Imaging technique with liquid crystals of halogen-containing 3,6-distyrylpyridazines (= 3,6-Bis(2-phenylethenyl)pyridazines)

Article abstract of DOI:10.1002/hlca.200800249

The 3,6-bis{(1E)-2-[4-(dodecyloxy)phenyl]ethenyl}pyridazine (1a) was regio- and stereoselectively brominated or chlorinated to the (Z,Z)-compounds 3 and 6, respectively. The conjugated core of these systems represents a mesogen for smectic (and nematic) l

Full text of DOI:10.1002/hlca.200800249

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Imaging Technique with Liquid Crystals of Halogen-Containing 3,6-
Distyrylpyridazines (¼ 3,6-Bis(2-phenylethenyl)pyridazines)
by Thorsten Lifka, Peter Seus, Annette Oehlhof, and Herbert Meier*
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10 – 14, D-55099 Mainz
(phone: þ 49 61313922605; fax: þ 49 61313925396; e-mail: hmeier@mail.uni-mainz.de)
The 3,6-bis{(1E)-2-[4-(dodecyloxy)phenyl]ethenyl}pyridazine (1a) was regio- and stereoselectively
brominated or chlorinated to the (Z,Z)-compounds 3 and 6, respectively. The conjugated core of these
systems represents a mesogen for smectic (and nematic) liquid crystals and, moreover, a chromophore
with an absorption at the UV/VIS border. Irradiation and efficient intersystem crossing to the triplet
state leads to isomerization reactions (Z,Z) ! (E,Z) in solution as well as in the thermotropic LC phases.
Since (E,Z)-configurations are only tolerated to a very small extent by the mesophases, the photoinduced
transition S ! N ! I of 6 can be used as an imaging technique.
1. Introduction. – Stilbenoid systems with an extended p conjugation, such as oligo-
and poly(1,4-phenylenevinylene)s (¼oligo- and poly(1,4-phenyleneethene-1,2-diyl;
OPV and PPV), represent a prominent class of compounds in material science. Their
tunable chromophores/fluorophores/electrophores permit various applications, for
example as electroluminescent and (photo)conductive or nonlinear optical materials
(for selected monographs and rewiews, see [1]).
However, apart from some small oligomers, these relatively rigid p systems are
barely suitable for the formation of liquid crystals (LCs) [2]. Recently, we found that
(E,E)-3,6-distyrylpyridazines (¼ 3,6-bis(2-phenylethenyl)pyridazines) 1 with one to
three alkoxy groups on the terminal benzene rings represent ideal compounds for the
formation of various different liquid crystals: N, SA, SB, SC, SE, SF/I, and Cub. [3]. We
attribute the high tendency of self-organization to the p stacking, the van der Waals
interaction of the chains and the transversal dipole moment. The first two effects are
also valid for the corresponding 1,4-distyrylbenzenes (¼1,4-bis(2-phenylethenyl)ben-
zenes) 2, but their self-organization tendency is much lower [2 – 4]. Therefore, the
transversal dipole moment seems to be the major effect.
The N-atoms of the pyridazine ring have still another effect. They bear two
(coupled) free electron pairs so that the electronic excitation at long wavelengths
ꢀ 2009 Verlag Helvetica Chimica Acta AG, Zꢁrich

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contains p ! p* and n_ ! p* transitions [5]. According to El Sayedꢂs rule [6],
exothermic singlet – triplet transitions S(pp*) ! T(np*) and S(np*) ! T(pp*) are
allowed. The 1,4-distyrylbenzenes 2 do not have this ability, and they do not show E !
Z photoisomerization reactions because the bond order of the olefinic C¼C bonds is
not lowered enough in the excited singlet state S1 [1b][7]. In the triplet state of the 3,6-
distyrylpyridazines 1, such isomerization routes are facile [3] and may be used for
imaging techniques with liquid crystals.
2. Results and Discussion. – We attempted now to enhance the intersystem-crossing
(ISC) rate of 3,6-distyrylpyridazines 1 by the introduction of heavy atoms such as Br-
or Cl-atoms. Scheme 1 shows that the reaction of 3,6-bis{(1E)-2-[4-(dodecyloxy)phe-
nyl]ethenyl}pyridazine (1a) with Br2 in AcOH/AcONa yielded the three products 3 – 5
in a ratio of 66 :33 :1 when 2 mol of Br2 were used for 1 mol of 1a. The separation of
3 – 5 was achieved by column chromatography.
Scheme 1
A spontaneous dehydrobromination occurs after the addition of Br2 to the olefinic
C¼C bond, i.e., 1a ((E,E)) ! 3 ((Z,Z)) and 4 ((E,Z))1). The additional electrophilic
1
)
Addition of Br2 to (E)-stilbene (¼1,1’-[(1E)-ethene-1,2-diyl]bis[benzene]) yields preponderantly
the meso-dibromo derivative (R,S). In the case of (E,E)-compound 1a, a mixture of the achiral
adducts with configuration (R,S,R,S)ꢀ(S,R,S,R) and the two chiral enantiomeric adducts with
configurations (S,R,R,S) and (R,S,S,R) can be expected to be the major products. Deprotonation
takes then place in a-position to the pyridazine ring because of the electron-withdrawing effect of
the heterocyclic ring.

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substitution at the electron-rich benzene ring leading to 5 ((Z,Z)) is a minor side
reaction as long as no Br2 excess is used. The dehydrobromination has obviously a very
high regioselectivity since an unsymmetric dibromo compound cannot be observed.
However, the question remains whether the Br-substituent is localized at the ꢃinnerꢂ or
at the ꢃouterꢂ olefinic C-atom of 3. The coupled 13C-NMR spectrum gives the answer.
The H-atom-bearing olefinic C-atom leads to a dt, which corresponds to the one-bond
coupling (1J) and to a three-bond coupling (3J) with the two Ho atoms of the
neighboring benzene ring. This result is only conform with structure 3.
The introduction of Cl-atoms into 1a (!(Z,Z)-compound 6) proved to be more
difficult. The direct chlorination with Cl2 led, by an attack on the olefinic C¼C bonds as
well as on the aromatic rings, to a scarcely stable mixture of products. Finally we
succeeded by application of SO2Cl2 as Cl-transfer reagent (Scheme 2); however, the
yield of the dichloro compound 6 in the (Z,Z) configuration was rather low (25%).
Scheme 2
Scheme 3 shows the catalytic hydrogenation of 1a to 7, which served for the
comparison of the photoactive olefinic systems to a photostable saturated system.
Scheme 3
Differential scanning calorimetry (DSC) revealed that the target (Z,Z)-compound
3 generates a mesophase between 86 and 1128. Compared to the starting (E,E)-system
1a, which forms smectic phases [3] between 139 and 2668, the LC temperature range of
3 is better suitable for applications. The mono-brominated compound 4 exhibits phase-
transition temperatures, which are lying between those of 1a and 3. According to the
textures obtained in the polarization microscope, all three compounds 1a, 3, and 4 form
smectic phases SC. The hydrogenated compound 7 generates also a smectic phase. Its
texture did not permit a more detailed classification. Fig. 1 summarizes the temper-
atures T and the corresponding enthalpies DH for the transitions between crystalline
(Cr), smectic (S), and isotropic (I) phases.
The chlorinated (Z,Z)-compound 6 exhibits a more complex phase behavior. Fig. 2
shows the DSC cooling curve. Two mesophases can be observed between the crystalline
phase (Cr) and the isotropic melt (I). The peak for the Cr! SC transition has, in the
second heating curve of the DSC, a maximum at 1298. The next phase transition SC ! N
occurs at 1378, and the clearing point N ! I lies at 1398. Due to the undercooling effect,

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Fig. 1. Phase transitions of the compounds 1a, 3, 4, and 7: onset temperatures [8] of the second heating
curves and the first cooling curves. The numbers in parentheses indicate the enthalpies DH in kJmolꢁ1
;
DH > 0 endothermic process, DH < 0 exothermic phase transition. A rate of 10 K minꢁ1 was used as
temperature gradient.
Fig. 2. DSC Measurement of (Z,Z)-compound 6: second cooling curve measured with a rate of 5 K minꢁ1
the phase transitions, in particular the crystallization, were observed at lower
temperatures in the cooling curve.
The characterization of the LC phases is based on the textures which were obtained
with the polarization microscope in a cooling process with a very low temperature
gradient of 0.1 K minꢁ1. Under crossed polarizers, the isotropic melt (I) appears black.
As soon as the first LC phase is formed, color appears due to the birefringence. A light
pressure on the phase of 6 causes short ꢃflashesꢂ in the structureless texture, which are
characteristic for a nematic phase N in a homeotropic arrangement. Tempering
generates then typical nematic droplets (Fig. 3,a). A ꢃschlierenꢂ texture evolves with

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beautiful singularities (Fig. 3,b). Further cooling leads in a broad temperature range, to
the SC texture (Fig. 3,c) until crystallization starts at ca. 1188 (Fig. 3,d). The nematic
phase N is extremely light-sensitive. Therefore, we used a green filter for its
measurements.
Fig. 3. Mesophases of 6 (scale 1:340): a) nematic droplets, b) singularity in the ꢀschlierenꢁ texture of the
nematic phase, c) texture of smectic phase SC at 1308, and d) starting crystallization at 1188
(Z,Z)-Dibromo compound 3 shows a photoisomerization in cyclohexane, which
leads fast to the photostationary state of the (Z,Z) and the less symmetrical (E,Z)
1
configuration. According to H- and 13C-NMR measurements, the presence of the
(E,E)-isomer can be excluded (Scheme 4)2). The UV/VIS-reaction spectra in Fig. 4
show the result of an irradiation into the long-wavelength band of 3 (l ꢂ 320 nm). The
decrease of this band with a lmax value of 363 nm is attended by an increase of the band
1
2
)
The detection limit in the H-NMR study was below 2%.

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between 220 and 300 nm. 1H-NMR Reaction spectra reveal a 50 :50 ratio of the (Z,Z)-
compound 3 and its stereoisomer (E,Z)-3 in the photostationary state. However,
subsequent irradiation of the 1:1 mixture with l 254 nm shifts the ratio back to a
somewhat higher portion of the (Z,Z)-isomer 33). The band between 220 and 300 nm
decreases then and the band between 300 and 420 nm increases. This wavelength
dependance renders the system suitable for an optical switching between states of
different (Z,Z)/(E,Z) ratio.
Scheme 4. Photoisomerization of (Z,Z)-Compounds 3 and 6 to Their (E,Z)-Isomers (E,Z)-3 and 6 and
(E,Z)-6, respectively
Fig. 4. UV/VIS Reaction spectra of the irradiation of (Z,Z)-compound 3 in cyclohexane with l ꢂ 320 nm
The isomer ratio can be easily determined by the integration of the s of the olefinic
H-atoms in the 1H-NMR spectra (CDCl3): the (Z,Z)-isomer 3 causes one s at d 8.55 and
3
)
The photostationary state is predominantly determined by the ratio e(3)/e((E,Z)-3).

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(E,Z)-3 two s, one at d 7.44 for the ꢃ(E)-sideꢂ and one at d 8.61 for the remaining ꢃ(Z)-
sideꢂ. The AA’BB’ spin pattern of the aromatic H-atoms is also shifted by the
isomerization from d 6.95 and 7.89 for 3 to d 6.69 and 6.94 for the ꢃ(E)-sideꢂ and d 6.94
and 7.87 for the ꢃ(Z)-sideꢂ in (E,Z)-3.
Compared to the photochemical behavior of 1a and other nonhalogenated 3,6-
distyrylpyridazines [3][4], compound 3 exhibits two major advantages: i) Due to the
enhanced intersystem crossing (ISC), the isomerization process of 3 is much faster. ii)
The sensitivity of the electronically excited states of 3 toward O2 is so low that
irradiations can be performed in oxygen-containing media.
A certain disadvantage of Br-containing compounds is due to the photochemical
CꢁBr cleavage in long-term irradiations. Therefore, we used compound 6 for the
irradiation of the LC phase since the CꢁCl bond has a much higher photostability.
Irradiations of the nematic phase at 1358 led to the isothermal formation of the
isotropic melt (Fig. 5,b). The polychromatic light of the polarization microscope is
already sufficient for this change which is based on the photoisomerizaton of the (Z,Z)-
isomer 6 to (E,Z)-6 (Scheme 4). Irradiations of the smectic phase at 128, 127, 125, 123,
120, and 1188 led in all cases to the isothermal formation of the nematic phase
(Fig. 5,a). The rate of the transformation decreased in this sequence of experiments
with decreasing temperature. We assume that higher mobility and lower order enhance
the chances for the isomerization in the constrained LC medium. We could never
observe a direct transformation of the SC phase to the isotropic phase I. Obviously, the
nematic phase N tolerates more ꢃwrongꢂ (E,Z) configurations than the higher ordered
SC phase.
Fig. 5. Isothermal phase transitions by irradiation of 6: a) SC phase irradiated at 128.08 on the left upper
part so that the nematic phase N is formed, and b) N phase irradiated so that the formation of the isotropic
phase I (black area) is started
The hydrogenated compound 7 does not contain a stilbenoid chromophore. It is
photostable in solution as well as in the liquid crystalline state. Compound 1a, which
contains olefinic C¼C bonds, shows only in solution the (E,E) ! (E,Z) isomerization.
In the LC state, 1a is surprisingly photostable [2][3]. The excitation energy can be

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transferred to the neighboring mesogens in the self-organized arrangement and finally
ends up in heat. We tried to use 7 as an inert host for photoactive guests such as 1a or 3.
However, phase separation turned out to be a major problem. Finally, we succeeded in
preparing a guest – host system of 90% 7 and 10% 1a. On cooling to 1428, the isotropic
mixture formed a smectic phase, and at 1328 another smectic phase, until at 998
crystallization started. The textures did not allow to determine the exact type of the S
phases. A macroscopic phase separation was not discernible. So we irradiated the LC
system at 1158. Although an energy transfer from the excited singlet or triplet states of
1a to 7 can be excluded for energetic reasons, a photoisomerization of 1a did not occur.
Either the constrained medium for the guest molecules 1a does not permit a
photoisomerization or the photostationary state is completely on the (E,E)-side.
3. Conclusions. – The 3,6-bis{(1Z)-1-bromo-2-[4-(dodecyloxy)phenyl]ethenyl}pyr-
idazine (3) and 3,6-bis{(1Z)-1-chloro-2-[4-(dodecyloxy)phenyl]ethenyl}pyridazine (6)
can be obtained by halogenation and spontaneous dehydrohalogenation from 3,6-
bis{(1E)-2-[4-(dodecyloxy)phenyl]ethenyl}pyridazine (1a). Both compounds form
thermotropic SC phases, 6 generates additionally a nematic phase N in a small
temperature range. Irradiation into the long-wavelength band (l ꢂ 320 nm) of the
(Z,Z)-dibromo compound 3 in cyclohexane leads to a photostationary state 3/(E,Z)-3/
(E,E)-3 50 :50 :0. Subsequent irradiation into the absorption band with higher energy
(l 254 nm) partly reverts the process, so that the (Z,Z) isomer 3 prevails. A triplet
mechanism is assumed for the isomerization routes. Since long-term irradiations of 3
revealed a certain photolability of the CꢁBr bond, the (Z,Z)-dichloro compound 6 was
used for irradiations of the LC phases. Thereby, isothermal phase transformations SC !
N and N ! I could be observed, which were effected by 6 ! (E,Z)-6 photoisomeriza-
tion processes. An increase of the (E,Z)/(Z,Z) ratio to a certain limit leads to the
breakdown of the SC phase. The N phase tolerates some more ꢃwrongꢂ configuration
(E,Z) until the isotropic melt I is generated. To the best of our knowledge, the SC ! N
process is the first photo-induced transformation between two LC phases [8]. Both
irradiation processes can be used as imaging techniques.
We are grateful to the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and
the Materialwissenschaftliches Forschungszentrum der Universitꢄt Mainz for financial support.
Experimental Part
1. General. CC ¼ Column chromatography. Phase transitions were measured with a differential
scanning calorimeter DSC-7 from Perkin-Elmer. UV/VIS: MCS-224/MCS-234 diode array spectrometer
from Zeiss. Polarization microscopy: Ortholux II from Leitz. The temp. was regulated with a FD-52
1
heating system from Mettler. H- and 13C-NMR Spectra: AM-400 spectrometer from Bruker; CDCl3 as
solvent and Me4Si as internal standard; d in ppm and J in Hz. FD-MS: Finnigan-MAT-95 spectrometer.
The elemental analyses were performed in the microanalytical department of the Institute of Organic
Chemistry of the Unviversity of Mainz.
2. Bromination of 3,6-Bis{(1E)-2-[4-(dodecyloxy)phenyl]ethenyl}pyridazine (1a). Pyridazine 1a
(5.0 g, 7.66 mmol) [3] and AcONa (82 mg, 1.0 mmol) were suspended in AcOH (550 ml) and refluxed,
while Br2 (2.45 g, 15.33 mmol) in AcOH (200 ml) was added dropwise. After 10 ꢁ 20 min refluxing, H2O
(50 ml) was added, and the mixture was extracted with several portions of CH2Cl2 until the AcOH phase
became colorless. The CH2Cl2 extracts were neutralized with aq. Na2CO3 soln., washed with H2O, and

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dried (MgSO4). CC (5 ꢃ 60 cm SiO2, CH2Cl2) gave, in this order, 5 (55 mg, 0.8%), 3 (3.42 g, 55%), and 4
(1.51 g, 27%).
Data of 3,6-Bis{(1Z)-1-bromo-2-[4-(dodecyloxy)phenyl]ethenyl}pyridazine (3): Yellow crystals
which melt after recrystallization from hexane at 868 to the smectic phase SC. 1H-NMR (CDCl3): 0.89 (t,
2 Me); 1.22 (m, 32 H, CH2); 1.45 (m, 4 H, CH2); 1.78 (m, 4 H, CH2); 4.01 (t, 2 CH2O); 6.95, 7.89 (AA’BB’,
8 arom. H); 8.00 (s, 2 heteroarom. H); 8.55 (s, 2 olef. H). 13C-NMR (CDCl3): 14.1 (Me); 22.7, 26.0, 29.2,
29.3, 29.4, 29.6, 31.9 (CH2, partly superimposed); 68.2 (CH2O); 114.4, 126.0, 131.8, 133.6 (arom. and olef.
CH); 115.7 (olef. C); 127.4, 157.3, 160.0 (arom. C). FD-MS: 810 (100, Mþ (Br2 isotope pattern)). Anal.
calc. for C44H62Br2N2O2 (810.8): C 65.18, H 7.71, N 3.46; found: C 65.23, H 7.78, N 3.61.
Data of 3-{(1Z)-1-Bromo-2-[4-(dodecyloxy)phenyl]ethenyl}-6-{(1Z)-2-[4-(dodecyloxy)phenyl]-
ethenyl}pyridazine (4): Yellow crystals which melt after recrystallization from hexane at 918 to the
1
smectic phase SC. H-NMR (CDCl3): 0.89 (t, 2 Me); 1.23 (m, 32 H, CH2); 1.45 (m, 4 H, CH2); 1.78 (m,
4 H, CH2); 4.01 (m, 2 CH2O); 6.92, 7.54 (AA’BB’, 4 arom. H (nonbrominated side); 6.94, 7.86 (AA’BB’,
4 arom. H (brominated side); 7.22, 7.67 (AB, 3J ¼ 16.3, 2 olef. H); 7.58, 7.92 (AB, 3J ¼ 8.4, 2 heteroarom.
H); 8.47 (s, 1 olef. H); assignments based on INDOR and NOE. 13C-NMR (CDCl3): 14.0 (Me); 22.6, 26.0,
29.2, 29.3, 29.5, 29.6, 31.9 (CH2, partly superimposed); 68.1, 68.2 (CH2O); 114.3, 114.9, 122.2, 123.9, 125.6,
128.8, 131.6, 133.0, 134.9 (arom., heteroarom., and olef. CH); 116.2 (CBr); 127.7, 128.7, 156.9, 157.0, 159.9,
160.2 (arom. C). FD-MS: 730 (100, Mþ (Br isotope pattern)). Anal calc. for C44H63BrN2O2 (731.9): C
72.21, H 8.68, N 3.83; found: 72.04, H 8.48, N 3.94.
Data of 3-{(1Z)-1-Bromo-2-[3-bromo-4-(dodecyloxy)phenyl]ethenyl}-6-{(1Z)-1-bromo-2-[4-(dode-
1
cyloxy)phenyl]ethenyl}pyridazine (5): Yellow powder. M.p. 938. H-NMR (CDCl3): 0.86 (t, 2 Me); 1.27
(m, 32 H, CH2); 1.45 (m, 4 H, CH2); 1.79 (m, 4 H, CH2); 3.99 (t, 1 CH2O); 4.06 (t, 1 CH2O); 6.92, 7.76,
8.18 (ABC, 3 arom. H); 6.94, 7.88 (AA’BB’, 4 arom. H); 7.99, 8.01 (narrow AB, 2 heteroarom. H); 8.48 (s,
1 olef. H, 1 H); 8.56 (s, 1 olef. H). 13C-NMR (CDCl3): 14.1 (Me); 22.7, 26.0, 29.0, 29.2, 29.3, 29.6, 29.9
(CH2, partly superimposed); 68.2, 69.3 (CH2O); 112.1, 115.2, 116.9 (CBr); 112.4, 114.4, 126.0, 126.1,
130.8, 131.8, 132.3, 133.9, 134.6 (arom., heteroarom., and olef. CH); 127.5, 128.8, 156.1, 157.0, 157.6, 160.1
(arom. and heteroarom. C). Due to the very low yield of this by-product, we refrained from further
characterizations.
3. Chlorination of 1a to 3,6-Bis{(1Z)-1-chloro-2-[4-(dodecyloxy)phenyl]ethenyl}pyridazine (6).
Pyridazine 1a (5.0 g, 7.66 mmol) was suspended at 778 in dry CCl4 (100 ml). A soln. of SO2Cl2 (1.0 g,
7.41 mmol) in dry CCl4 (50 ml) was added dropwise. Several small portions of dibenzoyl peroxide were
added to the boiling mixture. The originally yellow color turned orange, and the suspended particles
dissolved. When the generation of SO2 ceased, the mixture was cooled to r.t. and washed with aq. Na2CO3
soln. and H2O. Evaporation of the solvent gave an orange residue. TLC (SiO2, CH2Cl2): three products
with Rf 0.3, 0.5, and 0.65, the latter corresponding to 6 as major product. Recrystallization of the orange
residue from CH2Cl2 and then from hexane yielded yellow needles of 6 (1.40 g, 25% rel. to 1a), which
melted at 1298 to the smectic SC phase. An equimolar or excess amount of SO2Cl2 cannot be
recommended to get highly pure 6 (Z,Z). 1H-NMR (CDCl3): 0.86 (t, 2 Me); 1.26 (m, 32 H, CH2); 1.43 (m,
4 H, CH2); 1.78 (m, 4 H, CH2); 3.99 (t, 2 CH2O); 6.94, 7.87 (AA’BB’, 8 arom. H); 7.96 (s, 2 heteroarom.
H). 13C-NMR (CDCl3): 14.1 (Me); 22.7, 26.1, 29.3, 29.4, 29.6, 29.7, 31.9 (CH2, partly superimposed); 68.2
(CH2O); 114.6, 124.0, 124.1, 126.9, 129.7, 132.1, 156.3, 160.0 (arom., heteroarom., and olef. C). FD-MS:
720 (100, Mþ (Cl2 isotope pattern)). Anal. calc. for C44H62Cl2N2O2 (721.9): C 73.21, H 8.66, N 3.88; found:
C 73.09, H 8.52, N 3.96.
4. Hydrogenation of 1a to 3,6-Bis{2-[4-(dodecyloxy)phenyl]ethyl}pyridazine (7). A suspension of 1a
(200 mg, 0.3 mmol) and 10% Pd/C (22 mg)) in dry DMF (70 ml) was stirred at 1108 under a pressure of
3.0 bar (3 · 105 Pa) H2 for 18 h. H2O (600 ml) was added, and the mixture was extracted with CH2Cl2 (3 ꢃ
50 ml). Concentration of the dried (MgSO4) org. phase gave a residue, which was twice recrystallized
from hexane. Colorless needles of 7 (120 mg, 60%) were obtained which melted at 888 to the smectic
1
phase. H-NMR (CDCl3): 0.86 (t, 2 Me); 1.24 (m, 32 H, CH2); 1.42 (m, 4 H, CH2); 1.74 (m, 4 H, CH2);
3.02, 3.19 (AA’BB’, 8 H, CH2CH2 (linker)); 3.90 (t, 2 CH2O); 6.78, 7.05 (AA’BB’, 8 arom. H); 6.97 (s, 2
heteroarom. H). 13C-NMR (CDCl3): 14.1 (Me); 22.7, 26.1, 29.3, 29.4, 29.4, 29.6, 29.6, 29.6, 29.7, 31.9
(CH2); 68.1 (CH2O); 34.7, 38.0 (CH2 (linker)); 114.6, 126.3, 129.4 (arom. and heteroarom. CH); 132.8,

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157.6, 160.8 (arom. and heteroarom. C). EI-MS: 656 (48, Mþ), 655 (100), 487 (21), 107 (96), 57 (43), 43
(48). Anal. calc. for C44H68N2O2 (657.0): C 80.43, H 10.43, N 4.26; found: C 80.08, H 10.41, N 4.16.
5. Irradiation in Solution. A 0.5 · 10ꢁ4 m soln. of (Z,Z)-compounded 3 in cyclohexane (6.5 mg in
160 ml) was irradiated at r.t. with a Hanovia-450-W medium pressure mercury lamp equipped with a
Duran glass filter (l ꢂ 320 nm). The soln. was rigorously stirred, or N2 was purged through the soln., so
that the soln. was continously exchanged in the irradiation zone (monitoring by UV/VIS). As soon as the
photostationary state was reached, the soln. was concentrated, and the residue dissolved in CDCl3.
1H-NMR: several integrations of the signals of the olef. H-atoms and independently of the arom. H-
atoms (see General Part) were averaged and gave the ratio 3/(E,Z)-3 50 :50. The presence of (E,E)-3
could be excluded with a margin of error of ꢄ 2%.
When the soln. which represented the photostationary state was irradiated with l 254 nm (low-
pressure mercury lamp), the portion of (Z,Z)-isomer 3 was enriched from 50 to ca. 85% in the new
photostationary state. Irradiations with l 254 nm or l ꢂ 320 nm over a period of 48 h led to a partical
cleavage of the CꢁBr bonds and to a cross-linking of the material (fatigue effect).
6. Irradiation of the LC Phases. Compound 6 or the mixtures 7/1a or 7/6 10 :1 were molten to their
isotropic phase and studied in the polarization microscope by using the unfiltered polychromatic light of
the microscope or by additional irradiation with l 366 nm. A low temp. gradient yielded the desired temp.
ranges at 135, 128, 127, 125, 123, 120, and 1188 with an accuracy of ꢅ 0.3 K. The textures were then
photographed with a 400-ASA-Kodak-Ektachrom film.
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Received June 26, 2008