Alkoxy substituted (E,E)-3,6-bis(styryl)pyridazine-a photosensitive mesogen for liquid crystals

Article abstract of DOI:10.1016/j.tet.2008.04.033

(E,E)-3,6-Bis(styryl)pyridazines (3a-t) bearing 2, 4 or 6 alkoxy chains were prepared by applying the Siegrist reaction of 3,6-dimethylpyridazine (13) and the corresponding azomethines 10a-t. The transversal dipole moment of these calamitic compounds effe

Full text of DOI:10.1016/j.tet.2008.04.033

Tetrahedron 64 (2008) 6551–6560
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Alkoxy substituted (E,E)-3,6-bis(styryl)pyridazineda photosensitive
mesogen for liquid crystals
*
Thorsten Lifka, Georg Zerban, Peter Seus, Annette Oehlhof, Herbert Meier
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
(E,E)-3,6-Bis(styryl)pyridazines (3a–t) bearing 2, 4 or 6 alkoxy chains were prepared by applying the
Siegrist reaction of 3,6-dimethylpyridazine (13) and the corresponding azomethines 10a–t. The transversal
dipole moment of these calamitic compounds effects an extremely high tendency for self-organization in
thermotropic LC phases (N, SA, SB, SC, SE, SI/F, and Cub). The conjugated core structure represents moreover
a chromophore with a high photosensitivity for (E,E)%(E,Z) isomerization reactions: this property makes
the compounds interesting for optical imaging and switching techniques.
Received 25 January 2008
Accepted 8 April 2008
Available online 11 April 2008
Keywords:
Condensation reaction
Liquid crystals
Ó 2008 Published by Elsevier Ltd.
Photoisomerization
Self-organization
1. Introduction
and fluorescence bands and an increase of absorption and emission
intensity (3max and fluorescence quantum yield fF), it changes also
Stilbenoid compounds play an important role in materials
science because their photo- and electroactive chromophores/
fluorophores have tunable properties for various applications as
optical brighteners, laser dyes, scintillators, (photo)conductors,
photoresists, nonlinear optical (NLO) materials, electroluminescent
materials, etc.1,2 Moreover, stilbenes and related systems belong to
the best studied classes of compounds in organic photochemistry.2
They can show E/Z isomerization reactions, (oxidative) cyclizations,
cyclodimerizations, and crosslinking processes on irradiation.
Extensionofthe (E)-stilbenechromophoreto(E,E)-1,4-bis(styryl)-
benzene (1) (Scheme 1) provokes not only a red-shift of absorption
the photochemical behavior. Compound 1 has an extremely low
tendency for photoisomerization reactions (fE/Zz10ꢀ4).2
Therefore cyclization reactions cannot be observed. Preparative
[2pþ2p]cyclodimerization processes are also not usual; they occur
at most as by-reactions in photocrosslinkings on irradiation with
energy-rich UV light.2 Thus, 1 represents a rigid but relatively pho-
tostable mesogen for calamitic and phasmidic LC systems (liquid
crystals).3 A degradation of the mesophases is principally possible,
when hard UV light (254 nm) is used. Moreover, the conjugated
scaffold of 1 can be applied for photoinduced one-dimensional en-
ergyandelectrontransfer4,5 andforthirdorderopticalnonlinearity.6
To maintain all these properties, and to guarantee additionally
facile and effectively reversible optical switching processes, we
slightly changed the structure concept and involved N atoms in the
central ring, which should enhance the possibility of E/Z isomeri-
zations in the triplet state. Scheme 1 depicts the bis(styryl)azines
2–7, which contain 1–4 nitrogen atoms.
A
C
B
D
R
R
Theparent system, pyridine 2 (R¼H) and somesimple derivatives
(R¼p-CH(CH3)2, p-C6H5, o-OCH3, m-OCH3, p-OCH3, p-OC2H5, p-
N(CH3)2, and p-COOCH3) are known.7,8 Their flexible side chains are
not long enough to permit mesophase formation. An analogous
statement is valid for pyridazine 3, for which the parent system
(R¼H) and some derivatives (R¼o-F and p-F) were reported.9,10 The
pyrimidines 4 and the triazines 6 aredto our best knowledged
unknown. The parent pyrazine 5 (R¼H) and some of its derivatives
(R¼p-CH3, p-C2H5, p-C6H5, and p-OCH3) can be found in the litera-
ture.11–15 Until now, none of the azines 2–6 served as mesogen in
liquid crystalline compounds. Some years ago, we prepared the first
tetrazines of structure 7, the parent compound (R¼H) and some
1
2
3
4
5
6
7
A
CH
N
N
N
N
N
N
N
N
N
N
B
C
D
CH CH
CH CH
N
CH CH CH
N
CH
N
N
CH CH CH CH
CH
Scheme 1. 1,4-Bis(styryl)benzenes 1 and the related azines 2–7 with 1–4 nitrogen
atoms in the central ring (R¼H, alkyl, alkoxy, etc.).
* Corresponding author. Fax: þ49 6131 3925396.
E-mail address: hmeier@mail.uni-mainz.de (H. Meier).
0040-4020/$ – see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tet.2008.04.033

6552
T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
R1
R3
R1
R3
derivatives (R¼p-OH, p-OCOCH3, p-OC12H25
, m-OC12H25, and
p-OC12H25).16 3,6-Bis(4-dodecyloxystyryl)-1,2,4,5-tetrazine showed
a broad nematic phase in the temperature range between 228.5 and
156.1 ꢁC (DSC cooling curve). Whereas the solution photolysis of 7
led to the fragmentation of the tetrazine ring, the LC phase proved to
be rather photostable. A specialty of the tetrazine ring system is the
efficient internal conversion S2(pp*)/S1(np*), which causes the
loss of the typical stilbene photochemistry.
O
H
N
R2
NH2
R2
+
8
9
10a-t
12
CH3
6
CH3
CH3
H3C
7
5
1
8
4
H2N–NH2
Pd / C
Now, we decided to use the pyridazine structure 3 as mesogen.
The transversal dipole moment should enhance the tendency of
self-organization and the existence of pp* and np* states should
favor the formation of triplet states by intersystem crossing (ISC)
and consequently favor reversible E%Z photoisomerization
reactions.
N
N
O
O
N
3
- H2
N
9
11 N
CH3
10
2
N
H
CH3
CH3
CH3
11
12
13
R1
2. Results and discussion
R1
R3
N
N
R2
13
+ 10a-t
2.1. Synthesis of the (E,E)-3,6-bis(styryl)pyridazines 3a–t
R2
R3
(E,E)-
3a-t
The synthetic approach to the target compounds 3a–t is
illustrated in Scheme 2. The corresponding aldehydes 8a–t were
transformed with aniline (9) tothe azomethines(Schiff bases) 10a–t.
The condensation process was performed in the absence of solvents
and afforded quantitative yields when the generated water was
continuously removed under reduced pressure.
3,6-Dimethylpyridazine (13) was obtained from 2,5-hex-
anedione (11) and hydrazine. The intermediate tautomeric
dihydropyridazines generated the tricyclic dimer 12, which was
dehydrogenated to 13.9,17–19
3
a
b
c
d
e
f
R1
H
H
H
H
H
H
R2 OCH3 OC2H5 OC4H9 OC6H13 OC8H17 OC10H21
R3
H
H
H
H
H
H
3
g
h
i
j
The subsequent twofold Siegrist reaction20,21 13þ10a–t led
under kinetic control to highly pure (E,E) configurations of 3a–t.
Eventually present (E,Z) configurations were below 2%, the here
achieved detection limit in the 1H NMR spectroscopy. The yields of
3a–t vary strongly. Apart from the different electrophilicity of the
imine carbon atom, steric shielding of the reactive center, caused
by long alkoxy chains, and in particular the solubility of the com-
ponents seem to have a great effect. The highest yields (60–70%)
were obtained for 3d,g,l,m and the lowest yields (<10%) for
3b,c,e,i,j,o. An increasing number of alkoxy chains enhances the
solubility of 3 and 10 in many organic solvents.22 The length of the
chains has also a big effect on the solubility; it increases in CHCl3
from OCH3 to OC6H13 and decreases then for longer alkoxy chains,
whereby OC12H25 seems to represent another solubility maximum.
The 1H and 13C NMR data of the target compounds 3 are listed
R1
H
H
H
H
R2 OC12H25 OC18H37
H
H
H3C
O
CH3
O
H
C6H13
C6H13
R3
H
H
H
3
k
l
m
n
R1
H
H
H
H
R2 OC6H13 OC12H25 OC16H33 OC12H25
R3 OC6H13 OC12H25 OC16H33 OCH3
3
o
p
q
r
s
t
in Tables 1 and 2. The averaged chemical shifts of the central
p
R1 OCH3 OC6H13 OC10H21 OC12H25 OCH3
OCH3
system of 3a–t are illustrated in Figure 1. The alkoxy substituted
3,6-bis(styryl)pyridazines are conjugated donor–acceptor–donor
systems (D–A–D).1l This polarization is characterized by the 13C
shift difference Dd of the olefinic carbon atoms, which amounts to
11.3ꢂ2.3 ppm. The (E) configurations are proved by 3J(H,H)¼
16.2ꢂ0.2 Hz.
R2 OCH3 OC6H13 OC10H21 OC12H25 OC12H25 OC12H25
R3 OCH3 OC6H13 OC10H21 OC12H25 OCH3
OC12H25
Scheme 2. Preparation of the 3,6-bis(styryl)pyridazines (E,E)-3a–t. (Dimer 12 consists
of (1R,5R,6R)- and (1S,5S,6S)-1,4,7,9-tetramethyl-2,3,10,11-tetraazatricyclo[6.3.1.02,7]-
dodeca-3,9-diene).18
It can be assumed that the reaction of racemate 10i and 13 led
to a racemate (R,R)-3i/(S,S)-3i and the diastereomeric meso form
(R,S)-3i. The 1H NMR spectrum in CDCl3 contains for the CH3 group
on the chiral center a doublet at 1.30 ppm. Application of Pirkle’s
reagent [(R)-2,2,2-trifluoro-1-(9-anthryl)ethanol] causes a splitting
into two doublets of equivalent intensity for the R and the S con-
figured substructures. A differentiation between the diastereomers
is neither by this method nor with the shift reagent Eu(facam)3
possible.23 Since the reaction centers are far away from each other,
one can expect for 3i a statistical ratio (R,R)/(S,S)/(R,S)¼1:1:2.
In contrast to (E,E)-1,4-bis(styryl)benzenes 1,3 even short alkoxy
chains in p-position of the terminal benzene rings of 3 are sufficient
for the generation of mesophases. Figure 2 shows the temperature
ranges in which different phases of the p-substituted series 3a–h
with various OCnH2nþ1 groups exist. The mesomorphic behavior of
these compounds is outstanding. The decisive difference between
the rigid mesogens of 1 and 3 consists in the transversal dipole
moment of the heterocycles 3.
Compounds 3a–d (n¼1, 2, 4, 6) form nematic phases at high
temperatures. The upper limit of their temperature range
(T>300 ꢁC) is determined by starting decomposition. When tem-
peratures above 300 ꢁC are avoided, the second heating curves in
the differential scanning calorimetry (DSC) have the same maxima
2.2. Phase behavior of the (E,E)-3,6-bis(styryl)pyridazines 3a–t
The (E,E)-3,6-bis(styryl)pyrazine structure 3 proved to be an
ideal mesogen for the formation of thermotropic liquid crystals.24

T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
6553
Table 1
1H NMR data of the pyridazines 3a–t (
d values in CDCl3, TMS as internal standard)
Compound 3
Pyridazine (s, 2H)
Double bonds (AB, 4H)a
-CH
Benzene rings
(AA0BB0, ABC, A2)
Alkoxy groups
OCH3/2/1
a
b-CH
CH2 (m)
CH3 (t)
a
b
c
d
e
f
7.54
7.54
7.53
7.54
7.53
7.54
7.50
7.51
7.53
7.23
7.21
7.22
7.22
7.22
7.23
7.21
7.21
7.23
7.56
7.58
7.56
7.53
7.56
7.56
7.53
7.60
7.57
6.92, 7.53 (8H)
6.91, 7.52 (8H)
6.90, 7.52 (8H)
6.91, 7.56 (8H)
6.90, 7.52 (8H)
6.90, 7.52 (8H)
6.88, 7.48 (8H)
6.93, 7.53 (8H)
6.88, 7.51 (8H)
3.84 (s, 6H)
4.06 (q, 4H)
3.99 (t, 4H)
3.98 (t, 4H)
3.98 (t, 4H)
3.78 (t, 4H)
3.95 (t, 4H)
3.98 (t, 4H)
4.39 (m, 2H)
1.42 (6H)
0.98 (6H)
0.90 (6H)
0.89 (6H)
0.89 (6H)
0.83 (6H)
0.89 (6H)
0.87 (6H)
1.28 (d, 6H)
0.88 (12H)
1.78 (4H), 1.49 (4H)
1.79 (4H), 1.45 (4H), 1.36 (8H)
1.78 (4H), 1.45 (4H), 1.27 (16H)
1.78 (4H), 1.45 (4H), 1.22 (24H)
1.75 (4H), 1.40 (4H), 1.22 (32H)
1.80 (4H), 1.45 (4H), 1.25 (56H)
1.75 (2H), 1.42 (4H), 1.31 (20H)
1.55 (2H)
gb
h
i,j
k
lb
m
n
o
p
q
r
7.56
7.53
7.55
7.58
7.61
7.59
7.59
7.59
7.61
7.60
7.23
7.16
7.23
7.25
7.31
7.27
7.28
7.28
7.32
7.29
7.49
7.46
7.53
7.51
7.52
7.46
7.46
7.47
7.54
7.49
6.86, 7.08, 7.17 (6H)
6.81, 7.05, 7.10 (6H)
6.88, 7.08, 7.11 (6H)
6.87, 7.09, 7.16 (6H)
6.83 (4H)
4.02 (t, 4H)
4.05 (t, 4H)
3.93 (t, 4H)
3.97 (t, 4H)
4.02 (t, 4H)
4.05 (t, 4H)
3.92 (s, 6H)
4.05 (t, 4H)
3.89 (s, 6H)
3.89 (s, 12H)
3.97 (t, 4H)
4.01 (t, 8H)
3.98 (t, 4H)
4.00 (t, 8H)
3.97 (t, 4H)
4.01 (t, 8H)
3.88 (s, 12H)
4.00 (t, 4H)
3.88 (s, 6H)
4.02 (t, 4H)
4.05 (t, 4H)
1.85 (8H), 1.44 (8H), 1.27 (16H)
1.73 (8H), 1.39 (8H), 1.18 (64H)
1.85 (8H), 1.49 (8H), 1.28 (96H)
1.85 (4H), 1.45 (4H), 1.25 (32H)
0.80 (12H)
0.88 (12H)
0.89 (6H)
6.79 (4H)
1.74 (4H), 1.45 (12H), 1.34 (24H)
1.81 (8H)
1.75 (4H), 1.45 (12H), 1.28 (72H)
1.85 (8H)
1.74 (4H), 1.48 (12H), 1.25 (96H)
1.82 (8H)
1.72 (4H), 1.45 (4H), 1.29 (32H)
0.88 (18H)
0.88 (18H)
0.87 (18H)
0.88 (6H)
0.88 (12H)
6.80 (4H)
6.79 (4H)
s
6.84 (4H)
t
6.80, 6.81 (AB, 4H)
1.85 (4H), 1.45 (8H), 1.24 (64H)
1.72 (4H)
a
3J¼16.2ꢂ0.2 Hz.
b
Measurement in C2D2Cl4 at 50 ꢁC.
as the first heating curves. The onset temperatures (Fig. 3) of the
phase transitions can differ a little bit, because the peak shape of
the second heating curve can be somewhat different from that of
the first heating curve.
SC phase at 201 ꢁC (onset temperature) is characterized by the
typical generation of rhombic domains (upper part of Fig. 5).26
The SC phases of 3a–h, which are stable in a wide temperature
range, stimulated us to introduce a chirality center in the alkoxy
chains and to study the possibility of ferro- or antiferroelectric
phases. Therefore, we synthesized 3i [racemate of (R,R) and (S,S)
configurations and meso form (R,S)] and 3j, the pure (S,S) isomer.
The branching of the alkoxy chains led to a drastic decrease of the
phase transition temperatures. The (S,S) isomer 3j shows in the
second DSC heating curve, the formation of a smectic phase at
A butoxy chain is already sufficient for the self-organization in
a smectic phase. The strongly fluctuating textures of the nematic
systems are then transformed on cooling to textures in which
‘Schlieren’ regions can be seen as well as fan-shaped regions.
Homeotropic and homogenous oriented domains cause this co-
existence, which permits the assignment of the smectic phases as
SC.25 Further cooling leads for nꢃ6 to another phase transition with
112 ꢁC (
D
H¼22 kJ molꢀ1). The cooling process I/S has an onset
a very small enthalpy
DH (Figs. 2 and 3). The spacial impression of
temperature of 138 ꢁC (
D
H¼ꢀ2 kJ molꢀ1) and crystallization occurs
the fan-shaped structures disappears, transverse fissures appear
and a fine granulation becomes evident. The ‘Schlieren’ texture in
the homeotropic regions becomes in wide areas mosaic-like. Such
typical changes are based on the formation of hexagonal positions
of the mesogens, which were before fairly mobile in the SC phase.
The molecules can be tilted in the direction of a tip or a plane of
the hexagon. Without further distinction between these tilted
at 89 ꢁC (
D
H¼ꢀ21 kJ molꢀ1). Close to the crystallization/melting
peak toward lower temperatures are transitions between different
crystalline phases.
The racemate 3i melts at 124 ꢁC (
D
H¼25 kJ molꢀ1) to the SB
phase, which forms the isotropic phase at 144 ꢁC (
D
H¼3 kJ molꢀ1
)
and cooling leads then back at 130 ꢁC (
D
H¼ꢀ2 kJ) to the SB phase
and at 106 ꢁC (
D
H¼ꢀ23 kJ molꢀ1) to the first crystalline phase. (The
hexatic phases, we designate this phase as SF/I
.
25 Figure 4 illustrates
DSC processes were studied with rates of 10 and 3 K minꢀ1.) The
assignment of the smectic phase to the hexatic SB order is based on
typical mosaic structures of the texture.27
as an example the phase behavior of 3g (n¼12). The isotropic melt
(I) is transformed at 259.4 ꢁC (532.5 K) to the SC phase with an
exothermic transition enthalpy
(443.7 K), another exothermic phase transition (
occurs, which causes the above-mentioned and in Figure 4 visible
texture changes for the process SC/SF/I. Finally, the now more
viscous phase crystallizes at 129.7 ꢁC (402.8 K) with a strong exo-
D
H of ꢀ7 kJ molꢀ1. At 170.6 ꢁC
The slight modification of the alkoxy chains by introduction of
D
H¼ꢀ2 kJ molꢀ1
)
an a-CH3 group permits the molecules to adopt an arrangement
parallel to the director of the smectic layer, so that S*C, SC*A, and
S*F/I phases are not formed.
Increasing length of the alkoxy chains in 3a–h provokes a de-
crease of the melting and clearing points, which is attended by a
broadening of the mesophase range. Nevertheless, all phase tran-
sition temperatures of 3a–h are above 130 ꢁC, which is too high for
any application. Therefore, we introduced further alkoxy groups in
3- and 5-position of the terminal benzene rings. Compound 3k
thermic transfer enthalpy
D
H¼ꢀ34 kJ molꢀ1
.
Interestingly 3h (n¼18) does notshowthe directtransition I/SC,
which is observed for 3e–g (Fig. 3). A drastically enhanced
viscosity indicates at 216 ꢁC that the isotropic appearance of the
probe originates from a cubic mesophase, whose transfer to the

6554
T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
Table 2
13C NMR data of the pyridazines 3a–3g, and 3i,j–3t (
d
values in CDCl3, TMS as internal standard)
Compound 3
Pyridazine
CH
Double bonds
-CH
Benzene rings
CH
Alkoxy groups
OCH3/2/1
a
Cq
a
b-CH
Cq
CqO
CH2
CH3
a
b
c
d
e
123.5
123.5
123.5
123.5
123.5
156.4
156.4
156.4
156.4
156.5
123.1
123.0
123.0
123.0
123.1
133.8
133.8
133.8
133.8
133.9
114.3, 128.7
114.8, 128.7
114.8, 128.7
114.8, 128.6
114.9, 128.6
129.0
128.8
128.8
128.7
128.9
160.3
159.7
159.9
159.9
160.0
55.4
63.6
67.8
68.1
68.2
14.8
13.8
14.0
14.0
19.2, 31.1
22.6, 25.7, 29.2, 31.6
22.6, 26.0, 29.2, 29.3, 29.4,
31.8
f
123.5
124.6
123.4
123.4
124.1
123.4
123.4
123.5
123.4
123.4
123.4
123.5
123.4
156.7
156.6
156.4
156.5
156.6
156.5
156.5
156.4
156.4
156.4
156.5
156.4
156.5
123.0
122.9
123.0
123.1
123.1
123.3
121.4
124.7
124.4
124.4
124.4
124.6
124.5
133.9
134.6
133.8
134.1
134.9
134.3
134.2
134.4
134.5
134.6
134.6
134.5
134.5
114.9, 128.6
115.4, 129.0
116.0, 128.7
128.7
128.9
128.6
129.2
129.6
129.7
129.2
131.7
131.2
131.2
131.3
131.4
131.6
159.9
160.4
159.0
68.1
68.6
74.0
22.6, 26.0, 29.2, 29.3, 29.4,
29.6, 31.9
22.9, 26.3, 29.5, 29.6, 29.8,
29.9, 32.2
13.9
14.4
gb
i,j
k
l
22.6, 25.5, 29.2, 31.8, 36.4
14.1
19.7
14.1
111.7, 121.4
113.4
113.6, 121.9
114.7
112.8, 121.4
114.3
109.8, 121.4
112.9
153.3
149.4
150.0
151.1
149.8
150.7
149.8
149.9
139.4
153.6
139.3
153.2
139.6
153.4
139.6
153.4
138.8
153.8
139.1
153.3
153.9
69.2
69.4
69.9
71.1
69.6
69.8
56.1
69.2
56.2
61.0
69.1
73.5
69.3
73.6
69.3
73.6
56.3
73.7
56.3
69.3
73.7
22.6, 25.6, 25.7, 29.2, 29.3,
31.6
22.9, 26.3, 26.4, 29.7, 29.8,
29.9, 30.0, 31.1
22.6, 26.0, 29.3, 29.4, 29.6,
31.9
22.7, 26.0, 29.2, 29.3, 29.4,
29.5, 29.6, 29.7, 32.0
14.4
14.0
14.1
m
n
o
p
q
r
104.6
105.8
106.1
106.1
104.8
22.6, 25.7, 29.3, 30.2, 31.5,
31.7
22.6, 26.1, 29.3, 29.4, 29.6,
29.7, 30.9, 31.9
22.7, 26.1, 29.3, 29.4, 29.6,
29.7, 30.4, 31.9
22.7, 25.9, 29.4, 29.5, 29.7,
31.2
14.1
14.1
14.1
14.1
14.1
s
t
104.7
106.1
22.7, 26.0, 26.1, 29.3, 29.4,
29.5, 29.6, 30.3, 31.9
a
The CH2 signals of long alkoxy chains are often superimposed.
Measurement in C2D2Cl4 at 50 ꢁC.
b
0.07
134.4 0.6
7.53
with two hexyloxy groups on each benzene ring shows in the DSC
at 128.0 ꢁC (onset temperature) a relatively sharp melting point.
7.56 0.06
124.0
0.6
H
H
The crystalline phase passes over into the isotropic melt (
DH¼
156.5 0.2
52 kJ molꢀ1). Compounds 3l (p-OC12H25 and m-OC12H25) and 3m
(p-OC16H33 and m-OC16H33) behave in the first and second heating
1.5
R
130.2
H
N
N
7.24
0.08
1.7
123.1
272 (38)
250 (-29)
~310
~310
Figure 1. 1H and 13C NMR data of the styrylpyridazine substructure of 3a–t. (Mea-
surement in CDCl3, TMS as internal standard).
3a
K
I (decomp.)
I (decomp.)
N
230 (18)
220 (-15)
224 (20)
209 (-19)
3b
3c
K
K
N
258 (3)
256 (-3)
170 (2)
168 (2)
170 (1)
~310
N
SC
I (decomp.)
161 (19)
149 (-19)
158 (19)
283 (3)
SC
~305
N
3d
3e
K
SF/I
I (decomp.)
282 (-3)
285 (a)
K
K
K
SF/I
SF/I
SF/I
SC
I
I
I
150 (-21)
151 (27)
163 (-1)
183 (2)
280 (a)
293 (a)
SC
3f
145 (-26)
139 (36)
182 (-1)
168 (2)
292 (a)
266 (10)
SC
3g
3h
259 (-7)
130 (-34)
136 (44)
130 (-46)
171 (-2)
152 (3)
156 (-2)
211 (2)
SC
223 (2)
Cub
SF/I
K
I
201 (-2)
216 (-2)
Figure 2. Phases of (E,E)-3,6-bis[2-(4-alkoxyphenyl)vinyl]pyridazines 3a–3h
(OCnH2nþ1, n¼1, 2, 4, 6, 8, 10, 12, 18). K: crystalline, N: nematic, SC, SF/I: smectic, Cub:
cubic, and I: isotropic phase. (The transition temperatures refer to the heating curve in
the DSC).
Figure 3. Transitions of 3a–3h between crystalline (K), smetic (SF/I, SC), nematic (N),
and isotropic (I) phases. Upper transition: onset temperature of second heating curve,
ꢁC, (
D
H in kJ molꢀ1); lower transition: onset temperature of second cooling curve, ꢁC,
(
D
H in kJ molꢀ1); (a)
DH not determined.

T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
6555
Figure 5. Upper part: formation of the texture of the SC phase from the cubic phase of
3h at 213 ꢁC, scale 1:90. Lower part: texture of the SE phase of 3n, measured at 141 ꢁ
C
close to the clearing point, scale 1:90.
appropriate for the SA phase. The increased cylinder volume, caused
by the additional alkoxy chains, enhances the chances of the cisoid
conformation.
In contrast to the monotropic mesophase formation of 3l and 3m,
3n (p-OC12H25 and m-OCH3) exhibits enantiotropic phase transi-
tions. The crystalline state is transformed in the second heating
curve at 61 ꢁC (
D
H¼33 kJ molꢀ1) to the smectic phase and at 145 ꢁC
(
D
H¼50 kJ molꢀ1) the isotropic melt is generated. The cooling pro-
cess (measured with the same rate of 10 K minꢀ1) exhibits the I/S
Figure 4. Characterization of the mesophases of 3,6-bis[2-(4-dodecyloxyphenyl)-
vinyl]pyridazine (3g): (a) DSC diagram (second heating curve, 10 K minꢀ1), (b) cooling
curve, 10 K minꢀ1, and (c) textures obtained in the polarization microscope at 417 and
473 K, respectively. (Scale 1:115).
transition at 135 ꢁC (
crystalline state (S/K) at 56 ꢁC (
D
H¼ꢀ46 kJ molꢀ1) and the formation of the
D
H¼ꢀ33 kJ molꢀ1). It is remarkable
that the smectic mesophase of 3n exists in a much broader tem-
perature range than the mesophases of 3l and 3m. Moreover, the
transition enthalpy DH of 3n is higher for the clearing process S/I
curves similarly. They show at 122.2 ꢁC (
D
H¼92 kJ molꢀ1) and
than for the melting process. These results demonstrate, that the
smectic phase of 3n is highly ordered. The texture of 3n contains
a bulky mosaic structure, which points to an SE phase.29 Figure 5
(lower part) shows that this texture is preserved till the clearing
process starts. Compounds 3o–3t have alkoxy groups in p- and both
m-positions of the terminal benzene rings. The hexamethoxy sys-
tem 3o is a yellow solid, which decomposes above 170 ꢁC. Com-
pounds 3p and 3q exist at room temperature in highly viscous
smectic phases with clearing points at 77 and 96 ꢁC, respectively.
Crystallization cannot be observed in DSC down to ꢀ70 ꢁC. Figure 7
summarizes the phase transitions of 3r–t. Whereas the transition
122.6 ꢁC (
D
H¼117.5 kJ molꢀ1), respectively, such K/I transitions,
but in their cooling curves, smectic phases can be observed in
relatively narrow temperature ranges (3l: T [ꢁC] 123.7–106.4,
DH
[kJ molꢀ1] 4 and 96; 3m: T [ꢁC] 122.7–108.9, H [kJ molꢀ1] 4 and
D
124). The textures, obtained for these smectic phases, point to an SA
character.28 The models in Figure 6 illustrate the influence, which
the additional, long alkoxy chains in m-position can have on the
orientation of the molecules in the smectic phases. Whereas the
transoid arrangement of the two olefinic double bonds is favorable
for the SC phase, the cisoid arrangement seems to be more

6556
T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
OR
OR
OR
OR
RO
RO
N
N
N
N
N
N
N
N
OR
OR
RO
RO
OR
OR
SC
SA
Figure 6. Model for the arrangement of dialkoxy-substituted bis(styryl)pyridazines
3c–h in smectic SC phases (transoid double bonds in narrow cylindric volumina) and
tetralkoxy-substituted bis(styryl)pyridazines (3l–n) in smectic SA phases (cisoid double
bonds in wider cylindric volumina).
Figure 8. X-ray scattering of compound 3r in the SA phase at room temperature.
2.3. Light-sensitivity of the (E,E)-3,6-bis(styryl)pyridazines 3
14 (31)
4 (-26)
84 (5)
Compounds 3a–t exhibit strong long-wavelength absorption
bands with maxima between 360 and 380 nm. (E,E)-3,6-Bis[2-(4-
hexyloxyphenyl)vinyl]pyridazine (3d), for example, has a lmax
value of 372 nm (3max¼52,000 L molꢀ1 cmꢀ1) in benzene. The ab-
sorption of light of the visible and near UV region renders these
compounds light-sensitivity. Figure 8 shows UV/vis spectra of the
irradiation of 3d in a 1.96ꢄ10ꢀ4 M solution in oxygen-free benzene
with a Hanovia medium pressure lamp equipped with a Pyrex filter
I
I
3r
3s
3t
K
K
K
SA
S
82 (-5)
11 (15)
3 (-10)
91 (54)
66 (-50)
30 (12)
54 (38)
I
S,..,S
(l
ꢃ290 nm). Isosbestic points as well as the 1H NMR reaction
18 (-28)
52 (-12)
spectra reveal a clean process (E,E)-3d%(E,Z)-3d. The principally
possible further isomer (Z,Z)-3d can be excluded. According to the
1H NMR spectroscopy, the photostationary state hasdunder the
above-mentioned reaction conditionsdthe composition (E,E)-3d/
(E,Z-3d)¼54:46. This ratio was used to calculate the absorption
spectrum of the pure (E,Z) configuration of 3d, which is shown in
Figure 9.
Figure 7. Transitions of 3r, 3s, and 3t between crystalline (K), smectic (S), and isotropic
(I) phases. Upper transition: second heating curve in the DSC, onset temperature in ꢁ
(endothermic
H in kJ molꢀ1); lower transition: cooling curve in the DSC, onset
temperature in ꢁC (exothermic H in kJ molꢀ1). The rate of all measurements was
10 K minꢀ1
C
D
D
.
The 1H NMR spectrum of (E,Z)-3d corresponds to the reduced
symmetry. Table 3 shows the comparison of the data of (E,E)-3d and
(E,Z)-3d.30
enthalpies
idazine (3r) resemble the ‘normal’ behavior of compounds 3a–h
with a high H for K/S and a much lower H for S/I, but 3s shows
like3nthe opposite behavior, a muchlarger Hfor S/IthanforK/S,
DH of 2,6-bis[2-(3,4,5-tridodecyloxyphenyl)vinyl]pyr-
D
D
Now the crucial question is, how can these isomerization pro-
cesses be used in materials science? (E,Z) Configured 3,6-bis(styryl)-
pyridazines 3 are certainly not suitable for LC phases. Even small
D
which is again a strong indication for a highly ordered smectic
phase. The DSC measurements of 3t were much more complicated,
because the second heating and cooling curves exhibit several
closely lying smectic phases. The endothermic behavior of the
heating curve between 30 and 54 ꢁC is interrupted by an exothermic
part (reentrant phase).
According to the temperature range of the LC phases, com-
pounds 3p–3s are certainly the most suitable for applications, since
they exist at room temperature in smectic phases. In principle such
phasmidic systems could exist also in a columnar arrangement. In
order to exclude this possibility, we made for 3r an X-ray scattering
measurement, which is shown in Figure 8. The highest peak cor-
responds to a distance of 29.2 Å, which represents the thickness of
a smectic layer SA with highly flexible chains and relatively low
order. (Molecular models reveal that 3r would have a length of
about 45 Å, in the maximum stretched form.) Reflections for co-
lumnar or higher ordered smectic phases could not be found. The
halo at 2
q
¼20ꢁ corresponds to 4.5 Å and represents the average
Figure 9. UV/vis absorbance A of a 1.96ꢄ10ꢀ4 M solution of 3d in benzene: (E,E)- and
(E,Z)-3d, Ph.St.: photostationary state (E,E)/(E,Z)¼54:46.
intermolecular distance. The texture of 3r is in accordance with an
SA phase.24b,28

T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
6557
Table 3
1H NMR data of (E,E)-3d and (E,Z)-3d (
d values in C6D6, TMS as internal standard)
Compound
Pyridazine ring
(E)-Styryl group
(Z)-Styryl group
OCH2
Double bond
Benzene ring
Double bond
Benzene ring
(E,E)-3d
(E,Z)-3d
d
6.81
A2
7.36, 7.69
AB
16.1
6.83, 7.37
AA’BB’
3.61
Spin system
3J [Hz]
d
6.53, 6.99
AB
8.2
7.29, 7.53
AB
16.1
6.80, 7.30
AA’BB’
6.72, 7.03
AB
12.4
6.73, 7.18
AA’BB’
3.59, 3.61
Spin system
3J [Hz]
portions of ‘wrong’ configuration can disturb the self-organization
of mesogens. Processes I/LC, in which an isotropic phase is trans-
formed to an LC phase by the photoisomerization (E,Z)-3/(E,E)-3 of
a certain content of ‘wrong’ configuration, seem to be easily feasible.
However, the opposite process (E,E)-3/(E,Z)-3 may cause difficul-
ties in self-organized, constrained phases. The higher the order of
such LC phases is, the less likely are such photoisomerizations. The
great manifold of smectic phases of (E,E)-3 provides in this context
a very interesting research area.31
80 ꢁC in the absence of a solvent. The vacuum of 2.0–2.1 kPa served
for the removal of the generated water. IR or TLC control indicated
the end of the quantitative reaction. The excess aniline was then
removed at a reduced pressure of 0.1 kPa. The light yellow residue
could directly be used for the subsequent reaction step.
The majority of the Schiff bases 10a–t was identified by com-
parison with authentic samples: 10a,32 10b,33 10c,34 10d,35 10e,36
10f,37 10g,3 10h,38 10k,39 10l,40 10m,40 10o,20 10p,41 and 10r.40
4.2.1. N-[4-(1-Methylheptyloxy)benzylidene]aniline (10i)
3. Conclusion
Yellowish oil, yield: 7.73 g (quantitative) obtained from alde-
hyde 8i.42 1H NMR (CDCl3):
d 0.87 (t, 3H, CH3), 1.30 (d, 3H, CH3), 1.32
A large variety of 3,6-bis(styryl)pyridazines 3a–t with 2, 4 or 6
alkoxy groups on the terminal benzene rings were prepared as pure
(E,E)-isomers by using kinetically controlled Siegrist reactions. Even
short alkoxy chains (OCH3 or OC2H5) are sufficient to induce self-
organization in LC phases. Depending on the number and length of
the OCnH2nþ1 chains, nematic (N), smectic (SA, SB, SC, SE, and SI/F),
and cubic (Cub) orders were assigned to the thermotropic liquid
crystals. The characterization of the mesophases is predominantly
based on DSC measurements and observations of typical textures in
the polarization microscope. The conjugated structurewith a central
pyridazine ring renders compounds (E,E)-3 not only the character
of a rigid mesogen with a high aggregation tendency, it represents
also a chromophore (absorption at the UV/vis border), which con-
veys compounds 3 a highly photosensitive character for (E,E)%(E,Z)
isomerization reactions. The application of such processes in ma-
terials science will be reported in a forthcoming publication.
(m, 6H, CH2), 1.38 (m, 2H, CH2), 1.59 (m, 1H, CH2), 1.73 (m, 1H, CH2),
4.45 (m, 1H, CH), 6.95/7.82 (AA0BB0, 4H, p-phenylene), 7.18 (m, 3H,
phenyl), 7.38 (m, 2H, phenyl), 8.37 (s, 1H, CHN); 13C NMR (CDCl3):
d
14.0, 19.6 (CH3), 22.5, 25.3, 29.1, 31.7, 36.3 (CH2), 74.0 (CH), 115.7,
120.8, 125.4, 129.0, 130.5 (aromat. CH), 128.8, 152.2, 162.0 (aromat.
ꢅ
Cq), 159.6 (CHN); EIMS: m/z (%) 309 (16) [Mþ ], 197 (100), 110 (67),
93 (77), 66 (29), 43 (28). Anal. Calcd for C21H27NO (309.5): C 81.51, H
8.79, N 4.53; found: C 81.27, H 8.68, N 4.41. The chiral compound
(S)-10j was obtained in the same way from the corresponding
aldehyde (S)-8j.42
4.2.2. N-(4-Dodecyloxy-3-methoxybenzylidene)aniline (10n)
The preparation of 10n was started with the alkylation of 4-hy-
droxy-3-methoxybenzaldehyde (vanillin) with 1-bromododecane
to afford 4-dodecyloxy-3-methoxybenzaldehyde (8n),43 which
melted at 60 ꢁC (lit.41 57 ꢁC).1H NMR (CDCl3):
d 0.82 (t, 3H, CH3),1.23
(m,16H, CH2),1.45 (m, 2H, CH2),1.85 (m, 2H, CH2), 3.88 (s, 3H, OCH3),
4.05 (t, 2H, OCH2), 6.92/7.39/7.42 (ABC, 3H, aromat. H), 9.81 (s, 1H,
4. Experimental
4.1. General
CHO); 13C NMR (CDCl3):
d 14.0 (CH3), 22.6, 25.8, 28.9, 29.3, 29.3, 29.4,
29.5, 29.6, 29.6, 31.9 (CH2), 56.0 (OCH3), 69.2 (OCH2), 110.4, 111.5,
126.6 (aromat. CH),129.9,149.9,154.2 (aromat. Cq),190.7 (CHO). The
general procedure, described before, afforded then a quantitative
yield of 10n (9.88 g from 8.0 g aldehyde 8n), which is a yellow solid
Melting points were determined on a Bu¨chi 510 melting point
apparatus and are uncorrected. The DSC measurements were per-
formed on a Perkin–Elmer DSC 7. The IR reaction control was made
in neat phase with a Beckman Acculab spectrometer. A Zeiss MCS
224/234 diode array spectrometer was used for the UV/vis spec-
troscopy. The Bruker spectrometers AC 200, AC 300, and AM 400
served for the obtention of the 1H and 13C NMR spectra. CDCl3 was
used as solvent and TMS as internal standarddif not otherwise
stated. The mass spectra (EI and FD) were obtained on a Finnigan
MAT 95. Polarization microscopy was performed with a Leitz
Ortholux II microscope/Mettler FP-52 heating system. Small and
wide-angle X-ray scattering was measured with a Siemens D-500
and melted at 40 ꢁC. 1H NMR (CDCl3):
d 0.82 (t, 3H, CH3), 1.23 (m,
16H, CH2), 1.45 (m, 2H, CH2), 1.85 (m, 2H, CH2), 3.98 (s, 3H, OCH3),
4.08 (t, 2H, OCH2), 6.92 (m, 1H), 7.20 (m, 3H), 7.29 (m, 1H), 7.49 (m,
2H), 7.53 (s, 1H) [aromat. H], 8.44 (s, 1H, CHN); 13C NMR (CDCl3):
d
14.0 (CH3), 22.6, 25.8, 28.9, 29.3, 29.3, 29.4, 29.5, 29.6, 29.6, 31.9
(CH2, partly superimposed), 56.0 (OCH3), 69.2 (OCH2), 109.5, 111.6,
120.8, 124.2, 126.5, 129.0 (aromat. CH), 129.3, 149.8, 151.7, 152.3
ꢅ
(aromat. Cq), 159.8 (CHN); EIMS: m/z (%) 395 (21) [Mþ ], 65 (27), 54
(69), 43 (100). Anal. Calcd for C26H37NO2 (395.6): C 78.94, H 9.43, N
3.54; found: C 78.85, H 9.54, N 3.73.
diffractometer (DACO-MP computer interface, Cu Ka). The ele-
mental analyses were made in the Microanalytical Laboratory of
the Institute of Organic Chemistry, University of Mainz.
4.2.3. N-(3,4,5-Tridecyloxybenzylidene)aniline (10q)
Aldehyde 8q44 gave a quantitative yield (16.2 g) of 10q, a yel-
lowish oil. 1H NMR (CDCl3):
d 0.89 (t, 9H, CH3), 1.28 (m, 36H, CH2),
4.2. General procedure for the preparation of the Schiff bases
(N-benzylideneanilines) 10a–t
1.49 (m, 6H, CH2), 1.78 (m, 2H, CH2), 1.83 (m, 4H, CH2), 4.03 (t, 2H,
OCH2), 4.05 (m, 4H, OCH2), 7.12 (s, 2H), 7.18 (m, 2H), 7.21 (m, 1H),
7.37 (m, 2H) [aromat. H], 8.31 (s, 1H, CHN); 13C NMR (CDCl3):
d 14.0
The condensation reaction of the substituted benzaldehydes
8a–t (25 mmol) and aniline (9) (3.26 g, 35 mmol) was performed at
(CH3), 22.6, 26.1, 29.3, 29.4, 29.5, 29.6, 29.6, 29.7, 30.3, 31.9 (CH2,
partly superimposed), 69.2, 73.5 (OCH2), 107.3, 120.8, 125.6, 129.0

6558
T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
(aromat. CH), 131.3, 141.4, 152.2, 153.4 (aromat. Cq), 160.1 (CHN); FD
11.17. Application of the above described general procedure fur-
ꢅ
MS: m/z (%) 650 (100) [Mþ ]. Anal. Calcd for C43H71NO3 (650.0): C
nishes then the aldimine 10t in quantitative yield (3.0 g 10t from
79.45, H 11.01, N 2.15; found: C 79.11, H 10.92, N 2.19.
2.62 g 8t). Theyellow solid 10t melts at 67 ꢁC.1H NMR (CDCl3):
d 0.88
(t, 6H, CH3), 1.25 (m, 32H, CH2), 1.45 (m, 4H, CH2), 1.75 (m, 2H, CH2),
1.82 (m, 2H, CH2), 3.91 (s, 3H, OCH3), 4.03 (t, 2H, OCH2), 4.06 (t,
2H, OCH2), 7.04 (m, 2H), 7.23 (m, 3H), 7.49 (m, 2H) [aromat. H],
4.2.4. N-(4-Dodecyloxy-3,5-dimethoxybenzylidene)aniline (10s)
Aldehyde 8s45 afforded a quantitative yield (10.60 g) of 10s,
a viscous yellow oil, which solidifies at 5 ꢁC to a wax. 1H NMR
8.32 (s, 1H, CHN); 1H NMR (CDCl3):
d 14.0 (CH3), 22.6, 25.9, 26.0,
(CDCl3):
d
0.87 (t, 3H, CH3), 1.25 (m, 16H, CH2), 1.44 (m, 2H, CH2),
29.2, 29.3, 29.4, 29.6, 30.2, 31.6 (CH2, partly superimposed), 56.2
(OCH3), 69.6, 74.1 (OCH2), 106.8, 107.9, 121.1, 126.1, 129.2 (aromat.
CH), 131.8, 141.4, 153.4, 154.1 (aromat. Cq), 160.2 (CHN). EIMS: m/z
1.76 (m, 2H, CH2), 3.90 (s, 6H, OCH3), 4.02 (t, 2H, OCH2), 7.13 (s, 2H),
7.19 (m, 2H), 7.23 (m, 1H), 7.37 (m, 2H) [aromat. H], 8.32 (s, 1H,
ꢅ
CHN); 13C NMR (CDCl3):
d
14.0 (CH3), 22.6, 25.8, 29.3, 29.4, 29.6,
(%) 579 (32) [Mþ ], 71 (27), 57 (69), 43 (100). Anal. Calcd for
30.0, 31.8 (CH2, partly superimposed), 56.1 (OCH3), 73.5 (OCH2),
105.8, 120.8, 125.7, 129.0 (aromat. CH), 131.4, 140.3, 152.0, 153.7
C38H61NO3 (579.9): C 78.71, H 10.60, N 2.42; found: C 78.48, H 10.71,
N 2.21.
ꢅ
(aromat. Cq), 159.9 (CHN); EIMS: m/z (%) 425 (20) [Mþ ], 257 (93),
182 (100), 57 (41), 43 (60). Anal. Calcd for C27H39NO3 (425.3): C
76.20, H 9.24, N 3.29; found: C 76.03, H 9.35, N 3.50.
4.3. General procedure for the preparation of the
(E,E)-3,6-bis(styryl)pyridazines 3a–t
4.2.5. N-(3,4-Didodecyloxy-5-methoxybenzylidene)-aniline (10t)
The aldehyde 8t was prepared from ethyl 3,4,5-trihydroxy-
benzoate, which was first twofold alkylated with 1-bromododecane
to ethyl 3,4-didodecyloxy-5-hydroxybenzoate.40,46 Iodomethane
(1.56 g, 11.0 mmol) and the latter ester (4.97 g, 9.3 mmol) were
refluxed together with K2CO3 (4.1 g, 30.0 mmol) in 150 mL dry
acetone for 2 days. The filtered solution was evaporated and the
residue filtered over basic Al2O3 (7ꢄ6 cm) with CH2Cl2. Ethyl 3,4-
didodecyloxy-5-methoxybenzoate (4.20 g, 82%) was obtained as
A slow stream of dry, oxygen-free N2 was purged through a
solution of 3,6-dimethylpyridazine 1317,18 (542 mg, 5.0 mmol) and
aldimine 10a–t (10.0 mmol) in 150 mL dry DMF. After about 30 min,
KOH (3.3 g, 60 mmol) was added under vigorous stirring and the
mixture was heated to 60 ꢁC. After 1 h, 150 mL CH3OH was added at
0–5 ꢁC. The precipitated product was washed with H2O and CH3OH
and recrystallized. As indicated below, a column chromatography
proved to be useful in some cases. (The normally low yields can be
enhanced by an excess of 10a–t and higher reaction temperatures,
but the purification is then more difficult. In particular the (E,Z)-
isomers are then enriched to an appreciable amount.)
colorless solid, which melted at 59 ꢁC. 1H NMR (CDCl3):
d 0.88 (t, 6H,
CH3), 1.25 (m, 32H, CH2), 1.34 (t, 3H, CH3), 1.43 (m, 4H, CH2), 1.72
(m, 2H, CH2),1.79 (m, 2H, CH2), 3.85 (s, 3H, OCH3), 4.03 (t, 4H, OCH2),
4.35 (q, 2H, COOCH2), 7.24 (m, 2H, aromat. H); 13C NMR (CDCl3):
4.3.1. (E,E)-3,6-Bis[2-(4-methoxyphenyl)vinyl]pyridazine (3a)
Yield after recrystallization from toluene: 517 mg (30%) pale
yellow needles, which melt to the nematic phase at 270 ꢁC. EIMS:
d
14.0 (CH3), 14.3 (CH3, ester group), 22.7, 25.9, 26.1, 29.3, 29.4, 29.5,
29.7, 30.2, 31.9 (CH2, partly superimposed), 56.3 (OCH3), 60.9 (OCH2,
ester group), 69.3, 73.5 (OCH2), 106.9, 108.3 (aromat. CH), 125.2,
142.1, 152.8, 153.3 (aromat. Cq), 166.4 (CO). EIMS: m/z (%) 548 (52)
ꢅ
m/z (%) 344 (100) [Mþ ], 115 (23). Anal. Calcd for C22H20N2O2
(344.4): C 76.72, H 5.82, N 8.13; found: C 76.52, H 5.46, N 7.93.
ꢅ
[Mþ ], 380 (32), 212 (100), 57 (45), 43 (64). Anal. Calcd for C34H60O5
(548.8): C 74.41, H 11.02; found: C 74.54, H 11.08. The ester (4.0 g,
7.29 mmol), dissolved in 100 mL dry diethyl ether, was dropwise
added at 5 ꢁC to a suspension of LiAlH4 (0.276 g, 7.29 mmol, twofold
excess) in 50 mL dry diethyl ether. After refluxing for 3 h, 50 mL
H2O was added and the mixture was neutralized with 2 M H2SO4.
Extraction with diethyl ether and evaporation yielded the corre-
sponding alcohol, 3,4-didodecyloxy-5-methoxybenzyl alcohol,
which after recrystallization from toluene melted at 55 ꢁC. Yield:
4.3.2. (E,E)-3,6-Bis[2-(4-ethoxyphenyl)vinyl]pyridazine (3b)
Yield after recrystallization from toluene: 150 mg (8%) pale
yellow needles, which melt to the nematic phase at 230 ꢁC. FD MS:
ꢅ
m/z (%) 372 (100) [Mþ ]. Anal. Calcd for C24H24N2O2 (372.4): C 77.39,
H 6.49, N 7.52; found: C 77.31, H 6.47, N 7.61.
4.3.3. (E,E)-3,6-Bis[2-(4-butoxyphenyl)vinyl]pyridazine (3c)
Yield after recrystallization from toluene: 170 mg (8%) pale
yellow crystals, which melt to the nematic phase at 220 ꢁC. EIMS:
3.42 g (93%) colorless solid. 1H NMR (CDCl3):
d 0.88 (t, 6H, CH3), 1.25
ꢅ
(m, 32H, CH2),1.42 (m, 4H, CH2),1.71 (m, 2H, CH2),1.78 (m, 2H, CH2),
3.80 (s, 3H, OCH3), 3.92 (t, 2H, OCH2), 3.95 (t, 2H, OCH2), 4.58 (s,
m/z (%) 428 (100) [Mþ ], 372 (10), 57 (3). Anal. Calcd for C28H32N2O2
(428.6): C 78.47, H 7.53, N 6.54; found: C 78.07, H 7.32, N 6.23.
2H, CH2OH), 6.54 (s, 2H, aromat. H); 13C NMR (CDCl3):
d 14.0 (CH3),
22.7, 26.0, 26.1, 29.3, 29.4, 29.5, 29.6, 29.7, 30.2, 31.9 (CH2, partly
superimposed), 56.2 (OCH3), 65.6 (CH2OH), 69.1, 73.5 (OCH2), 104.1,
105.5 (aromat. CH),136.2,137.3,153.2,153.8 (aromat. Cq); EIMS: m/z
4.3.4. (E,E)-3,6-Bis[2-(4-hexyloxyphenyl)vinyl]pyridazine (3d)
Yield after recrystallization from toluene: 1.65 g (68%) pale
yellow crystals, which melt at 160 ꢁC to the smectic phase. EIMS:
ꢅ
ꢅ
(%) 506 (77) [Mþ ], 338 (40),170 (100), 69 (21), 57 (47), 43 (73). Anal.
m/z (%) 484 (100) [Mþ ], 400 (11), 316 (5). Anal. Calcd for
Calcd for C32H58O4 (506.8): C 75.84, H 11.54; found: C 75.87, H
11.68. The substituted benzyl alcohol (3.20 g, 6.3 mmol) was then
treated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1.58 g,
7.0 mmol) in 50 mL dry 1,4-dioxane. After 24 h stirring at ambient
temperature, the mixture was filtered, evaporated, and purified by
column filtration (7ꢄ6 cm SiO2, CH2Cl2). Aldehyde 8t was obtained
as a colorless solid (3.0 g, 94%), which melted at 61 ꢁC. 1H NMR
C32H40N2O2 (484.7): C 79.30, H 8.32, N 5.78; found: C 79.24, H 8.29,
N 5.90.
4.3.5. (E,E)-3,6-Bis[2-(4-octyloxyphenyl)vinyl]pyridazine (3e)
Yield after recrystallization from isopropanol: 136 mg (5%) pale
yellow crystals, which melt at 155 ꢁC to the smectic phase. EIMS: m/
ꢅ
z (%) 540 (100) [Mþ ], 428 (12), 57 (12). Anal. Calcd for C36H48N2O2
(CDCl3):
(m, 2H, CH2),1.81 (m, 2H, CH2), 3.88 (s, 3H, OCH3), 4.00 (t, 2H, OCH2),
4.03 (t, 2H, OCH2), 7.08 (s, 2H, aromat. H); 13C NMR (CDCl3):
14.0
d
0.88 (t, 6H, CH3),1.24 (m, 32H, CH2),1.42 (m, 4H, CH2),1.73
(540.8): C 79.96, H 8.95, N 5.92; found: C 79.85, H 8.95, N 5.66.
d
4.3.6. (E,E)-3,6-Bis[2-(4-decyloxyphenyl)vinyl]pyridazine (3f)
Yield after recrystallization from toluene: 358 mg (12%) pale
yellow crystals, which melt at 150 ꢁC to the smectic phase. EIMS:
(CH3), 22.6, 25.9, 26.0, 29.2, 29.3, 29.4, 29.6, 30.2, 31.9 (CH2, partly
superimposed), 56.2 (OCH3), 69.2, 73.6 (OCH2), 106.8, 108.1
(aromat. CH), 131.5, 143.5, 153.4, 154.0 (aromat. Cq), 191.0 (CHO).
ꢅ
m/z (%) 596 (100) [Mþ ], 456 (8), 316 (6), 57 (5). Anal. Calcd for
ꢅ
EIMS: m/z (%) 504 (26) [Mþ ], 336 (28), 168 (100), 57 (42), 43 (57).
C40H56N2O2 (596.9): C 80.49, H 9.46, N 4.69; found: C 80.25, H 9.31,
Anal. Calcd for C32H56O4 (504.8): C 76.14, H 11.18; found: C 76.08, H
N 4.78.

T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
6559
4.3.7. (E,E)-3,6-Bis[2-(4-dodecyloxyphenyl)vinyl]pyridazine (3g)
Yield after recrystallization from CHCl3: 2.25 g (69%) pale yellow
crystals, which melt at 139 ꢁC to the smectic phase. EIMS: m/z (%)
(464.5): C 67.23, H 6.08, N 6.03; found: C 66.91, H 6.27, N 6.34.
Application of the corresponding aldehyde with a CDO group led to
an analogous result. The deuterated compound was used to control
the assignment of the 1H and 13C NMR signals of the olefinic double
bonds.
ꢅ
652 (100) [Mþ ], 484 (25), 316 (13), 85 (11), 71 (17), 57 (46). Anal.
Calcd for C44H64N2O2 (653.0): C 80.93, H 9.89, N 4.29; found: C
80.84, H 10.04, N 4.28.
4.3.16. (E,E)-3,6-Bis[2-(3,4,5-trihexyloxyphenyl)-
4.3.8. (E,E)-3,6-Bis[2-(4-octadecyloxyphenyl)vinyl]pyridazine (3h)
Yield after recrystallization from CHCl3: 900 mg (22%) pale
yellow crystals, which melt at 135 ꢁC to the smectic phase. FD MS:
vinyl]pyridazine (3p)
Yield after recrystallization from isopropanol: 1.55 g (35%) yel-
low, highly viscous smectic phase, which shows a clearing point to
the isotropic phase at 77 ꢁC. Cooling down to ꢀ70 ꢁC, does not lead
ꢅ
m/z (%) 821 (100) [Mþ ]. Anal. Calcd for C56H88N2O2 (821.3): C 81.89,
ꢅ
H 10.80, N 3.41; found: C 81.79, H 10.78, N 3.58.
to a crystalline phase. FD MS: m/z (%) 885 (100) [Mþ ]. Anal. Calcd
for C56H88N2O6 (885.3): C 75.97, H 10.02, N 3.16; found: C 76.05, H
10.03, N 3.39.
4.3.9. rac-(E,E)-3,6-Bis{2-[4-(1-methylheptyloxy)phenyl]vinyl}-
pyridazine (3i)
The washed residue was purified by column chromatography
(3ꢄ40 cm SiO2, toluene/ethyl acetate 8:2). Recrystallization from
n-hexane yielded 244 mg (9%) pale yellow crystals, which melt at
4.3.17. (E,E)-3,6-Bis[2-(3,4,5-tridecyloxyphenyl)-
vinyl]pyridazine (3q)
The crude reaction product was purified by column chroma-
tography (40ꢄ3 cm SiO2, CH2Cl2) and subsequent crystallization
from ethanol. Yield: 0.61 g (10%) yellow, smectic phase, which does
not crystallize on cooling down to ꢀ70 ꢁC. The onset temperature in
the DSC for the transition to the isotropic phase is 96 ꢁC. FD MS: m/z
ꢅ
124 ꢁC to the smectic phase. EIMS: m/z (%) 540 (100) [Mþ ], 427
(38), 316 (58), 57 (40), 43 (57). Anal. Calcd for C36H48N2O2 (540.8): C
79.96, H 8.95, N 5.18; found: C 79.92, H 9.04, N 5.31.
ꢅ
4.3.10. (S,S)-(E,E)-3,6-Bis{2-[4-(1-methylheptyloxy)phenyl]vinyl}-
pyridazine (3j)
(%) 1121 (100) [Mþ ]. Anal. Calcd for C80H136N2O6 (1222.0): C 78.63,
H 11.22, N 2.29; found: C 78.40, H 11.62, N 2.27.
The purification of the crude product was performed as de-
scribed for 3i. Yield and spectroscopic data correspond to 3i,
however, the pale yellow crystals of 3j melt already at 104 ꢁC to the
smectic phase.
4.3.18. (E,E)-3,6-Bis[2-(3,4,5-tridodecyloxyphenyl)-
vinyl]pyridazine (3r)
The crude reaction product was purified by column chroma-
tography (60ꢄ3 cm SiO2, CH2Cl2) and recrystallized from iso-
propanol. Yield: 1.39 g (20%) yellow smectic phase, which shows
the clearing point at 84 ꢁC (onset temperature in the DSC). Cooling
leads to crystallization and a phase transition to the smectic phase
4.3.11. (E,E)-3,6-Bis[2-(3,4-dihexyloxyphenyl)vinyl]pyridazine (3k)
Yield after recrystallization from toluene, to which petroleum
ether (bp 40–70 ꢁC) was added till the hot solution became turbid:
1.68 g (49%) yellow crystals, which melt at 128 ꢁC to the isotropic
ꢅ
at 14 ꢁC. FD MS: m/z (%) 1390 (100) [Mþ ]. Anal. Calcd for
ꢅ
phase. EIMS: m/z (%) 684 (100) [Mþ ], 600 (22). Anal. Calcd for
C92H160N2O6 (1390.3): C 79.48, H 11.60, N 2.01; found: C 79.49, H
C
44H64N2O4 (685.0): C 77.15, H 9.42, N 4.09; found: C 77.01, H 9.38,
11.56, N 2.06.
N 4.17.
4.3.19. (E,E)-3,6-Bis[2-(4-dodecyloxy-3,5-dimethoxyphenyl)-
vinyl]pyridazine (3s)
4.3.12. (E,E)-3,6-Bis[2-(3,4-didodecyloxyphenyl)vinyl]-
pyridazine (3l)
Yield after recrystallization from petrol ether (bp 40–70 ꢁC):
0.46 g (12%) yellow crystals, which melt at 11 ꢁC to the smectic
Yield after recrystallization from CHCl3: 3.21 g (63%) yellow
crystals, which melt at 121 ꢁC to the smectic phase. FD MS: m/z (%)
ꢅ
phase. FD MS: m/z (%) 772 (100) [Mþ ]. Anal. Calcd for C48H72N2O6
ꢅ
1021 (100) [Mþ ]. Anal. Calcd for C68H112N2O4 (1021.7): C 79.94, H
(773.1): C 74.57, H 9.39, N 3.62; found: C 74.29, H 9.09, N 3.82.
11.06, N 2.74; found: C 80.26, H 11.07, N 2.71.
4.3.20. (E,E)-3,6-Bis[2-(3,4-didodecyloxy-5-methoxyphenyl)-
vinyl]pyridazine (3t)
4.3.13. (E,E)-3,6-Bis[2-(3,4-dihexadecyloxyphenyl)vinyl]-
pyridazine (3m)
Yield after recrystallization from n-hexane: 650 mg (12%) yellow
crystals, which melt at 30 ꢁC to the smectic phase. FD MS: m/z (%)
Yield after recrystallization from CHCl3: 3.73 g (60%) yellow
crystals, which melt at 118 ꢁC to the smectic phase. FD MS: m/z (%)
ꢅ
1081 (100) [Mþ ]. Anal. Calcd for C70H116N2O6 (1081.7): C 77.73, H
ꢅ
1245 (100) [Mþ ]. Anal. Calcd for C84H144N2O4 (1246.1): C 80.97, H
10.81, N 2.59; found: C 77.43, H 10.87, N 2.95.
11.65, N 2.25; found: C 81.20, H 11.57, N 2.41.
4.3.14. (E,E)-3,6-Bis[2-(4-dodecyloxy-3-methoxyphenyl)-
vinyl]pyridazine (3n)
4.4. Irradiation experiments
Yield after recrystallization from toluene: 570 mg (16%) yellow
crystals, which melt at 61 ꢁC to the smectic phase. FD MS: m/z (%)
Even in the daylight, solutions of 3 proved to be susceptible to
photoisomerization reactions. The study of the photochemistry of
(E,E)-3d was performed with a 2ꢄ10ꢀ4 M solution in oxygen-free
benzene (17.4 mg in 180 mL) at 30 ꢁC. Light source was a 450 W
Hanovia medium pressure lamp equipped with a Pyrex filter
ꢅ
712 (100) [Mþ ]. Anal. Calcd for C46H68N2O4 (713.1): C 77.48, H 9.31,
N 3.93; found: C 77.21, H 9.38, N 4.18.
4.3.15. (E,E)-3,6-Bis[2-(3,4,5-trimethoxyphenyl)-
vinyl]pyridazine (3o)
(
l
ꢃ290 nm). The emission range of the lamp reached from this
filter limit to the visible region, so that the whole long-wavelength
absorption of 3d (lmax¼372 nm, 3max¼52,000 L molꢀ1 cmꢀ1) was
covered. Magnetic stirring or a slow stream of oxygen-free N2
served for the mixing of the solution. In order to follow the progress
of the reaction, probes for the UV/vis spectroscopy were taken. As
soon as the photostationary state was obtained, the solvent was
evaporated and the residue dissolved in C6D6. Repeated 1H NMR
The crude reaction product was dried and extracted with
n-hexane in a Soxhlet apparatus. After evaporation of the solvent,
the residue was purified by column chromatography (30ꢄ3 cm
SiO2, CH2Cl2/CH3–CO–CH3 85:15). A yellow solid (185 mg, 8%)
was obtained, which decomposed slowly on heating above 170 ꢁC.
FD MS: m/z (%) 465 (100) [MþHþ]. Anal. Calcd for C26H28N2O6

6560
T. Lifka et al. / Tetrahedron 64 (2008) 6551–6560
17. Overberger, C. G.; Byrd, N. R.; Mesrobian, R. B. J. Am. Chem. Soc. 1956, 78,
1961–1965.
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(E,Z)-3d ratio, which amounted to 54:46 under the conditions used.
Acknowledgements
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21. Siegrist, A. E.; Liechti, P.; Meyer, H. R.; Weber, K. Helv. Chim. Acta 1969, 52,
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We are grateful to the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for financial support and to
Prof. T. Pakula (!) for valuable discussions about the LC phases.
22. The solubility of, for example, 3g in benzene at 45 ꢁC is very low (0.13 g Lꢀ1).
Two further dodecyloxy chains, present in 3l, enhance the solubility under the
same conditions to 1.2 g Lꢀ1
.
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