Synthesis and characterization of naphthalene-substituted triphenylene discotic liquid crystals

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Synthesis and characterization of

naphthalene-substituted triphenylene discotic liquid crystals

a

Department of Applied Chemistry, National Chiao Tung University Hsinchu, Taiwan 30050, Taiwan ROC

Published online: 06 Aug 2010.

To cite this article: Long-Hai Wu , N. Janarthanan & Chain-Shu Hsu (2001) Synthesis and characterization of naphthalene-substituted triphenylene discotic liquid crystals, Liquid Crystals, 28:1, 17-24, DOI: 10.1080/026782901462337

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Synthesis and characterization of naphthalene-substituted

triphenylene discotic liquid crystals

LONG-HAI WU, N. JANARTHANAN and CHAIN-SHU HSU* Department of Applied Chemistry, National Chiao Tung University Hsinchu,

Taiwan 30050, Taiwan ROC (Received 2 March 2000; accepted 23 May 2000)

A series of naphthalene-substituted triphenylene liquid crystals (LCs), viz. triphenylene-2,3,6,7,10,11-hexay l hexakis(6-alkenyloxy-2-naphthoate)s (HTPnN compounds) were syn-thesized and characterized. Alkenyloxy groups containing three to eleven carbon atoms were used as peripheral spacers in these liquid crystals. The discotic liquid crystals synthesized trap one to three water molecules depending on the length of their peripheral spacers. DiŒerential scanning calorimetry, polarizing optical microscopy and X-ray diŒraction measurements conŽ rm both nematic (N_d) and rectangular disordered columnar (Col_rd) phases for most of the synthesized discotic LCs. The clearing temperatures of these discotic LCs increase with increasing peripheral spacer length. A hybrid aligned HTP9N sample was investigated to evaluate its optical performance. Retardation values of this hybrid sample decrease with increasing wavelength and increase with increasing measuring angle.

1. Introduction (2) an ideal phase match with the LC employed so that good compensation results can be achieved over the Discotic liquid crystals were Ž rst reported by

entire visible region; (3) high optical transparency; and

Chandrasekhar et al. in 1977 [1]. Since then, many

(4) good uniformity [16]. In order to achieve the above liquid crystalline materials having disc-like shapes

requirements, various kinds of discotic LC with diŒerent have been synthesized and the Ž eld was reviewed by

birefringences are needed. Furthermore, according to the Chandrasekhar in 1993 [2]. Generally, traditional

literature [13–15], a uniform discotic compensation Ž lm hexa-alkyl substituted triphenylene discotic compounds

can only be prepared by aligning a discotic LC material [3–7] have the tendency to become polycrystalline at

below its N_d phase. Therefore, an ideal discotic LC

room temperature and form a columnar mesophase in

material should exhibit a wide N_dtemperature range so

which the disc-like mesogens are stacked closely and

as to have a wide processing window. So far, there are regularly within columns [8, 9] at elevated temperatures.

only a very limited number of discotic LCs that exhibit The situation changed in 1981 when

triphenylene-N_d phases and their N_d phase temperature ranges are

2,3,6,7,10,11-hexay l hexakis(4-n-alkoxy benzoate)s were

rather narrow.

found to exhibit N_d phases [10]. The eŒects of bulky

The aim of this study was to synthesize a new series lateral substituents on the mesomorphism of these

tri-of naphthalene-substitute d triphenylene derivatives, the phenylene benzoates were thoroughly investigated by

triphenylene-2,3,6,7,10,11-hexay l

hexakis(6-alkenyloxy-Goodby et al. [11, 12].

2-naphthoate )s. The naphthalene groups were intro-Recently nematic discotic liquid crystalline materials,

duced into the triphenylene core to alter the birefringence which exhibit negative birefringence, have demonstrate d

and facilitate the formation of the N_d phase [17].

their potential for applications as compensation Ž lm

The alkenyloxy groups were used as peripheral spacers for wide viewing angle liquid crystal displays (LCDs)

to manipulate the phase transitions and provide the [13–15]. Some of the desired properties for an ideal

possibility for further oxidation of the terminal vinyl optical compensation Ž lm for LCDs are: (1) an

appro-groups to form epoxy derivatives which can then be

priate phase retardation value,dDn (where d denotes the

photopolymerized to yield crosslinked networks. The

Ž lm thickness and Dn represents the Ž lm birefringence) _{mesomorphic properties were characterized by diŒerential}

ranging from l/8 to l/2, depending on the application;

scanning calorimetry (DSC), polarizing optical micro-scopy (POM) and X-ray diŒraction. The sample of HTP9N synthesized was aligned as a monodomain and * Author for correspondence;

e-mail: [email protected] its optical properties were also investigated. L iquid Crystals ISSN 0267-829 2 print/ISSN 1366-585 5 online © 2001 Taylor & Francis Ltd

http://www.tandf.co.uk/journals

18 L.-H. Wuet al.

2. Experimental (TMS) as internal standard. A Seiko/SSC/5200 diŒer-ential scanning calorimeter was used for determining 2.1. Materials

Allyl bromide, 6-hydroxy-2-naphthoi c acid, veratrole thermal transitions. Heating and cooling rates were

10ßC minÕ 1. A Carl-Zeiss Axiophot polarizing

micro-and all other reagents were obtained from Aldrich micro-and

used as received. 3-Bromo-1-butene , 4-bromo-1-pentene , scope equipped with a Mettler FP 82 hot stage and a

Mettler FP 80 central processor was used to analyse the 5-bromo-1-hexe ne, 6-bromo-1-hept ene, 7-bromo-1-octe ne

8-bromo-1-nonene , 9-bromo-1-decen e and 10-undecen- thermal transitions and observe the mesomorphase

textures. X-ray diŒraction measurements were made with 1-yl tosylate were prepared according to literature

pro-cedures [18, 19]. 6-Alkenyloxy-2-naphthoi c acids (nNA a Rigaku powder diŒractometer using nickel-Ž ltered

CuK

a radiation. Optical properties were studied using

compounds, n5 1–9) were synthesized by esteriŽ cation

of 6-hydroxy-2-n aphthoic acid with alkenyl halide accord- an Otsuka Multichannel retardation measuring system,

RETS-2000 (Osaka, Japan). ing to a literature procedure reported by our laboratories

[20]. 2,3,6,7,10,11-Hexahydroxytriphenylen e was

pre-pared according to the literature method reported by 2.3. Synthesis of triphenylene-2,3,6,7,10,11-hexay l

hexakis (6-alkenyloxy-2-naphthoate) s (HT PnN

Frederiket al. [21].

compounds, n5 1–9)

All HTPnN compounds were synthesized by the

2.2. T echniques

1_{H NMR spectra of all the compounds synthesized esteriŽ cation of 2,3,6,7,10,11-hexahydroxytriphenylen e}

with the corresponding 6-alkenyloxy-2-naphthoi c acids. were recorded on a Varian VXR-300 NMR spectrometer

(300 MHz) using CDC1₃as solvent and tetramethylsilan e An example for the synthesis of HTP9N is given below.

Figure 1. Synthetic routes to the naphthalene-substituted discotic liquid crystals HTPnN.

6-10-Undecenyloxy-2-naphthoi c acid (5 g, 0.015 mol ) 3. Results and discussion

3.1. Mesomorphic behaviour of 6-alkenyloxy-2-naphthoi c

was allowed to react with a two-fold excess of thionyl

chloride containing 5 drops of N,N-dimethylformamide acids (nNA compounds, n5 1–9)

Phase transition temperatures and phase transition in 50 ml of methylene chloride at room temperature for

2 h. The solvent and excess of thionyl chloride were enthalpies of the nNA compounds are summarized in

table 1. All the 6-(alkenyloxy)-2-naphthoi c acids syn-removed under reduced pressure to furnish the crude

acid chloride. The crude product was then dissolved in thesized have hydrogen bonded dimeric structures.

Compounds with short spacer lengths (1NA and 2NA) 50 ml of methylene chloride and added slowly to an

ice-cold solution of 2,3,6,7,10,11-hexahydroxytriphenylen e exhibit only a monotropic nematic phase. The Ž ve

compounds with medium spacer lengths (3NA–7NA) (0.661 g, 0.025 mol) and triethylamine (2.23 g, 0.022 mol)

in 50 ml of methylene chloride. The mixture was stirred give enantiotropi c nematic phases, and the other two

compounds with longer spacers (8NA and 9NA) give a for 12 h and then the solvent was removed by rotary

evaporation. The solid obtained was dissolved in ethyl smectic A phase besides a nematic phase. Figure 2

depicts the DSC thermogram s of 8NA. As can be seen,

acetate and washed with 5% aqueous K₂CO₃and water

and dried with anhydrous MgSO₄. Ethyl acetate was compound 8NA displays enantiotropi c nematic and

smectic phases. The clearing temperature decreases as removed under reduced pressure and the crude product

was puriŽ ed by column chromatograph y (silica gel, the number of carbon atoms increases as shown in

Ž gure 3, and a typical odd–even eŒect is also evident.

CH₂Cl₂as eluent) to yield 4.0 g (85%) of yellowish solid.

1_{H NMR (CDC1}

3, 300 MHz) d (ppm)5 1.32 (m, 72H,

( CH₂)₆ ) 1.76 (m, 12H, O CH₂ CH₂ ); 2.03 (m, 12H, 3.2. Characterization and thermal behaviour of

triphenylene-2,3,6,7,10,11-hexay l hexakis

(6-alkenyloxy-CH₂ CH CH₂ ); 3.89 (t, J5 6.3 Hz, 12H, O CH₂ );

4.94 (m, 12H, CH₂ CH ); 5.80 (m, 6H, CH₂ CH ); 6.59 2-naphthoate) s (HT PnN compounds, n5 1–9)

The1H NMR spectrum of HTP8N was representative

(s, 6H, H_arom); 6.76 (d, J5 8.9 Hz, 16H, H_arom); 7.01

(d,J5 8.8 Hz, 6H, H_arom); 7.15 (d,J5 9.0 Hz, 6H, H_arom); and showed a peak at 1.6 ppm which belongs to the

protons of the included water molecules. This peak did

7.76 (d, J5 9.2 Hz, 6H, H_arom); 8.24 (s, 6H, core); 8.54

(s, 6H, H_arom). not disappear when distilled, dry CDC1₃ was used as

Table 1. Phase transitions and transition enthalpy changes fornNA compounds: Cr5 crystalline, SmA5 smectic A, N5 nematic, I5 isotropic.

2nd heating Compound Transition temp./ßC (enthalpy change/kcal molÕ 1) 1st cooling

1NA Cr 206.6 (24.8) I I 206.4 (Õ _{0.05) N 152.4 (}Õ _{28.5) Cr} 2NA Cr 199.9 (33.1) I I 187.5 (Õ _{1.8) N 179.9 (}Õ _{28.5 ) Cr} 3NA Cr 151.5 (20.5) N 197.7 (3.3) I I 194.9 (Õ _{2.7) N 134.2 (}Õ _{20.4 ) Cr} 4NA Cr₁ 30.0 (0.7) Cr₂148.0 (12.1) N 191.1 (2.0) I I 184.7 (Õ _{2.3) N 133.7 (}Õ _{16.9 ) Cr} 2 122.4aCr1 5NA Cr₁ 34.0 (4.4) Cr₂148.0 (12.1) N 191.2 (2.0) I I 188.0 (Õ _{2.0) N 133.5 (}Õ _{11.2 ) Cr} 2 3.8 (Õ 4.3 ) Cr1 6NA Cr₁ 57.9 (6.5) Cr₂105.7 (0.2) Cr₃ 147.6 (15.3) N 180.9 (1.9) I I 178.0 (Õ _{2.2) N 133.9 (}Õ _{14.9 ) Cr} 3 94.2 (Õ 0.1 ) Cr2 43.7 (Õ 7.3 ) Cr1 7NA Cr₁ 34.0 (1.4) Cr₂102.4 (13.7) Cr₃139/1 (13.0) N 181.2 (2.2) I I 178.0 (Õ _{2.2) N 128.8 (}Õ _{12.7 ) Cr} 3 26.4 (Õ 13.6 ) Cr2 Õ 0.2 (Õ 0.3 ) Cr1 8NA Cr 129.5 (12.0) SmA 133.9 ( 0.8) N 174.2 (2.1) I I 172.2 (Õ _{1.8) N 131.3 (}Õ _{0.8) SmA 115.7 (}Õ _{3.9) Cr} 9NA Cr₁ 120.6 (30.2) Cr₂126.5a SmA 137.2 (0.4) N 171.0 (1.9) I I 168.3 (Õ _{2.0) N 133.4 (}Õ _{0.6) SmA 104.9 (}Õ _{10.3) Cr} 3101.7aCr259.3 (Õ 3.4) Cr1 a_{Overlapped transitions.}

20 L.-H. Wuet al.

Table 2. Elemental analysis data for HTPnN compounds. Compound Formula Calculated/% Found/%

HTP1N C₁₀₂H₇₂O₁₈H₂O C: 76.49 a H: 4.66 HTP2N C₁₀₈H₈₄O₁₈H₂O C: 76.85 C: 76.50 H: 5.14 H: 5.04 HTP3N C₁₁₄H₉₆O₁₈2H₂O C: 77.27 C: 77.10 H: 5.57 H: 5.77 HTP4N C₁₂₀H₁₀₈O₁₈2H₂O C: 76.90 C: 77.82 H: 6.02 H: 5.91 HTP5N C₁₂₆H₁₂₀O₁₈2H₂O C: 77.28 C: 77.46 H: 6.38 H: 6.40 HTP6N C₁₃₂H₁₃₂O₁₈3H₂O C: 77.62 C: 77.81 H: 6.71 H: 6.80 HTP7N C₁₃₈H₁₄₄O₁₈3H₂O C: 77.28 C: 77.21 H: 7.05 H: 7.01 HTP8N C₁₄₄H₁₅₆O₁₈3H₂O C: 77.60 C: 77.70 H: 7.33 H: 7.24 HTP9N C₁₅₀H₁₆₈O₁₈2H₂O C: 78.50 C: 78.38 H: 7.55 H: 7.53 Figure 2. DSC traces for 8NA (from top to bottom: Ž rst

cooling, second heating runs). _a

No satisfactory data are available.

Col_rd mesophases except for HTN1N, which shows no

mesophase on melting with decomposition. It is found that the clearing temperatures generally decrease with increasing number of carbon atoms in the peripheral alkenyl chains.

Compounds HTP2N–HTP6N present a similar thermal behaviour. Figure 4 depicts a representative DSC thermogram for compound HTP6N. The Ž rst

heating curve shows a melting transition at 170.3ßC, a

Col_rdto N_d transition at 201.0ßC and a N_d to isotropic

transition at 273.3ßC. The Ž rst cooling scan reveals an

isotropic to N_dtransition at 270.1ß C, and an N_dto Col_rd

transition at 131.2ß C, but no crystallization transition is

shown. When the sample was further cooled to room

temperature, the Col_rdtexture was frozen in and did not

Figure 3. Clearing temperatures ofnNAs as a function of n. _{change for more than one week. The second heating}

scan shows only a Col_rdto N_d transition at 198.2ßC and

an N_dto isotropic transition at 273.1ßC. Figure 5 depicts

solvent. The elemental analysis results for the HTPnN

compounds listed in table 2 conŽ rmed the inclusion of the N_d and Col_rdtextures shown by HTP6N.

HTP7N–HTP9N reveal similar thermal behaviours.

the water molecules. It is clear that the HTPnN

com-pounds include one to three water molecules depending Figure 6 presents the representativ e DSC thermogram for

HTP8N. The Ž rst heating scan shows a recrystallization on their spacer length. As a general trend, the number

of included water molecules Ž rst increases and then transition at 155.2ßC, a Col_rdto N_d phase transition at

174.2ßC and an N_d to isotropic phase transition of low

decreases as the carbon number of the peripheral alkenyl

chain increases. Frederik et al. [21] also reported this enthalpy at 224.9ßC, whereas the Ž rst cooling scan shows

only an isotropic to N_dphase transition at 223.6ßC. The

water-inclusion phenomenon.

Table 3 summarizes the phase transition temperatures second heating scan looks very similar to the Ž rst

heat-ing scan except for its broader recrystallization peak.

and phase transition enthalpies for the HTPnN

com-pounds. All these discotic compounds exhibit N_d and Figure 7 (a) shows the typical nematic schlieren texture

Table 3. Phase transitions and transition enthalpy changes for HTPnN compounds: Col_{rd 5} rectangular disorder and columnar mesophase, N_{d 5} discotic nematic mesophase, I5 isotropic phase.

2nd heating Compound Transition temp./ßC (enthalpy change/kcal molÕ 1) 1st cooling

HTP1N No transition was observed before decomposition. HTP2N Col_rd249.9 (1.67) N_d330.1aI I 325.1aN_d214.8 (Õ _{0.1 ) Col} rd HTP3N Col_rd227.4 (1.67) N_d327.5a_I I 324.2a_N d178.6 (Õ 2.00 ) Colrd HTP4N Col_rd227.4 (0.86) N_d288.8a_I I 285.7aN_d146.8 (Õ _{1.08 ) Col} rd HTP5N Col_rd222.0 (2.15) N_d286.5 (0.04) I I 280.1 (Õ _{0.04 ) N} d164.2 (Õ 2.37 ) Colrd HTP6N Col_rd198.2 (2.24) N_d273.1 (0.05) I I 270.1 (Õ _{0.04 ) N} d131.2 (Õ 2.89 ) Colrd HTP7N Col_rd174.2 (1.84) N_d235.7 (0.05) I I 233.8aN_d HTP8N Col_rd174.2 (1.84) N_d224.9 (0.05) I 233.6aN_d HTP9N Col_rd155.5 (0.45) N_d217.6 (0.05) I I 214.2a_N d a_{Transition observed via POM.}

peak in the small angle region. The wide angle diŒraction peak relates to the lateral distance between triphenylene cores, while the small angle diŒraction peak is attributed to the diameter of the triphenylene core. This conŽ rms

the formation of an N_d phase. By lowering the

temper-ature from 210 to 170ßC, the small angle diŒraction

peak splits into two peaks at 29.1 and 35.4 AÃ as shown

in curve B. In addition, the large angle region shows only a very broad diŒusion peak. This provides evidence for the formation of a rectangular disordered columnar phase [21].

In comparison with triphenylene-2,3,6,7,10,11-hexay l

hexakis (4-n-alkoxybenzoate )s, it is obvious that the

incorporation of six naphthalene moieties facilitates the Figure 4. DSC traces for HTP6N.

formation of the discotic nematic mesophase. Tinhet al.

[10] reported that triphenylene-2,3,6,7,10,11-hexay l exhibited by HTP8N. Upon cooling to room

temper-hexakis(4-decyloxybenzoate ) gave an N_d phase for a

ature, the nematic texture was frozen and recrystallized

range of only 21.0ßC. However, the corresponding

hexa-during the second heating scan, see Ž gure 7 (b). The high

naphthalene substituted triphylene discotic material molecular mass, which makes these discotic compounds

HTP8N gives a much broader N_drange of over 250.0ß C

turn into glassy states, may be responsible for the

as shown by the cooling scan process. inhibition of columnar mesophase formation during the

cooling processes.

The phase assignments for the HTPnN compounds

3.3. Optical anisotropy measurement

were conŽ rmed by X-ray diŒraction. Figure 8 shows the

Some optically anisotropic properties of HTP9N were temperature dependent X-ray diŒraction patterns obtained

next investigated. A small amount (0.1 g) of HTP9N was from a powder sample of HTP8N. Curve A, which was

dissolved in 0.5 ml of methyl ethyl ketone and

spin-obtained at 210ßC, shows a very broad diŒraction peak

in the wide angle region and a very strong diŒraction coated onto a rubbed polyimide surface. The sample

22 L.-H. Wuet al.

(a)

(b) (a)

(b)

Figure 7. Optical textures of HTP8N at (a) 187ßC, N_d; Figure 5. Optical textures of HTP6N at (a) 257ßC, N_d;

(b) 160ßC, frozen N_d(crossed polarizers, magniŽ cationÖ 200). (b) 160ßC, Col_rd(crossed polarizers, magniŽ cationÖ 200 ).

and azimuthal angles (0–Ô 50ß ). This monodomain

hybrid arrangement remained unchanged at room

tem-perature (about 25ß C) for over two months even though

it was not a crosslinked network.

Figures 11 and 12 depict, respectively, the

angle-dependent and wavelength-dependen t retardations

measured for the hybrid aligned HTP9N sample (with

azimuthal angle5 0ß ). Its retardation values are in the

range 20–90 nm. The retardation values increase with increasing measuring angle and decrease with increasing wavelength.

This mono-domai n aligned HTP9N sample shows diŒerent retardation values at a Ž xed angle from diŒerent directions. Figure 13 shows that the retardation value Figure 6. DSC traces for HTP8N. decreases gradually from 0 to Ô 50ß azimuthal angle

(with measuring angle5 50ß ). This indicates that the

HTP9N molecules are arranged in a biaxial way in

was annealed at 180ß C for 5 min to achieve a

mono-domain hybrid arrangement as shown in Ž gure 9. This which the discotic molecule has its plane inclined from

the plane of polyimide substrate at an angle varying hybrid structure was conŽ rmed by optical measurements

(see Ž gure 10) at several diŒerent viewing angles (10–60ß ) along the direction of the depth of the anisotropic layer.

Figure 10. Experimental set-up for measurement of optical properties.

Figure 8. X-ray power diŒractograms of HTP8N at (A) 210ßC, N_d; (B) 170ßC, Col_rd.

Figure 11. Retardation values of HTP9N for diŒerent wavelengths as a function of measuring angle.

Figure 9. Optical texture of monodomain prepared from HTP9N (crossed polarizers, magniŽ cationÖ 200).

From the above results, it is clear that these naphthalene-containing discotic liquid crystals can be used in pre-paring optical retardation Ž lms for improving the viewing angle of LCDs.

In order to form a cross-linked network, the HTPnN

compounds were oxidized with m-chloroperoxybenzoic

acid (MCPBA) to yield the epoxy derivatives which might then be photopolymerized to form a crosslinked network. Unfortunately, we only obtained mixtures of

the epoxy derivatives, and pure materials could not be _{Figure 12. Retardation values of HTP9N for diŒerent}

measuring angles as a function of wavelength. obtained from these epoxide mixtures.

24 Naphthalene-substitute d triphenylene L Cs

References

[1] Chandrasekhar, S., Sadashiva, B. K., and Saresh, K. A., 1977,Pramana, 9, 471.

[2] Chandrasekhar, S., 1993, L iq. Cryst., 14, 3.

[3] Collard, D. M., and Lillya, C. P., 1991, J. org. Chem.,

56, 6064.

[4] Boden, N., Bushby, R. J., Cammidge, A. N., and Martin, P. S., 1995, J. mater. Chem., 5, 1857.

[5] Rego, J. A., Kumar, S., Dmochowski, I. J., and Ringsdorf, H., 1996, Chem. Commun., 1031.

[6] Boden, N., Bushby, R. J., Cammidge, A. N., Duckworth, S., and Headdock, G., 1997, J. mater. Chem., 7, 601.

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[8] Levelut, A. W., 1979, J. Phys. L ett., 40, L81.

[9] Destrade, C., Tinh, N. H., Gasproux, G., Figure 13. Retardation values of HTP9N measured from diŒer- Malthete, J., and Levelut, A. M., 1981, Mol. Cryst.

ent azimuthal angles (measuring angle: 50ß, wavelength: liq. Cryst., 71, 111.

550 nm). [10] Tinh, N. H., Gasparous, H., and Destrade, C., 1981, Mol. Cryst. liq. Cryst., 68, 101.

[11] Hindmarsh, P., Hird, M., Styring, P., and

4. Conclusion

Goodby, J. W., 1993, J. mater. Chem., 3, 1117. A new series of naphthalene-substitute d LCs, viz.

[12] Hindmarsh, P., Watson, M. J., Hird, M., and

triphenylene-2,3,6,7,10,11-hexay l hexakis(6-alkenyloxy- _{Goodby, J. W., 1995, J. mater. Chem., 5, 2111.}

2-naphthoate )s were synthesized and characterized. All _{[13] Favre-Nicolin, C. D., and Lub, J., 1996,}

Macromolecules, 29, 6143.

the HTPnN compounds prepared, except HTP1N, gave

[14] Mori, H., and Bos, P. J., 1998, SID, 98, Dig., 830.

N_d and Col_rd phases. The results demonstrate that

[15] Mori, H., Itoh, Y., Nishiura, Y., Nakamura, T., and introducing six naphthalene groups into the triphenylene

Shinagawa, Y., 1997,SID, 97, Dig., 941.

core widens the mesomorphic temperature range. Most _{[16] Wu, S. T., 1995, Mater. Chem. Phys., 42, 163.}

of the HTPnN compounds obtained have very wide tem- [17] Praefcke, K., Kohne, B., and Singer, D., 1990, L iq.

Cryst., 7, 589.

perature range N_d phases. They could be easily aligned

[18] Percec, V., and Hahn, B., 1989, J. polym., Sci: polym. on glass substrate s which were coated with unidirectionall y

Chem., 27, 2367.

rubbed polyimide; a mono-domai n hybrid arrangement _{[19] Percec, V., Hsu, C. S., and Tomazos, D., 1988, J. polym.,}

was then obtained. These HTPnN compounds can be _{Sci: polym. Chem., 26, 2047.}

mixed with other UV-curable discotic monomers to [20] Lin, J. L., and Hsu, C. S., 1993, Polym. J., 25, 153.

[21] Frederik, C. K., Niels, C. S., Walther, B., and form mixtures for the fabrication of compensation Ž lms

Klaus, B., 1997, Synthesis, 1285. with desired retardation values.

[22] Zamir, S., Wachtel, E. J., Zimmermann, H. Z., and Dai, S., 1997, L iq. Cryst., 5, 689.

The authors are grateful to the National Science _{[23] Collings, P. C., and Hird, M., 1997, Introduction to}

Council of the Republic of China for Ž nancial support. _{L iquid Crystals (London: Taylor & Francis), pp. 79–92.}