Colloids and Surfaces

A: Physicochemical and Engineering Aspects 198–200 (2002) 917–922

Photo-induced in-plane alignment of LC molecules onlayer-by-layer self-assembled films containing azo dyesevaluated by attenuated total reflection measurements

Jun Ishikawa a, Akira Baba b,d, Futao Kaneko b,*, Kazunari Shinbo b,Keizo Kato a, Rigoberto C. Advincula c

a Graduate School of Science and Technology, Niigata Uni�ersity, Ikarashi 2-8050, Niigata 950-2181, Japanb Department of Electrical and Electronic Engineering, Niigata Uni�ersity, Ikarashi 2-8050, Niigata 950-2181, Japan

c Uni�ersity of Alabama at Birmingham, Department of Chemistry, Birmingham, AL 35294-1240, USAd Max Planck Institute for Polymer Research, Mainz 55128, Germany

Received 30 August 2000; accepted 11 February 2001

Abstract

The photo-induced in-plane alignment of nematic liquid crystal (LC) molecules, 5CB, has been investigated in a cellprepared with alternate layer-by-layer self-assembly of polyelectrolyte and low-molecular weight azobenzene dye filmson gold electrodes using the attenuated total reflection (ATR) measurement method. The ATR properties due toexcitation of surface plasmon polaritons were observed in the LC cell. They sensitively changed with irradiation oflinearly polarized visible light. The experimental results corresponded well with the calculated ATR curves at differentpolarization states. We have demonstrated that in-plane alignment of the LC molecules on the self-assembled filmscould also be controlled by the polarization direction of irradiated visible light on the LC cell. © 2002 ElsevierScience B.V. All rights reserved.

Keywords: Layer-by-layer self-assembled film; Azo dye; LC molecule; Attenuated total reflection; Photo-induced alignment

www.elsevier.com/locate/colsurfa

1. Introduction

Evaluating the characteristics of liquid crystal(LC) molecules in LC cells is very important inorder to control molecular alignment and to de-velop new photo-alignment systems. Attenuatedtotal reflection (ATR) or surface plasmon reso-nance (SPR) measurement [1] is one of the most

useful methods for evaluation of LC moleculesin-situ [2,3]. Tilt angles of LC molecules close tothe aligning layers and bulk LC molecules in cellscan be estimated by analyzing surface plasmonpolariton (SPP) excitations [4] and guided waveexcitation modes (GWEM) [5]. Recently, thephoto-induced alignment of LC molecules usingphoto-sensitive molecules such as azo dye hasbeen reported [6–8]. However, evaluation of in-plane alignment of LC molecules utilizing theATR method has rarely been reported until now

* Corresponding author. Tel./fax: +81-25-262-6741.E-mail address: fkaneko@eng.niigata-u.ac.jp (F. Kaneko).

0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

PII: S0927 -7757 (01 )01020 -2

J. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 917–922918

[9]. Thicknesses of thin films, complex dielectricconstants, in-plane alignment of LC molecules,and the profiles in cells can be evaluated using thewhole ATR curves [2,3,10].

In this report, photo-induced in-plane align-ments of nematic LC molecules, 5CB, have beeninvestigated in LC cells prepared with alternateDirect Red 80 (DR80: azo dye) and poly(dial-lyldimethylammonium chloride) (PDADMAC)layer-by-layer self-assembled films [11] on goldelectrodes using the ATR measurement method.From the ATR curves, in-plane switching proper-

ties and alignment of the LC molecules in the LCcells were evaluated during and after irradiationwith linearly polarized light.

2. Experimental details

Fig. 1 shows the Kretschmann configurationfor the ATR measurement, the LC cell and molec-ular structures of polycation (PDADMAC) andazo dye (DR80) used in this experiment. DR80contains azo groups exhibiting photo-isomeriza-

Fig. 1. The Kretschmann configuration for the ATR measurement, the LC cell and the molecular structures used in theseexperiments.

J. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 917–922 919

Fig. 2. Calculated ATR curves for the LC cell and in-planealignment models of the LC molecules.

termined from the ATR measurement using an-other type of prism (BK7, n=1.515) before thein-situ ATR measurements for the LC cell. Reflec-tivities in the ATR experiments were measured forthe LC cell as a function of the incident angle ofa He–Ne laser at 632.8 nm and also as a functionof irradiation time. The in-plane alignments of theLC molecules due to photo-isomerization ofDR80 have been investigated in-situ during andafter irradiation of polarized light of the halogenlamp at 300 W.

The dipping directions of the DR80 andPDADMAC on both the prism and the slide glasswere set in the Y direction in the LC cell withoutany irradiation. The nematic LC molecule was4-cyano-4�-n-pentylbiphenyl (5CB: Merck JapanCo.). A spacer (Mylar film) was used to fabricatethe cells as shown in Fig. 1.

Fig. 2 shows the calculated ATR curves due toexcitations of the SPP for four idealized cases,assuming that the alignment of the LC molecules,5CB, are planar, isotropic and homeotropic andthe cell has a semi-infinite thickness. The theoreti-cal ATR curves were calculated using a transfermatrix method. In these calculations, the refrac-tive index of the LC molecules was from a litera-ture value [13]. The theoretical ATR curvesdepend upon in-plane alignment of the LCmolecules, the tilt angles and the profiles in cells,and alignments of the LC molecules can bederived from the whole ATR properties [2,3].Large valleys in reflectivities of the ATR curveswere caused by the resonant excitations of theSPP. The ATR properties were calculated for thetwo-planar alignments, where the long axes of theLC molecules were parallel (n�) and perpendicular(n�) to the X-axis, respectively. If the azimuthalalignment of the LC molecules in the planaralignments changes from perpendicular to paral-lel, the ATR properties sensitively move to thehigher region in the incident angle as the refrac-tive indexes, i.e. the dielectric constants increasefrom n� to n�. The ATR curves for the home-otropic and the isotropic states were also shownin Fig. 2. The refractive index of the isotropicstate is assumed to be the average; niso�nave= (1/3)(n�

2+2n�2 )]1/2 [9].

tion and has been used for photo-induced align-ment of LC molecules [11,12]. PDADMAC wasused to prepare self-assembled films of well-defined thickness and order.

Half-cylindrical prisms (HOYA FDS90, n=1.85) were used for the ATR measurement. Goldfilms of approximately 40 nm thickness were vac-uum evaporated onto the flat side of the prism.Initial surface functionalization of the flat surfaceof the prism with the gold film involved treatmentwith 3-mercapto-1-propanesulfonic sodium salt,and the layer-by-layer adsorption of the DR80/PDADMAC self-assembled films was carried out.The procedure is as follows: the molecular layersof the PDADMAC and the DR80 were alter-nately deposited from these solutions with 0.01 Mon the coated prism and a slide glass as aligninglayers of the LC cell. These self-assembled filmshad 10 pair-layers on the gold-coated prism andthe slide glass.

The high dichroism of the DR80/PDADMACself-assembled films is due to the photo-isomeriza-tion of DR80 by irradiation of polarized light andhas been previously observed by polarized UV-visspectroscopy [11]. The complex dielectric con-stants and the thicknesses of the gold film and theDR80/PDADMAC self-assembled films were de-

J. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 917–922920

3. Results and discussion

Fig. 3(a) shows the experimental ATR curves inthe region of the resonant angles of the SPPexcitations for the LC cell at room temperatureexcept the curve during irradiation. The experi-mental procedure was as follows. Experimentalcurves were measured before irradiation of the

linearly polarized visible light. Curves 1–4 wereeach measured sequentially after 1-h irradiation ofthe visible light, where the angles between theX-axis of the cell and the direction of the polar-ized light (�D in Fig. 2) were set to be 0, 30, 60and 90° for the curves from 1 to 4, respectively.The ATR properties for only the aligning layerson the gold film did not exhibit such shifts in theincident angles that increased with the resonantangles. The results showed that the ATR measure-ments were very sensitive to the irradiation of thepolarized light, which induced alignment of theLC molecules in the cell. The resonant angles ofthe SPP excitations, that is, the angles at theminimum reflectivities, increased with polarizationangle �D of the irradiation light from 0 to 90° inthe X–Y plane. From the theoretical ATR curvesin Fig. 2, the results in Fig. 3(a) indicate that theLC molecules within the penetration length of theSPP evanescent fields [2,3] are aligned perpendicu-lar to the polarized direction of the irradiationlight. It was thought that the trans-phase de-creased in the polarized direction by cis– transtransitions or trans–cis– trans photo-isomeriza-tion of the azo groups of DR80 due to irradiationof linearly polarized visible light. The LCmolecules were aligned in the plane by the trans-phase in the perpendicular direction [14].

Fig. 3(b) shows the reflectivity change at 70.9°of the fixed angle in the ATR measurement as afunction of the irradiation time of the linearlypolarized light. It was also estimated that the LCmolecules were aligned in the Y-axis along thedipping direction before the irradiation. The LCcell was heated to about 40 °C during the irradia-tion, making the LC layer isotropic. This enabledthe irradiated light to pass through the LC layerto the self-assembled command layer on the prismside. The reflectivity after each irradiation gradu-ally increased with the polarized angle �D of theirradiation light. The change corresponded to theATR properties in Fig. 3(a). These changes werenot observed when only the self-assembled layerwas present, i.e. without LC molecules. Thus, thisclearly shows that the alignments of the LCmolecules were controlled by the polarized direc-tion of the irradiation light.

Fig. 3. (a) ATR curves for the LC cell during and afterirradiation of the polarized light at the polarized angles, 0, 30,60 and 90°, and (b) the reflectivity change caused by irradia-tion of the polarized light at the fixed angle 70.9° in the ATRproperty.

J. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 917–922 921

Fig. 4. The calculated ATR curves corresponding to theexperimental ones, that is, 1, 2, 3 and 4 in Fig. 3(a).

Fig. 4 shows the calculated ATR curves corre-sponding to the experimental curves for polariza-tions at conditions 1, 2, 3 and 4 in Fig. 3. Sinceexcellent fits were obtained when in-plane align-ment of the LC molecules and no distribution inthe thickness direction of the cell were assumed, itwas estimated that in-plane alignments of the LCmolecules occurred corresponding to the changeof polarization. The LC molecules were alignedby the trans-phase in the perpendicular directionto the polarized one of the irradiation light. Fig. 5shows a model of the preferred alignments of theLC molecules. It can be derived that the in-planealignments of the LC molecules within the pene-tration length of the SPP evanescent fields werealigned perpendicular to the polarized directionsof the irradiation light and that the tilt angle wasalmost 0°.

Fig. 5. A model of the in-plane alignments of the LC molecules.

J. Ishikawa et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 917–922922

4. Conclusion

Photo-induced alignments of LC molecules onalternate layer-by-layer self-assembled films con-taining azobenzene were evaluated using the in-situ ATR measurement. The ATR propertiesdue to the excitation of the SPP were observed,and sensitively changed with re-orientations ofthe LC molecules by means of irradiation withlinearly polarized visible light. In-plane align-ments of the LC molecules on the self-assembledfilms could be controlled by the polarized direc-tion of the irradiated light used for photo-iso-merization of the self-assembled films.

Acknowledgements

This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministryof Education, Science, Sports and Culture ofJapan.

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