Synthesis and photo- and electrophysical properties of conjugated copolyfluorenes with 7,8,10-triarylfluoranthene fragments in the main chain

Full text of DOI:10.1134/S0012500812020024

ISSN 0012ꢀ5008, Doklady Chemistry, 2012, Vol. 442, Part 2, pp. 23–29. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © M.L. Keshtov, E.I. Mal’tsev, S.I. Pozin, D.V. Marochkin, V.P. Perevalov, I.V. Blagodatskikh, P.V. Petrovskii, A.R. Khokhlov, 2012, published in Doklady
Akademii Nauk, 2012, Vol. 442, No. 4, pp. 486–492.
CHEMISTRY
Synthesis and Photoꢀ and Electrophysical Properties
of Conjugated Copolyfluorenes with 7,8,10ꢀTriarylfluoranthene
Fragments in the Main Chain
M. L. Keshtova, E. I. Mal’tsevb, S. I. Pozinb, D. V. Marochkinc,
V. P. Perevalovc, I. V. Blagodatskikha, P. V. Petrovskiia, and Academician A. R. Khokhlova
Received October 10, 2011
DOI: 10.1134/S0012500812020024
Polyfluorenes (PFs) are attractive photoꢀ and elecꢀ tion temperature, and improves the solubility and
thermal stability.
troluminescent polymers widely used in polymeric
blue lightꢀemitting diodes due to their high thermal
stability, good solubility in common organic solvents,
high quantum yield of fluorescence, and attractive
holeꢀtransport properties [1–5]. Nonetheless, polyꢀ
fluorenes have a number of drawbacks: a long route of
preparation of initial monomers, low yields (for examꢀ
ple, in the synthesis of boronic derivatives), poor elecꢀ
tronꢀtransport properties, instability of color purity
caused by formation of aggregates, excimers, and oxo
groups, which leads to the red shift of emission and
reduction of its efficacy [6–9]. Destruction caused by
crystallization and thermal decomposition during
operation of lightꢀemitting diodes proceed quickly
because of low glass transition temperature of the
polymers [10]. It has been recently shown [11, 12] that
the introduction of spirobifluorene in the structure of
polyalkylfluorene suppresses crystallization, prevents
luminescence quenching, increases the glass transiꢀ
To suppress interchain interactions and improve
thermal stability, we introduced pyrenefluoranthene
fragment, which combines high thermal stability with
improved holeꢀtransport properties of pyrene, in the
polymer chain of polyfluorene. In these studies, we
attempted to find a substitute for fluorene using
related fluoranthene derivatives as blue light emitters.
The partial or complete replacement of alkylfluorene
with phenylated fluoranthene derivatives in polyfluoꢀ
renes, owing to the presence of a large number of bulky
side groups, will hinder aggregation and completely
suppress excimer formation, with retention of high
electroluminescence in films.
To obtain new conjugated fluorantheneꢀcontaining
copolyfluorenes, we developed a new monomer,
1ꢀ[7,10ꢀbis(4'ꢀbromophenyl)fluoranthenꢀ8ꢀyl]pyrene
(
III). The synthesis of monomer III was carried out
according to Scheme 1 implying the transformation of
4ꢀbromophenylacetic acid into 1,3ꢀbis(4'ꢀbromopheꢀ
nyl)propanꢀ2ꢀone (I) using dicyclohexylcarbodiimide
(DCC) and N,Nꢀdimethylaminopyridine (DMAP) as
a catalyst, the reaction of resultant ketone with
acenaphthoquinone to give 7,9ꢀbis(4'ꢀbromopheꢀ
nyl)cyclopenta[a]acenaphthylenꢀ8ꢀone (II), and the
condensation of the latter with 1ꢀethynylpyrene in
chloronaphthalene.
a Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences, ul. Vavilova 28,
Moscow, 119991 Russia
b Frumkin Institute of Physical Chemistry
and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
c Mendeleev University of Chemical Technology
of Russia, Miusskaya pl. 9, Moscow, 125047 Russia
23

24
KESHTOV et al.
DCC, DMAP
1
3
4'
Br
Br
Br
CH2Cl2
(82%)
OH
O
O
I
O
O
O
4'
Br
Br
1''
8
9
8
7
9
4'
10
7
I
Br
Br
(98%)
KOH, EtOH
(95%)
1
2
6
5
II
III
3
4
Scheme 1.
ments, respectively (Fig. 1a). The 13C NMR spectrum
of compound III in the region of = 108–155 ppm
shows 44 peaks, 20 of which are quaternary, which
agrees well with the proposed structure (Fig. 1b).
The composition and the structure of the intermeꢀ
diate compounds and target products were confirmed
by the data of elemental analysis and IR and NMR
spectroscopy (see Experimental). In particular, the IR
spectra of compound II in the region 1700–1710 cm–1
show an intense absorption band typical for the C=O
δ
The synthesis of copolyfluorenes IVꢀ3–IVꢀ10 with
different content of 7,8,10ꢀtriarylfluoranthene fragꢀ
ment (from 3 to 10 mol %) was accomplished under
the Yamamoto reaction conditions according to
Scheme 2 with the aim to study the effect of this fragꢀ
1
group of the cyclopentadienone ring. The H NMR
spectra of compound III show all proton resonances in
the aromatic region within
δ = 8.2–6.7 ppm as three
doublets and five multiplets corresponding to the proꢀ
tons of the phenyl, fluoranthene, and pyrene fragꢀ ment on the optical properties of macromolecules.
7
2
x
+ y
Br
Br
Br
Br
1
9
H17C8 C8H17
III
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SYNTHESIS AND PHOTOꢀ AND ELECTROPHYSICAL PROPERTIES
25
H17C8 C8H17
y
x
IVꢀ3: x = 97 mol %, y = 3 mol %
IVꢀ5: x = 95 mol %, y = 5 mol %
IVꢀ10: x = 90 mol %, y = 10 mol %
Scheme 2.
Copolyfluorenes IV were obtained by the reaction homopolyfluorene (Тg = 66
°
С, Т10% = 410
°
С) and
of 2,7ꢀdibromoꢀ9,9ꢀdioctylfluorene with monomer
III in a toluene–DMF mixture at 80 for 72 h under
conditions of the Yamamoto reaction using Ni(COD)2
there is a trend for increase in these temperatures with
an increase in the molar content of monomeric unit
III in the main chain.
°С
(where COD is cyclooctadieneꢀ1,5) as a catalyst. The
The photophysical properties of the copolymers
were studied by UV and fluorescence spectroscopy, the
results are presented in Table 2 and Fig. 3. The maxꢀ
ima of the absorption spectra of copolymers IV are
rather close and, when the content of fluoranthene
units in polymer chain increases (from 3 to 10 mol %),
the main absorption peaks are shifted to the blue
region from 357 to 351 nm on account of interruption
of conjugation of polyfluorene chain by fluoranthene
units. All the copolymers show a strong fluorescence
in the blue region with maxima at 418 nm, which coinꢀ
cide with the value for homopolyfluorene (λmax
418 nm). The fluorescence intensity decreases when
the content of fluoranthene increases.
1
structure of copolymers IV was confirmed by H
1
NMR. The main differences between the H NMR
spectra of copolyfluorenes IVꢀ3, IVꢀ5, IVꢀ10 and
homopolyfluorene consist in the presence of two new
multiplet signals in the spectra of IV at
and 8.12–7.96 ppm corresponding to the residues of
monomer unit III (Fig. 2). The resonances at = 2.11,
δ = 8.29–8.13
δ
1.13, and 0.81 ppm are assigned to the aliphatic proꢀ
tons of the octyl groups of the fluoranthene fragment.
The integrated intensity ratio of the aromatic to aliꢀ
phatic protons for all the polymers agrees well with
proposed structure IV. The molecular weights and
polydispersity of the copolymers determined by GPC
vary within 4190078900 (
Mn), 113500–279200 (Mw),
The electrochemical properties of polymers IVꢀ3–
IVꢀ10 were studied by cyclic voltammetry (Table 2).
A typical cyclic voltammetric curve of copolyfluorene
IVꢀ10 is shown in Fig. 4. All the polymers display
reversible redox properties owing to their high electroꢀ
activity. Using the onset potentials of oxidation
and 2.7–3.5 (Mw Mn), respectively (Table 1). All the
/
copolymers are soluble in common organic solvents,
such as DMF, DMA, DMSO, toluene, and chloroꢀ
form.
onset
ox
onset
red
The thermal characteristics of copolyfluorenes IV
were studied by TMA and TGA (Table 1). The glass
transition temperatures of the polymers (Тg) found
(
E
) and reduction (
E
) and the empirical forꢀ
onset
ox
onset
red
mulas
E
HOMO = –(
E
+ 4.4) and
E
LUMO = –(
E
+
from TMA curves are in the range 108–142°С.
4.4), we found the HOMO and LUMO energies of
polymer molecules and the band gap of the polymers
The copolymers show a good thermal stability. The
temperatures of 10% weight loss (Т10%) determined by
TGA in an argon atmosphere and in air are within
(
Eegc ). These values are rather close to those for
homopolyfluorene [14]. The HOMO energies
HOMO) for the copolymers IVꢀ3–IVꢀ10 gradually
(
E
420–435 and 405–415°С, respectively. The data of
increase from –5.67 eV to –5.65 eV with the content
of fluoranthene fragments, while the hole affinity
Table 1 indicate that the introduction of 7,8,10ꢀtriꢀ
arylfluoranthene unit into the polymer chain leads to
an increase in the glass transition temperature and
decreases in the order: IVꢀ10
> IVꢀ5 > IVꢀ3. On the
thermal stability of the copolyfluorenes with respect to contrary, the LUMO energies ELUMO) of copolymers
DOKLADY CHEMISTRY Vol. 442
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26
KESHTOV et al.
δ
δ
1
13
Fig. 1. (a) H and (b) C NMR spectra of monomer III in CDCl .
3
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SYNTHESIS AND PHOTOꢀ AND ELECTROPHYSICAL PROPERTIES
27
Alk
H17C8 C8H17
x
y
IVꢀ5
Ar
CDCl3
9
8
7
6
5
4
3
2
1
0
δH, ppm
1
Fig. 2. H NMR spectrum of polymer IVꢀ5 in CDCl .
3
gradually enhanced with an increase in the content of fluoꢀ
ranthene fragments, which improves charge injection. It is
worth noting that the energies of the copolymers deterꢀ
mined from the electrochemical data (Eegc ) agree well with
IVꢀ3
2.84 eV) when the content of fluoranthene fragments
rises, impyling that their electron affinity grows in the order:
IVꢀ3 IVꢀ5 IVꢀ10. Thus, the electron and hole affinity is
–IVꢀ10 slightly decrease (from –2.71 eV to
⎯
<
<
Table 1. Molecularꢀweight and thermal characteristics of copolyfluorenes IV
T10%
,
°
C (TGA)
argon
Copolymer
Mn
×
10–4
Mw
×
10–4
Mw
/Mn
Tg,
°
C (TMA)
air
IVꢀ3
IVꢀ5
IVꢀ10
7.89
4.19
5.67
27.92
11.35
24.69
3.5
2.7
4.3
108
115
142
405
410
415
420
425
435
Table 2. Photophysical and electrochemical properties of copolyfluorenes IV
abs
max
fl
max
onset
red
onset
ox
λ
, nm a
λ
, nm b
E
, V c
E
, V d
Eegl , eV e
Eogpt , eV f
Copolymer
ELUMO, eV
ЕНОМО, eV
IVꢀ3
IVꢀ5
IVꢀ10
357
354
351
418
418
418
–1.69
–1.60
–1.56
–2.71
–2.80
–2.84
1.27
1.26
1.25
–5.67
–5.66
–5.65
2.96
2.86
2.81
2.98
2.98
2.98
a
b
c
d
–5
Absorption maxima of the copolymers in chloroform (1
× 10 M).
×
–5
Fluorescence maxima of the copolymers in chloroform (1
Determined from the onset potential of reduction.
Determined from the onset potential of oxidation.
10 M).
ec
e
LUMO
НОМО
Electrochemical energy gap width Eg
=
E
–
Е
.
abs
Optical band gap Eogpt =1240/
λ
.
onset
е
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28
KESHTOV et al.
1.0
1.0
1
2
3
4
5
0.5
0.5
1
'
3
'
4
'
2
'
0
0
750
350
400
450
500
550
, nm
600
650
700
λ
Fig. 3. Absorption spectra ((
1
) PF, (
2
= 1
)
IVꢀ3, (
3
)
IVꢀ5, (
4
)
IVꢀ10) and fluorescence spectra ((
1
') PF, (
2') IVꢀ3, (3') IVꢀ5, (4') IVꢀ10)
–5
of the copolymers in chloroform (
c
×
10 M) and (
5
) the electroluminescence spectrum of IVꢀ5 as a film.
I
, mA
1.00
0.75
0.50
0.25
0
−1500
−
500
500
−
2500
1500
2500
, mV
−0.25
−0.50
−0.75
−1.00
V
Fig. 4. Cyclic voltammogram of copolymer IVꢀ10 in a 0.1 M solution of tetrabutylammonium perchlorate in acetonitrile at the
scanning rate of 50 mV/s.
the values obtained from the absorption spectra (Eogpt ),
result from the presence of a small number of fluoꢀ
renone fragments in polymer chain as defects origiꢀ
nated from their admixture in the initial fluorene, and
on the other hand, it can be caused by the formation of
excimers between the fluorene fragments of polyfluoꢀ
rene [15]. In our case, if we rule out the presence of
fluorenone admixture in the initial monomer, it is
obvious that the close packing of macromolecules of
IVꢀ5 in the solid lightꢀemitting layer favors the formaꢀ
tion of excimers during electroluminescence. As it folꢀ
lows from the present study, the emission resulting
from the relaxation of excimers dominates in the emisꢀ
sion spectrum of the lightꢀemitting structure based on
IVꢀ5. Thus, the main channel of conversion of electriꢀ
cal energy into radiation is the formation of exciplex
Table 2.
The electroluminescence properties of polymer
IVꢀ5 were studied by introducing its as a lightꢀemitting
layer in the simplest lightꢀemitting diode structure.
The electroluminescence spectrum of polymer IVꢀ5
shows a wide intense band in the visible region at
526 nm (Fig. 3, spectrum 5) corresponding to green
light. The shortꢀwavelength range of the spectrum
contains several bands of low intensity with positions
corresponding to the photoluminescence spectrum of
polymer IVꢀ5 in solution. The presence of electroluꢀ
minescence bands in this region is determined by the
presence of fluoreneꢀcontaining fragments in IVꢀ5
molecule. The nature of the broad band at 526 nm is
well known. It was shown that, on the one hand, it can states followed by their radiative relaxation.
DOKLADY CHEMISTRY Vol. 442
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SYNTHESIS AND PHOTOꢀ AND ELECTROPHYSICAL PROPERTIES
29
7. List, E.J., Guentner, W.R., de Freitas, P.S., and
Scherf, U., Adv. Mater., 2002, vol. 14, no. 5, pp. 374–
378.
ACKNOWLEDGMENTS
This work was supported by the Russian Academy
of Sciences (complex programs of the Division of
Chemistry and Materials Science OKhꢀ3 and OKhꢀ2).
8. Lupton, J.M., Craig, M.R., and Meijer, E.W., Appl.
Phys. Lett., 2002, vol. 80, no. 24, pp. 4489–4492.
9. Wu, C.C., Lin, Y.T., Wong, K.T., et al., Adv. Mater.
2004, vol. 16, no. 1, pp. 61–65.
10. Wu, Ch. and Chen, Y., J. Mater. Chem., 2010, vol. 20,
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