Mesomorphic donor-acceptor-substituted 1,4-distyrylbenzenes

Article abstract of DOI:10.1515/znb-2009-1011

The (E,E)-distyrylbenzenes 1a, b with terminal donor-acceptor substitution were prepared by a sequence of four reaction steps including two Wittig-Horner reactions. The products form smectic mesophases, are photostable, and should permit an electric orien

Full text of DOI:10.1515/znb-2009-1011

Mesomorphic Donor-Acceptor-substituted 1,4-Distyrylbenzenes
Thorsten Lifka, Helga Kalbitz, and Herbert Meier
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10 – 14, 55099 Mainz, Germany
Reprint requests to Prof. Dr. H. Meier. Fax: +49-(0)6131-3925396.
E-mail: hmeier@mail.uni-mainz.de
Z. Naturforsch. 2009, 64b, 1183 – 1186; received July 8, 2009
The (E,E)-distyrylbenzenes 1a, b with terminal donor-acceptor substitution were prepared by a
sequence of four reaction steps including two Wittig-Horner reactions. The products form smectic
mesophases, are photostable, and should permit an electric orientation/reorientation, because of their
strong dipole character.
Key words: Liquid Crystals, Push-Pull Effect, Wittig-Horner Reaction
Introduction
Conjugated oligomers with terminal donor-acceptor
substitution attract increasing attention in organic
chemistry because of their outstanding and not sel-
dom unexpected properties in materials science [1 – 4].
Although a variety of oligo(1,4-phenylene-vinylene)s
with terminal donor-acceptor substitution (DAOPVs 1,
Scheme 1) are known [5 – 22], the number of ther-
motropic liquid crystals among them is very low and
is confined to monomers (n = 1), that means to certain
push-pull substituted (E)-stilbenes [23 – 31]. The be-
havior of conjugated molecules, which are highly polar
and form mesophases by self-organization, is very in-
teresting for various applications of liquid crystals [3].
A big disadvantage of stilbenes (n = 1), compared
to higher OPVs (n ≥ 2), is due to their light sensi-
tivity. Stilbenes show four types of photoreactions in
the first excited singlet state S1: E ꢀ Z isomerization,
concerted [2πs + 2πs] cycloadditions, [6πs] electro-
cyclic ring closures, and photo-crosslinking by statisti-
cal CC bond formation [32]. The extended conjugation
in OPVs (n ≥ 2) prevents in their S1 state the first three
processes and permits only the crosslinking by irradia-
tion with energy-rich UV light (λ = 254 nm) [32]. The
intersystem crossing (ISC) rate is normally very low in
OPVs (n ≥ 1), so that triplet photoreactions can also
be neglected.
Scheme 1. DAOPVs; donor-acceptor-substituted oligo(1,4-
phenylene-vinylene)s.
amino groups and cyano or nitro groups do not form
mesophases [3, 4]. We decided to study (E,E)-1,4-
distyrylbenzenes as calamitic mesogens with cyano
groups as acceptors A and long flexible alkoxy groups
as donors D. After all experience with liquid crystals,
these preconditions seemed to be the most promising.
The π-π interaction of (E,E)-1,4-distyrylbenzenes has
to be sustained by highly effective van der Waals inter-
actions of saturated chains.
Results and Discussion
The synthetic approach to the target compounds
1a, b started with the preparation of 4-((E)-2-p-tolyl-
vinyl)benzonitrile (4) by Wittig-Horner reaction of
aldehyde 2 and phosphonate 3 [33]. NBS bromination
afforded 5 which was transformed in an Arbusov re-
action with triethylphosphite 6 to the phosphonate 7.
Wittig-Horner reaction of 7 and 4-hexyloxybenzalde-
hyde (8a) [34, 35] or 4-dodecyloxybenzaldehyde (8b)
[35, 36] yielded the target compounds 1a and 1b, re-
spectively (Scheme 2). The nitriles 4, 5 and 7 form col-
orless crystals, which can be used directly for the sub-
sequent reaction step. Nevertheless, their purity was
checked by 1H NMR spectroscopy (Table 1). The por-
tion of Z configuration in all compounds including 1a,
b was below the detection limit of 5 %.
Among the multiple DAOPVs (n ≥ 2) which were
studied in the past, thermotropic LC phases could not
be found. Therefore, our chances to realize this concept
were low in the beginning. (E,E)-1,4-Distyrylbenzenes
with short alkoxy chains or small or large dialkyl-
c
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T. Lifka et al. · Mesomorphic Donor-Acceptor-substituted 1,4-Distyrylbenzenes
1
Compound Benzonitrile
moiety
Olefinic
double bond
AB
7.09/7.17
7.08/7.17
7.05/7.17
Other benzene
ring
Side chain
Table 1. H NMR spectra of the
nitriles 4, 5 and 7; δ values [ppm]
in CDCl3, TMS as internal stan-
dard.
AAꢀBBꢀ
3J (Hz)
16.2
16.2
16.3
AAꢀBBꢀ
4
5
7
7.54/7.60
7.53/7.62
7.53/7.58
7.18/7.41
7.39/7.41
7.28/7.45
2.38 (s, CH3)
4.50 (s, CH2)
1.23 (m, CH3)
3.12 (d, 2JP,H = 21.2 Hz, CH2P)
4.00 (m, OCH2)
O
O
KOH, DMF
N
N
C
C
+
CH3
(H5C2O)2P CH2
56 %
H
3
2
4
NBS
CH3
N
C
CH2Br
5
P(OC2H5)3
O
N
C
6
CH2 P(OC2H5)2
Product
R
Yield 4 → 1 (%) 27
1a
C6H13 C12H25
33
1b
7
O
H
RO
N
C
8a, b
Scheme 2. Preparation of the
push-pull-substituted (E,E)-1,4-
distyrylbenzenes 1a, b.
OR
KOH, DMF
1a, b
The DAOPVs 1a, b (n = 2) are green-yellow solids.
Their long-wavelength absorption maxima in CHCl3
solution are observed at 375 nm. The push-pull ef-
fect causes a bathochromic shift compared to the par-
ent system 1,4-distyrylbenzene,which has a λmax value
of 356 nm (CHCl3). The polarization of these rod-like
molecules can be seen by regarding the 13C chemical
shifts, in particular the ∆δ values of the olefinic car-
bon atoms. Fig. 1 depicts the data of 1a: ∆δ = 132.1–
126.2 = 5.9 ppm (acceptor side), ∆δ = 129.0– 125.9 =
3.1 ppm (donor side). Compound 1b shows the same
effect. The correlation of the signals with certain nu-
clei was based on INDOR, NOE and HMBC measure-
ments.
Fig. 1. 1H (underlined) and 13C chemical shifts of 1a (δ val-
ues in ppm in CDCl3, related to TMS as internal standard).
◦
ing generates smectic phases S at 253 and 245 C,
respectively. These phase transitions have by far the
highest ∆H values. The textures of the smectic phases S
consist of mosaic patterns with curved fissures, whose
number increases on further cooling to a smectic
phase Sꢀ with a very low mobility. We assume the for-
mation of higher ordered smectic phases, such as SB.
Recently performed small-angle X-ray scattering
The t◦arget compounds 1a, b are thermally stable up
to 320 C. Their DSC diagrams contain between 20
and 320 ◦C four reversible phase transitions, which are
listed in Table 2.
Cooling of the isotropic melt I in both cases leads
first to an isotropic, non-birefringent LC phase. The (SAXS) and polarized optical microscopy (POM) mea-
light in the polarization microscope is parallel to the surements of related compounds 1 (D = alkoxy, A = es-
optical axis of these rod-like molecules. Further cool- ter, n = 2) [37] demonstrated the problems to determine
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T. Lifka et al. · Mesomorphic Donor-Acceptor-substituted 1,4-Distyrylbenzenes
1185
Table 2. Phase transitions of compounds 1a and 1b between
the crystalline phase K at room temperature and the isotropic
melt I at 300 ◦C (DSC rate 10 K min−1).
7 instrument served for the differential scanning calorimetry.
Melting points were measured with a Bu¨chi melting point ap-
paratus and are uncorrected. A Leitz Ortholux II microscope
together with a Mettler FP-52 heating system was used for
the polarization microscopy. Elemental analyses were per-
formed in the microanalytical laboratory of the Chemistry
Department of the University of Mainz.
Compound
Onset transition Transition enthalpy
temperature T (◦C) ∆H (kJ mol−1
)
K
1a
Heating curve
Cooling curve
Heating curve
Cooling curve
176
192
251
295
2
3
19
1
(E)-4-[2-(4-Methylphenyl)vinyl]benzonitrile (4)
∼
Preparation accordi◦ng to ref. [33]. Colorless crystals,
yield 56 %, m. p. 178 C. – 13C NMR (CDCl3): δ = 21.3
(CH3), 110.3, 133.6, 138.6, 142.1 (aromat. Cq), 119.0 (CN),
125.7, 132.4 (olefin. CH), 126.7, 126.9, 129.6, 132.5 (aro-
mat. CH).
I
295
253
187
176
∼ − 1
∼ −16
− 3
∼ − 1
K
70
164
240
289
General procedure for the preparation of the (E,E)-4-
[2-(4-{2-[4-(alkoxy)phenyl]vinyl}phenyl)vinyl]benzo-
nitriles (1a, b)
1b
2
4
15
5
Nitrile 4 (6.0 g, 27.4 mmol) and N-bromosuccinim◦ide
(5.34 g, 30 mmol) were heated in 80 mL of CCl4 to 40 C.
Dibenzoylperoxide (0.1 g, 0.42 mmol) was added portion-
wise while the mixture was refluxed for 3 h. n-Hexane
(150 mL) was added to the hot-filtered solut◦ion. The bromi-
nated nitrile 5 crystallized overnight at 5 C. It was then
treated with triethyl phosphite 6 (4.16 g, 25 mmol) at 160 ◦C
for 1 h. The generated bromoethane was removed conti-
nously. Distillation in the vacuum (0.1 kPa) gave first some
excess phosphite (6) and then phosphonate 7, a light-yellow,
viscous oil, which started to crystallize at r. t. It was dissolved
together with 4-alkoxybenzaldehyde 8a or 8b (20.0 mmol) in
50 mL of DMF and then dropped to a vigorously stirred sus-
pension of KOH (2.8 g, 50 mmol) in 50 mL of DMF. Dur-
I
293
245
163
74
− 4
−15
− 4
− 3
precisely the molecular arrangement of such OPVs in
smectic phases. The mesophases of 1a, b are photo-
stable. The strong dipole character of these compounds
should enable an efficient electrical orientation or re-
orientation of the molecules in switching processes.
Moreover, high first-order hyperpolarizabilities (SHG)
can be expected. EFISHG measurements of a related
system 1 (n = 2, A = NO2, D = NR2), which does
◦
ing the addition the temperature was kept below 5 C and
◦
◦
not form mesophases, gave a very high value βZZZ
=
then slowly raised to 50 C. After 2 h at 50 C, 100 mL
(716 31) 10−50 C m3 V−2 [38].
of methanol was added. The green-yellow crystals formed
◦
at 0 C were filtered, washed with cold methanol and pu-
Conclusion
rified by column filtration (4 × 10 cm SiO2, CHCl3). The
total yield related to 4 amounts to 3.0 g (27 %) of 1a and
4.4 g (33 %) of 1b. The melting and clearing processes are
discussed in the theoretical part.
The push-pull-substituted (E,E)-1,4-distyrylbenz-
enes 1a, b with cyano and alkoxy termini can be
prepared by applying Wittig-Horner reactions. Hexyl-
oxy or dodecyloxy groups serve as electron donors
and enable the formation of smectic mesophases. The
compounds are photostable and owing to their strong
dipole character, they should permit an efficient electri-
cal switching process and high hyperpolarizabilities β.
(E,E)-4-[2-(4-{2-[4-(Hexyloxy)phenyl]vinyl}phenyl)vinyl]-
benzonitrile (1a)
IR (KBr): ν (cm−1) = 2930, 2230, 1595, 1580, 1505,
1250, 1170, 965, 835. – FD MS: m/z (%) = 408 (100)
[M]+. – C29H29NO (407.56): calcd. C 85.47, H 7.17, N 3.44;
found C 85.47, H 7.26, N 3.55.
Experimental Section
(E,E)-4-[2-(4-{2-[4-(Dodecyloxy)phenyl]vinyl}phenyl)vinyl]-
benzonitrile (1b)
1H and 13C NMR spectra were recorded on a Bruker AM
400 spectrometer. FD MS measurements were performed
with a Finnigan MAT 95 spectrometer. The UV/Vis spectra
IR (KBr): ν (cm−1) = 2900, 2220, 1595, 1580, 1505,
were recorded on a Zeiss MCS 320/340 and the IR spectra 1245, 1170, 960, 830. – 1H NMR (CDCl3): δ = 0.89 (m,
on a Beckman Acculab spectrometer. A Perkin-Elmer DSC- 3H, CH3), 1.2 – 1.4 (m, H, CH2), 1.45 (m, 2H, CH2), 1.78
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1186
T. Lifka et al. · Mesomorphic Donor-Acceptor-substituted 1,4-Distyrylbenzenes
(m, 2H, CH2), 3.98 (t, 2H, OCH2), 6.88/7.43 (AAꢀBBꢀ, mat. CH), 125.8, 126.3, 129.0, 132.1 (olefin. CH), 110.5,
4H, aromat. H, donor side), 6.09/7.09 (AB, 3J = 16.2 Hz, 129.8, 135.2, 138.3, 142.0 (aromat. Cq), 119.1 (CN), 159.2
2H, olefin. H, donor side), 7.07/7.19 (AB, 3J = 16.2 Hz, (aromat. CqO).
2H, olefin. H, acceptor side), 7.49 (m, 4H, aromat. H, mid-
Acknowledgement
dle), 7.56/7.62 (AAꢀBBꢀ, 4H, aromat. H, acceptor side). –
13C NMR (CDCl3): δ = 14.1 (CH3), 22.7, 26.1, 29.3,
29.4, 29.6, 29.7, 31.9 (CH2, partly superimposed), 68.2
(OCH2), 114.6, 126.7, 126.8, 127.3, 127.8, 132.5 (aro-
We are grateful to the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie for financial sup-
port.
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