perylenediimide 
chromophore appeared only in the UV spectra of PF 
and PFl, and no significant absorption was registered 
for PSi. However, for polymers PF and PFl, these ab-
sorptions determined by the chain segments containing 
perylenediimide units are much lower in intensity as 
compared with the absorption bands characteristic of 
diphenyl-1,3,4-oxadiazole or fluorene units. 
In thin films, both polymers displayed one large ab-
sorption maximum centred in the range of 296 –
 317 nm assigned to - * transitions involving oxadia-
zole chromophore, being slightly blue- or red-shifted 
with respect to the absorption of the isolated molecules. 
Two additional weaker absorption peaks were detected 
as well at 495 – 503 and 530 – 543 nm, being responsi-
ble for the absorption of perylenediimide only for PF 
and PFl. In the UV-vis spectrum the PSi very weak, 
 M.D. DAMACEANU, S. CHISCA, P.C. CONSTANTIN, M. BRUMA PROC. NAP 1, 01NDLCN09 (2012) 
 
 
01NDLCN09-4 
insignificant absorption peaks characteristic to 
perylenediimide unit were observed. Fig. 4 compares 
the UV spectra of the thin films made from PSi and 
PFl. 
 
300 350 400 450 500 550 600
0.05
0.10
0.15
0.20
0.25
0.30
A
b
s
o
rb
a
n
c
e
 (
a
.u
.)
Wavelength (nm)
 PSi
 PFl
 
 
Fig. 4 – UV-Vis absorption spectra of PSi and PFl thin films 
 
The fluorescence behaviour of these polymers was 
investigated by exciting with light of two different 
wavelengths, one characteristic to oxadiazole chro-
mophore and another one characteristic to perylene-
diimide chromophore. When excited with UV light of 
about 300 nm, the polymer solutions showed one 
broad weak emission in the UV domain, in the range 
of 368 – 375 nm, and two PL emission peaks within 
536 – 539 nm and 574 – 578 nm range, in the case of 
PF and PFl, which are due to perylenediimide. PSi 
showed only one strong emission peak at 524 nm, 
being characteristic to this chromophore. Since the 
absorption maximum of the diphenyl-oxadiazole 
chromophore is aprox. 304 – 306 nm and the 
perylenediimide moieties did not directly absorb the-
se wavelengths light, an excitation energy transfer 
(known as FRET phenomenon) from oxadiazole to 
perylenediimide occurred. Fig. 5 shows photolumi-
nescence spectra of these polymers in solution by 
excitation with light peaking at about 300 nm. The 
emission spectra in NMP solution excited by a 
492 nm light, showed two peaks at 534 – 537 nm and 
575 – 578 nm, which corresponded to the emission of 
perylenediimide moieties.  
The photoluminescent behavior of these copolyi-
mides changed significantly from solution to solid 
state. The light-emitting ability of these polymers 
was observed only by exciting at about 300 nm. The 
PL spectra present one emission maxima in the UV 
domain at 360 and 369 nm only for PSi and PF, re-
spectively, and several shoulders in the blue domain 
at 418, 444 – 457, 482 – 487 nm and 523 nm, due to 
the emission of diphenyl-oxadiazole/fluorene, imide 
and phenylene segments for all polymers. When the 
excitation was done with UV light of 490 or 520 nm 
no emission attributed to perylenediimide chromo-
phores that absorb this visible light was observed. 
This behavior can be ascribed to the high content of 
perylenediimide cromophore which leads to fluores-
cence quenching through aggregation. 
 
350 400 450 500 550 600
0
100
200
300
400
500
600
700
800
900
1000
P
L
 I
n
te
n
s
it
y
 (
n
m
)
Wavelength (nm)
 PF
 PSi
 PFl
 
 
Fig. 5 – Fluorescence spectra of perylenediimide-based copol-
ymers by excitation with UV light peaking at about 300 nm 
 
4. CONCLUSIONS 
 
The high-performance properties of these copoly-
mers provide an important guideline for the design of 
new materials that can be used to fabricate electro-
optical devices with high stability. 
 
AKNOWLEDGEMENTS 
 
The financial support provided by CNCSIS-
UEFISCDI through the Project PN II-RU, code TE_221, 
no. 31/2010 is acknowledged with great pleasure. 
 
 
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1. D. Wilson, H.D. Stenzenberger, P.M. Hergenrother, Poly-
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Applications (New-York: Dekker: 1996). 
3. P.M. Hergenrother, High Perform. Polym. 15, 34 (2003). 
4. Y. Kim, J.H. Keum, J.G. Lee, H. Lim, C.S. Ha, Adv. Mater. 
Opt. Electron. 10, 273 (2000).  
5. Y.T. Chern, W.H. Chung, J. Polym. Sci. Part A: Polym. 
Chem. 34, 117 (1996).  
6. L. Schmidt-Mende, A. Fechtenkotter, K. Müllen, 
E. Moons, R.H. Friend, J.D. MacKenzie, Science 293, 1119 
(2001). 
7. Y.J. Liu, P. Zhuang, H. Liu, Y. Li, F. Lu, H. Gan, T. Jiu, 
N. Wang, X. He, D. Zhu, Chem. Phys. Chem. 5, 1210 
(2004). 
8. S.G. Xu, M.J. Yang, S.K. Cao, React. Funct. Polym. 66, 471 
(2006).  
9. M.D. Iosip, M. Bruma, J. Robison, Y. Kaminorz, B. Schulz, 
High. Perform. Polym. 13, 133 (2001). 
10. J. D. Rich, Silylation method and organic silanes made 
therefrom, US Patent 4709054 (1987).  
11. T. Takekoshi, Polymer J. 19: 191 (1987).  
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High Performance Perylenediimide-Based Copolymers and Thin Films Made Therefrom

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 1, 01NDLCN09(4pp) (2012)   2304-1862/2012/1(1)01NDLCN09(4) 01NDLCN09-1  2012 Sumy State University High Performance Perylenediimide-Based Copolymers and Thin Films Made Therefrom  M.D. Damaceanu*, S. Chisca†, P.C. Constantin‡, M. Bruma§   "Petru Poni" Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, Iasi-700487, Romania  (Received 20 June 2012; published online 24 August 2012)  The paper describes the synthesis and characterization of three copolymers containing perylenediimide units and other chromophores in the main chain, as well as various flexible groups as siloxane, hex-afluoroisopropylidene or ether, with emphasis on their thermal and photo-optical properties. Very thin co-polymer films were obtained from these polymers with smooth and homogeneous surfaces that are self-organized into vertically segregated structures. Upon irradiation with light of different wavelengths the polymers in solution showed photoluminescence maxima in the UV, blue or green-yellow spectral range. In solid state only a weak emission in the UV domain occurred due to aggregation of perylenediimide cromo-phores which leds to fluorescence quenching. The FRET phenomenon between oxadiazole to perylene-diimide chromophores was observed to occur for which the oxygen bridge appear to be responsible.  Keywords: Poly(peryleneimide)s, Oxadiazole, Thin films, Optical properties, Photoluminescence.   PACS numbers: 68.60.Wm, 68.90. +g                                                             * damaceanu@icmpp.ro † stefan.chisca@icmpp.ro ‡ paulconstanyin@icmpp.ro § mbruma@icmpp.ro 1. INTRODUCTION  Polymeric materials are distinguished by low densi-ty, flexibility, versatile electronic properties, chemical inertness and low cost. As a result, they have been ex-tensively used in fields ranging from aerospace to mi-croelectronics, optoelectronics, composites and fiber optics. The growth of modern technology has posed a constantly increasing need for materials that can per-form well under harsh conditions, such as elevated temperatures. Polyimides have various outstanding characteristics such as good adhesion to metal, high thermal stability, low thermal expansion, excellent film-forming properties, and especially a high Tg, of above 200 °C, which make them potentially useful as stable materials for electronic applications [1-3]. Among the polyimides, those containing perylene units appear to possess one of the highest thermal sta-bility because of the condensed aromatic perylene ring. Perylene derivatives, with their promising electroactive and photoactive properties, have attracted a special interest for potential applications in organic molecular electronics. Perylene bisimides combine a strong ab-sorption in the visible region with a fluorescence quan-tum yields near unity and a high stability towards pho-tooxidation. Furthermore, perylene bisimides feature a relatively low reduction potential, which enables their use as an n-type semiconductor and as an electron ac-ceptor in photoinduced charge transfer reactions. Be-cause of their appealing properties, perylene mo-noimide and perylene bisimide derivatives have been utilized in various electronic and optical applications such as field-effect transistors, fluorescent solar collec-tors, electrophotographic devices, dye lasers, photovol-taic cells, and light-emitting diodes [4-8].  However, research using polyimides containing perylene in optoelectronic fields can rarely be found so far which may be due to the poor solubility of perylene-diimide polymers in common organic solvents. There-fore, the design and preparation of soluble poly(peryleneimide)s, to give satisfactory processability of products without a perceptible loss of favorable properties, is an important and urgent problem in the chemistry of high performance polymers. Here we report a study on the synthesis and proper-ties of new processable copolymers containing perylene, oxadiazole and various flexible or voluminous units in the main chain. Such polymers are expected to have a useful balance of material properties and processing capability, particularly for casting into very thin films for various applications in optoelectronics. The proper-ties of these copolyimides such as solubility, thermal stability, glass transition temperature, photo-optical as well as the morphology and quality of nanometric films made therefrom were investigated.  2. METHODS   2.1 Monomers  Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and hexafluoroisopropylidene-dianhydride (6FDA) have been purchased from Aldrich and used as received.  2,5-bis[4-(p-aminophenoxy)phenylene]-1,3,4-oxadi-azole, 1,3-bis(4-phthalic anhydride) tetramethyl-disiloxane and 1,3-bis(4-phthalic anhydride)-9,9-bis(p-hydroxyphenyl)fluorene were prepared following pub-lished procedures [9-11].  2.2 Polymers  Copolyimides PF, PSi and PFl have been prepared by one-step polycondensation reaction of the diamino-oxadiazole with a mixture of perylene-3,4,9,10-tetracarboxylic dianhydride and hexafluoroisopropy-lidene-, tetramethyl disiloxane- or fluorene-containing  M.D. DAMACEANU, S. CHISCA, P.C. CONSTANTIN, M. BRUMA PROC. NAP 1, 01NDLCN09 (2012)   01NDLCN09-2 dianhydride, in N-methylpyrrolidinone (NMP) and lith-ium chloride (LiCl) as solvent, at a concentration of 7 % total solids, under nitrogen stream, at high tempera-tures. The monomer ratio was adjusted up to soluble and film-forming polymers were obtained. In case of the two dianhydrides with different reac-tivity, the less active dianhydride monomer PTCDA was added in the first stage of reaction to the solution of diamine in NMP, and then the more active dianhy-dride monomer 6FDA, tetramethyl disiloxane- or fluo-rene-containing dianhydride was added to achieve a more even random incorporation of the two monomer units in the polymer chain. Owing to the low solubility of PTCDA in NMP  LiCl at room temperature, higher reaction temperature is necessary to make better solu-bility. The mixture was heated to 180 – 200 C under nitrogen for 6 h and then cooled to room temperature. The resulting dark red solution was poured into water, washed with plenty of water and finally treated with ethanol in a Soxhlet apparatus for 1 day in order to get rid of the unreacted monomers and high boiling sol-vent, followed by drying in an oven, under vacuum, at 100 C. The structure of copolyimides PF, PSi and PFl are shown in Fig. 1.   NOOOON NOOCCF3CF3 OONnxy zx : y : z = 2 : 1 : 1ONNOO PF (x : y : z = 3 : 1 : 2) NOOOONxyONNOO NOONOOSi OCH3CH3SiCH3CH3nz  PSi (x : y : z = 3 : 1 : 2) NOOOONnxyzONNOO NOONOOO O PFl (x : y : z =2 : 1 : 1)  Fig. 1 – Structure of copolyimides PF, PSi and PFl   2.3 Preparation of polymer films  Very diluted polymer solutions in NMP with con-centration of 0.5 – 1 % were used to obtain very thin films having the thickness in the range of nanometers onto silicon wafers, glass or quartz supports by drop-casting technique. These films, as deposited, were gradually heated from room temperature up to 200 C to remove the residual solvent and were used for atom-ic force microscopy (AFM) and photo-optical behaviour investigations.  2.4 Measurements  The infrared spectra of the polymers were recorded on FT-IR Bruker Vertex 70 Spectrophotometer in transmission mode, by using KBr pellets. The quality of very thin films as-deposited on silicon wafers was investigated by atomic force microscopy (AFM) using a Scanning Probe Microscopy Solver PRO-M, NT-MDT equipment made in Russia, in the semi-contact mode, semi-contact topography technique.  The thermal stability of the polymers was investi-gated by thermogravimetric analysis (TGA) using a STA 449 F1 Jupiter device (Netzsch, Germany) operat-ing at a heating rate of 10 K/min, in nitrogen, from 30 C to 900 C. The onset on the TG curve was consid-ered to be the beginning of decomposition or the initial decomposition temperature (IDT). The temperature of maximum rate of decomposition which is the maximum signal in differential thermogravimetry (DTG) curves was also recorded. The glass transition temperature (Tg) of the precipi-tated polymers was determined by using a DSC 200 F3 Maia (Netzsch, Germany) equipment. Approximately 4 to 6 mg of each polymer were crimped in aluminium pans and run in nitrogen atmosphere with a heat-cool-heat profile from 30 C to 200 – 250 C at 10 K/min. The mid-point temperature of the change in slope of the DSC signal of the second heating cycle was used to de-termine the glass transition temperature values of the polymers.  The UV-Vis absorption and photoluminescence spectra of polyimides were registered with Specord M42 apparatus and Perkin Elmer LS 55 apparatus, respectively, by using very diluted polymer solutions (aprox. 10 – 5 M) and very thin films.   3. RESULTS AND DISCUSSIONS   New oxadiazole-containing copolyperyleneimides, PF, PSi and PFl  containing various rigidity disturb-ing units as hexafluoroisopropylidene, siloxane or ether have been successfully prepared.  The structures of the present polymers were identi-fied by infrared spectra. In the IR spectra of all the polyimides characteristic imide ring absorption bands appeared at 1770 – 1780 cm-1 (asymmetrical C=O imide stretching), 1720 – 1730 cm-1 (symmetrical C=O imide stretching) and 720 – 730 cm-1 (imide ring defor-mation). Characteristic absorption peaks of 1,3,4-oxadiazole ring appeared at 960 – 970 cm-1 and 1012 – 1020 cm-1 (=C–O–C= stretching).  C-H linkage in aro-matic rings showed absorption peak at 3100-3120 cm-1 while the C-F linkage showed absorption peak in the range 1100 – 1300 cm-1. The incorporation of tetrame-thyldisiloxane units in the polymer chain was evi-denced by the clear absorption peaks in the range of 2960 – 2950 cm-1 (aliphatic C–H stretching, CH3 groups), 1250 – 1240 cm-1 (Si–CH3 deformation), 1120 – 1110 cm-1 (silicon–phenyl bond), 1070 – 1060 cm-1 (Si–O–Si asymmetric stretching) and 800 – 790 cm-1 (Si–CH3 stretching). Strong absorption bands in the range of 1250 – 1240 cm-1 in the FTIR spectra of copolyimides were assigned to the aromatic ether stretching.  It is known that conventional aromatic poly(peryleneimide)s are insoluble in easily accessible and safe organic solvents, being soluble only in m-cresol, due to the rigid nature of perylenediimide unit which dictates the overall shape of the corresponding macromolecules and thus facilitates the strong inter-chain interactions and due to the compact aggregation of the polymer chains which occurs during imidization that is carried out at high temperatures. Our copo-ly(peryleneimide)s are soluble in a convenient aprotic amidic solvent which is NMP, at a concentration of  HIGH PERFORMANCE PERYLENEDIIMIDE-BASED COPOLYMERS… PROC. NAP 1, 01NDLCN09 (2012)   01NDLCN09-3 0.5 – 1% due to the presence of the flexible 6F or silox-ane groups, or of voluminous fluorene units in the main chains. All these copolyimides are highly thermostable, as evaluated by thermogravimetric analysis, their initial decomposition temperature being above 475 C (Ta-ble 1). The temperature of 10% weight loss ranges from 482 C to 505 C. The temperature of maximum rate of decomposition was found in the domain of 520 C – 557 C. These data demonstrate that all the polyimides have high thermal stability, which is not affected by the introduction of the flexible 6F, siloxane or ether units in the main chain, while the solubility, hence the processability of the resulting copolymers are much improved due to the presence of these groups. Repre-sentative TG and DTG curves of these polymers are shown in Fig. 1.   Table 1 - Thermal properties of the polymers  Polymer  Tg ( C) IDT( C) T10%( C) Tmax( C) PF 267 475 482 530 PSi 187 488 501 520 PFl 304 483 505 557  Tg – glass transition temperature IDT – temperature of 5% weight loss T10% – temperature of 10% weight loss Tmax – temperature of maximum rate of decomposition  0 100 200 300 400 500 600 700 800 900406080100Temperature (oC)Mass/%488oC520oC-6-4-20DTG/(%/min)  Fig. 1 – TG and DTG curves of PSi  The glass transition temperature of the polyimides was evaluated by differential scanning calorimetry (DSC). All these polymers exhibit glass transition in the range of 187 – 304 C (Table 1). It can be seen that there is a large influence of the structure of the five-member dianhydride on the glass transition tempera-ture, the lowest being obtained in the case of tetrame-thyldisiloxane-containing dianhydride and the highest in the case of fluorene-containing dianhydride.   Very thin films having the thickness in the na-nometer range were deposited by drop-casting tech-nique onto silicon plates, by using diluted solutions of polymers  (concentration of 1%). The morphology of these films as-deposited onto silicon plates was studied by atomic force microscopy (AFM). The copolymer films are smooth and homogeneous and are self-organized into vertically segregated structures. The root mean square roughness (RMS) is in the range of 2 – 5 nm over a scanning area of 5  5 µm. A typical AFM image is shown in Fig. 2.    Figю 2 – AFM images of copolyimide PF (top: 2D image, down: 3D image)  Since 1,3,4-oxadiazole ring and perylene-diimide unit are known as light-emissive [8,12], the light-emitting properties of these polymers incorporating both perylenediimide and oxadiazole fluorophores were investigated. The light-emitting ability was assessed on the basis of the UV-Vis and photoluminescence spectra which were recorded for both polymer solutions in NMP and polymer films cast from NMP solutions, after excitation with light of different wavelengths.  Those copolymers in solutions show one strong UV absorption maximum in the range of 302 – 307 nm due to diphenyl-1,3,4-oxadiazole chromophore, and in the case of PFl there are three additional small peaks like fingers at 275, 286 and 298 nm specific to fluorene unit (Fig. 3).  300 350 400 450 500 550 6000.00.20.40.60.81.01.21.41.6Absorbance (a.u.)Wavelength (nm) PF PSi PFl  Fig. 3 – UV-Vis absorption spectra of colpolymers in solution  Three peaks at 452 – 462, 488 – 492 and 521 – 528 nm due to the absorption of perylenediimide chromophore appeared only in the UV spectra of PF and PFl, and no significant absorption was registered for PSi. However, for polymers PF and PFl, these ab-sorptions determined by the chain segments containing perylenediimide units are much lower in intensity as compared with the absorption bands characteristic of diphenyl-1,3,4-oxadiazole or fluorene units. In thin films, both polymers displayed one large ab-sorption maximum centred in the range of 296 – 317 nm assigned to - * transitions involving oxadia-zole chromophore, being slightly blue- or red-shifted with respect to the absorption of the isolated molecules. Two additional weaker absorption peaks were detected as well at 495 – 503 and 530 – 543 nm, being responsi-ble for the absorption of perylenediimide only for PF and PFl. In the UV-vis spectrum the PSi very weak,  M.D. DAMACEANU, S. CHISCA, P.C. CONSTANTIN, M. BRUMA PROC. NAP 1, 01NDLCN09 (2012)   01NDLCN09-4 insignificant absorption peaks characteristic to perylenediimide unit were observed. Fig. 4 compares the UV spectra of the thin films made from PSi and PFl.  300 350 400 450 500 550 6000.050.100.150.200.250.30Absorbance (a.u.)Wavelength (nm) PSi PFl  Fig. 4 – UV-Vis absorption spectra of PSi and PFl thin films  The fluorescence behaviour of these polymers was investigated by exciting with light of two different wavelengths, one characteristic to oxadiazole chro-mophore and another one characteristic to perylene-diimide chromophore. When excited with UV light of about 300 nm, the polymer solutions showed one broad weak emission in the UV domain, in the range of 368 – 375 nm, and two PL emission peaks within 536 – 539 nm and 574 – 578 nm range, in the case of PF and PFl, which are due to perylenediimide. PSi showed only one strong emission peak at 524 nm, being characteristic to this chromophore. Since the absorption maximum of the diphenyl-oxadiazole chromophore is aprox. 304 – 306 nm and the perylenediimide moieties did not directly absorb the-se wavelengths light, an excitation energy transfer (known as FRET phenomenon) from oxadiazole to perylenediimide occurred. Fig. 5 shows photolumi-nescence spectra of these polymers in solution by excitation with light peaking at about 300 nm. The emission spectra in NMP solution excited by a 492 nm light, showed two peaks at 534 – 537 nm and 575 – 578 nm, which corresponded to the emission of perylenediimide moieties.  The photoluminescent behavior of these copolyi-mides changed significantly from solution to solid state. The light-emitting ability of these polymers was observed only by exciting at about 300 nm. The PL spectra present one emission maxima in the UV domain at 360 and 369 nm only for PSi and PF, re-spectively, and several shoulders in the blue domain at 418, 444 – 457, 482 – 487 nm and 523 nm, due to the emission of diphenyl-oxadiazole/fluorene, imide and phenylene segments for all polymers. When the excitation was done with UV light of 490 or 520 nm no emission attributed to perylenediimide chromo-phores that absorb this visible light was observed. This behavior can be ascribed to the high content of perylenediimide cromophore which leads to fluores-cence quenching through aggregation.  350 400 450 500 550 60001002003004005006007008009001000PL Intensity (nm)Wavelength (nm) PF PSi PFl  Fig. 5 – Fluorescence spectra of perylenediimide-based copol-ymers by excitation with UV light peaking at about 300 nm  4. CONCLUSIONS  The high-performance properties of these copoly-mers provide an important guideline for the design of new materials that can be used to fabricate electro-optical devices with high stability.  AKNOWLEDGEMENTS  The financial support provided by CNCSIS-UEFISCDI through the Project PN II-RU, code TE_221, no. 31/2010 is acknowledged with great pleasure.   REFERENCES  1. D. Wilson, H.D. Stenzenberger, P.M. Hergenrother, Poly-imides (New York: Chapman and Hall: 1990). 2. M.K. Ghosh, K.L. Mittal, Polyimides: Fundamentals and Applications (New-York: Dekker: 1996). 3. P.M. Hergenrother, High Perform. Polym. 15, 34 (2003). 4. Y. Kim, J.H. Keum, J.G. Lee, H. Lim, C.S. Ha, Adv. Mater. Opt. Electron. 10, 273 (2000).  5. Y.T. Chern, W.H. Chung, J. Polym. Sci. Part A: Polym. Chem. 34, 117 (1996).  6. L. Schmidt-Mende, A. Fechtenkotter, K. Müllen, E. Moons, R.H. Friend, J.D. MacKenzie, Science 293, 1119 (2001). 7. Y.J. Liu, P. Zhuang, H. Liu, Y. Li, F. Lu, H. Gan, T. Jiu, N. Wang, X. He, D. Zhu, Chem. Phys. Chem. 5, 1210 (2004). 8. S.G. Xu, M.J. Yang, S.K. Cao, React. Funct. Polym. 66, 471 (2006).  9. M.D. Iosip, M. Bruma, J. Robison, Y. Kaminorz, B. Schulz, High. Perform. Polym. 13, 133 (2001). 10. J. D. Rich, Silylation method and organic silanes made therefrom, US Patent 4709054 (1987).  11. T. Takekoshi, Polymer J. 19: 191 (1987).  12. D. Yao, Z.Y. Wang, P.R. Sundararajan, Polymer 46, 4390 (2005).  

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High Performance Perylenediimide-Based Copolymers and Thin Films Made Therefrom

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