Polarised photoluminescence from oriented polymer liquid crystal films

Article abstract of DOI:10.1039/a605405f

Optical anisotropy in a new form of crosslinked liquid crystal network incorporating a fluorescent chromophore has been demonstrated. Linearly polarised absorption and fluorescence is observed in oriented thin films. From measurement of absorption and flu

Full text of DOI:10.1039/a605405f

Polarised photoluminescence from oriented polymer liquid crystal films
Andrew P. Davey,* Robert G. Howard and Werner J. Blau
Department of Physics, University of Dublin, T rinity College, Dublin 2, Republic of Ireland
Optical anisotropy in a new form of crosslinked liquid crystal network incorporating a fluorescent chromophore has been
demonstrated. Linearly polarised absorption and fluorescence is observed in oriented thin films. From measurement of absorption
and fluorescence anisotropy, the order parameter was found to be 0.65 and 0.80, respectively. Temperature dependent birefringence
measurements reveal that there is no appreciable loss of alignment up to 200 °C.
The last few years has seen many advances in the field of
organic optoelectronics, specifically, light emitting devices
using polymers and other smaller molecules.1 The performance
of such devices has improved considerably due to optimisation
of material interfaces, improvement of quantum yield and a
better understanding of charge transport processes in amorph-
ous organic systems. The point has now been reached where
more sophisticated devices, such as organic microcavity struc-
tures2 and polarised emitters,3 are beginning to be reported.
In any study of new systems, the starting point is characteris-
ation of the optical (absorption and photoluminescent) proper-
ties. This paper therefore presents the optical characterisation
of a new method of inducing polarised photoluminescence by
the use of aligned, crosslinked liquid crystal networks.
would preferentially emit light in a particular polarisation and
would exhibit good thermal stability properties. We therefore
set out to design and synthesise a liquid crystal with an
emissive functional core and crosslinkable side-groups. Our
aim was to utilise such a liquid crystal in an ordered thin
film form.
Synthesis and characterisation
In order to produce oriented films with linearly polarised
emission characteristics, a chromophore with a blue–violet
emissive core and acrylate functionalities (3) was synthesised.
The synthetic route to 3 is depicted in Scheme 1. A similar
molecule without any functionalities at the end of the alkyl
tails has been previously synthesised8 and the same general
method was used to prepare 2. Compound 3 was character-
ised by IR, 1H NMR, 13C DEPT NMR and UV–visible
spectroscopy.
To date, work in this area has employed a number of
different alignment techniques designed to induce polarised
emission. The first technique is controlled chemical vapour
deposition (CVD) which has been used to produce polycrystal-
line thin films of small conjugated organic molecules.4 The
second technique is Langmuir–Blodgett film deposition, which
was used to fabricate polarised polymer light emitting diodes.5
Finally, polarised luminescent films have been produced from
functionalised polymer liquid crystals by substrate rubbing-
induced alignment in the liquid crystal phase.6
Differential scanning calorimetry (DSC) showed that 3 exhi-
bits a small enthalpy phase transition at 36°C and a larger
transition (to the isotropic phase) at 80 °C on the heating cycle.
On the cooling cycle, however, no phase transitions could be
observed. This is possibly due to slow recrystallisation, since
on cooling overnight the phase transitions had returned.
Compound 3 exhibits strong blue–violet fluorescence in
dilute chloroform solutions. Fig. 2 shows the electronic absorp-
tion and fluorescence spectra of the compound in dilute toluene
solution. The quantum yield of fluorescence in chloroform was
determined by a comparative method9 and was found to be
60% in dilute solutions of cyclohexane.
These techniques have been applied with varying degrees of
success; there are still some problems to overcome. All of the
techniques have so far failed to produce particularly high
degrees of linear polarisation. The approach which we have
used in an attempt to overcome some of these problems is in
situ photopolymerisation of functionalised oriented liquid crys-
tals to produce highly aligned chromophore networks. This
technique has been studied for a number of years7 for potential
application in solid state liquid crystal display devices. Its
exploitation in other areas is still, however, limited.
To produce these oriented polymer networks, in situ photo-
polymerisation of macroscopically oriented mixtures of liquid
crystal (LC) diacrylates was used. Fig. 1 depicts the photopo-
lymerisation process in the liquid crystal phase. This technique
involves the macroscopic alignment of the LC diacrylates and
the ‘freezing-in’ of the orientation by photo-polymerisation.
Previous work has invariably employed molecules where the
saturated carbon chains are positioned at either end of the
long axis of the liquid crystal core.7 In our case however, the
alkyl chains are bonded across the short axis of the core (see
Fig. 1). Networks obtained by this method are highly
crosslinked and well ordered. This high degree of orientation
is both thermally and temporally stable.
Results and Discussion
In this study, we sought to utilise the long range ordering
characteristics of liquid crystals to produce a thin film which
Fig. 1 Schematic of in situ photopolymerisation
J. Mater. Chem., 1997, 7(3), 417–420
417

Fig. 3 Linear polarisation dependence of optical absorption (a) parallel
and (b) perpendicular to the director
through cross-polarisers under a microscope. Unfortunately,
no long-range ordering was observed in this intermediate
phase.
In order to produce a low viscosity nematic phase, 5% by
weight of 3 was mixed with 1,4-phenylenebis{4-[6-(acryloyl-
oxy)hexyloxy]benzoate}, a previously studied crosslinkable
nematic liquid crystal.10 The same proportion of photoinitiator
and thermal inhibitor as before was then added. This new
mixture exhibited a crystalline–nematic phase transition at
116°C and a nematic–isotropic transition at 150°C. Glass cells
containing this mixture were found to exhibit long range
ordering in the nematic phase. It was possible to ‘freeze-in’
this long range ordering by photopolymerisation. This pro-
cedure was found to be effective for loadings of up to 15% by
weight of 3.
Scheme 1 Reagents and conditions: i, HO(CH ) Br (2 equiv.), butanone,
2 6
K CO , heat; ii, PhCOCH (2 equiv.), piperidine, Pd(PPh ) , CuI,
2
3
3 4
90 °C, 36 h; iii, CH NCMeCOCl (2 equiv.), CH Cl , Et N
2
2
2
3
Polarised absorption and emission studies
In order to measure optical anisotropy in the film, polarisation
dependent UV–visible absorption spectra were recorded.
Spectra recorded for polarisations perpendicular and parallel
to the director and normalised for the polariser absorption are
shown in Fig. 3.
Clearly, the oscillator strength in absorption is concentrated
along the direction of rubbing. The order parameter S was
calculated by comparing peak absorption (355 nm) parallel
and perpendicular to the direction of rubbing, according to
eqn. (1),
A
−A
PA
PE
PE
S=
(1)
A
+2A
PA
where A and A are the values of absorbance parallel and
PA
PE
perpendicular to rubbing respectively. The value of S for
absorption was thus found to be 0.65, which compares favour-
ably with values found for similar non-fluorescent systems.10
It is noteworthy that there appears to be an extra absorption
band in the perpendicular absorption spectrum. The origin of
this band is not clear, it may perhaps be a weak vibrational
mode (bending mode) polarised primarily across the long axis
of the fluorescent molecule. Whatever the origin of the band,
its presence reduces the value of S as determined by absorption.
The emission spectra (for an excitation wavelength of 350 nm)
parallel and perpendicular to the rubbing direction are shown
in Fig. 4. There is obviously a strong linear polarisation of the
blue–violet emission. By way of comparison, the order param-
eter was again determined by comparing the numerically
integrated emission spectra for the parallel and perpendicular
directions according to eqn. (2), a modified form of eqn. (1),
Fig. 2 (a) Absorption and (b) fluorescence spectra of a toluene solution
of 3
Phase transition studies
The phase transitions of compound 3 were studied using DSC
and polarising microscopy. The material is crystalline at room
temperature and on heating shows a phase transition at 36 °C
to an unidentified intermediate phase. The transition from this
intermediate phase to the isotropic phase has an onset tempera-
ture of 69 °C and the peak is at 80 °C. The sample exhibits no
phase transitions when it is cooled from the isotropic phase
until below 30 °C, where it crystallises.
Long range ordering
A mixture of 3, photoinitiator (2 mol%; CIBA-Irgacure@ 651)
and thermal inhibitor (4-methoxyphenol; 0.1 mol%) was pre-
pared. A glass cell was filled with the mixture in molten form
by capillary action, and the sample was cooled overnight to
allow recrystallisation. The sample was heated to its intermedi-
ate state (50 °C) and any ordering in the film was observed
2
2
E
dl−
E
dl
PA
PE
P P
0
0
S=
(2)
2
2
E
dl+2
E
dl
PA
PE
P
AP B
0
0
418
J. Mater. Chem., 1997, 7(3), 417–420

Thin film preparation
Oriented photopolymerised thin films were produced by an
oriented rubbing method. A liquid crystal display type glass
cell of 10 mm thickness was fabricated. The inside walls of the
cell were coated with a thin layer (ca. 0.1 mm) of spun cast
Nylon 66 which was rubbed along one direction of the film
plane in order to induce alignment when the glass cell was
filled with liquid crystal material. Long range ordering could
not be induced in samples of compound 3. Thus ordered thin
films of 3 could not be produced.
Compound 3 (5% by weight) was mixed with 1,4-phenyl-
enebis{4-[6-(acryloyloxy)hexyloxy]benzoate}. Photoinitiator
(2 mol%; CIBA-Irgacure@ 651) and thermal inhibitor (4-meth-
oxyphenol; 0.1 mol%) was added. A glass cell containing the
mixture was brought to 130°C (nematic phase) and photopo-
lymerised by irradiation for 20 min with a low intensity UV
fluorescent lamp (4 W). The crosslinked film thus produced
was of very good optical quality and exhibited no sign of
phase separation or photodegradation. The same film was
used in all subsequent studies.
Fig. 4 Linear polarisation dependence of photoluminescence (a) par-
allel and (b) perpendicular to the rubbing direction
where E and E are the emission intensities parallel and
PA
PE
perpendicular to orientation respectively.
In this case, S was found to 0.80, a relatively high value.
The discrepancy between S determined by absorption and S
found by emission probably arises either from the different
measurement geometry or from the presence of the extra band
in the perpendicular absorption. In the case of absorption
transmitted light is monitored, whereas for fluorescence it is
light scattered from the sample surface which is detected.
Absorption and fluorescence measurements
Polarised absorption spectra were obtained using an ATI
UV–visible absorption spectrometer and a Rowi 55 mm polar-
iser. The polariser was placed between the spectrometer source
and the film sample. The parallel absorption spectrum was
recorded with the direction of polarisation parallel to the
rubbing direction of the film. The polariser was rotated 90°
and the perpendicular absorption spectra was recorded.
Polarised fluorescence spectra were measured using a Perkin
Elmer MPF-4413 spectrophotometer. The spectrophotometer
source was intrinsically polarised so no polariser was required.
The parallel and perpendicular spectra were recorded by
aligning the film at the appropriate angle (0 or 90°) with
respect to the polarised source. The film was placed at an
angle of 45° to both the excitation source and detector. The
excitation wavelength was 350 nm.
Variation of order with temperature
In order to monitor changes in order as a function of heating,
temperature dependent birefringence was measured using a
hot stage and a polarising microscope (equipped with a Leitz
type M tilting compensator). This method was specifically used
since thermal processes might affect the temperature dependent
emission characteristics independent of order. Fig. 5 shows the
variation of birefringence with temperature. There is little or
no change in order as temperature increases. This is due to
the very rigid nature of the crosslinked liquid crystal network.
Similar characteristics to this have been observed for other
acrylate functionalised liquid crystal networks.11
It is also interesting to note that the birefringence returns
to its original value on cooling to ambient temperature,
indicating that fluctuation of order is probably due to random
thermal motion rather than relaxation of the order.
Birefringence measurements
Measurements were performed using a Leitz polarising micro-
scope and a Leitz tilting compensator (type M at 546 nm).
The samples were heated using a microscope hot stage and a
Eurotherm temperature controller.
Experimental
Differential scanning calorimetry (DSC) measurements
Preparation of the fluorescent compound
Measurements were performed on a Perkin Elmer DSC-4. The
sample quantities were in the range of 10 mg. The heating and
cooling rates were 10 °C per minute, the measurements were
carried out under an inert nitrogen atmosphere.
All reactions were performed under an argon atmosphere. All
solvents were dried and degassed before use. Reagents were
used as supplied from the Aldrich Chemical Company. NMR
spectra were recorded in CDCl solution with an internal
3
Me Si standard.
4
Synthesis of compound 2. Compound 1 (1 mmol) and phenyl-
acetylene (2.1 mmol) were dissolved in 30 ml of piperidine.
Pd(PPh ) (0.046 g, 2 mol%) and copper() iodide (4 mg) were
3 4
then added and the mixture was stirred at 90 °C for 6 h.
Following cooling, the precipitated hydrobromide salts were
filtered and washed with hexane. The washings were combined
with the piperidine solution and the solvents were removed
under vacuum. The resulting solid was recrystallised from
butanone (0.42 g, 83%); 1H NMR: d 0.92–1.80 [m, 8H,
(CH ) ], 3.78 (t, 2H, OCH ), 3.83 (t, 2H, HOCH ), 6.81 (s, 1H,
2 4
2
2
Ar-H), 7.20–7.60 (m, 5H, Ar-H); 13C NMR: d 22.5, 25.4, 29.8
and 32.0 (CH ), 69.9 (OCH ), 72.4 (HOCH ), 86.3 and 95.0
2
2
2
(sp-C), 114.2 (Ar-C), 117.1 (Ar-C), 123.4 (Ar-C), 128.1 (Ar-C),
Fig. 5 Temperature dependence of birefringence: (#) heating and
(+) cooling
132.0 (Ar-C) and 154.0 (Ar-C).
J. Mater. Chem., 1997, 7(3), 417–420
419

Synthesis of compound 3. Compound 2 (0.5 mmol) was
dissolved in 20 ml of THF along with hydroquinone stabiliser
(3 mg) and triethylamine (1.1 mmol). The solution was heated
to reflux and methacryloyl chloride (1.1 mmol) dissolved in
5 ml of CH Cl was gradually added. Stirring was continued
absorption spectrum. Further studies on fabrication and testing
of electrically driven devices are in progress with a view to
producing highly polarised light emitting diodes.
This work was carried out under the EU ESPRIT programme
(LUPO project). The authors would like to thank F. M. Coyle
for help with DSC measurements.
2
2
for 2 h before the solution was allowed to cool to room
temperature. The mixture was then poured into 150 ml of
water and washed with saturated sodium carbonate (3×20 ml),
dried and the solvent evaporated in vacuo. The solid was
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J. Mater. Chem., 1997, 7(3), 417–420