Confined photoactive substructures on a chiral scaffold: The design of an electroluminescent polyimide as material for PLED

Article abstract of DOI:10.1039/b503284a

The optical and electroluminescent properties of a new chiral electroluminescent polyimide consisting of alternating conjugated segments and chiral units is described. The synthesized polymer exhibited high thermal stability (Td = 350°C), high electron af

Full text of DOI:10.1039/b503284a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Confined photoactive substructures on a chiral scaffold: the design of an
electroluminescent polyimide as material for PLED**{
Olivier J. Dautel,a Guillaume Wantz,b David Flot,c Jean-Pierre Lere-Porte,a Joe¨l J. E. Moreau,*a
Jean-Paul Parneix,b Franc¸oise Serein-Spiraua and Laurence Vignaub
Received 4th March 2005, Accepted 13th July 2005
First published as an Advance Article on the web 2nd September 2005
DOI: 10.1039/b503284a
The optical and electroluminescent properties of a new chiral electroluminescent polyimide
consisting of alternating conjugated segments and chiral units is described. The synthesized
polymer exhibited high thermal stability (Td 5 350 uC), high electron affinity, and good thin-film
forming ability. The C2 symmetry of the chiral unit induces a secondary structure which reduces
all types of intra and interchain interactions. The incorporation of a well-defined
phenyleneethynylene substructure within the chiral backbone allowed the fabrication of a
monolayer electroluminescent polymeric diode with high performance which compares most
favorably to related poly-phenyleneethynylene.
We describe here the first synthesis and electroluminescent
Introduction
characterization of a novel phenyleneethynylene-based poly-
During the past decade, several polymeric materials suitable
for light-emitting devices have been developed.1 Such devices
imide which combines thermal stability and a C2 chiral
structure with a confined chromophore. We focus on con-
require highly efficient luminescent materials with good
jugated oligophenyleneethynylenes which are chemically and
thermal and photochemical stability.
thermally more stable than polyphenylenevinylenes but usually
Although aromatic polyimides (APIs) have been extensively
studied as high-performance polymers for their unique
mechanical, dielectric and optical properties as well as thermal
and chemical stability,2 few attempts have been made to use
them for the purpose of light-emitting diodes. Uses are
limited to the transporting layer or dye doped matrix in
electroluminescent devices.3
afford low efficiency devices. Its incorporation in a chiral
backbone should allow control of the undesired inter- and
intrachain interactions.
Results and discussion
Synthesis and characterisation
On the other hand, chiral polymers today are of great
interest owing to their structure and supramolecular arrange-
ment controlling their optoelectronic properties.4 Recently,
conjugated polymers incorporating a C2 symmetry binaphtyl
unit were reported for diverse applications such as electro-
luminescence, non-linear optics, chiral sensors and asymmetric
catalysis.5 In a previous paper, we reported the synthesis of a
soluble chiral conjugated polyimine based on trans-(1R,2R)-
1,2-cyclohexyldiamine with good luminescence properties.6
The incorporation of a well-defined conjugated segment in
the chiral backbone prevents stacking of the conjugated
units minimizing the self-quenching processes of excitons.
Unfortunately, their low glass transition and HOMO–LUMO
levels precluded the fabrication of a polymeric light emitting
diode (PLED).
In order to avoid the high temperature treatment usually
required in the classical synthesis and casting of polyimides,2
we designed a different strategy involving low temperature
organometallic coupling of aromatic and conjugated units
(cf. Scheme 1). Thus, the imide function was first introduced
on the chiral unit by condensation of the bromophthalic
anhydride 17 and the trans-(1R,2R)-1,2-cyclohexyldiamine8 to
give the chiral halo-imide 3.{ Finally, 3 readily underwent
coupling with the conjugated 1,4-diethynyl-2,5-bis(octyloxy)-
benzene 49 in the presence of palladium catalyst and provided
high molecular weight conjugated polymer 6. The polymer was
separated from the low weight oligomers by a precipitation
with methanol from a THF solution and gel permeation
chromatography (THF, polystyrene standard) analysis showed
Mw 5 15000 and Mn 5 8500 (PDI 5 1.76). These values
correspond to an average of 11 repeating units per polymeric
chain. Unlike conventional fully aromatic polyimides, this new
material with a confined chromophore alternating with chiral
units is highly soluble in common organic solvents, highly
fluorescent and emits greenish blue light when irradiated.
The related low molecular weight model compound 5 was
synthesized for comparison in a similar manner using the halo-
imide 2 instead of 3.{
aHe´te´rochimie Mole´culaire et Macromole´culaire UMR CNRS 5076,
Ecole Nationale Supe´rieure de Chimie de Montpellier, 8 rue de l’Ecole
Normale 34296 Montpellier Cedex 05, France.
E-mail: olivier.dautel@.enscm.fr; joel.moreau@enscm.fr
bLaboratoire de Physique des Interactions Ondes-Matie`re (PIOM),
UMR CNRS 5501, Ecole Nationale Supe´rieure de Chimie et de
Physique de Bordeaux, 16 Av. Pey Berland 33607 Pessac Cedex, France
cEMBL, 6 rue Jules Horowitz, BP 181, 38042 Grenoble Cedex 9, France
{ Electronic supplementary information (ESI) available: NMR 1H and
13C spectra of 2, 3, 5 and 6. GPC chromatograms, TGA/DSC traces,
PXRD diagrams of 5 and 6. See http://dx.doi.org/10.1039/b503284a
Thermal stability is one of the most important requirements
in PLEDs, because the joule-heat generated by the device
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Scheme 1 Synthetic route of 5 and 6.
severely relaxes and damages organic materials. Thermogravi-
metry (TGA) of the conjugated segment 5 and the polymer 6
were studied in air with a heating rate of 10 uC min21.{ Both
products show a decomposition onset above 350 uC; the losses
of mass below this temperature are lower than 5%. Beyond
350 uC, a first weight loss, corresponding to the fragmentation
of the side chains (C8H17), was observed. After this stage,
degradation occurred at a slower rate. Thus, the polymer
shows a very good thermal stability bearing in mind that this
property could be optimized quite simply by modifying the
nature of side chains since the C–O bond constitutes a
breaking point.
with lower order and stacking of the conjugated segment in the
polymer compared to the isolated segment in the monomer.
Small needles of 5 were grown from the evaporation of a chloro-
form solution and microdiffraction patterns were recorded at
the Microfocus Beamline ID13 (European Synchrotron
Radiation Facility – ESRF, Grenoble). 5 crystallizes in a
¯
triclinic system with a P1 space group. One independent half
molecule is located on an inversion centre (Fig. 1).
The three benzene rings (A, B and B9) linked by triple bonds
are mostly coplanar with a dihedral angle C(15)–C(14)–C(10)–
C(9) 14.06(18)u. On the other hand, the plane defined by
the cyclohexane ring is orthogonal to this aromatic system
(B–A–B9) with a rotation angle C(23)–C(27)–C(14)–C(16)
90.47(12)u. Substitution in position 4 of phthalimides B and
B9 can be at the origin of two conformations, both groups B
and B9 can be in trans position relative to the B–A–B9 axis or in
cis position. The trans conformation appears to be the most
stable geometry in the solid state. In the crystal, molecules
form layers separated by side chains and the cyclohexyl
fragments along the c axis (Fig. 2). Each molecule belongs to a
Moreover, to use polyimide 6 as an electroluminescent layer,
it is necessary to detect the existence of a crystalline or glass
(Tg) transition. Indeed, this type of transition in the range
of operating temperatures of diodes affects their lifetime.
We studied the isolated segment 5 and the polymer 6 by
Differential Scanning Calorimetry (DSC) in a range of
temperatures from 2120 uC to 250 uC.{ Recordings were
carried out in nitrogen with a heating rate of 20 uC min21
.
DSC measurements on polymer 6 showed neither any crystal-
line transition nor Tg in the studied range of temperature. On
the other hand, the thermogram dedicated to compound 5
showed an endothermic transition at 211 uC signifying its
melt and an exothermic transition at 161 uC indicative of its
recrystallization. The stabilization of the polymer 6, in the
solid state, can be explained by the rigidity introduced by imide
functions but also by the cooperative effect of Van der Waals
interactions existing between side chains. These latter interac-
tions reduce polymeric inter-chain slips. In any case, consider-
ing its thermal, chemical and morphological stability, the
polymer is an excellent candidate for future applications.
Fig. 1 Molecular structure of compound 5: anisotropic displacement
˚
parameters are drawn at the 50% probability level. Distances [A] and
Structural properties
X-Ray diffraction diagrams of 6 only showed 3 broad
signals (2h 5 5.4, 12.2, 20.7u) characteristic of an amorphous
material.{ Conversely, 5 revealed several diffraction peaks
indicative of a crystalline product. This behavior is consistent
angles [u] (with standard deviations in parentheses): C(12)–C(13)
˚
1.216(2) A, C(13)–C(14) 1.433(2) A, C(18)–C(21) 1.504(2) A, C(19)–
˚
˚
˚
C(20) 1.503(2) A, C(15)–C(14)–C(10)–C(9) 14.06(18)u, C(23)–C(22)–
N(1)–C(21) 62.63(23)u, C(23)–C(27)–C(14)–C(16) 90.47(12)u.
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Fig. 2 Packing arrangement of 5: layers separated by the side chains and the cyclohexyl fragments developing along the c axis.
chain of interactions (dotted lines) which develops along the b
axis through phthalimide groups. The layers are stabilized by
p-stacking interactions in the ab plane between phthalimide
residues B and B9 (dotted lines) and between aryl rings A and
the triple bonds (broken lines) (Fig. 2).
In addition, unambiguous determination of the stereo-
structure of the bichromophoric system and, hence, consis-
tency of the (1R, 2R) absolute configuration and conformation
of the cyclohexyldiamine was given by circular dichroism
spectroscopy carried out on polymer 6. Indeed, the absolute
configuration of chiral b-bis-phthalimides is readily correlated
with the degenerate exciton Cotton effects resulting from the
intramolecular interaction of the electric-dipole-allowed transi-
tion moments of two vicinal phthalimide chromophores linked
by a cyclohexyldiamine (below 260 nm, p–p* band).10 The CD
spectrum of a 10 mM solution of 6 in dioxane (Fig. 3) displays
four sets of Cotton effects around 232 (exciton-type), 254, 298
and 404 nm which correspond to the position of the absorption
maxima in the UV spectrum (see below). Its molecular
ellipiticities were: [h]l 5 5.55 6 104 (222 nm) and 24.90 6
104 (242 nm), 24.22 6 104 (254 nm), 22.93 6 104 (298 nm),
˚
The perpendicular distance (3.25 A) between two molecules
and the shortest intermolecular contact distances C(17)–
˚
˚
C(15)(1+x, y, z) 3.275 A and C(16)–C(21)(12x, 22y, 12z) 3.378 A
indicate very strong intermolecular interactions and large
orbital overlap between neighbouring parallel molecules
although the overlap between aromatic rings is small. This
behaviour should result in an enhanced conjugation length due
to these intermolecular p-stacking interactions.
An indication of the structure of the polymeric material 6
can be deduced from the specific optical rotation and circular
dichroism measurements.
22.85 6 104 (404 nm). Based on correlations established by
20
The specific optical rotation of 6, [a]D 5 2452u (c 5 1,
11
Gawronski, the negative sign of the exciton Cotton effect
´
dioxane), was found to be higher in comparison with the
chiral bis-phthalimide 3 ([a]D20 5 2186.5u (c 5 0.97, CHCl3 or
dioxane). This value, three times higher, could be the result of
a secondary structure of helix type.
corresponding to the p–p* charge-transfer transition located at
232 nm (e 5 77300 M21 cm21), is in agreement with the (1R,
2R) absolute configuration and reveals that transition dipole
Fig. 3 CD spectra of 3 and 6 in dioxane at 20 uC (c # 10 mM ) and relation between the sign of the exciton Cotton effects and the absolute
configuration.
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moments of two vicinal phthalimide chromophores linked by a
cyclohexyldiamine are coupled. This implies that imides are in
close proximity allowing charge mobility (Fig. 3).
quantum yields of 5 and 6 were measured in chloroform
solutions. Very similar values were obtained (Wf5 5 0.28 and
Wf6 5 0.31).15
For polymer 6, the characteristic features of the di-imide
linkage are observed. The negative Cotton effect observed at
lowest wavelength is in full agreement with the exciton theory
of chiral bichromophoric systems as described by Harada and
Nakanishi.12 However, a bisignate CD effect is not observed in
the region of absorption of the p conjugated chromophore
(404 nm). For a helical polymer one would expect a so-called
bisignate Cotton effect at the p–p* transition.13
In the solid state, thin films of 5 and 6, realized by spin-
coating from 20 mg mL21 solutions in CHCl3 on a quartz
substrate, exhibited different behaviors. Compared to the
solution, the absorption of the isolated conjugated unit 5
showed a red-shift of the maximum wavelength (Dl 5 45 nm).
This behavior can be attributed to an enhanced conjugation
due to intermolecular p-stacking interactions in the solid state
as already deduced from the X-ray diffraction study. It is
worth noticing that the polymer showed identical absorption
bands in CH2Cl2 solution and in the solid state, signifying the
absence of interchain p-stacking interactions within the
polymeric solid. This is consistent with the presumed helical
structure deduced from the molecular modeling. The C2
symmetry, introduced by the chiral unit, results in the
transposition of the optical properties from the solution to
the solid. This behavior indicates that the conjugated segments
are fully isolated in the solid state. However CD measurements
showed that charge transport should be effective through the
overlap of the two vicinal phthalimide moieties. The use of
such a chiral polymer is a very convenient and interesting way
to circumvent problems arising from the recrystallization of
small molecules or aggregation of polymeric chains during
LED processing. Although phenyleneethynylenes are chemi-
cally and thermally more stable than phenylenevinylenes
(PPVs), they offer devices with poor performances
(300 Cd m22 for the best one compared to 2000 Cd m22 for
most PPVs).16 This is the result of their rod like structure
which favors fully aggregated and well packed chains giving
very low luminescence quantum yield efficiency. The coplanar
orientation of the conjugated polymer backbones is assumed
to lead to the formation of excimer complexes which provide
nonemissive decay channels for excited states. Nonemissive
orientations are more significant in materials with a higher
degree of long range order.17
Molecular modeling using the MM2 force field of polyimide
6 consisting of 11 repeating units (value deduced from the GPC
analysis) yielded two low-energy structures of C2 symmetry
which could be described by two types of helical shapes:
C-shape and S-shape.{ In both cases, the phthalimide groups
connected by the 1,4-diethynyl-2,5-bis(octyloxy)benzene
segment are in a trans geometry and the C or S conformations
are the result of a rotation of these phthalimides around the
C–N bond. The aromatic segments are fully isolated from each
other. Owing to the CD measurements, 6 certainly adopts less
ordered secondary structures. Indeed, due to the substitution
of the phthalimide group in position 4, other polymeric
arrangements could be considered, but in this case, their low
degree of symmetry results in an increase of their total energy.
This is supported by the crystal structure of 5 which reveals a
trans configuration of the phthalimide functions along the long
axis of the conjugated segment (cf. Fig. 1).
Optical and photoluminescence properties
The UV-vis spectra of 5 and 6 in solution (CH2Cl2) and as thin
films (obtained by spin coating from a 20 mg mL21 chloro-
form solution) are shown in Fig. 4. The confinement of the
conjugated chromophore between two chiral units resulted in
very similar optical absorption and emission properties for the
monomer 5 (CH2Cl2, labs_max 5 403 nm; lem_max 5 504 nm
(lex 5 400 nm)) and the polymer 6 [CH2Cl2, labs_max 5 406 nm;
lem_max 5 508 nm (lex 5 400 nm)] in solution. By comparing
the ratio of the fluorescence emission intensity maximum to
UV absorbance at the excitation wavelength used for the
sample with that of a standard,14 the relative fluorescence
Electroluminescence properties
Light emitting diodes have been made using polymer 6
and using monomer 5. 6 was dissolved in xylene which is a
commonly used solvent. The solution (30 mg mL21) is
homogeneous and transparent. 6 was highly soluble compared
to related PPVs. Conversely, 5 was not soluble in xylene.
Therefore, to enable the comparison of the LEDs, thin films of
5 were prepared by vacuum evaporation, whereas thin films of
6 were obtained by spin-coating. Both films were controlled
to be 100 nm thick. The structures of the LEDs, described
in Scheme 2, are as follows: ITO/PEDOT-PSS/6/Ca/Al and
ITO/PEDOT-PSS/5/Ca/Al.
LUMO and HOMO levels have been measured by cyclic
voltammetry at 23.64 eV and 25.76 eV, respectively, for 6
and at 23.04 eV and 25.46 eV, respectively, for 5 (Scheme 2).
From such electronic structures, it is believed that charge
carrier injection may differ. The energetic barrier at the anodic
interface is higher in a PLED based on 6 than in a device based
on 5. Hole injection is then easier in 5. At the cathodic
interface, electron injection from calcium is not limited by any
Fig. 4 UV-vis absorption and emission spectra of 5 and 6: solution in
CH2Cl2 and thin films.
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2.20 eV for 6. The observed light is blue and pale green for 5
and 6 respectively. The chromatic coordinates calculated from
the EL spectra are reported at x 5 0.27/y 5 0.55 for 5 and at
x 5 0.46/y 5 0.51 for 6 following the CIE-1964 chromaticity
diagram. The red shift observed only in the emission spectra
(photoluminescence and electroluminescence) of 6 could find
its origin in the fact that two imide functions grafted on the
same cyclohexyl fragment are in interaction. Indeed the C2
symmetry of the chiral unit bring them in close contact and
CD measurements showed that they are coupled in the excited
state. The red shift observed in the photoluminescence
spectrum of 6 from the solution to the solid state (Fig. 4,
Dl 5 10 nm) is even bigger in the electroluminescence
spectrum (Inset Fig. 5, Dl 5 40 nm) suggesting that the
electric field induced by the electroluminescence process
activates this exciton coupling. This effect related to built-in
electric fields has already been observed.18
Scheme 2 PLED and OLED structures of (a) 5 and (b) 6.
energetic barrier in both cases. As a consequence, charge
balance is better in 5 than in 6. On only this basis, higher
performances were expected with 5.
Fig.
5 shows the measured current–voltage-luminance
characteristics (I–V–L) of devices based on 5 and on 6. With
6, good luminance levels are obtained for voltages lower than
20 V (1500 Cd m22 @ 10.5 V) and the onset voltage is observed
at 5.5 V. With 5, a 27 V onset voltage is revealed. Much smaller
luminances are observed due to smaller current densities.
Values of 125 Cd m22 are only observed at 35 V. Such very
high applied voltages are consistent with poor performances of
5-based devices. As a consequence, the injection related data
reported above are not sufficient to predict the performance of
PLEDs. We believe that 5 present a poor ability to emit light
due to p-stacking which induces quenching of luminescence.
The use of 6 is therefore an interesting alternative to avoid
such p-stacking and to significantly increase LEDs perfor-
mances. Another parameter controlled by the use of 6 lies in
the quality of the electroluminescent layer deposited. Indeed,
the small molecule 5 tends to crystallise during the vacuum
evaporation process and generates some grain boundaries
known to be drastic for the OLEDs performances.
Conclusion
In summary, the synthesis and characterization of a new chiral
conjugated polyimide 6 was carried out. This new polymer is
soluble in common organic solvents and emits green light
upon irradiation. Its optical behaviour was also evaluated by
comparison with its respective repeat unit. The C2 symmetry of
the chiral unit induces secondary structures which reduce all
types of intra and interchain interactions. The high thermal
stability, the ability to cast thin polymer layers directly from
solution with excellent film-forming properties and efficient
EL make the chiral polyimide of potential interest for
technological applications. The incorporation of a well-defined
oligo-phenyleneethynylene structure within the chiral back-
bone allowed fabrication of monolayer electroluminescent
polymeric diodes with high performance, whereas the corres-
ponding poly-phenyleneethynylene is not efficient when used
as an emissive layer in electroluminescent devices.
Electroluminescence (EL) spectra have been measured with
both materials 5 and 6. They are centred at 510 nm and 564 nm
respectively. These EL spectra are in agreement with HOMO
and LUMO level positions deduced by cyclic voltammetry
since they provide a bandgap width of 2.45 eV for 5 and
Experimental
Materials and methods
All the reactions were performed under nitrogen atmosphere
using Schlenk tube techniques. The solvents were distilled
under nitrogen over Na–benzophenone (THF), KOH (NEt3)
and P2O5 (CH2Cl2) prior to use. Optically active trans-
(1R,2R)-1,2-cyclohexyldiamine was obtained enantiomerically
pure from the commercial racemic cis/trans mixture according
to literature procedure.19 4-Bromophthalic anhydride (1)
was prepared according to Hollingsworth’s procedure.20
1,4-Bis(octyloxy)-2,5-di(ethynyl)benzene (4) was synthesized
according to the reported procedure.21 Analytical data were in
good agreement with the literature. NMR (1H and 13C) spectra
were recorded on a Bruker AC 200 spectrometer and CDCl3
was used as solvent. Chemical shifts (d), reported in parts per
million, are relative to tetramethylsilane. Signal multiplicities:
s(singlet), bs(broad singlet), d(doublet), dd(doublet of
doublet), t(triplet), tt(triplet of triplet), m(multiplet). IR
spectra were recorded on a Perkin-Elmer 1000 FTIR spectro-
meter using KBr pellets. UV-vis spectra were recorded on a
Hewlett Packard 8453 spectrophotometer. Fluorescence
Fig. 5 I–V–L Curves of ITO/PEDOT/5 or 6/Ca/Al diodes. Inset:
electroluminescence and photoluminescence spectra of 6.
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spectra were recorded on a Hitachi F2500. Optical rotation
were measured on a Perkin-Elmer Polarimeter 241. Elemental
analyses were carried out by the Service Central de
Microanalyse du CNRS in Vernaison (France). Mass spectra
were measured on a JOEL MS-DX 300 mass spectrometer
utilising m-nitrobenzyl alcohol (3-NOBA) as the matrix. GPC
measurements were made on a Waters device equipped with
HR2 and HR3 styrragel columns using THF as eluent. Melting
points were measured with an Electrothermal 9100 apparatus.
the halo-phthalimide 2 (322.2 mg, 1.04 mmol) and the 1,4-
diethynyl-2,5-bis(octyloxy)benzene 4 (200 mg, 0.523 mmol)
was performed in the presence of Pd2dba3(CHCl3) (21.7 mg,
0.021 mmol), P(C6H5)3 (22 mg, 0.0837 mmol) and CuI (8 mg,
0.0418 mmol) in a mixture of THF (10 mL) and triethylamine
(5 mL). At 70 uC, coupling occurred and the reaction was
stirred for three days. The reaction mixture was filtered
and concentrated under vacuum. Further purification of the
residue by column chromatography on silicagel using a 1 : 1
mixture of cyclohexane and methylene chloride as eluent
provided 5 (420 mg, 96%) as a yellow solid. mp 5 205–207 uC;
1H NMR (CDCl3): d 7.90 (s, 2H), 7.77 (s, 4H), 7.02 (s, 2H),
4.09 (tt, J 5 2.8, 6.8 Hz, 2H), 4.02 (t, J 5 5 Hz, 4H), 2.15 (m,
4H), 1.24–1.90 (m, 40H), 0.83 (m, 6H); 13C NMR (CDCl3): d
167.82, 167.72, 153.86, 136.55, 132.35, 130.80, 129.35, 125.77,
122.98, 116.77, 113.74, 93.62, 90.13, 69.59, 51.07, 31.78, 29.83,
29.33, 29.31, 29.25, 26.06, 25.98, 25.08, 22.64, 14.07; UV-vis
(CH2Cl2, nm): labs_max 5 403; UV-vis (film on quartz, nm):
Synthesis
N-(4-Bromophthaloyl)-cyclohexylamine (2). To a solution of
bromophthalic anhydride 1 (1 g, 4.4 mmol) in toluene (150 mL)
was added cyclohexylamine (0.5 mL, 4.4 mmol) followed by
triethylamine (0.5 mL, 3.6 mmol). The mixture was refluxed
under a Dean–Stark trap for 24 h. The cold colorless solution
was evaporated and the residue was recrystallized from
MeOH. 1 g (yield 74%) of 2 was obtained as a white crystalline
labs_max 5 448; Fluo (lex 5 400 nm) (CH2Cl2, nm): lem_max
5
1
solid, mp 140 uC (MeOH); H NMR (CDCl3) d 1.29 (m, 2H),
504; UV-vis (film on quartz, nm): lem_max 5 508; Anal. Calcd
for C54H64N2O6: C, 77.48; H, 7.71; N, 3.35; O, 11,47. Found
C, 77.43; H, 7.83; N, 3.48; O, 11.10%; FAB (m/z): M+N 837 (90),
507 (100).
1.80 (m, 4H), 2.18 (m, 2H), 4.05 (tt, J 5 3.6, 12 Hz, 1H), 7.63
(d, J 5 8.0 Hz, 1H), 7.78 (dd, J 5 1.8, 8.0 Hz, 1H), 7.89 (d,
J 5 1.8 Hz, 1H); 13C NMR (CDCl3) d 24.97, 25.87, 29.69,
51.07, 124.30, 126.27, 128.48, 130.48, 133.61, 136.62, 166.89,
167.45; IR (KBr) 3084, 2932, 2856, 1768, 1708, 1369, 1351,
1092, 739 cm21; UV (MeCN) e 12 (301 nm), 16400 sh
(251 nm), 22000 sh (242 nm) 48300 (230 nm); Anal. Calcd
for C14H14BrNO2: C, 54.56; H, 4.58; N, 4.55; O, 10,38. Found
C, 54.63; H, 4.25; N, 4.42; O, 10.66%; m/z (FAB+) (%): 308
(M + H, 100), 309 (15), 310 (94).
Polymer (6). The polymerization of the chiral halo-
phthalimide 3 (278.3 mg, 0.523 mmol) and the 1,4-diethynyl-
2,5-bis(octyloxy)benzene
4 (200 mg, 0.523 mmol) was
performed in the presence of Pd2dba3(CHCl3) (21.7 mg,
0.021 mmol), P(C6H5)3 (22 mg, 0.084 mmol) and CuI (8 mg,
0.042 mmol) in a mixture of THF (15 mL) and triethylamine
(5 mL). At 70 uC the coupling occurred and provided 6 as an
orange solid after five days of heating. The polymer was
separated from the low weight oligomers by a precipitation
with methanol from a THF solution. This new material 6 was
soluble in common organic solvents (THF, CH2Cl2, CH3CN,
xylene) and gel permeation chromatography (THF, poly-
styrene standard) analysis showed Mw 5 15000 and Mn 5 8500
(PDI 5 1.76). These values correspond to an average of 11
repeating units per polymeric chain. [a]D 2452u (c 5 1, THF);
GPC: Mw 5 15000, Mn 5 8500 (PDI 5 1.76); 1H NMR
(CDCl3): d 7.72 (m, 6H), 6.97 (s, 2H), 5.00 (m, 2H), 3.99 (m,
4H), 2.41 (m, 2H), 1.23–1.89 (m, 30H), 0.81 (m, 6H); UV-vis
(CH2Cl2, nm): labs_max 5 406 ; UV-vis (film on glass, nm):
N,N9-Bis(4-bromophthaloyl)-(1R,2R)-1,2-diaminocyclohex-
ane (3). To a solution of bromophthalic anhydride 1 (4.54 g,
20 mmol) in toluene (150 mL) was added (1R,2R)-1,2-
diaminocyclohexane (1.15 g, 10 mmol) followed by triethyl-
amine (140 mL, 1 mmol). The mixture was refluxed under a
Dean–Stark trap for
4 h. A white precipitate of the
monophthalyol adduct formed. The reaction mixture was
cooled to room temperature and the white solid was filtered off
to give 700 mg (yield 22%) of the mono-addition product.
Concentration of the filtrate under reduced pressure gave a
colorless oil which solidified on standing. The bisphthaloyl
adduct was isolated by column chromatography on silicagel
using methylene chloride as eluent. Yield 1.770 g (33%) of 3,
mp 245 uC; Rf (silicagel, CH2Cl2) 0.4; 1H NMR (CDCl3) d 1.53
(m, 2H), 1.92 (m, 4H), 2.40 (m, 2H), 4.96 (m, 2H), 7.59 (m,
2H), 7.77 (m, 2H), 7.89 (m, 2H); 13C NMR (CDCl3) d 24.85,
29.12, 51.02, 124.64, 126.60, 128.84, 130.00, 132.50, 136.86,
166.50, 167.05; IR (KBr) 3081, 2933, 2861, 1769, 1719, 1374,
labs_max 5 406; Fluo (lex 5 400 nm) (CH2Cl2, nm): lem_max
5
508; UV-vis (film on glass, nm): lem_max 5 524; Anal. Calcd
for C48H52N2O6: C, 76.57; H, 6.96; N, 3.72; O, 12,75. Found
C, 74.04; H, 6.69; N, 3.81; O, 11.87%.
Crystal data for 5
20
1355, 1098, 739 cm21; [a]D 2186.5u (c 5 0.97, CHCl3 or
¯
˚
C54H64O6N2, triclinic, P1 (no. 2), a 5 5.254(3) A, b 5
THF); UV (MeCN) e 21800 (278 nm), 32600 sh (250 nm),
42500 sh (240 nm), 78000 (230 nm); Anal. Calcd for
C22H16Br2N2O4: C, 49.65; H, 3.03; N, 5.26; O, 12,03. Found
C, 50.89; H, 2.83; N, 5.32; O, 12.27%; m/z (FAB+) (%): 531
(M + H, 49), 533 (100), 535 (51).
˚
˚
14.407(4) A, c 5 16.820(3) A, a 5 112.813(4)u, b 5 90.918(43)u,
3
˚
c 5 99.076 (72)u , V 5 1154.72 A , T 5 100(2) K, R1 5 0.0658
for 4426 independent observed reflections [F . 4s(Fo)], S 5
0.982. Data collection for 11: Q scans, 2u oscillation range,
5 passes of 1.5 s exposure time per image, 90 images, l 5
˚
0.7305 A. CCDC reference number 256242. See http://
2-Cyclohexyl-5-{[4-[(2-cyclohexyl-1,3-dioxo-2,3-dihydro-
1H-isoindol-5-yl)ethynyl]-2,5-bis(octyloxy)phenyl]ethynyl}-
1H-isoindole-1,3(2H)-dione (5). The organometallic coupling of
dx.doi.org/10.1039/b503284a for crystallographic data in CIF
or other electronic format.
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 4446–4452 | 4451

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Characterization; ed. C. Feger, M. M. Khojasteh and
J. E. McGrath, Elsevier, Amsterdam, 1989.
Devices fabrication methods
3 (a) Y. Kim, J. G. Lee, D. K. Choi, Y. Y. Jung, B. Park, J. H. Keum
and C. S. Ha, Synth. Meth., 1997, 91, 329; (b) E. I. Mal’tsev,
M. A. Brusentseva, V. A. Kolesnikov, V. I. Berendyaev,
B. V. Kotov and A. V. Vannikov, Appl. Phys. Lett., 1997, 71,
3480; (c) Y. Kim, J. G. Lee, K. Han, H. K. Hwang, D. K. ChoiJung,
Y. Y. Keum, J. H. Kim, S. S. Park and W. B. Im, Thin Solid Films,
2000, 363, 263.
4 E. Peeters, M. P. T. Chrisliaans, R. A. T. Jansen, H. F. M. Schoo,
H. P. J. M. Dekkers and E. W. Meijer, J. Am. Chem. Soc., 1997,
119, 9909.
5 L. Pu, Chem. Rev., 1998, 98, 2405.
6 J. P. Le`re-Porte, J. J. E. Moreau, F. Serein-Spirau and S. Wakim,
Chem. Commun., 2002, 3020.
7 H. M. Norman, L. J. Kelley and B. E. Hollingsworth, J. Med.
Chem., 1993, 36, 3417.
The structure of the device includes a layer of Indium Tin
Oxide (ITO) which is the commonly used transparent electrode
for such applications with a sheet resistance of approximately
17 V/&. This substrate underwent a wet cleaning procedure of
successive 30 minute ultrasonic bath treatments in trichloro-
ethylene, ethanol and deionised water at room temperature.
Then, a layer of poly(styrene sulfonate) doped poly(3,4-
ethylene dioxythiophene) (PEDOT-PSS) was spun, from a
3 wt% water dispersion at 5000 rpm to form a 50 nm layer.
This conducting polymer layer was cured at 80 uC under rotary
pump vacuum for 1 hour. This layer improves hole injection
from the ITO to the HOMO level of the organic material and
increases the performance of the device. Then, for PLEDs
based on 6, the layer of 6 was spin-coated from the 30 mg mL21
solution in xylene at 1000 rpm to form a 100 nm-thick layer.
For PLEDs based on 5, the organic layer (100 nm) was
8 J. P. Larrow and E. N. Jacobsen, J. Org. Chem., 1994, 59, 1939.
9 J. P. Ruiz, J. R. Dharia, J. R. Reynolds and L. J. Buckley,
Macromolecules, 1992, 25, 849.
´
´
10 J. Gawronski, K. Kazmierczak, K. Gawronska, P. Skowronek,
J. Waluk and J. Marczyk, Tetrahedron, 1996, 52, 13201.
´
´
11 J. Gawronski, K. Kazmierczak, K. Gawronska, U. Rychlewska,
B. Norde´n and A. Holmen, J. Am. Chem. Soc., 1998, 120, 12083.
12 N. Harada and K. Nakanishi, Circular Dichroic Spectroscopy:
Exciton Coupling in Organic Stereochemistry, University Science
Books, Mill Valley, CA, 1983.
thermally evaporated at 1026 mbar at a rate of 1 nm s21
.
Finally, a calcium cathode was thermally evaporated under
vacuum (1026 mbar) through a shadow mask. This calcium
layer (y40 nm thick) was capped with an aluminium layer
(y150 nm thick) to minimize its oxidation. All devices
investigated here had an active area of 10 mm2. Samples
were then stored and characterized under inert atmosphere
(nitrogen glove box).
13 B. M. W. Langeveld-Voss, D. Beljonne, Z. Shuai, R. A. J. Janssen,
S. C. L. Meskers, E. W. Meijer and J.-L. Bre´das, Adv. Mater.,
1998, 10, 1343.
14 (a) J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991;
(b) W. H. Melhuish, J. Phys. Chem., 1961, 65, 229; (c) Here,
quinine sulfate was employed as a standard: quinine sulfate in
H2SO4 (1 N) at 25 ¡ 1 uC has been reported to have Wf 5 0.546
when excited at 365 nm. All sample solutions were freshly prepared
in dry chloroform and then used within 2 hours. The absorbance of
all sample solutions was controlled to be 0.07–0.09 by changing the
concentrations, to avoid an inner filter effect.
15 Due to the lack of reliable references, absolute fluorescence
quantum yields of 5 and 6 in the solid state were not determined.
16 C. Weder and M. S. Wrighton, Synth. Met., 1998, 97, 123.
17 C. Weder and M. S. Wrighton, Macromolecules, 1996, 29, 5157.
18 (a) I. H. Campbell, T. W. Hagler, D. L. Smith and J. P. Ferraris,
Phys. Rev. Lett., 1996, 76, 1900; (b) A. Bolognesi, G. Bajo,
Acknowledgements
The authors gratefully acknowledge financial support from the
‘‘Ministe`re de la Recherche’’ and the CNRS and the allocation
of beam time at the microfocus beamline (ID13) at the
European Synchrotron Radiation Facility–ESRF, Grenoble.
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Macromolecules, 1992, 25, 849.
4452 | J. Mater. Chem., 2005, 15, 4446–4452
This journal is ß The Royal Society of Chemistry 2005