Synthesis and Photoisomerization of mesogenic 4-alkoxystyrylpyridines

Article abstract of DOI:10.1134/S1070363211050173

Introduction of long-chain alkoxy substituents into molecules of styrylpyridines as potential ligands for photosensitive metal complexes endows them with liquid crystalline properties over a wide temperature range. trans-cis Photoisomerization of 4-alkoxy

Full text of DOI:10.1134/S1070363211050173

ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 5, pp. 937–943. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © O.A. Turanova, L.G. Gafiyatullin, O.I. Gnezdilov, A.N. Turanov, 2011, published in Zhurnal Obshchei Khimii, 2011, Vol. 81, No. 5,
pp. 850–856.
Synthesis and Photoisomerization of Mesogenic
4-Alkoxystyrylpyridines
O. A. Turanova, L. G. Gafiyatullin, O. I. Gnezdilov, and A. N. Turanov
Zavoiskii Kazan Physicochemical Institute, Kazan Research Center, Russian Academy of Sciences,
ul. Sibirskii trakt 107, Kazan, 420029 Tatarstan, Russia
e-mail: lightur@mail.ru
Received May 13, 2010
Abstract—Introduction of long-chain alkoxy substituents into molecules of styrylpyridines as potential ligands
for photosensitive metal complexes endows them with liquid crystalline properties over a wide temperature
range. trans–cis Photoisomerization of 4-alkoxystyrylpyridines in solution was revealed by NMR and
electronic absorption spectroscopy, and the kinetics of transformations of their molecular forms after irradiation
were estimated. Photoinduced trans→cis isomerization of 4-alkoxystyrylpyridines is characterized by a higher
rate, and intramolecular cyclization of their cis isomers after irradiation is slower, as compared to unsubstituted
analog.
DOI: 10.1134/S1070363211050173
Photoprocesses, i.e. processes induced by irradia-
tion in the visible and near-UV and near-IR regions,
are widespread in nature and extensively used in
technics [1]. Photochromic properties of many organic
compounds are determined by trans–cis isomerization
of their molecules. Such compounds are promising for
the design of recordable and rewritable optical media
and photoswitchable molecular devices [2–6].
sensitive blocks in compounds possessing photo-
switchable properties, in particular, magnetic param-
eters [13]. In this connection, of particular interest are
4-alkoxy-substituted styrylpyridines which are capable
of forming liquid crystalline phase (mesophase) and
acting as ligands in mesogenic spin-crossover metal
complexes. Therefore, it was important to estimate
differences in the kinetics of transformations of
mesogenic and non-mesogenic 4-styrylpyridines both
in the course of irradiation and after its termination.
4-Styrylpyridines are typical representatives of
compounds possessing an active C=C bond, which is
capable of changing molecular structure by the action
of external factors [7–9]. Variation of a substituent in
the 4-position could give rise to heteropolar molecules
which attract interest as potential materials for
nonlinear optics [10–12]. Such properties of 4-styryl-
pyridines make it possible to use them as photo-
In the present article we describe the synthesis of
mesogenic alkoxy-substituted 4-styrylpyridines and
their photoinduced isomerization. Unsubstituted 4-
styrylpyridine (I) was synthesized according to the
procedure described previously [14]. Its liquid
crystalline derivatives II and III were obtained by
O
R
C
+
CH3
N
R
CH CH2
OH
N
H
R
CH CH
N
_H2O
I_III
I, R = H; II, R = C8H17O; III, R = C12H25O.
937

938
TURANOVA et al.
(a)
(b)
Fig. 1. Textures of mesophases formed by compound III: (a) smectic SE and (b) smectic SB.
condensation of 4-methylpyridine with the cor-
responding 4-alkoxybenzaldehydes in acetic anhydride.
Schlieren texture and almost parallel inverse sides
(Fig. 1) and then into isotropic state. The phase
transition temperatures and yields of compounds I–III
are given in Table 1.
The structure of the newly synthesized compounds
1
was confirmed by elemental analysis, H NMR and
Irradiation of compounds I–III with UV light could
induce not only their trans–cis isomerization but also
subsequent intramolecular cyclization of the cis
isomer, as well as dehydrogenation of the cyclization
product in the presence of an oxidant (I2, FeCl3) or
atmospheric oxygen [5]. The photocyclization process
of stilbene was studied in most detail [16–22].
Figure 2 shows two regions of the 1H NMR spectra
of compound III. The spectra recorded in 10 h and
30 min after irradiation coincide with each other. This
means that a solution of III with a concentration of
UV spectroscopy, and MALDI-TOF mass spec-
trometry. 4-Alkoxy-substituted styrylpyridines ex-
hibited liquid crystalline properties over a wide
temperature range. The type of mesophase was
determined by examination of their characteristic
textures, i.e., optical patterns observed with the aid of
polarizing microscope in their thin films [15]. With
rise in temperature crystalline compounds II and III
produce smectic (E) mesophase (SE) with
characteristic striated texture, which is converted first
into less structured smectic (B) mesophase (SB) with
R
Hi
Hf
He
Hj
Hg
He
Hh
R
Hc
Hd
k1
k2
Hh
Hc
Hi
Ha
Hf
Hd
Hg
Hj
N
Ha
Hc
Hb
N
Hb
trans
cis
Hf
He
Hf
He
Hh
Hh
R
Hc
Hd
k3
k4
[O]
Hi
Ha
Hi
Ha
k5
Hg
Hj
N
N
R
Hj
Hb
Hb
A cycle
Dehydrogenation B cycle
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SYNTHESIS AND PHOTOISOMERIZATION OF MESOGENIC
939
1.2×10–3 M in 30 min reaches stationary state cor-
responding to the dihydro cyclic structure. Proton
Table 1. Phase transition temperatures and yields of 4-(4-
alkoxystyryl)pyridines I–III
1
signals in the H NMR spectrum of III (Table 2) were
Comp. no.
Phase transition temperatures, °C
Yield, %
assigned taking into account chemical shifts typical of
particular groups of protons, spin–spin coupling
constants, and signal intensities.
129 (Сr→I)
42
20
27
I
97 (Сr→SE), 127 (SE→SB), 165 (SB→I)
94 (Сr→SE), 117 (SE→ SB), 161 (SB→I)
II
III
Before irradiation, the chemical shifts of He and Hf
were as follows, δ ppm: 7.26 (He, trans), 7.71 (He, cis),
6.88 (Hf, trans), 6.33 (Hf, cis). The intensity ratio of
these signals was 0.8:0.2 (trans:cis). Upon irradiation
most part of molecules III is converted first into the cis
isomer and then into the cyclic structure with covalent
bond between the Cg and Cd atoms. Redistribution of
electron density as a result of formation of the third
ring leads to upfiedl shift of the He and Hf signals by
Δδ ≈ 0.50±0.05 ppm for the trans isomer, Δδ ≈
0.20±0.10 ppm for aromatic protons, and Δδ ≤
0.05±0.01 ppm from protons in the alkyl chain.
Exceptions are signals from He and Hf of the cis
isomer. The He signal is displaced upfield by ~1 ppm,
whereas the Hf signal shifts downfield by ~0.05 ppm
(Table 2).
The cyclization process continues when irradiation
is terminated (Fig. 2). As follows from the signal
intensity ratio (0.85:0.05:0.1), the fraction of the
cyclic isomer of III reaches 85%, while the
concentrations of the trans and cis isomers decrease to
5 and 10%, respectively. It should be emphasized that
no dehydrogenation typical of stilbene [16–21] was
observed in the examined system in the absence of
oxidant (k5 = 0) for at least several hours.
The electronic absorption spectra of compounds I–
III in the UV region contain several maxima (Fig. 3).
Absorption maxima in the region λ 220–230 nm are
obscured by the solvent (acetonitrile). The maxima at λ
305 and 322 nm correspond to the trans isomer of III
(Fig. 3a), taking into account that before irradiation
most part of 4-styrylpyridine molecules in solution
exist as trans isomers [13, 22]. The UV spectrum of II
was identical to the spectrum of III, while absorption
maxima in the UV spectrum of I were observed at
shorter wavelengths, λ 298 and 303 nm. The UV
spectrum of I in acetonitrile is usually described as a
characteristic absorption line at λ 299 nm [13]. The
presence of an electron-donating alkoxy substituent in
molecules II and III is responsible for electron density
redistribution in their molecules and red shift of the
absorption maxima belonging to the trans isomers of
II and III as compared to compound I (Fig. 3).
Variation of the number of methylene groups in the
alkoxy substituent does not lead to appreciable
variation of the position of absorption maxima in the
UV spectra.
The 1H NMR spectra of compound II recorded both
before and after irradiation were very similar to those
of compound III, except for the intensity of the
multiplet signal at δ 1.26 ppm belonging to methylene
protons in the alkoxy group (8H instead of 16H).
OCH2
Table 3 contains parameters of the 1H NMR spectra
of compound I, recorded under the same conditions as
the spectra of III. The spectrum of I before irradiation
coincided with the spectrum of trans-III with an
accuracy of 0.15 ppm (with account taken of the
absence of alkoxy group in I). This means that
compound I exists almost exclusively as trans isomer.
After UV irradiation over a period of 5 min, signals in
1
the H NMR spectra of I are also displaced upfield by
approximately the same value Δδ as that found for III.
However, the cyclization process is complete over the
irradiation period, and no appreciable variation of the
1H NMR pattern of I was observed in several hours
after irradiation.
δ, ppm
1
Fig. 2. Fragments of the H NMR spectra of compound III
in CDCl3 (1) before irradiation, (2) after UV irradiation over
a period of 5 min, and (3) in 30 min after the end of
irradiation.
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TURANOVA et al.
Table 2. 1H NMR parameters (chemical shifts δ, ppm, and coupling constants J, Hz) of a solution of compound III in CDCl3
After irradiation
Proton
Before irradiation
cis/trans
8.55 d (0.3H, J 5.8)
7.36 d (0.3H, J 6.0)
A
Ha, Hb
8.55 d (2H, J 5.8)
8.46 d (1.7H, J 5.8)
7.16 d (1.7H, J 6.0)
6.72 d (0.85H, J 12.4)
Hc, Hd
He
7.35 d (2H, J 6.0)
7.26 d (0.8H, J 16.8, trans)
7.71 d (0.2H, J 16.0, cis)
7.26 d (0.05H, J 16.8, trans)
7.71 d (0.1H, J 16.0, cis)
Hf
6.88 d (0.8H, J 16.8, trans)
6.34 d (0.2H, J 16.0, cis)
6.88 d (0.05H, J 16.8, trans)
6.33 d (0.1H, J 16.0, cis)
6.39 d (0.85H, J 12.4)
Hg, Hh
6.91 d (2H, J 8.8)
7.47 d (2H, J 8.8)
3.99 t (2H, J 6.7)
1.79 p (2H, J 6.7)
1.46 m (2H)
6.91 d (0.3H, J 8.8)
7.47 d (0.3H, J 8.8)
3.99 t (0.3H, J 6.7)
6.77 d (1.7H, J 8.8)
7.14 d (1.7H, J 8.8)
3.93 t (1.7H, J 6.7)
Hi, Hj
OCH2
OCH2CH2
OCH2CH2CH2
CH2
1.77 p (2H, J 6.7)
1.44 m (2H)
1.26 m (16H)
1.26 m (16H)
0.88 t (3H, J 6.7)
CH3
0.88 t (3H, J 6.7)
(a)
(b)
200
300
400
λ, nm
200
300
400
500
600
λ, nm
Fig. 3. (a) Electronic absorption spectra of a solution of compound III in acetonitrile (1) before irradiation and after irradiation with
UV light (λ 365 nm) over a period of 5 s in (2) 10 s and (3), 2, (4) 4, (5) 6, (6) 9, (7) 15, (8) 26, (9) 37, (10) 53, and (11) 69 min. (b)
Electronic absorption spectra of a solution of compound I in acetonitrile (1) before irradiation and after irradiation with UV light (λ
365 nm) over a period of (2) 5 and (3) 30 s in (2) 10 s and (3) 2, 4, 6, 8, 10, 15, 20, and 30 min.
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SYNTHESIS AND PHOTOISOMERIZATION OF MESOGENIC
941
1
Table 3. H NMR parameters (chemical shifts δ, ppm, and
coupling constants J, Hz) of a solution of compound I in
CDCl3
Irradiation of 4-styrylpyridines in solution with UV
light (λ = 365 nm) over a period of 5 s induces
photoisomerization, which is accompanied by
characteristic variation of the spectral pattern, as
shown in Fig. 3a for compound III. After irradiation
over a period of 5 s, the absorption maxima typical of
compound III sharply decrease in intensity. During the
first ~5 min after irradiation, the absorption intensity
slightly increases and then considerably decreases
(Fig. 3a). Such kinetic pattern is not typical of systems
where trans–cis–ring transformations occur [2, 22]; on
the other hand, it is theoretically possible [23]: it may
be described in terms of the scheme shown above
provided that k2 > k1 and k4 = 0. When the irradiation
time is prolonged from 5 to 10, 30, and 60 s, the
isomerization processes is complete during irradiation.
Regardless of the irradiation time, the UV spectra of
compound III, recorded after equilibration (cyclic
structure A and some amount of cis-III), were similar.
A different pattern was observed for compound I.
Figure 3b shows variations of the UV spectra of I after
irradiation for 5 and 30 s, other conditions being equal.
The intensities of all absorption maxima monotonically
decreases after irradiation. However, the equilibrium
state is determined by the irradiation time. The longer
the irradiation period, the greater the fractions of the
cis isomer and cyclization product A (Fig. 3b).
Proton
Ha, Hb
Before irradiation
8.59 d (2H, J 6.0)
After irradiation (A)
8.48 d (2H, J 6.0)
Hc, Hd
He
7.44 d (2H, J 6.4)
7.25 d (2H, J 6.4)
6.84 d (1H, J 16.4)
6.51 d (1H, J 16.4)
7.24 d (2H, J 7.6)
7.26 d. t (2H, J 7.2, 1.6)
7.23 t (1H, J 1.6)
7.35 d (1H, J 16.4, trans)
7.04 d (1H, J 16.4, trans)
7.40 d (2H, J 7.6)
Hf
Hg, Hh
Hi, Hj
HR
7.56 d. t (2H, J 7.2, 1.6)
7.43 t (1H, J 1.6)
The kinetics of the above transformations were
described with the aid of the following equation [23]:
A = l1exp{–k3τ} – l2exp{–(k2 – k1)τ}, where k3, k2, and
k1 are the rate constants for the corresponding
transitions [21], and l1 and l2 are quantities determined
by the initial isomer ratio and irradiation parameters.
The results of fitting for compound III are represented
by curves 1 and 2 in Fig. 4; k3 = (27±1)×10–5 s–1,
k2 – k1 = (7.6±2)×10–3 s–1 for λ 322 nm; k3
=
(57±1)×10–5 s–1, k2 – k1 = (7.0±2)×10–3 s–1 for λ 305 nm.
The correlation coefficient for both dependences is
|R| > 0.97.
Thus introduction of an electron-donating alkoxy
substituent into 4-styrylpyridine molecule accelerates
trans–cis isomerization upon irradiation (in 10 s after
5-s irradiation the absorption intensities of compounds
I and III drop down by 5 and 40%, respectively) and
slows down the cyclization step after irradiation. The
most probable reasons are different polarities of their
molecules in the first case and different rates of
thermodynamic equilibration in the system in the
second.
A, rel. units
These results are consistent with the NMR data
which indicate predominant formation of cyclic
product A. It should be taken into account that UV
spectra recorded from solutions with a concentration
lower by two orders of magnitude than in NMR
experiments. Insofar as absorption bands in the UV
spectra of compounds I–III are considerably over-
lapped, the first derivatives of the absorption curves
were plotted in order to describe the kinetics of
photoisomerization on a quantitative level. This is
quite admissible, for the absorption line width is
almost constant. Figure 4 shows the dependences of
the amplitudes of the first derivatives upon time.
t, s
Fig. 4. Dependences of the amplitudes of two components of
the first derivatives of the UV absorption curves of (1, 2)
compound III at (1) λ 322 nm and (2) λ 305 nm and (3, 4)
compound I at (3) λ 298 and (4) λ 303 nm upon time elapsed
after irradiation.
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TURANOVA et al.
The kinetics of transformations in the electronic
mass spectra (MALDI-TOF) were obtained on a Bruker
Ultraflex III MALDI-TOF/TOF mass spectrometer.
spectra of compound I may be described by the
simplified formula A = lexp{–kτ} for both λ 298 nm,
k = (7.0±0.6)×10–3 s–1, and λ 303 nm, k =
(6.8±0.7)×10–3 s–1. The results of fitting for compound
I are represented by curves 3 and 4 in Fig. 4. The
correlation coefficient for these dependences is |R| >
0.99. The rate constant k3 for compound I is higher by
almost an order of magnitude than that for compound
III, which confirms the above qualitative conclusions.
The textures and phase transition temperatures were
determined using a Boetius VEB Nagema polarizing
microscope equipped with a temperature-control unit.
The temperatures were measured with an accuracy of
±0.1°C.
4-Styrylpyridine (I) was synthesized according to
the procedure described in [14]. Mass spectrum: m/z
181.24. Found, %: C 85.09, 85.33; H 6.04, 6.39; N
7.41, 7.66. C13H11N. Calculated, %: C 86.15; H 6.12;
N 7.73.
Presumably, the difference in the kinetics of
photoisomerization of compounds I–III, as well as in
the behavior of components with different absorption
maxima, is determined to some extent by the existence
of rotamers (i.e., conformers differing by the dihedral
angle between the aromatic ring planes as a result of
rotation about quasi-single CH–Cring bond) [24] and by
the presence of some aggregated species in solution.
4-(4-Octyloxystyryl)pyridine (II). A mixture of
4.42 ml of 4-methylpyridine and 11.3 g of 4-octyl-
oxybenzaldehyde in 29 ml of acetic anhydride was
heated for 40 h under reflux. After cooling, the
precipitate was filtered off, recrystallized from hexane,
and washed with acetone. The product was a light
yellow crystalline powder. Mass spectrum: m/z 309.
Found, %: C 81.55, 81.70; H 8.77, 8.87; N 4.49, 4.89.
C21H27NO. Calculated, %: C 81.55; H 8.74; N 4.53.
To conclude, introduction of a long-chain alkoxy
substituent into styrylpyridine molecules gives struc-
tures capable of forming liquid crystalline mesophases
(smectic E and B) over a wide temperature range.
1
According to the H NMR and UV spectral data, 4-(4-
4-(4-Dodecyloxystyryl)pyridine (III) was syn-
thesized in a similar way. The light yellow precipitate
was recrystallized from ethanol. Mass spectrum: m/z
365. Found, %: C 81.81, 82.10; H 9.97, 10.17; N 3.69,
3.89. C25H35NO. Calculated, %: C 82.19; H 9.59; N
3.84.
alkoxystyryl)pyridines in solution undergo photoin-
duced trans→cis isomerization, and the kinetics of
these transformations were estimated after irradiation.
The rate of trans→cis isomerization in alkoxy-
styrylpyridines upon irradiation is considerably higher,
while the rate of intramolecular cyclization of their cis
isomer after irradiation is lower by an order of
magnitude, as compared to the unsubstituted analog.
ACKNOWLEDGMENTS
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 08-02-01348).
EXPERIMENTAL
The 1H NMR spectra were recorded at room
temperature (20°C) on a Bruker Avance 400 spec-
trometer [operating frequency 400 MHz; 90°-FID
followed by Fourier transform; pulse duration 9.50 μs,
delay 7.00 s, scan width 7 kHz (14.20 to –3.30 ppm),
scan number 8, 32000 points per FID] from solutions
in CDCl3 with a concentration of 1.2×10–3 M using 5-
mm ampules.
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