Emissive anisotropic polymeric materials derived from liquid crystalline fluorenes

Article abstract of DOI:10.1080/15421400490436250

The synthesis and characterization of photoluminescent crosslinking agents containing a fluorene core are described. They exhibit an efficient fluorescence and have a promesogenic molecular shape that depending on the substitution results in the development of thermotropic mesophases. A difunctionalized fluorene derivative has been incorporated into a liquid crystal host in order to produce polarized light emitting films. These films have been prepared by the in-situ photopolymerization technique.

Full text of DOI:10.1080/15421400490436250

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Emissive Anisotropic Polymeric
Materials derived from Liquid
Crystalline Fluorenes
M. Millaruelo a , L. Oriol a , J. L. Serrano a , M. Piñol
b & P. L. Sáez c
a Departamento de Química Orgánica, Facultad
de Ciencias-ICMA , Universidad de Zaragoza-CSIC ,
Zaragoza, Spain
b Departamento de Química Orgánica, Escuela
Politécnica Superior-ICMA , Universidad de
Zaragoza-CSIC , Huesca, Spain
c Departamento de Polímeros, Facultad de Ciencias
Químicas , Universidad de Concepción , Concepción,
Chile
Published online: 18 Oct 2010.
To cite this article: M. Millaruelo , L. Oriol , J. L. Serrano , M. Piñol & P. L.
Sáez (2004) Emissive Anisotropic Polymeric Materials derived from Liquid
Crystalline Fluorenes, Molecular Crystals and Liquid Crystals, 411:1, 451-466, DOI:
10.1080/15421400490436250
To link to this article: http://dx.doi.org/10.1080/15421400490436250
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 451=[1493]–466=[1508], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490436250
EMISSIVE ANISOTROPIC POLYMERIC MATERIALS
DERIVED FROM LIQUID CRYSTALLINE FLUORENES
M. Millaruelo, L. OriolÃ, and J. L. Serrano
ꢀ
´
Departamento de Quımica Organica, Facultad de Ciencias-ICMA,
Universidad de Zaragoza-CSIC, 50009-Zaragoza, Spain
~
M. Pinol
ꢀ
´
´
Departamento de Quımica Organica, Escuela Politecnica
Superior-ICMA, Universidad de Zaragoza-CSIC,
22071-Huesca, Spain
ꢀ
P. L. Saez
Departamento de Polımeros, Facultad de Ciencias Quımicas,
´
Universidad de Concepcion, Concepcion, Chile
´
ꢀ
ꢀ
The synthesis and characterization of photoluminescent crosslinking agents
containing a fluorene core are described. They exhibit an efficient fluorescence
and have a promesogenic molecular shape that depending on the substitution
results in the development of thermotropic mesophases. A difunctionalized flu-
orene derivative has been incorporated into a liquid crystal host in order to
produce polarized light emitting films. These films have been prepared by
the in-situ photopolymerization technique.
Keywords: fluorenes; liquid crystals; photoluminescence; photopolymerization; polarized
emission
INTRODUCTION
Over the last decade, the market for flat panel display technology has been
dominated by the liquid crystal displays for most applications where their
low cost, low power consumption, low operating voltage and light
weight constitute advantages not easily surpassed [1]. Nevertheless, the
Financial support from the CICYT, Spain projects MAT2002-04118-C02 and FEDER
ꢀ
financial support is gratefully acknowledged. P.L. Saez thanks AEIC (Spain) for his MUTIS
grant.
ÃCorresponding author: E-mail: loriol@unizar.es
451=[1493]

452=[1494]
M. Millaruelo et al.
combination of polarizers and color filters form the basis of numerous of
these commercial devices and such combination of elements reduces their
overall efficiency because part of the incident light is converted into ther-
mal energy. This limitation can be overcome by using emissive displays that
emit light without requiring absorbing polarizers, therefore improving their
brightness and viewing angle dependence. In the last few years, the devel-
opment of polarized light emitting films has gained attention because they
can provide linearly polarized emission, which is required for liquid crystal
display backlight applications.
Linearly polarized luminescence can be produced very efficiently from
the use of materials that are optically anisotropic and can be oriented in
a specific direction [2]. From the different ways of producing films contain-
ing highly aligned chromophores, liquid crystalline self-organization offers
an excellent control over microscopic orientation that can be macroscopi-
cally extended using appropriate processing techniques. Therefore, a com-
mon approach to prepare a polarized light-emitting film is based on the
incorporation of a luminophore into a polymeric liquid crystal matrix.
The chromophore must be a liquid crystal or at least compatible with the
liquid crystalline order and can be incorporated either as a guest or cova-
lently bonded to the matrix. In this context, in-situ photopolymerization of
oriented reactive liquid crystals provides an excellent, versatile and facile
method to produce such anisotropic films [3]. This strategy has already
been successfully applied to the production of oriented luminescent films
and it has been reported that highly cross-linked networks where the lu-
minophore is bonded to the polymeric matrix show properties that are
almost temperature independent and whose luminescence is thermally
stable and photostable [4,5].
One major problem encountered on the production of light-emitting
polarized films by in-situ photopolymerization is the selection of a lu-
minophore that combines in a single molecule all the requisite properties.
Amongst the requirements are thermal and photochemical stability, a nar-
row emission at the required wavelength and none of its functional groups
should interfere in the free-radical polymerization process. In addition,
the structure has to be compatible with the formation of mesophases.
This last condition is probably one of the most difficult to fulfill because
the structural criteria for achieving highly efficient luminophores are not
always the same than for developing liquid crystallinity. In this context,
fluorenes have been reported as blue-emitting luminophores with good
thermal stability and high quantum yield. In particular, a high research ef-
fort has been carried out on polyfluorenes as materials for light-emitting
diodes [6].
For some time, we have been interested on the preparation of cross-
linked anisotropic films with luminophores covalently bonded to the

Emissive Anisotropic Polymeric Materials
453=[1495]
FIGURE 1 Molecular structure and nomenclature of the reactive luminophores and
the reactive liquid crystal host.
polymeric matrix. The most common approach to this objective describes
the mono-functionalization of a luminophore that is incorporated into a
reactive matrix, which might contain a variable percentage of a non-emiss-
ive cross-linker [4,5,7,8]. A less common approach consists on the di-
functionalization of a luminophore that would also be able to act as a
cross-linking agent [9]. In that sense, electroluminescent networks have
been successfully prepared by photopolymerization of direactive fluor-
ene-based liquid crystals [10,11]. Therefore, in the present work, we de-
scribe the synthesis and characterization of fluorene-containing
dimethacrylates (RL-2 and RL-8) that have been used for the production
of anisotropic light-emitting films. The structure of these compounds is
shown in Figure 1. As the host reactive matrix we have used a monoacry-
late liquid crystal (C6Mm) whose synthesis and characterization is also
presented. We have investigated their thermal behavior and optical
properties in solution. In addition, the photopolymerization of polymeri-
zable mixtures and the photoluminescence of the polymeric films have
been studied.
EXPERIMENTAL SECTION
The synthetic routes followed for the preparation of the different mono-
mers are outlined in Figures 2 and 3.

454=[1496]
M. Millaruelo et al.
FIGURE 2 Synthetic pathway of the reactive luminophores.
Synthesis of the Fluorene-containing Monomers RL-2 and RL-8
2,7-dibromo-9,90-dialkylfluorenes (1)-2 and (1)-8
To a mixture of 2,7-dibromofluorene (42.8 mmol) and tetrabutylammo-
nium bromide (2.1 mmol) in toluene (80 mL) a 50% aqueous solution of
NaOH (80 mL) was added. Then, the required 1-bromoalkane (97.0 mmol)

Emissive Anisotropic Polymeric Materials
455=[1497]
FIGURE 3 Synthetic pathway of the reactive liquid crystal host C6Mm.
in toluene (20 mL) was added dropwise and the reaction was vigorously
stirred overnight at 60ꢀC. Once cooled down to room temperature, the
organic layer was separated and washed with water, dried over magnesium
sulphate and evaporated under vacuum. The residue was purified by flash
column chromatography on silica gel using hexane as eluant. The crude
product was purified by recrystallization from ethanol to yield the corre-
1
sponding product (60–65%). Characterization data for (1)-2. H NMR
[300 MHz, CDCl3, d (ppm)]: 7.51 (dd, J ¼ 7.5, 1.4 Hz, 2H), 7.47–7.39 (m,
4H), 1.97 (q, J ¼ 7.2 Hz, 4H), 0.29 (t, J ¼ 7.5 Hz, 6H). Characterization
1
data for (1)-8. H NMR [300 MHz, CDCl3, d (ppm)]: 7.51 (dd, J ¼ 7.5,
1.4 Hz, 2H), 7.47–7.39 (m, 4H), 1.92–1.87 (m, 4H), 1.24–0.97 (m, 20H),
0.81 (t, J ¼ 7.5 Hz, 6H), 0.64–0.48 (m, 4H).
2,7-Bis-(4-methoxyphenyl)-9,90-dialkylfluorenes (2)-2
and (2)-8
A mixture of the desired 2,7-dibromo-9,90-dialkylfluorene (3.6 mmol),
4-methoxyphenylboronic acid (10.7 mmol), sodium carbonate (22.0 mmol)
and tetrakis(triphenylphosphine)palladium(0) (0.29 mmol) was dissolved
in water (20 mL) and dimethoxyethane (100 mL). The reaction mixture
was heated at reflux temperature for 36 h. Then, the reaction was cooled

456=[1498]
M. Millaruelo et al.
and additional 4-methoxyphenylboronic acid (6.5 mmol) was added. The
reaction was heated at reflux temperature for 24 h. The reaction mixture
was cooled to room temperature and 2 N HCl (100 mL) was added. The
product was extracted with diethyl ether, washed with water, then washed
with brine and dried over MgSO4. The organic solution was concentrated
and the precipitated solid isolated by filtration. The product was purified
by recrystallization from 96% ethanol to yield the required product as a
white solid (61–63%). Characterization data for (2)-2. 1H NMR
[300 MHz, CDCl3, d (ppm)]: 7.74 (d, J ¼ 7.8 Hz, 2H), 7.61 (d, J ¼ 8.7 Hz,
4H), 7.55–7.51 (m, 4H), 7.00 (d, J ¼ 8.1 Hz, 4H), 3.86 (s, 6H), 2.09 (q,
J ¼ 7.3 Hz, 4H), 0.41 (t, J ¼ 7.2 Hz, 6H). Characterization data for (2)-
1
8. H NMR [300 MHz, CDCl3, d (ppm)]: 7.72 (d, J ¼ 7.8 Hz, 2H), 7.60 (d,
J ¼ 8.7 Hz, 4H), 7.55–7.51 (m, 4H), 7.00 (d, J ¼ 8.8 Hz, 4H), 3.84 (s,
6H), 2.03–1.98 (m, 4H), 1.21–1.05 (m, 20H), 0.78 (t, J ¼ 6.9 Hz, 6H),
0.75–0.63 (m, 4H).
2,7-Bis(4-hydroxyphenyl)-9,90-dialkylfluorenes (3)-2
and (3)-8
The appropriate 2,7-bis-(4-methoxyphenyl)-9,90-dialkylfluorene (2.7 mmol)
was dissolved in dichloromethane (80 mL) and then cooled to 0ꢀC. A
solution of boron tribromide (1 M in dichloromethane) (8 mmol) was added
dropwise under inert atmosphere and the mixture stirred overnight at room
temperature. Then, the solution was poured in water (300 mL) and stirred
for 2 h. The reaction was extracted with diethyl ether, washed with sodium
carbonate, dried over MgSO4 and the solvent removed under vacuum. The
crude was purified by flash column chromatography on silica gel using ethyl
acetate=hexane (3:7) as eluant to yield the desired product as a pale yellow
1
solid (80%). Characterization data for (3)-2. H NMR [300 MHz, CDCl3,
d (ppm)]: 7.72 (d, J ¼ 7.5 Hz, 2H), 7.57–7.49 (m, 8H), 6.94 (d, J ¼ 8.7 Hz,
4H), 2.09 (q, J ¼ 7.3 Hz, 4H), 0.41 (t, J ¼ 7.2 Hz, 6H). Characterization
1
data for (3)-8. H NMR [300 MHz, CDCl3, d (ppm)]. 7.73 (d, J ¼ 7.5 Hz,
2H), 7.58–7.45 (m, 8H), 6.94 (d, J ¼ 8.6 Hz, 4H), 2.03–1.98 (m, 4H),
1.20–1.05 (m, 20 H), 0.78 (t, J ¼ 6.9 Hz, 6H), 0.76–0.65 (m, 4H).
2,7-Bis[4-(6-hydroxyhexyloxy)phenyl]-9,90-dialkylfluorenes
(4)-2 and (4)-8
A mixture of the corresponding 2,7-bis(4-hydroxyphenyl)-9,90-dialkyl-
fluorene (1.3 mmol), K2CO3 (5.2 mmol) and 6-chlorohexanol (3.6 mmol)
in DMF (60 mL) was heated overnight at reflux temperature. After cooling
to room temperature, the reaction mixture was extracted with ethyl acet-
ate and the organic phase separated, washed with water and dried over
MgSO4. The solution was concentrated and the residue placed onto silica

Emissive Anisotropic Polymeric Materials
457=[1499]
gel and purified by flash column chromatography on silica gel using ethyl
acetate=hexane as eluant (4:1) to yield the required product as a white
solid (50–54%). Characterization data for (4)-2. 1H NMR [300 MHz,
CDCl3, d (ppm)]: 7.71 (d, J ¼ 7.6 Hz, 2H), 7.53–7.45 (m, 8H), 6.94 (d,
J ¼ 8.5 Hz, 4H), 4.01 (t, J ¼ 6.6 Hz, 4H), 3.57 (t, J ¼ 6.5 Hz, 4H), 2.09
(q, J ¼ 7.3 Hz, 4H), 1.82 (m, 4H), 1.72 (m, 4H), 1.53 (m, 8H), 0.40 (t,
1
J ¼ 7.2 Hz, 6H). H NMR [300 MHz, CDCl3, d (ppm)]: 7.73 (d, J ¼ 7.6 Hz,
2H), 7.55–7.45 (m, 8H), 6.96 (d, J ¼ 7.5 Hz, 4H), 4.01 (t, J ¼ 6.6 Hz,
4H), 3.58 (t, J ¼ 6.5 Hz, 4H), 2.04–1.82 (m, 8H), 1.72–1.53 (m, 12H),
1.22–1.06 (m, 20H), 0.75 (t, J ¼ 7.0 Hz, 6H), 0.69 (m, 4H).
2,7-Bis[4-(6-methacryloyloxyhexyloxy)Phenyl]-9,90-
dialkylfluorenes RL-2 and RL-8
Methacryloyl chloride (1.3 mmol) was added dropwise to a solution of
the corresponding 2,7-bis[4-(6-hydroxyhexyloxy)phenyl]-9,90-dialkylfluor-
ene (0.56 mmol) and triethylamine (1.3 mmol) in THF (50 mL) under argon
atmosphere. The reaction mixture was heated at reflux temperature for 3
days. Afterwards, the reaction was cooled and the precipitate filtered off.
The organic phase was washed with water, dried over magnesium sulphate,
filtered, and evaporated to dryness. The crude product was purified by
flash column chromatography on silica gel using hexane=ethyl aceta-
te=hexane (9:1 for RL-2 or 6:1 for RL-8) as eluant and subsequent recrys-
tallization from 96% ethanol to yield the compound as a white solid (49–
53%). Characterization data for RL-2. IR (nujol, cmꢁ1) m: 1716, 1737,
1
1607, 1515, 1404, 1320, 1296. H NMR [300 MHz, CDCl3, d (ppm)]: 7.72
(d, J ¼ 8.5 Hz, 2H), 7.55 (m, 8H), 6.97 (d, J ¼ 8.7 Hz, 4H), 6.09 (s, 2H),
5.54 (s, 2H), 4.16 (t, J ¼ 6.6 Hz, 4H), 4.00 (t, J ¼ 6.2 Hz, 4H), 2.07 (q,
J ¼ 6.9 Hz, 4H), 1.94 (s, 6H), 1.82 (m, 4H), 1.73 (m, 4H), 1.51 (m, 8H),
0.39 (t, J ¼ 7.3 Hz, 6H). 13C NMR [300 MHz, CDCl3, d (ppm)]: 167.53,
158.52, 150.72, 139.92, 139.59, 136.49, 134.07, 128.10, 125.22, 125.55,
121.07, 119.77, 114.76, 67.87, 64.64, 56.16, 32.84, 29.17, 28.56, 25.76,
18.32, 8.64. Anal. calcd. for C37H50O6: C, 75.22; H, 8.53. Found: C, 74.98;
H, 8.00. Characterization data for RL-8. IR (nujol, cmꢁ1) m: 1712, 1637,
1606, 1518, 1324, 1293. 1H NMR [300 MHz, CDCl3, d (ppm)]: 7.71 (d,
J ¼ 7.8 Hz, 2H), 7.54 (m, 8H), 6.97 (d, J ¼ 8.0 Hz, 4H), 6.09 (s, 2H),
5.54 (s, 2H), 4.16 (t, J ¼ 6.6 Hz, 4H), 4.00 (t, J ¼ 6.6 Hz, 4H), 2.02 (m,
4H), 1.94 (s, 6H), 1.82 (m, 4H), 1.70 (m, 4H),1.50 (m, 8H), 1.24–1.04 (m,
20H), 0.75 (t, J ¼ 7.0 Hz, 6H), 0.70 (m, 4H). 13C NMR [300 MHz, CDCl3, d
(ppm)]: 167.49, 158.54, 151.56, 139.55, 136.53, 134.18, 128.10, 127.99,
125.50, 125.14, 121.04, 119.77, 114.79, 67.89, 64.61, 55.14, 40.41, 31.72,
29.97, 29.12, 28.56, 25.75, 23.76, 22.52, 18.27, 13.98. Anal. calcd. for
C49H74O6: C, 77.53; H, 9.83. Found: C, 77.25; H, 9.41.

458=[1500]
M. Millaruelo et al.
Synthesis of the Liquid Crystal Host Monomer C6Mm
4-(6-Acryloyloxyhexyloxy)Benzoyl Chloride (5)
To a mixture of 4-(6-acryloyloxyhexyloxy)benzoic acid (6.4 mmol), a
catalytic amount of N,N-dimethylformamide, and 2,6-di-tert-butyl-4-
methylphenol (0.15 g) as thermal inhibitor in CH2Cl2 (15 mL), and thionyl
chloride (3.5 mmol) were added over a period of 20 min. After the addition
of thionyl chloride was complete the mixture turned yellowish. The
solution was stirred overnight and then CH2Cl2 and SOCl2 were removed
by distillation under reduced pressure. The product was used without
1
further purification (98%). H NMR [300 MHz, CDCl3, d (ppm)]: 8.03 (d,
J ¼ 9.0 Hz, 2H), 6.91 (d, J ¼ 9.0 Hz, 2H), 6.38 (dd, J ¼ 17.0, 1.0 Hz, 1H),
6.13 (dd, J ¼ 17.0, 10.0 Hz, 1H), 5.80 (dd, J ¼ 10.0, 1.0 Hz, 1H), 4.16
(t, J ¼ 7.0 Hz, 2H), 4.01 (t, J ¼ 7.0 Hz, 2H), 1.81 (q, J ¼ 7 Hz, 2H), 1.56–
1.41 (m, 4H).
4-octyloxy Benzoyl Chloride (6)
The compound was prepared by alkylation of 4-hydroxybenzoic acid and
subsequent formation of the corresponding acyl chloride as it was de-
scribed above.
Tert-butyl-diphenylsilyl 4-hydroxy-3-methylphenylEther(7)
Tert-butylchlorodiphenylsilane 3.85 mmol) was dropwise added to a
mixture of methylhydroquinone (3.85 mmol), triethylamine (3.85 mmol)
4-N,N-dimethylaminopyridine (0.15 mmol) in dichloromethane (30 mL)
under inert atmosphere. After vigorously stirring for 14 h, the solvent
was removed under vacuum the residue was purified by flash column chro-
matography on silica gel using hexane=ethyl acetate (25:1) as eluant to
1
yield the product (30%). H NMR [300 MHz, CDCl3, d (ppm)]: 7.71–7.68
(m, 6H), 7.43–7.31 (m, 6H), 6.58 (d, J ¼ 3.0 Hz, 1H), 6.44 (d, J ¼ 9.0 Hz,
1H), 6.41 (dd, J ¼ 9.0, 3.0 Hz, 1H), 4.28 (s, 1H), 2.07 (s, 3H), 1.07 (s, 9 H).
4-Hydroxy-2-methylphenyl 4-octyloxybenzoate (9)
4-octyloxybenzoyl chloride (1.65 mol) in dichloromethane (10 mL) was
added dropwise to a solution of tert-butyl-diphenylsilyl 4-hydroxy-3-
methylphenyl ether (1.66 mol) and (2.47 mol) of triethylamine (2.47 mol)
in dichloromethane (200 mL). The mixture was stirred at room tempera-
ture for three days and then was poured into a mixture of water
(10 mL), ice (10 g) and concentrated HCl (0.5 mL). The organic layer
was separated, washed with a saturated solution of NaCl, dried over MgSO4,
and the solvent was removed under pressure. The crude product was

Emissive Anisotropic Polymeric Materials
459=[1501]
purified by flash column chromatography on silica gel using dichloro-
methane=hexane (3:2) as eluant. Subsequent recrystallization in absolute
ethanol gave 4-[tert-butyl-diphenylsilyl]-2-methylphenyl 4-octyloxybenzo-
ate (8) as a white solid (40%). A round bottom flask was charged with
4-[tert-butyl-diphenylsilyl]-2-methylphenyl (1.35 mol) in THF (100 mL)
and tetrabutylammonium fluoride hydrate (1.35 mol). The solution was vig-
orously stirred for 5 h and finally water (150 mL) was added. The resulting
mixture was extracted with dichloromethane, washed with a brine solution,
dried over MgSO4 and finally, the solvent was removed under vacuum. The
residue was purified by flash column chromatography on silica gel with
using hexane=ethyl acetate (15:1) as eluant to render the required product
(75%). 1H NMR [300 MHz, CDCl3, d (ppm)]: 8.12 (d, J ¼ 8.6 Hz, 2H),
6.95 (d, J ¼ 9.4 Hz, 4H), 6.70–6.68 (m, 3H), 4.02 (t, J ¼ 6.6 Hz, 2H),
2.14 (s, 3H), 1.53 (s, 3H), 1.46 (m, 2H), 1.30–1.27 (m, 8H), 0.87 (t, J ¼
6.6 Hz, 2H).
1-[4-(6-acryloyloxyhexyloxy)Benzoyloxy]-2-methyl-4-
(4-octylbenzoyloxy)Benzene(C6Mm)
The compound was prepared from 4-(6-acryloyloxyhexyloxy)benzoyl
chloride and 4-hydroxy-2-methylphenyl 4-octyloxybenzoate as described
for 4-(tert-butyl-diphenylsilyl)-2-methylphenyl 4-octyloxybenzoate. The
crude reaction mixture was purified by flash column chromatography on
silica gel using dichloromethane=hexane (4:1) as eluant. Subsequent
recrystallization from 96% ethanol gave the product as a white solid
1
(87%). IR (nujol, cmꢁ1) m: 1729, 1605, 1580, 1254, 1182, 1163, 1069. H
NMR [300 MHz, CDCl3, d (ppm)]: 8.15 (d, J ¼ 7.3 Hz, 2H), 8.12 (d,
J ¼ 8.4 Hz, 2H), 7.00–7.20 (m, 3H), 6.96 (m, 4H), 6.40 (d, J ¼ 17.4 Hz,
1H), 6.11 (dd, J ¼ 17.2, 11.3, 1H), 5.81 (d, J ¼ 11.3 Hz, 1H), 4.17
(t, J ¼ 6.8 Hz, 1H), 4.04 (t, J ¼ 6.4 Hz, 4 H), 2.22 (s, 3H), 1.90–1.65
(m, 6H), 1.60–1.40 (m, 8H), 1.40–1.20 (m, 6 H), 0.88 (t, J ¼ 6.2 Hz, 3H).
Anal. calcd. for C38H46O8: C, 72.36; H, 7.35. Found: C, 72.01; H, 6.98.
Photopolymerization
Preparation of Photopolymerizable Mixtures
Samples for photopolymerization were prepared by dissolving the appro-
priate amounts of mono- and=or direactive compounds in freshly distilled
dichloromethane together with the photoinitiator Irgacure 784-DC (1%
weight) and the thermal inhibitor 2,6-di-tert-butyl-4-methylphenol
(200 ppm). The solvent was evaporated first at room temperature and then
at 30ꢀC under vacuum.

460=[1502]
M. Millaruelo et al.
Preparation of Films by In-situ Photopolymerization
Light-induced photopolymerization was carried out by irradiation of the
molten samples in commercial LC cells for planar alignment (Linkam
5 mm). The monomeric mixture was introduced by capillarity at 100ꢀC
and cooled down to the polymerization temperature. Uniaxial orientation
was checked by optical microscopy. The irradiation process was performed
using an OSRAM Ultravitalux 300 W lamp using a 400 nm long pass filter
between the lamp and the sample. Irradiation was maintained for 10 min
while the lamp was kept at 20 cm from the cell.
Techniques
IR spectra were measured on a ATI-Matsson Genesis Series FTIR from nujol
mulls between NaCl disks. 1H NMR and 13C NMR spectra were recorded on
a Varian Unit-300 spectrometer.
Mesogenic behaviour and transition temperatures were determined
using an Olympus BH-2 polarizing microscope equipped with a Linkam
THMS hot-stage central processor and a CS196 cooling system. Differential
scanning calorimetry (DSC) was performed using a DSC 2910 from TA
Instruments with samples sealed in aluminium pans and a scanning rate
of 10ꢀC=min under nitrogen atmosphere. Temperatures were read at the
onset of the peak. In-situ photopolymerization was studied by DSC using
a Perkin-Elmer 7 apparatus modified for light irradiation[12]. The sample
(2–4 mg) was placed in an aluminum pan that was irradiated under a nitro-
gen atmosphere using an OSRAM Ultravitalux lamp. A 400 nm long-pass
filter and water to filter the IR radition were placed between the sample
and the light source. The lamp was kept at 30 cm from the sample holder.
Optical absorption spectra were recorded with an UV-vis spectrophot-
ometer UV4-200 from ATI-Unicam in THF. Luminescence measurements
were performed using a Perkin-Elmer LS50B spectrofluorimeter. Spectra
were recorded in THF solution of ca. 0.01 optical density under excitation
in the absorption maximum [13]. Fluorescence quantum yields were de-
termined by comparing the integrated photoluminescence with that of a
reference with a known quantum yield. 9,10-Diphenylanthracene (99%,
Acros Organics) in cyclohexane was used as a reference (quantum yield
0.90). The method was validated by measuring the quantum yield of a pre-
viously reported luminophore [8]. Polarized luminescence measurements
were recorded using the same equipment with a sample holder designed
to measure LC cells [4]. The measurements were taken with the excitation
beam polarized in parallel to the rubbing direction of the cell and the detec-
tion polarizer either parallel or perpendicular, these two experimental con-
figurations are noted as vvv or vvh respectively.

Emissive Anisotropic Polymeric Materials
461=[1503]
RESULTS AND DISCUSSION
Synthesis and Characterization of the Monomers
The synthesis of the functionalized fluorenes was accomplished in several
steps starting from 2,7-dibromofluorene (Fig. 2). First, two alkyl chains
were incorporated at the 9 position of the fluorene system by a phase trans-
fer catalytic process. Alkyl chains with 2 and 8 carbon atoms were incorpor-
ated in order to establish their influence on the properties of the fluorene
derivatives. Short alkyl chains at the 9-position would not disturb the po-
tential mesomorphism by hindering lateral interactions [11] but long alkyl
chains may improve the solubility of the otherwise insoluble fluorenes.
The rigid core of the fluorene system was elongated by incorporation of
phenyl rings at the 2 and 7 positions by Suzuki coupling. In order to avoid
undesired lateral reactions, the flexible and methacrylic segments were in-
corporated in the last two steps of the synthetic sequence.
In relation to the host liquid crystal monomer C6Mm, the key step was
the incorporation of the asymmetrically substituted 2-methylhydroquinone
whose less hindered hydroxyl group at the 4-position was selectively pro-
tected using a bulky protective group such as the tert-butyl-diphenylsilyl
one. The remaining steps were adapted from the literature [14].
Thermal characterization data of the synthesized compounds is col-
lected in Table I. As might be expected RL-8 did not show liquid crystalline
properties. This fact is explained because the long lateral octyl chains pre-
vent the efficient formation of liquid crystalline aggregates and determines
its low melting temperature, 42ꢀC. However, RL-2 melts at higher tempera-
ture, 65ꢀC, and shows a monotropic smectic mesophase. The sample
showed no crystallization in the DSC scan on cooling. On the subsequent
heating the isotropization temperature is detected at 23ꢀC. When the sam-
ple is observed under the microscope the mesophase is relatively stable at
room temperature and crystallizes very slowly. In fact, when a sample of
RL-2 previously heated to the isotropic state, cooled down and maintained
TABLE I Properties of the Synthesized Monomers
Compound
Thermal transitions (ꢀC)a
kabs (nm)
kem (nm)
Ufl
RL-2
RL-8
K 65 (SmC 22) I
K 42 I
335
336
265
369, 388
372, 388sh
–
0.65
0.66
–
C6Mm
K 62 N 136 I
a Thermal properties of reactive luminophores were read in the first heating scan, and in se-
cond heating scan for C6Mn.

462=[1504]
M. Millaruelo et al.
at room temperature for 3 days was again studied by DSC, only the meso-
phase-isotropic state was detected. This very low tendency to crystallize
was also observed in RL-8. The reactive liquid crystal C6Mm exhibits a
N mesophase between 62 and 136ꢀC (measured in the second heating
scan).
In relation to the optical properties both fluorenes are very efficient
emitters. The maximum emission wavelengths have been determined at
388 and 372 nm for RL-2 and RL-8, respectively, in THF solutions of 0.01
optical density being the respective fluorescence quantum yields of 0.66
and 0.65.
In-situ Photopolymerization Study of Fluorene-containing
Liquid Crystal Mixtures
The first studies on photopolymerization of these materials have been car-
ried out with RL-8 in order to evaluate its orientation in a liquid crystalline
matrix despite its non-mesogenic nature. Photopolymerizable samples and
their liquid crystalline properties are collected in Table II. The samples con-
tain C6Mm blended with RL-8 in 5 or 10% molar percentage. They also
contain 1% (weight) of a photoinitiator of the titanocene type, Irgacure
784-DC1 from Ciba-Geigy, which absorbs between 400–500 nm where none
of the components of the mixture absorbs. The mixtures develop a N meso-
phase on between approx. 58 and 105ꢀC and no phase separation was
observed. The mesophase is stable on cooling, with no crystallization of
the sample at room temperature observed.
Photopolymerization of the mixtures was investigated by photo-DSC at
different temperatures both in the N and the isotropic liquid states. In all
cases, as deduced from the polymerization exotherm (Fig. 4a), the reaction
progresses at high rate on exposing the samples to the light source. The
conversions calculated from these exotherms were higher than 90% in
all cases. Photopolymerization rate against conversion was also evaluated
from the polymerization exotherms (Fig. 4b). From the plot, a high poly-
merization rate can be inferred at relatively low conversions thatslowsdown
TABLE II Thermal Transitions of the Photopolymerizable
Samples
Mixture
Thermal transitions (ꢀC)a
5% RL-8 þ 95% C6Mm
10% RL-8 þ 90% C6Mm
K 59 N 118 I
K 58 N 104 I
a First heating scan.

Emissive Anisotropic Polymeric Materials
463=[1505]
FIGURE 4 (a) Photo-DSC curve corresponding to the photopolymerization at 50ꢀC
– nematic phase- of the sample 10% RL-8 þ 90 % C6Mm; (b) polymerization rate in
percentage of C¼C reacted per second as a function of the conversion at different
ꢀ
ꢀ
ꢀ
~
~
&
temperatures: ( ) 30 C, nematic phase, ( ) 50 C nematic phase, ( )100 C nematic
ꢀ
&
phase and ( ) 125 C isotropic state.
as polymerization progresses. The autodeceleration phenomenon is more
marked at high temperatures either in the mesophase or the isotropic state,
therefore, indicating that the temperature of polymerization has a higher influ-
ence in comparison to the phase ordering [15].

464=[1506]
M. Millaruelo et al.
Anisotropic Films
Anisotropic films were produced by light-initiated polymerization of the
samples in cells for planar alignment. Molecular orientation was qualitat-
ively demonstrated by optical microscopy observing the film between
crossed polarizers.
To determine the order parameter, dichroism in the optical absorption
spectrum was measured using polarized UV-vis (Fig. 5a). Absorbance in
two orthogonal directions, one parallel to the molecular orientation and
FIGURE 5 (a) Polarized optical absorption spectra of the a the sample 5% RL-
8 þ 95% C6Mm photopolymerized at 50ꢀC – nematic phase- in a Linkam cell mea-
sured with light polarized parallel and perpendicular to the nematic director; (b)
Polarized photoluminescence of the same sample measured at room temperature.

Emissive Anisotropic Polymeric Materials
465=[1507]
the other perpendicular to it was measured which are related to the order
parameter by the following equation:
S ¼ ðAk ꢁ A?Þ=ðAk þ 2A?Þ
The difference between both spectra shows that there is a preferential
orientation of the luminophore. The order parameter has been roughly esti-
mated at about 0.6.
Polarized emission spectra of the films were measured at room tempera-
ture (Fig. 5b). First of all, it has to be mentioned that the emission wave-
length of films was slightly red-shifted when compared to solutions. This
fact has been related to a more planar structure of the excited state for
the film [16]. Again, the differences on the emission spectra corresponding
to vvv and vvh configurations (see experimental) reveal the existence of a
preferential orientation for the luminophore. From this spectra, the di-
chroic ratio on emission, defined from the maximum intensity as
R ¼ Ivvv=Ivvh
was calculated to be 6.5 and 5.7 for RL-2 and RL-8, respectively.
In conclusion, we have synthesized and characterized novel lumino-
phores, which can be blended with reactive liquid crystals and polymerized
to yield anisotropic films. These films showed a good orientation and polar-
ized photoluminescence. Current work is devoted to the study of the RL-2
mixtures as well as the study of the dependence of the luminescence inten-
sity on the temperature and the excitation wavelength irradiation.
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