Polarised phosphorescent emission in an organoplatinum(ii)-based liquid-crystalline polymer

Article abstract of DOI:10.1039/c2cc35085h

By combination of mesogenic rod-like cycloplatinated monomers based on 2,5-di(4-alkenyloxyphenyl)pyridine ligands with 1,1,3,3,5,5- tetramethyltrisiloxane, a phosphorescent, liquid-crystalline polymer was obtained, which showed polarised emission.

Full text of DOI:10.1039/c2cc35085h

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Polarised phosphorescent emission in an organoplatinum(II)-based
liquid-crystalline polymerw
´
Alvaro Dı
´
ez, Stephen J. Cowling and Duncan W. Bruce*
Received 16th July 2012, Accepted 30th August 2012
DOI: 10.1039/c2cc35085h
By combination of mesogenic rod-like cycloplatinated monomers
based on 2,5-di(4-alkenyloxyphenyl)pyridine ligands with 1,1,3,3,5,5-
tetramethyltrisiloxane, a phosphorescent, liquid-crystalline polymer
was obtained, which showed polarised emission.
phase of dinuclear Pt complexes that used an extended fluorene
core of the type popularised by Kelly, O’Neill and co-workers.5,8
However, in this regard, a particular target would be liquid
crystal polymers where alignment can be achieved and retained.
Within this context, most metal-containing OLED materials
developed to date are based on 2-phenylpyridine complexes of
iridium(III), however there are only a few cases in the literature
describing liquid-crystalline analogues, all of which are columnar.9
By using platinum(II) as the metal centre, mainly due to its
geometry, mesophases are more easily realised, and for that
reason, the potential of mesogenic platinum(II) derivatives in
OLEDs has been investigated in the last few years.6,7,10–12
That some of these complexes show nematic and smectic
phases presents a potential advantage as it is these phases that
are most easily aligned and, of the two, the SmA phase is
preferred owing to its greater degree of order.
In recent years, Organic Light-emitting Diode materials (OLEDs)
have been investigated actively, amongst other reasons, because
of their potential application in lighting products1 or backlighting
of liquid crystal displays (LCD), especially where molecular
orientation of the emission layer leads to a polarised output.2
However, the external quantum efficiency of traditional OLED
materials is limited by using organic chromophores where, in the
majority of cases, triplet–singlet transitions are forbidden giving
maximum efficiencies of 25%. The low degree of order in the
molecular packing of these amorphous molecules can also
compromise transport properties. By using metal complexes, the
high spin–orbit coupling leads to enhanced singlet–triplet mixing
and phosphorescent light emission can be achieved, increasing the
optimal efficiency of the device to an ideal value of 100%.3,4 In the
case of control of molecular packing, employment of the liquid
crystal state of materials gives rise to an easier alignment of the
mesophase, which can lead to polarised emission,5 and enhanced
charge transport mobility in the OLED device structure.
In this regard, we reported previously the synthesis and
characterisation of a family of calamitic 2,5-di(4-alkoxy-
phenyl)pyridine-based platinum(II) luminophores, which are very
efficient emitters (Flum r 70%) and show nematic and smectic A
phases.11,13 In these complexes, transition temperatures are
controlled efficiently by the incorporation, or not, of a fused
C5 ring, which is readily introduced through synthesis of the final
ligand from a 1,2,4-triazine precursor.11 Using these as a building
block, it is then possible to envisage their incorporation into
a semi-flexible, main-chain polymers, which would make for
materials capable of alignment leading to polarised emission.
Identifying main-chain polymers is important if order is to
be retained, which is often not the case with side-chain
analogues. As such, in order to obtain materials with moderate
transition temperatures, the preparation of semi-flexible derivatives
is essential. In this way, siloxane-linked materials were identified as
a productive way forward as is now described.
Conventional backlit displays require a polarised source,
which is achieved using a polariser at the back of the display.
These can absorb up to 50% of the light and, given the need
for an analyser, too, then up to 75% of the backlight intensity
can be absorbed by the polarisers alone. However, if the
backlight source were linearly polarised, then the need for
the back polariser would disappear, doubling the brightness
and halving the cost of polaroid. Consequently, polarised
emission from triplet emitters is a very interesting field of
research. The alignment necessary to achieve polarised alignment
can be realised in different ways, but a most attractive option is to
use liquid crystals, which present inherently ordered phases. With
metallomesogens, this has been explored only briefly,6 although
in some cases6b,c it is not exactly clear that dichroism was
measured on a liquid-crystalline sample. Nonetheless, Wang
et al. reported7 a highly dichroic system based on the columnar
The ligands were prepared following the method described
previously, which uses the intermediacy of 1,2,4-triazines14
that lead to the 2,5-disubstituted pyridines in an inverse
electron demand Diels–Alder reaction (Scheme 1).11 The
functionalised ligand, 2, was obtained by alkylation of the
diphenolic precursor (1) under Williamson ether conditions,
using 11-bromo-1-undecene. The final monomeric complex 4
was obtained through the preparation of the intermediate
species 3, by reaction with Na(acac). Complex 4 was obtained
in moderate yield after purification via a short silica column
using CHCl3 as eluent.
Department of Chemistry, University of York, Heslington, York
YO10 5DD, UK. E-mail: duncan.bruce@york.ac.uk
w Electronic supplementary information (ESI) available: Optical tex-
tures of ligands and details of synthesis. See DOI: 10.1039/c2cc35085h
c
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The mesomorphism of the ligands, monomers and polymers
was investigated by polarising optical microscopy. Thus, in
common with the related ligand with a saturated terminal
chain, the analogue of 2 without the fused C5 ring (Fig. S1,
ESIw) was polymesomorphic showing the following phase
sequence, in which the complexes were identified by their
characteristic textures (examples as Fig. S2, ESIw):
Cr Á 111 Á SmF Á 155.5 Á SmI Á 176.5 Á SmC Á 205 Á Iso
Scheme 1 Preparation of ligands and the mesogenic monomer. Con-
ditions: (i) C11H21Br/DMF/K2CO3/90 1C/12 h; (ii) cis-[PtCl2(dms)2]/
ethoxyethanol/125 1C/12 h; (iii) Na(acac)/CH3COCH3 : CH2Cl2
(2 : 1)/rt/12 h.
This is similar to that shown by the longer-chain, saturated
homologues except that a SmF phase appears below the SmI.
Compound 2 itself showed a less rich mesomorphism (suppressed
by the lateral fused ring) and an enantiotropic SmA phase was
found [Cr Á 91.5 Á SmA Á 114 Á Iso]. Complex 4 showed a
monotropic SmA phase [Cr Á 105 Á Iso Á 101 Á SmA], while its
analogue in which the ligand did not possess fused the C5 ring
(Fig. S1, ESIw) showed an enantiotropic SmA phase with signs of
decomposition on reaching the isotropic phase [Cr Á 149 Á SmA Á
230 Á Iso/dec].
The final polymer, 5, was prepared following the method
described by Donnio et al. (Scheme 2).15 Polymer 5 was a pale-
yellow solid and was obtained in high yield by addition of
MeOH as precipitant. The precipitation was repeated several
times in order to increase the purity of the final complex.
Further details of these experiments are given in the ESI.w
Note also that polymer 6, in which the pyridine ring of the
ligand does not bear a fused five-membered ring, was prepared
in the same way starting from the analogous diphenol.
GPC was used to characterise the two polymers, but for 6 this
proved unsatisfactory as a large proportion of the material was
very insoluble, so that even after stirring overnight in thf, much
solid remained and the resulting GPC was at best inconclusive.
Thus, we believe that higher oligomers of 6 are rather insoluble
and would, as such, be unsuitable for further processing. Indeed,
it was the insolubility of 6 that led to the decision to prepare 5 as
previous work11 had shown that the low-molar-mass analogues
with the fused ring were appreciably more soluble than those
without. GPC analysis of 5 showed a weight average molecular
weight was 22 180 g molÀ1 which, with a repeat unit mass of
1128 g molÀ1 suggested that a degree of polymerisation of just
under 20 with a polydispersity of 1.2.
In characterising polymer 6 by optical microscopy, the
sample was heated until what appeared to be its clearing point
at 200 1C, after which it was held just below that temperature in
order to develop a useful optical texture. However, prolonged
holding at this temperature led to decomposition of the material
and we were not able to determine its mesomorphism. Given
the high thermal stability of the monomeric analogues,11 we
attribute the instability to the siloxane fragment.
Polymer 5, however, was much better behaved and was
observed to melt at 64 1C, clearing at 150 1C; the mesophase
was readily identified as SmA (Fig. 1) from a texture developed
after holding just below the clearing point for 30 minutes.
In the literature, different methodologies describe how
linearly polarised luminescence may be induced in emitters.16
Here, nylon-6,6 was spin-coated onto two glass coverslip
substrates, which were dried at 100 1C and then rubbed
uniaxially with a dust-free cloth. A solid sample of 5 was then
deposited between the coverslips. After reaching the desired
phase at 149 1C, the phase was quenched to room temperature.
The temperature was then increased to 120 1C, the coverslip
on top was slid uniaxially over the one on the bottom and the
texture quenched again in a cold surface. As is illustrated in Fig. 2,
the optical properties of the aligned sample are different depending
on the degree of rotation of the microscope polarisers. Samples
were also prepared by filling pre-assembled cells (5 mm thickness,
homogeneous (planar) alignment) with molten polymer, but it
was evident from both microscopy and polarised fluorescence
Scheme 2 Preparation of the polymers 5 and 6; (i) toluene, 50 1C,
12 h, [PtCl2(COD)] (1 wt%).
Fig. 1 Optical micrograph (on cooling) of the SmA phase of polymer
5 at 149 1C.
c
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