1630
Chem. Mater. 1997, 9, 1630-1637
Asymmetric Discotic Liquid Crystals Based on Rufigallol K. S. Raja and S. Ramakrishnan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
V. A. Raghunathan Raman Research Institute, Bangalore 560080, India Received January 3, 1997X
Two series of asymmetric hexaethers (asymmetric by virtue of having alkyl chains of unequal lengths) of rufigallol, viz., R442n and R482n (1,5-dialkoxy-2,3,6,7-tetrabutoxy-9,10-anthraquinone and 1,5-dialkoxy-2,3,6,7-tetraoctyloxy-9,10-anthraquinone, respectively) were synthesized. These hexaethers exhibit a typical hexagonal columnar mesophase (Dh) as evidenced from X-ray diffraction and polarized light microscopic studies. Differential scanning calorimetric (DSC) studies suggest that the temperatures of isotropization (TD-i) and the molar isotropization entropy (∆Si) fall with increase in asymmetry. The fall in ∆Si with increase in asymmetry is significantly larger than what may be expected merely from increase in the relative alkyl chain content, suggesting that asymmetry causes a significant decrease in the extent of ordering in the columnar mesophase. However, X-ray diffraction studies suggests that the intercolumnar spacing (D) is not affected by the presence of asymmetry but depends only on the total number of alkyl chain carbon atoms (Cn) present in a molecule. Furthermore, from the linear variation of the disk area, πD2/4, versus Cn (which was shown to be system independent), it was possible to calculate the volume occupied by a single alkyl chain methylene (CH2) unit in the columnar mesophase. This value is significantly smaller than that estimated in liquid n-alkanes, suggesting that the alkyl chains are not truely liquidlike in such discotic coulmnar mesophases. A further interesting observation in these systems is that when the alkyl chains are very long, an additional ordering of the side chains suggestive of side-chain crystallization is observed.
Introduction Discotic liquid crystalline phases are exhibited by disklike molecules. A typical discotic molecule has an aromatic core to which are attached, via ester or ether links, six-to-eight alkyl chains of the same length.1 Such discotic molecules have been shown to form both nematic and coulmnar mesophases. In a typical columnar mesophase the molecules stack to form columns which in turn form a 2-D array, leading to various types of columnar mesophases namely, hexagonal (Dh), rectangular (Dr) and oblique (Dob). A simple model for the formation of such a discotic mesophase involves disordering of the side chains to attain a liquidlike state at the crystal-to-mesophase transition temperature, while the cores remain stacked and become disordered only at the isotropization temperature.2,3 Detailed vibrational spectroscopic studies carried out on such discotic systems have reaffirmed the validity of this model,4 though to a limited extent. Although many homologous series of disklike compounds have been shown to form Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Chandrasekhar, S. Liquid Crys. 1993, 14, 3. (2) Fontes, E.; Heiney, P. A. Phys. Rev. A 1988, 37, 1329. (3) Collard, D. M.; Lillya, C. P. J. Am. Chem. Soc. 1989, 111, 1829. (4) (a) Kardan, M.; Reinhold, B. B.; Hsu, S. L.; Thakur, R.; Lillya, C. P. Macromolecules 1986, 19, 616. (b) Yang, X.; Kardan, M.; Hsu, S. L.; Collard, D. M.; Heath, R. B.; Lillya, C. P. J. Phys. Chem. 1988, 92, 196. (c) Yang, X.; Waldman, D. A.; Hsu, S. L.; Nitzsche, S. A.; Thakur, R.; Collard, D. M.; Lillya, C. P.; Stidham, H. D. J. Chem. Phys. 1988, 89, 5950. X
S0897-4756(97)00019-7 CCC: $14.00
such mesophases,5 relatively few studies have been carried out to examine the effect side-chain structure has on their phase behavior. Some such studies include the effect of branch points,6,7 unsaturation,7 heteroatom substitution,8-10 and introduction of asymmetry using alkyl chains of different lengths.11,12 Tinh et al.12 reported the synthesis of asymmetric discotic molecules based on a triphenylene core by the cyclotrimerization of unsymmetrically alkylated catechol, followed by chromatographic separation of the various regioisomers formed. Apart from requiring tedious separation procedures, this approach offers very little control on the structure of the final products. The recent elegant approach developed by Boden et al.13 for the synthesis of triphenylene discotics, on the other hand, is much more versatile and permits the synthesis of the complete range of asymmetrically substituted triphenylenes. (5) For examples see: Chandrasekhar, S. Advances in Liquid Crystals; Brown G. H., Ed., Academic Press: New York, 1982; Vol. 5. Demus, A. Liquid Cryst. 1989, 5, 75. (6) Schouten, P. G.; van der Pol, J. F.; Zwikker, J. W.; Drenth, W.; Picken, S. J. Mol. Cryst. Liq. Cryst. 1991, 195, 291. (7) Collard, D. M.; Lillya, C. P. J. Am. Chem. Soc. 1991, 113, 8577 (8) Collard, D. M.; Lillya, C. P. J. Org. Chem. 1991, 56, 6064. (9) Lillya, C. P.; Collard D. M. Mol. Cryst. Liq. Cryst. 1990, 182B, 201. (10) Tabushi, I.; Yamamura, K.; Okada, Y. J. Org. Chem. 1987, 52, 2502. (11) Mullen, K.; Goltner, C.; Pressner, D.; Spiess, H. W. Angew Chem. Int. Ed. Engl. 1993, 32, 1660. (12) Tinh, N. H.; Bernaud, M. C.; Sigaud, G.; Destrade, C. Mol. Cryst. Liq. Cryst. 1981, 65, 307. (13) Boden, N.; Bushby, R. J.; Cammidge, A. N. J. Chem. Soc., Chem. Commun. 1994, 465.
© 1997 American Chemical Society
Asymmetric Discotic Liquid Crystals
While they have used this approach to demonstrate its feasibility and further exploited it to prepare discotic liquid-crystalline polymers,14 no detailed study to understand the effect of asymmetry on the mesophase structure and its properties has been reported. In this study, we report the synthesis of two series of asymmetric hexaethers of rufigallol (R4m2n), having four alkyl chains of one length (m) and two of another (n). The rufigallol core presents the advantage of unequal reactivity of the six phenolic groups. This permits the selective alkylation of four of the phenolic groups leaving behind the two intramolecularly hydrogenbonded ones. The asymmetry introduced by subsequent alkylation of the remaining two phenolic groups may be conceptualized as distorting a regular hexagon in two ways, which may be schematically represented as follows:
The trends observed in the enthalpy, entropy, and phase transition temperatures with the systematic increase in the asymmetry have been investigated by differential scanning calorimetric studies. The effect of this asymmetry on the structure of the mesophases exhibited by these compounds are studied by X-ray diffraction and polarized light microscopy. Experimental Section 1H
NMR spectra were recorded on a Bruker ACF-200 MHz spectrometer, using TMS/CDCl3 as internal reference. IR spectra were recorded using a Perkin-Elmer 781 spectrometer. Polarized light microscopic studies were carried out using a Leitz Ortholux II Pol-BK microscope equipped with a Mettler FP82HT hot stage. DSC studies were done using a Rheometric Scientific DSC Plus instrument. The thermograms were recorded, both during the heating and cooling runs, using a heating/cooling rate of 10 °C/min. A slow purge of nitrogen gas (5 mL/min) was maintained to prevent any possible oxidative degradation. The reproducibility of the thermograms was confirmed by recording the second and third heating runs. The X-ray diffraction patterns of the liquid-crystalline phases were recorded using an image plate (Marresearch, Germany); using Cu KR radiation. Synthesis. Rufigallol was prepared from Gallic acid using a previously reported procedure.15 It was then purified by hydrolysis of the recrystallized hexaacetate derivative. The alkyl halides were distilled prior to use. 2,3,6,7-Tetrabutoxy-9,10-anthraquinone (R44). Rufigallol (8.03 g, 0.0264 mol) was added to a round-bottom flask containing 750 mL of DMSO, 25 mL of NaOH (0.106 M), and 37.53 g of TBAB (0.1164 mol) under a flow of nitrogen gas. The mixture was stirred for 15 min, and 15.97 g (0.1165 mol) of bromobutane was added to it. The reaction was allowed to proceed at 80 °C with stirring under a nitrogen bladder for 24 h. The reaction mixture turned yellowish orange after about 9 h, and golden yellow needlelike crystals of rufigallol tetrabutyl ether (R44) precipitated out of the solution. The reaction mixture was filtered and the crude rufigallol tetrabutyl ether was recrystallized thrice from EtOH/CHCl3 to obtain it in pure form (yield 27%, mp 120 °C). Attempts to recover further R44 from the reaction mixture were futile. The tetraoctyl ether (R48) was also prepared using a similar (14) Boden, N.; Bushby, R. J.; Cammidge, A. N. J. Am. Chem. Soc. 1995, 117, 924. (15) Grimshaw, J.; Hawarth, R. D. J. Chem. Soc. 1956, 56, 4225.
Chem. Mater., Vol. 9, No. 7, 1997 1631 procedure using octyl bromide to give the required product in about the same yield (mp ) 111 °C). R44: 1H NMR (CDCl3) δ (ppm) 12.8 (s, 2H, phenolic OH), 7.42 (s, 2H, aromatic), 4.1-4.3 (m, 8H, Ar-O-CH2), 1.4-2.0 (m, 16H, alkyl protons), 0.9-1.1 (m, 12H, CH3). 1,5-Diacetoxy-2,3,6,7-tetrabutoxy-9,10-anthraquinone (R44DA). R44 (1 g, 1.9 mmol), 30 mL of pyridine, and 100 mL of CH2Cl2 were stirred in a round-bottom flask, and 20 mL of acetyl chloride was added to it dropwise using an additional funnel fitted with a CaCl2 guard tube. The reaction mixture was stirred at room temperature for 9 h and then washed with dilute HCl. The CH2Cl2 layer was further washed with water, dried over Na2SO4, and filtered and the solvent was removed using a rotary evaporator. The solid obtained was recrystallized twice from ethanol/CHCl3 to obtain rufigallol tetrabutyl ether diacetate (R44DA, yield ) 70%, mp ) 182 °C). R48DA was prepared using the same procedure (mp ) 123 °C). R44DA: 1H NMR (CDCl3) δ (ppm) 7.65 (s, 2H, aromatic); 4-4.2 (m, 8H, ArO-CH2); 2.46 (s, 6H, CH3CdO); 1.4-2.0 (m, 16H, alkyl protons); 0.9-1.1 (m, 12H, CH3). Typical procedure for R4m2n (R4425). R44DA (350 mg, 0.56 mmol), 1.3 g of K2CO3, 770 mg (7.22 mmol) of pentyl chloride, and 117 mg (0.2 mmol) of KI were taken in 10 mL of DMF and stirred under a N2 bladder at 100 °C for 3 days. The DMF was removed under reduced pressure and CHCl3 was added to the reaction mixture. The chloroform layer was washed once with water, then with brine, dried over Na2SO4, and filtered, and finally the solvent was removed using a rotary evaporator. The solid obtained was further purified by column chromatography using basic alumina as the stationary phase and eluted using hexane followed by CHCl3 to obtain the pure hexaether, R4425. The yield of the product obtained after recrystallization from EtOH was 77%. All the other members, of both the R442n and R482n series, were synthesized using essentially the same procedure to give the required product in yields ranging from 60 to 70%. R4425: 1H NMR (CDCl3) δ (ppm) 7.59 (s, 2H, aromatic); 4-4.2 (m, 12H, ArO-CH2); 1.1-2.0 (m, 28H, alkyl protons); 0.9-1.1 (m, 18H, CH3). The 1H NMR spectra of all the other asymmetric hexaethers exhibited essentially the same features with a single aromatic signal (at ca. 7.6 ppm); the only difference being the relative intensities of the aromatic and aliphatic proton signals, which matched the expected values.
Results The synthesis of the asymmetric hexaethers was achieved by a two-step alkylation procedure as shown in Scheme 1. The unequal reactivity of the six phenolic groups, two of which are less reactive by virtue of being intramolecularly hydrogen bonded to the adjacent quinone carbonyls, was exploited. Using 4 equiv of tetrabutylammonium hydroxide (TBAH) and 4.4 equiv of the alkyl halide in DMSO, the symmetric tetraether was prepared in ca. 30% yield.16 It was noticed that as the reaction progressed, golden yellow needles of the tetraether crystallizes out from the reaction mixture, probably precluding its further alkylation. Use of 4 equiv of NaOH along with 4.4 equiv of a phase-transfer catalyst, such as tetrabutylammonium bromide, was also found to work equally well and gave essentially similar yields of the tetraether. It may be added here that the tetraethers, as obtained from the reaction mixture, were essentially pure and was further purified easily by recrystallization from ethanol-chloroform. The tetraether, by virtue of its symmetry, exhibits only a single aromatic proton signal (Figure 1a). The residual (16) The symmetric tetramethyl ether was, in fact, first reported by Grimshaw et al.,15 who isolated it by treating rufigallol with excess ethereal diazomethane in order to establish the structure of rufigallol.
1632 Chem. Mater., Vol. 9, No. 7, 1997
Raja et al.
Scheme 1. (i) RBr, TBAB, NaOH, DMSO, 80 °C, 24 h; (ii) AoOCl, Py, CH2Cl2, 9 h; R(n)Br, K2CO3, 90 °C, 3 days
Figure 1. 1H NMR spectra of R44 (a), R44DA (b), and R4425 (c). (*) Peaks due to solvent/H2O.
phenolic protons are also seen at 12.8 ppm. The symmetric tetraether was converted to the corresponding diacetate in 70% yield using acetyl chloride/pyridine. The diacetate was then treated with the appropriate alkyl halide in the presence of K2CO3 in DMF at 90 °C to effect the second alkylation and give the asymmetric hexaether in ca. 70% yield.17 Any residual compounds containing unreacted phenolic groups was easily removed by passing through a basic alumina column. The 1H NMR spectra of the tetraether diacetate and a typical asymmetric hexaether are shown in Figure 1b,c. The diacetate shows an additional peak at 2.46 ppm due to the acetyl protons, while the asymmetric hexaether shows two types of alkoxymethylenes, Ar-O-CH2, in the region 4.0-4.2 ppm, in the approximate ratio of 2:1. Thus, it appears that the alkoxy protons in positions 1 and 5 are slightly deshielded because of its close proximity to the quinone carbonyls. It is also important (17) The symmetric hexaethers were first synthesized from rufigallol hexaacetate under similar conditions by Carfagna et al.18 Direct alkylation of the tetraether under strongly basic conditions is likely to lead to some scrambling of alkyl groups due to the lability of the alkyl ether linkages of rufigallol under these conditions.21
to note that, in all cases, we see a single aromatic proton signal which is a characteristic signature of the purity of the hexaethers. Two series of asymmetric hexaethers were synthesized, namely, R442n and R482n, which have four alkyl chains having 4 and 8 carbons (butyl and octyl), respectively, and two alkyl chains having n carbon atoms each. Thermal Data. The thermal data for the two series of asymmetric discotics, viz., R442n and R482n, are presented in Tables 1and 2, respectively. DSC thermograms of the asymmetric discotic molecules, belonging to the R442n and R482n series, are shown in Figures 2a,b, respectively. In a typical LC compound, say of R4826, the endotherms at 53.9 and 86.5 °C represent crystal f discotic (Tk-D) and discotic f isotropic (TD-i) transitions, respectively. The other transitions at lower temperatures are due to different crystal f crystal transitions. This was confirmed by viewing the samples under a polarized light microscope with a hot-stage. Several of the discotic samples exhibited such crystal f crystal transitions, which is not uncommon to this class of molecules.18 It may be added here that the relatively weak isotropization transitions, for the higher homologues (n > 9) in the R482n series, are not clearly visible in Figure 2b. A typical texture of the discotic mesophase is shown in Figure 3, which shows a flowerlike pattern characteristic of a hexagonal columnar mesophase. All the other liquid-crystalline members of the series also exhibited a similar texture confirming (18) Carfagna, C.; Ianneli, P.; Roviollo, A.; Sirigu, A. Liquid Cryst. 1987, 2, 611.
Asymmetric Discotic Liquid Crystals
Chem. Mater., Vol. 9, No. 7, 1997 1633 Table 1
sample R4423 R64 R4425 R4426 R4427 R4428 R4429 R44210 R44211 R44212
TK-D/K
TD-I/K
∆HD-Ka/kJ
mol-1
∆SD-Ka/kJ mol-1 K-1
∆HI-Da/kJ mol-1
∆SI-Da/kJ mol-1 K-1
0.017 0.004 0.020 0.029 0.055 0.040
17.68 16.86 15.13 12.55 11.34 7.42
0.045 0.043 0.042 0.035 0.032 0.022
406.4b 395.5c 378.5 357.0 355.8 350.2 350.3 311.6c
396.7c 396.2 391.7 381.0 365.1 352.5 332.8c 307.1b 312.0b
6.05 1.40 7.07 11.61 17.66 12.43
a Absolute values for the cooling runs. b Crystal-to-isotropic transition temperatures. c Transition temperatures are reported for the cooling runs for these monotropic samples.
Table 2 sample R4823 R4824 R4825 R4826 R4827 R68 R4829 R48210 R48211 R48212 a
TK-D/K
TD-I/K
328.4 326.9 312.9 310.0 324.1 305.2 311.2 323.0
353.2a 353.7a 346.9 359.5 364.4 369.0 356.4 330.9 329.5 343.1
∆HK-D/kJ
mol-1
4.79 6.21 2.70 1.48 7.69 34.01 96.35 102.95
∆SK-D/kJ mol-1 K-1
∆HD-I/kJ mol-1
∆SD-I/kJ mol-1 K-1
0.015 0.019 0.009 0.005 0.024 0.112 0.310 0.318
6.66 10.30 11.09 12.06 7.83 3.55 1.38 3.60
0.019 0.029 0.030 0.033 0.022 0.011 0.004 0.011
Crystal-to-isotropic transition temperatures.
Figure 3. Photograph of the discotic mesophase of R44210 at 330.4 K taken under crossed polarizers.
Figure 2. DSC thermograms (10 °C/min) of R442n (a, top) and R482n (b, bottom).
the hexagonal columnar nature of their mesophases. It may also be added that both the tetraethers, R44 and R48, were not liquid crystalline and exhibited only a single melting transition. A plot of the two transition temperatures, Tk-D and TD-i, versus n for the R442n
series is shown in Figure 4a. It is noticed that when 4 e n g 10, the samples exhibited LC behavior, but for other values of n they were not liquid crystalline. Furthermore, while the symmetric hexaether, R64, and R44210 were found to be monotropic, the rest were enantiotropic.19 The mesophase range in this series appears to increase at first with n and then decreases after attaining a maximum value for R4426. In the R482n series, a slightly different trend is observed. It is clear from Figure 4b that in this case the mesophase range is maximum for the symmetric hexaether R68 and decreases with increasing asymmetry, for n < 8 and n > 8. In the R442n series, since the symmetric R64 itself exhibited a very narrow (monotropic) mesophase range, the type I distortion (i.e., for n < 4) did not yield liquid-crystalline molecules. Thus, it appears that R64 has just sufficient alkyl chain content to stabilize the discotic columnar mesophase. Hence, the initial increase in the total alkyl chain content, albeit at the cost of introducing asymmetry, (19) R64 was found to be monotropic with a narrow mesophase range of ca. 1 °C, which is not in agreement with previously published work,18 that report the same compound as being enantiotropic with a mesophase range of about 24 °C.
1634 Chem. Mater., Vol. 9, No. 7, 1997
Figure 4. Plot of melting and isotropization temperatures versus the number of alkyl carbon atoms n of R442n (a, top) and R482n (b, bottom). (O) crystal-to-discotic transition temperature, (0) discotic-to-isotropic transition temperature, (4) crystal-to-isotropic transition temperature.
causes a stabilization of the mesophase resulting in an increase in the mesophase range. At larger values of n (from R4426 onward), the effect of asymmetry begins to dominate and causes a decrease in the mesophase range. In the R482n series, on the other hand, both the type I and type II symmetry distortions yielded discotic LC molecules, both of which destabilizes the mesophase as reflected by the decrease in their mesophase range. The extent of order present in the mesophase is often characterized by the isotropization entropy ∆Si (i.e., for the discotic f isotropic phase transition). To examine the effect of asymmetry, ∆Si is plotted versus n for both the asymmetric series and is shown in Figure 5a,b. In the case of the R442n series, ∆Si fell monotonically with increasing n, while in the R482n series, the isotropization entropy was largest for the symmetric case (R68) and decreased both for type I and type II modifications, i.e., for n < 8 and n > 8. However, a slight increase in ∆Si was observed for the R48212 (n ) 12) case.20 Another interesting observation for the molecules with (20) No explanation for this behavior is possible without further data points with larger values of n.
Raja et al.
Figure 5. Plot of molar isotropization entropy (∆Si) versus the number of alkyl chain carbon atoms n, for the series R442n (a,top) and for the series R482n (b, bottom).
large values of n is that, after controlled cooling (at 10 °C/min) from the isotropic melt, they exhibit a fairly large exotherm during the subsequent heating run (Figure 2). In many cases, this exotherm is interjected by other endotherms, due to crystal f crystal transitions, as seen in Figure 2. Furthermore, it was noticed that complete enthalpic recovery was not attained during the controlled cooling runs. This suggests that during the cooling cycle, the most stable crystalline form is not formed, but during the subsequent heating run an exothermic reorganization process occurs at a temperature when adequate alkyl chain mobility is attained. To establish that this behavior is not specific to the asymmetric series, the symmetric hexadodecyl ether, R612, was synthesized and was also shown to exhibit a similar behavior, although this was not reported by earlier workers.18 The plot of the melting entropy (∆Sm) (i.e., for crystal f discotic transition) with n, for the R482n series, is shown in Figure 6. This plot clearly indicates that there is indeed a drastic increase in ∆Sm beginning at n ) 9. It may be added here that R4829 is also the first molecule in the series that exhibits such an exothemic peak in the DSC scan of the second heating run (Figure 2b). In the R442n series, however, no prominent increase in ∆Sm is observed even
Asymmetric Discotic Liquid Crystals
Figure 6. Plot of molar melting entropy (∆Sm) versus the number of alkyl chain carbon atoms n for R482n.
Figure 7. Plot of the sum of entropies of melting and isotropization (∆Stotal) versus the number of alkyl chain carbon atoms n; (O) for the series R482n, (0) for the series R442n.
for the last LC member of the series, R44210 (not shown). However, to include the nonliquid crystalline members of the R442n series, if one were to plot the total entropy change, ∆S(total), on going from the crystalline to the isotropic phase for both the series (Figure 7), it is clear that for n g 9, an abrupt increase in the extent of order present in the crystalline phase of these discotic compounds does occur,22 probably suggestive of some kind of side-chain crystallization. Similar independent crystallization of alkyl side chains have been shown to occur in several polymeric systems after it was first reported by Kaufman et al.23 In general, the independent crystallization occurs when the alkyl chain length n g 8. The closest polymeric parallel to the discotic systems is that reported for long-chain poly(n-alkylglutamate)s,24 in which the polypeptide backbone forms a rigid helical structure (similar to the stacked aromatic cores) and the flexible alkyl side chains extend peripherally outward. In such systems, it was (21) Raja K. S.; Ramakrishnan, S., unpublished results. (22) A similar sharp increase in was also reported for the symmetric hexaalkanoates of rufigallol by Carfagna et al.: Carfagna, C.; Roviello, A.; Sirigu, A. Mol. Cryst. Liq. Cryst. 1985, 122, 151.
Chem. Mater., Vol. 9, No. 7, 1997 1635
noticed that when the alkyl chain length n g 10, independent crystallization of the alkyl chains in a typical parafinic crystal lattice occurred. It is interesting to note that the structural similarity extends further, in that the melting of the side chains in these polymers leads to the formation of a thermotropic mesophase, wherein the rigid helical cores are surrounded by a liquidlike parafinic segment, much like what is observed in the discotic mesophases. Confirmation of such side-chain crystallization in discotic systems can come only from further X-ray and possibly vibrational spectroscopic studies. X-ray Diffraction. The X-ray diffraction patterns of the mesophase exhibited by all the liquid-crystalline samples belonging to both the asymmetric series, R442n and R482n, is supportive of a discotic hexagonal columnar arrangement. A typical diffraction pattern of R48210, recorded at 40 °C (by cooling from isotropic state), is shown in Figure 8. A sharp inner ring corresponding to a d spacing of 21.1 Å is clearly evident, and also seen are two less intense outer rings corresponding to spacings of 12.3 and 10.6 Å, respectively. The d spacings of these three rings are in the ratio 1:1/ x3:1/2, which is typical of a hexagonal lattice. A fairly strong diffraction ring corresponding to the spacing of 3.6 Å, which corresponds to the interdisk spacing within a column and a diffuse chain reflection at ∼4.4 Å, suggesting a liquidlike structure of the alkyl chains, are also seen. The intercolumnar distance, D, calculated using the relation D ) d100/cos 30°, for the various asymmetric discotic molecules are listed in Table 3. In both the R442n and R482n series, it is evident that as n increases, the diameter (D) of the cylindrical column formed by the stacking of the discoidal molecules, also increases. If indeed cylindrical columns are formed by stacking of asymmetric discotic molecules, then these molecules will, in all probability, pack in a staggered manner one on top of the other. Assuming such a staggered packing, one can calculate the intercolumnar distance as the weighted average of the intercolumnar distances of the symmetric discotic molecules using the general formula
Dassy(nx,ny) ) [nxDsym(x) + nyDsym(y)]/(nx + ny)
(1)
where nx and ny are the number of alkyl chains of lengths x and y, respectively, and Dsym(x) and Dsym(y) are the intercolumnar distances for the symmetric discotic molecules with alkyl chain lengths x and y, respectively. The calculated values are in good agreement with the observed ones, as evident from Table 3. The above assumption is further justified by the fact that the experimentally observed intercolumnar spacing for pairs of discotic molecules, for which the total number of carbon atoms in the alkyl chains are the same, namely, R4427 and R65, R4825 and R67, and R44210 and R66, are essentially identical. This suggests that the intercolumnar distance depends only on (23) (a) Kaufman, H. S.; Sacher, A.; Alfrey, T.; Frakuchen, I. J. Am. Chem. Soc. 1948, 76, 6280. (b) Jordan, E. F., Jr.; Feldeisen, D. W.; Wrigley, A. N. J. Polym. Sci., Part A-1 1971, 9, 1835. (c) Jordan, E. F., Jr.; Artymyshyn, B.; Speca, A.; Wrigley, A. N. J. Polym. Sci., Part A-1 1971, 9, 3349. (d) Plate, N. A.; Shibaev, V. P. J. Polym. Sci., Macromol. Rev. 1974, 8, 117. (e) Hsieh, H. W. S.; Post, B.; Morawetz, H. J. Polym. Sci., Polym. Phys. Ed. 1976, 14, 1241. (24) (a) Watanabe, J.; Ono, H.; Uematsu, I.; Abe, A. Macromolecules 1985, 18, 2141. (b) Watanabe, J.; Ono, H.; Macromolecules 1986, 19, 1079.
1636 Chem. Mater., Vol. 9, No. 7, 1997
Raja et al.
Figure 8. X-ray diffraction pattern of R48210 at 313 K. Table 3 intercolumnar spacing (Å) obs
calcda
R442n 16.0, 4.4, 3.7 16.4, 4.4, 3.6 16.9, 4.4, 3.6 17.3, 4.5, 3.7 17.5, 4.4, 3.7 18.0, 4.5, 3.7
18.5 18.9 19.5 20.0 20.2 20.8
18.5 18.9 19.5 19.9 20.3 20.6
R482n 19.3, 4.4, 3.6 19.5, 4.4, 3.6 19.9, 4.5, 3.6 20.6, 4.5, 3.5 21.1, 4.4, 3.5 21.6, 4.5, 3.5 21.9, 4.6, 3.6
22.3 22.5 23.0 23.8 24.4 25.0 25.3
22.4 22.8 23.4 24.2 24.5 26.1c 25.3
sample
obs reflns (Å)
R4425 R4426 R4427 R4428 R4429 R44210 R4825 R4826 R4827 R4829 R48210 R48211 R48212
R6nb R64 R65 R66 R67 R68 R69 R610 R611 R612 R613
17.9 19.6 21.0 22.7 23.8 25.2 26.0 30.6c 28.2 28.5
a Calculated intercolumnar spacing obtained using formula 1. Intercolumnar spacing for series R6n taken from the literature.18 c This value, we believe, is incorrect due to the anomalously high value of the intercolumnar spacing reported in the literature.18 b
the total number of carbon atoms present in the alkyl segment and is not affected by the presence of asymmetry. Discussion In an attempt to understand the effect of asymmetry on the behavior of discotic LC molecules, we further compare the properties of the present homologous asymmetric series (R442n and R482n) with the symmetric series (R6n) reported by Carfagna et al.18 As stated earlier, the extent of order present in the mesophase is often characterized by the entropy change (∆Si) associated with the discotic f isotropic transition. In several symmetric discotic molecules it has been observed that as the volume fraction of the alkyl chain segment increases, the extent of order in the columnar
Figure 9. Plot of molar isotropization entropy (∆Si) versus total number of alkyl chain carbon atoms (Cn) for rufigallol hexaethers; (O) R6n, (0) R442n, (4) R482n .
mesophase Dh decreases and this is reflected in the decrease in the values of ∆Si as n increases.25 This was explained based on the hypothesis that the major part of the conformational freedom of the alkyl chains is gained upon melting, i.e., at TK-D, and isotropization causes further disordering of the stacked aromatic cores. Thus, as the volume fraction of the alkyl chains in the molecule increases, one may expect ∆Si to decrease.26 To make a meaningful comparison, ∆Si is plotted versus the total number of alkyl chain carbons (Cn), for all three series R442n, R482n, and symmetric R6n, in a single plot (Figure 9). It is clear from this plot that while ∆Si does decrease with increase in Cn, the decrease due to (25) A similar behaviour has also been observed in other discotics based on triphenylene,27 rufigallol,18,22 and benzene.28 (26) As was rightly pointed out by one of the reviewers, if isotropization corresponded only to the unstacking of the aromatic cores, one may expect the molar isotropization entropies ∆Si to be independent of the alkyl chain length. However, the experimentally observed drop in ∆Si in several symmetric discotic liquid crystals,25 appears to suggest that in these systems the alkyl chain carbon atoms close to the aromatic core may, in fact, be more conformationally restricted in the discotic phase than in the isotropic phase and hence also contribute to the observed change in entropy, ∆Si. This would also explain its rather steep dependence at smaller values of n and a tapering off at higher values (see Figure 9).
Asymmetric Discotic Liquid Crystals
Figure 10. Plot of disk area versus the total number of alkyl chain carbon atoms (Cn) for rufigallol hexaethers; (O) R6n, (0) R482n, (4) R442n.
Scheme 2. Schematic Representation of the Rufigallol Core and the Alkyl Segment Shell
the presence of asymmetry is far more drastic. Furthermore, in the case of the R482n series, a reverse trend is observed for n < 8, i.e., ∆Si is seen to decreases with decrease of Cn. This obviously implies that the effect of the molecular asymmetry dominates that due to decrease in the relative alkyl chain content, causing a net reduction in ∆Si. A well-accepted model for the formation of the discotic columnar mesophase is one where the disks stack one on top of another to form columns which then arrange themselves in a 2D array to form either a hexagonal (Dh), rectangular (Dr) or oblique (Dob) columnar mesophases. As already stated, it is also expected that the alkyl chains are in a fairly disordered liquidlike arrangement in the mesophase. A schematic representation of a single column formed by the asymmetric discotic molecules, as seen along the columnar axis, will therefore show two concentric circles due to the staggered arrangement the discotic molecules (Scheme 2). The inner circle would represent the rigid aromatic core, and the shell will contain the flexible alkyl chain segment. This simple model is supported by the fact that the diameter of the column (D) is insensitive to presence of asymmetry, but depends only on the total alkyl carbon count Cn. Furthermore, it may also be expected that the projected area of the column, πD2/4, will vary linearly with Cn. A plot of Cn versus πD2/4 (Figure 10) does indeed show a linear behavior. What is more interesting is that the data points for all the discotic molecules, immaterial of whether they belong to the asymmetric and the symmetric series, lie on the same straight line, which can be represented by the equation, area ) 73.4 + 7.54Cn. The intercept of 73.4 Å2 represents the area of the rigid core, which when
Chem. Mater., Vol. 9, No. 7, 1997 1637
calculated for rufigallol, using the longest molecular distance (as shown in Scheme 2) as the diameter, gave exactly the same value of 73.4 Å2.29 This excellent agreement further validates the simple geometric model of the columnar mesophase. The slope of the straight-line fit, on the other hand, represents the incremental area of the shell as a result of inclusion of a single methylene unit. Using the average interdisk distance of 3.6 Å (obtained from the X-ray data), one can calculate the incremental volume due to a single methylene unit as being 27.1 Å3. In simple linear hydrocarbons, the volume occupied by a single methylene unit has been estimated to lie between 24.5 and 26.5 Å3 in the cystalline solid (depending on the type of packing) and about 29.6 Å3 in the liquid state.30 The corresponding value in the discotic columnar mesophase, on the other hand, is estimated to be about 27.1 Å3, which is considerably smaller than that observed for liquid n-alkanes but significantly larger than that observed in the crystalline solid. Thus, it appears that the alkyl side chains do retain some degree of order in the columnar mesophase, which is consistent with the vibrational spectroscopic studies carried out on discotic benzene hexaalkanoate samples.4 In conclusion, we have shown that asymmetric hexaethers of rufigallol can be readily synthesized using a two-step alkylation process. The symmetric tetraethers intermediates can easily be prepared in large quantities, thereby making them excellent candidates for the synthesis of main-chain discotic polymers. Work along these lines is currently underway. While asymmetry due to the presence of unequal alkyl chain lengths in the discotic liquid-crystalline molecules, it appears, does reduce the extent of order in the columnar mesophase (as seen by a decrease in ∆Si), this reduction in order is not reflected in the intercolumnar distance. The latter is sensitive only to the total number of alkyl chain carbon atoms and not the manner in which they are linked. Hence, the decrease in order in the discotic columnar mesophase due to asymmetry must be intracolumnar in origin, although the exact nature of this disorder still remains to be understood. Further detailed X-ray studies would be required to probe the differences, in the short-range ordering within the columns, between the symmetric and asymmetric systems. The question of whether this insensitivity, of both the nature of the mesophase and the intercolumnar distances, to the presence of asymmetry can be generalized to other asymmetric discotic liquid crystals, such as those based on other cores, like triphenylenes, and also to ones that have branched alkyl chains, remains to be seen. Acknowledgment. We would like to thank DST, New Delhi, for providing the funds for the purchase of the DSC. We would also like to thank Drs. N. V. Madhusudhana and B. K. Sadhashiva for helping with the polarized light microscopy studies and also for the many useful discussions. We thank Dr. C. D. Bain for the useful discussions, specifically in relation to the methylene unit volumes in parafinic systems. CM970019+ (27) Destrade, C.; Malthete, M. C.; Tinh, N. H. J. Phys. (Paris) 1979, 40, 3. (28) Chandrasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana, 1977, 9, 471. (29) Value obtained using the Insight molecular modeling package. (30) Small, D. M. Physical Chemistry of Lipids; Plenum: New York, 1986.
