Novel discotic mesogen: Synthesis and liquid crystalline behavior of TAAB (tetrabenzo[b,f,j,n][1,5,9,13]tetraazacyclohexadecine) derivatives with long alkoxy chains

Article abstract of DOI:10.1039/a808739c

The self-condensation of 3,4-dialkoxy-2-aminobenzaldehyde (alkoxy = n-C₈H₁₇O, n-C₁₀H₂₁O and n-C₁₂H₂₅O) in the presence of HBF₄ gives the tetrafluoroborate salt of diprotonated oc

Full text of DOI:10.1039/a808739c

Novel discotic mesogen: synthesis and liquid crystalline behavior of TAAB
(tetrabenzo[b,f,j,n][1,5,9,13]tetraazacyclohexadecine) derivatives with long
alkoxy chains†
Seok Ho Kang,a Miok Kim,a Hyung-Kun Lee,a Yoon-Sok Kang,b Wang-Cheol Zinb and Kimoon Kim*a
a National Research Initiative Center for Smart Supramolecules, Department of Chemistry, and b Department of
Materials Science and Engineering, Pohang University of Science and Technology, San 31, Hyojadong, Pohang
790-784, South Korea. E-mail: kkim@postech.ac.kr
Received (in Cambridge, UK) 9th November 1998, Accepted 27th November 1998
The self-condensation of 3,4-dialkoxy-2-aminobenzalde-
hyde (alkoxy = n-C8H17O, n-C10H21O and n-C12H25O) in
the presence of HBF4 gives the tetrafluoroborate salt of
diprotonated octaalkoxy-TAAB which exhibits a hexagonal
columnar mesophase over a wide range of temperatures
including room temperature.
line-11,22-diium ion (B)‡ (Scheme 1).9 In fact, no metal-free
TAAB derivatives have been reported until now. Furthermore,
no liquid crystalline compound having a TAAB core is known.
Herein we describe the first successful synthesis of metal-free
TAAB derivatives having long alkoxy substituents on the
phenyl rings and their liquid crystalline behavior.
The synthesis of metal free, octaalkoxy-TAAB compounds
5a–c is shown in Scheme 2. 3,4-Dihydroxybenzaldehyde 1 was
alkylated with alkyl bromide (alkyl = n-octyl, n-decyl and n-
dodecyl) and subsequently converted into the corresponding
2-nitrobenzaldehyde by nitration. The key step for the synthesis
of TAAB derivatives is the reduction of the 3,4-dialkoxy-
2-nitrobenzaldehyde 3. The proper choice of reducing agent is
crucial because these reduction reactions are undoubtedly
complicated by competing inter- and intra-molecular condensa-
tion reactions. Attempts to reduce 3 with previously-used
reducing agents such as titanium(III) chloride and ferrous sulfate
gave none of the desired product, while sodium sulfide gave
only a 10% yield. However, the 3,4-dialkoxy-2-aminobenzalde-
hyde 4 could be prepared by hydrogenation of 3 with H2 and Pd/
C in 60–83% yield. In contrast to 2-aminobenzaldehyde, the
dialkoxyaminobenzaldehydes are stable at room temperature;
their stability increases with increasing alkyl chain length.
The self-condensation of 4 using HBF4 in refluxing EtOH
gives the tetrafluoroborate salt of diprotonated octaalkoxy-
TAAB (5a–c) in 14–21% yield. The products have been
characterized by NMR, UV–VIS, IR and mass spectroscopy and
Discotic liquid crystals are well-known examples of supra-
molecular assemblies based on self-organization.1 The colum-
nar arrangement of disk-like mesogens such as triphenylenes,
porphyrins and phthalocyanines is a promising architecture for
anisotropic materials with potential applications such as one-
dimensional conductors,2 photoconductors,3 molecular wires
and fibers,4 light emitting diodes5 and photovoltaic cells.6
We are interested in studying new discotic mesogens.7
Among the potential discotic mesogens we chose to examine
was the macrocyclic compound tetrabenzo[b,f,j,n][1,5,9,13]-
tetraazacyclohexadecine (TAAB). Metal complexes of TAAB
(A), which have been known for over three decades, are
synthesized by the self-condensation reaction of o-amino-
benzaldehyde in the presence of metal ions (Scheme 1).8
Without metal ion templates, however, the self-condensation
reaction produces various polycyclic compounds including the
eight-membered ring macrocyclic compound 4b,5,15,16-tetra-
hydrodibenzo[3,4:7,8][1,5]diazocino[2,1-b:6,5-b]diquinazo-
2+
gave satisfactory elemental analyses.§ Their simple 1H and 13
NMR spectral patterns are consistent with the 16-membered
C
N
N
N
N
MX2
RO
RO
RO
RO
HO
HO
M
A
i
ii
CHO
CHO
CHO
NO2
2
1
3
2X–
OR
M = CuII, NiII, ZnII etc.
OR
CHO
NH2
iii
RO
2+
RO
RO
RO
N
+HN
N
iv
CHO
NH2
H
N
NH+
OR
N+
HX
B
N+
N
OR
H
–
2BF4
2X–
RO
4
OR
HX = HClO4, HBF4, HCl etc.
5a R = C8H17
b R = C10H21
c R = C12H25
Scheme 1
† Synthetic and spectroscopic data for 2–5 and a photograph of the texture
of 5c under a polarizing microscope at 70 °C is available from the RSC web
site, see: http://www.rsc.org/suppdata/cc/1999/93
Scheme 2 Reagents and conditions; i, RBr, K2CO3, acetone; ii, HNO3,
NaNO2, CH2Cl2; iii, H2, Pd/C (10%), CH2Cl2, room temp.; iv, HBF4, EtOH,
reflux.
Chem. Commun., 1999, 93–94
93

Table 1 Phase behavior of 5a–ca
TAAB core has a saddle-shaped conformation similar to those
for metal complexes of unsubstituted TAAB reported earlier.8b
The saddle-shaped macrocycles are stacked along the column
with a mean interplanar distance of 3.39 Å. The columns are in
turn arranged in a two-dimensional hexagonal array.
Compound
Transition
K–Dhd
T/°C
DH/J g21
2.17
Lattice constant/Å
25.46
5a
5b
5c
24.3
271b
2.8
263b
22.5
249b
D
hd–I
K–Dhd
hd–I
K–Dhd
hd–I
In summary, we synthesized novel discotic liquid crystals
containing the macrocyclic TAAB core. The key to the
successful synthesis of the stable diprotonated TAAB core is the
introduction of electron–donating alkoxy substituents. The long
alkoxy chains stabilize not only the precursor 3,4-dialkoxy-
2-aminobenzaldehyde but also the diprotonated TAAB core
formed by self-condensation of the precursor. These macro-
cyclic compounds exhibit hexagonal columnar mesophases
over a wide range of temperatures including room temperature,
demonstrating that the saddle-shaped macrocyclic core may be
successfully used as a new discotic mesogen. These novel liquid
crystalline materials may have interesting physical properties
which are currently under investigation. The successful synthe-
ses of the metal-free TAABs open up new possibilities for
synthesizing a variety of metal complexes using these com-
pounds as ligands. We are also working along this line.
This work was supported by the Creative Research Initiative
Program (the Korean Ministry of Science and Technology). The
X-ray diffraction measurements were performed at the Pohang
Accelerator Laboratory (Beamline 3C2). We thank Mr Woo
Sung Jeon for molecular mechanical calculations and Professor
G. V. Smith for helpful suggestions in preparation of the
manuscript.
5.23
27.69
D
22.64
29.62
D
a Transition temperatures and enthalpies were determined by DSC (scan rate
10 °C min21). K, crystalline phase; Dhd, hexagonal columnar phase; I,
isotropic phase. b Transition observed only by polarizing microscopy.
Notes and references
Fig. 1 X-Ray diffraction pattern of 5c taken at room temperature.
‡ Old literature (ref. 8,9) refers to compound B as (TAAB)(HBF4)2 or
[(TAAB)H2][BF4], but these abbreviations are misleading because the
macrocycle core B is different from that of A.
TAAB core being in a diprotonated form. This is in sharp
contrast to the fact that the self-condensation of simple
2-aminobenzaldehyde under the same conditions yields B
having an eight-membered ring core (Scheme 1). This is the first
example of the self-condensation reaction of an o-amino-
benzaldehyde without metal ion templates leading to the
formation of the 16-membered macrocyclic TAAB core. The
electron-donating alkoxy substituents appear to stabilize the
diprotonated TAAB core in 5.
§ All the compounds have been fully characterized by 1H NMR, UV–VIS,
IR and mass spectroscopy and gave satisfactory elemental analyses.
Selected data for 5c: dH(CDCl3, 300 MHz) 0.87 (t, 24H, CH3), 1.44 [m,
144H, (CH2)9], 1.73 (m, 16H, CH2), 3.83 (q, 16H, OCH2), 6.42 (s, 4H, Ar),
6.58 (s, 4H, Ar), 7.96 (s, 4H, NNCH); dC(CDCl3, 75 MHz) 14.51, 23.13,
24.91, 26.56, 29.75, 29.89, 30.24, 30.33, 32.40, 69.39, 69.66, 99.92, 117.70,
119.87, 124.68, 138.44, 149.29, 154.71, 157.56; nmax(KBr)/cm21 1635
(CNC), 1555 (CNN); m/z (FAB) 1887.7 (M + H+ 2 2HBF4); Calc. for
C
124H214N4O8B2F8: C, 72.21; H, 10.46; N, 2.72. Found: C, 71.97; H, 10.32;
N, 2.84.
The diprotonated TAAB derivatives 5a–c show meso-
morphic behavior over a wide range of temperatures as revealed
by differential scanning calorimetry (DSC), polarizing micros-
copy and X-ray diffraction measurements (Table 1). For
example, 5c shows a columnar mesophase at room temperature
and transforms into an isotropic liquid at 249 °C. These
transitions are not observed by DSC due to the low isotropic
transition enthalpy, but are observed by polarizing microscopy.
In optical microscopy a pseudo-focal conic texture is observed,
which is typical for columnar mesophases. The compound starts
decomposing just above the clearing point.
In order to clarify the mesomorphism of these compounds,
we performed X-ray diffraction experiments at room tem-
perature. The X-ray diffraction pattern (Fig 1) of the mesophase
of 5c shows a set of sharp reflections in the small-angle region
which correspond to reciprocal spacings in a ratio of 1:A3:A4.
These peaks were assigned to the (100), (110) and (200)
reflections from a hexagonal arrangement with a lattice constant
a = 29.62 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 4–5 Å, which is related to the
liquid-like correlation between the molten aliphatic chains. The
second ring at 3.39 Å is presumably related to the periodic
stacking of the macrocyclic subunits within the columns. A
molecular mechanical calculation suggests that the diprotonated
1 S. Chandrasekar, in Advances in Liquid Crystals, ed. G. H. Brown,
Academic Press, New York, vol. 5, 1982; C. Destrade, P. Foucher, H.
Gasparoux, N-H. Tinh, A. M. Levelut and J. Malthete, Mol. Cryst. Liq.
Cryst., 1984, 106, 121; S. Chandrasekar, Liq. Cryst., 1993, 14. 3.
2 N. Boden, R. Bissell, J. Clements and B. Movaghar, Liq. Crystal. Today,
1996, 6, 1.
3 J. Simmerer, B. Glusen, W. Paulus, A. Kettner, P. Schuhmacher, D.
Adam, K. H. Etzbach, K. Siemensmeyer, J. H. Wendorff, H. Ringsdorf
and D. Haarer, Adv. Mater., 1996, 8, 815.
4 C. F. van Nostrum and R. J. M. Nolte, Chem. Commun., 1996, 2385;
E. J. Osburn, A. Schmidt, L. K. Chau, S. Y. Chen, P. Smolenyak, N. R.
Armstrong and D. F. O’Brian, Adv. Mater., 1996, 8, 926.
5 T. Christ, B. Glusen, A. Greiner, A. Kettner, R. Sander, V. Stumpflen, V.
Tsukruk and J. H. Wendorff, Adv. Mater., 1997, 9, 48.
6 C.-Y. Liu, H.-L. Pan, M. A. Fox and A. J. Bard, Science, 1993, 261,
897.
7 S. J. Kim, S. H. Kang, K.-M. Park, H. Kim, W.-C. Zin, M.-G. Choi and
K. Kim, Chem. Mater., 1998, 10, 1889.
8 (a) J. S. Skuratowicz, I. L. Madden and D. H. Busch, Inorg. Chem., 1977,
16, 1721; (b) A. J. Jircitano, R. I. Sheldon and K. B. Mertes, J. Am. Chem.
Soc., 1983, 105, 3022.
9 F. Seidel and W. Dick, Ber. Dtsch. Chem. Ges., 1927, 60, 2018; J. D.
Goddard and T. Norris, Inorg. Nucl. Chem. Lett., 1978, 14, 211; P. G.
Owston and L. S. Shaw, Acta Crystallogr., 1988, B44, 39.
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