Synthesis and optical properties of luminescent naphthalene-based liquid crystals

Article abstract of DOI:10.1080/15421400802219254

Luminescent naphthalene-based liquid crystals were newly synthesized and changes of the luminescent color through the phase transitions were characterized. We prepared the compounds by combining naphthalene and known mesogenic cores, oxadiazole and fluoro

Full text of DOI:10.1080/15421400802219254

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Synthesis and Optical
Properties of Luminescent
Naphthalene-based Liquid
Crystals
Takeshi Mori a & Masashi Kijima a
a Institute of Materials Science, Graduate School of
Pure and Applied Sciences, University of Tsukuba ,
Tsukuba, Ibaraki, Japan
Published online: 05 Apr 2011.
To cite this article: Takeshi Mori & Masashi Kijima (2008) Synthesis and Optical
Properties of Luminescent Naphthalene-based Liquid Crystals, Molecular Crystals and
Liquid Crystals, 489:1, 246/[572]-256/[582], DOI: 10.1080/15421400802219254
To link to this article: http://dx.doi.org/10.1080/15421400802219254
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Mol. Cryst. Liq. Cryst., Vol. 489, pp. 246=[572]–256=[582], 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802219254
Synthesis and Optical Properties of Luminescent
Naphthalene-based Liquid Crystals
Takeshi Mori and Masashi Kijima
Institute of Materials Science, Graduate School of Pure and Applied
Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan
Luminescent naphthalene-based liquid crystals were newly synthesized and
changes of the luminescent color through the phase transitions were characterized.
We prepared the compounds by combining naphthalene and known meso-
genic cores, oxadiazole and fluorocyanobiphenyl, using methylene spacers. Both
oxadiazole-naphthalene and fluorocyanobiphenyl-naphthalene series showed blu-
ish luminescence in all phases. Among them, the emission peaks of 3-fluoro-4-
cyano-40-[8-(6-hexyloxy-2-naphthyloxy)octyloxy]biphenyl in the liquid crystal and
isotropic liquid states showed significant red shifts compared to that in the crystal
state, which was due to specific interaction between the electron rich naphthalene
and electron deficient fluorocyanobiphenyl cores.
Keywords: cyanobiphenyl; naphthalene; oxadiazole; photoluminescence; UV-vis
absorption
INTRODUCTION
Liquid crystals have a specific property of self-organization in micro-
domains, which can lead to macroscopic orientation of molecules. Lumi-
nescent molecules having liquid crystalline property could be utilized
as intelligent materials functioning as polarized emission, organic
lasers, sensors and so on in novel optical and organic semiconductor
devices [1]. Up to the present, a variety of luminescent liquid crystals
has been reported, and some of them were applied to organic light
emitting diodes (OLEDs) [2–14]. The conjugated polymers such
This work was financially supported by the Sasagawa Scientific Research Grant from
The Japan Science Society. We would like to thank Chemical Analysis Division, University
of Tsukuba, for NMR, PL spectroscopic and elemental analysis data.
Address correspondence to Masashi Kijima, Institute of Materials Science, Graduate
School of Pure and Applied Sciences, University of Tsukuba, Tsukaba, Ibaraki, 305-8573,
Japan. E-mail: kijima@ims.tsukuba.ac.jp
246=[572]

Luminescent Naphthalene-Based Liquid Crystals
247=[573]
FIGURE 1 The structure of X-mNnCB series where X (15, 26), m and n (6, 8,
10) represent the linkage positions (1,5-, 2,6-) of naphthalene core, carbon
numbers of alkyl chains at the molecular terminal and inside, respectively.
as polyfluorenes and poly(p-phenylenevinylene)s that fluoresced
efficient in the solid state succeeded to develop polarized OLEDs by
the chain alignment [15–17]. However, reorientation of the emitting
material by an external force was not possible because of their specific
rigid polymer chain. We have studied to develop a novel emitting
device whose emitting property can be controlled by an external force
such as temperature, light, electric field, and magnetic field. We have
recently reported some photoluminescent naphthalene-based liquid
crystals (Figure 1) which showed bluish emission in crystal, liquid
crystal and isotropic liquid states, and a significant red shift of the
fluorescence spectrum in the liquid crystal state was observed [18].
However, their liquid crystal phases were unstable for further investi-
gations of optical properties of the liquid crystals. From that stand-
point, we focused on the synthesis of naphthalene-based liquid
crystals having more stable liquid crystalline phases to confirm the
spectral change of fluorescence through phase transitions. In this
study, we introduce two kinds of mesogenic cores, oxadiazole and
fluorocyanobiphenyl, into dialkoxynaphthalenes. The linkage used in
these syntheses is limited to ether, because the others such as ester
having carbonyl group will be unstable during operation when these
materials are used in devices such as OLEDs. In addition, the intro-
duction of such polar mesogenic moiety will have a significance to
assist carrier injections.
EXPERIMENTAL
Measurements
UV-vis and photoluminescent (PL) spectra of the naphthalene deriva-
tives in CHCl3 and in the crystal, liquid crystal and isotropic liquid
states sandwiched between quartz glasses with a temperature control-
ler were recorded on a U-3500 spectrophotometer (Hitachi) and a FP-
750 spectrofluorometer (Jasco). The thermal properties under argon

248=[574]
Takeshi Mori and Masashi Kijima
were investigated by a Extra-6000 differential scanning calorimetry
(DSC) system (Seiko) at a scanning rate of 10ꢀC=min in the heating
and cooling cycles. The textures of mesophases were observed by a
polarized optical microscope (POM) equipped with a temperature
controller.
Materials
General Procedure for Oxadiazole-Naphthalene (X-mNnOx)
Series
4’-Hydroxybiphenyl-4-carboxylic acid methyl ester 1 (0.300 g, 1.31 mmol)
and K2CO3 (0.272 g, 1.97 mmol) in DMF (2.00 ml) were reacted at 100ꢀC
for 10 min. To the reaction mixture was added dropwise 1-bromooctane
(0.380 g, 1.97 mmol), which was stirred at 100ꢀC overnight. The reaction
mixture poured into cold water was stirred at room temperature (r.t.)
overnight. After the filtration, a white solid was obtained. The resulting
white solid (0.420 g, 1.23 mmol) in KOH aqueous solution (5%, 13 ml) and
EtOH (30 ml) was heated under reflux for 26 h. After the mixture was
cooled to r.t., 12M HCl was added to neutralize the solution. The result-
ing white solid was filtered off, giving 2 (0.380 g, 95%).
The compound 2 (0.330 g, 1.01 mmol) and SOCl2 (0.515 ml, 7.06
mmol) in toluene (1 ml) were stirred at 100ꢀC for 26 h. After removing
the remaining SOCl2 in vacuo, a white solid of 3 (0.340 g, 98%) was
obtained.
The compound 3 (0.300 g, 0.870 mmol) in THF (2 ml) was added
dropwise to
a solution of 4-benzoic acid hydrazide (0.132 g,
0.870 mmol) in THF (2 ml) and Na2CO3 (0.0922 g, 0.870 mmol) in water
(2 ml). The reaction mixture was stirred at r.t. overnight, and then
poured into water. The resulting white solid was filtered and washed
with water to give 4 (0.380 g, 95%).
A solution of 4 (0.350 g, 0.760 mmol), SOCl2 (0.554 ml, 7.60 mmol)
and pyridine (0.0125 ml, 0.154 mmol) in toluene (2 ml) was stirred at
85ꢀC for 21 h. The reaction mixture was cooled to r.t. and extracted
with EtOAc=water. The organic layer was dried over Na2SO4, evapo-
rated in vacuo to give 5 (0.290 g, 86%).
To a stirred solution of 5 (0.500 g, 1.13 mmol), 6-bromo-hexane-1-ol
(0.308 g, 1.70 mmol) and PPh3 (0.446 g, 1.70 mmol) in dry THF (5 ml),
diethyl azodicarboxylate (DEAD) (40% in toluene: 0.296 g, 1.70 mmol)
was added dropwise under argon at 0ꢀC. The reaction mixture was
stirred at r.t. for 25 h, then extracted with CH2Cl2=water. The organic
layer was dried over Na2SO4, evaporated in vacuo. The residue was
purified on a silica gel column (EtOAc=hexane: 1=1), then recrystal-
lized from EtOAc to give 6 (0.500 g, 76%).

Luminescent Naphthalene-Based Liquid Crystals
249=[575]
To a stirred solution of 6-hexyloxy-2-hydroxynapthalene (0.0506 g,
0.207 mmol), K2CO3 (0.0286 g, 0.207 mmol) in dry DMF (3 ml), was
added 6 (0.150 g, 0.248 mmol) at 100ꢀC. The reaction mixture was stirred
at 100ꢀC for 25 h. The mixture was cooled to room temperature,
extracted with CH2Cl2=water. The organic layer was dried over Na2SO4,
evaporated in vacuo. The residue was purified on a silica gel column
(CH2Cl2=MeOH: 30=1), then recrystallized from CHCl3=hexane to
give 15-6N6Ox (0.130 g, 82%). In a similar way, 26-6N6Ox and
27-6N6Ox were obtained in 70 and 82% yields, respectively.
General Procedure for Cyanobiphenyl–Naphthalene Series
(X-mNnFCB)
To a stirred solution of 6-hexyloxy-2-hydroxynaphthalene (0.200 g,
0.819 mmol) and K2CO3 (0.170 g, 1.23 mmol) in DMF (5 ml), 6-bromo-
hexane-1-ol (0.223 g, 1.23 mmol) was added at 90ꢀC, which was stirred
at 90ꢀC for 2 d. The mixture was cooled to r.t., extracted with
EtOAc=water. The organic layer was dried over Na2SO4, evaporated
in vacuo. The residue was purified on
a silica gel column
(EtOAc=hexane: 1=1), then recrystallized from hexane to give a white
solid 9 (0.180 g, 64%). To a stirred solution of 9 (0.130 g, 0.377 mmol),
3-fluoro-4’-hydroxy-4-cyanobiphenyl (0.0670 g, 0.314 mmol) and PPh3
(0.0823 g, 0.314 mmol) in dry THF (3 ml), DEAD (40% in toluene:
0.0547 g, 0.314 mmol) were added dropwise under argon at 0ꢀC. The
reaction mixture was stirred at room temperature overnight, then
extracted with EtOAc=water. The organic layer was dried over
Na2SO4, evaporated in vacuo. The residue was purified on a silica
gel column (EtOAc=hexane: 1=1), then recrystallized from hexane to
give a white solid of 26-6N6FCB (0.140 g, 80%). In a similar way,
15-6N8FCB and 26-6N8FCB were obtained in 14 and 80% yields,
respectively.
RESULTS AND DISCUSSION
Synthesis
Oxadiazole-naphthalene derivatives abbreviated as X-mNnOx were
synthesized according to Scheme 1, where X represents the linkage
positions (1,5-, 2,6-, and 2,7-) of naphthalene core and m and n corre-
spond to carbon numbers of alkyl chains at molecular terminal and
inside, respectively. Hydroxylphenyloxadiazole (5) was prepared from
hydroxybiphenyl derivative (1) via hydrazide intermediate (4). Mitsu-
nobu reaction of 5 with x-bromoalcohol provided compound (6).
Combining 6 and monoalkoxynaphthol afforded X-mNnOx. Similarly,

250=[576]
Takeshi Mori and Masashi Kijima
SCHEME 1 Synthetic route of X-mNnOx.
cyanobiphenyl–naphthalene derivatives abbreviated as X-mNnFCB
were synthesized by combining monoalkoxynaphthol (9) with
4’-(x-hydroxyalkoxy)-3-fluoro-4-cyanobiphenyl using Mitsunobu reac-
tion (Scheme 2). All compounds have certain lengths of alkyl chains
as molecular terminals (m ¼ 6) and as linkers between two cores
(n ¼ 6, 8).
Thermal Properties
Phase transition behaviors of X-mNnOx and X-mNnFCB were investi-
gated by POM and DSC. The transition temperatures are summarized
in Table 1. Figure 2 shows DSC traces of 15-6N6Ox, 26-6N8FCB and
26-6N6FCB. All compounds showed thermotropic liquid crystalline

Luminescent Naphthalene-Based Liquid Crystals
251=[577]
SCHEME 2 Synthetic route of X-mNnFCB.
behaviors and notable difference depending on the linkage position
of the naphthalene core. 15-mNnOx showed enantiotropic phase tran-
sitions having smectic and nematic phases which were confirmed from
the enthalpies of phase transitions indicated in the DSC chart
(Figure 2) and the POM textures (a and b in Figure 3). On the con-
trary, 26-mNnOx and 27-mNnOx showed monotropic behaviors. This
tendency is a quite contrast to behaviors of X-mNnFCB, i.e.,
15-mNnFCB showed a monotropic phase transition while 26-mNnFCB
showed an enantiotropic one as well as previous results observed in
X-mNnCB depicted in Figure 1 [18]. Relation between the linkage pos-
ition of the naphthalene core and molecular shapes of the mesogen
core might influence the phase stability. The phase transition tem-
peratures of X-mNnOx are higher than those of X-mNnFCB, while
the transition temperatures observed in X-mNnFCB are lower than
those of X-mNnCB. Generally, phase transition temperatures of liquid
crystals are influenced by molecular interactions and crystallinity of
TABLE 1 Phase Transition Properties of X-mNnOx and
X-mNnFCB.
Compound
Transition temperature
15-6N6Ox
26-6N6Ox
27-6N6Ox
15-6N8FCB
26-6N6FCB
26-6N8FCB
C 132 S 145 N 149 I
C(142 S 155 N 171)a 185 I
C(129 N 132)a 159 I
C(61 N 71)a 115 I
C 98 S N 120 I
C 89 N 111 I
aObserved in a supercooling process.

252=[578]
Takeshi Mori and Masashi Kijima
FIGURE 2 DSC traces and of 15-6N6Ox, 26-6N6FCB and 26-6N8FCB and
the phase transition enthalpies (kJ=mol).
FIGURE 3 POM photographs of 15-6N6Ox (a: smectic at 140ꢀC, b: nematic at
145ꢀC) and 26-6N6FCB (c: smectic at 105ꢀC, d: mixture of smectic and nematic
at 110ꢀC) in a heating process.

Luminescent Naphthalene-Based Liquid Crystals
253=[579]
molecules. In this sense, introduction of the oxadiazole core that
increases rigidness and planarity results in raise of phase transition
temperatures. In contrast, monofluorination of the cyanobiphenyl
core that decreases symmetry of molecules results in lowering of
phase transition temperatures. At the same time, 26-mNnOx and
26-mNnFCB showed the highest phase transition temperatures in
each series, which suggests that 26-N molecules could have the stron-
ger molecular interaction than 15-N and 27-N molecules. It should be
noted that the temperature ranges of mesophases of X-mNnOx
and X-mNnFCB are wider than X-mNnCB in most cases, and distinct
smectic phases can be observed in certain cases (Figure 3).
Optical Properties
We chose 15-6N6Ox and 26-6N8FCB to investigate their optical
properties, because they showed enantiotropic behaviors with distinct
phase transitions. The optical properties in each state were investi-
gated by UV-vis and photoluminescence (PL) spectroscopies equipped
with a temperature controller. Their optical properties are summar-
ized in Table 2. All compounds showed bluish fluorescences in the
crystal, liquid crystal and isotropic liquid states, respectively.
Figures 4 and 5 show the UV-vis and PL spectra of 15-6N6Ox and
26-6N8FCB, respectively. In the UV-vis spectra in the solution state,
15-6N6Ox showed absorption peaks at 316 and 326 nm. These peaks
are attributed to absorptions at 1,5-dialkoxynaphthalene portion. On
the other hand, 26-6N8FCB showed a single peak at 306 nm with a
small shoulder at 346 nm. In this case, this shoulder is due to an
absorption at the 2,6-dialkoxynaphthalene portion.
In the PL spectra of 15-6N6Ox, an emission peak was observed at
387 nm in the solution and at 398 nm in the crystal state, respectively.
The PL spectrum of 15-6N6Ox in the liquid crystal state was similar to
that in the crystal state. Since we have preliminarily confirmed that
TABLE 2 The Optical Properties of 15-6N6Ox, 26-6N8FCB and N–FCB.
UV-vis (kmax
)
PL (kmax)
nm
nm
Compound
in CHCl3
in CHCl3
crystal
liquid crystal isotropic liquid
15-6N6OX
26-6N8FCB
N–FCB
316, 326
306 (346)
304 (346)
387
364
363
398, 410
424
420
401
459
–
–
457
458

254=[580]
Takeshi Mori and Masashi Kijima
FIGURE 4 PL spectra of 15-6N6Ox in the crystal state (solid line), in the
liquid crystal state (broken line) and UV-vis and PL spectra in CHCl3 solution
(open circle).
1,5-dialkoxynaphthalene showed an emission peak at around 480 nm
in the crystal state, the red shift observed in 15-6N6Ox is trivial. This
result indicates that introduction of the large oxadiazole core sup-
presses the excimeric interaction between the naphthalene moieties.
On the other hand, the PL spectra of 26-6N8FCB exhibited significant
changes through phase transitions, i.e., the emission peak in the
FIGURE 5 PL spectra of 26-6N8FCB in the states of crystal (solid line),
nematic (broken line), isotropic liquid (dot-dashed line), and UV-vis and PL
spectra in CHCl3 solution (open circle).

Luminescent Naphthalene-Based Liquid Crystals
255=[581]
FIGURE 6 UV-vis and PL spectra of N–FCB in the states of crystal (solid
line), isotropic liquid (dot-dashed line), and UV-vis and PL spectra in CHCl3
solution (open circle).
solution state observed at 364 nm largely red-shifted to 424 nm in the
crystal state. Furthermore, emission peaks in the liquid crystal and
isotropic liquid states were observed in a region of longer wavelength
at 459 nm and 457 nm, respectively. These results indicate that mol-
ecular interactions similarly work between 26-6N8FCB molecules in
the solid, liquid crystal and isotropic liquid states. To investigate what
interaction led to the significant red shifts, we prepared a mixture
sample (50=50 mol%) of 2,6-dialkoxynaphthalene and 40-octyloxy-3-
fluoro-4-cyanobiphenyl (N–FCB) and compared the UV-vis and PL
spectra. Because N–FCB only exhibited an unstable monotropic phase
transition in the supercooling process, we could obtain UV-vis and PL
spectra in solution, crystal and isotropic liquid states (Fig. 6). It is
interesting to note that the optical data of N–FCB well agree with
those of 26-6N8FCB. Since the characteristic red-shifts observed in
N–FCB during the phase transitions have never been observed
2,6-dialkoxynaphthalene and 40-octyloxy-3-fluoro-4-cyanobiphenyl by
themselves, it is concluded that there is a specific interaction between
electron rich dialkoxynaphthalene and electron deficient fluorocyano-
biphenyl cores.
CONCLUSIONS
In this study, we succeeded to synthesize luminescent naphthalene-
based liquid crystals having oxadiazole and fluorocyanobiphenyl cores.

256=[582]
Takeshi Mori and Masashi Kijima
Our objective is to synthesize novel luminescent liquid crystals
showing a remarkable spectral change through distinct phase transi-
tions. X-mNnOx and X-mNnFCB showed notable differences of phase
transition behaviors depending on the linkage positions of the naph-
thalene core. 15-6N6Ox, 26-6N6FCB and 26-6N8FCB showed enantio-
tropic behavior and their mesopheses were observed in the distinct
temperature regions. The phase transition temperatures of X-mNnOx
series were higher than those of X-mNnFCB series, which is due to the
rigid conjugated structure of the oxadiazole portion. All compounds
showed bluish emission in the crystal, liquid crystal and isotropic
liquid states. Especially, we observed that 15-6N6Ox showed little
PL spectral change through phase transitions but 26-6N8FCB showed
remarkable red shifts in the liquid crystal and isotropic liquid states.
From these results, it is obvious that there is little molecular interac-
tion between oxadiazole and naphthalene cores in 15-6N6Ox but the
specific interaction is present between fluorocyanobiphenyl and naph-
thalene cores in 26-6N8FCB. With this study, the materials having
change of PL colors through phase transitions will be initiated to
develop as a new class of luminescent materials.
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