In Situ Synthesis of Hexakis(alkoxy)diquinoxalino[2,3-a:2′,3′-c]phenazines: Mesogenic Phase Transition of the Electron-Deficient Discotic Compounds Chi Wi Ong,*,† Su-Chih Liao,† Tsu Hsing Chang,† and Hsiu-Fu Hsu‡ Department of Chemistry, National Sun Yat Sen University, Kaoshiung, Taiwan 804, and Department of Chemistry, Tamkang University, Tamsui, Taiwan 251
[email protected] Received December 18, 2003
2,3,8,9,14,15-Hexakis(alkoxy)diquinoxalino[2,3-a:2′,3′-c]phenazines with alkyl side chains varying from 6 to 12 carbon atoms were readily synthesized by the condensation of hexaketocyclohexane with the respective 1,2-bisalkoxy-4,5-diaminobenzene. Polarization microscopy and DSC studies showed all these compounds to exhibit a very wide mesophase range of over 150 °C. An interesting Dhd to Drd transition was observed for the octyl derivative 3b, as determined by X-ray diffraction measurements. The hexyl derivative showed three reduction potentials, suggesting that the HATN core maintained its electron-deficient characteristic and considered suitable as an n-doped discoticliquid crystalline material. Incorporation of six alkoxy chains did not override the electron deficiency of the HATN core. Introduction Recently, hexaazatriphenylene (HAT) having three π-deficient pyrazine nuclei at its core has been investigated as a new discogen with the potential to act as electron carrier.1 More impressively, intermolecular multiple hydrogen bonding was utilized to associate a hexaamide-HAT compound for the formation of columnar mesophases with a record setting disk-to-disk distance of 3.18 Å that resulted in a high carrier mobility.2 Although acceptor materials with high electron mobility are in demand, examples of acceptor discogens are limited.3 This could be due to avoidance of self-πcomplexation of the electron-deficient nature of the cores. Crystal structures of many electron-deficient heterocycles seem to support this view.4-13 However, hydrogen bond†
National Sun Yat Sen University. Tamkang University. (1) Uckert, F.; Tak, Y.-H.; Mullen, D.; Bassler, H. Adv. Mater. 2000, 12, 905. (2) Gearba, R. I.; Lehmann, M.; Levin, J.; Ivanov, D. A.; Koch, M. H.; Barbera´, J.; Debije, M. G.; Piris, J.; Geerts, Y. H. Adv. Mater. 2003, 15, 1614. (3) (a) Boden, N.; Bushby, R. J.; Headdock, G.; Lozman, O. R.; Wood, A. Liq. Cryst. 2001, 28, 139. (b) Kumar, S.; Rao, D. S. S.; Prasad, S. K. J. Mater. Chem. 1999, 9, 2751. (c) Boden, N.; Borner, R. C.; Bushby, R. J.; Clements, J. J. Am. Chem. Soc. 1994, 116, 10807. (4) Nishigaki, S.;Yoshioka, H.; Nakatsu, K. Acta Crystallogr., Sect. B 1978, 34, 875. (5) Huiszoon, C. Acta Crystallogr., Sect. B 1976, 32, 998. (6) Huiszoon, C.; van der Waal, W. B.; van Egmond, A. B.; Harkema, S. Acta Crystallogr., Sect. B 1972, 28, 3415. (7) Clearfield, A.; Sims, M. J.; Singh, P. Acta Crystallogr., Sect. B 1972, 28, 350. (8) van den Ham, D. M. W.; van Hummel, G. J.; Huiszoon, C. Acta Crystallogr., Sect. B 1978, 34, 3134. (9) Huiszoon, C.; van Hummel, G. J.; van den Ham, D. M. W. Acta Crystallogr., Sect. B 1977, 33, 1867. (10) van der Meer, H. Acta Crystallogr., Sect. B 1972, 28, 367. (11) Herbstein, F. H.; Schmidt, G. M. J. Acta Crystallogr. 1955, 8, 399. ‡
ing has been employed to force self-π-complexation to exhibit closely spaced dimmers with a one-half ring offset in the solid phase of hexacarboxamidohexaazatriphenylene.14 Reported examples of HAT-based liquid crystal possess a flexible unfused polyphenyl15 or succinamidepolyphenyl16 side chain that has the disadvantage of introducing excessive conformational freedom, whereby long-range three-dimensional order may be disrupted. Hexakis(alkylthio)diquinoxazlino[2,3-a: 2′,3′-c]phenazines were reported to exhibit a number of mesophases (two to five) depending on the chain length, but these cannot be unambiguously indexed.17 We have preliminarily reported the synthesis of the electron-deficient core of diquinoxazlino[2,3-a:2′,3′-c]phenazine (HATN) with six alkoxy chains and found the hexyloxy derivative to exhibit a columnar mesophase.18 Herein, we report the synthesis, solution electrochemistry, and supramolecular assembly of a series of hexakis(alkoxy)diquinoxazlino[2,3-a:2′,3′-c]phenazines (3) with longer electron-donating alkoxy side chains. As expected, the nature of the mesophase and the range of its mesomorphism are dependent on the side-chain length. The electrochemical property of 3 has been successfully carried out for the first time. Interestingly, the electron (12) Phillips, D. C.; Ahmed, F. R.; Barnes, W. H. Acta Crystallogr. 1960, 13, 365. (13) Phillips, D. C. Acta Crystallogr. 1956, 9, 237. (14) Beeson, J. C.; Fitzgerald, L. J.; Gallucci, J. C.; Gerkin, R. E.; Rademacher, J. T.; Czarnik, A. W. J. Am. Chem. Soc. 1994, 116, 4621. (15) Arikainen, E. O.; Boden, N.; Bushby, R. J.; Lozman, O. R.; Vinter, J. G.; Wood, A. Angew. Chem., Int. Ed. 2000, 39, 2333. (16) Pieterse, K.; Van Hal, P. A.; Kleppinger, R.; Vekemans, J. A. J. M.; Janssen, R. A. J.; Meijer, E. W. Chem. Mater. 2001, 13, 2675. (17) Kestenont, G.; De Halleux, V.; Lehmann, M.; Ivanov, D. A.; Watson, M.; Greets, Y. H. J. Chem. Soc., Chem. Commun. 2001, 2074. (18) Ong, C. W.; Liao, S.-C.; Chang, T. H.; Hsu, H.-F. Tetrahedron Lett. 2003, 44, 1477.
10.1021/jo035840l CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/01/2004
J. Org. Chem. 2004, 69, 3181-3185
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Ong et al. SCHEME 1.
Synthesis of Compounds 3a-d
TABLE 1. Influence of Alkoxy Chain Length on the
TABLE 2. Comparison of Tricondensation for Workup
Hydrogenation of 1,2-Bisalkoxy-4,5-dinitrobenzene R time yield (%)
N2H4, Pd/C H2, Pd/C (2atm) N2H4, Pd/C H2, Pd/C (2atm)
and in Situ Condition
2a
2b
2c
2d
C6H13 1 d, ∆ 6h 63 90
C8H17 1 d, ∆ 6h 60 90
C10H21 1 d, ∆ 12 h, ∆ 61 88
C12H25 2 d, ∆ 1 d, ∆ 40 90
deficiency in the HATN core is maintained even with the introduction of six alkoxy chains, and favorable aromatic stacking interaction leads to the formation of both hexagonal and rectangular columnar mesophase. Results and Discussion As reported, all attempts to synthesize the hexaalkoxy via nucleophilic substitution of the hexakis(chloro)diquinoxazlino[2,3-a:2′,3′-c]phenazine using various alkoxides afforded the desired 3 in very poor yield along with a complex mixture of products.18 We reported an alternative synthetic approach to 3 by the direct condensation of hexaketocyclohexane with 1,2-bisalkoxy-4,5-diaminobenzene of different chain lengths to afford the 2,3,8,9,14,15-hexaalkoxydiquinoxalino[2,3-a: 2′,3′-c]phenazines 3a-d, respectively (Scheme 1). The hexaketocyclohexane19 has to be freshly prepared; otherwise, the yield of the condensation product drops significantly. Reduction of 1,2-bisalkoxy-4,5-dinitrobenzene to produce 1,2bisalkoxy-4,5-diaminobenzene can be achieved using palladium on charcoal in the presence of hydrazine or hydrogen. It is noteworthy to point out that the length of time required for the reduction increases dramatically with the alkyl chain length due to the poorer solubility (Table 1). The most common condition used for the tricondensation reaction involved refluxing in acetic acid, but we have used ethanol in the presence of a small amount of acetic acid. This alternative synthetic condition allows the air-sensitive 1,2-bisalkoxy-4,5-diaminobenzene obtained from reduction in ethanol to be tricondensed directly with hexaketocyclohexane, thus avoiding unnecessary workup (Table 2). The compounds synthesized were easily purified by chromatographic separation on (19) Berger, P. J. Am. Chem. Soc. 1942, 64, 67.
3182 J. Org. Chem., Vol. 69, No. 9, 2004
R yield (%)
workup in situ
3a
3b
3c
3d
C6H13 30 75
C8H17 25 68
C10H21 24 65
C12H25 25 60
silica gel and recrystallization from acetone or ethanol, and the purity was confirmed using MALDI mass spectroscopy and elemental analysis. The liquid crystal properties of 3a-d studied using polarizing microscopy and differential scanning calorimetry are summarized in Table 3. For all series 3 compounds, no thermal decomposition was detected on the basis of reversible heating/cooling cycles. It is also noted that with longer alkoxy side chains 3b and 3c suffered severe supercooling; on the other hand, for the shorter chain analogue 3a, the temperatures and enthalpies for melting and crystallization are not much different. Previously, we have shown that the side chain length has to reach six carbon atoms, 3a, for the rectangular columnar-phase formation assigned by its mosaic texture and X-ray diffractomogram. In this work, all the compounds exhibited single enantiotropic mesophase except 3b in which two liquid crystal phases were detected. Compound 3b-d melted from isotropic to give a birefringent fluid phase with large regions of uniform extinction that is typical of columnar liquid crystal phase. The ease of homeotropic alignment in these compounds can be attributed to the large and flat aromatic core and suggests a low surface tension of the mesophases. An additional mesophase identifiable by a new texture with an in-grown needle feature when 3b was further cooled from the first columnar phase to 174 °C was also observed, and this fluid phase was maintained until it crystallized at 61 °C. The clearing temperatures are found to be chain-length dependent and decrease as the alkoxy chains lengthened. The small enthalpies of the columnar to isotropic transition are indicative of disordered mesophases. Surprisingly, compounds 3a-d exhibit a very wide mesophase range of over 150 °C. All the materials have been investigated by variabletemperature XRD experiments, and the results are summarized in Table 4. Only broad halos are observed at wide angle for all compounds at all mesophases,
Synthesis of Hexakis(alkoxy)diquinoxalino[2,3-a:2′,3′-c]phenazines
FIGURE 1. Variable-temperature XRD patterns of 3b. TABLE 3. Phase Behaviors of 3a-da compd
n
3a
6
phase transitions
K1 y\ z K2 y\ z Colrd y\ zI 92.1 (-31.59) 179.5 (-4.98) 225.6 (-1.78)
3b
8
K y\ z Colrd y\ z Colhd y\ zI b 61.3 (-36.90) 221.5 (-1.08)
97.1 (32.11)
186.6 (5.06)
114.5 (84.65)
229.9 (1.95)
224.4 (1.16)
176.0b
173.5 214.6 (1.41)
3c
10
K1 y\ z Colhd y\ zI 40.8 (-56.84) 212.5 (-1.28)
3d
12
K y\ z Colhd y\ zI 52.0 (-2.78) 206.1 (-1.46)
86.0 (76.38)
94.2 (106.87)
206.1 (2.35)
a The transition temperatures (°C) and enthalpies (in parentheses/kJ mol-1) were determined by DSC at 10 °C/min K, crystalline phase; Colr, rectangular columnar; Colh, hexagonal columnar; I, isotropic. n denotes the length of the alkoxy chains. b Observed by POM studies but not detected in DSC measurements.
TABLE 4. Variable-Temperature XRD Data for Compounds 3a-d compd
mesophase
3a
Colrd at 198 °C
3b
Colhd at 207 °C Colhd at 160 °C Colrd at 146 °C
lattice constant (Å)
3c
Colhd at 195 °C
a ) 40.00 b ) 19.15 a ) 25.51 a ) 24.93 a ) 43.68 b ) 22.20 a ) 42.70 b ) 21.76 a ) 27.94
3d
Colhd at 198 °C
a ) 29.44
Colrd at 128 °C
indicating a disordered liquidlike organization. The XRD variable-temperature measurements and POM of 3b are shown in Figures 1 and 2, and two columnar mesophases are revealed. When cooled from the isotropic melt, a hexagonal lattice with a dominant (100) reflection in the small angle region was identified and the d spacing shrinks with decreasing temperature. Upon further cooling, a rectangular columnar phase that was not detected by DSC measurements evolves and is assigned based on the two intense low angle peaks indexed to (110) and (200). Hence, for 3b, it appears that the Drd phase slowly transforms to a Dhd structure. This behavior has not been observed for the hexaazaphenylene system. The XRD measurements of 3c and 3d exhibit only the signal corresponding to a single hexagonal columnar mesophase. We have observed a crossover from the discotic rectangular phases (Drd) to discotic hexagonal phases (Dhd) with increasing side-chain length. The lattice constants of the compounds with hexagonal phase do not
obsd (calcd) spacing (Å)
Miller indices
halos obsd (Å)
20.00 17.28 22.09 21.59 21.84 19.79 21.35 19.39 24.20 13.87 (13.97) 25.50 14.68 (14.72)
(200) (110) (100) (100) (200) (110) (200) (110) (100) (110) (100) (110)
4.70, 3.58 4.58, 3.57 4.62, 3.66 4.64, 3.60 4.66, 3.61 4.64, 3.71 4.75, 3.53
correspond to the estimated intercolumn core-to-core distance, ca. 24.2, 26.8, and 29.3 Å for 3b, 3c, and 3d, respectively, when the cores are center-aligned within columns and the all-trans alkoxy chains are fully penetrating. The longer intercolumn distance observed could be attributed to the deviation from the central-aligned arrangement or less penetration of the side chains. More importantly, the deviation is chain-length dependent whereby an almost perfect match was found for 3d. Therefore, the hydrophobic interactions between the side chains override the conformational freedom, which disrupts long-range, three-dimensional order for chain length of up to 12 carbon atoms. No attempt was made to search for the upper limit of the chain length. The enforced intracolumnar stacking is also revealed by the corresponding average core-to-core correlation within columns observed as broad shoulders of 3.5∼3.7 Å at the wider angle side of the amorphous halos. These values are smaller than the 3.7∼4.0 Å reported for the thioether J. Org. Chem, Vol. 69, No. 9, 2004 3183
Ong et al.
FIGURE 2. POM of 3b.
analogues17 but are significantly larger than 3.18 Å for hexakis(alkylcarboxamido)hexaazatriphenylene with strong hydrogen bonds to enforce the intracolumnar stacking order which resulted in a high carrier-mobility.2 Nevertheless, the increasing core-core attraction of HATN units while maintaining minimal constrain of the six alkoxy side chains has encouraged molecular stacking and led to the formation of a liquid crystalline mesophase. To determine the ability of 3 to behave as an electron carrier mesogen, the redox properties of 3 were studied by cyclic voltammetry (CV), and the greatest drawback is the insolubility of these compounds in polar solvents. In dichloromethane using Pt electrodes and at ambient temperature, 3a showed three reduction waves at -1.25, -1.53, and -1.81 V (conditions: 0.05M Bu4NPF6 in dichloromethane, 200mV/s, V vs Ag/AgCl) associated, however, with two corresponding oxidation peaks in the reverse scanning. The intensity of the second oxidation peak is approximately twice that of the other peaks, indicating the coalescence of two oxidation processes into one. It should be mentioned that attempts to separate this two-electron oxidation wave into two peaks by varying the scan rate was not successful. We have revealed for the first time that the HATN core in 3a can behave as an electron-acceptor even with the incorporation of six alkoxy side chains. General speaking, 3a exhibits similar electron-accepting ability as HAT (-1.42 and -1.72 V in CH3CN vs SCE)20 but is less electron deficient than HAT-hexacarboxytriimide (-0.35 V in CH3CN vs SCE).16 Furthermore, the downfield 1H NMR aromatic signals at δ 7.85 of 3a also support the electronwithdrawing ability of the HATN core.
Furthermore, 3a-d can be rapidly constructed in a single step by the condensation of hexaketocyclohexane with 1,2-bisalkoxy-4,5-diaminobenzene. Future work will focus on the synthesis of new difunctionalized members of the family.
Experimental Section
In conclusion, we have shown that HATN-hexaalkoxy (3) can indeed behave as an electron-deficient liquid crystal material. Considering the fact that these compounds contain six electron-donating alkoxy groups, it is important to determine their ease of accepting electrons. Also for 3, we observe a transition from discotic rectangular phases to discotic hexagonal phases, and 3b possesses both phases.
General Procedure for the Preparation of 1,2-Bisalkoxybenzene. In a two-neck round-bottom flask (1 L) fitted with a condenser with a nitrogen inlet containing acetone (500 mL) were added catechol (10 g, 0.09 M) and potassium carbonate and the mixture stirred at room temperature for 1 h. After this time, the 1-bromoalkane (0.23 M) was added, and the reaction mixture was refluxed for 2 days. The solid residue from the reaction mixture was filtered, and the filtrate was evaporated to dryness. The crude product was then dissolved in ether and the ether layer washed with water and brine. The ether extract was dried over MgSO4 and evaporated. The excess 1-bromaoalkane was further removed by distillation under reduced pressure to give the respective 1,2-bisalkoxybenzene. 1,2-Bishexyloxybenzene: colorless oil (96% yield); 1H NMR (CDCl3, 200 MHz) δ 6.85 (s, 4H), 3.96 (t, 4H, J ) 6.6 Hz), 1.86-1.72 (m, 4H), 1.50-1.30 (m, 12H), 0.89 (t, 6H, J ) 6.2 Hz); MS m/z 278 (M+,
