Synthesis and photophysical properties of polymers containing a novel class of light emitters

Article abstract of DOI:10.1021/ma047424o

The monomers derived from blue-emission chromophores were investigated. The polymethacryclate and polyester based copolymers were obtained by using free radical polymerization or the Mitsunobu reaction. It was observed that all the copolymers, incorporati

Full text of DOI:10.1021/ma047424o

Macromolecules 2005, 38, 1531-1534
1531
Synthesis and Photophysical Properties of
Polymers Containing a Novel Class of Light
Emitters
Nicolas Leclerc, Marie-Christine Pasareanu, and
Andre´-Jean Attias*
Laboratoire de Chimie des Polyme`res, Universite´ Pierre et
Marie Curie, UMR CNRS 7610, 4, Place Jussieu,
75252 Paris Cedex 05, France
Received December 15, 2004
Revised Manuscript Received January 22, 2005
Introduction. The development of light-emitting
diodes (LEDs) based on π-conjugated organic materials
has been the subject of intense research due to their
potential applications in full-color, flat-panel organic
displays.1 Two key challenges currently remain for the
fabrication of high-performance organic electrolumines-
cent devices: design and synthesis of blue light-emitting
materials, on one hand,2 and of materials capable of
good electron injection and transport properties, on the
other one.3
Many photoluminescent conjugated polymeric materi-
als have been investigated, leading to polymeric light-
emitting devices (PLEDs). Among these polymers, blue
electroluminescence was observed from poly(p-phen-
ylene), polyfluorene, polycarbazole derivatives, and lad-
der-conjugated polymers.4 However, it should be noted
that for these conjugated homo- and copolymers (i) the
light-emitted wavelength depends on the extension of
the conjugated length, a parameter hard to control, and
(ii) most of these materials are better hole than electron
carriers. Therefore, to achieve blue emission and/or
electron injection, other types of emitting polymers have
been investigated: partially conjugated alternating
main-chain copolymers (altMCPs), in which a well-
defined active conjugated chromophore is covalently
bound along the main chain to nonconjugated seg-
ments,5 and side-chain polymers (SCPs), which have the
conjugated moiety linked via a spacer to a nonconju-
gated backbone.6
In this framework, there is a major challenge in
designing and synthesizing (i) chromophores exhibiting
efficient blue light emission, on one hand, and electron
injection ability (high electron affinity), on the other one,
as well as (ii) blue light-emitting polymers (altMCPs and
SCPs) based on these chromophores.
Figure 1. Emission spectra (a) of copolymers P1-P5 in CH2-
Cl2 solution, (b) of copolymers P2-P4 in thin film, and (c) of
copolymers P1 and P5 in thin film.
quantum efficiency of 2.9%. In this context, we were
interested in seeing whether it is possible to translate
results obtained with a molecular material as emitting
moiety to polymeric materials incorporating the same
emitting structure.
In the present communication, starting from I and
II as key emitting moieties, we report our first results
on syntheses of derived monomers (M1 and M2, respec-
tively) and copolymers bearing the active chromophore
in the main chain (P1) and as side group (P2-P4, P5,
and P5′) (Scheme 2). Copolymers were obtained by free
radical polymerization and the Mitsunobu reaction. We
also present some preliminary photophysical properties
of the copolymers in relation to their topology and
composition.
In previous papers,7 a novel class of conjugated
chromophores based on a 3,3′-bipyridine core was
described. It was shown that some of them (I and II in
Scheme 1) exhibit not only high electron affinity (EA ∼
3.0 eV), as expected from the electron deficiency of the
pyridinic ring, but also intense fluorescence emission
in the blue wavelength region. Then, an efficient pure
blue-emitting LED incorporating (I) was fabricated (CIE
coordinates: 0.148; 0.157).7b The main performances of
the device at 10 mA/cm2 are a luminous efficiency of
3.9 cd/A, a power efficiency of 1.4 lm/W, and an external
* Corresponding author: e-mail attias@ccr.jussieu.fr; Ph (+33)
1 44 27 53 02; Fax (+33) 1 44 27 70 89.
10.1021/ma047424o CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/10/2005

1532 Communications to the Editor
Macromolecules, Vol. 38, No. 5, 2005
Scheme 1. Syntheses of Monofunctionalized Chromophore (I) and Difunctionalized Chromophore (II)
Results and Discussion. The simplest way to obtain
an alternating main-chain copolymer was to synthesize
a polyester from the polymerization between an aro-
matic di(carbonyl chloride) and II as diol. As conven-
tional polyesterification methods failed, a polyconden-
sation via a Mitsunobu reaction was used leading to P1
(Scheme 2a).8 To prepare suitable polymer systems with
the emitting chromophore as pendant group, two syn-
thetic strategies have been used, leading to copolymers
P2-P4 and P5, P5′ respectively. For P2-P4, a free
radical polymerization process was chosen in combina-
tion with methyl methacrylate units as polymerizable
groups (Scheme 2b). For P5 and P5′, the synthetic route
involved either a Mitsunobu reaction or a conventional
polyesterification, respectively (Scheme 2c).
As shown in Scheme 1, the strategy for the obtention
of chromophores I and II is based on (i) the synthesis
of π-conjugated 6-substituted-3-bromopyridine building
blocks Br(1) and Br(2) incorporating the electron-rich
thienyl ring and (ii) borylation of Br(1) by using a
boronic diester.7 The resulting boronated building block
B(1) was then coupled with building blocks Br2 and Br1,
giving, after deprotection, chromophores I and II,
respectively, in 70-76% yield and multigram scale. The
monofunctional methacrylate monomer M1 was pre-
pared by direct esterification of I with methacryloyl
chloride, in a quantitative yield, as reported in Scheme
2b. As conventional esterification methods failed, the
difunctional monomer M2 was obtained by Mitsunobu
esterification between hydroxyl groups of I and acidic
groups of 3,5-dihydroxybenzoic acid (1), in a 75% yield.8a
Regarding the comonomers 1 and 2 (2,5-dibutoxy-
terephthalic acid and 2,5-dibutoxyterephthaloyl chlo-
ride, respectively), they were synthesized according to
procedures reported in the literature.9 The butoxy side
groups were introduced to ensure solubility of the
polymers. All these compounds have been characterized
by 1H and 13C NMR spectroscopies and elemental
analysis (see Supporting Information).
by the Mitsunobu reaction, using diethyl azodicarboxy-
late (DEAD) and triphenylphosphine (PPh3) in anhy-
drous tetrahydrofuran (THF) solvent at room temper-
ature (Schemes 2a and 2c, respectively). The use of the
Mitsunobu reaction, previously reported for the synthe-
sis of polyimides,8b,c has not been previously published
to obtain polyesters, at our knowledge. Regarding the
copolymer P5′, it was obtained by conventional polyes-
terification. Whatever the previous route, the comono-
mer feed ratio is 50:50. On the other hand, a series of
novel copolymers P2-P4 with emissive chromophore as
pendant group were obtained by copolymerizing M1 and
methyl methacrylate in the desired ratio, as indicated
in Scheme 2b, the free radical copolymerizations being
initiated with N,N-azobis(isobutyronitrile) (AIBN) in
toluene at 70 °C over 24 h. The copolymers P2-P4 were
characterized by 1H NMR. The relative integrals of the
two different units in the copolymers (signals for pyri-
dinic ring and OMe at ca. δ 8.6 and 3.6 ppm) allowed to
determine the molar ratio (Figure 1S, see Supporting
Information). These values are in broad agreement with
the feed ratio of monomers used in the copolymerization
as listed in Table 1.
The physical and photophysical properties of all the
copolymers are listed in Table 1. Concerning the poly-
esters obtained by polycondensation, the resulting
polymers P1, P5, and P5′ are partially soluble in THF.
Size exclusion chromatography (SEC) data (measured
against monodisperse polystyrene standards) indicate
that the number-average molecular weights (Mn) of the
soluble fraction are 9360, 5350, and 3110 g mol-1 with
a polydispersity (Ip) of 2, 1.2, and 1.4, respectively. These
low molecular weights can be partly related with the
low solubility of these polymers. Differential scanning
calorimetry (DSC) measurements indicate that P1, P5,
and P5′ have Tg of around 80 °C. It should be noted
that the synthetic route does not significantly influence
the characteristics of P5 and P5′. On the contrary, the
polymers obtained by free radical polymerization (P2-
P4) are highly soluble in common organic solvents, such
as THF and chloroform, and can be processed by spin-
coating to yield thin films. SEC measurements reveal
On one hand, two novel polyesters (P1 and P5) were
prepared from the polymerization reaction between the
diacid (1) and diol (chromophore II or M2, respectively)

Macromolecules, Vol. 38, No. 5, 2005
Communications to the Editor 1533
Scheme 2. Syntheses of Monomers M1 and M2 and Polymers P1, P2-P4, P5, and P5′
unimodal distributions, with number-average molecular
weights variying from 10 500 to 26 000 g mol-1 and
polydispersity ranging from 1.5 to 2.2. The Mn values
are higher than those of the polymers obtained by the
Mitsunobu reaction. All these polymers present a glass
transition temperature Tg higher than that of atactic
poly(methyl methacrylate) (PMMA). Moreover, it is
worth noting that all these polymeric materials are
amorphous, which should lead to a good mechanical
stability up to 100 °C. This parameter may become
particularly important for their future use in light-
emitting diodes.
The solution and solid-state optical properties of all
polymers have been investigated. Results are reported
in Table 1. In solution or as thin films, the absorption
spectra of all the polymers result in a strong and intense
low-lying absorption band around 390-400 nm, as
shown in Figure 1a,b.

1534 Communications to the Editor
Macromolecules, Vol. 38, No. 5, 2005
Table 1. Composition, Molecular Weights, and Thermal and Photophysical Properties of the Copolymers
Mn (×103 g
Mw (×103 g
absorption λmax (nm)
emission λmax (nm)
copolymers
mol-1
)
mol-1
)
Ip
DPn
Tg (°C)
solutionb and film
film
P1
9.4
10.5
11.3
26.0
5.3
18.6
17.2
17.0
56.6
6.3
2
12
119
123
155
5.7
81
103
108
107
75
383
391
392
390
385
385
535
455
476
528
537
537
P2 (M1/MMA: 0.8 mol %; 2 wt %)a
1.6
1.5
1.7
1.2
1.4
P3 (M1/MMA: 1.6 mol %; 8 wt %)a
P4 (M1/MMA: 17 mol %; 49 wt %)a
P5
P5′
3.1
4.4
3.3
77
a Calculated from 1H NMR data. b Determined in CH2Cl2.
Supporting Information Available: Syntheses details;
NMR and elemental analysis data. This material is available
free of charge via the Internet at http://pubs.acs.org.
In dilute solution (Figure 1a), upon radiative excita-
tion at 390 nm, P1-P5 exhibit an intense blue emission
with a maximum at 460 nm, as observed for the
chromophores I or II. More indicative are the emission
spectra of thin films prepared from the copolymers
(Figure 1b,c). For P2 and P3 the emission maximum is
located at 455 and 476 nm, respectively, in the blue
region, whereas for P1, P4, and P5 a yellowish emission
at 528 and 535 nm, respectively, is observed. This red
shift accompanied by large emission band could be
explained either by intramolecular aggregation between
laterals chromophore units within the same chain with
the increasing amount of chromophore (P4-P5) or by
interaction between chromophores in different main
chains (P1).
References and Notes
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Conclusion. We successfully designed and synthe-
sized monomers derived from blue-emitting chro-
mophores. Polymethacrylate- and polyester-based co-
polymers have been obtained by using free radical
polymerization or the Mitsunobu reaction, respectively.
All the copolymers, incorporating the fluorescent center
either as repeating units in main chain or as lateral
groups are soluble in organic solvents. Copolymers P2
and P3 emit blue light whereas P1, P4, and P5 are
yellowish emitters. All these results make these poly-
mers potential candidates for the fabrication of PLEDs.
Introducing longer alkyl chains into the terephthalic
derived comonomers and using controlled radical copo-
lymerizations should reduce inter- and intrachains
interactions and consequently allow to obtain blue light-
emitting polymers.
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