60
Chemistry Letters 2002
Synthesis and Liquid Crystal Phase Behaviour of 2-(4-Cyanophenyl)-7-n-alkylfluorenes:
Luminescent Mesogens
Frederick H. Boardman, David A. Dunmur,ꢀ Martin C. Grossel, and Geoffrey R. Luckhurst
Department of Chemistry, University of Southampton, SO17 1BJ, U.K.
(Received September 17, 2001; CL-010922)
A new range of nematic liquid crystals containing the 2-
a = H
b = C2H5
phenylfluorene core has been prepared which may be considered as
a model system for luminescent mesogens, which have potential
interest as new display materials.
i
c = C3H7
d = C4H9
e = C5H11
f = C6H13
g = C7H15
h = C8H17
i = C9H19
O
R
Fluorene containing materials are used as electroluminescent
materials for a new generation of flat panel displays,1 which are capable
of displaying coloured images at speeds compatible with their use as TV
monitors. The poly(dialkylfluorene)s used in such displays are
electroluminescent, and, by suitable chemical modification, they can
be made to luminesce at red, green and blue wavelengths.2 Liquid
crystalline luminescent materials are the subject of current research,
since they may offer new possibilities for light emitting elements in flat
panel displays.3
Mesogens containing biphenyl and terphenyl as core units have
been extensively used as components of liquid crystal display
mixtures.4 The fluorene moiety differs from a biphenyl unit in being
flat and slightly bent in the plane of the rings. Previous work on fluorene
containing mesogens showed them to exhibit mainly smectic phases or
very short nematic ranges.5 Lengthening the fluorene core by addition
of a p-cyanobenzene ring results in a wider mesophase range, and
addition of suitable terminal substituents yields stable nematic
mesophases.
The 2-(4-cyanophenyl)-7-alkylfluorenes 1b-i have been prepared
via the five step convergent synthesis outlined in Scheme 1. Fluorene
was monoacylated by a Friedel-Crafts procedure using a mixture of the
appropriate acid chloride, aluminium chloride and 1,1,2,2-tetrachloro-
ethane to give the products 2b-i in 69-85% yield.6 The 2-acylfluorenes
2b-i were then reduced using the Huang-Minlon modification of the
Wolff-Kishner reduction.7 Heating the compounds strongly with a
mixture of hydrazine hydrate, potassium hydroxide and diethylene
glycol gave the 2-alkylfluorene 3b-i in 38-89% yield. In some cases
sodium carbonate was used in place of potassium hydroxide, as the
strong caustic base led to unwanted side reactions by deprotonation of
the methylene bridge.8 Bromination of the alkyl compounds 3b-i was
effected by slow addition of a solution of bromine in chloroform. The
products 4b-i were obtained in crude yields of 52-83%. In some cases
this material was contaminated with up to 20% unbrominated starting
material. It was found that the impure samples could be carried forward
to the next step of the synthesis with no detrimental effects, as the
unbrominated material was inactive under the reaction conditions. Pure
material could, however, be isolated by subsequent recrystallizations,
but at the expense of yield.9 The parent compound 4a was prepared by
direct bromination of fluorene in 32% yield.10 Treatment of 4-
bromobenzonitrile with n-butyllithium at low temperature gave the
aryllithium intermediate, which was then quenched with triisopropyl-
borate to give the borate ester. Hydrolysis of the ester in dilute acid was
followed by precipitation from alkaline solution to give 4-cyanobenzo-
boronic acid 5 in 66% yield. The aryl bromide and boronic acid were
coupled using the Suzuki procedure.11 A mixture of 4, 5, tetrakis-
triphenylphosphine palladium(0), sodium carbonate solution and 1,2-
dimethoxyethane was heated at reflux. The coupled products 1a-i were
2b-i
ii
R
Br
CN
3b-i
iv-vi
iii
HO
HO
R
Br
B
CN
5
4a-i
vii
CN
R
1a-i
Scheme 1. Reagents and conditions: i, R0COCl, AlCl3, CHCl2CHCl2,
0
ꢁC; ii, NH2NH2.xH2O, KOHor Na2CO3, (HOCH2CH2Þ2O, reflux;iii, Br2,
CHCl3, 0 ꢁC then reflux; iv, n-BuLi, THF, À100 ꢁC; (v) (i-PrO)3B, THF,
À100 ꢁC; vi, H3Oþ, rt; vii, Na2CO3, Pd(PPh3Þ4, H2O, DME, reflux.
purified by filtration through a plug of silica followed by recrystalliza-
tion. In the early stages of this work the yields for this final step were
low, 3-21%. Refinements to the procedure resulted in much improved
yields of up to 57%.
The liquid crystal phase behaviour of the compounds prepared is
summarized in Table 1. Members of the series 1b-i exhibited
enantiotropic nematic phases, which were identified by polarized
optical microscopy. The heptyl (1g), octyl (1h) and nonyl (1i)
homologues also exhibited a smectic phase, identified as smectic A
from X-ray scattering experiments. Calorimetric data on the materials
obtained using a Perkin Elmer DSC 7 is also reported in Table 1. The
transition temperatures are compared with the structurally related 4-n-
alkyl-400-cyano-p-terphenyls12 in the final column of Table 1. Both
homologous series exhibit smectic A phases for the longer alkyl chain
lengths. The low values of the nematic to isotropic transitional entropy
changes recorded for the phenylfluorene series are consistent with the
bent shape of the molecules, which may have a tendency for local
biaxial ordering.
X-ray measurements were carried out on 2-(4-cyanophenyl)-7-
octylfluorene 1h using a Marr Research Image Plate Detector, and a
standard Cu Kꢀ X-ray source. The sample was contained in a
Lindemann tube, and the smectic layers aligned partially as concentric
cylinders in the sample tube. At 130 ꢁC, the measured layer spacing was
ꢀ
39 A, with a very weak scattering feature corresponding to a spacing of
ꢀ
22.8 A along the smectic layer normal. A calculation of the molecular
2
ꢀ
length (Cerius, MSI) gave a value of 23.2 A for the all trans extended
Copyright Ó 2002 The Chemical Society of Japan