Substituent effects on thermal and photo-alignment behavior of poly(ethylene imine)s carrying...

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Research ArticleReceived: 27 March 2010 Revised: 7 August 2010 Accepted: 5 September 2010 Published online in Wiley Online Library: 24 December 2010

(wileyonlinelibrary.com) DOI 10.1002/pi.2997

Substituent effects on thermaland photo-alignment behaviorof poly(ethylene imine)s carryingazobenzene side groupsMohammad Kamruzzaman,a Yutaka Kuwahara,a Tomonari Ogata,a

Seiji Ujiieb and Seiji Kuriharaa∗

Abstract

A new class of liquid-crystalline poly(ethylene imine)s (PEIs) having four differently substituted (–CN,–C4H9,–OCH3 and–NO2)azobenzene side-chain groups attached through alkyl spacer groups were successfully synthesized using a solutionpolycondensation reaction. The synthesized polymers were characterized using differential scanning calorimetry, polarizedoptical microscopy and X-ray diffraction. The photochemical, thermo-optical and photo-orientational behavior of the polymerswere investigated in detail. Spin-coated films of PEIs with azobenzene groups having–C4H9,–OCH3 and–NO2 substituentsshowed out-of-plane molecular orientation on annealing. Except for the PEI with an azobenzene group having –NO2 substituent,all polymers exhibited good photoresponsive properties upon irradiation with UV and visible light. Films of PEIs with azobenzeneside groups having–CN,–C4H9 and–OCH3 substituents showed reversible alignment behavior from random state to out-of-plane and from out-of-plane to random state on annealing and on irradiation with UV and non-polarized visible light. Thereversibility of the molecular orientation of PEIs from random state to out-of-plane and from out-of-plane to random stategreatly depended on the substituent attached to the azobenzene side-chain group.c© 2010 Society of Chemical Industry

Supporting information may be found in the online version of this article.

Keywords: azobenzene polymer; poly(ethylene imine)s; molecular orientation; liquid-crystalline properties; out-of-plane order

INTRODUCTIONAzobenzene-containing polymers have attracted considerableattention because of the possibility of changing the molecularorientation by irradiation with light of an appropriate wavelength,and their potential applications including in reversible optical stor-age, holographic gratings and optical switching.1,2 Azobenzenederivatives are known to undergo trans–cis photoisomerizationupon irradiation. This is not a simple switching process betweenthe two photochromic isomers, because a steady state is stronglydependent on substituents and the wavelength of the excita-tion. There are two typical liquid-crystalline systems as regardsthe photo-control of molecular orientation by means of photoi-somerization of azobenzene compounds: one is ‘photochemicalphase transition’ between a liquid-crystalline phase and a ran-dom state;3 – 5 the other is photo-alignment between in-plane andout-of-plane structures, which is called ‘command surface’.1,6 It isexpected that a larger change in the refractive index can be in-duced in the command surface system than in the photochemicalphase transition, because �(n|| − n⊥) is larger than �(n|| − n),where n|| is the extraordinary index and n⊥ the ordinary index. Thecommand surface system consists of an azobenzene monolayeron a glass substrate and low-molecular-weight liquid crystals.Therefore, the system requires a cell, and its memory stability isnot so high due to the thermal back reaction of azobenzene andthe fluidity of low-molecular-weight liquid crystals. In contrast, for

the photochemical phase transition system, polymeric materialscan be used to improve memory stability; however, to obtain�(n|| − n⊥) relating to the photochemical phase transition, analignment layer is required.

There are few papers on the spontaneous out-of-planemolecular orientation of polymeric liquid crystals.7 – 11 RecentlyBobrovsky et al. reported that a polyacrylate copolymer containing4-ethoxy-4′-hexoxyazobenzene and cholesterol groups as sidechains showed spontaneous out-of-plane molecular orientationon a glass substrate on annealing at 75 ◦C, whereas onlypartial out-of-plane molecular orientation was observed for thepolyacrylate homopolymer.9 Furthermore, it was reported thatsome poly(ethylene imine)s (PEIs) having azobenzene side groupsshow liquid-crystalline phases and align perpendicular to the

∗ Correspondence to: Seiji Kurihara, Department of Applied Chemistry andBiochemistry, Graduate School of Science and Technology, KumamotoUniversity, Kurokami 2-39-1, Kumamoto 860-8555, Japan.E-mail: [email protected]

a Department of Applied Chemistry and Biochemistry, Graduate School of ScienceandTechnology,KumamotoUniversity,Kurokami 2-39-1,Kumamoto860-8555,Japan

b Department of Applied Chemistry, Faculty of Engineering, Oita University,Dannoharu 700, Oita 870-1192, Japan

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Thermal and photo-alignment behavior of PEIs www.soci.org

Scheme 1. Synthetic routes to monomers and polymers.

substrate spontaneously on heating into an isotropic phase andsubsequent cooling without any alignment process.12,13 So ifone can control photochemically the molecular orientation ofPEIs between random state and out-of-plane structures, one canfabricate simply an optical switching system showing excellentmemory stability by coating PEIs on a substrate without anyalignment process.

In the work reported in this article, we synthesized PEIshaving variously substituted azobenzene groups attached throughmethylene spacer groups and investigated their photochemicalas well as thermal alignment behavior on annealing. We mainlyfocused on the influences of the substituents attached to theazobenzene group on thermal and photo-alignment behavior.

EXPERIMENTALMaterialsp-Cyanoaniline, 4-butylaniline, p-anisidine and p-nitroaniline werepurchased from Aldrich and Wako Pure Chemical Industries Ltd andwere used as received. 1,6-Dibromohexane was purchased fromTCI and PEI of low molecular weight (number-average molecularweight of ca 1800 g mol−1) was purchased from Polysciences Inc.Reagents were used without further purification, unless stated.N,N-dimethylformamide (DMF) was purchased from Wako PureChemical Industries Ltd and was dried with molecular sieves priorto use.

Synthesis of monomersAzobenzene derivatives 4-(6-bromo-n-hexyloxy)-4′-cyano/butyl/methoxy/nitroazobenezene were synthesized by familiar diazo-coupling reactions of p-cyanoaniline or 4-butylaniline orp-anisidine or p-nitroaniline (0.05 mol) with phenol (0.05 mol)in water and subsequent etherification with 1,6-dibromohexanein acetone at 70 ◦C for 48 h. The resulting azobenzene derivativeswere recrystallized from methanol twice.

Synthesis of polymersEach particular substituted azobenezene (4.2 mmol), PEI(4.0 mmol), potassium carbonate (6.0 mmol) and 50 mL of DMFwere added into a round-bottom flask equipped with a con-denser. With continuous stirring the reaction mixture was refluxedat 100 ◦C for 72 h. After the reaction, the mixture was filtered andhalf the amount of DMF was evaporated from the filtrate whichwas then poured into methanol. The resulting polymers (denotedPEI6X , where X represents the substituent on the azobenzeneside group, i.e. PEI6C, PEI64, PEI6M and PEI6N indicate, respec-tively, the use of–CN,–C4H9,–OCH3 and–NO2 substituents, asshown in Scheme 1) were purified by precipitation from chlo-roform/tetrahydrofuran (THF) into methanol two to three times.The removal of the monomer was monitored using thin-layerchromatography. Finally, the product was dried under reducedpressure for 24 h (Scheme 1).

CharacterizationThe phase transition behavior of the polymers was studied us-ing DSC (Seiko SSC-5020) with a heating rate of 10 ◦C min−1

and polarized optical microscopy (POM; Olympus BHSP po-larizing microscope; Mettler FP-80 and FP-82 hot stage andcontroller).

XRD was used to confirm the nature of the liquid-crystallinephases and to determine the spacing of the smectic layers. A RigakuRINT 2100/PC XRD machine (40 kV and 200 mA) equipped with aθ –θ wide-angle goniometer and scintillation detector was usedfor all XRD measurements, using Cu Kα radiation (λ = 1.5406 Å).XRD samples were made by casting solutions (40 mg of polymerin 1 mL of THF) onto a 1.5 × 1.5 cm2 glass substrate. Thick filmsof PEI6C, PEI64, PEI6M and PEI6N were heated to 115, 100, 85 and120 ◦C, respectively, for 10 min, cooled to room temperature andthen used for XRD analysis.

Thin films for thermo-optical experiments were obtained usingthe spin-coating technique with solutions in THF. Quartz glasssubstrates were used to prepare polymeric solid films. For drying,

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www.soci.org M Kamruzzaman et al.

Table 1. Reaction conditions and properties of PEIs

Reaction conditionsDegree of Phase transition

Polymer Substituent Temperature (◦C) Azo (mmol) PEI (mmol) substitution (%)a temperature (◦C)

PEI6C CN 100 4.2 4.0 63.7 G 38 S 115 N 147 I

PEI64 C4H9 100 4.2 4.0 64.5 G 12 S 102 N 148 I

PEI6M OCH3 100 4.2 4.0 65.8 G 33 S 98 N 111 I

PEI6N NO2 100 4.2 4.0 68.2 G 33 S 112 N 144 I

a Determined from UV-visible absorption spectra.Molecular weight of PEI ≈ 1800 g mol−1, reaction time = 72 h, reaction solvent (DMF) = 50 mL.

the spin-coated films were kept at room temperature for 1 h.Typical coating parameters were as follows: speed of rotation,3000 rpm; time of rotation, 30 s; polymer concentration, 4 wt%in THF.

Irradiation was performed using a 500 W high-pressure mercurylamp with appropriate cutoff filter for UV and visible light at roomtemperature. The orientational order was studied using polarizedUV-visible spectroscopy (PerkinElmer Lambda 650 spectrometer),because the transition moment of the trans isomer of theazobenzene moiety is directed along the long axis of this group.For this purpose the angular dependence of the absorbance wasmeasured. The values of order parameter, S, determined using thespectroscopic method were calculated according to9

S = A// − A⊥A// + 2A⊥

(1)

where A|| is the absorbance at the preferred direction and A⊥ is theabsorbance perpendicular to this direction. To study the out-of-plane thermal/photo-orientation phenomena we measured theangular distribution of the polarized absorption spectra at anangle of 45◦ to the film normal and calculated the order parameterusing Eqn (1).

RESULTS AND DISCUSSIONSynthesis and thermal characterization of PEI6XPEIs having variously substituted azobenzene groups attachedthrough methylene spacer groups (PEI6X) were synthesized. Tooptimize the reaction conditions we changed various parameterssuch as temperature, reaction time, molar ratio of azobenzenemonomer to PEI, and reaction solvent. The degree of substitutionof PEI6X is considerably affected by all of these parameters. Withan increase in reaction temperature and reaction time, the degreeof substitution increases, the best results being obtained at atemperature of 100 ◦C and reaction time of 72 h. At temperaturesgreater than 100 ◦C and longer reaction times (above 72 h), thedegree of substitution does increase a little but the solubilityof PEI6X decreases probably due to the formation of a gel. Sofinally we synthesized PEI6X by optimizing the reaction conditionsas follows: temperature, 100 ◦C; reaction time, 72 h; molar ratioof azobenzene monomer to PEI, 4.2 : 4.0; solvent (DMF), 50 mL.Using these reaction conditions, similar degrees of substitutionare obtained for the various polymers and all synthesized polymersshow good solubility in DMF, chloroform, THF and cyclohexanone.Table 1 summarizes the reaction conditions and properties of thesynthesized PEIs.

Figure 1. DSC thermograms of synthesized PEIs (PEI6X) obtained at10 ◦C min−1 for the second heating run.

The thermal characteristics of the synthesized PEIs were studiedusing DSC with a heating rate of 10 ◦C min−1 and POM analysis.Figure 1 shows the DSC thermograms of PEI6X for the secondheating scan. All polymers show two endothermic peaks with lowglass transition temperatures. On the basis of the DSC and POMmeasurements, these two peaks are assigned to smectic–nematicand nematic–isotropic thermal phase transitions. Also, allpolymers are stable up to 190 ◦C.

Thermal orientation behavior of PEI6X in solid filmsFigure 2 shows normalized UV-visible absorption spectra of PEI6Xin THF and in solid film form. Solid films of PEI6X with thicknessesfrom 200 to 300 nm were prepared using the spin-coating methodfrom THF solutions. In the spectra of samples in THF thereare strong absorption bands in the range from 350 to 380 nmcorresponding to π –π∗ transition (Fig. 2(a)). The wavelengths atabsorption maxima for all polymers both in solution and in filmform are listed in Table 2. Substitution with nitro group shifts theabsorption maxima to longer wavelength. This is due to electron-donating and electron-withdrawing substituents attached to bothends of the azobenzene groups, alkoxy and nitro, which induceconjugation in the aromatic system resulting in the red shift ofthe π –π∗ transition.14 – 16 The absorption maxima of the solidfilms are blue-shifted compared to those in THF, indicating theformation of H-aggregates. In addition, the intensity of the π –π∗absorption band relative to that at ca 250 nm, which is assigned

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(a)

(b)

Figure 2. Normalized UV-visible absorption spectra of PEI6X (a) in THFsolution and (b) in solid films.

Table 2. Absorbance and out-of-plane order parameter of polymersfor various conditions

Aπ−π∗/Aϕ−ϕ∗ Order parameter, S

Polymer

λmax,solution

(nm)

λmax,film(nm)

Afterspin

coatingAfter

annealingBefore

annealingAfter

annealing

PEI6C 363 344 1.41 0.91 0.009 0.044

PEI64 352 330 0.69 0.33 0.107 0.381

PEI6M 359 355 1.17 0.53 0.049 0.304

PEI6N 376 349 1.50 0.44 0.03 0.433

to the ϕ –ϕ∗ transition of the aromatic ring,10,17 varies, as shownin Fig. 2(b). It has been reported that the ϕ –ϕ∗ transition isinsensitive to orientation; consequently, the change in the intensityof the π –π∗ absorption band relative to the ϕ–ϕ∗ absorptionband (Aπ –π∗/Aϕ –ϕ∗ ) is related to the out-of-plane molecularorientation.10,11 Therefore, the change in the π –π∗ absorptionintensity and blue shift of the solid films may be attributed to theout-of-plane molecular orientation and consequent formation ofH-aggregates.10,18

In order to clarify the effect of annealing temperature onmolecular orientation, absorption spectra of PEI6X solid filmswere measured after annealing for 10 min at various temperatures

(a)

(b)

Figure 3. (a) Changes in absorbance at λmax = 355 nm of PEI6M single-layer film with annealing temperature. (b) Decrease in Aπ−π∗/Aϕ−ϕ∗ valuesof PEI6X single-layer films with annealing time.

ranging from 40 to 120 ◦C. As shown in Fig. 3(a), the absorbanceof PEI6M film at 355 nm decreases on heating to 120 ◦C. Similarresults were obtained for the other polymeric films (see supportinginformation). Figure 3(b) shows the change in Aπ−π∗/Aϕ−ϕ∗ for allpolymeric films as a function of annealing time. It is clear thatthe out-of-plane molecular orientation occurs in a few minutes forall solid films, and the final value of Aπ−π∗/Aϕ−ϕ∗ for the PEI6Cfilm is larger than those of the others, indicating less out-of-planemolecular orientation for PEI6C. In other words, the ordering ofout-of-plane molecular orientation depends significantly on thesubstituent attached to the azobenzene side-chain group. So,to quantify the thermo-optical anisotropy of PEI6X solid filmswe obtained the polarized absorption spectra, and explored theangular dependency of absorbance before and after annealingof the films. The experimental setup for UV-visible polarizedabsorption spectroscopy is shown in Scheme 2. Annealing ofthe PEI6X films brings about not only a decrease in absorbancecorresponding to the π –π∗ transition without polarization, butalso a change in the polarized absorption spectra, as shown inFigs 4(a) and (b) for the PEI6M film. Similar spectral changes wereobserved for the other polymers (see supporting information).The polarized absorption spectra were recorded at an angle of45◦ to the normal of the films, and with subsequent rotationof the films to obtain polar plots to evaluate the out-of-plane molecular orientation. Figure 5 shows polar plots for all


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Scheme 2. Experimental setup for the measurement of polarized absorp-tion spectra.

(a)

(b)

Figure 4. (a) UV-visible absorption spectra of PEI6M film before and afterannealing at 80 ◦C for 5 min. (b) Polarized UV-visible absorption spectra ofPEI6M film before and after annealing at 80 ◦C for 5 min. A|| donates theabsorption when the polarization direction is parallel to the film plane andA⊥ donates the absorption when the polarization direction is set at 45◦ toboth film normal and film plane.

synthesized polymers before and after annealing. In the case ofthe PEI6C solid film, no angular dependency is observed beforeand after annealing (Fig. 5(a)), indicating no significant changein the molecular orientation of azobenzene molecules on glasssubstrate by annealing. In contrast, PEI64, PEI6M and PEI6N solidfilms after annealing show significant angular dependency andthe shape of their corresponding polar plots is purely concave(Figs 5(b)–(d)), allowing an assessment of the order parameter.9

The order parameter of the annealed solid films decreases in theorder PEI6N (0.433), PEI64 (0.381), PEI6M (0.304) and PEI6C (0.044)as given in Table 2. Therefore, the out-of-plane molecular orderingis perhaps in this order.

The orientation of the azobenzene side-chain groups wasfurther investigated using XRD. Figure 6 shows the XRD patternsfor the four synthesized PEI films on glass substrate cast fromTHF, confirming that all of the polymers are smectic layerstructures.19 – 21 PEI6N and PEI64 exhibit a few strong diffractionpeaks in the small-angle region, whereas two weak peaks in thesmall-angle region corresponding to a layer structure are observedfor PEI6C and PEI6M. On the basis of the XRD patterns, the smecticlayer spacings were calculated using Bragg’s law, and the resultsare listed in Table 3.

Moreover, the molecular length of the azobenzene side-chaingroup (Scheme 3) for all PEIs was calculated using the MOPAC(PM3 level) method and compared with the values of smecticlayer spacings. The smectic layer spacings in all cases are slightlyhigher than the calculated values. We also calculated the valuesof L/L′ (where L is the smectic layer spacing and L′ is thecalculated molecular length of azobenzene side-chain group),which are given in Table 3. For PEI6C, PEI64 and PEI6M the valuesof L/L′ are comparatively large, indicating a loose packing ofmolecules compared to PEI6N. Thus, the molecular orientationbehavior of azobenzene molecules is found to be dependent onthe substituents present in the azobenzene side-chain groups,although the reason is not clear.

Photoisomerization behavior of PEI6X in solid filmsUpon non-polarized UV irradiation of spin-coated solid films ofPEIs, significant spectral changes are observed which are typical fortrans–cis photoisomerization, except for the PEI6N solid film.22 – 24

For PEI6M, photostationary state with a high content of the cisisomeric form is achieved in less than 1 min as shown in Fig. 7(a).Similar photoisomrization behavior is observed for PEI64 andPEI6M solid films. In contrast, in the case of PEI6N polymeric film,very little change in the absorption spectra occurs on prolongedUV irradiation. This may be explained in terms of rapid reversereaction from cis form to trans form of the azobenzenes havingelectron-donating and electron-withdrawing substituents at eachend of the azobenzene group.22,25,26

As a result of subsequent irradiation with visible light of 435 nm,the reverse changes occur establishing a steady state with a highconcentration of trans isomers (Fig. 7(b)). As with UV irradiation,PEI6N again shows less of a response upon visible light irradiation.The recovery of the initial spectrum is almost complete in allcases, and for PEI64 and PEI6M solid films the values of absorptionmaxima corresponding to the π –π∗ transition exceed the initialvalues (also see supporting information). This phenomenon isrelated to the out-of-plane molecular ordering of azobenzenemoieties on glass substrate. For irradiation longer than 10 s, forPEI64, PEI6M and PEI6C, a decrease in the absorption in bothregions corresponding to π –π∗ and n–π∗ transitions occurs asshown in Fig. 7(c). The rate of decrease in the absorption uponirradiation with visible light is less than that upon UV irradiation.In the first 5 or 10 s of irradiation with visible light, back cis–transphotoisomerization occurs, while upon continued irradiation, theazobenzene groups are oriented along the film normal due tophoto-orientation.9 In this case, the transition moment of thesegroups became oriented out of the plane of the films;9 as aresult, the probability of light absorption and, thus, optical densitymarkedly decreases.

Reversibility of alignment behavior of PEI6X in solid filmsTo explore the reversibility of molecular alignment of PEI6X fromrandom state to out-of-plane ordering and from out-of-plane


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(a) (b)

(c) (d)

Figure 5. Polar plots for (a) PEI6C (λ = 330 nm), (b) PEI64 (λ = 313 nm), (c) PEI6M (λ = 314 nm) and (d) PEI6N (λ = 332 nm) single-layer films before andafter annealing. Spectral measurements were made at 45◦ to film normal.

ordering to random state, we investigated the changes inalignment behaviors under various conditions. Figure 8 showsthe reversibility of the alignment behavior for all synthesizedPEIs for various conditions in terms of the order parameter,estimated by measuring polarized absorption spectra of all PEIs.All polymers show little out-of-plane ordering before annealing.Upon annealing, all polymeric films except PEI6C show high orderparameter values indicating out-of-plane molecular alignment,but the alignment tendencies are not the same. After UV irradiationof annealed films, the values of the order parameter for PEI6C, PEI64and PEI6M films decrease via cis–trans photoisomerization andalmost reach their initial levels. In other words, the azobenzenemolecules align randomly on the glass substrate except for PEI6N.Subsequent non-polarized visible light irradiation again increasesthe order parameter values, indicating the transformation of

molecular orientation of azobenzene molecules from randomstate to out-of-plane ordering due to photo-orientation. Againupon UV irradiation, the azobenzene groups align randomly onthe glass substrate. In this way, reversibility of the alignmentbehavior of azobenzene groups in the PEIs can be achievedby changing the conditions such as annealing and UV andvisible light irradiation. However, in the case of the PEI6Nfilm only a marked thermal alignment behavior is observed;upon UV and visible light irradiation no angular dependencyis observed.

CONCLUSIONSWe have reported the successful synthesis of PEIs havingazobenzene side-chain groups with various substituents and a


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Figure 6. XRD patterns of all synthesized polymers (PEI6X) in their liquid-crystalline state.

Table 3. Smectic layer spacings and molecular lengths of PEI6X

PolymerDistance 1,

L (Å)Distance 2

(Å)

Molecularlength,L′ (Å)a L/L′

PEI6C 31.30 15.54 23.8 1.32

PEI64 37.70 18.70 27.6 1.37

PEI6M 34.21 16.91 24.5 1.39

PEI6N 27.93 13.92 23.5 1.19

a Calculated using the MOPAC (PM3) method.

Scheme 3. Molecular model of PEI having a nitro-substituted azobenzeneside-chain group (PEI6N).

comprehensive study of the orientational behavior of azobenzenegroups in the polymeric films as induced by thermal treatmentand photoirradiation. All synthesized polymers showed liquid-crystalline characteristics. PEI64 and PEI6N showed high orderparameter values on annealing, indicating a well-pronouncedtendency for the development of out-of-plane order. Exceptfor the PEI6N polymeric film, the PEI films showed reversiblealignment behavior on annealing and on UV and non-polarizedvisible light irradiation. The results indicate that the molecularorientation behavior of the PEIs greatly depends on the structureof the azobenzene side-chain groups. In addition, photochemicalcontrol of the molecular orientation between random and out-of-

(a)

(b)

(c)

Figure 7. Changes of absorption spectra of PEI6M polymeric film during(a) UV irradiation (365 nm), (b) first 5 s of visible light irradiation and(c) further visible light irradiation (435 nm). The intensity of UV and visiblelight was 7.0 and 80 mW cm−2, respectively.

plane structures can be achieved by combination of thermal andphotochemical processes.

SUPPORTING INFORMATIONSupporting information may be found in the online version of thisarticle.


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Figure 8. Reversibility of alignment behavior of PEI6X for various experi-mental conditions (1, fresh film; 2, annealing; 3, UV irradiation; 4, visiblelight irradiation; 5, UV irradiation; 6, annealing; 7, UV irradiation; 8, visiblelight irradiation).

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