Rapid synthesis of new discotic liquid crystals based on diquinoxalino[2,3-a:2′,3′-c]phenazine containing hexakis(alkoxy) side arms

Article abstract of DOI:10.1016/S0040-4039(02)02862-9

The condensation of 1,2-bisalkoxy-4,5-diaminobenzene 2a-e derivatives with freshly prepared hexaketocyclohexane give hexakis(alkoxy)diquinoxalino[2,3-a:2′,3′-c]phenazines in good yield. DSC and polarization microscopy showed HATOC6 has both the crystalline (K) to mesophase (M) and the mesophase (M) to isotropic (I) phase transitions. Importantly, the alkoxy substituted compound synthesized showed an M-I transition because of greater stability, whereas the reported alkyl-thiol decomposes after the mesophase.

Full text of DOI:10.1016/S0040-4039(02)02862-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1477–1480
Rapid synthesis of new discotic liquid crystals based on
diquinoxalino[2,3-a:2%,3%-c]phenazine containing
hexakis(alkoxy) side arms
Chi Wi Ong,a,* Su-Chih Liao,a Tsu Hsing Changa and Hsiu-Fu Hsub
aDepartment of Chemistry, National Sun Yat Sen University, Kaoshiung, 804 Taiwan, ROC
bDepartment of Chemistry, Tamkang University, Tamsui, 251 Taiwan, ROC
Received 8 October 2002; revised 5 December 2002; accepted 13 December 2002
Abstract—The condensation of 1,2-bisalkoxy-4,5-diaminobenzene 2a–e derivatives with freshly prepared hexaketocyclohexane give
hexakis(alkoxy)diquinoxalino[2,3-a:2%,3%-c]phenazines in good yield. DSC and polarization microscopy showed HATOC6 has both
the crystalline (K) to mesophase (M) and the mesophase (M) to isotropic (I) phase transitions. Importantly, the alkoxy substituted
compound synthesized showed an M–I transition because of greater stability, whereas the reported alkyl-thiol decomposes after
the mesophase. © 2003 Elsevier Science Ltd. All rights reserved.
Hexaazatriphenylenes (HAT) are strong electron accep-
tors that contain three bidentate binding sites and have
been studied in the context of supramolecular
chemistry1 and photochemistry.2 Recently, a HAT hav-
ing three p-deficient pyrazine nuclei at its core was
investigated as a potential electron carrier discotic
mesogen and was anticipated to possess a low first
reduction potential that would facilitate electron injec-
tion and high electron mobility.3 Although acceptor
materials with high electron mobility are in demand,
examples of acceptor discogens are limited.4 Reported
examples of HAT based liquid crystals possess flexible
unfused polyphenyl5 or succinamide-polyphenyl6 side
chains that possess the disadvantage of introducing
excessive conformational freedom whereby long-range
three-dimensional order may be disrupted. Recently, a
fused HAT system, hexakis(alkylthio)diquinoxalino[2,3-
a:2%,3%-c]phenazine, was reported to exhibit a number of
mesophases (two to five) depending on the chain length
but these could not be unambiguously indexed.7
It has been reported that the incorporation of oxygen
into the side chain of hexakis(alkanoyloxy)benzenes has
an effect on both the crystalline to mesophase and the
mesophase to isotropic transitions.8 We now report the
successful synthesis of hexakis(alkoxy)diquinoxalino-
[2,3-a:2%,3%-c]phenazine (HATNOCn) and a preliminary
mesophase characterization of these compounds.
The commonly used method for the synthesis of the
hexaazatriphenylene (HAT) moiety involves the con-
densation of 1,2-diamino compounds with hexaketo-
* Corresponding author. E-mail: cong@mail.nsysu.edu.tw
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02862-9

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C. Ong et al. / Tetrahedron Letters 44 (2003) 1477–1480
Scheme 1. Synthetic route to HATNOCn; a: R=CH3 (n=1), b: R=C3H7 (n=3), c: R=C4H9 (n=4), d: R=C5H11 (n=5), e:
R=C6H13 (n=6).
nm and also a moderate emission at 459 nm in dilute
solution when excited at 308 nm.
cyclohexane. Furthermore, 2,3,8,9,14,15-hexachlorodi-
quinoxalino[2,3-a:2%,3%-c]phenazine, I, has been
reported to undergo further nucleophilic substitution by
an alkyl thiolate to yield the hexakis(alkylthio)di-
quinoxalino[2,3-a:2%,3%-c]phenazine, II.7 Similarly, we
have synthesized I and attempted unsuccessfully nucleo-
philic displacement of the chloride with alkoxides.
Polarizing optical microscopy (POM) and differential
scanning calorimetry (DSC) were used to check the
thermal behavior of HATNOC1, 3, 4, 5 and 6. Only
compound HATNOC6, having the hexyloxy side chain
exhibited an enantiotropic mesophase. The DSC plot
showed the crystal to liquid crystal (LC) phase transi-
tion and liquid crystal to isotropic phase transition in
the heating trace (Table 1). In the POM experiment, the
rectangular phase for compound HATNOC6 was easily
recognized by its mosaic textures and displayed a
prominent wedge-shaped defect pattern (Fig. 1). The
small enthalphies of the LC phase to isotropic transi-
tion is indicative of a disordered mesophase (Table 1).
Our results indicate that the alkoxy side chain has to
reach at least six carbon atoms in length, HATNOC6,
to attain discotic liquid crystal properties. More impor-
tantly, HATNOC6 is more stable than its reported
alkyl-thiol counterpart, decomposing at >300°C. The
alkoxy substituted diquinoxalino[2,3-a:2%,3%-c]phena-
zine, HATNOC6, has been found for the first time to
possess both transitions from the crystalline to
mesophase and from mesophase to isotropic phase. The
mesophase to isotropic transition was totally absent for
the reported alkyl-thio derivatives, which decomposed
at around 250°C.
Our synthetic route to the hexakis(alkoxy)di-
quinoxalino[2,3-a:2%,3%-c]phenazine
discotic
liquid
crystals, HATNOCn involved the condensation of 1,2-
bisalkoxy-4,5-diaminobenzenes with hexaketocyclo-
hexane.
Catechol was readily bis alkylated9 with the appropriate
alkyl bromide in high yield, and consecutive di-
nitration10 gave 4,5-bisalkoxy-1,2-dinitrobenzenes 1a–e
(Scheme 1). The time required for the hydrogenation
(H2, Pd/C; EtOH) of the dinitro- to the diamino-com-
pounds 2a–e was found to be highly dependent on the
length of the alkyl chain (6–10 h). The increasing alkyl
chain length decreases the solubility of the compounds
in ethanol, thus requiring longer reaction times. The
diamines obtained in high yield were unstable and had
to be used in situ. Condensation of 2a–e with freshly
prepared hexaketocyclohexane11 gave HATNOC1, 3, 4,
5 and 6 in yields ranging between 50 and 85%. The use
of commercially available hexaketocyclohexane gave
poor yields of the condensation products. The products
were rigorously purified by column chromatography
and thin layer preparative plate chromatography on
silica gel, followed by washing with hot acetone and
finally precipitation from chloroform using ethanol.
The XRD pattern for compound HATNOC6 at 198°C
supported the result from DSC and POM experiments.
1
Table 1. Phase behavior of HATNOC6
The H NMR chemical shifts (CDCl3) of the aromatic
protons for HATNOCn are around l 7.85, moving to a
higher field as compared to unsubstituted diquinox-
alino[2,3-a:2%,3%-c]phenazine and the hexakis(alkylthio)-
HAT derivatives. This implies that the alkoxy
substituents exhibit a greater electron-donating effect
into the discogen. These compounds, HATNOCn,
showed strong absorption peaks at 308, 341 and 432
Compound
Phase transitions
187.1 (1.32)
230.3 (0.52)
HATNOC6
K1 124.9X(15.51) K2
X
179.8 (−1.37) Colrd 226.0X(−0.52)I
92.1 (−8.81)
The transition temperatures (°C) and enthalpies (in parentheses/kJ
mol−1) were determined by DSC at 10°C/min. K, crystalline phase;
coloured, disordered rectangular columnar; I, isotropic.

C. Ong et al. / Tetrahedron Letters 44 (2003) 1477–1480
1479
Figure 1. Optical texture for HATNOC6.
,
Broad halos at ꢀ4.6 A are observed at wide angle for
Optical investigations were performed on a Nikon E600
POL with a Mettler FP90/FP82HT hot-stage system.
Transition temperatures and heats of fusion were deter-
mined at scan rates of 10°C/min by differential scan-
ning calorimetry using a Perkin–Elmer Pyris1. X-Ray
powder diffraction data were collected on the wiggler
beamline BL17A, using a triangular bent Si(111)
the mesophase indicating a disordered liquid-like orga-
,
nization. In addition, a shoulder at ꢀ3.6 A correspond-
ing to the average core-to-core correlation as also
observed. HATNOC6 was assigned
columnar phase based on the two intense low angle
peaks indexed to (110) and (200), attributed to an
a rectangular
,
monochromator and a wavelength of 1.32633 A. The
,
oblique unit cell with the parameters a=40.00 A and
sample in a 1 mm capillary was mounted on a Huber
5020 diffractometer. An air stream heater was equipped
at BL17A beamline.
,
b=19.15 A.
In summary, we have demonstrated that the het-
eroatom in the side chains of the disk like mesogen,
diquinoxalino[2,3-a:2%,3%-c]phenazine, affects both the
crystalline (K) to mesophase (M) and mesophase (M)
to isotropic (I) transition. Both the transition from
K–M and M–I was observed for the first time with
the alkoxy substituted diquinoxalino[2,3-a:2%,3%-c]-
phenazine, HATOC6. Future work will involve the
synthesis of longer alkoxy side chains of the same
family and the investigation of the charge carrier mobil-
ity of this family of compounds.
Acknowledgements
We thank the National Science Council of Taiwan for
financial support.
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General procedure for the preparation of HATNOC1, 3,
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1
HATNOC6: H NMR (CDCl3): l 7.85 (s, 6H), 4.30 (t,
12H), 2.07–1.93 (m, 12H), 1.67–1.35 (m, 36H), 0.95 (t,
18H); m/z (MALDI MS): 985.6 as a single peak. Anal.
calcd for C60H84N6O6: C, 73.14; H, 8.59; N, 8.53.
Found: C, 73.42; H, 8.35; N, 8.11%.

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