Langmuir 1992,8, 2279-2283
2279
Columnar Ordering of Liquid-Crystalline Discotics in Langmuir-Blodgett Films 0. Karthaus and H. Ringsdorf' Uniuersitiit Mainz, Institut fiir Organische Chemie, Becher- Weg 18-20,6500 Mainz, Germany
V. V. Tsukrukt and J. H. Wendorff Deutsches Kunststoff Institut, Schlossgartenstrasse 6, 6100 Darmstadt, Germany Received April 29, 1992. In Final Form: June 19,1992 The structures displayed by low molar mass and polymeric amphiphilic discotic liquid crystalline triphenylene derivatives in the bulk state and in Langmuir-Blodgett (LB) films were analyzed by X-ray scattering, optical microscopy, and DSC. The discotic triphenylene derivatives are characterized by the presence of two hydrophilic (OH containing)and four hydrophobic alkyl tails in the monomeric compound and two polar ester groups in the corresponding main chain polymer. An edge-on arrangement of the discotic molecules within a double-layer packing of the columns being parallel to the solid support is formed in the LB films of both compounds,while in the bulk state of discotic polymer only a nematic phase is detected.
Introduction Discotic liquid crystalline (LC) low molecular mass1,2 and polymeric compoundsM meet with increasing interest because of their unique anisotropic physical properties in the bulk state. Recently the formation of ordered monolayer structures at the air-water interface characterized by various types of arrangement of the discotic molecules (side-on, edge-on, etc.) has been observed.'-g However, the formation of ordered columnar Langmuir-Blodgett (LB) multimolecular films by transfer of monolayers from discoid compounds onto solid supports has just begun.g The elucidation of the molecular organization in columnar LB fiis is interesting since ordered multilayer films of discotic compounds can be expected to display anisotropic transport properties related to electron transport, energy transport, etc.le14 It could be shown for example that LB films obtained from triphenylene derivatives
possess in-plane photoconductivity caused by an arrangement of the columns parallel to the solid ~ u p p o r t . ~ ~ J ~ To elucidate the type of molecular ordering in columnar LB films, amphiphilic low molar mass and polymeric triphenylene derivatives (1 and 2) were investigated. The results of structural investigations on their LB films a well as on the bulk condensed state of these discotic LCs are reported. Up to now only spectroscopic and pressurearea investigations have been performed for such systems,1°J6 so that no direct structural information on the state of ordering is available.
Experimental Procedures Preparation of the Samples. The chemical structures of two amphiphilictriphenylene derivatives 1 and 2 are presented
1
* To whom all correspondence should be sent.
+ Present address: Instituteof Polymer Science,Akron University, Akron, OH 44325. Permanent address: Institute of Bioorganic Chemistry, Academy of Science, Kiev, 253094,Ukraine. (1)Chandrasekar,S.;Ranganeth, G. S. Rep. Prog. Phys. 1990,53,57. 1989,111, (2)Gregg, B. A.;Fox, M. A.; Bard, A. J. J. Am. Chem. SOC. 3024. (3)Kreuder, W.;Ringsdorf, H.; Tschirner, P. Makromol. Chem.Rapid
Commun. 1986,6,367. (4)Ringsdorf, H.;Wiisterfeld, R.; Zerta, E.; Ebert, M.; Wendorff, J. H. Angew. Chem., Int. Ed. Engl. 1989,28,914. (5)Green, M. M.; Ringsdorf, H.;Wagner, J.; Wiisterfeld, R. Angew. Chem., Int. Ed. Engl. 1990,29,1478. (6)Ringsdorf, H.;Voight-Martin, I.; Wendorff, J. H.; Wiisterfeld, R.; Zentel, R. In Chemistry and Physics of Macromolecules; Fisher, E. W., Shulz, R. C., Sillescu, H., Eds.; V C H Weinheim, Germany, 1991;p 211. (7)Albrecht, 0.; Cumming, W.; Kreuder, W.; Laschewsky, A.; Ringsdorf, H.Colloid Polym. Sci. 1986,264,659. (8) Laschewsky, A. Angew. Chem. Ado. Mater. 1989,101,1606. (9)Ortmann, E.; Wegner, G. Angew. Chem.,Znt. Ed. Engl. 1986,25, 1105. (10)Karthaus, 0.; Ringsdorf, H.; Urban, C. Makromol. Chem.,Macromol. Symp. 1991,46,347. (11)Boden, N.;Bushby,R.J.;Clementa,J.;Jesudason, M. V.; Knowles, P. F.;Williams,G.Chem. Phys. Lett. 1988,152,94. (12)van Keulen, J.; Warmerdam, T. W.; Nolte, R. J.; Drenth, W. Recl. Trau. Chim. Pays-Bas Belg. 1987,106,534. (13)Wannan, J. M.; de Haas, M. P.; van der Pol, J. F.; Drenth, W. Chem. Phys. Lett. 1987,139,207. (14)Tran-Thi,T. H.; Markovitai, D.; Even, R.; Simon, J. Chem. Phys. Lett. 1987,139,207.
-11
2
-11
The compounds were obtained as follows. 2,3-Bis((6-hydroxyhexyl)oxy)-6,7,lO,ll-tetrakis(pentyl-
0xy)triphenylene ( 1 ) . 1,2-Bis(pentyloxy)benzene(160 mmol) and 1,2-bis((6-hydroxyhexyl)oxy)benzene(25g, 80 mmol) were stirred in a mixture of 120 g of FeC13in 500 mL of sulfuric acid (70wt % ) for 24 h at room temperature. After the mixture was poured on ice, the slurry was extracted with chloroform and re(15)van der Auweraer,M.; Catry, C.; Chi, L. F.; Karthaus, o.;Knoll, W.; Ringsdorf, H.;Sawodny, M.; Urban, C. Proceedings of the Fifth International Conference on LB films, Paris, August 1991. Thin Solid Films, in press.
0743-746319212408-2279$03.00/0 0 1992 American Chemical Society
2280 Langmuir,
Karthaus et al.
VoZ.8,No. 9, 1992
crystallized from trifluoroacetic acid. The resulting bis(trifluoroacetic acid) ester of 1was purified by flash chromatography. . After purification the bis(trifluoroacetic acid) ester of 1 was hydrolyzed with 2 mL of water in 80 mL of boiling methanol. lH NMR (CDCl3,200 MHz): 7.8 ppm (s, 6 H, arom); 4.2 ppm (t,J = 7 Hz, 12 H,O-CH*);3.6 ppm (t, J = 7 Hz, 4 H, CH2-0H); 1.9 ppm (m, 12 H, O-CH2-CH2); 1.7-1.3 ppm (m, 28 H, Alkyl CH2); 0.9 ppm (t, J = 6 Hz, 12 H, CH3). Poly(2,3-bis((&hydroxyhexyl)oxy)-6,7,10,1l-tetrakis(penty1oxy)triphenylene)malonate (2). 2,3-Bis((6-hydroxyhexyl)oxy)-6,7,10,11-tetrakis(pentyloxy)triphenylene (1)(0.373 mmol) was melted with 60 mg (0.373 mmol) of malonic acid diethyl ester at 120 "C. After addition of 1 mol 96 of tetraisopropyl orthotitanate the polycondensation took place under reduced pressure. Several precipitations in methanol yielded the crude polymer which was purified by GPC (Sepnadex LH 60) in THF. M,,= 5800; M, = 9300. 1HNMR(CDC13,200MHz):7.8ppm-(s,6H,arom);4.2ppm (m,J = 7 Hz, 12H, O-CH2 and 4H, COO-CH2 and COOCH2-CH3 end groups); 3.35 ppm (s, 2 H, CO-cH2-CO); 1.9 ppm (m, 12 H, O-CHdH,); 1.7 ppm (m,4 H, COO-CH2-CH2 end groups); 1.7 ppm (m, 4 H, COO-CH2-CH2); 1.5 ppm (m, 24 H, alkyl-CH2); 1.2 ppm (t, COO-CH&H3 end groups); 0.95 ppm (t, 12 H, CH3). The monolayers were formed on a LB trough (Lauda) using pure MilliQ water as a subphase.lO The concentration of the spreading solution (chloroform) was 0.5 mg/mL. The LB films were prepared on clean glass slides which were hydrophobized using dichlorodimethylsilane. The transfer was performed near the collapse pressure of each component (see data in ref 10). From 5 to 20 layers were transferred in a Y-type deposition. A sketch of the deposition of monolayers from discotic compounds is presented
Table I. Phase Behavior of the Triphenylene Derivatives 1 and 2 in the Bulk S t a t e
phase transitions, T°C
compound monomer 1
## Ia ## 31
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Characterization of the Samples. The optical textures of the bulk compounds were observed with a Leitz polarzing microscope and thermogramswere obtained using a Perkin-Elmer DSC-4. The X-ray diagrams were recorded in reflection with a Siemens D-500 diffractometer using Cu Ka radiation. Models of molecular packing were constructed by means of the program INSIGHT (Biosym)16on a Silicon Graphics work station.
Results and Discussion Bulk State of Triphenylene Derivatives 1 and 2. The heating of monomer 1leads to a phase transformation from a crystalline to the isotropic state as apparent from the DSC curves and from optical microscopy (Table I). Cooling from the isotropic state results, on the other hand, in the formation of a monotropic LC phase at 66 "C and (16) INSIGHTII, Biosym. Tech. CA91121, San Diego, 1990.
to avoid overlapping).
crystallization at 35 "C. The optical textures in the LC state are characteristic of a smectic phase. This behavior differs compounds 1 studied from the symmetric substituted nonamphiphilic triphenylene derivatives with a common columnar phase of Dho type.3*496 Polymer 2 shows a glass transition at -10 "C and phase transformation from a nematic discotic state to the isotropic state at 31 "C during heating. Significant supercooling as was observed by optical microscopy and DSC (Table I). The X-ray diagrams of monomer 1 obtained at room temperature (in the crystalline state) are characterized by the occurrence of a large number of sharp peaks with different intensities which overlapped with a wide-angle diffuse halo arouud 21" (Figure 1). The cell parameters calculated from the positions of maxima are a = 2.1 nm, b = 7.0 nm, and c = 0.42 nm, and the correspondingindexing is presented in Table 11. The calculated density (0.9 g/cm3) is typical for discotic triphenylene derivatives.4J8 As can be seen from Table I1 all sharp maxima on the scattering curve can be indexed with good accuracy as even orders of reflections correspondingto the periodicity along the b direction (OkO reflections with k = 2n). The absence of or the very small intensities of the odd order reflections OkO (with k = 2n + 1)can be attributed to the existence of nonprimitive orthorhombic cell of body- or
Columnar Ordering of Liquid-Crystalline Discotics
Langmuir, Vol. 8, No. 9, 1992 2281
Table 11. Structural Data for the Bulk LC State of the ComDoundP indexation of reflexes 010 020 100 040 050 060 200 070 080 090 0100 0120
d-spacingsfor Bragg reflections, nm 1,25 O C 1,60O C , cooling 2,25 "C 6.6 vw 1.8d 3.46 3.4 0.84 wd 2.21 2.1 1.77 1.8 1.41 1.4 1.18 1.2 1.1 1.0 0.89 0.9 0.77 0.71 0.6 0.45d 0.42
001
0.44 d 0.36 w
7 nm
0.42 d
a Indexation corresponds to the model presented in Figure 2;d, diffuse halo; w, weak; vw,very weak.
face-centered type1' with random displacements (positional distortions) of second repeat units according to exact position as discussed in ref 18. Some additional weak reflections could be indexed as hOO reflexes with a periodicity of 2.1 nm (Table 11). The low intensity of these reflections indicates distortions of the translational ordering along the a edge. The intensive diffuse halo a t 0.45 nm (around 20°) is attributed to a liquidlike local packing of molecular fragments (flexible tails). This reflects a partial crystalline state of compound 1with the significant content of disordered fraction (shortrange ordered tails) as usually observed for discotic LCS.~ A spatial arrangement of the molecules of compound 1 a t room temperature which fits the calculated cell parameters is presented in Figure 2. In the model the repeat unit consists of two molecules characterized by a partial overlapping of the OH-containing tails forming an associated pair. Along the a direction the periodicity is determined by a dense edge-by-edge packing (columnar packing) of molecules (2.1-2.2 nm). Along the b direction a second pair of molecules is located around the center of the unit cell as required by body- and face-centered arrangements. Thus the periodicity in the b direction is determined by the distance between next nearest pairs; according to molecular models this distance amounts to 7 nm and corresponds to the main periodicity observed on the X-ray patterns. The cooling of the isotropic state of monomer 1 leads to the formation of the monotropic LC state (Table I). Only minor distortions of the layer packing of the molecules as compared to the partial crystalline state are connected with this transition while the main periodicity is preserved; the weaker reflections are observed and higher order reflections vanish (Figure 1, Table 11). The lateral packing of the discoid molecules, on the other hand, is rearranged strongly; a new peak appears at 24.3O in the wide-angle range on top of the diffuse halo (Figure 1). One thus observes a wide-angle scattering pattern typical for discotic columnar phases characteristic of a dense core-by-core intracolumnar packing (reflex at 0.36 nm) and liquidlike ordering of the flexible tails.6p18J9 The preservation of the layer packing described above for the partially crystalline phase (Figure 2) and the liquidlike intermolecular packing in layers can be explained as a (17)International Tables for X-ray crystallography: Kynoch Press: Birmingham. 1962. (18)Mbller, M.;Tsukruk,V.V.; Wendorff,J. H.; Bengs,H.; Ringsdorf, H.Liq. Cryst. 1992, 12, 17. ~
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Figure 2. Model of the double-layer packing of the discotic molecules and correspondingarrangement of the cell a, b, c edges (tails with circlesare hydrophilic(OH-containing)ones,the other chains are hydrophobic ones).
formation of a layered LC phase. Actually monomer 1 in LC state displays a type of layered structure with a doublelayer long-range one-dimensional ordering along the b direction and a liquidlike intralayer ordering with intermolecular distances of 2.1-2.2 nm for edge-by-edge contacts. Side-by-side contacts of the rigid cores of discotic molecules are characterized by distances of 0.36 nm and the correlation lengths (as estimated from the width of reflection) is 2 nm. Such structural parameters are typical for an intracolumnar ordering in columnar phases of triphenylene c o m p o u n d ~ . ~ ~ ~ J ~ Thus the structure displayed by the monomeric compound 1 corresponds to a "hybrid" of a lamellar and columnar structure as evidently detected for materials composed of both discotic and rodlike parts.lg But unlike the case considered here, such ordering is obviously controlled by the formation of associated pairs of molecules originating from local segregation of hydrophobic and hydrophilic flexible tails and hydrogen bonding between OH-containing tails. On the corresponding X-ray diagrams for polymer 2 only two broad halos at low angles and near 21° are detected in the whole temperature range (Figure 3). The d spacings are given in Table 11. The d spacing for the small-angle halo corresponds to a short-ranged edge-by-edge intermolecular packing. The wide-angle halo reflects the occurrence of a liquidlike side-by-side packing of the molecules with a mean distance of 0.42 nm. No defined columnar ordering is detected in polymeric discoticsystem 2. Thus in connection with the observed optical texture one can conclude that the polymers display a common discotic nematic phase. Langmuir-Blodgett Films. The X-ray scattering curves obtained for LB films of the polymer 2 and monomeric compound 1are characterized by the presence of two different scattering phenomena (Figures 4 and 51,
~~
(19) Kreuder,W.; Ringsdorf, H.; Herrmann-Schbnherr,0.; Wendorff, J. H. Angew. Chem. 1987,99, 1300.
Karthaus et al.
2282 Langmuir, Vol. 8, No. 9, 1992 I
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Figure 3. X-ray scatteringcurves for the bulk state of polymer 2 at differenttemperatures (curvesare displaced along they axis to avoid overlapping).
Table 111. Structural Characteristics of LB Films
n, no.of layers monomer 1 n = 20 polymer 2 n = 20 L
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sharp peaks at 2.3-2.5', 4.7-5.0°, and around 7 O superimposed on gradually damped oscillations. The sharp peaks are Bragg reflections resulting from the formation of ordered layer structures and their intensity is much higher than the intensity of gradual damping oscillations. The oscillations present the diffraction effects related to the overall thickness of the film (Kiessig fringe^).^+^^ The positions of the Bragg peaks correspond very well to (20)Rientord, F.; Benettar, J. J.; h i o , L.; Robin, P.; Blot, C.; de Kouchkovsky, R J. Phys. (Paris) 1987,48,679.
a
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second-, fourth-, and sixth-order reflections originating from the double-layer structure described above (Figure 2, Table 111). All odd orders of reflection are absent due to a symmetry. Thus the multilayer ordering in the LB film is very similar to one formed in the bulk LC state of monomer 1; separate layers are formed by pairs of disks in an edge-on arrangement and thus the columns lie parallel to the solid support (Figure 2). From the width of Bragg's reflections one can conclude that the translational correlation in layer ordering is expanded through the overall thickness of LB films studied. The thicknesses of layers are similar to the ones in the bulk state but slightly larger in the polymeric LB films (Table 111). The observation of a decreased scattering intensity of the LB films of polymer 2 and of a slightly larger thickness of the layers leads to the conclusion that the multilayered structure is characterized by a lower level of perfection evidently as a result of the presence of a disordered flexible macromolecular backbones. From the periodicity of the fringes one can calculate the overall thickness of the films taking the refraction of the X-ray beam at the air-film boundary into accountm (Table 111). As can be seen from these data the f i i thicknesses correspond very well (within an accuracy of (21) Feigin,L. A;Lvov, Yu. M. Makromol. Chem., Macmmol. Symp. 1988,15,259. (22) Jark, W.; Russel, T. P.; Comelli, G.; Stahr, J.; Erdellen, C.; Ringsdorf, H.; Schneider,J. Thin Solid F i l m 1991,199, 161. (23)Tippmann-Krayer,P.;Miihwald, H.; Lvov,Yu.M.Langmuir 1991, 7,2298.
Columnar Ordering of Liquid-Crystalline Discotics 2 ?6 to the ones calculated from the thickness of separate layers multiplied by the number of transferred layers. Thus a multilayer structure is formed in LB films displaying an edge-on arrangement of the discotic molecules for both compounds. The columns themselves lie parallel to the solid support. The cross section of one molecule within layer planes equals 0.84.9 nm2 (as calculated from the lengths of a and c edges) which corresponds well to the square per monomeric unit determined from the r-a (pressure-area) diagrams.'O Heating of the LB film of monomer 1 to 100 "C leads to a destruction of the initial ordering and to the formation of poorly ordered structures with a d spacing of 5.6 nm (Figure 41, which corresponds to a simple double-layer packing of the molecules. This layer structure melts close to 150 "C and is restored after cooling of the film to room temperature. A slightly different melting behavior was observed for the LB film from the polymeric compound 2. The internal layer structure melts at 80 OC but the overall thickness of the homogeneous films-38 nm thick-is still preserved (see weak fringes in Figure 5). A totally irreversible melting of the LB film takes place at about 170 OC. The observed effects correspond to the mechanism of melting of the internal layers of LB films discussed in ref 23.
Conclusions The monomeric compounds 1 containing a rigid triphenylene core and two hydrophilic and four hydrophobic flexibletails exhibits in the bulk LC state a "hybrid" layer and columnar phase characterized by a double-layer packing of associated pairs of disks and local columnar intralayer ordering. The structure shows a long-range
Langmuir, Vol. 8, No. 9, 1992 2283 ordering along the normal to the layer planes, a liquidlike edge-by-edge ordering, and a well-defined side-by side packing of the rigid cores (intercolumnar and intracolumnar ordering according to the terminology for a columnar discotic phases) within the layers. The corresponding polymeric compound 2 display a discoticnematic phase in the bulk state. The double-layer structure characteristic of the monomericcompound 1in the bulk LC state is preserved during the formation of the multilayer LB structures on the solid support. The discotic molecules are arranged in the multilayer film in an edge-on manner and the columns lie parallel to solid support. Translational correlationsoccur on over the total thickness of LB films. The same type of arrangement with a slightlylower perfection of ordering is formed in the LB films of polymeric compound 2 despite the existence of only a nematic ordering in the bulk state. Thus the induction of a well-ordered layered structure as a result of monolayer formation and transfer onto a solid support is observed for the discotic polymer 2. The heating of the LB films leads to the irreversible melting of the initial double-layer ordering of columns in both compounds. The character of the melting of the multilayered structures differs for monomeric and polymeric compounds 1 and 2; the melting of intrdilm layer packing leads to the destruction of the uniform film as a whole for the monomer while a homogeneous uniform film is preserved for the polymeric compounds 2 even if the internal layer structure is destroyed on melting.
Acknowledgment. V. V. Tsukruk acknowledges the financial support of the Alexander von Humboldt Foundation (AVH).
