Photochromic liquid-crystalline copolymers containing crown ether groups

Article abstract of DOI:10.1007/s11172-007-0384-6

An approach was developed for the synthesis of new multifunctional photosensitive liquid-crystalline copolymers of the acryl series containing azobenzene, ionophoric, and mesogenic groups in the same macromolecule. The phase behavior of the copolymers was studied. Most of these copolymers were demonstrated to form nematic mesophases. An increase in the concentration of crown-containing groups to 26 mol.% leads to amorphization of the copolymers. The influence of complexation of the crown ether groups of the copolymers with potassium perchlorate on the mesomorphic properties of the systems was investigated. A comparative study of the photooptical properties of the copolymers in solution and thin films was performed.

Full text of DOI:10.1007/s11172-007-0384-6

Russian Chemical Bulletin, International Edition, Vol. 56, No. 12, pp. 2414—2425, December, 2007
2414
Photochromic liquidꢀcrystalline copolymers containing crown ether groups

V. P. Shibaev, A. S. Medvedev, and A. Yu. Bobrovsky
Department of Chemistry, M. V. Lomonosov Moscow State University,
1 Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (495) 939 0174. Eꢀmail: lcp@genebee.msu.su
An approach was developed for the synthesis of new multifunctional photosensitive liquidꢀ
crystalline copolymers of the acryl series containing azobenzene, ionophoric, and mesogenic
groups in the same macromolecule. The phase behavior of the copolymers was studied. Most of
these copolymers were demonstrated to form nematic mesophases. An increase in the concenꢀ
tration of crownꢀcontaining groups to 26 mol.% leads to amorphization of the copolymers. The
influence of complexation of the crown ether groups of the copolymers with potassium perꢀ
chlorate on the mesomorphic properties of the systems was investigated. A comparative study
of the photooptical properties of the copolymers in solution and thin films was performed.
Key words: azobenzene, crown ether fragments, complexation properties, photochromic
properties, multifunctional combꢀshaped liquidꢀcrystalline copolymers.
In connection with the huge interest in the design of
nanomaterials, our recent attention has been drawn to the
5
problems of the synthesis of multifunctional polymers and
6
polymer composites consisting of differentꢀtype nanomeꢀ
terꢀsized components bearing various functions. These
are liquidꢀcrystalline (LC) copolymers containing meꢀ
sogenic side groups (they serve as models of the molecular
structures of lowꢀmolecularꢀweight liquid crystals, which
4
1
2
3
are, in turn, typical nanosized particles), as well as any
other substituents on a nanosize scale bearing a particular
functionality. These can be fragments of dyes or photoꢀ
chromic compounds, chiral optically active and nonlinꢀ
earꢀoptical groups, biologically active fragments, lumiꢀ
nescent and fluorescent groups, and electrically and magꢀ
netically active fragments.1,2
Fig. 1. Schematic representation of the macromolecule
of a multifunctional combꢀshaped copolymer containing
mesogenic (1), chiral (2), photochromic (3), and functional
groups (4) covalently bound to the main chain (5) by aliphatic
spacers (6).
The schematic representation of a hypothetical macꢀ
romolecule of a combꢀshaped LC copolymer containing
mesogenic, chiral, photochromic, and reactive functional
groups is shown in Fig. 1. Mesogenic groups that are
quantitatively dominant in this system and determine the
ability of the polymer to undergo selfꢀorganization and
form an LC phase are the key structural elements of these
macromolecules. Any of these molecular groups (taken
either individually or together with other groups) impart
the desired functional properties, such as photochromic,
electrical, optically active, etc., to the final LC polymers
at will of researchers.
Since such copolymers are generally synthesized by
the copolymerization of monofunctional nanosized monoꢀ
mers, the synthesis of these compounds is based on the
commonly used bottomꢀup method of the design of naꢀ
nomaterials.3—5 A similar principle of the selfꢀassembly is
observed in the living nature, where complex (multifuncꢀ
tional) protein macromolecules are assembled from 20
simple amino acids. The primary structure of protein molꢀ
ecules determines the further more complex processes
giving rise to the secondary and ternary structure.
In the case under consideration, complex structurally
organized and functionally integrated multifunctional
polymer LC systems are assembled from nanostructures
(monomers and functional fragments) with a size of tens
of nanometers. The tendency of mesogenic groups to unꢀ
dergo selfꢀorganization and form mesophases is the drivꢀ
ing force for the formation of such systems. The presence
of functional groups having different structures and difꢀ
ferent physicochemical properties has also a substantial
effect on the properties of LC polymers by imparting new
properties to these selfꢀorganized anisotropic systems. This
is finally responsible for their particular performance charꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2332—2242, December, 2007.
1066ꢀ5285/07/5612ꢀ2414 © 2007 Springer Science+Business Media, Inc.

Liquidꢀcrystalline crownꢀcontaining copolymers
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007 2415
acteristics and fields of application. These functional LC
polymers are often referred to the soꢀcalled smart materiꢀ
als, which meet the requirements imposed on modern
engineering devices. Such materials should consume the
minimum amount of energy, have a small size and weight,
a high efficiency, and can be able to be easily incorporatꢀ
ed into technological lines and systems.6,7
Among a wide range of smart materials, such as conꢀ
ducting and electroluminescent polymers, polymer cataꢀ
lysts, and biomimetic and fluorescent polymer systems,
particular attention is given to photochromic materials
and polymer sensors.
In the present study, we report a new approach to the
synthesis of photochromic LC polymers, which are charꢀ
acterized by the presence of both mesogenic groups proꢀ
viding the formation of a mesophase and the simultaꢀ
neous presence of photochromic and ionophoric groups
in a single monomeric unit capable of exerting a mutual
effect on the photochromic and complexation properties
of the polymers. Selected possible combinations of
mesogenic, photochromic, and ionophoric side groups
are presented in Fig. 2. However, we restricted ourselves
to the consideration of solely binary copolymers
(see Fig. 2, a, b).
It is well known that photosensitive systems play a conꢀ
siderable role in the living nature, to be more precise, in the
photosynthesis processes. In addition, various photosensiꢀ
tive materials have long been in use as integral parts of
various industrial and domestic apparatus and devices.
Photochromic materials lightꢀcontrolled at the moꢀ
lecular and supramolecular levels have attracted attention
of researchers working in various areas, such as synthetic
chemists, photochemists, and photophysicists.8,9 Due to
the progress in the nanoindustry of materials, a new field
of investigation, the nanophotonics, has been vigorously
developed.
In continuation of our studies on the photooptical
properties of photochromic LC polymers,11—13 we develꢀ
oped an approach to the synthesis of new multifunctional
combꢀshaped LC polymers bearing simultaneously three
types of functional fragments in the side chains, such as
mesogenic groups, photochromic azobenzeneꢀcontainꢀ
ing groups, and functional crownꢀcontaining fragments
capable of forming complexes with metal ions (Fig. 2).
It is well known that the introduction of azobenzeneꢀ
containing groups into combꢀshaped LC polymers allows
the design of lightꢀcontrolled polymer films and coatings,
which can be used with advantage for the optical informaꢀ
tion recording and storage due to the photoinduced
E—Zꢀisomerization reactions of azobenzene groups folꢀ
lowed by the cooperative photoorientation of the side
groups under polarized light.
The choice of the crownꢀcontaining fragment as an
ionophoric fragment was determined primarily by its high
activity with respect to the formation of coordination
bonds with metal ions, as well as by the ability of crownꢀ
containing molecules to undergo selfꢀassembly in soluꢀ
tion and in the solid phase giving rise to complex suꢀ
pramolecular structures.10
It should be noted that data on the synthesis and propꢀ
erties of crownꢀcontaining combꢀshaped LC polymers are
available in the literature. For example, the study of combꢀ
shaped LC copolymers of the acryl series containing
15ꢀcrownꢀ5 groups14 showed that the complexation of
macrocyclic fragments of the copolymer with metal ions
can be used as an efficient tool for controlling the mesoꢀ
morphic properties of polymers.
Investigations of lowꢀmolecularꢀweight photosensitive
crownꢀcontaining systems are of great interest. Photoꢀ
chromic fragments can substantially influence the comꢀ
plexation properties of the crown ether group15,16 and, on
the contrary, the complexation with metal salts can change
the photooptical properties of such systems (the kinetics,
the mechanism and the type of photochemical reactions,
and the absorption and fluorescence spectra).15,17,18 For
example, the Z isomer of azobenzeneꢀcontaining crown
ether was demonstrated15 to form stable complexes with
alkali metal ions (Na+, K+, Rb+, and Cs+), whereas the
E isomer of this compound cannot form such complexes.
In addition, a decrease in the rate of the thermal Z→E
a
b
c
1
1
1
2
2
1
1
3
3
3
Fig. 2. Schematic representation of the macromolecules of binary (a, b) and ternary (c) copolymers consisting of mesogenic (1),
photochhromic (2), and crownꢀcontaining fragments (3).

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Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007
Shibaev et al.
(X = 10 mol.%)
(X = 28 mol.%)
(X = 14 or 26 mol.%)
(X = 28 mol.%)
isomerization upon complexation with metals was docuꢀ
mented,15 a change in the isomerization rate being subꢀ
stantially dependent on the type of the metal ion.
To further develop procedures for the synthesis of difꢀ
ferentꢀtype crownꢀcontaining systems and improve our
knowledge and approaches to the synthesis of multifuncꢀ

Liquidꢀcrystalline crownꢀcontaining copolymers
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007 2417
tional systems, we synthesized a new type of photochroꢀ
mic ionophoric (crownꢀcontaining) LC polymers. The
structures of the macromolecules are schematically preꢀ
sented in Fig. 2, a, b. The structural formulas of the
resulting copolymers of the acryl series Pꢀ1ꢀX—Pꢀ4ꢀX are
shown below.
We studied the phase behavior and the photooptical
properties of the new copolymers and investigated the
influence of complexation of the copolymers with potasꢀ
sium perchlorate on their mesomorphic and photooptical
properties.
Experimental
The TLC analysis was carried out on Merck TLC Silica Gel
60F254 plates. The column chromatography was performed on
Fluka Silica Gel for Column Chromatography, 60.
Synthesis of monomers. The approach to the synthesis of
new monomers of the acryl series containing crown ether (monoꢀ
mer Mꢀ1) or photosensitive groups (monomers Mꢀ2, Mꢀ3, and
Mꢀ4) is illustrated in Schemes 1—4, respectively. The nematoꢀ
genic monomer 4ꢀ[6ꢀ(acryloyloxy)hexanoyloxy]phenylꢀ4ꢀmethꢀ
oxybenzoate (Mꢀ5) was synthesized according to the procedure
described earlier.19
Scheme 1
ABA is 4-[6-(acryloyloxy)hexyloxy]benzoic acid, DCC is dicyclohexylcarbodiimide.
Scheme 2
i. 2-(Hydroxymethyl)-18-crown-6.

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Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007
Shibaev et al.
1,4,7,10,13,16ꢀHexaoxacyclooctadecanꢀ2ꢀylmethyl 4ꢀ[6ꢀ
(acryloyloxy)hexyloxy]benzoate (Mꢀ1). Dicyclohexylcarbodiimꢀ
ide (0.29 g, 1.4 mmol) was added to a solution of ABA (0.29 g,
1 mmol),20 2ꢀ(hydroxymethyl)ꢀ18ꢀcrownꢀ6 (Merck) (0.35 g,
0.0012 mol), and 4ꢀdimethylaminopyridine (0.02 g) in anhyꢀ
drous THF (30 mL) cooled with ice water for 1.5 h. The reacꢀ
tion mixture was kept at ~20 °C for 24 h. The progress of the
reaction was monitored by TLC. After completion of the reacꢀ
tion, the precipitate of dicyclohexylurea that formed was filtered
off, and THF was evaporated. The product was purified by colꢀ
Scheme 3
i. 2ꢀ(Hydroxymethyl)ꢀ18ꢀcrownꢀ6.
Scheme 4

Liquidꢀcrystalline crownꢀcontaining copolymers
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007 2419
umn chromatography (toluene—ethyl acetate, 1 : 1, as the eluꢀ
ent). The yield was 0.34 g (60%). IR, ν/cm–1: 2936, 2856 (CH2);
1720 (C=O); 1632 (CH2=CH—); 1600, 1512 (C=C, Ar); 1256
(C—O—C, ArOR); 1168 (C—O—C). 1H NMR (CDCl3),
δ: 1.26—1.87 (m, 8 H, 4 CH2); 3.38—3.75 (m, 21 H, CH, 10
CH2O); 4.05—4.52 (m, 8 H, 4 CH2O); 5.77 and 6.35 (both d,
1 H each, CH2=CH—, J = 10.7 Hz, J = 17.3 Hz); 6.14 (dd, 1 H,
CH2=CH—, J = 10.4 Hz, J = 17.8 Hz); 6.91 (d, 2 H, Ar, J =
9.3 Hz); 7.9 (d, 2 H, Ar, J = 8.9 Hz).
In the last step, monomer Mꢀ3 was synthesized. For this
purpose, ABA (0.09 g, 0.0003 mol), compound 7 (0.2 g,
0.4 mmol), and 4ꢀdimethylaminopyridine (0.01 g) were disꢀ
solved in anhydrous THF (15 mL). Then DCC (0.09 g,
0.42 mmol) was added to the solution cooled with ice water for
1.5 h. The reaction mixture was kept at ~20 °C for 24 h. The
progress of the reaction was monitored by TLC. The precipitate
of dicyclohexylurea that formed in the course of the reaction
was filtered off, and THF was evaporated. The product was
purified by column chromatography (toluene—ethyl acetate,
1,4,7,10,13,16ꢀHexaoxacyclooctadecanꢀ2ꢀylmethyl 4ꢀ[(E)ꢀ
4ꢀ(2ꢀacryloyloxyethyl)(ethyl)aminophenyldiazenyl]benzoate
(Mꢀ2). Azo compound 1 was synthesized by the azoꢀcoupling
reaction with amine in an acidic medium analogously to the
procedure described earlier.21 In the second step, triethylamine
(2 mL) was added to a solution of azo compound 1 (3.1 g,
0.01 mol) in anhydrous THF (100 mL). Then acryloyl chloride
(1.35 g, 0.015 mol) was added dropwise to the reaction solution
cooled with ice water for 1 h. The progress of the reaction was
monitored by TLC. Within 12 h after the beginning of the reacꢀ
tion, the precipitate of triethylamine hydrochloride that formed
was filtered off, and THF was evaporated. The compound was
purified by column chromatography (chloroform—methanol,
10 : 1, as the eluent). The yield of compound 2 was 2 g (55%).
Compound Mꢀ2 was synthesized by mixing compound 2 (0.37 g,
1 mol), 2ꢀ(hydroxymethyl)ꢀ18ꢀcrownꢀ6 (Merck) (0.35 g,
1.2 mmol), and 4ꢀdimethylaminopyridine (0.02 g) in anhydrous
THF (20 mL). Then DCC (0.29 g, 1.4 mmol) was added to the
reaction solution cooled with ice water for 1.5 h. The reaction
mixture was kept at ~20 °C for 24 h. The progress of the reaction
was monitored by TLC. After completion of the reaction, the
precipitate of dicyclohexylurea that formed was filtered off,
and THF was evaporated. The product was purified by column
chromatography (chloroform—methanol, 7 : 1, as the eluent).
The yield was 0.29 g (45%). IR, ν/cm–1: 2934, 2853 (CH2);
1714, 1691 (C=O); 1627 (CH2=CH—); 1595, 1506 (C=C, Ar);
1378 (C—N); 1130 (C—O—C). 1H NMR (CDCl3), δ: 1.15 (t,
3 H, Me, J = 7.2 Hz); 3.32—3.75 (m, 25 H, CH, 12 CH2O);
4.21—4.60 (m, 6 H, 3 CH2); 5.78 and 6.37 (both d, 1 H each,
CH2=CH—, J = 10.5 Hz, J = 16.6 Hz); 6.12 (dd, 1 H,
CH2=CH—, J = 10.7 Hz, J = 17.5 Hz); 6.93 (d, 2 H, Ar,
J = 9.3); 7.75 (d, 4 H, Ar, J = 9.1 Hz); 8.18 (d, 2 H, Ar, J = 8.9 Hz).
4ꢀ[4ꢀ(1,4,7,10,13,16ꢀHexaoxacyclooctadecanꢀ2ꢀylmethoxyꢀ
carbonyl)phenyldiazenylphenyl 4ꢀ(6ꢀacryloyloxyhexyloxy)]ꢀ
benzoate (Mꢀ3). In the first step, azo dye 3 was synthesized by
the azoꢀcoupling reaction of pꢀaminobenzoic acid with phenol
in an alkaline medium.21 In the second step, the hydroxy group
of azo dye 3 was protected with chloroethoxyformate according
to a standard procedure.22 The third step involved the formation
of acid chloride 5 by the reaction of compound 4 with thionyl
chloride.22 In the fourth step, triethylamine (2 mL) was added
to a solution of 2ꢀhydroxymethylꢀ18ꢀcrownꢀ6 (0.29 g, 1 mmol)
in anhydrous THF (30 mL). Then acid chloride 5 (0.4 g,
1.2 mmol) was added dropwise to the reaction solution cooled
with ice water for 1 h. The progress of the reaction was moniꢀ
tored by TLC. Within 12 h after the beginning of the reaction,
the precipitate of triethylamine hydrochloride that formed was
filtered off, and THF was evaporated. The compound was puriꢀ
fied by column chromatography (chloroform as the eluent). The
yield of compound 6 was 0.4 g (70%).
1 : 1, as the eluent). The yield was 0.11 g (65%). IR, ν/cm–1
:
2926, 2861 (CH2); 1700 (C=O); 1610 (CH2=CH—); 1587, 1490
(C=C, Ar); 1263 (C—O—C, ArOR); 1118 (C—O—C). 1H NMR
(CDCl3), δ: 1.26—1.87 (m, 8 H, 4 CH2); 3.4—3.75 (m, 21 H,
CH, 10 CH2O); 4.00—4.52 (m, 8 H, 4 CH2O); 5.78 and 6.37
(both d, 1 H each, CH2=CH—, J = 11.0 Hz, J = 17.8 Hz); 6.15
(dd, 1 H, CH2=CH—, J = 10.0 Hz, J = 17.4 Hz); 7.01 (d, 2 H,
Ar, J = 9.6 Hz); 7.48 (d, 2 H, Ar, J = 9.2 Hz); 7.72 (d, 2 H, Ar,
J = 8.9 Hz); 8.15 (d, 6 H, Ar, J = 9.0 Hz).
Methyl
4ꢀ[4ꢀ(2ꢀacryloyloxyethyl)(ethyl)aminophenylꢀ
diazenyl]benzoate (Mꢀ4). Compound 2 (0.5 g, 1.4 mmol), anhyꢀ
drous methanol (0.1 mL, 2 mmol), and 4ꢀdimethylaminopyriꢀ
dine (0.03 g) were mixed in anhydrous THF (20 mL). Then
DCC (0.4 g, 2 mmol) was added to the solution cooled with ice
water for 1.5 h. The reaction mixture was kept at ~20 °C for
24 h. The progress of the reaction was monitored by column
chromatography. Then the precipitate of dicyclohexylurea that
formed was filtered off and THF was evaporated. The product
was purified by column chromatography (chloroform—methaꢀ
nol, 10 : 1, as the eluent). The yield was 0.29 g (55%). IR,
ν/cm–1
: 2931, 2849 (CH2); 1718, 1697 (C=O); 1630
(CH2=CH—); 1598, 1510 (C=C, Ar); 1381 (C—N); 1142
(C—O—C). 1H NMR (CDCl3), δ: 1.15 (t, 3 H, Me, J = 7.2 Hz);
3.35 (m, 4 H, 2 CH2); 3.87 (c, 3 H, OMe); 4.26 (t, 2 H, CH2,
J = 5.9 Hz); 5.78 and 6.37 (both d, 1 H each, CH2=CH—, J =
10.5 Hz, J = 16.6 Hz); 6.12 (dd, 1 H, CH2=CH—, J = 11.1 Hz,
J = 17.5 Hz); 6.93 (d, 2 H, Ar, J = 9.3 Hz); 7.75 (d, 4 H,
Ar, J = 9.5); 8.18 (d, 2 H, Ar, J = 9.0 Hz).
Synthesis of polymers (general procedure). All copolymers
were prepared by the radical copolymerization of monomers
Mꢀ1, Mꢀ2, Mꢀ3, or Mꢀ4 with Mꢀ5 initiated by azoisobutyric
acid dinitrile (2 wt.%). To purify the copolymers from lowꢀ
molecularꢀweight compounds and oligomeric products, the samꢀ
ples were refluxed in methanol for 6 h. The compositions of the
copolymers were determined by UV spectroscopy. Since the
copolymerization was carried out to high degrees of conversion,
the difference between the compositions of the final copolymers
and the starting monomeric mixtures was at most 4%.
Complexes of copolymers with potassium perchlorate (general
procedure). The copolymers were dissolved in anhydrous THF.
Then a solution of potassium perchlorate in anhydrous acetoniꢀ
trile was added to the solution of the copolymer. The resulting
solution was concentrated with slight heating. The complexes
thus prepared were kept in vacuo at 80 °C to remove the residual
solvent.
It is known that the stability constant of the complex of
18ꢀcrownꢀ6 ether with potassium perchlorate varies in the range
of 106—108 L mol–1 depending on the polarity of the solvent.
Hence, the equilibrium in the reversible reaction (Scheme 5) is
almost completely shifted to the right. The high stability conꢀ
stant is associated with the good match between the atomic
In the fifth step, the hydroxy group was deprotected with
ammonia to prepare phenol 6.22

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Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007
Shibaev et al.
radius of the potassium ion and the size of the cavity of the
18ꢀmembered ring in 18ꢀcrownꢀ6 ether (2.66 and 2.6—3.2 Å,
respectively23). Consequently, the complexes of the copolymers
with potassium perchlorate are formed in virtually quantitative
yields. In all cases, potassium perchlorate was introduced into
the polymers in an amount equivalent to the content of the
crown ether groups in the polymers.
Scheme 5
The newly synthesized crownꢀcontaining monomers
with different structures (monomers Mꢀ1, Mꢀ2, and Mꢀ3)
were used as the second component. The first monomer
(Mꢀ1) contains solely the crown ether group. More strucꢀ
turally complex monomer Mꢀ2 bears the photochromic
azobenzene fragment linked to the macrocyclic ionophoric
18ꢀcrownꢀ6 group by the ether bond. Monomer Mꢀ3 is
more extended and contains, along with the photochroꢀ
mic and crown ether fragments, an additional benzene
ring, resulting in an additional increase in the anisometry
of this monomer. All crownꢀcontaining compounds
Mꢀ1—Mꢀ3 were synthesized as described in the Experiꢀ
mental section and were chromatographically purified.
The molecular structures of these compounds were estabꢀ
i. 2
1
lished by NMR spectroscopy. The H NMR spectrum of
one of the monomers, Mꢀ3, is shown in Fig. 3.
Photochromic monomer Mꢀ4 was synthesized as a
model compound. This compound is an analog of Mꢀ2
and contains the methoxy group instead of the terminal
crown ether fragment.
Instrumental investigation. The phase behavior of the copolꢀ
ymers was studied with a LOMO Pꢀ112 polarizing microscope
equipped with a Mettler TAꢀ400 thermal analyzer. The photoꢀ
chemical properties were studied on the specially designed apꢀ
paratus equipped with a DRShꢀ250 highꢀpressure mercury lamp.
The light at a wavelength of 436 nm was obtained with an interꢀ
ference filter. The light intensity was 0.5 mW cm–2 (determined
with the use of a LaserMateꢀQ laser power meter (Coherent)).
The spectroscopic measurements were carried out in the UV—Vis
region on a Tidas (J&M) spectrometer. Films of the copolymers
for photooptical measurements were prepared by the spinꢀcoatꢀ
ing method. The molecular weights of the copolymers were deꢀ
termined by gel permeation chromatography on a Knauer chroꢀ
matograph equipped with an UV detector and LCꢀ100 columns
packed with the sorbent 100, 500, and 1000 Å with the use of
THF as the solvent (1 mL min–1) at 25 °C and using the polystyꢀ
rene standard.
The copolymers were synthesized by the radical copoꢀ
lymerization of monomers Mꢀ1—Mꢀ4 with the nematoꢀ
genic monomer. Their compositions were determined by
UV spectroscopy. For copolymers Pꢀ1, Pꢀ2, Pꢀ3, and
Pꢀ4, the content of the photochromic groups was 14, 26,
28, and 28 mol.%, respectively. For copolymers Pꢀ2ꢀ14
and Pꢀ3ꢀ28, the molecular weights estimated by gel perꢀ
meation chromatography are 6400 and 3000, respectively
(based on the polystyrene standard). This corresponds to
the degree of polymerization of ~10—15. Such a low deꢀ
gree of polymerization is attributed to the chainꢀtransfer
reaction to the N=N double bond of the photochromic
monomers.
Phase behavior of copolymers. Let us initially consider
the phase behavior of the simplest photochromic copolyꢀ
mer Pꢀ4ꢀ28 containing the nematic methoxyphenylbenꢀ
zoate group and the photochromic azobenzene fragments
in the side chains. The acryl homopolymer of the first of
the aboveꢀmentioned composites is known to form two
types of nematic mesophases, viz., the lowꢀtemperature
twoꢀdimensionally ordered mesophase (the nematic modꢀ
ification of TDK) and the usual nematic mesophase (N)
with the melting point T ~100 °C. The introduction of
28% azobenzene units into this polymer does not disturb
Results and Discussion
Synthesis of crownꢀcontaining monomers and polymers.
In all cases, photochromic and crownꢀcontaining copolꢀ
ymers were synthesized starting from the same acryl monoꢀ
mer Mꢀ5. The synthesis of the latter has been described
earlier.19 The homopolymer of this monomer forms two
types of nematic mesophases13 exhibiting the rather high
clearing (isotropization) temperature TI (~100 °C). Hence,
we used compound Mꢀ5 as the major nematic component
of all the copolymers synthesized in the present study.

Liquidꢀcrystalline crownꢀcontaining copolymers
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007 2421
f
c
b
e
a
d
δ
9
8
7
6
5
4
3
2
1
Fig. 3. 1H NMR spectrum of monomer Mꢀ3.
the character of the nematic packing of the side groups
but leads to a lowering of the isotropization temperature
to 81 °C. Such a substantial lowering of the isotropization
temperature of the copolymer compared to TI of the hoꢀ
mopolymer (100 °C) can be attributed to both a decrease
in the molecular weight of the copolymer (which is assoꢀ
ciated with the chainꢀtransfer reaction to the N=N douꢀ
ble bond in the synthesis of the copolymer) and the differꢀ
ence in the sideꢀchain length.
The phase behavior of the copolymers is presented
below (copolymers Pꢀ2ꢀ26 and Pꢀ2ꢀ26 (K+) are amorꢀ
phous due to which the phase transition does not occur);
N is the nematic phase and I is the isotropic melt; the
phase transition temperatures are given in degrees Centiꢀ
grade.
pization temperature of copolymer Pꢀ2ꢀ26 decreases to
84—85 °C; the further increase in the content of these
groups to 26% causes the complete amorphization of coꢀ
polymer Pꢀ2ꢀ26.
At the same time, an increase in the anisometry and
rigidity of the crownꢀcontaining mesogenic unit as a reꢀ
sult of incorporation of three benzene rings (Pꢀ3ꢀ28) diꢀ
minishes the disordering influence of the bulky crownꢀ
containing terminal fragment, resulting in the rise of the
isotropization temperature to 102 °C.
The investigation of the complexes of the copolymers
with potassium perchlorate showed that all samples are
characterized by the lowering of the isotropization temꢀ
perature compared to the corresponding starting samples
(see above).
Therefore, in spite of the fact that the complexation of
potassium perchlorate with the crown ether fragment leads
to the encapsulation of K+ in the macrocycle, the presꢀ
Copolymer
Pꢀ1ꢀ10
Phase behavior
N 93 I
N 81 I
N 84—86 I
N 75—77 I
N 102 I
Pꢀ1ꢀ10 (K+)
Pꢀ2ꢀ14
–
ence of the ClO4 counterion causes a substantial deꢀ
crease in the anisometry of the crownꢀcontaining fragꢀ
ment (Fig. 4), resulting in a decrease in thermal stability
of the mesophase.
Pꢀ2ꢀ14 (K+)
Pꢀ3ꢀ28
Pꢀ3ꢀ28 (K+)
Pꢀ4ꢀ28
N 83 I
N 81 I
The consideration of crownꢀcontaining copolymers
Pꢀ1ꢀ10, Pꢀ2ꢀ26, and Pꢀ3ꢀ28 revealed the characteristic
features of their phase behavior. First, the presence of
bulky crownꢀcontaining groups in the copolymers leads
to a substantial decrease in their thermal stability. The
introduction of 10% crownꢀcontaining units leads already
to a sharp lowering of the isotropization temperature (see
data for Pꢀ1ꢀ10). A complication of the structure of the
crownꢀcontaining side group by including the azobenꢀ
zene group in the monomeric unit leads to an even more
substantial distortion of the LC structure. Thus, in the
presence of 14% of crownꢀcontaining groups, the isotroꢀ
KClO4
Fig. 4. Molecular model of the monomeric crownꢀcontaining
unit of copolymer Pꢀ1ꢀ10 and its complex with potassium perꢀ
chlorate.

2422
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007
Shibaev et al.
Photooptical properties of copolymers and their comꢀ
plexes. The copolymers contain azobenzene groups, which
can undergo the reversible lightꢀinduced E—Z isomerizaꢀ
tion (Scheme 6). The forward reaction (E→Z) is induced
by light at the wavelength corresponding to the π—π*
transition of the dye. Due to instability of the Z isomer of
the azo compound, the reverse reaction proceeds spontaꢀ
neously but its rate can be increased under light irradiaꢀ
tion at the wavelength corresponding to the n—π* transiꢀ
tion or upon heating.
Let us consider the characteristic features of the phoꢀ
toisomerization of the photochromic groups of the copolꢀ
ymers in solution. The changes in the intensity of absorpꢀ
tion caused by irradiation of copolymer Pꢀ2ꢀ26 (curve 1)
in chloroform are presented in Fig. 6. The intensity of the
peak corresponding to the E isomer reaches the minimum
value within ~10 min after the beginning of irradiation
and then remains unchanged. The irradiation of the comꢀ
plex of copolymer Pꢀ2ꢀ26 with potassium perchlorate,
Pꢀ2ꢀ26 (K+), (curve 2) causes virtually no changes in the
absorption of the E isomer. This is evidence that the E→Z
isomerization does not proceed. The factors responsible
for the observed suppression of isomerization are considꢀ
ered below. It should be noted that the spectra of
the copolymers remain unchanged upon complexation
(Fig. 7); the position of the absorption maximum correꢀ
sponding to the π—π* transition of the azo group remains
unchanged.
Scheme 6
This phenomenon was observed also in films of the
complex of copolymer Pꢀ2ꢀ26 with potassium perchlorꢀ
ate. Figure 8 shows that the changes in the absorption in
thin films of copolymer Pꢀ2ꢀ26 (curve 2) and of the comꢀ
A
2
0.6
The changes in the absorption spectrum of Pꢀ2ꢀ26
(a solution in chloroform) under irradiation at λ = 436 nm
for 60 s are shown in Fig. 5.
It can easily be seen that the intensity of the peak at
450 nm rapidly decreases under irradiation. This peak
corresponds to absorption of the E isomer. In addition, a
slight increase in the intensity of the peak shoulder at
360 nm is observed. This is associated with an increase in
the content of the Z isomer of the azobenzene group.24
0.5
0.4
0.3
1
100
200
300
400
500
t/s
A
Fig. 6. Changes in the intensity of absorption at a wavelength of
450 nm after irradiation of solutions of the copolymers in chloꢀ
roform at λ = 436 nm for 10 min: Pꢀ2ꢀ26 (1) and complex
Pꢀ2ꢀ26 (K+) (2).
450 nm
A
0.6
0.7
A
0.6
0.4
1.0
0.5
0.8
0.6
0.2
20
40
t/s
0.4
2
350
450
550
650
750
λ/nm
0.2
1
Fig. 5. Changes in the absorption of Pꢀ2ꢀ26 (solution in chloroꢀ
form, the concentration is ~10–5 mol L–1) under light irradiaꢀ
tion at λ = 436 nm for 60 s. The changes in the intensity of
absorption during light irradiation at λ = 450 nm are shown in
the inset.
400
600
800
λ/nm
Fig. 7. Normalized absorption spectra of copolymer Pꢀ2ꢀ26 (1)
in chloroform and of its complex Pꢀ2ꢀ26 (K+) (2).

Liquidꢀcrystalline crownꢀcontaining copolymers
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007 2423
A
Since the photochromic group in copolymer Pꢀ2ꢀ26
contains the electronꢀreleasing and ꢀwithdrawing termiꢀ
nal substituents, two equilibrium forms can exist
(Scheme 8).
0.20
3
Form II is stabilized by the pushꢀpull effect due to the
presence of an electronꢀreleasing substituent (D) at one
end and an electronꢀwithdrawing substituent (A) at the
other end. The character of the N=N double bond beꢀ
comes less pronounced due to the electron density redisꢀ
tribution. Hence, the E→Z and Z→E isomerizations can
proceed by the rotation mechanism through mesomeric
form II as a result of rotation of the substituents about the
bond between the nitrogen atoms.
Mesomeric form II is formed both in the case of the E
and Z isomers. Apparently, the perchlorate ion additionꢀ
ally stabilizes this transition state by an ionic interaction.
Since the Z isomer of the azo compound is energetically
less favorable than the E isomer, this stabilization of the
transition state leads to a substantial increase in the reacꢀ
tion rate of the Z→E isomerization, and, consequently, to
the changes in the kinetics of the E→Z isomerization
under irradiation. Thus, the rate of the reverse thermal
transition from the Z to E isomer is so high that the
irradiation does not cause changes in the ratio between
the concentrations of the Z and E isomers.
0.15
0.10
2
1
200
400
600 t/s
Fig. 8. A decrease in the intensity of absorption at a wavelength
of 450 nm under irradiation at λ = 436 nm in films of copolymers
Pꢀ4ꢀ28 (1), Pꢀ2ꢀ26 (2), and Pꢀ2ꢀ26 (K+) (3).
plex of Pꢀ2ꢀ26 with potassium perchlorate (curve 3) unꢀ
der light irradiation at a wavelength of 436 nm. It can
easily be seen that the absorption of Pꢀ2ꢀ26 (K+) correꢀ
sponding to the E isomer of the azo compound remains
virtually unchanged.
To explain this phenomenon, it is necessary to conꢀ
sider the E—Z isomerization. The reaction can proceed
by two mechanisms, the soꢀcalled inversion and rotation
mechanisms25a (Scheme 7).
To confirm these conclusions, we performed a special
experiment. The results of this experiment are presented
in Fig. 9. A solution of model copolymer Pꢀ4ꢀ28 in chloꢀ
roform was irradiated with light at a wavelength of 436 nm
for 10 min until the photostationary state was attained.
Then a solution of potassium perchlorate, in which the
amount of the salt was equivalent to the amount of the
photochromic groups in the copolymer, was added dropꢀ
wise to the sample with continuous irradiation. In this
case, the concentration of the E isomer of the photochroꢀ
The substantial difference in the mechanisms is that
the isomerization by the rotation mechanism proceeds
through the bipolar transition state and is accompanied
by a rather large change in the volume, whereas the hyꢀ
bridization of the nitrogen atom by the inversion mechaꢀ
nisms does not require a considerable free volume. Elecꢀ
tronꢀreleasing and ꢀwithdrawing substituents in azobenꢀ
zene stabilize the bipolar transition state in the rotation
mechanism due to which it becomes energetically more
favorable.25b
Scheme 7
Rotation
E Isomer
Z Isomer
Inversion

2424
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007
Shibaev et al.
Scheme 8
mined from the spectroscopic data as (A – A∞)/(A0 – A∞),
where A is the absorption measured at the instant t, A0 and
A∞ is the absorption at t = 0 and t→∞, respectively.26 The
rate constant k can be determined from the parameters of
the plot in the coordinates (A – A∞)/(A0 – A∞) and t. The
kinetic curves characterizing the Z→E isomerization for
copolymers Pꢀ2ꢀ26 and Pꢀ4ꢀ28 are shown in Fig. 10. The
calculated rate constants have very similar values (5.6•10–3
and 5.8•10–3 s–1, respectively). Consequently, the kinetꢀ
ics of the isomerization reaction in films is independent of
the volume of the terminal substituent in the photochroꢀ
mic group.
To conclude, we developed procedures for the syntheꢀ
sis of new crownꢀcontaining photochromic acryl monoꢀ
mers bearing the azobenzene fragments and synthesized
binary multifunctional copolymers containing ionophoric
and photochromic groups starting from these monomers.
The phase behavior and the photooptical properties of the
copolymers were studied. The complexation of potassium
perchlorate with the crown ether groups of the copolyꢀ
mers was found to lead to the lowering of the clearing
temperature of the LC phase up to the complete amorꢀ
phization of the samples. This phenomenon is, apparentꢀ
ly, associated with a decrease in the anisometry of the side
crownꢀcontaining groups of the copolymers.
The characteristic features and the kinetics of the phoꢀ
tooptical behavior of the newly synthesized copolymers
was studied in detail. It was found that the complex of the
copolymer with potassium perchlorate Pꢀ2ꢀ26 (K+) does
not undergo E→Z photoisomerization under irradiation
in solution and thin films. This can be attributed to a
substantial increase in the rate of the reverse Z→E isomerꢀ
ization in the presence of potassium perchlorate. The inꢀ
vestigation of the kinetics of isomerization in films of the
copolymers showed that the reaction rate is independent
of the bulkiness of the terminal fragment of azobenzene
groups of the copolymers.
mic fragment rapidly increased, resulting in an increase in
the absorption up to almost the initial value within 12 s.
In other words, this indicates that, in the presence of
potassium perchlorate, the ratio between the concentraꢀ
tions of two isomeric forms of the azo fragment in copolyꢀ
mer Pꢀ4ꢀ28 is shifted toward the larger percentage of the
E isomer due to a high rate of the reverse thermal Z→E
isomerization.
To analyze the influence of the bulky crown ether
group on the kinetics of photoisomerization in amorꢀ
phized films of the copolymers, we compared the kinetics
of isomerization of Pꢀ4ꢀ28 containing the terminal methꢀ
oxy group and its crown ether analog Pꢀ2ꢀ26. We investiꢀ
gated the kinetics of the reverse thermal Z→E process.
The isomerization of azobenzene derivatives is known to
be the firstꢀorder reaction. In the general form, the kinetꢀ
ic equation for this reaction can be written as follows:
n/n0 = exp(–kt), where n and n0 are the running and
initial concentrations of the Z isomer, respectively, k is the
rate constant, and t is the time. The n/n0 ratio was deterꢀ
(А – А∞)/(A0 – A∞)
1.0
A
1
0.6
0.5
0.4
0.3
0.9
2
0.8
200
400
600
800 t/s
200
400
600
t/s
Fig. 10. Kinetic curve for the Z→E isomerization in the coordiꢀ
nates (A – A∞)/(A0 – A∞) and the time for films of Pꢀ4ꢀ28 (1)
and Pꢀ2ꢀ26 (2). The rate constants k for the reactions calculated
from the parameters of the curves are 5.3•10–3 and 5.7•10–3 for
Pꢀ4ꢀ28 and Pꢀ2ꢀ26, respectively.
Fig. 9. Photooptical behavior of a solution of copolymer Pꢀ4ꢀ28
in chloroform in the presence of potassium perchlorate under
light irradiation at λ = 436 nm; the instant of addition of KClO4
is indicated by an arrow.

Liquidꢀcrystalline crownꢀcontaining copolymers
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 12, December, 2007 2425
14. V. Percec, G. Johansson, and R. Rodenhouse, Macromoleꢀ
cules, 1992, 25, 2563.
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Received May 29, 2007;
in revised form September 17, 2007