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Surface phenomena in discotic liquid crystals investigated using polarized FTIR transmission spectroscopy
Materials Science and Engineering C 8–9 Ž1999. 283–289 www.elsevier.comrlocatermsec
Surface phenomena in discotic liquid crystals investigated using polarized FTIR transmission spectroscopy B. Orgasinska a , T.S. Perova b, K. Merkel a , A. Kocot a , J.K. Vij b
b,)
a Institute of Physics, UniÕersity of Silesia, Katowice, Poland Department of Electronic Engineering, Trinity College, UniÕersity of Dublin, Dublin 2, Ireland
Abstract Influence of uncoated and spin-coated surfaces of four different substrates on the bulk alignment of a discotic liquid crystal 2,3,6,7,10,11-hexapentyloxytriphenylene ŽHPT. was investigated using polarized FTIR transmission spectroscopy. The substrates investigated are: Si, ZnS, ZnSe and CaF2 . A planar-heterogeneous alignment of HPT has been found on the uncoated surfaces of Si and CaF2 substrates whereas HPT is planar-homogeneously aligned on similar surfaces of ZnS and ZnSe. In contrast, a homeotropic alignment of HPT is observed on the surfaces of substrates of Si, CaF2 spin-coated by nylon 6r6. Under certain experimental conditions of temperature and time variations, the anchoring transition from planar to homeotropic is observed for HPT. The observed alignments of LC and the anchoring transition are explained in terms of the topology and the interactions of the LC molecule with the surfaces of the substrate andror the polymer with which the substrate is spin-coated. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Discotic liquid crystals; Surface phenomena; Polarized FTIR transmission spectroscopy
1. Introduction Self-assembly and organization of supramolecules in discotic liquid crystals is a topic of enormous interest not only to physical chemists but also to biologists as these phenomena also occur in living organisms. A study on the methods of alignment of discotics and the reasons for achieving a particular alignment is a problem that needs to be solved in order to exploit the applications of these liquid crystals in devices and sensors. One of the best methods for achieving a particular alignment in discotic liquid crystals is the application of the magnetic field w1–3x. Goldfarb et al. w2x showed that a strong rotating magnetic field Ž; 2 T. is required to obtain a monodomain structure for hexa-heptyloxytriphenylene. Vauchier et al. w4x reported a planar orientation of some triphenylene derivatives in columnar phase on the perfect cleavage Ž001. surfaces of apophylite Ža lamellar tetragonal silicate. and the perfect cleavage of muscovite Ža monoclinic mica.. Kardan et al. w5x showed a homeotropic orientation of the core of some simple discotic liquid crystals Žbenzene-hexan-alkanoates derivatives. on metallic surfaces. In a previ)
Corresponding author. Tel.: q353-1608-1431; fax: q353-1677-2442; E-mail: [email protected]
ous work w6x we showed that ZnSe and Si substrates spin-coated by nylon 6r6 oriented some truxene and triphenylene derivatives differently. In this paper we investigate the influence of the surface and the structure of a substrate on the alignment of a discotic liquid crystal. The effect of structure and surfaces of ZnSe, ZnS, Si and CaF2 with and without the polymer Žnylon 6r6. spin-coated, on the orientation of a discotic liquid crystal was studied using polarized FTIR spectroscopy in its oblique and normal incidence of the IR radiation. The technique of oblique incidence of IR beam has been adapted to a study of the 3-D orientations of the liquid crystals molecules w7–9x. This technique had already been in extensive use for a 3-D study of molecules in L–B films w10,11x and molecular crystals w12,13x. Discotic liquid crystal used for these studies is 2,3,6,7,10,11-hexapentyloxytriphenylene ŽHPT.. The structure and phase sequences of HPT is shown in Fig. 1. This material exhibits highly viscous mesophase at temperatures between the crystalline solid and the isotropic liquid. Due to an incompatibility between the aromatic hydrocarbons and aliphatic chains, the disc-like molecules tend to pack themselves in the mesophase in segregated columns with the vector nˆ Ždirector direction. pointed along the columns axis Žsee Fig. 1.. HPT exhibits only a single hexagonal
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Fig. 1. Structure and phase sequences of hexapentyloxytriphenylene.
columnar phase w14x. In our previous works w6,15x, we found that the material orientates between the two optically flat Si substrates similarly to that between two glass plates. The substrates of Si, ZnS, ZnSe and CaF2 were selected to provide various types of interactions between different surfaces and the discotic material and being transparent to IR radiation. Since the alignment of rod-like molecules forming the nematic phase w16–19x depends on whether the substrate has one or more than one easy axis of orientations, it is therefore imperative to examine the dependence of any alignment of the discotic in the presence or otherwise of the easy axis of orientation of a substrate. For the crystalline-cut Ž110. of ZnSe substrate, one easy direction of orientation is found using X-ray diffraction w20–22x. Also ZnS substrate shows only one easy axis of orientation w20x. As already pointed out, Si is analogous to glass since a native oxide layer may exist on the surface of Si similar to that on glass. For glass substrates, however, the planar-heterogeneous orientation of triphenylene derivatives is observed using different techniques, X-rays, NMR, etc. w1–3x, it is therefore natural to accept a similar alignment of discotic on Si as on a glass substrate. The characteristics of CaF2 windows are rather similar to those of Si for the reason that three easy directions of orientation w20x exist instead of number of easy directions in the case of Si substrate.
slowly cooled down to room temperature at a rate of 0.1 K per minute. The IR cell was thermostated, in a hot-stage that was specially designed to fit the spectrometer, and to control temperature within "0.1 K using a programmable temperature controller. The IR spectra in the wavenumber range 600–4000 cmy1 are recorded for both the normal and the oblique incidence of an infrared beam using a Fourier transform infrared spectrometer FTS-Digilab 6000. For transmission measurements using oblique incidence of IR radiation, the hot-stage fitted in the spectrometer was also designed to rotate. Measurements were made over a temperature range of 300–450 K. Well separated bands in the IR spectra were fitted to a Lorentzian function to determine the frequency and amplitude of maximum absorbance. It was occasionally necessary to fit a complicated band structure in the wavenumber range 700–900 cmy1 with four sub-bands in order to fully characterize the perpendicular bands. The curve-fitting was carried out using a standard fitting program and a Voight function for the shape of the bands.
3. Polarized IR spectroscopy: normal and oblique incidence The transmission method with normal incidence of an IR beam ŽFig. 2 a. provides the orientations of different
2. Experimental The substrates of Si, ZnS, ZnSe and CaF2 , which are transparent to IR radiation, were optically polished. These were thoroughly rinsed with acetone and methanol before the preparation of cells. Some of these substrates were then spin-coated by 1% solution of nylon 6r6 in methanol Žthe layer thickness of ; 100 nm.. The discotic material HPT in its isotropic phase was sandwiched between the two windows of these substrates in order to obtain an IR transmission cell. Cells containing the discotic material between windows, uncoated and spin-coated with the polymer were first heated to a temperature of 400 K and then
Fig. 2. Schematic of polarized infrared transmission technique at normal Ž a. and oblique Ž b . incidence of light.
B. Orgasinska et al.r Materials Science and Engineering C 8–9 (1999) 283–289
segments of a molecule in the Y–Z plane Žplane of the window.. However, the information obtained is incomplete especially when we require a three-dimensional picture of the orientation of the functional groups. In the normal incidence, only components of transition moment parallel to the window plane give rise to the measured absorbance. In other words, we observe merely a projection of transition moments onto the window plane. The absorbance as a function of polarizer rotation angle v in the Y–Z plane can be defined by the following equation w23,24x A Ž v . s ylog 10yA 5 q Ž 10yA H y10yA 5 . sin2v 4 .
Ž 1.
The polarization angle, v , is measured with respect to the reference direction for which the maximum absorbance A 5 of the transition dipole moment is achieved, A H is the absorbance in the direction perpendicular to the reference direction. The method of oblique incidence w25,26x, shown in Fig. 2 b, can also provide the absorbance component which is not in the plane of the windows. This can be achieved by inclining a sample cell out of normal plane of incidence. The range of angles are restricted by a sample tilt angle to ; "608. The absorbance profile in X–Z plane Žsample tilt plane as shown in Fig. 2 b . is obtained by fitting of experimental data to the following equation w25,26x A Ž a . s A Z q Ž A X y A Z . sin2a
Ž 2.
where a is the angle between the electric field vector and the window plane. A Z and A X are the absorbance components in plane and normal to the window plane, respectively.
4. Results and discussion Two different types of alignments of the discotic LC with respect of the plane of the substrate can be observed. These are homeotropic Žor side-on. alignment, where the director of the columns Ž nˆ . is perpendicular to the windows, and planar Žor edge-on., where the director lies in the plane of the substrates Žsee Fig. 3.. For the side-on
Fig. 3. The orientation of the transition dipole moments for the in-plane and out-of-plane vibrations for the edge-on Ž a. and side-on Ž b . alignment of HPT in its columnar phase.
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alignment the aromatic core lies in the plane of the window and for the edge-on, the aromatic core is oriented perpendicular to this plane. The IR spectra are highly sensitive to the local structure and orientation of the liquid crystalline molecules. The IR studies for the discotic liquid crystals w5–8,15,27–30x have shown that the aromatic C–C stretching vibrations in the range of 1500–1600 cmy1 and the C–H out-of-plane vibrations close to 800– 880 cmy1 are the most sensitive indicators of ordering in discotics. This is due to the reason that for the C–C aromatic stretching vibrations the transition dipole moment is parallel to the plane of the core and the transition dipole moment for the C–H out-of-plane vibration is oriented perpendicular to the plane of the core Žsee Fig. 3.. The conclusion as to whether the alignment is edge-on or side-on depends on whether the intensity of the C–C aromatic stretching vibration is observed to be lower or greater than for the isotropic phase. Conclusions about the alignment of the discotic from the C–H out-of-plane deformation can also be drawn except that the opposite dependencies of the intensity of this band on phase are expected than for the in-plane C–C vibrations. Fig. 4 shows the IR spectra for the crystalline, columnar and the isotropic phases, obtained on the heating from the crystalline phase. These phases are characterized by IR spectroscopy due to large changes in the shape and position of some IR bands. These changes are especially apparent for vibrations with transition dipole moments oriented parallel and perpendicular to the plane of the core. Based on the infrared studies reported w5–8,15,27–30x, so far, several peaks in the spectra are identified as follows: Ža. C–H stretching modes near 2900 cmy1 ; Žb. benzene stretching modes near 1600–1500 cmy1 ; Žc. C–H deformation modes near 1465 and 1380 cmy1 ; Žd. aromatic out-of plane deformation modes in the range 800–880 cmy1 and the Že. C–H rocking mode of the methylene chain near 730 cmy1 . The bands at ; 1265 and 1170 cmy1 are probably due to the C–O–C stretching mode. Other bands are given no definite assignments. Here, we focus on the bands with the transition dipole moments oriented parallel and perpendicular to the core plane: 1617, 1517 and 828, 866 cmy1 , respectively. Fig. 3a and b show the distributions of the transition dipole moments for the in-plane and out-of-plane vibrations with respect to the plane of the window for both the edge-on and side-on alignment of the discotic material. For these cases of alignment, the absorbance as a function of the angle of the polarization can be predicted for both normal and oblique angles of incidence. For the side-on alignment ŽFig. 3b., the absorbance for the in-plane vibration should be independent of the angle of polarization, since the transition dipole moments for these type of vibrations are uniformly distributed in the window plane. The intensity of the out-of-plane vibrations Žwith the transition dipole moment perpendicular to the window plane. should be extremely low and would probably have the
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Fig. 4. IR spectra of hexapentyloxytriphenylene in the region of 750–1500 cmy1 in crystalline Žthin solid line., columnar Žbold line. and isotropic Ždotted line. phases.
same projections on the Y and Z axes. A strong angular dependence, however, is expected for the absorbance as a function of the oblique angle of incidence due to a difference between the projections of the transition moments parallel and perpendicular to the electric vectors of the field Žsee Figs. 2 and 3.. Furthermore, for the edge-on alignment, one can expect a large polarization dependence of the absorbance for both types of vibrations.
Large differences in the dependence of absorbance on the angle of polarization is observed for two sets of substrates. This is shown in Figs. 6 a and b for the C–C aromatic stretching vibrations Žat 1617 and 1517 cmy1 . and for the C–H out-of-plane deformations of HPT. For Si and CaF2 cells ŽFig. 6 a. no dichroism is observed whereas a pronounced dichroism is found in the Y–Z plane of the windows for ZnS cell ŽFig. 6 b .. Results of the polarization measurements for cells with ZnSe windows are also found to be similar to those with the ZnS windows. It should be noted here that for the band 866 cmy1 , the peak intensities for different polarizer positions were taken directly from the spectra, while peak intensities for the band 836 cmy1 were obtained after fitting of an observed complex band in the region 780–870 cmy1 . An example of the band fitting procedure for the polarizer angles of 08 and 908 is shown in Figs. 7a and b. The frequencies of a number of bands used in the analysis are identified from measurements in the crystalline and isotropic phases. The frequency maxima for these bands are: 818, 828, 836 and 866 cmy1 .
4.1. Uncoated substrates Fig. 5 shows the dependence of the C–C aromatic stretching vibrations on temperature, obtained on heating from the crystalline phase, for cells prepared with four different substrates without the polymer coating on the substrates. For cells that are investigated, the alignment within the temperature range of the discotic phase is observed to be edge-on since the intensity of the in-plane bands is lower than in the isotropic phase.
Fig. 5. The dependence of the peak intensity of C–C aromatic stretching vibration on temperature for HPT contained in between different untreated substrates. The thickness of the cells in mm is: Si-12; CaF2 -12; ZnS-17; ZnSe-10.
Fig. 6. The intensity distribution for in-plane Ž1517 cmy1 , 1617 cmy1 . and out-of-plane Ž836 cmy1 , 866 cmy1 . vibrations in the window plane ŽY–Z plane. for Ž a. Si and CaF2 cells and Ž b . for ZnS cell. Solid lines on the plot Ž b . show the calculated functions using Eq. Ž1. Žep. stands for experimental; Žcalc. is for calculated.
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angle. Results in Fig. 8 are illustrated for the 1517 cmy1 band for cells with ZnS and Si windows. These results are found to be adequate to the type of the substrates. From the above data, we conclude that for ZnSe and ZnS windows the orientation of the director of the columns is planar-homogeneous Žuniform orientation of the columnar axes. whereas for Si and CaF2 windows the orientation of nˆ is planar-heterogeneous Žcolumnar axes are distributed in the plane of the substrate.. These observations are consistent with the surface symmetry w20,22x, homogeneous for ZnSe and ZnS windows and heterogeneous for Si and CaF2 windows. 4.2. Nylon spin-coated substrates
Fig. 7. Example of the fitting procedure for the low-frequency band for HPT in ZnS cell for polariser position 08 Ž a. and 908 Ž b .. Figures in the insert show the frequency maximum value for appropriate bands.
Results for the oblique IR incidence were analysed for the two classes of substrates; Si and CaF2 on one side and ZnSe and ZnS on the other. While for HPT sandwiched between Si and CaF2 windows, a rather strong dependence of the band intensities on the sample tilt angle is observed; the results for ZnSe and ZnS windows on the contrary show a smaller dependence of the absorbance on the tilt
Fig. 8. Results of oblique IR measurements at polariser position 08. The intensity distribution in Z–X plane Žperpendicular to the window plane. for heterogeneous edge-on alignment ŽSi cell. and homogeneous edge-on alignment ŽZnS cell.. Solid and dashed lines show the calculated functions using Eq. Ž2. to fit the experiment in both cases.
Fig. 9 shows temperature dependence of the intensity of the C–C aromatic stretching vibrations for HPT cells assembled using nylon spin-coated substrates ŽSi, ZnS, CaF2 and ZnSe. as windows of cells. A temperature dependence of the absorbance for various bands is found to be dependent on the substrate. For Si and CaF2 windows, the alignment in discotic phase is side-on Žintensity of parallel bands in discotic phase greater than in the isotropic phase., whereas for ZnSe and ZnS windows the orientational transition from the edge-on to side-on alignment is observed with temperature w31x. The results are interpreted as follows. Since nylon spin-coated substrates usually favour a side-on alignment, this means that the long alkyl chains attached to the core of discotic molecules probably lie along the chains of the polymer, and as a result the benzene rings of the discotic are forced to lie flat on the polymer chains. For Si and CaF2 substrates, this type of alignment is observed in the entire range of the columnar phase. For ZnSe and ZnS substrates, however, the orientational transition to side-on orientation occurs at some temperatures within the columnar mesophase. For ZnSe, however, the topology seems to stabilize the edge on alignment over a wider range of the columnar phase. In
Fig. 9. Peak intensity of the C–C aromatic stretching vibrations Ž1617 cmy1 . vs. temperature for HPT contained between nylon coated windows. The thickness of the cells in mm being: Si-10; ZnSe-8; ZnS-9 and CaF2 -10.
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fact, it was observed by interferometry that nylon coated ZnSe substrate still shows substantial homogeneity of the surface topology w32x. The competition between the long range ŽVan der Waals. interactions, surface-potential created by the topology on the one side and the short range interaction of the discotic with the polymer on the other side compete with each other, and at a higher temperature the interaction forces win over the forces due to the topology. The homogeneous molecular distribution is found to provide a stronger bond than a heterogeneous one with the discotic LC sample since the thermal fluctuations are more likely to alter a random heterogeneous rather than a homogeneous structure. The IR transmission measurements for oblique and normal incidence of IR beam have revealed an edge-on alignment for nylon spin-coated ZnSe cell at a temperature of 355 K and side-on alignment at a temperature of 384 K. Figs. 10 a and b show the dependence of the absorbance for the edge-on alignment for bands around 1517 and 836 cmy1 on the angle of polarization. These figures also show a strong IR dichroism. These imply that an easy direction of orientation in ZnSe plays an important role for even nylon spin-coated substrate. Polarized measurements at a temperature of 384 K show no dependence of the peak intensity on the angle of polarization, confirming a side-on alignment.
Fig. 10. IR spectra of HPT contained between ZnSe windows Žcell thickness is 8 mm. for inplane Ž a. and out-of-plane Ž b . vibrations at polariser positions 08 and 908.
Fig. 11. IR spectra of HPT in ZnSe cell Žwith nylon coated windows. for edge-on Žat temperature 355 K. and side-on Žat temperature 384 K. alignment for normal Ždenotes as Ž00.. and oblique at 608 Ždenotes as Ž60.. incidence of IR beam at polariser position 08.
The results of the oblique incidence of IR, the measurements for the edge-on and side-on alignment for ZnSe cell are shown in Figs. 11 and 12. Fig. 11 demonstrates the changes in spectra for the sample tilt angles 08 and 608 for the side-on and the edge-on alignments of HPT molecules, whereas Fig. 12a and b show the dependence of peak intensity vs. tilt angle. The dependencies of the absorbance on the sample tilt angle are found to be different for these two cases.
Fig. 12. The intensity distribution for in-plane Ž1517 cmy1 , 1617 cmy1 . and out-of-plane Ž818 cmy1 , 866 cmy1 . vibrations in Z–X plane for HPT in ZnSe cell in Ža. edge-on Žat temperature 355 K. and Žb. side-on Žat temperature 384 K. alignment.
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5. Conclusions The polarized FTIR transmission spectroscopy in its normal and oblique incidence of IR radiation for studying the orientation of a discotic liquid crystal in 3-D has determined the alignment to be dependent on the substrate and its surface treatment. A large dichroism in the absorbance profile observed for ZnSe and ZnS substrates leads us to conclude that alignment of the columns of HPT discotic liquid crystal on these substrates is homogeneously planar. A preferable orientation of the director of the columns exists due to the presence of an easy direction of orientation on the substrate. The alignment of discotic liquid crystals on the untreated Si and CaF2 substrates have been found to be planar-heterogeneous in accordance with their surface and crystallographic properties. The results show that the substrates providing homogeneous alignment give rise to stronger interactions with the molecules of the discotic LC sample than those providing heterogeneous alignment. A thin nylon layer covering the substrate favours the side-on alignment. For Si and CaF2 substrates, the orientation is already converted to side-on in the entire temperature range of the discotic mesophase. For ZnSe and ZnS substrates, however, the orientational transition occurs at higher temperatures within the discotic mesophase. Acknowledgements The Foundation for Polish Science and the Foundation of the Polish–German Cooperation are thanked for granting the instrumentation ŽFTS-6000 and UMA-500 Bio-Rad. for this study. The Committee for Scientific Research ŽKBN. is thanked for funding this work through the grant 2P03B10116. Forbairt, Ireland, is thanked for partial funding this work in Dublin. References w1x A.M. Levelut, F. Hardouin, H. Gasparoux, C. Destrade, N.H. Tinh, J. Phys. 42 Ž1981. 147.
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w2x D. Goldfarb, Z. Luz, H. Zimmermann, J. Phys. 42 Ž1981. 1303. w3x W. Kranig, C. Boeffel, H.W. Spiess, Macromolecules 23 Ž1990. 4061. w4x C. Vauchier, A. Zann, P.L. Barny, J.C. Dubois, J. Billard, Mol. Cryst. Liq. Cryst. 66 Ž1981. 103. w5x M. Kardan, A. Kaito, S.L. Hsu, R. Takur, C.P. Lillya, J. Phys. Chem. 91 Ž1987. 1809. w6x T.S. Perova, J.K. Vij, Adv. Mat. 7 Ž1995. 919. w7x T.S. Perova, A. Kocot, J.K. Vij, Supramol. Sci. 4 Ž1997. 529. w8x T.S. Perova, A. Kocot, J.K. Vij, Ferroelectrics 214 Ž1998. 83. w9x J. Perez, M. Vandevyver, H. Strzelecka, M. Verber, C. Jallabert, A. Barrand, Liq. Cryst. 14 Ž1993. 1627. w10x P.-A. Chollet, Thin Solid Films 52 Ž1978. 343. w11x H. Nakahara, K.J. Fukuda, Colloid Interface Sci. 69 Ž1979. 24. w12x F. Kaneko, O. Shikai, H. Miyamoto, M. Kobayashi, J. Phys. Chem. 96 Ž1992. 10554. w13x F. Kaneko, O. Shikai, H. Miyamoto, M. Kobayashi, J. Phys. Chem. 98 Ž1994. 2185. w14x C. Destrade, P. Fouchier, H. Gasparoux, N.H. Tinh, A.M. Levelut, J. Malthete, Mol. Cryst. Liq. Cryst. 106 Ž1984. 121. w15x G. Kruk, A. Kocot, R. Wrzalik, J.K. Vij, O. Karthaus, H. Ringsdorf, Liq. Cryst. 14 Ž1993. 807. w16x J. Cognar, Mol. Cryst. Liq. Cryst. A5 Ž1982. 1, Suppl. Ser. w17x B. Jerome, Rep. Prog. Phys. 54 Ž1991. 391. w18x L.M. Blinov, V.G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials, Spring-Verlag, NY, 1994. w19x A.A. Sonin, The Surface Physics of Liquid Crystals, Gordon and Breach Publishers, Amsterdam, 1995. w20x G.V. Anan’eva, private communication. w21x T. Koyama, T. Yodo, K. Yamashita, J. Cryst. Growth 94 Ž1989. 1. w22x G.D. Anan’eva, A.A. Dunaev, T.I. Merkulyaeva, Russ. J. High Purity Compd. 4 Ž1995. 114. w23x W.G. Jang, C.S. Park, J.E. Maclennan, K.H. Kim, N.A. Clark, Ferroelectrics 180 Ž1996. 213. w24x A. Kocot, R. Wrzalik, J.K. Vij, Liq. Cryst. 21 Ž1996. 147. w25x P.G. Schmidt, J. Polym. Sci., Part A 1 Ž1963. 1271. w26x J.L. Koenig, S.W. Cornell, D.E. Witenhafer, J. Polym. Sci., Part A 2 Ž1967. 301. w27x X. Yang, S.A. Nitsche, S.L. Hsu, D. Collard, R. Thakur, C.P. Lillya, H.D. Stidham, Macromolecules 22 Ž1989. 2611. w28x W.K. Lee, P.A. Heiney, J.P. McCauley Jr., A.B. Smith III, Mol. Cryst. Liq. Cryst. 198 Ž1991. 273. w29x M. Rey-Lafon, C. Destrade, A.T. Hemida, Mol. Cryst. Liq. Cryst. 137 Ž1986. 381. w30x T.S. Perova, A. Kocot, J.K. Vij, Mol. Cryst. Liq. Cryst. 301 Ž1997. 111. w31x T.S. Perova, J.K. Vij, A. Kocot, Europhy. Lett. 44 Ž1998. 198. w32x T.S. Perova, J.K. Vij, D. Simpson, S. Kumar, Molecular Materials 12 Ž1999..
Surface phenomena in discotic liquid crystals investigated using polarized FTIR transmission spectroscopy B. Orgasinska a , T.S. Perova b, K. Merkel a , A. Kocot a , J.K. Vij b
b,)
a Institute of Physics, UniÕersity of Silesia, Katowice, Poland Department of Electronic Engineering, Trinity College, UniÕersity of Dublin, Dublin 2, Ireland
Abstract Influence of uncoated and spin-coated surfaces of four different substrates on the bulk alignment of a discotic liquid crystal 2,3,6,7,10,11-hexapentyloxytriphenylene ŽHPT. was investigated using polarized FTIR transmission spectroscopy. The substrates investigated are: Si, ZnS, ZnSe and CaF2 . A planar-heterogeneous alignment of HPT has been found on the uncoated surfaces of Si and CaF2 substrates whereas HPT is planar-homogeneously aligned on similar surfaces of ZnS and ZnSe. In contrast, a homeotropic alignment of HPT is observed on the surfaces of substrates of Si, CaF2 spin-coated by nylon 6r6. Under certain experimental conditions of temperature and time variations, the anchoring transition from planar to homeotropic is observed for HPT. The observed alignments of LC and the anchoring transition are explained in terms of the topology and the interactions of the LC molecule with the surfaces of the substrate andror the polymer with which the substrate is spin-coated. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Discotic liquid crystals; Surface phenomena; Polarized FTIR transmission spectroscopy
1. Introduction Self-assembly and organization of supramolecules in discotic liquid crystals is a topic of enormous interest not only to physical chemists but also to biologists as these phenomena also occur in living organisms. A study on the methods of alignment of discotics and the reasons for achieving a particular alignment is a problem that needs to be solved in order to exploit the applications of these liquid crystals in devices and sensors. One of the best methods for achieving a particular alignment in discotic liquid crystals is the application of the magnetic field w1–3x. Goldfarb et al. w2x showed that a strong rotating magnetic field Ž; 2 T. is required to obtain a monodomain structure for hexa-heptyloxytriphenylene. Vauchier et al. w4x reported a planar orientation of some triphenylene derivatives in columnar phase on the perfect cleavage Ž001. surfaces of apophylite Ža lamellar tetragonal silicate. and the perfect cleavage of muscovite Ža monoclinic mica.. Kardan et al. w5x showed a homeotropic orientation of the core of some simple discotic liquid crystals Žbenzene-hexan-alkanoates derivatives. on metallic surfaces. In a previ)
Corresponding author. Tel.: q353-1608-1431; fax: q353-1677-2442; E-mail: [email protected]
ous work w6x we showed that ZnSe and Si substrates spin-coated by nylon 6r6 oriented some truxene and triphenylene derivatives differently. In this paper we investigate the influence of the surface and the structure of a substrate on the alignment of a discotic liquid crystal. The effect of structure and surfaces of ZnSe, ZnS, Si and CaF2 with and without the polymer Žnylon 6r6. spin-coated, on the orientation of a discotic liquid crystal was studied using polarized FTIR spectroscopy in its oblique and normal incidence of the IR radiation. The technique of oblique incidence of IR beam has been adapted to a study of the 3-D orientations of the liquid crystals molecules w7–9x. This technique had already been in extensive use for a 3-D study of molecules in L–B films w10,11x and molecular crystals w12,13x. Discotic liquid crystal used for these studies is 2,3,6,7,10,11-hexapentyloxytriphenylene ŽHPT.. The structure and phase sequences of HPT is shown in Fig. 1. This material exhibits highly viscous mesophase at temperatures between the crystalline solid and the isotropic liquid. Due to an incompatibility between the aromatic hydrocarbons and aliphatic chains, the disc-like molecules tend to pack themselves in the mesophase in segregated columns with the vector nˆ Ždirector direction. pointed along the columns axis Žsee Fig. 1.. HPT exhibits only a single hexagonal
0928-4931r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 9 9 . 0 0 0 6 7 - 3
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Fig. 1. Structure and phase sequences of hexapentyloxytriphenylene.
columnar phase w14x. In our previous works w6,15x, we found that the material orientates between the two optically flat Si substrates similarly to that between two glass plates. The substrates of Si, ZnS, ZnSe and CaF2 were selected to provide various types of interactions between different surfaces and the discotic material and being transparent to IR radiation. Since the alignment of rod-like molecules forming the nematic phase w16–19x depends on whether the substrate has one or more than one easy axis of orientations, it is therefore imperative to examine the dependence of any alignment of the discotic in the presence or otherwise of the easy axis of orientation of a substrate. For the crystalline-cut Ž110. of ZnSe substrate, one easy direction of orientation is found using X-ray diffraction w20–22x. Also ZnS substrate shows only one easy axis of orientation w20x. As already pointed out, Si is analogous to glass since a native oxide layer may exist on the surface of Si similar to that on glass. For glass substrates, however, the planar-heterogeneous orientation of triphenylene derivatives is observed using different techniques, X-rays, NMR, etc. w1–3x, it is therefore natural to accept a similar alignment of discotic on Si as on a glass substrate. The characteristics of CaF2 windows are rather similar to those of Si for the reason that three easy directions of orientation w20x exist instead of number of easy directions in the case of Si substrate.
slowly cooled down to room temperature at a rate of 0.1 K per minute. The IR cell was thermostated, in a hot-stage that was specially designed to fit the spectrometer, and to control temperature within "0.1 K using a programmable temperature controller. The IR spectra in the wavenumber range 600–4000 cmy1 are recorded for both the normal and the oblique incidence of an infrared beam using a Fourier transform infrared spectrometer FTS-Digilab 6000. For transmission measurements using oblique incidence of IR radiation, the hot-stage fitted in the spectrometer was also designed to rotate. Measurements were made over a temperature range of 300–450 K. Well separated bands in the IR spectra were fitted to a Lorentzian function to determine the frequency and amplitude of maximum absorbance. It was occasionally necessary to fit a complicated band structure in the wavenumber range 700–900 cmy1 with four sub-bands in order to fully characterize the perpendicular bands. The curve-fitting was carried out using a standard fitting program and a Voight function for the shape of the bands.
3. Polarized IR spectroscopy: normal and oblique incidence The transmission method with normal incidence of an IR beam ŽFig. 2 a. provides the orientations of different
2. Experimental The substrates of Si, ZnS, ZnSe and CaF2 , which are transparent to IR radiation, were optically polished. These were thoroughly rinsed with acetone and methanol before the preparation of cells. Some of these substrates were then spin-coated by 1% solution of nylon 6r6 in methanol Žthe layer thickness of ; 100 nm.. The discotic material HPT in its isotropic phase was sandwiched between the two windows of these substrates in order to obtain an IR transmission cell. Cells containing the discotic material between windows, uncoated and spin-coated with the polymer were first heated to a temperature of 400 K and then
Fig. 2. Schematic of polarized infrared transmission technique at normal Ž a. and oblique Ž b . incidence of light.
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segments of a molecule in the Y–Z plane Žplane of the window.. However, the information obtained is incomplete especially when we require a three-dimensional picture of the orientation of the functional groups. In the normal incidence, only components of transition moment parallel to the window plane give rise to the measured absorbance. In other words, we observe merely a projection of transition moments onto the window plane. The absorbance as a function of polarizer rotation angle v in the Y–Z plane can be defined by the following equation w23,24x A Ž v . s ylog 10yA 5 q Ž 10yA H y10yA 5 . sin2v 4 .
Ž 1.
The polarization angle, v , is measured with respect to the reference direction for which the maximum absorbance A 5 of the transition dipole moment is achieved, A H is the absorbance in the direction perpendicular to the reference direction. The method of oblique incidence w25,26x, shown in Fig. 2 b, can also provide the absorbance component which is not in the plane of the windows. This can be achieved by inclining a sample cell out of normal plane of incidence. The range of angles are restricted by a sample tilt angle to ; "608. The absorbance profile in X–Z plane Žsample tilt plane as shown in Fig. 2 b . is obtained by fitting of experimental data to the following equation w25,26x A Ž a . s A Z q Ž A X y A Z . sin2a
Ž 2.
where a is the angle between the electric field vector and the window plane. A Z and A X are the absorbance components in plane and normal to the window plane, respectively.
4. Results and discussion Two different types of alignments of the discotic LC with respect of the plane of the substrate can be observed. These are homeotropic Žor side-on. alignment, where the director of the columns Ž nˆ . is perpendicular to the windows, and planar Žor edge-on., where the director lies in the plane of the substrates Žsee Fig. 3.. For the side-on
Fig. 3. The orientation of the transition dipole moments for the in-plane and out-of-plane vibrations for the edge-on Ž a. and side-on Ž b . alignment of HPT in its columnar phase.
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alignment the aromatic core lies in the plane of the window and for the edge-on, the aromatic core is oriented perpendicular to this plane. The IR spectra are highly sensitive to the local structure and orientation of the liquid crystalline molecules. The IR studies for the discotic liquid crystals w5–8,15,27–30x have shown that the aromatic C–C stretching vibrations in the range of 1500–1600 cmy1 and the C–H out-of-plane vibrations close to 800– 880 cmy1 are the most sensitive indicators of ordering in discotics. This is due to the reason that for the C–C aromatic stretching vibrations the transition dipole moment is parallel to the plane of the core and the transition dipole moment for the C–H out-of-plane vibration is oriented perpendicular to the plane of the core Žsee Fig. 3.. The conclusion as to whether the alignment is edge-on or side-on depends on whether the intensity of the C–C aromatic stretching vibration is observed to be lower or greater than for the isotropic phase. Conclusions about the alignment of the discotic from the C–H out-of-plane deformation can also be drawn except that the opposite dependencies of the intensity of this band on phase are expected than for the in-plane C–C vibrations. Fig. 4 shows the IR spectra for the crystalline, columnar and the isotropic phases, obtained on the heating from the crystalline phase. These phases are characterized by IR spectroscopy due to large changes in the shape and position of some IR bands. These changes are especially apparent for vibrations with transition dipole moments oriented parallel and perpendicular to the plane of the core. Based on the infrared studies reported w5–8,15,27–30x, so far, several peaks in the spectra are identified as follows: Ža. C–H stretching modes near 2900 cmy1 ; Žb. benzene stretching modes near 1600–1500 cmy1 ; Žc. C–H deformation modes near 1465 and 1380 cmy1 ; Žd. aromatic out-of plane deformation modes in the range 800–880 cmy1 and the Že. C–H rocking mode of the methylene chain near 730 cmy1 . The bands at ; 1265 and 1170 cmy1 are probably due to the C–O–C stretching mode. Other bands are given no definite assignments. Here, we focus on the bands with the transition dipole moments oriented parallel and perpendicular to the core plane: 1617, 1517 and 828, 866 cmy1 , respectively. Fig. 3a and b show the distributions of the transition dipole moments for the in-plane and out-of-plane vibrations with respect to the plane of the window for both the edge-on and side-on alignment of the discotic material. For these cases of alignment, the absorbance as a function of the angle of the polarization can be predicted for both normal and oblique angles of incidence. For the side-on alignment ŽFig. 3b., the absorbance for the in-plane vibration should be independent of the angle of polarization, since the transition dipole moments for these type of vibrations are uniformly distributed in the window plane. The intensity of the out-of-plane vibrations Žwith the transition dipole moment perpendicular to the window plane. should be extremely low and would probably have the
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Fig. 4. IR spectra of hexapentyloxytriphenylene in the region of 750–1500 cmy1 in crystalline Žthin solid line., columnar Žbold line. and isotropic Ždotted line. phases.
same projections on the Y and Z axes. A strong angular dependence, however, is expected for the absorbance as a function of the oblique angle of incidence due to a difference between the projections of the transition moments parallel and perpendicular to the electric vectors of the field Žsee Figs. 2 and 3.. Furthermore, for the edge-on alignment, one can expect a large polarization dependence of the absorbance for both types of vibrations.
Large differences in the dependence of absorbance on the angle of polarization is observed for two sets of substrates. This is shown in Figs. 6 a and b for the C–C aromatic stretching vibrations Žat 1617 and 1517 cmy1 . and for the C–H out-of-plane deformations of HPT. For Si and CaF2 cells ŽFig. 6 a. no dichroism is observed whereas a pronounced dichroism is found in the Y–Z plane of the windows for ZnS cell ŽFig. 6 b .. Results of the polarization measurements for cells with ZnSe windows are also found to be similar to those with the ZnS windows. It should be noted here that for the band 866 cmy1 , the peak intensities for different polarizer positions were taken directly from the spectra, while peak intensities for the band 836 cmy1 were obtained after fitting of an observed complex band in the region 780–870 cmy1 . An example of the band fitting procedure for the polarizer angles of 08 and 908 is shown in Figs. 7a and b. The frequencies of a number of bands used in the analysis are identified from measurements in the crystalline and isotropic phases. The frequency maxima for these bands are: 818, 828, 836 and 866 cmy1 .
4.1. Uncoated substrates Fig. 5 shows the dependence of the C–C aromatic stretching vibrations on temperature, obtained on heating from the crystalline phase, for cells prepared with four different substrates without the polymer coating on the substrates. For cells that are investigated, the alignment within the temperature range of the discotic phase is observed to be edge-on since the intensity of the in-plane bands is lower than in the isotropic phase.
Fig. 5. The dependence of the peak intensity of C–C aromatic stretching vibration on temperature for HPT contained in between different untreated substrates. The thickness of the cells in mm is: Si-12; CaF2 -12; ZnS-17; ZnSe-10.
Fig. 6. The intensity distribution for in-plane Ž1517 cmy1 , 1617 cmy1 . and out-of-plane Ž836 cmy1 , 866 cmy1 . vibrations in the window plane ŽY–Z plane. for Ž a. Si and CaF2 cells and Ž b . for ZnS cell. Solid lines on the plot Ž b . show the calculated functions using Eq. Ž1. Žep. stands for experimental; Žcalc. is for calculated.
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angle. Results in Fig. 8 are illustrated for the 1517 cmy1 band for cells with ZnS and Si windows. These results are found to be adequate to the type of the substrates. From the above data, we conclude that for ZnSe and ZnS windows the orientation of the director of the columns is planar-homogeneous Žuniform orientation of the columnar axes. whereas for Si and CaF2 windows the orientation of nˆ is planar-heterogeneous Žcolumnar axes are distributed in the plane of the substrate.. These observations are consistent with the surface symmetry w20,22x, homogeneous for ZnSe and ZnS windows and heterogeneous for Si and CaF2 windows. 4.2. Nylon spin-coated substrates
Fig. 7. Example of the fitting procedure for the low-frequency band for HPT in ZnS cell for polariser position 08 Ž a. and 908 Ž b .. Figures in the insert show the frequency maximum value for appropriate bands.
Results for the oblique IR incidence were analysed for the two classes of substrates; Si and CaF2 on one side and ZnSe and ZnS on the other. While for HPT sandwiched between Si and CaF2 windows, a rather strong dependence of the band intensities on the sample tilt angle is observed; the results for ZnSe and ZnS windows on the contrary show a smaller dependence of the absorbance on the tilt
Fig. 8. Results of oblique IR measurements at polariser position 08. The intensity distribution in Z–X plane Žperpendicular to the window plane. for heterogeneous edge-on alignment ŽSi cell. and homogeneous edge-on alignment ŽZnS cell.. Solid and dashed lines show the calculated functions using Eq. Ž2. to fit the experiment in both cases.
Fig. 9 shows temperature dependence of the intensity of the C–C aromatic stretching vibrations for HPT cells assembled using nylon spin-coated substrates ŽSi, ZnS, CaF2 and ZnSe. as windows of cells. A temperature dependence of the absorbance for various bands is found to be dependent on the substrate. For Si and CaF2 windows, the alignment in discotic phase is side-on Žintensity of parallel bands in discotic phase greater than in the isotropic phase., whereas for ZnSe and ZnS windows the orientational transition from the edge-on to side-on alignment is observed with temperature w31x. The results are interpreted as follows. Since nylon spin-coated substrates usually favour a side-on alignment, this means that the long alkyl chains attached to the core of discotic molecules probably lie along the chains of the polymer, and as a result the benzene rings of the discotic are forced to lie flat on the polymer chains. For Si and CaF2 substrates, this type of alignment is observed in the entire range of the columnar phase. For ZnSe and ZnS substrates, however, the orientational transition to side-on orientation occurs at some temperatures within the columnar mesophase. For ZnSe, however, the topology seems to stabilize the edge on alignment over a wider range of the columnar phase. In
Fig. 9. Peak intensity of the C–C aromatic stretching vibrations Ž1617 cmy1 . vs. temperature for HPT contained between nylon coated windows. The thickness of the cells in mm being: Si-10; ZnSe-8; ZnS-9 and CaF2 -10.
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fact, it was observed by interferometry that nylon coated ZnSe substrate still shows substantial homogeneity of the surface topology w32x. The competition between the long range ŽVan der Waals. interactions, surface-potential created by the topology on the one side and the short range interaction of the discotic with the polymer on the other side compete with each other, and at a higher temperature the interaction forces win over the forces due to the topology. The homogeneous molecular distribution is found to provide a stronger bond than a heterogeneous one with the discotic LC sample since the thermal fluctuations are more likely to alter a random heterogeneous rather than a homogeneous structure. The IR transmission measurements for oblique and normal incidence of IR beam have revealed an edge-on alignment for nylon spin-coated ZnSe cell at a temperature of 355 K and side-on alignment at a temperature of 384 K. Figs. 10 a and b show the dependence of the absorbance for the edge-on alignment for bands around 1517 and 836 cmy1 on the angle of polarization. These figures also show a strong IR dichroism. These imply that an easy direction of orientation in ZnSe plays an important role for even nylon spin-coated substrate. Polarized measurements at a temperature of 384 K show no dependence of the peak intensity on the angle of polarization, confirming a side-on alignment.
Fig. 10. IR spectra of HPT contained between ZnSe windows Žcell thickness is 8 mm. for inplane Ž a. and out-of-plane Ž b . vibrations at polariser positions 08 and 908.
Fig. 11. IR spectra of HPT in ZnSe cell Žwith nylon coated windows. for edge-on Žat temperature 355 K. and side-on Žat temperature 384 K. alignment for normal Ždenotes as Ž00.. and oblique at 608 Ždenotes as Ž60.. incidence of IR beam at polariser position 08.
The results of the oblique incidence of IR, the measurements for the edge-on and side-on alignment for ZnSe cell are shown in Figs. 11 and 12. Fig. 11 demonstrates the changes in spectra for the sample tilt angles 08 and 608 for the side-on and the edge-on alignments of HPT molecules, whereas Fig. 12a and b show the dependence of peak intensity vs. tilt angle. The dependencies of the absorbance on the sample tilt angle are found to be different for these two cases.
Fig. 12. The intensity distribution for in-plane Ž1517 cmy1 , 1617 cmy1 . and out-of-plane Ž818 cmy1 , 866 cmy1 . vibrations in Z–X plane for HPT in ZnSe cell in Ža. edge-on Žat temperature 355 K. and Žb. side-on Žat temperature 384 K. alignment.
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5. Conclusions The polarized FTIR transmission spectroscopy in its normal and oblique incidence of IR radiation for studying the orientation of a discotic liquid crystal in 3-D has determined the alignment to be dependent on the substrate and its surface treatment. A large dichroism in the absorbance profile observed for ZnSe and ZnS substrates leads us to conclude that alignment of the columns of HPT discotic liquid crystal on these substrates is homogeneously planar. A preferable orientation of the director of the columns exists due to the presence of an easy direction of orientation on the substrate. The alignment of discotic liquid crystals on the untreated Si and CaF2 substrates have been found to be planar-heterogeneous in accordance with their surface and crystallographic properties. The results show that the substrates providing homogeneous alignment give rise to stronger interactions with the molecules of the discotic LC sample than those providing heterogeneous alignment. A thin nylon layer covering the substrate favours the side-on alignment. For Si and CaF2 substrates, the orientation is already converted to side-on in the entire temperature range of the discotic mesophase. For ZnSe and ZnS substrates, however, the orientational transition occurs at higher temperatures within the discotic mesophase. Acknowledgements The Foundation for Polish Science and the Foundation of the Polish–German Cooperation are thanked for granting the instrumentation ŽFTS-6000 and UMA-500 Bio-Rad. for this study. The Committee for Scientific Research ŽKBN. is thanked for funding this work through the grant 2P03B10116. Forbairt, Ireland, is thanked for partial funding this work in Dublin. References w1x A.M. Levelut, F. Hardouin, H. Gasparoux, C. Destrade, N.H. Tinh, J. Phys. 42 Ž1981. 147.
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