Extended liquid-crystalline oligofluorenes with photo- and electroluminescence

Article abstract of DOI:10.1039/c0nj00404a

In this paper we report the synthesis and characterization of a series of low-molecular weight light-emitting molecules based on short oligofluorene cores, from one to three 9,9-dioctylfluorene units, elongated at the 2- and 7-positions by means of promes

Full text of DOI:10.1039/c0nj00404a

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www.rsc.org/njc | New Journal of Chemistry
Extended liquid-crystalline oligofluorenes with
photo- and electroluminescence
a
a
b
a
´
s del Barrio, Luiz Silvino Chinelatto Jr., Jose Luis Serrano, Luis Oriol,*
ol*a and Henk J. Bolinkc
Jesu
´
Milagros Pin
˜
Received (in Montpellier, France) 28th May 2010, Accepted 14th July 2010
DOI: 10.1039/c0nj00404a
In this paper we report the synthesis and characterization of a series of low-molecular weight
light-emitting molecules based on short oligofluorene cores, from one to three 9,9-dioctylfluorene
units, elongated at the 2- and 7-positions by means of promesogenic units. The effect of this
structural modification on the mesomorphic properties, as well as on the optical and
electrochemical properties, is studied. The possibility of using these molecules as emitters in
light-emitting diodes (LEDs) and polarized photoluminescent layers has been explored.
and a linearly polarized luminescence. Electroluminescence
Introduction
has also been extensively investigated. For electroluminescence,
a balanced charge injection of holes and electrons in the
luminescent material is crucial to achieve a high device
performance; therefore, materials with low ionization
potential (IP) and high electron affinity (EA) are required. In
general, most organic materials used as emitters for electro-
luminescence inject and transport holes more efficiently than
electrons due to their richness of p-electrons. The liquid-
crystalline, electronic and optical properties of fluorene-based
oligomers are primarily governed by the chemical structure of
the backbone, i.e. the size of the p-core, the type and number
of side chains, or incorporation of terminal chains and/or
functional groups.15–17 For instance, the absorption and
emission wavelengths are determined by the energy of the
optical band gap (DEgap). However, the injection of electrons
and holes is controlled by the HOMO (or ionization potential)
and LUMO (or electron affinity) levels. From the synthetic
point of view, a standard strategy to modify the optoelectronic
properties of a molecule is to include electron-withdrawing or
electron-donating groups in its aromatic core; thus, it is
possible to tune the HOMO and LUMO energy levels and
alter their electronic properties by choosing suitable functional
groups.18
Amongst linearly p-conjugated purely organic systems,
polyfluorenes and related oligomers have been much investi-
gated as photo- and electro-stimulated emissive materials for
flat-panel display applications. This relies on the fact that they
combine an almost unique set of chemical, thermal, optical,
and electronic features that can be easily tailored through
molecular design. From a structural point of view, poly-
fluorenes with extended p-conjugation and a stiff backbone
are ideal luminescent materials due to their high efficiency and
good thermal stability but also for their potential liquid
crystalline properties. In fact, many systematic structure–
activity reports showing the versatility of polyfluorenes can
be found in the literature.1–5
Polymers have the advantage of high Tg and good film
forming ability but also drawbacks concerning their high
viscosity and limited solubility. In this sense, oligofluorenes
with well-defined chain lengths and end groups represent an
unquestionable alternative to their polymer counterparts.6–8
In contrast to polymers, monodispersed oligomers are
characterized by structural uniformity and regularity that
generally turns into more predictable and reproducible
properties that can also be optimized more easily.9 However,
the synthesis of monodisperse oligomers involves a large
number of steps and lengthy purification procedures, and
has proven to be difficult in most cases.
From the point of view of the chemical structure, when
compared to the p-isoelectronic biphenyl unit, the fluorene
ring has a more planar and rigid structure with a lower level of
conformational freedom because of the presence of the
methylene bridge. This structural feature gives a red-shift in
the emission and improves the emission efficiency. However,
because the fluorene unit is slightly bent, the non-coaxial
structure of the 2,7-disubstituted fluorene core has an adverse
effect on the liquid-crystal properties. In addition, the usual
presence of alkyl chains or bulky side groups at the bridging
9-position of the fluorene units to increase the solubility in
organic solvents and to limit the formation of aggregates in the
solid state, lowers the melting point and imparts a high
tendency for glass formation.19 The adverse effect is that
substitution at the bridging methylene gives rise to larger
intermolecular distances, hindering the molecular packing
required for the development of liquid-crystallinity in short
The photoluminescent properties of a large number of
polyfluorenes and oligofluorenes have been described; in
particular, a large number of reports concern their use to
polarize the emission.6,10–14 When their intrinsic structural
anisotropy is combined with liquid-crystalline behavior, these
systems can be spontaneously oriented and macroscopically
aligned by surface forces, giving rise to optical anisotropies
a
´
´
Quımica Organica, Facultad de Ciencias-Instituto de Ciencia de
Materiales de Arago´n, Universidad de Zaragoza-CSIC,
50009-Zaragoza, Spain. E-mail: loriol@unizar.es, mpinol@unizar.es;
Fax: +34 976 761209; Tel: +34 976 762279
b
´
Quımica Organica, Facultad de Ciencias-Instituto de Nanociencia de
Aragon, Universidad de Zaragoza-CSIC, 50009-Zaragoza, Spain
´
´
c Instituto de Ciencia Molecular, Universidad de Valencia,
P.O. Box 22085, Valencia, Spain
c
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oligomers (in general, from dimers to pentamers) with low
length-to-breadth ratios.7,20 Moreover, structural changes of
the (phenylethenyl)fluorene have shown that large alkyl chains
can preclude either crystallization or the formation of a glassy
state at room temperature, providing fluid oils, which can
act to the detriment of the morphological stability of electro-
luminescent devices.21
in dimethoxyethane at 75 1C or toluene at 90 1C depending on
the solubility of the required boronic acids; the reaction was
initially conducted in toluene but due to the lack of solubility
of some boronic derivatives (in particular, boronic acids 3 and 4),
the more polar solvent dimethoxyethane was used.
Conjugated cores containing two fluorene units, CN-PFFP-CN,
CN-PPFFPP-CN and the intermediate HO-PFFP-OH, were
obtained by coupling the dibromide fluorene (2) with different
monoboronic derivatives (3, 4, or 5) in a B1 : 2.1 molar ratio.
The reaction of dibromide (1) in approximately equimolar
proportion with boronic acid (3) gives rise to the dicoupled
derivative with one fluorene unit CN-PFP-CN but also renders
the monocupled product CN-PF-Br, which in turn reacts with
the diboronate ester (6) to yield the oligofluorene homologue
with three fluorene units, CN-PFFFP-CN. Finally, the last
oligofluorene of this series, C5PPCOO-PFFP-OCOPPC5, was
prepared by an esterification reaction of the intermediate
HO-PFFP-OH with the acyl chloride of the 40-pentylbiphenyl-
4-carboxylic acid (7). Molecular structures were proved by
spectroscopic techniques, IR and NMR, as well as MALDI
spectrometry. Data are gathered in the Experimental section.
There are examples of liquid-crystalline fluorene analogues
of biphenyl with nematic and smectic mesophases over narrow
temperature ranges.22,23 Elongation of the p-system of one
fluorene core just by one or two additional 4-substituted
phenyl rings is enough to promote liquid-crystallinity, mainly
smectic phases, so long as the fluorene is not substituted at the
bridging 9-position or has appended short alkyl chains.24–27
Otherwise, if larger alkyl pendant chains are desired to
suppress crystallization and to lower the melting point, the
number of aromatic rings has to increase; this strategy has
been pursued in the literature to design a fluorene-based
liquid-crystal with luminescent or electronic charge transport
properties, which also have terminal alkyl chains in some cases
with reactive groups in order to provide liquid-crystalline
networks.15,19,28
Systematic studies of oligofluorenes have also been under-
taken and luminescent liquid-crystals have been described
consisting of three fluorene units with a methyl group and
flexible terminal chains showing smectic and nematic phases at
temperatures above 100 1C. If 4 or 5 fluorene units with longer
aliphatic lateral chains are required, synthesis becomes
exponentially more difficult.6
Thermal characterization of the oligofluorenes
Thermal stability was investigated by thermogravimetric
analysis (TGA) under nitrogen atmosphere; the data is
presented in Table 1. These oligofluorenes have good thermal
stability, the onset temperature of the thermal degradation
with associated mass loss being above 360 1C in all cases. It is
clear that the elongation of the aromatic core induces a
considerable increase in the stability of this series; therefore,
when the structurally related homologues CN-PFP-CN,
CN-PFFP-CN and CN-PFFFP-CN are compared the Tonset
of the weight loss goes up from 361 1C to 418 1C and 425 1C,
respectively.
Here, we report the synthesis and properties of a series of
novel molecules mainly based on two 9,9-dioctylfluorene
units whose rigid core has been extended with well-known
4-cyanophenyl or 4-cyanobiphenyl termini, or even phenyl
ester pro-mesogenic segments. From a structural point of
view, we are particularly interested on having long lateral
chains to promote glassy states and on the incorporation
of the 4-cyanophenyl terminal fragments to tailor liquid-
crystallinity, photoluminescence and electroluminescence
because its strong withdrawing character to enhance electron
transporting properties29 and its proclivity to promote the
Nematic liquid-crystalline phase, which is the preferred over
the Smectic phases because of its lower viscosity when
processing films is required.19 We report a detailed optical
and electrochemical characterization of these fluorene-based
luminophores and some preliminary work that shows how
they can be used for electroluminescent devices and polarized
photoluminescent layers.
Oligofluorenes were obtained as only partially crystalline
solids at room temperature that, once melted, had low
tendency towards crystallization. The absence of crystalliza-
tion was proved by DSC where, after cooling the sample from
the isotropic state at 10 1C/min, a ‘jump’ on the baseline of the
subsequent heating curve corresponding to the Tg was detected
in all cases (Fig. 1). Comparison of the CN-PFP-CN,
CN-PFFP-CN and CN-PFFFP-CN data shows that Tg
increases on increasing the length of the aromatic core.
Together, the increase on the number of the aliphatic chains
hinders the crystallization propensity. In this respect, the
compound CN-PFP-CN displays a cold crystallization process
above the Tg and a subsequent endothermic peak associated
with melting of the just-formed crystalline phase. This
situation is not reproduced with the higher homologues of
the series, which remain as amorphous solids.
Results and discussion
Synthesis and structural characterization of the oligofluorenes
When inspected under the polarizing optical microscope
(POM), compounds CN-PPFFPP-CN and C5PPCOO-PFFP-
OCOPPC5 are liquid-crystals. In both cases, well-defined
Schlieren and threaded-like textures associated with the
nematic phase are observed (see Fig. 2). The nature of the
mesophase was also confirmed by X-ray diffraction. A detailed
inspection of CN-PPFFPP-CN proves that it is a monotropic
liquid-crystal, i.e. the mesophase only forms on cooling the
Scheme 1 shows the structure of the target oligofluorenes and
the corresponding synthetic pathways. The oligofluorene-
based cores were built up by aryl–aryl Suzuki coupling
reaction between aryl halides and arylboronic acids or
arylboronates as reported for related oligofluorenes.6,30 The
reactions were conducted using tetrakis(triphenylphosphine)-
palladium(0) as the catalyst, and sodium carbonate as the base
c
2786 New J. Chem., 2010, 34, 2785–2795 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Scheme 1
Table 1 Thermal and thermodynamic data for the oligofluorenes
TGAa
Transition temperatures and enthalpiesb
1st heatingc 2nd heatingd
Tonset DTGA
Compound
CN-PFP-CN
361
418
425
418
391
C
C
C
C
C
108 (28.2)
I
I
I
I
g
g
g
g
g
15 [77 (ꢀ15.6)
C
106 (17.0)]
I
CN-PFFP-CN
CN-PFFFP-CN
CN-PPFFPP-CN
447, 488, 525
450, 581
451, 582
451, 533
130 (34.8)
85 (37.6)
117 (9.77)
142 (36.6)
17
26
32
29
I
I
N
N
105 (—)
250 (1.0)
I
I
C5PPCOO-PFFP-OCOPPC5 424
N
255 (—)
I
a
Thermogravimetry analysis. Tonset: Decomposition temperature given in 1C at the onset of the weight loss curve; DTGA: first derivative of the
b
weight loss curve. g: glass; C: crystal; I: isotropic liquid; N: nematic. Temperatures are given in 1C, values in brackets correspond to enthalpy
transitions given in kJ/mol. Heating rate 10 1C/min. Heating rate 20 1C/min.
c
d
isotropic liquid. Interestingly, because of the resistance to
crystallization, the nematic phase vitrifies on cooling below
the Tg, giving a mesomorphic glass, for which it is possible to
observe the mesophase on heating between Tg = 32 1C and
Ti E 105 1C. The glass crystallizes on standing above Tg.
For C5PPCOO-PFFP-OCOPPC5 the incorporation of the
promesogenic 40-pentylbiphenyl unit by an ester linkage
instead of a nitrile end group stabilizes the mesophase that
appears on heating in a wide temperature interval.
Following these observations, we can infer that the
p-elongation of the aromatic core with the 4-cyanophenyl
end group has not been successful up to the terfluorene core.
c
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Table 2 Optical properties of the oligofluorenes in THFa
b
c
Compound
labs (log e) DEgap lem
fem
CN-PFP-CN
342 (4.80) 3.28
362 (4.95) 3.08
372 (5.01) 3.02
364 (5.02) 3.07
386
0.69
CN-PFFP-CN
CN-PFFFP-CN
CN-PPFFPP-CN
412, 428 0.76
417, 438 0.78
417, 438 0.79
396, 416 0.61
C5PPCOO-PFFP-OCOPPC5 352 (4.99) 3.20
a
b
l in nm, e in Mꢀ1 cmꢀ1, DE in eV. Optical band gap calculated as
DEgap = 1240/lonset, where lonset is the low-energy absorption edge in
THF solution. Maximum wavelength of the emission registered
c
when excitation at labs
.
Fig. 1 DSC curves recorded at 20 1C/min after previous controlled
heating and cooling scans at 10 1C/min.
maxima together with an increase on the emission efficiency.
The optical band gap, DEgap, considered as the HOMO–
LUMO separation, was estimated from lonset of the low
absorption band, which for this series of compounds varied
from 3.28 to 3.02 eV with the value decreasing on increasing
the number of fluorene units on the aromatic core. This
predictable trend can be related to the narrowing of the
HOMO–LUMO gap on increasing the effective conjugation
length.
On comparing CN-PFFFP-CN and CN-PPFFPP-CN,
which have aromatic cores of the same-length, a hypsochromic
shift can be observed in both the absorption and emission
bands, together with increase of DEgap for the latter. This is
associated with the more planar and rigid structure of the
fluorene unit with a lower level of conformational freedom
than the p-isoelectronic biphenyl unit.
We should also emphasise the optical properties of
C5PPCOO-PFFP-OCOPPC5 when compared to the nitrile-
terminated homologous CN-PFFP-CN, which has the same
aromatic core. A hypsochromic shift on both absorption and
emission was observed, together with a smaller optical band
gap for C5PPCOO-PFFP-OCOPPC5. These differences are
ascribed to the moderate electron-donating character of the
terminal –OCOPPC5 ester group in comparison to the
electron-withdrawing character of the –CN group. Also,
the presence of terminal –CN groups allows better emission
yields than ester groups.
Fig. 2 Microphotographs of the nematic phase of the (a) threaded-
like texture observed for CN-PPFFPP-CN developed on cooling at
80 1C, and (b) the Schlieren texture observed for C5PPCOO-PFFP-
OCOPPC5 developed on cooling at 30 1C.
However, elongation of the bifluorene with promesogenic
units 4-cyanobiphenyl or phenyl 40-pentylbiphenyl-4-
carboxylate is a much better alternative in order to induce
liquid-crystallinity.
Compounds CN-PFFFP-CN and CN-PPFFPP-CN are
p-isoelectronic; they both have eight aromatic rings and
terminal nitrile groups but they differ in the number of
fluorene units. Their differences in the mesomorphic properties
can be related to the fact that CN-PFFFP-CN has two
additional bulky n-octyl lateral chains that decrease the
length/diameter ratio and hinder mesomorphism.
Cyclic voltammetry (CV) studies were carried out to
investigate the redox properties in solution, and relevant data
is collected in Table 3. Under our experimental conditions,
reduction features were not observed at potentials from 0 to
ꢀ2 V. However, one or two reversible/quasi-reversible oxida-
tion waves – yielding, sequentially, monoradical cation and
dication species – were registered for all the compounds. The
chain-length dependence of the oxidation potentials fits with
the behavior of monodisperse oligofluorenes with pendant
2-ethylhexyl chains in the C9 position described by Chi
et al.31 As we compare the data obtained by CV, it shows that
increasing the number of fluorene units leads to an increase of
the accessible oxidation states in related compounds, from one
in CN-PFP-CN, to two in CN-PFFP-CN and CN-PFFFP-CN
(see Fig. 3). In the last-mentioned compound, the two
oxidation couples can be clearly differentiated. A negative
shift of the half-wave oxidation potentials and a decrease
of the difference between successive half-wave potentials
(i.e., E21/2 ꢀ E11/2) are also observed on increasing the number
Electronic and electrochemical properties in solution
When considering applications in organic devices, evaluation
of the electronic and electrochemical properties is required.
The properties have been studied by UV-vis and fluorescence
spectroscopies and cyclic voltammetry.
Absorption and emission spectra of the compounds were
recorded in THF solution, and representative data are
collected in Table 2. All the compounds show unstructured
absorption bands located at 342–372 nm due to p–p*
transitions, and an efficient emission in the violet-blue region.
In particular, the emission quantum yield values of the nitrile
derivatives, fem E 0.7–0.8, are comparable to the value
reported for the poly(9,9-dioctylfluorene), fem E 0.8.30 As it
was expected, increasing the conjugation length on moving
from CN-PFP-CN to CN-PFFP-CN and CN-PFFFP-CN
gives a bathochromic shift on both absorption and emission
c
2788 New J. Chem., 2010, 34, 2785–2795 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Table 3 Cyclic voltammetry data in solutiona and energy level analysisb of the oligofluorenes
Compound
E11/2 (V)
E21/2 (V)
Eonset (V)
HOMO (eV)
LUMO (eV)
CN-PFP-CN
CN-PFFP-CN
CN-PFFFP-CN
CN-PPFFPP-CN
C5PPCOO-PFFP-OCOPPC5
1.63qr
1.38
1.27
1.30
1.56
—
1.74qr
1.49
1.55
1.32
1.21
1.24
1.50
ꢀ5.95
ꢀ5.77
ꢀ5.61
ꢀ5.64
ꢀ5.90
ꢀ2.67
ꢀ2.69
ꢀ2.59
ꢀ2.57
ꢀ2.70
1.56qr
1.77qr
a
E1/2: half-wave potential calculated as the midpoint between the cathodic and anodic peaks. E11/2: corresponds to the first oxidation couple and
E21/2: corresponds to the second oxidation couple. For quasi-reversible (qr) processes the values is only an estimate. Eonset: onset potential for the
b
first oxidation wave. HOMO and LUMO energy level values are given relative to the vacuum level. The HOMO was calculated from the oxidation
potentials assuming the energy level of Ag/AgCl to be 4.4 eV below the vacuum level. The LUMO was calculated from the optical band gap.
Fig. 3 Cyclic voltametry curves of 4-cyanophenyl-terminated oligo-
Fig. 4 HOMO–LUMO values depicted as the energy to the vacuum
level for the extended oligofluorenes compared to oligofluorenes
having the same number of aromatic rings.
fluorenes in dichloromethane.
of fluorene units, which can be related to the increase of the
p-conjugation length.
with a slightly discernible lowering of the HOMO and increasing
of the LUMO levels for the latter. Finally, for C5PPCOO-
PFFP-OCOPPC5 a larger stabilization of the HOMO level
than of the LUMO was observed when values were compared
to the nitrile-terminated homologous CN-PFFP-CN.
On the other hand, by comparing data obtained for
compounds CN-PFFFP-CN and CN-PPFFPP-CN it can be
observed that, in spite of being p-isoelectronic, the oxidation
potentials shift towards more positive values and also the
difference between successive half-potentials increases for
CN-PPFFPP-CN. The differences might be related to the
larger planarity of the aromatic core in CN-PFFFP-CN, which
provides a larger effective conjugation length, in agreement
with optical measurements.
The HOMO and LUMO levels of 9,9-dioctyloligofluorenes
(FF, FFF and FFFF according to our nomenclature) having
the same overall number of aromatic rings (4, 6 or 8) within
the core were also determined by combining CV and UV-vis
data.33 On comparing data for compounds having the
same number of rings, i.e. FF vs. CNPFPCN, or FFF vs.
CNPFFPCN, or FFFF vs. CNPFFFPCN and CNPPFFPPCN,
we can infer that the HOMO and LUMO levels are slightly
lower for the nitrile-terminated compounds than for the
corresponding 9,9-dioctyloligofluorenes (see Fig. 4).
The HOMO and LUMO energy levels referred to the
vacuum level have been estimated by combining electro-
chemical and optical data, and values are summarized in
Table 3 and depicted in Fig. 4. Obtaining absolute HOMO
and LUMO levels in this way is an issue in debate, but in this
case the values obtained are considered to provide a rough
approximation.32 Therefore, HOMO was determined from
the oxidation potential by the empirical relationship:
HOMO = ꢀ(Eonset + 4.4) eV where Eonset is the onset
potential for the first oxidation wave relative to the Ag/AgCl
reference electrode.29 The LUMO, which in our particular case
could not be obtained by electrochemical measurements, was
deduced from the optical band gap using the expression:
Electroluminescence properties
Double-layered devices with an ITO/PEDOT:PSS/oligofluorene/
Ba/Ag configuration were fabricated to investigate the
electroluminescent properties (see Table 4). The devices were
prepared by spin-coating a PEDOT:PSS hole-transporting
layer onto a glass substrate covered by an ITO anode. The
light-emitting layer was spin-cast onto the PEDOT:PSS layer,
and covered by a barium cathode. Silver was deposited on the
top of the device to protect the barium electrode. For the light-
emitting layer, CNPFPCN, CNPFFPCN and CNPFFFPCN
were blended with inert PS at 50% weight. It has been
described that handling solids is preferred to fluid oils in terms
of device performance and also that blending with polymers
LUMO = HOMO + DEgap
.
On the systematic extension of the aromatic core from
CN-PFP-CN to CN-PFFFP-CN, the HOMO energy level rises
from ꢀ5.95 eV to ꢀ5.61 eV. However, the LUMO levels are
essentially comparable, ranging from ꢀ2.7 to ꢀ2.6 eV. The
corresponding values estimated for the p-isolectronic
CN-PFFFP-CN and CN-PPFPP-CN are essentially similar,
c
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Table 4 Performance characteristics of ITO/PEDOT:PSS/oligofluorene/Ba/Ag electroluminescent devicesa
Emitter
Von (V)
Luminance (cd/m2)
Current efficiency (cd/A)
CN-PFP-CN
CN-PFFP-CN
CN-PFFFP-CN
7.5
7.0
6.5
10.2
45.8
3.6
0.08
0.30
0.71
a
Von: turn-on-voltage, defined as the voltage required to reach 0.1 cd/m2.
might reduce the undesired greenish emission in fluorenes.21
Our target compounds have Tg values at approximately room
temperature (15–26 1C); nevertheless, because the low Tg of
the oligomers limits the formation of good films, the emitter
was dispersed into inert PS to obtain good films that
do not lead to a short-circuit upon depositing the metal
counterelectrode.
The current–voltage–luminance (I–V–L) characteristics
show that the devices have turn-on voltages of 7.5–6.5 V, with
the turn-on voltage decreasing on decreasing the barrier for
hole injection from the ITO electrode (see Fig. 4). As it would
be expected, the luminophores with better photoluminescence
quantum yields in solution have better current efficiency, as
seen from the efficiency evolution in Fig. 5. A maximum
luminance of 45.8 cd/m2 at a current density 79.7 A/m2 was
achieved for the emitter CN-PFFP-CN with a turn-on voltage
of 6.9 V.
Fig. 6 Photoluminescence (solid symbols) in solution and electro-
luminescence (open symbols) spectra for CN-PFFFP-CN. The CIE
chromaticity coordinates of the electroluminescent device are
indicated.
Electroluminescence (EL) spectra were also recorded. In
Fig. 6 the EL spectra of CN-PFFFP-CN is shown and
compared to the photoluminescence (PL) in solution. The
bathochromic shift and broadness of the emission band on
the electroluminescence spectra when compared to the photo-
luminescence in solution is presumably due to a more planar
excited state in the solid state. Nevertheless, it has also to be
considered that the appearance of a long-wavelength emission
in polyfluorenes could be associated with the formation of
keto-defects at the fluorene bridge, which is related to
the deterioration of device performance.34 The CIE 1931
chromaticity coordinates of the device shown in Fig. 6 are
x = 0.20 and y = 0.24.
longitudinal axis of the molecule. Therefore, polarized photo-
luminescent films can be produced if the sample is uniformly
aligned in the liquid-crystal fluid state and the molecular
alignment can preserved in the solid state. To evaluate the
potential of these systems for polarized photoluminescence,
macroscopically anisotropic films with a guest–host configura-
tion were prepared by in situ photopolymerization of a host
non-emissive reactive liquid-crystalline matrix containing
fluorenes as an emissive guest (Table 5).
A liquid-crystal matrix consisting of monoacrylate and
diacrylate liquid-crystals was doped with 5 mol% of the
luminophore. The reactive mixtures were investigated by
POM with appropriate light filters and DSC, and photo-
polymerization verified by photo-DSC. They developed a
nematic phase between 65–60 1C and 120–140 1C, and were
homogeneous under POM inspection. Once in the molten
Polarized emission from liquid-crystalline networks
The fluorene core exhibits direction-dependent absorption and
emission, with the transition moment polarized along the
state, all of them showed
crystallization on cooling.
a small tendency towards
Table 5 Thermal properties of the investigated reactive blends and
emission properties of the anisotropic polymeric films
Thermal transitionsa lem (nm)b Rem
b
Luminophore
CN-PFP-CN
CN-PFFP-CN
CN-PFFFP-CN
CN-PPFFPP-CN
C5PPCOO-PFFP-OCOPPC5
C
C
C
C
C
55
60
54
55
55
N
N
N
N
N
123
120
119
127
140
I
I
I
I
I
410
430
440
434
420
7
6
5
8
8
a
Data collected from DSC curves corresponding to the first heating
scan at 10 1C/min rate. Transition temperatures given in 1C: C: crystal;
I: isotropic liquid; N: nematic. Maximum wavelength of the emission
b
and emission dichroic ratio (Rem) defined as the intensity ratio of the
parallel and perpendicular polarization of the polymeric films (IJ/I>).
Fig. 5 Current efficiency versus applied voltage for the EL devices
with a configuration ITO/PEDOT:PSS/oligofluorene/Ba/Ag.
c
2790 New J. Chem., 2010, 34, 2785–2795 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Anisotropic polymeric networks were produced by UV
photopolymerization using commercial cells for planar align-
ment. To achieve maximum ordering, cells were filled by
capillarity, heating the mixture into the isotropic liquid state,
followed by slowly cooling into the nematic phase. In the
course of the polymerization, transparent films with good
optical quality and a single-domain structure with good planar
orientation were obtained, as observed by POM. After testing
several temperatures (room temperature, 60 and 80 1C), the
blends were polymerized at 60 1C, because at this temperature
films with the highest optical anisotropy were processed.
The emission spectra of the films show similar bands that
are red-shifted when compared to those registered in diluted
solution, presumably due to emission from a more planar
structure in the solid state.6 Polarized emission spectra
were recorded both parallel to the rubbing direction in the
cell and perpendicular to it using polarized excitation parallel
to the rubbing orientation. Films exhibited direction-
dependent emission, maximum in the preferential orientation
direction in the cell (IJ), and minimum perpendicular
to it (I>), which indicates that the oligofluorenes align
parallel to the polarization direction of the incident UV
beam.
Fig. 7 Polarized photoluminescence spectra of films processed from
CN-PFFFP-CN (circles) and CN-PPFFPP-CN (squares) taken
parallel (open symbols) and perpendicular (solid symbols) to the
preferred molecular orientation direction.
R
em = 1.7–2.0 were determined which are substantially lower
than those obtained for the reported extended oligofluorenes
with the same number of aromatic rings. To interpret these
results, not only the effect of the core length and the number of
the alkyl chains must be considered, but also the effect of the
nitrile end groups on increasing the electronic polarizability of
the molecule.
The emission anisotropy ratio (Rem), defined as the quotient
between these two values, depends on several factors. The
polarization ratio depends on molecular aspects such as the
angle of the absorption and emission transition moments to
the long molecular axis, rotational diffusion, as well as macro-
scopic features such as the degree of order within the film.
Nevertheless, for comparative purposes, in films of this series
of structurally related fluorenes, Rem values can give some
information about the orientation.
Experimental section
Materials synthesis
All chemicals and reagents were used as received from their
commercial sources, except toluene, which was dried over
sodium and distilled. Reactions were conducted under argon.
2,7-Dibromo-9,9-di(n-octyl)fluorene (1) was obtained from
2,7-dibromofluorene using 1-bromooctane, NaOH and tetra-
(n-butyl)ammonium in 81% yield.26 2,70-Dibromo-9,9,90,90-
tetra(n-octyl)bifluorene (2) was obtained by bromination of
9,9,90,90-tetra(n-octyl)bifluorene using Br2/FeCl3 in 88%
yield.35 Boronic derivatives 4-cyanophenylboronic acid (3),
40-cyanobiphenyl-4-ylboronic acid (4), 4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)phenol (5) and 9,9-dioctylfluoren-2,7-
yl-bis(trimetilen)borate (6) were purchased from Aldrich.
On comparing values obtained for CN-PFP-CN, CN-PFFP-
CN and CN-PFFFP-CN, it can be inferred that, in spite of
their lack of liquid-crystallinity, due to their rod-like shape
these oligofluorenes are preferentially oriented within the
liquid-crystal matrix. Nevertheless, the emission anisotropy
lowers significantly on increasing the number of fluorene units
from Rem E 7 for CN-PFP-CN to Rem E 5 for CN-PFFFP-
CN. One should expect larger emission anisotropies on
increasing the p-length but, in this case, the number of n-octyl
chains increases from two to six, lowering the effective aspect
ratio and likely impeding the uniaxial molecular orientation.
Accordingly, compound CN-PPFFPP-CN gives rise to
emission anisotropy Rem E 8 compared to the mentioned
value of Rem E 5 of the p-isoelectronic CN-PFFFP-CN
(Fig. 7). This comparison underlines the adverse effect of the
long n-octyl chains on the degree of alignment.4 Nevertheless,
it can not be underestimated that the liquid-crystalline
properties of this compound can also favor the alignment
within the liquid-crystalline matrix. In this sense, a compar-
able emission anisotropy value of Rem E 8 was determined for
the liquid-crystal compound C5PPCOO-PFFP-OCOPPC5.
Therefore, it is expected that both factors (i.e. low number
of alkyl lateral chains and mesomorphism) contribute to
achieve the largest degree of orientational order.
General procedure for the Suzuki coupling30. The reaction
was conducted under argon using Schlenk techniques and dry
solvents, toluene or dimethoxyethane. Tetrakis(triphenyl-
phosphine)palladium(0) [Pd(PPh3)4] was purchased from
Aldrich and used as received. The experimental details for
the synthesis of 2,70-bis(4-cyanophenyl)-9,9,90,90-tetra(n-octyl)-
20,7-bifluorene CN-PFFP-CN, are given as an example.
A Schlenk tube was charged with Pd(PPh3)4 (0.05 mmol,
5% mol the amount of the boronic acid) and 2,70-dibromo-
9,9,90,90-tetra(n-octyl)bifluorene (2) (0.44 mmol) and purged
by several vacuum–argon cycles. Dimethoxyethane (3 mL)
was added via syringe and the mixture stirred at 75 1C for
30 min. Then, 4-cyanophenylboronic acid (3) (0.92 mmol)
dissolved in dimethoxyethane (3 mL) and 2 M Na2CO3
(2.3 mL, a five-fold molar amount with respect to the boronic
acid) was added also via syringe. The reaction was stirred
at 75 1C for 32 h. After cooling at room temperature, the
Finally, for the sake of comparison, oriented networks were
also processed from oligofluorenes FF, FFF and FFF under the
same experimental conditions. Anisotropy emission values of
c
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reaction crude was filtered through a pad of Celites and the
brownish solution diluted with water (3 mL). The aqueous
solution was extracted with ethyl acetate; the combined
organic layers were washed with water, dried over anhydrous
MgSO4 and evaporated to dryness. The isolated yellow oil was
purified by medium-pressure chromatography on silica gel
using hexane as eluant and gradually increasing the polarity
with dichloromethane. The desired compound CN-PFFP-CN
was obtained as a white solid (0.16 g, 37% yield).
CN-PFFP-CN: nmax (KBr) cmꢀ1 2224 (CN); 1602, 1509
(Ar, C–C); 813 (Ar); dH (400 MHz; CDCl3) 7.86–7.81 (m, 4H),
7.80 (d, J = 8.7 Hz, 4H), 7,77 (d, J = 8.7 Hz, 4H), 7.74–7.69
(m, 2H), 7.68–7,66 (m, 2H), 7.64–7.58 (m, 4H), 2.17–2.06
(m, 8H), 1.24–1.04 (m, 40H), 0.85–0.72 (m, 20H);
dC(100 MHz; CDCl3) 152.00, 151.84, 146.03, 141.44, 140.83,
139.52, 137.81, 132.53, 127.66, 126.29, 126.26, 121.48, 121.43,
120.28, 119.04, 110.51, 55.39, 40.27, 31.71, 31.38, 30.13, 29.12,
23.78, 22.53, 14.00; Found: C, 87.48; H, 9.33; N, 2.70%. Calc.
for C72H88N2: C, 88.11; H, 9.04; N, 2.85%; m/z (MALDI-TOF,
dithranol) 980.6 [M]+, calc. 980.69.
CN-PF-Br: nmax (KBr) cmꢀ1 2224 (CN); 1605, 1456
(Ar, C–C); 814 (Ar); dH (400 MHz; CDCl3) 7.78–7.72
(m 5H), 7.61–7.55 (m, 2H), 7.53–7.46 (m, 3H), 2.05–1.90
(m, 4H), 1.23–0.98 (m, 20H), 0.80 (t, J = 7.1 Hz, 6H),
0.68–0.55 (m, 4H); dC (100 MHz; CDCl3) 153.20, 151.45,
145.95, 140.76, 139.27, 183.35, 132.67, 130.16, 127.76, 126.48,
126.29, 121.62, 121.42, 121.45, 120.35, 119.03, 110.74, 55.69,
40.26, 31.75, 29.96, 29.26, 29.15, 23.77, 22.65, 14.1.0.
Synthesis of 2,70 0-bis(4-cyanophenyl)-9,9,90,90,900,90 0-hexa-
(n-octyl)-20,20 0,7,700-terfluorene CN-PFFFP-CN. The title
compound was obtained from the bromide CN-PF-Br and
the diboronate (6) in a 2.04 : 1 molar ratio using toluene as the
reaction medium. The compound was purified by medium-
pressure chromatography on silica gel using hexane as eluant
and gradually increasing the polarity with dichloromethane
(56% yield).
CN-PFFFP-CN: nmax (KBr) cmꢀ1 2227 (CN); 1604, 1460
(Ar, C–C) 813 (Ar); dH (400 MHz; CDCl3) 7.87–7.81 (m, 6H),
7.79 (d J = 8.8 Hz, 4H), 7.76 (d, J = 8.8 Hz, 4H), 7.73–7.64
(m, 8H), 7.63–7.56 (m, 4H), 2.18–2.01 (m, 12H), 1.23–1.01
(m, 60H), 0.87–0.68 (m, 30H); dC (100 MHz; CDCl3) 152.03,
151.84, 151.81, 146.12, 141.50, 141.08, 140.36, 140.17, 139.40,
137.85, 132.66, 127.74, 126.31, 126.28, 126.29, 121.50, 121.41,
120.34, 120.22, 120.04, 119.11, 110.57, 55.45, 55.38, 40.32,
40.21, 31.87, 31.76, 30.03, 29.97, 29.95, 29.26, 29.14, 23.91,
23.82, 23.78, 22.68, 22.54, 14.08, 14.02; Found: C, 88.44; H,
9.56; N, 2.11%. Calc. for C101H128N2: C, 88.54; H, 9.42; N,
2.04%; m/z (MALDI-TOF, dithranol) 1370.1 [M+H]+, calc.
1369.01.
Synthesis of 2,70-Bis(40-cyanobiphenyl)-9,9,90,90-tetra(n-octyl)-
20,7-bifluorene CN-PPFFPP-CN. The compound was obtained
from the dibromide (2) and boronic acid (4), and purified
following the described general procedure for Suzuki coupling
(23% yield).
CN-PPFFPP-CN: nmax (KBr) cmꢀ1 2226 (CN); 1604, 1461
(Ar, C–C), 814 (Ar); dH (400 MHz; CDCl3) 7.86–7.80 (m 8H),
7.79–7.75 (m, 8H), 7.74–7.66 (m, 8H), 7.66–7.62 (m, 4H),
2.18–2.02 (m, 8H), 1.21–1.03 (m, 40H), 0.85–0.71 (m, 20H),
dC (100 MHz; CDCl3) 151.82, 151.75, 145.24, 142.02, 140.67,
140.60, 139.88, 138.97, 137.87, 132.70, 127.82, 127.61, 127.51,
126.20, 126.00, 121.44, 121.35, 120.19, 120.15, 119.02, 110.84,
55.35, 40.46, 31.81, 30.03, 29.27, 29.23, 23.80, 22.65, 14.10;
Found: C, 88.32; H, 8.94; N, 2.17%. Calc. for C84H96N2:
88.99; H, 8.54; N, 2.47%; m/z (MALDI-TOF, dithranol)
1132.8 [M]+, calc. 1132.76.
Synthesis of 4,40-(9,9,90,90-tetrakis(n-octyl)bifluorenyl-7-yl)-
diphenol HO-PFFP-OH. This intermediate was prepared
following the described general procedure for Suzuki coupling
from dibromide (2) and boronate (5) in 1 : 2.10 molar ratio at
90 1C using toluene as the reaction media. Purification was
afforded by flash column chromatography on silica gel using
hexane–ethyl acetate 10 : 1 as eluant (35% yield).
HO-PFFP-OH: nmax (KBr) cmꢀ1 3424–3379 (OH); 1604,
1518 (Ar, C–C); 1258, 1201, 1060 (C–O); 812 (Ar); dH
(400 MHz; CDCl3) 7.77–7.52 (m, 10H), 7.02 (dd, J1 =
Synthesis of CN-PFP-CN and the intermediate CN-PFP-Br.
The reaction was conducted adapting the described general
procedure for Suzuki coupling. The title compounds were
prepared by reacting the dibromide (1) and the boronic acid
(3) in 1 : 1.05 molar ratio using dimethoxyethane as reaction
media. CN-PFP-CN (35% yield) and CN-PF-Br (25% yield)
were separated by medium-pressure chromatography on silica
gel using hexane as eluant and gradually increasing the
polarity with dichloromethane.
7.5 Hz, J2 = 1.7 Hz, 2H), 6.96 (dd, J1 = 7.5 Hz, J2
=
1.7 Hz, 2H), 2.10–2.05 (m, 4H), 1.23–1.11 (m, 20H), 0.84
(t, J = 6.9 Hz, 6H), 0.80–0.77 (m, 4H); dC (100 MHz; CDCl3)
155.11, 151.67, 139.65, 139.57, 134.49, 128.47, 125.62, 121.09,
119.94, 115.83, 55.26, 40.52, 31.84, 30.09, 29.27, 29.23, 23.88,
22.65, 14.14.
CN-PFP-CN: nmax (KBr) cmꢀ1 2226 (CN); 1604, 1465
(Ar, C–C), 812 (Ar); dH (400 MHz; CDCl3) 7.83 (d, J = 7.9
Hz, 2H), 7.76 (s, 8H), 7.61 (dd, J1 = 7.9 Hz, J2 = 1.5 Hz, 2H)
7.56 (d, J = 1.5 Hz, 2H), 2.09–2.01 (m, 4H), 1.20–1.01
(m, 20H), 0.78 (t, J = 7.0, 6H), 0.73–0.64 (m, 4H);
dC (100 MHz; CDCl3) 152.10, 145.92, 140.92, 138.45, 132.64,
127.77, 126.48, 121.69, 120.65, 119.05, 110.70, 55.50, 40.32,
31.72, 29.93, 29.14, 29.14, 23.85, 22.67, 14.09; Found: C, 87.35;
H, 8.40; N, 4.70%. Calc. for C43H48N2: C, 87.11; H, 8.16; N,
4.73%;. m/z (MALDI-TOF, dithranol) 592.3 [M]+, calc.
592.38.
Synthesis of 2,70-di(4(4(4-pentylphenyl)benzoyloxy)phenyl)-
9,9,90,90-tetrakis(n-octyl)-20,7-bifluorene
C5PPCOO-PFFP-
OCOPPC5. The compound HO-PFFP-OH (0.19 mmol,
0.18 g) and distilled triethylamine (0.68 mmol, 100 mL) were
dissolved in dried dichloromethane (6 ml) under argon
atmosphere. The acid chloride 4-(4-pentylphenyl)benzoyl
chloride (7) (0.45 mmol, 0.13 g) dissolved in the minimum
amount of dichloromethane was added and the reaction
stirred at room temperature for 4 h. The precipitate was
filtered off and the solvent removed under vacuum. The crude
was purified by medium-pressure chromatography on silica gel
c
2792 New J. Chem., 2010, 34, 2785–2795 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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using hexane as eluant and gradually increasing the polarity
with ethyl acetate to provide the title compound (58% yield).
C5PPCOO-PFFP-OCOPPC5: nmax (KBr) cmꢀ1 1736
(CQO); 1606, 1461 (Ar, C–C); 1263 (C–O); 806 (Ar); dH
(400 MHz; CDCl3) 8.30 (d, J = 8.0 Hz, 4H), 7.82 (d, J =
7.7 Hz, 4H) 7.78–7.71 (m, 8H), 7.71–7.57 (m, 12H), 7.35
(d, J = 8.3 Hz, 4H), 7.32 (d, J = 7.9 Hz, 4H), 2.68 (t, J =
7.7 Hz, 4H), 2.15–2.04 (m, 8H), 1.68 (q, J = 6.9 Hz, 4H),
1.42–1.32 (m, 8H), 1.22–1.05 (m, 44H), 0.92 (t, J = 6.2 Hz,
18H), 0.85–0.73 (m, 8H); dC (100 MHz; CDCl3) 165.20,
151.84, 151.75, 150.30, 146.31, 143.46, 140.58, 140.25,
139.99, 139.52, 139.24, 137.14, 130.76, 129.11, 128.25,
127.98, 127.27, 127.03, 126.13, 126.04, 122.07, 121.60,
121.42, 120.08, 120.00, 55.36, 40.47, 35.62, 31.85, 31.53,
31.21, 30.09, 29.28, 29.24, 23.92, 22.60, 22.61, 14.17, 14.02;
Found: C, 85.98; H, 8.96%. Calc. for C106H126O4: C, 86.95; H,
8.67%; m/z (MALDI-TOF, dithranol) 1464.0 [M+H]+, calc.
1462.97.
an Ag/AgCl electrode was used as a reference. The solutions
were purged with argon. Cyclic voltammograms were obtained
at room temperature with scan rates of 100 mV/s.
Preparation of linearly polarized photoluminescent films
Films of oligofluorenes in a nematic matrix were prepared by
in situ photopolymerisation of reactive liquid-crystalline
blends.25–27,37 Reactive mixtures consisted of a non-emissive
liquid-crystalline matrix and a 5% molar ratio of the fluorene-
based emissive guest. The liquid-crystalline matrix was composed
of 85% molar ratio of 1,2-di[4-(6-acryloyloxyhexyloxy)-
benzoyloxy]-2-methylbenzene (C6M) and 10% molar ratio
of 1-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methyl-4-(4-octyl-
benzoyloxy)benzene (C6Mm). C6M is a difunctional liquid-
crystalline monomer that causes cross-linking in the final
polymeric system; it has a nematic phase in the range
86–116 1C and shows a maximum absorbance at 268 nm
(in THF). C6Mm is a monofunctional liquid-crystalline
monomer that has a nematic phase between 53 and 136 1C
and a maximum absorbance at 265 nm (in THF). The photo-
polymerizable mixtures were prepared by dissolving C6M
(10% molar), C6Mm (85% molar), the oligofluorene (5%
molar), 1% (w/w) of the photoinitiator Irgacure 784-DC
(a blend of 30% bis(Z5-2,4-cyclopentadien-1-yl)-bis[2,6-
difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium with 70% Dicalite)
and 200 ppm of the thermal inhibitor 2,6-di-tert-butyl-4-
methylphenol in freshly distilled dichloromethane. The solvent
was evaporated at room temperature and residual solvent was
removed under vacuum. During their preparation, storage and
handling, the samples were suitably protected and kept away
from light. The thermal properties of the reactive mixtures
were determined by DSC and POM; they present a nematic
phase between 60 and 125 1C.
Instrumentation
Infrared spectra were measured on an ATI-Matsson Genesis
1
Series FTIR from either nujol mulls between KBr pellets. H
NMR and 13C NMR spectra were recorded on a Bruker
AV-400 operating at 400 or 100 MHz. Elemental analysis
was performed with a Perkin-Elmer 240C microanalyzer.
Matrix-assisted laser desorption ionization-time-of-flight mass
spectrometry (MALDI-TOF-TOF) was performed at the Sidi
Service of the Universidad Autonoma (Madrid) on a 4700
´
Proteomic analyzer from Applied Biosystems using dithranol
as the matrix.
The optical textures of the mesophases were studied with an
Olympus BH-2 polarizing microscope equipped with
a
Linkam THMS600 hot stage and a CS196 cooling system.
Differential scanning calorimetry (DSC) was performed using
a DSC Q2000 from TA Instruments with samples (B3 mg)
sealed in aluminium pans at a scanning rate of 10 or 20 1C minꢀ1
under a nitrogen atmosphere. Temperatures were read at the
maximum of the transition peaks, and the glass transition
temperature was read at the midpoint of the heat capacity
increase. Thermogravimetric analysis (TGA) was performed at
10 1C/min under nitrogen atmosphere using a TGA Q5000IR
from TA Instruments. TGA data are given as the onset of the
decomposition curve, and DTGA data is given at the maxima
of the weight derivative curve peaks.
Light-induced photopolymerization was carried out by
irradiation of the molten reactive sample with an OSRAM
Ultravitalux 300 W lamp using both a 400 nm long-pass filter
and an IR filter between the lamp and the sample. Irradiation
was maintained for 10 min.
Photopolymerization of the reactive mixtures was investi-
gated by photo-DSC using a Perkin–Elmer DSC-7 suitably
modified for light irradiation. The sample (2–4 mg) was placed
in an open aluminium pan and the experiment was carried out
under a nitrogen atmosphere to avoid oxygen inhibition. The
light source was at a distance of 30 cm from the sample and the
reference holders. The polymerization conversion was calcu-
lated of approx. 75% from the enthalpy content of the poly-
merization peak recorded on the DSC. Reference value was
78 kJ/mol as the polymerization enthalpy of an acrylate group.
Films were produced by irradiation of the molten sample
inside a commercial LC cell (Linkam 5 mm) in a thermostatic
stage at a controlled temperature. The reactive mixture was
introduced by capillary action at the mesophase temperature
(80–100 1C) and cooled down to the polymerization temperature
(60 1C). The irradiation source was located 20 cm from the cell.
UV-vis absorption spectra were measured with a UV4-200
from ATI-Unicam using 10ꢀ4–10ꢀ5 M solutions in THF.
Fluorescence spectra were measured with a Perkin-Elmer
LS50B spectrofluorimeter using solutions in THF of ca. 0.01
absorbance (about 10ꢀ6–10ꢀ7 M) under excitation at the
absorption maximum. Emission quantum yields (fem) were
measured relative to 9,10-diphenylanthracene (fem = 0.90 in
cyclohexane)36 from corrected emission spectra.
Cyclic voltammetry experiments in dichloromethane solu-
tions (10ꢀ3 M) containing tetrabutylammonium hexafluoro-
phosphate (0.1 M) as the supporting electrolyte were
performed using an EcoChemie Instrument. A three-electrode
cell was used where the working electrode was a graphite
electrode, a platinum wire was used as counter electrode and
Fabrication of the electroluminescence devices and measurements
EL devices were prepared by using a configuration ITO/
PEDOT:PSS/oligofluorene:PS/Ba/Ag. Devices were constructed
c
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by spin-coating a 100 nm layer of PEDOT:PSS onto an
indium–tin oxide (ITO) coated glass to aid the injection of
holes and to increase the device preparation yield. Then, a
100 nm emitting layer consisting of a 50 wt% blend of the
oligofluorene and polystyrene (PS) was also deposited by spin-
coating from a toluene solution. The thickness of the films was
determined using an Ambios XP1 profilometer. Finally, a
barium cathode (5 nm) capped with a silver layer (60 nm)
was thermally evaporated using a shadow mask under vacuum
(o1 ꢁ 10ꢀ6 mbar) using an Edwards Auto500 evaporator.
The metal evaporator is integrated into an inert atmosphere
glovebox (o0.1 ppm O2 and H2O).
photopolymerization. From all the investigated compounds,
the liquid-crystalline CN-PPFFPP-CN and C5PPCOO-PFFP-
OCOPPC5 are the most suitable luminophores as they combine
a set of structural features compatible with the formation
of mesophases and appropriate emissive properties. Optical
anisotropic films with emission polarization values of 8 have
been processed. These values are comparable to those
previously reported for shorter one-fluorene-based emitters
with a PFFPCN core25 where the fluorene is covalently bonded
by an hexyloxy spacer to an oriented network processed under
the same conditions, provided that is easier to synthesize
structurally symmetric compounds and taken into account that
the increase on the number of alkyl chains limits the emission
anisotropy. It is worth mentioning that a larger emission
polarization has been reported for a related fluorene without
side-alkyl chains to the detriment of the photostability of
the films.
To obtain good quality films, the emitting layer was
composed of 50 wt% of the corresponding oligofluorene in
PS (from Aldrich, Mn = 140 000, PI = 1.6). This composition
was selected after studying the EL properties of blends having
different compositions (from 25 to 80 wt% of oligofluorene).
Current density and luminance versus voltage were
measured using a Keithley 2400 source meter and a photo-
diode coupled to a Keithley 6485 pico-amperometer using a
Minolta LS100 camera to calibrate the photocurrent. External
quantum efficiencies (EQEs) were determined using an
integrated sphere coupled to an UDT instruments S370
Optometer. An Avantes luminance spectrometer was used to
measure the EL spectrum. Characterization was performed
in an inert atmosphere glovebox (o0.1 ppm O2 and H2O,
M. Braun).
Acknowledgements
The Universidad de Zaragoza acknowledges financial support
from MEC-FEDER Spanish project MAT2008-06522-C02-01
and from the Departamento de Ciencia, Tecnologı
Universidad of the Government of Aragon and FEDER
founding. J. del Barrio acknowledges his grant from the
Government of Aragon.
´
a y
´
´
Conclusions
References and notes
1 A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz and
A. B. Holmes, Chem. Rev., 2009, 109, 897.
In this paper we report the synthesis, characterization and
mesomorphic behavior of a series of novel fluorene-based
luminophores whose common structural feature is a short
oligofluorene central core extended at its ends with a
pro-mesogenic unit. The analysis of the presented structures
shows that the elongation of a bifluorene-based core with
40-cyanobiphenyl or phenyl 40-pentylbiphenyl-4-carboxylate
pro-mesogenic groups is an adequate strategy to promote
liquid-crystallinity. In this sense, the presented compounds
having a molecular core with a small number of aromatic rings
represent an approach better than the systematic elongation of
the aromatic core by fluorene units in order to minimize the
synthetic complexity. Also, this strategy is satisfactory to tune
the optical properties, providing efficient emitters in the
violet–blue region of the electromagnetic spectra, in particular
when electro-withdrawing cyano end-groups are found at the
end or the emitting core. However, the presence of ester
groups within the core is not as favorable regarding the optical
properties.
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c
2794 New J. Chem., 2010, 34, 2785–2795 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010 New J. Chem., 2010, 34, 2785–2795 2795