Electron-transporting and photopolymerisable liquid crystals

Article abstract of DOI:10.1039/b200526c

The synthesis of a novel, photopolymerisable liquid crystal (reactive mesogen) with a high mobility of electrons in the smectic C phase at room temperature is reported for the first time as a potential charge transport layer for OLEDs.

Full text of DOI:10.1039/b200526c

Electron-transporting and photopolymerisable liquid crystals
Panos Vlachos,a Stephen M. Kelly,*a Bassam Mansoorb and Mary O’Neill*b
a Organophotonics Research Group, Department of Chemistry, University of Hull, Hull, UK HU6 7RX.
E-mail: S.M.Kelly@hull.ac.uk; Fax: +44 (0)1482465210; Tel: +44 (0)1482465219
b Organophotonics Research Group, Department of Physics, University of Hull, Hull, UK HU6 7RX.
E-mail: M.Oneill@hull.ac.uk; Fax: +44 (0)1482465210; Tel: +44 (0)1482465246
Received (in Cambridge, UK) 15th January 2002, Accepted 8th March 2002
First published as an Advance Article on the web 21st March 2002
The synthesis of a novel, photopolymerisable liquid crystal
(reactive mesogen) with a high mobility of electrons in the
smectic C phase at room temperature is reported for the first
time as a potential charge transport layer for OLEDs.
spacer lengths. The penta-1,4-dien-3-yl derivative 1† shown in
Scheme 1 melts just above room temperature (Cr-SmC = 25
°C) to form an enantiotropic smectic C phase with a high
clearing point (SmC-I = 124 °C). The optical appearance
between crossed polarisers of the mesophase exhibited by
compound 1 contains the broken focal conic fan texture and the
Schlieren texture (with four-point brushes and no two-point
brushes) typical of a smectic C phase. The liquid crystal
transition temperatures and enthalpies of transition were
determined by a combination of optical microscopy and
differential scanning calorimetry. The low melting point of
compound 1 is attributable to the steric branching effect of the
two terminal diene end-groups and the presence of the flexible
aliphatic spacers between them and the molecular core. These
aliphatic spacers also serve to dilute the van der Waals forces
between the rigid aromatic cores. The advantageously low
melting point enables the charge mobility in a smectic phase, in
this case a smectic C phase, of a low-molar-mass calamitic
liquid crystal to be determined at room temperature for the first
time. Radiation from an argon ion laser at 300 nm with a total
fluence of 2.5 J cm22 was used, without a photoinitiator, to
Organic materials with large polyaromatic cores1–3 exhibit large
charge mobility in the columnar liquid crystalline (LC) state
with charge carrier mobilities (1021 cm2 V21 s21 > m > 1023
cm2 V21 s21) intermediate between those observed for organic
single crystals (m ≈ 1 cm2 V21 s21) and those of amorphous
conjugated polymers4,5 used in Organic Light-Emitting Diodes
(OLEDs).4 Holes represent the majority charge carrier, rather
than electrons, in most discotic liquid crystals due to their very
limited electron affinity and low ionisation potential, although
some electron deficient discotics have been reported.6 The
processing of materials in columnar mesophases is problematic.
There are also stability problems in regard to the fabrication of
multilayer OLEDs where the hole transport layer supports the
other layers. The charge carriers, even of photocurrent, in the
nematic phase, were found to be primarily ions.7,8 However, the
charge-carrier mobility in ordered lamellar smectic phases of
low-molar-mass organic compounds with a rod-like molecular
structure is surprisingly high (1022 cm2 V21 s21 > m > 1024
cm2 V21 s21), being comparable with that of LCs in columnar
mesophases, much higher that of the charge-transport materials
currently used in commercial OLEDs and undoubtedly elec-
tronic in nature.9,10 However, the smectic LCs investigated so
far are crystals at room temperature with high melting points.
Therefore, they are clearly not suitable for use in OLEDs, since
the presence of crystal grain boundaries results in charge
trapping and, potentially, dielectric breakdown.
We now report the synthesis of a new class of photo-
polymerisable liquid crystals (reactive mesogens) designed to
enable the study of charge transport through smectic phases at
room temperature for the first time. They were also intended for
use, after photochemical crosslinking, as stable electron
transport layers in efficient multilayer OLEDs. The most
favourable configuration of an OLED is a combination of at
least three discrete layers for hole transport, electron transport
and light emission. However, efficient electron transporting
materials are rare due to the low electron affinity of most
aromatic, i.e., electron rich, organic compounds.5 The reaction
pathway to the new electron transporting materials is short
enough to be of commercial relevance involving just six steps
altogether carried out in high overall yield, see Scheme 1. The
alkylation of 4-bromophenol with octyl bromide in a William-
son ether synthesis yields 1-bromo-4-octyloxybenzene, which
is converted under standard conditions to the corresponding
boronic acid. This is then reacted with 5-bromo-2-iodopyr-
imidine in a Suzuki aryl–aryl cross-coupling reaction to yield
2,5-bis(4-octyloxyphenyl)pyrimidine. An octyloxy chain is
used in order to render this diether sufficiently soluble in
appropriate organic solvents so that the next step can be carried
out. Dealkylation using boron tribromide yields the correspond-
ing bis-phenol, which is alkylated with the product of
commercially available w-bromoalkanoic acids and penta-
1,4-dien-3-ol, hexa-1,5-dien-3-ol and hepta-1,6-dien-4-ol to
produce a variety of symmetrical bis-dienes with different
Scheme 1 Reagents and conditions: i, C8H17Br, CH3COC2H5, K2CO3; ii, a,
BuLi, THF, b, B(OCH3)3, c HClaq; iii, Na2CO3, H2O, Pd(Ph3)4, CH3OC2-
H4OCH3; iv, a, BBr3, CH2Cl2, b, HClaq; v, BrC5H10CO2diene,
CH3COC2H5, K2CO3; diene = penta-1,4-dien-3-yl.
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photopolymerise and crosslink the diene end-groups of com-
pound 1 at 30 °C on a quartz substrate. The actual mechanism
of radical formation is currently under investigation and an
autocatalytic mechanism is suspected. The crosslinking of
reactive mesogens results in the formation of an insoluble liquid
crystalline polymer network as a thin solid film.11–16 We have
previously demonstrated the efficacy of this approach using
dienes as the polymerisable end-group for the fabrication of
bilayer OLEDs without an electron transport layer.13 We have
also shown recently that the polymerisation of reactive
mesogens with the penta-1,4-dien-3-yl end-group in particular
results in the formation of stable electroluminescent polymer
networks with superior properties to those of the non-
polymerised diene monomers.13,14 For example, it leads to an
increase by up to a factor of two in the magnitude of the hole
mobility of a fluorene based liquid crystal.15 This may be due to
favourable conformational changes of the aromatic cores due to
the formation of a rigid bicyclo[3.3.0]octane polymer back-
bone.16 The provision of an efficient photopolymerisable
reactive mesogen with a high electron charge transport would
be of use for multilayer OLEDs of all kinds, especially since it
is suspected that the presence of reactive residues from initiators
reduces the life-time of OLEDs.
The charge mobility through a thin ( ≈ 2 µm) layer of the
reactive mesogen 1 was determined by photocurrent time-of-
flight measurements with excitation from a pulsed nitrogen
laser. The values for the electron mobility are plotted against
temperature in Fig. 1. The magnitude of the mobility increases
with temperature, indicating a hopping mechanism between
molecules. The sudden order of magnitude decrease in the
electron mobility at the transition from the smectic C phase to
the isotropic liquid suggests that the order present in the smectic
phase is indeed responsible for the high charge mobility values.
The value for the electron mobility at 25 °C is reasonably high
(me = 1.5 3 1025 cm2 V21 s21) for an organic material. This is
probably due, at least in part, to the presence of the
electronegative nitrogen atoms in the pyrimidine ring of
compound 1. The value for the hole mobility is comparable (mh
= 1.8 3 1025 cm2 V21 s21 at 25 °C) to that found for electrons.
The mobility of a crosslinked sample was not measured because
photopolymerisation could not be achieved throughout a sample
of length 2 mm, as a result of the small absorption length at 300
nm. However, by analogy with previous results, photo-
polymerisation is expected to enhance the transport properties.
Optical microscopy also shows that crosslinking of a much
thinner film of compound 1 maintains the order of the smectic
phase below 25 °C.
We gratefully acknowledge the EPSRC and the University of
Hull for financial support and the analytical services of the
University of Hull for spectral data.
Notes and references
† All the compounds were thoroughly characterised with the help of spectral
and analytical data. Selected data for 1: nmax/cm21 2948, 2874, 1740, 1611,
1521, 1437, 1255, 1168, 831; dH(400 MHz, CDCl3) 8.92 (s, 2H), 8.41–8.38
(d, 2H), 7.54–7.51 (d, 2H), 7.03–6.98 (dd, 4H), 5.88–5.81 (m, 4H),
5.33–5.22 (m, 8H), 4.06–4.00 (dt, 4H), 2.42–2.38 (t, 4H), 1.88–1.50 (m,
12H); APCI-MS (m/z) 623; Calculated: C 73.05, H 7.10, N 4.48; Found: C
72.84, H 7.29, N 4.22%.
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Fig. 1 Electron mobility vs. temperature for compound 1.
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