Liquid crystal mono- and nano-layers covalently bonded to silicon and silica surface for alignment of LC layers

Article abstract of DOI:10.1080/15421406.2010.485062

We synthesized two types of liquid crystalline nano-layers covalently bonded to Si and Si/SiO₂ wafers. Monolayers of terminal and lateral low molecular weight mesogens exhibit spontaneous homeotropic orientation, while planar orientation is obtained by means of rubbing process. On the other hand a novel polymer liquid crystal nano-layer, obtained via atom transfer radical polymerization (ATRP) of acrylate monomer from Si/SiO₂ surface, exhibits planar orientation of liquid crystalline side chain moieties. Structure of such novel composites was confirmed by optical studies as well as spectroscopic methods, including ellipsometry. The goal was to obtain low molecular weight liquid crystals alignment just from the mentioned above, modified surfaces. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2010.485062

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Liquid Crystal Mono- and Nano-Layers
Covalently Bonded to Silicon and Silica
Surface for Alignment of LC Layers
Włodzimierz A. Stańczyk a , Anna Szeląg a , Jan Kurjata a , Edward
Nowinowski-Kruszelnicki b & Andrzej Walczak b
a Centre of Molecular and Macromolecular Studies , Polish Academy
of Sciences , Sienkiewicza, Łódź, Poland
b Military Technical University , Kaliskiego, Warsaw, Poland
Published online: 19 Aug 2010.
To cite this article: Włodzimierz A. Stańczyk , Anna Szeląg , Jan Kurjata , Edward Nowinowski-
Kruszelnicki & Andrzej Walczak (2010) Liquid Crystal Mono- and Nano-Layers Covalently Bonded to
Silicon and Silica Surface for Alignment of LC Layers, Molecular Crystals and Liquid Crystals, 526:1,
18-27, DOI: 10.1080/15421406.2010.485062
To link to this article: http://dx.doi.org/10.1080/15421406.2010.485062
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Mol. Cryst. Liq. Cryst., Vol. 526: pp. 18–27, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.485062
Liquid Crystal Mono- and Nano-Layers
Covalently Bonded to Silicon and Silica Surface
for Alignment of LC Layers
1
1
´
WŁODZIMIERZ A. STANCZYK, ANNA SZELA˛ G,
JAN KURJATA,1 EDWARD NOWINOWSKI-
KRUSZELNICKI,2 AND ANDRZEJ WALCZAK2
1Centre of Molecular and Macromolecular Studies, Polish Academy of
´ ´
Sciences, Sienkiewicza, Łodz, Poland
2Military Technical University, Kaliskiego, Warsaw, Poland
We synthesized two types of liquid crystalline nano-layers covalently bonded to Si
and Si=SiO2 wafers. Monolayers of terminal and lateral low molecular weight meso-
gens exhibit spontaneous homeotropic orientation, while planar orientation is
obtained by means of rubbing process. On the other hand a novel polymer liquid
crystal nano-layer, obtained via atom transfer radical polymerization (ATRP) of
acrylate monomer from Si=SiO2 surface, exhibits planar orientation of liquid
crystalline side chain moieties. Structure of such novel composites was confirmed
by optical studies as well as spectroscopic methods, including ellipsometry. The goal
was to obtain low molecular weight liquid crystals alignment just from the mentioned
above, modified surfaces.
Keywords Atom transfer radical polymerization; functional interfaces; liquid
crystals; self-assembled monolayers and nanolayers; surface organometallic
chemistry
Introduction
There is a significant interest in modification of silicon and silica via surface
organometallic chemistry (SOMC) [1,2]. Functional interfaces play nowadays
increasingly important role in micro- and nano-structuring technology, since the time
when the pathway of chemical reactions with solid silicon substrate was presented by
Chidsey [3,4] (alkenes=Si, alkynes=Si) and Whitesides [5,6] (alkanethiols=Au).
This developing trend of the search for novel self assembled monolayers
(SAM’s) is fueled by potential application of such organic=inorganic composites,
ranging from surfaces of controlled wettability [7], surface passivation of solar cells
[8], molecular recognition systems [9], molecular bioelectronics [10] and DNA chips
[11], to surface grafted molecular motors [12]. Our interest, in this respect, focuses on
´
Address correspondence to Włodzimierz A. Stanczyk, Centre of Molecular and
Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, Łodz 90-363, Poland.
´ ´
Tel.: þ48 42 6803208; E-mail: was@bilbo.cbmm.lodz.pl
18

Liquid Crystal Mono- and Nano-Layers
19
application of silicon wafers with LC modified surfaces as photoconducting and
orienting substrates in photonic devices [13].
Several synthetic methods have been developed for preparation of covalently
bonded organic mono- and nano-layers on silicon. In general, they lead to formation
of either Si–C or Si–O–Si bonds between organic moieties and surface silicon atoms
[1,2]. The latter ones are generated, once organic layers are grown from silica or
quartz [14].
A recent work by Ferringa [12] illustrates well the Si-O-Si bond formation as an
alternative pathway to hydrosilylation reaction (leading to Si-C bond between
organic group and the surface atoms [5,6]). Piranha solution generates Si-OH bonds
on the surface, which can be then condensed with an appropriate chloro- or alkox-
ysilane to give the organic moiety covalently bonded to the substrate (Si or silica).
The purpose of this study is formation of liquid crystalline (LC) mono- and
nano-layers having a stable chemical linkage with semiconductor (Si) and
semiconductror=insulator (Si=SiO2) surfaces. Such the modified wafers could find
application as alignment devices for opto-electronic elements, e.g., wave-guide
couplers (Fig. 1), splitters or sensors.
A similar approach has been presented in the past [15], concerning a liquid
crystalline film designed for LC flat panel displays. However, no details concerning
synthesis have been presented at that time and only formation an idealized, cross-
linked nanolayer structure has been claimed (Fig. 2).
Our present generic research concentrates on development of the most effective
methods of surface organometallic chemistry (SOMC), leading to novel organic
(LC)=inorganic hybrid materials and orientation studies of the LC layers deposited
Figure 1. Wave-guides coupler: 1 – wave-guides, 2 – glass (upper) and silicon (lower) plates, 3 –
electrodes (layers of ITO), 4 – nanolayer of covalently bonded liquid crystal, 5 – nematic
mixture of low molecular liquid crystals, 6 – polyimide layer.

´
W. A. Stanczyk et al.
20
Figure 2. An example of liquid crystal system network bonded via triethoxysilane.
over surfaces of such materials. We have synthesized three types of monolayers,
having covalently bonded liquid crystalline moieties of cyanobiphenyl, methoxyphe-
nyl benzoate and bis(methoxybenzoyl) type as well as acrylic polymer layer, having
cyanobiphenyl liquid crystalline core in the side chain. Orientation studies of LC
moieties, relative to the inorganic surfaces, were performed in order to evaluate
potential application of such materials in opto-electronic devices.
Experimental
Materials
Silicon wafers Si (100), and Si (100) with 300 nm layer of SiO2 were purchased from
Aldrich and Si-Mat Silicon Materials. Their surface was modified to generate Si-OH
and Si-H reactive groups, as described elsewhere [1,16]. Toluene and THF were dried
according to reported procedures [17] and stored under nitrogen. Copper(I) bromide,
copper(II) bromide, pentamethyldiethyltriamine (PMDTA), allyl 2-bromobutyrate,
triethoxysilane and chlorodimethylsilane were purchased from Aldrich and used as
received. 3-Chlorodimethylsilylpropyl 2-bromoisobutyrate was prepared by hydrosi-
lylation of allyl 2-bromoisobutyrate and chlorodimethylsilane as described in [18].
Tris[2-diethylaminoethyl]-amine (Me6TREN) was made according to [19] and
acrylate monomer-40-(7-undecenyloxy)-4-cyano-biphenyl acrylate (5), according to [20].
Mesogenic alkenes (Fig. 3) were made as follows: 40-cyano-4-(7-octenyloxy)
biphenyl (1) and 40-methoxyphenyl-4-(10-undecenyloxy)-benzoate (2) were prepared
by literature methods described, respectively, in [21] and [22].
5-[[[20-[[400-methoxybenzoyl]oxy]-methylene]-40-[(400methoxybenzoyl)-phenyl]
oxy]carbonyl]dec-9-ene (3a) and 5-[[[20-[[400-methoxybenzoyl]oxy]-methylene]-
40-[(400methoxybenzoyl)-phenyl]oxy]-carbonyl]but-3-ene (3b) were made by

Liquid Crystal Mono- and Nano-Layers
21
Figure 3. Structures of terminal and lateral liquid crystalline alkenes.
esterification of the substituted phenol 2-[[(40-methoxybezoyl)oxo]methylene]-4-
[(400-methoxybezoyl) oxy]phenol (4) [23] with 10-undecenoyl chloride (Aldrich)
and 4-pentenoyl chloride (Aldrich) (Fig. 4). Thus, 2-[[(40-methoxybezoyl)-oxo]-
methylene]-4-[(400-methoxybezoyl) oxy]phenol (2.76 g, 6.7 mmol) in 55 ml of dry
THF was stirred in a two-neck flask at room temperature, while solutions of
4-pentenoyl chloride (1.42 g, 8.2 mmol) in THF (5 ml) and triethylamine (1.09 g,
8.2 mmol) in THF (5 ml) were simultaneously added drop-wise to the solution. Once
the addition was completed the reaction mixture was stirred for 18 hours and a white
deposit of triethylammonium chloride precipitated. Then, the reaction mixture was
poured into 250 ml of cold water. The separated oil was crystallized from a mixture
of ethanol (120 ml) and toluene (20 ml) yielding 0.8 g (24.4%) of 5-[[[20-[[400-
methoxybenzoyl]oxy]methylene]-40-[(400methoxybenzoyl)-phenyl]oxy]carbonyl]-but-2-
ene (3a) as white crystals.
Anal. Found: C, 68.7; H, 5.4. C28H28O8 Calc.: C, 68.6; H, 5.3.
1
=
H NMR (CDCl3) d ppm: 2.4 (m, 2H, a, CH2CH CH2), 2.6 (m, 2H, b,
CH2CH2COO), 3.8 (s, 3H, c, OCH3), 3.9 (s, 3H, d, OCH3), 5.0–5.2 (m, 2H, e,
CH2¼CH), 5.3 (s, 2H, f, ArCH2O), 5.7–5.9 (m, 1H, g, CH¼CH2), 6.9 (d, 4 aromatic
H, h, ortho to OCH3), 7.0 (d, 2 aromatic H, i, ortho to OOC), 7.1 (s, 1 aromatic H, j,
ortho to CH2OOC), 8.0 (d, 4 aromatic H, k, meta to OCH3).
5-[[[20-[[400-Methoxybenzoyl]oxy]-methylene]-
40-[(400methoxybenzoyl)phenyl]oxy]-carbonyl]dec-9-ene (3b) was made by the above
procedure, using 10-undecenoyl chloride, to give 0.85 g (40%) of the acrylate (3b)
as white crystals.
Anal. Found: C, 72.3; H, 6.9. C38H28O8 Calc.: C, 71.0; H 6.7.
1
=
H NMR (CDCl3) d ppm: 1.2 (m, 10H, L, CH2CH2(CH2)5CH2CH CH2), 1.5
=
(m, 2H, m, CH2CH2(CH2)5CH2CH CH2), 2.0 (m, 2H, n, CH2CH CH2), 2.2 (m,
=
2H, o, OOCCH2CH2) 3.9 (s, 6H, p, OCH3), 4.9 (m, 2H, q, CH2¼CH), 5.1 (s, 2H,
=
r, ArCH2O), 5.8 (m, 1H, s, CH CH2), 7.0 (d, 4 aromatic H, t, ortho to OCH3),

´
W. A. Stanczyk et al.
22
Figure 4. Synthesis of lateral mesogenic alkenes.
7.3 (d, 2 aromatic H, u, ortho to OOC), 7.4 (s, 1 aromatic H, v, ortho to CH2OOC),
8.1 (d, 4 aromatic H, w, meta to OCH3).
Monolayer LC Films Formation
Hydrogen terminated flat silicon wafers ꢀ10 ꢁ 10 mm were etched chemically [1]
and subsequently reacted with toluene solutions (8 ml, 10 wt% of terminal alkenes
– {40-cyano-4-(7-octenyloxy) biphenyl (1), 40-methoxyphenyl-4-(10-undecenyloxy)-
benzoate (2) and the lateral 5-[[[20-[[400-methoxybenzoyl]oxy]-methylene]-40-
[(400methoxybenzoyl)phenyl]oxy]-carbonyl]dec-9-ene (3b)}. The heterogeneous
hydrosilylation process was carried out at 100ꢂC for 48 hours [16].
Hydroxy-terminated Si=SiO2 wafers were made in a CVD reactor at moist nitro-
gen flow (p ¼ 300 Pa, T ¼ 350ꢂC, t ¼ 10 min, I ¼ 0.2 A, P ¼ 80 W). Then ꢀ10 ꢁ 10 mm
shards were reacted with (EtO)3SiH, followed by hydrosilylation of lateral mesogen –
5-[[[20-[[400-methoxybenzoyl]-oxy]methylene]-40-[(400-methoxybenzoyl)phenyl]oxy]carbonyl]
but-3-ene (3a) [16].
The monolayers were characterized by goniometric and ellipsometric measure-
ments, ESCA, and AFM.
LC Polymer Layer Formation. ATRP from Si/SiO2 Surface
Atom transfer radical polymerization initiator – 3-chlorodimethylsilylpropyl 2-
bromoisobutyrate was anchored on hydroxy-terminated Si=SiO2 shards [14] and such
the system was used for polymerization of LC monomer – 40-(7-undecenyloxy)-4-
cyano-biphenyl acrylate (5) (Fig. 5), from such the modified surface. Thus, silicon

Liquid Crystal Mono- and Nano-Layers
23
(100) wafer shard, covered with 300 nm of SiO2 and functionalized with ATRP
initiator (dimethylsiloxylpropyl 2-bromoisobutyric group), was placed in a Schlenk
flask, in a toluene (5 ml) solution containing 40-(7-undecenyloxy)-4-cyano-biphenyl
acrylate (5) (0.5 g, 1.19 ꢁ 10ꢃ3 mol), CuBr (3.2 ꢁ 10ꢃ3 g, 2.3 ꢁ 10ꢃ5 mol), CuBr2
(0.5 ꢁ 10ꢃ3 g, 2.3 ꢁ 10ꢃ6) and PMDTA (2.1 ꢁ 10ꢃ3, 1.2 ꢁ 10ꢃ5 mol). The reaction
was carried out at 60ꢂC, under argon. At time intervals (1, 2, 3 and 22 hours) the shard
was withdrawn, washed, dried and the thickness of the generated layer was controlled
by ellipsometric spectroscopy. Analogous conditions were applied when Me6TREN
was used as the ATRP ligand. The growth of polymer was controlled after 1, 2, 20 and
23 hours (Fig. 7).
Orientation of liquid crystalline moieties against the surface, in nano- and
mono-layers was determined using experimental set-up shown in Figure 6.
Characterization
1H NMR was carried out with a Bruker DRX-500 MHz spectrometer using CDCl3
as the solvent. Phase transition temperatures were determined by DSC 2920 (TA
Instruments) and optical microscopic measurements. Surface morphology of LC
Figure 5. Synthesis of mesogenic acrylate monomer and its ATRP polymerization from
Si=SiO2 surface.

´
W. A. Stanczyk et al.
24
Figure 6. Experimental set-up for orientation studies: 1 – rotating table; 2 – silicon wafer;
3 – liquid crystal layer; 4 – glass reference plate; 5 – LED; 6 – broadband polarizing cube
beamsplitter; 7 – photodetector.
layers was followed using AFM (Nanoscope IIA, Digital Instruments) and their
elemental analysis by ESCA (Vega-Tescan Instrument). The layers were also
characterized by ellipsometry (V-VASE, J. A. Wollam Co. Inc.,) and contact angle
measurements (F70B Goniometer), using ‘‘DROP’’ program. The effect of LC modi-
fied silicon surface on orientation of nematic liquid crystal mixture was studied in a
sandwich cells of the experimental set-up, presented in Figure 6. The cells consisted
of the investigated silicon wafers, with covalently bonded low molecular weight LC
moieties, a standard mixture of low molecular weight LC’s nematics [24] and a
reference glass plates providing unidirectional planar or homeotropic orientation.
Figure 7. Progress of atom transfer radical polymerization followed by ellipsometric measure-
ments of the thickness of liquid crystalline nano-layer.

Liquid Crystal Mono- and Nano-Layers
25
The orienting layer on reference plates were prepared by spin coating with
commercially available polyimides SE 130 and SE 1211 (Nissan Chemical
Industries), respectively, for planar and homeotropic alignment, followed by heating
for 30 min at 80ꢂC and then for 60 min at 180ꢂC. Unidirectional planar orientation
was generated by rubbing, using rubbing cloth for liquid crystal panels YA-19R
(Yoshikawa Chemical Company). The liquid crystal layer thickness was fixed by
glass spacers PF-100 (Nippon Electric Glass).
Results and Discussion
Syntheses
We have synthesized two types of liquid crystalline layers covalently bound to silicon
(or silicon=silica) surface. The first group of liquid crystalline monolayers (with low
molecular weight liquid crystals) were made by hydrosilylation of terminal
mesogenic alkenes (1), (2), (3a, 3b), bearing respectively, cyanobiphenyl, methoxy
benzoate as well as phenol butyrate and undecenoate moieties with Si-H modified
silicon (100) wafers. Additionally, (3b) was also hydrosilylated on triethoxysilane
modified Si=SiO2 surface.
Another type of covalently anchored liquid crystalline layer was made by atom
transfer radical polymerization of liquid crystalline acrylate (5) from the Si=SiO2
surface.
The surface was modified with an ATRP initiator – 3-chlorodimethylsilylpropyl
2-bromoisobutyrate. ATRP gave the unique opportunity to obtain polymer layer
made of chains having very low polydispersity index (PDI ꢀ 1.1). In this case the
silane moiety with a single Si–Cl function was used for generation of SiꢃH on Si=
SiO2 surface. From our experience it appears that the typically used trifunctional sys-
tems, e.g., (EtO)3SiH, being very reactive, leads to complicated monolayer structure.
Apart from formation of the expected Si–O–Si bonds with hydrophilic surface
(Si–OH groups), they generate as well an irregular polysiloxane network, as a result
of hydrolytic condensation of ethoxy groups themselves. Thus, contrary to many
earlier reports [15,25,26], the chemically uniform layers are never formed once
multifunctional surface modifiers are used.
Orientation Studies
The sandwich cell made of silicon wafer and reference plate enclosed a liquid crystal
layer acts as reflective one and the normalized reflectance R for the cell with twist
?
angle U can be solved by Jones matrix method. Under the crossed-polarizer
configuration the reflectance of the cell can be described by the following Jones
2
matrices as R ¼ jMj , where:
?
2
4
3
5
cos X ꢃ i C2 sin x ꢃ/ sin X
x
X
M ¼ ½ cos b sin b ꢄ
sin X
X
/
cos X þ i C2 sin X
X
2
3
5
ꢀ
ꢁ
cos X ꢃ i C2 sin x
/
sin X
X
ꢃ sin /
cos /
x
4
ꢃ/ sin X
cos X þ i C2 sin X
X
X

´
W. A. Stanczyk et al.
26
b is the angle between the polarization axis and the front LC director, U is the twist
angle, X ¼ [U2 þ (C=2)2]1=2 and C ¼ 2pd ꢅ Dn=k, where d is the liquid layer thickness,
Dn the liquid crystal birefringence and k light wavelength in vacuum.
The analytical solution of this equation has the form [27]:
ꢂ
ꢃ
2
sin X
/
2
R ¼
?
C
ðsin 2b cos X ꢃ cos 2b sin XÞ
x
X
This solution allowed us to differentiate the orientation which have been promoted
by silicon wafer surface. For twisted or planar homogenous orientation, during cell
rotation the reflectance is function b. Once no reflectance modulation was observed
for the cell with homeotropicaly oriented reference substrate it led to conclusion that
the silicon plates promote homeotropic alignment. We have been able to obtain the
unidirectional planar orientation through standard rubbing process of the silicon
surface.
Conclusions
Thus, we have shown, that both homeotropic and planar alignment of liquid crystals
can be obtained on modified silicon or silicon=silica wafer surface. It can be reached
either by simple chemical bonding of both terminal and lateral low molecular
mesogenic moieties or by subsequent rubbing process. On the other hand, the liquid
crystalline polymer grown from Si=SiO2 surface generated planar arrangement of
side chain mesogens. The plot (Fig. 7) showing the growth of thickness of the
polymer layer against time demonstrated, that once PMDETA was used as a ligand
for Cu salts, at least 30 acrylate LC monomeric units were attached. Me6-TREN
ligand was less effective and it was found that only 20 monomeric units formed
the nano-layer. To our best knowledge it is the first case of the ATRP process
initiated from silicon surface, applied to a liquid crystalline monomer. This polymer-
ization method offers an unique opportunity of generating uniform macromolecular
chains, chemically linked to the surface and capable of transferring orientation onto
low molecular liquid crystals. The results of orientation studies were independently
confirmed by ellipsometric measurements. In all cases the detrmined thickness of
monolayers corresponded, within an experimental error, to the most extended
conformation of the respective liquid crystal moieties. The presented studies open
up a new way of creation semiconductor – liquid crystal structures, in which
photoinduced reorientation of liquid crystal layer can be controlled more efficiently,
compared to previously described system [13].
Acknowledgments
The authors wish to thank Ministry of Science and Higher Education for financing
this research within MSHE grant.
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