New metallomesogens derived from unsymmetric 1,3,4-thiodiazoles: synthesis, single crystal structure, mesomorphism, and optical properties

Article abstract of DOI:10.1016/j.tet.2007.09.031

A series of copper(II) complexes 1 derived from unsymmetric 1,3,4-thiadiazoles 2 exhibiting mesogenic properties are reported. All the precursors 2 and 3 exhibited smectic A or/and smectic C phases, whereas, copper complexes formed nematic, SmA or SmC pha

Full text of DOI:10.1016/j.tet.2007.09.031

Tetrahedron 63 (2007) 12437–12445
New metallomesogens derived from unsymmetric
1,3,4-thiodiazoles: synthesis, single crystal structure,
mesomorphism, and optical properties
Cheng-Tsung Liao,a Yueh-Ju Wang,a Chi-Shuen Huang,a Hwo-Shuenn Sheu,b
Gene-Hsiang Leec and Chung K. Laia,
*
aDepartment of Chemistry, National Central University, and Center for Nano Science Technology, UST, Chung-Li 32054, Taiwan, ROC
bNational Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan, ROC
cInstrumentation Center, National Taiwan University, Taipei 10660, Taiwan, ROC
Received 18 July 2007; revised 13 September 2007; accepted 13 September 2007
Available online 22 September 2007
Abstract—A series of copper(II) complexes 1 derived from unsymmetric 1,3,4-thiadiazoles 2 exhibiting mesogenic properties are reported.
All the precursors 2 and 3 exhibited smectic A or/and smectic C phases, whereas, copper complexes formed nematic, SmA or SmC phases.
The mesophases formed by derivatives 2 and 3 were probably attributed to the H-bondings induced both intramolecularly or/and intermole-
cularly between amide (–NH) and phenolic (–OH) groups. The crystal and molecular structures of mesogenic 2-(5-(2-(hexyloxy)naphthalene-
6-yl)-1,3,4-oxadiazole-2-yl)phenol (2; n¼6, m¼6) were determined by means of X-ray structural analysis. It crystallizes in the monoclinic
˚
˚
˚
˚
space group P-1, with a¼7.4255(18) A, b¼8.209(2) A, c¼17.315(5) A, and Z¼2. An intermolecular H-bond (d¼1.89 A) between N2 and
H1A atoms with an angle of 161.5ꢀ was observed. All molecules were packed as tilted layer arrangement and a p–p interaction (ca.
1
3.56 A) was observed. Variable temperature FTIR and H NMR spectroscopies were also used to probe the possible H-bondings formed
˚
in compound 2 (m¼0, n¼6). The fluorescent properties of these compounds 2 were examined. All lmax peaks of the absorption and photo-
luminescence spectra occurred at ca. 359–363 nm and 519–537 nm, respectively. Excited state intramolecular proton transfer (ESIPT)
reaction in this type of ortho-hydroxy-1,3,4-thiadiazole was also observed.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
were rarely reported. 1,3,4-Oxadiazole derivatives were in
fact considered as non-linear structures,8 therefore, reducing
the molecular symmetry would possibly lead to a lowering
of melting or/and clearing points.9 This was indeed observed
in our previous studies10 of mesogenic 1,3,4-oxadiazoles.
1,3,4-Oxadiazoles have also been linked to the pyridinium
or fluorinated moiety to obtain ionic liquid crystals11 for
optoelectronic applications.
1,3,4-Oxadiazole,1 considered as an important five-mem-
bered compound among the huge heterocyclic families, has
been studied as excellent candidate for material applications
during the past years due to its excellent thermal, chemical
stabilities and high photoluminescence quantum yields.
These properties, combined with its electron-deficient nature
and good electron accepting ability, have led to many poten-
tial applications in organic light-emitting diodes (OLEDs).
Various structural forms as core moiety such as low molecu-
lar weight1b,2 dendrimers3 and polymers4 have been studied.
Numerous examples of oxadiazole-based compounds have
been applied as electron-transporting materials1b,2 in OLEDs.
Most applications were focused on its electron-transporting
capability1b,2 or photoconductivity for advanced electronic
devices. Non-mesogenic 2,5-bis(4-naphthyl)-1,3,4-oxadi-
azole5 was found to be one of the best organic electron
conductors.
However, examples derived from 1,3,4-oxadiazole exhibit-
ing columnar phases10,12 were still limited. The metal-con-
taining liquid crystals (or metallomesogens),13 have been
explored and studied. Coordination compounds were con-
sidered as the largest category among metallomesogenic
materials. The two heteroatoms, N and O with free electron
pairs could provide active coordination sites for metal ions
or/and hydrogen bond acceptors14 to expand polymeric
frameworks. Non-mesogenic metal complexes15 such as
those of Ag,16 Ru or Cu were known. Earlier, we described
a series of palladium complexes17 derived from 1,3,4-oxa-
diazoles exhibiting columnar phases, and found most of
them were room temperature liquid crystals. On the other
hand, 1,3,4-thiadiazole has a core structure similar to
1,3,4-oxadiazole, different only in the oxygen atom being
replaced by sulfur atom. In addition, a less overall bent
On the other hand, examples of 1,3,4-oxadiazole6 or 1,3,4-
thiadiazole7 derivatives exhibiting mesomorphic properties
* Corresponding author. Tel.: +886 03 4259207; fax: +886 03 4277972.;
e-mail: cklai@cc.ncu.edu.tw
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.09.031

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C.-T. Liao et al. / Tetrahedron 63 (2007) 12437–12445
molecular structure of 1,3,4-thiadiazole was considered.
Sulfur atom is slightly larger in size and also more polarized
than oxygen atom. In this work, we report the preparation
and mesomorphic studies of a new series of copper(II) com-
plexes 1 derived from unsymmetric 1,3,4-thiadiazole. These
complexes were considered as rod-like in shape, and they all
formed N, SmA or SmC phases. The mesophases formed by
derivatives 2 and 3 were probably attributed to the H-bond-
ings induced both intermolecularly or/and intermolecularly
by amide (–NH) and phenolic (–OH) groups.
refluxing pyridine. All derivatives were isolated as white
solids or pastes depending on the carbon length attached.
Copper complexes were obtained by reactions of 1,3,4-thia-
diazoles and Cu(OAc)2 in refluxing CHCl3. The products
were recrystallized from CH2Cl2/CH3OH with yields rang-
ing from 74 to 85%. The complexes were also characterized
by FTIR spectroscopy and elemental analysis.
2.2. Mesomorphic properties of compounds 2 and 3
The mesomorphic properties of compounds 1, 2, and 3 were
characterized and investigated by differential scanning calo-
rimetry and polarizing optical microscopy. The phase transi-
tions and thermodynamic data of compounds 3 are listed in
Table 1. All compounds 3 except for three derivatives with
no lateral chain (R1, R2¼H in Scheme 1) attached on either
end of the molecules (n¼0 or m¼0) formed mesogenic
phases. Both the melting and clearing temperatures were in-
sensitive to the length of the side chains. This indicated that
the core–core interaction might be the controlling force than
van der Waal force in forming the mesophase in this type of
amide derivatives. The melting temperatures and clearing
2. Results and discussion
2.1. Synthesis
Two synthetic routes18 were applied to prepare 1,3,4-oxadi-
azole derivatives. In this work the synthetic procedures
of 2-(5-(2-(alkoxy) naphthalen-6-yl)-1,3,4-thiadiazol-2-yl)-
phenols 2 and their copper complexes 1 are given in Scheme
1. The 1,3,4-thiadiazole derivatives 2 were obtained by con-
densation reaction17 of hydrazides 3 and P2S5 (8.0 equiv) in
H2n+1CnO
OCmH2m+1
S
N
N
O
Cu
O
N
N
S
H2m+1CmO
OCnH2n+1
1
(n, m) = (0, 12), (6, 0), (12, 0), (12, 8), (12, 10),
(12,12), (12,14), (12,16)
R1
R1
a
COMe
CONHNH2
R2
R1
OH
OH
c
NHHN
R2
R2
b
O
O
OH
3
COOH
COCl
d
R2
R1
S
R2
R1
N
N
O
e
Cu
S
O
N
N
N
N
OH
S
2
R1
1; R1, R2 = H, alkoxy
R2
Scheme 1. Reactions and reagents: (a) N2H4 (10.0 equiv), refluxing in CH3OH, 48 h, 90%; (b) SOCl2 (10.0 equiv), refluxing in THF, 1.5 h, 95%; (c) triethyl-
amine (1.0 equiv), stirred in THF at rt, 70%; (d) P2S5 (8.0 equiv), refluxing in pyridine, 72 h, 65%; (e) Cu(OAc)2 (0.5 equiv), refluxing in CHCl3, 6 h, 70%.

C.-T. Liao et al. / Tetrahedron 63 (2007) 12437–12445
12439
Table 1. Phase transitions and enthalpiesa of compounds 3 Table 2. Phase transitions and enthalpiesa of 1,3,4-thiodiazole derivatives 2
n
m
n
m
166:1ð49:1Þ
173:9ð44:1Þ
ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ I
ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ I
3
0
6
12
2
0
6
12
151:9ð46:9Þ
176:7ð33:0Þ
153:8ð32:3Þ
197:8ð5:55Þ
183:3ð41:0Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ* ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmA )ꢁꢁꢁꢁꢁꢁ I
0
0
Cr )ꢁꢁꢁꢁꢁꢁ I
0
0
168:4ð32:0Þ
179:9ð33:9Þ
164:3ð40:8Þ
161:2ð44:6Þ
196:1ð5:63Þ
194:9ð5:66Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmA )ꢁꢁꢁꢁꢁꢁ I
144:6ð42:9Þ
ꢁꢁꢁꢁꢁꢁ*
12
12
12
12
12
12
Cr )ꢁꢁꢁꢁꢁꢁ I
12
12
12
12
12
12
167:8ð33:9Þ
199:6ð8:29Þ
190:9ð5:61Þ
b
b
153:9ð32:3Þ
153:4ð45:2Þ
213:1ð0:30Þ
217:9ð5:42Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
8
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
8
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ SmA )ꢁꢁꢁꢁꢁꢁ I
b
b
135:1ð25:6Þ
161:8ð27:8Þ
196:7ð7:74Þ
200:9ð7:01Þ
143:6ð45:7Þ
213:1ð0:09Þ
150:3ð44:8Þ
216:6ð4:44Þ
212:9ð7:59Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ* ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
10
12
14
16
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
10
12
14
16
128:3ð23:2Þ
150:9ð36:8Þ
192:6ð6:27Þ
195:8ð9:82Þ
136:9ð43:2Þ
148:1ð45:7Þ
210:6ð7:51Þ
204:2ð7:54Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ* ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
130:8ð30:3Þ
152:2ð35:4Þ
193:1ð8:98Þ
192:2ð7:90Þ
129:9ð45:2Þ
144:1ð51:6Þ
201:9ð7:45Þ
201:7ð8:16Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ* ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
129:1ð24:7Þ
149:9ð31:0Þ
189:9ð7:45Þ
186:2ð8:17Þ
127:6ð50:4Þ
144:6ð38:8Þ
200:6ð8:08Þ
198:5ð6:14Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
118:7ð36:7Þ
ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ I
127:0ð25:7Þ
183:3ð7:05Þ
194:2ð5:85Þ
a
a
n, m represent the carbon numbers in the alkoxy chains. Cr¼crystal
phases; SmA¼smectic A; SmC¼smectic C phase; I¼isotropic. The
transition temperatures (ꢀC) and enthalpies (in parenthesis, kJ/mol) are
determined by DSC.
n, m represent the carbon numbers in the alkoxy chains. Cr¼crystal
phases; SmC¼smectic C phase; I¼isotropic. The transition temperatures
(ꢀC) and enthalpies (in parenthesis, kJ/mol) are determined by DSC.
b
Determined by DSC at a scan rate of 5.0 ꢀC/min.
temperatures all ranged between 149.9 and 161.8 ꢀC, and
186.2 and 200.9 ꢀC on heating, respectively. The tempera-
tures of the mesophases ranged from 36.3 to 45.7 ꢀC The
phase was characterized and identified as smectic C phase.
A typical Schlieren texture under polarizing microscopy
was observed when cooled from isotropic liquid. This phase
behavior was also consistent with no observation of homeo-
tropic domain. A higher enthalpy (DH¼7.01–9.82 kJ/mol)
for the transition of SmC/I phase was obtained. The
SmC phase was also confirmed by powder XRD experi-
ments.
mesophases ranged from 62.6 (n¼12, m¼10) to 45.7 ꢀC
(n¼12, m¼16) on heating. A higher enthalpy (DH¼6.14–
8.16 kJ/mol) for the transition of SmC/I phase was
also obtained. The H-bondings induced by amide (–NH)21
or/and phenolic (–OH) group might be attributed to the
formation of the mesophase observed in compounds 2
and 3.
2.3. Mesomorphic properties of compounds 1
The phase transitions of compounds 2 are listed in Table 2.
All compounds 2 except for one derivative (n¼0, m¼12)
were mesogenic. Interestingly, two other derivatives (n¼6,
m¼0 and n¼12, m¼0), having either a shorter or an identical
molecular length were in fact mesogenic. This result indi-
cated that the mesophase was mainly controlled or influ-
enced by the relative carbon length attached on the
naphthalene over phenolic ring. On the other hand, the
formation of mesophase might be facilitated by the weak
p–p interaction between the two neighboring naphthalene
rings. This indeed was supported in the crystallographic
analysis of compound 2 (n¼6, m¼0), and a distance of
The phase transition thermodynamic data of copper com-
plexes 1 are summarized in Table 3. All copper complexes
were slightly decomposed upon heating above their clearing
temperatures. All compounds 1 formed mesogenic phases,
and the phases were identified as N, SmA or/and SmC
phases depending on the carbon length. The overall molec-
ular length of the copper complexes 1 increased upon coor-
dination with copper ion, therefore, the melting and the
clearing temperatures were all increased by ca. 40.7–
70.1 ꢀC and 60.0–70.8 ꢀC, respectively. Both the melting
and clearing temperatures were slightly decreased with car-
bon chain lengths. The melting temperatures and clearing
temperatures of compounds 1 ranged between 200.9 and
253.4 ꢀC, and 258.5 and 278.5 ꢀC on heating, respectively.
The temperature range of the mesophases ranged from
12.3 to 62.4 ꢀC. The derivative (n¼6, m¼0), with the shortest
overall molecular length exhibited N phase with the narrow-
est range (DT¼12.3 ꢀC). The phases were characterized and
identified as N, SmA and SmC phases based on the textures
observed by optical microscope (Fig. 1).
˚
3.56 A between two layers was obtained. These two deriva-
tives (n¼6, m¼0 and n¼12, m¼0) formed smectic A phases
with a short temperature range (DT¼14.5–33.7 ꢀC on heat-
ing) of mesophase. The derivative (n¼6, m¼0), the shortest
overall molecular length in the series has the narrowest range
(DT¼14.5 ꢀC) of phase temperature. The phase was con-
firmed and identified as smectic A phase. A fan texture
with homeotropic states under polarizing microscopy was
observed when cooled from isotropic liquid. The more
ordered phase, SmC appeared when the molecular length
increased (i.e., m or n increases). Both the melting and
clearing temperatures decreased as carbon length increases.
The melting temperatures and clearing temperatures ranged
between 153.4 and 144.6 ꢀC, and 217.9 and 198.5 ꢀC on
heating, respectively. The temperature range of the
2.4. Single crystal structure of 2-(5-(2-(hexyloxy)-
naphthalene-6-yl)-1,3,4-thiadiazol-2-yl) phenol 2
(n=6, m=0)
A single crystal isolated as brown needle of mesogenic com-
pound 2 (n¼6, m¼0) suitable for X-ray diffraction analysis

12440
C.-T. Liao et al. / Tetrahedron 63 (2007) 12437–12445
Table 3. Phase transitions and enthalpiesa of copper complexes 1
n
m
231:7ð32:4Þ
278:5ð1:83Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
264:0b
1
0
6
12
Cr )ꢁꢁꢁꢁꢁꢁ SmA )ꢁꢁꢁꢁꢁꢁ Id
213:1ð27:9Þ
253:4ð66:7Þ
265:7ð0:37Þ
ꢁꢁꢁꢁꢁꢁ* ꢁꢁꢁꢁꢁꢁ*
0
0
Cr )ꢁꢁꢁꢁꢁꢁ N )ꢁꢁꢁꢁꢁꢁ Id
250:0b
184:7ð17:1Þ
201:9ð59:7Þ
264:3ð5:88Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
261:0b
12
12
12
12
12
12
Cr )ꢁꢁꢁꢁꢁꢁ SmA )ꢁꢁꢁꢁꢁꢁ Id
180:9ð23:2Þ
279.0b
225.7 (38.4)
8
Cr
SmA
Id
218.0b
272.0b
SmC
219:7ð47:7Þ
276:0ð12:1Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
263:0b
10
12
14
16
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ Id
203:8ð40:4Þ
215:5ð45:8Þ
265:3ð9:32Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
260:0b
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ Id
205:0ð40:2Þ
209:3ð42:9Þ
262:7ð8:65Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ Id
199:0ð38:5Þ
200:9ð32:9Þ
250:0ð0:33Þ
258:5ð20:9Þ
ꢁꢁꢁꢁꢁꢁ*
ꢁꢁꢁꢁꢁꢁ*
Cr )ꢁꢁꢁꢁꢁꢁ SmC )ꢁꢁꢁꢁꢁꢁ Id
250:0b
289:6ð13:7Þ
a
n, m represent the carbon numbers in the alkoxy chains. Cr¼crystal phases;
SmA¼smectic A; SmC¼smectic C; N¼nematic phase; Id¼isotropic with
slight decomposition. The transition temperatures (ꢀC) and enthalpies (in
parenthesis, kJ/mol) are determined by DSC.
b
Obtained by optical microscope.
was slowly grown from CH2Cl2/CH3OH solution at room
temperature, and the crystal structure was investigated. Fig-
ure 2 shows the molecular structure with the atomic number-
ing scheme. When viewed from the side, the three planes of
thiadiazole, naphthalene, and benzene were nearly coplanar,
and this coplanar structure might facilitate the molecular
arrangement and the formation of the mesophases. The over-
all molecular shape is slightly bent, and the overall length
and the width of the molecule are measured to be ca.
˚
˚
22.32 A and 3.74 A. There are two molecules existing in
the unit cell, and the molecules are all tilted. The molecules
are arranged anti-parallel within the neighboring layers. A
perpendicular distance between the intermolecular benzenes
˚
within the layers is measured as ca. 3.56 A (Fig. 3), indicat-
ing that a very weak p–p interaction is possibly existent.
Interestingly, the crystallographic data indicated that the
phenolic-H atom (–OH) was conformationally located or
pointed out to the other side of the N1 and N2 atoms of
the thiadiazole ring and also at trans position away from
sulfur atom of the thiadiazole ring. Therefore, an intramole-
cular H-bonding between S(1) and H–O(1) atoms was not
possibly formed at room temperature. On the other hand,
˚
Figure 1. The optical textures observed in compounds. Top plate: N phase
by 1 (n¼6, m¼0) at 243 ꢀC, center plate: SmA phase by 1 (n¼12, m¼0) at
235 ꢀC, and bottom plate: SmC phase by 1 (n¼12, m¼10) at 227 ꢀC.
an intermolecular H-bonding distance of 1.89 A between
N2 and H1A atoms was observed, and its angle defined by
O1, H1A, and N2 atoms was measured to be 161.5ꢀ. This
molecular arrangement is consistent with the layer structures
observed by powder XRD and optical textures. A variable
temperature XRD experiment for compounds was also per-
formed to confirm the structure of the mesophases. A diffrac-
tion pattern with two peaks for compound 2 (n¼6, m¼0) at
2.5. Studies of H-bonding formation by 1H NMR and
FTIR spectroscopies
1
In this work we compare H NMR spectroscopic data as
˚
˚
d-spacings of 34.42 A and 17.22 A was obtained (Fig. 4).
The two peaks corresponded to the layer indices 001 and
002. A tilted angle in the smectic phase was roughly cal-
culated asw59ꢀ by d-spacing value and the molecular length
a function of concentration. In a typical experiment, a
0.1 mM solution of compound 2 (n¼6, m¼0) was prepared
by dissolving a sample in CDCl3 in order to reduce the mo-
lecular attraction.19 A sharp singlet occurred at 11.475 ppm,
assigned to phenolic –OH group was monitored, and this
˚
(ca. 40.06 A, obtained from MM2 simulation).

C.-T. Liao et al. / Tetrahedron 63 (2007) 12437–12445
12441
Figure 2. An ORTEP plot for compound 2 (n¼6, m¼0) with the numbering scheme, and the thermal ellipsoids of the non-hydrogen atoms are drawn at the 50%
ꢀ
ꢀ
ꢀ
˚
˚
˚
probability level. Parameters: triclinic; space group P-1; a¼7.4255(18) A, b¼8.209(2) A, c¼17.315(5) A; a¼101.245(13) , b¼92.501(13) , g¼95.804(17) ;
Z¼2; density (calcd)¼1.307 Mg/m3.
Figure 3. The weak p–p interaction between the layers (top) and the H-bonding (bottom) in compound 2 (n¼6, m¼0).
characteristic peak was not shifted upfield when dissolved in
more concentrated solution (3 orders of magnitude). The
concentration independence indicated that the H-bondings
are quite stable. The intermolecular H-bondings observed
in single crystallographic state might convert to intramolec-
ular H-bondings in solution state due to an increased
distance. Also the –OH signal was found to have no correla-
tion with concentration.
When a more polar solvent, DMSO-d6 was added,19b,20 the
characteristic phenolic peak started to appear upfield, i.e.,
d¼11.475 (CDCl3-d1/DMSO-d6: 1/0)/d¼11.354 (9/1)/
d¼11.097 (5/1), shown in Figure 5. In addition, the signal
started to broaden. However, as the ratio of CDCl3-d1/
DMSO-d6 was increased to 1/1, the peak was shifted to
d¼10.779. The Dd¼0.696 then reached the maximum. The
upfield shifting and the peak broadening were no longer
observed, and the peak was barely observed. The result indi-
cated that intermolecular H-bondings were indeed existent,
which might be explained as follows. The oxygen atom on
the DMSO has a higher electron density and is a stronger
electron donor. On the other hand, hydrogen atom of the
phenolic group in compound 2 (n¼6, m¼0) carries a partial
positive charge. A dipole–dipole interaction (d+/dꢁ)
occurred upon the addition of DMSO, which caused the
electron density enhancement on the hydrogen atom. The
original H-bondings were destroyed, and the signal appeared
upfield. Nevertheless, the characteristic signal which
appeared broader was attributed to the fast proton
exchange.21
25000
34.42 Å
20000
15000
17.22 Å
10000
4
5
6
7
8
5000
0
5
10
15
20
25
In order to confirm the H-bondings formed in crystal, liquid
crystal or/and isotropic state by compounds 2, variable
temperature FTIR spectra7b,22 of thin films were studied.
In general, the IR frequency of intermolecular H-bondings
2θ
Figure 4. X-ray powder diffraction pattern of compound 2 (n¼6, m¼0) at
180 ꢀC.

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C.-T. Liao et al. / Tetrahedron 63 (2007) 12437–12445
Figure 5. Plots of 1H NMR spectra observed for compound 2 (n¼6, m¼0) dissolved in CDCl3-d1/DMSO-d6 with a ratio of (a) 1/0 (top) (b) 9/1 (c) 5/1 (d) 1/1
(bottom).
by phenolic –OH group22e,23 occurred at ca. 3200–
3400 cmꢁ1, and intramolecular H-bondings then occurred
at lower region24 of 2800–3100 cmꢁ1. As seen from Figure 6,
a characteristic peak of intramolecular H-bondings which
occurred at ca. 3029 cmꢁ1, assigned to the phenolic –OH
group was not found to shift with temperature. However,
the total band area was decreased. This indicated that an
intramolecular H-bonding in compounds 2 was existent in
crystal, liquid crystal and isotropic states.
spectra for compounds 1 and 2 in CH2Cl2 solution are
presented in Figure 7. All compounds 2 exhibited a unique
intense absorption lmax peak at 363 nm except for compound
2 (n¼12, m¼0) which occurred at 359 nm, and these absorp-
tion spectra were very similar in shape because of their
structural similarity. The highest absorption peaks of all
compounds were not sensitive to the carbon length or/and
number of terminal chains. These absorption spectra corre-
sponded to the transitions of n/p* and p/p*. The emis-
sion luminescence spectra of compounds 2 measured in
CH2Cl2 are shown in Figure 7. A similar trend was observed
in the photoluminescence spectra. The emission peaks
occurred at 519–537 nm. The solution PL spectra were quite
similar in peak shape to UV–vis spectra except for compound
2 (n¼12, m¼0). Compound 2 (n¼12, m¼0) with no electron
donating group on the phenyl group has an absorption peak
red-shifted by ca. 18–23 nm. On the other hand, all the other
three derivatives n¼12, m¼8; n¼12, m¼12; n¼12, m¼16
were considered as electron withdrawing groups and were
then blue-shifted to the lower frequencies. However, the
quantum yields in CH2Cl2 were relatively low.
2.6. Optical properties
1,3,4-Oxadiazoles have been investigated as an excellent
candidate for light-emitting materials due to their photo-
physical and fluorescent properties. The UV–vis absorption
6
8
10
Cr
SmA
The PL spectra of compounds 2 (Fig. 7b) showed two intense
peaks, which were attributed to the excited state intramolec-
ular proton transfer23,25 (ESIPT). A longer wavelength emit-
ted by an excited phototautomer form than by an excited
normal form was often observed. The Stokes shift values
were found to be as large as 6000–12,000 cmꢁ1 for many
ESIPT compounds. The ESIPT reactions have been known
to occur in a variety of oxadiazole derivatives,22a as well
as other related compounds, such as ortho-hydroxybenzal-
dehydes, acetophenones, etc. Unfortunately, the noticeable
quenching of fluorescence has largely restricted the practical
applications of this type of organic materials. In our system,
the ESIPT reaction was enhanced or/and facilitated by an
intramolecular H-bonding. The two phototautomer forms,
enol and keto coexisting in the excited state, were attributed
to the two emission peaks on the PL spectra. The enol form
emitted light at a lower wavelength, while keto form emitted
light at higher wavelength. Without the ESIPT reactions the
12
I
14
2600
2800
3000
3200
3400
Wavenumber (cm-1)
-crystal -SmA -isotropic
183.3(41.03) 197.8(5.55)
164.3(40.75) 196.1(5.63)
Cr
SmA
I
Figure 6. Variable temperature FTIR spectra of compound 2 (n¼6, m¼0)
obtained during cooling process at a scan rate of 5.0 ꢀC/min.

C.-T. Liao et al. / Tetrahedron 63 (2007) 12437–12445
12443
2 (n = 12 m = 0)
2 (n = 12 m = 8)
2 (n = 12 m = 12)
2 (n = 12 m = 16)
60
50
40
30
20
10
0
2 (n = 12 m = 0)
2 (n = 12 m = 8)
2 (n = 12 m = 12)
2 (n = 12 m = 16)
519
363
0.5
0.4
0.3
0.2
0.1
0.0
537
359
418
414
250
300
350
wavelength (nm)
400
400
450
500
550
600
650
wavelength (nm)
Figure 7. Normalized UV and PL spectra of compounds 2 in CH2Cl2 at 25 ꢀC.
enol form returned directly to the ground state (S1/S0) by
emitting a shorter wavelength light, and keto form then un-
derwent a large Stokes shift. On the other hand, the ESIPT
molecules generated in the0excited state transferred to the
lower energy state (S1/S1 ), which were accompanied by
emission of the fluorescent light in a higher wavelength.26
refined by full matrix least-squares and difference Fourier
techniques with SHELXTL. Empirical absorption correc-
tions were applied with SADABS program. All non-hydro-
gen atoms were refined anisotropically. The powder
diffraction data were collected from the Wiggler-A beam
line of the National Synchrotron Radiation Research Center
˚
with the wavelength of 1.3263 A. Methyl 4-alkoxy-2-
hydroxybenzoates and 4-alkoxy-2- hydroxybenzoic acid
3. Conclusions
hydrazides were prepared by literature methods.17
A new series of heterocyclic 1,3,4-thiadiazoles 2 and copper
complexes 1 as core group are prepared and their mesomor-
phic properties investigated. The precursors 3 and 2 formed
SmC or/and SmA phases; however, copper complexes 1
formed N, SmA or/and SmC phases. The temperature range
of the mesophases formed by 1,3,4-oxadiazoles 2 and copper
complexes 1 remained about the same. The X-ray single
crystallographic analysis indicated that compounds 2
formed an intermolecular H-bonding in single crystalline
state. However, an intramolecular H-bonding was observed
in crystalline state, smectic phase and isotropic states, which
were also supported by FTIR spectra. Optical properties
evaluated for potential light-emitting materials were studied,
in which ESIPT reactions were observed in this system.
4.2. 6-Dodecyloxy-naphthalene-2-carboxylic acid (n=12)
Themixture of6-hydroxy-2-naphthoicacid (10.0 g, 0.053 mol),
1-bromododecane (13.21 g, 0.053 mol), and KOH (5.95 g,
0.106 mol) was dissolved in 250 mL of C2H5OH (95%),
and the solution was refluxed for 24 h. The solution was
cooled and neutralized with dilute HCl (1.5 M). The solvent
was removed under vacuum, and the white solids were col-
lected. The solids were dissolved in 50 mL of CH2Cl2 and
then extracted twice with water. The organic layers were
combined and dried over MgSO4. The products, isolated as
white solids, were obtained after recrystallization from
1
CH2Cl2/CH3OH. Yield 78%. H NMR (CDCl3): d 0.87 (t,
3H, –CH3, J¼6.69 Hz), 1.26–1.50 (m, 18H, –CH2), 1.78–
1.84 (m, 2H, –OCH2CH2), 4.09 (t, 2H, –OCH2, J¼
6.53 Hz), 7.13 (s, 1H, –C10H6), 7.16 (d, 1H, –C10H6,
J¼11.34 Hz), 7.70 (d, 1H, –C10H6, J¼8.64 Hz), 7.81 (d, 1H,
–C10H6, J¼8.97 Hz), 7.99 (d, 1H, –C10H6, J¼10.12 Hz),
8.49 (s, 1H, –C10H6). 13C NMR (CDCl3): d 13.92, 22.58,
26.05, 28.84, 29.17, 29.25, 29.32, 29.47, 29.52, 29.57,
31.85, 68.26, 106.68, 119.76, 125.66, 125.88, 126.64,
127.92, 130.65, 130.74, 137.21, 159.11, 166.90.
4. Experimental
4.1. General
1H and 13C NMR spectra were measured on a Bruker DRS-
200 spectrometer. DSC thermographs were obtained on
a Mettler DSC 822. All phase transitions are determined
at a scan rate of 10.0 ꢀC/min. Optical polarized microscopy
was carried out on Zeiss Axioplan 2 equipped with a hot
stage system of Mettler FP90/FP82HT. The UV–vis absorp-
tion and fluorescence spectra were obtained using JASCO
V-530 spectrometer and HITACHI F-4500 spectrophotome-
ter. Elemental analyses for carbon, hydrogen, and nitrogen
were conducted at Instrumentation Center, National Taiwan
University on a Heraeus CHN-O-Rapid elemental analyzer.
Data for all X-ray structures were collected using a
BRUKER SMART CCD diffractometer with graphite-
4.3. 6-(Dodecyloxy)naphthalene-2-carboxylic acid
N-(4-(dodecyloxy)-2-hydroxybenzoyl)hydrazide
(3; n=12, m=12)
The mixture of 6-(dodecyloxy)naphthalene-2-carboxylic
acid (1.5 g, 0.42 mmol) and SOCl2 (3.0 mL, 0.084 mol) dis-
solved in 3.0 mL of dry THF was refluxed for 1 h under
nitrogen atmosphere. The excess SOCl2 and THF were re-
moved under reduced pressure. The solids was redissolved
in 15 mL of dried THF, and transferred to a three-necked
flask. The solution was cooled in an ice bath for further
reaction. The three-necked flask was connected to two
˚
monochromated Mo Ka radiation (l¼0.71073 A) at
150ꢂ1 K. The structure was solved by direct methods and

12444
C.-T. Liao et al. / Tetrahedron 63 (2007) 12437–12445
addition funnels. The first funnel was charged with a solution
of methyl 4-dodecyloxy-2-hydroxybenzoic acid hydrazide
(1.42 g, 0.42 mmol) dissolved in 25 mL of dry THF, and
the second funnel was filled with a solution of TEA
(0.86 g, 0.84 mmol) dissolved in 20 mL of dry THF. Both
solutions in the two funnels were added dropwise to the
acid chloride solution at ice bath temperature. The solution
was stirred for 4 h. Water was poured to the solution, and
solids were collected. The products, isolated as white solids,
were obtained after recrystallization from CH2Cl2/MeOH.
Yield 70%. 1H NMR (CDCl3): d 0.86–0.89 (m, 6H, –CH3),
1.27–1.50 (m, 40H, –CH2), 1.77–1.87 (m, 4H, –OCH2CH2),
3.98 (t, 2H, –OCH2, J¼6.55 Hz), 4.10 (t, 2H, –OCH2,
J¼6.53 Hz), 6.45–6.47 (m, 2H, –C6H3), 7.15 (s, 1H,
–C10H6), 7.21 (d, 1H, –C10H6, J¼11.35 Hz), 7.43 (d, 1H,
–C10H6, J¼9.45 Hz), 7.77–7.83 (m, 3H, Ar–H), 8.31 (s,
1H, –C10H6), 9.01 (s, 1H, –NH), 9.09 (s, 1H, –NH), 11.54
(s, 1H, –OH). 13C NMR (CDCl3): d 13.94, 22.59, 25.92,
26.05, 29.00, 29.15, 29.26, 29.27, 29.33, 29.48, 29.51,
29.53, 29.56, 29.58, 31.85, 68.31, 68.39, 102.37, 105.08,
106.71, 108.00, 120.36, 123.61, 125.99, 126.83,127.43,
127.94, 127.98, 130.51, 137.00, 158.12, 159.21, 163.73,
164.79, 166.38. Anal. Calcd for C42H62N2O5: C 74.74, H
9.26, N 4.15. Found: C 74.52, H 9.25, N 4.13.
1.80 mmol) and P2S5 (3.20 g, 14.4 mol) was dissolved in
20.0 mL of pyridine. The solution was refluxed for 48 h.
The solution was then extracted with CH2Cl2 and washed
with aqueous KOH. The organic layers were collected and
washed again with aqueous HCl. The organic layers were
collected and dried over MgSO4. The products, isolated as
pale yellow solids, were obtained after recrystallization
from CH2Cl2/CH3OH. Yield 74%. 1H NMR (CDCl3):
d 0.87–0.89 (m, 6H, –CH3), 1.28–1.51 (m, 40H, –CH2),
1.77–1.87 (m, 4H, –OCH2CH2), 3.98–4.10 (m, 4H, –OCH2),
6.51 (d, 1H, –C6H3, J¼10.90 Hz), 6.59 (s, 1H, –C6H3),
7.14 (s, 1H, –C10H6), 7.20 (d, 1H, –C10H6, J¼11.2 Hz),
7.40 (d, 1H, –C10H6, J¼11 Hz), 7.77–7.80 (m, 2H, Ar–H),
8.05 (d, 1H, –C10H6, J¼10.00 Hz), 8.29 (s, 1H, –C10H6).
13C NMR (CDCl3): d 13.90, 22.57, 25.94, 26.05, 29.07,
29.18, 29.24, 29.27, 29.32, 29.47, 29.50, 29.55, 29.57,
31.84, 68.34, 68.41, 76.65, 76.90, 77.15, 102.30, 107.01,
107.59, 108.54, 120.28, 124.79, 124.93, 127.72, 127.96,
128.51, 130.10, 130.68, 136.25, 158.86, 159.40, 163.11,
165.44, 169.29. Anal. Calcd for C42H60N2O3S: C 74.96, H
8.99, N 4.16. Found: C 74.91, H 9.03, N 4.15.
4.7. 2-(5-(2-(Dodecyloxy)naphthalen-6-yl)-1,3,4-
thiadiazol-2-yl)phenol (2; n=12, m=0)
4.4. 2-Naphthoic acid N-(4-(dodecyloxy)-2-hydroxy-
benzoyl)hydrazide (3; n=0, m=12)
Light yellow solid, yield 78%. 1H NMR (CDCl3): d 0.88 (t,
3H, –CH3, J¼4.62 Hz), 1.28–1.52 (m, 18H, –CH2), 1.84–
1.86 (m, 2H, –OCH2CH2), 4.09 (t, 2H, –OCH2, J¼
6.55 Hz), 6.94 (s, 1H, –C6H4), 7.12–7.14 (m, 2H, Ar–H),
7.18–7.21 (m, 1H, –C6H4), 7.36 (s, 1H, –C10H6), 7.51 (d,
1H, –C10H6, J¼7.76 Hz), 7.77–7.80 (m, 2H, Ar–H), 8.02
(d, 1H, –C10H6, J¼9.90 Hz), 8.31 (s, 1H, –OH). 13C NMR
(CDCl3): d 13.89, 22.57, 26.05, 29.18, 29.24, 29.33, 29.50,
29.52, 29.55, 29.58, 31.84, 68.35, 107.01, 114.32, 118.10,
119.79, 120.36, 124.72, 124.81, 127.78, 128.16, 128.49,
129.57, 130.14, 132.56, 136.38, 157.49, 159.00, 166.69,
169.31. Anal. Calcd for C30H36N2O2S: C 73.73, H 7.43, N
5.73. Found: C 73.51, H 7.53, N 5.67.
1
White solid, yield 63%. H NMR (CDCl3): d 0.88 (t, 3H,
–CH3, J¼4.57 Hz), 1.27–1.53 (m, 18H, –CH2), 1.73–1.79
(m, 2H, –OCH2CH2), 3.93 (t, 2H, –OCH2, J¼6.53 Hz),
6.38–6.39 (m, 2H, –C10H7), 7.51–7.58 (m, 3H, Ar–H),
7.84–7.90 (m, 4H, Ar–H), 8.38 (s, 1H, –C10H7), 9.23 (s,
1H, –NH), 9.52 (s, 1H, –NH), 11.49 (s, 1H, –OH). 13C
NMR (CDCl3): d 13.89, 22.56, 25.91, 29.01, 29.23, 29.27,
29.47, 29.50, 29.54, 29.56, 31.84, 68.35, 102.37, 105.25,
107.85, 123.15, 126.95, 127.23, 127.76, 128.13, 128.28,
128.51, 128.74, 129.05, 132.62, 135.27, 159.21, 163.54,
164.77, 167.10. Anal. Calcd for C30H38N2O4: C 73.44, H
7.81, N 5.71. Found: C 73.27, H 7.78, N 5.68.
4.8. 5-(Dodecyloxy)-2-(5-(naphthalen-2-yl)-1,3,4-thia-
diazol-2-yl)phenol (2; n=0, m=12)
4.5. 6-(Dodecyloxy)naphthalene-2-carboxylic acid
N-(2-hydroxybenzoyl)hydrazide (3; n=12, m=0)
Light yellow solid, yield 70%. 1H NMR (CDCl3): d 0.89 (t,
3H, –CH3, J¼4.60 Hz), 1.28–1.48 (m, 18H, –CH2), 1.48–
1.81 (m, 2H, –OCH2CH2), 3.99 (t, 2H, –OCH2,
J¼6.55 Hz), 6.52 (d, 1H, Ar–H, J¼11.00 Hz), 6.58 (s, 1H,
Ar–H), 7.39 (d, 1H, –C10H6, J¼8.65 Hz), 7.53–7.54 (m,
2H, –C10H6), 7.85 (d, 1H, –C10H6, J¼5.15 Hz), 7.90–7.92
(m, 2H, Ar–H), 8.07 (d, 1H, –C10H6, J¼10.05 Hz), 8.37 (s,
1H, –C10H6), 11.56 (s, 1H, –OH). 13C NMR (CDCl3):
d 13.89, 22.57, 25.94, 29.07, 29.24, 29.28, 29.47, 29.50,
29.55, 29.57, 31.84, 68.42, 102.32, 107.51, 108.60,
124.29, 127.05, 127.25, 127.65, 127.88, 128.13, 128.63,
129.05, 130.72, 133.21, 134.65, 159.46, 163.22, 165.12,
165.12, 169.73. Anal. Calcd for C30H36N2O2S: C 73.73, H
7.43, N 5.73. Found: C 73.78, H 7.34, N 5.71.
Whitesolid, yield 79%. 1H NMR (CDCl3): d 0.88(t, 3H, –CH3,
J¼4.48 Hz), 1.16–1.52 (m, 18H, –CH2), 1.82–1.88 (m, 2H,
–OCH2CH2), 4.09 (t, 2H, –OCH2, J¼6.48 Hz), 6.85 (d, 1H,
–C10H6, J¼7.50 Hz), 6.95 (d, 1H, Ar–H, J¼8.35 Hz),
7.13–7.20 (m, 2H, Ar–H), 7.38 (s, 1H, –C10H6), 7.62 (d,
1H, –C10H6, J¼7.85 Hz), 7.73–7.83 (m, 3H, Ar–H), 8.30
(s, 1H, –C10H6), 9.19 (s, 1H, –NH), 9.76 (s, 1H, –NH),
11.21 (s, 1H, –OH). 13C NMR (CDCl3): d 13.89, 22.56,
26.04, 29.15, 29.23, 29.31, 29.51, 29.54, 29.56, 31.83,
68.35, 106.79, 112.58, 118.54, 119.20, 120.34, 123.62,
125.86, 127.42, 127.93, 128.12, 130.52, 134.84, 137.06,
159.28, 161.17, 164.96, 166.60. Anal. Calcd for C30H38N2O4:
C 73.44, H 7.81, N 5.71. Found: C 73.39, H 7.83, N 5.70.
4.9. Bis(5-(dodecyloxy)-2-(5-(2-(dodecyloxy)-
naphthalen-6-yl)-1,3,4-thiadiazol-2-yl)phenol)
copper(II) (1; n=12, m=12)
4.6. 5-(Dodecyloxy)-2-(5-(2-(dodecyloxy)naphthalen-6-
yl)-1,3,4-thiadiazol-2-yl)phenol (2; n=12, m=12)
The mixture of 6-(dodecyloxy)naphthalene-2-carboxylic acid
N-(4-(dodecyloxy)-2-hydroxy benzoyl)hydrazide (1.20 g,
A solution of 5-(dodecyloxy)-2-(5-(2-(dodecyloxy)naphtha-
len-6-yl)-1,3,4-thiadiazol-2-yl)phenol (0.15 g, 0.22 mmol)

C.-T. Liao et al. / Tetrahedron 63 (2007) 12437–12445
12445
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the solution of copper(II) acetate hydrate (0.022 g,
0.11 mmol) dissolved in 5.0 mL of CHCl3. The solution
was refluxed for 4 h. The brown solids were filtered and col-
lected. The products were obtained by recrystallization from
CH2Cl2/CH3OH. Yield 84%. Anal. Calcd for C84H118Cu-
N4O6S2: C 71.68, H 8.45, N 3.98. Found: C, 71.69, H
8.51, N 3.96. MS (FAB): 1407.4 (MH+).
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Acknowledgements
We thank the National Science Council of Taiwan, ROC
(NSC-94-2738-M-008-001)forgeneroussupportofthiswork.
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