(aromatic carbons). MS (EI, 70 eV) [m/z(rel. intensity)]:
462.3 (M+, 100); 445.1 ([M-OCH3]+, 40).
2,3,6,7,11,12-Hexakis(methoxy)dibenzo[a,c]phenazine (5):To a stirred solution of 4 (3.0g, 6.48 mmol) and vanadium(V)oxyfluoride in dichloromethane (15 mL) was added boron trifluoride (2.4 mL). The reaction mixture was stirred for 4 h at ambient temperature and poured into ice water. The aqueous layer was extracted with dichloromethane. The combineed organic layers were dried (Na2SO4) and the solvent was removed in vacuo. The resulting dark crude products were purified by column chromatography on silica gel (hexanes/ethyl acetate, 1:1, Rf
= 0.67) to provide a red solid 5 (52.56g, 69.3%).1H NMR (δ, ppm, CDCl3): 3.99 ~ 4.07 (m, 18H, OCH3); 7.42 (s, 4H, aromtic protons); 8.45 (m, 2H, aromatic protons).13C- NMR (δ, ppm, CDCl3): 56.36, 56.52, 56.72 (OCH3);
103.90, 106.04, 106.68, 123,47,125.86, 138.69, 138.88, 149.36, 151.44, 153.38 (aromatic carbons). MS (EI, 70 eV) [m/z(rel. intensity)]: 460.1 (M+, 100); 445.1 ([M-CH3]+, 20).
2,3,6,7,11,12Hexakis(hydroxy)dibenzo[a,c]phenazines(6) :Compound 5 (2.0g, 43.48 mmol) was dissolved in in a mixture of 30 mL of glacial acetic acid and 30 mL of hydrobromic acid (50%). It was refluxed with stirring for 18 h under a nitrogen atmosphere.The solvent was evaporated to afford 1.24g of dark solid. yield 76.0%.1H NMR (δ, ppm, THF-d8): 4.97 ~ 5.00 (m, 18H, OCH3); 4.98
~ 4.99 (m, 6H, OH); 7.14 ~ 7.22 (m, 4H, aromatic protons), 8.33 (s, 2H, aromatic protons. 13C-NMR (δ, ppm,THF-d8): 103.99, 105.04, 106.08, 123.87, 124.87, 123.87, 137.69, 138.69, 149.37, 151.45, 153.48 (aromatic carbons). MS (EI, 70 eV) [m/z(rel. intensity)]: 376.1 (M+, 100). IR (cm-1): 3100 ~ 3600 (broad, s, υOH).
General Procedure for the Preparation of 2,3,6,7,11,12- Hexakis(alkoxy)dibenzo[a,c]phenazine (7):
A mixture of K2CO3 (1.1g, 79.8 mmol), 2,3,6,7,11,12- Hexakis(hydroxy)dibenzo[a,c]phenazine 7 (0.5g, 13.3 mmol), and alkyl bromide (86.5 mmol) in DMF (20 mL) was heated at 120 oC for 24 h under a nitrogen atmosphere.
After cooling to room temperature, H2O (20 mL) was added, the layers were seperated and the aqueous layer was extracted with dichloromethane. The organic layer was washed with water, dried over sodium sulfate and concentrated.
2,3,6,7,11,12-Hexakis(octyoxy)dibenzo[a,c]phenazine (n = 8) : Purification of the crude porduct by column chromatography on SiO2 (hexanes/ethyl acetate 1:10) yielded 0.56g (40%) of a yellow solid. 1H NMR (δ, ppm, CDCl3): 0.81 ~ 0.84 (m, 18H, CH3); 1.25~1.36 (m, 48H, O(CH2)3(CH2)4CH3); 1.50~1.52 (m, 12H, O(CH2)2CH2); 4.17 ~ 4.20 (m, 8H, OCH2); OCH2); 4.26
~ 4.28 (m, 4H, OCH2 ); 7.51 (s, 2H, aromatic protons);
7.65 (s, 2H, aromatic protons); 8.68 (s, 2H, aromatic protons).13C-NMR (δ, ppm, CDCl3): 12.89, 13.08 (CH3);
25.10, 25.17, 25.20 (CH3CH2CH2); 27.38, 27.97 (CH3CH2CH2CH2); 28.28, 28.33, 28.39, 28.45, 28.48, 28.69 (Ar-OCH2(CH2); 30.84, 30.85, 30.86 (Ar- OCH2CH2); 67.00, 68.20, 68.70 (Ar-OCH2); 105.49, 105.61, 107.16, 122.76, 124.81, 137.74, 138.08, 148.76, 150.21, 152.02 (aromatic carbons). MS (EI, 70 eV) [m/z(rel. intensity)]: 1048.7 (M+, 20); 936.7 ([M-C8H17]+, 10); 823.7 ([M-C16H34+, 10); 711.5 ([M-C24H51]+, 10);
599.4 ([M-C32H68]+, 10); 487.2 ([M-C40H58]+, 10) Results and Discussion
As outlined in Scheme 1, 1,2-diamino-1,4- dimethoxybenzene (3), was obtaned by hydrogenation of an ethanolic solution of the corresponding 1,2-dinitro-4,5- dimethoxybenzene (2) compound in the presence of palladium on carbon (5%) at room temperature and 5 atm pressure. After removal of the catalyst an dsolvent, the residue was assumed to contain 60 % (w/w) of 1,2- diamino-1,4-dimethoxybenzene. Condensation of the fresh
: NSC -91-2216-E-011-023 : 91/08/01 ~ 92/07/31 :
:
Absract
Two novel homologous series of discotic liquid crystals, 2,3,6,7,12-hexaalkoxydibenzo[ a,c]phenazines (abbreviated as 2N-6Cn,sym; n = 3 ~ 12) and 2,3,6,7- tetraoctyloxy-11,12-dialkoxydibenzo[a,c]phenazines (abbreviated as 2N-4C8-2Cn,asym; n =3 ~ 12) (n = 3 ~ 12) with six alkyl side chain lengths varying from 3 to 12 carbons, have been synthesized and characterized. X- ray diffraction studies confirmed the mesophase assignment made by optical microscopy and DSC. Most of these samples display the typical signature of a two dimensional hexagonal lattice with a strong (100) peak and one weak (110) peak with relative d spacings of 1,
√3 respectively. The wide-angle region displays halos indicating liquid like correction between the rigid cores.
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
2 , 3 - B i s ( 3 , 4 - d i m e t h o x y p h e n y l ) - 6 , 7 - dimethoxyquinoxaline (4):1,2-bis(3,4- dimethoxyphenyl)ethane-1,2-dione 3 (4.09g, 12.38 mmol) was added to a freshly prepared solution of 1,2- diamino-4,5-dimethoxybenzene 2 (2.5g, 14.85 mmol) in acetic acid (25 mL). The reaction mixture was refluxed for 18 h. After the complectionof the reaction, the solvent was evaporated in vacuo. The crude mixture was dissolved in dichloromethane. The organic layer
was washed with water, dried (Na2SO4), and the solvent evaporated. After purification by flash chromatographyon SiO2 (hexanes/ethyl acetate, 1:5, Rf
= 0.5), compound 4 (4.12g, 71.7%) was obtained as pale yellow solid.1H NMR (δ, ppm, CDCl3): 3.94 (s, 18H, OCH3); 6.81~6.89 (m, 2H, benzene ring protons);
7.06~7.09 (m, 4H, benzene ring protons); 7.43 (s, 2H, quinoxaline protons).13C-NMR (δ, ppm, CDCl3): 56.22, 56.36,56.63 (OCH3); 110.63, 110.69,111,22, 113,27,122.97, 126.52, 126.78, 148.94, 149.65, 149.84
prepared o-diamine (3) with 1,2-bis(3,4- dimethoxyphenyl)ethane-1,2-dione in refluxing acetic acid gave the product of 2,3-bis(3,4-dimethoxyphenyl)- 6,7-dimethoxyquinoxaline (4) in 85% yield.
Intramolecular oxidative cyclization of (4) was achieved by stirring the compound with vanadiumoxy trifluoride in dichloromethane at room temperature to give 2,3,6,7,11,12-hexamethoxydibenzo[a,c]phenazine (5) in 65% yiield. The methoxy in (5) were removed with HBr in acetic acid, yielding the hexahydroxy intermediate (6) in 70%, which was alkylated under standard conditions using potassium carbonate in DMF at 80 oC to afford a homologous series of symmetric compounds of 2,3,6,7,11,12-hexaalkoxydibenzo[a,c]phenaz. The products have been characterized by NMR, UV-VIS, IR and mass spectroscopy. The representative 1H and 13C NMR spectra for (5), (6), and (7) are shown in Fig. 1, 2, qnd 3, respectively. The mesomorphic properties of 2,3,6,7,11,12-hexaalkoxydibenzo[a,c]phenazine 7 were studied by differential scanning calorimetry (DSC) and optical polarizing microscopy. The relationship between the phase transition temperatures and the number of carbons (n ) on peripheral alkoxy group for the series of discotic liquid crystals 7 (2N-6Cn,sym; n = 3 ~ 12) are shown in Fig. 5. Whereas the ethyl-substituted derivative (n = 2) did not show any mesomorphic properties, the compounds (n = 3 ~ 12) with longer alkoxy chains displayed liquid crystalline behaviour. As a typical example, the DSC thermograms of 2N-6Cn,sym (n = 8) is shown in Fig. 6. This dibenzo[a,c]phenazine was found to exhibit a liquid crystalline phase between 23.1 oC and 118.4oC; the enthalpies of these transitions were 39.9 J g-1 and 2.3 J g-1, respectively. This mesophase exhibits the characteristic dentritic texture of a columnar hexagonal phase when examined under a microscope (Fig.7). This assignment was confirmed by the X-ray diffraction pattern (Fig. 8), which has two peaks in the samall angle region that index to the (100) ans (110) reflections from a hexagonal arrangement with a lattice constant a = 25.29 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 4.38 Å, which is related to the liquid-like correlation between the molten aliphatic chains. The second ring at 3.54 Å is presumably related to the periodic stacking of the discotic subunits within the columns. The n =3 ~ 12 homologues of the series of asymmetric compounds (9) (2N-4C8-2Cn, asym) were prepared in 50 ~ 60%
yield by refluxing 1,2-Diamino-4,5-dialkoxybenzene (n
= 3 ~ 12) (8) with 2,3,6,7-
tetrakis(octyloxy)phenanthrene-9,10-dione in acetic acid. Representative 1H and 13C nmr spectra of compound (8) are shown in Fig.4. The thermal behaviour of compound 8 (2N-4C8-2Cn,asym; n = 5) was examined by DSC (Fig. 9). During the second heating, an endothermic peak at 75.2oC (∆H = 69.82 g J-1), which corresponding to the melting of the starting material into liquid-crystalline phase. At 155.5 oC (∆H = 6.13 g J-1), the mesophase clears into the isotropic liquid (Fig. 10). Moreover, in the first cooling, the isotropic-to- msophase was seen at 152.9 oC (∆H = 5.81g J-1). Upon further cooling, a mesophase-to-crystalline phase transition was also observed at 20.5 oC (∆H = 47.97 g J- 1). Fig. 11 shows a reprenentative X-ray diffractogram of the mesophase recorded at 106 oC of 8 (2N-4C8- 2Cn,asym; n = 5). The pattern has two peaks in the small-angle region which correspond to reciprocal spacings in a ratio of 1:√3 that index to the (100) and (110) reflections of a two dimensional hexagonal lattice (a = 20.21 Å). We also observed a broad diffuse scatterting halo in the wide angle region, corresponding to the liquid like disordered of the aliphatic chains, at
~4.54 Å. A plot of the lattice constant (a) versus the number of carbon atoms in the side chain is linear.
Asymmetric compounds of Series (2N-2C8-4Cn, asym; n
=3 ~ 9) displays a highly linear relation ship ( R2 = 0.998) with a slope of 0.619 Å/CH2 and an intercept of 20.32 Å.
(Fig. 12b). Compounds of symmetric series (2N-6Cn, sym; n =3 ~ 9) similarly have a slope of 1.625 Å/CH2 and an intercept of 12.77 Å ( R2 = 0.990)(Fig. 12a). The hexagonal lattice parameter (a), (23.34 Å), is somewhat smaller than the diameter of a stretched all-trans 2N-2C8- 4C5, asym molecule with n = 5 (27.99 Å) as calculated by molecular modeling. A similar behaviour was found for the 2N-6C8, sym comopound with n = 8, the hexagonal lattice parameter (a), (25.7 Å) is also smaller than the diameter of a stretched all-trans molecule (31.00 Å). The intercept of 12.77 Å for the compounds of symmetric series, represents the diameter of the rigid core, which when calculated for dibenzo[a,c]phenazine, using the longest molecular distance as the diameter, gave the value of 12.03 Å. On the other hand, molecular modelling for the asymmetric series 2N-2C8-4C5, asym molecule with n = 5 gives the approximate diameter of the molecular discs as 21.92 Å which corresponds to an inter-columnar distance of 20.32 Å, the diameter of the rigid core including an octyl side group measured by X-ray diffraction. These results indicate that the model for the formation of the discotic columnar mesophases is one where the dibenzo[a,c]phenazine disks stack one on top of another to form columns which then arrange themselves in a two-dimensional (2D) array to form a hexagonal (Colh) columnar mesophases (Fig. 13).
Conclusion
The demonstration that both homologues of symmetric compounds (2N-6Cn, sym)and asymmetric compounds (2N-4C8-2Cn, asym) exhibit stable, easily aligned columnar liquid crystalline mesogenic phases at relatively low temperatures suggests dibenzo[a,c]phenazine derivatives may be promising candidates for practical applications where formation of a broad columnar phase at room temperature is highly desirable. We are currently investigating the main chain polysiloxane based on the mesogenic core of dibenxo[a,c]phenazine of this seies in order to gain further insight into the phase behaviour.
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).
2
8
Fig.1: 1H and 13C nmr of the compound (5),
hexakis(methoxy)dibenzo[a,c]phenazine
.
Fig.3:1H and 13C nmr of discolti liquid crystal (7), hexakis(hexyloxy)dibenzo[a,c]phenazine
(2N-6Cn, sym; n = 6)
.
Fig.4: 1H and 13C nmr of discolti liquid crystal (8), 2,3,6,7-tetrakis(octyloxy)-11,12-
bis(pentyloxy)dibenzo[a,c]phenazine (2N-4C8-2Cn, sym; n = 5)
.
Fig.2: 1H and 13C nmr of the compound (6), hexakis(hydroxy)dibenzo[a,c]phenazine
.
Fig.6: DSC thermograms of compounds (a) 2N-6Cn, sym; n = 6 and
(b) 2N-6Cn, sym; n = 8
Half disk
Fig.7: POM of 2N-6Cn, sym; n = 8 (a) 115 oC; (b) 90 oC
Fig.9: DSC thermograms of compounds (a) 2N-4C8-2Cn, asym; n = 5 and (b) 2N-4C8-2Cn, asym; n = 7
Fig.10: POM of 2N-6Cn, sym; n
= 8 (a) 148 oC;
(b) 97 oC
Fig.5: Dependence of transitions temperatures on n, the number of carbons of alkoxy groups of the series of discotic liquid crystals, ( 2N-6Cn, sym; n = 3 ~ 12).
Fig.8: XRD patterns off discotic liquid crystals,
(a) 2N-6Cn, sym; n = 8 at 90 oC.
(b) 2N-6Cn, sym; n = 6 at 135 oC.
Fig.11: XRD patterns off discotic liquid crystals,
(a) 2N4C8-2Cn, asym; n = 5 at 122 oC.
(a) 2N4C8-2Cn, asym; n = 7 at 106 oC.
Fig.12: Plot of the lattice constant (a) vs n, the number of carbons of alkoxy groups of the series of discotic liquid crystals, (λ = 1.32633 A, NSRRC)
(a) ( 2N-6Cn, sym; n = 3 ~ 12), and (b) ( 2N-4C8-2Cn, sym; n = 3 ~ 10).
Discotic, antiphase
Fig.13: The most possible arrangement ofdibenzo[a,c]phenazine two neighboring molecules within a column resulted from antiparsllel alignment with respect to each other in order to achieve optimal space filling of the central core.XRD studies indicate that the lattice const. is only less than the calculated diameter of the discotic molecule.
a b
a
a
b b
2N-6Cn, sym; n = 6
2N-6Cn, sym; n = 8
2N-4C8-2Cn, asym; n = 5
2N-4C8-2Cn, asym; n = 7
a a
b b
4
It should be mentioned that William et al. studied extension the core size of hexadecyloxy-substituted triphenylene to the hexadecyloxy-substituted dibenzo[a,c]triphenylene. The benzo[a,c]triphenylene mesogen is liquid crystalline over a much broader temperature (35 ` 88 oC) than triphenylene, which forms a columnar hexagonal phase range from only 58 ~ 69 oC. Compared with the present work with the structurally related hexadecyloxydibenzo[a,c]triphenylene, lacking a hetrocyclic ring structure in the aromatic core,
It is interesting to compare the mesomorphic behaviour of the compound hexadecyloxydibenzo[a,c]phenazine with that of the structurally related hexadecyloxydibenzo[a,c]triphenylene derivative, lacking a hetrocyclic ring structure in the aromatic core. In contrast, the mesomorphic temperature range of the dibenzo[a,c]phenazine was found between 44.7 and 101.4 oC. Thus, introduction of the electron- deficient atoms to the mesogenic core of the
dibenzo[a,c]triphenylene to afford the dibenzo[a,c]phenazine moiety resulted
in an increase of the melting and clearing points together with similar width of the mesophase (∆T = ~55 oC).
Thus a structural model can be proposed in which the molecules are stacked on top of each other with very disordered chains forming the hydrophobic periphery of the column. Due to the C2 symmetry of dibenzo[a,c]phenazine two neighhboring molecules within a column are presumably resulted from the antiparallel alignment with respect to each other in order to achieve optimal space filling of the central core.
The pyrazine is an electron-deficient aromatic heterocyclewith two localized lone pair of electrons in two sp2orbital on the two nitrogen atoms.
