: NSC 92-2216-E-011 -012 : 920801-930731
: :
Absract
Three novel homologous series of discotic liquid crystals, 2,3,6,7-tetra(alkyloxy)-10,11- dinitrotriphenylene 3, 11,12-bis[3,4-dialkoxy)phenyl]- 2 , 3 , 6 , 7 - t e t r a ( a l k o x y ) - 1 0 , 1 3 - diazabenzo[b]triphenylenes 5 ( n =4 ~ 10) and 2,3,6,7,12,13,16,17-octa(alkyloxy)tetrabenzo[a,c,j,l]- 9,20-diazaanthracenes (n = 4 ~ 10) 6 with alkyl side chain lengths varying from 4 to 10 carbons, have been synthesized and characterized. Moreover, Cationic polymerization of the vinyl ether monmoer, 2,67- tris[3,4-di(octyloxy)phenyl]-3-[4-octyloxy-3-(ω- vinyloxyalkyloxy)phenyl]quinoxzline 9 initiated with CF3SO3H/(CH3)2S leads to novel tetraphenylquinoxaline-based discotic columnar side- chain liquid crystalline polymers 10
.
X-ray diffraction studies confirmed the mesophase assignment made by optical microscopy and DSC.The wide-angle region displays halos indicating liquid like correction between the rigid cores.
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Columnar liquid crystals, quinoxaline side-chain liquid crystalline polymers Introduction
Discotic liquid crystals are well known materials in the supramolecular chemistry due to their self- assembly characteristics such as columnar architectures. Since these unique structures normally enhance carrier mobility along column axis.
Therefore, the potentional applications of discotic liquid crystals in conducting and photoconducting systems, optical data storage, light emitting diodes, photovoltaic solar cells, gas sensors and other devices have been envisaged.1) Hexaalkoxytriphenylenes constitute one of the largest and most important classes of molecules that exhibit columnar mesophases in the field of discotic liquid crystals.
Encouraged by these results, we anticipated that functionalized dibenzo[a,c]phenazine derivatives might give access to a new class of discotic liquid crystals despite their possessing aromatic cores with one more heterocyclic ring structure of pyrazine than triphenylene. Although the charge generating, transporting and electroluminescent properties of several phenazine derivatives have recently been reported2), no attempts have been made to achieve liquid crystalline members of this class. We here report our achievements towards this goal.
Experimental Section
1,2-dibromo-4,5-dinitrobenzene, (1). 1,2-
Dibromobenzwne (40g, 0.17 mol) was added over 15 min to an ice-cold, stirring mixture of fuming nitric acid (100%, 114 mL) and sulfuric acid (98%, 70 mL). The solution was hwated under reflux for 10 h, cooled and poured into water. The solid precipitatethat slowly formed was filtered off after 48h and washed on the filter with water. The crude product was recrystallized from acetic acid to yield 35g (63.3 %) of 1.2-dibromo- 4,5-dinitrobenzene, m.p., 113 -115 oC.
1,2-Bis(3,4-dialkyloxyphenyl)-3,4-dinitrobenzene, (2) (n = 8) . A mixture of 1,2-dibromo-4,5-dinitrobenzene (0.3g, 0.992 mmol), (1) in toluene (8 mL) and 28 % aq.
Na2CO3 (1.5 mL) was stirred under a positive pressure of nitrogen. Tetrakis(triphenylphosphine)palladium (0) (012g, 1.0 mmol) was added followed by the 3.,4- di(octyloxy)phenylboronic acid (1.0g, 2.64 mmol) dissolved in THF (4 mL). The resulting mixture was heated under reflux for 24 h and allowed to cool to room temperature. The organic phase was seperated and the aqueous phase was extracted with ethyl acetate. The combined organic layers were washed with water, brine and dried (Na2SO4). The solvent was removed in vacuo and he resulting residue was subjected to silica gel column chreomatography (EtOAc: hexane = 1 : 20, Rf
= 0.30) to give 0.63 g of a yellow solid. Yield 76.0%.
1H NMR (δ, ppm, CDCl3): 0.88~0.92 (m, 12H, CH3);
1.30~1.40 (m, 32H, O(CH2)3(CH2)4CH3); 1.49~1.70 (m, 8H, O(CH2)2CH2); 1.80~1.85 (m, 8H, OCH2CH2); 3.69~4.00 (m, 8H, OCH2); 6.59 (s, 2H, Ar-H); 6.75 (s, 2H, Ar-H); 6.85 (s, 2H, Ar-H); 7.98 (s, 2H, Ar-H). 13C-NMR (δ, ppm, Benzene-d8): 14.03 (CH3); 22.61, 22.64 (CH3CH2); 25.93, 25.97, 26.01, 29.0, 29.17, 29.22, 29.27, 29.29, 29.34, 31.78 (CH2);
69.18, 69.32, 69.71, 69.01, 69.24 (ArOCH2); 113.24, 114.95, 121.0, 126.96, 129.88, 141.01, 145.59. 148.86, 149.64 (Aromatic carbons). MS(EI, 70 ev) [m/z rel.
intensity)]: 832.7 (M+, 90).
2,3,6,7-Tetra(alkyloxy)-10,11-dinitrotriphenylene, (3) (n = 8). To a stirred solution of 0.455 mmol of 1,2- Bis(3,4-dialkyloxyphenyl)-3,4-dinitrobenzene, (2) and 0.37 g (0.15 mL, 2.0 mmol) of boron trifluoride etherate in 40 mL of anhydrous dichloromethane was added 0.21g (1.66 mmol) of vanadium (V) oxyfluoride. The reaction mixture was stirred for 1 h at ambient temperature. To the resulting reaction mixture, was added 10 mL of water and then it was extracted with dichloromethane. The organic layer was washed with water, dried over sodium sulfate and reduced in vacuuo.
The resulting dark brown residue was subjected to silica gel column chreomatography (ethyl acetate : hexane = 10 : 1, Rf = 0.31) to give 031 g of a yellow solid. Yield 82.0 %.
11 , 1 2 - b i s [ 3 , 4 - d i ( a l k y o x y ) p h e n y l ] - 2 , 3 , 6 , 7 - tetra(alkyloxy)-10-13-diazabenzo[b]triphenylene. (5) (n = 8). 1H NMR (δ, ppm, Benzene-d8): 1.12~1.14 (m, 24H, CH3); 1.58 (m, 32H, CH3CH2(CH2)4); 1.70 (s, 16H, CH3CH2); 2.65~3.08 (m, 16H, ArOCH2CH2);
4.23~4.38 (m, 16H, Ar-OCH2). 13C-NMR (δ, ppm, Benzene-d8): 14.21 (CH3); 22.796 (CH3CH2); 28.84, 28.90, 29.76, 29.85 (CH2); 69.18, 69.32, 69.71, 69.79 70.20 (ArOCH2); 129.73 (CH); 139.43 (Ck); 141.66 (Cj); 152.37 (CI). MS(FAB) [m/z rel. intensity)]:
1458.1 (M+, 70); 1345.9 ([M-C8H17)]+,10); 1133.7 ([M-C24-H51]+,5).
2,3,6,7,12,13,16,17-octa(octyloxy[a,c,j.l]-9,20- diazaanthracene (6) discotic columnar liquid crystals , (7) discotic columnar liquid crystals (R = CnH2n+1; n
= 4 ~ 10). 1H NMR (δ, ppm, CDCl3): 0.88~0.92 (m, 12H, CH3); 1.30~1.40 (m, 32H, O(CH2)3(CH2)4CH3);
1.49~1.70 (m, 8H, O(CH2)2CH2); 1.80~1.85 (m, 8H, OCH2CH2); 3.69~4.00 (m, 8H, OCH2); 6.59 (s, 2H, Ar-H); 6.75 (s, 2H, Ar-H); 6.85 (s, 2H, Ar-H); 7.98 (s,
2H, Ar-H). 13C-NMR (δ, ppm, Benzene-d8): 14.12, 14.13 (CH2); 22.72, 22.73 (CH3CH2); 26.26, 26.35, 29.42, 29.52, 29.59, 29.64, 31.90, 31.95 (CH2); 69.21, 69.28, 69.50 (ArOCH2); 105.94, 107.00, 108.08, 108.77, 121,07, 123.06, 123.40, 124.49, 126.42, 131.09, 138.57, 141.85, 149.15, 149.35, 150.19, 151.78 (Aromatic carbons). MALDI-TOF MS 1456.1.
Poly(10) was synthesized by the cationic polymerization of 9 (0.3g, 0.2 mmol ) in methylene chloride (3 mL) using trifluoromethanesulfonic acid (6.5 mg) as initiator and methyl sulfide (17.0 mg) as Lewis base, at room temperature for 3 days. The polymerization was carried out in a round-bottomed flask with septum seals under nitrogen atmosphere. The polymerization was quenched with a 3 : 1 methanol- ammonium hydroxide solution and the polymer was purified by precipitation from chloroform into methanol. 1H and 13C NMR spectroscopy showed no residual vinyl functionality in the purifieed polymer as shown in Fig. 4.
Results and Discussion
The synthesis and characterization of a homologous series of discotic liquid crystals, the diphenanthro[9,10-b:g]quinoxalines 6, is reported in which the length of the peripheral alkoxy-substituted side groups is varied from 4 to 10 carbon atoms.
6,7,10,11-Tetra(alkyloxy)triphenylene-2,3-diamines 4 was chosen as a key synthon and prepared by hydrogenation of the corresponding nitro compound of 2,3,6,7-tetra(alkyloxy)-10,11-dinitrotriphenylenes 3 in ethyl acetate in the presence of palladium on carbon (5%) at room temperature and 5 atm pressure in ethyl acetate shown in Scheme I. Compounds 3 was prepared in a two-step synthesis by palladium-catalyzed cross- coupling (Miyaura-Suzuki cross-coupling) between the 3,4-di(alkoxy)phenylboronic acids and 1,2-dibromo- 4,5-dinitrobenzene 1 gave the compounds 1,2-bis(3,4- dialkyloxyphenyl)-3,4-dinitrobenzene 2 and subsequent aryl-aryl coupling with VOF3 in the presence of boron trifluoride-diethyl ether.
Condensation of the freshly prepared diamine 4 with
the compounds, 3, 5 and 6 exhibit enantiotropic columnar hexagonal phase behaviour as shown in Fig. 6, 9 and 12, respectively. The representative DSC curves for these discotic liquid crystals are shown in Fig. 5, 8 and 11. The dependence of transitions temperatures on n, the number of carbons of alkoxy groups of the series of discotic liquid
the appropriate dicarbonyl compound of bis(3,4- dialkoxyphenyl)-1,2-dione gave different side-chain length of 11,12-bis[3,4-dialkoxy)phenyl]-2,3,6,7- tetra(alkoxy)-10,13-diazabenzo[b]triphenylenes 5 varing from 4 to 10 carbon atoms. Similarly, aryl-aryl coupling of 5 with VOF3 in the presence of boron trifluoride- diethyl ether gave the novel discotic columnar liquid crystals, 2,3,6,7,12,13,16,17- octa(alkyloxy)tetrabenzo[a,c,j,l]-9,20-diazaanthracenes
crystals, TPP-NO2-n, BTDBP-n andODPDA-n (n = 4
~ 10) are shown in Fig. 14, 15 and 16.
The synthesis of the monovinyl ether, 2,67-tris[3,4- d i ( o c t y l o x y ) p h e n y l ] - 3 - [ 4 - o c t y l o x y - 3 - (ω - vinyloxyalkyloxy)phenyl]quinoxzline 9, fuctionalized with octa(alkoxy)-substituted quinoxalinee-based mesogenic core was prepared in a two-step synthesis by reaction of the benzil, 1- (3,4-dioctyloxy)-2-[4-(ω-hydroxyalkan-1-yloxy)-3- octyloxyphenyl]ethane-1,2-dione with diamine 7 to give the compound, 2-octyloxy-5-{3,6,7-tris[3,4- di(octyloxy)phenyl]quinoxalin-2-yl]phenol 8 and subsequent vinyl ether exchange reaction with butyl vinyl ether in the presence of aceto-(1,10- phenanthroline)palladium (II) as the catalyst.
Cationic polymerization of the vinyl ether monmoer, 2,67-tris[3,4-di(octyloxy)phenyl]-3-[4-
o c t y l o x y - 3 - ( ω -
vinyloxyalkyloxy)phenyl]quinoxzline 9 initiated with CF3SO3H/(CH3)2S leads to novel tetraphenylquinoxaline-based discotic columnar side-chain liquid crystalline polymers 10, as shown in Scheme II. Poly(10) shows glass transition temperarure at 58.5 oC and isotrpic temperature at 188 oC (Fig.17 ). The columnar mesophase of poly(10) was obseverd by OPM (Fig. 18) and confirmed by X=ray diffraction pattern (Fig.19).
Compound 3 (n = 8) was found to exhibit a liquid crystalline phase between 142.8 oC and 223.3 oC;
the enthalpies of these transitions were 55.9 J g-1 and 5.8 J g-1, respectively (Fig. 5). This mesophase exhibits the characteristic dentritic texture of a columnar hexagonal phase when examined under a microscope (Fig.6). This assignment was confirmed by the X-ray diffraction pattern (Fig. 7), which has one peak in the samall angle region that index to the (100) reflection from a hexagonal arrangement with a lattice constant a = 21.9 Å. In the wide-angle region, X-ray diffraction measurement also shows two broad and diffuse rings. The first one corresponds to a spacing of 4.72 Å, which is related to the liquid-like correlation between the molten aliphatic chains. The second ring at 3.53 Å
Fig.1: 1H and 13 C nmr of the compound( 3), 1 , 2 - B i s ( 3 , 4 - d i ( o c t y l o x y ) p h e n y l ) - 3 , 4 - dinitrobenzene.
8). In the coolimg process this compound exhibited a dentritic growth from the isotropic liquid, which is typical for the formation of a hexagonal columnar phase. Upon annealing, the mesophase defects changed to give a fan-like texture typical of a hexagonal disordered phase (Colh) (Fig. 9). The material did not crystalline on further cooling, and only showed a transitionto yhe isotropic liquid upon subsequent heating. By DSC, the first heating scan showed two major transitions; the first pwak corresponds to the transition from the crystal to the columnar mesophase and the second one corresponds to the transition to the isotropic liquid. On cooling, crystallizayion did not occur but a glass transition at - 20oC was detected and only the the peak related to the isotropization was observed on further heating scans.
However, when the DSC scan was repeated after one weak, the corresponding to the melting of the melting of the material was observed, showing that the crystallization of the compound takes place when it is left for a long period of time. The X-ray patterns of 5 taken at 110 oC, only one peak in the small angle region that index to the (100) reflection from a heagonal arrangement with a lattice constant a = 28.5 Å. In the high angle region only a diffuse halo at 4.56 Å characteristic of molten xhains is observed.
Compound 6 (n =8) was found to exhibit a liquid crystalline phase between 148.9 oC andand 222.0 oC;
the enthalpies of these transitions were 18.1 J g-1 and 1.9 J g-1, respectively. This mesophase exhibits the characteristic dentritic texture of a columnar hexagonal phase when examined under a microscope (Fig.12). This assignment was confirmed by the X-ray diffraction pattern (Fig. 13), which has two peaks in the samall angle region that index to the (100) ans
Fig.2: 1H and 13 C nmr of the compound (5), 11 , 1 2 - b i s [ 3 , 4 - d i ( a l k y o x y ) p h e n y l ] - 2 , 3 , 6 , 7 - tetra(alkyloxy)-10,13-diazabenzo[b]triphenylene..
(110) reflections from a hexagonal arrangement with a lattice constant a = 23.7 Å. In the wide-angle region, X-ray diffraction measurement also shows two broad and diffuse rings. The first one corresponds to a spacing of 4.67 Å, which is related to the liquid-like correlation between the molten aliphatic chains. The second ring at 3.60 Å is presumably related to the periodic stacking of the discotic subunits within the columns. In comparison of 5 and 6 (n = 8), the stronger interaction between the molecules for compound 6 by cyclization causes a change fromthe a hexagonal disordered mesophaseexhibited by the parent compound 5 to a hexagonal ordered phase, as well as increasing the transition temperatures.
Conclusions
Novel board-like liquid crystalline materials based on diphenanthro[9,10-b:g]quinoxaline or tetrabenzo[a,c,j,l]-9,10-diazaanthracene have been synthesized and characterized. On the other hand, cationic polymerization of the vinyl ether monmoer, 2,67-tris[3,4-di(octyloxy)phenyl]-3-[4-octyloxy-3-(ω- vinyloxyalkyloxy)phenyl]quinoxzline 9 initiated with
CF3SO3H/(CH3)2S leads to novel
tetraphenylquinoxaline-based discotic columnar side- chain liquid crystalline polymers 10
.
References
1) S. Chandraseekhar and S. Kumar in
"Hand Book of Liquid Crystals" (1988), Vol. 2B, Chapter VIII.
2) Uchida, M; Izumisawa,J; Furukawa,K;
Jpn. Kokai Tokkyo Koho JP 0913,025(1997).
Fig.3: 1H and 13C nmr of the compound (6), 2,3,6,7,12,13,16,17-octa(octyloxy[a,c,j.l]-9,20- diazaanthracene
.
Fig.4: 13C nmr of the vinly monomer compound (9), and side-chain columnar liquid crystalline poly(10) with polyethylene as the main chain.
Fig.5: DSC thermograms of compound.
2 , 3 , 6 , 7 - Te t r a ( a l k y l o x y ) - 1 0 , 1 1 - dinitrotriphenylene (3) (n =8).
Fig.6: POM of compound (3) n = 8 at 200 oC.
Fig.8: DSC thermograms of compound (5).11,12-bis[3,4-di(alkyoxy)phenyl]- 2,3,6,7-tetra(alkyloxy)-10,13-
diazabenzo[b]triphenylene. (3) (n =8).
Fig.9: POM of compound (5) n = 8 at 140 oC.
Fig.7: XRD patterns off discotic liquid crystals (3), n = 8 at 150 oC.
Fig.10: XRD patterns off discotic liquid crystals (5), n = 8 at 110 oC.
Fig.14: Dependence of transitions temperatures on n, the number of carbons of alkoxy groups of the series of discotic liquid crystals, TPP-NO2-n (n = 4 ~ 10).
Fig.11: DSC thermograms of compound (6).2,3,6,7,12,13,16,17-
octa(octyloxy[a,c,j.l]-9,20-diazaanthracene
Fig.12: POM of compound (6) n = 8 at 145 oC.
Fig.15: Dependence of transitions temperatures on n, the number of carbons of alkoxy groups of the series of discotic liquid crystals, BTDBP-n (n = 4 ~ 10).
Fig.16: Dependence of transitions temperatures on n, the number of carbons of alkoxy groups of the series of discotic liquid crystals, ODPDA-n (n = 4 ~ 10).
Fig.17: DSC thermograms of columnar liquid crystals of poly(10) (10), n = 8.
Fig.18: POM columnar liquid crystals of poly(10) (10), n = 8 at 115 oC.
Fig.13: XRD patterns off discotic liquid crystals (6), n = 8 at 200 oC.
Fig.19: XRD patterns of columnar liquid crystals of poly(10) (10), n = 8 at 90 oC.
