Liquid crystalline cyclic tetramethyltetrasiloxanes containing coumarin moieties

Article abstract of DOI:10.1039/b302857g

A series of new cyclic tetramethyltetrasiloxanes with coumarin units were synthesized. By a comparative study with their related vinyl terminated precursors, the liquid crystalline properties of the cyclic tetramethyltetrasiloxanes were investigated. The

Full text of DOI:10.1039/b302857g

View Article Online / Journal Homepage / Table of Contents for this issue
Liquid crystalline cyclic tetramethyltetrasiloxanes containing
coumarin moieties{
Yanqing Tian,*a{ Eiichi Akiyamab and Yu Nagasec
aSagami Chemical Research Center, 2743-1 Hayakawa, Ayase, Kanagawa, 252-1193, Japan
bTechnology Research Center, Ebara Research Co., Ltd., 4-2-1 Honfujisawa, Fujisawa,
Kanagawa, 251-8502, Japan
cDepartment of Applied Chemistry, School of Engineering, Tokai University, 1117
Kitakaname, Hiratsuka, Kanagawa, 259-1292, Japan
Received 12th March 2003, Accepted 11th April 2003
First published as an Advance Article on the web 28th April 2003
A series of new cyclic tetramethyltetrasiloxanes with coumarin units were synthesized. By a comparative
study with their related vinyl terminated precursors, the liquid crystalline properties of the cyclic
tetramethyltetrasiloxanes were investigated. The results show that the thermal properties of the cyclic
tetramethyltetrasiloxanes are stabilised through their increasing clearing temperatures and broadening
mesophases. The thermal properties of the cyclic tetramethyltetrasiloxanes show that introduction of a tail
group at the 3 position of the coumarin unit increases the thermal stability of the liquid crystalline order.
Increasing the length of the tail group causes the clearing temperatures of the cyclic tetramethyltetrasiloxanes to
decrease. The photochemical behavior of spin-coated films of the cyclic tetramethyltetrasiloxanes indicates the
photo-crosslinking of the coumarin units in the thin films.
cyclic tetramethyltetrasiloxanes with coumarin units and their
Introduction
vinyl terminated precursors through the change of the length of
the tail group at the 3 position of the coumarin ring. Typical
photo-chemical behavior of tetramethyltetrasiloxanes with
coumarin units was investigated by UV-vis absorption spectra
and fluorescence emission spectra with irradiation light of l w
300 nm.
Coumarin containing materials are very useful in many fields,
such as fluorescence materials and laser dyes,1 nonlinear optical
materials,2 photorefractive materials,3 photo-resists,4 inter-
mediates for medicine synthesis,5 and luminescent materials,6
etc. Therefore, the preparation of functional materials with
coumarin moieties has attracted much attention.7–9 Recently, it
has been also found that coumarin containing polymers can be
used as a new kind of photo-alignment layer for liquid crystals,
where the photodimerization of the coumarin unit occurs by
exposure to UV light and the direction of orientation of the
liquid crystals can be adjusted by the polarization direction of
the irradiation light.10–14 Furthermore, a kind of liquid crystal
display model has been realized by dispersion of coumarin dyes
into nematic liquid crystals.15–17
Recently, the preparation of liquid crystalline cyclic tetra-
methyltetrasiloxanes, cyclic pentamethylpentasiloxanes and
other cyclic siloxanes has caused scientific interest, because
these materials exhibit properties intermediate between low
molecular weight and polymeric liquid crystals.18–23 Due to
their well-defined molecular weights and their low viscosity as
compared to polymers, the liquid crystalline cyclic siloxanes
exhibit fast electro-optical switching properties.18,23 Further-
more, cyclic siloxanes can form high quality films upon spin
coating or casting from a solution, which is important for
optical measurement by using a film. Therefore functional
cyclic siloxanes have potential applications as optical storage
and optical notch materials.24–26
Experimental
Characterization
1H NMR spectra were taken using a Bruker-250 Spectrometer
operating at 250 MHz with TMS internal standard as a
reference for chemical shifts. FT-IR spectra were recorded by
using a Jasco-FTIR-5300 Spectrophotometer. UV-Vis absorp-
tion spectra were recorded by a JASCO V-550 Spectro-
photometer. Steady state fluorescence spectra were measured
on Hitachi F-4010 Fluorescence Spectrometer. Polymer films
were spin coated from 2 wt% CHCl3 solutions onto quartz
plates. Photo-chemical reactions were carried out by using a
500 W high pressure mercury lamp with a cut filter to remove
light of l v 300 nm. The irradiation energy was about
10 mJ cm22 s21 at 365 nm. Thermal transitions were deter-
mined by using a Seiko Electronic DSC-20 with a SSC-580
thermal controller at a scanning rate of ¡10 uC min21. Liquid
crystalline textures were observed under a Nikon Microphot-
UFX polarized optical microscopy (POM) with a Mettler FP-
82 hot stage and a FP-80 central processor. X-Ray diffraction
studies were carried out on a MAC Science MPX3 X-ray
diffractometer equipped with a thermal controller Model 5310.
Our aim is to prepare and investigate new functional liquid
crystals with coumarin units which display both photo-
reactivity and fluorescence properties.27,28 In this paper, we
report the preparation and thermal properties of a series of new
Materials
Methylene chloride and toluene were distilled twice over
CaH2 for the removal of trace amounts of water. Methyl
4-hydroxybenzoate, 5-bromo-1-pentene, 2,4-dihydroxybenz-
aldehyde, dimethyl malonate, diethyl malonate, dipropyl
malonate, dibutyl malonate, 7-hydroxycoumarin, 2,4,6,8-tetra-
methylcyclotetrasiloxane, cis-bis(benzonitrile)dichloroplatinum,
{Electronic supplementary information (ESI) available: characterisa-
tion data; textures of CS1 and CS8; table of layer spacings. See http://
www.rsc.org/suppdata/jm/b3/b302857g/
{Present address: Department of Materials Science and Engineering,
University of Washington, Box: 352120, Seattle, WA 98195-2120,
USA.
DOI: 10.1039/b302857g
J. Mater. Chem., 2003, 13, 1253–1258
This journal is # The Royal Society of Chemistry 2003
1253

View Article Online
dicyclohexylcarbodiimide (DCC), and other chemicals were
commerciallyobtainedandwereusedwithoutfurtherpurification.
Synthesis of alkyl 7-hydroxycoumarin-3-carboxylates (4). All
derivatives were prepared similarly and one representative
preparation is given for methyl 7-hydroxycoumarin-3-carboxy-
late, n ~ 1.
Synthesis
The synthesis of the cyclic tetramethyltetrasiloxanes with
coumarin moieties is outlined in Scheme 1.
Methyl 7-hydroxycoumarin-3-carboxylate (4, n ~ 1). 2,4-
Dihydroxybenzaldehyde (4.0 g, 29.0 mmol), dimethyl malonate
(5.0 g, 37.9 mmol) and 0.7 ml of piperidine were dissolved into
25 ml of methanol and heated at 90 uC for 6 hours. After the
mixture was cooled down, the precipitate was filtered out to
afford a pale yellow solid. Yield: 2.3 g (36%). H-NMR (ppm,
DMSO): d ~ 3.81 (3H, t, OCH3), 6.73 (1H, d, phenyl), 6.85
(1H, m, phenyl), 7.71 (1H, d, phenyl), 8.72 (1H, s, -CHL).
Synthesis of 4-(pent-4-enoxy)benzoic acid (2). This was
prepared according to a known procedure.29 1H-NMR (ppm,
CDCl3): d ~ 1.85–2.06 (2H, m, CH2), 2.15–2.36 (2H, m, CH2),
3.97 (3H, s, CH3), 4.04 (2H, t, OCH2), 5.05 (2H, m, CH2L), 5.86
(1H, m, LCH-), 6.92 (2H, d, phenyl), 8.05 (2H, d, phenyl).
1
Synthesis of vinyl terminated molecules with coumarin units (V)
Synthesis of dialkyl malonates (3). All derivatives were
prepared similarly and one typical example is given for dipentyl
malonate (3, n ~ 5).
All derivatives were prepared similarly and one representative
preparation is given for 7-[4-(pent-4-enoxy)benzoyloxy]cou-
marin (V0).
Dipentyl malonate (3, n ~ 5). A mixture of malonic acid
(5.02 g, 50 mmol), pentanol (10.0 g, 114 mmol) and p-tolu-
enesulfonic acid (80 mg) in benzene (100 ml) was heated at
reflux azeotropically for 3 hours. Then the mixture was cooled
down, washed with 5% sodium bicarbonate, and dried over
sodium sulfate. After vacuum distillation, 11.0 g of a colorless
liquid was obtained. Yield: 92%. b.p.: 132–135 uC/6 mmHg.
1H-NMR (ppm, CDCl3): d ~ 0.91 (6H, t, CH3), 1.13–1.45 (8H,
m, -CH2CH2-), 1.5–1.76 (4H, m, -CH2-), 3.37 (2H, s,
OOCCH2COO), 4.14 (4H, t, OCH2).
7-[4-(Pent-4-enoxy)benzoyloxy]coumarin (V0). A mixture of
compound 2 (2.6 g, 12.6 mmol), 7-hydroxycoumarin (2.1 g,
12.9 mmol), 4-(N,N-dimethylamino)pyridine (100 mg) and
DCC (3.0 g, 14.6 mmol) were suspended into 100 ml of dry
methylene chloride and stirred at room temperature overnight.
The insoluble solid was removed through filtration, the
solution was washed with 1 M aq. HCl, 5% aq. NaHCO3
and brine, dried over anhydrous Na2SO4. After the solvent was
removed under reduced pressure, the crude solid was
Scheme 1
1254
J. Mater. Chem., 2003, 13, 1253–1258

View Article Online
recrystallized twice from ethanol to afford 2.60 g of a white
solid in a yield of 57%. 1H-NMR (ppm, CDCl3): d ~ 1.86–2.08
(2H, m, CH2), 2.10–2.36 (2H, m, CH2), 4.07 (2H, t, OCH2),
5.05( 2H, m, CH2L), 5.85 (1H, m, LCH-), 6.41 (1H, d, phenyl),
6.98 (2H, d, phenyl), 7.15–7.24 (2H, m, phenyl), 7.51 (1H, d,
phenyl), 7.74 (1H, d, phenyl), 8.15 (2H, d, phenyl). Elemental
analysis: Calc. for C21H18O5: C, 71.98; H, 5.17%. Found: C,
72.01; H, 5.24%.
toluene. Some papers have already reported that the cyclic
tetramethyltetrasiloxanes could be prepared by hydrosilylation
of olefin terminated mesogens with 2,4,6,8-tetramethylcyclo-
tetrasiloxane by using dichloro(dicyclopentadienyl)platinum as
a catalyst.18–23 In this paper, we tried to use cis-bis(benzo-
nitrile)dichloroplatinum from our stores as the catalyst (a mole
ratio of about 1 : 103 of Pt : each monomer). The progress of
the reactions was monitored by FTIR spectroscopy, as a
decrease in the intensity of the Si–H band at about 2160 cm21
was easily detectable as hydrosilylation occurred. The reactions
were typically completed within 16 hours. Upon completion,
the mixture was purified by a column chromatography with a
mixture of chloroform and ethyl acetate as eluent to remove the
catalyst. Then the obtained product was washed twice with hot
ethanol to remove the excess unreacted vinyl terminated
compounds. The purity of the cyclic tetramethyltetrasiloxanes
with coumarin units was proven by thin layer chromatography
(one spot under 254 and 365 nm), FTIR (the disappearance
of the fingerprint absorption of Si–H bonds in the region of
2160 cm21), 1H-NMR (the disappearance of the chemical shifts
of the vinyl group at about 5.1 and 5.8 ppm, and the
appearance of the CH3–Si proton as a large broad singlet at
0.11 ppm) and element analysis.
Synthesis of cyclic tetramethyltetrasiloxanes with coumarin
moieties (CS)
All derivatives were prepared similarly and one representative
preparation of CS0 is given.
CS0. 7-[4-(Pent-4-enoxy)benzoyloxy]coumarin (V0, 0.72 g,
2 mmol) and cis-bis(benzonitrile)dichloroplatinum (1.3 mg,
0.0028 mmol) were evacuated for 1 h in a dried 25 ml round
bottom flask then filled with argon. 2,4,6,8-Tetramethylcyclo-
tetrasiloxane (100 mg, 0.42 mmol) in dry toluene (5 ml) was
added into the flask then heated at 90 uC with stirring for
16 hours. The reaction mixture was cooled down to room
temperature and passed through a silica column chromato-
graph with chloroform/ethyl acetate (3 : 1 by volume) as the
eluent to remove the catalyst. The obtained crude product was
washed twice with boiling ethanol in order to remove trace
amounts of unreacted vinyl terminal compound, and then the
residue was dissolved into a small amount of chloroform and
precipitated into methanol, and dried under vacuum to afford a
white powder. Yield: 200 mg (29.3%). 1H-NMR (ppm, CDCl3):
d ~ 0.11 (12H, br s, SiCH3), 0.45–0.78 (8H, br m, SiCH2-),
1.49–1.83 (24H, m, -CH2CH2CH2-), 4.02 (8H, t, OCH2), 6.41
(4H, d, phenyl), 6.94 (8H, d, phenyl), 7.14–7.25 (8H, m,
phenyl), 7.50 (4H, d, phenyl), 7.68 (4H, d, phenyl), 8.13 (2H, d,
phenyl). IR (KBr disk, cm21): 1732, 1704 (carbonyl, esters).
Elemental analysis: Calc. for C88H88O24Si4: C, 64.36; H, 5.40%.
Found: C, 64.64; H, 5.24%.
Thermal properties of the vinyl terminated compounds (V0–V8)
Table 1 lists the thermal transitions and the enthalpies of
V0–V8 from the combined results of DSC, POM and X-ray
diffraction. The compounds without a tail group at the
coumarin ring (V0) and with a short tail group at the 3
position of the coumarin rings (V1 and V2) do not show liquid
crystalline properties. The compound V3 is a monotropic liquid
crystalline compound. Using polarized optical microscopy,
schlieren texture was observed within a narrow temperature
range from 89 uC to 84 uC during the cooling process, then the
schlieren texture changed to a focal-conic texture as shown in
Fig. 1, confirming its monotropic nematic and smectic A
phases, respectively. V4, V5 and V8 are enantiotropic liquid
crystals with a smectic A phase which show focal-conic texture
under POM. However, interestingly, V6 and V7 only exhibit a
monotropic liquid crystalline phase (smectic A phase).
Results and discussion
Synthesis
When the alkyl 7-hydroxycoumarin-3-carboxylates (4, n ~ 1,
3–8) were prepared by the condensation of 2,4-dihydroxybenz-
aldehyde and dialkyl malonates by using ethanol as solvent, it
was found that the obtained product was a mixture of the
target compound and ethyl 7-hydroxycoumarin-3-carboxylate
(4, n ~ 2), and it was difficult to separate the two compounds.
Therefore, the condensations were carried out in the parent
alcohols of the dialkyl malonates at 90 uC for 6 hours with a
small amount of piperidine as catalyst. Under such conditions,
the pure title compounds were obtained easily.
Thermal properties of the cyclic tetramethyltetracycloxanes
(CS0–CS8)
CS0 is only a crystalline compound, whereas each of the cyclic
tetramethyltetracycloxanes with a tail group at the 3 position of
the coumarin ring (i.e. CS1–CS8) exhibits mesomorphic
properties. Fig. 2 presents the typical DSC curves of CS3
and CS8. The DSC scans of CS1, CS2 and CS3 show
transitions corresponding to crystal-to-mesophase (Tm) and
transitions corresponding to mesophase-to-isotropic liquid
state (Ti) during the heating process. The transitions corre-
sponding to mesophase-to-crystalline phase (Tc) can be
observed clearly during the cooling scan though a supra-
cooling effect was observed. The DSC curves of CS4–CS8
The cyclic tetramethyltetrasiloxanes (CS0–CS8) were
synthesized by hydrosilylation of the vinyl terminated com-
pounds (V0–V8) with 2,4,6,8-tetramethylcyclotetrasiloxane
(about 0.20 mole ratio as compared to each monomer) in
Table 1 Phase assignments, transition temperatures/uC, and transition enthalpies/J g21 of V0–V8 as determined by POM, DSC and X-ray
diffractiona
Compound
m, n
2nd heating process
1st cooling process
V0
V1
V2
V3
V4
V5
V6
V7
V8
0, —
1, 1
1, 2
1, 3
1, 4
1, 5
1, 6
1, 7
1, 8
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
113/93.8
140/132.7
123/121.4
109/101.9
82/3.7
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
93/288.6
112/2125
100/2108.6
59/257.6
44/224.6
43/256.8
68/288.5
77/2100.0
71/287.3
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
88/20.6
N
84/24.2
80/29.2
83/27.0
84/27.9
88/27.6
93/28.8
SmA
SmA
SmA
SmA
SmA
SmA
78/66.1
81/66.1
SmA
SmA
85/4.1
93/102.9
101/117.0
95/8.1
86/87.0
SmA
aCr: crystalline state; N: nematic phase; SmA: smectic A phase; I: isotropic state.
J. Mater. Chem., 2003, 13, 1253–1258
1255

View Article Online
heating, the glass transition and a cold crystallization followed
by a crystal-to-mesophase (Tm) transition and an isotropic
transition (Ti) were observed.
Identification of mesophases and transition temperatures
was conducted by examining the texture of a thin liquid crystal
film sandwiched between untreated glass substrates. Under
POM, focal-conic textures were observed easily on cooling
from the amorphous liquid into the liquid-crystalline state,
which classified the liquid crystalline phase of each compound
as a smectic A (SmA) phase. Mechanical shearing of the
specimens showed that the mesophase flowed easily and had a
relative low viscosity. And the fast formation of the focal-conic
textures after cooling from the isotropic liquid indicated that
the cyclic tetramethyltetrasiloxanes had properties more in
keeping with low molar mass materials than polymers. In the
cases of CS1–CS3, finger-printed stripes based on the focal-
conic textures were observed on cooling from their SmA
phases, which indicates their low temperature phases are
crystalline phases (Cr). In the cases of CS4–CS8, the formed
focal-conic textures in their SmA phases did not exhibit any
obvious change on further cooling and on the subsequent
heating process.
Fig. 3 gives the X-ray diffractograms of CS1–CS8 in their
smectic A phases. A sharp first diffraction peak (d001) in the
small angle region corresponding to a layered structure and a
broad halo reflection in the wide angle region around 2h # 20u
relative to the intermolecular distance were observed in their
mesophases, indicating that their mesophases are smectic
phases. Interestingly, the d-spacings of the cyclic tetramethyl-
tetrasiloxanes with the coumarin units decrease with increasing
tail group length. For CS4–CS8, the X-ray diffraction patterns
did not show any obvious changes after cooling from their
SmA phases to room temperature. And the patterns did not
change even after the samples were aged at room temperature
for more than one week, indicating that the smectic phase is
frozen at room temperature. This observation is in good
accordance with the DSC measurement.
Fig. 1 Typical textures of V3 in its liquid crystalline states on cooling.
a) Schlieren texture of nematic phase at 88 uC; b) focal-conic texture of
smectic A phase at 74 uC (magnification: 4006).
Fig. 2 The DSC curves of CS3 and CS8 for the heating and cooling
processes (¡10 uC min21). a) First heating of CS3, b) first cooling of
CS3, c) second heating of CS3; d) first heating of CS8, e) first cooling of
CS8, f) second heating of CS8.
reveal reversible transitions corresponding to Ti at high
temperatures during both heating and cooling cycles. However,
the crystallization transition is not observed during the cooling
cycles of the DSC runs, suggesting that these compounds freeze
their liquid crystalline states on cooling. Such a phenomenon
was observed by using several T-shaped dimesogenic com-
pounds.30 On subsequent heating of CS4–CS7, only Ti
was observed. In the case of CS8, a glass transition was
detected during the first cooling cycle. During the second
Fig. 3 The X-ray diffractograms of CS1–CS8 in their smectic A phases
on heating. a) CS1 at 185 uC; b) CS2 at 180 uC; c) CS3 at 170 uC; d) CS4
at 160 uC; e) CS5 at 145 uC; f) CS6 at 135 uC; g) CS7 at 128 uC; h) CS8
at 120 uC. The intensities of the first diffraction peaks of CS6, CS7 and
CS8 are much stronger (more than 20 times) than the other
compounds.
1256
J. Mater. Chem., 2003, 13, 1253–1258

View Article Online
Table 2 Phase assignments, transition temperatures/uC, and transition enthalpies/J g21 of CS0–CS8 as determined by POM, DSC and X-ray
diffractiona
Siloxanes
m, n
2nd heating process
1st cooling process
I
CS0
CS1
CS2
CS3
CS4
CS5
CS6
CS7
CS8
0, —
1, 1
1, 2
1, 3
1, 4
1, 5
1, 6
1, 7
1, 8
Cr
132/38.0
164/21.9
141/10.5
102/11.3
I
I
I
I
I
I
I
I
I
115/238.0
122/228.7
88/220.0
43/210.5
Cr
Cr
Cr
Cr
Cr1
Cr1
Cr1
153/6.7
116/7.0
75/5.6
Cr2
Cr2
Cr2
SmA
SmA
SmA
SmA
SmA
SmA
SmA
SmA
198/9.9
189/10.1
182/9.5
166/9.6
156/11.8
144/10.5
138/11.5
127/9.0
I
I
I
I
I
I
I
I
194/28.8
184/210.6
171/28.8
162/29.7
152/210.0
140/29.8
134/212.1
123/29.4
SmA
SmA
SmA
SmA
SmA
SmA
SmA
SmA
g
y15 uC
Cr
61/16.2
y10 uC
g
aCr1, Cr2, Cr3: crystalline state; SmA: smectic A phase; I: isotropic state; g: glassy state.
Table 2 gives the thermal properties of the cyclic tetra-
methyltetrasiloxanes (CS0–CS8) determined by the results of
DSC, POM and X-ray diffractometry. It can be concluded that
the introduction of a tail group at the 3 position of the
coumarin unit increase the thermal stability of liquid crystalline
order. Comparing the thermal properties of CS1–CS8, it is
found that the transition temperature at Ti decreases along
with the increasing length of the tail group. And comparing the
thermal properties of the vinyl terminated compounds with
their related cyclic tetramethyltetrasiloxanes as shown in
Table 1 and Table 2, it is seen that the cyclic tetramethyl-
tetrasiloxane leads to a stabilization of thermal properties
through the increased clearing temperature and mesophase
broadening.
SmA phases. A possible illustration of the influence of the
length of the tail group by using the ‘‘compact’’ structure of the
stereoisomers31,32 of the cyclic tetramethyltetrasiloxanes is
given in Fig. 4.
Photochemistry
Unlike their related vinyl terminated compounds, all of the
cyclic tetramethyltetrasiloxanes can form uniform and trans-
parent films. Fig. 5 shows the change of UV-Vis and
fluorescence emission spectra of CS8 film as a typical example
of the photochemical behavior upon irradiation at l w 300 nm.
During the irradiation, a continuous decrease of the absorption
at 340 nm (the p–p* transition of the coumarin unit) and
increase of the absorption band at 263 nm were observed with
an isosbestic point at 276 nm. This photo-reaction can be
explained by the consumption of the coumarin unit and the
formation of the dimer through the [2 1 2] cyclization of the
double bond, which has been confirmed by using coumarin
model compounds and photo-cleavage of the photo-irradiated
side-chain polymers with coumarin units by irradiation with
254 nm light.27 A schematic illustration of the photo-
dimerization is described in Fig. 5c.The photo-dimerization
can also be confirmed by the decreasing of the fluorescence
intensity of the coumarin chromophores (Fig. 5b).
As the photo-dimerization occurs in the cyclic tetramethyl-
tetrasiloxanes the photo-dimerization results in the cross-
linking within the film. The as-cast films are soluble in
chloroform before photo-irradiation. After 32 minutes
photo-irradiation, they became insoluble in chloroform.
Focal-conic textures could not be observed after irradiation,
however, birefringence from room temperature to 200 uC was
observed under POM, suggesting formed dimers in the
oligomers improve the thermal stability.
Layer structures investigated by X-ray diffraction
The layer spacings (d) of the vinyl terminated compounds
determined by X-ray diffraction increase slightly with increas-
ing length of the tail group. The ratios of the measured layer
spacing (d) to the calculated extended molecular length (L) of
V3–V8 are in the range of 0.9–1.0, which suggests their
monolayer arrangement styles. However, the d/L ratios of the
cyclic tetramethyltetrasiloxanes of CS1–CS8 are affected
significantly by the length of their tail groups (detailed data
are shown in the ESI{). For CS1 and CS2, the d/L ratois are
larger than 1.10, implying that each of them exhibits an
interdigitated bilayer smectic A phase. The ratios of the four
cyclic tetramethyltetrasiloxanes, CS3–CS6, are near 1.0,
suggesting their discrete monolayer smectic A phases. The d/L
ratios of CS7 and CS8 are smaller than 0.90, indicating some
interdigitation in their monolayer smectic A phases. Therefore,
we can conclude that the change of the length of the tail group
affects not only the thermal properties of the cyclic tetra-
methyltetrasiloxanes but also the arrangement styles in their
Fig. 4 Schematic representation of possible molecular arrangements resulting in different layer spacings. A) Interdigitated bilayer arrangement when
the tail group is short (such as CS1). In this case, the interdigitation is possibly due to the strong dipole–dipole interaction between the polar
coumarin mesogens. B) Discrete monolayer arrangement with suitable length of tail group (such as CS4). C) Monolayer smectic A phase with
interdigitation through a long tail group (such as CS8). In this case, the interdigitation is possibly due to the nonpolar interactions between the tail
groups. For simplification, we chose the compact structure32 for the illustration of the possible alignment of the SmA phase.
J. Mater. Chem., 2003, 13, 1253–1258
1257

View Article Online
Fig. 5 Changes in the UV-Vis absorption spectra (A) and fluorescence spectra (B) of films of CS8 upon irradiation with light of wavelength longer
than 300 nm. The possible mechanism of the photo-dimerization is given in (C). The fluorescence emission spectra were excited at 334 nm.
10 M. Schadt, H. Seiberle and A. Schuster, Nature, 1996, 381, 212.
11 M. Obi, S. Morino and K. Ichimura, Macromol. Rapid Commun.,
Conclusion
1998, 19, 643.
12 N. Kawatsuki, K. Goto and T. Yamamoto, Liq. Cryst., 2001, 28,
1171.
13 J. S. Dave, M. R. Menon and P. R. Patel, Liq. Cryst., 2002, 29,
In this paper, a series of novel cyclic tetramethyltetrasiloxanes
with coumarin units were prepared and investigated. The
results show that the lengths of the tail groups affect not only
their thermal properties but also the arrangement styles in their
smectic A liquid crystalline phases. The photo-chemical inves-
tigation shows that they could be photo-dimerized through
irradiation with light of wavelength longer than 300 nm.
543.
14 J. P. Orr, O. Mary, D. W. Lee, H. Paul, K. Stephen, M. Owen and
G. James, Chem. Mater., 2001, 13, 694.
15 H. Iwanaga, K. Naito, K. Sunohara and M. Okajima, Bull. Chem.
Soc. Jpn., 1998, 71, 1719.
16 G. Saielli, A. Polimeno, P. L. Nordio, P. Bartolini, M. Ricci and
R. Righini, J. Chem. Soc., Faraday Trans., 1998, 94, 121.
17 C. Martin, V. Monika and G. Martin, Chem. Phys. Lett., 2002,
355, 214.
18 H. Poths, E. Wischerhoff, R. Zental, A. Schonfeld, G. Henn and
F. Kremer, Liq. Cryst., 1995, 18, 811.
19 R. D. C. Richards, W. D. Hawthorne, J. S. Hill, M. S. White,
D. Lacey, J. A. Semlyn, G. W. Gray and T. C. Kendrick, J. Chem.
Soc., Chem. Commun., 1990, 95.
Acknowledgements
We would like to thank Prof. Tomiki Ikeda, Dr. Akihiko
Kanazawa, and Dr. Osamu Tsutsumi in Tokyo Institute of
Technology for their assistance in the UV-vis and fluorescence
measurements.
20 C. McHugh, D. W. Tomlin and T. J. Bunning, Macromol. Chem.
Phys., 1997, 198, 2387.
References
21 G. H. Mehl and J. W. Goodby, Angew. Chem., Int. Ed. Engl., 1996,
35, 2641.
22 J. W. Goodby, G. H. Mehl, I. M. Saez, R. P. Tuffin, G. Mackenzie,
R. Auzely-Velty, T. Benvegnu and D. Plusquellec, Chem.
Commun., 1998, 2057.
23 D. R. Medeiros, M. A. Hale, R. J. P. Hung, J. K. Leitko and
C. G. Willson, J. Mater. Chem., 1999, 9, 1453.
24 M. L. Tsai, S. H. Chen and S. D. Jacobs, Appl. Phys. Lett., 1989,
54, 2395.
25 R. Ortler, C. Brauchle, A. Miller and G. Riepl, Makromol. Chem.
Rapid Commun., 1989, 10, 5.
1
2
3
G. Jones II, W. R. Jackson, C. Y. Choi and W. R. Bergmark,
J. Phys. Chem., 1985, 89, 294.
G. A. Lindsay, J. M. Hoover, D. R. Yankelevich and A. Knoesen,
Polym. Prepr., 1993, 34, 771.
S. M. Silence, J. C. Scott, F. Hache, E. Ginsburg, R. Jenkener,
R. Miller, R. Twieg and W. E. Moerner, J. Opt. Soc. Am. B, 1993,
10, 2306.
S. Matsumoto, Y. Fukumuro, T. Maebashi and S. Kawakami,
Jpn. Kokai Tokkyo Koho JP 09297398, 1997; S. Matsumoto,
Y. Fukumuro, T. Maebashi and S. Kawakami, Chem. Abstr.,
1999, 128, 82147.
4
26 J. Pinsl, C. Brauchle and F. H. Kreuzer, J. Mol. Electr., 1987, 3, 9.
27 Y. Q. Tian, E. Akiyama, Y. Nagase, A. Kanazawa, O. Tsutsumi
and T. Ikeda, Macromol. Chem. Phys., 2000, 201, 1640.
28 Y. Q. Tian, E. Akiyama, Y. Nagase, A. Kanazawa, O. Tsutsumi
and T. Ikeda, Trans. Mater. Res. Soc. Jpn., 2000, 25, 1091.
29 H. Ringsdorf and A. Schneller, Makromol. Chem. Rapid Commun.,
1982, 3, 557.
30 J. W. Lee, X. L. Piao, Y. K. Yun, J. I. Jin, Y. S. Kang and
W. C. Zin, Liq. Cryst., 1999, 26, 1671.
31 M. S. Beevers and J. A. Semlyen, Polymer, 1971, 12, 378.
32 T. E. Mann, J. Haley and D. Lacey, J. Mater. Chem., 1999, 9,
353.
5
6
D. I. Brahmbhatt, S. Singh and K. C. Patel, Int. J. Polym. Mater.,
1997, 35, 145.
M. Uchida, M. Uchino, J. Izumisawa and K. Furukawa, Jpn.
Kokai Tokkyo Koho, JP 06298758, 1994; M. Uchida, M. Uchino,
J. Izumisawa and K. Furukawa, Chem. Abstr., 1997, 122,
105666.
S. Fomine, E. Rivera, L. Fomina, A. Ortiz and T. Ogawa,
Polymer, 1998, 39, 3551 and references therein.
Y. Chen and R. T. Hong, J. Polym. Sci. A. Polym. Chem., 1997,
35, 2999 and references therein.
Y. Chujo, K. Sada and T. Saegusa, Macromolecules, 1990, 23,
2693.
7
8
9
1258
J. Mater. Chem., 2003, 13, 1253–1258