Photochromic LC copolymers containing azobenzene and crown-ether groups

Photochromic LC Copolymers Containing Azobenzene andCrown-Ether Groups

VALERY SHIBAEV, ALEXEY MEDVEDEV, ALEXEY BOBROVSKY

Faculty of Chemistry, Moscow State University, Lenin Hills, Moscow, 119991 Russia

Received 2 May 2008; accepted 9 July 2008DOI: 10.1002/pola.22962Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: An approach for the creation of a novel family of multifunctional crown-ether-containing comb-shaped copolyacrylates consisting of chromophoric (azoben-zene), ionophoric (crown-ether), and mesogenic groups in the same macromoleculewas developed. Phase behavior of the copolymers was studied, and correlationbetween their molecular structure and thermal properties was established. Theincrease of crown-ether-containing groups’ concentration up to 26 mol % leads to dis-ruption of nematic order and formation of amorphous phase. Influence of copolymerscomplexation with potassium perchlorate on mesomorphic properties of such systemswas investigated. It was shown that complexation leads to decrease in mesophasethermostability due to the significant reducing of the side group anisometry by per-chlorate counter ion. The comparative investigations of photooptical properties andphotoorientation processes of copolymers and their complexes were also performed.An essential difference in kinetics of photooptical behavior was revealed; the bulkycrown-ether substituents decrease rotational mobility and prevent photoorientationprocess of azobenzene fragments diminishing photoinduced orientation and order pa-rameter. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6532–6541, 2008

Keywords: azobenzene; azopolymers; comb-shaped polymers; complexation; crownether; LC ionophoric copolymer; liquid-crystalline polymers; multifunctional;photochemistry; photochromic

INTRODUCTION

Because of the great interest to multifunctionalpolymeric materials, our attention in the last fewyears have been focused on the design and syn-thesis of the novel side-chain (comb-shaped) liquidcrystalline (LC) copolymers containing differenttype of mesogenic, chiral, photochromic, and com-bined chiral-photochromic side groups.1 It wasrepeatedly shown that such type of copolymers isvery promising with respect to an academic inter-est and their practical applications for optical

data recording and storage, optoelectronics, holog-raphy, and creation of light-controllable devices.

Figure 1(a) shows the schematic representationof a hypothetical macromolecule of the comb-shaped LC polymer containing mesogenic, chiral,photochromic, and functional groups. Key ele-ments of such macromolecule are the mesogenicgroups quantitatively dominating in this systemand defining the ability of polymer to self-organi-zation and formation of LC phase. Another sidegroups are responsible for other functional prop-erties, such as photochromism, optical activity,chemical reactivity, and so forth.

Taking into account all aforementioned andcontinuing our works in the area of the multifunc-tional LC polymers, we concentrated our attention

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 6532–6541 (2008)VVC 2008 Wiley Periodicals, Inc.

Correspondence to: A. Bobrovsky (E-mail: [email protected])

6532

on design of a novel series of LC copolymers con-taining three types of the side groups: mesogenic,photochromic azobenzene-based, and ionophoriccrown-ether-containing fragments capable of com-plex formation with metal ions [Fig. 1(b)].

It is well known that the incorporation of theazobenzene groups into the side-chain LC poly-mers allows one to obtain the light-controllablepolymer films and coatings which can be success-fully used for the optical data recording and stor-age. For example, the polarized light irradiationof such films leads to E-Z isomerization of azoben-zene groups followed by the cooperative photoor-ientation of azobenzene and mesogenic groupsinducing dichroism and high birefringence.2–13

In this study, we suggested a novel approachfor the creation of photochromic LC polymersbased on combination of the mesogenic and pho-tosensitive ionophoric groups in the same mac-romolecule capable for photoisomerizing and com-plexing with the metal ions. Figure 1(b) showsschemes of two copolymer structures synthesizedand studied by us.

As an ionophoric fragment, we have selected acrown-ether group because of its high complex for-

mation efficiency and tendency to self-organiza-tion in solutions and solid with formation of com-plex supramolecular structures.14–16

It should be pointed out that there are only afew articles concerning synthesis and propertiesof crown-ether-containing LC polymers.17–20 Inprevious work,17 several side-chain LC copolya-crylates with 15-crown-5-ether groups have beensynthesized, and it has been shown that theircomplex with the metal ions (Naþ) can be used asan efficient tool for regulation of mesomorphicproperties of copolymers.

Of particular interest is the investigation ofphotochromic low-molar-mass crown-ether sys-tems. Photochromic fragment can play significantrole in complexing properties of crown-ethermoiety21,22 and, vice versa, complex formationwith the metal ions may result in changes of thephotooptical and photophysical properties of suchsystem (kinetics, mechanism and type of photo-chemical reactions, absorbance and fluorescencespectra).21,23–25 For example, it was shown thatthe Z-isomer of the azobenzene-containing crown-ether may form very stable complexes with theions of alkali metals (Naþ, Kþ, Rbþ, Csþ), whereasthe E-isomer does not form any complexes. More-over, authors observed the decrease in rate of theZ-E isomerization after complex formation withthe metal ions, and rate change was dependent onthe type of the metal ion.21

Taking into account all aforementioned fea-tures of azobenzene and crown-ether polymersystems, we have synthesized a novel series ofthe binary comb-shaped copolyacrylates consist-ing of three types of the functional fragments—mesogenic (nematogenic) phenylmethoxyben-zoate groups AA, azobenzene-containing photo-chromes, and ionophoric crown-ether sidegroups. The molecular constitution of the syn-thesized copolymers and their composition arepresented below:

Figure 1. (a) Schematic representation of macro-molecule of side-chain copolymer containing meso-genic (1), photochromic (2), chiral (3), and functional(4) groups, covalently linked with backbone (5) by ali-phatic spacer (6); (b) schematic representation of twotypes of comb-shaped macromolecules consisting ofmesogenic (1), photochromic (2), and crown-ether (4)fragments.

PHOTOCHROMIC LC COPOLYMERS 6533

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

All copolymers have the same nematogenicphenylmethoxybenzoate side groups. CopolymersP1X contain nonphotosensitive crown-ethermoieties as another comonomer units, whereas co-polymer P2X, P3X, and P4X have the similarazobenzene photoactive fragments. In the case ofcopolymers P2X and P4X, the azobenzene frag-

ment has donor and acceptor auxochromic sub-stituents which favorable to the shift of absorb-ance to the longer wavelength in comparison withazobenzene moieties in P3X.

In this article, we report the study of phasebehavior and preliminary results on photoopticalproperty investigations of copolymers as well asan influence of complex formation on their meso-morphic properties.

EXPERIMENTAL

Monomers Synthesis

Synthesis details of novel crown-ether-containingmonomers were described elsewhere.26 Phenyl-benzoate acrylic nematogenic monomer AA wasobtained according to the procedure presented inthe article.27

Copolymers Synthesis

The copolymers were synthesized by radicalcopolymerization of monomers in benzene solu-tion at 65 8C; AIBN was used as an initiatingagent. The synthesized copolymers were purifiedby the repeated precipitation with methanol anddried in vacuum. Copolymers composition wasdetermined by UV spectroscopy. Because of thehigh conversion of polymerization process, the dif-ference between compositions of the final copoly-mers and the initial monomer mixtures did notexceed 4% (Table 1).

Table 1. Copolymerization Yield, Content of Crown-Ether Monomer in MonomersFeed, Composition of Copolymers, and Molar Mass Characteristics

CopolymerYield(%)

Crown-EtherMonomer

Concentrationin MonomersFeed, mol %

Crown-EtherGroups Contentin Copolymer,

mol % Mw Mw/Mn

P1-11 73 10 11a 17,850 2.2P1-19 68 20 19a 25,000 2.2P2-14 71 15 14 6,400 1.4P2-26 64 30 26 7,500 1.5P3-28 62 30 28 3,000 1.3P3-57 64 60 57 3,400 1.3P3-100 58 100 100 5,600 1.5P4-28 67 30 28 5,800 1.4

aConcentration of crown-ether groups was determined by H1 NMR.

6534 SHIBAEV, MEDVEDEV, AND BOBROVSKY


Preparation of Copolymers Complexes withPotassium Perchlorate

Complexes of the synthesized copolymers with po-tassium perchlorate were prepared according tothe following method. The sample of copolymerwas dissolved in anhydrous THF followed by addi-tion of potassium perchlorate solution in anhy-drous acetonitrile. Solvents were evaporated atslight heating. Finally, the polymer complex wasdried in vacuum at 80 8C. Because of the veryhigh binding constant (106–108) of 18-crown-6with potassium ions, one can expect that theequivalent amount of potassium perchlorate iscompletely involved in copolymer complex.

Physicochemical and Photooptical Investigations

Molecular masses of copolymers were determinedby GPC chromatography using instrument ‘‘Kna-uer’’ (UV detector; columns ‘‘LC-100’’ with sorbent100, 500, and 1000 A; solvent, THF (1 mL/min,25 8C, PS-standard).

The phase transitions of the copolymers andtheir complexes were studied by differential scan-ning calorimetry (DSC) with a Perkin–ElmerDSC-7 thermal analyzer (a scanning rate of 10 K/min).

The polarizing microscope investigations wereperformed using a Mettler TA-400 thermal ana-lyzer and a LOMO P-112 polarizing microscope.

Thin films of the copolymers and their com-plexes for photooptical experiments were obtainedby spin-coating technique using solutions of dif-ferent concentration in chlorophorm. For drying,the spin-coated films were kept at room tempera-ture for 1 day.

The photochemical properties were studiedusing a special instrument equipped with aDRSh-250 ultra-high pressure mercury lamp.Light with wavelength 365 or 436 nm wasselected using interference filter. To prevent heat-ing of the samples due to IR irradiation of thelamp, water filter was used. To obtain plane-par-allel light beam, quartz lens was used. The inten-sity of light was measured by LaserMate-Q(Coherent) intensity meter.

The orientational order was studied usingpolarized UV/visible spectroscopy, because thetransition moment of the E-isomer of azobenzenemoiety is directed along the long axis of thisgroup. For this purpose, the angular dependencesof the polarized light absorbance spectra were

measured using a photodiode array UV/visiblespectrometer (J and M) with a stepwidth of 108.

The dichroism values (D) were calculated fromthe spectra using the following equation:

D ¼ ðAk � A?Þ=ðAk þ A?Þ (1)

where Ak is the absorbance at the preferred direc-tion; A? is the absorbance perpendicular to thisdirection.

RESULTS AND DISCUSSION

Synthesis of Crown-Ether-Containing andPhotochromic Copolymers

In all the cases, acrylic monomer AA27 for synthe-sis of crown-ether-containing copolymers was used:

Homopolymer of this monomer forms only ori-entationally ordered nematic phase and has rela-tively high clearing temperature Tcl � 1008 (forlow-molar-mass fractions27,28) that was a reasonfor selection of the compound as nematogeniccomonomer for all copolymer synthesis.

As the second comonomers, a series of novelcrown-ether-containing monomers of the differentstructure were synthesized according to ourwork.26 One of them (incorporated in copolymerP1X) contains only a crown-ether group. Anothermore complicated monomer (component of P2X)consists of photochromic azobenzene group chemi-cally linked with a crown-ether fragment. Mole-cule of the third monomer (comprising copolymerP3X) has longer aromatic fragment whichincreases its anisometry. The photochromic mono-mer (incorporated in copolymer P4X) was synthe-sized as a model compound; it has similar photo-chrome as the second monomer but instead ofcrown-ether group contains methoxy group. Allsynthesized monomers were purified by columnchromatography and characterized by NMR H1.26

All copolymers were synthesized by the radicalpolymerization of crown-ether-containing mono-mers or photochromic monomers with nemato-genic monomer AA. Compositions of the finalcopolymers were determined by UV spectroscopyusing absorbance band of azobenzene chromo-phore. The content of photochromic side groups



and molar masses determined by GPC method(using PS standard) are presented in Table 1. Val-ues of molar masses correspond to degree of poly-merization of about 10–15. Such low degree of po-lymerization is explained by the high-chain trans-fer constant to double N¼¼N bond of photochromicmonomers. This effect is associated with the for-mation of a stable hydrazyl radical and describedearlier in number of publications.4,29,30

Phase Behavior of Copolymers and Complexes

Let us start with consideration of the phasebehavior of the simplest copolymer P4-28 contain-ing nematogenic phenylmethoxybenzoate andphotochromic azobenzene side groups. As waspreviously shown,27,28 homopolymer of nemato-genic phenylmethoxybenzoate-containing mono-mer with low degree of polymerization (�20 andless) forms only a nematic phase with clearingtemperature �100 8C. Introduction of 28 mol %of the photochromic azobenzene groups in the co-polymer does not disrupt the nematic order butleads to decrease of the clearing temperature till79 8C (Table 2). Such significant decrease in meso-phase thermostability is probably related to a dis-tinction in the length of nematogenic and photo-chromic side groups.

As for the phase behavior of copolymers P1X,P2X, and P3X, it should be emphasized that asignificant decrease in mesophase thermostabilityis due to the presence of bulky crown-ethergroups. As is seen from the Table 1 an incorpora-tion of only 11 or 14 mol % of crown-ether-contain-ing groups leads to the sharp decrease in clearingtemperature (see copolymers P1-11, P2-14). Fur-thermore, increase in crown-ether groups concen-tration to 26 mol % results in complete amorph-ization of copolymer (P2-26, Table 1).

At the same time, it is noteworthy that thelengthening of ‘‘aromatic fragment’’ in crown-ether-containing side group till three benzenerings diminishes the bulky crown-ether effect,and the clearing temperature increases up to102 8C (P3-28, Table 1).

Investigations of the complexes of copolymerswith potassium perchlorate showed clearly thegradual decrease in the clearing temperature incomparison with the initial copolymers withoutionophoric groups.

Incorporation of the potassium ion insidecrown-ether group reduces conformational mobil-ity of macrocycle. Moreover, perchlorate counterion leads to the significant reducing the sidegroup anisometry (Fig. 2). These structuralchanges result in decrease of mesophase thermo-stability. Similar effect was observed in nematiclow-molar-mass diazacrown-ether-containing bi-mesogenic substances.31

Study of E-Z and Z-E Isomerization of AzobenzeneUnits of Copolymers and Complexes

We now turn our attention to the preliminaryinvestigations of photooptical properties of syn-thesized photochromic copolymers and their com-plexes. Copolymers P2X, P3X, P4X contain azo-benzene groups capable of reversible E-Z

Table 2. Glass Transition, N-I TransitionTemperature and Enthalpy of Transition (DH)for the Copolymers and Complexes.

Copolymer

GlassTransition

(8C)

N-ITransition

Temperature(8C) DH (J/g)

P1-11 23 91 0.8P1-11(Kþ) 18 81a –a

P1-19 7 44 0.2P2-14 31 86 0.7P2-14(Kþ) 26 76a –a

P2-26 48 Amorphous –P2-26(Kþ) 44 Amorphous –P3-28 22 99 1.0P3-28(Kþ) 29 78 0.8P3-57 12 84 –a

P3-57(Kþ) –b Amorphous –a

P3-100 8 74 –a

P4-28 30 79 0.7

aEnthalpy of N-I transition is too low, and clearing tem-perature was determined by polarizing optical microscopy.

bGlass transition is too fuzzy and cannot be detected.

Figure 2. Molecular model of crown-ether-contain-ing monomer unit of copolymer P1-11 and its complexwith potassium perchlorate.



isomerizing under the blue light action (wave-length corresponding to the p-p* electronic transi-tion for chromophores of such structure32,33). The

back Z-E isomerization occurs thermally or underirradiation with the light corresponding to the n-p* electronic transition. For copolymers P2X andP4X containing push–pull type of azobenzenegroups, the rate of thermal isomerization is rela-tively high even at room temperature.33

Figure 3 shows the spectral changes of thinfilms of copolymers P2-26 and P3-28 under irra-diation with the wavelength corresponding to themain absorption bands. It is clearly seen that anabsorbance corresponding to the E-isomer underirradiation decreases. A slight increase of absorb-ance in the range of 360 nm (for copolymer P2-26)and �450 nm (for copolymer P3-28) also takesplace that concerns an increase of the Z-isomercontent.32,33

Let us consider some peculiarities of photoiso-merization process taking place in the films ofcopolymers and complexes. Irradiation of copoly-

mer P2-26 films with blue light (436 nm) leads tothe decrease in the azobenzene groups’ absorb-ance, but the same absorbance for complex P2-26(Kþ) is almost unchanged [Fig. 4(a)]. The concrete

reason of this phenomenon is still unclear, but wecan suppose decreasing of Z-isomer stability andits fast back isomerization into E isomer in thepresence of potassium perchlorate.

To check our assumption, we performed theexperiment with the same copolymer P2-26 inchlorophorm solution [Fig. 4(b)]. As seen from Fig-ure 4(b), at the beginning of irradiation the ab-sorbance of copolymer solution decreases (leftpart of the curve) very rapidly. Then, after ‘‘satu-ration’’ of absorbance values (close to 600 s of

Figure 3. Absorbance spectra of thin spin-coatedfilms of copolymers P2-26 (a) and P3-28 (b) beforeand after light irradiation. Monochromatic lightwavelength is (a) 436 nm (�0.8 mW/cm2), irradiationtime, 15 min; (b) 365 nm (�0.4 mW/cm2), 80 min.

Figure 4. (a) Change in absorbances of copolymer films P4-28, P2-26 at 450 nmduring irradiation (kirr ¼ 436 nm) and complex with potassium perchlorate P2-26(Kþ); (b) fast increase in absorbance after addition of potassium perchlorate solutionduring irradiation of copolymer P2-26 solution in chlorophorm (�10�5 mol/L).



irradiation) a drop of KClO4 solution in acetoni-tryle was added. An addition of KClO4 to the solu-tion in the course of irradiation results in largeand fast recovery of absorbance [right part of thecurve in Fig. 4(b)]. A subsequent prolonged irradi-ation does not lead to any change in the absorb-ance. The same effect was observed for the modelcopolymer P4-28 and does not depend on concen-tration of photochromic groups in macromole-cules. Such dramatic destabilization of Z-isomerwas observed only for copolymers with push–pulltype of azobenzene side groups (P2X and P4X)but does not take place in P3X.

A kinetics of back thermal Z-E isomerizationwas studied to compare crown-ether-containing co-polymer P2-26 with a ‘‘model’’ copolymer P4-28 inwhich crown-fragment was replaced by methoxy-end-group. For this purpose, spin-coated films ofcopolymers were irradiated by the blue light(436 nm) until stationary state achieving followedby absorbance spectra recording at 25 8C. It wasfound that kinetics of absorbance recovery obeysmonoexponential law [Fig. 5(a)] that is typical forazobenzene compounds in polymer matrix at tem-peratures near or higher than glass transition.34

Calculated from kinetic curves values of the Z-Eisomerization rate constants were almost the same(5.6 � 10�3 s�1 and 5.8 � 10�3 s�1, for P2-26 andP4-28, respectively). Thus, kinetics of back isomer-

ization does not depend on volume of subsistent.Annealing of the films prior to the irradiation stepdoes not change values of the constants.

For copolymer P3-28 and its complex withKClO4 [Fig. 5(b)] rate constant of Z-E isomeriza-tion are practically the same (10�3 s�1 and 9.7 �10�4 s�1, respectively, at 65 8C).

Such experimental observations allow one toconclude that copolymer matrices possess enoughfree volume for thermal back Z-E isomerization ofazobenzene moieties which does not depend onthe presence of the bulky terminal substituents.

Photoorientation Processes in Thin Spin-CoatedFilms of Copolymers and Their Complexes

It is well known that an irradiation of azobenzene-containing side-chain copolymers by the polarizedlight induces the cyclic E-Z-E isomerization proc-esses in azobenzene fragments that lead to thelight-induced cooperative orientation of photochro-mic and nonphotochromic side groups.1 In ourwork, we also raise the question as to whetherthere exists a photoinduced orientation of sidegroups inspite of the bulky crown-ether groupavailability?

Indeed, irradiation of the thin spin-coated filmsof copolymers and their complexes by polarizedlight in most cases induces the photoorientation

Figure 5. (a) Kinetic curves for back thermal Z-E isomerization of copolymers P2-26, P4-28 films at 25 8C. Spin-coated films of copolymers were irradiated by bluelight (436 nm) until stationary state achieving followed by absorbance spectra record-ing. (b) Kinetic curves for back thermal Z-E isomerization of copolymer P3-28 and itscomplex with KClO4 at 65 8C. Before absorbance spectra recording the films wereirradiated by UV-light (365 nm) until stationary state achieving. A0, At, and A1are the absorbances of the azobenzene chromophores at time t ¼ 0, current timet and t ! 1, respectively.



process resulting in orientation of azobenzene andmesogenic side groups in the direction perpendic-ular to the polarization axis. This effect is clearlyseen by an appearance of the noticeable lineardichroism (D � 0.2) in the azobenzene fragmentsabsorbance [Fig. 6(a,b)].

According to eq 1, we have calculated values oflinear dichroism and plotted kinetic curves ofdichroism growth for different copolymers andtheir complexes (Figs. 7 and 8). It is clearly seenfrom Figure 6 that dichroism value for copolymerP2-26 containing crown-ether fragment is morethan two times less in comparison with the‘‘model’’ copolymer P4-28. Evidently, bulky sub-stituent decreases rotational mobility, preventsphotoorientation process of azobenzene fragmentsand reduce photoinduced orientation and orderparameter (Fig. 7).

In the case of the copolymer P3-28, complexa-tion with KClO4 has the same pronounced effecton photoorientation process (Fig. 8) The existenceof perchlorate counter ion significantly decreasesthe side group anisometry (Fig. 2); introduction ofpotassium ion in crown-ether macrocycle dimin-ishes its conformational mobility.

It is interesting to note that the rate of thedichroism growth at the first stage of polarizedlight irradiation is the same for copolymers P2-26and P4-28 (first �2 min), as well as for copolymer

Figure 6. (a) Polarized absorbance spectra of co-polymer P2-26 before and after irradiation withpolarized nonfiltered light of mercury lamp (�1 mW/cm2) during 160 min.; (b) Polar plots of polarized ab-sorbance at 438 nm before and after irradiation.

Figure 7. Dichroism growth during polarizedlight irradiation [nonfiltered light of mercury lamp(�1 mW/cm2)] for crown-ether-containing copolymerP2-26 and its analog, P4-28.

Figure 8. Dichroism growth during polarizedlight irradiation [nonfiltered light of mercury lamp(�1 mW/cm2)] of copolymer P3-28 and its complexwith potassium perchlorate.



P3-28 and complex (up to �100 min) (Figs. 7 and8). In our opinion, initial increase of dichroism isassociated with photoselection process12,33 anddoes not involve cooperative rotational motion ofmesogenic and azobenzene fragments.

We did not find any photoorientation for filmsof complex based on copolymer P2-26 that is couldbe explained by suppression of E-Z isomerizationdiscussed in previous section.

Annealing of photooriented films of copolymersand complexes on the base of P2-26 and P3-28 attemperatures higher than glass transition tem-peratures vanishes linear dichroism to zero value.So-called ‘‘gain effect,’’ that is, heat-induced dra-matic increase in dichroism and birefringence offilms irradiated below glass transition tempera-ture, as previously observed in azobenzene-con-taining polymers (Stumpe and coworkers35,36 andother authors) was not found.

CONCLUSIONS

Synthesis methods of a novel family of crown-ether-containing photochromic acrylic monomerswere elaborated and new multifunctional LCcopolymers containing mesogenic, ionophoric(crown-ether), and photochromic (azobenzene) sidegroups were synthesized. Phase behavior and pho-tooptical properties of all copolymers synthesizedwere studied. It is found that incorporation ofcrown-ether fragments into nematic polymers andtheir complexation with potassium perchlorateresult in decrease of clearing temperature of ne-matic mesophase. This effect is associated withsignificant reducing of the side group anisometryby perchlorate counter ion. UV and visible lightirradiation of the copolymers and theirs complexesleads to the reversible E-Z isomerization process ofazobenzene fragments in both dilute solution andthin films. The kinetics of the back thermal Z-Eisomerization is not influenced by the presence ofbulky crown-ether fragment as was revealed bycomparison with model copolymer containingmethoxy-substituted azobenzene side groups. Thecomparative investigations of photooptical proper-ties and photoorientation processes of copolymersand their complexes with potassium perchlorate inthe thin films reveal an essential difference inkinetics of photooptical behavior: bulky crown-ether substituents decrease rotational mobilityand prevent photoorientation process of azoben-zene fragments disrupting photoinduced orienta-tion and order parameter. Rate of photoorientation

is significantly higher for copolymers containingpush–pull type azobenzene chromophores (P2Xand P4X) due to the higher isomerization ratesand, probably, dipole moments.

This research was supported by the Russian Founda-tion of Fundamental Research (05-03-33,193) and Pro-gramCOST-D35.

REFERENCES AND NOTES

1. Shibaev, V. P.; Bobrovsky, A.; Yu.; Boiko, N. I.Prog Polym Sci 2003, 28, 729–836.

2. Todorov, T.; Tomova, N.; Nikolova, L. Opt Com-mun 1983, 47, 123–126.

3. Eich, M.; Wendorff, J. H.; Makromol Chem RapidCommun 1987, 59, 467–471.

4. Kostromin, S. G.; Stakhanov, A. I.; Shibaev, V. P.Polym Sci Ser A 1996, 38, 1030–1039.

5. Eich, M.; Wendorff, J. H. J Opt Soc Am B 1990, 7,1428–1437.

6. Anderle, K.; Wendorff, J. H. Mol Cryst Liq Cryst1994, 243, 51–75.

7. Sekkat, Z. In Photoreactive Organic Thin Films,Sekkat, Z.; Knoll, W., Eds.; Academic Press: SanDiego, 2002; Chapter 3, pp 64–83.

8. Hore, D. K.; Natansohn, A. L.; Rochon, P. L.J Phys Chem B 2003, 107, 2197–2204.

9. Sekkat, Z.; Morichere, D.; Dumont, M.; Loucif-Saıbi, R.; Delaire, J. A. J Appl Phys 1992, 71,1543–1545.

10. Mueller, T. In Functional Organic Materials,Mueller, T.; Bunz, U., Eds.; Wiley-VCH: Wein-heim, 2007; Chapter 5, pp 179–218.

11. Santos, W.; Moura, A.; Povh, N.; Muniz, E.;Rubira, A. In Advanced Polymeric Materials, Bor-sali, R.; Soldi, V., Eds.; Wiley-VCH: Weinheim,2006; pp 150–160.

12. Shibaev, V.; Kostromin, S.; Ivanov, S. In Polymersas Electrooptical and Photooptical Active Media,Shibaev, V., Ed.; Springer: New York, 1996; pp37–110.

13. Mc-Ardle, C. In Applied Photochromic PolymerSystems, Mc-Ardle, C., Ed.; Blackie: New York,1992; Chapter 1, pp 3–98.

14. Eyring, E. M.; Petrucci, S. In Cation Binding byMacrocycles. Inoue, Y.; Gokel, G. W., Eds.; NewYork: Marcel Dekker, 1990; Chapter 4, pp 179–203.

15. Shinkai, S. In Comprehensive SupramolecularChemistry; Lehn, J.-M.; Ed., New York: PergamonPress, 1996; pp 671–700.

16. Alfimov, M. V.; Fedorova, O. A.; Gromov, S. P.J Photochem Photobiol A: Chem 2003, 158, 183–198.

17. Percec, V.; Johansson, G.; Rodenhouse, R. Macro-molecules 1992, 25, 2563–2565.

18. Wen, J. S.; Hsiue, G. H.; Hsu, C. S. MakromolChem Rapid Commun 1990, 11, 151–157.



19. Hsiue, G. H.; Wen, J. S.; Hsu, C. S. MakromolChem 1991, 192, 2243–2254.

20. Hsu, C.-S. Prog Polym Sci 1997, 22, 829–871.21. Shinkai, S.; Minami, T.; Kusano, Y.; Manabe, O. J

Am Chem Soc 1983, 105, 1851–1856.22. Shinkai, S.; Ogawa, T.; Kusano, Y.; Manabe, O.;

Kikukawa, K.; Coto, T.; Matsuda, T. J Am ChemSoc 1982, 104, 1960–1967.

23. Tahara, R.; Morozumi, T.; Nakamura, H.;Shimomura, M. J Phys Chem B 1997, 101, 7736–7743.

24. Barzykin, A.; Fox, M.; Ushakov, E.; Stanislavskii,O.; Gromov, S.; Fedorova, O.; Alfimov, M. J AmChem Soc 1992, 114, 6381–6385.

25. Kumar, G. S.; Neckers, D. C. Chem Rev 1989, 89,1915–1925.

26. Shibaev, V.; Bobrovsky, A.; Medvedev, A. RussChem Bull 2007, 12, 2332–2343.

27. Boiko, N.; Shibaev, V.; Kozlovsky, M. J Polym SciPart B: Polym Phys 2005, 43, 2352–2360.

28. Bobrovsky, A.; Boiko, N.; Shibaev, V. Macromole-cules 2006, 39, 6367–6370.

29. Nuyken, O.; Weidner, R.; Adv Polym Sci 1986, 73/74, 145–199.

30. Hallensleben, M. L.; Weichart, B. Polym Bull1989, 22, 553–556.

31. Minggui, X.; Jun, Q.; Feng, H.; Liangyu, W. MolCryst Liq Cryst 1991, 209, 309–318.

32. Brode, W.; Wyman, J. G. J Am Chem Soc 1952,74, 4641–4646.

33. Rau, H. In Photochromism Molecules and System,Durr, H.; Bouas-Laurent, H., Eds.; Elsevier: Am-sterdam, 2003; Chapter 4, pp 165–188.

34. Eisenbach, C. D. Makromol Chem 1978, 179,2489–2506.

35. Zebger, I.; Rutloh, M.; Hoffmann, U.; Stumpe, J.;Siesler, H. W.; Hvilsted, S. Macromolecules 2003,36, 9373–9382.

36. Meier, J. G.; Ruhmann, R.; Stumpe, J. Macromo-lecules 2000, 33, 843–850.



Photochromic LC copolymers containing azobenzene and crown-ether groups

Documents

Transcript of Photochromic LC copolymers containing azobenzene and crown-ether groups