Synthesis and investigation of photophysical and photochemical properties of new side-group liquid crystalline polymers containing coumarin moieties

Article abstract of DOI:10.1039/b408554j

Four coumarin (2-oxo-2H-1-benzopyran) containing liquid crystalline polymethacrylates possessing the same monomer formula but different linkage directions and/or construction directions of their mesogens into the same main-chain were prepared and investigated. Each of the four polymers exhibits a smectic A liquid crystalline phase. The change of construction and/or linkage directions significantly affected their liquid crystalline properties. Especially, the change of the construction directions of the mesogens caused different photophysical and photochemical properties.

Full text of DOI:10.1039/b408554j

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A R T I C L E
Synthesis and investigation of photophysical and photochemical
properties of new side-group liquid crystalline polymers
containing coumarin moieties
Yanqing Tian,*a{ Eiichi Akiyama,b Yu Nagase,c Akihiko Kanazawa,d
Osamu Tsutsumib and Tomiki Ikedad
aSagami Chemical Research Center, 2743-1 Hayakawa, Ayase, Kanagawa 52-1193,
Japan. E-mail: yqtian@u.washington.edu; Fax: 11 206 543 3100
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
dChemical Resource Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8503, Japan
Received 7th June 2004, Accepted 15th September 2004
First published as an Advance Article on the web 5th October 2004
Four coumarin (2-oxo-2H-1-benzopyran) containing liquid crystalline polymethacrylates possessing the same
monomer formula but different linkage directions and/or construction directions of their mesogens into the
same main-chain were prepared and investigated. Each of the four polymers exhibits a smectic A liquid
crystalline phase. The change of construction and/or linkage directions significantly affected their liquid
crystalline properties. Especially, the change of the construction directions of the mesogens caused different
photophysical and photochemical properties.
Introduction
Coumarin (2H-1-benzopyran-2-one) -containing materials are
very useful in many fields, such as fluorescent materials and
laser dyes, photo-refractive materials, nonlinear optical
materials, light harvesting materials, alignment layers for
liquid crystals, etc. because of its fluorescence and photo-
dimerization properties.1 Besides the above studies, we have
been studying coumarin containing liquid crystalline polymers
as new functional materials, and investigating the influence of
polymer structures on liquid crystalline properties, photophy-
sical and photochemical properties. We have reported the
controlling of liquid crystalline polymers with coumarin units
by using different molecular structures, polymer/oligomer
backbones and nanostructured block copolymers.2–5 Results
showed the lengths of tail groups and spacers, and even
positions of tail groups on the coumarin units affected liquid
Fig. 1 Chemical structures of the polymers. P1 and P2, or P3 and P4
crystalline properties significantly.
exhibit same mesogen constructions but opposite linkage directions of
mesogens to the main chain. P1 (P2) and P3 (P4) have different
mesogens constructions.
This paper deals with the syntheses and properties of four
novel side-group polymers containing coumarin moieties as
shown in Fig. 1. The mesogenic components of the polymers
were constructed from 4-hydroxy-benzoic acid and 7-hydroxy-
coumarin-3-carboxylic acid, each of which has two reactive
groups, i.e., a hydroxyl and a carboxylic group. Considering
the connection directions of the mesogens into the polymetha-
crylate main chain, we prepared a total of four polymers with
the same spacers and tail groups as shown in Fig. 1. As a result,
these polymers have the same monomer formula. However the
construction and/or linkage directions of mesogens are
opposite each other. Therefore, the four polymers are suitable
for clarifying the influence of the construction and linkage
directions of mesogens on their thermal properties. This study
is quite different from the traditional studies of the influence of
lengths of spacers and/or tail groups on the liquid crystalline
properties. Such a study might be profitable for much more
detailed understanding of the influence of molecular structures
on the liquid crystalline properties, and will open a new way for
the design and investigation of liquid crystals.
On the other hand, the liquid crystalline state provides a
wealth of possibilities for the assembly of molecules with
designed electronic properties into organized structures,
showing novel bulk properties. The molecular interaction
between mesogenic moieties plays an important role in
determining their physical properties. Studies of the photo-
physical properties through suitable techniques (UV–vis and/or
fluorescence spectra) together with the classic NMR, and/or
X-ray structural analyses6 are of particular importance for the
understanding of the dynamics and organizational phenom-
enon of the liquid crystalline polymers, and also for the
{ Present address: Department of Materials Science and Engineering,
University of Washington, Box: 352120, Seattle, WA 98195-2120,
USA, E-mail: yqtian@u.washington.edu, Fax: 11 206 543 3100
3 5 2 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 3 5 2 4 – 3 5 3 1
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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prediction and design of novel photonic and electronic
materials. On the other hand, the coumarins are well known
to undergo the [2 1 2] photodimerization through either singlet
or triplet excited states, which are strongly affected by not only
the molecular structures of the coumarins but also their
alignments.1 The combined studies of the photophysical and
photochemical properties of the liquid crystalline coumarin
polymers should provide the important information of the
alignment styles and interactions of the coumarin containing
mesogens and their photochemical processes and mechanisms
in their different thermal states. Therefore, besides the
syntheses and liquid crystalline properties of the coumarin
containing side-group polymers, the photophysical and photo-
chemical properties of the four novel polymers were also
compared and investigated.
carried out to determine the number-average (Mn) and weight-
average (Mw) molecular weights with a Tosoh HLC-802A
instrument by using THF as eluent, equipped with four
columns of TSK gels G5000H6, G4000H6, G3000H6 and
G2000H6. Standard polystyrenes were used to calibrate the
molecular weights. The mesophase 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 patterns were recorded on a MAC
Science MXP3 X-ray diffractometer, equipped with a thermal
controller Model 5301.
Materials
CH2Cl2 was distilled over CaH2 for the removal of trace
amounts of water. 11-Bromo-1-undecanol, 2,4-dihydroxyben-
zaldehyde, 4-hydroxybenzoic acid, methyl 4-hydroxybenzoate,
(S)-2-methyl-1-butanol, dicyclohexylcarbodiimide (DCC), etc.
were commercial products and were used without further
purification. Ethyl 7-hydroxy-coumarin-3-carboxylate,7 11-
hydroxyundecyl methacrylate,2 4-(11-hydroxyundecyloxy)ben-
zoic acid (3),8 4-(11-methacryloyloxy- undecyloxy)benzoic acid
(4),8 4-(S)-2-methylbutoxy)benzoic acid (8)9 and (S)-2-methyl-
butyl 4-hydroxybenzoate (16)10 were prepared according to the
literature.
Experimental
Characterization
1H–NMR spectra were obtained with a Bruker-250 Spectro-
meter operating at 250 MHz with TMS as an internal standard
for chemical shifts. FT-IR spectra were recorded on a Jasco-
FTIR Spectrophotometer. UV–vis absorption spectra were
recorded on a JASCO V-550 Spectrophotometer. Steady state
fluorescence spectra were measured by a Hitachi F-4010
Fluorescence Spectrometer. Polymer films for the UV–vis
and fluorescence spectroscopies were prepared by spin-coating
on to quartz glass substrates from their 2 wt.% of CHCl3
solutions. Photochemical reactions were carried out by using a
500 W high pressure mercury lamp with a cut filter to remove
the light of l v 300 nm. The irradiation energy was about
3 mJ cm22 s21 at 365 nm. Differential scanning calorimetry
(DSC) was conducted with a Seiko Electronic DSC-20 with
SSC-580 thermal controller at heating and cooling rates of
10 uC min21. Gel permeation chromatography (GPC) was
Synthesis of monomers
The synthetic routes of the monomers, M1, M2, M3 and M4,
are shown in Scheme 1, 2, 3 and 4. The typical preparative
procedures of the monomer M1 including its intermediates and
the characterization of the other monomers, M2, M3 and M4
are described as follows.
Di(S-2-methylbutyl) malonate (1). A mixture of malonic acid
(3.5 g, 33.6 mmol), (S)-2-methyl-1-butanol (6.5 g, 73.8 mmol)
and p-toluenesulfonic acid (40 mg) in benzene (140 mL) was
Scheme 1
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 3 5 2 4 – 3 5 3 1
3 5 2 5

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Scheme 2
refluxed azeotropically for 7 h. After it was cooled down, the
mixture was washed with 5 wt.% NaHCO3 aqua. and brine,
then dried. 6.8 g of a colorless liquid was obtained by
distillation. Yield: 94%. bp: 115–119 uC/5 mmHg. IR (neat,
cm21): 1740 (–COO–). 1H–NMR (CDCl3): d ~ 0.87–0.94
(12H, m), 1.2 and 1.4 (4H, m), 1.7 (2H, m), 3.39 (2H, s), 4.04
(4H, m).
2.0 g of M1 as a white solid was obtained after silica gel column
chromatography by using chloroform–ethyl acetate (20 : 1 by
volume) as eluent. Yield: 47%. IR (KBr disk, cm21): 2920, 2854
(aliphatic), 1755, 1737 and 1712 (–COO–), 1606 (phenyl).
[a]2D0 ~ 12.22 u L g21 dm21 (CHCl3, C ~ 1.35 g dL21).
1H–NMR (CDCl3): d ~ 0.93–1.86 (27H, m), 1.95 (3H, s), 4.06
and 4.16 (4H, t), 4.20 (2H, m), 5.55 and 6.10 (2H, s), 6.98 (2H,
d), 7.23 (2H, m), 7.66 (1H, d), 8.12 (2H, d), 8.52 (1H, s).
Elemental analysis: (C37H46O9) (634.8): Calcd. C 70.00, H 7.30;
Found C 69.64, H, 7.23.
(S)-2-Methylbutyl-7-hydroxy-2-oxo-2H-1-benzopyran-3-car-
boxylate (2). A mixture of 2,4-dihydroxybenzaldehyde (3.59 g,
26 mmol), 1 (6.86 g, 28 mmol) and piperidine (0.5 mL) in
ethanol (20 mL) was refluxed for 6 h. After the solvent was
removed under reduced pressure, the residue was purified by
silica gel column chromatography using hexane–ethyl acetate
(5 : 1 by volume) as eluent to afford 4.0 g of a white solid. Yield:
11-Methacryloyloxyundecyl-7-[4-(S-2-methylbutoxy)]benzoy-
loxy-2-oxo-2H-1-benzopyran-3-carboxylate (M2). IR (KBr
disk, cm21): 2962, 2920 and 2854 (aliphatic), 1759 and 1726
(–COO–), 1609 (phenyl). [a]2D0 ~ 14.47 u L g21 dm21 (CHCl3,
1
1
56%. H–NMR (CDCl3): d ~ 0.87–0.94 (6H, m), 1.2 and 1.4
(2H, m), 1.7 (1H, m), 4.12 (2H, m), 6.80 (1H, d), 6.93 (2H, m),
7.77 (1H, d), 8.62 (1H, s).
C ~ 4.02 g dL21). H–NMR (CDCl3): d ~ 0.95 (3H, t), 1.05
(3H, d), 1.30–1.93 (21H, m), 1.95 (3H, s), 3.86 (2H, m), 4.14 and
4.35 (4H, t), 5.54 and 6.09 (2H, s), 7.00 (2H, d), 7.23 (2H, m),
7.66 (1H, d), 8.13 (2H, d), 8.53 (1H, s). Elemental analysis:
(C37H46O9) (634.8): Calcd. C 70.00, H 7.30; Found C 69.74, H,
7.45.
(S)-2-Methylbutyl-7-[4-(11-methacryloyloxyundecyloxy)ben-
zoyloxy]-2-oxo-2H-1-benzopyran-3-carboxylate (M1). A mix-
ture of 2 (2.50 g, 8.3 mmol), 4 (2.50 g, 6.65 mmol), 4-N,
N-dimethylaminopyridine (100 mg) and dicyclohexylcarbodii-
mide (DCC, 2.5 g, 12 mmol) was stirred in CH2Cl2 (30 mL) at
room temperature for 12 h. After the insoluble solid was
removed by filtration, the solution was washed with 5% HCl
aqua., 5% NaHCO3 aqua. and brine, then dried over Na2SO4.
4-(11-Methacryloyloxyundecyloxycarbonyl)phenyl-7-[4-(S-2-
methylbutoxy)]-2-oxo-2H-1-benzopyran-3-carboxylate (M3). IR
(KBr disk, cm21): 2962, 2928 and 2854 (aliphatic), 1741 and 1716
(–COO–), 1609 (phenyl). [a]2D0 ~ 16.52 u L g21 dm21 (CHCl3,
C ~ 4.60 g dL21). 1H–NMR (CDCl3): d ~ 0.95 (3H, t), 1.07 (3H,
3 5 2 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 3 5 2 4 – 3 5 3 1

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Scheme 3
d), 1.30–1.94 (21H, m), 1.95 (3H, s), 3.89 (2H, m), 4.13 and 4.32
(4H, t), 5.54 and 6.09 (2H, s), 6.84 (1H, d), 6.94 (1H, m), 7.33 (2H,
m), 7.55 (1H, d), 8.11 (2H, d), 8.71 (1H, s). Elemental analysis:
(C37H46O9) (634.8): Calcd. C 70.00, H 7.30; Found C 69.78, H,
7.37.
P2: Yield: 50%. [a]2D0 ~ 14.66 u L g21 dm21 (CHCl3, C ~
0.60 g dL21). Mn ~ 3.85 6 104, Mw/Mn ~1.42. 1H–NMR
(CDCl3): d ~ 0.91–1.92 (32H, m), 3.79–3.90 (4H, m), 4.30 (2H,
t), 6.93 (2H, d), 7.20 (2H, m), 7.66 (1H, d), 8.05 (2H, d), 8.49
(1H, s).
P3: Yield: 80%. [a]2D0 ~ 17.34 u L g21 dm21 (CHCl3, C ~
1.66 g dL21). Mn ~ 4.60 6 104, Mw/Mn ~1.62. 1H–NMR
(CDCl3): d ~ 0.84–1.92 (32H, m,), 3.80–3.90 (4H, m), 4.20
(2H, t), 6.74 (1H, d), 6.80 (1H, m), 7.12 (2H, m), 7.45 (1H, d),
7.98 (2H, d), 8.60 (1H, s).
P4: Yield: 73%. [a]2D0 ~ 12.39 u L g21 dm21 (CHCl3, C ~
1.50 g dL21). Mn ~ 3.57 6 104, Mw/Mn ~1.57. 1H–NMR
(CDCl3): d ~ 0.83–1.75 (32H, m), 3.94–4.20 (6H, m), 6.65 (1H,
d), 6.80 (1H, m), 7.17 (2H, m), 7.44 (1H, d), 7.98 (2H, d), 8.58
(1H, s).
4-(S-2-Methylbutoxycarbonyl)phenyl-7-(11-methacryloylox-
yundecyloxy)-2-oxo-2H-1-benzopyran-3-carboxylate (M4). IR
(KBr disk, cm21): 2926 and 2854 (aliphatic), 1741 and 1716
(–COO–), 1604 (phenyl). [a]2D0 ~ 12.03 u L g21 dm21 (CHCl3,
1
C ~ 4.52 g dL21). H–NMR (CDCl3): d ~ 0.95 (3H, t), 1.07
(3H, d), 1.30–1.94 (21H, m), 1.95 (3H, s), 4.04 (2H, t), 4.14 (2H,
t), 4.17 (2H, m), 5.54 and 6.09 (2H, s), 6.84 (1H, d), 6.94 (1H,
m), 7.33 (2H, m), 7.55 (1H, d), 8.11 (2H, d), 8.71 (1H, s).
Elemental analysis: (C37H46O9) (634.8): Calcd. C 70.00, H 7.30;
Found C 69.83, H, 7.47.
Results and discussion
Polymerization
Thermal properties of polymers
0.5 g of each monomer, M1–M4, and azobis(isobutyronitrile)
(AIBN) were mixed in a polymerization tube. The ratio of the
Fig. 2 shows the DSC curves of the four polymers, P1–P4. In
the cases of P1 or P2, a glass transition (Tg) and a clearing
point (Ti) were clearly observed both on the heating and
cooling processes. Between the two transitions, fan-shaped
textures (Fig. 3a) were observed for P1 under POM, and
baˆtonnet textures were observed for P2, indicating that the
mesophase of each polymer was a smectic A (SmA) phase. A
glass transition, a melting point and a clearing point were
observed in the DSC curve of P3 on heating. On cooling, the
crystallization transition could also be observed clearly,
however, with a significant super-cooling effect. Under POM,
a baˆtonnet texture of P3 was observed on cooling as shown in
Fig. 3b, indicating a SmA phase of P3. A glass transition, a
initiator to each monomer, [I0][M0]21
, was 0.03. After
degassing the mixture, the tube was sealed and heated at 65–
70 uC for 18 h. The reaction mixture was then poured into an
excess amount of methanol to precipitate the polymer. The re-
precipitations were repeated twice from the chloroform
solution into methanol, and then the obtained polymers were
dried at 60 uC in vacuo.
P1: Yield: 70%. [a]2D0 ~ 12.62 u L g21 dm21 (CHCl3, C ~
1
1.60 g dL21). Mn ~ 4.04 6 104, Mw/Mn ~ 1.82. H–NMR
(CDCl3): d ~ 0.83–1.92 (32H, m), 3.88 (2H, t), 4.00–4.20 (4H,
m), 6.83 (2H, d), 7.12 (2H, m), 7.56 (1H, d), 7.97 (2H, d), 8.39
(1H, s).
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 3 5 2 4 – 3 5 3 1
3 5 2 7

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Scheme 4
pre-melt recrystallization, a melting point and a clearing point
were observed in the DSC curve of P4 on heating process,
whereas only the transition from isotropic phase to mesophase
and the glass transition were observed on cooling. This result
implies that both recrystallization and melting procedures of
P4 occur in the heating process. The similar thermal behavior
was also observed in the study of a side-group polymer with a
4-cyanobiphenyl mesogen.8 The optical texture of P4 was
almost the same as that of P3.
Fig. 4 illustrates typical X-ray diffraction patterns of these
polymers at their mesomorphic states together with those of P3
and P4 at room temperature. In the mesomorphic state of each
polymer, the smectic phase was identified from the sharp
diffraction peaks observed in the small angle region showing a
layer structure. In the case of P3 at room temperature, as two
diffusion peaks were also observed at the wide-angle region, it
would be a glassy state keeping the crystalline character.
However, no significant difference was seen between the X-ray
patterns of P4 at room temperature and the temperature range
of mesophase, suggesting that P4 be a glassy state keeping the
liquid crystalline properties at room temperature.
C* phases were not observed in the polymers though each
polymer has a chiral group, which would be due to the small
chiral tail group as compared with the large mesogen.
It was found that the difference of structures of side-groups
affected their thermal properties, especially the change of
construction directions of mesogens caused significant differ-
ence. Namely, P3 or P4 tends to exhibit a crystalline state and a
smectic A phase on heating, whereas P1 or P2 tends to form a
glassy phase and a SmA phase. It is also found that the Ti
transition temperature of P4 is 27 uC higher than P1, though
the two polymers have exactly the same tail groups and spacers.
The Ti transition temperature of P3 is 42 uC higher than P2.
We speculate that the dramatically different behaviors through
the changing of construction directions are due to the dipole
difference and the interactions between the mesogens. The
mesogenic interactions in P3 and P4 should be much stronger
than those in P1 and P2, which have been confirmed by the
photophysical studies.
Photophyscial properties
The phase behaviors of these polymers were summarized in
Table 1, which includes the extended molecular length (L) of
their monomers calculated using a MM2 force field method,
and the layer spacing (d) of polymers determined by X-ray
diffraction. The values of d/L of these polymers were near or
equal to 1.0, suggesting that each polymer exhibits a SmA
phase with a monolayer organization (SmA1 phase). Smectic
The linkage directions of the mesogens into the main chain
don’t affect the photophysical properties, resulting in P1 and
P2 having the same behaviors. Similar tendency was observed
between P3 and P4. However, the mesogen constructions affect
the photophysical properties significantly. Typical results of P1
and P4 were given in Fig. 5.
A solution of P1 exhibits two absorption maxima at 289 and
3 5 2 8
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Fig. 4 X-ray diffraction patterns of (a) P1 at 160 uC, (b) P2 at 95 uC,
(c) P3 at 140 uC, (d) P3 at r.t., (e) P4 at 190 uC, (f) P4 at r.t.
333 nm, whereas P4 exhibits only one absorption maximum at
358 nm. This difference can be attributed to the mesogens’
difference. The substituent at the 7 position of the coumarin
unit of P1 is benzoyloxy ester units, while that of P4 is
alkyloxy-ether units. The ether unit is a much stronger
donating group than the ester unit, therefore, the ether unit
has a stronger effect on the electron mobility of the coumarin
unit. As a result, the coumarin unit of P4 exhibits a stronger
electron resonance effect than P1, which caused the significant
red shift of the absorption spectrum of P4 as compared to P1.
And the enhanced electron resonance effects of the coumarin
unit in P4 resulted in the overlap of the transitions from SOMO
and HOMO, respectively, to LUMO.11–13
Fig. 2 DSC curves of the polymers on the second heating and the first
cooling scans (¡10 uC min21). (a) P1 on heating, (b) P1 on cooling, (c)
P2 on heating, (d) P2 on cooling, (e) P3 on heating, (f) P3 on cooling,
(g) P4 on heating, (h) P4 on cooling.
The absorption of P1 film is similar to that of its solution.
However, a blue shift is observed in the spectrum of P4 film as
compared to that of its solution, which can be attributed to
chromophore (mesogen) aggregation.14,15 The hypsochromism
of the P4 film implied that there was strong intermolecular
interactions among the chromophores. Further, the spectral
broadening and enhancement to the blue of the main band of
the P4 film as compared with its solution should be due to the
formation of parallel or nearly parallel H-aggregations of the
chromophores.15
Therefore, the above results show there are nearly no
considerable chromophore aggregations between the coumarin
units in P1 film; however there are strong chromophore
aggregations between the coumarin units in P4 film.
Photochemistry
The different mesogen constructions affected not only the
photophysical properties but also the photochemical properties
significantly. The comparison of the photochemical behaviors
of P1 and P4 thin films was illustrated in Fig. 6 with the
irradiation of l w 300 nm.
During the irradiation, continuous decrease of the absorp-
tion at 333 nm (the p–p* transition of the coumarin unit) and
increase of the absorption band at 263 nm were observed in P1
with an isobestic point at 276 nm. This photoreaction 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 the irradiation of
254 nm light.5 A schematic illustration of the photo-dimeriza-
tion is described in Fig. 6C. The photo-dimerization occurs in
the side-group of the coumarin polymer through both the
Fig. 3 Typical polarized optical micrographs of P1 and P3 (magni-
fication: 4006). (A) Broken fan-shaped texture of P1 at 160 uC, (B)
baˆtonnet texture of P3 at 140 uC.
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Table 1 Phase assignments, transition temperatures, and transition enthalpies (DH/J g21) at isotropic transition (Ti)
Code
Phase transition/uCa
DH at Ti
d/nmb
L/nmc
d/L
P1
P2
P3
P4
g 20 SmA 166 I (I 158 SmA 20 g)d
6.5 (26.6)d
4.8 (24.8)d
4.0 (24.8)d
8.9 (210.4)d
3.58
3.81
3.83
3.34
3.59
3.82
3.74
3.63
1.00
1.00
1.02
0.92
g 25 SmA 107 I (I 97 SmA 20 g)d
g 22 Cr 124 SmA 149 I (I 140 SmA 70 Cr 20 g)d
g 32 (83)e Cr113 SmA 193 I (I 187 SmA 30 g)d
a The transition temperatures were determined by DSC and the mesomorphic phases were determined by means of POM and X-ray diffracto-
metry. g: glassy state; SmA: smectic A phase; I: isotropic state; Cr: crystalline state. b The d-spacing, d, was determined by powder X-ray dif-
fraction in their SmA phases. c The extended molecular length of side-groups, L, which was calculated by MM2 force field method. d Cooling
process. e Recrystallization transition.
intramolecules and intermolecules. Especially, the photo-
dimerization through intermolecular bonds might result in
the crosslinking within the film. The as-cast films are soluble in
chloroform before photo-irradiation. After 32 min photo-
irradiation, they became insoluble. Focal-conic textures could
not be observed after irradiation, however, birefringence from
room temperature to 290 uC was observed under POM,
suggesting formed dimers in the polymer improve the thermal
stability.
For P4, the continuous decrease of the absorbance band at
340 nm (lmax) indicated the disappearance of the coumarin unit
due to the photoreactions. While the increase of the absorbance
band at 280 nm suggests the cyclobutane formation due to the
[2 1 2] cycloaddition reaction as known in the coumarin
system. The UV-irradiated films became insoluble, indicating a
significant amount of crosslinking, most probably via coumarin
dimerization. On the other hand, along with the irradiation, a
new absorption at longer wavelength centered around 455 nm
was observed. After photo-irradiation, the color of the film
changed to yellow. The above changes might be caused by the
photo-Fries rearrangement resulting in the formation of a
Fig. 5 UV–vis absorption spectra of polymers in their solution (5 6
1025 mol L21 in CH2Cl2) and films. (a) P1 in solution, (b) P1 film, (c)
P4 in solution, (d) P4 film.
Fig. 6 The photochemical behaviors of P1 (A) and P4 (B) films at room temperature. The light intensity at 365 nm was 3 mW cm22. The possible
mechanisms of the photochemical behaviors were given in C (dimerization) and D (rearrangement), respectively.
3 5 3 0
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hydroxycoumarin ketone as shown in Fig. 6D. Therefore, the
above results show that at least two kinds of photoreactions,
i.e. photo-Fries rearrangement and photo-dimerization,
occurred at such a condition. Several papers describe the
photo-Fries rearrangement and photo-dimerization by using
liquid crystalline poly-acrylcinnamates.14,16–19 However, it is
the first time to disclose this kind of photochemical behavior by
using liquid crystalline coumarin polymers.
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Conclusion
Four novel side-group liquid crystalline polymers with
coumarin units having the same monomeric formula but
different linkage and/or constructing direction of mesogens
were prepared and investigated. Their thermal behaviors
strongly depended on the linkage and construction directions
of their mesogens. This kind of study and comparison will open
a new way for liquid crystalline investigation. Interestingly,
different construction of mesogens affected not only their
thermal properties, but also photophysical and photochemical
properties. Polymers P1 and P2 tend to form a glassy phase
and a SmA phase, whereas P3 and P4 tend to exhibit a
crystalline state and a smectic A phase on heating. P1 and P2
films do not show significant mesogen aggregations, whereas
P3 and P4 films exhibit strong aggregations. The polymers P1
and P2 only exhibit simple photo-dimerization, whereas P3
and P4 show both the photo-dimerization and photo-Fries
rearrangement. Further studies of the photophysical and
photochemical properties of the liquid crystalline coumarin
polymers in their different thermal states will be reported in
forthcoming papers.
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