Molecular structure and pretilt control of photodimerized-monolayers (PDML)

Article abstract of DOI:10.1039/b403656e

We have studied the alignment of nematic liquid crystals on photo-sensitive chemisorbed monolayers. Surface modification and a single UV exposure at normal incidence resulted in photo-dimerized monolayers. A uniform, planar alignment of liquid crystals is realized on these surfaces. Chemical modification of the photo-sensitive chromophores of the monolayer allow fine-tuning of the pretilt. For a given alignment layer, there is a good correlation between the value of the pretilt and the polar properties of the liquid crystal used. Furthermore, the value of the pretilt depends on the chemical functionality at the outermost portion of the photo-alignment layer.

Full text of DOI:10.1039/b403656e

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A R T I C L E
Molecular structure and pretilt control of photodimerized-
monolayers (PDML)
Jawad Naciri,*a Devanand K. Shenoy,a Kirsten Gru¨enebergb{ and
Ranganathan Shashidharb
aCenter for Bio/Molecular Science and Engineering, Naval Research Laboratory, 4555
Overlook Avenue SW, Washington DC 20375, USA
bGeo-Centers, Inc. 4640 Forbes Blvd., Street 120, Lanham, MD 20706, USA
Received 10th March 2004, Accepted 2nd September 2004
First published as an Advance Article on the web 28th September 2004
We have studied the alignment of nematic liquid crystals on photo-sensitive chemisorbed monolayers. Surface
modification and a single UV exposure at normal incidence resulted in photo-dimerized monolayers. A
uniform, planar alignment of liquid crystals is realized on these surfaces. Chemical modification of the photo-
sensitive chromophores of the monolayer allow fine-tuning of the pretilt. For a given alignment layer, there is a
good correlation between the value of the pretilt and the polar properties of the liquid crystal used.
Furthermore, the value of the pretilt depends on the chemical functionality at the outermost portion of the
photo-alignment layer.
Pretilt can be generated by the following schemes: double
Introduction
exposure with polarized UV light with different polarizations
and angles of incidence,15 with a single exposure using
polarized UV light,16 or with a single exposure of poly(-
coumarin) using an oblique incidence of polarized UV light.8
Finite pretilts in photo-exposed polyimide films have also been
reported.17,18 A fine-tuning of the interactions between LC
molecules and alignment layer molecules was first shown in
rubbed polyimide films. Kobayashi et al.19 describe how alkyl
branches in polyimides contribute to the pretilt by interacting
with the LC molecules. A systematic dependence between
length of the branched alkyl chains and the pretilt was
observed, longer alkyl chains leading to higher pretilts. Also,
mixtures of polyimide with alkylamines gave finite pretilt
angles.20
The use of polyimides with fluorinated substituents yielded
high pretilts. The most successful substituents are fluorinated
alkyl side chains,21 trifluoromethyl groups22 and diamine
moieties containing hexafluoroisopropylidene groups with –
O– or –CH2– linkage.23 The attachment of the trifluoro-group
to the lateral benzene ring yielded higher pretilts compared to
its attachment to the polymer backbone.24
With the growing demand for liquid crystal displays (LCD) for
notebook computers, camcorders and digital cameras in the
past few years, there has been a strong effort to improve
production yields and display characteristics such as viewing
angle and multiplexibility. A number of potential problems
arise from the currently used rubbing techniques. The build up
of dust and static charges during the rubbing process leads to
failure of the devices.1 The high curing temperatures required
for polyimides can affect sensitive electronic parts such as color
filters. Incorporating multi-domains with small pixel sizes into
displays for high resolution and wide viewing-angle was found
difficult when using mechanical rubbing. To overcome these
problems, alternative alignment methods have been developed
which are not based on rubbing and photo-alignment
techniques. Several approaches have been reported: stretched
polymer films,2 Langmuir–Blodgett films,3 micro-grooves4 and
stamped polymers.5 However, photo-alignment methods have
caught the most attention from LCD manufacturers in recent
years due to the significant convenience of this non-contact
approach which eliminates problems stated earlier.
Two approaches for photo-alignment have been reported so
far. A guest–host approach was first reported by Gibbons
et al.,6 wherein dye molecules were incorporated into a liquid
crystal. Illumination with laser light reorients the dye molecules
and alignment of the liquid crystal molecules was achieved. In
the second approach spin cast polymers were exposed to UV
light. The photo-polymerized anisotropic surface yields planar
alignment of LC molecules.7–12
Twisted nematic (TN) as well as super-twisted nematic
(STN) devices require a finite pretilt in addition to the planar
alignment of the LC molecules. The pretilt angle is defined as
the angle between the optic axis of the LC molecules and the
substrate surface. This pretilt is necessary to prevent reverse tilt
domains in TN displays, stripe domains in STN-LCD and
surface stabilized ferroelectric LCD.13,14 Although most of the
photo-alignment methods lack the achievement of a finite
pretilt, recently published methods have solved this problem by
varying the processing conditions.
The relationship between the chemical structure of the liquid
crystal and the pretilt on polyimide was investigated by
Myrvold et al.25 When comparing cyano compounds and
dialkyl compounds it was found that the core of the liquid
crystal is tilted with respect to the surface in the first case and
parallel in the latter. Therefore increasing the length of the core
of the cyano LCs or increasing the longer alkyl chain in dialkyl
LCs respectively led to an increase in pretilt. They also found
that pretilts increase with increasing polarity of the liquid
crystal. LC mixtures with a high number of cyano groups
exhibit high polarity.
Fluorine and alkyl-substitutions were also used in photo-
alignment layers. These groups increased pretilt values in
poly(vinyl cinnamates) (PVCi), whereas unsubstituted PVCi
systems only showed pretilts of the order of 0.3u.26 UV
irradiation of PI surfaces with alkyl side chains led to pretilts of
the order of 6u.27 The influence of hydrophobic alkyl moieties
in the side chains of photo-aligned polysiloxanes on the
generation of oblique alignment has also been recognized.28
We reported the first alignment process that combines (a)
chemisorption of a self-assembled monolayer, (b) introduction
{ Currently at: Oblon, Spivak and Associates, 1940 Duke Street,
Alexandria, VA, USA
3 4 6 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 3 4 6 8 – 3 4 7 3
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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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

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found in the acid derivatives. A strong absorption at 1778 cm21
corresponding to the CLO stretch of the cinnamoyl chlorides
was detected.
Cleaning of the substrate surfaces
˚
ITO substrates with a coating of 1600 A SiO2 (referred to as
passivated ITO) were cleaned by soaking in chloroform, then in
a mixture of hydrochloric acid and water (50:50) solutions.
Each step was performed for 20 min and followed by two rinses
with methanol. The substrates were then soaked in concen-
trated sulfuric acid for 20 min. and washed with deionized
water until the rinsing water shows a neutral pH. The
substrates were than heated in deionized water for 20 min at
80–100 uC. The static water contact angle was measured to be
approximately 0u.
Scheme 2 Overview of chromophores.
Chemisorption and irradiation process
Surface chemistry
The reaction solution was prepared as follows: 94% (by
volume) of a 1 mM solution of acetic acid in anhydrous xylene
were combined with 1% of the cinnamate silylating material.
The freshly cleaned passivated ITO plates were used for the
three step procedure consisting of chemisorption, modification
of the monolayer and irradiation (Scheme 3). The self-assembly
process starts at the substrate surface 4 containing the
hydroxyl-groups. After formation of the 3-amino-propyl-
triethoxysilane (APS) monolayer 6, the substrates are washed
and thermally cured. In the next step the photosensitive
chromophore 3 reacts with the monolayer to give 7. The final
step consists of dimerization of adjacent monomers using
linearly polarized UV light l = 280nm at normal incidence. The
incident intensity was measured to be 1 mW cm22 and all
substrates were subjected to the same amount of light intensity
for a fixed time of 20 min which we found was sufficient to
cause a saturation of the pretilt measured. The photo-
dimerized monolayer (PDML) 8 gives rise to uniform planar
alignment of liquid crystal molecules with the alignment
direction orthogonal to the direction of polarization of incident
light.32,33
We have already reported studies of the monolayer growth
of APS and subsequent attachment of 4-octyloxycinnamoyl
chloride on silicon.37 One important finding was the fact that
the deposition time of APS was an important control
parameter for the formation of functionalized aminosilane
films. In this work we have determined the saturation value of
the surface coverage of passivated ITO with APS by measuring
the static water contact angle for different deposition times on
passivated ITO. The contact angle saturates after 30 min
(Figure 1). The contact angle for different substituted
alignment layers was measured (Table 1). As expected, the
contact angle increases with alkyl chain length.
The freshly cleaned substrates
4 were treated with 3-
aminopropyltriethoxysilane 5 solution at room temperature
until saturation of the static water contact angle. The substrates
6 were rinsed twice with methanol, dried and baked for 4 min at
120 uC. After cooling the substrates were rinsed twice with
anhydrous acetonitrile, treated with a solution of derivatized
cinnamoyl chloride 3 (60 mM) and diisopropylethylamine
(30 mM) in anhydrous acetonitrile and under the exclusion of
light and moisture for 1 h. The substrates 7 were rinsed twice
with acetonitrile and finally dried with a nitrogen stream and
stored in Teflon containers.
Irradiation of the substrates
A 500 W Oriel Mercury arc lamp (power supply 68810, lamp
housing 66033, bulb 6285) was used as the UV source. The UV
beam was collimated in the set up. A grating monochromator
(model# 77250) from Oriel was used to obtain the irradiation
wavelength of 280 nm with a bandwidth of 2.5 nm. A sheet
polarizer (model # 27320) from Oriel was used to linearly
polarize the UV light.
Fabrication of liquid crystal cells
Planar cells (20 mm) were fabricated using substrates treated
with the alignment layer. Mylar or glass spheres were used as
spacers. The substrates were mounted antiparallel and filled
with liquid crystals E7 or ZLI 4792 in the isotropic phase. The
liquid crystal cells were cooled to room temperature with a
After UV exposure, the substrates were mounted antiparallel
and filled with liquid crystals E7 or ZLI 4792 in the isotropic
phase.
temperature gradient of 0.5 uC min21
.
Pretilt
Results and discussion
The crystal rotation method38 is used to determine the pretilt
angle when the pretilt is small (0–10u).38 The pretilts were
measured using 20 mm thick planar samples. The method relies
on the determination of the optical phase retardation and the
transmitted intensity as a function of the angle of incidence of
the laser beam with respect to the normal to the cell walls. The
cell itself is rotated around an axis perpendicular to the optic
axis. The transmitted intensity oscillates as the angle of
incidence is changed. The intensity curve is symmetric about
a certain angle called the symmetry point, which is related to
the pretilt angle. Thus if the ordinary and the extraordinary
refractive indices and the cell thickness are known, the pretilt
angle can be calculated. The accuracy of the method is of the
order of 0.3u. A typical pretilt curve is shown in Fig. 2.
Synthesis of the chromophores
Scheme 1 illustrates the synthetic pathway used to obtain
derivatized cinnamic acid chlorides. 4-Hydroxy-cinnamic acid
1 is reacted with a series of alkyl bromide and twofold excess of
potassium hydroxide to yield the corresponding 4-alkyloxy-
cinnamic acids 2a. The acid chlorides of the chromophores 3
are obtained by reaction of the substituted cinnamic acids 2
with an excess of thionyl chloride in benzene. The other acid
chlorides obtained from the commercially available starting
materials were prepared following the same procedure
described above. We synthesized a variety of chromophores
that enabled us to control the pretilt for a given liquid crystal.
An overview of their structures is given in Scheme 2.
3 4 7 0
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 3 4 6 8 – 3 4 7 3

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Scheme 3 Chemisorption and irradiation process.
Fig. 1 Growth curve of the 3-amino-propyltriethoxysilane: time
dependence of the static water contact angle for passivated ITO.
Table 1 Static water contact angle of different substituted alignment
layers illustrates the different degrees of hydrophobicity of the
monolayers
Chromophore substituent
Contact angle/u
3a, R = H
64.5
54.9
58.8
63.3
71
80.1
83.4
88.1
92.2
67.6
52.8
60.1
3b, R = 4-NO2
3c, R = 3-NO2
3d, R = 2-NO2
3e, R = OCH3
3e, R = OC4H9
3e, R = OC6H13
3e, R = OC8H17
3e, R = OC10H21
3f, R = 4-F
Fig. 2 Pretilt curve.
substituents such as fluorine, trifluoro-methyl or difluoro-
methyl instead of the cyano group.
In the first set of experiments, altering the longitudinal dipole
moment of the alignment layer molecule varies the pretilt. This
can be illustrated by comparing the pretilt values of the liquid
crystal E7 on alignment layers with decreasing polarity as
shown in Table 2. To create a finite pretilt suitable for TN
applications (2–5u), it is useful to utilize the polar interactions
between cyano groups and nitro groups as is done in the
alignment layer with a nitro-substitution in the 4-position of
the phenyl ring. This results in a pretilt of 2u ¡ 0.3. By
changing the attachment of the nitro-substituent in the ortho
and meta positions (3-NO2, 2-NO2) the polarity of the
alignment layer decreases and so does the pretilt. An
unsubstituted chromophore (R = H) yields the least polar
alignment layer and therefore exhibits the smallest pretilt of
0.5u ¡ 0.3. The opposite trend is observed for the liquid crystal
mixture ZLI 4792 (Table 3). It should also be pointed out that
3g, R = 3.5-(F)2
3h, R = 4-CF3
Earlier studies have shown that molecular interactions
responsible for the alignment and pretilt of liquid crystal
molecules are complex. Typically these are polar, van der
Waals and steric interactions.7,8 The pretilt angle depends not
only on the alignment layer but on the liquid crystal itself. In
view of these parameters, we conducted four sets of experi-
ments. We systematically varied the structure of the chromo-
phore of the alignment layer as well as the type of liquid crystal.
The liquid crystal mixture E7 contains cyanobiphenyl groups
with a strong dipole moment. However, the liquid crystal
mixture ZLI 4792 consists of materials with less polar
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 7 1

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Table 2 Dependence of pretilt on the polarity of the chromophore
substituent for liquid crystal mixture E7
For instance, we have also shown earlier that these chemisorbed
layers used for obtaining a uniform alignment have polar
surface anchoring energies that are considered adequate for
device applications. Further, a systematic and detailed study of
the polar anchoring energies of such photo-dimerized mono-
layers have been published elsewhere.39 In this work, we show
how specific chemical substitutions at the outermost portion of
the photo-alignment layer correlate with the polar anchoring
energy.
Chromophore substituent
pretilt/ufor E7
R = 4-NO2
R = 3-NO2
R = 2-NO2
R = H
2
1.5
0.9
0.5
Table 3 Dependence of pretilt on the polarity of the chromophore
substituent for liquid crystal mixture ZLI 4792
Conclusion
Chromophore substituent
pretilt/u for ZLI 4792
We have developed a molecular level understanding of the
origin of pretilts in new photo-dimerized monolayers that are
irradiated at normal incidence. This understanding was based
on a study using two different classes of liquid crystal mixtures,
viz. the cyanobiphenyl based E7 class and the fluorinated
mixture ZLI 4792. We have demonstrated that, by introducing
substituents of different structure and polarity into the
chromophore of the self-assembled photo-alignment layer, a
pretilt is achieved. The pretilt is systematically tuned by altering
the molecular interactions at the interface between the
alignment layer and the liquid crystal molecules. These pretilts
are sufficient for some liquid crystal display modes used in the
display industry. Detailed characterization work has been
carried out on liquid crystal cells fabricated with these photo-
alignment layers. These results indicate that these photo-
alignment techniques are promising for device applications.
R = 4-NO2
R = 50% 4-NO2, 50% H
R = H
0
0.3
0.6
such variations in the dipolar interactions do not lead to any
significant pretilt values for ZLI 4792.
For the generation of a pretilt using the liquid crystal mixture
ZLI 4792 a different class of chromophores had to be
synthesized. We attached alkoxy chains to the chromophores
and utilized the concept of van der Waals interactions between
the substituents and the liquid crystal. Table 4 shows the
suitability of different alkoxy substituted alignment layers for
ZLI 4792. The pretilt increases as the chain length of the alkoxy
group increases. This trend reverses in the case of the polar
liquid crystal mixture E7 (Table 4).
It has been reported that fluorine substitution of photo-
polymers leads to high pretilts.17 As illustrated in Table 5, we
observe the same effect in the photo-dimerized monolayer. The
measured pretilts are of the order of 2u and compare to those
achieved by introduction of alkoxy chains of lower chain
length. It is important to note that these correlations between
liquid crystal properties and pretilt were observed with LC
mixtures consisting of many components. To make clear cut
correlations of such a nature would require single component
liquid crystals. Nevertheless, it is encouraging to see that the
general trends are consistent in both mixtures as observed.
It must be stated that the origin of the uniformity of pretilt is
not yet established. It is conceivable that because the surfaces
of the glass onto which these chemisorbed materials are
deposited are not atomically smooth, that surface roughness
may be causing a preferential bias of the tilt of molecules. We
have shown earlier that in those situations where a degeneracy
does arise, one could eliminate the non-uniformity of pretilt
through an electric field treatment.35 Besides pretilt, which is
the focus of this manuscript, other characterizations of such
alignment layers with display relevant liquid crystal mixtures as
well as single component liquid crystals have been performed.
Acknowledgements
We gratefully acknowledge the financial support of this work
by DARPA and the Office of the Naval Research, Washington
D.C. We also like to thank Martin Moore for the synthesis of
some starting materials. One of us (KG) is grateful to the
Alexander von Humboldt Foundation, Germany for the award
of the Feodor–Lynen Fellowship.
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Table 5 Pretilts of alignment layers with fluorinated substitution
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R = 4-F
R = 4-CF3
R = 3.5-(F)2
2.1
1.7
1.5
3 4 7 2
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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 7 3