of an UV-sensitive chromophore and (c) irradiation with
linearly polarized UV light to achieve uniformly planar
alignment of liquid crystal molecules.29–36 In this paper we
will demonstrate how a fine-tuning of the pretilt is possible by
varying the chemical structure of the chromophore in the
monolayer. Introducing fluoro-, trifluoro-methyl-, nitro- and
alkoxy- groups modifies the structure of the monolayer and its
polar properties. We investigated the interaction of the liquid
crystal mixtures E7 and ZLI 4792 with these monolayers. We
find that the value of the pretilt correlates to the polarity of the
liquid crystal for a given alignment layer.
Experimental
Materials
Cinnamic acid, 4-hydroxycinnamic acid, 4-methoxycinnamic
acid, 4-nitrocinnamic acid, 3-nitrocinnamic acid, 2-nitrocin-
namic acid, 4-fluorocinnamic acid, difluoro-3,5-cinnamic acid,
4-(trifluoromethyl)cinnamic acid, bromoalkanes (n = 1, 4, 6, 8,
10 where n is the number of carbons in the hydrocarbon
chains), aminopropyl-triethoxysilane, anhydrous THF, triethy-
lamine were obtained from Aldrich and used without further
purification. The liquid crystals, ZLI 4792 and E7 were
obtained from EM Industries and used as received. The ITO
˚
substrates with a coating of 1600 A of SiO2 (referred to as
passivated ITO) were obtained from Applied Films, Inc. Water
was purified with a Nanopure-Ultrapure water system to give a
resistivity of 18 MV cm. The deionized water was used for the
cleaning procedure, surface chemistry and static water contact
angle measurements.
Scheme 1 Reaction pathway for chromophores.
7.51 (d, 2 H, Ar–H), 7.74 (d, 1 H, CiLCHCOOH). Elemental
analysis, calculated for C13H16O3: C, 70.89; H, 7.32. Found: C,
70.72; H, 7.45.
Techniques
(2E)-3-[4-(hexyloxy)phenyl]-2-propenoic acid. Yield 78%;
mp: 149.6–151 uC; 1H NMR (CDCl3) dppm): 0.9 (t, 3 H,
CH3), 1.32 (m, 4 H, CH3(CH2)2), 1.45 (m, 2 H, O(CH2)2CH2),
1.80 (m, 2 H, OCH2CH2), 4.02 (t, 2 H, OCH2), 6.34 (d, 1 H,
CHLCHCOO), 6.90 (d, 2 H, Ar–H), 7.51 (d, 2 H, Ar–H), 7.76
(d, 1 H, CHLCHCOO). Elemental analysis, calculated for
C15H20O3: C, 72.55; H, 8.12. Found: C, 72.65; H, 8.04.
Analytical TLC was conducted on Whatman precoated silica
1
gel 60-F254 plates. 400 MHz H NMR spectra were recorded
on a Bruker DRX-400 spectrometer. All spectra were run in
CDCl3 or d-DMSO solutions. Infrared spectra were obtained
by using an FT Perkin–Elmer 1800 spectrophotometer and
KBr pellets. The static water (18 MV) contact angle was
measured using a goniometer from Rame–Hart, Inc. (model #
100-00-115). An automated measurement system using
National Instruments Labview (version 3.1) was used for the
electro-optic measurements. A white light source or a low
power He Ne laser was used as the source. The transmitted
luminance in the normally black or normally white mode was
measured using a detector from Graseby Optronics, Inc.
(model S370 Optometer).
(2E)-3-[4-(octyloxy)phenyl]-2-propenoic acid. Yield 82%;
mp: 146 uC; 1H NMR (CDCl3) dppm): 0.96 (t, 3 H, CH3),
1.32 (m, 4 H, CH3(CH2)2), 1.35 (m, 4 H, (CH2)2), 1.45 (m, 2 H,
O(CH2)2CH2), 1.80 (m, 2 H, OCH2CH2), 4.02 (t, 2 H, OCH2),
6.34 (d, 1 H, CHLCHCOO), 6.90 (d, 2 H, Ar–H), 7.49 (d, 2 H,
Ar–H), 7.70 (d, 1 H, CHLCHCOO). Elemental analysis,
calculated for C17H24O3: C, 73.88; H, 8.75. Found: C, 73.70; H,
8.65.
Synthesis
Typical synthesis steps leading to the preparation of 4-
alkyloxy-cinnamoyl chloride are shown in Scheme 1.
(2E)-3-[4-(decyloxy)phenyl]-2-propenoic acid. Yield 80%;
mp: 136 uC; 1H NMR (CDCl3) dppm): 0.92 (t, 3 H, CH3),
1.31 (m, 4 H, CH3(CH2)2), 1.36 (m, 8 H, (CH2)4), 1.45 (m, 2 H,
O(CH2)2CH2), 1.80 (m, 2 H, OCH2CH2), 4.02 (t, 2 H, OCH2),
6.34 (d, 1 H, CHLCHCOO), 6.90 (d, 2 H, Ar–H), 7.51 (d, 2 H,
Ar–H), 7.76 (d, 1 H, CHLCHCOO). Elemental analysis,
calculated for C19H28O3: C, 74.96; H, 9.27. Found: C, 74.88; H,
9.16.
4-Alkyloxy-cinnamic acid 2. 4-hydroxy-cinnamic acid
1(1 mmol), KOH (3 mmol) and a catalytic amount of KI
were dissolved in a mixture of ethanol/water (75/25%) and
refluxed for 1 h. Alkyl bromide (1 mmol) was added and the
reaction mixture was refluxed for 24 h. The solvent was
removed and the precipitate was acidified with concentrated
HCl. The crude product was filtered, washed with water, and
recrystallized from a mixture of ethanol/water (75:25). The final
product was dried under vacuum to yield 2aas a white solid.
Detail characterizations of each homologue are described
below.
Synthesis of the cinnamoyl chlorides 3. 1 mmol of the
cinnamic acid 2was dissolved in anhydrous benzene and stirred
in an aluminum foil covered flask under nitrogen. 3 mmol of
thionyl chloride was added and the mixture was refluxed
overnight. The solvent was removed under vacuum and the
product 3 recrystallized from hexane. The derivatives of
cinnamoyl chloride were obtained in a 90% yield. The products
were characterized by IR spectroscopy. All materials show no
broad absorption in the region of 3300–2500 cm21 typically
(2E)-3-[4-(butoxy)phenyl]-2-propenoic acid. Yield 80%; mp:
167–168 uC; 1H NMR (CDCl3) dppm): 0.99 (t, 3 H, CH3), 1.50
(m, 2 H, CH3(CH2)), 1.75 (m, 2 H, OCH2CH2), 4.02 (t, 2 H,
OCH2), 6.40 (d, 1 H, CHLCHCOOH), 6.95 (d, 2 H, Ar–H),
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 3 4 6 8 – 3 4 7 3
3 4 6 9